EP0745701B1 - Process for regenerating synthetic diaphragms - Google Patents

Description

The present invention relates to a method for the regeneration of Plastic diaphragms, especially for the regeneration of plastic diaphragms from chlor-alkali electrolysis.

In chlor-alkali electrolysis, the diaphragm process uses electrolysis cells used that use an iron cathode grid on which the Diaphragm material e.g. has been applied by vacuum deposition. Dimensionally stable anodes (DSA) are generally used as anode today, which are, for example, expanded metal grids made of titanium acts that are coated with ruthenium oxide / titanium oxide. After installation the anodes are expanded into the cell by the distance between the anode and cathode and thus the ohmic voltage drop as low as possible to keep.

The diaphragm process uses diaphragms made of different materials, such as asbestos. More recently, plastic diaphragms have also been used, which are produced by vacuum deposition of a fiber material and subsequent sintering. The fiber material can consist, for example, of PTFE fibers with embedded and adhering ZrO ₂ particles. Examples of such a fiber material are Polyramix® fibers (Oxytech) and Tephram® fibers (PPG Industries, Inc.).

Compared to asbestos diaphragms, the plastic diaphragms can be operated much longer. While an asbestos diaphragm typically has a service life of approximately 4,000 to 10,000 operating hours and is then replaced, plastic diaphragms can be used over a period of approximately 17,000 to 26,000 operating hours. With this longer operating time it can now happen that iron compounds that are contained in the trace range (<1 ppm) in the brine (NaCl solution), due to the steep gradient of the hydrogen ion concentration (pH value) in the diaphragm as an oxide (for example Fe ₂ O ₃ , Fe ₃ O ₄ ) not only deposit on the diaphragm as in the case of the asbestos diaphragm, but even grow through the plastic diaphragm in the form of veins or needles. These growths consist of a conductive iron oxide. These conductive growths lead to the fact that hydrogen can develop on the anode side of the diaphragm after a certain time (approximately after 1 to 3 years). Due to the increase in the hydrogen content in chlorine, there is a risk of a chlorine oxyhydrogen explosion after the explosion limits have been exceeded. For safety reasons, the cell must therefore be switched off when the hydrogen concentration is very high. In addition, the development of hydrogen on the anode side reduces the purity of the chlorine, which is also undesirable.

In addition to iron deposits, it can also cause calcium, strontium and some magnesium deposits occur, leading to reduced permeability or blockage of the diaphragm.

Asbestos diaphragms are due to the shorter operating time not to the growths as with the plastic diaphragms. For replacement the superficial deposits on asbestos diaphragms, for example in US 1,309,214 proposed using the asbestos diaphragms wash diluted lactic acid. This allows gelatinous deposits to be removed remove from magnesium and / or calcium hydroxide, which the Clog the diaphragm without corroding the iron parts or iron cathode, the iron oxide penetrations in plastic diaphragms can be but not in this way.

DE 19 56 291 proposes clogging of diaphragms by rinsing of the diaphragm with hydroxypolycarboxylic acids, such as citric acid, gluconic acid etc. to remove. This method is also suitable, superficial To partially remove deposits from iron oxides, removing them of iron oxide penetration in plastic diaphragms is due to this Way not possible.

In the Soviet patent application SU 808561 a method for Washing of asbestos diaphragms described during the electrolysis operation Hydrochloric acid metered into the cathode compartment and the pH up is lowered to pH = 7. This procedure can also be used remove superficial deposits and blockages of asbestos diaphragms, Iron growth cannot be removed satisfactorily. The method described is also uneconomical because it is larger Amounts of the valuable product sodium hydroxide neutralized and thus destroyed will. The SU 964024 therefore proposes a clean sodium chloride solution insert and remove the sodium carbonates beforehand. Since the Growth in the diaphragms mainly through special iron salts caused in the brine only in very small amounts would be a further reduction in the iron concentration in the brine considerable economic effort.

