Process for producing pure magnesium sulphite

A process for producing a pure magnesium sulphite from crude crystalline magnesium sulphite. Into a full suspension or flow containing magnesium sulphite crystals at least one further water-containing and heat-supplying flow is led. The heat-supplying flow rapidly increases the temperature of the suspension to above 80.degree. C. in less than 10 seconds, preferably in less than 2 seconds. The solid water-insoluble fraction is then separated from the resulting flow, and the pure product is recrystallized from the solution. The solution may be recycled into the process either for further purification or for use as at least one heat-supplying flow.

The invention relates to a process for producing pure magnesium sulphite 
from a crude flow of raw magnesium sulphite hexahydrate by maintaining a 
supersaturated metastable solution through rapid heating by a second flow, 
so that solid impurities are separated and a pure product is obtained by 
crystallization, without recrystallization of undesirable byproducts. The 
process may also be used to purify hexahydrate having admixtures of 
magnesium sulphate trihydrate. 
BACKGROUND OF THE INVENTION 
Magnesium sulphite is an important intermediate in many large-scale 
chemical processes, such as the desulphurization of gases and air 
pollutants by the magnesite process. However, magnesium sulphite is in 
many cases considerably contaminated by solid impurities from the starting 
magnesite and other impurities from flue gases. Industrial applications 
require pure magnesium sulphite, and purification methods have been 
developed to meet this need. 
The removal of impurities from easily soluble crystalline substances on the 
basis of solubility differences occurring at different temperatures is 
known. J. W. Mullin, Crystallization, CRC Press. These processes relate to 
the separation and subsequent crystallization of dissolved substances in 
solutions having concentrations that are as close as possible to the 
equilibrium determined by the temperature and pressure to which the 
solution is exposed. A disadvantage of this method is that it requires a 
considerable amount of time for the dissolving and crystallization steps, 
in order to obtain a sufficient yield. 
Magnesium sulphite is highly insoluble; only 0.7% by weight readily 
dissolves. The magnesite impurities contained in raw magnesium sulphite 
are also highly insoluble. The equilibrium concentrations of these 
materials in solution is thus very low. As a result, traditional 
separation methods based on differential solubility in water (as taught by 
Mullin) have not been successful, and are not used in the industry. 
It is also known that the solubility of some substances (such as 
MgSO.sub.3) is anomalous under certain temperature-dependent conditions, 
and that transient supersaturated metastable solutions can arise when 
these conditions are met. See Czechoslovak Author's Certificate No. 
209,952. However, such metastable solutions have been unpredictable, very 
short lived, difficult to control, and they rapidly revert to equilibrium 
solutions by crystallization of the excess. This phenomenon has not been 
efficient when used in a commercial process because of the serious 
recrystallization problem. The known process requires the rapid 
dissolution of magnesium sulphite crystals in water, and the rapid 
separation of impurities from the resulting warm solution. The preparation 
of this solution on an industrial scale has met with many difficulties. 
The required supersaturated solution could not be induced at all upon 
dissolution in a stirred heated charging reactor. Instead, 
MgSO.sub.3.6H.sub.2 O was immediately recrystallized into 
MgSO.sub.3.3H.sub.2 O. 
Dissolution in a through-flow tubular reactor produced a supersaturated 
solution, but only for a very short time after start-up. The concentration 
of the solution then decreased, and the device had to be taken out of 
operation after several hours because the piping became encrusted with 
MgSO.sub.3.3H.sub.2 O crystals. A major disadvantage of the known method 
resides in the undesirable conversion of hexahydrate into trihydrate, with 
the resulting accumulation of trihydrate in the pipes during heating. This 
causes interruption of the process for periodic and laborious cleaning of 
the pipes. 
The known process is successful only if the extraction and separation of 
impurities can be achieved rapidly, within the short life of the 
metastable solution. In addition, the known process is disadvantageous 
because it requires that the raw hexahydrate be free of magnesium sulphite 
trihydrate, since the presence of trihydrate produces a seeding effect 
which prohibits the extraction of hexahydrate and produces a poorly 
soluble trihydrate in industrial plants at temperatures of from 60.degree. 
to 120.degree. C. 
