The invention is directed to a process and its relating plant for thermal decom- position of aluminium chloride hydrate into aluminium oxide and gaseous hydro- gen chloride comprising the step (i) of partially decomposing the aluminum chloride hydrate into a decomposition zone by heating to a temperature between 600 and 800 °C and step (ii) calcining the partially decomposed aluminum chlo- ride hydrate in a calcining zone to aluminium oxide at a temperature between 900 and 1000 °C.

Aluminium chloride is often used to produce aluminium oxide with high purity. Therefore, aluminium chloride hydrate is allothermally heated in a rector which lead to a decomposition reaction:

AlCl ₃ -6 H ₂ 0 - >0,5 Al ₂ 0 ₃ +3HCl + 4,5H ₂ 0

Thus, a HCI rich gas is produces while the aluminium chloride hydrate is at least partially decomposed. Typically, this reaction takes place in a two-step system whereby the stage of heating the particles takes place in the first reactor, called decomposition reactor and the final product quality is obtained in the residence time reactor called calcination reactor. Such a process can be found in docu- ment WO 83/04017. This arrangement is particularly necessary for huge plant capacities since the high HCI concentration in the first reactor cannot be handled at reactor tempera- tures in the second reactor, while a high reaction temperature of the second reactor is necessary for a complete turnover. However, the separating of the process into two steps leads to a higher energy requirement. Especially when considering product flow rates of modern plants, i.e. AI2O3 1500-3500 tpd (ton per day) this is particularly necessary. Therefore, it is the aim of the invention to reduce the energy consumption.

This problem is solved with a process according to claim 1 .

Aluminum chloride hydrate is heated in a decomposition zone by heating to temperature between 600 and 800 °C, preferably 650 and 750 °C (step i) and afterwards passing the solid mixture of undecomposed aluminum chloride hy- drate and aluminium oxide in a calcining zone where the remaining aluminium chloride and/or aluminium chloride is oxidized to aluminium oxide at a tempera- ture between 850 and 1100 °C, preferably 900 and 1000 °C (step ii). The de- composition zone and the calcination zone are the only two zones for a heating above 400 °C, preferably, they are two separate reactors.

It is the essential part of the invention that the aluminium oxide is passed from the calcination zone into the decomposition zone where it is used as a heat transfer medium. Thereby, the decomposition zone works as a first cooler for the product received in the calcination zone. Moreover, residence time of the flow of solid is increased, which ensures a complete turnover. Finally, it is pos- sible to receive the two gas streams containing HCI.

It is preferred that the aluminium oxide being passed from step (ii) to step (i) is used as the only heat source for step (i). Therefore, the cooling effect for the aluminium oxide is maximized and energy consumption of the overall process is minimized.

In order to achieve the required degree of conversion in the decomposition zone, step i, the recirculation ratio, defined as massflow of solids from the calci- nation zone, step ii, to the decomposition zone divided by the alumina produc- tion rate (massflow), must be in the range of 5 to 50. This ratio depends on the operation temperatures in zone i and ii, as well as the feed quality and product quality targets.

In a preferred embodiment of the invention, step (i) and/or step (ii) takes place in a fluidized bed, particularly preferred step (i) and/or step (ii) takes place in a circulating fluidized bed. This leads to a most efficient heat and mass transfer.

In another embodiment, at least one of the two steps takes place in a rotary kiln which is more flexible regarding particle size of the aluminium chloride hydrate.

Moreover, off-gases containing at least 90 wt.-% of the produced hydrogen chloride from steps (i) and/or (ii) should preferably been quenched with water. Thereby, hydrochloric acid is produced which can be sold or used in another stage.

In this context, a passing of the hydrochloric acid from the quenching to a leach- ing step is particularly effective, since such a leaching is typically done as a previous step to produce of aluminum chloride hydrate.

In this regard, the off-gases containing at least 90 wt.-% of the produced hydro- gen chloride from steps (i) and (ii) are preferably passed separately to an off-gas treatment since they show different concentrations.

In an alternative or supplemental embodiment, the off-gas treatment contains at least one absorption step as an effective gas-cleaning.

As residence time is one of the most important aspects, the overall cumulative residence time of solids in step (i) and step (ii) is between 1 and 20 hours. Also, a circulating of several thousand tons per hour of material between the decom- position and the calcination reactor, the residence time of the solids is in the order of minutes. Passing the solids through the decomposition and the calcina- tion reactor several times, the overall residence time in each reactor is in the order of tens of minutes, while the total residence time of the solids within the whole dual fluidized bed reactor system including the decomposition reactor,

Moreover, if it is preferred that fuel is introduced in step (ii) for a direct heating and, therefore, an optimized heat transfer.

