How Did Silica Fume, Once A Waste, Become A Valuable Asset For Multiple Industries?

Silica fume, also known as condensed silica fume, is a type of dust formed by the rapid condensation following the oxidation of Si and SiO gases in flue gas. These gases are generated during the production of ferrosilicon alloys and metallic silicon in submerged arc furnaces. In recent years, with increasingly stringent environmental protection regulations, the output of silica fume has increased year by year. Direct discharge or stockpiling of silica fume can lead to environmental pollution and resource waste. Therefore, the effective utilization of such large quantities of silica fume has become an urgent issue for ferrosilicon smelting enterprises.

The main chemical component of silica fume is silicon dioxide (SiO₂), which primarily exists in an amorphous state, with a content of no less than 80% and low levels of impurities. It has a specific surface area of 20–28 m²/g, and more than 80% of its particles are smaller than 10 μm. Due to its high chemical activity, it readily reacts with alkalis. It also possesses advantages such as low density, high refractoriness, and strong reactivity. Accordingly, it is widely used in construction, refractory materials, metallurgy, ceramics, the chemical industry, and other fields.

This paper analyzes the physical and chemical properties of silica fume, a by-product of ferrosilicon smelting. It also explores the current status of its comprehensive recovery and utilization from the perspectives of generation, collection, and application, and proposes directions for the high-value utilization of silica fume.

01. Analysis of the Physicochemical Properties of Silica Fume

The chemical composition of the silica fume sample is shown in Table 1. As shown in Table 1, silica fume contains a large amount of SiO₂, along with small quantities of K₂O, Na₂O, free carbon, MgO, PbO, Al₂O₃, ZnO, CaO, iron oxides, and other components.

Due to differences in the raw materials used in submerged arc furnace production, the chemical composition of the by-product silica fume varies accordingly. Generally, the SiO₂ content in silica fume ranges from 80% to 96%. Silica fume appears off-white, gray, or dark gray in color, and its color is mainly related to the content of carbon and iron oxides.

Silica fume exhibits a uniform particle distribution, with particle sizes ranging from 0.75 to 11.8 μm and a narrow size distribution. Its median particle size is about 4 μm. It is mainly composed of spherical particles of different sizes, with obvious particle agglomeration. The X-ray diffraction pattern of silica fume shows typical broad diffuse peaks characteristic of a glassy state, indicating that silicon dioxide in silica fume exists in an amorphous form. This is attributed to the rapid condensation and formation process of silica fume, during which SiO₂ does not have sufficient time to form a crystalline structure.

2. Analysis of the Generation Process and Current Recovery Status of Silica Fume

2.1 Generation Process of Silica Fume

The formation process of silica fume during metallic silicon smelting is shown in Figure 1. Inside the electric furnace at 1700 ℃, silica rock or quartz is reduced and smelted with coke. SiO₂ is reduced to SiO and elemental silicon. During the reaction process, the molten silicon produced flows out from the bottom of the furnace, while unreduced SiO and part of the silicon escape in the form of vapor through the material layer. These gases are oxidized by air at the furnace mouth to form SiO₂ dust, which is discharged along with the flue gas.

The condensation of silica fume occurs at an extremely fast rate. Under such conditions, SiO₂ cannot crystallize sufficiently and ultimately forms amorphous SiO₂ dust. Meanwhile, some impurities contained in the silica rock also enter the flue dust during this process.

2.2 Analysis of the Current Recovery Status of Silica Fume

Silica fume dust is a type of fine, light, and easily airborne respirable particulate matter. Direct discharge can cause the dust to remain suspended in the air for a long time due to its poor settling performance, which seriously endangers human health and the surrounding ecological environment. Once inhaled, fine silica fume particles can directly enter the lungs and may induce pneumoconiosis, including lung cancer. Therefore, countries around the world have attached great importance to the recovery and resource utilization of silica fume dust.

Due to its fine particle size, low bulk density, poor moisture absorption, tendency to agglomerate, and difficulty in settling, silica fume is not suitable for wet dust removal and purification. The particle size and specific resistance of silica fume flue gas particles are generally 0.1–10 μm and 50 Ω·cm, respectively. However, electrostatic precipitation is only applicable to flue dust with a specific resistance below 20 Ω·cm and a particle size ranging from 1 to 200 μm. Therefore, the electrostatic dust removal process is also not suitable for silica fume recovery.

At present, dry dust removal technology is widely adopted for silica fume recovery, as illustrated in Figure 2. The quality of the finished silica fume is closely related to the dust removal efficiency of the equipment, which is influenced by factors such as wind pressure, air volume, carbon powder content, and flue gas temperature. The main bottlenecks restricting silica fume recovery at this stage include thermal expansion of high-temperature flue gas pipelines, difficulty in dust compaction, and delayed temperature control in the treatment system.

