A method of recovering sodium chromate

Abstract

The present application relates to a kind of methods for recycling chromate.The present application method includes the following steps: (1) the ion exchange resin is used to adsorb the wastewater containing chromium;(2) the resin after adsorption is eluted with alkaline solution, and the eluate is collected;(3) the collected eluate is adjusted to acidic pH, concentrated, and the chromate concentrate is obtained;(4) in the chromate concentrate, add sulfate precipitant, remove sulfate in the form of precipitation;(5) the obtained chromate solution is crystallized, dried;Wherein, the alkaline solution is sodium hydroxide solution, the concentration of the alkaline solution is 20-25%.The present application method can greatly reduce the cost required for evaporation concentration, improve the removal efficiency of sodium sulfate.

Description

Technical Field

[0001] This invention relates to the field of chemistry, and in particular to a method for recovering sodium chromate. Background Technology

[0002] Chromium plating is a common process in metal surface treatment. The process generates a large amount of chromium-containing wastewater, including pickling wastewater, rinsing wastewater, and wastewater from changing the electroplating bath solution. In electroplating industrial parks, chromium-containing wastewater from different electroplating plants is discharged into the park's wastewater treatment plant for unified treatment. The mixed wastewater typically contains 500–1000 mg / L of total chromium (of which hexavalent chromium accounts for approximately 80–90%), a pH value between 2 and 5, and also contains 3000–4000 mg / L of sulfate, 200–400 mg / L of nitrate, 300–500 mg / L of chloride, and 50–200 mg / L of copper, nickel, zinc, and other metal ions.

[0003] Most electroplating industrial parks in China typically use chemical precipitation to treat chromium-containing wastewater. This involves first using reducing agents such as iron powder, sodium sulfite, or ferrous sulfate to reduce hexavalent chromium in the wastewater to trivalent chromium. Then, alkaline agents such as lime or sodium hydroxide are added to adjust the pH to 7-9, converting the trivalent chromium into chromium hydroxide precipitate, while simultaneously removing copper, nickel, zinc, and other metal ions. This method is simple to operate and has a high metal ion removal rate, but it generates a large amount of chromium-containing sludge, which must be disposed of by qualified companies. This not only incurs high sludge disposal fees but also wastes a significant amount of valuable chromium resources.

[0004] To address this issue, some research has been conducted on the resource utilization of chromium-containing electroplating wastewater. For example, patent CN115677090 A describes a method for the resource utilization of chromium-containing electroplating wastewater, which mainly includes filtration to remove impurities, removal of hexavalent chromium using anion exchange resin, removal of trivalent chromium, copper, nickel, zinc, etc. using cation exchange resin, resin regeneration, and then sodium removal from the anion exchange resin solution through sodium ion exchange followed by evaporation and concentration to obtain a chromium plating solution. This solution is then treated with barium carbonate to remove sulfate ions before being reused in the electroplating process. However, the concentration of hexavalent chromium in the regenerated solution obtained by this patent is only 15307 mg / L. Furthermore, the removal efficiency of sulfate in the regenerated solution is low. According to the "Handbook of Chemical and Chemical Engineering Property Data", the solubility of barium carbonate, barium chromate, and barium sulfate in 100g of water at 20℃ is 2.3mg, 0.37mg, and 0.24mg, respectively. However, in actual operation, barium carbonate cannot be completely dissolved quickly. A small amount of dissolved barium ions react with sulfate ions to form barium sulfate, which adheres to the surface of barium carbonate particles and prevents further dissolution. At the same time, since the solubility of barium chromate is not much different from that of barium sulfate, co-precipitation of barium chromate and barium sulfate will occur, resulting in chromium loss. Patent CN 109336284 A describes a method for the deep treatment and recycling of chromium-containing electroplating wastewater. The method mainly includes: removing impurities using a precision filter; passing the wastewater through ion exchange fibers to remove hexavalent and trivalent chromium separately; discharging wastewater with chromium content meeting standards; desorbing the ion exchange fibers saturated with hexavalent chromium using a 4-6% sodium hydroxide solution; regenerating the fibers with a sodium chloride solution to obtain a sodium chromate solution for recycling. The patent does not mention the concentration or impurity content of the sodium chromate solution, but based on the principle of ion exchange, SO42-2020 is likely present. 2- Ion pair Cr2O7 2- or CrO4 2- The competitive adsorption of ions is quite severe, and the sulfate content in the regeneration solution is high, which will significantly affect the purity of the recovered sodium chromate product. Summary of the Invention

[0005] Based on this, the purpose of this invention is to provide a method for recovering sodium chromate with high chromium concentration and low sulfate content in ion exchange regeneration concentrate, which can greatly reduce the cost required for evaporation and concentration and improve the removal efficiency of sodium sulfate.

[0006] The technical solution of the present invention is as follows:

[0007] This invention provides a method for recovering chromate, comprising the following steps:

[0008] (1) Adsorb chromium-containing wastewater using ion exchange resin;

[0009] (2) The resin after adsorption is eluted and regenerated with an alkaline solution, and the eluent is collected;

[0010] (3) Adjust the pH of the collected eluent to acidic and concentrate it to obtain chromate concentrate;

[0011] (4) Add sulfate precipitant to chromate concentrate to remove sulfate by precipitation;

[0012] (5) Crystallize the obtained chromate solution and dry it;

[0013] The alkaline solution is a sodium hydroxide solution with a concentration of 20-25%.

[0014] In some embodiments, the ion exchange resin is a macroporous, strongly basic anion exchange resin.

