ADDING VALUE TO LIX / SX / EW OPERATIONS BY RECOVERING COPPER FROM BLEED, RAFFINATE AND OTHER PLANT

STREAMS

Richard E. Dixon HATCH Associates

Av. El Bosque 500, 12th Floor Santiago Chile

rdixon@hatch.cl

Rudi Fester, Carlos García, Carlos Contreras, Fernando Romero Compañía Minera Doña Inés de Collahuasi

Baquedano 902, Iquique-Chile

Ian D. Ewart Electrometals Technologies, Ltd.

206 – 3711 Delbrook Avenue North Vancouver BC V7N 3Z4

ABSTRACT

Copper SX/EW plants often bleed part of their electrowinning electrolyte to control

impurities and maintain consistent cathode quality. Depending on the specific situation at each site, the bleed is either returned to raffinate, to an SX extraction stage, to an SX wash stage, or to the PLS pond, to avoid losing the copper in the bleed. This practice can have undesirable effects on the overall process. An alternative is to electrowin all or most of the copper in the bleed, thus avoiding the recirculation of the copper in the bleed. In this manner, the bleed is converted to metallic copper without interfering with the SX circuit. This in turn reduces the copper concentration in the raffinate, improving global copper recovery because of improved leaching conditions.

INTRODUCTION

The main objective of an electrowinning tankhouse is to produce the largest possible amount of copper while maintaining high cathode quality. Achieving a high copper production implies applying a high current to the electrowinning cells. However, if the current density is increased beyond a certain point, undesirable reactions begin to occur, such as the degradation of organic carried over from the SX stage or the degradation of Galactosol (guar) that is usually added to the electrolyte to improve copper deposit quality. These reactions result in imperfections in the deposit and therefore lower copper quality. Additionally, depending on each site, chemical impurities begin to accumulate in the electrolyte, causing other problems such as those shown in Table 1:

Table 1 – Electrolyte Contaminant Issues

Contaminant Typical range Potential Problems at Higher Concentrations Cl 10 – 50 ppm Cathode Stripping Problems Fe 1-3 g/L Current Efficiency loss (2% to 3% per g/L of Fe) Mn 10 – 500 ppm Faster degradation of Lead Anodes

Because of these problems, many electrowinning tankhouses bleed a portion of their electrolyte in order to reduce the concentration of contaminants and to reach a steady state for the concentration of impurities that allows for acceptable operating conditions.

There is significant variability in this practice, as some tankhouses practically do not bleed any electrolyte and others bleed large amounts, as can be seen in Tables 2, 3 and 4, in which data are summarized for 30 Copper EW Tankhouses around the world from data compiled by Robinson [1] on SX/EW plants.

Table 2 – Typical Copper Electrowinning Bleed Volumes Bleed Amount (as % of total annual Cu production) Number of Plants % of Total 0 to 0.2% 6 20 0.2 to 0.4% 6 20 0.4 to 1% 6 20 1 to 2% 4 13.3 2 to 3% 3 10 3 to 4% 3 10 Over 4% 2 6.7 Total 30 100

Table 3 – Reason for Bleeding

Contaminant Number of Plants % of Total Fe 7 35 Fe + Cl 1 5 Fe + Mn 4 20 Cl 4 20 Cl + Mn 3 15 Mn 1 5 Total 20 100

Table 4 – Destination of the EW Bleed Destination Number of Plants % of Total E1 9 41 PLS 2 9 SX Wash 4 18 Raffinate 7 32 Total 22 100

It is noted that the total number of plants reporting is different in each table because information is not available for all plants about all aspects of their operation.

Recirculating the bleed has deleterious effect on the LiX – SX – EW operation. If the bleed is sent to the Raffinate pond, the increased concentration of copper in the raffinate causes a decrease in the leaching power of the raffinate (because the chemical potential of the leaching reaction decreases), which results in a lower recovery of copper from the heap.

If the bleed is returned to an extraction stage, the copper and acid concentrations are increased. As a result, the recovery in the SX stage is reduced, which is normally compensated for by increasing the extractant concentration, which in turn results in increased carryover and loss of organic.

If the bleed is sent to an SX wash stage, the acid concentration is increased and a loss of copper occurs, because the wash stage behaves like a stripping stage. In this case, the bleed also increases the amount of impurities, thus decreasing the efficiency of the wash stage.

Finally, if the bleed is sent to the PLS pond, the copper concentration entering the SX stage is increased, which means that it is necessary to increase the concentration of extractant, causing increased organic carryover and loss.

