Continuous electrowinning of zinc

Hydrometallurgy 54 Ž2000. 91–106 www.elsevier.nlrlocaterhydromet

Continuous electrowinning of zinc A.E. Saba ) , A.E. Elsherief Electro Metallurgy Lab. Central Metallurgical R&D Institute, P.O. Box 87, Helwan, Cairo, Egypt Received 1 March 1999; accepted 1 August 1999

Abstract Synthetic and pure zinc solution produced from laboratory leached oxidised zinc ores, under controlled temperature and pH were subjected to continuous elctrowinning operations until the least possible zinc concentration was reached. Conventional DC electrolysis technique, PC and PCR procedures were examined. The effect of organic additives and some of the impurity foreign cations were also investigated. Current efficiencies of more than 95% were obtained from acid sulphate solutions electrolysed at 45 mA cmy2 and 258C for DC and PC techniques. Electrowinning of zinc from relatively concentrated solutions Ž160 g dmy3 . could be achieved, successively, with acceptable current efficiencies down to a concentration of 40 g dmy3. Copper additives were found to decrease the current efficiency and worsen the quality of the cathode deposits. Manganese and silica were found to have limited effects on both current efficiency and morphology of the deposit. Iron was found to have a deleterious effect on both the current efficiency and the deposit features. Organic additives, gelatine and thiourea, have good leveling effects on the cathode deposits. Gelatine was found to improve the current efficiency especially in the presence of a mixture of foreign cations. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Zinc; Electrowinning; Current efficiency

1. Introduction Zinc ores, present at Um Gheig, in the eastern desert of Egypt, are found mainly as oxidised ore containing willemite ŽZnSiO4 ., hemimorphite ŽZnSi 2 O 7 ŽOH. 2 P H 2 O. and smithsonite ŽZnCO 3 .. This ore was leached by sulphuric acid in a two-step technique under controlled temperature and pH w1–3x. The coagulated silica produced by this

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treatment was filtered off. The leached liquor obtained contains, besides zinc, different amounts of Cu, Fe, Mn, Mg, Co and SiO 2 . Many of these cations were removed by the normal purification techniques w3x but traces of Fe, Cu, Mn and SiO 2 are still present. Some of these impurities, i.e., Cu and Fe are more noble than zinc, and will be co-deposited with zinc during electrolysis and serve as micro-cathodes upon which hydrogen is evolved w4–8x. They affect negatively the cathodic current efficiency ŽC.E.. and the quality of the electro-deposited zinc w9x. Winand w10x showed that the quality and type of the deposits depend on many factors such as the current density, temperature, pH, the presence of impurity cations and anions, inhibitors and type of substrate used. Das et al. w11x, showed that a considerable energy saving in the electrowinning processes could be achieved if the operating current density is increased, whilst maintaining high current efficiency ŽC.E.. and cathode quality. During electrowinning operations, zinc is depleted in the electrolyte solution, while acid is increased; this will affect badly the quality of the cathode deposit and greater acid mist formation, especially when using high current densities. Many types of organic additives have been investigated w6,12–17x by several authors, to improve the cathode quality and to prevent the acid mist formation w12,16,18x. Some of these additives were used as leveling agents. Appropriate amounts of these additives w6,8,14,19x were found to be necessary for the formation of fine grained, smooth and compact deposits. Usually the current density applied to the electrolysis process is restricted by the limiting current, beyond which serious deterioration in the quality of the cathode deposit is achieved. Pulsating current ŽPC. and periodic current reversal ŽPCR. are two techniques developed to permit the possibility of increasing the current density while decreasing the concentration of the metal-bearing solution during electrowinning, without affecting the cathode quality w20x. The interruption of the current flow, in PC, and the change of polarity, in PCR, greatly decrease the concentration gradients at the electrode–electrolyte interface, The effective current density with PC and PCR are equal to: i eff s i = TrT q T X for PC; and i eff s i = T y T XrT q T X for PCRwhere: i s current density, A my2 ; i eff s effective current density, A my2 ; T s forward time, seconds; T X s dead period or duration of reversal current, seconds. The factors and relations governing the different variables like the period of pulse, dead and reversal periods, concentrations, rate of diffusion, etc., have been thoroughly investigated w20x. The aim of this article is to study the continuous electrowinning of zinc from concentrated solutions, and the determination of the reduced zinc concentration at which electrowinning has to be stopped and the electrolyte has to be recycled as a leach solution. Also, a study of the effect of the presence of copper, iron, manganese and silica impurities, and gelatine and thiourea additives on the efficiency and morphology of the cathodic zinc deposits was made.

