SECTION V

ZINC AND LEAD PYROMETALLURGY

PRACTICE

Chapter 17

N O D U L I Z I N G OF Z I N C CALCINE AT EAGLE-PICHER INDUSTRIES

Galena, Kansas

Morris A. Dodd Manager, Metal Sa les

Chemicals and Metals Divis ion, Eagle-Picher I ndus t r i e s , Inc.

Abs t rac t

The nodulizing process described converts f i ne ly divided zinc ca lc ine i n t o a dense pu r i f i ed nodule. These zinc nodules a r e s u i t a b l e f o r d i r e c t use i n z inc reduction furnaces, espec ia l ly the hor izonta l r e t o r t type, f o r the production of metal o r American process z inc oxide furnaces f o r producing lead f r e e z inc oxide.

The bas ic reac t ions a r e ca r r ied ou t i n a ro ta ry k i l n f i r e d with f u e l o i l o r na tu r a l gas. The combustion gases t r a v e l countercurrent t o t he flow of the zinc bearing mater ia l . Vo l a t i l e impur i t i es a r e ca r r ied o u t of t he k i l n with t he combustion gases and a small port ion of t h e f i ne ly divided feed mater ia l . The gases a r e cooled and t he l a r g e r so l i d p a r t i c l e s a r e separated from the stream before the r e s u l t a n t fume i s co l lec ted i n a conventional c lo th f i l t e r baghouse. The fume coUected can be fu r t he r processed by conventional wet processes f o r t h e recovery of me t a l l i c values such a s l ead and cadmium. I n order t o nodulize t he ca lc ine properly i t i s e levated t o a point of i nc ip i en t fusion. The agglomerated mater ia l f a l l i n g from the k i l n i s cooled and then s ized by passing through a screening and crushing system cons i s t ing of a jau crusher and a r o l l crusher i n closed c i r c u i t with a v ibra t ing screen.

INTRODUCTION

The nodu l i z ing p rocess f o r t h e agglomeration of z inc o re was o r i g i n a l l y developed by t h e A t h l e t i c Mining Company a t i t s p l a n t i n F o r t Smith, Arkansas. The primary goal was t o produce a dense, porous product from r o a s t e d f l o t a t i o n z i n c c o n c e n t r a t e s which would be s u i t a b l e f o r sme l t ing by t h e h o r i z o n t a l r e t o r t p rocess . I n a d d i t i o n t o p repa r ing a product w i t h d e s i r a b l e p h y s i c a l c h a r a c t e r i s t i c s f o r sme l t ing , i t was a l s o d i scovered t h a t c e r t a i n v o l a t i l e i m p u r i t i e s could be removed and t h e r e s u l t a n t z i n c produced therefrom was of h igher q u a l i t y than had been p o s s i b l e wi th previous p r a c t i c e . I n 1953 t h i s process was l i c e n s e d t o Eagle-Picher I n d u s t r i e s f o r use a t t h e i r Roast ing and Acid P l a n t then being cons t ruc ted a t Galena, Kansas. Eagle-Picher eng inee r s , us ing m e t a l l u r g i c a l and eng inee r ing d a t a provided by A t h l e t i c Mining Company, designed and cons t ruc ted a nodu l i z ing u n i t which was f i r s t p laced i n ope ra t ion i n t h e l a t t e r p a r t of 1954. This f i r s t u n i t a t Galena was s u c c e s s f u l i n t h e produc- t i o n of a h igh p u r i t y nodule which was o r i g i n a l l y used f o r t h e produc- t i o n of z i n c oxide a t ~ a ~ l e - p i c h e r ' s p l a n t i n Hi l l sboro , I l l i n o i s . I n 1956 a second u n i t e x a c t l y l i k e the f i r s t was cons t ruc ted i n o rde r t h a t ~ a g l e - P i c h e r ' s z i n c sme l t e r a t Henryet ta , Oklahoma could be s u p p l i e d w i t h nodules a s raw m a t e r i a l f o r the product ion of s l a b z inc . Not only was t h e q u a l i t y of meta l improved by us ing nodules but hand l ing l o s s e s i n h e r e n t i n t h e shipment of z inc c a l c i n e were l a r g e l y e l imina ted .

S i n c e t h e nodu l i z ing process has been used t o supply raw m a t e r i a l f o r two d i f f e r e n t end p roduc t s made by d i f f e r e n t p rocesses , a campaign program was developed. Nodules produced f o r pigment usage were made from s e l e c t e d z i n c concen t ra t e s and t h e impur i ty l e v e l of t h e nodules emphasized t o a g r e a t e r degree than t h e p h y s i c a l p r o p e r t i e s . Con- v e r s e l y , nodules in tended f o r t h e product ion o f meta l by t h e h o r i z o n t a l r e t o r t p rocess were produced w i t h s t r i c t c o n t r o l a s t o the s i z e , appa ren t density. and p o r o s i t y . As i n many processes a c e r t a i n amount o f compromise between the optimum chemical and phys ica l c h a r a c t e r i s t i c s of t h e nodule i s necessary i n commercial p r a c t i c e .

