OA10181A - Process for the treatment of alumina trihydrate bauxite - Google Patents

Abstract

Treatment of alumina trihydrate bauxite by subjecting it to the action of caustic soda by the Bayer process. After crushing and desilication, a first fraction (3a) of at least 40 % by weight of said trihydrate bauxite is subjected at atmospheric pressure to the action of a decomposed liquor aliquot (12) recycled as a leaching liquor (13). The remaining bauxite fraction (3b) is subjected to the action of the remaining leaching liquor (13b) at 130-180 DEG C. The process then includes mixing the two suspensions (4a and 4b) so formed and diluting (D) them preferably using a washing water (20) countercurrent to the red mud (F) to form a third suspension (5), the Al2O3/Na2O g/1 concentration ratio Rp being 1.13-1.28 and the caustic soda concentration being 140-160 g Na2O/litre at a temperature of 100-107 DEG C. Finally, the supersaturated sodium aluminate solution (7) which is filtered (G) after separation by decantation (E) of the insoluble residues or red mud (19) of the suspension (5), is decomposed (H) in the presence of seeds.

Description

1 010181

METHOD FOR TREATING ALUMIMUM TRIHYDRATE BAUXITE

TECHNICAL FIELD The invention relates to a method for treating trihydrated alumina bauxite, according to the Bayer process, combining an alkaline attack with atmospheric pressure of a major fraction of said bauxite with an alkaline attack at a higher temperature of the remaining fraction of the same bauxite.

STATE OF THE ART

The Bayer process, widely described in the literature10, is the essential technique for producing alumina to be converted into aluminum by igneous electrolysis or to be used in the form of hydrate, transition alumina, calcined alumina, alumina or melted, in the many applications in the field of technical aluminas. According to this process, the bauxite ore is hot-etched by means of an aqueous solution of sodium hydroxide or "caustic soda" with appropriate concentration, thus ensuring the solubilization of the alumina and obtaining a supersaturated solution. sodium aluminate. After separation of the solid phase constituting the unseparated residue (sludge) from the ore, the supersaturated solution of sodium aluminate is seeded with alumina trihydrate particles in order to evoke the precipitation of alumina in the form of trihydrated alumina. The alumina-depleted sodium aluminate liquor, which is decomposed, is recycled to the etching stage after being optionally concentrated and recharged with sodium hydroxide to reconstitute a liquor, the so-called etching liquor, whose concentration is suitable for nature of the bauxite to attack.

It is well known to those skilled in the art that the conditions for treating alkali attack bauxites according to the Bayer process must be adapted according to the degree of hydration and the crystallographic structure of the alumina. It also contains the content and the content of the impurities present (silica, iron oxides, humic substances). Thus, bauxites containing monohydrate alumina (boehmite, diaspore) are treated at temperatures above 200 ° C and generally between 220 ° C and 300 ° C, while bauxites containing alumina in the form of trihydrate (hydrargillite also called gibbsite) are etched at temperatures below 200 ° C and generally between 100 ° C and 170 ° C.

In most cases, the extraction yields of the soluble aluminosilane are greater than 95% and the purity of the resulting sodium aluminate liquor (conditioning the purity of the subsequently precipitated trihydrated alumina) is satisfactory if we take the precautionary less to limit the level of these impurities (soluble silica, iron oxides, degraded humic substances, etc.) by selective treatment operations. This purification, especially in the case of lasilice, may however be more or less difficult to achieve depending on the mineralogical forms of the impurity present.

However, the choice of treatment conditions for an alumina trihydrate bauxite is not solely a function of the desired yield and purity criteria. Indeed, other factors, such as the importance of the investment necessary for the implementation of the process as well as its cost of exploitation, including the concept of productivity are to be taken into consideration. By productivity, it is necessary to understand the productivity of the supersaturated sodium aluminate liquor resulting from the alkaline attack, expressed as the quantity of alumina in grams per liter or in kg per m3 of liquor precipitated during decomposition in the presence of primer This product is determined by the product of the Na.sub.caustic concentration of said liquor by the difference in the ratios of concentration Rp = Al.sub.0 / g / l / Na.sub.2 O.sub.G / l between The inlet and outlet of the decomposition can reach or exceed 80 g Al / Og / liter (80 kg Al 2 O 3 / m3).

Thus, such a result can be obtained when the milled bauxite, generally having undergone a desilicating treatment, is attacked between 140 ° and 160 ° C. by a decomposed sodium aluminate liquor which is optionally concentrated by evaporation and whose Na 2 O 3 content is between 130 and 200. g / liter to reach end of attack a weight ratio Rp of the order of 1.20, which after decomposition, is lowered to about 0.60.

