U.S. patent application number 11/375246 was filed with the patent office on 2007-09-20 for process for the preparation of magnesia (mgo).
This patent application is currently assigned to COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH. Invention is credited to Rohit Harshadrai Dave, Kaushik Jethalal Langalia, Vadakke Puthoor Mohandas.
Application Number | 20070219081 11/375246 |
Document ID | / |
Family ID | 38518674 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070219081 |
Kind Code |
A1 |
Dave; Rohit Harshadrai ; et
al. |
September 20, 2007 |
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).sub.2 obtained from the reaction
of MgCl.sub.2 and lime, albeit indirectly, i.e., MgCl.sub.2 is
first reacted with NH.sub.3 in aqueous medium and the slurry is
then filtered with ease. The resultant NH.sub.4Cl-containing
filtrate is then treated with any lime, to regenerate NH.sub.3
while the lime itself gets transformed into CaCl.sub.2 that is used
for desulphatation of bittern so as to recover carnallite and
thereafter MgCl.sub.2 of desired quality required in the present
invention. The crude Mg(OH).sub.2 is dried and calcined directly to
produce pure MgO, taking advantage of the fact that adhering
impurities in the Mg(OH).sub.2 either volatilize away or get
transformed into the desired product, i.e., MgO.
Inventors: |
Dave; Rohit Harshadrai;
(Bhavnagar, IN) ; Langalia; Kaushik Jethalal;
(Bhavnagar, IN) ; Mohandas; Vadakke Puthoor;
(Bhavnagar, IN) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
COUNCIL OF SCIENTIFIC &
INDUSTRIAL RESEARCH
|
Family ID: |
38518674 |
Appl. No.: |
11/375246 |
Filed: |
March 15, 2006 |
Current U.S.
Class: |
502/60 |
Current CPC
Class: |
C01F 11/24 20130101;
C01F 5/08 20130101; C01P 2006/80 20130101; C01F 5/20 20130101; C01P
2002/72 20130101 |
Class at
Publication: |
502/60 |
International
Class: |
B01J 29/04 20060101
B01J029/04 |
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.MgCl.sub.2.6H.sub.2O), iii) recovering MgCl.sub.2 rich and
other salt free end bittern from step (ii), iv) further evaporating
end bittern of step (iii) to obtain crystalline
MgCl.sub.2.6H.sub.2O, v) seeding MgCl.sub.2.6H.sub.2O, obtained in
step (iv) either as such or after recrystallization, in solid or
solution form, with a small quantity of Mg(OH).sub.2 and treating
it with ammonia (NH.sub.3), vi) filtering the resultant slurry
obtained in step (v) to obtain the crude Mg(OH).sub.2 and
NH.sub.4Cl/residual NH.sub.4OH filtrate, vii) drying the above
crude Mg(OH).sub.2, followed by calcination to convert Mg(OH).sub.2
into MgO, and converting adhering MgCl.sub.2 into MgO and HCl gas,
and NH.sub.4Cl into sublimed vapor, viii) absorbing the hot
sublimed vapor of NH.sub.4Cl generated in step (vii) from the
calciner into the NH.sub.4Cl/residual NH.sub.4OH filtrate of step
(vi) to further enrich the filtrate in NH.sub.4Cl and also for
heating up the filtrate, ix) treating the above said hot filtrate
with lime to obtain CaCl.sub.2 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 CaCl.sub.2 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 29-32.sup.oBe'.
4. An improved process as claimed in claims 1-3, wherein the
carnallite (KCl.MgCl.sub.2.6H.sub.2O) obtained in step (ii) is
crystallized between 32-36.sup.oBe' either through solar or forced
evaporation and the end bittern of step (iii) having density of
35.5-36.0.sup.oBe' contains 450-460 gpl of MgCl.sub.2, 5-10 gpl of
NaCl, 5-10 gpl of KCl, 5-15 gpl of Ca, 0-5 gpl of sulphate, 6-7 gpl
Br.sup.-, 0.03% B.sub.2O.sub.3.
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.sup.- 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 MgCl.sub.2.6H.sub.2O in 60-80%
yield containing 0.020-0.015% B.sub.2O.sub.3 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 NH.sub.3 to MgCl.sub.2 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 MgCl.sub.2 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).sub.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.degree. 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.degree. C. for 2-3 h and
preferably by gradually ramping the temperature to expel adhering
NH.sub.4Cl, HCl (from adhering MgCl.sub.2.6H.sub.2O), H.sub.2O and
NH.sub.3 (from adhering NH.sub.4OH) at a temperature 600.degree.
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 MgCl.sub.2.6H.sub.2O 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 B.sub.2O.sub.3
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 MgCl.sub.2.6H.sub.2O of step (iv) has a
B.sub.2O.sub.3 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 MgCl.sub.2.6H.sub.2O has a
B.sub.2O.sub.3 impurity level in the range of 0.010-0.015%.
19. An improved process as claimed in claims 1-18, wherein the
B.sub.2O.sub.3 level in MgO can be made still lower through
appropriate treatment either of the precursor Mg(OH).sub.2 or of
the MgO itself.
20. An improved process as claimed in claims 1-19, wherein the
NH.sub.4Cl/NH.sub.4OH filtrate obtained as by-product of
Mg(OH).sub.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).sub.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
NH.sub.3 vapors generated in step (ix) is stripped out with air or
steam and is absorbed in a solution of MgCl.sub.2 by feeding into
the reaction chamber at a rate so as to maintain the desired mole
ratio of NH.sub.3 to MgCl.sub.2 for optimum reaction.
23. An improved process as claimed in claims 1-22, wherein the
solution obtained in step (ix) contains 20-30% CaCl.sub.2 and is
used directly in desulphatation reaction in step (i) or is
clarified through filtration and/or addition of acid to redissolve
Mg(OH).sub.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.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an improved process for the
preparation of magnesia (MgO). More particularly, the invention
relates to an improved process for the preparation of MgO of high
purity from Mg(OH).sub.2 without the need to wash Mg(OH).sub.2 or
MgO.
