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Innovative pilot for Silicon production with low environmental impact using secondary Aluminium and silicon raw materials

Periodic Reporting for period 1 - SisAl Pilot (Innovative pilot for Silicon production with low environmental impact using secondary Aluminium and silicon raw materials)

Reporting period: 2020-05-01 to 2021-10-31

SisAl Pilot aims to demonstrate a novel industrial process at TRL 6-7 for producing the Critical Raw Material (CRM) silicon (Si) in different qualities; Metallurgical Grade (MG-Si, >98%), high purity silicon (HP-Si, >99.9%), Solar Grade (SoG-Si, >6N) and Al-Si alloys – along with Metallurgical Grade Alumina (MGA) and/or High Purity Alumina (HPA). SisAl represents an environmentally and economically sustainable alternative to today’s carbothermic reduction process in the Submerged Arc Furnace (SAF), allowing Si production in an increasingly carbon-lean Europe.

For silicon production, the short-term solution of using bio-carbon sources as quartz reductants is challenging due to the extensive competition/high prices for this resource for use in chemicals, fuels etc. As such, and in light of increased focus on circular economy, the use of secondary aluminium sources as reductants is more relevant than ever. Underlying Si supply issues to the EU from China have also worsened, making EU industries vulnerable in both the short and long term. The power situation in China has meant that Si production has dropped and prices increasing more than 300% in less than two months during the fall of 2021.

The overall objective of SisAl Pilot is to scale up and demonstrate a new, carbon lean European technology to produce silicon and silicon alloys, along with metallurgical grade alumina (MGA) and high purity alumina (HPA), at TRL 6-7, validating raw materials and product quality, environmental impact and economic parameters to lay the ground for commercialisation.
A detailed raw material resource mapping and initial characterisation of potential raw materials to be used in the SisAl Pilot process have been conducted. Results from the raw material mapping together with results from detailed business case evaluation show that more attention on potential alumnium sources needs to be emphasized from now on.

Focus for the small-scale laboratory work in the pyrometallurgical part of the project has been on understanding the impact different raw materials and process parameters have on the SisAl process and to prepare for the pilot trials at Elkem, RWTH, FRey and Mintek. Based on results obtained at NTNU, Elkem has performed 22 pilot trials, with different types of raw materials and process parameters. Results from these verified that the small-scale experiments at NTNU are representative for large scale experiments in terms of raw material- and product composition as well as interaction with graphite crucible materials. Experimental work has been complimented by modelling work conducted by SIMTEC and ITMATI. Slags from Elkem has been sent to Mytilineos and NTUA Greece to be used in small scale and pilot scale hydrometallurgical tests and produced silicon (>98% Si) has been sent to Silicor for further purification to Si-SoG. NTUA has done modelling and lab scale screening of the optimum conditions for extracting alumina from the slags. Research performed included optimizing crystallization of alkaline leachable phases in produced slags, modelling and testing of alkaline leaching of slags, modelling of alumina precipitation from alkaline leach solutions, modelling and testing of acidic leaching of slags, modelling and testing of aluminium chloro-hydrates (ACH) precipitation from acidic leach solutions and refining ACH solutions to achieve high purity grades. Results over the first 18 months show that slags produced are/can be made suitable for both alkaline and acidic leaching; Aluminium leaching yields of 100% and 65% have been achieved in acidic and alkaline leaching respectively. Precipitation of alumina from the acidic solution produced and refined, has so far yielded a 99,9% pure alumina.

European short-term business cases were updated by BNW during the first reporting period with more detailed information on process yields and materials price data, giving overall positive economic performance estimates. The business foresight for making metallurgical grade alumina (MGA) from SisAl-made CaO-Al2O3 slags does however look less promising at this point than the initial estimates. Other commercial markets needing these slags in large volumes, e.g. secondary steelmaking and cement clinker are investigated in more detail in the coming project period.

Flowsheet models using HSC Sim/FACTSage and the environmental assessment of the SisAl process and the development of a data-sharing infrastructure has been established. The HSC Sim models done by HZDR comprise (i) individual flowsheets for pyro- and hydrometallurgical unit operations of the SisAl process for the production of silicon and alumina, and (ii) the conventional Submerged Arc Furnace (SAF) process for the production of silicon. The flowsheet models have generated the mass and energy balances for the SisAl and SAF processes. Then, a comparison of the environmental impacts between the conventional SAF and SisAl processes has been conducted by NTNU through a Life Cycle Assessment (LCA). The data required for the environmental impact assessment were obtained from the flowsheet models developed. This assessment has shown that the SisAl process has better environmental performance than the SAF process for almost all the impact categories considered.

Finally, a detailed dissemination and communication strategy as well as a dissemination kit has been created. Specifically, a homepage, social media accounts, a file sharing system (innovation place) and a promotion video of the SisAl concept have been made. Recently a series of film clips and a longer film about Elkem’s pilot experiments have been finalised and these will be realised on SisAl Pilots social media channels by the beginning of 2022. In addition, SisAl Pilot has been presented at the RMIS workshop, a clustering event organised by the European aluminium association (EAA) during fall 2021 and at different conferences such as Infacon XVI, RawMat, Silicon for the Chemical and Solar Industry XVI, CRU silicon Conference and Life Cycle Management Conference.
During the first period of the project strong focus has been on operational design including stirring method, charging method and raw material mixture. It has been demonstrated at 300 kg scale for the first time that aluminum dross (mixture of metal and oxide) is a suitable reductant for SiO2 containing slags. Dross is a by-product from the aluminium industry that contains considerable amount of valuable aluminum (typically 70-80%), additional industrial processes, with considerable cost and negative environmental impact, are employed today to recover parts of this aluminium. It has also been shown in pilot scale that sculls can be used to refine the produced Si-alloy from the SisAl process to MG-silicon (>99% purity). The European short-term business cases have been updated with more detailed information on process yields and materials price data, giving overall positive economic performance estimates. These updated cases have laid the ground for initial discussions with potential partners that could bring the SisAl Pilot process to the market.
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