Process for producing sodium aluminium tetrachloride

Abstract

A process for preparing sodium aluminium tetrachloride, the process comprising, under 5 anhydrous conditions and an oxygen-free atmosphere: in a first dosing stage, providing a reactor containing sodium aluminium tetrachloride and solid aluminium, and optionally solid sodium chloride, and bringing the reactor to a temperature of from about 150 °C to about 400 °C, thereby providing a reaction mixture comprising a liquid sodium aluminium tetrachloride phase; in a chlorine-dosing stage, contacting the reaction mixture with a chlorine-containing gas; and in an 0 optional further dosing stage, adding solid aluminium and / or solid sodium chloride to the reactor, wherein the optional further dosing stage is performed during or alternating with the chlorine-dosing stage, and wherein solid sodium chloride is added to the process in at least one of the first or further dosing stage.

Description

It has now been found that sodium aluminium tetrachloride can be prepared from sodium chloride, aluminium, and chlorine-containing gas in a one-pot synthesis. This new synthesis route not only avoids the lengthy separation process of the intermediate anhydrous aluminium trichloride discussed above but is also capable of preparing battery-grade sodium aluminium tetrachloride in high yield. Accordingly, the present invention is directed to a process for preparing sodium aluminium tetrachloride, the process comprising, under anhydrous conditions and an oxygen-free atmosphere: in a first dosing stage, - providing a reactor containing sodium aluminium tetrachloride and solid aluminium, and optionally solid sodium chloride; - bringing the reactor to a temperature of from about 150 °C to about 400 °C, thereby providing a reaction mixture comprising a liquid sodium aluminium tetrachloride phase; in a chlorine-dosing stage, contacting the reaction mixture with a chlorine-containing gas; and in an optional further dosing stage, adding solid aluminium and / or solid sodium chloride to the reactor, wherein the optional further dosing stage is performed during or alternating with the chlorine-dosing stage, and wherein solid sodium chloride is added to the process in at least one of the first or further dosing stage. Advantageously, by providing a liquid sodium aluminium tetrachloride phase in the reactor in the first dosing stage, the process of the present invention has been found to supply a reaction medium that not only ensures good interaction between the solid and gaseous reactants, but also dissolves the intermediate anhydrous aluminium trichloride as it is formed, thus preventing loss into the headspace of the reactor at temperatures above 160-170 °C, and enabling the overall reaction to be carried out in a single reactor. As a result, the presently disclosed process can be used to prepare high-quality sodium aluminium tetrachloride with reduced operational costs and lower capital investment compared to US 2004 / 0223902 and JPS52-129696. In the first dosing stage of the process, a reactor containing sodium aluminium tetrachloride and solid aluminium, and optionally solid sodium chloride, is provided, and is brought to a temperature of from about 150 °C to about 400 °C. The reactor may be brought to the required temperature range before or after it contains one or more of the solid aluminium, solid sodium chloride and sodium aluminium tetrachloride. Any suitable reactor can be provided for the process, such as, a stirred-tank reactor. The reactor may include a circulation loop for cooling and / or dosing purposes. When a circulation loop is included to remove heat from the reaction mixture, a heat exchange device may be provided in the circulation loop. In the first dosing stage of the process, two or more of the solid aluminium, solid sodium chloride and sodium aluminium tetrachloride may be pre-mixed before being dosed to the reactor. Alternatively, solid aluminium is contacted with sodium aluminium tetrachloride, and optionally solid sodium chloride, in the reactor. This contacting may be carried out before, after or at the same time as bringing the reactor to the required temperature range. Likewise, any or all of the solid aluminium, solid sodium chloride and sodium aluminium tetrachloride may be at, or above, this temperature range before they are contacted. Preferably, the sodium aluminium tetrachloride is at this temperature range when it is contacted with the solid aluminium and optional solid sodium chloride. When the solid aluminium is contacted with sodium aluminium tetrachloride, and optionally solid sodium chloride, in the reactor, the first dosing stage comprises, - contacting solid aluminium with sodium aluminium tetrachloride, and optionally solid sodium chloride, in a reactor; - bringing the reactor to a temperature of from about 150 °C to about 400 °C, thereby providing a liquid reaction mixture comprising a liquid sodium aluminium tetrachloride phase. The solid aluminium may be provided to the process in any suitable physical form, including as a powder, granules, needles, platelets, or a mixture thereof. Preferably, the aluminium is added as a powder having a particle size distribution of from 300 microns to 3000 microns, preferably of from 500 microns to 2000 microns, more preferably of from 500 microns to 1500 microns, as measured by dry sieving according to ISO 4497. When preparing battery-grade sodium aluminium tetrachloride, the purity of the aluminium used should be above 95 wt.