To avoid recirculating the electrolyte bleed, it would be necessary to remove most of the copper in the bleed. However, this is not common practice, because conventional electrowinning cells do not produce high quality copper when the copper concentration in the electrolyte is low, so that the bleed is not processed to remove copper and is instead recirculated to another part of the plant.

This paper describes a way to avoid recirculating the bleed by using EMEW electrowinning cells to remove the copper from the bleed. Earlier work by Escobar et al. [2] has shown that EMEW technology allows for the production of high purity copper from dilute copper solutions, maintaining Grade A quality copper down to copper concentrations of around 3 grams per liter. EMEW technology has important process advantages compared to traditional electrowinning technology, because it is not necessary to heat the copper electrolyte, or to add any chemical additives to the electrolyte. Additionally, there are Environmental advantages because the cells are closed and acid mist is not emitted to the atmosphere inside the electrowinning facility, thus avoiding the use of specialized clothing, respirators, and acid mist handling equipment.

In order to evaluate the feasibility of the concept of eliminating the recirculation of the EW bleed streams, the following results from pilot programs using EMEW technology carried out at Codelco Norte (Radomiro Tomic), Enami Salado, and Cia. Minera Doña Inés de Collahuasi results are presented.

EXPERIMENTAL

Pilot tests were carried out with an EMEW plant containing six full-scale EMEW plating cells and four full-scale EMEW powder cells. The cylindrical plating cells are 150 mm in diameter and 1500 mm in length. The internal DSA cylindrical anodes for plating cells are 50 mm in diameter. EMEW powder cells have a smaller electrode gap than plating cells to compensate for the lower conductivity of the electrolytes used with powder cells, which typically are run at a maximum copper concentration of 2500 ppm and a minimum copper concentration of 100 ppm. Powder cells are similar to plating cells, except that cathode diameters are 200 mm and DSA anode diameters are 180 mm. Current densities ranging from 200 to 1000 ampere per square meter were used. Copper concentrations varied between 45 and 2 grams per liter for runs using plating cells, and between 3000 and 100 ppm for runs using powder cells. Harvested copper cathode weights were between 12 and 30 kilos, which were analyzed to determine their level of impurities.

DISCUSSION

Codelco Norte Tests

Pilot tests were carried out at Codelco Norte using an EW bleed electrolyte with the following typical initial composition:

Table 5 – Codelco Norte Electrolyte Composition

H2SO4 175 to 210 Cu 36 – 42 g/L HNO3 4 – 60 ppm Cl 13 – 36 ppm Co 120 – 250 ppm Al 40 – 60 ppm Fe 1 – 1.2 g/L Fe ++ 10 – 160 ppm Mn 3 to 10

Table 6 shows the results of several runs where copper cathode was harvested at the end of each run. Different runs used varying copper concentration, varying current density and varying process flow rate. The first four runs were performed on the same electrolyte recirculating from a closed process tank. For the first run, the initial copper concentration was 39 grams per liter, and the copper concentration at the first copper harvest was 29 grams per liter. A current density of 560 ampere per square meter was maintained constant for the first run. After the first harvest, a second run was performed on the same solution at a lower current density, 417 ampere per square meter, and the copper concentration at the second harvest was 18.8 grams per liter. This procedure was repeated until a copper concentration of 0.8 grams per liter was achieved, with a total of four interruptions to harvest the accumulated copper. Runs 6 to 9 and 11 to 14 were carried out in a similar fashion.

Table 6 – Experimental Results for Codelco Norte Bleed No. Time Cu ini Cu end Volts Dens Flow Kg Effic g/T g/T g/T

Hrs g/l g/l A/m2 l/min cathode % S C O2 1 40.4 39.0 29.0 2.4 559 120 13.3 91 2 31 47 2 67.5 29.0 18.8 2.4 417 120 16.6 91 3 38 64 3 89.2 18.8 11.4 2.2 282 120 14.2 86 3 42 71 4 208.5 11.4 0.8 2.0 188 120 20.6 81 3 41 1465 46.0 38.9 30.6 3.7 891 120 23.5 88 9 39 70 6 47.9 40.4 32 3.3 825 120 22.5 87 3 22 62 7 68.2 32.2 24.0 2.8 614 200 23.9 88 2 29 37

8 65.5 23.9 13.5 2.8 605 200 22.8 89 2 22 43 9 61.0 13.3 1.9 2.7 528 200 20.6 98 15 32 71010 59.2 37.2 19.6 2.7 664 200 21.4 84 2 60 51 11 17.1 31.4 28.6 4.7 1130 200 11.4 89 7 53 11012 37.5 28.4 22.5 3.9 850 200 19.4 93 7 49 82 13 45.9 20.6 10.2 2.7 460 200 12.7 93 2 23 96 14 53.5 9.7 1.9 2.4 303 200 9.4 89 11 62 14015 33.8 38.4 32.3 3.8 1100 200 19.4 80 8 125 101

For each harvest, an analysis of impurities was carried out on the harvested copper. Table 6 also shows the levels of sulphur, carbon and oxygen for the harvested cathodes.

