Influence of electrolysis parameters on the silver content of cathodic copper

Hydrometallurgy 55 Ž2000. 17–33 www.elsevier.nlrlocaterhydromet

Influence of electrolysis parameters on the silver content of cathodic copper L. Burzynska ´ Department of Physical Chemistry and Electrochemistry, Faculty of Non-Ferrous Metals, UniÕersity of Mining and Metallurgy, Cracow, Poland Received 11 December 1998; accepted 13 October 1999

Abstract The effect of the following electrolysis parameters on the silver content in cathodic copper was investigated: temperature ranging from 30 to 608C; rate of electrolyte circulation ranging from 3 to 9 lrh; anode composition–silver content ranging from ca. 0.3 to ca. 1.0 wt.%; and oxygen content ranging from ca. 80 to ca. 1200 ppm. The silver content in the cathodic copper depends on temperature and its highest value has been reached at 608C. It depends also on the composition of anodes-on both silver and oxygen contents. However, it does not depend on the rate of the electrolyte circulation within the investigated range of this parameter. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrolysis parameters; Silver content; Cathodic copper

1. Introduction The silver content in anodes used in the industrial electrowinning of copper varies from a few tens to a few thousands of grams of silver per one tonne of copper. During the electrolysis, the main portion of silver remains in the anodic sludge. However, as a rule, the cathodic copper still contains up to several ppm of silver. The recovery of silver contained in copper to be refined remains an important problem of copper electrowinning. Taking into consideration the annual production of copper in Poland, the content of approximately 10 ppm of silver in the cathodic copper means the loss of a few tonnes of silver per year.

0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 7 0 - 5

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The problem of the reduction of silver content in cathodic copper is comprehensively discussed in literature w1–32x. Various theories and hypotheses are suggested to explain the mechanisms of anodic and cathodic processes. Silver can be transferred mechanically or electrochemically from the anode to the cathode during the electrolysis. To explain this fact, the following processes should be taken into consideration: Ži. reduction of silver ions on the cathode Žan electrode reaction.; Žii. deposition of metallic particles of silver on the cathode Žtransported together with circulating electrolyte.; Žiii. deposition of charged colloidal particles of silver on the cathode Žcataphoretic transport.. There exists a divergence of opinions concerning contribution of these processes to the transport of silver into the cathodic copper w1,13,19,20,22x. The influence of different parameters on the processes mentioned above shall be discussed in detail. Because of a very low concentration of silver ions in refining electrolyte ranging from 10y4 to 10y3 grdm3 Ag w14,15x, it should be expected that cathodic codeposition of silver with copper should take place within the diffusive area, i.e., within the range of limiting diffusion current. This has been confirmed in experiments w10,11,28,29x. Consequently, silver content in the cathodic copper deposited electrochemically depends on the ratio of limiting diffusion current and cathodic current density. It indicates that silver content in the cathodic copper should be proportional to the electrolyte flowrate Žfor a given geometry of the system., concentration of silver ions in the bath and temperature, and it should be inversely proportional to the cathodic current density. However, under industrial circumstances, it is observed as a rule that an increase of cathodic current density is accompanied with an increase of silver content in cathodic deposit. The copper electrowinning process is conducted under galvanostatic conditions. An increase of cathodic current density results in an increase of anodic current density and, consequently, an increase of concentration of silver ions in the bath. The anodic process is discussed below. Furthermore, as a rule, an increase of current density results in the increase of the electrolyte flowrate in order to avoid concentration polarisation. This, in turn, results in an increase of limiting diffusion current of cathodic reduction of Agq ions. The probability of an outflow of a portion of silver ions from the anode before reaching the cementation equilibrium increases. If under laboratory conditions the electrolyte flowrate is kept constant, no dependence between silver content in the deposit and the cathodic current density has been observed experimentally w1,13,19,20,30–32x. This means that the limiting diffusion current of cathodic reduction of Agq ions Žcaused by an increase of silver ions concentration in the bath. as well as the cathodic current density increase simultaneously. The ratio of these two quantities remains almost constant w2x. Application of diaphragms Ža collodium layer on the anode. causes reduction of silver content in cathodic copper accompanying an increase of applied current density w1,19,20,29x. Furthermore, the cathodic silver content is lower when diaphragms are used during the electrolysis process, than without them. These authors have drawn the conclusion that in that case, silver is transferred solely electrochemically onto the cathode. However, the diaphragm probably not only prevents mechanical entrainment of

