Title of Invention

A METHOD OF PRODUCING A ALUMINA TRIHYDRATE

Abstract

(57) Abstract: The present invention relates to a method of producing alumina trihydrate with a low silica content from Oibbsite containing bauxite of low reactive silica content. The present method involve two desilication steps, one step before extraction of alumina from the bauxite ie, a pre-desilication and an additional desilication step following alumina extraction i.e a post desilication. The method comprising the steps of a) heating a suspension of ground bauxite in an aqueous solution comprising sodium hydroxide b) increasing the sodium hydroxide concentration of the suspension, c) heating the suspension from step b), d) diluting the suspension for step c) with an aqueous solution and e) heating the suspension from step d) at a temperature greater than 80°C, to produce a suspension having a supersaturated, desilicated sodium aluminate liquor and insoluble solvent. PRICE: THIRTY RUPEES

Full Text

TITLE OF THE INVENTON METHOD OF TREATING ALUMINA TRIHYDRATE CONTAINING BAUXITE OF LOW REACTIVE SILICA CONTENT BACKGROUND OF THE INVENTION Field of the Invention; The present invention is a method for producing alumina trihydrate with a low silica content from Gibbsite-containing bauxite of low reactive silica content. The present method comprises two desilication steps which are effective in removing silica impurities: one step before extraction of alumina from the bauxite, a pre-desilication, and an additional desilication step following alumina extraction, a post-desilication. Description of the Background Art: The Bayer process, widely described in the specialized ■ literature, is the most important method used in the production of alvunina. Alumina produced by the Bayer process can be used in the hydrate state, as transition alumina, calcined alumina, sintered or melted alumina, and can also be transformed into alutninum by igneous electrolysis electrolysis. In the Bayer process, bauxite is digested at an elevated temperature with an aqueous solution of sodium hydroxide with a concentration sufficient to solubilize the alumina to produce a supersaturated solution of sodium aluminate. After separation of the solid phase comprising undigested ore (red mud), the supersaturated solution of sodium aluaiinate is seeded with particles of alumina trihydrate in order to induce precipitation of alumina as alumina trihydrate. The sodium aluminate liquor remaining after removing the precipitated alumina trihydrate, now poor in alumina, is known as spent liquor. This spent liquor is then concentrated and recharged with sodium hydroxide to provide a digestion liquor which can be recycled in the digestion step. It is well known to those of ordinary skill in the art that the conditions of the alkaline treatment of the bauxites in the Bayer process must be modified according to the degree of hydration and the crystallographic structure "of the alumina as well as the nature and the content of the impurities found in the bauxite such as silica, iron oxides, humic materials, etc. Accordingly, bauxites containing alumina in the monohydrate stage (bohemite, diaspore) are treated at a temperature higher than 200C, generally between 220oC and 300°C. Bauxites containing alumina in the trihydrate stage (hydrargillite, also called gibbsite) are digested at temperatures lower than 200°C, and even at atmospheric pressure, which is easier to implement and results in important savings in operating costs by eliminating the need for autoclaves and pressure reactors. Generally, the extraction yields of soluble alumina are higher than 9 5% and the purity of the resulting supersaturated sodium aluminate liquor, which influences the purity of the subsequently precipitated alumina trihydrate, is satisfactory if one is careful to limit the contents of these impurities by selective purification steps. The difficulty of these purifications, particularly int he case of silica, depends on the mineralogical forms of the impurities present. Silica can be present in bauxite in several mineralogical forms that are not equally soluble in the sodium hydroxide solution- Some mineralogical forms of which Kaolin (Al.,0,, 2SiO, 2H3O) is the most common are solubilized along with the alumina trihydrate. The term "reactive silica" is -commonly applied to that fraction of the silica present in the bauxite in one of these forms, counted as SiO., and usually accounts from 0.5 to 7% of the dry bauxite weight. In the presence of sodium hydroxide liquor, the reactive silica is first solubilized and then re-precipitated as sodium silico-aluminate. The concentration of soluble silica in the sodium hydroxide liquor is determined by the solubility equilibrium