AU2007314134B2

AU2007314134B2 - Method for alumina production

Info

Publication number: AU2007314134B2
Application number: AU2007314134A
Authority: AU
Inventors: Gregory Mills
Original assignee: Alcoa of Australia Ltd
Current assignee: Alcoa of Australia Ltd
Priority date: 2006-10-30
Filing date: 2007-10-24
Publication date: 2013-11-07
Anticipated expiration: 2027-10-24
Also published as: WO2008052249A1; BRPI0718257B1; BRPI0718257A2; CN105236458A; AU2007314134A1; CN101595057A; WO2008052249A9

Abstract

A method for the calcination of aluminium trihydroxide, the method comprising the steps of: directly contacting the aluminium trihydroxide with steam; and calcining at least a portion of the aluminium trihydroxide to alumina and/or aluminium oxyhydroxide.

Description

WO 2008/052249 PCT/AU2007/001617 -1 Method for alumina production Field of the Invention The present invention relates to a method for the calcination of aluminium trihydroxide. More specifically, the present invention relates to a method for the 5 calcination of aluminium trihydroxide using steam. Background Art The Bayer process is widely used for the production of alumina from aluminium containing ores, such as bauxite. The process involves contacting alumina containing ores with recycled caustic aluminate solutions, at elevated 10 temperatures, in a process commonly referred to as digestion. After cooling the solution, aluminium trihydroxide is added as seed to induce the precipitation of further aluminium trihydroxide therefrom. The precipitated aluminium trihydroxide, also known as hydrate or gibbsite, is separated from the caustic aluminate solution, with a portion of the aluminium trihydroxide being 15 recycled to be used as seed and the remainder recovered as product. The remaining caustic aluminate solution is recycled for further digestion of alumina containing ore. Depending on the method used to separate the aluminium trihydroxide and the aluminate solution, the recovered aluminium trihydroxide may not be completely dry and may contain both unbound and physically bound water. 20 In the context of the present specification, the term unbound water refers to water that maybe on the surface of the aluminium trioxide and the term physically bound water refers to water that may be held in, for example, interstitial pores of the aluminium trioxide. The recovered aluminium trihydroxide is heated to produce alumina, in a process 25 known as calcination. In addition to removal of the entrained water, water is a by product of the calcination reaction, as aluminium trihydroxide produces aluminium trioxide (A1 2 0 3 ), also known as alumina, according to the following reaction: WO 2008/052249 PCT/AU2007/001617 -2 2A1(OH) 3 -+ A1 2 0 3 +3H 2 0 The calcination reaction can produce a variety of distinct and measurable aluminous structures. These include aluminium trioxide structural forms and aluminium oxyhydroxide structural forms. The preferred composition of smelter 5 grade alumina contains mostly the, so-called, gamma form, but also quantities of other alumina phases (e.g. alpha, kappa, chi, etc.). In the context of the present specification, the term alumina shall be understood to encompass all structural forms or phases of aluminium trioxide including gamma alumina. In the context of the present invention, the term aluminium oxyhydroxide 10 shall be understood to encompass all structural forms or phases of aluminium oxyhydroxide including boehmite. In the context of the present specification, the term calcination shall be taken to encompass the complete or partial removal of the physically and chemically bound and unbound water entering with the aluminium hydroxide feed to the 15 calciner. The successive stages of calcination are: drying to remove unbound and physically bound water; heating in a furnace to remove chemically bound water; and product cooling. 20 Calcination is an energy intensive process. The water released as steam from the aluminium trihydroxide during calcination (- 0.53 t water/tA) is lost to the atmosphere through the stacks together with its latent heat energy (- 1.2 GJ/tA). Additional water is also released to the atmosphere as steam from the unbound water entering with the aluminium trihydroxide feed to the calciner and from the 25 products of combustion.

