Ferrosilicon 75 in Magnesium Production: Pidgeon Process

How Does Ferrosilicon 75 Work in Magnesium Production?

Ferrosilicon 75 is used as a reducing agent in the Pidgeon process, an industrial method for producing primary magnesium from dolomite. The process relies on the ability of silicon to remove oxygen from magnesium oxide under high-temperature vacuum conditions.

Magnesium oxide is chemically stable, so extracting metallic magnesium requires controlled reduction conditions. In the Pidgeon process, ferrosilicon 75 provides silicon for the reduction reaction, while calcined dolomite supplies magnesium oxide and calcium oxide.

Main Steps of the Pidgeon Process

1. Dolomite calcination: Natural dolomite is heated to decompose its carbonates, producing a calcined material containing CaO and MgO.
2. Raw material preparation: Calcined dolomite and ferrosilicon are crushed or ground to the required particle size and mixed. Some process formulations also include a controlled amount of fluorite.
3. Briquetting: The prepared mixture is pressed into briquettes to provide suitable material contact and handling characteristics.
4. Vacuum reduction: Briquettes are loaded into externally heated retorts. Reduction typically takes place at temperatures around 1,150–1,250°C under reduced pressure, depending on the equipment and process.
5. Magnesium condensation: Magnesium vapor leaves the reaction zone and condenses in the cooler section of the retort, forming crude magnesium deposits commonly called magnesium crowns.

Chemical Reaction of Ferrosilicon 75 in Magnesium Reduction

The overall reduction reaction can be represented as:

In this reaction, silicon removes oxygen from magnesium oxide, producing magnesium vapor. Calcium oxide participates in the formation of calcium silicate, which helps make the silicothermic reduction thermodynamically feasible under the process conditions.

Silicon is the primary reducing component of ferrosilicon 75. Iron does not directly reduce MgO in the overall reaction and largely remains in the solid residue after magnesium vapor is released.

The reaction is endothermic, requiring a continuous external heat supply during reduction. Vacuum conditions also help remove magnesium vapor from the reaction zone, supporting continued magnesium production.

Why Use Ferrosilicon 75 in the Pidgeon Process?

Ferrosilicon 75 is widely used because it combines a high silicon content with established industrial supply and a practical cost structure. Its suitability should be evaluated against alternative reducing agents and lower-silicon ferrosilicon grades, rather than assuming that the highest available silicon content always gives the best economic result.

1. High Silicon Content Reduces Alloy Demand

Silicon is the component consumed by the reduction reaction. Compared with FeSi72 or FeSi65, ferrosilicon 75 provides more silicon per tonne of alloy when grades are compared at their nominal silicon contents.

For the same theoretical silicon requirement, a higher-silicon alloy therefore requires less total alloy mass. This can reduce the amount of iron and other non-reducing material entering the briquetting and reduction system.

However, silicon content alone does not determine magnesium yield. The actual result also depends on briquette preparation, reduction temperature, vacuum conditions, raw material composition and reaction efficiency.

2. Practical Balance Between Silicon Content and Cost

Pure silicon metal contains more silicon than ferrosilicon 75, but its higher silicon concentration does not automatically make it more economical for every magnesium plant.

The purchase price, silicon utilization, processing requirements and magnesium recovery must be considered together. Ferrosilicon 75 offers an established industrial option for plants designed around ferrosilicon-based reduction.

Published magnesium metallurgy research also indicates that increasing the silicon content beyond approximately 75% does not necessarily produce a proportional increase in magnesium output. The benefit depends on the reduction conditions and material properties.

3. Cost per Kilogram of Silicon

When comparing ferrosilicon 75 with other ferrosilicon grades, the purchase price per tonne of alloy is only the starting point. A useful first calculation is the cost of the silicon contained in the material.

This formula compares the cost of contained silicon, not the cost of silicon that actually participates in reduction. For a production-level comparison, buyers should also evaluate processing costs, silicon utilization and magnesium recovery.

Ferrosilicon 75 Consumption per Tonne of Magnesium

The amount of ferrosilicon consumed per tonne of magnesium is an important factor in magnesium production costs. However, theoretical consumption and actual industrial consumption are different measurements and should not be used interchangeably.

Theoretical Ferrosilicon 75 Consumption

According to the overall Pidgeon-process reaction, one mole of silicon is required to produce two moles of magnesium.

Using the atomic masses of silicon (28.085) and magnesium (24.305), the theoretical silicon requirement is:

For ferrosilicon 75 containing exactly 75% silicon:

This is a stoichiometric minimum based on ideal conversion. It excludes incomplete reduction, unreacted silicon, handling losses and losses during magnesium recovery or subsequent processing.

Theoretical Consumption: Ferrosilicon 75 vs FeSi72

The following table compares the theoretical alloy requirements for producing one tonne of magnesium at different nominal silicon contents.

Note: Values are calculated from stoichiometry and nominal silicon content. They are not recommended operating consumption rates. Actual batch chemistry and process performance must be considered. Lower-silicon grades are included for mathematical comparison, not as guaranteed substitutes in a Pidgeon-process plant.

