Method for recovering sodium chromate

Abstract

The invention relates to a method for recovering chromate. The method comprises the following steps: (1) adsorbing the chromium-containing wastewater by using ion exchange resin; (2) eluting and regenerating the adsorbed resin by using an alkaline solution, and collecting an eluent; (3) adjusting the pH value of the collected eluent to be acidic, and concentrating to obtain a chromate concentrated solution; (4) adding a sulfate precipitator into the chromate concentrated solution, and removing sulfate in a precipitation manner; (5) crystallizing and drying the obtained chromate solution; wherein the alkali solution is a sodium hydroxide solution, and the concentration of the alkali solution is 20-25%. According to the method, the cost required by evaporation and concentration can be greatly reduced, and the removal efficiency of sodium sulfate is improved.

Description

Technical Field

[0001] The present invention relates to the field of chemistry, in particular to a method for recovering sodium chromate. Background Art

[0002] Chromium plating is a common process in metal surface treatment. A large amount of chromium-containing wastewater is generated during the chromium plating process, including pickling wastewater of plated parts, rinsing wastewater, and wastewater generated by changing the electroplating tank liquid. In the electroplating industrial park, the chromium-containing wastewater generated by different electroplating plants will be discharged to the park's wastewater treatment station for treatment. The mixed wastewater generally contains 500-1000 mg / L of total chromium (of which hexavalent chromium accounts for about 80-90%), with a pH value between 2 and 5, and 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 metal ions such as copper, nickel, and zinc.

[0003] Wastewater treatment plants in most domestic electroplating industrial parks typically use chemical precipitation to treat this type of chromium-containing wastewater. This involves first reducing the hexavalent chromium in the wastewater to trivalent chromium using reducing agents such as iron powder, sodium sulfite, or ferrous sulfate. Alkaline agents such as lime or sodium hydroxide are then added to adjust the pH to 7-9, converting the trivalent chromium into chromium hydroxide precipitate while simultaneously removing metal ions such as copper, nickel, and zinc. This method is simple to operate and offers a high metal ion removal rate, but it produces a large amount of chromium-containing sludge, which must be handled by qualified companies. This not only incurs high sludge disposal fees but also wastes valuable chromium resources.

[0004] To address this issue, some research is currently underway on the resource recovery of chromium-containing electroplating wastewater. For example, patent CN115677090 A describes a method for resource recovery of chromium-containing electroplating wastewater. The method involves filtering and removing impurities, using an anionic resin to remove hexavalent chromium, using a cationic resin to remove trivalent chromium, copper, nickel, and zinc, etc., followed by resin regeneration. The anionic resin regeneration solution is then de-sodiumized via sodium ion exchange and concentrated by evaporation to produce a chromium plating solution. The sulfate ions are then removed using barium carbonate before being reused in the electroplating process. However, the hexavalent chromium concentration in the regenerated solution obtained in this patent is only 15,307 mg / L. In addition, the removal efficiency of sulfate in the regeneration liquid is low. According to the "Chemical and Chemical Property Data Handbook" under 20°C conditions, the solubility of barium carbonate, barium chromate and barium sulfate in 100g of water are 2.3mg, 0.37mg and 0.24mg respectively. However, in actual operation, barium carbonate cannot be quickly dissolved completely. A small amount of dissolved barium ions react with sulfate ions to form barium sulfate, which will adhere to the surface of barium carbonate particles and prevent 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 introduces a method for deep treatment and recycling of electroplating chromium-containing wastewater, which mainly includes removing impurities with precision filters, removing hexavalent chromium and trivalent chromium respectively through ion exchange fibers, discharging wastewater with chromium content meeting the standard, desorbing the ion exchange fibers saturated with hexavalent chromium with 4-6% sodium hydroxide solution, and then regenerating them with sodium chloride solution to obtain sodium chromate solution for recycling. The patent does not mention the concentration and impurity content of the sodium chromate solution, but according to the ion exchange principle, SO4 2- Ion pair Cr2O7 2- or CrO4 2- The competitive adsorption of ions is severe and the sulfate content in the regeneration liquid 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 the present invention is to provide a method for recovering sodium chromate with a high chromium concentration and a low sulfate content in an 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 solutions of the present invention are as follows:

[0007] The present invention provides a method for recovering chromate, which comprises the following steps:

[0008] (1) Adsorbing chromium-containing wastewater with ion exchange resin;

[0009] (2) eluting and regenerating the adsorbed resin with an alkaline solution and collecting the eluate;

[0010] (3) adjusting the pH of the collected eluate to acidic and concentrating it to obtain a chromate concentrate;

[0011] (4) adding a sulfate precipitant to the chromate concentrate to remove the sulfate by precipitation;

[0012] (5) crystallizing the obtained chromate solution and drying it;

[0013] Wherein, the alkaline solution is a sodium hydroxide solution, and the concentration of the alkaline solution is 20-25%.

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

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

[0016] In some embodiments, the sulfate precipitating agent comprises a calcium-containing compound, an aluminum-containing compound, a silicon-containing compound, and a fluorine-containing compound.

[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 metaaluminate, 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 molar ratio of the aluminum content of the sulfate precipitant to the sulfate in the concentrate is 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 0.8-1.5:1, further 0.8-1.2:1, further 0.9-1.1:1.

[0019] In some embodiments, the molar ratio of the silicon content of the sulfate precipitant to the sulfate in the concentrate is 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 0.8-1.5:1, further 0.8-1.2:1, further 0.9-1.1:1.

[0020] In some embodiments, the molar ratio of the calcium content of the sulfate precipitant to the sulfate in the concentrate is 2 to 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, 8:1, preferably 3-6:1, further 3-5:1.

[0021] In some embodiments, the molar ratio of the fluorine content of the sulfate precipitant to the sulfate in the concentrate is 6 to 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, 20:1, preferably 10-20:1, further preferably 10-15:1.

[0022] In some embodiments, the amount of each element in the sulfate precipitant is as follows: the molar ratio of aluminum content to sulfate in the concentrated solution is 0.5-1.5:1, the molar ratio of silicon content to sulfate in the concentrated solution is 0.5-1.5:1, the molar ratio of calcium content to sulfate in the concentrated solution is 2-8:1, and the molar ratio of fluorine content to sulfate in the concentrated solution is 6-20:1.

[0023] Preferably, the amount of each element in the sulfate precipitant is as follows: the molar ratio of aluminum content to sulfate in the concentrated solution is 0.8-1.5:1, the molar ratio of silicon content to sulfate in the concentrated solution is 0.8-1.5:1, the molar ratio of calcium content to sulfate in the concentrated solution is 2-8:1, and the molar ratio of fluorine content to sulfate in the concentrated solution is 10-20:1.

[0024] Preferably, the amount of each element in the sulfate precipitant is as follows: the molar ratio of aluminum content to sulfate in the concentrated solution is 0.9-1.1:1, the molar ratio of silicon content to sulfate in the concentrated solution is 0.9-1.1:1, the molar ratio of calcium content to sulfate in the concentrated solution is 3-5:1, and the molar ratio of fluorine content to sulfate in the concentrated solution is 10-15:1.

[0025] In some embodiments, step (1) includes the following steps: preparing 3-5 ion exchange columns; first passing the chromium-containing wastewater through a first ion exchange column for adsorption; when the hexavalent chromium content of the effluent of 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 of the effluent of 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 feeding of the first ion exchange column and eluting and regenerating the first ion exchange column with an alkaline solution; when the hexavalent chromium content of the effluent of the last ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through the regenerated first ion exchange column, while stopping the feeding of the second ion exchange column, and regenerating alternately in sequence.

[0026] In some embodiments, the step (1) includes the following steps: first passing the chromium-containing wastewater through a first ion exchange column for adsorption; when the hexavalent chromium content of the effluent of 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 of the effluent of the second ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through a third ion exchange column; when the hexavalent chromium content of the effluent of the third ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through a fourth ion exchange column, while stopping the feeding of the first ion exchange column and eluting and regenerating the first ion exchange column with an alkaline solution; when the hexavalent chromium content of the effluent of the fourth ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through the regenerated first ion exchange column and stopping the feeding of the second ion exchange column, and regenerating alternately in sequence.

