JP2000070603A - Preparation for crystal using crystallizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a crystal product which is not easily broken, because of its hardness, and is a stable and larger crystal product, by reducing heating up for a condensation product just before crystallization of a crystal when concentrating liquid and forming a crystal. SOLUTION: When just before crystallization, heating up is reduced. That is to say, by checking the time, when the crystallization will soon begin, through measuring the density of liquid and then the heating up is reduced when the density becomes not less than about 80%, preferably not less than about 90% in comparison with at the time of the crystallization. The controlling of the heating up is generally carry out by controlling supplying steam. The reducing of heating up is required so as to keep slow the rate of concentration of the liquid, therefore the fast concentration rate of liquid is desired, preferably at 0.02 g/cm3/hr or more, more preferably about 0.05 g/cm3/hr or more and the upper limit is desired within a practical range until reducing the heating up.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the production of crystals, including the step of growing crystals in a crystallizer.

[0002]

2. Description of the Related Art Conventionally, there has been known a method of producing a crystal by concentrating a solution in a crystallizer, depositing and growing a crystal. As the crystals obtained by such a method, typically, nitrites such as sodium nitrite and potassium nitrite, nitrates such as sodium nitrate, and sulfates such as ammonium sulfate are known.

In the crystallization step, a mother liquor in which a target substance is dissolved is introduced into a crystallizer and heated to concentrate the liquid to precipitate crystals. , External circulation heating type and internal heating type. The external circulation heating type is provided with an external circulation heating device. The external circulation heating type apparatus has an advantage that the temperature at the heat exchanger outlet can be easily increased by reducing the external circulation amount as compared with the internal heating type apparatus. This makes it possible to increase the dissolving ability of the crystals in the solution. On the other hand, in the case of the internal heating system, the circulation amount can be adjusted only by the rotation speed of the stirrer, so that the operation state of the crystallization can is greatly changed.

FIG. 1 schematically shows a crystallization operation using an external circulation heating type crystallization apparatus. A solution containing no crystals is supplied from a storage tank 1 to a storage tank 2 as a mother liquor. The supplied liquid is stored in the crystallization can 3 via the storage tank 2. After storing to a predetermined amount, the external circulation pump 5 is started, and circulation is started via the crystallization can heater 4 with the valves 12 and 13 fully opened.

[0005] In order to perform concentration, the valve 9 is opened, and the heating amount is adjusted while watching the flow meter 8 so as to attain a target flow rate. If crystals are found on the viewing glass at the bottom of the crystallization can 3, the heating amount is adjusted by the valve 9 so as to reduce the heating amount to about half while watching the flow meter 8. After that, the liquid hydrometer 14
The vapor amount is adjusted by the valve 9 so that the target value is obtained, and the liquid is concentrated.

[0006] When the liquid hydrometer reaches the target specific gravity, the pump 6 is started to extract the slurry liquid in the crystallization can 3 and starts to be extracted. Then, only the solution is finally separated by centrifugal force by the separator 7, returned to the storage tank 2 again, and sent to the crystallization can 3 again. The crystals are processed as products.

Here, as a method for enlarging the crystal grains in the crystallizer 3, the amount of crystals in the crystallizer can is increased by opening the valve 13 and increasing the classification flow rate, that is, the flow rate at the lower portion of the crystallizer can. In this case, the residence time of the crystal is increased to increase the crystal size. This is because, when the valve 13 is opened, the classification flow rate increases, and the crystal size of the precipitated crystals increases. As a result, the ratio of the crystals that do not settle increases, and the residence time of the crystals in the crystallizer increases, thereby increasing the size of the crystals.

[0008]

In the conventional manufacturing method described above, even if a crystal is grown large in a crystallizer, the crystal grows in a state where the crystals easily adhere to each other as a result of increasing the crystal concentration. The structure was found to be in a very brittle and fragile state with cracks inside. For this reason, when taking out from a crystallization can, there is a problem that it is destroyed by a centrifugal pump or a separator and cannot be finally taken out from the solution as large crystals.

In addition, a large number of secondary nuclei are generated due to the destruction of the crystal, and it is difficult to increase the size of a stable crystal in the crystallizer. In other words, even if crystals can be grown large in the crystallizer, the product to be taken out is small and easily broken, and it is difficult to obtain a large crystal product stably. Was.

The present invention described below provides a method for producing a stable large crystal by suppressing the generation of secondary nuclei by producing a crystal which is not easily broken even when the crystal is enlarged, in a crystallizer. The purpose is.

