Method for preparing sodium chromate from chromium-containing wastewater

Abstract

The invention discloses a method for preparing sodium chromate from chromium-containing wastewater, comprising the steps of resin activation, resin adsorption elution and resin regeneration. The method can achieve high efficiency and low cost in preparing sodium chromate from chromium-containing wastewater by selecting appropriate resin and optimizing process parameters, without secondary pollution to the environment. The treated water has good quality and meets national emission standards. Moreover, a large amount of chromium-containing compounds can be recycled and utilized, thereby generating economic benefits.

Description

Technical Field

[0001] The invention relates to the technical field of industrial wastewater treatment, in particular to a method for preparing sodium chromate from chromium-containing wastewater. Background Art

[0002] A large amount of chromium-containing wastewater is discharged every day in the metallurgical industry, electroplating, leather making, paint, photography and other industries. The chromium in the chromium-containing wastewater mainly exists in the form of trivalent chromium ions, hexavalent chromium ions and complexes. Water-soluble hexavalent chromium compounds are strong carcinogens and have very strong toxic effects on the human body and the environment. China stipulates that the maximum allowable discharge concentration of hexavalent chromium in surface water is 0.5 mg / L. Therefore, the treatment of chromium-containing wastewater is particularly important. At present, the main treatment methods for chromium-containing wastewater are: chemical reduction method, electrochemical method, ion exchange method, activated carbon adsorption method and liquid membrane method; except for ion exchange method, other methods will produce trivalent chromium-containing wastewater, but the current ion exchange resin treatment of chromium-containing wastewater has the technical problems of low efficiency and high investment cost. Summary of the invention

[0003] In order to solve the above technical problems, the present application provides a method for efficiently preparing sodium chromate from chromium-containing wastewater, comprising the following steps:

[0004] Resin activation: soak the resin in 15-25wt% hydrochloric acid for 12-24h, then wash the resin with deionized water until the pH value of the eluate is stabilized between 4 and 6, then soak the resin in 15-25wt% sodium hydroxide solution for 12-24h, then wash the resin with deionized water until the pH value of the eluate is stabilized between 6 and 8, then soak the resin in 3-7wt% hydrochloric acid for 12-24h, then wash the resin with deionized water until the pH value of the eluate is stabilized between 4 and 6, and then put the resin into a columnar container to obtain a resin column; choosing the right soaking time is crucial to maintaining the best performance of the resin. On the one hand, a longer soaking time will extend the resin treatment cycle, and on the other hand, too long a strong acid or strong alkali treatment may damage the physical structure of the resin; if the soaking time is too short, it may lead to incomplete treatment, affecting the functionality and service life of the resin;

[0005] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 2-4 BV / h through a peristaltic pump. After the resin column turns yellow, a 4-12wt% sodium hydroxide solution is used at a flow rate of 2-6 BV / h to backwash the resin column until the resin in the resin column restores its own color. At this time, the eluate is a sodium chromate solution. The resin column is then washed with deionized water until the pH value of the eluate is stabilized between 6 and 8. The flow rate and concentration selected when backwashing the resin column with sodium hydroxide solution have a direct impact on the backwashing effect. A suitable flow rate can quickly take away the chromium ions adsorbed on the resin, reduce the chance of chromium ions being re-adsorbed on the resin, and improve the elution efficiency. If the flow rate is too fast, the sodium hydroxide will not be fully in contact with the resin, so that the chromium ions cannot be effectively eluted, and a too fast flow rate will also cause the resin bed to swell or be damaged, affecting the structural stability and service life of the resin column. If the flow rate is too slow, it will not only reduce the elution efficiency but also fail to provide sufficient power to maintain The gaps between the resin particles make the resin bed compacted, damaging the physical structure of the resin and slowing down the elution process. If the flow rate is too slow, the chromium ions eluted have more opportunities to be re-adsorbed back to the resin, especially when the sodium hydroxide concentration in the eluent is low, the chromium ion re-adsorption is more likely to occur, resulting in poor final elution effect. A sodium hydroxide solution of appropriate concentration helps to break the binding force between the chromium ions and the resin, making it easier for the chromium ions to be desorbed from the resin, and then forming a sodium chromate solution to be eluted. A sodium hydroxide solution with too high a concentration will not only corrode the resin, but also increase the cost of waste liquid treatment. A sodium hydroxide solution with too low a concentration is not enough to effectively destroy the binding force between the chromium ions and the resin, resulting in a decrease in the desorption efficiency of the chromium ions from the resin, so that some chromium ions remain on the resin, affecting the overall elution effect. If the elution is incomplete, the eluted chromium ions may be re-adsorbed to the resin in a sodium hydroxide environment with a lower concentration.

