A method for preparing iodine by using iodine-containing wastewater

Abstract

The application provides a method for preparing iodine element by using iodine-containing wastewater, relates to the technical field of LCD liquid crystal screen production wastewater treatment, and adopts treatment such as pH adjustment, reduction, concentration and oxidation, so that the iodine element in the iodine-containing wastewater is recycled and utilized continuously, the iodine resource is recycled and utilized to the maximum extent, the prepared iodine element has a purity of greater than or equal to 98 wt%, and a water content of less than or equal to 1.5 wt%, is high in purity, is better in economic benefit, and is free of by-products in the recycling and disposal process, the generated wastewater is treated and reused, there is no direct discharge of wastewater, zero discharge of wastewater is achieved, and resource recycling maximization is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of treatment of waste liquid in LCD liquid crystal screen production, and particularly relates to a method for preparing iodine element by using iodine-containing waste water. BACKGROUND

[0002] Liquid crystal display is one of the most widely used flat panel displays, which is composed of a liquid crystal unit and two polarizing plates arranged on both sides of the liquid crystal unit. The manufacturing process of the existing polarizing plate uses iodine, potassium iodide, boric acid and potassium thiosulfate, thus a large amount of iodine-containing waste water is generated.

[0003] The common treatment method of iodine-containing waste water is to use the difference in solubility to obtain iodine-containing crude boric acid solid through pretreatment, distillation and separation. However, the iodine-containing crude boric acid has low purity at this time, and generally has no economic value, so it is necessary to purify and recover industrial products as much as possible when treating iodine-containing waste water. Acidification is the most common method for purifying boric acid at present. Since boric acid is a weak acid, boric acid salt can be converted into boric acid by using strong acid. Although the operation is simple, the salt-containing filtrate obtained by this method has a large yield and is difficult to treat. The salt-containing filtrate is mainly composed of boric acid and inorganic salt, which is a high-salt-content acidic waste liquid. Generally, mixed solids are obtained by distillation, and are directly landfilled or subjected to extraction separation. The latter process requires strict requirements. At present, the treatment of iodine-containing waste water mainly aims at the recovery of boric acid, and the recovery of iodine is not clear.

[0004] CN109970073A discloses a method for preparing boric acid and potassium chloride from polarizing plate waste liquid, which comprises the following steps: beating, the polarizing plate waste liquid is concentrated by distillation to obtain iodine-containing crude boric acid, which is washed by beating with potassium sulfite solution, and solid-liquid separation is performed to obtain boric acid after beating; primary acidification, the boric acid after beating is adjusted to a pH value of 3, and cooling crystallization is performed to obtain finished boric acid and salt-containing boric acid solution; calcification, the salt-containing boric acid solution is heated and then calcium hydroxide is added to react to obtain calcium mud and salt-containing filtrate; secondary acidification, hydrochloric acid is added to the calcium mud to perform acidification until the pH value is 3, to obtain finished boric acid and calcium chloride solution; concentration, the salt-containing filtrate is distilled to obtain potassium chloride crude product, which is redissolved and impurities are removed, and then distilled to obtain high-purity potassium chloride product. The method uses calcium hydroxide to react with boron to generate calcium borate, and treats the salt-containing boric acid solution after boric acid extraction, but the method needs to be heated, and the consumption of calcium hydroxide is large. Moreover, the recovery of iodine is not clear.

[0005] CN109553116A discloses a method for purifying boronic acid-containing potassium iodide waste liquid, which comprises: removing impurities in the boronic acid-containing potassium iodide waste liquid with activated carbon, then concentrating the solution after removing impurities to a potassium iodide concentration of 25-35wt%, reacting boric acid in the concentrated solution with calcium hydroxide to generate calcium borate, and removing calcium ions in the remaining solution after solid-liquid separation by using chelating ion exchange resin. The focus of this method is still on the treatment of boric acid in the waste liquid and the conversion of boric acid into calcium borate, and the boric acid product is not effectively recovered, and the final product is a potassium iodide solution, and the iodine product, especially elemental iodine, is not recovered.

[0006] CN111498861A discloses a method for treating waste liquid of a polarizing plate, which comprises: pretreating and once acidifying the waste liquid of the polarizing plate to obtain boric acid and a salt-containing boric acid solution; and preparing potassium fluoroborate by twice acidifying the salt-containing boric acid solution with a solution containing hydrofluoric acid. This method focuses on the recovery of boron element and obtains potassium fluoroborate product by using hydrofluoric acid, but does not clearly show how to recover iodine in the solution, and the waste liquid treatment is not sufficient, and the enrichment of iodine can easily cause subsequent pollution problems.

