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The problems with producing a useful silica filler from the rice hulls exemplify the more general issue of turning waste into useful products. The silica present in the rice hulls is in a finely divided, hydrated, amorphous form and makes up 15-20% of the initial material. The remainder is mainly cellulose, lignin, and moisture. If combustion is used to turn the organic content into energy, then an ash results, which is mainly silica. Unfortunately, normal combustion conditions mean that this silica is dehydrated and fused into larger particles, with some crystallization occurring as well, and this product is thus unsuited to filler use. This difficulty can be overcome by more controlled and gentler burning which leaves behind an amorphous silica ash containing about 10% of carbon. Grinding of this ash yields a product with a particle size in the range 0.1-2 pm, a specific surface area of about 30 m2/g, and oil absorption of 30-40 ml/100 g. Due to the carbon content, it is dark colored. The applications for this product are very much dictated by the particle size and specific surface area. It is too fine to be considered for most thermoplastic and thermoset filler applications and is more suitable for use in elastomers. Unfortunately, the size is too large for good reinforcement, and it fits into a very special niche, that of the thermal blacks. This was the market that the US products of the 1970s, such as opal blacks, were aimed at (Rubber World 1977). For various reasons, this attempt did not prosper. One of the key reasons for lack of development is that a simpler alternative, use as a cement additive, was developed.

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