Calcination apparatus and process

Abstract

A process for calcining aluminum trihydrate (Al2O3. 3H2O) to form alumina (Al2O3) employs an electrical energy source instead of at least a portion of a natural gas energy source to generate heat for use in a calcining furnace. The process may include calcining aluminum trihydrate using at least one electrically heated fluidized bed reactor.

Description

Technical Field

[0001] The present invention relates to a process and apparatus for calcining aluminum hydroxide to form aluminum oxide. Background Art

[0002] Alumina (Al2O3) is produced in an alumina production plant, such as a Bayer process plant, and involves calcining aluminum hydroxide (Al2O3.3H2O - also known as alumina hydroxide, aluminum trihydrate, and hydrated aluminum oxide) to remove water.

[0003] The calcination of aluminum hydroxide is a thermal decomposition chemical reaction that proceeds endothermically according to the following reaction:

[0004] Al2O3.3H2O(s)→Al2O3(s)+3H2O(g).

[0005] A typical calciner for producing alumina has a reaction chamber that burns natural gas and oxygen to form heat and flue gas containing nitrogen, carbon dioxide, and steam. The heat generated in the reaction chamber by the combustion of natural gas and oxygen is used to drive water from aluminum hydroxide to form alumina.

[0006] The above description should not be taken as an admission of the common general knowledge in Australia or elsewhere. SUMMARY OF THE INVENTION

[0008] The applicant operates a natural gas-fired calciner to dehydrate aluminum hydroxide (Al2O3.3H2O) in the form of the ore gibbsite to aluminum oxide (Al2O3).

[0009] The dehydration process may involve the conversion of gibbsite to aluminum oxyhydroxide (boehmite), which in turn is converted to alumina.

[0010] The present invention provides an opportunity to reduce greenhouse gas emissions associated with using natural gas as an energy source for calcining by partially or completely replacing natural gas as the energy source for calcining aluminum hydroxide with electric heating derived from renewable energy sources.

[0011] The present invention also utilizes the following points.

[0012] - At least 35% of the energy required to calcinate aluminum trihydrate to aluminum oxide can be absorbed at or below 400°C.

[0013] - At least 80% of the energy required to calcine aluminum trihydrate to alumina can be absorbed at 700 °C or below 700 °C.

[0014] Broadly speaking, the present invention provides a process for calcining aluminum trihydrate (Al2O3·3H2O), such as gibbsite, to form alumina (Al2O3), the process comprising: (a) supplying aluminum trihydrate particles to an electrically heated first reactor and heating the aluminum trihydrate particles to a temperature of up to at least 280 °C, preferably up to at least 320 °C, more preferably up to at least 350 °C and not greater than 400 °C, and calcining at least a major portion of the aluminum trihydrate (Al(OH)3) to hydroxyaluminum oxide (γ-AlOOH or AlO(OH)) in the first reactor; and (b) supplying the hydroxyaluminum oxide particles from the first reactor to a second reactor, and heating the hydroxyaluminum oxide particles and calcining the hydroxyaluminum oxide to alumina.

[0015] The term "major portion" means at least 50%, suitably at least 60%, more suitably at least 70% of the number of aluminum trihydrate particles calcined to hydroxyaluminum oxide.

[0016] The term "reactor" is understood herein to mean a vessel or apparatus in which substances can undergo reactions, such as chemical reactions.

[0017] In some embodiments, the process may involve heating the aluminum trihydrate particles in the first reactor to a temperature of up to at least 350 °C.

[0018] In some embodiments, the process may involve heating the aluminum trihydrate particles in the first reactor to a temperature between 280 °C and 400 °C.

[0019] In some embodiments, the process may involve supplying at least 35% of the energy required to calcine aluminum trihydrate to alumina to the first reactor at a temperature of up to 350 °C.

[0020] In other embodiments, the process may involve supplying at least 80% of the energy required to calcine aluminum trihydrate to alumina to the first reactor at a temperature of up to 350 °C and to the second reactor at a temperature of up to 500 °C - 800 °C.

[0021] The energy required to convert hydroxyaluminum oxide to alumina in the second reactor can be obtained from any suitable source, such as natural gas, renewable energy, natural gas or hydrogen.