In order to regenerate plastic membranes or diaphragms, US-PS 5,133,843 proposed to clean them with aqua regia. Because of this In particular, deposits can be removed from the process. The diaphragm can be cleaned using this procedure however, all iron parts in the electrolysis cell, such as the cathode, destroyed. The diaphragm would have to be cleaned removed and detached from the cathode. A cleaning of the It would not be possible to use a diaphragm in the chlor-alkali electrolysis cell.

Japanese patent application JP 60077985, in which a method for Cleaning of electrolytic cells of the diaphragm type, in particular for Production of hydrogen from alkalis is used with mixtures described from acids and surfactants is only for cleaning a dismantled Asbestos diaphragms can be used because the corrosion of the iron and titanium parts cannot be avoided.

German Offenlegungsschrift 15 67 962 describes a method for Regeneration of an asbestos diaphragm, in order to protect the iron parts a corrosion inhibitor is used. Also leave after this procedure only superficial deposits become detached during growth of the diaphragm cannot be removed. Because asbestos as a material is not stable under strongly acidic conditions are also those in the published patent application proposed corrosion inhibitors to protect the cathode not sufficient when regenerating a plastic diaphragm. In addition, this method cannot prevent titanium corrosion will.

In US Pat. No. 3,988,223, the cleaning of plastic diaphragms is carried out Nafion® or Gore-Tex® with complexing agents such as EDTA (ethylenediamine tetraacetic acid = ethylenediaminetetraacetic acid), or ethylenediamine tetrapropionic acid (= ethylenediaminetetrapropionic acid). With the proposed Complexing agents are comparatively expensive compounds. The rinsing solution that occurs when cleaning the diaphragm due to the complexing agents contained therein, it cannot be treated untreated Wastewater can be given, so that additional costs for the complex Disposal.

The non-prepublished, priority EP-A-0 694 632 relates to the workup of diaphragms from electrolysis cells. For this purpose, the arrangement of cathode and Treated the diaphragm with a solution without separating the cathode from the diaphragm, the at least about 3% by weight hydrochloric acid and at least about 0.1% by volume one Contains corrosion inhibitor. Treatment is done by dipping the assembly into this medium and allowing the medium to flow through the diaphragm. After Treatment is removed from the solution with an aqueous solution Medium washed, baked at elevated temperature and after treatment with a Wetting agent dried.

Due to the difficulties in the prior art, is commonly used Regeneration of the plastic diaphragm of the cell completely removed and replaced with a new one. This procedure is costly because the Re-equipping the cell requires extensive work and that necessary new diaphragm material is very expensive. Also fall Landfill costs for the material that has become unusable. A cleaning of the plastic diaphragm must completely remove the iron impurities, otherwise a decrease in the hydrogen concentration after regeneration cannot be achieved permanently in chlorine. Because it is particular in the case of aged iron oxides to adhere very stubbornly deposits and growths that are difficult to dissolve is the Use of agents necessary, on the other hand, for the corrosion of iron and lead titanium parts in the cell.

The object of the present invention is therefore a method for regeneration of plastic diaphragms, where deposits and / or growth on or in the plastic diaphragm economically can be removed, especially without iron and / or Titanium parts corrode significantly and without residues that are difficult to dispose of arise. This object is achieved according to the present invention solved the objects defined in the independent claims; advantageous further developments are listed in the subclaims.

This task is accomplished through a regeneration process Plastic diaphragms solved, in which a mineral acid solution containing sodium chloride up to the saturation concentration with a Corrosion inhibitor is added, and the mixture thus obtained at a Temperature from 0 to 110 ° C, preferably 30 to 110 ° C, more preferably 40 to 80 ° C, in particular 50 to 70 ° C, about 0.1 to 84 hours, preferably 1 to 72 hours, in particular 2 to 24 hours through which the plastic diaphragm is passed. On the one hand, this method provides a possibility, too to remove stubborn, poorly soluble and streaky iron deposits, as well as the regeneration of the plastic diaphragm in to be able to perform in situ without having to remove the diaphragm, because adequate protection of the iron and titanium components can be achieved. Furthermore is not an expansion of the diaphragms in the preferred cell construction possible without destroying the diaphragms. Plastic diaphragms are preferred therefore regenerated in the cell. The regeneration in the Electrolysis cell saves time, costs and labor.

