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Zinc electrowinning from flue dusts at a secondary copper smelter and connected adhesion problems of the metal deposits
HydrometaUurgy, 19 (1987) 11-24 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
11
Zinc E l e c t r o w i n n i n g from Flue D u s t s at a S e c o n d a r y Copper S m e l t e r and C o n n e c t e d A d h e s i o n P r o b l e m s of the Metal D e p o s i t s R O L A N D K A M M E L , M U S T A F A G O K T E P E and H A R T M U T O E L M A N N Institut fiir MetaUurgie - - Metallhi~ttenkunde, Technische Universitiit Berlin, 1000 Berlin 12 (Germany) (Received June 12, 1986; accepted in revised form February 11, 1987)
ABSTRACT Kammel, R., G6ktepe, M. and Oelmann, H., 1987. Zinc electrowinning from flue dusts at a secondary copper smelter and connected adhesion problems of the metal deposits. Hydrometallurgy, 19: 11-24. The secondary copper smelter" Metallhtitte Carl Fahlbusch" (MCF), Rastatt, Federal Republic of Germany is discussed, with particular consideration of the MCF zinc electrowinning process from low-grade, chloride-containing flue dusts. The investigations performed were focussed on the adhesion of zinc deposits on the aluminium cathodes. With laboratory tests using the tankhouse electrolyte and an apparatus for measuring the adhesion force, the influence of the electrolyte impurities lead, cadmium, cobalt, nickel, tin and antimony on the adhesion behaviour and on the current efficiency have been determined. Field tests to affect the adhesion of the zinc deposits on aluminium cathodes by dip pretreatment of the cathodes with glue or sodium resinate reveal that with only small sodium resinate additions in the dipping bath better stripping can be achieved.
SMELTER PROCESS
In the secondary copper smelter "Metallhiitte Carl Fahlbusch" (MCF) [ 1 ] copper and copper alloy scrap, ashes, slimes, slags and other copper-containing materials have been treated as shown in the flow sheet (Fig. 1 ) in two blast furnaces, and the tapped molten black copper in a 10 t converter. The converter copper is then refined in a 40 t rotary furnace and cast into anodes for the electrolytic copper refinery with a capacity of 10,000 t electrolytic copper and 180 t anode slime per year. In a hearth furnace process a lead-tin alloy and zinc oxide are produced from the converter flue dust. The flue dusts from both the blast furnace and the hearth furnace are treated together with zinc oxides from other customers in an electrowinning zinc plant having a capacity of 10 t per day. The zinc-winning process has been developed by MCF and has been in industrial operation for 10 years. The smelter has been recently shut down due to the copper price situation. 0304-386X/87/$03.50
© 1987 Elsevier Science Publishers B.V.
12 Secondary
J Convertering-
materials
~ ~
Iron dusts containing
r-1 ----i Process II | Fluedusts l l" Silicia
copper
Spout slag
~-" ~
and Refining Slags
((1111
F!ue dusts to electrolytic
~__.(_
zlnc plant
I
"--3 z ~ Blackcopper JB1ackcopper i(Liq./sol.) Radiators | J~ _ Brass J ~.J Chips " ~, r'~
Flue dusts to
tin plant
Converter copper ~ i ~ (liq /sol ) ~._~..-~ Old copper ~J Tombac Anoderemnants
I~l
~I Flue dusts to electrolytic zinc plant
l
" Anode remnants rotary furnace Nickel sulfate
[ Anode Slime
Copper cathodes MFC
Nickel sulfate
plant
N~CJ4 • ~.5 Hz@
Fig. 1. Flowsheet of the secondary copper smelter in Rastatt. (1) wind box; (2) forehearth; (3) granulation channel; (4) waste gas; (5) secondary combustion chamber; (6) waste gas boiler; (7) waste gas cooling system; (8) bag house; (9) fan; (10) stack; (11) settling chamber; (12) burner; (13 ) casting machine; (14 ) cooling basin. ZINC ELECTROWINNING PROCESS
From the flow sheet of the M C F zinc winning process in Fig. 2 it can be seen t h a t the chlorine content of the flue dust (which can vary considerably, as
13
shown in Table 1 ) is removed in preliminary washing steps. The resulting rinse water and effluents are treated by neutralization, precipitation and filtration steps to meet effluent guidelines. The zinc content of the waste water is reduced by this treatment to less than 3.0 mg/l. Since the slimes of the waste-water treatment plant are recirculated to the blast furnace, dumping problems do not exist in this washing cycle. After having removed the chlorine content of the zinc oxide suspension to residual values of less than 0.02%, the zinc oxides are leached with spent elecl Flue Dusts from Shaft Furnace Rotary Furnace, Hearth Furnace Suppliers
Cu-Fe | c I Precipitatl°nJ!
