一、 Introduction

Owing to the protection of the environment ,which began in the 1970s, many countries have required silicon and silicon ferroalloy electric furnace flue dust discharge. Thus, increasing recovery of silica fume down, it is necessary for people to find ways to use the silica fume.

Resource development and utilization of silica fume research began in the 1950s, and in late sixties and early seventies,it has been applied in engineering. Currently in Northern Europe, Western Europe, North America, Japan, Australia, South Africa, Russia and other regions and countries, silica fume has been widely used in cement, concrete, fireproof material injection, rubber, chemical industry, and has developed a number of new product performance.

China's utilization of silica fume to carry out the work despite a late start, but progress is rapid. Such as high-strength concrete, silica fume shotcrete, composite to enhance the density agent, silica fume mortars, low cement materials and other research have been pouring through the ministerial or provincial, municipal technical appraisal, and has been in many engineering applications, technical and economic effect is remarkable. Some application technology is close to or reach the level of the early eighties abroad.

Second, the physical and chemical properties of silica fume
　　Silica fume in the furnace with silica fume in the production volume of more than 75% ferrosilicon and silicon, the resulting large number of highly volatile gases and of SiO2 and Si oxidation and condensation from the air quickly, and generally produce 10-20 % of the amount of silica fume.
1, the chemical composition
　　It's a high SiO2 content, up to 85-98% (see Table 1). Furnace gas also carries a small amount of impurities increased, such as the free C, Fe2O3, CaO, K2O, Na2O and so on. Silica fume color with C, Fe2O3 content increased and the color from white, gray to gray, dark gray change. In terms of its quality, SiO2 content, pale color as well. Generally believed that more than 75% SiO2 content of silica fume can be used in cement, concrete. China is currently produced by silica fume SiO2 content of 85% or more, is fully utilized.

Table 1: Chemical composition of silica fume (%)

2, fineness
　　Extremely fine particles of silica fume. From Table 2 shows, extreme 80% less than 1μm over. What is the average grain is a 0.1-0.15μm ultra-solid substance, silica fume, compared with other powdered substances, and its specific surface area of up to 20000-280000cm2 / g, about 50 times greater than the fly ash than cement greater than about 50-100 times. The manufacture and application of ultrafine particles is high-tech eighties. At present, zirconium oxide Al2O3, and other magnetic oxide and silicon carbide and ultrafine particles of silicon oxide ultrafine and other metals and alloys can be used for fine ceramics, high-performance powder, conductive materials, magnetic materials, sensors and other new materials. Therefore, the silica with ultrafine features, improve performance and enhance the material has a very important role.

3, particle shape

　　Silica fume in the condensation of gas, liquid, phase change process back by surface tension, was the formation of large and small round ball, and the surface is smooth, there may be two or more spherical particles stick coagulation in together. Material mixed with silica fume, this small smooth spherical body can play a lubricant used to reduce friction between the material particles, thus improving the workability of materials, water consumption can be reduced to improve the material properties.

4, the mineralogical structure

　　The formation of silica condensation process is extremely fast, so had a chance to form a crystalline SiO2, and its minerals are amorphous minerals. It is a great surface-active substances of volcanic ash, is a high quality of cement, concrete admixtures and additives with low cement castable. With X-ray diffraction analysis showed that the silica fume versed in X-ray diffraction diagram shows the typical characteristics of the dispersion of the glass-state peak.

5, other properties

1 Fire degrees (SiO2> 90% when) 1630 ℃
2 the proportion of 2.1-3.0g/cm3
3 Bulk density 200-250kg / m3
4. PH value of 6.7-8.0
5 resistivity (room temperature) 2.4 × 1014Ω.cm
6 natural inclination angle of 38-43O

Third, the silica packing, transport and storage

　　

Small proportion of the heap because of silica, natural inclination big, fluffy, easy flying, are unloaded from the shipping container is not easy, but expensive packaging and transportation costs. Currently, the countries of silica fume processing, storage and transportation of many forms, generally available the original powder, lumpish, bead-like use of bulk, bags and other forms of packaging, silica fume can also be made with the appropriate water mud or mud paste supply users.

Generally use the bulk of the dump truck with a canopy or enclosed cement tanker transport.

Bags are usually heavy bag 25kg, big bags of up to 300-500kg. Generally use paper or plastic bag. At present, China's Tangshan, Shanghai, Taiyuan, etc. Most of silica powder with a double plastic bag, bag weight 10-15kg.

