2. Chapter 32
W
A
E
L
Z TREATING O
F COMPLEX ZINC-LEAD ORES,
KILN PRODUCTS LIMITED, BERG AUKAS, SOUTH W
E
S
T AFRICA.
Harry E. Cross
Consulting Metallurgist, Gold F i e l d s of South Africa Limited,
Johannesburg, South Africa.
and
Frank 0. Read
Deputy Consulting Metallurgist, Gold F i e l d s of South Africa Limited,
Johannesburg, South Africa.
Abstract
Kiln Products commissioned a Waelz k i l n 4 metres i n diameter by
75 metres long, i n South West Africa i n March, 1969, t o recover zinc
i n a form s u i t a b l e f o r the production of e l e c t r o l y t i c zinc. The raw
m a t e r i a l c o n s i s t s of a mixture of zinc s i l i c a t e concentrates and zinc
r i c h t a i l i n g s purchased from an adjacent zinc mine - the mixture con-
t a i n i n g 2% t o 25% zinc. The chemical composition of the ore i s
tabulated and the i n i t i a l t e s t r e s u l t s a r e described. Due t o the
comparatively low temperature a t which the ore s t a r t s softening the
k i l n has c e r t a i n unusual f e a t u r e s t o enable the temperature t o be
controlled within the necessary l i m i t s . A d e t a i l e d description of
the k i l n , the starting-up problems and present operating conditions
and r e s u l t s a r e presented.
3. I n t r o d u c t i o n
The South West A f r i c a Company Limited have operated a mine a t Berg
Aukas, approximately 2 1 kilometres north-east of Grootfontein i n South
West A f r i c a since 1959. Lead-zinc vanadate concentrates and lead and
z i n c sulphide and z i n c s i l i c a t e concentrates a r e produced by f r o t h
f l o t a t i o n .
The z i n c s i l i c a t e (willemite) concentrate produced is of high
grade but t h e percentage recovery is low a s a n appreciable amount of
t h e z i n c s i l i c a t e i s i n t h e form of a f i n e slime not amenable t o f r o t h
f l o t a t i o n .
An a s s o c i a t e d company, K i l n Products Limited, have now commissioned
a Waelz k i l n on t h e same property, t o recover the z i n c from both t h e
zinc s i l i c a t e concentrate and a n accumulated dump of z i n c bearing
t a i l i n g s . The r e s u l t a n t zinc oxide fume is transported some 2 800
kilometres by road and r a i l , t o another a s s o c i a t e d company, Zinc Cor-
p o r a t i o n Limited, s i t u a t e d near Johannesburg, a s a source m a t e r i a l
f o r t h e production of e l e c t r o l y t i c zinc.
Description of ore mined a t Berg Aukas
Zinc occurs i n the ore mainly a s t h e mineral willemite 2Zn0.Si02.
Minor q u a n t i t i e s a r e present i n the form of s p h a l e r i t e and d e s c l o i z i t e .
The gangue m a t e r i a l is predominantly dolomite.
Treatment of Ore
Approximately 13 000 t o n s of ore a r e delivered t o t h e reduction
p l a n t monthly. Approximately 1 000 tons of barren waste a r e discard-
ed by manual s o r t i n g and t h e remaining 12 000 tons a r e crushed and
m i l l e d i n b a l l m i l l s t o a grind of approximately 7% minus 74 microns.
A f t e r thickening, a l e a d sulphide concentrate is produced by f l o t a t i o n
w i t h xanthate and Aerofloat 25, t h e z i n c being depressed by the addi-
t i o n of z i n c sulphate and calcium cyanide. The zinc is then a c t i v a t e d
by t h e a d d i t i o n of copper sulphate t o produce a zinc sulphide concen-
t r a t e . The t a i l i n g s from t h i s f l o t a t i o n a r e conditioned with lime
and calcium cyanide and a g a i n subjected t o f l o t a t i o n w i t h a mixture
of amyl and e t h y l x a n t h a t e s and Aerofloat 25, t o produce a lead vana-
d a t e concentrate.
The r e s u l t a n t t a i l i n g s , amounting t o some 10 000 t o n s per month
a r e deslimed i n a 522 mm cyclone i n closed c i r c u i t with a 13 m dia-
meter thickener. The cyclone overflow product discharges i n t o the
thickener. The f i n e slime overflowing t h e thickener amounts t o about
2 500 t o n s p e r month containing about 1
8
%
z i n c and about 474 lead.
T h i s i s impounded i n a temporary slimes dam f o r l a t e r treatment i n
4. 920 EXTRACTIVE
METALLURGY
OF LEADAND ZINC
the Waelz kiln.
The thickener underflow joins the feed to the cyclone. The cyclone
underflow i s subjected t o 2 stages of f l o t a t i o n t o produce a lead
carbonate ( c e r u s s i t e ) concentrate and a zinc s i l i c a t e concentrate
amounting t o about 3 000 tons per month.
For c e r u s s i t e f l o t a t i o n , sodium hydro-sulphide, i s used i n conjunc-
t i o n with amyl xanthate, Aeromine 3037 and T.E.B. f r o t h e r while
willemite i s floated with f u r t h e r additions of the same reagents but
excluding the xanthate.
The f l o t a t i o n t a i l i n g s containing about 6% zinc a r e discarded. The
c e r u s s i t e concentrate i s small i n quantity and is mixed with the
lead sulphide concentrate.
A s zinc s i l i c a t e concentrates a r e not popular with smelters and
fetch a much lower price per u n i t of contained zinc than do zinc
sulphide concentrates, i t was decided t o investigate the f e a s i b i l i t y
of t r e a t i n g these concentrates along with an e x i s t i n g t a i l i n g s dam
i n a Waelz Kiln to produce a zinc oxide fume s u i t a b l e a s a feedstock
f o r an e l e c t r o l y t i c zinc plant.
The Waelz Process
The Waelz process has i t s origin i n a process patented i n 1910 by
Edward Dedolph of B r i t i s h Columbia. However, i t was not u n t i l 1923
t h a t Krupp Grusonwerk i n collaboration with Metallgesellschaft A.G.
s t a r t e d developing the process t o the stage where i t could be applied
on a commercial scale f o r the f i r s t time a t Luenen i n Upper S i l e s i a
i n 1925. I n 1957, according t o M. ~ o ~ a c z l ,
28 Waelz Kilns were
operating i n Poland t r e a t i n g over 3 000 tons of ore per day.
The process which has been well described i n the l i t e r a t u r e 2, 3,
4 , 5 , 6 , 7 c o n s i s t s i n outline of t r e a t i n g zinc bearing materials
with coke o r a n t h r a c i t e i n a horizontal r o t a r y k i l n a t temperatures
ranging from 1 0 0 0 ° ~t o 1500°C. The zinc i s reduced, v o l a t i l i z e d ,
oxidised t o zinc oxide and f i n a l l y separated from the exhaust gases
by means of bag f i l t e r s or e l e c t r o s t a t i c precipitation.
I t i s commonly accepted t h a t the following reactions occur:-
ZnO + C --D C O + Zn (vapour)
Zn (vapour) + C O + O2 4 ZnO + C02
C + o2 -C02
ZnS + CaO + C -Zn (vapour) + CaS + CO
ZnS + FeO + C -Zn (Tapour) + FeS + C O
2 ( ~ n ,
Fe, M ~ ) O
+ Si02 -2 ( ~ n ,
Fe, M ~ ) O . S ~ O
2 ( ~ n ,
Fe, M ~ ) o . s ~ o ~
+ Fe Zn (vapour) + 2 ? F e , M n ) o . ~ i o ~
ZnO + A1203 -Zn0.A1203
5. 9. Zn0.A1203 + Fe + Zn (vapour) + Fe0.A120
lo. 3 ZnO.Fe2O + 3 C + 3 Zn0 + 2 Fe304 + 3 CO
11. ZnS + Fe?2 CU) -
+ Zn (vapour) + FeS ( C U ~ S )
12. 2 ZnO.SiO2 + 2 C -
+ 2 Zn (vapour) + SiO2 + 2 CO
13. ZnO + CO Zn (vapour) + C02
14. 2 ZnO.Si02 + 2 CO 2 Zn (vapour) + Si02 + 2 C02
15. C + C02 4 2 CO
Fig. 1
6
It i s obvious,however,from the work of Bodenstein , t h a t
no chemical reaction takes place d i r e c t l y between s o l i d s
a s shown i n equations 1, 3 or 4. Simultaneous reactions
a c t u a l l y occur i n the gaseous phase a s shown i n equations
13 and 15 and by analogy 14 and 15. The progress of
these p a r t i c u l a r reactions was c r i t i c a l l y examined by
Truesdale and waring8 t o determine which reaction was
r a t e controlling. They concluded t h a t i n the temperature
range 9000C t o 12000C reaction 13 proceeded r a t h e r more
rapidly than reaction 15, while reaction 14, the reduc-
t i o n of willemite by carbon monoxide, was by f a r the
slowest. This i s i l l u s t r a t e d i n Fig.1 below.
6. P i l o t Plant Waelz Kiln Tests
After preliminary laboratory scale t e s t s , Messrs. Fried Krupp
Industriebau were commissioned t o conduct a p i l o t plant scale in-
v e s t i g a t i o n i n t h e i r t e s t k i l n a t Rheinhausen near Duisburg i n West
Germany. For t h i s purpose approximately 70 tons of current slimes
t a i l i n g s , 40 tons of material reclaimed from the high grade dump, 2
tons of zinc s i l i c a t e concentrate and 30 tons of washed anthracite
duff from the Vryheid area i n Natal, were shipped t o t h e i r p i l o t
plant.
Chemical analyses of the zinc s i l i c a t e concentrate, reclaimed
slime from a high grade dump, and current slimes t a i l i n g s used f o r
the t e s t a r e shown i n Table 1 below:-
Table 1. Chemical Analyses of Zinc Bearing Feed Materials (%)
7. WAELZ
TREATING
OF COMPLEX
ZINC-LEAD
ORES 923
An a n a l y s i s of t h e a n t h r a c i t e duff used i s shown i n Table 1
1 below:-
Table 11. Analysis of Anthracite Duff
Krupp's t e s t k i l n was 1.2 metres i n diameter by 14 metres long with
a n e f f e c t i v e volume of 8.2 cubic metres. The k i l n was equipped with
6 thermocouples f i t t e d through t h e s h e l l and 2 s h e l l f a n s with s u i t -
able ducting t o permit additional a i r t o be introduced a t any of 5
d i f f e r e n t points. The i n c l i n a t i o n used was 2% and the speed of rota-
t i o n could be varied from 0.5 t o 1 rev/min. The charge t o the k i l n
consisting of the zinc bearing o r e , a n t h r a c i t e and recycled zinc
oxide ( ' p r e o x i d e ' ) was homogeneously mixed i n batches i n a concrete
mixer and the moisture content adjusted t o about 1% before being fed
t o the k i l n .
Fixed Carbon ($)
Volet iles
Ash
Total Sulphur
Screen analysis + 10 r
+ 5 r - 1 o m m
+ 3 m - 5mm
+ l m - 3am
- 1 1 m
Calorific value cal/g
Ash analvsis ($1
Si02
As203 + Ti02
CaO
u
O
Fe2O-j
Ash softening temperaturn
Ash melting point
Ash flow point
A
The exhaust gases passed through s e t t l i n g chambers t o permit entrain-
ed gangue material t o s e t t l e with some of the zinc and lead oxide i n
the form of 'preoxide', which was recycled. The oxide fume was
collected i n a bag f i l t e r .
