A method for purifying a chromium-containing mixed salt

Abstract

The application provides a purification method of a chromium-containing mixed salt, the mixed salt comprising sodium carbonate, sodium bromide and a heavy metal chromium compound, in the method, a high-purity sodium bromide aqueous solution is prepared from the mixed salt and chromium salt is recovered, and the method comprises the following steps: preparing a crude sodium bromide solution, preparing a chromium removal agent by using a barium compound or a calcium compound, removing chromium from the crude sodium bromide solution by using the chromium removal agent and collecting a filtrate A; and treating the filtrate A to obtain a high-purity sodium bromide aqueous solution. In the method, the sodium carbonate in the crude sodium bromide solution is all converted into sodium bromide; by using the method, the chromium in the sodium bromide is removed and converted into barium chromate or calcium chromate products; the obtained sodium bromide no longer contains the heavy metal chromium and can be directly used for preparing high-purity sodium bromide or preparing a sodium bromide aqueous solution.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of purification of mixed salt containing chromium, and particularly relates to a purification method of mixed salt containing chromium. BACKGROUND

[0002] Terephthalic acid (PTA) is an important organic synthetic monomer in the chemical industry, and is mainly used for producing polyethylene terephthalate (PET) and bottle sheet polyester; at present, the PTA production capacity in China has broken through 70 million tons, in order to ensure the quality of PTA products, mother liquor needs to be extracted in the PTA production process, and then sent to a film evaporator to recover acetic acid, and the distillation residue is discharged as waste residue, which accounts for about 0.45-0.6% of the total PTA production, the main components of the waste residue are organic acids, and there are also a small amount of cobalt, manganese, heavy metals, bromide and the like; in addition, in the PTA manufacturing process, in order to remove the blocking material in various equipment and pipelines, it is often necessary to use lye for cleaning, in addition, other waste materials brought out by running, spouting, dripping and leaking on site and in the system are finally discharged into a waste collection pool, these waste materials are also discharged into the PTA oxidation waste residue system, the waste residue is neutralized and dissolved in a sodium hydroxide or / and sodium carbonate aqueous solution, filtered, concentrated, and incinerated to obtain sodium carbonate and sodium bromide mixed salt, the mixed salt is dissolved in water, filtered, evaporated and crystallized, and separated to obtain sodium carbonate, and the mother liquor is recycled, when the content of sodium bromide in the mother liquor accumulates to a certain degree, the mother liquor needs to be extracted, otherwise sodium bromide will be left in the sodium carbonate crystallization, the extracted mother liquor generally contains 10-30% sodium carbonate and 22-41% sodium bromide; in addition, a large amount of metal ions such as iron, nickel and chromium are brought into the PTA oxidation residue in the incineration process, and part of the bromine in the residue is also converted into bromate in the incineration process, the iron and nickel in the PTA oxidation residue exist in the form of insoluble substances under alkaline conditions, and can be removed by filtration when the molten salt formed after incineration is dissolved, while the chromium compound in the molten salt exists in the form of CrO4 2- , and the bromate also enters the mother liquor, the content of the chromium compound (calculated as chromium) is 500-2600 mg / kg, and the content of the bromate is 200-1500 mg / kg.

[0003] In the application file with the application number CN202310729456.0 and the subject name of a high-efficiency treatment method for terephthalic acid oxidation residue, the recovery of bromine is to add the sodium carbonate crystallization mother liquor into a phosphoric acid or sulfuric acid aqueous solution, heat and collect the distillate to obtain crude hydrobromic acid and obtain sodium hydrogen phosphate or sodium sulfate products. The method has the disadvantages that a large amount of chromium and bromate and the like are brought into the incineration process, resulting in that the products of sodium hydrogen phosphate and sodium sulfate are high-pollution waste due to the existence of a large amount of chromium, and the recovered hydrobromic acid cannot be directly used as a catalyst due to the existence of a large amount of free bromine.

[0004] Chromate ion is a heavy metal ion that is very important in environmental protection. Chromate is widely used in industries such as electroplating, dyes, alkali-chlorine, chemical fertilizers, petrochemistry, leather tanning, textiles, papermaking, and steel. Because chromate ion is highly toxic, it has potential risks of causing cancer or inducing mutations in humans and animals. Therefore, it is of great significance to remove chromium.

[0005] For chromium removal, existing chromium-containing wastewater treatment processes mainly use chemical reduction precipitation method, electrolytic reduction method, adsorption desorption method, and ion exchange method. Among them, the chemical reduction precipitation method is to add a reducing agent to the chromium-containing wastewater under acidic conditions, to reduce Cr 6+ to Cr 3+ , and then add lime or sodium hydroxide to generate chromium hydroxide Cr(OH)3 precipitate under alkaline conditions, thereby removing chromium. The electrolytic reduction method is to pass chromium-containing wastewater into an electrolytic cell, using iron as the anode material, under the action of two electrode potentials, the anode continuously dissolves to produce reducing ferrous ions, which reduce Cr 6+ to Cr 3+ , and Cr 3+ is precipitated as chromium hydroxide Cr(OH)3 to achieve the purpose of chromium removal. The adsorption desorption method is to use a special adsorbent to adsorb chromium in wastewater. The ion exchange method is to use ion exchange resin to selectively adsorb hexavalent chromium in wastewater to achieve chromium removal.

[0006] The disadvantages of the above methods are that they all need to reduce hexavalent chromium ions to trivalent chromium ions under acidic conditions, which is complex and the chromium products formed still need to be treated as solid waste.

[0007] In the application file with application number CN202411397273.4 and subject name "A chromium ultra-low emission treatment method and device for chromium-containing wastewater", a chromium removal method and device are disclosed. This method optimizes and improves the disadvantages of various chromium removal methods and devices, but it still needs to use ferrous sulfate or ferrous chloride as a reducing agent to reduce hexavalent chromium to trivalent chromium, and then use a complex treatment process and device to remove chromium. The process is complex and introduces many impurities.