[0006] Resin regeneration: Use 3-7wt% hydrochloric acid at a flow rate of 2-6BV / h to wash the resin column until the pH value of the eluate is less than 7, then stop washing, and then continue to soak in 3-7wt% hydrochloric acid for 12-24h, and finally wash the resin with deionized water until the pH value of the eluate is stabilized between 4 and 6.

[0007] Preferably, a method for preparing sodium chromate from chromium-containing wastewater comprises the following steps:

[0008] Resin activation: soak the resin in 15wt% hydrochloric acid for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 4, then soak the resin in 15wt% sodium hydroxide solution for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, then soak the resin in 3wt% hydrochloric acid for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 4, and then put the resin into a columnar container to obtain a resin column;

[0009] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 2BV / h through a peristaltic pump. After the resin column turns yellow, 8wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 4BV / h until the resin column resin restores its own color. At this time, the eluate is sodium chromate solution. The resin column is then washed with deionized water until the pH value of the eluate is equal to 7;

[0010] Resin regeneration: Use 5wt% hydrochloric acid at a flow rate of 4BV / h to wash the resin column until the pH value of the eluate is less than 7, then stop washing, then continue soaking in 5wt% hydrochloric acid for 12h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 4.

[0011] Preferably, a method for preparing sodium chromate from chromium-containing wastewater comprises the following steps:

[0012] Resin activation: soak the resin in 20wt% hydrochloric acid for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 5, then soak the resin in 20wt% sodium hydroxide solution for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 7, then soak the resin in 5wt% hydrochloric acid for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 5, and then put the resin into a columnar container to obtain a resin column;

[0013] Resin adsorption and elution: The chromium-containing wastewater is pumped into the resin column through a peristaltic pump at a flow rate of 3BV / h. After the resin column turns yellow, a 4wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 2BV / h for 2 bed volumes (BV) to initially desorb the weakly bound chromium ions; then an 8wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 4BV / h for 3 bed volumes (BV) to further desorb more chromium ions; finally, a 12wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 6BV / h. Rinse the resin column until the resin column restores its own color to ensure that all adsorbed chromium ions are effectively desorbed. At this time, the eluent is a sodium chromate solution. Then wash the resin column with deionized water until the pH value of the eluent is equal to 7. Gradient elution can more effectively desorb chromium ions of different binding strengths by gradually increasing the alkali concentration, thereby improving the elution efficiency, reducing the eluent volume, and obtaining a higher purity sodium chromate solution. At the same time, it avoids the resin from being exposed to strong acids or strong bases for a long time, which helps to protect the resin structure and extend the service life of the resin.

[0014] Resin regeneration: Use 5wt% hydrochloric acid at a flow rate of 4BV / h to wash the resin column until the pH value of the eluate is less than 7, then stop washing, then continue soaking in 5wt% hydrochloric acid for 18h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 5.

