Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The pre~ent invention relates to a proce~s for
producing magnesia ~rom magneaiuDI-containing carbonate ores,
which comprise~ leachin~ the ore~ u~th ~ulfuric acid~ neutrali-
zing the obtained solution w1th magne~ia, separating undi~ol-
ved impuritie~, crystallizing magne~ium sul~ate and thermally
decompo~ing the latter to form magnesia and S02~ recovering
3ulfuric acid ~rom the sulfur dio~lde produced in th~ decom-
po~ing ~tep and recycling the sul~uric acid to the leaching
~tage.
The magnesium chloride contained in sea water has
mainly been used as 8 ~ource of high-purity magne~ia. An
increa~e in concentration i3 ~ucceeded by thermal decomposition~
which may be carried out, e.g~, in accordance with the German
Patent Specification 878~801 and result~ in a formation o~
magnesia and hydrogen chloride ga~, ~his practioe has the
di~advantage that the resulting oxide has a purity of about
97~ and `i9 conta~inated with boron compound~. A further
disadvantage resides in the consumption of large quantities o~
energy required to increa~e the concentration of the originally
highly dilute solutionsO
Another proce~s of producing magne~ia relies on
large deposit~ of carbonate ores. Accord~ng to the U.S. Patent
Specification 2,381,053~ the ores are ground and in an aqueous
medium are treated ~ir~t with sul~ur dioxide and then with air.
This results in a formation o~ magnesium ~ul~ite as an inter-
mediate product and then o~ magne~ium ~ulfate, ~hich i~ ~iltered
from ~olid re~idues and is concentrated by evaporation and
- finally thermally decompo~ed. The sulfur dioxide formed in the
decomposing step i9 recycled to the first treating stage. ~his
practice ha~ the important disadvantage that the treatment with
sulfur dioxide and the ~ubsequent treatment with air result in
solid-ga~ reac-tion~ vlhich are rela-tively 810w and requ~re a
high surplus of reactants, particularly in the second ~tage.
Another process has b~en proposed which also relies
on magne~ium-contalning c~rbonate ores and in which the ores
are leached in a surplu~ of hot sulfuric aoid until the
magne~ium sulf~te concentration exceeds 60~ of the saturation
concentration~ then th~ freQ acid i~ neutralized with magn~sia,
the ~olid r~sidue~ are separated, and the magnesium ~ulfatecrysta
li~ed and i~ ~inally thermally craoked (Opened German Spe¢i~ication
2~159~973), Sulfurlo a¢id can be recovered from the ~a~ produced
by cracking and may be re-used for leaching.
~bough this prooess ha~ considerable advantages
compared to the one above mentioned, particulàrly beoause a much
higher reaction rate i9 achieved during the leaching stage, the
further proces~ing of the magnesium sulfate solution requires
a multi-step heat treatment, for instance by spray drying,
~inal drying, and decompoYition or by crystallization, drying,
and decomposition. Besides, large quantities of dust trhich are
ver~ difficult to separate are formed during the thermal
deoompo~ition of magnesium sulfate from which~all water has
been removed. Moreover~ only a rotary kiln has been described
as a decomposing unit; sùch a kil~ has a low throughput rate
per unit o~ the reactor volume and due to the dis~ipation of
radiant heat has a poor thermal efficiency.
- ~he sub~ect invention proposes to provide a process
~hich is free of the disadvantages of the known proces~es and
which has a high throughput rate and can be carried out in a
particularly simple manner, with a high heat econom~ and
yields a pure product of uniform quality.
Thi~ purpose is accomplished in that the process of
the kind defined fir~t hereinbefore is carried out according
to the inventi~n in such a manner that
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a) the crystallization iseffectcd under superatmospheric pressure
and at temperatures above 150C,
b) the resulting crystals are separated from the mother liquor
under such conditions that the subsequent cooling results
in a magnesium sulfate which contains 1.5-4 moles of water of
crystallization,
c) the magnesium sulfate in such a fineness that it is suitable
for being decomposed in a fluidized bed, is dewatered and/or
heated in a suspension-type-heat-exchanger, and fed through
a separator to a fluidized-bed reactor operating at a temperatu-
re in the range of from about 900 to 1200 C, the exhaust gases of
which are used to operate the suspension-type-heat-exchanger,
d) the reaction product is taken from the fluidized-bed furnace
and is fed to a multi-stage fluidized-bed cooler, which is
operated with air as fluidizing gas,
e) the fluidizing air leaving the fluidized-bed cooler is fed to
the fluidized-bed furnace, and
f) the heat required for the reaction is generated by an appro-
ximately stoichiometric combustion of fuel, which has been
charged into the fluidized-bed furnace.
