US3467586A - Rejuvenation of diaphragms for chlor-alkali cells - Google Patents

Description

Sept. 16, 1969 M. P. GROTHEER ET AL 3,467,586

REJUVENA'I'ION OF DIAPHRAGMS FOR CHLOR-ALKALI CELLS Filed April 12, 1965 United States Patent 3,467,586 REJUVENA'IION 0F DIAPHRAGMS FOR CHLOR-ALKALI CELLS Morris P. Grotheer and Edward H. Cook, Jr., Lewiston,

N.Y., assignors to Hooker Chemical Corporation,

Niagara Falls, N.Y., a corporation of New York Filed Apr. 12, 1965, Ser. No. 447,210 Int. Cl. B01k 3/10 US. Cl. 204-98 8 Claims ABSTRACT OF THE DISCLOSURE Asbestos diaphragms which have become less permeable toward the electrolyte in a chlor-alkali electrolytic cell may be rejuvenated by passing an aqueous solution of an acid through the clogged diaphragm until the efiluent exhibits a pH below about 8. The acid treatment may be conducted without disassembly of the cell. The cell, subsequent to acid treatment of the asbestos diaphragm, displays a normal brine level and operates at a reduced cell voltage.

This invention relates to chlor-alkali cells and more particularly to the rejuvenation of porous asbestos diaphragms used in chlor-alkali cells.

Caustic soda, chlorine and hydrogen are produced by electrolyzing alkali metal chlorides in diaphragm cells through which an aqueous solution of alkali metal chloride is passed. The diaphragm used in such a cell serves to maintain separate from each other the hydrogen and chlorine produced by the electrolysis. The diaphragm is of a material which is sufficiently porous to permit the flow of brine through it and which does not insulate the cathode from the anode nor greatly increase the voltage applied to the cell.

One of the most suitable diaphragm materials used in chlor-alkali cells is as'bsetos. Although asbestos provides a good diaphragm material, it will eventually become restricted by impurities contained in the brine and in concrete cell boxes which are often employed, thus limiting the diaphragm life to about one-half to one-quarter of the life of the graphite anodes used in such cells. It would be most desirable to have a diaphragm life equal to the anode life. However, during electrolysis, various salts such as magnesium and calcium are deposited on the diaphragm and the deposited salts impede the flow of brine through the diaphragm eventually causing the height of the brine within the cell to increase and finally reducing the output of the cell. Soon, the diaphragm loses its permeability by becoming so plugged or clogged that it is best to replace it. The frequency of diaphragm plugging has previously made it necessary to renew the diaphragm about one to three times for each replacement of the anodes, depending on the amount and nature of the impurities in the brine.

The replacement of the diaphragm previously required the disassembly of the cell by removing the cell top and the cathode section on which is deposited the asbestos diaphragm. The diaphragm was then removed from the cathode and replaced with a newly deposited diaphragm prior to reassembling the cell. In actual practice, spare cathode sections were maintained as replacements to limit the shutdown time of the cell.

Because of the frequency of replacing asbestos diaphragms and the labor involved in disassembling the cell, removing the old diaphragm from the cathode and redepositing a new diaphragm, it has been most desirable to find a means of extending the life of the asbestos diaphragms and thereby to eliminate or reduce the number of. occasions on which the cell must be disassembled.

3,467,586 Patented Sept. 16, 1969 ice It is an object of this invention to provide a method of extending the useful life of asbestos diaphragms used in chlor-alkali cells. It is another object of this invention to provide a method of rejuvenating asbestos diaphragms used in chlor-alkali cells without disassembling the cell. These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

In accordance with the invention a method for rejuvenating asbestos diaphragms used in chlor-alkali cells is provided comprising washing an asbestos diaphragm of restricted permeability with an aqueous solution of a water-soluble acid having an ionization constant greater than 10 wherein said washings are continued until said aqueous solution, when passed through said diaphragm is of a pH of less than about 8.

The present invention provides a method of removing precipitated calcium hydroxide, magnesium hydroxide and the like from the diaphragm pores, thus increasing the diaphragm permeability. The increased permeability results in cells again being cap-able of operation at regular feed rates and at lower cell voltages. The process is readily accomplished in situ, thus eliminating the need for disassembly of the cell and greatly reducing the time, materials and manpower previously required for diaphragm renewal. A further and unexpected benefit of the present method of diaphragm rejuvenation is a lowering of the cell voltage and the continued operation of the cell, after renewal, at a lower cell voltage for an extended period of time.

