Smelting Differences Between FeSi45 and FeSi75

Quick Answer: FeSi45 and FeSi75 require different charge compositions and reduction conditions in submerged arc furnace (SAF) production. Compared with FeSi45, FeSi75 requires substantially more quartzite and carbon reductant per tonne of alloy, while the required iron input is lower. This changes the furnace's material balance, heat demand, silicon monoxide (SiO) circulation and reduction-zone conditions. Typical closed-furnace reference data show electricity consumption of approximately 4.8 MWh/t for FeSi45 and 8.8 MWh/t for FeSi75Al1. These are industry reference figures, not guaranteed consumption values for every furnace. Actual performance depends on raw material quality, furnace design, charge resistivity, electrode operation and silicon recovery.

For producers, the main difference is not simply the final silicon percentage. The target grade determines how much silica must be reduced, how much iron enters the charge and how the carbon balance must be controlled. Understanding these relationships helps operators evaluate raw material consumption, furnace stability, power demand and finished alloy chemistry when switching between FeSi45 and FeSi75.

The silicon content of the finished alloy directly affects the proportions of quartzite, iron-bearing materials and carbon reductants required for production. FeSi45 contains substantially more iron relative to silicon than FeSi75, while FeSi75 requires a larger silica reduction load for each tonne of finished alloy.

The following comparison combines typical closed-furnace material consumption data with the main metallurgical differences between the two grades.

Data reference: Typical materials and energy demand for smelting ferrosilicon in closed electric furnaces, reported per tonne of alloy. The high-silicon reference is FeSi75Al1, not every commercial FeSi75 designation. Values are indicative industry data and should not be treated as ZhenAn production records or guaranteed operating specifications.

Three differences are particularly important:

• Raw material balance: FeSi75 requires substantially more quartzite and coke, while FeSi45 requires more iron-bearing material.
• Energy demand: The higher silicon target in FeSi75 requires more extensive silica reduction and a greater energy input per tonne in the reference data.
• Silicon recovery: Iron content, SiO circulation and furnace conditions influence the amount of silicon retained in the final alloy.

These figures describe a typical reference furnace balance. Actual consumption varies with quartzite composition, reductant quality, iron source, furnace capacity and operating practice.

For available commercial grades and product specifications, see our Ferrosilicon Product Range.

Raw material consumption provides a more useful way to compare FeSi45 and FeSi75 smelting than looking at the finished silicon percentage alone.

In a submerged arc furnace, quartzite supplies silicon, carbonaceous materials provide the reducing agent, and iron-bearing materials supply the iron needed to form the alloy. The proportions must be adjusted to the target composition and the actual chemical analysis of each raw material.

Why Does FeSi75 Require More Quartzite?

The answer is the higher silicon content of the finished alloy. Producing one tonne of FeSi75 requires more silicon to be recovered from silica than producing one tonne of FeSi45.

Quartzite is primarily composed of silicon dioxide (SiO₂). During smelting, the silica is reduced by carbon to produce silicon and carbon monoxide. A higher silicon target therefore requires a greater silica input and a correspondingly larger carbon requirement.

The actual quartzite consumption is also affected by:

• SiO₂ content and mineral impurities in the quartzite.
• Silicon recovery and SiO losses in furnace off-gas.
• Quartz particle size and thermal stability.
• The amount of silica retained in intermediate and slag phases.

Why Does FeSi45 Require More Iron?

FeSi45 contains a lower proportion of silicon than FeSi75, so a larger proportion of the finished alloy must come from iron.

Steel scrap, iron chips and other suitable iron-bearing materials can provide this iron. Their chemical composition matters because unwanted chromium, phosphorus, sulphur and other residual elements can enter the finished alloy.

Iron is not merely a diluent. Its presence also affects silicon activity and the equilibrium of SiO-related reactions during smelting. This metallurgical effect is one reason iron content is important when evaluating silicon recovery.

Why Does FeSi75 Require More Coke?

Carbon is consumed during silica reduction. As the silicon target increases, the furnace must reduce more SiO₂, which increases the theoretical carbon requirement.

