design of comminution circuits rowland 1982

Chapter 23

SELECTION OF ROD MILLS, BALL MILLS,

PEBBLE MILLS AND REGRIND MILLS

CHESTER A. ROWLAND, JR.

MANAGER, GRINDING PROCESS DEVELOPMENT AND GRINDING MILL APPLICATIONS

ALLIS-CHALMERS CORPORATION

INTRODUCTION

Comminution is g e n e r a l l y a feed prepa- r a t i o n s t e p f o r subsequent process ing s t ages ; except ions be ing when a f i n a l product such a s agg rega t e s , s p e c i f i c a - t i o n sand, Por t land Cement, and s i m i l a r products i s produced. Grinding, t h e f i n e product phase of comminution, re- q u i r e s a l a r g e c a p i t a l investment and I. f r equen t ly i s t h e a r e a of maximum usage of power and wear r e s i s t a n t ma te r i a l s .

Grinding is most f r e q u e n t l y done i n ro- t a t i n g drums u t i l i z i n g l oose g r ind ing media, l i f t e d by t h e r o t a t i o n of t h e drum, t o break t h e o r e s i n v a r i o u s com- b ina t i ons of impact, a t t r i t i o n and abras ion t o produce t h e s p e c i f i e d product. Grinding media can be the o r e i t s e l f (autogenous g r i n d i n g -- primary and secondary), n a t u r a l o r manufactured non-metallic media (pebble m i l l i n g ) o r manufactured m e t a l l i c media -- s t e e l rods, steel o r i r o n b a l l s .

This chapter w i l l d i s c u s s gene ra l m i l l design and t h e s p e c i f i c de s ign and ap- p l i c a t i o n of t h e fo l lowing t ypes of tumbling g r ind ing m i l l s .

Overflow Rod M i l l s F igure 1 Pe r iphe ra l Discharge Rod M i l l s

F igu re s 2 and 3 393

Compartment M i l l s Rod and B a l l F igu re 6 B a l l F igu re 6a

Pebble M i l l F igu re 6 b

Overflow B a l l M i l l s F i g u r e 8 Diaphragm (Grate Discharge) B a l l

M i l l s F i g u r e 9

GENERAL MILL DESIGN

A. L ine r s

The i n t e r i o r s u r f a c e of g r i n d i n g m i l l s exposed t o g r i n d i n g media and /o r t h e m a t e r i a l be ing ground a r e p ro t ec t ed from wear and cor ro- s i o n by rubber , m e t a l l i c , a combi- n a t i o n of rubber and m e t a l l i c , o r non-metal l ic wear r e s i s t a n t mater- i a l s .

B. Dr ives

Economics a t t he time of p l a n t de- s i g n and m i l l purchase de t e rmine t h e d r i v e t o be used.

The s imp le s t d r i v e i s t h e low speed synchronous motor w i t h speeds i n t he range of 150 t o 250 RPM connected t o t he m i l l p inion- s h a f t by e i t h e r an a i r c l u t c h o r f l e x i b l e coupling.

DESIGN, INSTALLATION OF COMMINUTION CIRCUITS

Grinding m i l l s e s s e n t i a l l y draw c o n s t a n t power, t h u s a r e w e l l s u i t e d f o r u s e of synchronous mo- t o r s w i t h power f a c t o r c o r r e c t i o n c a p a b i l i t i e s a s d r i v e motors . A n e t of approximate ly 120 t o 130% of running t o r q u e i s r e q u i r e d t o cascade t h e c h a r g e i n t h e s e m i l l s . The p u l l i n t o r q u e i s a b o u t 130 t o 140%- w i t h t h e p u l l o u t t o r q u e t o keep t h e motor i n - s t e p ( in -phase) g e n e r a l l y i n e x c e s s of 150%.

When m i l l s a r e s t a r t e d a c r o s s - t h e - l i n e t h e s t a r t i n g and p u l l - i n t o r q u e s r e s u l t i n i n r u s h c u r r e n t s exceed ing 600% which r e s u l t i n p o s s i b l y h i g h v o l t a g e drops . To d e l i v e r 130% s t a r t i n g t o r q u e t o t h e m i l l t h e motor d e s i g n must t a k e i n t o a c c o u n t t h e maximum an- t i c i p a t e d v o l t a g e drop. Motor t o r q u e d e c r e a s e s a s t h e decimal f r a c t i o n of t h e v o l t a g e a v a i l a b l e squared. E.g., a motor r a t e d 160% s t a r t i n g t o r q u e w i t h a 10% system v o l t a g e d r o p w i l l d e l i v e r 160% x ( 1 0 0 % - 1 0 % ) ~ o r 129.6% t o r q u e t o

100 i t s o u t p u t s h a f t .

When i t i s n o t p o s s i b l e o r p r a c t i - c a l t o s t a r t a f u l l y loaded syn-

speed range, i f power f a c t o r tor- r e c t i o n i s n o t requ i red i n d u c t i o n motors can be used; s q u i r r e l cage when t h e r e i s no r e s t r i c t i o n on i n r u s h c u r r e n t ; s l i p r i n g when a s low s t a r t and low i n r u s h c u r r e n t i s requi red . A i r c l u t c h e s can a l - s o be used t o e a s e s t a r t i n g problems wi th s q u i r r e l cage motors.

I n some a r e a s of t h e world induc- t i o n motors and s t a r t e r s a r e l e s s expens ive t h a n synchronous motors a t a s a c r i f i c e of motor e f f i c i e n c y and power f a c t o r c o r r e c t i o n .

Dual d r i v e s , t h a t i s two p i n i o n s d r i v i n g one g e a r mounted on t h e m i l l , can become economical f o r b a l l m i l l s drawing more than 3500 t o 4000 horsepower (2600 t o 3000 k i l o w a t t s ) . A t t h i s t ime, s i n g l e p i n i o n d r i v e s w i t h r a t i n g s of 6000 horsepower i s a p r a c t i c a l l i m i t .

F u r t h e r developments of t h e low f requency , low speed synchronous motors w i t h t h e r o t o r mounted on t h e m i l l s h e l l o r a n e x t e n s i o n of t h e m i l l t r u n n i o n s could improve t h e c o s t p i c t u r e f o r t h e s e "gear- l e s s d r i v e s " , making them p r a c t i - c a l f o r l a r g e b a l l m i l l s .

chronous motor a c r o s s - t h e - l i n e i t i s p o s s i b l e t o u t i l i z e t h e m o t o r ' s C r i t i c a l speed , which i s t h e speed

a t which t h e c e n t r i f u g a l f o r c e i s pu l l -ou t t o r q u e t o s t a r t t h e m i l l . By u s i n g a c l u t c h , n o r m a l l y a n a i r s u f f i c i e n t l y l a r g e t o cause a

c l u t c h , between t h e motor and t h e s m a l l p a r t i c l e t o adhere t o t h e

m i l l , t h e motor i s brought up t o s h e l l l i n e r s f o r t h e f u l l revolu-

synchronous speed b e f o r e t h e t i o n of t h e m i l l . C r i t i c a l speed

c l u t c h i s e n e r g i z e d . I f t h e motor i s determined from t h e fol lowing:

h a s a n a d e q u a t e amount (175% o r g r e a t e r ) of p u l l - o u t t o r q u e t h e pu l l -ou t t o r q u e s t a r t s t h e m i l l w i thout major d i s r u p t i o n s of t h e e l e c t r i c a l s y s tem.

S i n c e t h e energy r e l e a s e a t i n i - t i a l c a s c a d e of t h e m i l l c h a r g e i s a n i n v e r s e f u n c t i o n of a c c e l e r a - t i o n t ime , a minimum a c c e l e r a t i o n t ime o f 6 t o 1 0 s e c o n d s o r more i s recommended t o p r e v e n t damage t o t h e m i l l o r t h e m i l l founda t ion .

Using a speed r e d u c e r between t h e motor and p i n i o n s h a f t p e r m i t s us- i n g motors hav ing speeds i n t h e range of 600 t o 1000 RPX. I n t h i s

Where D i s m i l l d iameter i n s i d e l i n e r s s p e c i f i e d i n meters .

Nc i s c r i t i c a l speed i n RPM.

When D i s s p e c i f i e d i n f e e t :

P e r i p h e r a l speed , which doesn' t i n f l u e n c e m i l l power but i s a fac- t o r i n l i n e r wear and t o a n e x t e n t media wear , h a s t o be cons idered

ROD, BALL, PEBBLE, REGRIND MILLS 395

i n m i l l design. It can be d e t e r - mined by t h e f o l l o w i n g e i t h e r a s mete rs per minute o r a s f e e t p e r minute.

where

Np = P e r i p h e r a l speed D = Diameter i n s i d e l i n e r s W = M i l l speed i n rpm

To r e l a t e c r i t i c a l speed and pe- r i p h e r a l speed a s m i l l d i a m e t e r s i n c r e a s e , t h e a v e r a g e recommended speed a s percen t of c r i t i c a l speed d e c r e a s e s a s shown i n Table I. These a r e gu ide l i n e s f o r i n i t i a l p l a n t des ign . Actua l speeds may d i f f e r from t h e s e t o s u i t s p e c i f i c o r e and economic c o n d i t i o n s t h a t a p p l y t o t h e s p e c i f i c p l a n t .

11. ROD MILLS

When g r i n d i n g t o a c o a r s e product s i z e i n t h e range of 80% p a s s i n g 2.0 mm t o 80% p a s s i n g 0.5 mm (sometimes f i n e r ) rod m i l l s a r e normally used. The f e e d s i z e can be a s c o a r s e a s 80% p a s s i n g 20 mm and a s f i n e a s 80% p a s s i n g 4 mm.

A rod m i l l i s a tumbling m i l l i n which r o d s a r e t h e g r i n d i n g media. See F i g u r e s 1, 2 and 3.

Rod m i l l s a r e u s u a l l y used i n wet g r i n d i n g a p p l i c a t i o n s . For t h e f i n e r c o a r s e g r i n d s wet over f low ( F i g u r e 1) rod m i l l s a r e used and f o r t h e c o a r s e r g r i n d s c e n t e r pe- r i p h e r a l d i s c h a r g e rod m i l l s (Fig- u r e 3 ) a r e used. The l a t t e r c a s e be ing f o r p r o d u c t s where a minimum of ex t reme f i n e s a r e d e s i r e d such a s f o r s p e c i f i c a t i o n sand.

Dry g r i n d i n g i n rod m i l l s i s gen- e r a l l y n o t recommended. Dry mate- r i a l f lows poor ly and c a u s e s rod s w e l l i n g which l e a d s t o rod break- a g e and rod t a n g l i n g . Dry rod m i l l s a r e used f o r s p e c i a l a p p l i - c a t i o n s such a s g r i n d i n g coke b r e e z e i n i r o n o r e s i n t e r i n g p l a n t s f o r g r i n d i n g cement c l i n k e r ( a n energy s a v e r bu t h i g h c a p i t a l c o s t ) . Dry g r i n d i n g rod m i l l s a r e u s u a l l y des igned f o r end p e r i p h e r - a l d i s c h a r g e ( F i g u r e 2 ) b u t c a n b e c e n t e r p e r i p h e r a l d i s c h a r g e (F ig- u r e 3). Except i n c a s e s such a s cement c l i n k e r d r y rod m i l l s a r e i n e f f i c i e n t power wise and s u b j e c t t o mechanica l problems p a r t i c u l a r - l y rod t a n g l i n g .

Table I

Average % of Critical Speed

Compeb

78-75

75-72

72-70

70-68

Mill Diameter Inside Liners

Meters

0.91-1.83

1.83-2.74

2.74-3.66

3.66-4.57

4.57-5.49

% O f Critical Speed

Feet - 3-6

6-9

9-1 2

12-15

15-18

Rod Mil 1s

76-73

73-70

70-67

67-64

-

Ball Mills

80-78

78-75

75-72

72-69

69-66

Rodpeb

75-72

72-70

70-68

- -


Grinding of coke breeze i n iron ore s in ter plants i s a spec ia l ap- p l i ca t ion . The coke usual ly has some moisture and the horsepower per tonne i s higher than calcu- lated. Swelling of the rod charges increases with increasing moisture so i t i s necessary t o use a lower rod charge than the normal 40% of m i l l volume. Table I1 i s a

FIGURE 3. Center Peripheral Discharge Rod Mill. specia l capacity table for grinding coke breeze.

Table I1

Rod M i l l i n g Coke Breeze

* Capaci ty F igures M e t r i c Tonnes Approximate f o r 28 nun x 0 Feed S ize , 3 mm x 0 Product S i ze

-

G r i ndi ng Condi t ions

a, L 2 a, w Ol VI L '7 a 0s z v a-eH In* I-IN

a, L 3 a, w Dl VI L -7 a o r z v a-ea-e 00 I-I ~7

a, L 2 aJ 0 Cn VI L 'r a o r z v = w? In In

~7

Approx. Capac i ty

(TPH ) *

5-112-6 7-8-112 10-10-1 12 14-15-112 19-112-28

22-23 24-26 29-31 31-33 34-36 36-38

9-9-112 12-13-112 17-17-112

24-25 29-30 36-37 42-43 48-50 53-54 61-63

13-112-14 19-19-112

27-28 37-38 45-46 55-57 66-67 75-76 81-82 95-97

M i l 1 S i z e

5 ' x 10 ' 5 ' x 1 2 ' 6 ' x 12 ' 7 ' ~ 1 2 ' 7 ' ~ 1 4 ' 8 ' x 13 ' 9 ' ~ 1 2 ' 9 ' ~ 1 4 '

9-112' x 13 ' 9-112' x 1 4 ' 9 - 1 / 2 ' ~ 1 5 '

5 ' x 9 ' 5 ' x 1 2 ' 6 ' x 1 2 ' 7 ' x 1 1 1 7 ' ~ 1 3 ' 8 ' ~ 1 2 ' 8 ' x 1 4 ' 9 ' x 13 ' 9 ' ~ 1 4 '

9 - 1 / 2 ' ~ 1 4 '

5 ' x 8 ' 5 ' x 1 1 1 6 ' x 12 ' 7 ' ~ 1 0 ' 7 ' x 1 2 ' 8 ' x 11' 8 ' x 1 3 ' 9 ' ~ 1 2 ' 9 ' x 13 '

9-112' x 13 '

G r i nd ing Charge

(Ki lograms)

6,800 8,200

12,100 17,100 20,100 23,800 26,300 30,900 34,600 37,300 40,100

7,600 10,400 15,300 19,300 23,000 26,700 31,300 37,700 40,800 45,800

7,800 11,100 17,500, 20,500 24,800 29,100 34,500 40,400 44,000 49,900

Horsepower Req'd.

77 93

134 205 240 299 339 398 431 466 500

7 6 103 147 207 246 304 356 407 440 516

7 3 102 159 202 245 300 356 405 440 516

Motor

7 5 100 150 200 250 300 350 400 400 450 500

7 5 100 150 200 250 3 00 350 4 00 450 500

7 5 100 150 200 250 300 350 4 00 450 500

398 DESIGN, INSTALLATION OF COMMINUTION CIRCUITS

To p r e v e n t most c o n d i t i o n s l e a d i n g t o rod c h a r g e t a n g l i n g , t h e gener - a l l y recommended r e l a t i o n s h i p of rod l e n g t h t o m i l l d i a m e t e r i n s i d e l i n e r s i s 1.4 t o 1.6. When t h i s r a t i o becomes l e s s t h a n 1.25 t h e r i s k o f t a n g l i n g i n c r e a s e s r a p i d - l y . F o r rod m i l l s l a r g e r t h a n 3.8 m e t e r s (12.5 f e e t ) i n d i a m e t e r rod a v a i l a b i l i t y and q u a l i t y have t o b e c o n s i d e r e d . T a b l e 111 g i v e s rod l e n g t h t o m i l l d i a m e t e r r a t i o s f o r t h e l a r g e r d i a m e t e r rod m i l l s .