In order to improve the transport efficiency of silica fume powder, some manufacturers took increasing the frequency measures. Such as Canada, Sweden and the United States and some factories to the silica powder with water tank factory Ban Cheng mud (water and silica fume, 50%). Norway in columns made of mud than plants containing 35% silica fume, and delivered directly to the concrete plant. In this way lower transport costs, handling is not dust, a good working environment; easy pumping, high degree of mechanization.

Silica powder into a ball more than the traditional method of the disc into a ball machine, Norway, the Soviet Union, Japan and other countries have reported. With paper waste, silica fume sodium, NaCl, alkali or alkaline earth metal hydroxides such as aqueous solution or water as binder, pellet diameter is made of 5-25mm, usually as a silica raw material iron furnace return . Some rely on their own strength pellet binder bond strength, and some need to roasting.

The late seventies, a Scandinavian company with a high alkaline agent and silica powder with the production of beads used in concrete admixtures to "Corrocem" for a product appear on the market. After 1982, bead-like admixture with new development, the Norwegian company Elkem use of different alkaline agents, high alkaline agent and silica fume powder bonded to the performance of different admixtures for cement beads, large concrete , and other mineral ingredients (including low cement castable, etc.), a commodity to "EMSAC" named supply market. This is extremely easy to use additives, can be dissolved in the water after the break, while mixed with silica powder, high alkaline agent played the same effect. At present, some countries have developed this technology.

Diameter silica beads 0.5mm, from its heap of bulk specific gravity of silica fume powder 0.2t/m3 to 0.6t/m3. It will enable the packaging and transportation costs by about 60%. Handling efficiency, reduce storage space and area, but also improving the working conditions and environment.

In recent years, the Ministry of Construction Research Institute and Shanghai Metallurgical number of units of increasing the frequency of silica fume also made a lot of research. Ministry of Metallurgical Research Institute developed the compound to enhance building density agent in 1987 passed the technical appraisal. This additive for cement mortar and concrete alone.

Fourth, the comprehensive utilization of silica fume

1, for the cement industry

Norway, Iceland, Canada and other countries of the ferroalloy plant to recover part of the silica fume as a cement raw materials are reported. Its application as follows:

1 for material batching of cement, the Soviet Union patent that the raw materials in the production of Portland into ,18-25% with silica fume, cement kilns can improve the yield and quality of cement, kiln capacity by 10-20%; health 28 days after the cement from 47Pa to 54-56Mpa.

2 produce a mixture of cement, cement companies in Iceland made with spherical silica fume and grinding clinker production of about 6% silicon powder containing a mixture of cement, the application effect is very good. 1982 Canada Rafah agent (Lafarge) cement mixed with silica fume in the production of clinker in Portland cement mixture has entered the market. Canada has developed a standardized mixture of cement, mixed cement, silica fume incorporation requires less than 10%, silica fume SiO2 content greater than 85%, loss on ignition <6%.

3 production of ultra-dense cement from Denmark, Japan Electrical Industry Co., Ltd. Brindisi off (DENSIT) the company introduced the "Brindisi by special law," named after the ultra-dense cement. This mainly by the silica fume cement, cement clinker and other special additives formulated Canada. It is made with high density concrete, increase its strength than ordinary concrete, 2-3 times, with good corrosion resistance, insulation resistance, abrasion resistance, impermeability, frost resistance and chloride ions on the block performance and so on. Denmark has been the cement for corrosion, wear and heavy machinery plant food requirements of insulation and the ground and stop the corrosion layer of chloride ions, hydraulic concrete projects, you can also replace part of metal products. Danish Institute for Corrosion Research Center is using this cement coating for corrosion protection of steel; Atomic Energy Research Institute are studying it for the protection of radioactive waste isolation materials.

2, for mortar and concrete

In construction, building materials currently operating in the application of silica fume is the main use of channels. Since 1983, in North America, Europe, Japan and others have begun extensive use of silica fume concrete. According to 1986 data reported that approximately 40,000 tons per year Viagra for silica fume concrete, the equivalent of 50 million tons of cement mixed with silica fume. Canada 1981-1985 were used silica fume concrete 1 million m3.

Mixed with silica fume in concrete can play much effect, Norway PerA.jahren made a valid index value of 1-6 (Figure 1), using 1 kg of silica fume can save 1-6 kg of cement in most cases , the effective index values between 2-5.