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fine silver crystals from the anode, but also it reduces the rate of transport of Agq ions into the electrolyte and, consequently, simplifies the approach to the state of cementation equilibrium on the anode surface. Mechanical deposition of metallic silver particles on the cathode is conditioned by their transport, which depends on the geometry of electrodes and bath system, location of the electrolyte inlet and outlet, direction and the rate of electrolyte flow and the size of transported particles. There is a lack of quantitative information on the possibility of cataphoretic transport. Fischer et al. w18x carried out preliminary investigations on the application of the electro-acoustic method ŽESA, Matec. for the measurements of quantities related to the electrokinetic phenomena occurring at the phase boundary: colloidal silver particles– electrolyte ŽCuSO4 , H 2 SO4 .. The measuring method is based on the electrophoretic process used also as a base of two other methods: ‘‘Zeta-Sizer’’ ŽMalvern. and ‘‘Particle Charge Detector’’ ŽPDC, Mutec ¨ .. However, the two last methods cannot be used for measuring the properties of silver’s sols in refining electrolyte because of very high ionic strength and high conductivity of this electrolyte. This is also a limitation for application of the method suggested by these authors w18x. Fischer et al. w18x investigated the slime samples after industrial electrowinning of copper. The centrifuged slime separated from the industrial electrolyte was introduced in different amounts into the electrolyte containing 50 grdm3 Cu2q and 10 grdm3 H 2 SO4 . The authors w18x stated that the majority of colloidal silver particles are charged positively and that the presence of copper sulphate and sulphuric acid intensifies the dispersion of particles. The particles of dispersed colloids consist of 10 3 to 10 9 atoms or molecules w34x. It seems that these investigations prove significant participation of cataphoretic transport in the electrolyte with composition mentioned above. On the other hand it is doubtful to draw conclusions from the described investigations of the behaviour of colloids in refining electrolyte. These investigations have been performed in a solution containing 10 grdm3 H 2 SO4 while the refining electrolyte contains ca. 200 grdm3 H 2 SO4 . The experiments performed by these authors w18x on the samples of synthetic slime obtained in a reaction of Cuq ions with Agq ions showed that in this case silver particles smaller than 1 mm are formed. These particles are charged positively. The source of silver in the system is silver contained in anodes. Hitherto an opinion was accepted that a part of silver in copper anodes exists in a form of selenides and tellurides w4–6x. However, based upon thermodynamic calculations and structural investigations of synthetic alloys, Grejver w7x showed the existence of only Cu 2 Se and Cu 2Te even for silver content up to 1%. In Grejver’s w7x opinion the silver selenides and tellurides present in slime were formed as a result of secondary reactions occurring during the electrolysis. By comparing the results of structural investigations Želectron microscope, synthetic alloys containing up to 1% of silver. with calculated values of silver solubility in copper at low temperatures, Sedzimir w2x drew a conclusion that silver exists in anodes mainly in a form of supersaturated solid solution. Only single-phase systems occurred. ŽSilver solubility in saturated solutions of silver in copper extrapolated to lower temperatures is equal to 1.4 " 0.6 and 7.1 " 2.8 g Agrtonne at 298 and 333 K, respectively w2x..

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Grauman et al. w3x obtained similar results using X-ray diffraction analysis of synthetic alloys of copper with silver. This was a single-phase system. Since silver exists in the form of supersaturated solution, the anodic co-dissolution of silver and copper occurs. At the first stage of this process silver passes into the solution in the form of ions and then it takes part in secondary reactions w2x. Because of anodic dissolution of silver Žfrom the supersaturated solution., there appear the zones of saturated solid solution Žsolid solution of silver in copper.. The concentration of silver ions in the state of equilibrium with the saturated solution is lower than in the case of equilibrium with the supersaturated solution. Therefore, silver dissolving from the supersaturated solution cements on the surface of the saturated solid solution w2x. The reaction of cementation of silver ions with metallic copper belongs to the sequence of consecutive reactions and it could be expressed in the form of the following Eq. Ž1.:

™ Cu

Cu q 2Agq

2q

q 2Ag

Ž 1.