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2. Experimental The electrolysis cell consisted of a Pyrex 500 ml beaker with a Perspex cover holding the electrodes and powder feeder w3,20x. Aluminum strips, 40 cm2 surface area, were used as cathodes. Two platinum sheets, measuring 4 = 5 cm were used as anodes placed on either side of the cathode, sheathed in bags of polypropylene cloth to prevent MnO 2 sludge from contaminating the bath, when manganese salts were added to the electrolyte. All experiments were performed at 25 " 18C at constant stirring. The electrolyte used, 250 ml, was prepared by the double leaching of Egyptian oxidised zinc ore, purified by air, to oxidise the iron content, and then with the zinc powder addition technique. The solution obtained after the purification steps usually contains iron, 25 mg dmy3 , copper, 0.2 mg dmy3 , manganese, less than 0.1 mg dmy3 and silica in negligible amounts. Sulphuric acid and distilled water were added to get the desired electrolyte composition. Foreign cations i.e., iron, copper and manganese, as the sulphate salts while silica as SiŽOH. 2 were added to the purified solution to get the desired composition. Gelatine and thiourea were dissolved in the electrolyte before experiments. All the chemicals used were of analytical grade. During electrolysis, the concentration of zinc was adjusted by the addition of prepared ZnŽOH. 2 through the feeder, at intervals of time, so that the concentrations of zinc and sulphuric acid in the electrowinning cell were kept constant. An electronic pulse-control unit was used w20x which provides pulse duration from 1 to 180 s for ‘‘on’’ and ‘‘off’’ or ‘‘reversal’’ intervals. Cyclic voltammetry measurements were carried out by the aid of a Wenking Potentioscan POS 73. They were conducted in a Pyrex glass cell employing a carbon rod counter electrode. The working electrode was an aluminum wire mounted in epoxy resin, with a cross-section area of 0.049 cm2 . Potentials were measured relative to a mercury–mercurous sulphate electrode using Luggin capillary and a bridge filled with the working solution. The voltage and current changes were followed through a system connected to the potentioscan. Prior to the electrolysis experiments, the electrodes were degreased with acetone and washed thoroughly with distilled water. The cathodic current efficiency was determined as the ratio of weight of metal actually deposited by passing a definite quantity of electricity ŽW X . to the theoretical weight that would be deposited by that quantity of electricity according to Faraday’s law ŽW .: W s I = t = 32.69r26.8 A h where I s electric current passed in amperes; T s time in hours; 32.69 s equivalent weight of zinc; 26.8 s quantity of electricity needed to produce an electro-chemical change of 1 g-equivalent of a substance. If W X is the weight of the metal actually deposited at the cathode, then Current efficiencys Ž W XrW . = 100. The surface morphology and microstructure of the electro-deposited samples were examined by scanning electron microscope ŽSEM..

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3. Results and discussions 3.1. Cyclic Õoltammetry Cyclic voltammetry experiments of zinc electro-deposition from 120 g dmy3 Zn and 50 g dmy3 H 2 SO4 solution, on aluminum substrate, were carried out. The cathodic potential was varied in the range from y900 to y1700 mV at a scan rate of 20 mV sy1 . The linear sweep curves that represent the onset of cathodic deposition and redissolution of the deposited zinc are shown in Fig. 1. The cathodic curve can be attributed to both zinc deposition and hydrogen evolution reactions. Zinc deposition commences at y1550 mV. The crossover potential, the point of zero net current, is reached at y1435 mV, representing the equilibrium potential. The anodic curve, on the other hand, corresponds to the dissolution or stripping of the deposited zinc. When stripping is complete, the anodic peak reaches the original starting potential. The region BCD is properly called a nucleation hysteresis loop w21x. Cyclic voltammogram due to current sweep, at a scan rate of 16 mA sy1 , with the same conditions is given in Fig. 2. The difference in the potentials of zero current in the cathodic and anodic direction of the sweep reflects the changes in the potentials due to changes in zinc concentration near the aluminum substrate. Also the change in the

