Calcination apparatus and process using hydrogen gas

Abstract

A process for calcining aluminum hydroxide (Al2O3.3H2O) to form aluminum oxide (Al2O3), such as in an alumina plant, such as a Bayer process plant, is disclosed. The process includes combusting hydrogen and oxygen and producing steam and heat, and using the heat to calcine the aluminum hydroxide and form aluminum oxide and more steam. An apparatus is also disclosed.

Description

Technical Field

[0001] This invention relates to a process and equipment for calcining aluminum hydroxide to form alumina in an alumina production plant, such as a Bayer process plant. Background Technology

[0002] The production of alumina (Al2O3) in alumina production facilities such as the Bayer process involves calcining aluminum hydroxide (Al2O3·3H2O – also known as aluminum hydroxide, aluminum trihydrate, and hydrated alumina) to remove water.

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

[0004] 2Al2O3.3H2O(s)→2Al2O3(s)+3H2O(g)

[0005] A typical calciner for alumina production has a reaction chamber where natural gas and oxygen are burned to generate heat and flue gas containing N2, CO2, and steam. The heat generated in the reaction chamber by the combustion of natural gas and oxygen is used to expel water from aluminum hydroxide to form alumina. Due to heat losses during calcination, the amount of energy supplied to the reaction chamber is significantly more than theoretically required. Some of the heat generated in the reaction chamber is transferred to steam in the flue gas. However, attempting to capture the heat from the flue gas to reduce the amount of energy required for calcination may be technically difficult or prohibitively expensive.

[0006] The above description should not be regarded as an endorsement of common knowledge in Australia or elsewhere. Invention Overview

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

[0009] The present invention is based on the inventor’s understanding that by using hydrogen as a combustion fuel to completely or partially replace natural gas, considerable advantages can be achieved by calcining aluminum hydroxide in the applicant’s calcining furnace, among which the advantage is the reduction of greenhouse gas emissions associated with the calcination process.

[0010] Furthermore, the present invention is based on the understanding that the calcination process can be advantageously operated by using oxygen instead of air to produce pure steam flue gas.

[0011] The inventors also recognized that the calcination process (combustion of hydrogen and air / oxygen) can be carried out in a separate reaction chamber from the current calcining furnace, and the resulting steam and heat can be used to calcine gibbsite (or other forms of aluminum hydroxide) and form alumina and further steam. The use of a separate reaction chamber has advantages in retrofitting current calcining furnaces.

[0012] Some of the steam discharged from the calcining furnace can be recycled to facilitate material fluidization and the transfer of material and heat. The remaining steam can be used elsewhere in the refinery.

[0013] In a broad sense, the present invention provides a process for calcining aluminum hydroxide (Al2O3.3H2O), such as gibbsite, to form alumina (Al2O3), for example, in an alumina plant such as the Bayer process, the process comprising: supplying hydrogen and oxygen to a reaction chamber and causing the hydrogen and oxygen to burn and generate steam and heat; and using the heat to calcine the aluminum hydroxide and form alumina and further steam.

[0014] The term "reaction chamber" is understood in this text to refer to the chamber used for the calcination reaction of aluminum hydroxide to aluminum oxide.

[0015] The advantage of burning hydrogen is that it eliminates the need for hydrocarbon fuel sources such as natural gas. As mentioned above, this can help reduce carbon-based emissions from the calcination process.

[0016] Furthermore, the process can operate using only oxygen as the oxygen source, thus avoiding the complete use of air (i.e., a gas mixture of 78% nitrogen and 21% oxygen). This is an advantage in reducing the volume of gas processed in the unit.

[0017] The process can be operated using oxygen-enriched air, and depending on the amount of enrichment, the amount of nitrogen required is reduced compared to operating with air.

[0018] As mentioned above, another advantage of burning hydrogen and oxygen is the opportunity to generate steam, which can be beneficially used in the process and / or other unit operations in alumina units such as the Bayer process.

[0019] The process may also include venting steam from the process and then transferring at least some of the vented steam back into the process, for example, into a reaction chamber. The vented steam transferred back into the process may be at least 30% by volume of the vented steam, typically at least 40%. Thus, any heat retained in the steam after combustion can be transferred back into the process, for example, back into the reaction chamber. Transferring at least some of the vented steam from the process to the reaction chamber helps reduce the amount of energy required to calcine an additional amount of aluminum hydroxide supplied to the reaction chamber. The steam may also facilitate the fluidization and / or transport of aluminum hydroxide and / or alumina through the process, for example, through the reaction chamber.

[0020] As mentioned above, steam is produced by the combustion of hydrogen and oxygen.

[0021] Steam can also be generated in the reaction chamber by dehydrating aluminum hydroxide into aluminum oxide.

[0022] The process may include maintaining the steam at a temperature higher than the condensation temperature of the steam under the operating conditions of the process.

[0023] Typically, the condensation temperature of steam at atmospheric pressure is 100°C.

[0024] This process can be carried out at atmospheric pressure or below.

[0025] Alternatively, the process can be carried out without placing the reaction chamber at a pressure higher than that generated by operating the process as described above, i.e., by supplying hydrogen and oxygen to the reaction chamber and causing the hydrogen and oxygen to burn and generate steam and heat, and using the heat to calcine aluminum hydroxide and form aluminum oxide and more steam.

[0026] More specifically, the process can be carried out without constructing the reaction chamber as a pressure vessel.

[0027] The steam generated in the reaction chamber can serve as a transport gas in the process, i.e., a fluidizing gas, for example, to transport particulate aluminum hydroxide and / or alumina into and / or out of the reaction chamber.

[0028] The steam generated in the reaction chamber can serve as a heat transfer medium in the process.

[0029] The process may also include transferring at least some of the steam generated in the reaction chamber (and / or elsewhere in the process) to an alumina production apparatus for use in the production of alumina in an apparatus used for processes other than calcining aluminum hydroxide into alumina.

[0030] For example, at least some of the steam from the process can be used in the process within the unit, such as during the digestion of bauxite or the evaporation of Bayer liquor.

[0031] Before being used in other processes, the steam used in the process in the unit can be upgraded, for example, by using a mechanical steam recompression unit or a thermal steam recompression unit.

[0032] Hydrogen can have a purity of >99%.