86.0
5.6
8.4
1.17
0.0%
5.7
23.7
50.0
20.6
7.4
40.3
31.6
3.5
2.7
16.7
1110"~
1250'~
1360°c
Test work was conducted over a period of 4 weeks during which time
over 60 tons of zinc bearing material averaging 21.7% zinc and 3.8%
lead were t r e a t e d i n the kiln. The residue discharged averaged 2.03%
zinc and 0.01% lead. The Waelz oxide produced averaged 63.3% zinc
and lo.% lead, with a magnesium content ranging from 0.4% t o 1.%.
A t y p i c a l a n a l y s i s i s shown i n Table 1
1
1
8. Table 111. Tvpical a n a l y s i s Waelz Oxide
produced i n Krupps p i l o t plant
Anthracite duff was used a s the reductant a t the r a t e of approxi-
mately 3
% of t h e weight of the new zinc bearing feed. The follow-
ing conclusions were reached:-
( a ) the burden s t a r t e d softening a t about 1 1 0 0 ~ ~
( b ) thus t o avoid excessive formation of a c c r e t i o n s the operating
temperature should not exceed llOO°C
( c ) it was possible t o v o l a t i l i z e a t l e a s t 90% of t h e zinc present
without exceeding 1 1 0 0 ~ ~
( d ) as a source of a d d i t i o n a l h e a t , gas would be preferable t o
e i t h e r o i l o r powdered c o a l , due t o the s o f t e r flame obtainable.
( e ) although a c c r e t i o n s were t o be expected, i t should be possible
t o t r e a t the raw m a t e r i a l t e s t e d by t h e Waelz Process, provided
operating conditions could be s u f f i c i e n t l y c a r e f u l l y controlled.
A s t h e Waelz oxide produced i n the t e s t k i l n had a bulk density of
only 0.83 g/cm3 i t was obvious t h a t bulk transport by normal road
and railway v e h i c l e s would present considerable problems. It was
therefore decided t o p e l l e t i z e the oxide using a d i s c p e l l e t i z e r and
adding a small q u a n t i t y of water only. The bulk d e n s i t y was thereby
increased t o about 1.7 g/cm3.
A bulk sample of t h i s p e l l e t i z e d m a t e r i a l was returned t o South
Africa f o r t e s t i n g purposes i n a p i l o t e l e c t r o l y t i c z i n c plant.
Test work showed t h a t although the magnesium and f l u o r i n e might
prove troublesome, good zinc recoveries could be expected and no
insuperable d i f f i c u l t i e s were l i k e l y t o be encountered.
9. A s a r e s u l t of t h e above, t h e decision was taken t o i n s t a l l a k i l n
a t Berg Aukas, with a nominal capacity of 10 000 tons p e r month of
new z i n c bearing feed.
K i l n Products - Description of Waele Plant
Messrs. Lurgi (s.A. ) ( p r o p r i e t a r y ) Limited were commissioned t o
design, supply and e r e c t a k i l n capable of t r e a t i n g 120 000 metric
t o n s p e r annum of new z i n c bearing feed c o n s i s t i n g of 20 000 tons of
z i n c s i l i c a t e and 100 000 tons of t a i l i n g s of a n a l y s i s s i m i l a r t o
those shown i n Table 1.
Location
The p l a n t i s s i t u a t e d adjacent t o t h e Berg Aukas mine a t a n a l t i -
tude of 1 450 m above sea l e v e l . Maximum ambient temperature i s 40°C
and t h e average annual r a i n f a l l is 625 mm e s s e n t i a l l y within t h e 3
months of summer. The nearest r a i l h e a d i s 21 kilometres d i s t a n t .
A flow plan of the p l a n t i n s t a l l e d i s shown i n Fig. 2.
Kiln
The k i l n i t s e l f i s 4 metres i n diameter by 75 metres long. The
s h e l l i s f a b r i c a t e d of welded mild s t e e l p l a t e , q u a l i t y ~ ~ ~ . 1 5 0 1 / 1 5 1
Grade 2BA, 29 mm t h i c k reinforced t o 60 mm f o r t h e f o u r running
t y r e s and t h e d r i v i n g gear sections.
The running t y r e s , 5 160 mm i n diameter and 770 mm wide a r e of
c a s t s t e e l , ~ ~ ~ . 5 9 2 / 1 9 5 0
Grade B q u a l i t y , welded t o the reinforced
s e c t i o n s of t h e s h e l l . The e i g h t supporting r o l l e r s , 1 300 mm i n
diameter a r e a l s o made of c a s t s t e e l and run on r o l l e r bearings which
a r e mounted on concrete p e d e s t a l s approximately 4 metres above ground
leve1.
The k i l n i s mounted a t a n i n c l i n a t i o n of 2% t o t h e horizontal.
The f i r s t , t h i r d and f o u r t h s e t s of supporting r o l l e r s numbered from
the f e e d end a r e adjusted such t h a t t h e supporting surfaces of each
p a i r a r e p a r a l l e l t o each o t h e r but a r e inclined t o t h e f a c e of the
r i d i n g r i n g by a n amount of about 0.15 mm. This has t h e e f f e c t of
causing t h e k i l n t o move up the slope towards the feed end. A p a i r
of h o r i z o n t a l r o l l e r s r o t a t i n g about a v e r t i c a l a x i s a r e r i g i d l y
mounted a t No.2 r i d i n g r i n g , one on each s i d e of i t , t o r e s t r i c t the
maximum h o r i z o n t a l movement of the k i l n t o about 50 mm. When No.2
r i d i n g r i n g reaches the t o p p o s i t i o n , o i l i s applied t o numbers 1,
3 and 4 r i d i n g r i n g s , which causes t h e k i l n t o s l i p back t o t h e lower
position. By wiping o f f the excess l u b r i c a n t , the k i l n begins t o
climb again towards t h e upper position. This complete cycle i s per-
formed once d a i l y .
10. COKE STOCKPILE COKE AND
OAS PLANT
2- PLLLETIZINO
WAELZ
OXIDE
PELLETS
A N D STACK
Fig. 2
11. The k i l n i n l e t section i s tapered over the f i r s t 1.1 metres from
a n opening of 1.9 metres. This tapered portion was o r i g i n a l l y lined
with s p e c i a l l y shaped tapered Lusite f i r e b r i c k s , but due t o severe
s p a l l i n g , these were l a t e r replaced by castable r e f r a c t o r y approxi-
mately 150 mm i n thickness which has since given good service.
The feed t o the k i l n i s introduced by gravity v i a a n inclined
s t e e l tube about 5 metres long and 500 rnm i n diameter with a wall
thickness of 25 mm. Sealing a i r i s blown down the feed tube t o
prevent the escape of hot k i l n gases, by means of a 4 k
W f a n capable
o f delivering 2 000 I4m3/h a t a pressure of 200 mm wg. The a c t u a l
amount used i s coritrolled by manual adjustment of a c a l i b r a t e d
b u t t e r f l y valve.
Sealing of the r o t a r y k i l n i n l e t end i s achieved by means of 2
s l i p r i n g s , of c a s t i r o n and c a s t s t e e l respectively, s l i d i n g against
each other. One r i n g i s attached e x t e r n a l l y t o the periphery of the
k i l n and the o t h e r , which i s s t a t i o n a r y , c o n s i s t s of separate seg-
ments held up against the r o t a t i n g r i n g by means of 16 pneumatic
pistons.
The k i l n o u t l e t section i s double walled and provided with a
labyrinth type s e a l cooled by means of a 10 k
W fan. The s h e l l dia-
meter i s reduced by 610 mm i n two s t e p s over the l a s t 4 metres and
a s the brick-lining thickness i s maintained a t 229 mm throughout,
the i n t e r n a l diameter of the discharge end of the k i l n i s reduced t o
2.87 metres.
Eight a i r fans were o r i g i n a l l y i n s t a l l e d on the s h e l l of the k i l n
a t 19, 25, 33, 37, 42, 46, 50 and 56 metres respectively from the
feed end, f o r blowing a d d i t i o n a l a i r i n t o the k i l n via tubes of
s p e c i a l heat r e s i s t a n t s t e e l containing 25% chromium and 12% nickel.
The tubes a r e approximately 400 mm i n diameter with a wall thick-
ness of 20 mm. The discharge nozzle i s 200 mm i n diamter and design-
ed and positioned so a s t o ensure t h a t the a i r i s directed horizon-
t a l l y a t the centre of the k i l n , counter-current t o the main gas flow,
i . e . towards the k i l n discharge end. Sight g l a s s e s were subsequently
f i t t e d on the bends on the a i r d u c t s between the fans and the tube t o
permit v i s u a l examination of the tube during operation. Each fan has
a rated capacity of 2 000 ~my/ha t a pressure of 200 watergauge, and
i s driven by a 4 k
W motor, power being supplied by means of copper
s l i p r i n g s attached t o the k i l n . The a c t u a l amount of a i r blown i n a t
each point can be controlled by manual adjustment of a c a l i b r a t e d
b u t t e r f l y valve.
The k i l n was o r i g i n a l l y provided with 6 platinum-rhodium thermo-
couples f i t t e d tnyough the s h e l l and protruding a maximum of 50 mm
inside the brick-lining. These were connected t o s l i p r i n g s near the
feed end of the k i l n from where impulses were transmitted t o a 12
point temperature recorder i n the control room. The transmission
12. t i m e s and the p o s i t i o n s of the thermocouples were s o arranged t h a t
each thermocouple produced 2 recordings per r e v o l u t i o n - once when
the thermocouple was embedded i n t h e burden a t about a 7.0 o'clock
p o s i t i o n and t h e o t h e r when i n the d i a m e t r i c a l l y opposite position.
By t h i s means i t had been hoped t o o b t a i n both a burden temperature
and a gas temperature. However, i t was soon obvious t h a t t h e two
temperatures recorded were p r a c t i c a l l y i d e n t i c a l a s t h e r e was i n s u f f i -
c i e n t time between readings f o r t h e thermocouple t o reach equilibrium.
It was decided l a t e r t o i n t e r s p e r s e 5 a d d i t i o n a l thermocouples and a t
the same time a l t e r the design s o t h a t t h e thermocouple could be
allowed t o protrude up t o 360 mm w i t h i n the k i l n , i f desired. Thermo-
couples a r e t h e r e f o r e s i t e d a t the following p o s i t i o n s from t h e feed
end: -
8, 20, 29, 34, 40, 44, 47, 50, 53, 59 and 67 metres. The thermo-
couple a t 8 m s t i l l recorded burden and g a s temperatures - a l l the
r e s t recorded burden temperatures only.
The k i l n o u t l e t head is of welded s t e e l c o n s t r u c t i o n l i n e d with
heat r e s i s t a n t r e f r a c t o r i e s and is mounted on a n undercarriage with
c a s t o r s t o enable i t t o be moved back out of t h e way when necessary.
A l a r g e hinged door provides ready a c c e s s t o t h e k i l n . A s the k i l n
i s operated with t h i s door closed, two 100 mm diameter s i g h t g l a s s e s
a r e provided f o r viewing t h e c o n t e n t s of the k i l n during operation.
