Recovery of valuable substances

A method for producing hydroxyapatite, said method comprising the steps of:

• (a) adding oxalic acid to a spent electroless nickel plating solution to precipitate nickel oxalate therefrom, said spent plating solution comprising a water-soluble nickel salt, an organic material complexing agent for the nickel salt, a hypophosphite, a phosphite and an alkali metal ion;
• (b) separating the nickel oxalate precipitate from the solution obtained from step (a);
• (c) adding a mineral acid and a calcium compound to the solution obtained from step (b), wherein said calcium compound is selected from the group consisting of calcium oxide, calcium hydroxide, calcium carbonate, calcium sulfate, calcium chloride and a calcium salt of an organic acid;
• (d) evaporating water from the solution obtained from step (c) to obtain dry solids and firing said solids in air (1) to convert the alkali metal ion into an alkali metal salt of the mineral acid added in step (c), (2) to convert the phosphorus value of the spent solution of step (a) into hydroxyapatite and (3) to remove the organic material through pyrolysis, thereby forming a mixture of alkali metal salt and hydroxyapatite;
• (e) contacting the mixture of alkali metal salt and hydroxyapatite obtained from step (d) with water to form an aqueous solution of the alkali metal salt including undissolved hydroxyapatite;
• and
• (f) recovering the hydroxyapatite.