J.E. Dutrizac

The effect of seeding on the rate of precipitation of ammonium jarosite and sodium jarosite

Abstract The beneficial effect of jarosite seed on the rate of precipitation of ammonium jarosite and sodium jarosite was confirmed, and it was shown that the initial rate of precipitation increases linearly with the amount of seed present. A minimum degree of agitation is necessary to suspend the seed, but higher agitation rates are without further effect. Furthermore, the presence of seed extends the range of jarosite precipitation to lower pH values and to lower temperatures than is the situation in the absence of seed. The presence of seed accelerates the initial precipitation rate at all pH values and temperatures and generally eliminates any induction period. The activation energy for sodium jarosite precipitation in the presence of seed is 106 kJ/mol. In the presence of sodium jarosite seed, the rate of sodium jarosite precipitation is nearly independent of ferric ion concentrations >0.03 M Fe 3+ when the rate is expressed as a percentage of the initial Fe 3+ concentration. A more pronounced ferric ion concentration dependence is noted in the absence of seed. Increasing Na 2 SO 4 concentrations cause the rate of sodium jarosite precipitation, in the presence or absence of seed, to increase to ∼0.2 M Na 2 SO 4 but to decrease at higher Na 2 SO 4 concentrations. This dependence seems to be related mostly to the changing sodium ion concentration, as the rate is nearly independent of ZnSO 4 concentrations up to 2.0 M in the presence or absence of seed.

The precipitation of hematite from ferric chloride media

The precipitation of hematite from ferric chloride media was systematically investigated in a series of autoclave experiments. The minimum temperature to form hematite in the absence of hematite seed is ∼ 125°C; akaganeite, β-FeO · OH, is precipitated at lower temperatures for the 65% Fe, < 0.5% Cl and less than 0.1% of Na, Ca, Zn or Pb. The precipitated hematite is very fine, but its filterability is greatly improved by the presence of hematite seed.

Calcium sulphate solubilities in simulated zinc processing solutions

Abstract Calcium sulphate solubilities in simulated zinc processing solutions, as well as the densities of the corresponding saturated solutions, were determined on heating and cooling in a series of experiments carried out from 20 to 95 °C. In dilute ZnSO 4 media, increasing H 2 SO 4 concentrations in the range of 0.0–0.6 M H 2 SO 4 strongly increase the solubility of calcium sulphate, but have little additional effect for acid concentrations >0.6 M H 2 SO 4 . In more concentrated zinc sulphate solutions, however, increasing acid concentrations in the range from 0.0 to 2.0 M H 2 SO 4 slightly decrease the solubility of calcium sulphate. In dilute H 2 SO 4 media, the solubility of calcium sulphate decreases with increasing ZnSO 4 concentrations, and the decrease is most pronounced for concentrations in the range 0.0–0.2 M ZnSO 4 . Higher ZnSO 4 concentrations further depress the solubility at temperatures below 70 °C, but have a complex effect at higher temperatures. Increasing ferric sulphate concentrations to 1.0 M Fe(SO 4 ) 1.5 in 0.3 M H 2 SO 4 –1.15 M ZnSO 4 media depress the solubility of calcium sulphate to a modest extent. The addition of up to 1.25 M MgSO 4 also decreases the solubility of calcium sulphate in 0.3 M H 2 SO 4 –1.15 M ZnSO 4 media. In contrast, the solubility of calcium sulphate in 2.5 M ZnSO 4 –0.4 M MgSO 4 –0.18 M MnSO 4 solutions, at pH values ranging from 3.6 to 4.6, is not significantly affected by the presence of up to 0.25 M Na 2 SO 4 or up to 0.09 M (NH 4 ) 2 SO 4 . Supporting mineralogical studies showed that the solubility depends strongly on whether the saturating solid phase is gypsum (CaSO 4 ·2H 2 O) or anhydrite (CaSO 4 ). The transition from gypsum to anhydrite was found to depend on the temperature, the H 2 SO 4 concentration and the total sulphate concentration of the solution. Unlike the solubilities, which varied in a complex manner, the densities of the calcium sulphate-saturated solutions increased systematically with increasing concentrations of any of the solution species and decreased slightly with increasing temperature.

The mineralogical characterization of lead-acid battery paste

Abstract The paste component of spent lead-acid batteries was subjected to mineralogical examination to identify the phases present and their morphological relationships. The battery paste consists principally of PbSO4 and PbO2 (both tetragonal and orthorhombic forms) together with lesser amounts of Pb2O(SO4), Pb2O3, Pb metal, gypsum and silicates. Although both PbO2 and Pb2O3 are detected as inclusions in PbSO4, they occur principally as large oxide intergrowths. The major occurrence of Pb2O(SO4) is as masses of intergrown fibrous crystals. Lead metal occurs principally as inclusions in large particles of PbSO4. Most of the Sb in the battery paste is present in solid solution in PbO2, Pb2O3 and Pb2O(SO4). Reaction of the battery paste with Na2CO3 solution rapidly converts the PbSO4 to Pb3(CO3)2(OH)2 and then to NaPb2(CO3)2(OH). Although Pb2O(SO4) and Pb metal react slowly in Na2CO3 media, neither PbO2 nor Pb2O3 are significantly affected.

The behaviour of cadmium during jarosite precipitation

The behaviour of cadmium during the precipitation of ammonium jarosite has been investigated as a function of the cadmium concentration, the ZnSO4 concentration and the pH of the synthesis solutions. The Cd content of the jarosite seems to increase in a linear manner with increasing Cd concentrations in the solution, but the extent of Cd incorporation is always low. Less than 100 ppm Cd is present in the precipitates made from solutions containing up to 3,000 mg/l Cd, and similar Cd contents were realized in the presence of 75 g/l Zn and for all pH values from 0.9 to 2.0. Mass balance calculations showed that < 1% of the Cd reported to the ammonium jarosite precipitates under all conditions studied; virtually 100% of the Cd remained in the final solution. Ammonium jarosite precipitates made in the presence of 0.1–40 g/l each of Cd and Zn incorporated 5–10 times more Zn than Cd. Low concentrations of Cd have a negligible effect on the morphology of the ammonium jarosite precipitates, but high concentrations of CdSO4 and/or ZnSO4 appear to promote jarosite crystallization. Finally, the laboratory results were used to rationalize the behaviour of cadmium during jarosite precipitation in commercial zinc operations.

The behaviour of thiocyanate and cyanate during jarosite precipitation

Abstract The behaviour of thiocyanate and cyanate during the precipitation of potassium jarosite was investigated. Thiocyanate strongly complexes dissolved ferric ion and thereby reduces the amount of jarosite precipitated. The composition of the jarosite, however, is virtually independent of the thiocyanate concentration, and