Chapter 19

THE VERTICAL RETORT Z I N C SMELTER

AT NEW JERSEY Z I N C COMPANY Depue, I l l i n o i s

L. D. F e t t e r o l f , Technical A s s i s t a n t t o D i r e c t o r , Research and Development-Palmerton

W. R. Bechdolt , Manager of t h e Depue P l a n t

V. S t i l o , A s s i s t a n t Manager of t h e Depue P l a n t

J. A. Motto, P a t e n t s Department-Palmerton

Abs t rac t

The New J e r s e y Zinc Company opera tes a t Depue, I l l i n o i s , an i n t e g r a t e d z i n c smel t ing p l a n t us ing t h e v e r t i c a l r e t o r t re- duc t ion process . The o v e r a l l ope ra t ion conpr i ses green con- c e n t r a t e r o a s t i n g , s i n t e r i n g , v e r t i c a l r e t o r t smel t ing and meta l r e f in ing . I n t h e r o a s t i n g opera t ion , t h e f i n e concen t ra te i s p e l l e t i z e d , t h e d r i e d p e l l e t s f l u i d i z e d and autogenously r o a s t e d with a i r i n a " f l u i d column" tr i th minimal format ion of d u s t r e q u i r i n g r e c i r c u l a t i o n . The roas ted p e l l e t s and any assoc ia ted r o a s t e r d u s t a r e agglomerated i n a r o t a t i n g drum and l a i d down, i n combination with c o a l f o r f u e l , on a Llwight-Lloyd s i n t e r i n g machine which conver ts t o a semi-fused nass - a phys ica l s t a t e more s u i t a b l e f o r v e r t i c a l r e t o r t smel t ing. The smel t ing p l a n t of 27 v e r t i c a l r e t o r t s r a t e d a t 8 T z i n c each pe r day on average r e q u i r e s t h e following major process ing s t e p s - mix p r e p r a t i o n and b r i q u e t t i n e , b r i q u e t coking, v e r t i c a l r e t o r t r educ t ion , vapor condensation and s l a b c a s t i n g of metal . k por t ion of z i n c production i s converted t o 99,99+ percant p u r i t y by b o i l i n g and f r a c t i o n a l d i s t i l l a t i o n i n columns c o n s i s t i n g of e x t e r n a l l y heated, superinposed t r a y s of high-conductivity r e f r a c t o r y .

P l a t e I

A e r i a l View of Depue P l a n t w i t h V e r t i c a l R e t o r t P l a n t i n Foreground - L e f t

514 EXTRACTIVE METALLURGY OF LEAD AND ZINC

In t roduc t ion

The New Je r sey Zinc Company has operated a z inc smel te r a t Depue, I l l i n o i s , s i n c e 1905, the l o c a t i o n being 90 mi les southwest of Chicago i n proximity t o a concentra ted z inc metal market. The geographic s i t - u a t i o n with respec t t o raw m a t e r i a l s i s a favorable compromise i n view of t h e s c a t t e r e d sources of t h e d i v e r s e raw m a t e r i a l s required.

P r i o r t o 1929, z i n c smel t ing was accomplished by t h e then conven- t i o n a l h o r i z o n t a l r e t o r t process involving use o f thousands o f small r e t o r t s opera t ing a s ind iv idua l u n i t s . I n 1929, h igh capac i ty v e r t i - c a l r e t o r t s were placed i n opera t ion and smel t ing capac i ty has s i n c e been expanded on t h i s bas i s .

A major expansion was completed i n 1967 with c o n s t r u c t i o n of a l l f a c i l i t i e s required f o r an i n t e g r a t e d z i n c smel t ing p l a n t - namely, z i n c concen t ra te r o a s t i n g , s i n t e r i n g , z inc smel t ing, metal r e f i n i n g and cas t ing .

PART A - FLUID COLUMN ROASTING

The z inc concen t ra tes a v a i l a b l e today a r e predominately z inc s u l - f i d e which mineral form r e q u i r e s r o a s t i n g t o z inc oxide f o r amenabil- i t y t o reduc t ion with carbon. With a few p o s s i b l e except ions , t h e r e a r e t h r e e somewhat s i m i l a r r o a s t i n g methods c u r r e n t l y i n use - a l l based on ox ida t ion dur ing suspension i n a medium of a i r . The choice f o r Depue was t h e F lu id Column r o a s t i n g process developed by The New Jersey Zinc Company and l i censed t o Sherbrooke Meta l lu rg ica l Company i n Por t Maitland, Canada. This method d i f f e r s from o t h e r s i n t h a t t h e green concen t ra te i s p e l l e t i z e d (through 4 and on 28 mesh - Tyler Standard) prel iminary t o roas t ing . There is l i t t l e degradat ion o f p e l l e t s dur ing f l u i d a c t i o n with the r e s u l t t h a t carryover of gas- en t ra ined dus t from the r o a s t i n g chamber i s minimized a s compared with i n excess o f 50 percent d u s t carryover dur ing f l a s h roas t ing . Furthermore, t h e coarse c a l c i n e may b e crushed t o a s i z e s u i t a b l e f o r smel t ing mix with avoidance of s i n t e r i n g whereas t h e very f i n e c a l c i n e o f o t h e r r o a s t i n g methods must be s i n t e r e d t o provide s u i t - a b l e s i z i n g . I n a d d i t i o n , p e l l e t r o a s t i n g o f f e r s t h e p o s s i b i l i t y of opera t ion wi th over 90 percent e l imina t ion of lead and cadmium. This provides smel te r feed f o r d i r e c t product ion of z i n c a t lead and cadmium l e v e l s which meets some market requirements wi thout r e f i n i n g . I f e l imina t ion r o a s t i n g i s n o t p r a c t i c e d , roas ted p e l l e t s c o n s t i t u t e i d e a l feed fo r s i n t e r i n g machine e l imina t ion o f lead and cadmium a t g r e a t l y increased s i n t e r i n g machine capac i ty .

Raw Mate r ia l s and Handling

The m a t e r i a l s involved a r e p r imar i ly green concen t ra tes , r e c i r c u - l a t e d m a t e r i a l s i n the forms of green p e l l e t f i n e s and r o a s t e r d u s t , ben ton i t e b inder , s u l f u r i c a c i d f o r r e a c t i o n t o produce z inc s u l f a t e

binder and t h e water r equ i red f o r p e l l e t formation. The green o r e concen t ra tes come from Company mines wi th supplement by purchased concen t ra tes . The z i n c grades of t h e concen t ra tes va ry widely; s u l - f u r con ten t s vary some; i r o n may range from 1-12 pe rcen t ; lead and cadmium d e v i a t e widely from average but i n a low percentage range. An average concen t ra te a n a l y s i s i s a s follows:

Zn - 52% Minerals Fe - 10 ZnS - 77.4% S - 32 FeS - 15.7 PE - 0.65 Cd - 0.22 Gangue - 6

Concentra tes and b e n t o n i t e a r e received i n boxcars o r covered hopper-bottom c a r s which a r e unloaded i n t o a t r a c k hopper - thence v i a b e l t conveyors - e l e v a t o r - b e l t conveyor t o e i t h e r of two cov- e red s t o r a g e bu i ld ings o r d i r e c t l y t o process supply bins i n t h e mix p r e p a r a t i o n bu i ld ing . A d i s t r i b u t i o n b e l t s u p p l i e s t h e fol lowing i n - 1 i n e b i n s :

Bentoni te Bin - 65 T Capacity Concentrate Bins - Two a t 185 T Each

Company concen t ra tes a r e g e n e r a l l y coa r se and a r e segregated i n one of t h e 185 T b ins . Concentrate i s drawn from t h e b i n v i a t a b l e d i scharge t o feed a Hardinge A i r C l a s s i f i e r type Thermo-Mill which g r inds t o a f ineness b e t t e r than 65 percent through 325 mesh. This f ineness i s requ i red f o r good p e l l e t i n g performance and q u a l i t y pe l - l e t s . The m i l l , r a t e d a t 18.5 ~ / h r , i s d r i v e n by a 260 KW motor and i s swept wi th ho t a i r t o permit g r ind ing of concen t ra tes con ta in ing up t o 5 pe rcen t mois tu re . A l l t h e c o n t r o l s , d u s t c o l l e c t i o n equip- ment e t c . a s soc ia ted wi th a modern m i l l a r e provided.

The m i l l e d concen t ra te i s s t o r e d i n a 180 T b i n which, followed by a 135 T green p e l l e t f i n e s b i n and a 135 T r o a s t e r d u s t b in , forms wi th t h e b i n s previously mentioned an i n - l i n e s e r i e s of s i x b ins .

Mix Prepara t ion S e c t i o n

The b i n s descr ibed d i scharge v i a c a l i b r a t e d r o t a r y s t a r o r t a b l e f eeders t o a common c o l l e c t i n g b e l t i n t h e p ropor t ions r equ i red f o r t h e s e l e c t e d mix composition. A mix composit ion designed t o produce p e l l e t s a t 27 percent s u l f u r con ten t may average about a s follows:

I Weight, % ~ / h r ~ / d a *

Green Zinc Concentrate 64.5 20.5 410 Dry Green P e l l e t Fines 14 4 .5 90 Roaster Dust 11 3.7 74 Bentoni te 1 0.21 4.2 S u l f u r i c Acid (as H2S04) 1 .5 0.32 6 .4 Water (added l a t e r ) 8 3 60

I *Based on 20-hour opera t ing day.

The r e c i r c u l a t e d green p e l l e t f i n e s comprise t h e crushed o v e r s i z e and t h e through 28 mesh f i n e s r e s u l t i n g from screening d r y green p e l l e t s f o r proper r o a s t e r feed s i z i n g . The r e c i r c u l a t e d r o a s t e r dus t r e p r e s e n t s t h e b o i l e r , cyclone and p r e c i p i t a t o r d u s t s recov- ered from dust-bear ing gas l eav ing t h e r o a s t e r . Bentoni te , although s u b j e c t t o some a d d i t i o n l i m i t a t i o n s , i s an e x c e l l e n t b inder provid- ing g r e a t assis+aAlce t o pe l l a t i zg a.d t o pai ie ' i s t r e t g t h . ?Le fd2: t i~ : of s u l f u r i c a c i d i s t o r e a l i z e t h e bonding p r o p e r t i e s of z i n c s u l f a t e fonned by ac id r e a c t i o n wi th z i n c oxide i n r e c i r c u l a t e d c a l c i n e dus t .

