EP0092562A1

EP0092562A1 - Chlorination of an aluminous material

Info

Publication number: EP0092562A1
Application number: EP82903251A
Authority: EP
Inventors: Anthony John Wickens; John Harry Wallice Turner
Original assignee: Mineral Process Licensing Corp BV
Current assignee: Mineral Process Licensing Corp BV
Priority date: 1981-10-28
Filing date: 1982-10-27
Publication date: 1983-11-02
Also published as: ZA827753B; WO1983001612A1; IT1158027B; IT8249375A0

Abstract

Procédé multi-étapes de production d'un chlorure d'aluminium par chloruration d'un matériau alumineux, par exemple de bauxite, de préférence en utilisant du chlore et de l'oxyde de carbone ou phosgène, consistant à condenser le produit gazeux, à séparer un mélange d'aluminium et de chlorures ferriques des chlorures d'autres éléments présents dans le matériau alumineux et de l'anhydride carbonique, à séparer le chlorure d'aluminium du chlorure ferrique au moyen d'une réduction sélective, à séparer le chlorure ferreux solide et à récupérer du chlorure d'aluminium sensiblement exempt de fer dans la phase de vapeur permettant d'alimenter une cellule électrolytique, et à oxyder le chlorure ferreux pour produire de l'oxyde de fer et du chlore pouvant être exploités industriellement, le chlorure étant recyclé vers l'étape de chloruration.A multi-step process for producing an aluminum chloride by chlorinating an aluminous material, for example bauxite, preferably using chlorine and carbon monoxide or phosgene, consisting in condensing the gaseous product, to separating a mixture of aluminum and ferric chlorides from the chlorides of other elements present in the aluminous material and carbon dioxide, separating the aluminum chloride from the ferric chloride by means of selective reduction, separating the chloride ferrous solid and to recover aluminum chloride substantially free of iron in the vapor phase to power an electrolytic cell, and to oxidize ferrous chloride to produce iron oxide and chlorine that can be exploited industrially, the chloride being recycled to the chlorination step.

Description

TITLE: Chlorination of an aluminous material

This invention relates to a process for the chlorination of an aluminous material, particularly bauxite. More specifically this invention relates to a process for the chlorination of an aluminous material such as bauxite to give aluminium chloride of sufficient purity for it to be suitable for use as the cell feed for the electrolysis of aluminium chloride to give aluminium metal. According to another aspect the invention provides a process whereby by-products of the primary production of aluminium chloride from an aluminous material such as bauxite may be easily recovered and, if desired, converted into valuable components for recycling.

The absence of satisfactory industrial processes for the separation of iron chloride from aluminium chloride and for recovering the chlorine values^ from iron chloride has discouraged the use of aluminous material such as bauxite as the raw material for the manufacture of aluminium chloride for subsequent electrolysis to aluminium metal. Instead of chlorinating bauxite, most aluminium producers use the non-chloride route to obtain aluminium metal. The one company known to be using the chloride route to aluminium metal operates a process involving chlorination of alumina recovered from bauxite by the energy intensive and costly Bayer process in a substantially pure form. Using this relatively high cost feed it is doubtful whether the additional though lesser cost of the carbochlorination process to produce aluminium chloride suitable for use as a cell feed can be justified. For this reason the

O F aluminium industry as a whole lacks the incentive to change to the chloride route for the production of aluminium metal.

Apart from the economical and technical dis- advantages of chlorinating the alumina produced by the Bayer route instead of directly chlorinating bauxite, there is the serious environmental disadvantage -of producing and having to dispose of the large quantities of red mud arising as a by-product from the Bayer process. Accordingly, for economic and environmental reasons there is the need for the direct production of a technically acceptable grade of aluminium chloride from an aluminous material such as bauxite by a process which also allows for the separation of the resulting associated iron chlorides and the recovery of chlorine values from the iron chloride thus separated.

Carbochlorination of bauxite to produce aluminium chloride (for uses other than as electrolytic cell- feed) has been fairly widely used industrially over the past 60 years or so. The best known examples are the processes employing the treatment of bauxite with chlorine and carbon, or with mixtures of chlorine and carbon monoxide or phosgene. These processes, however, relied on sources of bauxite with a very low content of iron oxide. In the case of the process employing mixtures of chlorine and carbon monoxide or phosgene, pure gibbsite was used when supplies of bauxite low in iron oxide were no longer available. This would not be economically acceptable for the production of aluminium chloride for use as electrolytic cell-feed.