'f'OonO ~, Slime to Shaft Furnace
Clean Effluent
~ Elecfrolyfe~
CuR~idue Shoff .L to
~,
'v. Pb Concentrate Furnace Cd ResidueIEIecfrowmm~ to Shaft for Sole Furnace E-Zfnc
Fig. 2. Flow sheet of the MCF process for electrowinning of zinc from flue dusts. TABLE 1
Composition of the flue dusts ( % ) treated in the electrowinning zinc plant Zn C1 Fe Bi
40-65 1-12 0.1-1.5 0.01-0.05
Pb Cd Ni Co
6-20 0.5-0.8 0.1-1 0.001-0.005
Cu Sn Sb Ag
1-6 1-2 0.01-0.05 20-40 g/t
14
trolyte in an agitation tank. Depending on the composition of the flue dust charge, the resulting leach residues can contain up to about 25% lead, 10% zinc, 6% copper, 2% tin and 100 g Ag/t. Copper and zinc are extracted from these residues by an acid-leaching step for subsequent recovery. Copper is fed as a precipitate to the blast furnace and zinc as zinc sulphate solution to the cell house; in this way, a closed loop is achieved. The lead concentrate thus obtained contains the total input of lead, tin and silver. This intermediate product is converted, after drying in a blast furnace operation, to a silver-bearing lead bullion which is either solidified and sold or desilvered and detinned whereby the silver-zinc-lead skims formed by the Parkes process can be further treated in the copper converter. The tin-bearing flue dusts are transferred to the hearth furnace. The zinc sulphate leach solution from the agitated leaching tank is subjected to a two-stage refining process, yielding a copper-bearing precipitate, which is returned to the blast furnace, and a cadmium precipitate containing approximately 20% Cd and 80% Zn, which is sold. In Table 2 the composition of th~ MCF process electrolyte is shown in comparison with two electrolytes of other zinc producers, which were also tested in the course of these investigations. This compilation of data reveals that the impurity and, especially, the iron levels differ considerably. The usual electrolyte type produced from roasted zinc concentrates, with a combined jarosite process for further zinc extraction, e.g. the electrolyte from Preussag AG, Nordenham, may have iron concentrations below 10 mg/1. The Cinkur AG, Turkey electrolyte,which is extracted from rotary kiln furnace calcine (with low iron content) has only a relatively low iron concentration. Due to the tin contents in the MCF flue dusts (Table 1) relatively high Fe 3+ concentrations in the leach liquor have to be adjusted, e.g. by Fe2 (S04) 3" H20 addition, for optimum coprecipitation of tin during the iron hydroxide precipitation step. Therefore, the iron content of the MCF electrolyte is several times higher than that of usual SHG (special high grade) electrolytes. Higher chlorine contents in the MCF and Cinkur electrolytes lead to increased corrosion of the lead anodes and consequently to increased lead concentrations in the electrolyte. TABLE 2 Composition of zinc electrolytes of plants with flue dust and different calcines as raw materials
Metallhtitte C. Fahlbusch Cinkur Turkey Preussag
Zn H2SO4 (g/l) (g/l)
Cu Cd Ni Fe Sb Mn Pb Co As (mg/1) (rag/l) (mg/l) (mg/l) (mg/1) (rag/l) (mg/1) (mg/1) (mg/1)
46.2
189.6
0.1
4.2
2.37
40.9
n.d.
55
164
0.4
0.5
0.1
3.5
n.d.
56
176
0.05
0.06
0.004
8.4
0.003
n.d.: not determined.
255
7.32
0.66
traces
n.d.
n.d.
n.d.
n.d.
6900
0.15
0.004
traces
15 From the purified MCF acidic zinc sulphate solution zinc is then recovered in a 40-cell electrowinning plant having a capacity of about 10 t / d electrolytic zinc. Each cell is equipped with 20 aluminium cathodes and 21 lead anodes. With the usual applied current density of 500 A / m 2 the current efficiency amounts to 84% and the cell voltage to 3.15 V. The zinc deposits are stripped manually, remelted and cast to bars. With the various zinc reclamation steps zinc recoveries up to 93% in the total plant can be achieved. Furthermore, with the integration of the zinc electrowinning process in the MCF operation, recycling without significant losses of metal values can be improved and more readily marketable products such as electrolytic zinc and copper, silver skims and precious metals-containing anode slimes, cadmium precipitate, lead products, tin/lead alloys, nickel sulphate, slag for blasting operations etc. are produced in this secondary smelter operation. ADHESION OF ZINC DEPOSITS The adhesive strength of electrolytic metal deposits, which mainly depends on the degree of chemical bonding and mechanical interactions between the substrate and the electrodeposit, is of great importance in the field of electrowinning and refining as well as electroplating of metals. In electroplating high adhesive strength of metal layers is essential to protect the substrate. For continuous electrowinning processes [ 2 ] the adhesion should be low enough that the metal deposits peel off from the substrate after having reached a certain thickness. In electrowinning and refining processes where the electrodeposited metal has to be stripped off from the cathode plates a medium adhesive strength is desired. One of the poorly understood problems in the zinc electrowinning process is the unexpected and sometimes suddenly occurring variation in the intensity of the zinc deposit adhesion on the aluminium cathodes. Insufficient adhesion can involve untimely fall-off of the cathodic zinc plate, whereas so-called "sticky" adhesion may either cause unfavourable working conditions during the manual or mechanical stripping or make even the redissolution of the zinc deposits by dipping the cathodes in the electrolyte necessary. From plant information it is known that sticking zinc losses which do not exceed 1% can be considered as acceptable. Medium and uniform adhesion values of the deposits are therefore of great importance with respect to manual or mechanical stripping operations as well as for the efficiency of metal recovery. Data from the literature [ 3-7] and investigations performed in our department [8-11 ] indicate that adhesion behaviour of zinc on aluminium is not only controlled by a few main factors which can be easily adjusted in plant operation, but is also influenced by minor or overlapping changes, e.g. of operating conditions and electrolyte composition. Results from research work on the laboratory scale with synthetic and tech-
16
nical zinc sulphate electrolytes as well as investigations performed at plant sites reveal that the adhesion increases with the surface roughness of the cathode, the electrolyte temperature, and the fluorine or chlorine content and other impurity contents. LABORATORY AND PLANT INVESTIGATIONS
The laboratory electrolysis unit consists, as shown schematically in Fig. 3, of five electrolysis cells (1), each measuring 200 X 100 X 170 mm. Each cell is equipped with two silver alloyed lead anodes (0.75 wt% Ag; 190 X 90 X 6 mm ) and one cathode of semihard rolled aluminium plate (99.5% Al; 195 X 95 X 4 mm). In order to avoid concentration gradients in the cell the anodes are perforated with 20 holes comprising 15% of the total area. The electrolyte inlet is arranged at the centre of the bottom of each cell and flows out at the upper edge into a tank (2) with a capacity of 120 1. A thermostat (3) with heat exchangers of flexible PVC pipes maintained an electrolyte temperature of 33 ° C. The electrolyte circulation is achieved through centrifugal pumps ( 4,5 ) of polypropylene. The current density was 530 A/m 2.
Fig. 3. Electrolysis installation for bench-scale investigation. (1) electrolysis cells; (2) tank; (3) thermostat; (4,5) pumps; (6) valve; (7) flow meter.
