AU2004280272A1 - Method for reducing fouling of heat exchangers in a Bayer circuit - Google Patents

Description

C"est votre traduction ! Informatique- Web Aronautique Automobile Techniqu, -Manml dutiliation M6dical - Pharmaceutique . ' Commercial - Marketing VERIFICATION OF TRANSLATION A.R.T. International - 26, rue Carnot 95410 Groslay, France hereby declares as follows: 1. That we are well acquainted with both the English and French languages, and 2. That the attached document is a true and correct translation made by us to the best of our knowledge and belief of: International patent application NO PCT/FR2004/002538 filed on October 8, 2004 Dated this 29th of March 2006 B.P. 18 95410 GROSLAY T61: 01.39.34.70.70 Fax: 01.39.34.70.77 01 S.A. au capital de 40 000 E - R.C.S. B 392 830 337 S1)10 ()CAIAifll AIV 1 METHOD FOR REDUCING FOULING OF HEAT EXCHANGERS IN A BAYER CIRCUIT Technical field The present invention relates to a method for reducing fouling of heat exchangers in the digestion circuit of the Bayer process. It relates more particularly to reducing the volume of siliceous deposits 5 in heat exchangers in respect of the aluminate liquor before it is directed to digest the bauxite, with evaporators and other heat exchangers treating the liquor alone, in particular prior to the mixing thereof with the bauxite. Exchangers that allow the slurry -resulting from 10 mixing the ground bauxite with the aluminate liquor (dry matter content typically above 50 g/l) to be heated up to the digestion temperature, are not therefore concerned. Prior art 15 The Bayer process is a well-known process for the production of aluminium trihydrate. It comprises a generally wet - grinding step for the bauxite, possibly a desilication step, then a step of digestion of said bauxite by a high concentration caustic soda liquor, 20 typically a sodium aluminate liquor, which will be called hereinafter digestion liquor. Digestion leads to the formation of a slurry, which, after dilution, is settled and filtered so as to separate the insoluble residues from the sodium aluminate liquor. The latter, 25 supersaturated in alumina, is then decomposed in the presence of a primer to form alumina trihydrate precipitates then, after separation of the precipitated alumina trihydrate, concentrated by evaporation so as to increase its caustic concentration and recycled as 30 digestion liquor.

2 It is well-known that the conditions for digesting the bauxite are to be adapted on the one hand according to the level of hydration and the mineralogical structure of the alumina contained in the ore and on the other hand 5 according to the nature and content of the impurities (particularly silica, iron oxides and titanium) present in said bauxite. The extraction yields of alumina that can be made soluble in the digestion liquor, expressed as a proportion of the alumina trihydrate contained in the 10 ore, may exceed 95%, if the unfavourable influence of some impurities, particularly silica, is well controlled. Silica may be present in the bauxite in a number of mineralogical forms differently soluble in the digestion liquor. Some mineralogical forms, of which kaolin (A1203, 15 2 SiO2, 2 H20) is the most widespread species, are made soluble during the digestion of the bauxite. The part of the silica present in the bauxite in one of these forms is commonly known as "reactive silica". The reactive silica is therefore made soluble in the digestion liquor 20 and the dissolved silica content reaches a level of supersaturation such that the silica re-precipitates in the form of slightly soluble sodium alumino-silicate. Good control of silica precipitation is therefore vital if it is required to produce alumina trihydrate that is 25 free from siliceous impurities. On the other hand, a high level of silica supersaturation in the liquor leads to significant fouling at some points in the Bayer process, particularly in the heat exchangers. This fouling causes deterioration 30 in the performance of the heat exchanger and thereby compels frequent stops for the equipment to be cleaned. Generally, cleaning means removing the siliceous scale by acid attack.

