GB2138792A - Production of zeolite A - Google Patents

Abstract

Process for the manufacture of zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, wherein use is made of a pulp of silicated gel originating from the treatment of blast-furnace slag with waste acid, said pulp being made basic by addition of caustic soda as well as sodium aluminate originating from the treatment of an alumina gel with caustic soda and/or originating from sodium aluminate obtained during the surface treatment of aluminium; by incorporating, into the silica, alumina in a proportion defined by the molar ratio of silica to alumina of between 60 and 1.2.

Description

SPECIFICATION Process for the manufacture of zeolite A and products obtained Zeolite A is a compound having ion exchanger characteristics which is incorporated especially in washing powder formulations.

It is well known to prepare zeolite A by reacting silica, alumina and caustic soda in an appropriate stoichiometric ratio. This zeolite A must satisfy certain physical criteria, amongst which there may be mentioned, in particular, the whiteness or reflectance, the particle size and the sequestering power.

For a production process to be viable on the industrial scale, it is also necessary for the operating parameters, especially the separability of the by-products and the desired products to be easily controllable.

Zeolite A is currently produced by relatively expensive processes starting from costly raw materials. The aim of the present invention is to produce a zeolite A of good quality, in particular for a common use, namely as an ingredient of washing powders.

The process of the invention is based on the use of reactants which are less expensive than those normally used, i.e. firstly on the use of waste sulphuric acid, in particular as produced in the manufacture of titanium dioxide by the "sulphate" process, which has hitherto generally been discharged into the sea, and secondly on the recovery of aluminium scrap solutions, in particular those produced in the surface treatment (anodic oxidation) of aluminium.

The main characteristic of the invention is the fact that use is made of pulp of silicated gel originating form the treatment of blast-furnace slag with waste acid, said pulp being made basic by addition of caustic soda, and sodium aluminate orginating from the treatment of an alumina gel with caustic soda and/or originating from sodium aluminate obtained during the surface treatment of aluminium, by incorporating, into the silica, alumina in a proportion defined by the molar ratio of silica to alumina of between 60 and 1.2, preferably between 35 and 1.7.

The extrapolation to practical use of the indicated raw materials for the purpose of producing a zeolite A satisfying the industrial requirements make it necessary to employ a series of specific techniques, which will be described in greater detail below and which constitute further characteristics of the present invention.

According to the invention, it is envisaged, in particular, to employ a sequence of simple operations for using the abovementioned reactants in order to achieve the desired result, namely a reduced cost price for a zeolite A of good quality.

It is known that granulated slag, obtained by rapid cooling of the molten slag when it leaves the blase furnace, constitutes a valuable source of silica and of aluminium. There are also different sources of waste sulphuric acid, in particular the source resulting from the manufacture of titanium dioxide or the source containing waste sulphuric acid consisting, for example, of the exhausted pickling acids.

The compositions of waste acid originating from the manufacture of titanium dioxide depend especially on the composition of the titanium ore leached. The average concentration of sulphuric acid varies between 10 and 20% and the major impurity is ferrous sulphate, which can reach a proportion of 20 g of iron per litre. The acid contains other impurities of less importance, such as Al, Mg, Cr, V, Ti and Mn.

The pickling acid residues are also characterized by a high proportion of iron.

However, iron constitute a particularly trouble-some pollutant because it tends ultimately to colour the zeolite A produced.

In some cases it is advantageous to enrich the waste acid with aluminium; this aluminium can be a waste from the anodising of this metal.

Moreover, it has been observed that the conditions of dissolution of the slag must be chosen so as to keep the silicic acid in a stable state having a low degree of polymerization.

The reaction of the slag, which is a basic reactant, with the acid proceeds according to the following equation: (CaO.SiO2.0.2AI203.0.35MgO)+ 1.95 H2SO4+ 1.05 H20 < slag H2SiO3 + 0.2AI2(SO4)3 + 0.6MgS04 + CaS04.2K20.

The neutralization of the slag is homogeneous and the dissolution of the various constituents takes place at the same rate. The reaction products are soluble in water except for the gypsum, which precipitates. The dissolution is exothermic.

In practice, however, it is found that easy separation of the gypsum by filtration is only possible provided that the particle size of the slag treated is less than 0.4 mm and preferably less than 0.2 mm. One operating condition of the process of the invention thus consists generally in grinding the granulated slag to the particle size indicated.

The slag is dissolved at constant pH (1.5), the reactor being fed simultaneously with a slag pulp (50% of solids) and the waste acid at the appropriate flow rates. The temperature of the dissolution medium can vary between 60 and 70 C.

In order to keep the degree of polymerization of the silicic acid as low as possible, care will be taken to comply with the following operating parameters: -the pH of the reaction must be regulated so that the medium remains distinctly acid, the stability of the silicic acid being a maximum at pH 1.5; beyond pH 2, the silica polymerizes rapidly; -the concentration of the silicic acid is advantageously regulated so as to be close to the optimum concentration of 25 g of SiO2 per litre.This may involve diluting the waste acid in accordance with the proportion of H2 504. In general, dilution approximates to the ratio 1/1 in the case of concentrated waste acid from the production of TiO2; and -as the rate of polymerization of silicic acid increases with temperature, because the synthesis of silica gel takes place at 40"C, it is generally necessary to cool the dissolution medium.

