Method of treating an alkaline granular carbonatable material
The invention concerns a method of treating an alkaline granular carbonatable material which contains aluminium metal and which has in particular a pH of at least 10. The method comprises an oxidation step wherein at least a portion of said aluminium metal is oxidized by contact with moisture. The aluminium should be oxidized to avoid swelling problems when using the granular material as aggregate. In the method according to the invention this oxidation is accelerated by providing at least one oxidizing agent in said moisture, which oxidizing agent has a higher redox potential than the water contained in said moisture. The method further comprises a carbonation step wherein the granular carbonatable material is at least partially carbonated to lower the pH thereof. In this way the formation of ettringite, which may also release aluminium ions which causing further swelling problems, can be avoided in the granular material or any ettringite present therein can be destabilized.
1. Method of treating an alkaline granular carbonatable material which contains aluminium metal and which has a pH of at least 10, which method comprises an oxidation step wherein at least a portion of said aluminium metal is oxidised by contact with moisture, characterised in that said oxidation is accelerated by providing at least one oxidising agent in said moisture, which oxidising agent has a higher redox potential than the water contained in said moisture, and in that the method comprises a carbonation step wherein the granular carbonatable material is at least partially carbonated.
2. Treatment method according to claim 1 , wherein the alkaline granular carbonatable material is carbonated until it has a pH lower than 10.
3. Treatment method according to claim 1 , wherein said carbonation is performed with carbon dioxide.
4. Treatment method according to claim 3 , wherein said carbonation is accelerated by treating the alkaline granular carbonatable material with a medium containing more than 1 wt. % of carbon dioxide.
5. Treatment method according to claim 1 , wherein said oxidising agent is selected from the group consisting of hypochlorites, peroxides, permanganates, perchlorates perborates and mixtures thereof.
6. Treatment method according to claim 1 , wherein said oxidising agent is dissolved in an aqueous medium which is applied onto the alkaline granular material.
7. Treatment method according to claim 1 , wherein said accelerated oxidation is at least partially performed after having lowered the pH of said moisture to a pH lower than 9.
8. Treatment method according to claim 7 , wherein the pH of said moisture is lowered to a pH lower than 9 by performing the carbonation step at least partially before and/or during the oxidation step.
9. Treatment method according to claim 1 , wherein the accelerated oxidation is performed at least partially before the carbonation.
10. Treatment method according to claim 1 , wherein the moisture content of the granular material is less than 90% by dry weight during the accelerated oxidation.
11. Treatment method according to claim 1 , wherein said carbonation is performed in a rotary drum.
12. Treatment method according to claim 1 , wherein said granular carbonatable material contains at least bottom ash.
13. Treatment method according to claim 1 , comprising a prior step of removing part of the aluminium metal by eddy currents.
14. Treatment method according to claim 1 , wherein the granular carbonatable material comprises at the start of said oxidation step at least 0.1% by dry weight of aluminium metal.
15. Treatment method according to claim 1 , wherein said carbonatable material is in the granular state during said carbonation step and during said oxidation step, and comprises during these steps particles with a size greater than 1 mm.
16. Treatment method according to claim 15 , wherein said carbonatable material contains particles with a size greater than 2 mm during the carbonation and oxidation steps.
17. Treatment method according to claim 1 , further comprising a pelletisation step during which the particles of said carbonatable material are agglomerated so as to obtain a coarser granular material, which pelletisation step is performed prior to and/or during the carbonation step.
18. Treatment method according to claim 1 comprising the further step of using the carbonated and oxidised granular material as a construction aggregate to produce a layer which is not bonded by means of cement.
19. Treatment method according to claim 12 , wherein the alkaline granular material contains municipal waste incinerator bottom ash.
20. Treatment method according to claim 2 , wherein the alkaline granular material is carbonated until it has a pH lower than 9.5.
21. Treatment method according to claim 20 , wherein the alkaline granular material is carbonated until it has a pH lower than 9.
22. Treatment method according to claim 4 , wherein said medium is a gas.
23. Treatment method according to claim 4 , wherein said medium contains more than 5 wt. % of carbon dioxide.
24. Treatment method according to claim 23 , wherein said medium contains more than 10 wt. % of carbon dioxide.
25. Treatment method according to claim 10 , wherein the moisture content of the granular material is less than 70% by dry weight during the accelerated oxidation.
26. Treatment method according to claim 25 , wherein the moisture content of the granular material is less than 50% by dry weight during the accelerated oxidation.
27. Treatment method according to claim 16 , wherein said carbonatable material contains particles with a size greater than 4 mm during the carbonation and oxidation steps.