Mineralogical Characteristics of Copper Electrorefining Anode Slime

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Mineralogical Characteristics of Copper Electrorefining Anode Slime and Its Leached Residues Jhumki Hait,† R. K. Jana,*,† and S. K. Sanyal‡ National Metallurgical Laboratory, Jamshedpur-831007, India, and Chemical Engineering Department, Jadavpur University, Kolkata-700032, India

A new approach for the treatment of copper electrorefining anode slime based on hydrometallurgical route involving sulfuric acid leaching with additives such as manganese dioxide and sodium chloride has been developed at National Metallurgical Laboratory (NML), Jamshedpur, India. Mineralogical characterization of the anode slime and the residues after various leaching treatments, carried out by XRD, SEM, and EDX studies, are reported in this paper. The XRD studies of the untreated anode slime sample revealed the presence of CuSO4‚5H2O, NiO, CuSe, BaSO4, and (Cu0.2Ni0.8)O phases. NiO crystals and selenide rings were found in the SEM studies. After sulfuric acid leaching, the residues under X-ray diffraction analysis showed the absence of the (Cu0.2Ni0.8)O phase. When MnO2 was added during sulfuric acid leaching, the new phases MnSe and CuSeO3‚2H2O were found in XRD studies of the residue. The general view of the residue under SEM revealed a number of porous structures favoring the ash-diffusion-controlled reaction in leaching. Leaching with both MnO2 and NaCl in the sulfuric acid medium resulted in precipitated AgCl in the residue, as observed by XRD studies. The porous AgCl mass observed under SEM confirmed the ash-diffusion-controlled nature of the leaching reaction. Introduction Anode slime coming from the electrorefining of copper and generated at the bottom of the electrorefining cell contains valuable elements such as Cu, Ni, Se, Te, Ag, Au, and platinum group metals. Several processes to recover these metals based on pyro-, pyrohydro-, hydro-, and hydropyrometallurgical routes have been reported in the literature.1-5 All of these processes have been developed depending on the composition and mineralogy of the anode slime. The present investigation is based on a new hydrometallurgical process, described in detail elsewhere.6 This process involves leaching of anode slime in a sulfuric acid medium in the presence of additives such as MnO2 and NaCl. Sulfuric acid was selected as the leaching medium because it is a good lixiviant and is widely used in various industries. To increase the recovery of metals from the anode slime, MnO2 was added to the leachant to act as an oxidizing agent. Further, NaCl was also added to the system so that chlorine gas, which is also a powerful oxidizing agent, generated in situ in the reaction of MnO2 and NaCl with sulfuric acid, could improve the recovery of metals. In this paper, the mineralogical characteristics of the untreated anode slime and of the residues after various leaching treatments are investigated to elucidate the complex leaching behavior of anode slime. Studies made using the XRD and SEM-EDX techniques indicate how the different phases are formed during leaching. Prior to the presentation of these results, a literature review on the characteristics of anode slimes and their leached residue is included in this paper to comprehend the subject better. * To whom correspondence should be addressed. Address: Non-ferrous Process Division, National Metallurgical Laboratory, Jamshedpur-831007, India. Fax: 091-0657-2270527. Tel.: 091-0657-2271806. E-mail: [email protected]. † National Metallurgical Laboratory. ‡ Jadavpur University.