A disadvantage of an earlier proposal is the lack of any satisfactory step either for the separation of iron chlorides from aluminium chloride, or for the recovery of chloride values from the iron chloride in conjunction with the conversion of the iron residue to an environmentally acceptable or saleable form. One suggested method for the removal of iron chloride from the aluminium chloride has involved the use of metallic aluminium to reduce ferric chloride, FeCl_ , to ferrous

5 chloride, FeCl₂, which would deposit from the vapour phase leaving gaseous aluminium chloride, AlCl^, of sufficient purity to be fed to the electrolytic cell. This method, however, has obvious economic disadvantages, in that it effectively reduced the yield of aluminium

10 obtained from the electrolytic cell.

We have now found a process for the -chlor¬ ination of an aluminous. material, particularly bauxite, to give aluminium chloride of sufficient purity that it* is suitable for use as the cell feed for electroylsis.

15 The present invention thus provides a multi¬ stage process for the production of aluminium chloride which comprises

(A) chlorinating an aluminous material, for example, bauxite, preferably using chlorine and carbon monoxide or

20 phosgene or a mixture thereof;

(B) subjecting the resulting gaseous product to con¬ densation whereby a mixture of aluminium and ferric chlorides is separated from the chlorides of other elements present in the aluminous material, such as

25 titanium and silicon tetrachlorides, and from carbon dioxide;

(C) separating the aluminium chloride from the ferric chloride by subjecting the mixture to selective reduction, separating solid ferrous chloride and

30 recovering substantially iron-free aluminium chloride

"<u in the vapour phase suitable for feeding to an ^* electrolytic cell, and

(D) oxidising the resulting ferrous chloride with oxygen or an oxygen-containing gas at an elevated

35 temperature to produce iron oxide and chlorine and

c:_ recycling the chlorine to stage (A) .

The selective reduction in stage (C) whereby aluminium chloride is separated from ferric chloride may be accomplished using carbon monoxide as the reducing agent or metallic iron.

The process according to the invention is particularly economical in that reactants required for one stage of the multi-stage process can be generated from another stage. For example, according to one particular embodiment carbon monoxide which is preferably employed in the first stage (A) for the chlorination of the aluminous material is obtained in a further reaction stage (E) by reaction of the carbon dioxide off-gas from the condensation stage with a suitable source of carbon at _an elevated temperature. The carbon monoxide obtained from the carbon dioxide off-gas in stage (Ξ) may also be used in a subsequent reaction stage, (C) according to one embodiment, where the aluminium and ferric chlorides are separated by reduction with carbon monoxide.

According to the invention, the ferrous chloride resulting from the separation of the aluminium chloride is subjected to a further reaction stage (D) where the ferrous chloride is oxidised with oxygen or an oxygen-containing gas at an ^'elevated temperature preferably in the presence of at least one salt of an alkali metal or alkaline earth metal. This oxidation reaction produces iron oxide and chlorine which is recycled to the initial chlorination stage (A) . More¬ over, the oxidation reaction of stage (D) may be operated under conditions such that the iron oxide produced is in the form of an industrially useful form of iron oxide suitable, for example, for pigmentary application or for use as a filler.

Cl-fr According to a still further example, where separation of the ferric and aluminium chlorides in stage (C) is accomplished using carbon monoxide one of the by-products of the reduction is phosgene, COCl-. which can conveniently be recycled for use in the chlorination stage (A) .

In accordance with another possibility the iron oxide, or a part thereof, obtained in stage (D) , by dechlorination of ferrous chloride, may be subjected to further reduction in stage (F) , for example using carbon monoxide, to give the metal which may be recycled to stage (C) when the separation of the ferric and aluminium chloride (C) is accomplished using metallic iron. The carbon monoxide required for this reaction may be obtained from the carbon monoxide generation stage (E) as previously described.

Thus the present invention not only provides a process for the production of aluminium chloride suitable for use as feed for an electrolysis cell but also provides a process whereby chlorine values can be recovered from iron chloride by-product and used in the initial chlorination stage of the invention. The invention moreover provides a multi-stage process whereby reactants -such as carbon monoxide, phosgene and metallic iron can be generated from one process stage and recycled for use in another stage. The invention also lends itself to the production of a potentially valuable by-product, iron oxide.

