NO155276B - PROCEDURE FOR CATALYTIC DIVISION OF HYPOOCLORITE AND CATALYST FOR APPLICATION BY THE PROCEDURE. - Google Patents

Description

Hypochlorite ions in aqueous solution are corrosive

on many metals and is highly toxic to all organisms that live in water. Industrial waste discharges containing aqueous hypochlorite are the result of many processes such as in the manufacture of hypochlorite and dry bleaching. Before this waste can be discharged into public water

is treatment necessary to remove hypochlorite ions.

Different methods that include photochemical, thermal

and chemically induced cleavages have been proposed to remove hypochlorite from dilute aqueous solution. For application on a large industrial scale, chemical methods are the most commonly used. Chemical methods, which include the use of e.g. H 2 O 2 , NaSH, HC 1 and SO 2 are all expensive when dealing with very large quantities of dilute aqueous hypochlorite. Waste treatment systems that consume large quantities of these chemicals create a significant financial burden on the processes they take part in.

There is a need for an ecologically efficient and economically sound method for the decomposition of large quantities of dilute hypochlorite. One basis for such a system is the decomposition of hypochlorite by heterogeneous fixed-bed catalysts to give chloride ion and oxygen. A number of such catalysts including the oxides and hydroxides of iron, copper, magnesium, nickel and cobalt have been described in the literature. Of these catalysts, those made from cobalt are the most active.

Due to certain practical disadvantages, fixed-bed catalysts have not been widely used commercially for the decomposition of hypochlorite. E.g. the high alkalinity of the hypochlorite solution causes the binding support for most tableted and extruded catalysts to disintegrate, reducing the catalyst in whole or in part to a fine slurry. Due to the problems associated with the recovery and recycling of finely divided catalyst particles in aqueous media, this technology has not found widespread application. Also, when fixed-bed catalysts are exposed to waste solutions containing both calcium ions and hypochlorite, such as waste from the dry bleaching process, the catalyst rapidly loses activity due to calcium carbonate decomposition in the catalyst pores. Reactivation of inactivated catalyst is difficult.

Accordingly, it is an aim of the present invention to provide an efficient and economically sound method for splitting hypochlorite contained in aqueous industrial waste streams, including those containing dissolved and suspended calcium salts. A further object of the present invention is to provide a catalyst for use in the present invention which is efficient, non-polluting and has a long life.

The present invention relates to catalyst pellets

with the ability to split hypochlorite, which consists of a powdered active catalyst and a resin binder as described in claim 4. The catalyst pellets according to the present invention are particularly

suitable for the treatment of waste streams that contain hypochlorite in solid layers and have improved resistance to disintegration compared to known tablet

ter tea and extruded catalysts. Method of catalytic cleavage with the catalyst described here, such as in fixed-bed cleavage of hypochlorite, constitutes another object of the present invention, and is described in the characterizing part of claim 1.

The aqueous solutions containing hypochlorite which

can be treated according to the method of the present invention and with the catalyst pellets described herein, can be any aqueous solution containing hypochlorite ions such as hypochlorous acid or salts of hypochlorous acids, especially the alkali metal and alkaline earth metal salts.

A common source of aqueous streams containing hypochlorite ions is the waste water from purification in a chlorinated

plant where the non-condensable tail-gases are cleaned with a caustic solution to prevent residual chlorine from escaping. in the atmosphere. This cleaning stream contains alkali metal hypochlorite which must be decomposed before it is discharged into public water. Other sources of non-

waste water containing hypochlorite that can be treated

les according to the method of the present invention occurs during the production of chlorine caustic and dry bleaching.

The method of treatment of various chemical streams

in a fixed bed reactor is well known and as such does not form part of this invention. Similarly, a number of materials suitable for the cleavage of the hypochlorite ion are known. These do not form part of the invention either.

It is the particular catalyst form and its application

by splitting hypochlorite which forms the basis of the present invention. In essence, known catalysts for splitting hypochlorite are formed into pellets with an organic resin binder in a matrix and these pellets are used in the known method for splitting hypochlorite in a solid layer.

