Zeolites are being used increasingly in recent times, wherein for a number of applications it is not essential for the zeolite to be obtained with a high degree of purity.

The object of the present invention is to provide a method for preparing a zeolite product, wherein use can be made of cheaper raw materials, wherein a zeolite product is obtained with a degree of purity and definition which are possibly lower than those of traditional synthetic zeolites but which are better than those of natural zeolites, in combination with a zeolite product of much higher quality.

The present invention therefore provides a method for preparing zeolite of two different qualities, wherein a hydroxide solution is added to fly ash and the thus resulting mixture is separated into fly ash residue and silica extract, wherein the fly ash residue is then mixed with a hydroxide solution to form a reaction mixture, the reaction mixture is heated to a temperature between 80"C and 1500C for a period of 10 to 50 hours, wherein the fly ash is held continuously in suspension by stirring, and fly ash residues containing the zeolite product are subsequently separated from the process water.

The silica extract is zeolitized in the same manner after addition of the correct quantity of

aluminium hydroxide suspension in order to synthesize pure zeolite (>99W).

It is possible to carry out the various process steps simultaneously and/or sequentially.

Another preferred embodiment of this process relates to the zeolitization of fly ash from the combustion of domestic refuse (WIP fly ash) and chemical waste (CWIP fly ash), which produces a zeolite- immobilizer. The zeolitization serves to immobilize the heavy metals present in excess in these fly ashes and proceeds similarly to the described basic process.

However, because these types of fly ash generally contain too little aluminium and silicon for zeolitization, additional aluminium and silicon is added for this purpose to the reaction mixture in the form of an aluminium hydroxide suspension, being an industrial residual product, and amorphous silica or silicon-rich coal fly ash and/or process water remaining after the zeolitization of the fly ash.

The various preferred embodiments are elucidated in the following description with reference to the annexed figures, in which: figure 1 shows a schematic view of the first method (basic process) according to the present invention, and figure 2 shows a schematic view of a second method (first preferred embodiment) according to the present invention.

As stated in the preamble, the present invention provides a method for providing zeolite products from diverse fly ashes. This means that said fly ashes, which have either a small positive value (coal fly ash) or a high negative value (WIP and CWIP fly ash), can be sold as high-grade product on the environmental technology market (as absorbent) or, in the case of WIP and CWIP fly ash, can be sold more easily on the building market for bulk applications.

For this purpose a pretreatment of the fly ash is carried out, wherein the fly ashes are washed with a water and specific organic extraction agents to remove a substantial part of the possibly present heavy metals which are usually to be found in fly ash, and also a part of the components which are inactive in the zeolite synthesis (particularly calcium, magnesium, sulphur and phosphor). These substances are also usually present in fly ashes. A large part of the iron present in the fly ash is also removed via magnetic separating techniques.

This situation is shown at 2 in figure 2. Pretreated fly ash 3 thus results.

According to the basic process, the pretreated fly ash 3 is subjected to a zeolitization process, which is shown in figure 1 in the box 4. For this purpose hydroxide solution is fed to the pretreated fly ash. The hydroxide solution is a so-called 2-molar solution which is preferably formed by an NaOH or a KOH solution or a mixture thereof with a ratio of 2.5 liquid and solids.

It is herein possible to optimize the ratio of silicon and aluminium or to increase the concentration of these two elements in the reaction mixture. This will certainly be necessary for the zeolitization of WIP and CWIP fly ashes as referred to in preferred embodiment 2.

In the case of coal fly ash, and certainly Dutch coal fly ash, this is generally not necessary. As stated above in the case of preferred embodiment 2, a residual product from the aluminium-processing industry, obtainable as residue at a low price from the said industry, can be used as additional aluminium source and amorphous silica or specific fly ashes with a high silicon content from an electricity power station can be used as additional silicon source. The quantity of the relevant materials required to obtain a zeolite of a determined type is shown in table 1.

Table 1 Overview of the required molar ratios of the components determining zeolite type for the synthesis of zeolite Na-P1, ZK-19 and K-G in the basic process Product Na2O K2o Awl203 SiO2 H2O zeolite Na-Pl 1.1 0-0.1 1 3.2 58 zeoiite ZK-19 0.8 0.4 1 3.2 58 zeolite K-G 0-0.6 1.2-0.6 1 3.2 58 The reaction mixture is then incubated at a temperature of between 800C and 1500C at autogenous pressure, preferably and subject to the reactors available as closely as possible to 1000C or 1500C, wherein the fly ash is held continuously in suspension by stirring. The incubation time lies between 10 and 50 hours, and depends on the chosen temperature and stirring speed. In one example the incubation time amounted to 24 hours at 930C and 200 revolutions per minute, and in another example 12 hours at 1500C and stirring at 5 revolutions per minute.