For product-level comparisons, see our ferrosilicon 75 lumps and FeSi72 specifications.

Actual Ferrosilicon 75 Consumption in Magnesium Production

In practical magnesium production, ferrosilicon consumption is higher than the theoretical minimum because not all silicon contributes to recovered magnesium.

A figure of approximately 1.2 tonnes of ferrosilicon 75 per tonne of magnesium appears in historical descriptions of the Pidgeon process, including a 2012 patent application comparing conventional production with an alternative silicothermic method.

However, 1.2 t/t Mg should not be treated as a universal operating standard. Actual consumption varies with raw material quality, reduction efficiency, silicon utilization, magnesium recovery and the basis used to report production.

For a reliable comparison, always identify whether consumption is measured per tonne of crude magnesium crown or finished magnesium ingot, and whether it covers a single reduction charge or a longer production period.

What Affects Ferrosilicon Consumption?

These factors influence the actual ferrosilicon requirement per tonne of recovered magnesium. They do not change the ideal stoichiometric ratio of one silicon atom for every two magnesium atoms in the overall reaction.

Ferrosilicon 75 Specifications for Magnesium Production

When purchasing ferrosilicon 75 for magnesium production, the grade name alone is not enough to define an acceptable shipment. Silicon content, impurity limits, particle size and batch consistency should be agreed before ordering.

A commercial ferrosilicon 75 designation may cover different chemistry options depending on the applicable standard and supplier specification. Our published product information includes different silicon and aluminium limits for specific subgrades. Buyers should therefore confirm the exact contractual silicon range and the corresponding batch Certificate of Analysis (COA).

Impurity requirements should be based on the magnesium plant's actual production process and final product specifications. The presence of an element in ferrosilicon does not necessarily mean the same amount will enter the condensed magnesium, because impurity transfer depends on the reduction and condensation conditions.

For more information about chemical limits, inspection and COA requirements, read our ferrosilicon 75 specification guide.

Ferrosilicon Particle Size for Magnesium Production

Particle size selection should distinguish between the delivered ferrosilicon size and the prepared feed size used in reduction. These are not necessarily the same.

Ferrosilicon 75 Lump Size for Magnesium Plants

Ferrosilicon may be delivered as lumps or crushed material. Common commercial lump sizes include 10–50 mm and 10–100 mm, but the appropriate size depends on the plant's milling equipment and feeding requirements.

For plants that grind ferrosilicon before mixing and briquetting, the delivered lump size mainly affects mill compatibility, throughput, handling and material losses.

Ferrosilicon 75 lumps for industrial raw material preparation.

Prepared Ferrosilicon Size for Briquetting

In the Pidgeon process, ferrosilicon is typically ground before being mixed with calcined dolomite. Fine and sufficiently uniform particles help distribute the reducing agent through the briquette and provide contact between reactants.

However, smaller particles are not automatically better. Excessive grinding can increase energy consumption, dust generation and handling difficulties. Fine material may also be more susceptible to oxidation under unsuitable storage or processing conditions.

The target powder size should therefore be defined by the plant's established grinding, mixing and briquetting process rather than by a single universal particle-size recommendation.

Should Magnesium Plants Buy Lumps or Powder?

Plants with suitable milling equipment may prefer ferrosilicon 75 lumps because they can control the final particle-size distribution during preparation. Purchasing pre-crushed or powdered ferrosilicon may reduce on-site grinding, but it introduces different requirements for dust control, storage, packaging and feeding.

When requesting a quotation, specify both the incoming material size and the prepared feed size required by your process.

For further guidance, see our ferrosilicon particle size guide.

How to Evaluate Ferrosilicon 75 for Magnesium Smelting

A supplier comparison should consider more than the quoted price per tonne. The objective is to determine whether the material meets your specifications and supports consistent magnesium production at an acceptable total cost.

Step 1: Verify the Chemical Composition

Request a COA for the actual ferrosilicon 75 batch. Confirm silicon content and all specified impurity limits against your purchase requirements. Where necessary, compare the supplier's results with independently sampled material.

Step 2: Confirm Particle Size and Handling

Check whether the delivered material is compatible with your crushing, grinding and feeding equipment. Review the size distribution, fines content, packaging and handling requirements.

Step 3: Conduct Comparable Reduction Trials

When testing a new supplier or grade, keep key process conditions as consistent as practical. Record the calcined dolomite composition, charge ratio, grinding conditions, briquette preparation, reduction temperature, vacuum level and reduction time.

Measure the resulting magnesium production and actual ferrosilicon consumption using the same reporting basis as your existing process.

Step 4: Compare Cost per Tonne of Magnesium

The lowest ferrosilicon 75 purchase price does not necessarily provide the lowest magnesium production cost. Compare the quantity of alloy consumed, processing and handling costs, magnesium recovery and material-related losses.

Use the trial results to establish whether the new material provides a measurable advantage over your current approved ferrosilicon supply. A single successful reduction charge is not sufficient evidence of consistent performance.

Ferrosilicon 75 for Magnesium Production: FAQs