[0007] In recent years, considering energy and environmental aspects, researchers have begun to recover potassium iodide from boron-containing iodine wastewater and further utilize the recovered potassium iodide. However, due to the low purity of the recovered potassium iodide or the introduction of new impurities in the recovery process, the recovered potassium iodide cannot be fully utilized, the recovery rate of iodine is less than 95%, and a large amount of distillate water generated in the recovery of potassium iodide is directly disposed at the water station, increasing the pressure of the wastewater treatment station to dispose wastewater, and the part of water resources cannot be recycled.

[0008] In view of the problems in the prior art, the present application provides a method for preparing elemental iodine from iodine-containing wastewater. SUMMARY

[0009] In view of the problems in the prior art, the present application provides a method for preparing elemental iodine from iodine-containing wastewater.

[0010] To achieve this purpose, the present application adopts the following technical solutions:

[0011] The present application aims to provide a method for preparing elemental iodine from iodine-containing wastewater, which comprises the following steps:

[0012] (1) adding alkali to the iodine-containing wastewater to adjust the pH value to 7.5-8.5 to obtain an alkaline adjustment liquid;

[0013] (2) adding the alkaline adjustment liquid obtained in step (1) to a reducing agent to obtain a reduction mixture;

[0014] (3) sequentially subjecting the reduction mixture obtained in step (2) to concentration and first solid-liquid separation to obtain distillate water, an alkali metal iodide mother liquor and a boric acid product;

[0015] (4) adding acid to the alkali metal iodide mother liquor obtained in step (3) to adjust the pH value to ≤1 to obtain an acid adjustment liquid;

[0016] (5) adding an oxidizing agent to the acid adjustment liquid obtained in step (4) while maintaining the pH value of the system at ≤1, and subjecting the system to second solid-liquid separation to obtain a filtrate and an iodine product; the filtrate is returned to step (1) for further treatment.

[0017] In the present application, according to the source of the iodine-containing wastewater, which generally contains iodine, alkali metal iodide and alkali metal boric acid salt, alkali is first added to adjust the pH value to 7.5-8.5 to obtain a weakly alkaline adjustment liquid, which neither causes the boric acid to become a boric acid salt nor provides an alkaline environment for the subsequent addition of a reducing agent for reduction reaction, so that the iodine in the iodine-containing wastewater is completely converted into alkali metal iodide, then concentrated to precipitate boric acid with low solubility, and the boric acid product is obtained through first solid-liquid separation, and finally the pH value is adjusted to ≤1 by adding acid, an oxidizing agent is added while maintaining the pH value of the system at ≤1, and the iodine product is obtained through second solid-liquid separation; the method described in the present application continuously recycles the iodine element in the iodine-containing wastewater by using a specific process, so that the iodine resource is maximally recovered and utilized, the iodine product prepared has a purity of ≥98wt%, a water content of ≤1.5wt%, a high purity and better economic benefits, and no by-products are produced during the recovery and disposal process, the generated wastewater is treated and reused, no wastewater is directly discharged, zero wastewater discharge is achieved, and resource recovery maximization is achieved.

[0018] It should be noted that, in the present application, alkali and acid are added in sequence to generate corresponding inorganic salts, such as potassium sulfate, and when the content of the corresponding inorganic salt in the filtrate reaches the saturation concentration, the corresponding inorganic salt product can be prepared after impurity removal.

[0019] As a preferred technical solution of the present application, the source of the iodine-containing wastewater in step (1) includes iodine-containing wastewater generated in the production process of polarizing plates.

[0020] Preferably, the composition of the iodine-containing wastewater in step (1) includes iodine, alkali metal iodide and alkali metal boric acid salt.

[0021] Preferably, the content of elemental iodine in the iodine-containing wastewater in step (1) is 0.001-0.2 wt%, such as 0.001 wt%, 0.005 wt%, 0.01 wt%, 0.03 wt%, 0.05 wt%, 0.07 wt%, 0.1 wt%, 0.13 wt%, 0.15 wt%, 0.18 wt% or 0.2 wt%, etc., the content of alkali metal iodide is 0.01-2 wt%, such as 0.01 wt%, 0.05 wt%, 0.1 wt%, 0.5 wt%, 1 wt%, 1.5 wt% or 2 wt%, etc., the content of alkali metal borate is 0.5-4 wt%, such as 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt% or 4 wt%, etc., but not limited to the listed values, and other values not listed in the above range are also applicable.

[0022] As a preferred technical solution of the present application, the alkali in step (1) is potassium hydroxide solid and / or potassium hydroxide solution.

[0023] Preferably, the effective purity of the potassium hydroxide solid is ≥99 wt%.

