IL32525A - Electrolysis of dilute brine - Google Patents

Description

Electrolysis of dilute brine HOOKER CHEMICAL COSPORATION C. 30823 BACKGROUND OF THE INVENTION The economics of the diaphragm type electrolytic chlor-alkali cell operation in conventional operation today generally dictate the use of saturated or nearly saturated feed brines in a nearly pure state. In most instances, this requirement for nearly pure saturated brine is commercially satisfied by formulating the feed brine solution to further purify the solution. However, sources of dilute impure brines abound on the earth making it highly desirable that techniques be developed 0 to tap these supplies of cell feed brine as directly as possible, without la^ge expenditure for purification and concentration.

Among the innumerable sources of dilute impure brines which present potential sources for diaphragm type chlor-alkali cell feed brines the following sources are exemplary: 1. inland -salt lakes such as the Dead Sea which contain ? brine of about 5 to 1$ percent concentration · 2. brines resulting from partial concentration by solar evaporation or ion exchange membrane cells 3· sea water made up to a concentration between about to 15 percent sodium chloride h. dilute brine effluents from chemical operations euch as those resulting from the reaction of sodium hydroxide with halogenated organic compounds in the production of alcohols, and . dilute brines pumped from underground brine pools.

These available brine sources present potential feed brines, for diaphragm type chlor-alkali cells with impurities such as the sulfate, phosphate, nitrate and carbonate anions as well as organic impurities such as glycerine and its precursors. The objectionable inorganic anions may be characterized as highly hydrated oxygen containing anions.

BRIEF SUMMARY OF THE INVENTION In accordance with this invention it has been discovered that under certain specific conditions impure dilute brines may be economically electrolyzed in a diaphragm chlor-alkali cell equipped with anodes with present an active surface on an electrically conductive substrate metal. By substrate metal it is intended to encompass those metals and metal alloys which will become passivated when polarized anodically and remain passive well beyond the anodic potential needed to convert a chloride ion to chlorine. The phenomenon of passivity in this connection is discussed in an article by Greene, appearing in the April, 1962 issue of Corrosion-National Association of Corrosion Engineers , pages 136t to ll{2t, wherein reference may be made to Figure 1 which describes a typical active-passive transition of a metal toward a corrosive medium. The metal substrate employed in the electrodes applicable in this invention will not pass into the transpassive range until a potential is reached which is considerably higher than that needed to produce chlorine from the l remains passive during metals in a generic sense are the "valve" metals (with the exclusion of certain metals which obviously are inapplicable such as aluminum,, zirconium, and the like). Titanium, tantalum or niobium are acceptable substrate V of titanium of intermediate strength. Alloys of titanium may be employed as long as the alloy "sets the criterion of passivity set forth in the preceding paragraph. For example, titanium alloys of aluminum, vanadium, palladium, chromium or tin may be employed in which the latter metals are present as less than about 10 percent of the alloy.

The surface of the substrate metal may be made active by various methods. For example, a conductor such as a noble metal (preferably platinum) may be deposited on the surface of the substrate metal by methods known to the art. Mixtures of noble metals and platinum may be used to activate the surface of the metal substrate. The preferred surface metal mixture or alloy is one containing more than about S>0 per cent platinum. Likewise, noble metal oxides may be used alone or in combination with noble metals to form the active electrode surface. By noble metal, it is intended to include the platinum and palladium triads of the periodic table with the exclusion of osmium. Thus, ruthenium, rhodium, palladium, iridium and platinum represent noble metals which are especially applicable in their metallic form, alloys thereof and' as oxides.

With specific reference to platinum-titanium electrodes or platinized titanium electrodes, the titanium substrate used is the commercially pure grade of intermediate strength or a titanium alloy of aluminum, vanadium, palladium, chromium or tin in which about 90 percent or more of the alloy is titanium, The platinum-titanium or noble metal oxide-titanium electrodes are acceptable as well as the platinum or noble metal oxide surfaced titanium or tantalum clad copper electrodes wherein the titanium or tantalum is applied to the copper core by mechanical coating or with electrically conductive adhesive materials.

