WO2022187903A1

WO2022187903A1 - Method for calcination

Info

Publication number: WO2022187903A1
Application number: PCT/AU2022/050199
Authority: WO
Inventors: Raymond Chatfield; Brian HAGGERTY
Original assignee: Alcoa Of Australia Limited
Priority date: 2021-03-12
Filing date: 2022-03-11
Publication date: 2022-09-15
Also published as: BR112023018375A2; AU2022231808A1

Abstract

A method for calcination of hydrated alumina, the method comprising the steps of: heating the hydrated alumina in a dehydrating zone to reduce the water content of the hydrated alumina and provide partially calcined alumina; and heating the partially calcined alumina in a calcining zone to provide alumina, wherein the step of heating the hydrated alumina in a dehydrating zone uses at least in part, stored thermal energy, electrical energy, renewable energy or nuclear energy including combinations thereof and the step of heating the partially calcined alumina in a calcining zone uses at least in part, electrical energy or chemical potential energy including combinations thereof, and where the hydrated alumina is heated at least in part with stored thermal energy, the method is alternately operable in a thermal energy charging state and in a thermal energy discharging state.

Description

Method for calcination

TECHNICAL FIELD

[0001 ] A method for calcination of hydrated alumina.

BACKGROUND ART

[0002] The Bayer process is a cyclic process where bauxite ore is digested in a hot caustic solution to dissolve the alumina bearing minerals. Undissolved solids are separated and the liquor cooled to precipitate hydrated alumina also known as aluminium trihydroxide.

[0003] The recovered hydrated alumina is heated at approximately 1000 °C to produce alumina, in a process known as calcination. In addition to removal of the entrained water, water is a byproduct of the calcination reaction, as hydrated alumina produces aluminium trioxide (AI2O3) also known as alumina.

[0004] 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 grade alumina contains mostly the, so-called, gamma form, but also quantities of other alumina phases (e.g., alpha, kappa, chi, etc.).

[0005] Calcination is an energy intensive process. The water released as steam from the hydrated alumina during calcination (-0.53 t water/t alumina) is lost to the atmosphere through the stacks together with its latent heat energy calcination (-1 .2 GJ/t alumina).

[0006] Additional water is also released to the atmosphere as steam from the unbound water entering with the hydrated alumina feed to the calciner and from the products of combustion.

[0007] 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 of the material referred to was part of the common general knowledge as at the priority date of the application. [0008] 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 shall be understood to encompass all structural forms or phases of aluminium oxyhydroxide including boehmite.

[0009] 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 hydrated alumina feed to the calciner.

[0010] 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 any other integer or group of integers.

[0011 ] Throughout the specification, unless the context requires otherwise, the word "solution" or variations such as "solutions", will be understood to encompass slurries, suspensions and other mixtures containing undissolved and/or dissolved solids.

[0012] 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 modifications. The invention also includes all of the steps, features, compositions and compounds referenced to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

[0013] The present invention is not to be limited in scope by the specific embodiments 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 hereby incorporated by reference.

SUMMARY OF INVENTION

[0014] In accordance with the present invention, there is provided a method for calcination of hydrated alumina, the method comprising the steps of: heating the hydrated alumina in a dehydrating zone to reduce the water content of the hydrated alumina and provide partially calcined alumina; and heating the partially calcined alumina in a calcining zone to provide alumina, wherein the step of heating the hydrated alumina in a dehydrating zone uses at least in part, stored thermal energy, electrical energy, renewable energy or nuclear energy including combinations thereof and the step of heating the partially calcined alumina in a calcining zone uses at least in part, electrical energy or chemical potential energy including combinations thereof, and where the hydrated alumina is heated at least in part with stored thermal energy, the method is alternately operable in a thermal energy charging state and in a thermal energy discharging state.

[0015] In accordance with the present invention, there is provided a method for calcination of hydrated alumina, the method comprising the steps of: heating the hydrated alumina in a dehydrating zone to reduce the water content of the hydrated alumina and provide partially calcined alumina; and heating the partially calcined alumina in a calcining zone to provide alumina, wherein the step of heating the hydrated alumina in a dehydrating zone uses at least in part, stored thermal energy, electrical energy, renewable energy or nuclear energy including combinations thereof, and the step of heating the partially calcined alumina in a calcining zone uses at least in part, electrical energy or chemical potential energy including combinations thereof, wherein the stored thermal energy is provided by a thermal material and the method comprises the further step of: retaining thermal material in a retaining system prior to heating the hydrated alumina. [0016] In one form of the invention, the method is also alternately operable in a thermal energy steady state.

[0017] In accordance with the present invention, there is provided a method for calcination of hydrated alumina, the method comprising the steps of: heating the hydrated alumina in a dehydrating zone to reduce the water content of the hydrated alumina and provide partially calcined alumina; and heating the partially calcined alumina in a calcining zone to provide alumina, wherein the step of heating the hydrated alumina in a dehydrating zone uses at least in part, stored thermal energy, electrical energy, renewable energy or nuclear energy including combinations thereof and the step of heating the partially calcined alumina in a calcining zone uses at least in part, electrical energy or chemical potential energy including combinations thereof, and where the hydrated alumina is heated at least in part with stored thermal energy, the method is alternately operable in a net energy increasing state, and in a net energy decreasing state.

[0018] In one form of the invention, the method is also alternately operable in a net energy steady state.

[0019] In the context of the present specification, the term hydrated alumina shall be understood to encompass all hydroxide and oxyhydroxide forms of aluminium including gibbsite and boehmite.

[0020] It will be appreciated that the hydrated alumina, the partially calcined alumina and the alumina may be provided in numerous structural forms.

[0021] In the context of the present specification, the term partially calcined alumina shall be taken to encompass the complete or partial removal of the physically and chemically bound and unbound water.

[0022] Without being limited by theory, it is believed that the calcination of partially calcined alumina removes physically bound water followed by water of crystallisation. Partial removal of water of crystallisation can result in the formation of different aluminum-containing species including aluminium trioxide in a variety of structural forms and aluminium oxyhydroxide in a variety of structural forms. Aluminium trioxide structural forms may include gamma, alpha, kappa and chi.

[0023] The partially calcined alumina may be provided with varying degrees of hydration. In one form of the invention, the partially calcined alumina is substantially aluminium monohydrate. Preferably, the partially calcined alumina is boehmite.

[0024] In the context of the present specification, the term chemical potential energy shall be understood to refer to the energy stored in the chemical bonds of one or more substances. In one form of the invention, the chemical potential energy is provided in the form of aluminium powder. When contacted with steam, aluminium powder releases large amounts of energy in addition to hydrogen and alumina. Said energy may provide at least in part, energy for the calcination step.

[0025] In one form of the invention, the chemical potential energy is provided in the form of combustion energy.

[0026] In the context of the present specification, the term combustion energy shall be understood to refer to energy resulting from combustion, a chemical reaction between two substances usually including oxygen.

[0027] Advantageously, retaining the thermal material in a retaining system enables the stored thermal energy to be accessed as required.

[0028] In one form of the invention, the thermal material is retained for at least 10 minutes. In one form of the invention, the thermal material is retained for at least 20 minutes. In one form of the invention, the thermal material is retained for at least 60 minutes. In one form of the invention, the thermal material is retained for at least 2 hours minutes. In one form of the invention, the thermal material is retained for at least 4 hours. In one form of the invention, the thermal material is retained for at least 8 hours. In one form of the invention, the thermal material is retained for at least 12 hours. In one form of the invention, the thermal material is retained for at least 24 hours. In one form of the invention, the thermal material is retained for at least 2 days.

