JP2587742B2 - Method for removing halogenate ions from aqueous alkali metal halide solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing halide ions from an aqueous alkali metal halide solution.

[0002]

2. Description of the Related Art As one method of producing sodium hydroxide, for example, a method of electrolyzing raw salt water obtained by dissolving a raw material salt in water by an ion exchange membrane method or the like is widely used. In this method, first, sodium carbonate and sodium hydroxide are added to raw salt water and fresh saline (brine water obtained in an electrolytic cell described later) in a reaction tank, and calcium ions and magnesium ions contained in the raw salt water are added. To crystal particles of calcium carbonate and magnesium hydroxide, then add a polymer coagulant to aggregate the crystal particles to form a floc, further perform solid-liquid separation, and then filter the supernatant, for example, a chelate resin Then, the raw salt water (supernatant) is electrolyzed in an electrolytic cell to obtain sodium hydroxide.

Meanwhile, a solution containing sodium chloride (hereinafter referred to as "fresh salt water") remains in the anode chamber of the above-mentioned electrolytic cell, and this fresh salt water is returned to the reaction tank as return salt water and mixed with raw salt water. However, during electrolysis, a large amount of chlorate ions such as ClO ⁻ and ClO ₃ ⁻ are mixed in the salt water, and it is necessary to remove these chlorate ions. The reason for this is that when chlorate ions are taken into the brine purification process, the chelate resin in the filter section will be degraded and the filter function will be degraded, and the ion exchange membrane will be degraded and the electrolytic performance will be greatly reduced. This is because they will let you.

[0004] Therefore, conventionally, in order to remove chlorate ions in the salt water, a method in which a solution in which a sulfite or a thiosulfate is dissolved is added to a salt water containing chlorate to cause a reaction, or activated carbon is used. A reduction method such as a method in which fresh salt water is brought into contact with hydrogen, and a method in which hydrochloric acid is added to decompose the mixture are employed.

[0005]

However, in the method of reducing chlorate ions using sulfate, sulfate ions remain in the salt water, so that the fresh salt water is returned to the salt water refining process so that the final product, hydroxylation, is obtained. Sulfate ions are mixed into sodium as impurities. For this reason, when the purity of sodium hydroxide is a problem,
₂ SO ₄ ) is removed, but in this case, the equipment for removing bow nitrate is required and the cost is increased. In the method using activated carbon, the removal rate of chlorate ions is low. In the method using hydrochloric acid, about 10 times mol of hydrochloric acid must be added per 1 mol of chlorate ions, so that unreacted hydrochloric acid is neutralized. Alkalis are required, which increases costs.

The present invention has been made under such circumstances, and it is an object of the present invention to efficiently remove halogenate ions in an aqueous alkali metal halide solution, for example, chlorate ions contained in salt water. It is to provide a way to do it.

[0007]

According to the present invention, a large number of child particles made of a water repellent substance are adhered to the surface of a base particle made of a platinum group metal, and a part of the child particles is welded to a substrate made of a water repellent substance. And an aqueous solution of an alkali metal halide containing halogen acid ions and hydrogen gas are simultaneously brought into contact with the reduction catalyst.

The above reduction catalyst can be produced, for example, as follows.

First, a solution in which a platinum group metal salt is dissolved in an organic solvent such as ethanol, and a suspension obtained by dispersing a powder of the first water repellent substance in a dispersion medium containing water or alcohol containing a surfactant. To obtain a coating solution.

On the other hand, for example, a porous substrate made of a second water repellent substance having a softening point equal to or close to the softening point of the first water repellent substance is prepared. It is preferable to use the same substance as the first and second water-repellent substances, and for example, a fluororesin such as PTFE (polytetrafluolefin) can be used. Then, the substrate is immersed in the mixed solution, or the mixed solution is applied or sprayed on the substrate to coat the mixed solution on the surface of the substrate (including the inner surface when porous), and then the substrate is dried. . As a result, crystal particles of the platinum group metal salt are deposited on the surface of the substrate, and a large number of PTFs are formed on the surface of the crystal particles.
The child particles of E aggregate and adhere.