[0015] In some embodiments, the macroporous strong base anion exchange resin has a skeleton of styrene-divinylbenzene copolymer and a functional group of quaternary ammonium group; preferably, the quaternary ammonium group is (-N(CH3)3OH).

[0016] In some embodiments, the sulfate precipitant includes calcium-containing compounds, aluminum-containing compounds, silicon-containing compounds, and fluorine-containing compounds.

[0017] In some embodiments, the calcium-containing compound is selected from at least one of calcium carbonate, calcium oxide, and calcium hydroxide; the aluminum-containing compound is selected from at least one of aluminum oxide, aluminum hydroxide, sodium aluminate, and sodium hexafluoroaluminate; the silicon-containing compound is selected from at least one of silicon dioxide and fluorosilicic acid; and the fluorine-containing compound is selected from at least one of fluorosilicic acid, sodium fluoride, and sodium hexafluoroaluminate.

[0018] In some embodiments, the aluminum content of the sulfate precipitant and the molar ratio of sulfate in the concentrate are 0.5 to 1.5:1, for example, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1.0:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1. Preferably, it is 0.8-1.5:1, more preferably 0.8-1.2:1, and more preferably 0.9-1.1:1.

[0019] In some embodiments, the molar ratio of silicon content of the sulfate precipitant to sulfate in the concentrate is 0.5–1.5:1, for example, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1.0:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, or 1.5:1. Preferably, it is 0.8–1.5:1, more preferably 0.8–1.2:1, and more preferably 0.9–1.1:1.

[0020] In some embodiments, the molar ratio of calcium content in the sulfate precipitant to sulfate in the concentrate is 2–8:1, for example, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, or 8:1. Preferably, it is 3–6:1, more preferably 3–5:1.

[0021] In some embodiments, the molar ratio of fluorine content of the sulfate precipitant to sulfate in the concentrate is 6–20:1, for example, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, or 20:1. Preferably, it is 10–20:1, and more preferably 10–15:1.

[0022] In some embodiments, the amounts of each element in the sulfate precipitant are as follows: the molar ratio of aluminum to sulfate in the concentrate is 0.5–1.5:1, the molar ratio of silicon to sulfate in the concentrate is 0.5–1.5:1, the molar ratio of calcium to sulfate in the concentrate is 2–8:1, and the molar ratio of fluorine to sulfate in the concentrate is 6–20:1.

[0023] Preferably, the amounts of each element in the sulfate precipitant are as follows: the molar ratio of aluminum to sulfate in the concentrate is 0.8–1.5:1, the molar ratio of silicon to sulfate in the concentrate is 0.8–1.5:1, the molar ratio of calcium to sulfate in the concentrate is 2–8:1, and the molar ratio of fluorine to sulfate in the concentrate is 10–20:1.

[0024] Preferably, the amounts of each element in the sulfate precipitant are as follows: the molar ratio of aluminum to sulfate in the concentrate is 0.9–1.1:1, the molar ratio of silicon to sulfate in the concentrate is 0.9–1.1:1, the molar ratio of calcium to sulfate in the concentrate is 3–5:1, and the molar ratio of fluorine to sulfate in the concentrate is 10–15:1.

[0025] In some embodiments, step (1) includes the following steps: preparing 3-5 ion exchange columns; first, adsorbing chromium-containing wastewater through a first ion exchange column; when the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through a second ion exchange column; when the hexavalent chromium content in the effluent from the second ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through a third ion exchange column, and so on, until the effluent passes through the last ion exchange column, while stopping the feed to the first ion exchange column and eluting and regenerating the first ion exchange column with an alkaline solution; when the hexavalent chromium content in the effluent from the last ion exchange column exceeds 0.01-0.5 mg / L, passing its effluent through the regenerated first ion exchange column, while stopping the feed to the second ion exchange column, and so on, alternating between the two ion exchange columns.

[0026] In some embodiments, step (1) includes the following steps: chromium-containing wastewater is first adsorbed through a first ion exchange column; when the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.01-0.5 mg / L, the effluent is passed through a second ion exchange column; when the hexavalent chromium content in the effluent from the second ion exchange column exceeds 0.01-0.5 mg / L, the effluent is passed through a third ion exchange column; when the hexavalent chromium content in the effluent from the third ion exchange column exceeds 0.01-0.5 mg / L, the effluent is passed through a fourth ion exchange column, while the feed to the first ion exchange column is stopped, and the first ion exchange column is eluted and regenerated with an alkaline solution; when the hexavalent chromium content in the effluent from the fourth ion exchange column exceeds 0.01-0.5 mg / L, its effluent is passed through the regenerated first ion exchange column, while the feed to the second ion exchange column is stopped, and the regeneration is repeated alternately.

[0027] Preferably, when the hexavalent chromium content of the effluent exceeds 0.05-0.15 mg / L, the effluent is passed through a subsequent ion exchange column.

[0028] Preferably, when the hexavalent chromium content of the effluent exceeds 0.1 mg / L, the effluent is passed through a subsequent ion exchange column.

[0029] In some embodiments, step (2) includes the following steps: regenerating and eluting the saturated ion exchange resin with an alkaline solution of 1.5-2.5 times the resin volume, and washing with water of 1.5-2.5 times the resin volume, and collecting the eluent and washing solution.

[0030] Preferably, step (2) includes the following steps: regenerating and eluting the adsorbed ion exchange resin with an alkaline solution of 1.8-2.2 times the resin volume, and washing with water of 1.8-2.2 times the resin volume, and collecting the eluent and washing solution. Preferably, step (2) includes the following steps: regenerating and eluting the adsorbed ion exchange resin with an alkaline solution of 2 times the resin volume, and washing with water of 2 times the resin volume, and collecting the eluent and washing solution.

