Introduction
Silicon is intrinsic in our lives due to its conductivity, numerous applications, and abundance in our natural world. Silicon is used both in its natural elemental form and as a man-made product; the most common of which being silica and silicone (silica forms naturally but is often artificially produced for purity). Pure silicon is a metalloid (an element that falls between the categories of metal and non-metal) and is widely used in semiconductors for electronics, and solar panels. Silica is oxidised silicon, and owing to its purity and transparency, is frequently used for glass and optical fibres. Silicone, possibly the most familiar silicon product in our day-to-day lives, is the most versatile of the three. It is a synthetic polymer, coming in both solid and liquid forms, and is used for a range of purposes: from kitchen utensils to lubricants. Like many seemingly abundant resources on Earth, we have been taking silicon for granted, and the negative impacts are becoming more and more apparent.
Extraction and Refinement
Silicon is obtained through the mining of sands and gravels with high silicon dioxide content. These sands and gravels are commonly referred to as silica, silica sand, and quartz sand. They are predominately mined by the use of open pit or dredging mining methods. Open pit mining is your traditional above ground quarrying, and dredging involves sucking up sand from waterbeds.
Once the quartz has been excavated, it is transported via lorries or conveyor belts, then sieved, washed, and finally dried. This part of the process is called quartz mining. The second step is to extract the silica sand from the quartz. This process depends on the purity of the quartz, whether it is high quality (>98% silicon), industrial-grade (95% silicon) or low quality (65% silicon). If the quality is industrial-grade or of low quality, it will go through beneficiation processes such as magnetic separation, flotation, and gravity separation to remove impurities. If the quartz is of high purity, there is no need for the beneficiation process, it simply needs to be transported and dried.
Electronics require metallurgical-grade silicon (MG-silicon), which has a purity of >99%. Solar panels have an even higher purity of 99.99999%. MG-silicon is extracted from the silica sand through a process called refining. High-quality quartz enables this step to be reached sooner and easier. Unfortunately, there are few places in the world that have access to quartz deposits of >98% purity, meaning predominately the beneficiation process must be completed. Consequently, the carbon footprint for producing high-quality silica sand from quartz, for MG-silicon production, is more than doubled by going through the beneficiation process. A paper from Elsevier [1], calculates that in the USA emissions for 1kg of silica sand production jump from 0.02kg CO₂e to 0.05kg CO₂e following beneficiation.
The refining process to obtain MG-silicon is largely standardised globally, thus having essentially the same CO₂e footprint from direct emissions. Refining, in principle, involves heating the silica sand and carbon in an electric furnace at temperatures approaching 2200°C, requiring between 1000-1500MJ per 1kg of silicon. The difference in the emissions from refining is dependent on the energy’s fuel source. Renewable energies have the lowest footprint, and coal sourced electricity has the highest. A paper written in the Journal of Sustainable Metallurgy [2], estimates that the CO₂e of refining 1kg of silicon ranges from 5.5-17kg depending on the fuel source.
The shipping of silicon and silicone is also worth considering. Often the products we buy are not wholly manufactured in our own countries, especially in the UK where our silicon products are frequently shipped from Asia. Although we should consider all contributors to a product’s LCA, regarding silicon and silicone, the haulage is relatively insignificant. From mining to refining, silicon can have a footprint as high as 17.05kg CO₂e / kg MG-Si. On top of that, the footprint of manufacturing silicone from silicon can range from 6.12-9.5 kg CO₂e / kg silicone, depending on the desired end product [3]. Whereas shipping 1kg of silicon/silicone from Beijing to Southampton by sea, with a filled large shipping vessel, only emits 0.22kg CO₂e. I have used the most common shipping method as an example, which happens to have the lowest footprint of the 4 most used methods. If air haulage was used however, the footprint would be far higher: 435g CO₂e/ton-km compared to 5g emitted by sea [4].
Day to day products
The silicon products we are most familiar with in our day-to-day lives are in our kitchens. Think silicone spatulas, spoons, and cake moulds. But the silicon that is the most intrinsic in our lives, is probably the silicon semiconductors in microchips. Without these, we wouldn’t have phones, computers, and even modern-day cars.
Since 2020, we have been experiencing a global chip shortage, affecting a wide range of industries with long delays and a sharp rise in the value of second-hand items. Although the shortage was not caused by a lack of silicon (instead by COVID-19, conflict, and severe weather), it highlights a reliance on virgin materials.
We all get annoyed when our particular branded phone packs out after 2 years, and we label it as poor value for money. This phenomenon is called ‘planned obsolescence’. Although infuriating for the consumer, it is actually quite a sustainable practice. It forces you to return your electrical items so their parts can be recycled and reused in other devices. This is an example of good practice that we should carry out with all our electrical items to help prevent resource shortages, such as the global chip shortage. Not only will our resource management be more sustainable, but we will also reduce our carbon footprint through the reduction of mining and refining. This is not a practice that is synonymous with electronics either, we should be trying to do it with all our unneeded items.
Conclusion
It is important to know the impact of the products we buy, and it is not always easy to do so, often due to a lack of data. We can only work with what we have and try to do our best with it. When buying silicon products, look out for where they were manufactured; the country of origin will have different distances to ship and use different energy fuel sources. If possible, opt for electronics that use recycled materials or better yet purchase refurbished second-hand devices. If we do not start to halt our virgin silicon consumption, it will become yet another finite resource on the endangered list, as well as a continued source of unnecessary emissions. Fortunately, silicon has great potential to be circular, we as a planet just need to be quicker at improving our practices.
References
- Seyed M. Heidari and Annick Anctil (2022) Country-specific carbon footprint and cumulative energy demand of metallurgical grade silicon production for silicon photovoltaics. Available at: Country-specific carbon footprint and cumulative energy demand of metallurgical grade silicon production for silicon photovoltaics (uottawa.ca)
- Greenhouse gas emissions from silicon production – development of carbon footprint with changing energy systems (September 2021) php (ssrn.com)
- Silicon-Chemistry Carbon Balance, an assessment of greenhouse gas emissions and reductions (April 2012) [Online] Available: SIL_exec-summary_en.pdf (silicones.eu)
- Shell – Greenhouse Gas Emissions of Shipping [Online] Available: Greenhouse gas emissions in shipping | Shell Global
Written By Rufus Henderson