The c o l l e c t i n g b e l t ca r ry ing t h e proport ioned mix components feeds a pug m i l l i n t o which t h e s u l f u r i c a c i d and water a r e metered t o b r ing t h e mois tu re con ten t t o 7-8 percent . Thorough mixing i s accomplished with good b inder d i s t r i b u t i o n accompanied by mix cond i t ion ing r e s u l t - ing i n improved p e l l e t i z i n g .

I P e l l e t i n g Operation

The pug m i l l d i scharge is conveyed by b e l t t o two Dravo p e l l e - t i z e r d i s c s which may be fed s i n g l y o r i n p a r a l l e l a t c o n t r o l l e d r a t e s ranging from 13-18 ~ / h r per d i s c . The 3.65 m diameter d i s c s a r e operated a t 7.5 rpm with a p i t c h of 47O. The mix on a r r i v a l a t t h e d i s c i s d e f i c i e n t i n mois ture f o r p e l l e t i n g and f i n e water sprays a r e played on t h e r o l l i n g bed i n amount requ i red t o mainta in an op- timum p e l l e t i n g cond i t ion which corresponds t o about 9 percent H20. The o b j e c t i v e i s maximum product ion of through 4 and on 28 mesh pe l - l e t s of good s t r e n g t h a s required f o r b e s t r o a s t e r opera t ion ( f l u i d - i z a t i o n , s u l f u r e l imina t ion and minimum dus t ing) .

Rather c l o s e a t t e n t i o n t o p e l l e t i z e r opera t ion i s required and t o t h i s end t h e p e l l e t i z e r s a r e loca ted near t h e c e n t r a l p l a n t c o n t r o l room so t h a t opera to r s may monitor and perform remote c o n t r o l manip- u l a t i o n s . The d i s c s d i scharge cont inuously t o a b e l t feeding a t r a v e l i n g sc reen f o r removal of l a r g e lumps, and thence v i a b e l t conveyor t o t h e d ryer .

P e l l e t Dryer and Screens

The Wyssmont Dryer, 8 m high by 6.7 m diameter , c o n s i s t s of a s e r i e s of 32 superimposed t r a y s mounted on a v e r t i c a l s h a f t assembly

which r o t a t e s a t about 1 rpm. The wet p e l l e t s fed t o t h e top t r a y of the d r y e r a t a maximum r a t e of 32 ~ / h r , a r e d i s t r i b u t e d on t h a t t r a y and progress downwardly from t r a y t o t r a y by a c t i o n of s t a t i o n - a r y l e v e l e r s and sc rapers . Drying i s accomplished b y , h o t combustion gas and a i r from an e x t e r n a l source which i s d i s t r i b u t e d through s i d e p o r t s top t o bottom and i s c i r c u l a t e d w i t h i n t h e d r y e r by s h a f t - mounted turbofans running a t 100 rpm. The top-zone d ryer temperature is maintained a t 6 5 0 ' ~ by automatic r e g u l a t i o n of t h e n a t u r a l gas burned e x t e r n a l l y a s descr ibed. Lower l e v e l d ryer temperatures a r e he ld by c i r c u l a t i n g gas c o n t r o l s responding t o temperature.

Retent ion t ime i n the d ryer i s about 30 minutes which is ample to d ry from 9 percent mois ture t o t h e prescr ibed maximum of 0.5 percent . E x i t gas i s p r a c t i c a l l y dus t f r e e and i s vented t o atmosphere.

The d r i e d p e l l e t s discharged from the bottom t r a y a r e received by a n a t u r a l frequency v i b r a t i n g conveyor which feeds a bucket e l e v a t o r d e l i v e r i n g t o e i t h e r one o r both of two Derr ick, low amplitude, high frequency v i b r a t i n g screens arranged t o s e p a r a t e i n t o four f r a c t i o n s :

1. Plus 1.25 cm - crushed i n a hammemill and re turned t o t h e green f i n e s b i n i n t h e mix p repara t ion bui ld- ing.

2. Minus 1.25 cm and p lus 4 mesh - passed through a r o l l c rusher and re turned t o sc reens f o r recovery of on- s i z e agglomerates.

3. Minus 4 and p lus 28 mesh product - de l ive red v i a con- veyor t o a 410 T p e l l e t s t o r a g e b i n o r t o a 275 T p e l l e t feed b i n supplying t h e r o a s t e r . The recovery of t h i s product f r a c t i o n i s about 85 percent of the p e l l e t i z e d mix.

4 . Minus 28 mesh - represen t s p e l l e t breakage and a t - t r i t i o n f i n e s - re tu rned t o t h e green f i n e s b in i n t h e mix p repara t ion bui lding.

The p e l l e t product (item 3 ) i n t h e p e l l e t s t o r a g e b i n i s r e c i r c u l a t e d t o t h e screens be fore t r a n s f e r t o t h e r o a s t e r feed b in t o i n s u r e d u s t - f r e e feed.

F l u i d Column Roaster

There a r e two f l u i d column r o a s t e r s i n t h e p l a n t - each 11 m long, 4 . 3 m high a s shown i n the c ross - sec t ion drawing, F igure 2. The h e a r t h zone i s 68.5 cm wide extending t h e f u l l length o f the r o a s t e r . A t t h e he igh t of 112 cm, t h e s idewal l s f l a r e a t an angle of 50° to a width of 3.65 m , which width is maintained t o t h e top of t h e r o a s t e r .

EMERGENCY OUTLET PIPE

TUYERES

Figure 2

Cross Sec t ion of F l u i d Column Roaster

A r o a s t e r opera tes wi th a charge of p e l l e t s about 1.2 m deep a s a s t a t i c bed which expands t o about 2.15 rn depth when proper ly f l u - id ized by t h e a i r suppl ied f o r f l u i d i z a t i o n and t h e ox ida t ion reac- t i o n . The maximum design a i r volume i s 290 m3/min represen t ing 25 percent over theory requirement f o r ox ida t ion of t h e s u l f i d e min- e r a l s . With good f l u i d i z a t i o n , t h e p e l l e t s a r e uniformly d i s t r i b - u ted i n gas suspension and temperature uniformity p r e v a i l s throughout

P e l l e t s i z e d i s t r i b u t i o n must be w i t h i n r e a s o n a b l e b u t n o t c r i t i c a l l i m i t s f o r good f l u i d i z a t i o n . For example - a c o a r s e p e l l e t cha rge w i l l r e s u l t i n bed i n a c t i v i t y w h i l e a n e x c e s s i v e l y f i n e cha rge causes v i o l e n t bed a c t i v i t y and i n t o l e r a b l e d u s t l o s s from t h e r o a s t e r . Good p e l l e t i z e r o p e r a t i o n f o r t u n a t e l y w i l l a lmost a u t o m a t i c a l l y r e - s u l t i n s a t i s f a c t o r y p e l l e t s i z e d i s t r i b u t i o n ave rag ing a s fo l lows :

i-4 Mesh - 1% -4 and +10 - 60

-10 and +14 - 20 -14 and +28 - 12 -28 - 7

The c o n d i t i o n s d e s c r i b e d m a i n t a i n a bed t empera tu re o f 1273-1323O~, a good range f o r e f f e c t i v e r o a s t i n g wi thou t t h e t h r e a t o f c l i n k e r i n g a ~ ~ c a d e i i t ;.it;-. :i:r.ar ;oLi?eraCdre.

The r o a s t e r s o p e r a t e i ndependen t ly w i t h p e l l e t s s u p p l i e d t o each from t h e r o a s t e r f eed b i n v i a e i t h e r o f two r o t a r y s t a r f e e d e r s , r a t e d a t 11 ~ / h r . The f eed i s in t roduced through a p i p e a t one end o f t h e chamber j u s t above t h e expanded bed a s shown i n F i g u r e 2. At t h e o p p o s i t e end o f t h e r o a s t e r , r o a s t e d p e l l e t s a r e d i scha rged through e i t h e r o f two overf low p i p e s a t t h e same l e v e l . An i n v e r t e d r e f r a c t o r y dam a c r o s s t h e r o a s t e r w i t h r e s t r i c t e d underf low pas sage i n t e r v e n e s t h e feed and d i s c h a r g e ends f o r p reven t ion o f s h o r t - c i r c u i t i n g .

Each r o a s t e r i s s u p p l i e d w i t h a i r by i t s own E l l i o t t s team t u r - b i n e blower r a t e d a t 355 KW and 370 m3/min and d r i v e n by 2.8 M N / ~ ' (28 atm) steam f u r n i s h e d by t h e r o a s t e r b o i l e r s . The a i r i s c a r r i e d through a main heade r and i s d i s t r i b u t e d i n accordance w i t h r o a s t e r requi rements among e i g h t wind-boxes below t h e h e a r t h o f each r o a s t e r and among supply p i p e s a long t h e s i d e s . There a r e 360 t u y e r e s spaced o v e r t h e h e a r t h and t h e s e supply about 8 5 p e r c e n t o f t h e a i r r e q u i r e - ment a t about 30 k ~ / m ~ ( 0 . 3 atm) and 52 m / s e x i t v e l o c i t y . Each t u - y e r e i s a 2 cm i n s i d e d i ame te r p i p e ex t end ing v e r t i c a l l y through t h e r e f r a c t o r y - f a c e d h e a r t h p l a t e s and each d e l i v e r s a i r l a t e r a l l y from s i x 0 .8 cm d iame te r h o l e s l o c a t e d n e a r t h e t op . Supplementary s i d e - w a l l a i r i s i n j e c t e d l a t e r a l l y through t u b e s on 30 cm c e n t e r s and l o c a t e d 38 cm above t h e h e a r t h . N a t u r a l ga s may b e mixed w i t h s i d e a i r f o r combustion t o p rov ide h e a t f o r s t a r t - u p , t o s u s t a i n tempera- t u r e a t low r o a s t i n g r a t e o r t o compensate f o r s u l f u r d e f i c i e n c y i n f eed .