It will be apparent that it is possible to optimize the relevant parameters.

After the reaction has ended, the zeolite product is separated from the process water, indicated at 5 in figure 1, whereafter the remaining zeolite product can be washed in step 6 and 8 and dried in step 9. After drying 9 the zeolite product 11 can be pelletized and be sold as absorbent on the environmental technology market.

It is herein noted that about two-thirds of the glass fraction in the original coal fly ash is converted into zeolite. The zeolite product contains roughly 60% zeolite, 15% mullite, 5W quartz and 20% residual glass, assuming a fly ash from a Dutch electricity power station.

The cation-exchanging capacity of the zeolite product obtained in the basic process amounts to about 3 meq/g, assuming a sodium-ammonia exchange. It is pointed out that without pretreatment of the fly ash a product is obtained with a lower cation-exchanging capacity of about 2.5 meq/g. The said quantities of zeolite, residual glass, mullite and quartz and the cation-exchanging capacity can of course vary slightly depending on the glass content of the fly ash used and the grain size distribution thereof.

After the zeolite synthesis has ended, the process water contains per litre about 0.5 mol hydroxide, 5,000 to 15,000 mg silicon and less than 50 mg aluminium and can hereby be circulated and thus used for: 1. the production of the zeolites of the types Na-P1, ZK-19 or K-G from the production of coal fly ash with the above described process in step 4.

2. the production of pure zeolite analogously to the synthesis of pure zeolites from the silica extract, as in the 2-stage zeolitization process described hereinbelow; 3. the production of a zeolite-immobilizer from WIP and CWIP fly ash, as in preferred embodiment 2.

The above described process is designated as the 1-stage zeolite preparation process. It is however possible to apply a 2-stage process as referred to in the first preferred embodiment. Such a 2-stage process is shown in figure 2. This process corresponds with the process steps of the 1-stage process as shown in figure 1, with the proviso that the pretreated fly ash 3 is subjected to silica extraction by adding hydroxide solution 7, as designated at 14.

This essentially forms the first part of the zeolitization process. This is a so-called induction period lasting between four and eight hours depending on the chosen temperature and the stirring speed. During this induction period growth cores for zeolite crystallization are formed, though not yet the zeolite

crystals themselves. At the end of this period, just before the zeolite crystal formation begins, the silicon concentration in the process water reaches a maximum value of 10,000 to 20,000 mg silicon per litre, depending on the type of hydroxide solution used and the stirring speed. By interrupting the zeolitization process at this moment by separating off the process water (first stage), a silica extract and a fly ash residue are obtained. Both parts, fly ash residue and silica extract, are then converted separately and simultaneously into zeolites in the second stage. In the case of Dutch fly ash, 10% to 20% of the SiO2 present in the fly ash is dissolved by the silica extraction.

A separation of solid and liquid thereafter takes place in step 15.

The remaining solid, the fly ash residue 16, is subsequently subjected to the same steps 4,5,6,8 and 10 as in the 1-stage process.

The silica extract 17 obtained in step 15 is used for the synthesis of pure zeolites of the types A, X, Y, ZK-19 or K-G. For this purpose the Na2O;K2O; Al203;S iQ2 ratio of the silica extract is optimized by adding aluminium hydroxide or an aluminate solution thereto in a quantity which depends on the silicon concentration of the silica extract. For synthesis of the diverse types of zeolite, components determining the zeolite types must be present in the ratios shown in table 2.

Table 2 Overview of the required molar ratios of the components determining zeolite type for the production of zeolite A, X, Y, ZK-19 and K--G from the silica extract product Na2O - K2o Awl203 SiO2 H2O zeolite A 2 0.3 1 2 300 zeolite X 4 0.2 1 3 450 zeolites 8 0.2 1 10-14 800-1600 zeolite ZK-19 1-2.5 1-2.5 1 2-4 200-400 zeolite K-G 0.2 2.4 1 4-5 300-400 When a residual product is used from the aluminium ore industry, being an aluminate solution, the molar ratios differ slightly because the residual product includes much sodium and moreover contains a complexing substance which holds aluminium in solution. Applicable here are the molar ratios as shown in table 3.

