Micro silica fume is the dust formed by the oxidation of Si and SiO gas in the process of producing ferrosilicon alloy and metallic silicon in the submerged arc furnace with air and rapidly condensing, also known as silica fume (or agglomerated silica fume). With the strengthening of environmental protection, the output of micro-silica fume is increasing year by year. If it is directly discharged or discarded, it will cause environmental pollution and waste of resources. Therefore, how to recycle these huge amounts of micro-silicon powder has become an urgent solution for ferrosilicon smelting enterprises. The problem.
Micro silica fume is a by-product of ferrosilicon and metallic silicon smelting. With its excellent and magical properties, it is widely used in metallurgical pellets, special concrete, special cement, refractory materials, chemical products and other fields.
1. Characteristics of micro silica fume
The main chemical composition of micro-silica fume is SiO2, in which SiO2 mainly exists in amorphous phase (or amorphous SiO2), the content is ≥80%, the impurities are few, the specific surface area is 20~28㎡/g, and the particle size is less than 10μm. It accounts for more than 80%, has high chemical activity, is easy to react with alkali, and has the characteristics of light weight, high refractoriness and strong activity, and is widely used in construction, refractory materials, metallurgy, ceramics, chemical industry and other fields.
2. Formation of microsilica
3. The harm of micro silica fume
Micro-silica dust is a kind of inhalable particles that are small, light and easy to float. If it is discharged directly, the dust will not easily settle.
Floating in the air, it seriously affects human health and the surrounding environment. Micro-silica dust particles can directly enter the lungs after being inhaled by the human body, which can cause lung cancer and other dust diseases.
4. High value-added utilization of micro-silica fume
Generally speaking, the higher the grade of SiO2 in the microsilica, the higher the added value.
(1) for the concrete industry
Concrete mixed with microsilica has the characteristics of high strength, good adhesion and cohesion, and can increase the molding thickness. In water conservancy and hydropower projects such as long-span bridges and offshore oil drilling platforms, micro-silica fume concrete can improve its impermeability, corrosion resistance and abrasion resistance. In the process of road construction, micro silica fume can greatly improve the early strength and wear resistance of concrete.
Micro silica fume can improve the compressive, tensile, impact strength and wear resistance of concrete. This is because the micro-silica fume does not participate in the curing reaction, which improves the bonding strength and compactness of each component in the concrete, and reduces the harm of Ca(OH)2. Micro silica fume and resin are easy to mix and not easy to agglomerate, which can effectively reduce precipitation and stratification. Micro-silica dust can reduce the expansion and shrinkage of concrete, and has good insulation and arc resistance to concrete. Generally speaking, concrete mixed with silica fume does not chemically react with most acids and alkalis, which enhances the corrosion resistance of concrete.
(2) as cement admixture
Micro-silica fume is used as a blending material to produce special cement. Special cement mixed with micro-silica fume can be made into dense concrete whose strength is 2~3 times that of ordinary concrete. It has good wear resistance, corrosion resistance, impermeability, insulation, frost resistance and resistance to chloride ions. blocking performance, etc.
The strength of cement increases significantly with the addition of micro-silica fume content. Micro-silica fume can effectively improve the compactness and secant elastic modulus of cement, and significantly improve the corrosion resistance and creep of cement. Volume quality will decrease. Therefore, when incorporating microsilica into cement, attention should be paid to the reduction of water absorption and volume quality.
(3) Used in the refractory industry
Micro-silicon dust has excellent properties such as high refractoriness, and is widely used in the refractory industry, mainly for the preparation of high-temperature ceramics, ladle materials, high-temperature wear-resistant materials, breathable bricks, refractory castables, etc.
Micro silica fume can improve the fluidity, bulk density and strength of refractory materials, improve the cohesion and high temperature performance of refractory materials, prolong the service life of refractory materials, and realize internal digestion and clean production of dust in ferrosilicon and industrial silicon smelting enterprises.
(4) Metallurgical pellets
In the metallurgical industry, most companies use microsilica as a return material. Using the mixed pelletizing ore of silica and micro-silica powder as the raw material for reduction of silicon by electric furnace can achieve normal silicon recovery rate and constant energy consumption per unit product. Wetting the micro-silica fume with water and agglomerating it into pellets of about 4cm can be directly reduced and smelted by electric furnace without roasting and drying. The pellets can also be sintered at high temperature, and there are no problems such as bursting during the sintering process, and the product sinter has high strength.
The micro-silica fume and pulp waste liquid are mixed into pellets, and the electric furnace is reduced and smelted. The production proves that because the pellets are stronger than ordinary materials, they are not easy to be broken during transportation. The micro-silica fume slurry recovered by the wet method of silicon smelting in electric furnace was used as the return silicon source, and it was mixed with chrome ore to make pellets.
(5) Preparation of nano-silica (nano-SiO2)
Zhang Deyi et al. used low-grade micro-silica fume as the silicon source, and prepared nano-SiO2 with a grade of more than 97% through the process of “acid leaching purification-HF acid dissolution-hydrolysis-drying”, and its BET specific surface area and average particle size were 210㎡/ g and 90nm. Pei Xinyi et al. added silica fume into quicklime and calcined it to prepare white carbon black by acid leaching, and prepared white carbon black with a purity of 99.9%, an average particle size of 11.15 μm and a whiteness of 96. Zhu Huixian et al. used micro-silica fume and calcium oxide as raw materials, and prepared acidic silica sol with good stability by mixing acid, adding HF for iron removal, high temperature calcination and water dissolving.
However, the research on the preparation of silica by using micro-silica dust mainly focuses on the preparation process, product purity and performance control. However, the produced product is precipitated silica, which has a large performance gap with the silica produced by the gas phase method.
(6) Preparation of metal silicon
BARATI et al. used micro-silica powder with a grade of 97.5% as the silicon source, and prepared silicon metal by acid leaching, low-temperature calcination, magnesium thermal reduction, and acid leaching after reduction. The silicon content in the prepared metal silicon is more than 99%, and the B and P contents are respectively less than 3g/t and 12g/t, and the quality is better than that of metallurgical silicon, and it is a high-quality raw material for preparing solar grade silicon. Therefore, the use of microsilicon powder to prepare silicon metal is an important direction for its high-value utilization.
(7) Preparation of high-performance adsorbents
Micro silica fume has a large specific surface area and developed pores, and has a strong adsorption effect. The adsorption material prepared from micro-silica fume has a good adsorption effect on heavy metals, CO2 and oil and gas. It can be predicted that the adsorption material prepared by micro-silica fume will have a wide range of applications in the treatment of waste water, waste gas and waste liquid. Turning highly polluted dust into an efficient reagent for environmental governance is a major innovation in the high-value utilization of microsilica.
(8) Preparation of gel material
Since micro-silica fume can be calcined at high temperature or alkali-dissolved to prepare water glass, no matter whether micro-silica powder or water glass is used as the silicon source to prepare silicon aerogel, the high value-added utilization of micro-silica powder can be realized. Silicon aerogel prepared from micro-silica fume as a silicon source has high porosity, high strength, low density, good thermal insulation performance, and non-toxic characteristics, and is expected to be widely used in aerospace, construction, medicine and other industries.