CH619430A5 - Process for the extraction of an aluminium sulphate from an impure solution containing it - Google Patents

Abstract

The solution of composition: Sulphate of: A1 (expressed as Al2O3)...6.88 % Fe (expressed as Fe2O3)...0.195 % Na (expressed as Na2O)....1.5 % Ti (expressed as TiO2)....0.113 % K (expressed as K2O).....1 % H2SO4 uncombined............29.7 % at 120 DEG C, is brought to 78 DEG C by evaporation under vacuum, leading to the departure of 55 g of water per 1000 g of initial solution. The solution is introduced at 78 DEG C at a charge of 1056 l/h continuously into a first crystalliser maintained at 70 DEG C. A suspension of Al2(SO4)3.0.5 H2SO4.11 to 12 H2O is formed. This suspension is passed successively into three other crystallisers in which it is maintained, each time, for three hours at 60 DEG C, 50 DEG C and 40 DEG C. At the outlet of the fourth crystalliser, the suspension is filtered and a cake is recovered comprising crystals of composition Al2(SO4)3.0.5 H2SO4.11.5 to 12 H2O having very low contents of impurities.

Description

The present invention relates to a method for precipitating an aluminum sulphate of formula

A12 (S04) 3 • 0.5 H2SO4 • 11 to 12 H2O

from a hot solution resulting from the attack of an aluminous material with a sulfuric solution and the elimination of the attack residue.

Numerous processes have been described in which an aluminous material, for example a silicoaluminous material, is attacked by a sulfuric solution. The hot solution resulting from such an attack mainly contains aluminum sulphate,

but also excess free sulfuric acid and other metallic sulfates such as iron, titanium, and possibly potassium, sodium and / or magnesium sulfates, the nature and amount of which obviously depend on the composition of the treated material and the conditions of the attack.

For the extraction of the aluminum element from such solutions, various methods have been proposed in which the aluminum is precipitated in the form of a sulfate or in the form of a chloride which can then be decomposed in order to alumina production or sold.

The literature has mentioned numerous aluminum sulphates: neutral anhydrous sulphate, neutral sulphates hydrated at 4, 6, 9, 12,16 or 18 H2O, basic sulphates, various acid sulphates, for example at 1 or 'A H2SO4 and 12 H2O. (See for example “Journal of Metals”, July 1966, p. 811-818, or “Linke”, 4th ed. Volume 1, Van Nostrand Company Inc. 1958, p. 206 et seq.).

Some authors doubt the existence of some of these sulfates, because, in general, their crystallization is slow and their filterability poor; the cake which results from their separation, even by filtration, is impregnated with a large quantity of mother liquors which render the analyzes uncertain, even after washing. When one passes from a pure solution to an industrial solution which, as has just been said, contains other dissolved sulphates, the presence of a significant quantity of mother liquors has, in addition to the difficulties of separation which 'It involves the disadvantage of leaving in the separated solid mass unwanted quantities of metal sulphates, some of which require complex or costly operations, to be subsequently completely separated from the aluminum sulphate. In fact, this separation is hardly total: it requires, most of the time, numerous successive operations to achieve an alumina sufficiently pure for metallurgical uses.

The present invention aims to provide a method for precipitating a well crystallized aluminum sulphate capable of giving, when it is separated from the mother liquors, a cake containing relatively few mother liquors and easily washable. The licensee found that the sulfate of the formula

A12 (S04) 3 • 0.5 H2SO4 • 11 to 12 H2O

could be suitable for this purpose. Its precipitation requires well defined conditions which, in particular, avoid the precipitation of other aluminum sulphates. The difficulties encountered in avoiding such unwanted simultaneous precipitation of other aluminum sulphates are great, since certain areas of precipitation are close, there are metastable equilibria and numerous states of supersaturation, and the precipitation rates of each of the various possible sulfates, especially from supersaturated solutions, are different, which contributes to the simultaneous precipitation of several sulfates whose mixture is difficult to filter and washable.

The method according to the invention is characterized in that said solution is introduced into the first of at least two crystallizers at a temperature at least equal to that of this first crystallizer, its composition being such that, brought to the temperature of said crystallizer, it is in a state of supersaturation and the figurative point of its composition on a diagram plotting on the abscissa the percentage in free H2SO4 and on the ordinate the percentage in AI2O3 is inside a triangle whose apex A is the representative point of the precipitated sulfate and the other vertices B and C of which have respectively 39 and 56% and the ordinate 1 and 0.5%, in that this first crystallizer is maintained at a temperature at most equal to 80 ° C, in that the suspension is circulated from one crystallizer to another, each crystallizer being maintained at a temperature lower than that of the previous one, the temperature of the last crystallizer being at least equal to 15 ° C , and in that the crystals are separated from the solution.

It should be noted that the coordinates of the vertices B and C of the triangle ABC which has just been defined may be slightly modified by the presence in the solution of other sulfates than aluminum sulfate.

The composition of the solution to be treated can be brought within the range previously defined by any means known per se such as: evaporation, addition of water, addition of sulfuric acid.

It is advantageous to wash the crystals separated from the residual solution.

It has been said that one seeks, in each crystallizer, to obtain a concentration of aluminum sulphate in solution close to the static equilibrium at the temperature of this crystallizer. By static equilibrium is meant the concentration which would be reached after a very long residence time, and below which the crystallization does not continue. Due to the circulation of the suspension, a composition of the mother liquors is established which, according to the invention, is kept close to static equilibrium, in order to avoid precipitation of other types of aluminum sulphates. To obtain the desired proximity, one plays on the residence time in each crystallizer and on its temperature.