[0008] In the patent file with application number US06334011 and subject name "A method for removing dissolved chromium from a solution", it is provided that hydrated iron oxide FeO(OH) is processed into a black material with high surface area and porosity, which is used as an adsorbent to adsorb chromate or dichromate ions in aqueous solution. Each gram of the adsorbent can only adsorb 1 to 25 milligrams of hexavalent chromium ions, and the adsorption capacity is not significant.

[0009] There is no literature report on the method of using sodium bromide aqueous solution containing sodium carbonate, bromate, and chromium to prepare sodium bromide and recover chromium salt. SUMMARY

[0010] In view of this, the present application provides a purification method of mixed salt containing chromium, wherein a mother liquor containing sodium carbonate, sodium bromide and heavy metal chromium is used to prepare high-purity sodium bromide aqueous solution and recover chromium salt.

[0011] The technical solution of the present application is as follows:

[0012] A purification method of mixed salt containing chromium, wherein the mixed salt contains sodium carbonate, sodium bromide and heavy metal chromium compound, and the purification method comprises the following steps:

[0013] S1, pretreatment

[0014] The mixed salt solid or aqueous solution is added into a neutralization reactor, hydrobromic acid with a mass concentration of 20-48% is added, the pH of the solution is adjusted to 8-11, and a crude sodium bromide solution is prepared;

[0015] The addition of hydrobromic acid can neutralize sodium carbonate and convert it into sodium bromide, and can also adjust the pH of the solution;

[0016] S2, preparation of chromium removal agent

[0017] A barium compound or a calcium compound is added into a dispensing tank, hydrobromic acid or / and water is added for dissolution, and then water is added to prepare a solution with a mass concentration of 10-50%, which is the chromium removal agent;

[0018] S3, chromium removal

[0019] According to the molar ratio of chromium to barium compound or calcium compound of 1.0:1.08-2.0, the chromium removal agent prepared in step S2 is added into the crude sodium bromide solution prepared in step S1, after the addition is completed, the solution is heated, settled, filtered and separated, and washed with water, to obtain a barium chromate or calcium chromate precipitate, and collect the filtrate A;

[0020] S4, the filtrate A collected in S3 is added into a reactor, hydrobromic acid is added to adjust the pH of the solution to less than 5, so that bromate in the solution is converted into free bromine, then a reducing agent is added, the solution is heated and the temperature of the solution is controlled at 50°C-boiling temperature, so that the free bromine is converted into bromide, then a sodium carbonate aqueous solution with a mass concentration of 10-22% or a saturated sodium carbonate aqueous solution is added, the pH of the solution is adjusted to 8-14, and then the solution is settled, filtered, the filtrate B and the precipitate B are collected, and the precipitate B is recycled and utilized;

[0021] Sodium carbonate is an alkaline compound, after being added, it can adjust the pH of the solution and also act as a precipitant to generate precipitate B with the excess barium compound or calcium compound in the chromium removal agent which does not participate in the chromium removal reaction, and the precipitate B is barium carbonate or calcium carbonate;

[0022] The filtrate B is added with hydrobromic acid to adjust the pH to 6-8, to obtain a high-purity sodium bromide aqueous solution; the content of sodium bromide in the sodium bromide aqueous solution is controlled to be below 46%.

[0023] Preferably, in step S1, the hydrobromic acid is industrial-grade hydrobromic acid with a mass concentration of 40-48%; the content of sodium bromide in the final product can be relatively increased, and the evaporation amount is reduced when preparing sodium bromide crystals, which is more energy-saving.

[0024] In step S1, the pH of the solution after neutralization is controlled to be 8-11; when the pH is less than 8, bromate and CrO4 2- are easily converted into free bromine and Cr 3+ , thereby affecting the formation of chromium salt precipitate from 6-valence chromium in step S2; when the pH is greater than 11 and continues to increase, it has no effect on the precipitation of Cr 6+ ; therefore, the pH in the neutralization stage S1 is controlled to be 8-11 in the present application.

[0025] Preferably, in step S2, when the barium compound is barium bromide, barium chloride and barium acetate, and the calcium compound is calcium bromide and calcium chloride, water is added for dissolution; when the barium compound is barium oxide, barium hydroxide or barium carbonate, and the calcium compound is calcium oxide, calcium hydroxide or calcium carbonate, 48% mass concentration hydrobromic acid is first added for dissolution, and then water is added to adjust the concentration of barium bromide or calcium bromide; the molar ratio of the barium compound or the calcium compound to the hydrobromic acid is 1:2.02-2.1; the hydrobromic acid is industrial-grade.

