5.8 Metallurgical Methods for Metals Recovery
143
5.8.2.6 Electrochemical Methods
Compared to the leaching methods, the electrochemical processes have higher energy
efficiency, less environmental impact and less chemical usage. Oxidizing agents can
be used to dissolve the metals at the anode and then reduce the dissolved metals at
the cathode [12]. Electrochemical recovery can be divided into electrodeposition,
electrodialysis and electrosorption. The electrodeposition method uses an electric
current to reduce the dissolved metal ions and produce a coherent metal coating at
the electrode. The most used anode materials in this process include Pb–Ag alloys
and DSA-type Ti/IrO 2 –Ta 2 O 5 and the most employed cathode materials are stainless
steel, copper, or titanium plates. Electrosorption or Capacitive Deionization (CDI) is
a method in which charged metal ions are forced to transport toward the electrodes
and an electric double layer is generated for further adsorption. Carbon materials,
metal oxides/sulfide, conductive polymers and related hybrid/derivative nanomaterials are commonly used as electrode materials in this method. Electrodialysis (ED)
is an electricity-driven membrane separation process in which aqueous ions selectively transport through an ion-exchange membrane and form concentrated and dilute
solutions in two compartments [26].
5.8.3 Recovery of Precious Metals
Depending on the composition of e-waste, precious metals can be recovered from
the leachate by using cementation, solvent extraction or liquid–liquid extraction,
adsorption on activated carbon, ion exchange, electrodeposition, etc. [20, 25], as
detailed below.
5.8.3.1 Solvent Extraction or Liquid–Liquid Extraction
Solvent extraction or liquid–liquid extraction involves the separation of a compound
or metal complex based on their relative solubilities in two different immiscible
liquids, usually water and an organic solvent. Different organic extracting solutions
such as cationic, anionic or solvating type can be used for the recovery of precious
metals, based on their dissolution characteristics [12, 25]. The most common anionic
extractants are amides and amines for the extraction of vanadium, gold, iridium,
rhodium and tungsten; the most used extractants are methyl-iso-butyl ketone for
gold, amide for iridium, diamine for platinum and palladium, and di-2-ethylhexyl
phosphoric acid (DEHPA) for zinc and cadmium, etc. Operating conditions such as
the extractant solution composition, pH, concentration, and the aqueous-to-organic
volume ratio, play an important role in determining the efficiency of the process [25].
Solvent extraction methods provide high selectivity and purity of the targeted
metal, excellent extraction efficiency and the ability to process large quantities of
solutions at a time. However, the solutions are often volatile, inflammable, toxic and
143
5.8.2.6 Electrochemical Methods
Compared to the leaching methods, the electrochemical processes have higher energy
efficiency, less environmental impact and less chemical usage. Oxidizing agents can
be used to dissolve the metals at the anode and then reduce the dissolved metals at
the cathode [12]. Electrochemical recovery can be divided into electrodeposition,
electrodialysis and electrosorption. The electrodeposition method uses an electric
current to reduce the dissolved metal ions and produce a coherent metal coating at
the electrode. The most used anode materials in this process include Pb–Ag alloys
and DSA-type Ti/IrO 2 –Ta 2 O 5 and the most employed cathode materials are stainless
steel, copper, or titanium plates. Electrosorption or Capacitive Deionization (CDI) is
a method in which charged metal ions are forced to transport toward the electrodes
and an electric double layer is generated for further adsorption. Carbon materials,
metal oxides/sulfide, conductive polymers and related hybrid/derivative nanomaterials are commonly used as electrode materials in this method. Electrodialysis (ED)
is an electricity-driven membrane separation process in which aqueous ions selectively transport through an ion-exchange membrane and form concentrated and dilute
solutions in two compartments [26].
5.8.3 Recovery of Precious Metals
Depending on the composition of e-waste, precious metals can be recovered from
the leachate by using cementation, solvent extraction or liquid–liquid extraction,
adsorption on activated carbon, ion exchange, electrodeposition, etc. [20, 25], as
detailed below.
5.8.3.1 Solvent Extraction or Liquid–Liquid Extraction
Solvent extraction or liquid–liquid extraction involves the separation of a compound
or metal complex based on their relative solubilities in two different immiscible
liquids, usually water and an organic solvent. Different organic extracting solutions
such as cationic, anionic or solvating type can be used for the recovery of precious
metals, based on their dissolution characteristics [12, 25]. The most common anionic
extractants are amides and amines for the extraction of vanadium, gold, iridium,
rhodium and tungsten; the most used extractants are methyl-iso-butyl ketone for
gold, amide for iridium, diamine for platinum and palladium, and di-2-ethylhexyl
phosphoric acid (DEHPA) for zinc and cadmium, etc. Operating conditions such as
the extractant solution composition, pH, concentration, and the aqueous-to-organic
volume ratio, play an important role in determining the efficiency of the process [25].
Solvent extraction methods provide high selectivity and purity of the targeted
metal, excellent extraction efficiency and the ability to process large quantities of
solutions at a time. However, the solutions are often volatile, inflammable, toxic and