Dear readers, welcome back to the SULTAN blog. I am Chiamaka Belsonia Opara, the SULTAN researcher (ESR9) responsible for extracting metals from SULTAN tailings with the help of microorganisms. In this blog, we will discuss one of the most interesting applications of biotechnology: biomining of gold. Yes, microorganisms can also be used to mine gold from ores! Your expensive gold necklaces, rings and earrings may have been produced as a result of the beneficial activities of some incredible microorganisms that you will soon get to know as you read further.
First, to get a general overview of the Biomining technology, I recommend that you read my colleague’s (Tamara-ESR8) blog on biomining.
Biomining is the use of microorganisms and their products to catalyse the extraction of metals from ores and waste materials. If you have read articles on biomining, you undoubtedly came across two terms always used in biomining, namely ‘bioleaching’ and ‘biooxidation’. You probably thought they meant the same thing and could be used interchangeably. Well, not really! The two terms are slightly different in the context of their applications in biomining, though the fundamental principles behind each technique are the same. Biomining of gold is a very good example to understand these two terms perfectly well.
Having now understood that biomining is the general term used to classify all biotechnological processes that require microorganisms to extract metals from ores, let’s continue. Biooxidation is a technique in biomining that uses microorganisms for the pretreatment of the ore by degrading the minerals that hinder the leaching of your metal(s) of interest. Bioleaching is a technique in biomining, which uses microorganisms to solubilise your metal(s) of interest . Therefore, you can see that the difference lies in what happens to your metal(s) of interest. If your metal of interest is to be dissolved in solution by microorganisms, then the correct term to use is ‘bioleaching’. While you use the term ‘biooxidation’, if microorganisms are used to remove minerals that obstruct your metal of interest. This metal of interest can then be solubilised in a subsequent process. Both biooxidation and bioleaching techniques are applied most often in biomining of gold.
Refractory gold ores
Rich gold ores containing minerals such as Calaverite (AuTe2), sylvanite ((Ag.Au)Te2), and petzite (Ag3AuTe2) can be easily leached via cyanidation, resulting in high recovery of gold. However, some kinds of gold ores, called refractory gold ores, are not susceptible to conventional cyanidation alone. This could be due to one or both of the following reasons :
(a) The gold is present as small particles fully encapsulated, or ‘locked’, within other minerals, usually sulphides such as arsenopyrites (FeAsS) and pyrites (FeS2). The physical and chemical concept of gold locked in gangue minerals are illustrated in figure 1A and 1B respectively below.
(b) Despite not physically locking the gold, some minerals such as sulphides and carbonaceous material, when present in the same ore, can chemically interfere with the dissolution of gold by cyanide.
Figure 1: Schematic diagram showing the concepts of: [A] gold physically locked in other minerals; [B] gold chemically locked in pyrite interstitial lattice site .
Bioprocessing of Refractory Gold Ores
Bioprocessing of gold integrates both biooxidation and bioleaching processes. Bioleaching in this case can be more specifically referred to as biocyanidation. The general process flow sheet for the bioprocessing of refractory gold ores is illustrated in figure 3 below.
(A) Biooxidation of gold:
Now that we have seen how refractory gold ores could be quite challenging in terms of gold yields and productivity, I am sure you are curious to know how this challenge is overcome in the mining industry. Various pretreatment methods such as pressure oxidation and roasting have been used to liberate refractory gold. However, these methods are capital intensive and require high operational costs. Biooxidation is another pretreatment method that is widely applied because it is less polluting of the environment and less demanding of energy and capital. Biooxidation involves the use of iron- and/or sulphur-oxidizing acidophilic chemolithotrophic bacteria such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Sulfobacillus thermosulfidooxidans and Leptospirillum ferrooxidans, to oxidize or break down the sulphide matrix surrounding the gold, resulting in improved accessibility of gold to cyanidation. During this biological pretreatment stage, gold is not leached, rather, the sulphide minerals are oxidised to soluble metal sulfates and sulfuric acid, while the gold becomes more susceptible to dissolution in cyanidation .