[0033] The process may include connecting an oxygen source to fluid communication with the reaction chamber.

[0034] The process may include connecting a hydrogen source to fluid communication with the reaction chamber.

[0035] After the process has reached steady-state conditions, the process may include venting flue gas from the reaction chamber, which is at least 85%, typically at least 90%, and more typically at least 95% steam by volume.

[0036] The term "steady-state condition" is understood herein to mean that the process has completed its start-up phase and is operating at or above a predetermined operating state within the control parameters instructing the unit operator to operate stably. Control parameters can be any suitable control parameters selected by the unit operator, including temperatures at different points in the process. An example of a control parameter is a temperature at or above the condensation temperature of the steam.

[0037] The flue gas can be 100% steam.

[0038] The present invention also provides a calcination apparatus for carrying out the process described above.

[0039] The present invention also provides a process for starting an apparatus for calcining aluminum hydroxide to form alumina, the calcination apparatus including a reaction chamber, the process comprising: a preheating step that heats the reaction chamber until predetermined steady-state conditions are reached, and then starting to supply aluminum hydroxide to the reaction chamber, and calcining the aluminum hydroxide to form alumina.

[0040] The predetermined steady-state conditions may include temperatures at or above the condensation temperature of the steam.

[0041] The preheating step is not limited to burning hydrogen and oxygen in the reaction chamber.

[0042] The preheating step may include burning any suitable fuel source, including hydrocarbon fuel, in the reaction chamber, or burning it outside the reaction chamber and transferring heat to the reaction chamber.

[0043] As a specific example, the steam generated in an alumina production unit can be used to heat the reaction chamber during the preheating step.

[0044] In the preheating step, the reaction chamber can be heated by transferring at least some of the generated steam to the reaction chamber.

[0045] Changing operating conditions after steady-state conditions have been reached for combustion of hydrogen and oxygen in the reaction chamber may include providing a gas feed that increases the proportion of hydrogen over a predetermined time period.

[0046] The present invention also provides a process for calcining aluminum hydroxide, such as gibbsite, to form alumina (Al2O3), for example, in an alumina plant such as a Bayer process plant, the process comprising: burning hydrogen and oxygen to generate steam and heat, using the heat to calcine aluminum hydroxide to form alumina and further steam, and using the steam generated by the combustion as a transport gas in the process.

[0047] The process described in the preceding paragraph may also include venting steam from the process and then transferring at least some of the vented steam back into the process.

[0048] The described process may include burning hydrogen and oxygen in a reaction chamber to generate steam and heat, and calcining aluminum hydroxide in a reaction chamber to form aluminum oxide.

[0049] Alternatively, the process may include burning hydrogen and oxygen in a reaction chamber to generate steam and heat, and transferring the steam and heat to a second reaction chamber and calcining aluminum hydroxide in the second reaction chamber to form aluminum oxide.

[0050] This process can be applied to existing calcination equipment that operates using natural gas as a fuel source and air as an oxygen source for combustion of the fuel.

[0051] Existing devices can be appropriately modified to use hydrogen as a fuel source and oxygen (typically only oxygen) as an oxygen source for the combustion of the fuel source.

[0052] Furthermore, existing apparatus can be modified so that steam discharged from the process is transferred to the reaction chamber and serves as a transport gas, and optionally as a heat transfer medium.

[0053] The present invention also provides an apparatus for calcining aluminum hydroxide to form aluminum oxide, the apparatus comprising:

[0054] - A reaction chamber, which is configured to calcine aluminum hydroxide to form aluminum oxide;

[0055] - Hydrogen and oxygen sources are used to generate heat, which is used to calcine aluminum hydroxide in the reaction chamber and produce aluminum oxide and vapor.

[0056] - Alumina exports, and

[0057] -Including the outlet of flue gas containing steam.

[0058] The device may include a pipeline for supplying steam discharged via the flue gas outlet to the device, for example, to a reaction chamber.

[0059] The device may include: a first reaction chamber for calcining aluminum hydroxide to form aluminum oxide and vapor; and a second reaction chamber for burning hydrogen and oxygen to generate heat for use in the first reaction chamber.

[0060] These two second reaction chamber options can be advantageous regardless of whether the calcination process of the present invention is modified to an existing calcination apparatus.

[0061] In this case, the existing reaction chamber can continue to be used as the chamber for calcining aluminum hydroxide, and the second reaction chamber can be specially designed to burn hydrogen and oxygen, and is positioned close to and operably connected to the existing device to supply heat to the existing reaction chamber.

[0062] The present invention also provides an apparatus for producing alumina, such as a Bayer process apparatus, the apparatus comprising the apparatus described above for calcining aluminum hydroxide to form alumina. Brief description of the attached diagram

[0064] Embodiments of the invention are further described with reference to the non-limiting accompanying drawings, in which:

[0065] Figure 1 An example of a typical Bayer process flow diagram for the production of alumina is illustrated.

[0066] Figure 2 An embodiment of the apparatus for calcining aluminum hydroxide according to the present invention is illustrated;

[0067] Figure 3 The illustration shows another embodiment of the apparatus for calcining aluminum hydroxide according to the present invention, although it is not the only alternative embodiment;

[0068] Figure 4 The illustration shows an embodiment of the calcination apparatus according to the present invention, which is based on Figure 3 An embodiment of the apparatus for calcining aluminum hydroxide is shown in the figure; and

[0069] Figure 5The XRD results are from the test work of calcining gibbsite in a steam environment according to the present invention.

[0070] Description of the implementation plan

[0071] The following description is in the context of calcining aluminum hydroxide, such as the mineral gibbsite, in an alumina plant in the form of a Bayer process to form alumina.

[0072] It should be noted that the present invention is not limited to calcining aluminum hydroxide in a Bayer process unit to form alumina, but extends to any unit used for the production of alumina, wherein calcination is a process step in such a unit.

[0073] Figure 1 The flowchart shown includes the following process steps:

[0074] • Boiling bauxite in a caustic alkali solution 3.

[0075] • Purification (solid / liquid separation of residues from mother liquor).

[0076] • Precipitation of aluminum hydroxide (aluminum oxide hydrate).

[0077] • Return the used liquid to the cooking process, for example via evaporation step 17.