It is observed that the curve of the dissolution kinetics of slag is characterized by a rapid step and a slow step. A quasi-equilibrium is reached after a contact time of 30 minutes and the efficiency of dissolution corresponds to 85%. Advantageously, the retention time is set at 30 minutes and the dissolution system consists of two vats, the material passing through the first in 10 minutes (rapid step) and staying in the second for 20 minutes (slow step).

Stirring in the vats keeps the slag in suspension in order to achieve a high degree of dissolution ( 85%). As will be indicated below, it is advantageous for the gypsum suspensions obtained by flotation during the separation of the gel/gypsum mixtures to be recycled to the dissolution stage, this recycling assisting the formation of larger crystals and reducing the number of filtrations.

The dissolution slurry is filtered so as to separate the gypsum from the solution of silicic acid; the gypsum cake is washed in order to recover all the filtrate.

The filterability of the gypsum is characterized by the value of the S.C.F.T. (standard cake formation time); this parameter is defined as then time required to form a 1 cm thick cake under a differential pressure of one atmosphere. The average S.C.F.T. value of the gypsum is 20 + 5 seconds and this value diagnoses the very good filter-ability of the gypsum.

The gypsum cake is washed with a quantity of water corresponding to filling of the pore volume, and the filtrate is totally recovered. The gypsum produced is suitable for recycling to a cement factory provided that it has a high degree of dryness (85-90%). To achieve this, the gypsum can be separated off in two steps, the slurry being filtered on a thickening filter under pressure and the gypsum cake then being dried on a filter press.

After the gypsum formed during the dissolution of the slag has been filtered off, the silicic acid resulting from this dissolution is precipitated using calcium carbonate, with the concomitant precipitation of a further quantity of gypsum, according to the equation: nSi(OH)4+AI3+ + 1.5804- + 1.5CaCO3 ((HO)3SiO)nAl(OH)2 + 1 .4CaSO4.2H20 + 1.5 CO2 + (n-3)H2O.

It is apparent that, to obtain a zeolite A of acceptable whiteness, it is necessary to use a calcium carbonate in which the proportion by weight of Foe203 is less than 0.1% and is preferably 0.02% or less.

According to a characteristic of the invention, in order to be able to filter off the insoluble silica gel and gypsum easily, alumina is incorporated into the silica in a proportion defined by the molar ratio of silica to alumina of between 60 and 1.2, preferably between 35 and 1.7, and particularly between 35 and 20 or 1.7 and 2 according to the particular embodiments of the invention.

The precipitation of the silicate gel is carried out at pH between-2.8 and 4.7.

A first embodiment of the invention consists in precipitating a silicate gel rich in silica, the ratio of silica to alumina being at least equal to 20. This precipitation is carried out preferably at pH 3.2 by simultaneous addition of the silicate gel solution of calcium carbonate. Incorporatiqn of the correct quantity of alumina into the silica is achieved by careful regulation of the pH of the precipitation. Below pH 2.8, the precipitation of the silica is incomplete; the gel obtained is unfilterable. Above Ph 4, an excessive quantity of alumina is incorporated into the silica. The gel is precipitated at 40"C without heat being supplied. The silica sol must be cooled because a reaction temperature above 40"C would cause the precipitation of iron oxide.

The precipitation of the silica gel is very rapid, but it is advantageous to keep the reactants in the reactor for one hour in order to ensure complete dissolution of the calcium carbonate.

The stirring system must permit homogenization of the reaction medium in order to prevent the calcium carbonate from settling out in the bottom of the reactor. Under such conditions, the efficiency of the precipitation of silica reaches 100%.

The gel/gypsum mixture is then separated from the mother liquor by vacuum filtration; the filtrate is kept because it contains utilizable aluminium sulphate, the recovery of which will be described below. The cake is washed on the filter with water at pH 2 in order to reduce the proportion of iron as much as possible. A sequence of at least three washings is preferable and the volume of water for each washing corresponds to the pore volume of the cake. The washing can be carried out using the water from the process, after purification and acidification.

It is found that the filterability of the cake of the gel/gypsum mixture corresponds to an S.C.F.T. value of the order of 40 seconds. In view of the quality required from the washing, it is appropriate to use a band filter (under vacuum).

The two constituents of the mixture are then separated by flotation of the gypsum. This operation is carried out on a pulp whose solids content preferably reaches 120 g/litre and whose pH is preferably between 3.0 and 3.5.

The flotation technique for separating the gypsum and the silica gel is an elegant and effective method of separating the components under conditions which, at first sight, may seem necessarily to cause considerable difficulties as a result of the presence of the silica gel.

The Applicant Company has observed in this respect that the use of dodecylamine (laurylam ine) as the collector, by itself or mixed with other amines having a fatty hydrocarbon chain (for example a C16 chain), gives separation results which cannot be obtained with other compounds which are also used as collectors under other circumstances.