Calc ine f o r t h e nodu l i z ing system a t Galena i s produced by a s i n g l e l a r g e suspension r o a s t e r o f t h e Cominco type .. The r o a s t e r has a s t u b s h a f t w i th two d ry ing h e a r t h s a t t h e bottom. F l o t a t i o n concen t ra t e s from Eagle-Picher concen t ra t ing m i l l s (and sometimes purchased z i n c concen t ra t e s ) a r e p r e - d r i e d i n a Hardinge r o t a r y d r i e r where any blending of o r e s i s accomplished. S ince t h e o p e r a t i o n of a f l a s h r o a s t e r r e q u i r e s a v e r y f i n e f eed , the completely dry o r e from the d ry ing h e a r t h s i s b a l l m i l l e d and a i r c l a s s i f i e d be fo re being blown i n t o t h e top of t h e r o a s t e r . Ca lc ines from t h e h e a r t h , t h e waste h e a t b o i l e r , cyclones and C o t t r e l l s a r e blended t o g e t h e r , cooled and t r a n s p o r t e d by screw and e l e v a t o r t o s t o r a g e hoppers. Cleaned r o a s t e r gas i s used t o produce 94% s u l f u r i c a c i d i n a Monsanto des igned c o n t a c t a c i d p l a n t .

PROCESS METALLURGY

M e t a l l u r g i c a l l y s p e a k i n g , t h e n o d u l i z i n g p r o c e s s c o n s i s t s of s imply h e a t i n g z i n c c a l c i n e a t a c o n t r o l l e d r a t e t o t h e p o i n t 'of i n c i p i e n t f u s i o n s o t h a t t h e h e a t and t h e r o l l i n g a c t i o n w i t h i n t h e k i l n produces a g e n e r a l l y s p h e r i c a l nodule which i s dense bu t a t t h e same t ime po rous . Dur ing t h e h e a t i n g p e r i o d and a t t h e p o i n t o f maximum t empe ra tu r e i n t h e k i l n t h e v o l a t i l e i m p u r i t i e s o f t h e z i n c c a l c i n e a r e removed and swept o u t w i t h t h e bu rne r g a s e s . Those c o m o n e l emen t s g e n e r a l l y removed a r e s u l f u r , c h l o r i n e , l e a d and cadmium. P a r t o f t h e s u l f u r i n t h e c a l c i n e e x i s t s a s z i n c s u l f a t e formed i n t h e r o a s t e r . Such s u l f a t e s u l f u r i s removed by t he rma l decompos i t i on s i n c e t h e maximum t e mpe ra tu r e o f t h e nodu l e s ( abou t 1 2 3 0 ' ~ ) is w e l l above t h e decompo- s i t i o n p o i n t of z i n c s u l f a t e . S u l f i d e s u l f u r is removed by s imp le o x i d a t i o n t o S02. S u l f u r , a s w e l l a s l e a d and cadmium, a r e a l s o removed by t h e double decompos i t ion r e a c t i o n s o f l e a d and cadmium s u l f i d e s w i t h t h e i r r e s p e c t i v e o x i d e s t o produce t h e m e t a l l i c mo lecu l e and s u l f u r d i o x i d e . These v a p o r i z e d m e t a l s a r e immediately o x i d i z e d and swept from t h e k i l n by t h e bu rne r ga se s .

The major chemica l r e a c t i o n s i n t h e n o d u l i z e r a r e a s f o l l o w s :

2 ZnSOq -- 2 ZnO + 2 SO2 + 0 2 ZnS + 02 1- ZnO + SO2 PbS + 2 PbO = 3 Pb + SO2 CdS + 2 CdO 3 Cd + SO2

S i n c e t h e o p e r a t i n g t empe ra tu r e is w e l l above t h e v o l a t i l i z a t i o n p o i n t of cadmium, a v e r y h i g h p e r c e n t a g e o f cadmium removal f rom t h e c o n c e n t r a t e s c an be expec t ed . Lead w i l l a l s o be removed f rom t h e nodu l e s b u t t h e p e r c e n t a g e o f removal is g e n e r a l l y less t h a n t h a t f o r cadmium and much more c a r e must be t aken i n t h e o p e r a t i o n i n o r d e r t o e f f e c t a p p r o p r i a t e l e a d removal .

C h l o r i n e i n t h e c a l c i n e w i l l p robab ly be p r e s e n t as z i n c c h l o r i d e which w i l l s imply be v o l a t i l i z e d and removed w i t h t h e s u l f u r d i o x i d e and m e t a l l i c o x i d e s . F i g u r e 1 on t h e nex t page shows t h e vapo r p r e s s u r e s o f t h e v a r i o u s compounds p r e s e n t i n t h e k i l n and i n d i c a t e s t h e r e l a t i v e e a s e w i t h which cadmium and l e a d can be removed from t h e c a l c i n e .

