Method for producing basic aluminum sulphate (III)

The present invention relates to a method for producing sulphate-poor, polynucleate aluminium hydroxide sulphate complexes of the general formula EQU [Al(OH).sub.x (SO.sub.4).sub.y (H.sub.2 O).sub.z ].sub.n in which n is an integer; x is 0.75-2.0; y is 0.5-1.12; x+2y is 3; z is 1.5-4 when the product is in solid form, and z>>4 when the product is in the form of an aqueous solution, an amorphous aluminium hydroxide being admixed with aluminium sulphate and/or sulphuric acid to y=0.5-1.12; preferably 0.5-0.75.

DESCRIPTION 
1. Technical Field 
The present invention relates to a method for producing polynucleate basic 
aluminium sulphate which gives in aqueous solution positively charged 
polynucleate complexes having charge-neutralizing properties in systems 
containing suspended or colloidal, negatively charged particles. 
The object of the present invention is to provide a rational method for the 
industrial production of polynucleate basic aluminium sulphate. 
2. Background Art 
A number of aluminium products which contain polynucleate aluminium ions in 
solution are known to the art. These products are the result of a 
requirement for more effective chemicals in, for example, the fields of 
water purification, paper sizing, and plant de-watering. When used in 
these particular fields, the polynucleate products exhibit far superior 
qualities than do previously used simple compounds, due to the higher 
charge of the metal ions. In principle, two different kinds of aluminium 
products have been developed with the aim of improved efficiency in the 
aforesaid context; these products being firstly chloride-based basic 
aluminium compounds and secondly sulphate-based aluminium compounds. Of 
the first mentioned group there was developed first a polyaluminium 
chloride () of the general formula 
EQU [AlCl.sub.x (OH).sub.3-x ].sub.n 
where x is &lt;3, normally 1-2. 
These compounds and methods for their manufacture have been described in, 
for example, SE,B,7, 201 333-7, SE,B,7 405 237-4, SE,B,7 412 965-1, SE,B,7 
503 641-8 and DE,A,2 630 768. 
The other kind of aluminium chloride solutions, C, which are also based 
on a context of the same polynucleate complexes, have general summary 
formuli which can be written thus: 
EQU [AlCl.sub.3.xAOH].sub.n 
where A is alkali metal, and 
EQU [AlCl.sub.3.(x/2)B(OH).sub.2 ].sub.n 
where B is an alkali earth metal, n is a positive integer, and x is a value 
within the range of 1-2.7. 
Polynucleate aluminium-chloride solutions of the C-type are found 
described in FR,Al, 7 512 975, according to which the solutions are 
produced by alkalizing aluminium-chloride solutions with solutions of 
alkali hydroxide. According to this publication, however, it has not been 
possible to produce clear, stable solutions other than in a highly diluted 
state. By "stable solution" is meant here and in the following a solution 
which does not change with respect to composition and properties, not even 
when stored for extended period of time. Thus, it is stated in the 
publication that under certain conditions, it is possible to obtain a 
solution with up to 0.40 moles of aluminium per liter. The stability of 
this solution, however, is greatly limited, and it is necessary to inject 
the solution directly into the water to be purified. It is clearly evident 
from the publication, and in particular from the working examples, that 
solutions having aluminium concentrations in excess of about 0.1 mole per 
liter cannot be expected to be effective and stable in the C-solutions 
known according to the publication. 
Sulphate-based basic aluminium compounds are found described in EP,A, 
79850039-3; EP,A, 80850033-4 and SE,A, 8101830-1. 
These products have to a greater or lesser extent polynucleate metal-ions 
in solution, and are therefore effective water-purifying substances. The 
sulphate-based products can also be used for purposes other than for 
purifying water, where the presence of the polynucleate metal ions favours 
the achievement of desired effects. 
In certain cases, however, it is desirable, and at times even necessary, to 
restrict the supply of sulphate ions to the greatest possible extent with 
applications made in the aforementioned fields, particularly when 
purifying drinking water. This is particularly important in systems which 
are repeatedly used and cleansed in order to eliminate the risk of the 
accumulation of sulphate in tissues, and in water from the aspect of 
concrete corrosion. This, therefore, applies to certain water-purifying 
systems where there is a lack of water, rendering it necessary to use the 
water for as long as possible, with intermediate water-purifications. 
After having been cleansed ten times with conventional aluminium sulphate 
or compositions having corresponding sulphate contents (FALS), the 
sulphate content of such water is so high that the water will attack the 
water-piping system resulting in troublesome leakages. The problem of 
sulphate-ion enrichment has now also manifested itself in the papermaking 
industry, in which for environmental reasons the water system is closed to 
a large extent. Excessively high percentages of salts in the paper stock 
seriously disturb the manufacturing processes. 
