CS200523B2 - Method for removal of calcium ions from chlorides solutions - Google Patents

Abstract

Calcium ions are removed from chloride solns., esp. solns. contg. over 10% MgCl2, by adding sulphate ions to a soln. at pH 2-7 and below 40 degrees C., esp. at 15-25 degrees C., i.e. room temp.; the pptd. calcium sulphate is removed from the soln. The sulphate employed is pref. in the stoichiometric ratio to the Ca++ ions present, and is obtd. from conc. or dil. H2SO4, or solid metal sulphates or solns. of the latter. If the initial acid soln. contains sesquioxides or silica, the soln. is pref. heated and a neutralising agent added, e.g. MgO, to ppte. these cpds.; the soln. is then cooled and the sulphate ions added; the pptes. can be removed separately or together from the soln. Used for the purificn. of MgCl2 solns., which are then thermally decomposed to obtain MgO and gas from which HCl can be mfd. The process may also be applied to other chloride solns.,.

Description

The invention relates to a process for the removal of calcium ions from chloride solutions, in particular from magnesium chloride solutions, which are then converted by pyrolysis to magnesium oxide (periclase) for the manufacture of refractory bricks, as well as hydrogen chloride gas returning to water or aqueous solutions. as hydrochloric acid into the process (HCl recycling) to dissolve fresh feedstock, for example magnesite MgCO 3.

Said process is selected as known to remove disturbing impurities from a chloride solution such as iron, aluminum and the like by chemical means, for example by increasing the pH.

Removal of calcium impurities is a particular problem. Attempts have been made in the art to wash calcium chloride from periclase with water, since calcium chloride does not decompose during heat treatment for thermodynamic reasons. Although scrubbing is possible, the process is associated with high costs for drying the material - after scrubbing and cleavage of the resulting Mg (OH) 2 hydrate. It has therefore been sought to remove calcium prior to pyrolysis of the magnesium chloride solution by chemical precipitation. The oxalic acid precipitation has failed, in particular, due to the too high price of oxalic acid. From a chemical and economic point of view, carbonate precipitation was obvious, but surprisingly, calcium carbonate with a solubility of about 5 g Ca ⁺⁺ / 1 was not sufficiently insoluble in a concentrated magnesium chloride solution. By increasing the pH to the alkaline region, the solubility can be reduced, but Mg (OH) 2 is already lost. Considering all circumstances, the practice therefore always reverts to calcium precipitation in the form of calcium sulfate. A special variant of this process is also the subject of the following process.

Calcium sulfate precipitation and filtration has so far taken advantage of the fact that the solubility in water decreases strongly at higher temperatures and therefore precipitates when hot. Giant. 1 shows. (according to the table in D‘ANS-LAX, Taschenbuch fur Chemiker. und Physiker, 2nd ed. 1949 Berlin, p. 927) ratios in aqueous solutions without foreign salts. Below -40 ° C there exists as the < th &quot; gypsum of Ca-SCO. 2ΗζΟ with a slightly positive tendency to dissolve; above 40 ° C anhydrite II CaSO 3 (without crystal water) with strongly decreasing dissolution tendency. Foreign salts, especially chlorides, increase the solubility of both gypsum and anhydrite, so that it is unpredictable whether the area of the gypsum with its positive or anhydrite with its negative dissolution tendency is widened.

However, there are references in the literature in the field of geological exploration of crystallized sea salts, i.e. the behavior of salts in highly concentrated and saturated sodium chloride solutions. Giant. 2 (according to ULLMANN'S EnzyklopSdie der etchnischen Chemie, 3rd ed. 1957, Munchen, Vol. 8, p. 107, Fig. 8) shows that in concentrated sodium chloride solutions · the existence area of anhydrite II is expanding strongly - from 40 ° C to 15 ° C and below, which means that the solubility of gypsum in the presence of NaCl increases more significantly than anhydrite II. This, moreover, also complies with the general rules in such systems, since, as the water concentration decreases, the existence of salts with a high content of crystalline water in favor of salts with a low content of crystalline water or without crystalline water is also generally suppressed.

