KR100307728B1 - Separation method of boric acid - Google Patents

Abstract

The boric acid separation method of the present invention is a method for separating boric acid from a boric acid-containing liquid, in particular from a liquid waste of a nuclear power plant, wherein the boric acid-containing liquid is continuously supplied to the reactor (1 or 10) in a non-alkali atmosphere. The boric acid is characterized in that the boric acid is concentrated and the boric acid is removed by steam which is continuously discharged from the reactor 1 or 10.

Description

Separation method of boric acid

The present invention relates to a process for separating boric acid from a liquid containing boric acid, in particular from the waste liquid of a nuclear power plant.

The first order of a nuclear power plant using water under pressure comes in direct contact with nuclear fuel, and although the water is very chemically pure, the water contains low GBq of radionuclides per cubic meter. To adjust the reactivity, usually up to 0.25% of boron is added to this water in the form of boric acid.

At this time, only a part of the primary water is contained in the wastewater. Nuclear power plants contain boric acid that requires treatment and emit thousands of cubic meters of waste water each year with some radioactivity. This wastewater is usually evaporated after the addition of base. Evaporation is generally considered to be the method of calculating the maximum radiation removal factor. That is, the vapor contains no or very few radionuclides except tritium. Residual concentrates containing about 50% of dry residue are charged to concrete and stored in containers at suitable places.

Processing of concentrates, in particular to keep them, requires a high cost, the cost being proportional to the volume of the concentrate. For this reason, not only the radiation removal coefficient but also the volume coefficient, ie the volume ratio between the wastewater and the concentrate, is very important.

Because of the problem of crystallisation, the volume reduction coefficient upon evaporation is also limited due to the presence of boron which accounts for the largest part of the dry residue in the form of boric acid or borate. In addition, the presence of boric acid may prevent charging waste into concrete. For this reason evaporation is carried out in an alkaline atmosphere.

Therefore, by removing boric acid from the waste water, the volume reduction coefficient becomes larger, and the volume of the waste can be greatly reduced. Further, if necessary, boric acid can be added to the primary water again.

Selective ion exchangers can be used to remove all boric acid from the wastewater, but this method is difficult to implement on an industrial scale. Regeneration of ion exchange resins and recovery of boric acid are particularly problematic.

Other useful methods include a method of evaporating the wastewater, then forming volatile boric acid esters such as trimethylborate, followed by distillation. Such a method is known from DD-A-293 219, whereby butyl alcohol is added to the concentrate after evaporation of the waste water to esterify the boric acid and then distilled off the volatile boric acid ester. Can be. These methods are difficult to execute continuously and are also time consuming. Alkalineization steps are carried out before evaporation and reoxidation is carried out to generate a large amount of salt and a large amount of waste.

It is an object of the present invention to overcome these drawbacks and to provide a very simple and relatively economical and particularly suitable method for industrial use for removing boric acid from a liquid containing boric acid.

This object is achieved by continuously supplying a liquid containing boric acid to a reactor in a non-alkaline atmosphere, then evaporating the boric acid with water vapor, and continuously discharging this water vapor from the reactor when the boric acid concentration in the water vapor is high.

This method is based on the fact that boric acid can be contained in water vapor because boric acid evaporates in water vapor.

It is preferred that the liquid is continuously fed and the vapor with elevated boric acid concentration is continuously discharged to keep the contents of the reactor substantially constant.

Concentrates remaining in the reactor can be discharged continuously, but are preferably discharged discontinuously.

In the first embodiment of the present invention, the liquid containing boric acid is added as a solution, and most of the water vapor is formed in the reactor by heating, so that the reactor acts as an evaporator.

Non-volatile impurities remain in the concentrate in the reactor. Initially, the boric acid is contained in the concentrate in large amounts because the boric acid content in the vapor phase is less than the boric acid content in the liquid, but over time, the boric acid content of the water vapor discharged will be as high as the boric acid content in the supplied liquid. The boric acid content of the concentrate no longer increases. Thus, the volume reduction factor is no longer limited by the presence of boric acid in the wastewater.

Water vapor can be used to supply heat to the reactor.

Thus, some of this supplied water vapor can absorb and release some of the boric acid.

In a second embodiment of the invention, the concentrate is introduced into the reactor as a boron containing liquid, and at least most of the water vapor for evaporating the boric acid is obtained from the outside of the reactor. Thus, water vapor is continuously supplied to the reactor from the outside and brought into contact with the concentrate.

The advantage of this method is that it is possible to use evaporators which are commonly available in existing devices for evaporating boron-containing waste water in nuclear power plants. Since the reactor is assembled behind the evaporator generating the concentrate, the reactor can also be small when processing a small volume of concentrate. If the concentrate is alkaline, acid can be added to make it alkaline.

In these two embodiments, the pressure in the reactor is moderately higher than atmospheric pressure and the temperature is maintained above 100 ° C.

The distribution coefficient, i.e. the ratio of boric acid content of the vapor to the boric acid content of the liquid in the reactor, increases with increasing temperature.

