Biobeneficiation development for the reduction of potassium and phosphorus from Sishen iron ore

Abstract

High levels of elements such as sodium (Na), potassium (K) and phosphorus (P) in iron ore minerals are known to reduce the quality and price of these minerals. South Africa, as one of the world largest exporter of iron ore, is affected by this problem. Both potassium (K) and phosphorus (P) are peculiar to South African iron ore. The present study has therefore focussed on developing an environmentally friendly biological method for lowering the levels of K and P in iron ore minerals. Short and long term experiments were set up to isolate, identify, screen and test potential bioleaching bacteria and fungi from different environmental samples. The study started by investigating the possible relationship that exists between weathering and bioleaching processes. The investigation was intended to provide relevant information on the natural role of microorganisms such as ectomycorrhizal (ECM) fungi in the mining environment. The experiments involved the use of both mycorrhizal and non-mycorrhizal Pinus patula seedlings for the weathering of iron ore minerals. Four types of ECM fungi were used, namely Pisolithus tinctorius (PT), Paxillus involutus (PI), Laccaria bicolor (LB) and Suillus tomentosus, (ST). From the results, ectomycorrhizal weathering can be said to be species-specific and significantly influenced by fungal type and particle size. In addition, it was also discovered that both mycorrhizal and non-mycorrhizal roots can participate in weathering processes. Further investigations of ECM fungi when not in symbiosis, were carried out to know how or if they can be potential candidates to mobilise K and P from iron ore minerals. The experimental set up involved in vitro pure cultures of four different ECM fungi, namely Pisolithus tinctorius (PT), Paxillus involutus (PI), Phialocephala fortini (PFR), and Suillus tomentosus (ST). In addition, the treatments involved the use of five different particle sizes of each ore type. The results obtained indicated the potential of the ECM fungi to mobilise P and K from the two iron ore types though at different levels. Factors such as ore type, particle size, organic acid production and attachment of the fungi to the iron ore were all found to influence the mobilisation of nutrients from these ores. Another experiment that addressed some of the limitations encountered with the use of pure cultures of ECM fungi was conducted. Isolated indigenous fungal pure cultures from the surfaces of iron ore minerals were screened for their abilities to solubilise minerals by lowering the levels of K and P. These isolates were identified molecularly as close relatives of three genera that included Penicillium, Alternaria (2 isolates) and Epicoccum for isolates FO, SFC2/KFC1 and SFC2B respectively. The identified Penicillium sp. turned out to be the only phosphate solubiliser among these isolates. Direct bioleaching capability of the fungus was compared to that of its metabolite. At the end, the metabolite showed better K removal than the direct use of the fungi. Interpretation of these results indicates possible relationship between K and P removal, and the organic acids production by this fungus. Other factors such as particle size and mineral type were also found to significantly influence the leaching process. Additional experiment was conducted to investigate the indigeous bacteria and their potentials in reducing the K and P contents of iron ore minerals. A total of 23 bacterial strains that belong to Proteobacteria, Firmicutes, Bacteroidetes and Actinobateria were isolated from the iron ore minerals and identified with molecular methods. All the bacterial isolates were screened for their potential as mineral solubilisers. Only eight of the isolates were selected and used in shake flask experiments that contained both KGT and SK mineral types as their sources of K and P. The experiment showed that all the eight isolates have potentials to produce organic acids especially high levels of gluconic acid but lower quantities of acetic, citric and propanoic acid. Scanning electron microscopy (SEM) and fourier transform infrared (FITR) analyses also helped to uncover the role that biofilm and extracellular polymeric substances could play in mineral solubilisation. Finally, an investigation of a new method for reduction of K and P levels of iron ore minerals was carried out, focussing on the use of cheap resources as well as septic conditions. The study involved the use of fermented spoilt grape fruits (Vitis sp.) and the solution from the product utilised in shake-flask experiments. Treatments involved two types of iron ore minerals (KGT and SK) and two different particle sizes. The result suggests the significant effect of particle size, time and organic acids on the reduction of K and P from the iron ore minerals. The important part of this finding is the discovery of a cheap microbial energy source (spoilt grape) that can be further exploited for full biobeneficiation of iron ore minerals. Another advantage of this method is the fact that the experiment can be conducted under non–sterile conditions, making it a system that can be operated outdoor.