6.8 m e t e r s (20 f e e t ) i s a p r a c t i - c a l l i m i t on t h e l e n g t h of good q u a l i t y r o d s ( t h a t i s r o d s t h a t w i l l s t a y s t r a i g h t i n t h e m i l l and w i l l b r e a k i n t o p i e c e s t h a t w i l l d i s c h a r g e from t h e m i l l when worn). T h i s l e n g t h i s a f u n c t i o n of rod q u a l i t y and p r o d u c t i o n l i m - i t s imposed by t h e rod manufac- t u r e r s .

The m i l l l e n g t h i n s i d e end l i n e r s measured a l o n g t h e s u r f a c e of t h e s h e l l l i n e r s shou ld be 0.1 t o 0.15 m e t e r s (4" t o 6 " ) l o n g e r t h a n t h e r o d s , s o t h a t t h e r o d s w i l l f i t i n t h e l e n g t h of t h e g r i n d i n g chamber w i t h o u t t i p p i n g o r l a y i n g a c r o s s t h e charge . A s l o p e a s s t e e p a s p o s s i b l e shou ld be used f o r rod m i l l head (end) l i n e r s t o p r e v e n t unsuppor ted ends of r o d s from pro- t r u d i n g from t h e charge and be ing b roken under impact from o t h e r r o d s .

The rod s p e c i f i c a t i o n s g i v e n i n T a b l e I V can be cons ide red a s a minimum s p e c i f i c a t i o n . B e t t e r rod q u a l i t y , which reduces b reakage , a l l o w s wearing t h e r o d s t o a s m a l l e r s i z e and which can reduce r o d o p e r a t i n g c o s t s , i s a v a i l a b l e . The b e t t e r q u a l i t y r o d s a r e gener- a l l y recommended when u s i n g 100 mm ( 4 " ) d i a m e t e r r o d s a n d / o r t h e l a r g e r d i a m e t e r rod m i l l s .

Table I11

Rod Mil 1 Diameter - Rod Length

I Mil 1 Diameter I Rod Inside

Meters

3.81

3.96

4.11

4.27

Meters

4.76

4.95

5.14

5.34

25 D

Feet -

15.6

16.2

16.9

17.5

Length L = 1.4

Meters

5.33

5.54

5.75

5.98

6.19

6.40

6.61

6.83

7.04

D

Feet

17.5

18.2

18.9

19.6

20.3

21.0

21.7

22.4

23.1

ROD, BALL, PEBBLE, REGRIND MILLS

Table I V

Minimum Rod S p e c i f i c a t i o n s

Gr ind ing m i l l rods should be hard enough t o remain s t r a i g h t throughout t h e i r e n t i r e l i f e , y e t they cannot be so b r i t t l e as t o break up a t coarse sizes.

When rods are t o o s o f t , t hey a r e sub jec t t o bending i n t h e m i l l . Bending causes premature breakage and tanglement o f rods. Tangled rods make m i l l c lean ing d i f f i c u l t and hazardous, and cause c o s t l y downtime.

Mater i a1 of t he f o l 1 owing chemical ana l ys i s i s recommended: I CHEMICAL ANALYSIS I Carbon

Manganese 0.60 t o 0.90% I S i l i c o n 0.15 t o 0.30% I Sulphur 0.05% Max. I Phosphorous 0.04% Max.

PHYSICAL REQUIREMENTS I Rods should a l s o have t h e f o l l o w i n g phys ica l requirements: I - Rods are t o be spec ia l commercial s t ra igh tened. I

Rods are t o be hot sawed t o l e n g t h where m i l l ( s t e e l ) f a c i l i t i e s permit . I f hot sawing i s not poss ib le , use an abras ive c u t t i n g wheel o r machine cu t b o t h ends t o proper length.

- A1 1 g r i nd ing m i l 1 rods should be 152 mm (6 inches) s h o r t e r i n l e n g t h than t h e working l eng th o f t h e rod m i l l .

The feed ends of rods wear i n t o a long tapered "spear-shaped" pro- f i l e , while the d i scha rge ends wear i n t o more of a c o n i c a l shape. Approximately t h e middle two t h i r d s of t he rod l eng th eventua l - l y wears i n t o an e l l i p t i c a l shaped sec t ion . Small p i eces of broken rods can accumulate i n t h e m i l l before being discharged. The

tapered wear and accumulation of broken rods reduces t h e bulk den- s i t y of t h e m i l l charge , and thus m i l l power.

The rod charge bulk d e n s i t y g iven i n Table V can be used t o de t e r - mine t h e power a rod m i l l w i th a worn-in charge should draw. Bulk d e n s i t y i s v a r i a b l e , s u b j e c t t o


Table V

Bulk Dens i ty Worn-In Rod Charges

New Rods

Worn-I n Charge

M i l l Diameter

Meters Feet

0.91-1.83 3-6

1.83-2.74 6-9

2.74-3.66 9-12

3.66-4.57 12-15

Bul k Densi ty KG Per Cubic Meter Lbs Per Cubic Foot

c a r e g i v e n a rod charge ( " c u l l i n g " o u t broken and t h i n r o d s ) , and ex- p e r i e n c e i n d i c a t e s m i l l d i a m e t e r a l s o has a n e f f e c t on bu lk d e n s i t y of t h e worn-in charge . The l a r g e r t h e d i a m e t e r of t h e rod m i l l t h e l e s s p r a c t i c a l " c u l l i n g " of t h e c h a r g e becomes, t h u s more broken and worn r o d s i n t h e charge reduc- i n g t h e b u l k d e n s i t y of t h e charge . Rod m i l l s normal ly c a r r y a rod charge from 35 t o 40% of m i l l vo l - ume. They c a n c a r r y up t o a 45% charge . The l i m i t s on c h a r g e l e v e l a r e : keep ing t h e f e e d end t r u n n i o n open so f e e d w i l l go i n t o t h e m i l l and keeping t h e rod charge low enough so r o d s w i l l n o t work i n t o t h e d i s c h a r g e end t r u n - n i o n opening , where t h e y can t i p and c a u s e rod t a n g l i n g .

Rod m i l l s a r e normal ly f e d by s p o u t f e e d e r s a s shown i n F i g u r e 4. To g e t t h e p r o p e r f low of f e e d i n t o t h e m i l l a minimum head of 1.5 m e t e r s ( 5 f e e t ) i s r e q u i r e d above t h e m i l l c e n t e r l i n e t o t h e bot tom of t h e f e e d hopper t o which t h e f e e d e r i s a t t a c h e d .

Heavy d u t y s i n g l e wave s h e l l li- n e r s c a s t of e i t h e r a l l o y s t e e l (manganese s t e e l i s no t recommended) o r wear r e s i s t a n t a l l o y e d c a s t i r o n a r e most f r e q u e n t l y used i n rod m i l l s . The number of l i f t e r s t o t h e c i r c l e i s u s u a l l y e q u a l t o approximately 6.6 D i n m e t e r s ( f o r D i n f e e t d i v i d e 6.6 D by 3.3). These l i n e r s have 65 mm (2.5") t o 90 mm (3.5") h i g h waves above 65 mm t o 75 mm (3") l i n e r s . Rubber backing can be used between t h e l i n e r s and s h e l l t o p r o t e c t t h e s h e l l from washing and corro- s i o n . However, w i t h rubber back- i n g c a r e must be taken wi th t h e l i n e r b o l t s p e c i f i c a t i o n s and s e a l e r assembly t o a s s u r e t h e li- n e r s w i l l s t a y t i g h t and no t move on t h e s h e l l . T h i s c r e a t e s l e a k y l i n e r b o l t s and causes t h e b o l t h o l e s i n t h e s h e l l t o wear i n t o a n e longa ted shape.

There a r e m o d i f i c a t i o n s such a s t h e two p i e c e l i n e r - l i f t e r d e s i g n t h a t can be used i n s t e a d of t h e s i n g l e wave l i n e r . Rubber s h e l l l i n e r s have been s u c c e s s f u l l y ap- p l i e d i n t h e s m a l l e r diameter rod


m i l l s running a t slow speeds. When using rubber l i n e r s c a r e must be g iven t o using good q u a l i t y rods and c u l l i n g broken and t h i n rods from t h e charge. Rubber li- n e r s can he lp reduce t h e n o i s e l e v e l emanating from a rod m i l l .

End l i n e r s a r e g e n e r a l l y a t h i c k , smooth l i n e r c a s t of a l l o y s t e e l . Impacting from t h e rod charge , which has a l a t e r a l movement i n t he m i l l , r e q u i r e s g r e a t c a u t i o n

i n us ing wear r e s i s t a n t c a s t i r o n end l i n e r s . Rubber l i n e r s can be used w i t h c a u t i o n a s t hey can be s u b j e c t t o damage from t h e sharp ends on worn rods. Except when us ing rubber l i n e r s t h e r e should be a rubber backing between t h e head l i n e r s and t h e heads. End l i n e r s should be smooth w i t h no waves o r l i f t e r s a s t h e s e can d i s - r up t rod a c t i o n and cause rod t ang l ing .

FIGURE 4. Spout Feeder


The gene ra l de s ign t r end i s t o have a minimum s lope on end liners . Some m i l l s l e s s t han 3.35 meters i n d iameter have v e r t i c a l end l i n e r s t o h e l p keep t h e rods K W ~ = s t r a i g h t i n t h e m i l l . In l a r g e r m i l l s t h e s l ope i s from 3" t o 7 " t o keep t he weight of t h e end li- n e r s t o a weight t h a t can be sup- ported i n t he m i l l . One m i l l man- u f a c t u r e r has used end l i n e r s w i th a 20" s lope .

Overflow rod m i l l s can be equipped w i th trommels t o remove broken p i ece s of rods and tramp o v e r s i z e from t h e rod m i l l d i scharge . The

The fol lowing equat ion i s used t o determine t he power t h a t a rod m i l l should draw.

where

kWr = Kilowat t s per met r ic tonne of rods (1000 kg.)

D = M i l l d iameter i n s i d e l i n e r s i n meters.

Vp = F r a c t i o n of m i l l volume loaded with rods.

C, = F r a c t i o n of c r i t i c a l - d i scha rge end of a n overflow rod speed. m i l l can be enc losed i n a housing which w i l l he lp c o n t a i n t he no i se In terms of m i l l d iameter i n f e e t and sp l a sh coming from t h e m i l l . and rod charge i n sho r t tons (2000 A door should be provided a t t h e pounds) t h e equa t ion becomes: end of t h e housing which can be opened f o r charg ing rods. Suf f i- c i e n t c l e a r space a t t h e d i s c h a r g e K W ~ = 1.07D0.34 (6.3 - 5.4 Vp) Ca (3a) end of t h e m i l l should be al lowed f o r charg ing rods. See F igu re 5.

FIGURE 5. Rod M~lI/Ball Mill Installation: shows space for rod charging


Table V I l is ts many of t h e common s i z e rod m i l l s g iv ing speed, loading and power da t a . The power i s i n horsepower a t t h e m i l l p inion- sha f t . For d i f f e r e n t l e n g t h rod m i l l s power v a r i e s d i r e c t l y a s rod length. For d i f f e r e n c e between new and worn l i n e r s i n c r e a s e power draw by 6X, and ad j u s t f o r bulk dens i ty per Table V.

The rod compartment of a rod-ball compartmented m i l l , s e e F igure 6 , i s t h e same a s a n overflow rod m i l l . When wet rod m i l l i n g a non- abras ive mineral o r m a t e r i a l t o prepare feed f o r a wet open cir-

c u i t b a l l m i l l and when t h e same d iameter m i l l s can be used mechan- i c a l l y , i t i s f e a s i b l e t o make t h e two m i l l s i n t o one mul t i - compartment m i l l . These m i l l s a r e found wet g r ind ing cement raw ma- t e r i a l and a r e a l s o used f o r g r ind ing bauxi te i n a c a u s t i c so- l u t i o n . The c a l c u l a t i o n s r e l a t i v e t o t h e rod compartment a r e t h e same a s f o r a s e p a r a t e rod m i l l . S ince i t i s harder t o r ep l ace rods t h e t i m e i n t e r v a l between adding new rods i s a month o r more, some a d d i t i o n a l reserve must be added t o a l l ow t h e rod compartment t o perform i t s work a t a lower than normal (40%) rod charge.

FIGURE 6. RodIBall (RODPEB) Compartmented Mill

Ta

ble

VI

Rod

Mil

1 P

ower

at

Mil

1 P

i nio

nsh

aft

(H

orse

pow

er)

Rod

M

il 1

D

iam

eter

M 0.91

1.

22

1.52

1.

83

2.13

2.

44

2.59

2.

74

2.89

3.20

3.

35

3.51

3.

66

3.81

3.

96

4.12

4.

27

4.42

4.

57

Ft

3.0

4.0

6.0

7.0

8.0

8.5

9.0

9.5

3.05

10.0

4.27

10

.5

11.0

11

.5

12.0

12

.5

13.0

13

.5

14.0

14

.5

15.0

Rod

M

il 1

Le

ngth

M

1.22

1.

83

5.02

.44

3.05

3.

35

3.66

3.

66

3.66

3.

96

4.57

4.

88

4.88

4.

88

5.49

5.

79

5.79

6.

10

6.10

6.

10

Ft 4 6 8 10

11

12

12

12

13

14

15

16

16

16

18

19

19

20

20

20

Rod

L

en

gth

(L

1

L/D

1.4

1.57

1.73

1.

62

1.53

1.

44

1.38

1.

41

4.1

11

3.5

1.4

41

6.8

67

.04

93

1.

47

1.50

1.

43

1.37

1.

48

1.50

1.

44

1.46

1.

41

1.36

M

1.07

1.

68

2.29

2.

90

3.20

3.

51

3.51

3.

51

3.81

4.42

4.

72

4.72

4.

72

5.34

5.

64

5.64

5.

94

5.94

5.

94

Ft

3.5

5.5

9.5

10.5

11

.5

11.5

11

.5

12.5

14.5

15

.5

15.5

15

.5

17.5

18

.5

18.5

19

.5

19.5

19

.5

Mil

l S

peed

RPM

36.1

30

.6

7.5

1.6

72

5.7

71

.23

63

23

.1

21.0

19

.4

18.7

17

.9

17.4

16.2

15

.9

15.5

15

.1

14.7

14

.3

14.0

1.3

.6 13

.3

13.0

Bul

k D

en

sity

R

od

% CS

74.5

74

.7

70.7

69

.9

69.3

69

.0

67.5

67

.6

66.4

66

.8

66.6

66

.4

66.0

65

.6

65.5

64

.9

64.6

64

.3

Cha

rge

kg/m

s

5847

58

47

5847

58

47

5766

57

66

5766

57

66

5606

56

06

5606

56

06

5606

56

06

5446

54

46

5446

54

46

5446

54

46

Rod

Cha

rge

Wei

ght

FPM

284

336

399

428

457

470

470

483

501

517

528

538

547

555

565

570

579

586

- lb

/ft3

365

365

365

365

360

360

360

360

350

350

350

350

350

350

340

340

340

340

340

340

Me

tric

Ton

nes

Mil

l P

ower

% V

olu

me

tric

L

oa

din

g

35

1 40

1

45

7 1 8 23

25

26

57

61

11

4 12

2 12

8 18

1 19

4 20

4 27

5 29

5 31

0 31

8 34

1 35

9 34

4 36

9 38

8 41

6 44

6 47

0 50

7 54

4 60

9 65

3 68

7 73

5 78

8 82

9 81

9 87

8 92

4 90

6 97

2 10

23

1093

11

1234

12

64 1

356

1426

13

85 1

486

1562

15

80 1

695

1783

% 35

1.0

2.25

6.

