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Total world production of wollastonite except the USA is in the 600 Ttpa range. China accounted for approx. 300 Ttpa and India for 210 Ttpa. The USA is the third biggest producer of wollastonite; data of production are not published (Virta 2013).
Wollastonite is available in lumps and flours down to an average particle size of approx. 2 ^m. Neat grades as well as surface-treated grades are used depending on the application.
Wollastonite is unique in being the only white and acicular, nonmetallic mineral. During crushing and grinding, it forms acicular particles having an aspect ratio (length vs. diameter ratio) in the range of 1:3 up to 1:15. The structure of the wollastonite particles depends partly on geological conditions of genesis but also to a large extent on the grinding technology. With specific processing technologies, powders with exceptional acicular structure can be achieved. Figures 2 and 3 show two wollastonite products prepared by different methods. A nearly blocklike wollas- tonite (TREMIN 283-product series by HPF - The Mineral Engineers) with an average aspect ratio of approx. 3:1 is shown in Fig. 2. These are the so-called low- aspect-ratio (LAR) wollastonites. A wollastonite product with an exceptional acicular structure (TREMIN 939-product series by HPF - The Mineral Engineers) - including
Fig. 2 Scanning electron micrograph: micronized LAR wollastonite flour (magnification 1:1,000)
Fig. 3 Scanning electron micrograph: micronized HAR wollastonite flour (magnification 1:1,000)
fractions with aspect ratio of up to 30:1 - is displayed in Fig. 3. This type is called high-aspect-ratio (HAR) wollastonite.
Because of the particle shape and - frequently also for special applications - surface treatment with silanes or silane-based compounds, wollastonite powders have excellent reinforcing properties. The mineral fiber itself is said to have a tensile strength of approx. 2,700-4,100 MPa.
Usual brightness of the processed mineral flour is about 90 (Y), so the neutral color and high brightness make it applicable in most polymer systems.
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