CS257665B1 - A method for purifying crude magnesium sulfites - Google Patents

Abstract

Purification of crude magnesium sulfite by separating solid impurities from a metastable solution of magnesium sulfite hexahydrate with a high concentration of up to 10 wt. %, from which a very pure salt of up to 99.8 wt. % of magnesium sulfite is obtained by crystallization, is carried out by rapid heating of the entire volume of the suspension of magnesium sulfite in water or in magnesium sulfate, carried out in such a way that the temperature exceeds 80 °C in a time shorter than 10 seconds, preferably shorter than Z seconds.

Description

The invention solves the purification of the crude magnesium sulphite hexahydrate, possibly with the addition of magnesium sulphite trihydrate.

Magnesium sulphite is an important intermediate step in some large-volume chemical processes, such as the desulphurization of magnesite gases. However, such magnesium sulphite is often heavily contaminated with natural magnesite impurities and flue gas impurities. drizzle applications require pure magnesium sulphite and purification methods have been developed for this purpose. Magnesium sulphite hydrates are in low stability in their equilibrium with their solution. 209 952 and 251 456 utilizes the ability of magnesium sulfite hexahydrate to form a highly stable, high-concentration solution up to 10% by weight. from which, after separation of the solid impurities, very pure salt (99.8% by weight) crystallizes. magnesium sulphite - calculated on the anhydrous basis, either by cooling the hexahydrate or by keeping the solution at elevated temperature, the magnesium sulphite hexahydrate is converted into a metastable solution by homogeneously heating the entire volume of the magnesium sulphite suspension.

Magnesium sulphite trihydrate does not form an inetastable solution, therefore known processes assume the absence of magnesium sulphite trihydrate in the starting crude hexahydrate as necessary.

condition for the process. Indeed, if trihydrate is present, it precludes the elimination of hexahydrate as a result of its vaccination effect, which results in the formation of sparingly soluble magnesium sulphite trihydrate in industrial plants when the system is heated to a temperature of 60 - 120 ° C, necessary to form a metastable solution; , the metastable solution is formed, but only for a short period of time, and the device overgrown with magnesium sulfite trihydrate crystal ingrowth.

In real industrial processes according to these known processes, magnesium sulfite hexahydrate is converted to trihydrate whenever it is operated at higher temperatures (e.g., flue gas desulfurization) above 42 ° C, when magnesium sulfite trihydrate is already a thermodynamically stable phase. However, magnesium sulfite hexahydrate formed after recrystallization of this trihydrate almost always contains an admixture of undesired trihydrate.

The above-mentioned drawbacks are overcome by the solution according to the invention which is based on the rapid heating of the suspension of crude hexahydrate in water or in magnesium sulphate solution, so that the temperature exceeds 80 ° C in less than 10 seconds, preferably less than 2 seconds. Rapid temperature exceeding 80 ° C can be achieved, for example, by suddenly mixing the heat-carrying stream (s) with the stream of the crude magnesium sulfite hexahydrate suspension.

Preferably, water vapor having a high enthalpy is used as a heat transfer stream. It is also possible to use a solution after crystallization of pure magnesium sulphite hexahydrate or trihydrate. The solution after crystallization of the hexahydrate is cold and must therefore be heated with a stream. The solution after crystallization of the trihydrate is warm and its heating no longer requires little heat, so in this case energy is saved.

If the sludge of impurities contains a high content of magnesium sulphite trihydrate, the trihydrate is subjected to hydration in an aqueous or magnesium sulphate suspension, and the resulting crystals of impure magnesium sulphite hexahydrate after separation from the solid impurities sludge by sedimentation or other treatment techniques are recycled. This achieves an overall utilization rate of magnesium sulfite of 95 to 98

The process of the invention takes advantage of the fact that the known crystalline form of magnesium sulfite trihydrate is stable up to 80 ° C, while other magnesium sulfite hydrate is stable above this temperature. The present trihydrate admixture cannot be used as a nucleating agent when the temperature is rapidly exceeded 80 ° C.

The process according to the invention makes it possible to purify the crude magnesium sulphite hexahydrate even with an admixture of its trihydrate, which was known to be unprocessable according to known methods. For example, it is possible to purify the crude hexahydrate containing up to 40 ‘mol. trihydrate of the total magnesium sulphite content.

However, the process according to the invention is also important for the purification of the crude trihydrate-free magnesium sulphite hexahydrate, since the sudden mixing of the hexahydrate suspension with the stream (s) bringing heat thereby achieving a rapid temperature exceeding 80 ° C increases the lifetime of the metastable solution. then safely performed on technical equipment. An advantage is also the reduction of process failure due to the formation of deposits of magnesium sulfite trihydrate in pipes and apparatuses.

and

Example ¹

The suspension of crude hexahydrate, which was formed by crystallization of the crude trihydrate at 30 ° C for 4 hours, contained 26.3%. wt. solids suspended in a solution containing 11 wt.% magnesium sulphate and 1 wt. magnesium sulphite. Solid phases included

71.9% hexahydrate; trihydrate and 10.7 masses, in meta (a stable solution of magnesium sulfite of insoluble compounds of Ca, Fe, Mg, Si and other elements derived from natural magnesite.

Of the total magnesium sulphite content, 24.5 mol% was present. trihydrate. The suspension was mixed in a continuous mixer with a solution containing 9 wt. of magnesium sulfate and water vapor at a rate of 15 liters / min of suspension and 10 liters / min of solution. The mixer was a 1000 mm long vertical tube with an internal diameter of 50rom, terminated at the bottom with a bottom tangent and provided with three tangential sockets for the slurry, solution and steam supply. At the other end of the mixer, 25 cm above the bottom of the tube, a thermometer was built into the outflow. The steam flow rate was automatically controlled so that the exit stream temperature was 89-91 ° C. The resulting suspension of solid impurities in the meta-stable magnesium sulfite hexahydrate solution was fed from the mixer to a centrifuge where the sludge was separated. the iletastable solution contained 5.5 wt. magnesium sulphite. Of the total magnesium sulphite in the feed suspension, it was transferred to a metastable solution of $ 68, leaving the residue with insoluble substances in the sludge as a trihydrate.

The process according to the invention can be used in the production of cellulose and in the production of pure magnesium oxide, since the recrystallized magnesium sulphite according to the process of the invention is very pure, the maximum content of main impurities being in mass: Ca 0.1; Fe 0.05; Vin 0.05. It is thus suitable for use in the magnesium bisulfite process for the production of cellulose and for the purification of the magnesium sulfite produced by the magnesite. the method of combustion of flue gases in thermal power plants,

Claims (1)

Process for purifying a crude magnesium sulfite by separating solid impurities from a solution of magnesium sulfite hexahydrate in a metastable state, characterized in that heating the suspension of the magnesium magnesium sulfite hexahydrate or a mixture thereof with magnesium sulphite trihydrate in water or magnesium stearate solution is carried out so rapidly less than 10 seconds, preferably; less than 2 seconds, the temperature has exceeded 80 ° C.