The anodes may be assembled within the diaphragm type chlor-alkali cell in any manner known to the art. The anode current conductor connections may be isolated from the corrosive contents of the electrolytic cell by bituminous materials such as mastic or synthetic resin sealants. In this regard, m especially advantageous anode assembly is that disclosed by M. P. Grotheer in copending application S. N. (ID 1388 for Electrolytic cell structure, filed of even date herewith, wherein an anode assembly is disclosed which is applicable to monopolar and bipolar chlor-alkali cell operation. The anode assembly provided therein is a unitary electrode assembly which involves a metallic (preferably steel) base plate or backer plate to which spacer bars of an electrically conductive material such as platinized-titanium, aluminum alloys and preferably copper is attached by welding or tinning to the steel at predetermined intervals based upon the desired pitch of the anodes . When the spacer bar is constructed from, platinized. titanium, the use of sealants may be dispensed with as protective means against attack by the corrosive materials which contact it. The spacer bar is disposed in such a manner that the attached anodes will be aligned with abutting edges vertically situated within the cell unit. The spacer bar contains holes through which pass the bolts running parallel to the base plate. The holes through the spacer bars are preferably slotted, at an angle downwardly extending from the vertical bar surface. The number of anodes that may be attached to the spacer bars by pressure bars depends upon the designed height of the cell where the anodes are horizontally attached to the spacer bar in a vertically disposed bank of anodes, or the cell width where the anodes are vertically attached to the spacer bar in a bank extending across the cell.

The pressure bars, one being drilled and countersunk, the other being provided with threaded holes-, act, in conjunction with the bolt running through them, the anode and spacer bar, as an electrode clamping device. In the clamped position, the electrical resistance through the anode-spacer bar contact is a function of the pressure developed at the contacting surfaces. Hence, the resistance developed through the clamped connection of anode and spacer bar may be controlled by regulating the pressure applied by the clamping bolts. Consideration must also be given to the thermal expansion of the spacer bar during operation of the cell in which temperatures above 5° C. are common. In practice, the bolts may be of a suitable metal or metal alloy, to compensate for the expansion of the spacer bars and pressure bars. Likewise, the pressure bars may be made of any suitable material such as steel.

Any corrosion resistant sealant known to the art may be placed over the connecting members between each electrode. For example, natural or synthetic rubbers may be employed by themselves, in combination or in conjunction with other resins. Bituminous materials may be employed if desired and the phenol-formaldehyde resins and polyester resins are acceptable sealants. Especially good sealants may be derived from the reaction of a polyhydric alcohol with a Diels-Alder adduct of hexahalocyclopentadiene and an alpha, beta unsaturated dicarboxylic acid, such as are disclosed in U. S. 3,2l6,88ii. The sealants employed in this invention may be advantageously highly filled with such materials as sand, SiO , graphite particles or other inert materials.

The electrolytic cells contemplated by this invention are those conventionally used in the electrolysis of sodium chloride solutions. The electrolytic cell comprises a cell top, a cell bottom, sidewalls, an anode compartment and a cathode compartment separated by a porous diaphragm which may be of deposite asbestos. The brine is fed into the anode compartment from which it flows through the diaphragm into the cathode compartmen .

Chlorine and hydrogen are withdrawn from the anode and cathode compartments, respectively. The cell liquor containing sodium hydroxide, sodium chloride and other impurities is withdrawn from the cathode compartment.

When employing a dilute brine feed in accordance with the instant invention, the feed rate may be that employed in conventional cell operation. However, it is preferred to introduce a dilute feed brine into the cell at a rate higher than that of flow through the diaphragm, while recycling the excess brine with makeup. The advantages of recycle are to maintain a relatively constant anjflyte chloride concentration and pH. In operation, a dilute feed brine containing from about 3.5 to V~> per cent alkali metal chloride is fed to the anode compartment of the diaphragm cell equipped with electrodes described above. An oxygen containing ear in concentrations as ulfate ion. To achieve sa s ac ory curren e c ency e conversion of brine to caustic should be kept below £0 per cent of the original brine feed concentration. Hence, the caustic concentration in the catholyte will vary between about 1ζ to 100 grams per liter. The electrolytic cell may be efficiently operated within the temperature range of about 80° C up to the boiling point of the brine (which depends As an example of a specific application of the instant invention, its integration with a sea water desalination operation involves : 1. Treatment of sea water with an alkaline-carbonate containing solution to precipitate a portion of the dissolved calci as calcium carbonate, 2. Adjustment of the pH of the treated sea water to abou neutrality, ■ 3. Evaporation to recover potable water, k. Treatment of the brine feed for the electrolytic cell -with an alkaline-carbonate containing solution to remove calcium and magnesium as the carbonate and hydroxide, respectively, >. Electrolysis of the treated brine directly or optiona after further concentration of brine or addition of make up sodium chloride to produce hydrogen, chlorine, .and caustic cell liquor, 6. Recombination of a portion of the hydrogen and chlorine generated in Step 5 "to form hydrochloric acid for use in Step 2, supra, 7. Calcination of calcium carbonate precipitated in Step 1 to produce calcium oxide and carbon dioxide, 8. Combination of the sodium hydroxide containing causti product of Step $ with carbon dioxide from Step 6 to produce an alkaline carbonate containing mixture for treatment of sea water specified in Steps 1 and I4 .

