KR890005297B1 - Method for the solidification of waste water - Google Patents

Abstract

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Description

Solidification Treatment of Radioactive Wastewater

The present invention mainly relates to a process for weight loss treatment of wastewater containing boric acid and / or borate and radioactive material which is extracted from pressurized water.

In a pressurized water-type light water reactor as one type of nuclear reactor, water containing boric acid, which easily absorbs neutrons as primary cooling water, is used, and the output of the reactor is controlled in accordance with the increase and decrease of the concentration of boric acid in the cooling water. Therefore, from this cooling water system, wastewater containing boric acid and radioactive particulates mixed during use of the cold water or the neutralized wastewater is discharged. The radioactive solids in these wastewaters need to be separated from the water, solidified into blocks, and so-called weight loss treatments to withstand the final disposal method such as ocean dumping or soil dumping, or intermediate storage or transportation on the island. For this purpose, various methods are disclosed as the solidification method. For example, such a method is disclosed in Japanese Patent Laid-Open No. 52-92100, Japanese Patent Laid-Open No. 52-104700, Japanese Patent Laid-Open Publication No. 54-34000, Japanese Patent Laid-Open Publication No. 55-34397 and the like. All three of them are solidified by dispersing some of the concentrated wastewater or powder obtained by drying after concentration of the wastewater in a solidifying aid (hereinafter referred to as self-hardening) having a property of solidifying itself, such as cement or thermosetting synthetic resin. I'm using the method. Therefore, these conventional methods use a solidification aid, so that the weight is increased by at least the solidification aid, so that the degree of loss of radioactive wastewater is insufficient. Japanese Patent Laid-Open Publication No. 55-34397 describes a method of powdering solids in wastewater. The method of solidifying the powder obtained is merely a mechanical compression method, and the strength of the solids is temporarily limited. Insufficient amount to withstand the storage of the powder, and the powder obtainable at this time has no magnetic hardening, so the hardening aid is still required to improve the strength.

Accordingly, an object of the present invention is to propose a method of solidifying boric acid and radioactive material contained in wastewater without using a self-hardening property solidifying aid, and having a sufficiently high compressive strength against sea dumping.

The present invention is prepared by preparing a wastewater containing boric acid and / or borate and a radioactive substance at a pH of 7 or higher, followed by addition reaction of at least one compound selected from the group consisting of magnesium, calcium and barium. It is a solidification method of radioactive wastewater characterized in that the solid is prepared into a slurry containing 20 to 80% of water on a wet basis and the slurry is solidified.

Next, the method of this invention is demonstrated about the case where calcium hydroxide is used as an example of a calcium compound. First, the raw wastewater containing boric acid was adjusted to a pH value of 7 or more using concentrated sodium hydroxide (hereinafter, this process is called a pH adjusting process). Calcium hydroxide is added and reacted to the liquid after pH adjustment so that calcium weight may be about 0.5 times with respect to the weight ratio of boric acid in a liquid. (Hereinafter, this step is referred to as reaction fixing.) In this reaction step, the boric acid group and calcium hydroxide in the raw wastewater react and the liquid becomes a slurry. The slurry is adjusted so that the water content in the slurry is 20-80% based on the moisture content by means of filtration and concentration, and the adjusted slurry is poured into a desired mold and allowed to stand for 24 hours or more. By this curing step, the slurry can be solidified into a block having a desired shape. The compressive strength of the solid product taken out from the mold after completion of the curing process is 150 kg / cm ² or more, which is sufficient for sea dumping or soil picking.

In the series of processes described above, as the alkaline substances used in the pH adjustment process, known alkaline substances such as sodium hydroxide, potassium hydroxide and calcium hydroxide can be used alone or as a mixture, and it is the simplest to use a concentrated aqueous solution or solid of sodium hydroxide. . In this pH adjustment step, the waste water temperature is appropriate from room temperature to 120 ° C., and can be carried out even below room temperature or above 120 ° C., but does not require cooling below room temperature or heating above 120 ° C. in particular. Moreover, although 7 or more is preferable as a pH value after pH adjustment, 9-12 are especially suitable.