[0031] In some embodiments, the collection of the eluent and washing solution includes the following steps:

[0032] First, 20-30% of the eluent is used as the first eluent and mixed into the original chromium-containing wastewater for re-adsorption treatment.

[0033] The 40-60% volume of eluent obtained after elution is then used as the second eluent for subsequent concentration.

[0034] The remaining eluent and the 70-80% washing solution are then used as the third eluent, which is used as a substitute for alkali solution for subsequent resin elution.

[0035] The final 20-30% volume of the washing solution is used as the fourth eluent and as a solvent to prepare the alkaline solution.

[0036] In some embodiments, the collection of the eluent and washing solution includes the following steps:

[0037] The first 22-27% volume of eluent (about 1 / 2 the volume of resin) is used as the first eluent and mixed into the original chromium-containing wastewater for re-adsorption treatment.

[0038] Then, the 45-55% volume of eluent obtained after elution (approximately 1 resin volume) is used as the second eluent for subsequent concentration.

[0039] The remaining eluent (approximately 1 / 2 resin volume) and the 72-77% washing solution (approximately 3 / 2 resin volume) were then used as the third eluent, serving as a substitute for the alkali solution for subsequent resin elution.

[0040] The final 22-27% volume of the washing solution (approximately 1 / 2 the volume of the resin) is used as the fourth eluent and as a solvent to prepare the alkaline solution.

[0041] In some embodiments, the collection of the eluent and washing solution includes the following steps:

[0042] The first 25% volume of eluent obtained from elution is used as the first eluent and mixed into the original chromium-containing wastewater for re-adsorption treatment.

[0043] The 50% volume of eluent obtained after elution is then used as the second eluent for subsequent concentration.

[0044] The remaining eluent and the 75% washing solution were then used as the third eluent, which was used as a substitute for the alkali solution for subsequent resin elution.

[0045] The final 25% volume of the washing solution was used as the fourth eluent and as a solvent to prepare the alkaline solution.

[0046] In some embodiments, step (3) concentration includes: adjusting the pH of the obtained second eluent to 1-4 with chromic acid, and then evaporating and concentrating it to obtain a sodium chromate concentrate with a hexavalent chromium content of 300-600 g / L. Preferably, the pH is adjusted to 2-4. Preferably, a sodium chromate concentrate with a hexavalent chromium content of 400-500 g / L is obtained.

[0047] In some embodiments, the chromium-containing wastewater used in step (1) is first filtered and then adsorbed using an ion exchange resin.

[0048] In some embodiments, the chromium-containing wastewater passes through the ion exchange column at a rate of 10–20 BV / h.

[0049] In some embodiments, the crystallization in step (5) includes: evaporating and crystallizing a sodium chromate solution to obtain a sodium chromate slurry with a hexavalent chromium content of 800-900 g / L, cooling and crystallizing, separating the wet sodium chromate product, mixing the centrifuged mother liquor into the second eluent, and continuing the subsequent processing steps.

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

[0051] (1) The present invention uses a sodium hydroxide solution with a concentration of 20-25% for elution. In particular, combined with the macroporous strong base anion exchange resin of the present invention, an eluent with a sodium chromate concentration of more than 100 g / L is obtained for the first time, which greatly reduces the cost required for concentration.

[0052] (2) In addition, the present invention uses a sulfate precipitant with a specific formulation for the first time, which can react with sulfate to undergo a double salt precipitation reaction and does not react with chromate ions. Without causing chromium loss, the present invention can achieve efficient removal of sulfate ions from sodium chromate concentrate, overcoming the defects of high sulfate content or serious chromium loss in the prior art. Attached Figure Description

[0053] Figure 1 The sodium chromate lyophilized powder and crystalline sodium chromate prepared in Example 1 are shown. Detailed Implementation

[0054] To facilitate understanding of the present invention, a more complete description will be given below with reference to embodiments, of which preferred embodiments are provided. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided so that a thorough and complete understanding of the disclosure of the present invention will be achieved.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0056] The present invention will be further described in detail below with reference to specific embodiments.

[0057] The raw materials used in the following examples are all commercially available or prepared by conventional methods.

[0058] Mixed wastewater typically contains 500–1000 mg / L of total chromium (of which hexavalent chromium accounts for about 80–90%), a pH value between 2 and 5, and also contains 3000–4000 mg / L of sulfate, 200–400 mg / L of nitrate, 300–500 mg / L of chloride, and 50–200 mg / L of copper, nickel, zinc and other metal ions.

[0059] Example 1

[0060] A method for recovering industrial-grade sodium chromate from chromium-containing electroplating wastewater includes the following steps:

[0061] (1) Ion exchange resin adsorption of hexavalent chromium: The electroplating wastewater containing chromium is pumped into a filter press to filter out suspended solids and solid particles.

[0062] The filtered and impurity-removed chromium-containing electroplating wastewater is then fed through four sets of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong-base anion exchange resin, model D201) at a feed rate of 10-20 BV / h. The wastewater first enters the first ion exchange column for adsorption. When the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.1 mg / L, the effluent is pumped into the second ion exchange column. Similarly, when the hexavalent chromium content in the effluent from the second and third ion exchange columns exceeds 0.1 mg / L, the effluent is pumped into the third and fourth ion exchange columns. When the wastewater enters the fourth ion exchange column, the feed to the first ion exchange column is stopped, and it is eluted and regenerated. When the hexavalent chromium content in the effluent from the fourth ion exchange column exceeds 0.1 mg / L, it is pumped into the regenerated first ion exchange column, and the feed to the second ion exchange column is stopped. This alternating regeneration process continues. The effluent is then discharged into a wastewater treatment plant for further treatment.