91

13.1

20

.0

29.0

33

.0

36.0

42

.7

51.5

61

.4

72.5

79

.7

82.7

10

4 12

0 13

0 14

7 15

9 17

1

Sh

ort

Ton

s

Dia

(D

) In

sid

e

New

% 35

1.1

2.48

7.

62

14.4

22

.0

32.0

36

.4

39.7

47

.1

56.8

67

.7

79.9

87

.8

91.1

11

5 13

2 14

3 16

2 17

5 18

8

M 0.76

1.

07

641.

37

1.68

1.

98

2.29

2.

44

2.55

2.

70

5722

.85

3.00

3.

15

3.31

3.

46

3.61

3.

76

3.92

4.

07

4.22

4.

37

1935

20

91

1715

18

53

Vo

lum

etr

ic

40

1.13

2.

58

7.95

15

.0

22.8

33

.2

37.7

41

.1

48.8

59

.0

70.1

82

.8

90.7

99

.8

119

137

148

169

181

194

Lin

ers

F

t

2.5

3.5

4.5

5.5

6.5 7.5

8.0

8.35

8.

85

9.35

9.

85

10.3

5 10

.85

11.3

5 11

.85

12.3

5 12

.85

13.3

5 13

.85

14.3

5 18

40

1988

Lo

ad

ing

45

1.27

2.

9 8.

89

16.8

25

.6

37.4

42

.5

45.5

54

.9

63.8

78

.9

93.5

10

3 11

2 13

4 15

4 16

6 19

0 20

4 21

9

Vo

lum

etr

ic

40

1.25

2.

84

8.76

16

.5

25.1

36

.6

41.6

45

.3

53.8

65

.0

77.3

91

.3

100

110

131

151

163

186

200

214

Lo

ad

ing

45

1.4

3.2

9.8

18.5

28

.2

41.2

46

.8

50.1

60

.5

70.3

87

.0

103

1.13

12

3 14

8 17

0 18

3 20

9 22

5 24

1


The v a r i o u s rod m i l l m a n u f a c t u r e r s have d i f f e r e n t e q u a t i o n s f o r de- t e r m i n i n g t h e power rod m i l l s draw, b u t a l l come c l o s e t o t h e same c a l c u l a t e d power draw, as g i v e n i n T a b l e V I .

BALL MILLS

When g r i n d i n g t o i n t e r m e d i a t e pro- d u c t s i z e s i n t h e r a n g e of 80% p a s s i n g 0.5 mm t o 80% p a s s i n g 7 5 mic romete r s and t o f i n e p r o d u c t s , t h o s e f i n e r t h a n 80% p a s s i n g 7 5 m i c r o m e t e r s , b a l l m i l l s a r e used e x c e p t when d r y g r i n d i n g m a t e r i a l s

which a r e c o n s i d e r e d t o be non- a b r a s i v e , where r o l l e r m i l l s (Fig- u r e 7 ) c a n be more economical ly used. A b a l l m i l l i s a tumbling m i l l i n which m e t a l l i c b a l l s o r s p e c i a l shapes a r e t h e g r i n d i n g media , s e e F i g u r e s 8 and 9 .

B a l l m i l l s c a n be used f o r e i t h e r wet g r i n d i n g o r d r y g r i n d i n g . Normally e i t h e r fo l lowing u n i t p r o c e s s e s o r t h e m a t e r i a l i t s e l f w i l l de te rmine whether wet o r d ry g r i n d i n g shou ld be used.

FIGURE 7 Roller Mill


FIGURE 8. Overflow Ball Mill.

FIGURE'S. D~aphragrn (Grate) D~scharge Ball MIII, Dry Grinding Type


The f e e d t o d r y g r i n d i n g b a l l m i l l s must b e d r y c o n t a i n i n g l e s s t h a n 1% m o i s t u r e by we igh t . There i s a l o s s i n e f f i c i e n c y when t h e f e e d c o n t a i n s s u f f i c i e n t m o i s t u r e t o s low down t h e f l o w r a t e o r c a u s e c o a t i n g of t h e g r i n d i n g media a n d / o r m i l l l i n e r s . Drying c a n be accomplished i n one of t h e f o l l o w i n g ways :

A. S e p a r a t e d r y e r - r o t a r y - f l u o - s o l i d s , e t c .

B. Drying i n t h e m i l l by drawing h o t g a s e s t h r o u g h t h e m i l l w i t h a p a r t i a l o r t o t a l a i r sweeping.

C. Combined d r y e r and b a l l m i l l w i t h t h e d r y e r b e i n g t h e f i r s t compartment and t h e b a l l m i l l t h e second compartment. The b a l l m i l l compartment i s a i r swept because g a s e s from t h e d r y e r a r e p u l l e d t h r o u g h i t .

D. Drying i n t h e a i r s e p a r a t o r where t h e f e e d goes t o t h e a i r s e p a r a t o r . Hot g a s e s a r e drawn t h r o u g h t h e a i r s e p a r a - t o r d r y i n g t h e new f e e d . Drying a l s o o c c u r s i n t h e b u c k e t e l e v a t o r used t o con- vey t h e m i l l d i s c h a r g e , which i s h o t , and t h e new f e e d t o t h e a i r s e p a r a t o r .

Except f o r f u l l y a i r swept m i l l s , d r y g r i n d i n g b a l l m i l l s a r e sup- p l i e d w i t h low l e v e l d i s c h a r g e d i - aphragms ( F i g u r e 1 0 ) . With a i r swept and p a r t i a l a i r swept m i l l s , t h e a i r volume and v e l o c i t y w i l l be t h a t r e q u i r e d t o c a r r y t h e c o a r s e s t p a r t i c l e s i z e d e s i r e d from t h e m i l l . C l a s s i f i c a t i o n t o c l o s e t h e m i l l c i r c u i t i s u s e d , t h e r e f o r e t h e l a r g e s t p a r t i c l e t o b e swept from t h e m i l l w i l l be l a r g e r t h a n t h e d e s i r e d p r o d u c t s i z e . Diaphragm d i s c h a r g e m i l l s w i l l r e q u i r e a s u f f i c i e n t a i r draw t o keep t h e m i l l under n e g a t i v e p r e s s u r e t o p r e v e n t d u s t from l e a k i n g from t h e m i l l a round t h e f e e d e r and d i s c h a r g e hous ing . Dry

g r i n d i n g b a l l m i l l s u s e spou t f e e d e r s ( s e e F i g u r e 4 ) w i t h a i r s e a l s . A rule-of-thumb f o r d e t e r - min ing t h e a i r r e q u i r e d f o r d u s t c o n t r o l i s :

Closed C i r c u i t 5.5 c u b i c m e t e r s p e r hour per horsepower of m i l l power.

Open C i r c u i t 5.0 c u b i c m e t e r s p e r hour per horsepower of m i l l power.

Being f r e e of t h e l i m i t s imposed o n rod m i l l s by t h e rods , b a l l m i l l s have more v a r i a t i o n s i n l e n g t h t o d i a m e t e r r a t i o s , r ang ing from s l i g h t l y less t h a n 1:l t o some g r e a t e r t h a n 2 : l . There a r e n o f i x e d r u l e s on t h e proper L/D r a t i o s t o u s e a s t h e s e v a r y w i t h t h e c i r c u i t used , o r e type , f e e d s i z e and o v e r a l l g r i n d i n g r e q u i r e - ments . T a b l e VII g i v e s some rough g u i d e l i n e s showing, based upon p a s t e x p e r i e n c e , t h e g e n e r a l L/D r a t i o s used i n t h e a p p l i c a t i o n of b a l l m i l l s .

Depending upon t h e s i z e of make-up b a l l s used , a d v e r s e b a l l segrega- t i o n , t h a t i s l a r g e b a l l s going t o t h e d i s c h a r g e end and smal l b a l l s t o t h e f e e d end , c a n occur a s t h e L/D becomes l a r g e r . Th i s b e g i n s t o occur a s t h e make-up b a l l s i z e i s l a r g e r t h a n 65 mm (2.5").

Gr ind ing b a l l s can be made of f o r g e d o r c a s t s t e e l o r c a s t i r o n . The q u a l i t y depends upon t h e s o u r c e of supp ly . While n o t a l - ways t r u e , f r e q u e n t l y t h e b e t t e r q u a l i t y b a l l s a r e fo rged s t e e l . G e n e r a l l y b a l l s a r e s p h e r i c a l bu t t h e y c a n b e i n v a r i o u s c y l i n d r i - c a l , c o n i c a l o r o t h e r i r r e g u l a r shapes . B a l l s v a r y c o n s i d e r a b l y i n h a r d n e s s w i t h s o f t b a l l s having B r i n n e l l h a r d n e s s e s i n t h e range o f 350 t o 450, and t h e ha rd b a l l s have h a r d n e s s e s i n e x c e s s of 700. A rule-of-thumb s u b j e c t t o argu- ment is: " t h e h a r d e r t h e b a l l t h e b e t t e r i t s l i f e " (p rov ided i t i s n o t t o o b r i t t l e and b reaks o r becomes t o o h i g h l y po l i shed and t o o smooth t o n i p t h e m a t e r i a l being g round) .


Local economics and grinding a p p l i c a t i o n the deciding f a c t o r s

t h e s p e c i f i c gene ra l ly a r e

i n s e l e c t i n g the b a l l s t o use. The b a l l s giv- ing the lowest ope ra t ing c o s t and best performance a r e gene ra l ly se- lec ted . This need not be t he low- e s t priced b a l l s a v a i l a b l e nor t he ones g iv ing t h e lowest wear r a t e , but can be a compromise between the two extremes.

Bal l s should be s o l i d wi th a rea- sonably uniform hardness through t h e e n t i r e ba l l . They should wear i n a r e l a t i v e l y uniform pa t t e rn . An i n d i c a t o r of good b a l l wear i s when the worn b a l l s d ischarging from the m i l l a r e around 16 mm (518") o r smal ler i n s i z e and a r e

polygon shaped having a s many a s 8 t o 12 s u r f a c e s , which can be s l i g h t l y concave. Evidence of broken b a l l s i s found when p i eces of b a l l s a r e being d ischarged, some a s c i r c u l a r d i s c s , some a s h a l f rounds, some c re scen t shaped. P i eces of worn o r broken b a l l s w i th h o l e s i n them i n d i c a t e poor q u a l i t y b a l l s wi th sand i n c l u s i o n s and/or blow ho le s and/or hollow c e n t e r s .

For c a l c u l a t i n g the power t h a t a b a l l m i l l w i l l draw, forged s t e e l and c a s t s t e e l b a l l s a r e assumed t o weigh 4646 kilograms per cub ic meter (290 pounds per cubic f o o t ) w i th c a s t i r o n b a l l s weighing 4165 kg pe r m3 (260 pcf) .

ON PLATE)

! R ING

HE AD

FIGURE 10. Discharge Diaphragm


Table V I I

Ball Mil 1 L I D Ratio - Application General Guidelines

1 Type o f G r i nd ing

Wet

Wet

Wet o r Dry

Wet o r Dry

D ry

D ry

Feed 80% Passing S ize

M i c r o m t e r s

5,000 t o 10,000

900 t o 4,000

F i n e Feeds - Regr ind

F ine Feeds - Open C i r c u i t

5,000 t o 10,000

900 t o 4,000

Top B

M i l l i m e t e r

60 t o 90

40 t o 50

20 t o 30

20 t o 50

60 t o 90

40 t o 50

11 S ize

Inches

2.5 t o 3.5

1.8 t o 2.0

I 314 t o 1-114

314 t o 2

2.5 t o 3.5

1.8 t o 2.0

LID Ra t i o

B a l l m i l l s n o r m a l l y c a r r y a b a l l m i l l s o p e r a t i n g i n c l o s e d c i r c u i t c h a r g e occupy ing from 40 t o 45% of w i t h a i r c l a s s i f i e r s o r mechanical t h e m i l l volume, b u t c a n c a r r y up a i r s e p a r a t o r s . t o a 50% o r s l i g h t l y h i g h e r charge . F i g u r e 11 shows t h e rela- On open c i r c u i t wet o r d r y g r i n d - t i o n s h i p of m i l l power and vo lu - i n g drum f e e d e r s a s shown on Fig- m e t r i c l o a d i n g . F o r p l a n t capa- u r e 13 can be used. c i t y and d e s i g n p u r p o s e s , b a l l m i l l s a r e f r e q u e n t l y s e l e c t e d based upon c a r r y i n g 40% b a l l c h a r g e s w i t h t h e m i l l s and d r i v e s des igned t o c a r r y h i g h e r c h a r g e s i f r e q u i r e d .

Spout f e e d e r s used w i t h wet g r i n d - i n g m i l l s a l l o w arrangement where t h e c l a s s i f i e r underf low f lows by g r a v i t y i n t o t h e spou t f e e d e r hop-

However, d r y g r i n d i n g b a l l m i l l s n o r m a l l y c a r r y from a 35 t o 40% b a l l c h a r g e w i t h a i r swept m i l l s c a r r y i n g e v e n a lower c h a r g e n e a r e r t o 30% of m i l l volume.

P l a n t d e s i g n , c a p i t a l c o s t and o p e r a t i n g c o s t s a l l i n f l u e n c e b a l l m i l l f e e d e r s e l e c t i o n . When a 45 t o 50% b a l l c h a r g e i s t o be c a r - r i e d i n a wet g r i n d i n g b a l l m i l l , g e n e r a l l y a d o u b l e scoop f e e d e r a s shown i n F i g u r e 1 2 i s used. T h i s i s a more e x p e n s i v e f e e d e r t h a n a s p o u t f e e d e r as shown i n F i g u r e 4 , which c a n a l s o b e used t o f e e d b a l l m i l l s . When a wet g r i n d i n g b a l l m i l l i s c l o s e d c i r c u i t e d w i t h a r a k e o r s p i r a l c l a s s i f i e r , a scoop f e e d e r i s r e q u i r e d t o f e e d t h e c i r c u l a t i n g l o a d i n t o t h e m i l l . Wi th c y c l o n e c l a s s i f i e r s s p o u t f e e d e r s c a n be used. Spout f e e d e r s a r e used on d r y g r i n d i n g

pe r . T h e r e f o r e , t h e cyc lone c l a s - s i f i e r s must be i n s t a l l e d h i g h enough t o o b t a i n t h e head r e q u i r e d f o r t h i s f low i n t o t h e hopper. T h i s can r e s u l t i n h i g h pumping h e a d s and pump power t o pump t h e m i l l d i s c h a r g e t o t h e cyc lone c l a s s i f i e r s .

F i g u r e 14 shows a s i n g l e s t a g e b a l l m i l l i n s t a l l a t i o n u s i n g dou- b l e scoop f e e d e r s w i t h cyc lone c l a s s i f i e r s i n s t a l l e d a t a b o u t t h e h o r i z o n t a l c e n t e r l i n e of t h e m i l l . Depending upon t h e r a d i u s and w i d t h of t h e scoops and t h e capa- c i t y ( i n c l u d i n g c i r c u l a t i n g l o a d ) doub le scoop f e e d e r s consume from 2 5 t o 30 k i l o w a t t s (30 t o 40 horsepower) . T h i s arrangement reduces pumping head and power con- s i d e r a b l y and must be ba lanced a g a i n s t h i g h e r maintenance c o s t f o r t h e scoop f e e d e r .


60 -

2b i5 i0 i5 4b 45 50 i5 66b

% OF MILL VOLUME OCCUPIED BY BALL OR PEBBLE CHARGE

FIGURE1 1. Grinding Mill Power vs Loading.