Preliminary tests revealed that adhesion values of zinc deposits on aluminium substrate were much higher at cathodes brushed across compared with those ground longitudinally. Therefore, aluminium cathodes used in the subsequent runs were wet ground longitudinally with emery paper (grit 320), then cleaned and dried. After this procedure the edges of the cathodes were covered with PVC clips to prevent zinc deposits growing together. The electrolyte used during these investigations was tapped from the feed line of the tankhouse in a quantity sufficient for several test runs. After the usual 8 hour duration of a test the decrease in zinc concentration amounted to about 5 g/l, which corresponds approximately to that in industrial operations. The adhesion strength of the zinc deposits was determined by means of a stripping device which had been designed and built in our workshop (Fig. 4 ). It consists of a movable and a fixed traverse resembling a tensile-testing
17
s
7 /
3
~
j Fig. 4. Stripping device for the laboratory-scale electrodes. (1) chuck on the fixed traverse; (2) cathode; (3) shearing wire; (4) chuck on the mobile traverse; (5) dynamometer; (6) amplifier; (7) motion pickup; (8) amplifier; (9) X-Yrecorder.
machine. The fixed traverse is linked via a spherical joint with a strain gauge and is suspended at the centre of the device. It comprises a holder facilitating attachment of the cathode. Guide pulleys at the columns of the stripping device assure vibration-free movement of the traverse. An electromotor drives the lower traverse via a worm gear at a speed of 35 m m / m i n along the columns in the downwards or upwards direction. The jig (attached like a cutter via two movable spindles to the mobile traverse) comprises tightened steel wires at either side of the cathode. Clamping of the jig to the aluminium cathode facilitates adjustment of the wires vertically to the upper edge of the zinc deposit. The downward motion initiates peeling off of the deposit by the wires. The stripping power is recorded continuously via an amplifier with an X - Y recorder as a force-length diagram. Planimetering of these diagrams yields an average adhesion value for each cathode. Dividing this value by the adhesion width (in this case two times the deposit width) gives the adhesion strength of the deposits in N/m. As an example for metal contents in zinc electrolytes which reduce the adhesion strength of the deposits, the results with increasing nickel additions to the MCF tankhouse electrolyte are illustrated in Fig. 5. Based on the experience of the test runs [ 8,9 ] the adherence values are grouped on the left side of the diagram as extremely low (fall-off type deposits), low, normal, high and extremely high (sticky deposits), whereas in the upper part, the electrolyte composition of the plant feed is given.
18
From the results it can be seen that nickel additions up to more than 2 mg/1 lead to a marked decrease from high to low adhesion values. Previous investigations [4-9] have shown that these changes of deposit adherence with increasing impurity levels can be related to alterations of the deposit morphology or crystal orientation. Zinc is generally deposited from high-purity and addition-free electrolytes in the form of large hexagonal platelets with random angles to the aluminium substrate. Investigations with the scanning electron microscope of zinc deposits after stripping indicate that increasing nickel contents cause less coarse and more randomly oriented crystallization, which leads to a decrease of adherence. Increasing nickel contents enhance the pitting and corroding of the deposits, and a significant decrease in current efficiency is unavoidable. Additions of cobalt, arsenic, antimony and tin have a similar influence on adherence and current efficiency [10]. Increasing adherence values have been obtained by increasing the cadmium and lead levels of the tankhouse feed. From Fig. 6 it can be seen that lead additions up to 7 mg/1 lead to maximum adhesion. The deposits become more ductile and greyish whereas with increasing cadmium contents (up to 5 mg/1) bright, compact and partially brittle deposits are produced [ 10 ]. No changes with respect to the current efficiency with either metal have been observed. Scanning electron micrographs confirm that zinc deposit morphology and orientation is very sensitive to variation of the lead level [ 5 ] and show increas-
e,trem, y \ \ high
-
=
~
Zo EH2Mn Cu
~
g/L
s, 122~ ls3to~1
\\
19 108211,17162 po~o~oo61= lopo~] ~1 1~66
\\\
200
rag, I
\
?
100 ~ o
--
extremely low
0
2 Ni
Eoncentration [mg/l]
Fig. 5. Effect of nickel on zinc deposit adhesion.
3
19
\\
300
E z
?00
.c:
J
r0
¢,-
t/n HzSO~MnCu Co Cd Ni Fe Sb Sn I°b As El F g/I / mq/I
] 2
3
I
I
J
i
/~
5
6
7
J 8
9
10
11
12
Pb Concentration [rng/I] Fig. 6. Effect of lead on zinc deposit adhesion.
ingly basal planes and thus less randomly oriented zinc crystallization. This parallel and uniform growth of deposits on the aluminium substrates induces increased adhesion. DIPPING PRETREATMENT OF CATHODES
Aluminium cathode surfaces are usually brushed after stripping to remove sticking deposits and to ensure lower adherence values. Besides this convenient and economic pretreatment technique, several techniques are known for controlling the adherence of zinc deposits, e.g. by a sealing treatment of the aluminium cathodes after anodisation. With the aim of investigating whether an easy and inexpensive dipping pretreatment of the aluminium cathodes in aqueous solutions containing parting compounds influences the adhesion strength favourably, plant site tests with production cathodes were performed. After preliminary tests on the laboratory scale two parting additives were chosen: gum arabic, because it is frequently added to the zinc electrolyte in order to improve the electrolysis conditions and to achieve smooth compact deposits; and sodium resinate, which has shown favourable parting effects for copper deposits [ 12,13 ] and is nowadays applied on an industrial scale. In these plant tests the zinc deposits were stripped every 24 hours by means
20 of a stripping device which is schematically shown in Fig. 7. The equipment handles cathodes of industrial size and comprises two steel rods of 20 m m diameter m o u n t e d with a pivot on a platform. These steel rods can be positioned arbitrarily at a distance of approximately 10 cm parallel to each other. Prior to any stripping operation the upper edges of the deposits are detached slightly to position and fix the steel rods between the cathode and the deposit. On upward motion of the cathodes by means of a crane the rods separate the deposits from the substrate. A strain gauge at the crane hook continuously records the forces. By transmitting the test data via an amplifier to an X - t recorder it was possible to plot a force-length diagram, the evaluation of which yields an average adhesive strength in N / m . Results pertaining to dipping p r e t r e a t m e n t with gum arabic are presented in Fig. 8. It can be seen t h a t gum arabic contents of up to 15 g/1 in the dip solution cause an increase in adhesion. To decrease the adhesion below the base values, concentrations ranging from 20 g/1 to 25 g/l are necessary. However, in this concentration range the parting film thickness increases in such a way that unfavourable polarization p h e n o m e n a occur at the cathode. Sodium resinate addition to the aqueous dip solutions lowers the adherence of zinc deposits at concentrations up to 5 g/l, as shown in Fig. 9. This pretreatm e n t method, even under unfavourable electrolysis conditions, enabled achievement of low adherence values for easy stripping. The dipping technique is simple and inexpensive if a byproduct of the paper industry with about 50%
1
2 F?
@ Fig. 7. Stripping device for the technical plant investigations. (1) dynamometer; (2) cathode; (3) electrodeposit; (4) amplifier; (5) X - t recorder; (6) spindles.
