Transformation of bentonite used in green sand molds during metal casting process and its relevance in sand reclamation

Abstract

Green sand molds in foundry use bentonite as a binder. During the casting process, the heat transferred to the surrounding sand in the mold leads to the degradation of bentonite structure, thereby forming new types of deposits on the surface of sand grain. This results in deactivation of binder and unsuitability of sand for its reuse. In the present work, useful insights are provided into this transformation that may help in developing technology to reclaim the sand for its reuse in foundry. The green sand and bentonite were subjected to high temperatures over the range 500 °C–1100 °C, for varying periods (10–120 min) and analyzed. It was found that the binder in the used sand existed in four different forms. These were active clay, loosely bound dead clay, strongly bound sintered clay and high-temperature phases; the latter three forms need to be removed in the reclamation process. While a simple mechanical attrition method can remove the loosely bound dead clay, it fails in removing the other two forms, that are referred as oolitic deposits of sintered clay and high-temperature phases. More aggressive attrition or chemical treatment may be necessary to facilitate their removal. Analytical methods such as FTIR, DSC, SEM, and X-ray diffraction were used to show the differences between clay binder and oolitic deposits and how the transformation takes place from one form to the other as a function of temperature. Furthermore, the kinetics of oolitic formation was examined and the results showed that the formation of oolitic deposits follows temperature-dependent first-order kinetics.