FIGURE 12. Single Scoop Feeder with Ball Charging Drum. (Double Scoop is available)

ROD, BALL, PEBBLE, REGRIND MILLS 41 3

Make-up g r i nd ing b a l l s a r e fed t o t h e m i l l a s r equ i r ed through t h e m i l l f e ede r with t h e m i l l i n oper- a t i on . Ba l l s may feed d i r e c t l y through a spout , bu t should not be fed i n t o a scoop box because of pos s ib l e jamming and s e r i o u s m i l l damage. Scoop f e e d e r s u s u a l l y have a c e n t r a l b a l l feed p ipe o r a smal l b a l l charging drum t o accom- p l i s h t h i s .

There a r e many d i f f e r e n t de s igns and s t y l e s of b a l l m i l l l i n e r s . A s wi th g r i nd ing b a l l s l o c a l economics and u l t i m a t e l y ope ra t i ng c o s t s determine t h e b e s t de s ign and m a t e r i a l t o use. The i n i t i a l s e t of l i n e r s i s r a r e l y t h e f i n a l des ign s e l ec t ed . Based upon i nd i - v i d u a l exper ience , m i l l super in- t enden t s develop p r e f e r ences f o r l i n e r des igns . The fo l lowing i s g iven a s a gu ide f o r t h e i n i t i a l s e t of l i n e r s .

A. For 65 mm (2.5") and smaller top s i z e b a l l s f o r c a s t me t a l l i n e r s use double wave l i n e r s w i th t h e number of l i f t e r s t o t h e c i r c l e approximately 13.1 D i n meters ( f o r D i n f e e t d i v i d e 13.1 D by 3.3). Wave he igh t from 40 t o 65 mm (1- 518" t o 2-112") above t he li- n e r t h i cknes s , l i n e r t h i ck - n e s s i s from 40 t o 50 mm (1- 518" t o 2"). Rubber l i n e r s of t h e i n t e g r a l molded de s ign fo l l ow t h e c a s t meta l des ign . I f using t h e r e p l a c e a b l e l i f t e r bar de s ign i n e i t h e r meta l o r rubber t h e number of l i f t e r s should be about 3.3 D i n me t e r s ( f o r D i s i n f e e t d i v i d e 3.3 D by 3.3) w i th t h e l i f t e r he igh t above t h e l i n e r s about tw ice t h e l i n e r t h i cknes s .

The use of double wave li- n e r s , p a r t i c u l a r l y when u s ing 50 mm (2 " ) o r l a r g e r b a l l s , may show a l o s s of 5% o r so

i n t h e m i l l power draw u n t i l t h e waves wear i n and t h e b a l l s can n e s t between t h e l i f t e r s . When l i n e r s , and

double wave l i n e r s i n p a r t i - c u l a r , wear w i th circumferen- t i a l grooves, s l i p p i n g of t h e charge i s i n d i c a t e d , and t h i s warns of a c c e l e r a t e d wear. When t h e top s i z e b a l l i s s m a l l e r than 65 mm (2.5") and m i l l speed i s less than 72% of c r i t i c a l wear r e s i s t a n t c a s t i r o n s can be used. For o t h e r cond i t i ons a l l o y e d c a s t steel i s recommended.

Rubber l i n e r s a r e w e l l s u i t e d t o t h i s same a r e a and n o t on ly reduce o p e r a t i n g c o s t s bu t can reduce n o i s e l e v e l s . Due t o t h e t empe ra tu r e of t h e f e ed and t h e h e a t gene ra t ed i n g r i nd ing going i n t o t h e f e e d , t h e t empe ra tu r e s i n d r y g r i n d i n g m i l l s a r e t o o h igh t o a l l ow t h e u se of r ubbe r l i n e r s .

B. S i n g l e wave l i n e r s a r e recommended f o r l a r g e r s i z e b a l l s (60 mmI2.5" and l a r g e r ) . The number of t h e l i f t e r s t o t h e c i r c l e equa l s app rox ima te ly 6.6 D i n me t e r s ( f o r D i n f e e t d i v i d e 6.6 D by 3.3). The l i n e r s a r e from 50 t o 65 mm t h i c k (2" t o 2.5") w i t h t h e waves from 60 t o 75 mm (2.5" t o 3") above t h e line r s . The r e p l a c e a b l e l i f t e r ba r de s ign made of e i t h e r me t a l o r rubber i n abou t t h e same de s ign p r o p o r t i o n s can b e used. There cou ld be a l o s s i n power w i t h rubbe r li- n e r s of up t o 10%. Rubber l i n e r s a r e no t recommended when t h e b a l l s i z e i s l a r g e r t han 75 mm.

Because of t h e impact ing from t h e l a r g e b a l l s , s i n g l e wave l i n e r s f o r b a l l m i l l s a r e u s u a l l y made from a l l o y e d


s t e e l o r s p e c i a l wear r e s i s t - a n t a l l o y e d c a s t i r o n s . Be- cause of t h e d i f f i c u l t y of ba lanc ing growth and wear w i t h work h a r d e n i n g , manganese s t e e l i s used i n f r e - q u e n t l y and t h e n w i t h extreme c a r e t o a l l o w f o r growth.

There a r e c a s e s where double wave l i n e r s have been used a s replacement l i n e r s f o r s i n g l e wave l i n e r s . T h i s r e q u i r e s a s t u d y of wear p a t t e r n s m i l l power, c a p a c i t y and o p e r a t i n g c o s t s .

C. C l a s s i f y i n g l i n e r s have been used i n b a l l m i l l s t o pu t t h e l a r g e r b a l l s a t t h e feed end and t h e s m a l l e r b a l l s a t t h e d i s c h a r g e end. S p i r a l s h e l l l i n e r s such a s shown i n Fig- u r e 9 have been used success - f u l l y . The s p i r a l i s a n ad- vancing s p i r a l .

The s q u a r e m i l l l i n e r a l s o known a s a c l a s s i f y i n g l i n e r g i v e s a s q u a r e c o n f i g u r a t i o n t o t h e i n s i d e of t h e m i l l . There a r e a s e r i e s of o f f s e t t i n g c i r c u m f e r e n t i a l rows which r e t a r d t h e movement of b a l l s and a l l o w a b e t t e r ma- t e r i a l f low and f i l l i n g of t h e b a l l charge . There have been s u c c e s s e s and f a i l u r e s w i t h t h i s t y p e of l i n e r . It does r e d u c e m i l l volume and cause a r e d u c t i o n i n t h e power drawn by t h e m i l l . I n some c a s e s a r e d u c t i o n i n power p e r tonne h a s o c c u r r e d , improving g r i n d i n g e f f i c i e n - cy. I n o t h e r c a s e s t h e r e h a s been a c o r r e s p o n d i n g reduc- t i o n i n c a p a c i t y w i t h t h e re- d u c t i o n i n power draw.

D. End l i n e r s f o r b a l l m i l l s conform t o t h e s l o p e of t h e m i l l head and c a n be made of r u b b e r , a l l o y e d c a s t s t e e l o r w e a r - r e s i s t a n t c a s t i r o n . To p r e v e n t r a c i n g and e x c e s s i v e wear end l i n e r s f o r b a l l m i l l s a r e f u r n i s h e d w i t h in -

t e g r a l r a d i a l r i b s o r with r e p l a c e a b l e l i f t e r s o r with both.

E. When a g r a t e d i scharge i s used, t h e g r a t e s and wear p l a t e s a r e normally perpendi- c u l a r t o t h e m i l l a x i s whi le t h e d i s c h a r g e pans conform t o t h e s l o p e of t h e m i l l head. The g r a t e s and wear p l a t e s a r e normally made from a l - loyed wear - res i s tan t c a s t s t e e l o r rubber. They a r e r ibbed t o prevent rac ing and e x c e s s i v e wear. The d i s - c h a r g e r s and pans a r e gener- a l l y made from e i t h e r wear- r e s i s t a n t c a s t i r o n o r rub- b e r , o r wear - res i s tan t f a b r i - c a t e d s t e e l .

S l o t plugging can be a problem i n g r a t e d i s c h a r g e m i l l s . Whether t h e g r a t e s a r e made of meta l o r rubber t h e s l o t s should have ample r e l i e f t apered toward t h e d i scharge s ide . T o t a l a n g l e s 7 t o 10 degrees (3.5 t o 5 degrees per s i d e ) a r e commonly used. Metal g r a t e s o f t e n have a s m a l l l ead- in pocket o r re- c e s s which can f i l l i n w i t h peened meta l r a t h e r than have t h e s l o t peen shu t . With t h e proper combination of meta l i n t e r n a l s and rubber s u r f a c e s , rubber g r a t e s have f l e x i b i l i t y t h a t tend t o make them s e l f c lean ing and y e t n o t f a i l due t o f l ex ing . Rubber g r a t e s cannot be used i n d r y g r i n d i n g b a l l m i l l s .

F. Except when using rubber li- n e r s , t h e m i l l s u r f a c e s a r e covered w i t h a p r o t e c t i v e rubber o r p l a s t i c m a t e r i a l t o p r o t e c t t h e s u r f a c e s from pulp r a c i n g and cor ros ion . T h i s i s done i n wet g r i n d i n g m i l l s . S ince dry gr ind ing m i l l s g e t h o t due t o h e a t from g r i n d i n g rubber l i n e r s and rubber m a t e r i a l s u s u a l l y cannot be used. In dry


gr ind ing m i l l s no backing ma- t e r i a l i s used, however, a p l a s t i c m a t e r i a l s i m i l a r t o t h a t used t o s e t concaves i n a primary c r u s h e r o r concave r i n g s i n a cone c ru she r ; o r z inc i s used t o f i l l t h e space between l i n e r s .

The fol lowing e q u a t i o n i s used t o determine the power t h a t wet g r i nd ing overf low b a l l m i l l s should draw.

where

kWb = Kilowat t s per m e t r i c tonne of b a l l s (1000 kg).

D = M i l l d iameter i n s i d e l i n e r s i n meters.

V p = Frac t i on of m i l l volume loaded w i th b a l l s .

Cs = Frac t i on of c r i t i c a l speed. Ss = B a l l s i z e f a c t o r .

I n terms of m i l l d i ame te r i n f e e t and power per s h o r t t o n a t (2000 pounds) of b a l l charge Equat ion 4 becomes:

For m i l l s l a r g e r t han 3.3 me te r s (10 f e e t ) diameter i n s i d e l i n e r s t h e top s i z e of t h e b a l l s used a f - f e c t s t h e power draw by t h e m i l l . Th i s i s c a l l e d t h e b a l l s i z e fac- t o r S,.

where

B = B a l l s i z e i n m i l l i m e t e r s D = M i l l d iameter i n s i d e

l i n e r s i n meters . Ss = Kilowat t s per m e t r i c

tonne of b a l l s .

I n terms of b a l l s i z e i n i nches and m i l l d iameter i n f e e t and power pe r s h o r t t on of b a l l charge e q u a t i o n 5 becomes:

To de te rmine t h e power t h a t a wet g r i n d i n g , low l e v e l g r a t e d i s - charge m i l l should draw m u l t i p l y kWb by 1.16 and f o r a d ry gr ind- i n g , f u l l g r a t e d i s c h a r g e m i l l m u l t i p l y by 1.08.

These same equa t i ons a r e used t o c a l c u l a t e t h e power t h a t each b a l l m i l l compartment of a mu l t i - compartment b a l l m i l l should draw. The t o t a l power i s t h e sum of t h e power c a l c u l a t e d f o r each of t h e s e p a r a t e compartments.

Fo r s p e c i a l a p p l i c a t i o n s such a s wet g r i n d i n g cement raw m a t e r i a l s , b a u x i t e i n c a u s t i c s o l u t i o n s and o t h e r c l ay - l i ke m a t e r i a l s c o n s u l t t h e m i l l manufac turers , s i n c e t h e s e m a t e r i a l s a f f e c t t h e power draw by b a l l m i l l s .

Tab le V I I I l i s t s e s s e n t i a l l y "square" b a l l m i l l s g i v i n g m i l l speed a s pe r cen t of c r i t i c a l , weight of a 40% b a l l cha rge , t o p b a l l s i z e and c a l c u l a t e d power draw. B a l l m i l l power changes i n d i r e c t p ropo r t i on t o m i l l l e n g t h . The power i s horsepower a t t h e m i l l p i n i o n s h a f t i n s i d e new s h e l l l i n e r s . I nc r ea se power f o r worn s h e l l l i n e r s by 6%. There a r e in - d i c a t i o n s t h a t rubber l i n e r s may cause from a 5 t o 10% l o s s i n m i l l power.

The v a r i o u s b a l l m i l l manufac- t u r e r s have d i f f e r e n t e q u a t i o n s f o r de te rmin ing t he power b a l l m i l l s draw, bu t a l l come c l o s e t o t h e same c a l c u l a t e d power draw.

Opera t ing d a t a a v a i l a b l e f o r wet g r i n d i n g b a l l m i l l s 5.49 m e t e r s (18.0 f e e t ) d iameter i n s i d e s h e l l , show t h e m i l l s w i l l draw t h e c a l - c u l a t e d power, however, when t h e

Tab

le V

III

Bal

l M

ill

Pow

er a

t M

ill

Pin

ion

shaf

t (H

orse

pow

er)


b a l l charge exceeds 35% of m i l l volume t h e m i l l s d o n o t use t h e power f o r e f f i c i e n t g r i n d i n g . A b a l l charge between 32 and 35% i n 5.49 mete rs (18.0 f e e t ) d i a m e t e r wet g r i n d i n g b a l l m i l l s seems t o be a maximum t o o b t a i n a n e f f i - c i e n t use of power. The same problem has n o t occur red i n d r y g r i n d i n g b a l l m i l l s of t h e same d iameter .

IV. COMPARTMENTED MILLS

Compartmented m i l l s have two o r more compartments us ing b a l l s , pebbles , o r rods a s g r i n d i n g media combining two o r more g r i n d i n g s t a g e s i n t o one u n i t . These m i l l s a r e p a r t i c u l a r l y popula r i n t h e cement i n d u s t r y , bu t c a n be used wherever s t a g e m i l l i n g i s r e q u i r e d wi thout a n i n t e r m e d i a t e s e p a r a t i o n o r c l a s s i f i c a t i o n s t e p . R e c e n t l y , p e r i p h e r a l d i s c h a r g e s have reap- peared i n t h e Cement I n d u s t r y per- m i t t i n g d i s c h a r g i n g ground m a t e r i - a l from each compartment f o r s t a g e c l a s s i f i c a t i o n between t h e s t a g e s of t h e compartmented m i l l s t o reduce overgr ind ing .

The Rodpeb M i l l , F i g u r e 6 , i s a wet g r i n d i n g multi-compartment m i l l w i t h rods i n t h e f i r s t compartment and b a l l s i n t h e second compartment. S e v e r a l minor com- promises a r e necessary when s p e c i - f y i n g t h e m i l l . The d i a m e t e r i s t h e same f o r bo th compartments. The o p e r a t i n g speed i n t e rms of p e r c e n t of c r i t i c a l speed i s a compromise being on t h e h i g h s i d e f o r rod m i l l i n g and t h e low s i d e f o r b a l l m i l l i n g . Rodpeb m i l l s a r e most f r e q u e n t l y o p e r a t e d i n open c i r c u i t . The u s u a l a p p l i c a - t i o n i s t o g r i n d m a t e r i a l s w i t h low a b r a s i v e c h a r a c t e r i s t i c s . Adding replacement r o d s and c lean- i n g out broken r o d s and worn r o d s i s more t ime consuming t h a n i n rod m i l l s . The two p r i n c i p a l a p p l i c a - t i o n s have been f o r wet g r i n d i n g raw m a t e r i a l s f o r cement making k i l n s and g r i n d i n g of b a u x i t e i n c a u s t i c s o l u t i o n s .

