JP7048950B2 - Method for treating polarizing plate manufacturing waste liquid - Google Patents

Description

The present invention relates to a method for treating a polarizing plate manufacturing waste liquid. More specifically, the present invention relates to a method for treating a polarizing plate production waste liquid that recovers potassium iodide from the waste liquid produced in the production of the polarizing plate.

In the production of the polarizing plate, iodine, potassium iodide (KI), and a boron-containing compound (for example, boric acid) are used in a large amount in dyeing, cross-linking, stretching steps, and the like. Therefore, the waste liquid produced in the production of the polarizing plate (hereinafter referred to as the waste liquid for producing the polarizing plate) contains a certain amount or more of such a compound. In recent years, from the viewpoint of environmental problems, sustainable development, etc., it has been studied to recover KI from the polarizing plate manufacturing waste liquid and reuse the recovered KI. However, such recovered KI has a problem that the boron-containing compound is not sufficiently reduced and the quality is insufficient for reuse.

Japanese Unexamined Patent Publication No. 2001-314864 Japanese Unexamined Patent Publication No. 2006-23325

The present invention has been made to solve the above-mentioned conventional problems, and a main object thereof is to provide a method for treating a polarizing plate manufacturing waste liquid capable of recovering potassium iodide having sufficient quality for reuse. It is in.

The method for treating the polarizing plate manufacturing waste liquid according to the embodiment of the present invention is a method for recovering potassium iodide from the polarizing plate manufacturing waste liquid. The method for treating the polarizing plate production waste liquid is a first concentration step of evaporating and concentrating the polarizing plate production waste liquid to produce a first precipitate containing mainly a boron-containing compound and polyvinyl alcohol; the first precipitate. The first solid-liquid separation step of solid-liquid separating the first precipitate from the waste liquid for producing a polarizing plate containing the above; the first solid-liquid separation step; the first filtrate is evaporated and concentrated to form iodide. A second concentration step, which produces a second precipitate predominantly containing potassium; a second solid-liquid separation of the second precipitate from the first filtrate containing the second precipitate. A second solid-liquid separation step of producing a filtrate; a filtrate discharge step of discharging at least a part of the second filtrate to the outside of the system; and recovering potassium iodide from the separated second precipitate. , With recovery steps;
In one embodiment, the method for treating the polarizing plate production waste liquid is an aqueous solution preparation step of dissolving the second precipitate in water to prepare an aqueous solution mainly containing potassium iodide; evaporating the aqueous solution. A third concentration step of concentrating to produce a third precipitate containing potassium iodide with a higher purity than the second precipitate; the third from the aqueous solution containing the third precipitate. A third solid-liquid separation step of solid-liquid separation of the precipitate of the above; the recovery step comprises recovering potassium iodide from the separated third precipitate.
In one embodiment, in the method for treating the polarizing plate manufacturing waste liquid, the first filtrate is replenished with the second filtrate not discharged from the system, and the replenished liquid is added to the second concentration step. Including offering to.
In one embodiment, the boron concentration in the recovered potassium iodide is 0.01% by weight or less.
In one embodiment, the method for treating the polarizing plate manufacturing waste liquid is to cool-crystallize the polarizing plate manufacturing waste liquid after evaporation concentration between the first concentration step and the first solid-liquid separation step. Further includes a cooling crystallization step.

According to the embodiment of the present invention, it is possible to realize a method for treating a polarizing plate manufacturing waste liquid capable of recovering potassium iodide having sufficient quality (purity) for reuse.

It is a block diagram explaining the apparatus which can be used for the treatment method of the polarizing plate manufacturing waste liquid by one Embodiment of this invention, and the procedure of the treatment method in combination. It is a figure which shows the mutual solubility of potassium iodide and boric acid.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments.