The effluent from a desalination process contains sodiu chloride in concentrations generally above 3.$% by weight and normally within the range of 7 to 12$ by weight. This effluent also contains sodium sulfate in a concentration up to. about 5% and normally within the range of 3 to h% by weighty This effluent has generally in the past been discarded as a worthless by-product 6 diaphragm chlor-alkali cell because of low currei.t efficiencies.

However, the sulfate containing effluent from a desalination operation may be used as the source of dilute brine in the performance of the instant invention.

The preliminary treatment of sea water with partially or totally carbonated cell liquor serves to precipitate a portion of the dissolved calcium. Subsequent correction of the pH of the sea water to near neutrality before it is introduced into the evaporator affords such a decrease in calcium content that a larger percentage of potable water may be removed from a given amount of sea water before scaling begins.

The economically advantageous on-site availability of an alkaline-carbonate containing reagent for pre-treatment of sea water, coupled with the advantage of utilizing the evaporator effluent to produce valuable products makes this process decidedly superior to present operations.

DETAILED DESCRIPTION OF THE INVENTION An integrated saline water conversion plant-effluent electrolysis operation herein described may best be understood Dereference to the accompanying drawing which is self-explanatory.

A series of experiments Was run to determine the effect of anolyte sodium chloride concentration on both anode current efficiency and on cell voltage. Both graphite anodes and platinized-titanium anodes were used.

To illustrate the electrolysis of dilute feed carbonate to precipitate calcium and magnesium. After treatment, this brine contained 3l grams per liter sodium chloride, 3.8$ sodium sulfate, 12 parts per million calcium oxide, and 7 parts per million magnesium oxide. This solution and treated plant brine containing a corresponding amount of sodium sulfate was used for the laboratory test. In these experiments a laboratory size version of a Hooker type diaphragm cell was used. Anode current densities were maintained at 0.9 amperes per square inch.

All the experiments were started using in excess of 300 grams per liter (gpl) sodium chloride in the brine. During the course of the experiments, approximately 2 weeks, the brine was diluted with distilled water to obtain data at the lower salt concentrations. The ratio of sodium sulfate to sodium chloride would remain constant as, a result of these dilutions. The data collected in the following Tables I and II represents examples of the use of platinized-titanium anode (Type B-Englehard Coating). The platinized-titanium anodes were bolted directly to a copper bus current conductor. A highly filled polyester was used as the sealant to insulate the anode connection from the anolyte.

