Chris A. du Plessis

Biomineralization of metal-containing ores and concentrates

Biomining is the use of microorganisms to extract metals from sulfide and/or iron-containing ores and mineral concentrates. The iron and sulfide is microbially oxidized to produce ferric iron and sulfuric acid, and these chemicals convert the insoluble sulfides of metals such as copper, nickel and zinc to soluble metal sulfates that can be readily recovered from solution. Although gold is inert to microbial action, microbes can be used to recover gold from certain types of minerals because as they oxidize the ore, they open its structure, thereby allowing gold-solubilizing chemicals such as cyanide to penetrate the mineral. Here, we review a strongly growing microbially-based metal extraction industry, which uses either rapid stirred-tank or slower irrigation technology to recover metals from an increasing range of minerals using a diversity of microbes that grow at a variety of temperatures.

Extraction, isolation and NMR data of the tetraether lipid calditoglycerocaldarchaeol (GDNT) from Sulfolobus metallicus harvested from a bioleaching reactor

The successful extraction and isolation of the hydrolysed tetraether lipid calditoglycerocaldarchaeol (GDNT) from Sulfolobus metallicus, a key thermophilic bioleaching archaeon, is described. The archaeal biomass was recovered directly from a thermophilic (68 degrees C) bioleaching tank reactor used to extract nickel from a pentlandite mineral concentrate. The initial Soxhlet extraction method employed was scaled to a bench-scale extraction procedure suitable for the preparation of gram-scale quantities of GDNT. The GDNT so obtained was analysed by 1D- and 2D-NMR techniques, providing the first complete 13C and 2D-NMR data-set for GDNT, including that for the intact underivatised calditol moiety. The study demonstrates the feasibility of recovering high-quality GNDT from thermophilic archaeal-mediated bioleaching reactors. The recovery of these lipids at relatively low cost, as a by-product from bioleaching reactors used in the metals processing industry, has important implications for future tetraether lipid availability and costs.

Development of respirometry methods to assess the microbial activity of thermophilic bioleaching archaea.

Respirometry methods have been used for many years to assess the microbial activity of mainly heterotrophic bacteria. Using this technique, the consumption of oxygen and evolution of carbon dioxide for heterotrophic carbon catabolism can be used to assess microbial activity. In the case of autotrophic bioleaching bacteria, carbon dioxide is used as a carbon source resulting in the consumption of both oxygen and carbon dioxide. The use of such respirometry techniques at high temperatures (up to 80 degrees C) for the investigation of bioleaching Archaea, however, poses particular difficulties. At these elevated temperatures, the solubility of oxygen into the liquid phase is particularly poor. This work details specific methods by which high temperature constraints are overcome while monitoring the activity of thermophilic Archaea using a Micro-Oxymax respirometer (Columbus Instruments). The use of elevated headspace oxygen concentrations, in order to overcome low oxygen solubility, is demonstrated as well as the effect of such elevated oxygen concentrations on microbial oxygen consumption rates. The relative rates of oxygen and carbon dioxide consumption are also illustrated during the oxidation of a chalcopyrite concentrate. In addition, this paper details generic methods by which respirometry data can be used to quantify inhibitory effects of a compound such as Na(2)SO(4). The further use of such data in predicting minimum hydraulic reactor retention times for continuous culture bioleaching reactors, as a function of concentration of potentially inhibitory compounds, is also demonstrated.

Reduction and Complexation of Copper in a Novel Bioreduction System Developed to Recover Base Metals from Mine Process Waters

An integrated bio-processing scheme was devised and tested in the laboratory for recovering copper, or other base metals, from pregnant leach solutions (PLS) using a two-step process involving both iron-reduction, and sulfate-reduction for H2S generation and sulfide precipitation, as a potential alternative to conventional SX-EW. Reduction of ferric iron in the PLS was achieved using iron-reducing Acidithiobacillus spp. and Sulfobacillus thermosulfidooxidans in column reactors containing elemental sulfur as electron donor. Analysis of the column reactor effluents showed not only that most of the ferric iron was reduced to ferrous, but also that all of the copper (II) had been reduced to copper (I), i.e. cuprous copper. This copper (I) appeared to be complexed as it was not oxidized when exposed to ferric iron nor precipitated as a copper-sulfide when exposed to either sodium sulfide or H2S. The data suggested that copper (II) was reduced and the resulting copper (I) complexed, with both reactions probably mediated by sulfur oxy-anions produced indirectly by the bacteria, in the anoxic sulfur column bioreactors. It was also noted that copper (I) produced chemically by reduction of copper (II) by hydroxylamine was more toxic to axenic cultures of Acidithiobacillus spp. and Sb. thermosulfidooxidans than was the copper (I) in the column effluent liquors.

Respirometry Studies of the Bioleaching of a Copper Ore in a Large Isothermal Column

During large-scale column tests at BHP Billiton’s Johannesburg Technology Centre (JTC) during 2005/6 on a low-grade copper ore, the concentrations of both oxygen and CO2 were continuously monitored in feed and exit gas as well as at various intermediate positions over the height of the column. This paper describes results from a test run at 40 °C fed with an air stream enriched to between 1000 and 2000 ppm CO2. Oxygen consumption very closely tracks iron and copper leaching. CO2 is consumed rapidly from the bottom up, resulting in significant depletion midway through the column, even though an enriched feed was used. Oxidation rates decline in CO2 depleted zones, but were not observed to cease completely. This rate of decline is postulated to be linked to a slowly decaying population unable to regenerate itself. Comparison between O2 and CO2 consumption rates shows a linear correlation beyond a minimum oxidation rate. This minimum rate corresponds to a non-growth maintenance energy requirement, and the slope of the linear correlation to the growth yield. Both are functions of available CO2 in the range 50 to 1000 ppm, with maintenance declining and yield increasing. The findings of this study imply that CO2 supplementation in bioheaps will stimulate microbial growth and CO2 consumption, but not necessarily increase the rate of oxygen uptake and hence leaching. Absence of CO2 is expected to result in gradual population decline, but a degree of oxidation continues on the basis of maintenance. In tall heaps CO2 depletion with height is likely and may therefore result in impaired leaching in the upper zones.