FIG. 1 is a block diagram illustrating a combination of an apparatus that can be used in a method for treating a polarizing plate manufacturing waste liquid according to one embodiment of the present invention and a procedure for the treatment method. As shown in FIG. 1, the treatment device for the polarizing plate manufacturing waste liquid includes a first concentrator, a first solid-liquid separation device, a second concentrator, a second solid-liquid separation device, and further, if necessary. Accordingly, an aqueous solution preparing device, a third concentrating device, and a third solid-liquid separating device are provided. The illustrated example is a form including a third solid-liquid separation device. Practically, the polarizing plate manufacturing waste liquid treatment device is provided with a raw water tank (waste liquid pit) upstream of the first concentrating device. If necessary, the polarizing plate manufacturing waste liquid treatment device may further include a cooling crystallization device (not shown) between the first concentrator and the first solid-liquid separation device. Hereinafter, a typical example of a method for treating the polarizing plate manufacturing waste liquid using such a polarizing plate manufacturing waste liquid treatment apparatus will be described.

First, the polarizing plate production waste liquid is introduced from the raw water tank into the first concentrating device and evaporated and concentrated (first concentrating step). The polarizing plate manufacturing waste liquid is a waste liquid generated in the manufacturing of the polarizing plate. In the production of the polarizing plate, a large amount of iodine, potassium iodide (KI), and a boron-containing compound (typically boric acid (H ₃ BO ₃ )) are used in dyeing, cross-linking, stretching steps, and the like. Further, a polyvinyl alcohol (PVA) -based resin film is used as the raw material of the polarizing plate. Therefore, the polarizing plate production waste liquid is typically iodine, KI, a boron-containing compound (typically boric acid (H ₃ BO ₃ )), PVA, and impurities generated or mixed in the production of the polarizing plate. including. KI and H ₃ BO ₃ are contained in the polarizing plate manufacturing waste liquid, typically in the form of ions. The pH of the polarizing plate manufacturing waste liquid is typically 3.5 to 8.0. Since the polarizing plate production waste liquid contains boric acid (H ₃ BO ₃ ), it is usually acidic, but it may be neutral or weakly alkaline (for example, pH 7.0 to 8.0). In order to suppress corrosion of the processing apparatus, a neutralizing agent such as potassium hydroxide may be added to the polarizing plate manufacturing waste liquid.

As the first concentrator, any suitable configuration can be adopted as long as the polarizing plate production waste liquid can be concentrated by evaporation. Specific examples include an evaporation concentrator such as a heat pump type, an ejector drive type, a steam type, and a flash type. The first concentrator may be used alone or in combination of two or more. When two or more concentrators are used in combination, all the concentrators may be of the same type or different types of concentrators may be combined. In one embodiment, a heat pump type concentrator is arranged in the front stage (upstream), and a flash type concentrator for further evaporating and concentrating the polarizing plate production waste liquid concentrated by this device is arranged in the rear stage (downstream). May be good. Evaporation concentration is typically performed by heating the polarizing plate production waste liquid to a predetermined temperature (for example, 70 ° C.) to evaporate the water content. The water vapor generated by the first concentrator is cooled by self-condensation by heating or heat exchange with cooling water to become condensed water (not shown). In order to suppress the oxidation of free iodine generated during the concentration of the polarizing plate production waste liquid, the first concentrator may be purged with nitrogen.

