Process for preparing sodium silicate alkali solution depleted of sodium salt and enriched in silica

The present invention relates to an improved process for obtaining sodium silicate alkali solution depleted of sodium salt and enriched in silica from a mother liquor recovered after isolation of molecular sieves and more particularly, the present invention relates to a process for recycling mother liquor obtained after the isolation of molecular sieves for the preparation of fresh molecular sieves or as a binder for producing Fluid Catalytic Cracking (FCC) catalyst.

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What claimed is:

1. A process for preparing a sodium silicate alkali solution depleted of sodium salt and enriched in silica from a mother liquor obtained after isolation of molecular sieves andsuitably recycled further for preparing fresh sodium alumino silicate molecular sieves or as a binder for preparing Fluid Catalytic Cracking (FCC) catalyst, said process comprising steps of: a. recovering the mother liquor left after isolation ofmolecular sieves in a crystallizer; b. optionally adding to said mother liquor of step (a), a mineral acid in the range of 1-75% moles equivalent of Na.sub.2 O moles present in the mother liquor with stirring; c. effecting crystallization of sodiumsalt by cooling the mother liquor of step (b) to a temperature ranging from 5 to 25.degree. C. for a period ranging from 30 minutes to 48 hours, and d. filtering the crystalline sodium salt to yield the sodium silicate alkali solution depleted of sodiumsalt and enriched in silica.

2. A process as claimed in claim 1, wherein the mother liquor of step (a) is obtained after recovery of molecular sieves selected from the group consisting of zeolite A, zeolite X, zeolite Y, Mordenite, Beta, Omega. ZSM-5 and ZSM-11.

3. A process as claimed in claim 1, wherein the mother liquor of step (a) has the composition in the molar ratio of: 0.5-9 Na.sub.2 O:0-5 Na.sub.2 SO.sub.4 :0.001-0.05 Al.sub.2 O.sub.3 :2-20SiO.sub.2 :50-1000 H.sub.2 O.

4. A process as claimed in claim 1, wherein in step (b), the mineral acid is added to the mother liquor for the conversion of Na.sub.2 O to its sodium salt.

5. A process as claimed in claim 4, wherein the mineral acid is selected from the group consisting of sulfuric acid, orthophosphoric acid, hydrochloric acid and nitric acid.

6. A process as claimed in claim 1, wherein in step (b) the pH of the mother liquor after treatment with mineral acid is in the range 9.0-12.5.

7. A process as claimed in claim 1, wherein in step (d) the sodium salt is a pure salt selected from the group consisting of sodium sulfate, sodium chloride, sodium phosphate, sodium nitrate, and mixtures thereof.

8. A process as claimed in claim 1, wherein the sodium silicate alkali solution depleted of sodium salt and enriched in silica has the composition in the molar ratio of: 0.5-7 Na.sub.2 O:0-2.5 Na.sub.2 SO.sub.4 :0.001-0.05 Al.sub.2 O.sub.3 :2-20SiO.sub.2 :50-1000 H.sub.2 O.

9. A process as claimed in claim 1, wherein to prepare fresh sodium alumino silicate molecular sieves, the sodium silicate alkali solution depleted of sodium salt and enriched in silica of step (d) is topped with a suitable source of silica,soda and alumina to obtain a reaction mixture which is suitable for preparing fresh sodium alumino silicate molecular sieve.

10. A process as claimed in claim 9, wherein molar ratio of the composition of the topped sodium silicate alkali solution is: 0.5-9 Na.sub.2 O:1-5 Na.sub.2 SO.sub.4 :1 Al.sub.2 O.sub.3 :2-20 SiO.sub.2 :50-1000 H.sub.2 O.

11. A process for preparing FCC catalyst using the sodium silicate alkali solution depleted of sodium salt and enriched in silica prepared by the process of claim 1, said process comprising: (a) adding clay to dilute mineral acid followed byaddition of sodium silicate alkali solution enriched in silica with stirring to form a clay-silica sol having a pH of about 2.5; (b) adding a fine slurry of zeolite having pH 4.0 to the silica-clay sol of step (a) with vigorous stirring and adjustingthe pH to about 2.5-3.1 to obtain a silica-clay-zeolite slurry having 20-35 wt % silica, 30-60 wt % clay and 15-40 wt % zeolite; (c) spray drying the slurry of step (b) to obtain microspheres; (d) washing the microspheres of step (c) with hotdemineralized water and exchanging using an ammonium salt solution in the temperature range of 80-95.degree. C. to obtain ammonium exchanged catalyst having residual soda less than 0.40 wt %; (e) contacting the ammonium exchanged catalyst of step (d)with a rare earth metal chloride solution in the temperature range of 60-80.degree. C. to obtain rare earth loaded catalyst having 1-2.5 wt % of rare earth oxide, and (f) washing the rare earth loaded catalyst of step (e) with DM water to remove freechloride ions and drying the same to obtain the FCC catalyst.

12. A process as claimed in claim 11, wherein in step (a) the dilute mineral acid is sulfuric acid.

13. A process as claimed in claim 11, wherein in step (b) the zeolite slurry is prepared by mixing and milling the zeolites selected from the group consisting of zeolite A, zeolite X, zeolite Y, Mordenite, Beta, Omega, ZSM-5 and ZSM-11 withsuitable quantity of DM water and 10% aqueous sulphuric acid.

14. A process as claimed in claim 10, wherein in step (d) the ammonium salt solution used is selected from the group consisting of ammonium sulfate, ammonium chloride and ammonium nitrate.

15. A process as claimed in claim 10, wherein in step (e), the rare earth metal chloride solution is selected from chlorides of lanthanide series either alone or in combination.