21 160
_f
1/+0 120 E
Z
J
J
_ .,./~~ ~ 6o~ 100
~ o ,o
•c ~
.
.
.
"× \ "~~~~,
.
~
__
zn
0
jH~SO~Iculed INi IFe ]Sb IMo IPb ]Co
/+9,/+ 05,3 "0," I 2,31 2,r'/11~.,2 ]
]13 111,1510,51
r,-~,:es
-
Concenfrofion of gum arabic in parring sotufion [g/t] Fig. 8. Effect of pretreatment of the cathodes in a parting solution with gum arabic.
160" 1~0 120 ~-~
100
=~
80
-13
~"
6O
~
- 0 , - ~ , . 2
0
~.o 20
S 10 15 20 25 30 35 Concentration of Sodium resinate in parting Solution[g/l]
/,0
Fig. 9. Effect of the pretreatment of the cathodes in a parting solution with sodium resinate.
22
Fig. 10. SEM photographs of zinc deposits. (a) without pretreatment; (b) dip treatment in sodium resinate solution of cathodes. sodium resinate - - price approximately 60 US c/kg - - is applied. From comparison of scanning electron photographs (Fig. 10) it can be seen that platelets of hexagonal shape with partly random orientation are deposited (Fig. 10a) without dip pretreatment. The zinc deposition on the thin remaining parting layer of sodium resinate solution on the cathode after dip t r e a t m e n t leads to a
23 dominating fine needle-type crystallization (Fig. 10b) with various orientations. Due to the high density of nuclei only a few hexagonal platelets can be observed, which explains the decrease in adherence of the deposits. Insufficient data are available to show if the adherent parting layer additionally reduces corrosion and increases the life time of the aluminium cathodes. CONCLUSIONS Investigations on the influence of metal impurities on the adhesion behaviour of zinc deposits on aluminium cathodes were performed with a tankhouse zinc sulphate electrolyte produced by leaching flue dust in the MCF secondary copper smelter. The results show that additions of nickel, cobalt, arsenic, antimony and tin in certain ranges decrease the adherence of zinc deposits, whereas increasing the lead and cadmium content of the electrolyte induces an increase in adhesion strength. Accompanying scanning electron microscope investigation shows that high adherence can be related to a dominating growth of hexagonal platelets parallel to the substrate whereas field-oriented and more randomly oriented columnar deposits have a weak adherence. Plant site tests conducted with the aim of controlling the zinc deposit adhesion even under critical electrolysis conditions indicate that sufficient reduction of the adhesive strength can be achieved by easy and inexpensive dip pretreatment of the cathodes in aqueous solutions with sodium resinate as parting addition. Further investigations on plant sites will be necessary to gain more data on the influence of the most important electrolysis parameters as well as the impurity concentration and the synergic effect of several impurities on the adhesion behaviour of zinc deposits. ACKNOWLEDGEMENTS The authors express their gratitude to the Deutsche Forschungsgemeinschaft, Bonn - - Bad Godesberg, for financial support and the Metallhiitte Carl Fahlbusch GmbH, Rastatt, for the allowance of and assistance during the plant investigations. They also acknowledge most thankfully Mr. Jiirgen Krampitz and Mr. Harry Milbrandt from the Department workshop for the construction of the electrolytic and stripping units, the experimental assistance of Dipl.Ing. Mehmet Giilbas and Dipl.-Ing. Mustafa GtingSr as well as the discussions with Dipl.-Ing. Hasan Giindiiz Eran. REFERENCES 1 Oelmann,H., Die Metallhtitte Carl Fahlbusch -- eine Recyclinghiittemit vollst~ndiger Aufarbeitungder Flugsta'ube,Erzmetall, 36 (1978) 229-232.
24 2
3
4 5
6 7 8 9 10 11
12 13
Kammel, R., Eran, H.G. and Lieber, H.-W., Review and outlook in continuous electrowinning and recovery processes from aqueous solutions, International Syumposium on Hydrometallurgy, AIME Meeting, Atlanta, March 1983, pp. 647-657. Kerby, L.R., Impurity effects in zinc electrowinning, Proceedings International Conference on Application of Polarization Measurements in the Control of Metal Deposition, Victoria, Canada, May 1982. Fukubayashi, H.H., O'Keefe, T.J. and Cinton, W.C., Effect of impurities and additives on the electrowinning of zinc, U.S. Bur. Mines., Rep. Invest. No. 7966 (1974) 26. MacKinnon, D.J. and Brannen, J.M., Zinc deposit structures obtained from high purity synthetic and industrial acid sulphate electrolytes with and without antimony and glue additions, J. Appl. Electrochem., 7 (1977) 451-459. Kerby, R.C., Jackson, H.E., O'Keefe, T.J. and Wang, Y.M., Evaluation of organic additives for use in zinc electrowinning, MetaU. Trans., 8B (December 1977) 661-668. Adrianne, P., Scoyer, J. and Winand, R., Zinc electrowinning-- A comparison of adherencereducing pretreatment for aluminium cathode blanks, Hydrometallurgy, 6 (1980) 159-169. Saadat, M.H., Untersuchungen tiber die Haftung von elektrolytischen Zinkniederschl~gen auf Aluminiumkathoden, Dissertation, TU Berlin, 1976. Kammel, R. and Saadat, M.H., Die Haftung von Elektrolytzink auf Aluminium-Kathoden, Metall, 30 (1976) 551-555. Kammel, R., GSktepe, M. and Gtilbas, M., Einfluss von Verunreinigungen auf das Haftungsverhalten von Elektrolytzink, Erzmetall, 37(6) (1984) 289-294. Kammel, R. and GSktepe, M., Effect of electrolytic impurities and aluminium cathode dip treatment on zinc deposit adhesion, Proc. AIME, Energy Reduction Techniques in Metal Electrochemical Processes, 1985, pp. 281-216. Nelson, W., McGregor, J.G., Snow, R. and Delhees, D., A new look at the series system of electrorefining copper, AICC Metals Ltd., London, 1977. Kammel, R., GSktepe, M. and Bor, F.Y., Haftungsverhalten von Elektrolytkupfer auf vorbehandelten Kathodenunterlagen, Metall, 37(11) (1983) 1103-1109.