When the polarizing plate production waste liquid is concentrated in the first concentrating device, a boron-containing compound (typically boric acid (H ₃ BO ₃ )) contained in the waste liquid is precipitated. Since the boron-containing compound contained in the waste liquid is substantially boric acid, the mechanism will be described below with boric acid as a representative example. FIG. 2 is a diagram showing the mutual solubility of KI and H ₃ BO ₃ in% by weight with respect to the solution. When the concentrations of KI and H ₃ BO ₃ contained in the polarizing plate manufacturing waste liquid at the time of introduction into the first concentrator are represented by point A, when the polarizing plate manufacturing waste liquid is evaporated and concentrated at 70 ° C., KI and H ₃ The concentration of BO ₃ moves to point B. The KI and H ₃ BO ₃ are maintained in a state of being dissolved in the polarizing plate manufacturing waste liquid while moving from the point A to the point B. Then, when the polarizing plate production waste liquid is further evaporated and concentrated while maintaining the temperature at 70 ° C., precipitation of H ₃ BO ₃ is started, and the concentrations of KI and H ₃ BO ₃ are B points while forming crystals of H ₃ BO ₃ . Move from to point C. In the polarizing plate production waste liquid after concentration at point C, KI is unsaturated and has a high concentration (for example, 20% by weight or more, about 50% by weight in the illustrated example), while H ₃ BO ₃ has a low concentration (in the illustrated example). About 10% by weight). Further, since the polarizing plate manufacturing waste liquid contains PVA, by evaporating and concentrating the polarizing plate manufacturing waste liquid at a high temperature as described above, PVA is further polymerized, for example, to become colloidal. As a result, most of H ₃ BO ₃ and PVA contained in the polarizing plate manufacturing waste liquid become sludge. The result is a first precipitate predominantly containing a boron-containing compound (typically boric acid) and PVA. The first precipitate may contain impurities other than H ₃ BO ₃ and PVA.

Then, if necessary, the polarizing plate production waste liquid containing the first precipitate is collected in a cooling crystallization device (not shown) arranged between the first concentrator and the first solid-liquid separation device. Cool crystallization (cooling crystallization step). Any suitable configuration can be adopted as the cooling crystallization apparatus. Specific examples include a jacket type, vacuum type and other cooling crystallizers. For cooling crystallization, it is preferable to cool the polarizing plate production waste liquid to preferably 45 ° C. or lower, more preferably 40 ° C. or lower, still more preferably 30 ° C. or lower, and particularly preferably to around room temperature (for example, 25 ° C.). By performing the cooling crystallization, boric acid is further crystallized, so that the boric acid concentration in the polarizing plate manufacturing waste liquid can be further reduced. More specifically, in FIG. 2, the concentrations of KI and H ₃ BO ₃ that have moved to point C are further crystallized by boric acid by moving to point D by cooling crystallization that cools to 25 ° C. ( In the illustrated example, the boric acid concentration is 5% by weight or less at point D). If the concentration of H ₃ BO ₃ in the polarizing plate manufacturing waste liquid is sufficiently reduced in the first concentration step, the cooling crystallization step may be omitted.

Next, the polarizing plate manufacturing waste liquid containing the first precipitate is introduced into the first solid-liquid separation device, and the first precipitate is solid-liquid separated from the polarizing plate manufacturing waste liquid (first solid-liquid separation step). .. As a result, the first precipitate is removed from the polarizing plate production waste liquid, and the first filtrate is produced. By removing the first precipitate from the polarizing plate production waste liquid, H ₃ BO ₃ is removed, for example, about 60% to 90%, and PVA is removed, for example, about 60% to 90% in the first filtrate. ing. Any suitable configuration can be adopted as the first solid-liquid separation device. Specific examples include a filtration device (for example, a pressure filtration (filter press) device, a vacuum filtration device, a centrifugal filtration device), and a centrifuge device (for example, a decanter type centrifuge device).

The separated first precipitate is a crystal mainly composed of H ₃ BO ₃ , and contains PVA and some KI crystals. The separated first precipitate is typically discharged out of the system. The first precipitate (H ₃ BO ₃ main crystal) is, for example, manufactured by washing and recovering using condensed water generated by the first concentrator, for example, to manufacture semiconductors, LEDs, and the like. It can be reused in the process. Further, since the cleaning waste liquid after cleaning contains the KI crystals, the cleaning waste liquid may be introduced into a second concentrator described later together with the first filtrate.