By way of illustration, it may be mentioned that it can be envisaged to use a mixture of tallow amine and coconut amine as the collector for gypsum, in a quantity which depends on the quantity of gypsum produced. The efficiency of separation of the gypsum is excellent (99%); the loss of silica gel through entrainment by the gypsum is low (8%). The proportion of solids in the gel pulp reaches 10%.

In terms of the operation, the thickened pulp is kept as such. The gypsum pulp is recycled to the slag dissolution step.

Under the abovementioned working conditions, the solution for synthesis of the alumina gel contains utilizable aluminium in a proportion of the order of 6.3 g litre.

This aluminium, carried by the mother liquor of the silica gel, is therefore precipitated selectively by adding calcium carbonate. The overall precipitation reaction is represented by the equation: n+ n n n n (A] (OH) .S04-] + -Ca CO3. (n + 1,5) H20oAI (OH)3 + -Ca S04.2H20 + -CO2 3-n 2 2 2 2 A precipitate of gypsum is therefore formed at the same time as the alumina gel.

According to a characteristic of the invention, it is apparent that the subsequent separation of the gypsum by flotation is facilitated if, prior to the precipitation of the alumina, a quantity of soluble silicic acid is added to the solution containing the aluminium. The addition of a small quantity of soluble silicic acid corresponds approximately to the molar ratio: sio2 =0.1 Al203 provides the gel with surface properties which enable the gypsum to be separated off by flotation.

Various techniques can be used for precipitating the alumina in the form of a gel.

This precipitation is advantageously carried out in two stages. In a first stage, which takes place at a constant pH of the order of 3.9, a solution of aluminium and calcium carbonate are added simultaneously (reaction time of the order of 2 hours). The requirements regarding the purity of the calcium carbonate used are less stringent than for the precipitation of the silica gel.

It is apparent, in particular, that a proportion of Fe2O3 which can reach values of the order of 0.08% does not result in any serious disadvantage.

The precipitation is then completed (in thirty minutes) in a second stage, at a pH of the order of 4.6, by adding caustic soda contained in the mother liquor of the zeolite A. This technique ensures a good reactivity of the calcium carbonate and makes it possible to purge the zeolite A crystallization circuit; the consumed volume of mother liquor of the solution A is equivalent to the purge required to balance the materials (water and impurities) in the system.

Care will be taken to ensure that the pH of the second step does not exceed 4.6; otherwise an iron oxide will also precipitate. The precipitation does not require heat to be supplied; the temperature of the reaction medium reaches at most 40"C, which is the temperature of the mother liquor of the silica gel. The stirring must be sufficient to keep the solid phase in suspension. The efficiency of the precipitation of the alumina reaches 100% under the conditions indicated.

The mixture of alumina gel/gypsum is filtered off and washed.

The pulp of alumina gel is characterized by a filterability whose S.C.F.T. value can vary between 1 and 3 minutes. Filtration under pressure on a thickening filter is adequate on account of the small proportion (5%) of solids in the suspension. The cake is washed to remove the soluble salts, the washing being carried out on the filter under pressure.

The two constituents of the mixture are then separated by flotation of the gypsum. The operation is carried out on a pulp whose solids content is fixed at 1 20 g/litre and whose pH is between 4.5 and 5. The water from the process is used for pulping, after purification. The gypsum pulp collected after flotation is preferably recycled to the slag dissolution step.

Advantageously, the mixture of tallow amine and coconut amine is used as the gypsum collector in a quantity which depends on the quantity of gypsum produced.

As a variant of the technique described, a second method of precipitation of the alumina gel is possible.

This method consists in precipitating the metals from the waste solution by means of waste aluminiate, at a constant pH of 10, and then in selectively precipitating the aluminium, at a constant pH of about 4.3, by means of the suspension obtained in the previous step. The gel obtained also has a good filterability.

The second embodiment of the invention is to precipitate a silicate gel rich in aluminium, the molar ratio of silica to alumina being between 20 and 1.2 and preferably between 2 and 1.2.

This high content of aluminium can be obtained if the waste acid is enriched with aluminium beforehand, for example by adding anodisation wastes thereto.

The precipitation of aluminium-rich silicate gel is obtained at pH between 4 and 4.7, preferably at pH 4.2 by simultaneous additions of the silicic acid solution and of calcium carbonate.

The gel is precipitated at 40"C with the addition of heat. The silica sol must be cooled since a reaction temperature above 40"C would cause the precipitation of iron oxide.

The precipitation of silicated gel is very fast, but it is advantageous to keep the reactants for an hour in the reactor in order to ensure that calcium carbonate is completely dissolved.

The agitation system must make it possible to homogenize the reaction medium to prevent the settling of calcium carbonate at the bottom of the reactor. Under such conditions the yield of the precipitation of silicate gel reaches 100%.

The gel-gypsum mixture is then separated from the mother liquor by filtration in vacuo. The cake is washed with water at pH 2 so as to reduce the iron content to the maximum. The filterability of the gel-gypsum mixture is characterised by an S.C.F.T. of 22 seconds.