3 Problem posed Current desilication treatments are intended in the first place to prevent the trihydrate being polluted during decomposition. Most of these treatments, described 5 for example in the applications US 3 413 087 (Reynolds), US 4 426 363 (Sumitomo), US 5 118 484 and US 6 086 834 (Alcan), EP 0 203 873, EP 0 739 309 and EP 0 765 290 (Aluminium Pechiney), are applied to the liquor before and/or after digestion. They translate into the formation 10 of compounds, of the sodium alumino-silicate type, known as DSP (desilication products), which are then eliminated with the insoluble residues of the digestion (red mud). Despite the different desilication processes employed, the silica level is still too high in the 15 liquor passing through the heat exchangers. These significant supersaturation conditions lead to the formation of fouling deposits which reduce the efficiency of the heat transfer. Patent application WO 96/06043 (Comalco), dealing 20 more particularly with the double digestion of a boehmite rich bauxite, shows in passing that, in order to reduce the dangers of scaling, lime can be introduced into the spent liquor upstream of the heat exchangers intended to heat the liquor prior to boehmitic digestion. 25 US patent 5 415 782 (Nalco) describes a treatment for the spent liquor by the addition of biological additives of high molecular weight in order to control the deposition of desilication products on the surfaces of the equipment in a Bayer circuit. 30 The applicant has therefore sought an additional solution which allows a further reduction in the level of silica concentration of the liquor prior to digestion, at least in the heat exchangers treating the liquor in the digestion circuit.

