GB2059793A - Catalyst pellets with resin binder for decomposition of hypochlorite and method of use - Google Patents

Abstract

Catalysts for use in decomposition of aqueous hypochlorite in fixed beds are in the form of pellets comprising a substance which is known to catalyze decomposition of hypochlorite and an organic resin binder, e.g., polyvinylidene fluoride.

Description

SPECIFICATION Catalyst pellets with resin binder for decomposition of hypochlorite and method of use Hypochlorite ions in aqueous solution are corrosive to many metals and are highly toxic to aquatic life. Industrial waste streams containing aqueous hypochlorite are produced by many processes such as in the manufacture of chlor-caustic and dry bleach. Before these waste streams can be released into public waters, they require treatment to remove hypochlorite ions.

Various method including photochemically-, thermally-, and chemically-induced decompositions have been proposed for removing hypochlorite from dilute aqueous solution. For large scale industrial application, chemical methods are most commonly used. Chemical methods, which include the use of H202, NaSH, HCI, and SO2, for example, are all expensive when very large quantities of dilute aqueous hypochlorite are involved. Waste treatment systems which consume large quantities of these chemicals create a substantial economic burden on processes which they support.

There is a need for an ecologically efficient and economically sound method for decomposing large quantities of dilute hypochlorite.

One basis for such a system is the decomposition of hypochlorite by heterogeneous fixedbed catalysts to give chloride ion and oxygen.

A number of such catalysts comprising the oxides and hydroxides of iron, copper, magnesium, nickel, and cobalt have been described in the literature. Of these catalysts, those prepared from cobalt are the most active.

Because of certain practical drawbacks, fixed-bed catalysts have not seen widespread commercial application for hypochlorite decomposition. For example, the high alkalinity of hypochlorite solution causes the binder support of most tableted and extruded catalysts to disintegrate, reducing the catalyst totally, or in part, to a fine slurry. Because of the problems associated with recovery and recycle of finely divided catalyst particles in aqueous media, this technology has not seen widespread application. Also, when fixed-bed catalysts are exposed to waste solutions containing both calcium ions and hypochlorite, such as waste from dry bleach manufacture, the catalysts rapidly loses activity due to calcium carbonate deposition in the catalyst pores. Reactivation of blinded catalyst is difficult.

In accordance with this invention a novel catalyst composition is provided which enables an efficient and economically sound method for decomposing hypochlorite contained in aqueous industrial waste streams, including those containing dissolved and suspended calcium salts.

The catalyst compositions of this invention comprise pellets of a substance capable of catalyzing the decomposition of hypochlorite embedded in an organic resin matrix. The catalyst pellets of this invention are particularly suitable for treating waste waters containing hypochlorite in fixed beds and have improved resistance to disintegration compared to known tableted and extruded catalysts. The use of the catalyst described herein in fixedbed decomposition of hypochlorite constitutes a second aspect of the present invention.

The aqueous solutions containing hypochlorite which can be treated in accordance with the process of this invention and with the catalyst pellets described herein may be any aqueous solution which contains hypochlorite ions such as hypochlorous acid or salts of hypochlorous acids particularly the alkali metal and alkaline-earth metal salts.

One common source of aqueous streams containing hypochlorite ions is the waste water from scrubbing in a chlorine liquefaction plant where the non-condensable "tail gases are scrubbed with caustic solution to prevent residual chlorine from entering the atmosphere. This scrubbing stream contains alkali metal hypochlorite which must be decomposed before discharge into public waters.

Other sources of aqueous waste waters containing hypochlorite which can be treated by the method of the present invention occur in the manufacture of chlor-caustic and dry bleach.

The method of treating various chemical streams in a fixed bed reactor is well-known and does not, as such, contitute a part of this invention. Similarly, a variety of materials suitable for the decomposition of hypochlorite ion are known. These too are not part of the present invention. It is the special form of the catalyst and its use in decomposition of hypochlorite that forms the basis of this invention.

Essentially, known catalysts for decomposition of hypochlorite are formed into pellets with an organic resin binder in a matrix and these pellets are utilized in the known process of decomposing hypochlorite in a fixed bed.

Substances which are suitable for catalyzing the decomposition of hypochlorite include oxides or hydroxides of iron, copper magnesium, nickel or cobalt. However, any substance serving this function can be adapted for use in the catalyst pellets described herein.

