Processing of wollastonite and its application to coatings

I. Overview

　　

Wollastonite is a chain metasilicate normally in fiber or acicular shapes. Its special crystalline structure of crystal morphology has decided its good insulation as well as high whiteness, good dielectric properties and satisfactory heat and weather resistance, enabling it to be widely applied to ceramics, chemicals, metallurgy, papermaking, plastics, coatings, etc.

　　

II. Processing of Wollastonite

　　

(I) Ore dressing

　　

Wollastonite is generally dressed by hand sorting, magnetic separation only, floatation only or combined magnetic-floatation (or electric separation) processes. The purpose of ore-dressing is to mitigate the Fe content and to separate the calcite in it.

　　

(1) Hand sorting

　　

Rich ores are hand picked or crushed at the ore-dressing site and loaded onto the belt conveyor where wollastonite-rich ores are hand sorted. Hand sorting is best suitable for ores of satisfactory quality.

　　

(2) Magnetic separation only

　　

Main associated minerals with wollastonite rocks, such as diopside and garnet, are weakly magnetic minerals whereas wollastonite hardly displays any magnetism. So dry or wet strong magnetic separation will separate the wollastonite content from these associates to get rid of a large part of Fe-bearing minerals and increase the purity of the product.

　　

(3) Floatation only

　　

Based on the difference of surface physical and chemical properties between wollastonite and calcite, floatation will also enable effective separation and thereby higher purity of the wollastonite product.

　　

(4) Combined magnetic – floatation process

　　

This method applies to low-grade wollastonite. First, weakly magnetic minerals are separated by dry or wet magnetic process. Then the wollastonite content is separated from the calcite, quartz and other minerals by floatation. Separation of wollastonite from calcite, quartz and other minerals is achieved in either of the following ways.

　　

(1) Reverse flotation protocol using anionic collector

　　

As calcite is a carbonate mineral while wollastonite and quartz are silicate minerals, it is possible to float and separate calcite by changing the pH of the ore slurry media and inhibiting wollastonite with conditioner according to the difference between these minerals in surface electricity before quartz and wollastonite are further floated and separated. Normally oxidized paraffinum sodium salt is used as the calcite collector while sodium silicate as the wollastonite and quartz inhibitor.

　　

(2) Direct floatation protocol using cationic collector

　　

This protocol conditions the surface electricity of wollastonite and calcite so that they have potential of different signs to enable wollastonite to be floated out first by cationic collector through static adsorption while calcite is left in the floatation tank as tailings. The floatation is carried out in two steps. First, wollastonite is floated out by amine collector together with silicate minerals as foam product while calcite is left in the tank. Then, the silicate impurities are floated out by mixed amine and anionic collector while wollastonite is recovered as in-tank product.

　　

(II) Crushing and the processing of acicular wollastonite

　　

Wollastonite powder is a short-fiber inorganic powder. In some of the applications like ceramics, microcrystalline glass and metallurgical protection slag where it is used as asbestos substitute, paper pulp substitute, plastic or rubber reinforcing fillings and some coating fillings, not only the observation and distribution of its size but also the slenderness ratio of its fiber particles are specified. Wollastonite powders of higher slenderness ratios (>10) provide higher applicable and economic values as they can be used as substitute for high-grade reinforcing fillings for high polymer-based composite materials like asbestos fiber, paper fiber as well as plastic and rubber. For this reason, processing high slenderness ratio acicular wollastonite powders has become one of the main deep processing techniques for wollastonite.

 　　

At present, 400 mesh below the ordinary wollastonite powder is used mostly Raymond mill and ball mill for processing, the main use of Raymond mill. In order to ensure that the content of large particles do not exceed the standard, the classification or sieving equipment. This production process is used mostly dry.

 

High ratio of length to diameter of the main production equipment and wollastonite wollastonite are: mechanical impact mill, centrifugal mill (like a whirlwind or hurricane ultrafine mill, LFS centrifugal mill), fluidized bed type airflow crusher etc.. The long, high mechanical impact mill, centrifugal autogenous mill, generally applicable to the production of 400~1000 mesh around the diameter ratio of wollastonite acicular powder; fluidized bed type airflow crusher is suitable for the production of 1250 mesh (D97 ≤ 10 μ m) ultrafine needle-shaped wollastonite powder. Production of these two types of equipment dual dry. If the wollastonite in wet purification (flotation or wet high intensity magnetic separation) after ultra-fine grinding processing, can be used wet process, wet grinding to product fineness requirements, then dehydrating (filtration and drying)

　　

Wollastonite application:

　　

1.Application of wollastonite in plastic and rubber, wollastonite unique needle-like fiber, with insulation, good wear resistance, and high refractive index. Plastic, rubber products better filling material. Features and performance: wollastonite can improve the impact strength, enhance the liquidity and improve the tensile strength, impact strength, tensile strength and modulus of linear shrinkage rate.

　　

2.Application of wollastonite in paper products, wollastonite, after special processing technology can still keep its unique acicular structure, the added silicon ash powder white paper, improve the whiteness, opacity (surface coverage), evenness, smoothness, adaptability, decrease the quantity of transverse differential and wet cardboard deformation, improve the printing adaptability, and can greatly reduce the use of other raw materials, reduce the cost of paper products in general.

　　

3.Application of wollastonite in ceramics, adding the right amount of wollastonite in ceramic raw materials, can greatly reduce the sintering temperature, shorten the time of firing, achieve a low-temperature sintering. Saving a lot of fuel, reduce product cost; at the same time, the product can improve the mechanical properties of the reduced product cracks and warping, increase luster glaze, improve green strength, so as to improve the qualified rate of the products.

　　

4.Wollastonite instead of asbestos used in building the isolation plate, wollastonite powder is a non-toxic, tasteless, no radioactive and other advantages to gradually replace the harmful to human body health of asbestos, become the new raw material of environmental protection building materials in the new century. After special processing technology can still keep its unique acicular structure, the added calcium silicate board, wollastonite acicular powder fire board materials such as impact resistance, flexural strength, abrasion resistance is improved greatly.

 

5.Application of wollastonite in coatings, wollastonite has a reinforcing good, which can improve the coating's toughness and durability, and can keep the paint surface flatness and good gloss. Because the wollastonite with high hardness and wear resistance, so that the coating can obtain better mechanical strength, increased durability, enhanced adhesion and corrosion resistance, anti-corrosion coatings in general used for primer. With the development of deep processing of wollastonite production enterprise technological progress, wollastonite because of their excellent weather resistance, good coloring brightness is more and more used in latex paint. 

 

①Application in exterior wall paint of high hardness, wollastonite, Mohs hardness of 4.5 ~ 5, low thermal expansion coefficient, belongs to inert filler weatherability, the acicular structure with crack resistance, high whiteness, good color palette color brightness high, as an important packing more and more use paint for exterior wall.

 

②Application of interior wall latex paint, wollastonite because the original color slants yellow, not much, wall latex paint, now with wollastonite production enterprise deep processing technology advances, white degree can reach above 90, (such as the mirror mount wollastonite JLH-SW6 the maximum brightness Jinlong Hui can be up to 92) can be used for interior wall paint. The wollastonite acicular structure can improve the PVC latex paint for interior wall porosity and improve the hiding power. Acicular wollastonite can improve the scrub resistance, data shows, joined the 5-8% wollastonite, wall latex paint can improve the scrub resistance 100 times. In addition to acicular wollastonite can reduce the side angle light, improve the compressive optic, improve the performance of construction.

　　

③Application of the primer, wollastonite sheets or acicular structure, can form a network structure, has the good permeability.