3262 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
The silicon cell sheets in Figure 2(a) are more problem-
atic. The polyvinyl fluoride back sheet is still attached, so
any high-temperature process above 400°C would result in
a fluorine exhaust. Today NPC Inc. sends these cell sheets
to copper smelters to recover silver and copper. However,
smelting does not recover lead, silicon, or tin. Its recovery
rate for silver is low at about 65% based on conversations
with silicon module recyclers. It generates about $3/mod-
ule in additional revenue for a total of about $7/module:
• ~$2/module for aluminum
• ~$0.70/module for copper
• ~$1.30/module for glass
• ~$3/module for silver
This pyrometallurgical process leaves out about $3/
module in potential revenue:
• ~$1.50/module for lost silver
• ~$1/module for silicon
• ~$0.50/module for tin
• A few cents for copper and lead
Smelting is not designed to recover metals from silicon
cells (It is designed to reduce copper sulfides to copper). The
cell sheets are mixed with copper ores and go through three
high-temperature steps in smelting, converting, and casting
furnaces. The produced 90% pure copper after casting is
then electrorefined to achieve a 99.9% purity. The anode
sludge from copper electrorefinning is further processed by
electrowinning to recover silver and other valuable metals.
It is generally believed that a hydrometallurgical process is
more suitable to recover metals from silicon cells, although
such a process does not exist today.
TECHNICAL BARRIERS TO CHEMICAL
RECYCLING
The silicon cells in Figure 2(a) are still covered under the
encapsulant. For effective metal leaching, the encapsulant
must be removed first. The only practical method today
for encapsulant removal is thermal decomposition above
500°C in a furnace (Tao et al., 2023). This method presents
two challenges. First, it requires a large furnace to process
thousands of silicon cell sheets per batch. Such a furnace is
not commercially ready and would emit large amounts of
carbon dioxide. Second and more critically, the polyvinyl
fluoride back sheet still attached to the cell sheet releases
a fluorine exhaust during thermal decomposition, and no
one wants to deal with gaseous fluorine. This is probably the
biggest barrier to chemical recycling of silicon cells today.
Once the encapsulant is removed and the silicon cells
are exposed, chemical cell recycling can proceed as outlined
in Figure 3. The metals including silver electrode, cop-
per wire, and solder of lead and tin are leached out first
(Figure 3(a)). The non-silicon layer including aluminum
back electrode and silicon nitride antireflection layer are
then etched off (Figure 3(b)). Finally the silicon is recov-
ered either as metallurgical-grade silicon or solar-grade sili-
con (Figure 3(c)). To obtain solar-grade silicon, the heavily
phosphorus-doped emitter and heavily aluminum-doped
back surface field must be etched off. Only the p-type
base (the yellow part in Figure 3(c)) qualifies as solar-grade
silicon.
Although there is no commercial process to recover
metals from silicon cells, there are a large number of
attempts to develop such a process (Deng et al., 2019
Farrell et al., 2020 Tao et al., 2020 Wang et al., 2022
Figure 2. Photos of separated (a) silicon cell sheets and (b) glass sheets by NPC’s hot-knife tool (NC Inc., 2023a)
The silicon cell sheets in Figure 2(a) are more problem-
atic. The polyvinyl fluoride back sheet is still attached, so
any high-temperature process above 400°C would result in
a fluorine exhaust. Today NPC Inc. sends these cell sheets
to copper smelters to recover silver and copper. However,
smelting does not recover lead, silicon, or tin. Its recovery
rate for silver is low at about 65% based on conversations
with silicon module recyclers. It generates about $3/mod-
ule in additional revenue for a total of about $7/module:
• ~$2/module for aluminum
• ~$0.70/module for copper
• ~$1.30/module for glass
• ~$3/module for silver
This pyrometallurgical process leaves out about $3/
module in potential revenue:
• ~$1.50/module for lost silver
• ~$1/module for silicon
• ~$0.50/module for tin
• A few cents for copper and lead
Smelting is not designed to recover metals from silicon
cells (It is designed to reduce copper sulfides to copper). The
cell sheets are mixed with copper ores and go through three
high-temperature steps in smelting, converting, and casting
furnaces. The produced 90% pure copper after casting is
then electrorefined to achieve a 99.9% purity. The anode
sludge from copper electrorefinning is further processed by
electrowinning to recover silver and other valuable metals.
It is generally believed that a hydrometallurgical process is
more suitable to recover metals from silicon cells, although
such a process does not exist today.
TECHNICAL BARRIERS TO CHEMICAL
RECYCLING
The silicon cells in Figure 2(a) are still covered under the
encapsulant. For effective metal leaching, the encapsulant
must be removed first. The only practical method today
for encapsulant removal is thermal decomposition above
500°C in a furnace (Tao et al., 2023). This method presents
two challenges. First, it requires a large furnace to process
thousands of silicon cell sheets per batch. Such a furnace is
not commercially ready and would emit large amounts of
carbon dioxide. Second and more critically, the polyvinyl
fluoride back sheet still attached to the cell sheet releases
a fluorine exhaust during thermal decomposition, and no
one wants to deal with gaseous fluorine. This is probably the
biggest barrier to chemical recycling of silicon cells today.
Once the encapsulant is removed and the silicon cells
are exposed, chemical cell recycling can proceed as outlined
in Figure 3. The metals including silver electrode, cop-
per wire, and solder of lead and tin are leached out first
(Figure 3(a)). The non-silicon layer including aluminum
back electrode and silicon nitride antireflection layer are
then etched off (Figure 3(b)). Finally the silicon is recov-
ered either as metallurgical-grade silicon or solar-grade sili-
con (Figure 3(c)). To obtain solar-grade silicon, the heavily
phosphorus-doped emitter and heavily aluminum-doped
back surface field must be etched off. Only the p-type
base (the yellow part in Figure 3(c)) qualifies as solar-grade
silicon.
Although there is no commercial process to recover
metals from silicon cells, there are a large number of
attempts to develop such a process (Deng et al., 2019
Farrell et al., 2020 Tao et al., 2020 Wang et al., 2022
Figure 2. Photos of separated (a) silicon cell sheets and (b) glass sheets by NPC’s hot-knife tool (NC Inc., 2023a)