Next, the first filtrate obtained above is introduced into a second concentrator and evaporated and concentrated (second concentration step). As described above, the concentration of H ₃ BO ₃ and PVA contained in the first filtrate is sufficiently reduced by the first concentration step, and KI is dissolved in the first filtrate at a high concentration. Further concentration in the concentrator results in supersaturation of KI. As a result, a second precipitate containing mainly KI can be produced. More specifically, the second precipitate is a KI-based crystal containing some H ₃ BO ₃ and PVA. If the concentration ratio is increased in order to increase the yield of KI, the amount of PVA contained in the second precipitate also increases, so that the second precipitate (KI crystal) can be washed to remove PVA. preferable. As the second concentrator, any suitable configuration may be adopted as in the first concentrator. In one embodiment, a flash type concentrator may be employed. The second concentration can be terminated, for example, when the specific gravity of the concentrated solution becomes about 1.1 to 1.4 times the specific gravity of the first filtrate before concentration.

Next, the first filtrate (slurry liquid) containing the second precipitate is introduced into the second solid-liquid separation device, and the second precipitate is solid-liquid separated from the first filtrate (second). Solid-liquid separation step). As a result, the second precipitate is separated from the first filtrate to produce a second filtrate. As the second solid-liquid separation device, any suitable configuration may be adopted as in the case of the first solid-liquid separation device.

The separated second filtrate typically contains KI, H ₃ BO ₃ , and PVA. The KI concentration in the second filtrate is about 30% by weight to 55% by weight, the H ₃ BO ₃ concentration is about 1% by weight to 5% by weight, and the PVA concentration is substantially all in the third concentration step described later. Is small enough to remove. In one embodiment, a portion of the second filtrate is discharged out of the system without returning to the second concentration step. In this case, the ratio of the second filtrate discharged to the outside of the system is preferably 1% by weight to 50% by weight, more preferably 5% by weight to 40% by weight, based on the entire second filtrate. , More preferably 10% by weight to 33% by weight. In this case, the second filtrate that is not discharged from the system is supplemented with the first filtrate. The liquid supplemented with the first filtrate (mixture of the second filtrate and the first filtrate) is subjected to the second concentration step and the subsequent solid-liquid separation step. The replenishment amount (weight) of the first filtrate is typically larger than the discharge amount (weight) of the second filtrate. More specifically, the replenishment amount of the first filtrate is "the discharge amount of the second filtrate + the water to be evaporated in order to make the specific gravity of the first filtrate the specific gravity of the second filtrate in the second concentration step. Can correspond to "quantity". By discharging a part of the second filtrate to the outside of the system, the boron concentration in the finally recovered KI can be significantly reduced. Conventionally, KI recovered from polarizing plate manufacturing waste liquid often does not have sufficient quality (purity) for reuse, but the present inventors have found that the cause is boron in KI (substantially). Was a boron-containing compound). Then, as a result of trial and error, without reusing a part of the second filtrate as a means for reducing the boron concentration (which can correspond to the content of the boron-containing compound) in the recovered KI (substantially, the second). It has been found that it is useful to discharge to the outside of the system (without returning to the concentration step). More specifically, since the second filtrate contains KI, those skilled in the art will reuse all of the second filtrate in order to increase the KI recovery rate. It was found that the decrease in KI purity due to H ₃ BO ₃ is more dominant than the improvement in KI recovery rate due to such reuse. That is, the embodiment of the present invention is based on a technical idea in the opposite direction to the common general technical knowledge of the industry obtained by trial and error, and therefore, the effect is an unexpectedly excellent effect. In another embodiment, all of the second filtrate may be drained out of the system. In this case as well, the same effect as described above can be obtained. The second filtrate discharged to the outside of the system may be discarded as it is, and is not the first concentration step (that is, the immediately preceding concentration step (second concentration step) but the previous concentration step). ), Or it may be returned to the stage prior to the first concentration step (for example, the raw water tank). This makes it possible to further suppress an increase in the boron concentration in the recovered KI. Preferably, the second filtrate discharged out of the system is returned to a stage (for example, a raw water tank) prior to the first concentration step.

