A mineralogical study of nodulated copper cathodes

J.E..Dutrizac and T.T. Chen CANMET 555 Booth Street Ottawa, Canada KIA OGI

ABSTRACT

Mineralogical studies were carried out on nodulated copper cathodes from three primary refineries to characterize the nodular growths and to elucidate the causes of the nodulation. Nodulation is often initiated at the surface of the starter sheet or the stainless steel plating blank, although a layer of smooth copper sometimes is deposited before nodulation commences. In some instances, the "roots" of the nodules exhibit a pronounced dendritic texture that is associated with an abundance of cavities. Slimes particles are not usually associated with these growth features which lead to a globular surface deposit. The globules sometimes develop into larger nodules, and this type of nodulation is likely caused by improper addition agent concentrations. The nodules on most cathodes, however, exhibit "roots" at the contact with the substrate that are associated with microcavities and large clusters (>40 pm) of slimes particles. The slimes constituents are commonly Ag powder, PbSO, and Cu,(Se,Te) but not AgCu(Se,Te) or Ag,(Se,Te). The size of the slimes clusters, rather than their composition, appears to be the important factor causing the copper to grow into nodules. Tiny individual slimes particles themselves do not appear to cause cathode nodulation.

Proceedings of Copper 99-Cobre 99 International Conference

Volume III-Electrorefining and Electrowinning of Copper Edited by J.E. Dutrizac J. Ji and V. Ramachandran The Minerals, Metals & Materials Society, 1999

INTRODUCTION

The Inco Copper Cliff Copper Refinery has a capacity of 170,000 t!y, and plates onto conventional copper starter sheets (1). The Kidd Metallurgical Division of Falconbridge Limited produces 145,000 t/y of copper by deposition onto stainless steel plating blanks (2). Although the CCR Refinery of Noranda Inc. traditionally plated onto copper starter sheets, the company has recently adopted stainless steel plating blank technology for its entire production of 360,000 t!y ofcathode (3). All three refineries normally produce high purity copper having a good physical appearance. Occasionally, however, varying degrees of cathode nodulation occur in all three operations, and the nodulation can affect all parts of the cathode deposit. The nodules sometimes grow to several centimeters in size and the presence of such large surface features makes the handling and stacking of the cathodes more difficult. The formation of nodules fiequently traps quantities of electrolyte and slimes particles, thereby reducing the purity of the copper product. Furthermore, the nodulation also causes an uneven distribution of the current density, a reduction in current efficiency, and hence, an increase in the operating cost of the refinery.

Several mechanisms have been postulated to explain cathode nodulation; these include insufficient mass transfer, improper concentrations of the addition agents, and suspended conducting particles. In an electrolyte fiee of additives, nodulation may be caused by insufficient mass transfer (4); in this case, gas sparging sometimes reduces the degree of nodulation. An improper ratio of thiourea, glue and chloride in the electrolyte can also produce nodulation, and it is known that the optimum concentrations of the addition agents change as the current density increases (5,6,7). Nodulation is also reported to be caused by suspended conductive particles, such as anode slimes (8,9). Consequently, those parameters which enhance the suspension of particulate matter, such as increased electrolyte density or viscosity, the evolution of gases at the anode and a high slimes fall, could indirectly promote cathode nodulation. It is believed that once a nodulated surface develops, the localized current density, and hence the copper deposition rate, increases abruptly resulting in the further rapid growth of the nodules.

Cathode nodulation is clearly undesirable, and all three.copper refineries would. like to 'eliminate the nodulation problem. A first step towards the. elimination of cathode nodulation is a clear identification of its causes. To this end, CANMET recently carried out detailed mineralogical investigations of nodulated copper cathodes from the three refineries to elucidate the various causes of the nodulation and to suggest possible means to resolve the nodulation problem. The results .of those studies are sumrnariied in this report.

ELECTROREFINING AND ELECTROWINNING OF COPPER

EXPERIMENTAL Samples

Two nodulated copper cathodes were supplied by Inco's Copper Cliff Copper Refinery. The first sample (Inco-4) was a severely nodulated cathode obtained after 5 days of plating at a current density of 180 AM, and1 the second sample (Inco-2) was a similar cathode collected after 6 days of plating. Nodulated areas from the tops and mid-sections of both cathodes were chosen for study. To provide complementary information, a sample of the suspended slimes was collected from about 2 cm below the electrolyte surface prior to the removal of the Inco- 1 sample from the cell.