Substances suitable for catalyzing the hypochlorite decomposition include oxides or hydroxides of iron, copper, magnesium, nickel or cobalt.

The resin binder which forms the other essential component

of the catalyst pellets according to the present invention consist of organic resins, which are thermoplastic and consist of solid polyolefins or halogenated polyolefins. It is only essential that the resin is relatively stable for long periods of time in contact with hypochlorite and that it has the ability to form a pellet that is relatively stable against mechanical disintegration as well as chemical disintegration in use. In particular, it has been found that polyolefins, halogenated polyolefins and polyvinylidene-

halide polymers are suitable. Representative of these materials are polyethylene, polypropylene, polytetrafluoroethylene and polyvinylidene fluoride.

The ratio between the substance capable of splitting hypochlorite and the organic resin binder can vary greatly, but generally it will be within the ratio of 100:1 to 1:10. Ratios of 1:1 to 15:1 are generally preferred. The weight ratio of approx. 5:1 has been found to be suitable where the substance capable of catalyzing the hypochlorite decomposition is cobalt oxide and the organic resin is any of a number of thermoplastic polymers. The essential criteria for the selection of a suitable ratio are that sufficient organic resin must be present to provide a matrix that is stable to mechanical handling and that the amount of organic resin does not exceed that which will allow penetration of catalyst pellets by the hypochlorite solution.

The size of the pellets is not extremely critical. Consideration must be given to the fact that the pellets are easy to handle and are portable. Therefore, extremely large pellets are undesirable, because of the possible difficulty in hypochlorite penetration and the consequent efficient use of the catalyst component. Pellets in a cylindrical shape with a diameter of approx. 3.2 mm and a length of about 4.8 mm has been found to be suitable for use according to the present invention. Smaller catalyst particles of the granular type have also been used with greater efficiency than the pellets due to the relatively larger available surface area. The preferred particle size is 18 - 35 mesh.

The pellets are produced so that finely divided catalysts for splitting hypochlorite are dispersed throughout the organic resin matrix. One method of achieving this is to grind powdered catalysts and powdered organic resin, e.g. in a ball mill, and form the intimately mixed pulverized mixture into tablets or pellets by compressing them in a conventional machine and then sintering the pellets at or around the softening temperature of the organic resin. It is desirable that the heating takes place at a temperature sufficiently high for sintering to take place, but not so high that the physical form of the pellets is destroyed. The catalyst particles of granular type have been produced by crushing the pellets and sieving to a specific size range. An equivalent catalyst can be prepared directly by extrusion followed by sintering.

The present invention can be used for splitting hypochlorite waste water containing calcium ions. This presents particular problems, as calcium ion apparently contributes to catalyst deactivation by splitting calcium carbonate in the "catalyst pores". In a particular aspect according to the present invention, it has been found advantageous to remove calcium ion by precipitating the calcium as an insoluble salt, such as calcium carbonate, which is removed before the hypochlorite solution is allowed to come into contact with the catalyst. However, it is also possible to treat the hypochlorite solutions containing calcium ion directly and periodically regenerate the catalyst.

The following examples will further illustrate the production of the catalyst pellets according to the present invention and their use in a fixed-bed system for splitting hypochlorite solutions.

EXAMPLE 1

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

To 15 g of the aforementioned powder, 5 g of polyvinylidene fluoride casting powder ("Kynar 401") is added. The mixture is placed in a ball mill size 000 together with 1/4 of the full capacity of ceramic balls and ground for 1 hour. The powdered mixture is tableted into cylindrical tablets approx. 3.2 mm in diameter and 4.8 mm long at 676 kgp/cm 2 and the resulting tablets are sintered in an oven at 180°C for 1 hour. The polymeric matrix tablets are highly active with respect to hypochlorite cleavage and retain their physical integrity indefinitely under reaction conditions.

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

Simulated industrial hypochlorite wastewater was prepared by dissolving 32.4 g of potassium hypochlorite (69.4% available chlorine), 166 g of sodium chloride, and 74.1 g of potassium chloride in 1000 mL of distilled water. The resulting solution was clarified by precipitation and the hypochlorite content determined by titration with sodium thiosulphate (~1.25% available chlorine). Other concentrations were prepared by successive dilutions.

