EA022436B1 - Process and plant for producing alumina from aluminum hydroxide - Google Patents

Abstract

In the production of alumina from aluminum hydroxide, a) aluminum hydroxide is purified with washing water in a hydrate filter, b) the purified aluminum hydroxide is at least partly dried and/or precalcined in at least one preheating stage, c) this pretreated aluminum hydroxide is calcined in a fluidized-bed reactor to obtain alumina, d) the alumina obtained is cooled in at least one indirect cooling stage using water as coolant, e) the steam (D) obtained from the cooling water due to the heat transfer in the indirect cooling stage is separated from the liquid fraction (A) of the exit stream from the cooling stage (E), f) and at least one partial stream (T) of the liquid fraction (A) is guided to the hydrate filter and used there as washing water for purifying the aluminum hydroxide in the hydrate filter. In accordance with the invention, an additional water stream (Z) is added to the partial stream (T) of the liquid fraction (A) guided to the hydrate filter and the mixing ratio of the two streams (T, Z) is adjusted such that the washing water stream (W) resulting therefrom has a constant maximum temperature value below the boiling point of water and the volume flow required by the hydrate filter as washing water.

Description

The present invention relates to a method and apparatus for producing metal oxides from metal salts, in particular alumina from aluminum hydroxide, where aluminum hydroxide (also called aluminum oxide trihydrate or hydrate) is first purified by washing water in a hydrate filter, then purified aluminum hydroxide, at least partially dried and / or pre-calcined during at least one pre-heating step, subsequently this pre-treated aluminum hydroxide is calcined in the fluidized bed reactor, producing alumina, and the obtained alumina are cooled with water as a refrigerant during at least one indirect cooling stage, then the steam obtained from the cooling water due to heat transfer in the indirect cooling stage is separated from the liquid fraction of the stream leaving cooling steps, and where at least one part of the liquid fraction stream is directed to a hydrate filter and used there as washing water to purify aluminum hydroxide in the hydrate filter.

Obtaining aluminum oxide is usually carried out by the so-called Bayer method. In this method, minerals, primarily aluminum containing bauxite, are ground and mixed with a solution of sodium hydroxide (ΝαΟΗ). Insoluble residues, primarily red mud, mainly consisting of iron oxide, are separated from dissolved aluminum hydroxide in the form of sodium aluminate (Να | Α1 (ΟΗ) ₄ |). Pure aluminum hydroxide Α1 (ΟΗ) ₃ then precipitates from the diluted alkaline solution of aluminate. This solid hydroxide was separated by filtration and washed. Subsequently, the conversion of aluminum hydroxide to aluminum oxide (Α1 ₂ Ο ₃ ) is carried out by calcination.

Calcination of aluminum hydroxide requires a very high energy consumption. In conventional methods, an energy consumption of approximately 3000 kJ / kg of produced aluminum oxide is required. By combining heat sources and heat absorbers, they are trying to reduce the energy needed for the method and, thus, increase profitability, as well as improve the ecological balance.

A method of energetically more efficient production of alumina from aluminum hydroxide is known, for example, from EP 0861208 B1 or from EE 102007014435 A1. Here, the wet aluminum hydroxide is first dried in a first suspension heat exchanger and preheated to a temperature of about 160 ° C. After separation in a cyclone separator, the solid particles are fed to a second slurry heater, in which they are further dried with exhaust gas from a recycle cyclone in a circulating fluidized bed. The dried solids are then loaded into a circulating fluidized bed fluidized bed reactor and calcined at temperatures of about 1000 ° C. to obtain alumina. Part of the preheated aluminum hydroxide stream is diverted after the first suspension preheater (EP 0861208 B1) or after the second suspension preheater (EE 102007014435 A1) and mixed with hot alumina recovered from the recirculation cyclone of the circulating fluidized bed. The hot product mixture is then cooled in a multi-stage suspension cooler in direct contact with air, and then fed to the final cooling in a fluidized bed cooler. To efficiently use the energy released during cooling, this fluidized bed chiller is equipped with a variety of chambers. The fluidization of the fluidized bed in the calcination reactor is carried out by means of a fluidized gas (primary air), which is preheated in one of the chambers of the fluidized bed cooler to a temperature of approximately 188 ° C. In suspension heat exchangers, for the first cooling of the product, the air is additionally heated to about 525 ° C. in direct heat exchange with alumina and then fed as secondary air to a fluidized bed calcination reactor.

