OA13269A - Process for reducing the fouling of heat exchangers of a Bayer circuit. - Google Patents

Abstract

The invention relates to a Bayer process for the production of aluminium trihydrate comprising: a) a milling step (B) for the bauxite (10), b) a step of digestion (A) of said bauxite by a sodium aluminate liquor, c) a liquid/solid separation step (C and F), d) a decomposition step (D) for said supersaturated aluminate liquor e) after separation of the precipitated aluminium trihydrate (30), a step of concentration (E) by evaporation of said sodium aluminate liquor for recycling as digestion liquor, said process being characterised in that a calcium based compound (11, 111, 11', 111'), belonging to the group of tricalcinated aluminates, is introduced into said sodium aluminate liquor. The introduction of said calcium compound into the aluminate liquor is carried out between the exit thereof from the decomposition chain and the mixing thereof with the milled bauxite upstream of a heat exchanger.

Description

1 13269

METHOD FOR REDUCING THE HEAT EXCHANGER ENCRASSMENT OF A BAYER CIRCUIT

TECHNICAL FIELD 5

The present invention relates to a method for reducing the fouling of heat exchangers of the Bayer chain driver. It relates more particularly to the reduction of the volume of the silica deposits in the heat exchangers on the aluminate liquor before the latter is directed towards the attack of the bauxite: evaporators and other heat exchangers treating the liquor alone, in particular before its mixing with bauxite. Exchangers which make it possible to heat up to the attack temperature the suspension resulting from the mixing of the milled bauxite with the aluminate liquor (typically a solids content of more than 50 g / l) are therefore not concerned.

STATE OF THE TECH NIC

The Bayer process is a well-known process for the production of aluminum trihydrate. It comprises a step of grinding - usually wet - of labauxite, possibly a step of desilication, then a stage of attack of said bauxite by a sodium liquor with highcaustic concentration, typically a sodium aluminate liquor, which we will call thereafter attack liquor. The etching leads to the formation of a suspension which, after dilution, is decanted and filtered to separate the insoluble residues from the sodium aluminate liquor. The latter, supersaturated with alumina, is then decomposed in the presence of a primer to form precipitates of alumina trihydrate and, after separation of the precipitated alumina trihydrate, concentrated by evaporation to increase its caustic concentration and recycled as a liquor. attack. -2- 13269

It is well known that the conditions of attack of bauxite are to be adapted on the one hand according to the degree of hydration and the mineralogical structure of the alumina contained in the ore and on the other hand according to the nature and the content of the impurities ( especially silica, iron oxides and titanium) present in said bauxite. The extraction yields of the solubilizable alumina in the attack liquor, expressed in proportion to the alumina trihydrate contained in the liner, may exceed 95%, if the adverse influence of certain impurities, in particular silica, is well controlled. ίο Silica can occur in bauxite in several mineralogical forms that are also soluble in the attack liquor. Certain mineralogical forms, including kaolin (Al2O3, 2SiO2, 2H2O) is the most widespread species, solubilize during the attack of bauxite. We call

I commonly "reactive silica" the share of silica present in bauxite in one of these forms. The reactive silica is solubilized in the etching liquor and the dissolved silica content reaches a supersaturation level such that the silica re-precipitates in the form of weakly soluble sodium silico-aluminate. A good control of the precipitation of silica is therefore essential if it is desired to produce alumina trihydrate free of siliceous impurities. On the other hand, a high level of silica supersaturation in the liquor leads to significant fouling in some parts of Bayer, especially at the level of the heat exchangers. These fouling degrade the performance of the heat exchange and thus impose frequent stops for cleaning the devices. Generally, the cleaning process involves removing siliceous scale by acid attack. 1 3269 -3-

PROBLEM

The present desiccation treatments are intended primarily to prevent pollution of the trihydrate during the decomposition. Most of these treatments, described for example in 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 (Aluminum Pechiney), are carried out on the liquor before and / or after attack. They result in decomposed formation, of sodium silico-aluminates type, called DSP ("dessilicationίο producfs"), which are then eliminated with the insoluble residues of the attack (red mud).

Despite the various desilication processes used, the level of desilice is still too high in the liquor which passes through the heat exchangers. These large supersaturation conditions lead to the formation of fouling deposits which reduce the efficiency of heat transfer.