[0022] The process may involve heating the hydroxyaluminum oxide particles in the second reactor to a temperature of up to 720 °C, suitably up to 600 °C, and calcining the hydroxyaluminum oxide in the second reactor.

[0023] The process may involve heating the aluminium oxyhydroxide particles in the second reactor at a temperature of up to 760°C, suitably up to 700°C, and calcining the aluminium oxyhydroxide in the second reactor.

[0024] The process may involve providing 20% to 50% of the energy required to calcine the aluminium trihydrate to alumina in the first reactor at temperatures up to 400°C.

[0025] The process may involve providing 40% to 90%, suitably 70% to 90%, of the energy required to calcine the aluminium trihydrate to alumina in the second reactor at a temperature of up to 760°C.

[0026] The process may result in a loss on ignition (LOI) of less than 6% alumina from the second reactor, suitably around 4% LOI.

[0027] The process may include providing the remainder of the energy required to calcine the aluminum trihydrate to alumina in a third reactor following the second reactor, typically in order to obtain target product qualities, such as desirable alumina surface properties.

[0028] The electricity used to power the electrically heated first reactor may be generated from any suitable source, for example from a renewable source such as wind, solar, hydro or geothermal.By using a renewable source, greenhouse gas emissions may be further reduced.

[0029] The first reactor may be a fluidized bed reactor.

[0030] A "fluidized bed reactor" is understood herein to mean a reactor of the type that involves the passage of a fluid through solid particulate material at a velocity sufficient to suspend the solid material and cause it to behave as if it were a fluid.

[0031] Advantages of using a fluidized bed reactor include higher heat exchange efficiency compared to a fixed bed, and better temperature control due to turbulent gas flow and rapid circulation.

[0032] The first reactor may be a first fluidized bed reactor that is electrically heated via jacketed electrical elements in direct contact with the aluminum trihydrate within the reactor.

[0033] The first reactor may be a first fluidized bed reactor electrically heated by a coil element or an induction element.

[0034] The first fluidized bed reactor may be configured to promote direct contact between aluminum trihydrate particles supplied to the reactor and electric heating elements in the reactor.

[0035] The electrical components in the first fluidized bed reactor may be operated at any suitable voltage.

[0036] For example, the voltage can be in the range of 200V - 600V, suitably at the higher end of this range.

[0037] The second reactor can be electrically heated.

[0038] The electricity used to power the electrically heated second reactor can be generated from any suitable source, such as from renewable sources such as wind, solar, hydro or geothermal. By using renewable sources, greenhouse gas emissions can be further reduced. However, it is also envisaged that the second reactor can be heated by any other means, such as by the combustion of fuels such as natural gas or hydrogen.

[0039] The second reactor can be a second fluidized bed reactor.

[0040] The second reactor can be a furnace.

[0041] The second reactor can be a second fluidized bed reactor that is electrically heated via a jacketed electrical element in direct contact with aluminium trihydroxide within the reactor.

[0042] The electrical element in the second fluidized bed reactor can operate at any suitable voltage.

[0043] For example, the voltage can be in the range of 400V - 600V, suitably at the higher end of this range.

[0044] The first fluidized bed reactor and the second fluidized bed reactor can be configured to promote direct contact between the aluminium trihydroxide particles supplied to the reactor and the electrically heated elements within the reactor.

[0045] The process can involve heating the boehmite particles from the first reactor, for example by heat transfer from the hot waste gas from the process, and supplying the heated boehmite particles to the second reactor, and heating and calcining the boehmite particles in the second reactor.

[0046] The process can involve transferring the calcined boehmite particles in the second reactor to a third reactor, and heating and calcining the calcined boehmite particles to alumina in the third reactor.

[0047] The third reactor can be electrically heated.

[0048] The electricity used to power the electrically heated third reactor can be generated by renewable sources, such as wind, solar, hydro or geothermal. However, it is also envisaged that the third reactor can be heated by any other means, such as by the combustion of fuels such as natural gas or hydrogen.

[0049] The third reactor can be a fluidized bed reactor.

[0050] The third reactor can be a furnace.