Three cathode deposits from the Kidd Metallurgical Division of Falconbridge Limited were studied. The first sample (Kidd- 1) was obtained after 16 h of plating onto a stainless steel blank at a current density of about 250 A/m2. Many 100-500 pm nodules were dispersed over an otherwise smooth and fine-grained copper deposit. The Kidd-2 sample was collected after 24 h of plating. This deposit was generally smooth, but contained several 1-2 mm nodules dispersed randomly over the surface. The Kidd-3 sample was obtained by plating copper for 36 h onto a piece of a milled and polished Kidd anode. The -0.9 mm thick cathode copper deposit was smooth and fine grained; no nodulation was evident although several pin holes -1 mm in diameter were detected in the deposit.

Four samples were provided by 'the CCR Refinery of Noranda Inc. Two of the samples (CCR-1 and CCR-2) were starting sheets produced using -22 h plating cycles and conventional copper plating blanks. Both samples were extensively nodulated and multiple nodule growth was common. The nodules on the CCR- 1 sample were 2-3 mm in diameter and were often loosely attached to the copper matrix; those on the CCR-2 sample were 1-2 mm in diameter and were firmly adherent. The CCR-3 sample was another copper starter sheet made on a copper plating blank; several sinall nodules were present near the solution level, although most of the deposit was smooth. The fourth sample (CCR-4) was a copper starter sheet made by plating for -22 h at about 260 A/m2 onto a stainless steel blank. The deposit was relatively uniform but contained numerous tiny semi-globules and many pin holes dispersed over the entire surface.

Mineralogical Techniques

Special efforts were necessary to prepare the nodulated cathodes for study. After direct examination of the nodulated samples using an optical stereomicroscope andlor the scanning electron microscope (SEM) to select the areas for study, the samples were sawed to a convenient size. The sawn pieces were mounted at right angles to the deposit surface using liquid epoxy, and were ground and polished to expose the contact zone between the nodule and the underlying copper. The sections were examined using optical microscopy and scanning electron microscopy with backscattered electron (BSE) or secondary electron images to determine whether any slimes particles or impurity phases were present at the "roots" of the nodules or inside the nodules. The sections were then lightly ground to

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remove -100 pm of the surface and were repolished; these sections were re-examined with the SEM. Repeated grinding and polishing of the samples were carried out across the "roots" of the nodules to increase the possibility of detecting any impurity particles around the contact zones. When impurity particles were detected, the sections were analyzed in detail using the scanning electron microscope with energy dispersive X-ray analysis (EDX) and the electron microprobe.

The suspended anode slimes sample, which was collected using a suction device, was filtered, water washed and dried at room temperature. The slimes were studied initially by X-ray diffraction analysis to identify the major crystalline phases. Polished' sections were prepared and ,the samples were examined with a scanning electron, microscope equipped with an energy dispersive X-ray analyzer to determine the chemical species and their morphologies. In this regard, extensive use was made of backscattered electron (BSE) images to differentiate the various chemical species. Details of the mineralogical procedures have been reported elsewhere (10).

RESULTS AND DISCUSSION

Suspended Slimesin the Inco Refinery . .

Figure 1 illustrates the morphologies of the suspended slimes in the Inco refinery that are generally similar 'to those detected in the Kidd and CCR operations. In general, the slimes occur as large clusters or agglomerates which are composed of a diversity of species. The tiny bright grains are Ag powder, the ring-like particles are selenides, the triangular- shaped particle is an octahedral crystal of NiO and the matrix is the oxidate phase, (Cu,Ni)SO,.nH,O or Cu-Sn arsenate. Some of the bright particles are PbSO,, but Cu20 is rare or entirely absent. Figure 2 shows another view of the suspended slimes particles. The tiny bright grains are mostly Ag powder, the spheroidal and ring-like particles'are selenides, the platy hexagonal-shaped crystal is Cu-Sn-Ni oxide, the matrix is mainly (Cu,Ni)SO,.nH,O, and the two largedark grains are K-AI silicate. It is apparent from both photomicrographs that Ag powder is an important constituent of the suspended slimes, and the silver is believed to form mostly by the reaction of low concentrations of silver ion with the abundant cuprous ions present in the electrolyte (1 1,12).

Ag' + Cu' - Ago + Cu2' (1) Overall, the suspended solids contain major amounts of Cul(Se,Te),(Cu,Ag),,(Se,Te), PbSO,, Ag powder and NiO, as well as minor or trace amounts of AgCu(Se,Te), Cu-Sn-Ni oxide, K-Ca-AI silicate, Al silicate, K-A1 silicate, Cu-Sn arsenate, Ni-Fe-Sn oxide, BaSO,, Cu-Pb-As-Bi oxide and an oxidate phase of Cu-Ag-Se-Pb-Ni-As0,-SO, composition. Significantly, the Ag-rich selenides, which are a major constituent of; the bulk anode slimes fiom the Inco refinery (139, are but a minor species in the suspended slimes. The implication is that the suspended slimes were recently generated and released from the anode surface. If the suspended slimes had been in contact with the electrolyte for a prolonged period of

