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

1. 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.

2. The method according to claim 1 , wherein at least a part of said iron hydroxide used in said step of adding a base comprises at least a part of said filtered iron hydroxide obtained in said step of filtering iron hydroxide from said second leach solution.

3. The method according to claim 1 , comprising the further steps of:

adding an acid to at least a deviated part of said second leach solution after said step of separating precipitated calcium phosphate, causing precipitation of aluminium hydroxide; and

separating said precipitated aluminium hydroxide from said deviated part of said second leach solution.

4. The method according to claim 2 , comprising the further steps of:

adding an acid to at least a deviated part of said second leach solution after said step of separating precipitated calcium phosphate, causing precipitation of aluminium hydroxide; and

separating said precipitated aluminium hydroxide from said deviated part of said second leach solution.

5. The method according to claim 3 , wherein said acid used in said step of adding an acid to said deviated part of said second leach solution comprises carbonic acid or gaseous carbon dioxide, wherein said method comprises the further steps of:

adding lime to said deviated part of said second leach solution after said step of separating said precipitated aluminium hydroxide, causing precipitation of calcium carbonate;

separating said precipitated calcium carbonate from said deviated part of said second leach solution; and

recycling said deviated part of said second leach solution after said separating of said precipitated calcium carbonate to be used in said step of dissolving phosphate compounds by an alkaline solution.

6. The method according to claim 4 , wherein said acid used in said step of adding an acid to said deviated part of said second leach solution comprises carbonic acid or gaseous carbon dioxide, wherein said method comprises the further steps of:

adding lime to said deviated part of said second leach solution after said step of separating said precipitated aluminium hydroxide, causing precipitation of calcium carbonate;

separating said precipitated calcium carbonate from said deviated part of said second leach solution; and

recycling said deviated part of said second leach solution after said separating of said precipitated calcium carbonate to be used in said step of dissolving phosphate compounds by an alkaline solution.