If at the end of the attack, even higher Rp values (1.30 to 1.35) are tested, during the cooling and decantation of lasuspension resulting from the attack, the problem of retrogradation in the form of hydragillite is confronted part of the solubilized alumina, that is to say to a precipitation by spontaneous nucleation of the alumina solubilized alumina trihydrate. This retrogradation may also be activated by the presence of the fine particles of the sludge suspension acting as a decomposition starter during decantation of the red sludge so that: the loss of yield by retrogradation can exceed 5% making losing any interest in the search for a Rp very high.

Furthermore, obtaining etching temperatures between 130 ° and 200% requires the use of autoclaves or pressurized reactors (tubular reactor type) generating both large investments and operating costs very significantly increased by the energy consumed and maintenance costs. This is the reason why atmospheric attack processes, known as "atmospheric processes", have been developed which, with the exception of driving conditions carried out near 100 ° C, at atmospheric pressure, extend the other conditions of labauxite attack with trihydrate. Given the simplicity of the means to implement, maintenance and maintenance costs and the importance of investments are significantly reduced. It is on this principle that several wastewater treatment plants are located near the ore extraction site in Africa or India. 4 010181

In addition to the increase in residence times resulting from the slowing down of the desilication kinetics, that is to say the removal of a significant amount of soluble or reactive silica by precipitation in an insoluble form, on the other hand the kinetics of dissolution of the extractable or solubilizable alumina, the major disadvantage of atmospheric processes is their low productivity, which generally exceeds 75 kg / m3. Indeed, in the vicinity of 100 ° C it is not possible to exceed Rp values greater than 1.10 without risking cause, by excessive oversaturation, a significant retrogradation of alumina hydrargillite lprs cooling and decantation of the suspension made at temperatures often below 100 ° C.

PROBLEM

For a person skilled in the art using an atmospheric process for treating trihydrate bauxite, obtaining a productivity of at least 80 kg / m 3 equivalent to that of the higher temperature etching processes without increasing other operating costs or significant investment, remains a poorly resolved problem.

SUBJECT OF THE INVENTION The invention relates to a process for treating trihydrated alumina bauxite according to the Bayer process combining an atmospheric pressure-induced alkaline attack of at least 40% by weight of said bauxite with an alkaline attack at a higher temperature of the fraction. remaining of the same bauxite. This method, which we will call by simplification '' 'in the following description "method of mixed attack" or "procedémixte", provides a satisfactory solution to the previously evoked problem that must solve the skilled person.

More specifically, the invention relates to a treatment process, sodium salt trihydrate debauxite, according to the Bayer process, comprising the steps of grinding and desilication in the presence of an aliquot of decomposed sodium aluminate liquor, and then placing said milled and desilicated bauxite in contact with the remainder of the decomposed liquor, optionally concentrated, to form a suspension which, after dilution, is decanted and filtered to separate the solid fraction from the red sludge of the sodium aluminate liquor, said liquor then being decomposed in the presence of recycled amorphous as etching liquor after separation of the trihydrated alumina, characterized in that: a first fraction of at least 40% by weight of trihydrated and desilicated bauxite is contacted at the atmospheric pressure at a temperature between 100 ° C and 1C7 ° C and for at least one hour with, an aliquot a decomposed liquor and optionally concentrated by evaporation or etching liquor, whose ratio ratio Rp is between 0.5 and 0.7 and the concentration of caustic is between 180 and 220 g. Na 2 O / liter, to form a suspension whose weight ratio Rp is between 1 and 1,20 and the concentration of caustic soda is between 160 and 200 gNa ,, 0 / liter, said first suspension after slight dilution may be subject to further desilicating or post-desilicating by maintaining the temperature at about 100 ° C for at least 2 hours, the remaining fraction of milled and desilicated trihydrate bauxite, not exceeding 60% by weight, is contacted for at least 5 hours minutes between 130 ° C and 180 ° C with the remainder of the decomposed liquor and optionally concentrated by evaporation, or etching liquor, to form a second suspension whose weight ratio Rp is between 1.30 and 1.55 and the concentration in caustic soda is between 160 and 220 g Na 2 O / liter, the two suspensions thus obtained, whose after-attack temperatures are respectively between 90 ° C and 100 ° C for the first time. spension and 110 ° C and 130 ° C for the second suspension after relaxation, areintimately mixed and diluted with the washing water against the current of red sludge to form a third suspension whose Rp concentration ratio is between 1.13 and 1 28 and the concentration of calcium hydroxide is between 140 and 175 g Na 2 O 2, at a temperature of between 100 and 107 ° C, the third suspension, the Rp after separation of sludge decantation, should remain between 1.13 and 1.23 is optionally diluted to adjust its caustic concentration before filtration to between 140 and 160 g Na 2 O / liter, and then decomposed to the primerpresence according to the prior art for precipitating alumina trihydrate which is separated by filtration of the decomposed liquor whose concentration ratio Rp is between 0.5 and 0.7 and the concentration of caustic salt is between 140 and 160 g Na 2 O / liter, - a decomposing liquor fraction e is recycled directly to the étapede wet grinding of bauxite in process head at a rate of environ0,6 m3 per tonne of bauxite to be ground, while the rest of liqueurdécomposée is concentrated by evaporation and recycled as liqueurd'attaque.