The second, independent consecutive reaction is the reduction of silver ions with cuprous CuŽI. ions according to the formula Ž2.:

™ Ag q Cu

Agqq Cuq

2q

Ž 2.

The secondary reactions Ž1. and Ž2. can occur on the anode or in the near-electrode layer. The cuprous CuŽI. ions necessary for the reaction Ž2. are formed w15,24x as a product of acidic dissolution of cuprous oxide Cu 2 O existing in anode used in electrowinning process — reaction Ž3.:

™ 2Cu q H O The CuŽI. ions may also appear as a result of the reaction: Cu q Cu ™ 2Cu Cu 2 O q 2Hq 2q

q

2

q

Ž 3. Ž 4.

The present work is aimed at the determination of optimal conditions of the electrolysis in order to reduce the amount of silver in the cathodic deposit. It seemed purposeful to arrange the experiments under the conditions similar to the industrial processes. Therefore, the electrolysis processes were run with vertical set-up of hanging electrodes and using the electrolyte with composition corresponding to the refining bath. The electrolyte was not doped with silver. In accordance with industrial conditions, the anodes were the only source of silver.

2. Details The investigations have been performed at constant current density of 200 Arm2 and for fixed initial composition of the electrolyte: 0.630 " 0.003 M Cu2q ions and 1.80 " 0.05 M H 2 SO4 . Anodes with various silver and oxygen contents were used in these experiments. The composition of anodes is given in Table 1. Oxygen content in the samples was determined using LECO type analyser.

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Table 1 Analytically determined silver and oxygen content in anodes Sample no.

wwt.%x Ag

wppm O 2 x

1 2 3 4 5

0.2933"0.0050 0.5224"0.0008 0.9652"0.0100 0.39 a 0.41a

130"11 81"11 108 400"26 1182"33

a Samples 4 and 5 were used in the experiments as anodes of high oxygen concentration and therefore silver content in these samples has not been determined more precisely.

The initial electrolyte did not contain silver ions. The variable parameters were: Ži. composition of anodes ŽTable 1.; Žii. temperature: 30, 45 and 60 " 0.28C; Žiii. electrolyte flowrate: 3, 6 and 9 lrh Želectrolyte volume 1 dm3 .. Direction of electrolyte flow was from the cathode to the anode; Živ. time of electrolysis. The following quantities were determined as a function of time: Ži. concentration of silver ions in the electrolyte; Žii. mass of slime remaining on the anode and mass of slime contained in the electrolyte; Žiii. anode potential, electrolysis voltage; Živ. silver content in the cathodic deposit. Furthermore, a morphology of the slime Žfrom the reaction of cementation or reduction of silver ions by means of cuprous CuŽI. ions. has been analysed using a scanning electron microscope. Samples of the electrolyte for chemical analyses were sampled by gravimetric method. Copper content was determined iodometrically while sulphuric acid concentration was measured by titration with ammonia solution using methyl orange. The concentration of silver in the electrolyte Žsilver passes into the solution as a result of dissolution of copper anodes containing silver. was determined using the atomic absorption method. A system with two electrodes was used. The surface area of the anode and the cathode was equal, which means that the anodic and cathodic current densities were equal. The active surfaces were solely the internal planes of electrodes, parallel to each other. The rear and side planes of electrodes were insulated. In this way the electric field lines were parallel. Ti-plate was used as a cathode. The surface area of the electrodes was ca. 9 cm2 . A perpendicular electrolytic tank made of high-pressure polyethylene ŽPE-LD. 14 = 11 = 11 cm was used. The total volume of the electrolyte constituted 1 dm3. Potential of the anode was measured against sulphate–mercury electrode w33x immersed in 1.80 M H 2 SO4 solution Žacid concentration was equal to that of the refining electrolyte.. The Luggin capillary tube was applied. This capillary was placed as close as possible to the anode surface, however taking care that it should not interfere significantly with electric field lines. The anode potential was measured using a digital millivoltmeter. The copper coulometer was connected in series with the measuring system in order to determine the charge passing the system. The results obtained are presented separately for cathodic and anodic processes.