Both l e a d and cadmium removal are dependent upon a n adequa t e s u p p l y of s u l f i d e s u l f u r i n t h e f e e d m a t e r i a l . I n a c t u a l p r a c t i c e i t has been found t h a t a b o u t 3% s u l f i d e s u l f u r w i l l e f f e c t s a t i s f a c t o r y removal o f l e a d and cadmium from t h e normal o r e s . I f f o r some r e a s o n t h e f e e d m a t e r i a l i s p a r t i c u l a r l y h i g h i n l e a d and cadmium o r i f i t is d e s i r e d t o produce a p a r t i c u l a r l y p u r e nodule , a somewhat h i g h e r s u l f i d e s u l f u r f e e d t o t h e n o d u l i z e r may be n e c e s s a r y . It might be p o i n t e d o u t t h a t t o o much s u l f i d e s u l f u r , however, i s d i f f i c u l t t o remove and may appear t o some e x t e n t i n t h e f i n a l nodu l e . At t h i s p o i n t t h e s u l f u r i s p a r t i c u l a r l y d e l e t e r i o u s s i n c e i t w i l l i n h i b i t

Figure 1. Vapor Pressures of Se lec ted Compounds

the recovery of z inc i n e i t h e r the z inc oxide o r the zinc metal furnace. Nodule dens i ty produced w i l l vary from 1000 t o 2320 kg/ cu.m. depending upon t h e type of concen t ra tes being processed. If a s u f f i c i e n t l y high grade z inc concen t ra te i s used fo r r o a s t i n g , the z inc content of an average nodule w i l l be from 68% t o 72% w i t h a s u l f u r con ten t of l e s s than .5% and a lead con ten t of .l% t o .5%. The cadmium content of the nodules w i l l genera l ly range from .01% t o .04% unless an ore of p a r t i c u l a r l y high cadmium content i s being t r e a t e d . Over the years some high grade f l o t a t i o n concentra tes have been t r e a t e d which produced nodules having a z inc content of 75% with a lead con ten t of l e s s than .01% and a cadmium content of l e s s than .005%. Conversely, nodules made from some high i r o n fo re ign concen- t r a t e s have produced nodules a s low as 62% i n z inc content .

Af te r the nodulized o r e f a l l s from the k i l n i t is cooled, crushed and screened i n order t o produce mate r ia l of appropr ia te s i z e f o r smel t ing. Generally speaking, the most s a t i s f a c t o r y s i z e f o r ordinary smel t ing opera t ions has been considered t o be 6.35 mm. The screening and s i z i n g process w i l l n a t u r a l l y produce some f i n e s by abras ion, even though the o re has been completely agglomerated i n the k i l n i t s e l f . A small percentage of f i n e s i n the f i n a l product has not genera l ly been considered d e l e t e r i o u s t o the even tua l use of the nodules.

DESCRIPTION OF PHYSICAL PLANT

At Galena c a l c i n e produced by t h e r o a s t e r i s s t o r e d i n t h r e e l a r g e e l e v a t e d hoppers having a nominal c a p a c i t y o f 500 m e t r i c tons o f c a l c i n e each. These c a l c i n e hoppers a r e equipped w i t h c o l l e c t i n g screws and a f eed screw system which d e l i v e r s c a l c i n e t o t h e nodu- l i z i n g k i l n s themselves . The k i l n i s f e d by t h e s imple expedient of a l lowing c a l c i n e t o s l i d e down an i n c l i n e d s t a i n l e s s s t e e l p i p e 20.4 cm i n d iameter . This feed p ipe extends i n t o t h e end of t h e k i l n and t e rmina te s a s h o r t d i s t a n c e above t h e l i n i n g i n o rde r t h a t t he f eed w i l l be d e p o s i t e d a s q u i e t l y a s p o s s i b l e . The top of t h e f e e d p i p e pas ses through a b r i c k a r c h over a d u s t chamber a t t h e f eed end of t h e k i l n . Some p o r t i o n of t h e f eed i s swept o u t of the k i l n a lmost immediately w i t h t h e burner gases b u t s e t t l e s i n t h e chamber and i s r e t u r n e d t o t h e f e e d system.

