Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/46—Purification of aluminium oxide, aluminium hydroxide or aluminates
  • C01F7/47—Purification of aluminium oxide, aluminium hydroxide or aluminates of aluminates, e.g. removal of compounds of Si, Fe, Ga or of organic compounds from Bayer process liquors
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
  • C01F7/0613—Pretreatment of the minerals, e.g. grinding
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F7/00—Compounds of aluminium
  • C01F7/02—Aluminium oxide; Aluminium hydroxide; Aluminates
  • C01F7/04—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom
  • C01F7/06—Preparation of alkali metal aluminates; Aluminium oxide or hydroxide therefrom by treating aluminous minerals or waste-like raw materials with alkali hydroxide, e.g. leaching of bauxite according to the Bayer process
  • C01F7/062—Digestion

Definitions

• the invention relates to a process for the manufacture of alumina trihydrate by alkaline attack of ' bauxite according to the Bayer process, comprising the grinding and then the attack of said ground bauxite by contacting with a sodium aluminate liquor, the attack leading to the formation of a suspension which is treated to separate them . insoluble residues of the sodium aluminate liquor, the aluminate liquor thus obtained is then decomposed and then recycled as attack liquor after having been separated from the alumina trihydrate precipitated during decomposition.
• the invention relates more particularly to a method for reducing the content of dissolved silica in the aluminate liquor circulating in the Bayer chain.
• Control of the content of dissolved silica in the aluminate liquor is essential to avoid scaling of the pipes in the Bayer circuit and to control the purity of the trihydrate produced. Depending on the nature of the bauxite treated, this control must be carried out with greater or less vigilance. Paradoxically, it is with bauxites having a low reactive silica content that this control proves to be the most delicate to carry out.
• a first type of desilication is carried out on the entire liquor itself. It is carried out for example just after the attack with a sufficient residence time so that the quantity of dissolved silica reaches a critical threshold of supersaturation, which initiates the desilication reaction by precipitation of siliceous compounds and thus leads to a lowering of the concentration in silica in the liquor to an acceptable level just before separation of the insoluble residues. It can also be carried out before the attack, by locally introducing precipitation germs: for example, in the process described by US 6086834 (ALCAN), where the liquor must cross a bed composed of a mixture of sand and sodalite or in that described in French application No. 03 1 1909 (ALUMINUM PECHINEY), where a calcium-based compound, typically comprising hydrogrenates, is introduced into the aluminate liquor upstream of heat exchangers.
• a second type of desilication is carried out before attack by bringing the ground bauxite into contact with an alkaline solution.
• the patents US 3481 705, AU 0474596 and US 4426363 indicate that this alkaline solution can be a sodium hydroxide solution, optionally mixed with sodium carbonate or a potassium hydroxide solution, or, preferably, an aliquot aluminate liquor from the Bayer chain. In practice, the latter, much more economical, is used: a small part of the decomposed liquor is directed to the wet grinding of bauxite. The silica contained in bauxite is dissolved in this aliquot of liquor, which quickly reaches a threshold of supersaturation.
• the present invention relates to an improvement in the predilining treatment.
• the predessilication treatment can be considered as a preventive treatment, carried out on bauxite entering the Bayer.
• the effectiveness of such a treatment only proves that it prevents the liquor before decomposition reaching a certain critical concentration threshold for dissolved silica - typically a silica / caustic soda ratio close to 0.7 % for plants with a caustic soda concentration of around 155 g / l (we will note below such ratios related to the weight of caustic soda "% ctq" or, in this case, "% Si02 ctq"] - which allows to control in particular the purity of the trihydrate produced.
• a perfect predessilication would consist in transforming all the silicate silicate phases of bauxite (aolinite for example) into a phase (DSP) as stable as possible under the attack conditions.
• the silica concentration of the liquor at the attack output would be at most equivalent to that of the solubility of the DSP (solubility between 0.5 and 0.9% ctq depending on the attack conditions).
