GB1603084A

GB1603084A - Zeolites

Info

Publication number: GB1603084A
Application number: GB2304977A
Authority: GB
Original assignee: English Clays Lovering Pochin Co Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1977-05-31
Filing date: 1977-05-31
Publication date: 1981-11-18
Also published as: FR2392934B1; FR2392934A1; DE2823927A1; CA1101825A; ES470368A1; DE2823927C2

Description

(54) IMPROVEMENTS IN OR RELATING TO ZEOLITES (71) We. ENGLISH CLAYS LOVERING POCHIN & COMPANY LIMITED. a British company. of John Keav House. St. Austell. Cornwall. do herebv declare the invention. for which we pray that a patent may be granted to us. and the method bv which it is to be performed. to he particularly described in and by the following statement: This invention relates to zeolites and. more particularly. is concerned with a process for preparing a svnthetic zeolite. zeolite A.

Zeolites are crystalline. hydrated aluminosilicates of group I or group II elements and consist of a three-dimensional network of AlO4 and SiO4 tetrahedra linked to each other by sharing all of the oxygen atoms. Many zeolites are useful as molecular sieves and may also have high cation exchange capacities.

A svnthetic zeolite which has an especially high cation exchange capacity is known as zeolite A. Zeolite A can be represented be the formula: x Na2O. Al2O3. y SiO2. z H2O wherein N ranges from ().8 to 1.'. v ranges from 1.S to 2.() and z ranges from 0 to 5.1. By virtue of its high cation exchange capacity zeolite A is very useful for softening water by exchanging its Na+ ions for Ca2+ and Mg2+ ions in hard water. It has been proposed to use zeolite A in detergent compositions to replace the condensed phosphates which have hitherto been used for precipitating calcium and magnesium ions. because the use of phosphate in detergents has been found to have a damaging effect on the environment.

Processes for producing zeolites from metakaolin are disclosed in United States Patent Specification Nos. 2,992,068: 3,065,054; 3,100,684; 3,114,603; 3,119,659; 3,185,544; 3,370,917; 3,433,587; 4,034,058; 4,075,028 and 4.089,929.

According to the present invention there is provided. in a process for preparing zeolite A from a mineral of the kandite group which process comprises the following steps: (a) calcining a mineral of the kandite group which has associated therewith substantially no surface iron and not more than 0.5% by weight of interstitial iron, the calcination being carried out at a temperature in the range of from 550 C to 925 C and for a time such that the loss on ignition for 2 hours at 1000 C of a sample of the calcined mineral is less than 1% by weight, to form a calcined kandite mineral;; (ii) preparing an aqueous mixture of the calcined kandite mineral and an aqueous solution of sodium hydroxide of concentration not less than SM. the proportions of calcined kandite mineral and sodium hydroxide in the aqueous mixture being such that the molar ratio Na2O SiO2 is in the range of from 0.5 to 1.5: (c) heating said aqueous mixture at a temperature in the range of from 60 C to 100 C for a time in the range of from 2 to 8 hours to form the desired zeolite A: and (tl) separating crystalline zeolite A from the aqueous mixture: the improvement which comprises subjecting said mineral of the kandite group. before carrying out step (a), to high energy milling for a time and under conditions such that the kandite mineral is rendered substantially free of aggregates and no further significant improvement in the brightness of the kandite mineral can be obtained by milling.

The mineral of the kandite group used as a starting material in the process of the present invention can be, for example. nacrite, dickite or halloysite, but advantageously is kaolinite.

It has now been found that in order to obtain the best brightness and cation exchange capacity in the zeolite A, the mineral of the kandite group should be substantially free of aggregates. To achieve this, the mineral should be subjected to high energy milling prior to calcination until no further improvement in the brightness of the mineral may be obtained bv milling. The amount of energy that needs to be dissipated in the mineral for this purpose depends upon the type of mill used. but it has been found that in general it should be at least 80 kJ.kg- and preferably is in the range of from 100 to 400 kJ.kg'.

A further improvement in the brightness and cation exchange capacity of a zeolite A can, under certain circumstances, be obtained by subjecting the calcined kandite mineral formed in step (a) to high energy milling until no further significant improvement in the brightness of the calcined kandite mineral can be obtained by milling.

