Ferrosilicon Chemical Composition and Grades
Quick Answer: Ferrosilicon is composed mainly of silicon and iron, while elements such as Al, Ca, Mn, Cr, P, S and C may also be controlled in the purchase specification. Grades such as FeSi75, FeSi72 and FeSi65 are primarily distinguished by silicon content, but the grade name alone does not describe the complete chemistry. For actual purchasing, compare the agreed composition limits with the batch COA rather than relying only on the grade number.
What Is the Chemical Composition of Ferrosilicon?
Ferrosilicon, commonly abbreviated as FeSi, is an iron-silicon ferroalloy. Silicon is the principal alloying element, while iron forms most of the remaining composition.
Depending on the grade and downstream process, the specification may also control:
- Al – Aluminum
- Ca – Calcium
- Mn – Manganese
- Cr – Chromium
- P – Phosphorus
- S – Sulfur
- C – Carbon
The importance of each element depends on the process. A standard deoxidation application may tolerate a broader impurity range, while a steel grade with tighter chemistry control may require lower Al, P, S, C or other specified elements.
Chemical Composition of Common Ferrosilicon Grades
Ferrosilicon grades are mainly distinguished by silicon content. The table below gives a commercial reference for several common grades.
| Grade | Si ≥ | Al ≤ | Ca ≤ | Mn ≤ | Cr ≤ | P ≤ | S ≤ | C ≤ |
|---|---|---|---|---|---|---|---|---|
| FeSi75 | 75% | 1.5% | 1.0% | 0.5% | 0.5% | 0.04% | 0.02% | 0.20% |
| FeSi72 | 72% | 2.0% | 1.0% | 0.5% | 0.5% | 0.04% | 0.02% | 0.20% |
| FeSi70 | 70% | 2.0% | 1.0% | 0.6% | 0.5% | 0.04% | 0.02% | 0.20% |
| FeSi65 | 65% | 2.0% | 1.0% | 0.7% | 0.5% | 0.04% | 0.02% | 0.20% |
The values above should be treated as a commercial reference. Exact limits can vary with the ordered grade and contract specification, so the final acceptance criteria should be confirmed before shipment.
For available grades and particle-size options, see our ferrosilicon specifications.
What Do FeSi75, FeSi72 and FeSi65 Mean?
The grade number mainly indicates the silicon-content level of the alloy.
- FeSi75 contains the highest silicon level among these three common grades.
- FeSi72 contains slightly less silicon per tonne.
- FeSi65 contains less silicon and therefore requires a larger alloy addition to supply the same amount of Si.
The grade name should not be interpreted as a guarantee that every FeSi75 batch contains exactly 75.00% silicon. The actual result can vary within the accepted specification.
If your main question is which grade to choose rather than how the chemistry is defined, see our FeSi75, FeSi72 and FeSi65 comparison.
Why Do Ferrosilicon Impurity Limits Matter?
Silicon determines the main alloying value of FeSi, but the smaller elements can determine whether the material is acceptable for a particular melt.
Aluminum and Calcium
Al and Ca may require tighter control when additional aluminum or calcium affects the required steel chemistry, inclusion-control practice or downstream process.
Phosphorus and Sulfur
P and S contribute to the total phosphorus and sulfur balance of the melt. When the final steel specification has limited margin, the contribution from ferroalloy additions should be included in the chemistry calculation.
Carbon
Carbon can become important for grades with a narrow final carbon range. In these cases, the C content of the ferrosilicon should be included together with other charge and alloy additions.
Manganese and Chromium
Mn and Cr may also be controlled when unwanted carry-over from ferrosilicon needs to be limited.
This is why two products sold under the same FeSi75 designation may not be equivalent for every application. The relevant impurity limits should be agreed before price comparison.
Grade Specification vs Batch COA
The grade specification defines the composition limits the product must meet. The COA reports the actual test results of a particular batch.
For example, an FeSi75 specification may require:
- Si ≥75%
- Al ≤1.5%
- P ≤0.04%
- S ≤0.02%
A batch can test comfortably inside all four limits and still be sold under the same FeSi75 grade.
For repeat orders, comparing several recent COAs can also show whether actual Si and impurity values remain reasonably stable between production lots.
Ferrosilicon Grade and Product Form Are Different
Chemical grade and physical form should be specified separately.
FeSi75, FeSi72 and FeSi65 describe chemical grades.
By contrast, the following describe physical form or particle size:
- Ferrosilicon lump
- Ferrosilicon granule
- Ferrosilicon powder
A FeSi75 composition can therefore be supplied in different physical forms without changing its chemical grade.
If your process requires fine material, see our ferrosilicon powder specifications.
FeSiMg should also be treated separately. It contains magnesium and is primarily used for nodularization in ductile iron production rather than being another ordinary FeSi75/72/65 grade.
Ferrosilicon Product Appearance
Visual inspection can help confirm the supplied form, approximate particle size, excessive fines and obvious contamination, but it cannot determine the chemical grade.
The actual Si and impurity values should still be verified from the batch test report rather than judged from color or appearance.
What Should Be Included in a Ferrosilicon Purchase Specification?
A complete purchase specification should define more than simply "FeSi75" or "FeSi72."
| Item | What to Confirm |
|---|---|
| Grade | FeSi75, FeSi72, FeSi65 or another required grade |
| Si content | Required minimum or agreed silicon range |
| Impurity limits | Al, Ca, P, S, C, Mn, Cr or other process-critical elements |
| Particle size | Required lump, granule or powder size range |
| Fines / oversize | Tolerance where size consistency affects charging or handling |
| Batch documentation | COA or other agreed test report linked to the supplied lot |
| Packing | Bag type and net weight required for the shipment |
When several quotations are compared, make sure they use the same chemistry and particle-size requirements. A lower tonne price may not represent the same material if impurity limits or size tolerances are different.
Need to Confirm a Ferrosilicon Grade or Composition?
Send your required FeSi grade, Si range, impurity limits, particle size, quantity and destination port. We can compare the requested specification with available grades and provide recent batch COA data before quotation.
FAQ About Ferrosilicon Chemical Composition
What is ferrosilicon mainly made of?
Ferrosilicon consists mainly of silicon and iron. Commercial specifications may also control Al, Ca, Mn, Cr, P, S and C depending on the grade and downstream process.
What is the difference between FeSi75 and FeSi72?
The primary difference is silicon content. FeSi75 contains more silicon per unit mass than FeSi72, although impurity limits and actual batch chemistry should also be compared before substitution.
Is FeSi75 exactly 75% silicon?
No. FeSi75 is a grade designation rather than a guarantee that every batch contains exactly 75.00% Si. The actual accepted range depends on the purchase specification.
Why can two FeSi75 batches have different compositions?
A grade defines acceptable limits rather than one fixed chemistry. Two batches can both meet the FeSi75 specification while having different actual Si, Al, Ca or other values within those limits.
Which impurities are commonly checked in ferrosilicon?
Common controlled elements include Al, Ca, P, S and C, while Mn and Cr may also be specified. The required limits depend on the final steel or foundry process.
Is ferrosilicon powder a separate grade?
No. Powder mainly describes physical form and particle size. Ferrosilicon powder can be produced from different chemical grades, so composition and size should be specified separately.
Should I check the COA when buying ferrosilicon?
Yes. The grade gives the required limits, while the COA shows the actual chemistry reported for the supplied batch. For repeat purchasing, several recent COAs can also help evaluate consistency.