WO 2008/052249 PCT/AU2007/001617 -3 In prior art systems, large quantities of fuel are used to vaporise unbound water and heat the dried aluminium trihydroxide whereby it passes through a series of intermediate crystalline forms prior to reaching the final desired alumina form. Direct heat transfer equipment and methods for drying and decomposition of 5 aluminium trihydroxide to form an alumina product and steam include the use of static calciners, kiln calciners, electrical resistance heating, hot oil or salt baths, heating by inductance, lasers, plasmas, microwave radiation and combustion of fuels. In flash and kiln calcination processes, combustion gases from the furnace 10 section of the calciner mix directly with the hydrated substance being calcined. After calcination, the gases are separated, e.g. through cyclones and electrostatic precipitators for dust capture. The stack gases are a mixture of the products of combustion and water vapour. US5336480 teaches the calcination of aluminium trihydroxide by indirectly heating 15 the aluminium trihydroxide in a pressurised container and collection of the steam released. The specification teaches that a bed of the aluminium trihydroxide exhibits self-fluidising behaviour on heating due to steam release from the aluminium trihydroxide upon decomposition. According to US5336480, steam is generated for supply to the digester and 20 evaporator portions of the Bayer process by heating aluminium trihydroxide in a decomposer to drive off unbound, physically bound and some chemically bound water from the aluminium trihydroxide. More specifically, the aluminium trihydroxide is heated indirectly in tubes by hot exhaust gases. The water evolved in the tubes in the gaseous state flows upwardly and is said to fluidise the particle 25 beds in the tubes. This has been termed self-fluidising, as the fluidising gas comes from the particles themselves. During start-up, steam from an auxiliary steam source may be used to fluidise particles in the tubes, until self-fluidisation is achieved.

Received 28 August 2008 -4 Advantageously, the present invention permits the utilisation of smaller pressure containers that the method of US5336480, which requires a large pressure container; a consequence of the relatively low gas-side heat transfer coefficient. Further consequences of the large size for this unit include the need to 5 accommodate thermal expansion and difficulties associated with uniformly distributing the solids. The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any 10 of the material referred to was part of the common general knowledge in Australia as at the priority date of the application. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of 15 any other integer or group of integers. Throughout the specification, unless the context requires otherwise, the word "steam" will be understood to encompass dry steam (steam which does not contain water held in suspension mechanically), wet steam (steam which contains water held in suspension), saturated steam (steam at the temperature of the 20 boiling point which corresponds to its pressure) or superheated steam (steam heated to a temperature 'higher than the boiling point corresponding to its pressure). Other definitions for selected terms used herein may be found within the description of the invention and apply throughout. Unless otherwise defined, all 25 other scientific and technical terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the invention belongs. Amended Sheet

IPEA/AU

Received 28 August 2008 -5 Disclosure of the Invention Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and 5 modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features. The present invention is not to be limited in scope by the specific embodiments 10 described herein, which are intended for the purpose of exemplification only. Functionally equivalent products, compositions and methods are clearly within the scope of the invention as described herein. The entire disclosures of all publications (including patents, patent applications, journal articles, laboratory manuals, books, or other documents) cited herein are 15 hereby incorporated by reference. Inclusion does not constitute an admission is made that any of the references constitute prior art or are part of the common general knowledge of those working in the field to which this invention relates. In accordance with the present invention, there is provided a method for the calcination of aluminium trihydroxide, the method comprising the steps of: 20 directly contacting the aluminium trihydroxide with steam; and calcining at least a portion of the aluminium trihydroxide to alumina and/or aluminium oxyhydroxide. wherein at least a portion of the steam is sourced from the calcination of the at least a portion of the aluminium hydroxide. 25 It will be appreciated that the alumina and the aluminium oxyhydroxide may be provided in numerous structural forms. Amended Sheet

IPEA/AU

Received 28 August 2008 -6 Without being limited by theory, it is believed that the present invention may be utilised to control the alumina and aluminium oxyhydroxide structural forms so produced. Preferably, the method comprises the further step of: 5 heating the. aluminium trihydroxide by directly contacting the aluminium trihydroxide with steam. Preferably, the method comprises the further step of: treating the sourced steam to increase its temperature and/or pressure prior to directly contacting the aluminium trihydroxide with the steam. 10 It will be appreciated that the temperature and pressure of the steam will determine whether the steam contains any entrained liquid water. The temperature of the steam is preferably at least about 250 *C. Without being limited by theory, it is believed that the steam usage to achieve the desired level of decomposition is reduced by increasing the steam supply temperature. More 15 preferably, the temperature of the steam is about 480 'C. Without being limited by theory, it is believed that where the temperature of the steam is greater than about 480 0C, the calciner may require construction from specialised materials such as nickel-chromium alloys. In a highly preferred form of the invention, the temperature of the steam is greater than 480 *C. 20 Preferably, the pressure of the steam is greater than atmospheric pressure. In a highly preferred form of the invention, the pressure of the steam is greater than about 6 bar. Preferably, the pressure and temperature of the steam exiting the calcination unit are suitable for further use in a bauxite refinery. In a highly preferred form of the 25 invention, the pressure of the steam exiting the calcination unit is greater than about 6 bar. Amended Sheet

