GB2337515A - Removing fluoride impurity from sodium carbonate - Google Patents

Abstract

A method of purifying anhydrous soda ash containing sodium fluoride as an impurity so as substantially to remove the sodium fluoride, e.g. to level less than 0.03% w/w comprises the steps of (i) milling the crude, anhydrous soda ash; (ii) admixing the milled, crude anhydrous soda ash with a saturated solution of sodium carbonate under conditions forming a heated admixture at a temperature which is below about 112‹C and which provides formation of sodium carbonate monohydrate crystals; (iii) effecting a separation by size of sodium fluoride and the sodium carbonate monohydrate crystals; (iv) heating an aqueous slurry of the sodium monohydrate crystals from (iii) to a temperature above about 112‹C at which the sodium carbonate monohydrate crystals are converted into the anhydrous form; (v) cooling the slurry to a temperature below about 112‹C so as to convert the anhydrous crystals from step (iv) to sodium carbonate monohydrate crystals; (vi) effecting a separation by size of the sodium carbonate monohydrate crystals from step (v) from sodium fluoride, and (vii) recovering the sodium carbonate monohydrate crystals.

Description

1 2337515 REMOVAL OF FLUORIDE IMPURITIES FROM SODA ASH The present

invention relates to the removal of fluoride impurities from soda ash (sodium carbonate) and more particularly but not exclusively to a process for the benefication of fluoride-containing trona for the production of pure soda ash therefrom.

Trona is a mineral comprising sodium sesquicarbonate (Na2C03.NaHCO3.2H20) and is found in a number of deposits around the world, e.g. in Wyoming (U.S.A.), Magadi (Kenya), Tanzania (Lake Natron), Turkey, Venezuela and Egypt.

Trona is used for the manufacture of soda ash by well established techniques which generally comprise milling the trona (which has been mined or dredged depending on the location of the trona deposit), and calcining the crushed material to produce soda ash. The product (soda ash) thus produced generally incorporates a few percent of impurities which were present in the original trona deposit and is therefore less pure than the product of the Ammonia Soda process for producing sodium carbonate. Various purification processes have been proposed (see below) for producing a purer form of trona-derived soda ash. Nevertheless, the impurities do not adversely influence a number of uses of the soda ash (e. g. steel making, glass making) and therefore trona-derived soda ash can generally be used without further purification.

It is a characteristic of the trona deposits found in Africa, and particularly with the deposit at Magadi (Kenya), that they incorporate sodium fluoride as barnacle-like encrustations on the trona crystals. The encrustations comprise large numbers of small individual crystals of sodium fluoride and the encrustations degrade into their constituent crystals during the initial milling of the trona. The sodium fluoride is 2 present in the soda ash produced from the trona at a level of 0.5% to 3% (more typically 1 % to 1. 5 %) by weight.

This sodium fluoride does not adversely affect processes in which the soda ash is used but since many of these processes (e.g. steel making, glass making) involve high temperatures the sodium fluoride gives rise to fluoride containing emissions which are undesirable from the environmental point of view. (This disadvantage is not associated with soda derived from major trona deposits in other parts of the world (e.g. Wyoming, USA) since sodium fluoride is not present in such deposits).

There is therefore a need for a process capable of being used in the production of soda ash from trona containing sodium fluoride so as to remove the latter impurity compound.

As indicated above, various purification processes which may be applied to the production of soda ash from trona are already known.

US-A-2 887 360 (Hoekje) discloses a process for purifying sodium carbonate (e.g. as obtained from trona) by removing specific soluble components therefrom. The process comprises forming an aqueous slurry of sodium carbonate monohydrate and heating the slurry to a temperature (usually above 112T) such that the "nondissolved" hydrated sodium carbonate suspended in the slurry is converted substantially completely to anhydrous sodium carbonate, subsequent to which the slurry is cooled (usually to a temperature of 3 5'C to less than 1 12'C) to produce once again a slurry of sodium carbonate monohydrate crystals. These crystals are then separated from the mother liquor to produce a higher purity product than the starting materials. Examples of impurities which may be removed by this method are stated to include sodium chloride, sodium sulphate and boron compounds but there is no reference to the removal of sodium fluoride. Furthermore the purification of soda ash obtained from a trona deposit containing sodium fluoride is not described.

