CA1224201A - Process for the manufacture of zeolite a and product obtained - Google Patents

Abstract

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 alu-mina gel with caustic soda and/or originating from sodium aluminate obtained during the surface treatment of alumi-nium; by incorporating, into the silica, alumina in a pro-portion defined by the molar ratio of silica to alumina of between 60 and 1.2, preferably between 35 and 1.7.

Description

- la -STATEMENTS OF I~iENTION

In accordance with the present invention there is provided a process for manuacturing zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, comprising the steps of:
a) treating blast furnace slag with waste sulfuric acid, which has a maximum pH of 1.5 to avoid substantial polymerization of silica and which is enriched with aluminium in solution in order to obtain a preparation;
b) precipitating from said preparation between pH 4 and 4.7 an aluminium-rich silicate gel by adding in a reaction vessel said preparation and calcium carbonate, said aluminium-rich silicate gel further comprising in admixture gypsum resulting from the reaction of sulfate ions from said waste sulfuric acid and calcium from said calcium carbonate;
c) separating said gypsum ~rom step b);
d) neutralizing said gypsum-free, aluminium-rich silicate gel at pH 10 and producing a neutral suspension e) filtering said neutral suspension to obtain a filtration cake and washing said cake;
f~ digesting said filtration cake with caustic soda to result in a mother liquor;
g) crystallizing zeolite A and separating the same from the mother liquor as a crystalline solid.

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Also in accordance with the invention there is provided a process for manufacturing zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, comprising the steps of:
a~ treating blast furnace slag in a reaction vessel with waste sulfuric acid produced in the manufacture of titanium dioxide by the "sulphate" process and having a concentration in H2SO4 of 10 to 20 weight % at a pH comprised between 0 and 1,5 thus avoiding polymerization of silica into silicagel and the addition into the reaction product of said blast furnace slag with said waste sulfuric acid of aluminum generated from the treatment of aluminum anodization waste in order to obtain a mixture;
b) adding calcium carbonate in the said reaction vessel to the said mixture in order to adjust the pH between 4 and 4,7 causing the precipitation of an aluminum - rich silicate gel t said gel further comprising in admixture gysum resulting from the reaction of sul~ate ions from said waste sulfuric acid and calcium ions from said calcium carbonate;
c) separating said gypsum from said aluminum-rich silicate gel by flotation thus producing a gypsum free and aluminum-rich silicate gel;
d) adding to said gypsum free and aluminum-rich silicate gel a ~asic reacting agent to obtain a pH equal to 10 and producing a neutral suspension of aluminum-rich silicate;
e) filtering said neutral suspension of aluminum-rich silicate to obtain a solid filtration residue and washing said solid filtration residue;

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f) digesting said solid filtration residue cake with caustic soda;
g) crystallizing zeolite A and separating the same from mother liquor as a crystalline solid.

Further in accordance with the invention there is provided an improved process for manufacturing zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, the process including the reaction of a silica gel with an alumina gel, the improvement comprising incorporating into the silica gel prior to the reaction of the gels alumina in a proportion defined by the molar ratio of silica to alumina of between 60 and 1.2, and incorporating silica into the alumina gel in a proportion defined by the molar ratio of silica to alumina between 1.2 and 2.

The main characteristic of the invention is the fact that usa i8 made of pulp of 0ilicated gel origina~
ting from the treatment of blast-furnace slag with waste acid, said pulp being made basic by addition of cau~tic soda~and sodium aluminate orginating from the treatment of an alumina gel with cau~tic soda and~or originating from sodium aluminate o~tained during the surface treat-ment of ~luminium, by incorporating, into the silica, alumina in a proportion defined by the molar ratio of si-lica to alumina of between 60 and 1.2, preferably between 35 and 1.7.
The 0xtrapolation to pracSical use of the indi-cated raw materials for the purpose of producing a zeoli-te A sati~fying the indu~trial requirements makes it ne-ce~sary to employ a series of specific technique6, which will be described in greater detail below and which cons-titute further characteristics o t~e present invention.
~ ccording to the invention, it is envi~aged, in particular, to employ a sequence of simple oparations 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 qlag, obtained by rapid cooling of the molten slag when it leaves the blast furnace, constitutes a valuable source of silica and of aluminium. There are also different sources of waste sul-phuric acid, in particular the source resulting from the manufacture of titanium dioxide or the source containing wa~te ~ulphuric acid consisting, for example, of the 2~L20~

exhausted pickling acids.
The composi~ions of wa3te &cid origina~ing fro~
the manufacture of titanium dioxide depend especially on the composition of the titanium ore leached. The average concentratlon of sulphuric acid varles between 10 and 20 and the malor impurity is ferrous sulphate, which csn reach a proportion of 20 g of iron per litre. The acid contains other impurlties of less importance, such as Al, Mg, Cr, ~, Ti and Mn.
The pickling acid re~idueQ arP also characterized by A high proportion of iron. 1 1 Howe~er, iron constitutes a particularly trouble- ¦
some pollutant because it tends ulti~at~ly to colour the ' zeolite A produced.
In some cases lt i8 advantageou3 to enrich the ;
wa~te acid with aluminium; thls aluminiu~ can be a waste from the anodising of this metal~ ;
MoreGver, it ha~ been observed that ~he conditions of dlssolution of ths slag must be ~hosen 80 as to keep 20 the ~illcic acid in a stable state having a low degree of '~
polymerization.
The reaction of the sla~, which i~ a basic reac- ¦
eant9 with the acid proceeds according to the following equation~
(CaO.SiO2 0.2A1203 0-35MgO) ~ 1-95 H2S4 + 1~05 H20 -slag H2si3 + 0-2A12(S04)3 ~ 0.6MgS04 4 2 The neutralization of the slag i8 homogeneous and the dlssolution of the variou~ constituents takes place at : i ~2;~