The expanded volume o f p e l l e t s i n a r o a s t e r i s about 28 m3 and t h e ave rage r e t e n t i o n t ime c a l c u l a t e s t o o v e r 2 hou r s . P e l l e t s o f 28 mesh s i z e obv ious ly r o a s t t o low S c o n t e n t i n a much s h o r t e r t ime. P e l l e t s o f 4 mesh do n o t r o a s t t o comparable s u l f u r c o n t e n t even a t f u l l r e t e n t i o n t ime. I n a d d i t i o n t o t h e p e l l e t s i z e f a c t o r - (1) some c o n c e n t r a t e s r o a s t t o low S c o n t e n t more r e a d i l y t han o t h e r s , (2) f i n e c o n c e n t r a t e s i z e i s f a v o r a b l e t o r o a s t i n g r a t e and (3) h igh

ben ton i te binder con ten t has an adverse e f f e c t due t o lowered p e l - l e t permeabi l i ty .

The concen t ra te blends fed t o t h e r o a s t e r genera l ly produce c a l - c i n e s having analyses w i t h i n t h e following l i m i t s :

Zn - 58-59% Fe - 10-12 Pb - 0.5-0.8 Cd - 0.2-0.3 To ta l S - 1.35-2.1 F a u l t S - 0.6-1.5 S u l f i d e S - 0.4-1.25

The d i f f e r e n c e between t o t a l s u l f u r and f a u l t s u l f u r ( t h a t which should i n theory be e l imina ted by ox ida t ion o r decomposition) i s ass ignab le t o non-decomposable s u l f a t e s o f calcium and magnesium. The z inc s u l f a t e con ten t of c a l c i n e i s low but shows up i n more im- p o r t a n t amounts i n d u s t and fume c o l l e c t e d l a t e r from t h e r o a s t e r gas stream. The e l imina t ion of lead and cadmium during conventional r o a s t i n g i s small and, i n view of a 17 percent weight shr inkage on r o a s t i n g , t h e lead and cadmium analyses of c a l c i n e s l i g h t l y exceed those of t h e composite green concen t ra tes . The lead and cadmium e l imina ted a r e recovered a s descr ibed l a t e r .

It is sometimes advantageous t o produce low lead and cadmium c a l - c i n e bu t t h e Depue p l a n t has not y e t operated with t h i s ob jec t ive . El iminat ion r o a s t i n g was demonstrated on a 50 T F lu id Column r o a s t e r a t Palmerton, Pennsylvania, dur ing development of t h e process and has been p r a c t i c e d c o m e r c i a l l y by Sherbrooke Meta l lu rg ica l Company with e x c e l l e n t r e s u l t s .

Calc ine d i scharges cont inuously from t h e r o a s t e r a t about 1273OK through t h e overflow pipe a t a r a t e i n conformity wi th t h e charge r a t e . Af te r pass ing gas s e a l s , t h e c a l c i n e i s discharged t o a ro- t a r y drum coo le r immersed i n water bath which coo l s t h e c a l c i n e t o 373-423OK. From t h e c o o l e r , t h e c a l c i n e i s moved p rogress ive ly by v i b r a t i n g conveyor, bucket e l e v a t o r and b e l t t o t h e mix house. A t t h i s p o i n t , t h e p e l l e t s may be crushed t o f i n e r s i z i n g f o r d i r e c t u s e i n smel ter mix o r may be s i n t e r e d f o r lead and cadmium el imina- t ion.

Waste Heat Boi le r s and Roaster Dust Col lec t ion

The 290 m3/min of a i r (25 percent i n excess of theory) requ i red f o r a 10.5 ~ / h r r o a s t e r feed r a t e produces 270 m3 of r o a s t e r gas a t 1273-1323'~ wi th a composition approximately 11.6 percent S02, 4 .4 percent 02 and 84 percent N2. Cooling of the gas and c o l l e c t i o n of en t ra ined d u s t , amounting t o 10-15 percent of feed, a r e accomplished i n a s e r i e s o f u n i t s a s fol lows - waste hea t b o i l e r , cyclone, i n t e r - vening ho t fan and e l e c t r o s t a t i c p r e c i p i t a t o r .

522 EXTRACTIVE METALLURGY OF LEAD AND ZINC

Each r o a s t e r has a waste h e a t b o i l e r , e s p e c i a l l y designed by Bab- cock & Wilcox, b u i l t i n t e g r a l w i th t h e r o a s t e r and equipped wi th 400 m2 of h e a t i n g s u r f a c e inc lud ing b o i l e r tubes forming t h e r o a s t e r roof and extending i t s f u l l length . Up t o 11,400 kg of s a t u r a t e d steam per hour a r e produced a t 2.8 M N / ~ ~ (28 atm) , t h e steam being used mainly f o r t h e tu rb ine -d r iven f l u i d i z i n g a i r blowers. Each b o i l e r has s i x s o o t blowers and 11 l a n c e doors f o r l anc ing . The u n i t and i t s a s s o c i a t e d f a c i l i t i e s a r e equipped wi th t h e dev ices r e - qu i red f o r automat ic ope ra t ion .

Gas l eaves t h e b o i l e r a t 6 2 0 ° ~ having depos i t ed about one t h i r d o f i t s d u s t load i n d u s t c o l l e c t hoppers which d i scha rge cont inuously through Western P r e c i p i t a t o r Company motor ized, double-f lap va lves . The d u s t c o l l e c t , having lead and cadmium c o n t e n t s s l i g h t l y h i g h e r than c a l c i n e p roduc t , r e p r e s e n t s ca r ryover m a t e r i a l ranging from f i n e p e l l e t s t o c o a r s e d u s t .

The gas passes through c l o s e coupl ing t o two 1.2 m d iameter Buel l cyclones i n p a r a l l e l which e f f e c t i v e l y p r e c i p i t a t e d u s t a t a p res - s u r e drop of <700 ~ / m 2 (5 mm Hg) f o r t h e gas volume handled. The d u s t c o l l e c t i s discharged from t h e b o i l e r d u s t hoppers i n amount c o n s t i t u t i n g two t h i r d s of t h e t o t a l d u s t e x i t i n g t h e r o a s t e r . Its s i z i n g i s l a r g e l y through 80 mesh w i t h 20 pe rcen t through 400 mesh. S u l f a t e s u l f u r , lead and cadmium a r e 2-5 t imes h ighe r than i n c a l - c i n e product.

Next i n c i r c u i t is a Robinson h o t f a n (700 m3/min r a t i n g - 59 KW) w i th 8 .5 mm Hg s u c t i o n a t t h e i n l e t t o m a i n t a i n 1 mm Hg s u c t i o n a t r o a s t e r e x i t . The fan r e q u i r e s p e r i o d i c s a n d b l a s t i n g t o remove ac- c r e t i o n s .

Gas i s moved by t h e fan t o a Wheelabrator (Lurgi) E l e c t r o s t a t i c P r e c i p i t a t o r w i t h an i n l e t temperature of about 5 7 0 ° ~ , an e x i t t e m - p e r a t u r e of 5500K and g r a c t i c a l l y no p r e s s u r e drop through t h e u n i t . With a r a t i n g of 570 m /min, t h e p r e c i p i t a t o r e f f i c i e n c y i s h igh and r e s i d u a l s o l i d s i n t h e gas a r e l a r g e l y removed. They amount t o about 3 percen t o f t h e t o t a l s o l i d s l eav ing t h e r o a s t e r i n t h e gas stream. The m a t e r i a l i s fume-like i n n a t u r e being e s s e n t i a l l y <35 microns wi th 35 pe rcen t <5 micron s i z e . Zinc s u l f a t e , l ead and cadmium w i l l run 5-10 t imes t h a t o f c a l c i n e product - t h e f i r s t named being ve ry h igh due t o s u l f a t i o n of t h e z i n c oxide fume dur ing slow cool ing. The two l a s t named a r e h igh because of s e l e c t i v e t r a n s p o r t a s fume from t h e r o a s t e r . Under e l i m i n a t i o n r o a s t i n g c o n d i t i o n s (h igher temperature and ve ry low excess a i r ) , t he amount of p r e c i p i t a t o r c o l l e c t and i t s l ead and cadmium con ten t s a r e g r e a t l y inc reased .

Although t h e gas l eav ing t h e r o a s t e r has an SO2 con ten t of 11.5- 12 p e r c e n t , t h e a n a l y s i s drops t o about 10 pe rcen t l eav ing t h e p re - c i p i t a t o r . This i s due t o d i l u t i o n by a i r i n l eakage a t d u s t d i s - charge v a l v e s on a l l u n i t s . The s i t u a t i o n is being improved but does n o t , i n f a c t , pose a problem i n a c i d product ion o t h e r than t h e

p o s s i b l e l imi ted volume capac i ty of t reatment u n i t s i n t h e gas sys- tem beyond the p r e c i p i t a t o r .

The p r e c i p i t a t o r fume i s discharged through gas va lves t o screw conveyors feeding a drag conveyor s i m i l a r l y s e r v i c i n g dus t d i scharge from b o i l e r s and cyclones. Dust c o l l e c t s (bag f i l t e r s ) from var ious p o i n t s i n t h e p l a n t a r e a l s o routed t o t h e common drag conveyor, thence t o bucket e l e v a t o r and aga in a drag conveyor r e t u r n i n g t h e combined d u s t s t o t h e r o a s t e r d u s t b in i n t h e mix p repara t ion bui ld- ing f o r r e c i r c u l a t i o n i n p e l l e t feed mix.