In general, several successive crystallizers are used, the temperatures of which can range between 80 and 15 ° C. For the same starting liquor, the total amount of crystallized aluminum sulphate is all the more important as the temperature of the last crystallizer is lower. However, we may prefer not to go below 30 to 40 ° C, to avoid inconvenience5

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lower temperatures, in particular those due to the increase in the viscosity of the liquid phase of the suspension, which results in an increase in the proportion of mother liquors in the separated cake during filtration of the final suspension.

The number and the volume of the crystallizers are chosen for a given flow rate according to the speed of crystallization of the sulfate

A12 (S04) 3 • 0.5 H2SO4 • 11 to 12 H20

at each temperature and the degree of proximity desired at each temperature between the concentration of aluminum sulphate of the liquid phase of the suspension present in a crystallizer and the concentration corresponding to the static equilibrium at the temperature of this crystallizer. For this, it is possible to use crystallizers of different volumes with identical temperature differences between the successive crystallizers; the residence time is then different from one to the other. For technological reasons, it is generally preferred to use successive crystallizers of the same volume; under these conditions, better productivity is obtained, for a given overall residence time, by choosing neighboring temperature differences between a crystallizer and the next crystallizer. The content of Al2O3 in the form of aluminum sulphate for each of the liquid phases is, as has been said, close to that which corresponds to the static equilibrium under the conditions in which this phase is found. For example, between 40 and 60 ° C, it will be preferred that the content of Al2O3 in a liquid phase does not exceed by more than 0.7% the content which corresponds to the static equilibrium; at 70 ° C, it is preferable that this difference does not exceed 1.3%.

It may be advantageous to recycle crystals, as primers, mainly in the first crystallizer; such recycling is generally not of practical interest for the following crystallizers which receive a suspension.

The advantages of operating according to the conditions which have just been defined will appear without ambiguity, if we consider the attached figures which represent, on the indicated scale, the crystalline masses obtained, in the case of FIG. 1, by crystallizing, by cooling, without special precautions, an aluminum sulphate solution whose composition was inside the triangle ABC defined above, and, in the case of FIG. 2, by treating according to the present process an aluminum sulphate solution of composition similar to the previous one. It is clear that in the case of FIG. 1, a mixture of different crystalline species, very entangled and difficult to filter, in the case of FIG. 2, crystals of the same species, correctly formed and easily filterable.

For example, we had a composition solution:

Sulfates of:

Al (expressed as AI2O3) 6.88%

Fe (expressed as Fe203) 0.195%

5 Na (expressed as Na2O) 1.5%

Ti (expressed as TÌO2) 0.113%

K (expressed in K20) 1%

H2SO4 not combined 29.7%

at a temperature of 120 ° C.

The temperature of this solution was brought back to 78 ° C. by evaporation under vacuum, resulting in the departure of 55 g of water per 1000 g of initial solution. After this concentration, its free H2SO4 and AI2O3 contents gave it, in a free H2SO4 / AI2O3 diagram, a figurative point having as abscissa 15 31.5% and as ordinate 7.2%.

On the other hand, there were a series of four crystallizers of unit volume 3500 1 with double wall, provided with a mechanical stirring device and arranged in cascade with respect to each other, the full of one flowing into the next. The solution at 78 ° C. was introduced at a rate of 10561 / h continuously into the first crystallizer which was maintained at the temperature of 70 ° C. A suspension of Al2 (S04) 3 • 0.5 H2SO4 was formed. 11 to 12 H20.

After an average residence time of 3 h, the suspension flowed into the second crystallizer maintained at the temperature of 60 ° C. The crystallization continued with an average residence time of 3 h. By overflowing, the suspension flowed into the third crystallizer whose temperature was maintained at 50 ° C. The residence time in this crystallizer was 3 h. By overflow, the suspension flowed into a fourth crystallizer, the temperature of which was maintained at 40 ° C. On leaving this latter crystallizer, the suspension was directed to a rotary filter. 760 kg / h of a cake were collected comprising 145 kg / h of mother liquors and 615 kg / h of crystals having the composition

A12 (S04) 3 • 0.5 H2S04 • 11.5 to 12.5 H20

that we could wash easily. These crystals, similar to those of FIG. 2, had very low levels of impurities in the form of sulfates of:

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Fe (expressed as Fe203) 0.0089%

Na (expressed as Na20) 0.2%

K (expressed in K20) 0.1%

Ti (expressed as TÌO2) 0.07%.

R

1 sheet of drawings

Claims (2)

619,430 2 CLAIMS 1. Precipitation process for an aluminum sulphate of formula A12 (S04) 3 ■ 0.5 H2SO4 • 11 to 12 H2O from a hot solution resulting from the attack of an aluminous material by a sulfuric solution and the elimination of the attack residue, characterized in that said solution is introduced into the first of at least two crystallizers at a temperature at least equal to that of this first crystallizer, its composition being such that, brought to the temperature of said crystallizer, it is in a supersaturation state and the figurative point of its composition on a diagram plotting on the abscissa the percentage in free H2SO4 and on the ordinate the percentage in AI2O3 is inside a triangle of which a vertex A is the representative point of the above-mentioned sulfate and whose other vertices B and C have respectively for abscissa 39 and 56% and for ordinate 1 and 0.5%, in that this first crystallizer is maintained at a temperature at most equal to 80 ° C., in that the suspension is circulated from one crystallizer to another, each crystallizer being kept at a temperature lower than that of the previous one, the temperature of the last crystallizer being at least equal to 15 ° C, and in that the crystals are separated from the solution. 2. Method according to claim 1, wherein the temperature of the last crystallizer is between 30 and 40 ° C.