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In the past 20 years, the consumption of wollastonite has been increasing by 10% every year. It is not only used in ceramic industry, but also used in coating and paper industry. Its consumption gap i
發布時間: 2022-09-05 01:53:53
In the past 20 years, the consumption of wollastonite has been increasing by 10% every year. It is not only used in ceramic industry, but also used in coating and paper industry. Its consumption gap is also changing. It is of great economic significance to continuously develop the industrial application field of wollastonite.
1. Acicular crystallization habit
Most of the natural wollastonite is needle shaped or fibrous, and its length is about seven times the diameter. After crushing and grinding, it still has needle like structure. Through research, wollastonite extends into columnar and fibrous along the b-axis, and the cleavage is also parallel to the b-axis. The calcium oxygen octahedron extends along the axis b and is connected by a common edge. The direction of the edge is fixed, so the internal tendency of octahedron structure is straight. Synthetic wollastonite is annular structure, generally in granular form. The aggregate is in a radial or parallel column shape, with the unique color contrast of other symbiotic minerals, which is rich in decoration.
The fine ceramic green body is pressed by mixing powder and clay. Because the needle shaped crystals are interwoven in other raw materials, the binding force between particles is increased, so that the green body has higher drying strength and molding quality. Because this is not the most compact packing, and the water absorption of the green billet is high (23-27%), large tonnage presses are used abroad to improve the compaction degree. Non compact packing can provide a channel for the rapid escape of internal moisture, which makes it have the characteristics of fast drying speed, so as to meet the requirement that the moisture content of green billets is less than 0.5% during rapid kiln burning.
The ceramic mold made of wollastonite and alumina has high strength and service life several times longer than the gypsum mold. Using wollastonite as the raw material for grouting can increase the stability of the slurry. It can also be used as the filler of the coating and has good stability. It can also replace 25% asbestos as cement asbestos products. These are all related to the needlelike shape and the interwoven structure it forms. It is reported that the ultimate bending strength of 700-1000 special concrete made of wollastonite and Portland cement can be doubled.
2. Brightness and Luminescence
Pure wollastonite is bright white, and when mixed with other impurities, it is gray to grayish brown. The reflectivity of wollastonite with purity of 90% and particle size less than 325 meshes is 92-96%.
Its white and bright color is the main reason why it is used in the paint industry. Its good thermal stability and needle structure can improve the leveling property of the coating. The wollastonite used in Xu's material industry in the United States accounts for 30% of the total consumption. It is particularly suitable for the production of white and light colored coatings. For thermal control coatings with low absorption emission ratio, the choice of pigments and fillers is based on the premise of whiteness, high incidence and optical stability. The vast majority of pigments will change color seriously after vacuum ultraviolet irradiation, while wollastonite has good stability.
In China, wollastonite is used to replace titanium dioxide (5%) in latex paint and 30-40% in water-based paint. Wollastonite accounts for 9% of the vinyl acetate emulsion paint made in the United States.
Wollastonite glows yellow orange under ultraviolet light; Blue fluorescence can be obtained by 365 μ m excitation. Wollastonite containing manganese can excite bright green. This luminous property is used to make luminous colored enamel.
3. Chemical properties
The composition of wollastonite is generally close to that of pure calcium silicate, which can be widely used instead of synthetic calcium silicate. It can react slowly in 1 to 4 hydrochloric acid, and can be completely decomposed into flocs in concentrated hydrochloric acid. The flocs are superfine (<0.5 μ m) and have high chemical activity. Research conducted in Canada shows that it is better to use acid treated wollastonite as paper filler than to use kaolin. The suspension with a concentration of 10% is alkaline, which is not suitable for coating filler in acidic medium, but it can neutralize some acidity, such as acetic acid in polyvinyl acetate, thus facilitating the diffusion and stability of pigments.
In the glass industry, wollastonite is used instead of limestone to reduce chlorine escape and bring in silica. In the ceramic industry, if it is used to replace calcite or chalk in calcium glaze, the glaze will have good luster, but the process problems such as strong reaction between body and glaze and less glass phase in glaze should be solved. Wollastonite can provide calcium oxide for the welding flux of the electrode, and at the same time, it can bring in silicon dioxide. The obtained high alkaline slag can reduce the burning pores and other defects at the joints, and the addition amount is 10-20%.
4. Thermal properties
Wollastonite expands 1.6 * 10-6 degrees along the b-axis within the temperature range of 20-1000 ℃, and reaches 6.7 * 10-6 degrees when it contains impurities. The expansion of synthetic calcium silicate below 900 ℃ is 10.7 * 10-6 degrees.
Wollastonite has high thermophysical and thermochemical stability, which is conducive to rapid firing of glazed tiles at low temperatures. The crystalline phases of silica fume glazed tiles are all minerals with very low thermal expansion rate, which is conducive to rapid cooling. Wollastonite is used to make ceramic grinding wheels and porous filter ceramics because of its thermal stability, impact resistance and melting aid. Porous filter ceramics made in the Soviet Union have a total porosity of 40-50%. They use heat-resistant fiberboard made of wollastonite, and the working temperature is above 1000 ℃.
5. Electrical properties
Wollastonite has low dielectric loss and high resistivity, so it is used to prepare high-frequency insulating materials. In the United States, wollastonite is used to make vacuum tube coatings. Wollastonite can also be used as the filler of polypropylene, and its insulation effect is better than that of asbestos and talcum powder.
6. Purity and Selectability
The use of wollastonite is closely related to its purity. In foreign countries, Fe2O3 in concentrate is required to be less than 1%, calcite content is required to be less than 10-12%. In China, ceramic industry requires Fe2O3+TiO2 to be less than 2%, calcite content is required to be less than 10-15%, and impurities content ＞ 0.5% cannot be used as coatings.
Removal of impurities requires beneficiation. The key point of ore washability evaluation is the nature and dissemination characteristics of impurities. Hubei Daye wollastonite can be used for building ceramics after hand selection. Hubei Lijiashan and Fengshandong skarn type ores, after classified desliming and two-stage wet high intensity magnetic separation, have a significant reduction in impurities and obvious effect. In order to remove impurities, flotation, gravity separation, photoelectric separation and other mineral processing methods are also adopted abroad.