The patent application WO96 / 06043 (Comalco), dealing more particularly with the double attack of a boehmite-rich bauxite, indicates incidentally that, in order to reduce the risk of scaling, lime can be introduced into the decomposed liquor upstream. heat exchangers for heating the liquor prior to the boehmitic attack. U.S. Patent 5,415,782 (Nalco) discloses treatment of the decomposed liquor by the addition of high molecular weight organic additives to control the de-sanding product deposit on the surfaces of Bayer equipment. The Applicant has therefore sought a complementary solution which makes it possible to further lower the level of the silica concentration of the liquor before etching, at least at the level of the heat exchangers which process the liquor in the driver circuit.

OBJECT OF THE INVENTION The object of the invention is a Bayer process in which a calcium-based compound belonging to the group of tricalcium aluminates is introduced into the aluminate aliquer, the introduction of said calcium-based compound being carried out in said aluminate liquor between its exit from the decomposition chain and its mixture with milled bauxite, upstream of a heat exchanger for increasing the temperature of said aluminate liquor. The group of tricalcium aluminates includes: o certain hydrogrenates, which are hydroxyl silicates of the general formula X 3 Y 2 (SiO 4) 3-x (OH) 4, for which X = Ca and Y = Al; in the Bayer procedure, it is customary to represent these hydrogrenates by the following general formula: 3 XO, Y 2 O 3, (6-2k) H 2 O, k SiO 2 where X = Ca, Y = Al and where k is between 0 and 3 preferably between 0 and 2, more preferably between 0 and 1; the hydrogrenaf corresponding to k = 0 is the tricalcium aluminate hexahydrate. substituted hydrogrenates from the preceding family, for example by partial substitution of calcium (up to 10 atomic%) or aluminum (up to 20 atomic%) with another metal: calcium can be replaced by iron (ferrous ion), manganese or magnesium, aluminum can be replaced by chromium or iron (ferric ion); The agent (s) for introducing said calcium-based compound into the aluminate label is (are) located between the exit of the decomposition chain - at this point, the liquor, before being concentrated by evaporation, is called "decomposed liquor" - and the mixture with labauxite - upstream of the entry into the first attack tank. To be effective at the exchanger, the addition must be carried out upstream or at the inlet of this exchanger on the whole or on an aliquot having aproportion as much as possible of the aluminate stream to pass through said exchanger.

The heat exchangers acting on the aluminate liquor before it is directed towards the attack of bauxite are at least of two types: they are either evaporators used to increase the caustic concentration of the decomposed liquor. and recycling it as an attack liquor, or heat exchangers that treat the liquor alone before mixing it with the bauxite, in particular to warm it up before the attack. Any other type of exchanger, for example an exchanger used to cool the aluminate liquor alone, could also be concerned by the invention. The addition of compound is preferably carried out in one place, for example upstream of the heat exchange device located furthest upstream of the part of the circuit between the decomposition outlet and the mixture with the bauxite. In general, the plants comprise series of exchangers and it is advantageous to locate the point of introduction of the calcium-based compound upstream of this series of exchangers, that is to say either enamont of the evaporation chain. or upstream of the preheat chain of the liquor, if it exists. When it is intended to preheat the liquefier alone to a temperature close to the etching temperature, it may be advantageous to do the ojout at the entrance of this heating chain. Said calcium-based compound is introduced into the decomposed liquor or etching liquor in a proportion of preferably between 0.1 g CaO / liter aluminate and 50 g CaO / liter aluminate. A lower proportion would not reduce the risk of scaling. The upper limit is indicated primarily for economic reasons.

Said calcium-based compound reacts with a portion of the silica contained in the calcium and it is converted into a hydrogrenate with a higher silica content. Typically, a hydrogrenate introduced into the liquor with a k near 0 is transformed into a hydrogrenate having a k close to 1.