In its tests, the plaintiff was able to find that the fact that the The performance of the atmospheric etching process of the trihydrate bauxites could be significantly improved, in particular in terms of productivity, without requiring expensive additional investments or fundamental modifications of the basic process.

This, by simple enrichment of the supersaturated liquor resulting from the atmospheric attack of bauxite (whose Rp can scarcely exceed 1, 10), with the aid of a highly supersaturated liquor so little stable (Rp generally between 1.35 and 1.50) resulting from the higher temperature parallelate attack of a fraction of the same trihydrate bauxite. The intimate mixture of these two liquors immediately after attack makes it possible, after a slight dilution, to obtain, at the least cost, a supersaturated soda-sodium aluminate preserving a sufficient stability at the temperature where the decantation takes place and. after decomposition a high productivity thanks to the best compromise between the 3 essential characteristics which regulate the supersaturation before decantation: temperature, initial Rp and concentration in caustic soda namely:

- Temperature: 100 ° C to 107 ° C - Initial Rp: 1.13 to 1.25 7 010181 - Concentration in caustic soda: 140 to 160 g Ma ^ O / liter

The applicant has also been able to ascertain during the treatment of alumina trihydrate bauxite according to the mixed etching process of the invention that the usual operation of desilication of bauxite generally carried out before attack and of a maximum duration of ten or so hours. In fact, hours should be carried out over more than 30 hours, or even 40 hours, with trihydrate bauxites with a low silica-active weight content and generally not exceeding 2%. This is intended to achieve an acceptable soluble silica scrubbing rate, ie, SiO 2 sol / Na 2 O caust 0.70%, in the sodium sulphate aluminate liquor from which the decomposition of alumina trihydrate is precipitated. in the presence of primer. This problem of the abnormal increase of desilication times, and therefore residence times, very often encountered during the treatment of low-silica-reactive trihydrate bauxites is related to the very slow kinetics of precipitation in the form of sodium silico-aluminate. insoluble reactive silica solubilized in the alkaline liquor; kinetics all the more slowly as bauxite contains less reactive silica to shift the equilibrium of the reactant towards the formation of the insoluble compound.

To avoid annihilating the advantages afforded by the mixed attack according to the invention of the trihydrate bauxites for too long a period of time, the applicant has developed a specific treatment in the particular case of low reactive silica bauxites. Double desilication consisted of pre-sessilication of the bauxite under the usual conditions, that is to say, limited in time to about ten hours, then a desilication after attack, or post-desilication, of a few hours of the only fraction. of ore etched at atmospheric pressure to obtain, after this double drying of this fraction, a weight content of double silica, based on Na.sub.2 O.sub.2 0.50%; This makes it possible to ensure that the liquorsursurate to be decomposed is a mixture of liquors resulting from attacks at atmospheric pressure and at medium pressure, with a weight content of SiO 2 sol / Nagd caust. not exceeding the required limit of 0.70% and this without altering the extraction efficiency of the solubilizable alumina or the liquorproductivity. The invention will be better understood from the following detailed description of the embodiment of the invention with reference to FIG. 1, schematic representation of the mixed etching method in its usual version and FIG. 2 schematic representation of the process. of attack according to the "double desilication" variant, that is to say, associating with the predesilication of bauxite a desilication after attack or post-desilication of the fraction of bauxite attacked at atmospheric pressure.

According to FIG. 1, the alumina trihydrate bauxite 1 is placed in contact with an aliquot 14 of sodium aluminate decomposed liquor 11, the Rp of which is preferably between 0.57 and 0.65 and the concentration of caustic soda inclusive of preferably between 150 and 160 g Na 2 O / liter at 1600 to 1800 kg of dry ore to be treated per m3de liqueur to form after wet grinding to a slurry 2 which undergoes desilication B by heating between 90 ° C and 105 ° C for a sufficient time, generally between 5 and 10 hours, to dissolve the solubilizable silica (in particular the kaolin Al 2 O 3, 2SiO 2 · 2 O 0) contained in the bauxite and reprecipitate it in the form of desilicoaluminate of sodium which is very slightly soluble but of which the kinetics of precipitation is slow.