[0063] (2) Resin elution and regeneration: The saturated resin is regenerated using a 20-25% sodium hydroxide solution (twice the resin volume). The resin bed is then washed with water (twice the resin volume) until the effluent becomes colorless. The eluent and washing solution are collected separately. Specifically, the first eluent (the residual liquid in the resin bed after being replaced by the alkali solution, yellow in color, about 1 / 2 the resin volume) is collected in the first collection tank and mixed with the original water for re-adsorption treatment; the second eluent (the high-concentration chromium-containing waste liquid eluted, orange-red in color, about 1 the resin volume or with a neutral pH in the eluent) is collected in the first collection tank. The first eluent (the eluent with a relatively high concentration of hexavalent chromium, bright yellow in color, about twice the volume of resin) is collected in the second collection tank and used as a substitute for alkali solution for subsequent resin elution; the second eluent (the eluent with a relatively low concentration of hexavalent chromium, yellow in color, about half the volume of resin, and the elution stops when the color of the eluent becomes colorless) is collected in the fourth collection tank and used to dissolve caustic soda flakes or as a dilution solution for 30-50% liquid alkali to prepare a 20-25% sodium hydroxide solution.

[0064] (3) Evaporation and concentration: The pH of the collected second eluent is adjusted to 2-4 with chromic acid, and then pumped into an evaporator for evaporation and concentration to obtain a sodium chromate concentrate with a hexavalent chromium content of 400-500 g / L.

[0065] (4) Sulfate removal: Sulfate removal agent (calcium oxide, silicon dioxide, aluminum oxide, sodium fluoride, in which the molar ratio of aluminum and silicon content to sulfate is 1:1, the molar ratio of calcium content to sulfate is 4:1, and the molar ratio of fluoride content to sulfate is 14:1) is added to sodium chromate concentrate to remove sulfate in the form of double salt precipitation;

[0066] (5) Evaporation, crystallization and drying: The sodium chromate solution with most of the sulfate removed is pumped into the evaporator for evaporation and crystallization to obtain sodium chromate slurry with a hexavalent chromium content of 800-900 g / L. The sodium chromate wet product is obtained by cooling crystallization and centrifugation separation. The centrifugation mother liquor is mixed into the second collection tank for subsequent processing steps.

[0067] The wet sodium chromate was dried to obtain a sodium chromate product that meets the requirements of the standard "HG / T 4312-2012 Industrial Sodium Chromate".

[0068] Example 2

[0069] A method for recovering industrial-grade sodium chromate from chromium-containing electroplating wastewater includes the following steps:

[0070] (1) Ion exchange resin adsorption of hexavalent chromium: The electroplating wastewater containing chromium is pumped into a filter press to filter out suspended solids and solid particles.

[0071] The filtered and impurity-removed chromium-containing electroplating wastewater is then fed through four sets of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong-base anion exchange resin, model Lanxess MonoPlus MP500) at a feed rate of 10-20 BV / h. The wastewater first enters the first ion exchange column for adsorption. When the hexavalent chromium content of the effluent from the first ion exchange column exceeds 0.1 mg / L, the effluent is pumped into the second ion exchange column. Similarly, when the hexavalent chromium content of the effluent from the second and third ion exchange columns exceeds 0.1 mg / L, the effluent is pumped into the third and fourth ion exchange columns. When the wastewater enters the fourth ion exchange column, the feed to the first ion exchange column is stopped, and it is eluted and regenerated. When the hexavalent chromium content of the effluent from the fourth ion exchange column exceeds 0.1 mg / L, it is pumped into the regenerated first ion exchange column, and the feed to the second ion exchange column is stopped. This alternating regeneration process continues. The effluent is discharged into the wastewater treatment plant for further treatment.

[0072] (2) Resin elution and regeneration: The saturated resin is regenerated using a 20-25% sodium hydroxide solution (twice the resin volume). The resin bed is then washed with water (twice the resin volume) until the effluent becomes colorless. The eluent and washing solution are collected separately. Specifically, the first eluent (the residual liquid in the resin bed after being replaced by the alkali solution, yellow in color, about 1 / 2 the resin volume) is collected in the first collection tank and mixed with the original water for re-adsorption treatment; the second eluent (the high-concentration chromium-containing waste liquid eluted, orange-red in color, about 1 the resin volume or with a neutral pH in the eluent) is collected in the first collection tank. The first eluent (the eluent with a relatively high concentration of hexavalent chromium, bright yellow in color, about twice the volume of resin) is collected in the second collection tank and used as a substitute for alkali solution for subsequent resin elution; the second eluent (the eluent with a relatively low concentration of hexavalent chromium, yellow in color, about half the volume of resin, and the elution stops when the color of the eluent becomes colorless) is collected in the fourth collection tank and used to dissolve caustic soda flakes or as a dilution solution for 30-50% liquid alkali to prepare a 20-25% sodium hydroxide solution.

[0073] (3) Evaporation and concentration: The pH of the collected second eluent is adjusted to 2-4 with chromic acid, and then pumped into an evaporator for evaporation and concentration to obtain a sodium chromate concentrate with a hexavalent chromium content of 400-500 g / L.