[0011]

Means for Solving the Problems The present inventors have made intensive studies in view of the above problems. As a result, the present inventors have found that a crystal that is large and hard to break can be obtained by performing a crystallization operation by a specific method described below, and arrived at the present invention. That is, the present invention is characterized in that (1) in the production of crystals including the step of concentrating and crystallizing a solution, when concentrating the solution before the crystals are formed, the amount of heating is reduced immediately before the crystals are precipitated. (2) In the production of crystals including the step of concentrating and crystallizing a solution, when concentrating a solution before crystals are formed, determining the amount of heat to be introduced by the specific gravity of the solution. (3) In the production of crystals including the step of concentrating and crystallizing the solution, when concentrating the solution before crystals are formed, the concentration rate is set to 0.02 g / cm ³ / (4) In the production of crystals including the step of concentrating and crystallizing the solution, before extracting the crystals generated in the crystallizer, the liquid must be extracted for 1 hour. Production of crystals characterized by being performed more than once Law, resides in.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The present invention relates to the production of crystals including the steps of concentrating and crystallizing a solution. The entire crystallization operation can be performed using the apparatus outlined in FIG. 1 as described in the prior art. Here, the present invention is characterized in that, when concentrating a solution before crystals are generated, the amount of heating is reduced immediately before the crystals are precipitated.

The term “immediately before” means a stage before the crystal is deposited, and when it is said in terms of specific gravity, it is 80% or more, preferably 90% or more of the specific gravity of the liquid at the time of crystal deposition. this is,
As the mother liquor in the state introduced at the start of the operation is heated and concentrated, the specific gravity of the liquid gradually increases and crystals are precipitated. The heating amount is reduced when the specific gravity of the liquid becomes 80% or more, preferably 90% or more of the specific gravity at the time of crystal precipitation.

Further, since it is only necessary to reduce the heating amount when the specific gravity of the liquid reaches the above-mentioned state, it is not always necessary to measure the specific gravity of the liquid every time. There is no problem even if it is confirmed that the amount of steam is reduced. Since the specific gravity of the liquid at the time of crystal precipitation can be obtained from its physical property value, the concentration at which crystals precipitate from the internal temperature of the crystallizer and the solubility of the substance in the liquid may be obtained, and the specific gravity may be converted from the relationship between the concentration and the temperature. .

[0015] The heating amount is generally controlled by controlling the amount of steam introduced into the heating device. In the concentration operation described in the section of the prior art, before the precipitation of crystals on the sight glass below the crystallizer can 3, the valve 9 is closed while the heating amount is reduced while looking at the flow meter 8 as much as possible. You may.

It is necessary that the amount of reduction in heating can achieve the effect of keeping the rate of concentration of the liquid at the time of crystal precipitation low. Here, it is desirable that the concentration rate of the liquid is efficiently increased by rapidly concentrating the solution until the time point when the heating is reduced, preferably 0.02 g / cm ³ / hr or more, more preferably 0.05 g / cm ³ / hr or more. / Cm ³ / hr or more, but there is no particular upper limit, and it may be selected within a practical range on the apparatus. On the other hand, after the time when the heating is reduced, the concentration rate of the liquid is 0.0%.
It is preferable to keep the content to 1 g / cm ³ / hr or less.

It is preferable to select the amount of reduction of the vapor so that the concentration rate of the liquid is in such a range before and after the point of the decrease in the heating, and the amount of reduction of the vapor for this purpose is preferably 1 m of the liquid. 0.025 to 0.1 t / hr, more preferably 0.05 to 0.075 t / hr per ³ is appropriate. If the amount of reduction is too large, the concentration time is significantly affected. on the other hand,
If the amount of reduction is not sufficient, the effect of improving the crystal hardness may not be sufficiently obtained.

By reducing the amount of heating immediately before crystal deposition, the amount of primary nuclei generated can be reduced.
Even if the heating amount is reduced at a stage earlier than the above timing, there is no problem in the properties of the obtained crystals, but it takes time to concentrate the liquid and it is not efficient. On the other hand, if the amount of heating is reduced after the generation of crystals, the object of the present invention of obtaining large and hard crystals cannot be achieved. For this reason, it is necessary to reduce the amount of heating immediately before crystal deposition.

However, there are cases where the generation of primary nuclei cannot be sufficiently reduced only by controlling the amount of heating before the above-described crystal deposition. In this case, after the primary nucleus is generated, the valve 15 installed at the lower part of the crystallizer is opened, and the crystal in the crystallizer is extracted to the outside together with the liquid.
There is no particular limitation as long as the crystal can be extracted.) afterwards,
Since the amount of crystals increases in the concentration stage, the same operation is repeated several times before the start of production in order to reduce the amount of crystals as much as possible.