However, the coke charge cannot be determined from the silica quantity alone. Operators must also consider:

• Fixed carbon content of the reductant.
• Carbon consumed in intermediate reactions.
• Carbon lost through gas reactions and other furnace losses.
• Ash and volatile matter in the reductant.
• The carbon balance required to maintain the desired furnace condition.

Adding excess carbon does not automatically increase silicon recovery. An unsuitable carbon balance can change SiC formation, furnace resistance, gas generation and burden permeability.

FeSi45 and FeSi75 are produced through carbothermic reduction in submerged arc furnaces. Although the overall chemical reaction is often written as a single equation, industrial smelting involves several intermediate reactions between silica, carbon, silicon monoxide and silicon carbide.

1. Overall Reduction Reaction

The simplified overall reaction is:

SiO₂ + 2C → Si + 2CO

Silicon dioxide reacts with carbon at high temperature to produce silicon and carbon monoxide. The reaction is strongly endothermic, so electrical energy is required to maintain the high-temperature reduction zone.

This equation describes the overall material balance, but it does not fully represent the actual reaction pathway inside the furnace.

2. Silicon Monoxide Formation

Silicon monoxide (SiO) is an important intermediate in ferrosilicon smelting. It forms in the high-temperature region and moves through the burden with furnace gas.

A representative reaction involving silica and silicon carbide is:

2SiO₂ + SiC → 3SiO(g) + CO(g)

SiO can participate in further reactions as it moves through different temperature regions. Depending on local conditions, it may be recovered through reactions with carbon or silicon carbide, or it may escape with furnace off-gas.

3. Silicon Carbide Formation

Silicon carbide is an important intermediate product in the furnace. One representative formation reaction is:

SiO(g) + 2C(s) → SiC(s) + CO(g)

Silicon carbide can subsequently react with silicon monoxide to produce liquid silicon:

SiC(s) + SiO(g) → 2Si(l) + CO(g)

These reactions explain why carbon availability, SiO circulation and temperature distribution must be considered together. Silicon carbide is not simply an unwanted by-product; it participates in the reaction sequence that produces the alloy.

4. Why Reaction Conditions Matter for Different Grades

FeSi75 requires a higher silicon concentration in the finished alloy. This changes the equilibrium conditions and the amount of silica that must pass through the reduction process.

FeSi45 has a higher iron proportion in the final alloy. The iron-bearing liquid phase influences silicon activity and can help reduce SiO losses.

Consequently, the two grades cannot be compared only by the temperature of the furnace or the amount of coke charged. The complete reaction environment, including temperature, gas composition, carbon balance and iron content, determines the actual production result.

Iron plays two roles in ferrosilicon production: it supplies the iron component of the finished alloy and influences the thermodynamic conditions under which silicon is recovered.

When reduced silicon enters an iron-bearing liquid phase, it forms an Fe-Si solution. The presence of iron lowers the activity of silicon compared with pure liquid silicon.

This changes the equilibrium conditions of silica reduction and the partial pressure of SiO required for the reaction. Under suitable conditions, SiO losses through the upper furnace can therefore be reduced.

Why FeSi45 Can Have a Higher Silicon Yield

Typical closed-furnace reference data report a silicon yield of approximately 98–99% for FeSi45, compared with approximately 91–93% for FeSi75Al1.

The higher iron proportion in FeSi45 contributes to the different silicon activity and SiO equilibrium. However, these reported yields should not be interpreted as a fixed result of grade selection alone.

Actual silicon recovery also depends on:

• Silica quality and actual SiO₂ input.
• Furnace temperature and reaction-zone conditions.
• Carbon balance and reductant reactivity.
• SiO gas circulation and recovery in the burden.
• Furnace sealing, gas flow and operating stability.

For production management, silicon yield should be calculated from measured silicon input and the silicon recovered in the finished alloy, using a consistent material-balance boundary.

The electrical behavior of the furnace burden is an important factor in submerged arc furnace operation. Current passes through the burden and the high-temperature reaction region, while the electrodes supply electrical energy to maintain the reduction process.

The resistance of the charge depends on more than the final silicon grade. It is influenced by the physical and electrical properties of the raw materials, their particle-size distribution, temperature, porosity and contact between particles.