TABLE I Platinized Titanium Anode 0.9 Amperes per Square Inch Feed Brine Anolytc Cntholyte Temp Cell gpl aCl pH u gpl NaCl pH gpl NaCl gpl NaOH _ 'C Vol age 314 6.25 264 2.95 224 58 92 2.9 274 7.25 243 2.8 205 78 95 3.0 231 7.5 201 2.8 145 66 93 3.05 211 6.8 165 ' 3.45 125 64 93 3.15 173 6.5 137 3.8 90 72 905 3.2 152 6.5 120 3.85 76 66 96 '·. 3.3 124 6.8 86 3.8 52 62 95 3.45 140 6.7 105 3.75 65 61 94 3.35 136 7.1 91 3.7 61 62 93 3.4 136 6.7 91 3.5 51 59 93. 3.45 133 6.6 83 3.7 1 4S 62 95 3.45 129 7.2 87 3.7 49 5S 96 3.45 144 6.8 92 3.4 65 56 .96. 3.45 132 7.2 92 3.7 50 61 97 3.45 135 7.3 92 ' 3.6 53 ' 62 96 3.45 132 7.2 90 3.9 50 63 96 3.5 Initial Feed Brine contains 3.8 per cent Na2SC¾ Source of 0¾ gas comes from Feed Brine TABLE II Platinized Titanium Anode 0.9 Amperes Per Square Inch c were.cas n ea emp oy ng a copper us current TABLE III Graphite Anode - Electrical Connection Cast i 0.9 Amperes per Square Inch Feed Brine Anolyte Catholytc Temp Cell gpl.NaCl pH • gpl NaCl PH gpl NaCl gpl NaOH °C Voltage 31 7.7 284 1.8 264 48 . 95 3.2 300 7. 265 1.8 231 78 95 3.2 238 7.3 215 1.7 172 64 93 3.3 198 7.7 168 1.7 135 62 92 3.5 149 7.5 102 2.6 89 64 97 3.55 153 7.6 108 2.7 40 98 97 3.6 136 7.8 82 2.4 40 80 93 3.65 139 7.6 92 2.1 46 78. 94 3.65 121 7.8 75 2.1 43 70 94 3.8 81 7.9 40 1.75 13 44 9.6 4.3 81 7.7 41 1.4 9 54 97 4.4 It may be readily seen from the results obtained by the use of a platinized-titanium anode, the results of which are compiled in Tables I and II, that high anode current efficiencies, above 9$%, were obtained down to an anolyte sodium chloride concentr of 60 to 70 grams per liter. Below anolyte cell concentrations of £0 grams per liter sodium chloride, anode current efficiencies decreased rapidly.

Higher current efficiencies observed with platinized- titanium anodes at lower anolyte sodium chloride concentrations may be explained as a result of the higher oxygen oyervoltage on platinum when compared to graphite, and by the fact that carbon complexes form with some oxygen containing anions such as sulfate and phosphate on graphite anodes and accelerate graphite consumption.

The compiled data set forth in Table III illustrates the use of a graphite anode to exemplify an accelerated graphite consumption in the presence of sodium sulfate which adversely affects the anode efficiency. Carbon dioxide to oxygen ratios range from 2:1 to 22:1, even when the anode current efficiencies were above 9$%. At current efficiencies of 95% or greater, the graphite consumption was calculated to be between 6 to lU lbs. per ton of chlorine. At an anode efficiency of 90% (l$0 grams per liter sodium chloride in anolyte), the graphite consumption was about 20 lbs. per ton of chlorine. Below an anolyte concentration of 120 grams per liter sodium chloride, the graphite consumption increased to between 30 to 100 lbs. per ton of chlorine.

In contrast to high and uneconomical graphite loss, the from the noble metal electrodes was under 2 grams per ton of ine. During this experiment, conditions were maintained whereby noble metal loss was held at a rate between 0.1 and 2 grams per ton of chlorine; this level of noble metal consumption produces chlorine and its by-products at costs acceptable to a commercial plant. To minimize platinum losses, rectifiers are chosen which will provide a minimum percentage ripple in the direct current .

Similarly, it was found that by operating under the conditions specified that cell voltages were obtained ranging from 3.2 to U.U volts at the current density of 0.9 amperes per square inch. The voltages obtained are sufficiently low to make commercial operations economically feasible.

In each of the series of experiments tabulated in Tables through III, voltages increased as the anolyte sodium chloride concentration decreased. Cell voltages associated with platinized-titanium anodes are lovrer than those associated with graphite anodes.

The voltage increase, as the anolyte salt concentration decreases, appears to be independent of the type of anode used. The current efficiencies are based on gas analysis and are not corrected for chlorine los due to the solubility of chlorine in the anolyte or the resultant chlorate in the catholyte.

For equivalent conditions, the anolyte pH was higher in experiments using platinized-titanium anodes than in experiments using graphite anodes.

