potential changes in the physical ... - · pdf fileplatinum 2012 343 j ... potential changes...

The Southern African Institute of Mining and Metallurgy

Platinum 2012

343

J. Engelbrecht

POTENTIAL CHANGES IN THE PHYSICAL BENEFICIATION PROCESSES

THAT CAN IMPROVE THE RECOVERY GRADE OR COSTS FOR THE

PLATINUM GROUP METALS

J. Engelbrecht Multotec (Pty) Ltd

Abstract

Understanding the deportment of minerals in size fractions has already led to

improvements in the flow sheets for platinum minerals beneficiation.

There are, however, more potential changes in the flow sheets that may improve the

recovery of platinum group metals or reduce the costs of beneficiation before refining.

Based on known technology of pre-concentration, milling, gravity concentration,

flotation, and magnetic separation a number of potential alterations or additions can

be considered and motivated based on the mineral deportment in size fractions.

Introduction

The concept of using the deportment of minerals at different sizes to assist in the

understanding of the behavior of the liberation process and beneficiation responses of

the platinum group metal (PGM) minerals is not new. Fine milling of the tailings or the

concentrate to increase recovery and/or grade has been successfully implemented at a

number of platinum plants1.

There may, however, be other technologies that can lead to cost reductions or

improvements in grade and/or recovery.

Costs and revenue

Table I shows the value chain of PGM beneficiation and the associated costs. It is clear

that mining can contribute substantially from a cost reductions point of view while the

liberation or milling and beneficiation processes can contribute with an increase in

revenue, particularly if the recoveries can be improved.

Cost reduction will also be significant if grades can be improved through pre-

concentration or better concentration prior to further refining, due to the high energy

costs of refining.


Platinum 2012

344

Table I- PGM processing – Costs

Parameter Mining

Milling

and

flotation

Smelting

and

converting

Base

metal

refining

Precious

metal

refining

Total

Percent of

total cost 65 – 75 9 – 12 6 7 4 – 5 100

PGE grade 4 – 6

g/t

100 – 600

g/t

640 – 6000

g/t

30 – 65

% >99.8% -

PGE recovery

(%) - 80 – 90 95 – 98 >99 98 – 99 75 – 85

Concentration

ratio - 30 - 80 20 75 2 200 000

Lonmin 3rd

Quarter 2010 Production Report

Table II shows the value of the PGM minerals as well as copper and nickel in various

ore types. A one percentage point increase in the recovery of platinum is estimated to

result in U$85.6 million additional revenue per annum, and nearly double that value if

all the associated minerals are taken into consideration for the South African mines.

There is therefore a good financial incentive to improve the recoveries of the PGM

minerals.

Current operating costs are between US$10 to US$14 per ton of fresh feed, and it is a

simple calculation to show that the increase in revenue per percentage recovery varies

between US$1.75 and US$3 per ton treated, depending on which reef is mined. This

implies that a reasonable expenditure is justified in order to improve recovery.

Th

e S

ou

the

rn A

fric

an

In

stit

ute

of

Min

ing

an

d M

eta

llu

rgy

Pla

tin

um

20

12

34

5

Ta

ble

II-

Me

tal

va

lue

s in

va

rio

us

ore

ty

pe

s

P

rice

as

on

01

Ma

y

20

12

($

)