[0074] (4) Sulfate removal: Sulfate removal agent (calcium oxide, silicon dioxide, aluminum oxide, sodium fluoride, the molar ratio of aluminum and silicon content to sulfate in the sulfate removal agent is 0.9:1, the molar ratio of calcium content to sulfate is 4.5:1, and the molar ratio of fluoride content to sulfate is 13:1) is added to sodium chromate concentrate to remove sulfate in the form of double salt precipitation;

[0075] (5) Evaporation, crystallization and drying: The sodium chromate solution with most of the sulfate removed is pumped into the evaporator for evaporation and crystallization to obtain sodium chromate slurry with a hexavalent chromium content of 800-900 g / L. The sodium chromate wet product is obtained by cooling crystallization and centrifugation separation. The centrifugation mother liquor is mixed into the second collection tank for subsequent processing steps.

[0076] The wet sodium chromate was dried to obtain a sodium chromate product that meets the requirements of the standard "HG / T 4312-2012 Industrial Sodium Chromate".

[0077] Example 3

[0078] A method for recovering industrial-grade sodium chromate from chromium-containing electroplating wastewater includes the following steps:

[0079] (1) Ion exchange resin adsorption of hexavalent chromium: The electroplating wastewater containing chromium is pumped into a filter press to filter out suspended solids and solid particles.

[0080] The filtered and impurity-removed chromium-containing electroplating wastewater is then fed through four sets of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong-base anion exchange resin, model D296) at a feed rate of 10-20 BV / h. The wastewater first enters the first ion exchange column for adsorption. When the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.1 mg / L, the effluent is pumped into the second ion exchange column. Similarly, when the hexavalent chromium content in the effluent from the second and third ion exchange columns exceeds 0.1 mg / L, the effluent is pumped into the third and fourth ion exchange columns. When the wastewater enters the fourth ion exchange column, the feed to the first ion exchange column is stopped, and it is eluted and regenerated. When the hexavalent chromium content in the effluent from the fourth ion exchange column exceeds 0.1 mg / L, it is pumped into the regenerated first ion exchange column, and the feed to the second ion exchange column is stopped. This alternating regeneration process continues. The effluent is then discharged into the wastewater treatment plant for further treatment.

[0081] (2) Resin elution and regeneration: The saturated resin is regenerated using a 20-25% sodium hydroxide solution (twice the resin volume). The resin bed is then washed with water (twice the resin volume) until the effluent becomes colorless. The eluent and washing solution are collected separately. Specifically, the first eluent (the residual liquid in the resin bed after being replaced by the alkali solution, yellow in color, about 1 / 2 the resin volume) is collected in the first collection tank and mixed with the original water for re-adsorption treatment; the second eluent (the high-concentration chromium-containing waste liquid eluted, orange-red in color, about 1 the resin volume or with a neutral pH in the eluent) is collected in the first collection tank. The first eluent (the eluent with a relatively high concentration of hexavalent chromium, bright yellow in color, about twice the volume of resin) is collected in the second collection tank and used as a substitute for alkali solution for subsequent resin elution; the second eluent (the eluent with a relatively low concentration of hexavalent chromium, yellow in color, about half the volume of resin, and the elution stops when the color of the eluent becomes colorless) is collected in the fourth collection tank and used to dissolve caustic soda flakes or as a dilution solution for 30-50% liquid alkali to prepare a 20-25% sodium hydroxide solution.

[0082] (3) Evaporation and concentration: The pH of the collected second eluent is adjusted to 2-4 with chromic acid, and then pumped into an evaporator for evaporation and concentration to obtain a sodium chromate concentrate with a hexavalent chromium content of 400-500 g / L.

[0083] (4) Sulfate removal: Sulfate removal agent (calcium oxide, silicon dioxide, aluminum oxide, sodium fluoride, in which the molar ratio of aluminum and silicon content to sulfate is 1.1:1, the molar ratio of calcium content to sulfate is 3.5:1, and the molar ratio of fluoride content to sulfate is 15:1) is added to sodium chromate concentrate to remove sulfate in the form of double salt precipitation;

[0084] (5) Evaporation, crystallization and drying: The sodium chromate solution with most of the sulfate removed is pumped into the evaporator for evaporation and crystallization to obtain sodium chromate slurry with a hexavalent chromium content of 800-900 g / L. The sodium chromate wet product is obtained by cooling crystallization and centrifugation separation. The centrifugation mother liquor is mixed into the second collection tank for subsequent processing steps.

[0085] The wet sodium chromate was dried to obtain a sodium chromate product that meets the requirements of the standard "HG / T 4312-2012 Industrial Sodium Chromate".

[0086] Example 4

[0087] A method for recovering industrial-grade sodium chromate from chromium-containing electroplating wastewater includes the following steps:

[0088] (1) Ion exchange resin adsorption of hexavalent chromium: The electroplating wastewater containing chromium is pumped into a filter press to filter out suspended solids and solid particles.

[0089] The filtered and impurity-removed chromium-containing electroplating wastewater is then fed through four sets of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong-base anion exchange resin, model D201) at a feed rate of 10-20 BV / h. The wastewater first enters the first ion exchange column for adsorption. When the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.1 mg / L, the effluent is pumped into the second ion exchange column. Similarly, when the hexavalent chromium content in the effluent from the second and third ion exchange columns exceeds 0.1 mg / L, the effluent is pumped into the third and fourth ion exchange columns. When the wastewater enters the fourth ion exchange column, the feed to the first ion exchange column is stopped, and it is eluted and regenerated. When the hexavalent chromium content in the effluent from the fourth ion exchange column exceeds 0.1 mg / L, it is pumped into the regenerated first ion exchange column, and the feed to the second ion exchange column is stopped. This alternating regeneration process continues. The effluent is then discharged into a wastewater treatment plant for further treatment.