The separation of the two compounds of the mixture is next carried out by gypsum flotation; the operation is carried out with a pulp the solids content of which reaches 1 20 g/litre and the pH of which is preferably between 4.2 and 5.

The yield of the gypsum precipitation is excellent (99%); the loss of silicate gel entrained with the gypsum is low (8%). The silicate gel pulp (a (10% solids) is kept as such while the gypsum pulp is recycled to the slag dissolution.

The zeolite A is then synthesized in three steps: -conditioning of the reactants; -formation of the reaction mixture; -crystallization, filtration and conditioning of the zeolite A.

The invention allows the zeolite A to be synethesized in two ways depending on whether the reaction mixture is prepared either from silica gel and alumina gel or from an aluminium-rich silicate gel.

In a first particular method of synthesis according to the invention, the reaction mixture is formed from an alkaline suspension consisting of the silica gel, sodium aluminate and caustic soda. The formulation of the mixture is fixed by the following molecular ratios: SiO2 Na2O H20 41.7; = 1.8 and - = 33.

Al203 SiO2 Na2O It is apparent that the order in which the reactants are added in the formation of the zeolite A is important for obtaining good results.

The conditioning of the reactants consists in neutralizing, with caustic soda, the silica and alumina gels precipitated in an acid medium, and then in causing attack to take place in the caustic soda.

The silica and alumina pulps (100 g of gel/litre) obtained from the gypsum flotation steps are first neutralized to pH 10, respectively by means of a fresh caustic soda brine and solutions of waste aluminate originating from anodic oxidation works. The nuetralization is carried out at ambient temperature up to pH = 10. The neutralization of the alumina gel causes precipitation of the aluminium carried by the solutions of waste aluminate; this aluminium is completely recovered. The neutral pulps are dried on a filter press to give a degree of dryness equal to at least 35%. To balance the water, the hydration volume of the filter cakes is replaced with an equivalent volume of zeolite A synthesis solution.

The silica gel is attacked at ambient temperature in a caustic soda solution consisting of synthesis water enriched in sodium hydroxide. The quantity of sodium hydroxide used corresponds to the molar ratio: Na2O =0.8 SiO2 The gel is attacked rapidly (3 minutes) but incompletely, and an insoluble residue of silica remains.

The alumina gel is dissolved at ambient temperature in a caustic soda solution consisting of the mother liquor enriched in sodium hydroxide. The quantity used for enrichment corresponds to the sodium hydroxide consumed by the purge. The gel is dissolved in 1 5 minutes. The iron and the silica associated with the alumina gel are only very slightly soluble in an alkaline medium; they form a precipitate which contaminates the sodium aluminate solution.

This residue is filtered off under pressure and washed. The efficiency of the dissolution is 90%; the composition of the solution obtained is characterized by the molar ratio: Na2O - =2 Al2O3 The formation of the reaction mixture comprises mixing of the reactants and predigestion.

It is apparent that there are two possible methods of adding the reactants.

In the case of a batch process, it is advisable to add the reactants of the zeolite A formulation in the following order: a. sodium aluminate solution; b. basic silica pulp.

The reactants can also be mixed in a continuous operation. The alkaline silica pulp and the sodium aluminate solution are fed simultaneously, in this case, into the mixing vat and the flow rates of the reactants are regulated so as to produce the formulation of the mixture and to keep it constant.

In both cases, the stirring is regulated so as to permit rapid homogenization of the reactants, because the operation produces a thick gel.

In a second particular method of synthesis according to the invention, the reaction mixtures consists of an alkaline suspension obtained from an aluminium-rich silicate gel and caustic soda.

The formulation of the mixture is fixed by the following molar ratios: SiO2 < 1.7 Al203 Na20 =1.8 and sio2 H20 - 33 Na2O The conditioning of the aluminium-rich silicate gel pulp consists in its being neutralised with caustic soda to pH 10. The mother liquor from the crystallisation of the zeolite A may be advantageously employed for the neutralisation; this procedure makes it possible to remove the chromium and vanadium present as trace elements. The neutral pulp is then filtered; the water volume of the cake is replaced with an equivalent volume of solution from the synthesis of the zeolite A to equilibrate the water balance.

The aluminium-rich silicate gel is then digested, at ambient temperature, with a caustic soda solution consisting of the mother liquor from crystallisation enriched with caustic soda.

The quantity of soda employed corresponds to the molar ratio Na2O =1.8 sio2 According to the two methods of preparation described, the reaction mixture is directed into a digesting vat, where it stays for 1 2 hours at ambient temperature.

Finally, the zeolite A is caused to crystallize by heating the reaction mixture for 2 hours at 85to; good stirring ensures homogenization of the medium, the solids content of which reaches 16%. The efficiency of the crystallization is 100%.

The crystalline solid is separated from the mother liquor by filtration, the filterability of the zeolite pulp being characterized by an S.C.F.T value of 2 minutes. The solid is filtered off and washed with deionized water until the pH of the pulp is 10.5. The moisture content of the cake filtered off under vacuum can reach 58%.

