Hazards associated with the use of bone ash in contact with molten aluminum

Abstract

Bone ash (calcium hydroxyapatite or simply calcium phosphate) has traditionally been used in the casthouse to fill cracks, patch holes and cover “make and break” or moveable joints. It has many attractive attributes including its ease of use, low cost and non-wetting characteristics. Bone ash itself is non-toxic and environmentally benign. However recent evidence indicates that bone ash can be reduced upon contact with aluminum alloys to produce metal phosphides. Metal phosphides can in turn react with water or water vapor to liberate phosphine (PH3) a highly dangerous and toxic gas. This paper reviews the observations and experiments that lead to this conclusion and discusses Novelis’ search to identify a satisfactory substitute.