The slurry 3 after grinding and desilication is split into 2 fractions:

A first fraction 3a representing at least 40% and preferably 50% to 65% by weight of the bauxite to be treated 1, is brought into contact Ca, for 2 to 5 hours and at a temperature between 103 ° C and 107 ° C with an aliquot 13a of etching liquor 13, the Rp of which is between 0.57 and 0.65 and the concentration of caustic soda of between 195 and 215 g / l. At the end of the attack, a first suspension 4a is obtained, the Rp of which is between 1.05 and 1.15 and the content of calcium hydroxide is between 175 and 185 g Na 2 O / liter. The remaining fraction corresponding to not more than 60% and preferably 35% to 50% by weight of the bauxite to be treated is brought into contact with Cb for 10 to 30 minutes in an autoclave heated between 140 ° C. and 155 ° C. with the other aliquot 13b of the etching liquor 13 to form a second suspension 4b whose Rp is between 1.30 and 1.50 and the concentration of caustic Na.sub.2 O.sub.1 is between 175 and 195 gNa.sub.2 / liter.

The mixture and dilution of the suspensions 4a and 4b is carried out in the next step D. More precisely the suspension 4a resulting from the atmospheric attack, already slightly cooled between 95 ° C. and 100 ° C., is intimately mixed with the suspension 4b, which is autoclave attack, whose temperature after successive detents is still about 120 ° C which allows to heat both the suspension too cold 4a atmospheric attack and the dilution liquor 20? from the countercurrent washing F of red mud. This dilution liquor containing less than 100 g of Na 2 O / liter is intended to lower the caustic soda concentration of the mixture of suspensions 4a and 4b in a concentration range more adapted to the bonneréalisation of the subsequent operations of decantation and especially decomécomposition.

Thus the suspension resulting from the mixing and dilution step D, whose temperature is between 100 ° C. and 105 ° C. with a ratio Rp of between 1.135 and 1.25 and a concentration of caustic soda of between 140 and 160 g Na 2 O / liter is decanted E to separate> in known manner the red sludge 19 discharged 21 after backwashing successively by the washing water 17 trihydrate production alumina 16, then with pure water 18.

The liquor 6 from decantation whose Rp must remain between 1.13 and 1.23 undergoes G security filtration to remove the remaining suspended sludge particles in suspension. The clarified filtrate 7 of supersaturated sodium aluminate liquor is decomposed by the presence of recycled trihydrate primer according to the prior art. Alumina hydrate in suspension in the decomposed liquor is separated by filtration to a large extent (about 90% by weight) recycled as primer 10 and partly minor (about 10%) extracted for production after washing. 15. The residue 12 of the filtrate 11 of decomposed Sodium Alumiriate liquor of between 0.57 and 0.65 and a caustic concentration of between 15 ° C. and 160 g. an aliquot 14 for wet milling and desilication of bauxite at 1600 to 1800 kg of dry ore per m 3 of liquor is concentrated by evaporation to provide the etching liquor 13 of milled and milled bauxite, the characteristics of which have been already been specified.

It is obviously advantageous in the mixed process to work with high soda concentration (according to the European Bayer process) and to carry out settling in high performance settling tanks in order to reduce the specific energy consumption but above all to further improve the productivity by limitation of residence times, cooling times and consequently of the staggering.

Moreover, the mixed process according to the invention, compared with the processes of the prior art, allows the same weight of ore to significantly reduce the volumes of product treated in an autoclave between 140 ° C and 160 ° C. Thus, in an example according to the preferred version in which 35% to 50% by weight of the bauxite is etched in an autoclave, the volume v of etching liquor used is significantly lower than that V of the atmospheric process. It is indeed decreasing according to the weight ratios Rp targeted at the end of the attack in the two processes (approximately 1.10 for the atmospheric process and 1.425 for the mixed process), so that v = V (1.10 - 0 , 6) = 0.6 VΓΚ425 - 0.6)

Compared to the volumes v 'and V of the corresponding suspensions, it is about 0.6 V, thus translating a volume reduction of the order of 30% of the flows of material and thus limiting to a flow volume of 21 to 30% of the total material flow the autoclaving of the 35 to 50% by weight of the trihydrate bauxite.

In terms of investment, this observation is very important for the man in the business, because the cost of implementing some pressurized reactors intended to treat only 21 to 30% of the total flow of material is largely offset by the economy. investment, made elsewhere on the whole process, because of the reduction of specific volumess products treated at each stage.

In the variant, according to FIG. 2, adapted to the treatment of bauxites with a low content of reactive silica, the operations of grinding A, predesilication B, then the attacks of the two fractions 3a and 3pi respectively are carried out strictly according to that of FIG. the atmospheric pressure Ca for fraction 3a and autoclave Cb between 140 ° C and 155 ° C for fraction 3b. At the end of these two attacks conducted in parallel, the following results are obtained: a first suspension 4a whose Rp is between 1.05 and 1.15 and the concentration of caustic soda is between 185 and 195 gNa 2 O / liter.