11
Zinc E l e c t r o w i n n i n g from Flue D u s t s at a S e c o n d a r y Copper S m e l t e r and C o n n e c t e d A d h e s i o n P r o b l e m s of the Metal D e p o s i t s R O L A N D K A M M E L , M U S T A F A G O K T E P E and H A R T M U T O E L M A N N Institut fiir MetaUurgie - - Metallhi~ttenkunde, Technische Universitiit Berlin, 1000 Berlin 12 (Germany) (Received June 12, 1986; accepted in revised form February 11, 1987)
ABSTRACT Kammel, R., G6ktepe, M. and Oelmann, H., 1987. Zinc electrowinning from flue dusts at a secondary copper smelter and connected adhesion problems of the metal deposits. Hydrometallurgy, 19: 11-24. The secondary copper smelter" Metallhtitte Carl Fahlbusch" (MCF), Rastatt, Federal Republic of Germany is discussed, with particular consideration of the MCF zinc electrowinning process from low-grade, chloride-containing flue dusts. The investigations performed were focussed on the adhesion of zinc deposits on the aluminium cathodes. With laboratory tests using the tankhouse electrolyte and an apparatus for measuring the adhesion force, the influence of the electrolyte impurities lead, cadmium, cobalt, nickel, tin and antimony on the adhesion behaviour and on the current efficiency have been determined. Field tests to affect the adhesion of the zinc deposits on aluminium cathodes by dip pretreatment of the cathodes with glue or sodium resinate reveal that with only small sodium resinate additions in the dipping bath better stripping can be achieved.
SMELTER PROCESS
In the secondary copper smelter "Metallhiitte Carl Fahlbusch" (MCF) [ 1 ] copper and copper alloy scrap, ashes, slimes, slags and other copper-containing materials have been treated as shown in the flow sheet (Fig. 1 ) in two blast furnaces, and the tapped molten black copper in a 10 t converter. The converter copper is then refined in a 40 t rotary furnace and cast into anodes for the electrolytic copper refinery with a capacity of 10,000 t electrolytic copper and 180 t anode slime per year. In a hearth furnace process a lead-tin alloy and zinc oxide are produced from the converter flue dust. The flue dusts from both the blast furnace and the hearth furnace are treated together with zinc oxides from other customers in an electrowinning zinc plant having a capacity of 10 t per day. The zinc-winning process has been developed by MCF and has been in industrial operation for 10 years. The smelter has been recently shut down due to the copper price situation. 0304-386X/87/$03.50
© 1987 Elsevier Science Publishers B.V.
12 Secondary
J Convertering-
materials
~ ~
Iron dusts containing
r-1 ----i Process II | Fluedusts l l" Silicia
copper
Spout slag
~-" ~
and Refining Slags
((1111
F!ue dusts to electrolytic
~__.(_
zlnc plant
I
"--3 z ~ Blackcopper JB1ackcopper i(Liq./sol.) Radiators | J~ _ Brass J ~.J Chips " ~, r'~
Flue dusts to
tin plant
Converter copper ~ i ~ (liq /sol ) ~._~..-~ Old copper ~J Tombac Anoderemnants
I~l
~I Flue dusts to electrolytic zinc plant
l
" Anode remnants rotary furnace Nickel sulfate
[ Anode Slime
Copper cathodes MFC
Nickel sulfate
plant
N~CJ4 • ~.5 Hz@
Fig. 1. Flowsheet of the secondary copper smelter in Rastatt. (1) wind box; (2) forehearth; (3) granulation channel; (4) waste gas; (5) secondary combustion chamber; (6) waste gas boiler; (7) waste gas cooling system; (8) bag house; (9) fan; (10) stack; (11) settling chamber; (12) burner; (13 ) casting machine; (14 ) cooling basin. ZINC ELECTROWINNING PROCESS
From the flow sheet of the M C F zinc winning process in Fig. 2 it can be seen t h a t the chlorine content of the flue dust (which can vary considerably, as
13
shown in Table 1 ) is removed in preliminary washing steps. The resulting rinse water and effluents are treated by neutralization, precipitation and filtration steps to meet effluent guidelines. The zinc content of the waste water is reduced by this treatment to less than 3.0 mg/l. Since the slimes of the waste-water treatment plant are recirculated to the blast furnace, dumping problems do not exist in this washing cycle. After having removed the chlorine content of the zinc oxide suspension to residual values of less than 0.02%, the zinc oxides are leached with spent elecl Flue Dusts from Shaft Furnace Rotary Furnace, Hearth Furnace Suppliers
Cu-Fe | c I Precipitatl°nJ!
'f'OonO ~, Slime to Shaft Furnace
Clean Effluent
~ Elecfrolyfe~
CuR~idue Shoff .L to
~,
'v. Pb Concentrate Furnace Cd ResidueIEIecfrowmm~ to Shaft for Sole Furnace E-Zfnc
Fig. 2. Flow sheet of the MCF process for electrowinning of zinc from flue dusts. TABLE 1
Composition of the flue dusts ( % ) treated in the electrowinning zinc plant Zn C1 Fe Bi
40-65 1-12 0.1-1.5 0.01-0.05
Pb Cd Ni Co
6-20 0.5-0.8 0.1-1 0.001-0.005
Cu Sn Sb Ag
1-6 1-2 0.01-0.05 20-40 g/t
14
trolyte in an agitation tank. Depending on the composition of the flue dust charge, the resulting leach residues can contain up to about 25% lead, 10% zinc, 6% copper, 2% tin and 100 g Ag/t. Copper and zinc are extracted from these residues by an acid-leaching step for subsequent recovery. Copper is fed as a precipitate to the blast furnace and zinc as zinc sulphate solution to the cell house; in this way, a closed loop is achieved. The lead concentrate thus obtained contains the total input of lead, tin and silver. This intermediate product is converted, after drying in a blast furnace operation, to a silver-bearing lead bullion which is either solidified and sold or desilvered and detinned whereby the silver-zinc-lead skims formed by the Parkes process can be further treated in the copper converter. The tin-bearing flue dusts are transferred to the hearth furnace. The zinc sulphate leach solution from the agitated leaching tank is subjected to a two-stage refining process, yielding a copper-bearing precipitate, which is returned to the blast furnace, and a cadmium precipitate containing approximately 20% Cd and 80% Zn, which is sold. In Table 2 the composition of th~ MCF process electrolyte is shown in comparison with two electrolytes of other zinc producers, which were also tested in the course of these investigations. This compilation of data reveals that the impurity and, especially, the iron levels differ considerably. The usual electrolyte type produced from roasted zinc concentrates, with a combined jarosite process for further zinc extraction, e.g. the electrolyte from Preussag AG, Nordenham, may have iron concentrations below 10 mg/1. The Cinkur AG, Turkey electrolyte,which is extracted from rotary kiln furnace calcine (with low iron content) has only a relatively low iron concentration. Due to the tin contents in the MCF flue dusts (Table 1) relatively high Fe 3+ concentrations in the leach liquor have to be adjusted, e.g. by Fe2 (S04) 3" H20 addition, for optimum coprecipitation of tin during the iron hydroxide precipitation step. Therefore, the iron content of the MCF electrolyte is several times higher than that of usual SHG (special high grade) electrolytes. Higher chlorine contents in the MCF and Cinkur electrolytes lead to increased corrosion of the lead anodes and consequently to increased lead concentrations in the electrolyte. TABLE 2 Composition of zinc electrolytes of plants with flue dust and different calcines as raw materials
Metallhtitte C. Fahlbusch Cinkur Turkey Preussag
Zn H2SO4 (g/l) (g/l)
Cu Cd Ni Fe Sb Mn Pb Co As (mg/1) (rag/l) (mg/l) (mg/l) (mg/1) (rag/l) (mg/1) (mg/1) (mg/1)
46.2
189.6
0.1
4.2
2.37
40.9
n.d.
