Note: Descriptions are shown in the official language in which they were submitted.
1093~7
This invention relates to a process of producing
aqueous solutions containing 220-320 g/l magnesium chloride and
having a calcium chloride content amounting to 4-6% by weight of
the magnesium chloride content, for use in the production of MgO
by thermal decomposition, particularly spray roasting.
The production of synthetic magnesia, i.e. MgO, from
magnesium chloride-containing aqueous solutions by thermal de-
composition and particularly by spray roasting has progressively
increased in importance in recent times. This is due to various
reasons. One reason is the fact that the demand for pure magnesia
and particularly for very pure magnesia increases steadily and
that magnesia which contains 99-99.5% MgO can be produced by such
a thermal decomposition whereas magnesia recovered, e.g., from
contaminated magnesite by various conventional physical dressing
processes, such as heavy-liquid separation and/or flotation, has
a much lower MgO content. Another reason for the increasing
interest in the production of magnesia by thermal decomposition
is the fact that this process enables the utilization of naturally
occurring brines and abraum salts or solutions which result from
the dressing of MgO-containing minerals and contain magnesium
chloride.
It has now been found that aqueous solutions which
contain 220-320 g/l magnesium chloride and have a calcium chlor-
ide content of 4-6% by weight of the magnesium chloride content
are particularly suitable for the production of MgO by thermal
decomposition, particularly spray roasting, e.g., in a so-called
Aman reactor, and that such aqueous solutions can advantageously
be produced from solutions having a certain content of calcium
chloride. Thus the invention relates to a process which serves
to produce aqueous solutions of the kind stated and which essen-
tially resides in that an aqueous solution which contains 220-
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~.
1093Z77
320 g/l magnesium chloride and has a calcium chloride content inexcess of 6% by weight of the magnesium chloride content is re-
acted with carbon dioxide and with MgO in the form of caustic
magnesia or magnesium hydroxide, and the precipitated calcium
carbonate together with any additional magnesium chloride is
removed. In that reaction, calcium chloride is transformed into
calcium carbonate, and there is dissolved
- la -
10~33277
from the caustic magnesia or the magnesium hydroxide Mg in form of magnesium
chloride in a quantity which is proportional to the reacted Ca. The result-
ing solution may be subjected to thermal decomposition, e.g., in an Aman
reactor, as such or after ha~ing been concentrated. If the solutions contain
10-25 g/l calcium chloride, the thermal decomposition will result in a
magnesia which is rather coarse-grained, as this is preferred. In this context,
the tern "caustic magnesia" refers to a magnesia which has been obtained by
heating, e.g., raw magnesite or magnesium hydroxide to a temperature of about
600-1100C.
The starting material used in the process according to the invention
may consist, e.g., of a solution which has been obtained by the action of
gaseouq hydrogen chloride on an aqueous slurry of raw magnesite and/or waste
magnesite. In a special embodiment, the starting material consists of a
solution formed by reaction of calcium chloride-containing hydrochloric acid
with raw magnesite. This measure enables a utilization of the calcium
chloride-containing spent washing liquors which become available in the usual
process of producing magnesia in a spray reactor by a thermal decomposition
of solutions that contain magnesium chloride and calcium chloride and a
subsequent washing of the resulting mixture of MgO and calcium chloride. In
this way, the chloride losses which are due to the removal of CaC12 from the
system, can be minimized.
The invention will be described more fully with reference to the
following examples.
Example 1:
Hydrochloric acid having a concentration of about 20% (19~ g HCL/l,
pH below 0.1) and containing calcium chloride is fed at a rate of 1963 kg/h
HCl and 118 kg/h CaC12 in 9110 kg/h H20 into a vessel which has an acid-resist-
ing lining and is provided with a stirrer. The vessel is then fed with raw
magnesite at a rate of 2746.2 kg/h and subsequently with caustic magnesia at
a rate of 238.8 kg/h. Said raw magnesite and caustic magnesia have the
1093Z7~7
following compositions in percent:
Raw Caustic
magnesite magnesia
MgO 37 8 72.2
CaO 3 9 4 0
SiO2 6.9 4.6
2 3 3.5 3 0
A1203 1.O 1.O
Mn304 0.1 0.1
Cr203 0.1 0.1
Ignition loss 46.7 15.0
The pH-value is increased above 1 by the addition of the raw
magnesite, and to 5-6 by the subsequent addition of the caustic magnesia. In
a batch process, the raw magnesite and the caustic magnesia are dissolved
at a temperature of 80-85C in about 2 hours. At the same time, a mixture
of air and chlorine gas is introduced to oxidize the ferrous compounds pre-
sent to ferric compounds so that the hydroxides of iron, aluminium and
manganese (sesquioxides) are precîpitated.