The overall process according to the invention thus avoids the disadvantages of the prior proposals discussed hereinbefore and has certain additional advantages which make the process particularly economical and industrially feasible.

The invention is described by way of example

-j≥ ff with reference to the accompanying flowsheets illustrated in Figures 1 and 2 which show two embodiments of the process according to the invention; and with reference to Figures 3 and 4 which illustrate apparatus suitable for carrying out the reactions of stages (C) and (D) respectively. In these embodiments bauxite is the aluminous material used as starting material.

With reference to Figure 1, the process comprises the following reaction stages:- Stage A

In a first stage (A) bauxite (identified as BX _.in the flowsheets) is chlorinated using chlorination reagents which comprise chlorine together with carbon monoxide, or phosgene (C0C1-) , or mixtures of chlorine, carbon. onoxide and phosgene. Chlorin¬ ation of the alumina content of the bauxite then occurs according to the following chemical equations:

1₂0 + 3CO + 3C1₂ > 2A1C1₃ + 3C0_2< . (1)

A1₂0₃ + 3C0C1₂ 2A1C1₃ ÷ 3CO₂..(2)

The Fe^O.., TiO- and SiO- contents of the bauxite, are similarly chlorinated to give FeCl,, TiCl₄ and SiCl. respectively.

The chlorination reaction may be carried out in either a fixed-bed type of reactor, as has been de¬ scribed in the literature for example by Wurster (Zeitschrift fur Angewandte Chemie 1930, 43, 877-880) and Hille and Durrwachter (Angewandte Chemie, I960, pp. 73-79) , or it may be performed in a fluid-bed reactor. In order both to increase the reactivity of the bauxite and to remove water which, on subsequent chlorination, could lead to the loss of some chlorine as HCl, the bauxite is^'preferably calcined prior to the chlorination

* step. The temperature chosen for calcination will depend to some extent on the increase in reactivity and decrease in water content desired, but is likely to be in the temperature range 600 - 1100°C. The temperature employed for chlorination will depend on the reaction rate obtainable, compared with the service life of containment refractories, but will generally fall within the range 550°C-1100°C.. The chlorination reaction may be carried out batchwise or continuously. Stage B

In a second stage (B) some of the chlorides by-produced in the first stage (A) , and particularly titanium and silicon tetrachlorides (ii) , are removed by condensation. The product gases leaving the chlorination reactor (Stage A) comprise a mixture of carbon dioxide, aluminium chloride (AlCl,) , ferric chloride (FeCl₃) , titanium tetrachloride (TiCl.) , and silicon tetrachloride (SiCl.) . These gases pass to Stage (B) , which is a two- stage condenser, the operating principle of which is based on the difference in volatilities between FeCl, (boiling-point 315°C) , AlCl., (Sublimation temperature 177.8°C), TiCl₄ (boiling-point 136.4°C) and SiCl₄ (boiling-point 57.6°C). Due to the well-known vapour-phase association between FeCl., and AlCl., to give FeAlCl_fi, it is not possible to separate these two species by a simple condensation step. Accordingly, the first stage of the condenser is maintained at a temperature below 177.8°C (the sublimation temperature of AlCl,) , but substantially above the boiling-point of TiCl. (136.4°C). This results in the removal of both (i) FeCl, and AlCl., from the vapour-phase, whilst the (ii) TiCl₄ , SiCl₄ and carbon dioxide pass to the second stage of the condenser. The first stage of the condenser may take any suitable form. A preferred form of condenser particularly suitable for the continuous deposition in solid form of FeCl,- and AlCl,-containing species from the vapour-phase comprises a fluidised- bed of a suitable inert material, maintained at a suitable temperature to ensure substantially complete removal of the FeCl, and AlCl,-containing species from the vapour-phase. With this type of condenser, continuous removal of the product can be achieved by maintaining a continous bed overflow. The second stage of the condenser may again take any suitable form, and is maintained at a temperature low enough to ensure substantially complete removal of the remaining chlorides, TiCl. and SiCl^,, from the vapour-phase. Condensation of TiCl₄ and SiCl. as a liquid mixture may be achieved by maintaining the temperature of the second-stage substantially below the boiling-point of SiCl. (57.6°C). Alternatively separation of TiCl. and SiCl_d may be provided for by operating the second stage of the condenser at a temperature within the range between the boiling-points fo SiCl. and iCl^ (57.6°C and 136.4°C respectively) with a number of fractionating stages.

