Thomas Feldmann

Stability of continuously produced Fe(II)/Fe(III)/As(V) co-precipitates under periodic exposure to reducing agents.

Arsenic mobilized during ore processing necessitates its effective removal from process effluents and disposal in environmentally stable tailings. The most common method to accomplish this involves co-precipitation with excess ferric iron during lime neutralization. The precipitates produced are stable under oxic conditions. This may not be true, however, under sub-oxic or anoxic conditions. In this context, the potential stabilizing role of ferrous iron on arsenic removal/retention becomes important. As such, this work investigates the removal and redox stability of arsenic with ferrous, ferric and mixtures of both. The stability of produced solids is monitored in terms of arsenic release over time. It was found that ferrous was very effective for arsenic (V) removal with Fe(II)/As(V)=4, reducing its concentration down to <15 ppb via the apparent formation of ferrous arsenate. The presence of Fe(II) seemed to favor an oxidation path toward goethite (and possibly scorodite) formation in the aged bench-scale tailings. When pH and Eh were regularly adjusted with lime and sulfite or sulfide, slightly higher arsenic amounts were released (1-5 mg L(-1)); ferrous again was found to oxidize. Hence, it is concluded that Fe(II)/Fe(III)/As(V) co-precipitates are quite robust against incidental reducing agent exposure.

Hydrochloric Acid Regeneration via Calcium Sulfate Crystallization for Non-Ferrous Chloride Leaching Processes

Regeneration of hydrochloric acid constitutes a key operation in many new chloride hydrometallurgical processes, including polymetallic Ni-Cu sulfides, laterite ores or extraction of rare earths from apatite ores. For leaching processes that generate a spent calcium chloride solution, one novel option for regenerating HCl involves reaction of CaCl2 liquor with concentrated H2SO4 at low temperatures (T<100°C). Due to the favorable price and availability of sulfuric acid this constitutes a viable option. Therefore, key considerations concern the process kinetics of the crystallization carried out by stage-wise mixing of solutions to control the precipitation reaction, which leads to formation of different calcium sulfate phases depending on concentrations and temperature. Crystal growth and phase transformation kinetics determine the efficiency of required solid-liquid separation steps and optimum residence time in individual process stages. In this paper highlights from our work defining appropriate operating conditions and results from crystallization kinetic studies will be presented.