EP0093319B1 - Process for the electrolytic production of chlorine and caustic soda from salt containing sulphate - Google Patents

Description

Various processes are available for the electrolytic production of chlorine and sodium hydroxide solution: the diaphragm, the amalgam and more recently the membrane process. Usually, in particular in the latter two methods, the brine is circulated, part of the salt being decomposed as it passes through the electrolytic cell and the brine depleted in the electrolytic cell being strengthened again by dissolving solid salt.

The present invention relates in particular to processes for the electrolytic production of chlorine and sodium hydroxide solution from sulfate-containing salt, in which the brine is circulated.

The impurities in the salt are enriched in the brine circuit if they are not removed by continuous cleaning operations except for residual levels that are tolerable during electrolysis. After passing through the electrolysis cell, the brine is acidified and dechlorinated. In the further course of the brine cycle, there are process and plant-related variants. The majority of large-scale plants are still amalgam plants, most of them have open salt dissolving stations. In this case, an alkalization of the brine is required before reactivation in order to safely avoid emissions of residual chlorine. Usually, the necessary excess of sodium hydroxide solution is added at the time of re-saturation, so that the hydroxides of magnesium and iron also precipitate out during the dissolving process. Calcium carbonate and barium sulfate are then precipitated by adding barium compounds and soda. Soda and caustic soda are used in excess in order to achieve the most complete precipitation possible. Barium compounds are added in the deficit so that they are fully implemented. A certain sulfate content in the brine is tolerable. In the membrane process, the tolerable limit of a few g / l is determined by the membranes currently available. It is expected that sulfate-resistant membranes will also be available in the future. In the case of the amalgam process with graphite anodes, the graphite burnup, which increases with the sulfate content, determines the upper limit. When using titanium anodes, relatively high sulfate contents of up to 30 g sulfate / l can be controlled in the amalgam process.

From FR-A-2376907 a process for sulfate removal without precipitation chemicals is known for converting sodium sulfate-containing sodium chloride salt into a NaCl brine which can be used in the diaphragm process and is largely reduced in sulfate content. For this purpose, the sulfate-containing salt is partially dissolved in water, resulting in an aqueous solution containing Na ₂ SO ₄ / NaCl and solid, pure NaCl. Pure Glauber's salt is separated from the solution by cooling crystallization, the remaining NaCl solution, which is reduced in sulfate content, can be recycled into the electrolysis.

It is also known from EP-A-8470 to prevent an accumulation of chlorate in the brine circuit by partial flow treatment of an alkali halide-containing solution with hydrochloric acid, halogen oxyacids / salts being broken down to an acceptable level.

The object of the present invention is to provide a process which does not require the relatively expensive barium compounds for sulfate precipitation and in which there are also no difficulties in removing the other impurities such as magnesium and iron.

It has been found that the sulfate can be removed from the circulating brine under certain additional conditions as a Glauber's salt by cooling crystallization without adversely affecting the electrolysis process.

• a) aus dem Solekreislauf ein erster Teilstrom abgezweigt wird, zu dem die zum Ansäuern der Sole notwendige Salzsäure zugegeben wird, der Teilstrom nach einer Verweilzeit die zur Zersetzung des bei der Elektrolyse gebildeten, in dem Teilstrom enthaltenen Chlorats ausreicht, mit dem Solehauptstrom vereinigt wird, und
• b) aus dem Solestrom ein zweiter Teilstrom abgezweigt wird, der zweite Teilstrom derart gekühlt wird, daß eine dem gesamten in die Sole eingeführten Sulfat entsprechende Menge Na₂S0₄ . 10H20 auskristallisiert, der Kristallbrei abgetrennt und der zweite Teilstrom mit dem Sole-Hauptstrom wiedervereinigt wird, und
• c) daß die Sole zur Wiederaufstärkung durch Zugabe von Natriumhydroxid auf einen pH-Wert von 6 bis 8,5 eingestellt wird.

The present invention now relates to a process for the production of chlorine and sodium hydroxide solution from sulfate-containing sodium chloride salt by electrolysis, in which a sulfate-containing sodium chloride brine is circulated between depletion of the sodium chloride brine in the electrolysis cell and re-strengthening of the brine by dissolving sulfate-containing salt, the After leaving the electrolysis cell, the brine is dechlorinated by acidification with hydrochloric acid and the sulfate introduced with the salt into the brine and other impurities are removed from the brine, characterized in that

• a) a first partial stream is branched off from the brine circuit, to which the hydrochloric acid necessary for acidifying the brine is added, the partial stream is combined with the main brine stream after a dwell time which is sufficient to decompose the chlorate formed in the partial stream during electrolysis, and
• b) a second partial stream is branched off from the brine stream, the second partial stream is cooled in such a way that an amount of Na ₂ S0 ₄ corresponding to the total sulfate introduced into the brine. 10H20 crystallized, the crystal slurry separated and the second partial stream is reunited with the brine main stream, and
• c) that the brine is re-strengthened by adding sodium hydroxide to a pH of 6 to 8.5.