The calcined kandite mineral is obtained in an appropriate form by calcining the mineral at a temperature in the range of from 550 C to 925 C for a time, preferably ranging from one quarter of an hour to twenty hours. such that the loss on ignition for 2 hours at 1000 C of a sample of the calcined mineral is less than 1.0% by weight, and preferably is less than 0.1 r? bv weight. Advantageously the mineral is calcined at a temperature above 800"C, but not greater than 925 C. for a time in the range of from 1/2 to 6 hours. If calcination is carried out at temperatures above 925 C the product is a defect spinel which on treatment with sodium hydroxide solutions gives zeolite Y and not zeolite A.If calcination is carried out at temperatures below 55() C the product formed on treating the calcined material with sodium hydroxide is zeolite HS.

The amount of iron. or other discolouring impurities, present in the mineral of the kandite group from which the calcined kandite mineral is obtained should be as small as possible. Iron, for example, may be present in a kaolin in two different forms: it may be on the surface of the kaolinite particles as impurity particles or it may be within the crvstal lattice of particles in the kaolin as interstitial impurities. In the first form, the iron is accessible to reagents and should be removed as completely as possible. for example by treating the kaolin with excess of a reducing agent. such as sodium or zinc dithionite.

sodium sulphite or hydrazine. In the second form. the amount of iron. or other discolouring impurity present. may be reduced by separating from the kaolinite as much as possible of the discolouring impurities such as mica. tourmaline, montmorillonite and impure forms of titania which are present in the kaolin. The amount of discolouring impurities present may be reduced by various methods but two convenient methods are (a) to pass a deflocculated aqueous suspension of the kaolin through a magnetic separator and (b) to remove substantially all of the particles having an equivalent spherical diameter larger than a given value. good results being obtained if all particles having an equivalent spherical diameter larger than 2 m are removed.It has been found to be necessarv for the mineral of the kandite group to have associated therewith substantially no surface iron and not more than 0.5% bv weight of interstitial iron.

In step (b) of the process of the invention the concentration of the aqueous solution of sodium hydroxide is preferably in the range SM to about 6M (corresponding to molar ratios of H2O Na2O ranging from 23 to about 19). although there can be used solutions with higher concentrations of sodium hydroxide. The proportions of calcined kandite mineral and sodium hydroxide in the mixture are preferably such that the molar ratio NanO/SiOr is in the range of from o.X to 1.2.

After step (b), and before step (c), one or more operations may be performed to remove from the mixture anv discolouring foreign ions which have been dissolved from the calcined kandite mineral bv the concentrated sodium hydroxide solution: such operations may comprise. for example. heating the aqueous mixture to a temperature in the range of from 61) to 1()()'C. preferably to a temperature in the range of from 9() C to 9n'Css in order to dissolve the discolouring foreign ions. the major part of the discolouring foreign ions forming a colloidal suspension which can be removed from the aqueous suspension of calcined kandite mineral bv filtration. or treating the aqueous mixture at a temperature in the range ot from 15() to 1()() C with a reducing agent. such as sodium or zine dithionite. or sodium sulphite or hydrazine. or with a complexing agent for the discolouring foreign ions, such as phosphoric acid. or an amino compound. After such an operation the treated aqueous mixture is advantageouslv diluted with water until the concentration of sodium hydroxide is in the range of from about SM to about 6M.

In step (c) of the process of the invention the temperature to which the aqueous mixture is heated is preferably in the range of from 81)to 1()() C, and most preferably is in the range of from 81) to ')() C. and the mixture is advantageously maintained at such a temperature for a time in the range of from 7 to 6 hours.

In step (d) of the process of the invention the crystallised zeolite A is conveniently separated from the aqueous mixture bv filtration or centrifugation.