The products of the reaction of said calcium-based compound with the silica dissolved in the liquor are entrained with the suspension in the attack devices and then in the liquid / solid separation devices (typically decadders). They are evacuated with all the insoluble residues of the attack (red mud). The introduction of a tricalcium aluminate formed outside the Bayerou chain already formed elsewhere in the Bayer chain is preferred to the introduction of lime into the aluminate liquor directly upstream of the heat exchangers, as it is In fact, if the lime reacts with the aluminate liquor to give compounds belonging to the family of tricalcium aluminates, they can lead to inadvertent deposits on the walls of the reactor. The quantity of lime to be added should be carefully checked to not exceed a critical solids content of the liquor at the heat exchanger level, beyond which there could be a risk of inadvertent deposit or accelerated wear. by abrasion descanalisations. On the other hand, this addition leads to an increase in the overall consumption of lime in the Bayer circuit since the solid products of the reaction, which are in the attack liquor, are rapidly mixed with the bauxite and are evacuated with the red sludge. In a preferred embodiment of the invention, the hydrogrenates formed by safety filtration are used through which the supersaturated liquor passes before being directed to the decomposition chain. At this stage, the liquor has less than 200 mg / l of solid particles difficult to filter. To facilitate the filtration, lime or tricalcium aluminate is poured into the supersaturated liquor in addition to these filters. The adjuvant is transformed into a hydrogrenate which grows progressively into silica. It is retained in the filter cake, taken and re-introduced into the decomposed liquor upstream of a heat exchanger. In view of the increase of the temperature and the lower content of alumina, the said hydrogrenat sees its effectiveness in absorbing silicide to increase strongly. In this way, a product is used which has not been upgraded so far and the overall consumption of lime in the Bayer circuit is not changed. The effect of the reaction of said calcium-based compound with a portion of the silica contained in the liquor has the effect of reducing the equilibrium content of the dissolved silica at the exchanger level. Now, it is known that the scale of scale is substantially proportional to the square of the silica supersaturation, that is to say the difference between the effective concentration of silicateissoute and the concentration at equilibrium. It is also known that the scale of scale is all the more important as the temperature prevailing in the seine liquor is high. The effect of the addition according to the invention of said calcium base compound will therefore be more effective on heat exchangers operating at high temperature. The heat exchanger may be an evaporation device for deconcentrating the liquor to turn it into a liquor. The first example given below describes a European type Bayer process for attacking a diaspore bauxite where the caustic concentration of the liquorpasse typically evaporates from 160 g Na2O! liter of aluminate at 240 gNa 2 O / liter of aluminate in evaporators operating at 145 ° C -150 ° C. The heat exchanger can also be a preheating device, which makes it possible to bring the liquor to the appropriate temperature before attack, which varies according to the type of bauxite. The second example given below describes an American type Bayer process for attacking a trihydrate bauxite where the liquor is separately heated in exchangers. In this particular case, it may be more advantageous to introduce the calcium base compound downstream of the evaporators but upstream of the liquor heat exchangers, so that there is no loss by entrainment with the aliquot. the aiuminate liquor directed to grinding, the totality of the adjuvant then being devoted to the prevention of scaling exchangers which operate at a significantly higher temperature than the evaporators.

As indicated above, hydrogrenates from the security filtration cake are used in a preferred embodiment of the invention. Safety filtration is carried out on the overflow of the settlers, that is to say on the aliquin of supersaturated aiuminate that is directed towards the decomposition chain. This leaching is commonly used at the level of this safety filtration. tricalcium aluminate as filtration aid. The latter, facilitating the filterability of residual sludge present at the overflow decadders, is found in the form of hydrogrenat low silica load in the filter cake. Instead of evacuating said cake with the sludge, it is advantageous to recycle it by introducing it into the alumina liquor which has an exchanger. It is preferably recycled continuously in the form of a suspension containing on the order of 1000 g / l of dry matter. Onrecycle this filter cake in whole, possibly partially (lereste being for example evacuated with the solid residues of the attack), the essential thing being not to exceed a critical content of dry matter of the aliqueur at the level of the heat exchanger, beyond which there could be a risk of inadvertent deposition or accelerated wear through abrasion of the pipes. Preferably, the liquor intended to pass through said exchanger is intended to have a solids content of less than 100 g / l. Deprèférence still, for more security, one seeks not to exceed 3g / l.

FIG. 1 schematically illustrates a Bayer bauxite cycle treatment method locating the preferred points for the introduction according to the invention of said calcium-based compound.

FIG. 2 schematically illustrates a process for treating Bayer-cycle adsorbed bauxite in which the introduction, according to a preferred embodiment of the invention, of the cake of the safety filtration is carried out at the level of an evaporator.

