Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~197663
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The present invention relates to the pro-
duction of potassium sulfate from magnesium sulfate.
More specifically, it deals with an improved economlcal
process for producing K2S04 from the intermediate
(NH4)2S04 generated in the course of recovering useful
magnesium salts from magnesium sulfate.
PRIOR ART
The method of recovery of magnesium compounds
from magnesium sulfate is well known (for example US
Patent 3 338 667) and calls for the action of ammonia
and ammonium carbonate on the starting magnesium sulfate
to give hydromagnesia and ammonium sulfate according to
the following equation:
g o4 3(NH4)2C03 + 2N~140H + 3H20
3MgC03 Mg(OH)2 3H20 + 4(NH4)2 4
The hydromagnesia is quite insoluble and is
separated from the solution by filtration. The solution
of (NH4)2S04, containing the excess (NH4)2C03 and NH40H
required for the complete precipitation of hydromagnesia
is then the starting material for the production of
K2S04 .
It is obvious that the production of K2S04
from (NH4)2S04 can be achieved by a displacement
reaction starting from the abundant and relatively cheap
KCl, in accordance with the following reaction:
(NH4)2S04 + 2KCl ` K2S04 + 2NH4Cl
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This basic reaction has been reported previ-
ously with similar starting materials such as calcium
sulfate (US Patent 3 445 185), mixtures of magnesium
sulfate and potassium sulfate (US Patent 3 436 175, US
Patent 3 843 772~, or even magnesium-ammonium phosphate
(US Patent 1 820 160).
In these processes, the displacement of the
equilibrium in favor of the precipitation of K2SO4 is
achieved in two ways.
In a first approach, the addition of methanol
or other organic solvents or organic substitutes induce
the precipitation of the desired K2SO4 GUS Patents
3 445 185, 3 843 772, 3 436 175, 2 906 603). Such pro-
cesses are not of commercial interest because of the
high cost of these organic materials or solvents and the
problems associated with their recovery. This recovery
of organic solvents involves distillation of large
amounts of water, a very expensive operation and is
never really quantitative. Furthermore, such a pro-
cedure would necessary be reflected in a very adverse
way on the price of the resulting K2SO4.
Another process involves the precipitation of
K2SO4 by makiny the solution very rich in ammonia (US
Patents 1 820 160, 2 882 128). It is known (J. Am.
Chem. Soc., 59, 2096 (1937)) that K2SO4 is rather
insoluble in concentrated solution of ammonia. This
approach is also well suited to the ammonia/ammonium
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sulfate solution obtained from the recovery of hydro-
magnesia since an excess of ammonia and ammonium carbon-
ate is needed in order to assure the quantitative pre-
cipitation of hydromagnesia and, therefore, is already
present in the ammonium sulfate solution.
However, the procedures as described in these
two patents are not operational for the following
reasons. On one hand, high concentrations of ammonia
are required to insure the completeness of precipi-
10 tation. This is shown by the results of Hill and Loucks(J. Am. Chem. Soc., 59, 2096, (1937)~ reported in
Table I.
TABLE I
SOLUBILITY OF K2SO4
IN AQUEOUS AMMONIA
Weight Weight
96 NH3 % K2S04
0 10.80
13.90 1.286
18.20 0.639
22.35 0.421
24.83 0.220
27.04 0.1~9
On the other hand, the commercial solutions
of NH40H are of the order of 30% maximum. In order to
raise the concentration of NH3 to 279~ in the NH40H
solution with 30% NH3, it will be necessary to use ex-
cessively large volumes of 3096 NH3 solution. That would
make the whole operation quite impractical, part1cularly
~9~6~3
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at the recovery stage of ammonia since the starting
volume of the ammonium sulfate solution would then have
been increased by a factor of the order of ten with the
addition of the required 30~ NH3 solution.
Of course, gaseous ammonia could be used. But
at the recovery stage, this ammonia would be again in
solution and only a complex and expensive process, as
known to those familiar in the art, could be used in
order to recycle the NH3 in the anhydrous form.
SUMMARY OF THE INVENTION
In accordance with the present invention,
there is now provided an improved process which avoids
the necessity of adding unduly large volumes of fresh
ammonia solution to the starting ammonium sulfate
solution. Essentially, this process involves the use of
the starting ammonium sulfate solution as a dissolving
agent for the gaseous ammonia obtained from a subsequent
recycling step thereby to produce an ammonium sulfate
solution enriched in ammonia and in a suitable volume
for subsequent reaction with potassium chloride and pre-
cipitation of the desired potassium sulfate thus also
producing a volume suitable for the economic recovery of
the gaseous ammonia from the filtrate obtained from said
reaction.
DETAILED DE5CRIPTION OE THE INVENTION
More specifically, it is the essence of the
present invention to provide a process for obtaining a
~L~976~3
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high yield of potass.ium sulfate by the reaction of an
ammonia enriched solution of ammonium sulfate and
potassium chloride, the ammonia enriched solution of
ammonium sulfate being of a volume size such that after
removal of the precipitated potassium sulfate, the
volume of the filtrate comprising ammonium chloride and
ammonium hydroxide is such that the recovery of ammonia
is commercially feasible.
It has also been found, in accordance with the
present invention, that by using an ammonium sulfate
solution as an absorbent for the recycled ammonia, huge
volumes of water do not have to be used to produce an
ammonia enriched solution of ammonium sulfate in oppo-
sition to the volume of water which would be required to
attain an ammonia enriched solution of ammonium sulfate
of the same ammonia concentration if a commercial
ammonia solution was used. According to the novel
improved process the present invention there is also
provided a volume of filtrate which can be economiCally
treated to recover ammonia suitable for recycling in the
first step of the process of the present invention.