BACKGROUND OF THE INVENTION
[0002] Magnesia is an important compound that finds application in
various industries. Magnesium oxide has the highest melting point
of the moderately priced oxides and is therefore an important raw
material for refractory bricks and other materials. It is the only
material apart from ZrO.sub.2 that can withstand long-term heating
above 2000.degree. C.
[0003] Reference may be made to Ullmann's Encyclpedia, 6.sup.th
Edition (electronic version) wherein it is stated that: "The
increased demands made on refractory materials as a result of
higher operating temperatures and shorter tap to tap times in
metallurgical furnaces and reactors can only be met by pure,
high-density magnesia sinters. Small quantities of "contaminants"
are disadvantageous if they form low-melting eutectics with MgO
(e.g., with CMS at 1485.degree. C. or with C.sub.2F at 1200.degree.
C. because this leads to deterioration of mechanical properties
(e.g., strength and volume stability) at high temperatures.
High-quality sinters therefore have a low content of high-melting
silicate phases (such as C.sub.2S), a low B.sub.2O.sub.3 content,
and a high degree of direct periclase-periclase contact (without
intermediate silicate phases).
[0004] Magnesia bricks have a high heat storage capacity and a high
thermal conductivity. They are used in efficient off-peak storage
heaters. The heat generated by a heating element is transferred to
the magnesia brick and increases its temperature. Thermal
conductivity is increased by a high periclase content and a low
porosity. The specific heat is only slightly lowered by SiO.sub.2
and Al.sub.2O.sub.3, but is significantly lowered by CaO,
Cr.sub.2O.sub.3, and Fe.sub.2O.sub.3. The bricks should not contain
free CaO (risk of hydration) or crystal phases with different
modifications.
[0005] MgO can be pressure hydrated to form Mg(OH).sub.2. It can
also be converted into anhydrous MgCl.sub.2 through the reaction of
eq. 1 (Electrolytic Production of Magnesium, Kh. L. Strelets, Keter
Publishing House Jerusalem Ltd., 1977, p 28)
MgO+Cl.sub.2+CO.fwdarw.MgCl.sub.2+CO.sub.2+70.8 cal/mole (eq.
1)
and the anhydrous MgCl.sub.2 can be converted into Mg and Cl.sub.2
by electrolysis (eq. 2)
MgCl.sub.2.fwdarw.Mg+Cl.sub.2 (eq.2).
[0006] Alternatively, MgO can be thermally reduced with Si to
obtain Mg.
[0007] Reference is made to Ullmann's Encyclopedia wherein it is
reported that magnesia can be prepared by the decomposition of
magnesite (MgCO.sub.3). The main drawback of this method is that
magnesite ore can have high levels of impurity. The highest quality
magnesites, particularly those for refractory applications, are
needed for a magnesia product with a high MgO content, a
CaO:SiO.sub.2 mass ratio of 2-3, and low contents of
Fe.sub.2O.sub.3 and Al.sub.2O.sub.3. The presence of accompanying,
low-melting minerals can adversely affect the properties of the
sintered magnesia.
[0008] Reference may also be made to a publication entitled
"Magnesite--A market survey" published by Indian Bureau of Mines,
Nagpur and "Magnesite" in Indian Minerals Year Book, Vol.--2 (1989)
Published By Indian Bureau of Mines, Nagpur. Page--698 to 699,
wherein magnesia is prepared by calcination of naturally occurring
magnesite deposits. The drawback of this process is that magnesite
ores contain varying amount of silica, iron oxide, alumina, and
lime as silicates, carbonates, and oxides. Selectively mined ore is
passed through various beneficiation methods like crushing and size
separation, Heavy media separation, and froth flotation to reduce
lime and silica content prior to calcination. Magnetic separation
reduces iron concentration but is effective only when the iron is
present in the form of discrete ferromagnetic minerals rather than
as ferrous carbonate. Due to all this, high purity magnesia is
difficult to produce by this process and most such magnesia has
less than 95% purity.
[0009] Reference is made to the Sulmag II process (W. S. Ainscow:
"Aufbereitung von Magnesit zu hochwertiger Sintermagnesia," TIZ 110
(1986) no. 6, 363-368. Sulmag II the Sinter Magnesite Process,
Sulzer Brothers Ltd., Winterthur, Switzerland) for producing
light-burned caustic magnesia in a gas suspension kiln from
low-magnesite ores. Dissolved magnesium chloride is obtained by
selective extraction with recycled NH.sub.4Cl solution (eqs. 3, 4)
and all insoluble impurities are removed through filtration.
Needle-shaped crystals of nesquehonite (MgCO.sub.3.3H.sub.2O) are
precipitated out in the reactor and filtered. Caustic magnesia with
a high specific surface area is obtained by heating the
nesquehonite.
MgCO.sub.3.fwdarw.MgO+CO.sub.2 (eq. 3)
2NH.sub.4Cl+MgO+H.sub.2O+Contaminants.fwdarw.2
NH.sub.4OH+MgCl.sub.2+Tailings (eq. 4)
MgCl.sub.2+(NH.sub.4).sub.2CO.sub.3+3H.sub.2O.fwdarw.MgCO.sub.3.3H.sub.2-
O.dwnarw.+2NH.sub.4Cl (eq. 5)
[0010] The process has not been applied for production of MgO via
Mg(OH).sub.2 which would give magnesia of desired characteristics
suitable for refractory applications whereas direct production from
the nesquehonite would give product of very low bulk density.
[0011] Reference is made to the preparation of MgO from
Mg(OH).sub.2 by calcination. Reference is also made to Kirk Othmer,
Encyclopedia of Chemical Technology, 4.sup.th Ed., Vol. 15, p 690
wherein it is stated that "To precipitate and recover magnesium
hydroxide from solutions of magnesium salts, a strong base is
added. The more commonly used base is calcium hydroxide derived
from lime (CaO) or dolime (CaO--MgO). Sodium hydroxide is used as a
precipitant if a product having low CaO content is desired.
