WO1997007057A1

WO1997007057A1 - Method for producing anhydrous rare earth chlorides

Info

Publication number: WO1997007057A1
Application number: PCT/SE1996/001024
Authority: WO
Inventors: Johan SUNDSTRÖM
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-08-17
Filing date: 1996-08-19
Publication date: 1997-02-27
Anticipated expiration: 1998-02-17
Also published as: AU6760696A; SE9502865D0

Abstract

Method for producing essentially pure and anhydrous rare earth metal halides by dehydration of their hydrated salts, characterized by the continuous or batch-wise feeding of hydrated rare earth halides in a fluidised bed system comprising one reactor or several reactors coupled in series; introducing a gaseous drying agent at an elevated temperature causing dehydration of hydrated rare earth halides and obtaining rare earth halides of specified maximum water content free from oxide impurities. The method may also comprise a terminal step connected to the fluidised bed system for desired water contents below 0.05 moles water per mole formula unit.

Description

METHOD FOR PRODUCING ANHYDROUS RARE EARTH CHLORIDES

The present invention relates to a method for producing anhydrous rare earth chlorides, and more particularly to a method for the dehydration of hydrated rare earth chlorides.

As rare earth metals or rare earths are denoted the elements 57 to 71 in group lllb of the periodic system, and usually element No. 21, scandium, and No. 39, yttrium, are also included. These metals are expected to be of a strongly increasing importance in the future, especially as alloying elements in intermetallic compounds. Already at present, the sales of Nd-Fe-B magnets amount to 625 million US dollars, and this amount is increasing by about 12-15 % per year. Other examples of uses for rare earth metals which are or are expected to become commercially important are La-Ni hydrides for i.a. electrical storage batteries and storage of hydrogen, and Tb-Dy-Fe magnetostrictive materials. The rare earth metals are also expected to become increasingly important as alloying elements in special alloys, especially then of aluminum, magnesium, titanium and high-strength steel. A further use for the rare earth elements is as a doping material for optical fibres.

Because of the relatively recent interest for metallurgical manufacturing processes for rare earth metals, there is as yet no clear picture of the commercially important manufacmring processes. The scientific and patent literature contains a very wide range of suggested processes, and it can be predicted that many of these processes will not become commercially interesting.

In the suggested processes for the preparation of rare earth metals, the starting material is usually a rare earth metal compound of the fluoride, oxide or chloride type. For the commercial production of metals, electrolysis of rare earth chlorides is probably the dominating memod at present. The rare earth metal chlorides have many advantageous properties which make their use as a raw material for the preparation of the metals interesting. However, the hygroscopic properties have been regarded as a difficult problem, as they require a pretreatment step for obtaining pure and anhydrous halides.

A rather important aspect of the industrial production of rare earth metals is the preceding recovery of raw materials for the metal winning process. Rare earth chlorides are attractive raw materials both for electrolytic and metallothermic processes. However, pure and anhydrous rare earth chlorides are not easily recovered. If a cheap and environ¬ mentally acceptable method for the production of rare earth chlorides is developed, extraction of rare earth metal by the chloride route will become very attractive.

Preparation of anhydrous rare earth chlorides has been performed from different starting material and with the use of a wide variety of different chlorinating agents (GMELIN HANDBOOK (1982), part C4a, Chapt. 11.2.6). However, most of the preparation techniques were designed for laboratories, and aspects about time, cost and environment were not regarded. With respect to industrial production, these aspects are very significant.

A conventionally known method practiced industrially is the direct chlorination of oxides by reaction with carbon and chlorine. However, this method is connected with the release of environmentally unacceptable chlorinated hydrocarbons. Furthermore, due to the high temperature used (1100-1200°C) reactor materials are severely corroded during operation. Corrosion of reactor materials together with contamination from reactant carbon result in a deterioration of the quality and purity of the rare earth chloride product.