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3. Results and discussion 3.1. Anodic processes The mass of silver ‘‘released’’ during the electrolysis depends on its content in anodic copper and on the quantity of charge passing the system. It can be calculated using these data. The calculations were performed assuming that only copper had dissolved. In this case the following formula Ž5. is valid: mAg s

MCu a 200F

It

Ž 5.

where mAg is the calculated mass of silver, MCu is the molar mass of copper, a is the silver content in anode w%x, F is the Faraday constant, I is the current intensity and t is the time of electrolysis. Similar calculations were performed under the assumption that both components Ži.e., supersaturated solid solution of silver in copper. dissolved into the solution. For this case, the following dependence was obtained: mAg s

MAg MCu a F 2 MAg Ž 100 y a . q MCu a

It

Ž 6.

where MAg is the molar mass of silver; other symbols as given above. Fig. 1 presents the dependence of silver mass on the amount of charge passing the system. Both quantities were converted to a unit surface. The results are related to anodic dissolution of anodes containing 0.2933% of silver at 308C. The straight line ‘‘1’’ in this graph visualises the calculated mass of silver. Independently of the assumed mechanism, Ži.e., dissolution of copper only or dissolution of two components., the

Fig. 1. Total mass of silver in slime Ždetermined experimentally. and mass of silver contained in the dissolved anodic copper as a function of charge passing through the system and the electrolyte flowrate. Both quantities: silver mass and charge amount, were converted to a unit surface. Temperature 308C, anode: 0.2933% Ag, 130 ppm O 2 . r-correlation coefficient. The equation of straight line ‘‘2’’ is: mAg rSs Ž0.0017"0.0088.q Ž0.00023"0.00004. QrS.

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straight line ‘‘1’’ exhibits almost identical slope. The slope calculated from Eq. Ž5. is 0.965 = 10y3 wmg Cy1 x while that obtained from Eq. Ž6. is 0.9672 = 10y3 wmg Cy1 x. The calculated values are given for a s 0.2933 wt.% Ag. This similarity is caused by low content of silver in anodic copper. In the case of silver content in anodes equal to 0.9652% the difference of slopes of straight lines calculated for Eqs. Ž5. and Ž6. is also very small and the values obtained were 3.1776 = 10y3 wmg Cy1 x and 3.1983 = 10y3 wmg Cy1 x for Eqs. Ž5. and Ž6., respectively. The straight line ‘‘2’’ in Fig. 1 presents the quantity of silver contained in anodic slime, determined analytically. During each experimental run, the silver content was determined in the slime taken mechanically from the anode, in the slime obtained after filtration of the electrolyte after measurements, and in the slime deposited on the bottom of the electrolytic tank. The values shown in Fig. 1 represent the total amount of silver in the slime. Fig. 2 presents the dependence of silver mass on the amount of charge for identical anode at 608C. The silver content in the slime can be expressed as a ratio of the slopes of these two straight lines: a xra t ; where a x is the slope of the straight line ‘‘2’’ for the fixed electrolyte flowrate x or for the average value, and a t is slope of the straight line ‘‘1’’. Fig. 3 illustrates the temperature dependence of the a xra t ratio for anodes containing 0.2933% Ag. It follows from this graph that silver content in slime determined analytically increases with temperature increase. No dependence was observed between silver content in slime and the electrolyte flowrate for the investigated range of this variable. The regularities described above were observed for all investigated anodes, compositions of which are given in Table 1. Fig. 4 shows the temperature dependence of the a xra t ratio for anodes containing 0.9652% Ag. It follows from the comparison of Figs. 3 and 4 that silver content in slime depends probably on the anode composition. The type of this dependence is different for anodes containing 0.2933 wt.% Ag and 130 ppm oxygen than for anodes containing

