Harnessing the sun’s potential in mid-stream silicon processing

In the second instalment of the Australian Mining Review’s critical minerals series, we speak with Australian Critical Minerals Research and Development Hub manager Lucy O’Connor, ANSTO high purity quartz project manager Neilesh Syna and ANSTO high purity quartz project technical lead Robert Fowles for a dive deep into emerging mid-stream processing pathways for silicon in order to achieve net zero emissions by 2050.

Deployment of solar power is essential to the success of Australia’s energy transition. The nation stands to be a leader in this space, as we are equipped with the vast solar resources needed to meet global demands. However, the country is entirely reliant on overseas supply chains to produce solar photovoltaic (PV) technology used for solar panels.

To prepare for the energy transition and insulate itself from the risk of disruption in the global market, Australia needs to amplify its mid-stream silicon processing capabilities, as highlighted in Australia’s national science agency, CSIRO’s From Minerals to Materials: An Assessment of Australia’s Critical Minerals Mid-Stream Processing Capabilities.

Despite a global oversupply of the polysilicon used for solar technologies, the energy transition will still require a 10 to 12-fold increase on current production capacity, according to CSIRO. This increase is reliant on the improvement in production capabilities for lower grade silicon, which is used as feedstock, as well as improvements in mid-stream polysilicon processing.

According to the CSIRO report, Australia will need to pilot and scale up its technology, accelerate emerging technologies and grow Australian IP or establish new capabilities in emerging technologies.

Ms O’Connor says that until now, there have been modest domestic research, development and demonstration (RD&D) activity to further develop silicon production in Australia.

“International collaboration and RD&D efforts will be required to deliver near-term commercial outcomes and to develop the capabilities for long-term step-change innovations,” she said.

Stakeholders in the supply chain

But investment is being made, with multiple interest groups shifting focus to RD&D across the silicon supply chain.

“The Australian Critical Minerals Research and Development Hub is funding research that will provide technologies to support development of the silicon industry in Australia,” said Ms O’Connor.

“Geoscience Australia is funded to identify mineral systems and regions that have the greatest potential to supply raw feed stock material suitable to support the production of silicon.

“ANSTO is developing processing routes for high purity quartz (HPQ) production from Australian quartz and silica sand projects.

“More research is needed to unlock technical improvements in metallurgical refining of solar-grade silicon, and alternative lower energy, lower footprint technologies, as well as technoeconomic and market analyses to assess the viability of onshore production.

“RD&D is also critical to further develop mid?stream activities using, for example, chemical vapour deposition techniques in Australia and will require strong collaboration with overseas equipment providers.”

Both mature and emerging technologies have room for improvement and these improvements must be made to grow the global silicon supply, achieve decarbonisation efforts and establish a diverse silicon industry.

Future of RD&D

To produce silicon, quartz must be reduced. However, the process of reduction is emissions intensive. As a result, improvements in existing processes as well as development of alternative processes have been identified.

Applications of silicon depend on the purity of the material, which is improved through the refining processes.

Mr Fowles explains the two primary routes for silicon refining.

“There’s one route which can produce the metallic silicon, which gets further produced to make the electronic components and photovoltaic cells,” he said.

“Then there’s the high end which can be produced from the beneficiated HPQ all the way through to the high-grade photovoltaic cells.

“At the moment, the HPQ refers to what we call 3N type purity, which is sufficient for carbothermic reduction to metallic silicon, and is generally then used for items to provide aluminium alloys and other battery products.

“From there, they can also reduce an oxidiser to a polycrystalline which we call polymorphic silicon which can be used in both electronics and solar panels.

“But that is just general grade for general use in Australia.

“Highly purity quartz, or 4N7, is 99.997% pure and used to make fuse glass crucibles, a crucial precursor to making the wafer polycrystalline for solar cells.

“Those are higher quality photovoltaic cells that they use in solar farms.”

Metallurgical and battery grade silicon

According to the IEA, demand for silicon will increase from 816.2kt in 2022 to 2,200.5kt by 2050, driven by solar PV and electric vehicles, making metallurgical silicon a significant material of interest.

Ms O’Connor says Australia currently has one silicon refinery, at Kemerton in WA, owned by Simcoa.

“It uses a traditional carbothermic electric arc process to produce metallurgical grade silicon, at about 99% purity,” she said.

“This is used to make aluminium alloys but can be purified even further to make solar grade polysilicon or semiconductor grade.

“There are a number of Australian companies planning both new smelters and the production of polysilicon.”

Traditionally, silica quartz is reduced within a furnace to produce metallurgical grade silicon. However, concerns around emissions have highlighted opportunities for innovation in this carbothermal reduction process.

Carbothermal reduction

Carbothermal reduction is a mature technology used to reduce compounds into metals with carbon as a reductant. In silicon production, quartz is reduced in arc or electric arc furnaces at temperatures of 2000 °C to produce metallurgical grade silicon of 99-99.8% purity.