As a compound added in a reaction process, it is good to contain magnesium, calcium, and / or barium in the compound. Any anionic group bound to these elements in the additive may be any of OH (including oxides as anhydrides of compounds having these groups), chlorine ions, carbonic acid groups, acidic carbonic acid groups, nitric acid groups, sulfuric acid groups, acidic sulfuric acid groups, phosphoric acid groups, 1 or 2 It is easy to find out by hydrogen phosphate group etc. These compounds may be added to the solution after the pH adjustment process as an aqueous solution or powder, but do not increase the total amount of the treated water, so it is preferable to add these compounds as powders or products having a low moisture content. Moreover, it is preferable to make the addition amount of these compounds into the quantity which contains 0.2-1.0 mol of the said metal per mol of boric acid groups in a liquid. Moreover, these compounds may add 1 type of these compounds, and may mix and add 2 or more types so that the said addition amount may be satisfied. As for the temperature of the liquid at the time of addition, 30-90 degreeC is suitable and it is preferable to add, stirring normally. Moreover, in reaction with waste water after addition of these compounds, the temperature of 40-120 degreeC and the reaction time of 0.5 to 3 hours are preferable. Even if the temperature is outside the above temperature range and the time is 2 hours or more, there is no particular effect on the curing reaction described below. Moreover, it is preferable to carry out stirring during this reaction. Insoluble solids in the slurry obtained as a result of the above reaction have electric magnetic hardness in the presence of water. However, when the method of the present invention is applied, the boric acid group content in the wastewater serving as a raw material usually varies between 4 to 20% by weight, so that the concentration of insoluble solids in the slurry produced after the reaction also varies significantly. Therefore, it is preferable to make effective use of the inner volume of the mold after adjusting the water content in the slurry before carrying out the next curing step.

As a means for reducing the moisture contained in the slurry, well-known filtration means such as centrifugation, gold mesh or follicles, or many well-known means such as evaporation, concentration or drying can be used. The water content in the slurry after the water content is reduced is preferably 20 to 80% by weight in terms of both the fluidity of the slurry and the effective use of the inner volume of the mold. The optimum value depends on the type and particle size distribution of the added electric compound. .

By directly filling the slurry after adjusting the amount of water in the mold and leaving it to stand, the solid content in the slurry is solidified to obtain a solidified block of a desired shape. If moisture is 10% or less, a high strength block is not obtained. In addition, when dehydration by filtration or the like results in a very small amount of water and thus poor fluidity as a slurry, fresh water or filtered water is added to prepare a slurry having good fluidity within the above water content. I can charge it in a mold. The suitable temperature of the curing process is 10 to 50 ° C., for example, when it is left at a temperature of 20 ° C., it reaches a compressive strength of about 150 kg / cm ² per day, and the strength is improved according to the extension of the subsequent leaving time. In addition, if left in contact with air at a temperature of 50 ℃ to reach the compressive strength of the 150kg / cm ² in about 2 hours it can shorten the curing time. When adjusting the moisture of the slurry, it is also possible to use a cylindrical rotary dryer with or without a stirring blade inside or a band dryer for dehydrating drying on a gold mesh belt, but it is not necessary to dry the moisture content to 20% or less.

It is not yet clear why the hardening phenomenon occurs in the hardening process of this invention. However, according to the present invention, boric acid contained in the waste water without using a thermosetting resin such as cement, epoxy resin, unsaturated polyester resin or polymerizable monomer which polymerizes and solidifies as a self-hardening solidifying aid in the conventional method is used. Solids containing borates and other radioactive materials can be solidified into blocks, as in later examples. Therefore, according to the present invention, instead of using about 16,000 kg of cement or 800 kg of expensive synthetic resin per ton of boric acid group, the addition of 400-600 kg of inexpensive electric compound has a sufficiently high compressive strength. Cargo can be obtained, and the weight and volume of solid cargo can be reduced, and the cost of solidification and the transport of solid cargo can be reduced.

In the final disposal of the solidification block obtained by the present invention such as soil tying or seawater throwing, the elution of radioactive elements contained in the block to the underground or seawater is reduced, so the outer periphery of the block is closely adhered to the concrete or synthetic resin shell. It is possible to enclose or impregnate the obtained block with a low viscosity uncured unsaturated polyester resin, a polymerizable monomer, etc., then impregnate the polymerizable monomer, etc. and then carry out a polymerization reaction to reduce the water permeability of the block. It is preferable to apply the resin means. In the case of close enveloping by the former outer shell, for example, the inner surface of the metal drum, except for the inlet of the slurry drum, is coated with a cured product of concrete or unsaturated polyester resin, and the slurry is filled in the cavity inside the inner drum. The method of curing and closing the slurry inlet with concrete or unsaturated polyester resin after curing is simple. In case of the latter impregnation polymerization method, the block obtained by the present invention is left indoors, for example, at room temperature for about one month, and the free water in the portion close to the surface of the block is dried and removed. It is preferable to degas and to impregnate in an electrically unsaturated polyester resin or polymerizable monomer which is already blended with a catalyst for polymerization, to impregnate these resins or monomers in the interior of the block, and then to carry out polymerization curing at a temperature of normal temperature to 50 ° C. Preferable polymerizable monomers include styrene monomer, methacrylic acid ester and the like.

And in this invention, the compound structure of boric acid and boric acid contained in the raw wastewater and the wastewater under treatment differs depending on the history before these boric acid and boric acid is discharged as wastewater together with water, or the concentration temperature when discharged. Although not clear in detail, it is generally thought to include many other boric acids and their derivatives of chemical structure. In view of such circumstances, in this invention, boric acid and a boric acid salt mean the boric acid type compound which has a following formula as a single or a mixture.