~he sa~e ra~e. The reactlon products are fioluble in wa~er except for ~he gypsum, which preclpitates. The dissolution i8 ~xothermicO
In practice~ however, it ig ound that easy separa~
tion of the gypsum by filtration i9 only pos~ible provided tha~ the partlcle size of the slag treated is less ehan 0.4 mm and preferably less than 0.2 mm. One operating condition of the process of the lnvention thus consists generally ln grinding the granulated slag to the particle ~ize lndicated.
The slag is dissolved at constant pH (1.5), the reactor belng fed simuleaneously wlth a slag pulp t50X of solids) and the waste ac~d at the sppropriate flow rates.
The temperature of the dis~olution medium can vary between 60 and 70C.
In order to keep the degree of polymerlzation of the slllcic acid as low as possibleS care will be taken to comply with the followlng operatlng parameters:
- the pH of the reaction must be regulated so that the ~edlum remalns distinctly acid, the stability of the sillc~c acid being a ~a~imum at pH 1.5; beyond pH 2, the silica polymerizes rapidly;
- the concentration of the 8ilicic ac~d is advantageously regulated 80 aa to be close to the optimum concentration of 25 g of SiO2 per litre. This may involve diluting the waste acid ln accordance with the proportion of H2S04.
In general, dilution appro~lmates to the ratio 1/1 in the case of concentrated waste acid from the production of TiO2;
and ~ 2~
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~ as the ra~e of polymerization of silicic acid increases with temperature9 because the synthesla of silica gel takes place at 40C~ lt ia generally necessary to cool the dissolution medium.
It is obRerved that the curve of the dissolution kinetics of slag i9 characterized by a rapld step and a slow atep. A quasi-equilibrium is reached after a contact - time of 30 minutes and the efficiency of dissolution cor~
responds to 85%. Advantageously, the retention time is lO set at 30 minutes and the dissolution system consists of two vats, the material passing through the first in lO
minutes (rapid seep) and staying ~n the second for 20 minutes ~low step).
Stirring in the vats keeps the slag in su6pension 15 in order to achieve a high degree of dissolution ( 85%).
A3 will be indicated below, it is advantageous for the gyp~um suspensions obtained by flotation during the separa-tion of the gel/gypsum mixtures to be recycled to the di~solution stage, this recycllng as~i6ting the formation 3 0 of larger cry~tals and reducing the number of filtrat~on6.
~he dissolut~on alurry ls filtered so as to sepa~ate the gypsum from the solution of sillcic acid; the gypsum cake is washed in order to recover all the filtrate.
The filterability of ehe gypsum is characteri~ed 25 by the value of the S.C.F.T. (standard cake formation time);
thia parameter ia defined as ehe eime rsquired eo form a l cm thick cake under a differential pressure of one atmo~-phereO The average S.C.F.T. value of the gypsum is 20 +
5 seconds and this value diagnoses the very good filter-. I

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ability of the gypsumO
The gypsum cake i~ washed wlth a quantity of water corresponding to filling of the pore ~olume, and the filtrate is totally recovered. The gypsum produced is suitable for recycling ~o a cemen~ factory provided that it has a hlgh degree of dryness (85-90~)o 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 dlssolution o the slag has been flltered off, the silicic acid resulting from ~his dlssolution ls precipitated uslng calclum car-bonate, with the concomltant precipitation of a further quantity of gypsum, according to the equation:
nSi(OH)4 + A13+ 1 1.5S04 + l.SCaCQ3 -((HO)3SiO)nAl(OH)2 + 1.4CaS04.2H20 + 1.5 C0 (n-3)H2o .
It is apparent that, to obtain a zeolite A of acceptable whi~eness, it is necessary to use a calcium carbonate in which the proportion by weight of ~e203 is ~ess than 0.1% and is preferably O oO2% or lessO
~ According to a characteristic of the lnventiQn, in order to be able to filter off the insoluble silica gel and gypsum easily, alumina is incorporated into the silica 25 in a proportion defined by the molar ratio of slllca to alumina of between 60 and 1.2, preferably between 35 and 1.7, and particularly between 35 and 20 or 107 and 2 according to the particular embodiments of the invention.
The precipitation of the silicate gel is carried L2~

out at p~l beeween 2.8 and 407.
A first embodlment of the invention conalsts ln preclpltatlng n silicsta gel rich in 8illc8, the ratio of sllica to alumina being at least equal to 20. Thls preclpi-tation is carried out preferably at p~ 3.2 by simultaneous S addition of the sllicate gel solution of calcium carbonate.
Incorporation of the correct quantlty of alumlna lnto the silica i8 achleved by careful regulation of the pH of the precipitation. Below pH 2.8, the precipitation of the silica is incomplete; the gel obtained is unfilter-able. Above pH 4, Pn excesslve quantity of alumina isincorporated lnto the silica. The gel is precipitated at 40C withou~ heat being suppliPd. The silica sol must be cooled because a reaction temperature abo~e 40C
would cause the precipitation of iron oxideO
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 dissolu-tion of the calcium carbonate.
The stirring system must permit homogenizatIon of the reaction medium in order to prevent the calcium car-bonate from settling out in the bot~om of ~he reactor~
Under such conditions, the efficiency of the precipitatlon of silica reaches 100~.

