Rice Hull Ash
Rice hulls or husks are the outer coating on rice grains and provide protection to the seeds during the growing period. They contain large amounts of amorphous silica and lignin and are not very digestible for humans. As a result, the hulls are usually removed when rice is harvested. The hulls are usually burnt as a source of energy and leave behind an ash known as RHA (rice hull ash), which is largely silica.
This is a very significant waste stream, estimated at over three million tons of silica worldwide. Moreover, it can be regarded as sustainable due to the plant origin. As such, it has been much studied as a raw material for many applications, including as polymer filler.
Recovering the silica in a suitable form for direct polymer applications is not easy and requires careful pyrolysis, which results in the retention of some carbon (see ? Chap. 16, “Fillers from Organic Sources” for more details). This led to attempts in the USA to commercialize it as a substitute for some carbon blacks in the 1970s (Haxo and Mehta 1975), but these did not succeed. Research has continued however, and the use of rice hull ash in polymers was reviewed by D.S. Chaudhary et al. (2004), while R. Srilathakutty et al. described the successful use in microcellular shoe soles (2002). More recently E.P. Aysawarya et al. (2012) have described its potential for use in HDPE. One of the difficulties is that an alternative, relatively high value, use in cement products has emerged as a competitor and the ash cannot now always be regarded as a waste resource.
Some of the tire majors are also seeing it as a useful raw material for use in the manufacture of precipitated silica for tire use. This involves dissolution as sodium silicate and reprecipitating. The substitution of rice hull ash for sand or other crystalline silica normally used is claimed to reduce energy consumption, as lower temperatures can be employed. Ghosh and Bhattacherjee (2013) and Bhuiyan et al. (2011) also describe advantages of making precipitated silicas in this way.