of the sodium silico-aluminate after a very long time. During the industrial treatment of a bauxite containing alumina trihydrate, it is rare that the solubility equilibrium of the sodium silico-aluminate be reached. Usually, the silica concentration in the sodium liquor is greater, and even much greater, than the solubility equilibrium of the sodium silico-aluminate. This concentration is linked, consequently, to the soliibility equilibrium of the sodium silico-aluminate and at the same time to its kinetic precipitation. This kinetic precipitation is slowed when the bauxite contains low amounts of reactive silica, because the precipitation of the sodium silico-aluminate is favored by the presence of reaction product. In the Bayer cycle, the concentration of soluble silica in the alumina-enriched liquor after digestion of the bauxite is an important parameter because it determines that of the recycled digestion liquor as well as the level of silica impurity in the alumina product. It is therefore desirable to combine with the alumina extraction, a process known as "desilication" of the sodium aluminate liquor in order to reduce the silica concentration in this liquor, and therefore the level of the silica impurity in the alumina product. In order to allow the formation of insoluble sodium silico-aluminates, whose kinetic precipitation is relatively slow, desilication times of several hours are necessary, usually not exceeding ten hours, however This desilication process can be done during the digestion of the bauxite but preferentially during a distinct operation either preceding or following the digestion. See U.S. 4,426,363, FR 1506516 and U.S. 3,413,087, all incorporated herein by reference. These desilication processes all place crushed bauxite in contact with all or part of the decomposed Bayer liquor, with a Na2O concentration between 190 and 200 g/liter and at temperatures between 80C and 200C, according to the nature of the bauxite to be treated. They usually afford satisfactory desilication performances with bauxite comprising less than 3% reactive silica based on the weight of dry bauxite. On the other hand, using prior art processes for the desilication of alumina trihydrate containing bauxite in which the reactive silica content"is less than 3% and often between 0.5 to "1.5%, requires a desilication time of at least 30 hours, or three times the usual desilication time, either before or after digestion of the bauxite. This long desilication period is necessary to reduce the concentration of solvible silica in the supersaturated sodium aluminate solution so that the ratio of soluble SiO2/Na2O is less than 0.90%, preferably less than 0.70%. Under such conditions, the advantages expected from the digestion process occurring at atmospheric pressure are eliminated by the notable reduction of treatment capacity which can only be compensated by scaling up the process. We note that the methods of EP 0203873 and U.S. 4,650,653, both incorporated herein by reference, of increasing the rate of the kinetic precipitation of sodium silico-aluininate by lowering the sodixxm hydroxide concentration to less than 12 0 g/liter in of the solution decomposed liquor used in the pre-desilication step, cannot be applied effectively in the present case because they are contrary to the desired goals herein including: -Preserving the existing production capacity and avoiding any significant increase of the time spent in the reactor and also any volume increase by dilution of the products circulating in the production line. -Obtaining a productivity of at least 70 kg Al2O, per m of supersaturated sodium aluminate liquor. This productivity, P, is the product of the concentration, C, of sodium hydroxide in said liquor and the variation, ARp, of the concentration ratios Rp = soluble AI2O3 (g/l)/Na20(g/l) , between the beginning and the end of the decomposition. It is advantageous to keep the sodium hydroxide concentration in the digestion as high as possible, since this concentration also determines the maximum value of Rp before decomposition and therefore the magnitude of ARp. In fact, during the dilution and the decantation of the suspension obtained following digestion of the bauxite, the risks of precipitation by spontaneous nucleation of part of the alumina trihydrate, known as retrogradation, are greatest when the sodium hydroxide concentration is low. In the present case, the maximum Rp before decomposition cannot be greater than 1.05 which practically limits the productivity to 70 kg Al2O2/m for a final Rp usually between 0.5 and 0.6. -Obtaining an extraction yield of the potential soluble alumina of at least 95%, which is similar to the yields usually obtained with the other