[0078] • Calcining aluminum hydroxide to form aluminum oxide.

[0079] refer to Figure 1 The bauxite, which has been crushed to a suitable particle size distribution, is then transferred to the cooking step 5.

[0080] Step 5 of the cooking process in the diagram is essentially two steps: (a) a pre-disilication step, which pre-reacts any clay or other minerals containing highly reactive silica in the bauxite and initiates the formation of desilication products (DSP); and (b) cooking, in which, depending on the type of bauxite, the slurry formed in step (a) is heated to between ~140°C and 280°C, wherein alumina and reactive silica dissolve and silica is redeprecipitated as DSP, which comprises caustic alkali, alumina, and silica.

[0081] The output of cooking step 5 is transferred to purification step 7, which produces solid and liquid outputs.

[0082] The solid output from purification step 7 is transferred as a stream [4] to washing step 9 and forms residue 11, which is transferred as a residual stream from washing step 9 [6].

[0083] The liquid output, namely Bayer liquor, more specifically pregnant Bayer liquor, is transferred as a stream[1] to precipitation step 11.

[0084] In precipitation step 11, the Bayer solution is gradually cooled from approximately 80°C to 65°C in a series of large containers. The dissolved alumina precipitates as aluminum hydroxide (Al₂O₃·3H₂O).

[0085] The output slurry from precipitation step 11 is transferred to hydrate sorting and washing step 13, and the aluminum hydroxide crystals are thermally washed.

[0086] The output of hydrate sorting and washing step 13 is:

[0087] (a) The used liquid, which is transferred as a stream [2] to the evaporation step 17 and then to the cooking step 5 for use in that step,

[0088] (b) Washed aluminum hydroxide (Al₂O₃·3H₂O) crystals, which are transferred to calcination step 15 and calcined in this step to remove water and produce the output alumina (Al₂O₃) product; and

[0089] (c) The hydrate washing filtrate, which is transferred as a stream [3] to the causticization step 19.

[0090] The causticizing step 19 produces a causticizing flow that becomes part of the residue 11 [7].

[0091] Figure 2 An embodiment of an apparatus for calcining aluminum hydroxide (Al2O3·3H2O) in calcination step 15 to form aluminum oxide (Al2O3) is shown.

[0092] exist Figure 2 In the device 23, there is a reaction chamber 25 for calcination reaction to form aluminum oxide from aluminum hydroxide.

[0093] The reaction chamber 25 can be any suitable chamber used for the calcination reaction of aluminum hydroxide to aluminum oxide.

[0094] For example, reaction chamber 25 can be a rotary kiln or a gas suspension calciner chamber.

[0095] The process of the present invention does not need to be operated under high pressure conditions, and therefore, the reaction chamber 25 does not need to be a pressure vessel.

[0096] The reaction chamber 25 is in fluid communication with a hydrogen source 27, an oxygen source 29 (which in this embodiment is only oxygen) and an aluminum hydroxide source 31. The reaction chamber 25 includes an inlet and a transfer line for supplying these feed materials to the reaction chamber 25. The reaction chamber 25 includes an alumina discharge line 33 for discharging the alumina formed in the reaction chamber 25. The reaction chamber 25 also includes an outlet line 35 for discharging the flue gas generated in the reaction chamber 25.

[0097] Hydrogen and oxygen from hydrogen source 27 and oxygen source 29 are fed into reaction chamber 25 and combusted there, generating heat and flue gas. The heat drives water away from aluminum hydroxide to form aluminum oxide and steam. The flue gas, including steam, is discharged from reaction chamber 25 via flue gas line 35.

[0098] If the fuel source is not limited to hydrogen and includes other fuels, such as natural gas (as is possible in some embodiments of the invention), the flue gas stream will contain steam plus other components, such as CO2. However, when hydrogen is the only fuel source and is burned only with oxygen in reaction chamber 25, steam is the only component in flue gas line 35. Producing only steam means that other flue gas components, such as CO2 and N2, do not need to be separated before the steam can be reused. Separating flue gas into separate components is generally technically difficult, and / or is prohibitively expensive when seeking to separate steam from flue gas.

[0099] In one embodiment, hydrogen source 27 has a purity of >99%. Water expelled during calcination is also present in flue gas line 35. In this way, two steam sources exist in device 23, the first from the combustion of hydrogen and oxygen, and the second from the dehydration of aluminum hydroxide. In some embodiments, oxygen is supplied in stoichiometric excess relative to hydrogen to ensure complete combustion of hydrogen. When oxygen is supplied in stoichiometric excess, the flue gas in the flue gas supply line may have trace amounts (e.g., <5%) of oxygen. Typically, any excess oxygen used for hydrogen combustion is kept to a minimum.

[0100] As mentioned above, by using only hydrogen and only oxygen for combustion to generate heat for reaction chamber 25, device 23 produces no CO2 or other carbon-based emissions. If the hydrogen is sourced from a renewable source, device 23 can significantly reduce its carbon footprint compared to devices that rely on natural gas for calcination.

[0101] exist Figure 2In the embodiment shown, the flue gas line 35 is divided into and includes a flue gas transfer line 37 in fluid communication with the reaction chamber 25. The flue gas transfer line 37 transfers at least some of the flue gas (which is typically at least substantially steam) from the flue gas line 35 to the reaction chamber 25. Up to 30% of the heat generated in the reaction chamber 25 is lost to the environment when the heat in the flue gas is not captured and is instead released into the environment. The advantage of transferring at least some of the steam back to the reaction chamber 25 via the flue gas line 35 is that the heat lost to the environment by releasing the steam is transferred back to the reaction chamber 25 in another manner. In this way, the steam can act as a heat transfer medium when the heat from the steam can be used elsewhere in the device 23. Using the flue gas transfer line 37 to return the steam to the reaction chamber 25 can also help reduce the amount of hydrogen required to maintain the reaction temperature of the reaction chamber 25, since the steam contributes heat to the reaction chamber 25.

[0102] It should be noted that in some cases, depending on the calcination conditions, a small amount of solids may be present in the flue gas (i.e., steam), even after the flue gas has passed through a solids filtration unit (e.g., a bag house and / or an electrostatic precipitator). If a small amount of solids is present in the flue gas, additional filtration steps can be performed to remove the solids before at least some of the steam is transferred back to the reaction chamber 25 via the flue gas line 35.

