CS263071B1 - Process for preparing a solution for crystallization of potassium sulfate - Google Patents

Abstract

A supersaturated solution of potassium chromium sulfate or a supersaturated solution containing the products of dissolution of farochrome in sulfuric acid and potassium arbutate is subjected to electrolysis on a graphite or titanium electrode, so that ηα 1 liter of the processed solution falls during electrolysis to at least 500 C.

Description

The present invention relates to a process for preparing a solution for the crystallization of potassium chromium sulfate.

Potassium chromium sulfate dodecahydrate crystallizes very reluctantly from the aqueous solution. The slow formation of crystal nuclei is due to the nature of chromium ions, which form various multinuclear complexes, with oxygen and sulfate bridges. The presence of such complexes in the solution is accompanied by a green color, while the potassium chromate dodecahydrate crystals are violet. The rate of formation of violet hexachlorochromite ions, and hence the creation of the necessary conditions for the construction of the crystal lattice, is dependent on the rate of bridge breakdown. The spontaneous establishment of phase equilibrium in the formation of the crystalline phase in such a system takes several tens of days. Such a long crystallization time on an industrial scale is very unpleasant and undesirable and is often shortened to the detriment of the yield of one operation.

The literature, including the patent, mentions a number of ways to accelerate the transition of green to violet: pH reduction, addition of chromium ions, action of sulfur dioxide under pressure, addition of certain mineral acids (nitric acid),

- 2263 071 age of certain organic acids (formic acid), addition of some inorganic salts (sodium or potassium nitrate), addition of some organic salts (ootane, phthalate), exposure to light or reduction of boiling in the dissolution of potassium sulphate. The action of some of these factors is indisputable (lowering of pH). The importance of other measures has been experimentally verified or theoretically justified to varying degrees.

The acceleration of the transition of the green modification to the violet and hence the acceleration of crystallization can be achieved by the action of chromium ions, which can be added to the solution before crystallization or directly electrolytically generated therein as described in jap. U.S. Pat. No. 60-69128. The Japanese patent cited generally describes the manufacture of chromium alum, starting with the removal of iron from the solution formed by dissolving ferochrome (a chromium-iron alloy containing other impurities) in sulfuric acid. The iron is separated by crystallization upon addition of additional salt to the system (e.g., ammonium sulfate) in the form of a double ammonium iron sulfate, which is a by-product of the process. The treated ferrous solution is subjected to electrolysis for as long as there is a potential difference on the electrodes. In electrolysis, the catholyte and anolyte are separated by a membrane. After electrolysis, a sulfur salt or sulfur dioxide is added, followed by crystallization.

The disadvantage of the Japanese patent is the much higher consumption of added sulphate (e.g., ammonium, potassium) associated with the production of the corresponding double ferrous sulphate. Perhaps with the exception of ammonium ferrous sulfate (Mohr's salt), the market for other double salts will be difficult. Problematic is also the addition of sulfites, which in addition to higher raw material costs

263 071 can accumulate in the system and contaminate both products when cyclically conducting the mother liquor recovery process after ferrous sulfate crystallization.

A simpler process for preparing a solution for crystallizing potassium chromium sulphate by generating chromium ions electrolytically in a membrane or diaphragm electrolyzer according to the present invention is characterized in that the supersaturated potassium chromium sulphate solution or supersaturated solution containing ferrochromic solution in sulfuric acid and potassium sulphate is subjected to el. on a graphite or titanium electrode so that at least 500 ° C per 1 liter of the solution to be treated is crystallized after electrolysis.

The advantage of the solution according to the invention, in particular in comparison with the Japanese patent, is that a solution can be crystallized which, in addition to the required components, also contains a significant iron content without affecting the quality of the product, i.e. potassium chromium sulfate dodecahydrate. Nor is it necessary to carry out an additional reduction of the solution with sulfite ions or sulfur dioxide. The reduction in crystallization time required to eliminate the same amount of alum under the conditions described by the present invention and the Japanese patent is comparable.