In general, the higher the SiO₂ content in silica fume, the greater its added value. The grade of silica fume is influenced by raw materials, production processes, equipment, process parameters, and operating conditions. For instance, during the smelting of ferrosilicon and industrial silicon, appropriately reducing the ash content of coke used as the reducing agent can lower the contents of Al₂O₃ and CaO in flue gas silica fume, thereby increasing the SiO₂ content.

3. Research Progress on the Application of Silica Fume

As early as the 1940s, Elkem in Norway conducted systematic research on the recovery, production, and comprehensive application of silica fume and has long remained a leader in this field. Since then, research institutions both domestically and internationally have explored the application of silica fume in the concrete industry, cement, metallurgy, chemical engineering, ceramics, composite materials, and other sectors.

3.1 Application in the Concrete Industry

Concrete mixed with silica fume exhibits high strength, excellent adhesion, and good cohesion, and allows for increased casting thickness. In hydraulic and hydropower projects such as long-span bridges and offshore oil drilling platforms, silica fume-blended concrete can improve impermeability, corrosion resistance, and abrasion resistance. In road construction, silica fume can significantly enhance the early strength and wear resistance of concrete.

Studies have been conducted on the influence of silica fume on the properties of fast-hardening polymer-modified concrete. The results show that concrete strength increases with the addition of silica fume, reaching a maximum at a silica fume dosage of 4%. When 10% silica fume is incorporated into inorganic geopolymer concrete, the tensile strength, compressive strength, and flexural strength increase by 12.8%, 6.9%, and 11.5%, respectively.

Silica fume can improve the compressive strength, tensile strength, impact resistance, and wear resistance of concrete. Since silica fume does not participate in the hydration reaction, it enhances the bonding strength and compactness among the various components of concrete and reduces the adverse effects of calcium hydroxide (Ca(OH)₂).

Silica fume mixes easily with resin and is not prone to agglomeration, which effectively reduces sedimentation and delamination. It can lower the expansion and shrinkage rate of concrete and impart favorable insulating properties and arc resistance to concrete. In general, silica fume-blended concrete is chemically resistant to most acids and alkalis, significantly enhancing its corrosion resistance.

3.2 Use as a Cement Blending Material

In some foreign countries, silica fume is used as a mineral admixture in cement production. Canada stipulates that cement blended with silica fume shall contain more than 85% silica and have a loss on ignition below 6%. In addition, countries such as the former Soviet Union, the United States, and Japan also use silica fume as a blending material for producing special cements.

Special cement incorporating silica fume can be used to prepare dense concrete with 2 to 3 times the strength of ordinary concrete. It exhibits excellent wear resistance, corrosion resistance, impermeability, electrical insulation, frost resistance, and superior chloride ion penetration resistance.

A study was conducted on the engineering performance of peat block ground using cement–silica fume precast slabs. The finite element method was applied to analyze the stress distribution of the precast slabs and peat ground. The results show that such precast slabs can significantly improve the strength and bearing capacity of soft peat ground.

When silica fume and polypropylene fibers are added together into cement, the composite incorporation can effectively improve the cement's strength, crack resistance, compactness, and durability, while reducing its water absorption and bulk density.

The strength of cement increases significantly with increasing silica fume dosage. Silica fume can effectively enhance the compactness and secant elastic modulus of cement and greatly improve its corrosion resistance and creep properties. However, it may also lead to a decrease in water absorption and bulk density. Therefore, attention should be paid to reducing water absorption and bulk density when incorporating silica fume into cement.

3.3 Application in the Refractory Industry

Due to its excellent properties, such as high refractoriness, silica fume is widely used in the refractory industry. It is mainly applied in the manufacture of high-temperature ceramics, ladle lining materials, high-temperature wear-resistant materials, purging bricks, refractory castables, and other products.

Under conditions with an Al2O3 to SiO2 molar ratio of 3:2.5 and a sintering temperature of 1450 ℃, high-purity porous mullite ceramics were prepared from silica fume via pressureless sintering combined with gel-casting technology. The compressive strength reaches 260.93 MPa, with a porosity of 21%.

It has been shown that silica fume can improve the fluidity, bulk density, and mechanical strength of refractories, optimize their setting behavior and high-temperature properties, and extend their service life. Furthermore, it enables the utilization of flue dust and supports cleaner production in ferrosilicon and industrial silicon smelting enterprises.

3.4 Application in Metallurgical Pelletizing

Some North American enterprises mix silica rock with silica fume for pelletizing and use the pellets as raw materials for silicon smelting via electric furnace reduction. Practical results show that the silicon recovery rate remains normal and the energy consumption per unit product remains unchanged.

Elkem in Norway wets silica fume with water and agglomerates it into pellets of about 4 cm in diameter. Without roasting or drying, these pellets can be directly fed into electric furnaces for reduction smelting. The pellets can also be high-temperature sintered without cracking or other defects during the sintering process, and the resulting sintered material exhibits high strength.