types of bauxite. This implies not only a very complete digestion of the ore but also the prevention of any retrogradation that could lead to a significant decrease in yield of alumina, which can be 5 to 10% even 20% of the final product, -Limiting the soluble silica content in the supersaturated liquor before decantation and decomposition, measured by the weight ratio of soluble siO2/Na2O, to less than 0-90%, and preferably less than 0.70%, to guarantee a silica content in the precipitation alumina trihydrate less than 100 ppm. Accordingly the present invention provides a method of producing alumina trihydrate comprising the steps of (a) heating a suspension of ground bauxite in an aqueous solution comprising sodium hydroxide, soluble alumina, and soluble silica, wherein the ratio, Rp, defined as soluble Al2O3(g/1)/Na20 (g/'I) is 0.5 to 0.7, the weight content of soluble SiO2 /Na20 is less than or equal to 0.9%, and the concentration of sodium hydroxide is 140 to 170 gNa2O/liter, and the concentration of dry material in the suspension is greater than or equal to 0.7 ton/m, for at least 30 minutes, at a temperature less than or equal to108C, at atmospheric pressure, to effect desilication; (b) increasing the sodium hydroxide concentration of the suspension from (a) by adding a digestion liquor, wherein Rp is 0.5 to 0.7 and the sodium hydroxide concentration is 180 to 220 g Na20/litre; (c) heating the suspension from (b) at a temperature less than or equal to 1 OS'C, at atmospheric pressure, for a period of time sufficient to extract at least 95% of the extractable alumina trihydrate in said bauxite, affording a supersaturated sodium aluminate suspension; (d) diluting said supersaturated suspension from (c) such that Rp is 1.05 to 1.17 and the sodium hydroxide concentration is 140 to 180 g Na20/liter; (e) heating the suspension fiom (d) at a temperature less than or equal to 108'C, at atmospheric pressure, for a period of time greater than or equal to 2 hours in order to reduce the weight content of soluble Si02/Na20 to less than 0.9%; (0 removing the insoluble solid from the suspension from(e) by decanting said suspension and washing the remaining insoluble solid after decantation with an aqueous solution, affording a supersaturated sodium aluminate liquor, wherein Rp is 1.05 to 1.17, the concentration of sodium hydroxide is 140 to 180 g Na2 0/liter, and the weight content of soluble Si02/Na20 is less than 0.9%. (g) cooling and decomposing said supersaturated sodium aluminate liquor in the presence of said particles of alumina trihydrate, affording a suspension of a alumina trihydrate in decomposed sodium aluminate liquor, wherein Rp is 0.5 to 0.7, and the concentration of sodium hydroxide is 140 to 180g Na20/liter; and (h) separating said alumina trihydrate from (g) by filtering; washing said filtered alumina trihydrate with an aqueous solution, affording alumina trihydrate, wherein the silica content is less than 100 ppm. The invention will now be described in more detail with reference to n the accompanying drawings, in which; Fig. 1 is a schematic diagram of the present invention. The present method for treating alumina trihydrate containing bauxite with a low reactive silica content achieves all of the above-stated goals. The present method is based on the surprising observation that by combining a desilication step prior to digestion of the bauxite, i.e., a pre-desilication, with a desilication step after bauxite digestion, i.e., a post-desilication, one can obtain an alumina trihydrate extraction yield of greater than or equal to 95%, a sodium aluminate liquor productivity greater than or equal to 70 kg AlO2/m, and a weight ratio of soluble Si02/Na20 in the liquor lesss than 0.9%. Thus, alumina trihydrate can be produced in high yield with a silica content below 100 ppm. In the present process, the total desilication time is generally lO hours, which is the same amount of time used in the desilication of other bauxites. Specifically, the present method involves pre-desilication of ground bauxite suspended in a solution, preferably a decomposed sodium aluminate liquor, wherein Rp is 0.5 to 0.7 and the sodium hydroxide concentration is 140 to 17 0 g NaO/liter, by heating at a temperature less than or equal to 108°C for at least 3 0 minutes, preferably at atmospheric pressure, while keeping the concentration of dry material in the suspension greater than 0.7 ton/mFollowing pre-desilication the bauxite is digested with a digestion liquor, preferably a decomposed sodium aluminate liquor, wherein Rp is 0.5 to 0.7 and the sodixm hydroxide concentration is 18o to 220 g Na2O/liter, for a period of time sufficient to extract at least 95% of the alumina trihydrate from the