3 US-A-3 498 744 (Frint et al) discloses a process for producing soda ash from trona by calcining and milling the latter to produce anhydrous crude "soda ash" which is then added to a hot, saturated solution of sodium carbonate whereupon the anhydrous product dissolves and - recrystallises as a hydrated form of sodium carbonate (preferably the monohydrate). During this stage, very fine insoluble matter (muds and shales) is released. The hydrated sodium carbonate may be classified whereby most of the insoluble components are removed from the mother liquor. There is however no disclosure in this prior U.S. patent that the process described therein can be used for the removal of sodium fluoride. Furthermore the only specific trona disclosed in Wyoming trona which as indicated above does not contain sodium fluoride as an impurity.

GB-A-1 502 850 (Allied Chemical Corporation) discloses a process similar to that of US-A-3 498 744 for removing sodium fluoride present as in impurity in sodium carbonate. More specifically, GB-A-1 502 850 proposes that sodium carbonate obtained by calcination of trona obtained from the Lake Magadi trona deposit may be treated (to reduce sodium fluoride level) by adding the sodiu-m carbonate to an aqueous solution which is saturated in both sodium carbonate and sodium fluoride resulting in the formation of crystalline sodium carbonate monohydrate and sodium fluoride solids. Subsequently the crystalline sodium carbonate monohydrate and sodium fluoride solids are separated from each other, e.g. by classification. However as disclosed in the Example of GB-A-1 520 850 the resultant "purified" sodium carbonate still contains 0.4% by weight of sodium fluoride. Such a fluoride level is not acceptable if the soda ash is to be used in glass making since the resulting fluoride-containing emissions contravene current environmental legislation. Thus new glass- making plants require a soda ash containing less than 0.033% fluoride to meet current international standards.

4 WO-A-9622398 (Environmental Projects, Inc) discloses a method of purifying trona by calcining to produce a crude product, obtaining various size fractions from the latter and treating at least some of the fractions using a dissolution and crystallisation process similar to that disclosed in US-A-3 498 744 (Frint) - see above. WO-A-9622398 proposes that this dissolution/crystallisation process may be modified by heating the sodium monohydrate crystals obtained to a temperature above about 11 2T to convert the monohydrate crystals to the anhydrous form subsequent to which the anhydrous crystals are cooled for conversion back to the monohydrate form. It is however stated in WO-A-9622398 that the additional step of heating to above 112T and subsequent cooling is preferably not adopted since it is believed that sufficient purity can be obtained without the extra crystallisation step. Furthermore, there is no specific disclosure in WO-A-9622398 of application of the modified dissolution/crystallisation process to trona from a deposit containing sodium fluoride as impurity. Additionally there is no disclosure of application of the process of WOA-9622398 to removal of fluoride impurities.

According to the present invention there is provided a method of purifying anhydrous soda ash containing sodium fluoride as an impurity, the method comprising the steps of (i) milling the crude, anhydrous soda ash; (ii) admixing the milled, crude anhydrous soda ash with a saturated solution of sodium carbonate under conditions forming a heated admixture at a temperature which is below about 112T and which provides formation of sodium carbonate monohydrate crystals; (iii) effecting a separation by size of sodium fluoride and the sodium carbonate monohydrate crystals; (iv) heating an aqueous slurry of the sodium monohydrate crystals from (iii) to a temperature above about 112'C at which the sodium carbonate monohydrate crystals are converted into the anhydrous form; (v) cooling the slurry to a temperature below about 112T so as to convert the anhydrous crystals from step (iv) to sodium carbonate monohydrate crystals; (vi) from step (v) from sodium fluoride, and effecting a separation by size of the sodium carbonate monohydrate crystals (vii) recovering the sodium carbonate monohydrate crystals.