Russian enterprises mix silica fume with pulp waste liquid to prepare pellets for electric furnace reduction smelting. Production practice has verified that these pellets have higher strength than ordinary raw materials and are not easily broken during transportation.

A Nordic ferrochrome enterprise uses slurry of silica fume recovered from wet dust removal in electric furnace silicon smelting as a recycled silicon source and mixes it with chromium ore for pelletizing. Production results indicate that silica fume can significantly improve the bonding performance of chromium ore pellets.

In the metallurgical industry, most enterprises use silica fume only as recycled feed material. Although this method reduces environmental pollution caused by silica fume, it does not fully exploit its excellent properties and is considered a low-value utilization approach. In metallurgical applications, greater attention should be paid to the high-value utilization of silica fume.

3.5 Applications in the Chemical Industry, Composite Materials, and Other Fields

3.5.1 Preparation of Nano-Silica (Nano-SiO₂)

At present, the market price of silica fume in China ranges from 1,200 to 1,500 yuan per ton, while the price of white carbon black ranges from 3,500 to 35,000 yuan per ton. The efficient and low-cost conversion of silica fume into white carbon black has therefore become an important research focus.

Research on the preparation of white carbon black from silica fume mainly focuses on production processes, product purity, and performance control. However, the products obtained are precipitated silica, whose properties differ significantly from those of fumed silica. There are relatively few studies on the efficient conversion of silica fume into soluble silicon, as well as on the preparation of high-value-added food-grade white carbon black (fumed silica) using silica fume as a raw material.

3.5.2 Preparation of Metallic Silicon

Using silica fume with a purity of 97.5% as the silicon source, metallic silicon can be prepared through a series of processes, including acid leaching, low-temperature calcination, magnesiothermic reduction, and post-reduction acid leaching.

The silicon content of the prepared metallic silicon exceeds 99%, while the contents of boron (B) and phosphorus (P) are less than 3 g/t and 12 g/t, respectively. Its quality is superior to that of conventional metallurgical silicon, making it a high-quality raw material for the production of solar-grade silicon.

Therefore, the preparation of metallic silicon from silica fume represents an important development direction for the high-value utilization of silica fume.

3.5.3 Preparation of High-Performance Adsorbents

Silica fume has a large specific surface area and a well-developed pore structure, which gives it strong adsorption capacity. Water glass can be prepared by dissolving silica fume, and hydrophobic silica gel synthesized via the sol-gel method can be applied to oil and gas recovery and adsorption processes.

Adsorbent materials derived from silica fume exhibit excellent adsorption performance for heavy metals, carbon dioxide, oil and gas, and other pollutants. Therefore, it is foreseeable that such adsorbents will have broad application prospects in the treatment of wastewater, waste gas, and waste liquids.

Converting highly polluting dust into high-efficiency environmental treatment agents represents a major innovation in the high-value utilization of silica fume.

3.5.4 Application in Gel Materials

Silica aerogel with a narrow particle size distribution and high purity was synthesized via the acid addition–sol–gel method after mixing silica fume with calcium oxide and calcining. Silica aerogel can also be prepared using the acid addition–sol–gel process with water glass as the silicon source. By incorporating asbestos fibers into water glass and applying carbonization and drying treatments, the resulting silica aerogel exhibits excellent strength and thermal insulation performance.

Since water glass can be produced from silica fume through high-temperature calcination or alkali dissolution, the preparation of silica aerogel using either silica fume or water glass as the silicon source enables the high-value utilization of silica fume.

Silica aerogel derived from silica fume features high porosity, high strength, low density, excellent thermal insulation properties, and non-toxicity. It is expected to be widely applied in the aerospace, construction, pharmaceutical, and other industries.

04. Conclusions and Prospects

In recent years, with the rapid development of the ferrosilicon industry and increasingly stringent environmental regulations, the amount of smelting flue dust generated from ferrosilicon production has increased year by year. Silica fume is widely used in traditional and relatively low-value applications such as special cement, special concrete, refractory materials, and metallurgical pellets. However, research on the high-value-added comprehensive utilization of silica fume remains insufficient. Most white carbon black prepared from silica fume is precipitated silica with relatively low added value, and its overall performance still needs further improvement.

Metallic silicon produced from silica fume exhibits higher quality than ordinary metallurgical silicon, making it a high-quality raw material for the production of solar-grade silicon. Adsorbents synthesized from silica fume show excellent adsorption capacity for heavy metals, carbon dioxide, oil and gas, and other pollutants, demonstrating broad application prospects in environmental remediation. Silica aerogel prepared using silica fume as the silicon source offers superior overall performance, particularly its non-toxicity, which gives it greater practical application value.

Future utilization of silica fume should focus on resource recycling, comprehensive utilization, and high-value upgrading. Deep processing of silica fume should be strengthened to optimize its physicochemical properties, thereby expanding its application scope to high-end fields such as electronics, aerospace, pharmaceuticals, colored tires, toothpaste, rubber, and other industries.