[0090] (2) Resin elution and regeneration: The saturated resin is regenerated using a 20-25% sodium hydroxide solution (twice the resin volume). The resin bed is then washed with water (twice the resin volume) until the effluent becomes colorless. The eluent and washing solution are collected separately. Specifically, the first eluent (the residual liquid in the resin bed after being replaced by the alkali solution, yellow in color, about 1 / 2 the resin volume) is collected in the first collection tank and mixed with the original water for re-adsorption treatment; the second eluent (the high-concentration chromium-containing waste liquid eluted, orange-red in color, about 1 the resin volume or with a neutral pH in the eluent) is collected in the first collection tank. The first eluent (the eluent with a relatively high concentration of hexavalent chromium, bright yellow in color, about twice the volume of resin) is collected in the second collection tank and used as a substitute for alkali solution for subsequent resin elution; the second eluent (the eluent with a relatively low concentration of hexavalent chromium, yellow in color, about half the volume of resin, and the elution stops when the color of the eluent becomes colorless) is collected in the fourth collection tank and used to dissolve caustic soda flakes or as a dilution solution for 30-50% liquid alkali to prepare a 20-25% sodium hydroxide solution.

[0091] (3) Evaporation and concentration: The pH of the collected second eluent is adjusted to 2-4 with chromic acid, and then pumped into an evaporator for evaporation and concentration to obtain a sodium chromate concentrate with a hexavalent chromium content of 400-500 g / L.

[0092] (4) Sulfate removal: Sulfate removal agent (calcium carbonate, silicon dioxide, aluminum hydroxide, sodium fluoride, in which the molar ratio of aluminum and silicon content to sulfate is 0.8:1, the molar ratio of calcium content to sulfate is 6:1, and the molar ratio of fluoride content to sulfate is 12:1) is added to sodium chromate concentrate to remove sulfate in the form of double salt precipitation;

[0093] (5) Evaporation, crystallization and drying: The sodium chromate solution with most of the sulfate removed is pumped into the evaporator for evaporation and crystallization to obtain sodium chromate slurry with a hexavalent chromium content of 800-900 g / L. The sodium chromate wet product is obtained by cooling crystallization and centrifugation separation. The centrifugation mother liquor is mixed into the second collection tank for subsequent processing steps.

[0094] The wet sodium chromate was dried to obtain a sodium chromate product that meets the requirements of the standard "HG / T 4312-2012 Industrial Sodium Chromate".

[0095] Example 5

[0096] A method for recovering industrial-grade sodium chromate from chromium-containing electroplating wastewater includes the following steps:

[0097] (1) Ion exchange resin adsorption of hexavalent chromium: The electroplating wastewater containing chromium is pumped into a filter press to filter out suspended solids and solid particles.

[0098] The filtered and impurity-removed chromium-containing electroplating wastewater is then fed through four sets of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong-base anion exchange resin, model D201) at a feed rate of 10-20 BV / h. The wastewater first enters the first ion exchange column for adsorption. When the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.1 mg / L, the effluent is pumped into the second ion exchange column. Similarly, when the hexavalent chromium content in the effluent from the second and third ion exchange columns exceeds 0.1 mg / L, the effluent is pumped into the third and fourth ion exchange columns. When the wastewater enters the fourth ion exchange column, the feed to the first ion exchange column is stopped, and it is eluted and regenerated. When the hexavalent chromium content in the effluent from the fourth ion exchange column exceeds 0.1 mg / L, it is pumped into the regenerated first ion exchange column, and the feed to the second ion exchange column is stopped. This alternating regeneration process continues. The effluent is then discharged into a wastewater treatment plant for further treatment.

[0099] (2) Resin elution and regeneration: The saturated resin is regenerated using a 20-25% sodium hydroxide solution (twice the resin volume). The resin bed is then washed with water (twice the resin volume) until the effluent becomes colorless. The eluent and washing solution are collected separately. Specifically, the first eluent (the residual liquid in the resin bed after being replaced by the alkali solution, yellow in color, about 1 / 2 the resin volume) is collected in the first collection tank and mixed with the original water for re-adsorption treatment; the second eluent (the high-concentration chromium-containing waste liquid eluted, orange-red in color, about 1 the resin volume or with a neutral pH in the eluent) is collected in the first collection tank. The first eluent (the eluent with a relatively high concentration of hexavalent chromium, bright yellow in color, about twice the volume of resin) is collected in the second collection tank and used as a substitute for alkali solution for subsequent resin elution; the second eluent (the eluent with a relatively low concentration of hexavalent chromium, yellow in color, about half the volume of resin, and the elution stops when the color of the eluent becomes colorless) is collected in the fourth collection tank and used to dissolve caustic soda flakes or as a dilution solution for 30-50% liquid alkali to prepare a 20-25% sodium hydroxide solution.

[0100] (3) Evaporation and concentration: The pH of the collected second eluent is adjusted to 2-4 with chromic acid, and then pumped into an evaporator for evaporation and concentration to obtain a sodium chromate concentrate with a hexavalent chromium content of 400-500 g / L.

[0101] (4) Sulfate removal: Sulfate removal agent (calcium hydroxide, fluorosilicic acid, sodium aluminate, sodium fluoride, the molar ratio of aluminum and silicon content to sulfate in the sulfate removal agent is 1.2:1, the molar ratio of calcium content to sulfate is 4:1, and the molar ratio of fluorine content to sulfate is 10:1) is added to sodium chromate concentrate to remove sulfate in the form of double salt precipitation;

[0102] (5) Evaporation, crystallization and drying: The sodium chromate solution with most of the sulfate removed is pumped into the evaporator for evaporation and crystallization to obtain sodium chromate slurry with a hexavalent chromium content of 800-900 g / L. The sodium chromate wet product is obtained by cooling crystallization and centrifugation separation. The centrifugation mother liquor is mixed into the second collection tank for subsequent processing steps.