IPEA/AU

WO 2008/052249 PCT/AU2007/001617 -7 Preferably, the steam is superheated steam. In one form of the invention, the method comprises the further step of: subjecting the aluminium trihydroxide, where present, and the mixture of alumina and aluminium oxyhydroxide to a second calcination stage. 5 The step of subjecting the aluminium trihydroxide and the mixture of alumina and aluminium oxyhydroxide to a second calcination stage may be performed by any means known in the art including direct calcination with combustion fuel gases, indirect calcination with combustion fuel gases, direct calcination with steam, indirect calcination with steam, solar, electrical resistance and microwave 10 calcination. Preferably, the step of subjecting the aluminium trihydroxide and the mixture of alumina and aluminium oxyhydroxide to a second calcination stage is performed in a separate calcination unit. Where the second calcination stage comprises direct calcination with combustion 15 fuel gases, the second calcination stage may be conducted in a gas suspension calciner. It will be appreciated that the temperature of the second calcination stage will be determined by the required alumina product properties. For example, it is known that alumina for ceramics applications requires calcination temperatures about 20 1250 0C whilst conventionally alumina calcination for the production of smelter grade alumina is conducted at about 850 to 1100 C. It is believed that the second calcination stage may be conducted at a lower temperature. Preferably, the second calcination stage is conducted at temperatures between about 600 and 950 *C.

WO 2008/052249 PCT/AU2007/001617 -8 Without being limited by theory, it is believed that direct steam calcination produces different alumina structure(s) to that produced in the first stage of conventional calcination. Where the method comprises the step of: 5 subjecting the aluminium trihydroxide and mixture of alumina and aluminium oxyhydroxide to a second calcination stage, the method preferably comprises the further step of: heat recovery from the second calcination stage. In one form of the invention, the step of: 10 heat recovery from the second calcination stage, comprises the step of: heat recovery in a boiler to generate and reheat steam. Preferably, the steam is used in the step of: directly contacting the aluminium trihydroxide with steam. 15 Preferably, the method comprises the further step of: preheating the aluminium trihydroxide. In one form of the invention, the step of: preheating the aluminium trihydroxide; is performed concurrently with the step of: WO 2008/052249 PCT/AU2007/001617 -9 directly contacting the aluminium trihydroxide with steam. In a second form of the invention, the step of preheating the aluminium trihydroxide; is performed prior to the step of: 5 directly contacting the aluminium trihydroxide with steam. Preferably, the method comprises the further step of: drying the aluminium trihydroxide. In one form of the invention, the step of: drying the aluminium trihydroxide; 10 is performed concurrently with the step of: directly contacting the aluminium trihydroxide with steam. In a second form of the invention, the step of drying the aluminium trihydroxide; is performed prior to the step of: 15 directly contacting the aluminium trihydroxide with steam. Advantageously, the step of drying the aluminium trihydroxide can decrease energy consumption. In one form of the invention the method comprises the further step of dewatering the aluminium trihydroxide; WO 2008/052249 PCT/AU2007/001617 -10 prior to the step of directly contacting the aluminium trihydroxide with steam. Preferably, the step of dewatering the aluminium trihydroxide; 5 comprises the step of: dewatering the aluminium trihydroxide in a pressure filter, which may include the use of steam. It will be appreciated that where steam is used subsequently in the plant, it may be necessary to treat said steam to remove, for example, entrained particles. 10 Brief Description of the Drawings The present invention will now be described, by way of example only, with reference to one embodiment thereof, and the accompanying drawing, in which: Figure 1 is a schematic flow sheet showing a Bayer Process circuit; and Figure 2 is a schematic flow sheet showing how a method in accordance with 15 the present invention may be utilised in a Bayer Process circuit. Best Mode(s) for Carrying Out the Invention Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and 20 modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