Another method proposed for large scale production (Y.S. Kim, F. Planinsek, B. J. Beaudry and K. A. Gschneider, Jr. ; The Rare Earths in Modern Science and Technol¬ ogy, ed. by McCarthy, Rhyne and Silber, Vol. 2 (1980)) is the direct conversion of the rare earth oxide by reaction with ammonium chloride. However, large amounts of over- stoichiometric ammonium chloride had to be used in order to reach conversion degrees over 97% and sublimation of excess ammonium chloride in vacuum was a necessary post- treatment. The method seems to be inefficient with respect to time and extractant, particularly if a product of high purity is desired. In general, dehydration of hydrated rare earth chlorides is a rather difficult procedure because of the great amount of water that must be removed from the product by transport in the gas phase. Because of the occurrence of hydrolytic reactions between water and rare earth chloride, many dehydration operations have resulted in products highly contaminated with oxide chlorides and oxides. Admission of hydrogen chlorides in the gas phase has been used as a means to suppress hydrolytic reactions (G. Haeseler and F. Matthes; J. Less-Corn. Met., 1965, vol 9, pp 133-151). In EP-A-0 395 472, dehy¬ dration was performed by purging mixmres of dry air, hydrogen chloride and chlorine gas through a packed bed of rare earth chlorides. However, the method is considered to be inefficient for industrial purposes, mainly because of the batch- wise processing and the excessive reaction times necessary. Moreover, sigmficant amounts of chlorinating agents were consumed making the method less attractive. Another serious disadvantage of this method appears by the big temperature difference between the heating source (walls of the reactor) and die bulk of the heat consuming charge. This difference grows in magnimde if a higher heating rate is desired and for a larger reactor size. Since the reaction rate will be substantially higher in the wall vicinity, water vapour migrates out to colder parts where it eventually may condense and dissolve the chloride powder, thereby decreasing the reactive surface area of the charge and severely disturbing the production. This may be avoided by very slow heating rates but it leads to very long start-up times for a larger scale production. The lack of control of such a reactor also renders the ability to avoid hydrolytic reactions more difficult.

BRIEF DESCRIPTION OF THE INVENTION AND ITS OBJECT The object of the present invention is to produce pure and anhydrous rare earth chlorides with a high productivity, while minimizing the use of chlorinating reactants; minimizing the temperamre (thus minimizing energy consumption); minimizing the environmental effect and using as simple a process as possible. Another important object is to achieve process control and taking advantage of new fundamental knowledge on the dehydration kinetics of rare earth chlorides.

According to the present invention, there is provided a method for producing rare earth chlorides by dehydration of hydrated rare earth chlorides, characterized by the continuous or batch-wise feeding of hydrated rare earth chlorides in a fluidised bed system comprising one reactor or several reactors coupled in series; introducing dry gas com¬ posed of air, air/hydrogen chloride, nitrogen/hydrogen chloride, another inert gas/hydro¬ gen chloride or pure hydrogen chloride; at an elevated temperamre causing dehydration of hydrated rare earth chlorides and obtaining rare earth chlorides of specified maximum water content free from oxide impurities. The method may also comprise a terminal step connected to the fluidised bed system for desired water contents below z=0.05 (where z represents the number of moles of crystallized water per mole of formula unit). The dehydration in the terminal step may be conducted in various ways. In a batch-wise performance, which for example is preferred in the small scale production or in the alternately production of different products, the terminal step is preferably characterized by a packed bed reactor of rare earth chlorides slowly purged with gas of previously described composition. In a continuous performance, usually preferred in large scale productions, the terminal step is for example characterized by a rotary drum dryer. DESCRIPTION OF THE DRAWINGS

Figure 1. shows the estimated equilibrium water partial pressures for the dehy¬ dration of DyCl₃ ^• 6H₂O.

Figure 2. shows the batch- wise dehydration of DyCl₃ ^• H₂O of 220°C using a fluidised bed. Figure 3. is a block diagram showing an example of the fluidised bed system. DESCRIPTION OF THE CHEMICAL BACKGROUND TO THE CHOICE OF GAS COMPOSITION

Because of the addition of hydrogen chloride gas to the fluidising gas, hydrolytic reactions may be completely avoided, thereby making it possible to produce rare earth chlorides free from oxide impurities. The necessary partial pressure of hydrogen chloride is given by a certain equilibrium ratio, (PHcι^/PH2_θ)eq* which must be exceeded in order to avoid hydrolytic reactions. The magnimde of (PHcι H2_θ)eq **^s dependent upon the specific rare earth element, the reaction step of dehydration and the temperature. In general, the magnimde of (Pπci'^/PH2_θ)e_q increases with increasing atomic number of the lanthanide and with decreasing hydrated water content (z). However, thermodynamic equations describing this ratio as a function of the temperamre have only been established in some limited cases. Therefore, in most cases (PHcι^/PH2_θ)eq *^^{as t0 be} determined empirically by pilot plant tests. At higher temperamres ( >200°C) oxygen in air becomes increasingly reactive according to d e following reaction:

RC1₃ + 1/2 O₂ (g) → ROC1 + Cl₂ (g) (1)

However, if HCl is injected in excess amounts of what is needed to maintain the (PHCi PH2_θ)eq"^rati°' ^me above reaction is counteracted by d e following reaction: ROC1 + 2HC1 (g) → RC1₃ + H₂O (g) (2) Therefore, in order to avoid me use of excess amounts of hydrogen chloride and the production of Cl₂-gas (according to reaction 1) to the outlet gas, air should be avoided in cases where the reaction temperamre is higher than 200°C.