%, preferably above 98 wt.% and more preferably above 99 wt.% or above 99.5 wt.% (on trace metals basis). The aluminium may be obtained from a new source, or it may be recycled from other applications or processes. The solid sodium chloride provided to the process should have a maximum water content of 1 wt.%, preferably 0.5 wt.%, more preferably 0.1 wt.%. This low water content can be achieved, for example, by drying the sodium chloride before use in any batch or continuous drying device known in the art, such as an oven, flash dryer or fluid bed drier. When preparing battery-grade sodium aluminium tetrachloride, the purity of the sodium chloride used should be above 98 wt.%, preferably above 99 wt.% and more preferably above 99.5 wt.%. The sodium aluminium tetrachloride may be provided to the process as a liquid or solid. After the initial process start-up, the sodium aluminium tetrachloride required in the first dosing stage may be obtained by the process, with at least a part of the sodium aluminium tetrachloride being kept in the reactor during a discharge stage (see below). When preparing battery-grade sodium aluminium tetrachloride, the purity of the sodium aluminium tetrachloride provided for start-up should be above 98 wt.%, preferably above 99 wt.% and more preferably above 99.5 wt.% (on trace metals basis). The first dosing stage may further include the action of evacuating the reactor to an absolute pressure below about 5000 Pa, preferably below about 3000 Pa, more preferably below about 2000 or 1500 Pa. Evacuating the reactor before contacting the reaction mixture with a chlorinecontaining gas can increase the efficiency of the chlorine-dosing stage. As mentioned above, in the first dosing stage, the reactor is brought to a temperature of from about 150 °C to about 400 °C, preferably of from about 200 °C to about 350 °C, preferably from about 220 °C to about 320 °C, and more preferably from about 230 °C to about 300 °C. At this temperature, the reaction mixture provided by the first dosing stage comprises solid aluminium (and optional solid sodium chloride) suspended in a liquid sodium aluminium tetrachloride phase. Typically, the reaction mixture provided by the first dosing stage has a total solids concentration of at least 3 vol%, at least 5 vol%, at least 10 vol% or at least 15 vol%, and at most 40 vol% or at most 35 vol%. That is, the reaction mixture provided by the first dosing stage may have a total solids concentration of from 3 to 40 vol% in the liquid sodium aluminium tetrachloride phase, preferably of from 5 to 40 vol%, more preferably of from 5 to 35 vol%. In the chlorine-dosing stage of the process, the reaction mixture provided by the first dosing stage is contacted with a chlorine-containing gas, resulting in the formation of a chlorinated reaction mixture (i.e., a reaction mixture into which chlorine has been partly absorbed or dissolved). Throughout this stage, the reactor temperature is preferably maintained at about 150 °C to about 400 °C, preferably at about 200 °C to about 350 °C, preferably at about 220 °C to about 320 °C, and more preferably at about 230 °C to about 300 °C, by heating and / or cooing the reactor as required. The chlorine-containing gas may be provided in a continuous manner or intermittently in at least 2, preferably at least 4, more preferably at least 6, and most preferably at least 10 portions. The duration of each portion and the time intervals between the portions may be the same or different. If many portions are added at short intervals, or if portions are provided for long durations, continuous dosing is approached. Continuous dosing, which is the preferred manner of dosing, may be performed at constant or variable rate. The rate at which the chlorine-containing gas is continuously dosed is preferably controlled by an automated pressure-controlled dosing system. Preferably, the chlorine-containing gas is provided to the process to bring the reactor to an absolute pressure of from about 0.20 MPa to about 2 MPa, preferably from about 0.25 MPa to about 1.5 MPa, more preferably from about 0.28 MPa to about 1 MPa. With intermittent dosing, this may be achieved by adding a portion of the chlorine-containing gas when the reactor pressure is detected to fall below a predetermined threshold within the range, while with continuous dosing, the rate of addition of the chlorine-containing gas can be varied to maintain a relatively constant reactor pressure. The chlorine-containing gas stream provided to the process may be chlorine gas (Cl2). In order to prevent build-up of inerts during the reaction, the chlorine gas preferably contains less than 5wt.% impurities, more preferably less than 1 wt.