WO 2008/052249 PCT/AU2007/001617 - 11 Figure 1 shows a schematic flow sheet of the Bayer process circuit 10 for a refinery using a single digestion circuit comprising the steps of: digestion 12 of bauxite 14 in a caustic solution; liquid-solid separation 16 of the mixture to residue 18 and liquor 20; 5 aluminium trihydroxide precipitation 24 from the liquor 20; separation of aluminium trihydroxide 24 and liquor 26; and calcination 31 of the aluminium trihydroxide 24 to alumina 30 and water 32. In accordance with the present invention, aluminium trihydroxide 24 is fed into a duct and heated by direct contact with steam 34 at approximately 480 to 650 0C 10 and greater than 6 bar, best seen in Figure 2. The steam 34 heats and dries the aluminium trihydroxide 24. The mixing of the steam 34 and aluminium trihydroxide 24 results in the release of chemically bound water as water vapour relative to the level of energy available in the mixed streams. The resulting steam released mixes with the heating steam 34. 15 The heat exchange between the aluminium trihydroxide 24 and the steam 34 decomposes a portion of the aluminium trihydroxide 24 and provides a mixture of aluminium hydroxides, oxyhydroxides and various alumina forms 36 depending on the temperature of the recycled steam and the final temperature of the solids. The decomposition of the aluminium trihydroxide 24 results in an increase in the 20 steam mass flow 38 exiting the mixing phase. Sufficient steam is added so that the mixture temperature is about 300 *C. The steam 38 and solids 36 are separated using gas/solids separation technology such as filtration. The overall energy reduction resulting from pressure calcination is through using the released water vapour as process steam. 25 The separated steam 38 is split into two streams 40 and 42. Stream 40 passes through a recirculation blower 44 to a steam re-heater 46 via stream 48, heated to WO 2008/052249 PCT/AU2007/001617 - 12 about 480 to 650 0C and returned as stream 34 and mixed with further aluminium trihydroxide 24. Stream 42 which is actually the water released from the aluminium trihydroxide is cooled to the plant steam requirement which is achieved by water addition 50 5 which creates further steam 52 and the steam discharged to the plant steam circuit. The solids 36 from the gas/solids separation step are passed from the pressurised stage to a Gas Suspension Calciner (GSC) 54 operating at approximately atmospheric pressure where they are heated to about 850 0C to achieve the 10 target product quality. Hot gases 56 from the GSC 54 are used in the steam re-heater 46 to heat the recirculated steam 48 to the recirculation temperature of about 480 to 650 *C. Due to the heat balance, it may be necessary to add fuel 58 to the steam re heater 46. Water 60 is added for further heat recovery from the hot gases prior to 15 the hot gas being discharged to the stack 61. Hot Alumina 62 is cooled with cooling air 64 and water 66 prior to discharging as product 68. The cooling air 64 is therefore preheated prior to passing 70 to the GSC 54. Fuel 72 (e.g. gas or oil) is added to the GSC 54 to maintain the required 20 temperature. The water streams 66, 60 may be used for a variety of purposes in the plant as required by the heat balance. For example, the heated water may be used to cool the process steam (i.e. as stream 50) or these water streams may be used to supplement the recirculated steam flow 34. 25 In a conventional gas suspension calciner, the products of combustion from the calcining stage are used to dry and preheat the aluminium trihydroxide. The products of combustion are also used to provide partially counter current heat WO 2008/052249 PCT/AU2007/001617 -13 exchange in the calcining section. Where the present invention utilises a second calcination stage employing products of combustion from a gas suspension calciner, it is not possible for the products of combustion to provide partially counter current heat exchange in the calcining section. Consequently, products of 5 combustion from the second calcination stage are used to reheat the steam required for the pressure calcination section as shown in Figure 2. As the temperature of the gas suspension calciner is about 850 0C, there may not be sufficient energy available in the products of combustion to provide all of the steam and it may be necessary to provide further fuel and air to provide the 10 required heat to reheat the recycled steam. As shown in Figure 2, steam is recycled from the pressure calcination stage through the steam re-heater 46. To achieve an appropriate heat balance, significant quantities of steam are required at 480 to 650 C. To provide sufficient heat to the pressure calcination section, steam should be recycled and in doing 15 so, the steam inlet temperature into the steam re-heater as low as practical. A further issue is that, depending on the level of decomposition achieved in the first pressure stage, there may be less energy consumed in the gas suspension chamber. When the level of decomposition in the pressure stage is high there will be less energy required in the second stage and therefore there is less air 20 required and therefore there is insufficient air to cool the alumina to a low enough temperature to