From EP 0245751 B1, a method for carrying out endothermic processes with fine-grained solids is known, in which the generated heat is also used in the best way within the whole process. During the calcination of aluminum hydroxide, a part of the feed stream is fed into an indirectly heated pre-heater and then introduced into the electrostatic precipitator together with directly supplied raw materials. The solids are then fed from the electrostatic precipitator through two series-connected preheating systems into a circulating fluidized bed in which the solids are fluidized by a fluidizing gas and calcined at a temperature of about 1000 ° C. The solids stream recovered from the circulating fluidized bed is cooled in an indirect fluidized bed cooler, which is the first cooling stage, and then fed to the second and third cooling stages, which are again carried out each time in fluidized bed coolers, for further cooling of the solid product. The primary air heated in the first fluidized bed cooler is introduced into the fluidized bed reactor as a fluidized air with a temperature of approximately 520 ° C., while the fluidized air from the fluidized bed cooler is supplied to the calcination reactor with a fluidized bed as secondary air with a temperature of 670 ° C. The heat transfer medium from the second fluidized bed cooler is supplied to the indirect pre-heater for the starting material as a heating medium with a temperature of 200 ° C, and then after cooling to

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160 ° C. is recycled to the inlet of the second fluidized bed cooler.

Another heat sink in the process is heating water for filtration, designed to purify aluminum hydroxide. Crude aluminum hydroxide, in particular one that is obtained after precipitation from an alkaline solution of aluminate, is washed before being fed to the first preheating step. In particular, in order to remove adhering soda, warm wash water is used, since the solubility of impurities improves at elevated temperatures. However, this wash water should not reach the boiling point, as otherwise it would evaporate.

In LI 2005237179 A1, the calcined reactor exhaust gas is used as a heat source to heat the wash water to filter alumina hydrate. According to the chemical equation

2A1 (OH) s -> A1 _g O ₃ + ZN ₂ O water is formed in this reactor during calcination.

The exhaust gas discharged from the fluidized bed reactor is thus a mixture of an inert fluidizing gas from the reactor and steam produced in the reaction. The water that is condensed from this mixture has a temperature of approximately 83 ° C and is recycled to the alumina hydrate filter as wash water. In such a method, however, it is disadvantageous that due to the relatively low concentration of water (approximately 50%) in the exhaust gas from the calcining unit, it is not possible to achieve a higher temperature of the condensate water, and thus, cleaning in the hydrate filter is not carried out under optimal conditions, but it is at water temperature just below the boiling point.

Another possibility of obtaining pre-heated wash water for the hydrate filter is the removal of cooling water from the indirect cooling stage, removal of the evaporated fraction and recirculation of the liquid fraction into the hydrate filter. However, this method has the disadvantage that it cannot be adapted to the dynamic conditions of the process. When the temperature or mass flow rate of the material to be calcined is increased, the fraction of the amount of heat generated at the corresponding cooling stage will also increase. As a result, the cooling water in the cooling step will evaporate either completely or at least to such a degree that there will not be enough wash water for the hydrate filter.

Therefore, it is an object of the present invention to provide a hydrate filter with as warm water as possible for filtration when operating under non-stationary conditions.

In accordance with the present invention, this problem is solved in a method comprising the features of claim 1. An additional stream (K) of water is added to part (A) of the stream of liquid fraction (A) leaving the cooling stage, which is sent to the hydrate filter, the mixing ratio of the two streams being controlled so that the stream (A) of washing water formed from of them, has a constant maximum temperature below the boiling point and the volumetric flow rate required for rinsing water for the hydrate filter. At normal pressure in the hydrate filter, this maximum temperature is between 90 and 100 ° C., with a value of 95 ° C. being preferred and a value of 97 ° C. particularly preferred. When the mass flow rate of alumina to be cooled is increased, or the temperature of the solid particles to be cooled is increased, more steam (E) is generated. The temperature of the wash water stream (A) is controlled by adding a water stream (Ζ) so that the temperature drops below a constant maximum temperature or even at a high evaporation rate, the volumetric flow rate does not become less than the value needed to filter the hydrate. It has been found that it is particularly advantageous to carry out an indirect cooling step in a fluidized bed cooler with many separate chambers. In order to make particularly efficient use of the amount of heat contained in the still hot alumina, the amount of heat obtained in the first cooling chamber is used to preheat the hydrate in the hydrate dryer by indirect heat transfer. The cooling water from the second fluidized bed chamber is used to preheat the primary air of the process as described in EP 0245751 B1, and the cooling water from the third chamber is used to preheat the wash water stream for the hydrate filter in accordance with the invention.

Preferably, cooling water passing through the indirect cooling step is supplied at an overpressure, and cooling water expands after passing through the indirect cooling step. Thus, phase transitions of the refrigerant and the associated reduced heat transfer during the cooling stage can be avoided. For example, if the amount of energy that is transferred to the cooling water fluctuates due to a temporary increase in the mass flow of alumina or due to the higher temperature of the supplied alumina, more steam is generated. Since evaporation consumes a lot of energy, the amount of corresponding steam does not increase much and does not affect the constant amount and temperature of the wash water necessary for continuous operation in the process. It turned out that the amount (Ό) of steam in excess of the minimum amount of steam is beneficial for filtration and residual moisture content in the hydrate.

