Process for the Purification of Electrolysis Brine

The brines used for the electrolysis of alkali metal chlorides by the Amalgam process have to be freed from troublesome constituents, for example SO.sub.4.sup.2.sup.- -, Ca.sup.+.sup.+-, Mg.sup.+.sup.+-ions. In known processes, SO.sub.4.sup.2.sup.- is precipitated with Ba.sup.+.sup.+; Ca.sup.+.sup.+ is precipitated with CO.sub.3.sup.2.sup.- ; and Mg.sup.+.sup.+ is precipitated with OH.sup.-. The precipitation mixture generally consists of barium carbonate, soda and sodium hydroxide. Of these three components, barium carbonate is the most expensive. For this reason, attempts have already been made to precipitate the SO.sub.4.sup.2.sup.- with Ca.sup.+.sup.+. In this known process, some of the sulfate is precipitated in the form of CaSO.sub.4 . 2 H.sub.2 O in the brine by the addition of CaCl.sub.2 or Ca(OH).sub.2, and then separated off. The brine thus treated, which has a calcium content of at least 0.5 g/l, is introduced into the electrolysis cell. However, the presence of such large quantities of Ca.sup.+.sup.+ in the electrolysis cell is a considerable disadvantage.

In another known process (described in Brine Purification For The Mercury Cell Process, Paper presented at the Cleveland Meeting of Electrochemical Society, May 1966), the sulfate content of the brine is controlled through the precipitation of CaSO.sub.4 . 2 H.sub.2 O. To this end, substantially half the sulfate is precipitated with an excess of Ca.sup.+.sup.+-ions in a component stream of the brine, generally amounting to about 1/10 of the total quantity of brine, the CaSO.sub.4 . 2 H.sub.2 O deposit formed is separated off in the component stream and finally the component stream freed from the sulfate is recombined with the main brine stream. Since approximately 1.1 to 4 moles of Ca.sup.+.sup.+, based on sulfate in the component stream, are used during precipitation of the sulfate in the component stream, depending upon the sulfate content, the excess calcium, which would be troublesome during electrolysis, is precipitated in the form of CaCO.sub.3 after the component streams have been combined by the addition of sodium carbonate to which up to 5% by weight of NaOH may be added. Precipitation is carried out in such a way that, after precipitation, the concentration of CO.sub.3 .sup.-.sup.- in the solution does not exceed 1 g/l, while the pH-value does not exceed pH 11. In addition to Ca.sup.+.sup.+, which emanates from the CaSO.sub.4 . 2 H.sub.2 O precipitation step, Mg.sup.+.sup.+. which may be present as an impurity in the brine, is co-precipitated during this precipitation. Following this second precipitation, the brine thus treated is filtered, Mg(OH).sub.2 - containing CaCO.sub.3 being removed from the circuit as residue. The purified brine, in which required sulfate ion concentrations can be adjusted in this way (these concentrations are generally of the order of 5 g/l to 20 g/l), may then be introduced into the electrolysis cell.

Although it is possible with this known process also to reduce the Ca.sup.+.sup.+-concentration to such an extent that Ca.sup.+.sup.+ no longer occurs as a troublesome factor in the electrolysis cell, this process involves the use of considerable quantities of calcium chloride as precipitant in the SO.sub.4 -precipitation stage and of soda in the main precipitation stage. Both precipitants are lost in the form of Mg(OH).sub.2 -containing calcium carbonate.

The present invention provides a process for purifying electrolysis brine by dividing the electrolysis brine into component streams, precipitating sulfate substantially quantitatively in the form of CaSO.sub.4 . 2 H.sub.2 O in a first precipitation step by adding calcium chloride to one of the component streams, separating off the deposit, combining the component stream freed from the deposit with the other component stream, precipitating the calcium excess and any magnesium present substantially quantititatively by the addition of sodium carbonate and alkali hydroxide, separating off the CaCO.sub.3 - deposit optionally containing Mg(OH).sub.2 from the brine and introducing the purified brine into the electrolysis cell, wherein CaSO.sub.4 . 2 H.sub.2 O separated off after the first precipitation stage is converted under pressure with sodium hydroxide and CO.sub.2 into CaCO.sub.3 and Na.sub.2 SO.sub.4 . 10 H.sub.2 O, Na.sub.2 SO.sub.4 . 10 H.sub.2 O is separated off, CaCO.sub.3 is converted with hydrochloric acid into CaCl.sub.2 and CO.sub.2, optionally with addition of the CaCO.sub.3 obtained in the second precipitation stage and separated off, and the resulting acid CaCl.sub.2 -containing solution is recycled for the precipitation of SO.sub.4 -ions.