In one embodiment, KI is recovered from the separated second precipitate (KI crystals). The second precipitate can preferably be washed with an aqueous KI solution. By washing with an aqueous solution of KI, the impairment of the obtained KI can be reduced as compared with the case of washing with water. This is because the dissolution of the obtained KI in the cleaning liquid can be suppressed. From the viewpoint of suppressing the dissolution of the obtained KI, the concentration of the KI aqueous solution as a cleaning liquid is preferably a high concentration close to the saturation concentration. In addition, such cleaning can efficiently remove residual PVA. Most of the PVA contained in the polarizing plate manufacturing waste liquid is removed as the first precipitate, and the PVA removed as the first precipitate has a large molecular weight, so that the remaining PVA has a high molecular weight. It's a small one. Therefore, even if PVA remains in the KI crystal, it can be efficiently removed by washing.

As described above, KI can be recovered from the polarizing plate manufacturing waste liquid.

In one embodiment, if necessary, instead of recovering KI from the second precipitate, the second precipitate is introduced into an aqueous solution preparation device to prepare an aqueous solution mainly containing KI (aqueous solution preparation). Step), the aqueous solution may be subjected to a third concentration step and a third solid-liquid separation step described later. Any suitable configuration can be adopted as the aqueous solution preparation device. For example, the second precipitate may be dissolved in water in a tank. The aqueous solution can be prepared so that the concentration of the second precipitate is preferably 40% by weight to 60% by weight. If the concentration is within such a range, H ₃ BO ₃ , PVA and impurities can be efficiently removed in the third concentration step described later, and finally a very pure KI can be obtained. Can be done. If necessary, the obtained aqueous solution may be passed through a predetermined filter. By passing the aqueous solution through the filter, impurities and the like can be removed more efficiently.

Next, the aqueous solution obtained above is introduced into a third concentrator and evaporated and concentrated (third concentration step). As mentioned above, the concentration of H ₃ BO ₃ and PVA contained in the aqueous solution is further reduced as compared with the first filtrate, and the ratio of H ₃ BO ₃ , PVA and impurities to KI is significantly smaller than that of the first filtrate. Further concentration in the third concentrator can produce a third precipitate containing KI crystals with a higher purity than the second precipitate. As the third concentrator, any suitable configuration may be adopted as in the first and second concentrators. The third enrichment may be terminated when any suitable indicator reaches a predetermined value. Substantially, since the third concentration is continuously performed, when the index reaches a predetermined value, an aqueous solution (concentrated solution) containing the third precipitate is subjected to the third solid-liquid separation step. , The following aqueous solution may be supplied to the third concentrator. Specific examples of the index include the specific gravity of the concentrated solution and the ratio of the third precipitate to the aqueous solution in the concentrated solution. For example, when the specific gravity of the concentrated solution is used as an index, the third concentration can be completed when the specific gravity of the concentrated solution becomes about 1.1 to 1.4 times the specific gravity of the aqueous solution before concentration. If the specific gravity of the concentrate is too large, the removal of the boron-containing compound may be insufficient. If the specific gravity of the concentrate is too small, the purification and recovery efficiency of KI may be insufficient. All of the concentrated liquid may be subjected to the third solid-liquid separation step, or a part of the concentrated liquid may be left in the third concentrating device.

Next, the aqueous solution containing the third precipitate is introduced into the third solid-liquid separation device, and the third precipitate is solid-liquid separated from the aqueous solution (third solid-liquid separation step). As the third solid-liquid separation device, any suitable configuration may be adopted as in the case of the first and second solid-liquid separation devices. The third filtrate obtained by solid-liquid separation of the third precipitate from the aqueous solution may be returned to the third concentration step or may be discharged to the outside of the system as it is. In one embodiment, as in the case of the second filtrate, a part of the third filtrate can be discharged out of the system. As a result, the boron concentration in the finally recovered KI can be significantly reduced. The third filtrate discharged to the outside of the system may be discarded as it is, may be subjected to the first concentration step together with the polarizing plate production waste liquid, or may be subjected to the second concentration step together with the first filtrate. It may be returned to the stage prior to the first concentration step (for example, the raw water tank). This makes it possible to further suppress an increase in the boron concentration in the recovered KI. Preferably, the third filtrate discharged out of the system is returned to a stage (for example, a raw water tank) prior to the first concentration step.

