Production of calcium carbonate

1. A method of producing calcium carbonate from lime comprising the steps of:

(i) providing an aqueous solution comprising 10% to 35% by weight of dissolved polyhydroxy compound and 1% to 5% by weight of dissolved calcium hydroxide (expressed as Ca(OH) 2 ) and having a pH of at least 11.5;

(ii) treating the solution prepared in step (i) to remove solids including suspended solids;

(iii) dispersing carbon dioxide through the solution of step (ii) so as to form a mixture containing calcium carbonate, said dispersing initially producing a reduction in the pH of the mixture, and subsequently producing a rise in the pH of the mixture;

(iv) terminating the dispersion of carbon dioxide and adding an alkaline agent at a time after the beginning of the rise in the pH of the reaction mixture in step (iii) but before any subsequent reduction in the pH of the reaction mixture to 9.5, the alkaline agent being added to maintain a pH for the mixture of at least 9.5, and

(v) recovering precipitated calcium carbonate.

2. A method as claimed in claim 1 wherein in step (i) said aqueous solution comprises 20% to 30% by weight of dissolved polyhydroxy compound and 2 to 4.5% by weight of dissolved calcium hydroxide (expressed as Ca(OH) 2 ).

3. A method as claimed in claim 2 wherein in step (i) said aqueous solution comprises 23% to 27% by weight of dissolved polyhydroxy compound and 3% to 4% by weight of dissolved calcium hydroxide (expressed as Ca(OH) 2 ).

4. A method as claimed in claim 3 wherein in step (i) said aqueous solution comprises about 25% by weight of dissolved polyhydroxy compound and 3.4% to 3.9% by weight of dissolved calcium hydroxide (expressed as Ca(OH) 2 ).

5. A method as claimed in claim 1 wherein in step (iii) of the method the amount of carbon dioxide dispersed through the solution is at least the stoichiometric amount required to convert the dissolved calcium to calcium carbonate.

6. A method as claimed in claim 5 wherein during step (iii) there is a transition from a first reaction phase to a second reaction phase marked by a decrease in the rate at which the reaction mixture is able to absorb carbon dioxide and wherein 70% to 85% by weight of the total amount of the stoichiometric amount of carbon dioxide is dispersed in the first reaction phase and the remainder is dispersed in the second reaction phase.

7. A method as claimed in claim 5 wherein during step (iii) there is a transition from a first reaction phase to a second reaction phase marked by formation of a gel and wherein 70% to 85% by weight of the total amount of the stoichiometric amount of carbon dioxide is dispersed in the first reaction phase and the remainder is dispersed in the second reaction phase.

8. A method as claimed in claim 6 wherein in the first reaction phase the carbon dioxide is dispersed at the maximum rate at which it can be absorbed by the reaction mixture.

9. A method as claimed in claim 8 wherein in the second reaction phase the carbon dioxide is dispersed at the maximum rate at which it can be absorbed by the reaction mixture, said dispersion rate in the second reaction phase being less than in the first reaction phase.

10. A method as claimed in claim 1 wherein in step (iv) the rise in pH begins at a pH value of 10.2 to 10.8.

11. A method as claimed in any one of claim 1 wherein in step (iv) dispersion of carbon dioxide is terminated prior to addition of the alkaline agent.

12. A method as claimed in claim 1 wherein, in step (iv) the alkaline agent provides a pH for the product mixture of at least 10.

13. A method as claimed in claim 12 wherein, in step (iv) the alkaline agent provides a pH for the product mixture of at least 10.5.

14. A method as claimed in claim 1 wherein the polyhydroxy compound is of the formula:

HOCH 2 (CHOH) n CH 2 OH

wherein is n is 1 to 6.

15. A method as claimed in claim 14 wherein the polyhydroxy compound is sorbitol.

16. A method as claimed in claim 1 wherein the lime is produced by calcination of limestone.

17. A method as claimed in claim 1 wherein the lime is carbide lime.

18. A method as claimed in claim 1 wherein the lime is Paper Sludge Ash.

19. A method as claimed in claim 1 wherein the alkaline agent is prepared in accordance with the procedure of steps (i) and (ii) as defined above.

20. A method as claimed in claim 1 wherein a solution of polyhydroxy compound is separated from the calcium carbonate in step (v) and is treated with carbon dioxide to reduce its pH to a value of less than 8, and the treated solution is recycled to step (i) of the method.

21. A method as claimed in claim 20 wherein the treatment of the solution of the polyhydroxy compound with carbon dioxide is effected to provide a pH of 7-8 for the solution of the polyhydroxy compound.

22. A method as claimed in claim 20 wherein the solution of the polyhydroxy compound is additionally concentrated by evaporation prior to recycling to step (i) of the method.

23. A method as claimed in claim 7 wherein in the first reaction phase the carbon dioxide is dispersed at a maximum rate at which it can be absorbed by the reaction mixture.

24. A method as claimed in claim 23 wherein in the second reaction phase the carbon dioxide is dispersed at a maximum rate at which it can be absorbed by the reaction mixture, said dispersion rate in the second reaction phase being less than in the first reaction phase.