A method is preferably provided in which the mineral acid solution in a concentration of 0.3 to 20% by weight, in particular 2 to 10 % By weight is used. The use of a is particularly preferred Acid, at least partially, preferably exclusively from hydrochloric acid exists as a mineral acid. The use of hydrochloric acid prevents that foreign ions enter the cell, which can then be Rinsing would have to be removed again. In principle would be for the implementation cleaning, of course, another mineral acid, for example Sulfuric acid, suitable.

In a further preferred method, the mineral acid solution contains bis to 250 g / l sodium chloride. By adding the sodium chloride, the Cleaning effect of this mixture increases. So it is e.g. when adding NaCl possible to reduce the concentration of hydrochloric acid (e.g. by 9% to 2%), the solution still having a sufficient cleaning effect owns.

In another preferred method of the present invention is in the mixture of the mineral acid solution with the corrosion inhibitor 0.005 to 5% by weight, preferably 0.05 to 0.5% by weight, of corrosion inhibitor provided, the percentages by weight refer to the mixture of the mineral acid solution refer to the corrosion inhibitor as 100 wt .-%. This dosage of the corrosion inhibitor leads to protection of the iron parts in the Electrolytic cell.

In another preferred method of the present invention uses a corrosion inhibitor that contains at least one alkinol. Prefers a corrosion inhibitor can also be used, which at least contains an alkinol and preferably with 1 to 25 wt .-% of an amine and / or 0.1 to 3 wt .-% of a surfactant is added, the weight percentages refer to the corrosion inhibitor as 100% by weight. This Alkynols can, for example, alkynediols, such as butynediol, 3-hexyne-2,5-diol, 3,6-dimethyl-4-octyne-3,6-diol, 2,5-dimethyl-3-hexyne-2,5-diol, or else Propargyl alcohol or hexinol (3-hexin-2-ol) or ethinylcyclohexanol. These alkynols can be amines, such as hexamethylenetetramine, Ethylhexylamine, diethylhexylamine or other primary, secondary or tertiary Amines can be added. The alkynols act as a monomer for the formation of a corrosion-inhibiting coating on the iron parts, to be protected.

In a preferred method of the present invention, a surfactant used, which contains a quaternary ammonium compound. Organic ammonium compounds with quaternary nitrogen atoms, for example quaternary ammonium compounds with in particular long alkyl chains, such as for example distearyldimethylammonium chloride (DSDMA), Protectol KLC 80® or Protectol KLC 50® (BASF) or Pluradyne CI 1066® (BASF Corp.). Particularly preferred mixtures of alkynols with amines and / or quaternary ammonium compounds comprise about 98% butynediol and 2% hexamethylenetetramine or, for example, about 97.8% butynediol plus about 2% hexamethylenetetramine plus about 0.2% Protectol KLC 50®.

In another preferred method of the present invention, the mineral acid solution contains about 500 to 5000 ppm copper or iron salts. Water-soluble Fe III or Cu II salts are preferably added to the mixture of the mineral acid solution and the corrosion inhibitor. This can be achieved, for example, by additionally adding, for example, iron chloride to the mixture of the mineral acid solution with the corrosion inhibitor, or by pumping the mixture of the mineral acid solution with the corrosion inhibitor through the diaphragm in a circle. The iron-containing deposits detached from the diaphragm thereby provide Fe ³⁺ compounds, which then act as a corrosion inhibitor with respect to the titanium, of which the anodes in particular have larger proportions in uncoated form.