ELECTROREFINING AND ELECTROWINNING OF COPPER

Fig. 1 - Morphology of the suspended Fig. 2 - Morphology of the suspended slimes in the Inco electrolyte. 1,- slimes in the Inco electrolyte. 1 - NiO, 2- Ag, 3- selenide, 4- Ag, 2- selenide, 3- Cu-Sn-Ni oxidate phase (matrix), 5- oxide, 4- NiO, 5- PbS04.6- (Cu,Ni)SOd.nHzO (Cu,Ni)S04.nH20, 6- K-A1

silicate

Fig. 3 - Secondary electron .micrograph Fig. 4 - BSE micrograph showing the show,ing the general morpho- general' morphology of a logy of the copper nodules in nodule in the Kidd-1 sample. the Kidd-31 sample. 1- copper matrix, 2- plating

blank (removed$, 3- slimes particles

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.time, as would 'bethe case if thezsuspended slimes originated from the bottom of the refining cell, the selenides would ,have been enriched in silver,. as, suggested by .the following equations:

Cu,(Se,Te) + Ag' - AgCu(Se,Te) + Cu'

Ag' + AgCu(Se,Te) - Ag,(Se,Te) + Cut (3)

On the other:hand; the suspended slimes musthave been in contact with the electrolyte long. ,enough for the.Cu,O .phase, which is a minor species in the slimes attached to the ,face of the Inco anodes (1 3), to dissolve according to the following reactions:

As will be discussed in detail later, the constituents of the suspended slimes are similar to those found at the "roots" of many of the cathode nodules. The Ag powder, Cu- rich selenides and PbSO, particles are abundant in the suspended slimes, and'these are the common constituents of the "roots" of the cathode nodules.

Slimes-Related Cath~de~Nodulation

The Kidd-1 sample, which was obtained after 16 h of plating, represents the earliest stage of nodulation considered in this investigation, and was studied to try to characterize the "root" responsible for nodule growth. Figure 3 shows the typical morphology of the nodules which developed on the fine-grained and flat copper matrix. The nodules are 100- 200 pm in size, and seem to occur randomly on the copper matrix. Multiple growths of the nodules are common, and many of the nodules in this sample exhibit a somewhat irregular form. Figure 4 shows a cross-section of one of the nodules, and illustrates a common morphology. The nodule and the copper matrix appear to exist as a single mass; no physical boundary is discemable between them. A cluster of slimes particles, however, is present at the base or "root" of the nodule. The slimes particles, typically Ag powder, Cy(Se,Te) and PbSO,, are randomly embedded in a compact copper matrix. Significantly, no cavities are detected in the copper mass, although abundant cavities are often associated with the nodules. The thickness of the copper deposit is about 450 pm; the slimes particles are detected approximately 150 pm from the start of the copper deposit and they extend into the nodule itself. It appears that the attachment of a cluster of slimes particles to a pre-existing smooth copper deposit caused the development of the nodule in this sample.

Figure 5 shows another common morphology of the nodules in the Kidd- 1 sample.. A cluster of slimes particles, approximately 40pm in size, occurs at the beginning of the copper deposit and near the contact between the copper deposit. and the stainless steel; plating blank. No other slimes particles aredetected in the nodule or in the copper deposit adjacent

ELECTROREFINING AND E ,LECTROWINNING OF COPPER

Fig. 5 - BSE micrograph of another nodule in the Kidd- 1 sample. 1- copper matrix, 2- plating blank (removed), 3- slimes particles

Fig. 6 - ;BSE micrograph illustrating the morphologies of the slimes particles shown in Figure 5. 1- copper, 2- Cuz(Se,Te), 3- PbS04, 4- Ag

Fig. 7 - Secondary electron micrograph showing the morphology of the copper deposit of the ,Kidd-3 sample and the presence of holes at the surface. 1- copper matrix, 2- 'hole, 3- groove, 4- striation

Fig. 8 - BSE micrograph of the interface between the cathode deposit, the anode copper.plating blank and a hole in the 'Kidd-3 sample. 1- hole, 2- cathode copper, 3- anode copper (plating blank, 4- Cu20+Cu2(Se,Te)+(Cu,Pb,As) oxide inclusions, 5-.interface.

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to the nodule. The nodule and the copper deposit occur as a single mass; no boundary can be discerned between them and no cavities are present. It appears that the slimes particles occur at the "root" of this nodule. Figure 6 shows the detailed morphology of the slimes cluster illustrated in Figure 5. The slimes cluster, approximately 40 pm in size, appears to be a single entity, with PbSO, and Ag particles closely associated with the larger Cu,(Se,Te) structure. The slimes particles seem to be embedded in a compact copper matrix, which superficially resembles a fragment of Kidd anode copper (14).