bauxite sample at an elevated temperature, preferably at atmospheric pressure. Following digestion the suspension is diluted wherein Rp is 1.05 to 1.17 and the sodium hydroxide concentration is 140 to 180 g Na2O/liter. This diluted suspension is then sub2ected to a post-desilication step by heating at a temperature less than or equal to 108•'C, usually between 95 to 105"C, for at least two hours, preferably at atmospheric pressure. In many desilication experiments using alumina trihydrate containing bauxite with a low reactive silica content, notably of African or Indian origin, the we have obtained unsatisfactory results using only one isolated desilication step either before or after digestion of the bauxite. On the other hand, combining the effects of the pre-desilication and the post-desilication steps of the present invention allows one to treat without any capacity limitation, by digestion at atmospheric pressure, alumina trihydrate containing bauxite and a low reactive silica content, with performances in terms of yields of digestion and silica purification rates equivalent to the one obtained with the other bauxite containing alumina trihydrate, and also to considerably increase the productivity of the liquors to be decomposed since it is possible to reach Rp values of 1.12 to 1,15 without any risks of retrogradation. The present invention also affords a liquor productivity of at least 80 kg of AlO2 per m of liquor, therefore much higher than the one of the prior art which does not usually exceed 70 kg of AI2O3 per mDESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be better understood from the following detailed description of its preferential implementation with reference to Figure 1, a schematic representation of the present invention. The bauxite containing alumina trihydrate 1 in which the reactive silica content is less than 3% and usually comprised between 0.5 and 1.5%, is placed in contact with an aliquot 14 of decomposed sodium aluminate liquor 11, wherein the ratio Rp is preferentially between 0.57 and 0.67 and sodium hydroxide concentration is 150 to 160 g Na2O/liter, and a weight content of soluble Si02/Na20 not exceeding 0.7%, in order to obtain, after wet grinding A, a thick suspension 2 which undergoes a pre-desilication B by heating, preferentially between 100 and 105°C, for 1 to 6 hours and which concentration in dry material stays preferentially between 0.9 and 1.1 tnr. pr y cf suspension at the end of the pre-desilication. It is important to work with a sufficient concentration in dry material of the suspension because below 0.7 ton per m, one observes a significant decrease in the rate of sodium silico-aluminate precipitation. The thick, pre-desilicated suspension 3 is then digested C, at atmospheric pressure and at a temperature not exceeding lOBC, preferentially between 103 and I07°c, for 1 to 3 hours, by the remaining part 12 of the deeoWposed liquor 13 previously concentrated by evaporation N, called digestion liquor, wherein Rp is preferentially between 0.57 and 0,67, and the sodiuia hydroxide concentration is ad2usted to 19 5 to 215 g NaO/liter. At the end of the digestion C done at atmospheric pressure one obtains a new suspension 4, wherein Rp is preferably between 1-09 and 1.15 and the sodium hydroxide concentration is usually between 175 and 190 g Na2O/liter. This is ad2usted by dilution so that the sodium hydroxide concentration is between 140 and 180 g NaO/liter but preferably between 160 and 170 g Na2O/liter, with, for example, a fraction 20a of the back water washes of the insoluble mud or the water washes of the alumina trihydrate production. It is essential, to ad2ust carefully the sodium hydroxide concentration of the suspension before the post-desilication D. In fact; a sodium hydroxide concentration higher than 180 g Na2O/liter increases the solubility limit of the reactive silica without improving the kinetic precipitation of the soluble silica in the solution into sodium silico-aluminate, maKing it impossible to lower the ratio soluble silica/NaO of the suspension 4 to less than 0.90% without a significant increase of the pre-desilication time (more than 15 hours). Conversely, too high of a dilution of the liquor ( 1-05. For this reason it is recoHunended that a sodiu2n hydroxide concentration of 140-155 g NaO/liter be used in the present invention. It is recommended to aim for Rp values between 1.05 and 1.10 to avoid important yield drops due to retrogradation, which can decrease the liquor productivity to values less than 70 kg Al2Oa/m- The post-desilication D of the diluted suspension 4 is then conducted for preferably 4 to 8 hours at a temperature between 100 and lOSC in order to lower the soluble SiOa/NaaO Weight