In one form of the invention, the thermal material is retained for at least 3 days. In one form of the invention, the thermal material is retained for at least 4 days. In one form of the invention, the thermal material is retained for at least 5 days. In one form of the invention, the thermal material is retained for at least 6 days. In one form of the invention, the thermal material is retained for at least 7 days. In one form of the invention, the thermal material is retained for at least 14 days.

[0029] In one form of the invention, the thermal material is retained for up to 10 minutes. In one form of the invention, the thermal material is retained for up to 20 minutes. In one form of the invention, the thermal material is retained for up to 60 minutes. In one form of the invention, the thermal material is retained for up to 2 hours minutes. In one form of the invention, the thermal material is retained for at least up to 4 hours. In one form of the invention, the thermal material is retained for up to 8 hours. In one form of the invention, the thermal material is retained for up to 12 hours. In one form of the invention, the thermal material is retained for up to 24 hours. In one form of the invention, the thermal material is retained for up to 2 days. In one form of the invention, the thermal material is retained for up to 3 days. In one form of the invention, the thermal material is retained for up to 4 days. In one form of the invention, the thermal material is retained for up to 5 days. In one form of the invention, the thermal material is retained for up to 6 days. In one form of the invention, the thermal material is retained for up to 7 days. In one form of the invention, the thermal material is retained for up to 14 days.

[0030] In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 10 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 20 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 30 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 60 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 1 hour. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 2 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 4 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 6 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 12 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 24 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 1 day. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 2 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 3 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 4 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 5 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 6 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 7 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for at least 14 days.

[0031] In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 10 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 20 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 30 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 60 minutes. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 1 hour. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 2 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 4 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 6 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 12 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 24 hours. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 1 day. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 2 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 3 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 4 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 5 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 6 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 7 days. In one form of the invention, the method comprises the additional step of retaining sufficient stored thermal energy to accommodate continuous heating of the hydrated alumina for up to 14 days.

[0032] In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 10 minutes before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 20 minutes before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 60 minutes before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 2 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 4 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 8 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 12 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 24 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 2 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 3 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 4 days before operating the method in a thermal energy discharging state.

In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 5 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 6 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 7 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for at least 14 days before operating the method in a thermal energy discharging state.

[0033] In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 10 minutes before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 20 minutes before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 60 minutes before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up 1 to hour before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 2 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 4 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 8 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 12 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 24 hours before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 2 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 3 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 4 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 5 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 6 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 7 days before operating the method in a thermal energy discharging state. In one form of the invention, the step of operating the method in a thermal energy charging state continues for up to 14 days before operating the method in a thermal energy discharging state.

[0034] In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 10 minutes before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 20 minutes before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 60 minutes before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 2 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 4 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 8 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 12 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 24 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 2 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 3 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 4 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 5 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 6 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 7 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for at least 14 days before operating the method in a thermal energy charging state.

[0035] In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 10 minutes before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 20 minutes before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 60 minutes before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up 1 to hour before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 2 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 4 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 8 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 12 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 24 hours before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 2 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 3 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 4 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 5 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 6 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 7 days before operating the method in a thermal energy charging state. In one form of the invention, the step of operating the method in a thermal energy discharging state continues for up to 14 days before operating the method in a thermal energy charging state.

[0036] In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 10 minutes before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 20 minutes before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 60 minutes before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 2 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 4 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 8 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 12 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 24 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 2 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 3 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 4 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 5 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 6 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 7 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for at least 14 days before operating the method in a net energy decreasing state.

[0037] In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 10 minutes before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 20 minutes before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 60 minutes before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up 1 to hour before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 2 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 4 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 8 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 12 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 24 hours before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 2 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 3 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 4 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 5 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 6 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 7 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy increasing state continues for up to 14 days before operating the method in a net energy decreasing state.

[0038] In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 10 minutes before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 20 minutes before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 60 minutes before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 2 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 4 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 8 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 12 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 24 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 2 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 3 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 4 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 5 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 6 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 7 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for at least 14 days before operating the method in a net energy increasing state.

[0039] In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 10 minutes before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 20 minutes before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 60 minutes before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up 1 to hour before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 2 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 4 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 8 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 12 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 24 hours before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 2 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 3 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 4 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 5 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 6 days before operating the method in a net energy increasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 7 days before operating the method in a net energy decreasing state. In one form of the invention, the step of operating the method in a net energy decreasing state continues for up to 14 days before operating the method in a net energy decreasing state.

[0040] During the thermal energy charging state, electricity may be drawn from an external supply. During the net energy increasing state, electricity may be drawn from an external supply. Preferably, the electricity is drawn for a period that is suitable. Suitability may be determined by a number of factors including price, availability, demand or by agreement.

[0041 ] Electricity availability may be relevant where the electricity is derived from renewable sources. Some renewable energy sources such as solar and wind are not generally continuously available.

[0042] Local electricity demand can impact electricity supply from a grid. During periods of high demand, sufficient electricity may not be available to operate the thermal energy charging state or the net energy increasing state.

[0043] Local electricity supply may be classified according to demand with the price being reflective thereof. For example, periods of high demand may be classified as peak demand or peak electricity and may be associated with higher prices. Periods of high demand may be classified as off-peak demand or off-peak electricity and may be associated with lower prices. There may further be periods between high and low demand, referred to as shoulder periods with intermediate pricing.

[0044] It will be appreciated that access to off-peak electricity will be impacted by the relevant regulatory electricity supply. For example, over a 24 hour period, there may be one period of off-peak electricity supply or there may be two or more. In addition, either side of the off-peak period, there may be shoulder periods. The availability of off-peak electricity may be different on weekends and on working days. It may also change with the seasons of the year.

[0045] For example, the electricity supply in Perth, Western Australia from Monday to Friday is charged at peak prices from 3 pm to 9 pm, at off-peak prices from 9 pm until 7 am and at shoulder prices from 7 am to 3 pm. Operating the method of the present invention in Perth, Western Australia may entail operating the thermal energy charging state from 9 pm until 7 am thereby stockpiling hot thermal material. During this time, hot thermal material may still be used to heat the hydrated alumina. At 7 am, the electricity supply to the thermal material can be reduced or terminated and the system can run from 7 am until 9 pm in the thermal energy discharging state. During this period, energy to heat the hydrated alumina is provided by the stockpiled thermal material.

[0046] In transitioning between the thermal energy charging state and the thermal energy discharging state, the external electricity supply may be gradually decreased.

As the electricity supply decreases, the method will transition from the thermal energy charging state to the thermal energy discharging state transitioning through the steady state as it does so.

[0047] In transitioning between the thermal energy discharging state and the thermal energy charging state, the external electricity supply may be gradually increased. As the electricity supply increases, the method will transition from the thermal energy discharging state to the thermal energy charging state transitioning through the steady state as it does so.

[0048] In transitioning between the net energy increasing state and the net energy decreasing state, the external electricity supply may be gradually decreased. As the electricity supply decreases, the method will transition from the net energy increasing state to the net energy decreasing state transitioning through the steady state as it does so.

[0049] In transitioning between the net energy decreasing state and the net energy increasing, the external electricity supply may be gradually increased. As the electricity supply increases, the method will transition from the net energy decreasing state to the net energy increasing state transitioning through the steady state as it does so.

[0050] The thermal energy may be provided in the form of a heated thermal material.

[0051 ] The thermal material may indirectly or directly heat the hydrated alumina.

[0052] The stored thermal energy may be provided in the form of heat that is either sensible or latent. Sensible heat corresponds to thermal storage in a single phase where the temperature of the thermal material varies with the amount of stored energy.

