151
vapour pressures existing among various metals to improve the selective recovery of
desired metals (Townsend 2011). Furthermore, Zhang and Xu (2016) reported an
enhanced metal extraction when pyrometallurgical technology was combined with
some mild extracting reagent such as ammonia or chloride.
Recently, investigators have focused on energy recovery from E-waste to compensate the high energy demand. This is achieved through pyrolysis of plastic in the
presence of suitable catalysts to produce oil with high calorific value when compared to commercial fuel (Sharuddin et al. 2016). Also, plastic components of the
E-waste under the influence of catalyst have been shown to form aromatic oil
(gasoline) when pyrolyzed (Muhammad et al. 2015). For instance, the plastics from
equipment containing cathode ray tubes (CRTs) and also plastic waste from refrigeration equipment have been successfully converted to derivatives of aromatic
hydrocarbons. Addition of the Y zeolite and zeolite ZSM-5 to the pyrolysis process
resulted in a reduced concentration of styrene, but appreciable concentrations of
benzene and its derivatives (toluene and ethylbenzene) were found in the product oil
(Muhammad et al. 2015). Using microwave-aided pyrolysis on the plastic fraction
of E-waste produced dense and viscous liquid fractions with a high concentration of
useful chemicals such as xylenes and styrenes (Rosi et al. 2018).
Though pyrolysis oil could be obtained from the application of pyrolysis technology on WEEE-based plastics, the presence of the brominated flame retardants
(BFRs) makes the process problematic (Wang and Xu 2014). Technology that is
gasification-based and supercritical fluids methods have been suggested to achieve
effective recycling with minimal impact on the environment when compared with a
process such as heating at very high temperature. Other components such as glass
from the cathode ray tube or liquid display glass can be reutilized sometimes to
produce some precious metals such as tin and indium. Nonetheless, before any
option can be used for the recycling at a commercial scale, it is very important to
conduct an environmental impact assessment to ascertain the effect in the long run
(Wang and Xu 2014).
8.1.3.1.2 Hydrometallurgy for Selective Metal Recovery
Compared with pyrometallurgy, hydrometallurgical methods have become desirable treatments for E-waste towards metal extraction/recovery due to reduced gas
emission from the latter than the former. Other merits of hydrometallurgy include
cost-effectiveness and ease of operation especially under laboratory settings
(Chauhan et al. 2015). At a smaller scale, they may present a better control of the
processes, thereby ensuring higher efficiency of metal recovery (Chauhan et al.
2018). In this method, alkaline or acidic leaching agents are used to wash the
E-waste in order to dissolve and recover the metal of interest. This is accompanied
by different forms of physical-chemical methods to finalize metal extraction (Soare
et al. 2016). Chauhan et al. (2018) have shown that solvents especially halides, cyanides, thiourea and thiosulfates could be used for the leaching metals from ores.
8 E-Waste Management from Macroscopic to Microscopic Scale
vapour pressures existing among various metals to improve the selective recovery of
desired metals (Townsend 2011). Furthermore, Zhang and Xu (2016) reported an
enhanced metal extraction when pyrometallurgical technology was combined with
some mild extracting reagent such as ammonia or chloride.
Recently, investigators have focused on energy recovery from E-waste to compensate the high energy demand. This is achieved through pyrolysis of plastic in the
presence of suitable catalysts to produce oil with high calorific value when compared to commercial fuel (Sharuddin et al. 2016). Also, plastic components of the
E-waste under the influence of catalyst have been shown to form aromatic oil
(gasoline) when pyrolyzed (Muhammad et al. 2015). For instance, the plastics from
equipment containing cathode ray tubes (CRTs) and also plastic waste from refrigeration equipment have been successfully converted to derivatives of aromatic
hydrocarbons. Addition of the Y zeolite and zeolite ZSM-5 to the pyrolysis process
resulted in a reduced concentration of styrene, but appreciable concentrations of
benzene and its derivatives (toluene and ethylbenzene) were found in the product oil
(Muhammad et al. 2015). Using microwave-aided pyrolysis on the plastic fraction
of E-waste produced dense and viscous liquid fractions with a high concentration of
useful chemicals such as xylenes and styrenes (Rosi et al. 2018).
Though pyrolysis oil could be obtained from the application of pyrolysis technology on WEEE-based plastics, the presence of the brominated flame retardants
(BFRs) makes the process problematic (Wang and Xu 2014). Technology that is
gasification-based and supercritical fluids methods have been suggested to achieve
effective recycling with minimal impact on the environment when compared with a
process such as heating at very high temperature. Other components such as glass
from the cathode ray tube or liquid display glass can be reutilized sometimes to
produce some precious metals such as tin and indium. Nonetheless, before any
option can be used for the recycling at a commercial scale, it is very important to
conduct an environmental impact assessment to ascertain the effect in the long run
(Wang and Xu 2014).
8.1.3.1.2 Hydrometallurgy for Selective Metal Recovery
Compared with pyrometallurgy, hydrometallurgical methods have become desirable treatments for E-waste towards metal extraction/recovery due to reduced gas
emission from the latter than the former. Other merits of hydrometallurgy include
cost-effectiveness and ease of operation especially under laboratory settings
(Chauhan et al. 2015). At a smaller scale, they may present a better control of the
processes, thereby ensuring higher efficiency of metal recovery (Chauhan et al.
2018). In this method, alkaline or acidic leaching agents are used to wash the
E-waste in order to dissolve and recover the metal of interest. This is accompanied
by different forms of physical-chemical methods to finalize metal extraction (Soare
et al. 2016). Chauhan et al. (2018) have shown that solvents especially halides, cyanides, thiourea and thiosulfates could be used for the leaching metals from ores.
8 E-Waste Management from Macroscopic to Microscopic Scale