The Compartmented B a l l M i l l , Fig- u r e 6 a c o n s i s t s of two o r more g r a t e d i s c h a r g e b a l l m i l l s i n se- r i e s b u i l t i n t o one r o t a t i n g assembly. Power drawn by t h e s e m i l l s c a n be as h i g h a s 12,000 HP. M i l l d i a m e t e r s a r e a s l a r g e a s 5.5 m e t e r s (18.0 f e e t ) . M i l l l e n g t h s a r e a s g r e a t a s 15 m e t e r s (50.0 f e e t ) . The most f r e q u e n t use i s f o r d r y g r i n d i n g cement c l i n k e r t o produce P o r t l a n d Cement. They c a n a l s o be used f o r d r y g r i n d i n g cement raw m a t e r i a l s when i t i s n o t economical o r i s n o t p r a c t i c a l t o use r i n g r o l l e r m i l l s ( s e e F i g u r e 7 ).

Multi-Compartment Dryer - M i l l . ( F i g u r e 1 5 ) T h i s c i r c u i t u t i l i z e s - - a t h r e e compartment m i l l w i t h c l a s s i f i c a t i o n between t h e gr ind- i n g s t a g e s . The d r y e r ( f i r s t com- par tment ) i s l o c a t e d i n t e r n a l t o t h e l a r g e d i a m e t e r t r u n n i o n bear- i n g and m a t e r i a l i s " f l a s h d r i e d " by h i g h volume a i r swept g a s e s . Dryer l i f t e r s a r e s p e c i a l l y de- s i g n e d t o promote d r y i n g e f f i c i e n - cy. A Double R o t a t o r m i l l sepa- r a t e s t h e d r y e r and c o a r s e g r ind- i n g compartment from t h e f i n e g r i n d i n g compartment by a per iph- e r a l d i s c h a r g e . T h i s d i s c h a r g e i s l a r g e enough t o h a n d l e h i g h volume of v e n t g a s e s w i t h r e l a t i v e l y h i g h g r a i n l o a d i n g and t h e m a t e r i a l p roduc t of b o t h compartments.

Double R o t a t o r m i l l s h a n d l e mois- t u r e s up t o 7 % w i t h p r e h e a t e r k i l n e x h a u s t g a s e s and up t o 14 t o 15% w i t h h i g h t e m p e r a t u r e g a s e s . G r i t s e p a r a t o r s a r e u t i l i z e d t o remove t h e c o a r s e m a t e r i a l b e f o r e e n t e r - i n g a c y c l o n e c o l l e c t o r which pre- c e d e s t h e baghouse o r e l e c t r o s t a t - i c c o l l e c t o r . A i r s e p a r a t o r re- j e c t s a r e r e t u r n e d t o t h e f i n e g r i n d i n g compartment o r , i f de- s i r e d , a p o r t i o n c a n be r e t u r n e d t o t h e c o a r s e g r i n d i n g end of t h e m i l l .

Wet g r i n d i n g compartmented b a l l m i l l s a r e r a r e l y used. The p r i n - c i p a l use i s f o r wet open g r i n d - i n g of cement raw m a t e r i a l s i n wet


A

FIGURE 15. Doublerotator Mill

process cement p l an t s . Because of h igh f u e l consumption very few wet cement p l a n t s a r e being b u i l t to- day. The same type of m i l l can be used f o r wet open c i r c u i t g r i n d i n g of h igh grade i r o n o r e o r coa r se i r o n o r e concen t r a t e s t o make feed f o r p e l l e t i z i n g . (The more common i r o n o r e p e l l e t i z i n g feed app l i ca - t i o n s a r e d ry g r ind ing open c i r - c u i t m i l l s . ) The main purpose f o r t h e multi-compartment m i l l i s t o mainta in b a l l s eg rega t ion by use of a d i v i s i o n head and t o h e l p pulp f low through t h e m i l l .

Wet g r ind ing of cement raw mater i - a l s i n a compartmented b a l l m i l l has proven a good a p p l i c a t i o n f o r g r ind ing a i d s designed t o make a s heavy a s p o s s i b l e s l u r r y t o reduce t h e amount of water t o be d r i v e n o f f i n t h e k i l n .

Dry g r ind ing compartmented b a l l m i l l s can c a r r y up t o 50% by volume b a l l charges but more f r e - quen t ly a r e used wi th 30 t o 35% b a l l charges , a n a r e a where t h e g r ind ing e f f i c i e n c y seems t o be b e s t . Depending upon m i l l performance and t h e d r i v e l i m i t a - t i o n s , d i f f e r e n t b a l l charges by

percent loading can be ca r r i ed i n d i f f e r e n t compartments t o balance t h e grinding work a s required i n each compartment.

To determine t h e power t h a t multi- compartment m i l l s draw use the equat ions t o c a l c u l a t e the power t h a t rod and b a l l m i l l draws. When using a combined dryer and gr inding m i l l , add the power required f o r a r o t a r y dryer of t h e same diameter and length running a t the same speed a s the m i l l . Note normally r o t a r y dryers run a t cons iderably lower speeds.

V. PEBBLE MILLS

Within t h e g l a s s , ceramic, cleans- e r s and chemical i n d u s t r i e s t h e r e a r e requirements p r inc ipa l ly f o r d ry gr inding m i l l s where t he re i s l i t t l e , i f any, i r o n contamination due t o l i n e r and media wear. These m i l l s a r e ca l l ed pebble m i l l s and use e i t h e r na tu ra l peb- b l e s o r manufactured pebbles o r s l u g s a s g r ind ing media. The m i l l s a r e l i n e d wi th s i l e x blocks o r wi th ceramic l i n e r s . See Fig- u r e 6 b . The l i n i n g can e i t h e r be smooth o r can be made t o have


Table I X

Pebble M i l 1s Power

l i f t e r s . Ceramic l i n e r s can be made i n va r ious wave p a t t e r n s . These m i l l s have d i s cha rge d i a - phragms normally made from wear r e s i s t a n t a l loyed c a s t i r o n s o r s t e e l s . The power t h a t t h e s e m i l l s draw i s a func t i on of t h e bulk d e n s i t y of t h e media, which i s a func t i on of t h e shape of t h e media.

Table I X shows t h e power drawn by some t y p i c a l s i z e pebble m i l l s based upon f l i n t pebbles from France. M i l l 40 t o 45% load ing of media. Power i s d i r e c t l y propor- t i o n t o l e n g t h and v a r i e s a s t h e d iameter i n s i d e l i n e r s t o t h e 2.3 power. It i s recommended t o r e f e r s p e c i f i c a p p l i c a t i o n problems t o t h e m i l l manufac turers .

Size o f M i 11

7 ' x 1 0 ' 7 ' x 12 ' 7 ' x 14 ' 7 ' x 1 6 ' 7 ' x 18 ' 7 ' x 20 ' 7 ' x 2 2 '

8 ' x 1 2 ' 8 ' x 1 4 ' 8 ' x 1 6 ' 8 ' x 1 8 ' 8 ' x 2 0 1 8 ' x 22 ' 8 ' x 2 4 '

9 ' x 1 4 ' 9 ' x 16 ' 9 ' x 18 ' 9 ' x 2 0 ' 9 ' x 2 2 ' 9 ' x 24 ' 9 ' x 2 6 '

1 0 ' x 1 6 ' 1 0 ' x 18 ' 1 0 ' x 20 ' 1 0 ' x 2 2 ' 10 ' x 2 4 ' 10 ' x 2 6 '

11' x 18 ' 11' x 2 0 ' 11' x 2 2 ' 11' x 2 4 ' 11' x 2 6 ' 11 ' x 2 8 '

4 "

H.P.

7 1 84 9 7

111 124 137 151

120 140 159 178 197 217 235

188 213 239 264 291 315 342

277 310 343 378 409 444

392 434 478 517 561 603

L i n i n g M i l l R.P.M.

23.0 23.0 23.0 23.0 23.0 23.0 23.0

21.0 21.0 21 .O 21.0 21.0 20.0 21.0

19.6 19.6 19.6 19.6 19.6 19.6 19.6

18.6 18.6 18.6 18.6 18.6 18.6

17.6 17.6 17.6 17.6 17.6 17.6

3 "

H.P.

77 92

106 120 135 149 165

128 149 170 191 213 234 255

198 227 255 284 312 340 367

293 329 367 403 439 474

414 462 507 553 596 642

1-1/4"

H.P.

82 100 117 134 151 168 185

139 163 186 210 234 258 282

215 246 277 309 341 372 403

315 355 396 436 477 518

444 495 546 597 647 696

L i n i n g M i l l R.P.M.

22.6 22.6 22.6 22.6 22.6 22.6 22.6

20.6 20.6 20.6 20.6 20.6 20.6 20.6

19.3 19.3 19.3 19.3 19.3 19.3 19.3

18.3 18.3 18.3 18.3 18.3 18.3

17.3 17.3 17.3 17.3 17.3 17.3

L i n i n g M i l 1 R.P.M.

22.0 22.0 22.0 22.0 22.0 22.0 22.0

20.0 20.0 20.0 20.0 20.0 20.0 20.0

19.0 19.0 19.0 19.0 19.0 19.0 19.0

18.0 18.0 18.0 18.0 18.0 18.0

17.0 17.0 17.0 17.0 17.0 17.0


P e b b l e m i l l s can be used f o r e i t h e r open o r c l o s e d c i r c u i t g r i n d i n g . Normally t h e s e a p p l i c a - t i o n s c a l l f o r d r y g r i n d i n g . When d r y g r i n d i n g t o t h e v e r y f i n e s i z e r a n g e s a l l p a s s i n g 1 0 m i c r o n s o r f i n e r some m a t e r i a l s d e v e l o p f l u i d f l o w c h a r a c t e r i s t i c s . I n t h e s e c a s e s t h e m i l l s h o u l d have a n o v e r f l o w d i s c h a r g e t h e same a s i n wet m i l l s . There a r e c a s e s where wet g r i n d i n g i s used p a r t i c u l a r l y when making f e e d f o r f l o t a t i o n o r some o t h e r wet b e n e f i c i a t i o n method.

V I . SECONDARY AUTOGENOUS MILLS (PEBBLE MILLS)

Secondary au togenous g r i n d i n g

a r e des igned f o r a 45 t o 50% volu- m e t r i c charge l o a d but normal o p e r a t i n g l e v e l i s around 40%.

Pebb le m i l l s a r e good a p p l i c a t i o n s f o r rubber and wear r e s i s t a n t c a s t i r o n l i n e r s . Rubber l i n e r s usual- l y have s e p a r a t e l i f t e r s w h i l e t h e wear r e s i s t a n t c a s t i r o n and c a s t s t e e l l i n e r s a r e of t h e i n t e g r a l s i n g l e wave d e s i g n . With rubber t h e r e can be up t o a 10% l o s s i n t h e power drawn by t h e m i l l w i t h a cor respond ing l o s s i n c a p a c i t y .

The feed f o r secondary autogenous m i l l s i s normal ly e i t h e r rod m i l l p roduc t o r pr imary autogenous m i l l p roduc t . There a r e a few in- s t a n c e s where i t h a s been b a l l

m i l l s , a l s o r e f e r r e d t o a s p e b b l e m i l l p roduc t and one c a s e where i t m i l l s , u s e p e b b l e s from t h e o r e a s g r i n d i n g media. The s i z e d o r e c a n e i t h e r be s c r e e n e d o u t of t h e o r e s t r e a m i n a c r u s h i n g p l a n t o r c a n be e x t r a c t e d f rom a p r imary a u t o g - enous m i l l . They a r e used i n s t e a d of a b a l l m i l l when t h e p e b b l e s i n t h e o r e make good g r i n d i n g media and t h e wear r a t e w i l l be l e s s t h a n t h e r a t e a t which p e b b l e s c a n be o b t a i n e d . P e b b l e s a r e f e d t o t h e m i l l i n a c o n t r o l l e d manner t o m a i n t a i n a c o n s t a n t power draw. It i s g e n e r a l l y a n i n t e r r u p t e d f e e d w i t h t h e on-off f r e q u e n c y g r e a t enough t o r e q u i r e a u t o m a t i o n of t h e p e b b l e f e e d i n g system. The a v e r a g e p e b b l e s i z e r a n g e i s from 40 mm (1 .5" ) t o 75 mm ( 3 " ) .

Secondary a u t o g e n o u s m i l l s gener - a l l y a r e used f o r wet c l o s e d c i r - c u i t g r i n d i n g . To g i v e a f l o w g r a d i e n t t h r o u g h t h e m i l l t h e y have low l e v e l g r a t e d i s c h a r g e s . T h i s p r e v e n t s t h e t endency f o r s m a l l p e b b l e s and p e b b l e c h i p s t o f l o a t on t h e s l u r r y i n t h e m i l l , r e d u c i n g g r i n d i n g e f f i c i e n c y .

M i l l power i s a f u n c t i o n o f t h e b u l k d e n s i t y o f t h e p e b b l e s and of t h e s i z e of t h e o r e media (peb- b l e s ) .

M i l l speed i s u s u a l l y be tween 70 and 75% of c r i t i c a l speed . M i l l s

was t h e p roduc t of a n i n t e r m e d i a t e au togenous m i l l (lump m i l l ) . An i n t e r m e d i a t e autogenous m i l l is a secondary t y p e m i l l f e d a c o a r s e f e e d ( r o d m i l l f eed s i z e o r s l i g h t l y l a r g e r ) which i s ground by p e b b l e s i n t h e 100 mm (4" ) t o 175 mm (7" ) s i z e range.

V I I . REGRIND MILLS

B a l l m i l l s which a r e used t o g r i n d rougher c o n c e n t r a t e s , midd l ings , f i n a l c o n c e n t r a t e s o r t a i l i n g s a r e c a l l e d r e g r i n d m i l l s ; because t h e y a r e f e d m a t e r i a l t h a t has been ground, t h e n t r e a t e d i n one o r more p r o c e s s i n g s t e p . R e l a t i v e l y s m a l l s i z e b a l l s a r e used. They c a n be used i n open o r c l o s e d c i r - c u i t . The d i s c u s s i o n s on b a l l m i l l s f i t s r e g r i n d m i l l s .

V I I I . ORE TESTING FOR MILL SELECTION

A f t e r t h e g r i n d requ i rements a r e e s t a b l i s h e d , t e s t i n g f o r t h e se- l e c t i o n of comminution c i r c u i t s and m i l l s i z e c a n be i n i t i a t e d and c a n depending upon t h e a p p l i c a t i o n i n c l u d e a l l o r some of t h e fol low- i n g :

- Pr imary Autogenous Media Competency

- Pr imary Autogenous and Semi-Autogenous P i l o t P l a n t

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5. F i n e s from e a c h c l a s s i - f i c a t i o n s t a g e .

k. Power drawn by e a c h m i l l (mo- t o r i n p u t ) .

1. Motor and d r i v e e f f i c i e n c y of e a c h m i l l .

m. S i z e of and t y p e of g r i n d i n g media used i n e a c h m i l l .

n. Speed i n rpm f o r e a c h m i l l . o. L i n e r d e s i g n and c o n d i t i o n i n

e a c h m i l l . p. Media wear r a t e .

q. L i n e r wear r a t e i f t e s t i s r u n l o n g enough t o o b t a i n li- n e r wear d a t a .

r . % c i r c u l a t i n g l o a d f o r e a c h s t a g e t h a t i s c l o s e c i r - c u i t e d .

During g r i n d i n g tes ts o b t a i n sam- p l e s of m i l l f e e d f o r g r i n d a b i l i t y tests s o t h a t work i n d i c e s c a l c u - l a t e d from t h e p i l o t p l a n t d a t a c a n be compared t o g r i n d a b i l i t y t e s t r e s u l t s . O p e r a t i n g work i n - d i c e s c a n be o b t a i n e d u s i n g p i l o t p l a n t d a t a i n t h e f o l l o w i n g equa- t i o n :

where

W i o = O p e r a t i n g Work I n d e x W = kwh p e r t o n ( c a n be

m e t r i c , s t a n d a r d , o r l o n g )

P = P r o d u c t s i z e which 80% p a s s e s i n mic romete r s .