Other apparently promising methods of obtaining a useful metastable 
solution have not been successful. Indirect heating in a continuous heat 
exchanger results in rapid fouling and clogging of the exchanger with 
incrustations of trihydrate. Precipitate also accumulates in a batch 
reactor, or in a batch reactor used in combination with a heat exchanger. 
In order to obtain the most efficient use of the metastable solution, it 
is necessary to minimize (and if possible completely avoid) the gratuitous 
conversion of hexahydrate into trihydrate. This objective is particularly 
difficult in the known processes, because the temperatures they disclose 
encourage the formation of an undesired stable trihydrate solid in 
equilibrium with the desired liquid solution. Trihydrate precipitation and 
its later removal have therefore become recognized as necessary though 
undesirable features of the art processes. 
SUMMARY OF THE INVENTION 
According to the invention, pure magnesium sulphite can be obtained from 
raw magnesium sulphite hexahydrate, based on the recrystallization of 
magnesium sulphite from a metastable solution of magnesium sulphite 
hexahydrate, by a process wherein a suspension of raw hexahydrate in water 
or a magnesium sulphate solution is heated above 80.degree. C. within a 
period of less than 10 seconds, preferably 2 seconds. Solid impurities are 
then separated from the resulting metastable solution, and the isolated 
product is processed into pure magnesium (II) salt. The rapid heating is 
achieved by abruptly mixing a heat supplying flow or flows with a primary 
flow of the raw hexahydrate in suspension. The heat supplying flow can 
preferably be water vapor and/or a recycled solution remaining after the 
recrystallization of pure magnesium sulphite from the metastable solution. 
The metastable magnesium sulphite solution is cooled before 
recrystallizing the hexahydrate or the hexahydrate with trihydrate, but 
the magnesium sulphite trihydrate is allowed to crystallize from the hot 
metastable solution without intentional cooling. Another metastable 
solution producing additional pure product can be prepared by reheating to 
above 80.degree. C., as described above. 
According to the professional literature, cf., D. Trendafeloff et al., 
Chim. Ind. Vol. 46 No. 10, page 438 (Sofia: 1974), magnesium sulphite 
dihydrate is stable at temperatures above 80.degree. C. According to the 
invention, magnesium sulphite trihydrate is stable below 80.degree. C., 
but if this temperature is rapidly exceeded, the trihydrate cannot act as 
an undesirable seeding agent. Thus, the present invention permits the 
purification of raw hexahydrate with an admixture of trihydrate --which 
cannot be achieved according to the method provided in Czechoslovak 
Inventor's Certificate No. 209,952. This improvement broadens the 
substrates available for use in the process, and provides for advantageous 
and efficient recirculation of the solution remaining after isolation of 
the product. 
For example, it is possible to purify (a) raw hexahydrate containing up to 
40 mol. % of trihydrate from the overall magnesium sulphite content, or 
(b) trihydrate after recrystallization to hexahydrate. Since magnesium 
sulphite trihydrate becomes thermodynamically stable at about 40.degree. 
C. and above, its formation is frequent in industrial processes, such as 
the magnesite process of desulphurization of flue gases, and the process 
of the invention is therefore quite important when industrially applied. 
The present process is also important for purifying hexahydrate without any 
trihydrate admixture. The invention provides for a rapid mixing of the 
hexahydrate suspension with a second heat supplying flow or flows, and the 
very rapid temperature increase to above 80.degree. C. prolongs the life 
of the metastable solution in a manner not heretofore known in the art. 
Once the prolonged metastable solution is produced, the separation of 
solid impurities is a relatively simple matter. The process is also 
advantageous because it decreases the rate of failure of the industrial 
purification apparatus, by preventing sedimentation of trihydrate in the 
pipes. 