Having disclosed the invention it will be apparent to thos skilled in the art that various modifications and changes may be made which do not depart from the true spirit of this contribution. -15- 32525/2

Claims (2)

1. A process for the electrolysis of a dilute brine solution containing between 3.5 to about 15 percent alkali metal chloride and up to 5 percent, of total solids in the feed brine solution, of an oxygen containing impurity, expressed as sodium sulfate, said impurity comprising at least one member selected from the anions sulfate, phosphate, carbonate, nitrate and organic materials, which comprises applying a decomposition voltage across said solution at a temperature from about 80°C. to the boiling point of said solution, between an anode and a cathode, separated by a diaphragm into an anode compartment and a cathode compartment, to afford an anode current density of at least 0.8 amperes per square inch, flowing said brine solution through said diaphragm at a rate sufficient to electrolyze less than about 50 percent of the alkali metal chloride present in the initial feed brine solution, to produce a catholyte containing from 15-100 grams per liter alkali metal hydroxide, said anode comprising (a) a metal substrate selected from the group consisting of titanium, tantalum, niobium, titanium-cM copper, tantalum-clad copper niobium-clad copper, and alloys of titanium in which less than about 10 percent of said alloy is a member of the group consisting of aluminum, vanadium, chromium, palladium, tin and mixtures thereof; and (b) an active surface selected from at least one member of the group consisting of a noble metal and a noble metal oxide.

2. The process of Claim 1, in which the brine contains between -16- 32525/2 -3- ϊ ~~ ' A process for the production of chlorine, caustic and " 1 hydrogen comprising 3 (a) adjusting the pH of an alkali metal chloride brine containing calcium, magnesium and sulfate impurities with an 5 alkaline-carbonate containing reagent to partially precipitate 6 * ' calcium carbonate, ** 7 ; (b) acidification of the brine from Step (a) to obtain 8 a near neutral pH, 9 (c) concentration of the brine to, roduce a solution 10 containing between about 5» to 15 percent alkali metal chloride ·* 11 (d) precipitation of the remaining calcium and magnesium 12 as the carbonate and hydroxide, respectively, from at least a portion 13 of said solution by treatment with an alkaline-carbonate containing l * reagent l · .(e) electrolysis of the treated solution to produce 16 chlorine, caustic and hydrogen in a diaphragm type electrolytic cell 17 at a temperature between about 80°C. to the boiling point of said 18 solution to produce an alkali metal hydroxide concentration in the 19 catholyte equivalent to less than 50 percent conversion of the alkali 20 metal hydroxide, said electrolytic cell being equipped with anodes 21 comprising 32525/2 (1) a metal substrate selected from the group consisting of titanium, tantalum, niobium, : titanium-clad copper, and alloys of titanium in which less than about 10 percent of said · alloy is a member of the group consisting of aluminum, vanadium, chromium, palladium, tin and mixtures thereof; and (2) an active surface selected from at least one member of the group consisting of a noble metal and noble metal oxide. The process of claims in which the hydrogen produced in the electrolysis Step (e) is recombined with chlorine from Step (e) to produce HCl which is used to acidify the brine in*Step (b). The process of claim 3 in which the calcium carbonate precipitated in Step" (a) is calcined to produce CaO and CO2 and the CO is reacted with a portion of the caustic produced in Step (e) to produce an alkaline NagCO^ containing solution which is recycled to adjust the pH in Step (a) and precipitate calcium and magnesium values is Step (d). 1-6- . The process of claim;3 in which the alkali metal chloride brine of Step (a) is sea water.' The process of claim.3 in which the Step (c) concentration is b evaporation to produce potable water. 8» An integrated process of water desalination and brine electrolysis which comprises supplying the dilute effluent containing from about 5 to 15 percent alkali metal chloride from a wate desalinize,tion process from which calcium and magnesium have been removed to a diaphragm type ehlor-alkali cell, equipped with anodes comprising? (a) a metal substrate selected from the group consisting of titanium, tantalum, niobium, titanium-clad copper, tantalum-clad copper, niobium-clad copper, and alloy of titanium in which less than about 10 percent of said alloy is a member of the group consisting of aluminum, vanadium, chromium, palladium, tin and mixtures thereof; and (b) an active surface selected from at least one member of the group consisting of a noble metal and noble metal oxide, elec olyzing said dilute effluent, at a temperature from about 80°C to the boiling point of said dilute effluent, reacting at least a portion of the caustic product with CO^ to produce an alkaline containing solution for pre cipitating calcium and magnesium and reacting at least a portion of the chlorine and hydrogen from the electrolytic cell to adjust the pH of the water to be treated prior to evaporation, to produce potable water, chlorine and caustic. For tb^Applicants