F = Feed s i z e which 80% p a s s e s i n mic romete r s .

The a p p l i c a t i o n of g r i n d i n g c i r - c u i t and e q u i p m e n t - r e l a t e d f a c t o r s d i s c u s s e d l a t e r a r e a p p l i e d t o Wio t o p u t i t on t h e same b a s i s a s g r i n d a b i l i t y tes t r e s u l t s . T h i s a l l o w s a d i r e c t c o m p a r i s o n of p i - l o t p l a n t test r e s u l t s and g r i n d - a b i l i t y tes t r e s u l t s . I n a d d i t i o n t o t h e s e , b e s u r e t o a p p l y motor and d r i v e e f f i c i e n c y f a c t o r s s o t h a t t h e p i l o t p l a n t m i l l power

d a t a i s r e f e r r e d t o t h e m i l l pin- i o n s h a f t o r t o t h e m i l l s h e l l (measured power d a t a i s g e n e r a l l y e l e c t r i c a l energy i n t o t h e motor) . N e t power f o r p i l o t p l a n t m i l l s i s o b t a i n e d by o b t a i n i n g t h e t a r e power f o r t h e m i l l a t t h e begin- n i n g and end of t h e t e s t pe r iod . T h i s s u b t r a c t e d from t h e g r o s s power measured g i v e s n e t power. T h i s c a n be used t o g e t n e t power p e r ton. To c o n v e r t n e t power p e r t o n t o power a t t h e p i n i o n s h a f t add 2.5% t o t h e n e t power p e r t o n .

With t h e d i f f i c u l t y i n o b t a i n i n g a c c u r a t e p i l o t p l a n t d a t a , p a r t i - c u l a r l y power d a t a , Bond c l o s e d c i r c u i t g r i n d a b i l i t y t e s t r e s u l t s o f t e n g i v e t h e more a c c u r a t e d a t a f o r s e l e c t i n g rod and b a l l m i l l s . G r i n d a b i l i t y test work should span t h e feed and p roduc t s i z e s of t h e proposed g r i n d i n g c i r c u i t . The g r i n d a b i l i t y t e s t work g e n e r a l l y recommended f o r pr imary m i l l i n g c i r c u i t s i n c l u d e s :

A. Bond rod m i l l g r i n d a b i l i t y t e s t s a t 1 0 o r 1 4 mesh f o r Work Index.

B. For e a c h b a l l m i l l g r i n d i n g s t e p , a Bond b a l l m i l l g r ind- a b i l i t y t e s t a t one mesh s i z e c o a r s e r t h a n t h e t o p s i z e t h a t w i l l produce t h e d e s i r e d 80% p a s s i n g s i z e and a t t h e mesh s i z e , o r j u s t f i n e r t h a n t h e mesh s i z e , t h a t w i l l produce t h e d e s i r e d 80% p a s s i n g s i z e .

C. I f 50 mm x 7 5 mm ( 2 " x 3") o r e lumps a r e a v a i l a b l e a n impact c r u s h i n g Work Index t e s t .

D. I f 30 mm x 20 mm (1-114" x 314") o r e i s a v a i l a b l e a n a b r a s i o n index test.

F o r r e g r i n d b a l l m i l l i n g o r b a l l m i l l i n g of rougher c o n c e n t r a t e s produced i n t h e g r i n d i n g c i r c u i t r u n b a l l m i l l g r i n d a b i l i t y t e s t s a s o u t l i n e d i n B above. S i n c e t h e


Bond g r i n d a b i l i t y t e s t r e q u i r e s a r a t i o of r e d u c t i o n of a b o u t 6 : l t o o b t a i n a c c u r a t e r e s u l t s i t may be necessary t o run t h e t e s t a t a f i n e r s i z e than r e q u i r e d by t h e s p e c i f i e d gr ind o r i f t h i s cannot be done a s p e c i a l r e g r i n d gr ind- a b i l i t y t e s t w i l l have t o be run.

This w i l l g ive a good c r o s s sec- t i o n of t h e g r i n d a b i l i t y of t h e o r e and w i l l a l l o w f o r a c c u r a t e c a l c u l a t i o n s of t h e g r i n d i n g power requ i red .

The ba lance of t h i s d i s c u s s i o n w i l l be a n example demons t ra t ing t h e s e l e c t i o n of a primary gr inding c i r c u i t and a r e g r i n d c i r c u i t where a l l of t h e o r e i s ground t o t h e requ i red produc t s i z e i n t h e primary c i r c u i t . For g r i n d i n g c i r c u i t s where c o n c e n t r a t i o n i s inc luded i n t h e c i r c u i t t h e b a s i c approach i s t h e same a s g i v e n i n t h e example, c o n s i d e r i n g e a c h s t a g e a s a s e p a r a t e e n t i t y and ad- j u s t i n g f o r new f e e d r a t e s and feed s i z e (which c o u l d be d i f f e r - e n t t h a n t h e r a t e and produc t s i z e from t h e preceding s t a g e ) .

IX. EQUATIONS USED TO DETERMINE GRINDING POWER

T h i s f i r s t s t e p i n s e l e c t i n g g r i n d i n g m i l l s i s t o d e t e r m i n e t h e power needed t o p roduc t t h e de- s i r e d g r i n d . The b a s i c e q u a t i o n used f o r t h i s is t h e Bond Equa- t ion .

where

W = Kwh p e r s h o r t ton. Wi = Work Index. P = Product s i z e i n microns

which 80% passes . F = Feed s i z e i n microns

which 80% p a s s e s .

Note: W c a n be p e r m e t r i c tonne i f t h e Work Index i s on t h e b a s i s of m e t r i c tonnes .

The power determined from e q u a t i o n 7 i s f o r t h e f o l l o w i n g s p e c i f i c c o n d i t i o n s .

A. Rod M i l l i n g - wet , open c i r - c u i t g r i n d i n g i n a 2.44 mete r ( 8 ' ) d i a m e t e r i n s i d e l i n e r s rod m i l l .

B. B a l l M i l l i n g - w e t , c l o s e d c i r c u i t g r i n d i n g i n a 2.44 mete r ( 8 ' ) d i a m e t e r i n s i d e l i n e r s b a l l m i l l .

C. Power c a l c u l a t e d i s t h e power r e q u i r e d a t t h e p i n i o n s h a f t o f t h e m i l l , which i n c l u d e s m i l l b e a r i n g s and g e a r p i n i o n l o s s e s b u t does n o t i n c l u d e motor l o s s e s o r l o s s e s i n any o t h e r d r i v e component, s u c h a s r e d u c e r s and c l u t c h e s .

The f e e d f o r Bond G r i n d a b i l i t y T e s t s is:

Rod m i l l i n g o r e c rushed t o minus 13,200 micrometers (0.530") o r f i n e r . B a l l m i l l i n g o r e c rushed t o minus 3,350 micrometers ( 6 mesh) o r f i n e r which have been used t o es- t a b l i s h optimum rod and b a l l m i l l f e e d s i z e s .

There a r e e i g h t e f f i c i e n c y f a c t o r s t o be a p p l i e d t o t h e c a l c u l a t e d g r i n d i n g power t o a l l o w f o r v a r i a - t i o n s from t h e s p e c i f i e d condi- t i o n s and optimum f e e d s i z e s .

EF1 Dry Grinding.

EF2 Open C i r c u i t B a l l M i l l i n g .

EF3 Diameter E f f i c i e n c y F a c t o r .

EFq Overs ized Feed.

EFg F i n e Gr ind ing i n b a l l m i l l s t o p roduc t s i z e s f i n e r t h a n 80% p a s s i n g 200 mesh ( 7 5 micrometers ) .

EFg High o r low r a t i o of reduc- t i o n rod m i l l i n g .


EF7 Low R a t i o of r e d u c t i o n b a l l m i l l i n g .

EF8 Rod M i l l i n g .

The m u l t i p l i e r s f o r t h e e f f i c i e n c y f a c t o r s a r e d e t e r m i n e d by t h e f o l - lowing:

EFl - Dry G r i n d i n g - f o r t h e same r a n g e of work, d r y g r i n d i n g r e q u i r e s 1 . 3 t i m e s a s much power a s wet g r i n d i n g .

EF2 - Open C i r c u i t Gr ind ing -when g r i n d i n g i n open c i r c u i t b a l l m i l l s , t h e amount of e x t r a power r e q u i r e d , com- p a r e d t o c l o s e d c i r c u i t b a l l m i l l i n g , i s a f u n c t i o n of t h e d e g r e e of c o n t r o l re-

NOTE: In s e l e c t i n g m i l l s f o r new o p e r a t i o n s , where t h i s fac- t o r i s l e s s t h a n 1.0 i t i s sometimes n e g l e c t e d and i s used a s a s a f e t y f a c t o r y . In t h e example i t w i l l be a p p l i e d .

EFq - Oversized Feed - when being f e d a c o a r s e r than optimum f e e d , t h i s f a c t o r a p p l i e s t o rod m i l l i n g and b a l l m i l l - ing . However, t h e most f r e - quen t u s e i s found i n con- j u n c t i o n w i t h s i n g l e s t a g e b a l l m i l l i n g . T h i s i s t h e one e f f i c i e n c y f a c t o r t h a t i s r e l a t e d t o Work Index a s i s s e e n i n t h e fo l lowing e q u a t i o n :

q u i r e d on t h e p r o d u c t produced. The i n e f f i c i e n c y f a c t o r s f o r open c i r c u i t g r i n d i n g a r e g i v e n i n T a b l e X. The b a s i c c a l c u l a t i o n i s t o t h e d e s i r e d 80% p a s s i n g s i z e p e r t h e Bond E q u a t i o n a f t e r which t h e EF2 f a c t o r i s a p p l i e d .

EF3 - Diamete r E f f i c i e n c y F a c t o r - u s i n g t h e b a s e m i l l d i a m e t e r of 2.44 m e t e r s ( 8 ' ) i n s i d e l i n e r s , t h e d i a m e t e r e f f i - c i e n c y f a c t o r c a n be c a l c u - l a t e d from t h e f o l l o w i n g :

When D i s i n m e t e r s :

When D i s i n f e e t :

T a b l e X I g i v e s a t a b u l a t i o n of t h e EF3 f a c t o r s f o r some o f t h e more common m i l l d i - a m e t e r s i n b o t h t h e i m p e r i a l and m e t r i c measur ing sys - tems.

F where R, = Ratio of reduction = - ( 1 0 )

P

Fo = Optimum feed s i z e Rod m i l l i n g : FO = 16,000(%)aJ

B a l l mil1ing:Fo = 4000 13 O" (12) ( W I 1

Use t h e Work Index from a g r ind- a b i l i t y t e s t a t t h e d e s i r e d g r i n d f o r W i i n e q u a t i o n 9. For equa- t i o n 11, u s e e i t h e r t h e Work Index from a Bond impact t e s t o r a rod m i l l g r i n d a b i l i t y t e s t , whichever i s h i g h e r . For e q u a t i o n 12, u s e t h e Work Index from a rod m i l l g r i n d a b i l i t y t e s t , s i n c e t h i s more r e p r e s e n t s t h e c o a r s e f r a c t i o n of t h e feed ; i f n o t a v a i l a b l e then u s e t h e b a l l m i l l g r i n d a b i l i t y t e s t r e s u l t s .

EF5 - F i n e n e s s o f Grind F a c t o r - t h i s a p p l i e s t o f i n e g r ind- i n g when t h e 80% p a s s i n g s i z e o f t h e p roduc t i s f i n e r t h a n 75 micrometers (200 mesh). The e q u a t i o n t o de- t e r m i n e t h i s is :


Table X

Open Circuit Inefficiency Multiplier

E F g - High o r Low Ra t io of Reduc- t i o n Rod Mi l l i ng - t h e equa- t i o n to be used , u n l e s s Rr i s between Rro = -2 and Rro = +2 is:

Product Size Control Reference % Passing

50

6 0

70

8 0

9 0

9 2

95

9 8

where R,, = 8 + D (15)

Inefficiency Multiplier

1.035

1.05

1.10

1.20

1.40

1.46

1.57

1.70

L = Rod Length

This f a c t o r always a p p l i e s t o low r a t i o s of r educ t ion . Although i t s a p p l i c a t i o n t o h igh r a t i o s of r educ t ion i s not always needed, i t should be u t i l i z e d f o r m i l l s i z e s e l e c t i o n whenever W i from t h e rod m i l l and b a l l m i l l g r i n d a b i l i t y tests exceed 7.0.

EF7 - Low Ratio of Reduction B a l l M i l l - the need t o use t h i s

f a c t o r does no t occur very o f t e n as i t on ly a p p l i e s t o b a l l m i l l i n g when the Ra t io of Reduction i s less than 6 . Thi s shows up p a r t i c u l a r l y i n r eg r ind ing c o n c e n t r a t e s and t a i l i n g s . The equa t ion f o r t h i s is:

E F ~ =2 (R, - 1.35) + 0.26 2 (R, - 1.35) (16)

E F g - Rod Mi l l i ng - a s tudy of rod m i l l o p e r a t i o n s shows t h a t rod m i l l performance i s a f - f e c t e d by t h e a t t e n t i o n g iven t o p r e p a r a t i o n and f eed ing i n a uniform top s i z e feed s i z e t o t h e m i l l and t h e c a r e g i v e n t o main- t a i n i n g t he rod charge. Th i s e f f i c i e n c y f a c t o r has n o t been d e f i n i t e l y d e t e r - mined. I n s e l e c t i n g rod m i l l s based upon power ca l - c u l a t e d from g r i n d a b i l i t y t e s t s , t h e fo l l owing proce- du re h a s been recommended:


1 ) When c a l c u l a t i n g rod m i l l power f o r a rod- mi l l ing-only appl ica- t i on , use a n inef f i c iency f a c t o r of 1.4 when t he f eed i s t o be prepared w i th open c i r c u i t c ru sh ing , and use 1.2 when t h e feed i s t o be prepared w i th c lo sed c i r c u i t c rush ing . The m i l l d i ame te r , low o r high r a t i o of r educ t i on , and o v e r s i z e feed fac- t o r s a l s o must be ap- p l i e d t o t h e c a l c u l a t e d g r i nd ing power.

2 ) When c a l c u l a t i n g rod m i l l power f o r a rod m i l l - b a l l m i l l c i r c u i t , d o n o t a l l ow f o r im- provement i n t h e b a l l m i l l performance due t o r e c e i v i n g rod m i l l feed. I f t h e rod m i l l feed i s produced w i t h open c i r - c u i t c ru sh ing , app ly a 1.2 i n e f f i c i e n c y f a c t o r t o t h e power c a l c u l a t e d f o r t h e rod m i l l i n g s t a g e on ly . I f t h e rod m i l l f eed w i l l cons i s - t e n t l y be t h e same, such a s produced w i th c lo sed c i r c u i t c ru sh ing , do n o t apply a rod m i l l i n e f f i - c iency f a c t o r . The m i l l d i ame te r , low o r h igh r a t i o of r e d u c t i o n , and o v e r s i z e feed f a c t o r s should be a p p l i e d t o t h e c a l c u l a t e d g r i nd ing power.

X. EXAMPLE G R I N D I N G POWER CALCULA- TIONS AND GRINDING MILL SELECTION

PROBLEM NO. 1

S e l e c t rod m i l l s , b a l l m i l l s and pebble m i l l s a s r e q u i r e d f o r t h e fo l lowing c i r c u i t s .

