Abstract: The present invention is directed to a method for treating a sludge containing substantial amounts of chromium, aluminum, calcium, iron, and phosphorous. The method comprises forming a slurry of the sludge; reacting the slurry with an acid to dissolve substantially all the aluminum and chromium and provide a solids residue comprising mostly calcium sulfate; separating the chromium/aluminum-containing solution from the solids residue; raising the pH of the chromium/aluminum solution to precipitate chromium as chromium hydroxide; and separating the chromium hydroxide from the remaining aluminum-containing solution. The chromium hydroxide is then enriched with an alkali or earth alkali compound and calcined in an oxidizing atmosphere to convert the chromium to a chromate. The calcine is leached in an aqueous medium at a controlled pH to yield a high concentration, high purity chromate solution suitable for producing metallic chromium or a variety of chromium chemicals.

Abstract: A method is provided for treating sludge containing substantial amounts of chromium, aluminum and iron and residuals of other elements. The method comprises forming a slurry of the sludge in water at a temperature ranging from ambient to 150.degree. F. with the specific gravity of the slurry ranging from about 1.05 to 1.25, and adding a mineral acid to the slurry to provide a pH ranging from about 0.1-3 to dissolve selectively the aluminum and the chromium and leave a solids residue containing gangue material comprising an oxidized iron compound, calcium sulfate, calcium fluoride, calcium silicate among other solids. The pH of the solution is controlled at range of about 2 to 3.5 to precipitate undesirable elements, including iron, without substantially adversely affecting the dissolved chromium and aluminum.

Abstract: Metal values and alumina are recovered from spent, usually oily, catalysts by oxygen pressure leaching with sodium hydroxide and/or sodium aluminate to dissolve molybdenum, vanadium and/or tungsten and provide a solid, filterable residue containing alumina and cobalt and/or nickel, the residue is digested with sodium hydroxide to give a sodium aluminate solution and a residue enriched in nickel and/or cobalt, alumina is recovered from the sodium aluminate solution as a solid and the remaining supernatant solution is recycled to the oxygen pressure leaching step wherein the alumina content of the aluminate solution is precipitated and a bleed for metals in the aluminate solution is provided.

Abstract: Selenium is precipitated from solutions containing at least one metal selected from the group consisting of nickel, cobalt, copper, iron, zinc, manganese, magnesium, chromium and cadmium by adding chromous sulfate to the solution to precipitate selenium therefrom.

Abstract: Selenium is precipitated from solutions or slurries containing nonferrous metals, particularly nickel by contacting the solution or slurries with a semimetallic chromous precipitant of the nonferrous metal, particularly nickel, which has a nonferrous metal to chromium ratio between about 10:1 and about 200:1 and which has a metallic content between about 50% and about 90%. The semimetallic chromous precipitant of the nonferrous metal is prepared by hydrogen reducing a solution of the nonferrous metal or slurries containing chromous salts in amount to provide the aforementioned nickel to chromium ratios.

Abstract: A method is provided for the recovery of cobalt from nickel solutions, wherein cobalt is separated from said solution as a precipitate containing cobaltic hydroxide precipitate and also containing nickel, the method comprising dissolving the precipitate for the subsequent recovery of cobalt therefrom by reducing trivalent metal in said precipitate to the divalent state. This is achieved by forming an aqueous slurry of said precipitate acidified with sulfuric acid to a pH ranging from about 0.1 to 2, subjecting the precipitate of said aqueous slurry to electrolytic reduction at the cathode of an electrolytic cell having an insoluble anode, said precipitate being isolated from said anode during said electrolytic reduction and continuing the electrolytic reduction of said precipitate from the trivalent to the divalent state and effect the dissolution thereof.

Abstract: A method is provided for effecting the dissolution of a cobaltic hydroxide-containing precipitate which may also contain nickel, wherein an aqueous slurry of the precipitate containing trivalent metal is acidified to a pH of from about 0.1 to 2, and the aqueous slurry then treated with an organic reductant to effect substantial reduction of the trivalent metal to the divalent state and hence the dissolution of said precipitate.

Abstract: A method is provided for decreasing the silicon content of relatively pure nickel and/or cobalt powder, wherein silicon is present as an impurity substantially in the oxidic state. The method comprises mixing the metal powder in an aqueous solution of an alkali metal hydroxide, e.g. sodium hydroxide, with the temperature of the solution ranging from ambient to below boiling and maintaining the powder in contact with the solution until the silicon content of the powder has been reduced to the desired level.