In U.S. Pat. No. 4,238,347 there is proposed a method for producing a basic 
aluminium sulphate which is poor in sulphate and which has the formula 
EQU Al(OH).sub.x (SO.sub.4).sub.y (H.sub.2 PO.sub.4).sub.z (H.sub.2 O)).sub.w 
where 
x is 0.75-1.5; 
y is 0.7-1.07; 
z is 0-0.2; and 
w is 2.0-4.2, 
x+2y+z being equal to 3. 
The compounds are produced by reacting aluminium sulphate with ground 
calcium carbonate, optionally in the presence of phosphoric acid, and by 
filtering the reaction mixture to separate the gypsum formed. It has been 
found, however, that the carbon dioxide departing from the reaction with 
the calcium carbonate, together with the gypsum, creates enormous problems 
with respect to process techniques, due to the foam formed, which, 
especially with high aluminium-sulphate concentrations, cannot be broken 
up. It is even impossible to break up effectively the foam when foam 
inhibitors are present. In fact, the use of foam inhibitors should be 
avoided, since they act as a contaminant in the different uses to which 
the aluminium sulphate is put. The calcium carbonate also reduces the 
stability of the product. 
In SE,A, 8104149-3 there is described an improved method for producing 
sulphate-poor polynucleate aluminium hydroxide complexes of the formula 
EQU [Al(OH).sub.x (SO.sub.4).sub.y (H.sub.2 O).sub.z ].sub.n 
where 
n is an integer 
x is 0.75-2.0; 
y is 0.5-1.12; 
x+2y is 3 and 
z is 1.5-4, z being .ltoreq.4 when the product is in solid form and z being 
&gt;&gt;4 when the product is in solution, 
the aluminium sulphate being admixed with one or more of the compounds from 
the group CaO, Ca(OH).sub.2, BaO, Ba(OH).sub.2, SrO, Sr(OH).sub.2 in 
aqueous solution to the reaction to form the given compound, whereafter 
the resultant alkali earth metal sulphate precipitate is separated out and 
the residual solution optionally vapourised. The resultant solution, which 
can be obtained with a high basicity, OH/Al.ltoreq.2.0, has splendid 
properties, but the manufacture of the solution results in a water, the 
alkali earth metal sulphate, which is undesirable, since it constitutes up 
to 30% of the reaction mixture at a basicity of 2.0. A search has 
therefore been made for other methods which give no waste or which yield a 
residual product which has its own market value. 
BRIEF DESCRIPTION OF THE INVENTION 
It has now been found possible to eliminate these technical problems and to 
provide in a rational and simple manner a sulphate-poor, basic aluminium 
sulphate of the general formula 
EQU [Al(OH).sub.x (SO.sub.4).sub.y (H.sub.2 O).sub.z ].sub.n 
where 
n is an integer 
x is 0.75-2.0, preferably 1.5-2.0 
y is 0.5-1.12, preferably 0.5-0.75 
x+2y is 3 
z is 1.5-4, when the product is in solid form and 
z is &gt;&gt;4, when the product is in the form of an aqueous solution, 
the invention being characterised in that a solution of an aluminium salt 
is neutralised to pH 5-7 for precipitating amorphous aluminium hydroxide, 
which is separated out and then admixed with sulphate ions in the form of 
aluminium sulphate and/or sulphuric acid to a y-value of 0.5-1.12, 
preferably 0.5-0.75, this product optionally being converted to a solid 
product. 
Further characteristic features of the invention are set forth in the 
accompanying claims. 
Particularly preferred compounds are those in which x is 1.6-1.8. y is 
0.6-0.7; and in solid form, in which case z is 2.0-2.5. 
The compound exists in the form of a polynucleate complex in aqueous 
solution and it should also exist in the same form in the solid product. 
THE BEST MODE OF CARRYING OUT THE INVENTION 
The compound according to the invention can be produced in accordance with 
the following: 
Amorphous aluminium-hydroxide is produced by neutralising an aluminium 
salt, particularly aluminium sulphate, aluminium chloride, aluminium 
nitrate or sodium aluminate, to pH 5-7. In this respect, a solution of 
aluminium sulphate, aluminium chloride or aluminium nitrate, which is 
acidic, is admixed with hydroxyl-ions, in the form of sodium hydroxide, 
potassium hydroxide, for example, to obtain the aforesaid pH, whereupon 
amorphous aluminium hydroxide is precipitated, this precipitate being 
isolated, for example, by filtering to form a filter cake, which can 
attain a substance content of about 20% (5-6% Al) and about 80% H.sub.2 O. 
The yield with respect to aluminium is about 100%. A solution of sodium 
aluminate, NaAlO.sub.2, which is alkaline, is admixed with hydrogen ions, 
in the form of sulphuric acid, hydrochloric acid or nitric acid, for 
example, to obtain the aforesaid pH, whereupon amorphous aluminium 
hydroxide is precipitated, the precipitate being isolated in the aforesaid 
manner. The yield is 100%. 