In both examples, boric acid can be recovered in an economical way from the vapor containing boric acid exiting the reactor.

The recovery can be done in a fractionating column.

In addition, boric acid may be washed from the vapor containing boric acid in a wash column.

As long as boric acid is recovered from the steam and water vapor from the outside of the reactor is used, this water vapor can be used well without concentration by flowing in a closed loop through the reactor and equipment for extracting boric acid. have.

Other special features and advantages of the present invention will become apparent from the following description of a method for separating boric acid from a liquid according to the present invention. The description is given by way of example only and does not limit the invention in any way.

The apparatus shown in FIG. 1 is used for separating boric acid from low-radioactive wastewater of a nuclear power plant of the type using pressurized water, according to the method of the present invention, which acts as an evaporator and serves as a non-alkaline atmosphere. The waste water is continuously evaporated under pressure in the reactor 1, and the vapor having a high boric acid concentration is continuously discharged from the reactor.

Waste water containing boric acid is supplied from storage tank 4 via filter 3 by pump 2 and then sent to heat exchanger 5 in reactor 1. The steam formed in the reactor 1 is continuously discharged to the distillation column 7 via the pressure regulating valve 6, in which the steam is discharged to the upper condenser 8 and the boric acid is lowered. To be discharged. This boric acid is reheated in the heat exchanger 9, a part of which is introduced back into the distillation column 7. Some of the concentrate of the condenser 8 is returned to the distillation column 7 but most of it is supplied to the heat exchanger 5 as a primary liquid.

This method is used to observe evaporation of boric acid into water vapor in the form of non-bonded H ₃ BO ₃ , whereby the experimentally based distribution (D) is the mole fraction of boric acid in the liquid. It can be specified as the ratio of the mole fraction of boric acid in the steam to the mole fraction.

In contrast to conventional methods, in order to evaporate boric acid, the wastewater is made non-alkaline and the atmosphere in the reactor is non-alkaline, i.e. acidic or substantially neutral. Usually, the wastewater has a predetermined pH value, but if necessary, an acid or a base such as sulfuric acid can be supplied to the storage tank 4 via the pipe 15. The method of the present invention does not work properly if the pH value is too high. However, too low a pH value should also be avoided as it involves corrosion problems. Suitable pH values for the wastewater are between 5 and 7.5, with 6 to 7 being suitable.

The distribution ratio D is smaller than 1, but the value increases as the temperature rises. The distribution ratio D at about 100 ° C., which is the boiling point at atmospheric pressure, is 0.0025, but at about 180 ° C., the value has already risen to 0.03. In order to obtain a temperature above the boiling point at atmospheric pressure, the reactor 1 is under pressure, preferably at temperatures between 150 and 180 ° C., at a pressure between 5.0 and 10.0 bar, for example at a temperature of 175 ° C. and at a pressure of 7.6 bar. It must be operated under

The pressure is obtained by the pump 2. In order to obtain the above temperature of about 180 ° C., the pressure in the reactor 1 needs to be about 9.0 bar.

The reactor 1 is operated at a constant temperature and pressure and a constant liquid volume.

Wastewater, which is already about 25 ° C., can be heated to about 98 ° C. by a heat exchanger. Further heating is achieved by the introduction of heat, which can be obtained by various other methods such as, for example, the supply of superheated steam, most of which is used to heat and evaporate the liquid in the reactor. If necessary, a portion of the heat can flow through the liquid and flow out of the reactor 1 together with the liquid containing boric acid.

When the apparatus is started up, since the distribution ratio D indicating the distribution ratio of boric acid between the gas phase and the liquid is less than 1, the boric acid content in the vapor phase is smaller than the boric acid content in the liquid. The liquid in reactor 1 is therefore initially high in boric acid content and very few will evaporate into water. The boric acid content in the steam will continue to increase over time and will equilibrate after a while. The boric acid content in the steam will be equivalent to the boric acid content in the liquid introduced, ie waste water. The boric acid content of the concentrate no longer increases, and as a certain amount of liquid is used in the reactor, all of the boric acid introduced into the reactor with the wastewater moves to the gas phase and is discharged from the reactor (1).

Among other components, non-volatile and radioactive impurities remain both at the bottom of the concentrate in the reactor 1, and if necessary, these impurities are discharged continuously, preferably occasionally (intermittently) to the bottom of the reactor 1. . Therefore, this increase in impurity concentration is no longer limited by boric acid concentration.

Thus a very large volumetric reduction factor of the wastewater is obtained, which is no longer limited by the presence of boric acid in the wastewater stream.

The steam flowing out of the reactor 1 is discharged via the pressure regulating valve 6. The steam exiting the distillation or fractionation column 7 operating under atmospheric pressure is effectively separated into pure water vapor and concentrated boric acid solution. The column 7 is adjusted so that the boron concentration of the heat exchanger 9 is 7500 ppm, which concentration is the concentration of the boric acid solution used for the production of the primary water of the nuclear power plant.