The resin binder which forms the second essential component of the catalyst pellets of this invention can be any of a wide variety of organic resins both thermoplastic and thermosetting. It is merely essential that the resin be relatively stable for long periods of time under contact with hypochlorite and that it be capable of forming a pellet which is relatively stable to mechanical disintegration as well as chemical disintegration in use. Thermoplastic resins are preferred. In particular, it has been found that polyolefins, halogenated polyolefins, and polyvinylidene halide polymers are suitable. Representative of these material are polyethylene, polypropylene, polytetrafluoroethylene and polyvinylidene fluoride.

The ratio between the substance capable of decomposing hypochlorite and the organic resin binder can vary widely but generally will be within the ratio of 100:1 to 1:10. Ratios of 1:1 to 1 5:1 are generally preferred. The weight ratio of about 5:1 has been found to be suitable where the substance capable of catalyzing the decomposition of hypochlorite is cobalt oxide and the organic resin is any one of a variety of thermoplastic polymers.

The essential criteria for selecting an appropriate ratio are that sufficient organic resin must be present in order to provide a matrix which is stable to mechanical handling and that the amount of organic resin is not in excess of that which will allow permeation of the catalyst pellets by the hypochlorite solution.

The size of the pellets is not extremely critical. Consideration should be given to ease of handling and the permeability of pellets.

Therefore, extremely large pellets are undesirable because of the possible difficulty of permeation by hypochlorite and the consequent efficient utilization of the catalyzing ingredient. Pellets in a cylindrical form having a diameter of about one-eight inch and the length of about three-sixteenths of an inch have been found to be suitable for use in this invention. Smaller granular-type catalyst particles have also been used with greater efficiency than the pellets because of the relatively larger available surface area. The preferred particle size is 18-35 mesh.

The pellets are prepared so that finely divided catalysts for decomposing hypochlorite is intimately dispersed in the organic resin matrix. One method for accomplishing this is to grind powdered catalysts and powdered organic resin, for example, in a ball-mill, forming the intimately mixed powdered composition into tablets or pellets by compacting them in a conventional machine and then sintering the pellets at or about the softening temperature of the organic resin. It is desirable that heating be conducted at a temperature high enough for sintering to take place but not so high that the physical form of the pellet is destroyed. The granular-type catalyst particles have been made by crushing the pellets and sieving to a selected size range.

An equivalent catalyst can be made directly by extrusion followed by sintering.

This invention can be used in decomposing hypochlorite waste liquors which contain calcium ion. This presents special problems since calcium ion apparently contributes to catalyst deactivation by depositing calcium carbonate in the "pores of the catalyst". In a special aspect of this invention it has been found advantageous to remove calcium ion by precipitation of the calcium as an insoluble salt, such as calcium carbonate, which is removed prior to allowing the hypochlorite solution to contact the catalyst. However, it is also possible to process hypochlorite solutions containing calcium ion directly and to periodically regenerate the catalyst.

The following Examples will further illustrate the preparation of the catalyst pellets of this invention and their use in a fixed bed system for decomposition of hypochlorite solutions.

EXAMPLE 1 A silica supported cobalt oxide powder was prepared by slowly precipitating cobalt hydroxide from an aqueous solution of cobalt nitrate containing suspended kieselguhr by the addition of base. The product was water washed, dried and calcined at 450 C for 2 hours. The resulting powder contained 35% cobalt by weight as cobalt oxide.

To 15 g of the above powder is added 5 g of polyvinylidene fluoride molding powder (Kynar 401). The mixture is placed in a size 000 ball mill along with 1 full capacity of ceramic balls and milled for 1 hour. The powdered mixture is tableted into cylindrical tablets approximately 1 /8th inch in diameter and 3/16ths inch long at 9,600 Ibs/in2 and the resulting tablets sintered in an oven at 1 80 C for 1 hour. The polymer matrix tablets are very active for hypochlorite decomposition and retain their physical integrity indefinitely under reaction conditions.

A fixed-bed catalytic reactor was charged with 100 grams of catalyst. Simulated industrial hypochlorite waste liquor (prepared as described below) treated for removal of soluble calcium (0.499% available chlorine) was passed through the reactor at a rate of 2.25 mls/min. At 25 C a vent solution containing 0.058% available chlorine was obtained, corresponding to an 88.4% conversion of hypochlorite to chloride ion and oxygen.

