|Title of Invention||
"AN IMPROVED PROCESS FOR THE PRODUCTION OF FLYASH BASED SODALITE"
|Abstract||Flyash based sodalite (FAS) having silica to alumina molar ratio of 0.9 - 1.5 is prepared from a reaction mixture of flyash and caustic soda. The reaction mixture has specified properties of Na2O, SiO2, AI2O3 and H2O. A caustic soda to flyash ratio greater than 1.2 yields exclusively sodalite product at fusion temperature of 550 - 600°C and crystallisation time of 4-10 hrs. Morphological characterisation using scanning electron microscopy reveals cubic structure as corroborated with XRD data. The exchange capacity (340-360 meq/100 g) and surface area compare well with that reported in literature. The surface area and exchange capacity indicates their uses as a low-cost ion exchanger, water softener and conditioner etc.|
|Full Text||The present invention relates to an improved process for the production of flyash based sodalbe. More specifically, the invention relates to the production of sodalite with its uniqueness being attributed to use of flyash as a raw material.
Sodalite unit comprises of 24 tetrahedral arranged in six four rings (or four faces) and eight six rings (or six faces). Close packings of the sodalite units give the framework structure of sodalite, a naturally occurring mineral with channels of six rings which are too small to give zeolite properties. However, an octahedral array of sodalite units linked via oxygen bridges generates zeolite A,X, & Y. The pore size of sodalite is very narrow (2.2A°). In spite of their particularly favorable composition sodalite shows the greatest deviation from theoretical ion exchange capacity. The sodalite thus prepared is useful as ion exchanger. Sodalite has been used for encapsulation / immobilisation of radioactive waste (U.S.Patent No. 5340506). A novel application of sodalite material for development of photochromic and / or cathochromic material as sensory materials for transmission device has been reported recently (U.S.Patent No. 4490286). Another application which is of much interest is the preparation of other zeolites using sodalite as a source material (U.S.Patent No. 5340563). Removal of contaminants (Cl, CO3 etc.) from strongly alkaline solutions using sodium alumino silicate with simultaneous formation of sodalite has been reported (U.S.Patent No. 4289629). Presently the alumiosilicate zeolites are prepared by subjecting silica and alumina source to highly alkaline conditions at a temperature of 100-110°C. One of the shortcoming of the prior art processes is the requirement of a more expensive silica such as silica sol or particulate silica
and alumina source. This has been resolved by using flyash as a substitute for sodium silicate & aluminate. Information available on synthesis methods of sodalite using waste materials is scanty. Kaolinite has been used to synthesize sodalite hydrate by hydrothermal treatment using caustic soda (Somani R.S., Oza P.M. &Pandya V.P., 1987 (25) pp 373-374). Stamboliev et. al have reported the synthesis of zeolite-A and hydroxy sodalite using silicic tuff. (Stamboliev CH., Scapora N., Bergk K.H. and Porsch M. Zeolites, 1985 pp 155-160). Bayer's liquor / aluminium residues is also being increasingly used for synthesis of sodalite. Subotic et. al have detailed the mechanistic aspects of transformation of zeolite-A into hydroxy sodalite in a concentrated solution of sodium hydroxide. (Subotic B., Skrtic I., Smit and Sekovanic L.J., Cryst. Growth, 1980 (50) pp 498-508 and Subotic and Sekovanic L. J. Cryst Growth 1986 (75) pp 561-572).
The emphasis of present invention is thus on the preparation of crystalline aluminocilicate viz. Sodalite using a cost-effective process for the use of flyash as a raw material.
In the present invention sodalite was prepared by mixing sodium hydroxide and flyash in a ratio of 1.2-2.0 The reaction mixture was placed in an inert vessel and heated to about 500-600°C. The fused mass was cooled, milled again, mixed with water and then stirred vigorously for 10 to 30 minutes.
The amorphous aluminosilicate gel thus prepared was then subjected to crystallisation for 4-8 hrs at 90-100°C. The solid crystalline product was then recovered by filtration, washed with water and dried at temperature of 50-80°C.