By dilution, for example with a portion of 20a of the dilution water from the countercurrent washing of the insoluble sludge or bleaching of the production alumina trihydrate, the caustic soda concentration of this slurry 4a is adjusted between 160 and 180 g. O / liter and preferably between 165 and 175 g Na 2 O / liter. It is then carried out by maintaining at a temperature of between 97 and 103 ° C. for more than 2 hours and preferably 3 to 5 hours the post-desilication of this suspension 4a, whose soluble silicon content, based on sodium hydroxide Na 2 O, is lowered between , 40 and 0.50% to give the suspension 4'a. A second suspension 4b whose Rp is between 1.30 and 1.50, the concentration of caustic Na.sub.2 O is between 175 and 195 gNa.sub.2 O / liter and the level of soluble silica referred to NagO is between 0.degree. 80 and 0.90%.

The mixture and dilution of the suspensions 4'a and 4b is carried out in the next step D. More specifically, the suspension 4'a issue of the atmospheric attack, already slightly cooled between 95 and 100 ° C, is intimately mixed with the suspension 4b, resulting from the autoclave attack, whose temperature after successive detents is still about 120 ° C, which allows to heat both the cold suspensiontrop 4'a atmospheric attack and a part 20b of Dilution indicator 20 from the countercurrent washing F of the red sludge. This dilution liquor 20b containing less than 100 g of Na 2 O / liter is intended to lower the caustic soda concentration of the mixture of suspensions 4'a and 4b in a concentration range slightly less than 175 g Na 2 O / l more suitable for bonneréalisation of the subsequent operations of decantation. Thus, the suspension resulting from the mixing and diluting step D, whose temperature is between 100 and 105 ° C with a ratio Rp of between 1.135 and 1.25, a caustic concentration of between 160 and 175 g / l. liter and a rate SiO ^ / Na ^ O & 0.70% is put endécantation E to separate in a known manner the sludge 19 put -> discharge 21 after backwashing successively by washing water 17 production alumina trihydrate 16, then the waterpure 18.

The liquor 6 from decantation, whose Rp must remain between 1.13 and 1.23, is again diluted by a third portion 20c of the dilution liquor 20 from the countercurrent washing of the red sludge and then undergoes filtration. G safety to remove sludge finesparticles still in suspension. The supernatant clarified filtrate of saturated sodium aluminate, the concentration of which must not exceed 160 g / l, is decomposed H in the presence of recycle hydride primer recycled according to the prior art. The end of the operating cycle is performed in exactly the same way as previously described from FIG.

It should be noted that the level of silica controlled between 0.60 and 0.65% in diluent 5 after dilution D makes it possible to guarantee in the trihydrate) alumina precipitated at the decomposition stage less than 100 ppm of silica referred to Al 1 O 2. and this without substantially modifying the capacities necessary for the desilication envisaged to implement usually the mixed etching process of bauxite with trihydrate, which therefore reserves all its advantages, as can be seen from the examples 13

0101SI following applications namely: - high productivity greater than 80 kg Al ^ O ^ / m ^ - a bauxite extraction yield greater than 95% - reduced operating costs and especially investment.

EXAMPLES OF APPLICATION

The first 3 examples of application were made from 3 identical spots of 100 tons of Indian origin trihydrate bauxite composition (dry) following:

Al203%: 44.7 Si0? %: 2,7Fe203%: 26,6 Ti02%: 2,4 of which SiO2 reagent%: 2,3perte on fire%: 23,4

Example 1

The entire first batch of 100 tonsatmosphérique according to the prior art. was treated by etching - wet milling and desilication in the presence of a 14 deliquid aliquot decomposed at a rate of 1660 kg of bauxite per m3 of liquor for 10 hours at 100 ° C. - Liquor attack 13 after evaporation:

Rp - 0.61 Na2O g / l = 200.3 - Attack at 106/107 ° C for 2 hours in an atmospheric batch batch reactor after grinding and desilication. - Suspension 4a after attack:

Rp -1.10 Na20 g / 1 = 180.2

Temperature 105 ° C - Suspension 5 after dilution of the suspension 4a

Rp: 1.08 Na 2 O / g = 150.4 14 010181

Temperature 89 ° C - Liquor 7 before decomposition

Rp - 1.03 Na2O g / l = 150.4 - Decomposition in the presence of alumina trihydrate primer between 62 ° C and 55 ° C. - Decomposed liquor 11 before recycling as attack liquor

Rp: 0.61 Na2 O / g = 156.8

That is, a productivity of (1.03-0.61) 150.4 = 63.2 g / l (63.2 kg / m 3).