The output from Stage (B) therefore comprises three distinct product streams, viz. (i) a solid mixture of AlCl, and FeCl., from the first stage, (ii) liquid TiCl. and SiCl^ from the second stage, and (iii) carbon dioxide gas from the second stage. Stage C

In a third stage (C) , ferric chloride is separated from the desired aluminium chloride. Separation of the FeCl, from the AlCl, is accomplished by chemical reduction of the FeCl, to F Cl₂, which has a much lower volatility than FeCl, and therefore can be separated by deposition as a solid

C from the vapour-phase. In this embodiment of the invention, the reducing agent used is carbon monoxide. The reduction step is represented by the following chemical equation:

CO + 2FeCl₃ -> C0C1₂ + 2FeCl₂ (3)

The reaction may be carried out in the vapour-phase, at a temperature. above the boiling-point of FeCl, (315°C) but below the melting-point of eCl₂ (673°C) . Within this temperature range, reduction of FeCl, to FeCl- according to equation (3) results in the deposition of FeCl- in solid form from the vapour-phase, leaving substantially iron-free AlCl₃ in the vapour-phase, suitable for feeding to the electrolytic cell.

The feasibility of the reaction according to equation (3) above was demonstrated using the apparatus shown in Fig. 3. Carbon monoxide was passed via tube 1 into a charge of ferric chloride 2 held in a silica crucible 3 by means of silica wool plugs 9 mounted in a silica envelope 4. The envelope 4 was fitted with a sheathed thermocouple 5 inserted into the ferric chloride bed 2 and a side-arm 6 leading to a condenser

7 and gas absorption train (not shown) . The silica envelope 4 and side-arm 6 were heated by means of electrically-powered heating tape

8 wrapped around the -outer surfaces, the resultant FeCl, bed temperature being monitored by the thermo¬ couple 5. The experimental procedure was as follows:- Carbon monoxide gas from a gas cylinder was passed at a metered flow-rate via tube 1 through the FeCl, charge 2, whilst supplying power to the heating tape 8 to bring the FeCl, bed temperature up to the desired value. The run was allowed to proceed at a constant -temperature for a timed period, after which the CO flow was stopped and the apparatus allowed to cool to room temperature in a stream of dry nitrogen, after which the contents of the reaction bed and condenser were removed and analysed. The results of a typical run are given below:

Bed Temperature 320°C

CO flow-rate 500 ml/min

Initial wt. FeCl, charge: 25g

10 Wt. deposit in condenser after run : 18.02g

Analysis of the condenser deposit showed that 89.6% by weight of the total iron content was present as ferrous iron, 10.4% was present as ferric iron, i.e. ca. 80%

,15 conversion of FeCl3, to FeCl2, had been achieved.

The required feed of gaseous FeCl, and

AlCl, for reaction (3) may be obtained by the vaporisation of the mixed condensed FeCl, - AlCl, solids from Stage (B) , and feeding the resulting vapour

20 stream into a suitable reactor into which carbon monoxide is also fed. The desired reactor configuration may take several possible forms. A particularly suitable form is a cyclone reactor, into which the carbon monoxide gas and mixed FeCl- - AlCl, vapours

25 are fed tangentially. This reactor configuration provides for continuous operation, with good mixing of the reacting gases and a relatively long residence time per unit of reaction volume, and facilitates the discharge of the solid FeCl- from the bottom of the

30 reactor.

Alternatively, depending on the desired rate of reduction and the preferred method of handling the solid FeCl₃" AlCl, product from Stage (B) , Reaction 3 may also be performed by contacting carbon ,_ monoxide gas with the solid FeCl., -AlCl,, followed by

OI..PI heating of the solid product FeCl- - AlCl, to sublime the AlCl, leaving solid FeCl₂ behind. This method of conducting the reaction is particularly suitable when the solid FeCl, - AlCl, from Stage B is obtained as the overflow from a fluidised-bed condenser. The over¬ flow material, comprising a suitable inert bed material coated with the solid mixture of FeCl, and A1C1₃, is transferred to the reduction reactor which again comprises a fluidised-bed, fluidisation being provided by the flow upwards through the bed of the reactant carbon monoxide. The overflow from the fluidised- bed reduction reactor, comprising a solid mixutre of FeCl- and AlCl,, is then subjected to a further heating stage to sublime the AlCl, in substantially iron- free form.