[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 adsorption saturated ion exchange resin with an alkaline solution of 1.5-2.5 times the volume of the resin, washing it with water of 1.5-2.5 times the volume of the resin, and collecting the eluate and the washing solution.

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

[0031] In some embodiments, collecting the eluate and the cleaning solution comprises the following steps:

[0032] The first 20-30% volume of the eluate is used as the first eluate and mixed into the original chromium-containing wastewater for re-adsorption treatment;

[0033] Then the 40-60% volume of the eluted eluate is used as the second eluate for subsequent concentration;

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

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

[0036] In some embodiments, collecting the eluate and the cleaning solution comprises the following steps:

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

[0038] Then the 45-55% volume of the eluted eluate (about 1 times the volume of the resin) is used as the second eluate for subsequent concentration;

[0039] Then the remaining eluent (about 1 / 2 times the volume of the resin) and the 72-77% washing liquid (about 3 / 2 times the volume of the resin) are used as the third eluent as an alkali solution substitute for subsequent resin elution;

[0040] The final eluted 22-27% volume of the washing solution (about 1 / 2 times the volume of the resin) is used as the fourth eluent and as a solvent for preparing the alkaline solution.

[0041] In some embodiments, collecting the eluate and the cleaning solution comprises the following steps:

[0042] The first 25% volume of the eluate is used as the first eluate and mixed into the original chromium-containing wastewater for re-adsorption treatment;

[0043] Then the 50% volume of the eluted eluate is used as the second eluate for subsequent concentration;

[0044] The remaining eluent and 75% of the washing liquid are then used as the third eluent to replace the alkali solution for subsequent resin elution.

[0045] The last 25% volume of the washing liquid eluted is used as the fourth eluent and is used as a solvent to prepare the alkaline solution.

[0046] In some embodiments, step (3) of concentrating comprises adjusting the pH of the obtained second eluate to 1-4 with chromic acid, and concentrating by evaporation to obtain a sodium chromate concentrate having a hexavalent chromium content of 300-600 g / L. Preferably, the pH is adjusted to 2-4. Preferably, the obtained sodium chromate concentrate has a hexavalent chromium content of 400-500 g / L.

[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 to 20 BV / h.

[0049] In some embodiments, the crystallization in step (5) includes: evaporating and crystallizing the sodium chromate solution to obtain a sodium chromate slurry having a hexavalent chromium content of 800 to 900 g / L, cooling and crystallizing, separating and obtaining a wet sodium chromate product, centrifuging the mother liquor and mixing it into a second eluent, and continuing with 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, and in particular, in combination with the specific macroporous strong basic anion exchange resin of the present invention, an eluent with a sodium chromate concentration greater than 100 g / L is obtained for the first time, greatly reducing the cost required for concentration.

[0052] (2) In addition, the present invention adopts a sulfate precipitant of a specific formula for the first time, which can react with sulfate to form a double salt precipitation reaction and does not react with chromate ions. It can achieve efficient removal of sulfate ions from sodium chromate concentrate without causing chromium loss, thus overcoming the defects of high sulfate content or serious chromium loss in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 The sodium chromate freeze-dried powder and crystalline sodium chromate prepared in Example 1. DETAILED DESCRIPTION

[0054] To facilitate understanding of the present invention, the present invention will be described more fully below with reference to the embodiments. Preferred embodiments of the present invention are provided below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided for the purpose of providing a more thorough and comprehensive understanding of the disclosure of the present invention.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit 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 is 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 generally 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 metal ions such as copper, nickel, and zinc.

[0059] Example 1

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

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

[0062] The chromium-containing electroplating wastewater after filtration and impurity removal is then passed through four groups of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong basic anion exchange resin, model D201) at a feed rate of 10 to 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 feeding of the first ion exchange column is stopped and the column is eluted and regenerated. When the hexavalent chromium content of the effluent from the fourth ion exchange column exceeds 0.1 mg / L, the column is pumped into the regenerated first ion exchange column and the feeding of the second ion exchange column is stopped. The regeneration is repeated alternately. The effluent is discharged into a wastewater treatment station for subsequent treatment.