[0015] Preferably, a method for preparing sodium chromate from chromium-containing wastewater comprises the following steps:

[0016] Resin activation: soak the resin in 25wt% hydrochloric acid for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, then soak the resin in 25wt% sodium hydroxide solution for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 7, then soak the resin in 7wt% hydrochloric acid for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, and then put the resin into a columnar container to obtain a resin column;

[0017] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column through a peristaltic pump at a flow rate of 4BV / h. After the resin column turns yellow, a 4wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 4BV / h for 2 bed volumes (BV); then a 10wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 4BV / h for 3 bed volumes (BV); finally, a 12wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 6BV / h until the resin column restores its own color. At this time, the eluate is a sodium chromate solution, and the resin column is washed with deionized water until the pH value of the eluate is equal to 7;

[0018] Resin regeneration: Use 7wt% hydrochloric acid to wash the resin column at a flow rate of 6BV / h until the pH value of the eluate is less than 7, then stop washing, then continue soaking in 7wt% hydrochloric acid for 24h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 6.

[0019] Preferably, the resin is any one of an iminodiacetic acid (IDA)-based chelating resin or a thiourea-based chelating resin. The iminodiacetic acid (IDA)-based chelating resin contains an iminodiacetic acid functional group, and the thiourea-based chelating resin contains a thiourea functional group, both of which can effectively capture trivalent chromium ions and can achieve efficient adsorption under appropriate pH conditions.

[0020] Preferably, the iminodiacetic acid (IDA) based chelating resin particles have a diameter of 0.5 to 0.6 mm and a molecular formula of R-CH2N(COOH)2.

[0021] Preferably, the thiourea-based chelating resin has a diameter of 0.5 to 0.6 mm and a molecular formula of R-SCNH(NH2). Selecting a suitable resin pore size can ensure that the resin can not only effectively capture chromium ions, but also maintain good physical properties such as mechanical strength and a long service life.

[0022] The beneficial effect is that the present application realizes efficient and low-cost preparation of sodium chromate from chromium-containing wastewater by selecting suitable resins and optimizing process parameters, without secondary pollution to the environment. The water quality after treatment is good and meets national emission standards. Moreover, a large amount of chromium-containing compounds can be recycled and utilized, thereby generating economic benefits. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below in conjunction with specific embodiments so that those skilled in the art can understand the present invention more clearly.

[0024] The following embodiments are only used to illustrate the present invention, but are not limited to the scope of the present invention. Based on the specific embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work belong to the protection scope of the present invention.

[0025] In the examples of the present invention, unless otherwise specified, all raw material components are commercially available products well known to those skilled in the art; in the examples of the present invention, unless otherwise specified, the technical means used are conventional means well known to those skilled in the art.

[0026] Source of raw materials:

[0027] Chromium-containing wastewater comes from Dongguan Daming Electroplating Co., Ltd., where the hexavalent chromium ion concentration in the chromium-containing wastewater is 68 mg / L and the pH value of the chromium-containing wastewater is 1;

[0028] Concentrated hydrochloric acid was purchased from Guangdong Qiming Chemical Technology Co., Ltd.;

[0029] Sodium hydroxide was purchased from Cangzhou Jinglong Chemical Products Co., Ltd.;

[0030] IDA-based chelating resin, with a diameter of 0.5-0.6 mm and a molecular formula of R-CH2N(COOH)2, was purchased from Shanghai Kaiping Resin Co., Ltd.

[0031] Thiourea chelating resin, with a diameter of 0.5-0.6 mm and a molecular formula of R-SCNH(NH2), was purchased from Shanghai Kaiping Resin Co., Ltd.;

[0032] Example 1

[0033] This embodiment provides a method for preparing sodium chromate from chromium-containing wastewater, comprising the following steps:

[0034] Resin activation: soak the resin in 15wt% hydrochloric acid for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 4, then soak the resin in 15wt% sodium hydroxide solution for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, then soak the resin in 3wt% hydrochloric acid for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 4, and then put the resin into a columnar container to obtain a resin column;

[0035] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 2BV / h through a peristaltic pump. After the resin column turns yellow, 40L of the chromium-containing wastewater is used, and 8wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 4BV / h until the resin column resin restores its own color. At this time, a total of 25L of sodium chromate solution is collected, and then the resin column is washed with deionized water until the pH value of the eluate is equal to 7. At this time, the hexavalent chromium ion concentration in the eluate is 0.12mg / L;

[0036] Resin regeneration: Use 5wt% hydrochloric acid to wash the resin column at a flow rate of 4BV / h until the pH value of the eluate is less than 7, then stop washing, then continue to soak with 5wt% hydrochloric acid for 12h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 4. The resin selected in this embodiment is an iminodiacetic acid (IDA)-based chelating resin with a diameter of 0.5-0.6mm and a molecular formula of R-CH2N(COOH)2.

