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14 Recovery of Precious Metals from Electronic and Other Secondary Solid Waste

of bioleaching, research on the dissolution kinetics, metal leaching yields, and

processes to improve the efficiency of bioleaching process.

14.2

What Is Bioleaching?

There is a growing need to dispose e-waste generated from electronic products

when they function or life ceases. Recovery of metals from printed circuit board

(PCB) is presently done through processes, such as bio metallurgical, mechanical,

pyrometallurgical, and hydrometallurgical technologies [6]. There is growing

attention toward biohydrometallurgy for the extraction of metals. In this process,

microorganisms and their metabolites are found to play a vital role for solubilization

of metals into aqueous phase and subsequently their recovery. Microbial agents

involved in bioleaching are grouped into two types based on the requirement of

metabolism as chemolithotrophs and chemoorganotrophs. The mode of action

of these microbes in solutions is as follows: initially, microorganisms bring about

oxidation and reduction reactions in the solution. This is followed by formation

of acid from either organic or inorganic route and ends with the leaching of metal

from the sulfide matrix [7].

The process is carried out under acidic conditions (pH between 1.5 and 3). This

pH condition aids the growth of the microbes, thus improving the solubilization

and separation of metals. These technologies are easy to regulate and maintain as

they work under mild conditions at ambient temperature and pressure. Microbial

processes for the metal extraction from low grade ores were first discovered at

sulfide mines for field-scale processing and were referred to as bioleaching or

biomining. The first scientific proof of metal solubilization by microorganisms

was identified by Colmer and Hinkle in year 1947. In 1950s, copper was extracted

from mine dump using microorganisms for the first time at Kennecott Copper

Corporation [8]. Currently, at the industrial scale, bioleaching is applied to process

substantial portion of many minerals/ores. Advancement in the field of molecular

biology has helped scientists to recognize the various genera of microbes playing a

vibrant role in bioleaching.

14.2.1

Mechanism of Bioleaching

The three principles involved in the metabolization of metal on minerals are:

acidolysis, complexolysis, and redoxolysis. In Acidolysis, the metal solubilization

using microorganisms is carried out through the process of formation of organic or

inorganic acids like the production of gluconic acid by Penicillium simplicissimum,

citric acid by Aspergillus niger, and sulfuric acid by Acidithiobacillus ferrooxidans

and Acidithiobacillus thiooxidans. In these processes, protons and oxygen that

combine with water and metal get detached from the surface. This process is partic-

ularly important in fungal bioleaching and is usually fast [9]. Redoxolysis is divided