What Determines Charge Electrical Resistance?

The following factors should be evaluated when comparing FeSi45 and FeSi75 charge designs:

A more iron-rich FeSi45 charge may have different electrical characteristics from a high-silicon FeSi75 charge. However, the direction and magnitude of the change must be evaluated for the actual raw material combination and furnace configuration.

Electrode Position During FeSi45 and FeSi75 Production

Electrode position affects the location and distribution of electrical energy inside the furnace. It also influences the current path and the concentration of heat in the reaction region.

It is not technically correct to assume that FeSi45 always requires shallow electrode insertion and FeSi75 always requires deep insertion. The effective working position depends on furnace design, charge resistance, electrode diameter, electrical load and burden condition.

When changing the production grade, operators should monitor:

• Electrode movement and slipping behavior.
• Current and voltage stability.
• Electrode consumption and paste behavior.
• Burden level and surface condition.
• Temperature distribution and tapping behavior.

The operating target should be established from the furnace's own electrical and production data rather than from a universal FeSi45-to-FeSi75 conversion rule.

Stable gas movement through the furnace burden is essential for continuous ferrosilicon production. Carbon monoxide and silicon monoxide participate in the reduction reactions, while the upward movement of furnace gas also transfers heat through the charge.

If the burden becomes too compact or contains excessive fines, gas permeability may deteriorate. This can affect heat distribution, SiO recovery and the stability of furnace operation.

Why Particle Size Distribution Matters

Quartzite, coke and other carbon reductants must be prepared to particle sizes appropriate for the furnace design. A suitable size distribution helps maintain the required void structure and provides consistent contact between the raw materials.

Excessive fines can fill the spaces between larger particles and restrict gas movement. On the other hand, an unsuitable proportion of oversized particles can create irregular packing and inconsistent reaction contact.

Particle size should therefore be controlled as part of the complete charge design, not treated as an isolated purchasing specification.

Feeding Differences Between FeSi45 and FeSi75

The feed composition must change when the target grade changes. FeSi75 requires a larger silica and carbon input per tonne of alloy in the reference charge balance, while FeSi45 requires a higher iron input.

Operators should therefore adjust the batch proportions according to the calculated material balance and the actual consumption of the furnace.

Feeding frequency and batch size should be determined by the furnace's charging system, burden settlement, gas flow and operating response. Neither grade has a universal feeding frequency that applies to every plant.

Furnace Poking and Burden Loosening

Furnace poking and burden loosening are operational measures used to address specific furnace conditions. They should not be applied at a fixed frequency simply because the furnace is producing FeSi45 or FeSi75.

Before taking corrective action, operators should evaluate:

• Uneven burden settlement or surface buildup.
• Changes in gas flow or furnace pressure.
• Electrode movement and electrical instability.
• Localized sintering or excessive fines accumulation.
• Changes in tapping behavior and metal production.

Any corrective operation should follow the plant's established furnace safety and operating procedures.

Two furnaces producing the same ferrosilicon grade can have different raw material consumption and energy requirements because their quartzite and reductant properties are not necessarily the same.

Quartzite Quality

Quartzite is the primary silicon-bearing raw material. Its SiO₂ content determines the theoretical silicon input, while mineral impurities influence the chemical balance of the charge and the finished alloy.

When evaluating quartzite, producers should consider:

• SiO₂ content: Determines the amount of available silicon in the raw material.
• Al₂O₃: Contributes aluminium-bearing impurities to the furnace system.
• CaO and MgO: Influence the composition of oxide phases and slag.
• Fe₂O₃: Contributes iron-bearing material and must be included in the charge balance.
• Particle size and strength: Influence burden packing, gas flow and resistance to thermal breakdown.

Carbon Reductant Quality

Coke, coal, charcoal and other carbonaceous materials can be used in ferrosilicon furnace charges. The selection and blending of reductants affect carbon supply, charge resistivity, gas permeability and reaction kinetics.

A reductant with high fixed carbon is not automatically the best choice. Reactivity, electrical resistance, ash content and mechanical strength must be evaluated together.

For FeSi45 and FeSi75 production, the carbon blend should be selected according to the required carbon balance and furnace operating characteristics.