The gel/gypsum mixture is then separated from the mother liquor by vacuu~ filtration; the flltrate is kept because it contains utilizable aluminium sulphate, the re-covery of which will be described below. The cake is washed on the filter with water at pH 2 in order to reduce ., the proportlon of iron a~ much as possibl~ A sequence of at least three washings is preferable and the volume of water for each washlng corresponds to the pore volume of the cakeO The washing can be carried out using the water from the process9 after purlfication and acidlflcation.
It i9 found that the filterability of ~he cake of the gel/gypsum mixture corresponds to an S~C~FoTo value ~ of the order of 40 seconds. In view of the quality required from the washlng, 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 lS 3.0 and 3.5~
The flotation technique for separating the gypsum and the 6ilica gel i8 an elegant and effective method of separating the components under conditions which, at first sight~ may seem nece~sarily to cause conRiderable diffi-culties as a result of the presence of the silica gel.
The Applicant Company has observed in th~s respectthat the use of dodecylamine (laurylamine) as the collector, by itself or mixed with other amines having a fatty hydro-carbon chain (for example a C16 chain), gives separation results which cannot be obtained with other compounds which are also used as collectors under other circumstancesO
By way of illus~ration, 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 ~.~2~

which depends on ehe quantity of gypfiUm produced. The effi-clency of separation of the gypsu~ is excellent ~99%~; the loss of silira gel through entralnment by the gypsum is lo~
(8%)o The proportion of sollds in tllP gel pulp reaches 10%.
In t~r~s o~ the ope~ation, the thickened pulp is kept as such. The gypsum pulp i8 recycled to the slag dissolution step.
- Under the abovementioned working contitions, the solution for ~ynthesls of the alumina gel contains utili-zable aluminium in a proportion of the order of 6.3 g litre.
This aluminium, carried by the mother liquor of the silica gel, is thereEore precipitated selectively by addlng calcium carbonate. The overall precipitation re-a~tion is represented by the equation:
[ l (OH)3_n n S04 ] + n Ca Co3~ (n + 1,5) H2O~Al (OH)3 +
n Ca S04n2H~0 + n CO2 A precipitate of gypsum is therefore formed at the same tlme AS the alumina gel.
According to a characteristic of the invention, it is apparent that the sub~equent separation of the gypsum by flotation is facilitated if, prior to the precipitation of the alumina, a quantity of ~oluble 8ilicic acid i8 ; added to the solution containing ~he aluminlu~. The addition of a small quantity of soluble silicic acid correspo~ding approximately to the molar ratlo:
Si2 = O.l provides the gel with surface properties which enable the gypsum ~o be separated off by flota~ion~
Various techniques can be used for preclpitating the alumina in the form of a gel, This precipitation is advantageously carried out in two stages. In a first stage, which take~ place at a constant pH of the order of 3.9, a solution of sluminiu~
and calcium carbonate are added slmultaneously (reaction time of the order of 2 hours). The requlrements regarding the purity of the calcium carbonate used are less strin-gent than for the precipitatlon of the silica gel. I~ isapparent, in particular, that a proportion of Fe~03 which can reach values of the order of 0.08~ does no~
re~ule in any serious disadvantage.
The precipitation is then completed (ln 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 zeo`lite A crystallizati~n circuit; the consumed volume of mother llquor of the solution A i9 equivslent to the purge requlred to balance the materials (water and impurities) in the system.
Care will be taken to ensure that the p~ of the ~econd step does not exceed 4.6; otherwise an iron oxide will also precipitate. The precipitatlon does not require heat to be supplied; the temperature of the reaction medium reaches a~ most 40C, which is the temperature of the mother liquor of ~he silica gel. The stirring must be sufficient to keep the solid phase in suspension.

The efficiency of the precipltation of the alumina reaches 100% under the conditlons indicated.
The ~ixture of alumina gellgypsum i8 flltered off and washed.
The pulp of alumina gel i5 characterized by a filterabillty whos~ S.C.F.T~ value can vary between 1 and

3 minutes. Filtration under pressure on a thickening fllter is adequate on account of the small proportion (5%) of solids in the suspensionO 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 120 g/litre and whose pH is between 4.5 and 50 The water from the process is used for pulping, after purifica- ;
tion~ The gypsum pulp collected after flotation i8 preferably recycled to the slag dlssolution 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.
A~ a variant of the technlque described, a second method of precipitation of the alumina gel is po6sible.
This method consists in precipitating the metals from the waste solution by means of waste aluminate, at a constant pH of 10, and then in selectively precipitating the aluminlum, 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 fllterability.