The b a s i c n o d u l i z i n g u n i t c o n s i s t s of a r o t a r y k i l n 2.625 m i n s i d e d iameter and 39.37 m long. The k i l n s h e l l i s made of 15.875 mm mi ld s t e e l p l a t e except f o r a 1 m s e c t i o n a t t he d i scha rge end which i s 19.05 mrn s t e e l . The k i l n s have two t i r e s w i t h a 35.56 cm f a c e running on s i n g l e t r u n i o n r o l l e r s f o r a f a c e w i t h 40.46 cm. The d r i v e mecha- nism i s by means of a conven t iona l e l e c t r i c motor and speed r educe r d r i v i n g a 16 t o o t h p i n i o n and a 30.48 cm f a c e main gea r c o n t a i n i n g 148 t e e t h . Although a u x i l i a r y d r i v e s f o r use d u r i n g power i n t e r r u p - t i o n s were cons idered , none was provided. Through t h e y e a r s t h e d e c i s i o n t o omit t h i s equipment has been proven t o be economically and t e c h n i c a l l y j u s t i f i a b l e . The s l o p e of t h e k i l n i s s e t a t approxi - mately 2.385' and t h e nominal r o t a t i o n a l speed i s 1 .33 rpm. O r i g i n a l l y t h e f eed end of t h e k i l n was f i t t e d w i t h a r e f r a c t o r y dam made of s p e c i a l r e f r a c t o r y b locks . This c o n s t r u c t i o n was somewhat u n s a t i s - f a c t o r y over t h e y e a r s and has more r e c e n t l y been r e p l a c e d w i t h a s e t of p r o p r i e t a r y c a s t i n g s c o n s i s t i n g of p i e shaped a l l o y s t e e l segments b o l t e d d i r e c t l y t o t h e k i l n s h e l l . The o r i g i n a l i n s t a l l a t i o n of t h e s e c a s t i n g s i s s t i l l i n e x c e l l e n t c o n d i t i o n a f t e r cont inuous o p e r a t i o n f o r some e i g h t y e a r s . The o t h e r k i l n has a l s o r e c e n t l y been s o modi- f i e d . At t h e d i scha rge end of t h e k i l n a s e t of h igh a l l o y "T" shaped c a s t i n g s a r e used t o r e t a i n the l i n i n g . These c a s t i n g s have a l i f e expectancy of s e v e r a l y e a r s be fo re warpage makes f u r t h e r u t i l i z a t i o n imposs ib l e . The o r i g i n a l c a s t i n g s were 38.1 mm t h i c k but have been r ep laced by ones 19.05 mm t h i c k which d i s s i p a t e h e a t f a s t e r and warp l e s s , t hus prolonging u s e f u l l i f e . The k i l n i s l i n e d w i t h 15.24 cm f i r s t q u a l i t y f i r e b r i c k us ing conven t iona l k i l n l i n e r s .

A s the nodules f a l l from the k i l n they pass through a heavy g r i z z l y made of r a i l r o a d r a i l s w i t h openings of about 1 5 cm. The g r i z z l y s e r v e s t o p r o t e c t a v i b r a t i n g conveyor which conveys t h e nodules t o a high tempera ture e l e v a t o r . The e l e v a t o r d e l i v e r s t h e product t o an un l ined c o o l i n g drum 1.968 m i n d iameter and 19.68 m long. At

t h e end o f t h e coo l ing drum t h e r e i s a heavy duty jaw c r u s h e r t o reduce t h e l a r g e p i e c e s . A f t e r pas s ing through t h e c r u s h e r , t he nodules go over a v i b r a t i n g s c r e e n , t h e o v e r s i z e of which i s i n

General view of nodulizing kiln.

closed c i r c u i t with the heavy duty r o l l crusher . A l l the product i s thereby reduced t o a maximum s i z e of 6.35 mm and emptied i n t o the product e l e v a t o r which d e l i v e r s the nodules t o two 500 met r i c ton f i n i s h e d s t o r a g e hoppers. These s to rage hoppers a r e e l e v a t e d i n such a manner t h a t nodules can be loaded i n t o covered hopper c a r s through a simple loading pipe using g r a v i t y t o accomplish loading. This system was devised because the nodules a r e being de l ive red t o two s e p a r a t e p l a n t s f o r even tua l use.

The k i l n i s f i r e d w i t h a dual f u e l f a n mix burner having a capaci ty 8 of 4.9 x 10 J / s . The nominal f i r i n g r a t e i s about 2.1 x lo8 J / s

us ing n a t u r a l gas. The f a n mix burner has a d e s i r a b l e h e a t p a t t e r n r e s u l t i n g i n adequate temperature c o n t r o l throughout the l eng th of the k i l n . Also, an important p a r t of t h e temperature c o n t r o l i s the amount o f induced d r a f t app l i ed t o the k i l n i t s e l f . Af te r the burner gases and d u s t and fume pass through the k i l n i n t o the d u s t chamber a t the end they go through a cyclone 3.937 m i n diameter f o r the removal o f l a r g e s o l i d p a r t i c l e s of c a l c i n e . From the cyclone the fume i s conveyed through a s t e e l t r a i l t o a s tandard automatic bag- house. The fume i s c o l l e c t e d cont inuously using g l a s s f i b e r bags (because o f h igh temperature) . The fume f a l l s from the bags i n t o a l a r g e holding hopper beneath the bagroom c e l l s from whence i t i s conveyed by means of a screw conveyor t o a holding hopper. The c l e a n a i r pass ing through the bags goes through an induced d r a f t f an and i s exhausted i n t o the atmosphere.