[0024] Preferably, the concentration of the potassium hydroxide solution is 32-45 wt%, such as 32 wt%, 33 wt%, 34 wt%, 35 wt%, 36 wt%, 37 wt%, 38 wt%, 39 wt%, 40 wt%, 41 wt%, 42 wt%, 43 wt%, 44 wt% or 45 wt%, etc., but not limited to the listed values, and other values not listed in the above range are also applicable.

[0025] It should be noted that in the present application, the alkali metal iodide in the iodine-containing wastewater is mainly potassium iodide, so potassium hydroxide solid and / or potassium hydroxide solution is preferably used in the alkali treatment, so that after the elemental iodine is reduced to iodide, the alkali metal iodide is all potassium iodide.

[0026] As a preferred technical solution of the present application, the reducing agent in step (2) includes potassium sulfite solid and / or potassium sulfite solution.

[0027] Preferably, the content of the potassium sulfite solid is ≥95 wt%.

[0028] Preferably, the concentration of the potassium sulfite solution is ≥30 wt%.

[0029] As a preferred technical solution of the present application, the amount of the reducing agent added in step (2) is 1.2-1.3 times the theoretical amount, for example, 1.2 times, 1.21 times, 1.23 times, 1.25 times, 1.27 times, 1.28 times or 1.3 times, etc., but not limited to the listed values, and other values not listed in the above range are also applicable.

[0030] As a preferred technical solution of the present application, the end point of the concentration in step (3) is that the content of alkali metal iodide is ≥ 35 wt%.

[0031] Preferably, the concentration in step (3) includes distillation concentration.

[0032] Preferably, the concentration ratio of the concentration in step (3) is (25-26):1, for example, 25:1, 25.3:1, 25.5:1, 25.7:1, 25.9:1 or 26:1, etc., but not limited to the listed values, and other values not listed in the above range are also applicable.

[0033] Preferably, the first solid-liquid separation in step (3) is centrifugal separation.

[0034] As a preferred technical solution of the present application, the distillate water in step (3) is subjected to RO membrane filtration, which can further remove possible residual suspended solids and macromolecular contaminants, and the obtained fresh water is used for cooling tower make-up water or recycling, and the obtained concentrated water is returned to step (1) for further treatment.

[0035] As a preferred technical solution of the present application, the pH value in step (4) is adjusted to 0.1-0.5, for example, 0.1, 0.2, 0.3, 0.4 or 0.5, etc., but not limited to the listed values, and other values not listed in the above range are also applicable.

[0036] It should be noted that in the present application, the pH value in step (4) is adjusted to ≤ 1, preferably to 0.1-0.5, because a strong acidic environment is more conducive to the oxidation of iodine ions to iodine element, but when the pH value is greater than 1, the rate of oxidation of iodine ions to iodine element is much lower than that when the pH value is ≤ 1; in addition, since boric acid has been separated out as much as possible to obtain boric acid product before acid addition, the boric acid in the system will not precipitate to affect the purity of iodine element.

[0037] Preferably, the acid in step (4) is sulfuric acid with a concentration of 40-50 wt%, for example, 40 wt%, 41 wt%, 42 wt%, 43 wt%, 44 wt%, 45 wt%, 46 wt%, 47 wt%, 48 wt%, 49 wt% or 50 wt%, etc., but not limited to the listed values, and other values not listed in the above range are also applicable.

[0038] Preferably, the acid is added at a rate of 180-220 L / h, such as 180 L / h, 190 L / h, 200 L / h, 210 L / h or 220 L / h, but not only the listed values, other values not listed in the above range are also applicable.

[0039] Preferably, the acid is added with constant stirring, and the stirring rate is controlled at 25-30 rpm, such as 25 rpm, 26 rpm, 27 rpm, 28 rpm, 29 rpm or 30 rpm, but not only the listed values, other values not listed in the above range are also applicable.

[0040] As a preferred technical solution of the present application, the oxidizing agent in step (5) is a hydrogen peroxide solution with a concentration of 30-35 wt%, such as 30 wt%, 31 wt%, 32 wt%, 33 wt%, 34 wt% or 35 wt%, but not only the listed values, other values not listed in the above range are also applicable.

[0041] Preferably, the amount of oxidizing agent added in step (5) is 1-1.5 times the theoretical amount, such as 1 times, 1.1 times, 1.2 times, 1.3 times, 1.4 times or 1.5 times, but not only the listed values, other values not listed in the above range are also applicable.

[0042] Preferably, the oxidizing agent is added at a rate of 130-170 L / h, such as 130 L / h, 135 L / h, 140 L / h, 145 L / h, 150 L / h, 155 L / h, 160 L / h, 165 L / h or 170 L / h, but not only the listed values, other values not listed in the above range are also applicable.

[0043] Preferably, the oxidizing agent is added with constant stirring, and the stirring rate is controlled at 35-40 rpm, such as 35 rpm, 36 rpm, 37 rpm, 38 rpm, 39 rpm or 40 rpm, but not only the listed values, other values not listed in the above range are also applicable.