• the authors E.TIZON, Ph. CLERIN, B.
• the Applicant has therefore sought, with the aim of increasing the efficiency of w predesilication, a process which would make it possible to reduce the solubility of the DSP formed while ensuring maximum transformation of the soluble siliceous phases into DSP.
• a first object of the invention is a process for the manufacture of alumina trihydrate by alkaline attack of bauxite according to the Bayer process, comprising the grinding and then the attack of said ground bauxite by contacting with a liquor of aluminate of sodium, the attack leading to the formation of a
• 25 ground bauxite is, before attack, brought into contact with an aqueous sodium solution, characterized in that an aqueous sodium solution which, at least partially, from the Bayer chain and which is treated so is used for said pre-silica treatment that its caustic concentration is between 60 g Na20 / I and 220 g Na20 / I and
• the weight content of carbonates and sulphites or sulphates is expressed independently of the associated cations.
• the carbonate content can be expressed in terms of weight of CO3 per unit volume.
• the caustic concentration expressed in terms of weight of Na 2 0 per unit volume we will denote it “% CO3 ctq”.
• the sulphite or sulphate content can be expressed in terms of weight of SO3 per unit volume. Relative to the caustic concentration expressed in terms of weight of Na 0 per unit volume, we will denote it "% S0 3 ctq".
• the concentrations of carbonate impurities and sulphates or sulphites in Bayer liquors depend on the nature of the attacked bauxites, which is why it is difficult to define critical concentrations of impurities by absolute values. The important thing is to avoid the presence of these impurities, or at least to reduce them significantly, at the Bayer chain stage where they are likely to be the most harmful.
• the impurity content of the aqueous sodium hydroxide solution used for the predesilication should have a carbonate concentration of less than 6% CO3 CTQ and suJfates or sulphites less than 1% SO3 ctq.
• the applicant noted with surprise that, among the DSPs obtained, the compounds belonging to the sodalite family were in fact not all as stable as the suggested specialized literature. In particular, it observed that the sodalites were all the more stable on attack as they contained less carbonate, sulphate or sulphite type ions and more aluminate ions.
• the sodalites formed are represented with the following general formula: NazO 2Si0 2 Al 2 0 3 l / 3 (Na 2 Y)
• a mole of silica corresponds to one sixth of a mole Na 2 Y, which behaves like a "cage” trapping a carbonate, sulfate or sulfite ion, or two chloride or aluminate ions.
• the Applicant has found that the stability of the sodalites during the attack could strongly depend on the nature of the tones coming from the aqueous solution used for the pre-desiccation treatment and trapped by the Na 2 Y cages.
• the sodalites comprising aluminate ions are the most stable.
• Those comprising carbonate ions and / or sulphate or sulphite ions are the least stable.
• the chloride ions also have a negative influence on the stability of the sodalite during the attack.
• a preferred embodiment of the invention consists in using for the predessilication an aqueous sodium solution, the chloride content of which is also less than half the chloride content of the decomposed liquor.
• the chloride content can be expressed in terms of weight of Cl per unit of volume. Relative to the caustic concentration expressed in terms of weight of Na2 ⁇ per unit volume, we will denote it "% Cl ctq".
• % Cl ctq Relative to the caustic concentration expressed in terms of weight of Na2 ⁇ per unit volume
• the chloride content of the aqueous sodium solution used for predilining must be less than 2% Cl ctq.
• Aluminate liquor Bayer has a content of impurities (% S03 ctq,% C03 ctq or% Cl ctq) almost identical whatever its point of sampling
• the aqueous sodium solution used for predilining has traditionally been an aliquot of the decomposed aluminate liquor.
• the aqueous sodium solution used for predessilication comes, at least partially, from the Bayer chain and is treated so as to contain less impurities such as sulfates or sulfites, carbonates or chlorides than the circulating aluminate liquor in the Bayer chain.
