144
5 Recovery of Metals from Electronic Waste
not recyclable in a long term. These drawbacks of solvent extraction methods have
led to increased interest in solid-phase extraction, as described below [12, 25].
5.8.3.2 Solid-Phase Extraction
Solid-phase extraction involves separating a desired cation or anion from a complex
sample matrix by concentrating/adsorption onto a surface of a sorbent. It is more
efficient than liquid–liquid extraction and reduces the solvent used and operating
time. It consists of a solid surface and a complexing agent on the solid surface to
adsorb the selected compounds from a solution. When the solution is passed through
the solid surface, some of the compounds are retrained on the solid surface and are
removed from the solution. The selectivity towards a specific metal ion depends on
the characteristics of the complexing agent. The most commonly used solid-phase
extractant is Amberlite IRA 400, SuperLig or AnaLig materials [12]. The SuperLig
products are used in Molecular Recognition Technology (MRT), which is a kind of
solid-phase extraction. The MRT methods are highly selective, non-ion exchange
and environmentally friendly processes for the separation of selective metals. The
SuperLig materials can bind selectively with ions and absorb specific precious metals.
As an example, SuperLig 2, SuperLig 133 and SuperLig 190 are selective for the
recovery of Pd
2+ , Pt and Rh, from other precious metals, respectively [12].
Despite the advantages of solid-phase extraction compared to the liquid–liquid
extraction method, this method is expensive due to costly solid-phase extractants. It
is also hard to find an extractant with high adsorption capacity, excellent selectivity
and recyclability, all at the same time [12].
5.8.3.3 Electrodeposition
Electrodeposition involves the deposition of a thin coating of metal oxide or salt on
the surface of a conductor substrate through electrolysis of a solution that contains the
metal ion or complex. This process has been extensively used for refining base metals
and has been extended to recover precious metals from e-waste. Metal ions from the
leaching solution are deposited at the cathode and are reduced by the following
reaction [27]:
M
n+
(aq) + ne
−
→ M(s)
(5.8)
where M is the metal and n is the valence of the metal. The anode is inert in the
electrolyte with the following reaction [27]:
4OH
−
(aq) → O 2 (g) + 2H 2 O + 4e
−
(5.9)
Metals such as lead, tin and copper can be recovered by electrodeposition after
leaching. In order to achieve a high-efficiency recovery of lead and copper, the
5 Recovery of Metals from Electronic Waste
not recyclable in a long term. These drawbacks of solvent extraction methods have
led to increased interest in solid-phase extraction, as described below [12, 25].
5.8.3.2 Solid-Phase Extraction
Solid-phase extraction involves separating a desired cation or anion from a complex
sample matrix by concentrating/adsorption onto a surface of a sorbent. It is more
efficient than liquid–liquid extraction and reduces the solvent used and operating
time. It consists of a solid surface and a complexing agent on the solid surface to
adsorb the selected compounds from a solution. When the solution is passed through
the solid surface, some of the compounds are retrained on the solid surface and are
removed from the solution. The selectivity towards a specific metal ion depends on
the characteristics of the complexing agent. The most commonly used solid-phase
extractant is Amberlite IRA 400, SuperLig or AnaLig materials [12]. The SuperLig
products are used in Molecular Recognition Technology (MRT), which is a kind of
solid-phase extraction. The MRT methods are highly selective, non-ion exchange
and environmentally friendly processes for the separation of selective metals. The
SuperLig materials can bind selectively with ions and absorb specific precious metals.
As an example, SuperLig 2, SuperLig 133 and SuperLig 190 are selective for the
recovery of Pd
2+ , Pt and Rh, from other precious metals, respectively [12].
Despite the advantages of solid-phase extraction compared to the liquid–liquid
extraction method, this method is expensive due to costly solid-phase extractants. It
is also hard to find an extractant with high adsorption capacity, excellent selectivity
and recyclability, all at the same time [12].
5.8.3.3 Electrodeposition
Electrodeposition involves the deposition of a thin coating of metal oxide or salt on
the surface of a conductor substrate through electrolysis of a solution that contains the
metal ion or complex. This process has been extensively used for refining base metals
and has been extended to recover precious metals from e-waste. Metal ions from the
leaching solution are deposited at the cathode and are reduced by the following
reaction [27]:
M
n+
(aq) + ne
−
→ M(s)
(5.8)
where M is the metal and n is the valence of the metal. The anode is inert in the
electrolyte with the following reaction [27]:
4OH
−
(aq) → O 2 (g) + 2H 2 O + 4e
−
(5.9)
Metals such as lead, tin and copper can be recovered by electrodeposition after
leaching. In order to achieve a high-efficiency recovery of lead and copper, the