-->

Πέμπτη, 27 Ιουλίου 2017

Microbial assisted leaching of nickel laterites

Bioleaching of non-sulphide nickeliferous ores using heterotrophic microorganisms, phD thesis by P.G. Tzeferis, 1991 



Dr Tzeferis' phd thesis (1986-1991) was one of the very first in heterotrophic/fungal laterite leaching research studies worldwide
Nickel is an important metal in human life and in the industry. In recent years, the world nickel demand has been driven by soaring steel production, particularly in China. With the rapid growing demand for nickel coupled with the depletion of high grade sulphide reserves, low-grade nickel ores, which cannot be economically processed by conventional metallurgical processes, become increasingly important sources of nickel. Laterite ore, which is often considered as a low-grade nickel ore, contains several kinds of metal elements including nickel, cobalt, iron, silicon, aluminium, and chromium; and thus, constitutes an alternative source of nickel.

The nickel present in nickel laterites is not usually present as discrete minerals, but as cations substituted within manganese oxides, goethite, and/or clays. Because of this, it is difficult to upgrade the ore by beneficiation. As a result, nickel laterites are traditionally processed using pyrometallurgical and hydrometallurgical methods. In recent years, microbiological leaching has been found to be a promising novel technology for recovering valuable minerals from traditionally difficult-toprocess ores.

Microbial leaching of low-grade ores offers many advantages over other conventional methods due to its relative simplicity, requiring mild operating conditions, low capital costs, low energy input, relatively unskilled labour requirements, and being environmentally friendly. Because of the importance of microbial leaching, recent advances in microbial assisted leaching of nickel laterites are discussed in this paper with emphasis on fungal  and chemolithotrophic microorganisms (G. Simate, 2009).

Microbial leaching of non-sulphide ores, especially oxides and silicates, represents a new challenge. For these ores, including laterites, chemoorganotrophic leaching using fungi is necessary. This is because the organic acids metabolically produced by fungi have a dual effect of providing hydrogen ions for acidolysis of minerals and complexing metals due to their chelating capacity (Tzeferis, 1994; Gadd, 2001). Therefore, the leaching of nickel laterites by fungi, which are heterotrophic microorganisms has continued to be the main focus of research in the recent past. In fact, several other heterotrophic microorganisms, which include both bacteria and fungi species areknown for their leaching capabilities, especially of oxidic, siliceous or carbonaceous material (Willscher and Bosecker, 2003). These microorganisms, in direct contrast to autotrophs, ingest biomass to obtain their energy and nutrition. The heterotrophs have an absolute dependence on the biological products of autotrophs. This is because they obtain their carbon for growth solely by feeding on the carbon produced by autotrophs (e.g., dead plants and dead organic matter). Aspergillus and Penicillium are the two widely studied strains of fungi that can be used in the microbial leaching.

Metal leaching by fungi generally involves an indirect process with microbial production of organic acids, amino acids, and other metabolites. Four mechanisms have been identified: (i) acidolysis, (ii) complexolysis, (iii) redoxolysis (Berthelin, 1983), and (iv) bioaccumulation (Weed et al., 1969).The following are some of the possible reactions that can take place to finally produce nickel ions (Tzeferis, 1992):
The use of fungi in a quest to recover nickel from low-grade nickel laterite ores has been studied by several researchers (Bosecker, 1986; McKenzie et al., 1987; Alibhai et al., 1993; Tzeferis, 1994; Valix et al., 2001a,b). 

However, commercial application of these microorganisms has been less successful due to process inefficiencies such as poor metal recovery (Tang and Valix,2006). This problem is particularly prevalent in limonit ore, characterised by high iron content in the form of goethite (FeOOH) and nontronite, characterised by clay minerals consisting of illite, kaolinite, and chlorite (Tang and Valix, 2006). The effectiveness of these microorganisms was found to depend on their ability to produce hydroxycarboxylic acids (citric, lactic, gluconic, pyruvic and tartaric), and also other metabolites, which are excreted in culture media (Tzeferis, 1994; Castro et al., 2000; Le et al., 2006), and their resistant to heavy metals (Burgstaller and Schinner, 1993; Le et al., 2006). 

The results of these studies showed that microbiological leaching is more effective compared to chemical leaching. The more favourable results obtained in the bioleaching process suggest that microbiological activity, apart from bio-acid production is participating in the leaching process (Valix et al., 2001a). It is suggested, for example, that fungal hyphae physically attach onto surfaces of minerals with possibility of high acid concentration formed at hyphal tips reacting directly with adjacent mineral surfaces without greatly affecting the pH of the bulk medium (Alibhai et al., 1993).

The studies by Tzeferis and Agatzini-Leonardou (1994), Castro et al. (2000), and Valix et al. (2001a) clearly showed differences in the leachability of different minerals (and/or the metal present in it), i.e., the mineralogy of the ores had signicant effects on the metal recovery and selectivity of the leaching process. Extraction of nickel appeared more effective from the silicate ores, the saprolite and weathered saprolite, and least effective from limonite ores (Valix et al., 2001a,b).

Apart from the effect of mineralogy, Tang and Valix (2006) showed that nickel and cobalt dissolution were also dependent on acid activity, oxygen reduction potential, and pulp density. While previous studies showed that various acids exhibit differences in effectiveness in dissolving metals from laterites (Alibhai et al., 1993; Burgstaller and Schinner, 1993; Tzeferis, 1994; Tzeferis and Agatzini-Leonardou,1994; Tzeferis, 1997, Castro et al., 2000), the results by Tang and Valix (2006) suggested that the extent of metal dissolution is dependent on the acid activity (hydronium ion concentration) rather than the type of metabolic acids used. Other results by Tang and Valix (2006) also suggested that highly oxidizing conditions found at higher pulp densities suppressed metal dissolution. These results emphasises the importance of pH, ORP, and pulp density control during the bioleaching of nickel laterite ores.

Summary of the fungal leaching of nickel laterites.
Table 4 shows a summary of some of the studies conducted, showing the types of ore or mineralogical makeup of nickel laterite, fungi species, types of acid produced, and a range of recoveries obtained. It is also clear from Table 4 that citric and oxalic acids were the two well established products of fungal metabolism. Citric acid was the most effective and oxalic acid was the least effective leaching agent (Tzeferis, 1994Tzeferis, 1995). A possible explanation for this could be that oxalic acid precipitates the leached nickel as nickel oxalate, which is known to have a very low solubility (Tzeferis, 1994).

Available from:Geoffrey S. Simate:The fungal and chemolithotrophic leaching of nickel laterites—Challenges and opportunities (PDF Download Available)