[0103] To prevent condensation of the steam in flue gas line 35 and flue gas transmission line 37, lines 35 and 37 are maintained at a temperature above the steam condensation temperature. In one embodiment, the steam condensation temperature is 100°C. In another embodiment, the steam in flue gas line 35 is superheated steam, i.e., >100°C. In yet another embodiment, the steam temperature is maintained at 160°C or above. Maintaining the steam temperature above 100°C, such as at approximately 160°C, helps prevent steam condensation. Preventing steam condensation also helps reduce the likelihood of condensed steam causing alumina to “adhere” to the walls and surfaces of reaction chamber 25 and surrounding structures. Preventing steam condensation in flue gas line 35 helps prevent the steam density from dropping below a threshold that would prevent the steam in flue gas line 35 from acting as a fluid flow medium, such as a transport gas. The latent heat required to decompose the steam uses a significant amount of energy, therefore maintaining the temperature of device 10 above the steam condensation temperature helps reduce or eliminate energy-intensive steam heating steps.

[0104] The condensation temperature of the steam depends on the pressure of the flue gas line 35. Generally, as the pressure of the flue gas line 35 increases, the condensation temperature of the steam also increases. As mentioned above, in the embodiment, the equipment 10 operates at atmospheric pressure, such as at about 1 atmosphere.

[0105] Figure 3 Another example of an apparatus 100 for calcining aluminum hydroxide to form aluminum oxide is shown, although it is not the only other example.

[0106] Equipment 100 is similar to Figure 2 Equipment 23 of the implementation scheme.

[0107] In this respect, device 100 includes a reaction chamber 112 similar to that of device 10, a hydrogen source 114, an oxygen source 115, an aluminum hydroxide source 116, an alumina discharge line 117, a flue gas discharge line 118, and a flue gas transmission line 120.

[0108] exist Figure 3 In the embodiment shown, aluminum hydroxide is supplied from aluminum hydroxide source 116 to reaction chamber 112 via dryer 124. Dryer 124 removes at least some surface-bound water from the aluminum hydroxide and forms at least some dried aluminum hydroxide upstream of reaction chamber 112. The partially dried aluminum hydroxide is then calcined in reaction chamber 112 to form alumina.

[0109] exist Figure 3 In the embodiment shown, after calcination in reaction chamber 112, the alumina is then transferred to heat recovery device 128, which recovers heat from the alumina. Heat recovery device 128 can be any suitable form of equipment. This heat recovery helps cool the alumina and form cooled alumina, while retaining heat within device 100. The cooled alumina is fed through a discharge line 117 for further processing, such as packaging and transportation.

[0110] exist Figure 3 In the embodiment shown, a dust recovery apparatus 126, such as a bag filter chamber, is in fluid communication with a dryer 124. A flue gas line 118 extends from the dust recovery apparatus 126.

[0111] exist Figure 3In the embodiment shown, flue gas transmission line 120 is in fluid communication with flue gas line 118 and heat recovery device 128. At least a portion of the steam in flue gas line 118 is transmitted to heat recovery device 128 via flue gas transmission line 120. The steam transmitted to heat recovery device 128 is used as a transport gas or fluid medium to facilitate the transmission of aluminum hydroxide and / or alumina through device 100. The steam entering heat recovery device 128 travels to reaction chamber 112, dryer 126, and then through dust recovery device 126. This direction of steam travel is indicated by arrow 132. When aluminum hydroxide is introduced into dryer 124, dust and other fine particulate matter are carried by the steam and transmitted to dust recovery device 126. Since aluminum hydroxide and alumina travel in a direction generally opposite to the flow of steam passing through heat recovery device 128, reaction chamber 112, and dryer 126 (i.e., opposite to the direction of steam travel 132), the net flow of aluminum hydroxide and alumina through device 100 is generally countercurrent to the flow of steam. However, it should be noted that localized cocurrent flows of aluminum hydroxide and / or alumina and steam may exist within dryer 124, reaction chamber 112, and heat recovery device 128, but generally, a net countercurrent flow of aluminum hydroxide and / or alumina and steam may exist.

[0112] The flue gas line 118 is divided into two lines. The first line is the flue gas transmission line 120 described above, which supplies steam to the heat recovery unit 128. The second line supplies steam as a steam source 130 for external use of the unit 100.

[0113] Steam source 130 can be used to supply steam to other equipment in the Bayer process plant.

[0114] For example, steam source 130 can be used by a cooker during bauxite cooking, during the evaporation of used Bayer liquor, during causticizing to remove impurities from the Bayer process, and in an evaporator / steam generator to supplement low-pressure steam. In this way, device 100 can be used as a steam generator. The dashed line extending from steam source 130 represents the fact that, in some embodiments, the steam is not stored or discharged, but instead used elsewhere.

[0115] In some embodiments, the steam source 130 also includes a mechanical vapor recompression compressor and / or a thermal vapor recompression compressor to “boost” the steam source 130 to higher pressures. For example, mechanical vapor recompression can boost steam from 1 atmosphere to 5 atmospheres, and thermal vapor recompression can boost steam from 5 atmospheres to >10 atmospheres.

[0116] To control the relative flow rate of steam in the flue gas transmission line 120 and the steam source 130, a control valve 134 is installed at the connection between the flue gas transmission line 120 and the steam source 130. The control valve 134 can be operated manually or automatically to control the relative flow rate of steam in the flue gas transmission line 120 and the steam source 130. The relative flow rate of steam in the flue gas transmission line 120 and the steam source 130 can be determined by the operating conditions of the equipment 100 and the heat requirement for calcination.

[0117] The use of excess steam generated by device 100 can help improve the efficiency of other equipment and apparatus located in and around a Bayer process unit that requires steam to operate.

[0118] It should be noted that, although oxygen source 115 and hydrogen source 114 are in Figure 3 The feed materials are shown connected to and directly supplied to reaction chamber 112, but oxygen source 115 and hydrogen source 114 only need to be in fluid communication with reaction chamber 112. Therefore, oxygen source 115 and / or hydrogen source 114 can be connected to the upstream side of reaction chamber 112 instead of being directly connected to the reaction chamber.