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To ensure a constant flow rate of cooling water within the respective cooling stage, fresh water (P) is added to the residual stream (K) remaining after separation of a portion of the liquid fraction stream; the residual stream (K) is the difference between the total stream (E) and the allotted steam (Ό) and part (T) of the stream. The mixed stream (M) obtained by mixing the streams (A) and (P) is at least partially recycled to the indirect cooling step as a cooling stream (K). To simplify the temperature control in the installation, the cooling stream (K) can always be supplied at a constant volumetric flow rate and / or at a constant temperature. Due to the ability to flexibly add fresh water stream (P), the volumetric flow rate and / or temperature of the cooling water (K) in the cooling stage can, however, also be controlled depending on the amount and / or temperature of the alumina to be cooled.

However, the residual liquid fraction (K) can also first be pumped into the storage tank and mixed there with fresh water (P), according to which, the water tank can be placed in this storage tank, the possible temperature range of which lies between the fresh water temperature and the temperature of the residual stream (K).

To simplify the principle of regulation, the stream (Ζ) of water added to part (T) of the stream of liquid fraction (A) can also be taken from fresh water.

It is especially advantageous when this flow (Ζ) of water for regulating the temperature and volumetric flow rate of the washing water (XV) is part of the flow from the mixed stream (M) pumped into the storage tank and mixed there with fresh water, since this can be achieved higher flow temperature (Ζ) of water without additional heating.

In terms of energy, it is especially advantageous when the hydrate filter is provided with a casing for supplying steam, by means of which the hydrate can be subjected to first drying already during filtration. In a preferred aspect of the invention, this casing for supplying steam at least partially works with steam (Ό), which is obtained from the cooling water of the indirect cooling stage, since in this way the required energy can be reduced for further preliminary drying stages.

The invention also relates to a plant for producing aluminum oxide from aluminum hydroxide, which is suitable for implementing the described method and includes the features of claim 8. The installation includes at least one hydrate filter, in which the aluminum hydroxide is purified by washing water, at least one preheating step, in which the purified aluminum hydroxide is at least partially dried and / or preliminarily calcined, in a fluidized bed reactor, wherein the pretreated aluminum hydroxide is calcined to obtain alumina and at least one indirect cooling stage with a cooling circuit through which water circulates as a refrigerant, in cooled torus obtained alumina. After the indirect cooling stage, a steam separation device is located to separate the gaseous and liquid fractions of the cooling water. A return pipe connects the cooling circuit of the indirect cooling stage to a pipe supplying washing water to the hydrate filter, where, in accordance with the present invention, there is a control device after the steam separator that controls the supply of washing water with a constant maximum temperature below the boiling point of the water and the volumetric flow rate, required for rinsing water for the hydrate filter, so that it controls the proportions of the part (V) of the stream directed to the filter for hydrate, and additional water flow (Ζ). In addition, the control device is connected to the input of the cooling circuit through a pipeline.

In accordance with an embodiment of the invention, a storage tank is located at the beginning of the pipeline at the inlet of the cooling circuit, which at the same time can be used as a water source to control the temperature and amount of washing water supplied to the hydrate filter.

In accordance with one aspect of the present invention, the hydrate filter is provided with a casing for supplying steam for partially drying the alumina hydrate, the casing for supplying steam being connected to a steam outlet by a steam separator through a conduit. As a result, the obtained steam can be used at a process point where fluctuations in the quality and quantity of steam have practically no effect on process control.

It is especially advantageous when the heat exchanger is placed in the pipeline coming from the control device and leaving the hydrate filter. During the start-up of the installation, when there is still no hot alumina in the indirect cooling stage, this heat exchanger nevertheless provides washing of the already loaded hydrate filter with warm washing water. In principle, this heat exchanger can also be located in another place, for example, between a steam separator and a control device, whereby the water recirculated both to the hydrate filter and to the cooling stage is heated, and therefore, the temperature is controlled at the cooling stage itself occurs in a narrow temperature range from the very beginning.

Further developments, advantages, and applications of the invention may also be taken from the following description of embodiments and drawings. All features described and / or depicted constitute the subject of the present invention by themselves or in any combination, regardless of their inclusion in the claims or in reverse links.

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In the drawings:

in FIG. 1 schematically shows an apparatus for implementing the method of the invention;

in FIG. 2 schematically shows an apparatus for carrying out the method according to the invention in accordance with a second embodiment;

in FIG. 3 schematically shows an apparatus for implementing the method of the invention in accordance with a third embodiment;

in FIG. 4 shows the course of individual flows in conjunction with a cooling stage;

in FIG. 5 shows a reduction in residual moisture as a function of the relative amount of steam used.

According to the flowchart of the method of the invention, as shown in FIG. 1, a slurry that contains crude aluminum hydroxide (A1 (OH) ₃ ) is loaded into the hydrate filter 1 and cleaned there with wash water from line 51. Preferably, the hydrate filter is provided with a casing for supplying steam, where the hydrate is already partially dried by filtration. The filtrate is discharged.