[0044] As a preferred technical solution of the present application, the method comprises the following steps:

[0045] (1) adding alkali to adjust the pH value of the iodine-containing wastewater to 7.5-8.5, wherein the iodine-containing wastewater is obtained from the production of polarizing plates, and the iodine-containing wastewater contains 0.001-0.2 wt% of elemental iodine, 0.01-2 wt% of alkali metal iodide and 0.5-4 wt% of alkali metal borate, and the alkali is potassium hydroxide with an effective purity of 99 wt% or more and / or a potassium hydroxide solution with a concentration of 32-45 wt%, to obtain an alkaline adjustment solution;

[0046] (2) adding a reducing agent to the alkaline adjustment solution obtained in step (1), wherein the reducing agent is potassium sulfite with a purity of 95 wt% or more and / or a potassium sulfite solution with a concentration of 30 wt% or more, and the amount of the reducing agent is 1.2-1.3 times the theoretical amount, to obtain a reduction mixture;

[0047] (3) concentrating the reduction mixture obtained in step (2) by distillation, wherein the end point of the distillation concentration is that the alkali metal iodide content is 35 wt% or more, and the concentration ratio is (25-26):1, and then centrifuging to obtain distillate water, an alkali metal iodide mother liquor and a boric acid product; wherein the distillate water is filtered by an RO membrane, and the obtained fresh water is used for cooling tower water replenishment and reuse, and the concentrated water is returned to step (1) for further treatment;

[0048] (4) adding acid to adjust the pH value of the alkali metal iodide mother liquor obtained in step (3) to 1 or less, wherein the acid is sulfuric acid with a concentration of 40-50 wt%, the acid is added at a rate of 100-220 L / h, and the acid is added with constant stirring at a stirring rate of 25-30 rpm, to obtain an acid adjustment solution;

[0049] (5) adding an oxidizing agent to the acid adjustment solution obtained in step (4) while maintaining the pH value of the system at 1 or less, wherein the oxidizing agent is hydrogen peroxide solution with a concentration of 30-35 wt%, the amount of the oxidizing agent is 1-1.5 times the theoretical amount, the addition rate of the oxidizing agent is 130-170 L / h, and the oxidizing agent is added with constant stirring at a stirring rate of 35-40 rpm, to obtain a filtrate and an elemental iodine product by a second solid-liquid separation; and the filtrate is returned to step (1) for further treatment.

[0050] Compared with the prior art, the method has at least the following beneficial effects:

[0051] (1) The method adjusts the pH value, reduces, concentrates and oxidizes, and continuously recycles the iodine element in the iodine-containing wastewater, so that the iodine resource is maximally recovered and utilized.

[0052] (2) The purity of the iodine element prepared by the method is greater than or equal to 98 wt%, and the water content is less than or equal to 1.5 wt%, which is high in purity and better in economic benefit;

[0053] (3) The method generates no by-products in the recycling process, and the generated wastewater is treated and reused, without direct discharge of wastewater, achieving zero discharge of wastewater and maximizing resource recycling. BRIEF DESCRIPTION OF DRAWINGS

[0054] Figure 1 is a process flow diagram of a method for preparing iodine element using iodine-containing wastewater provided by one specific embodiment of the present application. DETAILED DESCRIPTION

[0055] The technical solutions of the present application will be further illustrated by specific embodiments in combination with the accompanying drawings.

[0056] To better illustrate the present application and facilitate understanding of the technical solutions of the present application, the following are typical but non-limiting embodiments of the present application:

[0057] The specific embodiment part of the present application provides a method for preparing iodine element using iodine-containing wastewater, and the process flow diagram of the method is shown in Figure 1 The method comprises the following steps:

[0058] (1) Adjust the pH value of the iodine-containing wastewater to 7.5-8.5 by adding alkali to obtain an alkaline adjusted solution;

[0059] (2) Add a reducing agent to the alkaline adjusted solution obtained in step (1) to obtain a reducing mixed solution;

[0060] (3) The reducing mixed solution obtained in step (2) is sequentially subjected to concentration and first solid-liquid separation to obtain distillate water, alkali metal iodide mother liquor and boric acid product; the distillate water is subjected to RO membrane filtration to obtain fresh water, which is subjected to cooling tower make-up water and reuse, and the concentrated water is returned to step (1) for further treatment;

[0061] (4) Add acid to the alkali metal iodide mother liquor obtained in step (3) to adjust the pH value to less than or equal to 1 to obtain an acid adjusted solution;

[0062] (5) Add an oxidizing agent to the acid adjusted solution obtained in step (4) and maintain the pH value of the system to be less than or equal to 1, and then perform second solid-liquid separation to obtain a filtrate and iodine element product; the filtrate is returned to step (1) for further treatment.