Me

ren

sky

Ore

UG

2 o

re

Pla

tre

ef

ore

g/t

$

/t o

f o

re

ma

ss %

g/t

$

/t o

f o

rem

ass

%

g/t

$

/t o

f o

re

ma

ss %

Pt

15

70

3

.25

1

63

.79

5

9

2.4

61

24

.19

4

11

.26

63

.59

42

Pd

6

80

1

.38

3

0.0

62

5

2.0

44

4.6

1

34

1.3

83

0.1

74

6

Rh

1

35

0

0.1

7

7.1

6

3

0.5

42

3.4

4

9

0.0

93

.91

3

Ru

1

15

0

.44

1

.62

8

0

.72

2.6

71

20

.12

0.4

4

4

Ir

10

85

0

.06

1

.92

1

0

.11

3.9

81

.9

0.0

20

.82

0

.8

Au

1

55

0

0.1

8

8.7

5

3.2

0

.02

1.1

90

.4

0.1

5

.07

3

.4

To

tal

PG

M+

Au

5.4

8

21

3.3

0

99

.25

.89

20

0.0

89

8.3

2.9

7

10

4.0

0

99

.2

Av

era

ge

gra

de

s o

f th

e i

nd

ivid

ua

l p

reci

ou

s m

eta

ls i

n M

ere

nsk

y,

UG

2 a

nd

Pla

tre

ef

ore

s a

nd

th

eir

cu

rre

nt

po

ten

tia

l v

alu

e2 M

ark

et

pri

ces

are

as

on

1 M

ay

20

12

P

rice

as

on

01

Ma

y

20

12

($

)

Me

ren

sky

Ore

UG

2 o

re

Pla

tre

ef

ore

% i

n o

re

$/t

of

ore

m

ass

%%

in

ore

$/t

of

ore

ma

ss %

%

in

ore

$

/t o

f

ore

ma

ss %

Ni

17

50

0

0.1

3

22

.75

62

0

.07

1

2.2

5

80

0.3

6

63

.00

6

7

Cu

8

54

0

0.0

8

6.8

3

38

0

.01

8

1.5

42

00

.18

1

5.3

7

33

To

tal

Ba

se M

eta

ls

0.2

1

29

.58

1

00

0

.09

1

3.7

9

10

0

0.5

4

78

.37

1

00

To

tal

B M

eta

ls +

PG

M +

Au

2

42

.88

2

13

.87

1

82

.37

Av

era

ge

co

nte

nt

of

me

tals

in

Me

ren

sky

, U

G2

an

d P

latr

ee

f o

res

an

d t

he

ir c

urr

en

t p

ote

nti

al

va

lue

. M

ark

et

pri

ces

are

as

on

1 M

ay

20

122


Platinum 2012

346

Mineralogy

Owing to the low concentration and small particle sizes of PGM minerals and the mineralogical

complexity of PGM ores, advances in equipment and processes were required before

meaningful mineralogical observations could be made.

The main characteristics of the three PGM ore types in South Africa are summarized in Table III.

Table III-Main characteristics of the Bushveld Complex PGM ore types3

Characteristics Merensky Reef Platreef UG2 Reef

Thickness (m) 0.9 – 1.2 3 – 90 0.45 – 0.75

Grade

PGM

(g/t)

Ni (%)

Cu (%)

5 – 9

0.13

0.08

3 – 4

0.36

0.18

6 – 7

0.07

0.018

Gangue minerals

50 – 80 % pyroxene

20 – 40 % plagioclase

3 – 5 % chromite

0.5 – 5 % talc

80 – 90 % pyroxene


3 – 5 % chromite

0.5 – 3 % talc

60 – 90 % chromite

5 – 25 % pyroxene


1 – 5 % talc

PGM grain size

(μm) 20 – 150 40 – 200 3 - 10

PGM minerals

30 – 40 % Cooperite

(PtS) + Braggite (Pt,Pd)S

10 – 30% Kotulskite

(PdTe) + Michenerite

(PdBiTe)

10 – 15% Ru phases

5 – 8% Sperrylite

(PtAs2)

3 – 6%

Isoferroplatinum

(Pt3Fe)

3% Au/Ag phases

Moncheite

[(Pt,Pd)(Bi,Te)2 – PtTe2]

+ Merenskylte

[(Pd,Pt)(Bi,Te)2 –

PdTe2]>>Sperrylite

(PtAs2)>

Isoferroplatinum

(Pt3Fe)> Braggite

(Pt,Pd)S

Cooperite (PtS) >

Laurite (RuS2) >

Braggite (Pt,Pd)S>

Malanite

(CuPt1.5lr0.5S4)>

Isoferroplatinum

(Pt3Fe)> Sperrylite

(PtAs2)

PGM analysis

Pt (%)

Pd (%)

Rh (%)

Ru (%)

Ir (%)

Os (%)

Au (%)

59

25

3

8

1

0.8

2.5

42

46

3

4

0.8

0.6

3.4

41

34

9

12

1.9

1.7

0.4


Platinum 2012

347

Table IV gives the mineral associations for the main streams for a UG2 beneficiation process1.

