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

adequate supply of oxygen needed for their growth. At laboratory scale, oxygen is

used for aeration. On larger scales, supply of oxygen is tough for stack leaching [11].

Efficiency, metal extraction, and count of organisms are also affected by carbon diox-

ide, and it is used as energy source. It also affects leaching process and ferrous ion

oxidation.

14.2.4

Advantages of Bioleaching Over Other Methods

Bioleaching is considered to be environment friendly compared to other conven-

tional processes. Due to its flexibility, waste toxicity is reduced and also valuable

resources can be recovered. Pyrometallurgy and hydrometallurgy were applied for

the extraction of metals from minerals or secondary sources other than microbial

leaching, and both processes are found to be efficient but still have limitations

[12]. One drawback with hydrometallurgy method is that it involves usage of

concentrated acids or bases, and thus, the acid waste generated has to be managed

at higher downstream processing cost [11]. Similarly, due to usage of high energy

in pyrometallurgy process, operating at a 1500–1700 ^∘C is considered as inefficient

and also is related to the emission of harmful gases like sulfur dioxide [13]. Both

the approaches fail because of economics, account of energy, and environment,

whereas in bioleaching, harmful gases are not emitted and are energy efficient.

14.2.5

Limitation of Bioleaching Over Other Methods

While bioleaching approaches have a lot of advantages, a number of drawbacks are

also encountered that can be solved by applying appropriate strategies. Limitation

of this process includes – (i) slow reaction rate compared to other methodology,

(ii) microbes involved in process may be contaminated with toxic metals from the

waste, and (iii) commercially high cost and low yield due to waste refractory proper-

ties [14]. A low metal yield is obtained as the iron hydroxides and jarosite are formed

on ore surface which causes the oxidants to diffuse slowly to ore surface [15]. There-

fore, if bioleaching technique is not strategically planned the process construction,

material grinding cost and operating cost may make it uneconomical.

14.3

E-Waste, What Are They?

Electronic waste is also known as e-waste and refers to discarded electronic devices

such as mobile phones, computers, printers, televisions, cell phones, printers, CD

players, personal digital assistants (PDAs), fax machines, and many other electronic

devices commonly used in homes, offices, institutions, etc. Technically, e-waste

refers to discarded electronic products having primary functions with various

components and circuits which are discarded when they reach end of their life

due to loss of functions. E-waste is completely different from the other domestic or

industrial waste as they contain precious metals and other compounds such as lead,

cadmium, arsenic, mercury, polyvinyl chloride (PVC), etc. which when discarded