Subsequently, the mixture is coated on the surface of the substrate and dried in the same manner, and this operation is repeated to form a particle layer having a thickness of, for example, 0.5 to 10 μm on the surface of the substrate.
Thereafter, the substrate is heated in a hydrogen gas to around the softening point of PTFE, for example, at 250 to 310 ° C., to reduce the platinum group metal salt to the platinum group metal. The drying of the substrate can be incorporated in this heat treatment without special treatment.

At this time, the PTFE is softened and melted, and the platinum group metal particles are bonded to each other by welding of the PTFE child particles, and the child particles and the base are integrated by welding to obtain a reduction catalyst. FIG. 2 is a diagram showing such a reduction catalyst, wherein 1 is a base particle made of a platinum group metal, and 2 is PTFE.
Numeral 3 is a substrate, and 4 is a welded portion.

Here, as the substrate, a cut wool mesh body of PTFE, a Raschig ring or a tube can be used. Then, for example, in a treatment tower, a catalyst layer is formed so as to close the passage by the reduction catalyst, and an alkali metal halide aqueous solution containing a halogenate ion (hereinafter, referred to as “liquid to be treated”) is formed in the treatment tower. The gas flows from the upper part to the lower part of the bed, and the hydrogen gas is supplied from the lower part, so that the liquid to be treated and the hydrogen are simultaneously brought into contact with the reduction catalyst.

[0014]

In the contact area (three-phase interface) between the platinum group metal, hydrogen gas and the liquid to be treated, Na in the liquid to be treated,
ClO ₃ is reduced by hydrogen gas to H ₂ O,
When hydrogen gas is present on the surface of the water-repellent substance, the water is repelled, so that the reaction region becomes a contact portion between the mother particle and the child particle.

Here, for example, when platinum group metal particles are simply attached to a substrate made of a water-repellent substance, only the contact area between each particle and the substrate becomes a reaction region. The contact area of a large number of child particles adhered to the surface becomes a reaction area, so that a large reaction area can be obtained. As a result, a catalyst activity as high as, for example, platinum black can be obtained, and the three-phase interface reaction can be efficiently performed. Can be done

The platinum group metals include Pt, Ir,
Rh, Ru or the like may be used as a single substance, or may be used as an alloy formed by combining two or more kinds.

The water-repellent substance is not limited to a fluororesin.

[0018]

FIG. 1 is a schematic view showing an example for carrying out the method of the present invention. In this example, a liquid inlet 51 and a liquid outlet 52 are provided at an upper end and a lower end, respectively. Using a treatment tower 5 with an inner diameter of 10 cm and a height of 1.5 m, a fixed catalyst is filled near the center between two upper and lower mesh bodies made of, for example, a fluororesin so as to block the passage by filling the catalyst with the catalyst. 6 and the processing tower 5
A perforated plate 7 that allows only gas to pass therethrough is disposed at the bottom, and a hydrogen gas supply unit 8 is provided below the perforated plate 7 to constitute a fresh brine treatment apparatus.

[0019] Next will be described preparation of the reduction catalyst, using a solution of chloroplatinic acid _{(H 2 PtCl 6 · 6H 2} O), and ethanol solution 50ml containing Pt2g, P
The suspension was mixed with 12 g of a suspension in which 4 g of TFE powder was dispersed, and a PTFE cut piece (5 mm in width and 5 to 1 in length) was added to the mixture.
After immersing 200 g of a substrate having a thickness of 0 cm and an average thickness of 100 μm, the substrate is dried, and immersion and drying are repeated 25 to 30 times to obtain a particle layer having an average thickness of about 1.2 μm. Thereafter, this was heated at 280 ° C. for 60 minutes in a hydrogen gas atmosphere furnace,
As a result, the platinum salt is reduced to platinum metal, and a part of the PTFE powder adhering to the surface and the base are welded to each other by softening and melting, whereby a reduction catalyst is obtained.