discharge it to conveyor belts etc. Consequently, it is necessary to recover heat from the partially cooled product and this can also be in the form of process steam or heating boiler feed water. It will be appreciated that a number of heat recovery devices may be employed to 25 recover otherwise waste heat to normal stack temperatures in the range of 150 0C and from the alumina. All of these heat recovery devices could be a single waste heat boiler with various zones. The embodiments of the invention were formulated, evaluated and refined using a combination of inhouse models built on chemical engineering first principles and 30 tuned to existing Bayer unit operations, an extensive database of Bayer properties WO 2008/052249 PCT/AU2007/001617 -14 and thermodynamic data, Bayer operating experience and flowsheet models built within ASPEN PIusTM, ASPEN Technology Inc. software process simulation software with state-of -the-art physical properties packages, including added Bayer process properties and unit operations built inhouse. 5 It will be appreciated that the numbers provided below in relation to flow rates and temperatures are specific to the models and embodiments used and are influenced by parameters put into the models. Investigations have highlighted that the practical application of direct steam calcination can produce approximately 0.39 tonnes of plant steam per tonne of 10 alumina, (0.32 tonnes directly from the decomposition of aluminium trihydroxide and an additional 0.07 tonnes of steam from 4.5 % unbound water in the feed material). It should be noted that the steam from the waste heat recovery is not included in the above 0.39tonnes of steam generated. The maximum amount of steam generated is achieved when all of the bound 15 water is removed in the pressurised stage however the conditions and flow rate of steam required make this impractical. Preliminary studies have highlighted (see Tables 1) that the practical minimum limit is achieved when approximately optimum performance is achieved when approximately 60% of the bound water is released from the aluminium trihydroxide decomposition in the pressure 20 calcination section and provides the following results for a plant with a capacity of 100 tph of alumina. * Plant Steam produced 32 tph at 8 bar (absolute) and 220 0C In conventional gas suspension calciner operations, alumina is produced at benchmark of approximately 3.0 GJ/t (for gas fired plant) and steam at 25 approximately 2.57 GJ/t. Therefore, utilising current equipment to produce the same levels of alumina and steam, the energy consumed would be; the energy reduction to produce the above steam is therefore 2.57 * 0.39 = 1.0 GJ/t of alumina WO 2008/052249 PCT/AU2007/001617 -15 Based on the above calculations, the present invention offers a significant potential to reduce plant energy costs. While the above highlights significant savings, it is anticipated that further savings may be achieved through further optimisation. 5 Table 1 below shows the results from data and process models. In each case the targeted steam condition exiting the system to the plant is 8 bar pressure and 220 0 C. Steam Temp deg C 480 550 650 480 480 480 Steam Recycle t/t of alumia 6.17 3.83 2.48 2.98 3.85 4.93 Rate Pressure stage deg C 360 360 360 330 340 350 exit temp Pressure Boehmite % 91.56 91.56 91.56 34.22 69.77 69.77 Stage Gibbsite % 8.44 8.44 8.44 65.78 30.23 30.23 composition Gamma % 0 0 0 0 0 0 Steam produced based on 4.5% t/t of alumina 0.39 0.39 0.39 0.21 0.26 0.33 unbound water in feed Energy from Steam at 2.57 GJ/t of alumina 1.01 1.01 1.01 0.53 0.68 0.85 GJ/t I I I I Table 1. Data and process modelling results. 10 Compared to a conventional gas suspension calciner where the energy consumption is approximately 3 GJ/t of A1 2 0 3 for gas fired units (2.9 for oil fired units), the above represents a significant saving in energy. The saving needs to be evaluated as a comparison of the energy to produce alumina in a conventional calciner plus the energy to produce the same quantity 15 of steam in a conventional boiler operation. Utilising the model where 0.39 tonnes of steam per tonne of alumina are produced from the pressure stage, from the feed aluminium trihydroxide by direct contact with steam, a 100 tph calciner would require the recirculation of approx 248 tph of WO 2008/052249 PCT/AU2007/001617 -16 steam at approx 650 *C. The main heat exchange duct would be approximately 1.3 m in diameter with a target velocity of 10 ms- 1 . In a conventional gas suspension calciner for a 100 tph plant, the diameter would normally be about 2.7 m. Furthermore, the second stage equipment would also be smaller. It will be 5 appreciated that the present invention will utilise smaller equipment than conventional gas suspension calciner technology. The recycling of large volumes of high temperature steam is believed to be best achieved through the use of turbo machinery. It will be appreciated that as turbo machinery should have clean steam, it may be necessary to filter the steam. 10 Calculations have shown that calcination using direct steam heating has the potential to recover energy in the order of 1 GJ/t of alumina, through the production of up to 0.39 tonnes of process steam per tonne of alumina.