[0026] In step S2, the barium compound is selected from barium bromide, barium oxide, barium hydroxide or barium carbonate, and the calcium compound is selected from calcium bromide, calcium oxide, calcium hydroxide or calcium carbonate. When these low-cost raw materials are used to prepare the solution, the cost of raw materials can be reduced, and the prepared chromium removal agent can be directly used. In the present application, the water-insoluble barium compound and the water-insoluble calcium compound are dissolved by first adding hydrobromic acid and then diluting with water, so as to ensure complete dissolution of the barium compound or the calcium compound and to minimize the introduction of excessive water into the system. For example, barium oxide and barium hydroxide can be directly dissolved in water, but the solubility of barium oxide and barium hydroxide at room temperature is less than 3%. Therefore, when water is used as a solvent for direct dissolution, a large amount of water will be introduced into the system, and the concentration of the prepared sodium bromide solution will be low, which will result in high energy consumption, low yield and high transportation cost during the preparation of crystals in the later stage. The molar ratio of barium oxide or barium hydroxide or barium carbonate or calcium oxide or calcium hydroxide or calcium carbonate to hydrobromic acid is 1:2.02-2.1. When the barium compound in the chromium removal agent is barium chloride and barium acetate, and the calcium compound is calcium chloride, water can be directly used for dissolution, but a large amount of chloride ions and acetate ions will be introduced into the system. Therefore, the preferred barium compound in the present application is barium oxide, barium hydroxide or barium carbonate, and the preferred calcium compound is calcium oxide, calcium hydroxide or calcium carbonate. The above barium compound or calcium compound is dissolved in an aqueous hydrobromic acid solution and then diluted with water to a mass concentration of 10-50%, thereby obtaining the chromium removal agent. When the molar ratio of barium oxide or barium hydroxide or barium carbonate or calcium oxide or calcium hydroxide or calcium carbonate to hydrobromic acid is greater than 1:2.02, incomplete dissolution or turbidity of the solution will occur. When the molar ratio of barium oxide or barium hydroxide or barium carbonate or calcium oxide or calcium hydroxide or calcium carbonate to hydrobromic acid is less than 1:2.1, increasing the amount of hydrobromic acid has no effect on dissolution, but excessive hydrobromic acid will affect the pH value of the solution during the removal of chromium. Therefore, the molar ratio of barium oxide or barium hydroxide or barium carbonate or calcium oxide or calcium hydroxide or calcium carbonate to hydrobromic acid is controlled to be 1:2.02-2.1. The present application can also directly use barium bromide or calcium bromide to prepare the chromium removal agent by dissolving in water. The above two methods are beneficial to obtaining a more pure sodium bromide solution after the removal of barium and calcium.

[0027] Preferably, in step S3, after the chromium removal agent of step S2 is added to the crude sodium bromide solution prepared in step S1, the solution is heated and the temperature is controlled at 50°C-boiling temperature to promote the growth of precipitated particles, improve the filtration efficiency and filtration effect. Then, the heating is stopped and the solution is allowed to settle for 20-120 min. During the filtration and separation, one of pressure filtration, suction filtration or centrifugation is used.

[0028] In step S3, the barium compound or the calcium compound in the chromium removal agent reacts with CrO4 2-The reaction is carried out to generate BaCrO4 or CaCrO4 precipitate, so as to remove the chromium in the solution; when the molar ratio of chromium to barium compound or calcium compound is greater than 1.0:1.08, the barium compound or calcium compound is insufficient, and the CrO4 2- in the solution is not removed completely; when the molar ratio of chromium to barium compound or calcium compound is less than 1.0:2.0, the amount of barium compound or calcium compound is continuously increased, and the CrO4 2- in the solution is not removed any more; therefore, the molar ratio of chromium to barium compound or calcium compound is selected as 1.0:1.08-2.0 in the present application.

[0029] Preferably, in step S4, the reducing agent is at least one of formaldehyde, hydrazine hydrate, formic acid, sodium formate, oxalic acid, sodium oxalate, ascorbic acid, ferrous salt, sulfite, sulfide, and the reducing agent can reduce the free bromine.

[0030] Preferably, in order to avoid the impurities which are not easy to separate from entering the solution, the reducing agent is one of formaldehyde, hydrazine hydrate, formic acid, sodium formate, oxalic acid, sodium oxalate, ascorbic acid; wherein the molar ratio of free bromine to formaldehyde or hydrazine hydrate is 1.0:0.54-1.0, and the molar ratio of free bromine to formic acid or sodium formate or oxalic acid or sodium oxalate or ascorbic acid is 1.0:1.05-2.0.

[0031] Preferably, in step S4, before preparing the filtrate B, the pH of the solution is adjusted to 9-12, and the solution is left to stand for 20-120 min.

[0032] In step S4, the pH of the solution is adjusted to be less than 5, preferably less than 3, by adding hydrobromic acid, so as to convert the bromate and bromine ion in the solution into free bromine under this condition; when a small amount of CrO4 2- still remains in the solution, it will also be converted into Cr2O7 2- ; by adding formaldehyde, hydrazine hydrate, formic acid, sodium formate, oxalic acid, sodium oxalate, ascorbic acid, ferrous salt, sulfite, sulfide into the solution, the free bromine and Cr2O7 2- can be reduced; in order to avoid the impurities which are not easy to separate from entering and simplify the operation, it is more preferred to add one of formaldehyde, hydrazine hydrate, formic acid, sodium formate, oxalic acid, sodium oxalate, ascorbic acid, and wherein the molar ratio of free bromine to formaldehyde or hydrazine hydrate is 1.0:0.54-1.0, and the molar ratio of free bromine to formic acid or sodium formate or oxalic acid or sodium oxalate or ascorbic acid is 1.0:1.05-2.0; the free bromine (Br2) and Cr2O7 2- in the solution are reduced into Br - and Cr 3+When the molar ratio of free bromine to formaldehyde or hydrazine hydrate is greater than 1.0:0.54 or the molar ratio of free bromine to formic acid or oxalic acid or ascorbic acid is greater than 1.0:1.05, free bromine still remains in the solution; when the molar ratio of free bromine to formaldehyde or hydrazine hydrate is less than 1.0:1.0 or the molar ratio of free bromine to formic acid or sodium formate or oxalic acid or sodium oxalate or ascorbic acid is less than 1.0:2.0, with the increase of the amount of formaldehyde, hydrazine hydrate, formic acid, sodium formate, oxalic acid, sodium oxalate or ascorbic acid, free bromine and chromium are not detected, therefore, the molar ratio of free bromine to formaldehyde or hydrazine hydrate is 1.0:0.54-1.0 and the molar ratio of free bromine to formic acid or sodium formate or oxalic acid or sodium oxalate or ascorbic acid is 1.0:1.05-2.0 is selected;

[0033] In step S4, the pH of the solution in which free bromine is removed is adjusted to 8-14 by adding a sodium carbonate aqueous solution with a mass concentration of 10-22% or a saturated sodium carbonate aqueous solution, so as to convert the excess Ba 2+ or Ca 2+ into BaCO3 or CaCO3 precipitates, thereby being removed by standing, settling and filtering, so that the sodium bromide solution is purified; at this time, if there is Cr 3+ that has been reduced in the solution, it will also be converted into Cr(OH)3 precipitates and thereby be removed; the final sodium bromide aqueous solution obtained is detected, and the concentration of chromium in the solution is all below 0.15 mg / kg or not detected; when the concentration of Cr 3+ in the solution is below 0.15 mg / kg, 0.1-3% of activated carbon in terms of solution mass is added, and the solution obtained after filtering and separating is detected and found that there is no chromium.