[0053] The thermal material may be provided as a liquid or a solid. Non limiting examples of a liquid thermal material include carbon dioxide, water, oil, molten metals such as molten sodium, molten potassium or molten eutectic mixtures of sodium- potassium or lead-bismuth and molten salts. Non limiting examples of a solid thermal material include non-molten salts, rock, metal or non-molten compounds such as alumina or partially calcined alumina.

[0054] The thermal material may be provided in the form of an organic fluid such as an oil. It will be appreciated that an organic fluid must have appropriate operating temperature. A non-limiting example of a suitable fluid is Dowtherm^® A from Dow^®.

[0055] In one form of the invention, the thermal material is a molten salt. Advantageously, molten salts have high boiling points, low viscosity, low vapour pressure and high volumetric heat capacities.

[0056] Preferably, the molten salt is an ionic salt of an alkali metal such as those comprising fluorides, chlorides, carbonates, nitrates, nitrites. It will be appreciated that the molten salt may contain more than one salt.

[0057] Suitable salts include sodium nitrate NaNC>3 and potassium nitrate KNO3 which have melting points between 300-500°C and volumetric heat capacities between 1670 - 3770 kJ/m^3oC. Sodium hydroxide NaOH has a melting point of 320 °C and can be used up to 800 °C, but is highly corrosive. Solar salt, 60 % NaNC>3 and 40 % KNO3 may also be used. Commercially available "HITEC" salt consists of potassium nitrate (53 % by weight), sodium nitrite NaNC>2 (40 % by weight), and sodium nitrate (7 % by weight) with a liquid temperature range of 149 - 538 °C. [0058] The thermal material may be heated with electrical energy, renewable energy, nuclear energy or combinations thereof.

[0059] It will be appreciated that the stored thermal energy results from heating the thermal material directly or indirectly with electricity.

[0060] Preferably, the step of heating the thermal material with electricity utilises off- peak electricity.

[0061] In one form of the invention, at least a portion of the electricity is generated from a renewable power source and/or a nuclear energy source. Renewable power sources may include wind, solar, hydro, tidal, geothermal, biomass and hydrogen. In an alternate form of the invention, the electricity is generated from nuclear energy.

[0062] In one form of the invention, all of the electricity is generated from a renewable power source

[0063] In one form of the invention, the electricity is sourced from a supply grid. At least a portion of the supply grid electricity may be generated from renewable power sources and/or nuclear energy sources. Renewable power sources may include wind, solar, hydro, tidal, geothermal, biomass and hydrogen.

[0064] In one form of the invention, the thermal material is heated directly with energy from a renewable energy source.

[0065] In one form of the invention, the thermal material is heated with solar energy using with solar towers or solar troughs to concentrate the solar radiation for direct use on hydrated alumina.

[0066] Advantageously, thermal materials can retain their heat for a number of hours without (continuous) energy input. During peak electricity demand periods, electricity to the thermal material can be shut off without significant temperature reduction. Peak electricity demand is generally in the early evening, such as between 5 pm and 8 pm. Understanding that larger volumes of molten material can retain heat for longer periods, it is believed that a three or four hour period can be compensated for.

[0067] Preferably, the stored thermal energy has a carbon intensity of less than 0.5 t CC MWhr. In one form of the invention, the electricity has a carbon intensity of less than 0.4 t CCWMWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.3 t CCWMWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.2 t CCWMWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.1 t CCWMWhr.

[0068] Advantageously, the method of the present invention may decrease the carbon output of the alumina refinery. The term decrease shall be understood to encompass reducing the carbon output relative to an equivalent alumina refinery solely utilising fossil fuel fired calcination as is known in the art. The term decrease shall also be understood to encompass the retrofit of an existing alumina refinery with the present invention as well as the construction of a new alumina refinery with the present invention.

[0069] There may further be provided a heat transfer medium to transfer the heat from the thermal material to the hydrated alumina. The use of a heat transfer medium is particularly applicable where the thermal material is a solid such as alumina.

[0070] The thermal material may be heated with electrical energy, renewable energy, nuclear energy or combinations thereof.

[0071] The heat transfer medium is preferably provided as a fluid. Non limiting examples of a heat transfer medium include gases such as carbon dioxide, water, oil, molten metals such as molten sodium, molten potassium or molten eutectic mixtures of sodium-potassium or lead-bismuth and molten salts.

[0072] The heat transfer medium may be provided in the form of an organic fluid such as an oil. It will be appreciated that an organic fluid must have appropriate operating temperature. A non-limiting example of a suitable fluid is Dowtherm^® A from Dow^®.

[0073] In one form of the invention, the heat transfer medium is a molten salt. Advantageously, molten salts have high boiling points, low viscosity, low vapour pressure and high volumetric heat capacities.

[0074] Preferably, the molten salt is an ionic salt of an alkali metal such as those comprising fluorides, chlorides, carbonates, nitrates, nitrites. It will be appreciated that the molten salt may contain more than one salt. [0075] Suitable salts include sodium nitrate NaNC>3 and potassium nitrate KNO3 which have melting points between 300-500°C and volumetric heat capacities between 1670 - 3770 kJ/m^3oC. Sodium hydroxide NaOH has a melting point of 320 °C and can be used up to 800 °C, but is highly corrosive. Commercially available "HITEC" salt consists of potassium nitrate (53 % by weight), sodium nitrite NaNC (40 % by weight), and sodium nitrate (7 % by weight) with a liquid temperature range of 149 - 538 °C.

[0076] In one form of the invention, the heat transfer medium is heated by the thermal material.

[0077] In one form of the invention, the heat transfer medium is heated using electricity.

[0078] Preferably, the step of heating the heat transfer medium utilises off-peak electricity.

[0079] In one form of the invention, at least a portion of the electricity is generated from a renewable power source and/or a nuclear energy source. Renewable power sources may include wind, solar, hydro, tidal, geothermal, biomass and hydrogen. In an alternate form of the invention, the electricity is generated from nuclear energy.

[0080] In one form of the invention, all of the electricity is generated from a renewable power source

[0081] In one form of the invention, the electricity is sourced from a supply grid. At least a portion of the supply grid electricity may be generated from renewable power sources and/or nuclear energy sources. Renewable power sources may include wind, solar, hydro, tidal, geothermal, biomass and hydrogen.

[0082] In one form of the invention, the step of heating the heat transfer medium uses at least in part, stored thermal energy, electrical energy, renewable energy sources or nuclear energy

[0083] In one form of the invention, the heat transfer medium is heated directly with energy from a renewable energy source. [0084] In one form of the invention, the heat transfer medium is heated with solar energy using with solar towers or solar troughs to concentrate the solar radiation for direct use on hydrated alumina.

[0085] The retaining system may be provided in the form of one or more storage vessels.

[0086] Retaining the thermal material in at least one storage vessel may include monitoring the temperature of the thermal material for the period of time the thermal material is retained in at least one storage vessel. It will be appreciated that the temperature of the thermal material should not drop significantly during the period of time the thermal material is retained in the at least one storage vessel.

[0087] It will be appreciated that the mass of the thermal material in the at least one storage vessel should have sufficient heat capacity to account for periods of time where the energy supply for heating the thermal material is interrupted.

[0088] The thermal material is retained in at least one vessel prepared from materials with appropriate chemical and thermal resistivity. For example, stainless steel is expected to be appropriate for a molten salt up to about 600 °C. At higher temperatures, a refractory lining would be considered.

[0089] The storage vessel may be provided with electrical heating elements.