The individual stages of the process according to the invention are described in more detail in the following. First of all, part of the impure brine, depending upon the SO.sub.4 -concentration, for example about 1/10 for a concentration of 8 g/l, is branched off and SO.sub.4 precipitated by the addition of calcium chloride solution in a quantity corresponding to the sulfate freshly introduced through the addition of make-up sodium chloride for regenerating the brine weakened in concentration during electrolysis. The calcium sulfate dihydrate precipitated is separated off from the brine, for example in a settling vessel or in a filtration unit. The brine freed from the deposit is then reintroduced into the main stream. The residual SO.sub.4 .sup.-.sup.--concentration in the component stream treated with Ca.sup.+.sup.+-ions is adjusted with an excess of Ca.sup.+.sup.+ corresponding to the solubility product for CaSO.sub.4 . 2 H.sub.2 O. This amounts to about 1.1 to 4 and preferably about 1.5 to 2.5 moles, based on the SO.sub.4 to be precipitated in the component stream. The entire quantity of brine, including the component stream substantially freed from SO.sub.4, is then treated with soda and sodium hydroxide, the calcium introduced during regeneration of the brine weakened in concentration during electrolysis and the excess of calcium, which was added in the form of calcium chloride for precipitating the sulfate, being precipitated in the form of CaCO.sub.3, while the Mg.sup.+.sup.+ present is precipitated in the form of Mg(OH).sub.2. In this precipitation stage, soda or sodium hydroxide is added in such quantities that, on completion of precipitation, the solution is left with an excess of CO.sub.3 .sup.-.sup.- of up to about 1 g/l, preferably about 0.2 to 0.5 g/l of carbonate, and with a pH-value of no higher than about 11. The brine containing the deposit is introduced for example into a filtration unit in order to separate the deposit and is then introduced into the electrolysis cell, optionally after pH-adjustment to between pH 4 and pH 8 by the addition of hydrochloric acid.

The filter residue, which essentially contains calcium carbonate and magnesium hydroxide, is reacted substantially stoichiometrically with hydrochloric acid at temperatures in the range from about 25.degree.C to 80.degree.C to form CaCl.sub.2, MgCl.sub.2 and gaseous CO.sub.2. The CO.sub.2 liberated during the reaction may be treated in a washing tower with NaOH-containing solutions in order in this way to reform the soda required for precipitating the Ca.sup.+.sup.+.

However, in order, in the case of a dilute soda solution, which on account of the solubility of Na.sub.2 CO.sub.3 in water at room temperature cannot be adjusted to a concentration of any higher than about 200 to 250 g/l of Na.sub.2 CO.sub.3, to avoid the presence of a relatively large quantity of water which would interfere with electrolysis in the closed brine circuit, it is best to dilute about 50 % standard commercial-grade sodium hydroxide with brine until an approximately 15 to 20% solution of NaOH in the brine is obtained, followed by cooling to approximately 10.degree.C and separation of the sodium chloride precipitated (which may be redissolved at the salt-dissolving stage), the NaCl/NaOH-solution obtained (about 19 to 24% of Na.sub.2 CO.sub.3, about 10 to 12 % of NaCl) being used for absorbing the CO.sub.2. In this case, it is only the water present in the 50% sodium hydroxide solution which is introduced into the brine.

A 10 to 25% by weight CaCl.sub.2 solution, which may optionally contain magnesium, is preferably prepared in the manner described. In order to separate off the magnesium, alkali hydroxide, preferably sodium hydroxide, is added to the solution until a pH in the range from about 8 to 10 is obtained. Magnesium is thereby precipitated in the form of magnesium hydroxide and may be separated off by filtration.

The CaSO.sub.4 . 2 H.sub.2 O obtained during the precipitation of SO.sub.4 may be converted into CaCO.sub.3 with about 50 % sodium hydroxide in a stoichiometric ratio with addition of CO.sub.2 under slightly elevated pressure, preferably about 0.2 to 1 atm gauge, and at temperatures in the range from about 25.degree. to 80.degree.C. The Na.sub.2 SO.sub.4 . 10 H.sub.2 O formed as secondary product may be washed out with water. The pure calcium carbonate may be added to the calcium carbonate containing magnesium hydroxide, which accumulates during precipitation of the excess calcium, and correspondingly converted into calcium chloride and CO.sub.2 or, optionally, may be separately converted directly into calcium chloride and CO.sub.2.