Since a combustion of fuel (gaseous or liquid) takes place within the calcination reactor, it is important to establish the required combustion conditions, which is ensured by good solid mixing and secondary and/or tertiary air penetration up to the center of the reactor, particularly considering that calcination reactor cross- sections may become as big as 50-100 m ² or more in order to match the product flow (up to 3500 tons per day or above) as it is typical for a Bayer process.

An essential aspect of the current invention is the controlling of the two reactors since he additional recycling line passing solids from the calcination reactor back into the decomposing reactor gives additional process parameters. There- fore, a stream of solids is withdrawn from step (i) via a downstream downer and is fluidized at the bottom of the downer by supplying a conveying gas. So, it is transported to a higher level via a riser branching off from the downer. The flow of the stream of solids conveyed through the riser is varied by the variable sup- ply of the conveying gas, wherein temperature or pressure or load of step (i) or step (ii) is used as a variable to be controlled and the volume flow of the convey- ing gas is used as an actuating variable of a control circuit. This enables a con- trolling without any mechanical parts like valves being in contact with the hot solids. Moreover, it is preferred that that the off-gas stream of step (i) and/or step (ii) is asymmetrically withdrawn. The increased plant size also leads to increased reactor sizes, In this regard, it is necessary to ensure a constant off-gas flow without any acid accumulation.

The invention also covers a plant with the features of claim 15, especially for operating a process according to any of claims 1 and 14.

Such a plant for thermal decomposition of aluminium chloride hydrate into alu- minium oxide and gaseous hydrogen chloride comprising a decomposition reac- tor for decomposing the aluminum chloride hydrate by heating to a temperature between 600 and 800 °C and a calcination reactor for calcining the partially decomposed aluminum chloride hydrate in the calcining zone to aluminium oxide at a temperature between 850 and 1200 °C. According to the invention, a circulating conduit for passing the aluminium oxide from the calcination reactor to the decomposing reactor is foreseen whereby the decomposing reactor is designed such that the aluminium oxide is used as a heat transfer medium.

Particularly, it is preferred that the decomposition reactor and/or the calcination reactor, preferably the calcination reactor is rectangular shaped with a length to width ratio > 1 , which enables large dimension of the reactor. So, high through- put rates can be achieved, for which the current invention is particularly efficient. Modern day AI ₂ O ₃ production rates cannot be realized cylindrical calcination reactors fed with AIC ₃ 6H ₂ O. Reason is that large diameters (> 8 m) would not allow appropriate solid mixing and secondary air penetration which are neces- sary for appropriate combustion taking place within these reactors. Due to the smaller width of the preferred reactor, secondary air penetration and gas-solid mixing issues are drastically improved thus allowing the construction of calcina- tion reactors of > 40 m ² size (e.g. 100 m ² ). Further aims, features, advantages and applications of the invention will become apparent from the following description of the accompanying drawings. All of the described and/or depicted features, by themselves or in any combinations, form the subject matter of the invention, independently of whether they are defined in the individual claims or their dependencies.

In the drawings: Fig. 1 shows a process flow diagram of the method in accordance with the invention,

Fig. 2 shows a first control system for a process according to the invention, Fig. 3 shows a second control system for a process according to the inven- tion,

Fig. 4 shows a third control system for a process according to the invention and

Fig. 5 shows a forth control system for a process according to the invention

Fig. 1 shows the principal design underlying the invention. Wet aluminium chlo- ride hydrate is passed via conduit 1 1 1 into the decomposition reactor 1 10. Fur- ther, additional oxygen, preferably by air, is introduced via conduit 1 12 and quench water is added via conduit 1 13.

The produced rich HCI gas with an HCI content of above 30 vol.-% is passed via conduit 1 14 in an HCI absorption stage 130, preferably featuring at least two stages. From there, HCI solution is withdrawn via conduit 131 , while first off-gas is withdrawn via conduit 133. Process water is added via a conduit 132. Cooling water is injected by conduit 136 and withdrawn via conduit 137. Via conduit 1 15, a mixture of already calcined AI2O3 and AICl _6' 6H ₂ 0 is passed into calcination reactor 120. Therein, a gaseous fuel and/or liquid fuel is intro- duced via conduit 126. Conduit 125 feeds an oxygen source into the calcination reactor 120. Via conduit 127, quench water is added. Via conduit 124 and 128, cool water is recirculated in and out of the calcination reactor 120. The final calcined product AI2O3 is withdrawn via conduit 123.

Via conduit 122, a lean HCI gas with an HCI content of less than 7 vol.-% is passed into a second stage of the HCI absorption 130. From there, an off-gas is withdrawn via conduit 134, while process water is added via conduit 138. Cool- ing water is recirculated via conduit 139 and 135.

It is the basic idea of the invention that via conduit 121 , parts from the calcina- tion reactor 120 are passed into the decomposition reactor 110. Alternatively but not shown, it is also possible to have only one withdrawing of AI ₂ O ₃ , from which a recycling stream into the decomposition reactor 1 1 0 is branched off.

