Producing high grade silicon metal from quartz and secondary silicon metal by Northern Silicon

Producing high grade silicon metal from quartz and secondary silicon metal by Northern Silicon

Northern Silicon (NOSI) has developed a process for producing high grade silicon metal from quartz and silicon metal that can be used in solar cell production. NOSI plans to build a full-scale plant in Norway and license its technology and know-how in other countries.

The process has several advantages compared to available solutions in the market today:

• Substantially less emissions of CO₂ (long term goal to produce CO₂ neutral silicon metal by using carbon black from biogas)
• Reduced operational costs as the process requires substantially less energy to operate
• Lower investment costs and footprint.

In ICARUS, NOSI is responsible for the Pilot B: Integrating closed-loop value chain in Solar Silicon route. This pilot aims at re-introducing Si-kerf, graphite and silica waste products into the silicon value chain; develop agglomeration and pre-treatment methods for the waste materials to make them suitable for charging to furnaces in terms of purity and material handling; develop proper recipes for mixtures of virgin and secondary material as well as process modification for Si production and refining, build and operate at industrial scale a demonstrator for the entire value chain from silicon waste to Si-metal.

Pyro-metallurgical process for producing Solar Grade Silicon

NOSI owns the rights to a pyro-metallurgical process to upgrade metallurgical silicon and/or secondary silicon metal for use in solar panels. This solution could produce silicon special grade silicon with up to 99.9999999% Si at a lower price than conventional processes.

Objectives, Challenges and answers

Pilot B aims at recovering kerf / silicon, graphite, quartz from crucibles using NOSI’s process technology to produce a silicon metal that can be re-used in solar panels. Two main challenges need to be addressed to reach the main objective of the pilot:
1. PV panels used extremely high purity silicon – NOSI’s process need to purify the waste to minimum 99.99999% silicon
2. The kerf is doped with Phosphorous and Boron, two elements that are costly to remove but need to be reduced to less than 0,2ppm each

In ICARUS, the NOSI technology will benefit from relatively high-purity ancillary materials, graphite/black carbon, silica and Si-kerf. The Submerged Arc Furnace (SAF) will be used during the initial pilot trials to investigate the relative differences between selected parameters (e.g. the quality of secondary raw materials used on the final silicon product quality). The focus will be set on the following secondary raw material streams: silicon: kerf-loss, silica crucible waste and graphite waste from discarded furnace parts.

To overcome these challenges, NOSI’s process includes 3 main steps:

1. A pre-treatment, where the input material is mixed using predefined recipes, and pellets are made of the fine/dusty material, like Si kerf, silica and graphite/carbon black.
2. A reduction and purification, consisting of carbothermic reaction of silica in a SAF. Main impurities are removed by evaporation. The good quality of silica from crucibles is introduced at this stage. Recycled graphite fractions might be used instead of carbon black as reduction agent.
3. A purification, consisting of a holding furnace and a directional solidification furnace. The molten Si is injected with Cl and heated to remove impurities, allowing to achieve 8N SoG-Si. The Si kerf is introduced during this stage.

 

ICARUS’s first results

The first SAF experiment was carried out in cooperation with SINTEF and based on natural quarts pebbles plus NOSI briquettes as the reduction agent. During the test period, natural quarts were replaced by crushed Pot Scrap. The process worked and more than 1000 kg Silicon was produced.
After installing a briquetting machine in the NOSI test center, they carried out briquetting tests, still in cooperation with SINTEF. The most promising recipe was chosen to be produced and used in the second SAF test run. In this briquette, quarts powder was replaced by pulverized Pot Scrap and produced in accordance with the NOSI know-how. Approximately 1400 kg Quarts/carbon briquettes was produced and in addition circa 500 kg kerf briquettes.
SINTEF carried out the SAF reduction process based on Pot Scrap pebbles and ICARUS briquettes. In the last 20 hours of the test run, kerf briquettes were added to the material mix charged to the furnace. In total, 7 silicon tapping were achieved and 100 kg silicon produced.

 

Kerf briquettes ready for processing	The inducation furnace, removing impurities using 2 types of gas	Directional solidification

 

The NOSI briqueting mix and technique works well for kerf and crucible quartz and is key to achieving an energy-efficient process. Graphite from electrodes is not reactive enough to replace carbon as a reduction agent. Silica from crucibles is well suited to produce standard silicon metal and may replace virgin material but contains too many impurities to be a viable source in making high purity silicon metal. Kerf is very promising as raw material to make silicon wafers – Testing remains.

Next steps

In the coming months, the refining process shall be developed and tested out in addition to an small industrial sized campaign to prove the commercial potential of recycling of waste from solar industry by NOSI technology.