The ~econd embodiment of the invention ls to pre-cipitate a silicate gel rlch in aluminium, the molar ratio of silica to alumir~a being between 20 and 102 and prefer ably between 2 and 1.2. This high content of alumlnium can be obtained if the waste acid i9 enriched with aluminium beforehand, for example by adding anodisation wastes thereto.
The precipltation of aluminium-rich silicate gel i8 obtained at p~ between 4 and 4.7, preferably at pH 4.2 by simultaneous additions of the 6ilicic acid solution and of calclum carbonate.
The gel is precipitated at 40C with the addition of heatO The silica sol must be cooled since a reaction temperature above 40C would cause the prec~pitation of iron oxide.
The precipitation of s~ica~ gel is very fast, but ~t i~ advantageous to keep the reactants for an hour in the reactor in order to ensure that calclum carbonate is com-pletely dissolved.
The agitation system must make it possible to homogenlze the reaction medium to prevent the Qettling 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 i9 washed with water at p~ 2 so as to reduce the lron 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 operatlon i8 ~arried out wlth a pulp the solids content of which reaches 120 g/litre and the pH of which is preferably between

4.2 and 5.
The yleld of the gypsum preclpitatlon is excellent (99%); the 1098 of sllicate gel entrained with the gypsum ifi low (8%). The silicate gel pulp (10% sollds) is kept as such while the gypsum pulp is recycled eo the slag dissolutlon. ' The zeolite A 18 then synthesi ed in three steps:
- conditloning of the reactants;
- formation of the reaction mixture;
- crystallization~ filtration and conditioning of the zeolite A.
- 15 The lnvention allows the ~eolite A to be syn-thesized in two ways depending on whether the reaction mixture is prepared either from silica gel and alumina gel or from an alum~nium-rich silicate gel.
In a first particular method of ~ynthesis accord-ing to the lnventlon, the reaction mixture i8 fo~med fro~ an alkaline suspension conslstlng of the ~ilica gelJ
sodium aluminate and cau~tic soda. The formulation of the mixture is fixed by the following molecular ratios:
SiO2 Na20 H20 - ~ 1.7 ; = 1.8 and = 33.
2 3 SiO2 Na2 It i8 apparent ehat the order in which the re-actants are added in the formation of the zeolite A is important for obtaining good re6ults.
The conditioning of the reactants consists in ~ , ~L2~42~

neutralizing, with caustic ~oda, the silica and alu~lna gel~ precipltated in an acid medium3 and then in causing attack to take place in the caustic soda.
The silica and alumina pulp5 (100 g of gel/litre) obtained from the gypsum flotatlon steps are first neutralized to pH 10, respectively by means of a fresh caustic soda brine and solutions of waste aluminate - origina~ing from anodic oxidation works. The neutraliza-tion iR carred out at ambient temperature up to pH ~ 10.
The neutrali~a~ion of the alumina gel causes precipitatio~
of the aluminium carried by the solutions of waste aluminate; this aluminium is completely recovered. The neutral pulp8 are dried on a filter press to give a degree of dryness equal to at least 35%. To balance he water, the hydration volume of the fllter cake~ 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 quant~ty of sodium hydroxlde u~ed corresponds to the molar ratio:
Na20 = 0.8 sio2 The gel is attacked rapidly (3 minutes) but lncompletPly~ and an insoluble residue of silica remains.
The alumina gel is dissolved at amb~ent temperature in a caustic soda solution consisting of the mother liquor enriched in sodium hydroxide. The quantity used for en-richment corresponds to the sodium hydroxide consumed by the purge. The gel is dissolved in 15 minutes. The iron 2~l and the allica associated with the alumlna gel are only very slightly soluble in a~ alkaline medium; they form a preclpitate which contaminates the fiodium aluminate 801u tion.
This residue is filtered off under pressure and washed. The efficiency of the dissolution $s 90%; the composition of the solution obtained is characterized by the molar ratio:
Na2 2 The formstion of the reaction mix$ure comprlses mixlng of the reactants and predigestion.
It is apparent that there are two possible methods of sdding 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:
aO sodium aluminate solution;
b. basic ~ilica pulp.
The reactants can also be mixed in a continuous operation. The alkaline sllica 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 50 as to produce the formulation of the mixture and to keep it constant.
In both cases> the stirring ls regulated so as to permit rapid homogenization of the reactants, because the operation produces a thick gel.
In a second particular method of synthesis accord-ing to the invention, the reaction mlx~ure consists of an alkaline su~pension obtalned from an aluminium-rich slli-cate gel and caustlc soda.
The formulation of ~he mixture 1~ fixed by the following molar ratios:

sio2 - ~ 1~7 Al2 3 Na 0 2 = 1.8 ~nd s i o2 2 = 33 The condltioning of the alumlnium-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 neut-ralisation; thi~ procedure makes it possible to remove ~he chromium and vanadium present as trace elements. The ~;
neutral pulp is then filtered; the water volume of the cake i9 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 amblent temperature, with a caustic soda solution con-sisting of the mother liquor from crystallisation enriched ~lth caustic sodaO
The quantity of ~oda employed corresponds to the molar ratlo Na20 j = 1.B

According to the two methods of preparation des-cri~ed9 ~he reaction mixture i8 directed in~o a digestlng vat, where it stays for 12 hours at ambient temperature.
Finally, the zeolite A is caused to crystallize by heating the reaction mixture for 2 hours at 85C;
good stirring ensure3 homogenizatlon of the medium, the solids content o which reaches 16%. The efficlency of the crystallization is 100%.
The crystalline solid i3 æeparated from the mother liquor by filtration, the filterabllity of the zeolite lS pulp being chsracterized by an S.C.F.T. vslue of 2 minutes.
The solld is flltered off and washed with deionized water until the pH of the pulp is 10.5. The moisture content of t`he cake flltered of under vacuum can reach 58Z.
- The zeolite can be ~upplied ln two different 20 forms: ;
- either in suspension, a mixture of sodium polyacrylate (fluidizing agent for atomization~ and phosphate, pre-ferably sodium polyphosphate, belng added to the filtered pulp in a proportion of 0.3% to give a fluld suspenslon . !