Another advantageous method of the present invention provides that the diaphragm additionally with water and / or a sodium chloride solution rinsed, in particular rinsed. With the water used it is preferably pure water. By rinsing the Diaphragms are residues of acid or hydrochloric acid, iron salts, etc. washed out. A sodium chloride solution is advantageously used for this rinse used, because in this case the freshener is filled later Brine into the cell does not pose the risk of uncontrolled dilution There is residual water. In addition, the cell is filled with fresh brine anyway.

In an additional method of the present invention are preferred as alkynols also alkyne monools, in particular propargyl alcohol or Ethinylcyclohexanol, with an alkinol preferably in one concentration greater than 30% by weight, typically in a concentration greater than 80 % By weight is used. The percentage by weight relates on the total alkynols used as 100% by weight. These corrosion inhibitors are more effective in preventing corrosion of iron. They can preferably be used wherever cells are used be in which anodes are provided, which are completely with a Ruthenium-titanium oxide layer are coated. In addition, in this case the addition of iron salts can be dispensed with. Effective corrosion inhibitors are mixtures containing alkyne monools, for example propargyl alcohol or ethinylcyclohexanol, as the main component. These corrosion inhibitors are particularly suitable for use in mixtures that are not dissolved Have iron salts. The mixture of mineral acid solution with the In this case, the corrosion inhibitor should only be used once. A preferred mixture for inhibiting iron corrosion includes, for example a mixture of about 2% Protectol KLC 80®, about 1% ethinylcyclohexanol, about 8% ethylhexylamine or diethylhexylamine, and about 89% Propargyl alcohol. Another advantageous mixture comprises about 2% Pluradyne CI 1066® and about 98% propargyl alcohol.

In another preferred method of the present invention the mixture through an electrolytic cell without prior removal of Diaphragm and electrode conducted. So in this way it is possible that To regenerate the diaphragm without having to remove the diaphragm. Such an in-situ cleaning of the diaphragm saves time, costs and Workload. A complex removal of the diaphragm from the cell and it is therefore no longer necessary to remove the diaphragm material.

In addition, an advantageous method according to the present invention be provided for the regeneration of plastic diaphragms, a process as described above several times in succession or at least two procedures as described above are used in succession will. By using this procedure in a row you can for example the diaphragms with different mixtures of mineral acid solutions with different corrosion inhibitors at different Temperatures applied consecutively for different lengths of time the advantages of the individual process parameters can be combined so that for the present impurity optimal combination of individual processes and process parameters is provided. The individual processes or process steps can also by rinsing the diaphragm with a rinsing solution, in particular with pure water or a sodium chloride solution, separated from each other will.

The present invention is based on the following examples, in which further preferred features and combinations of features or embodiments the invention are described, are explained in more detail.

Comparative Example 1

A chlor-alkali cell (e.g. a monopolar cell from Diamond Shamrock), in their anode gas a high hydrogen concentration (> 4 vol.%) was measured, was switched off and the solution therein drained. Then an 8% hydrochloric acid, the 0.2 wt .-% Korantin BH® (corrosion inhibitor from BASF AG based on butynediol and hexamethylenetetramine) contains, preheated to 40 ° C and at the Pumped anode side into the cell. After the cell is completely filled solution was pumped in further and removed on the cathode side and returned to the reservoir. This process took 24 hours continued, keeping the temperature of the hydrochloric acid at 50 ° C.

Immersed in the mixture of hydrochloric acid and the corrosion inhibitor Pieces of titanium electrodes showed no weight loss. Also immersed Pieces of iron cathodes showed weight loss after 24 hours from about 1%.

After the regeneration was complete, the acid was drained off and the electrolysis cell charged with fresh brine. The electrolysis of this brine provided Chlorine, which contained less than 0.2% by volume of hydrogen.

