Paper -1

Quality Control of Copper Cathodes: An overviewby Balachandran.P.Kamath, B. Latchhayya,

Kiran Prasad Shetty *

Paper presented at the seminar on

“Testing & Quality Control of Copper & Copper Alloy Products”held at Mumbai on 8-9 December, 1999.

Introduction:Copper Electro refining has been established as an industry since 1871 at Nevark, in United States. The basic operating process has remained the same, but there has been tremendous improvement in its application. All operations have become automated. The permanent cathode technology has been one of the largest single factors responsible for improvement of quality.

The main driving force for improvement of cathode quality has been the market demand for better quality of rods. Till the first half of the century, a copper with a purity of 99.95 % was considered good. Today, the copper quality is not decided by the purity level, but by the type and level of individual impurities

This paper discusses some basic aspects, which are important to a Refinery operator. It also dwells into certain critical facets of electrorefining, which although well known are not felt important by the production personnel. It may be emphasized that in refinery the problems get out of hand over a period of time and it takes equally long time to bring back the operations to normalcy. The initial indications of problems are innocuous but suddenly they take a turn in the form of poor efficiency, poor quality, high scrap percentages or higher power consumption.

Mechanisms affecting Cathode Quality:Theoretically, the cathodes produced should be absolutely pure under ideal conditions. This is because the impurities which are more reactive than copper dissolve but do not deposit. Impurities which are less reactive than copper do not dissolve at all and settle at the bottom of the cell as slimes. In real practice, however, the ideal conditions do not prevail under industrial

conditions and the quality is affected. The primary reasons are as under:

a) Co-deposition with copper

b) Entrapment of electrolyte

c) Entrapment of slimes.

Co deposition of impurities:

It is well known that each element has an electrode potential for deposition. The Electro deposition potential for copper is much higher than the impurities with the exception of Silver. Co deposition of impurities along with copper takes place under the following set of conditions:

Very low copper content in electrolyte

Presence of silver in electrolyte in dissolved form

High voltage conditions due to passivation etc.

Lowering copper concentration of electrolyte to less than 20 gms/ Litre causes deposition of impurities. This may take place in De copperising or Liberator cells but not in Electro refining cells.

Silver dissolves to a small extent in Electrolyte and gets electro deposited since it’s electrode potential is higher than Copper. This however is limited to below 10 ppm normally. Chlorine ion causes the Silver to precipitate as Silver Chloride which is highly insoluble in aqueous electrolyte.

Entrapment of electrolyte:

Entrapment of electrolyte is another parameter causing deterioration in Cathode quality.

*THE AUTHORS ARE WITH STERLITE INDUSTRIES (I) LTD.,COPPER DIVISION,SILVASSA.1

Nickel, Arsenic and Iron are some of the impurities, which get entrapped along with the electrolyte. Entrapment is enhanced due to rough and spongy cathode deposit. To prevent this occurrence, one should keep the level of these impurities below certain limits. Also, the deposition condition in cells should be such that spongy and nodular deposits are not formed.

Entrapment of slimes.

Presence of suspended particles in electrolyte causes the cathode to deteriorate. Besides, slime causes nodulation in cathodes, which causes entrapment of more slimes and electrolyte. The entrapment of slimes also causes more nodules to form unless the reagent addition is satisfactory.

Measurement of Cathode Quality Parameters:1. Visual Inspection :

The first and simplest method is to carry out regular visual inspection of cathodes during production. In case of ISA process, observing the operation of the Cathode Washing and stripping machine would give the best indication to the process operator.

It is well known that use of certain addition agents give striations on cathode surface. Flow pattern has a big bearing on the deposit, which can be to some extent seen by looking at the striations. Daily inspection of cathodes while stripping can give clear indications of flow patterns in cells, problems of deposition if any and even some indications about the consistency of addition agent presence in cells. It is absolutely essential that all persons connected with electrorefining inspect the stripping on a day to day basis and correct various operating parameters regularly, albeit making very small changes.

2. Metallographic Examination

To have a better control over the cathode growth, the crystal structure of the cathode can be viewed using microscope. A sample is taken at right angles to the direction of growth, polished and etched to reveal the grain structure. The structure will indicate whether the reagents are added at correct level and if the reagent delivery system is functioning correctly. Power interruption lines, slime and electrolyte

entrapments are also visible. Figure – 1 gives the microscopic structure of fine grain copper deposit.

Fig – 1: Microscopic structure offine grain copper deposit.

3. Analysis :

Several methods of analysis are used for cathodes. However, the segregation of impurities in cathodes has made it very difficult to get a representative sample of the cathode lot. The distribution of impurities is not uniform, tending to concentrate at top, bottom and edges. Different impurities show bias at different locations. Many methods have been suggested but none of them have been accepted universally. The problem is in getting a representative sample and not of analysis. Hence the method of sampling cathode will influence the level of impurities found.