It has been found that the cake of zeolite A crystals obtained on completing step (d) should preferably be washed free of dissolved species by reslurrying the cake in water at room temperature. or at a temperature up to 100 C, and filtering or centrifuging the slurry a second time. It is important not to wash the zeolite A to such a degree that the concentration of the sodium ions in the solution falls to a very low level. If this happens the brightness of the zeolite A begins to fall off again. It has been found to be best to wash the zeolite A until the sodium ion concentration in the washing solution is in the range of from 500 to 10,000 ppm (grams of sodium per million millilitres of solution).

The invention is illustrated by the following Examples.

Example I A kaolin was treated with sodium dithionite to remove surface iron associated therewith.

the amount of interstitial iron present in the kaolin being less than 0,5% by weight. The kaolin was milled for a time sufficient to dissipate in the clay more than 100 kJ.kg-1 of clay.

The kaolin was then calcined at a temperature of 600 C for 16 hours and the loss on ignition for two hours at 1000 C of a sample of the calcined clay was found to be below 0.1% by weight. The metakaolin thus obtained was pulverised and 80g. of the powdered product was mixed with 90 ml of a 15M sodium hydroxide solution. They resultant aqueous mixture was heated to a temperature within the range 990 - 95 C. The molar ratio Na2O/SiO2 in the mixture was 0.94. A colloidal suspension of ferric hydroxide was formed and was separated from the metakaolin by coarse filtration. The colloidal ferric hydroxide was removed by fine filtration, and the clear filtrate diluted until the concentration of sodium hydroxide in the solution was equivalent to 3M. and it was then remixed with the metakaolin.The resultant slurry was heated to a temperature of 990 - 95 C for 5 hours and the zeolite A which crystallised during this period was separated by filtration, washed by resuspension in water.

filtered again and dried. The zeolite A was found to have a reflectance of 90,8 when measured at 457 nm wavelength in accordance with I.S.O. Standards Nos. 2469, 2470 and 2471.

By comparison a sample of zeolite A prepared in accordance with the procedure described in United States Patent Specification No. 3,114,603 (wherein metakaolin was mixed with approximately 2M sodium hydroxide solution) was found to have a reflectance at 457 nm wavelength o 87.1 according to the I.S.O. Standards.

Example 2 80 g of metakaolin prepared as described in Example 1 were mixed with 90 ml of a 15M sodium hydroxide solution at room temperature; and the iron which was dissolved from the metakaolin was precipitated as colloidal ferric hydroxide. The colloidal precipitate was treated with sodium dithionite solution to reduce the iron to the ferrous state and then with orthophosphoric acid to form ferrous phosphate which is stable and only very slightly coloured. The slurry was then diluted with water until the concentration of sodium hydroxide in the solution was equivalent to 3M, and the diluted slurry was heated to a temperature of 90-95 C for 5 hours. The zeolite A which crvstallised during this period was separated by filtration, washed by resuspension in warm water, filtered again and dried.

The zeolite A was found to have a reflectance at 457 nm wavelength of 91.6 according to the I.S.O. Standards.

Example 3 80 mg of metakaolin prepared as described in Example 1, was mixed with 90 ml of a 10M sodium hydroxide solution and heated at a temperature of 90 - 95 C for 5 hours. Zeolite A crystallised during this period and was separated by filtration. washed by resuspension in warm water, filtered again, and dried. The zeolite A was found to have a reflectance at 457 nm wavelength of 90.7 according to the I.S.O. Standards.

Example 4 A partially refined kaolin from South Devon having a particle size distribution such that 76% by weight consisted of partieles having an equivalent spherical diameter smaller than a m was treated in aqueous suspension at a pH of 2.8 with 101b of sodium dithionite per ton of dry kaolin (4.5 kg per tonne) in order to remove subbstantially all the surface iron from the kaolin. The pH was then raised to 4.5 with sodium hydroxide. The suspension of treated kaolin was then deflocculated with 0.13% by weight. based on the weight of dry kaolin, of a sodium silicate dispersing agent having an SiO2/Na2O molar ratio of 3.4 and the pH was adjusted to 8.5 with sodium hydroxide.The deflocculated suspension was then subbjected to gravitational sedimentation for a time sufficient to sediment substantially all of the particles having an equivalent spherical diameter larger than 2 m wherebv the interstitial iron present in the kaolin was reduced to less than 0.5% by weight.