[0103] The wet sodium chromate was dried to obtain a sodium chromate product that meets the requirements of the standard "HG / T 4312-2012 Industrial Sodium Chromate".

[0104] Example 6

[0105] A method for recovering industrial-grade sodium chromate from chromium-containing electroplating wastewater includes the following steps:

[0106] (1) Ion exchange resin adsorption of hexavalent chromium: The electroplating wastewater containing chromium is pumped into a filter press to filter out suspended solids and solid particles.

[0107] The filtered and impurity-removed chromium-containing electroplating wastewater is then fed through four sets of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong-base anion exchange resin, model D201) at a feed rate of 10-20 BV / h. The wastewater first enters the first ion exchange column for adsorption. When the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.1 mg / L, the effluent is pumped into the second ion exchange column. Similarly, when the hexavalent chromium content in the effluent from the second and third ion exchange columns exceeds 0.1 mg / L, the effluent is pumped into the third and fourth ion exchange columns. When the wastewater enters the fourth ion exchange column, the feed to the first ion exchange column is stopped, and it is eluted and regenerated. When the hexavalent chromium content in the effluent from the fourth ion exchange column exceeds 0.1 mg / L, it is pumped into the regenerated first ion exchange column, and the feed to the second ion exchange column is stopped. This alternating regeneration process continues. The effluent is then discharged into a wastewater treatment plant for further treatment.

[0108] (2) Resin elution and regeneration: The saturated resin is regenerated using a 20-25% sodium hydroxide solution (twice the resin volume). The resin bed is then washed with water (twice the resin volume) until the effluent becomes colorless. The eluent and washing solution are collected separately. Specifically, the first eluent (the residual liquid in the resin bed after being replaced by the alkali solution, yellow in color, about 1 / 2 the resin volume) is collected in the first collection tank and mixed with the original water for re-adsorption treatment; the second eluent (the high-concentration chromium-containing waste liquid eluted, orange-red in color, about 1 the resin volume or with a neutral pH in the eluent) is collected in the first collection tank. The first eluent (the eluent with a relatively high concentration of hexavalent chromium, bright yellow in color, about twice the volume of resin) is collected in the second collection tank and used as a substitute for alkali solution for subsequent resin elution; the second eluent (the eluent with a relatively low concentration of hexavalent chromium, yellow in color, about half the volume of resin, and the elution stops when the color of the eluent becomes colorless) is collected in the fourth collection tank and used to dissolve caustic soda flakes or as a dilution solution for 30-50% liquid alkali to prepare a 20-25% sodium hydroxide solution.

[0109] (3) Evaporation and concentration: The pH of the collected second eluent is adjusted to 2-4 with chromic acid, and then pumped into an evaporator for evaporation and concentration to obtain a sodium chromate concentrate with a hexavalent chromium content of 400-500 g / L.

[0110] (4) Sulfate removal: Sulfate removal agent (calcium hydroxide, silicon dioxide, sodium hexafluoroaluminate, sodium fluoride, the molar ratio of aluminum and silicon content to sulfate in the sulfate removal agent is 1.1:1, the molar ratio of calcium content to sulfate is 3:1, and the molar ratio of fluorine content to sulfate is 16:1) is added to sodium chromate concentrate to remove sulfate in the form of double salt precipitation;

[0111] (5) Evaporation, crystallization and drying: The sodium chromate solution with most of the sulfate removed is pumped into the evaporator for evaporation and crystallization to obtain sodium chromate slurry with a hexavalent chromium content of 800-900 g / L. The sodium chromate wet product is obtained by cooling crystallization and centrifugation separation. The centrifugation mother liquor is mixed into the second collection tank for subsequent processing steps.

[0112] The wet sodium chromate was dried to obtain a sodium chromate product that meets the requirements of the standard "HG / T 4312-2012 Industrial Sodium Chromate".

[0113] Example 7

[0114] The difference from Example 1 is that macroporous weakly basic resin D310 is used.

[0115] Example 8

[0116] The difference from Example 1 is that the concentration of the sodium hydroxide solution is 15%.

[0117] Example 9

[0118] The difference from Example 1 is that the concentration of sodium hydroxide is 30%. Because the concentration of sodium hydroxide is too high, the density of the sodium hydroxide solution is greater than the density of the resin. The resin floats on the sodium hydroxide solution, making it difficult to complete resin adsorption and regeneration elution.

[0119] Comparative Example 1

[0120] The difference from Example 1 is that no sulfate precipitant is added.

[0121] Comparative Example 2

[0122] The difference from Example 1 is that barium carbonate is used instead of sulfate precipitant, and the molar ratio of barium carbonate to sulfate is 1:1.

[0123] The concentrations of hexavalent chromium in the second eluent of the above embodiments and comparative examples, as well as the concentrations of sulfate in the concentrates before and after sulfate removal, are detailed in Table 1. The ion detection methods of this invention all employ conventional detection methods. The hexavalent chromium concentration detection method is the ferrous ammonium sulfate titration method. The sulfate concentration detection method is the gravimetric method as specified in HG / T4312-2012 Industrial Sodium Chromate.

[0124] Table 1

[0125]

[0126] The results are shown in Table 1. The method described in this invention can directly regenerate a sodium chromate solution with a hexavalent chromium content exceeding 90 g / L, significantly reducing the cost of subsequent evaporation and concentration. Simultaneously, a unique sulfate ion precipitant is used, which, upon addition to the sodium chromate concentrate, forms a double salt precipitate with the sulfate ions in the concentrate, achieving efficient removal of sulfate ions from the concentrate without loss of chromium.