- Rod M i l l - B a l l M i l l - S i n g l e S tage B a l l M i l l - Rod M i l l - Pebble (Secondary

Autogenous) M i l l

- B a l l M i l l t o Grind Primary Semi-Autogenous M i l l Product

- Regrind B a l l M i l l

Feed r a t e t o t he primary m i l l c i r - c u i t i s 500 m e t r i c tonnes pe r hour i nc lud ing f a c t o r f o r a v a i l a b i l i t y . Feed r a t e t o r eg r i nd m i l l i s 40 m e t r i c tonnes pe r hour.

Rod m i l l f eed and feed f o r s i n g l e s t a g e b a l l m i l l w i l l be prepared w i t h c lo sed c i r c u i t c ru sh ing . The feed s i z e s f o r t he v a r i o u s m i l l s w i l l be:

- Rod Mi l l i ng : minus 25 mm w i th 80% pass ing 18 mm.

- S i n g l e s t a g e B a l l M i l l : minus 12 mm 80% pass ing 9.4 mm.

- B a l l M i l l and Pebble M i l l f o l - lowing Rod M i l l and B a l l M i l l f o l l owing Primary Autogenous o r Semi-Autogenous M i l l : minus 2 mm 80% pass ing 1.2 mm.

- Regrind B a l l M i l l : 80% pas s ing 210 micrometers .

The c i r c u i t s a r e a l l wet g r i n d i n g type . A l l b a l l o r pebble m i l l s a r e c l o s e d c i r c u i t w i t h t h e excep- t i o n of t h e r eg r ind m i l l which w i l l be open c i r c u i t f o r t h i s example.

Pebble s i z e f o r pebble m i l l i n g p l u s 30 mm minus 70 mm w i t h a peb- b l e consumption of 30 m e t r i c t onnes pe r hour which i s 6% of t h e c i r c u i t p roduct ion r a t e .

The s p e c i f i e d g r i n d s a r e : p r imary g r i n d i n g c i r c u i t 80% pas s ing 175 micrometers , r e g r i n d c i r c u i t 80% pas s ing 45 micrometers .

Bench s c a l e g r i n d a b i l i t y t e s t re- s u l t s a r e t o be used f o r g r i n d i n g power c a l c u l a t i o n s . The t e s t re- s u l t s t o be used i n t h e example a r e :

Bond Impact Crushing .............. Work Index 11.5 Rod M i l l G r i n d a b i l i t y T e s t ......... (Wi) a t 10 mesh 13.2


B a l l M i l l G r i n d a b i l i t y T e s t (Wi) a t 6 5 mesh ......... 11.7 ........ (Wi) a t 100 mesh 12.1 (Wi) a t 325 mesh o n r e g r i n d m i l l f e e d .... 14.0 ........ A b r a s i o n Index (Ai) 0.215

The f o l l o w i n g d e m o n s t r a t e s t h e u s e of t h e Bond Work I n d e x Method t o d e t e r m i n e t h e power r e q u i r e d t o produce t h e d e s i r e d g r i n d . A f t e r t h e g r i n d i n g power h a s been d e t e r - mined by t h i s o r o t h e r methods, t h e m i l l ( s ) t h a t w i l l draw t h e re- q u i r e d power c a n be s e l e c t e d . For f i n a l m i l l s i z e recommendat ions a l l t h e p r o c e s s d e s i g n d a t a and c o n t r o l l i n g economic and g e o l o g i c f a c t o r s s h o u l d be g i v e n t o g r i n d - i n g m i l l m a n u f a c t u r e r s and t h e i r recommendations o b t a i n e d .

With t h e g rowth of m i l l s i z e s and changing economic s i t u a t i o n s new f a c t o r s i n f l u e n c i n g g r i n d i n g power c a l c u l a t i o n s and m i l l s i z e s e l e c - t i o n a r e becoming known and more w i l l become known. The g r i n d i n g m i l l m a n u f a c t u r e r s a r e a good s o u r c e f o r t h e a p p l i c a t i o n of t h i s c o n t i n u a l l y growing t e c h n o l o g y i n - c l u d i n g t h e p r a c t i c a l a p p l i c a t i o n of t h e newer a p p r o a c h e s and math model ing b e i n g deve loped t h r o u g h i n d u s t r i a l and academic r e s e a r c h .

A. Rod M i l l s

F = 18,000 m i c r o m e t e r s P = 1 ,200 m i c r o m e t e r s Wi = 13.2

= 2.83 kwh/s. t o n

E f f i c i e n c y F a c t o r s :

EF1 d o e s n o t a p p l y .

EF2 d o e s n o t a p p l y .

EF3 d e t e r m i n e a f t e r power c a l c u l a t i o n s i s com- p l e t e d .

Fo i s less t h a n F, s o a p p l y EF4.

EF5 does n o t apply.

EF6 w i l l n o t app ly assuming r a t i o of r e d u c t i o n w i l l be w i t h i n Rro _f 2. How- e v e r , Rro w i l l be d e t e r - mined a f t e r m i l l s i z e s e l e c t i o n .


The rod m i l l f eed w i l l be p repared by c l o s e d c i r c u i t c r u s h i n g and t h e rod m i l l w i l l be i n a r o d m i l l - b a l l m i l l ( o r p e b b l e m i l l ) c i r c u i t w i t h no i n t e r m e d i a t e c o n c e n t r a t i o n s t a g e s o no EF8 f a c t o r need be a p p l i e d . I f i t were j u s t a rod m i l l i n g c i r - c u i t o r i f t h e r e were a n i n t e r m e d i a t e concen t ra - t i o n s t a g e between t h e rod m i l l and t h e b a l l m i l l a 1.2 f a c t o r would

Convers ion s h o r t t o n t o m e t r i c tonne 1.102 K i l o w a t t s t o horsepower 1.341 1.341 x 2.83 x 1.06 x 1.102 =

4.43 Hphlmetr ic tonne 4.43 x 500 = 2215 HP

R e f e r r i n g t o T a b l e V I two m i l l s w i l l be r e q u i r e d . The p r e l i m i n a r y r o d m i l l s e l e c t i o n would be a 3.66 m e t e r ( 1 2 f o o t ) i n s i d e s h e l l 3.46


meter (11.35 f o o t ) d iameter i n s i d e new s h e l l l i n e r s . Re fe r r i ng t o Table X I t h e EF3 (Diameter E f f i - c iency) f a c t o r i s 0.931.

Refer r ing t o Table V I t h e 3.66 m x 4.88 m rod m i l l w i t h 4.72 m (15.5 f t . ) long rods c a l c u l a t e s t o draw 972 HP when ca r ry ing a 40 pe r cen t rod charge wi th a worn-in bu lk d e n s i t y of 5606 kg p e r cub i c meter (350 pounds per cub i c f o o t ) . 1031 HP i s required. The re fo r e , in - c r ea se m i l l l e n g t h by 0.3 me t e r s ( 1 f o o t ) .

Note: Rod m i l l power i s d i - r e c t l y p r o p o r t i o n a l t o rod length .

Therefore , u se two 3.66 meter (12 f o o t ) d iameter i n s i d e she11 3.46 meter (11.35 f o o t ) d iameter i n s i d e new s h e l l l i n e r s by 5.18 meter (17.0 f o o t ) long over f low rod m i l l s w i th a 40 pe r cen t by m i l l volume rod charge w i t h 5.02 meter (16.5 f o o t ) long rods.

Therefore , EF6 assumption i s confirmed.

These m i l l s a r e r equ i r ed t o prepare b a l l m i l l f eed .

With pebb l e m i l l i n g t h e pebble p o r t i o n of the product does no t go through t he rod m i l l t h u s t h e rod m i l l feed r a t e i s reduced by 30 me t r i c tonnes pe r hour (6% of 500 me t r i c tonnes pe r hour ) .

Therefore , use two 3.66 meter (12 f o o t ) d iameter i n s i d e s h e l l

3.46 meter (11.35 f o o t ) i n s i d e new s h e l l l i n e r by 4.88 meter (16 f o o t ) long over f low rod m i l l s w i t h a 40 p e r c e n t by m i l l volume rod charge w i th 4.72 meter (15.5 f o o t ) long rods.

The re fo r e , EF6 assumption is a l - so confirmed here.

B. B a l l M i l l s : Rod Mi l l -Bal l M i l l C i r c u i t

F = 1,200 micrometers P = 175 micrometers Wi = 11.7 ( t h e g r i n d a b i l i t y

work index a t 100 mesh d i d no t show any unusua l c h a r a c t e r i s t i c s t h a t would re- q u i r e s p e c i a l c o n s i d e r a t i o n ) .

= 5.47 kWh/s. t o n

The on ly e f f i c i e n c y f a c t o r t h a t a p p l i e s i s t h e d i ame te r e f f i c i e n c y f a c t o r EF3.

By r e f e r r i n g t o Table V I I i t i s obvious t h e m i l l i s l a r g e r t han 3.81 me te r s (12.5 f e e t ) d i ame te r i n s i d e l i n e r s s o t h e EF3 p e r Table I X i s 0.914.

S i n c e 2 rod m i l l s a r e re- q u i r e d use 2 b a l l m i l l s ( 1 b a l l m i l l pe r rod m i l l ) , t h i s i s t h e s imp le s t c i r c u i t t o o p e r a t e and c o n t r o l o r au to- mate and r e p r e s e n t s a lower c a p i t a l c o s t than t h e one rod


m i l l two b a l l m i l l s a l l of t h e same d i ame te r c i r c u i t .

R e f e r r i n g t o Table V I I t h e g e n e r a l g u i d e l i n e s i n d i c a t e t h e b a l l m i l l s should have a n LID i n t h e a r e a of 1.5. Re- f e r r i n g t o Table V I I I a 4.12 meter (13.5 f o o t ) by 3.96 meter (13.0 f o o t ) over f low b a l l m i l l w i t h a 40 pe r cen t by m i l l volume b a l l charge , new s h e l l l i n e r s and 64 mm (2.5") d iameter b a l l s draws 1266 HP.

3.96 x 1.46 = 5.78 meters (18.96 f e e t )

T h e r e f o r e , u se two 4.12 meter (13.5 f o o t ) d iameter i n s i d e s h e l l 3.93 meter (12.9 f o o t ) d i ame te r i n s i d e new l i n e r s by 5.79 meter (19.0 f o o t ) long over f low b a l l m i l l s wi th a 40% by volume b a l l charge.

For lower o p e r a t i n g c o s t s , s l i g h t l y b e t t e r e f f i c i e n c y , and b e t t e r m i l l a v a i l a b i l i t y t h e c u r r e n t p r a c t i c e f avo r s over f low b a l l m i l l s , however, t h e r e a r e some o p e r a t o r s t h a t p r e f e r g r a t e d i s cha rge m i l l s . R e f e r r i n g t o Table VIII a 3.96 meter (13.0 f o o t ) d i a - meter by 3.96 meter (13.0 f o o t ) diaphragm b a l l m i l l w i t h a 40% by m i l l volume b a l l cha rge , new s h e l l l i n e r s and 50 mm (2") d iameter b a l l s draws 1311 HP.

3.96 x 1.41 = 5.58 me te r s (18.3 f e e t )

T h e r e f o r e , u se two 3.96 meter (13.0 f o o t ) d iameter i n s i d e s h e l l 3.78 meter (12.4 f o o t ) d i ame te r i n s i d e new l i n e r s by

5.79 meter (19.0 f o o t ) long diaphragm ( g r a t e ) d i scharge b a l l m i l l s wi th a 40% by volume b a l l charge.

C. Ba l l Mi l l s : S ing le S tage

The feed t o t h e s tandard Bond b a l l m i l l g r i n d a b i l i t y t e s t i s minus 6 mesh. Thus, t h e coa r s e r f r a c t i o n of a minus 112" s ing le -s tage b a l l m i l l feed is no t included i n t he feed t o t h e g r i n d a b i l i t y b a l l m i l l . The minus 112" feed t o a s tandard Bond rod m i l l g r i n d a b i l i t y t e s t , however, does inc lude t he coarse frac- t i o n of a s ing le -s tage b a l l m i l l feed. To ob t a in t he complete g r i n d a b i l i t y p r o f i l e (Wi vs s i z e ) of a n o r e when s e l e c t i n g a s ing le -s tage b a l l m i l l , i t i s recommended t h a t bo th rod and b a l l m i l l grind- - a b i l i t y t e s t s be made.

I f t h e r e i s a d i f f e r ence i n t h e work i n d i c e s obtained from the rod m i l l and the b a l l m i l l g r i n d a b i l i t y t e s t s , which f r equen t l y occurs , t hen , p a r t i c u l a r l y i f the rod m i l l t e s t work index i s h ighe r , a two-step ca lcu la - t i o n should be made t o de te r - mine t h e requi red gr ind ing power. The rod m i l l work index should be used t o calcu- l a t e power requi red from the p l a n t b a l l m i l l feed s i z e t o 80% pass ing 2100 microns. The c a l c u l a t i o n f o r t he power requi red t o gr ind from 2100 microns t o t he de s i r ed produc t s i z e i s made using the work index from t h e b a l l m i l l g r i n d a b i l i t y t e s t . The sum of t h e s e two g i v e s t h e t o t a l uncor rec ted power per ton requ i red f o r g r ind ing .

F = 9,400 micrometers P = 175 micrometers

Wi = Rod m i l l t e s t 13.2.

Wi = B a l l m i l l t e s t 11.7.

Step one:


= 1.52 kWh/s. t o n

S t ep two:

= 6.29 kWh/s. t o n

Tota l : 1.52 + 6.29

= 7.81 kWh/s. t o n

= 5766 HP, uncor rec ted .

E f f i c i ency Fac to r s :


EF7 does no t apply .

EF3 Mi l l s w i l l be l a r g e r than 3.81 meter i n d iameter so use 0.914.

Fo i s l e s s than F, s o app ly EF4.

EFg, EFg, EF7 and EF8 do no t apply .

Use 2 m i l l s

Per Table V I I t h e L/D should be around 1.25. Re fe r r i ng t o Table VIII a 5.03 meter (16.5 f o o t ) d iameter by 4.88 meter

(16.0 f o o t ) over f low b a l l m i l l w i t h a 40% by m i l l volume b a l l charge , new s h e l l l i n e r s and 64 mm (2.5") d i a - meter b a l l s draws 2370 HP.

4.88 x 1.25 = 6.1 me te r s (20.0 f e e t )

The re fo r e , use two 5.03 meter (16.5 f o o t ) d i ame te r i n s i d e s h e l l 4.85 meter (15.9 f o o t ) d i ame te r i n s i d e new l i n e r s by 6.1 meter (20.0 f o o t ) long over f low b a l l m i l l s w i th a 40% by volume b a l l charge .

The a l t e r n a t e g r a t e d i s c h a r g e m i l l can be a s f o l l ows : Re- f e r r i n g t o Table VIII f o r s i z i n g a g r a t e d i s c h a r g e m i l l a 4.72 meter (15.5 f o o t ) d i a - meter by 4.57 meter (15.0 f o o t ) g r a t e d i s c h a r g e b a l l m i l l w i t h a 40% by m i l l volume b a l l charge , new s h e l l l i n e r s and 65 mrn (2.5") d i a - meter b a l l s draws 2269 HP.

4.57 x 1.3 = 5.93 me te r s (19.5 f e e t )

The re fo r e , use two 4.72 meter (15.5 f o o t ) d i ame te r i n s i d e s h e l l 4.54 meter (14.9 f o o t ) d i ame te r i n s i d e new l i n e r s by 6.1 meter (20.0 f o o t ) long diaphragm ( g r a t e ) d i s c h a r g e b a l l m i l l w i t h a 40% by vol - ume b a l l charge.