K i l n Drive
The k i l n i s driven by a c a s t s t e e l ~ ~ ~ . 5 9 2 / 1 9 5 0
Grade C g i r t h g e a r
400 mm wide, made i n two s e c t i o n s and a t t a c h e d t o t h e s h e l l by 12
t a n g e n t i a l s p r i n g p l a t e s . Two synchronized v a r i a b l e speed d.c.
motors each 70 kW, with t h y r i s t o r s and diodes f o r voltage r e g u l a t i o n ,
operate separate reduction gearboxes which enable the k i l n speed of
r o t a t i o n t o be v a r i e d over the range 0.25 t o 1.0 r e v o l u t i o n s p e r
minute. Two a d d i t i o n a l emergency 22 k
W motors, powered by a Deutz
d i e s e l operated 80 kVA synchronous g e n e r a t o r , operating through addi-
t i o n a l gearboxes a r e capable of r o t a t i n g t h e k i l n a t 0.0625 revolu-
t i o n s p e r minute. S u i t a b l e instrumentation ensures t h a t t h i s u n i t
comes i n t o operation automatically i n case of a power f a i l u r e , t o
keep t h e k i l n r o t a t i n g and t o keep c e r t a i n other e s s e n t i a l s e r v i c e s
i n operation.
Residue Disposal
The residue discharged from t h e k i l n drops down a transverse
i n c l i n e d chute l i n e d w i t h f i r e b r i c k , i n t o a quench pond from where
i t i s raked up a n i n c l i n e d s t e e l covered slope by means of a manually
c o n t r o l l e d motorized s c r a p e r winch, and discharged v i a a 50 mm
g r i z z l e y onto a n i n c l i n e d rubber conveyor b e l t , 610 mm i n width. The
s c r a p e r bucket has a vcd-ume of 0.7 m3 and the t o t a l l i f t is about
13. 9 metres. The winch i s powered by a 35 kW motor.
A s originally designed, the discharge chute was open t o the atmos-
phere, which i n e f f e c t l e f t the discharge end of the k i l n a l s o com-
pletely unsealed and permitted an uncontrollable amount of a i r t o be
drawn i n t o the kiln. This was eventually remedied by sealing the dis-
charge chute by means of hinged horizontally overlapping s t e e l s l a t s
each about 500 mrn wide, continued down t o below the water-level i n
the quench pond.
An elevated s t e e l walkway runs the e n t i r e length of the k i l n on
one side, t o provide access t o the s h e l l fans and the s h e l l thermo-
couples, etc.
Gas Producer
The producer gas plant, operating on bituminous coal nuts, nomin-
a l l y 25 mrn t o 40 mm i n size i s capable of delivering 2 000 ~m?/h of
gas a t 300°C t o 350°C with c a l o r i f i c value of approximately 1 800
kcal/~m3, i.e. a maximum of 3.6 x lo6 kcal per hour, t o a burner
c e n t r a l l y positioned a t the discharge end. Flexible connections per-
m i t the angle of the burner t o be a l t e r e d r e l a t i v e t o the a x i s of the
kiln. Primary combustion a i r is delivered t o the burner by a separate
fan with a maximum capacity of 3 500 &3/h a t 600 mm water gauge,
driven by a 10 kW motor. The a c t u a l volume can be varied by manual
adjustment of a calibrated b u t t e r f l y valve, and is recorded i n the
control room. The coal is screened immediately p r i o r t o use through
a 19 mm screen t o remove any f i n e s which a r e used a s a reductant and
mixed with the coke and anthracite. Maximum coal consumption is
about 20 tons per day. A typical analysis of the coal used i s given
i n Table 1 V below:-
Table 1V. Typical Analysis of Bituminous Coal
( ~ i r
Dry l3asisl
Calorific valw
Ash
Volatile material
Flxed Carbon
Total Sulphur
Aah m i o n twpemtura
Noiatum
6 950 c
a
l
k
10.1 $
25.9 $
61.2 $
0.6
lye0 C
2.8 $
14. 930 EXTRACTIVE
METALLURGY
OF LEADAND ZINC
The producer gas has a t y p i c a l a n a l y s i s a s shown i n Table V below:-
Table V. Typical Analysis of Producer Gas
Fig. 3
A general view of the k i l n showing the driving gear,
the a i r i n l e t ducts and the gas producer plant i n
t h e back ground is shown i n Fig.3.
Brick Lining
The bricks used f o r l i n i n g the k i l n measure 229 mm by 152 mm by
76 mm tapering down t o about 69 mm and a r e l a i d end on t o give a
l i n i n g 229 mm thick. Two q u a l i t i e s of bricks were used o r i g i n a l l y ;
'Superlinf f o r the f i r s t 30 m from the feed end and 'Lusite' f o r
the remaining 45 m. Subsequently 'Jumbof bricks were a l s o used as
replacements. Typical analyses a r e shown i n Table V 1
15. Table V1. Typical Analyses of Superlin,
Lusite and Jumbo Bricks
s i q k
~ 1 2 0 9
~ 0 ~ 0 ~ s
Tio2k
c a d
n d
Alkalis
Porosity $
Bulk density g / a
3
Permeability ( c
.
g
.
s
. units)
Cold crushing strength kg/cm2
Luaite
- -
44 - 46
51 - 53
1
.
2 - 1
.
7
0
.
5 - 0
.
7
0
.
3 - 0
.
5
0
.
3 - 0
.
5
Trace
I 14 - 18
2.40
1 0.10
350 - 850
Jumbo
S t e e l rings made of 25 mm f l a t mild s t e e l 100 mm deep were welded
i n t o the s h e l l a t about 4 t o 5 metres i n t e r v a l s t o prevent movement
of the bricks r e l a t i v e t o the s h e l l . A cardboard expansion j o i n t
3 mm thick was i n s e r t e d at approximately 1.5 metre i n t e r v a l s . Relin-
ing i s now done by the plant operators a t the r a t e of 2 metres per
2 bricklayers per 8 hour s h i f t , provided they a r e not kept waiting
f o r materials. The lower half of the k i l n i s lined f i r s t . These
bricks a r e then kept i n position by means of mechanical jacks while
the k i l n i s rotated s u f f i c i e n t l y t o allow the r e s t of the bricks t o
be l a i d .
Recovery of Oxide from Exhaust Gases
The gases emerging from the feed end of the k i l n pass through
s e t t l i n g chambers t o remove entrained s o l i d s , a cooling tower t o
reduce the temperature 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 s where the
oxide is p r e c i p i t a t e d , before f i n a l l y being discharged t o atmosphere
v i a a stack.
S e t t l i n g Chambers:
The three f l u e gas s e t t l i n g chambers a r e each approximately 5 m
long by 7 m wide by 6 m high with hopper type bottoms. They a r e con-
s t r u c t e d of s t e e l p l a t e and the walls a r e lined with 27 cm of normal
duty f i r e b r i c k containing about 3% A1203. The roof i s f l a t and i s
insulated by about 23 c
m of heat r e s i s t a n t mortar, held i n place by
means of s t a i n l e s s s t e e l reinforcing mesh. Three large doors a r e
f i t t e d f o r providing access f o r inspection and there a r e eight
hatches each about 0.5 m2 i n s i z e which a r e designed t o blow off i n
case of a n explosion i n the chamber. Preoxide was discharged
16. 932 EXTRACTIVE
METALLURGY OF LEADAND ZINC
o r i g i n a l l y from each compartment v i a a s t a r f e e d e r i n t o a pneu-
matic t r a n s f e r system d e l i v e r i n g i n t o a storage s i l o which i n t u r n
discharged v i a s t a r f e e d e r on t o t h e main feed b e l t immediately
ahead of t h e pugmill.
T h i s system which had been designed t o operate under vacuum gave
so much t r o u b l e due t o blockages i n the d e l i v e r y l i n e t h a t i t was
l a t e r a l t e r e d t o operate a s a pressure system. T h i s was s t i l l u n s a t i s -
f a c t o r y and a s r e s u l t of continual blockages and l a r g e dust l o s s e s ,
wan abandoned u n t i l i t was eventually modified t o f u n c t i o n a s a
pressure system embodying t h e various refinements described l a t e r i n
the paragraph d e a l i n g with pneumatic t r a n s f e r .
A i r i s provided by a Rootes type blower with a displacement of
1500 m3/h a t a pressure d i f f e r e n c e of 0.41 kg/cm2. The d e l i v e r y duct
i s 125 mm i n diameter w i t h r e i n f o r c e d bends. Approximately 95% of
t h e dust i s c o l l e c t e d i n a 17O cyclone, approximately 550 mm i n d i a -
meter, t h e remaining 5% f i n a l l y 'being removed i n a bag f i l t e r , a l l
t h i s dust being discharged i n t o t h e storage s i l o ahead of t h e pugmill.
The bag f i l t e r c o n t a i n s 2 8 socks with a t o t a l f i l t e r i n g a r e a of
approximately 40 m2. The maximum o p e r a t i n g temperature f o r t h e type
of bags used i s approximately 150°C.
Cooling Tower: A f t e r l e a v i n g t h e s e t t l i n g chambers t h e f l u e gases
a r e cooled i n a n evaporative c o o l e r by t h e c o n t r o l l e d i n j e c t i o n of
spray water. The cooling tower i s 6.5 m i n diameter and approximately
24 m i n h e i g h t , constructed of 8 mm mild s t e e l p l a t e .
The cooling tower was designed t o c o o l a maximum of 65 000 Nm3/h
of g a s from a n i n l e t temperature of about 5 5 0 ' ~ t o a n o u t l e t tempera-
t u r e of about 2 0 0 ' ~ - i . e . a temperature g r a d i e n t of about 350°C.
Seven s e t s of sprays each containing 6 nozzles a r e f i t t e d a t t h e top.
The nozzle design i s such t h a t excess water which i s not sprayed i n t o
the tower i s bypassed back from t h e nozz1.e i n t o a r e t u r n water mani-
f o l d . The amount of water sprayed i n t o t h e tower i s c o n t r o l l e d by
a d j u s t i n g the opening of a valve on the r e t u r n manifold, thereby in-
c r e a s i n g o r decreasing t h e pressure on the nozzles. This c o n t r o l i s
e f f e c t e d automatically from the temperature of t h e gas leaving t h e
c o o l i n g tower. Two o t h e r automatic c o n t r o l s a r e provided t o operate
i n a n emergency, viz:-
0
( a ) i f t h e temperature of t h e exhaust g a s exceeds 300 C a n emer-
gency f l a p a t t h e top of t h e tower operated by means of a
motorized winch opens t o atmosphere, t o avoid damage t o t h e
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 s .
( b ) i f t h e volume of gas passing through t h e exhaust s t a c k drops
below 30 000 Nm3/h o r i f the temperature of t h e g a s a t t h e
i n l e t t o the c o o l i n g tower drops below 220°C, t h e water supply
t o t h e nozzles i s shut off completely t o prevent t h e accumula-
t i o n of unevaporated water i n t h e tower, and a s i g n a l l i g h t
17. f l a s h e s i n the control room. The amount of water consumed here
i s normally about 110 l i t r e s per minute - maximum output of the
pump is 250 l i t r e s per minute a t a maximum operating pressure of
40 kg/cm2. S e t t l e d dust drops i n t o a screw conveyor, the trough
of which has a hinged f l a p a t the base, t o f a c i l i t a t e cleaning.
By changing the direction of r o t a t i o n of the screw, oxide which
s e t t l e s i n the tower can be diverted e i t h e r t o the preoxide pneu-
matic t r a n s f e r system o r t o the Waelz oxide system, depending on
its quality. Provision i s a l s o made f o r discharging the dust on
t o the f l o o r i n case of emergency.
E l e c t r o s t a t i c Precipitators: Two horizontal e l e c t r o s t a t i c precipi-
t a t o r s designed and supplied by Lurgi a r e i n s t a l l e d f o r normal opera-
t i o n i n p a r a l l e l , f o r a maximum gas flow of 85 000 ~m3/h. I n emer-
gency e i t h e r precipitator can be isolated and the f u l l gas stream
routed through the other. Collection efficiency of 99.5% i s guaran-
teed f o r an ;inlet gas dust content of above 50 g/~m3. A t a lower in-
l e t gas dust content, the cleaned gas dust content is not t o exceed
300 mg per ~ m 3 . With only one p r e c i p i t a t o r on l i n e the collection
efficiency drops t o about 95%.