Mixed attack according to the invention of the 2nd batch of 100 tons of trihydrated'alumine:

Half of the 50 tonne batch was treated by atmospheric attack. The other half of the 50 tonne batch was treated by autoclave attack at 155 ° C. - wet grinding and desilication under the conditions of Example 1. - etching liquor 13 after evaporation

Rp = · 0.59 Na2O g / l - Attack at 105/106 ° C for 2 hours in an atmospheric reactor of 50 tonnes of milled and desilicated bauxite. - Suspension 4a after atmospheric attack

Rp - 1.11 Na2O g / 1 = 184.8

Temperature 105 ° C - Attack at 155 ° C for 10 minutes in an autoclave of the other 50 tons of milled desilicate bauxite. 010181 - Suspension 4b after autoclave attack and partial relaxation

Rp - 1.46 Na2 O / g = 203.2

Temperature ^ 120 ° C - Suspension 5 after mixing and dilution suspensions 4a and 4b

Rp - 1.23 Na2O g / 1 - 148.4

Temperature 104 ° C - Liquor 8 before decomposition

Rp = 1.22 Na 2 O / g = 148.4 Decomposition in the presence of alumina trihydrate primer between 62 ° C. and 55 ° C. - Decomposed liquor 11 before recycling as attack liquor

Rp = 0.59 Na2Og / 1 = 157

That is a productivity of (1.22 - 0.59) 148.4 = 93.5 g / l (93.5 kg / m3).

Example 3

Mixed attack according to the invention of the third batch of 100 tons of trihydrated alumina. 65% of the scit batch 65 tonnes was treated by atmospheric attack 35% of the lot or 35 tonnes was treated by autoclave attack at 145 ° C. - wet grinding and desilication under the conditions of Example 1 - etching liquor 13 after evaporation

Rp - 0.57 Na 2 O - 196.1 - attack at 104/105 ° C for 4 hours in an atmospheric reactor of 65 tonnes of desilicated milled bauxite. 16 010181 - Suspension 4a after atmospheric attack

Rp = 1.10 Na £ 0 g / l - 175.8

Temperature 106 ° C - attack at 145 ° C for 15 minutes in an autoclave of 35 tonnes of milled debauxite desilicate - Suspension 4b after autoclave attack

Rp - 1.40 Na2O g / 1 - 192.1

Starting off temperature after partial expansion: 120 ° C - Suspension 5 after mixing and dilution of suspensions 4a and 4b Rp - 1.175 Na 2 O / g = 145.8

Temperature 101 ° C - Liquor 8 before decomposition

Rp - 1.17 Na2O g / l = 145.7 - Decomposition in the presence of alumina trihydrate primer between 62 ° Cr; and 55 ° C. - Decomposed liquor 11 before recycling as attack liquor

Rp = 0.57 Na 2 O / g = 153.2

That is a productivity of (1,17 - 0,57) 145,7 = 87,5 g / l (87,5 kg / m3) Ό Example 4 2nd 4th example of application associating the double desilication with the mixed attack, a was made from a lot of 100 tons of alumina trihydrate of Indian origin with low reactive silage content having the following composition (dry): j ·. Al203%: 45.45 Si02%: 2.57 of which Si02 reagent:% 1.30

Fe 2 O 3%: 25.13 TiO 2%: 2.58.

The conditions of grinding, predesilication and then atmospheric pressure and medium pressure attacks are strictly the same as those of Example 2, on the one hand obtaining a suspension 4a resulting from the atmospheric attack and containing:

Na 2 O - 193 g / 1 Rp = 1.09 SiO 2 / Na 2 O = 0.66% ·> on the other hand a suspension 4b resulting from the medium pressure attack and containing:

Na 2 3 3 = - 178.6 g / l Rp = 1.35 SiO 2 / Na 2 O = 0.87%,

The suspension 4a is then subjected to a post-desilication in the following conditions: after a slight dilution with a washing water 20a sludge with precipitated alumina trihydrate for production, adjust the concentration to 170 g Na 2 O / liter, the suspension 4a is maintained under stirring at 100 ° C. for 4 hours, the resulting Lasuspension 4'a title then:

Na2O = 171 g / 1 Rp - 1.09 SiO2 / Na2O = 0.50%.

Thanks to the excellent purification rate in silica, the suspension resulting from the mixture of the suspensions 4'a (approximately 60% of the total volume) and 4b (approximately 40% of the total volume) titre before dilution 0 anddécantation E:

Na 2 O = 187.2 g / 1 Rp - 1.19 SiO 2 / Na 2 O - 0.64%. It should be noted that by only pre-silicating for 10 hours at 100 ° C. the SiO 2 / Na 2 O level is 0.76% and that a duration of 39 hours under these conditions is necessary to lower the SiO 2 level. / 0.65% Na2O.

