Process for the preparation of magnesia (MgO)

Abstract

The present invention provides an improved process for the preparation of MgO of high purity >99% from salt bitterns via intermediate formation of Mg(OH)2 obtained from the reaction of MgCl2 and lime, albeit indirectly, i.e., MgCl2 is first reacted with NH3 in aqueous medium and the slurry is then filtered with ease. The resultant NH4Cl-containing filtrate is then treated with any lime, to regenerate NH3 while the lime itself gets transformed into CaCl2 that is used for desulphatation of bittern so as to recover carnallite and thereafter MgCl2 of desired quality required in the present invention. The crude Mg(OH)2 is dried and calcined directly to produce pure MgO, taking advantage of the fact that adhering impurities in the Mg(OH)2 either volatilize away or get transformed into the desired product, i.e., MgO.

Description

Claims

1. An improved process for the preparation of MgO, the said process comprising the steps of: i) desulphating brine or bittern with calcium chloride,ii) evaporating the clarified brine/bittern after separation of gypsum to separate out the common salt and carnallite (KCl.MgCl2.6H2O),iii) recovering MgCl2 rich and other salt free end bittern from step (ii),iv) further evaporating end bittern of step (iii) to obtain crystalline MgCl2.6H2O,v) seeding MgCl2.6H2O, obtained in step (iv) either as such or after recrystallization, in solid or solution form, with a small quantity of Mg(OH)2 and treating it with ammonia (NH3),vi) filtering the resultant slurry obtained in step (v) to obtain the crude Mg(OH)2 and NH4Cl/residual NH4OH filtrate,vii) drying the above crude Mg(OH)2, followed by calcination to convert Mg(OH)2 into MgO, and converting adhering MgCl2 into MgO and HCl gas, and NH4Cl into sublimed vapor,viii) absorbing the hot sublimed vapor of NH4Cl generated in step (vii) from the calciner into the NH4Cl/residual NH4OH filtrate of step (vi) to further enrich the filtrate in NH4Cl and also for heating up the filtrate,ix) treating the above said hot filtrate with lime to obtain CaCl2 solution and ammonia vapor,x) using the ammonia vapor obtained in step (ix) in a process step (v) to complete the loop while using the by-product CaCl2 solution in step (i).

2. An improved process as claimed in claim 1 wherein the bittern used in step (i) is obtained from ocean brine, sea brine, sub-soil brine or lake brine.

3. An improved process as claimed in claims 1&2, wherein the sulphate-containing bitterns used in step (i) are desulphated in the density range of 29-32oBe′.

4. An improved process as claimed in claims 1-3, wherein the carnallite (KCl.MgCl2.6H2O) obtained in step (ii) is crystallized between 32-36oBe′ either through solar or forced evaporation and the end bittern of step (iii) having density of 35.5-36.0oBe′ contains 450-460 gpl of MgCl2, 5-10 gpl of NaCl, 5-10 gpl of KCl, 5-15 gpl of Ca, 0-5 gpl of sulphate, 6-7 gpl Br−, 0.03% B2O3.

5. An improved process as claimed in claims 1-4, wherein the end bittern of step (iii) is preferably debrominated so as to recover bromine and simultaneously reduce the Br− impurity in debrominated bittern to <0.5 gpl.

6. An improved process as claimed in claims 1-5, wherein the pristine end bittern of step (iii) is used for MgO recovery, preferably debrominated and used without crystallization of step (iv).

7. An improved process as claimed in claims 1-6 wherein the end bittern of step (iii) is used with or without debromination and is evaporated as per the procedure of step (iv) to reduce the volume by 20-25% to crystallize out the MgCl2.6H2O in 60-80% yield containing 0.020-0.015% B2O3 impurity and is free from other salts

8. An improved process as claimed in claims 1-7, wherein the ammonia used for the initialization of reaction in step (v) is an aqueous ammonia solution containing 20-25% ammonia (w/w).

9. An improved process as claimed in claims 1-8, wherein the mole ratio of NH3 to MgCl2 used in step (v) is in the range 0.5:1 to 2.0:1, preferably in the range 1.1:1 to 2.0:1 to obtain the residual MgCl2 level in the filtrate of <1.5% and preferably <0.5%.

10. An improved process as claimed in claims 1-9, wherein the filtration operation used in step (vi) is carried out with ease on a Nutsche filter or rotary disk filter or filter press.

11. An improved process as claimed in claims 1-10, wherein the filtration operation used in step (vi) is carried out in a centrifuge.

12. An improved process as claimed in claims 1-11, wherein the drying and calcinations operation used in step (vii) is carried out directly or alternatively after washing the crude Mg(OH)2 with a minimum quantity of water and additives to remove a part of the adhering impurities and rest during calcination.

13. An improved process as claimed in claims 1-12, wherein the drying operation used in step (vii) is carried out at a temperature of 70-150° C. in either a conventional oven or a solar oven to yield soft white lumps that crumble easily into a powder.

14. An improved process as claimed in claims 1-13, wherein the calcination operation used in step (vii) is carried out in a muffle furnace at a temperature of about 900° C. for 2-3 h and preferably by gradually ramping the temperature to expel adhering NH4Cl, HCl (from adhering MgCl2.6H2O), H2O and NH3 (from adhering NH4OH) at a temperature 600° C., to yield MgO of high purity.

15. An improved process as claimed in claims 1-14, wherein the MgO obtained in step (vii) has a purity of 98.0-98.9% when produced directly from the end bittern of step (iii) and a purity in the range of 99.1-99.7 when prepared from crystallized or recrystallized MgCl2.6H2O obtained in step (iv).

16. An improved process as claimed in claims 1-15, wherein the MgO obtained from end bittern of step (iii) has a B2O3 impurity level in the range of 0.10-0.12%.

17. An improved process as claimed in claims 1-16, wherein the MgO obtained from crystallized MgCl2.6H2O of step (iv) has a B2O3 impurity level in the range of 0.060-0.080%.

18. An improved process as claimed in claims 1-17, wherein the MgO obtained from recrystallized MgCl2.6H2O has a B2O3 impurity level in the range of 0.010-0.015%.

19. An improved process as claimed in claims 1-18, wherein the B2O3 level in MgO can be made still lower through appropriate treatment either of the precursor Mg(OH)2 or of the MgO itself.

20. An improved process as claimed in claims 1-19, wherein the NH4Cl/NH4OH filtrate obtained as by-product of Mg(OH)2 preparation in step (vi) contains 0.5-2.0% Mg and preferably, 0.5-1.0% Mg to minimize the formation of Mg(OH)2 during treatment with lime.

21. An improved process as claimed in claims 1-20, wherein the lime used in step (ix) is either hydrated lime or quicklime in the form of a solid or solid suspension.

22. An improved process as claimed in claims 1-21, wherein the NH3 vapors generated in step (ix) is stripped out with air or steam and is absorbed in a solution of MgCl2 by feeding into the reaction chamber at a rate so as to maintain the desired mole ratio of NH3 to MgCl2 for optimum reaction.

23. An improved process as claimed in claims 1-22, wherein the solution obtained in step (ix) contains 20-30% CaCl2 and is used directly in desulphatation reaction in step (i) or is clarified through filtration and/or addition of acid to redissolve Mg(OH)2 prior to executing in step (i).

24. An improved process for the preparation of MgO, substantially as herein described with reference to the examples and drawing accompanying this specification.