The major losses of PGM minerals in the tails were through PGM minerals locked in or attached

to silica. This problem was addressed through the introduction of the fine grinding of the silica-

rich fraction through MIG IsaMillTM

technology, predominantly by Anglo American Platinum 1.

The second major loss is fine liberated PGM minerals.

Table IV- Mineral associations for typical Amandelbult UG2 process samples4

Association Feed Concentrate Tailings Tailings

<10 μm

Tailings

>10 μm

Tailings

>53 μm

Liberated 49.2 53.1 31.3 82.3 18.5 2.4

Enclosed in BMS* 23.6 15.8 4.7 4.1 9.7 1.6

Attached to BMS 7.9 12.7 0.3 0.4 0.6 -

PGM/BMS/Silicate 5.6 6.0 7.7 - 5.6 15.5

Enclosed in Silicate 7.5 8.4 36.0 2.7 44.3 57.0

Attached to Silicate 0.6 2.7 9.3 3.5 13.9 6.8

Enclosed in Oxide 4.8 1.3 7.6 4.3 6.0 11.7

Attached to Oxide 0.8 - 3.1 2.7 1.4 5.0

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0

Middlings 702 14.6 7.0 8.3 14.8 -

Locked 43.6 32.1 61.7 9.4 66.7 97.6

*BMS – base metal sulphides

Liberation

Most of the modern flow sheets for UG2 ores incorporate a multiple liberation / beneficiation

principle and are referred to as MF2 or MF3 flow sheets, as shown in Figures 1 and 2. Excellent

reviews and summaries of applications are available in the literature1. The principle is also used

for all sources of PGMs, i.e. Merensky, UG2, and Platreef.


Platinum 2012

348

Figure 1-MF2 Circuit

Figure 2-MF3 circuit


Platinum 2012

349

Figure 3 shows a complex four-stage circuit for a UG2 plant. The pre-concentration stage was

most probably successful due to the mining method, whereby excess material had to be mined

beyond the reef itself as dictated by the mining method, and economic and safety

considerations.

Figure 3-Four-stage UG2 processing flow sheet

The question is whether there is additional potential for improvement with significant

economic benefits.

Pre-concentration

Sorting technologies are currently under investigation in addition to dense media separation

(DMS) to remove barren and/or uneconomic ore as early as possible, and should be

implemented if economically justified.

Regardless of the process used for pre-concentration, it may be economically attractive to carry

out the process underground to reduce the energy costs of hoisting the ore to the surface, and

use the reject stream as backfill material.

Milling

Besides assessing the capital and operating costs of fully autogenous (FAG), semi-autogenous

(SAG), ball, and stirred milling, the industry has already begun implementing inter-particle

breakage technology (IPBT) through the use of high-pressure grinding roll (HPGR) technology1.

The major potential advantages of IPBT technology are:

1) Reduction in energy consumption

2) Preferential breakage on grain boundaries.


Platinum 2012

350

The preferential breakage on grain boundaries is of particular interest to the platinum industry.

Mineralogical investigations (Table IV) of all the sources showed a preference of the PGM

minerals to occur on grain boundaries or within base metal sulphides.

In the case of the UG2 Reef, the chromite spinel crystals are mostly barren of valuable PGMs

and the grain sizes are coarser than 120 μm5. If the chromite can be removed at a coarser size,

a substantial saving in milling energy can be realized, provided the losses of PGMs are

minimized.

If the primary breakage is done through IPBT equipment and the secondary stage uses

conventional indiscriminate crushing, most of the advantages can be lost. It will be better to

continue to use the compression technology and use the advantages as far as possible.