[0034] For the BaCO3 or CaCO3 obtained in step S4, a sample is taken, industrial-grade hydrobromic acid and water are added to dissolve and detect the chromium content therein; when there is no chromium, it is directly returned to the S2 process for recycling; when chromium is detected, the BaCO3 or CaCO3 is dissolved by industrial-grade hydrobromic acid and water, and then the chromium is removed by activated carbon adsorption before being returned to the S2 process for recycling.

[0035] Compared with the prior art, the present application has the following beneficial effects:

[0036] 1. The method of the present application realizes the conversion of all sodium carbonate in crude sodium bromide into sodium bromide.

[0037] 2. By using the method of the present application, chromium in sodium bromide is removed and converted into barium chromate or calcium chromate products.

[0038] 3. The sodium bromide obtained by the present application no longer contains heavy metal chromium and can be directly used for preparing high-purity sodium bromide or preparing a sodium bromide aqueous solution.

[0039] 4、The method provided by the application has the advantages that the whole purification process does not cause impurities to enter the sodium bromide product. DETAILED DESCRIPTION

[0040] In order to enable those skilled in the art to better understand the technical solutions in the present application, the technical solutions in the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts shall fall within the protection scope of the present application.

[0041] In the present application, the percentage is a mass percentage, min: minute; h: hour;

[0042] In the present application, the test method of chromium is ICP method; the determination of free bromine adopts volumetric method or colorimetric method; the determination of sodium bromide adopts volumetric method.

[0043] In the present application, the mother liquor containing sodium carbonate and sodium bromide obtained after the sodium carbonate is obtained by dissolving, evaporating and crystallizing from the sodium carbonate and sodium bromide mixed salt obtained by incinerating the xylene oxidation residue is used to prepare high-purity sodium bromide aqueous solution and recover chromium salt.

[0044] Example 1

[0045] S1, pretreatment

[0046] 500g of sodium bromide mother liquor is taken, in which the content of sodium carbonate is 17.1%, the content of sodium bromide is 38.2%, and the content of chromium is 2249mg / kg, and then the sodium bromide mother liquor is added into a neutralization reaction kettle, and then a hydrobromic acid solution with a mass concentration of 48% is added to adjust the pH of the solution to 8.68, the amount of the hydrobromic acid (mass concentration 48%) is 270g, and a crude sodium bromide solution is obtained;

[0047] S2, preparation of chromium removal agent

[0048] 27g of barium carbonate with a purity of 98% is added into a blending tank, 47.7g of a hydrobromic acid solution (the hydrobromic acid solution is a commercial product) with a mass concentration of 48% and 30g of water are added, stirring and dissolving, until no bubbles are generated, after the dissolving is completed, water is added to 100g, and a 40% barium bromide solution, that is, a chromium removal agent, is obtained; the molar ratio of barium carbonate to hydrobromic acid is 1:2.1;

[0049] S3, chromium removal

[0050] 24.6g of the chromium removal agent is added into the crude sodium bromide solution of S1, and the molar ratio of chromium to barium bromide is 1.0:1.53;

[0051] After the addition is completed, the solution is heated to boiling; then the heating is stopped, and the solution is left to stand and settle for 60min;

[0052] The precipitate was separated by suction filtration, and the precipitate A and filtrate A were collected; the precipitate A was washed with water for 3 times to obtain 6.08 g of wet solid barium chromate; the washing liquid and filtrate A were combined to obtain a mixed liquid A;

[0053] S4,

[0054] The mixed liquid A was added into a reaction kettle, and the pH of the solution was adjusted to 1.25 by adding hydrobromic acid with a mass concentration of 48%, and the amount of hydrobromic acid (with a mass concentration of 48%) added was 5.0 g, and the stirring reaction was performed for 30 min;

[0055] It was determined that the free bromine content was 0.14% and no chromium was detected; then 4.63 g of formic acid solution with a mass concentration of 10% was added, and the molar ratio of free bromine to formic acid was 1.0:1.5, so that the bromate and bromine ion in the solution were converted into free bromine under this condition;

[0056] Boiling was performed by heating, and stirring reaction was performed for 20 min; the pH of the solution was adjusted to 9.67 by adding saturated sodium carbonate aqueous solution (containing 26.5% of sodium carbonate), and the amount of sodium carbonate aqueous solution added was 6.11 g; the stirring was stopped, and the solution was allowed to stand for 30 min; suction filtration was performed, and the filtrate B and the precipitate B were collected; the precipitate B was 2.53 g, which was recycled and used in the process of S2;

[0057] The pH value of the filtrate B was adjusted to 7.21 by adding hydrobromic acid with a mass concentration of 48%, and the amount of hydrobromic acid added was 3.52 g, so that the high-purity sodium bromide aqueous solution was obtained, and the sodium bromide content in the solution was 45.7%.