Fig. 1. Potential–sweep cyclic voltammograms for zinc deposition and stripping on Al electrode Ž0.049 cm2 ., from 120 g dmy3 Znq50 g dmy3 H 2 SO4 solution at 258C Žscan rate 20 mV sy1 ..

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Fig. 2. Current–sweep cyclic voltammograms for zinc deposition and stripping on Al electrode Ž0.049 cm2 ., from 120 g dmy3 Znq50 g dmy3 H 2 SO4 solution at 258C Žscan rate 16 mV sy1 ..

character of the aluminum substrate due to the deposition of zinc and the changes in its behavior to the evolution of hydrogen is considered. 3.2. Potentiostatic Fig. 3 shows typical results obtained by the potentiostatic method. Three different curves were obtained by maintaining the cathode potential constant at y1400, y1500 and y1600 mV. The results obtained show that as long as the response current is smaller than the limiting current, i l , its value does not change with time. A continuous steady value was obtained at y1400 mV. A gradual increase in current was recorded at y1500 mV, at the first stage of electrolysis, and then semi-steady state was reached. A sharp increase in current was recorded at y1600 mV, showing that the response current is in the region of the limiting current. This sharp increase in the current was related to the progressive formation of a rough deposited zinc, with a large surface area.

Fig. 3. Current–time curves for zinc deposition and stripping on Al electrode Ž0.049 cm2 ., from 120 g dmy3 Znq50 g dmy3 H 2 SO4 solution at 258C, at constant potential of: Ž1. y1400, Ž2. y1500 and Ž3. y1600 mV.

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3.3. GalÕanostatic The cathodic polarization of zinc electrowinning on aluminum substrate when being subjected to different polarizing currents, ranging from 30 to 60 mA cmy2 , were recorded as a function of time. Experiments were performed under two sets of conditions, without stirring and with moderate stirring of the solutions. The results are depicted in Fig. 4. With stirred solutions, at 30, 45 and 60 mA cmy2 , curves I, II and III were obtained, respectively, representing continuous potential plateaus at y1590, y1700 and y1720 mV, respectively. With non-stirred bath, at 45 mA cmy2 , the recorded potential plateau is found at y1710 mV and lasts after 65 s, curve IV, followed by a sharp increase in the cathodic polarization. This can be explained on the premise that the diffusion layer is rapidly increased due to the slow zinc cation replenishment near the electrode surface as a result of the decrease in convection of the ions in the solution. Also, the accumulation of hydrogen bubbles on the electrode surface results in a sudden increase in the cathodic overpotential. 3.4. Current efficiency The change in cathodic current efficiency with variable applied current densities, ranging from 30 to 60 mA cmy2 , was studied with electrolytes containing different concentrations of zinc ions and free sulphuric acid. The concentration of zinc and acid were kept constant. Experiments were performed for 3 h. The results obtained are summarised in Table 1. The above results show that the current efficiency is high, within the current densities applied, for a wide range of concentrations for both zinc and sulphuric acid. The highly convenient results were achieved with a current density of 45 mA cmy2 . The current

Fig. 4. Potential–time curves for zinc deposition on Al electrode at: Ž1. 30, Ž2. 45, Ž3. 60 mA cmy2 with stirred solution and Ž4. 45 mA cmy2 with non-stirred solution.