Fig. 2. Total mass of silver in slime Ždetermined experimentally. and mass of silver contained in the dissolved anodic copper as a function of charge passing through the system and the electrolyte flowrate. Both quantities: silver mass and charge amount, were converted to a unit surface. Temperature 608C, anode: 0.2933% Ag, 130 ppm O 2 . r s correlation coefficient. The equation of straight line ‘‘2’’ is: mAg rSs Ž0.0082"0.0146.q Ž0.00046"0.00007. QrS.

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Fig. 3. Analytically determined share of silver Žslime. contained in the dissolved part of the anode as a function of temperature and electrolyte flowrate; a x — slope of the straight line ‘‘2’’ for a given electrolyte flowrate Ž x . or for the average value; a t — slope of the straight line ‘‘1’’. The straight line ‘‘1’’ and ‘‘2’’ are presented in Figs. 1 and 2. Anode: 0.2933% Ag, 130 ppm O 2 .

0.9652 wt.% Ag, and 108 ppm oxygen. In the case of higher silver content and lower oxygen content in anodes, higher silver content at 608C was found analytically in slime. However, there is no simple dependence between silver content in the slime and temperature or anode composition Žsee Fig. 5.. Fig. 5 illustrates the changes of the a xra t ratio for anodes containing different amounts of oxygen. The results are presented for 458C. An increase of oxygen content in anodes causes significant reduction of silver content in the slime determined analytically. It is a surprising result because, according to the Eq. Ž2., one should expect

Fig. 4. Analytically determined share of silver Žslime. contained in the dissolved part of the anode as a function of temperature and electrolyte flowrate; a x — slope of the straight line determined experimentally; a t — slope of the straight line resulting from Eqs. Ž4. and Ž5.. Anode: 0.9652% Ag, 108 ppm O 2 .

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Fig. 5. The a x r a t ratio as a function of oxygen content in anodes. Temperature: 458C. The numbers near the measurement points refer to the silver composition in anodes.

favourable conditions for transfer of silver into the slime. Thus, a hypothesis that silver creates mainly colloidal particles was not confirmed. Furthermore, the silver content in cathodic copper is lower when high-oxygen anodes are used ŽFig. 9., i.e., the contribution from the cataphoretic or mechanical transfer of silver did not increase. The three different slime fractions have been investigated. Within the experimental period of time, the slime adhering to the anode constituted the main portion. On the other hand, the mass of slime circulating in the electrolyte was often comparable to the mass of slime forming a deposit on the bottom of the tank. The morphology of the slime taken out from the anode was analysed by means of a scanning electron microscope. This slime was obtained during the run at 458C with current density equal to 200 Arm2 , using anodes containing 0.9652% Ag. The electrolysis lasted 9 h. Fig. 6 presents a microstructure of the anodic slime. It constitutes an agglutination of large grains with highly developed surface. The X-ray local analysis proved that only silver is present in this slime.

Fig. 6. Anode slime microstructure.

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Fig. 7. Dependence between oxygen and silver content in anodes calculated for the boundary case, i.e., when all the Cuq ions Žfrom reaction Ž3. react with Agq ions according to the reaction Ž2...

The electrode potential of the anode Žmeasured against the sulphate-mercury electrode. did not depend on the anode composition, temperature, electrolyte flowrate, time of electrolysis Žwithin the scatter of measurement results. and they reached yŽ305 " 10. mV. 1 The potential of sulphate–mercury electrode in 1.80 M solution of H 2 SO4 is 656 mV against normal hydrogen electrode. The electrolysis voltage decreases with an increase of temperature as a result of rising conductivity of the electrolyte. At 308C, the electrolysis voltage was 0.20 " 0.02 V and at 608C it dropped to 0.12 " 0.02 V. Anodes with various oxygen contents were used in the experiments. Oxygen exists in anodes in a form of Cu 2 O, which dissolves under zero-current conditions according to the reaction with acid, see Eq. Ž3.. In a boundary case — if the total amount of Cuq ions created in this process would react with Agq ions according to Eq. Ž2. — the following dependence between metallic silver content and oxygen content in anodes is valid:% Ag s 1.3485 = 10y3 ppm O 2 . Such a case is presented in Fig. 7. It follows from this graph that if the anodes contain, e.g., ca. 0.3% Ag then 250 ppm O 2 in the anode will be, theoretically, sufficient for the precipitation of metallic silver. It should be underlined, that Cuq ions may take part in three different consecutive reactions: Eq. Ž2.;