The invention has been based on our finding that sodium fluoride impurity in soda ash may be substantially completely removed by the procedure of steps (i) to (vii). More particularly we have found that the crystallisation properties of sodium fluoride are such that step (ii) yields a slurry in which a proportion of the sodium fluoride is present as insoluble crystals of a size sufficiently different from that of the sodium carbonate monohydrate whereby this proportion of the sodium fluoride may be readily separated on the basis of size, e.g. using conventional classification techniques. Substantially the remainder of the sodium fluoride is subsequently removed by employing steps (iv) to (vi).

The invention is particularly applicable to the purification of crude soda ash obtained by calcining trona from a fluoride-containing deposit thereof (e.g. the Magadi deposit in Kenya or the deposits from Lake Natron in Tanzania). As applied to such a crude soda ash containing 0.5% to 3% (e.g. 1% to 1.5%) by weight of sodium fluoride impurity it is possible to obtain soda ash having a purity greater than 99.8%.

6 If the method of the invention is to be applied to the purification of crude soda ash obtained from a naturally occurring trona deposit then it is preferred that the trona be calcined with a small amount (e.g. 0.3% by weight) of oxidising agent such as sodium nitrate to destroy organic matter. This is conventional practice in the processing of trona.

In step (i) of the process, a milling operation is effected so that sodium carbonate and sodium fluoride degrade into their constituent crystals. We prefer that the milling be effected to ensure a size below 150gm although for optimum removal of sodium fluoride milling should be effected to be a size less than 1 0Ogm.

After the milling operation, it is preferred that the milled material is heated to a temperature of 1OWC - 20WC (preferably about 200') prior to being subjected to step (ii) to enhance the reactivity of the soda ash feed.

Step (ii) of the process of the invention is based on the procedure disclosed in US-A-3 498 744 (Frint) the disclosure of which is incorporated herein by reference. The purpose of step (ii) is to grow relatively large (e.g. at least 250-30Ogm mean size) crystals of sodium carbonate monohydrate so that they may be separated, by size, from the smaller sodium fluoride crystals (and other fine insoluble impurities),

The parameters of the process as disclosed in US-A-3 498 744 are applicable mutatis mutandis to step (ii) of the present process.

It is preferred that, in step (ii), that the admixture of the crude anhydrous soda ash and the saturated solution of sodium carbonate is heated to (or is otherwise at) a temperature above 35'C (but below 112 'C) to ensure formation of sodium carbonate monohydrate. The size of the monohydrate crystals produced is determined by residence time, slurry density and circulation rate in the crystalliser in which the process is effected. We prefer that the slurry density be in the range 30-50%w/w 7 sodium carbonate monohydrate, more preferably 40-50%w/w and ideally about 45%w/w. The residence time in the crystalliser is preferably 2 to 4 hours.

The slurry from step (ii) is then subjected (in step (iii)) to a separation by size so that sodium fluoride crystals are separated from the larger sodium carbonate monohydrate crystals. This separation may be effected by a conventional classification technique. We prefer to use an inclined screw classifier preferably adopting back-washing with a saturated sodium carbonate solution. The screw classifier is preferably operated to produce a "cake" in which the amount of residual liquor is in the range 30 to 35%w/w.

Steps (iv) and (v) of the process of the invention are based on the procedure disclosed in US-A-2 887 360 (Hoekje), the disclosure of which is incorporated herein by reference. We do however prefer to use lower slurry densities than those disclosed in US-A-2 887 360. Thus, for example, whereas this prior specification specifies that the sodium carbonate slurry to be treated should contain at least 33%, and usually 4060% by weight of sodium carbonate monohydrate, we prefer in step (iv) that the slurry contains less than 35%w/w of sodium carbonate monohydrate. Preferably the slurry contains 15 to 30%w/w, and more preferably about 20%w/w, by weight of the sodium carbonate monohydrate and is heated in an autoclave to a temperature in the range of 112'C to 150'C (more preferably 115-120'C). This process results in the conversion of sodium carbonate monohydrate to anhydrous sodium carbonate which, in step (v), is once again converted back into the monohydrate form.