Characterisation of FAS
Calcium Binding Capacity
The calcium binding capacity (CBC) of aluminocilicates was determined as follows:
1 liter of aqueous solution containing 0.5g of CaCb and adjusted to a pH of 10 with dilute NaOH, was mixed with 1 g of alumino-silicate (FAS). The suspension was then stirred vigorously for 15 minutes at room temperature (29-30°C). After filtration the residual hardness of the filtrate was determined. From the difference between hardness of the original solution and filtrate the CBC is calculated as meq/100g. The Flyash based sodalite (FAS) samples were fused with HNO3 and analysed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) for Na2O and AI2O3 while SiO2 was estimated using conventional method. Crystallographic and Morphological Observations
The surface morphology of the zeolite was examined using Jeol-64 scanning electron microscope (SEM). Powder XRD analysis was employed to monitor zeolite formation process, using CuKα, and CoKα as source of x-rays. d-spacing values reported in Joint Committee on Powder Diffraction Systems (JCPDS) file (37-476) for sodalite were used as standard for comparison.
The formation of sodalite was observed when the fused mass was used after a short aging time (mixing after its inclusion in water). This zeolite phase selectivity has been attributed to local high alkali concentration effect, motivating the formation of the high soda phase zeolite (sodalite). However, at higher mixing period, formation of zeolite A, X and Y was observed.
Similarly at caustic soda: flyash ratio greater than 1.2, formation of sodalite was observed while at caustic : flyash ratio of 1.2, and less, formation of zeolite A, X and Y was predominant. Thus, it may be concluded that excessive or alkali enriched system leads to formation of sodalite.
Main advantages of FAS synthesis:
• Provides an inexpensive alternate to commercial grade sodalite
• Economically viable and technically non-tedious process
• Tackles at least partially the adverse environmental effects
envisaged for flyash
• High value utilisation of flyash
Accordingly the present invention provides an improved process for the production of flyash based sodalite which comprises mixing thoroughly caustic soda and flyash in a ratio of 1.2 to 2.0, heating the said mixture at 500-600°C in inert atmosphere for 1 to 2 hrs, cooling, milling, mixing with water for 10-30 minutes then subject to crystallization at 90 to 100°C for 4 to 10 hrs, followed by filtration and drying to get sodalite having calcium binding capacity of said sodalite ranging from 200-380 meq/100g and molar ratio SiO2/ AI2O3 =2.0,Na2O/SiO2=0.66 Na2 O/H2O=0.833.
Caustic soda and flyash are mixed in the weight ratio ranging from 1.2 to 2.0. Milling is carried out to have particle size of 45 to 50µ, crystallization is effected for 4 to 8 hrs and drying of sodalite is effected at 50-80°C.
A comparison of the characteristics of FAS with the commercial sodalite is provided in Table 1.
The following examples illustrate the influence of different parameters viz. temperature, NaOH/flyash ratio, mixing time and crystallisation time but does not restrict the scope of the present invention:
Pre-weighed sample of flyash (20g) and sodium hydroxide (24g) were milled and placed in a vessel inert towards the reaction mixture and heated to about 550°C for 1 hr. The fused mass was cooled, milled and mixed thoroughly with 200 ml of distilled water for 10 minutes. The amorphous alumino-silicate gel so obtained was then subjected to crystallisation for 8 hrs. The solid crystalline product was recovered by filtration, washed with water and oven dried at 50°C for one hour. The CBC is 340 meq/100g and SiO2/AI2O3 mole ratio is around 1.94. X-ray diffraction, d-spacing values reported for sodalite in JCPDS (Joint Committee Powder Diffraction Systems) file (37-476) compare well with FAS and are as follows:
The same process was repeated as in example 1 except for variation in caustic soda : flyash ratio which was varied from 1.4-2.0. The samples were designated as FAS-2(1.4), FAS-3(1.6) and FAS4(2.0). The exchange capacity, and SiO2/AI2O3 ratio are detailed below in Table 3.
d-spacing values for sodalite reported in JCPDS file (37-476) matches very closely with FAS-2, FAS-3, FAS-4 and are as follows:
1. An improved process for the production of flyash based sodalite which
comprises mixing thoroughly caustic soda and flyash in a ratio of 1.2 to
2.0, heating the said mixture at 500-600°C in inert atmosphere for 1 to 2
hrs, cooling, milling, mixing with water for 10-30 minutes then subject to
crystallization at 90 to 100°C for 4 to 10 hrs, followed by filtration and
drying to get sodalite having calcium binding capacity of said sodalite
ranging from 200-380 meq/100g and molar ratio SiO2/ AI2O3
=2.0,Na2O/SiO2=0.66 Na2 O/H2O=0.833.
2. An improved process for the production of flyash based sodalite
substantially as here in described with reference to the examples.
|Indian Patent Application Number||277/DEL/2000|
|PG Journal Number||01/2009|
|Date of Filing||16-Mar-2000|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110 001, INDIA.|
|PCT International Classification Number||C01B 33/26|
|PCT International Application Number||N/A|
|PCT International Filing date|