[0051] The third reactor can be a fluidized bed reactor which is electrically heated via a jacketed electrical element in direct contact with aluminum trihydroxide within the reactor and which has an electrical heating element within the reactor.

[0052] The first reactor can be a fluidized bed reactor which is electrically heated by a coil element or an induction element.

[0053] The process can involve heating the calcined boehmite particles in the third reactor to a temperature of up to 1000 °C.

[0054] The process can involve providing 90% - 100% of the energy required to calcine aluminum trihydroxide to alumina at a temperature of up to 1000 °C in the third reactor.

[0055] The process can result in a loss on ignition (LOI) of less than 1%, suitably less than 0.8% LOI, from the third reactor.

[0056] The process can involve drying the feed aluminum trihydroxide particles by heat transfer from the hot waste gas from the process, separating the dried feed aluminum trihydroxide particles from the waste gas stream, and supplying the dried feed aluminum trihydroxide particles to the first reactor.

[0057] The present invention also provides an apparatus for calcining aluminum hydroxide (Al2O3·3H2O) to form alumina (Al2O3), the apparatus comprising: (a) a first reactor configured to heat aluminum trihydroxide particles to a temperature of up to 400 °C and to calcine at least a major portion of the aluminum trihydroxide to boehmite (γ - AlOOH or AlO(OH)) in the first reactor; and (b) a second reactor for heating and calcining the boehmite particles from the first reactor to alumina.

[0058] The first reactor can be electrically heated. The electricity used to power the electrically heated first reactor can be generated from a renewable source such as wind, solar, hydro or geothermal. By using a renewable source, greenhouse gas emissions can be further reduced.

[0059] The first reactor can be a first fluidized bed reactor.

[0060] The first reactor can be a first fluidized bed reactor having an electrical heating element within the reactor.

[0061] The electrical heating element of the first fluidized reactor can comprise: (a) a metal tube, and (b) an electrical heating element within the tube.

[0062] The second reactor can be electrically heated. The electricity used to power the electrically heated second reactor can be generated from renewable sources such as wind, solar, hydro, or geothermal. By using renewable sources, greenhouse gas emissions can be further reduced. However, it is also contemplated that the second reactor can be heated by any other means, such as by the combustion of fuels such as natural gas or hydrogen.

[0063] The second reactor can be a second fluidized bed reactor.

[0064] The second reactor can be a second fluidized bed reactor having an electric heating element within the reactor.

[0065] The electric heating element of the second fluidized bed reactor can be any suitable element.

[0066] The electric heating element of the second fluidized bed reactor can include an electric heating element within a thermally conductive housing.

[0067] The electric heating element of the second fluidized bed reactor can include: (a) a metal tube, and (b) an electric heating element within the tube.

[0068] The first reactor and the second reactor can be connected in a countercurrent configuration. In this configuration, the flow of the gas is opposite to the flow of the aluminum trihydroxide particles. For example, the flue gas leaving the second reactor is directed towards the first reactor, while the flow of the aluminum trihydroxide particles is directed from the first reactor towards the second reactor. Brief Description of the Drawings

[0070] Embodiments of the present invention are further described with reference to the non-limiting drawings, in which:

[0071] Figure 1 An example of a conventional calcination apparatus is illustrated;

[0072] Figure 2 Illustrated is Figure 1 a schematic version of

[0073] Figure 3 An embodiment of an apparatus for calcining aluminum hydroxide according to the present invention is illustrated in schematic form;

[0074] Figure 4 Another embodiment of an apparatus for calcining aluminum hydroxide according to the present invention is illustrated in schematic form, but not the only other embodiment;

[0075] Figure 5 Another embodiment of an apparatus for calcining aluminum hydroxide according to the present invention is illustrated in schematic form, but not the only other embodiment;

[0076] Figure 6Shows a horizontal cross - section of the lower half of a fluidized bed reactor used in an embodiment of a calcination device according to the present invention;

[0077] Figure 7 Shows the use in Figure 6 A plan view of a cartridge including a bundle of electric heating devices used in a fluidized bed reactor, as described above, which fluidized bed reactor is used in an embodiment of a calcination device according to the present invention; and

[0078] Figure 8 Schematically shows the counter - current configuration of two fluidized bed reactors used in an embodiment of a calcination device according to the present invention.