[0058] Example 2

[0059] The difference between Example 1 and Example 2 is that,

[0060] In step S2, the 40% barium bromide solution prepared by reacting barium carbonate with hydrobromic acid solution in Example 1 was replaced by directly using barium bromide to dissolve in water to prepare a solution containing 40% barium bromide;

[0061] In step S3, after the precipitate A was washed with water for 3 times, 6.01 g of wet solid barium chromate was collected;

[0062] In step S4, the 4.63 g of formic acid solution with a mass concentration of 10% in Example 1 was replaced by 6.97 g of sodium formate solution with a mass concentration of 10%, and the molar ratio of free bromine to sodium formate was 1.0:1.5;

[0063] After the treatment of the present example, the sodium bromide content in the obtained high-purity sodium bromide aqueous solution was 45.5%.

[0064] Example 3

[0065] S1, pretreatment

[0066] Take 500g of sodium bromide mother liquor, which contains 19.4% sodium carbonate, 35.8% sodium bromide, and 2032mg / kg chromium, and add it to a neutralization reactor. Then add a 30% hydrobromic acid solution to adjust the pH of the solution to 10.81. The amount of 30% hydrobromic acid used is 490g. A crude sodium bromide solution is obtained.

[0067] S2, preparation of chromium removal agent

[0068] Add 21g of 98% pure barium oxide to a preparation tank, add 46.6g of 48% hydrobromic acid, and add water to 100g to dissolve, obtaining a 40% barium bromide solution, which is the chromium removal agent. The molar ratio of barium oxide to hydrobromic acid is 1:2.05.

[0069] S3, chromium removal

[0070] Add 20.3g of chromium removal agent to the crude sodium bromide solution from S1, and the molar ratio of chromium to barium bromide is 1.0:1.4.

[0071] After the addition is complete, heat the solution to boiling. Then stop heating and let it settle for 60 minutes.

[0072] Separate the precipitate by suction filtration, and collect the precipitate A and the filtrate A. Wash the precipitate A with water 3 times to obtain 5.50g of wet solid barium chromate. Combine the water washings and the filtrate A to obtain a mixed solution A.

[0073] S4,

[0074] Add the mixed solution A to a reactor, adjust the pH of the solution to 1.17 with a 30% hydrobromic acid solution, and add 10.0g of 30% hydrobromic acid. Stir for 20 minutes.

[0075] Determine that the free bromine content is 0.13% and no chromium is detected. Then add 0.26g of hydrazine hydrate, with a molar ratio of free bromine to hydrazine hydrate of 1.0:0.64, to convert the bromate and bromide ions in the solution to free bromine under these conditions.

[0076] Heat to boiling, stir for 20 minutes, adjust the pH of the solution to 9.55 with a 10% sodium carbonate aqueous solution, and add 15g. Stop stirring and let it stand for 30 minutes. Suction filter and collect the filtrate B and the precipitate B. The precipitate B is 1.71g, which is recycled and used in the S2 process.

[0077] Add a 30% hydrobromic acid solution to the filtrate B to adjust the pH to 6.95, and add 2.52g to obtain a high-purity sodium bromide aqueous solution, which contains 36.5% sodium bromide.

[0078] Example 4

[0079] S1, pretreatment

[0080] Take 500g of sodium bromide mother liquor, which contains 30.4% of sodium carbonate, 22.2% of sodium bromide, and 1273mg / kg of chromium, and add it to a neutralization reactor. Then add 40% hydrobromic acid to adjust the pH of the solution to 9.73. The amount of 40% hydrobromic acid used is 575g, and a crude sodium bromide solution is obtained.

[0081] S2, preparation of chromium removal agent

[0082] Add 21g of barium hydroxide with a purity of 98% to a preparation tank, add 57.67g of 48% hydrobromic acid, and add water to 100g to dissolve, obtaining a 50% barium bromide solution, which is the chromium removal agent. The molar ratio of barium hydroxide to hydrobromic acid is 1:2.03.

[0083] S3, chromium removal

[0084] Add 7.65g of chromium removal agent to the crude sodium bromide solution of S1, and the molar ratio of chromium to barium bromide is 1.0:1.05.

[0085] After the addition is complete, heat the solution to boiling, then stop heating and let it stand for 120min.

[0086] Vacuum filtration is used to separate the precipitate, and precipitate A and filtrate A are collected. Precipitate A is washed with water 3 times to obtain 3.34g of wet barium chromate solid. The washing liquid and filtrate A are combined to obtain a mixed solution A.

[0087] S4,

[0088] Add the mixed solution A to a reactor, adjust the pH of the solution to 1.91 with 40% hydrobromic acid, and add 8.1g of 40% hydrobromic acid. Stir for 30min.

[0089] The free bromine content is about 0.04% and the chromium ion content is 5.87mg / kg. Control the solution temperature to 73℃, then add 1.0g of ascorbic acid, and the molar ratio of free bromine to ascorbic acid is about 1.0:2.0. The bromate and bromine ions in the solution are converted into free bromine under this condition, which is reduced to Br - , and Cr2O7 2- is reduced to Cr 3+ .

[0090] Stir the reaction for 20 min, then add 20% sodium carbonate aqueous solution to adjust the pH of the solution to 13.50, the amount of which is 15 g, stand for 120 min, vacuum filter, collect the filtrate B and the precipitate B; wherein the precipitate B is 0.31 g; through detection, no chromium ion is detected in the filtrate B, and after the precipitate B is dissolved by adding industrial-grade hydrobromic acid and water and then adsorbed by activated carbon, the effluent is returned to the S2 process for reuse when no chromium is detected.

[0091] Add 40% hydrobromic acid to the filtrate B to adjust the pH value to 6.57, the amount of which is 10.1 g, to obtain a high-purity sodium bromide aqueous solution, wherein the sodium bromide content is 37.3%.