content to less than 0.9% and usually less than 0.7%, without any significant changes in the sodium hydroxide concentration or the Rp value. The resulting suspension 5 is then decanted E in order to separate, using a well-known method, the red muds 19 placed in discharged 21 after back-washes successively with the water wash 17 of the production alumina trihydrate 16 and pure water 18. The liquor 6 resulting from the decantation E, in which Rp is preferably between 1,08 and 1.15, is again diluted by a fraction 2 0b of the dilution liquor coming from the back-wash washes of the red muds before undergoing a security filtration G in order to eliminate the fine particles of red muds remains the in suspension. The clear filtrate 7 of supersaturated liquor of sodium aluminate, wherein the sodium hydroxide concentration does not exceed 160 g NaO/liter, is cooled and decomposed H in the presence of seed particles of alumina trihydrate lO recycled according to the prior art. The alumina trihydrate in suspension 8 in the deeoaposed liquor is separated by filtration 2 to be for the main part (90% of the weight approximately) recycled as seed 10 and for the minor part 9 (10% approximately) extracted for the production 16 after water 15 washes K. The remainder 12 of the filtrate 11 of decomposed sodium, aluminate liquor, wherein Rp is between 0.55 and 0.67 and the sodium hydroxide concentration is between 150 and 160 g NaO/liter, after taking an aliquot 14 destined to the wet grinding and the desilication of the bauxite/ is concentrated by evaporation M to give the digestion liquor 13 suitable for digesting ground b..uxite as previously described. One will note that the ratio of soluble Si02/Na,O is usually ad2usted to between 0,60 and 0.70% in the liquor 5 after dilution D, allowing for less than 100 ppm of silica in the AlO2 product. This is achieved without modifying the desilication capacities compared to prior art processes as total time spent in the reactor is approximately ten hours- This also without decreasing the extraction yields of the soluble alumina which are greater than 95%. The 7 Examples described hereinafter have been carried out with a homogeneous lot of 60 tons of bauxite of Guinean origin containing alumina trihydrate with a low reactive silica content having the following (dry) composition : Examples 1 and 2 uses standard conditions of pre-desilicatlon and of post-desilication, respectively, of bauxites containing alumina trihydrate as described in the prior art of treatment at atmospheric pressure of bauxites with trihydrate and a low reactive silica content. Examples 3 to 7 involve the use of the present invention for treating the same bauxite sample. EXAMPLE I Pre-desilication according to the prior art and atmospheric digestion of a 5-ton fraction from the 60-ton lot of Guinean bauxite previously characterized comprising: Wet grinding and desilication with an aliquot of decomposed liquor and 1000 kg of dry material per m of thick suspension for 10 hours at 100°C- decomposed liquor characteristics : Rp = 0.58 Na2O; 152 g/1 soluble Si02/Na;0: 0.69% Digestion at 107"C for 1 hour and 3 0 minutes in an atmospheric reactor by the remaining part of the concentrated deeoirosed liquor or digestion liquor of concentration 201 g NaO/liter aiming for an Rp value of 1.05. Characteristics of the supersaturated liquor after digestion, dilution and decantation of the suspension resulting"from the digestion: Although the digestion yield is creater than 95% and the productivity is on the order of 70 kg AISO./E for a final Rp of 0.58, the content of soluble silica is redhibitory for the precipitated alumina trihydrate product as well as for the spent liquor recycled as digestion liquor. In order to satisfy that criteria of the present invention another 5-ton fraction of the bauxite lot undeirwent a prolonged pre-desilication treatment for a total duration time of 3 0 hours while keeping all the other conditions identical. The characteristics of the supersaturated liquor after dilution and decantation are the following: These characteristics are acceptable but require a threefold increase in time necessary for desilication. EXAMPLE II Post-desilication according to the prior art after atmospheric digestion of another 5-ton fraction of the same Guinean bauxite lot comprising: Wet grinding with an aliquot of spent liquor having the same characteristics as in Example I. Digestion at 107"C for 2 hours in an atmospheric reactor by the remaining part of the concentrated spent liquor, aiming for an Rp value of 1.06. After dilution of the suspension resulting from the digestion to 145 g Na,0/liter, a post-desilication of 8 hours at 100°C. Characteristics of the supersaturated liquor