One special embodiment of the process according to the invention is described in the drawing which is a flow sheet of the process in which the reference numerals used have the following meaning:

1 -- electrolysis cell;

2 -- brine regeneration;

3 -- NaCl addition;

4 -- first component stream;

5 -- second component stream;

6 -- calcium sulfate dihydrate precipitation vessel;

7 -- calcium chloride input;

8 -- settler;

9 -- calcium sulfate dihydrate removal;

10 -- combined brines;

11 -- calcium carbonate and magnesium hydroxide precipitation;

12 -- sodium hydroxide addition;

13 -- sodium carbonate addition;

14 -- filtration;

15 -- brine feed to cell;

16 -- calcium carbonate and magnesium hydroxide dissolution;

17 -- HCl input;

18 -- sodium hydroxide input;

19 -- magnesium hydroxide separation;

20 -- calcium chloride solution;

21 -- calcium sulfate dihydrate dissolution;

22 -- CO.sub.2 input;

23 -- NaOH input;

24 -- separation of calcium carbonate and sodium sulfate hydrate;

25 -- sodium sulfate hydrate removal;

26 -- calcium carbonate dissolution;

27 -- hydrochloric acid input;

28 -- 50 % sodium hydroxide solution;

29 -- brine input;

30 -- container;

31 -- cooler;

32 -- sodium chloride separation;

33 -- sodium chloride recycling;

34 -- absorption; and

35, 36, 37 -- CO.sub.2 -pipe

More particularly, the brine leaves the electrolysis cell 1 with a sodium chloride content of approximately 280 g/l and enters the dissolving stage 2, where for example solid sodium chloride is added through conduit 3 to regenerate the brine to 320 g of sodium chloride per liter. The brine is then divided up into the component streams 4 and 5. The component stream 4 is treated in a precipitation vessel 6 with calcium chloride introduced through conduit 7 in order to precipitate calcium sulfate dihydrate which is removed in the settling vessel 8 through conduit 9. The component streams are combined in conduit 10 and Ca.sup.+.sup.+ and Mg.sup.+.sup.+ are precipitated in vessel 11 through the introduction of sodium hydroxide and sodium carbonate, respectively, through conduits 12 and 13. The soda solution 13 is prepared by mixing brine through conduit 29 with 50% sodium hydroxide 28 in the container 30, the NaCl precipitated is centrifuged off 32 after cooling 31 and the salt precipitated is recycled through conduit 33 to the brine regeneration stage. CO.sub.2 is introduced into an absorption stage 34 from vessel 16 via conduits 35 and 36 and the requisite Na.sub.2 CO.sub.3 -solution formed in this way 13. After filtration in 14, the filtered brine may be returned to the electrolysis cell through conduit 15. The deposit containing calcium carbonate and magnesium hydroxide is dissolved in 16 by the addition of hydrochloric acid through 17, after which the pH-value is adjusted to about pH 9.5 through sodium hydroxide introduced through 18, resulting in the precipitation of magnesium hydroxide which may be removed in the filter 19. The resulting calcium chloride may be reused through conduits 20 and 7 for the precipitation of calcium sulfate dihydrate. The calcium sulfate dihydrate removed in settler 8 through conduit 9 is treated under pressure in vessel 21 with CO.sub.2 introduced through pipe 35 and input 23 and sodium hydroxide introduced through input 22, resulting in the formation of calcium carbonate and sodium sulfate hydrate. In vessel 24, the soluble sodium sulfate hydrate is separated by washing from the insoluble calcium carbonate which, in vessel 26, is converted into calcium chloride by the addition of hydrochloric acid through input 27 and may be reused through input 7 for the reprecipitation of calcium sulfate dihydrate. CO.sub.2 formed in vessel 26 may be introduced through pipe 37 into the absorption stage 34.