The major mechanisms of these acidophilic bacteria in biooxidation are the oxidation of Fe2+ to Fe3+ and the oxidation of reduced inorganic sulphur compounds to sulphuric acid. Then the generated Fe3+ and sulphuric acid cause oxidative and proton attacks respectively on the ore, resulting in the leaching out of other metal contaminants from the refractory gold ore. These chemolithotrophs obtain their energy by using reduced sulphur compounds and/or ferrous iron as electron donor and oxygen as electron acceptor . One of the challenges of biooxidation is the generation of intermediate sulfide species that could be cyanide-consuming and gold-passivating. In addition, the resulting product from biooxidation is highly acidic (pH ≈ 1.8) and requires adjustment by a significant amount of alkali in preparation for cyanidation. Biooxidation is also not able to prevent preg-robbing during cyanidation, which is caused by the carbonaceous matter present in the ore. This is because these acidophilic bacteria get their carbon requirements from fixing atmospheric CO2 (autotrophic mode of nutrition), thereby not metabolizing the inherent carbonaceous matter in the ore [3, 6]. Biooxidation of gold ores has been successfully applied commercially in various mines around the world such as the Fairview mine in Barberton, South Africa and the Ashanti Goldfields, Sansu, Ghana (figure 2 below); using the commercial BIOX® plant (a stirred tank bioreactor).
Figure 2: BIOX® plant (960 t/d, 24 X 1000m3 stirred tank bioreactor) used for the treatment of refractory gold ores in Ashanti Goldfields, Ghana . These are the largest tanks in use for biotechnology.
(B) Bioleaching/biocyanidation of Gold:
The pretreated gold ore from biooxidation is subsequently treated or leached by cyanidation. Cyanidation is the process of treating or dissolving the ore in cyanide to extract gold (or silver). However, cyanidation is a hazardous process for both the environment and the mine workers. Consequently, Biotechnology offers a safer alternative called biocyanidation and has received considerable attention by the scientific community and the mining industry. Biocyanidation is the extraction of gold (and/or) or metals by treating ores with biologically produced cyanide (biogenic cyanide).
I will now introduce you to my favorite group of bacteria called cyanogenic bacteria. Cyanogenic bacteria are bacteria that can produce cyanide as a secondary metabolite during their early stationary phase of growth. These bacteria form hydrogen cyanide (HCN) directly from glycine using the HCN synthase enzyme, which stoichiometrically forms HCN and CO2 via the oxidative decarboxylation of glycine as shown in equation 1 below . HCN is mainly formed in alkaline and aerobic conditions. The most commonly used cyanogenic bacteria are Chromobacterium violaceum, Pseudomonas fluorescens and Bacillus megaterium.
Biocyanidation is an indirect process whereby cyanide produced as a metabolite by cyanogenic bacteria dissolves gold from minerals by forming soluble metallic complexes as shown in the Elsner’s equation below . Other microbial products such as amino acids (e.g Alanine, Aspartic acid and serine) have also been found to form soluble gold complexes in alkaline media .
Figure 3: The overall process flow chart of bioprocessing (biooxidation and biocyanidation) of gold-bearing refractory ore .
Biomining is a well-established technology for the extraction of base and precious metals and is envisaged to become more relevant in the mining industry in the future. This is due to its competitive cost-effectiveness for the processing of low grade ores and refractory gold ores, which are the ores of the future, as most of the accessible rich ore bodies, have already been depleted. Low grade ores also include mining wastes such as the waste rocks and tailings deposits being reprocessed in the SULTAN project. Biomining seems to be the most promising, economically viable method for the extraction of metals from these low grade ores and is therefore being studied in the SULTAN project. Advantages of biomining as compared to other mining technologies (such as pyrometallurgy) include low capital investment, ecofriendliness (low carbon foot prints), low energy and atmospheric pressure requirement. The major drawback of biomining is the relatively slow time it takes to mobilize metals from ores.
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