The crystallization under a superatmospheric pressure
and àt temperatures above 150 C is known to afford the advantage
that the magnesium sulfate which is formedhas a low contènt
of water of crystallization.
The subsequent separation of the crystals from the
mother liquor should be effected in such a manner that the
subsequent cooling of the crystals results in a magnesium sulfate
containing 1.5-4 moles of water of crystallization. As the crys-
tals are cooled, the magnesium sulfate is known to take up any
water which is present until an equilibrium has been reached
between the saturated solution and the undissolved solids at the
existing temperature. In order to inhibit during
-
A'
this cooling the forma~ion of a ~ulfate having more than 4
mole~ water of cry~tallization, care ~u~t be taken during the
separation of the crystal~ that the quanti-ty of mech~nically
adhering mother liquor doe~ not exceed the largest quantity
which is permi~sible for the formation of a hydrate which
contains 4 moles of water of crystallization. On the other
hand~ when the magne~ium ~ul~ate cry~talli~e~ a9 a monohydrate~
the mother liquor must no~ be removed to su¢h an extent that
le~s water i~ a~ailable than is reqtlired to form a hydrate
which contains at least 1.5 moles of ~vater.
The ~eparation of the ~rystals from the mother liquor
may be effected under normal pre~sure. ~he methods o~ separation
used for this purpose should suitably ensure a fast separation
between the solids and the sQlution so that a ~ormation of a
~alt having an excessively high content of crystallization
or a redi~olution of the previously crystallized magnesium
~ulfate i~ a~oided. It will be particularly desirable to separate
the crystal~ from the mother liquor under pressure and particu-
larly to use for thi~ purpose the pressure which pre~ails during
the crystallization
The orystals which have been ~eparated from the
mother liquor are desirably cooled in such a manner that a
friable product is obtained. Cooling rolls or cooling belts are
particularly suitable for thi~ purpo~e.
The main reason for performing crystallization, separa-
tion of cry~tals, and cooling o~ crystals under carefully
controlled conditions resides in the surpri~ing fact that a
magne9ium sulfate which hss 1.5-4 moles of water of crystalli-
zation has excellent mechanical properties for the further
proces~ing, ~hich compri~es dewatering and/or heating in
~uspension-type-heat exchangers and decompo~ition in a fluidized
bed. The solids remain granular and flowable and the particle~
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lJ~
exhibit virtual]y no tendency to disintegrate and form du~t.
The cooled magnesium ~ulfate should be generally
disintegrated to the fineness required ~or the fluidized-bed
decompo~ition. ~hi~ disintegration i9 3uitably accompli~hed in
hammer mills, impact mills or pinned disc ~ttrition mills.
~he magnesium sulfate is then charged into a ~uspension-type-
heat-exchanger, in which the magnesium sulfate is dewatered
and/or preheated in conjunction with a cooling of ga~es and,
through a ~eparator, i~ fed to a fluidized-bed re~ctor~ which
is operated at a temperature o~ about 900~1200C. ~he exhaust
gase~ from the ~luidized-bed reactor are used to feed the
~uspension-type heat-exchanger,
~he solids then enter a fluidized-bed cooler, ~hich
comprises à plurality of separate cooling stages flo~n throu~h
in succes~ion. In that cooler, the solids deliver heat in
steps to a fluidizing air, which is thus heated. ~he heated air
i9 then fed to the ~luidi~ed-bed reactor, where the heat content
of the air is utilized in the decomposition step.
The heat required for the decompo~ition ~tep is
generated by an approximately stoichiometric combustion of fuel
charged into the fluidized-bed reactor. ~he term "approximately
stoichiometric" means that there should be no or only a slight
excess of air.
~he fuel may consist of a conventional fuel oil
and/or fuel gas as vell as high-sul~ur fuel oils and ~ul~ur-
containing distillation residues~ i.e.~ products or waste mate-
rials which can otherwi~e be proces~ed only with difficulty.
Contrary to expectations, it has been found that the sulfur-
containing impurities of these fuels do not contaminate the pro-
duct but enter the ga~ pha~e as sulfur dioxide.
Particularly owing to the approximately stoichiometric
combustion and the heat economy (recycling of heat to the
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decomposition ~tcp) the exhau~t ga~ produced in the decomposing
~tep has a high concentration of ~ulfur dio~ide.