By virtue of the process according to the invention, it is possible with advantage to remove the sulfate content of the salt required for electrolysis using an inexpensive by-product, because CaCl.sub.2 accumulates as secondary product during recovery of the soda. Since the Na.sub.2 SO.sub.4 obtained during reaction of the CaSO.sub.4 is a product which is in short supply, no unusable secondary products are formed in this process. Since most of the Ca and CO.sub.3 are cycled, the only reagents required are hydrochloric acid and sodium hydroxide. Since the Cl content of the hydrochloric acid used is recovered in the form of Cl.sub.2 during electrolysis, hydrochloric acid may be profitably used here for degrading excess HCl, in other words purification of the electrolysis brine is essentially carried out with hydrochloric acid and sodium hydroxide only:

The process according to the invention is illustrated by the following Examples:

1. Sulfate Precipitation

In a technical institution, two electrolysis cells for the electrolysis of alkali metal chlorides by the Amalgam process at a load of 65 kA were supplied with approximately 7 m.sup.3 /h of saturated brine. The diluted brine after electrolysis was brought up to strength by addition of 260 kg/h of rock salt (approximately 1 % of SO.sub.4, approximately 0.2 to 0.3 % of Ca). A component stream of about 600 l/h was separated off from the main stream and introduced into the sulfate precipitation stage. The component stream entered a container approximately 3 meters long, 0.3 meter wide and 0.3 meter deep, at its bottom end. 13 l/h of an approximately 32% Ca Cl.sub.2 solution were added at this point. The stream of brine which flowed off at the upper end then had a solids content of 0.1 g/l. The CaSO.sub.4 . 2 H.sub.2 O (4.5 kg/h) precipitated accumulated at the bottom of the container and was periodically discharged. The brine which entered the sulfate precipitation stage, like the circulating brine, had a sulfate content of approximately 8 g/l, while the brine which left the precipitation stage after the settler had a content of about 4 g of SO.sub.4 .sup.-.sup.-/l, while the Ca-content amounted to between 2 and 2.4 g/l, which corresponds to a solubility product of CaSO.sub.4 of about 2.0 to 2.4 .sup.. 10.sup..sup.-3 (Mole/l).sup.2 at 70.degree.C.

2. CaCO.sub.3 Precipitation

The main stream and component stream were recombined just in front of the precipitation vessel, 50 l/h of an Na.sub.2 CO.sub.3 solution, according to (4) hereinbelow, and a little 50 % sodium hydroxide running in. The input was adjusted so that, on completion of filtration, the brine contained an excess of about 400 mg of Na.sub.2 CO.sub.3 and 50 mg of NaOH. The precipitated brine was pumped through a filter (14 m.sup.2). The residence time was up to 16 hours. The purified brine was introduced into the cell.

The residue of the filter was introduced into concentrated 30% acid until a pH-value of about 4 was obtained. The suspension was adjusted to pH 8 with sodium hydroxide and then filtered. The Mg remained quantitatively on the filter together with the insoluble fractions (graphite, Fe(OH).sub.3 etc). The solution running off had a concentration of from 28 to 32%, based on CaCl.sub.2, and could be reused for dissolution of the sulfate.

3. Working up CaSO.sub.4

The CaSO.sub.4 residue from the sulfate precipitation stage was made into a paste with NaOH (50% by weight) in a stoichiometric ratio in the form in which it accumulates, i.e. without drying, in a screw apparatus and was reacted with CO.sub.2 under a slight excess pressure, the reaction being accompanied by spontaneous heating. The almost solid cake was then leached with hot dilute Na.sub.2 SO.sub.4 solution, the solution being cycled between a crystallizer and the leaching stage. In the crystallizer, the Na.sub.2 SO.sub.4 obtained from the CaSO.sub.4 crystallized out almost quantitatively in the form of Na.sub.2 SO.sub.4 . 10 H.sub.2 O providing the leaching temperature is as high as possible, i.e. above 80.degree.C, and the crystallization temperature as low as possible, i.e. below 32.degree.C and preferably in the range from about 0.degree. to 10.degree.C.

4. Preparation of the Soda Solution

Brine containing 300 g/l of NaCl and 50% sodium hydroxide solution were mixed in a container in a ratio of 1 : 0.4 while cooling to 10.degree.C. After the NaCl precipitated had been separated, the NaOH content amounted to 200 g/l and the residual NaCl content to 153 g/l. This solution was gassed with CO.sub.2 in an absorption tower until the Na.sub.2 CO.sub.3 content amounted to 256 g/l and the NaOH content to 4 g/l, the sodium chloride content remained unchanged. This solution was used for precipitating the Ca.sup.+.sup.+ in the brine in step 2.

It will be appreciated that the instant specification and examples are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.