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## Semiconductor recycling plant case study of GaAs photovoltaic manufacturing - case study

, 18:41, 9 July 2012
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[[category:industrial ecology]]
Current Production methods of GaAs semiconductors produce a large amount of waste. To determine the amount of waste produced during production of GaAs solar cells, we must first determine the amount of GaAs per Area/Watt<sub>peak</sub>. The calculations are seen below.
'''Amount of Material GaAs per Area'''
Size of Panels = 10 inches x 9 inches<ref name="">
Number of Cells per Panel = 18<ref name=""></ref>

Area of Cell = (90 in<sup>2</sup>/18)*.00064516 m<sup>2</sup>/in<sup>2</sup> [((Area Panel)/(Number Cells))*(conversion from in.<sup>2</sup> to m<sup>2</sup>)]

Area of Cell = .0032258 m<sup>2</sup>

GaAs per Cell = 10 grams<ref name=""></ref>

GaAs per Area = 10 g /.0032258 m<sup>2</sup> [(GaAs/Cell)/(Area Cell)]
GaAs per Area = 3100 g/m<sup>2</sup>
Amount of Material per Area = 3100 g/m<sup>2</sup>
'''Amount of Material GaAs per Watt'''
Peak Power per Cell = 880mW <ref name=""></ref>

Peak Power per Area = 880mW / .0032258 m<sup>2</sup> [(Power/Cell)/(Area Cell)]

Peak Power per Area = 272.8 W/m<sup>2</sup>

GaAs per Watt<sub>peak</sub> = 3100 g/m<sup>2</sup> / 272.8 W/m<sup>2</sup> [(GaAs/Area)/(Power/Area)]
GaAs per Watt<sub>peak</sub> = 11.4 g/W<sub>peak</sub>
GaAs per Watt<sub>peak</sub> = 11.4 g/W<sub>peak</sub>
===Metalorganic Chemical Vapour Deposition Waste===
Average Material Utilization Efficiency = 30%<ref name="">V.M. Fthenakis, B. Bowerman, Environmental Health and Safety (EHS) Issues in III-V Solar Cell Manufacturing, National PV EHS Assistance Center (http://www.bnl.gov/pv/files/pdf/art_168.pdf)</ref>

'''Waste GaAs per Area'''

Waste GaAs per Area = (3100 g/m<sup>2</sup>/.3)-3100g/m<sup>2</sup> [(GaAs/Area)/(Percentage Waste)-(GaAs/Area)]
Waste GaAs per Area = 7233 g/m<sup>2</sup>
'''Waste GaAs per Area = 7233 g/m<sup>2</sup>''''''Waste GaAs per Watt<sub>peak</sub>'''
Waste GaAs per Watt<sub>peak</sub> = (11.4 g/W<sub>peak</sub>/.3)-11.4g/W<sub>peak</sub> [(GaAs/Watt<sub>peak</sub>)/(Percentage Waste)-(GaAs/Watt<sub>peak</sub>)]
'''Waste GaAs per Watt<sub>peak</sub> = 26.6 g/W<sub>peak</sub>'''
===Molecular Beam Epitaxy Waste===
Average Material utilization efficiency for Ga = 55%

'''Waste Ga per Area'''
Grams of Ga per Area = 3100 g/m<sup>2</sup>*69.723/(69.723+74.922) [(GaAs/Area)*(Wt% Ga)]

Grams of Ga per Area = 1494.3 g/m<sup>2</sup>
Waste Ga per Area = (1494.3 g/m<sup>2</sup>/.55)-1494.3g/m<sup>2</sup> [(Ga/Area)/(Percentage Waste)-(Ga/Area)]
Waste Ga per Area = 1222.6 g/m<sup>2</sup>
'''Waste Ga per Area = 1222.6 g/m<sup>2</sup>'''
'''Waste Ga per Watt<sub>peak</sub>'''
Grams of Ga per Watt<sub>peak</sub> = 11.4 g/Watt<sup>peak</sup>*69.723/(69.723+74.922) [(GaAs/Watt<sup>peak</sup>)*(Wt% Ga)]

Grams of Ga per Watt<sub>peak</sub> = 5.495 g/Watt<sup>peak</sup>

Waste Ga per Watt<sub>peak</sub> = (5.495 g/W<sub>peak</sub>/.55)-5.495g/W<sub>peak</sub> [(Ga/Watt<sub>peak</sub>)/(Percentage Waste)-(Ga/Watt<sub>peak</sub>)]
'''Waste Ga per Watt<sub>peak</sub> = 4.496 g/W<sub>peak</sub>'''
'''Waste As Calculations'''
Material utilization efficiency for As = 10-20%<ref name =""></ref>
Average Material utilization efficiency for As = 15%
Average Material utilization efficiency for As = 15%
'''Waste As per Area'''