After purification, aluminum hydroxide is introduced through line 5 into hopper 1 ', whereby fluctuations in the addition of the recovery product can be compensated. From there, the hydrate is introduced through line 2 into the hydrate dryer 3, in which the hydrate is heated to a temperature of approximately 100 110 ° C by indirect heat exchange with a liquid heat transfer medium, in particular water, and dried almost completely based on a moisture content of, for example, 6% . The dried hydrate is then fed to the slurry heat exchanger 4 of the first preheating stage and preheated to a temperature of from 100 to 200 ° C.

Through the bypass pipe 5 ', part of the hydrate stream can be directly fed to the slurry heat exchanger 4 past the hydrate dryer 3. The size of the part of the flow is controlled by a control valve 6, which can be positioned in the pipe 2 or the bypass pipe 5. The bypass flow is controlled depending on the temperature of the exhaust gas in order to keep the energy loss as low as possible. If more hydrate is fed to the hydrate dryer 3, then the temperature of the exhaust gas from the slurry heat exchanger 4 increases, since more moisture (water) is removed in the hydrate dryer 3 and evaporated not only in the subsequent slurry heat exchanger 4. When a small amount of hydrate is fed into a hydrate dryer 3, a larger amount of wet hydrate is supplied to the slurry heat exchanger 4, and the temperature of the exhaust gas decreases accordingly. The exhaust gas stream coming from the second preheating stage picks up the solid particles introduced into the suspension heat exchanger 4, heats them, and pneumatically introduces them through the pipeline 7 into the inlet region of the electrostatic gas scrubber (EGO) 8, which is a preliminary separator. In the electrostatic precipitator 8, the gas is purified and at a temperature of from 110 to 170 ° C, preferably from 120 to 140 ° C, is discharged through a pipe not shown in the drawing.

The solid particles removed from the electrostatic scrubber 8 are delivered through a conduit 9 to a second suspension heat exchanger 10 of the second preheating stage, where the solid particles are picked up by a stream of gas leaving the third preheating stage, heated to a temperature of from 150 to 300 ° C. and fed to a separation cyclone 12 through conduit 11. The exhaust gas stream from the separation cyclone 12 is fed into the suspension heat exchanger 4 through conduit 13 in order to heat the hydrate and transfer it to the eleme ically electrostatic filter.

Through the conduit 14, solids from the separation cyclone 12 are introduced into the third suspension heat exchanger 15 (third preheating stage), picked up by the gas stream leaving the recirculation cyclone of the circulating fluidized bed, and then dehydrated and at least partially dehydrated (pre-calcined), obtaining monohydrate of aluminum oxide (chemical formulas A1 ₂ O ₃ -H ₂ O or A1OOH), hereinafter referred to as monohydrate, at a temperature of from 200 to 450 ° C, in particular from 250 to 370 ° C.

Through the pipeline 17, the gas and solid particles flow is fed into the separation cyclone 18, in which, in turn, the gas and solid particles are separated, where the solid particles are discharged downward through the pipeline 19, and the exhaust gas is introduced into the second suspension heat exchanger 10 of the second preliminary stage heating up.

In the second, and especially in the third, stage of preheating, the preliminary calcination of aluminum hydroxide is thus carried out. The preliminary calcination in the framework of the present invention should be understood as partial dehydration or cleavage of compounds, such as, for example, HCl and Lox. Calcination, on the other hand, refers to complete dehydration or cleavage of compounds, such as, for example, §O ₂ .

After the separation cyclone 18, which goes after the third suspension heat exchanger 14, the solid particles are separated by means of the device described, for example, in ΌΕ 102007014435 A1. Through conduit 19, a main stream containing from about 80 to 90% by weight of the solid particle stream is fed to a fluidized bed reactor 20 in which the solid particles are calcined and dehydrated to alumina (Al ₂ O ₃ ) at a temperature of from 850 to 1100 ° With, in particular, approximately 950 ° C.

The fuel necessary for calcination is supplied through the fuel pipe 21,

- 4 022436 which is located at a small height above the lattice of the reactor 20 with a fluidized bed. Streams of oxygen-containing gas necessary for combustion are supplied through the supply pipe 22 as fluidizing gas (primary air) and through the supply pipe 23 as secondary air. As a result of the gas supply, a relatively high density of the suspension is obtained in the lower region of the reactor between the grate and the secondary gas supply 23, and a relatively low density suspension is obtained above the secondary gas supply 23. After conventional compression, the primary air is supplied to the fluidized bed reactor 23 at a temperature of approximately 80 ° C without further heating. The secondary air temperature is approximately 550 ° C.