55
164
0.4
0.5
0.1
3.5
n.d.
56
176
0.05
0.06
0.004
8.4
0.003
n.d.: not determined.
255
7.32
0.66
traces
n.d.
n.d.
n.d.
n.d.
6900
0.15
0.004
traces
15 From the purified MCF acidic zinc sulphate solution zinc is then recovered in a 40-cell electrowinning plant having a capacity of about 10 t / d electrolytic zinc. Each cell is equipped with 20 aluminium cathodes and 21 lead anodes. With the usual applied current density of 500 A / m 2 the current efficiency amounts to 84% and the cell voltage to 3.15 V. The zinc deposits are stripped manually, remelted and cast to bars. With the various zinc reclamation steps zinc recoveries up to 93% in the total plant can be achieved. Furthermore, with the integration of the zinc electrowinning process in the MCF operation, recycling without significant losses of metal values can be improved and more readily marketable products such as electrolytic zinc and copper, silver skims and precious metals-containing anode slimes, cadmium precipitate, lead products, tin/lead alloys, nickel sulphate, slag for blasting operations etc. are produced in this secondary smelter operation. ADHESION OF ZINC DEPOSITS The adhesive strength of electrolytic metal deposits, which mainly depends on the degree of chemical bonding and mechanical interactions between the substrate and the electrodeposit, is of great importance in the field of electrowinning and refining as well as electroplating of metals. In electroplating high adhesive strength of metal layers is essential to protect the substrate. For continuous electrowinning processes [ 2 ] the adhesion should be low enough that the metal deposits peel off from the substrate after having reached a certain thickness. In electrowinning and refining processes where the electrodeposited metal has to be stripped off from the cathode plates a medium adhesive strength is desired. One of the poorly understood problems in the zinc electrowinning process is the unexpected and sometimes suddenly occurring variation in the intensity of the zinc deposit adhesion on the aluminium cathodes. Insufficient adhesion can involve untimely fall-off of the cathodic zinc plate, whereas so-called "sticky" adhesion may either cause unfavourable working conditions during the manual or mechanical stripping or make even the redissolution of the zinc deposits by dipping the cathodes in the electrolyte necessary. From plant information it is known that sticking zinc losses which do not exceed 1% can be considered as acceptable. Medium and uniform adhesion values of the deposits are therefore of great importance with respect to manual or mechanical stripping operations as well as for the efficiency of metal recovery. Data from the literature [ 3-7] and investigations performed in our department [8-11 ] indicate that adhesion behaviour of zinc on aluminium is not only controlled by a few main factors which can be easily adjusted in plant operation, but is also influenced by minor or overlapping changes, e.g. of operating conditions and electrolyte composition. Results from research work on the laboratory scale with synthetic and tech-
16
nical zinc sulphate electrolytes as well as investigations performed at plant sites reveal that the adhesion increases with the surface roughness of the cathode, the electrolyte temperature, and the fluorine or chlorine content and other impurity contents. LABORATORY AND PLANT INVESTIGATIONS
The laboratory electrolysis unit consists, as shown schematically in Fig. 3, of five electrolysis cells (1), each measuring 200 X 100 X 170 mm. Each cell is equipped with two silver alloyed lead anodes (0.75 wt% Ag; 190 X 90 X 6 mm ) and one cathode of semihard rolled aluminium plate (99.5% Al; 195 X 95 X 4 mm). In order to avoid concentration gradients in the cell the anodes are perforated with 20 holes comprising 15% of the total area. The electrolyte inlet is arranged at the centre of the bottom of each cell and flows out at the upper edge into a tank (2) with a capacity of 120 1. A thermostat (3) with heat exchangers of flexible PVC pipes maintained an electrolyte temperature of 33 ° C. The electrolyte circulation is achieved through centrifugal pumps ( 4,5 ) of polypropylene. The current density was 530 A/m 2.
Fig. 3. Electrolysis installation for bench-scale investigation. (1) electrolysis cells; (2) tank; (3) thermostat; (4,5) pumps; (6) valve; (7) flow meter.
