TECHNICAL FIELD

The present disclosure relates to a method of temporarily modifying a permeability of an electrolyte permeable diaphragm for the chlor-alkali electrolytic cell.

BACKGROUND

The diaphragm cell process is a well-known method and widely employed in the production of chlorine and caustic soda (sodium hydroxide). The diaphragm cell process includes chlor-alkali electrolytic cell having both an anode area and a cathode area that are separated by an electrolyte permeable diaphragm. Brine introduced into the anode compartment flows through the electrolyte permeable diaphragm into the cathode compartment. The anode acts to produce chlorine gas, while the cathode acts to produce hydrogen gas and hydroxide. Dilute caustic soda and dilute brine are continuously removed from the cathode compartment.

The electrolyte permeable diaphragm is typically formed of a porous mixture of asbestos and polymers. In one embodiment, the electrolyte permeable diaphragm is formed by depositing asbestos and polymer fibers on a foraminous structure, such as a cathode, by drawing the diaphragm from a slurry of the asbestos and polymer fibers onto the surface of the cathode member. The foraminous structure may be a mesh type material (e.g., with holes). The life of these asbestos based electrolyte permeable diaphragms had presented no particular problems because the graphite anodes which were widely used where typically replaced before the cathode. When the electrolytic cells were down for replacement of the anodes the electrolyte permeable diaphragms were also replaced. The metal anodes being used, however, have a life substantially longer than the graphite anodes, thus making longer lasting electrolyte permeable diaphragms desirable. An additional reason for considering asbestos-free electrolyte permeable diaphragms is the environmental concern about the use of asbestos.

In response to this need, non-asbestos electrolyte permeable diaphragms were developed, where such non-asbestos electrolyte permeable diaphragms deliver comparable performance (caustic soda concentration, cell voltage and caustic current efficiency) to the asbestos diaphragms after initial start-up of the chlor-alkali electrolytic cell. However, one of the challenges of this non-asbestos electrolyte permeable diaphragm is its tendency to "self-tighten," which leads to a rapid increase in caustic soda concentration in cell effluent and a decrease in the caustic current efficiency. In extreme cases of "self-tightening", the chlor-alkali electrolytic cell process must be shut down after as little as two weeks, as the electrolyte permeable diaphragm becomes too tight to operate.

One approach to address the issue of self-tightening is to produce non-asbestos diaphragm electrolytic cells that are initially more permeable. Even though non-asbestos diaphragms do not display the self-tightening phenome, they are initially very permeable.

However, even though the electrolyte permeable diaphragm can be initially produced to be more permeable such that the caustic soda concentration would be in a desired range after start-up, the initial permeability of the electrolyte permeable diaphragm in the chlor-alkali electrolytic cell would be so high that it is almost impossible to start the diaphragm cell process without major hardware modification in the plant. As such, non-asbestos diaphragms are treated but only after the initial start-up with e.g. magnesium chloride and/or insoluble micro-size materials such as clays or organic or inorganic fibers to plug the diaphragm so that the produced caustic will be in the desired range. As those non-asbestos diaphragms are initially very permeable and sometimes too permeable, a safe start-up of the production cell is often not guaranteed without major hardware modification. As such, there is a need in the art for an electrolyte permeable diaphragm that can allow for the chlor-alkali electrolytic cell to be started without major hardware modification to the plant.

SUMMARY

The present disclosure provides for an electrolyte permeable diaphragm that allows a diaphragm cell process to be started without major hardware modification to the plant. More specifically, the present disclosure provides an electrolyte permeable diaphragm for a chlor- alkali electrolytic cell, where the electrolyte permeable diaphragm is modified to temporarily reduce the permeability of the electrolyte permeable diaphragm prior to the initial start-up of the diaphragm cell process (e.g., prior to current being applied to the diaphragm cell). For example, temporarily may include a period of 5 mins to 48 hours prior to initial start-up of the electrolyte permeable diaphragm. Temporarily may further include a period of 5 mins to 72 hours after initial start-up of the electrolyte permeable diaphragm.

The method of temporarily modifying the permeability of the electrolyte permeable diaphragm includes installing the electrolyte permeable diaphragm in a chlor-alkali electrolytic cell having an anode compartment and a cathode compartment. The electrolyte permeable diaphragm includes a foraminous structure, particulate material on the foraminous structure, and an alkali compound on the particulate material. Using an aluminum-containing brine a precipitate is formed on the particulate material, which temporarily changes or modifies the permeability of the electrolyte permeable diaphragm. The aluminum-containing brine includes 0.01 to 2 grams of aluminum chloride per liter of the aluminum-containing brine, a pH having a value of 0 to 5 and is used at a temperature of 0 to 60 °C. Exposing the electrolyte permeable diaphragm to the aluminum-containing brine causes aluminum hydroxide to precipitate on the particulate material. The precipitate of aluminum hydroxide forms on the particulate material deposited on the foraminous structure from a reaction of the alkali compound on the particulate material with the aluminum chloride. The presence of the aluminum hydroxide precipitate modifies the permeability of the electrolyte permeable diaphragm.

The aluminum hydroxide precipitate is removed from the electrolyte permeable diaphragm as the chlor-alkali electrolytic cell progresses through its start-up. During start-up a production brine (a brine that does not include the aluminum chloride) is added to the chlor- alkali electrolytic cell, where the production brine begins to replace the aluminum-containing brine. During the start-up a flow of the production brine is provided, where the production brine flows through the electrolyte permeable diaphragm from the anode compartment and the cathode compartment. The chlor-alkali electrolytic cell is energized as the production brine flows through the electrolyte permeable diaphragm from the anode compartment and the cathode compartment, causing the production of caustic soda in the cathode compartment.

As the chlor-alkali electrolytic cell operates, caustic soda is produced in the cathode compartment. This in turn causes the diaphragm to become alkaline and/or more alkaline (e.g., a higher pH), e.g., due to back-migration of the caustic soda from cathode compartment to anode compartment, making the aluminum hydroxide soluble, which was precipitated before from the reaction of aluminum chloride and metal hydroxide on the electrolyte permeable diaphragm. The aluminum hydroxide in its soluble state is then washed from the electrolyte permeable diaphragm as the production brine flows through the electrolyte permeable diaphragm. The aluminum hydroxide is therefore removed from the electrolyte permeable diaphragm thereby changing the permeability of the electrolyte permeable diaphragm. Specifically, as the pH of the electrolyte permeable diaphragm increases (e.g., becomes more alkaline) the precipitate of aluminum hydroxide reacts to form a hydroxoaluminate. The flow of the production brine to the anode compartment washes the hydroxoaluminate from the chlor-alkali electrolytic cell, thereby changing the permeability of the electrolyte permeable diaphragm.

Depositing the alkali compound on the particulate material of the electrolyte permeable diaphragm can be accomplished by exposing the electrolyte permeable diaphragm to an alkaline solution of the alkali compound. In one embodiment, exposing the electrolyte permeable diaphragm to an alkaline solution of the alkali compound is done by filling the anode

compartment and the cathode compartment of the chlor-alkali electrolytic cell with the alkaline solution of the alkali compound, where the alkali compound deposits on the particulate material of the electrolyte permeable diaphragm. Another example includes exposing the electrolyte permeable diaphragm to an alkaline solution of the alkali compound by spraying the alkaline solution of the alkali compound onto the electrolyte permeable diaphragm to deposit the alkali compound on the particulate material of the electrolyte permeable diaphragm. Other techniques include, but are not limited to, adding the alkali compound on the particulate material during the production of the electrolyte permeable diaphragm, where the alkali compound is a part of the slurry that produces the electrolyte permeable diaphragm.

For the various embodiments, the alkali compound is an alkali hydroxide. The alkali hydroxide can be selected from the group of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide or combination thereof. The electrolyte permeable diaphragm can include 10 to 500 grams of the alkali compound per square meter of the foraminous structure.