Claims

1. A method for the calcination of aluminium trihydroxide, the method comprising the steps of: directly contacting the aluminium trihydroxide with, steam; and calcining at least a portion of the aluminium trihydroxide to alumina and/or aluminium oxyhydroxide, wherein at least a portion of the steam is sourced from the calcination of the at least a portion of the aluminium trihydroxide; and treating the sourced steam to increase its temperature prior to directly contacting the aluminium trihydroxide.

2. A method for the calcination of aluminium trihydroxide according to claim 1, wherein the method comprises the further step of: heating the aluminium trihydroxide by directly contacting the aluminium trihydroxide with steam.

3. A method for the calcination of aluminium trihydroxide according to claim 1 or claim 2, wherein the method comprises the further step of: treating the sourced steam to increase its pressure prior to directly contacting the aluminium trihydroxide with the steam.

4. A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the temperature of the steam is at least about 250 *C.

5. A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the temperature of the steam is at least about 480 *C.

6. A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the temperature of the steam is between about 480 'C and 650 0 C. 18

7. A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the pressure of the steam is greater than atmospheric pressure.

8. A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the pressure of the steam is greater than about 6 bar.

9. A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the pressure of the steam exiting the calcination unit is greater than about 6 bar.

10.A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the steam is superheated steam.

11.A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the method comprises the further step of: subjecting the aluminium trihydroxide, where present, and the mixture of alumina and aluminium oxyhydroxide to a second calcination stage.

12.A method for the calcination of aluminium trihydroxide according to claim 11, wherein the step of subjecting the aluminium trihydroxide and the mixture of alumina and aluminium oxyhydroxide to a second calcination stage is performed by direct calcination with combustion fuel gases, indirect calcination with combustion fuel gases, direct calcination with steam, indirect calcination with steam, solar, electrical resistance and microwave calcination.

13.A method for the calcination of aluminium trihydroxide according to claim 11 or claim 12, wherein the step of subjecting the aluminium trihydroxide and the mixture of alumina and aluminium oxyhydroxide to a second calcination stage is performed in a separate calcination unit.

14.A method for the calcination of aluminium trihydroxide according to any one of claims 11 to 13, wherein the second calcination stage is conducted in a gas suspension calciner. 19

15.A method for the calcination of aluminium trihydroxide according to claim 14, wherein the second calcination stage is conducted at about 850 C.

16.A method for the calcination of aluminium trihydroxide according to any one of claims 11 to 15, wherein the method comprises the further step of: heat recovery from the second calcination stage.

17.A method for the calcination of aluminium trihydroxide according to claim 16, wherein the step of: heat recovery from the second calcination stage, comprises the step of: heat recovery in a steam re-heater to generate and reheat steam.

18.A method for the calcination of aluminium trihydroxide according to claim 17, wherein the steam is used in the step of: directly contacting the aluminium trihydroxide with steam.

19.A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the method comprises the further step of: preheating the aluminium trihydroxide.

20.A method for the calcination of aluminium trihydroxide according to claim 19, wherein the step of: preheating the aluminium trihydroxide; is performed concurrently with the step of: directly contacting the aluminium trihydroxide with steam.

21.A method for the calcination of aluminium trihydroxide according to claim 19, wherein the step of preheating the aluminium trihydroxide; is performed prior to the step of: 20 directly contacting the aluminium trihydroxide with steam.

22.A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the method comprises the further step of: drying the aluminium trihydroxide.

23.A method for the calcination of aluminium trihydroxide according to claim 22, wherein the step of: drying the aluminium trihydroxide; is performed concurrently with the step of: directly contacting the aluminium trihydroxide with steam.

24.A method for the calcination of aluminium trihydroxide according to claim 22, wherein the step of: drying the aluminium trihydroxide; is performed prior to the step of: directly contacting the aluminium trihydroxide with steam.

25.A method for the calcination of aluminium trihydroxide according to any one of the preceding claims, wherein the method comprises the further step of dewatering the aluminium trihydroxide; prior to the step of directly contacting the aluminium hydroxide with steam.

26.A method for the calcination of aluminium trihydroxide according to claim 25, wherein the step of: dewatering the aluminium trihydroxide; comprises the step of: dewatering the aluminium trihydroxide in a pressure filter, which may include the use of steam. 21

27.A method for the calcination of aluminium trihydroxide as hereinbefore described with reference to the accompanying Figures.