[0063] (2) Resin elution and regeneration: Use 20-25% sodium hydroxide solution with a volume of 2 times the resin volume to regenerate the adsorption saturated resin, and use 2 times the resin volume of water to wash the resin bed until the effluent becomes colorless, and collect the eluent and cleaning liquid separately. Specifically, the first eluent (the residual liquid of the resin bed replaced by the alkali solution, the color is yellow, about 1 / 2 times the resin volume) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; the second eluent (the eluted high-concentration chromium-containing waste liquid, the color is orange-red, about 1 times the resin volume or the pH value of the eluted effluent is neutral) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; The eluted hexavalent chromium is collected in the second collection tank and enters the subsequent evaporation and concentration stage; the third eluate (the eluted hexavalent chromium concentration is relatively high, the color is bright yellow, and the volume is about 2 times the resin volume) is collected in the third collection tank and used as an alkali solution substitute for the subsequent resin elution; the fourth eluate (the eluted hexavalent chromium concentration is relatively low, the color is yellow, and the volume is about 1 / 2 times the resin volume. The elution is stopped when the color of the effluent 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 caustic soda to re-prepare a 20-25% sodium hydroxide solution;

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

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

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

[0067] The wet sodium chromate product is 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 electroplating chromium-containing wastewater comprises the following steps:

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

[0071] The chromium-containing electroplating wastewater after filtration and impurity removal is then passed through four groups of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong basic anion exchange resin, model Lanxess resin MonoPlus MP500) at a feed rate of 10 to 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 feeding of the first ion exchange column is stopped and the column is eluted and regenerated. When the hexavalent chromium content of the effluent from the fourth ion exchange column exceeds 0.1 mg / L, the column is pumped into the regenerated first ion exchange column and the feeding of the second ion exchange column is stopped. The regeneration is repeated alternately. The effluent is discharged into a wastewater treatment station for subsequent treatment.

[0072] (2) Resin elution and regeneration: Use 20-25% sodium hydroxide solution with a volume of 2 times the resin volume to regenerate the adsorption saturated resin, and use 2 times the resin volume of water to wash the resin bed until the effluent becomes colorless, and collect the eluent and cleaning liquid separately. Specifically, the first eluent (the residual liquid of the resin bed replaced by the alkali solution, the color is yellow, about 1 / 2 times the resin volume) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; the second eluent (the eluted high-concentration chromium-containing waste liquid, the color is orange-red, about 1 times the resin volume or the pH value of the eluted effluent is neutral) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; The eluted hexavalent chromium is collected in the second collection tank and enters the subsequent evaporation and concentration stage; the third eluate (the eluted hexavalent chromium concentration is relatively high, the color is bright yellow, and the volume is about 2 times the resin volume) is collected in the third collection tank and used as an alkali solution substitute for the subsequent resin elution; the fourth eluate (the eluted hexavalent chromium concentration is relatively low, the color is yellow, and the volume is about 1 / 2 times the resin volume. The elution is stopped when the color of the effluent 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 caustic soda to re-prepare a 20-25% sodium hydroxide solution;

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

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

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

[0076] The wet sodium chromate product is 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 electroplating chromium-containing wastewater comprises the following steps:

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

[0080] The chromium-containing electroplating wastewater after filtration and impurity removal is then passed through four groups of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong basic anion exchange resin, model D296) at a feed rate of 10 to 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 feeding of the first ion exchange column is stopped and the column is eluted and regenerated. When the hexavalent chromium content of the effluent from the fourth ion exchange column exceeds 0.1 mg / L, the column is pumped into the regenerated first ion exchange column and the feeding of the second ion exchange column is stopped. The regeneration is repeated alternately. The effluent is discharged into a wastewater treatment station for subsequent treatment.