For the embodiments, magnesium chloride, magnesium hydroxide or a combination thereof can be included in the aluminum-containing brine. Providing magnesium chloride, magnesium hydroxide or a combination thereof can include providing up to 500 grams of magnesium chloride, magnesium hydroxide or a combination thereof per square meter of the foraminous structure.

The present disclosure also includes the electrolyte permeable diaphragm itself, where the electrolyte permeable diaphragm includes a foraminous structure, particulate material on the foraminous structure, and aluminum hydroxide on the particulate material. As discussed herein, the aluminum hydroxide is the reaction product of an alkali compound on the particulate material and aluminum chloride. In another example, aluminum sulfate can be also used instead of aluminum chloride. The particulate material on the foraminous structure can be formed from a variety of materials. For example, the particulate material on the foraminous structure can include a polyfluoroethylene fiber; a water-wettable, inert, inorganic, micron-size material; and a polymer dispersion sintered with the polyfluoroethylene fiber and the water-wettable, inert, inorganic, micron-size material. In one embodiment, the polymer dispersion is a polyfluoroethylene dispersion. The water-wettable, inert, inorganic, micron-size material can be selected from the group of talc, metal silicates, alkali metal titanates, alkali metal zirconates, magnesium aluminates and combinations thereof.

In an additional example, the particulate material on the foraminous structure includes zirconium oxide and poly(tetrafluoroethylene) in fibrous and particulate forms. In another example, the particulate material on the foraminous structure includes a contiguous sheet of pulverized expanded vermiculite. The particulate material on the foraminous structure can further include a fibrous polymer reinforcement. A particulate polymer binder can also be incorporated in the fibrous polymer reinforcement. In one embodiment, the particulate polymer binder is a fluoropolymer. For the various embodiments, the electrolyte permeable diaphragm can include 2000 to 6000 grams of the particulate material per square meter of the foraminous structure.

The present disclosure also includes an electrolyte permeable diaphragm, as provided herein, formed by any one of the methods provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a partial view of an electrolyte permeable diaphragm according to the present disclosure.

Fig. 2 is a partial view of an electrolyte permeable diaphragm according to the present disclosure.

The images seen in the Figs, are not to scale.

DETAILED DESCRIPTION

The present disclosure provides for an electrolyte permeable diaphragm that allows a diaphragm cell process to be started without major hardware modification to the plant. More specifically, the present disclosure provides an electrolyte permeable diaphragm for a chlor- alkali electrolytic cell, where the electrolyte permeable diaphragm is modified to temporarily reduce the permeability of the electrolyte permeable diaphragm prior to the initial start-up of the diaphragm cell process.

The method of temporarily modifying the permeability of the electrolyte permeable diaphragm includes installing the electrolyte permeable diaphragm in a chlor-alkali electrolytic cell having an anode compartment and a cathode compartment. The chlor-alkali electrolytic cell is used in a diaphragm cell process for electrolysis of sodium chloride (NaCl) to produce chlorine (Ch) gas and sodium hydroxide (NaOH, also known as caustic soda). The diaphragm cell process is conducted on a brine (e.g., an aqueous solution of sodium chloride) which produces sodium hydroxide, hydrogen (H2) gas and chlorine gas. As an alternative to sodium chloride, other metal chlorides such as potassium chloride can be used with the method of the present disclosure.

In the diaphragm cell process, the electrolyte permeable diaphragm separates a cathode compartment and an anode compartment. Typically, production brine is introduced into the anode compartment and flows into the cathode compartment. When energized, chloride ions are oxidized at the anode to produce chlorine, and at the cathode, water is split into caustic soda and hydrogen. So, chlorine gas is produced in the anode compartment, while hydrogen gas and dilute caustic soda in the diluted brine are produced in the cathode chamber. One function of the electrolyte permeable diaphragm is to prevent the reaction of the caustic soda in the cathode chamber with the chlorine in the anode compartment or more importantly between the hydrogen and chlorine.

The production brine flowing in the diaphragm cell process may be alkaline, but it could be acidic.

Referring to Fig. 1, there is shown an electrolyte permeable diaphragm 100 (partial view) of the present disclosure. The electrolyte permeable diaphragm 100 includes a foraminous structure 102, particulate material 104 on the foraminous structure 102, and an alkali compound 106 on the particulate material 104. As discussed herein, the foraminous structure 102 is a fenestrated electrode that is coated with the particulate material 104 in a slurry process by lowering the foraminous structure 102 into a slurry of the particulate material and depositing the particulate materials on the surface of the foraminous structure 102 by drawing the slurry onto the foraminous structure 102 using a vacuum.

As discussed herein, the electrolyte permeable diaphragm 100 of the present disclosure is used in a method of temporarily modifying the permeability of the electrolyte permeable diaphragm in a chlor-alkali electrolytic cell. The permeability of the electrolyte permeable diaphragm is temporarily modified by first installing the electrolyte permeable diaphragm 100 between an anode compartment and a cathode compartment of the chlor-alkali electrolytic cell. The electrolyte permeable diaphragm 100 is then exposed to an aluminum-containing brine in the chlor-alkali electrolytic cell. In one embodiment, exposing the electrolyte permeable diaphragm 100 to the aluminum-containing brine is accomplished by filling the anode compartment of the chlor-alkali electrolytic cell with the aluminum-containing brine. When the aluminum-containing brine passes through the electrolyte permeable diaphragm 100 into the cathode compartment the aluminum reacts in the alkaline environment created by the alkali compound 106 on the particulate material 104 to form aluminum hydroxide (Al(OH)3), which precipitates in the diaphragm and reduces the permeability of the diaphragm. Filling the chlor- alkali electrolytic cell with the aluminum-containing brine can stop when the cell is filled. In an additional embodiment, the aluminum-containing brine can be circulated between the anode chamber and the cathode chamber (or vice versa) to create a brine hydraulic pressure across the anode and cathode compartment so that the aluminum-containing brine flows through the electrolyte permeable diaphragm 100 to fill the cathode compartment, where a precipitate of aluminum hydroxides forms on the particulate material on the foraminous structure from a reaction with the aluminum chloride (AlCh).

Aluminum chloride is selected for temporary reduction of diaphragm permeability due to its ability to be soluble at an acidic pH range and a basic pH range and insoluble (as aluminum hydroxide, Al(OH)3) in a pH range positioned between these two solubility ranges. Specifically, in a temperature range of 0 to 60 °C, aluminum chloride is soluble in an aqueous solvent having a pH of less than 5.5, and an aqueous solvent having a pH 8 or greater (soluble as Α1(ΟΗ)4 ^" )· Aluminum chloride, however, forms a precipitate (insoluble aluminum hydroxide Al(OH)3) in an aqueous solvent at a pH in a range of 5.5 to 7. This ability to change solubility based on the pH allows for the permeability of the electrolyte permeable diaphragm 100 to be modified as discussed herein. It is also appreciated that other metal halides, oxides, and/or hydroxides having similar properties as those seen in aluminum chloride can be used in the present disclosure.

The aluminum-containing brine includes 0.01 to 2 grams of aluminum chloride per liter of the aluminum-containing brine. For example, the aluminum-containing brine includes 0.01 to 1 grams of aluminum chloride per liter of the aluminum-containing brine. For example, the aluminum-containing brine includes 0.05 to 0.5 grams of aluminum chloride per liter of the aluminum-containing brine, and for another example the aluminum-containing brine includes 0.1 to 0.3 grams of aluminum chloride per liter of the aluminum-containing brine. Other examples of the aluminum-containing brine include 0.1 to 1.2 grams of aluminum chloride per liter of the aluminum-containing brine and for another example 0.5 to 1.1 grams of aluminum chloride per liter of the aluminum-containing brine. In one example the aluminum-containing brine includes 1 gram of aluminum chloride per liter of the aluminum-containing brine. The aluminum- containing brine can also include 2 to 100 grams of aluminum (from the aluminum chloride) per square meter of the foraminous structure. For example, the aluminum-containing brine includes 3 to 20 grams of aluminum (ion from the aluminum chloride) per square meter of the foraminous structure. As provided herein, the square meter of the foraminous structure is measured based on the peripheral edge(s) of the foraminous structure.