D. B a l l M i l l : Fol lowing Autoge- nous o r Semi-Autogenous P r i - mary M i l l

I f t h e product s i z e from t h e primary autogenous o r semi- autogenous m i l l i s t h e same a s from a rod m i l l , t h e b a l l m i l l c a l c u l a t i o n and s i z e se- l e c t i o n i s t h e same a s covered under s e c t i o n B


above. I f t h e b a l l m i l l f e e d s i z e i s d i f f e r e n t from t h i s , t h e same p r o c e d u r e a s covered by e i t h e r s e c t i o n B o r C (whichever a p p l i e s ) s h o u l d be used t o d e t e r m i n e g r i n d i n g power, and b a l l m i l l s i z e se - l e c t i o n .

The p r o d u c t from a p r imary au togenous m i l l o r a semi- au togenous mil.1 c a n have a d i f f e r e n t p a r t i c l e s i z e d i s - t r i b u t i o n t h a n a p r o d u c t w i t h t h e same 80% p a s s i n g s i z e made i n a rod m i l l o r f i n e c r u s h i n g cone c r u s h e r o r i n c r u s h i n g r o l l s and t h u s pos- s i b l y i t c o u l d have a d i f f e r - e n t work index . I n s e l e c t i n g b a l l m i l l s t o g r i n d p r imary a u t o g e n o u s o r semi-autogenous m i l l s p r o d u c t i f p r imary m i l l p r o d u c t i s a v a i l a b l e u s e a sample of t h i s f o r t h e g r i n d - a b i l i t y t e s t s n e c e s s a r y t o d e t e r m i n e t h e power r e q u i r e d f o r g r i n d i n g t h i s p r o d u c t i n a b a l l m i l l .

I f good p i l o t p l a n t d a t a i s a v a i l a b l e , s o t h a t a n oper- a t i n g work i n d e x based upon n e t power i s a v a i l a b l e ( s e e E q u a t i o n 6 ) , t h i s c a n b e used t o d e t e r m i n e b a l l m i l l power.

P r imary au togenous o r semi- a u t o g e n o u s m i l l p r o d u c t c a n c o n t a i n a s i z e a b l e q u a n t i t y of g round m a t e r i a l f i n e r t h a n t h e d e s i r e d b a l l m i l l p r o d u c t s i z e . By f e e d i n g t h i s t o a c l a s s i f i c a t i o n s t a g e , t h i s p r o d u c t s i z e m a t e r i a l i s r e - moved w i t h t h e c o a r s e mater- i a l from t h e c l a s s i f i e r ( s a n d s o r under f low) b e i n g f e d t o t h e b a l l m i l l . T h i s i s b a l l m i l l f e e d . It can o n l y b e i d e n t i f i e d a s s u c h when t h e b a l l m i l l w i l l be o p e r a t e d i n c l o s e d c i r c u i t w i t h a s e p a r a t e c l a s s i f i c a - t i o n s t a g e . I n t h i s c a s e t h e n , t h e g r i n d a b i l i t y work r e q u i r e d t o d e t e r m i n e power r e q u i r e d f o r b a l l m i l l i n g c a n

be done on samples of t h e c o a r s e m a t e r i a l from t h e p r i - mary c l a s s i f i e r . I n t h i s c a s e , t h e b a l l m i l l f eed r a t e i s based upon t h e amount of c o a r s e m a t e r i a l coming from t h e pr imary c l a s s i f i e r and n o t on t h e feed r a t e t o t h e c i r c u i t .

The more common c i r c u i t h a s t h e pr imary m i l l p roduc t being f e d t o t h e c l a s s i f i e r used t o c l o s e t h e b a l l m i l l c i r c u i t . I n t h i s c a s e , u s e t h e pr imary m i l l c i r c u i t pro- d u c t s f o r g r i n d a b i l i t y t e s t s and pr imary m i l l feed r a t e a s t h e feed r a t e t o t h e b a l l m i l l c i r c u i t .

I f samples from p i l o t p l a n t t e s t s a r e n o t a v a i l a b l e i t w i l l be n e c e s s a r y t o use sam- p l e s of t h e o r e f o r g r ind- a b i l i t y t e s t s which may o r may n o t b e t h e same a s t h e p roduc t from t h e pr imary autogenous o r semi-autogenous m i l l . I n t h i s c a s e , t h e feed r a t e t o t h e b a l l m i l l i s t h e same a s t h e feed r a t e t o t h e pr imary m i l l .

E. Pebble M i l l : (Secondary Au- togenous) Rod M i l l Pebble M i l l C i r c u i t

The c a l c u l a t i o n f o r d e t e r - mining g r i n d i n g power f o r Pebb le m i l l i n g (secondary autogenous) c a n be t h e same as f o r b a l l m i l l i n g from rod m i l l p roduc t s i z e t o t h e de- s i r e d s p e c i f i e d s i z e , ne- g l e c t i n g t h e d iamete r e f f i - c i e n c y f a c t o r i f l e s s t h a n 1.0.

To t h i s add t h e power re- q u i r e d t o wear t h e pebb les down t o rod m i l l product s i z e ( p e b b l e m i l l f eed s i z e ) .


F = 70,000 micrometers P = 1,200 micrometers Wi = 13.2

= 3.31 kWh/s. t o n

The i n e f f i c i e n c y f a c t o r t o a l l ow f o r t h e i n e f f i c i e n t use of power i n wearing down from pebble s i z e t o rod m i l l produc t s i z e i s 2.0. Feed r a t e f o r pebbles 30 mtph.

S e l e c t two 2200 HP pebble m i l l s . For s p e c i f i c s i z - ing r e f e r t o m i l l manufac- t u r e r s f o r recommendations a s they have p r o p r i e t a r y equa t ions f o r c a l c u l a t i n g m i l l power draw t a k i n g in- t o account t h e v a r i o u s o r e media and pulp f a c t o r s in - vo lved , which a r e propr i - e t a r y f a c t o r s w i th each manufacturer .

F. Regrind Ba l l M i l l

F = 210 micrometers P = 45 micrometers Wi = 14.0

= 11.21 k ~ h / s . t o n

E f f i c i e n c y Fac to r s :

EF1 does no t apply .

EF2 Many r e g r i n d o p e r a t i o n s a r e c losed c i r c u i t , bu t assume t h i s one i s open c i r c u i t and 8 0 p e r c e n t pass ing g r i n d w i l l be

t h e c o n t r o l l i n g p o i n t . Re fe r t o Table X. The EF2 f a c t o r i s 1.2.

EF? Because b a l l s w i l l be - s m a l l e r than 40 mm (1.5") and o t h e r minor f a c t o r s n e g l e c t EF3 un- l e s s m i l l d iameter i s l e s s t han 2.44 meter (8.0' ) diameter i n s i d e l i n e r s .

EF4 does no t apply.

EF5 Grind is 80 p e r c e n t pass ing 45 micrometers .


EF7 Rr = 2 1 0 + 45 = 4.67 which i s l e s s t h a n 6.


Feed r a t e t o r eg r i nd m i l l 40 m e t r i c tonnes pe r hour.

R e f e r r i n g t o Table V I I t h e L /D c a n be between 1.75 and 2.10 o r even g r e a t e r . Re- f e r r i n g t o Table V I I I a 3.05 me te r ( 10 f o o t ) by 3.05 me te r (10 f o o t ) over f low b a l l m i l l w i t h a 40% by m i l l volume b a l l charge , new l i n e r s and 50 mm (2") b a l l s draws 491 HP . T h i s m i l l c o n t a i n s 37.3 met- r i c tonnes of b a l l s . S i n c e 30 mm d i ame te r b a l l s w i l l be used i n s t e a d of 50 mm d i a - me t e r b a l l s f o r which t h e b a l l m i l l power was ca l cu - l a t e d , t h e r e w i l l be a de- c r e a s e i n t h e power t he m i l l w i l l draw. Using e q u a t i o n 5

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m i l l volume charge w i t h new l i n e r s and a t l e a s t a 36 per- c e n t charge w i t h worn l i n e r s . It may be d e s i r e d t o u t i l i z e more of t h e a v a i l a b l e m i l l volume a s t h e l i n e r s wear so a h i g h e r c h a r g e can be s p e c i - f i e d f o r worn l i n e r s . Speci- f i c a t i o n s c a n a l s o c a l l f o r t h e d r i v e and motor t o be r a t e d t o a l l o w us ing p i n i o n s w i t h one l e s s and two more t e e t h , t h u s a l lowing f o r changing m i l l speed i f i t i s found n e c e s s a r y t o b a l a n c e t h e c i r c u i t , i n c r e a s e capa- c i t y , s u i t changing o r e char- a c t e r i s t i c s , e t c .

X I . SELECTION OF GRINDING MEDIA SIZES AND ESTIMATING STEEL CONSUMPTION

The e q u a t i o n f o r s e l e c t i o n of t h e l a r g e s t d iameter rod f o r t h e i n i - t i a l charge and f o r t h e make-up charge is:

R = Diameter of rod i n m i l l i - me te rs

F = Feed s i z e 80% p a s s e s i n microns

W i = Work Index Sg = S p e c i f i c G r a v i t y

Cs = F r a c t i o n of c r i t i c a l speed. D = Diameter i n s i d e s h e l l l i n e r s

i n meters .

With R i n inches and d i a m e t e r (D) i n f e e t e q u a t i o n 17 becomes:

Table X I 1 g i v e s t h e e q u i l i b r i u m s t a r t - u p rod charge f o r t o p rod s i z e s from 125 mm ( 5 " ) t o 65 mm (2.5") .

The e q u a t i o n f o r s e l e c t i o n of t h e l a r g e s t d iameter b a l l f o r t h e i n i -

t i a l c h a r g e and f o r t h e make-up c h a r g e is:

B = Diameter of b a l l i n m i l l i m e t e r s .

NOTE: Except f o r K which i s g i v e n below a l l o t h e r t e rms t h e same a s f o r e q u a t i o n 17.

B a l l M i l l K F a c t o r

M i l l Type and S t e e l o r C . I . Gr ind ing C i r c u i t B a l l s

K

Wet-Overflow-Open C i r c u i t 3 50 Wet-Overflow-Closed C i r c u i t 350 Wet-Diaphragm-Open C i r c u i t 330 Wet-Diaphragm-Closed C i r c u i t 330 Dry-Diaphragm-Open C i r c u i t 335 Dry-Diaphragm-Closed C i r c u i t 335

With B i n i n c h e s and d i a m e t e r (Dl i n f e e t e q u a t i o n 18 becomes:

T a b l e X I 1 1 g i v e s t h e e q u i l i b r i u m s t a r t - u p b a l l charge f o r t o p b a l l s i z e s from 115 mm (4.5") t o 40 mm (1 .5" ) .

These two e q u a t i o n s g i v e t h e l a r g e s t d i a m e t e r of t h e g r i n d i n g media r e q u i r e d . S i n c e t h e c a l - c u l a t e d s i z e i s no t a lways a n a v a i l a b l e s t a n d a r d s i z e s e l e c t t h e n e a r e s t l a r g e r s i z e a v a i l a b l e . A c t u a l o p e r a t i n g e x p e r i e n c e may d i c t a t e a change from t h e c a l c u - l a t e d s i z e . T h e o r e t i c a l l y i t i s a lways a d v i s a b l e t o u s e a g r a d e d c h a r g e a s a rep lacement c h a r g e . Using a g raded charge o f t e n i s n o t p r a c t i c a l . The l o s s i n e f f i c i e n c y by n o t u s i n g a g raded make-up c h a r g e g e n e r a l l y c a n n o t b e nea- s u r e d . I n some c a s e s , i t i s o n l y n e c e s s a r y t o add t h e l a r g e s t s i z e


Table X I 1

Start-Up Equil i b r i a Grinding Rod Charges, Percent Weight

Table XI11

Start-Up Equ i l ib r i a Grinding Ball Charges, Percent Weight

115 (4.5") 100 (4 .0" )

9 0 (3 .5" ) 75 (3.0") 6 5 (2 .5" ) 5 0 (2 .0" ) 4 0 (1.5") 25 (1 .0" )

TOTAL Pct

Make-Up Ball s Fed

S izes , M = B 115

4.5" 100 4.0"

9 0 3.5"

75 3.0"

65 2.5"

50 2.0"

4 0 1.5"


media ca lcula ted a s make-up. Op- e ra t ing r e s u l t s w i l l i n d i c a t e the necess i ty of using more than one s i z e of media i n the make-up charge.

The best f i gu res f o r media and li- ner consumption come from a c t u a l operating experience and a s oper- a t ing data i s generated t h i s should be used t o e s t a b l i s h wear r a t e s .

Actual t e s t i n g i n l abora to ry s c a l e equipment t o determine the abra- s ion c h a r a c t e r i s t i c s of an o r e i s d i f f i c u l t , and a v a i l a b l e t e s t s a r e guides f o r es t imat ing purposes but a r e not completely accura te . One abras ion test measures the weight l o s s of a s t e e l paddle continuous- l y impacting f a l l i n g o re p a r t i c l e s f o r a prescribed time period under standard condit ions. From t h i s i s developed a measurement c a l l e d an abras ion Index, A i . From p lan t da t a , empirical equat ions corre- l a t i n g with A i were developed t o be used t o es t imate rod, b a l l , m i l l l i n e r and crusher l i n e r wear r a t e s . These equat ions a r e :

Wet Rod Mil l s :

Rods kglkw-hr = 0.159 (A, - 0.020)0.2 (19)

Liners kglkw-hr = 0.0159 (A, - 0.015)0.3 (20)

Wet Bal l M i l l s :

B a l l s kglkw-hr = 0.159 (A, - 0.015)0.34 (21)

Line r s kglkw-hr = 0.01 18 (A, - 0.015)03

(22)

Mult ip ly equat ions 19 , 20, 21 and 22 by 2.2 t o g e t pounds per k i lo - wat t .

These formulas give e s t i m a t e s of wear r a t e s which can be used a s a guide. They a r e sub jec t t o such f a c t o r s a s m i l l speeds, percent volumetric loading, a l l o y of gr inding media and l i n e r s , opera t - ing p r a c t i c e s , e t c .

Table X I V l i s t s average ab ras ion i n d i c e s f o r the more commonly o r e , minera ls and ma te r i a l s ground i n rod m i l l s , b a l l m i l l s o r pebble m i l l s .

This chap te r is a compi la t ion of previous work done by Fred C. Bond, D. M. Kjos, D. R. Olson, C. A. Rowland and o t h e r s i n t h e Grinding M i l l Sec t ion of t he Allis-Chalmers Corporation, Ce- ment, Mining and Metals Systems Divis ion . It r e f l e c t s exper ience gained i n obta in ing opera t ing d a t a i n ope ra t ing p l an t s and i n study- ing published opera t ing data .


Tab le X I V

Average Abras ion I n d i c e s

M a t e r i a l

A1 umi num Oxide B a s a l t B a u x i t e B e r y l 1 i um Ore Cement C l i n k e r Cement Raw M i x Clay, C a l c i n e d Copper-Nickel M a t t e Copper-Nickel Ore Copper Ore Copper-Si 1 v e r Ore Do1 omi t e Fel dspar Ferro-Chrome A1 1 oy Ferro-Manganese Ful l e r s E a r t h Go ld Ore G r a n i t e Gravel I r o n Ore, U n i d e n t i f i e d H e m a t i t e L i m o n i t e M a g n e t i t e T a c o n i t e

Lead Z i n c Ore L imes tone Magnesi t e Marb le Molybdenum Ore N i c k e l Ore O i l Sha le Phosphate Rock Q u a r t i z e S c h i s t Shale S i 1 i c a Rock S i l v e r Ore S l a g S l a t e Stone T i n e Ore Trap Rock

T n t a l

No. Tes ts

1 Average 1 Range