In the use of silica fume mortar and concrete are generally used simultaneously with the reducing agent, primarily played two roles: ① cement digestion process produces Ca (OH) 2 can react with silica, resulting in lower C / S ratio of CSH hydrates, constitute less with or without Ca (OH) 2, is not easy to penetrate the cement structure, so that mortar and concrete dense and uniform. Huang Chengyi and feldman and tests showed that 10% mixed, 30% silica fume cement paste, Ca (OH) 2 content is very reduced, 30% silica-doped samples, within seven days has completely disappeared . ② mortar, concrete, silica fume and superplasticizer added, filling the cement particles, the mortar, concrete, apparent porosity decreased, pore structure improved further improve the performance of concrete. In these experiments, the control sample is 10% silica fume and 30% of the sample 90 days after the apparent porosity was 17.02%, 14.81%, 11.71%, each of which is less than 0.0175μm 6.72% 10.35% , 7.54%.

Fresh silica fume concrete performance characteristics:

A. viscous, in the concrete construction process is not easy to transport and isolation layer, and rock, old concrete, bricks and other materials, bonding, good for a small amount of shotcrete construction rebound. Containing silica fume reached 20-30%, the viscosity can be used for underwater construction.
B. Small water secretion, silica fume concrete in tap water, the surface is not bleeding, difficult to produce Shen shrinkage cracks in the concrete surface is smooth, the silica fume concrete to early conservation.
 

The performance characteristics of hardened concrete:

A. Studies in the laboratory preparation of high strength for the 100-150Mpa compressive strength of silica fume concrete-pei. Denmark prepared a 225-255Mpa compressive strength of concrete. China's Shanghai-listed with the preparation of the compressive strength of concrete aggregate for the concrete 125pa. Silica fume concrete compressive strength than ordinary concrete can increase 20,080%, other properties such as flexural strength, tensile strength has greatly improved.
B. Physical and chemical properties to improve the density increase as silica fume concrete, porosity reduction, micro-pores are more cement in the Ca (OH) 2 does not exist to reduce or improve the quality of concrete to resist the penetration of the outside world, erosion and other capabilities. Dense silica fume concrete can reduce the rate of carbonation, there are large resistivity, it can protect steel and buried pieces. Silica fume concrete can exist in K2O and Na2O combination of alkaline substances, thereby eliminating or reducing the alkali aggregate reaction and. Avoid damage to the project. In addition, it has a good antifreeze, oil, acid and other properties.

At present, the silica fume concrete has been applied in the following places:

1 high-rise building construction
2 for anti-erosion wear, leakage of the Agency in 1986, China Gezhouba gates have been used in mortar silicon repair and 2000-3000m3 reservoir seepage.
3 permeability of the Agency for China's high-strength high-strength precast Shanghai Shanghai Huangpu Jiang Yuejiang tunnel construction project.
4 for resistance to sulfate, perchlorate, and improve the durability of steel rust 1976-1978 Swedish Agency built the pier by the impact of the waves; built in the coastal water of China's oil platform.
5 for the anti-wear, frost, corrosion and other projects.

3, for low cement castable

Low-cement castable was developed in the seventies a new type of high performance refractory castable, first developed by the French. It is widely used in various industrial high temperature furnaces, have a good technical and economic effect. 1983, Pittsburgh, United States General Refractories Company for dry low-cement castable blast furnace iron gully cover lining the main out, life is three times the original ordinary castable from 75-100 to 300-400 furnace furnace.

Ministry of Metallurgical Research Institute since 1985, construction began of a low cement castable, 1988, through the ministry-level appraisal. It has been in Baosteel, Shougang thermal equipment use, and some have been imported from Japan-made materials instead of materials.

Low-cement castable and castable general use calcium aluminate cement. , But before the distribution into the silica fume and superplasticizer, its cement company 4-8%, and the water is only 5-8%, 15-20% of the amount of cement than the latter, 10-15% water, plus a large rate reduction. Low-cement castable can not only save 50-70% high aluminate cement, and the performance also improved. Metallurgical Research Institute of the Ministry of Construction of low-cement castable test results are as follows:

Lower porosity and pore structure of cement castable material with ordinary casting after heating at 1400 ℃ for apparent porosity contrast, the former was 20.4%, while the latter was 26.4%, an increase of nearly 30%. Determination of pressure pump with low cement castable in the total porosity is lower than normal casting material. Heating after 800 ℃ 110 ℃ than ordinary castables drying increased by nearly 40%. From the pore size distribution, the normal casting compound is greater than the pore size of 100A low cement castable 2-3 times smaller than the aperture 1000A low cement castable only 30-50%.