[0092] Example 5

[0093] S1, pretreatment

[0094] Take 500 g of sodium bromide mother liquor, wherein the sodium carbonate content is 10.2%, the sodium bromide content is 40.7%, and the chromium content is 2646 mg / kg, add it to a neutralization reactor, then add 20% hydrobromic acid to adjust the pH of the solution to 8.25, the amount of which is 387 g, to obtain a crude sodium bromide solution;

[0095] S2, preparation of chromium removal agent

[0096] Add 17.0 g of calcium hydroxide with a purity of 98% to a blending tank, add 77.08 g of 48% hydrobromic acid, and add water to 100 g for dissolution to obtain a 45% calcium bromide solution, which is a chromium removal agent; wherein the molar ratio of calcium hydroxide to hydrobromic acid is 1:2.03;

[0097] S3, chromium removal

[0098] Add 15.28 g of the chromium removal agent to the crude sodium bromide solution of S1, and the molar ratio of chromium to calcium bromide is 1.0:1.35;

[0099] After the addition is completed, heat the solution to 83°C; then stop heating and stand for 30 min;

[0100] Filter the precipitate by compressed air, collect the precipitate A and the filtrate A; wash the precipitate A with water for 3 times to obtain 4.41 g of wet solid calcium chromate; combine the washing liquid and the filtrate A to obtain a mixed liquid A;

[0101] S4,

[0102] Add the mixed liquid A to a reaction kettle, adjust the pH of the solution to 1.33 by adding 20% hydrobromic acid, the amount of which is 10.1 g, and stir the reaction for 20 min;

[0103] The free bromine content was determined to be 0.34%, and no chromium was detected; then 4.79 g of oxalic acid was added, and the molar ratio of oxalic acid to free bromine was 2.0:1.0;

[0104] The solution was heated to boiling, and stirring was performed for 20 min. A 10% sodium carbonate aqueous solution was added to adjust the pH of the solution to 13.12, and the amount of the sodium carbonate aqueous solution added was 24 g. The stirring was stopped, and the solution was allowed to stand for 120 min. The precipitate was filtered by compressed air, and the filtrate B and the precipitate B were collected. The precipitate B was 1.02 g.

[0105] The pH of the filtrate B was adjusted to 6.35 by adding a 20% hydrobromic acid, and the amount of the hydrobromic acid added was 12.3 g. Thus, a high-purity sodium bromide aqueous solution was obtained, and the sodium bromide content was 33.3%.

[0106] Example 6

[0107] S1, pretreatment

[0108] 500 g of a sodium bromide mother liquor was taken, in which the sodium carbonate content was 30.4%, the sodium bromide content was 22.2%, and the chromium content was 1273 mg / kg. The sodium bromide mother liquor was added to a neutralization reactor, and then a 40% hydrobromic acid was added to adjust the pH of the solution to 9.50. The amount of the 40% hydrobromic acid added was 575 g, and a crude sodium bromide solution was obtained.

[0109] S2, preparation of a chromium removal agent

[0110] 22.96 g of calcium carbonate with a purity of 98% was added to a preparation tank, 79.7 g of a 48% hydrobromic acid was added to dissolve the calcium carbonate, and then water was added to 100 g. Thus, a 45% calcium bromide solution, which was the chromium removal agent, was obtained. The molar ratio of the calcium carbonate to the hydrobromic acid was 1:2.1.

[0111] S3, chromium removal

[0112] 6.53 g of the chromium removal agent was added to the crude sodium bromide solution of S1, and the molar ratio of chromium to calcium bromide was 1.0:1.2.

[0113] After the addition was completed, the solution was heated to boiling. Then, the heating was stopped, and the solution was allowed to stand for 30 min.

[0114] The precipitate A and the filtrate A were collected by vacuum filtration. The precipitate A was washed with water for 3 times, and 2.06 g of a wet solid calcium chromate was obtained. The washing liquid and the filtrate A were combined, and a mixed liquid A was obtained.

[0115] S4,

[0116] The filtrate A was added to a reactor, and the pH of the solution was adjusted to 1.50 by adding a 40% hydrobromic acid. The amount of the 40% hydrobromic acid added was 8.2 g, and the stirring was performed for 30 min.

[0117] The free bromine content was 0.04%, and no chromium was detected. The solution temperature was controlled at 55°C, and then 0.51 g of ascorbic acid was added, with a molar ratio of ascorbic acid to free bromine of 1.05:1.0;

[0118] Boiling was performed, stirring was performed for 20 min, a 20% sodium carbonate aqueous solution was added to adjust the pH of the solution to 10.50, the amount of the 20% sodium carbonate aqueous solution added was 7 g, stirring was stopped, and standing was performed for 120 min. The filtrate B and the precipitate B were collected by vacuum filtration. The precipitate B was 0.3 g.

[0119] A 40% hydrobromic acid was added to the filtrate B to adjust the pH to 7.50, and the amount of the 40% hydrobromic acid added was 6.5 g. Thus, a high-purity sodium bromide aqueous solution was obtained, and the sodium bromide content of the high-purity sodium bromide aqueous solution was 37.6%.

[0120] Example 7

[0121] The difference between Example 7 and Example 6 is that,

[0122] In step S2, the 45% calcium bromide solution prepared by reacting calcium carbonate with hydrobromic acid in Example 6 was replaced by directly using calcium bromide to dissolve in water to prepare a solution containing 45% calcium bromide.

[0123] In step S3, after the precipitate A was washed with water for 3 times, 2.04 g of a water-containing solid calcium chromate was collected.

[0124] After the treatment in this example, the sodium bromide content of the obtained high-purity sodium bromide aqueous solution was 37.7%.