Table 3 Overview of the required molar ratios of the components determining zeolite type for the production of zeolite A, X, Y, ZK-19 and K-G from the silica extract when aluminium- containing residual product is used product Na2O K2O A1203 SiO2 H2o zeolite A 6.5-7.2 0.3 1 2.3-2.6 540 zeolite X 5.8 0.2 1 2.2 370 zeolite Y 14.9-36.5 0.6 1 20 1200 zeolite ZK-19 1-2.5 1-2.5 1 2-4 400 zeolite K-G 1.4-2.2 3.3 1 4 400 Production of the zeolites takes place by incubating the optimized silica extract for 24 to 48 hours at 9O0C, which is shown in step 18. Stirring is herein only necessary at the start to homogenize the mixture. After the reaction has ended, the zeolite product, which consists almost entirely of zeolite, is separated from the process water in step 20. The yield amounts to about 100 g zeolite per kg fly ash. The remaining process water (with 1000-2000 mg silicon and 50-200 mg aluminium per litre) can be used as hydroxide source for: 1. the production of zeolite from power station fly ash as according to the described 1-stage zeolitization process (step 4); 2. the production of a zeolite-immobilizer from WIP and CWIP fly ash, as described below.

As in the 1-stage process elucidated with reference to figure 1, the remaining solid is then washed in step 21, separated again into liquid and solid in step 22 and subsequently dried in step 24.

A variant of the part of this preferred embodiment relating to the silica extraction is a continuous process wherein (washed) coal fly ash is mixed with the desired hydroxide solution in a continuous flow

reactor. The average residence time of the process water is the same as said induction period (4 to 8 hours, depending on the chosen temperature). Aluminium hydroxide suspension is added to the process water from the reactor, being a silica extract, in order to precipitate silicon as amorphous aluminium silicate. This precipitate is separated off and the process water is returned to the reactor again. The average residence time of the fly ash amounts to 10 to 40 hours and is modified such that silicon concentrations in the silica extract remain high enough for a cost-effective process. A proportion of the process water is periodically replaced. Left over are a fly ash residue, a silicon aluminium precipitate and process water. In this variant these latter two together form the silica extract. Further zeolite formation from the fly ash residue and the silica extract proceeds in a manner analogous to that from the fly ash residue and silica extract from the batch-wise silica extraction described above.

The zeolitization as performed with the silica extract can also be carried out with the process water from the 1-stage zeolitization process. The only difference is that due to the lower silica concentrations in the process water the yield of pure zeolites will also be lower, i.e. about 50 g zeolite per kg fly ash, than when the silica extract is used.

The fly ash residue obtained in the separation of the silica extract can be used for: 1. a second silica extraction; or 2. the synthesis of zeolite Na-P1, zeolite ZK-19 or zeolite K-G as according to the described 1-stage zeolitization process.

To the above described 1- and 2-stage zeolitization process can be linked a process for the production of a zeolite-immobilizer from WIP and CWIP fly ashes, which produces a variant of preferred embodiment 2. The process water obtained from the zeolitization power station fly ash in the 1- and 2-stage process is

herein used as additional silicon and hydroxide source in addition to other forms of (reactive) alumina and silica and a hydroxide solution. Addition of extra alumina and silica to the reaction mixture is necessary to obtain the desired zeolite yield in the immobilizer (more than 50%), because the aluminium and silicon contents of WIP and CWIP fly ashes are relatively low (1W to 10% Awl203/ 10t to 24t SiO2) In order to form a zeolite-immobilizer with zeolite Na-P1 or hydroxy-sodalite a reaction mixture is required in which the components determining zeolite type occur in the molar ratios as stated in table 4.

Table 4 Overview of the required molar ratios of the components determining zeolite type for the production of a zeolite-immobilizer from WIP and CWIP fly ash product Na2O K2O AI,O, SiO2 H2O zeolite Na-P1 2-4 0-0.5 1 3-5 100-130 hydroxy-sodalite 1.5-4 0-0.5 1 2-4 100-150 As in the zeolitization of power station fly ashes, the obtained reaction mixture is incubated at a temperature of 80 to 1500C and autogenous pressure, wherein the solid phase is held in suspension by stirring. The incubation time amounts to 10 to 50 hours and depends on the chosen temperature and the stirring speed (for instance 24 hours at 95"C and 200 revolutions per minute, 12 hours at 1500C and 5 revolutions per minute).

Once the reaction period has ended, the zeolite product is separated from the process water, washed and dried. The zeolite product contains about 60t to 80% zeolite and 20% to 40t residual fly ash and other new compounds (including salts and amorphous phases).

The quality of the product in terms of environmental protection is such that the product complies at the very least with the requirements laid down in the waste disposal regulations for classification in category 4. For some fly ashes, depending on the concentrations of heavy metals present, it is possible that the zeolite-immobilizer fulfills the requirements laid down in the building materials regulations for category 2 bulk products.