In addition to the aluminum chloride, the aluminum-containing brine includes water and 10 to 310 grams and/or 200 to 300 grams of sodium chloride per liter of aluminum-containing brine. The aluminum-containing brine has a pH having a value of 0 to 5. For example, the aluminum-containing brine has a pH having a value of 1 to 5, a pH having a value of 1 to 4, and/or a pH having a value of 3 to 4. The aluminum-containing brine is used in the chlor-alkali electrolytic cell at a temperature of 0 to 60 °C. For example, the aluminum-containing brine is used in the chlor-alkali electrolytic cell at a temperature of 5 to 40 °C, and/or at a temperature of 10 to 30 °C.

In an additional embodiment, magnesium chloride, magnesium hydroxide or a combination thereof can be included in the aluminum-containing brine at a concentration of 0.01 to 0.2 grams per liter of the aluminum-containing brine. Including magnesium chloride, magnesium hydroxide or a combination thereof in the aluminum-containing brine can help to prolong the effect of the temporary reduction of permeability discussed herein. When magnesium chloride, magnesium hydroxide or a combination thereof is provided in the aluminum-containing brine it can include providing 0.01 up to 500 grams of magnesium (ion from magnesium chloride, or magnesium hydroxide or a combination thereof) per square meter of the foraminous structure. In one example, when magnesium chloride, magnesium hydroxide or a combination thereof is provided in the aluminum-containing brine it can include providing 0.01 to 10 grams of magnesium (ion from magnesium chloride, or magnesium hydroxide or a combination thereof) per square meter of the foraminous structure. In another example, when magnesium chloride, magnesium hydroxide or a combination thereof is provided in the aluminum-containing brine it can include providing 5 grams of magnesium (ion from magnesium chloride, or magnesium hydroxide or a combination thereof) per square meter of the foraminous structure. In another example, when present, 0.01 to 3 grams of magnesium are present per square meter of the foraminous structure.

As discussed herein, exposing the electrolyte permeable diaphragm 100 to the aluminum- containing brine causes a precipitate of aluminum hydroxide to form on the particulate material on the foraminous structure. The precipitate of aluminum hydroxide forms on the particulate material on the foraminous structure from a reaction of the alkali compound on the particulate material with the aluminum chloride. The precipitate of aluminum hydroxide on the particulate material acts to reduce the permeability of the electrolyte permeable diaphragm 100 as compared to the electrolyte permeable diaphragm 100 without the precipitate of aluminum hydroxide. So, using the aluminum-containing brine causes a precipitate to form on the particulate material, which temporarily changes or modifies the permeability of the electrolyte permeable diaphragm. The degree to which the presence of the precipitate on the particulate material changes the permeability of the electrolyte permeable diaphragm 100 can be determined using changes in hydraulic differential pressure of the brine flow through the diaphragm.

In an additional exemplary embodiment, it is possible to provide an insoluble additive in the aluminum-containing brine if the initial permeability of the electrolyte permeable diaphragm 100 is too high. The insoluble additive is selected from the group of clay, pulverized non- asbestos diaphragm, fibrous material or a combination thereof. In one embodiment, if an insoluble additive such as clay, pulverized non-asbestos diaphragm or fibrous material is added to the aluminum-containing brine, it is added at a concentration of 0.1 to 2 grams per liter of the aluminum-containing brine. In another embodiment, 5 to 200 grams of the insoluble additive can be used in the aluminum-containing brine per square meter of the electrolyte permeable diaphragm. In another embodiment, when present, 10 to 60 grams of the insoluble additive can be used in the aluminum-containing brine per square meter of the electrolyte permeable diaphragm.

Clay could be any one of fine-grained natural rock or soil material that combines one or more clay minerals with traces of metal oxides and organic matter, for example vermiculite, sepiolith, kaolin, attapulgite, bentonite and so on. Pulverized non-asbestos diaphragm could be any non-asbestos diaphragm, for example, those diaphragms described in the U.S. Patent Nos. 4,426,272; 4,606,805; 5,685,755; 4, 170,538; 4,720,334; 6,059,944; 6,299,939; U.S. Patent Pub. Nos. 2006/0042936 Al; 2008/0289956 Al or PCT Publication WO 2009/071565 A2. The fibrous material could any organics or inorganic fibers, for example, PTFE fiber and ceramic fibers.

The change in permeability is, however, temporary in that after start-up of the diaphragm cell process the precipitate on the particulate material dissolves into the production brine (brine that does not contain the aluminum chloride) as the pH of the production brine changes due to the production of the caustic soda. Specifically, the aluminum hydroxide precipitate is removed from the electrolyte permeable diaphragm as the chlor-alkali electrolytic cell progresses through its start-up. During the start-up of the chlor-alkali electrolytic cell, the aluminum-containing brine is replaced with the production brine. To do this, a flow of the production brine is provided through the chlor-alkali electrolytic cell (e.g., from the anode compartment into the cathode compartment through the electrolyte permeable diaphragm), where the production brine does not include the aluminum chloride. The presence of the aluminum hydroxide precipitate on the particulate material 104 helps to initially reduce the permeability of the electrolyte permeable diaphragm at the beginning of the start-up process.

For the present disclosure, the flow of the production brine at the beginning of the startup process is sufficient to provide a hydraulic differential pressure across the electrolyte permeable diaphragm to a brine height of 10 to 50 cm. For example, the flow of the production brine can be from 6 to 20 1/h/m ² , which is sufficient to provide a brine height of 20 to 80 cm.

As the production brine (200 to 310 grams per liter of NaCl, a pH of 0-14, and/or 7-12) begins to flow through the electrolyte permeable diaphragm the chlor-alkali electrolytic cell is energized, causing the production of caustic soda in the cathode compartment. As the chlor- alkali electrolytic cell operates to produce caustic soda in the cathode compartment, the diaphragm becomes alkaline and/or or more alkaline, e.g., due to back-migration of hydroxide ions (OH ^" ) from cathode compartment to the anode compartment during the operation of the chlor-alkali electrolytic cell. As the diaphragm becomes alkaline and/or more alkaline, the aluminum hydroxide on the particulate material 104 begins to dissolve into the electrolyte flux from anode to cathode compartment through the diaphragm, and is flushed out by electrolyte flux. Specifically, as the diaphragm becomes alkaline and/or more alkaline the precipitate of aluminum hydroxide reacts to form a hydroxoaluminate. The flux of the electrolyte from the anode compartment to the cathode compartment through the diaphragm washes the hydroxoaluminate from the chlor-alkali electrolytic cell, thereby changing the permeability of the electrolyte permeable diaphragm.

For the various embodiments, energizing the chlor-alkali electrolytic cell to produce caustic in the cathode compartment includes providing a current density in the chlor-alkali electrolytic cell of 0.5 to 3 kiloampere (KA) per square meter of the foraminous structure. The chlor-alkali electrolytic cell can be operated with the production brine at a temperature in the range of 40 to 90 °C, and/or from 60 to 80 °C. Providing the flow of the production brine to the anode compartment provides for a caustic soda concentration in an effluent leaving the cathode compartment of 5 to 15 weight percent sodium hydroxide based on the total weight of the effluent. In an exemplary embodiment, providing the flow of the production brine to the anode compartment provides for a caustic soda concentration in an effluent leaving the cathode compartment of 8 to 12 weight percent sodium hydroxide based on the total weight of the effluent. The anolyte can have a pH of 1 to 5, where a pH of 2 to 4 is an example.

Referring again to Fig. 1, the alkali compound 106 is shown on the particulate material 104. The alkali compound 106 provides alkalinity to the electrolyte permeable diaphragm 100, where such alkalinity can be introduced either during or after the manufacture of the electrolyte permeable diaphragm 100. For the various embodiments, the alkali compound is an alkali hydroxide. The alkali hydroxide can be selected from the group of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide or combination thereof. For example, the alkali hydroxide is sodium hydroxide. The alkaline compound 106 may be introduced during production of the electrolyte permeable diaphragm 100, but may be introduced later after completion of the production of the electrolyte permeable diaphragm 100.