The zeolite can be supplied in two different forms: -either in suspension, a mixture of sodium polyacrylate (fluidizing agent for atomization) and phosphate, preferably sodium polyphosphate, being added to the filtered pulp in a proportion of 0.3% to give a fluid suspension containing 40% of solids, -or in powder form, the zeolite pulp being dried to give a finely disperse powder after drying.

The invention will be described in greater detail with reference to the examples below, which are intended to illustrate the invention without implying a limitation.

In the attached drawings, flow-sheets are given which represent the process-units according to the Examples.

Figs 1 and 2 refer to Example 1 and reprsent respectively the acid neutralization and the synthesis of the gels, and the zeolite A synthesis.

Figs. 3 and 4 refer to Example 2 and represent respectively the silica-aluminium synthesis and the zeolite A synthesis.

EXAMPLE 1 1. DISSOLUTION OF THE SLAG The granulated slag is dissolved in a waste acid originating from the manufacture of titanium dioxide; thu composition of these two reactants is given below: Granulated slag Waste acid Constituent Proportion, % Constituent Proportion, g/litre SiO2 34.13 H2SO4 184.4 Al203 12.63 Fe2+ 19.16 MgO 8.39 Al 4.17 CaO 34.86 Mg 5.13 Fe2O3 0.96 Na 0.84 MnO 0.54 V 0.96 K 1.37 Cr 0.26 Na 0.94 TiO2 3.61 TiO2 0.78 S 1.4 A pulp of ground slag (0 < 0.2 mm) is used and the waste acid is diluted prior to dissolution.

The gypsum suspension obtained after separation from the silica and alumina gels are added to the pulp of ground slag.

Dissolution is carried out at pH 1.5, in a continuous operation, in a system consisting of two reactors (15 and 28 litres) each equipped with 3 baffles and stirred so as to keep the solids in suspension. The reactants are introduced into the first vat by means of peristaltic pumps.

The slag feed is constant while the acid feed is subject to the measurement of the pH. The contents of the 1st vat overflow into the second, the system being regulated so that the average residence time in the reactors is 30 minutes. These conditions make it possible to obtain a dissolution efficiency of 85%.

The consumption per hour of the reactants is as follows: -slag pulp comprising: 6.65 kg of ground slag, 4.38 kg of gypsum and 11.03 litres of water; -acid solution comprising: 37.8 litres of acid and 28.33 litres of dilution water.

The production per hour of silicic acid pulp is 77.16 litres; the solids content amounts to 15.5%.

The acid pulp is filtered and the cake is washed on a vacuum filter (pressure reduction = 0.3 bar) at a rate of 1.3 litres per m2 per second; under these conditions, the thickness of the cake is 10.5 mm. The composition of the silicic acid separated from the gypsum is indicated below: Constituent Proportion, g/litre SiO2 25 Al 7.03 Fe2+ 10.55 Mg 6.53 TiO2 1.85 Cr 0.14 V 0.49 Na 0.99 K 0.86 2. PRECIPITATION OF THE SILICA GEL The precipitation of the silica gel is carried out at pH 3.2 by adding calcium carbonate to the silicic acid solution. The calcium carbonate is used in the form of a 25% suspension; the preferred particle size is less than 55 microns and the proportion of iron oxide does not exceed 0.02%.

The precipitation installation is made up of two vats having a useful volume of 40 litres; they are equipped internally with 3 baffles and a heating element making is possible to carry out the precipitation at 40"C. Stirrers in the vats keep the solid phase completely in suspension. The reactants are introduced simultaneously into the first vat by means of pumps; the synthesis mixture then overflows into the second vat. The flow rates of the reactants are regulated so as to keep the gel in the synthesis medium for one hour. A system of electrodes measures the precipitation pH and governs the control regulating the flow rate of calcium carbonate.

The consumptions per hour of the reactants are 77.16 litres of silicic acid and 0.698 kg of calcium carbonate. The pulp of silica gel and gypsum produced has a solids content of 5%. The inorganic mixture is filtered off and washed with acidified water (pH 2) on a vacuum filter; washing is continued until the iron has been totally eliminated.

This solid/liquid separation is carried out at a rate of 0.37 litre of pulp per m2 per second; the pressure reduction in the filter is 0.3 bar. Under these conditions, the thickness of the cake is 11 mm.

The separation of the silica gel and the gypsum by flotation is carried out on a pulp containing 1 20 g of solids per litre and the separation requires a rough operation and a finishing operation.

A mixture of tallow amine (Ct6) and coconut amine (C12), in respective proportions of 1 50 and 600 g per tonne of product to be separated, is ued as the collector; the frothing agent is pine oil.

Taking account of the separation efficiency, the weights per hour of the products separated off are as follows: -silica gel 2.33 kg.

-gypsum 2.33 kg.

The volume of the mother liquor containing the aluminium is 77.16 litres.

