Åke Sandström

Bioleaching of a complex sulphide ore with moderate thermophilic and extreme thermophilic microorganisms

Abstract Biooxidation of a fine-grained, complex zinc and gold-containing sulphide ore has been performed in a series of experiments at bench scale with 201 leaching volume in a series of three continuously stirred reactors. A mixed culture of moderate thermophilic bacteria was used for bioleaching at 45°C and a mixed culture of extreme thermophilic archea were used for bioleaching at 65°C. The leaching yields for zinc were in the range 80–87% with the moderate thermophilic bacteria and 96–98% with the extremely thermophilic microorganisms. It was found that, to obtain a high zinc recovery with a low degree of pyrite oxidation, a fine particle size was essential. Changes in retention time did not influence zinc solubilisation to any greater extent. Due to a high limestone content in the ore, the bioleaching was acid consuming. The acid consumption was strongly dependent on the throughput of ore in the leaching system. Recoveries of gold and silver, of ∼90% and 60–80%, respectively, after cyanidation of the bioleaching residue were obtained, irrespective of experimental conditions.

Acid leaching of coal and coal-ashes☆ ☆

Twelve Turkish lignites and the corresponding ashes were leached in sulfuric acid (pH 1.0, 25 8C) for 14 days. Asphaltite from Silopi amended the coals. The conditions mimic treatment in the effluent from bioleaching of sulfidic mineral concentrates, but the results are equally valid for an isolated leaching process. The extended time meant that we approached equilibrium and maximum extraction. The coals have limited neutralizing capacity. H2SO4 (1.0‐ 2.0 l, 1 M) was needed to stabilize 1 kg coal at pH 1.0 (liquid:solid ratio 10:1), but the coal-ashes required 18.0 ‐24.0 l/kg dry solid, which is the neutralizing equivalent to CaO. Leaching of dominant inorganic phases consume acid, but our interest is merely to remove trace elements present as dopants. We removed large fractions of Mg and Mn, but Al, K and Na extractions were limited by the presence of stable minerals and bimetallic oxides. The formation of the latter is driven by combustion at high temperatures. Alumina, normally not stable at pH 1.0, was protected from the effluent by the organic phase in coal. Fe leaching varied and appeared to be a marker for different chemical occurrences in the solids. Cd, V, Zn, U and Th were leached to near 80% from the ashes, but considerably less from the coals. Co and Ni extractions were near 60%, but not always higher from the ashes compared with the coals. Cu yields increase following ashing and reached ca. 60%. Ti, Ba, and Cs were not leached. We suggest that direct acid leaching is of interest to limit the deleterious impact of ash deposits and to recirculate metals from the ash. Ash may partly replace limestone in hydrometallurgical processing, but, more importantly, metal ions extracted from ash may be fed into the metal recovery stages of such processes. It is particularly interesting to leach Co, Cu, Ni and Zn, besides Mn, V and the environmentally hazardous Cd, U and Th. Leaching of whole coals is well motivated for domestic use-lump sizes around 18 ‐ 50 mm, or slightly smaller, 10 mm, if mandated by practical residence times—where generally no other measures are taken to protect the local environment. q 2003 Elsevier Science Ltd. All rights reserved.

Full potential calculations on the electron bandstructures of Sphalerite, Pyrite and Chalcopyrite

The metal contents of ore can be as low as 0.4%m This means sophisticated methods of enrichment have to be applied. Better understanding of the processes of flotation and leaching may lead to highe ...

Fine-particle characterization - An important recycling tool

This paper summarizes the results from different research projects on fine-particle characterization of metallurgical residues, such as hydroxide sludge, electric-arc furnace dust, and copper-converter dust. These studies demonstrated that characterization is essential for developing new recycling strategies. The basic knowledge of the chemical and physical properties of the residues obtained by the characterizations also provides necessary information on potential problems that could be encountered during the processing of such materials.

A study on the toxic effects of chloride on the biooxidation efficiency of pyrite

Bioleaching operations in areas with limited chloride-free water and use of ashes and dust as neutralizing agents have motivated to study the chloride toxicity and tolerance level of the microorganisms. Biooxidation of pyrite using chloride containing waste ash compared with Ca(OH)(2)+NaCl as neutralizing agent was investigated to evaluate the causes of low pyrite oxidation. Both precipitation of jarosite as well as the toxic effect of chloride on the microorganisms were responsible for lower pyrite recoveries. Another study with sudden exposure of chloride during pyrite biooxidation, addition of 4 g/L was lethal for the microorganisms. Addition of 2g/L chloride resulted in precipitation of jarosite with slightly lower pyrite recovery whereas the addition of 3g/L chloride temporarily chocked the microorganisms but activity was regained after a short period of adaptation. Population dynamics study conducted on the experiment with 3g/L chloride surprisingly showed that Leptospirillum ferriphilum, which was dominating in the inoculum, completely disappeared from the culture already before chloride was added. Sulphobacillus sp. was responsible for iron oxidation in the experiment. Both Acidithiobacillus caldus and Sulphobacillus sp. were adaptive and robust in nature and their numbers were slightly affected after chloride addition. Therefore, it was concluded that the microbial species involved in the biooxidation of pyrite vary in population during the different stages of biooxidation.