It is preferred that step (v) is effected at a temperature of 95-100'C and using a crystal residence time of at least 1.5 hours (preferably about 3 hours) and a slurry density of 15% to 30%w/w monohydrate (preferably about 20%) The product of step (v) is then subjected to a size separation operation to remove further remaining sodium fluoride and fine impurities from the larger 8 monohydrate crystals. This separation may be a classification technique as described above for step (iii).

The process of the invention may include at least one repeat of steps (iv)- (vi) to achieve the required degree of purity for the final product.

Sodium carbonate monohydrate crystals may be recovered (step (vii)) using conventional techniques, e.g. a centrifuge. A washing operation may be effected in the centrifuge to remove fluoride impurity dissolved in the mother liquor. If desired, the recovered sodium carbonate monohydrate may be calcined to produce anhydrous soda ash.

The process as described above is capable of producing sodium carbonate monohydrate (or calcined soda ash produced therefrom) having a sodium fluoride content less than 0.03%w/w.

The invention will be further described by way of example only with reference to the accompanying Fig 1 which is a flowchart illustrating the method of the present invention.

Fig 1 illustrates a process for producing sodium carbonate monohydrate substantially free of sodium fluoride from a feed stock comprised of socalled "calcined ash" obtained by calcining trona extracted from a sodium fluoride containing trona deposit.

In the process of Fig 1, the calcined ash initially admixed with ca 0.3% sodium nitrate and re-calcined in a rotary calciner 1 with a residence time greater then 10 minutes and a discharge temperature of 400-500'C in order to destroy organic material which could interfere with the subsequent crystallisation stages of the process (producing badly-shaped crystals and a poor colour product).

9 After leaving the rotary calciner 1, the ash is fed to a pin-mill 2 in which it is milled to a size of less than 10Ogm. This milling operation ensures that sodium fluoride can be efficiently removed in the subsequent stages of the process (as described more fully below) and also that the surface area, and hence reactivity of the ash is increased.

The milled ash is then passed via a heated screw-feeder 3 in which the temperature of the ash is raised from ambient to 100-200'C, preferably ca 200'C to enhance the reactivity of the soda ash. From the heater 3, the ash is fed at a controlled rate into a well-mixed crystalliser vessel 4 containing a mother liquor (supplied from a mother liquor tank-not shown) comprising Na2C03, NaCI, Na2SO, and NaF. This liquor is saturated with respect to sodium carbonate and sodium fluoride and is typically at a temperature of 90'C. The levels of sodium sulphate and sodium chloride are determined by the levels in the feed stock and the purge rate from the system. It is in the crystalliser 4 that step (ii) as discussed above is effected.

By way of example, the working volume of the crystalliser 4 may be 150 litres and adequate mixing ensured by incorporating an equal-area draft tube (not shown) with a marine impellor.

Within the crystalliser 4, relatively large crystals of sodium carbonate monohydrate are grown. The size of the crystals is determined by the residence time, slurry density and circulation rate in the crystalliser. We prefer that the sodium carbonate monohydrate crystals are grown to a mean size of at least 250-30Ogm and this may be achieved using a slurry density of 30-50%w/w, preferably 40-50% w/w (e.g. ca 45%w/w) with a residence time of 2 to 4 hours. The size of the thus formed monohydrate crystals (250-30Ogm) is to be contrasted with the size of the sodium fluoride crystals(< 1 0Ogm).

The solid (heated) ash is fed into the ascending flow in the outer annulus (defined by the draft tube) of the vessel and removed via an outlet at an angle of 45' to the wall. The solids feed rate to the crystalliser will typically be 0.7kg/min. The rate of the removal of the slurry may be controlled by a timer linked to a ball-valve.