Example 2

A cell in the chlor-alkali electrolysis was switched off and the solution therein was drained off. The diaphragm was then rinsed for 2 hours at 70 ° C. with an aqueous solution of approximately 2% hydrochloric acid, approximately 250 g / l sodium chloride, approximately 0.5% Korantin BH and approximately 0.1% Fe ³⁺ ions. The diaphragm was then rinsed with pure water for about an hour.

The weight loss of the iron cathode was between 0.5 and 1.5% by weight, and the titanium corrosion was less than 0.02% weight loss. The ferrous deposits were completely, i.e. over 98% from which Diaphragm removed.

Comparative Example 3

The solution contained therein was drained from a switched off chlor-alkali electrolysis cell. The diaphragm was then rinsed with an aqueous solution containing about 8% hydrochloric acid, 0.5% Korantin BH® and about 0.1% Fe ³⁺ ions at 70 ° C for about 2 hours. In a second step, the diaphragm was rinsed with an aqueous solution containing about 8% hydrochloric acid, about 0.5% Korantin BH® and about 0.1% Fe ³⁺ ions at 50 ° C for 24 hours. The diaphragm was then rinsed with pure water for about an hour.

The weight loss of the iron cathode was between 1 and 2% by weight and the titanium corrosion was less than 0.02% weight loss. The ferruginous Storage was complete, i.e. over 98% from which Diaphragm removed.

The present invention thus provides a method for regeneration been provided by plastic diaphragms that not only capable growth of iron deposits in plastic diaphragms is economical to remove without corroding the iron and / or titanium parts, it also avoids residues that are difficult to dispose of, that could pollute the environment.

Claims (14)

1. A process for the regeneration of plastic diaphragms, which comprises mixing a mineral acid solution which contains sodium chloride up to the saturation concentration with a corrosion inhibitor and passing the mixture thus obtained through the plastic diaphragm at from 0 to 110°C for from 0.1 to 84 hours.
2. A process as claimed in claim 1, wherein the mineral acid solution is employed in a concentration of from 0.3 to 20% by weight.
3. A process as claimed in claim 1 or 2, wherein the mineral acid used is an acid which consists at least partially of hydrochloric acid.
4. A process as claimed in one of claims 1 to 3, wherein the mineral acid solution contains up to 250 g/l of sodium chloride.
5. A process as claimed in one of claims 1 to 4, wherein from 0.005 to 5% by weight of corrosion inhibitor is provided in the mixture of the mineral acid solution with the corrosion inhibitor, the percentage by weight data being based on the mixture of the mineral acid solution with the corrosion inhibitor as 100% by weight.
6. A process as claimed in one of claims 1 to 5, wherein a corrosion inhibitor is used which contains at least one alkynol.
7. A process as claimed in claim 6, wherein a corrosion inhibitor is used which contains an alkynediol, and is mixed with from 1 to 25% by weight of an amine and/or 0.1 to 3% by weight of a surfactant, the percentage by weight data being based on the corrosion inhibitor as 100% by weight.
8. A process as claimed in claim 7, wherein the amines are amines from the group consisting of hexamethylenetetramine, ethylhexylamine and diethylhexylamine.
9. A process as claimed in one of claims 6 to 8, wherein at least one surfactant contains a quaternary ammonium salt.
10. A process as claimed in one of claims 1 to 9, wherein the mineral acid solution contains from approximately 500 to 5000 ppm of copper and/or iron compounds.
11. A process as claimed in one of claims 1 to 10, wherein the diaphragm is additionally rinsed with water and/or a sodium chloride solution.
12. A process as claimed in one of claims 1 to 11, wherein the alkynols also contain alkynemonools in a concentration of greater than 30% by weight, the percentage by weight datum being based on the total alkynols used as 100% by weight.
13. A process as claimed in one of the preceding claims, wherein the mixture is passed through an electrolysis cell without prior dismantling of diaphragm and electrode.
14. A process for the regeneration of plastic diaphragms, wherein a process as claimed in one of the preceding claims is used several times in succession or at least two processes as claimed in one of the preceding claims are used in succession.