According to the present invention, trihydrate deposits are prevented by 
homogeneous heating of the entire volume of the flow, so that subsequent 
precipitate removal is not necessary. Instead of conventional contact 
heating, a second watercontaining flow, such as steam or heated water is 
used. The second flow is introduced to the first flow, or liquid phase, of 
aqueous MgSO.sub.3.6H.sub.2 O in an amount and temperature chosen to 
produce, upon intermixing of the two phases, a flow with a temperature in 
excess of 80.degree. C. (preferably 90.degree.-120.degree. C.). The 
magnesium sulphite hexahydrate concentration in the initial suspension is 
selected so that the MgSO.sub.3 content is in excess of 2% (preferably 7%) 
by weight after addition of the second phase (water vapor) and its 
condensation and intermixing with the first phase. Water vapor is a 
preferred heat-supplying flow because of its high enthalpy. In a cyclical 
process, according to the invention, the heat-supplying flow can be 
obtained from the warm solution remaining after the crystallization of 
pure product, thereby minimizing the amount of energy needed to produce 
the desired heated flow. 
A particular advantage of the process in accordance with the invention is 
the fact that heating does not take place through the mediation of a heat 
exchanging surface whereupon MgSO.sub.3.3H.sub.2 O and further salts would 
crystallize out as a result of overheating. The whole volume of the 
suspension is homogeneously heated. The heating of the suspension to the 
required temperature is also practically immediate, so that the life of 
the metastable solution of magnesium sulphite is prolonged for efficient 
separation of solid impurities. Another advantage of the invention is that 
the necessary apparatus is relatively small and simple. 
Steam can be used for heating the suspension of magnesium sulphite crystals 
because the consumption of steam is low due to its high thermal content; 
and the solution is only negligibly diluted. To prevent undue thickening, 
it is advantageous to dilute the flow to a content of from 2 to 15% by 
weight of MgSO.sub.3 in the final outlet flow. It is then advantageous to 
lead overheated hot water or waste warm water into the flow of suspension 
containing magnesium sulphite crystals. The introduction of steam can be 
completely abolished, or if it is used, it may be used only for equalizing 
thermal balance. Besides water and steam, aqueous solutions can also be 
used, preferably solutions of MgSO.sub.3 or Mg(HSO.sub.3).sub.2 or 
MgSO.sub.4 or a combination of these salts. This enables waste diluted 
solutions to be employed, which originate with magnesium sulphite 
processing. In this manner, the excess components are reintroduced and 
recirculated into the process. 
A liquid phase cleared entirely or to a substantial extent of solid water 
insoluble impurities is considered a pure magnesium sulphite solution. 
Dissolved salts such as Mg(HSO.sub.3).sub.2 are not prejudicial to further 
treatment or industrial use, and are not considered impurities. 
The obtained pure supersaturated magnesium sulphite solution can change 
spontaneously and relatively quickly into a stable state accompanied by 
the formation of magnesium sulphite crystals. Either MgSO.sub.3.6H.sub.2 O 
or MgSO.sub.3.3H.sub.2 O or a mixture of both, if need be, will result, 
depending upon the protocol. The crystals are extremely pure and contain 
up to 99.8% by weight of MgSO.sub.3 (calculated in the anhydrous state). 
Other salts, such as MgSO.sub.4 and Mg(HSO.sub.3).sub.2 remain in the 
solution. 
The metastable solution of hexahydrate can be processed to pure magnesium 
(II) salt in various ways. The most simple method involves the processing 
of pure magnesium sulphite. If the metastable solution is quickly cooled, 
a hexahydrate is obtained at a temperature below 40.degree. C. If the 
metastable solution is slowly cooled, a hexahydrate/trihydrate mixture is 
obtained at a temperature below 40.degree. C. If the temperature is 
maintained continuously at above 40.degree. C., pure trihydrate is 
obtained, while at temperatures above 80.degree. C., magnesium sulphite 
hydrate is obtained. 