The instrument for analysis which are adopted are:

1. DC Arc spectrometer

2. Spark Spectrometer.

3. ICP

4. Atomic Absorption Spectrophotometer

The two most commonly used sources for detection of impurities in copper cathodes are low voltage DC arc spectrometer and high voltage AC spark spectrometer

DC arc spectrometer continuous to be an important spectroscopic technique for solid samples, while other techniques such as ICP

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and Atomic absorption work well for liquid samples. The digestion necessary to put the solid sample in to a liquid will in many cases dilute the sample beyond the detection limits obtainable with these techniques. In addition, some sample resists digestion even in strong acids.

a) DC Arc Spectrometer

Principle:The essential parts of a spectrograph are a slit, an optical system and a camera for recording the spectrum. The light from the source of radiation passes through the slit, which is a narrow vertical aperture, then through the optical system, which includes a prism or grating. An image of the slit is produced by means of lenses at the point where the light is recorded. One such image is produced for each radiation having a specific wavelength, and the result is a series of vertical line images, which constitute the spectrum of the element being investigated. The optical system may be either of glass or quartz; the latter transmits in the ultraviolet region, where many useful lines occur, as well as in the visible region

The holders for arc and spark excitation are housed in a metal box fitted with a safety shield.

b) Atomic Emission Spectrometer

Principle:In atomic emission spectroscopy, a minute part of the sample is vaporized and thermally excited to the point of atomic emission. An electric arc or spark supplies the energy required for these processes, or more recently by a laser or plasma composed of inert gas. The atomic spectrum emitted by the sample is used to determine its elemental composition. The wavelength at which the intensity measurement is made identifies the element, whereas the intensity of the emitted radiation quantifies its concentration.

The instrument may be divided into three major components.

1. The sampling device and source

2. The spectrometer.

3. The detector and reader.

The Sampling device and the source depend on the type of the sample and the analytical data desired. As the emission spectrum emerges from the source it is focussed on to the spectrometer’s entrance slit, where it is dispersed in to its component wavelengths. The optical elements permit site selection within the source discharge and thus eliminate the need to move the entire source assembly. The use of mirrors, properly placed also eliminates the chromatic & spherical aberrations encountered with lens optics. At the exit slits of spectrometer, the radiation is sensed by a photodetector.

No single source is best for all applications. Factors that influence the selection of an excitation source are the concentration of the elements being determined, the vapor pressures or volatility’s of these elements, the excitation potentials of the atomic lines used in the analysis and the physical condition of the sample. For solid samples, arc excitation is more sensitive, while spark excitation are more stable. Plasma sources are choices for solutions and gaseous samples.

Factors governing the cathode quality.1. Anode composition and behavior of

impurities during electrolysis:

Copper anodes of different origins will have different chemical compositions, thereby different behavior during electrolysis. Major impurities in copper anodes are Ag, As, Au, Bi, Fe, Ni, Pb, S, Sb, Se, and Te apart from oxygen. The accumulation of these impurities can lead o serious complications in electrorefining. Typical anode composition is given below( Table-1).

Table-1 : Typical Anode composition

Element Concentration

Cu 99.3 % - 99.7 %

Oxygen 1200–1500 ppm

Sulphur < 20 ppm

Iron < 20 ppm

As, Ag, Bi, Sb, Pb, Ni

Should be as consistent as possible from cast to cast.

Ag/Se Molar ratio >1

As (Bi+Sb) molar ratio

> 2

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Anode composition plays a vital role in cathode quality. The approximate distribution of the principal elements contained in anode between cathodes, electrolyte and slime is given below in %. ( Table-2).

Table – 2 : Distribution of anode impurities in copper electrorefining .( in %)

Element Cathode

Electrolyte Slime

Gold 1-1.5 - 98.5-99

Silver 2-3 - 97-98

Copper 98 1.9 0.1

Se & Te 1-2 - 98-99

Lead 1-5 - 95-99

Nickel 15 75 10

Sb 20-30 20 5-60

Sn 20-30 20 50-60

S 3-5 - 95-97

Fe 10-20 80 10-20

Zn 2 94 4

Al 3 77 20

Bi - 22 79

AS 10 70 20

Impurities can be divided into following groups according to their behavior in the electrolysis process.

Ni, Zn, Co, Fe :

These are the metals that are more electronegative than the copper and pass almost completely in to the solution, thereby gradually accumulating in the electrolyte. During the electrolytic refining of copper, nickel accumulates in the electrolyte faster than the other impurities, since its content in the anodes is always greater than the content of any elements of this group. Nickel, iron and zinc contained in the electrolyte in comparatively large quantities do not affect the composition of the cathodes but may give rise to unfavorable effects in the electrolysis of copper. Sometimes small amount of iron and nickel do nevertheless appear in the cathode copper. This is explained by the fact that iron and nickel sulphates remain in the cathode either with the electrolyte in the form of inclusions in the pores and inter crystalline cavities or on account of the poor washing of electrolyte from the cathodes. Fig–

2 gives the microscopic structure of cathode deposit entrapped with electrolyte.