Roaster and Boi le r Heat Balances

Heat balances applying t o r o a s t e r opera t ion a t capac i ty r a t i n g (10.5 ~ / h r feed a t 27 pe rcen t s u l f u r and 25 percent excess a i r ) a r e a s follows:

Roaster (one u n i t )

Heat I n (oxidat ion r e a c t i o n s ) 9,650,000 kg c a l / h r

Heat Out Gases a t 13230K 6,100,000 kg c a l / h r So l ids a t 1 3 2 3 ' ~ 1,300,000 II

Radiat ion Loss a t 5% 480,000 I I

To Boi le r Tubes i n Roaster Roof 1,770,000 11

Boiler : 1323-620°K

Recovery from Gas and Dust 4,520,000 kg c a l / h r Recovery Via Roaster Roof Tube 1,770,000 I I

6,290,000 kg c a l / h r

Equivalent Steam Production

PART B - SINTERING

Although t h e p e l l e t e d r o a s t from t h e F lu id Column r o a s t e r s , a f t e r r educ t ion t o proper s i z i n g , i s an e n t i r e l y s a t i s f a c t o r y feed f o r t h e v e r t i c a l r e t o r t s , t h e p r a c t i c e a t Depue inc ludes s i n t e r i n g f o r lead and cadmium e l imina t ion . Figure 3 d e p i c t s t h e flow of m a t e r i a l s ,

Roasted p e l l e t s , r e t u r n s i n t e r and sometimes r o a s t e r dus t a r e mixed wi th 6 percent a n t h r a c i t e dus t coa l i n a modified pug m i l l 1.1 m wide x 0.8 m deep x 4.3 m long. This mix i s e levated t o a p e l l e t i z i n g drum 2 m O.D. x 3.6 m long. The p e l l e t e d mix discharges t o a chute and by means o f a swinging spout i s fed uniformly ac ross

F i g u r e 3

Flow Shee t o f Depue S i n t e r P l a n t

t h e g r a t e s of t h e 1.0 m wide x 10 m long, downdraft Dwight-Lloyd s i n t e r i n g machine. As i t e n t e r s t h e n a t u r a l gas f i r e d i g n i t i o n box, the bed i s 15 cm high; t h e lower 4 cm is h e a r t h l a y e r comprising r e - t u r n s i n t e r l a r g e r than 1 .5 cm. Jus t be fore t h e d i scharge end of t h e machine, t h e top 9 cm of t h e bed i s shaved o f f by a r o t a t i n g s c a l p e r . This top l a y e r from which about 80 percent of t h e cadmium and 40 percent of t h e lead have been e l imina ted , c o n s t i t u t e s t h e s i n t e r e d product con ta in ing approximately 60 percent z inc , 0.35 per- cen t lead and 0.05 percent cadmium. Fluid Column roas ted p e l l e t s have proved t o be an e x c e l l e n t s i n t e r feed i n t h a t t h e c u r r e n t s i n - t e r i n g capac i ty of 11 ~ / h r is more than twice t h a t previously ob- t a i n e d w i t h f l a s h r o a s t .

The lower p o r t i o n of t h e bed no t removed by t h e s c a l p e r , i s d i s - chargkd o f f t h e end of t h e machine t o a s e t of crushing r o l l s and then fed t o a s i n g l e deck v i b r a t i n g screen. The o v e r s i z e (+1.5 cm) i s re turned d i r e c t l y a s h e a r t h l a y e r whi le t h e unders ize i s recycled t o t h e modified pug m i l l f o r incorpora t ion i n t o t h e feed.

Suct ion on t h e t h r e e wind-boxes i s 10-25 mm Hg water wi th an a i r flow of 700 m3/min. Dust and fume a r e removed from t h e waste gases be fore d i scharge t o the s t a c k by a cyclone and C o t t r e l l e l e c t r o s t a t i c p r e c i p i t a t o r . The fume c o l l e c t e d i n t h e C o t t r e l l con ta ins about 20 percent each, z i n c , lead and cadmium.

PART C - VERTICAL RETORT SMELTING

A v e r t i c a l r e t o r t p l a n t comprising e i g h t r e t o r t s was placed i n opera t ion i n July 1929. This followed c l o s e l y t h e cons t ruc t ion and opera t ion of a s i m i l a r p l a n t i n Palmerton, Pennsylvania, which was t h e outcome of i n t e n s i v e development work over s e v e r a l previous years . The h o r i z o n t a l r e t o r t p l a n t s , then opera t ing i n Depue and Palmerton, were becoming an t iqua ted wi th decreasing p r o f i t a b i l i t y . It was necessary, t h e r e f o r e , t o develop a continuous, mechanized process employing l a r g e smel t ing u n i t s of h igh product ion capac i ty wi th a t t e n d i n g good economics.

The Depue p l a n t has been expanded from time t o time and 27 r e - t o r t s a r e now i n operat ion. P l a t e I is an a e r i a l view of t h e com- p o s i t e p l a n t wi th t h e v e r t i c a l r e t o r t p l a n t showing i n t h e fo re - ground extreme l e f t .

General Descr ip t ion of V e r t i c a l Re to r t Process

The advantages of t h e v e r t i c a l r e t o r t process d e r i v e from t h e mechanical handl ing of m a t e r i a l s i n t o and from a l a r g e , cont inu- ously operated r e t o r t producing z i n c and r e a c t i o n products of un i - form composition a t constant r a t e s w i t h high thermal e f f i c i e n c y and recovery. A fundamental requirement of t h e process i s t h a t t h e smel t ing charge be suppl ied t o t h e r e t o r t i n a form conducive t o

526 EXTRACTIVE METALLURGY O F LEAD AND ZINC

t h e r a p i d and e f f i c i e n t t r a n s m i s s i o n and u t i l i z a t i o n o f h e a t deve l - oped by combustion o f gas i n f i r i n g chambers a d j a c e n t t h e h igh - c o n d u c t i v i t y , r e f r a c t o r y s i d e w a l l s of t h e r e t o r t . The r e q u i r e d form o f t h e c h a r g e i s a l a r g e , l o a f shape produced by r o l l - b r i q u e t t i n g a s p e c i a l l y p repa red mix. The mix i n g r e d i e n t s a r e s o s e l e c t e d , s i z e d and p ropor t ioned a s t o meet t h e requi rement o f maintenance o f b r i - q u e t i n t e g r i t y throughout t h e r e d u c t i o n p roces s .

See V e r t i c a l R e t o r t Flow Sheet - Figure 4 .

The mix i n g r e d i e n t s mee t ing p r e s c r i b e d s p e c i f i c a t i o n s a r e p repa red by an i n t e n s i v e c o n d i t i o n i n g t r ea tmen t f o r development o f t h e mix p l a s t i c i t y r e q u i r e d f o r r o l l - b r i q u e t t i n g . The g reen b r i q u e t s from t h e p r e s s have s t r e n g t h p e r m i t t i n g g r a v i t y f low v i a g r i z z l y o n t o t h e moving g r a t e o f a cok ing furnace . The cok ing fu rnace i s au togenous ly o p e r a t e d , t h e combustion o f v o l a t i l e from t h e bi tuminous c o a l i n t h e mix supp ly ing t h e h e a t requi rement . T h i s c o a l i n t h e b r i q u e t con- v e r t s t o coke forming a con t inuous phase o f coke s t r u c t u r e which bonds t h e mass t o produce a tough, i n d u r a t e d b r i q u e t capab le o f w i t h s t a n d i n g subsequent t r a n s p o r t and hand l ing .

Hot coke b r i q u e t cha rges are e l e v a t e d t o t h e r e t o r t cha rge f l o o r i n weighed h o i s t bucke t s which a r e emptied on a t i m e s chedu le i n t o t h e t o p e x t e n s i o n o f t h e r e t o r t t o occupy t h e s p a c e provided by t h e con t inuous wi thdrawal of reduced b r i q u e t s from t h e bottom.

During downward pas sage through t h e v e r t i c a l r e t o r t w i t h r e t e n - t i o n t ime c o r r e l a t e d wi th t h e r e d u c t i o n r e a c t i o n , t h e r e d u c t i o n h e a t i s s u p p l i e d by i t s t r a n s m i s s i o n through t h e h i g h - c o n d u c t i v i t y s i d e - w a l l s from t h e combustion chambers e x t e r n a l t h e s i d e w a l l s . The z i n c vapor and r e a c t i o n g a s e s produced f low upwardly through t h e r e t o r t and t h e dez inced b r i q u e t s a r e e x t r a c t e d a t t h e bottom. The vapor and g a s e s e scape v i a a d u c t l e a d i n g from an upper e x t e n s i o n o f t h e r e t o r t and a r e drawn i n t o a z i n c vapor condenser from which t h e l i q u i d z i n c i s withdrawn f o r c a s t i n g o r r e f i n i n g . The permanent gases e scap ing t h e condenser a r e coo led and c l eaned and p iped t o t h e r e t o r t f i r i n g chambers f o r supplementary f u e l .

Mix Materials

Mix Composit ion

Mix composi t ion i s impor t an t inasmuch a s a n a l y s e s , p r o p o r t i o n s and s i z i n g s a r e v a r i a b l e s t o which b r i q u e t q u a l i t y and subsequent p r o c e s s i n g behav io r a r e c r i t i c a l l y r e l a t e d . A u n i v e r s a l mix cannot be s p e c i f i e d b u t a t y p i c a l composi t ion i s about a s fo l lows:

Roasted Zinc Concen t r a t e ( g e n e r a l l y s i n t e r ) 60% Bituminous Coal 2 5 A n t h r a c i t e F i n e s 5 P l a s t i c R e f r a c t o r y Clay 10 S u l f i t e Liquor 1

Figure 4

Vertical Retort Plant Flow Sheet

The z i n c i n t h e coked b r i q u e t s from t h e above m i x should f a l l i n t h e range of 40-43 p e r c e n t . Changes i n z i n c g rade o f o r e , i r o n oxide c o n t e n t o f o r e and o t h e r f a c t o r s may d i c t a t e mix r e v i s i o n i n con fo r - mi ty w i t h t h e z i n c i n coke requi rement .

