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CONVOCATORIA TRABAJOS TÉCNICOS Y DE INVESTIGACIÓN

ENCUENTRO DE OPERADORES

PERUMIN – 30 Convención Minera

Arequipa, 12 al 16 setiembre de 2011

Remoción de arsénico y antimonio desde muestras de enargita usando sulfuro de sodio Arsenic removal from enargite samples using sodium sulphide

F. Parada Torres – E. Asselin Department of Materials Engineering The University of British Columbia

Resumen

Muestras de enargita que contienen

aproximadamente 40 % Cu, 12 % As y 0.5

% de Sb fueron expuestas a un solución

conteniendo hidróxido de sodio y sulfuro de

sodio bajo diferentes condiciones. La

remoción de As y Sb es rápida alcanzando

casi un 100 % en menos de 2 horas en

algunos casos y con casi nula

solubilización de cobre. El residuo sólido

libre de arsénico y antimonio contiene todo

el cobre inicial y es apto para ser tratado

vía fundición. Las nuevas fases formadas

presentan un tamaño de partícula muy fino,

probablemente con poca cristalinidad lo

que hace difícil su identificación. Algunas

fases detectadas incluyen bornita, digenita

y NaCu5S3. La Remoción de arsénico

desde la solución fuerte puede llevarse a

cabo mediante cristalización por

enfriamiento.

Abstract

Enargite samples containing approximately

40 % Cu, 12 % As and 0.5 % Sb were

treated using a solution containing sodium

hydroxide and sodium sulphide under

different conditions. Removal of As and Sb

is fast reaching almost 100 % in less than 2

hours in some cases with practically no

copper being solubilised. The solid residue

produced is suitable for smelting. The new

phases formed present very fine particle

size, perhaps with poor crystallinity, which

makes them difficult to identify. Some

phases found include bornite, digenite and

NaCu5S3. Partial removal of arsenic can be

achieved through crystallisation via cooling.

Background

The presence of arsenic in copper concentrates is undesirable due to the inability of smelters to efficiently remove it, especially at concentrations higher than 2 % (Castro 2008). Arsenic can end up in the final copper product thus hindering its quality. A maximum value of 0.5 % of arsenic in copper concentrates seems to be accepted by most smelters without penalties (Filippou 2007). The alkaline sodium sulphide leaching of enargite provides a means of selectively removing arsenic and antimony, thus producing a clean copper concentrate suitable for smelting. Several authors have studied this process (Nadkarni 1975-1988, Anderson 1994-2005-2008, Curreli 2009, Tongamp 2009) reporting efficient removal of arsenic and antimony. It has also been reported that sodium hydroxide is used to ensure the stability of sulphide ions in solution, which should react with enargite to dissolve arsenic as sodium thioarsenate (Anderson 2008, Curreli 2009). The reaction that is usually proposed for the leaching of enargite in sulphide solutions is as follows (Nadkarni 1988):

432243 2332 AsSNaSCuSNaAsSCu (i)

However, a much higher value for the second dissociation constant of H2S has been reported, which means that sulphide ions would hydrolize and produce hydrosulphide, which then would react with

enargite (Giggenbach 1971, Licht 1988):

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NaHSNaOHOHSNa 22 (ii)

Therefore the leaching reaction can now be re-written as:

OHAsSNaSCu

NaOHNaHSAsSCu

2432

43

323

332 (iii)

This reaction has been reported by Tongamp et al (Tongamp 2009) when leaching enargite with sodium hydrosulphide. The purpose of this paper is to review the leaching of enargite using sodium sulphide, compare results with previous findings and provide new details regarding its chemistry that could affect the process and take these details into account in order to optimise it.

1 Procedure

Leaching tests were performed batchwise in a 200 ml glass jacketed cell. The leach solution was prepared by dissolving sodium hydroxide first and later adding sodium sulphide. The solution was heated up to the desired temperature using a circulating water bath. Once the desired temperature was reached, 10 grams of enargite sample were added. Samples were drawn periodically and sent for ICP analysis. Solid residues were analysed using ICP, XRD and scanning electron microscopy. A phase composition for the feed is given on Table 1:

Mineral Ideal formula Weight %

Enargite Cu3AsS4 60.4

Quartz SiO2 5.90

Tennantite (Cu,Ag,Fe,Zn)12As4S13 4.90

Covellite CuS 3.00

Pyrite FeS2 25.7

2 Results

Note: NaOH concentrations consider the

hydrolization of sodium sulphide

2.1 Effect of Temperature

Leaching of enargite was tested at 50, 65, 80 and 95ºC. Arsenic and antimony removal was noticeably enhanced as temperature increased as seen in Figures 1 and 2 suggesting a chemically controlled process.

Figure 1: Effect of temperature on As removal at 500 RPM, P80 30 μm, 3.5 M NaOH and 1.0 Na2S after 2 hours.

Figure 2: Effect of temperature on Sb removal at 500 RPM, P80 30 μm, 3.5 M NaOH and 1.0 Na2S after 2 hours.

2.2 Effect of agitation and particle size

Maintaining particles in suspension is key in hydrometallurgical reactors. In this case agitation does not have a noticeable effect on the dissolution of arsenic and antimony, thus suggesting the process is not controlled by mass transfer in the stagnant film and supporting the idea that it is chemically controlled.