after decantation of the suspension resulting from the In order to satisfy the criteria of the present invention it was necessary to implement, on another 5~ton fraction of the same bauxite lot, a post-desilication treatment of a duration time of 32 hours while Keeping all the other conditions identical, thus reducing the SiO2/Na,0 ratio to 0.71% which is entirely satisfactory but at the expense of a four-fold increase in the desilication time. EXAMPLE III This is the first example of the process according to the present invention which comprises combining a very short 1-hour pre-desilication to either a post-desilication of a duration time of 7 hours with dilution following the usual conditions in the range of 145 g Na2O/liter or to a post-desilication of a duration tiiae of 7 hours also but with a weak dilution in order to keep a high sodium hydroxide concentration during the post-desilication, that is to say in the order of 165 g Na,0/liter. These treatments done on a lO-ton fraction of the Guinean bauxite lot previously characterized comprise: Wet grinding and pre-desilication with an aliquot of decomposed liquor and 1000 kg of dry material per m of thick suspension for 1 hour at 100°C, spent liquor characteristics: Rp - 0.58 NaO: 152 g/1 SiO so/NaO: 0-72% Digestion at 107"C for 1 hour 30 rainutes in an atmospheric reactor by the remaining part of the concentrated spent liquor or digestion liquor of concentration 2 03 g Na2O/liter aiming for an Rp value of 1.11, A) On one half of the suspension resulting from the digestion, ad2ustment of the concentration by water dilution to 145 g NaO/liter and post-desilication of 7 hours at 100°C. Characteristics of the supersaturated liquor after decantation of the suspension resulting from the the digestion, ad2ustment of the concentration by weak water dilution to 165 g NaO/liter and post-desilication of 7 hours at 100 Characteristics of the supersaturated liquor after decantation of the suspension resulting from the short pre-desilication with a normal post-desilication for a total duration time of 1 + 7 = 8 hours is surprisingly much more efficient than isolated treatments of-pre-desilication or post-desilication for an equivalent length of time, as described in the prior art. In addition to effectively removing silica impurities and affording high alumina extraction yields, the productivity of the present method is clearly superior in the variable B of Example 2 specially when after dilution of the liquor to 150 g Na.O / liter and aiming to a final Rp The variable A, noticeably more impressive than variable B in terms of desilication because of the diluti effect, is limited in terms of the Rp value due to a slic degradation of the alumina during the post-desilication which is expressed by an appreciable decrease in the extraction yield of 2.5%, compared to variable B. EXAMPLE IV This Example consists of combining a short 3-hour pre-desilication, with either a 7-hour post-desilication with previous dilution using the usual conditions to approximately 145 g Na2O/liter, or to a 7-hour post-desilication with a weaK dilution, that is a previous dilution to approximately 165 g NaO/liter. These treatments were conducted on a lO-ton fraction of the Guinean bauxite lot previously characterized, and comprise Wet grinding and pre-desilication with an aliquot of decomposed liquor and 1000 kg of dry material per m of thick suspension for 3 hours at 100c. The spent liquor characteristics are identical t the one of Example III. Digestion at 107°C for 1 hour and 30 minutes in an atmospheric reactor by the remaining part of the concentrated spent liquor or digestion liquor identical to the one of Example III aiming for an Rp value of 1.15. A) On one half of the suspension resulting from the digestion, adjustment of the concentration by water dilution to 14 5 g Na20/liter and post-desilication of 7 hours at 100°C. Characteristics of the supersaturated liquor after decantation of the suspension resulting from the After ad2ustment of the liquor concentration to approximately 150 g NaO/liter and aiming for an Rp value a1 the end ot the decomposition of 0.57, one registers a very high productivity of the liquor (87 kg Al2O2/m) according to variable IV B, as well as a significant decrease in the silica content. It is noted that only variable IV B allows alumina retrogradation to be avoided whereas variable IV A leads to a significant decrease in extraction yield of the soluble alumina. One also notes that the total desilication time of 10 hours (3 hours + 7 hours) is similar to the one used for the treatment of classical bauxite containing alumina trihydrate. EXAMPLE V This Example consists of combining an average 6-hour pre-desilication, to