The solubility of cobalt in iron silicate slags at 1573 K

The solubility of cobalt in non-ferrous smelting slags has been studied at 1573 K. The study involved both sulfur-free and sulfur-containing systems. In the sulfur-free systems, the solubility of cobalt in slag was examined by equilibrating iron silicate slag with liquid Co-Au-Fe or solid Co-Fe alloys under an oxygen pressure of either 10^-9 or 10^-10 atmospheres as controlled by mixture of CO/CO2 gas. Three series of experiments were carried out to evaluate the respective effects of cobalt content, iron/silica ratio and oxide additives on the solubility of cobalt in slags. The melts were contained in either silica, alumina, cobalt or magnesium oxide crucibles. The cobalt solubility in slag was interpreted in terms of an activity coefficient of cobalt oxide, calculated from a knowledge of the activity of cobalt in the alloy, the oxygen pressure in the gas phase and the temperature of equilibration. The activity coefficient of ferrous oxide in the slag was calculated in a similar way. In the systems containing sulfur, a novel experimental method was employed to study the effect of sulfur on the activity coefficient of cobalt oxide in slag at 1573 K. Iron silicate slag was equilibrated with a liquid Co-Au-Fe alloy and a cobalt matte, with the oxygen and sulfur pressures controlled by mixtures of CO/CO2/SO2 gas at 1573 K. The method was novel since the presence of a low sulfur alloy phase, in addition to matte, allowed the metal activities in the system to be defined. In this way, the activities of cobalt oxide and the activity of cobalt sulphide could be calculated when the respective oxygen and sulfur pressures were defined. Thermodynamic analysis indicated that the solubility of cobalt in slag containing sulfur could be attributed to a solubility of cobalt sulphide in addition to cobalt oxide. The respective activity coefficients of these species were calculated for the slag along with the activity coefficient of cobalt sulphide in the matte. The data were combined to quantify the distribution coefficient of cobalt between slag and matte as a function of mole fraction of cobalt sulphide in matte as well as the sulfur and oxygen partial pressures. The effect of iron/sulfur ratio in nickel matte on the distribution coefficients of both cobalt and nickel was investigated finally by the equilibration of nickel matte (with minor cobalt), iron silicate slag and a Ni-Co-Fe-Au alloy, system was allowed to find its own equilibrium under argon, but the oxygen pressure was calculated indirectly and correlated with the change in matte composition