.

containing 40% of solids, - or ~n powder forrn, the zeollte pulp being dried to glve & finely disperse powder after drylng.
The lnvention will be described in greater detail with reference to the ~xamples below, which are intended to illustrste the invention without implying a limitation.

1. DISSOLUTION OF THE SLAG
ThP granulated slag is dissolved in 8 waste acid orlglnating from the manufacture of titanium dioxlde;
~he composition of these ~wo reactants is given below:

Granulated slag Waste acid Constituent Proportion, % Constituent Proportion, g/litre SiO2 34.13 H2S04 184.4 A123 12.63 Fe2+ 19.16 MgO 8.39 Al 4.17 CaO 34.86 Mg 5.13 Fe23 0~96 Na 0.84 MnO 0.54 V 0.96 K 1.37 Cr 0.26 Na 0.94 T102 3.61 TiO~ 0.78 S 1.4 2~

A pulp of ground s18g (~C 0.2 mm) i9 used asld ehe waste acid is diluted prior to dissolutionO The gypsu~
suspensions obtained after separation from the ~ilica and alumlns gels are added to the pulp of ground slag.
Dissolution is carried out at pH 1.5, in a continuous cperation9 in a system consisting of two re-actors (15 and 28 litres~ each equipped with 3 baffles - and stirred so as to keep the solids ln suspension. The reactants are introduced into the first vat by means of peristaltic pumps.
The ~lag feed is constant ~hile the acid feed is sub~ect to the measurement of the pH. The contents of the ls~ vat overflow into the second, the system being regulated so that the average residence time in the re-15 actors is 30 minutes. These conditions make it possibleto obtain a dissolution efficiency of 85%.
The consumption per hour of the reactants ls as follows:
- slag pulp comprising: 6 ~65 kg of ground ~lag, 4.38 k~
of gypsum and 11~03 litres of water;
- acid solution comprising: 37.8 litres of acid and 28.33 litres of dilution water.
Th~ production per hour of silicic acid pulp is 77.16 litres; the solids content amounts to 15~5%o The acid pulp ls filtered and the cake ls washed on a vacuum filter ~pressure reduction ~ 0.3 bar) at a ra~e of 1.3 litres per m2 per second; under these condltlons, the thickness of the cake is 10~5 mm. The composition of ~he silicic acid separated from the gypsum is indicated below:

Constituent Proportlon9 g/litre . _ SiO2 25 Al 7003

5 Fe2+ 10055 - ~g 6.53 TiO2 l o85 Cr 0.14 V 0.49 Na 0.99 K 0 ~ 8 6 ; ~
2. PRECIPITATION OF THE SILICA GEL
The precipitation of ths silica gel is carried out at pH 302 by adding calcium carbonate to the silicic acid solut~on. The calcium carbonate is used in the form of a 25Z suBpensiOn; the preferred particle slze ls less - than 55 microns and the proportion of iron oxide does not exceed 0.02~. , The precipitatlon lnstallation is made up of two vats havlng a u3eful volume of 40 litres; they are equipped internally with 3 baffles and a heating element making lt possible to carry out the precipitation at 40C.
Stirrers in the vats keep the solid phase completely in suspenslon. The reactants are introduced slmultaneously lnto the first vat by means of pumps; the synthesis mix-ture then overflows into the second vat. The flow rates ~2~

of the reactants are regulated 80 as to keep the gel in the synthesis medium for one hour. A system of elec-trodes measures the precipitation pH and governs the control regulating the flow rate of calcium carbonate.
S The consumptions per hour of the reactants are 77016 litres of sllicic acid and 0.698 kg of calcium carbonateO 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.
Thls solid/liquid separation is carried out at a rate of 0.37 litre~ of pulp per m2 per second; the pressure reduction in the fllter is 0.3 bar. Under these conditlons, the thickness o the cake is 11 mm.
The separation of the silica gel and the gypsum by flotat$on is carried out on a pulp conta~ning 120 g of solids per litre and the separation requires a rough operation and a finiRhing opera~ion. A mixture of tallow amine (C16) and coconut amine (C12), in respective proportions of 150 and 600 g per tonne of product to be separated, is used as the collector; the frothing agent i~ pine oilO
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 ~22~

aluminium is 77.16 lltresO
The composit:Lons of the silica gel and the mother llquor are given below:

.
Silica gel Mother liquor Con~tituent Proportion, % Con~tituent Proportion~ g/litre Sio2 78.1 Al 6.3 A123 3.81 Fe2+ 10.55 Loss on ignition 8.7 Mg 6.53 Cr23 0.17 Mn 0.3Q
Fe23 0.08 K 0.86 TiO2 8.85 Na 0.99 V205 0.34 3~ PRECIPIT-ATION OF THE ALUMINA GEL ;
Prior to the precipitation of the slumina g&l, a ~llicic acid solutlon is added to the mother liquor con~
taining the aluminium so as to give the following molar ratio:
sio2 - = 0.1 2Q The precipitation is carried oue in two stage~ ~n an installation similar to that used for preclpi~ating the silica gel.
In a f.trst stage, the pH is lncreased to 3.9 by adding a 25~ suspension of calcium carbonate; the particle size of the carbonate is less than 55 microns.