Each precipitator i s 17.5 metres long and 5 metres wide and i s con-
structed of 5 mm s t e e l p l a t e with s u i t a b l e s t i f f e n e r s t o withstand a
negative pressure of 120 mm wg. The positively charged grounded
c o l l e c t i n g electrodes a r e of the plate type about 5 metres i n height
and spaced about 250 mm apart with the negatively charged discharge
electrodes suspended c e n t r a l l y between them a t about 160 mm intervals.
Mechanical rapping devices a r e provided f o r both positive and nega-
t i v e electrodes. Each precipitator contains three e l e c t r o s t a t i c
f i e l d s connected i n sequence. Each power pack i s rated a t 95 k
V
A and
has a cabinet type switchboard with t h y r i s t o r control and automatic
power control.
The precipitated dust c o l l e c t s i n the hopper type bottoms from
where it i s removed continuously by means of Redler conveyors. A s
o r i g i n a l l y supplied, these Redlers each discharged v i a individual
s t a r feeders i n t o a pneumatic transfer system operating under vacuum.
This arrangement gave endless trouble due t o blockages and was l a t e r
modified a s described i n the paragraph dealing with the pneumatic
t r a n s f e r system.
A i r f o r the pneumatic t r a n s f e r system is provided by a 1 500 m3/h
Rootes blower powered by a 30 k
W motor. The dust delivery column i s
125 ma i n diameter and i s constructed of mild s t e e l with suitably re-
inforced bends. Approximately 95% of the dust i s collected i n a
cyclone and the remaining 5% i n a bag f i l t e r with a t o t a l f i l t e r i n g
area of 40 m2. Both products g r a v i t a t e i n t o e i t h e r of two conical
bottom storage s i l o s , each with a nominal capacity of 100 tons.
Exhaust Fan: An a x i a l flow exhaust f a n with vane control and
servo motor i s i n s t a l l e d between the p r e c i p i t a t o r s and the f i n a l
18. s t a c k , which i s 2.2 metres i n diameter and 26 metres i n height.
The f a n w a s supplied by Kiihnle, Kopp and Kausch of Frankenthal-
Pfalz and has a rated capacity of 175 000 m3/h a t 2 0 0 ' ~ and 131.5 mm
wg - it i s driven by a 100 kW motor. I n l e t vane control operates i n
conjunction with the automatic draught c o n t r o l l e r .
Fig. 4 below shows the cooling tower i n the centre with the electro-
s t a t i c p r e c i p i t a t o r s and s e t t l i n g chambers on e i t h e r side.
Fig. 4
P e l l e t i z i n g Plant: Waelz oxide i s discharged from each s i l o v i a
a s t a r feeder and screw conveyor t o two p e l l e t i z i n g d i s c s 4 metres i n
diameter. By the a d d i t i o n of about 1% of water, a s a f i n e spray,the
powder i s turned i n t o p e l l e t s ranging from about 6 mrn t o 20 mm i n
diameter - no other a d d i t i v e s a r e found t o be necessary. The "green"
p e l l e t s , a s produced, without any drying, have a crushing strength
of about 15 t o 20 kg per p e l l e t . Each d i s c i s f i t t e d with a 30 kW
motor. The angle of i n c l i n a t i o n can be a l t e r e d i n the range 300 t o
60° by operating a handwheel, and the angle normally used i s 45O t o
the horizontal. The d i s c s a r e operated a t a speed of 6 revolutions
per minute and t h i s can be varied only by changing pulleys. The
r a t e of output is'approximately 8 tons per hour per p e l l e t i z i n g disc.
The p e l l e t s a r e discharged on t o a conveyor b e l t f o r delivery t o a
storage shed and a r e subsequently handled by means of f r o n t end
loaders.
19. Control Room
An air-conditioned c o n t r o l room i s provided with a mimic flow dia-
gram of the p l a n t , and the necessary i n d i c a t i n g l i g h t s , recorders,
switches, alarms, e t c . t o enable the p l a n t t o be remotely c o n t r o l l e d
from t h i s position. I n p a r t i c u l a r the following instruments a r e pro-
vided: -
( a ) A Schenk b e l t weighing machine f o r new zinc bearing feed, which
records i n the c o n t r o l room, and i n t e g r a t e s the tonnage.
( b ) 2 Schenk reductant proportioning weigh f e e d e r s and recording and
i n t e g r a t i n g equipment.
( c ) K i l n draught c o n t r o l l e r and recorder.
( d ) 12 point k i l n temperature recorder.
( e ) K i l n r a t e of r o t a t i o n i n d i c a t o r and recorder.
( f ) K i l n main d r i v e kilowatt recorder.
( g ) Airflow r a t e s , pressure and temperature recorders, e t c . f o r pro-
ducer gas p l a n t .
( h ) Recorder of oxygen i n k i l n exhaust gas.
( i ) Recorder f o r gas temperatures a t i n l e t and o u t l e t of cooling
tower with c o n t r o l equipment.
( j ) I n d i c a t i n g instruments f o r m i l l i amps and voltages i n 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 s .
( k ) Exhaust g a s temperature i n d i c a t o r s .
( 1 ) High l e v e l i n d i c a t o r s f o r Waelz oxide bins.
Operati o n
Reductants
Unwashed a n t h r a c i t e duff from t h e Vryheid a r e a i n Natal and coke
f i n e s from ISCOR (IRON & STEEL CORPORATION)
a t Van d e r B i j l Park a r e
used a s reductants. Current a.nalyses a r e shown i n Table V 1 1
20. Table V11. Typical Analyses Unwashed Anthracite
Duff and Coke Fines
These products a r e r a i l e d i n bulk approximately 2 800 kilmetres
t o Grootfontein, the nearest railhead t o the k i l n , and then trans-
f e r r e d t o road trucks by means of C a t e r p i l l a r 622B f r o n t end loaders,
t o traverse the f i n a l 21 kilometres t o the kiln. Both products a r e
stockpiled i n bulk i n the open, the height of the stockpile being
about 3 metres. A s required, the reductant is removed by means of
f r o n t end loaders and dropped onto a variable speed p l a t e feeder with
an e f f e c t i v e width of 1 metre, which discharges onto a n inclined con-
veyor b e l t 610 mm wide and d e l i v e r s onto a n elevated reversible con-
veyor a t a maximum r a t e of 20 tons per hour, which drops the product
i n t o e i t h e r of 2 storage s i l o s , each with a capacity of 150 tons.
Each storage s i l o i s equipped with a Schenk weigh feeder which can
be s e t to automatically d e l i v e r a preset percentage of the weight of
the new zinc bearing feed material passing over the b e l t weighto-
meter on i t s way i n t o the kiln. Alternatively each weigh feeder can
be adjusted from the control room t o d e l i v e r reductant a t a steady
pre-selected r a t e . Each weigh feeder i n d i c a t e s , i n the control room
the tons per hour being fed, and i n t e g r a t e s the tonnage. The reduc-
t a n t drops on the zinc bearing feed material on the main feed b e l t
which discharges i n t o a pugmill to provide homogeneous mixing of the
reductant with the ore.
Sice Analyais
+ 5mm
- 5 + 3 m
- 3 + 2 mm
- 2 + 1 m
- 1 + 0.5 mm
- 0.5 mm
Ash
Voletiles
Fixed Carbon
Ash fusion temp.
Calorific value cal
Bulk density
Zinc bearing feed
Zinc s i l i c a t e concentrate is delivered d a i l y t o an open stock-
Unwashed
Anthracite
duff
1 s
22
18
21
11
15
9 . f i
10.5
78.2
1 4 0 0 ~ ~
7 500
0.88
Coke
Fines
lllr
11
11
24
18
25
16.65
1.1
81.6
1400°c
7 000
0.69
21. p i l e , using a Shawnee-Poole t r a c t o r - t r a i l e r u n i t w i t h a carrying
capacity of 10 tons. The product a s delivered contains about 12%
moisture. It i s removed from t h e s t o c k p i l e by means of a f r o n t end
loader a s required and dropped on t h e same p l a t e f e e d e r a s used i n t e r -
m i t t e n t l y f o r reductant, f o r d e l i v e r y i n t o the zinc s i l i c a t e storage
s i l o w i t h a capacity of 450 tons. The s i l o i s equipped with a 2 metre
diameter t a b l e f e e d e r , r o t a t i n g a t 0. 9 revolutions p e r minute,'
capable of d e l i v e r i n g up t o 10 tons per hour on t o the main feed b e l t
t o t h e pugmill.
Slime i s reclaimed from the slimes dam some 500 metres from the
k i l n by means of f r o n t end loaders digging i n from t h e side. The dam
i s about 6 metres i n h e i g h t and p e r i o d i c a l l y i t has been considered
prudent t o collapse t h e f a c e when i t becomes too n e a r l y v e r t i c a l , by
b l a s t i n g . The f r o n t end l o a d e r s d e l i v e r i n t o Shawnee-Poole t r a c t o r -
t r a i l e r u n i t s a s used f o r the z i n c s i l i c a t e concentrate, and d e l i v e r
t o a d a i l y s t o c k p i l e under cover, alongside the z i n c s i l i c a t e p l a t e
feeder. The b i n above t h e feeder has a capacity of 90 tons and i s
f i l l e d p e r i o d i c a l l y during t h e course of each s h i f t from the stock-
p i l e alongside, using a f r o n t end loader. The p l a t e f e e d e r can be
a d j u s t e d t o d e l i v e r a t a steady r a t e ranging from about 5 t o about
25 t o n s p e r hour, on t o the main feed b e l t .
The main feed b e l t i s equipped w i t h a weightometer which records
t h e r a t e of feed i n t o n s per hour and i n t e g r a t e s t h e t o t a l tonnage.
The mixture of zinc s i l i c a t e , reclaimed slime, reductant and pre-
oxide i s delivered on t o a g r i z z l e y which permits the f i n e s t o drop
i n t o a pugmill while t h e p l u s 50 mm lumps, stones and f o r e i g n bodies
a r e bypassed d i r e c t l y on t o feed b e l t receiving t h e mixed product
from t h e pugmill. The pugmill was supplied by Karl Handle and Sons
and i s about 2.5 m long by 0.7 m wide. It i s equipped with two
p a r a l l e l s h a f t s carrying blades which r o t a t e i n opposite d i r e c t i o n s
t o produce a mixing and propelling motion. It i s driven by a 30 k
W
motor.
S t a r t up procedure
Drying out of the brickwork was commenced on 19th February, 1969.
The following procedure was adopted:-
( a ) The k i l n was r o t a t e d o n e q u a r t e r t u r n every twenty minutes,
using t h e a u x i l i a r y d r i v e only, i . e . r a t e of rotation:-
0.0625 rev/min.
( b ) Producer gas was burnt a t the discharge end using t h e standard
burner, a t a r a t e s u f f i c i e n t t o r a i s e t h e temperature of the
brickwork a t Numbers 1 and 3 thermocouples 67 and 59 m respect-
i v e l y from t h e f e e d end t o 1 5 0 ' ~ i n 24 hours, and the tempera-
t u r e was then held steady a t 1500C f o r 5 days.
22. ( c ) Temperature was then increased a t the r a t e of 50°C per hour t o
200°C and held there f o r one day.