In a preferred embodiment of the lnvention, the mag-
ne~ium sulfate is decompo3ed in an expanded fluidized bed
reactor having a mean fractional voidage f~0.7, the solids are
separated in a recycling cyclone and a major portion of the
~olid~ are recycled into the fluidi2ed bed reactor. ~or this
purpose a fluidizing air is introduced into the fluidized-bed
reactor at such a high ~elocity that the gase~ produced in the
decompo~ition step and the solid~ ~orm a highly expanded
fluidized-bed, in which the solid~ are highly agitated and all
or a ma~or portion of the solids are discharged with the ~ases
A ma~or portion of the dischar~ed solids are then recycled to
the fluidized-bed reactor. The proportion o~ recycled ~olid~
may be selected so that the residence time is kept within the
` required range.
According to another desirable feature of the invention,
the fluidized-bed reactor for decomposing the magnesium sulfate
is supplied with ~econdary air above the inlet for the fluidi-
zing air. In this embodiment the ratio of fluidizing air to
secondary air ~hould be in the range of approximately 1:2 to 5:1
and the fuel should be charged between the inlets for the
- f~uidizing air and secondary air.
As a result of the supply of air in two component
streams, the reactor volume is divided into a reducing lower
zone disposed below the secondary air conduit and a neutral
upper zone disposed over the secondary air conduit, The tempe-
rature in these zones i9 virtually the same owing to the
inten~e circulation of solid~ in the fluidized-bed reactor.
This practice afforas the advantage that the decomposition
tak~ place under reducing conditions, which are more favorable,
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" ~.
and neverthele~s thc fuel i9 fully uti].lzed a3 a re~ult of the
afterburning above the secondary air inlet.
In connection with the reaction~ of decompo~ition it
i9 known to u~e an expanded ~luidiæed-bed reactor "~hich i~
preceAed by 8 suspension type-heat exchanger ~tage and
~ucceeded by a ~luidi~ed-bed cooler. Thi~ practi¢e ha~ been
fully explained in the Opened German Specification 1,76?~628.
(corre~p. to Canadian Appln. 50 460).
Because certain sulfur losse~ are lnevitable in such
a proce~s, these losse~ are de~irably compensated by the
production of sulfur dioxide in the decomposition ~tep. ~o
this end, the rate a~ ~hich sulrur dioxide i~ produced in the
decomposition ~tep should be ~ufficient for the production of
the sulfuric acid required for the leaching stage. I~ conven-
tional fuel oils and/or ~uel gase~ are used as fuel or the
wa~te fuel doe~ not contain enough sul~ur ~or the production
Or the additional sulrur dio~ide required, additional elementary
sulrur and/or hydrogen sulfide may be supplied.
The exhaust gas formed in the decompo~ition step is
purified in conventional manner, e.g., by hot gas filters, by
electrostatic purification of gas, by Venturi-tube scrubbers,
and by wet-type electrostatic precipitators ~or fine purifica-
tion, and is then catalytically processed to form sulfuric
acia.
The in~ention will be explained more fully and by
way of example with reference ~ the flow scheme.
The leaching stage 1 is supplied with magnesium-
con~ining carbonate ore through conduit 7, sulfuric acid
through conduit 8, and water through conduit 9 and, if de~ired,
conduit 27. After the treatment ha3 been carried out for a
sufficiently long time and the ~urplus ~ulfuric acid ha~ been
ltJ~
neutralized with magne~ia, an insoluble re~idue i9 removed
through conduit 10. The magne~ium sulfate solution flow~ through
conduit 11 to the pre~ure crystalli~ation unit 2, from which wa-
ter vapor-laden exhaust ga~e~ are withdrawn through conduit 12.
The mother liquor i9 filtered and then recycled through conduit
13 to the leaching stage 1. Th~ cooled magnesium sulfate
cry~tal~ contain 1.5-4 ~oles of water o~ cry~talliæation a~d
are di~integrated to the required fineness in a fluidized-bed
and through conduit 14 are then fed to the decompo~ition unit
3, which ig fed ~ith ruel oil through conduit 15, air through
conduit 16, and elementary sulfur through conduit 17. The hot
magnesia leaves the decomposition unit 3 through conduit 18
and in a ~luidized-bed cooler 19 i~ cooled by a heat exchange
wlth air supplied through conduit 20, and i9 discharged at 21.
The heated air flows through the above-mentioned conduit 16 into
the decomposition unit 3.
The exhaust gas which has been formed in the decom-
position step and which i~ used ~or pre-dewatering and/or
preheating i~ fed through conduit 22 to a gas puri~ier 4, in
which water and dust are removed. Dust i9 discharged through
conduit 23. The condensed water may contain small amount~ of
sul~uric acid and flows through conduit 24 first to the washing
unit 6, in wh~h the filter residue is washed, which has been
withdrawn through conduit 10 from the leaching stage 1. The
water i~ then recycled through 9 into the leaching stage 1. ~he
~ashed filter residue is discharged through 26 and discarded.