Through the connecting conduit 24, a suspension of gas and solid particles is fed into a cyclone 16 for recirculation of the circulating fluidized bed, in which further separation of the solid particles and gas is carried out. After the solids are removed from the recycle cyclone 16 through a conduit 25 with a temperature of approximately 950 ° C, they are introduced into the mixing tank 26. Through the bypass conduit 27, a part of the stream separated under the separation cyclone 27 and consisting mainly of monohydrate is also fed into the mixing tank 26 with a temperature of approximately 320-370 ° C. In the mixing tank 26, a mixing temperature of about 700 ° C. is maintained in accordance with the mixing ratio of the hot alumina stream supplied through line 25 and the monohydrate stream supplied through the bypass line 27. The two product streams are mixed in the mixing tank 26, which includes a fluidized bed , in order to also completely calcine the monohydrate fed through the bypass pipe 27 to produce alumina. A very long retention time of up to 30 minutes, preferably up to 60 minutes, leads to excellent calcination in the mixing tank. However, a retention time of less than 2 minutes, in particular 1 minute, or even less than 30 seconds, may be sufficient.

The resulting product is fed from the mixing tank 26 into the first suspension cooler formed by the ascending pipe 28 and the cyclone separator 29. Through the pipe 23, the exhaust gas from the cyclone separator 29 is fed into the fluidized bed reactor 20 as secondary air, solid particles are fed into the second suspension cooler, formed by the ascending pipe 30 and the cyclone separator 31 and finally into the third suspension cooler formed by the ascending pipe 32 and the cyclone separator 33. Gas flow through separate coolers suspension is passed countercurrently to the solids through conduits 34 and 35.

After passing through the last suspension cooler, the obtained alumina is subjected to final cooling in a fluidized bed cooler 36 equipped with three or four cooling chambers. Alumina enters the first chamber 36a at a temperature of approximately 300 ° C and heats the liquid heat transfer medium, in particular water, to a temperature of from 140 to 195 ° C, preferably from 150 to 190 ° C, in particular from 160 to 180 ° C. Through the circulation pipe 37, the heated heat transfer medium is supplied to the hydrate dryer 3 in order to dry the metal salt (hydrate) by indirect heat transfer.

After passing through the hydrate dryer 3, the heat transfer medium is recycled through the circulation line 37 to the first stage 36a of the fluidized bed cooler with a temperature of from 100 to 190 ° C, preferably from 120 to 180 ° C, in particular from 140 to 170 ° C. The pressure in the heat transfer circuit is preferably controlled in such a way as to avoid condensation of the heat transfer medium in the hydrate dryer 3, and it is at a level of about 0.1 to 5 MPa (1 to 50 bar), in particular 0.2 to 4 MPa (from 2 to 40 bar). In the downstream chamber 36b, the alumina is further cooled by a heat transfer medium supplied in countercurrent, preferably water. The heat transfer medium can be used to preheat the primary air that is blown into the fluidized bed reactor 20 through a conduit 22.

In the third heating chamber 36c, the heat transfer medium has a temperature between 100 and 140 ° C, preferably from 110 to 135 ° C, and particularly preferably about 120 ° C. Through line 41 it is fed to a steam separator 42, in which the steam is separated from the liquid fraction. Through line 43, this steam can be fed into the hydrate filter 1 or into its casing 1 'for supplying steam, and already here the hydrate can be subjected to first preliminary drying.

Through line 44, the liquid fraction is taken from the steam separator 42. The control device 50 withdraws a portion of this liquid fraction through conduit 45 and mixes it with an additional water stream, which is supplied to the control device 50 through conduit 52. The newly formed flow is mixed in such a way that a certain temperature value is established, preferably 95 ° C, and more preferably 97 ° C, with fluctuations of ± 2 ° C, preferably ± 1 ° C, and particularly preferably ± 0.5 ° C. In addition, the flow of wash water directed to the hydrate filter 1 through line 51 has a certain volumetric flow rate. In the pipe 51, there is a heat exchanger 54, which heats the washing water to the desired temperature when the cooling stage 36c cannot provide enough energy, as is the case, for example, when starting processes.

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A fraction of the liquid stream discharged through conduit 45 is fed through conduit 46 to a storage and mixing tank 47, into which additional fresh water is supplied via conduit 48. By conduit 49, a mixture of the liquid fraction from the cooling step and fresh water can be withdrawn from tank 47 to storage, and then partially introduced through the pipe 52 into the control device 50 to control the required maximum temperature and volumetric flow rate of the wash water for the hydrate filter 1. Through conduit 53, the residue is again fed to the cooling circuit of the cooling stage 36c as a cooling medium, and it has been found to be particularly favorable when its volumetric flow rate remains constant and, in a preferred aspect, it also has a constant temperature. As a control variable, the temperature of the wash water entering the hydrate filter 1 is used.

The pressure in the cooling circuit of the cooling chamber 36c can either be kept constant at 0.5 MPa (5 bar), or adjusted depending on the flow rate and / or temperature of the cooling water after passing through the chamber 36c.

