Table of Contents
The ability to process complex precious metal sulfide concentrates containing antimony and arsenic, in an economic and environmentally safe manner, is an important problem facing the mining industry in the United States. Preliminary experiments have been performed to evaluate the possibility of treating a complex sulfide concentrate to recover antimony and precious metals while fixing the arsenic and sulfur as calcium arsenate and calcium sulfate.
The proposed process is shown schematically in Figure 1 and involves an adaptation of previously known technology. The roast step is performed with sodium carbonate in order to trap the fugitive sulfur dioxide, antimony and arsenic. As all of the furnace gas, dust and residue pass through a caustic water solution; the arsenic, antimony and sulfur should be completely contained.
Roasting:
Sb2S3(s) + 7O2(g) + 3Na2C03(s) = Sb2O5(s) + 3Na2SO4(s) + 3CO2(g)…………………………………………(1)
2FeAsS(s) + 8O2(g) + 2Na2C03(s) = Fe2O3(s) + As2O5(s) + 2Na2SO4(s) + 2CO2(g)……………………..(2)
Arsenic Leaching:
As2O5(S) + 6NaOH(aq) = 2Na3AsO4(aq) + 3H2O(aq)………………………………………………………………(3)
Na2SO4(s) = Na2SO4(aq)……………………………………………………………………(4)
Arsenic and Sulfate Precipitation:
2Na3AsO4(aq) + 3CaCl2(s) = Ca3(AsO4)2(s) + 6NaCl(aq)…………………………………………….(5)
Na2SO4(aq) + CaCl2(s) = CaSO4(s) + 2NaCl(aq)…………………………………………………(6)
Antimony Reduction:
Sb2O5(s) + 5CO(g) – 2Sb(s) + 5CO2(g)……………………………………………………..(7)
C(s) + CO2(g) = 2CO(g)…………………………………………………………………………..(8)
Antimony Oxide Production:
Sb(l) = Sb(g)…………………………………………………………………………………..(9)
4Sb(g) + 3O2(g) = 2Sb2O3(s)………………………………………………………….(10)
If the chemical reactions proceed as proposed, then the stochiometric requirements of reagents would be as follows (Table 1.):
The disposal of the calcium arsenate and calcium sulfate may be possible in a tailings pond environment (in certain locations). Alternative disposal methods would involve disposal at an approved site or treatment of the residue to produce a marketable arsenic product.
The presence of chloride ions, through recycle, in the arsenic leach stage may lead to the solubility of some of the antimony as antimony oxychloride.
It is possible that both sodium arsenate and sodium antimonate will be formed during the roasting operation. This would not be a problem in the leaching step, as the sodium arsenate is desired and the sodium antimonate is insoluble. Sodium antimonate may be a problem during the reduction step due to the formation of a sodium oxide slag, which is very corrosive and will require the addition of more fluxing agents to counteract the basic nature of the sodium oxide.
Experimental Procedures
The roasting experimental system is shown schematically in Figure 2. The reactor system consists of a tube furnace, temperature controller, gas flow system, ice condenser trap, and hydrogen peroxide-water absorption system. The air flow rate to the furnace was controlled in order to control the oxidation reaction, which is highly exothermic.
Experimental Results
Preliminary experiments were performed using hydrated lime, caustic soda and soda ash to compare the relative abilities to retain the sulfur in the roast calcines. The results are given in Table 2.:
Sb2S3(s) + 4NaOH(s) = Na2SbS3(s) + NaSbO2(s) + 2H2O(g)…………………………………….(11)
4Sb2S3(s) + 3Na2CO3(s) = Sb2O3(s) + 6NaSbS3(s) + 3CO2(g)…………………………………………….(12)
Sb2S3(s) + 2Na2CO3(s) = Na3SbS3(s) + NaSbO2(s) + 2 CO2(g)…………………………………………..(13)
Stibnite roasting, without soda ash, in air was performed and the rate of sulfur lost to the gas phase tracked.
Some experiments were performed using caustic soda in place of soda ash during stibnite roasting. One result was generated using: 2 grams of – 150+200 stibnite, 1.25 times the stoichiometric amount of NaOH, 823 K (550 C), 90 minutes, and an air flow rate of 130 ml/min.