At this time, it is desirable that the standard of the amount of the extracted crystal is 5 wt% or more, particularly 10 wt% or more of the amount of the crystal before the extraction. Since the mother liquor is replenished to the crystallizer by an amount reduced by the withdrawal, the concentration of the crystal is reduced.

The concentration rate for concentrating the liquid is desirably as slow as possible by checking the indicated value of the liquid hydrometer 14 in order to avoid a rapid increase in the amount of crystals. By slowing down the concentration rate in this way, it is possible to limit the growth of crystals in the form of agglomeration and agglomeration, and it is possible to make the structure hard to have cracks inside the crystal and to harden the crystal.

The concentration rate after the precipitation of the crystals is preferably 0.1.
03 g / cm ³ / hr or less, particularly preferably 0.02 g
/ Cm ³ / hr. If it is faster than this, the hardness of the obtained crystal may not be sufficient.

Apart from this, in order to increase the size of the crystal, the operation of increasing the classification flow rate by opening the valve 13 as described above to increase the residence time of the crystal in the crystallizer is performed as described above. It is as follows. However, as described above, on the other hand, an increase in the classification flow rate leads to generation of a large amount of secondary nuclei due to friction between crystals and between the crystal and the apparatus.

For example, the presence of a large amount of secondary nuclei generated due to such causes hinders the growth of the crystal, so that it is necessary to remove these secondary nuclei.

As a method for removing secondary nuclei, it is possible to increase the temperature difference between the inlet thermometer 10 and the outlet thermometer 11 of the heater 4. Specifically, it is suitable to provide a difference of 2 ° C. or more, more preferably 5 ° C. or more, between the inlet temperature and the outlet temperature. By doing so, the dissolving ability of the crystal is increased, and the secondary nucleus generated can be stably dissolved even if the crystal is enlarged, so that a large and hard crystal can be obtained stably.

Specifically, the outlet valve 12 of the crystallization can heater 4 is closed to increase the outlet temperature 11.
At this time, it is necessary to adjust the opening of the valve 12 so that the opening of the valve 9 does not become close to the full opening. If the opening is fully opened, the indicated value of the flow meter 8 will soon decrease, and the heating amount required for production cannot be secured.

If a sufficient temperature difference cannot be obtained by performing this operation, there is a possibility that crystals larger than crystal nuclei that do not need to be dissolved may be contained in the liquid. In order to improve the ability to separate crystals in the
Try reducing the amount of crystals in the crystallization can to some extent.

In order to reduce the amount of circulating liquid in the heater, the amount of steam necessary for evaporating water from the crystallizer can be secured.
Adjust while watching the steam flow meter of the heater. If sufficient dissolution capacity cannot be secured by this operation, reduce the amount of crystals brought into the heat exchanger by lowering the crystal suspension density of the crystallizer, and improve the heat exchange capacity to further increase the circulating liquid volume. What is necessary is just to decrease and to increase the temperature difference of the liquid side inlet-outlet of a crystallization can heater.

This makes it possible to stably dissolve the increased secondary nuclei even in a state where the crystal is enlarged, and to produce a large and hard crystal.

Crystals can be grown sufficiently by keeping the amount of crystals in the crystallizer high and lengthening the crystal residence time in the crystallizer. The generation of secondary nuclei increases by increasing the amount of crystals in the crystallizer and increasing the size of the crystals. This leads to an increase in the variation of the crystal grain size in the crystallizer. If the secondary nucleus is dissolved by the above method, such a change in the crystal grain size can be reduced.

As is apparent from the following examples, the effect of obtaining large and hard crystals according to the present invention is remarkably found in sodium nitrite. However, it can be considered that the present invention can prevent the formation of a weak structure of crystals by keeping the amount of crystals small in the crystallization operation and by agglomeration and growth of crystals, thereby leading to formation of large and hard crystals. Therefore, it is considered that the present invention can be obtained by a crystallization operation and can be applied to all substances in which large and hard crystals are desired. By producing a hard crystal characterized by limiting the amount of crystal nuclei present in the crystallizer by performing the above operations, the crystal structure in which the crystal nuclei adhere to each other and the crystal structure is prevented from being brittle. Can be.

[0032]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1 A crystallization operation was performed by the method of the present invention using a DP type crystallizer schematically shown in FIG. 1 and using a 40 wt% aqueous solution of sodium nitrite as a mother liquor. The specific gravity of the mother liquor initially introduced into the crystallizer is 1.3. Steam was introduced into the crystallization can heater 4 as an external circulation heating device at a rate of 0.15 t / hr per 1 m ³ of the aqueous solution to start heating. The temperature difference between the inlet and outlet of the crystallization heater was 10 ° C.
It was 20 g / cm ³ / hr.

