DE1961779A1 - Process for cleaning salt solutions for chlor-alkali electrolysis - Google Patents

Description

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1A-37 233

Description of the patent application

Dr, -Ing · Hans Müller,
Männedorf, Switzerland

concerning

Process for cleaning salt solutions for chlor-alkali electrolysis

The invention relates to a method for purifying salt solutions for chloralkali electrolysis,

The salt solutions used in the chlor-alkali electrolysis should consist of the purest possible solutions of sodium chloride. or potassium chlorine. Additions of other metal ions or anions, especially sulfate ions, caused by impurities in the crude salt, are undesirable because they impair the purity of the electrolysis products chlorine and sodium hydroxide or potassium hydroxide solution and make expensive subsequent cleaning necessary attack the most commonly used graphite anodes. The contaminating metal ions are therefore removed by precipitation, e.g. with NaQH and Na ₂ CO, as carbonate or hydroxide and the sulphate ions as barium sulphate. Subsequent to the precipitation, there is a filtration through filter presses or leaf filters of various designs, which, however, because of the fine and often kolloidalen- · precipitation clog quickly, therefore need large filter surfaces, yet often have little life, so that \ affect the economy through large workload j, i ·

009831/1489

If the expensive titanium anodes, which are insensitive to sulphate, have recently been increasingly used instead of graphite anodes are, the work for the barium sulfate precipitation is eliminated, but is due to the increased effort for the anodes at least partially outweighed. Even if you do not continue to use the brine that has once been used. the cost increases. For economic and wastewater technical reasons · is in most cases a Circulation required. In many places, it is therefore not possible to do without filtration.

It has therefore already been proposed to use filter aids during the filtration in order to increase the service life of the filter elements to extend. As this filter aid through the precipitation are contaminated, the filter cake is thrown away. The loss of the filter aid in turn increases the price of the procedure.

The invention is based on the object of a method specify the cleaning of the salt solutions to be used in the chlor-alkali electrolysis by precipitation and subsequent filtration, with which a longer running time of the filter systems can be achieved with a higher specific filter performance and through ■ saving of filter aids can increase the efficiency of the filtration.

The method according to the invention solves the problem for one such a method in that the filtration is carried out using a precoat filter agent which is obtained from the residues a previous brine purification by separating the filtration-inhibiting Shares was gained.

Even if that could be thought of, in the simplest case to use the precipitates themselves as filter media, so will it turns out that the hoped-for success will not materialize,

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BAD ORSQiNAL

because the precipitates are also deposited in a sludge-like and colloid manner Contain parts that quickly clog the filter layer

To separate these fine and especially the finest filtration-inhibiting substances, the precipitates (precipitation residues) must be subjected to wet classification _f z, B * in hydrocyclones, by decanting, by centrifugation or by sieving with or without simultaneous vibration treatment (vibration sieving).

If, in the filtration nor a special precoat filter aid to what particularly other than the known filter aids siliceous base (diatomite, perlite) and coal plastics in fine grain size to 10 - prove to 200 / um grain size to be advantageous _g eoverden this Filter aid thereby recovered ^ dsß nan the contaminating metal ions fall as chemically easily soluble precipitate, then filtered with the addition of the insoluble filter aid, then the kickstand is chemically freed from the precipitate - the carbonates and / or hydroxides, preferably by treatment with hydrochloric acid - the undissolved filter aid after washing with water by sieving or wet classification or also by drying converted back into the condition suitable for use. The residues to be reused as precoat filter aids can expediently contain barium sulfate precipitated in the electrolysis salt solution if anodes made of platinum-coated titanium are used for the electrolysis.

In practical application of the method according to the invention was for example found that even when using ^Licher% filter aid usual filter only specific services in the customary purification by precipitation and filtration

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200 to 1500 l / m ² .h can be achieved, while when using the precipitation residues freed from the filtration-inhibiting finest fractions by wet classification as a filter medium, capacities of 3000 to 5000 l / m.h are achieved with the addition of special filter aids such as plastic powder, in addition to the cleaned precipitation residues, even higher outputs of up to 8000 l / m.h were achieved with a simultaneous extension of the running time.