4 Subject matter of the invention The subject matter of the invention is a Bayer process in which a calcium based compound belonging to 5 the group of tri-calcium aluminates is introduced into the aluminate liquor, said calcium based compound being introduced into said aluminate liquor between the exit thereof from the decomposition chain and the mixing thereof with the ground bauxite, upstream of a heat 10 exchanger intended to increase the temperature of said aluminate liquor. The group of tri-calcium aluminates brings together: o some hydrogarnets, which are hydroxylated silicates of general formula X3 Y2 (SiO4) 3 -x(OH) 4 X, 15 for which X=Ca and Y=Al; in the Bayer process, it is customary to represent these hydrogarnets by the following general formula: 3 XO, Y 203, (6-2k) H20, k SiO2 where X=Ca, Y=Al and where k is between 20 0 and 3, preferably between 0 and 2, still better between 0 and 1; the hydrogarnet corresponding to k=0 is hexahydrated tri calcium aluminate. o substituted hydrogarnets of the previous group, 25 for example by partial substitution of the calcium (up to 10% atomic) or of the aluminium (up to 20% atomic) by another metal: the calcium can be replaced by iron (ferrous ion), manganese or magnesium, the aluminium can be replaced by 30 chromium or iron (ferric ion); The point or points of introduction of said calcium based compound into the aluminate liquor is (are) located between the exit from the decomposition chain - at this point, the liquor, before being concentrated by 5 evaporation, is known as "spent liquor" - and the mixing with the bauxite - upstream of the input into the first digestion tank. To be effective in the exchanger, the addition must be made upstream or at the input of this 5 exchanger to the totality or to an aliquot presenting as large a proportion as possible of the aluminate flow that is to pass through said exchanger. Heat exchangers acting on the aluminate liquor before it is directed to digest the bauxite are of two 10 types at least: they may either be evaporators used to increase the caustic concentration of the spent liquor and to recycle it as digestion liquor, or heat exchangers that treat the liquor alone before it is mixed with the bauxite, particularly in order to heat it prior to 15 digestion. Any other type of exchanger, for example an exchanger used to cool only the aluminate liquor, could also be concerned by the invention. The compound is added preferably at a single point, for example upstream of the heat exchange device located 20 the most upstream from the part of the circuit between the exit from decomposition and the mixing with the bauxite. Generally speaking, the equipment comprises series of exchangers and it is advantageous to locate the introduction point of the calcium based compound upstream 25 of this series of exchangers, in other words either upstream of the evaporation chain, or upstream of the liquor preheating chain, if there is one. When provision is made to preheat only the liquor up to a temperature close to the digestion temperature, it may prove 30 advantageous to make the addition only at the input of this preheating chain. Said calcium based compound is introduced into the spent liquor or the digestion liquor preferably in a proportion of between 0.1 g CaO/litre of aluminate and 50 6 g CaO/litre of aluminate. A lower proportion would not allow any reduction in the risk of scaling. The upper limit is given mainly for economic reasons. Said calcium based compound reacts with a part of 5 the silica contained in the liquor and it is converted into a hydrogarnet with a higher silica content. Typically, a hydrogarnet introduced into the liquor with a k close to 0 is converted into a hydrogarnet that has a k close to 1. 10 The products of the reaction of said calcium based compound with the silica in solution in the liquor are carried off with the slurry into the digestion devices then into the liquid/solid separation devices (typically settling tanks). They are discharged with all the 15 insoluble residues of the digestion (red mud). The introduction of a tri-calcium aluminate formed outside the Bayer process or already formed at another point in the Bayer process is preferred to the introduction of lime into the aluminate liquor directly 20 upstream of the heat exchangers, as is shown in the application WO96/06043. Indeed, although lime reacts with the aluminate liquor to give compounds belonging to the tri-calcium aluminate family, these may lead to inappropriate depositions on the walls of the reactor. It 25 would indeed be necessary to control extremely carefully the quantity of lime to be added so as not to exceed a critical dry matter content of the liquor in the heat exchanger, beyond which there might well be a risk of exposure to excessive deposition or accelerated wear and 30 tear by abrasion of the pipes. On the other hand, this addition brings about an increase in the overall consumption of lime in the Bayer circuit since the solid products of the reaction, which are in the digestion liquor, are rapidly mixed with the bauxite and are 7 discharged with the red mud. In a preferred embodiment of the invention, use is made of the hydrogarnets formed in the safety filter through which the pregnant liquor passes before being directed to the decomposition chain. 5 At this stage, the liquor has less than 200 mg/l of solid particles that are difficult to filter. To facilitate filtration, lime or tri-calcium aluminate is poured into the pregnant liquor upstream of these filters. The additive is converted into a hydrogarnet which gradually 10 becomes enriched with silica. It is retained in the filter cake, removed and reintroduced into the spent liquor upstream of a heat exchanger. By virtue of the increase in temperature and the smaller alumina content, said hydrogarnet sees its silica absorbing efficiency 15 increase considerably. A product is thus used which hitherto was not valued and the overall consumption of lime in the Bayer circuit is not altered. The consequence of the reaction effect of said calcium based compound with a part of the silica 20 contained in the liquor is to reduce the equilibrium dissolved silica content in the exchanger. In fact, it is known that the rate of scaling is substantially proportionate to the square of the silica supersaturation, in other words the difference between 25 the actual concentration of dissolved silica and the equilibrium concentration. It is also known that the rate of scaling gets faster the higher the temperature prevailing within the liquor. The effect of the addition according to the invention of said calcium based compound 30 will therefore be more effective in relation to heat exchangers that operate at high temperature. The heat exchanger may be an evaporation device allowing the liquor to be concentrated in order to convert it into a digestion liquor. The first example 8 given below describes a Bayer process of the European type intended to digest a diaspore bauxite where the caustic concentration of the liquor moves through evaporation typically from 160 g of Na20 / litre of 5 aluminate to 240 g Na20 / litre of aluminate in evaporators operating at 145*C - 150 0 C. The heat exchanger may also be a preheating device, which allows the liquor to be brought to the appropriate pre digestion temperature, which may vary according to 10 the type of bauxite. The second example given below describes a Bayer process of the American type intended to digest a trihydrate bauxite where the liquor is heated separately in exchangers. In this particular case, it may prove to be more advantageous to introduce the calcium 15 based compound downstream of the evaporators but upstream of the liquor preheating exchangers, so that there is no loss by carrying off with the aliquot of the aluminate liquor sent for grinding, since the totality of the additive is then dedicated to preventing the scaling of 20 the exchangers which operate at a much higher temperature than the evaporators. As indicated above, use is made in a preferred embodiment of the invention of the hydrogarnets from the safety filter cake. The safety filter is fitted to the 25 overflow of the settling tanks, in other words to the pregnant aluminate liquor that is directed towards the decomposition chain. Lime or tri-calcium aluminate are commonly used, in this safety filter, as a filtration additive. This facilitates the filterability of the 30 residual sludge present in the overflow of the settling tanks and is found in the form of a hydrogarnet with a low content of silica in the filter cake. Instead of discharging said cake with the sludge, it is advantageous to recycle it by introducing it into the aluminate liquor 9 upstream of an exchanger. It is recycled preferably continuously in the form of a slurry having a solid content of about 1000 g/l. This filtration cake is recycled in its entirety, possibly partially (the 5 remainder being for example discharged with the solid residues from the digestion), the main thing being not to exceed a critical solid content of the liquor in the heat exchanger, beyond which there might be a danger of exposure to excessive deposition or accelerated wear and 10 tear by abrasion of the pipes. Preferably, for the liquor intended to pass through said exchanger, a solid content is targeted that is below 100 g/l. Preferably still, for greater security, efforts are made not to exceed 3 g/l. Figure 1 shows diagrammatically a method for 15 treating bauxite in the Bayer cycle that identifies the preferred points for the introduction according to the invention of said calcium based compound. Figure 2 shows diagrammatically a method for treating diaspore bauxite in the Bayer cycle in which the 20 safety filtration cake is introduced, according to a preferred embodiment of the invention, into an evaporator. Figure 3 shows diagrammatically a method for treating bauxite in the Bayer cycle in which the safety 25 filtration cake is introduced, according to a preferred embodiment of the invention, into the digestion liquor preheating chain. Detailed description of the invention 30 Figure 1 shows diagrammatically a typical Bayer cycle in which the bauxite 10 is ground by wet grinding (B), here in the presence of an aliquot L2 of the digestion liquor LO. An aliquot Ll, which represents the greatest part of the digestion liquor LO, typically more 10 than 80%, is not directed towards the wet grinding of the bauxite. Two cases arise: Case 1: only this aliquot is heated (L'1) in a pre 5 heating chain P'I1. The aliquot thus preheated (L'I10) is then mixed with the slurry S'l produced by wet grinding the bauxite and directed towards the digestion chain A. case 2: this aliquot is added (L''I1) to the slurry S''1 produced by wet grinding and therefore having a very 10 high solid content - in other words including a great number of solid particles - so as to obtain a less viscous slurry S''"10 which is preheated in a chain P''1. The slurry thus preheated (S''"100) is introduced into the digestion chain A. 15 Digestion generally occurs under pressure. The digestion chain then presents itself in the form of a series of autoclaves in which the slurry has to circulate. At the end of digestion, the slurry S2 is directed 20 towards a liquid/solid separation device (C), typically a settling tank in which the insoluble residues are separated by gravitation of the liquor: the insoluble residues are discharged in the form of red mud 20 while the overflow L3 still containing some particles is 25 subject to filtration F, so-called "red filtration" or "safety filtration". The filtrate L4 is the pregnant aluminate liquor that is directed towards the decomposition chain D to precipitate the alumina trihydrate 30. At the exit from the decomposition chain 30 D, the spent liquor L5 is concentrated by evaporation (E) so as to be redirected towards digestion (digestion liquor LO). According to the invention, a calcium based compound belonging to the group including lime and tri-calcium 11 aluminates can be introduced into the depleted liquor or into the digestion liquor, preferably at the introduction point 11, in other words into the spent liquor L5, before it is concentrated by evaporation to be converted into 5 digestion liquor or at the introduction point 11' into the aliquot L'1 of the digestion liquor, when this is to be preheated on its own (P'1). Example 1 (figure 2) 10 Example 1 deals with the recycling of the safety filtration cake upstream of evaporation. Figure 2 shows diagrammatically a method for treating diaspore bauxite in the Bayer cycle. Unless otherwise indicated, the different elements involved in 15 the Bayer cycle are given the same reference numbers as in figure 1. In this example, the slurry S''10, resulting from mixing the principal aliquot L''"l of the digestion liquor LO with the slurry S''"l produced by wet grinding (B) the diaspore bauxite 210, is preheated prior to 20 digestion (A). The digestion, intended to dissolve the diaspore (alumina monohydrate) contained in the bauxite is carried out at a high temperature (257 0 C) in an autoclave with a liquor with a typical caustic concentration of 240 g Na20/litre. After decomposition 25 (D), the spent liquor L5 has a caustic concentration of no more than 160 g Na20/litre and must be concentrated by evaporation in order to recover the digestion concentration (240 g Na20/litre). The chain of evaporators (E) employed for this 30 purpose operates at a temperature of between 145 and 1500C. Hexahydrated tri-calcium aluminate is used as a filtration additive 100 for safety filtration F. The pregnant liquor in fact comes from the overflow of the 12 settling tanks and still contains a certain quantity of solid particles. These generally prove to be highly agglutinative, and, in order not to block the weft of the filters, a filtration additive is added which alters the 5 physicochemical behaviour of these fine particles. Lime or tri-calcium aluminate is generally used, particularly hexahydrated tri-calcium aluminate. Safety filtration includes a number of filters that operate under pressure in parallel, one or more of them being taken out of the 10 circuit for removal of the filtration cake and cleaning. On average, a particle of filtration additive remains for about 4 hours in contact with the pregnant liquor and becomes a hydrogarnet, which gradually becomes enriched with silica. The filtration cake 111, intended until the 15 present invention to be discharged with the insoluble residues, is recovered and introduced upstream of the evaporation chain E. The advantage of the hydrogarnet addition can be felt on the one hand at evaporation (E), where the 20 increase in silica substitution leads to a reduction in the level of silica in the liquor, and on the other hand at digestion (A), when it reduces the quantity of DSP ("desilication products") formed, therefore the loss in caustic soda. 25 At the evaporator input, the liquor has a caustic concentration of 170g/l, a silica content (expressed as a percentage of caustic soda) equal to 0.732%ctq and a saturation index Rp (expressed by the weight relationship: 30 concentration of dissolved alumina (gAl203/l) equal to 0.62. caustic concentration (g Na2 / 10.62. caustic concentration (gNaO2/1) 13 At evaporation at 1500C, silica substitution passes from 0.2% per unit mass to 2.7% per unit mass. Recycling the filtration cake upstream of evaporation translates into a drop in the level of silica in the liquor from 5 0.732%ctq - the abbreviation "%ctq" expresses the dissolved silica content through the relationship concentration of dissolved silica (g SiO2 / 1) x 100 - to 0.700%ctq, caustic concentration (g NaO2/1) i.e. a drop of 0.032%ctq. The square of the silica supersaturation is then divided by 1.13, which translates 10 into a drop in the rate of fouling by siliceous deposits (proportionate to the square of the silica supersaturation). Digestion (A) is carried out at 2570C in an autoclave. At digestion, silica substitution increases up 15 to a content of 8% per unit mass. This corresponds to an additional collection of silica by the hydrogarnet of 1.25 kg SiO2/t A1203. The silica collected by the hydrogarnet is not used for the formation of DSP, which translates into a gain in caustic soda consumption of 20 0.6 kg Na20/t A1203. After digestion, the slurry S2 is cooled and directed to a settling tank under atmospheric pressure (C). The insoluble residues are discharged in the form of red mud 20 while the overflow L3, still loaded with some 25 particles, is subject to safety filtration F. In safety filtration, the saturation index Rp of the pregnant liquor is equal to 1.22. Hexahydrated tri-calcium aluminate 100 is introduced upstream of the filter at a rate of 0.95 g CaO per litre of liquor. The filtration 30 cake 111 thus formed is recycled in the form of a sludge having a solid content of 1000 g/l, directed toward the introduction point upstream of the evaporation chain (E).

