Ferrosilicon 72 and 75 for Steelmaking: Grades and Deoxidation Practice

Quick Answer: FeSi72 vs FeSi75 for Steelmaking

Ferrosilicon 72 (FeSi72) and Ferrosilicon 75 (FeSi75) are silicon-based alloy additions used in steelmaking for deoxidation and silicon adjustment. The main difference is their silicon content: FeSi75 contains 75% to less than 80% Si under the PG FeSi75Al1.5 designation of GB/T 2272-2020, while FeSi72 contains 72% to less than 75% Si under the PG FeSi72Al2.0 designation.

For the same mass of alloy, FeSi75 supplies more silicon than FeSi72 when compared at equivalent recovery rates. However, FeSi75 is not automatically the better choice for every steel grade. The appropriate grade depends on the target silicon content, initial dissolved oxygen, silicon recovery, aluminium and other impurity limits, addition method and delivered alloy cost.

In steelmaking, both grades can be used for precipitation deoxidation, while appropriately sized ferrosilicon material can also be used in certain slag-reduction practices. The key purchasing decision is to match the actual chemical specification and particle size to the steelmaking process rather than selecting a grade by its silicon percentage alone.

What Do FeSi72 and FeSi75 Grade Numbers Mean?

The numbers 72 and 75 refer to the nominal minimum silicon content of the respective commercial grades. A higher silicon grade supplies more silicon per tonne of alloy, which can reduce the mass of material needed to deliver a specified silicon input, depending on actual silicon recovery.

However, the grade number does not describe the complete chemical composition. Aluminium, calcium, manganese, chromium, phosphorus, sulphur and carbon limits can differ according to the specific grade designation and applicable standard.

Under GB/T 2272-2020, ordinary ferrosilicon includes several chemical-composition designations. For example, PG FeSi75Al1.5 and PG FeSi72Al2.0 have different aluminium limits and other specified impurity limits. Buyers should therefore confirm the complete designation instead of treating all FeSi75 or FeSi72 products as chemically identical.

We supply different ferrosilicon grades according to the agreed chemical specification and product requirements. See our Ferrosilicon 75 product information and Ferrosilicon 72 product information for the corresponding product details.

Chemical Composition of FeSi72 and FeSi75

The following table compares two representative ordinary ferrosilicon designations listed in Table 2 of GB/T 2272-2020. It is important to retain the complete grade suffix because the aluminium designation affects the permitted chemical composition.

Standard reference: GB/T 2272-2020, Table 2, ordinary ferrosilicon grades. The chemical limits above refer specifically to PG FeSi75Al1.5 and PG FeSi72Al2.0, not every available FeSi75 or FeSi72 designation.

A blank or unspecified element in the standard's corresponding grade row must not be interpreted as zero content. If your steelmaking process requires a maximum limit for an element that is not separately specified, include that requirement in the purchase specification and obtain the supplier's confirmation.

Other designations, including PG FeSi75Al2.0, PG FeSi75Al2.5, PG FeSi72Al1.5 and PG FeSi72Al2.5, are also covered by the standard. The buyer should select the designation according to the required silicon content and impurity limits.

Why Aluminium Content Matters in Ferrosilicon

Aluminium is an important purchasing consideration when selecting ferrosilicon for steelmaking. Although aluminium is itself a strong deoxidizing element, uncontrolled aluminium input can affect the inclusion chemistry and final steel composition.

When a steel grade has a strict residual aluminium limit, the buyer should calculate the potential aluminium contribution from the ferrosilicon addition. The relevant figure is the actual aluminium content reported on the batch COA, not simply the maximum value printed in the grade designation.

For example, PG FeSi75Al1.5 permits a maximum aluminium content of 1.5%, while PG FeSi72Al2.0 permits up to 2.0%. These are specification ceilings, not typical measured values for every batch.

Before placing an order, compare the actual Si, Al, Ca, P and S results with the steel grade's chemical requirements. Where additional residual-element restrictions apply, agree on them separately with the supplier.

How Does Ferrosilicon Deoxidize Molten Steel?

During steelmaking, oxygen remains dissolved in molten steel after oxidation reactions in the furnace. If it is not controlled, dissolved oxygen can react with carbon during solidification and contribute to gas evolution, porosity and other quality problems.

Ferrosilicon supplies silicon, which has a strong affinity for oxygen. Once the alloy dissolves in molten steel, dissolved silicon reacts with dissolved oxygen to form silica.