[0090] In one form of the invention, there are provided two storage vessels. The two storage vessels are adapted to retain thermal material at different temperatures. In the context of the present specification, the storage vessel adapted to retain thermal material at the lower of the two temperatures is designated the low temperature storage vessel and the storage vessel adapted to retain thermal material at the higher of the two temperatures is designated the high temperature storage vessel.

[0091] The hotter thermal material is adapted for heating the hydrated alumina in a dehydrating zone. It will be appreciated that the step of heating the hydrated alumina in a dehydrating zone decreases the temperature of the thermal material. After the step of heating the hydrated alumina in a dehydrating zone, this cooled thermal material is transferred to the low temperature storage vessel. [0092] The hydrated alumina may be heated directly or indirectly with electrical energy. Where the hydrated alumina is heated with electrical energy, the electrical energy may be used to heat electrical elements in direct contact with the hydrated alumina.

[0093] Preferably, the step of heating the hydrated alumina with electrical energy utilises off-peak electricity.

[0094] In one form of the invention, at least a portion of the electrical energy is generated from a renewable power source and/or a nuclear energy source. Renewable power sources may include wind, solar, hydro, tidal, geothermal, biomass and hydrogen. In an alternate form of the invention, the electricity is generated from nuclear energy.

[0095] In one form of the invention, all of the electrical energy is generated from a renewable power source.

[0096] In one form of the invention, the electrical energy is sourced from a supply grid. At least a portion of the supply grid electrical energy may be generated from renewable power sources and/or nuclear energy sources. Renewable power sources may include wind, solar, hydro, tidal, geothermal, biomass and hydrogen.

[0097] . It will be appreciated that electrical energy may require storage prior to use.

This is particularly relevant for periods of high power demand. Storage may be provided in the form of a battery.

[0098] In one form of the invention, the hydrated alumina is heated directly with energy from a renewable energy source.

[0099] In one form of the invention, the hydrated alumina is heated with solar energy using with solar towers or solar troughs to concentrate the solar radiation for direct use on hydrated alumina.

[00100] Preferably, the step of heating the hydrated alumina in a dehydrating zone is conducted at a temperature of between 250 °C and 600 °C. More preferably, the step of heating the hydrated alumina in a dehydrating zone is conducted at a temperature of between 350 °C and 500 °C. In one form of the invention, the step of heating the hydrated alumina in a dehydrating zone is conducted at a temperature of about 450 °C. [00101 ] In one form of the invention, the step of: heating the hydrated alumina in a dehydrating zone, is conducted under a pressure greater than atmospheric pressure.

[00102] Where the step of heating the hydrated alumina in a dehydrating zone is conducted under a pressure greater than atmospheric pressure, the pressure is preferably about 600 -1000 kPa gauge. More preferably, the pressure is about 700 - 850 kPa gauge. It will be appreciated that high temperature refineries may operate at higher pressure up to 7000 kPa.

[00103] In one form of the invention, the hydrated alumina is indirectly heated with an indirect heat exchanger. Suitable indirect heat exchangers include shell and tube heaters and plate heat exchangers. The hydrated alumina may be heated in a fluidised bed. Alternatively, or in addition thereto, the hydrated alumina may be heated with electrical elements.

[00104] Preferably, the step of heating the partially calcined alumina in a calcining zone is conducted at a temperature of between 500 °C and 1200 °C.

[00105] In one form of the invention, the step of heating the partially calcined alumina in a calcining zone is conducted at a temperature of between 500°C and 1000 °C. In one form of the invention, the step of heating the partially calcined alumina in a calcining zone is conducted at a temperature of between 500°C and 900 °C. In one form of the invention, the step of heating the partially calcined alumina in a calcining zone is conducted at a temperature of between 500°C and 800 °C. In one form of the invention, the step of heating the partially calcined alumina in a calcining zone is conducted at a temperature of between 500°C and 700 °C. In one form of the invention, the step of heating the partially calcined alumina in a calcining zone is conducted at a temperature of between 500°C and 600 °C.

[00106] It will be appreciated that the step of heating the partially calcined alumina in a calcining zone may not calcine all of the partially calcined alumina.

[00107] Advantageously, the step of heating the partially calcined alumina in a calcining zone may be conducted at lower temperatures than in traditional calcination. The concentrated steam that evolves can act as a mineraliser to lower the temperature required for calcination.

[00108] In one form of the invention, the step of: heating the partially calcined alumina in a calcining zone, is conducted under a pressure greater than atmospheric pressure.

[00109] Where the step of heating the partially calcined alumina in a calcining zone, is conducted under a pressure greater than atmospheric pressure, the pressure is preferably about 600 - 1000 kPa gauge. More preferably, the pressure is about 700 - 850 kPa gauge.

[00110] In one form of the invention, the partially calcined alumina is directly heated with electricity, for example, with electrical elements. In an alternate form of the invention, partially calcined alumina is heated by combustion, for example, with hydrogen, natural gas or fuel oils or combinations thereof.

[00111] It will be appreciated that where hydrogen is the combustion fuel, a source of oxygen will be required.

[00112] The partially calcined alumina may be heated in a fluidised bed. The fluidised bed is preferably heated with electrically heated elements.

[00113] In one form of the invention, the method comprises the additional step of: heating the hydrated alumina in a drying zone prior to the step of heating the hydrated alumina in a dehydrating zone.

[00114] Preferably, the step of heating the hydrated alumina in a drying zone is conducted at a temperature of between 100 °C and 300 °C. More preferably, the step of heating the hydrated alumina in a dehydrating zone is conducted at a temperature of between 150 °C and 250 °C. In one form of the invention, the step of drying the hydrated alumina in a dehydrating zone is conducted at a temperature of about 190 °C.

[00115] In one form of the invention, the step of: heating the hydrated alumina in a drying zone, is conducted under a pressure greater than atmospheric pressure.

[00116] Where the step of heating the hydrated alumina in a drying zone, is conducted under a pressure greater than atmospheric pressure, the pressure is preferably about 600 - 1000 kPa gauge. More preferably, the pressure is about 700 - 850 kPa gauge.

[00117] The step of drying the hydrated alumina may remove at least some free water and may remove some water of crystallisation.

[00118] The step of heating the hydrated alumina in a drying zone may remove at least some free water and may remove some water of crystallisation.

[00119] It will be appreciated that the step of heating the hydrated alumina in a drying zone will generate water vapour. Said water vapour may be utilised in the Bayer refinery where appropriate, depending on the pressure and temperature. In one form of the invention, the water vapour is compressed up to about 800 kPa and 220 °C and returned to the refinery. In an alternate form of the invention, the water vapour is recycled to the drying zone to facilitate the step of drying the hydrated alumina. In an alternate form of the invention, the water vapour is used to fluidise the solids in the dehydration zone, the calcining zone, and/or recycled to fluidise the solids in the drying zone.

[00120] It will be appreciated that the water vapour may need to be compressed to be utilised. In one form of the invention, the step of compressing the water vapour is performed by mechanical vapour recompression.

[00121 ] It will be appreciated that the step of heating the hydrated alumina in a dehydrating zone will generate water vapour. Said water vapour may be utilised in the Bayer refinery where appropriate, depending on the pressure and temperature. In one form of the invention, the water vapour is compressed up to about 800 kPa and 220 °C and returned to the refinery. For a high temperature refinery, the water vapour may be compressed up to about 6000 kPa and 300 °C. In an alternate form of the invention, the water vapour is passed to the drying zone to facilitate the step of drying the hydrated alumina. In an alternate form of the invention, the water vapour is used to fluidise the solids in the drying zone, the calcining zone, and/or recycled to fluidise the solids in the dehydration zone. [00122] It will be appreciated that the water vapour may need to be compressed to be utilised. In one form of the invention, the step of compressing the water vapour is performed by mechanical vapour recompression

[00123] It will be appreciated that the step of calcining the partially calcined alumina will generate water vapour. Said water vapour may be utilised in the Bayer refinery where appropriate, depending on the pressure and temperature. In one form of the invention, the water vapour is compressed up to about 800 kPa and 220 °C and returned to the refinery. Alternatively, or in addition thereto, the water vapour is passed to the drying zone and/or the dehydrating zone.