The Kidd-3 sample was generated by plating copper onto a smoothly milled Kidd copper anode. This test was done to ascertain whether individual slimes particles, on an otherwise flat copper surface, caused cathode nodulation. In this experiment, the deposited copper was consistently smooth and nodul'e-free, although numerous pin holes were evident in the deposit. Figure 7 illustrates the morphology of two of these holes which are nearly circular in cross-section; they seem to start as a tiny point and gradually expand with prolonged plating thickness. Therhorizontal striations.on the interior walls of the holes likely imply an irregular release of gas during copper deposition. A groove originating from the hole and pointing vertically upward (the sample is shown 'upside down in Figure 7) is commonly present on the surface of the copper deposit. The orientation of the groove and the shell-like morphology of the hole with the larger>dimension pointing upward imply that these are gas vent holes. No impurity or slimes particles are detected in any of the holes. Figure 8 shows the detailed morphology of a hole near the surface of the plating blank. The lower-right portion of the photomicrograph is the anode copper plating blank and the left portion is the deposited copper. Clusters of Cu,O-Cu,(Se,Te)-(Cu,Pb,As) oxide particles occur along the grain boundaries of the anode copper. The pin hole developed from the surface of the polished anode copper plating blank; that is, the gas is released from the beginning of electrolysis and at the contact between the anode copper starter sheet and the cathode deposit. An impurity cluster in the anode copper is exposed at the polished surface of the plating 'blank. Based on the morphology of the copper deposited on the surfaces of the impurity clusters and that on the surface of the inclusion-free metal, it is concluded that the tiny individual slimes particles embedded in the surface of the plating blank do not affect the morphology of the cathode deposit and do not cause cathode nodulation.

Figure 9 provides a general view of the cross-section of a copper nodule formed at the solution level of an Inco cathode (Inco-1). Numerous slimes particles and tiny cavities are detected at the "root" of the nodule, and these features are shown in greater detail in Figures 10 and 11. Numerous tiny Ag grains are embedded in the "root" of the nodule, and many (Cu,Ag),(Se,Te) particles are present on the surfaces of the cavities which are otherwise totally enveloped by thelmass of the nodule. Figure 12 illustrates the cross-section of another cathode nodule occuning at the solution level of the Inco cathode (lnco-1). The "root" of this nodule contains several tiny cavities, which are totally enveloped by the mass of electrodeposited copper. Numerous slimes particles, such as Ag powder, selenides, PbSO,, Cu-Sn-Ni oxide, NiO and Cu,(Se,Te) particles with Cu-Pb-As-Bi oxide cores are present in the cavities (Figure 13). The morphologies of these slimes particles are very similar to those of the suspended slimes collected from near the surface of the Inco

ELECTROWFINING AND EZ ,ECTROWINNING OF COPPER

Fig. 9 - Cross-section of a cathode nodule occurring at ,the solution level of an Inco cathode (lnco-1). 1- nodule, 2- cathode, 3- impurity particles, 4- cavity

Fig. 10 - Detailed morphology of the "root" of the cathode nodule shown in Figure 9. 1- cavity, 2- Ag, 3- Cu2(Se,Te) + FbS04, 4- copper

Fig. 1.1 - Detailed morphology of the cavity at the "root" of the cathode nodule shown in Figure 9. 1- Cu2(Se,Te), 2- Ag, 3- PbSO., or Cu-Pb-As-Bi oxide

Fig. 1'2 - Cross-section of a cathode nodule occurring at the solution, level of the Inco cathode (Inco- 1). 1- nodule, 2- cavity, 3- cathode

392 VOLUME III

Fig. 13 - Detailed motpho'logy of a cavity Fig. 14 - Cross-sect.ion of a nodule in the cathode nodule shown in occurring ,at the middle of Figure 12. 1- Ag, 2- the Inco cathode (Inco-2). 1- Cu2(Se,Te), 3- Cu-Sn-Ni oxide nodule, 2- cathode, 3- slimes + NiO, 4- ,PbS04. 5- copper particles + tiny cavities

Fig. 15 - Detailed morphology of the Fig. 16 - BSE micrograph of a nodule at "root" of the nodule shown in the solution, level of the CCR-3 Figure 14. 1 - NiO + Cu-Sn-Ni sample. 1- cavity, 2- plating oxide, 2- Ag, 3- cavity, 4- blank, 3- slimes particles, 4- copper, 5- porous copper copper dendrites

ELECTROREFINING AND ELECTROWINN1N.G OF COPPER

electrolyte (Figures 1 and 2). In fact, the association of Cu-Pb-As-Bi oxide with Cu,(Se,Te), and not with AgCu(Se,Te) or Ag,cSe,Te), implies that the "root" consists of a cluster of slimes particles spalled relatively recently from the anode surface. In the bulk anode slimes on the bottom of the refining cells, the selenide species are significantly Ag-rich; e.g., AgCu(Se,Te), (Cu,Ag),(Se,Te), or Ag,(Se,Te).

