GB2128977A

GB2128977A - Process for the manufacture of basic aluminium chlorosulphates

Info

Publication number: GB2128977A
Application number: GB08328341A
Authority: GB
Inventors: Bernard Gurtner; Jean Guerin
Original assignee: Ugine Kuhlmann SA
Current assignee: Ugine Kuhlmann SA
Priority date: 1982-10-26
Filing date: 1983-10-24
Publication date: 1984-05-10
Also published as: GB2128977B; JPS59116125A; FR2534897A1; NL192450B; CH660724A5; BR8305886A; ES8406058A1; ES526742A0; CA1248733A; AU2053783A; NL8303672A; PT77516B; PT77516A; FR2534897B1; JPH0367967B2; IT8368104A0; NL192450C; DE3338624A1; GB8328341D0; IT1167350B

Abstract

Process for the manufacture of basic aluminum chlorosulphate corresponding to the general formula: [Aln(OH)3n-m-2p Clm (SO4)p]z (I> in which: 3n-m-2p DIVIDED 3n is from 0.4 to 0.7, p is from 0.04 to 0.25 n, and m/p is from 8 to 35, z being >/= 1, which process comprises reacting basic aluminium chloride with basic aluminium sulphate, both heated beforehand, at a temperature of from 80 to 120 DEG C, the relative proportions of the constituents being chosen so as to give a product corresponding to formula (I) above. The basic aluminium chlorosulphates prepared according to the invention are suitable for use in water purification.

Description

SPECIFICATION Process for the manufacture of basic aluminium chlorosulphates The present invention relates to a process for the preparation of basic aluminium chlorosulphate salts of the general formula:

in which the basicity of the aluminium chlorosulphate, defined by the ratio

is from 40 to 70%.

These products are generally aqueous solutions having a concentration of 1 to 35 mols of alumina per litre, but they can also be presented in the form of a more or less hydrated solid containing more than 30% by weight of soluble alumina.

These basic aluminium chlorosulphates are especially useful for water purification.

It is known to prepare basic aluminium chlorosulphates by the following procedure described in French Patent 2036685. Thus, hydrargillite is attacked with a mixture of hydrochloric acid and sulphuric acid, and the neutral chlorosulphate is brought to the desired basicity by partial neutralisation and consolidated by means of alkali metal carbonate, hydroxide or bicarbonate. This type of process does indeed make it possible to prepare the most suitable compounds, but has the disadvantage of giving a considerable amount of alkaline earth metal sulphate, for example calcium sulphate, as a by-product, which presents pollution problems.

There are other variants of the above process as described in French Patent No.2281895 in which the chloride ion is provided by calcium chloride and not in the form of hydrochloric acid. However, with this type of process, the problem of effluents is further exacerbated.

It is for this reason that attempts have been made to overcome this shortcoming of the prior art by carrying out the basification step in a different way: the aluminium chlorosulphate or chloride is treated with powdered aluminium metal. Apart from the economic handicap arising from the use of aluminium powder, this process does not make it possible to obtain basic aluminium chlorosulphate salts having the polymeric structure desired for water treatment.

Finally, a process has been proposed in which the basification is carried out by bringing a mixture of aluminium chloride, aluminium hydroxide and sulphuric acid to a temperature of 170 C, under pressure, the excess unreacted products being recycled if appropriate (French Patent 2,125,337). This procedure is similar to the basification technique already known which consists in attacking an excess of hydrargillite with hydrochloric acid, this reaction also being carried out under pressure in order to accelerate its kinetics (French Patent 2,174,114). In this procedure, it is necessary to add the sulphuric acid before the basiciation stage, in order to obtain a basic chlorosulphate having a greater efficiency in water treatment than a similar salt basified in the absence of sulphuric acid.

We have now unexpectedly found that it is possible to prepare a basic aluminium chlorosulphate having good characteristics in water treatment by carrying out the basification stage under pressure in the absence of sulphuric acid, contrary to what might have been expected from the teaching of the prior art.

It was surprising to find that, after the aluminium hydroxide has been dissolved in a concentrated solution of aluminium chloride, under pressure, the reaction mixture can be stabilised, that is to say the basic aluminium chloride can be kept in solution, by the addition of a more dilute solution of slightly basic aluminium sulphate. The procedure for performing this stabilisation is as follows: after the supersaturated solution of basic aluminium chloride has been brought back to its boiling point at atmospheric pressure, a freshly prepared solution of basic aluminium sulphate, obtained by attacking an excess of aluminium hydroxide with sulphuric acid at atmospheric pressure, is added, the temperature of the latter solution being as close as possible to that of the basic chloride.

