Experiences gathered during reclamation of used water glass and bentonite sands in extra low and ambient temperature

Abstract

A study is reported in which cryogenic temperatures are applied to intensify the mechanical reclamation of used foundry sand in experimental equipment that reproduces the abrasion, rubbing and crushing found in industrial processing. Results for used silica sands with bentonite and silicate (water-glass plus ethylene glycol) binders carried out at approximately –70°C and ambient temperature are presented. The reclaimed material was characterised by pH value, ADV (acid demand value) and Na2O content, which usefully define ease of reclamation, bending sand strength and compressive sand strength. Mechanical–cryogenic reclamation clearly improved the output for both sand types, utilising the hygroscopic behaviour of moulding sands to intensify the release of sand from the binder. The presence of a 'freezing' agent changes the volume of both binder and sand grains, leading to increased stresses on the surface and inside the grains, combined with higher brittleness of the binder material. Compared with current practice, the cryogenic procedure reduces reclamation times and confers environmental benefits. However, if not adequately controlled, mechanical–cryogenic reclamation of sands with water-glass or bentonite binders may lead to much quicker wear of the sand grains and formation of large volumes of dust.