% impurities. Alternatively, the chlorinecontaining gas may be a purified hydrogen chloride-containing gas stream, containing, for example, at least 95 vol%, preferably at least 99 vol%, of hydrogen chloride. The contacting of the reaction mixture with the chlorine-containing gas may be carried out in manners known in the art. In general, the mixing conditions should be sufficient to ensure intimate contact and proper mass-transfer between the chlorine-containing gas and the liquid medium. Preferably, the liquid medium is stirred such that chlorine-containing gas in the headspace of the reactor is pulled into the reaction mixture, e.g., by a self-suction stirrer, a hollow shaft impeller, or venturi jet mixer. Alternatively, the chlorine-containing gas could be bubbled through the liquid reaction mixture. When the reactor includes a circulation loop, the reaction mixture may be contacted with the chorine-containing gas in a liquid / gas contacting device provided in the loop, such as a static mixer, packed column, spray column or venturi mixer, to which the chlorine-containing gas is fed. Without being bound by theory, once absorbed into the reaction mixture, the chlorine is thought to react with the suspended solid aluminium to form anhydrous aluminium chloride, which dissolves in the liquid sodium aluminium tetrachloride phase. Once dissolved in the liquid sodium aluminium tetrachloride phase, the anhydrous aluminium chloride can react with any suspended solid sodium chloride to form the product sodium aluminium tetrachloride. Because the overall reaction is suggested to proceed via the two consecutive steps discussed above, the process can include more than one dosing stage. In particular, it has been found that dissolved anhydrous aluminium chloride will stay in the liquid sodium aluminium tetrachloride phase until a sufficient level of solid sodium chloride is present for further reaction. Thus, the process may include a further dosing stage, in which solid aluminium and / or solid sodium chloride is added to the reactor. This further dosing stage may be performed during or alternating with the chlorine-dosing stage. In the first and / or further dosing stage, the solid aluminium and / or solid sodium chloride may be provided to the reactor in a single portion, in a continuous manner or intermittently in at least 2, preferably at least 4, more preferably at least 6, and most preferably at least 10 portions. The duration of each portion and the time intervals between the portions may be the same or different. If many portions are added at short intervals, or if portions are provided for long durations, continuous dosing is approached. For the overall reaction to be completed, solid sodium chloride must be added to the process in at least one of the first dosing stage or the further dosing stage. Typically, the molar ratio of the total amount of solid sodium chloride and solid aluminium used in the process is from 0.8:1.2 to 1.2:0.8, preferably 0.9:1.1 to 1.1:0.9, and more preferably 0.95:1.05 to 1.05:0.95. After sufficient chlorine-dosing, the sodium aluminium tetrachloride product can be recovered. Thus, the process of the invention may further comprise: in an optional purging stage, removing chlorine-containing gas from the reactor; and in a recovery stage, discharging at least a part of the sodium aluminium tetrachloride from the reactor. In the optional purging stage, the chlorine-containing gas may be removed from the reactor by displacing with an inert gas, such as nitrogen. In the recovery stage, at least a part of the sodium aluminium tetrachloride may be discharged from the reactor at a temperature of from about 150 °C to about 400 °C, preferably from about 160 °C to about 350 °C, more preferably from about 160 °C to about 250 °C. At these temperatures, sodium aluminium tetrachloride is liquid, and thus, more easily handled. The recovered sodium aluminium tetrachloride can then be sent for further treatment, e.g., filtration or centrifugal separation, as required. In the recovery stage, preferably at least 10 wt.%, preferably at least 25 wt.%, more preferably at least 40 wt.% of the total sodium aluminium tetrachloride (i.e., that introduced in the first dosing stage and that prepared by the process) is retained in the reactor for the first dosing stage of a next production run. Accordingly, as described above and shown in Figure 1, the process for preparing sodium aluminium tetrachloride of the present invention may be carried out in a single reactor and include, under anhydrous conditions and an oxygen-free atmosphere: in a first dosing stage, - providing a reactor containing sodium aluminium tetrachloride and solid aluminium, and optionally solid sodium chloride; - bringing the reactor to a temperature of from about 150 °C to about 400 °C, thereby providing a reaction mixture comprising a liquid sodium aluminium tetrachloride phase; in a chlorine-dosing stage, contacting the reaction mixture with a chlorine-containing gas; in an optional further dosing stage, adding solid aluminium and / or solid sodium chloride to the reactor, in an optional purging stage, removing chlorine-containing gas from the reactor; and in a recovery stage, discharging at least a part of the sodium aluminium tetrachloride from the reactor, wherein the optional further