[Si] + 2[O] = (SiO₂)

In this metallurgical notation:

• [Si] represents silicon dissolved in molten steel.
• [O] represents oxygen dissolved in molten steel.
• (SiO₂) represents the silica-containing oxide product.

The reaction is the chemical basis of silicon deoxidation. However, forming silica does not mean that all oxide inclusions have already been removed from the steel. The subsequent behavior of the inclusions is also important.

From Silicon Addition to Inclusion Removal

The practical sequence of ferrosilicon deoxidation can be understood in five stages:

1. Alloy addition: Sized FeSi is introduced into molten steel according to the shop's addition procedure.
2. Heating and dissolution: The alloy heats up and dissolves, supplying silicon to the steel bath.
3. Oxygen reaction: Dissolved silicon reacts with dissolved oxygen to form silica-containing products.
4. Inclusion movement: Oxide particles may collide, aggregate and move toward the steel-slag interface.
5. Slag absorption: Suitable slag conditions allow inclusions to be absorbed and removed from the steel bath.

The final cleanliness of the steel depends on more than the amount of silicon added. Inclusion size, bath circulation, slag composition, residence time and secondary refining conditions all influence the removal process.

For this reason, steelmakers should evaluate FeSi performance using actual silicon recovery and final steel quality rather than assuming that a higher silicon grade automatically produces cleaner steel.

Precipitation vs. Diffusion Deoxidation

Ferrosilicon can participate in different deoxidation practices depending on where the material is added and how the oxygen is controlled. Precipitation deoxidation and diffusion deoxidation should not be treated as the same operation.

1. Precipitation Deoxidation

Precipitation deoxidation takes place when ferrosilicon is added directly to molten steel. After dissolution, silicon reacts with dissolved oxygen and produces silica-containing inclusions.

In a typical ladle operation, the addition point, particle size and stirring conditions influence how quickly the alloy dissolves and how the resulting inclusions behave.

The main process considerations include:

• Silicon input: The actual Si content determines the theoretical silicon supplied by each tonne of alloy.
• Recovery rate: Oxidation, slag interaction and material losses influence the amount of silicon retained in the steel.
• Addition position: The location and timing of the addition influence dissolution and mixing.
• Particle size: The size distribution affects heating, dissolution and feeding behavior.
• Inclusion removal: Slag conditions and steel circulation affect the removal of deoxidation products.

FeSi72 and FeSi75 can both be selected for precipitation deoxidation. The decision depends on the required silicon input, actual recovery, residual-element limits and procurement cost.

2. Diffusion Deoxidation

Diffusion deoxidation focuses on reducing the oxidizing potential of the slag rather than relying solely on direct precipitation of oxide products inside the steel bath.

In certain ladle refining practices, a silicon-containing reductant, including appropriately specified ferrosilicon material, is added to the slag. Silicon can react with iron oxide in the slag, as represented by the simplified reaction:

2(FeO) + [Si] = 2[Fe] + (SiO₂)

Here, (FeO) represents iron oxide in the slag, while the reaction produces metallic iron and silica. By lowering the FeO content under suitable conditions, the slag becomes less oxidizing.

This changes the oxygen equilibrium between the steel and slag. When the chemical potential and mass-transfer conditions are favorable, oxygen can move from the steel into the slag, where it is subsequently controlled by the refining process.

The effectiveness of diffusion deoxidation depends on:

• Initial FeO and MnO levels in the slag.
• Slag basicity and chemical composition.
• Temperature and treatment time.
• Reducing-agent composition and particle size.
• Slag-steel contact and mass-transfer conditions.

Fine ferrosilicon may be used in some slag-reduction practices, but its use and addition rate must follow the specific steel plant's process procedure. Powder addition also requires suitable dust and handling controls.

Precipitation and Diffusion Deoxidation: Practical Comparison

These two methods are not interchangeable in every operation. The appropriate practice depends on the refining route, target steel cleanliness, slag design and plant operating procedure.

When Should Steelmakers Choose FeSi72 or FeSi75?

FeSi72 and FeSi75 should be compared according to the actual steelmaking requirement rather than the assumption that a higher grade is always superior.

FeSi75 supplies a higher silicon fraction per tonne of alloy. FeSi72 may offer a lower purchase price per tonne, but the actual economic advantage depends on its delivered price, chemical composition and silicon recovery.