[0079] Description of the Embodiment

[0080] The following description is made in the context of calcining aluminum trihydrate such as the mineral gibbsite to form alumina.

[0081] Figure 1 And Figure 2 Shows an example of a conventional calcination device 10.

[0082] The device 10 includes more than one zone, namely: a drying zone A, a heating zone B, and a cooling zone C.

[0083] In the drying zone A, aluminum trihydrate (gibbsite) particles are supplied to the device 10 via an inlet and heated by direct contact with combustion gases at a low temperature between 100 °C and 340 °C to remove unbound water and physically bound water. Then the dehydrated aluminum trihydrate particles are supplied to the heating zone B by gravity and the inlet.

[0084] In the heating zone B, the dehydrated aluminum trihydrate particles are heated to between 900 °C and 1100 °C to remove chemically bound water, resulting in alumina particles. Then the alumina particles are supplied to the cooling zone C.

[0085] In the cooling zone C, the alumina particles are cooled. Heat can be recuperated using a heat exchanger and used in other areas of the device 10.

[0086] At the center of the operation of the device 10 is a reactor in the form of a furnace 12 that burns natural gas, which burns natural gas or other fossil fuels to generate heat. In the heating stage B, the heat is used to remove the chemically bound water in the aluminum trihydrate particles. The temperature generated in the furnace 12 is between 900 °C and 1100 °C.

[0087] The apparatus 10 includes more than one cyclone 14a-1, 14a-2 in a drying zone A and cyclones 14c-1, 14c-2, 14c-3 in a cooling zone C.

[0088] Figure 1 The cyclones 14c-1, 14c-2, 14c-3 in Figure 2 are collectively referred to as 14c in Figures 3 - 5 and the same is true in the case described below.

[0089] In the case of cyclone 14a-1, the purpose of the cyclone is to facilitate: (a) heat transfer between the flue gas and the feed aluminum trihydroxide particles and the partially calcined aluminum trihydroxide particles, and (b) separation of the particles from the flue gas.

[0090] In the case of cyclone 14a-2, the purpose of the cyclone is to facilitate: (a) heat transfer between the flue gas and the partially calcined aluminum trihydroxide particles, and (b) separation of the particles from the flue gas.

[0091] In the case of cyclones 14c-1, 14c-2, 14c-3, the purpose of the cyclones is to facilitate: (a) heat transfer between the air and the alumina particles, and (b) separation of the particles from the flue gas, wherein the heat transfer cools the alumina particles and heats the air.

[0092] Each cyclone includes a conical housing having a tangential side inlet, an upper outlet, and an opposite lower outlet. A vortex is generated within the conical housing, which entrains the particles entering via the inlet. Most of the particles leave each cyclone via the lower outlet, while a small amount of particles, mainly the lighter ones, are carried upward by the vortex and leave the cyclone via the upper outlet and enter the electrostatic precipitator 18, see Figure 2 .

[0093] The apparatus 10 also includes more than one holding container 16a, 16b, 16c to control the residence time of the particles in each zone.

[0094] The present invention provides an opportunity to reduce greenhouse gas emissions associated with using natural gas or other fossil fuels as the energy source for calcination by: using electric heating from renewable energy sources to partially or fully replace natural gas as the energy source for calcining aluminum hydroxide.

[0095] The present invention takes advantage of the point - 35% of the energy required to calcine aluminum trihydroxide to alumina can be absorbed at temperatures below 400 °C, suitably 350 °C.

[0096] For example, in Figure 3In embodiments of the present invention, the present invention takes advantage of the above points by calcining at a temperature lower than the temperature in the conventional calcination apparatus of Figure 1 and Figure 2 at least a major portion of the feed aluminum trihydroxide particles. This in turn provides an opportunity to use electrical power, such as electrical power generated from renewable energy sources, to replace at least a portion of the natural gas used to generate energy in the apparatus of Figure 1 and Figure 2 .

[0097] Figure 3 FIG. shows an embodiment of a calcination apparatus 20 according to the present invention.