The state of the liquid in the crystallization can was determined by measuring the specific gravity of the liquid. When the specific gravity reached 1.37, the amount of steam introduced into the crystallization heater was reduced to 0.05 t / hr per 1 m ³ of the aqueous solution. The theoretical specific gravity at the time of crystal precipitation is 1.42. In addition, in the concentration stage until the pump 15 is started and the removal of the crystal as a product is started, in order to adjust the amount of crystals, one liquid is poured from the crystallization can.
It was extracted three times at 0% each.

After crystal deposition, crystals were collected at the inlet and outlet of the pump 6 and dried. The flow rate distribution of the obtained crystal was measured and is shown in FIG. Crystals were also collected from the crystallization can and the outlet of the separator 7 and dried. The correlation between the particle sizes of the obtained crystals was measured and is shown in FIG.

Comparative Example 1 The same operation as in Example 1 was performed except that the operation of reducing the amount of steam and extracting the crystal was not performed. As in Example 1, crystals were collected at the inlet and outlet of the pump 6, and at the crystallization can and at the outlet of the separator 7, and dried. The correlation between the particle size distribution of the obtained crystals and the particle size is shown in FIGS. 2 and 3, respectively.

Referring to FIG. 2, which compares the crystal grain sizes at the inlet and the outlet of the pump 6 of Example 1 and Comparative Example 1, the ratio of the large-diameter crystals is reduced at 600 μm in the method of Comparative Example 1. The proportion of small grain crystals increases proportionately. This clearly shows that the crystal was severely broken by the pump.
On the other hand, in the method of Example 1, although the ratio of crystals having a large grain size decreases similarly at 700 μm, it can be seen that the ratio of crystals having a diameter of 400 μm or less hardly changes.

This indicates that the strength of the crystal was increased by the method of the present invention.

Further, comparing the crystallized can 3 with the operation method of Comparative Example 1 shown in FIG. 3 and the crystal average particle size at the outlet of the centrifugal separator 7, even if the crystal in the crystallized can was increased to 700 μm. It can be seen that the crystal particle diameter at the outlet of the separator does not become too large. On the other hand, when the average crystal grain size at the outlet of the crystallization can 3 and the outlet of the centrifugal separator 7 in the operation method of the embodiment 1 shown in FIG. Can be confirmed.

This also indicates that the method of the present invention hardened the crystal and made it harder to break.

[0041]

According to the present invention, a large crystal that is hard and not easily broken can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing an example of a crystallization apparatus that can be used in the present invention.

FIG. 2 is a diagram showing the particle size distribution of the sodium nitrite crystals obtained in Example 1 and Comparative Example 1 at the inlet and the outlet of the pump.

FIG. 3 is a view showing the particle size of the sodium nitrite crystal obtained in Comparative Example 1.

FIG. 4 is a graph showing the particle size of the sodium nitrite crystal obtained in Example 1.

[Explanation of symbols]

 1: Storage tank 1: Storage tank 2: Crystallization can 4: Crystallization can heater 5, External circulation pump 6: Slurry extraction pump 7: Centrifuge 8: Heated steam flow meter 9: Heated steam control valve 10: Heater inlet thermometer 11: Heater outlet thermometer 12: Heater outlet flow control valve 13: Classification flow control valve 14: Crystallizer can hydrometer 15: Crystallizer can crystal extraction valve

Claims (5)

[Claims] In a method for producing crystals comprising concentrating and crystallizing a solution, when concentrating a solution before crystals are formed,
A method for producing a crystal, comprising reducing a heating amount immediately before a crystal is precipitated. 2. A method for producing crystals, comprising concentrating and crystallizing a solution, comprising:
A method for producing a crystal, characterized in that the amount of heat introduced is determined by the specific gravity of the solution. 3. A method for producing crystals, comprising concentrating and crystallizing a solution, comprising: concentrating a solution before crystals are formed;
A method for producing crystals, wherein the concentration rate is 0.02 g / cm ³ / hr or less. 4. A method for producing crystals comprising concentrating and crystallizing a solution, wherein the liquid is withdrawn at least once before taking out the crystals formed in the crystallizer. . 5. A method for producing a sodium nitrite crystal comprising concentrating and crystallizing a solution, wherein the amount of heating is reduced immediately before the crystal is precipitated when the solution before the crystal is formed is concentrated. Method for producing crystals.