The ga~ which contain~ sul~ur dioxide i~ fed through
25 to the sulfuric acid-producing plant 5 and i9 converted
therein into sulfuric acid, which i9 rec~cled through conduit
8 to the leaching ~tage 1.
Example
The leaching ~tage 1 is fed through conduit 7 at a
r~te of 2.3 metric tons per hour ~th raw magne~ite having
the following com~osition by weight:
80~o Mg C03
4% CaC0
2% ~e23
14% acid insoluble substance, particularly silica-
tes and is fed through conduit 8 with concentrated ~ulfuric
acid at a rate of 2.05 metric tons per hour (calculated as
100% acid), ~he leaching stage 1 i9 al~o fed through conduits
9 and 13 with washing water ~rom the washing unit 6, at a rate
of 1.22 metric tons per hour, and with mother liquor from the
pressure crystallisation unit 2 at a rate of 3.45 metric tons
per hour (0.65 metrio ton of magnesium sulfate and 2.8 metric
ton~ of water per hour). Additional water at a rate of 0.58
metric ton per hour is fed through conduit 27.
After a leaching treatment at 80C for two hours,
the slurry is neutralized with magnesia, and ~olids at a rate
of 0.65 metric ton per hour are withdrawn through conduit 10,
. washed in the washing unit 6 with water at 8 rate of 1.22 metric
tons per hour, and finally discharged through 26.
At a rate of 8 metric tons per hour (consi~ting of
3.04 metric tons of magnesium sulfate and 4.96 metric tons
of water per hour) the solution is fed at 80C through 1.2 into
the pressure crystallization unit 2. At a temperature o~ 210C,
corre~ponding to a pressure of 18 kg/cm2 sbove atmospheric
pressure, water vspor-laden exhau~t ga~es at a rate of 1.26
metric tons per hour are withdrawn through 12 and mother liquor
at a rate of 3.45 metric tons per hour is withdrawn through
13. The filter residue obtained at a rate of 3.29 metric tons
per hour is cooled to form hydrated magnesium ~ulfate at the
same rate. The magne~ium ~ulfate produced per hour consists of
2,39 metric tons of anhydrous magnesium sulfate and 0.9 metric
ton of ~ater of crystallization, corresponding to a content of
.5 moles of ~ter of crystallization.
When the material ha~ been ground to the fineness
of 100% below 1 mm which i~ required in a fluidized-bed, the
material enters the decompo~ition unit 3~ ~hich consists of a
~uspension-type-heat-exchanger and a fluidized-bed reactor.
Initially, in the suspension-type-heat-exchanger~ the magne~ium
sulfate i8 dewatered and preheated by the exhau~t gases from
the fluidized-bed reactor, whioh are at 1130C and by said
treatn~nt are cooled to 350C. The pre-dewatered and heated
magne~ium sul~ate then enters the fluidized-bed reactor, which
i9 operated at 1130C ~nd fed with air at a rate of 4000
stanaard cubic meter~ per hour, fuel oil at a rate of 0,45
metric ton per hour~ and elementary sulfur at a rate of 23kg/h.
~he air is used as fluidizing air and secondary air in equal
part~. ~he magnesia which has been formed in the fluidized-bed
reactor enters the fluidized-bed cooler 19, in which it is
; cooled to 130C as it flows in cross-and countercurrent to the
~luidizing air and to the air which i9 to be indirectly heated
The air stream~ leaving the fluidized-bed cooler have been heated
therein to 400C (~luidizing air) and 120C (indirectly heated
air) and are fed to the fluidized-bed reactor as secondary air
and as fluidizing air.
; Exhaust gase3 at 350C leave the su~pension-type-heat-
exchanger ~tage of the decompo~ition unit 3 at a rate of 4500
standard cubic meter~ of dry gas per hour and have the
~ollowing composition by volume:
N2 ?1 5%
C2 15.9%
S2 10.5%
2 2,1%
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~hey flow through conduit 22 into a ga9 puri.fier 4, in Ythich
water i9 separated at a rate of 1.22 metric tons per hour,
~he water i~ recycled lnto the leachin~ ~tage 1 through condult
24 and wa~hing unit 6. The sul~ur dioxid~-containing, purified
ga~ at 40C i~ fed through 25 into the sulfuric acid plant 5,
in which sulfuric acid is recovered at the rate of 2,05 metric
ton~ per hour which i9 required in the leaching ~tage 1.
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