Under some conditions, it may be found that sublimation of the bed overflow material from the fluid bed reduction of solid FeCl, - AlCl, with carbon monoxide gives AlCl, with a somewhat higher iron ^' content than the AlCl, from the vapour-phase reaction described earlier. Should this be the case, the sublimate from the bed overflow may be passed through a tube packed with iron wire or the like to remove residual traces of FeCl, which might be carried over in the sublimate. Alternatively, the AlCl, sublimate could be passed through and used to fluidise.a bed of metallic iron produced, according to the proposals in Stage F (described hereinafter with reference to the flowsheet. Figure 2) . It may be preferable to initiate the reaction exemplified in Equation (3) , whether performed as a vapour-solid reaction or as a homogeneous vapour-phase reaction, by the presence of sulphur or a sulphur chloride, as described in U.S. Patent 4,140,746. The separation of FeCl, from A1C1₃ according to this invention based on Equation (3) has the advantage, not found with other methods of^* separating FeCl, from AlCl,, of providing phosgene, C0C1₂, as a useful by-product for recycle to the bauxite chlorination reactor (Stage A). Moreover, the requirement of a source of carbon monoxide for use as the reducing agent is met, as will be described later under Stage (E) , by the passage of carbon dioxide (obtained from Stage (A) and after passing through the two-stage condenser (Stage B) over heated carbon. The method of separating FeCl, from AlCl, according to this embodiment of the invention is therefore eminently suitable when seen in the context of the overall process, the aim of which is to provide AlCl, of eletroclytic cell-feed purity, starting with the chlorination of an ^"aluminous material such as bauxite. Not only does the separation step recover part of the chlorine content of the FeCl, in useable form, but is also precludes the need to use expensive reducing agents, such as metallic sodium, zinc or aluminium,which are not obtainable as by-products from any of the various Process Stages. Stage D

In Stage (D) , chlorine values are recovered from the ferrous chloride produced in Stage (C) , and, recycled to the chlorination Stage (A) . The fourth Stage (D) may also be used to by-produce an industrially useful form of iron oxide.

Thus, the ferrous chloride by-product from Stage (C) is subjected in Stage (D) to oxidation with oxygen or an oxygen-containing -gas at an elevated temperature in the presence of at least one salt of an alkali metal or alkaline earth metal.

The preferred salts are those with which ferrous chloride forms melts which are most suitable to meet the particular requirements of the oxidation process

O-.-PI tøfe VV....11..00 used. Sodium chloride is one of the preferred salts and is used to exemplify, in the following reaction equations, the nature of the oxidation reaction. However, other salts may also be used to satisfy the specific requirements of the process according to the invention and to achieve the specific properties which may be required of the iron oxide produced as a co- product. Examples of other suitable salts include sodium sulphate, sodium carbonate, sodium metaborate, sodium tetraborate, sodium chlorate, potassium chloride, potassium carbonate, lithium chloride, magnesium chloride, magnesium sulphate, calcium carbonate and calcium chloride and mixtures thereof. The oxidation reaction according to the present invention is postulated in the following equations using, as example, sodium chloride as the salt.

6NaCl + 6FeCl₂ - 6NaCl.FeCl₂ (4) (MP 801°C) (MP 673°C) (MP 419°C)

+ 43_0₂ -> 6NaCl + 3Fe₂0₃ +

6C1₂ (5) or, perhaps, in overlapping stages, thus:

6NaCl + 6FeCl₂ ^*> 6NaCl.FeCl₂ + 1%0₂ ^*> 2NaCl + Fe₂0₃ +

4NaCl.FeCl_ (6)

+ 30₂ -> 6NaCl + 3Fe₂0₃ +

6C1₂ (7)

The recovery of chlorine values from FeCl- using the oxidation process of Stage (D) has been demonstrated on a laboratory-scale using the apparatus shown schematically in Fig. 4.