When purifying raw hexahydrate with a trihydrate admixture, the yield of 
the process is less than for a single extraction of trihydrate-free 
hexahydrate. When the initial content of the raw material is from 5 to 40 
mol. % trihydrate, about 10 to 50 mol. % of magnesium sulphite is lost as 
sludge. In the absence of a trihydrate admixture, the losses to sludge are 
only 2 to 5%. When the magnesium sulphite trihydrate content of the sludge 
is high, the trihydrate may be subjected to hydration in suspension in an 
aqueous or magnesium sulphate solution. This results in crystallization of 
contaminated hexahydrate, which may be separated from the sludge by 
sedimentation, or by other known means, and which may be repeatedly 
extracted according to the invention. In this way, the final yield of 
magnesium sulphite may reach 95 to 98%. 
The process according to the invention may be employed in the production of 
cellulose and pure magnesium oxide, since the recrystallized magnesium 
sulphite produced by the process is very pure. The maximum weight 
percentages of the main contaminents are: 0.1% Ca; 0.05% Fe; and 0.50% Mn. 
The invention can also be used in connection with waste gas 
desulphurization. The invention is suitable for the magnesium bisulphite 
process, which uses ferrous types of magnesite having an iron content 
above 5% by weight of magnesium raw material. In prior methods the iron 
content limit was 1.5% by weight. The process according to the invention 
also permits the purification of magnesium sulphite produced in the 
magnesia scrubbing process used for the removal of sulphur dioxide from 
the products of coal combustion in power stations; the process results in 
the production of magnesium oxide having a purity over 98.5% by weight. 
Such magnesium oxide can be used for producing clinker MgO, as well as for 
other chemical purposes.

DETAILED DESCRIPTION 
FIG. 1 shows the behavior of the H.sub.2 O --MgSO.sub.3 system relative to 
temperature. Part A represents the known solubility curve of magnesium 
sulphite in water. See, e.g., Gmelin Handbuch der anorganischen Chemie, 
8th Ed., p. 208. Part B illustrates the solubility characteristics of 
metastable magnesium sulphite according to the present invention. 
Curve 1 represents the maximum concentration of an aqueous MgSO.sub.3 
solution in equilibrium with solid MgSO.sub.3.6H.sub.2 O. Curve 2 
represents the equilibrium of aqueous MgSO.sub.3 at higher temperatures, 
in excess of 42.degree. C. The maximum equilibrium concentration is about 
one percent by weight of MgSO.sub.3 at about 42.degree. C. The 
concentration decreases at both higher and lower temperatures. 
Curve 3 represents an ephemeral metastable and supersaturated solution of 
MgSO.sub.3, which may be induced at temperatures above 42.degree. C. The 
supersaturated solution rapidly reverts to the saturated equilibrium given 
by curve 2 with precipitation of MgSO.sub.3.3H.sub.2 O. Curve 3 reaches a 
maximum heretofore reliable concentration of 2 percent at temperatures 
above 65.degree. C. 
Section B of FIG. 1 shows experimental points corresponding to the 
concentration of MgSO.sub.3 in solutions prepared according to the 
invention. It is possible, as dislosed herein, to achieve a repeatable 
metastable solution with an MgSO.sub.3 concentration of about 8 percent by 
weight. 
The invention is further described according to a number of examples. It 
will be understood by those in the art that these examples are 
illustrative, and do not serve to limit the scope of the invention or the 
appended claims. 
EXAMPLE 1 
A suspension of crude magnesium sulphite having a temperature of 35.degree. 
C. was led into piping having a length of 4 m at a rate of 1420 kg/hr by 
using a pump. 