[0098] Figure 3 The calcination apparatus 20 in Figure 1 and Figure 2 differs from the conventional apparatus 10 shown in that Figure 1 and Figure 2 the containment vessel 16a in Figure 1 and Figure 2 is replaced with an electrically heated first fluidized bed reactor 25a that typically operates at about 600V. The remaining equipment of apparatus 20 is the same as the equipment in the apparatus of Figure 3 where the same reference numerals are used as in Figure 1 and Figure 2 .

[0099] Transfer the hydroxyaluminum oxide particles from the first fluidized bed reactor 25a to the cyclone separator 14a-2.

[0100] In the first fluidized bed reactor 25a, the aluminum trihydroxide particles from the cyclone separator 14a-1 are heated to a temperature between 320 °C and 400 °C, suitably about 350 °C. This causes at least a major portion, typically at least 50%, of the aluminum trihydroxide to be partially calcined to hydroxyaluminum oxide (γ-AlOOH or AlO(OH)).

[0101] 20%-50% of the energy required to calcine aluminum trihydroxide to aluminum oxide is provided by heating the aluminum trihydroxide particles to a temperature between 320 °C and 400 °C in the first fluidized bed reactor 25a.

[0102] The incorporation of the electrically heated first fluidized bed reactor 25a effectively replaces Figure 1 and Figure 2 20%-50% of the natural gas-derived energy required by the apparatus 10 shown in, which results in a significant reduction in the associated greenhouse gas emissions.

[0103] In Figure 3The peak power demand of the electrically heated first fluidized bed reactor 25a in the apparatus 20 is about 35% of the total energy required to calcine to the target surface area.

[0104] The remaining energy required to form the alumina is provided by the furnace 12.

[0105] Figure 4 and Figure 5 Shows two other embodiments of the calcination apparatus 30 according to the present invention.

[0106] Figure 4 and Figure 5 The embodiments utilize the following points.

[0107] - 35% of the energy required to calcine aluminum trihydroxide to alumina can be absorbed at temperatures below 400°C, suitably about 350°C - as described above with respect to Figure 3 the embodiments.

[0108] - 80% of the energy required to calcine aluminum trihydroxide to alumina can be absorbed at temperatures below 800°C, suitably about 600°C.

[0109] It should be noted that the last part of the energy required to complete the calcination is mainly needed to obtain the desired alumina surface properties. This energy is absorbed at temperatures above 760°C. The temperature is inversely proportional to the residence time at that temperature.

[0110] Figure 4 and Figure 5 The calcination apparatus 30 in the two embodiments in Figure 3 differs from the apparatus 20 in Figure 3 in that: the second fluidized bed reactor 25b is located between the cyclone separator 14b and the furnace 12. The second fluidized bed reactor 25b typically operates at about 600V. The second fluidized bed reactor 25b is electrically heated.

[0111] In Figure 4 and Figure 5 in the second fluidized bed reactor 25b of the two embodiments, the aluminum trihydroxide particles from the cyclone separator 14a-2 are heated to a temperature of about 700°C. This results in further calcining the aluminum trihydroxide to at least hydroxyaluminum oxide (γ-AlOOH or AlO(OH)) and to alumina (Al2O3).

[0112] Thus, heating the aluminum trihydroxide particles in the electrically heated second fluidized bed reactor 25b can provide an additional 40% - 60% of the energy required to calcine the aluminum trihydroxide to alumina.

[0113] The remaining approximately 10% - 30% of the energy required to form the alumina is provided by Figure 4 andFigure 5 is provided by the furnace 12 shown in

[0114] Thus, in Figure 4 and Figure 5 the combination of the electrically heated first fluidized bed reactor 25a and the second fluidized bed reactor 25b in the embodiments of Figure 1 and Figure 2 improves the thermal efficiency of the calcination process by providing 70%-90% of the energy required to calcine aluminum trihydrate at a lower temperature than in the conventional devices of

[0115] Furthermore, the fluidized bed reactors replace 70%-90% of the total energy derived from natural gas or other fossil fuels with electrical energy that can be sourced from renewable sources, which can result in a significant reduction in the associated greenhouse gas emissions.