dosing stage is performed during or alternating with the chlorine-dosing stage, and wherein solid sodium chloride is added to the process in at least one of the first or further dosing stage. The process of the present invention can be performed continuously, semi-continuously or batchwise, preferably semi-continuously or batch-wise. When performed batch-wise, a predetermined amount of solid aluminium and solid sodium chloride can be added to the process in the first or further dosing stage, and the chlorine-dosing stage can be run until there is reduced (or no further) uptake of chlorine observed. Alternatively, a predetermined amount of solid aluminium, solid sodium chloride and chlorine-containing gas can be used in the process, based on calculated mass balance. When operating continuously or semi-continuously, a predetermined amount of solid aluminium, solid sodium chloride and chlorine-containing gas can be introduced to the process whilst continuously or intermittently withdrawing sodium aluminium tetrachloride from the reactor, so as to maintain a desired level of liquid inside the reactor. The process is performed under anhydrous conditions and an oxygen-free atmosphere. For the avoidance of doubt, the term “an oxygen-free atmosphere” is used herein to mean an atmosphere containing less than 1 vol% oxygen. The process is run under an oxygen-free atmosphere to prevent oxidation reactions, for example, of the aluminium. For the avoidance of doubt, the term “anhydrous” is used herein to mean containing less than 1 wt.% water, preferably less than 0.5 wt.%, more preferably less than 0.1 wt.%. Anhydrous conditions are also important to prevent reaction of sodium aluminium tetrachloride and the intermediate anhydrous aluminium trichloride with water. For this reason, the solid sodium chloride should be dry (max 1 wt.%, preferably max 0.5 wt.%, more preferably max 0.1 wt.% water). The anhydrous conditions and oxygen-free atmosphere required for the process can be obtained, for example, by purging the reactor with an inert gas (such as nitrogen, argon, or helium), priorto the first dosing stage. Removal of water may be enhanced by heating the reactor to a temperature applied during the first dosing stage and / or by evacuating the reactor to pressures below atmospheric pressure. A preferred embodiment of the present process is illustrated schematically in Figure 1. Here: A is a reactor, B is a purification section, and C is a scrubber. In the process, liquid sodium aluminium tetrachloride is added, via 1, solid aluminium is added, via 2, and solid sodium chloride is added, via 3, into reactor A. The reactor A is heated to a temperature between 240-270 °C and evacuated while gently stirring the resulting liquid reaction mixture. Chlorine gas is fed into the reactor A, via 5, and the stirring speed increased such that the chlorine gas is pulled into the liquid reaction mixture. The reactor temperature is maintained via heating and / or cooling of the reactor wall and / or internal coils or via heat exchange devices installed in a circulation loop of the reactor, as required (not shown). When no further chlorine uptake is observed the chlorine feed is stopped and nitrogen gas is fed into the reactor A, via 4. The displaced chlorine gas is purged from the reactor A and fed, via 7, to scrubber C. At least a part of the molten sodium aluminium tetrachloride is discharged from the reactor A and fed, via 6, to the purification section B, where it is purified to a high purity sodium aluminium tetrachloride product. The sodium aluminium tetrachloride produced by the present process can be used in batteries. For example, as electrolyte in molten salt metal-halide battery cells, including ZEBRA (ZeroEmission Battery Research Actives) cells. It is noted that various elements of the present invention, including but not limited to preferred ranges for the various parameters, can be combined unless they are mutually exclusive. The invention will be elucidated by the following examples without being limited thereto or thereby. Examples Example 1: One-pot preparation of sodium aluminium tetrachloride from aluminum, sodium chloride and chlorine A 0.5-liter reactor was filled with 96.3 g NaCI (> 99.9 %, dried at 700 °C for 15 hours) and 44.5 g Al (Sigma Aldrich, granular, < 1 mm, 99.7 % trace metals basis). The reactor was heated to 240 °C and evacuated to 15 mbar. 242.8 g NaAICI4 (obtained by the experimental procedure as described in US 2017 / 0050860 A1) was heated in a dosing vessel to 240 °C and then transferred to the reactor. The reactor was heated to 260 °C and evacuated while gently stirring the mixture with a hollow shaft impeller. After closing the loop to the vacuum pump, the reactor was pressurized with chlorine gas to a pressure of 3 bar. The impeller speed was increased to 1100 rpm and chlorine gas was mixed into the reaction mixture through the self-suction impeller. Chlorine dosing was pressure controlled and the flow was monitored during the reaction time. The reactor pressure was increased to 5 bar after 100 minutes, thus increasing the chlorine uptake. The reaction temperature was maintained between 260 and 270 °C via cooling of the reactor wall. After 2.5 hours the reaction run was stopped as no further chlorine uptake was observed. The molten product was unloaded from the reactor and 535.9 g of NaAICI4 product was recovered. The amount of material remaining at the wall of the reactor was not quantified. Based on the mass balance, the theoretical chlorine uptake based on aluminium was calculated to be 175.4 g and the theoretical product yield of NaAICI4 product was calculated to be 558.9 g. The measured amount of product drained from the reactor was 535.9 g, resulting in an overall yield of 96 %. By attributing the difference between the theoretical product weight and the actual product weight (i.e., 23 g of NaAICI4) to less than complete chlorine uptake, the minimum Al conversion was calculated according to: (Theoretical chlorine uptake - Theoretical product yield + Actual measured product yield) / Theoretical chlorine uptake. In this example, the minimum total Al conversion was calculated to be 87 %. This is a minimum value as material remaining at the wall of the reactor or lost via the gas phase was not accounted for. This example shows that it is possible to prepare sodium aluminium tetrachloride in a one-pot process from aluminium, sodium chloride and chlorine at relatively moderate temperature conditions and with sufficiently high conversion and yield. Example 2: One-pot preparation of sodium aluminium tetrachloride from aluminum, sodium chloride and chlorine (with two NaCI loadings') A 0.5-liter reactor was filled with 16.1 g NaCI (> 99.9 %, dried at 700 °C for 15 hours) and 15.0 g Al (Sigma Aldrich, granular, < 1 mm, 99.7 % trace metal basis). The reactor was heated to 220 °C and evacuated to 15 mbar. 318.5 NaAICI4 (obtained by the experimental procedure as described in US 2017 / 0050860 A1) was heated in a dosing vessel to 220 °C and then transferred to the reactor. The reactor was evacuated to 15 mbar and stirred with a hollow shaft impeller. After closing the loop to the vacuum pump, the reactor was pressurized with chlorine gas to a pressure of 3 bar. The reaction temperature was set at 250 °C. After 1 hour of reaction, the chlorine feed was stopped and an additional 16.0 g NaCI was loaded. Chlorine dosing was started again, and the pressure set point was set at 4 bar. After another hour the reaction run was stopped as no further chlorine uptake was observed. During the reaction the cumulative chlorine uptake was determined based on the measured chlorine flowrate. The cumulative chlorine uptake was 17.5 nL, corresponding to 55.5 g chlorine. The molten product was unloaded from the reactor and 419.1 g of reaction product was recovered. Based on the mass balance, the theoretical chlorine uptake was calculated to be 59.1 g and the theoretical product yield of NaAICI4 product was calculated to be 424.7 g. Thus, the overall yield of the reaction is 99%. When calculating the minimum total Al conversion according to the method in Example 1, the conversion was found to be 91%. For a more accurate approach, the Al conversion was calculated directly from the cumulative chlorine uptake according to: Measured cumulative chlorine uptake / Theoretical chlorine uptake. Based on the cumulative chlorine uptake, an Al conversion of 94% was calculated. [Remark: the chlorine flowrate showed some fluctuations, thus decreasing the accuracy of the cumulative chlorine uptake], A further method to qualify the reaction product is to determine the product composition. Here, the aluminium and sodium content of the reaction product was measured using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Agilent 5110). Energy Dispersive X-ray Fluorescence (EDXRF; Rigaku NEXCG) was used to measure the chlorine content as well as the sodium content. Table 1 shows the analytical results and the corresponding molar ratios that can be calculated from the normalized weight percentage. Table 1 Compound ICP-EOS wt.% EDXRF wt.% Normalized wt.% Molar ratio compared to Al Na 11 11.9 11.3 1 Al 13.9 / 14.0 13.8 1 Cl 76.1 74.9 4 From Table 1 it can be concluded that within the accuracy of the analytical measurements the molar composition of the product corresponds to the molar composition of sodium aluminium tetrachloride, confirming that the obtained reaction product is indeed sodium aluminium tetrachloride. This Example shows that almost complete conversion to sodium aluminium tetrachloride can be reached according to the present process. Whilst the invention has been described with reference to an exemplary embodiment, it will be appreciated that various modifications are possible within the scope of the invention. In this specification, unless expressly otherwise indicated, the word ‘or’ is used in the sense of an operatorthat returns a true value when either or both of the stated conditions is met, as opposed to the operator ‘exclusive or’ which requires that only one of the conditions is met. The word ‘comprising’ is used in the sense of ‘including’ rather than to mean ‘consisting of. All prior teachings acknowledged above are hereby incorporated by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge in Europe or elsewhere at the date hereof.

