Quartzite Mining for Silicon

Mine/Company	Output	Country
Covia		US
Creswick Quartz
HPQ Materials		Australia
Imerys		France
Mineracao Santa Rosa
Sibelco Group
The Quartz Corp

Metallurgical Silicon Refining

Metallurgical silicon (MGSi) takes crushed quartzite and quartz sand as feedstock to produce MGSi at 98-99% purity. About x% of metallurgical silicon is used to produce solar grade and electronic grade silicon.

Crushed quartzite is mixed with carbon in the form of coke (coal that has been heated in the absence of oxygen). Woodchips are added to the charge as well; they serve both as a carbon source and a physical bulking agent that allows gasses and heat to circulate better in the furnace.

Purification starts by mixing powdered metallurgical silicon with hot, gaseous hydrochloric acid. This reaction produces trichlorosilane.

Si + 3HCl → SiHCl₃+ H₂

Finally, the pure SiHCl₃ is reacted with hydrogen at 1100°C for ~200 – 300 hours to produce a very pure form of silicon.

The reaction takes place inside large vacuum chambers and the silicon is deposited onto thin polysilicon rods to produce high-purity polysilicon rods of diameter 150-200mm. The process was first developed by Siemens in the 1960s and is often referred to as the Siemens process.

The resulting rods of semiconductor-grade silicon are broken up to form the feedstock for the ingot crystallisation process. The production of semiconductor grade silicon requires a lot of energy.

While silicon is the prevalent material for wafers used in the electronics industry, other compound III-V or II-VI materials have also been employed. Gallium arsenide (GaAs), a III-V semiconductor produced via the Czochralski method, gallium nitride (GaN) and silicon carbide (SiC) are also common wafer materials, with GaN and sapphire being extensively used in LED manufacturing.

Using provided raw material and emissions quantities for the smelting/refining process, the impact of silicon smelting on carbon footprints was estimated and suggests that silicon solar panels and semiconductors account for more atmospheric carbon dioxide than they save. Excluding the silicon smelting process from carbon footprint calculations undermines the integrity of the silicon solar panel industry, and challenges the "green" claim of the proposed silicon smelter, and the "green" claim of silicon solar panels as well.

Mine/Company	Output	Country
Donghai Shihu Quartz
Hemlock Semiconductor		US
MEMC Electronic Materials
Mississippi Silicon
Mitsubishi Materials
Osaka Titanium Technologies
REC Silicon		US
Tokuyama Corporation		Japan
Wacker Chemie		Germany

Polysilicon Production

To remove the 0.5% to 1.5% of impurities contained in metallurgical-grade (MG) silicon, the Siemens process creates trichlorosilane (SiHCl3, or briefly TCS), a highly volatile liquid, as intermediate product.

For that purpose, MG silicon is ground up into small particles which react with hydrogen chloride (HCl). The resulting TCS has a low boiling point of 31.8 degrees centigrade (°C) so that it can be purified in tall distillation columns relatively easily.

Silicon is then deposited from the TCS on highly pure, slim silicon filaments that are electrically heated to up to 1,150 °C in a steel bell-jar reactor (see image on the top of this page) until they have grown to polysilicon rods with a diameter of 15 to 20 cm. This energy-intensive step is called chemical vapor deposition (CVD). The long rods are broken into small chunks.

The by-product silicon tetrachloride (SiCl4, or briefly STC) is recycled to TCS mostly through hydrochlorination: STC is fed along with hydrogen (H2) and MG silicon particles into the reactor for TCS production.

Depending on how thoroughly TCS is distilled and whether impurities on the surface of the polysilicon chunks are etched off, different levels of polysilicon purity can be achieved:

solar grade for multicrystalline cells (multi grade): 99.99999% (7N) to 99.999999% (8N); solar grade for monocrystalline cells (mono grade): 9N to 10N; electronic grade for semiconductors: 10N to 11N.