4. A process of separating alumina from the siliceous material contained in alumina-bearing sand comprising the steps of slurrying the sand with water and sodium hypochlorite in an amount sufficient to cause separation of the siliceous material from the alumina and to permit settling of heavy siliceous material, settling and separating said heavy siliceous material from the slurry to obtain a suspension of alumina and lighter siliceous material, and centrifuging said suspension to reduce the siliceous material content of said suspension.

7. In a process of separating gibbsite from the silica contained in gibbsite-bearing sand, the steps comprising slurrying the sand with water and an aqueous solution of sodium hypochlorite having a concentration of from 0.003 to 6 percent by weight in the water phase of the slurry to cause separation of the silica from the gibbsite and to permit settling of heavy silica values, and settling and separating said heavy silica values from the slurry.

8. A process of separating gibbsite from the silica contained in gibbsite-bearing sand comprising the steps of slurrying the sand with water and an aqueous solution of sodium hypochlorite having a concentration of from 0.003 to 6 percent by weight in the water phase of the slurry, settling and separating heavy silica values from the slurry to obtain a suspension of gibbsite and lighter silica values, and centrifuging said suspension to reduce the silica content of said suspension.

10. In a process of separating alumina from the siliceous material contained in alumina-bearing sand, the steps comprising slurrying the sand with water and a reagent consisting essentially of sodium pyrophosphate and ammonium hydroxide to cause separation of the siliceous material from the alumina and to cause settling of heavy siliceous material from the slurry, the concentration of the sodium pyrophosphate in the water phase of the slurry being in the range of from 0.003 to 0.10 percent by weight and the concentration of the ammonium hydroxide in the water phase of the slurry being in the range of from 0.003 to 6 percent by weight, and settling and separating said heavy siliceous material from the slurry to obtain a suspension of alumina and lighter siliceous material.

14. In a process of separating alumina from the siliceous material contained in alumina-bearing sand comprising the steps Of slurrying the sand with water and trisodium phosphate, the concentration of the trisodium phosphate in the water phase of the slurry being in the range of from 0.003 to 6 percent by weight, and settling and separating heavy siliceous material from the slurry to obtain a suspension of alumina and lighter siliceous material.

17. In a process of separating gibbsite from the silica contained in gibbsite-bearing sand, the steps comprising slurrying the sand with water and a dilute solution of an oxidizing reagent in an amount sufficient to cause separation of the silica from the alumina and to permit settling of heavy silica values, agitating the slurry, and settling and separating said heavy silica values from the slurry to obtain a suspension of alumina and lighter silica values.

21. In a process of separating gibbsite from the silica contained in gibbsite-bearing sand, the steps comprising slurrying the sand with water and a reagent in an amount sufficient to cause separation of the gibbsite from the silica and to permit settling of heavy silica values, said reagent being selected from the group consisting of sodium hypochlorite, sodium pyrophosphate and ammonium hydroxide, and trisodium phosphate, and settling and separating said heavy silica values from the slurry to obtain a suspension of gibbsite and lighter silica values.