The suspension of refined kaolin was flocculated (by reducing the pH of the suspension to 4 with hydrochloric acid). filtered and the filter cake dried at 80 C. The dry kaolin was then milled for a time which had been found by experience to give the maximum increase in brightness of the powder. The milled powder was divided into three portions (I). (2) and (3). which were calcined in a muffle furnace under the following conditions: Portion (1) at 600 C for 16 hours Portion (2) at 750 C for 4 hours Portion (3) at 870 C for 4 hours.

Each portion of metakaolin thus obtained was then milled for a time sufficient to achieve the maximum incrcase in brightness of the powder. 80 g. samples of each portion of milled metakaolin were then stirred into: (a) 450 ml of 3M sodium hydroxide solution (molar ratio H2O/Na2O = 38.1), or (b) 270 ml of 5M sodium hydroxide solution (molar ratio H2O/Na2O = 23.2) and maintained for 3 hours at 85 C to convert the metakaolin into zeolite A. In each case the molar ratio Na2O/SiO2 was about 0.94. Zeolite A was separated from the solution and, in each case, the zeolite A was washed with water until the sodium ion concentration in the washing solution was in the range 500 to 10.000 ppm.

The reflectance to light of wavelengths 457 nm and 574 nm was measured for the dry zeolite A according to ISO Standards Nos. 2469, 2470 and 2471. The cation exchange capacity of the zeolite A was also measured by vigorously stirring lg of dry zeolite A for 15 minutes at 22 C in 1 litre of a solution containing 0.594 g of calcium chloride (300 mg. of CaO per litre), the pH of the solution being adjusted to 10 with dilute sodium hydroxide.

The suspension was then filtered and the concentration of calcium ion in the aqueous phase determined by titration with EDTA. The amount of calcium adsorbed by the zeolite A (the cation exchange capacity) was then found bv difference and expressed as milliequivalents of calcium ion per 100g of dry zeolite A (meg./100g).

The results obtiined are set forth in Table I below: TABLE 1 Calcining Conditions 3M sodium hydroxide 5M sodium hydroxide % reflectance cation % reflectance cation to light of ex. cap. to light of ex. cap.

wavelength wavelength 457nm 574nm meg/100g 457nm 574nm meg/100g 600 C for 16 hours 88.1 94.4 527 89.0 94.4 541 75()^C for 4 hours 88.5 94.5 5()1 89.7 94.X 551 870 C for 4 hours 89.1 94.7 532 90.5 95.3 563 These results show that the properties of the zeolite A are improved as the calcining temperature of the initial kaolin is raised within the range of from 550 - 925 C and also that better results are obtained when the metakaolin is reacted with 5M, rather than 3M. sodium hydroxide.

Example 5 A further sample of the same initial kaolin as was used in Example 4 was treated with sodium dithionite in the same manner in order to remove substantially all the surface iron from the kaolin. The suspension of treated kaolin was then divided into three portions.

One portion was deflocculated with 0.13% by weight. based on the weight of dry kaolin, of the same sodium silicate dispersing agent as was used in Example 4. and the pH was adjusted to 8.5 with sodium hydroxide I'he deflocculated suspension was then subjected to a particle size classification by gravitational sedimentation for a time sufficient to sediment substantially all of the particles having an equivalent spherical diameter larger than 2 m whereby the interstitial iron present in the kaolin was reduced to less than 0.5% by weight.

The suspension of refined kaolin was then flocculated by reducing the pH to 4 with hydrochloric acid.

A second portion was deflocculated in the same way as the first portion. but was then subjected to a magnetic separation process to remove impurities containing interstitial iron.

The suspension was passed three times through a separating chamber packed with ferromagnetic stainless steel wool with a voidage of 95% by volume and in a magnetic field of intensity 13 kilogauss (1.3 tesla). Magnetisable impurities, such as iron-bearing mica and tourmaline, were retained in the packing leaving purer kaolinite in the suspension. In this way the interstitial iron present in the kaolin was again reduced to below 0.5% by weight. This suspension was then flocculated by reducing the pH to 4 with hydrochloric acid.

