Patent EA-002798-B1 (EA)
Title: Process for bayer liquors causticisation and apparatus therefor
Inventor: ROSENBERG STEVEN PHILIP
IPC: C09K23/00
Filed: 1999-09-13
Abstract: 1. An improved process for the causticisation of Bayer liquors in an alumina refinery, the process including the steps of: reacting lime with aluminate ions in a Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions; and heating said hydrocalumite species in contact with a Bayer liquor under controlled conditions so as to cause the hydrocalumite species to react with the liquor to form calcium carbonate, aluminate ions and hydroxyl ions, whereby a causticised Bayer liquor is obtained and wherein the efficiency of lime utilisation is substantially increased and alumina losses minimised. 2. An improved process for the causticisation of Bayer liquors as defined in Claim 1, wherein the Bayer liquor employed in the first reaction involving the formation of the hydrocalumite species has been pre-causticised whereby the C/S ratio of the precausticised liquor can also be further increased, where C is a sum of sodium aluminate and sodium hydroxide concentrations and S is an actual sodium carbonate concentration. 3. An improved process for the causticisation of Bayer liquors as defined in Claim 2, wherein the first reaction is performed in a Bayer liquor with a moderately high A/C ratio and low free caustic, where A is alumina concentration. 4. An improved process for the causticisation of Bayer liquors as defined in Claim 3, wherein said Bayer liquor has an "S" concentration of between 40 and 350 g/L, and an A/C ratio of between 0.2 and 0.95. 5. An improved process for the causticisation of Bayer liquors as defined in Claim 4, wherein the liquor has an "S" concentration of betveen 120 and l60 g/L, and an A/C ratio greater than 0.55. 6. An improved process for the causticisation of Bayer liquors as defined in Claim 5, wherein a residence time required for the completion of the first reaction is between 5 and 30 minutes, in the presence of a suitable inhibitor. 7. An improved process for the causticisation of Bayer liquors as defined in Claim 6, wherein the first reaction involving the formation of a hydrocalumite slurry is performed at temperatures between about 25 degree C and 100 degree C. 8. An improved process for the causficisation of Bayer liquors as defined in Claim 7, wherein best performance with most Bayer liquors is obtained if the temperature is maintained between about 70 degree C and 80 degree C. 9. An improved process for the causticisation of Bayer liquors as defined in Claim 8, wherein the first reaction occurs in a Bayer liquor which is subject to agitation. 10. An improved process for the causticisation of Bayer liquors as defined in Claim 1, wherein the second reaction involving the heating of the hydrocalumite species in contact with a Bayer liquor is performed at temperatures between about 100 degree C and 180 degree C. 11. An improved process for the causticisation of Bayer liquors as defined in Claim 10, wherein the second reaction is performed under conditions of low shear at about 120 degree C. 12. An improved process for the causticisation of Bayer liquors as defined in Claim 1, wherein the process further includes the step of adding a suitable inhibitor to the Bayer liquor at a suitable point prior to heating the hydrocalumite species whereby undesirable reaction of the hydrocalumite species to form TCA is inhibited. 13. An improved process for the causticisation of Bayer liquors as defined in Claim 12, wherein said inhibitor is a complexing agent and/or surfactant which is capable of being adsorbed at active sites on the surface of the hydrocalumite species to restrict the diffusion of active species at these sites. 14. An improved process for the causticisation of Bayer liquors as defined in Claim I3, wherein suitable surfactants include sugars such as sucrose and glucose, and polysaccharides such as starch. 15. An improved process for the causticisation of Bayer liquors as defined in Claim 13, wherein anionic organic surfactants are employed as said inhibitor. 16. An improved process for the causticisation of Bayer liquors as defined in Claim 15, wherein said anionic organic surfactants are selected from the group consisting of the following materials, their salts, and derivatives: any anionic homopolymers or copolymers (e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate functional groups), hydroxamic acids, humic and tannic acids, lignosulphonates, fatty acids, sulphonated carboxylic acids, carboxylic acids, and polyhydroxy carboxylic acids. 17. An improved process for the causticisation of Bayer liquors as defined in Claim 1, wherein the hydrocalumite slurry formed in the first reaction is subject to solid/liquid separation and the separated hydrocalumite solids are reacted with a fresh liquor to be causticised via said second reaction. 