Many sands with ideal grain size and distribution for the fabrication of sand molds for metal casting contain carbonate minerals. The presence of carbonate minerals in the sand results in a reaction of the molten metal to release carbon dioxide which creates deformities in the casting.
The carbonate minerals can be removed via flotation with a tall oil fatty acid collector (TOFA) such as AERO 704 and AERO 726 promoters at a pH of 7.0 or greater.
The sand should be thoroughly washed of slimes and organic matter. The sand enters a conditioner where the pH is adjusted to be alkaline. It is very important that the percent solids in the conditioner be maintained at or near 70%. The tall oil fatty acid should be added to the conditioner at a dosage of 0.25 – 0.75 Kg per tonne of dry solids and the sand conditioned for a minimum of five minutes. The conditioned feed should be diluted to 30-35% solids in the flotation cells for optimum removal of the carbonate minerals. If excessive sand losses are noted in flotation, the pH can normally control the losses through adjustment of one to two pH units. If the losses persist, the addition of sodium silicate at a dosage of 0.25-0.5 Kg per tonne in the conditioner will eliminate the losses.
Cytec offers its AERO 700 series of formulated tall oil fatty acid promoters for carbonate flotation which are more selective and more effective in that dosages are often reduced by as much as 50 percent, and the heavy residue that collects on the flotation equipment associated with the use of a tall oil fatty acid collector is eliminated. These products are much more effective in obtaining a consistent ADV (Acid Demand Value) for foundry operators.