Fig. 2 shows the current invention with its basic control system. Therefore, from a feeding belt 1 , the wet aluminium chloride hydrate is passed into a preheating section 2. From there, the educt is fed via conduits 17, 17' into the decomposi- tion reactor 10, which is a fluidized bed reactor. Fluidizing gas is fed in via con- duit 63. A gas-solid-mixture is withdrawn from the decomposition reactor 10 and passed via conduit 12 into a cyclone 13. Via conduit 1 1 , hot off-gases from the cyclone 13 are fed into the preheating section 2 for utilizing the enclosed ener- gy-

The separated solids are passed from cyclone 13 via conduit 14 into a so called seal pot 15, wherein they are fluidized by gas, which was branched off from conduit 17. The gas is injected via conduit 18 and blower 19 into the seal pot 15. As only parts of the solids are fluidized, whereby the amount of fluidized solids can be controlled by the added gas flow, these solids are withdrawn form an overflow via conduit 16 and recirculated into decomposition reactor 10.

Further, via an underflow solids are passed from decomposition reactor 10 via conduit 71 into a seal pot 72, where they come to the bottom and are fluidized by gas injected via conduit 73 and controlled via blower 74. Also here, parts of the solids are withdrawn from the overflow and passed via conduit 75 into calci nation reactor 70, which is also preferably built as a fluidized bed reactor. Fluid izing gas is injected into calcination reactor 70 via conduit 62. Via conduit 61 , a secondary or tertiary gas is brought into reactor 20. Secondary and / or tertiary air is used for combustion purposes and to utilize the heat from cooling down the solid product (air through conduit 51 ) and from the reactor off gases (air through conduit 61 ).

Preferably, the calcination reactor 20 features at least two possibilities to with- draw the gas-solid-mixture, which are most preferably arranged asymmetrically. Flowever, also only one withdrawing is possible. The first withdrawing conduit 21 pass a gas-solid-mixture into a cyclone 22, from which the off-gases are with- drawn via conduit 33 into the preheating stage 2, where their heat is used to preheat the feed.

The aluminium oxide is withdrawn via conduit 23 into a seal pot 24. The bottom of the seal pot is fluidized by a gas injected via conduit 26 and blower 27. Solids from the overflow are withdrawn via conduit 25 and repassed into the decompo- sition reactor 10. This recirculation is the essential idea of the invention.

Moreover, a final product is withdrawn via conduit 28 and also passed into a cyclone 29. Therein, the off-gases are withdrawn via conduit 34 and also passed to the preheating stage 2 to preheat the aluminium chloride hydrate. The solid part in the cyclone 29 are withdrawn via conduit 41 and passed into a seal pot 24, where they are fluidized via a gas injected via conduit 34 and blow- er 44. From the overflow, parts of the solids are withdrawn via conduit 46 and recirculated into the calcination reactor 20. Moreover, other parts of the solids are withdrawn via conduit 45 and passed into cooling section 50..

In the cooling section 50, the AI ₂ O ₃ particles can also be fluidized via fluidizing gas injected via conduit 53 and blower 54 and are withdrawn via conduit 52. Off- gases are recirculated via conduit 51 into the calcination reactor 20, where they can also be used as a secondary or tertiary gas.

From the feeding section 2 they are further passed into a gas preheater 31 , which is used to preheat the fluidizing and/or secondary air streams of conduit 61 , 62 and 63. These gas streams are fed in via at least on, preferably three conduits 35 and the respective blower(s) 36 which can be each for each gas stream or one common.

From the gas-gas preheating stage 31 , the off-gas is further passed into an HCI absorption stage 30 to produce HCI. It is preferred that the gas-solid preheating stage 2, the gas-gas preheating stage 31 and the HCI absorption stage have at least two preferably three stages to handle each off-gas separately. So, HCI with different concentrations can be produced and withdrawn via lines 38 and the respective blowers. However, it is also possible to have only one stage and, therefore, an HCI with a mixed concentration.

A possible control loop is presented in Figure 3. The temperature of the decom- position reactor is controlled through varying the speed of calcination transfer side blower (38b). Varying the speed of blower 38b results to a differentiated gas flow between calcination transfer cyclone 29 (see Figure 3) and calcina- tion recycle cyclone 22, e.g. increasing the speed of blower 25 will increase the gas flow through calcination transfer cyclone 29 and reduce the gas flow through the calcination recycle cyclone 29.

This control logic holds true even if the calcination reactor is equipped with more than one pair of cyclones.