Mg(OH).sub.2.fwdarw.MgO+H.sub.2O (eq. 6)
[0012] Reference may be made to the Paper entitled "Carbonation of
Aqueous Suspensions containing Magnesium Oxides or Hydroxides" by
Robert L. Evans and Hillary W. St. Clair in "Industrial and
Engineering Chemistry" 1949, 41(12), 2814-2817, wherein a
modification of the Pattinson process (carbonation of magnesium
hydroxide to magnesium bicarbonate) is described. A suspension of
magnesium hydroxide is carbonated to form a metastable solution of
magnesium bicarbonate. After the separation of insoluble
impurities, the solution is decarbonated by heating or aeration and
the magnesium carbonate precipitates as trihydrate, the penta
hydrate or the basic carbonate. The precipitate is recovered from
the solution by filtration and converted to magnesium oxide by
thermal decomposition. The main drawback of the process is that the
process is very sensitive to the partial pressure of carbon dioxide
and to the temperature. The stability of the metastable solution of
magnesium bicarbonate decreases markedly as the temperature rises
above normal room temperature. Moreover, the bulk density of the
MgO would be too low for refractory applications.
[0013] Reference may be made to the paper "Chemical Engineering
Problems in the Sea Water Magnesia Process" read by H. W. Thorp and
W. C. Gilpin at a meeting of the chemical engineering group, held
in the Apartment of the geological society, Burlington House,
London, W. I. on Tuesday, Oct. 25, 1949 wherein the recovery of
magnesia from sea water lies in the difficulty of precipitating the
magnesium hydroxide in a form which will settle rapidly and which
will yield a sludge easy to de-water. It is realized that each ton
of magnesia must be separated from some 300 tons of water, which
amount does not include any used for washing the precipitate. It is
necessary to ensure the minimum contamination by lime; the sea
water is treated prior to the removal of the magnesium hydroxide,
with a small proportion of lime to precipitate the bicarbonate ion
as calcium carbonate.
[0014] Reference is made to Ullmann's Encyclopedia wherein the
production of MgO from seawater and brines is described. 470
m.sup.3 of seawater are required to produce 1 t of MgO; in practice
600 m.sup.3 are needed. The process is based on the precipitation
of magnesium hydroxide (solubility in water 0.0009 wt %) by
addition of calcium hydroxide (solubility 0.185 wt %):
Mg.sup.2+2Cl.sup.-+Ca(OH).sub.2.fwdarw.Mg(OH).sub.2.dwnarw.+Ca.sup.2++2C-
l.sup.-
[0015] The main drawbacks of the above process are that a supply of
freshwater (>40 m.sup.3 per tonne MgO) is required to wash the
Mg(OH).sub.2 and to produce the milk of lime. High-purity limestone
or dolomite deposits should be available in the vicinity; they are
calcined and slaked to provide Ca(OH).sub.2 as the precipitating
agent and should therefore contain minimal quantities of elements
that form insoluble carbonates, sulfates, etc. The freshwater also
requires to be decarbonated. Unless specially treated, caustic and
sintered magnesia produced from seawater usually contain ca. 0.2%
B.sub.2O.sub.3 and small amounts of CaO, SiO.sub.2,
Al.sub.2O.sub.3, and Fe.sub.2O.sub.3 derived from the limestone or
wastes in the seawater. The B.sub.2O.sub.3 content of the magnesia
is also generally lowered to ca. 0.05% by using a 5-12% excess of
lime for precipitation (overliming); this increases the pH to 12
and minimizes the adsorption of boron.
[0016] Reference may also be made to the paper "Recovery of
Magnesium Compounds from Sea Water" by W. C. Gilpin and N. Heasman"
in "Chemistry and Industry", 6 Jul. 1977, 567-572, wherein the
process of recovering magnesia from seawater and the problems with
the process are clearly outlined. The drawbacks of the process are
similar to those described above.
[0017] Reference may also be made to the technique of
pyrohydrolysis. MgCl.sub.2-rich brine is purified to remove bromide
and traces of boron and then fed via steel pipes into the spray
nozzles of the reactor. It is sprayed into the cylindrical,
externally insulated reactor at ca. 600.degree. C. The water
evaporates from the atomized brine droplets leaving a perforated
chloride crust which reacts with the steam to form MgO and HCl. The
crude product is washed with water and hydrated in a stirred tank,
and then concentrated in a thickener. The resulting slurry is
difficult to filter and is washed and dewatered in a two-stage
vacuum drum filter. The calcined product typically
contains.gtoreq.99.5 wt % MgO, <1 wt % CaO, .ltoreq.0.05 wt %
SiO.sub.2, .ltoreq.0.05 wt % Fe.sub.2O.sub.3, .ltoreq.0.005 wt %
Al.sub.2O.sub.3, and .ltoreq.0.01% B.sub.2O.sub.3; its specific
surface area is 2-50 m/g, the loose bulk density ranges from 0.8 to
0.2 g/cm.sup.3. The main drawback is that the spray calcined
product needs to be washed to remove unreacted MgCl.sub.2 and
soluble salts and once again subjected to calcination. Spray
calcination is an energy intensive process and choking up of
nozzles can pose a problem. Reference may be made to the U.S. Pat.
No. 4,255,399 Dated Mar. 10, 1981 entitled "Process for the
Recovery of Magnesium Oxide of high Purity" by Grill et. al,
wherein magnesium oxide is obtained by thermal decomposition of a
magnesium chloride brine previously purified. Concentrated
magnesium chloride is decomposed in a thermal reactor where hot
gases convert it into magnesium oxide and hydrochloric acid. The
problems no doubt would be similar to those stated above.