The process of the present invention is used primarily to rejuvenate a diaphragm when indications have occurred that the diaphragm is becoming restricted. The restriction of flow through the diaphragm is a result of a loss in permeability and is readily apparent since a constant flow of liquor through the diaphragm requires a higher hydrostatic head. This increase in the hydrostatic head is readily observed by an increase in the liquid level in the anolyte compartment. Eventually, the feed rate of brine to the cell is lessened to offset the decreasing rate at which brine passes through the cell. Finally, the restriction of flow through the diaphragm reduces the cell efliciency to the point where it is most desirable to renew the diaphragm.

The invention is further described with reference to the drawing which is a partial sectional view of a typical deposited diaphragm type, chlor-alkali cell.

The cell 10 comprises a base plate 12 on which are positioned anodes 14; cathode section 16 which comprises mesh cathode 18 on which is deposited a porous diaphragm 20; and cell top 22 in which is located brine feed means 24. Mesh cathod 18 is interconnected, forming a continuous opening which forms catholyte compartment 26. The normal flow of brine within the cell is by gravity flow, wherein brine enters the anolyte compartment 28 of cell 10 by means of brine feed 24, and fiows through diaphragm 20 and mesh cathode 18 into catholyte compartment 26. Subsequently, the brine flows out the catholyte compartment by means of drain or perc pipe 30.

In the in situ acid washing of the diaphragm, it is preferred to pass the acid wash through the cell in the same direction or manner of flow.

To rejuvenate the diaphragm by the present method, the flow of brine to the cell is discontinued and the electric current is short-circuited around the cell. The cell is then drained, preferably in a manner such that the catholyte compartment is thoroughly drained. The anolyte compartment, which contains the brine, does not need to be entirely drained. However, since the catholyte compartment contains a solution which has about 12 to 15 percent caustic therein, the thorough draining of this com partment will reduce the amount of acid used in rejuvenating the diaphragm.

Although the rejuvenation is conveniently accomplished within the cell, it is to be understood that a similar rejuvenation by the method of this invention can also be accomplished by disassembling the cell and carrying out the process of this invention on the removed cathode and deposited diaphragm.

After draining of the catholyte compartment, acid is added to the anolyte compartment. The acid added to the anolyte compartment can be added in admixture with brine and when a strong and/ or concentrated acid is used, it is often preferred to dilute the acid with brine and/or water during the washing procedure.

The acid used in the treatment of the diaphragm is any water-soluble acid having an ionization constant greater than The term water-soluble acid means an acid, either organic or inorganic, which is at least sufficiently soluble in water at degrees centigrade, to make a one normal solution. Thus, many organic and inorganic acids are useful in the present process. Such acids as hydrochloric acid, acetic acid, formic acid, sulfuric acid, phosphoric acid, phosphorous acid, and the like, are typical of the aids used, the preferred acid being hydrochloric acid. When acids other than the preferred hydrochloric acid are used, the rejuvenated diaphragm is preferably rinsed to remove the residual anion prior to placing the cell back into operation. However, when hydrochloric acid is used, rinsing is of no consequence since the chloride ion is common with the ions of the system.

Because the cathode, on which the asbestos diaphragm is normally deposited, is frequently constructed of steel or another metal subject to corrosion, and other metal parts may be used within the cell, it is usually desirable to use a corrosion inhibitor with the acid. Such a corrosion inhibitor is especially desirable when it is expected, that over a period of time, a number of diaphragm rejuenvations will be made.

Numerous corrosion inhibitors are known to be effective in reducing or eliminating the corrosive action of acid solutions when in contact with metals such as steel. Thus, water soluble corrosion inhibitors, which are soluble in the acid solution used in the present method, can be used. Typical inhibitors are primary, secondary and tertiary amines and hydroxyl amines, heterocyclic amines such as pyridine and lauryl pyridinium chloride, nitriles, ureas, aldehydes such as furfural, chromates, sugars and molasses, glycerine, quinolines and the like. Short chain tertiary amines and tertiary hydroxyl amines having carbon chains of l to about 8 carbon atoms are frequently used, with triethanolamine and trimetholamine being particularly satisfactory. Also preferred are pyridine compounds and substituted pyridines such as lauryl pyridinium chloride and the like.

The inhibitor is used in an inhibiting amount sufficient to effectively reduce corrosion of metals, such as a steel cathode, during the acid wash. Such an inhibiting amount varies from about 0.01 percent to about 10 percent of corrosion inhibitor by weight of the acid solution.

The acid added to the cell through the anolyte compartment is added in an amount sufficient to solubilize the deposited calcium, magnesium and other compounds which impede liquid flow through the diaphragm thereby removing them with the acid solution. It has been found that when the acid-wash solution passing through the diaphragm reaches a pH of about 8 or less, most of the compounds which have reduced the permeability of the diaphragm have been solubilized. It is preferred, however, that the effluent liquor flowing through the diaphragm reach a pH of about 7 to 4. Achieving this pH is an indication that sufiicient acid has been added to the anolyte compartment. Thus, the effluent from the catholyte compartment preferably reaches a pH of less than 7 and may drop as low as 1, or less.