Grams of As per Area = 3100 g/m<sup>2</sup>*74.922/(69.723+74.922) [(GaAs/Area)*(Wt% As)]

Grams of As per Area = 1605.7 g/m<sup>2</sup>
Waste As per Area = (1605.7 g/m<sup>2</sup>/.15)-1605.7g/m<sup>2</sup> [(As/Area)/(Percentage Waste)-(As/Area)]
Waste As per Area = 9099 g/m<sup>2</sup>
'''Waste As per Area = 9099 g/m<sup>2</sup>'''
'''Waste As per Watt<sub>peak</sub>'''

Grams of As per Watt<sub>peak</sub> = 11.4 g/Watt<sup>peak</sup>*74.922/(69.723+74.922) [(GaAs/Watt<sup>peak</sup>)*(Wt% As)]

Grams of As per Watt<sub>peak</sub> = 5.9 g/Watt<sup>peak</sup>

Waste As per Watt<sub>peak</sub> = (5.9 g/W<sub>peak</sub>/.15)-5.9g/W<sub>peak</sub> [(As/Watt<sub>peak</sub>)/(Percentage Waste)-(As/Watt<sub>peak</sub>)]
Waste As per Watt<sub>peak</sub> = 33.4 g/W<sub>peak</sub>
'''Waste As per Watt<sub>peak</sub> = 33.4 g/W<sub>peak</sub>'''=Recycling==
==Recycling==
[[File:Recycling process flowchart.jpg|thumb|left|Proposed process for recycling GaAs waste]]
===Collecting Waste===
Semiconductor waste from MBE occurs mainly as a solid waste coating the reactor walls and parts, while the MOCVD process creates non solid waste in the form of exhaust vapors drawn off of the epitaxial reactors.
===='''Solid Waste===='''
The coating of the reactor walls and parts forms a solid waste material, which can be collected by simply scraping it off off reactor components. The solid waste material collected in this manner may be contaminated with dopants such as Si, Zn, C and Cr, as well as with GaAsP, arsenic oxides, and phosphorous oxides. These dopants however usually only have concentration levels of only 10<sup>18</sup><ref name=""></ref> atoms/cc.
The energy collection cost for this solid waste is assumed to be negligible, as the solid waste needs to be removed in current production processes.
===='''Non-solid Waste===='''
The collection of semiconductor waste from exhaust vapors is not as straight forward as the collection of solid waste material. In order to collect the waste from the exhaust, the exhaust vapors undergo a series of "scrubbing" processes, in which cooled water would be sprayed into the vapor, cooling it off until the relative temperature of the desired components in the exhaust reach a temperature where it changes from a gas to either liquid or solid phase.<ref name="">Potter, G. U.S. Patent 4,008,056, 1977 (http://patft.uspto.gov/netacgi/nph-Parser?Sect2=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=1&f=G&l=50&d=PALL&RefSrch=yes&Query=PN%2F4008056)</ref> This material would then be collected and, depending upon the contaminants (which would include the dopants used to create the thin films), would undergo multiple purification process in order to reach the desired purity for reuse.
Energy Cost = .003182 kW-hrs/g
===='''Collection Efficiency===='''
Since the primary methods of material loss are accounted for above, it can be assumed that, while a certain loss of materials will always occur, most of the materials will be captured and re-purified. Materials will be lost in the water scrubbing method, as not all of the waste materials will be condensed into liquid and so some reusable gases will escape into the atmosphere despite the taken precautions. The solid wastes will undoubtedly be captured entirely, with little to no loss occurring. However, when the purification process takes place, some Ga and As will undoubtedly be lost, as defects and impure materials will have to be removed from the system in order to purify the elements to their required level of purity. It can be assumed that about 80-90% of the materials will be reusable though, depending upon the standards of quality enacted in processing the waste materials.
[[File:GaAs recycling process.jpg|thumb|left|Proposed recycling process of GaAs semiconductor material]]
===='''Non-Solid Waste recycling and re-purification===='''
Before the non-solid waste produced from the MOCVD process can be recycled and re-purified, it must first be turned into solid waste.
====='''Non-Solid Waste to Solid Waste Conversion====='''
The waste water obtained from the water scrubbing process is adjusted to a pH between 11.5 and 12 with the use of sodium hydroxide. After the pH is correctly adjusted, a soluble calcium salt is added to the mixture. This soluble calcium salt reacts with the arsenate, and produces a calcium arsenate precipitate which can be removed using a centrifuge. After the calcium arsenate precipitate is removed the pH is readjusted to between 6 to 8 using sulfuric acid, which causes gallium hydroxide to precipitate out, which can be collected and filtered<ref name=""></ref>.
The energy cost of converting the non-solid waste to solid waste can be estimated by determining the energy cost of the centrifuge process.
Energy cost Centrifuge = 18.5 kW-hrs<refname="centrifuge">http://www.uscentrifuge.com/decanter-test-module.htm</ref>
Estimated Capacity = 2000 grams GaAs (Mixed into water)
Energy cost Centrifuge per gram = .00925 kW-hrs/g
====='''Purifying Gallium Hydroxide====='''
To obtain gallium instead of the solid waste gallium hydroxide, a process of electrowinning can be performed. To perform electrowinning the gallium hydroxide is redissolved in a caustic solution, and electrodes are placed into the material. When a current passes between the electrodes, the gallium metal is deposited onto a electrode and is collected<ref>Recycling-Metals. Web. 9 Oct. 2011. <http://minerals.usgs.gov/minerals/pubs/commodity/recycle/recycmyb02r.pdf>.</ref>.