When FeSi75 Is the More Suitable Option

FeSi75 is worth considering when the steelmaking process requires a higher silicon input per tonne of alloy. Its higher silicon content can reduce the total alloy mass needed to supply a given amount of silicon, provided that the actual recovery rate is comparable.

For example, if a steel plant has limited alloy feeding capacity, using a higher-silicon grade may reduce the mass of material that must be charged to deliver the target silicon input.

However, the actual choice must still account for aluminium, phosphorus, sulphur and other residual-element requirements. A high silicon percentage alone does not establish suitability for a particular steel grade.

When FeSi72 Is the More Suitable Option

FeSi72 may be more economical when its delivered price is sufficiently lower and the lower silicon content still meets the steelmaking requirement.

For plants with flexible silicon adjustment and no special requirement for a higher silicon grade, FeSi72 can be a practical choice. The purchasing decision should compare the cost of recovered silicon rather than only the price per tonne of alloy.

Where a steel grade has strict impurity limits, a lower-aluminium or specially controlled FeSi72 designation may also be considered if its complete chemistry meets the agreed specification.

Silicon Recovery and Cost per Unit of Silicon

Comparing FeSi72 and FeSi75 only by their quoted prices per tonne can produce a misleading purchasing decision. The more useful comparison is the delivered cost of the silicon that is actually recovered in the steel.

1. Theoretical Silicon Input

Using the minimum silicon content of each representative grade, one tonne of alloy supplies the following theoretical silicon quantities:

This comparison uses the minimum grade values. Actual silicon input should be calculated using the silicon content reported on the batch COA.

2. Calculate the Cost per Tonne of Recovered Silicon

Use the following formula to compare the economic value of different ferrosilicon offers:

Cost per tonne of recovered Si = Delivered alloy price ÷ (Actual Si fraction × Si recovery rate)

Where:

• Delivered alloy price: The purchase cost per metric ton, including the relevant freight and delivery costs.
• Actual Si fraction: The silicon content shown on the batch COA, expressed as a decimal.
• Si recovery rate: The proportion of the silicon input retained in the steel under the relevant operating conditions.

For a fair comparison, use the same plant-specific recovery basis where appropriate, or use measured recovery data for each grade if the operating results differ.

Do not assume a universal recovery rate for FeSi72 or FeSi75. Temperature, addition position, particle size, slag conditions, stirring and process losses all affect actual recovery.

For a more detailed calculation of the required alloy mass, see our Ferrosilicon 75 Addition Rate Calculation guide.

What Determines Ferrosilicon Addition in the Ladle?

The amount of FeSi required for a ladle addition cannot be determined from the grade number alone. The calculation must account for the initial steel composition, target silicon content and actual silicon recovery.

The main variables are:

For a preliminary mass-balance estimate, the required FeSi addition can be expressed as:

FeSi addition = Steel mass × Required Si increase ÷ (Actual FeSi Si fraction × Recovery rate)

When using this formula, express steel mass in kilograms and silicon contents as mass fractions. The result is an initial estimate, not a substitute for a plant-specific addition model. Actual calculations should account for the process route, other silicon inputs, material losses and the final steel mass balance.

Ferrosilicon Particle Size for Different Deoxidation Methods

Particle size influences how ferrosilicon is fed, heated and dissolved. A grade with suitable chemistry may still be unsuitable if its particle size does not match the plant's addition equipment.

For direct ladle additions, larger lumps or screened granules may be selected according to the charging system. Fine material can be used in specific slag-treatment or injection practices, but excessive fines may increase dust generation and material losses.

The listed sizes are purchasing examples, not universal operating requirements. Buyers should confirm nominal size, undersize tolerance, oversize tolerance and fines content before ordering.

Read our Ferrosilicon Particle Size for Steelmaking guide for a more detailed comparison of particle sizes and feeding requirements.

Quality Control and Batch Verification

For steelmaking applications, chemical composition and physical size should be verified separately. A grade designation indicates the applicable composition limits, but the actual batch results determine whether the delivered material meets the purchase specification.

1. Chemical Composition Verification

Request a batch-specific Certificate of Analysis (COA) showing the actual chemical results. The elements to check depend on the selected grade and the requirements of the steel being produced.

• Silicon (Si): Confirm the actual silicon content.
• Aluminium (Al): Check the maximum permitted contribution to the steel.
• Calcium (Ca): Verify the agreed limit where relevant.
• Phosphorus (P) and sulphur (S): Check against the steelmaking specification.
• Manganese (Mn), chromium (Cr) and carbon (C): Confirm applicable residual-element limits.