Zinc Ores

The o r e s used a t Depue a r e g e n e r a l l y of t h e r o a s t e d s u l f i d e t ype and a r e l a r g e l y ob ta ined from s c a t t e r e d New J e r s e y Zinc Company mines supplemented by some purchased o r e s . Zinc o r e g rades and components such a s i r o n , l e a d , cadmium, gangue o x i d e s , e t c . va ry c o n s i d e r a b l y . The named m e t a l s a f f e c t t h e composi t ion o f t h e z i n c m e t a l produced and t h e p h y s i c a l c o n d i t i o n o f t h e cha rge i n t h e r e t o r t may be a£ - f e c t e d by t h e f u s i b i l i t y c h a r a c t e r i s t i c s o f t h e gangue.

Bituminous Coal

The bituminous c o a l used i s a c a r e f u l l y s e l e c t e d type g e n e r a l l y ana lyz ing 38-42 p e r c e n t v o l a t i l e m a t t e r , c h a r a c t e r i z e d by v e r y low chemical ly combined H20 a s mined and by t h e development o f v e r y h igh f l u i d i t y a t 670-725'~. Coal used f o r m e t a l l u r g i c a l coke is hope- l e s s l y i n f e r i o r a s i t i s f a r t o o weakly coking f o r 25 p a r t s t o bond about 75 p a r t s o f o r e and o t h e r non-cohes ive c o n s t i t u e n t s . The c o a l r e q u i r e d forms a l i q u i d phase i n t h e b r i q u e t du r ing cok ing , which e f f e c t i v e l y envelops o r e and a n t h r a c i t e g r a i n s and c o n v e r t s t o s t r o n g l y bonding coke s t r u c t u r e a t h i g h e r tempera ture . A ha rd , s h a t t e r - r e s i s t a n t b r i q u e t i s t h e r e b y produced.

A n t h r a c i t e Coal

S ince t h e z i n c o r e c o n t e n t o f mix may n o t exceed a l e v e l f u r n i s h - i n g about 43 pe rcen t z i n c i n coke and t h e bi tuminous c o a l c o n t e n t i s l i m i t e d t o 25 p e r c e n t because o f t h e a d v e r s e e f f e c t o f an exces s on coke s t r u c t u r e , i t i s neces sa ry t o i n c l u d e a s m a l l p r o p o r t i o n o f a n t h r a c i t e ("dust c o a l " - e s s e n t i a l l y through 14 mesh and on 200 mesh) a s a d i l u e n t . It i s something more t h a n a n i n e r t f i l l e r a s i t improves t h e p h y s i c a l s t r u c t u r e o f t h e b r i q u e t and augments t h e carbon s u p p l i e d by t h e bi tuminous c o a l .

Binders

The mix p l a s t i c i t y r e q u i r e d f o r b r i q u e t t i n g i s provided by b ind - e r s . About 8-9 p e r c e n t c l a y and 1 p e r c e n t s u l f i t e l i q u o r a r e c u r - r e n t l y used. The c l a y i s o f t h e h i g h l y p l a s t i c , r e f r a c t o r y t y p e and i t s s u i t a b i l i t y may be e v a l u a t e d by l a b o r a t o r y f i r i n g o f test b a r s . The s u l f i t e l i q u o r used i s o f t h e n e u t r a l i z e d , 50 pe rcen t s o l i d s type. It makes a n impor tant c o n t r i b u t i o n t o coking behavior

- -

even i n modest amount.

Mater ia l s S i z i n g

The major m i x c o n s t i t u e n t s , z inc o r e and bituminous c o a l , have optimum s i z i n g a t a small percentage plus 14 mesh and about 30 per- cent through 200 mesh. This s i z i n g represen t s a compromise as f i n e r s i z i n g produces much harder res idue b r ique t s t r u c t u r e but aggravates any tendency toward coking d e f e c t s such a s cracking, s p a l l i n g and s u r f a c e s h e l l i n g - p a r t i c u l a r l y i n c l a y binder b r ique t s - because of d e n s i t y of s t r u c t u r e .

The bituminous coa l is ground t o s i z e s p e c i f i c a t i o n i n two Penn- sy lvan ia Crusher Company hammer m i l l s . The z inc o r e i n roasted pel- l e t o r s i n t e r e d p e l l e t form is reduced t o s i z e i n two rod m i l l s .

Raw Mater ia l s Handling

The mix m a t e r i a l s a s descr ibed a r e stocked i n conventional, round, concre te s t a v e s i l o s - two f o r s i n t e r s , one f o r a n t h r a c i t e coa l and a double compartment b i n f o r crushed c l a y and coke f ines . Each b in d e l i v e r s by feeder t o a weigh b e l t and i n t u r n t o a c o l l e c t o r b e l t which continuously d e l i v e r s t h e proportioned m a t e r i a l s t o a r o t a r y drum mixer. Three b ins provide f o r segregated s t o r a g e of d i f f e r e n t bituminous coa l s which, a f t e r crushing, a r e de l ive red t o one of two crushed coa l bins . Crushed coa l i s fed independently v i a weigh feeders t o t h e mixer. S u l f i t e l iquor binder i s metered i n t o t h e mixer.

Mix Prepara t ion

The r o t a r y mixer discharge t r a v e l s by b e l t conveyor i n t o t h e f i r s t of a s e r i e s of chasers o r Chilean M i l l s . Four chasers a r e ava i lab le ; t h e s e a r e operated i n s e r i e s of two o r th ree t o avoid s h o r t - c i r c u i t i n g . The chaser is a massive u n i t 3.7 m i n diameter i n which two 10 T r o l l s a r e d r iven over an 8-10 cm bed of m i x of about 2 T. Plows s e t on cross-arms a r e s o adjusted a s t o r e t u r n mix i n t o t h e pa th of t h e r o l l s and t o r e g u l a t e the discharge through a c e n t r a l opening.

Water i s added t o the f i r s t chaser t o p l a s t i c i z e t h e mix and t o t h e second and t h i r d i f required. The chaser power requirement i s a good index of p l a s t i c i t y . Optimum p l a s t i c i t y is ind ica ted by maxi- mum power - usua l ly 275 t o 300 A a t 440 V. The optimum b r i q u e t a b i l - i t y of mix wi th c l a y binder i s obtained wi th 6-7 percent water.

The h ighes t poss ib le chasing r a t e c o n s i s t e n t wi th mix br ique t - a b i l i t y and green br ique t q u a l i t y is sought. Clay binder m i x i s favorable t o chasing r a t e and a r a t e of 8+ T per chaser pe r hour may be maintained.

- Plate I1

View of Chasing Section of Mix House

Briquetting

The chased mix is belt conveyed to a holding bin with a bottom discharge opening superimposed over a roll-briquetting press into which mix feeds by gravity. The rolls are 61 cm in diameter, and run intermittently at about 10 rpm and produce 6 T of 6.4 cm x 2.4 cm pillow-block briquets per hour. The function of the press is to furnish continuous, uniform and densified feed to a final press with avoidance of pocket starvation.

The preliminary press discharges into a small hopper over the final "loaf" briquet press and feeds thereinto by gravity as the press assimilates the feed. The loaf briquet measures 6.3 x 10 x 7.6 cm thick (rounded surfaces) and weighs about 0.5 kg. The mass of the briquet is near ideal for the reduction conditions in the vertical retort as is the shape considering the shape limitations of roll-briquetting.

The loaf press rolls are 61 cm in diameter with two or three pockets staggered across the faces of the rolls. The press is driven by an 11 KW motor coupled by Reeves Vari-Drive to the press to produce up to 127 T of green briquets per day.

The briquets from the final press drop a minimum distance to an inclined grizzly down which they slide, with dropout of fines, to the inlet of the close-coupled coking furnace.

Briquet Coking

Coking treatment of briquets is required to convert the rela- tively weak green briquet to one having the strength to withstand the rough treatment incident to transport to and introduction into the vertical retort. In addition to insuring coke briquet-quality measured by maintenance of briquet form during retort passage, the bituminous coal pays additional dividend in providing volatile for combustion to support a thermally self-sustaining coking operation. Furthermore, the low temperature coke developed is uniquely reactive in the reduction chemistry of the vertical retort.

The coking furnace is essentially a large refractory combustion chamber housing a series of downwardly inclined, alloy step-grates. Alternate grates have a slow reciprocating movement which imparts a forwarding impulse to the bed of briquets from feed to discharge ends. Green briquets are fed continuously from the press grizzly with the briquetting rate and grate speed so correlated as to pro- vide a thin, uniformly distributed bed of briquets on the coker grate.

Upon arrival in the coker, the entrant briquet bed heats up, dis- tillation of volatile from coal proceeds and autogenous operation is maintained by combustion with air supplied both above and through

Plate I11

Loaf Briquetting Press and Densified Feed Above

t h e b r ique t bed from below. Temperatures a r e so c o n t r o l l e d t h a t b r i q u e t s a r e l a r g e l y v o l a t i l e - f r e e upon discharge but a r e under z inc reduc t ion temperature. The g r a t e s d i scharge i n t o a holding hopper of about 3 T c a p a c i t y wherein t h e soaking h e a t may expel r e s i d u a l v o l a t i l e .

Coked b r i q u e t s a r e drawn from t h e holding chamber on a time schedule by r o l l - d i s c h a r g e mechanism onto a t r a v e l i n g g r i z z l y and thence i n t o coke buckets holding 1.35 T. Any f i n e s fomed i n coking pass t h e g r i z z l y thus providing an e s s e n t i a l l y whole b r ique t product. The weighed coke buckets a r e h o i s t e d t o t h e r e t o r t charging f l o o r and a r e a v a i l a b l e t o each ind iv idua l r e t o r t a s t h e burden demand requ i res .

