Silica fume is a mineral composed of primary particles < 1 µm of amorphous, non-crystalline,  silicon dioxide (SiO2), produced during the manufacture of silicon or ferrosilicon. This tightly controlled electro-metallurgical process involves the carbothermic reduction of quartz in the production of Silicon and FeSi alloys. Silica fume is formed when SiO gas, given off as the quartz is reduced, mixes with oxygen. Here SiO is oxidised to SiO2 condensing into the spherical particles of silica fume, forming the major part of the smoke or fume from the furnace.

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At ambient temperature, silica fume is a fine odourless powder with a putrity range of 85-98 % depending on raw materials and furnace parameters. The colour varies from white to black, depending on the carbon content. In order to save on transport and storage costs and to avoid dust formation in the use of the product, silica fume is often densified.

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What is Silica Fume (Microsilica)?

Silica Fume Introduction

Silica fume (also known as microsilica) is very fine noncrystalline silica by-product resulting from the reduction of high-purity quartz and carbon in electric arc furnaces during the production of elemental silicon or alloys containing silicon.

Silica fume consists of very fine vitreous particles with a surface area ranging from 15 to 30 m2/g, with particle approximately 100 times smaller than the average cement particle. Because of its extreme fineness and high silica content, silica fume is a highly effective pozzolanic material.

Silica fume is used in concrete to improve its properties. It has been found that silica fume greatly improves compressive strength, bond strength, and abrasion resistance; reduces permeability; and therefore helps in protecting reinforcing steel from corrosion.

Silica Fume Production

Raw Materials

• Quartz
• Carbon
  • Coke
  • Coal
  • Wood Chips

Desired Reaction

SiO2 + 2C = Si + 2CO

Smelting Furnace

Temperature 2000℃

off-gas

Baghouse Filter

Silicon Metal

As-Produced Silica Fume

Overview of Facility

Capture as-produced silica fume by bag house

After being collected over the furnace, the silica fume must be transferred, cooled, and physically trapped. The large pipe on the left is bringing the silica fume from the furnaces. The vertical elements are cyclones that are used to remove oversize and other unwanted materials. The large building is the bag house where the fume is captured.

Densification silo

Densification silo for production of densified form of silica fume — The as-produced fume is brought into this silo. Compressed air is used to aerate and tumble the fume particles. Electrostatic charges develop and cause the individual particles to agglomerate. Once the densified silica fume reaches the desired bulk density, it is taken from this silo for packaging or shipping.

Applications

Silica fume can fill the pores between cement particles. It reacts with calcium hydroxide in water to form calcium silicate hydrate (C-S-H) gels, and also with magnesium oxide and water to form magnesium silicate hydrate (M–S–H) gels. For cement-based concrete, mortar, mixing the right amount of silica fume can significantly improve its compressive strength, flexural strength, impact resistance, corrosion resistance, wear resistance; as well as reduce its permeability.

Silica fume products are also used in the refractory industry, of which high-grade undensified silica fume is currently the most used. As an additive of refractory products, silica fume improves their flow ability, strength, durability, and performance under high-temperature conditions.

For Other Applications

· To produce silicon carbide.
· Used for fertilizers, pesticides, etc., as a chemical dispersant to prevent some synthetic powders from caking or enhance effects.
· As a metallurgical pellet binder.

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SiO2 Content Comparison

• Collected from ferrosilicon: SiO2 > 85%.
• Collected from high purity silicon: SiO2 > 92%.
• Collected from silicon metal: SiO2 > 94%.

The difference of SiO2 content results in different performance in concrete, like compressive strength, chloride permeability, etc. So does their application: low-grade (low SiO2 content) microsilica can be used for substructures mix (for example: 40Mpa), but for higher strengths or more durable concrete, high-grade microsilica should be selected.

Densification and Bulk Density

According to the different degrees of densification, silica fume can be divided into undensified (as-produced) silica fume, semi-densified silica fume, densified silica fume, with bulk density around 180 kg/m3, 300-360 kg/m3, 630-650 kg/m3, respectively.

Except for bulk density, there is no difference in specifications among the three types. Densified silica fume is commonly used in construction like ready-mixed concrete, while undensified silica fume is often used in refractory, shotcrete, fertilizer. Because the as-produced silica fume has a lower density, its transport cost is higher.

Color

The color of silica fume ranges from light gray to dark gray. And there is also white silica fume with both SiO2 and ZrO2 content.

Factors that influence the color of silica fume:

• Raw material, reductant: Since SiO2 is colorless, the color is influenced by other content such as carbon and iron oxide. Under normal conditions, silica fume collected from silicon metal, high purity silicon has darker colors than that collected from ferrosilicon.
• Time: Silica fume produced from smelters just start operating has darker colors than from smelters that have operated for more than 2 or even 6 months.
• Bulk density: Typically, the higher the density, the darker the color.
• Temperature in smelters.
• Conditions of smelters.

Reference of colors:

• Densified SF

• Undensified SF

• Zirconia SF

The color of silica fume collected from the same smelter in a day is the same.

Normally, a densification silo contains 24 ton of base material with bulk density of 300kg/m3. And around 10 ton per day silica fume will be added in the silo, evenly mix with the base material to densify. An above-40-minute densification process can ensure that the color of silica fume collected in a day is consistent.

The densification silo discharges also about 10 ton quantity once a day, retaining 24 ton as base material.

Chlorine Content

Silica fume chlorine content from low to high: collected from silicon metal, ferrosilicon and high purity silicon.

• From silicon metal: < 0.02%;
• From ferrosilicon metal and high purity silicon: normally < 0.5%. Calcium chloride is added if chlorine content exceeds 0.5%.

Viscosity

viscosity from high to low: from silicon metal > high purity silicon > ferrosilicon. (due to different reductants)

Factors that influence the color of silica fume:

• Bulk density: The higher the density, the lower the viscosity.
• Content of coal tar: The lower the content, the lower the viscosity.

pH Value

The pH value of silica fume is mainly influenced by the sulfur content in coal tar or petroleum coke. With coke high in sulfur, the pH of silica fume will be low.

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