a standard 7-hour pre-desilication following dilution to approximately 165 g Na2O/liter. The desired Rp being 1.15, only the tests with weak dilution of the suspension resulting from the digestion were done considering the retrogradation observed in Example IV A for such an Rp value. This treatment done on a 5-ton fraction of the Guinean bauxite lot previously characterized, and comprises: Wet grinding and pre-desilication with an aliqxiot of spent liquor and 900 kg of dry material per m of suspension for 9 hours at 105"C. The characteristics of the spent liquor are identical to those of the liquor of Example III. Digestion at 107°c for 1 hour and 30 minutes in an atmospheric reactor by the remaining part of the concentrated spent liquor or digestion liquor identical to the one of Example III while aiming for an Rp value of 1.15. Characteristics after decantation of the suspension Using a total desilication time of 13 hours, one obtains a good purification with respect to the reactive silica in the liquor while keeping an excellent yield, in spite of the high Rp. EXAMPLE VI This Example was designed to determine the highest value of Rp acceptable in the present method invention by combining a long 9-hour pre-desilication, with a standard 7-hour post-desilication with a previous very weak dilution to approximately 175 g NaO/liter in order to limit the retrogradation risks since the desired Rp is 1,17. This treatment was conducted on a 5-ton fraction of the Guinean bauxite lot previously characterized, and comprises: Wet grinding and pre-desilication with an aliquot of spent liquor and 900 kg of dry material per m' of suspension for 9 hours at 105°C. The characteristics of the spent liquor are identical to those of the liquor of Example III. Digestion at 107"C for 2 hours and 30 minutes in an atmospheric reactor by the remaining part of the concentrated spent liquor or digestion liquor identical to the one of Example III while aiming for an Rp value of 1.17. Post-desilication for 7 hours at 100°C of the suspension resulting from the digestion after ad2ustment of the sodium hydroxide concentration to 175 g NaO/liter. Characteristics after decantation of the suspension resulting from the post-desilication: One observes a yield loss of almost 10% compared to the results of Examples III, IV and V which leads to an important decrease in the Rp value. This confirms the obligation to limit the application of the present process to Rp values not exceeding 1.15, even with a relatively high sodium hydroxide concentration before post-desilication. EXAMPLE VII This Example combines an average 5-hour pre-desilication, with a reduced 4-hour post-desilication with weak dilution to approximately 165 g Na2O/liter while aiming for an Rp value of 1.12. This treatment was done on a 10-ton fraction of the Guinean bauxite lot previously characterized, and comprises: Wet grinding and pre-desilication with an aliquot of spent liqfuor and 110 0 kg of dry material per m of thick suspension for 5 hours at 100*C. The characteristics of the spent liguor are identical to those of the liguor of Example III. Digestion at 107"C for 1 hour and 3 0 minutes in an atmospheric reactor by the remaining part of the concentrated decomposed liguor or digestion liquor identical to the one of Example III while aiming for an Rp i/alue of 1.12. Post-desilication for 4 hours at 105°C of the suspension resulting from the digestion and after ad2ustment of the sodium hydroxide concentration to 170 g a2O/liter. After diluting the liquor to approximately 150 g Na2/liter until Rp - 0.57 one obtains a productivity of 82.5 kg Al2O3/m3 The choice of average time conditions for both the pre-desilication and the post-desilication demonstrates that one can achieve an excellent compromise between the silica purification, the extraction yield, and the productivity which is greater than 8 0 kg AlO2/xn of liquor, using a total desilication time not exceeding 9 hours- This application is based on French Application No. 95-04609 filed March 31, 1995, incorporated herein by reference- WE CLAIM: 1. A method of producing alumina trihydrate comprising the steps of (a) heating a suspension of ground bauxite in an aqueous solution comprising sodium hydroxide, soluble alumina, and soluble silica, wherein the ratio, Rp, defined as soluble Al2O3(g/1)/Na20 (g/1) is 0.5 to 0.7, the weight content of soluble SiO2 /Na2O is less than or equal to 0.9%, and the concentration of sodium hydroxide is 140 to 170 g Na2O/liter, and the concentration of dry material in the suspension is greater than or equal to 0.7 ton/m3, for at least 30 minutes, at a temperature less than or equal to108oC, at atmospheric pressure, to effect desilication; (b) increasing the