In chambers 36a-366, the fluidized bed is supported by secondary air, which is supplied through conduit 39 with a temperature of from 80 to 100 ° C. Secondary air is then removed from the fluidized bed cooler 36 and used as conveying air in a third slurry cooler. Secondary air is supplied through a slurry cooler in countercurrent to the flow of solid particles discharged from the fluidized bed reactor 20, where it is heated before being fed into the fluidized bed reactor 20 through a conduit 23. Through the conduit 40, additional air can be sent to the cooling stage 36 . Instead of air, pure oxygen or oxygen-enriched air with an oxygen content of 21 to 100 vol.% Can also be supplied through a pipe 39 and / or 40.

In FIG. 2 shows a simplified diagram of a calcination device in which aluminum hydrate and other metals can be calcined. By analogy with FIG. 1, the hydrate suspension is loaded into the filter 1 and washed with water from the pipe 51. It is also preferred here that the filter is provided with a casing for supplying steam to which steam is supplied through the pipe 51, whereby the material obtained by filtration is already partially dried. The filtrate is drained and the resulting hydrate is sent to the hopper 1 'through the pipeline 5. From there it can be used evenly to load the installation through the pipeline 5'.

This installation includes a suspension heat exchanger 4, from which the material is introduced into the filtering device 8 through a pipe 7. Through a pipe 9, it is sent from there to a further suspension heat exchanger 15, which is connected to a separation cyclone 18 through a pipe 1.7

Through conduit 19, preheated and dried material is then fed to the calcination reactor 20. This reactor is connected to the recycle cyclone 16 via conduit 24. It is also advantageous to design the reactor as a fluidized bed reactor and to introduce heated fluidizing gas into the reactor through conduit 22. The pretreatment and calcination conditions substantially correspond to those described in FIG. 1, in relation to the calcination of aluminum.

The solids exiting the recycle cyclone 16 through conduit 25 and the solids separated through the bypass conduit 27 below the separation cyclone 18 are introduced into the mixing tank 26. In this mixing tank 26, the mixing temperature is controlled in accordance with the mixing ratio between the hot oxide stream, which is fed through a conduit 25, and a hydrate stream that is fed through a bypass conduit 27, and the hydrate is calcined. To ensure good mixing, it has proven favorable when solids are present in the mixing tank 26 as a circulating fluidized bed.

Through conduit 35, solids are then introduced into a cyclone separator 33, which is connected to a multi-stage fluidized bed cooler 36. Cooler chambers 36 may be used to preheat various streams in the process. The outline shown here corresponds to the outline shown in FIG. one.

Through a pipe 41, water heated in one of the chambers is supplied to a steam separator 42, in which the steam is separated from the liquid fraction. Through line 43, this steam can be supplied to the hydrate filter 1.

Through line 44, the liquid fraction is taken from the steam separator 42 and introduced into the control device 50. It also draws part of this liquid fraction through the pipe 45 and mixes it with an additional stream of water, which is supplied to the control device 50 through the pipe 52. In the newly formed stream, thus, it is possible to set a specific temperature value, preferably ^C 95 C, more preferably 97 ° C, with a range ± 2 ° C, preferably ± 1 ° C, and particularly preferably ± 0.5 ° C.

Through conduit 51, the wash water stream is directed to the hydrate filter 1, with a heat exchanger 54 located in conduit 51, which can heat the wash water to the desired temperature when it does not yet have the required temperature.

A fraction of the liquid stream not discharged through line 45 is fed through line 46 to tank 47 for storage and mixing. Fresh water is additionally supplied to this mixing tank by

- 6 022436 to the pipeline 48. Through the pipeline 49, water can be drained from the storage tank 47, and then a portion can be supplied to the control device 50 via the pipe 52 to control the required maximum temperature and volumetric flow rate of the wash water for the hydrate filter 1. Through line 53, the residue is again fed into the cooling circuit of the cooling stage 36c as a cooling medium. The control variable is the temperature of the wash water entering the hydrate filter 1 through line 51.

FIG. 3 corresponds to that of FIG. 2, except that after the calcination reactor 20 and the mixing tank 26, not one but two suspension heat exchangers 29, 33 are located, which are connected to each other via a pipe 35.

in FIG. 4 schematically illustrates individual flows within a unit consisting of a cooling stage 36c, a hydrate filter 1, and a related cooling circuit system. In the cooling stage 36c, warm alumina is preferably introduced into the fluidized bed chamber. If the cooling stage is designed as a cooling stage with a fluidized bed, fluidizing gas is supplied to it, as shown in FIG. 4. Above the fluidized bed can pass a stream of additional gas. The flow E withdrawn from the cooling stage 36c contains a total flow of refrigerant heated in the cooling stage. In the steam separator 42, a gaseous fraction is withdrawn from this stream E as a steam stream Ό, and a liquid fraction is withdrawn as a stream A. It is advantageous that stream E is under overpressure and expands to normal pressure in the steam separator 42 and in the unit located upstream , respectively. The liquid fraction A withdrawn from the steam separator 42 is divided into a portion T of the stream and a residual stream K. Fraction T is that fraction which is ultimately recycled to the hydrate filter 1 as wash water.