1

A current density of 200 Arm2 did not cause changes of stationary potentials of copper electrode; also there was no effect of ca. 1 wt.% content of silver on the anode potential.

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reaction of anodic oxidation of CuŽI. to CuŽII. and reaction Ž4. — in this case the consecutive reaction can be a cementation of silver ions with metallic copper. 3.2. Cathodic processes As an example, Fig. 8 presents the silver content in cathodic copper ppm wAgx as a function of the charge passing the unit surface QrS wC cmy2 x and the electrolyte flowrate. These results were obtained for anodes containing 0.5224 wt.% Ag, 81 ppm O 2 at 458C. It follows from Fig. 8 that silver content in cathodic copper does not depend on the amount of charge passing the system. It means that it does not depend also on the duration of electrolysis performed under galvanostatic conditions. Furthermore, no dependence between silver content in cathodic copper and the electrolyte flowrate has been observed. The silver content in cathodic copper constitutes 7.1 " 3.4 ppm. For all the analysed anodes with a given composition, neither dependence of cathodic silver content on the amount of charge passing the system nor on the electrolyte flowrate was observed within the investigated range of these variables. Fig. 9 presents the silver content in cathodic copper as a function of temperature and the amount of silver in anodes Žwt.%.. It can be seen that an increase of silver content in anodes from ca. 0.3 up to ca. 1.0 wt.% results in almost twice higher silver content in cathodes at 608C and more than twice higher silver content at 308C. However, it seems that temperature is the crucial parameter for the silver content in the cathode. At 608C, the silver content in cathodic copper is higher than 10 ppm Žca. 0.3 wt.% Ag in the anode. and it exceeds more than 20 ppm Žca. 1.0 wt.% Ag in the anode.. At 308C and 458C, lower values of silver

Fig. 8. Analytically determined silver content in the cathodic copper wppm Agx as a function of charge passing through unit surface QrS wC cmy2 x and electrolyte flowrate. Temperature 458C. Anode: 0.5224% Ag, 81 ppm O2 .

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Fig. 9. Silver content in the cathodic copper wppm Agx as a function of Ag content w%x in anodes and a function of temperature. The dark points are related to the anodes with high oxygen content.

content in the cathode are observed. In the case of anodes containing ca. 0.3 wt.% Ag decrease of electrolysis temperature to 458C would result in reduction of silver content in cathodic copper to ca. 3 " 1 ppm. Fig. 10 presents the dependence of silver content in cathodic copper on oxygen content in anodes. If oxygen content in anodes increases, a decrease of cathodic silver content is observed.

Fig. 10. Silver content in the cathodic copper wppm Agx as a function of oxygen content in anodes wppm O 2 x. Temperature 458C.

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3.3. Analytical concentration of Ag q ions in the electrolyte Concentration of silver ions in the electrolyte is presented in Fig. 11 as a function of silver content in anodic copper, temperature and the electrolyte flowrate. The obtained experimental results are encumbered with a significant error Žstandard deviation.; however, certain conclusions can be drawn. Ži. In the case of anodes containing ca. 1% of silver, the concentration of silver ions in the electrolyte practically does not depend on the electrolyte flowrate, independently on the temperature. Žii. At 308C in the case of anodes containing 0.2 to ca. 0.5% of silver lower flowrates correspond to the lower concentrations of silver ions in the electrolyte. It seems that these conditions are favourable for reaching the state of cementation equilibrium. Žiii. At elevated temperatures also for the anodes containing smaller amounts of silver Žca. 0.2 to ca. 0.5% Ag., the concentration of Agq ions in the electrolyte does not depend on the electrolyte flowrate. Živ. At 458C a minimum of the concentration of Agq ions in the electrolyte is observed. The rate of electrolyte circulation has influence — first of all — on the colloidal transport of silver and on the value of Agq limiting diffusion current. The black circles in Fig. 11 represent the results obtained for anodes with high oxygen content. Within the investigated range of the variables, i.e., temperature, anode composition and electrolyte circulation rate, the concentration of silver ions in the electrolyte did not depend on the time of electrolysis process.