A sequential two-step process using moderately and extremely thermophilic cultures for biooxidation of refractory gold concentrates

In many cases, the use of extreme thermophiles, like the archeon Sulfolobus metallicus, in a continuous bioleaching process of gold concentrates is limited by the arsenic content in the feed. In this work, a sequential two-step bioleaching process for gold-containing refractory pyrite/arsenopyrite concentrates has been investigated for the possibility of lowering the toxicity of arsenic with respect to the extremely thermophilic culture. In the first stage, a moderately thermophilic culture was used followed by the extremely thermophilic S. metallicus in the second stage. It was found that the S. metallicus culture survives higher arsenic concentrations than expected when the concentrate was pre-oxidized at a lower temperature. Thus, with this sequential two-step bioleaching process, it is possible to reduce the toxicity of the released arsenic. Therefore, the use of higher pulp densities of arsenic-containing minerals is enabled. When the leached mineral residues were subjected to cyanidation, cyanide consumption and thiocyanate formation were significantly lower after the second stage. In addition, a somewhat higher gold and silver grade was found in the residue from the concentrate ultimately oxidized by S. metallicus.

Effect of chloride on ferrous iron oxidation by a leptospirillum ferriphilum-dominated chemostat culture

Biomining is the use of microorganisms to catalyze metal extraction from sulfide ores. However, the available water in some biomining environments has high chloride concentrations and therefore, chloride toxicity to ferrous oxidizing microorganisms has been investigated. Batch biooxidation of Fe2+ by a Leptospirillum ferriphilum‐dominated culture was completely inhibited by 12 g L−1 chloride. In addition, the effects of chloride on oxidation kinetics in a Fe2+ limited chemostat were studied. Results from the chemostat modeling suggest that the chloride toxicity was attributed to affects on the Fe2+ oxidation system, pH homeostasis, and lowering of the proton motive force. Modeling showed a decrease in the maximum specific growth rate (µmax) and an increase in the substrate constant (Ks) with increasing chloride concentrations, indicating an effect on the Fe2+ oxidation system. The model proposes a lowered maintenance activity when the media was fed with 2–3 g L−1 chloride with a concomitant drastic decrease in the true yield (Ytrue). This model helps to understand the influence of chloride on Fe2+ biooxidation kinetics. Biotechnol. Bioeng. 2010; 106: 422–431.

Bacterial ferrous iron oxidation of acid mine drainage as pre-treatment for subsequent metal recovery

Mining activities at Falu copper mine in Sweden started around the year 1080 AD and continued until 1993. During all these centuries, the acid mine drainage has caused low pH values and high metal ...

Leaching Behaviour of Industrial Oxidic By-Products: Possibilities to Use as Neutralisation Agent in Bioleaching

In this study chemical leaching with sulphuric acid has been performed on 10 selected oxidic by-products in order to determine their neutralising capacity. The ultimate aim with this work is to replace the lime or limestone normally used in bioleaching operations to maintain pH at 1.5, the optimum pH-level for bioleaching microorganisms, with oxidic by-products. The investigated by-products includes three ashes from combustion for energy production, five slag samples from ore and scrap based steelmaking, an EAF dust and mesa lime from a paper and pulp industry, slaked lime (Ca(OH)2) was used as reference material. The neutralising potential of the by-products were evaluated by leaching them with sulphuric acid and comparing the amount of acid needed to that of the reference. Most of the by-products examined had good neutralisation potential and some had even higher capacities than Ca(OH)2. Neutralisation kinetics were lower for some slag products due to slow dissolution of some of the silicates present, but kinetics are considered good enough since stirred tank bioleaching is a relatively slow process. Zinc recoveries from the zinc containing materials were high, which thus is an additional benefit if these materials were to be used for neutralisation in a bioleaching process for zinc recovery.

Modelling and process optimisation of antimony removal from a complex copper concentrate

The modelling and optimization for the alkaline sulphide leaching of a complex copper concentrate containing 1.69% Sb and 0.14% Sn were studied. Response surface methodology, in combination with central composite face-centred design (RSM-CCF), was used to optimise the operating parameters. The leaching temperature, sulphide ion concentration and solid concentration were chosen as the variables, and the response parameters were antimony and tin recovery, and the time required to achieve 90% Sb dissolution. It was confirmed that the leaching process was strongly dependent on the reaction temperature as well as the sulphide ion concentration without any significant dependence on the solid concentration. Furthermore, a mathematical model was constructed to characterise the leaching behaviour. The results from the model allow identification of the most favourable leaching conditions. The model was validated experimentally, and the results show that the model is reliable and accurate in predicting the leaching process.