The alkali compound 106 can be deposited on the particulate material 104 of the electrolyte permeable diaphragm 100 in a variety of ways. For example, the alkali compound 106 is deposited on the particulate material 104 of the electrolyte permeable diaphragm 100 by exposing the electrolyte permeable diaphragm 100 to an alkaline solution of the alkali compound. In one embodiment, exposing the electrolyte permeable diaphragm to an alkaline solution of the alkali compound is done by filling the anode compartment and the cathode compartment of the chlor-alkali electrolytic cell with the alkaline solution of the alkali compound, where the alkali compound deposits on the particulate material of the electrolyte permeable diaphragm. In another example, exposing the electrolyte permeable diaphragm to an alkaline solution of the alkali compound is done by spraying the alkaline solution of the alkali compound onto the electrolyte permeable diaphragm to deposit the alkali compound on the particulate material of the electrolyte permeable diaphragm. Other techniques include, but are not limited to, adding the alkali compound on the particulate material during the production of the electrolyte permeable diaphragm, where the alkali compound is a part of the slurry that produces the electrolyte permeable diaphragm.

The amount of alkali compound 106 present on the particulate material 104 can range from 0.1 to 20 weight percent based on the total weight of the electrolyte permeable diaphragm 100. For example, the amount of alkali compound 106 present on the particulate material 104 can range from 3 to 15 weight percent based on the total weight of the electrolyte permeable diaphragm 100. For example, the amount of alkali compound 106 present on the particulate material 104 can range from 5 to 10 weight percent based on the total weight of the electrolyte permeable diaphragm 100. Using the techniques discussed herein, the electrolyte permeable diaphragm can include 10 to 500 grams of the alkali compound on the particulate material 104 per square meter of the foraminous structure.

Referring now to Fig. 2, there is shown an embodiment of an electrolyte permeable diaphragm 200 (partial view) of the present disclosure, which includes the foraminous structure 202 and the particulate material 204 on the foraminous structure 202, as discussed herein. The electrolyte permeable diaphragm 200 seen in Fig. 2 also shows the aluminum hydroxide 208 on the particulate material 204, as discussed herein.

The particulate material on the foraminous structure can be formed from a variety of materials and using a variety of methods, as discussed herein. For example, the particulate material on the foraminous structure can include a polyfluoroethylene fiber; a water-wettable, inert, inorganic, micron-size material; and a polymer dispersion sintered with the

polyfluoroethylene fiber and the water-wettable, inert, inorganic, micron-size material. In one embodiment, the polymer dispersion is a polyfluoroethylene dispersion. The water-wettable, inert, inorganic, micron-size material can be selected from the group of talc, metal silicates, alkali metal titanates, alkali metal zirconates, magnesium aluminates and combinations thereof. In an additional example, the particulate material on the foraminous structure includes zirconium oxide and poly(tetrafluoroethylene) in fibrous and particulate forms. In another example, the particulate material on the foraminous structure includes a contiguous sheet of pulverized expanded vermiculite. The particulate material on the foraminous structure can further include a fibrous polymer reinforcement. A particulate polymer binder can also be incorporated in the fibrous polymer reinforcement. In one embodiment, the particulate polymer binder is a fluoropolymer. Examples of such electrolyte permeable diaphragms include the non-asbestos diaphragms discussed and disclosed in U.S. Patent No. 4,426,272, which is incorporated herein by reference in its entirety. Other examples of electrolyte permeable diaphragms include those discussed and disclosed in U.S. Patent Nos. 4,606,805; 5,685,755; 4, 170,538; 4,720,334;

6,059,944; 6,299,939; U.S. Patent Publication Nos. 2006/0042936 Al; 2008/0289956 Al; and Publication WO 2009/071565 A2, all of which are incorporated herein by reference in their entirety. For the various embodiments, the electrolyte permeable diaphragm can include 2000 to 6000 grams of the particulate material per square meter of the foraminous structure.

An example of an electrolyte permeable diaphragm for use in the present disclosure includes those formed using an aqueous slurry containing as one of the particulate materials 104, at least 50 percent by weight of a water-wettable, inert, inorganic, micron-size material. The inorganic material is water-wettable in order to be readily dispersed in the aqueous media. Also, the other particulate materials included in the slurry are hydrophobic in nature so that a hydrophilic or water-wettable material is needed to enable the resulting electrolyte permeable diaphragm to be wetted by the aqueous electrolyte in the electrolytic cell. Such particulate materials include talc, various metal silicates, the alkali metal (including magnesium) titanates and zirconates, and magnesium aluminates such as spinel. By "micron-size" is meant those materials whose average diameter may vary from about 0.2 micron up to as much as about 10 microns, with particles of about 1.5 microns being an example. The inorganic material should constitute at least about 50 percent of the weight of the particulate materials in the slurry and may range as high as about 95 percent. Exemplary diaphragms contain from about 70 to about 90 percent of inorganic materials, e.g., with about 81 percent.

A second particulate material included in the slurry is polyfluoroethylene fiber. The term "polyfluoroethylene," as used herein, is meant to include any polymer of a halogenated ethylene wherein the halogen atoms consist of at least one fluorine atom and the balance, if any, chlorine. The fluorine atoms appear to impart stability to the polymer when used in electrolytic cells. A material for use in the present embodiment is polytetrafluoroethylene.

The word "fiber" refers herein to a product in elongated form which may or may not be branched or feathered. These fibers may have a diameter of from about 1 to about 10 microns and may vary in length from about 1/32 inch to about 1/2 inch, where fibers of about 1/4 inch in length are an example. These fibers make possible the deposition of the particulate materials on the foraminous structure from a slurry and add to the structural integrity of the resulting diaphragm. An example of polyfluoroethylene fibers is material that is sold by Toray

Fluorofibers (America) Inc., Decatur, AL. The polymeric fibers employed in the present embodiment frequently occur commercially in the form of "floe" or bundles of fibers which need to be combed apart or separated for proper dispersion in the aqueous slurry for the most advantageous practice of the present embodiment.

Enough of the fibrous material is used to enable deposition of the particulate materials on the foraminous structure when drawing the diaphragm from the slurry. The amount of polyfluoroethylene fiber needed will vary with the particular inorganic material employed, and may range from about 1 to about 20 percent of the total weight of the particulate materials in the slurry, with about 5 to about 20 percent being an example, and with about 9 percent being another example for inorganic materials such as talc.

The third particulate material included in the slurry of the present disclosure is a polyfluoroethylene dispersion. This dispersion (sometimes referred to as a latex) comprises very small droplets of polyfluoroethylene dispersed in an aqueous medium, which usually includes various wetting or dispersing agents. Examples of such a dispersion include the

polytetrafluoroethylene dispersions sold by Chemours under the trade names PTFE DISP XL. The polyfluoroethylene dispersion functions as a binder for the inert, inorganic particles and is used in minor amounts ranging from about 2 percent to about 30 percent, of the weight of particulate materials in the slurry, with about 5 to about 30 percent being an example, and with amounts approximating 10 percent being another example. Again, the amount of polymeric dispersion required will vary with the particular inorganic material used in the slurry.

