Bismuth Sulfide Concentrate Low Temperature Alkaline Melting Process

A new process for bismuth sulfide concentrate alkaline smelting at low temperature was presented in this paper to avoiding the shortcomings of the traditional pyrometallurgical process for bismuth extraction, which including high smelting temperature, high energy and material consumption, high production cost and severe environmental pollution. In the process, the raw bismuth was produced one step at the temperature of 780℃～830℃in the smelting system of soda and caustic soda. Sulfur in the bismuth sulfide concentrate was partially transformed into Na₂SO₄ and the others mostly transformed into Na₂S. Molybdenum and tungsten in the concentrate could easily be recovered as sodium molybdate and sodium tungstate in the slag.The thermodynamic analysis of major elements in the reaction for alkaline smelting was calculated. The calculation results show that both self-reduction and carbon-reduction can proceed spontaneously in the process of bismuth sulfide alkaline smelting at low temperature, and the carbon-reduction was the main reaction. There were three possible approaches for carbon-reduction. By comparing theΔG^θof each possible approaches, it is found that the reactions follow the second approach and third approach when the temperature was lower and higher than 900K, respectively. The results also shown that AgS, PbS, Cu₂S formed Ag, Pb, Cu in the alkali smelting medium; While the FeS₂, ZnS directly entered into slag and sulfur without any reaction occured.The effects of W_NaOH/W_Na2CO3 solvent, alkaline dosage, temperature, duration on the direct recovery of bismuth and the quality of raw bismuth were investigated using the unitary factor testing in the laboratory scale. The optimal conditions were W_NaOH/W_Na2CO3= 20/133, alkaline dosage of 1.64 times the theoretical value, temperature of 780～830℃, time of 1.5h.The pilot tests and contrast tests have been done under the optimal conditions, and the results showed the tests had good effects, the direct recovery rate of bismuth and the content of bismuth was 94.43%, 98.00%, respectively. The contrast tests show that the new process is significantly superior to the conventional pyro-refining bismuth technology. The analysis of input and output of sulfur and beryllium showed that the efficiency of sulfur retention was 99%, which means that the pollution of sulfur dioxide was basically eliminated; the structure of beryllium in the raw material wasn't destroyed, mostly reserved in the leaching residue, having no water pollution.The new process has advantage of sharp decrease of smelting temperature, which not only reducing the energy consumption but also eliminating the environmental pollution of low concentration SO₂, greatly increasing the direct recovery rate of bismuth. It has a great importance for the technology of bismuth smelting.