[0018] Reference is made to U.S. Pat. No. 6,776,972, DT. 17-08-2004
wherein Vohra et al. have described the use of HCl gas generated
from spray pyrolysis for reaction with limestone to prepare
CaCl.sub.2 which can then be used to desulfate sea/sub-soil bittern
for the facile production of carnallite double salt wherefrom KCl
can be produced. The problem of spray calcination, however,
remains.
[0019] Reference may be made to "Encyclopedia of chemical
Reactions" compiled and edited by C. A. Jacobson page No. 427,
Reaction No.--IV--1757 and further reference may be made to
"Encyclopedia of chemical Technology" third edition. Vol. 14,
edited by Herman F. Mark et. al. page No. 624 wherein addition
compounds with ammonia include MgCl.sub.2.6NH.sub.3,
MgCl.sub.2.2NH.sub.3 and MgCl.sub.2.NH.sub.3 are obtained by the
reaction between anhydrous MgCl.sub.2 and NH.sub.3 gas in a closed
system. It will be evident that magnesium complexes only form under
certain specific conditions only.
[0020] Reference may be made to a paper "Effect of pH on the Growth
of Mg(OH).sub.2 Crystals in an Aqueous Environment at 60.sup.0 C"
by V. A. Phillips et. al. in "Journal of Crystal Growth" 41 (1977)
228-234 wherein magnesium hydroxide was precipitated at 60.sup.0 C
at various constant pH levels in the range 8.7 to 12.5 from
magnesium chloride and ammonium hydroxide. The results showed that
the particle morphology, average diameter, diameter to thickness
ratio and surface area varied with pH. No mention is made of any
process of preparation of MgO from the hydroxide.
[0021] Reference may be made to "Preparation of magnesium hydroxide
flame retardant by ammonia method." by Li, Kemin; Zhang, Li,
Wujiyan Gongye, (33(2), 14-16 (Chinese) 2001 Wujiyan Gongye
Bianjib; CA 135:115882; CA Section: 78 (Inorganic Chemicals and
Reactions), wherein the flame retardant was prepared by allowing
bittern after recycling K.sub.2SO.sub.4 to react with NH.sub.4OH,
hydrothermal treatment to obtain Mg(OH).sub.2, treating by surface
treatment, washing, drying, and crushing. The content of
Mg(OH).sub.2 of the flame retardant was 97%. No mention is made of
any process that produces MgO from the crude unwashed
Mg(OH).sub.2.
[0022] Reference may be made to "Recovery of magnesium hydroxide,
gypsum and other products from natural and technical brines, in
particular from final lyes of potash works." by Krupp, Ralf
(Germany) (Ger. Offen. DE 10154004 A1 15 May 2003, 9 pp. (German);
CA 138:371080), wherein, recovery of Mg(OH).sub.2 and gypsum from
MgSO.sub.4- and MgCl.sub.2-containing brines results by
precipitation of Mg-ions with NH.sub.3 or NH.sub.4OH. Gaseous
NH.sub.3 is recovered by addn. of CaO and recycled for the pptn.
step. The method allows the manufacture of Mg(OH).sub.2 without
impurities such as Fe, Mn, Al, and Ca. However, although not stated
explicitly, the preparation of pure Mg(OH).sub.2 would no doubt
have involved the washing of the solid to remove adhering
NH.sub.4Cl, MgCl.sub.2, etc. No mention is made of any process that
produces MgO from the crude unwashed Mg(OH).sub.2.
[0023] Reference may be made to "One-step process for manufacture
of magnesium hydroxide" by Wang, Fuwen; Zhang, Jun; Liu, Jianhua;
Dong, Yijun (Shandong Haihua Group Corp., Ltd., Peop. Rep. China).
Faming Zhuanli Shenqing Gongkai Shuomingshu CN 1332117 A 23 Jan.
2002, 7 pp. (Chinese). (People's Republic of China). Bittern
containing MgCl.sub.2 and ammonium hydroxide[mol ratio of
MgCl.sub.2/ammonia=1/(1.3-2.0)] are reacted at 45-90.degree. for
5-30 min, filtered, washed, dried, and pulverized to give solid
magnesium hydroxide. No mention is made of the preparation of MgO
from the crude Mg(OH).sub.2.
[0024] Reference may be made to the paper by J. A. Fernandez-Lozano
entitled "Utilization of Seawater Brines for the Production of High
Purity Magnesium Oxide and Magnesium Hydroxide" published in the
Proceedings of the Fifth International Symposium on Salt--Northern
Ohio Geological Society, 1979, pp 269-279 wherein the author has
stated that Mg(OH).sub.2 obtainable from the reaction of
MgCl.sub.2-rich seawater brine and ammonia can be made of high
purity by washing the hydroxide and that, in principle, MgO of high
purity can be obtained as a result. No mention is made of the
possibility of obtaining high purity MgO directly from the crude
hydroxide. Neither is there any mention of the recycle of ammonia
to make the process economically viable.
OBJECTIVES OF THE INVENTION
[0025] The main object of the present invention is to provide an
improved process for the preparation of magnesia from magnesium
chloride via intermediate formation of magnesium hydroxide wherein
no washing is required either of the magnesium hydroxide or of the
magnesia while at the same time obtaining MgO of very high
purity.
[0026] Yet another object of the present invention is to dispense
with the tedious process of washing of magnesium hydroxide resorted
to in the prior art.
[0027] Yet another object is to use aqueous ammonia in a reaction
with magnesium chloride.
[0028] Yet another object is to drive the equilibrium-controlled
reaction between magnesium chloride and ammonia to 85% formation of
Mg(OH).sub.2 under ambient conditions, through use of a small
excess of ammonia.
[0029] Yet another object is to recover the magnesium hydroxide
precipitate thus formed by rapid filtration and subjecting the
crude solid directly to calcination after drying.
[0030] Yet another object is to exploit the sublimation property of
ammonium chloride to expel it during calcinations and to obtain
directly the magnesia of high purity.