18. An improved process for the causticisation of Bayer liquors as defined in Claim 17, wherein the causticised Bayer liquor obtained from the second reaction involving the heating of the hydrocalumite species is subject to cooling and solid/liquid separation, and wherein at least a portion of the clarified liquor is returned to the first reaction. 19. An improved process for the causticisation of Bayer liquors as defined in Claim 18, wherein the solids separated from the Bayer liquor from the second reaction include substantially pure calcium carbonate which is recalcined to regenerate the lime, for re-use within the refinery, thereby further reducing the lime requirements of the refinery. 20. An improved process for the causticisation of Bayer liquors in an alumina refinery, the process including the steps of: adding a suitable inhibitor to a Bayer liquor to stabilize the formation of a hydrocalumite species during causticisation to inhibit undesirable reaction of the hydrocalumite species to form TCA, whereby the attainable C/S ratio of the liquor can be increased. 21. An improved process for the causticisation of Bayer liquors as defined in Claim 20, wherein said inhibitor is a complexing agent and/or surfactant which is capable of being adsorbed at active sites on the surface of the hydrocalumite species to inhibit the diffusion of active species at these sites. 22. An improved process for the causticisation of Bayer liquors as defined in Claim 21, wherein suitable surfactants include sugars such as sucrose and glucose, and polysaccharides such as starch. 23. An improved process for the causticisation of Bayer liquors as defined in Claim 21, wherein anionic organic surfactants are employed as said inhibitor. 24. An improved process for the causticisation of Bayer liquors as defined in Claim 23, wherein said anionic organic surfactants are selected from the group consisting of the following materials, their salts, and derivatives: any anionic homopolymers or copolymers (e.g. polyacrylic acid and its co-polymers with acrylamide, or polymers bearing hydroxamate functional groups), hydroxamic acids, humic and tannic acids. lignosulphonates, fatty acids, sulphonated carboxylic acids, carboxylic acids, and polyhydroxy carboxylic acids. 25. An improved process for the causticisation of Bayer liquors as defined in Claim 20, wherein the improved process further comprises the step of heating the liquor during causticisation to temperatures within the range 100 degree C to 180 degree C. 26. An improved process for the causticisation of Bayer liquors as defined in Claim 25, wherein the liquor is heated to between 120 degree C and 140 degree C. 27. An improved process for the causticisation of Bayer liquors in an alumina refinery, the process including the steps of: obtaining a pre-causticised Bayer liquor, and reacting lime with aluminate ions in said pre-causticised Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions whereby the C/S ratio of the precausticised liquor can be further increased. 28. An apparatus for the improved causticisation of Bayer liquors in an alumina refinery, the apparatus including: a conventional reactor for causticising a Bayer liquor; and a trim reactor for reacting lime with aluminate ions in the causticised Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions whereby the C/S ratio of the causticised liquor can be increased. 29. An apparatus in an alumina refinery, the apparatus including: a primary reactor for reacting lime with aluminate ions in a Bayer liquor under controlled conditions of low to moderate temperature to form substantially only a hydrocalumite species and hydroxyl ions; and a secondary reactor wherein said hydrocalumite species have been subject to heating in contact with a Bayer liquor under controlled conditions so as to cause the hydrocalumite species to react with the liquor to form calcium carbonate, aluminate ions and hydrohyl ions, whereby a causticised Bayer liquor is obtained and wherein the efficiency of lime utilisation is substantially increased and/or alumina losses are minimised. 30. An apparatus as defined in Claim 29, wherein said primary reactor is a stirred tank reactor in which adequate mixing of the lime and the Bayer liquor occurs to promote the first reaction. 31. An apparatus as defined in Claim 30, wherein said secondary reactor is a stirred tank reactor. 32. An apparatus as defined in Claim 30, wherein said secondary reactor is a pressurised tube reactor. 33. An apparatus as defined in Claim 29, wherein the apparatus further comprises means for separating the solid hydrocalumite species and the liquor from the primary reactor before reacting the hydrocalumite species in the secondary reactor with a fresh liquor. 34. An apparatus as defined in Claim 33, wherein the liquor causticised in the secondary reactor is used as the feed liquor for the primary reactor, whereby the C/S ratio of the caust