The o r i g i n a l gas coo l ing system c o n s i s t e d o f 16 coo l ing loops 1 m i n diameter and 13.125 m t a l l mounted on a continuous hopper from the s e t t l i n g chamber t o t h e baghouse. These loops provided excess ive coo l ing t o the e x i t gas, b r ing ing i t below t h e dew po in t and causing severe co r ros ion t o t h e coo l ing system and baghouse. Therefore , these loops were rep laced wi th t h e p resen t cyclone and t r a i l which served t o s impl i fy opera t ion and maintenance and make i t p o s s i b l e t o mainta in gas temperature w e l l above t h e dew p o i n t a t t h e baghouse i n l e t .

The o r i g i n a l v i b r a t i n g conveyor handl ing t h e h o t nodules has been rep laced wi th a l a r g e r and s t ronger u n i t t o more e f f e c t i v e l y combat the hea t and abras ion o f the product as i t f a l l s from the k i l n . Minor modi f i ca t ions have a l s o been made i n the crushing and screening c i r c u i t

The flow diagram on the succeeding page shows the flow of z inc c a l c i n e through t h e c i r c u i t and the pa th taken by the fume and gases e x i t i n g t h e k i l n .

OPERATING TECHNIQUE

Feed r a t e t o the k i l n i s c o n t r o l l e d a t the c a l c i n e feed hoppers by a d j u s t i n g a feed c o n t r o l ga te on each hopper. Since a l l t h r e e hoppers a r e connected by a double s e t of s c r e w s , i t i s poss ib le t o mix m a t e r i a l s vo lumet r i ca l ly from any combination of hoppers.

Figure 2 . Process F l o w Diagram

R e t e n t i o n t ime i n t h e k i l n i s about one hour under normal o p e r a t i n g c o n d i t i o n s . The burner and d r a f t (measured by a r eco rd ing d r a f t gauQa i n t h e t ra i l j u s t beyond t h e d u s t chamber) a r e i n i t i a l l y s e t t o a t t a l n a product tempera ture o f about 1200°C and a n e u t r a l d r a f t c o n d i t i o n a t t h e d i s c h a r g e end of t h e k i l n . As soon a s nodules a r e d i scha rged at a s t eady r a t e , t he p roduc t tempera ture about 5 m i n s i d e t h e k i l n i s de termined by use o f an o p t i c a l pyrometer . The i r r e g u l a r s i z e and t h e a b r a s i v e n e s s and tempera ture o f t h e nodules p r o h i b i t t h e use of a thermocouple f o r c o n t r o l purposes . Exper ience has shown a p o r t a b l e o p t i c a l pyrometer t o be a most s a t i s f a c t o r y c o n t r o l dev ice . S ince t h e o p e r a t i o n o f t h e sys tem i s cont inuous and comple te ly mechanized, t h e o p e r a t o a s main d u t i e s are t o check t h e f eed r a t e , t h e d r a f t and t h e nodule tempera ture . Samples a r e taken each hour and examined f o r p h y s i c a l c h a r a c t e r i s t i c s and s e n t t o t h e l a b o r a t o r y f o r a n a l y s i s i f t i g h t chemical c o n t r o l is r e q u i r e d . The expe r i enced o p e r a t o r can de termine immediately i f t h e p rope r tempera ture has been a t t a i n e d . C o r r e c t l y f i r e d nodules w i l l be dense and d u s t l e s s bu t s t i l l porous a s can r e a d i l y be de termined by dropping a few drops of water on t h e nodule s u r f a c e . O v e r f i r e d nodules w i l l p r e s e n t a ha rd g l a s s y s u r f a c e .

Ope ra t ing expe r i ence has shown t h a t s t r i c t d r a f t c o n t r o l i s neces sa ry . I f any exces s a i r i s p u l l e d through t h e k i ln , t h e r e s u l t a n t h e a t l o s s causes a h ighe r f i r i n g r a t e and a h ighe r gas v e l o c i t y through t h e k i l n r e s u l t i n g i n a h igh deg ree o f removal of c a l c i n e from t h e k i l n p l u s e x c e s s i v e f u e l c o s t s . F l a s h r o a s t e d c a l c i n e i n g e n e r a l i s ve ry f i n e and eve ry e f f o r t needs t o be made t o r e t a i n t h e c a l c i n e i n t h e k i l n u n t i l h igh enough tempera ture i s r eached t o cause t h e p a r t i c l e s t o c o a l e s c e . As a c o r r e l a r y a h igh v e l o c i t y d e l i v e r s c o n s i d e r a b l e amount o f c a l c i n e through t h e c o o l i n g sys tem t o t h e baghouse and d i l u t e s t he q u a l i t y o f t h e fume produced. Th i s baghouse fume c o n t a i n s t h e l e a d and cadmium u n i t s removed from t h e c a l c i n e and i s subsequent ly t r e a t e d by conven t iona l wet methods f o r t h e recovery o f t h e s e two v a l u a b l e me ta l s . Over t h e y e a r s two o t h e r t y p e s o f bu rne r s have been f i t t e d on the n o d u l i z i n g k i l n s . One of t h e s e was a 100% premixing type which pe rmi t t ed a ve ry a c c u r a t e me te r ing o f t h e gas and a i r , wh i l e t h e o t h e r was a l s o a f u l l y p ro - p o r t i o n a l bu rne r - sys t em o f t h e nozz l e mixing v a r i e t y . Both o f t h e s e bu rne r systems worked s a t i s f a c t o r i l y b u t i n t h e l ong r u n gave no advantage over t he o r i g i n a l f a n mix system, which has been r e t u r n e d t o a c t i v e o p e r a t i o n i n b o t h o f the k i l n s a t Galena.