Intensity characteristics of the strength of low cement castable, whether low, medium and high-temperature phases of moderate growth, while the average strength of castables in the temperature decreased significantly. Two kinds of castable hot flexural strength as follows:

Line changes in the heating, low cement castable shrinkage is lower than normal casting material, to 1500 ℃, the emergence of low-cement castable 0.1-0.8% of the micro-expansion, so the material will not crack.

Refractoriness was determined that low cement castable refractoriness of 1790-1830 ℃, higher than normal casting material (1690-1710 ℃).

Softening point load test results are as follows:

　　Thermal expansion at 1400 ℃ for the first two are basically the same castable. 1400 ℃ after slight contraction of the low cement castable then begin to swell, 1500 ℃ when the expansion rate of 0.5%, while the ordinary castable after a sharp contraction in 1400 ℃, 1500 ℃ when the expansion rate has been negative (-1.8%).

　　Erosion resistance test with converter slag heated to 1450 ℃, temperature 3 hours, the erosion of low-cement castable about 4CM2, better than ordinary castable 7CM2. So far, Ye Jian General Hospital has developed a variety of the DJ series and grade of cement castable used for many projects.

1. DJ-C165Z, DJ-C17Z two kinds of low-cement castable blast furnace iron ditch Baosteel have to cover. When the original casting material imported from Japan usual life of 20 days, has been used in all domestic instead of low-cement castable, the average life expectancy of 20 days. Up to 26 days, saving a lot of foreign exchange.
2. DJ-C70, DJ-60 for low-cement castable reheating furnace of Shougang 250 and 430 Tsui burning brick oven. 250 not enter the furnace of life of 9 months (the original use of only two weeks), increased nearly 20 times.
3. DJ-C60 low cement castable for billet heating furnace of Shougang to open 500 high-temperature gas stream erosion by the pressure of serious wear and tear parts, one year is still intact.
4. DJ-C60 for a low cement castable Institute of Nanjing Chemical Industry Company CO furnace lining life of 68 days, than the original corundum tube life increased 10-20 times.
According to available information reports, low cement castable is widely used in foreign countries, has been used in coke ovens, sintering, iron making, steel making, rolling, non-ferrous metallurgy, oil and chemicals, cement, glass, ceramics and power generation industries.
 
4, other uses

1 Shanghai Spark chemical production silicate water glass has been used in raw silica fume. Simplify the process with silica fume in the case of production of sodium silicate modulus greater than 4. Modulus sodium silicate neutral 4 above, can be used in high temperature coating, steel and so on. Shanghai Baosteel has been the use of this water glass.

2  for organic compounds containing silica filler dishes similar Sio2 with silica content, like the other two and have similar physical properties, and thus silica fume can be used as rubber, resin and other organic compounds of the filler material.

3 silicate bricks of raw materials for the Soviet Union to improve Portland's quality of application of silica fume, in the trial of Harry Cove with 5% silica fume into the silicate brick. By brick strength, frost resistance and water absorption index can be obtained by the equivalent number of M200 blocks. Such as silica fume with the intake increased to 7%, the bricks of the label can be increased to M250-M300.

4 for agricultural fertilizer and improved varieties of soil silica fume mixed with potassium hydroxide or potassium carbonate can be made slow heating agricultural fertilizer potassium silicate, it is less volatile loss, to protect the soil and promote root development of crop, pest and disease suppression. Silica fume also be used as fertilizer anti-caking agent, Norway, France and Italy this approach works well to prevent the fertilizer particle agglomeration.

5 silica fume clumps made for recycled used melted. The United States has offered to 20-25mm pellet form by 5-20% silica into the furnace by smelting proved that adding a large number of pellets, the stove life of three weeks, and did not find silicon recovery unit to reduce power consumption increases in the . Norway, Aiken used to reconcile the water into 10-15mm pellet, 800-1200 ℃ in a vertical furnace sintering, will not burst and there is enough strength to return to the use of raw materials for smelting. Soviet Union, it concludes with a paper pulp waste liquid agent, pellet strength can be broken by bad luck rather than by the proportion of 30% into the furnace used. A ferroalloy plant in Sweden for the use of silica fume use of chrome ore pellets binder.
In short, the silica fume was once considered a serious pollution of the environment of smoke, now used by the national research has proven that it is a valuable commodity. With in-depth knowledge and practice and development, will find new ways to use and continue to improve its value.