Fundamental research about the dehydration kinetics has revealed information of profound significance for me performance of the present method; it has been revealed that reactions proceed close to equilibrium in me fluidised bed even at high gas flow rates and low bed loads. Therefore, d e total reaction rate (moles of H₂O per mole rare earth and second) may be calculated by the following equation:

_m (1- H2O) where G represent the molar gas flow rate, p_m represents the molar bed load and p^o represents a value close to the equilibrium water partial pressure of the specific reaction.

Thus, it is possible to speed up die reaction rate of dehydration by simply increasing the temperamre and gas flow dirough the fluidised bed, which infers that a high productivity of die process may be achieved. However, in order to avoid hydrolysis, me temperature, which corresponds to a specific water partial pressure in die bed, should be below iat prescribed by me (P_Hcι _H2θ)_eq"^rati° ^anc *^e prevailing partial pressure of hydrogen chloride in the bed atmosphere. Therefore, a maximum temperamre (T^) and a maxi¬ mum rate of dehydration is reached when d e fluidised bed condition satisfies the prescri¬ bed equilibrium ratio of hydrolysis while using inlet fluidising gas composed of 100% of dry hydrogen chloride.

The fluidised bed has turned out to exhibit properties which are of great advanta¬ ge for die dehydration process. For example, a very high gas flow can be used in a fluidising bed, making it possible to speed up me reaction rate of dehydration to a great magnitude; me turbulent mixture of gas and particles in the bed has greatly improved the access of hydrogen chloride to d e reacting particle surfaces, thereby essentially improving me ability to avoid hydrolysis; and die isothermal state, which is closely attained in the fluidised bed, has made it possible to run the process in a controlled fashion. All told, dehydration in a fluidised bed offers ie possibility to combine high productivity with obtaining of high purity rare earth chlorides. DETAILED DESCRIPTION OF THE INVENTION

The dehydration may be performed in one or several stages of fluidised beds judged by d e dehydration scheme. Several investigations have been performed about me dehydration scheme of rare earth chlorides. Most of e rare earth chlorides precipitate from their samrated water solutions as hexahydrates (z=6), excluding La, Ce and Pr which precipitate as heptahydrates (z=7). The dehydration scheme from z=6 (or 7) to 1 may be composed of two or several steps occurring at appreciable rates between tempera¬ mres of 50 to 200°C. The last step from z=l to 0 occurs at appreciable rate between temperamres of 150 to 400°C. However, the specific temperature intervals of reaction are more narrow for the dehydration of d e individual rare earth chlorides. For example, me dehydration of DyCl₃- 6H₂O has been investigated by die means of fluidising bed technique, and d e following dehydration scheme was discovered: Step 1: DyCl₃ ^• 6H₂O → DyCl₃ ^• 3H₂O + 3H₂O (g) Step 2: DyCl₃ ^• 2H₂O → DyCl₃ ^• H₂O + 2H₂O (g) Step 3: DyCl₃ ^• H₂O → DyCl₃ + 2H₂O (g) The first step of reaction was proceeding at an appreciable rate between tempera¬ tures of 100 to 130°C, while during die second step diis temperamre interval was between 110 and 140°C, and for the third step between 220 and 250°C. Equilibrium water partial pressures were estimated for die three reactions (figure 1). Thereby it was possible to predict die reaction rates according to equation 3. Figure 2 shows die batch- wise dehydra- tion of DyCl₃* H₂O at 220°C using a fluidised bed. From z= l to about 0.05-0.10 the reaction rate is constant and given by equation 3, but subsequentiy it retards to a slow convergence towards z=0. This behaviour is similar for all rare earth chlorides. Appar- endy, during die last mode of reaction some slow rate mechanism controls the reaction rate, so at diis moment processing conditions should be adjusted accordingly, for example by slow purging of gas through a packed bed.