[0037] Example 2

[0038] This embodiment provides a method for preparing sodium chromate from chromium-containing wastewater, comprising the following steps:

[0039] Resin activation: soak the resin in 20wt% hydrochloric acid for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 5, then soak the resin in 20wt% sodium hydroxide solution for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 7, then soak the resin in 5wt% hydrochloric acid for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 5, and then put the resin into a columnar container to obtain a resin column;

[0040] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 3BV / h through a peristaltic pump. After the resin column turns yellow, 40L of the chromium-containing wastewater is used, and the resin column is backwashed with a 4wt% sodium hydroxide solution at a flow rate of 2BV / h for 2 bed volumes (BV); then, an 8wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 4BV / h for 3 bed volumes (BV); finally, a 12wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 6BV / h until the resin column recovers its own color. At this time, a total of 18L of sodium chromate solution is collected, and then the resin column is washed with deionized water until the pH value of the eluate is equal to 7, and the hexavalent chromium ion concentration in the eluate is 0.28mg / L;

[0041] Resin regeneration: Use 5wt% hydrochloric acid to wash the resin column at a flow rate of 4BV / h until the pH value of the eluate is less than 7, then stop washing, then continue soaking with 5wt% hydrochloric acid for 18h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 5. The resin selected in this embodiment is an iminodiacetic acid (IDA)-based chelating resin with a diameter of 0.5-0.6mm and a molecular formula of R-CH2N(COOH)2.

[0042] Example 3

[0043] This embodiment provides a method for preparing sodium chromate from chromium-containing wastewater, comprising the following steps:

[0044] Resin activation: soak the resin in 25wt% hydrochloric acid for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, then soak the resin in 25wt% sodium hydroxide solution for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 7, then soak the resin in 7wt% hydrochloric acid for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, and then put the resin into a columnar container to obtain a resin column;

[0045] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 4BV / h through a peristaltic pump. After the resin column turns yellow, 40L of the chromium-containing wastewater is used, and the resin column is backwashed with a 4wt% sodium hydroxide solution at a flow rate of 4BV / h for 2 bed volumes (BV); then, the resin column is backwashed with a 10wt% sodium hydroxide solution at a flow rate of 4BV / h for 3 bed volumes (BV); finally, the resin column is backwashed with a 12wt% sodium hydroxide solution at a flow rate of 6BV / h until the resin column recovers its own color. At this time, a total of 13L of sodium chromate solution is collected, and then the resin column is washed with deionized water until the pH value of the eluate is equal to 7, and the hexavalent chromium ion concentration in the eluate is 0.17mg / L;

[0046] Resin regeneration: Use 7wt% hydrochloric acid to wash the resin column at a flow rate of 6BV / h until the pH value of the eluate is less than 7, then stop washing, then continue to soak with 7wt% hydrochloric acid for 24h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 6. The resin selected in this embodiment is an iminodiacetic acid (IDA)-based chelating resin with a diameter of 0.5-0.6mm and a molecular formula of R-CH2N(COOH)2.