From the crystalliser 4, the slurry discharges into a screw classifier 5 operating at a speed sufficient to deliquor the crystals of sodium carbonate monohydrate. The contents of the screw classifier 5 are backwashed with a saturated sodium carbonate liquor from the clarified mother liquor stock tank in order to remove much of the fine solids, especially the sodium fluoride. The purity of the wet solids which are in the form of a cake discharging from the screw of the classifier 5 depends of the efficiency of the solid-liquid separation and on the washing efficiency. Typically the amount of residual liquor in the cake is in the range 30 to 35% w/w.

The cake from classifier 5 is then passed to a tank 6 in which it is slurried with clarified mother liquor which is saturated with respect to sodium carbonate and sodium fluoride and which is typically at a temperature at least 5'C below the boiling point of the mother liquor but preferably greater than 60'C. Ideally the temperature may be about 90'C. Within the tank 6, the solids concentration is typically in the range 20 to 30% w/w (preferably toward the lower end of this range) in order to prevent scaling and blockages.

From tank 6, the slurry comprising the sodium carbonate monohydrate crystals is fed forward to three autoclaves 7 in series (only one autoclave being shown) in which the temperature is raised progressively to 11 5-120'C. Each autoclave 7 may, for example, have a capacity of 20 litres and be stirred by a twin marine impellor operating at 950rpm. to prevent formation of scale without pulverising the crystals. The total residence time of the crystals in the three autoclave vessels may be about 30 minutes. Within the autoclaves 7, the sodium carbonate monohydrate crystals are converted into anhydrous form.

11 The slurry from the autoclaves is fed into a forced circulation crystalliser 8 in such a manner that the feed does not flash. The crystalliser preferably operates at a temperature in the range 9 5 - 1 OOT and converts the anhydrous product from autoclave 7 back to sodium carbonate monohydrate. The residence time in the crystalliser 8 determines both the particle size distribution and the shape of the final product. We prefer to employ a minimum residence time of 3.0 hours.

Slurry from crystalliser 8 is discharged at a controlled rate using a Moyno pump (not shown) into a second inclined screw classifier 9 which, like classifier 5, is back-washed with clarified mother liquor. Within classifier 9, the sodium carbonate monohydrate crystals are deliquored. Typically, the residual liquor content of the crystals discharged from classifier 9 is in the range 30 to 35% w/w.

To obtain a further degree of purification, the sodium carbonate monohydrate discharged from classifier 9 may be subjected to a repeat of the process described in relation to that occurring in plant components 69 (i.e. conversion into the anhydrous form and re-conversion into the monohydrate form). This repeat process is effected using plant components 10-13 in which the same process steps are carried out as for components 69.

Deliquored sodium carbonate monohydrate crystals discharged from classifier 13 is then passed to tank 14 where they are resuspended in clarified mother liquor (saturated with respect to sodium carbonate and sodium fluoride) to form a 25% w/w slurry which is then fed to a pusher centrifuge 15 for a final deliquoring' operation. The crystals may be washed during the final centrifuging operation and the crystals leaving the centrifuge may have a residual liquor content of about 5% w/w.

12 Using the procedure as described, the sodium carbonate monohydrate discharged from the centrifuge 15 may have a sodium fluoride content less than 0.03% w/w.

The monohydrate crystals from centrifuge 15 may be calcined in a proprietary furnace to give a soda ash with a purity of greater than 99. 8%.

Examples of the purity and form of soda ash which can be produced by the process as described above are as follows:

Total alkalinity Sodium fluoride Sodium chloride Sodium sulphate Insolubles (in water) Angle of repose Hardness Pouring Density Median size > 99.8 %w/w < 0.03 %w/w < 0.03 %w/w < 0.02 %wlw < 0.07 %w/w 30 0 6.5 % 1.033 kg/m' 241 ptm The invention will be illustrated by the following non-limiting Examples EXAMPLE1

Magadi Industrial Grade ash containing 1.2% sodium fluoride, 0.3% sodium sulphate, 0.2% sodium chloride and 0.4% of water-insoluble matter was blended with 0.3% sodium nitrate, and recalcined for ten minutes in a rotary calciner with a discharge temperature of 400-4500C. The freshlycalcined ash was then milled to -10Optm in a pin-mill, and stored in drums. This material was then passed into a heated screw feeder, in which its temperature was raised from ambient to 200'C.