Thus, it is found that double desilication is the only industrial means which allows, at a high concentration of Na.sub.2, to rapidly lower the silica content in the liquor from the atmospheric attack at a sufficiently low SiO.sub.2 / NaO.sub.2 content of 0.5%. in order to compensate for the systematically high silica content (SiO 2 / Na 2 O between 0.8 and 0.9%) of the liquor resulting from the average pressure attack, and thus to maintain the SiO 2 / Na 2 O 2 ratio required for the mixing of these liquids. 2 liquors destined to be decomposed. 010181 18

After diluting D with an aliquot 20b of washing water 20, the suspension 5 with a titre of Na <0.0 = 169.5 g / l; Rp = 1.18 and SiO 2 / Na 2 O 3 0.63% is decanted The liquor 6 from decantation E is rediluted by another aliquot 20c of wash water to lower the caustic concentration to less than 160. Na 2 O / liter After filtration 6 and before decomposition H, the liquor 7 is then titrated:

Na 2 O - 149 g / l Rp = 1.16 SiO 2 / Na 2 O - 0.59

By decomposition in the presence of alumina trihydrate primer between 62 and 65 ° C, a decomposed liquor 11 is obtained, titrating: Na 2 O ~ 157 g / l Rp = 0.58 and SiO 2 / Na 2 O = 0.58 which is recycled as attack liquor.

The liquor productivity is therefore: (1.16 ·· 0.58) 149 - 86.4 g / l (86.4 kg / nr 3) therefore greater than 80 g / l and this while maintaining an overall yield of A solubilizable alumina extraction of greater than 96% and a silica weight in the precipitated alumina trihydrate less than 50 ppm.

ADVANTAGE OF THE PROCESS

In addition to the fact that productivities of the order of 90 kg / m 3, equivalent to those of high temperature etching processes and better than those of atmospheric processes are obtained with the mixed process according to the invention, the latter has the advantage of can be easily integrated into an atmospheric process. Thus, in the context, for example, of an increase in the capacity of a process line using the atmospheric process, the excess capacity can be easily obtained by the use of a small auxiliary chain of autoclaves which operating under the conditions of the mixed process of the invention, it will optimize the productivity of the whole of the existing chain but will also reduce the overall amount of the investment, as a result of the reduction in the specific volumes of treated materials which is allowed. the mixed process. Indeed the cost of acquisition of additional means of pressure attack, including autoclaves preferably tubular reactors ior because of the abrasivity overpensions very concentrated in dry matter and soda, is widely compensated by the savings made on the number additional devices (decanter, decomposer, atmospheric reactors, 1 (<evaporators ...) that should be implemented in the context of an atmospheric etching process according to the prior art.

Claims (6)