Despite the employment of electric furnaces, carbothermal reduction is an energy intensive process due to its use of carbon reductants.

State of play

Although this technology is mature, the process is being innovated overseas to reduce energy, cost and emissions and target battery grade silicon.

Globally, research is being focused into improvements in arc furnace technology. These improvements include desulphurisation options for furnaces that utilise coal reductants, nitrous oxide emissions reductions and heat recovery for improved efficiency.

Pathways for sustainable charcoal sourcing and the use of biomass-based briquettes to replace low ash coal are also potential pathways that will reduce emissions intensity of the technology.

Alternative low-cost silica sources such as sand, rice husks, diatomaceous earth are also being researched. Novel carbothermal reduction processes such as induction furnaces can be developed to process these alternative materials.

Simcoa is Australia’s only commercial producer of metallurgical grade silicon using carbothermal reduction. Although Australia is commercially active, there are not high levels of RD&D in this area.

Metallothermic reduction

Metallothermic reduction is a mature technology used to obtain metal from ore. Rather than using carbon as a reducing agent, a reactive metal — such as aluminium, magnesium, zinc, sodium metal or potassium metal — is used to reduce ore to its elemental form.

This process features low emissions and low costs due to its omission of carbon as an agent. However, for it to be considered net zero, the metals used would also have to be produced at net zero emissions. This is a challenge, as some of the potential metals have high emissions in terms of production or extraction.

This technology is not in commercial use at present but there has been successful pilot scale demonstration globally.

Gas-based reduction

The replacement of carbon with hydrogen and methane as reduction agents are also being investigated. The use of these gasses can further reduce emissions for silica quartz reduction.

Gas-based reduction is an emerging technology, still in its infancy. Because silica is very stable, reduction with hydrogen requires extremely high temperatures. Processes using plasma technology have shown early potential to reduce silicon oxides at lower temperatures.

State of play

Although gas-based reduction for other metals is being researched globally in industries like steelmaking, the silicon technology is still being explored at a lab scale.

Australia can leverage its capabilities in hydrogen or other metals reduction technologies to improve its low RD&D activity in this area. One group that is doing so is Calix, with backing from the Australian Renewable Energy Agency (AREA). The company is designing a hydrogen direct reduced iron demonstration.

Solar and semiconductor grade silicon

Polysilicon is a high purity, refined silicon used for solar PV and electronic device manufacturing, which have sensitive requirements as impurities affect performance.

Dr Syna says this grade of silicon is essential for energy creation and consumer goods needed in Australia.

“Photovoltaics are for energy creation whereas semiconductors are used to produce chips that go into high end devices or general consumer devices,” he said.

“Once you go into semiconductors you are in another space altogether.

“Some of that can fall under a defence application so the sovereignty aspects start to become more critical.”

Dr Syna says presently, solar panel manufacturing is predominantly taking place in China.

“You have the element of energy and labour which China can do cheaply, but Australia is also all about the other factors that go into production like environmental concerns,” he said.

“Outside China you’ll be looking at using all those parameters to deliver a much more sustainable product, but you also need to be able to compete financially in that space.”

This is where Australia is looking to innovate.

Attaining such high grades of silicon needed for these applications requires the purification of metallurgical-grade silicon feedstock using chemical vapour deposition (CVD) methods.

One CVD method, the Siemens process, has remained the dominant method for silicon feedstock production for decades, holding a 90% share over the market, according to CSIRO. However, metallurgical techniques such as refining, leaching and directional solidification can be used with or as an alternative to these traditional CVD methods.

Chemical vapour deposition

CVD is a mature technology used to produced high purity and advanced materials. It passes a metal in vapour form through a reactor, where the metal is deposited onto a surface as a film, powder or crystal.

Alternative processes to Siemens include the fluidised bed reactor (FBR) which makes up 6% of the market. FBR is expected to make up to 20% of the market over the next 10 years, according to CSIRO.

In FBR, metallurgical silicon is converted to trichlorosilane (TCS) or silane gas. The gas then reacts with hydrogen. Following the reaction, high purity silicon is deposited on a surface to obtain polysilicon in the form of granules. The FBR process is continuous, meaning it could be less energy intensive than Siemens.

While CVD techniques are well-established globally, innovation is still ongoing. New CVD techniques are being demonstrated internationally at the lab scale.

Australia has no polysilicon production onshore and lacks significant RD&D activity.  As a result, the Federal Government has allocated up to $1b for its Solar SunShot program which will develop domestic solar PV manufacturing capabilities from production to module assembly. Additionally, the Queensland Government is supporting project Green Poly, aimed at manufacturing polysilicon and wafers.

RD&D should be concentrated on improving reactor design and streamlining CVD processes to reduce cost and improve energy efficiency.

State of play

More RD&D will be needed across silicon grades for Australia to fulfill its role in diversifying the global silicon supply chain. By increasing domestic activity in RD&D for mid-stream processing, Australia can not only secure its energy future but capitalise on the exponential demands for these silicon products.