The res~dence time of the reac~ants ln the reactor~ i8 two hours and the temperature re~ches 40C. In a second stage, the pH i9 lncrea3ed to 4.6 by adding caustlc soda orlglnating from the ~ynthesls wa~er of the zeolite A. The consumptlons per hour of reactants are 77.16 litres of alumlnium solutlon, to which are added 2.16 litres of a solutlon of 9ilicic acid (25 g/litre), - 1.247 kg of calcium carbonate and 0.3 kg oE caustlc soda.
The pulp of alumlna gel and gypsum produced has a solids content of 5.2~; the lnorganlc mixture is filtered off and washed .
The filtration, which is followed by washing, is carriedout at a rate of 0.53 litre of pulp per m2 .
per secondi the pressurereduction in the filter isO.3 bar. Under these conditions, the thickness of the cake is 8 mm.
The separation of the s~lica gel and the gypsum by flotation is carried out on a pulp containing 120 g of solids per litre; the separation requires a rough opera-tion and two finishing operations.
A Wedag machlne, model ~N 935¦4, equ~pped with a6 litre cell is uæed for the flotationO
A mixture of tallow am~ne 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 plne oil.
Taking account of tha separation efficiency~ the weight~ per hour of the products separated off are as follows:

~l22~

- alumina gel 1.75 kg - gypsum 2~14 kg.

The volume of the waste ~olution is 77.16 lltre~O

The composition of the alumina gel produced i~ given below:

.
Constituent Proportion, %

.
A12357.71 Fe232.45 S035.72 LOB8 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 $emperature~ The characteristics o ~he neutralization reaction aFe indicated below:

S102 gel A1203 gel .
weight treated 2.33 kg 1~94 kg pH of the pulp before neutrallzation 3-3O5 4,5-5 neutralizing agent NaOH brine NaOH, waste 365.ô g/litre aluminate (80 g -- ~224~
of NaOH/litre and 160 g of A1203/litre) consumptlon, g/mol 6.77 40.8 The neutralized gels are filtered and the water with which the cakes are impregnated is exchanged with - the mother liquor of the zeoli~e A.
The consumptions of mother liquor are re6pectively 4.33 litres and 3.6 litres for the silica gel and the aluminium~
b. Attack of the gels.
The neutrali~.ed gels are sttacked in a batch operation at ambient temperature in caustic soda under tbe condit1on~ listed below:

.
SiO2 gel Al23 gel reaceant NaOH brine, zeolite mother 365.8 g/litre liquor enriched in NaOH
consu~ption 4.14 litres 20.33 litr~s +
514 g of NaOH
composition of the solution Na20 3 O.B Na20 ~ 2 SiO2 A12 3 Clarification of the solution _ +

~2;~

The sodlum aluminate solu~lon is clarlfied ~o remove the insoluble impurities (lron oxide and sillca~
2~ ~ixlng of the reactants and predigestion.
The two liquid6 obtalned by attack of the gels are pumped simultaneously into a vat equipped with 3 baffles and are stirred in order to produce a homogeneous mlxture of the t~o streams. The flow rates of the re-actants are balanced so as to keep the formulation of the mixeure constantl this formulation being represented by the following molar ratios:
SiO2 ~ 1.7; Na20 ~ 1.8 and H~0 ~ 33.

A123 SiO2 Na20 The reaction mixture ls aged overnight at ambient temperature.
3. Crystallization.

I

The reaction mixture is heated for 2 hours ae 85CJ the reactor ls sti-rred in order to keep the solid phase in suspension.
The solid ls 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 i8 identified by X-ray diffraction; its chemical composition is given below:

ConstituentProportion, %

.
SiO2 32.31 A123 31.05 Na2Q 18003 H20 12.90 TiO2 4.30 Fe23 O.OS

The ma~n characteristic~ are given belo~:
reflectance : 91 ~unter coord~nates 10 odour ` : none particle slze distribution below lO~ : 94X

6~ o 82 5/u : ~0~
15 p~ snhydrous ln aqueous ~uspension~ . 10.4 weight 1088 after 50 minutes at 800C : 21Z
sequestering power at 25C per ~ of anhydrous zeolite after 15 minutes : 150 mg of CaO
The zeolite A obtained can be used as a seques-terlng agen~ for calcium ln a wa~hing powder formulation.