sodium hydroxide concentration of the suspension from (a) by adding a digestion liquor, wherein Rp is 0.5 to 0.7 and the sodium hydroxide concentration is 180 to 220 g Na2O/litre; (c) heating the suspension from (b) at a temperature less than or equal to 108oC, at atmospheric pressure, for a period of time sufficient to extract at least 95% of the extractable alumina trihydrate in said bauxite, affording a supersaturated sodium aluminate suspension; (d) diluting said supersaturated suspension from (c) such that Rp is 1.05 to 1.17 and the sodium hydroxide concentration is 140 to 180 g Na2O/liter; (e) heating the suspension from (d) at a temperature less than or equal to 108oC, at atmospheric pressure, for a period of time greater than or equal to 2 hours in order to reduce the weight content of soluble SiO2/Na2O to less than 0.9%; (f) removing the insoluble solid from the suspension from(e) by decanting said suspension and washing the remaining insoluble solid after decantation with an aqueous solution, affording a supersaturated sodium aluminate liquor, wherein Rp is 1.05 to 1.17, the concentration of sodium hydroxide is 140 to 180 g Na2 O/liter, and the weight content of soluble SiO2/Na2O is less than 0.9%. (g) cooling and decomposing said supersaturated sodium aluminate liquor in the presence of said particles of alumina trihydrate, affording a suspension of a alumina trihydrate in decomposed sodium aluminate liquor, wherein Rp is 0.5 to 0.7, and the concentration of sodium hydroxide is 140 to 180g Na2O/liter; and (h) separating said alumina trihydrate from (g) by filtering; washing said filtered alumina trihydrate with an aqueous solution, affording alumina trihydrate, wherein the silica content is less than 100 ppm. 2. The method as claimed in claim 1 wherein a fraction of said decomposed sodium aluminate liquor is recycled to step (a) and the remaining fraction of said decomposed sodium aluminate liquor is concentrated by evaporation, followed by addition of sodium hydroxide, affording a digestion liquor, wherein Rp is 0.5 to 0.7 and the sodium hydroxide concentration is 180 to 220 g Na2O/liter; and then said digestion liquor is recycled to step (b). 3. The method as claimed in claim 1 wherein the weight content of reactive silica is less than 3% of the dry weight of said bauxite. 4. The method as claimed in claim 1 wherein the weight content reactive of silica is 0.5 to 1.5% of the dry weight of said bauxite. 5. The method as claimed in claim 1 whereas the aqueous solution of (a) is a decomposed sodium aluminate liquor, wherein Rp is 0.57 to 0.67 and the concentration of sodium hydroxide is 150 to 160 g Na2O/liter. 6. The method as claimed in claim 1 wherein step (a) is conducted at a temperature of 100 to 105 °C for 1 to 6 hours. 7. The method as claimed in claim 1 wherein the suspension of (a) has a concentration of dry material of 0.9 to 1.1 ton/m3. 8. The method as claimed in claim 1 wherein said digestion liquor is a concentrated, decomposed sodium aluminate liquor, wherein Rp is 0.57 to 0.67 and the sodium hydroxide concentration is 195 to 215 Na2O/1iter. 9. The method as claimed in claim 1 wherein step (c) is conducted at a temperature of 103 to 107°C for 1 to 3 hours. 10. The method as claimed in claim 1 wherein the sodium aluminate enriched suspension from (c ) has an Rp value of 1.09 to 1.15 and a sodium hydroxite concentration of 175 to 190 Na2O/liter. 11. The method as claimed in claim 1 wherein the diluted suspension from (d) has a sodium hydroxide concentration of 160 to 170 g Na2O/liter. 12. The method as claimed in claim 1 wherein the dilution of step (d) is achieved with aqueous washes of the remaining insoluble solid of step (f). 13. The method as claimed in claim 1 wherein the dilution of step (d) is achieved with aqueous washes of the alumina trihydrate product of a Bayer process. 14. The method as claimed in claim 1 wherein step (f) further comprises a security filtration of the decanted sodium aluminate liquor. 15. The method as claimed in claim 1 wherein step (e) is conducted at a temperature between 100 to 105oC for 4 to 8 hours. 16. The method as claimed in claim 1 wherein the supersaturated sodium aluminate liquor from step (f) has a weight content of soluble Si02/Na2O less than or equal to 0.7%. 17. A method of producing alumina trihydrate substantially as herein described with reference to the accompanying drawing. DUPLICATE

Documents:

0487-mas-1996 abstract.pdf

0487-mas-1996 claims.pdf

0487-mas-1996 correspondence-others.pdf

0487-mas-1996 correspondence-po.pdf

0487-mas-1996 description(complete).pdf

0487-mas-1996 form-1.pdf

0487-mas-1996 form-26.pdf

0487-mas-1996 form-4.pdf

0487-mas-1996 form-9.pdf

0487-mas-1996 petition.pdf