The third portion was not subjected to further treatment.

Each portion was filtered and the filter cake obtained dried at 80 C. The dry clays were milled in the manner described in Example 4, and were then calcined to produce metakaolin in a muffle furnace at 600 C for 16 hours. In each case the metakaolin was subjected to further milling as described in Example 4. 80g samples of each metakaolin were then stirred into 270 ml of 5M sodium hydroxide (Na2O/SiO2) ratio = 0.94) and maintained for 3 hours at 85 C to convert the metakaolin into zeolite A. The zeolite A was separated from the solution and, in each case, the zeolite A was washed with water until the sodium ion concentration in the washing solution was in the range 50(3 to 10.000 ppm. The zeolite A was then filtered and dried.

The reflectance to light of wavelengths 457 and 574 nm was measured (according to the appropriate ISO Standards) for the treated initial kaolin, the metakaolin and the zeolite A.

The results obtained are set forth in Table II below.

TABLE 11 Treatment of Treated Kaolin Metakaolin Zeolite A initial kaolin C reflectance to light of wavelength Ar7nm 574nm 457nm 574nm 457nm 574nm sodium dithionite onlv 85.7 88.9 X3.6 90.9 84.5 91.X sodium dithionite + 87.3 90.9 88.5 93.9 88.4 93.9 magnetic separation sodium dithionite + particle size 86.7 90.3 87.6 93.4 88.3 93.9 classitication These results show that substantial improvements in the whiteness of the zeolite A are achieved if the initial kaolin is treated prior to calcination in order to remove iron-containing impurities.

Example 6 A partially refined kaolin from Cornwall having a particle size distribution such that 76% by weight consisted of particles having an equivalent spherical diameter smaller than 2 m was suspended in water and the suspension was divided into two portions.

One portion was treated with sufficient sulphuric acid to reduce the pH to 7.8 and then with 6Ib of sodium dithionite per ton of dry kaolin (2.7 kg tonne) in order to remove substantially all of the surface iron from the kaolin.

The other portion was not treated with sodium dithionite.

Both portions were filtered and the filter cakes obtained dried at 80 C. The dry kaolin was milled until to further improvement in brightness was obtained and each portion was then subdivided into three portions. (1). (') and (3). which were calcined in a muffle furnace under the following conditions: Portion (1) at 600 C for 16 hours Portion (2) at 750 C for 4 hours Portion (3) at 870 C for 4 hours In each case the metakaolin was subjected to further milling as described in Example 4.

80 g samples of each metakaolin were then stirred into 270 ml of 5M sodium hydroxide and maintained for 3 hours at 85 C to convert the metakaolin into zeolite A. The zeolite A was separated from the solution and, in each case, the zeolite A was washed with water until the sodium ion concentration in the washing solution was in the range 500 to 10.000 ppm. The reflectance to light of wavelength 457 nm was measured for each dry zeolite A and the results obtained are set forth in Table III below.

TABLE III Calcined at 600 C 750 C 870 C Treatment of % reflectance to light of 457 nm initial kaolin wavelength Sodium dithionite 83.4 84.3 85.6 No sodium dithionite 75.5 77.7 82.0 These results show that the brightness of the zeolite A is greatly improved if the initial kaolin is treated to remote surface iron. especially when the kaolin is to be calcined at a relatively low temperature.

Example 7 A further sample of the same initial kaolin as was used in Example 4 was treated in the manner described in Example 4. the treated refined and milled kaolin being calcined in the muffle furnace for 87()0C for 4 hours to form metakaolin. XO g samples of the milled metakaolin were stirred into 270 ml of 5M sodium hydroxide solution and maintained for 3 hours at 85 C to convert the metakaolin into zeolite A.

Samples of the zeolite A were washed with hot water to different extents so that varying amounts of sodium ion remained in the final washing solution. The samples of zeolite A were then filtered and dried and the reflectance to light of wavelength 457 nm was measured according to the appropriate ISO Standards. The results obtained are set forth in Table IV below.