The present invention therefore provides a process for the manufacture of basic aluminium chlorosulphate corresponding to the general formula:

in which:

is from 0.4 to 0.7, p is from 0.04 to 0.25 n, and m/p is from 8 to 35, z being 3 1, which process comprises reacting basic aluminium chloride with basic aluminium sulphate, both heated beforehand, at a temperature of from 80 to 12000. The reaction is rapid: the reaction time is usually from a few minutes to one hour.

The relative proportions of the constituents are determined so as to give a product having the formula (I) above.

The basic aluminium chloride which can be used according to the present invention can be obtained by any suitable means. In particular, it can be obtained in the following manner, which is in itself known: hydrochloric acid is reacted with alumina in proportions such that, after reaction, the solution of aluminium chloride obtained is close to supersaturation, and this solution is then reacted with a 10 to 150% excess of aluminium hydroxide, relative to the theoretical amount required for the desired basicity, in a stirred medium, for a maximum period of 5 hours, at a temperature of 140 - 16500 and under a pressure of 0.5 to 3 bars.

The basic aluminium sulphate which can be used according to the present invention can be prepared by any suitable means. In particular, it can be prepared in the following manner, which is in itself known: a solution of sulphuric acid is treated with a 10to 100% excess of aluminium hydroxide, relative to the stoichiometry of the neutral sulphate, and the suspension obtained is heated and kept at boiling point for 15 to 30 minutes.

More particularly, the process, which can be a continuous or batch process, can be carried out in the following manner: hydrochloric acid having a concentration by weight equal to or greater than 30%, and preferably 33%, and aluminium hydroxide (for example hydrargillite) are introduced into a stirred reactor in proportions such that, after reaction, the solution of aluminium chloride obtained is close to saturation; the solution, which can contain suspended aluminium hydroxide if the previous reaction has been carried out with an excess of this reactant, is then transferred into a pressure-resistant reactor, into which aluminium hydroxide is also introduced so as to have a 10 to 150% excess, and preferably a 70 to 120% excess, relative to the theoretical amount required for the desired basicity.The reaction medium is stirred and heated at a temperature of 140 - 165or, under a pressure of 0.5 to 3 bars, for a maximum period of 5 hours, but preferably for 2 to 3 hours depending on the fineness of the hydroxide used; these operating conditions make it possible to achieve maximum dissolution of the aluminium hydroxide. The reaction mixture is then brought back to atmospheric pressure, the temperature then being 120 - 12500, depending on the initial cocentrations of chloride, the degree of dissolution of the hydroxide and the water content of the latter.

Simultaneously to the basification of the aluminium chloride, the basic aluminium sulphate intended for stabilising the basic chloride is synthesised. A solution of sulphuric acid is treated with a 10 to 100% excess, and preferably a 30 to 70% excess, of aluminium hydroxide, relative to the stoichiometry of the neutral sulphate, and the suspension obtained is heated and then kept at the boil for 15 to 30 minutes; this period varies according to the fineness of the hydroxide used.After dilution with water in order to fluidise the reaction medium, this slightly basic aluminium sulphate, the temperature of which is then within the range 70 to 100 C, is poured into the basic aluminium chloride, the temperature of which is 120 - 12by, in proportions such that the chlorosulphate obtained corresponds to the general formula:

in which n, m and p are as defined above and z is 3 1.

To ensure maximum dissolution of the aluminium hydroxide in the solution of aluminium chloride, it is necessary to add an approximately 100% excess thereof, in one or more portions, the particles with a size of less than 30 microns contributing preferentially as they dissolve to making the reaction medium basic. The unreacted aluminium hydroxide is recovered during filtration, which produces the final solution of basic chlorosulphate; depending on the excess used, its amount can be sufficient to manufacture a further solution of aluminium chloride, the remainder being provided by fresh hydroxide. This solution is then rendered basic with fresh hydroxide added in excess, in one or more portions.

This excess amount can vary from 10 to 150% and preferably from 70 to 120%, depending on the reactivity and the moisture content of the hydroxide. In fact, a very moist hydroxide added in too large an excess has the effect of diluting the solution of aluminium dhloride and reducing its boiling point, and hence its temperature of attack, which must then be compensated by increasing the pressure in the stirred autoclave.

After stabilisation of the basic chloride by the solution of sulphate, the basic chlorosulphate is diluted, cooled and filtered to remove the excess hydroxide, by techniques known to those skilled in the art.

The aluminium hydroxide used in the process according to the invention can originate from different sources. It can be a freshly prepared hydroxide, but it will be preferred, for economic reasons, to use hydrargillite originating from the Bayer process for the extraction of alumina from bauxite, in moist form (water content of the order of 35%) or in dry form. Depedning on the acceptable levels of impurities, it is also possible to make direct use of an alumina ore, for example bauxite, in untreated form or after prior calcination.