The apparatus comprises a gas-tight silica envelope 1 having a silica crucible base 2 and being

_CHH provided with a tube 3 for input of oxidising gas (0₂ or air) , a sheathed thermocouple 4 and outlet 5 for the C1-/0- off-gases. The apparatus is adapted to be heated by means of a resistance-wound tube furnace 6. In a typical run, a mixed charge of FeCl- and an alkali (or alkaline) earth metal salt 7 was melted and brought up to the desired reaction temperature under nitrogen in the gas-tight silica envelope 1, which was heated by the resistance-wound tube-furnace 6. The melt temperature was measured by means of the sheathed thermocouple 4 immersed in the melt 7. Oxidation was then commenced by bubbling oxygen or air through the melt via tube 3 at a measured flow-rate for a timed period, the off-gas 5 being scrubbed with sodium hydroxide solution. At the end of the run the reactor and contents were allowed to cool under nitrogen, followed by water leaching and chemical analysis of the reaction products.

The results of a batch oxidation run performed in the apparatus shown in Fig. 4 using a melt of molar composition 0.5 FeCl-, 0.52 NaCl, 0.48 N ₂S0. are shown in the following Table 1. The results show the excellent chlorine recovery (91%) attained. The melt composition selected for this run is particularly suitable for batch operation in a simple sparged fused salt-bath reactor, since the added alkali-metal salts (NaCl and Na-SO_d) are present in the ratio corresponding to the NaCl-Na-SO, eutectic composition (eutectic temperature 625°C) . This permits substantially complete oxidation of the FeCl- content to give Cl_ and Fe-0, without solidification of the reactants, at a temperature substantially below the melting-points of NaCl or Na_?S0₄ (801°C and 884°C, respectively) . TABLE 1

Charge wt . ( g) 267. 9 0. .52 NaCl,

Oxidation time (mins ) 300 ^'

Temperature ( °C) 657

0 flow-rate (ml/min) 300

Total wt Cl₂ evolved (g) 53.33

Theoretical wt. Cl- for 56.68 100% conversion (g

% Cl- recovered 91

Wt. Fe-0_ recovered (g) 62.3

The iron oxide co-product of the oxidation reaction was separated off by a simple water-leaching procedure, followed by filtration, water-washing and drying. If desired, the soluble salts can be recovered for re-cycling. Further water-washing of the iron oxide is needed to remove traces of absorbed chloride-. This can be achieved by elutriation with water which can be used to classify the product into fractions varying in particle size and suitable for different pigmentary applications or, for example, for use as a filler.

Thus, the present invention not only provides a method of recovering chlorine from iron chloride which chlorine can be recycled to the bauxite chlorination Stage (A) , but also provides a method of producing a potentially industrially useful iron oxide. By virtue of the present invention a by-product which has to date caused considerable problems of disposal is transformed into a readily available starting material for two valuable industrial products.

The optimum reaction conditions for Stage (D) of the process depend, among other things on the particular salt which is employed. The oxidation reaction may be effected in the melt or on the particul¬ ate solids.

Furthermore, by appropriate selection of salt composition, temperature and oxygen flow rate, it is possible to achieve a balance, between the rate of melting of the salt mix with the iron chloride and the rate of oxidation of the mixed salt melt to solid iron oxide and gaseous chlorine. This makes it possible to select reaction conditions which combine the advantages of both the liquid melt and particulate solid processes and minimise the disadvantages of both. Stage E

In this process stage the^' carbon monoxide which is required both for the bauxite chlorination stage (Stage A) and for the separation of FeCl, and A1C1₃ (Stage C) is generated by the reaction of the carbon at an elevated temperature, according to the chemical equation:

C0₂+C ^** 2C0 (8)

A suitable source of carbon is coke or coal char.

The carbon monoxide-forming reaction as represented by equation (8) is endothermic. Various methods are availalbe for supplying the necessary heat, for example the coke or coal char bed, which may be either a fixed-bed or a fluidised-bed, may be heated directly by inserting electrodes into the charge and passing on electric current through the charge, as has been described, for example, for a fluidised-bed reactor in U.S. Patent No. 2,921,840, and for a fixed-bed reactor in U.K. Patent No. 001,657. An alternative method of supplying process heat is to admit a mixture of oxygen and carbon dioxide to the carbon monoxide generator, the oxygen reacting exothermically with the coke or coal char according to the equations:

C + 0₂ -> C0₂ (9)

2C + 0₂ 2C0 (10)

This method of operation has been described in ^"J.K. Patent No. 521,415. Generally, in order to obtain satisfactory carbon dioxide conversions to carbon monoxide by admitting oxygen with the carbon dioxide, the carbon monoxide .generator must be operated at a temperature high enough to cause fusion or slagging of the ash from the carbon source employed, as 5nas been described,^" for example, by von Frederscorff and Elliott in "Chemistry of Coal Utilisation, Supplement Volume" (John Wiley & Sons, New York, 1963).