In order to avoid a situation in which the washing water in the hydrate filter 1 boils during filtration and, therefore, is no longer available for the cleaning process, an additional stream меш is mixed into the part T of the stream, and the mixed fraction is so high that the temperature of the total stream of washing water A obtained by mixing flows T and имеет has a fixed temperature of approximately 95 ° C, preferably 97 ° C, but in any case below the boiling point of water. In addition, the volumetric flow rate of the wash water remains constant.

The fraction of the liquid stream A, used as washing water, is fed into the storage tank 47 as a residual stream K. It is mixed there with fresh water from stream P. The mixture withdrawn from the storage tank 47, namely, mixed stream M, is partially used in stream quality Ζ.

The difference between the flows M and Ζ is fed back to the indirect cooling stage 36c as a cooling stream K. The volumetric flow rate of this cooling stream K is kept constant.

In a typical installation, about 3 th ⁻¹ steam is obtained in the third cooling chamber 36c at full load. For safety reasons, parts of the unit connected to the third cooling chamber 36c should be designed so that all water from the cooling circuit can evaporate. This value is the result of multiplying the amount of water supplied as cooling water, the temperature difference that occurs during the cooling stage, and the heat capacity of water at an average temperature in the cooling chamber 36c. With a quantity of water of 72 t-h ^-1 , a temperature difference of 48 ° C and an average heat capacity of 4.2 kJ-kg ^-1 -K ^-1 , the calculated amount of energy is 14.5 GJ-h ^-1 , which corresponds to a vapor amount of 7960 m ^3- h ^-1 (at NU). Thus, all valves should be designed for a load of approximately 8000 m ³ -h ^-1 (at n.o.) steam.

In FIG. 5 shows a decrease in residual moisture in the hydrate as a function of the amount of steam used, and this amount of steam is indicated relative to the amount of solid particles used. Thus, by using large quantities of steam, the residual moisture in the hydrate can be reduced, which leads to stabilization of the process, since this prevents the introduction of large quantities of water into the process. An advantageous reduction in hydrate moisture, thus, leads to a reduction in energy consumption for the calcination process.

Example.

The values in the table refer to the circuit shown in FIG. 4. Columns 2–9 indicate the corresponding mass flow rates per hour, while columns 10–16 indicate the temperatures of the respective flows. The table shows the size of the individual streams and their corresponding temperatures under various conditions, in particular, at different volumetric flows of washing water in the hydrate filter. If less water is required in the hydrate filter 1, large fractions are collected in the storage tank 47, with the same total volume.

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Mass flow rates and temperature values in a circuit according to the method of the invention

m kg-h ' ¹ Ζ, kg-h ' ¹ K, kg / h ' ¹ E, KG'CH ' ¹ A kg-h ' ¹ νν, kg-h ' ¹ R, KG'CH ' ϋ, kg / h ' ¹ T (M) FROM T <E>, 'FROM TOLO 'FROM T (O) 'FROM T (8) ‘, 'FROM T (A) ‘, 'FROM one 8200 9169 72831 72831 72355 81524 82000 476 52 106 97 103 161 66 2 8200 9135 72865 72865 71919 73862 74808 946 56 109 97 103 161 67 3 8200 9080 72920 72920 71508 66287 67698 1411 61 113 97 103 161 67 4 8200 9017 72983 72983 71110 58794 60667 1873 65 116 97 103 161 67 5 3200 8815 73185 73185 70910 51361 53636 2275 70 119 97 103 161 67 6 8200 8649 73351 73351 70639 43989 46680 2712 74 122 97 103 161 67 7 8200 8416 73584 73584 70441 36592 39735 3143 78 125 97 103 161 68 8 8200 8240 73760 73760 70141 29283 32901 3618 82 129 97 103 161 68 nine 8200 7902 74098 74098 70009 21904 25993 4089 87 132 97 103 161 68 10 8200 6779 75221 75221 70713 12851 18358 4508 91 134 97 103 161 88

* T (8): Steam temperature at boiling point.

* T (A): Aluminum outlet temperature.