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Table 1 Current density, mA cmy2

Efficiency %

Bath composition g dmy3 Zn

g dmy3 H 2 SO4

30 45 60 30 45 60 30 45 60 30 45 60 30 45 60 30 45 60

94.5 96.1 97.2 95.8 96.6 96.9 96.3 97.2 97.0 92.3 92.8 92.1 83.1 88.3 87.5 61.5 63.2 64.2

160 160 160 120 120 120 65 65 65 50 50 50 40 40 40 40 40 40

40 40 40 50 50 50 90 90 90 150 150 150 80 80 80 270 270 270

efficiency decreased dramatically when zinc content in the electrolyte decrease to less than 40 g dmy3 , and especially when sulphuric acid increases Ž) 80 g dmy3 . in these experiments. A long term experiment, using a fixed volume and one electrolyte, with a starting composition of 120 g dmy3 zinc and 50 g dmy3 H 2 SO4 , was performed to determine the minimum zinc concentration solution suitable for electrowinning, with acceptable current efficiency. The electrolyte composition, Zn and H 2 SO4 , were analysed and determined, at different intervals, during the experiment. The results obtained are summarised in Table 2. This experiment shows that continuous removal of zinc from the electrolyte, down to around 40 g dmy3 , were achieved with current efficiency of more than 85%. Beyond this concentration a very low current efficiency was obtained. Accordingly, solutions with this range of zinc concentration Ž- 40 g dmy3 . should be transferred to the leaching vessel, to make use of its high acid content, for further dissolution of the ore.

Table 2 Current density, mA cmy2

Period, hour

Efficiency %

45 45 45 45

3 4 4 3

96.6 93.4 86.3 53.1

Bath composition Zn g dmy3 H 2 SO4 g dmy3 start end of experiment end of experiment end of experiment

120 99.2 69.2 41.2

50 98.4 140.4 181.6

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From the results of the above table, the variation of the zinc ion concentration with time is depicted in Fig. 5. This figure reveals that at high concentrations the depletion rate of zinc is approximately linear and tails off as the concentration of zinc reaches 40 g dmy3 . After this the concentration decays near exponentially according to the well-known expression for a reactor with a diffusion-limited reaction w22x. 3.4.1. Morphological examinations The surface morphology of the deposits electrowon from different concentrated electrolytes, at a current density of 45 mA cmy2 , were inspected by SEM. These pictures show that zinc occurs mainly in acicular crystalline form parallel to the substrate. At moderate concentrations Ž160 g dmy3 Zn and 40 g dmy3 H 2 SO4 . mossy structures were seen around the edges. By changing the concentration, i.e., lowering zinc and increasing acid concentrations, these features were changed, where the mossy-like structures predominate. Fig. 6a shows that the acicular and needle-like platelets are still present beside the mossy structure. As the zinc concentration decreases and the acid increases, more fine deposits, composed of the same features, but in finer form were obtained. On increasing the applied current density to 60 mA cmy1 , Fig. 6b, coarser platelets were obtained. This can be related to the absorption and adsorption of the evolved hydrogen, a process which increases with the increase of the applied current density on the active sites and thus preventing the formation of increased number of nucleation sites. This enables the zinc to be deposited on lesser number of nuclei that cause the deposited crystals to grow. The above mentioned morphology can be explained according to Winand w10x who shows that in the absence of organic additives, the inhibition intensity may be charac-

Fig. 5. Concentration variations of zinc and sulphuric acid in the bulk solution during a long term electrolysis experiment Žc.d.s 45 mA cmy2 ..

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Fig. 6. Scanning electron micrographs of zinc deposits from: Ža. 40 g dmy3 Znq80 g dmy3 H 2 SO4 at 45 mA cmy2 ; Žb. 50 g dmy3 Znq150 g dmy3 H 2 SO4 at 60 mA cmy2 . =1000.

terised by the relation between the exchange current density, which is a function of the apparent cathodic current density J and the diffusion limiting current density Jdl . He showed that the lower the J, the higher the inhibition intensity and the smaller the crystals deposited. He also showed w13x that, with high concentrated electrolytes, the value of Jrc Žwhere c is the concentration. is low, and the crystals formed are ‘‘FI’’ Žfield oriented isolated crystals. and ‘‘BR’’ Žoriented repeated crystals.. With the