[0119] exist Figure 3 In this embodiment, the upstream side of reaction chamber 112 is opposite to the direction of arrow 132, i.e., it faces the heat recovery device 128. For example, in one embodiment, oxygen source 115 is connected to heat recovery device 128.

[0120] With this arrangement, the oxygen transferred from oxygen source 115 to reaction chamber 112 via heat recovery device 128 can act as a cooling fluid, which helps cool the alumina in or near output line 117. Simultaneously, the oxygen is heated before entering reaction chamber 112. Similarly, instead of oxygen source 115, hydrogen source 114 can be connected to heat recovery device 128. As an alternative, both oxygen source 115 and hydrogen source 114 can be connected to heat recovery device 128.

[0121] When oxygen source 115 and / or hydrogen source 114 are connected to the upstream side of reaction chamber 112, steam from return line 120 that is transferred to heat recovery device 128 is used to transfer oxygen and / or hydrogen to reaction chamber 112 for combustion.

[0122] Devices 10 and 100 are illustrated in exemplary form only. These are examples of two implementations among many possible implementations.

[0123] Understandable, Figure 3Features such as reaction chamber 112, heat recovery device 128, and dryer 126 can be formed from many different components, and reaction chamber 112, heat recovery device 128, and dryer 126 can have different stages. For example, reaction chamber 112 can have a primary heating stage and a secondary heating stage. Heat recovery device 128 can also have multiple cooling stages, such as a series of interconnected cyclone separators, which help purify alumina at different stages.

[0124] Figure 3 The embodiment of the device 100 shown can be formed as a newly constructed device or by modifying existing calcination equipment in an alumina plant.

[0125] As mentioned above, one retrofit option includes providing a separate, specially designed reaction chamber for the combustion of hydrogen and oxygen, and being located near and operatively connected to the existing calcination equipment to supply heat to the existing reaction chamber.

[0126] Regarding retrofit options, existing calcination equipment typically vents flue gas to the atmosphere and has a natural gas supply connected to the reaction chamber. Air is typically used as the oxygen source and is transferred to the reaction chamber via heat recovery equipment such as 128. Air is also typically used as a transport fluid. Existing calcination equipment does not have a flue gas return line 120, an oxygen source 115, and a hydrogen source 114.

[0127] In the implementation plan, the process of modifying the calcination equipment includes installing a flue gas transmission pipeline 120, so that the flue gas flow, for example 118, is in fluid communication with the reaction chamber 112. Figure 2 As shown in the diagram, the flue gas transmission line 120 is in fluid communication with the reaction chamber 112 via a heat recovery device 128. Then, a hydrogen source, such as 114, and an oxygen source, such as 115, are connected to the reaction chamber 112.

[0128] because Figure 3 The device 100 shown requires the use of steam as a transport gas or fluid medium to facilitate the transfer of aluminum hydroxide and / or alumina through the device 100; therefore, the device 100 should ideally be at or above the condensation temperature of the steam. The condensation temperature of the steam is approximately 100°C, although this does depend on the operating pressure of the device 100. In an embodiment, the device 100 is maintained at or above 160°C.

[0129] To start Figure 3The apparatus 100 shown is used to calcine aluminum hydroxide. Before the aluminum hydroxide is supplied to the reaction chamber, the reaction chamber needs to be heated in a preheating step to be at or above a predetermined operating state as a steady state. In an embodiment, the predetermined operating state is a temperature at or above the condensation temperature of the steam. Heating the reaction chamber 112 above the condensation temperature of the steam can be achieved by burning oxygen and hydrogen in the reaction chamber 112 to generate heat. Once sufficient heat has been generated, the reaction chamber 112 should be above the condensation temperature of the steam. The steam generated by the combustion of hydrogen and oxygen can be transferred to the reaction chamber 112, for example, via a flue gas return line 120, to heat the reaction chamber 112.

[0130] In this implementation, to prevent the reaction chamber 112 from being flooded by condensed steam before it reaches or exceeds the steam condensation temperature, the reaction chamber 112 is typically heated to a temperature above the steam condensation temperature during the start-up phase via a preheating option rather than by combustion of pure hydrogen and oxygen within the reaction chamber 112. Once the reaction chamber 112 is heated to a temperature above the steam condensation temperature, the operating conditions can be altered, and hydrogen and oxygen can then be combusted in the reaction chamber 112 to generate heat and steam. The steam generated in the reaction chamber 112 can then be used to heat other components of the apparatus 100.

[0131] In one implementation, at least reaction chamber 112 is preheated during the start-up phase with an external heat source, such as steam from another location in the Bayer process unit, prior to the combustion of hydrogen and oxygen. For example, steam generated during the cooking of bauxite can be transferred to reaction chamber 112 via steam source 130, return line 120, and heat recovery equipment 128.

[0132] In the implementation scheme, preheating of reaction chamber 112 during the start-up phase includes burning natural gas and oxygen in reaction chamber 112 to generate heat. Once reaction chamber 112 is at or above the condensation temperature of steam, the operating conditions are changed, and hydrogen is burned instead of natural gas with oxygen.

[0133] The conversion from natural gas to hydrogen can be gradual. For example, preheating reaction chamber 112 can begin with 100% natural gas, and over a period of time, or when predetermined reaction chamber conditions are met, hydrogen can be used to replace a portion of the natural gas until it is completely replaced. Natural gas can be completely replaced just before reaction chamber 112 reaches the predetermined operating conditions.

[0134] Alternatively, during the start-up phase, reaction chamber 112 may be preheated by burning a lean hydrogen fuel mixture, which is then converted into a rich hydrogen fuel mixture until a predetermined operating state is achieved, at which point the rich hydrogen fuel mixture is exchanged with 100% hydrogen.

[0135] In one embodiment, the reaction chamber 112 is heated to a temperature at or above the condensation temperature of the steam by heating upstream of the reaction chamber 112, such as at the location of the heat recovery device 128, and allowing heat to be transferred into the reaction chamber 112.

[0136] Preheating reaction chamber 112 during the start-up phase can be achieved by combining different heating processes. For example, an external heat source can be used and reaction chamber 112 can be preheated by burning oxygen and hydrogen or by burning a mixture of oxygen and a fuel containing natural gas.