Anspruch [en] A method for the treatment of alumina hydrate bauxite by etching, according to the Bayer process, comprising the grinding and desilication steps in the presence of an aliquot of sodium aluminate liquor, and then contacting the said bauxite milled and desilicated with the remainder of the decomposed liquor, optionally concentrated, to form a suspension which, after dilution, is decanted and filtered to separate the solids fraction of red sludge from the supersaturated sodium aluminate liquor which it then decomposed in the presence of a primer and then recycled as etching liquor after separation of the trihydrate of alumina, characterized in that; - A first fraction of at least 40% by weight of trihydrated and desilicated bauxite is contacted at atmospheric pressure at a temperature between 100 ° C and 107 ° C and> for at least 1 hour with an aliquot of decomposed liquor and optionally concentrated by evaporation or etching liquor, whose weight ratio Rp is between 0.5 and 0.7 and the concentration of caustic soda is between 180 and 220 gNa20 / liter, to form a first suspension, the ratio of which) the weight ratio Rp is between 1 and 1.20 and the concentration of caustic soda is between 160 and 200 g Na 2 O / l, - the remaining fraction of milled and silicated trihydrate bauxite, not exceeding 60% by weight, is contacted for at least 5 minutes between 130 ° C and 180 ° C with the remainder of the decomposed label and optionally concentrated by evaporation or etiqueur etching, to form a second suspension whose Rp is equal ratio com taken between 1.30 and 1.55 and the corcentration ensoude caustic is between 160 and 220 g Na ^ Otre - the 2 suspensions thus obtained, whose postappaque temperatures are respectively between 90 ° C and 100 ° C for the 1st suspension and 110 ° C and 130nC for the second suspension after stirring, are intensely mixed and diluted with the backwash water of the red sludge to form a third suspension whose ratio of Rp concentrations is between 1.13 and 1.28 and the concentration of caustic soda is between 140 21 010182 and 175 g Na 2 O / liter, at a temperature between 100nC and 107 ° C, - said 3rd suspension, of which ^ e Rp after separation of sludge decantation, must remain between 1, 13 and 1.23 is optionally rediluted to adjust its concentration of caustic before filtration between 14C and 160 g Na ^ u / liter, anddécomposée in the presence of primer according to the prior art to precipitate trihydrate of alumina which is separated by filtration from the decomposed liquor whose Rp concentration ratio is between 0.5 and 0.7 and the concentration of caustic soda is between 140 and 160 g Na 2 O / liter. - A fraction of decomposed liquor is recycled directly to the wet milling stage of the bauxite at the head of the process raiseth about 0.6 m3 per ton of bauxite to grind, while the remainder v. of decomposed liquor is concentrated by evaporation and then recycled as an attack liquor. 2) Process according to claim 1 characterized in that preferably 50 to 65% by weight of the bauxite is etched at atmospheric pressure and the remaining fraction of 35 to 50% is etched under pressure autoclave. 3) Method according to claim 1 or? characterized in that the weight ratio Rp of the liquor aliquot decomposed and concentrated by evaporation, or etching liquor is between 0.57 and 0.65 and its concentration of caustic soda is between 195 and 215 g Na20 / liter. 4) Process according to any one of claims 1 to 3 characterized in that the contact time at atmospheric pressure between 103'C and 107 ° C of the 1st fraction of bauxite "trihydratebrushed and desilicate is between 2 and 5 hours. 5) Process according to any one of Claims 1 to 4, characterized in that the weight ratio Rp of the first suspension resulting from the atmospheric attack of the 1st fraction of bauxite is between 1.05 and 1.15 and its The concentration of caustic soda is preferably from 175 to 185 grams per liter. 6) Process according to any one of claims 1 to 5 characterized in that the autoclaving time between 140 ° C and 9 155 ° C of the remaining fraction of milled and desilicate trihydrate bauxite is between 10 and 30 minutes. 7) Process according to any one of claims 1 to 6 characterized in that the weight ratio Rp of the second suspension resulting from the autoclave attack of the remaining fraction of bauxite is between 1.30 and 1.50 and its concentration in caustic soda is between 175 and 195 g Na 2 O / liter. 8) Process according to any one of claims 1 to 7 characterized in that the Rp weight ratio of the third suspension resulting from mixing the two previous suspensions after dilution with a portion of the wash water against the flow of red sludge is between 1.135 and 1.25 and its concentration in caustic soda is between 160 and 175 g Na 2 O / liter. 9) Process according to claim 8 characterized in that the temperature of the third suspension is between 100 ° C. and 105 ° C. 10) Process according to any one of Claims 1 to 9, characterized in that, after separation by filtration of the alumina trihydrate, the weight ratio Rp of the decomposed liquor is between 0.57 and 0.65 and its concentration in caustic soda. is between 150 and 160 g / l. 11. Process according to claim 1 or 10, characterized in that the fraction of decomposed liquor of Rp of between 0.57 and 0.65 and of concentration of caustic soda of between 150 and 160 g / l are recycled to the head wet milling step to form a slurry at a rate of 1600 to 1800 kg dry ore per m of decomposed liquor; 12. Method according to any one of claims 1, 10 or 11 characterized in that the desilicatation of the thick slurry wet grinding is carried out between 90 and 105 ° C for 5 to 10 hours. 13. Process according to any one of Claims 1 to 5 and 10 to 2, characterized in that, after slight dilution, the suspension resulting from the atmospheric attack is subjected to a new desilication or post-desilication by keeping the temperature at approximately 100 ° C. C. The process according to claim 13, wherein the duration of the post-desilication is between 2 and 8 hours and preferably between 3 and 5 hours. 15. Process according to claim 13, characterized in that the concentration of the suspension resulting from the atmospheric attack is adjusted by diluting between 160 and 180 g Na 2 O / liter and preferably between 165 and 175 g Na 2 O 3. /liter. 16. The method of claim 15 characterized in that the dilution of said suspension is carried out by a portion of the dilution water from the countercurrent washing insoluble sludge or washing the production alumina trihydrate. 17. A process according to any one of claims 1 to 5 and 10 to 16characterized in that the solubilized SiO 2 content by weight added to caustic Na 2 O in the suspension resulting from the attack at atmospheric pressure is between 0.40 and 0.50%. 18. Process according to any one of Claims 1 to 17, characterized in that the solubilized SiO 2 content by weight relative to the sodium hydroxide Na 2 O in the mixture of the two suspensions resulting respectively from the attacks at atmospheric pressure and at medium pressure. is less than or equal to 0.70%.