Preliminary tests revealed that adhesion values of zinc deposits on aluminium substrate were much higher at cathodes brushed across compared with those ground longitudinally. Therefore, aluminium cathodes used in the subsequent runs were wet ground longitudinally with emery paper (grit 320), then cleaned and dried. After this procedure the edges of the cathodes were covered with PVC clips to prevent zinc deposits growing together. The electrolyte used during these investigations was tapped from the feed line of the tankhouse in a quantity sufficient for several test runs. After the usual 8 hour duration of a test the decrease in zinc concentration amounted to about 5 g/l, which corresponds approximately to that in industrial operations. The adhesion strength of the zinc deposits was determined by means of a stripping device which had been designed and built in our workshop (Fig. 4 ). It consists of a movable and a fixed traverse resembling a tensile-testing
17
s
7 /
3
~
j Fig. 4. Stripping device for the laboratory-scale electrodes. (1) chuck on the fixed traverse; (2) cathode; (3) shearing wire; (4) chuck on the mobile traverse; (5) dynamometer; (6) amplifier; (7) motion pickup; (8) amplifier; (9) X-Yrecorder.
machine. The fixed traverse is linked via a spherical joint with a strain gauge and is suspended at the centre of the device. It comprises a holder facilitating attachment of the cathode. Guide pulleys at the columns of the stripping device assure vibration-free movement of the traverse. An electromotor drives the lower traverse via a worm gear at a speed of 35 m m / m i n along the columns in the downwards or upwards direction. The jig (attached like a cutter via two movable spindles to the mobile traverse) comprises tightened steel wires at either side of the cathode. Clamping of the jig to the aluminium cathode facilitates adjustment of the wires vertically to the upper edge of the zinc deposit. The downward motion initiates peeling off of the deposit by the wires. The stripping power is recorded continuously via an amplifier with an X - Y recorder as a force-length diagram. Planimetering of these diagrams yields an average adhesion value for each cathode. Dividing this value by the adhesion width (in this case two times the deposit width) gives the adhesion strength of the deposits in N/m. As an example for metal contents in zinc electrolytes which reduce the adhesion strength of the deposits, the results with increasing nickel additions to the MCF tankhouse electrolyte are illustrated in Fig. 5. Based on the experience of the test runs [ 8,9 ] the adherence values are grouped on the left side of the diagram as extremely low (fall-off type deposits), low, normal, high and extremely high (sticky deposits), whereas in the upper part, the electrolyte composition of the plant feed is given.
18
From the results it can be seen that nickel additions up to more than 2 mg/1 lead to a marked decrease from high to low adhesion values. Previous investigations [4-9] have shown that these changes of deposit adherence with increasing impurity levels can be related to alterations of the deposit morphology or crystal orientation. Zinc is generally deposited from high-purity and addition-free electrolytes in the form of large hexagonal platelets with random angles to the aluminium substrate. Investigations with the scanning electron microscope of zinc deposits after stripping indicate that increasing nickel contents cause less coarse and more randomly oriented crystallization, which leads to a decrease of adherence. Increasing nickel contents enhance the pitting and corroding of the deposits, and a significant decrease in current efficiency is unavoidable. Additions of cobalt, arsenic, antimony and tin have a similar influence on adherence and current efficiency [10]. Increasing adherence values have been obtained by increasing the cadmium and lead levels of the tankhouse feed. From Fig. 6 it can be seen that lead additions up to 7 mg/1 lead to maximum adhesion. The deposits become more ductile and greyish whereas with increasing cadmium contents (up to 5 mg/1) bright, compact and partially brittle deposits are produced [ 10 ]. No changes with respect to the current efficiency with either metal have been observed. Scanning electron micrographs confirm that zinc deposit morphology and orientation is very sensitive to variation of the lead level [ 5 ] and show increas-
e,trem, y \ \ high
-
=
~
Zo EH2Mn Cu
~
g/L
s, 122~ ls3to~1
\\
19 108211,17162 po~o~oo61= lopo~] ~1 1~66
\\\
200
rag, I
\
?
100 ~ o
--
extremely low
0
2 Ni
Eoncentration [mg/l]
Fig. 5. Effect of nickel on zinc deposit adhesion.
3
19
\\
300
E z
?00
.c:
J
r0
¢,-
t/n HzSO~MnCu Co Cd Ni Fe Sb Sn I°b As El F g/I / mq/I
] 2
3
I
I
J
i
/~
5
6
7
J 8
9
10
11
12
Pb Concentration [rng/I] Fig. 6. Effect of lead on zinc deposit adhesion.
ingly basal planes and thus less randomly oriented zinc crystallization. This parallel and uniform growth of deposits on the aluminium substrates induces increased adhesion. DIPPING PRETREATMENT OF CATHODES
Aluminium cathode surfaces are usually brushed after stripping to remove sticking deposits and to ensure lower adherence values. Besides this convenient and economic pretreatment technique, several techniques are known for controlling the adherence of zinc deposits, e.g. by a sealing treatment of the aluminium cathodes after anodisation. With the aim of investigating whether an easy and inexpensive dipping pretreatment of the aluminium cathodes in aqueous solutions containing parting compounds influences the adhesion strength favourably, plant site tests with production cathodes were performed. After preliminary tests on the laboratory scale two parting additives were chosen: gum arabic, because it is frequently added to the zinc electrolyte in order to improve the electrolysis conditions and to achieve smooth compact deposits; and sodium resinate, which has shown favourable parting effects for copper deposits [ 12,13 ] and is nowadays applied on an industrial scale. In these plant tests the zinc deposits were stripped every 24 hours by means
20 of a stripping device which is schematically shown in Fig. 7. The equipment handles cathodes of industrial size and comprises two steel rods of 20 m m diameter m o u n t e d with a pivot on a platform. These steel rods can be positioned arbitrarily at a distance of approximately 10 cm parallel to each other. Prior to any stripping operation the upper edges of the deposits are detached slightly to position and fix the steel rods between the cathode and the deposit. On upward motion of the cathodes by means of a crane the rods separate the deposits from the substrate. A strain gauge at the crane hook continuously records the forces. By transmitting the test data via an amplifier to an X - t recorder it was possible to plot a force-length diagram, the evaluation of which yields an average adhesive strength in N / m . Results pertaining to dipping p r e t r e a t m e n t with gum arabic are presented in Fig. 8. It can be seen t h a t gum arabic contents of up to 15 g/1 in the dip solution cause an increase in adhesion. To decrease the adhesion below the base values, concentrations ranging from 20 g/1 to 25 g/l are necessary. However, in this concentration range the parting film thickness increases in such a way that unfavourable polarization p h e n o m e n a occur at the cathode. Sodium resinate addition to the aqueous dip solutions lowers the adherence of zinc deposits at concentrations up to 5 g/l, as shown in Fig. 9. This pretreatm e n t method, even under unfavourable electrolysis conditions, enabled achievement of low adherence values for easy stripping. The dipping technique is simple and inexpensive if a byproduct of the paper industry with about 50%
1
2 F?
@ Fig. 7. Stripping device for the technical plant investigations. (1) dynamometer; (2) cathode; (3) electrodeposit; (4) amplifier; (5) X - t recorder; (6) spindles.