[0137] Figure 4 It shows the basis Figure 3 An embodiment of the calcination equipment 200 of the equipment 100 shown herein.

[0138] The following summary outlines device 100 and Figure 4 The relationship between the components of device 200:

[0139] • The reaction chamber 112 of the device 100 is the calcining section 212a in the apparatus 200.

[0140] • The dryer 124 of the device 100 is the drying section 224a in the device 200.

[0141] The heat recovery device 128 in device 100 is the heat recovery section 228a in device 200.

[0142] The dust recovery device 126 in the device 100 is the dust recovery section 226a in the device 200, which is in the form of a bag filter chamber 226.

[0143] • The aluminum hydroxide source 116 in device 100 is the hydrate input section 216 in device 200.

[0144] • The oxygen source 115 and hydrogen source 114 in device 100 are the oxygen input section 215 and hydrogen input section 214 in device 200, respectively.

[0145] • The return line 120 in device 100 is the return steam line 220 in device 200.

[0146] • The output line 117 in device 100 is the alumina outlet 217 in device 200.

[0147] exist Figure 4 In the apparatus 200 shown in the diagram, the steam flows from the outlet 217 to the bag filter chamber 230 in a left-to-right direction. Therefore, the alumina outlet 217 is upstream of the reaction chamber 212, and the bag filter chamber 226 is downstream of the reaction chamber 212.

[0148] Drying section 224a includes a cyclone separator 240. Aluminum hydroxide is fed into hydrate inlet 216, where the steam flow described above through device 200 carries the aluminum hydroxide to the cyclone separator 240. During the transport from inlet 216 to cyclone separator 240, at least some, and typically most, of the surface-bound water is removed from the aluminum hydroxide. Cyclone separator 240 purifies the aluminum hydroxide, and dust and other unwanted fine particulate matter are transferred to bag filter chamber 226.

[0149] The purified aluminum hydroxide is then transferred from cyclone separator 240 to calcination section 212a.

[0150] The calcination section 212a has cyclone separators 242a and 242b located downstream of the reaction chamber 212. Purified aluminum hydroxide is fed from cyclone separator 240 in the drying section 224a to a location upstream of cyclone separator 242b, where steam then transports the purified aluminum hydroxide downstream to cyclone separator 242b for further purification. The further purified aluminum hydroxide (and any alumina formed as a result of calcination in cyclone separator 242b) is then transported to the reaction chamber 212. Hydrogen inlet 214 and oxygen inlet 215 are immediately upstream of the reaction chamber 212. Hydrogen and oxygen are fed into the reaction chamber 212 through their respective inlets 214 and 215, where they are combusted to generate heat and steam. The heat calcines the aluminum hydroxide to form alumina in the reaction chamber 212. Steam is also generated in the reaction chamber 212 by the dehydration (i.e., calcination) of the aluminum hydroxide. Vapor is also generated by the evaporation of surface moisture on the aluminum hydroxide in the drying section 224a. For example, the surface moisture of the aluminum hydroxide is typically about 6% w / w. Most of the aluminum hydroxide present in the reaction chamber 212 is converted to aluminum oxide in the reaction chamber.

[0151] Then, the alumina, along with any remaining purified aluminum hydroxide, is transferred from reaction chamber 212 to cyclone separator 242a, where the remaining purified aluminum hydroxide is calcined and forms alumina.

[0152] The calcination of most, or at least 80%, of purified aluminum hydroxide usually occurs in reaction chamber 212.

[0153] The steam generated in reaction chamber 212 is transferred through a device to bag filter chamber 226. This transfer of steam from reaction chamber 212 to bag filter chamber 226 facilitates at least partially the transfer of aluminum hydroxide from hydrate inlet 216 to cyclone separator 240. Upon exiting bag filter chamber 226, the steam is split into return steam line 220 and steam source 230.

[0154] After alumina has been formed in reaction chamber 212, it is then transferred to heat recovery stage 228a. Heat recovery stage 228a has multiple cyclone separators 244 that purify and cool the alumina. The alumina passes through a final cyclone separator 246 before passing through alumina outlet 217. Return steam line 220 is in fluid communication with the final cyclone separator 246. Steam in return steam line 220 fluidizes and transports the alumina and aluminum hydroxide in device 200. Example

[0155] Example 1 - Modeling Calcination Apparatus 200

[0156] Figure 4 The calcination apparatus 200 shown was modeled using SysCAD to determine the flow rates of the various input and output sections used in the apparatus 200.

[0157] In one example, 4.51 t / h of H2 and 38.2 t / h g of O2 are supplied to reaction chamber 212, and 284 t / h of aluminum hydroxide is fed into input section 216.

[0158] H2 and O2 are burned to produce 187 t / h of steam.

[0159] The steam value of 187 t / h also includes the steam generated by the dehydration of aluminum hydroxide in reaction chamber 212.

[0160] Before aluminum hydroxide enters reaction chamber 212, the dehydration of aluminum hydroxide in drying stage 224a and calcination stage 212a means that the total amount of steam generated and transferred from calcination stage 212a and drying stage 224a to bag filter chamber 226 is 287 t / h.

[0161] 284 t / h of aluminum hydroxide is converted into 205 t / h of aluminum oxide.

[0162] 114 t / h of steam is transported through return steam line 220 to serve as a transport gas for particulate matter such as aluminum hydroxide and alumina.

[0163] The apparatus used to calcine aluminum hydroxide using natural gas to form alumina has an energy requirement of about 3 GJ / h, while apparatus 200 has an energy requirement of about 2.9 GJ / h.

[0164] It should be noted that the theoretical energy requirement for converting aluminum hydroxide to aluminum oxide in device 200 is approximately 1.8 GJ / h to 2.0 GJ / h, and the difference between the theoretical energy requirement and the actual energy requirement is due to energy losses, such as heat losses.

[0165] However, the calculation does not take into account the fact that the steam generated by device 200 can be used elsewhere to reduce the energy demand of auxiliary equipment in the alumina refinery, and therefore the use of device 200 can help improve the overall energy efficiency of the alumina refinery.