Figure 14 illustrates a cross-section of another cathode nodule occurring at the middle portion of an Inco cathode (Inco-2). The "root" of this nodule, which is shown in Figure 14, contains several'cavities and many tiny impurity particles. The cavities are totally enclosed by the nodule. Detailed examination of the "root" revealed the presence of Ag powder, crystals of NiO associated with Cu-Sn-Ni oxide and Cu,(Se,Te) particles with Cu- Pb-As-Bi oxide cores embedded in the copper matrix, whereas the cavities appear to be free of impurity particles (Figure 15). The presence of Cu,(Se,Te) particles with Cu-Pb-As-Bi oxide cores in the copper matrix and the association of many Ag particles suggests that the slimes cluster, which' appears to serve as the nucleus for the growth of the nodule, was freshly liberated fiom the anode surface.

Figure 16 shows the tiny copper nodules present.at the solution level of the CCR-3 sample. The nodules exhibit distinctivebanding and radiating growth textures. Numerous cavities arepresent at the "roots" of the nodules, and the "roots" occur at the contact between the deposit .and the copper plating blank. The radiating copper texture seems to have developed fiom the very inception of electrodeposition. Slimesparticles such as Ag powder, Cu,(Se,Te), AgCu(Se,Te), PbSO,, a Pb-Sb-Bi-Cu-S-0 phase, Cu,O and ,(Cu,Ni)SO, are present in the cavities or are.embedded in the copper deposit. Figure 17 illustrates a cluster . .. . , , . ,.

.* ..

ofslimes particles detected at the:"root" of one.ofthe nodules. Silver powder and various .. ..., :

. .

selenides are abundant, but Cu,O, PbSO, Sn0,and (Cu,N,i)SO,.are .also detected:: The . . . ,... ,.. . implication is thata largecluster of floating slimes particles became attached to the,top of .. . ..,": .?:

. . .-I: the copper starter sheet very early in the re'fining cycle and caused the observed nodulation ._ ,. ..+> , .. . . ...

. ,..

The adhesion of "large" clusters of slimes particles on the cathode surface results in enhanced localized copper deposition that can lead to cathode nodulation. Large clusters of slimes particles, suspended in the electrolyte or floating on the surface of the electrolyte, can be transported to the cathode surface by the electrolyte flow or by gas bubbles. Flloating slimes clusters may lead to nodulation at the solution level, whereas suspended slimes clusters may cause nodulation anywhere on the face of the cathode . The cavities at the "roots" of the nodules may indicate that the slimes clusters were transported by gas bubbles, or they may indicate copper deposition around the porous slimes clusters. There is little evidence that individual slimes species, such as discrete particles of Cu-Ag selenide, isolated grains of Ag powder or crystals of PbSO,, cause cathode nodulation. In fact, many such slimes particles are present as individual grains, and even as tiny agglomerated grains, on the surfaces of the cavities, but copper nucleation and growth did not occur on such particles. Likewise, the cross-sections of the nodules show no evidence of the development of individual nodules or radiating copper features fiom the individual slimes particles. Rather, a single nodule seems to develop from the entire slimes cluster.

394 VOLUME IIJ

Fig. 17 - BSE micrograph of a slimes Fig. 18 - General morphology of a cluster at the "root" of a cathode nodule on the Kidd- nodule in the CCR-3 sample. 2 sample. 1- Ag in (Cu,Ni)S04, 2- Ag in Cu20, . 3 - SnOz, 4- Cu2(Se,Te), 5- PbS04, 6- copper, 7- cavity, 8- Ag

Fig. '$9 - BSE micrograph showing the Fig. 20 - Detailed :morphology of the contact zone between a slimes particles shown ,in nodule and the initial c.opper Figure 19. 1- Cuz(Se,Te), 2- deposit for the Kidd-2 Ag powder, 3- PbS04, 4- sample. 1- slimes particles, 2- cavities cavity zone, 3- nodule, 4- initial copper deposit, 5- stainless steel plating blank (removed)

