**4.1 Bioleaching of uranium**

Recent study shows that elements like uranium, copper, gold, zinc and other elements are commercial focus of bioleaching and biooxidation [34]. Many studies indicate microbial leaching is more important in low-grade ore, ore sample collected from Mianhuakeng uranium mine located in northern Guangdong province in China, leached by heap, by mixed microorganism of Acidithiobacillus Ferrooxidans and Leptospirillum Ferriphilum with 88.3% leaching efficiency [35]. Uranium leaching takes places by indirect mechanism, as Acidithiobacillus Ferrooxidans does not directly interact with uranium minerals. The role of Acidithiobacillus Ferrooxidans in uranium leaching is the best example of the indirect mechanism. Bacterial activity is limited to oxidation of pyrite and ferrous iron. The process involves periodic spraying or flooding of worked-out stops and tunnels of underground mines with

*Bio Hydrometallurgical Technology, Application and Process Enhancement DOI: http://dx.doi.org/10.5772/intechopen.94206*

lixiviant [4]. The pH of lixiviant was optimized during the bioleaching of uranium from low grade Indian silicate-apatite ore with 0.024% of U3O8. This study uses Acidithiobacillus Ferrooxidans for leaching and biochemically generated ferric ions as an oxidant, optimizing particle, pulp density and redox potential results 98% uranium bioleaching. In this indirect bioleaching of uranium, the bacteria generate ferric sulfate and pyrite is oxidized by a lixiviant, within acidic environment the oxidations of ferrous ion to ferric ions process executed by the bacteria is fasters than chemical oxidation [36]. In case of uranium bioleaching the main drawback is to oxidize uranium (IV) since it insoluble but on this bioleaching process when ferrous sulfate produced in the process, then re-oxidized to ferric sulfate which enzymatically oxidize uranium (IV) to uranium (VI) by the energy produced by this reaction. A case study in India at Jaduguda mines proofs that use of biogenic ferric sulfate produced by the strain which was then used for efficient uranium extraction and cause no harm to the environment, while extracting uranium, use of reduced MnO2 in Bacfox process to generate biogenic ferric sulfate, results passed air saturated ferrous sulfate solution over Acidithiobacillus Ferrooxidans which is absorbed on solid surface [36]. Since the permeability of the ore surface is a factor, the above study uses a process called "rubblizing" that increase fragmenting of ore in place which can be applied in the extraction of sulfide mineral, gold and uranium. While isolating the bacteria from mine water, the isolation media and H2SO4 consumption during isolation, pH variation and temperature were determinate factors, the microbial cell count and the growth of (A. Ferrooxidans) determines by rate of oxidation of iron from Fe2+ to Fe3+, so while leaching if the amount of Fe2+ decrease means the bacteria is using it as energy source to convert it to Fe3+, uranium bioleaching depends on the synergic effects Fe3+ and proton produced by the bacterial [37] that process uses either of the two energy sources to growth iron or sulfur. The reaction of making insoluble uranium to soluble form is as follows [38].

$$\text{UO}\_2 + \text{Fe}\_2(\text{SO}\_4)\_3 \to \text{UO}\_2\text{SO}\_4 + 2\text{FeSO}\_4.\tag{19}$$

Studies indicate that microbial cell count and pulp density ranges 5–30% (w/v), particle size <75 μm has brought an optimum ore leaching but it should be clear that each ore has its own distinct behavior and no size fits all, meaning results indicated here might be different for another ore sample due to ore elemental composition, crystal structure, grade, topography and surface tension.

#### **4.2 Bioleaching of copper**

The ore is loaded on a water-resistant surface or ore is piled on an impermeable surface until a dump of suitable dimension forms. After leveling the top, then spraying a leach solution onto the dump is followed [4]. These dump is a habitat of heterogeneous microorganism. Dump can have variety particles sizes, where the bacterial annexation, which is anaerobic (microaerophilic), thermophilic begins from the top.