Simulated industrial hypochlorite waste liquid was prepared by dissolving 32.4 g of calcium hypochlorite (69.4% available chlorine), 166 g of sodium chloride, and 74.1 g of calcium chloride in 1000 ml of distilled water. The resulting solution was clarified by settling and the hypochlorite content determined by titration with sodium thiosulfate (-1.25% available chlorine). Other concentrations were made by successive dilutions.

Calcium-free hypochlorite solution was prepared by treating the above simulated industrial hypochlorite waste liquor with a stoichiometric amount of sodium carbonate (one mole of carbonate per mole of calcium). The resulting suspension ws clarified by settling and the clear supernatant liquor, after filtering, was treated with the catalyst.

EXAMPLE 2 This example is identical to Example 1 with the exception that nickel is substituted for cobalt.

EXAMPLE 3 Ths example is identical to Example 1 with the exception that polyethylene powder is substituted for polyvinylidene fluoride powder and the resulting tablets were sintered in an oven at 1 20 C for 1 hour.

The resulting polymer matrix tablets are active for hypochlorite decomposition and retain their physical integrity indefinitely under reaction conditions.

EXAMPLE 4 This example is identical to Example 1 with the exception that tetrafluorethylene is substituted for polyvinylidene fluoride powder and the resulting tablets were sintered in an oven at 270 C for 1 hour.

The resulting polymer matrix tablets are active for hypochlorite decomposition and retain their physical integrity indefinitely under reaction conditions.

Catalyst samples prepared according to procedures described in Example 1 through 4 were evaluated using both 1% sodium hypochlorite solution (calcium free) and simulated industrial hypochlorite waste liquor. With 1% sodium hypochlorite solution, no fall off in catalytic activity was measured over four weeks of continous operation. No disintegration of the catalyst was noted and total cobalt in the vent liquor was less than 0.5 ppm.

When simulated industrial hypochlorite waste liquor was used as feed to the reactor, substantial catalytic activity was lost within 48 hours. Catalyst deactivation was attributed to calcium carbonate deposition in the "pores of the catalyst". The catalyst activity was restored using the procedure described in Example 5.

EXAMPLE 5 A fixed-bed catalytic reactor of cross sectional area 2.5 cm2 was charged with 100 grms (-100 cm3) of the cobalt oxide/Kynar catalyst used in Example 1. Calcium containing simulated industrial hypochlorite waste liquor (~1.25% available chlorine) prepared according to Example 1 was passed through the reactor at a rate of 2.5 mls/min (25 C).

After one day of continuous operation, 93% of the hypochlorite fed to the reactor was being converted to chloride ion and oxygen.

After two days the conversion was 88%, after three days the conversion was 83%, after four days the conversion was 77% and after five days the conversion was 71%. At this point the catalyst was regenerated by purging with fresh water at a rate of 300 mls/min for three hours. Hypochlorite waste was again passed over the catalyst at a rate of 2.5 mls/min. Hypochlorite conversion after regeneration was 96.5%. Continuous operation followed by regeneration when conversions fall below 80% was continued for 64 days with no indication that this procedure cannot be continued indefinitely without a loss of catalyst efficiency.

Claims (8)

1. A pelleted catalyst composition comprising pellets of a substance capable of catalyzing the decomposition of hypochlorite embedded in an organic resin matrix.

2. A composition according to claim 1, wherein said organic resin matrix is a sintered thermoplastics resin.

3. A composition according to claim 2, wherein said organic resin is a polyolefin, halogenated polyolefin or a polyvinylidene halide polymer.

4. A composition according to claim 1, 2 or 3, wherein said substance capable of catalyzing the decomposition of hypochlorite is an oxide or a hydroxide or iron, copper, magnesium, nickel or cobalt.

5. A composition according to any one of claims 1-4, wherein the weight ratio of said substance capable of catalyzing the decomposition of hypochlorite to said organic resin in said pellets is in the range 1:1 to 15:1.

6. A process for the treatment of aqueous hypochlorites solutions to decompose the hypochlorite therein, which comprises contacting the solution with a pelleted catalyst composition as claimed in any one of the preceding claims.

7. A process according to claim 6, wherein said solution is contacted with said pelleted composition in a fixed bed.

8. A process according to claim 7, wherein said aqueous hypochlorite solution contains calcium ion and a pretreatment step is conducted by removing the calcium as an insoluble salt prior to decomposition of the hypochlorite.