Continuous electrowinning of zinc
?.
Hydrometallurgy54200091–106
www.elsevier.nl r locate r hydromet Continuous electrowinning of zinc
A.E.Saba),A.E.Elsherief
Electro Metallurgy Lab.Central Metallurgical R&D Institute,P.O.Box87,Helwan,Cairo,Egypt
Received1March1999;accepted1August1999
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 than95%were obtained from acid sulphate solutions electrolysed at45mA cm y2and258C for DC and PC techniques.Electrowin-
?y3.
ning of zinc from relatively concentrated solutions160g dm could be achieved,successively, with acceptable current efficiencies down to a concentration of40g dm y3.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 https://www.sodocs.net/doc/1f16273974.html,anic 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.q2000Elsevier 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 ZnSiO,hemimorphite ZnSi O OH P H O and
42722?.
smithsonite ZnCO.This ore was leached by sulphuric acid in a two-step technique 3
w x
under controlled temperature and pH1–3.The coagulated silica produced by this )Corresponding author.Fax:q20-202-5010639;e-mail:rucmrdi@https://www.sodocs.net/doc/1f16273974.html,
0304-386X r00r$-see front matter q2000Elsevier Science B.V.All rights reserved.
?.
PII:S0304-386X9900061-4
()
92
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106
treatment was filtered off.The leached liquor obtained contains,besides zinc,different
amounts of Cu,Fe,Mn,Mg,Co and SiO.Many of these cations were removed by the
2
w x
normal purification techniques3but traces of Fe,Cu,Mn and SiO are still present.
2
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 w x?. hydrogen is evolved4–8.They affect negatively the cathodic current efficiency C.E.
w x w x
and the quality of the electro-deposited zinc9.Winand10showed 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.
w x
Das et al.11,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 C.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
w x high current densities.Many types of organic additives have been investigated6,12–17 by several authors,to improve the cathode quality and to prevent the acid mist formation w x
12,16,18.Some of these additives were used as leveling agents.Appropriate amounts w x
of these additives6,8,14,19were 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 PC and periodic current reversal PCR are two techniques developed to permit the possibility of increasing the current density while decreasing the concentration of the metal-bearing solution during electrowinning,with-
w x
out affecting the cathode quality20.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 s i=T r T q T X
eff
for PC;and
i s i=T y T X r T q T X
eff
for PCRwhere:i s current density,A m y2;i s effective current density,A m y2;
eff
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 w x
investigated20.
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.
()
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–10693
2.Experimental
The electrolysis cell consisted of a Pyrex500ml beaker with a Perspex cover holding
w x2
the electrodes and powder feeder3,20.Aluminum strips,40cm surface area,were
used as cathodes.Two platinum sheets,measuring4=5cm 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 elec-trolyte.All experiments were performed at25"18C at constant stirring.The electrolyte used,250ml,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,25mg dm y3,copper,0.2mg dm y3,manganese,less than0.1mg dm y3and 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 Si OH were added to the purified solution to get the desired composi-
2
tion.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 Zn OH through the feeder,at intervals of time,so that the concentrations of 2
zinc and sulphuric acid in the electrowinning cell were kept constant.
w x
An electronic pulse-control unit was used20which provides pulse duration from1 to180s for‘‘on’’and‘‘off’’or‘‘reversal’’intervals.
Cyclic voltammetry measurements were carried out by the aid of a Wenking Potentioscan POS73.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 of0.049cm2.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
?X.
actually deposited by passing a definite quantity of electricity W to the theoretical weight that would be deposited by that quantity of electricity according to Faraday’s law ?.
W:
W s I=t=32.69r26.8A h
where I s electric current passed in amperes;T s time in hours;32.69s equivalent weight of zinc;26.8s quantity of electricity needed to produce an electro-chemical change of1g-equivalent of a substance.
If W X is the weight of the metal actually deposited at the cathode,then
?.
Current efficiency s W X r W=100.
The surface morphology and microstructure of the electro-deposited samples were
?.
examined by scanning electron microscope SEM.
()
94
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106
3.Results and discussions
3.1.Cyclic?oltammetry
Cyclic voltammetry experiments of zinc electro-deposition from120g dm y3Zn and
50g dm y3H SO solution,on aluminum substrate,were carried out.The cathodic 24
potential was varied in the range from y900to y1700mV at a scan rate of20mV s y1. 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 y1435mV, 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
w x
nucleation hysteresis loop21.
Cyclic voltammogram due to current sweep,at a scan rate of16m A s y1,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
?2. Fig.1.Potential–sweep cyclic voltammograms for zinc deposition and stripping on Al electrode0.049cm, y3y3?y1.
from120g dm Zn q50g dm H SO solution at258C scan rate20mV s.
24
()
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–10695
?2. Fig.2.Current–sweep cyclic voltammograms for zinc deposition and stripping on Al electrode0.049cm, y3y3?y1.
from120g dm Zn q50g dm H SO solution at258C scan rate16mV s.