To d e group of rare earth chlorides belong all die landianides, viz. La-Lu and Y and Sc. The gas flow may be composed of a mixmre of dry air and hydrogen chloride, or nitrogen and hydrogen chloride, or anodier inert gas and hydrogen chloride or pure hydrogen chloride. Hydrated rare earth chlorides may be injected as a solution or preferentially as a well-defined particle fraction contaimng a minimum amount of dust. The invention may be carried into practice in various ways and some embodi¬ ments will now be described by way of example with reference to the accompanying drawings (performed as a block diagram in figure 3) showing a system for practising a me od according to die present invention.

The following embodiment comprises the dehydration of rare earth chlorides to pure and anhydrous chlorides.

As shown in figure 3, hydrated rare earth chloride feedstock is supplied at a predetermined rate from a storage tank by means of a hopper 1 to a reactor 10. Dry gas (preferably air), containing a minor content of hydrogen chloride necessary to avoid hydrolysis, is injected dirough a heater 5 to a temperamre necessary to keep a fluidised bed 13 at d e desired temperamre. By a transfer pipe 2, bed material is by a controlled rate transferred continuously into a reactor 11. A dry mixmre of an inert gas (preferably nitrogen) and hydrogen chloride having a partial pressure necessary to avoid hydrolysis is injected dirough a heater 6 to a temperamre necessary to keep a fluidised bed 14 at die desired temperature for the reaction of the last step of dehydration. Through a transfer pipe 3 bed material is by a controlled rate transferred continuously into a reactor 12, which constitutes the terminal step. The terminal step is preferably characterized by a rotary drum dryer. A further example of die terminal step is characterized by a packed bed tower dirough which dry gas of preferably die same composition as injected dirough d e heater 6 is slowly conducted in counter-current flow to die packed bed, which is heated from me outside walls of d e reactor to preferably d e same temperature as in the reactor 11. The final product is discharged by a controlled rate from a discharge pipe 4. Elutriated dust may be re-circulated (for example by CFB technique) or captured by (some) filters and recycled at an earlier stage in the process. Oudet gas from 7, 8 and 9 may be washed wid respect to hydrogen chloride to produce hydrochloric acid or alternatively, it may be dried (for example through some desiccator) and recirculated. The last alternative is preferred as hydrogen chloride is diereby not consumed in the system. In the first of the two successive reaction vessels, each one containing me fluidized bed of d e hydrated salts to be dehydrated, die average molar ratio between the water of hydration and die rare earth halide is maintained between 1 and 3, and in the second reaction vessel the average molar ratio is maintained not higher than 0.2. The temperature in the first reaction vessel is maintained between 50 and 200°C, preferably between 80 and 160°C, and in die second reaction vessel between 150 and 400°C, prefer¬ ably between 150 and 300°C.

When performing the dehydration steps in a single reaction vessel me average molar ratio between die water of hydration and d e rare earth metal halide is maintained no higher than 0.2 and the temperamre in me reaction vessel is maintained between 150 and 400°C, preferably between 150 and 300°C.

A iospheric pressure may be used in me reaction system, but die use of elevated or reduced pressure is widiin die scope of diis invention. The total gas pressure is preferably maintained between 10 kPa and 1 MPa, more preferably between atmospheric pressure and 300 kPa. The inside walls of reactors and gas conduits may be of stainless steel or some other resistant material. The outside walls of die fluidised bed reactors should be heated or isolated in order to minimize heat losses and avoid condensation of water vapour on the inside walls.

In a reaction system constructed as described above, hydrated rare earth metal chlorides are fed into fluidised bed reactors and made to fluidise by injected dry gas of a composition and a temperamre of pertinence to die reaction step of dehydration. Accordingly, dehydration occurs at a high rate and rare earth chlorides of specified maximum water content and free from oxide impurities are produced from die fluidised bed system. Also, in the present invention a last step may be implemented for desired water contents below 0.05 moles water per mole formula unit, for example in which a gas having about the same composition and temperamre as in ie preceding fluidised bed reactor, is by counter-current flow slowly passed dirough a reactor, for example a packed bed reactor.

The above process has been described witii reference to chlorides of rare earths and hydrogen chloride. However, it is to be understood diat the present invention is also applicable to the production of odier halides of rare earths and d e corresponding hydro¬ gen halide, except d e fluorides of rare earths, which do not form hydrates.