Fume from t h e n o d u l i z e r may c o n t a i n from 5% t o 15% cadmium and from 10% t o 20% l e a d , depending upon t h e type of o r e s be ing t r e a t e d . As has been i n d i c a t e d e a r l i e r , primary c o n t r o l of l e a d and cadmium e l i m i n a t i o n i s made by adjus tment o f t h e s u l f i d e s u l f u r c o n t e n t i n t h e feed . Secondary c o n t r o l c a n be e x e r c i s e d by t h e o p e r a t o r by v a r i a t i o n i n t h e f eed r a t e , k i l n atmosphere and tempera ture g r a d i e n t . It i s a l s o p o s s i b l e t o p u t a r educ ing agen t such a s c o a l i n t h e f eed and thereby e f f e c t some h ighe r pe rcen tage o f removal. Th i s p a r t i c u l a r

o p e r a t i o n may be o f i n t e r e s t i f t h e r e i s a d e s i r e t o produce a p a r t i - c u l a r l y low s u l f u r nodule and a t t h e same t ime e f f e c t a good degree

of cadmium and lead removal. It might be pointed out t h a t i n order t o permit the double decomposition s u l f i d e r eac t i on t o take place i n the k i l n , i t i s necessary t o exclude p r a c t i c a l l y a l l of the oxygen i n t he feed ha l f of the k i l n . Otherwise, s u l f i d e s u l f u r i n the feed i s simply burned t o SO2 and escapes the k i l n without taking place i n any fu r t he r meta l lu rg ica l reac t ions . It might a l so be added t h a t t h i s burning of s u l f u r c r ea t e s an exothermic reac t ion a t the wrong place and may cause the feed mater ia l t o overheat and s t i c k t o the k i l n l i n ing . This l o c a l overheating and s t i ck ing w i l l produce r i ngs i n t he k i l n and may i n f a c t s t o p the operat ion completely. High s u l f i d e s u l f u r s i n the feed o r e r r a t i c and varying s u l f i d e s u l f u r w i l l a l so cause continued d i f f i c u l t y i n the nodulizer operation. Varying s u l f u r contents makes i t almost impossible f o r the k i l n operator t o maintain an even temperature through the k i l n . This s i t u a t i o n w i l l a f f e c t the lead and cadmium removal and su l fu r removal and the dens i ty of the f i n a l nodule produced. The presence of a s u l f u r f i r e i n the k i l n o r odor of SO2 a t discharge end i s a quick ind ica t ion of excessive s u l f u r i n the feed. For s im i l a r reasons the feed r a t e i t s e l f must be maintained a t a constant r a t e once the f i r i n g and d r a f t condi t ions have been es tab l i shed .

Some years ago, one k i l n was fed with pe l l e t i z ed ca lc ine f o r a period of about four months. This t e s t work showed a s l i g h t increase i n nodule densi ty , improved feed r a t e con t ro l , l e s s mechanical ca r ry- over t o the baghouse and some decrease i n accret ions i n the k i l n . However, these advantages were not deemed s u f f i c i e n t t o compensate f o r the added cos t of p e l l e t i z i n g and the p ro j ec t was discontinued. It i s poss ib le t h a t with today's improved p e l l e t i z i n g techniques r e a l economy might be had by using pe l l e t i z ed feed.

Another technique explored i n the pa s t was the p o s s i b i l i t y of automatic temperature con t ro l . For t h i s purpose a con t ro l instrument was ac t i va t ed by thermocouples through the k i l n s h e l l and re f rac tory l i n ing . These couples were located a t each end and cen te r of the ac tua l nodulizing zone and were connected through s l i p r i ngs t o the instrument. This procedure was a l so abandoned because accre t ions and breakage of thermocouple housing frequent ly rendered the system inoperat ive.