The new method can be implemented in such a way that extensive automation enables 1st and the implementation of the procedural steps in each case regularly and can be done safely.

The process according to the invention is based on a flow diagram shown in the accompanying drawing of the process for carrying out the process required system is shown as an example.

The system includes a built-up with a arranged in a plurality of superimposed _ \ disc-shaped filter elements rotatable filter elements package equipped Zentrifugalreinigungsfliter 1, one with a mixing tool - here vibration mixer with oscillating magnetic drive - equipped and arranged in the bottom part horizontal, finely perforated sieve treatment tank 2, a container 5 for turbid Sole (Trübsol ”) voip precipitation tank or the electrolysis cells of a chlor-alkali electrical and lysis system, a container 6 for pure, filtered sol” (Reiaisple) and a container 7 for hydrochloric acid. . ■. *

The 'kept constantly filled' by means of float flow control Container 5 located sludge brine is fed to the filter 1 by a pump, the pure brine in the control with overflow equipped to feed the electrolysis cells 6 headed. After exhaustion of the filter elements (clogging and Flow reduction) the filter container is filled with air through '

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ORIGINAL INSPECTED

a line 9 is depressed, then the residue (filter cake) deposited on the horizontal filter elements is thrown off by rapid rotation and emptied from the lower part of the filter housing through the opening of a slide 10 into the processing container 2. In this first hydrochloric acid is pumped from the container 7 through lines 11 and 12 and the residue is intimately mixed with the acid by means of the vibration mixer 3. Hydroxides and carbonates dissolve from the residue in the acid. The latter is conveyed back into the acid tank 7 from the processing tank 2 by supplying compressed air via a line 13. Washing water is now introduced into the treatment tank 2 via a line 14, in which def is now suspended from the part of the filter residue (filter aid and / or barium sulfate) undissolved in hydrochloric acid using the mixing device and freed from acid residues. The washing water is then also pressed out of the treatment container 2 by means of compressed air through the sieve £, through which even the finest portions of the residue that are undesirable in the filter aid pass through Pure brine from the container 6 and the lines 11 and 15 mixed and slurried in it with the mixing tool 3, fed by a pump 16 via lines 17 and 18 to the centrifugal cleaning filter 1 to form a new basic subsoil layer for the following filtration. A continuous metering of filter aid can also be carried out through the same line in cases of need - to extend the filter run time, by continuously supplying suspended filter aid via a line 18 during the inflow of the cloudy brine by pump 8.

The process sequence can be controlled automatically by program control units.

1058 claims

009831 / U89

Claims (6)

Claims fiJ Process for the purification of in the chlor-alkali electrolysis salt solutions to be used by precipitation and subsequent filtration, characterized in that that the filtration is carried out using a precoat filter medium, which is from the residues of a previous Brine purification was obtained by separating the filtration-inhibiting components. 2. The method according to claim 1, characterized. ^ © indicates that das.Anschwemm filter medium consists of precipitation residues, the finest proportions of which by wet classification removed. 3. The method according to 'claim 1, characterized in that the contaminating in the brine cleaning Metal clays precipitated as a chemically easily soluble precipitate, filtered with the addition of an insoluble filter aid, then the residue is chemically freed from the precipitate and the filter aid remaining undissolved for separated the reuse in a subsequent filtration will. 4, method according to claim 1 and 3, characterized in that that the metal ions as Korboüä.- · and / or hydroxides are precipitated and hydrochloric acid is used as a chemical solvent; Lrd " 5. The method according to claim 3, characterized in that as a filter aid Fine-grain plastic, especially about 10 is turned. 009831/1489 6. The method according to claim 1 and one of claims 3 »^ or 5, characterized in that the residue removed from a filter with controllable discharge Residues in a with a mixing device and arranged in the lower part, essentially horizontal, a fine sieve-equipped processing container, in which it is first treated with acid and then with water, then mixed with pure brine and the residues, which now only consist of the insoluble filter aid, in the brine is slurried to form a new precoat filter layer are introduced into the filter. 7. Method according to one of Claims 1 to 6, characterized in that the precoat filter means reusable residues in the electrolytic saline solution contain precipitated barium sulfate. 009831/1489 Blank page