The present invention will now be illustrated with reference to the following non- limiting examples.

EXAMPLE 1

A Pyrex glass reaction vessel with an inner diameter of 10 cm and provided widi a silica filter for gas distribution, tiiereby constituting a fluidised bed reactor, was charged widi 500 g of DyCl₃ ^• 6H₂O powder of particle sizes below 150 μm. The external surface of the reaction vessel was isolated witii fibrous glass to avoid condensation on the inside wall.

The inlet gas, composed of dry air and 0.3 % HCl, was flowing at 30 l/min and making the powder to fluidise. When die inlet gas was heated to 300°C, d e temperamre of the fluidised bed rose steadily up to about 100 ^*C, where it converged into temperamre plateaus at about 120°C and 130°C, which were reflecting the heat consumed by die two first reaction steps of dehydration. After about 80 minutes from the start, a new rise of temperature occurred, which indicated tiiat the monohydrate had been obtained. The inlet gas was then heated to 350°C and its composition was changed to mtrogen, flowing at 25 l/min, and hydrogen chloride admitted to flow wid an increasing rate witii respect to bed temperamre. A temperamre plateau was reached at about 225°C. At tiiat temperature, hydrogen chloride was admitted to flow at 5.2 l/min, making up about 17 % of total inlet gas flowing at about 30 l/min. The bed temperamre was held constant for about 45 minutes, when a new rise of temperamre indicated the end of die reaction.

The dysprosium chloride powder dius produced had a total weight of 331 g, a water content of 0.08 moles water per mole formula unit (0.5 w%) and was free from oxide impurities. The total time of the reaction was 2 hours and 20 minutes. 8 % of the charged amount was carried away over d e freeboard. EXAMPLE 2

Example 1 was repeated except tiiat particle sizes were between 150 and 425 μm, and die inlet gas during d e first stage of dehydration (i.e. from z=6 to 1) was flowing at 45 l/min.

The monohydrate was obtained after about 50 minutes making up a total time of 1 hour and 50 minutes for d e whole reaction. The dysprosium chloride tiius produced had a total weight of 352 g, a water content of 0.09 moles per mole formula unit (0.6 w%) and was free from oxide impurities. 2 % of die charged amount was carried away over die freeboard.

EXAMPLE 3

Example 1 was repeated except tiiat 400 g of TbCl₃- 6H₂O of particle sizes between 180 and 355 μm was used. During the last stage of dehydration (z= l to 0) die inlet gas, containing 20 % hydrogen chloride, was heated to obtain a temperamre plateau at about 228°C.

The total time of die reaction was about 1 hour and 45 minutes. The terbium chloride thus produced had a total weight of 280 g, a water content of 0.08 moles per mole formula unit (0.5 w%) and was free from oxide impurities. 2 % of the charged amount was carried away over die freeboard. EXAMPLE 4

Example 1 was repeated except tiiat 40 g of DyCl₃- H₂O of particle sizes between 150 and 250 μm was used. The inside diameter of die fluidised bed reaction vessel was 6 cm. The inlet gas was pure hydrogen chloride and was admitted to flow at 5.2 l/min. The inlet gas was heated to obtain a temperamre plateau at about 240°C during about 10 minutes

The total time of the reaction was about 25 minutes. The dysprosium chloride dius produced had a total weight of 37 g, a water content of 0.07 moles per mole formula unit (0.5 w%) and was free from oxide impurities. 1 % of the charged amount was carried away over die freeboard. EXAMPLE 5

A fluidised bed system composed of two reaction vessels made of Pyrex glass of 6 cm inner diameter was continuously fed witii dysprosium chloride hexahydrate powder at a rate of 2 g/min. The particle sizes were between 150 and 355 μm. The outiets 2 and 3 in figure 3 were kept at levels so as to maintain bed loads of 53 g in the first reactor and 40 g in the second reactor at stationary conditions. A gas mixture of 99.5 % dry air and 0.5 % HCl was injected at 6 l/min and heated to keep die bed in d e first reactor at 130°C; while in the second reactor pure hydrogen chloride gas was flowing at 4.5 l/min and heated to keep d e bed temperamre at 240°C. At the outiet port, 4, dysprosium chloride of 0.11 moles hydrated water per mole formula unit (=0.73 w%) and free from oxide impurities was continuously collected at a rate of 1.4 g/min. EXAMPLE 6

2000 g of DyCl₃ powder of particle sizes below 425 μm and an average water content of about 0.10 moles water per mole formula unit was charged into a packed bed reactor of Pyrex glass widi an inner diameter of 10 cm. The inlet gas was composed of nitrogen, injected at 0.3 l/min, and HCl, injected at 0.1 l/min. By heating from die outside walls die temperature in the reactor reached 225-230°C and tiie total time of the reaction was 24 hours.