The compositions of the silica gel and the mother liquor are given below: Silica gel Mother liquor Constituent Proportion, % Constituent Proportion, g/litre SiO2 78.1 Al 6.3 Al203 3.81 Fe2+ 10.55 Loss on ignition 8.7 Mg 6.53 Cr203 0.17 Mn 0.30 Fe203 0.08 K 0.86 TiO2 8.85 Na 0.99 V205 0.34 3. PRECIPITATION OF THE ALUMINA GEL Prior to the precipitation of the alumina gel, a silicic acid solution is added to the mother liquor containing the aluminium so as to give the following molar ratio: SiO2 = =0.1 Al203 The precipitation is carried out in two stages in an instaliation similar to that used for precipitating the silica gel.

In a first stage, the pH is increased to 3.9 by adding a 25% suspension of calcium carbonate; the particle size of the carbonate is less than 55 microns. The residence time of the reactants in the reactors is two hours and the temperature reaches 40"C. In a second stage, the pH is increased to 4.6 by adding caustic soda originating from the synthesis water of the zeolite A.

The consumption per hour of reactants are 77.16 litres of aluminium solution, to which are added 2.1 6 litres of a solution of silicic acid (25 g/litre), 1.247 kg of calcium carbonate and 0.3 kg of caustic soda.

The pulp of alumina gel and gypsum produced has a solids content of 5.2%; the inorganic mixture is filtered off and washed.

The filtration, which is followed by washing, is carried out at a rate of 0.53 litre of pulp per m2 per second; the pressure reduction in the filter is 0.3 bar. Under these conditions, the thickness of the cake is 8 mm.

The separation of the silica gel and the gypsum by flotation is carried out on a pulp containing 120 g of solids per litre; the separation requires a rough operation and two finishing operations.

A Wedag machine, model MN 935/4, equipped with a 6 litre cell is used for the flotation.

A mixture of tallow amine and coconut amine is used as the collector in a proportion of 450 g of mixture per tonne of solid to be separated; the frothing agent is pine oil.

Taking account of the separation efficiency, the weights per hour of the products separated off are as follows: -aluminia gel 1.75 kg -gypsum 2.14 kg.

The volume of the waste solution is 77.16 litres. The composition of the alumina gel produced is given below: Constituent Proportion, % Al203 57.71 Fe2O3 2.45 SO3 5.72 Loss on ignition 30.65 Insoluble matter 1.54 4. SYNTHESIS OF THE ZEOLITE A 1. Conditioning of the reactants.

a. Neutralization of the gels.

The silica and alumina pulps (10% of solids) are neutralized to pH 10 by means of caustic soda. The neutralization is carried out in a continuous operation at ambient temperature. The characteristics of the neutralization reaction are indicated below: SiO2 gel Alp03 gel weight treated 2.33 kg 1.94 kg pH of the pulp before neutralization 3-3.5 4.5-5 neutralizing agent NaOH brine NaOH, waste 365.8 g/litres aluminate (80 g of NaOH/litre and 160 g of Al2O3/ litre) consumption, g/mol 6.77 40.8 The neutralized gels are filtered and the water with which the cakes are impregnated is exhanged with the mother liquor of the zeolite A.

The consumptions of mother liquor are respectively 4.33 litres and 3.6 litres for the silica gel and the aluminium.

b. Attack of the gels.

The neutralized gels are attacked in a batch operation at ambient temperature in caustic soda under the conditions listed below: SiO2 gel Al203 gel reactant NaOH brine, zeolite mother 365.8 g/litres liquor enriched in NaOH consumption 4.14 litres 20.33 litres + 514 g of NaOH composition of the solution Na2O = 0.8 Na2O = 2 SiO2 Al203 Clarification of the solution + The sodium aluminate solution is clarified to rempve the insoluble impurities (iron oxide and silica).

2. Mixing of the reactants and predigestion.

The two liquids obtained by attack of the gels are pumped simultaneously into a vat equipped with 3 baffles and are stirred in order to produce a homogeneous mixture of the two streams.

The flow rates of the reactants are balanced so as to keep the formulation of the mixture constant, this formulation being represented by the following molar ratios: SiO2 = 1.7; Na2O=1.8and H20 = 33.

Al2O3 SiO2 Na2O The reaction mixture is aged overnight at ambient temperature.

3. Crystallization.

The reaction mixture is heated for 2 hours at 85"C; the reactor is stirred in order to keep the solid phase in suspension.

The solid is separated from the mother liquor by vacuum filtration; the crystals are washed with deionized water until the pH of the pulp is 10.5.

The product obtained is zeolite A, which is identified by X-ray diffraction; its chemical composition is given below: Constituent Proportion, % SiO2 32.31 Al203 31.05 Na2O 18.03 H20 12.90 TiO2 4.30 Fe2O3 0.05 The main characteristics are given below: reflectance : 91 Hunter coordinates odour : none particle size distribution below 10,u : 94% : : 82% : : 50% pH (1% anhydrous in aqueous suspension) : 10.4 weight loss after 50 minutes at 800"C : 21% sequestering power at 25'C per g of anhydrous zeolite after 1 5 minutes : 1 50 mg of CaO The zeolite A obtained can be used as a sequestering agent for calcium in a washing powder formulation.