KI is recovered from the separated third precipitate (KI crystal). The third precipitate can be washed with an aqueous KI solution as in the case of the second precipitate. The effect of washing with the KI aqueous solution is the same as that of the second precipitate.

By further performing the third concentration step and the third solid-liquid separation step as described above, KI with higher purity can be recovered.

If necessary, a fourth concentration step and a fourth solid-liquid separation step may be further performed. Specifically, a third precipitate is dissolved in water to prepare an aqueous solution, and the aqueous solution is subjected to a fourth concentration step and a fourth solid-liquid separation step to obtain a fourth precipitate (KI crystal). You may collect it. The fourth precipitate (KI crystals) may contain a higher purity KI than the third precipitate. The fourth concentration step and the fourth solid-liquid separation step are the same as the third concentration step and the third solid-liquid separation step, respectively. Further high-purity KI can be recovered by further performing the fourth concentration step and the fourth solid-liquid separation step.

The boron concentration in the recovered KI is preferably 0.01% by weight or less, more preferably 0.009% by weight or less, still more preferably 0.008% by weight or less, and particularly preferably 0. 0075 Weight% or less. The smaller the boron concentration is, the more preferable it is, and the lower limit thereof may be, for example, 0.001% by weight. According to the embodiment of the present invention, as described above, by discharging at least a part of the second filtrate to the outside of the system without reuse, the boron concentration is very low (that is, the purity is very high). ) KI can be recovered. Such high-purity KI has sufficient quality for reuse. In addition, in this specification, "boron concentration" means the amount (%) of boron with respect to the total weight of recovered KI. The amount of boron can be measured, for example, by inductively coupled plasma (ICP) analysis.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.

<Example 1>
The polarizing plate production waste liquid generated in the actual production of the polarizing plate is subjected to a first concentration step, a cooling crystallization step, a first solid-liquid separation step, a second concentration step, a second solid-liquid separation step, and an aqueous solution preparation. The process, the third concentration step, and the third solid-liquid separation step were performed in this order, and the KI in the polarizing plate production waste liquid was recovered.

In the first concentrating step, the heat pump type concentrating device was arranged in the front stage (upstream), and the flash type concentrating device was arranged in the rear stage (downstream). In the first concentration step, the polarizing plate production waste liquid was evaporated and concentrated while maintaining the temperature at 70 ° C. The cooling crystallization step was carried out by cooling the temperature from 70 ° C to 25 ° C. The first concentration step and the cooling crystallization step produced the first precipitate.
The first solid-liquid separation step was performed using a centrifuge. As a result, the first precipitate was removed from the polarizing plate production waste liquid to obtain a first filtrate. The first precipitate was discharged out of the system.
In the second concentration step, the first filtrate was evaporated and concentrated using a flash type concentrator while maintaining the temperature at 70 ° C. This produced a second precipitate.

The second solid-liquid separation step was performed using a centrifuge. As a result, the second precipitate was separated from the first filtrate to obtain a second filtrate. A portion of the second filtrate (in this example, about 33% by weight based on the total second filtrate) was discharged out of the system as it was (ie, without reuse). The second filtrate that was not discharged from the system was replenished with the first filtrate and returned to the second concentration step for reuse.
In the aqueous solution preparation step, the second precipitate was dissolved in water to prepare an aqueous solution. The concentration of the second precipitate in the aqueous solution was adjusted to about 50% by weight.
In the third concentration step, the aqueous solution was evaporated and concentrated while maintaining the temperature at 70 ° C. using a flash type concentrator. This produced a third precipitate.
The third solid-liquid separation step was performed using a centrifuge. As a result, the third precipitate was separated from the aqueous solution. The obtained third precipitate was washed with a KI saturated aqueous solution to obtain KI crystals. The boron concentration in the obtained KI crystals was 0.0072% by weight. About 10% by weight of the third filtrate obtained in the third solid-liquid separation step was discharged from the system, and the rest was returned to the third concentration step.