21 160
_f
1/+0 120 E
Z
J
J
_ .,./~~ ~ 6o~ 100
~ o ,o
•c ~
.
.
.
"× \ "~~~~,
.
~
__
zn
0
jH~SO~Iculed INi IFe ]Sb IMo IPb ]Co
/+9,/+ 05,3 "0," I 2,31 2,r'/11~.,2 ]
]13 111,1510,51
r,-~,:es
-
Concenfrofion of gum arabic in parring sotufion [g/t] Fig. 8. Effect of pretreatment of the cathodes in a parting solution with gum arabic.
160" 1~0 120 ~-~
100
=~
80
-13
~"
6O
~
- 0 , - ~ , . 2
0
~.o 20
S 10 15 20 25 30 35 Concentration of Sodium resinate in parting Solution[g/l]
/,0
Fig. 9. Effect of the pretreatment of the cathodes in a parting solution with sodium resinate.
22
Fig. 10. SEM photographs of zinc deposits. (a) without pretreatment; (b) dip treatment in sodium resinate solution of cathodes. sodium resinate - - price approximately 60 US c/kg - - is applied. From comparison of scanning electron photographs (Fig. 10) it can be seen that platelets of hexagonal shape with partly random orientation are deposited (Fig. 10a) without dip pretreatment. The zinc deposition on the thin remaining parting layer of sodium resinate solution on the cathode after dip t r e a t m e n t leads to a
23 dominating fine needle-type crystallization (Fig. 10b) with various orientations. Due to the high density of nuclei only a few hexagonal platelets can be observed, which explains the decrease in adherence of the deposits. Insufficient data are available to show if the adherent parting layer additionally reduces corrosion and increases the life time of the aluminium cathodes. CONCLUSIONS Investigations on the influence of metal impurities on the adhesion behaviour of zinc deposits on aluminium cathodes were performed with a tankhouse zinc sulphate electrolyte produced by leaching flue dust in the MCF secondary copper smelter. The results show that additions of nickel, cobalt, arsenic, antimony and tin in certain ranges decrease the adherence of zinc deposits, whereas increasing the lead and cadmium content of the electrolyte induces an increase in adhesion strength. Accompanying scanning electron microscope investigation shows that high adherence can be related to a dominating growth of hexagonal platelets parallel to the substrate whereas field-oriented and more randomly oriented columnar deposits have a weak adherence. Plant site tests conducted with the aim of controlling the zinc deposit adhesion even under critical electrolysis conditions indicate that sufficient reduction of the adhesive strength can be achieved by easy and inexpensive dip pretreatment of the cathodes in aqueous solutions with sodium resinate as parting addition. Further investigations on plant sites will be necessary to gain more data on the influence of the most important electrolysis parameters as well as the impurity concentration and the synergic effect of several impurities on the adhesion behaviour of zinc deposits. ACKNOWLEDGEMENTS The authors express their gratitude to the Deutsche Forschungsgemeinschaft, Bonn - - Bad Godesberg, for financial support and the Metallhiitte Carl Fahlbusch GmbH, Rastatt, for the allowance of and assistance during the plant investigations. They also acknowledge most thankfully Mr. Jiirgen Krampitz and Mr. Harry Milbrandt from the Department workshop for the construction of the electrolytic and stripping units, the experimental assistance of Dipl.Ing. Mehmet Giilbas and Dipl.-Ing. Mustafa GtingSr as well as the discussions with Dipl.-Ing. Hasan Giindiiz Eran. REFERENCES 1 Oelmann,H., Die Metallhtitte Carl Fahlbusch -- eine Recyclinghiittemit vollst~ndiger Aufarbeitungder Flugsta'ube,Erzmetall, 36 (1978) 229-232.
24 2
3
4 5
6 7 8 9 10 11
12 13
Kammel, R., Eran, H.G. and Lieber, H.-W., Review and outlook in continuous electrowinning and recovery processes from aqueous solutions, International Syumposium on Hydrometallurgy, AIME Meeting, Atlanta, March 1983, pp. 647-657. Kerby, L.R., Impurity effects in zinc electrowinning, Proceedings International Conference on Application of Polarization Measurements in the Control of Metal Deposition, Victoria, Canada, May 1982. Fukubayashi, H.H., O'Keefe, T.J. and Cinton, W.C., Effect of impurities and additives on the electrowinning of zinc, U.S. Bur. Mines., Rep. Invest. No. 7966 (1974) 26. MacKinnon, D.J. and Brannen, J.M., Zinc deposit structures obtained from high purity synthetic and industrial acid sulphate electrolytes with and without antimony and glue additions, J. Appl. Electrochem., 7 (1977) 451-459. Kerby, R.C., Jackson, H.E., O'Keefe, T.J. and Wang, Y.M., Evaluation of organic additives for use in zinc electrowinning, MetaU. Trans., 8B (December 1977) 661-668. Adrianne, P., Scoyer, J. and Winand, R., Zinc electrowinning-- A comparison of adherencereducing pretreatment for aluminium cathode blanks, Hydrometallurgy, 6 (1980) 159-169. Saadat, M.H., Untersuchungen tiber die Haftung von elektrolytischen Zinkniederschl~gen auf Aluminiumkathoden, Dissertation, TU Berlin, 1976. Kammel, R. and Saadat, M.H., Die Haftung von Elektrolytzink auf Aluminium-Kathoden, Metall, 30 (1976) 551-555. Kammel, R., GSktepe, M. and Gtilbas, M., Einfluss von Verunreinigungen auf das Haftungsverhalten von Elektrolytzink, Erzmetall, 37(6) (1984) 289-294. Kammel, R. and GSktepe, M., Effect of electrolytic impurities and aluminium cathode dip treatment on zinc deposit adhesion, Proc. AIME, Energy Reduction Techniques in Metal Electrochemical Processes, 1985, pp. 281-216. Nelson, W., McGregor, J.G., Snow, R. and Delhees, D., A new look at the series system of electrorefining copper, AICC Metals Ltd., London, 1977. Kammel, R., GSktepe, M. and Bor, F.Y., Haftungsverhalten von Elektrolytkupfer auf vorbehandelten Kathodenunterlagen, Metall, 37(11) (1983) 1103-1109.