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decrease of zinc concentration, Jrc increases, and FI begins to predominate with dendritic powder; and hydrogen evolved in much larger amounts, consuming electricity and decreasing current efficiency. 3.4.2. Gelatine additions Different ratios of gelatine from 50 mg dmy3 to 2 g dmy3 were added to different electrolytes with zinc concentration ranging from 160–120 and 50 g dmy3 H 2 SO4 . The addition of gelatine at such concentrations was found not to affect the current efficiency obtained, agreeing with that obtained with electrolytes free of gelatine. On the other hand, gelatine was found to smoothen the electrodeposited zinc, Fig. 7. This effect is highly pronounced with gelatine concentration of 50 mg dmy3 . The deposited zinc become fine grained with much smaller crystals. The positive effect of gelatine on the structure of the cathodic zinc is related to its nature, which is a protein high polymer of amino acids linked by peptide chain Ž –CO–NH– . w15x. Small amounts of gelatine are enough to improve the quality of the cathodic deposit. Such long chain peptide forms adsorbed layers which increase the cathodic polarisation of zinc and thus improve the quality of the metal deposit. This polarising effect of gelatine was studied by the same authors w3x by cyclic voltammetry using acidified zinc sulphate electrolytes and in the presence of different ratios of gelatine ranging from 50 to 2000 mg dmy3 . The results obtained showed that the addition of gelatine to the acid sulphate zinc solution caused a marked progressive polarisation. The polarisation was very pronounced at 50 mg dmy3 gelatine addition. On further increase of the gelatine additions, the rate of increasing polarisation was found to decrease. This high polarisation was related, on the same basis

Fig. 7. Scanning electron micrographs of zinc deposits electrowon from 50 g Znq150 g H 2 SO4 q50 mg gelatine Iy1 at 45 mA cmy2 , X s1000.

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mentioned before, to the build up of adsorbed layers of gelatine at the electrode surface, which hinders the further diffusion of zinc ions. 3.4.3. Thiourea addition As gelatine is reported w13x to have no influence on preventing the nodulation growth of electrodeposits, thiourea, which has been reported to decrease nodular growth w14x, was added in addition to gelatine in a concentration of 0.01 g dmy3 . The surface morphology of the zinc precipitate was found to be good, as with gelatine alone. The current efficiency was not affected with experiments up to 19 h Žunder conditions of constant zinc and acid concentration.. Thiourea was expected to lower the possibility of nodular formation for long-run industrial scale processes. 3.4.4. Effect of PC and PCR As PC and PCR techniques are used to improve the morphology of the electrodeposits, especially at high current densities, these two techniques were tested with the electrodeposition of zinc. A time span of 50 s forward current and 5 s for ‘‘cut off’’ or ‘‘reversing’’ the current polarity for the PC and PCR techniques were selected and used. This pulse duration, 50 s, was chosen as less than the time required to reach the transition time, with non-stirred solution, determined earlier i.e., 65 s, Fig. 4. Galvanostatic electrowinning experiments were studied using the PC and PCR techniques. The potential-time relationship obtained, on applying 45 mA cmy2 for the forward period, are given in Fig. 8Ža. and Žb. for the PC and PRC, respectively. With PC technique, Fig. 8 curve Ža., on passing the current the cathodic polarisation is increased to a sufficiently negative value forming a plateau at about y1720 mV. On switching off the current, the potential rapidly reaches the ZnrZn2q equilibrium value, y1480 mV, determined earlier ŽFig. 1.. It was found that the 5 s off period is sufficient

Fig. 8. Potential–time curves for zinc deposited on Al electrode Ž0.049 cm2 ., from 120 g dmy3 Znq50 g dmy3 H 2 SO4 at 45 mA cmy2 , with pulse cycles of 50:5 s. Ža. PC and Žb. PCR.

