Post on 20-Jul-2016
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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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