JPS6347516B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for evaporating various waste liquids using a centrifugal thin film evaporator. It is suitable for highly concentrated or dry powdering.

Radioactive waste fluid generated at nuclear power plants includes:
There is a solution (hereinafter referred to as concentrated waste liquid) that is obtained by evaporating and concentrating waste liquid that contains sodium sulfate as the main component and impurities such as seawater components and corrosion products, and a liquid that is in a suspended state is powder. There are various types of slurries (hereinafter referred to as suspensions) such as granular resin slurry, granular resin slurry, and filter aid slurry containing cellulose as a main component. Conventionally, among these radioactive waste liquids, concentrated waste liquid has a solid content of approximately 20wt.
% concentrated waste liquid is mixed with cement in drums to solidify it, and the suspension liquid is stored safely within the power plant in stainless steel tanks, but the cumulative storage amount of these waste liquids is increasing. has become a problem. In recent years, a method has been proposed to reduce the volume of these waste liquids to the maximum extent possible by using a centrifugal thin-film evaporator to dry and powder these waste liquids, and treat and dispose of them as a safe solidified substance that does not pollute the environment. ing.

An example of the structure of a centrifugal thin film evaporator is shown in FIGS. 1 and 2. A heating jacket cylinder 2 is disposed outside the heat transfer cylinder 1.
is provided, and a heat medium inlet 3 and a heat medium outlet 4 are attached. A rotating shaft 5 is mounted inside the heat transfer shell 1 and supported by an upper bearing box 6 and a lower bearing box 7, and a liquid distributor 8 and a rotary blade 9 are provided.
It rotates via a drive wheel 10. A liquid supply inlet 11 and an evaporated material outlet 12 are provided in the upper part of the heat transfer cylinder 1, and a powder outlet 13 is provided in the lower part.

The waste liquid is supplied into the heat transfer shell 1 from the liquid supply inlet 11, is uniformly distributed in the circumferential direction by the liquid distributor 8, and flows down the inner surface of the heat transfer shell 1 by gravity to the rotating shaft 5 and the rotor blades 9. It is pushed and spread on the inner surface of the heat transfer cylinder 1 by the action of centrifugal force due to rotation. In this way, a thin film of waste liquid is formed on the inner surface of the heat transfer cylinder 1. The waste liquid formed into a thin film is heated by a heat medium flowing through a heating jacket cylinder 2 provided outside the heat transfer cylinder 1, and flows down by gravity while being evaporated and concentrated. Due to this action, the waste liquid is finally turned into dry powder and is discharged from the powder outlet 13.
When the purpose of the evaporation treatment of waste liquid is to highly concentrate it, the powder outlet 13 is converted into a concentrated slurry by adjusting the flow rate of the heating medium, the amount of liquid supplied, etc.
(In this case, the name is slurry outlet 13). On the other hand, the evaporated matter evaporated inside the heat transfer cylinder 1 is discharged from the evaporated matter outlet 12 and treated by an evaporated matter processing device (not shown).

In the concentrated waste liquid containing sodium sulfate as a main component, dissolved components are precipitated by evaporation in the centrifugal thin film evaporator, resulting in a slurry state. As evaporation and concentration proceed further, the slurry dries and becomes powder.
In this slurry region, deposits are observed to adhere to the inner surface of the heat transfer cylinder 1. Centrifugal thin film evaporator
The rotary blades 9 are constantly sliding on the heat transfer surface, and this scraping action prevents deposits from growing as scale. This effect of preventing scale growth is mainly achieved by the scraping force and sliding speed of the rotor blade 9, and if both of these are below the limit values, scale growth will begin within a short period of time and thermal conductivity will deteriorate. , the evaporation capacity decreases. Further, even when the system is designed under appropriate conditions to prevent scale growth, the evaporation capacity gradually decreases due to the accumulation of a small amount of scale during long-term operation. Therefore, in order to maintain good evaporation capacity, it is necessary to periodically clean the inner surface of the heat transfer cylinder 1. A conventional method for cleaning the inner surface of the heat transfer cylinder 1 is to supply a cleaning fluid (for example, water) into the heat transfer cylinder 1 from the liquid supply inlet 11,
A method is used to wash away the scale that has grown on the inner surface. However, in the conventional cleaning method, it is essential to stop the evaporation processing function of the centrifugal thin film evaporator during the cleaning operation, which reduces waste liquid treatment efficiency. Further, there were major drawbacks such as the need for a separate cleaning fluid and the generation of cleaning waste liquid after cleaning. It should be noted that increasing the scraping force and sliding speed of the rotor blades 9 more than necessary in order to prevent scale adhesion and growth is not a very desirable measure because it increases the power and increases the size of the mechanical equipment.