For the present embodiment, a slurry is made up by adding the particulate materials above-described to an aqueous medium to obtain a concentration of about 50 to about 100 grams of particulate material per liter of aqueous medium. This addition is accompanied by sufficient stirring to obtain uniform wetting and dispersion of the particulate materials. Cell effluent is an exemplary aqueous medium, since it is readily available and also because it produces diaphragms having excellent porosity. Cell effluent from electrolytic cells used in the production of sodium hydroxide and chlorine normally contains from about 50 to about 200 grams per liter of sodium hydroxide and from about 260 to about 160 grams per liter of sodium chloride. As used herein "cell effluent" includes synthetically produced cell effluent, that is, any aqueous media to which caustic (e.g. 32% or 50% caustic or solid NaOH) and solid salt may be added in the amounts normally found in the effluent from the chlor-alkali electrolytic cell. The aqueous medium for the slurry may be distilled or deionized water or water to which no salt or caustic has been added, or it may be water containing a number of inorganic salts or caustic materials added thereto, with or without various wetting agents. In practicing the process of the present embodiment, a foraminous structure, which is usually a hollow, finger-type electrode, is lowered into the slurry and the particulate materials deposited on the surface of the electrode by drawing the slurry therethrough by means of a vacuum applied to the interior of the electrode. During the course of the drawing process, the foraminous structure may be removed at various stages from the slurry, with the vacuum still being applied to dry the diaphragm and to consolidate the mass and make it sufficiently strong to be handled. These steps of dipping the diaphragm and drying it are continued until a diaphragm of the desired thickness and weight has been deposited on the foraminous structure.

After the diaphragm has been deposited on the electrode, the diaphragm coated electrode is dried overnight in an oven at a temperature between about 100 °C and about 120 °C. Any suitable means for drying the diaphragm coated electrode may be employed, however, as long as a substantial portion of the water is removed from the diaphragm. The next step comprises heating the diaphragm until the polyfluoroethylene dispersion particles sinter or soften to the point that they adhere to one another and to the inert, inorganic particles and polyfluoroethylene fibers. It is probable that some of the polyfluoroethylene fibers soften to the point that they too adhere to other particulate materials in the diaphragm. This sintering step increases the structural integrity of the diaphragm. To sinter diaphragms made from polytetrafluoroethylene these diaphragms are heated to temperatures approximating 350 °C for about one-half hour to affect this sintering or softening. The temperature and the time may vary, of course, with the melting point of the particular polyfluoroethylene employed. It will be apparent to one skilled in the art that the drying and sintering of the diaphragm may be a continuous procedure, beginning at the lower drying temperatures and then increasing the temperature to effect sintering.

Another specific example of an electrolyte permeable diaphragm for use in the present disclosure as generally discussed above includes those formed using vermiculate. Vermiculite is a naturally-occurring mineral of lamellar structure which is obtained by mining. It is a monoclinic hydrated magnesium silicate generally illustrated by the empirical formula

Mg3Si40io(OH)2. χΗ ₂ 0. When heated quickly to about 250 to 300 °C vermiculite undergoes a rapid and large (up to 30 times original volume) expansion in a direction parallel to the C-axis, which is normal to the lamellar platelet structure. It is this expanded vermiculite which is ground-up (pulverized) for use in one embodiment of the electrolyte permeable diaphragm of the present disclosure. A convenient method for pulverizing the expanded vermiculite is by a cement mixer, mortar mixer, nip roller, hydropulper, or Waring blender, depending on what final average particle size is desired. Ordinarily, the vermiculite particles of greatest interest in the present disclosure will be of a size which, prior to being expanded, will pass through a 16-mesh screen (U.S. standard sieve size). The step of pulverizing breaks platelets apart from other platelets in the expanded lamellar structure and even breaks up platelets themselves. Ordinarily, and desirably, the platelet fragments after the pulverizing step have dimensions in the c-axis which are only 1/2 or less the dimensions in the a-axis (normal to the c-axis).

Binders for the vermiculite, if desired, can include water-soluble binders that exhibit low shrinkage upon drying, such as polyvinyl alcohol or an aliphatic resin copolymer, or even a dextrine/glycerol mixture. In an example, a heat-bondable polymer can be used, where the heat- bondable polymer becomes heat-plastified, or will at least heat-sinter. Various polyolefins may be employed, including olefins and diolefins which have organic or inorganic substituents; vinyl and acrylic type olefins are included. For example, the polymer is a fluoropolymer in order to supply additional chemical resistance.

Some thermoplastic fluoropolymer latexes, such as polyvinylidene fluoride, have a negative surface charge in water; so does vermiculite. Hence in a slurry the two will repel each other and the plastic resists attaching to the vermiculite when heated to effect bonding. When the slurry is acidified, for example with HC1, the plastic will attach to the vermiculite.

The fluorocarbon polymer may be solid, particulate polymers or copolymers of tetrafluoroethylene, trifluoroethylene, or dichlorodifluoroethylene or may be fluorinated ethylene/propylene copolymer commonly known as FEP. Also, a copolymer of

ethylene/chlorotrifluoroethylene known as Halar® may be used. For example, the fluorocarbon polymer is polyvinylidene fluoride, fluorinated ethylene/propylene copolymer, or

polytetrafluoroethylene. For example, the fluorocarbon polymer is polytetrafluoroethylene.

In an exemplary embodiment, pulverized expanded vermiculite is combined in aqueous slurry with a polymer, especially a fluorocarbon polymer and the resulting slurry is deposited on foraminous structure 102. The polymer is then heat-sintered to add mechanical and chemical stability to the deposited mineral. The weight ratio of vermiculite/polymer is for example in the range of about 20: 1 to 1 : 1 and in another example the ratio is about 4: 1. The slurry may also contain minor amounts of processing aids such as surfactants, wetting agents, or dispersing agents, or modifiers, such as pH-adjusters, inorganic metal compounds, e.g., T1O2, CaCCb, MgO and CaO. Such processing aids or modifiers may be employed in order to help disperse the polymer and the vermiculite uniformly in the aqueous medium and to impart certain porosity features to the diaphragm.

The polymer aqueous slurries or dispersions may be commercially available and generally contain such processing aids or modifiers as stabilizers, surfactants or dispersing agents. Such polymer dispersions may also be prepared for use in the present disclosure by dispersing particulate or fibrous polymer in an aqueous medium by using wetting agents, surfactants, dispersing agents, or stabilizers which help to disperse the polymers and/or stabilize such dispersions.

The vermiculite and polymer slurry may be deposited on the foraminous structure 102 by being vacuum-drawn. By vacuum-drawn it is meant that a slurry of the diaphragm ingredients (vermiculite, polymer and/or modifiers) is contacted with one side of the foraminous structure 102 and a "vacuum" (reduced pressure) is applied to the other side to pull the solids tightly into place against the foraminous structure 102 while pulling the liquid on through.

Other methods of depositing the diaphragm onto the foraminous structure 102 include the use of gravity flow or positive pressure to force the slurry against a porous surface, thereby depositing the solids in the form of a mat or web while the liquid flows on through the porous surface. The mat or web of diaphragm material may be prepared on a surface other than the foraminous structure 102 (such as by using a Fourdrinier process) and then transferred to the foraminous structure 102.

In general, an exemplary method of preparing embodiments of the present diaphragms for use in a chlor-alkali electrolytic cell is as follows:

1. The pulverized expanded vermiculite and the fibrous and/or particulate polymers

(the particulate materials) are intimately mixed and slurried in an aqueous medium. The aqueous slurry also contains any modifiers, surfactants and/or acidifiers, as desired. The total amount by weight of the combined polymer fibers and particles may be from about 0 parts to about 200 parts per hundred parts of total vermiculite, for example the amount is about 5 to 100 parts with about 20 to 50 parts being another example. The polymer content may be in the form of fine particles, fibers, or a mixture of both. For example, the polymer content is substantially particulate rather than fibrous. 2. The slurried particulate materials are deposited on the foraminous structure 102 to the desired weight, e.g., 0.5 grams to 3.0 grams per in ² , and dried. For example, the weight is about 1 to about 2 grams per in ² of the foraminous structure 102.

3. The particulate materials on the foraminous structure 102 are subjected to a sufficient amount of heat to cause sintering or bonding of the polymer particles (when they are present in the mixture); pressure may be applied, if desired, either by placing a positive force against the diaphragm or by using a vacuum (reduced pressure) on the other side of the foraminous structure 102 which will draw the particulate materials of the diaphragm tightly against the foraminous structure 102 during the sintering operation. The amount of heat will depend, to a large extent, on which polymer is being used; the sintering temperature (or softening temperature) of the desired polymer is easily determined experimentally or is available in publications. If no binder or fibers are used in the vermiculite, the need for heat-bonding is obviated and the diaphragm may be only de-watered, thereby forming a sheet.

