Coating processes – Foraminous product produced
Reexamination Certificate
2000-04-28
2001-10-09
Bell, Bruce F. (Department: 1741)
Coating processes
Foraminous product produced
C427S247000, C204S296000
Reexamination Certificate
active
06299939
ABSTRACT:
DESCRIPTION OF THE INVENTION
The present invention relates to a method of preparing a liquid-permeable asbestos-free diaphragm. An asbestos-free liquid-permeable base mat is formed on a foraminous structure, e.g., a foraminous cathode structure, and water-insoluble inorganic particulate material is deposited on and within the base mat by drawing a liquid topcoat slurry comprising an aqueous medium, water-insoluble inorganic particulate material and alkali metal polyphosphate through the base mat. Diaphragms made by the method of the present invention are useful in electrolytic cells, e.g., cells used to electrolytically convert aqueous alkali metal halide to aqueous alkali metal hydroxide and halogen.
The electrolysis of alkali metal halide brines, such as sodium chloride and potassium chloride brines, in electrolytic cells is a well known commercial process. Electrolysis of such brines results in the production of halogen, hydrogen and aqueous alkali metal hydroxide. In the case of sodium chloride brines, the halogen produced is chlorine and the alkali metal hydroxide is sodium hydroxide. The electrolytic cell typically comprises an anolyte compartment containing an anode, and a separate catholyte compartment containing a cathode assembly. The cathode assembly is typically comprised of a cathode and a liquid-permeable diaphragm, which partitions the electrolytic cell into the anolyte and catholyte compartments.
The electrolysis of brine typically involves charging an aqueous solution of the alkali metal halide salt, e.g., sodium chloride brine, to the anolyte compartment of the cell. The aqueous brine percolates through the liquid permeable diaphragm into the catholyte compartment and then exits from the cell. With the application of direct current electricity to the cell, halogen gas, e.g., chlorine gas, is evolved at the anode, hydrogen gas is evolved at the cathode and aqueous alkali metal hydroxide is formed in the catholyte compartment from the combination of alkali metal cations with hydroxide anions.
For the cell to operate properly it is required that the diaphragm, which partitions the anolyte and catholyte compartments, be sufficiently porous to allow the hydrodynamic flow of brine through it, while at the same time inhibiting the back migration of hydroxyl ions from the catholyte compartment into the anolyte compartment. The diaphragm should also (a) inhibit the mixing of evolved hydrogen and chlorine gases, which can pose an explosive hazard, and (b) possess low electrical resistance, i.e., have a low IR drop. Historically, asbestos has been a common diaphragm material used in these so-called chlor-alkali electrolytic diaphragm cells. Subsequently, asbestos in combination with various polymeric resins, particularly fluorocarbon resins (the so-called polymer-modified asbestos diaphragms), have been used as diaphragm materials.
Due in part to possible health and safety issues associated with air-borne asbestos fibers in other applications, the development of asbestos-free diaphragms for use in chlor-alkali electrolytic cells has been an area of ongoing investigation. Such diaphragms, which are often referred to as synthetic diaphragms, are typically fabricated from non-asbestos fibrous polymeric materials that are resistant to the corrosive environment of the operating chlor-alkali cell. Such materials are typically perfluorinated polymeric materials, e.g., polytetrafluoroethylene (PTFE). These synthetic diaphragms may also contain various other modifiers and additives, such as inorganic fillers, pore formers, wetting agents, ion-exchange resins and the like. Examples of U.S. Patents describing synthetic diaphragms include U.S. Pat. Nos. 4,110,153, 4,170,537, 4,170,538, 4,170,539, 4,253,935, 4,311,566, 4,666,573, 4,680,101, 4,720,334, 5,188,712, and 5,192,401.
It is known that synthetic diaphragms for chlor-alkali cells having improved performance can be prepared by coating and/or impregnating them with inorganic materials. However, the surface and/or the degree of inorganic particulate impregnation of such coated diaphragms can be less than uniform. In some instances, the nonuniformity of the coated diaphragm may result in lower than desired electrolytic cell efficiencies, e.g., low caustic efficiencies in the case of chlor-alkali cells.
U.S. Pat. No. 5,612,089 describes a method of preparing asbestos-free diaphragms for use in chlor-alkali electrolytic cells. The diaphragms of the '089 patent are described as being prepared by establishing a liquid permeable asbestos-free base mat on a cathode structure, drawing through the base mat a liquid dispersion comprising inorganic particulate material dispersed in alkali metal chloride brine containing a wetting amount of organic surfactant, and drying the formed diaphragm.
U.S. Pat. No. 5,683,749 describes the preparation of asbestos-free diaphragms for chlor-alkali electrolytic cells used in chlor-alkali cells. The '749 patent describes preparing an asbestos-free diaphragm by forming an asbestos-free base mat on a cathode structure, drawing through the base mat a slurry of inorganic particulate material dispersed in a strongly alkaline alkali metal hydroxide solution, and drying the formed diaphragm.
U.S. Pat. Nos. 3,980,547, 4,003,811, 4,048,038, 4,110,189 and 4,132,189 describe the electrokinetic separation of clay particles from an aqueous suspension of clay particles. The suspension of clay particles is described in the '547, '811, '038, U.S. Pat. Nos. 4,110,189 and 4,132,189 as being formed by dispersing clay particles in water with tetrasodium pyrophosphate.
In accordance with the present invention there is provided a method of forming a liquid-permeable asbestos-free diaphragm for use in an electrolytic cell, said method comprising:
(a) forming on a foraminous structure a liquid-permeable diaphragm base mat of asbestos-free material comprising fibrous synthetic polymeric material resistant to the environment of said electrolytic cell;
(b) drawing through said diaphragm base mat a liquid topcoat slurry comprising an aqueous medium, water-insoluble inorganic particulate material, and alkali metal polyphosphate, thereby to deposit inorganic material on and within said diaphragm base mat; and
(c) drying the resultant liquid-permeable asbestos-free diaphragm.
Other than in the operating examples, or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be understood as modified in all instances by the term “about.”
DETAILED DESCRIPTION OF THE INVENTION
The water-insoluble inorganic particulate material that is present in the topcoat slurry may be selected from (i) oxides, borides, carbides, silicates and nitrides of valve metals, (ii) clay mineral, and (iii) mixtures of (i) and (ii). As used herein and in the claims, the term “valve metal” is meant to be inclusive of vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, titanium, tungsten and mixtures thereof. Of the valve metals, titanium and zirconium are preferred in the present invention. Of the valve metal oxides and valve metal silicates, valve metal oxides are preferred, e.g., titanium dioxide and zirconium oxide.
Clay minerals that may be present in the topcoat slurry in the method of the present invention include those that are naturally occurring hydrated silicates of metals, such as aluminum and magnesium, e.g., kaolin, meerschaums, augite, talc, vermiculite, wollastonite, montmorillonite, illite, glauconite, attapulgite, sepiolite and hectorite. Of the clay minerals, attapulgite and hectorite and mixtures thereof are preferred for use in the method of the present invention. Such preferred clays are hydrated magnesium silicates and magnesium aluminum silicates, which may also be prepared synthetically.
The mean particle size of the water-insoluble inorganic particulate material of the topcoat slurry may vary, but is typically in the range of from 0.1 microns to 20 microns, e.g., from 0.1 microns to 0.5 micr
DuBois Donald W.
Maloney Bernard A.
Bell Bruce F.
Franks James R.
Millman Dennis G.
PPG Industries Ohio Inc.
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