[0031] Yet another object to simultaneously convert the adhering
MgCl.sub.2 into MgO and HCl vapor.
[0032] Yet another object is to recover the ammonium chloride and
HCl vapors by absorbing these in the ammonium chloride/ammonia
filtrate obtained from the magnesium hydroxide-forming reaction
thereby increasing the ammonium chloride concentration.
[0033] Yet another object is to use of lime for the regeneration of
ammonia.
[0034] Yet another object is to have a continuous process wherein
the operations of magnesium hydroxide preparation, filtration,
drying, calcination, and regeneration of ammonia are performed
simultaneously to obtain the desired high purity yield of MgO and
CaCl.sub.2-rich liquor as by-product.
[0035] Yet another object is to utilize the by-product CaCl.sub.2
liquor, with or without clarification, for desulphatation of raw
bittern as described in the prior art, to facilitate
crystallization of carnallite double salt and also to minimize
sulphate impurity in end bittern.
[0036] Yet another object is to utilize the end bittern containing
440-480 gpl of MgCl.sub.2, which is nearly free of other
impurities, as a raw material directly for the preparation of MgO
having 98% purity.
[0037] Yet another object is to obtain high yields MgO having
purity 99% and very low B.sub.2O.sub.3 impurity directly upon
calcination, useful for refractory applications.
[0038] Still another object is to produce high purity magnesium
products such as milk of magnesia, magnesium metal, Mg(OH).sub.2
fire retardant, etc., utilizing the MgO of the present
invention.
SUMMARY OF THE INVENTION
[0039] The aim of the present invention is directed to provide an
improved process for the preparation of MgO of high purity from
salt bitterns via intermediate formation of Mg(OH).sub.2 obtained
from the reaction of MgCl.sub.2 and lime, albeit indirectly, i.e.,
MgCl.sub.2 is first reacted with NH.sub.3 in aqueous medium and the
slurry is then filtered with ease. The resultant
NH.sub.4Cl-containing filtrate is then treated with any lime,
preferably the most inexpensive lime, to regenerate NH.sub.3 while
the lime itself gets transformed into CaCl.sub.2 that is used for
desulphatation of bittern so as to recover carnallite and
thereafter MgCl.sub.2 of desired quality required in the present
invention. The crude Mg(OH).sub.2 is dried and calcined directly to
produce pure MgO, taking advantage of the fact that adhering
impurities in the Mg(OH).sub.2 either volatilize away or get
transformed into the desired product, i.e., MgO.
DETAIL DESCRIPTION OF THE INVENTION
[0040] Accordingly, the present invention provides an improved
process for the preparation of MgO, the said process comprising the
steps of: [0041] i) desulphating brine or bittern with calcium
chloride, [0042] ii) evaporating the clarified brine/bittern after
separation of gypsum to separate out the common salt and carnallite
(KCl.MgCl.sub.2.6H.sub.2O), [0043] iii) recovering MgCl.sub.2 rich
and other salt free end bittern from step (ii), [0044] iv) further
evaporating end bittern of step (iii) to obtain crystalline
MgCl.sub.2.6H.sub.2O, [0045] v) seeding MgCl.sub.2.6H.sub.2O,
obtained in step (iv) either as such or after recrystallization, in
solid or solution form, with a small quantity of Mg(OH).sub.2 and
treating it with ammonia (NH.sub.3), [0046] vi) filtering the
resultant slurry obtained in step (v) to obtain the crude
Mg(OH).sub.2 and NH.sub.4Cl/residual NH.sub.4OH filtrate, [0047]
vii) drying the above crude Mg(OH).sub.2, followed by calcination
to convert Mg(OH).sub.2 into MgO, and converting adhering
MgCl.sub.2 into MgO and HCl gas, and NH.sub.4Cl into sublimed
vapor, [0048] viii) absorbing the hot sublimed vapor of NH.sub.4Cl
generated in step (vii) from the calciner into the
NH.sub.4Cl/residual NH.sub.4OH filtrate of step (vi) to further
enrich the filtrate in NH.sub.4Cl and also for heating up the
filtrate, [0049] ix) treating the above said hot filtrate with lime
to obtain CaCl.sub.2 solution and ammonia vapor, [0050] x) using
the ammonia vapor obtained in step (ix) in a process step (v) to
complete the loop while using the by-product CaCl.sub.2 solution in
step (i).
[0051] In an embodiment of the present invention the bittern used
in step (i) is obtained from ocean brine, sea brine, sub-soil brine
or lake brine.
[0052] In yet another embodiment the sulphate-containing bitterns
used in step (i) are desulphated in the density range of
29-32.sup.oBe'.
[0053] In yet another embodiment the carnallite
(KCl.MgCl.sub.2.6H.sub.2O) obtained in step (ii) is crystallized
between 32-36.sup.oBe' either through solar or forced evaporation
and the end bittern in step (iii) having density of
35.5-36.0.sup.oBe' contains 450-460 gpl of MgCl.sub.2, 5-10 gpl of
NaCl, 5-10 gpl of KCl, 5-15 gpl of Ca, 0-5 gpl of sulphate, 6-7 gpl
Br.sup.-, 0.03% B.sub.2O.sub.3.
[0054] In yet another embodiment the end bittern of step (iii) is
preferably debrominated so as to recover bromine and simultaneously
reduce the Br.sup.- impurity in debrominated bittern to <0.5
gpl.
[0055] In yet another embodiment the pristine end bittern of step
(iii) is used for MgO recovery, preferably debrominated and used
without crystallization of step (iv).
[0056] In yet another embodiment 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 MgCl.sub.2.6H.sub.2O in 60-80% yield containing
0.020-0.015% B.sub.2O.sub.3 impurity and is free from other
salts.
[0057] In yet another embodiment the ammonia used for the
initialization of reaction in step (v) is an aqueous ammonia
solution containing 20-25% ammonia (w/w).