Characteristics of Anode Slime. The particles in the copper electrorefining slime originating from the fire refining of blister copper have a heterogeneous chemical composition. They are three-dimensional and have smooth surfaces, round edges (many of them have a spherical shape), and a crystalline structure, which favor their rapid deposition on the bottom of the cell.7 The slime behavioral phenomena that occur during the electrorefining process are influenced by variables such as the chemical composition of the anode (especially lead) and the current density.8 The slimes of the refining cell are agglomerated into phases such as CuSO4‚5H2O and copper-nickel sulfates that persist despite the anode slime washing steps employed.9,10 Because of their fine-grained, heterogeneous, soft, and often amorphous nature, hydrometallurgical byproducts such as copper refinery slimes are difficult to characterize.11 Despite the difficulties, a number of references12-21 exists for the determination of a large number of mineralogical phases in the anode slime. Characteristics of Leached Residues of Anode Slime. Mineralogical characterization of leached residues of anode slime was extensively carried out by Chen and Dutrizac.22 In H2SO4-leached residues, PbSO4, CuSe, and Ag2Se were found to be present as major phases. The residue also contained significant amounts of CuSeO3‚2H2O formed as a result of the oxidation of selenium during leaching.22 In the leaching of anode slime in 4 M NaCl, precipitation of an oxidate phase containing major amounts of copper with minor amounts of Pb, Ag, SO4, and Cl was detected.22 The XRD study of the NaCl-leached residue also showed the presence of a Pb-Se-Cl phase formed by the dissolution of PbSO4 in the raw anode slime. In the case of the leaching of anode slime in 30% acetic acid, major amounts of PbSO4, CuSe, and Ag2Se with varying amounts of CuSeO3‚2H2O phases were observed in the residue. The X-ray diffraction analysis of the residue

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Figure 1. XRD pattern of untreated anode slime.

from the ammonium acetate leaching of anode slime indicated the presence of significant amounts of PbSO4 along with major quantities of PbSeO3, CuSe, and Ag2Se. Reaction of the raw anode slimes in 2 M Na2CO3 solution converted all of the PbSO4 present in the slime to complex carbonates. The X-ray diffraction analysis of the residue revealed the presence of major amounts of NaPb2(CO3)2(OH), together with minor quantities of Ag2Se, AgCuSe, and CuSe. The difference in the morphology of NaPb2(CO3)2(OH) crystals and PbSO4 in the raw slime indicated very rapid dissolution of PbSO4 in the Na2CO3 solution and immediate precipitation as mats of NaPb2(CO3)2(OH).23 Experimental Section The anode slime used for the experiments was obtained from the copper electrorefining plant of the Indian Copper Complex, Ghatsila, India. Representative samples of the anode slime were used for leaching experiments and characterization studies. The XRD analyses of the anode slime and the leached residue were carried out on a Siemens D-500 X-ray diffractometer using Cu KR radiation. The scanning speed was maintained at 2θ 2°/min. The different phases were identified from the XRD patterns with the help of JCPDS cards. To identify the various phases and their morphologies, a scanning electron microscope (model JEOL-840 A) equipped with an energy-dispersive X-ray analyzer (EDX-Kevex) was used. The powder slime was investigated by imaging both secondary electrons and backscattered electrons. The leached residues of anode slime were produced during leaching as described earlier.6 Results and Discusion Mineralogical Characterization. (1) Characteristics of Untreated Anode Slime. The anode slime sample used for the experiments contained 12.29% Cu, 36.76% Ni, 10.5% Se, 3.38% Te, 1.54% Ag, and 0.1% Au. The XRD pattern of the sample (Figure 1) indicated the presence of significant amounts of copper sulfate

Figure 2. Secondary electron image showing the general structure of the untreated anode slime (loose powder mount): (1) NiO, (2) selenide, (3) PbSO4.

Figure 3. Backscattered electron micrograph showing a selenide particle in the untreated anode slime (polished section mount): (1) selenide, (2) NiO.

(CuSO4‚5H2O), nickel oxide (NiO), copper selenide (CuSe), and barium sulfate (BaSO4) phases, in addition to a variety of other minor phases in the slime. X-ray diffraction analysis also indicated the presence of (Cu0.2Ni0.8)O in the untreated anode slime, which is perhaps the solid solution of copper in the NiO phase. SEM-EDX analysis of the as-received anode slime indicated the presence of various phases in a variety of morphologies. Figure 2 presents the secondary electron image of the untreated anode slime.

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aggregates, a few were found to be closely associated with the selenides. A particle of PbSO4 was also found to be embedded in a NiO crystal. Figure 3 shows a single NiO particle and a selenide particle in a polished section of a mounted sample. In general, the selenide shells as observed by EDX analysis were filled with copper oxide. One of the copper selenide spheroids present in the raw anode slime, however, was found to be filled with selenium (Figure 4). The selenium might have been formed by selective dissolution of Cu and/or Ag from the original selenide.16 Figure 4. Backscattered electron micrograph showing a selenide structure in the untreated anode slime (polished section mount): (1) selenide, (2) Se.