Calcium-free hypochlorite solution was prepared by treating the pre-simulated industrial hypochlorite wastewater with a stoichiometric amount of sodium carbonate (1 mol carbonate per mol calcium). The resulting suspension was clarified by precipitation, and the clear supernatant after filtration was treated with the catalyst.

EXAMPLE 2

This example is identical to example 1 except that nickel is used instead of cobalt.

EXAMPLE 3

This example is identical to Example 1 except that polyethylene powder is used instead of polyvinylidene fluoride powder, and the resulting tablets were sintered in an oven at 120°C for 1 hour.

The resulting polymeric matrix tablets are active with respect to hypochlorite cleavage and retain their physical integrity indefinitely under the reaction conditions.

EXAMPLE 4

This example is identical to example 1 except that polytetrafluoroethylene is used instead of -For doI yvinylidene fluoride powder, and the resulting tablets were sintered in an oven at 270°C for 1 hour.

The resulting polymeric matrix tablets are active with respect to hypochlorite cleavage and retain their physical integrity indefinitely under the reaction conditions.

Catalyst samples prepared according to the methods described in examples 1-4 were evaluated using both 1% sodium hypochlorite solution (calcium-free) and simulated industrial hypochlorite wastewater. With 1% sodium hypochlorite solution, no drop in catalytic activity was measured during 4 weeks of continuous operation. No disintegration of the catalyst was observed and total cobalt in the effluent was less than 0.5 ppm.

When simulated industrial hypochlorite wastewater was used as feed to the reactor, substantial catalytic activity was lost within 48 hours. Catalyst deactivation was attributed to calcium carbonate cleavage in the "catalyst pores". Catalyst activity was recovered using the method described in Example 5.

EXAMPLE 5

A fixed-bed catalytic reactor with a cross-sectional area of 2.5 cm<2> was fed with 100 g (-^100 cm<3>) of cobalt oxide/Kynar catalyst which was used in example 1. Calcium-containing simulated industrial t hypochlorite-free general water (~ 1.25% available chlorine) prepared according to Example 1 was passed through the reactor at a rate of 2.5 ml/min (25°C). After one day of continuous operation, 93% of the hypochlorite feed to the reactor had been converted into 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 stage, the catalyst was regenerated by flushing with fresh water at a rate of 300 ml/min for three hours. Hypochlorite precipitates were again passed over the catalyst at a rate of 2.5 ml/min. The 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 process could not be continued indefinitely without loss of catalyst effectiveness.

Claims (6)

1. Method for catalytic splitting of aqueous hypochlorite solutions by bringing the solutions into contact with a metal oxide or hydroxide catalyst, characterized in that a porous catalyst matrix is used consisting of pellets or granules of a mixture of a powdered oxide or hydroxide of a metal which is iron, copper, magnesium, nickel or cobalt, and a powdered thermoplastic organic resin consisting of solid polyolefins or halogenated polyolefins, which powder mixture is compacted and then thermally sintered at a temperature close to the softening temperature of the resin. 2. Method according to claim 1, characterized in that a mixture of polyvinylidene fluoride and cobalt oxide or cobalt hydroxide is used as catalyst. 3. Method according to claim 1, characterized in that a catalyst is used where the ratio between metal oxide or hydroxide and the organic resin is in the range 1:1 to 15:1. 4. Porous catalyst matrix, characterized in that it consists of pellets or granules of a mixture of a powdered oxide or hydroxide of a metal selected from the group consisting of iron, copper, magnesium, nickel and cobalt, and powdered polyvinylidene fluoride, which mixture of powders is has been compacted and then thermally sintered at or around the resin's softening temperature, whereby a porous catalyst matrix is formed which is stable in aqueous hypochlorite solutions. 5. Catalyst according to claim 4, characterized in that it is in the form of pellets with a length in the range 3.2 - 4.8 mm. 6. Catalyst according to claim 4 or 5, characterized in that the sintered pellets are crushed and then sieved to a particle size in the range 18 - 35 mesh.