Recycling Cullet from Quartz Ceramic Shells Used in Investment Casting

The use of low-temperature quartz modifications in pulverized form creates additional risks in the manufacture of molds, since polymorphic transformations decrease their crack resistance. In practice, individual layers of the shell wall or the mold as a whole often crack and even disintegrate. In many casting shops, the lining is preliminarily roasted. That may somewhat improve the consequences of polymorphic transformations in the quartz, but steady heating of the molds through the filler to reduce the odds of cracking prolongs the process and increases the energy consumption. The best-known approach to minimizing the cracking and disintegration of shell molds on roasting is to replace the filler: instead of quartz dust, it is possible to use disperse quartz sand of polyfractional composition, sillimanite, pulverized alumosilicate, globular corundum, or fused quartz. However, those materials are expensive and their use is inconsistent with the requirements of resource conservation in casting. A possible material might be silica-based ceramic cullet from the shells used in steel and aluminum investment casting. At present, such cullet is not recycled. It is dumped or used as filler in the flasks used for shaping shells. Chemical and phase analysis shows that the ceramic cullet formed after removing the steel and aluminum castings contains not only quartz and high-temperature tridymite and cristobalite phases (the base) but also 5–10% iron and iron scale and 3–5% aluminum and its oxides. The use of such ceramic cullet in shell production eliminates repeated polymorphic transformation of the quartz in roasting and sealing the molds, which would change the volume, density, and crystal lattice of the material. That increases the crack resistance and strength of the shells and minimizes the waste of the castings produced. Residual iron and aluminum and their oxides improve the utility of the mold. Tests of this recycling method in production conditions confirm its effectiveness.