Fig. 11. Concentration of silver ions in the electrolyte wmg Agr1000 g solutionx as a function of temperature, electrolyte flowrate and anode type. The dark points are related to the measurements performed for anodes with higher oxygen content.

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When the amount of silver determined analytically in the slime, in the electrolyte and in the cathodes is compared with that resulting from the total amount of charge passing the system, it can be stated that the silver balance does not fit in. Silver balance is discussed below. The amount of silver determined analytically in the slime is lower at lower temperatures ŽFigs. 3 and 4.. As a preliminary hypothesis it has been assumed that when temperature increases, the amount of silver in its colloidal form decreases in the electrolyte, and that the atomic absorption method makes possible determination of silver in ionic form, only. Therefore, analytical investigations have been performed in order to check the possibility of the occurrence of chemical as well as ionic interference or matrix effects in the Ag–Agq–CuSO4 –H 2 SO4 system. The results of the following investigations were compared: Ža. standard samples of silver in nitric acid solution Ž1:1.; Žb. standard samples of silver diluted at different ratios with refining electrolyte Žintroduction of CuSO4 , and H 2 SO4 into the samples.: Žc. dilution of investigated samples with distilled water or HNO 3 Ž1:1.; Žd. the samples of the electrolyte used in analytical measurements were oxidised with H 2 O 2 of different concentrations, then evaporated and diluted with H 2 O or HNO 3 ; Že. the samples of the electrolyte were dried, then HNO 3 was added, samples were dried again and again HNO 3 was added. These analyses were performed using parallel samples Židentical.. The results of these investigations were not dependent on the procedure followed. It refuted the hypothesis that not all amount of silver in the electrolyte was determined — i.e., both ionic and colloidal silver was determined by atomic absorption. As already mentioned, the shares of three different slime fraction have been investigated. One of them was the slime adhering to the anode. It was not possible to separate it from the surface of the electrode quantitatively. After taking the slime off, dark silver-enriched spots became visible on the anode surface. It has been stated that the slime-to-anode adherence weakens at elevated temperatures. Taking the slime off mechanically at higher temperatures was much easier; however, also in this case, the slime could not be removed quantitatively. A series of investigations have been performed in order to determine silver content in slime remaining on the anode surface. Measurements were carried out using anodes containing 0.2933 wt.% Ag at 30 and 608C for the electrolyte flowrate 3 lrh. All the hitherto analyses have been performed and — additionally — a thin layer has been cut off from the anode surface. In this layer copper content as well as the total silver content resulting from the initial silver content in the anode material, plus silver enriching the surface were determined. Once these values are known from experimental measurements, a silver balance can be formulated which takes into account: the silver content in slime Žthree fractions., the cathodic silver content, the silver content in the electrolyte, and the amount of silver enriching the anode surface Žthe cut layer.. The total silver content determined analytically can be expressed as follows: ÝmAg s mAg ,slime q mAg ,cath q mAg ,electr q mUAg ,slime

Ž 7.

where mAg,slime is the mass of silver in three slime fractions defined by the straight line equation as a function of Q, the charge passing the system, mAg,cath is the mass of silver