The present invention will now be illustrated by the following Examples.

Example 1 In a stirred reactor, 550 g of 40% strength hydrochoric acid (specific gravity 1.198) were poured onto 162 g of moist hydrated alumina containing 62.8% of Al203. After the violence of the reaction had been subdued by virtue of a heating-cooling means suitable for keeping the temperature at 40 - 60 C, the mixture was heated gradually to 1 050C and 324 g of moist hydrated alumina were then poured in. While continuing to heat the mixture, boiling occurred at 123 C at atmospheric pressure. The reactor was then placed under an air pressure of 1.2 bars; boiling occurred again when the temperature of the reactor reached 147 C. The heating was moderated so as to maintain incipient boiling for 3 hours.Despite the excess of hydrated alumina used, no thickening of the suspension, which would be due to a precipitation of basic chloride, took place. The heating was stopped, the pressure was released slowly to atmospheric pressure and a hot solution (at about 80 to 90 C) of basic sulphate was poured in, with stirring. The basic sulphate solution was prepared in the following manner. 91 g of 70% strength sulphuric acid were poured onto 55 g of hydrated alumina. The mixture was heated at boiling for 15 minutes, with stirring, and 30 g of cold water were poured in, followed by 220 g of hot water for dilution.

The contents of the 1 litre reactor were transferred into a stirred 2 litre vessel. 700 g of water at 400C were then poured in, the mixture was stirred for 15 minutes to cool to 40 C, and the suspension was filtered. After less than 12 minutes, 1,895 g of a solution with a specific gravity of 1.225 at 20 C were collected, and diluted with 136 g of water to bring the specific gravity down to 1.206. The solid residue weighed 207 g and contained 57% of Al203 and 2.0% of chloride ion. It essentially consists of excess alumina and of impregnating solution.

The solution of specific gravity 1.206 contained 10.0% of chloride ion, 10.5% of alumina and 3.0% of total sulphate ion.

Example 2 184 g of the above residue were poured into 550 g of 40% strength hydrochloric acid in order to manufacture the solution of aluminium chloride, and this was then basified as above with 324 g of moist hydrated alumina, boiling being maintained for 3 hours at 146 C under 1.2 bars.

Furthermore, the basic sulphate was prepared with the remaining residue, to which 43 g of hydrated alumina was added. After boiling and dilution with 30 g of cold water and 220 g of water heated to 90 C, this suspension was poured into the solution of basic chloride, and the mixture was then diluted with 830 g of water at 40 C. Filtration produced 200 g of residue containing 57.7% of Awl203 and 1.93% of chloride ion, and 2.050 g of a solution of specific gravity 1.213,which was brought down to 1.206 with 58 g of water. The 2,108 g of final solution contained 9.95% of chloride ion, 10.6% of Awl203 and 3.0% of S04 ion.

As can be seen, complete consumption of the alumina has been achieved in manufacturing solutions of basic aluminium chlorosulphate, whereas, in some processes involving neutralisation with lime or calcium carbonate, the portion not attached at 105 C in a medium of hydrochoric acid and sulphuric acid is lost with the bulky residue of gypsum formed.

Claims

1. Process for the manufacture of basic aluminium chlorosulphate corresponding to the general formula: [ AIn (OH)3n-m~2pClm (S04)4 Z in which: 3n-m-2p 3n is from 0.4 to 0.7, p is from 0.04 to 0.25 n, and m/p is from 8 to 35, z being 3 1, which process comprises reacting basic aluminium chloride with basic aluminium sulphate, both heated beforehand, at a temperature of from 80 to 1 200C, the relative proportions of the constituents being chosen so as to give a product corresponding to formula (I) above.

2. Process according to Claim 1, in which the said basic aluminium chloride is obtained by reacting hydrochloric acid with alumina in proportions such that, after reaction, the solution of aluminium chloride obtained is substantially supersaturated, and then reacting this solution of aluminium chloride with a 10 to 150% excess of aluminium hydroxide, relative to the theoretical amount required for the desired basicity, in a stirred medium, for a maximum period of 5 hours, at a temperature of 140 to 165 C and under a pressure of 0.5 to 3 bars.

3. Process according to Claim 1 or 2, in which the said basic aluminium sulphate is obtained by reacting sulphuric acid with a 10 to 100% excess of aluminium hydroxide, relative to the stoichiometry of the neutral sulphate, the solution being heated and then kept at boiling point for 15 to 30 minutes.

4. Process according to Claim 1, substantially as described with reference to the foregoing Examples.

5. A basic aluminium chlorosulphate whenever prepared by a process as claimed in any one of the preceding claims.