Figure 5. Backscattered electron micrograph showing a barium sulfate particle in the untreated anode slime (polished section mount): (1) BaSO4.

In this figure, several octahedral NiO crystals and selenide rings can be observed. Although most of the NiO particles are free as single crystals or crystal

The presence of large grains of BaSO4 (Figure 5) was confirmed by EDX studies. Mold washing material is generally ascribed to be the source of BaSO4.10 (2) Characteristics of Residue after Leaching of Anode Slime in Sulfuric Acid Medium. The leaching of anode slime in sulfuric acid medium without any additive was carried out at 30, 60, and 80 °C under atmospheric conditions.6 A maximum of 58% of the Cu and 9.5% of the Te was leached out at 80 °C after 240 min of leaching with 3.6 M sulfuric acid. Recovery of other metals was negligible. The residue (containing 6.45% Cu, 45.05% Ni, 3.82% Te, 5.51% Se, 1.92% Ag, and 0.12% Au) of this experiment was subjected to mineralogical characterization. X-ray diffraction analysis of the leached slime (Figure 6) indicated the presence of major amounts of NiO, CuSO4‚5H2O, BaSO4, SiO2, Cu4SeTe, and Cu3.8Ni. Leaching with H2SO4 in the presence of air resulted in a progressive decrease in the Cu content of the a node slime, and therefore, a corresponding increase in the concentrations of other species was observed. During sulfuric acid leaching, selective dissolution of copper from the (Ni1-xCux)O matrix possibly took place

Figure 6. XRD pattern of the residue after sulfuric acid leaching of anode slime.

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Figure 7. Backscattered electron micrograph showing a selenide particle as an irregular mass in H2SO4-leached residue (polished section mount): (1) selenide.

sulfuric acid leached residue. However, dissolution of NiO did not take place under the experimental conditions applied because of its refractory nature. The presence of CuSO4‚5H2O in the residue even after H2SO4 leaching might be due to the intergrowth of CuSO4‚5H2O with silica gel, which retarded its dissolution.22 The protection of CuSO4‚5H2O by other slime constituents is also possible.22 Silica gel was formed by the acid decomposition of the numerous silicates present in the raw anode slime.24 BaSO4 present in the anode slime did not dissolve in the sulfuric acid medium, and hence, it was detected in the residue again, as confirmed by the SEM studies. As in the SEM studies, where the selenides in the raw anode slime were observed to exist as compact masses, spheroidal particles, or tubelike grains, a few similarly shaped selenide particles were also noticed in the sulfuric acid leached residue. Figure 7 shows a selenide particle as an irregular mass that consists mainly of copper, selenium, tellurium, and silver, as revealed by EDX analysis. Some particles with a significant reduction in copper content were also observed in the leached residue, which indicated the dissolution of copper in the sulfuric acid medium following the reaction22

2AgCuSe(s) + O2(g)+ 2H2SO4(aq) f Ag2Se(s) + Se(s) + 2H2O + 2CuSO4(aq) (2) Figure 8. Backscattered electron micrograph showing a typical particle in H2SO4 leached residue (polished section mount): (1) lead-copper silicate, (2) selenide.

according to the following reaction24

(Ni1-xCux)O + xH2SO4 f (1 - x)NiO + xCuSO4 + xH2O (1) Therefore, the (Ni1-xCux)O phase that was present in the untreated anode slime was not identified in the

A typical occurrence of the lead-copper silicate enveloped by a selenide particle is presented in Figure 8 for the acid-leached residue. This material was possibly formed during anode casting by the reaction of silicon with lead and copper oxide in molten copper.22 (3) Characteristics of Residue Obtained after Leaching of Anode Slime in Sulfuric Acid Medium with Manganese Dioxide Additive. To increase the leaching efficiency of metals/metalloids, manganese dioxide, an oxidative agent, was added during sulfuric acid leaching of anode slime. The recovery of the metals improved to 90% of the Cu, 74% of the Te, and 42% of

Figure 9. XRD pattern of the residue after sulfuric acid leaching of anode slime with MnO2 additive.