On the other hand, in the case of suspensions such as the resin slurry and filter aid slurry, the suspended matter does not precipitate due to evaporation in the centrifugal thin film evaporator, but only the water content evaporates and becomes dry. Since the particles do not firmly adhere to each other and do not adhere to the inner surface of the heat transfer cylinder 1, there is no decrease in thermal conductivity and no decrease in evaporation capacity.

The present invention prevents scale adhesion and growth on the heat transfer surface seen in the solution, eliminates periodic cleaning, enables long-term continuous operation, and improves the operating rate of the centrifugal thin film evaporator, that is, the waste liquid treatment efficiency. The purpose is to

In the present invention, when a solution that causes scale adhesion and a suspension that does not adhere to scale are evaporated using the same evaporator, the processing characteristics of each are determined through operation, and the results show that the problem of scale adhesion in the solution When the suspension is subsequently evaporated in the evaporator, the scale adhering to the heat transfer cylinder during the evaporation process of the suspension will be redissolved in the suspension, and the scale will be removed. It is based on what has been experimentally confirmed. Based on this result, the solution and suspension were evaporated alternately.
It is characterized by preventing scale adhesion.

A system diagram of an evaporation treatment apparatus as an embodiment of the present invention is shown in FIG. 3, and the treatment of waste liquid in a boiling water nuclear power plant will be explained as an example.

A condensate demineralizer (not shown) is installed to purify the primary cooling water of boiling water nuclear power plants. Due to the decline in processing capacity, the powdered resin slurry is periodically stored in waste resin tanks. Furthermore, when the capacity of granular ion exchange resin decreases, it is regenerated with caustic soda and sulfuric acid and used repeatedly, but when regeneration is no longer effective, it is stored in a waste resin tank as granular resin slurry. On the other hand, the caustic soda and sulfuric acid used for regeneration are mixed to form sodium sulfate, which is temporarily stored in a recycled waste liquid tank and then evaporated and concentrated in a concentrator using a shell-and-tube heat exchanger to form sodium sulfate. The concentration of the main solid content is approximately 20wt%
Reduce the volume until . In addition, floor drain waste liquid used within the power plant is similarly evaporated and concentrated, and these waste liquids are temporarily stored in a tank as concentrated waste liquid.

In FIG. 3, the concentrated waste liquid, which is a solution with a solid content concentration of about 20 wt%, is transferred from a temporary storage tank (not shown) to a solution supply tank 14. The resin slurry, which is a suspension, settles in the transport path,
The suspension is transferred to the suspension supply tank 15 with a solid content of about 10 wt% to prevent sedimentation. The solution supply tank 14 and the suspension supply tank 15 are constantly stirred and mixed by a circulation pump 16.
To remove concentrated waste liquid, open the switching valve A17 and use the liquid supply pump 18.
The liquid is supplied to the centrifugal thin film evaporator 21 at a predetermined supply rate through the liquid supply pipe 20 by the flow control valve 19. In the centrifugal thin film evaporator 21, the concentrated waste liquid is evaporated, dried and powdered by the above-mentioned action,
The volume is reduced to a fraction of that and transferred to the powder hopper 22, where it is safely solidified by a plastic solidification processing device. On the other hand, the evaporated material passes through the evaporated material pipe A23 and is made into clean evaporated material by the mist separator 24, and then passes through the evaporated material pipe B25 and is converted into condensed water by the condenser 26 and sent to other liquid processing systems through the condensed material outlet 27. be guided. Non-condensable gas in the evaporated material is led to the pent gas treatment system through the gas outlet 28. The mist separator 24 has a cleaning liquid pipe A29.
The condenser 26 is provided with a cleaning liquid pipe B30, which is an outlet for the liquid used to clean the evaporated matter, and a cooling water pipe 31
will be provided.