Fig – 2: Microscopic structure of Cathode deposit entrapped with electrolyte.

When the electrolyte contains large amount of nickel, iron and zinc sulphates, the solubility of copper sulphate decreases, and the solution becomes supersaturated with copper sulphate; as a result its crystal may separate. Consequently the concentration of copper ions in the layer of electrolyte adjacent to the cathode may become too low and hydrogen will be released at the cathode together with the copper. The current efficiency will consequently decrease and the quality of cathode deposit will deteriorate.

It is necessary to ensure that the electrolyte contains little of the these metals

Ag, Se, Te, Au:

These metals are more electropositive than copper and almost completely pass on to the slime. Metals which do not dissolve in sulphuric acid solution (Pb, Sn) and also relatively week electrochemically active compounds of Cu2Te, Cu2Se and Cu2S also belongs to this group. . Analysis of slime indicates predominately silver bearing compounds Ag2Se, (AgCu)2Se and their analogous tellurides. To prevent physical inclusion of these compounds in copper cathode, it is required to maintain the atomic ratio Ag / (Te+Se) at more than 2.

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For silver contamination of cathode there exists the possibility of electrolyte deposition at the cathode limited by the solubility of silver in the electrolyte. When the anode contains a large amount of silver some of it can pass in to the solution in the form of AgSO4. In order to prevent silver from accumulating in the electrolyte and possibility of codeposition along with the copper, it is precipitated in slime as AgCl by adding Chloride Ion Cl-.

Factor affecting the level of selenium contamination level in cathode copper is the co deposition of selenium. Normally the concentration of Se is low (<O.2ppm), but care should be taken to avoid any contamination, particularly by silver refinery leach liquors. Selenium forms complex with thiourea in the ratio Se (IV) : CS (NH)2::1:4 particularly when there is insufficient silver in anodes to precipitate silver as Ag2Se. Formation of these compounds could increase the solubility of certain selenides and at the same time is absorbed on the cathode surface. High concentrations of Te in the electrolyte result in contaminated copper with poor drawing properties.

Gold does not dissolve in the electrolyte and completely goes to the slime.

As, Bi, Sb:

These metals have potentials very near to the copper and are most harmful, as they readily pass on to the solution and remains in the solution and co deposits with the copper under conditions of the low copper concentration, high concentration of these metals in the electrolyte and high current density. The presence of these impurities reduces the electrical conductivity, ductility and malleability of copper. The incorporation of As, Sb and Bi in cathode can be due to one or more of the following: anode slime occlusion, electrolyte occlusion, direct precipitation of impurities from solution apart from co deposition(3).

Arsenic presents in electrolyte as AsO43-. When

the concentration of bismuth and animony in electrolyte is low arsenic does not have appreciable effect on the electrolytic process. But if the electrolyte contains higher concentration of Bi and Sb , then it forms insoluble compounds Bi2O3.As2O5, and

arsenates BiAs04, SbAsO4 which separates from the electrolyte in the form of fine particles, forming floating slime. This slime adheres to top of the cathode forming top nodules and contaminates them.

Fig – 3: Solubilities of As and Sb in electrolyte.

The Sb in the anode dissolves in the electrolyte in the trivalent state. . Then in the presence of dissolved oxygen in the electrolyte, Sb (III) oxidises in to Sb (V) State. Studies reveal that Sb (V) contributes significantly in floating slime formation. The floating slime formation could be controlled, if As/(Sb+Bi) molar ratio is maintained at more than 2. The solubility of arsenate compounds is strongly influenced by temperature. One investigation has shown that a temperature decrease from 60OC to 50OC results in 20% decrease in the solubility product for antimony arsenate.(5).

Ideally cathode quality will be enhanced by maintaining low Sb concentration and by maintaining an electrolyte well under the saturation limit of arsenates. Fig - 1 shows the solubilities of Sb and As in electrolyte

One method for controlling the floating slime is the Bolidens process. The Bolidens process for eliminating the float slimes is based on preventing the oxidation of Sb (III) to Sb (V). It was found that oxidation of, As (III) to As (V) is more rapid than is Sb oxidation. Thus addition of As (III) to electrolyte will minimise the formation of Sb (V) preventing float slime formation. The arsenic additions will have a similar effect on Bi concentrations. Increasing content of arsenic in anodes also helps.(5)

Other processes for removing Sb and Bi are detailed in Electrolyte purification section.

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OXYGEN:Oxygen present in the anode as copper oxide (Cu2O). This chemically dissolves by electrolyte, as opposed to the electrochemical dissolution of metallic copper.

Cu2O + H2SO4 CuSO4 + H20 + Cu

This copper which is accumulated in the electrolyte must be removed, so that copper concentration remains constant and passivation of anodes is avoided. This is achieved by using liberator cells. More on oxygen and passivation is discussed in coming sections.