This gallium metal can be processed further using the gallium zone refinement described in the next section.
====='''Purifying Calcium Arsenate====='''
The calcium arsenate can be further processed using the sublimation refining process described in the next section.
===='''Solid Waste recycling and re-purification===='''
The recycling of GaAs solid waste can be accomplished in three steps, Thermal Separation, Sublimation Refining of Arsenic, and Gallium Zone Refinement.
====='''Thermal Separation====='''
[[File:Thermal Separation Furnace schematic.jpg|thumb|right|Thermal Separation Furnace]]
Thermal separation is the process of separating gallium and aresenic using elevated temperatures and reduced pressures. To begin separation solid GaAs waste is placed into a graphite or SiC crucible inside of a thermal separation furnace. The furnace is heated to above 1050 degrees C for 2-3 hours, and the pressure inside is reduce to less than 1 torr. At these elevated temperatures and reduced pressures, arsenic is able to be separated out as a condensable vapor, leaving a gallium-rich residue in the crucible. Due to the low melting temperature of gallium compared to other elements, the gallium-rich residue left in the crucible is composed of a liquid gallium fraction, and a slag fraction. Filtering out the slag from the gallium-rich liquid before cooling, and condensing the arsenic vapors produces fairly pure gallium and arsenic solids.
Energy cost thermal separation per gram = .001663 kW-hrs/g
====='''Sublimation Refining of Arsenic====='''
[[File:Purification of Arsenic.jpg|thumb|left|Purification of As]]
Although thermal separation separates gallium and arsenic fairly effectively, the arsenic can still be contaminated with more volatile contaminants like carbon. To further purify, the arsenic is put through through the process of sublimation refining. During sublimation refining, the arsenic is heated slightly above 610 degrees C, the sublimation temperature of arsenic. This heating is performed in an inert gas stream such as nitrogen, and the arsenic is transferred to and recondensed in a second chamber. To obtain higher purity arsenic, this process can be accomplished multiple times. This method works because arsenic and the impurities within will differ in partial pressures and volatility.
Energy cost of As purification per gram = .001102 kW-hrs/g
====='''Gallium Zone Refining====='''
[[File:Purification of Gallium.jpg|thumb|right|Purification of Ga]]
Gallium zone refining is a purification technique for gallium based upon low expectations of impurities. The process only works due to the near pure gallium obtained from thermal separation. To further purify, gallium is placed in a gallium refining pan with a spiral groove in the pan bottom, and cooled to 0 degrees C. A heat lamp is used to heat specific zones of the gallium sample, and the pan is slowly rotated. Based upon the temperature difference of solid and liquid in the sample, impurity segregation will occur, and impurities will segregate to both ends of the spiral groove in the gallium refining pan.
The energy cost associated with gallium zone refining can be estimated as the cost to spin, cool, and heat, and spin the gallium.
The energy cost for spinning will be assumed to be the same as for the centrifuge, but the capacity will be increased, as almost pure gallium is being spun.
Energy cost spinning =18.5 kW-hrs<ref name="centrifuge"></ref> Estimated Capacity =20000 grams GaAs (Almost pure Gallium) Energy cost spinning per gram =18.5 kW-hrs/ 20000 grams [(Energy cost)/(Capacity)]  Energy cost spinning per gram =Purity.000925 kW-hrs/g  Energy cost of refrigeration per batch is estimated as normal daily use, due to having to bring from hot to cold. Energy cost of refrigeration = 3.1 kW-hrs<ref>http://www.fypower.org/ind/tools/products_results.html?id=100210</ref> Estimated Capacity = 20000 grams GaAs Energy cost of refrigeration per gram = 3.1 kW-hrs / 20000 grams [(Energy cost)/(Capacity)]  Energy cost of refrigeration per gram = .000155 kW-hrs / gram   The energy cost of heating will be based upon furnace values used previously, however they will be quartered to account for spot heating. Energy cost of furnace per gram =.001653 kW-hrs/g Energy cost of heating per gram =.001653 kW-hrs/g / 4  Energy cost of heating per gram =.00041325 kW-hrs/g  Total Energy cost gallium zone refining per gram =.00041325 kW-hrs/g + .000144 kW-hrs/g + .000925 kW-hrs/g [(Cost of heating/g) + Cost of spinning/g)+(Cost of refrigeration/g)]  Total Energy cost gallium zone refining per gram =.001483 kW-hrs/g  '''Purity'''
The contamination level of Gallium and Arsenic after the above processes are completed can be as low as 5-50 ppb, or in other words the material is 99.99999% pure<ref>Bautista, R. G. 2003. Processing to Obtain High-Purity Gallium. JOM., 55: 23–26.(http://atmsp.whut.edu.cn/resource/pdf/1994.pdf)</ref>. This purity meets the requirements for both semiconductor grade, 99.99%, and solar grade, 99.9999%.
Using the material waste values produced through various methods of GaAs thin-film production, we can determine the necessary capacity of a feasible recycling plant.
===='''Supporting a MOCVD plant===='''
The amount of material per Watt Peak wasted using MOCVD was determined to be 26.6 g/W<sub>peak</sub>. Using this value, and the size of the supported manufacturing plant, we can determine the daily required capacity of a recycling plant.
Required Recycling Capacity = 72876.7 kg/day