[0127] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0128] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for recovering chromate, wherein, Includes the following steps: (1) Adsorb chromium-containing wastewater using ion exchange resin; (2) The resin after adsorption is eluted and regenerated with an alkaline solution, and the eluent is collected; (3) After adjusting the pH of the collected eluent to 1-4, concentrate it to obtain chromate concentrate; (4) Add sulfate precipitant to chromate concentrate to remove sulfate by precipitation; (5) Crystallize the obtained chromate solution and dry it; The alkaline solution is a sodium hydroxide solution with a concentration of 20-25%. The ion exchange resin is a macroporous, strongly basic anion exchange resin. The sulfate precipitant comprises: a calcium-containing compound, an aluminum-containing compound, a silicon-containing compound, and a fluorine-containing compound; the amounts of each element in the sulfate precipitant are as follows: the molar ratio of aluminum to sulfate in the concentrate is 0.5~1.5:1, the molar ratio of silicon to sulfate in the concentrate is 0.5~1.5:1, the molar ratio of calcium to sulfate in the concentrate is 2~8:1, and the molar ratio of fluorine to sulfate in the concentrate is 6~20:

1. Step (2) includes the following steps: regenerating and eluting the adsorbed ion exchange resin with an alkaline solution of 1.5-2.5 times the resin volume, then washing it with water of 1.5-2.5 times the resin volume, and collecting the eluent and washing solution; the collection of the eluent and washing solution includes the following steps: first, 20-30% of the eluent volume is used as the first eluent and mixed into the original chromium-containing wastewater for re-adsorption treatment; then, 40-60% of the eluent volume is used as the second eluent for subsequent concentration; then, the remaining eluent volume and 70-80% of the washing solution are used as the third eluent as an alkaline solution substitute for subsequent resin elution; finally, 20-30% of the washing solution is used as the fourth eluent as a solvent for preparing the alkaline solution.

2. The method according to claim 1, wherein, The macroporous strong base anion exchange resin has a skeleton of styrene-divinylbenzene copolymer and a functional group of quaternary ammonium group.

3. The method according to claim 2, wherein, The quaternary ammonium group is (-N(CH3)3OH).

4. The method according to claim 1, wherein, The calcium-containing compound is selected from at least one of calcium carbonate, calcium oxide, and calcium hydroxide; the aluminum-containing compound is selected from at least one of aluminum oxide, aluminum hydroxide, sodium aluminate, and sodium hexafluoroaluminate; the silicon-containing compound is selected from at least one of silicon dioxide and fluorosilicic acid; and the fluorine-containing compound is selected from at least one of fluorosilicic acid, sodium fluoride, and sodium hexafluoroaluminate.

5. The method according to claim 1, wherein, The amounts of each element in the sulfate precipitant are as follows: the molar ratio of aluminum to sulfate in the concentrate is 0.8~1.5:1, the molar ratio of silicon to sulfate in the concentrate is 0.8~1.5:1, the molar ratio of calcium to sulfate in the concentrate is 2~8:1, and the molar ratio of fluorine to sulfate in the concentrate is 10~20:

1.

6. The method according to claim 5, wherein, The amounts of each element in the sulfate precipitant are as follows: the molar ratio of aluminum to sulfate in the concentrate is 0.9~1.1:1, the molar ratio of silicon to sulfate in the concentrate is 0.9~1.1:1, the molar ratio of calcium to sulfate in the concentrate is 3~5:1, and the molar ratio of fluorine to sulfate in the concentrate is 10~15:

1.

7. The method according to claim 1, wherein, Step (1) includes the following steps: Prepare 3-5 ion exchange columns; first, adsorb the chromium-containing wastewater through the first ion exchange column; when the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.01-0.5 mg / L, pass the effluent through the second ion exchange column; when the hexavalent chromium content in the effluent from the second ion exchange column exceeds 0.01-0.5 mg / L, pass the effluent through the third ion exchange column, and so on, until the effluent passes through the last ion exchange column. At the same time, stop feeding the first ion exchange column and regenerate the first ion exchange column with an alkaline solution; when the hexavalent chromium content in the effluent from the last ion exchange column exceeds 0.01-0.5 mg / L, pass its effluent through the regenerated first ion exchange column, and at the same time stop feeding the second ion exchange column, and so on, alternating between the two ion exchange columns.

8. The method according to claim 7, wherein, Step (1) includes the following steps: chromium-containing wastewater is first adsorbed through a first ion exchange column. When the hexavalent chromium content in the effluent from the first ion exchange column exceeds 0.01-0.5 mg / L, the effluent is passed through a second ion exchange column. When the hexavalent chromium content in the effluent from the second ion exchange column exceeds 0.01-0.5 mg / L, the effluent is passed through a third ion exchange column. When the hexavalent chromium content in the effluent from the third ion exchange column exceeds 0.01-0.5 mg / L, the effluent is passed through a fourth ion exchange column. At the same time, the feed to the first ion exchange column is stopped, and the first ion exchange column is eluted and regenerated with an alkaline solution. When the hexavalent chromium content in the effluent from the fourth ion exchange column exceeds 0.01-0.5 mg / L, its effluent is passed through the regenerated first ion exchange column. At the same time, the feed to the second ion exchange column is stopped, and the regeneration is carried out alternately.

9. The method according to any one of claims 1-8, wherein, Step (3) involves evaporation and concentration to obtain a sodium chromate concentrate with a hexavalent chromium content of 300-600 g / L.

10. The method according to any one of claims 1-8, wherein, The chromium-containing wastewater used in step (1) is first filtered and then adsorbed using ion exchange resin.

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