Ferrosilicon is the highest-volume ferroalloy in steelmaking and one of the least discussed. It is bought in bulk, specified in a line or two, and only becomes interesting when something goes wrong with it. This piece is about what sits behind that line.
What the alloy is actually doing
FeSi does two jobs, and they are not the same job.
In steelmaking it is a deoxidiser. Liquid steel at tap carries dissolved oxygen that would otherwise form blowholes and oxide inclusions as it solidifies. Silicon has a strong affinity for oxygen, so adding FeSi strips it out as silica, which floats into the slag. A secondary role is alloying, where silicon is a specified element in the finished grade rather than a processing aid.
In cast iron it is an inoculant. Added late, in the ladle or the stream, finely sized FeSi provides nucleation sites that control how graphite forms as the iron solidifies. Get it right and you get the mechanical properties the casting was designed for. Get it wrong and you get chill, hard spots and scrap.
The same FeSi 75 can be a commodity deoxidiser or a precision inoculant depending on who is buying. That difference explains most of the price spread you will see quoted for a single grade.
Why sizing and consistency matter more than the headline number
Buyers tend to negotiate on silicon content because it is the number on the certificate. In practice, two other things drive whether a cargo performs:
- Size distribution. A deoxidiser going into a furnace can tolerate a wide spread. An inoculant cannot: too fine and it burns off before it nucleates, too coarse and it does not dissolve in time. The fines fraction is where disputes usually start.
- Trace elements. Aluminium and calcium are not contaminants in every application. In some inoculant grades they are deliberately present and specified. In clean steel grades, aluminium above spec is a rejection.
A cargo that meets the silicon spec and fails on sizing is a cargo you cannot use, and the certificate will not tell you that in advance.
The production side
FeSi is made by carbothermic reduction of quartz in a submerged arc furnace. The inputs are quartzite, a carbon reductant and steel scrap or iron ore for the iron units. The dominant input cost is electricity, at roughly 8,000 to 9,000 kWh per tonne of FeSi 75 depending on furnace efficiency.
That single fact explains most of the industry's geography. Production concentrates where power is cheap, long-term and reliable: hydro-heavy regions, countries with subsidised industrial tariffs, and places with stranded generation. It also explains why FeSi supply responds to power policy and seasonal hydrology rather than to steel demand alone.
| DRIVER | WHAT IT CHANGES | HOW FAST |
|---|---|---|
| Industrial power tariffs | Producer cash cost, furnace run rates | Weeks to months |
| Hydrology in hydro-powered regions | Seasonal output in the wet and dry seasons | Seasonal |
| Quartzite and reductant availability | Input cost and achievable grade | Months |
| Export duties and quotas | Which origins are economic into which market | Immediate on announcement |
Grades in practice
The grade ladder is mostly about how much silicon you are paying to ship.
| GRADE | SI | TYPICAL USE |
|---|---|---|
| FeSi 75 | 72–80% | The workhorse. Steel deoxidation and alloying, and the base for most inoculants |
| FeSi 72 | 72–75% | Where a tighter band is specified |
| FeSi 70 | 70–72% | Regional preference, often a logistics decision |
| FeSi 65 | 65–70% | Less common, specific process fits |
| FeSi 45 | 40–47% | Foundry applications and some specialised steel practice |
Buying a lower grade is rarely cheaper per unit of silicon delivered. You pay to freight iron you do not need. The exception is where furnace practice or a specific casting process genuinely calls for it.
What a buyer should ask for
A usable enquiry contains five things. Most enquiries contain two.
- The grade, with the silicon band you can accept rather than the one you prefer
- The maximum on aluminium, carbon, phosphorus and sulphur, if any of them matter to you
- The size range, and the maximum acceptable fines percentage
- The quantity and whether it is a trial, a spot cargo or a term requirement
- The destination port and your preferred Incoterm
That is enough to go to producers with. Without the size range and the fines limit, any quote you receive is provisional, because the producer is guessing at the hardest part of your specification.