In the evaporation process of concentrated waste liquid with a dissolved concentration of solids of about 20 wt%, the area where the dissolved component, that is, sodium sulfate, precipitates and adheres to the heat transfer surface and grows as scale is necessary for evaporation, as shown in Figure 4. This is approximately the center 1/3 area H _B of the height H _E of the heat transfer cylinder 1 determined from the heat transfer area. In region H _A , the solution is concentrated by evaporation, but since the saturated solubility of solids has not reached approximately 32 wt%, there is no scale deposition due to precipitation of dissolved components. Area H _B
In this case, some of the precipitated dissolved components adhere to the heat transfer surface, albeit in very small amounts. This adhesion deteriorates heat conduction in the region H _B , and the region H _B gradually becomes longer due to long-term operation. In the H _C region, the concentrated waste liquid changes from semi-solid to dry powder, and almost no solid matter is observed. The area _HD is the margin of the heat transfer part, and is a completely dry powder area with no solid matter attached. Practically speaking, the height of the entire heat transfer cylinder is set to H _F , and the evaporation capacity is designed to be sufficient.

Precipitated scale causes a decrease in evaporation capacity, so it is desirable to suppress its growth or remove it as much as possible.However, since cleaning by shutting down the operation will cause a decrease in the operating rate of the equipment, it is necessary to remove the scale while continuing the evaporation process. A method of operation that removes it is preferred. Third
In the figure, if scale adhesion occurs after the concentrated waste liquid has been evaporated for a certain period of time, the switching valve A17 is closed and the switching valve B32 is opened instead, and the resin slurry is evaporated in the same way as the concentrated waste liquid. Process. In the evaporation process of resin slurry, all the water present at the height H _E of the heat transfer cylinder necessary for evaporation has the ability to dissolve the melt. Therefore, in region H _B , the scale of the precipitates in the concentrated waste liquid treated in the previous stage is gradually dissolved by the water in the resin slurry, and strictly speaking, the scale of the precipitates from H _B to H It is _evaporated and discharged as a dry powder. In this way, the scale in area H _B is completely dissolved and removed. After this, the heat transfer surface becomes a completely clean surface, and after a predetermined amount of resin slurry has been processed, the switching valve B3
2 is closed, and the switching valve A17 is opened instead to perform the evaporation process of the concentrated waste liquid.

In another embodiment of the present invention, a filter aid slurry is selected instead of the resin slurry and transferred to the suspension supply tank 15, in the same manner as described above.

In addition, as an example of a solution in which a soluble substance is dissolved, sodium sulfate is shown in this example, but in addition, sodium borate solution, Na, K, Ka,
A solution of a compound such as Mg that is powdery at room temperature will evaporate, similar to a solution of sodium sulfate.
Since it indicates a dry state, it can be treated in the same way. Also,
Suspensions containing insoluble substances include resin slurries, filter auxiliary slurries, and solid substances such as powders at room temperature that do not form into solutions. It can be treated in the same way as it indicates evaporation and dryness.

As described above, in the present invention, since the solution and the suspension can be evaporated alternately, it is possible to prevent the precipitation scale of solid content due to the evaporation of the solution, and it is possible to operate for a long period of time, improving the operating rate. Furthermore, since cleaning is almost unnecessary, the generation of cleaning waste water can also be minimized.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal sectional view of a centrifugal thin film evaporator, Fig. 2 is a cross sectional view thereof, Fig. 3 is a system diagram of an evaporation device showing a preferred embodiment of the present invention, and Fig. 4 is a centrifugal thin film evaporator. FIG. 3 is a vertical cross-sectional view showing a scale adhesion growth region near the heat transfer cylinder. DESCRIPTION OF SYMBOLS 1...Heat transfer cylinder, 2...Heating jacket cylinder, 9...Rotary blade, 12... Evaporated material outlet, 13...Powder outlet, 14...
Solution supply tank, 15...Suspension supply tank, 17
...Switching valve A, 32...Switching valve B.

Claims (1)

[Claims] 1. A solution in which a soluble substance that causes scale attachment through evaporation treatment is dissolved and a suspension in which an insoluble substance that does not cause scale attachment through evaporation treatment is suspended are prepared using a centrifugal thin film evaporator. A method for evaporating waste liquid, characterized in that, after the solution causing scale adhesion is evaporated, the suspension without scale adhesion is subsequently evaporated. 2 After evaporating the solution for a predetermined time, switching between the solution and suspension, evaporating the suspension for a predetermined time, and then switching between the suspension and solution and evaporating the solution for a predetermined time, which are repeated. A method for evaporating waste liquid according to claim 1, characterized in that: 3. Claim 1, wherein the solution is a concentrated waste liquid containing sodium sulfate as a main component, and the suspension is a desired one of a resin slurry or a filter aid slurry, or The method for evaporating waste liquid according to item 2.