Rice hull ash has been used widely, in steel mills, as an insulating cover on tundishes and ladles containing molten steel. The rice hull ash is a good insulator because it is inexpensive, it flows over and covers the steel surface well, and it does not get crusty or lumpy during use. As a commodity, rice hull ash is produced by Agrilectric in Lake Charles, La., as the end product of the combustion of rice hulls in a boiler which feeds a power plant.
The major problem with rice hull ash is that, because of its low bulk density (15 to 20 lb/ft.sup.3) and small particle size, some of the ash becomes airborne when it comes in contact with hot metal. The resulting dust is believed not to be a health hazard, but it is annoying to operating personnel.
Previous attempts to improve the properties of rice hull ash for molten steel insulation have met with limited success. Although it is known to produce rice hull ash briquettes with 1) sodium silicate, 2) starch, 3) cement dust/starch and 4) lime/molasses binders, these briquettes do not break down (i.e., spontaneously subdivide into smaller pieces), spread or insulate well when transferred to the surface of molten steel. It is also known to produce rice hull ash pellets with an industrial molasses binder, and such pellets do break down, spread and insulate well on the molten steel surface. However, pellets containing molasses binder smoke excessively when exposed to liquid steel temperatures. Applicants are also aware of other rice hull ash pelletizing attempts which have failed, which failure is probably also attributable to the composition of the binder. These unacceptable binders include lime, lime/lignosol admixtures, starch, starch/lignin admixtures, and wood pulp. (For example, U.S. Pat. No. 4,440,575 to Daussan et al. discloses pelletizing rice hull ash with wood pulp or with admixtures containing a starchy binder and clay. Unfortunately, these and all other organic binders smoke excessively during use, yielding an unsatisfactory insulation material.)
Accordingly, a need remains for a pelletized rice hull ash product which overcomes the disadvantages of the prior art.