Due to the separate processes that arsenic and gallium must go through, a capacity for each should be calculated. It is assumed that a 1:1 mole ratio exist for MOCVD waste.
Required Recycling Capacity As = 72876.7 kg/day *74.922/(69.723+74.922) [(Total Capacity)*(Wt% As)]144.645

'''Required Recycling Capacity As =37748.1 kg/day'''

Required Recycling Capacity Ga = 72876.7 kg/day *69.723/(69.723+74.922) [(Total Capacity)*(Wt% Ga)]

'''Required Recycling Capacity Ga = 35128.6 kg/day'''
===='''Supporting a MBE plant===='''
The amount of material per Watt Peak wasted using MBE was determined to be 4.496 g/W<sub>peak</sub> for Ga and 33.4 g/W<sub>peak</sub> for As. Using these values, and the size of the supported manufacturing plant, we can determine the daily required capacity of a recycling plant.
Required Ga Recycling Capacity = (4.496 g/W<sub>peak</sub> *1 GW)/(1000 *365)[((Waste/W<sub>peak</sub>)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]

'''Required Ga Recycling Capacity = 12318 kg/day'''

Required As Recycling Capacity = (33.4 g/W<sub>peak</sub> *1 GW)/(1000 *365)[((Waste/W<sub>peak</sub>)*(Plant capacity))/((Gram to Kilogram conversion)*(Days/Year)]

'''Required As Recycling Capacity = 91506.8 kg/day'''
Depending upon the solar production technique supported, the equipment requirements will vary slightly for recycling.
===='''Supporting a MOCVD plant===='''
The following equipment is necessary to recycle waste from a MOCVD plant.
-Gallium Zone Refiner
===='''Supporting a MBE plant===='''
The following equipment is necessary to recycle waste from a MBE plant.
-Gallium Zone Refiner

<font color=red>Energy for As separation and purification:
91506.8(kg/day) --> 3.81278*10^6(g/hr)
(3.81278*10^6(g/hr))/(33.4(g/W))=114155 (W/hr) --> 114.155kW/hr - As to support an MBE plant.
114.155(kW/hr)*0.001102(kw-hr/g)*91506.8(kg/day)=11511.6(kW/day)

Energy required for Ga zone refining:
12318(kg/day) --> 513250(g/hr)
513250(g/hr)/4.496(g/W)=114157(W/hr) --> 114.157(kW/hr) - Ga to support MBE plant
114.157(kW/hr)*0.001483(kw-hr/g)*12318(kg/day)=2085.37(kW/day)

Total energy required:
11511.6(kW/day)+2085.37(kW/day)=13596.97kW/day
</font>

===Material Flow Chart===
[[File:Recycling process flowchart.jpg|Proposed process for recycling GaAs waste]]
==Safety==