Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-04-28
2001-10-02
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S252000, C204S263000, C205S510000, C205S512000
Reexamination Certificate
active
06296745
ABSTRACT:
DESCRIPTION OF THE INVENTION
The present invention relates to an improved method of operating a chlor-alkali electrolytic cell. In particular the present invention relates to a method of operating a chlor-alkali cell in which water-insoluble inorganic particulate material and alkali metal-polyphosphate are added to the anolyte compartment of the electrolytic cell during operation of the cell.
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. The electrolytic cell typically further comprises a liquid-permeable diaphragm, which partitions the electrolytic cell into the separate 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 ions with hydroxyl ions.
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.
During the operation of a chlor-alkali cell, the porosity of the diaphragm typically increases resulting in, for example, reduced current efficiency, the production of overly dilute alkali metal hydroxide, the back migration of hydroxyl ions from the catholyte compartment into the anolyte compartment, and an increased risk of the mixing of evolved hydrogen and chlorine gases. To adjust and optimize the porosity of the diaphragm, inorganic particulate materials, such as clay minerals, are typically added periodically to the anolyte compartment during operation of the cell.
U.S. Pat. No. 5,567,298 describes a method of making chlorine and alkali metal hydroxide in an electrolytic cell of the type wherein a liquid permeable asbestos-free diaphragm separates the catholyte and anolyte compartments. The '298 patent describes increasing the current efficiency of the cell by the sequential steps of (a) adding clay mineral to the anolyte compartment of the cell, (b) lowering the pH of the anolyte by the addition of an inorganic acid, and (c) maintaining the anolyte at the lowered pH for a time sufficient to restore the cell to a predetermined current efficiency.
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 patents as being formed by dispersing clay particles in water with tetrasodium pyrophosphate.
It would be desirable to develop improved methods of operating electrolytic cells used for the production of chlorine and alkali metal hydroxide, i.e., chlor-alkali electrolytic cells. In particular, it would be desirable to develop improved methods of increasing the current efficiency of a chlor-alkali cell during its operation.
In accordance with the present invention there is provided a method of operating an electrolytic cell, said method comprising:
(a) providing an electrolytic cell having a catholyte compartment containing a cathode, an anolyte compartment containing an anode, and a liquid-permeable diaphragm separating said catholyte and anolyte compartments;
(b) introducing alkali metal chloride brine into said anolyte compartment;
(c) applying an electrical potential across said cathode and anode;
(d) withdrawing hydrogen gas and an aqueous solution comprising alkali metal hydroxide from said catholyte compartment, and chlorine gas from said anolyte compartment; and
(e) adding water-insoluble inorganic particulate material and alkali metal polyphosphate to said anolyte compartment while said electrolytic cell is operating, thereby increasing the current efficiency of said electrolytic cell.
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 added to the anolyte compartment in the method of the present invention is typically selected from valve metal oxides, valve metal silicates, clay minerals and mixtures thereof. 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 added to the anolyte compartment 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 that is added to the anolyte compartment 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 microns. In an embodiment of the present invention, the water-insoluble inorganic particulate material is an attapulgite clay. An attapulgite clay product having a mean particle size of about 0.1 microns and available from Engelhard Corporation under the trademark, “ATTAGEL®” has been found to be particularly useful in the practice of the method of the present invention.
The water-insoluble inorganic particulate material is typically added to the anolyte compartment in an amount sufficient to provide the desired diaphragm permeability and current efficiency. The amount of inorganic particulate material added may vary depending on, for example, electrolytic cell operating characteristics, cell geometry and cell capacity. Typically, water-insoluble inorganic particulate material is added to the anolyte compartment in an amount of from 10 grams to 120 grams per square meter of diaphragm surface area, e.g., from 20 grams to 60 grams per square meter of diaphragm surface area. As used herein and in the claims, the “diaphragm surface area” is calculated from the dimensions of the diaphragm, for example, a 10 cm×10 cm diaphragm has a calculated surface area of 100 square centimeters (cm
2
).
The alkali metal polyphosphate that is added to the anolyte compartment in the method of the present inven
DuBois Donald W.
Maloney Bernard A.
Bell Bruce F.
Franks James R.
Millman Dennis G.
PPG Industries Ohio Inc.
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