Here, chlorate (NaClO ₃ ) is added in an amount of 0.5
g / l of fresh salt water is added with hydrochloric acid, and the fresh salt water is circulated in the treatment tower 5 at a flow rate of 0.5 l / hr through the inlet 51 and the outlet 52 and from the hydrogen gas supply unit 8. When hydrogen gas was passed through the inside of the treatment tower 5 at a flow rate of 60 l / hr, the concentration of NaClO ₃ in fresh salt water was as shown in FIG.
As shown. However, the solid line 1 is the temperature of fresh salt water at 75 ° C., pH is 0, and the dotted line 2 is the temperature of fresh salt water at 65 ° C., p
H is 0, solid line 3 is the temperature of fresh salt water at 75 ° C. and pH is 1.5
It is a result in the case of having set it as. From this result, NaCl
O ₃ is understood to decompose 60% to 90%.

Further, fresh salt water was allowed to flow in the treatment tower 5 for three months as described above, and then hydrogen gas was supplied while the fresh salt water was flowing in the same manner to determine the decomposition rate of NaClO ₃ .
Again, a degradation rate of 60-90% was obtained. Therefore, it is understood from this experiment that the activity of the reduction catalyst has not declined.

FIG. 4 is an explanatory diagram showing the overall configuration of a brine purification apparatus for obtaining sodium hydroxide by brine purification by applying the method of the present invention. In FIG. 4, reference numeral 91 denotes a reaction tank. In this reaction tank 91, sodium carbonate and sodium hydroxide are added to raw salt water and fresh salt water to convert calcium ions and magnesium ions into crystal particles. 92 is
A floc forming tank for adding a polymer flocculant to agglomerate crystal particles, 93 is a sedimentation separation tank for sedimenting and separating the aggregated crystal particles, 94 is a filter section for filtering remaining crystal particles, and 95 is a remaining filter part. An ion-exchange resin tower 96 for removing calcium ions and magnesium ions from the ion-exchange resin is an electrolytic cell. In this embodiment, a chlorate ion removing device 98 composed of a treatment tower as shown in FIG. 1 is provided so as to perform a chlorate ion removal treatment on the fresh salt water discharged from the anode chamber of the electrolytic cell 96. Before the fresh salt water is supplied to the removing device 98, hydrochloric acid is added to adjust the pH to 1-2, and bubbling with air is performed in the dechlorination tower 97. The reason why the hydrochloric acid is added and the bubbling is performed is to reduce the concentration of dissolved chlorine gas in the fresh salt water.
The fresh salt water treated by the chlorate ion removing device 98 is p
After the pH is adjusted in the H adjusting tank 99, the pH is returned to the raw salt dissolving tank, and is sent to the reaction tank 91 again.

In the above, in the present invention, the same effect can of course be obtained by using a fluidized bed in which a reduction catalyst is suspended, instead of forming a fixed bed with a reduction catalyst.

The present invention is not limited to salt water, and can be applied to the case of removing halogenate ions from other alkali metal (including alkaline earth metal) halide aqueous solutions.

[0025]

According to the present invention, since a large number of child particles made of a water-repellent substance are adhered to the surface of a base particle made of a platinum group metal, as described in the section of (action), The reaction area can be made large, and therefore, the catalytic activity is high, and the treatment for removing halogenate ions in the liquid to be treated can be performed efficiently.

Moreover, since the platinum group metal is firmly bound to the substrate through the welded portion of the water repellent material, physical peeling or peeling due to chemical deterioration can be suppressed, and the catalyst can be used for a long time without replacing the catalyst. The processing can be performed over a period of time.

When a platinum group metal layer is simply formed on the surface of a substrate made of a water-repellent substance, physical separation easily occurs, and the activity is immediately reduced.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of an apparatus used in the present invention.

FIG. 2 is an explanatory view schematically showing a reduction catalyst used in the method of the present invention.

FIG. 3 is a graph showing a change in the concentration of NaClO ₃ .

FIG. 4 is an overall configuration diagram showing a salt water purification apparatus to which the present invention is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Base particle made of platinum group metal 2 Child particle made of water-repellent substance 3 Substrate 5 Treatment tower

Claims (1)

(57) [Claims] A reducing catalyst comprising a plurality of water repellent substances adhered to the surface of mother particles made of a platinum group metal, and a part of the child particles welded to a substrate made of a water repellent substance; A method for removing halogenate ions from an aqueous alkali metal halide solution, comprising simultaneously contacting an aqueous alkali metal halide solution containing halogenate ions and hydrogen gas with a reduction catalyst.