SULPHUR:

Although anode sulphur content does not have any direct effect on the cathode sulphur, sulphur content in the anode should be prevented for any spewing at the desired oxygen level.

2. Current Density

Current density (CD) is an important parameter in copper electrorefining as operating at higher current densities lead to decrease in purity of copper deposit. CD should be optimized to get higher production rate and at the same time higher copper cathode purity. The affect of CD on electrorefining can be explained as below(2).

At higher CD, copper deposition at cathode increases, but at the same time it could lead to dendritic growth at cathode surface. This causes slime and electrolyte to get entrapped on the cathode and lowers cathode purity.

Also at higher CD , chances of anode passivation are more. The anodes produce Cu+

+ ions at faster rate then they convect away leading to a higher concentration of Cu++ ions at anode surface resulting in a layer of copper sulphate. The passivation of anodes having higher oxygen and low As/(Sb+Bi) ratio can be partially overcome by operating at lower current densities.

At higher CD, chances of impurities such as As, Bi and Sb , co depositing with the copper is more.

3. Electrode Alignment

Electrode alignment has a significant effect on cathode purity as it determines the current density across electrodes. If the electrodes are not spaced equally, more current passes through the section where the distance between anode and cathode is less, due comparatively lesser resistance and less current through the section where distance is more. This result in uneven distribution of current across cell and localized high current density areas are formed resulting in the detrimental effects as discussed in previous section. It is important to keep the anode and cathode contacts with the busbar cleaner, as it is another cause for uneven distribution of current.

Also presence of any bent plates can entrap slime deteriorating cathode purity.

4. Electrolyte composition

Copper refining electrolyte normally has following composition

Table – 3 : Electrolyte Composition

Concentrations

Copper 40-50 gpl

Sulphuric acid 160-180 gpl

Total Sulphates < 300 gpl

Nickel <15 gpl

Arsenic < 15 gpl

Iron < 3 gpl

Sb < 500 ppm

Bi <300 ppm

Chloride 25-50 ppm

And various other impurities dissolved from anodes. Apart from this they also contain additives added for facilitating smooth deposition of copper. The temperature is maintained AT 62-66OC and continuously circulated at required rate. It is essential that electrolyte composition be maintained at optimum level and impurities at as low as possible to avoid any harm to the electrorefining and thereby cathode contamination.

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The copper concentration in electrolyte during copper refining is primarily depends on the anode oxygen content. Copper ions in solution must be maintained to ensure copper is always available for deposition at cathode surface, at level far in excess of impurities. Using liberator cells controls copper concentration in electrolyte.

If copper concentration is lowered, the impurities start depositing along with the copper and contaminate the cathode. Too high concentration of copper ions results in crystallization of copper sulfate and chances of anode passivation also increases. This also increases power consumption.

Acid concentration is maintained in the electrolyte as the electrical resistance of cell depends to a considerable degree on the concentration of sulphuric acid in the electrolyte. If the acidity of electrolyte is too high then following unfavorable processes occurs

The dissolution of anode increases and consequently copper ion concentration in electrolyte increases. This leads to an increase in number of cells with insoluble anodes, which is undesirable since in these cells the cathode purity is low and power consumption increases. The chemical dissolution at the top of cathode also increases and this leads to broken cathode

The passage of silver in to the solution increases and some of it may be deposited on the cathodes.

The concentration of As3+ in solution increases.

Studies(5) reveal that the solubility of Bi in the electrolyte is strongly dependent on the sulphuric acid concentration. . The variation in the concentration of bismuth in the electrolyte as function of acid concentration is given in Fig - 4 .

An increase in the acidity of electrolyte by only 20gpl (from 150 gpl to 170 gpl) may result in the sharp increase of the Bi content in the electrolyte. This rapid change suggests that, in an electrolyte already

saturated with Bi, a drop in acid concentration will induce precipitation of Bi.

Fig – 4: Solubility of Bi in electrolyte.

The average impurity levels in the cathodes at various acid concentrations in the electrolyte are given in Fig –5 & 6 for Se, Sb, Bi, Pb & Ni.

Fig–5 & 6 : Variations of cathode impurities atdifferent electrolyte acid concentrations.

The figure indicates that for most of the cases the impurity levels in the cathode pass through a minimum at an acid concentration in the vicinity of 160 gpl. This trend appears to apply both to the elements which contaminate the

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cathode mainly by slime occlusion such as Se and Pb, and for which the contamination occurs to a greater extent by electrolyte entrapment.

The contamination of the cathodes at low acidities (100-125 gpl) appears to have been caused by the poor morphology of deposits. The tendency for increasing contamination of cathode at higher acidity values of 175 –200 gpl can be attributed to increased viscosity and density of the electrolyte, causing a decrease in the limiting current density and in the rate of slime settlement. Acid level is maintained by adding concentrated sulphuric acid to the system.