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contained in the cathode, also being a function of the charge, Q, mAg,electr is the mass of silver in the electrolyte, and mUAg,slime is the mass of silver in the cut layer of the anode, related to silver enriching the anode surface. For 308C, Eq. Ž7. can be expressed as follows: ÝmAg s Ž 0.0183 " 0.0671 . q Ž 0.00023 " 0.00003 . Q q q 3.29 = 10y7 Ž ppm Ag . Q q Ž 0.12 " 0.09 . relectr q Ž 2.4958 " 0.0560 . Ž 8. where relectr is the electrolyte density, relectr s 1.197 grcm3, Q is the amount of charge, Q s 3870 C, wppm Agx is the silver content in the cathode, wppm Agx s 4.1 " 2.5. The silver mass calculated from Eq. Ž8. amounts to 3.5530 " 0.3502 mg Ag, while that one calculated from Eq. Ž5. is equal to 3.737 mg. This means that the silver balance is correct. The analytically determined portion of silver constitutes 95.1 " 9.3%. For 608C and an identical composition of the anode, the following dependence was obtained: ÝmAg s Ž 0.1042 " 0.1131 . q Ž 0.00044 " 0.00005 . Q q q 3.29 = 10y7 Ž 13 " 4 . Q q Ž 0.12 " 0.05 . relectr q Ž 1.6127 " 0.0500 .

Ž 9. wppm Agx is the silver content in the cathode; wppm Agx s 13.0 " 4.0. The silver mass calculated from Eq. Ž9. for Q s 3890 C is equal to 3.5887 " 0.4225 mg Ag. It constitutes 96.0 " 11.3% of silver resulting from this amount of charge passing the system.

4. Summary Considering the laboratory investigations performed with synthetic anodes doped with silver and oxygen, the following conclusions can be drawn. Ža. Co-deposition of copper and silver occurs on the cathode, Žb. An increase of concentration of silver ions in the electrolyte is accompanied with an increase of silver content in cathode deposit. It proves that electrochemical deposition of silver within the diffusive range occurs. Žc. No influence of the electrolyte flowrate Žin the range from 3 to 9 lrh. on silver content in cathodic copper was observed for a given system geometry and the flow direction. It seems that a certain combined effect can be observed in the experiments but the problem itself is much more complicated. According to the conclusion Žb. it should rather be expected that the amount of silver deposited electrochemically in cathodic copper is proportional to the electrolyte flowrate Ždiffusive range.. Furthermore, it should make the transport of silver particles easier, including the possibility of broken-out of fine slime particles from the anode, i.e., the amount of silver mechanically incorporated in the cathode should increase. It means that these two independent processes should sum. Based upon the experimental results, another model of mechanical incorpo-

L. Burzynskar Hydrometallurgy 55 (2000) 17–33 ´

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ration of silver particles in the cathode can be suggested. This process may occur if a silver particle remains on the cathode for a certain period of time and the growing copper layer incorporates it. If the electrolyte flowrate is high, the silver particle will be pulled out from the cathode prior to the possible incorporation. Therefore, the amount of silver deposited mechanically should decrease with increasing flowrate. In turn, the amount of silver deposited electrochemically on the cathode will increase if the electrolyte flow is intensified. The superposition of these two effects is the reason why the silver content in cathodic copper remains constant independent of the flowrate. Žd. At elevated temperatures, the concentration of silver ions in the electrolyte increases. Že. At elevated temperatures an increase of silver content in the cathodic copper is observed. If the temperature does not exceed 458C, it is possible to obtain the cathodic copper containing ca. 3 ppm Ag. Žf. The co-dissolution of silver and copper takes place in the anodic process. Žg. The following reactions or consecutive processes occur on the anode-electrolyte phase boundary: Ži. reaction of cementation of silver ions with metallic copper, Žii. reaction of reduction of silver ions by Cuq ions, Žiii. anodic oxidation of Cuq ions to Cu2q ions, Živ. transport of Cuq ions into the bulk of the electrolyte. The reactions Ži. and Žii. are responsible for anode slime formation. Žh. At lower temperatures, the enrichment of the surface of anodes in silver becomes more and more intensified.

Acknowledgements I would like to express many thanks to the ‘‘Polish Copper’’ Copper Mining and Metallurgy Group, Joint Stock for financial support of these investigations.

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