[0081] (2) Resin elution and regeneration: Use 20-25% sodium hydroxide solution with a volume of 2 times the resin volume to regenerate the adsorption saturated resin, and use 2 times the resin volume of water to wash the resin bed until the effluent becomes colorless, and collect the eluent and cleaning liquid separately. Specifically, the first eluent (the residual liquid of the resin bed replaced by the alkali solution, the color is yellow, about 1 / 2 times the resin volume) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; the second eluent (the eluted high-concentration chromium-containing waste liquid, the color is orange-red, about 1 times the resin volume or the pH value of the eluted effluent is neutral) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; The eluted hexavalent chromium is collected in the second collection tank and enters the subsequent evaporation and concentration stage; the third eluate (the eluted hexavalent chromium concentration is relatively high, the color is bright yellow, and the volume is about 2 times the resin volume) is collected in the third collection tank and used as an alkali solution substitute for the subsequent resin elution; the fourth eluate (the eluted hexavalent chromium concentration is relatively low, the color is yellow, and the volume is about 1 / 2 times the resin volume. The elution is stopped when the color of the effluent 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 caustic soda to re-prepare a 20-25% sodium hydroxide solution;

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

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

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

[0085] The wet sodium chromate product is 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 electroplating chromium-containing wastewater comprises the following steps:

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

[0089] The chromium-containing electroplating wastewater after filtration and impurity removal is then passed through four groups of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong basic anion exchange resin, model D201) at a feed rate of 10 to 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 feeding of the first ion exchange column is stopped and the column is eluted and regenerated. When the hexavalent chromium content of the effluent from the fourth ion exchange column exceeds 0.1 mg / L, the column is pumped into the regenerated first ion exchange column and the feeding of the second ion exchange column is stopped. The regeneration is repeated alternately. The effluent is discharged into a wastewater treatment station for subsequent treatment.

[0090] (2) Resin elution and regeneration: Use 20-25% sodium hydroxide solution with a volume of 2 times the resin volume to regenerate the adsorption saturated resin, and use 2 times the resin volume of water to wash the resin bed until the effluent becomes colorless, and collect the eluent and cleaning liquid separately. Specifically, the first eluent (the residual liquid of the resin bed replaced by the alkali solution, the color is yellow, about 1 / 2 times the resin volume) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; the second eluent (the eluted high-concentration chromium-containing waste liquid, the color is orange-red, about 1 times the resin volume or the pH value of the eluted effluent is neutral) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; The eluted hexavalent chromium is collected in the second collection tank and enters the subsequent evaporation and concentration stage; the third eluate (the eluted hexavalent chromium concentration is relatively high, the color is bright yellow, and the volume is about 2 times the resin volume) is collected in the third collection tank and used as an alkali solution substitute for the subsequent resin elution; the fourth eluate (the eluted hexavalent chromium concentration is relatively low, the color is yellow, and the volume is about 1 / 2 times the resin volume. The elution is stopped when the color of the effluent 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 caustic soda to re-prepare a 20-25% sodium hydroxide solution;

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

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

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

[0094] The wet sodium chromate product is 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 electroplating chromium-containing wastewater comprises the following steps:

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

[0098] The chromium-containing electroplating wastewater after filtration and impurity removal is then passed through four groups of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong basic anion exchange resin, model D201) at a feed rate of 10 to 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 feeding of the first ion exchange column is stopped and the column is eluted and regenerated. When the hexavalent chromium content of the effluent from the fourth ion exchange column exceeds 0.1 mg / L, the column is pumped into the regenerated first ion exchange column and the feeding of the second ion exchange column is stopped. The regeneration is repeated alternately. The effluent is discharged into a wastewater treatment station for subsequent treatment.

[0099] (2) Resin elution and regeneration: Use 20-25% sodium hydroxide solution with a volume of 2 times the resin volume to regenerate the adsorption saturated resin, and use 2 times the resin volume of water to wash the resin bed until the effluent becomes colorless, and collect the eluent and cleaning liquid separately. Specifically, the first eluent (the residual liquid of the resin bed replaced by the alkali solution, the color is yellow, about 1 / 2 times the resin volume) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; the second eluent (the eluted high-concentration chromium-containing waste liquid, the color is orange-red, about 1 times the resin volume or the pH value of the eluted effluent is neutral) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; The eluted hexavalent chromium is collected in the second collection tank and enters the subsequent evaporation and concentration stage; the third eluate (the eluted hexavalent chromium concentration is relatively high, the color is bright yellow, and the volume is about 2 times the resin volume) is collected in the third collection tank and used as an alkali solution substitute for the subsequent resin elution; the fourth eluate (the eluted hexavalent chromium concentration is relatively low, the color is yellow, and the volume is about 1 / 2 times the resin volume. The elution is stopped when the color of the effluent 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 caustic soda to re-prepare a 20-25% sodium hydroxide solution;