A second embodiment of the invention s illustrated with reference to Figure 2. In this embodiment. Stages (A) (chlorination of bauxite), B (condensation of titanium and silicon tetracr.lorides) , D (dechlorination of ferrous chloride) and Ξ (generation of carbon monoxide from carbon dioxide and carbon) are as described in the first embodiment of the invention described with reference to Figure 1. However,. Stage (C) is affected using metallic iron to separate hs aluminium and iron chlorides and an additional stage (Stage F) is employed in which ferric oxide is reduced with carbon monoxide to give metallic iron. 5zis embodiment may advantageously be employed whe for any reason it is preferred not to recycle phosgene

^' 5—>' (COCl_?) to Stage (A) (chlorination of bauxite) . Stage (C) of this embodiment and the addition Stage (F) are as described below. Stage F

In this stage ferric oxide is reduced with carbon monoxide to give metallic iron. The underlying chemical principle of this stage is the well-known ability of carbon monoxide to reduce ferric oxide to metallic iron according to the chemical equation:

Fe₂0 + 3C0 3C0₂ + 2Fe (11)

The reaction is carried out at an elevated temperature. A description of various investigations of the reaction as represented by Equation (11) , which forms the basis of the production of iron from iron ore in a blast furnace, is given in Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Volume XIII (Part 2) _r p. 813. At a reaction temperature of ca. 905°C, essentially complete reduction of ferric oxide to metallic iron occurs.

As incorporated in the second embodiment of this invention, part of the ferric oxide from Stage (D) (dechlorination of ferrous chloride) is fed to a reduction reactor which may take any suitable form e.g. a fixed- or fluidised-bed, where reaction occurs according to Equation (11) with carbon monoxide provided from Stage (E) (generation of carbon monoxide from carbon dioxide and carbon) . If a fluidised-bed reactor is employed it is possible to provide for a continuous feed of ferric oxide and a continuous bed overflow of metallic iron. The metallic iron, which is produced in extremely reactive form, is fed to Stage (C) in which FeCl, and AlCl₃ are separated using metallic iron as described hereinafter. The by-product carbon dioxide may be recycled to Stage (E) but is preferably put to stack . Stage G

As in the first embodiment, separation of the FeCl, from the AlCl,. is again accomplished by chemical reduction of the FeCl₃ to FeCl-, which has a much lower volatility than FeCl, and is therefore deposited as a solid from the vapour-phase. In this embodiment of the invention, the reducing agent used is metallic iron from Stage (F) (reduction of ferric oxide with CO to give metallic iron) . The reduction step is represented by the following chemical equation:

2FeCl₃ + Fe ^*? 3FeCl₂ (12)

As employed in this.embodiment of the invention, the reaction is advantageously carried out at a temperature above the boiling-point of eCl₃ (315°C) but below the melting-point of FeCl- (673°C) . Within this temperature range, reduction of FeCl, to FeCl- according to Equation (12) results in the deposition of FeCl- in solid form from the vapour-phase, leaving a substantially iron-free AlCl, vapour which is fed to the electrolytic cell for the production of aluminium metal. As in the first embodiment of this invention illustrated in Figure 1, the required feed of gaseous FeCl, andAlCl, for the reaction represented by Equation (12) may be obtained by the vaporisation of the mixed condensed FeCl₃ - AlCl, solids from Stage (B) , and feeding the resulting vapour stream into a suitable reactor where the vapour comes into contact with, and is reduced by, the metallic iron from Stage (F) . The desired reactor configuration may again take several possible forms, e.g. a fixed- or fluidised-bed. De- position of ferrous chloride in solid form may occur

o^y.ιι , Vvll-ϋ on, or adjacent to, the metallic iron. If a fluidised- bed type of reactor is used, continuous operation may be achieved by providing for a continuous solids over¬ flow from the fluidised-bed. The FeCl- by-product from the reaction is fed to Stage (D) (dechlorination of ferrous chloride) for conversion to iron oxide and recovery of elemental chlorine, the latter being recycled to Stage (A) (chlorination of bauxite) .