[0063] Example 1

[0064] The present embodiment provides a method for preparing iodine element using iodine-containing wastewater, which comprises the following steps:

[0065] (1) adding alkali to adjust the pH value of the iodine-containing wastewater to 8, the iodine-containing wastewater being from the polarizer production process; the iodine-containing wastewater contains 0.002 wt% of elemental iodine, 2 wt% of alkali metal iodide, and 2.5 wt% of alkali metal borate; the alkali is potassium hydroxide solution with a concentration of 32 wt%, to obtain an alkaline adjustment liquid;

[0066] (2) adding a reducing agent to the alkaline adjustment liquid obtained in step (1), the reducing agent being potassium sulfite solution with a concentration of 30 wt%, and the amount of the reducing agent being 1.2 times the theoretical amount, to obtain a reduction mixture;

[0067] (3) distillation concentration of the reduction mixture obtained in step (2) in an MVR concentration device, the distillation condition being a temperature of 95°C and a pressure of -0.06 MPa, and the end point of the distillation concentration being an alkali metal iodide content of 35 wt%, and the concentration ratio being about 25:1; then centrifugal separation of the concentrated liquid in an automatic centrifuge, to obtain fraction water, alkali metal iodide mother liquor, and boric acid product; wherein the fraction water is subjected to RO membrane filtration, the obtained fresh water is used for cooling tower make-up water and reuse, and the obtained concentrated water is returned to step (1) for further treatment;

[0068] (4) adding acid to adjust the pH value of the alkali metal iodide mother liquor obtained in step (3) to 1, the acid being sulfuric acid with a concentration of 50 wt%, the adding rate being controlled at 200 L / h, and the adding process being continuously stirred at a stirring rate of 30 rpm, to obtain an acid adjustment liquid;

[0069] (5) adding an oxidizing agent to the acid adjustment liquid obtained in step (4) and maintaining the pH value of the system at ≤1, the oxidizing agent being hydrogen peroxide solution with a concentration of 32 wt%, the amount of the oxidizing agent being 1.2 times the theoretical amount according to the molar ratio of iodine element to hydrogen peroxide of 1:1.2, the adding rate being controlled at 150 L / h, the adding process being continuously stirred at a stirring rate of 40 rpm, and black-brown solid being continuously precipitated with the addition of hydrogen peroxide solution; the pH value of the system is maintained at 1 by adding sulfuric acid with a concentration of 50 wt% during the reaction process, the filter cake is controlled to have a water content of <2 wt% by suction filtration, to obtain a filtrate and elemental iodine product; the filtrate is returned to step (1) for further treatment; when the potassium sulfate content in the filtrate reaches the saturated concentration, potassium sulfate product can be prepared after impurity removal.

[0070] Example 2

[0071] The present embodiment provides a method for preparing elemental iodine by using iodine-containing wastewater, the method comprising the following steps:

[0072] (1) adding alkali to adjust the pH value of the iodine-containing wastewater to 7.5, wherein the iodine-containing wastewater is obtained from the production of polarizing plates, the content of elemental iodine in the iodine-containing wastewater is 0.002 wt%, the content of alkali metal iodide is 2 wt%, and the content of alkali metal borate is 4 wt%, the alkali is potassium hydroxide solution with a concentration of 32 wt%, and an alkaline adjustment solution is obtained;

[0073] (2) adding the alkaline adjustment solution obtained in step (1) to a reducing agent, wherein the reducing agent is potassium sulfite solution with a concentration of 35 wt%, the adding amount of the reducing agent is 1.3 times of the theoretical adding amount, and a reduction mixture is obtained;

[0074] (3) distillation concentration of the reduction mixture obtained in step (2) in an MVR concentration device, wherein the distillation condition is 95°C and-0.06 MPa, the end point of the distillation concentration is that the content of alkali metal iodide is 38 wt%, and the concentration ratio is about 26:1, and then the concentrated solution is subjected to centrifugal separation in an automatic centrifuge, and fraction water, alkali metal iodide mother liquor and boric acid product are obtained; wherein the fraction water is subjected to RO membrane filtration, the obtained fresh water is used for cooling tower water replenishment and reuse, and the obtained concentrated water is returned to step (1) for treatment again;

[0075] (4) adding acid to adjust the pH value of the alkali metal iodide mother liquor obtained in step (3) to 1, wherein the acid is sulfuric acid with a concentration of 50 wt%, the adding rate is controlled at 200 L / h, the adding process needs constant stirring, and the stirring rate is 30 rpm, and an acid adjustment solution is obtained;