Energy consumption is one of the main differences between the two grades. The higher silicon content of FeSi75 requires a larger silica reduction load and a greater amount of carbon reductant per tonne of finished alloy.

In the typical closed-furnace reference data presented earlier, the reported electricity consumption is:

The difference is calculated from the reported reference values: (8.8 - 4.8) / 4.8 × 100 ≈ 83.3%.

This comparison illustrates the difference in typical energy demand per tonne of alloy. It does not establish that every FeSi75 furnace consumes exactly 83% more electricity than every FeSi45 furnace.

Why Does FeSi75 Require More Electrical Energy?

Several factors contribute to its higher reference energy consumption:

• Higher silica reduction load: More silicon must be recovered from quartzite per tonne of finished alloy.
• Greater carbon consumption: More reducing carbon is required for the larger silica input.
• High-temperature reactions: Silicon recovery involves endothermic reactions and a high-temperature reaction zone.
• SiO circulation and losses: The amount of silicon monoxide recovered or lost affects overall energy and material efficiency.

Does FeSi45 Always Produce More Alloy per Hour?

Not necessarily. A lower reference energy consumption per tonne does not automatically mean that a furnace will produce more alloy per hour.

Hourly output depends on the installed transformer capacity, actual electrical load, furnace geometry, charging system, tapping cycle, raw material quality and the operating availability of the furnace.

For a meaningful production comparison, use the same reporting period and measure:

• Electricity consumption per tonne of saleable alloy.
• Daily or monthly finished alloy output.
• Raw material consumption per tonne.
• Silicon recovery rate.
• Off-spec production and material losses.

These indicators provide a more reliable comparison of actual furnace efficiency than grade names alone.

Stable furnace operation must ultimately produce an alloy that meets the agreed chemical specification. The main composition challenge differs according to the target silicon grade and the raw materials used.

Silicon Content Control

For FeSi45, the iron-to-silica ratio and carbon balance must be controlled to maintain the required silicon range. Excessive iron input or insufficient silica reduction can lower the finished silicon content.

For FeSi75, a larger proportion of the charge must undergo silica reduction. Inadequate reducing conditions, unsuitable carbon supply or increased SiO losses can prevent the finished alloy from reaching its target silicon content.

Impurity Control

Aluminium, calcium, phosphorus, sulphur and other elements can enter the alloy through quartzite, carbon reductants and iron-bearing raw materials.

The main control measures include:

• Check the chemical analysis of each major raw material.
• Control the proportion of different iron sources.
• Monitor ash and impurity inputs from carbon reductants.
• Maintain a consistent batch composition.
• Verify the finished alloy using representative sampling and chemical analysis.

Do not assume that changing from FeSi45 to FeSi75 automatically produces a particular aluminium, phosphorus or calcium content. The final chemistry depends on the actual charge materials, reduction conditions and grade specification.

Finished Product Verification

After tapping, the alloy should be sampled and tested against the agreed product specification. The inspection should confirm silicon content and the relevant impurity limits before the batch is accepted.

For commercial deliveries, the batch Certificate of Analysis (COA) should correspond to the actual material supplied.

Switching between FeSi45 and FeSi75 requires more than changing the silicon target in the production plan. The complete charge and furnace operating conditions must be reviewed.

The following checklist identifies the main items that should be reassessed before a grade change.

The exact adjustments should be determined from the furnace's material balance, electrical response and operating records. A fixed conversion formula between FeSi45 and FeSi75 cannot replace a plant-specific charge calculation.

Technical References

• Theoretical Investigation of the Impact of Impurities in Recycled Silicon Used for the Production of Ferrosilicon – Includes typical raw material and energy demand for closed-furnace ferrosilicon production.
• Ferrosilicon – ScienceDirect Topics – Reference data on charge materials, energy consumption, silicon yield and Fe-Si reduction behavior.
• Changes in Quartz During Heating and the Possible Effects on Si Production – Discusses quartz transformations and their implications for silicon and ferrosilicon production.
• Ferrosilicon Smelting – INFACON XII – Discusses furnace reaction zones, SiO and SiC formation, and the mechanisms of silicon recovery.