( d ) S h e l l fans were s t a r t e d up and s u f f i c i e n t a i r admitted t o main-
t a i n the a i r tube temperatures a t 200°C.
( e ) When the temperature i n the s e t t l i n g chambers reached 2000C, i t
was maintained there f o r 4 days.
( f ) Cooling tower f l a p was opened and the main fan s t a r t e d and ad-
justed t o i t s minimum delivery r a t e .
( g ) Kiln temperature was s t e a d i l y increased a t the r a t e of 50% per
hour u n t i l the brickwork a t Nos. 1 and 3 thermocouples reached
7OO0C.
( h ) Kiln was then rotated continuously on a u x i l i a r y drive.
( i ) Temperature was raised t o 9 0 0 ~ ~
a t the r a t e of 5 0 ' ~ per hour.
( j ) Anthracite was then fed i n t o the k i l n followed by zinc bearing
feed when the temperature had reached 1 0 0 0 ~ ~ .
The above programme took 13 days t o complete. Once the brickwork
had been thoroughly dried out the subsequent s t a r t i n g up procedure
was streamlined t o enable zinc bearing feed t o be introduced approxi-
mately 18 hours a f t e r s t a r t i n g preheating.
During the f i r s t year of operation i t was necessary t o cool the
k i l n down and cease operations on 13 occasions. Current start-up
procedure, i f there i s no brickwork t h a t requires slow drying out,
i s a s follows:-
( a ) The cooling tower f l a p i s l e f t open.
The ducts t o the p r e c i p i t a t o r s a r e closed.
The s h e l l fans a r e operated with b u t t e r f l y valves s u f f i c i e n t l y
open t o allow only a t r i c k l e of a i r t o pass.
The main f a n i s stationary.
The k i l n i s rotated 4 t u r n every 20 minutes on a u x i l i a r y drive.
The gas burner i s s t a r t e d up a t about half maximum capacity.
By gradually increasing the gas burning r a t e and closing the
cooling tower f l a p , the temperature of the gas a t the i n l e t t o
the cooling tower i s raised t o 200°C over a period of about 6
hours.
( b ) The k i l n i s then rotated continuously on main drive a t minimum
r a t e of 0.25 rev/min.
The f l a p on the cooling tower i s closed completely, the ducts
t o the p r e c i p i t a t o r s a r e opened, the s h e l l fans a r e opened one
o r two notches and the main f a n i s s t a r t e d up with i n l e t vanes
opened l e s s than 1% i n i t i a l l y .
23. Anthracite duff i s fed t o the k i l n a t the r a t e of 2 tons per
hour.
( c ) About 5 hours l a t e r the a n t h r a c i t e reaches the discharge end of
t h e k i l n where i t i s i g n i t e d by the gas burner. The temperature
then r i s e s rapidly and when the gas temperatures a t the i n l e t t o
the cooling tower reaches 650°C, zinc bearing feed is introduced,
a t a n i n i t i a l r a t e of 5 tons per hour, with 50% by weight of
a n t h r a c i t e and coke. This i s usually about 18 hours a f t e r the
i n i t i a l l i g h t i n g up.
( d ) The feed r a t e i s gradually increased over the following 12 hours
t o maximum r a t e .
Results of Operation
Operating r e s u l t s obtained t o d a t e a r e summarized i n Table V111.
Zinc bearing feed containing about 14% zinc was introduced i n t o
the k i l n f o r the f i r s t time on 10th March, 1969. D i f f i c u l t y was ex-
perienced i n maintaining the temperature, which eventually a f t e r 7
days dropped so low t h a t it was decided t o cool down and inspect the
k i l n . Except f o r some s p a l l i n g of bricks i n the feed cone and some
physical damage t o some of the thermocouples, the k i l n was i n good
condition. During t h i s campaign the feed t o the k i l n consisted of 8
tons per hour of reclaimed slime and 15 t o 20% of washed anthracite.
During the next 3 months there were 5 f u r t h e r campaigns ranging
from 4 days t o 27 days i n length. I n each case the k i l n was even-
t u a l l y closed down due t o excessive buildup of accretions. On various
occasions shock treatment was t r i e d whereby t h e k i l n was rapidly
cooled f o r a couple of hours and then heated up again a s rapidly a s
possible i n a n attempt t o cause t h e accretions t o crack and drop off.
Some success was achieved with comparatively short r i n g s , but where
the accretions extended over a few metres o r more, the treatment was
q u i t e i n e f f e c t i v e . I n these cases the k i l n had t o be cooled down
completely t o enable t h e accretions t o be removed manually. The
worst accretions occurred during the f o u r t h campaign i n May, 1969,
when one p a r t i c u l a r a c c r e t i o n was found t o be over 8 metres long
ranging i n thickness from 1 t o 1&
metres. The t o t a l weight of ac-
c r e t i o n s removed from t h e k i l n on t h i s occasion exceeded 450 tons.
A t y p i c a l view of a c c r e t i o n s prized o f f , alongside an i n t e r n a l a i r
tube, i s shown i n Fig. 5
25. WAELZ
TREATING
OF COMPLEX
ZINC-LEAD
ORES
Fig. 5
The thicker the accretions the longer the k i l n takes t o cool off
s u f f i c i e n t l y t o s t a r t work inside. This period ranges from about
76 hours with a comparatively clean k i l n t o 3 days o r more. Even
a f t e r 3 o r 4 days of cooling, the thicker accretions a r e s t i l l red
hot inside. 'Jack hammers' and 'tommy bars' have t o be used f o r
prizing off the accretions which a r e then trundled along t o the k i l n
discharge end i n a wheel-barrow, f o r disposal.
During t h i s period, characterized by the formation of accretions,
the amount of reductant used ranged from about 15% t o about 25% by
weight of the zinc bearing feed. For most of the time the discharge
chute was open t o atmosphere which rendered i t p r a c t i c a l l y impossible
t o prevent an excessive volume of a i r from being drawn i n t o the k i l n
and cause the formation of accretions near the discharge end. Con-
s t a n t trouble was a l s o experienced with ma1 operation of the cooling
tower, r e s u l t i n g i n a flood of water pouring i n t o the cooling tower
oxide, turning i t i n t o a paste which could not be handled by the
screw conveyor o r the s t a r feeder.
Both pneumatic t r a n s f e r systems a l s o continually blocked and ren-
dered steady operation impossible. Dust and p r e c i p i t a t o r oxide were
continually being e i t h e r dumped on the ground o r blown i n t o the atmos-
phere making i t impossible t o obtain any semblance of a metallur-
g i c a l balance.
A t t h i s stage discharge chute was then e f f e c t i v e l y sealed, the
percentage of reductant added was increased and the temperature of
operation was d e l i b e r a t e l y kept low i n a n endeavour t o prevent the
formation of accretions. This resulted i n higher zinc values i n the
residue but did prolong the length of the campaigns. The cooling
26. tower and both pneumatic t r a n s f e r systems continued t o give trouble.
During campaign No.8 which s t a r t e d during J u l y , 1969, the q u a n t i t y
of reductant added a s a percentage of the new zinc bearing feed was
increased s u b s t a n t i a l l y t o over 5%. This enabled higher operating
temperatures t o be used without a r a p i d buildup of l a r g e a c c r e t i o n s .
From t h i s period onwards k i l n stoppages were brought about almost
e n t i r e l y by mechanical breakdowns of one type o r another, and not
p r i m a r i l y by the n e c e s s i t y t o remove a c c r e t i o n s . It was found t h a t
although a c c r e t i o n s s t i l l tended t o form, they were v e r y f r i a b l e and
b r i t t l e and g e n e r a l l y f e l l down of t h e i r own accord when they grew
too b i g because t h e burden contained s o much unburnt coke. This
raisedanother problem, however, which p e r i o d i c a l l y caused a n increase
i n the z i n c content of t h e residue and a l s o reduced the percentage
z i n c i n the Waelz oxide produced. I f a r i n g formed anywhere, the
burden dammed up behind i t u n t i l the r i n g collapsed when the accumu-
l a t i o n rushed through the k i l n and was discharged before g e t t i n g hot
enough f o r t h e zinc t o be v o l a t i l i z e d . Furthermore, a s a r e s u l t of
the a d d i t i o n a l a g i t a t i o n , a g r e a t dust cloud was formed, much of which
was swept out with the exhaust gases and polluted the Waelz oxide.
A t t h i s s t a g e p e l l e t i z i n g of Waelz oxide was temporarily suspended
a t t h e request of Zinc Corporation, who were having considerable d i f -
f i c u l t y i n handling and d i s s o l v i n g the p e l l e t s supplied. A t l e a s t
5% of t h e p e l l e t s had broken down t o powder by t h e time t h e profluct
had been off-loaded a t t h e i r s i l o s . Furthermore, t h e r e were a cer-
t a i n number of oversize p e l l e t s up t o 50 mm i n diameter and hard
lumps of agglomerated m a t e r i a l . The moisture content was about 7%.
A t K i l n Products, a s a temporary procedure, the Waelz oxide from the
storage s i l o s was discharged onto the f i n a l p e l l e t conveyor b e l t by-
passing t h e p e l l e t i z i n g d i s c s , dropped onto t h e ground and loaded
i n t o a 24 ton t i p p i n g t r u c k by means of a f r o n t end l o a d e r , f o r trans-
p o r t t o the railway s i d i n g a t Grootfontein. Here t h e product was
tipped onto t h e loading platform and loaded i n t o open railway t r u c k s ,
l i n e d with 0.15 mm polythene sheeting, with t h e a i d of a f r o n t end
loader. The t r u c k s were covered with waterproofed t a r p a u l i n s p r i o r
t o despatch. The powder w a s found t o have a bulk d e n s i t y of about
1.25 g/cm3, which was considerably higher than had been a n t i c i p a t e d
from t h e o r i g i n a l p i l o t p l a n t work, when t h e bulk d e n s i t y was only
0.83 g/cm3. Despite t h e primitive handling f a c i l i t i e s f o r t h i s pro-
d u c t , d u s t i n g and dust l o s s e s were not n e a r l y a s s e r i o u s a s had been
expected.
Due t o continual blockages i n both t h e pneumatic t r a n s f e r systems
and the r e s u l t a n t i n t e r f e r e n c e with production, Lurgi undertook t o
redesign both systems. A s t h i s could not be implemented f o r s e v e r a l
months, various temporary arrangements had t o be made, i n order t o
keep producing. A l l preoxide, cooling tower oxide and p r e c i p i t a t o r
oxide were discharged onto the ground and handled by means of f r o n t
end l o a d e r s , the f i r s t two products being returned t o t h e k i l n feed
with the reclaimed slime while the p r e c i p i t a t o r oxide was despatched
27. without being p e l l e t i z e d . The preoxide from t h e s e t t l i n g chambers
was discharged a t a temperature i n excess of 500°C and presented the
biggest problem. It was found necessary t o deposit t h i s product on
an intermediate stockpile t o allow it t o cool, and then spray water
onto i t t o wet i t before returning i t t o the feed b e l t . Otherwise
when the hot dust was dropped onto the reclaimed slime containing
about 1% of moisture so much steam was generated t h a t a large dust
cloud resulted.
A t the beginning of November the pneumatic t r a n s f e r system f o r the
p r e c i p i t a t o r oxide was overhauled and recommissioned temporarily, and
the product was again pelletized. Two important a l t e r a t i o n s were
made t o the pneumatic system which eliminated most of the e a r l i e r
trouble, viz:-
1. Oxide was delivered i n t o the system a t one point only, i n the
f i r s t instance, by operating only one p r e c i p i t a t o r a t a time and
l a t e r when operating both p r e c i p i t a t o r s i n p a r a l l e l , by f i t t i n g
a time switch which allowed oxide from only one p r e c i p i t a t o r t o
be discharged i n t o the system a t a time, by stopping a l t e r n a t e
Redler conveyors a t the bottom of the p r e c i p i t a t o r s f o r 15 min-
u t e s p e l l s .