[0058] In yet another embodiment the mole ratio of NH.sub.3 to
MgCl.sub.2 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
MgCl.sub.2 level in the filtrate of <1.5% and preferably
<0.5%.
[0059] In yet another embodiment the filtration operation used in
step (vi) is carried out with ease on a Nutsche filter or rotary
disk filter or filter press.
[0060] In yet another embodiment the filtration operation used in
step (vi) is carried out in a centrifuge. In yet another embodiment
the drying and calcination operation used in step (vii) is carried
out directly or alternatively after washing the crude Mg(OH).sub.2
with a minimum quantity of water and additives to remove a part of
the adhering impurities and rest during calcination.
[0061] In yet another embodiment the drying operation used in step
(vii) is carried out at a temperature of 70-150.degree. C. in
either a conventional oven or a solar oven to yield soft white
lumps that crumble easily into a powder.
[0062] In yet another embodiment the calcination operation used in
step (vii) is carried out in a muffle furnace at a temperature of
about 900.degree. C. for 2-3 h and preferably by gradually ramping
the temperature to expel adhering NH.sub.4Cl, HCl (from adhering
MgCl.sub.2.6H.sub.2O), H.sub.2O and NH.sub.3 (from adhering
NH.sub.4OH) at a temperature 600.degree. C., to yield MgO of high
purity.
[0063] In yet another embodiment 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 MgCl.sub.2.6H.sub.2O obtained
in step (iv).
[0064] In yet another embodiment the MgO obtained from end bittern
of step (iii) has a B.sub.2O.sub.3 impurity level in the range of
0.10-0.12%.
[0065] In yet another embodiment the MgO obtained from crystallized
MgCl.sub.2.6H.sub.2O of step (iv) has a B.sub.2O.sub.3 impurity
level in the range of 0.060-0.080%.
[0066] In yet another embodiment the MgO obtained from
recrystallized MgCl.sub.2.6H.sub.2O has a B.sub.2O.sub.3 impurity
level in the range of 0.010-0.015%.
[0067] In yet another embodiment the B.sub.2O.sub.3 level in MgO
can be made still lower through appropriate treatment either of the
precursor Mg(OH).sub.2 or of the MgO itself.
[0068] In yet another embodiment the NH.sub.4Cl/NH.sub.4OH filtrate
obtained as by-product of Mg(OH).sub.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).sub.2 during treatment with lime.
[0069] In yet another embodiment the lime used in step (ix) is
either hydrated lime or quicklime in the form of a solid or solid
suspension.
[0070] In yet another embodiment the NH.sub.3 vapors generated in
step (ix) is stripped out with air or steam and is absorbed in a
solution of MgCl.sub.2 by feeding into the reaction chamber at a
rate so as to maintain the desired mole ratio of NH.sub.3 to
MgCl.sub.2 for optimum reaction.
[0071] In still another embodiment the solution obtained in step
(ix) contains 20-30% CaCl.sub.2 and is used directly in
desulphatation reaction in step (i) or is clarified through
filtration and/or addition of acid to redissolve Mg(OH).sub.2 prior
to executing in step (i).
[0072] The inventive step of the present invention lies in the
preparation of magnesia from magnesium chloride via intermediate
formation of magnesium hydroxide wherein no washing is required
either of the magnesium hydroxide or of the magnesia while at the
same time obtaining MgO of very high purity.
[0073] The important features of the present invention are:
[0074] (1) Recognising that MgO production from Mg(OH).sub.2 via
the conventional process of treatment with lime or caustic soda
suffers from the drawback of poor filterability and requirement of
large quantity of fresh water for repeated washing, and that when
lime is used because of its lower cost, insoluble matter in the
lime such as silica, CaCO.sub.3, etc., can contaminate the
Mg(OH).sub.2 and, consequently, the MgO derived from Mg(OH).sub.2
by calcination.
[0075] (2) Reasoning thereafter that, where MgO is the required
product, it may be possible to devise a scheme to obtain MgO in
pure form without, as such, spending effort in the purification of
Mg(OH).sub.2.
[0076] (3) Conceptualising thereafter that it would be desirable to
devise a scheme whereby in a single calcination operation
Mg(OH).sub.2 is converted into MgO and simultaneously the product
is "self-purified" by eliminating undesired impurities.
[0077] (4) Recognising that Mg(OH).sub.2 formation from the
reaction of MgCl.sub.2-rich end bittern--which is almost free of
other impurities--and NH.sub.3 would fit in well with the proposed
scheme of (3) above since adhering NH.sub.3 to product would be
driven out, adhering NH.sub.4Cl would sublime away, and adhering
MgCl.sub.2 will also simultaneously convert into MgO while HCl gas
would be driven away under the calcination condition as it is
evident from the thermograms of FIG. 1.
[0078] (5) Recognising that, since NH.sub.4Cl is highly soluble in
water, the hot sublimed NH.sub.4Cl can be absorbed into
NH.sub.4Cl-containing filtrate obtained in (4) above to raise the
NH.sub.4Cl concentration of the solution and also its temperature.
Recognising further that released HCl, NH.sub.3 and water from (4)
above would also get absorbed in the same filtrate.
[0079] (6) Recognising that the ammonia recovery step of the Solvay
Process can be adopted to liberate NH.sub.3 from aqueous NH.sub.4Cl
through reaction with any inexpensive lime and the liberated
NH.sub.3 can be recycled by directly absorbing it into aqueous
MgCl.sub.2 to produce Mg(OH).sub.2 once again.
[0080] (7) Recognising that the concentrated NH.sub.4Cl solution
obtained above would get converted into a CaCl.sub.2 solution
during the reaction with lime and this solution can be filtered to
remove insoluble matter, or clarified in other ways, to yield a
clear concentrated CaCl.sub.2 solution that is fit for use
[0081] (8) Recognising that the CaCl.sub.2 solution can be used to
desulphate sea bittern and the resultant bittern, after separation
of gypsum, can be evaporated to crystallize out carnallite (from
which MOP can be recovered) leaving end bittern rich in MgCl.sub.2
and free of other impurities, as reported in the prior art.