14 The recycling of the hydrogarnet at evaporation leads to a solid content of the spent liquor of 2.16 g/l, not dangerous for the evaporation plant. Moreover, recycling the aluminate liquor with the spent liquor 5 leads to a rise in Rp of only 0.001. The impact on production is therefore minimal. Example 2 (figure 3) Example 2 deals with the recycling of the safety 10 filtration cake upstream of the preheating chain. In this example, the slurry S'1 produced by wet grinding the bauxite 310 in the presence of an aliquot L2 of the digestion liquor LO is subject to pre-desilication treatment G before being injected (S'10) into the 15 digestion chain A. The principal aliquot L'1 of the digestion liquor LO is preheated separately in the preheating chain P'1. Unless otherwise indicated, the different elements participating in the Bayer cycle are given the same reference numbers as in figure 1. 20 The calcium based compound 111' is, as in the previous example, the safety filtration cake F. It is introduced upstream of the liquor preheating chain (P'1). In this way, there is no loss of compound by carrying off with the aliquot L2 of the aluminate liquor sent for 25 grinding (B), since the totality of the additive is then dedicated to preventing scaling of the exchangers in the preheating chain (P'1) which operate at a much higher temperature than the evaporators (E). Hexahydrated tri-calcium aluminate is employed as a 30 filtration additive 100 in safety filtration F. The filtration cake 111', intended until the present invention to be discharged with the insoluble residues, is recovered and introduced upstream of the preheating chain P'l.