[0076] (5) adding an oxidizing agent to the acid adjustment solution obtained in step (4) and maintaining the pH value of the system to be ≤1, wherein the oxidizing agent is hydrogen peroxide solution with a concentration of 32 wt%, the adding amount of the oxidizing agent is 1.2 times of the theoretical adding amount according to the molar ratio of iodine element to hydrogen peroxide of 1:1.2, the adding rate is controlled at 150 L / h, the adding process needs constant stirring, and the stirring rate is 40 rpm, and with the addition of hydrogen peroxide solution, black-brown solid is constantly precipitated; the pH value of the system is maintained to be 1 by adding sulfuric acid with a concentration of 50 wt% during the reaction process, the filter cake is controlled to have a water content of <2 wt% by suction filtration, and a filtrate and elemental iodine product are obtained; the filtrate is returned to step (1) for treatment again; after the potassium sulfate content in the filtrate reaches the saturated concentration, potassium sulfate product can be prepared by impurity removal.

[0077] Example 3

[0078] The embodiment provides a method for preparing elemental iodine by using iodine-containing wastewater, and the method comprises the following steps:

[0079] (1) adding alkali to adjust the pH value of the iodine-containing wastewater to 8.5, wherein the iodine-containing wastewater is generated in the production process of polarizing plates; the content of elemental iodine in the iodine-containing wastewater is 0.003 wt%, the content of alkali metal iodide is 0.1 wt%, and the content of alkali metal borate is 0.5 wt%; the alkali is potassium hydroxide solution with a concentration of 32 wt%, and an alkaline adjustment liquid is obtained;

[0080] (2) adding the alkaline adjustment liquid obtained in step (1) to a reducing agent, wherein the reducing agent is potassium sulfite solution with a concentration of 35 wt%, and the adding amount of the reducing agent is 1.2 times of the theoretical amount, and a reduction mixture liquid is obtained;

[0081] (3) distillation concentration of the reduction mixture liquid obtained in step (2) is carried out in an MVR concentration device, wherein the distillation condition is 95℃ and-0.06 MPa, and the end point of the distillation concentration is that the content of alkali metal iodide is 38 wt%, and the concentration ratio is about 26:1; then the concentrated liquid is subjected to centrifugal separation in an automatic centrifuge, and fraction water, alkali metal iodide mother liquor and boric acid product are obtained; wherein the fraction water is subjected to RO membrane filtration, and the obtained fresh water is used for cooling tower water replenishment and reuse, and the obtained concentrated water is returned to step (1) for treatment again;

[0082] (4) adding acid to adjust the pH value of the alkali metal iodide mother liquor obtained in step (3) to 0.5, wherein the acid is sulfuric acid with a concentration of 50 wt%, the adding rate is controlled at 200 L / h, and the adding process needs constant stirring with a stirring rate of 30 rpm, and an acid adjustment liquid is obtained;

[0083] (5) adding an oxidizing agent to the acid adjustment liquid obtained in step (4) and maintaining the pH value of the system to be ≤1, wherein the oxidizing agent is hydrogen peroxide solution with a concentration of 35 wt%, the adding amount of the oxidizing agent is 1.2 times of the theoretical amount according to the molar ratio of iodine element to hydrogen peroxide of 1:1.2, the adding rate is controlled at 150 L / h, the adding process needs constant stirring with a stirring rate of 40 rpm, and black-brown solid is constantly precipitated with the addition of hydrogen peroxide solution; the pH value of the system is maintained at 0.5 by adding sulfuric acid with a concentration of 50 wt% during the reaction process, the filter cake is controlled to have a water content of <2 wt% by suction filtration, and a filtrate and elemental iodine product are obtained; the filtrate is returned to step (1) for treatment again; when the potassium sulfate content in the filtrate reaches the saturated concentration, potassium sulfate product can be prepared after impurity removal.

[0084] Comparative Example 1

[0085] The present comparative example provides a method for preparing elemental iodine by using iodine-containing wastewater, wherein the difference from Example 1 is that the pH value of the system is not controlled during the reaction process in step (5), and the reaction is not continued until the pH value of the system rises to 6.

[0086] The filtrate obtained in the step (5) of the present comparative example is darker than the filtrate obtained in the examples 1 and 2, because the iodine element is not completely reduced.

[0087] Comparative example 2

[0088] The present comparative example provides a method for preparing iodine element by using iodine-containing wastewater, and the difference from the example 1 is that the pH value of the iodine-containing wastewater is adjusted to 6.5 by adding alkali in the step (1).

[0089] Comparative example 3

[0090] The present comparative example provides a method for preparing iodine element by using iodine-containing wastewater, and the difference from the example 1 is that the pH value of the iodine-containing wastewater is adjusted to 9 by adding alkali in the step (1).