FIG. 3 schematically illustrates a Bayer Bauxite treatment process in which the introduction of the security filtration cake according to a preferred embodiment of the invention is carried out in the preheating chain of the etching liquor.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically illustrates a typical Bayer cycle in which the bauxite is milled by wet milling (B), here in the presence of an L 2 aliquot of the LO liquor. An aliquot L1, which represents the largest portion of the L1 etching liquor, typically more than 80%, is not directed to wet milling of the bauxite.

There are two cases:

case 1: this aliquot is heated alone (L'I) in a preheating chainΡΊ. The aliquot thus preheated (L'10) is then mixed with the suspension S'I resulting from the wet grinding of the bauxite and directed towards the chain of attack A. - 1 3269 case 2: this aliquot is added ( 1 ") to the suspension S" 1 resulting from wet milling and therefore very rich in dry matter - that is to say comprising a large number of solid particles - to obtain a suspension S "10 lessviseux which is preheated in a chain P 1. The suspension thus preheated (S "100) is introduced into the chain of attack A. The attack is generally under pressure. The chain of attack then presents itself in the form of a series of autoclaves in which the suspension must circulate.

At the end of the attack, the suspension S 2 is directed towards a liquid / solid separation device (C), typically a decahtor in which the insolublic residues are separated by gravitation from the liquor: the insoluble residues are discharged in the form of red mud while the overflow L3, still charged with some particles is subjected to the filtration F, called "red filtration" or "security filtration". The filtrate L4 is the supersaturated aluminate liquor which is directed towards the decomposition chain D to precipitate the alumina trihydrate 30. At the outlet of the decomposition chain D, the depleted liquor L5 is concentrated by evaporation (E) to be redirected to attack (liquor attack LO).

According to the invention, it is possible to introduce a calcium-based compound belonging to the group comprising lime and tricalcium aluminates, in the depleted liquor or in the liquor of attack, preferably at the point of introduction 11, that is to say in the depleted liquor L5, before the latter is concentrated by evaporation to be converted into attack liquor or still at the point of introduction 11 'in the aliquot L'I of the liquor of attack, when this is must be preheated alone (ΡΊ). EXAMPLE 1 (FIG. 2) 1326 Example 1 deals with the recycling of the security filtration cake upstream of the evaporation. Figure 2 schematically illustrates a process for treating Bayer cycle adsorbed bauxite. Unless otherwise indicated, the various elements involved in the Bayer cycle are repeated with the same references as in FIG. 1. In this example, the suspension S "10 which results from the mixing of the main aliquot L" 1 of the etching liquor 10 LO with the slurry S-1 obtained from the wet grinding (B) of the bauxite at diaspore 210. The etching, intended to dissolve the diaspore (alumina monohydrate) contained in the bauxite, is carried out at high temperature. temperature (257 ° C) autoclaved with a typical caustic liquor of 240 g Na 2 O / liter After decomposition (D), the L5 decomposed clarifier only has a caustic concentration of 160 g Na 2 O / liter and must be concentrated by evaporation to recover the attack concentration (240 g Na2O / liter).

The evaporator chain (E) used for this purpose operates at a temperature between 145 and 150 ° C. 20

Tricalcium aluminate hexahydrate is used as defiltration admixture 100 for safety filtration F. The supersaturated liquor indeed comes from the overflow of the settling tanks and still contains a certain amount of solid particles. These are generally very sticky and, in order not to clog the screen of the filters, a filter aid is added which modifies the physicochemical behavior of these fine particles. In general, lime or tricalcium aluminate is used, in particular tricalcium aluminate hexahydrate. The safety filtration comprises a plurality of filters operating in parallel pressure, one or more of which are removed from the circuit for removal of the filter cake and cleaning. On average, a particulate filter aid remains about 4 hours in contact with the supersaturated liquor and becomes a hydrogrenate which progressively enriches silica. Filtration cake 111, intended until the present invention to be evacuated with insoluble residues, is recovered and introduced upstream of the evaporation chain E. The interest of the addition of hydrogrenate is appreciated on the one hand by the evaporation (E), where the increase of the silica substitution leads to a reduction of the silica level in the liquor, and on the other hand to the attack (A), where it decreases the quantity of DSP ("desilication products") formed, so the loss of soda. At the inlet of the evaporator, the liquor has a caustic concentration of 170 g / l, a silica content (expressed as a percentage of sodium hydroxide) equal to 0.732% etq and a saturation index Rp (expressed as the weight ratio:

The concentration of dissolved alumina (g A12O3 / 1) concentration of caustic soda (g Na / 0) was 0.62. At evaporation at 150 ° C., the silica substitution increases from 0.2% by weight to 2.7% by weight. The recycling of the filter cake upstream of the evaporation is translated by a lowering of the silica level in the liquor by 0.732% and the abbreviation "% ctq" expresses the dissolved silica content by the concentration ratio of dissolved silica (g SiO 2 (1) xlOO - at 0.700% wt, ie a drop in concentration of caustic soda (g Na 2 O / l) 0.032% wt. The square of the silica supersaturation is then divided by 1.13, which corresponds to a decrease in the rate of fouling by the silicious deposits (proportional to the square of the silica supersaturation). The attack (A) is carried out at 257 ° C. in an autoclave. On the attack, the substitution in silica increases to a content of 8% by mass. This corresponds to an additional adaptation of silica with the hydrogrenate of 1.25 kg SiO 2 / t Al 2 O 3. The silica captured by the hydrogrenate is not used for the formation of the DSPs, 13269, which results in a gain. on the consumption of soda of 0.86 kg Na2O / 1Al2O3.

After etching, the suspension S2 is cooled and directed to a decanter under atmospheric pressure (C). The insoluble residues are evacuated as red debau 20 while monitoring L3, still charged with some particles is subjected to the security filtration F. At the filtration security level, the saturation index Rp of the supersaturated liquor is equal at 1.22. The filter of the tricalcium aluminate hexahydrate 100 has a feed rate of 0.95 g CaO per liter of liquor. The fiiteration cake 111 thus formed is recycled in the form of sludge with a solids content of 1000 g / l, directed towards the point of introduction upstream of the evaporation chain (E).

The recycling of the hydrogrenate to evaporation leads to a solids content of the decomposed liquor of 2.16 g / l, which is not dangerous for the evaporation plant. In addition, the recycling of aluminate liquor with the decomposed liquor leads to a rise of Rp of only 0.001. The impact on production is therefore minimal. EXAMPLE 2 (FIG. 3) Example 2 deals with the recycling of the safety filter cake upstream of the preheating chain.

In this example, the suspension S'I resulting from the wet milling of the bauxite 310 in the presence of an aliquot L2 of the liquor L0 is subjected to a preessilatiatafion treatment G before being injected (S'I0) in the chain Attack A. The main aliquot The I of the L0 attack liquor is preheated in the preheating chain ΡΊ. Unless otherwise indicated, the various elements involved in the Bayer cycle are repeated with the same references as in Figure 1. 1 3269 - 14-

The calcium-based compound 111 'is, as in the previous example, the cake of the safety filtration F. It is introduced upstream of the preheating (ΡΊ) chain of the liquor. In this way, there is no loss of compound 5. by entrainment with the aliquot L2 of the aluminate liquor directed towards the grinding (B), the totality of the adjuvant then being devoted to the prevention of scaling of the exchangers of the chain, of preheating (ΡΊ) which operate at a temperature significantly higher than evapora te u rs (E).

io

Tricalcium aluminate hexahydrate is used as defiltration admixture 100 for safety filtration F. The filter cake HT, intended for the present invention to be discharged with the insoluble residues, is recovered and introduced upstream of the chain. preheating ΡΊ. The advantage of the addition of hydrogrenate is appreciated on the preheating chain (ΡΊ) and also on the attack (A) At the inlet of the preheating chain, the liquor has a caustic concentration of g / L, a silica content (expressed as a percentage of caustic) equal to 0.583% etq and a saturation index Rp equal to 0.70.

Preheating ΡΊ is carried out at 220 ° C. At this temperature, the silica substitution increases from 0.2 mass% to 6 mass%. The recycling of the filter cake upstream of the preheating chain results in a decrease of the silica level in the liquor from 0.583% to 0.543% by weight, ie a decrease of 0.04% by weight. The square of the silica supersaturation is then divided by 1.45, which is then translated by a reduction in the rate of fouling by siliceous deposits (proportional to the square of the silica supersaturation). The interest of the introduction of a hydrogrenate is more significant than in the case of evaporators, operating at a lower temperature. The attack (A) is carried out at 253 ° C. in an autoclave. On the attack, the substitution in silica increases to a content of 8% by mass. This corresponds to a further adaptation of silica with the hydrogrenate of 0.24 kg SiO2 / 1Al2O3. The silica collected by the hydrogrenate is not used for the formation of the DSPs, which results in a gain on the sodium hydroxide consumption of 0.16 kg Na 2 O / t Al 2 O 3.

