The effect of temperature and culture history on the attachment of Metallosphaera hakonensis to mineral sulfides with application to heap bioleaching

Temperatures in excess of 60 °C are required for efficient bioleaching of chalcopyrite. Within heaps, colonisation of the mineral with thermophilic archaea is important in reaching and maintaining these high temperatures. The effect of temperature and culture history on the attachment of Metallosphaera hakonensis, an extreme thermophilic acidophile identified as a key player in heap bioleaching, to sulfide concentrates and low-grade ore was investigated in shake flasks and packed beds. Attachment studies were conducted at 25 °C, 45 °C and 65 °C. The results show a clear relationship between increasing temperature and attachment efficiency for both suspended and packed bed systems. Attachment at 25 °C was low. Increasing the temperature to 45 °C improved attachment efficiency by between 50% and 100% while a further increase to 65 °C improved attachment by an additional 20-50%. Cells cultured on elemental sulfur as energy source prior to contacting showed 1.3 times greater affinity for the mineral concentrate than those cultured on sulphide mineral concentrates or ferrous sulphate. In contrast to previous studies using mesophilic organisms the selective attachment of Metallosphaera to sulfide minerals, relative to gangue, was less pronounced. Attachment efficiency was lower in the packed bed system which more closely mimicked flow through a heap. The cell surface properties surface charge and hydrophobicity as well as metabolic activity were investigated to provide insight into the observed phenomena. The data suggest that retention of thermophiles within the heap could be enhanced by a secondary inoculation following elevation of the temperature above 40 °C by the mesophilic pioneer species.     

Respirometry studies of bioleaching of low-grade chalcopyrite ore using six acidophilic strains

Respirometry was used to study the growth and activity of six pure cultures of acidophilic bioleaching strains grown on a concentration series of low-grade chalcopyrite ores under various pH conditions. Sulfolobus metallicus, Acidithiobacillus ferrooxidans, Acidianus brierleyi and Leptospirillum ferriphilum were able to grow on a very low-grade ore (equivalent to 0.1% Cu content). However, the two sulphur-oxidising bacteria Acidithiobacillus caldus and Acidithiobacillus thiooxidans grew poorly on the low-grade ore. Growth rates of all strains, except for perhaps S. metallicus at highest ore grades, displayed growth that was limited by substrate availability on this low-grade ore (0.5% Cu content in the ore). The decrease in solution pH from 3.0 to 1.0 enhanced both the cell growth and Cu dissolution.     

The efficiency of indigenous and designed consortia in bioleaching stirred tank reactors

The efficiency of bioleaching is dependant on the establishment of an active microbial community. There is debate as to whether an indigenous microbial community is superior to one composed of microbial strains selected for specific physiological traits. The bioleaching efficiency of three microbial communities was studied: the indigenous community of a commercial bioleaching system (KCCL), a reconstituted consortium of the four major organisms which comprise KCCL that had been ‘un-adapted’ through a period of continuous maintenance in synthetic media (KCCR) and a specifically designed consortium of bioleaching organisms (KCCD). Acidithiobacillus caldus was unable to re-establish itself in the reconstituted, un-adapted consortium. However, the bioleaching rate of this consortium improved over time, and its overall performance was very similar to that of the indigenous community. This was despite the absence of an obligate sulfur-oxidising species, which resulted in the generation of substantially less acid. The performance of the designed consortium was poor, and the results implied that bioleaching consortia (mesophiles or moderate thermophiles) cannot be assembled ‘off-the-shelf’, at least not without a substantial period of adaptation.     

Extracellular polymeric substances (EPS) from bioleaching systems and its application in bioflotation

The use of EPS-producing heterotrophic and chemolithotrophic bacteria in bioflotation processes have been investigated for the last two decades. These studies have mostly been confined to laboratory microflotation tests using pure cultures. In this study the use of EPS, extracted from bioleaching consortia, to float chalcopyrite was evaluated and key process parameters such as pH, flotation time and the concentration of collector, bacterial cells and EPS were optimised. Analyses of the EPS extracted from various bioleach systems indicated that the EPS consisted mainly of carbohydrates, proteins and uronic acids. Microflotation tests using free EPS yielded a chalcopyrite recovery of 77% when chalcopyrite was floated alone and 70% during the separation of a mixture of pure chalcopyrite and pure pyrite compared to 32% when using SIBX only. The results obtained suggested that the flotation of chalcopyrite could be significantly increased in the presence of EPS extracted from bioleaching populations.     

Some quantitative data on bacterial attachment to pyrite

Bacterial attachment and biofilm formation are important for microbial bioleaching of metal sulfides, however, many details of the role played by bacteria are still unknown. Attachment as the first step in biofilm formation is critical. Our investigations are focused on these processes to control bacterial bioleaching processes.In moderately thermophilic pure cultures of Acidithiobacilluscaldus and Leptospirillumferriphilum, only the latter one showed significant attachment and biofilm formation to metal sulfides. In pure cultures of the mesophilic strains Acidithiobacillusspp., Leptospirillumferrooxidans, Ferrimicrobiumacidiphilum and a novel undescribed leaching organism designated SPIII/3, attachment was significant by L. ferrooxidans, At. thiooxidans and SPIII/3.In general, interaction of different species in mixed cultures resulted in increased attachment, increased production of extracellular polymeric substances (EPS) and leaching. Bacteria like Leptospirillumspp. enhanced the attachment of other species in mixed biofilms. Another finding is that large areas of the surface of minerals remain uncolonized, whereas at some places bacteria attach in clusters.Based on these findings we conclude that attachment and thereby bacterial leaching can be improved by the production of EPS by several strains especially in combination with Leptospirillumspp.     

Possibilities for Co(III) dissolution from an oxidized ore through simultaneous bioleaching of pyrite

Solubilisation of Co(III) from a heterogenite met in copper cobaltiferous oxide ore has been realized through reductive leaching using ferrous iron generated via bio-oxidation of pyrite. Biotic and abiotic experiments at various pulp densities and redox potentials have been performed and results compared. Cobalt leaching at elevated redox potential is possible, offering cost reduction benefits due to reduced consumption of ferrous iron. At elevated potential of 625 mV, however, the initial rate of cobalt leaching has been found as 115 mg/(g ore)⋅(24 h), lower than the rate of 865 mg/(g ore)⋅(24 h) registered at 505 mV. Less stochiometric amount of ferrous iron was required when cobalt leaching was coupled to pyrite bioleaching, with 75% of cobalt recovered for 12 h at the optimally found conditions. It could be inferred that the Fe³⁺–Fe²⁺ cycle exists and is efficiently maintained through bacterial presence in the studied system.