24
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.3shows typical results obtained by the potentiostatic method.Three different curves were obtained by maintaining the cathode potential constant at y1400,y1500 and y1600mV.The results obtained show that as long as the response current is
smaller than the limiting current,i,its value does not change with time.A continuous
l
steady value was obtained at y1400mV.A gradual increase in current was recorded at y1500mV,at the first stage of electrolysis,and then semi-steady state was reached.A sharp increase in current was recorded at y1600mV,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.
?2.y3 Fig.3.Current–time curves for zinc deposition and stripping on Al electrode0.049cm,from120g dm y3?.?.?.
Zn q50g dm H SO solution at258C,at constant potential of:1y1400,2y1500and3y1600mV.
24
()
96
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106
3.3.Gal?anostatic
The cathodic polarization of zinc electrowinning on aluminum substrate when being subjected to different polarizing currents,ranging from30to60mA cm y2,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,at30,45and60mA cm y2,curves I,II and III were obtained, respectively,representing continuous potential plateaus at y1590,y1700and y1720 mV,respectively.With non-stirred bath,at45mA cm y2,the recorded potential plateau is found at y1710mV and lasts after65s,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 from30to60mA cm y2,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 for3h.The results obtained are summarised in Table1.
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 of45mA cm y2.The current
?.?.?.y2 Fig.4.Potential–time curves for zinc deposition on Al electrode at:130,245,360mA cm with ?.y2
stirred solution and445mA cm with non-stirred solution.
()
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–10697 Table1
Current density,Efficiency Bath composition
y2y3y3
mA cm%g dm Zn g dm H SO
24
3094.516040
4596.116040
6097.216040
3095.812050
4596.612050
6096.912050
3096.36590
4597.26590
6097.06590
3092.350150
4592.850150
6092.150150
3083.14080
4588.34080
6087.54080
3061.540270
4563.240270
6064.240270
efficiency decreased dramatically when zinc content in the electrolyte decrease to less y3?y3.
than40g dm,and especially when sulphuric acid increases)80g dm in these
experiments.
A long term experiment,using a fixed volume and one electrolyte,with a starting
composition of120g dm y3zinc and50g dm y3H SO,was performed to determine
24
the minimum zinc concentration solution suitable for electrowinning,with acceptable
current efficiency.The electrolyte composition,Zn and H SO,were analysed and
24
determined,at different intervals,during the experiment.The results obtained are summarised in Table2.
This experiment shows that continuous removal of zinc from the electrolyte,down to around40g dm y3,were achieved with current efficiency of more than85%.Beyond this concentration a very low current efficiency was obtained.Accordingly,solutions
?y3.
with this range of zinc concentration-40g dm should be transferred to the leaching vessel,to make use of its high acid content,for further dissolution of the ore.
Table2
Current density,Period,Efficiency Bath composition
y2y3y3
mA cm hour%Zn g dm H SO g dm
24
45396.6start12050
45493.4end of experiment99.298.4
45486.3end of experiment69.2140.4
45353.1end of experiment41.2181.6
()
98
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106
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 reaches40g dm y3.After this the concentration decays near exponentially according to the well-known
w x
expression for a reactor with a diffusion-limited reaction22.
3.4.1.Morphological examinations
The surface morphology of the deposits electrowon from different concentrated electrolytes,at a current density of45mA cm y2,were inspected by SEM.These pictures show that zinc occurs mainly in acicular crystalline form parallel to the
?y3y3. substrate.At moderate concentrations160g dm Zn and40g dm H SO mossy
24 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 to60mA cm y1,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.
w x The above mentioned morphology can be explained according to Winand10who 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 ?y2.
experiment c.d.s45mA cm.
()
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–10699
?.y3y3
Fig.6.Scanning electron micrographs of zinc deposits from:a40g dm Zn q80g dm H SO at45mA
24 y2?.y3y3y2
cm;b50g dm Zn q150g dm H SO at60mA cm.=1000.
24
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 J.He
dl showed that the lower the J,the higher the inhibition intensity and the smaller the
w x
crystals deposited.He also showed13that,with high concentrated electrolytes,the ?.
value of J r c where c is the concentration is low,and the crystals formed are‘‘FI’’?.?.
field oriented isolated crystals and‘‘BR’’oriented repeated crystals.With the
()
100
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106
decrease of zinc concentration,J r c 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 from50mg dm y3to2g dm y3were added to different
electrolytes with zinc concentration ranging from160–120and50g dm y3H SO.The
24 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 of50mg dm y3.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
?.w x
amino acids linked by peptide chain–CO–NH–15.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 w x
authors3by cyclic voltammetry using acidified zinc sulphate electrolytes and in the presence of different ratios of gelatine ranging from50to2000mg dm y3.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 at50mg dm y3 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 from50g Zn q150g H SO q50mg
24 gelatine I y1at45mA cm y2,X s1000.