ELECTROREFTNING AND ELECTROWINNING OF COPPER

The nodules obtained from different parts of the cathode or from different cathodes exhibit the same general morphology and contain the same impurity species. Often, the arrangement of the slimes particles at the "roots" of the ilodules (e.g., Figures 10 and 55) is superficially similar to that of the grain-boundary inclusions in an uncorroded copper anode (cf. Figure 8). In fact, it was initially postulated that sinall fragments of the copper anode were transported to the cathode and contributed to the nodulation. This belief was further strengthened by the abundance of silver powder and Cul(Se,Te), rather than AgCu(Se,Te), particles at the "roots" of the nodules; these species are prevalent in the anode and at its surface, but are relatively rare in the bulk slimes. Subsequent studies, however, did not support this hypothesis. Firstly, the slimes particles in the "roots" are smaller than those in the grain-boundary inclusions in the anodes, and the copper grains which they appear to delineate are also smaller than the copper crystals in the anode. Secondly, the slimes particles detected in the "roots" of the nodules are generally devoid of Cu,O whereas copper oxide is an abundant constituent of the grain-boundary inclusions. Lastly, the electron microprobe- determined composition of the copper metal, even at points between the individual slimes particles, is not that of anode copper. Specifically, low Ni contents (

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close association of the slimes particles with the cavities, and hence an earlier surface of the nodule, 'may imply that these slimes particles were simply trapped on the surface of the nodule. On the other hand, they may represent part of a larger cluster of slimes particles which initiated the growth of the nodule.

Figure 21 illustrates the general' morphology of the CCR-I sample, which is a starter sheet formedon a copper blank. The surface of this deposit is extensively covered with 2-3 mm nodules and many of the nodules consists of multiple growths. Most of the nodules sit in "craters" formed in the starter sheet, and inany of the nodules appear to be only loosely attached to the starter sheet.

More than 70 nodules in the CCR- 1 sample were examined, and many of them contain slimes particles at the contact zone between the nodule and the copper matrix or between the nodule and the plating blank. Figure 22 illustrates the typical morphology of these nodules as seen in cross-section. Multiple growths of the nodules are evident. The dark region with the sharp straight boundary at the bottom of the figure represents the original location of the copper plating blank from which the starter sheet was stripped. The dark spaces between or within the nodules are voids originally present during electrolysis, and such cavities are common in this sample. The nodule in the center developed directly on the surface of the copper plating blank. Slimes particles are commonly present on the surfaces of the cavities, but are rare within the nodules themselves. Theldetailed morphology of a cavity which is located near the contact zone between the nodule and the copper plating blank is shown in Figure 23. The dark regions are cavities near the surface of the nodule; the tiny bright particles are Ag powder, the bright grains are PbSO, and the ring-like particle is Cu,(Se,Te). The slimes species are similar to those identified in the Kidd and Inco cathode nodules. Based on the mozphology, it appears that the slimes particles may have been trapped on the surfaces of the nodules, especially in the cavities between the surfaces of the nodules and the copper starter sheet. On the other hand, the nodule may have developed from a large slimes cluster part of which remained on the surface of the nodule. There is no indication that any of the individual slimes particles caused the nucleation and growth of the copper nodules, despite the obvious abundance of such individual slimes particles on the surface of the copper during the early stages of copper deposition.

Non-Slimes Related' Nodulation

The CCR-2 sample is extensively nodulated. The surface of this sample is covered by nodules which are 1-2 mrn in diameter, and are similar in habit to those shown in Figure 21. In contrast to the nodules on the CCR-1 sample, however, these nodules adhere firmly to the starter sheet and no "craters" are evident. Figure 24 shows the general morphology of this sample, and the multiple growth of the nodules is evident. Nodules grow on top of other nodules, creating spaces (between thein. In contrast to the CCR-1 sample, where most of the nodules developed directly from the sudace of the plating blank, nodulation in the CCR-2 sample began after a small amount of smooth copper deposition had taken place. In most cases, a layer of copper approximately 50- 100 pm thick was plated before nodulation

ELECTROREFINING AND E IEECTROWINNf.NG OF COPPER

Fig. 21 - General morphology of the cathode nodules on the CCR- 1 sample.

Fig. 22 - .BSE micrograph showing the typical morphology of the nodules in ihe CCR- 1 sample. 1- nodule, 2- copper plating blank (removed), 3- open space, 4- slimes particles in cavity zone

Fig. 23 - Detailed morphology of the cavity zone shown in Figure 22. 1- copper, 2- cavity, 3- Ag powder,, 4- Cu2(Se,Te), 5- PbS04

Fig. 24 - BSE micrograph showing the typical morphology of the nodules in the CCR-2 sample. 1- nodule, 2- open space, 3- copper plating blank (removed), 4- beginning of the nodular growth

occurred. Despite repeated grinding and polishing of this sample to try to expose the "roots" - of the nodules on the copper matrix, no impurity particles were detected in this sample. More than 60 such nodules were examined in detail, but no slimes particles were detected in any of them.