• the aqueous sodium solution used for predessilication is an aliquot of decomposed liquor, it is freed, in whole or in part, of the impurities which it contains, typically carbonate, sulphate or sulphite ions and possibly chlorides by a specific treatment. purification such as membrane filtration or precipitation.
• the aqueous sodium solution can also result from the mixture of an aqueous solution of sodium hydroxide with an aluminate liquor sampled at a point in the Bayer circuit where it has as few impurities as possible.
• the aqueous sodium hydroxide solution can be obtained by diluting commercially available sodium hydroxide (which we will call by also following "pure soda” or "new soda") in water whose content of impurities is controlled. This water can come from condensates resulting from the evaporation of water in the stages of evaporation or attack; it can also be hot water from the purging of the factory boilers or even raw water.
• the aluminate liquor intended to be mixed with the aqueous sodium hydroxide solution is preferably an effluent from a washing chain: a causticizing clear, a first scrubber or the filtrate or washing scrub hydrate.
• the clear from the causticization process which is often carried out on a sludge washing clear, has a caustic soda concentration of 30 to 60 g / l. Most of these clears have an identical level of impurities expressed in% ctq, but they also have the advantage of having a lower concentration of caustic soda than the decomposed liquor (typically (30-70 g / 1 instead of 160- 180 g / 1), which makes it possible to limit the overall amount of impurities introduced during predilining.
• the clear resulting from the caustification process has the advantage of having a lower carbonate content than that of the rest of the cycle. (between 4 and 8% CO3 ctq).
• the aqueous sodium solution used for the predessilication treatment is often also used for the wet grinding of bauxite: a first aliquot is taken for grinding bauxite and the second aliquot is added to the whole thus crushed of so as to obtain a suspension with a high dry matter content, typically between 300g / l and 1500g / l.
• the predessilication treatment consists in putting, before attack, the ground bauxite and an aqueous sodium solution whose caustic concentration is between 60 g Na20 / I and 220 g Na20 / I, preferably between 60 g Na20 / I and 180 g Na20 / I.
• a minimum caustic at the end of predessilication, depending on the attacked bauxite, between 60 g Na20 / I and 140 g Na20 / I is recommended to obtain the most stable sodalite possible (beyond this minimum caustic, the stability of sodalite does not depends more on the caustic).
• the suspension resulting from the mixture of the ground bauxite and the aqueous sodium solution is maintained for a period of less than 24 hours, typically between 1 and 24 hours, preferably between 5 and 24 hours, at a temperature between 80 ° C. and 150 ° C.
• the quantity of aqueous sodium solution is adjusted so that the dry matter content of said suspension is between 800 and 1500 g / l.
• an additive making it possible to reduce the viscosity of the suspension, such as the additive DREWO (registered trademark).
• the highest possible dry matter concentration (within the limits of the minimum viscosity and caustic constraints) is desirable so as to limit the introduction of impurities to the predilining.
• the aqueous solution used for the predesilication treatment is obtained by dissolving pure commercial soda in water (sodium or not) sufficiently low in carbonates and sulphates or sulphites, and possibly in chlorides, for that the final contents recommended in the context of the invention are respected.
• sodium hydroxide pure commercial soda
• This sodium hydroxide is traditionally introduced into the aluminate liquor, directly or indirectly, at the outlet of evaporation.
• the indirect introduction of sodium hydroxide consists for example in using the liquor coming from the unclogging of the decomposers or of certain filters (safety filter for example). Indeed, the walls of the decomposers rip off, certain filters become clogged over time and they must be regularly dismantled for cleaning. If a pure aqueous solution of sodium hydroxide is used to remove the hydrate crusts which have settled there, the liquor resulting from the treatment, enriched in dissolved alumina, can then be poured into the aluminate liquor.
• this sodium hydroxide can now be used for predilining.
• the injection point of the pure soda (or unclogging liquor) is moved by placing it in the supply (total or partial) of the aqueous sodium solution intended for the predessilication treatment.