With these results, a graph of copper concentration and sulphur content versus current density can be constructed, which is shown in Figure 1. Each bar on the graph shows the initial and final copper concentration for a particular run at a given current density, and the number inside the bar shows the sulphur content of the copper harvested for that run.

Figure 1 – Sulphur levels in Copper Cathodes produced at a given Current density and within a given Copper concentration range.

With the help of this graph, the ideal working conditions to assure the production of Grade A copper from EW bleed streams can be defined for any concentration of copper in the bleed stream, in the range of 36 grams per liter of copper to 2 grams per liter of copper. It is seen that at high copper concentrations, above 30 grams per liter, a very high current density of 800 ampere per square meter can be used and the resulting copper will be of high purity, Grade A. In contrast, if the copper concentration is low, for example 4 grams per liter, a current density no greater than 250 ampere per square meter should be used if Grade A copper is desired as a product. Therefore, the optimum configuration of a plant to treat an EW bleed streams should consider at least two different electrowinning stages, one operating at a very high current density where most of the copper is produced, and one operating at a more modest current density. The conclusion is that as copper concentration decreases, current density must be decreased in order to maintain high copper quality.

Enami Salado Tests:

Similar tests as those described above for Codelco Norte were carried out at the ENAMI El Salado plant. The composition of the electrolyte used was very similar to that of Codelco Norte shown in Table 5. The results of the Enami Salado work are shown in Tables 7 and 8 below. Table 7 shows the operational parameters for each run and Table 8 shows the chemical analysis of the impurities in the copper harvested for each run:

Table 7 – Experimental Results for Enami Salado Bleed Run Current Flow Concentration Voltage Mass

Density Cu gpl Cu gpl Volts Cu (amp/m2 ) (m3/h) Initial Final (Kg) 1 450 9 33.0 18.2 2.4 19.9 2 350 9 18.2 7.0 2.2 15.4 3 580 9 35.3 14.8 2.6 32.0 4 300 9 14.8 3.0 2.2 22.8 5 700 9 34.2 22.0 2.8 16.4 8 600 9 39.1 22.3 2.6 34.0

10 350 9 22.3 3.71 2.25 25.3 11 800 10 42.78 28.13 2.9 19.4

Table 8 – Cathode quality from Enami Salado Bleed Run Current Flow Concentration S O2 Current

Density Cu g/L Cu g/L Efficiency (amp/m2 ) (m3/h) Initial Final g/T g/T %

1 450 9 32.95 18.2 8 120 94 2 350 9 18.2 6.95 4 170 94 3 580 9 35.3 14.78 5 30 92 4 300 9 14.78 3.02 5 150 93 5 700 9 34.18 22.03 1 20 97 8 600 9 39.1 22.3 2 15 94 10 350 9 22.3 3.71 2 40 91 11 800 10 42.78 28.13 2 20 95

Cia. Minera Doña Inés de Collahuasi Tests:

Three different substrates were used for EMEW trials at Collahuasi. The first was Electrowinning bleed, similar to that presented above for Codelco Norte and Enami Salado, the second was raffinate, and the third was an effluent from the SX/EW plant, combining cell cleaning effluent, spills, etc.

Electrowinning Bleed

For the tests carried out at the Oxides plant at Compañía Minera Doña Inés de Collahuasi, the bleed electrolyte used with the EMEW pilot plant had the composition shown in Table 8:

Table 8 – Collahuasi Electrolyte Composition Parameter Concentration Cu 36 gpl H2SO4 200 gpl Fe 1.3 gpl Mn 30 ppm Al 150 ppm Cl 20 ppm Co 140 ppm

The results obtained for the bleed stream are shown below in Tables 9 and 10:

Table 9 – Experimental Results for Collahuasi Bleed Run 1 2Current Density, Amp/m2 400 300Flow, m3/h 9 9Initial Copper, gpl 33.1 13.1Final Copper, gpl 13.1 3.6Voltage ,V 2.6 2.4Initial Acid, gpl 199 241Final Acid, gpl 241 255Run Time, h 121 143Cathode Mass, Kg 28 24Current Efficiency, % 96 93Conversion Energy, Kwh/Kg Cu 2.2 2.1