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Figure 3: Effect of agitation on As removal at 95ºC, P80 30 μm, 3.5 M NaOH and 1.0 Na2S after 2 hours.

Figure 4: Effect of agitation on Sb removal at 95ºC, P80 30 μm, 3.5 M NaOH and 1.0 Na2S after 2 hours. Particle size, on the other hand, has a more noticeable effect. As particle size is decreased extraction increases. Figures 3 to 6 show the results.

Figure 5: Effect of particle size on As removal at 95ºC, 500 RPM, 3.5 M NaOH and 1.0 Na2S after 2 hours.

Figure 6: Effect of particle size on Sb removal at 95ºC, 500 RPM, 3.5 M NaOH and 1.0 Na2S after 2 hours.

2.3 Effect of sodium hydroxide and

sodium sulphide

According to reaction (i) sulphide would react with enargite to solubilise arsenic and produce chalcocite. In this case hydroxide is assumed only to raise the pH to avoid hydrolization of sulphide (Anderson 2008, Curreli 2009). However there seems to be some disagreement regarding the chemistry of sulphide in solution and it has been reported that sulphide ions would exist in solution only at pH values of 17 or perhaps higher (Giggenbach 1971, Licht 1988), which supports the idea that reaction (iii) represents the leaching procedure more accurately. Considering these facts it seems important to study how the leaching process takes place when modifying hydroxide to sulphide ratios. Results shown in Figures 7 and 8 suggest that both reagents (hydroxide and sulphide or hydrosulphide) are acting in the leaching procedure. In fact, it can be seen that when hydroxide is increased up to 3.5 M and sulphide is lowered to 0.5 M, As and Sb removal is almost identical when hydroxide is decreased to 2.0 M and sulphide is increased to 1.0 M. This fact can help to find an optimal ratio between hydroxide and sulphide, especially considering that sulphide is a much more expensive reagent.

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Figure 7: Effect of NaOH and Na2S concentration on As removal at 95ºC, 500 RPM, and P80 30 μm.

Figure 8: Effect of NaOH and Na2S concentration on Sb removal at 95ºC, 500 RPM, and P80 30 μm.

2.4 Behaviour of copper

Some advantages of this process include its relatively fast kinetics and the possibility of using atmospheric conditions. However one key feature is its selectivity; copper and other metals such as zinc and silver are practically 100 % left in the solid residue, which becomes an upgraded copper concentrate. Table 2 shows a comparison between the feed and the solid residue after leaching.

Table 2: Effect of NaOH and Na2S concentration on Cu, Fe, Zn and Ag after leaching at 95ºC, 500 RPM and P80 of 30 μm.

Sample Cu (%)

Fe (%)

Zn (%)

Ag (%)

Head 38.00 12.00 0.316 0.0193

3.5 M NaOH 1.0 M Na2S

48.48 12.70 0.362 0.0243

2.0 M NaOH 1.0 M Na2S

48.49 15.66 0.346 0.0239

1.1 M NaOH 1.0 M Na2S

42.39 12.45 0.330 0.0241

3.0 M NaOH 0.5 M Na2S

49.17 15.87 0.384 0.0246

1.5 M NaOH 0.5 M Na2S

45.65 14.64 0.342 0.0229

2.5 Arsenic precipitation

During the leaching procedure arsenic and antimony are solubilised as thio compounds. These compounds appear to have solubilities very sensitive to changes in temperature, making them suitable for crystallization via cooling (Nadkarni 1988). Tables 3 and 4 show the main arsenic species detected and the main crystallisation parameters, respectively. Table 3: Main crystallised arsenic species

Sodium sulphide Arsenate Hydrate Na3AsO2S2•11H2O

Sodium Sulphide Arsenate Hydrate Na3AsO2S2•7H2O

Sodium Sulphide Arsenate Hydrate Na3AsS4•8H2O

Table 4: Crystallisation parameters

Precipitation parameters As Sb

Highest concentration seen (M) 1.30 0.032

Concentration after precipitation (M) 0.70 0.030

Content in solid precipitate (%) 11.8 0.320

Average removal from solution (%) 42.0 4.500

Removal of arsenic can reach approximately 40 % via crystallization of sodium thioarsenates. This method does not remove all the arsenic and antimony from solution, but it is a simple procedure and the solution can be recirculated back to the leaching stage.

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3 Conclusions

Arsenic and antimony from an enargite sample can be leached using sodium hydroxide and sodium sulphide to produce a clean copper concentrate that can be suitable for smelting. Copper, iron, zinc and silver remain almost completely in the solid residue, however the new copper phases formed are difficult to determine with XRD.Partial removal of arsenic and antimony from solution can be achieved by crystallization. This depends on the concentration of As and Sb in the PLS. The process presents an alternative to treat high As/Sb copper concentrates without the need of high temperature or high pressure and does not produce volatile As or Sb compounds.

Acknowledgements

The authors wish to acknowledge the financial support of Newmont Mining Corporation and the Natural Sciences and Engineering Research Council of Canada (NSERC). REFERENCES Anderson, C.G. and L.G. Twidell, (2008),

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