• the quantity of pure soda necessary to desiccate bauxite coincides with the quantity of soda necessary to compensate for the losses of soda of the Bayer circuit.
• this case does not generally arise with an atmospheric attack, medium pressure, or simple high pressure, because the quantity of soda necessary to compensate for the losses is significantly lower than that corresponding to the flow of aqueous sodium solution used for predessilication.
• this preferred method of the invention can advantageously be applied at least in the following three cases:
• the liquor aliquot used for wet grinding and pre-desiccation of bauxite at gibbsite is replaced by a solution aqueous sodium hydroxide, obtained by dissolving pure commercial soda in water sufficiently poor in carbonates and sulphates or sulphites, and possibly in chlorides.
• FIG. 1 illustrates the kinetics of dissolution of the silica during the attack with sodalites synthesized under different predesilication conditions, namely by using an aqueous sodium solution used containing more or less impurities.
• FIG. 2 schematizes the steps of a particular method of the method according to the invention, implemented in the context of the improvement of the attack with sweetening described by EP 0 960069.
• FIG. 3 illustrates three kinetics of dissolution of the silica during the attack with sweetening shown diagrammatically in FIG. 2, each of these curves being relative to the use of a particular aqueous sodium solution intended for the predessilication of gibbsite.
• FIG. 4 schematizes the steps of a particular modality of the method according to the invention, implemented temporarily in the context of a "classic" Bayer method.
• Example 1 - Dissolution kinetics (FIG. 1) Different sodalites were synthesized for 24 hours under predesilication conditions chosen so as to evaluate the effect of the presence of impurities in the aqueous sodium solution.
• the impurity contents of the various sodium solutions used are as follows:
• V 0% C03 ctq, 0% S03 ctq, 0% Cl ctq
• sodalites were then attacked with hydrate under the following attack conditions: Temperature: 145 ° C.
• a monohydrate bauxite 1 after a first wet grinding Al in the presence of an aliquot 20a of attack liquor 20, the Rp of which is preferably between 0.55 and 0.65 and the sodium hydroxide concentration is between 160 and 220 g Na20 / I, is introduced with another aliquot 20b of the attack liquor 20 in autoclaves to form a suspension which is heated to a temperature between 220 and 300 ° C for a time sufficient to dissolve the monohydrate d alumina, typically between 0.2 and 1 hour.
• the suspension 2 resulting from this first attack under pressure A2 the supersaturation of which in alumina is characterized by an Rp of between 1.05 and 1.20, is partially cooled and relaxed (Cooling / Relaxation NI).
• a pulp 6 ′ resulting from the wet grinding L of a bauxite with trihydrate 13 is injected under pressure (“Sweetening” N2). Before being injected into the suspension 2, the crushed pulp 6 'is maintained for 8 hours at 100 ° C, so as to produce a predissilication P of said pulp.
• a first aliquot 30a of the aqueous sodium solution 30 is used for the wet grinding of the bauxite to trihydrate 13 and the second aliquot 30b is poured into the suspension which results from the wet grinding, with a quantity adjusted to reach a dry matter content of 1000g / l.
• the injection is preferably carried out between 190 ° C and 130 ° C in the regulator which corresponds to this temperature and which provides a residence time of at least two minutes.
• the suspension 3a enriched in alumina thanks to the injection of the crushed pulp 6 ', is then cooled and expanded to atmospheric pressure. It has an Rp of between 1.20 and 1.40 and its concentration of caustic soda is between 180 and 240 g Na20 / I. It is subjected to a first dilution by adding a fraction 7b of diluted liquor from the washing of the sludge. Its concentration is lowered to a value between 160 and 175 g Na20 / 1.
• the aluminate liquor 4a is diluted (dilution M) with the washing waters 7c filtered (filtration O) of the residues 5.
• the diluted solution 4b has a sodium hydroxide concentration of between 140 and 155 g Na20 / 1. It is then cooled and decomposed (D) between 80 ° C and 50 ° C, in the presence of fine particles of recycled precipitate 10 serving as a primer.