Although the acid solution may be merely added to the anolyte compartment, it has been found to be preferable to agitate the solution within the anolyte compartment to lessen stratification. Because of the lack of sufficient room within the anolyte compartment to effect complete stirring or mixing, sufficient mixing may be diflicult. However, it has been found that complete mixing can be conveniently accomplished by use of an aspirator on the line feeding liquid into the cell and extending the feed line to the bottom of the anolyte compartment thereby forcing a turbulent liquid-air flow into the anolyte compartment and causing thorough mixing thereof.

After reaching the desired pH, the catholyte compartment is drained of acidic solution and preferably, the anolyte compartment is also drained. The best results are obtained by completely removing all of the acid liquor from the anolyte compartment. When acids other than hydrochloric are used, the diaphragm and cell are preferably rinsed with brine or water to remove the anions. The cell is then filled with brine and is ready to return to service. Since the drainings from the cell contain acid values, these drainings can be used in subsequent acid washings, or as dilution solution therefor.

The invention will be readily understood with reference to the examples which illustrate certain preferred embodiments in the present invention. Unless otherwise indicated, all temperatures are in degrees centigrade and all parts are by weight.

EXAMPLES 1-7 The procedure of the present invention was followed in rejuvenating a number of asbestos deposited diaphragms in production size Hooker S1 cells. The cells treated had been run for varying periods of time ranging from 49 to 117 days and were scheduled for diaphragm renewal due to the high liquid level in the cell and the reduced brine feed rates.

The cell rejuvenation procedure commenced by shutting down the cell using a jumper switch to cut off the electrical power. The perc pipe, which is the means for regulating the liquid level in the cathode compartment, was lowered thereby draining the cathode compartment. Using a brine feed, the brine level within the anolyte compartment was maintained at a level above that of the diaphragm thereby keeping it covered with brine. After about 30 minutes, the cathode compartment was completely drained.

In preparation for the treatment of each cell, 530 grams of triethanolamine and 200 grams of lauryl pyridinium chloride were added to 12 liters of 33 percent muriatic acid. The acid solution was then diluted with about an equal part of water, prior to using as the diaphragm Wash.

With the perc pipe in the lowered position, the prepared acid solution was added through the brine feed opening. During the acid addition, suflicient brine was also added to the cell so as to maintain a liquid level in the anolyte compartment of several inches above the top of the diaphragm. The brine-acid feed was fed with sufficient force to aid in mixing and dispersing the acid throughout the anolyte compartment. The pH of the solution flowing out of the perc pipe was periodically checked with pH paper. Also, the perc pipe was alternately raised and lowered during the acid cleaning oper ation to fill and flush out the catholyte compartment.

Acid solution was added to the anolyte compartment over a period of about 30 to 45 minutes until a pH of about 5 was observed in the drainings from the catholyte compartment. Occasionally, a pH as low as 5 was not observed in the catholyte drainings after all of the prepared acid had been added. In such an instance, if a pH of about 8 had been reached, the diaphragm was considered to have been rejuvenated.

A pH of between 6 and 4 was observed in the drainings of the catholyte compartment of most cells upon the addition of 75 to 100 percent of the prepared acid. Upon reaching the pH of about 5, acid addition was discontinued and the anolyte and catholyte compartments were drained. The anolyte compartment was best drained by siphoning the cell using a hose. Unless most of the acid was removed from the anolyte compartment, the life of the rejuvenated diaphragm was shortened in that a high liquid level resulted soon after the cell was placed back into operation.

After draining the cells, the cells were refilled with fresh brine and were then ready to be placed back into service. Table I summarizes the results obtained in acid washing the diaphragms of production cells as described:

TABLE I Cell voltage After washing (day Before the acid wash of the present invention reduces the cell voltage and aids in maintaining the lower cell voltage for an extended period of time, which benefit increases the power efliciency.

In addition to using the preferred hydrochloric acid as the wash solution, other acids as previously described can be used, particularly sulfuric acid, phosphoric acid, phosphorous acid, acetic acid and formic acid. When using acids other than hydrochloric acid, greater or lesser quantities of acids may be needed depending upon the particular acid ionization constant and the concentration of acid. Stronger acids generally require lesser quantities of acid. In each instance, the proper amount of Brine level in inches Days after acid wash 1 5 10 20 30 washing 1 4 10 50 In each of the above examples, the brine level shown prior to acid washing was the estimated level of the brine within the cell. This level indicates the loss of diaphragm permeability and the retardation of the flow of liquid through the diaphragm. The level was estimated by measuring the anolyte liquid level in inches as shown on the site glass, in excess of the normal level, in relationship to the degree the brine feed rate had been decreased.

