Why amorphous FeO-SiO2 slags do not acid-leach at high temperatures.

It has been shown previously that amorphous FeO-SiO2 slags are not amenable to high pressure oxidative acid leaching - unlike their crystalline counterparts. Independent studies of glass and silicate mineral dissolution at ambient conditions suggest that acid attack can be hindered by the formation of a passive silica layer. The current work extends this finding to the case of high temperature dissolution of amorphous FeO-SiO2 slags by providing evidence for the formation of a passive silica layer within slag particles under high pressure oxidative acid leaching conditions (250°C, 70g/L initial H2SO4, 0.62MPa [90psi] O2). Based on the percolation model of glass dissolution, a mechanism of amorphous slag leaching is proposed.

Co-treatment of converter slag and pyrrhotite tailings via high pressure oxidative leaching.

High pressure oxidative acid leaching (HPOXAL) was successfully applied to slow-cooled converter slags from Vale's operations in Sudbury (Ontario, Canada). Extractions of Ni, Co and Cu exceeded 90% within 15-20 min and levelled at 95-97% after 45 min at 250°C, 90 psi O(2) overpressure and 70 g/L initial H(2)SO(4). Pyrrhotite tailings with ∼ 0.6% Ni content were also tested as a source of sulphuric acid in high pressure oxidation. Co-leaching of pyrrhotite tailings with converter slags at the same temperature, oxygen partial pressure and equivalent stoichiometric H(2)SO(4) was found to have kinetics similar to that of leaching with sulphuric acid. Lowering the addition of pyrrhotite tailings (and hence, the acidity) was found to have a detrimental effect on the kinetics of leaching and final extractions (especially at 250°C), and cause precipitation of metal sulphates. Continuous on-line acidity measurements were facilitated in experiments with an electrodeless conductivity sensor. It was shown that acid plays a major role in the conversion of fayalite to hematite and silica, and the dissolution of the base metals, while oxygen overpressure (or dispersion efficiency) determines the rate of acid generation and re-generation.

Cleaning of waste smelter slags and recovery of valuable metals by pressure oxidative leaching.

Huge quantities of slag, a waste solid product of pyrometallurgical operations by the metals industry are dumped continuously around the world, posing a potential environmental threat due to entrained values of base metals and sulfur. High temperature pressure oxidative acid leaching of nickel smelter slags was investigated as a process to facilitate slag cleaning and selective dissolution of base metals for economic recovery. Five key parameters, namely temperature, acid addition, oxygen overpressure, solids loading and particle size, were examined on the process performance. Base metal recoveries, acid and oxygen consumptions were accurately measured, and ferrous/ferric iron concentrations were also determined. A highly selective leaching of valuable metals with extractions of >99% for nickel and cobalt, >97% for copper, >91% for zinc and <2.2% for iron was successfully achieved for 20 wt.% acid addition and 25% solids loading at 200-300 kPa O(2) overpressure at 250 degrees C in 2h. The acid consumption was measured to be 38.5 kg H(2)SO(4)/t slag and the oxygen consumption was determined as 84 kg O(2)/t slag which is consistent with the estimated theoretical oxygen consumption. The as-produced residue containing less than 0.01% of base metals, hematite and virtually zero sulfidic sulfur seems to be suitable for safe disposal. The process seems to be able to claim economic recovery of base metals from slags and is reliable and feasible.