Chloride concentration is maintained in the electrolyte due to following reasons.

It prevents the passing of silver to the solution by precipitating it as silver chloride.

Chloride ions work in combination with the thiourea to reduce the grain size of copper deposit via a copper-chloride-thiourea complex.

It also precipitate out antimony and arsenic as antimony oxychloride and arsenic chloride and pass these impurities in to slime

Low chloride levels causes soft cathodes due to grain refinement. High chloride levels causes large columnar based crystals and pile of relatively long, needle like crystals resulting in increase in shorts. Chloride level in electrolyte is monitored daily and are maintained by adding hydrochloric acid.

The another important parameter to be monitored is the total sulphates (total concentration of sulphates associated with the dissolved metals in electrolyte.). If the total sulphate levels go above 300 gpl, there is a strong chance for passivation.

Passivation occurs when copper is prevented from coming off the anode because of copper sulfate barrier at anode surface. This is most undesirable as Passivation causes severe short growth apart from higher power consumption. This leads to areas of high current density and therefore poor cathode quality. Anode affected by passivation will have scrap with uneven surface.

Passivation can be controlled only by reducing sulphates, either by dilution or removal. A slight increase in temperature also increases solubility of copper sulphte, thereby reducing passivation. One more solution for this problem is the periodical reversal of refining current.

Apart from this electrolyte contains major impurities such as nickel, arsenic and iron. Higher the concentration of these impurities higher is the chances of their inclusion in copper deposit. Electrolyte also contains other impurities such as Bi, Sb. In order to keep the level of these impurities it is important to go for electrolyte purification.

5. Electrolyte temperature:

Electrolyte temperature is one important parameter to be controlled in electrorefining. Higher the temperature, higher is the mobility of ions in the electrolyte and, consequently the conductivity increases. Apart from this temperature control is necessary for the following reasons (1)

At higher temperatures the cathode deposit crystal structure becomes much coarser, and the density of cathode decreases.

Too high temperature results in increased solubility of silver in electrolyte and silver co deposits in cathode along with the copper contaminating the copper and silver loss.

High temperature decomposes glue at faster rate and could lead to glue shortage at cathode surface resulting in rough growth.

Low temperature lowers solubility of copper, thereby increasing chances of anode passivation.

Low temperature also increases electrolyte density, so anode slimes have a tendency to float increasing chances of slime inclusion in cathode deposit.

Lower the temperature, higher is the electrolyte viscosity, so electrolyte moves unevenly in cells and anode slime gets entrained in the electrolyte.

Low surface temperature of the electrolyte resulting due to uncovering of cells may

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result in protruding horizontal growth at solution line.

Considering the above facts it is desirable to maintain the temperature in the range of 62-66OC.It is worthy to note here that a section is be cut in, only when the electrolyte temperature is at minimum 550C to avoid any precipitation of impurities at cathode mother blank surface.

6. Electrolyte circulation and flow

Electrolyte circulation plays a vital role in electrorefining of copper. Any major variation in this could lead to cathode deposit contamination. The continuation circulation of electrolyte at desired rate is necessary to:

Maintain optimum electrolyte temperature

Low flow rates provide insufficient reagents and mixing, thereby contributing to rough growth at cathode surface. Also circulation of electrolyte helps in replenishing addition reagents which have been consumed / degraded.

Fig – 7 : Effect of flow rate on deposit at various current densities.

During electrorefining of copper, the solution near the anode is constantly being enriched with copper ions, while that near the cathode is being depleted. Eventually the time arises when the amount of copper ions becomes insufficient for normal electrolysis; the deposition of antimony and arsenic may begin resulting in cathode contamination.

Higher flow rates disturb the slime layer at cell bottom and chances of slime inclusion in the cathode increases. Attempts are therefore made to maintain a circulation rate where the electrolyte is well mixed but the slime is not disturbed.

Hence, given the importance of flow rate, following points be followed to ensure target flow rates are achieved and maintained

Fig – 8 : Change of amount of slime in electrolyte with flow rate

It is important for operating personnel to have a constant watch on electrolyte circulation and flow rate and always be maintained at optimum rate of 25 LPM. A V-notch provision at cell outlet will enable to have a proper control over flow rate.

A look at the cathode deposit and the direction of striations gives complete picture of flow pattern in the cells.

Apart from this as the electrolyte is fed to cells from a common head tank, it is necessary to have all the cells be aligned and kept at same level, so that each cell could experience similar electrolysis conditions.

It is advisable to cut off the current, whenever circulation is stopped or variation in flow rate is observed. Analysis of cathodes taken from the cell where flow is stopped indicates higher contamination due to Bi, Sb, and As. especially at cathode top with dark spots

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7. Colloids addition and control

The grain refining and leveling agents (Colloids) are added to the electrolyte as they assist in smooth, dense deposition of copper deposit. These colloids act as grain refining and leveling agents, without which cathode deposits would be dendrite and soft, entrapping electrolyte and anode slimes.