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Figure 10. Backscattered electron micrograph showing the general view of the resiude obtained after H2SO4 leaching with MnO2 addition (polished section mount).

Figure 11. Backscattered electron micrograph showing various particles in the residue (polished section mount): (1) porous lump, (2) BaSO4, (3) NiO.

the Se when the anode slime was leached with 3.6 M H2SO4 at 80 °C with addition of 0.575 M MnO2.6 The silver recovery was negligible, and no gold was recovered. The composition of the leached residue was found to consist of 1.91% Cu, 48.97% Ni, 9.43% Se, 1.39% Te, 2.17% Ag, and 0.12% Au. X-ray diffraction analysis (Figure 9) of the residue from the leaching of the anode slime in sulfuric acid medium with manganese dioxide as the additive revealed the presence of NiO, BaSO4, MnSe, and CuSeO3‚ 2H2O as the major phases. The absence of the CuSO4‚ 5H2O phase indicated the dissolution of major amounts of copper, which can be further substantiated by its good leaching recovery. Figure 10 shows the general view of the residue under a scanning electron microscope. It can be observed that there was a decrease in the number of ring structures after leaching. The EDX analyses at the inner and outer portions of a typical ring structure demonstrate the presence of manganese and absence of copper in the inner part of the particle, which indicates the diffusion of the reacting species through the porous outer layer of the particle. Figure 11 represents a porous lump that was possibly formed during the oxidation dissolution of Se.22 The composition of the porous lump found by EDX studies consisted predominantly of selenium. Therefore, it seems that Se dissolved but reprecipitated as CuSeO3‚ 2H2O.22 This phase was also identified in XRD studies (Figure 9). Selenium deposits, which also possibly formed by chemical reaction 2 mentioned earlier, were found in the outer layer of the ring structure, as observed in the SEM-EDX studies.

Figure 12. Backscattered electron micrograph showing the lumpy structures in the residue (polished section mount): (1) SiO2, (2) lumpy structure.

SEM studies also revealed the presence of a number of lumps (Figure 12) in the residue having the same composition as the ring structure except for copper. The EDX results in Figure 13 show the composition of the lump particles, which mainly consisted of Se, Ag, and Te. The lumps might have been formed by the agglomeration of different ring structures during leaching of copper. (4) Characteristics of Residue Obtained after Leaching of Anode Slime in Sulfuric Acid Medium with Manganese Dioxide and Sodium Chloride Additives. As the recovery of the metals was not satisfactory in the previous experiments, the next set of experiments was done with the addition of both manganese dioxide and sodium chloride into the sulfuric acid. Here, chlorine generated by the following reaction was expected to act as an oxidizing agent

MnO2(s) + NaCl(s) + H2SO4(aq) f MnSO4(aq) + NaSO4(aq) + Cl2(g) + H2O (3) With these additives, 90% of the Cu, 80% of the Se, 79% of the Te, and 77% of the Au were extracted during leaching.6 The leached residue contained 2.13% Cu, 62.54% Ni, 3.64% Se, 1.23% Te, 2.67% Ag, and 0.41% Au. X-ray diffraction analysis (Figure 14) indicated the presence of major amounts of AgCl, NiO, and BaSO4, along with lesser amounts of elemental copper and selenium in the residue. Elemental selenium might have been produced by the oxidation reaction of Ag2Se,18 as follows

Ag2Se(s) + 1/2O2(g) + 2H+(aq) f 2Ag+(aq) + H2O + Se(s) (4) The formation of a new AgCl phase took place through the precipitation of dissolved silver upon the addition of sodium chloride. Figure 15 presents a general view of the residue from the leaching of the anode slime in sulfuric acid medium with manganese dioxide and sodium chloride as the additives. SEM studies revealed that AgCl precipitated as irregular masses without any external cryatallographic features. Figure 16 shows a AgCl precipitate along the outer surface of a ring structure. EDX analysis (Figure 17) indicated that the composition of the outer layer mainly consisted of Ag, Cl, and Se. The composition of the inner part of the ring structure consisted of Si and Ni (Figure 18).