It is also a feature of the present invention
that by using an enriched ammonia solution of ammonium
sulfate there is first obtained a higher yield of
potassium sulfate. Secondly, because of the lower
volume of filtrate after removal of the precipitated
potassium sulfate there is provided a most economic
~7~6~3
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procedure for recovering ammonia for recycling purposes
in the present invention.
The recovery operation of the ammonia is also
simplified by adding lime to the resulting ammonium
chloride solution thus increasing the basicity of the
filtrate allowing for the liberation of further amounts
of ammonia from the chloride ion and facilitating the
operation by decreasing in a very substantial way the
volume of water to be distilled to reclaim the ammonia.
The reactions involved in the various steps of
the present invention for the formation of potassium
sulfate and recovery of ammonia are illustrated as
follows:
(NH4)2S04 + 2KCl + NH40H(excess) >
K2S04~ + 2NH~Cl + NH40H(excess)
2NH4C1 + NH40H(excess) + l.lCa(OH)2 >
CaC12 + O.lCa(OH)2 NH40H(excess)
The advantages of operating in the presence of
an excess of lime are illustrated in Table II where the
amount of water to be distilled in order to recover
ammonia from its solution is shown. The advantage of
operating in the presence of lime is obvious from the
examination of these data.
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TABLE II
Percentage of a 25% NH3 Percentage of evaporated
solution distilled (%) ammonia
_
In the presence In the absence
. of CaO of CaO
_ _ _
1.9 53 65
73 81
9 90 87.5
96.96 ~9
99.64 89.5
99.8
99.92
It can be noted that in the presence of an
excess of Ca(OH)2, after the distillation of 20% of the
water from the NH3 solution, the recovery of ammonia is
essentially complete whereas without Ca(OH)2, only 90%
of the ammonia has been recovered after distillation of
20% of the ammonia solution.
THE AMMONIA ENRICHMENT
The starting stock solution of ammonium
sulfate has an ammonia content of from 13 to 17~ by
weight. After its enrichment with gaseous ammonia its
ammonia concentration is about from 29 to 31% weight
without significant increase in volume, with a content
of about 30% by weight of ammonia being preferred.
76~3
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The enrichment step must be carried out prior
to the reaction with potassium chloride to allow for the
full enrichment in ammonia of the ammonium sulfate
solution. This is done by passing the ammonium sulfate
solution in a scrubbing tower with gaseous ammonia
circulating in countercurrent of the ammonium sulfate
solution. It should be appreciated that the reaction
between the ammonium sulfate and the potassium chloride
must not take place in the scrubbing tower because the
immediate precipitation of potassium sulfate would
prevent the circulation of the ammonia and the solution
by obturating the trays or the lining of the scrubbing
tower.
It will also be appreciated that since the
absorption of the ammonia in the ammonium sulfate
solution is an exothermic reaction the scrubbing tower
must therefore be equipped with cooling units to favour
the absorption of the ammonia at low temperatures. It
is well obvious to those skilled in the art that pre-
cipitation must be avoided in such a system.
DESCRIPTI02~ OF DRAWING
A stock solution 10 of ammonium sulfate con-
taining ammonium carbonate obtained from the carbonation
of a magnesium sulfate solution 11 or from any other
source is cooled to a temperature between about 5 and
10C in a heat exchanger 12 to improve the capacity of
the ammonium sulfate to dissolve ammonia. After
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g
cooling the ammonium sulfate solution is introduced
into a scrubbing tower 14 equipped with plates or other
standard lining so that ammonia vapors 16 admitted
countercurrently are absorbed by the ammonium sulfate
solution in the scrubbing tower 14. Since there is a
substantial heat accompanying the solution of the
ammonia 16 it is useful to have a heat exchanger or
equivalent arrangement at the level of each plate in
order to keep the ammonia scrubbing solution at a
temperature of from 5 to 10C, preferably about 7C.
The ammonium sulfate solution saturated with ammonia 17
is withdrawn and directed to a reactor 18 to which is
then added potassium chloride 20 thereby causing immedi-
ate precipitation of potassium sulfate. The reaction
mixture from the reactor 20 is subjected to filtration
21 or any suitable separation means to separate the
slurry of potassium sulfate 22 from the mother liquor
24. The slurry 22 of potassium sulfate from the solid
is then washed with water 26 saturated with ammonia and
the washing 28 are then recycled to reactor 18 while the
solid potassium sulfate 30 is recovered.
The mother liquors 24 are then directed to a
two-stage stripper 32 where the temperature is raised to
55C and then to 90C. From the zone 32A of the two-
stage stripper 32 heated at 55C there is recovered a
portion of ammonia gas 34 which is directed to an
ammonia reservoir 36. From the zone 32B of the two-
~766~3
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stage stripper 32 heated at 90C there is recovered ahot ammonia solution 38 which is distilled in flash
boiler 40 to which lime 42 has been added. Calcium
chloride 44 is formed while after distillation of about
25~ of the volume of water there is obtained an essen-
tially quantitative recovery of gaseous ammonia 46 which
after cooling in a heat exchanger 4~ can be returned to
the ammonia reservoir 36. The ammonia 16 can be return-
ed to the scrubbing tower 14 for the saturation of a new
batch of magnesium sulfate stock solution 10. The two-
stage stripper 32 will also yield a mixture of ammonia
and carbon dioxide 50 which upon cooling below 60C will
give ammonium carbonate, which can be directed to the
carbonation of the starting magnesium sulfate solution
11 .
EXAMPLE 1
The process of the present invention will be
more readily understood by referring to Table III which
illustrates the various steps described herein and in
Figure 1.
~97~63
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