Reduction Re tor t s

Construct ion and F i r i n g

The smel t ing r e t o r t i s a t a l l r e f r a c t o r y s t r u c t u r e c o n s i s t i n g o f two t h i n , p a r a l l e l s idewal l s , two massive endwalls, an upper r e f r a c - t o r y ex tens ion superimposed on t h e four w a l l s and a lower ex tens ion o f metal cons t ruc t ion . The p a r a l l e l s i d e w a l l s a r e 11.5 cm i n th ick- ness spaced 30.5 cm a p a r t i n s i d e and, toge ther wi th t h e endwalls, enc lose t h e muff le space o f 30.5 cm x 1.85 m o r 2.44 m c ross s e c t i o n through which t h e b r i q u e t charge descends by g r a v i t y . The ends of t h e s idewal l s a r e recessed i n t o v e r t i c a l s l o t s i n t h e endwalls s o t h a t t h e four monol i th ic w a l l s may move independently t o accnmnodate t h e expansion and s t r a i n s induced i n w a l l s of 10 m and 11.3 m he igh t . The product ion c a p a c i t y of a r e t o r t obviously depends upon t h e flow of hea t through t h e w a l l s , consequently use of t h e most h ighly con- d u c t i v e r e f r a c t o r y - namely, s i l i c o n carbide. The r e f r a c t o r y b r i c k is a s t andard shape (usua l ly 6.35 x 11.5 x 23 cm, tongue-ahd-groove type) bonded by high temperature, s i l i c o n ca rb ide mortar.

A r e t o r t i s heated by combustion o f n a t u r a l gas i n a f i r i n g cham- ber e x t e r n a l each s idewal l . The gas i s i n j e c t e d downwardly through f i v e p o r t s i n t h e roof of t h e f i r i n g chamber. Ai r f o r combustion i s admitted i n c o n t r o l l a b l e volumes through a i r p o r t s a t 9 o r 10 l e v e l s . Qui te uniform temperatures p r e v a i l from top t o bottom of t h e chamber us ing t h e p rogress ive combustion f i r i n g technique wi th near-perfect combustion a t t a i n e d a t t h e bottom. Natural gas i s supplemented, t o t h e ex ten t o f about 30 percent o f t h e t o t a l h e a t requirement, by carbon monoxide r e a c t i o n gas from r e t o r t reduct ion. This gas e n t e r s t h e f i r i n g chambers through d i s t r i b u t i o n p o r t s j u s t below t h e roof o f t h e combustion chamber.

The temperature maintained i n t h e f i r i n g chamber is a t a s e l e c t e d l e v e l between 1550° and 1600°K a s d i c t a t e d by r e t o r t cond i t ion , pro- duc t ion demand, e t c . Combustion gases e x i t through f l u e s a t t h e bottom of t h e f i r i n g chamber and a r e moved by s t a c k s u c t i o n through three-pass recupera to r s of t h e concurrent type. With combustion gas a t about 1 5 7 0 ° ~ , t h e recupera to r s preheat a i r f o r r e t o r t f i r i n g t o about 8 2 5 ' ~ wi th d i scharge f l u e gas a t about 1 1 ~ 0 ~ ~ .

Plate IV

Withdrawal of Coked B r i q u e t s i n t o Coke Bucket

P l a t e V

Top View of R e t o r t and F i r i ng Chambers a t Roof Line

R e t o r t Opera t ion and Reduction

A weighed p o r t i o n of h o t coke b r i q u e t s i s charged i n t o t h e upper ex tens ion (volume about 45 pe rcen t of t h a t of t h e heated r e t o r t ) through a charge cap a t t h e top about once a n hour and occupies t h e space c r e a t e d by cont inuous d i scha rge o f r e s i d u e b r i q u e t s a t t h e bottom. The unheated ex tens ion i s known a s t h e charge column and performs s e v e r a l func t ions : t h e occupant b r i q u e t s a b s t r a c t h e a t from t h e h o t gas ascending from t h e r e t o r t , some r e f l u x i n g and s e l e c t i v e condensat ion o f l ead t a k e p l a c e , t h e r e a c t i o n C + C02 -+ 2C0 i s pe r - m i t t e d t o occur i n a zone t o o low i n temperature f o r ZnO + CO -+ Zn + C02 and e n t r a i n e d z i n c oxide fume and charge d u s t which a r e d e t r i - menta l t o z i n c vapor condensat ion a r e q u i t e e f f e c t i v e l y f i l t e r e d from the gas .

A s t h e b r i q u e t charge e n t e r s t h e hea ted r e t o r t w i th e x t e r n a l w a l l s a t 1573 '~ , t h e b r i q u e t s ad jacen t t o t h e w a l l s h e a t r a p i d l y and t h e product ion of z i n c vapor begins v i a t h e r e a c t i o n given above. Al- though ,z inc oxide r e d u c t i o n may begin a t a r e l a t i v e l y low temperature , a p r a c t i c a b l e r a t e probably r e q u i r e s a temperature o f 1273'~. The s u b s t a n t i a l thermal head r e s u l t s i n r a t h e r r ap id r e d u c t i o n of b r i - que t s a d j a c e n t t o t h e w a l l s and probable f i n a l a t t a inmen t o f an av- e rage b r i q u e t temperature nea r 1 5 2 3 ' ~ a s r educ t ion comes t o nea r - completion a t t h e bottom of t h e r e t o r t .

The b r i q u e t s i n t h e mid-zone o f t h e charge 15 cm remote from t h e w a l l s a r e n a t u r a l l y pena l i zed i n a v a i l a b i l i t y o f h e a t and i t i s t h i s c i rcumstance which fundam&ntally r e q u i r e s t h a t t h e charge b e b r i - que t t ed and t h a t t h e b r i q u e t s pass through the r e t o r t w i thou t ap- p r e c i a b l e breakage o r format ion o f f i n e s . The i n t e r s t i c e s provided by b r i q u e t s i n s u r e s t h e p e n e t r a t i o n of r a d i a n t h e a t a s w e l l a s f r e e passage o f h o t gas f o r h e a t i n g by convection. It i s w e l l known t h a t b r i q u e t degrada t ion i n t h e r e t o r t imposes an unacceptable z i n c pro- d u c t i o n pena l ty because o f t h e impediment t o hea t t r a n s f e r a s w e l l a s t h e d i f f i c u l t i e s caused by charge hang-up, by o b s t r u c t i o n of gas flow and o t h e r f a c t o r s . I n a d d i t i o n t o t h e above, t h e dense s t r u c - t u r e o f a sound b r i q u e t f avors h e a t t r a n s f e r by conduction from s u r - f a c e t o i n t e r i o r - a d i s t i n c t advantage i n comparison w i t h f i n e , l oose charge. To f u r t h e r emphasize b r i q u e t q u a l i t y , i t i s noted t h a t t h e coke s t r u c t u r e must p e r s i s t through t h e dezinced b r i q u e t s t a g e corresponding t o a r e s i d u e b r i q u e t of only 40 pe rcen t of i t s o r i g i n a l coke mass.

The r e s i d u e b r i q u e t s emerging from t h e r e t o r t should be reduced t o 2s-3 pe rcen t z i n c r e p r e s e n t i n g 97 pe rcen t e l imina t ion . The r e s - idue from Depue-type m i x w i l l c o n t a i n 30-35 pe rcen t carbon and prac- t i c a l l y a l l t h e i r o n , copper, s i l v e r and gold i n t h e f eed . A l a r g e percentage of t h e l ead i n feed w i l l a l s o be p resen t wi th h igh lead i n feed but the p ropor t ion dec reases a s lead i n feed dec reases .

P l a t e V I

Hot Coke B r i q u e t Charge E n t e r i n g Vertical Retort

The r e s i d u e l eav ing t h e heated r e t o r t e n t e r s a lower ex tens ion of f a b r i c a t e d metal . I ts r a t e of withdrawal i s regu la ted by a r o l l ex- t r a c t o r , t h e r a t e being synchronized wi th t h e r e t e n t i o n time i n t h e r e t o r t r equ i red f o r 97 percent z i n c e l imina t ion . The r e s i d u e b r i - que t s drawn by t h e e x t r a c t o r f a l l i n t o a ba th of wa te r i n c losed sys- tem f o r quenching and a r e cont inuously removed by an i n c l i n e d screw.

About 0.25 m3/min a i r i s admitted i n t o t h e bottom ex tens ion of t h e r e t o r t which a f t e r s h o r t a scen t produces corresponding volumes of n i t r o g e n and carbon monoxide. The purpose of t h e extraneous gas i s t o prevent back-d i f fus ion o f z i n c vapor which would otherwise condense ou t on cold res idue .

Condensation of Zinc Vapor and Recovery of Liquid Zinc, Blue Powder and React ion Gas

The gaseous r e a c t i o n products formed i n t h e r e t o r t r i s e i n t o t h e charge column a t a r a t e of about 4.25 m3/min (2.44 x 10.7 m r e t o r t ) and wi th t h e approximate composit ion - 40 pe rcen t z i n c vapor , 45 pe rcen t carbon monoxide, 8 pe rcen t hydrogen and 7 pe rcen t n i t r o g e n p l u s some carbon d iox ide i n smal l b u t important amount. The hydro- gen and n i t r o g e n have o r i g i n i n a i r , steam and r e s i d u a l v o l a t i l e i n coke. The gases e x i t nea r t h e top of t h e charge column i n t o a down- wardly s l o p i n g , r e f r a c t o r y condui t which t e rmina tes i n a z i n c vapor condenser.

The problems i n h e r e n t i n t h e condensat ion of z i n c vapor stem from t h e equ i l ib r ium cond i t ions i n t h e system of gaseous r e a c t i o n products which favor r e o x i d a t i o n of z i n c vapor by carbon d iox ide wi th dec reas - ing temperature. I f t h e back- reac t ion , Zn + C02 + ZnO + 2C0, i s no t suppressed dur ing coo l ing f o r condensat ion, t h e r e is produced a co- p ious q u a n t i t y of m e t a l l i c and p a r t i a l l y oxidized z i n c powder which s u b t r a c t s correspondingly from l i q u i d z i n c production.