2. Particle Size Inspection

For screened ferrosilicon, the inspection report should identify the agreed nominal size range and the percentage of undersize and oversize material. This is particularly important for automated feeding systems and applications where consistent dissolution behavior is required.

3. Sampling and Independent Inspection

The sampling method, inspection frequency and acceptance criteria should be agreed before shipment. If independent verification is required, buyers can arrange third-party inspection to check chemical composition, particle size, packaging and shipping condition before dispatch.

4. Documentation Checklist

For more details, read our Ferrosilicon 75 COA and Certificate of Analysis guide.

What Information Is Needed for a Ferrosilicon Quotation?

To obtain a quotation that matches your steelmaking process, provide the required grade, chemical composition, particle size and order details. This allows the supplier to evaluate the correct product specification rather than quote only against a general FeSi72 or FeSi75 designation.

Frequently Asked Questions

Q: What is the main difference between FeSi72 and FeSi75?
A: The main difference is their silicon content. Under GB/T 2272-2020, PG FeSi75Al1.5 contains 75.0% to less than 80.0% Si, while PG FeSi72Al2.0 contains 72.0% to less than 75.0% Si. Their permitted aluminium and other impurity limits also differ.

Q: Which ferrosilicon grade is better for steelmaking deoxidation?
A: Neither grade is universally better. FeSi75 supplies more theoretical silicon per tonne of alloy, while FeSi72 may offer a lower purchase price. The choice depends on target silicon, recovery rate, impurity limits, addition method and delivered cost.

Q: Does FeSi75 remove more oxygen than FeSi72?
A: At equivalent mass and silicon recovery, FeSi75 supplies more silicon and therefore has greater theoretical silicon deoxidation capacity per tonne of alloy. Actual oxygen removal also depends on initial dissolved oxygen, reaction conditions, slag interaction and silicon recovery.

Q: What is precipitation deoxidation with ferrosilicon?
A: It is a process in which ferrosilicon dissolves in molten steel and supplies silicon. The dissolved silicon reacts with dissolved oxygen to form silica-containing oxide inclusions, which must subsequently be removed through appropriate refining and slag practices.

Q: How does diffusion deoxidation using ferrosilicon powder work?
A: In certain slag-treatment practices, fine ferrosilicon acts as a reducing agent and reacts with iron oxide in the slag. Reducing slag oxidation can support oxygen transfer from the steel to the slag. The result depends on slag chemistry, temperature and treatment conditions.

Q: Why is aluminium content important in ferrosilicon?
A: Aluminium contributes to the residual chemistry of the steel and can influence oxide inclusion composition. When aluminium is tightly controlled, select a grade with an appropriate maximum Al limit and verify the actual batch result using the COA.

Q: How does silicon recovery affect FeSi72 and FeSi75 consumption?
A: Lower silicon recovery means more alloy is required to deliver the same amount of silicon to the steel. The calculation should use the actual silicon content of the alloy and recovery data from the relevant plant process.

Q: What particle size is suitable for ladle deoxidation?
A: The appropriate size depends on the ladle addition equipment, addition position, feeding rate and operating procedure. Lumps and screened granules are common commercial forms, but the final size range should be agreed with the steel plant.

Q: Can low-aluminium ferrosilicon be used for clean steel production?
A: Yes, if its complete chemical composition meets the steelmaking specification. Low aluminium content can help control aluminium input, but final cleanliness also depends on deoxidation practice, inclusion modification, slag control and refining conditions.

Q: Can ferrosilicon be used as a desulphurizing agent?
A: Ferrosilicon is primarily used for deoxidation and silicon alloying. It should not be treated as a general-purpose desulphurizing agent. Sulphur removal requires suitable slag chemistry and a dedicated desulphurization practice.

Q: Is ordinary ferrosilicon the same as a ductile iron nodularizer?
A: No. Ordinary ferrosilicon can be used as an inoculant in cast iron production, while ductile iron nodularization generally requires a magnesium-bearing treatment alloy. These materials perform different metallurgical functions.

Get FeSi72 and FeSi75 for Steelmaking

We supply ferrosilicon grades for steelmaking and related metallurgical applications. When requesting a quotation, specify your target silicon content, aluminium and other impurity limits, particle size, quantity and destination port.

Our team can coordinate product specifications, batch COA requirements, particle size options and export packaging according to your order requirements.

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