(70aa) Study of the Preparation of Ultrafine High Aspect Ratio Wollastonite Powders

1 Wollastonite Resources in China

Wollanstonite is one of richest nonmetallic mineral resources in China. Wollastonite deposits occur as economically important mineral resources in many parts of China, from Heilongjiang Province in the northern China to the Hainan Province in the south of the country, and from Jiangxi Province in the east to the Xinjiang Autonomous Region in the western China. Generally speaking, wollastonite resource in China has three characteristics as follows: abundant resources, high quality and favourable conditions for mining.

2 Sample Preparation and Analysis

Twenty tonnes of  wollastonite rock samples were supplied by the Xinyu Southern Wollastonite Mining Company. Samples came as hand-picked specimens with length of 0 to 40 mm.

2.1 Sample Preparation

Samples were first crushed to desirable sizes using a laboratory scale crusher by applying a compression force onto the sample. Large specimens were peeled along the long crystal axis during the crushing process. This crushing method can effectively protect and keep the fibrous crystal structure of wollastonite. In order to meet the particle requirements for subsequent application purposes, 0-40 mm rock samples were crushed into 0-5 mm following the procedures shown in Figure 1.

Figure 1    Sample Preparation Processing

2.2 Sample Analysis

2.2.1 Chemical Analysis

Wollastonite is a Ca-bearing meta-silicate. The chemical composition can be present as Ca (SiO3), with 48.3% of CaO and 51.7% of SiO2. Natural wollastonite is present as white to grey coloured crystals with glassy to pearl luster, density of 2.78 to 2,91 g/cm³, and hardness of 4.5 to 5.0. Wollastonite normally occurs as various crystal shapes such as needle and fibrous assemblage. However, it tends to keep its fibrous crystal structure appearance even if it was crushed into very fine particles. The chemical analysis results of this wollastonite sample are given in Table 1.

Table 1    The chemical analysis results of this wollastonite sample(%)

The chemical analysis shows that the chemical composition is close to its stoichiometric composition, suggesting this sample is similar to pure wollastonite, with SiO₂ and CaO very close to the theoretical CaSiO3 composition.

2.2.2 X-ray Diffraction Analysis

The X-ray diffractogram (XRD) of this sample is given in Figure 2. By comparing to JCPDS card, semi-quantitative XRD results suggested this sample contains 98~99% of wollastonite with 1~2% of impurity minerals (mainly Ca-Fe-bearing garnet).

Figure 2    The XRD of the Sample

3 Industrial Experiments

Wollastonite can be crushed into various shapes by differential pressures applied onto the sample during different crushing procedures.

In the superfine grinding process, wollastonite was hit and pressured by various forces coming from different directions due to different grinding methods. Therefore, the shapes and appearance of the final products of superfine ground differ from each other. From the currently known pulverizers, the grinding forces that applied to the sample surface during the superfine ground are punching, extrusion press, mill, shearing stress, friction, etc.

In the punch mill, such as mechanical punch crusher and air current mill, this is how they work. The mill generate a punch forces which hit sample from different directions; when the energy generated from punching process is greater than the energy needed for breaking up the sample, sample is ground. The punching force generated from this type of grinders make sample particles fiercely collide, friction and shear each other in the grinding chamber. Because of these punching effects, extremely intensive stress is created and applied to sample particles; internal stress wave is also created and disseminated within the particles. The intensive stresses in places with internal defections, cracks and cleavages, and crystal surface, make particles first break up along these weak places.

In the process of grinding, the combination of extrusion and shearing stresses are applied to sample particles. Because of the shearing stresses loaded to the sample, particles intend to be easily peeled off in the direction paralleled to the cleavages and cracks of the particles. For the fibrous wollastonite, a right amount of shearing stress could peel off wollastonite into single fibre pieces.

The same as punching grinding process, shearing and friction stresses also have the characteristics of randomly distributing during the grinding process. However, shearing and friction stresses always have a tendency of peeling off particles in the directions paralleled to cleavages and cracks, which are the weakest bond strength planes within the particle. Therefore, with right amount of shearing stress, particles can be peeled off into single fibres.

Taking into account the crystal characteristics of wollastonite, customers requirements and various ground mechanisms resulted from different grinding equipments, samples were superfine ground following the procedures given in Figure 3. The main stresses applied to sample particles are shearing and friction stresses. Aided with some chemical agents such as coupling agents, active ions created from superfine ground interact with the functional groups of those chemical agents. Superfine modified wollastonite, which can be used as functional mineral materials, with desirable high aspect ratios (length to diameter ratio) were successfully achieved in this study.

Figure 3    Superfine ground and surface modified processing of wollastonite

4 Product testing

4.1 Product Particle size analysis

Product Particle size analysis was measured by Sympatec-Helos/BF laser granularity instrument. The results are given in Table 2. The granularity distributing curve are shown in Figure 5.

Table 2             The results of product particle size analysis

Figure 4   The granularity distributing curve

4.2 Scanning Electron Microscopy Analysis

JEOL JSM-35CF Scanning Electron Microscopy (SEM) was employed to examine the powder products. The SEM examination was undertaken under temperature of 20^oC and humidity of 70%. SEM photomicrographs were shown in Figures 5a to 5b.

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Figure 5    SEM photomicrographs

5 The Application in Engineering Plastic Industry

The application tests of the wollastonite superfine powder were undertaken in Baliang Petroleum and Chemical Institute, China. Wollastonite/glass fibre mixture were added as the reinforcement agent into Nylon 6, mixed together and produced into a reinforced Nylon 6. Test results are given in Table 3.

Table 3            Wollastonite/glass fibre reinforcement Nylon 6

The above results (Table 3) indicate that addition of either wollastonite or glass fibre into Nylon 6 have better reinforcing effects than the others. However, if only glass fibre was added into nylon 6, the resultant reinforced nylon 6 has relatively smaller modulus of deformation. If only wollastonite was added, the resultant reinforced nylon 6 has less punch-resistance strength and less tensile strength. However, when certain proportions of wollastonite and glass fibre were added as a reinforcement agent into nylon 6 matrix in this study, the resultant nylon 6 tends to have better reinforcing effects. This is because the blending of these two reinforcement agents (wollastonite and glass fibre) can lead to the interaction between these two filling powders; in return the interaction between these two filling powders can reduce the self-coalescent effects among the function groups of the same reinforcement filling agent. Furthermore, the interaction may also facilitate the exchange of energy and various forces resulted from these two filling agents, and eventually improve the dissemination property, and accomplish the best reinforcement effects of either filling powder. The test results suggested that the best overall reinforcement effects for nylon 6 were achieved by adding a wollastonite and glass fibre by a ratio of 25 to15 into the nylon 6 matrix, the resultant reinforcing material become mostly strengthened.

6 Conclusions

There are abundant wollastonite reserves in China. Some of the wollastonite deposits can be easily processed into fibrous wollastonite needle-shaped crystals. This type of wollastonite deposits occur in certain parts of China. For example, the wollastonite deposits mined by Xinyu South Wollastonite Mining Company, Jiangxi Province, characterizes with premium quality and some outstanding transformation properties.

Wollastonite possesses special physical and chemical properties. With further investigation into its mineral structure and combined with the surface modifying treatment techniques, wollastonite can be used as new reinforcing filling material in the rubber and other engineering industries.

After testing in wollastonite/glass fibre as reinforcing fillings into nylon 6 matrix in this study, it was found that adding wollastonite can significantly improve some material properties.

All tests were carried out in industrial scale facilities in this study. The satisfactory results of this study provide a technical foundation for designing and optimizing any future wollastonite process plant with low energy-consumption and simple flow-on procedures.