Rice Hull Ash
This has already been discussed in the sustainable fillers section “Wood Flour.”
Spent Oil Refinery Catalysts
Zeolite-based fluid bed cracking catalysts used in petroleum refining are a significant waste product (estimated at over 300,000 tons per annum worldwide). They have been reported as having potential for use as a synergistic component of intumescent flame retardant formulations for use in polymer systems (L.R. Demoura Estevao et al. 2005).
Precipitated Silica from Geothermal Power Plants
Geothermal energy generation is a well-established global technology, which is expected to increase significantly over the next decade. The water from the wells can contain significant amounts of dissolved silica, which causes operational problems. There are various ways of controlling this, but one way, which is starting to attract interest, is to remove the silica, either as a concentrated dispersion or as a precipitate. The quantities involved are large; for instance, it is estimated that the amount of silica that precipitates out of geothermal waste water in Iceland amounts to over 40,000 tons annually. One potential use for the precipitate is as an alternative to conventional precipitated silica used in polymer and coating applications. At least two companies are already working on commercial scale recovery and use of this silica (see www.geosilica.com and www.environmetals.co.nz).
Silica Fume (Microsilica) (CAS Number 69012-64-2, EINECS Number 273-761-1)
This is yet another source of by-product silica and is an amorphous, submicron, powder arising from silicon and ferrosilicon alloy production. Silica Fume consists of spherical particles with an average particle diameter of 150 nm. Despite the name, it should not be confused with fumed silica, which is a totally different type of silica of much smaller size.
There has been some interesting industrial work on the use of silica fume as a polymer filler. However, this has not really progressed commercially, due to limited availability and the emergence of relatively high value uses as a specialty cement additive. Indeed, this highlights one of the problems faced in the search for recycled fillers. The value in use often cannot compete with alternative nonfiller applications.