[0116] To maximize heat recovery and limit power consumption, Figure 4 and Figure 5 the first fluidized bed reactor 25a and the second fluidized bed reactor 25b in

[0117] In Figure 8 one such arrangement shown in

[0118] the flue gas leaving the fluidized bed reactor 25b in line 22 is directed to the cyclone separator 14a-2, and the flue gas leaving the fluidized bed reactor 25a in line 24 is directed to the cyclone separator 14a-1. This flue gas flows countercurrently to the aluminum trihydrate particles through the fluidized bed reactors 25a, 25b.

[0119] Figure 5 The embodiments of Figure 4 show additional heating means that are not part of the embodiments of

[0120] This additional heating means 26 is located within the containment vessel 16b to provide additional thermal energy to calcine the aluminum trihydrate.

[0121] - Proof of concept in the laboratory - Phase 1 (-35% use of natural gas-derived energy) - Conversion of gibbsite to boehmite in a reactor with a fluidized bed and 240V electrical elements.

[0122] - Engineering of the full-scale reactor for Phase 1 (-35% use of natural gas derived energy)

[0123] - Proof of concept in the laboratory - Phase 2 (- 80% using natural gas derived energy) - Conversion of gibbsite to alumina in a series of 2 reactors with fluidized bed and electrical elements.

[0124] Table 1 below provides a summary of the results of the proof-of-concept in the laboratory pilot phase 2.

[0125] The table shows that an LOI below 5% is achieved only when the second reactor temperature is above 500°C, indicating that the two-reactor setup works.

[0126] Table 1-

[0127]

[0128] Figure 6 A horizontal cross-sectional view of the lower half of an embodiment of a fluidized bed reactor 25 used in an embodiment of a calcining apparatus according to the present invention is shown.

[0129] The fluidized bed reactor 25 can be used as one or both of the first fluidized bed reactor 25a and the second fluidized bed reactor 25b, as previously described. Figures 3 - 5 described.

[0130] like Figure 6 As shown, the fluidized bed reactor 25 includes a vertically oriented vessel 40 having one or more inlets 42 , a lower outlet 44 , and one or more electrical heating devices 46 therein.

[0131] In some embodiments, in embodiments where the cross-section of the fluidized bed reactor is circular, vessel 40 has an outer diameter of about 3.6 m.

[0132] It should be noted that the present invention extends to any suitable shape and size of fluidised bed reactor, for example a reactor having a rectangular cross section.

[0133] like Figure 6 As shown in FIG, a fluidized bed reactor 25 includes more than one cartridge 56a, 56b, 56c, 56d in a vessel 40, each cartridge having attached thereto more than one electric heating device 46, referred to as a bundle 55. Each cartridge is configured to be removable from the fluidized bed reactor 25 and replaceable with another cartridge. An advantage of this arrangement is that more than one electric heating device 46 can be quickly replaced without having to overhaul the entire fluidized bed reactor 25. However, each electric heating device 46 in a bundle 55 can be individually wired to enable individual replacement of a single damaged electric heating device 46.

[0134] Figure 7 The barrel 56 is shown (which is Figure 6 56a, 56b, 56c, 56d in the example of the embodiment of the present invention), the cartridge 56 includes a frame 57 that holds the bundle 55 of the electric heating devices 46. The frame 57 is slidable relative to its mounting location within the vessel 40 of the fluidized bed reactor 25. In some embodiments, the frame 57 includes 57 wheels or bearings to help move the frame 57 into and out of position within the fluidized bed reactor 25.

[0135] Each electric heating device 46 includes a metal tube 47 that houses an electric heating element 49 and other electronic componentry, such as a thermocouple, an active terminal, a neutral terminal, and a ceramic spacer.

[0136] It is generally preferred to have small electric heating devices 46 with low energy input per heating device 46 rather than larger electric heating devices 46 with higher energy input per heating device 46. In some embodiments, each fluidized bed reactor has between 500 and 800 electric heating devices.

[0137] Each electric heater 46 is supported relative to the frame 57 at two locations, namely, in the middle section of the electric heater 46 and at the ends of the electric heater 46. Supporting the electric heater 46 in this manner limits sagging and vibration of the electric heater 46.