In case of an independent pressure drop control in the calcination reactor 20, as shown in figure 4, the pressure drop of the decomposition reactor 10 is con- trolled through controlling the speed of the blower 19 fluidizing the control seal pot 15. So, changes of the air flow entering the seal pot 15 leads to changes of the solid fraction within the riser (solid up-flow section) of the seal pot 15 and therefore to alteration of the pressure drop through the riser of the seal pot 15. Since the pipe leading to the transfer seal pot 15 is short and is connected near the bottom of the decomposition reactor 10, the alteration of the pressure drop through the seal pot riser leads an alteration of the ability of the decomposition reactor 10 to withhold material and hence its pressure drop. Hence, decreasing seal pot aeration, i.e. decreasing the speed of the decomposition transfer seal pot blower 19 leads to an increase of the decomposition reactor 10 pressure drop and hence of the residence time ratio between the decomposition reactor 10 and the calcination reactor 20.

Example

The example uses a plant design shown in any of figures 2 to 5. Therein, 530 tph of AICb-6H20 including 8 wt.-% moisture enter the gas-solid pre-heating section 2 through the feed belt 1 where they are dried and heated to a tempera- ture of 650 - 800 °C before entering the decomposition reactor 10.

Part of the decomposition of the AIC ₃ 6H2O occurs within the gas-solid pre- heating sections. Solids enter the decomposition reactor 10 which operates at a temperature of 750 °C and a pressure drop of 150 mbar. Solids circulate inter nally within the decomposition reactor 10 with use of the decomposition recycle cyclone 13 and the decomposition recycle seal pot 15 . The decom- position recycle seal pot 15 is fluidized through the decomposition recycle seal pot blower 19.

The diameter of the decomposition reactor 10 can be up to 8 m, while its veloci- ty may range from 4-6 m/s thus making it a turbulent to fast fluidized bed, with the ability of achieving the above mentioned pressure drop. The tempera- ture of the decomposition reactor 10 is maintained through the control loop mentioned above, i.e. by adjusting the speed of the calcination transfer seal pot blower 19. The pressure drop of the decomposition reactor 10 is maintained based on the control loop mentioned above. In the case that another non- mechanical valve would be used for the seal pot (in other sources also termed as loop seal) such as the L-valve, V-valve, J-valve the aeration of that valve would control the decomposition reactor pressure drop. If a mechanical valve would be used e.g. cone valve, slide valve, the opening of that valve would determine the decomposition reactor pressure drop.

2800 tph partly converted solids are transferred to the calcination reactor 20. Part of the calcination reactor entrainment proceeds enters the calcination trans- fer cyclone 22 and enters the decomposition reactor 10 through the calcination transfer seal pot 23, which is fluidized through the calcination seal pot blow- er 42.

The calcination reactor 20 operates at a pressure drop of 50 mbar, which is set by adjusting the opening of the discharge device and at a temperature of 950 °C, which is controlled by the input of liquid/gaseous fuel within the system. Efficient combustion is assured through the rectangular calcination reactor design shown. For the scale at hand the calcination reactor would have to operate with three pairs of cyclones, three pairs of respective seal pots down- stream these cyclones and three pairs of blowers fluidizing the above mentioned seal pots. Approximately, 100 tph exit through the discharge device and enter the cooling section 50, where they are cooled to < 80°C before being discharged to the atmosphere. This is possible, since a much higher solid stream enters the calcination recycle seal pot 24 after exiting the calcination recycle cyclone 22.

Overall, approximately 520.000 Nm ³ /h air is needed for the process. For energy efficiency the streams are pre-heated to a temperature of 450 °C.

Since the AICl ₃ -6H ₂ 0 feed enters the decomposition reactor 10 first, and the residence time ratio is equal to 3 when taking into account the noted pressure drops and by constructing a decomposition and calcination reactor of similar area it can be claimed that the AIC ₃ -6H ₂ 0 conversion within the decomposition reactor 10 is 3 times higher than that of the calcination reactor 20 leading to a HCI rich gas exiting the decomposition reactor 10 of > 20-40 vol.-% and a HCI lean gas exiting the calcination reactor 20 <7 vol.-%. These concentrations are excellent in order to produce a HCI solution with concentration of > 30 vol.-% within HCI absorption.

List of reference numerals

1 feeding belt

2 gas-solid preheating

10 decomposition reactor

1 1 ,12 conduit

13 cyclone

14 conduit

15 seal pot

16 18 conduit

19 blower

20 calcination reactor 21 conduit

22 cyclone

23 conduit

24 seal pot

25,26 conduit

27 blower

28 conduit

29 cyclone

30 HCI absorption

31 gas-gas preheating

33 35 conduit

36 blower

37 conduit

38 blower

41 conduit

42 seal pot

43 conduit

44 blower 45 conduit

50 cooler

51 53 conduit

54 blower

61 63 conduit

71 conduit

72 seal pot

73 conduit

74 blower

75 conduit

1 10 decompostion reactor

1 11 116 conduit

120 calcination reactor

130 HCI absorption

131 139 conduit