When producing the aforesaid compound from aluminium sulphate and sulphuric 
acid a certain amount of the sulphate will be found in the aluminium 
hydroxide, bound to the surfaces thereof.

EXAMPLE 1 
An aluminium hydroxide produced by neutralising aluminium sulphate with 
sodium hydroxide and filtering to attain an aluminium content of 4.2 grams 
was admixed with 93 grams of aluminium sulphate having a total aluminium 
content of 8.4 grams, and was heated to 80.degree. C. for 30 minutes. In 
this way there was obtained a clear solution of a sulphate-poor, 
polynucleate aluminium hydroxide sulphate complex containing 4.9% Al; 
19.0% SO.sub.4 ; OH/Al=0.8; .rho.=1.29 kg/dm.sup.3. 
EXAMPLE 2-3 
Solutions having the following data were prepared in accordance with 
Example 1 above. 
______________________________________ 
Starting Materials 
Alumi- End Product 
Filter cake nium sulphate 
Al SO.sub.4 .rho. 
Ex g g Al g g Al % % OH/Al kg/dm.sup.3 
______________________________________ 
2 103 6.3 70 6.3 4.8 15.6 1.2 1.25 
3 138 8.4 47 4.2 4.6 11.7 1.6 1.20 
______________________________________ 
EXAMPLES 4-6 
Solutions were prepared in accordance with Example 1, with the exception 
that sodium aluminate and sulphuric acid were the starting materials. 
______________________________________ 
Starting Materials 
Alumi- End Product 
Filter cake nium sulphate 
Al SO.sub.4 .rho. 
Ex g g Al g g Al % % OH/Al kg/dm.sup.3 
______________________________________ 
4 76 4.0 93 8.4 5.0 19.3 0.8 1.30 
5 121 6.3 70 6.3 5.0 15.8 1.2 1.26 
6 162 8.3 47 4.2 4.6 12.6 1.5 1.22 
______________________________________ 
EXAMPLE 7 
Amorphous aluminium hydroxide was prepared in accordance with Example 1 
above. An aluminium sulphate solution containing 0.17 mole sodium citrate 
per mole aluminium was added to the amorphous aluminium hydroxide. The 
resultant sulphate-poor, polynucleate aluminium hydroxide sulphate complex 
solution was stable for at least four month at room temperature, without 
any tendency of aluminium hydroxide precipitating. 
The solution is unstable when stored, and the aluminium compound present 
decomposes to form insoluble aluminium salts. The solution can be 
stabilised, however, by adding thereto, 1.0-2.0% sodium heptonate. 
Other suitable stabilising agents are sodium citrate, sodium acetate, 
sodium tartrate, sodium carbonate or combinations thereof. Other salts of 
citrate, acetate, tartrate and carbonate can also be used. Other 
.alpha.-hydroxy carboxylic acids having stabilising qualities, in addition 
to citric acid, and tartaric acid, include lactic acid, glycolic acid and 
hydroxy succinic acid. These .alpha.-hydroxy acids are added in a quantity 
of up to one half equivalent, although a smaller quantity, also exhibits 
good stabilising effects. For example, an extremely good stabilising 
effect is obtained with 0.08 mole citric acid per mole aluminium. 
The resultant 5 to 7%-solutions, can be diluted down to a concentration of 
2% calculated as aluminium. Lower concentrations reduce the stability of 
the solutions excessively, even when a stabiliser is added, and should 
therefore be avoided. 
As beforementioned, the 5 to 7%-solutions, produced in accordance with the 
aforegoing are unstable in the absence of stabilisers, due to the 
prevailing states of equilibrium, conversion to water-insoluble, aluminium 
compounds taking place, as beforementioned. The conversion rate is 
dependent upon temperature, this rate increasing with increasing 
temperature. 
Despite this, however, it is possible to vapourise the solution to form a 
stable, solid, dry product, which is soluble in water and which will 
provide solutions having properties which are comparable with the 
properties of the original solution. Although the stability of the 
solution obtained when re-dissolving the solid product may be impaired, 
the stability of the solution is quite sufficient for practical, technical 
use, for example for use in the manufacture of paper. 
When vapourising the solution, the temperature shuld not exceed 70.degree. 
C., and the vapourising time should be restricted as far as possible, by 
suitable selection of a vapourising means, such as a thin-film evaporator, 
with good air ventilation. Such an evaporator produces a brittle cake, 
which disintegrates into an X-ray amorphous product. The products produced 
in accordance with the invention possess good properties with respect to 
neutralising the charges in colloidal systems, which results in good 
coagulation and rapid flocculation in water-purifying processes, and good 
retention in papermaking processes. Since the product is poor in sulphate, 
it can be used to advantage in papermaking industries having closed 
white-water systems, to eliminate undesirable accumulation of sulphate 
ions. Excessively high quantities of sulphate ions block the desired 
surface-chemical reactions.