The sulfate contents of the soda ash roast experiments were determined. The results for the temperature tests is given in Figure 12. There is a minimum at 723 K (450 C) and a maximum at 823 K (550 C). This is thought to be due to two competing reactions.
Sb2S3(s) + 3Na2CO3(s) + 6O2(g) = Sb2O3(s) + 3Na2SO4(s) + 3CO2(g)…………………………………….(14)
4Sb2S3(s) + 3Na2CO3(s) = Sb2O3(s) + 6NaSbS2(s) + 3CO2(g)…………………………………..(15)
Sb2S3(s) + 2Na2CO3(s) = Na3SbS3(s) + NaSbO2(s) + 3CO2(g)…………………………………..(16)
Sb2O3(s) + Na2CO3(s) = 2NaSbO2(s) + CO2(g)…………………………………………………………(17)
The melting point of stibnite is 823 K (550 C) and the molten stibnite apparently accelerates the rate of sulfate formation. The later set of reactions apparently are important at the higher temperatures.
Arsenopyrite – Soda Ash Roasting
Arsenopyrite was roasted with soda ash to evaluate the containment of sulfur and arsenic and the solubility of the arsenic in the roast calcine (as one of the objectives is to roast the mineral in such a way to make the arsenic soluble). The following reaction was proposed:
2FeAsS(s) + 7O2(g) + 5Na2CO3(s) = Fe2O3(s) + 2Na3Aso4(s) +2Na2SO4(s) + 5CO2(g)…………………….(18)
The effect of the arsenopyrite particle size on sulfur retention is shown in Figure 18. There is very little effect. The arsenic retention was greater than 99.9% over the experimental range.
Arsenic Leaching
The amounts of soluble arsenic in the roaster calcines were determined by leaching them in distilled water with and without pH adjustments.
The amount of soluble arsenic increased with increasing soda ash in the roast as is shown in Figure 21. This is due to the enhanced formation of sodium arsenate with increasing amounts of soda ash during roasting.
The effect of the final pH of the water leach on the soluble arsenic is shown in Figure 25. The solubility is enhanced at low and high pH values. The high pH reaction is thought to be the conversion of ferrous (or ferric) arsenate with the sodium hydroxide,
FeAsO4(s) +3NaOH(s) = Na3AsO4(s) + Fe(OH)3(s)………………………………………………….(19)
Arsenic Precipitation
The reactions of interest are thought to be:
HAsO2(aq) + Ca(OH)2(aq) = CaAsO20H(s) + H2O(aq)………………………………………(20)
NaAsO2 (aq) + Ca(OH)2(aq) = CaAsO2OH(s) + NaOH(aq)………………………………..(21)
NaAsO2(aq) + CaCl2(aq) + H2O(aq) = CaAsO20H(s) + NaCl(aq) + HCl(aq)……………………………(22)
CaCl2(aq) + 2NaOH(aq) = 2NaCl(aq) + Ca(OH)2(aq)……………………………………………………..(23)
HCl(aq) + NaOH(aq) = NaCl(aq) + H2O……………………………………………..(24)
The effect of adding calcium chloride on the arsenic precipitation is shown in Figure 28. Again the arsenic is effectively precipitated at CaCl2 additions equivalent to the stoichiometric requirements. The concentrations in solution are shown in Figure 29.
Na3AsO4(aq) + 2Ca(OH)2(aq) = Ca2AsO40H(s) + 3NaOH(aq)……………………………(25)
Na2SO4(aq) + Ca(OH)2(aq) + 2H2O(aq) = CaSO4.2H2O(s) + 2NaOH(aq)…………..(26)
Na3AsO4(aq) + 2CaCl2(aq) + 2H2O(Aq) = Ca2AsO40H(s) + 3NaCl(aq) + HCl(aq)….(27)
Na2SO4(aq) + CaCl2(aq) + 2H2O(aq) = CaSO4.2H2O(s) + 3NaCl(aq) + HCl(aq)…….(28)
HCl(aq) + NaOH(aq) = NaCl(aq) + H2O………………………………………………….(29)
