Method of producing calcium carbonate from waste and by-products

1. A method for producing calcium carbonate and vanadium comprising the steps of;

a) extracting alkaline industrial waste and by-products using as a first extraction solvent an aqueous solution of a salt formed from a weak acid and a weak base, whereby a first residue is allowed to settle and a calcium-rich first filtrate is formed,

b) filtering, whereby the first filtrate is separated from the first residue,

c) carbonation of the calcium-rich first filtrate using a carbonation gas, whereby calcium carbonate precipitates and a second filtrate is formed,

d) filtering, whereby the calcium carbonate is separated from the second filtrate,

e) dissolving the first residue using a second extraction solvent, whereby a vanadium-lean second residue is allowed to settle and a vanadium-rich third filtrate is formed, and

f) filtering, whereby the second residue is separated from the third filtrate.

2. The method of claim 1 , wherein the first extraction solvent is an aqueous solution of ammonium acetate (CH 3 COONH 4 ).

3. The method of claim 1 , wherein the second extraction solvent is an aqueous solution of ammonium dihydrogen phosphate.

4. The method of claim 1 , wherein the alkaline industrial waste or by-product is iron- and steelmaking slag selected from the group of blast furnace slag, steel converter slag, desulphurization slag and ladle slag of the iron- and steelmaking industry.

5. The method of claim 1 , wherein the gas used in the carbonation is CO 2 , a CO 2 -containing gas, or a CO 2 -containing flue gas of the iron- and steelmaking industry.

6. The method of claim 1 , wherein the carbonation is carried out by bubbling the carbonation gas through the first filtrate or by spraying the filtrate into a flue gas scrubber.

7. The method of claim 1 , wherein a stream of CO 2 -lean gas, used for carbonation, is conducted away from the carbonation step, which stream of CO 2 -lean gas contains a minor amount of evaporated first extraction solvent, which solvent is condensed and recycled to step a) to be used as the first extraction solvent.

8. The method of claim 1 , wherein the second filtrate separated from the formed calcium carbonate in step d) is recycled to step a) to be used as first extraction solvent.

9. The method of claim 1 , wherein steps a and e) are carried out at a temperature of 10-90° C.

10. The method of claim 1 , wherein the vanadium-enriched third filtrate is subjected to electrolysis to produce metallic vanadium.

11. The method of claim 9 , wherein steps a), c) and e) are carried out at 20-70° C.

12. The method of claim 9 , wherein steps a), c) and e) are carried out at 20-25° C.