4. The resulting diaphragm is placed into position in the electrolytic cell and, in some cases, is "pre-wetted" by being soaked with a water-soluble wetting agent such as detergent, surfactant, methanol, or acetone to make the diaphragm less hydrophobic. Then it is generally flushed with water, anolyte, or brine after which the alkali compound is applied to the particulate materials 104 deposited on the foraminous structure 102 as discussed herein.

Another example of an electrolyte permeable diaphragm for use in the present disclosure includes those formed using zirconium oxide as a principal constituent of the diaphragm, with polytetrafluoroethylene (PTFE) in fibrous and powdered forms being the other constituent materials of the diaphragm and with one or both of the PTFE materials (that is, the PTFE fibers and powdered PTFE) having a thin, durable coating of an ion-containing polymer placed thereon. The thin, durable coating of an ion-containing polymer can be formed using one of three methods.

The first method is described in U.S. Pat. No. 5,993,907, which is incorporated herein by reference in its entirety. In the context of the referenced coating process, the chemically-resistant materials to be coated are contacted with a colloidal, surface active dispersion of an ion- containing polymer and then the dispersion-wetted materials (while still wetted with the colloidal dispersion or solution (excess dispersion can be removed from contact with the chemically- resistant materials)) are contacted with a solution of a salt or of a strongly ionizing acid which is of a sufficient concentration to cause a preferably essentially continuous adherent coating of the ion-containing polymer to be formed on the surface of the chemically-resistant materials.

The salt-contacting step is in this first embodiment may be conducted through the preparation of a NaCl or Na ₂ CCb based draw slurry included the dispersion-wetted materials, and the diaphragm drawn therefrom is dried and bonded, with the bonding step providing an annealing of the coated materials whereby the adhesion of the coating to the materials is enhanced as compared to an unannealed, coated material.

A second, exemplary embodiment employs a coating process as described in U.S. Pat. No. 5,716,680, which is incorporated herein by reference in its entirety. The incorporated application describes a solventless coating process which involves adding a colloidal, surface active dispersion in water of a perfluorocarbon ionomer and a salt or a strongly ionizing acid to a vessel containing a polymeric chemically-resistant substrate such as PTFE, with the salt or acid being added in an amount such that a solution results of a sufficient ionic strength to cause an adherent, preferably essentially continuous coating of the perfluorocarbon ionomer to be formed on the surface of the powdered and/or fibrous PTFE under conditions of high shear or significant agitation, and subjecting the dispersion, salt or acid and PTFE materials to such conditions whereby a thin, durable coating of the perfluorocarbon ionomer is formed on the PTFE materials.

In the context of the present disclosure, conventionally the salt employed is NaCl or Na ₂ CCb for forming a draw slurry including the coated PTFE or other chemically-resistant material to be incorporated in the diaphragm, and the remaining diaphragm constituents are incorporated with the salt solution/ionomer dispersion/PTFE mixture to form the draw slurry directly. Thereafter the slurry is drawn through a foraminous support to form a diaphragm thereon, and the diaphragm dried and bonded as in the first embodiment.

In a third, exemplary embodiment, a process is provided as more fully described in U.S.

Pat. No. 5,746,954, which is incorporated herein by reference in its entirety, for manufacturing a diaphragm for use in a chlor-alkali diaphragm cell which comprises coating a substrate which is to be incorporated into the diaphragm and with respect to which an improvement in

hydrophilicity is desired (for example, PTFE fibers or powder, or a fiber composite of the type described in U.S. Pat. No. 4,853, 101 to Hruska et al. which includes PTFE fibers or fibrils) with the thermoplastic, sulfonyl fluoride precursor of the known perfluorosulfonic acid form and perfluorosulfonate salt form ionomers via an aqueous surface active dispersion containing the precursor, forming an aqueous draw slurry including the coated substrate with sodium carbonate or sodium chloride, drawing a diaphragm from the draw slurry through vacuum deposition on a diaphragm support, drying and then bonding the diaphragm under bonding conditions, and only thereafter hydrolyzing the sulfonyl fluoride precursor within the bonded diaphragm to its perfluorosulfonate, sodium salt form ionomer through contact with sodium hydroxide.

An exemplary, first embodiment of a process for making a ZrC /PTFE fibers/PTFE particulate diaphragm employs a solventless, essentially completely water-based coating process described in U.S. Pat. No. 5,993,907 for placing a thin, durable coating of a lower equivalent weight, perfluorosulfonate ionomer on one or both of the PTFE fibers and the PTFE particulate employed as a binder in the diaphragm. This solventless coating process can be carried out in several ways depending on the ionomer type employed and the nature of the dispersion to be used. For example, for the ionomers which have been produced by The Dow Chemical Company with a shorter side-chain (acid-form) structure:

wherein the ratio of a:b is typically about 7 to 1, an integrated coating process would initially and involve the preparation of a dispersion in water of from about 1 to about 3 percent by weight of a perfluorosulfonic acid form ionomer having an equivalent weight of from about 550 to about 1000, and especially from about 550 to about 800 inclusive, by stirring the selected ionomer solids in a closed vessel at temperatures of from about 170 to about 200 °C, a pressure of from about 110 pounds per square inch, absolute (psia), and over a time frame of from about 1 to about 3 hours to provide yields of dispersed ionomer solids on the order of from about 70 percent to about 95 percent or greater for an 800 equivalent weight ionomer. For example, a powdered ionomer in the desired equivalent weight is combined with water in a closed vessel, and heated to a temperature of from about 180 to about 185 °C with stirring for about 2 hours, with the pressure being on the order of 145 to about 165 psia. In another example, an available alcohol/water-based dispersion could be conventionally processed to remove the alcohol.

Where the ionomer is a perfluorosulfonic acid ionomer of the Nafion™ type, initially a dispersion could be prepared in water of up to about 10 percent of an ionomer of an equivalent weight of from 550 to 1500, according to the process and under the conditions specified in U.S. Pat. No. 4,433,082 to Grot, or more commonly a commercially-available alcohol/water-based dispersion will again be conventionally processed to remove the alcohol.

The resulting dispersion is then added to a PTFE powder, for example, which will preferably have been subjected to intensive shearing in water to produce uniformly-sized PTFE particles, or to presheared PTFE fibers, or to a mixture of PTFE in particulate form and in the form of fibers. The mixture is then subjected to high shear conditions generally corresponding to a blade tip speed on the mixer used of 800 ft/minute (240 meters/minute) or greater, for a time sufficient to coat the PTFE substrate with the ionomer and achieve a uniform slurry, with care being taken to not create such heat by excessive mixing/shearing as might cause the coated PTFE to begin to clump together. The liquids in question are to be added to the PTFE, as opposed to the PTFE being added to the water or dispersion.

The resulting ionomer to PTFE solids ratio will generally be about 0.005 to 1 by weight or greater, for example being from about 0.005 to 1 to about 0.015 to 1 and for example being approximately 0.015 to 1, with sufficient ionomer and PTFE being present for a given volume of water to achieve adequate shearing of the solids and coating of the PTFE by the ionomer. This minimum solids level can reasonably be expected to vary with different tip speeds and different mixing conditions and with different equipment, but can be determined through routine experimentation.

Those skilled in the diaphragm art will appreciate at this point, that because there is no need for a rinse step to remove the lower alcohol solvent from the coated PTFE material, the ionomer coated PTFE is, e.g., then contacted with the requisite salt solution in the preparation of a NaCl- or Na ₂ CCb -based aqueous draw slurry incorporating the ionomer coated PTFE materials and the zirconium oxide, in the draw vat for drawing a diaphragm.