[0166] Although the detailed description mentions calcining aluminum hydroxide to form alumina, the described equipment and process are applicable to the calcination of other materials such as gypsum. For example, gypsum, selenite, and / or basalt can be fed into reaction chamber 112 to form anhydrous gypsum.

[0167] Example 2 - Simulating steam conditions (similar to the conditions for hydrogen-oxygen steam generation) to calcine gibbsite. Alumina

[0168] As mentioned above, the applicant operates a calcining furnace that burns natural gas to dehydrate aluminum hydroxide (Al2O3.3H2O) in the form of gibbsite into alumina (Al2O3), and the present invention is made by the inventors in the context of these calcining furnaces.

[0169] One difference between the current conditions in the applicant's natural gas combustion furnace and the calcination process and equipment of the present invention is the use of a hydrogen-oxygen flame according to the present invention.

[0170] The properties of hydrogen-oxygen flames include a combustion temperature significantly higher than that of natural gas-air flames (Table 1), and hydrogen burns in a pale blue flame, resulting in minimal heat transfer via radiation.

[0171] Table 1: Approximate Flame Temperature

[0172] fuel Combustion with air (°C) Combustion with oxygen (°C) natural gas 1940 2760 hydrogen 2130 2800

[0173] For hydrogen-oxygen The primary heat transfer mechanism of a flame is through convection and conduction via the steam generated during combustion.

[0174] These heat transfer mechanisms allow hydrogen-oxygen The flame is contained within the calcining apparatus or externally within a separate reaction chamber (as described above), thereby generating steam and heat that are then transferred to the calcining apparatus, allowing the vast majority of the solids in the calcining apparatus to reach the target temperature.

[0175] By employing a separate hydrogen combustion chamber, the risks associated with high-temperature zones (associated with hydrogen-oxygen flames) in calcination equipment are at least substantially eliminated.

[0176] Despite the comments in the preceding paragraphs, it should be noted that both options—including the hydrogen-oxygen flame within the calcination apparatus or externally within a separate reaction chamber—are viable options.

[0177] Another difference between the current conditions in the applicant's natural gas-fired calcining furnace and the calcination process and equipment of the present invention is the gas composition in the calcining equipment. If oxygen and hydrogen are burned, the flue gas from the calcining furnace will be pure steam, and if oxygen-enriched air is used, the flue gas will be a combination of nitrogen and steam.

[0178] Some studies have shown that the thermal decomposition rate of gibbsite is negative relative to water vapor concentration, which means that the generated water vapor hinders further calcination of gibbsite. Conversely, there is a view that the high water vapor pathway can proceed unimpeded through the boehmite pathway, γ pathway, δ pathway, θ pathway, and finally the α pathway.

[0179] Industrially, gibbsite calcination is carried out in flash calciners and bubbling fluidized bed or circulating fluidized bed (CFB) reactors.

[0180] CFB technology can be scaled up without compromising product quality because the recycling of solids in the CFB results in uniform temperature distribution and consistent product quality, both at high capacities and during periods of load variation.

[0181] The main components of the CFB calcination process are two preheating stages, one calcination stage, and two cooling stages. The entire residence time from when the feed material is introduced into the process to when the alumina product is discharged is typically about 20 minutes. The CFB calcination furnace usually operates in the range of 900°C to 1000°C, depending on the product quality target. The material is held at the target temperature for 6 minutes.

[0182] The main purpose of this embodiment is to simulate steam conditions (similar to the conditions under which hydrogen-oxygen steam is produced) in order to calcine gibbsite into alumina under conditions that repeat those of a typical circulating fluidized bed calciner.

[0183] The testing was conducted in a laboratory-scale circulating fluidized bed reactor.

[0184] Methodology

[0185] An 85mm diameter CFB reactor with an external electric furnace was used to test the calcination of gibbsite in a steam environment.

[0186] Before each test, gibbsite is dried at 105°C to remove any free moisture. The dried solid is then placed in a pressure feeder.

[0187] The furnace is heated to the target temperature. Nitrogen gas is introduced into the system at low flow rates at the following points:

[0188] • Pressure feeder.

[0189] • Recirculation line loop seal.

[0190] • Sampling point at the bottom of the furnace.

[0191] These nitrogen streams are needed to prevent vapor from condensing in the colder parts of the system and causing blockages.

[0192] Steam is then introduced at the target flow rate, and once the temperature inside the reactor has stabilized, ~1.5 kg of solids is introduced into the system via a pressure feeder.

[0193] Once the solids have reached the target temperature, they are held in the system for the required duration in a collection flask at the bottom of the furnace before being sampled.

[0194] Nitrogen gas is introduced into the flask to help cool the solid in an inert atmosphere and to displace the vapor from the solid before water condenses in the collection flask.

[0195] result

[0196] To simulate the combustion of hydrogen and oxygen in the applicant's calcining furnace, the following test conditions were used:

[0197] Table 2: Summary of Test Operating Conditions

[0198] Temperature (°C) Gas flow rate (L / min) Gas flow rate (L / min) Duration of stay (min) steam <![CDATA[N2]]> 950 25 30 30

[0199] Due to the small size of the equipment and high ambient heat loss, vapor condensation occurs at the alumina exhaust port, leading to alumina blockage during testing. For this reason, nitrogen is introduced as an inert gas in increased quantities to prevent vapor condensation and material blockage.

[0200] Once the material blockage is resolved using an inert gas flow, gibbsite is calcined, resulting in the following:

[0201] X-ray diffraction (XRD)

[0202] XRD was used to identify the alumina phase formed during the calcination process. Figure 5 The image shows feature maps of the two submitted samples.

[0203] from Figure 5 The following can be seen:

[0204] 1. The trihydrate gibbsite is mainly calcined into γ-alumina and θ-alumina phases - which meets the applicant's furnace-grade alumina product quality specifications.

[0205] 2. A small amount of gibbsite is calcined into trace amounts of α-alumina phase - this meets the applicant's furnace-grade alumina product quality specifications.

[0206] Loss on Ignition (LOI)

[0207] Loss on ignition was used to determine the amount of gibbsite that transformed into the alumina phase described above. From this, it can be seen that:

[0208] 1. The surface moisture of alumina is negligible, with a residual water content of <0.05%.

[0209] 2. The conversion of gibbsite to alumina is ~99.7% complete - which meets the applicant's furnace-grade alumina product quality specifications.