The only equipment currently available that can use IPBT to grind fine is the Loesch Mill and

Horomill. Both utilize dry processing and can be used to grind down to 80% -20 μm, as used in

the cement Industry.

IPBT can be used for all three ore types. Platreef ore is hard and tough and is suitable for IPBT.

In the case of the UG2 ore, IPBT will most probably be used to liberate the chromite spinel

crystals, which can then be removed effectively as a secondary product through a wet process.

Such a flow sheet is shown in Figure 4, where the chromite spinel crystals are removed through

spiral concentrators. This is not a new technique, and the major criticism in the past was that

spiraling led to the loss of PGM minerals to the concentrate, as well as circuit balance problems.

This can be addressed in a number of ways:

a) Flotation prior to gravity concentration

b) Use of thickeners to balance the water circuit

c) Use of a special spiral that captures the liberated heavy PGM minerals in a high-grade

concentrate which can be returned to the main circuit.


Platinum 2012

351

Figure 4-Flow sheet incorporating fine grinding and spirals for chromite removal

Milling and classification

The effect of classification efficiency on mill capacity and the particle size distribution of the

product is well documented 6.

The mineral industry in general was slow to adapt this principle, but it has been successfully

applied in the industrial minerals sector 7.

In simple terms, the advantages of efficient classification and circulating load are based on the

principle of break and remove as soon as possible. This implies that both the residence time in

the mill, or more correctly - the residence time distribution and the efficiency of the classifier,

will determine the overall effect.

Figure 5 summarizes the effect of classification efficiency and mill capacity (constant P80) as a

function of circulating load. The potential increase in mill capacity is approximately 20% if one

compares current operating conditions with potential operating conditions. Figure 6 shows the

effect on the particle size distribution for gold ore 8.

��

��

��

��

��

�

��

��

��

��

��

��

��

Cu

mu

la

tiv

e %

P

as

sin

g

��

��

1

��

�

9

9

8

7

6

5

4

3

2

1

1

5

4

3

2

1

��

��

9.8

5

5

5

5

55

5

5

5

5

0

5

�

��

8

��

�

1

2

��

��

40

11

24

��

� �

%

%

0%

��

��

CL

CL

% C

��

��

10

L

L

CL

��

�

��

��

��

��

��

�

��

��

��

��

��

R

C

��

��

��

��

RO

Cla

�

��

��

10

��

SIN

ass

��

��

��

00

�

N -

sifi

�

��

��

��

RA

ca

��

��

��

AM

tio

�

��

��

ML

on e

��

��

LER

eff

��

� ��

R

fici

�

��

1

en

��

000

ncy

� �

0

y =

��

10

� ��

00%

��

%

��

1

��

100

��

99

9

8

7

6

5

4

3

2

1

1

5

4

3

2

1

000

��

9.8

95

85

75

65

55

45

35

25

15

10

5

4

3

2

1

0

�

��


Platinum 2012

353

Hydrocyclones have been the preferred classifier in the mineral industry, but the hydraulic

effect may be a disadvantage for the process. A good example is in the processing of UG2 ore,

where the over-grinding of the chromite spinel crystals is not productive but will be the result

of the hydraulic effect in the hydrocyclone, whereby the heavier particles will be classified finer.

The disadvantage of the hydraulic effect can be overcome by a physical classifier such as the

vibrating screen. Unfortunately, the screening area required and the associated capital and

running cost has been an implementation barrier, as was the case with the micro-screens at the

Crocodile River concentrator. This can be addressed by a combination of a hydraulic and a

physical classifier as shown in Figure 7. Instead of screening the complete circulating load, only

the cyclone overflow is screened to produce the correct particle-size distribution for the lighter

fractions.

Figure 7-Combination of physical and hydraulic classification

Flotation

Flotation is the preferred beneficiation process for PGM minerals. Flotation is a complex

process, and the recovery of a specific mineral will depend mainly on the following

characteristics:

• Hydrophobicity

• Particle size

• Degree of liberation.


