148
5 Recovery of Metals from Electronic Waste
5.8.4 Biometallurgical Processing
The pyrometallurgical and hydrometallurgical methods are conventional routes for
the recovery of metals from e-waste. However, these methods have the drawbacks of
difficult control, secondary waste production and high cost and environmental/health
risks. Specifically, the pyrometallurgical methods require high energy demand and
produce atmospheric pollutants such as dioxins and furans due to the presence of
some flame retardants in electronic equipment. In addition, they have high capital
and operating costs, low selectivity and harsh thermal treatment conditions. The
hydrometallurgical methods have the drawbacks of being slow and hazardous, with
negative environmental impacts and high operating costs. They also require pretreatment of e-waste, which leads to the necessity of treatment and management of
large amounts of acidic wastewater effluent [19].
Biometallurgical processes are emerging and very promising technologies.
Compared to the conventional processes, the biometallurgical methods have a simpler
operation, higher efficiency, lower operating costs and energy consumption, less
environmental impacts, milder operating conditions (atmospheric pressure and room
temperature) and easier management. They are based on the concept of hydrometallurgical processes differing in using microorganisms for the production of reagents to
recover metals. The microorganisms interact with and depend on metals to perform
their cellular functions through sorption, reduction, oxidation and sulfide precipitation reactions [11, 19]. There are numerous R&D opportunities in this area. Currently,
research is focused on the recovery of copper, nickel, cobalt, zinc, gold and silver.
However, complete recovery of gold and silver has not been achieved so far [11, 20].
The biometallurgical methods are suitable for recovering metals from low-grade
ores, mine tailings and contaminated soils or e-waste. However, due to the presence
of microorganisms, they have some disadvantages that limit their industrial applications, as follows [33]: (1) Rigorous conditions are required for the bio-leaching
processes and the microorganisms are sensitive to the changes in the environmental
conditions of the culture media; (2) Just like the hydrometallurgical methods, the
recovery procedures for the biometallurgical methods such as microbial cultivation,
bio-leaching, solvent extraction, separation and precipitation, are usually very long
and slow processes; (3) Recovery of metals from the leaching solution is a difficult
process, as some of the microorganisms produce organic acids as leaching agents,
leading to a difficult separation procedure.
Two main biometallurgical methods for the recovery of metals are bioleaching and
biosorption, to be detailed in the following sections. Bioleaching involves the mobilization of metal cations from insoluble materials through biological oxidation and
complexation reactions. This method has been traditionally used for the extraction
of gold and copper from ores and has been successfully applied for the recovery of
other metals such as Co, Mo, Ni, Pb and Zn from sulfidic ores. Biosorption is defined
as the adsorption of metals using biomass-based adsorbents such as algae, bacteria,
yeasts and fungi. The process involves a complex physical or chemical interaction
between the charged surface of microorganisms and ions in a solution [11, 17, 20].
5 Recovery of Metals from Electronic Waste
5.8.4 Biometallurgical Processing
The pyrometallurgical and hydrometallurgical methods are conventional routes for
the recovery of metals from e-waste. However, these methods have the drawbacks of
difficult control, secondary waste production and high cost and environmental/health
risks. Specifically, the pyrometallurgical methods require high energy demand and
produce atmospheric pollutants such as dioxins and furans due to the presence of
some flame retardants in electronic equipment. In addition, they have high capital
and operating costs, low selectivity and harsh thermal treatment conditions. The
hydrometallurgical methods have the drawbacks of being slow and hazardous, with
negative environmental impacts and high operating costs. They also require pretreatment of e-waste, which leads to the necessity of treatment and management of
large amounts of acidic wastewater effluent [19].
Biometallurgical processes are emerging and very promising technologies.
Compared to the conventional processes, the biometallurgical methods have a simpler
operation, higher efficiency, lower operating costs and energy consumption, less
environmental impacts, milder operating conditions (atmospheric pressure and room
temperature) and easier management. They are based on the concept of hydrometallurgical processes differing in using microorganisms for the production of reagents to
recover metals. The microorganisms interact with and depend on metals to perform
their cellular functions through sorption, reduction, oxidation and sulfide precipitation reactions [11, 19]. There are numerous R&D opportunities in this area. Currently,
research is focused on the recovery of copper, nickel, cobalt, zinc, gold and silver.
However, complete recovery of gold and silver has not been achieved so far [11, 20].
The biometallurgical methods are suitable for recovering metals from low-grade
ores, mine tailings and contaminated soils or e-waste. However, due to the presence
of microorganisms, they have some disadvantages that limit their industrial applications, as follows [33]: (1) Rigorous conditions are required for the bio-leaching
processes and the microorganisms are sensitive to the changes in the environmental
conditions of the culture media; (2) Just like the hydrometallurgical methods, the
recovery procedures for the biometallurgical methods such as microbial cultivation,
bio-leaching, solvent extraction, separation and precipitation, are usually very long
and slow processes; (3) Recovery of metals from the leaching solution is a difficult
process, as some of the microorganisms produce organic acids as leaching agents,
leading to a difficult separation procedure.
Two main biometallurgical methods for the recovery of metals are bioleaching and
biosorption, to be detailed in the following sections. Bioleaching involves the mobilization of metal cations from insoluble materials through biological oxidation and
complexation reactions. This method has been traditionally used for the extraction
of gold and copper from ores and has been successfully applied for the recovery of
other metals such as Co, Mo, Ni, Pb and Zn from sulfidic ores. Biosorption is defined
as the adsorption of metals using biomass-based adsorbents such as algae, bacteria,
yeasts and fungi. The process involves a complex physical or chemical interaction
between the charged surface of microorganisms and ions in a solution [11, 17, 20].