They most often act as inhibitors. They adsorb on the cathode surface, where they take part in the electrochemical deposition of copper. Some of these additives restrict the growth of existing crystals and thus force the deposition to take place via the formation of new nuclei. The amount of colloids added depends on various parameters such as composition of electrolyte, the current density and others. As said earlier regular look at the physical appearance and quality of copper deposit gives good indication of colloids addition.

Addition rates may vary at different refineries according to the cathode deposit morphology. If the continual delivery of reagents stops, coarse crystal growth occurs after about 4 hrs. If the stoppage is remedied within that time, fine equipped growth resumes, with no discernible loss of cathode quality(4).

Glue {Gelatin}:

Glue consists of large protein molecules and possesses a strong positive charge and gets adsorbed in to the areas in cathode surface, where there is a strong negative charge or high spots. These spots occur where there is a high current flow and copper deposition rate is more. Being a poor conductor of electricity it acts as a inhibitor, preventing any further deposition on these high spots thus enabling cathode to a smoother surface and less shorts. This eliminates entrapment of slime or electrolyte in these areas. Glue is also consumed by hydrolysis in the presence of acid and at high temperature especially during the first half hr of addition. . It is necessary to have a continuous dosing of glue. Inefficient glue supply results in pyramidal copper deposits. However oversupply of glue makes it less effective in cathode surface leveling apart from increasing power consumption.

An instrument called COLLAMAT (practiced in Norddeutsche Affinerie) could monitor glue content in the electrolyte. Using COLLAMAT it

is possible to optimize the glue addition rate, and to have knowledge about flow conditions and glue supply in the cell

Thiourea {C (H2N)2S} :

It is a white crystalline material and dissolves readily in water and helps in smooth deposition of copper by reducing the grain size, increasing hardness, brightness and cathode density. It gets adsorbed to the cathode surface and controls the size and shape of crystals. Studies also reveal that thiourea acts in conjunction with the chlorine ions forming a complex and thus facilitating smooth cathode surface. Thiourea is also capable of prohibiting the polarising effect of gelatine on the deposition mechanism of copper. It is likely that thiourea acts as a solvent for gelatine.

Striations on the cathode surface are the best indication for the thiourea addition rate. Striations are the series of small ridges on the cathode and follow the same direction as electrolyte flow. Excess thioure addition causes deep striations and can entrap slime or electrolyte, thus reducing the cathode purity. Thiourea does not decompose, but it is apparently consumed in connection with the deposition of copper.

Since thiourea is a sulphur bearing compound, higher addition rate could lead to sulphur contamination of cathode. Cathode flexing sound and its strippability gives a good indication for thiourea addition. Thiourea in electrolyte can be monitored by reatrol system

Avitone:

It is a sulphonate surfactant and enhances the action of gelatin by allowing the protein to uniformly spread out and adsorb over greater area.

8. Suspended solids and electrolyte filtration

The solid particles contaminating the cathodes are either anode slimes containing elements or compounds which are not dissolved during the electrolytic process or are formed in the bulk of electrolyte through the increasing concentration of insoluble compounds. The extent of cathode contamination due to the occlusion of anode slimes may depend on various electrorefining

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parameters such as anode composition, the morphology of the cathode deposit, anode passivation and the characteristics of the slime i.e. its specific gravity and particle size.

Insoluble solids formed in the bulk of electrolyte are generally rich in As, Bi and Sb. Depending on their respective concentration in electrolyte; these elements can for from wide range of insoluble compounds such as antimony and bismuth arsenate (floating slimes). These compounds may have a detrimental effect on the quality of copper deposit.

Floating slime can also be formed by contamination of electrolyte with oil, lubricants or any other petroleum products.

Apart from floating slime there exist suspended particles. The particulate can be classified in to two groups as follows.

Nodule producers: Cu, C, Sb, Se, Ag

Non producers of nodules :PbSO4, PbO2,

Cu2Se, CaSO4.

Electrically conducting and semi conducting particulates causes nodules on copper cathodes whereas non-conducting particulates do not. Surface characteristics of deposits grown at various temperatures and with various paticulates present at standard conditions is given below in Table – 4.

Table – 4: Effect of various particulates on Cathode deposit.

Particulate 500C 700C

Cu Moderately smooth nodules

Very few nodules

C Very rough nodules

Round and moderately rough nodules

Sb Moderately rough nodules

Quite smooth and rounded nodules

Se Round and smooth nodules

Round and smooth nodules

Ag No nodules No nodules

The extent of entrapment of particulate depends much more on the deposit quality than on the availability of contaminating particulate.

Increasing temperature decreases solution density and viscosity, thereby enabling slime to settle at the bottom.