Table10 – Experimental Results (Analytical) for Collahuasi Bleed Element Ag As Bi Cd Co Cr Fe Mn Ni P ppm ppm Ppm ppm ppm ppm ppm ppm Ppm ppmConcentration < 0.2 < 0.1 < 0.1 < 0.2 < 0.8 < 1 < 0.8 < 0.5 < 0.8 < 1 Element Sb Se Si Te Zn Sn O Cl C S ppm Ppm Ppm ppm ppm ppm ppm ppm Ppm ppmConcentration < 0.2 < 0.1 < 5 < 0.1 < 2 < 0.2 85 < 5 20 5

The results for Codelco Norte, Collahuasi and Salado show that it is possible to electrowin the copper concentration to values below 4 grams per liter and obtain high quality copper at good efficiencies. For the Collahuasi case, if one considers that the Raffinate flowrate in the plant is on the order of 1000 m3/h and that the feed to an EMEW plant to remove copper from the bleed would be approximately 6 m3/h at 36 grams per liter of Copper, then the production of copper cathodes from the EMEW circuit would be on the order of 150 Kg/h. Therefore, the reduction of the copper concentration in the Raffinate would be approximately 0.2 gpl. This means that if the current concentration of copper in the Raffinate were 500 ppm, the copper concentration in the Raffinate would drop to 300 ppm if the EW bleed were treated using EMEW cells. This has important effects on the overall copper recovery, because a lower Raffinate copper concentration improves leaching conditions and allows for a greater global recovery of copper from the leach pads, as shown by Dixon et al. [3], Dixon et al. [4], Aguad et al. [5], García et al. [6] and García et. al. [7].

Raffinate Results

Tests were also conducted at Collahuasi using EMEW powder cells on Raffinate having the composition shown in Table 11:

Table 11. Collahuasi Raffinate Parameter ConcentrationCu 1.46 gpl H2SO4 20 gpl Total Fe 18 gpl Fe+2 16 gpl

Table 12 shows the operational conditions used for the runs using EMEW powder

cells with Raffinate at low copper concentrations. The applied current density was 600 Ampere /m2, the flowrate was 2.5 m3/h and cell voltage was 3.8 volt.

Table 12. Powder Results with EMEW cells on Raffinate Run Number 5 6 7 8 9 Current Density, Ampere/m2 600 600 600 600 600 Flowrate, m3/h 2.5 2.5 2.5 2.5 2.5 Initial Copper, gpl 1.46 0.5 0.34 0.23 0.17 Final Copper, gpl 0.5 0.34 0.23 0.17 0.13 Voltage, V 3.7 3.9 3.9 3.8 3.8 Run time, Hr. 8 5 15 16 28

From Table 12 it can be seen that the copper concentration in the Raffinate is

reduced from 1.46 grams per liter to 130 ppm over 5 consecutive runs on the same electrolyte. The change in the copper concentration for the first run is larger than the change in copper concentration for the following runs, suggesting that a practical limit for the depletion of copper in the Raffinate is on the order of 200 ppm, as further run time produces a relatively small change in copper concentration. These results show that another way to reduce copper concentration in the Raffinate is to electrowin copper directly from the Raffinate using EMEW powder cells, which is a direct method, as compared to the method discussed above, which is an indirect reduction of copper concentration by electrowinning copper from the EW bleed using EMEW plating cells.

Due to indications of ferrous oxidation from earlier evidence gathered in laboratory work and field tests, a separate run was performed on the Raffinate after the copper was depleted to the 130 ppm level to examine the change in the distribution of Ferrous / Ferric concentration, as shown in Table 13:

Table 13. Change in the Distribution of Fe (II) / Fe (III) in the Raffinate t

(h)Fe t (gpl)

Fe II (gpl)

Fe III (gpl)

0 18 16 2 31 17 13 4 44 16 13 4 55 18 13 5

The results in Table 13 show that the concentration of Ferric ion increases and that the concentration of Ferrous ion decreases with consecutive runs. In a similar fashion than what happens with the copper concentration in the Raffinate as discussed above, the initial change in Ferric concentration is greater than the change at the end of the sequence of runs. These results show that EMEW cells could be used to produce ferric ion to improve leaching conditions at plants that need greater concentrations of Ferric ion to leach copper or other metals. A calculation using data obtained from the run indicates that the energy needed to produce one kg of Ferric ion starting from a Ferrous solution is on the order of 1.8 Kw Hr., which would allow for an economical production of Ferric ion. The results with Raffinate indicate that it may be possible to simultaneously deplete copper and oxidize ferrous to ferric in Raffinate streams via direct electrowinning of the Raffinate, thus providing a large increase in Raffinate leaching power by having a low copper, high ferric Raffinate returning to the heap leaching operation.