After stripping, the suspension S2 is cooled and directed to a decanter under atmospheric pressure (C). The insoluble residues are discharged in the form of red debaues 20 while the L3 overflow, still charged with some particles is subjected to the security filtration F. At the filtration security level, the saturation index Rp of the supersaturated liquor is equal to 1 22. The filtrate of the tricalcium aluminate hexahydrate 100 is fed in at the rate of 0.46 g CaO per liter of liquor. The filter cake ΠΤ thus formed is recycled in the form of sludge at 1000 g / l dry matter, directed to the point of introduction upstream of the preheating chain (ΡΊ).

The recycling of the hydrogrenat to the preheating chain of the liquor leads to a solids content of the liquor of the order of 1 g / l. This dry matter does not pose a risk to the evaporation chain. In addition, the recycling of aluminate liquor with the decomposed liquor leads to a rise of Rp of less than 0.001. The impact on production is therefore minimal. 25 1 K.Z- -16-

BENEFITS o decrease of Ια frequency of the stops of the exchangers for cleaning and maintenance; increase their lifespan.

5 o in the case where the cake of the safety filtration, consisting essentially of hydrogrenats, is used, the consumption of sodium hydroxide is reduced, the losses of sodium hydroxide are reduced with the evacuation of the DSPs; the mechanical effect of cleaning the exchange walls by the solid compounds. 10

Claims (10)

Claims 1. A Bayer process for producing aluminum trihydrate comprising: a) a milling step (B) of the bauxite (10); b) a step of attacking (A) said bauxite with a sodium aluminate liquor; c) a liquid-solid separation step, typically by decantation (C) and filtration (F), for separating the insoluble residues (20) from the alumina-saturated sodium aluminate liquor (L4); d) a decomposition step (D) of the supersaturated aluminate liquor (L4) to form alumina trihydrate precipitates (30); e) after separation of the precipitated alumina trihydrate, a step of deconcentration (E) by evaporation of the alumina-reduced sodium aluminate liquor (L5) to recycle it as the etching liquor (LO, L1, L ') 1, L'10, L "1, L2); said method being characterized by introducing into said sodium aluminate liquor a calcium-based compound (11, 11 ') belonging to the group of tricalcium aluminates, introduction of said calcium-based compound being effected in said sodium aluminate liquor between its exit from the decomposition chain and its mixing with the milled bauxite, upstream of a heat exchanger (Ε, ΡΊ) for increasing the temperature of said sodium aluminate liquor. 2) A Bayer process according to claim 1 wherein said calcium compound is a hydrogrenate having the general formula XO, Y2O3, (6-2k) K2O, k SiO2, X being calcium, optionally partially substituted with iron, manganese or magnesium, Yrepresentant aluminum, optionally partially replaced by chromium or iron, k being between 0 and 3. 30 1 3269 -18- 3) Bayer process according to claim 2 wherein k is between 0 and 2. 4) Bayer method according to claim 2 wherein k is between 0 and 5 1. 5) Bayer process according to claim 1 or 2 wherein said calcium base compound is introduced upstream or at the inlet of a heat exchanger on all of the flow of sodium aluminate to pass through said io exchanger. 6) Bayer process according to any one of claims 1 to 5 whereinquedit said calcium-based compound is introduced in one place upstream of the heat exchange device located furthest upstream of the part of the liquor circuit. sodium aluminate between the outlet of the decomposition and the mixture with bauxite. 7. The Bayer process according to claim 6 wherein said calcium-based compound (11, 111) is introduced upstream of the evaporation chain (E). 8) Bayer process according to claim 6 wherein said compound based decalcium (11 ', 11 T) is introduced upstream of the preheating chain of the sodium aluminate liquor. 25 9) Process according to any one of claims 1 to 8 wherein saidcomposé based on calcium is introduced into the sodium aluminate liquor with a proportion of between 0.1 g CaO / liter of aluminate and 50 g CaO / liter of aluminate. 30 The Bayer process according to any one of claims 1 to 9 wherein said calcium-based compound (111, 111 ') is derived from the recycling of the safety filtration cake (F) when lime or of the aluminatetricalcique is used as filtering additive.