The s p l a s h condenser a s used a t Depue l a r g e l y overcomes condensa- t i o n problems a s compared wi th e a r l i e r condensers which produced about 12 pe rcen t b lue powder. Blue powder i s decreased t o 3-5 per- c e n t (sometimes l e s s ) by reason of (a) coo l ing vapor r a p i d l y t o avoid r e e x i d a t i o n , (b) r e g u l a t i n g condenser temperature t o prevent f r e e z i n g of z i n c m i s t , (c) inc reas ing t h e s u r f a c e of l i q u i d z inc i n con tac t wi th vapor and (d) a s s i m i l a t i n g a l a r g e p ropor t ion of b l u e powder formed i n t o a ba th of l i q u i d z i n c by i n t e n s i v e scrubbing wi th t h a t l i q u i d medium.

The r e t o r t gases pass through t h e condui t quickly wi thou t much coo l ing , thus reducing b lue powder format ion, b e f o r e e n t r y i n t o t h e condenser. The condenser i s a n e longated r e f r a c t o r y chamber holding a ba th of z i n c mainta ined a t f ixed l e v e l by continuous overflow and a t about 7 7 3 ' ~ by coo l ing c o i l s which submerge o r emerge by thermo- s t a t i c response. A motor-driven, i n c l i n e d s h a f t passes through t h e remote end of t h e condenser and t e rmina tes i n a 35 cm diameter

P l a t e VII

Removal o f Water-Quenched Residue V i a Inc l ined Screw t o Transport B e l t

F i g u r e 5

Lengthwise Cross S e c t i o n o f Sp l a sh Condenser

impel ler d ipping i n t o t h e z i n c bath. Rotat ion of t h e impel ler a t 400 rpm produces a shower of l i q u i d z i n c i n such profusion t h a t t h e c r o s s s e c t i o n of t h e chamber i s e f f e c t i v e l y cur ta ined by t h e l i q u i d spray. The e n t r a n t z inc vapor and accompanying permanent gases must t r a v e r s e t h e shower wi th such intimacy of con tac t t h a t i m e d i a t e cool ing and condensation a r e e f f e c t e d with t h e condensate jo in ing t h e bath.

A small amount of b lue powder, no t a ss imi la ted by t h e ba th , f l o a t s thereon and i s p e r i o d i c a l l y skimmed o f f . A somewhat l a r g e r amount accompanies t h e s t a c k e x i t gases - perhaps 3 t o 4 percent including t h e z inc vapor no t condensible a t 773OK.

Gases a r e moved through t h e condenser by s u c t i o n developed by a water eductor i n a scrubber-tower system which scrubs out t h e en- t r a i n e d b lue powder and c leans t h e gas f o r use a s supplementary f u e l of 11.1 w/m3 (2650 kcal/m3) content . The separa ted b lue powder c o l l e c t s i n a trough and i s p e r i o d i c a l l y removed, p a r t i a l l y d r i e d and used a s s e a l a n t over t h e charge caps on t h e r e t o r t s . Upon open- i n g t h e charge caps f o r b r i q u e t charging, t h e b lue powder e n t e r s t h e r e t o r t s and i s r a t h e r promptly f l a shed o f f a s z i n c vapor.

Metal Handling and Cas t ing

The molten z i n c cont inuously overflows from t h e condensers i n t o a launder; one launder s e r v i c e s 13 r e t o r t s , whi le another c o l l e c t s from 14 r e t o r t s . Each launder d e l i v e r s t o a 15 T holding pot . Each holding pot i s connected t o a heated trough by which metal is t r a n s - f e r r e d t o t h e r e f i n i n g bui lding. Each of t h e four lead column sup- p ly po t s is fed from t h i s trough. Constant l e v e l i n t h e troughing system and i n t h e supply p o t s i s maintained by a dam i n t h e trough beyond t h e lead c o l m n feed po in t s . About h a l f o f t h e troughed metal overflows t h e dam and down a chute t o a 160 T, gas - f i r ed , reverbera- t o r y holding furnace on t h e ground l e v e l . This metal , des ignated a s VFZ, has an as-produced composition averaging 0.3 percent l ead , 0.10 percent cadmium and 0.01 percent i r o n wi th smal le r contents of minor impuri t ies . The lead and cadmium con ten t s vary depending on t h e o r e being processed.

A s i m i l a r 160 T, g a s - f i r e d , r everbera to ry holding furnace i s pro- vided f& t h e r e f i n e d z inc . F l e x i b i l i t y i s provided i n t h e handling of molten z i n c so t h a t composition may be adjusted. For example, r e - f ined and VFZ metal may be blended i n an a u x i l i a r y holding pot t o give in te rmedia te compositions. Lead, aluminum o r o t h e r metals may be blended wi th VFZ metal t o produce compositions requ i red f o r ho t - d i p galvanizing and o t h e r s p e c i a l requirement uses .

Each of t h e 160 T holding p o t s feeds , v i a v e r t i c a l l i f t r o t a r y pump, a Shepherd c a s t i n g machine and Shepherd s t acker . The i n t e - g ra ted machines c a s t s l a b s and s t a c k i n s tandard shipping u n i t s . Each u n i t i s r a t e d a t 16.5 ~ / h r but i s operated a t 13.5 T. The

c a s t i n g machine i s equipped wi th 132 molds forming 25 kg s l a b s wi th t h e molds arranged i n such sequence t h a t four l eg - s l abs a r e f i r s t c a s t t o provide t h e base course l a i d by t h e s t a c k i n g machine. The 40 molds fol lowing c a s t f l a t s l a b s which t h e s t a c k e r ar ranges i n 10 rows of four each on t h e l e g - s l a b s i n such o r i e n t a t i o n a s t o g i v e s t a b i l i t y t o t h e s tacked u n i t . Upon completion o f t h e 11-row s t a c k , t h e 1100 kg u n i t i s removed by f o r k l i f t and t h e c y c l e is repea.ted.

M e t a l l u r g i c a l Data and Performance

Raw, Wet Mix Coke (about 40% Zn) Dry Residue Zinc Product ion Kg Zinc p e r m2 Heated R e t o r t Area p e r Day Zinc Recovery on R e t o r t Input Overa l l Zinc Recovery Average R e t o r t Operat ing Time Natural Gas *Thermal Ef f i c i ency of V e r t i c a l R e t o r t

575 T/da 490 T/da 195 T/da 182 T/da 160+ 95%

92-93% 9 5%

374.5 m3/T Zinc 4 5%

*An e f f i c i e n c y f i g u r e of 45 pe rcen t has been c a l c u l a t e d f o r Depue r e t o r t s based s o l e l y on t h e t h e o r e t i c a l thermal r e - quirements of t h e ZnO reduc t ion r e a c t i o n a t 1 3 7 3 O ~ ( to - g e t h e r wi th t h e h e a t necessary t o r a i s e t h e preheated charge t o t h i s temperature) i n r e l a t i o n t o t h e a c t u a l n a t u r a l gas consumption r a t e o f 375 m3 p e r ton o f z inc . This e f f i c i e n c y f i g u r e i s low because t h e Depue h e a t requirement i s penal- ized by t h e h igh i r o n ox ide con ten t o f t h e o r e and t h e r e l a - t i v e l y poor e f f i c i e n c y of t h e concurrent r ecupera to r s used f o r a i r p rehea t . Giving due c r e d i t t o t h e process , t h e corresponding e f f i c i e n c y of Palmerton r e t o r t s which r e c e i v e o r e of lower i r o n con ten t and which o p e r a t e wi th counter- c u r r e n t r ecupera t ion may be a s h igh a s 70-75 pe rcen t .

P l a n t Equipment and S p e c i f i c a t i o n s

The t a b u l a t i o n fol lowing g i v e s some d e t a i l s a s t o p l a n t equip- ment, s p e c i f i c a t i o n s , c a p a c i t i e s e t c . s i n c e t h e foregoing t e x t has been r a t h e r l i m i t e d t o coverage o f genera l p l a n t and process .

Raw Mate r i a l s S to rage Bins

Bin - Contents Capaci ty Discharge

1A Clay 140 m3 F e e d - 0 - ~ e i g h t / ~ e l t Conveyor 1 B Coke F ines 140 m3 V i b r a t i n g Trough/Belt Conveyor 2 Zinc Ore 280 m3 Feed-0-we igh t /~e l t Conveyor 3 Zinc Ore 280 m3 ~ e e d - 0 - w e i g h t / ~ e l t Conveyor 4 A n t h r a c i t e Coal 280 m3 Feed-0-weight/Belt Conveyor

Bituminous Coal Bins

Bin - Contents Capaci ty Discharge

205 ) Uncrushed Coal 46 m3) 205A) f o r ?/I M i l l 46 m3)

45 cm Pan Conveyor

206 Uncrushed Coal 153 m3 107 cm Apron-Feeder, f o r 112 M i l l V a r i a b l e Speed

204 ) Milled Coal 56 m3) 50 crn Feed-0-Weight 2 04A) 56 m3) and Redler Conveyor

Hannner M i l l s f o r Bituminous Coal

Pennsylvania Crusher - 75 HP - 4.5 ~ / h r ?I2 Pennsylvania Crusher - 125 HP - 8.2 T/hr

A t r a t i n g s g iven, both c rush t o nominal -14 mesh and 30 pe rcen t -200 mesh. Dust Co l l ec t ion : Model 70-A American Dustube C o l l e c t o r .

Rod M i l l s f o r ~ i n t e r / ~ r e Gr ind ing

Marcy M i l l s (2) - 93 KW - 3.7 rn Long x 1 .5 m Diameter - 11 ~ / h r

Rotary Blender f o r P re l imina ry Mixing

Rotary Drum - 1 .5 m Diameter x 11 m - 19 KW

Chasers f o r Mix Condi t ioning

Vulcan I r o n Works - four i n o p e r a t i o n wi th v a r i o u s p a r a l l e l - s e r i e s pa ths p o s s i b l e - 150 KW - 4 m pan diameter - each has two 10 T m u l l e r s a t 1 .6 m d iameter x 1 m wide - 21 rpm - r a t e of 8+ T pe r c h a s e r pe r hour . T r a n s f e r of chased mix t o b r i q u e t p res sed by b e l t conveyor.