Example 1 indicated abrine level of about 24 inches prior to cleaning. This cell had been run for 117 days. After an acid washing, the brine level returned to normal and remained there for an extended period of time. The cell voltage was not measured.

The cell of Example 2 had a brine level of 26 inches and a cell voltage of 3.9 volts. This cell had been in continuous operation for 90 days. After acid washing, the brine level returned to normal and remained there for an extended period of time. In addition, an unexpected benefit in a reduced cell voltage of 0.4 volt was observed. This lower cell voltage also continued for an extended period of time.

The cell of Example 3 had a brine level of 24 inches before cleaning and a cell voltage of 3.8 volts. The cell had been run continuously for 87 days. The acid washing of the diaphragm returned the brine level to normal and reduced the cell voltage by 0.3 volt.

The cell of Example 4 had been run for 61 days and had a brine level of 18 inches. The cell voltage was 4.0 volts. After acid washing the asbestos diaphragm, the brine level was reduced to normal and the cell voltage was lowered by 0.3 volt.

The cell of Example 5 had been run for 93 days and had a brine level of 23 inches. The cell voltage was 4.0 volts. After acid washing the diaphragm, the cell voltage was reduced to 3.6 volts and the brine level was reduced to normal.

The cell of Example 6 had been run for 77 days and had a brine level of 19 inches. Acid washing of the diaphragm reduced the brine level to normal and reduced the cell voltage from 3.9 volts to 3.6 volts.

The cell of Example 7 had been run for 49 days and had reached a brine level of 20 inches. The voltage of this cell was 3.9 volts. After acid washing, the voltage was reduced by 0.3 volt and the brine level returned to normal.

As is clearly illustrated in the examples, the diaphragm renewal of the present invention provides a simple process for renewing diaphragms of chlor-alkali cells with greatly reduced labor and material costs. In addition,

acid to be used is readily determined by observing the pH of the effluent from the catholyte compartment during the acid wash.

It will also be apparent to those skilled in the art that numerous known corrosion inhibitors can be used, other than those specifically cited.

While there have been described various embodiments of the invention, the methods and elements described are not intended to limit the scope of the invention, since it is realized that changes therein are possible. It is intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same result in substantially the same or equivalent manner. It is intended to cover the invention broadly in whatever form its principles may be utilized.

What is claimed is:

1. A method for rejuvenating an asbestos diaphragm of restricted permeability and reducing the operating voltage of an electrolytic chlor-alkali cell which comprises passing an aqueous solution of HCl through said diaphragm until the efiluent exhibits a pH below about 8.

2. The method of claim 1 wherein said acid is passed through said diaphragm until an efiluent pH of about 7 to 4 is achieved. A

3. The method of claim 1 wherein said aqueous solution of HCl contains a corrosion inhibitor.

4. The method of claim 3 in which said solution contains between about 0.01 to 10 percent corrosion inhibitor by Weight of HCl.

5. The method of claim 3 in which said corrosion inhibitor is at least one member selected from the group consisting of soluble primary, secondary, and tertiary amines and hydroxyl amines; heterocyclic amines; nitrides; ureas; aldehydes; chromates; sugars and molasses, glycerine and quinolines.

6. The method of claim 3 in which said corrosion inhibitor is at least one member selected from the group consisting of pyridine, lauryl pyridinium chloride, urea, furfural, molasses, glycerine, quinoline, triethanolamine, trimethanolamine and tertiary amines and hydroxylamines of 1 to 8 carbon atoms.

7. A method of rejuvenating in situ, asbestos diaphragms used in chlor-alkali cells comprising draining the catholyte compartmentsof a chlor-alkali cell, washing the asbestos diaphragm of restricted permeability by adding an aqueous solution of hydrochloric acid and from 0.01 to 10 percent by weight based on the hydrochloric acid of a corrosion inhibitor to the anode compartment of said cell, agitating said solution in said anode References Cited compartment, passing said acid solution through the diaphragm to the cathode compartment until the efiluent UNITED STATES PATENTS pH in the cathode compartment is between about 7 and 4 1,309,214 7/19 9 Moore 4... 2 4-100 and removing excess acid solution from the anode and 5 cathode compartments prior to placing said cell back JOHN MACK, Pflmary Examlner into Operatiofl- D. R. JORDAN, Assistant Examiner 8. The method of claim 7 1n which sa1d COIIOSIOII inhibitor is at least one member selected from the group U,S, CL X,R

consisting of trimethanolamine, triethanolamine and 10 204 295 lauryl pyridinium chloride.

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