The s h e l l a t t he discharge end of the k i l n , being subjected t o the highest temperature i n t he process, w i l l eventual ly b e l l and make i t impossible t o hold the k i l n l i n e r s i n place. This be l l i ng process a l so warps the cas t ings and makes t h e i r f u r t he r use impossible. The k i l n l i n e r s l a s t i nde f in i t e l y s i nce the operat ing temperature of the k i l n i s somewhat below the sof tening po in t of f i r s t qua l i t y f i r e b r i c k and there i s l i t t l e , i f any, chemical a t t a ck . However, the product adheres t o the br ick l i n i n g and must f requent ly be removed by jack hauunering. This cleaning of the k i l n , plus some spa l l i ng ac t i on as the k i l n temperature i s brought up and down, r e s u l t s i n eventual f a i l u r e of the l in ing , espec ia l ly i n the l a s t half of the k i l n . Refractory replacement cos t i s not a l a rge item

bu t i t w i l l be necessary t o r e p l a c e s e v e r a l r i n g s of l i n e r s du r ing any one o p e r a t i n g y e a r . During long p e r i o d s o f o p e r a t i o n of t h e k i l n i t is a lmost imposs ib le t o avoid t h e format ion o f r i n g s a t one time o r ano the r . These r i n g s i n h i b i t t he f low o f product through t h e k i l n and must, o f cour se , from time t o t i m e be removed. The o r i g i n a l concept o f t h e o p e r a t i o n was t o shoo t t h e r i n g s o u t u s ing an i n d u s t r i a l gun and a l e a d s l u g about 1 3 m i n d iameter . However, t h i s procedure was n o t always s a t i s f a c t o r y so t h e p r a c t i c e was ceased and now t h e k i l n i s cooled and any r i n g s o r excess ive adherence t o t h e s i d e of t h e k i l n i s removed by j ack h m e r . I n e f f e c t , t h e s e r i n g s a r e product and s o they a r e passed on through t h e c rush ing and s c r e e n i n g system i n t o t h e f i n a l s t o r a g e hopper.

Due t o t h e ext remely a b r a s i v e n a t u r e of t h e f i n i s h e d nodule t h e c rush ing and sc reen ing system a r e s u b j e c t t o r a t h e r r a p i d wear and c o n s t a n t replacement of wearing p a r t s . The s h e l l of t h e c o o l i n g drum i t s e l f has been comple te ly worn through and has had t o be r e p l a c e d i n i t s e n t i r e t y . Repa i r s t o t h e l i n i n g i t s e l f have been nominal a s i n d i c a t e d above and maintenance of t i r e s , t r u n i o n s and d r i v e mechanism c o n s i s t e n t w i th good k i l n p r a c t i c e i n gene ra l . There i s c o r r o s i o n of t h e t r a i l from t h e cyclone t o the baghouse brought about by t h e SO2 c o n t e n t of t h e gas and t h e water vapor produced by t h e gas burner . This c o r r o s i o n would probably be minimal were t h e k i l n i n cont inuous o p e r a t i o n , however, t h e shutdowns f o r r e p a i r s t o t h e s c r e e n i n g and c rush ing system and f o r t h e removal of r i n g s from the k i l n permi t coo l ing and condensat ion of t h e t r a i l system wi th r e s u l t a n t c o r r o s i o n due t o the SO2 con ten t of t h e gases .

Manpower requi rements f o r normal o p e r a t i o n a r e one man pe r s h i f t pe r nodu l i z ing k i l n p lus one bagroom a t t e n d a n t d u r i n g t h e day s h i f t on ly . Another man no t a member of t h e o p e r a t i n g crew loads f i n i s h e d nodules i n t o hopper c a r s on a f i v e s h i f t pe r week b a s i s s i n c e s t o r a g e c a p a c i t y i s ample t o hold t h e weekend product ion . E x t r a h e l p i s provided t h e o p e r a t i n g crew when i t i s necessary t o c l e a n t h e k i l n and c rush the a c c r e t i o n s removed.

I n Table I a r e g iven r e p r e s e n t a t i v e a s says of t h e c a l c i n e f e d t o t h e nodu l i z ing k i l n and t h e z i n c nodules and baghouse fume produced from such f e e d m a t e r i a l . Except f o r t he l a s t s e t , t h e c a l c i n e was produced from a b lend of z i n c concen t r a t e s des igned t o produce a uniform product t o be used i n t h e p roduc t ion o f m e t a l l i c z inc . The l a s t example i s t y p i c a l of a low cadmium o r e used t o produce nodules f o r z i n c oxide manufacture .

As i n d i c a t e d above, v a r i a t i o n s i n s u l f i d e s u l f u r c o n t e n t and /o r tempera ture can produce undes i r ab le r e s u l t s . I n Table I1 a r e shown

t y p i c a l r e s u l t s which may r e s u l t from t h e s e f a c t o r s .