It will thus be appreciated from the fore- going descriptions of the two embodiments of the process, and the corresponding flowsheets shown in Figures 1 and 2, that this invention comprises a most skilful synthesis from several process stages to give an overall process particularly applicable to the production of electrolytic cell-feed aluminium chloride from the c.hlorination of bauxite.

The various process stages employed are organised, as indicated in the flowsheets in such a way that maximum use is made of the .by-products from each stage to provide the reactants for other essential process stages. Consequently the process as a whole provides imporant economic savings and environmental advantages when compared with known methods of producing aluminium chloride of sufficient purity to be used as the cell-feed for the electrolytic production of aluminium metal.

,- R Q-.- '

Claims

CLAIMS :

1. A process for the production of aluminium ^• chloride which comprises

(A) chlorinating an aluminous material;

(B) subjecting the resulting gaseous product to condensation whereby a mixture of aluminium and ferric chlorides is separated from the chlorides of other elements present in the aluminous material, and from carbon dioxide;

(C) separating the aluminium chloride from the ferric chloride by subjecting the mixture to selective reduction, separating solid. ferrous chloride and recovering substantially iron-free aluminium chloride in the vapour phase suitable for feeding to an electrolytic cell; and

(D) oxidising the resulting ferrous chloride with oxygen or an oxygen containing gas at an elevated temperature to produce iron oxide and chlorine and recycling the chlorine to stage^' (A) .

2. A process according to claim 1, wherein, in stage A, the chlorination is effected using chlorine and carbon monoxide or phosgene or a mixture thereof.

3. A process according to claim 1 or 2 , wherein, in stage C, aluminium chloride is separated from ferric chloride using carbon monoxide as a reducing agent.

4. A process according to claim 3, wherein phosgene produced in stage C is recycled to stage A for use in chlorinating the aluminous material.

5. A process according to claim 1 or 2 , wherein, in stage C of the process aluminium chloride is separated from ferric chloride using metallic iron as a reducing agent.

6. A process according to claim 1, wherein, in stage D, ferrous chloride is oxidised with oxygen or an oxygen-co taining gas at an elevated temperature in the presence of at least one salt of. an alkali metal or alkaline earth metal.

7. A process for the production of aluminium chloride which comprises:

(A) chlorinating bauxite in the presence of chlorine and carbon monoxide;

(B) subjecting the resulting gaseous product to condensation whereby a mixture of aluminium and ferric chlorides is separated from the chlorides of other elements present in the bauxite including titanium and silicon tetra.chlorides and from carbon dioxide;

(C) separating the aluminium chloride from the ferric chloride by subjecting the mixture to selective re¬ duction using carbon monoxide to produce phosgene and ferrous chloride, separating solid ferrous chloride and recovering substantially iron-free aluminium chloride in the vapour phase suitable for feeding to an electrolytic cell;

(D) oxidising the resulting ferrous chloride with oxygen or an oxygen-containing gas at an elevated temperature in the presence of at least one salt of an alkali or alkaline earth metal to produce iron oxide and chlorine and recycling the chlorine to stage A; and

(E) passing carbon dioxide produced in stage A over heated carbon and recycling the resulting carbon monoxide to stage A and/or to stage C.

8. A process for the production of aluminium chloride which comprises:

(A) chlorinating bauxite in the presence of chlorine and carbon monoxide;

o_.: VyV> ^V, (B) subjecting the resulting gaseous .product to con¬ densation whereby a mixture of aluminium and ferric chlorides is separated from the chlorides of other elements present in the bauxite including titanium and silicon tetrachlorides and from carbon dioxide;

(C) separating the aluminium chloride from the ferric chloride by subjecting the mixture to selective re¬ duction using metallic iron to produce ferrous chloride, separating solid ferrous chloride and recovering substantially iron-free aluminium chloride in the vapour phase suitable for feeding to an electrolytic cell;

(D) oxidising the resulting ferrous chloride with oxygen or an oxygen-containing gas at an elevated temperature in the presence of. at least one salt of an alkali or alkaline earth metal to produce iron oxide and chlorine and recycling the chlorine to stage A;

(E) passing carbon dioxide produced in stage A over heated carbon and recycling the resulting carbon monoxide to stage A and/or to stage F; and

(F) reducing the iron oxide produced in stage D with carbon monoxide to give metallic iron which is re¬ cycled to stage C.

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