The relationship between properties and uses of Wollastonite
In the past two decades, the consumption of wollastonite is increasing by 10% every year. It is not only used in ceramic industry, but also in coating and paper industry, and its consumption gap is also changing. It is of great economic significance to continuously develop the industrial application field of wollastonite.
1. Acicular crystal habit
Most of the natural Wollastonites are acicular or fibrous, and their length is about seven times of the diameter. After crushing and grinding, it still has needle like structure. The results show that the wollastonite extends along the b axis into columnar and fibrous shapes, and the cleavage is parallel to the b axis. The Ca-O octahedron extends along the axis B and is connected by a common edge. The direction of the ridge is fixed, so the inner tendency of octahedron structure is straight. The synthetic wollastonite is of ring structure and is generally granular. The aggregate is radial or parallel columnar, and it is rich in decoration with the special color of other symbiotic minerals.
Because the needle like crystals are interwoven in other raw materials, the binding force between particles is increased, which makes the blank have higher drying strength and molding quality. Because this is not the most compact packing, and the water absorption rate of the green slab is high (23-27%), large tonnage press is used abroad to improve the compaction degree. The non compact packing can provide a channel for the internal moisture to escape quickly, which makes it have the characteristics of fast drying speed, so as to meet the requirement that the moisture content of the green body is less than 0.5% when the kiln is fired quickly.
The ceramic mold made of wollastonite and alumina has high strength and service life several times higher than that of gypsum mold. Using wollastonite as the raw material for grouting can increase the stability of slurry, and it can also be used as the filler of coating, with good stability. It can also replace 25% of asbestos as cement asbestos products. All of these are related to the needle shape and its interwoven structure. 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 luminosity
Pure wollastonite is bright white. When mixed with other impurities, it is gray gray brown. The purity is 90%, and the reflectivity of wollastonite with particle size less than 325 mesh is 92-96%.
White and bright color is the main reason why it is used in coating industry. Its good thermal stability and needle like structure can improve the leveling property of the coating. Wollastonite used in the Xu 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 selection of pigments and fillers is based on the premise of white color, high incidence and optical stability. Most of the pigments will change color seriously after vacuum ultraviolet irradiation, but wollastonite has good stability.
In China, wollastonite is used to replace titanium dioxide (5%) in latex paint, and 30-40% in waterborne paint. Wollastonite accounts for 9% of the vinyl acetate latex paint made in the United States.
Wollastonite glows yellow orange under ultraviolet light, and blue fluorescence can be induced by 365 μ m excitation. Wollastonite with manganese can excite bright green. This luminescent property is used to make luminescent color enamel.
3. Chemical properties
The composition of wollastonite is generally close to pure calcium silicate, which can be widely used instead of synthetic calcium silicate. In 1:4 hydrochloric acid, it can react slowly. In concentrated hydrochloric acid, it can decompose completely and become floccules. The floccules are super fine (< 0.5 μ m) with high chemical activity. The research in Canada shows that the wollastonite treated with acid is better than kaolin. The suspension with concentration of 10% is alkaline, which is not suitable to be used as the coating filler of acid medium, but it can neutralize some acidity, such as acetic acid in polyvinyl acetate grease, which is conducive to the diffusion and stability of pigment.
In the glass industry, replacing limestone with wollastonite not only reduces the phenomenon of chlorine escape, but also brings silica. In the ceramic industry, it can replace the calcite or chalk in the calcium glaze. The glaze has a good luster, but it needs to solve the technical problems such as strong reaction between the body and the glaze and less glass phase on the glaze. Wollastonite can provide calcium oxide for the flux of electrode, and bring in silica at the same time. The high alkaline fulvic slag can reduce the burning pores and other defects at the joint, and the addition amount is 10-20%.
4. Thermal properties
The expansion of wollastonite along the b-axis is 1.6 * 10-6 degrees in the range of 20-1000 ℃, and it can reach 6.7 * 10-6 degrees when containing impurities. The expansion of synthesized calcium silicate below 900 ℃ is 10.7 * 10-6 degrees.
Wollastonite has high thermophysical and thermochemical stability, which is conducive to the rapid firing of glazed tiles at low temperature. The crystalline phases of the glazes are all minerals with low thermal expansion rate, which is conducive to rapid cooling. Because of its thermal stability, impact resistance and melting aid, wollastonite is used to make ceramic grinding wheel and porous filter ceramic. Porous filter ceramics made in the Soviet Union have a total porosity of 40-50%. They use wollastonite to make heat-resistant fiberboard, 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 electron tube coating. Wollastonite can also be used as filler of polypropylene, and its insulation effect is better than that of asbestos and talcum powder.
6. Purity and selectivity
The use of wollastonite is closely related to its purity. It is required that Fe2O3 in concentrate is less than 1%, calcite content is less than 10-12%, Fe2O3 + TiO2 content is less than 2%, calcite content is less than 10-15%, and impurity content is more than 0.5% in China's ceramic industry, which can not be used as coating.
Removal of impurities requires beneficiation. The key point of ore washability evaluation is the properties of impurities and their imbedding characteristics. Daye wollastonite, Hubei Province