[0082] (9) The process of the invention dispenses with the need for
any water for washing and enables the overall desired reaction:
MgCl.sub.2(aq)+CaO.fwdarw.MgO+CaCl.sub.2
to be achieved indirectly as follows:
MgCl.sub.2(aq)+2NH.sub.3.fwdarw.Crude
Mg(OH).sub.2.dwnarw.+2NH.sub.4Cl(aq)
Crude
Mg(OH).sub.2[Mg(OH).sub.2/NH.sub.4Cl.sub.2/MgCl.sub.2/NH.sub.3].fw-
darw.MgO+NH.sub.4Cl+NH.sub.3+HCl+H.sub.2O
2NH.sub.4Cl(aq)+CaO.fwdarw.CaCl.sub.2(aq)+2NH.sub.3
with great practical consequences in terms of (a) ease of
processing, (b) elimination of requirement of any fresh water for
washing, (c) utilization of inexpensive CaO as the consumable base
for Mg(OH).sub.2 production, (d) preparation of MgO with 98% purity
without any washing operation, and (e) utilizing by-product
CaCl.sub.2 for desulphatation of bittern so as to obtain muriate of
potash (via carnallite) and end bittern containing nearly pure
MgCl.sub.2 that is used directly for Mg(OH).sub.2 preparation or
evaporated further to obtain MgCl.sub.2.6H.sub.2O.
[0083] The following examples are given by the way of illustration
and therefore should not be construed to limit the scope of the
invention.
EXAMPLE 1
[0084] 200 g of MgCl.sub.2.6H.sub.2O (0.985 moles) was reacted with
75.53 g of NaOH (1.890 moles). The residue obtained on filtration
was washed with water. The mass of the wet cake was 270.65 g. Of
this 235.34 g was dried at 110.degree. C. to obtain 77.98 g of dry
residue. 60 g of this dry residue was calcined to obtain 22.55 g
MgO having>99% purity. The loss on ignition was 62.42%
(theoretical LOI=30.88%). The observations from this example are
that, even though MgO of high purity is obtainable, this is
achieved only through elaborate and tedious washing.
EXAMPLE 2
[0085] 250 g of MgCl.sub.2.6H.sub.2O (1.232 moles) was reacted with
190.8 g of 23.8% (w/w) ammonia solution (2.671 moles) and the
slurry was stirred for 15 min and then filtered. Filtration was
found to be very facile. The residue was washed with 280 mL of pure
water to obtain 152 g wet cake which, on drying at 110.degree. C.
yielded a weight of 43.40 g. Of this, 31.83 g was calcined at
900.degree. C. to give 21.13 g of MgO (58.46% yield w.r.t.
MgCl.sub.2) having>99.31% purity. The loss on ignition was
33.71% (theoretical LOI=30.88%) indicating that the dried material
prior to calcinations was essentially pure Mg(OH).sub.2. This
example establishes that no complexed form of ammonia exists in the
product mixture and that all impurities are therefore of adhering
nature. These are expected to be MgCl.sub.2, NH.sub.3 and
NH.sub.4Cl besides some minor impurities, if any, which may be
present in the solid MgCl.sub.2.6H.sub.2O. It will be further
evident that the major adhering impurities would either be
converted into MgO or volatilize away on calcination and,
therefore, if the Mg(OH).sub.2 is not thoroughly washed; indeed,
even if it is not washed at all, the MgO that can be expected to be
obtained of high purity.
EXAMPLE 3
[0086] 100 gm of MgCl.sub.2.6H.sub.2O (AR grade) (0.493 moles)
having available MgCl.sub.2 content of 46.8 gm was mixed under
stirring with 73.44 ml (NH.sub.3=23.84% w/w; sp. Gr. 0.91)) of
ammonia liquor (0.937 moles) of specific gravity 0.91. The mixture
was allowed to stand for two hours. The resultant slurry could be
readily filtered by vacuum filtration. Wet cake weighing 51.5 gms
and 92 ml of filtrate having specific gravity of 1.12 and chemical
composition Ca=ND %, Mg=2.68%, Cl=20.38% was obtained. The wet cake
was washed with 50 ml water. The wash filtrate composition is,
Ca=ND, Mg=0.83%, Cl=5.44%. The cake was dried at 110 C to obtain
dry mass of 43.43 gm which was further calcined at 900 C to obtain
13.78 gm (0.344 moles; 70% yield) of MgO showing purity of 99.35%,
CaO=ND and Cl=0.37% and B.sub.2O.sub.3=0.012%. It will be evident
from this example and the powder XRD of FIG. 2 that MgO purity
matching that of Example 1 is obtainable if the MgCl.sub.2 taken is
free of impurity salts as is the case when AR grade MgCl.6H.sub.2O
is used. It will be evident from this example that the contention
in Example 2 that good quality MgO can be obtained even without any
washing of the Mg(OH).sub.2 is borne out by the result
obtained.
EXAMPLE 4
[0087] 200 ml (0.958 moles of MgCl.sub.2) of end bittern having
specific gravity of 1.324 and chemical composition Ca=0.504% (w/v),
Mg=11.50%, SO.sub.4=ND, Na=0.41%, K=0.4%, B.sub.2O.sub.3=0.032, was
mixed under stirring with 123.8 gm solution of ammonia liquor of
specific gravity 0.91 having ammonia content as NH.sub.3=23.84%,
i.e., 1.736 moles NH.sub.3. The mixture was allowed to stand for
seven hours. The resultant slurry was filtered by vacuum
filtration. Wet cake weighing 109.52 gms and 242 ml of filtrate
having specific gravity of 1.12 and chemical composition Ca=0.40%,
Mg=2.75%, Cl=20.74% was obtained. The wet cake was dried and
calcined at 900 C to obtain 23 gm (0.575 moles; 66.24% yield w.r.t.