()
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106101
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
w x
As gelatine is reported13to have no influence on preventing the nodulation growth
w x of electrodeposits,thiourea,which has been reported to decrease nodular growth14, was added in addition to gelatine in a concentration of0.01g dm y3.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 to19h under 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 electrode-posits,especially at high current densities,these two techniques were tested with the electrodeposition of zinc.
A time span of50s forward current and5s 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.,65s,Fig.4.
Galvanostatic electrowinning experiments were studied using the PC and PCR techniques.The potential-time relationship obtained,on applying45mA cm y2for the
?.?.
forward period,are given in Fig.8a and b for the PC and PRC,respectively.
?.
With PC technique,Fig.8curve a,on passing the current the cathodic polarisation is increased to a sufficiently negative value forming a plateau at about y1720mV.On switching off the current,the potential rapidly reaches the Zn r Zn2q equilibrium value,
?.
y1480mV,determined earlier Fig.1.It was found that the5s off period is sufficient
?2.y3
Fig.8.Potential–time curves for zinc deposited on Al electrode0.049cm,from120g dm Zn q50g y3y2?.?.
dm H SO at45mA cm,with pulse cycles of50:5s.a PC and b PCR.
24
()
102
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106
to restore the equilibrium potential of zinc.By repeating the above cycle,semi-steady cathodic potential of y1720mV was reached in subsequent pulses.
?.
Results obtained with PCR technique,Fig.8curve b,show that when the polarity of the cathode was reversed,for5s the potential rapidly passed the crossover potential ?.
y1480mV and reached y1250mV.Cathodic polarisation values remained almost constant,;–1720mV,with repeated cycles.
The two techniques were then examined for both the current efficiency and precipi-tate morphology.Experiments were conducted for3h.The results obtained with different bath compositions are summarised in Table3.
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 condi-tions 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 Table4.
The table shows similar general trend to that obtained with DC experiments,Table2. Solutions with;65g Zn dm y3and high acid concentrations of about130g dm y3, should not be subjected to further electrowinning,otherwise very low current efficien-cies 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
Table3
Current density,Technique Period,Efficiency Bath composition
y2y3y3
mA cm hour%g dm Zn g dm H SO
24
60DC396.016040
60PC398.75
60PCR381.27
45DC396.612050
45PC397.0
60DC363.240270
60PC372.8
60PCR348.14
()
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106103 Table4
Current density,Period,Current Starting bath composition
y2y3y3
mA cm hour efficiency%g dm Zn g dm H SO
24 45398.7120.050
494.496.384.9
486.565.7129.2
455.238.0170.0
a
E20.4196.0
a E s End of experiment.
HSO y and SO2y anions are strongly adsorbed at the cathode surface during the‘‘off 44
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 w x
14,20.
3.4.5.Effect of foreign cations
3.4.5.1.Effect of copper.Three different copper concentrations namely6.2,12.5and20 mg dm y3Cu were made up in a solution containing120g dm y3zinc and50g dm y3
H SO in order to determine their effect on the efficiency and quality of electro-de-
24
posited zinc.Electrolysis was performed for4h,at a current density of45mA cm y2 and at258C.
The current efficiencies were96%,91%and88%,respectively.Cathodic polarisation
w x
of zinc in the presence of copper was found by the authors3to 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 from50to150mg Fe dm y3as were w x
studied earlier3,show that the cathodic current efficiency of zinc deposition decreases with the increase of the iron concentration in the electrolyte.With50mg dm y3Fe,the current efficiency obtained was82%,under the same conditions applied with copper additions.A more dramatic decrease in efficiency was obtained with150mg dm y3Fe, where an efficiency of53%was obtained.This decrease in efficiency was related to the reverse redox system of Fe2q r Fe3q 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-de-posited zinc.Coarse crystalline deposits of large separate polyhedral needles and
()
104
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106
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,0.9and1.8g
y3w x
dm Mn,caused little increase in the polarisation of zinc in sulphate solutions3.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 1.5–3g dm y3,was reported to be required in the electrolyte to minimise the lead anode
w x
corrosion8.
Colloidal silica,in the proportions of33to165mg dm y3SiO,was added and found
2
w x
to give little decrease in cathodic polarisation3.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 electrode-posited.Zinc platelets with random orientations were clearly observed.
The presence of33mg dm y3silica gave compact,smooth and round edges of the electro-zinc precipitate obtained.
3.4.5.4.Effects of mixture of foreign cations.A mixture containing10mg Cu,1.8g Mn and165mg dm y3silica 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 from50g dm y3Zn q150g dm y3H SO q40mg dm y3Fe at80mA cm y2.=1000.
24
()
A.E.Saba,A.E.Elsherief r Hydrometallurgy54200091–106105
?.
decreasing effect on the current efficiency82.5%.The electrodeposit obtained was full of cavities and holes.Small fine-grained,scattered nodules and spongy dendrites were also found.
Addition of gelatine,50mg dm y3,improves the current efficiency of the electrode-?.
posited zinc88.3%.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 of40gm dm y3with a sulphuric acid content of)150gm dm y3. Addition of gelatine,in the range of50mg dm y3was 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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