[0210] discuss

[0211] The above results indicate that, under conditions generated by a hydrogen-oxygen flame, steam allows gibbsite to be calcined into alumina.

[0212] Furthermore, the results indicate that the produced alumina is suitable for meeting the applicant's furnace-grade alumina specifications.

[0213] Furthermore, the formation of large amounts of γ-alumina and θ-alumina supports the expected calcination pathway under high vapor conditions:

[0214] Boehmite → gamma alumina → delta alumina → theta alumina → alpha alumina

[0215] Although the phases enclosed in parentheses were not directly observed, technical literature suggests that these phases may have been present during the decomposition reaction.

[0216] Because the small-scale nature of the equipment allows vapor to condense on surfaces exposed to the atmosphere, nitrogen is needed for the purposes described above in managing material handling issues during testing. This is not expected to be a problem for scaling up.

[0217] Many modifications can be made to the embodiments of the present invention described above without departing from the spirit and scope of the invention.

[0218] As an example, although the embodiments of the invention described with respect to the accompanying drawings involve the combustion of hydrogen and oxygen in a reaction chamber to generate steam and heat, and the calcination of aluminum hydroxide in the reaction chamber to form aluminum oxide, the invention is not limited thereto, but extends to embodiments operating in the case of two reaction chambers, one for the combustion of hydrogen and oxygen to generate steam and heat, and the second for the calcination of aluminum hydroxide to form aluminum oxide, wherein heat and steam are transferred to the second reaction chamber for this purpose.

Claims

1. A process for calcining aluminum hydroxide Al2O3·3H2O to form aluminum oxide Al2O3, comprising: Hydrogen and oxygen are supplied to the reaction chamber, and the hydrogen and oxygen are burned to produce steam and heat; Aluminum hydroxide is fed into steam discharged from the reaction chamber to transport the aluminum hydroxide and at least some surface-bound water is removed from the aluminum hydroxide during transport to form at least partially dried aluminum hydroxide, and the heat is used to calcine the at least partially dried aluminum hydroxide to form alumina, wherein the removal of at least some surface-bound water and the calcine generate more steam; as well as At least some of the steam generated in and discharged from the reaction chamber is transferred to the reaction chamber.

2. The process according to claim 1, further comprising maintaining the steam at a temperature above the steam's condensation temperature.

3. The process according to claim 1 is carried out under atmospheric pressure.

4. The process according to claim 1, wherein the steam generated in the reaction chamber serves as the transport gas in the process for transporting particulate aluminum hydroxide and / or alumina into and / or out of the reaction chamber.

5. The process according to claim 1, wherein the steam generated in the reaction chamber serves as the heat transfer medium in the process.

6. The process according to claim 1 further includes transferring at least some of the steam generated in the reaction chamber to an alumina production apparatus for use in the apparatus.

7. The process of claim 1, wherein after the process has reached steady-state conditions, the process includes discharging flue gas from the reaction chamber, the flue gas being at least 95% steam by volume.

8. The process according to claim 1, wherein after the process has reached steady-state conditions, the process includes discharging flue gas from the reaction chamber, the flue gas being 100% steam by volume.

9. The process according to claim 1, wherein the oxygen is supplied as oxygen and not as air or as oxygen-enriched air.

10. The process according to claim 1, wherein the oxygen is supplied as oxygen-enriched air.

11. A process for calcining aluminum hydroxide Al₂O₃·3H₂O to form aluminum oxide Al₂O₃, wherein the calcination is performed in a Bayer process unit, the process comprising: Hydrogen and oxygen are burned in a reaction chamber to generate steam and heat. Aluminum hydroxide is fed into the steam discharged from the reaction chamber to transport the aluminum hydroxide and at least some surface-bound water is removed from the aluminum hydroxide during transport to form at least partially dried aluminum hydroxide. The heat is used to calcine the at least partially dried aluminum hydroxide to form alumina. The removal of at least some surface-bound water and the calcification generate more steam. The steam generated by the combustion is used as the transport gas in the process. The process also includes venting steam from the reaction chamber and then transferring at least some of the vented steam back to the reaction chamber.

12. The process according to claim 11, comprising calcining the aluminum hydroxide in the reaction chamber to form aluminum oxide.

13. The process of claim 11, comprising burning hydrogen and oxygen in a reaction chamber to generate steam and heat, and transferring the steam and heat to a second reaction chamber and calcining aluminum hydroxide in the second reaction chamber to form aluminum oxide.

14. The process of claim 11, wherein the oxygen is supplied as oxygen and not as air or as oxygen-enriched air.

15. The process of claim 11, wherein the oxygen is supplied as oxygen-enriched air.

16. An apparatus for carrying out the process of any one of the preceding claims.

17. An apparatus for calcining aluminum hydroxide Al₂O₃·3H₂O to form aluminum oxide Al₂O₃, said apparatus comprising: A reaction chamber configured to calcine aluminum hydroxide to form aluminum oxide; A hydrogen source and an oxygen source are provided to generate heat, which is used to calcine aluminum hydroxide in the reaction chamber and produce aluminum oxide and flue gas including steam; an outlet for the aluminum oxide; and an outlet for the flue gas. and pipelines for supplying flue gas discharged through the flue gas outlet to the reaction chamber. The apparatus is arranged such that aluminum hydroxide is fed into flue gas venting from the reaction chamber to transport the aluminum hydroxide and remove at least some surface-bound water from the aluminum hydroxide during transport to form at least partially dried aluminum hydroxide, wherein the at least partially dried aluminum hydroxide is calcined in the reaction chamber.

18. The device according to claim 17, comprising: The first reaction chamber is used to calcine aluminum hydroxide to form aluminum oxide and vapor; And a second reaction chamber, which is used to burn hydrogen and oxygen and generate heat for use in the first reaction chamber.

19. The apparatus of claim 17, wherein the oxygen source is configured to supply oxygen and not supply air or oxygen-enriched air.

20. The apparatus of claim 17, wherein the oxygen source is configured to supply oxygen-enriched air.

21. An apparatus for producing alumina, the apparatus being a Bayer process apparatus, the apparatus comprising the equipment for calcining aluminum hydroxide to form alumina according to any one of claims 17 to 20.

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