For example, the draw slurry employed in constructing the electrolyte permeable diaphragm 100 with this coating process will have a slurry solids concentration between about 190 and about 250 grams per liter, and/or of about 250 grams per liter to about 280 grams per liter and higher, with the higher concentrations generally having been found to result in higher caustic current efficiencies. The slurry will generally contain from about 60 weight percent to about 81 weight percent of zirconium oxide (typically having a particle size between about 0.85 microns and about 1.7 microns), from about 14 to about 31 percent of a PTFE particulate (for example, Teflon™ 7C granular PTFE from E.I. DuPont de Nemours & Company, Inc., having an average particle size of about 30 microns), and from about 5 to about 9 weight percent of PTFE fibers (for example, as shown in the referenced, commonly-assigned application, bleached 0.25 inch long, 3.2 denier PTFE fibers). For example, , from about 75 to 76 weight percent will be zirconium oxide, with from 14 to 16 percent of the particulate PTFE and from 6 to 8 weight percent of PTFE fibers.

Sodium carbonate may be used as the draw carrier, at a concentration in water which will typically be from about 3 percent by weight to about 20 percent by weight. A suspending agent may be used also, with the suspending agent being aluminum chloride or xanthan gum. The concentration of the suspending agent will be sufficient to keep the zirconium oxide in suspension, for example, between about 1.0 and about 1.8 grams per liter.

The electrolyte permeable diaphragm 100 is vacuum drawn on the foraminous structure 102. For example, the drawing is accomplished at temperatures, for example, of from about 20 to about 38 °C, and with flow control of residual slurry through the vacuum flow line of the draw vat to prevent pin-holing of the electrolyte permeable diaphragm 100. The electrolyte permeable diaphragm 100 is thereafter dried by continuing application of a vacuum thereon and by oven drying, or simply by oven drying. A slow, uniform drying is desired in any event to avoid blistering of the diaphragm at the exemplary drying temperatures of from about 40 °C to about 110 °C, and where oven drying is employed the electrolyte permeable diaphragm 100 may be placed in a position in the drying oven wherein the air flow surrounding the diaphragm is relatively free and uniform.

Upon completion of the drying cycle, the electrolyte permeable diaphragm 100 is bonded in a bonding over at temperatures between about 330 °C and about 355 °C, with exemplary temperatures being from 330 °C up to about 345 °C and especially being controlled at about 335 °C for the bonding of diaphragms including PTFE which has been provided with a

perfluorosulfonate, sodium form ionomer coating (as in the first and second processes for making the contemplated diaphragms, the second process being described hereafter). The sintering of the diaphragm is accomplished by slowly ramping up to the desired temperature (e.g., at about 2 °C per minute), maintaining this temperature for a period of time, for example, about one half hour, and then slowly cooling the diaphragm at a rate for example of about 2 °C per minute.

The present disclosure also includes the following embodiments: Embodiment 1 that includes an electrolyte permeable diaphragm, comprising a foraminous structure; particulate material on the foraminous structure; and aluminum hydroxide on the particulate material.

Embodiment 2, where for the electrolyte permeable diaphragm of Embodiment 1 the aluminum hydroxide is a reaction product of an alkali compound on the particulate material and aluminum chloride.

Embodiment 3, where for the electrolyte permeable diaphragm of Embodiment 1 the particulate material on the foraminous structure includes: a polyfluoroethylene fiber; a water- wettable, inert, inorganic, micron-size material; and a polymer dispersion sintered with the polyfluoroethylene fiber and the water-wettable, inert, inorganic, micron-size material.

Embodiment 4, where for the electrolyte permeable diaphragm of Embodiment 1 the polymer dispersion is a polyfluoroethylene dispersion.

Embodiment 5, where for the electrolyte permeable diaphragm of Embodiment 1 the water-wettable, inert, inorganic, micron-size material is selected from the group consisting of talc, metal silicates, alkali metal titanates, zircon oxide, alkali metal zirconates, magnesium aluminates and combinations thereof.

Embodiment 6, where for the electrolyte permeable diaphragm of Embodiment 1 the particulate material on the foraminous structure includes zirconium oxide and

poly(tetrafluoroethylene) in fibrous and particulate forms.

Embodiment 7, where for the electrolyte permeable diaphragm of Embodiment 6 the particulate material on the foraminous structure is coated with an ion-containing polymer of the formul where the ratio of a:b is about 7: 1, or wherein the coating is of a thermoplastic, sulfonyl fluoride polymer precursor of such an ion-containing polymer.

Embodiment 8, where for the electrolyte permeable diaphragm of Embodiment 1 the particulate material on the foraminous structure includes a contiguous sheet of pulverized expanded vermiculite. Embodiment 9, where for the electrolyte permeable diaphragm of Embodiment 8 the particulate material on the foraminous structure further includes a fibrous polymer

reinforcement.

Embodiment 10, where for the electrolyte permeable diaphragm of Embodiments 8 and 9, there is incorporated therein a particulate polymer binder.

Embodiment 11, where for the electrolyte permeable diaphragm of Embodiment 10 the particulate polymer binder is a fluoropolymer.

Embodiment 12, where for the electrolyte permeable diaphragm of Embodiments 1 through 11 the electrolyte permeable diaphragm includes 2 to 100 grams of aluminum per square meter of the foraminous structure.

Embodiment 13, where for the electrolyte permeable diaphragm of Embodiments 1 through 12 the electrolyte permeable diaphragm includes 2000 to 6000 grams of the particulate material per square meter of the foraminous structure.

Embodiment 14, where for the electrolyte permeable diaphragm of Embodiments 1 through 13 the electrolyte permeable diaphragm includes an insoluble additive selected from the group consisting of clay, pulverized non-asbestos diaphragm, fibrous material or a combination thereof.

Embodiments 15, which is a method of temporarily modifying a permeability of an electrolyte permeable diaphragm, comprising: installing the electrolyte permeable diaphragm in a chlor-alkali electrolytic cell having an anode compartment and a cathode compartment, wherein the electrolyte permeable diaphragm includes a foraminous structure; particulate material on the foraminous structure; and an alkali compound on the particulate material;

exposing the electrolyte permeable diaphragm to an aluminum-containing brine that includes: 0.01 to 2 grams of aluminum chloride per liter of the aluminum-containing brine; a pH having a value of 0 to 5; and a temperature of 0 to 60 °C, wherein a precipitate of aluminum hydroxide forms on the particulate material on the foraminous structure from a reaction with the aluminum chloride; providing a flow of a production brine through the chlor-alkali electrolytic cell, wherein the production brine does not include the aluminum chloride; and energizing the chlor-alkali electrolytic cell to produce caustic in the cathode compartment and wherein the electrolyte permeable diaphragm becomes alkaline as the chlor-alkali electrolytic cell operates.

Embodiment 16, where for the method of Embodiment 15 the particulate material on the foraminous structure includes: a polyfluoroethylene fiber; a water-wettable, inert, inorganic, micron-size material; and a polymer dispersion sintered with the polyfluoroethylene fiber and the water- wettable, inert, inorganic, micron-size material.

Embodiment 17, where for the method of Embodiment 16 the polymer dispersion is a polyfluoroethylene dispersion.

Embodiment 18, where for the method of Embodiment 16 the water-wettable, inert, inorganic, micron-size material is selected from the group consisting of talc, metal silicates, alkali metal titanates, alkali metal zirconates, magnesium aluminates and combinations thereof.

Embodiment 19, where for the method of Embodiment 15 the particulate material on the foraminous structure includes zirconium oxide and poly(tetrafluoroethylene) in fibrous and particulate forms.

Embodiment 20, where for the method of Embodiment 15 the particulate material on the foraminous structure includes a contiguous sheet of pulverized expanded vermiculite.

Embodiment 21, where for the method of Embodiment 20 the particulate material on the foraminous structure further includes a fibrous polymer reinforcement.

Embodiment 22, where for the method of Embodiments 20 or 21 there is incorporated therein a particulate polymer binder.

Embodiment 23, where for the method of Embodiment 22 the particulate polymer binder is a fluoropolymer.

Embodiment 24, where for the method of Embodiment 15 the electrolyte permeable diaphragm includes 2000 to 6000 grams of the particulate material per square meter of the foraminous structure.