15 The advantage of adding the hydrogarnet can be felt in the preheating chain (P'1) and also at digestion (A). At the preheating chain input, the liquor has a caustic concentration of 155 g/l, a silica content 5 (expressed as a percentage of caustic soda) equal to 0.583%ctq and a saturation index Rp equal to 0.70. Pre-heating P'l is carried out at 2200C. At this temperature, silica substitution passes from 0.2% per unit mass to 6% per unit mass. Recycling the filtration 10 cake upstream of the preheating chain translates into a drop in the level of silica in the liquor from 0.583%ctq to 0.543%ctq, i.e. a drop of 0.04%ctq. The square of the silica supersaturation is then divided by 1.45, which translates into a drop in the rate of fouling by 15 siliceous deposits (proportionate to the square of silica supersaturation). The advantage of introducing a hydrogarnet is more significant than in the case of evaporators, operating at a lower temperature. Digestion (A) is carried out at 2530C in an 20 autoclave. At digestion, silica substitution increases up to a content of 8% per unit mass. This corresponds to an additional collection of silica by the hydrogarnet of 0.24 kg SiO2/t Al2o3. The silica collected by the hydrogarnet is not used for the formation of the DSP, 25 which translates into a gain in caustic soda consumption of 0.16 kg Na20/t A1203. After digestion, the slurry S2 is cooled and directed to a settling tank under atmospheric pressure (C). The insoluble residues are discharged in the form of 30 red mud 20 while the overflow L3, still loaded with a few particles is subject to safety filtration F. At safety filtration, the saturation index Rp of the pregnant liquor is equal to 1.22. Hexahydrated tri-calcium aluminate 100 is introduced upstream of the filter at a 16 rate of 0.46 g CaO per litre of liquor. The filtration cake 111' thus formed is recycled in the form of sludge at 1000 g/l of dry matter, directed towards the introduction point upstream of the preheating chain 5 (P'1). Recycling the hydrogarnet in the liquor preheating chain leads to a solid content of the liquor of about 1 g/l. This small amount of solid content does not generate any risk to the evaporation chain. Moreover, 10 recycling the aluminate liquor with the spent liquor leads to a rise in the Rp of below 0.001. The impact on production is therefore minimal. Advantages 15 o drop in the frequency of stopping the exchangers for cleaning and maintenance; increase in their life cycle o in the event of the safety filtration cake, constituted mainly of hydrogarnets, being used, the 20 consumption of caustic soda is reduced, by reducing losses of caustic soda with the discharge of the DSP o mechanical effect of cleaning the exchange walls by solid compounds.