The CCR-4 sample was plated onto a stainless steel blank. In this instance, the copper surface is more or less smooth; it exhibits a uniform globular morphology with -0.5 mm globules covering the entire surface. Numerous~circular pin holes are present (cf. Figure 7) and these originate both from the plating blank and from the deposited copper itself. No inclusions or slimes particles are detected in any of the holes. It is believed thatthe pin holes represent vents which allow the escape of the gases evolved at the cathode during commercial electrolysis.

Figure 25 illustrates the globular morphology of the nodules on the surface of the CCR-4 sample. In the vicinity of the globule (left half of photo) there are abundant cavities and an extensive radiating-dendritic copper texture, which is characteristic of this type of growth in the vicinity of the plating blank. Small copper globules are present in the cavities and also in the nodulated matrix. Particles of Fe-Cu-Cr-Ni sulphate and Cu-Ni sulphate are detected in some of the cavities; the presence of Fe and Cr in the sulphate phase implies the superficial leaching of the stainless steel plating blank. The non-nodulated part of the deposit (right side of the photomicrograph) is compact and more uniform; the surface in contact with the plating blank is smooth. Figure 26 illustrates the morphology of the globule in greater detail, and its radiating-dendritic texture is evident.

Figure 27 shows the surface of the deposit in the CCR-4 sample which was in direct contact with the stainless steebplating blank. Abundant tiny radiating sphemles are evident, and the implication is that excessive copper nucleation occurred at the surface of the stainless steel. The result is a globular-dendritic texture with abundant cavities between the growing copper sphemles and dendrites. Although trace amounts of Fe-Cu-Cr-Ni sulphate or Cu-Ni sulphate are present in the cavities, significantly no anode slimes particles such as Ag powder, PbSO, or Cu,(Se,Te), were detected in this sample. Occasionally, particles of Cu,O were evident, and in this regard, Figure 28 shows a semi-globular copper growth with a strongly dendritic texture. Patches of Cu,O are present between some of the copper dendrites, but the morphology of the Cu,O is very different from that detected in either the CCR anodes or CCR anode slimes 61 5). The conclusion is that the Cu,O formed directly at the cathode, presumably by the reaction of electrochemically produced cuprous ions with water.

The extensive radiating dendritic copper growth, the abundant cavities and the heterogeneous copper growth morphologies at and near the contact with the stainless steel

ELECTROREFINING AND ELEClXOWINNING OF COPPER

Fig. 25 - Cross-section of the copper Fig. 26 - deposit (CCR-4 sample) showing its semi-globular morphology. 1- cavity, 2- radiating dendritic copper, 3- plating blank (removed), 4- particles of Fe-Cu-Cr-Ni sulphate

Cross-section of the copper nodule (CCR-4 sample) showing radiating dendrites. 1- cavity, 2- radiating dendrites, 3- Fe-Cu-Cr-Ni sulphate, 4- plating blank (removed), 5- tiny copper spherule

Fig. 27 - BSE micrograph showing the Fig. 28 - BSE micrograph of Cu20 morphology of the copper particles in a semi-globular originally in contact with the copper mass of the CCR-4 stainless steel' plating blank of sample. 1- Cu20, 2- cavity, the CCR-4 sample. 1- .Fe-Cu- 3- Fe-Cu-Cr-Ni sulphate, 4- Cr-Ni sulphate, 2- tiny copper copper dendrites, 5- plating grains, 3- radiating copper blank (removed) spherules. 4- organic, 5- cavity

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plating blank indicate that the globular morphology commences fiom the very beginning of electrolysis. Slimes particles were not detected in this sample, and this observation conf im that the uniform globular morphology which extensively covers most of the surface of the copper deposit is likely caused by variations in the electrolysis conditions, such as improper ratios of the addition agents, locally high current densities, poor electrolyte circulation or even the superficial corrosion of the stainless steel plating blank.

CONCLUSIONS

Mineralogical studies, carried out on nodulated cathodes from three primary copper refineries, suggested two causes of cathode nodulation. The more prevalent is the attachment of large clusters of slimes particles on the surface of the cathode; the second and less common cause seems to be related to improper ratios of the addition agents. Nodulation is often initiated at the surface of the copper starter sheet or the stainless steel plating blank, although less commonly a thin layer of smooth copper is /deposited before nodulation commences.