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

[0101] (4) Sulfate removal: adding a sulfate remover (calcium hydroxide, fluosilicic acid, sodium metaaluminate, sodium fluoride, wherein the molar ratio of aluminum and silicon to sulfate in the sulfate remover is 1.2:1, the molar ratio of calcium to sulfate is 4:1, and the molar ratio of fluorine to sulfate is 10:1) to the sodium chromate concentrate to remove the sulfate in the form of double salt precipitation;

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

[0103] The wet sodium chromate product is 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 electroplating chromium-containing wastewater comprises the following steps:

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

[0107] The chromium-containing electroplating wastewater after filtration and impurity removal is then passed through four groups of ion exchange columns (quaternized styrene-divinylbenzene copolymer macroporous strong basic anion exchange resin, model D201) at a feed rate of 10 to 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 feeding of the first ion exchange column is stopped and the column is eluted and regenerated. When the hexavalent chromium content of the effluent from the fourth ion exchange column exceeds 0.1 mg / L, the column is pumped into the regenerated first ion exchange column and the feeding of the second ion exchange column is stopped. The regeneration is repeated alternately. The effluent is discharged into a wastewater treatment station for subsequent treatment.

[0108] (2) Resin elution and regeneration: Use 20-25% sodium hydroxide solution with a volume of 2 times the resin volume to regenerate the adsorption saturated resin, and use 2 times the resin volume of water to wash the resin bed until the effluent becomes colorless, and collect the eluent and cleaning liquid separately. Specifically, the first eluent (the residual liquid of the resin bed replaced by the alkali solution, the color is yellow, about 1 / 2 times the resin volume) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; the second eluent (the eluted high-concentration chromium-containing waste liquid, the color is orange-red, about 1 times the resin volume or the pH value of the eluted effluent is neutral) is collected in the first collection tank and mixed into the raw water for re-adsorption treatment; The eluted hexavalent chromium is collected in the second collection tank and enters the subsequent evaporation and concentration stage; the third eluate (the eluted hexavalent chromium concentration is relatively high, the color is bright yellow, and the volume is about 2 times the resin volume) is collected in the third collection tank and used as an alkali solution substitute for the subsequent resin elution; the fourth eluate (the eluted hexavalent chromium concentration is relatively low, the color is yellow, and the volume is about 1 / 2 times the resin volume. The elution is stopped when the color of the effluent 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 caustic soda to re-prepare a 20-25% sodium hydroxide solution;

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

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

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

[0112] The wet sodium chromate product is 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 the 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%. Since the sodium hydroxide concentration is too high, the density of the sodium hydroxide solution is greater than the density of the resin, and 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 the sulfate precipitant, and the molar ratio of barium carbonate to sulfate is 1:1.

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

[0124] Table 1

[0125]

[0126] The results, as shown in Table 1, demonstrate that the method of the present invention not only directly regenerates a sodium chromate solution with a hexavalent chromium content exceeding 90 g / L, significantly reducing subsequent evaporation and concentration costs, but also utilizes a unique sulfate ion precipitant. Upon addition to the sodium chromate concentrate, the precipitant reacts with the sulfate ions in the concentrate to form a complex salt precipitate, achieving efficient sulfate ion removal from the concentrate without loss of chromium.

[0127] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned 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 above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for recovering chromate, wherein: The following steps are involved: (1) Adsorbing chromium-containing wastewater with ion exchange resin; (2) eluting and regenerating the adsorbed resin with an alkaline solution and collecting the eluate; (3) adjusting the pH of the collected eluate to acidic and concentrating it to obtain a chromate concentrate; (4) adding a sulfate precipitant to the chromate concentrate to remove the sulfate by precipitation; (5) crystallizing the obtained chromate solution and drying it; Wherein, the alkaline solution is a sodium hydroxide solution, and the concentration of the alkaline solution is 20-25%.

2. The method according to claim 1, wherein The ion exchange resin is a macroporous strongly basic anion exchange resin.

3. The method according to claim 2, wherein: The skeleton of the macroporous strong basic anion exchange resin is a styrene-divinylbenzene copolymer, and the functional group is a quaternary ammonium group; preferably, the quaternary ammonium group is (-N(CH3)3OH).