Composition of suspension: 
20% by weight of solid phase 
80% by weight of liquid phase 
Composition of solid phase: 
70% by weight of MgSO.sub.3.6H.sub.2 O 
30% by weight of solid impurities 
Composition of liquid phase: 
0.5% by weight of MgSO.sub.3 
0.1% by weight of Mg(HSO.sub.3).sub.2 
4.4% by weight of MgSO.sub.4 
Steam having a pressure of 0.6 MPa was lead into the piping immediately 
after the pump. The steam supply was controlled by a valve according to 
the temperature indicated by a thermometer which was disposed in the 
piping at the exit end, the temperature of the liquid being controlled so 
that the temperature of the outflowing suspension was 100+/-4.degree. C. 
Steam consumption was approximately 100 kg/hr. The delivery end of the 
piping was connected to a continuous sedimentation centrifuge which 
yielded 200 kg/hr of thickened brown suspension and 1300 kg/hr of nearly 
clear suspension. The composition of the thickened suspension was as 
follows: 
41% by weight of solid phase 
59% by weight of liquid phase 
Composition of the pure supersaturated solution was as follows: 
9.0% by weight of MgSO.sub.3 
0.1% by weight of Mg(HSO.sub.3).sub.2 
4.2% by weight of MgSO.sub.4 
0.5% by weight of solid impurities. 
EXAMPLE 2 
A raw hexahydrate suspension obtained by 4 hours of recrystallization of 
raw trihydrate at 30.degree. C. contained 26.8% by weight of solid phases 
suspended in a solution containing 11% by weight of MgSO.sub.4 and 1% by 
weight of MgSO.sub.3. Solid phases contained in a metastable magnesium 
sulphite solution are, by weight, 71.9% MgSO.sub.3.6H.sub.2 O; 17.4% 
MgSO.sub.3.3H.sub.2 O; and 10% insoluble compounds of Ca, Fe, Mg, Si, and 
other elements found in natural magnesite. The trihydrate portion of the 
overall magnesium sulphite content was 24.5 mol. %. The suspension was 
then mixed in a continuous mixer with solution containing 9% by weight of 
MgSO.sub.4 and 1% be weight of MgSO.sub.3 as well as with water vapor, the 
suspension flow speed having been 15 liters/min. and the solution flow 
speed having been 10 liters/min. The water vapor through-flow was 
automatically controlled so as to maintain a temperature of from 
89.degree. to 91.degree. C. in the mixer. A suspension of solid 
impurities in a metastable solution of magnesium sulphite hexahydrate in 
the mixer was conveyed from the mixer to a centrifuge, for separation of 
the sludge. The metastable solution contained 5.5% by weight of 
MgSO.sub.3. From the total amount of magnesium sulphite in the initial 
suspension, 68% was converted into metastable solution, while the residue 
remained together with the insoluble substances as trihydrate. 
EXAMPLE 3 
A raw hexahydrate suspension obtained by 4 hours of recrystallization of 
raw trihydrate at 30.degree. C. contained 29.2% by weight of solid phases 
suspended in a solution containing 11% by weight of MgSO.sub.4 and 1% by 
weight of MgSO.sub.3. Solid phases contained in a metastable magnesium 
sulphite solution are, by weight, 70% MgSO.sub.3.6H.sub.2 O; 18.7% 
MgSO.sub.3.3H.sub.2 O; and 11.3% insoluble compounds. The trihydrate 
portion of the overall magnesium sulphite content was 26.4 mol. %. The 
suspension was then mixed in a mixer with a solution containing 9% by 
weight of MgSO.sub. 4 and 1% by weight of MgSO.sub. 3 as well as with 
water vapor, the suspension flow speed having been 15 liters/min. and the 
MgSO.sub.4 solution flow speed having been 12 liters/min. The water vapor 
through-flow was automatically controlled so as to maintain a temperature 
of from 89.degree. to 91.degree. C. in the mixer. A suspension of solid 
impurities in a metastable solution of magnesium sulphite hexahydrate in 
the mixer was conveyed from the mixer to a centrifuge, for separation of 
the sludge. The metastable solution contained 5.6% by weight of 
MgSO.sub.3. From the total amount of magnesium sulphite in the initial 
suspension, 67% was converted into metastable solution, while the residue 
remained together with the insoluble substances as trihydrate.