TABLE IV Sodium ion concentration 'i, reflectance to (ppm) in final light of 457 nm washing solution wavelength 9() 87.() 151) 87.7 250 89.3 800 90.5 950 89.8 7()()() 9(1.' 50000 88.7 Example 8 A further sample of the same initial kaolin as was used in Example 4 was treated with sodium dithionite and then subjected to gravititional sedimentation for a time sufficient to sediment substantially all of the particles having an equivalent spherical diameter larger than 2 m, in the manner described in Example 4. In this way substantially all the surface iron was removed and the interstitial iron content was reduced to below 0.5% by weight.

The suspension of refined kaolin wis flocculated with hydrochloric acid. filtered and the filter cake dried at X() C. The dry kaolin was then divided into two portions. A and B.

Portion A was milled under conditions which had been found to give maximum brightness and portion A was not milled. Both portions A and B were then calcined for 4 hours at 870"C. The metakaolin produced bv calcining portion A was again divided into two portions. Af I ) and A(2). Portion A( I ) was milled for a time which had been found to give maximum brightness of the metakaolin and portions A(2) and B were not milled. 80g samples of each metakaolin were then stirred into 270 ml of 5M sodium hydroxide and maintained for 3 hours at 85 C to convert the metakaolin into zeolite A. In each case the zeolite A was washed with water until the sodium ion concentration in the washing solution was in the range 500-10,000 ppm.The reflectance to light o wavelengths 457 and 574 nm were measured for each dry zeolite A according to the appropriate ISO Standards and the results are set forth in Table V below: TABLE V size reflectance to light of wavelengths Portion 457 nm 574 nm A(1) 90.5 95.3 A(2) 89.6 95.0 B X3.9 89.2 These results show that milling the kaolin before calcining. and milling the metakaolin both contribute to the ultimate brightness of the zeolite A. but the more important contribution to the brightness improvement is made by milling the kaolin before calcining.

Example 9 A sample of halloysite having a particle size distribution such that 90% by weight consisted of particles having an equivalent spherical diameter smaller than 1 m and substantially no surface iron and a total iron content of 0.2% by weight of Fe2O3 was milled for a time such that further milling gave no further improvement in the brightness of the dry powder. and the powder was divided into two portions (1) and (2). which were calcined in a muffle furnace under the following conditions.

Portion (1) at 600 C for 4 hours, and Portion (2) at 870 C for 4 hours Each portion of calcined halloysite was then milled for a time sufficient to achieve the maximum increase in brightness of the powder. 8(1 g samples of each portion of calcined halloysite were then stirred into: (a) 450 ml of 3M sodium hydroxide solution. and (b) '7() ml of SM sodium hydroxide solution for 3 hours at 85 C to convert the calcined halloysite into zeolite A. In each case the zeolite A was washed with water until the soclium ion concentration in the washing solution was in the range 500 - 10000 ppm.

The reflectance to light of wavelengths 457 and 574 nm and the cation exchange capacity were measured tor each dry zeolite A according to the methods described in Example 4.

The results are set forth in l'able VI below.

TABLE VI Calcining conditions 3M sodium hydroxide 5M sodium hydroxide % reflectance of reflectance to light of Cation to light of Cation wavelength Ex-cap wavelength Ex-cap 457nm 574nm meg/100g 457nm 574nm meg/100g 600 C for 4 hours 83.3 90.4 500 84.4 91.1 450 70 C for 4 hours 89.2 93.7 260 90.7 95.0 490 WHAT WE CLAIM IS: In In a process for preparing zeolite A from a mineral of the kandite group by the steps of: (a) calcining a mineral of the kandite group to form a calcined kandite mineral. the mineral of the kandite group having associated therewith substantially no surface iron and not more than 0.5% by weight of interstitial iron and the calcination being carried out at a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

zeolite A was washed with water until the sodium ion concentration in the washing solution was in the range 500-10,000 ppm. The reflectance to light o wavelengths 457 and 574 nm were measured for each dry zeolite A according to the appropriate ISO Standards and the results are set forth in Table V below: TABLE V size reflectance to light of wavelengths Portion 457 nm 574 nm A(1) 90.5 95.3 A(2) 89.6 95.0 B X3.9 89.2 These results show that milling the kaolin before calcining. and milling the metakaolin both contribute to the ultimate brightness of the zeolite A. but the more important contribution to the brightness improvement is made by milling the kaolin before calcining.