2. A pressure switch was i n s t a l l e d i n the a i r duct adjacent t o the
blower which tripped out the Redlers whenever the pressure rose
too high, t o allow time f o r the system t o c l e a r i t s e l f . A s soon
a s t h i s happened the pressure dropped and the Redler automatic-
a l l y s t a r t e d up again.
With a b i t more experience and perseverance i t was found possible t o
produce p e l l e t s of r e l a t i v e l y uniform s i z e and by c a r e f u l supervi-
s i o n of the handling, t o prevent excessive subsequent breakage of
the p e l l e t s .
A t about the same time temporary arrangements were made t o spray
s u f f i c i e n t water onto the dust emerging from each of the 3 s e t t l i n g
chambers and from the cooling tower, t o t u r n the product i n t o a
watery s l u r r y which was allowed t o g r a v i t a t e i n t o a submerged sump
and pumped i n t o s e t t l i n g paddocks nearby. After s e t t l i n g and drying
f o r a few days, the preoxide was reclaimed by means of f r o n t end
loaders and returned t o t h e c i r c u i t with the reclaimed slime. This
system, although i t necessitated double handling and rendered it
more d i f f i c u l t t o c a l c u l a t e the percentage recovery, worked reason-
ably s a t i s f a c t o r i l y u n t i l February when i t s t a r t e d r a i n i n g heavily.
The whole settlement a r e a became a bog and i t was impossible t o use
the f r o n t end loader. It then became necessary t o r e v e r t t o the
e a r l i e r system of dropping the hot dust onto the f l o o r . For a few
days, however, a temporary pneumatic t r a n s f e r system was introduced
t o recycle the cooling tower oxide only, by discharging t h e dust
plus c a r r i e r a i r d i r e c t l y down the feed chute i n t o the k i l n with the
new feed. Mechanically t h i s worked s a t i s f a c t o r i l y but it was noted
28. during t h i s time t h a t the zinc content of the Waelz oxide produced
was c o n s i s t e n t l y l e s s than 6@, possibly due t o excessive recircula-
t i o n of dust.
During campaign No.13 i t became progressively more d i f f i c u l t t o
maintain a negative pressure a t the discharge end of t h e k i l n , u n t i l
eventually i t was necessary t o s t o p production and close down. Typi-
c a l pressures a t various p o i n t s i n the c i r c u i t measured by means of
water manometers immediately p r i o r t o t h e end of campaign No.13 a r e
shown i n Table lX, with t y p i c a l pressures subsequently obtained during
campaign No. 14.
Table 1X. Water Gauge Pressures - mm w g
It was found t h a t the duct d e l i v e r i n g t h e g a s from the s e t t l i n g
chambers t o t h e cooling tower was completely blocked with d u s t which
had s e t t l e d on the b a f f l e s within the uppermost bend.
Towards ecd
Campign No. 13
Campaign No. 14
A few days a f t e r t h e s t a r t of campaign No.14 i t became necessary
t o s t o p t h e feed and allow the k i l n t o cool down s u f f i c i e n t l y f o r a
few days t o enable a l a r g e accumulation of oxide t o be removed from
the cooling tower. This had deposited i n the tower presumably over
a long period of time, i n t h e form of a stalagmite, t h e base of
which blanked off more than h a l f t h e base of t h e cooling tower. The
deposit was about 10 metres i n height and had consolidated t o such
a n e x t e n t t h a t i t took 4 days t o remove, using shovels and pickaxes.
The t o t a l weight removed was nearly 200 tons.
Much of t h e trouble experienced i n c o n t r o l l i n g the temperature i n
the cooling tower i s caused by the hardness of t h e water used. A
t y p i c a l water a n a l y s i s i s shown i n Table X.
Wain fan
, s u c t i o n
- 110
- 35
I n l e t t o
Precipita-
tors
- 90
- 20
Cooling
Tower
- 90
- 15
S e t t l i n g
Chambers
- 10
- 1 0
K i l n
Discharge
- 4
- 4
29. WAELZ
TREATING
OF COMPLEX
ZINC-LEAD
ORES
Table X. Analysis of Process Water a t Kiln Products
Scale accumulates i n the e n t i r e system which i n the f i r s t instance
reduces the volume of water a v a i l a b l e a t the nozzles. Correct func-
tioning of the nozzles i s hampered not only by the buildup of scale
i n the nozzles themselves, but a l s o by loose b i t s of scale lodging
i n t h e i n l e t ports. When t h i s happens,large drops of water a r e dis-
charged which do not vaporize f a s t enough t o prevent the accumula-
t i o n of water a t the base of the tower. This wets the oxide and
t u r n s it i n t o a paste which cannot be transported by the equipment
provided. It then becomes necessary t o open the f l a p s below the
screw conveyor and drop the product onto the floor. While operating
with the f l a p s open much of the cooling tower oxide i s a s p i r a t e d
i n t o the p r e c i p i t a t o r s thus lowering the zinc content of the Waelz
oxide.
Total dissolved s o l i d s
Total a l k a l i n i t y , a s CaC03
Total hardness, a s CaCO3
Calcium, a s Ca
Magnesium, a s M
g
Fluorine, a s F
Chloride, a s C 1
Sulphate, a s SO4
PH
Steps a r e now being taken t o soften the water by the controlled
a d d i t i o n of lime, followed by f i l t r a t i o n .
459 milligrams p e r litre
439
406
7.8
92.7
0.3
0.1
0.1
7.1
Kiln Gun
A s i t i s common practice i n the cement industry t o use k i l n guns
f i r i n g lead s l u g s t o break down rings, a WesternRingblaster k i l n gun
was purchased. This gun has a bore of approximately 23 mm and f i r e s
85 gram s o l i d lead slugs a t a muzzle v e l o c i t y of 500 metres/second.
It i s mounted with r e c o i l springs on a s u b s t a n t i a l g u n support. It
has been found t h a t comparatively narrow rings up t o about l m i n
width can be e f f e c t i v e l y collapsed by c u t t i n g a longitudinal s l o t
r i g h t through the ring. This usually requires a t l e a s t 500 rounds
of ammunition. The gun i s q u i t e i n e f f e c t i v e with rings which a r e
e i t h e r much wider than t h i s o r more than about 20 metres from the
discharge end of the k i l n .
30. Comments on D i f f i c u l t i e s Encountered
Accretions
Samples of a c c r e t i o n s removed were submitted t o the laboratory,
with samples of the willemite concentrate and reclaimed slime being
charged t o t h e k i l n , f o r determination of the temperature a t which
these products s t a r t e d melting. The softening points a r e shown i n
Table X 1 below:-
Table X1. Softening temperature of various products
Portion of one t y p i c a l accretion approximately 14 c
m t h i c k was
examined mineralogically. It was found t o have a layered s t r u c t u r e
marked by texture and a l s o compositional differences. The s p e c i f i c
g r a v i t y ranged from 3.26 f o r t h e l a y e r adjacent t o the k i l n l i n i n g ,
Product
Villeaite concentrate
Reclaimed slime
Accretion eru~pleA
Accretion sample B
Accretion sample C
Kiln residue
t o 7.68 f o r t h e surface layer. The corresponding zinc contents were
8.25% and 28.@ respectively. The chief minerals i d e n t i f i e d were
diopside, m e l i l i t e , p e r i c l a s e and calcium oxide, with unconverted
willemite and globules of m e t a l l i c zinc i n the layers nearer the
surface. Periclase and calcium oxide a r e obviously t h e formation
products of dolomite a t elevated temperatures. Diopside and m e l i l i t e
a r e complex s i l i c a t e s of calcium, magnesium and aluminium. Diopside
probably begins t o form a t a temperature between 6500C and 800°C
and i s s t a b l e up t o about 1 3 5 0 ~ ~ .
M e l i l i t e i s a high temperature
mineral and begins t o form a t around 800°C t o 9 0 0 ~ ~
and i s s t a b l e
up t o a temperature of about 1 4 0 0 ~ ~ .
SofteningoT~peratul*t
C
1150
1150
1120
1200
1300
1150
Pneumatic t r a n s f e r systems
The pneumatic t r a n s f e r systems were designed t o work under suc-
t i o n a t a maximum vacuum of about 0.5 atmospheres. I n each case a
Rootes blower evacuated the warm a i r from a bag f i l t e r and dis-
charged t o atmosphere. A i r was allowed t o be drawn i n t o the con-
veying pipeline immediately behind the furthermost dust feeding
points. I n the case of the preoxide, dust was fed v i a s t a r feeders
31. i n t o the s e t t l i n g chambers. The dust c o l l e c t e d i n the cyclone was
fed i n t o a screw conveyor and moistened with water before a l s o being
dropped down the feed chute. Two such systems were i n s t a l l e d , one a s a
standby. The preoxide bag f i l t e r which thus became redundant was re-
i n s t a l l e d a s a standby u n i t i n the Waele oxide c i r c u i t .
Thus modified, the preoxide pneumatic t r a n s f e r system has worked
sploothly. However, the screw conveyor under the cooling tower proved
t o be under capacity and tripped out several times on overload before
the s h a f t eventually broke, a f t e r l e s s than two weeks i n operation.
The modified p r e c i p i t a t o r oxide system was still not capable of
operating a t the required capacity of 8 tons per hour without f r e -
quent choking. Consequently t h e following f u r t h e r modifications were
c a r r i e d out during A p r i l , 1970:-
( a ) The cross Redler, newly i n s t a l l e d between t h e two p r e c i p i t a t o r s ,
was removed and i n s t a l l e d below t h e cooling tower i n place of
t h e screw conveyor.
( b ) Each 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 was provided with a completely
separate pneumatic t r a n s f e r system.
A i r i n l e t tubes
The o r i g i n a l a i r i n l e t tubes were constructed of a 25% chromium,
1%nickel a l l o y i n 3 p a r t s , vie:-
( a ) the end portion approximately 60 cm i n length with the dis-
charge nozzle
( b ) t h e tubular middle s e c t i o n approximately 130 cm long, which was
spun c a s t and
(-c) t h e flanged end s e c t i o n approximately 40 cm long.
These three portions were welded together.
The a i r i n l e t tubes s t a r t e d f a i l i n g from about August, 1969, when
No.1 a i r tube was found t o have a circumferential crack j u s t over
half a metre from t h e end p a r a l l e l t o the o r i g i n a l weld. a s can be
seen i n Fig. 6
32. Fig. 6
T h i s was welded i n p o s i t i o n , but by the end of campaign No.8 about
a month l a t e r , t h e c r a c k had opened up a g a i n and t h e metal was so
s e v e r e l y c r y s t a l l i z e d t h a t i t became necessary t o remove the tube,
a f t e r 146 days i n a c t u a l operation. This tube was not replaced a t
t h i s stage.
No.? a i r tube a c t u a l l y broke o f f somewhere near t h e weld a t the
base during No.9 campaign and was a l s o not replaced - i t had been
i n operation f o r 161 days. By t h i s stage both Nos. 2 and 4 a i r
tubes had developed c i r c u m f e r e n t i a l cracks s i m i l a r t o No.1 and had
t o be welded. The next stoppage w a s brought about by the breakage
of No.4 a i r tube. It and No.2 and 5 a i r tubes were removed a f t e r
196 days i n a c t u a l operation. No.2 had cracked c i r c u m f e r e n t i a l l y
but No.5 had a c t u a l l y abraded away t o such a n e x t e n t t h a t a hole
had appeared near t h e extremity.
