Abstract: A grinding aid for use in an abrasive material comprising abrasive grains and a grinding aid consists of particles obtainable by crushing a cold bath from the manufacture of metallic aluminium by reducing Al.sub.2 O.sub.3. This grinding aid is produced by a method wherein the cold bath is crushed, optionally in several steps, whereby particles of an optional free metallic aluminium residue are rolled to flakes, following which the crushed material is screened to optionally sort out the formed aluminium flakes, and then the residual material is recovered as a product or is optionally subjected to further grinding and optionally screening to provide a material which preferably has a grain size distribution corresponding to 100% by weight less than 100 micrometers. This grinding aid lends itself for use in the product of abrasive materials.

Abstract: Aluminum and fluorine are recovered from fluorine containing waste materials from the production of aluminum metal (FCWM) as ALF.sub.2 OH hydrate by a process comprising: leaching FCWM with dilute sulphuric acid at a pH value within the range of 0-3; if needed adding aluminum in an acid soluble form; adjusting the pH value of the aqueous phase to a value within the range 3.7-4.1 at the end of the leaching period by addition of a caustic aqueous solution; precipitating silica at a temperature not exceeding 60.degree. C.; separating the reaction mixture into a solid phase containing precipitated silica and non-soluble residues of the FCWM and a purified solution; and precipitating AlF.sub.2 OH hydrate from the purified solution at a temperature within the range 90.degree.-100.degree. C. and at a pH value not exceeding about 4.1 by controlled continuous addition of an aqueous caustic solution. Reducing and/or non-oxidizing operating conditions are maintained during all these process steps. The precipitated AlF.