13 The soda ash was then fed at a rate of 0.7kg/h into a well-mixed crystalliser containing a mother liquor comprising Na2C03, NaCI, Na2S04 and NaF at a temperature of WC. The liquor was saturated with respect to Na2C03 and NaF The working volume of the crystalliser was 150 litres. Adequate mixing was ensured by use of an equal-area draft tube and a marine impellor. The crystal residence time was 2 hours, and the slurry density was 45% w/w sodium carbonate monohydrate. The median size of the crystals so formed was 260gm, and the amount of material smaller than 10Ogm was 5.5%.

The slurry was discharged into an inclined screw classifier, the contents of which were back-washed with a saturated sodium carbonate liquor from the clarified mother liquor stock tank in order to remove much of the fine solids, especially the sodium fluoride. The amount of residual liquor in the cake was 35% w/w, and the insoluble fluoride content was 0. 43% NaE The solids from the screw classifier were slurried with clarified mother liquor saturated with respect to sodium carbonate and sodium fluoride at a temperature of WC. The slurry was then fed to three autoclaves in series, in which the temperature was raised progressively to 115-120'C. The vessels each had a capacity of 20 litres, and were stirred by a twin marine impellor. The total residence time of the crystals in these vessels was 30 minutes.

The slurry from the autoclaves was fed into the rising leg of an externallypumped forced-circulation crystalliser operating at 95-100'C. The crystal residence time was 1.7 hours, and the slurry density was 20% wlw.

The slurry was then discharged into a second inclined screw classifier in order to dewater the crystals, and back-washed with clarified mother liquor. The residual 14 liquor content of the crystals leaving this unit was 32% w/w, and the insoluble fluoride content was 0.20% NaE The autoclave recrystallisation, the recrystallisation of the anhydrous sodium carbonate and the screw classification stages were then repeated a further two times in order to achieve the desired degree of purification.

The dewatered crystals were then re-suspended in clarified mother liquor saturated with respect to sodium carbonate and sodium fluoride, and fed as a 25% w/w slurry to a pusher centrifuge, where they were dewatered to a residual liquor content of 5% w/w. The median size of the final sodium carbonate crystals was 220gm. The crystals were washed during the centrifuging operation, and then calcined. The median size of the final sodium carbonate crystals was 220im. The sodium fluoride content of the crystals was <0.03%.

Finally, the monohydrate crystals were calcined in a proprietary furnace to give a soda ash with a purity of >99.8%.

Pilot, bench scale and laboratory work have all indicated that optimisation of the operating parameters on each of the above stages can give improvements in monhydrate recovery and purity at each stage. Example demonstartes such an improvement for the first stage of the process.

EXAMPLE2

The first stage of the process described above was repeated, but this time using a slurry density of 40% instead of 45% w/w in the first of the crystallisation stages. The median size of the crystals fell from 260 to 220gm, the -10Ogm fraction increased to 7.7% and the sodium fluorride conetnet (of this product of this first stage) was 0.35%.

Claims (30)

1. A method of purifying anhydrous soda ash containing sodium fluoride as an impurity, the method comprising the-steps of (i) milling the crude, anhydrous soda ash; (ii) admixing the milled, crude anhydrous soda ash with a saturated solution of sodium carbonate under conditions forming a heated admixture at a temperature which is below about 112T and which provides formation of sodium carbonate monohydrate crystals; (iii) effecting a separation by size of sodium fluoride and the sodium carbonate monohydrate crystals; (iv) heating an aqueous slurry of the sodium monohydrate crystals from (iii) to a temperature above about 112T at which the sodium carbonate monohydrate crystfils are converted into the anhydrous form; (v) cooling the slurry to a temperature below about 112T so as to convert the anhydrous crystals from step (iv) to sodium carbonate monohydrate crystals; (vi) effecting a separation by size of the sodium carbonate monohydrate crystals from step (v) from sodium fluoride, and (vii) recovering the sodium carbonate monohydrate crystals.