_{x M m} O yB _l O _n (OH) _k ) z (H ₂ O)

Where x is 0 or an integer of 1 to 3, y is an integer of 1 to 5, z is an integer of 0 or 1 to 10, m is 1 or 2, l is an integer of 1 to 10, n is 0 or 1 to 15 K is an integer of 0 or 1-4, M is a monovalent or divalent metal atom, O is an oxygen atom, B is a boron atom, H is a hydrogen atom.

In the present invention, the filtered water at the time of the post-reaction filtration as described above contains almost no radioactive material. Generally, the filtered water is often discharged after neutralization. If it is contained, the filtered water can be concentrated by a known method alone or together with raw wastewater, and the method of the present invention can be applied again.

Example 1

Dissolving 400 g of solid caustic soda in 10 l of simulated wastewater containing 1.3 kg of ortho boric acid gives a pH of 9.1. 650 gr of calcium hydroxide powder was added to this solution, and the reaction was carried out at 60 DEG C under stirring for 1 hour. Immediately after the reaction, suction filtration was performed using a buchner funnel. The moisture content of the cake obtained was 30% on the basis of the moisture content (hereinafter equal to) and the cake weight was about 2.7 kg. In this example, the cake has a poor fluidity, so the new container is added with 700 gr of water to increase the moisture content to about 44%, and the continuous diameter of 40 mm × 40 mm × 160 m / m is specified in JIS R-520. It was injected into two molds and cured for 1 day. Then, this initial hardened | cured material was taken out to the mold, and hardening was performed for 27 days at 20 degreeC in the glass container. The uniaxial compressive strength was measured on day 28, and the average value was 550 kg / cm ² .

As a comparison, in order to solidify the simulated wastewater 10l by the method disclosed in Japanese Patent Application Laid-Open No. 52-92100, the addition of about 1.5 kg of hydrated lime, about 5 kg of Portland cement, and about 1.5 kg of water glass and solidification containing 1.3 kg of boric acid is required. It is clear that both the weight and the volume of water are rarely from this invention.

Example 2

If the same experiment as in Example 1 is conducted and suction is performed after suction filtration for a long time, the cake is dried. The uniaxial compressive strength of the solidified water obtained by adding the filtrate to the cake dried by 3% of moisture to 50% of moisture content, and performing the operation similar to Example 1 after that was 570 kg / cm <^{2> on} average.

Example 3

The reaction mixture was filtered and filtered in the same manner as in Example 2, except that 500 gr of solid caustic calories were used instead of 400 gr of solid caustic soda in Example 1, and 790 gr of barium hydroxide powder was used instead of 650 gr of calcium hydroxide powder. The moisture of the cake obtained by inhalation for a long time was 5%, and the uniaxial compressive strength after curing as in Example 2 was measured as a slurry having a water content of 30% by adding fresh water thereto, and the average was 610 kg / cm ² .

Example 4

The initial cured product, which was subjected to the initial curing as in Example 1, was allowed to stand for 28 days at room temperature in an indoor atmosphere and then cured, and then degassed in a glass sealed container for 30 seconds under a vacuum of 10 mm mercury. As a methyl methacrylate monomer containing 1.0% of methyl ethyl ketone hydroperoxide as an agent and 0.25% of cobalt naphthenic acid as a polymerization accelerator, a vacuum was broken to impregnate the methyl methacrylate monomer with the cured product. This impregnated hardened | cured material was left to stand at 60 degreeC for 24 hours, and the superposition | polymerization of methyl methacrylate was advanced. After that, the uniaxial compressive strength was measured and showed a value of 610 kg / cm ² .

Example 5

100 gr of solid caustic soda and 700 gr of calcium hydroxide powder were added to 10 l of simulated wastewater containing 1.56 kg of sodium tetraborate and 3.0 gr of cesium chloride, and six initial cured products were prepared by the same operation as in Example 1. Curing was carried out in a vitreous container for 100 days, and then used as a test sample. Curing was carried out for 28 days at room temperature as in the other three examples, followed by impregnation polymerization of methyl methacrylate, followed by curing for 70 days, and then measurement. The uniaxial compressive strength thus obtained was 520 kg / cm ^{2 in} which methyl methacrylate was not present in the impregnation polymerization, whereas the impregnation polymerization of this monomer was 600 kg / cm ² . In addition, three cylindrical test samples having an outer diameter of 45 mm and a height of 44 mm prepared as described above were immersed in 700 ml of water at 20 ° C. for 100 days, respectively, and the dissolution test of cesium in water was performed. The amount of cesium eluted was analyzed by atomic absorption method. As a result, an average of 48 mg in 100 days in the absence of impregnation polymerization of the monomer was 4 mg in the impregnation polymerization of the monomer.

Claims (1)

In the method of treating wastewater containing boric acid and / or borate and radioactive material, the wastewater after adjustment to pH7 or higher is added to and reacted with at least one compound selected from the group consisting of compounds containing magnesium, calcium and barium. A method for solidifying radioactive wastewater, characterized in that the solid content is adjusted to a slurry containing 20 to 80% of water on a wet basis and solidified.