The reaction results in a solution which contains magnesium chloride
and calcium chloride as well as insoluble matter, including the precipitated
hydroxide~ or sesquioxides respectively. This solution is transferred from
the dissolving vessel into a second vessel, in which calcium carbonate is
to be precipitated. The transfer is effected at a rate of 2365 kg/h mgC12,
345 kg/h CaC12 and 558 kg/h insoluble matter in 9594 kg/h H20. C02 formed
as a result of the dissolving of the raw magnesite, is simultaneously fed
from the dissolving vessel into the second vessel at a rate of 1328 kg/h.
Alternatively, C02 in the form of combustion gases may be used. MgO in the
for~ of caustic magnesia which is highly contaminated with silicates is added
at a rate of 83 kg/h MgO (83 kg/h insoluble matter) to the solution then con-
tained in the second vessel. The reaction in the second vessel takes about
1093Z77
one hour. The action of carbon dioxide and caustic magnesia in the second
vessel results in the precipitation of calcium carbonate at a rate of 208
kg/h. This calcium carbonate is withdrawn together with insoluble matter,
which amounts to 641 kg/h and includes sesquioxides. C02 becomes available
in the second vessel at a rate of 1281 kg/h.
The solution obtained in this second vessel is transferred at a
rate of 2563 kg/h MgC12 and 118 kg/h CaC12 in 9594 kg/h H O into a spray
roasting reactor, in which the thennal decomposition is effected at a temp-
erature of 700-900C. HCl at a rate of 1963 kg/h, water at a rate of 9110
kg/h, and as end product 1083 kg/h MgO with 118 kg/h CaCl can be withdrawn
from the reactor. The MgO can be freed from the calcium chloride by being
washed with water, and the resulting spent washing liquor may be re-used in
the process if this is desired. The hydrochloric acid recovered in the spray
reactor may be used in the dissolving vessel to dissolve new material and, if
desired, may previously be absorbed in that CaC12-containing spent washing
liquor.
Ex~mple 2_
11,0?3 kg hydrochloric acid having a concentration of about 20 %
(pH below 1, 1963 kg HCl, 9110 kg H O) are fed into a vessel which has an
acid-resistinglining and is provided with a stirrer. 2746.2 kg raw magnesite
and thereafter 238.8 kg caustic magnesia are then added to the hydrochloric
acid. The raw magnesite and the caustic magnesia have the respective com-
positions stated in Example 1.
The pH-value of the hydrochloric acid is increased to 1.1 by the
addition of the raw magnesite and to 5.3 by the subsequent addition of
caustic magnesia. The temperature at the reaction is 80-85 C. During the
feeding of the caustic magnesia, simultaneously a mixture of air and chlorine
gas is fed into the vessel, so that the sesquioxides are precipitated.
The solution obtained in the dissol~ing vessel contains 2365 kg
MgC12, 231 kg CaC12, and 558 kg insoluble matter and sesquioxides in 9594 kg
1093Z77
B ~ha~e~/
H20 and is ~*h~ into a second vessel, in which calcium carbonate is to be
precipitated and which is fed at the same time with 1328 kg C02 from the dis-
solving vessel. The feeding of 82 kg caustic magnesia, which contains 41 kg
insoluble matter and sesquioxides, into the second vessel results in a preci-
pitation of 101 kg calcium carbonate within about 1 hour. This calcium car-
bonate is withdrawn together with 599 kg insoluble matter and sesquioxides.
A solution of 2462 kg MgC12 and 118 kg CaC12 in 9594 kg H20 is
withdrawn from the second vessel and is transferred into a spray roasting
reactor, in which a thermal decomposition is effected at a temperature of
700-900 C. 1888 kg HCl, 9110 kg water, and an end product consisting of a
mixture of 1041 kg MgO and 118 kg CaC12 are withdrawn from the reactor.