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to restore the equilibrium potential of zinc. By repeating the above cycle, semi-steady cathodic potential of y1720 mV was reached in subsequent pulses. Results obtained with PCR technique, Fig. 8 curve Žb., show that when the polarity of the cathode was reversed, for 5 s the potential rapidly passed the crossover potential Žy1480 mV. and reached y1250 mV. Cathodic polarisation values remained almost constant, ; –1720 mV, with repeated cycles. The two techniques were then examined for both the current efficiency and precipitate morphology. Experiments were conducted for 3 h. The results obtained with different bath compositions are summarised in Table 3. These results show that the current efficiencies obtained, with all the experiments conducted, are high with the PC technique, even compared with those obtained with the DC experiments. PCR, as expected, gives lower efficiencies. This is related to the re-dissolution of the cathodic zinc electrodeposited during the reverse cycle, which represents a negative use of current and energy. A long-term experiment with PC technique was performed, under the same conditions applied before, to determine the lowest concentration at which electrowinning of zinc should be stopped, when applying the PC technique. The results obtained are summarised in Table 4. The table shows similar general trend to that obtained with DC experiments, Table 2. Solutions with ; 65 g Zn dmy3 and high acid concentrations of about 130 g dmy3 , should not be subjected to further electrowinning, otherwise very low current efficiencies will be obtained. Such solutions should be returned for the second stage of leaching of the ore. 3.4.4.1. Morphological examinations. The main advantage of using PC and PCR techniques is their production of highly compact zinc deposits. These deposits are found to be more homogeneous and adherent than those obtained with the DC experiments. The morphological features obtained are the same for both PC and PCR. The zinc deposits obtained were found to have smaller crystals with smooth edges than those obtained with DC technique and can be compared with those obtained with the presence of gelatin, Fig. 7. This can be related to the chemical dissolution effect acting on the crystal edges during the cutting off or reversing the current. Also, during electrolysis, the

Table 3 Current density, mA cmy2

Technique

Period, hour

Efficiency %

Bath composition g dmy3 Zn g dmy3 H 2 SO4

60 60 60 45 45 60 60 60

DC PC PCR DC PC DC PC PCR

3 3 3 3 3 3 3 3

96.0 98.75 81.27 96.6 97.0 63.2 72.8 48.14

160

40

120

50

40

270

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Table 4 Current density, mA cmy2

Period, hour

Current efficiency %

Starting bath composition g dmy3 Zn g dmy3 H 2 SO4

45

3 4 4 4

98.7 94.4 86.5 55.2

120.0 96.3 65.7 38.0 E a 20.4

a

50 84.9 129.2 170.0 196.0

E s End of experiment.

HSO4y and SO42y anions are strongly adsorbed at the cathode surface during the ‘‘off time’’, in the PC technique. This blocks the growth centers at the cathode and forces the creation of new nuclei at each new pulse, with the formation of fine-grained deposits w14,20x. 3.4.5. Effect of foreign cations 3.4.5.1. Effect of copper. Three different copper concentrations namely 6.2, 12.5 and 20 mg dmy3 Cu were made up in a solution containing 120 g dmy3 zinc and 50 g dmy3 H 2 SO4 in order to determine their effect on the efficiency and quality of electro-deposited zinc. Electrolysis was performed for 4 h, at a current density of 45 mA cmy2 and at 258C. The current efficiencies were 96%, 91% and 88%, respectively. Cathodic polarisation of zinc in the presence of copper was found by the authors w3x to decrease with the copper concentration in the solution. In addition, copper was found to activate the dissolution of electrodeposited zinc. This explains the decrease in efficiency obtained with the increase of copper content in the electrolyte. The morphological examination of electrodeposited zinc in the presence of copper, shows that the deposit is dull and dark. This effect is more pronounced with the increase of copper content in the electrolyte. This can be explained on the basis that copper is reduced to the metallic state, electrolytically, prior to zinc and co-deposited with it. Copper was found to decrease the grain size of the electrodeposited crystals of zinc. 3.4.5.2. Effect of iron. The voltammograms obtained for zinc electrodeposition, in the presence of different proportions of iron ranging from 50 to 150 mg Fe dmy3 as were studied earlier w3x, show that the cathodic current efficiency of zinc deposition decreases with the increase of the iron concentration in the electrolyte. With 50 mg dmy3 Fe, the current efficiency obtained was 82%, under the same conditions applied with copper additions. A more dramatic decrease in efficiency was obtained with 150 mg dmy3 Fe, where an efficiency of 53% was obtained. This decrease in efficiency was related to the reverse redox system of Fe 2qrFe 3q taking place at the two electrodes, consuming high amounts of electricity. Iron was found to have a pronounced effect on the morphology of the electro-deposited zinc. Coarse crystalline deposits of large separate polyhedral needles and