[0091] The iodine element products obtained in the above examples and comparative examples are tested for purity, including iodine content, water content and other impurities, and the test results are shown in Table 1.

[0092] Table 1

[0093] Item Iodine content / wt% Water content / wt% Other impurities / wt% Example 1 98.5 1.2 0.3 Example 2 98.4 1.3 0.3 Example 3 98.5 1.3 0.2 Comparative Example 1 93.2 5.5 1.3 Comparative Example 2 98.4 1.3 0.3 Comparative Example 3 83.1 5.3 11.6

[0094] From Table 1, the following points can be seen:

[0095] (1) The method for preparing iodine element by using iodine-containing wastewater provided by the present application, by using a specific process, continuously recycles the iodine element in the iodine-containing wastewater, so that the iodine resource is maximally recovered and utilized, and the iodine element product with purity ≥98wt% and water content ≤1.5wt% is prepared, realizing the preparation of high-quality iodine element product;

[0096] (2) Comparing the example 1 with the comparative example 1, because the pH value of the system is not controlled to be less than 1 in the reaction process in the step (5) of the comparative example 1, the potassium iodide is not fully oxidized to iodine element, and the potassium iodide remains in the iodine element, so that the water content of the iodine element is high;

[0097] (3) Comparing the example 1 with the comparative example 2, because the pH value of the iodine-containing wastewater is adjusted to 6.5 by adding alkali in the step (1) of the comparative example 1, the reducing agent is decomposed in the weak acid system, and the reduction is reduced, which not only causes the waste of chemical reagents, but also causes part of the iodine element to enter the boric acid, resulting in unqualified boric acid. Although it does not significantly affect the purity of the iodine element, it reduces the yield of the iodine element, and the purity of the boric acid is also unqualified, so that the method described in the comparative example 2 does not have industrial promotion value;

[0098] (4)Comparing Example 1 with Comparative Example 3, since the pH of the iodine-containing wastewater is adjusted to 9 in step (1) of Comparative Example 1, the system is relatively strong in alkalinity, which causes the boric acid in the system to form borate, resulting in a large amount of borate in the concentrated solution, and a large amount of boric acid is precipitated when the acidity is adjusted, which causes the content of elemental iodine to decrease.

[0099] The above embodiments illustrate the detailed structural features of the present application, but the present application is not limited to the above detailed structural features, i.e., it does not mean that the present application must rely on the above detailed structural features to be implemented. It should be understood by those skilled in the art that any improvement of the present application, equivalent replacement of the components selected by the present application, addition of auxiliary components, selection of specific modes, etc., all fall within the protection scope and disclosure scope of the present application.

[0100] The above describes the preferred embodiments of the present application in detail, but the present application is not limited to the specific details in the above embodiments, and within the technical concept scope of the present application, various simple modifications can be made to the technical solutions of the present application, and these simple modifications all belong to the protection scope of the present application.

[0101] In addition, it should be noted that each specific technical feature described in the above specific embodiments can be combined in any suitable manner without contradiction, and in order to avoid unnecessary repetition, the present application will not further describe various possible combination manners.

[0102] In addition, various different embodiments of the present application can also be combined in any manner, as long as it does not deviate from the idea of the present application, and it should also be considered as disclosed by the present application.

Claims

1. A method for producing elemental iodine using iodine-containing wastewater, characterized by comprising the steps of, The method comprises the following steps: (1) adjusting the pH value of iodine-containing wastewater to 7.5-8.5 by adding alkali to obtain an alkaline adjusted solution; the iodine-containing wastewater is generated in the production process of polarizing plates, and the composition of the iodine-containing wastewater comprises elemental iodine, alkali metal iodide and alkali metal borate; (2) adding a reducing agent to the alkaline adjusted solution obtained in step (1) to obtain a reducing mixed solution; (3) sequentially subjecting the reducing mixed solution obtained in step (2) to concentration and first solid-liquid separation to obtain distillate water, alkali metal iodide mother liquor and boric acid product; (4) adjusting the pH value of the alkali metal iodide mother liquor obtained in step (3) to ≤1 by adding acid to obtain an acid adjusted solution; (5) adding an oxidizing agent to the acid adjusted solution obtained in step (4) while maintaining the pH value of the system ≤1, and subjecting the system to second solid-liquid separation to obtain filtrate and elemental iodine product; the filtrate is returned to step (1) for further treatment.

2. The method of claim 1, wherein, In step (1), the content of elemental iodine in the iodine-containing wastewater is 0.001-0.2 wt%, the content of alkali metal iodide is 0.01-2 wt%, and the content of alkali metal borate is 0.5-4 wt%.

3. The method of claim 1, wherein, In step (1), the alkali is added in the form of potassium hydroxide solid and / or potassium hydroxide solution.