EXAMPLE 2 1. DISSOLUTION OF THE SLAG The granulated slag is dissolved in a waste acid originating from the manufacture of titanium dioxide; the composition of these two reactants is given below: Granulated slag Waste acid Constituent Proportion, % Constituent Proportion, g/l SiO2 34.13 H2SO4 205.4 Al2O3 12.63 Fe2+ 19.16 MgO 8.39 Al 4.06 CaO 34.86 Mg 5.8 Fe203 0.96 Na 0.84 MnO 0.54 V 0.68 K 1.37 Cr 0.25 Na 0.94 TiO2 2.9 TiO2 0.78 S 1.4 Before being used for the dissolution, the concentration of the waste acid in aluminium has been improved by addition of a waste product originating from the aluminium anodisation; said waste has the following composition: NaOH:102 g/l; Al203:172 g/l. The aluminium concentration enhancing is such that a molar ratio silica to alumina of 1.49 is obtained in the solution resulting from the dissolution of slag.Practically speaking, to one litre of waste acid, 0.1 litre of a solution containing 102 g/l NaOH and 1 72 g/l Al2O3 are added.

Aluminium enhanced waste acid thus obtained Constituent Proportion, g/litre H2SO4 130.24 Fe2+ 17.64 Al 12.75 Mg 5.22 Na 6.62 V 0.61 Cr 0.22 TiO2 2.61 The dissolution of the slag is performed under conditions similar to those described in Example 1.

The consumption per hour of the reactants is as follows: -slag pulp comprising: 5.75 kg of ground slag, 9.18 kg of gypsum and 11.2 litres of water; -acid solution comprising: 48.5 litres of acid and 10.29 litres of dilution water.

The production per hour of silicic acid pulp is 70 litres; the solids content amounts to 21.8 %.

The acid pulp is filtered and the cake is washed on a vacuum filter (pressure reduction = 0.3 bar) at a rate of 1.0 litre per m2 per second; under these conditions, the thickness of the cake is 10.5 mm. The composition of the silicic acid separated from the gypsum is indicated below: Constituent Proportion, g/litre SiO2 22.77 Al 13.73 Mg 7.12 TiO2 2.37 Cr 0.15 V 0.42 Na 5.09 K 0.97 2. PRECIPITATION OF THE SILICA TED GEL.

The precipitation of the silicated gel is carried out at pH 4.2 by addiging calcium carbonate to the silicic acid solution. The precipitation of the silicated gel is performed under conditions similar to these of Example 1.

The consumptions per hour of the reactants are 70 litres of silicic acid and 5.34 kg of calcium carbonate. The pulp of silicated gel and gypsum produced has a solids content of 20 %. The mineral mixture is filtered off and washed with acidified water (pH 2) on a vacuum filter: washing is continued until the iron has been totally eliminated.

This solid/liquid separation is carried out at a rate of 2.0 litre of pulp per m2 per second; the pressure reduction in the filter is 0.3 bar. Under these conditions, the thickness of the cake is 11 mm.

The separation of the silica gel and the gypsum by flotation is carried out on a pulp containing 120 g of solids per litre and the separation requires a rough operation and a finishing operation.

A mixture of tallow amine (cut6) and coconut amine (C12), in respective proportions of 300 and 1 200 g per tonne of product to be separated, is used as the collector; the frothing agent is pine oil.

Taking account of the separation efficiency, the weights per hour of the products separated off are as follows: -silica gel 4.67 kg -gypsum 7.58 kg.

The volume of the mother liquor is 70 litres.

The compositions of the silica gel and the mother liquor are given below: Silica gel Mother liquor Constituent Proportion, % Constituent Proportion,g/l SiO2 31.38 Al203 36.27 Fe2+ 12.70 Loss on ignition 24.37 Mg 7.32 Cr203 0.14 Fe2O3 0.33 K 5.09 TiO2 2.9 Na 0.97 V2o5 0.66 3. SYNTHESIS OF THE ZEOLITE A 1. Conditioning of the silica ted gel.

a. Neutralization.

The pulp of silicated gel (10% of solids) is neutralized to pH 10 by means of the mother liquor of the crystallization. The neutralization is carried out in a continuous operation at ambient temperature. The characteristics of the neutralization reaction are indicated below: weight treated 4.67 pH of the pulp before neutralization 4.2-5 neutralizing agent NaOH from mother liquor of crystallization at 97.3 g/l consumption, gNaOH/mole SiO2 19.6 The neutralized gel is filtered and the water with which the cake is impregnated is exhanged with the mother liquor of the zeolite A.

The consumption of mother liquor is 4.9 1.

b. Attack of the silicated gel and predigestion.

The neutralized gel is attacked in a batch operation at ambient temperature in caustic soda under the conditions listed below: Reactants : NaOH brine (365.8 g/l and mother liquor of the crystallization (97.3 g/l) Consumptions : NaOH brine: 3.96 1 Mother liquor of the crystallization: 21.2 1.

The NaoH brine is added to the silicated gel, after dispersion of the mixture, the mother liquor of the crystallization is added. The thus obtained suspension is stirred and aged for 1 2 hours.

2. Crystallization The reaction mixture is heated for 2 hours at 85"C; the reactor is stirred in order to keep the solid phase in suspension.

