Silicon metal and ferrosilicon (FeSi) are produced industrially by carbothermic reduction of silica in submerged-arc furnaces. The most important point for a raw-material buyer is straightforward: a material with a high SiO₂ content is not automatically a good metallurgical furnace feed.
In these processes, the silica source is commonly supplied as lump quartz or quartzite, not as generic fine sand. Chemistry remains essential, but lump size, generation of fines, thermal strength, softening behaviour, reducibility and lot consistency can also affect furnace performance.
1. Silica sand, quartz or quartzite: clarify the physical form first
The commercial term “silica sand” covers very different materials. For glass, filtration or mortars, a fine or medium granular product can be perfectly suitable. A silicon-metal or ferrosilicon furnace operates under different constraints.
The USGS states that silica as quartz or quartzite is used to produce silicon ferroalloys and silicon metal. SINTEF technical work and metallurgical literature likewise describe quartz/quartzite charges combined with carbon reductants in submerged-arc furnaces.
This distinction prevents a common purchasing mistake: comparing only the SiO₂ percentage of fine sand with that of lump metallurgical quartz.
For Si/FeSi applications, the question is not only “how much SiO₂?”, but also “in what form, at what size and with what furnace behaviour?”.
2. Why fines and sizing matter in the furnace
A submerged-arc furnace must allow solids and gases to move through the burden. A material that breaks down significantly during handling or heating can generate additional fines.
SINTEF guidance on silicon raw materials identifies size and strength of quartz/quartzite as process requirements. Excessive fines can affect gas flow through the charge. Undersized material may therefore be screened out before furnace feeding, depending on the producer’s process.
The literature reports lump-size ranges used industrially, but these should not be converted into a universal specification. The correct distribution depends on furnace design, carbon reductants, operating strategy and preparation equipment.
Useful supplier data therefore include:
- nominal lump size or particle-size distribution;
- fines fraction before shipment;
- change in fines after handling;
- mechanical strength;
- behaviour after heating when required by the customer.
3. Chemical purity: high SiO₂, but especially a controlled impurity profile
Silicon metal and ferrosilicon do not have identical requirements. In ferrosilicon production, an iron source is intentionally added to the charge. For silicon metal, controlling impurities introduced by quartz, reductants, electrodes and refractories becomes especially important.
Elements monitored may include Al, Fe, Ca, Ti, B and P, depending on the product and process. Universal limits should not be published for each of them because specifications change with metal grade and the producer’s refining capability.
Work by Dal Martello and co-authors also shows that impurities do not all behave in the same way during reduction. Some can transfer toward silicon, others toward slag or the gas phase. Initial quartz chemistry alone therefore cannot perfectly predict final metal composition.
4. Thermal behaviour can distinguish two chemically similar quartz sources
Two lots with similar chemistry can behave differently during heating.
Studies of metallurgical quartz commonly examine:
- thermal strength and fines generation;
- softening and melting behaviour;
- mineralogical changes during heating;
- reducibility;
- microstructure and inclusions;
- interaction with the other charge materials.
A study published in Metals notes that quartz and quartzite sources with similar chemical composition are not necessarily equivalent for FeSi or silicon production. Physical and metallurgical properties should therefore complement the chemical certificate when the application is demanding.
5. Why slag and mineral impurities remain a process issue
Si and FeSi production has sometimes been described as nearly “slag free”. More recent work nevertheless shows that slag phases can accumulate inside the furnace.
A 2025 paper in the International Journal of Minerals, Metallurgy and Materials reports formation of CaO–SiO₂–Al₂O₃ slag systems and notes that slag accumulation can interfere with the flow of material and gases. Quartz impurities contribute to the composition of these phases.
This does not mean that one simple CaO or Al₂O₃ limit can predict furnace performance. The purchasing message is instead that mineralogy and impurity profiles must be interpreted in the context of the actual process.
6. What should a certificate of analysis contain?
For initial supplier qualification, a certificate of analysis should be linked to an identifiable lot and state the test methods used.
Depending on the requirement, the dossier may include:
| Control | Why it matters |
|---|---|
| SiO₂ | Confirms the main silica content |
| Fe, Al, Ca, Ti and other impurities | Helps assess compatibility with the product and process |
| B and P when critical | Important elements for some silicon grades |
| Particle size / lump size | Affects burden preparation and permeability |
| Fines | Indicator of handling behaviour and material strength |
| Moisture | Affects delivered mass and burden preparation |
| Lot traceability | Links analysis, delivery and industrial trial |
For major elements, XRF can be suitable in many contexts. For some trace elements, more sensitive methods such as ICP-OES or ICP-MS may be required. The analytical method should match the decision level required, not simply the instrument available.
7. Supplier qualification: do not move directly from a lab sample to an annual contract
A cautious qualification route is to proceed in stages:
- review the technical data sheet and lot certificate;
- confirm chemistry with an appropriate method;
- inspect form, sizing and fines;
- evaluate mechanical or thermal strength if required by the producer;
- run a representative industrial trial;
- compare several lots before accepting a long-term supply specification.
This matters because one chemically excellent sample does not by itself guarantee stable long-term production.
8. Silicon metal and ferrosilicon: same foundation, different requirements
Both processes rely on reduction of SiO₂ with carbon in a submerged-arc furnace, but ferrosilicon also includes an iron source.
Silicon metal generally requires cleaner silica feed because the final product contains far less iron than FeSi. That still does not justify copying a generic “high-purity” specification: the purchasing specification should be tied to the metal grade, burden blend and producer’s refining route.
A useful commercial document is therefore not a promise such as “99.x% SiO₂ = silicon grade”. It is a complete raw-material specification covering chemistry, sizing, physical behaviour, test methods and consistency criteria.
9. What a buyer should send to the supplier
To make a technical enquiry efficient, specify:
- application: silicon metal or ferrosilicon grade;
- target chemical composition;
- limits for critical impurities;
- required form: lumps, crushed material or other preparation;
- particle-size range or lump size;
- fines limit when applicable;
- maximum moisture if contractual;
- lot quantity and annual volume;
- expected sampling and test methods;
- delivery destination.
The more precise the specification, the lower the risk that a supplier proposes a material that is “rich in silica” but unsuitable for the furnace.
Conclusion
For silicon metal and ferrosilicon, qualifying a silica feed requires much more than checking SiO₂ percentage. Lump quartz/quartzite sizing, mechanical and thermal strength, impurity profile and consistency between lots should be considered together.
A fine silica sand can be excellent for another industry while being unsuitable for a Si/FeSi furnace. Conversely, chemically pure quartz may still require behaviour testing before industrial qualification.