[0125] Example 8

[0126] S1, pretreatment

[0127] 500 g of a sodium bromide mother liquor was taken, the sodium carbonate content of the sodium bromide mother liquor was 30.4%, the sodium bromide content of the sodium bromide mother liquor was 22.2%, and the chromium content of the sodium bromide mother liquor was 1273 mg / kg. The sodium bromide mother liquor was added to a neutralization reactor, and then a 40% hydrobromic acid was added to adjust the pH of the solution to 9.82. The amount of the 40% hydrobromic acid was 575 g. Thus, a crude sodium bromide solution was obtained.

[0128] S2, preparation of a chromium removal agent

[0129] 14.2 g of calcium oxide with a purity of 98% was added to a preparation tank, 85 g of a 48% hydrobromic acid was added for dissolution, and then water was added to 100 g. Thus, a 50% calcium bromide solution, which was a chromium removal agent, was obtained. The molar ratio of calcium oxide to hydrobromic acid was 1:2.02.

[0130] S3, chromium removal

[0131] 5.88 g of chromium removal agent was added into the crude sodium bromide solution of S1, and the molar ratio of chromium to calcium bromide was 1.0:1.2;

[0132] After the addition was completed, the solution was heated to 50°C; then the heating was stopped, and the solution was allowed to settle for 120 min;

[0133] The precipitate A and the filtrate A were collected by vacuum filtration; the precipitate A was washed with water for 3 times to obtain 2.05 g of wet solid calcium chromate; the water washing liquid and the filtrate A were combined to obtain a mixed liquid A;

[0134] S4,

[0135] The mixed liquid A was added into a reaction kettle, and the pH of the solution was adjusted to 1.35 by adding 8.1 g of hydrobromic acid (mass concentration: 40%) under stirring for 30 min;

[0136] The solution was determined to contain 0.04% of free bromine and no chromium; the temperature of the solution was controlled at 55°C, and then 0.23 g of formaldehyde (mass concentration: 37%) was added, and the molar ratio of formaldehyde to free bromine was 2.0:1.0;

[0137] The solution was heated to boiling, and then 7 g of sodium carbonate aqueous solution (mass concentration: 20%) was added to adjust the pH of the solution to 10.51; the stirring was stopped, and the solution was allowed to settle for 120 min; the filtrate B and the precipitate B were collected by vacuum filtration; the precipitate B was 0.31 g;

[0138] Hydrobromic acid (mass concentration: 40%) was added into the filtrate B to adjust the pH to 7.48, and the addition amount was 6.5 g; thus, a high-purity sodium bromide aqueous solution was obtained, which contained 37.6% of sodium bromide and no chromium.

[0139] Example 9

[0140] S1, pretreatment

[0141] 500 g of sodium bromide mother liquor was taken, which contained 17.6% of sodium carbonate, 25.1% of sodium bromide, and 586 mg / kg of chromium; the mother liquor was added into a neutralization reaction kettle, and then hydrobromic acid (mass concentration: 48%) was added to adjust the pH of the solution to 12.98; the addition amount of the hydrobromic acid (mass concentration: 48%) was 273 g; thus, a crude sodium bromide solution was obtained;

[0142] S2, preparation of chromium removal agent

[0143] Barium bromide was directly dissolved in water to prepare a solution containing 50% of barium bromide, which was the chromium removal agent;

[0144] S3, chromium removal

[0145] The chromium removal agent 6.70 g is added to the crude sodium bromide solution of S1, and the molar ratio of chromium to barium bromide is 1.0:2.0;

[0146] After the addition is completed, the solution is heated to boiling; then the heating is stopped, and the solution is settled for 60 min;

[0147] The precipitate A and the filtrate A are collected by vacuum filtration; the precipitate A is washed with water for 3 times to obtain 1.64 g of wet solid barium chromate; and the water washing liquid and the filtrate A are combined to obtain a mixed liquid A;

[0148] S4,

[0149] The mixed liquid A is added into a reaction kettle, and the pH of the solution is adjusted to 1.51 by adding 15.0 g of hydrobromic acid with a mass concentration of 48%, and the stirring reaction is performed for 30 min;

[0150] It is determined that the free bromine content is 0.05%, and no chromium is detected; then 1.44 g of formic acid solution with a mass concentration of 10% is added, and the molar ratio of free bromine to formic acid is 1.0:1.3;

[0151] The solution is heated to boiling, and the stirring reaction is performed for 20 min; then the pH of the solution is adjusted to 9.50 by adding 13.7 g of sodium carbonate aqueous solution with a mass concentration of 22%, and the solution is settled for 60 min; the filtrate B and the precipitate B are collected by vacuum filtration; wherein the precipitate B is 1.32 g;

[0152] The pH value of the filtrate B is adjusted to 7.01 by adding 2.2 g of hydrobromic acid with a mass concentration of 48%, and a high-purity sodium bromide aqueous solution is obtained, wherein the sodium bromide content is 37.7%.

[0153] The chemical equation related to the present application is as follows:

[0154] BaBr2+ CrO4 2- =BaCrO 4 ↓ +2Br -

[0155] CaBr2+ CrO4 2- =CaCrO 4 ↓ +2Br -

[0156] BaBr2+ Na2CO3= BaCO 3↓ +2NaBr

[0157] CaBr2+ Na2CO3= CaCO 3↓ +2NaBr

[0158] Ba0+ 2HBr=BaBr2+H2O

[0159] Ba(OH)2+ 2HBr = BaBr2+ 2H2O

[0160] BaCO3+ 2HBr - = BaBr2+ CO2↑+ H2O

[0161] CaO + 2HBr = CaBr2+ H2O

[0162] Ca(OH)2+ 2HBr = CaBr2+ 2H2O

[0163] CaCO3+ 2HBr = CaBr2+ CO2↑+ H2O

[0164] Na2CO3+ 2HBr = 2NaBr + CO2↑+ H2O

[0165] BrO3 - - 5Br - - 6H + = 3Br2+ 3H2O

[0166] 2Br2+ HCHO + 2H2O = CO2↑+ H2O + 4HBr

[0167] Br2+ HCOO - → 2Br - + H + + CO2↑

[0168] N2H4•H2O + 2Br2= N2↑+ H2O + 4HBr -

[0169] C2O4 2- + Br2→ 2CO2↑+ 2Br -

[0170] C6H8O6+ Br2→ C6H6O6+ 2HBr.