Two new tubes of somewhat d i f f e r e n t design, c a s t i n one piece
by Scaw Metals from a n a l l o y containing 25% chromium and 20% n i c k e l
were f i t t e d i n the Nos. 2 and 5 p o s i t i o n s while p o s i t i o n s Nos. 1 ,
3 and 4 were blanked o f f . No.2 Scaw tube had t o be removed a f t e r
only 25 days of operation due t o severe cracking both longitudin-
a l l y and c i r c u m f e r e n t i a l l y . The No.2 p o s i t i o n was then a l s o blanked
o f f .
A f t e r a f u r t h e r 27 days of operation it became necessary t o s t o p
t h e k i l n due t o b r i c k damage a t t h e feed end and a l s o a severe block-
age i n the duct t o t h e cooling tower. No.5 Scaw tube appeared t o
be i n good condition but No.6 o r i g i n a l tube a f t e r 248 days i n use
had developed a small c i r c u m f e r e n t i a l crack and a bulge. It was,
however, decided t o operate one more campaign before replacing i t .
33. Nos. 7 and 8 tubes were s t i l l apparently i n good condition. It was
decided t o put back one tube nearer the discharge end t o f a c i l i t a t e
b e t t e r d i s t r i b u t i o n of a i r . A s a n experiment a new Lurgi tube was
i n s t a l l e d i n the No.1 position with the a i r i n l e t port rotated
through 180° so t h a t the discharge was directed towards the feed
end. A s no rings developed anywhere near No.1 f a n , i t was decided
a t the end of campaign No.14 t o operate with Nos. 1, 2 and 4 fans
discharging towards the k i l n feed end and Nos. 7 and 8 fans dis-
charging i n the opposite direction. By t h i s stage No.6 a i r tube had
f a i l e d completely, a f t e r 280 days i n operation. Scaw tube No.5 a f t e r
84 days i n use a l s o had t o be removed due t o metal f a i l u r e .
It i s of i n t e r e s t t o note that Komley and ~ a r e e v ' lrecommend, a s
r e s u l t of an investigation a t the Chelyabinsk Waelz Kiln, 40 m i n
length t r e a t i n g leach plant residues, t h a t i t i s highly desirable
t h a t a i r should be introduced i n t o the k i l n stagewise i n proportion
t o the oxygen demand i n the p a r t i c u l a r zone. I n the case of the
above k i l n , they consider t h a t 20% t o 25% of the t o t a l a i r should
be supplied a t a point 21 metres from the feed end and t h a t addi-
t i o n a l a i r i s 3 l s o required a t 27 t o 30 metres from the feed end.
Kubyshev e t a 1 report t h a t i n the treatment of oxidized zinc ores
i n a k i l n a t Achisaisk, improved r e s u l t s were obtained by introduc-
ing 1 800 t o 2 500 m3/h of a i r a t a pressure of 7 t o 8 atmospheres,
a t the discharge end of the k i l n , the high pressure being required t o
ensure t h a t the a i r travelled well down the k i l n before the oxygen
could be depleted.
Brick Lining
Right from the s t a r t trouble was experienced with the brick-
l i n i n g i n the feed cone t o the kiln. I n i t i a l l y these bricks spalled
r a t h e r severely and then s t a r t e d dropping out. Patching was q u i t e
i n e f f e c t i v e and eventually a t the end of November, 1969, the conical
portion was c a s t i n position using refractory material REFCAST S.P.,
with t y p i c a l properties a s shown i n Table X 1 1 . The f i r s t 3 rows of
bricks adjacent t o the cone were a l s o replaced using Superlin bricks.
A f t e r 52 days the c a s t material s t i l l appeared t o be i n good condition
but the bricks had a l l f a l l e n out again and had t o be replaced.
34. T a b l e X 1 1 . R e f c a s t S.P. - T y p i c a l P r o p e r t i e s
The f i r s t s i g n s of b r i c k f a i l u r e i n t h e main body of t h e k i l n were
n o t i c e d a f t e r 196 days o f o p e r a t i o n . The zone e x t e n d i n g from 42.5
m e t r e s t o 49.6 m e t r e s from t h e f e e d end was then replaced u s i n g
L u s i t e b r i c k s as b e f o r e b u t i n c l u d i n g 2 rows of S u p e r l i n e s a t e s t .
The L u s i t e b r i c k s t h a t had t o be replaced had a l l f a i l e d i n t h e
middle. The t o p 5 of 1 0 crns of e a c h b r i c k were s t i l l extremely
h a r d , but had a c q u i r e d 3 p u r p l i s h c o l o u r and t h e zone 5 t o 1 0 crns
deep, i n c o n t a c t w i t h t h e s t e e l s h e l l had turned n greyish-black
c o l o u r b u t was a l s o s t i l l hard. The middle p o r t i o n was completely
f r i a b l e and impregnated w i t h s l a g , g l o j u l e s o f m e t a l end carbon.
T h i s i s i l l u s t r a t e d i n F i g . 7 below - the s u r f a c e of t h e b r i c k s i n
c o n t a c t w i t h t h e burden can be s e e n i n t h e t o p l e f t hand c o r n e r and
t h e s t e e l s h e l l of t h e k i l n i n t h e bottom r i g h t hand c o r n e r .
i
S i 0 2 33.8 C
a
O 14.9
A1203 42.4 ago 0.1
Fe 0
2 3 6.9 Loss on ignition 0.5
Ti02 1.5
Brick density
A i r d r i e d 2.24 g/cm3
Fired to 1250'~ 2.08 g/cm3
-
A i r dried 425 k&lcm2
Fired to 1 2 5 0 ~ ~ 140 d c m 2
Linear c h a w aft=
- 0.5 to 1
%
.
35. Fig. 7
A f t e r a f u r t h e r 25 days another 1.5 metres were renewed from
49.6 metres t o 51.1 metres from t h e feed end. 27 days l a t e r 1.6
metres were renewed from 42.5 metres t o 40.9 metres from the
f e e d end.
A t the end of campaign No.14, 1.4 metres of t h e l i n i n g , v i z . from
42.5 t o 43.9 metres from f e e d end, which had previously been renewed
a f t e r 196 days i n operation, had t o be replaced f o r t h e second time
a f t e r 84 days i n operation. This time S u p e r l i n b r i c k s were used.
The two t e s t rows of S u p e r l i n b r i c k s previously f i t t e d i n t h i s
same zone s u p e r f i c i a l l y s t i l l appeared t o be i n good condition.
This w a s confirmed by test d r i l l i n g , a s shown i n Table X.
Using a dianond d r i l l , approximately 70 mm i n diameter, 6 t e s t
h o l e s were d r i l l e d i n b r i c k s a t s e l e c t e d p o i n t s i n the k i l n and the
bore c o r e s examined. The r e s u l t s a r e tabulated i n Table X l l l
36. 952 EXTRACTIVE
METALLURGY
OF LEADAND ZINC
Table X 1 1 1 . Bore Cores from Brick Lining
Prom a c r i t i c a l examination of the type of b r i c k f a i l u r e , i t
Conditions of cony
i n colour tetndiry t~rrrds
black
- suspect oarbon i.pir(oation.
core broken, but no sign of
carbon i.pm&ntation.
appears t h a t t h e damage has been caused by the p e n e t r a t i o n of gases
followed by some chemical and o r physical changes w i t h i n the body
of t,he b r i c k . Normally the s u r f a c e of t h e b r i c k i n c o n t a c t w i t h
the burden w i l l have a temperature i n t h e region of l000oC t o 1 2 0 0 ' ~
while the end i n c o n t a c t w i t h the s h e l l i s a t a temperature of 200
t o 300'~.
5
6
The carbon tieposition r e a c t i o n i s w e l l known and may occur i n the
temperature range 4 0 0 ' ~ t o 7 0 0 ' ~ a s follows:-
This r e a c t i o n appears t o be c a t a l y s e d by the presence of i r o n
oxides and could r e s u l t i n t h e d e p o s i t i o n of carbon a l o n g t h e j o i n t s
and i n the pores of the b r i c k s , s e t t i n g up s t r e s s e s which cause crack-
i n g of t h e l i n i n g .
32.5
22
Zinc vapour could a l s o p e n e t r a t e the b r i c k s t o a p o i n t where the
temperature i s s u f f i c i e n t l y low t o cause condensation of t h e vapour.
The presence of globules of m e t a l l i c zinc i n t h e c e n t r e of the
b r i c k s i s c l e a r proof t h a t t h i s has occurred.
A f u r t h e r r e a c t i o n involving zinc vapour i s p o s s i b l e . The C02
produced by the carbon deposition r e a c t i o n , while d i f f u s i n g back
i n t o t,he k i l n could r e a c t with t h e zinc vapour p e n e t r a t i n g t h e b r i c k s
and. cause re-oxidr:ti.on by the r e a c t i o n .
Lusite
Superlin
280
280
Dark grey i n colour, but no sign
of carbon impmgantion.
Light grey with mauve ti- - no
sign of carbon i.pmgmtion
37. Zn (vapour) + C02 --+ZnO + CO
The z i n c oxide produced by t h i s r e a c t i o n would a l s o d e p o s i t w i t h i n
t h e b r i c k and behave i n t h e same f a s h i o n a s the carbon produced by
the carbon d e p o s i t i o n r e a c t i o n .
F u r t h e r l a b o r a t o r y work i s a t present being conducted t o confirm
the above assumptions. However, i t i s s i g n i f i c a n t t o note t h a t the
S u p e r l i n b r i c k s i n t h e 42 m (from f e e d end) zone have stood up much
b e t t e r than the a d j a c e n t Lusite b r i c k s which had t o be replaced a f t e r
84 days. I n f u t u r e Jumbo b r i c k s w i l l be used a s these have s t i l l
lower p o r o s i t y and permeability c h a r a c t e r i s t i c s and a lower i r o n con-
t e n t , a s can be seen from Table V 1 .
Thermocouples i n Kiln
The only thermocouple i n the k i l n t h a t h a s continued t o provide
r e l i a b l e readings i s No.11 which i s approximately 8 metres from t h e
feed end, where a temperature of 400°c t o 500°C i s recorded. It i s
suspected t h a t t h i s reading i s probably h i g h e r than t h e t r u e tempera-
t u r e of the burden and lower than t h e g a s temperatures a t t h i s p o i n t .
It does, however, provide a u s e f u l guide f o r o p e r a t i o n a l purposes.
The o t h e r thermocouples very r a r e l y record a r e l i a b l e temperature
f o r more than a few days; t h e r e a f t e r they e i t h e r become embedded
i n a n a c c r e t i o n and t h e n record a n erroneously low temperature o r
they become p h y s i c a l l y damaged by l a r g e lumps of f a l l e n a c c r e t i o n
r o l l i n g around. A p o r t a b l e thermocouple i s used f o r determining
t h e temperature of the r e s i d u e discharge from the k i l n . At hourly
i n t e r v a l s i t i s i n s e r t e d , f o r a few minutes, through a guide tube
s u i t a b l y positioned t o allow t h e residue t o cascade over t h e end of
t h e thermocouple. This has been found t o be more r e l i a b l e than using
a n o p t i c a l pyrometer, due t o the i n t e r f e r e n c e o f t e n caused by exces-
s i v e l y dusty c o n d i t i o n s within t h e k i l n .
The s h e a t h s of the thermocouple a r e f a b r i c a t e d from a 26% Cr, 5%
N i a l l o y and t h e r e has been no evidence of the s h e a t h s e i t h e r being
abraded o r eroded away nor of any a c t u a l metal f a i l u r e .