2. A method as claimed in claim 1 wherein the anhydrous soda ash is obtained by calcining trona obtained from a sodium fluoride containing deposit thereof.

16 A method as claimed in claim 2 wherein the trona is from the Magadi deposit in Kenya.

4 A method as claimed in any one of claims 1 to 3 wherein the crude soda ash contains 0.5% to 3% by weight of sodium fluoride.

A method as claimed in claim 4 wherein the crude soda ash contains 1% to 1.5% by weight of sodium fluoride.

6. A method as claimed in any one of claims 1 to 5 wherein, in step (i), the crude, anhydrous soda ash is milled to a size less than 150pim.

7. A method as claimed in claim 6 wherein the crude, anhydrous soda ash is milled to a size less than 1 0OVim.

8. A method as claimed in anyone of claims 1 to 7 wherein the milled material from step (i) is heated to a temperature of 1 OOT - 200T prior to step (ii).

9. A method as claimed in any one of claims 1 to 7 wherein the admixture of the crude anhydrous soda ash and the saturated solution of sodium carbonate is heated to (or is otherwise at) a temperature above 3 5T and below 11 2T.

10. A method as claimed in any one of claims 1 to 9 wherein the slurry density in step (ii) is in the range 30-50%w/w sodium carbonate monohydrate.

11. A method as claimed in claim 10 wherein, in step (ii), the slurry density is in the range 40-50%w/w sodium carbonate monohydrate.

12. A method as claimed in claim 11 wherein, in step (ii) the slurry density is about 45%w/w sodium carbonate monohydrate.

13. A method as claimed in any one of claims 1 to 12 wherein the crystal residence time in step (ii) is 2 to 4 hours.

14. A method as claimed in any one of claims 1 to 13 wherein, in step (ii) the sodium carbonate monchydrate crystals grow to a mean size of at least 250-300im.

15. A method as claimed in any one of claims 1 to 14 wherein step (iii) is effected using an inclined screw classifier.

16. A method as claimed in claim 15 wherein the screw classifier is backwashed with a saturated sodium carbonate solution.

17. A method as claimed in claim 15 or 16 wherein the screw classifier is operated to produce a "cake" in which the amount of residual liquor is in the range 25 to 40%w/w.

18. A method as claimed in any one of claims 1 to 16 wherein, in step (iv) the slurry contains 15 to 30%w/w sodium carbonate monohydrate.

19. A method as claimed in any one of claims 1 to 18 wherein, in step (iv) the slurry is heated to a temperature in the range 1 12T to 1 SOT.

20. A method as claimed in claim 19 wherein, in step (iv) the slurry is heated to a temperature in the range 11 5T to 120T.

21. A method as claimed in any one of claims 1 to 20 wherein, in step (v), the slurry is at a temperature of 95-100T.

18

22. A method as claimed in any one of claims 1 to 21 wherein, in step (v), the crystal residence time is at least 1.5 hours.

23. A method as claimed in claim 22 wherein, in step (v), the crystal residence time is about 3 hours.

24. A method as claimed in any one of claims 1 to 23 wherein, in step (v), the slurry density is 15% to 30% w/w of the sodium carbonate monohydrate.

25. A method as claimed in claim 24 wherein, in step (v) the slurry density is about 20% w/w monchydrate.

26. A method as claimed in any one of claims 1 to 25 wherein step (vi) is effected using an inclined screw classifier.

27. A method as claimed in claim 26 wherein the screw classifier is backwashed with a saturated solution containing sodium carbonate.

28. A method as claimed in any one of claims 1 to 27 which includes at least one repeat of steps (iv)-(vi).

29. A method as claimed in any one of claims 1 to 28 wherein step (vii) is effected by centrifugation.

30. A method as claimed in any one of claims 1 to 29 comprising the further step of calcining the sodium carbonate monohydrate crystals form step (vii) to form calcined ash.