Claims

1. A process for preparing sodium aluminium tetrachloride, the process comprising, under anhydrous conditions and an oxygen-free atmosphere:in a first dosing stage,- providing a reactor containing sodium aluminium tetrachloride and solid aluminium, and optionally solid sodium chloride;- bringing the reactor to a temperature of from about 150 °C to about 400 °C, thereby providing a reaction mixture comprising a liquid sodium aluminium tetrachloride phase;in a chlorine-dosing stage, contacting the reaction mixture with a chlorine-containing gas; andin an optional further dosing stage, adding solid aluminium and / or solid sodium chloride to the reactor,wherein the optional further dosing stage is performed during or alternating with the chlorine-dosing stage, and wherein solid sodium chloride is added to the process in at least one of the first or further dosing stage.

2. A process as claimed in claim 1, wherein, in the chlorine-dosing stage, the reaction mixture is contacted with the chlorine-containing gas at an absolute pressure of from about 0.20 MPa to about 2 MPa, preferably from about 0.25 MPa to about 1.5 MPa, more preferably from about 0.28 MPa to about 1 MPa.

3. A process as claimed in claim 1 or claim 2, wherein, in the first dosing stage, the reactor is evacuated to an absolute pressure below about 5000 Pa, preferably below about 3000 Pa, more preferably below about 2000, and more preferably below about 1500 Pa.

4. A process as claimed in any preceding claim, wherein, in the first dosing stage, the reactor is brought to a temperature of from about 200 °C to about 350 °C, preferably from about 220 °C to about 320 °C, more preferably from about 230 °C to about 300 °C.

5. A process as claimed in any preceding claim, wherein the solid aluminium is added to the process in the form of aluminium powder having a particle size distribution of from 300 microns to 3000 microns, preferably of from 500 microns to 2000 microns, more preferably of from 500 microns to 1500 microns, as measured by dry sieving according to ISO 4497.

6. A process as claimed in any preceding claim, wherein the reaction mixture provided by the first dosing stage has a total solids concentration of from 3 to 40 vol% in the liquid sodium aluminium tetrachloride phase, preferably of from 5 to 40 vol%, more preferably of from 5 to 35 vol%.

7. A process as claimed in any preceding claim, wherein the molar ratio of the total amount of solid sodium chloride and solid aluminium used in the process is from 0.8:1.2 to 1.2:0.8, preferably 0.9:1.1 to 1.1:0.9, more preferably 0.95:1.05 to 1.05:0.95.

8. A process as claimed in any preceding claim, further comprising,in an optional purging stage, removing chlorine-containing gas from the reactor; and in a recovery stage, discharging at least a part of the sodium aluminium tetrachloride from the reactor.

9. A process as claimed in claim 8, wherein the recovery stage is performed at a temperature of from about 150 °C to about 400 °C, preferably from about 160 °C to about 350 °C, more preferably from about 160 °C to about 250 °C.

10. A process as claimed in claim 8 or claim 9, wherein, in the recovery stage, at least 10 wt.%, preferably at least 25 wt.%, more preferably at least 40 wt.% of the total sodium aluminium tetrachloride is retained in the reactor for the first dosing stage.

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