Platinum 2012

354

Table IV shows that the losses of PGMs to the tailings for UG2 ores are due mainly to locked

particles in silicates and liberated PGM particles. Losses in locked particles have been

addressed by finer grinding1. The detail of the liberated PGM minerals shows that the major

loss occurs in the -10 μm fraction (+80%) 4. The mineralogical analyses also confirmed that the

PGM minerals are predominantly in the 2 – 6 μm fraction 5

.

It is well known that the flotation rate of fine particles is slow, based on particle-bubble collision

models. An excellent review has been published by Dai et al. 9. Experimental results and the

model developed by Dukhin et al. 10

showed clearly that fine particles do float and that smaller

bubbles increase the rate of flotation.

In order to improve the recovery and/or the rate of flotation, the effects of bubble size and the

intensity of shear forces have to be considered, as shown in Figure 8 Newcombe et al. 11

.

Figure 8-The effect of turbulence and bubble size on the particle size in flotation

Sedimentation

Imp

ell

er

spe

ed

Bubble size

Fine

Coarse

Sedimentation

Detachment

Detachment

No

lift

No

lift

Low rec

Low rec


Platinum 2012

355

In order to float fine particles, a different reagent mix may be required because the mechanism

of adhesion and the surface area are different compared to coarse particles. Similarly, the

flotation cell conditions will be different, with lower-intensity agitation required for the

recovery of fine particles, which is not conducive for coarse particles. Although a split flotation

circuit was tried and abandoned (1)

, the concept of fine particle flotation after coarse flotation

or after classification may prove to be a viable economic option.

The advantage of froth washing to improve the grade in column flotation has been proven and

has also been introduced in conventional flotation cells. This is certainly a proven technique to

improve the grade of the concentrate in flotation by removing entrained and entrapped

particles, and should be used.

Preferential shedding occurs whereby fast-floating particles ’crowd’ slow-floating gangue

particles off the surfaces of bubbles, and can be used to recover high-grade froth products if

applicable.

Preferential shedding and froth washing can therefore be used to recover higher grade

products and limit contamination by entrained and entrapped gangue particles, as well as slow

floating-gangue components in the froth.

Magnetic concentration

Pyrrhotite and pentlandite, as well as certain PGM minerals, are magnetic 12,13

and equipment is

available to recover the fine platinum minerals.

Pyrrhotite is normally slow floating and can account for significant losses in the final tails if

insufficient residence time is available in flotation.

High-intensity wet magnetic separation techniques like WHIMS and SLON can be used to

recover some of the magnetic minerals lost to the tailings. Typical recovery-size relationship for

hematite is shown in Figure 9 14

. This implies that both the very fine and the oversize magnetic

particles that were not recovered through the flotation process may be recovered by magnetic

separation.


Platinum 2012

356

Figure 9-Recovery as a function of size for hematite with WHIMS

Gravity concentration

Batch centrifugal gravity concentrators have been developed for gold and were followed by

continuous machines like the Knelson and Falcon concentrators. Sepro Mineral Systems Corp

has developed an ultrafine (UF) separator, and the results for a tin recovery application are

given in Figure 10 15

. The recoveries for heavier PGM minerals should be higher and this could

be an attractive option for the small liberated PGM minerals. According to the manufacturer

the removal of coarse particles and dilute feed conditions before treatment are essential.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

Re

cov

ery

of

He

ma

tite

Particle size micron


Platinum 2012

357

Figure 10-Recovery of Sn as a function of size

Conclusion

Despite the fact that advanced flow sheets are already used in the PGM beneficiation process,

there seem to be additional technologies that can further improve the recoveries or grades of

PGM mineral products or can reduce costs. The application of a specific technology will depend

on the economic viability of the process.

A summary is given in Table V of different concepts that can potentially be applied in the

beneficiation of PGM minerals for different ore types.

Table V

Merensky Ore UG2 Ore Platreef Ore

Pre-concentration √ √ √

Classification efficiency in milling √√ √ √√

Inter-particle breakage √√√ √√√ √√√

Fine particle flotation √√√ √√√ √√√

Magnetic separation √ √ √

Gravity concentration √√√

Fine gravity concentration √ √√√ √

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

P i l i

Particle size

Re

cov

ery

of

Sn


Platinum 2012

358

References

1. Rule, C.M. and Plint, N. What will the typical PGM concentrator look like? Keynote

address: 8th International Comminution Symposium (Comminution 2012), Vineyard Hotel,

Cape Town, 16–20 April 2012.