The present invention accordingly relates to the combination of Glauber's salt cooling crystallization precipitation, chlorate removal and the precipitation of the further impurities. The embodiment of the process according to the invention is particularly preferred in which, after the brine has been re-strengthened, the calcium ions introduced as an impurity with the salt are first precipitated as calcium carbonate and then further impurities are precipitated by adding sodium hydroxide, and that Precipitated is filtered off together. This results in an excellent filterable brine.

The possible accumulation of chlorate in the brine circuit can be avoided within a few minutes by decomposing the chlorate with a relatively high excess of hydrochloric acid. The chlorate removal from a partial stream is sufficient to keep the chlorate content in the brine circuit at a tolerable value. In particular, it is sufficient to keep the partial flow for the chlorate removal in the order of magnitude for the amount of hydrochloric acid necessary for the acidification for the dechlorination. The volume flow of the first partial flow is preferably about half to double the volume flow of hydrochloric acid. 15 to 25%, more preferably about 20%, hydrochloric acid is preferably used. Hydrochloric acid diluted in this way is still sufficient to bring about chlorate decomposition. With more concentrated hydrochloric acid, solid sodium chloride can precipitate. In a technical process, to dilute the hydrochloric acid, condensate from the chlorine cooling is expediently branched off, which is returned to the brine anyway for environmental reasons. The partial brine flow in which the chlorate removal takes place is, for example, approximately 0.5 to 2% of the main brine flow.

If the removal of chlorate from the brine circuit is ensured, Glauber's salt precipitation can be carried out according to known laws without any problems. The strong dependence of the solubility of the Glauber's salt in concentrated brine on the temperature is known. For example, the solubility of Glauber's salt, expressed in g SOJI, is approx. 15 at 5 ° C and approx. 10 at 0 ° C.

The parameters determining the Glauber's salt cooling crystallization precipitation, such as the partial flow quantity and the temperature to which cooling is carried out, can be specified in a simple manner for the other parameters present in a technical electrolysis cell.

If, for example, a sulfate content of 25 g / l is tolerable in an electrolysis system, a sodium chloride salt with 0.5% sulfate content is also used to strengthen a brine that has been depleted in the electrolysis cell by 40 g / l, so a partial flow of 2% of the brine flow to 5 ° C to cool so that the sulfate introduced when 40 g of salt are dissolved per 1 brine is removed. With more cooling, the partial flow for the cooling crystallization can be kept smaller. The brine in this second partial stream is preferably cooled to temperatures between -5 and 10 ° C., particularly preferably to temperatures between 0 and 5 ° C.

This provides a safe process which does not influence the other electrolysis conditions and which, without the use of precipitation chemicals, ensures sufficient sulfate removal from the brine, the precipitated sulfate also being obtained in the form of a directly usable sulfate compound as Glauber's salt.

Claims (6)

1. Process for the production of chlorine and sodium hydroxide solution from sulphate-containing sodium chloride salt by electrolysis, in which a sulphate-containing sodium chloride brine is circulated between the reduction of the content of sodium chloride in the brine in the electrolytic cell and the reconcentration of the brine by dissolving - sulphate-containing salt therein, the brine being dechlorinated, after leaving the electrolytic cell, by acidification with hydrochloric acid and the sulphate and other impurities introduced into the brine together with the salt being removed from the brine, characterised in that

a) a first partial stream is branched off from the brine cycle, to which partial stream the hydrochloric acid necessary for acidifying the brine is added, the partial stream is united with the brine main stream after a residence time which is sufficient for the decomposition of the chlorate which is formed during the electrolysis and is contained in the partial stream, and

b) a second partial stream is branched off from the brine stream, the second partial stream is cooled such that a quantity of Na₂SO₄ · 10H₂O corresponding to the total quantity of sulphate introduced into the brine is crystallised out, the crystal slurry is separated off and the second partial stream is reunited with the brine main stream, and

c) in that, for the reconcentration, the brine is adjusted to a pH value of 6 to 8.5 by adding sodium hydroxide.

2. Process according to claim 1, characterised in that, after the reconcentration of the brine, the calcium ions introduced as an impurity together with the salt are first precipitated in the form of calcium carbonate by the addition of soda and then other impurities are precipitated by the further addition of sodium hydroxide and the precipitates are filtered off jointly.

3. Process according to claim 1 or 2, characterised in that a sulphate content of 10 to 30g/i, preferably 15 to 25 g/I is maintained in the brine cycle.

4. Process according to one of claims 1 to 3, characterised in that the brine in the second partial stream is cooled to - to 10 °C, preferably 0 to 5 °C.

5. Process according to one of claims 1 to 4, characterised in that 15 to 25 % strength hydrochloric acid is introduced into the first partial stream.

6. Process according to one of claims 1 to 5, characterised in that the volumetric stream of the first partial stream is equal to about half to double the hydrochloric acid necessary for the acidification of the brine for the dechlorination.