4. The method according to claim 1, wherein The sulfate precipitant includes: a calcium-containing compound, an aluminum-containing compound, a silicon-containing compound and a fluorine-containing compound; 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 metaaluminate 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 4, wherein The dosage of each element in the sulfate precipitant is as follows: the molar ratio of aluminum content to sulfate in the concentrated solution is 0.5-1.5:1, the molar ratio of silicon content to sulfate in the concentrated solution is 0.5-1.5:1, the molar ratio of calcium content to sulfate in the concentrated solution is 2-8:1, and the molar ratio of fluorine content to sulfate in the concentrated solution is 6-20:1; Preferably, the amount of each element in the sulfate precipitant is as follows: the molar ratio of aluminum content to sulfate in the concentrated solution is 0.8-1.5:1, the molar ratio of silicon content to sulfate in the concentrated solution is 0.8-1.5:1, the molar ratio of calcium content to sulfate in the concentrated solution is 2-8:1, and the molar ratio of fluorine content to sulfate in the concentrated solution is 10-20:

1.

6. The method according to claim 5, wherein: The dosage of each element in the sulfate precipitant is as follows: the molar ratio of aluminum content to sulfate in the concentrated solution is 0.9-1.1:1, the molar ratio of silicon content to sulfate in the concentrated solution is 0.9-1.1:1, the molar ratio of calcium content to sulfate in the concentrated solution is 3-5:1, and the molar ratio of fluorine content to sulfate in the concentrated solution is 10-15:

1.

7. The method according to claim 1, wherein The step (1) comprises the following steps: preparing 3-5 ion exchange columns; first passing the chromium-containing wastewater through a first ion exchange column for adsorption; when the hexavalent chromium content of the effluent of 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 of the effluent of the second ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through a third ion exchange column, and so on, in sequence, until the effluent passes through the last ion exchange column, while stopping the feeding of the first ion exchange column and eluting and regenerating the first ion exchange column with an alkaline solution; when the hexavalent chromium content of the effluent of the last ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through the regenerated first ion exchange column, while stopping the feeding of the second ion exchange column, and regenerating alternately in sequence; Preferably, the step (1) comprises the following steps: first passing the chromium-containing wastewater through a first ion exchange column for adsorption; when the hexavalent chromium content of the effluent of 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 of the effluent of the second ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through a third ion exchange column; when the hexavalent chromium content of the effluent of the third ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through a fourth ion exchange column, while stopping the feeding of the first ion exchange column and eluting and regenerating the first ion exchange column with an alkaline solution; when the hexavalent chromium content of the effluent of the fourth ion exchange column exceeds 0.01-0.5 mg / L, passing the effluent through the regenerated first ion exchange column and stopping the feeding of the second ion exchange column, and regenerating alternately in sequence.

8. The method according to claim 1, wherein The step (2) comprises the following steps: regenerating and eluting the adsorbed ion exchange resin with an alkaline solution of 1.5-2.5 times the volume of the resin, washing it with water of 1.5-2.5 times the volume of the resin, and collecting the eluate and the washing solution.

9. The method according to claim 8, wherein The collecting of eluent and cleaning solution comprises the following steps: The first 20-30% volume of the eluate is used as the first eluate and mixed into the original chromium-containing wastewater for re-adsorption treatment; Then the 40-60% volume of the eluted eluate is used as the second eluate for subsequent concentration; The remaining eluent and 70-80% of the washing liquid are then used as the third eluent, which is used as an alkali solution substitute for subsequent resin elution. The last 20-30% volume of the washing liquid eluted is used as the fourth eluent and is used as a solvent to prepare the alkaline solution.

10. The method according to any one of claims 1 to 9, wherein: The concentration in step (3) comprises: adjusting the pH of the obtained second eluate to 1-4 with chromic acid, and evaporating and concentrating to obtain a sodium chromate concentrate having a hexavalent chromium content of 300-600 g / L; and / or, The chromium-containing wastewater used in the step (1) is first filtered and then adsorbed using an ion exchange resin.

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