Table of Contents
The gradual depletion of high-grade sulphide mineral deposits has turned the attention of the mineral industry to the recovery of metals from the oxides and silicates. Anionic (fatty acid) and cationic collectors have in some cases made flotation of non-sulphides possible, although in general, flotation of oxide ores is not as yet an economic process.
Sulphidization of Metal Oxides and Silicates
Sulphidization with hydrogen sulphide gas was carried out in a horizontal tube furnace (Fig. I). The temperature of the furnace was regulated by means of a Variac and determined with a chromel-alumel thermocouple. The constant temperature zone in the furnace was about 6″ in length, and it was within this zone that the experiments were carried out.
Samples of reagents or minerals, placed in a 3″ alundum boat, were inserted within the constant temperature zone of the furnace. Nitrogen gas (6 ppm O2 content) was passed through the tubes at room temperature to remove air in the tube, and continued to pass whilst the furnace was heating up. When the required temperature was reached, hydrogen sulphide was allowed to enter the tube and the supply of nitrogen was shut off.
When H2S, either alone or with nitrogen, is passed through a heated tube at the temperatures employed in this investigation, elemental sulphur is deposited in the cool discharge end of the tube. This is due to decomposition of the H2S:
8 H2S (g) ↔ 8 H2(g) + S8(g): k298°K = 1.7 x 10 -26
k500°K = 2.6 x 10 -14
k1000°K= 4.8 x 10 -5
In contrast to the above, when certain metallic oxides were treated at temperature with H2S, both water vapor and sulphur were evolved.
X-ray analysis of the products of sulphidization of cupric oxide indicated the presence of Cu2S (chalcocite), CuS (covellite) and CuS (digenite), with chalcocite being the predominant mineral. Since the quantity of Cu2S as indicated by the x-ray patterns was very much higher than that of the other two minerals, Cu2S was taken as the approximate chemical composition of the sulphide product. The effects of time and temperature on the conversion of CuO, calculated as Cu2S formed, are summarized in Table 1 and Figure 4.
b. Sulphidization of Reagent Grade Litharge with Hydrogen Sulphide
The overall reaction of PbO with H2S may be represented as follows:
PbO + H2S(g) → PbS + H2O(g)
This is confirmed by the chemical analysis of the products and by the x-ray patterns obtained. When partially converted products were analyzed by x-ray methods, the presence of PbO in addition to PbS was indicated.
The effects of time and temperature on the conversion of PbO are summarized in Table 2 and Fig. 5. The percent conversion was calculated as the ratio
%S in Product/%S in PbS
As mentioned earlier, the hand-picked specimens of chrysocolla contained some azurite and malachite, which made it difficult to establish the actual reactions that took place. The matter was further complicated by the formation of more than one sulphide, as in the case of sulphidization of cupric oxide. Chrysocolla is a hydrated copper silicate, and its water is removed at about 110°C. We can regard its composition as CuSiO3 and write the following possible reactions:
A. CuSiO3 + H2S(g) → CuS + SiO2 + H2O(g)
B. 6 CuSiO3 + 4H2S(g) → 3 Cu2S + 6 SiO2 + 4 H2O(g) + SO2(g)
C. 8 CuSiO3 + S8(g) → 4 Cu2S + 8 SiO2 + 4SO2(g)
Flotation of Artificial Sulphide Minerals
Flotation was carried out with purified potassium amyl xanthate at a pH of 10.5. C.P. grade NaOH was used as the pH regulator. Flotation time was 5 minutes. It is evident that the recovery of copper increases with an increase in the time of sulphidization of the chrysocolla.
One gram samples of products of sulphidization of lead oxide were floated using purified potassium amyl xanthate at a pH of 8.5. C.P. grade CaO was used as the pH regulator. The recovery is reported as the ratio of the weight of the floated fraction to the weight of the original sample.
When pyrite is used for sulphidizing, the required temperature is that at which elemental sulphur is liberated and the pyrite is converted to pyrrhotite.