[00124] In an alternate form of the invention, the water vapour is used to fluidise the solids in the drying zone, the dehydration zone, and/or recycled to fluidise the solids in the calcining zone.

[00125] It will be appreciated that the water vapour may need to be compressed to be utilised. In one form of the invention, the step of compressing the water vapour is performed by mechanical vapour recompression

[00126] At least some of the water vapour generated from at least two of the steps of; heating the hydrated alumina in a drying zone, heating the hydrated alumina in a dehydration zone; and heating the partially calcined alumina in a calcining zone; and may be combined.

[00127] Generated water vapour may be treated, returned to the refinery or used elsewhere in calcination.

[00128] The step of treating the captured water vapour may comprise compressing the captured water vapour.

[00129] The step of compressing the captured water vapour may be repeated. There may be a plurality of compression steps to attain the desired water vapour condensing temperature. [00130] In one form of the invention, the step of compressing the captured water vapour is performed by mechanical vapour recompression. Advantageously, mechanical vapour recompression reduces reliance on fossil fuel generated sources of heat to provide high condensing temperature steam.

[00131 ] Mechanical vapour compression may be performed by centrifugal compressors, axial flow compressors or turbo compressors. Centrifugal compressors may be classified as high speed or low speed compressors.

[00132] There may be provided a plurality of mechanical vapour compressors in series wherein compressed steam from the first compressor in the series is passed to the second compressor for further compression. This process may be repeated until the steam is at the desired condensing temperature.

[00133] It will be appreciated that a power source is required to compress the captured water vapour. Preferably, said power source is a low carbon power source. In one form of the invention, the power source is a renewable power source.

[00134] In one form of the invention, the electricity is generated from a renewable power source. Renewable power sources may include wind, solar, hydro, tidal, geothermal, biomass and hydrogen. In an alternate form of the invention, the electricity is generated from a zero carbon power source such as hydrogen or nuclear. The electricity may be stored in a battery after generation and prior to use.

[00135] In one form of the invention, the compressor is electrically powered with electricity with a carbon intensity of less than 0.5 t CCWMWhr. In one form of the invention, the electricity has a carbon intensity of less than 0.4 t CCWMWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.3 t CCWMWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.2 t CCWMWhr. In an alternate form of the invention, the electricity has a carbon intensity of less than 0.1 t CCWMWhr.

[00136] Compressed water vapour may be used to fluidise solids in one or more of the drying zone, the dehydrating zone or the calcining zone.

[00137] In one form of the invention, the method comprises the additional step of: recovering heat from the calcined alumina in a first stage heat recovery zone.

[00138] The heat recovered from the calcined alumina may be utilised in the refinery.

In addition, thereto or separate therefrom, the heat recovered from the calcined alumina is used to heat a liquid or solid heat transfer material. In one form of the invention, the liquid heat transfer material is provided in the form of a high temperature oil. Suitable high temperature oils include Dowtherm. Alternatively, heat may be transferred through solid walls or pipes made of steel or other suitable heat transfer solids. The heat transfer material may be utilised in the step of drying the hydrated alumina.

[00139] In addition, the heat recovered from the calcined alumina may be used to heat a heat transfer fluid such as a molten salt. The heat transfer fluid may be utilised in the step of dehydrating the hydrated alumina.

[00140] It will be appreciated that the heat recovered from the first stage heat recovery zone should be hot enough to the heat the drying zone and the dehydration zone.

[00141] In one form of the invention, the method comprises the additional step of: recovering heat from the calcined alumina in a second stage heat recovery zone.

[00142] Preferably, the first stage heat recovery zone precedes the second stage heat recovery zone.

[00143] Preferably, the first stage heat recovery zone is hotter than the second stage heat recovery zone.

[00144] The heat recovered from the calcined alumina may be utilised in the refinery.

In one form of the invention, the heat recovered from the calcined alumina is used to heat a heat transfer fluid such as water. In one form of the invention, the water may be flashed to generate water vapour. Said water vapour may be utilised in the refinery where appropriate.

[00145] In accordance with the present invention, there is provided a system for calcination of hydrated alumina, the system comprising: dehydrating means to reduce the water content of the hydrated alumina and provide partially calcined alumina; and calcining means to heat the partially calcined alumina and provide alumina, wherein the dehydrating means uses at least in part, stored thermal energy, electrical energy, renewable energy or nuclear energy including combinations thereof; the calcining means uses at least in part, electrical energy or combustion energy including combinations thereof; and wherein the stored thermal energy is provided by a thermal material and the thermal material is retained in a retaining system prior to heating the hydrated alumina.

[00146] The retaining system may be provided in the form of one or more storage vessels.

[00147] In one form of the invention, the dehydrating means is provided in the form of a heat exchanger such as a shell and tube heat exchanger.

[00148] In one form of the invention, the calcining means is provided in the form of an electric calciner.

[00149] In one form of the invention, the system further comprises a drying means to reduce the water content of hydrated alumina. The drying means may be provided in the form of a cyclone.

[00150] In one form of the invention, the system further comprises a high temperature heat exchanger to recover heat from the alumina formed in the calciner. In one form of the invention, the high temperature heat exchanger is provided in the form of a plurality of cyclones.

[00151 ] In one form of the invention, the system further comprises a low temperature heat exchanger to recover heat from the alumina from the high temperature heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS [00152] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;

Figure 2 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;

Figure 3 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;

Figure 4 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;

Figure 5 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;

Figure 6 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised;

Figure 7 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised; and

Figure 8 is a schematic flow sheet showing how a method in accordance with an embodiment of the present invention may be utilised.

DESCRIPTION OF EMBODIMENTS

[00153] 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 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. [00154] 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; hydrated alumina precipitation 24 from the liquor 20; separation of hydrated alumina 24 and liquor 26; and calcination 31 of the hydrated alumina 24 to alumina 30 and water 32.

[00155] In accordance with a description of the present invention as shown in Figure 2, hydrated alumina 24 is pressurised up to about 800 kPa gauge in a lock hopper 50 and passed to a drying zone provided in the present embodiment as a pre-heater 52. The damp hydrated alumina 24 is heated at about 180-250 °C. At 250 °C, about 25 % of the physically and chemically bound water is removed.

[00156] The dried hydrated alumina 54 is passed to a dehydrating zone provided in the present embodiment as a decomposer 56. The energy provided to the dehydrating zone 56 may be provided in a number of ways including stored thermal energy, electrical energy, renewable energy sources or nuclear energy.

[00157] The decomposer 56 is an indirect shell and tube heat exchanger. Holding times will vary but times in the order of 15 minutes are considered appropriate. At temperatures of about 430 °C, about 60 % of the physically and chemically bound water is removed in the dehydrating zone. At temperatures of 500 °C, about 67 % of the physically and chemically bound water is removed.

[00158] The partially calcined alumina 58 is passed to a calcining zone 60 to remove water of crystallisation. The energy provided to the calcining zone 60 may be provided in a number of ways such as an electrically heated calciner, or a calciner heated by combustion of hydrogen, natural gas or fuel oils or combinations thereof.