Continuous filtration of the electrolyte helps in reducing the total suspended solids content in the electrolyte. It is desirable to filter 100 % of the electrolyte entering the electrolytic cells to maintain the suspended solids well within the limit of less than 2 ppm. It is important for the production personnel to monitor the efficiency of polishing filter, the filter cloth conditions etc. on regular basis and to take the corrective actions immediately as any negligence on this part could lead to increase in suspended solids in the system thereby reducing the cathode purity due to slime entrapment. The efficiency of the filter is monitored by measuring the suspended solids at inlet and out let of polishing filter.

One more possible method for decreasing the suspended solids content in the electrolyte is the addition of flocculating agents. Flocculating agents consists of large organic molecules, which have anionic, cationic or non-ionic characteristics. In some refineries flocculent such as Prercol-351 (Colloids Canada Inc), Superfloc-210 (Cyanamid Canada Inc.) are used to settle down the suspended solids to bottom slime layer. For flocculating agents to be successful, it is essential that its addition does not have any affect on cathode morphology.

The suspended solid content in electrolyte need to be analysed daily to have a better control over their content in the electrolyte. Fig-9 shows the microstructure of copper deposit with slime entrapment.

Fig – 9: Microscopic structure of cathode deposit with slime entrapment

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9. Other factors.

Power disturbances: - Cathode quality may deteriorate if there occurs frequent power interruption, resulting in formation of electrolyte layer on the cathode surface and further deposition taking place over the this layer. It is advisable to increase the current in increments to avoid any drastic changes taking place. Also temperature of the electrolyte to be maintained to avoid precipitation of impurities at the cathode surface whenever the current is switched off. Fig - 10 shows the power out lines in cathode deposit microstructure.

Fig – 10 : Microscopic structure of cathode deposit with “power out” lines.

Electrolyte volume shrinkage and drastic changes in electrolyte parameters could also result in poor quality of cathode. Volume shrinkage will result in increase in concentration of impurities thus harming the electrorefining process.

Cathode washing is important, as it is necessary to remove the adherent electrolyte salts on the cathode surface. Also sometimes dilute suplphuric acid is added to the wash water to maintain the level at 2 gpl as it helps in removing the oxide layer. Also cathode mother blanks should have minimum temperature of 65oC before placing in electrolyte, as there is chances of electrolyte impurities precipitating out at surface, when cold plates are put in to the electrolyte.

Electrolyte PurificationMany impurities pass on to the electrolyte when anode dissolves and over a period of time they accumulate in the electrolyte and when the concentration of these impurities exceeds the limiting concentration, they deteriorate the electrorefining process leading to poor cathode quality. Following methods are used in controlling the impurities.

1. Electrolyte Bleeding

Bleeding of electrolyte, low in copper helps in removing major soluble impurities of copper (derived from soluble Cu2O), arsenic, nickel, iron, antimony and bismuth. This can be achieved by:

Copper – Electrodeposited in liberator cells using insoluble lead anodes (Electrowinning) in three stages.

Arsenic - Co deposited with the copper during second and third stages of Electrowinning. A major proportion of it could be removed by solvent extraction. The arsenic is recovered in solution, which is then combined with copper sulphate to produce copper arsenate byproduct for sale.

Antimony/Bismuth – Co deposited with the copper during second and third stages of Electrowinning. A significant portion of this also precipitates out on cell walls and pipes, from where it could be removed. Also some selective removal processes are available for removing Sb and Bi, which are discussed in coming section.

Nickel/iron - Crystallised in the form of crude hydrated nickel sulphate by evaporating the decopperised electrolyte in evaporating pans. The concentrated black acid is sent back the electrolyte circuit to maintain the acid balance. A small of this is discard to prevent a gradual build up of Ca, K, Mg and Na ions.

Surface electrolyte bleeding process.(6)

At the top 10 mm layer below the surface of electrolyte inside the cell, the copper concentration diminishes very suddenly to about 35 gpl. The difference between this portion and the bottom portion is nearly 10 gpl. However impurity concentration is same at all layers. This enables us to take the surface

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electrolyte from the rest of the solution, which has already been decopperised by 15 –20 %. If this surface electrolyte, low in copper is sent to the purification section, the quantity of electrolyte purified would be increased by around 20 % and the impurity removal quantity will also be higher.

Acid recovery and neutralization process

Electrolyte is bled for electowinning cells at less than 2 gpl copper and around 250 gpl acid concentration. The electrolyte is passed through sorption chamber containing ion exchange resins, where acid is adsorbed in resins and resultant electrolyte is sent to neutralisation section. The acid could be recovered from the resins by desorption.

The electrolyte from sorption chamber is then neutralised in a mixing tank using caustic or Soda ash. The pH is maintained at 10-11. During neutralisation most of the impurities are converted to their respective hydroxides or carbonates and precipitates out from solution. The tank content is filtered in a filter press. The impurity sludge collected from filter press is bagged. The clear liquid is then subjected to evaporation, to recover sodium sulphate sludge.