NH.sub.3) MgO showing purity of 98.5%, CaO=0.99%, Cl=0.7% and
B.sub.2O.sub.3=0.106%. It will be evident from this example that
the contention in Example 2 that MgO of very good quality can be
obtained without any washing of the Mg(OH).sub.2 holds even when
end bittern of the quality described above is used. Nevertheless,
the purity of the MgO is somewhat lower than achieved with washed
Mg(OH).sub.2 obtained through NaOH precipitation following the
conventional process of Example 1. As will be shown in an example
below, bittern of this quality is obtainable by desulphatation of
the raw bittern using CaCl.sub.2 generated in the process of the
invention.
EXAMPLE 5
[0088] 1 L of bittern of Example 4 was partially evaporated by
forced evaporation to reduce its volume to 800 ml. The resultant
mass was cooled to room temperature followed by filtration to
obtain 619.7 gm crystalline magnesium chloride having chemical
composition, Ca=0.22%, Mg=11.17%, B.sub.2O.sub.3=0.0147% and 370 ml
of filtrate having specific gravity 1.338 and
B.sub.2O.sub.3=0.0657%. 500 gm (2.327 moles) of this magnesium
chloride was mixed with 5 gm (0.08 moles) seeds of Mg(OH).sub.2
along with 400 ml of solution of ammonia having NH.sub.3
concentration of 23.84% (w/w) (5.104 moles) The mixture was allowed
to stand for 2 hours. The slurry was vacuum filtered to obtain
283.56 gm wet cake and 512.00 ml of filtrate containing ammonium
chloride having specific gravity 1.08 and Ca=0.12%, Mg=1.54%. The
cake was dried and calcined at 900 C to obtain 80.5 gm (2.012
moles; 86.48%) of MgO having MgO content of 99.09%, CaO=0.38% and
Cl=0.23%. This example teaches us that the MgCl.sub.2-rich end
bittern of Example 4 can be used for the preparation of
MgCl.sub.2.6H.sub.2O that can yield MgO of >99% purity and
containing only 0.0737% B.sub.2O.sub.3 even without any manner of
washing either of the Mg(OH).sub.2 or of the MgO. It would be
evident that recrystallization of MgCl.sub.2.6H.sub.2O would
further improve the purity of MgO.
EXAMPLE 6
[0089] 9.543 g of the MgO of Example 3 was subjected to simple
water wash and re-calcined to yield 9.371 g of purified MgO wherein
the Cl impurity was absent, the B.sub.2O.sub.3 impurity was lowered
from 0.0121% to 0.0061%, and the MgO purity improved from 99.3% to
99.7%. The present example gives an indication of the feasibility
of making MgO of exceptional purity if such purity were to be
required for highly demanding applications.
EXAMPLE 7
[0090] 100 ml of the filtrate containing ammonium chloride and
having specific gravity of 1.08 obtained in Example-5 above was
mixed with 32 gm of hydrated lime [Ca(OH).sub.2=89.90%] on a water
bath. The contents were heated to expel ammonia generated from the
reaction of lime and ammonium chloride. The ammonia generated was
allowed to absorb in 200 ml of end bittern of Example-4. The
mixture from the lime still for ammonia and calcium chloride
generator was filtered to obtain 99 ml of calcium chloride liquor
having CaCl.sub.2 content of 24.51% and Mg content of 0.12%. The
slurry containing magnesium hydroxide prepared by absorption of
ammonia was filtered and dried at 110.degree. C. to obtain 12.00 g
of solid which on calcinations at 900 C gave 4.66 gm of MgO (54%
yield w.r.t. CaCl.sub.2 generated and, consequently, NH.sub.3
generated) and having MgO purity of 98.9%. This example teaches us
the recycle of ammonia for Mg(OH).sub.2 production, with
concomitant generation of calcium chloride useful for
desulphatation of bittern.
ADVANTAGES OF THE PRESENT INVENTION
[0091] The main advantage of the present invention is that
Mg(OH).sub.2 obtained from the reaction of MgCl.sub.2 and ammonia
in aqueous medium, which is very easy to filter, requires no
further purification and can directly yield MgO of high purity,
thereby largely eliminating the tedious nature of work up
encountered in the conventional process of preparation of MgO from
purified Mg(OH).sub.2.
[0092] Another advantage is the conservation of fresh water which
is a scarce commodity in some of the regions where production of
MgO from bittern, integrated with recovery of other marine
chemicals, is intended.
[0093] Yet another advantage is that the quality of MgO obtained
from the present invention surpasses, in many cases, the quality of
MgO obtained from Mg(OH).sub.2 prepared following the conventional
process as described in the prior art.
[0094] Yet another advantage is that the process of the present
invention also compares favorably opposite other methods of
producing MgO, such as through pyrohydrolysis of MgCl.sub.2 and
decomposition of magnesium carbonate. In the former case, two
calcination steps along with a difficult washing step are involved,
besides the operational complexity, whereas in the latter case, the
MgO obtained is either too impure or suffers from the problem of
low bulk density that makes it unsuitable for refractory
applications.
[0095] Yet another advantage is that the easier work up and
improved product quality--over that realized with the conventional
process involving MgCl.sub.2 and lime--are not at the expense of
higher raw material cost since the net reaction is still the same,
i.e., MgCl.sub.2 and lime are the consumable raw materials while
MgO and CaCl.sub.2 are the products.
[0096] still another advantage is that not only is lime the
cheapest base but the CaCl.sub.2 generated as co-product from the
reaction of lime and intermediate NH.sub.4Cl is useful in
desulfating sea bittern that yields MgCl.sub.2 of desired quality
required for MgO production by the process of the present
invention, besides also enabling the production of KCl through
carnallite formation.
* * * * *