4. The method of claim 3, wherein, The effective purity of the potassium hydroxide solid is ≥99 wt%.

5. The method of claim 3, wherein, The concentration of the potassium hydroxide solution is 32-45 wt%.

6. The method of claim 1, wherein, In step (2), the reducing agent comprises potassium sulfite solid and / or potassium sulfite solution.

7. The method of claim 6, wherein, The content of the potassium sulfite solid is ≥95 wt%.

8. The method of claim 6, wherein, The concentration of the potassium sulfite solution is ≥30 wt%.

9. The method of claim 1, wherein, In step (2), the adding amount of the reducing agent is 1.2-1.3 times the theoretical adding amount.

10. The method of claim 1, wherein, In step (3), the end point of the concentration is that the content of alkali metal iodide is ≥35 wt%.

11. The method of claim 1, wherein, In step (3), the concentration comprises distillation concentration.

12. The method of claim 1, wherein, In step (3), the concentration ratio of the concentration is (25-26):

1.

13. The method of claim 1, wherein, In step (3), the first solid-liquid separation is centrifugal separation.

14. The method of claim 1, wherein, In step (3), the distillate water is subjected to RO membrane filtration, the obtained fresh water is used for cooling tower make-up water and reuse, and the obtained concentrated water is returned to step (1) for further treatment.

15. The method of claim 1, wherein, In step (4), the pH value is adjusted to 0.1-0.5 by adding acid.

16. The method of claim 1, wherein, In step (4), the acid is added in the form of sulfuric acid with a concentration of 40-50 wt%.

17. The method of claim 1, wherein, In step (4), the adding rate of the acid is controlled to be 100-220 L / h.

18. The method of claim 1, wherein, In step (4), the acid is added with constant stirring, and the stirring rate is controlled to be 25-30 rpm.

19. The method of claim 1, wherein, In step (5), the oxidizing agent is hydrogen peroxide solution with a concentration of 30-35 wt%.

20. The method of claim 1, wherein, In step (5), the adding amount of the oxidizing agent is 1-1.5 times the theoretical adding amount.

21. The method of claim 1, wherein, In step (5), the adding rate of the oxidizing agent is 130-170 L / h.

22. The method of claim 1, wherein, In step (5), the oxidizing agent is added with constant stirring, and the stirring rate is controlled to be 35-40 rpm.

23. The method of claim 1, wherein, The method comprises the following steps: (1) adding alkali to adjust the pH value of iodine-containing wastewater to 7.5-8.5, wherein the iodine-containing wastewater is obtained from the production of polarizing plates, the content of elemental iodine in the iodine-containing wastewater is 0.001-0.2 wt%, the content of alkali metal iodide is 0.01-2 wt%, and the content of alkali metal borate is 0.5-4 wt%, the alkali is potassium hydroxide solid with an effective purity of ≥99 wt% and / or potassium hydroxide solution with a concentration of 32-45 wt%, and an alkaline adjustment solution is obtained; (2) adding a reducing agent to the alkaline adjustment solution obtained in step (1), wherein the reducing agent comprises potassium sulfite solid with a content of ≥95 wt% and / or potassium sulfite solution with a concentration of ≥30 wt%, the adding amount of the reducing agent is 1.2-1.3 times the theoretical adding amount, and a reduction mixture is obtained; (3) concentrating the reduction mixture obtained in step (2) by distillation, wherein the end point of the distillation concentration is that the content of alkali metal iodide is ≥35 wt%, the concentration ratio of the concentration is (25-26):1, and centrifugal separation is then performed, and fraction water, alkali metal iodide mother liquor and boric acid product are obtained; wherein the fraction water is subjected to RO membrane filtration, the obtained fresh water is used for cooling tower water replenishment and reuse, and the obtained concentrated water is returned to step (1) for further treatment; (4) adding acid to adjust the pH value of the alkali metal iodide mother liquor obtained in step (3) to ≤1, wherein the acid is sulfuric acid with a concentration of 40-50 wt%, the adding rate of the acid is controlled to be 100-220 L / h, the acid is added with constant stirring, and the stirring rate is controlled to be 25-30 rpm, and an acid adjustment solution is obtained; (5) adding an oxidizing agent to the acid adjustment solution obtained in step (4) while maintaining the pH value of the system to be ≤1, wherein the oxidizing agent is hydrogen peroxide solution with a concentration of 30-35 wt%, the adding amount of the oxidizing agent is 1-1.5 times the theoretical adding amount, the adding rate of the oxidizing agent is 130-170 L / h, the oxidizing agent is added with constant stirring, and the stirring rate is controlled to be 35-40 rpm, and filtrate and elemental iodine product are obtained by second solid-liquid separation; the filtrate is returned to step (1) for further treatment.

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