[0138] Positioning the electric heating element 49 inside the metal tube 47 provides a number of advantages. First, it protects the heating element 49 from the environment within the fluidized bed reactor 25. Second, it makes the electric heating element 49 easier to handle and replace. Third, it improves the efficiency of heat transfer between the electric heating element 49 and the aluminum trihydrate particles because the metal tube 47 absorbs the radiant heat generated by the electric heating element 49, which would otherwise be dissipated by the air in the fluidized bed reactor 25.

[0139] In summary, the electric heating device 46 provides the following advantages:

[0140] 1. The metal tube 47 protects the heating element 49 from external elements such as steam and moisture.

[0141] 2. The metal tube 47 is more rigid than the heating element 49 and can therefore be supported inside the container 40 more easily than the heating element 49 itself.

[0142] 3. The metal tube 47 can be more easily disconnected and removed as a unit, thus providing a more convenient means of replacement.

[0143] The metal tube 47 has an outer diameter (OD) between 40 mm and 70 mm, suitably 44.2 mm. In some embodiments, the metal tube houses between three and six electrical heating elements 49.

[0144] For full phase angle control of three-phase power, a star connection for the active terminals (not shown) is preferred over a delta connection because the star connection is more robust than the delta connection. In a delta connection, unlike a star connection, if one of the heating elements 49 in a group fails, this can cause the other heating elements 49 in the group to overheat and thus fail more quickly.

[0145] In the example of three heating elements 49, each heating element 49 is connected to a common neutral terminal, with the opposite ends of each element connected to the active terminals. A grounding cable (not shown) is connected to the terminals. In another example, if six heating elements 49 are employed, two heating elements 49 are paired in parallel with one active terminal.

[0146] The current consumption of the heating element 49 in amperes is calculated by dividing the required power in watts by the voltage. Then, for each individual active terminal, the amperage is divided by the number of heating elements 49. For example, for a 30 kW heating element 49, the amperage required for each active connection;

[0147] = 30000 (W) ÷ 240 (V) = 125 amperes per heating element 49

[0148] = 125 (amperes) ÷ 3 = 42 amperes per active connection.

[0149] In the example where a fluidized bed reactor has eight cylinders, each cylinder housing 66 electrical heating devices 46 of 35 kW, i.e., a total of 528 electrical heating devices 46, the total power input is 18 MW.

[0150] In use, the heat generated by the electrical heating element 49 is transferred to the metal tube 47, and the metal tube 47 is cooled by heating the aluminum trihydrate. Thus, for a set amperage supplied to the electrical heating element 49, the temperature of the metal tube 47 is set by the rate of heat transfer to the aluminum trihydrate. In use, the temperature of the electrical heating element 49 rises until it supplies the energy set by the amperage to the metal tube 47. The maximum temperature of the electrical heating element 49 can be set and controlled by changing the amperage, or the hydrate temperature in the reactor can be used to control the amperage.

[0151] For example, if the electric heating element 49 is supplied with a total of 125 amperes, i.e., 30 kW on a 240 V circuit, the temperature of the electric heating element 49 increases until the energy of 30 kW can be carried away by the hydrate. Since the energy to break the water molecules apart is more than 1.6 times the energy required to boil water, once the hydrate reaches approximately 300 °C, the heat transfer rate into the hydrate is very high. To reach this temperature in aluminum trihydrate, the temperature of the metal tube 47 must be higher than the temperature in the aluminum trihydrate.

[0152] The thermocouple provides over-temperature protection.

[0153] The ceramic gasket is made of high alumina ceramic cast to the required dimensions.

[0154] Many modifications may be made to the embodiments of the invention described above without departing from the spirit and scope of the invention.

Claims

1. A process for calcining aluminum trihydrate (Al2O3·3H2O) to form aluminum oxide (Al2O3), comprising: (a) Supplying aluminum trihydrate particles to an electrically heated first reactor and heating the aluminum trihydrate particles to a temperature of at least 280°C and not greater than 400°C, and calcining at least a major portion of the aluminum trihydrate to hydroxyaluminum oxide (γ-AlOOH or AlO(OH)) in the first reactor; and (b) Supplying the hydroxyaluminum oxide particles from the first reactor to a second reactor, and heating the hydroxyaluminum oxide particles and calcining the hydroxyaluminum oxide to aluminum oxide.