The f i r s t two s e t s i n Table I1 show t h a t a very h igh s u l f u r c o n t e n t i n t h e c a l c i n e w i l l r e s u l t i n e x c e l l e n t l e a d e l i m i n a t i o n bu t t h e excess ive s u l f u r i n t h e nodules cannot be t o l e r a t e d by t h e s m e l t e r .

Table I. Representat ive Assays of-Reelar Production

S u l f i d e S u l f u r Zn Pb C d S u l f u r i n SO&

Calcine 69.2% 1.57% .58% 2.92% -37% Nodules 71 .O .28 .037 .55 .05 Fume 45.1 20.1 12.32 .60 4.29

Calcine 70.0 .68 .40 3.7 .1 Nodules 71.3 .ll .032 .50 .08 Fume 33.1 25.4 14.4 .60 6.8

Calcine 69.1 1.18 .61 2 .O .6 Nodules 71.4 .15 .013 .55 .07 Fume 41.6 22.0 13.5 1.7 5.1

Calcine 68.1 .45 .13 3.21 -27 Nodules 70.9 .017 .002 .47 .06 Fume 43.8 14.2 4.92 .83 5.2

Table 11. Representative Assays of Undesirable Operation

S u l f i d e S u l f u r Zn Pb C d S u l f u r i n SO& - -

Calcine 68.6% .5 3% -34% 4.98% .33% Nodules 7 1 . 1 .029 .010 2.26 .07 Fume 55.9 10.1 5.5 .30 3.8

Calcine 66.2 1.33 .46 7.3 .60 Nodules 70.8 .02 Tr 3.18 .02 Fume 52.2 12.3 5.9 . l o 3.4

Calcine 68.6 1.20 .66 2.47 .52 Nodules 71.8 .019 Tr 1 .3 .03 Fume 55.5 13.0 7.1 .92 3.24

Calcine 68.5 1.03 .49 4.22 .45 Nodules 70.6 .035 .005 2.34 .03 Fume 46.2 18.6 7.7 .50 3 .5

Calcine 68.9 .86 .31 4.0 . 3 Nodules 72.0 . 0 36 .005 .33 .02 Fume 58.6 7.1 3.5 .26 3.57

Calcine 69.2 .73 .37 3.27 .33 Nodules 71.9 .013 Tr .21 .02 Fume 62.9 8 .8 4.7 .60 2.14

Examples t h r e e and four a l s o i n d i c a t e undes i r ab le r e s u l t s brought about by improper temperature and d r a f t c o n t r o l i n the nodu l i z ing k i l n i t s e l f . Again e l i m i n a t i o n i s good bu t excess ive s u l f u r remains i n t h e f i n a l product .

The l a s t two examples show r e s u l t s from o p e r a t i o n s i n which c o a l has been added t o t h e c a l c i n e feed. I n t h e s e cases e l i m i n a t i o n o f l ead and cadmium i s e x c e l l e n t and the nodules produced a r e q u i t e s a t i s f a c t o r y . However, t h e fume has been d i l u t e d by an excess amount of z i n c (reduced by t h e carbon) and i s thus a much l e s s d e s i r a b l e product f o r f u r t h e r t rea tment .

SUMMARY

Nodulizing o f z inc c a l c i n e has proven t o be t h e most e f f i c i e n t method o f agglomerating t h e s u p e r f i n e f l a s h r o a s t e d m a t e r i a l . Pro- duc t ion per u n i t over s e v e r a l y e a r s has averaged 150 f_ 5 tons per day. This c a p a c i t y has exceeded r o a s t e r p roduc t ion enough t o permit thorough c l ean ing o f t h e k i l n s when r e q u i r e d and replacement of worn equipment when needed. A majo r i ty of t h e unscheduled i n t e r r u p t i o n s i n o p e r a t i o n have been occasioned by f a i l u r e of t h e h o t shaker conveyor o r some component of t h e c rush ing and s i z i n g equipment.

Treatment o f new types of o r e s has from t i m e t o t i m e r e q u i r e d adjustment i n f i r i n g techniques . It i s l i t e r a l l y t r u e t h a t each z i n c concen t ra t e i s s l i g h t l y d i f f e r e n t from another and any v a r i a t i o n i n t h e r e f r a c t o r y q u a l i t y w i l l n e c e s s i t a t e a s l i g h t l y a l t e r e d nodu- l i z i n g t rea tment . However, t h e s e d i f f i c u l t i e s have n o t prevented economic and m e t a l l u r g i c a l success of t h e nodu l i z ing k i l n s . The use of the z i n c nodules a t consuming p l a n t s has improved the q u a l i t y of t h e end products and i n some cases inc reased c a p a c i t i e s and recovery .

Acknowled~ements: The author g r a t e f u l l y acknowledges the inva luab le a s s i s t a n c e of t h e Galena, Kansas p l a n t s t a f f and t h e Research Depart- ment, Jop l in , Missour i , i n t h e p r e p a r a t i o n of t h i s manuscript .