Instead of a distillation or fractionation column, a washing column can be used to recover the boric acid from the steam.

The apparatus shown in FIG. 2 is used to implement another embodiment of the method according to the invention. This example is essentially distinguished from the first example in that the method of the present invention is not applied to a relatively diluted boric acid solution but to a concentrated boric acid solution. The water vapor required to absorb and discharge the evaporated boric acid is no longer obtained by evaporation in the reactor, so that substantially all of the required water vapor is added to the reactor 10, in which case the reactor is a contactor, preferably Is a countercurrent contact column.

Inside the reactor 10, the same temperature, pressure and pH conditions as in the first embodiment are applied.

The concentrate is introduced from the top of the reactor 10 and the hot and high pressure water vapor introduced into the bottom of the reactor 10 is allowed to flow in the reverse direction.

This concentrate, which contains little boron and can volatilize an amount, can be removed continuously or discontinuously from the reactor 10. Water vapor filled with evaporated boric acid is discharged from the top of the reactor (10) via the demister (11) to the scrubbing column (12), in which the boric acid is reacted with water flowing in reverse direction at a low flow rate. This is washed from the steam. This flow rate is determined by the predetermined concentration of recovered and purified boric acid.

Residual water vapor that does not contain boric acid is fed to a heat exchanger (13) in which lost heat is compensated for, and finally pumped to high temperature and high pressure water vapor using a pump (14) and introduced into the reactor (10) again. The concentrate is then heat evaporated to absorb boric acid from the concentrate.

According to this embodiment, the existing concentrate obtained by waste water evaporation in a nuclear power plant can, on the one hand, be separated into an evaporated concentrate with very little or no boron and, on the other hand, a concentrated solution of boric acid. No special evaporator needs to be installed. Only the apparatus as shown in FIG. 2 arranged in connection with the existing evaporator is needed. As a concentrate and, thus, a very small flow rate, are used, the result can be very small. Water vapor is hardly concentrated or released, minimizing energy consumption.

In today's use, the base may be added to the vapor so that the pH of the concentrate can be at least 8, in which case until the pH value is at most 8, preferably at most 7, before introducing the concentrate into the reactor 10. Acids such as sulfuric acid must be added. At this time, a significant amount of salt is formed and remains in the concentrate in reactor 10.

The method described above significantly reduces the amount of radioactive waste. Boric acid can also be recovered and reused.

The present invention is not limited at all by the above-described embodiment. Conversely, various modifications may be made to these embodiments as long as they are within the scope of the present invention.

1 is a block diagram showing an apparatus for implementing the method of removing boric acid according to the present invention;

2 is a block diagram according to another embodiment of the present invention, similar to FIG.

<Explanation of symbols for the main parts of the drawings>

1: reactor

4: storage tank

5: heat exchanger

7: distillation column

8: condenser

10: reactor

12: washing column

Claims (11)

In a method for separating boric acid from a boric acid-containing liquid such as a waste liquid of a nuclear power plant, the boric acid-containing liquid is continuously supplied to a reactor (1 or 10) in a non-alkaline atmosphere, and the boric acid is evaporated using water vapor, and the water vapor thereof. The boric acid separation method of removing boric acid by discharging this steam from a reactor (1 or 10), when the concentration of boric acid in a genus becomes high. The method of separating boric acid according to claim 1, wherein the liquid is continuously introduced and water vapor having a high boric acid concentration is continuously removed so that the contents of the reactor remain substantially in a certain amount. The process for separating boric acid according to claim 1 or 2, characterized in that the concentrate remaining in the reactor (1 or 10) is removed discontinuously. The liquid according to claim 1 or 2, wherein a liquid containing boric acid is added as a solution, and the reactor 1 is operated as an evaporator, so that most of the water vapor is formed in the reactor 1 by heat introduction. Boric acid separation method. The method of separating boric acid according to claim 4, wherein heat is supplied to the reactor (1) by introducing water vapor into the reactor (1). The process according to claim 1 or 2, wherein the concentrate is introduced into the reactor (10) as a liquid containing boron, and most of the steam for evaporating the boric acid is obtained from outside of the reactor (10), which is the reactor (10). And continuously introduced into the reactor (10) from the outside, and contacting the concentrate. 3. The method of claim 1, wherein the temperature and pressure of the reactor (1 or 10) are maintained at 100 ° C. or higher and at least atmospheric pressure, respectively. 4. 8. The process of claim 7 wherein the temperature and pressure of the reactor (1 or 10) are maintained at pressures between 150 and 180 ° C and between 5 and 10 bar, respectively. 3. The method of claim 1, wherein the boric acid is recovered from a vapor containing boric acid exiting the reactor (1 or 10). 4. 10. The method of claim 9, wherein the boric acid is recovered by distillation and fractionation column or washing column. 10. The method of claim 9, wherein the water vapor flows in a closed loop through the reactor (10) and the boric acid removal device (12) without being concentrated.