<Comparative Example 1>
KI crystals were obtained in the same manner as in Example 1 except that all of the second filtrate was returned to the second concentration step and all of the third filtrate was returned to the third concentration step for reuse. .. The concentration of the boron-containing compound in the obtained KI crystal was 0.2456% by weight.

Claims (8)

It is a method for treating a polarizing plate manufacturing waste liquid that recovers potassium iodide from the polarizing plate manufacturing waste liquid.
With the first concentration step of evaporating and concentrating the polarizing plate production waste liquid to produce a first precipitate mainly containing a boron-containing compound and polyvinyl alcohol;
With the first solid-liquid separation step of separating the first precipitate from the polarizing plate manufacturing waste liquid containing the first precipitate into a solid-liquid separation to produce a first filtrate;
With a second concentration step, the first filtrate is evaporated and concentrated to produce a second precipitate predominantly containing potassium iodide;
With the second solid-liquid separation step, in which the second precipitate is solid-liquid separated from the first filtrate containing the second precipitate to produce a second filtrate;
With the filtrate discharge step of discharging at least a part of the second filtrate to the outside of the treatment apparatus of the polarizing plate manufacturing waste liquid ;
A recovery step of recovering potassium iodide from the second precipitate by washing the separated second precipitate with an aqueous solution of potassium iodide and removing the residual polyvinyl alcohol;
Including
The second filtrate discharged to the outside of the polarizing plate manufacturing waste liquid treatment apparatus is not returned to the second concentration step and is not reused .
A method for treating waste liquid for manufacturing polarizing plates. The method for treating a polarizing plate manufacturing waste liquid according to claim 1, wherein in the filtrate discharging step, 10% by weight to 33% by weight of the second filtrate is discharged to the outside of the polarizing plate manufacturing waste liquid treating apparatus . An aqueous solution preparation step of dissolving the second precipitate in water to prepare an aqueous solution mainly containing potassium iodide;
With the third concentration step of evaporating and concentrating the aqueous solution to produce a third precipitate containing potassium iodide with a higher purity than the second precipitate;
With a third solid-liquid separation step of solid-liquid separating the third precipitate from the aqueous solution containing the third precipitate;
Including
The recovery step comprises recovering potassium iodide from the third precipitate by washing the separated third precipitate with an aqueous solution of potassium iodide and removing the residual polyvinyl alcohol.
The method for treating a polarizing plate manufacturing waste liquid according to claim 1 or 2. The method for treating a polarizing plate manufacturing waste liquid according to claim 3, wherein the aqueous solution is prepared so that the concentration of the second precipitate is 40% by weight to 60% by weight in the aqueous solution preparation step. The method for treating a polarizing plate manufacturing waste liquid according to claim 4, further comprising passing the aqueous solution through a filter before the third concentration step. Any of claims 1 to 5, wherein the first filtrate is replenished with the second filtrate not discharged to the outside of the treatment apparatus for the polarizing plate manufacturing waste liquid , and the replenished liquid is subjected to the second concentration step. The method for treating a polarizing plate manufacturing waste liquid according to the above. The method for treating a polarizing plate manufacturing waste liquid according to any one of claims 1 to 6, wherein the boron concentration in the recovered potassium iodide is 0.01% by weight or less. Any of claims 1 to 7, further comprising a cooling crystallization step of cooling crystallization of the polarizing plate manufacturing waste liquid after evaporation concentration between the first concentration step and the first solid-liquid separation step. The method for treating a polarizing plate manufacturing waste liquid according to the above.

Images