P re l imina ry Br ique t P resses

New J e r s e y Zinc Company Design - s i x i n o p e r a t i o n - 7.5 KW - gear reducer and c h a i n d r i v e - 5 t o 13 rpm - i n t e r m i t t e n t o p e r a t i o n a s r e q u i r e d - r o l l s o f s p e c i a l a l l o y mix - 51 cm d iamete r - 4 pock- e t s wide i n l i n e a c r o s s f a c e o f r o l l - b r i q u e t s i z e 5 x 5 x 3 .2 cm.

F i n a l Loaf Br iquet P r e s s e s

New J e r s e y Zinc Company Design - s i x i n o p e r a t i o n - 11 KW - Reeves d r i v e wi th r educe r - 1.8 t o 4 . 5 rpm - cont inuous o p e r a t i o n a t speed conforming t o coker requirement - r o l l s o f c a s t manganese s t e e l - 61 cm diameter - two pockets wide and s t aggered - b r i q u e t s i z e 10.5 x 7.25 x 5.1 cm t h i c k - b r i q u e t weight about 0.6 kg and product ion about 118 T/da.

Coking Furnaces

F u l l e r Company ( s t e p - g r a t e c l i n k e r coo le r ) - s i x a v a i l a b l e - 3.7 KW wi th var ia-speed d r i v e - i n s i d e dimensions 2.45 m x 6.7 m - 21 g r a t e s - each e i g h t bol ted s e c t i o n s t o form 40 cm x 244 cm g r a t e - some s p e c i a l a l l o y - a l t e r n a t e f ixed and r e c i p r o c a t i n g g ra tes . Coked b r i q u e t holding w e l l - 3 m h igh x 2.4 m t o 1.9 m wide x 1.3 m t o 0.7 m deep - 6.7 m3 volume holding 7 T coke - 17 minute drawing cyc le . Production: 90-110 ~ / d a . Ind iv idua l b r i q u e t weight: 0.45- 0.5 kg.

V e r t i c a l Re to r t s

Nominal Bat- Heated Production

Nos. t e r y S i z e Levels Area Rating*

Tota l f o r 27 R e t o r t s a t 100% Operating Time - 200 T/da

*Assumes Normal I r o n i n S i n t e r

Condensers

Refractory Construct ion - 190 cm long x 58 cm wide i n s i d e and 40 cm deep bath holding 4.7 T z i n c including e x t e r n a l cool ing wel l . Impel ler - g r a p h i t e , 36 cm diameter wi th seven r a d i a l f i n s d r iven by 3.7 KW motor a t 400 rpm - 7 8 0 ° ~ bath temperature.

Recuperators

Surface Combustion Company - r e f r a c t o r y t i l e cons t ruc t ion - con- c u r r e n t flow with t h r e e combustion gas passes - a l l t h r e e f l u e s wide - bottom pass four f l u e s deep - upper passes t h r e e f l u e s deep - a i r flow v e r t i c a l l y upward. A i r p rehea t - 820°K. E x i t f l u e gas - 1 1 2 0 ~ ~ .

Zinc Pumps

New J e r s e y Zinc Company Design - v e r t i c a l s h a f t type with s p e c i a l l i f t impe l le r - g r a p h i t e o r s p e c i a l a l l o y - capac i ty of 13.5 ~ / h r and over.

PART D - VERTICAL REFINING

About one-half of t h e z inc metal produced on t h e v e r t i c a l r e t o r t s i s re f ined t o 99.995 percent p u r i t y by means of t h e continuous, f r a c - t i o n a l d i s t i l l a t i o n process which was developed a t about t h e same time a s t h e v e r t i c a l r e t o r t process. I n t h e v e r t i c a l r e f i n i n g proc- e s s , impur i t i e s b o i l i n g a t temperatures h igher than z inc such a s lead and i r o n , a r e removed i n a f i r s t s t age ; lower b o i l i n g compo- nents such a s cadmium, a r e removed i n a second s tage . The Depue z inc r e f i n e r y comprises four f i r s t - s t a g e columns and two second- s t a g e columns; t h e former a r e normally r e f e r r e d t o a s lead columns o r b o i l e r s whi le t h e l a t t e r a r e c a l l e d cadmium columns. 90 ~ / d a re - f ined z inc a r e produced a t Depue. The flow of m a t e r i a l s through t y p i c a l lead and cadmium columns i s shown i n Figure 6.

Each column comprises a s e r i e s of about 50 superposed, mono- l i t h i c , r ec tangu la r t r a y s made of bonded s i l i c o n carbide. Trays 60 cm x 120 cm a r e i n u s e a t Depue. Each t r a y i s adapted t o con- t a i n a pool of meta l , t h e he igh t of which i s s e t by an overflow t o a rec tangu la r opening near one end of t h e t r a y . The t r a y s a r e i n - s t a l l e d s o t h a t t h e openings on succeeding t r a y s a r e 180° a p a r t . Consequently, t h e metal overflowing from one t r a y i s caught on t h e one immediately below, t h e descending flow including a t r a v e r s e of each t r a y . The to r tuous path f o r t h e cascading l i q u i d required by t h e a l t e r n a t e d openings i s t h e only one o f fe red the ascending vapors , r e s u l t i n g i n more e f f e c t i v e con tac t between vapor and l i q u i d .

Heat i s suppl ied e x t e r n a l l y t o t h e lower p o r t i o n of each column by combustion gases . The lower por t ion of a lead column c o n s i s t s of a s e r i e s of t r a y s d i f f e r e n t from those descr ibed above i n t h a t they have a r a i s e d c e n t r a l por t ion whereby a trough i s formed around t h e per iphery of t h e t r a y capable of con ta in ing a he igh t of metal much g r e a t e r than t h a t on a f l a t t r a y . Increased h e a t t r a n s - f e r and b o i l i n g capac i ty r e s u l t from t h i s design which providss fo r contact of l i q u i d metal wi th a maximum proport ion of heated su r face .

The lower p a r t of each column i s enclosed by a r e f r a c t o r y walled combustion chamber. Natural gas i s introduced through p o r t s i n t h e roof along each of the longer s i d e s of t h e t r a y s . About 15 percent of t h e combustion a i r i s i n s p i r a t e d through t h e burner openings. Progress ive combustion of t h e gas f o r more uniform temperatures i s achieved by supplying preheated a i r a t each of t h r e e v e r t i c a l l e v e l s through f l u e s and p o r t s i n t h e wa l l s of t h e combustion chamber. The h o t combustion gases are .exhausted through r e f r a c t o r y recupera to r s i n countercurrent r e l a t i o n t o t h e flow of t h e incoming a i r . Pre- h e a t s a t Depue a r e normally i n excess of 1073'~.

As ind ica ted i n t h e previous s e c t i o n , molten z inc overflows from t h e r e t o r t condensers through a launder t o a holding pot and thence by a heated trough t o t h e lead column supply pots . The constant l e v e l maintained i n the trough and supply pots a ssures uniform feed

CADMIUU COLUMN

LEAD COLUMN ZINC-CAOYIUY

VAPOR ALLOY

,ZINC AND CADUIUU

FEED DV HEATED TROUGH

FROM RETORTS SECTION- CONDENSER

COUDUSTIO

RECYCLE

L E A D AND IRON,-ZINC DROSS

Figure 6

Flow Sheet of Vertical Refining Process

548 EXTRACTIVE METALLURGY OF LEAD AND ZINC

t o the lead columns, an e s s e n t i a l element f o r good performance. In each lead column, about 25 ~ / d a of z inc p lus e s s e n t i a l l y a l l of the cadmium is vaporized from t h e feed t o t a l i n g 40 ~ / d a . The remaining 15 T i n which t h e l ead , i r o n and o t h e r high-boi l ing impur i t i e s have become concentra ted, flows through t h e column and out t h e bottom t o a two-compartment, holding pot. Cooling i n the f i r s t compartment e f f e c t s separa t ion and s e t t l i n g of lead and an i ron-zinc d ross . The l iqua ted z inc overflows t o a second compartment from which i t i s re - cycled t o the column. The 40 ~ / d a of feed t o each column c o n s i s t s of t h e 15 T of r ecyc le p lus 25 T of new metal .

The z inc and cadmium vapors emanating from t h e heated s e c t i o n , undergo f u r t h e r p u r i f i c a t i o n from t h e h igher -bo i l ing meta l s by rec - t i f i c a t i o n dur ing t h e i r ascent through t h e r e f l u x s e c t i o n of t h e lead column. These vapors e x i t t o the lead column condensers, box- l i k e s t r u c t u r e s made of s i l i c o n ca rb ide b r i c k , where a condensation t o l i q u i d metal occurs.

The condensed z inc containing t h e cadmium from two lead columns i s fed t o one cadmium column f o r removal of cadmium and o t h e r low- b o i l i n g impur i t i e s . The cadmium columns d i f f e r from t h e lead c o l - umns i n t h a t they c o n s i s t e n t i r e l y of the previously descr ibed f l a t t r a y s s i n c e removal of only t h e comparatively small proport ion of low-boiling impur i t i e s r e q u i r e s much l e s s b o i l i n g capaci ty . Addi- t i o n a l l y , t o i n s u r e adequate r e f l u x , a condenser surmounts t h e co l - umn of t r ays . Adjust ing i n s u l a t i o n on t h i s condenser, a f f o r d s con- t r o l over t h e amount of r e f l u x and a l s o t h e cadmium content o f t h e cadmium-zinc a l l o y taken as product i n t h e smal ler condenser which follows t h a t a top t h e column. At Depue, about 350 kg of z inc con- t a i n i n g 15 percent cadmium a r e c o l l e c t e d from each cadmium column per day. The z inc metal i s s u i n g from t h e bottom of t h e cadmium column c o n s t i t u t e s t h e re f ined z inc product amounting t o about 96 percent of t h e z inc i n the new feed. The remaining 4 percent ex- cept f o r about 0.75 pe rcen t , i s accounted f o r i n t h e skimmings, i ron-z inc d r o s s , lead and cadmium-zinc a l l o y by-products. The re - f ined z inc contains 0.001-0,002 ~ e r c e n 2 lead, 3.3005-3.3357 perzea: ii-a:. :, ,; : -331-3.002 :serce;;t cn6ni.c.