Dump leaching used to pretreat low-grade, refractory- sulfidic gold ores and to leach copper from chalcocite ores while ore grade is low with values ranges between 0.1–0.5%. Copper can be obtained from ore rocks from the mound then washed with dilute H2SO4 to facilitate the oxidation process of mineral by acidophiles, which is followed by cementation process where copper is precipitated from the drainage with scrap iron since it primary iron oxidizing process [39]. Check the leaching process of copper sulfide chalcocite (Cu2S), which occurs with pyrite (FeS2), leaching is due to ferric ion reacts with copper sulfide mineral processes ferrous and copper ions in solution.

$$\text{Cu}\_2\text{S} + 4\text{Fe}^{3+} \rightarrow 4\text{F}^{2+} + 2\text{Cu}^{2+} + \text{S.} \tag{20}$$

$$\text{FeS}\_2 + \text{8H}\_2\text{O} + \text{14Fe}^{3+} \rightarrow \text{15Fe}^{2+} + 2\text{SO}\_4^{2-} + \text{16H}^\*. \tag{21}$$

In these regions indirect leaching by ferric sulphate also prevails. The exterior of the dump is at ambient temperature and undergoes changes in temperature reflecting seasonal and diurnal fluctuations. Many different microorganisms have been isolated from copper dumps, some of which have been studied in the laboratory. These include a variety of mesophilic, aerobic iron and sulfur oxidizing microorganisms; thermophilic iron and sulfur oxidizing microorganisms; and anaerobic sulphate reducing bacteria. In copper leaching the concentration of target metal by itself is an important variable, copper concentration (100–300 mM range) is values cause difficulty for the microorganism to operate, selecting the microorganism is one of the mechanisms of copper resistant, Acidithiobacillus Ferrooxidans can resist copper concentration and strong acidic environment [40]. Thiobacillus Ferrooxidans was the main product observed after a culture study, from an ore or leach solution for the identification of composition of bacterial population and incase of low ferrous ions, it was Leptospirillum Ferrooxidan was observed, the study shows that utilization of ferrous iron as energy source is dominated by the previous bacteria as the culture shows. *Pseudomonas aeruginosa*, where heterotrophic bacteria produce various organic acids in an appropriate culture medium is used in copper leaching [41]. The addition of salt in bioleaching of copper resulted process enhancement, after designing the bioreactor the bioleaching of copper was enhanced in both stirred tank or shack flask by adding sodium chloride in leach solution, increasing the dissolution of Fe3+ that eventually reduces precipitation [29] addition of some elements might result inhibition of bioleaching process, fluorine in solution increase the viscosity of leach liquor that result inhibition of bioleaching [42]. It is important to understand the microbiology, which is responsible or identify a means to study bulk activity of microorganism, these features are oxygen uptake in solid and liquid samples, redox potential, pH, ferrous iron concentration and temperature. Microbial leaching has also direct relation with enrichment and culture from solution of ores. Acidithiobacillus Thiooxidans, Acidithiobacillus Ferrooxidans, and Leptospirillum Ferrooxidans have been cultured where the process run at an ambient temperature and the strain of bacterial related to the microorganism mentioned here [27, 43]. Leach solutions enriched with copper exit at the base of the dump and are conveyed to a central recovery facility. In most large-scale operations the leach solution, copper-bearing solution pumped into large cementation units containing iron scrapings for cementation and then electrolysis followed [4]. It was in Chile and Australia the commercial bio heap leaching of copper started mass production. And the first bioleach heap copper extraction plant is in China [44]. The copper extracted percentage can be calculated as,

E = Copper content in the solution/copper content in the sample X 100% [41].

#### **4.3 Bioleaching of gold**

Acidophilic bacteria are able to oxidize gold containing sulphidic ore, such a process can be ameliorated by conventional process of cyanidation, these basically reduces the complexation by increasing the capability of microorganisms to reach to the target metal. Certain sulphidic ores containing encapsulated particles of elemental gold, resulting in improved accessibility of gold to complexation by leaching agents such as cyanide. Relative to other conventional process and pretreatments like roasting, smelting and pressure oxidation, bio-oxidation demands

#### *Bio Hydrometallurgical Technology, Application and Process Enhancement DOI: http://dx.doi.org/10.5772/intechopen.94206*

less cost and no harm to nature [7]. Though it is under study a commercial biooxidation and bio heap leaching of gold prior cyanide extraction. It is the bacteria, Acidithiobacillus Ferrooxidans used to oxidize the sulphide matrix for gold recovery. Prior to extraction, gold ore must be bio-oxidize by the bacteria. In this process refractory sulphidic gold ores contain mainly two types of sulphides: pyrite and arsenopyrite where silver ion was used as a catalyst in acidic environment. Since gold is usually finely disseminated in the sulphide matrix, the objective of biooxidation of refractory gold ores is to break the sulphide matrix by dissolution of pyrite and arsenopyrite and extract 95% of iron and arsenic, the residue of both filtered through a vacuum pump. The consumption of cynide is much higher while biooxidation, the study suggested that using thiourea instead of cyanide is much less toxic but since the process require high consumption of thiourea cost increase steadily, consumption of thiourea reduced by using different agents like SO2, bisulfite, cystine, cystine with oxygen during extraction process [45].