Despite the entire implementation of the preparation of the solution for crystallization according to the invention, and thus, in the electrolytic generation of chromium ions alone, foreign ions are not introduced into the solution. At the cathode, chromium ions are reduced to chromium ions, or hydrogen is excreted, which in the state of birth can easily reduce chromium ions to chromium ions. Excessive cathode excretion, even at high current densities, practically does not occur as a result4

263 071 to high chromium ion concentration in supersaturated solution ·

Separate anode space in solution does not produce chromate ions, which could immediately oxidize chromate ions. An anode of platinum titanium or a lead alloy with one percent of silver produces oxygen that does not pollute the atmosphere. Electrolysis takes place at a small inter-electrode distance, power consumption and heating of electrolytes are not great, even when working with a very concentrated solution. The acceleration of crystallization of potassium chromium sulfate dodecahydrate depends on the concentration of the supersaturated solution. The more concentrated the solution, the smaller but still significant the acceleration of crystallization.

The invention can be utilized without major modifications to existing production equipment. Directly into the solution in the crystallizer, the electrodes are immersed together with the anode chamber, the volume of which is substantially smaller than the volume of the crystallizer. it is also possible to use a relatively small electrolyzer whose cathode space supersaturated solution only flows during continuous crystallization.

The size of the electrode surfaces and the electrolysis surface is chosen so that a charge of at least 500 G is charged per 1 liter of supersaturated solution. For example, for a 10,000 l solution, 24 hours of electrolysis and a current density of 0.1 A / cm · For the flow cell, the residence time of the solution in the cathode compartment and the current load must be selected by analogy so that at least 500 C per liter of solution is again obtained.

The anode compartment contains a dilute solution of sulfuric acid or a salt thereof. The whole process can be run discontinuously or continuously.

263 071

Example 1

A closed laboratory electrolyzer with separate electrode compartments and vertically placed electrodes was used for the electrolytic generation of chromium ions. The cathode was of titanium, the anode of platinum titanium. 60 cm · Both electrodes were made of titanium wires with a diameter of 3 mm and a length of 5 cm, which were fixed to a titanium beam with 2 mm gaps. The inter-electrode distance was 2 cm. The anolyte was 0.5 L of a dilute HgSO4 solution (50 ml conc. HgSO4 / L) which was separated from the catholyte by a NAPION 427 cation exchange membrane. The current load was 6 A for 10 minutes, i.e. 0.1 A / cm, terminal voltage approx. 6 V.

The catholyte was prepared by dissolving 1.2 kg of potassium alum

After boiling for 2 hours, the solution was cooled to room temperature and adjusted to 1 liter. Half of the solution was used as a catholyte in the above-described electrolyzer (solution B), the other half of solution (solution A) was not electrolytically treated. Crystallization of potassium chromium sulfate dodecahydrate in both solutions was observed. The process was as follows:

Crystallization time (day) Crystals weight (g)

1 day

Day 2 · Day 3

Day 6

7th day

97.6

149.7

158-, 3

269.9

310.4

438.5

513.2 482.1

530.3

496.6

B / A

The data shows that crystallization of solution B is more intense than crystallization of solution A.

263 071

Example 2

The catholyte was prepared by dissolving potassium chromium alum and ferrous sulfate in distilled water by boiling for 2 hours. This gave 1.5 L of a solution with a density of 1590 kg / nr (at 20 ° C) containing 6.25 wt. Cr and 2.32 wt. Pe. The solution was divided into three equal portions. One of them (solution C) was used as a catholyte in the same electrolyzer as in Example 1. The current load was 10 A for 1 minute. The second part (solution D) was treated in the same electrolyser at a current load of 10 A for 5 minutes. The third part (solution E) was not electrolytically treated. The amount of crystals deposited from each part of the solution was as follows:

Crystallization time (hour) Mass C of crystals D (G) E 20 May 63 70 0 44 147 149 60 66 215 210 140 92 260 237 203

SUBJECT OF THE INVENTION

Claims (1)

SUBJECT OF THE INVENTION A process for preparing a solution for the crystallization of potassium chromium sulphate by chromate ion generation electrolytically in a membrane or diaphragm electrolyser, characterized in that the supersaturated potassium chromium sulphate solution or supersaturated solution containing the products of ferochromium dissolution in sulfuric acid and potassium sulphate is electrolyzed on graphite or titanium per 1 liter of the solution to be treated was electrolysed at least 500 G, after which the treated solution was crystallized.