Example 9 A sample of halloysite having a particle size distribution such that 90% by weight consisted of particles having an equivalent spherical diameter smaller than 1 m and substantially no surface iron and a total iron content of 0.2% by weight of Fe2O3 was milled for a time such that further milling gave no further improvement in the brightness of the dry powder. and the powder was divided into two portions (1) and (2). which were calcined in a muffle furnace under the following conditions.

Portion (1) at 600 C for 4 hours, and Portion (2) at 870 C for 4 hours Each portion of calcined halloysite was then milled for a time sufficient to achieve the maximum increase in brightness of the powder. 8(1 g samples of each portion of calcined halloysite were then stirred into: (a) 450 ml of 3M sodium hydroxide solution. and (b) '7() ml of SM sodium hydroxide solution for 3 hours at 85 C to convert the calcined halloysite into zeolite A. In each case the zeolite A was washed with water until the soclium ion concentration in the washing solution was in the range 500 - 10000 ppm.

The reflectance to light of wavelengths 457 and 574 nm and the cation exchange capacity were measured tor each dry zeolite A according to the methods described in Example 4.

The results are set forth in l'able VI below.

TABLE VI Calcining conditions 3M sodium hydroxide 5M sodium hydroxide % reflectance of reflectance to light of Cation to light of Cation wavelength Ex-cap wavelength Ex-cap 457nm 574nm meg/100g 457nm 574nm meg/100g 600 C for 4 hours 83.3 90.4 500 84.4 91.1 450 70 C for 4 hours 89.2 93.7 260 90.7 95.0 490 WHAT WE CLAIM IS: In In a process for preparing zeolite A from a mineral of the kandite group by the steps of: (a) calcining a mineral of the kandite group to form a calcined kandite mineral. the mineral of the kandite group having associated therewith substantially no surface iron and not more than 0.5% by weight of interstitial iron and the calcination being carried out at a

temperature in the range of from 550"C to 925"C and for a time such that the loss on ignition for 2 hours at 1000"C of a sample of the calcined kandite mineral is less than 1% by weight; (b) preparing a mixture of the calcined kandite mineral and an aqueous solution of sodium hydroxide of concentration not less than 5 molar, the proportions of calcined kandite mineral and sodium hydroxide in the mixture being such that the molar ratio Na.OlSiO2 is in the range of from 0.5 to 1.5; (c) heating said mixture at a temperature in the range of from 60"C to 1000C for a time in the range of from 2 to 8 hours: and (d) separating crystalline zeolite A from the mixture; The improvement which comprises subjecting said mineral of the kandite group. before carrying out step (a). to high energy milling for a time and under conditions such that the kandite mineral is rendered substantially free of aggregates and no further significant improvement in the brightness of the kandite mineral can be attained by milling.
2. A process according to claim 1, wherein before carrying out step (a) said kandite mineral is milled for a time and under conditions such that the energy dissipated in said kandite mineral is not less than 80 kJ.kg-l of mineral.
3. A process according to claim 2. wherein before carrying out step (a), the mineral of the kandite group is milled for a time and under conditions such that there is dissipated in said mineral an amount of energy in the range of from 100 to 400 kJ.kg-' of mineral.
A. A process according to claim 1, 2 or 3 wherein said mineral of the kandite group is a kaolinitic clav.
5. A process according to claim 1 2, 3 or 4, wherein the calcined kandite mineral is milled until no further significant improvement in the brightness of the calcined kandite mineral can be obtained by milling.
6. A process according to claim 1. 2, 3. 4 or 5, wherein the separated crystalline zeolite A obtained after carrying out step (d) is washed in water until the concentration of sodium ions in the washing solution is in the range of from 500 to t0,00() grams of sodium per million millilitres of solution.
7. A process for preparing zeolite A according to claim 1 and substantially as described in any one of the foregoing Examples.
8. Zeolite A whenever prepared by the process claimed in any one of the preceding claims