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upward-growing zinc pyramidal dendrites were obtained, Fig. 9. This worsening of the properties of the zinc deposit, in addition to the decrease in the current efficiency, shows that iron should be removed, if possible, from the electrolyte during the leaching step of the ore. 3.4.5.3. Effects of manganese and silica. The presence of manganese, 0.9 and 1.8 g dmy3 Mn, caused little increase in the polarisation of zinc in sulphate solutions w3x. This effect may be attributed to the accumulation of the manganese cations near the cathode surface, which hinder the approach of zinc cations. The proportions of manganese added were found to have no effect on the current efficiency of the electrodeposited zinc, under the conditions applied. The presence of manganese, on the other hand, in the range of 1.5–3 g dmy3 , was reported to be required in the electrolyte to minimise the lead anode corrosion w8x. Colloidal silica, in the proportions of 33 to 165 mg dmy3 SiO 2 , was added and found to give little decrease in cathodic polarisation w3x. It was found to have nearly no effect on the current efficiency in the range studied. The presence of manganese was found to decrease the size of zinc grains electrodeposited. Zinc platelets with random orientations were clearly observed. The presence of 33 mg dmy3 silica gave compact, smooth and round edges of the electro-zinc precipitate obtained. 3.4.5.4. Effects of mixture of foreign cations. A mixture containing 10 mg Cu, 1.8 g Mn and 165 mg dmy3 silica was added to the electrolyte. This mixture was found to have a

Fig. 9. Scanning electron micrographs of zinc electrodeposits in presence of Fe cations at different current densities, electrowon from 50 g dmy3 Znq150 g dmy3 H 2 SO4 q40 mg dmy3 Fe at 80 mA cmy2 . =1000.

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decreasing effect on the current efficiency Ž82.5%.. The electrodeposit obtained was full of cavities and holes. Small fine-grained, scattered nodules and spongy dendrites were also found. Addition of gelatine, 50 mg dmy3 , improves the current efficiency of the electrodeposited zinc Ž88.3%.. The deposit obtained becomes continuous and covers the whole electrode surface, with small grain sizes. A transition from dendritic to boulder type and the marks of dendritic growth disappear. Using the PC technique yields very fine, smooth and compact Zn deposits. Moss formations formed with free electrolytes, or powdery and dendritic and nodules deposits obtained with the presence of mixtures of foreign cations were suppressed.

4. Conclusions The effects of zinc, free sulphuric acid concentrations and current density on the current efficiency and morphology of electrodeposited zinc were studied. The minimum zinc concentration at which electrowinning should be stopped was determined and found to be in the range of 40 gm dmy3 with a sulphuric acid content of ) 150 gm dmy3 . Addition of gelatine, in the range of 50 mg dmy3 was found to improve the quality of electrodeposited zinc. PC and PCR techniques were examined. PC was found to be more suitable for the electrowinning of zinc than the PCR, as it improves both the current efficiency and quality of the deposit. The effect of the presence of Cu, Fe, Mn and silica in the electrolyte media was studied. Copper and iron, even at low concentrations, were found to decrease the current efficiency and worsen the quality of the electrodeposited zinc. Both of them should be removed during the purification steps to avoid their harmful effect.

Acknowledgements The authors gratefully acknowledge the considerable assistance of Mrs. N. Elhusseiny in the experimental work.

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106 w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x

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