1. DISSOLUTION OF THE SLAG
-The granulated slag is dissolved in a waste acid originating from the manufacture of titanium dio-xide i the composition of these two reactants is given5 below:

Granulated slag Waste acid . Constituent Proportion, ~ ~nstituent Proportion g/l .
Si2 34 13 H2SO4 205.4 2 312.63 Fe2+ 19.6 MgO 8.39 Al 4.06 CaO 34.86 Mg 5.8 2 3 0.96 Na 0.84 MnO 0.54 V 0.68 K 1.37 Cr 0.25 Na 0.94 TiQ2 2.9 TiO2 0.78 S 1.4 ~ Before being used for the dissolution, the con-centration of the waste acid in aluminium has been im-proved by addition of a wa6te product originating from the aluminium anodisation ; said waste has the follo-wing composition: NaOH : 102 g/l; Al2O3 : 172 g/l.
The aluminium concentration enhancing is suCh thata molar ratio silica to alumina of l.49 is obtained in the solution resulting from the dissolu-tion of slag. Practically speaking, to one litre of waste acid, 0.1 litre of a solution containing 102 g/l NaOH and 172 g~l Al2O3 are added.
~uminium enhanced waste acid thus obtained . . . _ 5Constituent Proportion, g/litre 2 4 130.24 Fe2+ 17.64 Al 12.75 Mg 5,22 Na 6.62 V 0.61 Cr 0.22 TiO2 2.6l The dissolution of the slag is performed under conditions similar to those described in Example 1O
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 i6 70 litres ; the solidscontent amounts to 21.8 %.
The acid pulp is filtered and the cake is washed on a ~acuum 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:

ConstltuentProportion, 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 SILICATED GEL.
The precipitation of the silicated gel is car ried out at pH 4.2 by addiging calcium carbonate to the silicic acid solution.The precipitation of the silica-ted 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 carbo-nate. The pulp of silicated gel and gypsum produced has a solids content of 20 %. The mineral mixture is filte-red off and washed with acidified water (pH 2) on a va-cuum 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 ~2~

these conditions~ the thickness of the cake i8 11 mm~
The separation of the silica gel and the gypsum by f lotation i8 carried out on a pulp containing 120 g of solids per litre and the separation requires a rough operation and a ~inishing operation.A mixture of kallow amine (C16) and coconut amine (C12), in respective proportions of 300 and 1200 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 mo-ther liquor are given below :

Silica gel Mother liquor Constituent Proportion, ~ Constituent Proportion,g/l . .. . . _ _ . . ... . _ .
SiO2 31.38 Al23 36.27 Fe2+ 12.70 Loss on ignition24.37 Mg 7~32 Cr23 0.14 2 3 K 5.09 TiO2 2.9 Na 0.97 V25 0.66 3. SYNTHESIS OF THE ZEOLIT~ A
1. Conditioning of the silicated gel.
a. Neutralization.
The pulp of silicated gel (10% of solids) is neutralized to pH 10 by means of the moth~r liq~or of the crystalli7ation. The neutralization is carried out in a continuous operation at ambient temperature. The characteristics of the neutralization reaction are in-dicated below :

.. _ . _ . . ..... . _ . _ .. . _ weight treated 4.67 pH of the pulp befora 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 exchanged with the mother liquor of the zeolite A.
The consumption of mother liquor is 4.9 l.
b. Attack of the silicated gel and predigestion.
The neutralized gel is attacked in a batch ope~
ration 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) Consumption~ : NaOH brine : 3.96 Motherliquor of the crystallization : 21.2 1.

~o~ o~

Ihe ~oH brine i.s added to the silicated gel, a~ter .

dispersion of the mixture, the mother liquor of the crystallization is added. The thus obtained suspension is stirred and aged for l2 hours.
2. Crystallization The reaction mixture i8 heated for 2 hours at 85 C j 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 ara 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 composi-tion as in Example 1.
The main characteristics are given below :
reflectance : 91 Hunter coordinates odour : none particle size distribution below 10 ~ : 95 %
6 ~ : 94 %
4 ~ ; 90 %
pH t1 % 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 15 minutes : 151 mg of CaO.
The zeolite A obtained can be used as a seques-tering agent for calcium in a washing powder formula-tion.

3~
In the attached drawings, flow-sheets are given which represent the process~units according to the Examples.
Figures 1 and 2 refer to Example 1 and represent .
respectively the acid neutrali~ation and the synthesis of the gels, and the ~eolite A synthesis.
Figures 3 and 4 refer to Example 2 and represent respectively the silico-aluminium synthesis and the zeolite A synthesis.

Claims (26)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for manufacturing zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, comprising the steps of:
a) treating blast furnace slag with waste sulfuric acid, which has a maximum pH of 1.5 to avoid substantial polymerization of silica and which is enriched with aluminium in solution in order to obtain a preparation;
b) precipitating from said preparation between pH 4 and 4.7 an aluminium-rich silicate gel by adding in a reaction vessel said preparation and calcium carbonate, said aluminium-rich silicate gel further comprising in admixture gypsum resulting from the reaction of sulfate ions from said waste sulfuric acid and calcium from said calcium carbonate;
c) separating said gypsum from step b);
d) neutralizing said gypsum-free, aluminium-rich silicate gel at pH 10 and producing a neutral suspension;
e) filtering said neutral suspension to obtain a filtration cake and washing said cake;
f) digesting said filtration cake with caustic soda to result in a mother liquor;
g) crystallizing zeolite A and separating the same from the mother liquor as a crystalline solid.

2. The process according to claim 1, wherein ground blast-furnace slag, which has a particle size of less than 0.4 mm, is dissolved with said waste sulfuric acid at a pH of between 0 and 1.5 and wherein the gypsum formed by the reaction of calcium ions in said slag and sulfate ions of said sulfuric acid is separated off by filtration of the solution.