Claims (10)

1. Bayer process for the production of aluminium trihydrate comprising: a) a grinding step (B) for the bauxite (10); b) a step of digestion (A) of said bauxite by a 5 sodium aluminate liquor; c) a liquid/solid separation step, typically by settling (C) and filtration (F) allowing the insoluble residues (20) of the sodium aluminate liquor supersaturated in alumina (L4) to be 10 separated; d) a decomposition step (D) for the pregnant aluminate liquor (L4), to form alumina trihydrate precipitates (30); e) after separation of the precipitated alumina 15 trihydrate, a step of concentration (E) by evaporation of the alumina depleted sodium aluminate liquor (L5) for recycling as digestion liquor (L0,L1,L'1,L'10,L''"1,L2); said process being characterised in that the calcium 20 based compound (11, 11') that is introduced into said sodium aluminate liquor belongs to the group of tri calcium aluminates, said calcium based compound being introduced into said sodium aluminate liquor between the exit thereof from the decomposition chain and the mixing 25 thereof with the ground bauxite, upstream of a heat exchanger (E,P'1) intended to increase the temperature of said sodium aluminate liquor.

2. Bayer process according to claim 1 wherein said calcium based compound is a hydrogarnet meeting the 30 general formula 3 XO, Y203, (6-2k) H20, k SiO2, with X meaning calcium, possibly partially replaced by iron, manganese or magnesium, Y meaning aluminium, possibly 18 replaced partially by chromium or iron, and k being between 0 and 3.

3. Bayer process according to claim 2 wherein k is between 0 and 2. 5

4. Bayer process according to claim 2 wherein k is between 0 and 1.

5. Bayer process according to claim 1 or 2 wherein said calcium based compound is introduced upstream or at the input of a heat exchanger into the totality of the 10 sodium aluminate flow that is to pass through said exchanger.

6. Bayer process according to any one of claims 1 to 5 wherein said calcium based compound is introduced at a single point upstream of the heat exchange device located 15 the most upstream from the part of the sodium aluminate liquor circuit between the exit from decomposition and the mixing with the bauxite.

7. Bayer process according to claim 6 wherein said calcium based compound (11, 111) is introduced upstream 20 of the evaporation chain (E).

8. Bayer process according to claim 6 wherein said calcium based compound (11', 111') is introduced upstream of the sodium aluminate liquor preheating chain.

9. Process according to any one of claims 1 to 8 25 wherein said calcium based compound is introduced into the sodium aluminate liquor with a proportion of between 0.1 g CaO/litre of aluminate and 50 g CaO/litre of aluminate.

10. Bayer process according to any one of claims 1 30 to 9 wherein said calcium based compound (111, 111') comes from the recycling of the safety filtration cake (F) when lime or tri-calcium aluminate is employed as a filtration additive.