2. Von Gruenewaldt. The mineral resources of the Bushveld Complex. Mineral Science and

Engineering, vol. 9, no. 2, Apr. 1977. pp. 83–96.

3. Newell, A.J.H. The processing of platinum group metals (PGM). Pincock, Allen & Holt

Perspectives, no 89, Mar. 2008.

4. Rule, C., and De Waal, H. IsaMillTM

Design improvements and operational performance at

Anglo Platinum. Metplant 2011. Plant Design & Operating Strategies – World’s Best

Practice, Perth, Western Australia, 8–9 August 2011. Australasian Institute of Mining and

Metallurgy, Carlton, Australia. Pp. 176 – 192.

5. Penberthy, C.J. The effect of mineralogical variation in the UG2 chromitite on recovery of

platinum-group elements. PhD thesis, University of Pretoria. December 2001.

6. Engelbrecht, J.A. The effect of classification efficiency on closed circuit grinding. The role

of the Practical Metallurgist Symposium, 1989. Mine Metallurgical Managers Association,

South Africa. pp. 16 – 44.

7. Roettle, J. Improving grinding performance with high efficiency classification. 8th

International Comminution Symposium (Comminution 2012), Vineyard Hotel, Cape Town,

16–20 April 2012.

8. Guest, R.N. A laboratory investigation of the importance of the circulating load in the

control of particle size distribution. SA Mechanical Engineer, Feb. 1972. pp. 46 – 51.

9. Dai, Z., Fornasiero, D., and Ralston, J. Particle-bubble collision models – a review.

Advances in Colloid and Interface Science, vol. 85, 2000. pp. 231 – 256.

10. Dukhin, S.S., Kretzschmar, G., and Miller, R. Dynamics of Adsorption of Liquid Interface.

Ch. 10. Elsevier, Amsterdam, 1995.

11. Newcombe, B., Bradshaw, D., and Wightman, E. Flash flotation … and the plight of the

coarse particle. Minerals Engineering, vol. 34, Jul. 2012. pp. 1 – 10.


Platinum 2012

359

12. Vermaak, M.K.G. Fundamentals of the flotation behavior of palladium bismuth tellurides.

PhD thesis Faculty of Engineering, Built Environment and Information Technology,

University of Pretoria. May 2005..

13. De Villiers, J. The composition and crystal structures of pyrrhotite: A common but poorly

understood mineral. Proceedings of Mintek 75, Randburg, South Africa, 4–5 June 2005.

14. Forssberg, K.S.E. and Kostkevicius, N.R. Comparative pilot scale tests with wet high

intensity magnetic separators. Erzmetal, vol. 35, no. 6, 1982. pp. 284 - 293.

15. McAlistar S. Private communication 2012.

The Author

Johan Engelbrecht, Director International Business Division, Multotec (Pty) Ltd

Johan Engelbrecht is currently the Director for the International Business Division for the

Multotec Group of companies. He, as a qualified Metallurgical Engineer, was responsible for the

formation of Multotec Process Equipment (Pty) Ltd, which has become well known worldwide

for cyclones, spirals, samplers and magnetic separators. Multotec is regarded as a world leader

in dense medium cyclone separation as well as spirals and sampling in the Coal Industry. It is

therefore not surprising that Multotec Process Equipment received the award for leader in R &

D in the Technology Top 100 awards in 2006 and the Group of Companies the Exporter of the

Year for Gauteng award in 2007. Johan was Coalman of the year in 1998 and has been awarded

lifetime membership by the South African Coal Processing Society. He is well known worldwide

and has presented numerous papers at various conferences.


Platinum 2012

360

potential changes in the physical ... - · pdf fileplatinum 2012 343 j ... potential changes...

Documents