The dysprosium chloride thus produced had a total weight of 1987 g, a water content of less tiian 0.01 moles per mole formula unit (< 0.067 w%) and was free from oxide impurities.

Claims

1. A metiiod for producing essentially pure and anhydrous rare earth metal halides, except fluorides, by dehydration of dieir hydrated salts, wherein a bed of said hydrated salts is contacted with a gaseous drying agent at an elevated temperamre, characterized in at a) a bed of one or more hydrated halide salts of rare earth metals is arranged in a reaction vessel; b) a stream of said gaseous drying agent is injected at die bottom of said bed, and is caused to flow at an elevated temperature upwards dirough said bed at such a velocity and in such an amount as to cause the bed to fluidize, while dehydrating die hydrated salts in the bed; c) said stream of gaseous drying agent is maintained until die rare earth metal halides in die bed are essentially anhydrous; and d) essentially pure and anhydrous rare earth metal halide or halides are discharged from said reaction vessel.

2. A method according to claim 1, characterized in tiiat the gaseous drying agent consists of air, one or more inert gases, hydrogen halide or any mixture of these.

3. A method according to claim 1 or 2, characterized in that the partial pressure of hydrogen halide in die gaseous drying agent in any place in die fluidized bed is maintained higher than the value prescribed by the tiiermodynamic equilibrium for hydrolysis.

4. A metiiod according to any one of claims 1-3, characterized in that it is carried out in a batch-wise manner.

5. A method according to claim 4, characterized in tiiat the temperamre, velocity and/or composition of the gaseous drying agent are varied during the dehydration process, such that the temperamre of the bed is rising during the process.

6. A method according to claim 5, characterized in that the temperamre is raised to a value not exceeding 400°C.

7. A metiiod according to any one of claims 1-3, characterized in that it is carried out in a continuous manner, wherein particles or a solution of said hydrated salts are continuously charged into die reaction vessel, while essentially pure rare earth halide salt is continuously discharged from said reaction vessel.

8. A method according to claim 7, characterized in that the average molar ratio between the water of hydration and die rare earth metal halide in the reaction vessel is maintained no higher than 0.2.

9. A method according to claim 7 or 8, characterized in that the temperature in the reaction vessel is maintained between 150 and 400°C, preferably between 150 and 300°C.

10. A metiiod according to claim 7, characterized in that the fluidized bed is divided into a plurality of beds, which are arranged in series and witii each bed in its own reaction vessel, tiie particles or solution of the hydrated salts being continuously charged into the first reaction vessel and being subsequently transferred to die following reaction vessel or vessels, and die essentially pure and anhydrous rare earth metal halide is continuously discharged from d e final reaction vessel.

11. A method according to claim 10, characterized in tiiat the temperamre, velocity and/or composition of the gaseous drying agent are maintained at different values in die different reaction vessels.

12. A method according to any one of claims 10 and 11, characterized in that the dehydration process is carried out continuously in two successive reaction vessels, each one containing a fluidized bed of die hydrated salts to be dehydrated.

13. A method according to claim 12, characterized in tiiat the average molar ratio between the water of hydration and the rare earth halide is maintained between 1 and 3 in die first reaction vessel, and not higher than 0.2 in die second reaction vessel.

14. A metiiod according to claim 12 or 13, characterized in that the temperamre in the first reaction vessel is maintained between 50 and 200°C, and in d e second reaction vessel between 150 and 400°C.

15. A metiiod according to claim 14, characterized in that the temperamre in the first reaction vessel is maintained between 80 and 160°C, and in die second reaction vessel between 150 and 300°C.

16. A mediod according to any one of claims 1-15, characterized in that the total gas pressure in the reaction system is maintained between 10 kPa and 1 MPa.

17. A method according to claim 16, characterized in tiiat the total gas pressure in the reaction system is maintained between atmospheric pressure and 300 kPa.

18. A method according to any one of claims 1-17, characterized in tiiat the drying gas is recirculated and is dried before it is re-injected into the fluidized bed or beds.