Patent EA-002862-B1 (EA)
Title: Process for the treatment of alumina, a bayer process incorporating treatment process of alumina and feedstock for producing alumina treated by said process
Inventor: HOLLITT MICHAEL JOHN
IPC: C01F7/0613
Filed: 1999-09-28
Abstract: 1. A process for the treatment of an alumina process feedstock prior to transferring the treated feedstock to an alumina process for extracting refined alumina from the treated feedstock, which treatment process includes the following steps: heating the alumina process feedstock to a temperature of 400 degree C to 650 degree C by direct contact with combustion gas, and cooling the heated feedstock to a temperature at which it can be handled and fed to the alumina process; which process further includes controlling the contact time of the alumina process feedstock at temperatures in the above range in step of heating to ensure decomposition of trihydrate alumina and monohydrate alumina present, by dehydration, while not substantially forming monohydrate alumina from trihydrate alumina, or  reducing residual bound water to the extent that extraction is adversely affected, or allowing sufficient time for contact with watex vapour that slower water vapour dependent loss of extractability is experienced; and which process further includes limiting the maximum temperature of incoming gases to a stage of the process in which the alumina process feedstock reaches the above temperature range of 400 to 650 degree C (hereinafter referred to as "the highest temperature stage"). 2. A process fox the treatment of an alumina process feedstock prior to transferring the treated feedstock to an alumina process for extracting refined alumina from the treated feedstock, which treatment process includes the following steps: heating the alumina process feedstock to a temperature of 400 degree C to 650 degree C by direct contact with combustion gas, and cooling the heated feedstock to a temperature at which it can be handled and fed to the alumina process; which process further includes controlling the contact time of the alumina process feedstock at temperatures in the above range in step of heating to ensure decomposition of trihydrate alumina and monohydrate alumina present, by dehydration, while not substantially forming monohydrate alumina from trihydrate alumina, or reducing residual bound water to the extent that extraction is adversely affected, or allowing sufficient time for contact with water vapour that slower water vapour dependent loss of extractability is experienced; and which process further includes limiting the heat input into the highest temperature stage of the process by ensuring that the main thermal load associated with thermal dehydration of the alumina process feedstock is taken up in an earlier stage or stages into which combustion gas, at a temperature above that of the inlet gas of the highest temperature stage, is added. 3. The treatment process defined in Claim 1 further includes limiting the heat input into the highest temperature stage of the process by ensuring that the main thermal load associated with thermal dehydration of the alumina process feedstock is taken up in an earlier stage or stages into which combustion gas, at a temperature above that of the inlet gas of the highest temperature stage, is added. 4. The treatment process defined in Claim 1 includes preheating the alumina process feedstock prior to supplying the feedstock to step of heating. 5. The treatment process defined in Claim 2 or Claim 3 wherein the step of limiting the heat input into the highest temperature stage of the process includes preheating the alumina process feedstock in the earlier stage or stages. 6. The treatment process defined in Claim 4 or Claim 5 includes preheating the alumina process feedstock to a temperature of less than 400 degree C with combustion gas and causing at least partial thermal dehydration of the feedstock and thereby reducing the thermal load, water vapour pressure and the required contact time in step of heating 7. The treatment process defined in Claim 6 includes diluting the combustion gas with outlet gas from the highest temperature stage and preheating the alumina process feedstock with the resultant gas mixture. 8. The treatment process defined in Claim 7 wherein the mixture of the combustion gas and the outlet gas from the highest temperature stage is at a temperature of less than 750 degree C  9. The treatment process defined in any one of the <DP=4preceding Claims wherein step (a) includes heating the alumina process feedstock to the temperature of 400 degree C to 650 degree C by direct contact with combustion gas and air. 10. The treatment process defined in any one of the preceding Claims includes producing combustion gas in excess air and supplying the combustion gas and air to step of heating. 11. The treatment process defined in any one of the preceding Claims wherein the maximum gas temperature in contact with solids in the highest temperature stage of the process is less than 750 degree C. 12. The treatment process defined in Claim 11 wherein the maximum gas temperature in contact with solids in the highest temperature stage of the process is less than 700 degree C. 13. The treatment process defined in Claim 12 wherein the maximum gas temperature in contact with solids in the highest temperature stage of the process is less than 680 degree C. 14. The treatment process defined in any one of the preceding Claims wherein the average particle size of the alumina process feedstock supplied to step (a) is relatively fine to ensure that particle shells are not overheated or otherwise affected by water vapour. 15. The treatment process defined in any one of the preceding Claims wherein the particle size distribution of the alumina process feedstock supplied to step (a) is narrow so that very fine particles are not overheated or otherwise affected by water vapour while the thermal treatment of the cores of coarser particles is completed. 16. The treatment process defined in any one of the preceding Claims includes milling the alumina process feedstock prior to supplying the feed stock to step of heating. 17. The treatment process defined in any one of the preceding Claims wherein the alumina process feedstock supplied to step of heating contains no more than 5% by weight of material which would be retained on a 5mm aperture. 18. The treatment process defined in any one of the preceding Claims wherein the alumina process feedstock supplied to step of heating contains no more than 5% by weight of material which would be retained on a 2mm aperture. 19. The process defined in any one of the preceding Claims wherein the alumina process feedstock supplied to step of heating contains no more than 5% by weight of material which would be retained on a 1mm aperture. 20. The treatment process defined in any one of the preceding Claims wherein the alumina process feedstock supplied to step of heating contains no more than 30% by weight of a material which would pass a 20 micron aperture. 21. The treatment process defined in any one of the preceding Claims wherein the alumina process feedstock contains no more than 20% by weight of material which would pass a 20 micron aperture. 22. The treatment process defined in any one of the preceding Claims wherein the alumina process feedstock contains no more than 10% by weight of material which would pass a 20 micron aperture. 23. The treatment process defined in any one of the preceding Claims includes dividing the alumina process feedstock on the basis of temperature sensitivity or water vapour sensitivity and selectively introducing the divided feedstock streams into step of heating having regard to the sensitivity or sensitivities. 24. The treatment process defined in Claim 18 includes introducing a trihydrate alumina fraction of the alumina process feedstock into step of heating where the combustion gas is sufficiently cool so that there is reduced potential for loss of extractability from use of hotter gases to decompose the alumina trihydrate in this fraction. 25. The treatment process defined in Claim 23 includes introducing a fine fraction of the alumina process feedstock into step of heating where the combustion gas is sufficiently cool so that there is reduced potential for loss of extractability from use of hotter gases to decompose the fines in this fraction. 26. A Bayer process substantially as described herein incorporating the treatment process defined in any one of the preceding Claims. 27. A treated alumina process feedstock produced by the process defined in any one Claims 1 to 25.