[00159] The calcined alumina 62 is passed to a high temperature heat recovery stage 64, provided in the present embodiment as an indirect heat exchanger such as a shell and tube variety. Calcined alumina 62 enters the heat recovery stage 64 at about 850 °C exits as partially cooled alumina 66 at about 230 °C. [00160] The partially cooled alumina 66 is depressurised in a lock hopper 68 to atmospheric pressure and passed a low temperature heat recovery stage 70, provided in the present embodiment as an indirect heat exchanger such as a shell and tube variety. Partially cooled alumina 66 enters the recovery stage 70 at about 230 °C and exits as cooled alumina 72 at about 90 °C. The final product of cooled, aluminium oxide 72 may be further treated as required.

[00161 ] Recovered heat 90 from the high temperature heat recovery stage 64 is used to heat a heat transfer fluid, such as steam or synthetic oil such as Dowtherm^® A which in turn, provides heat to the pre-heater 52. Cooled heat transfer fluid 92 from the pre heater 52 is returned to the high temperature heat recovery stage 64 for heating. The heat transfer between the pre-heater and the high temperature heat recovery stage 64 is a closed loop with minimal heat loss.

[00162] Recovered heat 100 from the low temperature heat recovery stage 70 may be used in the refinery. In the present embodiment, it is used to heat a water stream which in turn, is used to wash the hydrated alumina on the filter and heat the slurry of hydrated alumina 24 and liquor 26 to separation. Cooled water 102 is returned to the low temperature heat recovery stage 70 for heating. Trim cooling in a cooling tower may be required.

[00163] Water vapour 110 from the pre-heater 52 may be utilised in the refinery. It may be necessary to compress the water vapour 110 before use.

[00164] Water vapour 114 from the decomposer 56 and water vapour 116 the calciner 60 are combined and passed to the pre-heater 52 to heat the hydrated alumina 24.

[00165] Alternatively, or in addition thereto, water vapour 116 from the calciner may be passed to the decomposer 56.

[00166] In accordance with an embodiment of the present invention as shown in Figure 3, hydrated alumina 24 is pressurised up to about 800 kPa gauge in a lock hopper 50 and passed to a drying zone provided in the present embodiment as a pre heater 52. The damp hydrated alumina 24 is heated at about 180-250 °C. About 25 % of the physically and chemically bound water is removed at 250 °C. [00167] The dried hydrated alumina 54 is passed to a dehydrating zone provided in the present embodiment as a decomposer 56. The decomposer 56 is an indirect shell and tube heat exchanger. Holding times will vary but times in the order of 15 minutes are considered appropriate. At temperatures of about 430 °C, about 60 % of the water is removed in the dehydrating zone. At temperatures of 500 °C, about 67 % of the physically and chemically bound water is removed.

[00168] The partially calcined alumina 58 is passed to a calcining zone, provided in the present embodiment as an electrically heated calciner 60 operating at about 850 °C to remove water of crystallisation. Depending on the temperature and residence time, a small amount of the water of crystallisation may not be removed.

[00169] The calcined alumina 62 is passed to a high temperature heat recovery stage 64, provided in the present embodiment as an indirect heat exchanger such as a shell and tube variety. Calcined alumina 62 enters the heat recovery stage 64 at about 850 °C exits as partially cooled alumina 66 at about 230 °C.

[00170] The partially cooled alumina 66 is depressurised in a lock hopper 68 to atmospheric pressure and passed a low temperature heat recovery stage 70, provided in the present embodiment as an indirect heat exchanger such as a shell and tube variety. Partially cooled alumina 66 enters the recovery stage 70 at about 230 °C and exits as cooled alumina 72 at about 90 °C. The final product of cooled, aluminium oxide 72 may be further treated as required.

[00171] The decomposer 56 is heated with thermally stored energy from a high temperature molten salt storage vessel 82. Molten salt 76 exits the vessel 82 at about 530 °C and indirectly heats the hydrated alumina in the decomposer 56. Cooled molten salt at about 300 °C 78 is returned to a low temperature molten salt storage vessel 80.

[00172] The method cycles sequentially and continuously through the two operating states, the charging state and the discharging state, alternatively known as the net energy increasing state and the net energy decreasing state respectively. During the charging state, off-peak electricity may be used to heat the molten salt either as it is transferred from the low temperature storage vessel 80 to the high temperature storage vessel 82 or in the high temperature storage vessel 82 itself. During this charging state, the thermal material is being continuously cycled through the heat exchanger to heat the hydrated alumina in the decomposer 56. Despite the continuous deployment of hot molten salt, the amount of molten salt in the high temperature molten salt storage vessel 82 increases and the amount of molten salt in the low temperature molten salt storage vessel 80 decreases.

[00173] During the discharging sate, no or only a small amount of external energy is supplied to the molten salt. However, molten salt from the high temperature molten salt storage vessel 82 continues to heat the hydrated alumina in the decomposer 56.

During the discharging state, the amount of molten salt in the low temperature molten salt storage vessel 80 increases and the amount of molten salt in the high temperature molten salt storage vessel 82 decreases.

[00174] It will be appreciated that the volume of the high temperature molten salt storage vessel 82 must be sufficient to retain sufficient hot molten salt for the duration of the discharging state. For example, if local off-peak electricity is available from 10 pm until 6 am, then the charging state may last for about 10 hours and the discharging state may last for about 14 hours. In addition, even during the charging state, hot molten salt is being removed from the high temperature molten salt storage vessel 82.

[00175] In addition to using off-peak electricity to heat the molten salt during the charging state, off-peak electricity may also be used to directly heat the hydrated alumina during the charging state.

[00176] The method may also utilise a third state, the steady state. During the steady state, a small amount of electricity (off-peak or otherwise) may be used to account for the amount of hot molten salt leaving the high temperature molten salt storage vessel 82. In this state, the amount of material in the high temperature molten salt storage vessel 82 remains substantially constant. Alternatively, the steady state may be briefly operated when transitioning from the other two states.

[00177] Recovered heat 90 from the high temperature heat recovery stage 64 is used to heat a heat transfer fluid, such as steam or Dowtherm^® A which in turn, provides heat to the pre-heater 52. Cooled heat transfer fluid 92 from the pre-heater 52 is returned to the high temperature heat recovery stage 64 for heating. The heat transfer between the pre-heater and the high temperature heat recovery stage 64 is a closed loop with minimal heat loss. [00178] Recovered heat 100 from the low temperature heat recovery stage 70 may be used in the refinery. In the present embodiment, it is used to heat a water stream which in turn, is used to wash the hydrated alumina on the filter 124 and heat the slurry of hydrated alumina 24 and liquor 26 to separation. Cooled water 102 is returned to the low temperature heat recovery stage 70 for heating. Trim cooling in a cooling tower may be required.

[00179] Water vapour 110 from the pre-heater 52 may be utilised in the refinery. It may be necessary to compress the water vapour before use.

[00180] Water vapour 114 from the decomposer 56 and water vapour 116 the calciner 60 are combined and passed to the pre-heater 52 to heat the hydrated alumina 24.

[00181 ] Alternatively, or in addition thereto, water vapour 116 from the calciner may be passed to the decomposer 56.

[00182] Traditional calciners and other heating units are designed to operate in a steady state thermal equilibrium, i.e., energy in = energy out at all times. Figure 3 depicts box 128 which defines the portion of the system to which the terms the net energy increasing state and the net energy decreasing state refer. Energy enters the box 128 in the form of hot hydrated alumina 54 and energy exits the box in the form of partly calcined alumina 58 and steam 114. During the thermal energy charging state, external energy is supplied to the system defined by box 128. During charging, the system defined by box 128 is in a net energy increasing state. During the thermal energy discharging state where no or low external energy is supplied, the system defined by box 128 is in a net energy decreasing state. During the thermal energy steady state, the overall energy within the system is substantially constant, barring short term fluctuations due to inherent process instability.

