Production of phosphate compounds from materials containing phosphorus and at least one of iron and aluminium

A method for the production of phosphate compounds, comprising the steps of:

• dissolving a raw material comprising sludge ash, comprising phosphorus, aluminium and iron, in a liquid comprising at least mineral acid;
• separating insoluble residues from said dissolving step, thereby forming a primary leach solution;
• adding a base comprising iron hydroxide to said primary leach solution, after said step of separating insoluble residues, causing precipitation of phosphate compounds comprising iron and aluminium from said primary leach solution;
• removing said precipitated phosphate compounds from said primary leach solution;
• dissolving said phosphate compounds by an alkaline solution into a second leach solution;
• filtering iron hydroxide from said second leach solution;
• adding lime into said second leach solution after said step of filtering iron hydroxide, causing precipitation of calcium phosphate;
• separating precipitated calcium phosphate from said second leach solution;
• recycling said second leach solution after said step of separating precipitated calcium phosphate to be used in said step of dissolving phosphate compounds by an alkaline solution.