• the alumina trihydrate grains thus obtained are classified so that the largest 11 are recovered for washing H with water 14 and calcination K in order to provide the production alumina 12, while the finest remain within the recycled primer 10.
• the decomposed liquor 8 is then concentrated by evaporation E. Once concentrated, the liquor is used as attack liquor 20.
• suspension 2 - resulting from the attack on monohydrate bauxite - circulates in the installation at the rate of 650-1500 m 3 / h
• the flow of suspension 6 'associated with sweetening is of the order of 30 to 80 m 3 / h
• the pure aqueous solution of sodium hydroxide is introduced into water 15 with a flow rate of between 5 and 10 m 3 / h.
• FIG. 3 illustrates three curves representative of kinetics of dissolution of silica during the attack with sweetening of a bauxite with diaspore, each of these curves being relative to the use of a different sodium solution during predessilication sweetening.
• the caustic concentration is high (around 220 g Na20 / I) and the critical concentration threshold for dissolved silica which it is preferable not to exceed in the liquor at the attack output is here l '' order of 0.85% Si02 ctq - 0.90% Si02 ctq.
• the curve referenced (X) plots the kinetics of dissolution of the silica in the liquor when the predilining was done with an aliquot of decomposed liquor. After passing through a very high maximum content (1.02% Si02 ctq), the content decreases slowly, remaining clearly above the critical threshold.
• the curve referenced (XI) plots the kinetics of dissolution of the silica in the liquor when the predissilication was done using a pure aqueous solution of sodium hydroxide diluted with raw water. After passing through a maximum content of 0.90% Si02 ctq, the content decreases by passing below the critical threshold.
• the curve referenced (XII) traces the kinetics of dissolution of the silica in the liquor when the predilining was done using an aqueous solution of sodium hydroxide containing impurities in concentration identical to that of the decomposed liquor.
• This last curve which gives intermediate results, makes it possible to quantify the respective effects of the content of alumina dissolved in the sodium solution and of the content of impurities of this same sodium solution.
• a gibbsite 101 bauxite is subjected to wet grinding B in the presence of a sodium solution which is usually an aliquot of the attack liquor 120.
• the ground bauxite undergoes a predessilication treatment P (typically 8 hours at 100 ° C. ), usually using an aliquot of the attack liquor 120.
• the suspension 102 resulting from attack A is subjected to a post-desilication Q then to a dilution by adding a fraction 107b of diluted liquor coming up from the washing of the sludges .
• the resulting suspension 103 is then subjected to decantation CI in the presence of a flocculating agent and finally to filtration C2 to separate the liquor 104a from sodium aluminate from
• the aluminate liquor 104a is diluted (dilution M) with the washing water 0 107c filtered (filtration O) of the residues 105.
• the diluted solution 104b has a sodium hydroxide concentration of between 140 and 155 g Na20 / I. It is then cooled and decomposed (D) between 80 ° C and 50 ° C, in the presence of fine particles of recycled precipitate 110 serving as a primer.
• the alumina trihydrate grains thus obtained are classified so that the largest 111 are recovered for washing H with water 114 and calcination K in order to provide the production alumina 112, while the finest remain within the recycled primer 110.
• the decomposed liquor 108 is then concentrated by evaporation E. Once concentrated, the liquor is used as attack liquor 120.
• This addition is then carried out at the level of the wet grinding B of the gibbsite bauxite 101 and of the predessilication P with the supply of soda 109 '.
• the quantity of pure sodium hydroxide required to desiccate bauxite being considerably greater than the quantity of sodium hydroxide necessary to compensate for the losses of sodium hydroxide from the Bayer circuit, this addition can only be temporary.
• the addition is carried out here in two stages: a first aliquot 130a of the aqueous sodium solution 130 is used for wet grinding of bauxite and the second aliquot 130b is poured into the suspension which results from wet grinding, with an amount adjusted to reach a dry matter content of 1000g / l.

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