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Figure 13. EDX analysis at location 2 in Figure 12.

Figure 14. XRD pattern of the residue after sulfuric acid leaching of anode slime with MnO2 and NaCl additives.

The presence of comparatively lesser amounts of copper, tellurium, and selenium revealed the extent of leaching that took place in the presence of manganese dioxide and sodium chloride additives in the sulfuric acid medium. The ash-diffusion-controlled reaction of Se and Te, earlier established during leaching studies,6 was perhaps due to the presence of the porous mass of AgCl in the leached residue.

SEM studies (Figure 19) followed by EDX analysis showed the presence of barium sulfate as free particles and sometimes attached to AgCl particles. The presence of a large number of NiO particles was also detected in the SEM studies. This corroborated the leaching results, where the nickel recovery was barely 4%.6 Even though gold was reported in the chemical analysis of the anode slime, no gold phase could be

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Conclusions

Figure 15. Backscattered electron micrograph showing the general view of the residue (polished section mount).

Figure 16. Backscattered electron micrograph showing the ring structure with AgCl at the outer surface (polished section mount).

identified either by XRD studies or by SEM studies of the untreated anode slime and leached residues, perhaps because of gold’s presence in minor quantity.

Figure 17. EDX analysis at position a in Figure 16.

To investigate the leaching behavior of the various phases present in anode slime, characterization of anode slime and its leached residues was carried out using the XRD and SEM-EDX techniques. The findings presented herein have helped to elucidate some of the issues of anode slime leaching in sulfuric acid medium with/ without additives. (1) XRD studies of the untreated anode slime indicated the presence of CuSO4‚5H2O, NiO, CuSe, BaSO4, and (Cu0.2Ni0.8)O phases. Several octahedral NiO crystals and selenide rings were found in SEM studies of the sample. (2) X-ray diffraction analysis of the sulfuric acid leached residue revealed the presence of all of the phases that were in the untreated anode slime except (Cu0.2Ni0.8)O. In SEM-EDX studies of this residue, some agglomerated selenide particles were observed, along with a significant reduction in the copper content, indicating the dissolution of copper in the sulfuric acid medium from the selenide particles. (3) The presence of NiO, BaSO4, MnSe, and CuSeO3‚ 2H2O was detected in the XRD analysis of the residue of the anode slime leached in the sulfuric acid with manganese dioxide additive. The absence of the CuSO4‚ 5H2O phase in the XRD pattern indicated the dissolution of major amounts of copper. SEM studies of this residue revealed a number of porous structures and lumps and lesser numbers of ring structures. EDX studies showed that the lumps had the same composition as the ring structures, except for the copper content, which was lower. EDX studies also revealed the presence of manganese and the absence of copper in the inner part of the ring structures.

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Figure 18. EDX analysis at position b in Figure 16.

Literature Cited

Figure 19. Backscattered electron micrograph showing BaSO4 attached to AgCl (polished section mount): (1) porous structure, (2) BaSO4, (3) NiO.

(4) Major amounts of AgCl, NiO, and BaSO4 were found in XRD studies of the residue obtained after leaching of the anode slime in sulfuric acid medium in the presence of manganese dioxide and sodium chloride. SEM studies revealed the AgCl precipitate to consist of irregular masses. Some of the masses were found to precipitate along the outer surface of the ring structures. The porous nature of the AgCl precipitate possibly contributed to the ash-diffusion-controlled nature of the reaction. EDX studies indicated the presence of no or small amounts of copper, tellurium, and selenium, which verified the good leaching recovery of these metals. Acknowledgment The authors are thankful to M/S ICC, Ghatsila, India, for providing adequate anode slime samples for this work to be performed.

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Received for review July 1, 2003 Revised manuscript received January 15, 2004 Accepted January 18, 2004 IE0305465