3. The process according to claim 1, wherein said waste sulfuric acid for the treatment of blast-furnace slag is enriched with aluminum generated from the treatment of aluminum anodization waste.

4. The process according to claim 1, wherein the precipitation of an aluminum-rich silicate gel is caused by adding calcium carbonate containing about 0.1% Fe2O3 by weight.

5. The process according to claim 1, wherein in said aluminum-rich silicate gel, the molar ratio of silicia to alumina ranges from about 1.2 to 2.

6. The process according to claim 5, wherein said molar ratio of sieca to alumina is achieved by enhancing aluminum concentration in said waste acid using wastes resulting from aluminum anodization.

7. The process according to claim 1, including separating gypsum in step c by flotation, the gypsum being collected in the froth.

8. The process according to claim 7, employing dodecylamine selected from the group consisting of tallow amine, coconut amine, amine having a fatty hydrocarbon chain and mixtures thereof, for flotation of the gypsum.

9. The process according to claim 7, wherein said flotation of gypsum is carried out on a suspension having a solid content of about 120 mg/litre and having a pH between 3.0 and 5Ø

10. The process according to claim 1, wherein digesting is performed by keeping the filtration cake in a digesting vat containing an alkaline medium for 12 hours at room temperature.

11. The process according to claim 1, wherein the crystallizing is performed by heating for 2 hours at 85°C.

12. The process according to claim 1, wherein the zeolite A
obtained in step g) is washed until the pH of the pulp is 10.5.

13. The process of claim 2, wherein said particle size is less than about 0.2 mm.

14. A process for manufacturing zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, comprising the steps of:

a) treating blast furnace slay in a reaction vessel with waste sulfuric acid produced in the manufacture of titanium dioxide by the "sulphate" process and having a concentration in H2SO4 of 10 to 20 weight % at a pH comprised between 0 and 1,5 thus avoiding polymerization of silica into silicagel and the addition into the reaction product of said blast furnace slag with said waste sulfuric acid of aluminum generated from the treatment of aluminum anodization waste in order to obtain a mixture;
b) adding calcium carbonate in the said reaction vessel to the said mixture in order to adjust the pH between 4 and 4,7 causing the precipitation of an aluminum - rich silicate gel, said gel further comprising in admixture gysum resulting from the reaction of sulfate ions from said waste sulfuric acid and calcium ions from said calcium carbonate;
c) separating said gypsum from said aluminum-rich silicate gel by flotation thus producing a gypsum free and aluminum-rich silicate gel;
d) adding to said gypsum free and aluminum-rich silicate gel a basic reacting agent to obtain a pH equal to 10 and producing a neutral suspension of aluminum-rich silicate;
e) filtering said neutral suspension of aluminum-rich silicate to obtain a solid filtration residue and washing said solid filtration residue;
f) digesting said solid filtration residue cake with caustic soda;
g) crystallizing zeolite A and separating the same from mother liquor as a crystalline solid.

15. The process according to claim 14, wherein ground blast-furnace slag, which has a particle size of less than 0.4 mm is dissolved with waste sulfuric acid at a pH of between 0 and 1.5 and wherein the gypsum formed by the reaction of calcium ions in said slag and sulfate ions of said sulfuric acid is separated off by filtration of the solution.

16. The process according to claim 14, wherein said waste sulfuric acid for the treatment of blast-furnace slag is enriched with aluminum generated from the treatment of aluminum anodization waste.

17. The process according to claim 14, wherein the precipitation of an aluminum-rich silicate gel is caused by adding calcium carbonate containing about 0.1% Fe2O3 by weight.

18. The process according to claim 14, wherein in said aluminum-rich silicate gel, the molar ratio of silicia to alumina ranges from about 1.2 to 2.

19. The process according to claim 18, wherein said molar ratio of siica to alumina is achieved by enhancing aluminum concentration in said waste acid using wastes resulting from aluminum anodization.

20. The process according to claim 14, employing dodecylamine selected from the group consisting of tallow amine, coconut amine, amine having a fatty hydrocarbon chain and mixture thereof for flotation of the gypsum.

21. The process according to claim 14, wherein said flotation of gypsum is carried out on a suspension having a solid content of about 120 mg/litre and having a pH between 3.0 and 5Ø

21. The process according to claim 14, wherein digesting is performed by keeping the filtration cake in a digesting vat containing an alkaline medium for 12 hours at room temperature.

22. The process according to claim 14, wherein the crystallizing is performed by heating for 2 hours at 85°C.

23. The process according to claim 14, wherein the zeolite A obtained in step g) is washed until the pH of the pulp is 10.5.

24. The process of claim 15, wherein said particle size is less than about 0.2 mm.

25. An improved process for manufacturing zeolite A by reacting silica, alumina and caustic soda in an aqueous medium in an appropriate stoichiometric ratio, the process including the reaction of a silica gel with an alumina gel, the improvement comprising incorporating into the silica gel prior to the reaction of the gels alumina in a proportion defined by the molar ratio of silica to alumina of between 60 and 1.2, and incorporating silica into the alumina gel in a proportion defined by the molar ratio of silica to alumina between 1.2 and 2.

26. The process of claim 25, wherein the ratio of silica to alumina is between 35 and 1.7.