The nodules on many of the cathodes show a "root" at the contact with the substrate that is associated with micro cavities and large clusters of slimes particles. In these instances, it is concluded that the'nodulation is initiated by the slimes clusters. For example, a deposit fiom the Kidd Metallurgical Division of Falconbridge Limited, obtained after only 16 h of deposition ontotstainless steel blanks in the commercial circuit, showed the presence of clusters of Ag powder, Cu,(Se,Te) and PbSO, particles at the roots of the nodules. Likewise, two nodulated cathode samples fiom Inco's Copper Cliff Copper Refinery, after 5 or 6 days of plating onto copper starter sheets, consistently exhibitedl micro cavities and large slimes clusters at the roots of the nodules. The slimes particles are Ag powder, NiO, Cu,(Se,Te), 'PbSO,, Cu,(Se,Te) with Cu-Pb-As-Bi cores and Cu-Sn-Ni oxide. One of the CCR deposits obtained after 22 h of plating showed several' nodules at the solution level which seem to have originated fiom large clusters of slimes particles. The individual slimes particles in the clusters were Ag powder, Cu,(Se,Te), PbSO,, Cu,O, (Cu,Ni)SO, andl SnO,. Significantly, Ag-rich selenides and Cu,O, which are abundant in the slimes h m the bottom of the refining cells, are not detected at the "roots" of the nodules. In fact, the compositions and morphologies of the slimes particles at the "roots" of the nodules are similar to those of the slimes clusters suspended in the electrolyte that were sampled at the time the cathode deposits were obtained. The occurrence of micro cavities and large clusters of slimes particles at the "roots" of the nodules implies that this type of nodulation is caused by nucleation and growth on the large clusters. That is, the size of the slimes clusters, rather than their composition, appears to be the more important factor leading to nodule growth. Tiny individual slimes particles likely do not cause nodulation, and this conclusion is supported by the observation that smooth deposits are formed on milled anode copper plating blanks despite an abundance of individual slimes particles at the surface of this material.

In other instances, however, the nodulation does not seem to be caused by the presence of slimes particles or large clusters of slimes particles. In' this regard, one of the

ELECTROREFINING AND ELECTROWINNING OF COPPER

starter sheets provided by the CCR Refinery of Noranda Inc. contained abundant globule-like surface growths.and circular pin holes dispersed across the surface. Although more than 60 globules were examined, no slimes particles were detected, and no slimes particles were found in the. tiny holes: In the regions of the.globular morphology, the.copper surface in contact with the stainless steel plating blank is rough, and. there amabundant cavities. The texture of the copper is that of radiating dendrites or micro spherules, and it is believed that the dendrites initiated,the globular growths. Nodulation starts from the plating blank, and no slimes particles are detected in the deposit, effectively excluding the possibility of nodulation initiated by suspended slimes. Another starter sheet provided by the CCR Refinery was extensively covered by tiny nodules up to 3 mm ,in size. Multiple nodule growths are common and the nodules adhere firmly to the copper sheet. For this.sample, a small amount of uniform copper deposition occurred prior to the development of the nodulation. Although over 50 nodules were examined, no slimes particles were detected, and the conclusion is that the nodulation in this .sample is caused by variations in the electrolysis conditions, such as improper ratios of the addition agents.

REFERENCES

1. J.M. Schloen and W.G. Davenport, "Electrolytic Copper Refining - World Tankhouse Operating Data". C o ~ ~ e r '95 - Cobre '95: Electrorefinine and -, W.C. Cooper, D.B. Dreisinger, J.E. Dutrizac,H. Hein and G. Ugarte, eds., Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Quebec, 1995,3-25.

2. J. Lenz, Personal communication. Kidd Metallurgical Division of Falconbridge Limited, Tirnmins, Ontario, 1999.

3. 0. Pogacnik, M. Guilbert, H. Persson, Y. Fiset and C. Belanger, "Permanent Cathode Modernization at the Noranda-CCR Refinery". Copper '99 - Cobre '99: Electrorefining and Electrowinning of Copper, this volume.

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T.T. Chen and J.E. Dutrizac, "A Mineralogical Study of the Deportment and Reaction. of Silver During Copper Electrorefining?'. Metall. Trans. 20B, 1989, 345-36 1.

H. Bombach, K. Hein and D. Schab, "Studies on the Cu+ and 0, Content of Copper Sulphate Electrolytes DuringlCopper Refining Electrolysis". Errmetall 48, 1995, 703-7 1 1.

T.T. Chen and J.E. Dutrizac, "Mineralogical Characterization of Anode Slimes - 11. Raw Anode Slimes fiom Inco's Copper Cliff Copper Refinery". Can. Metall: Duarterlv 27, 1988, 97-105.

T.T. Chen and J.E. Dutrizac,"Mineralogical Characterization of Anode Slimes from the Kidd Creek Copper Refinery". The Electrorefininp and Winnine of Copper, J.E. H o h a n n , R.G. Bautista, V.A. Ettel, V. KudrylLand R.J. Wesely, eds., Minerals, Metals and Materials Society, Warrendale; Pennsylvania, 1987, 499-525.

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15. T;T. Chen and J.E. Dutrizac, "The Behaviour of Silver During the Electrorefining of Copper". Precious Metals '89, M.C. Jha and S.D. Hill, eds., Minerals, Metals and Materials Society, Warrendalej Pennsylvania, ,1~988,377-390.