Effluent Results

Two runs were performed on a mixture of effluents from the Collahuasi plant. The solution contained effluents from EW cell cleaning, electrolyte spills, and other streams from the SX plant. This solution was analyzed for copper and acid only.

Table 14. Collahuasi Effluent Parameter ConcentrationCu 21.7 gpl H2SO4 110 gpl

Tables 15 and 16 show the operational parameters and the analysis of impurities in

the harvested cathodes for EMEW plating runs on Collahuasi Effluent:

Table 15. Parameters for Copper recovery from Effluent with EMEW plating cells Run 3 4 Current Density, Ampere /m2 400 300 Flow Rate, m3/h 9 9 Initial Cu Concentration, gpl 21.7 13.6

Final Cu Concentration final, gpl 13.6 4 Initial Voltage, V 2.7 2.4 Initial Acid, gpl 110 117 Final Acid, gpl 117 135 Run Time, h 103 119 Cathode Weight, Kg. 23 19 Current Efficiency, % 94% 90% Energy Use, KWh/Kg Cu 2.4 2.3

Table 16. Analysis of impurities in Copper harvested from Collahuasi Effluent Element Ag As Bi Cd Co Cr Fe Mn Ni P ppm ppm ppm ppm ppm Ppm ppm ppm ppm ppm Results < 0.2 < 0.1 < 0.1 < 0.2 < 0.8 < 1 < 0.8 < 0.5 < 0.8 < 1 Element Sb Se Si Te Zn Sn O Cl C S ppm ppm ppm ppm ppm Ppm ppm ppm ppm ppm Results < 0.2 < 0.1 < 5 < 0.1 < 2 < 0.2 N/A < 5 20 5

These results show that EMEW plating cells can be used to produce high-grade

copper from electrolytes that are contaminated as compared to the cleaner electrolytes normally used in copper electrowinning.

CONCLUSIONS • A tool is available that allows for the recovery of copper contained in Electrowinning

bleed streams. By removing this copper, the recirculation of the bleed stream to other parts of the circuit and the problems that this practice causes can be avoided.

• It is shown that EMEW technology can produce high quality copper cathodes with

high current efficiencies from Electrowinning bleed streams, and that copper can be depleted (while maintaining high copper quality) to concentration levels that conventional technology has difficulty achieving.

• It is suggested that by using this method, operational conditions of existing Solvent

Extraction / Electrowinning circuits can be improved. This is because it is likely that by truly purging the Electrowinning bleed stream, with no indirect returns, the system impurity level is lowered, which would allow the existing EW stage to operate at a higher current and increase the production of copper while maintaining quality.

• By allowing for a larger bleed without the potential for loss of copper, the electrolyte

quality in the existing electrowinning circuit can be improved, thus increasing the percentage of high quality cathode copper produced in the existing circuit and decreasing the percentage of copper not meeting quality specifications.

• The capacity of the SX stage is slightly increased because copper is not recirculated,

so that SX capacity is increased in an amount equal to the copper contained in the bleed, which can be greater than 4% for some copper producers.

• The use of this method allows for a reduction of Raffinate copper concentration.

From the results, it can be estimated that a typical reduction of copper concentration in the Raffinate is on the order of 200 ppm.

• The method allows for a plant expansion of the same magnitude as the copper

contained in the bleed at a comparatively low cost, because it is not necessary to improve upon the SX stage facilities, the investment is reduced to adding enough electrowinning capacity to treat the bleed. This is true however, if and only if the mine can produce and the leaching facilities can process the additional ore needed.

• The copper concentration in the raffinate can be reduced directly with EMEW

powder cells. In this case however, the product is copper powder and not grade A cathode.

• Ferrous ion in the Raffinate can be oxidized to Ferric ion with reasonable

efficiencies. It is likely that simultaneous copper depletion and Ferrous oxidation to produce a low copper, high Ferric Raffinate can be achieved.

• For the production of cathode copper from electrolyte bleed streams and from waste

stream, the energy use is on the order of 2.2 KwHr per kilo of copper deposited, and the electrical efficiency is on the order of 94%.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the valuable contributions of Verónica Escobar (Codelco Norte), David Olguín (Enami El Salado), and Rodrigo Valdés (Enami El Salado).

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3. Dixon S., (2004). "Definition of Economic Optimum for the Leaching of High Acid

Consuming Copper Ores". Minerals and Metallurgical Processing 21(4), pp. 198-201.

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"Lixiviación de minerales sulfurados de Cobre" Hydromet 2002, Antofagasta, Octubre 2002.

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