FeSi 75 and FeSi 70 differ by about five percentage points of silicon. That sounds like a detail, and on a single heat it is. Across a year's consumption it is a real number — and the way most buyers compare the two grades gets it backwards.
What separates them
| FeSi 75 | FeSi 70 | |
|---|---|---|
| Silicon | 74–80% | 70–75% |
| Iron | balance, roughly 20–25% | balance, roughly 25–30% |
| Aluminium | Typically 1.5–2.5% max, specified to requirement | |
| Carbon | Typically 0.1–0.2% max | |
| Density | Both close to 3.0 g/cm³ — both float on liquid steel | |
Metallurgically they behave the same way. Silicon is silicon: it deoxidises identically whichever grade carries it. There is no quality difference in the sense buyers sometimes assume — FeSi 70 is not a lower quality product, it is a different silicon-to-iron ratio.
What differs is how many tonnes you handle to deliver the same silicon.
The comparison that matters
Compare delivered cost per unit of silicon, not price per tonne of alloy. The calculation is simple and it is the one that decides the question.
Take a worked example. Suppose FeSi 75 is quoted at 1,000 currency units per tonne delivered, at 76% silicon, and FeSi 70 at 940 per tonne, at 72% silicon.
- FeSi 75: 1,000 ÷ 0.76 = 1,316 per tonne of silicon
- FeSi 70: 940 ÷ 0.72 = 1,306 per tonne of silicon
On those numbers FeSi 70 is marginally cheaper per silicon unit, despite the lower headline grade. Move the FeSi 70 price to 960 and the answer flips. This is why the comparison has to be run on each quotation rather than settled once.
Then adjust for what the tonnage costs you
Cost per silicon unit is the starting point, not the finish. To deliver the same silicon, FeSi 70 requires roughly 5–6% more material by weight. That extra tonnage carries real costs:
- Freight, which for imported material is usually the largest of these and is charged by weight
- Handling and storage — more bags, more movements, more space
- Charging time, which matters where the tapping window is tight
- Duty, where it is assessed on weight or value
For material moving a long distance, freight alone often closes a small gap in the per-silicon-unit price. For material bought locally, it may not.
When each grade wins
FeSi 75 tends to win when…
- Freight is a significant share of delivered cost — long sea routes, inland destinations, or both
- Your tapping window is short and charging time is a genuine constraint
- Storage or handling capacity is limited
- You are adding large amounts of silicon, so the tonnage difference compounds
- You want fewer bags to sample, weigh and track
FeSi 70 tends to win when…
- The delivered price per silicon unit is genuinely lower and freight is a small share of the total
- Supply of FeSi 75 is tight and FeSi 70 is more readily available at a workable price
- You want a slightly finer degree of control on small additions, where the lower concentration makes a modest weighing error less consequential
Practical points if you switch
Substituting one for the other is straightforward, but three things need attention.
Recalculate the addition weight. Obvious, and still the most common error. Moving from 76% to 72% silicon means roughly 5.5% more alloy for the same silicon. Update the standing recipe, not just the first heat.
Re-check your sizing specification. Sizing is set independently of grade, but a new supplier often means a new size distribution. Recovery is more sensitive to sizing and fines than to the difference between these two grades, so confirm the fraction and the fines limit rather than assuming they carry over. Our guide to ferro silicon as a deoxidiser covers what actually drives recovery.
Watch the iron balance on tight grades. FeSi 70 brings proportionally more iron. On the great majority of steels this is irrelevant — you are adding iron to iron. On a few tightly balanced alloy grades it is worth a check.
A note on the trace elements
Neither grade designation says anything about aluminium, calcium, carbon or phosphorus, and those are frequently what actually matters. Aluminium in particular varies between producers and affects both inclusion chemistry and, in foundry work, nucleation behaviour.
If you are comparing quotations, compare the full specification. Two suppliers both offering "FeSi 75" can be offering materially different products, and the difference will not show up in the grade name. Our specification checklist covers what a complete purchase order should state.
In short
There is no universally better grade. FeSi 75 reduces the tonnage you move and handle for a given silicon input, which usually wins where freight is significant. FeSi 70 can be the cheaper source of silicon when the market puts it there.
Run the cost-per-silicon-unit calculation on every quotation, add your freight and handling cost per tonne, and let that decide. Do not let the larger number in the grade name decide it for you.
We manufacture ferro silicon at Durgapur, West Bengal in both grades. Send us your requirement with tonnage and destination and we will quote both so you can compare properly.
Frequently asked questions
Is FeSi 75 always better value than FeSi 70?
No. FeSi 75 delivers more silicon per tonne handled, which cuts freight and handling per silicon unit, but the delivered price per silicon unit is what decides it and that moves with the market. Run the calculation on each quotation rather than assuming.
Can I substitute one grade for the other without changing anything else?
Not quite. The addition weight has to be recalculated for the different silicon content, and the iron balance shifts slightly. For most deoxidation work the substitution is straightforward once the weight is corrected.