For centuries people have been using microbes to their advantage, turning grapes into wine, milk into cheese, and cabbage into sauerkraut. People benefit from what microbes do naturally: They eat and digest organic compounds, changing the chemical makeup of one product and turning it into a completely different yet tasty food or drink. Now microbes, in form of biomining, are providing efficient helping hand for extraction of heavy metals from sub-graded ores and minerals (Fig. 14.7).

Biomining is the interaction between metals and microbes with the specific aim of converting insoluble metal sulfides to soluble metal sulfates. Bioleaching has been defined as the dissolution of metals from their mineral sources by certain naturally occurring microorganisms or the use of microorganisms to transform elements so that the elements can be extracted from a material when water is filtered through it. So, it is the application of microbial process in the mining industry for economic recovery on a large scale [7] (Fig. 14.8).

Fig. 14.7 Biomining

In short, biomining is a term that describes the processing of metal containing ores and concentrates of metal containing ores using microbiological technology. It is often called bioleaching.

By convention bioleaching has been divided into two approaches:

• Direct bioleaching

• Indirect bioleaching

Direct bioleaching entails an enzymatic attack by the bacteria on components of the mineral that are susceptible to oxidation. In the process of obtaining energy from the inorganic material the bacteria cause electrons to be transferred from iron or sulfur to oxygen. In many cases the more oxidized product is more soluble. It should be noted that the inorganic ions never enter the bacterial cell; the electrons released by the oxidation reaction are transported through a protein system in the cell membrane and then (in aerobic organisms) to oxygen atoms, forming water. The transferred electrons give up energy, which is coupled to the formation of adenosine triphosphate (ATP), the energy currency of the cell.

Indirect bioleaching, in contrast, does not proceed through a frontal attack by the bacteria on the atomic structure of the mineral. Instead the bacteria generate ferric iron by oxidizing soluble, ferrous iron; ferric iron in turn is a powerful oxidizing agent that reacts with other metals, transforming them into the soluble oxidized form in a sulfuric acid solution. In this reaction ferrous iron is again produced and is rapidly reoxidized by the bacteria. Indirect bioleaching is usually referred to as bacterially assisted leaching. In an acidic solution without the bacteria, ferrous iron is stable and leaching mediated by ferric iron would be slow. T ferrooxidans can accelerate such an oxidation reaction by a factor of more than a million.

Biomining is applied using four different engineered methods:

• Dump bioleaching

• Heap bioleaching

• Heap minerals biooxidation

• Stirred-tank bioleaching

• Minerals biooxidation

Dump bioleaching extracts copper from sulfide ores that are too low grade to process by any other method. This process has been used since the mid-1950s.

Heap bioleaching, which has been used since the 1980s, extracts copper from crushed sulfide minerals placed on engineered pads.

Heap minerals biooxidation pretreats gold ores in which the gold particles are locked in sulfide minerals, significantly enhancing gold recovery.

Stirred-tank bioleaching extracts base metals from concentrates of metal containing sulfide ores.

Stirred-tank minerals biooxidation enhances gold recovery from mineral concentrates in which the gold is locked in sulfide minerals [8].

Advantages

The advantages of biomining process over chemical leaching are:

(i) Biomining is a way to exploit low grade ores and mineral resources located in remote areas that would otherwise be too expensive to mine.

(ii) It is more environmentally friendly than the conventional (smelting) method, since it uses less energy and does not produce SO2 emissions. This also translates into profit, as the companies have to spend huge sums finding ways of limiting their SO2 emissions.

(iii) Less landscape damage occurs, since the bacteria grow naturally. Native bacteria can operate over a wide temperature range between 20 and 55°C. Other materials for the process are also natural such as air and water.

(iv) The bacteria breed on their own, i. e. they are self-sustaining. Since there is no need to pay for heating and chemicals required in a conventional operation, companies may be able to reduce the price of metal production by nearly a half.

(v) It is a less energy intensive process.

(vi) It is simpler and therefore cheaper to operate and maintain, as no technical specialist is needed to operate complex chemical plants [9].

(vii) Even the dumps left behind after traditional mining processes can be reprocessed to extract residual metal [10].

So, biomining is the process of extracting valuable metals from ores and mine tailings with the assistance of microorganisms. It is a green technology that can help mine valuable metals with minimal impact on the environment. It requires low energy, causes low gaseous emission and is not labor intensive.