2. Selective removal Processes.

a) Chelating Resin Adsorption Process.(6)

Since antimony & bismuth are the main component of the scales present in the pipes and of floating slimes, it can be called as the most troublesome impurity in electrorefining. Unitica Co. developed a chelating resin (UR-3300) which removes Sb and Bi effectively by adsorption (proposed by Tamano refinery of Hibi Kyodo Smelting Company). As the electrolyte passes through the packed column of chelating resin, Sb and Bi are selectively removed. They are then desorbed by means of hydrochloric acid, and then recovered in the form of a highly concentrated antimony chloride solution by means of evaporation. Purified electrolyte is then sent back to the circulation section.

This process doubles the capacity for removing antimony from electrolyte..

b) Activated carbon solvent adsorption

Activated carbon is known and is widely used as adsorbent, but this is rarely utilized in strong acid solution such as copper electrolyte. Sumitomo Copper refinery developed a technique for separation of antimony from electrolyte using activated carbon under specific conditions. The presence of As plays a major role in this adsorption as antimony gets adsorbed in the form of Sb AsO4.

c) Solvent extraction

Zeneca Inc. has developed a reagent called SBX-50, which is suitable for selective removal of bismuth and antimony through solvent extraction method. Sbx-50 is a long chain alkyl ester of phosphoric acid. BHP copper uses this method for Sb and Bi removal.

d) Norddeutsche Process(5)

It is based on the selective adsorption of As, Sb and Bi from electrolyte with stannic acid. The process not only helps in eliminating float slime formation by reducing the Sb concentration but the SbAsO4 and Bi AsO4 equilibrium are broken. The resultant electrolyte becomes undersaturated and arsenate precipitation is eliminated.

e) Barium Carbonate Treatment(7).

This process to remove bismuth in electrolyte was developed in Outokumpu’s Pori Copper refinery. The process is based on the co-precipitation of bismuth from the electrolyte by means of poorly soluble barium sulphate.

The feed electrolyte is fed to a mixing tank; in to which barium carbonate is added. Barium sulphate precipitates, when barium carbonate decomposes under the effect of the acid content in the electrolyte. The forming sulphates co precipitates bismuth and part of antimony . The precipitate is separated by filtration and the filtered electrolyte is sent back to the circulation tank.

The disadvantage of this method is the disposal of barium sulphate, which contains Bi and unreacted barium carbonate. (Salts of heavy metals Pb, Sr can also be used instead of Ba salt.)

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f) Detellurisation

High concentrations of Te in the electrolyte result in contaminated copper, which has poor drawing properties. If the refinery operates with the near zero discharge bleeds, the Te which is dissolved in the decopperising operation of slimes (leaching) could return to the refinery. The tellurium is removed by cementation on Cu at 950C in up flow fixed bed reactor.

Apart from above other processes used for selective removal of impurities whenever impurity level in electrolyte exceeds specified limit are

Combined solvent extraction of As, Sb and Bi using hydroxamic acid and stripping using H2S.

Removal of Bi from electrolyte using Superlig materials ( Superlig 83)involving macrocyclic affixed to silica supports,

Addition of SO2 to reduce pentavalent Sb to trivalent state and the addition of chelating agents for Sb and Bi removal (Hitachi process).

ConclusionFrom the above discussions, it can be concluded that the high cathode quality can be obtained if following parameters are maintained.

Cu++ irons constant availability at cathode surface

Optimum current density and electrode alignment

Availability of appropriate quantity of additives over cathode surface constantly

Low concentration of impurities in electrolyte.

Uniform electrolyte temperature and flow in electrolytic cells

Absence of slime particles in electrolyte.

Acknowledgement:The authors wish to express their deep sense of gratitude to the management of Sterlite Industries (I) limited, for giving total support to us with our endeavor to improve quality of our cathode and wire rods to international standards. We also grateful to them for granting the permission to attend this seminar and present the papers.

References:

1. A.K.Biswas and W.G.Davenport, “Extractive metallurgy of copper”, Pergamon Press, 1976.

2. C.L.Mantell.”Electrochemical Engineering”

3. T.B.Braun, J.R.Rawling, and K.J.Richards, “Factors affecting the quality of electrorefined cathode copper.

4. 0.Forsen, L.Zhu and A.E.Antila.,” Electrochemical Characterization of gelatine , thiourea and chloride ions in the electrorefining of copper”.

5. V.Baltazar and P.L.Claessens, “Factors affecting the cathode purity during copper electrorefining” (AIME, Las Vegas, 1976).

6. T.Shibata, M.Haschiuchi, and T.Kato, ”Tamano refinery’s new processes for removing impurities from electrolyte”.

7. Olli V.J.Hyarinen, “Process for selective removal of bismuth and antimony from an electrolyte especially in electrolytic refining of copper”.

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