[00183] In accordance with a second embodiment of the present invention as shown in Figure 4, the hydrated alumina 24 is pressurised in the lock hopper 50 and passed to a cyclone 130 for drying. The dried hydrated alumina 132 is passed to the pre-heating zone 52. The dried hydrated alumina 54 at about 250 °C is passed to a dehydrating zone provided as a decomposer 56. The partially calcined alumina 58 at about 430 - 500 °C is passed to a calcining zone provided as an electrically heated calciner 60. The calcined alumina 62 at about 850 °C is passed to a high temperature heat recovery stage 64. The partially cooled alumina 66 is depressurised in a lock hopper 68 and passed a low temperature heat recovery stage 70. The final product of cooled, aluminium oxide 72 may be further treated as required.

[00184] Recovered heat 100 from the low temperature heat recovery stage 70 is used to heat a water stream which in turn, is flashed in a bank of one or more flash tanks (only two shown) 140, 142. A portion of the heated water 144 may bypass the flash tanks and be used to directly wash the hydrated alumina cake on the filter. Water vapour 146 from the flash tank 140 may be used to wash the hydrated alumina cake on the filter 124. Water vapour 148 from the final flash tank 142 may be passed through a bank (only one shown) of mechanical vapour recompressors 150 to increase its pressure to just above atmospheric pressure and used to heat the slurry of hydrated alumina 24 and liquor 26 on the filter 124.

[00185] Cooled water 102 is returned to the low temperature heat recovery stage 70 for heating. A portion of the cooled water may be passed to a trim cooling circuit 152 prior to returning to the low temperature heat recovery stage 70.

[00186] Water vapour 110 from the pre-heater 52, water vapour 114 from the decomposer 56, water vapour 116 from the calciner 60 and water vapour 140 from the high temperature heat recovery stage 64 may be combined and mixed with the hydrated alumina 24 entering the cyclone 130.

[00187] At least a portion 153 of the water vapour from the calciner 60 may be returned to the refinery.

[00188] At least a portion of the water vapour 134 from the cyclone 130 may be returned to the refinery. Alternatively, or in addition thereto, at least a portion of the water vapour 134 from the cyclone 130 is passed through a bank (one being shown) of mechanical vapour recompressors 156 and used to fluidise the material in any or all of the pre-heater 52 the decomposer 56, the calciner 60 and the high temperature heat recovery stage 64. It will be appreciated that the water vapour should be compressed and be above its condensing temperature.

[00189] In accordance with a third embodiment of the present invention as shown in Figure 5, the hydrated alumina 24 is pressurised in the lock hopper 50 and passed to a first cyclone 130 for drying and a second cyclone 150 for further drying and heat recovery. The dried hydrated alumina 132 at about 250 °C is passed to a dehydrating zone provided as a decomposer 56. The partially calcined alumina 58 at about 430 - 500 °C is passed to a calcining zone provided as an electrically heated calciner 60. The calcined alumina 62 at about 850 °C is passed to a high temperature heat recovery stage 64 provided as a series of cyclones 174, 172, 164. The partially cooled alumina 66 is depressurised in a lock hopper 68 and passed a low temperature heat recovery stage 70. The final product of cooled, aluminium oxide 72 may be further treated as required.

[00190] Recovered heat from the low temperature heat recovery stage 70 is used to heat a water stream 100 which in turn, is flashed in a bank (only two shown) of flash tanks 140, 142. A portion of the heated water 144 may bypass the flash tanks and used to directly heat the slurry 124 of hydrated alumina 24 and liquor 26. Water vapour 146 from the flash tanks may be used to heat the slurry 124 of hydrated alumina 24 and liquor 26. Water vapour 148 from the final flash tank 142 may be passed through a bank (only one shown) of mechanical vapour recompressors 150 to increase its pressure to just above atmospheric pressure and used to heat the slurry 124 of hydrated alumina 24 and liquor 26.

[00191 ] Cooled water 102 is returned to the low temperature heat recovery stage 70 for heating. A portion of the cooled water may be passed to a trim cooling circuit 152 prior to returning to the low temperature heat recovery stage 70

[00192] Water vapour 160 from the first cyclone 130 is passed to the feed of the cyclone 164. Water vapour 162 from the second cyclone 150 is used to heat the hydrated alumina 24 before it enters the first cyclone 130.

[00193] Water vapour 114 from the decomposer 56, water vapour 116 from the calciner 60 and water vapour 164 from the cyclone 174 are combined and used to heat the hydrated alumina 166 entering cyclone 150. Part of the water vapour may be split to cyclone 130 to achieve a higher degree of drying in cyclone 130.

[00194] The water vapour 168 from the cyclone 164 may be returned to the refinery. Alternatively, or in addition thereto, water vapour 168 is passed through a bank (one being shown) of mechanical vapour recompressors 136 and used to fluidise material in any or all of the decomposer 56, the calciner 60 and the second cyclone 172. [00195] Water vapour 170 from cyclone 172 is passed to the feed 62 to cyclone 174. Water vapour 176 from cyclone 174 is passed to the feed to cyclone 150. Water vapour from cyclone 150 is passed to the feed of cyclone 130.

[00196] At least a portion of the water vapour 178 from the cyclone 172 may be passed through a bank (one being shown) of mechanical vapour recompressors 179 and used to fluidise material in any or both of the decomposer 56 and the calciner 60.

[00197] At least a portion of the water vapour 180 from the cyclone 150 may be passed through a bank (one being shown) of mechanical vapour recompressors 181 and used to fluidise material in any or both of the decomposer 56 and the calciner 60.

[00198] At least a portion of the water vapour 182 from the cyclone 174 may be passed through a bank (one being shown) of mechanical vapour recompressors 183 and used to fluidise material in any or both of the decomposer 56 and the calciner 60.

[00199] A fourth embodiment of the present invention is shown in Figure 6. The flow sheet in Figure 6 is essentially the same as that shown in Figure 2 apart from being conducted at atmospheric pressure. In such an embodiment, lock hopper 50 and lock hopper 68 are not required.

[00200] Water vapour 110 from the pre-heater 52, water vapour 114 from the decomposer 56 and water vapour 116 the calciner 60 are combined and passed through a bank (one being shown) of mechanical vapour recompressors 180 to compress the steam to refinery pressure requirements prior to being returned to the refinery.

[00201] A fifth embodiment of the present invention is shown in Figure 7. The flow sheet in Figure 7 is essentially the same as that shown in Figure 6. At least a portion 188 of recovered heat 90 from the high temperature heat recovery stage 64 may be used to provide steam to the pressure filter 124.

[00202] A sixth embodiment of the present invention is shown in Figure 8. In this embodiment, the thermal material is provided in the form of alumina itself.

[00203] The partially calcined alumina 58 at about 430 - 500 °C is passed through a lock hopper 200 to reduce the pressure and passed to the lower temperature storage vessel 202. It is transferred to the higher temperature storage vessel 204 for heating to 650 -1200 °C by off peak electricity or other energy source, including renewable energy to undergo calcination and store thermal energy. The calcined alumina is passed to a heat exchanger 206 to heat the molten salt. Molten salt exits 76 the vessel 82 at about 530 °C and indirectly heats the hydrated alumina in the decomposer 56. Cooled molten salt at about 300 °C 78 is returned to the heat exchanger 206. In addition to the above and as required, electricity may be used to directly heat units 56, 76, 78 and/or 206.