The solid is separated from the mother liquor by vacuum filtration; the crystals are washed with dionized water until the pH of the pulp is 10.5.

The product obtained is zeolite A, which is identified by X-ray diffraction; its chemical composition as in Example 1.

The main characteristics are given below: reflectance : 91 Hunter coordinates odour : none particle size distribution below 10,u : 95% : : 94% : : 90% pH (1% anhydrous in aqueous suspension) : 10.4 weight loss after 50 minutes at 800"C : 21% sequestering power at 25"C per g of anhydrous zeolite after 1 5 minutes : 1 51 mg of CaO.

The zeolite A obtained can be used as a sequestering agent for calcium in a washing powder formulation.

Claims (23)

1. Process for the manufacture of zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, wherein use is made of a pulp of silicated gel originating from the treatment of blast-furnace slag with waste acid, said pulp being made basic by addition of caustic soda and sodium aluminate originating from the treatment of an alumina gel with caustic soda and/or originating from sodium aluminate obtained during the surface treatment of aluminium, by incorporating, into the silica,alumina in a proportion defined by the molar ratio of silica to alumina of between 60 and 1.2, preferably between 35 and 1.7.

2. Process according to claim 1, wherein the ground slag, which has a particle size of less than 0.4 mm and preferably of less than 0.2 mm, is dissolved at a pH of between 0 and 2 and preferably about 1,5, and wherein the gypsum formed is separated off by filtration of the solution.

3. Process according to claim 1 or 2, wherein the precipiation of a silica gel is caused by adding a calcium carbonate containing a proportion by weight of Fe2O3 of about 0.1 % and preferably of about 0.02 % or less.

4. Process according to any one of claims 1 to 3 wherein alumina is incorporated into the silica, in a proportion defined by the molar ratio of silica to alumina between 35 and 20.

5. Process according to any one of claims 1 to 4, wherein the precipitation of the silica gel is carried out at a pH of between 2.8 and 4 and preferably of about 3.2.

6. Process according to any one of claims 1 to 5, wherein the gypsum is separated from the mixture of silica gel and gypsum by flotation, the gypsum being collected in the froth.

7. Process according to any one of claims 1 to 6, wherein the precipitation of the aluminium contained in the mother liquors from the synthesis of the silica gel is caused in the presence of a proportion of soluble silica which is such that the molar ratio of silica to alumina is about of 0.1.

8. Process according to claim 7, wherein the separation of the mixture of alumina gel and gypsum is carried out by flotation, the gypsum being collected in the froth.

9. Process according to claim 8, wherein the flotation operation for the separation of the gypsum from the mixture of alumina gel and gympsum is carried out on a pulp whose solids content is about 120 g/litre and having a pH between 4.5 and 5.

10. Process according to claim 8 or 9, wherein dodecylamine, by itself or mixed with other amines having a fatty hydrocarbon chain, is used as the collector for the gypsum.

11. Process according to claim 10, wherein mixture of tallow amine and coconut amine is used.

1 2. Process according to claim 10 or 11, wherein the gypsum pulp separated off is recycled to the slag dissolution step.

1 3. Process according to any one of claims 1 to 12, wherein the precipitation of the alumina gel is carried out by adding calcium carbonate in two successive steps, the first consisting of the simultaneous addition of an aluminium solution and calcium carbonate at a constant pH of about 3.9, and the second, which takes place at a constant pH of about 4.6, being carried out by adding caustic soda originating from the mother liquor of the zeolite A.

14. Process according to any one of claims 1 to 12, wherein the precipitation of the alumina gel is carried out by precipitating the metals from the waste solution, at pH 10, with a caustic soda solution possibly containing waste aluminate, and then by selectively precipitating the aluminium, at pH 4.3, by means of the suspension obtained in the first step.

1 5. Process according to any one of claims 1 to 14, wherein the aluminate solution is added to the basic silica pulp either batchwise according to the following order of addition: a) sodium aluminate solution b) basic silica pulp, or continuously by simultaneously mixing these two reactants in a mixing vat.

1 6. Process according to any one of claims 1 to 6, wherein alumina is incorporated into silica, in a proportion defined by the molar ratio of silica to alumina between 1.2 and 2.

1 7. Process according to claim 16, wherein the molar ratio of silica to alumina is achieved by enhancing the aluminium concentration of the waste acid used for treating the blast-furnace slag by means of wastes resulting from aluminium anodisation.

18. Process according to any one of claims 1 6 to 1 7 wherein the flotation operation for separation of the gypsum is carried out on a pulp whose solids content reaches 1 20 mg/litre and having a pH between 3.0 and 3.5.

19. Process according to anyone of the claims 6 to 18 wherein dodecylamine, by itself or mixed with other amines having a fatty hydrocarbon chain, is used as the collector for the gypsum.

20. Process according to claim 19, wherein a mixture of tallow amine and coconut amine is used.

21. Process according to anyone of claims 5 to 20 wherein the gypsum pulp separated off is recycled to the slag dissolution step.

22. Process substantially as hereinbefore described with reference to the examples.

23. Product obtained by the process of any one of claims 1 to 22.