Dimitrios Filippou

RECOVERY OF METAL VALUES FROM COPPER—ARSENIC MINERALS AND OTHER RELATED RESOURCES

Copper is often associated with arsenic in mixed sulphide minerals such as enargite (Cu3AsS4) and tennantite (Cu12As4S13). Enargite, in particular, is the principal mineral in many deep epithermal copper–gold deposits. Most mining companies avoid exploiting such resources, because the arsenic can become a serious environmental liability or may considerably reduce the resource value due to hefty treatment charges. The few enargite deposits that have been exploited so far are usually rich in gold and silver. The first challenge in the exploitation of copper–arsenic sulphides is the effective separation of arsenic phases from other valuable minerals. In the last decade, though, it was shown that this is possible by pulp-potential adjustment (oxidative conditions) combined with pH adjustments (basic conditions) prior to flotation. In this way, two types of concentrate can be produced: one rich in arsenic and another low in arsenic but rich in other valuable metals. Arsenic-rich concentrates have traditionally been processed pyrometallurgically by reduction roasting to gaseous arsenic sulphide, which is then converted to arsenic trioxide. New pyrometallurgical technologies for the treatment of copper–arsenic sulphides include sulphidization roasting, sulphidization roasting and halogenation, and carbothermic reduction to copper arsenide. The hydrometallurgical treatment of copper–arsenic-antimony resources has been done by atmospheric leaching in alkaline sodium-sulphide solutions. Ultrafine grinding and ferric oxidation at atmospheric pressure, total pressure oxidation at temperatures above 220°C, and bacterial leaching have recently been tried on copper–arsenic sulphides, some with considerable success.

Acidity, valency and third-ion effects on the precipitation of scorodite from mixed sulfate solutions under atmospheric-pressure conditions

The present study is focused on the precipitation of scorodite from mixed sulfate media at 95 °C under atmospheric pressure. In particular, this study explores the effects of acidity (pH), valency [Fe(II)/Fe(III), As(III)/As(V)], and solution composition (third cation/anion) on the yield, crystallinity, and stability (leachability) of scorodite precipitates. Thus, it was found that the precipitation of crystalline scorodite can be achieved without stringent pH control once the precipitation has started. Nonetheless, the selection of the initial pH is critical to avoid the formation of an amorphous precipitate. A leachability as low as 0.5 mg/L As at pH 5 and 22 °C (TCLP-like test) is obtained when the initial molar ratio Fe(III):As(V) is increased to 3:1, but the precipitation yield is very low. When Fe(II) is used as excess iron, the precipitate solubility drops to 0.2 mg/L As with a yield exceeding 80 pct in 2.5 hours. The stability of the product is not measurably affected by the presence of Cu2+, Zn2+, Ni2+, Co2+, Mn2+, SO42−, and NO3−. The presence of PO43−, however, leads to the formation of crystalline phosphate-containing scorodite precipitates of somewhat reduced stability. In most cases, the TCLP leachability of the precipitate was found to be between 1 and 3 mg/L As, and never exceeded the regulatory limit of 5 mg/L As.

INNOVATIVE HYDROMETALLURGICAL PROCESSES FOR THE PRIMARY PROCESSING OF ZINC

The zinc industry has been in the forefront of hydrometallurgical developments for almost a century. The development of pressure leaching in the 1980s and the development of atmospheric direct leaching in the 1990s for treating zinc-sulphide concentrates have resulted in a number of zinc-refinery expansions without an increase in roasting capacity. Likewise, solvent extraction techniques are now used for the hydrometallurgical treatment of zinc-oxide ores on an industrial scale. The treatment of poor and complex zinc-sulphide resources has been extensively studied using processes as diverse as pyrolusite leaching and heap bioleaching. Finally, the precipitation of relatively pure zinc oxide at the mine site, a process that has been proven to be technically feasible very recently, may significantly change the paradigm of primary zinc production.

Gas–Liquid Oxygen Mass-transfer; from Fundamentals to Applications in Hydrometallurgical Systems

Oxygen gas is used as a reactant in many hydrometallurgical processes. However, oxygen is sparingly soluble in aqueous electrolyte solutions, and the driving force for its masstransfer from the gas to the aqueous phase is also very low. Hence, the mass-transfer of oxygen depends significantly on the interfacial area between the gas and the liquid phase, which in turn depends on the mixing conditions inside the reactor. A change in the solution composition or the presence of solids can further alter the rate of oxygen masstransfer. All such phenomena related to the gas-liquid oxygen mass-transfer in hydrometallurgical unit operations are critically reviewed in the present article – from the basic thermodynamics and kinetics of oxygen dissolution in aqueous solutions to the most recent advances in oxygen mass-transfer systems.