2. The process according to claim 1, comprising heating the aluminum trihydrate particles in the first reactor to a temperature of at least 350°C.

3. The process according to claim 1, comprising heating the aluminum trihydrate particles in the first reactor to a temperature between 280°C and 400°C.

4. The process according to any one of the preceding claims, comprising supplying at least 35% of the energy required to calcine the aluminum trihydrate to aluminum oxide to the first reactor at a temperature of up to 350°C.

5. The process according to any one of the preceding claims, comprising supplying at least 80% of the energy required to calcine the aluminum trihydrate to aluminum oxide to the first reactor at a temperature of up to 350°C and to the second reactor at a temperature of up to 500°C - 800°C.

6. The process according to any one of the preceding claims, comprising heating the hydroxyaluminum oxide particles in the second reactor to a temperature of up to 720°C and calcining the hydroxyaluminum oxide in the second reactor.

7. The process according to any one of claims 1 - 5, comprising heating the hydroxyaluminum oxide particles in the second reactor to a temperature of up to 760°C and calcining the hydroxyaluminum oxide in the second reactor.

8. The process according to any one of the preceding claims, comprising supplying 20% - 50% of the energy required to calcine the aluminum trihydrate to aluminum oxide to the first reactor at a temperature of up to 350°C.

9. The process according to claim 8, comprising supplying 40% - 90% of the energy required to calcine the aluminum trihydrate to aluminum oxide to the second reactor at a temperature of up to 760°C.

10. The process according to claim 9, comprising supplying the remaining portion of the energy required to calcine the aluminum trihydrate to aluminum oxide in a third reactor after the second reactor, typically in order to obtain a target product quality, such as a desirable aluminum oxide surface property.

11. The process according to claim 10, comprising heating the aluminum trihydrate particles in the third reactor to a temperature of up to 1000°C.

12. The process according to any one of the preceding claims, wherein one or both of the first reactor and the second reactor is a fluidized bed reactor.

13. The process according to any one of the preceding claims, wherein the first reactor and the second reactor are arranged in a countercurrent configuration.

14. The process according to any one of the preceding claims, wherein the second reactor is an electrically heated reactor.

15. The process according to any one of the preceding claims, wherein the first reactor and the second reactor are configured to promote direct contact between the aluminum trihydroxide particles supplied to the reactors and the electrical heating elements in the reactors.

16. The process according to any one of the preceding claims, comprising heating the boehmite particles from the first reactor and supplying the heated boehmite particles to the second reactor, and heating and calcining the boehmite particles in the second reactor.

17. The process according to any one of the preceding claims, comprising drying the feed aluminum trihydroxide particles by heat transfer from the hot exhaust gas from the process, separating the dried feed aluminum trihydroxide particles from the waste gas stream, and supplying the dried feed aluminum trihydroxide particles to the first reactor.

18. An apparatus for calcining aluminum hydroxide (Al2O3·3H2O) to form alumina (Al2O3), the apparatus comprising: (a) an electrically heated first reactor configured to heat aluminum trihydroxide particles to a temperature of up to at least 280 °C and not greater than 400 °C, and calcine at least a major portion of the aluminum trihydroxide to boehmite (γ-AlOOH or AlO(OH)) in the first reactor; and (b) a second reactor for heating and calcining the boehmite particles from the first reactor to alumina.

19. The apparatus according to claim 18, wherein the first reactor is a first fluidized bed reactor having electrical heating elements in the reactor.

20. The apparatus according to claim 19, wherein the electric heating element of the first fluidized reactor comprises: (a) a metal tube, and (b) electrical heating elements within the tube.

21. The apparatus according to any one of claims 18 to 20, wherein the second reactor is a second fluidized bed reactor having electrical heating elements within the reactor.

22. The apparatus according to claim 21, wherein the electric heating element of the second fluidized bed reactor comprises: (a) a metal tube, and (b) electrical heating elements within the tube.

Images