[00204] At least a portion 208 of the cooled calcined alumina from the heat exchanger is passed to the high temperature heat recovery stage 64 for further cooling as described above.

[00205] The method cycles sequentially and continuously through the two operating states, the charging state and the discharging state, alternatively known as the net energy increasing state and the net energy decreasing state respectively. During the charging state, off-peak electricity or renewable sourced energy may be used to heat the partially calcined alumina either as it is transferred from the low temperature storage vessel 202 to the high temperature storage vessel 204 or in the high temperature storage vessel 204 itself. During this charging state, the alumina heat transfer material is being continuously cycled through the heat exchanger 206 to heat the molten salt which in turn heats the hydrated alumina in the decomposer 56.

[00206] During the discharging state, no or only a small amount of external energy is supplied to the alumina. However, alumina from the high temperature storage vessel 204 continues to heat molten salt 206. During the discharging state, the amount of alumina in the low temperature storage vessel 202 increases and the amount of alumina in the high temperature storage vessel 204 decreases.

[00207] It will be appreciated that the volume of the high temperature storage vessel 204 must be sufficient to retain sufficient hot alumina for the duration of the discharging state. For example, if local off-peak electricity is available from 10 pm until 6 am, then the charging state may last for about 10 hours and the discharging state may last for about 14 hours. In addition, even during the charging state, hot alumina is being removed from the high temperature alumina storage vessel 204. [00208] In addition to using off-peak electricity to heat the alumina during the charging state, off-peak electricity may also be used to directly heat the hydrated alumina during the charging state.

[00209] The method may also utilise a third state, the steady state. During the steady state, a small amount of electricity (off-peak or otherwise) may be used to account for the amount of hot alumina leaving the high temperature t storage vessel 204. In this state, the amount of material in the high temperature storage vessel 204 remains substantially constant. Alternatively, the steady state may be briefly operated when transitioning from the other two states.

[00210] Figure 8 depicts box 220 which defines the portion of the system to which the terms the net energy increasing state and the net energy decreasing state refer.

Energy enters the box 220 in the form of hot hydrated alumina 54 and energy exits the box in the form of calcined alumina 208 and steam 114 and 216. During the thermal energy charging state, external energy is supplied to the system defined by box 220. During charging, the system defined by box 220 is in a net energy increasing state. During the thermal energy discharging state where no or low external energy is supplied, the system defined by box 220 is in a net energy decreasing state. During the thermal energy steady state, the overall energy within the system is substantially constant, barring short term fluctuations due to inherent process instability.

[00211 ] A stream of desuperheating water 210 is added to the water vapour 212 from the hot storage vessel 204 and pressurised in a bank of mechanical vapour compressors 214. The heated water vapour 216 is mixed with water vapour stream 114 from the decomposer 56.

Claims

1. A method for calcination of hydrated alumina, the method comprising the steps of: heating the hydrated alumina in a dehydrating zone to reduce the water content of the hydrated alumina and provide partially calcined alumina; and heating the partially calcined alumina in a calcining zone to provide alumina, wherein the step of heating the hydrated alumina in a dehydrating zone uses at least in part, stored thermal energy, electrical energy, renewable energy or nuclear energy including combinations thereof and the step of heating the partially calcined alumina in a calcining zone uses at least in part, electrical energy or chemical potential energy including combinations thereof, and where the hydrated alumina is heated at least in part with stored thermal energy, the method is alternately operable in a thermal energy charging state and in a thermal energy discharging state.

2. A method for calcination of hydrated alumina according to claim 1 , wherein the thermal energy is provided by a thermal material.

3. A method for calcination of hydrated alumina according to claim 2, wherein the thermal material is a liquid or a solid.

4. A method for calcination of hydrated alumina according to claim 3, wherein the liquid thermal material is selected from the group comprising carbon dioxide, water, oil, molten metals and molten salts.

5. A method for calcination of hydrated alumina according to claim 3, wherein the liquid thermal material is a molten salt.

6. A method for calcination of hydrated alumina according to claim 3, wherein the solid thermal material is selected from the group comprising non-molten salts, rock, metals and non-molten compounds including alumina or partially calcined alumina.

7. A method for calcination of hydrated alumina according to claim 3, wherein the solid thermal material is alumina.

8. A method for calcination of hydrated alumina according to claim 3, wherein the solid thermal material is partially calcined alumina.

9. A method for calcination of hydrated alumina according to claim 2, wherein there is further provided a heat transfer medium to transfer the heat from the thermal material to the dehydrating zone.

10. A method for calcination of hydrated alumina according to any one of claims 2 to 9, wherein the thermal material is heated using electricity.

11. A method for calcination of hydrated alumina according to claim 10 wherein the electricity is off-peak electricity.

12. A method for calcination of hydrated alumina according to claim 10 or claim 11 , wherein the electricity is generated from a renewable power source.

13. A method for calcination of hydrated alumina according to any one of claims 2 to 12, wherein the method comprises the further step of: heating the thermal material; and retaining the thermal material in at least one storage vessel, prior to heating the hydrated alumina.

14. A method for calcination of hydrated alumina according to claim 13, wherein there are provided two storage vessels.

15. A method for calcination of hydrated alumina according to any one of the preceding claims, wherein the method comprises the additional step of: heating the hydrated alumina in a drying zone prior to the step of heating the hydrated alumina in a dehydrating zone.

16. A method for calcination of hydrated alumina according to claim 15, wherein the steps of: heating the hydrated alumina in a drying zone prior to the step of heating the hydrated alumina in a dehydrating zone; heating the hydrated alumina in a dehydrating zone to reduce the water content of the hydrated alumina and provide partially calcined alumina; and heating the partially calcined alumina in a calcining zone to provide alumina, generate water vapour.

17. A method for calcination of hydrated alumina according to claim 16, wherein the water vapour is used to fluidise solids in the drying zone, the dehydrating zone and/or the calcining zone.

18. A method for calcination of hydrated alumina according to any one of the preceding claims, wherein the method comprises the additional step of: recovering heat from the calcined alumina in a first stage heat recovery zone.

19. A method for calcination of hydrated alumina according to claim 18, wherein the method comprises the additional step of: recovering heat from the calcined alumina in a second stage heat recovery zone.

20. A system for calcination of hydrated alumina, the system comprising: dehydrating means to reduce the water content of the hydrated alumina and provide partially calcined alumina; and calcining means to heat the partially calcined alumina and provide alumina, wherein the dehydrating means uses at least in part, stored thermal energy, electrical energy, renewable energy or nuclear energy including combinations thereof; the calcining means uses at least in part, electrical energy or chemical potential energy including combinations thereof; and wherein the stored thermal energy is provided by a thermal material and the thermal material is retained in a retaining system prior to heating the hydrated alumina.

21. A system for calcination of hydrated alumina in accordance with claim 20, wherein the retaining system comprises at least one storage vessel.

22. A system for calcination of hydrated alumina in accordance with claim 21 or claim 22, wherein the dehydrating means is provided in the form of a heat exchanger such as a shell and tube heat exchanger.

23. A system for calcination of hydrated alumina in accordance with any one of claims 20 to 22, wherein the calcining means is provided in the form of an electric calciner.

24. A system for calcination of hydrated alumina in accordance with any one of claims 20 to 23, wherein the system comprises a drying means to reduce the water content of hydrated alumina.

25. A system for calcination of hydrated alumina in accordance with any one of claims 20 to 24, wherein the system further comprises a high temperature heat exchanger to recover heat from the alumina formed in the calciner.

26. A system for calcination of hydrated alumina in accordance with claim 25, wherein the system further comprises a low temperature heat exchanger to recover heat from the alumina from the high temperature heat exchanger.