Embodiment 25, where for the method of Embodiments 15 through 24 the alkali compound is an alkali hydroxide.

Embodiment 26, where for the method of Embodiment 25 the alkali hydroxide is selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide or combination thereof.

Embodiment 27, where for the method of Embodiments 15 through 26 the electrolyte permeable diaphragm includes 100 to 500 grams of the alkali compound per square meter of the foraminous structure.

The following examples are set forth by way of illustration and not by way of limitation, it being understood that the present disclosure is limited only as defined in the claims attached hereto. EXAMPLES

Heat Treated Diaphragm-Coated Cathode

The following examples use a heat treated diaphragm-coated cathode. To one liter of cell effluent (approximately 120 grams per liter of sodium hydroxide and 180 grams per liter of sodium chloride in water) add 9 grams of polytetrafluoroethylene fibers. Stir these fibers into the cell effluent until thoroughly wetted. Then add 65 grams of powdered talc whose particles have a median average dimension of about 2.2 microns. Again, rapidly stir the talc until the cell effluent wets the individual talc particles. Add a dispersion of polytetrafluoroethylene particles in water (60 weight % polytetrafluoroethylene in a basic water solution containing a wetting agent) to the slurry containing the fibers and the talc, in an amount sufficient to provide 18 grams of the finely dispersed polytetrafluoroethylene. Mix the slurry to obtain a uniform, thick creamy consistency. The particulate material in the slurry comprises 70.6 weight % talc, 19.6 weight % polytetrafluoroethylene dispersion and 9.8 weight % polytetrafluoroethylene fibers.

The test cathode is a 193 cm square sheet of 3 mm thick carbon steel having 2.5 mm perforations. Place the cathode in a drawing pan, attach means for drawing a vacuum of about 88 to about 95 Kpa to the drawing pan and apply the vacuum. Immerse the cathode in the drawing pan in the slurry, where the particulates in the slurry drawn are drawn to the face of the cathode by the vacuum on the pan to form a diaphragm thereon. After 240 seconds, remove the cathode-pan assembly from the slurry and smooth the diaphragm surface with a spatula where needed. Vacuum dry the cathode-pan assembly for 20 minutes. Remove the diaphragm coated cathode from the drawing pan and dry overnight (about 16 hours) in an oven at a temperature between 100 °C and 120 °C. The diaphragm weight was 60 grams and has a cross-sectional area of about 193 cm ² .

Place the dried diaphragm coated cathode into an oven. Increase the oven temperature by about 50 °C at a time in half hour steps until an oven temperature of 350 °C is reached. After holding the temperature at 350 °C for 20 min, allow the diaphragm coated cathode to cool to room temperature (23 °C) to form the heat treated diaphragm-coated cathode. The heat treated diaphragm-coated cathode contains about 10 weight percent NaOH (about 350 grams NaOH per square meter of the diaphragm) and 5 weight percent carbonate. Many samples were produced in the same way, and the weight of each sample varied between 10 to 30%, but the permeability could vary ± 100%, which is not surprised because of the manual control of the drawing process. Cell Body

The heat treated diaphragm-coated cathode was assembled in a cell body using a facing gasket, exposing about 150 cm ² of the cathode. Position the cathode opposite a dimension stable ruthenium-titanium anode.

Comparative Example A

Fill the anode compartment full with alkaline brine (water containing about 300 grams per liter of sodium chloride, pH 12), and let the brine flow through diaphragm to fill the cathode compartment. After the both compartments are filled, bring the hydraulic differential pressure across the diaphragm to 42 cm brine height. The brine flow through the diaphragm at the hydraulic differential pressure of 42 cm brine height was 41 1/h/m ² .

Example 1

Repeat Comparative Example A, with a new sample and with the following changes. Fill the cell body with brine (water containing about 300 grams per liter of sodium chloride) containing 0.15 grams of aluminum chloride per liter of brine and having a pH of 3.7, resulting in 4 grams of aluminum chloride per square meter of diaphragm. Bring the hydraulic differential pressure across the diaphragm to 42 cm brine height. The brine flow through the diaphragm at the hydraulic differential pressure of 42 cm brine height was 12 1/h/m ² . The brine flow through the diaphragm as compared to Comparative Example A is significantly reduced.

Comparative Example B

Again, with a new diaphragm sample assembled into the cell body, and fill the cell body described above with alkaline brine (water containing about 300 grams per liter of sodium chloride, pH 12). Bring the hydraulic differential pressure across the diaphragm to 30 cm brine height. The brine flow through the diaphragm at the hydraulic differential pressure of 30 cm brine height was 64 1/h/m ² . Energize the cell to about 0.65 kA/m ² , after which the flow gradually drops due to a phenome called self-tightening. Twelve hours later, the flow rate is down to 8.8 1/h/m ² , given a NaOH concentration of 10 weight % in the cell effluent.

Example 2 Repeat Comparative Example B, with a new diaphragm, but with the following changes. Fill the cell with a brine (water containing about 300 grams per liter of sodium chloride) containing 0.15 grams of aluminum chloride per liter of brine and 0.83 grams of clay (Attagel® 40 - BASF), resulting in 5 grams of aluminum chloride per square meter of diaphragm and 50 grams of clay per square meter of diaphragm. Bring the hydraulic differential pressure across the diaphragm to 30 cm brine height, where the initial brine flow is 14 1/h/m ² . After start-up and self-tightening, the flow rate decreased to 8 1/h/m ² , given a NaOH concentration of 10.2% in the cell effluent. As compared to Comparative Example B, the initial brine flow rate for Example 2 is significantly lower.

Comparative Example C

Fill the cell body described above with alkaline brine (water containing about 300 grams per liter of sodium chloride, pH 12), where the initial brine flow through the diaphragm is 80 1/h/m ² at a hydraulic differential pressure of 30 cm brine height. Add to the alkaline brine 0.15 grams of aluminum chloride per liter of brine, resulting in up to 20 grams of aluminum chloride being present in the alkaline brine per square meter of diaphragm. There was no or only minimal reduction in brine flow through the diaphragm at a hydraulic differential pressure of 30 cm brine height.

Comparative Example D

After the experiment in the Comparative Example C, the cell is flushed with production brine (pH 12, and no aluminum chloride) for 4 hours to remove all aluminum chloride added before, and is drained. The diaphragm in the cell has a pH of the brine (about 12) and contains less than 1 g NaOH per square meter of the diaphragm. Re-fill the cell with the aluminum chloride containing brine as in Example 1. The brine flow through the diaphragm is about 72 1/h/m ² at a hydraulic differential pressure of 30 cm brine height, almost same as the flow in the Comparative Example C

Example 3

The diaphragm in the comparative Example D is flushed again with production brine for 4 hours, and drained. The cell is then re-filled with 10 weight percent sodium hydroxide (NaOH) aqueous solution and soaked for 2 hours at room temperature and then drained. The diaphragm contains now about 200 grams NaOH per square meter. Re-fill the cell with the aluminum chloride containing brine as in Example 1. The brine flow through the diaphragm is then reduced to 15 1/h/m ² at a hydraulic differential pressure of 30 cm brine height, which is much lower than the flow in the Comparative Example C and D.

Example 4

Prepare a heat treated diaphragm-coated cathode having about 350 g NaOH per square meter of diaphragm. Flush the diaphragm with a production brine to remove the alkalinity. Soak the diaphragm in a 10 weight percent NaOH solution, and dried again. About 200 grams of NaOH per square meter is estimated in the diaphragm. Install the diaphragm into the cell body and fill the cell with the aluminum chloride containing brine as in Example 1. The brine flow through the diaphragm is about 14 1/h/m ² at a hydraulic differential pressure of 30 cm brine height. It will become apparent from the above detailed description of the disclosure, as well as the Examples above set forth, that many variations and modifications may be made in the particular embodiments of the disclosure set forth herein without departing from the disclosure. Other variations and modifications of the present disclosure will become apparent to those skilled in the art, and the present disclosure is to be limited only as set forth in the following claims.