XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3263
Tao et al., 2023). Those attempts tend to focus on silver
and silicon recovery, with little attention paid to recovery of
lead, tin, and copper. It is important to recognize that lead
is a must-recover metal as it is toxic. If not removed from
the recycling sludge, it can poison the environment when
landfilled. With a chemical process, one must be mind-
ful of the quantity and hazard of the chemicals involved,
while maintaining high material recovery rates. Moreover,
it is desirable to recover pure metals instead of alloys or
compounds, as pure metals can be directly reused in new
modules, while alloys and compounds require further pro-
cessing before they become useful, resulting in additional
process cost and chemical waste.
In the reported recipes for chemical cell recycling,
nitric acid (HNO3) is the most common leaching agent for
the metals: silver, lead, tin, and copper. Hydrofluoric acid
(HF) is required to remove the silicon nitride layer. Nitric
acid is a broad-spectrum leaching agent dissolving all four
metals, so it requires selectivity in subsequent processes to
separate and recover pure metals from the nitric leachate.
In addition, tin precipitates in nitric acid as tin dioxide
(SnO2) which requires additional steps to obtain metallic
tin (Hwang et al., 2017). Hydrofluoric acid is nasty, but
there is no other effective method to etch off the silicon
nitride layer at room temperature. Hydrofluoric acid is cur-
rently used in the production of silicon cells, so the indus-
try has experience with it.
Once the metals are dissolved into the leaching agent,
one can use various hydrometallurgical methods to recover
pure metals such as solvent extraction and precipitation.
It is reminded that further processing is needed as solvent
extraction and precipitation do not produce pure metals.
Electrowinning can directly produce pure metals from a
leachate, without requiring additional steps or chemicals
(Hwang et al., 2017). However, electrowinning from a
nitric leachate leads to low recovery rates for silver and cop-
per at 65% and 80%, respectively.
SIGNIFICANT PROGRESS IN CHEMICAL
RECYCLING
Polymer removal (including encapsulant and back sheet)
and metal recovery from silicon cells will be discussed here.
Polymer Removal
It has long been established that the polyvinyl fluoride
back sheet can be peeled off by softening the encapsulant at
200°C (Bruton et al., 1994), and the exposed ethylene vinyl
acetate can then be safely burned off by thermal decompo-
sition. However, no commercial tool is available today for
peeling off the back sheet. Recently, mechanical milling has
been successfully applied to back sheet removal (Fiandra et
al., 2019), and the remaining encapsulant is then removed
by thermal decomposition. The optimum conditions
for complete decomposition of ethylene vinyl acetate are
500°C for 1 hour. A common question for both methods is
how well do they work on deformed and broken modules?
The reported experiments were both performed on intact
modules.
For encapsulant removal, Yan et al. (Yan et al., 2020)
reported potassium hydroxide (KOH) plus ethanol
(CH3CH2OH) to dissolve ethylene vinyl acetate. This is
a new method as encapsulant dissolution had been using
either an organic solvent or nitric acid, but those methods
are not suitable for commercial-scale deployment (Tao et
al., 2023). Potassium hydroxide is more environmentally
friendly as it does not emit carbon dioxide and is less haz-
ardous than many organic solvents and nitric acid. Small
pieces of silicon modules were placed in the basic solution
and heated to 200°C for 3 hours in an autoclave. Although
100% detachment of silicon cells from glass was observed,
the kinetics needs improvement to enable at or near room
temperature operation without a pressurized container.
An interesting method to separate encapsulant from
silicon cells is laser debonding (Li et al., 2022). This is the
first physical method reported for encapsulant removal,
Figure 3. Chemical recycling of silicon solar cells: (a) dissolution of metals, (b) removal of non-silicon layers, and (c) recovery
of silicon
Tao et al., 2023). Those attempts tend to focus on silver
and silicon recovery, with little attention paid to recovery of
lead, tin, and copper. It is important to recognize that lead
is a must-recover metal as it is toxic. If not removed from
the recycling sludge, it can poison the environment when
landfilled. With a chemical process, one must be mind-
ful of the quantity and hazard of the chemicals involved,
while maintaining high material recovery rates. Moreover,
it is desirable to recover pure metals instead of alloys or
compounds, as pure metals can be directly reused in new
modules, while alloys and compounds require further pro-
cessing before they become useful, resulting in additional
process cost and chemical waste.
In the reported recipes for chemical cell recycling,
nitric acid (HNO3) is the most common leaching agent for
the metals: silver, lead, tin, and copper. Hydrofluoric acid
(HF) is required to remove the silicon nitride layer. Nitric
acid is a broad-spectrum leaching agent dissolving all four
metals, so it requires selectivity in subsequent processes to
separate and recover pure metals from the nitric leachate.
In addition, tin precipitates in nitric acid as tin dioxide
(SnO2) which requires additional steps to obtain metallic
tin (Hwang et al., 2017). Hydrofluoric acid is nasty, but
there is no other effective method to etch off the silicon
nitride layer at room temperature. Hydrofluoric acid is cur-
rently used in the production of silicon cells, so the indus-
try has experience with it.
Once the metals are dissolved into the leaching agent,
one can use various hydrometallurgical methods to recover
pure metals such as solvent extraction and precipitation.
It is reminded that further processing is needed as solvent
extraction and precipitation do not produce pure metals.
Electrowinning can directly produce pure metals from a
leachate, without requiring additional steps or chemicals
(Hwang et al., 2017). However, electrowinning from a
nitric leachate leads to low recovery rates for silver and cop-
per at 65% and 80%, respectively.
SIGNIFICANT PROGRESS IN CHEMICAL
RECYCLING
Polymer removal (including encapsulant and back sheet)
and metal recovery from silicon cells will be discussed here.
Polymer Removal
It has long been established that the polyvinyl fluoride
back sheet can be peeled off by softening the encapsulant at
200°C (Bruton et al., 1994), and the exposed ethylene vinyl
acetate can then be safely burned off by thermal decompo-
sition. However, no commercial tool is available today for
peeling off the back sheet. Recently, mechanical milling has
been successfully applied to back sheet removal (Fiandra et
al., 2019), and the remaining encapsulant is then removed
by thermal decomposition. The optimum conditions
for complete decomposition of ethylene vinyl acetate are
500°C for 1 hour. A common question for both methods is
how well do they work on deformed and broken modules?
The reported experiments were both performed on intact
modules.
For encapsulant removal, Yan et al. (Yan et al., 2020)
reported potassium hydroxide (KOH) plus ethanol
(CH3CH2OH) to dissolve ethylene vinyl acetate. This is
a new method as encapsulant dissolution had been using
either an organic solvent or nitric acid, but those methods
are not suitable for commercial-scale deployment (Tao et
al., 2023). Potassium hydroxide is more environmentally
friendly as it does not emit carbon dioxide and is less haz-
ardous than many organic solvents and nitric acid. Small
pieces of silicon modules were placed in the basic solution
and heated to 200°C for 3 hours in an autoclave. Although
100% detachment of silicon cells from glass was observed,
the kinetics needs improvement to enable at or near room
temperature operation without a pressurized container.
An interesting method to separate encapsulant from
silicon cells is laser debonding (Li et al., 2022). This is the
first physical method reported for encapsulant removal,
Figure 3. Chemical recycling of silicon solar cells: (a) dissolution of metals, (b) removal of non-silicon layers, and (c) recovery
of silicon