[0171] 2Cr2O7 2- + 16H + + 3HCHO = 4Cr 3+ + 3CO2↑+ 11H2O

[0172] Cr2O7 2- + 11H + + 3HCOO - = 2Cr 3+ + 3CO2↑+ 7H2O

[0173] 2Cr2O7 2- + 3N2H4•H2O + 16H+ = 4Cr 3+ + 3N2+ 17H2O

[0174] Cr2O7 2- + 3C2O4 2- + 14H + = 2Cr 3+ + 6CO2↑+ 7H2O

[0175] Cr2O7 2- + 3C6H8O6+ 8H + = 2Cr 3+ + 3C6H6O6+ 7H2O

[0176] Although the present application has been described in detail by reference to the preferred embodiments, it is to be understood that the application is not limited to the particulars of the preferred embodiments. Various modifications and changes can be made to the application without departing from the spirit and scope of the application. It is intended that the scope of the application be defined by the appended claims and their equivalents.

Claims

1. A method for purifying a chromium-containing mixed salt, characterized in that, The mixed salt comprises sodium carbonate, sodium bromide, and a heavy metal chromium compound. The purification method includes the following steps: S1, Preprocessing Add the mixed salt solid or aqueous solution to the neutralization reaction vessel, add hydrobromic acid with a mass concentration of 40-48%, adjust the pH of the solution to 8-11, and prepare a crude sodium bromide solution. S2, Preparation of chromium removal agent Add barium or calcium compounds to a mixing tank, add hydrobromic acid and / or water to dissolve them, and prepare a solution with a mass concentration of 10-50%, which is the chromium removal agent; S3, chromium removal According to the molar ratio of chromium to barium or calcium compounds of 1.0:1.08-2.0, the chromium removal agent prepared in step S2 is added to the crude sodium bromide solution prepared in step S1. After the addition is completed, the solution is heated, allowed to stand and settle, filtered and separated, and washed with water to obtain barium chromate or calcium chromate precipitate. Filtrate A is collected. S4. Add the filtrate A collected in S3 to the reaction vessel, add hydrobromic acid to adjust the pH of the solution to <5, so that the bromate and bromide ions in the solution are converted into free bromine. Then add a reducing agent, heat the solution and control the solution temperature at 50℃ to boiling temperature, so that the free bromine is converted into bromide ions. Then add a 10-22% sodium carbonate aqueous solution or saturated sodium carbonate aqueous solution to adjust the pH of the solution to 8-14. Let it stand, filter, and collect the filtrate B and precipitate B. Precipitate B is recycled. Add hydrobromic acid to filtrate B to adjust the pH to 6-8 to obtain a high-purity sodium bromide aqueous solution; the sodium bromide content in the sodium bromide aqueous solution is controlled below 46%.

2. The purification method for chromium-containing mixed salts as described in claim 1, characterized in that, In step S2, when the barium compound is barium bromide, barium chloride, or barium acetate, and the calcium compound is calcium bromide or calcium chloride, water is added to dissolve it; when the barium compound is barium oxide, barium hydroxide, or barium carbonate, and the calcium compound is calcium oxide, calcium hydroxide, or calcium carbonate, 48% hydrobromic acid is added first to dissolve it, and then water is added to adjust the concentration of barium bromide or calcium bromide. The molar ratio of the barium compound or calcium compound to the hydrobromic acid is 1:2.02-2.1; the hydrobromic acid is industrial grade.

3. The purification method for chromium-containing mixed salts as described in claim 1, characterized in that, In step S3, after adding the chromium removal agent from S2 to the crude sodium bromide solution prepared in step S1, the solution is heated and the temperature is controlled at 50℃ to boiling point; then heating is stopped and the solution is allowed to settle for 20-120 minutes; during filtration separation, one of pressure filtration, vacuum filtration or centrifugation is used.

4. The purification method for chromium-containing mixed salts as described in claim 1, characterized in that, In step S4, the reducing agent is at least one of formaldehyde, hydrazine hydrate, formic acid, sodium formate, oxalic acid, sodium oxalate, ascorbic acid, ferrous salt, sulfite, and sulfide.

5. The purification method for chromium-containing mixed salts as described in claim 4, characterized in that, The reducing agent is one of formaldehyde, hydrazine hydrate, formic acid, sodium formate, oxalic acid, sodium oxalate, or ascorbic acid; wherein the molar ratio of free bromine to formaldehyde or hydrazine hydrate is 1.0:0.54-1.0, and the molar ratio of free bromine to formic acid, sodium formate, oxalic acid, sodium oxalate, or ascorbic acid is 1.0:1.05-2.

0.

6. The purification method for chromium-containing mixed salts as described in claim 1, characterized in that, In step S4, before preparing filtrate B, the pH of the solution is adjusted to 9-12 and allowed to stand for 20-120 minutes.

7. The purification method for chromium-containing mixed salts as described in claim 1, characterized in that, In step S4, hydrobromic acid is added to adjust the pH of the solution to <3.

8. The purification method for chromium-containing mixed salts as described in claim 1, characterized in that, In step S4, the obtained barium carbonate or calcium carbonate is sampled and dissolved in industrial-grade hydrobromic acid and water to detect the chromium content. If no chromium is detected, it is directly returned to process S2 for recycling. If chromium is detected, the barium carbonate or calcium carbonate is dissolved in industrial-grade hydrobromic acid and water, and then the chromium is removed by activated carbon adsorption before being returned to process S2 for recycling.