Supporting r o l l e r s
I n mid January, 1970, one of t h e r o l l e r s supporting the t h i r d
r i d i n g r i n g stopped r o t a t i n g with t h e k i l n . It was found t h a t the
s h a f t had sheared. The k i l n was operated without t h i s r o l l e r f o r a
week, without any apparent d e l e t e r i o u s e f f e c t s , while a new s h a f t
was being f i t t e d . During A p r i l , about 3 months l a t e r , t h e same
r o l l e r a g a i n stopped r o t a t i n g . It was found t h a t t h e r o l l e r had
moved on the s h a f t , so the whole assembly was renewed. Investiga-
t i o n s a r e proceeding t o e s t a b l i s h the cause of the f a i l u r e .
38. a t 4 different points, into one common conveying pipeline which dis-
charged into a cyclone, the overflow from the cyclone being f i l t e r e d
i n a bag house. In the case of the Waelz oxide, oxide was delivered
into the conveying pipeline a t 2 different points v i a s t a r feeders,
and was collected i n e i t h e r of two dust cyclones each situated imme-
diately above a storage silo. The cyclone exhaust gas was f i l t e r e d
i n a bag f i l t e r . The systems suffered from the following defects:-
( a ) the conveying pipelines choked frequently
(b) thebends and the cyclone were very rapidly abraded away parti-
cularly i n the case of the preoxide
( c ) the cyclone dust outlet choked frequently
( d ) the d u s t outlet from the f i l t e r choked frequently
( e ) the f i l t e r bags themselves charred brown and appeared t o shrink
and pull off the f i t t i n g s .
( f ) the f i l t e r bags actually burst, often a t the seams
( g ) a s no s t a r feeders were f i t t e d between the cyclones and the
respective storage s i l o s , these s i l o s became part of the vacuum
system and caused a great deal of i n e r t i a i n the c i r c u i t .
Hence i f any incipient blockage occurred a t the feed point into
the system, there was a considerable time lag before the vacuum
i n the system increased sufficiently t o c l e a r the blockage.
Before t h i s happened, the c i r c u i t had invariably blocked up com-
pletely. This was particularly bad i n the case of the Waelz
oxide c i r c u i t due t o having the two 100 ton s i l o s a s part of
the vacuum system.
A s a f i r s t modification the suppliers of the equipment decided t o
a l t e r both systems t o work under pressure instead of under vacuum,
a s t h i s would overcome the time lag problem. However, a s most of the
s t a r feeders had not been designed t o work under pressur5,this scheme
did not work e i t h e r , a s sufficient a i r leaked back through the s t a r
feeders t o interfere with the discharge of the dust i n t o the convey-
ing system. Dust a l s o escaped through the various seals and dis-
charged copiously into the atmosphere.
During March, 1970, a cross Redler was i n s t a l l e d t o pick up the
oxide from the two Redlers under the e l e c t r o s t a t i c precipitators and
deliver i t via a K i l l e r pump i n t o the pneumatic system operating
under pressure. A t the same time one Redler was installed t o collect
the preoxide from the 3 s e t t l i n g chambers and also the cooling tower
oxide and deliver the combined load i n t o the pneumatic system a t one
point. This also worked under pressure and discharged i n t o a dust
collecting cyclone positioned immediately above the feed end of the
kiln. The gas leaving the top of the cyclone was discharged back $
,
.
39. Recovery of coke from the residue
I n order t o maintain a granular, f r e e flowing discharge from the
k i l n i t has been found necessary t o maintain a f r e e carbon content
of about 35% i n the residue. Even i f accretions do form a t i s o l a t e d
' s p o t s due t o localized overheating, accretions with t h i s amount of
surplus coke, a r e usually s u f f i c i e n t l y f r i a b l e t o break away of t h e i r
own accord from time t o time. Under normal conditions over 9
% of
the residue w i l l pass through a 6 mm screen. A s can be seen from
Table V111, the amount of reductant a c t u a l l y consumed i n the k i l n i s
of the order of 15 t o 20% of the weight of new zinc bearing feed;
whereas, the a c t u a l a d d i t i o n is i n the range of 50 t o 6
%
. P i l o t
p l a n t s c a l e jigging t e s t s conducted on the residue showed t h a t i f
t h e residue were reduced t o a l l minus 6 mm i n s i z e it should be
possible t o recover 75% of the carbon i n the residue i n t o a product
containing 70 t o 75% carbon, which could be re-used i n the k i l n i f
t h i s could be a t t a i n e d on a plant s c a l e the l o s s i n the residue
would be equivalent t o about 10% of the weight of the new feed which
would reduce the o v e r a l l consumptions of new coke o r a n t h r a c i t e t o
about 25 t o 30%.
A s a temporary expedient a second-hand 1 metre square twin hutch
Yuba j i g was i n s t a l l e d i n February, 1970, t o recover a s much coke
a s possible. Due t o inadequate screening f a c i l i t i e s , i n s u f f i c i e n t
hutch water and various other obvious drawbacks, t h i s plant has
operated very i n e f f i c i e n t l y but has nevertheless managed t o re-
cover up t o 50% of the carbon i n the residue i n t o a product contain-
i n g about 70% carbon. This has been fed back i n t o the k i l n without
any d e l e t e r i o u s r e s u l t s . I n the l i g h t cf the experience gained, a
f u l l s c a l e plant i s now being designed.
S t a t i s t i c a l Data
Volume of gas discharged
Under normal operating conditions, i t i s estimated t h a t the dis-
-
t r i b u t i o n and source of gases a r e approximately a s follows i n Table
X1V:-
40. Table X1V. Source of Gases Exhausted up
the Stack - Typical Conditions
Sealing a i r fan delivering down feed chute 2 000 k3/h
Producer gas consumed 1 330
Water vapour ex feed 1 620
C02 from decomposition of carbonates i n feed 1 280
Water vapour ex sprays i n cooling tower 6 360
A i r introduced via s h e l l fans 17 170
A i r introduced a s combustion a i r N i l
A i r drawn i n through end of k i l n a s r e s u l t
of draught 16 610
Hiscellaneous a i r leaks nminnlly estimated 2 000
-
Total gas exhausted up stack 48 370
-
The a c t u a l vclme of gases pazsing through the k i l n , from the
above f i g u r e s would be 38 000 Nm~/h. Assurnirg an average gas tem-
perature o f sag 1000oC, the volume of gas passing through the k i l n
would be approxinately 176 000 rnJ/h, equivalent t o a l i n e a r v e l o c i t y
o
!
' approxi.mn:ely 5.3 m per second.
Oxygen content of k i l n gases
N
o f a c i l i t i e s ? r e a v s i l a b l e f o r measuring the oxygen c o r t e n t of
the gas withir, the kilri i t s e l f . The r'igures shown i n Table XV below
r e f e r t o the gas i n the s e t t l i n g chamber and the gas discharged u~
the stack respectively. As can be seen from Table X1V the gas i n
the s e t t l i n g chamber includes about 2 000 NIDS/~of a i r blown down
the feed chute, which d i l u t e s 38 000 Nrn3/h of gases emerging from
the k i l n rind would account f o r approximately 1
% of oxygen i n the
mixture.
T:ible XV. ';.ypical Analysis of Exhaust Gases
Stnck
7.0
15.0
Trace
-
% O2
% co2
$ co
S e t t l i n g Chamber
5.3
17.1
Trace
41. Labour requirements
The p l a n t operates with the following labour complement:-
Manager
P l a n t foreman
~ e c r e
tary/s torekeeper
Part-time ~ ~ ~ i s t / c l e r k
S h i f t Operators ( 2 / s h i f t )
Dayshift Operators
Relief Operator
Foreman Mechanic
Other Mechanics
Manual Labourers
Water, e l e c t r i c power and producer gas consumption
When operating a t the r a t e of 10 000 t o n s of new zinc bearing
f e e d per month the consumption of w a t e r , e l e c t r i c power and producer
g a s a r e a s shown i n Table XV1 below:-
Table XV1. Water, e l e c t r i c power and
producer gas consumption
Preoxide and cooling tower oxide and Waelz oxide
There is no ready means of determining the tonnage of the f i r s t
two products, but spot checks from time t o time have i n d i c a t e d t h a t
the preoxide (from the s e t t l i n g chambers) amounts t o about 4 tons
per hour and the cooling tower oxide about 2 tons per hour. Typi-
c a l metal c o n t e n t s a r e shown i n Table X V l l below:-
por t o n
new feed
1 2
27
180
Water ( t o n s )
E l e c t r i c power (klfh)
Producer gas Xu3
Table XV11. Typical analyses of preoxide and cooling oxide (%)
Der month
12 000
270 000
1 800 000
Preoxide
Cooling tower oxide
Zinc
32.8
4
4
.
0
Lead
6.9
8.3
KR
3.2
2.7
Ca
5.3
4
.
6
42. A t y p i c a l a n a l y s i s of Waelz oxide c u r r e n t l y produced i s shown i n
Table X V l l l below:-
Table XV111. Typical Analysis of Waelz Oxide ($1
Conclusions
One year of o p e r a t i n g experience has shown t h a t a s a t i s f a c t o r y
e l i m i n a t i o n of z i n c can be achieved without excessive t r o u b l e due
t o t h e formation of a c c r e t i o n s , t h a t a s a t i s f a c t o r y grade of Waelz
oxide can be produced, and t h a t t h e l i f e of t h e r e f r a c t o r y b r i c k
l i n i n g w i l l be w i t h i n t h e expected l i m i t s .
The f a c t t h a t n e i t h e r t h e designed r a t e of production nor the
a n t i c i p a t e d percentage recovery have been maintained is due l a r g e l y
t o t h e inadequacy of t h e pneumatic t r a n s f e r systems. Vigorous s t e p s
a r e now being taken t o remedy t h i s p o s i t i o n .
Acknowledgment
Thanks a r e due t o M r . R.R.M. Cousens, Technical D i r e c t o r of Gold
F i e l d s of South A f r i c a Limited, f o r permission t o p u b l i s h t h i s paper.
References
1. Bogacz M. , "Fire-ref i n i n g of Zinc-Lead o r e s i n Poland," f rom
M a t e r i a l s of the 'Technical Conference i n Warsaw on Questions
of Production Techniques f o r Zinc, lead and Associated Metals,'
Moscow 1957, pp. 129 - 149.
2. Johannsen F. , "The Advance i n t h e development of t h e Waelz
Process," Metal1 und E r z , 1927, 24 (17) 425.
3. Hoffman R., "The Waelz Process,'' Transactions of the American
I n s t i t u t e of Mining and M e t a l l u r g i c a l Engineers, Vol. 76, 1928.
43. 4. H a r r i s W .E., "The Waelz Process,'l American I n s t i t u t e of Mining
and Metallurgical Engineers Transactions, Vo1.121, 1936, Rocky
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5. Jensen C .W. , "The Waelz Process ,I1 Mining Magazine, Vol. 92, 1955,
PP. 73 - 79.
6. Bodenstein M . , "The Mechanism of the Metallurgical production of
Zinc," Transactions of the American Electrochemical Society (1927),
51, 449.
7. Komlev G.A. and Gareev V.N., "Chemism of the Waelz method of
processing Zinc Cakes," Tsvetnge met all^ The Soviet Journal of
Non-ferrous Metals, March 1964, pp. 22 - 29.
8. Truesdale E.C. and Waring R.K., "Relative Rates of Reactions
involved i n Reduction of Zinc Ores," AIME Transactions, I n s t i t u t e
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