List of numbers in the drawings:

- filter for hydrate 1 'hopper;

- pipeline;

- hydrate dryer;

- suspension heat exchanger;

5, 5 '- pipeline;

- control valve;

- pipeline;

- electrostatic precipitator;

- pipeline;

- suspension heat exchanger;

- pipeline;

- separation cyclone;

- pipeline;

- pipeline;

- suspension heat exchanger;

- cyclone recirculation;

- pipeline;

- separation cyclone;

- pipeline;

- fluidized bed reactor;

- fuel pipeline;

- supply pipeline;

- supply pipeline;

- connecting pipeline;

- pipeline;

- mixing tank;

- bypass pipeline;

- ascending pipeline;

- cyclone separator;

- ascending pipeline;

- cyclone separator;

- ascending pipeline;

- cyclone separator;

- pipeline;

- pipeline;

- fluidized bed cooler (several chambers); 36a-4 — fluidized bed cooler chambers 36;

- circulation pipe;

- pipeline;

- pipeline;

- pipeline;

- pipeline;

- steam separator;

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- pipeline;

- pipeline;

- pipeline;

- pipeline;

- storage tank;

- pipeline;

- pipeline;

- regulating device;

- return pipe;

- pipeline;

- pipeline;

- heat exchanger;

A is the liquid fraction;

Ό - steam flow;

E is the flow of refrigerant;

P is the flow of fresh water;

M is a mixed stream;

K is the residual flow;

T is part of the stream;

No. - flow of wash water; Ζ - additional stream.

Claims (11)

1. The method of producing alumina from aluminum hydroxide, where: a) aluminum hydroxide is purified by washing water in a hydrate filter; b) the purified aluminum hydroxide is at least partially dried and / or pre-calcined during at least one pre-heating step; c) this pretreated aluminum hydroxide is calcined in a fluidized bed reactor to obtain alumina; d) the resulting alumina is cooled during at least one indirect cooling step using water as a refrigerant; d) steam (Ό) obtained from cooling water due to heat transfer at the stage of indirect cooling is separated from the liquid fraction (A) of the stream leaving the cooling stage (E); and e) at least one part (T) of the liquid fraction stream (A) is sent to the hydrate filter and used there as washing water to purify aluminum hydroxide in the hydrate filter, characterized in that to the part (T) of the liquid fraction stream ( A), sent to the hydrate filter, add an additional stream (Ζ) of water and the mixing ratio of the two streams (T, Ζ) is controlled in such a way that the stream (No.) of washing water formed from them has a constant maximum temperature below the boiling point water and volumetric flow th is required for rinsing water for a hydrate filter. 2. The method according to claim 1, characterized in that the passage of cooling water through the indirect cooling stage is carried out at an overpressure and the cooling water expands after passing through the indirect cooling stage. 3. The method according to claim 1 or 2, characterized in that fresh water (P) is added to the residual stream (K) remaining after separation of part (T) of the liquid fraction stream (A) and the resulting mixed stream (M), at least at least partially recycled to the cooling step. 4. The method according to claim 3, characterized in that the residual stream (K) is pumped into the storage tank and mixed there with fresh water (P). 5. The method according to any one of the preceding paragraphs, characterized in that the additional stream of water (Ζ) consists of fresh water. 6. The method according to claim 3 or 4, characterized in that the flow (Ζ) of water for regulating the temperature and volumetric flow rate of washing water (No.) is part of a mixed stream (M). 7. The method according to any one of the preceding paragraphs, characterized in that the hydrate filter is provided with a casing for supplying steam, in which at least partially use steam (Ό) obtained from cooling water at the stage of indirect cooling. 8. Installation for producing aluminum oxide from aluminum hydroxide according to the method according to any one of the preceding paragraphs, including: a) a filter (1) for a hydrate in which aluminum hydroxide is purified with wash water; b) at least one pre-heating agent (10, 12) for partially drying and / or calcining the purified aluminum hydroxide; c) a fluidized bed reactor (20) for calcining the pretreated aluminum hydroxide and producing aluminum oxide; d) at least one fluidized bed cooler (36) for indirectly cooling the resulting alumina with water as a refrigerant; e) a steam separation device (42) installed after the fluidized bed cooler (36) for separating gaseous and liquid fractions of cooling water; f) a pipeline (44, 45, 51), a connecting device (42) for separating steam and a filter (1) for a hydrate, characterized in that in the pipeline (44, 45, 51) there is a control device (50) for regulating the flow of flushing water with a constant maximum temperature below the boiling point of the water and the volumetric flow rate required for rinsing water for the hydrate filter (1) by adjusting the proportion of the portion (V) of the stream directed to the hydrate filter and the additional stream (Ζ) of water, and the fact that the regulatory device (5 0) is connected to a cooler (36) with a fluidized bed for indirect cooling through a pipe (53). 9. Installation according to claim 8, characterized in that a tank (47) is attached to the pipeline (53) for storage as a water source for an additional stream (Ζ) of water. 10. Installation according to claim 8 or 9, characterized in that the hydrate filter (1) is provided with a casing for supplying steam for partial drying of aluminum hydroxide, this casing for supplying steam is connected to the steam outlet of the device (42) for separating steam through a pipeline (43 ) 11. Installation according to any one of claims 8 to 10, characterized in that a heat exchanger (54) is located in the pipeline (51). FIG. one - 10 022436 - »V FUEL 'optional, secondary air primary air condensate ¹ condensate cooling water cooling water FIG. 2 FIG. 3 - 11 022436