Accordingly, calcium sulphate usually precipitates not only from gypsum solutions, but also from other solutions of other salts in the hot, since the temperature range of anhydrite II, with its negative solubility dependence on temperature, generally appears to increase in such solutions.

However, this does not change the technological difficulty in filtering the precipitated calcium sulfate, since the filter surfaces tend to overgrow. It sees the kinetic effect of the idea that the precipitation of the last fractions requires a long time.

The object of the present invention is therefore that, for the removal of calcium ions from chloride solutions, in particular magnesium chloride solutions containing more than 10% MgCl2, the addition of sulfate ions and the separation of the precipitated calcium sulfate are carried out by adding the sulfate ions a. the actual precipitation takes place with stirring at a pH of 2 to 7 and at a constant temperature below 40 ° C, in particular at 15 to 25 ° C. The surprising advantage of this method, which overcomes the common preconception of the need for hot precipitation, is that at least keeping the characteristics - pH between 2 and 7, maintaining a constant temperature and also stirring. for solutions. magnesium chloride - there is no seemingly kinetic conditional additional precipitation of calcium sulfate and the feared becomes an easily manageable operation without a noticeable tendency to clog the filter surfaces.

Methodological laboratory tests have also shown what is likely to be the whole phenomenon. Giant. 3 shows the surprisingly positive dependence of calcium sulfate. at a solution temperature of 70 g Mg ++ / l at a pH of about 6.5. The function can be expressed as follows:

[g Ca ++ / 1] = 0.907 + [° C]:

: 400 + [° C] -2: 9333

In other words: at least in magnesium chloride solutions, but probably also in many other higher concentration soil solutions, the solubility ratios are again the opposite; it is beneficial to precipitate calcium sulfate at a lower temperature, and the "kinetically" interpreted effect of filter ingrowth is certainly often due to cooling, i.e., a decrease in the solubility on the filter surface.

In the process according to the invention, concentrated sulfuric acid or dilute sulfuric acid has proved to be a precipitant, but also. metal sulphates, for example magnesium sulphate in solid or dissolved form. Stirring is recommended during the addition. After a few minutes you can already filter

It is also possible to remove the sesquioxides and silicic acid with them by increasing the pH of the chloride solution, for example by adding magnesium oxide before or during the precipitation of the calcium sulfate. In most cases, however, it is preferred to precipitate the sesquioxides hot, for example at 90 ° C, for the sake of better filterability, and then cold calcium. However, both precipitates can be removed together and not each separately.

Example

AND

The contaminated magnesium chloride solution, which still contains free hydrochloric acid, precipitates the sesquioxides and any silicic acid present at 90 DEG C. under agitated magnesia stirring and is filtered. The solution now contains 274.1 g / l MgCl 2, 16.6 g / l CaCl 2, and has a pH of approximately 6.5. Solid MgSO4 was used as the solid precipitant. 7H2O in equivalent amount to Ca ⁺⁺ . The salt is added under vigorous stirring after cooling to about 20 ° C. After 5 minutes stirring is stopped and after a further 5 minutes it is filtered.

The clear filtered solution no longer became cloudy after hours and days and contains 274 g / l MgCl2, 2.8 g / l CaCl2 and 2.4 g / l SOt. This corresponds to a spray pyrolysis product containing 96% MgO, 2.3 CaCl 2 and 1.7% SOT-.

Within the scope of the invention, the subject may be. the process may also be used for chloride solutions other than magnesium chloride solutions, for example aluminum chloride solutions. ..

Claims (1)

OBJECT OF THE INVENTION Process for removing calcium ions from chloride solutions, in particular magnesium chloride solutions containing more than 10% by weight. MgC12, the addition of sulphate ions and the separation of the precipitated calcium sulphate, characterized in that the addition of sulphate ions and the actual precipitation is carried out with stirring at a pH of 2 to 7 and at a constant temperature below 40 ° C, in particular at 15-25 ° C .