[0047] Example 4

[0048] This embodiment provides a method for preparing sodium chromate from chromium-containing wastewater, comprising the following steps:

[0049] Resin activation: soak the resin in 15wt% hydrochloric acid for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 4, then soak the resin in 15wt% sodium hydroxide solution for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, then soak the resin in 3wt% hydrochloric acid for 12h, then wash the resin with deionized water until the pH value of the eluate is equal to 4, and then put the resin into a columnar container to obtain a resin column;

[0050] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 2BV / h through a peristaltic pump. After the resin column turns yellow, 40L of the chromium-containing wastewater is used, and 8wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 4BV / h until the resin column resin restores its own color. At this time, a total of 28L of sodium chromate solution is collected, and then the resin column is washed with deionized water until the pH value of the eluate is equal to 7. At this time, the hexavalent chromium ion concentration in the eluate is 0.22mg / L;

[0051] Resin regeneration: Use 5wt% hydrochloric acid to wash the resin column at a flow rate of 4BV / h until the pH value of the eluate is less than 7, then stop washing, then continue to soak with 5wt% hydrochloric acid for 12h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 4. The resin selected in this embodiment is a thiourea-based chelating resin with a diameter of 0.5-0.6mm and a molecular formula of R-SCNH(NH2).

[0052] Example 5

[0053] This embodiment provides a method for preparing sodium chromate from chromium-containing wastewater, comprising the following steps:

[0054] Resin activation: soak the resin in 20wt% hydrochloric acid for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 5, then soak the resin in 20wt% sodium hydroxide solution for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 7, then soak the resin in 5wt% hydrochloric acid for 18h, then wash the resin with deionized water until the pH value of the eluate is equal to 5, and then put the resin into a columnar container to obtain a resin column;

[0055] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 3BV / h through a peristaltic pump. After the resin column turns yellow, 40L of the chromium-containing wastewater is used, and the resin column is backwashed with a 4wt% sodium hydroxide solution at a flow rate of 2BV / h for 2 bed volumes (BV); then, an 8wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 4BV / h for 3 bed volumes (BV); finally, a 12wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 6BV / h until the resin column restores its own color. At this time, a total of 24L of sodium chromate solution is collected, and then the resin column is washed with deionized water until the pH value of the eluate is equal to 7, and the hexavalent chromium ion concentration in the eluate is 0.44mg / L;

[0056] Resin regeneration: Use 5wt% hydrochloric acid to wash the resin column at a flow rate of 4BV / h until the pH value of the eluate is less than 7, then stop washing, then continue to soak with 5wt% hydrochloric acid for 18h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 5. The resin selected in this embodiment is a thiourea-based chelating resin with a diameter of 0.5-0.6mm and a molecular formula of R-SCNH(NH2).

[0057] Example 6

[0058] This embodiment provides a method for preparing sodium chromate from chromium-containing wastewater, comprising the following steps:

[0059] Resin activation: soak the resin in 25wt% hydrochloric acid for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, then soak the resin in 25wt% sodium hydroxide solution for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 7, then soak the resin in 7wt% hydrochloric acid for 24h, then wash the resin with deionized water until the pH value of the eluate is equal to 6, and then put the resin into a columnar container to obtain a resin column;

[0060] Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 4BV / h through a peristaltic pump. After the resin column turns yellow, 40L of the chromium-containing wastewater is used, and the resin column is backwashed with a 4wt% sodium hydroxide solution at a flow rate of 4BV / h for 2 bed volumes (BV); then, the resin column is backwashed with a 10wt% sodium hydroxide solution at a flow rate of 4BV / h for 3 bed volumes (BV); finally, the resin column is backwashed with a 12wt% sodium hydroxide solution at a flow rate of 6BV / h until the resin column recovers its own color. At this time, a total of 15L of sodium chromate solution is collected, and then the resin column is washed with deionized water until the pH value of the eluate is equal to 7, and the hexavalent chromium ion concentration in the eluate is 0.29mg / L;

[0061] Resin regeneration: Use 7wt% hydrochloric acid to wash the resin column at a flow rate of 6BV / h until the pH value of the eluate is less than 7, then stop washing, then continue to soak with 7wt% hydrochloric acid for 24h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 6. The resin selected in this embodiment is a thiourea-based chelating resin with a diameter of 0.5-0.6mm and a molecular formula of R-SCNH(NH2).

[0062] It can be found from the above Examples 1 to 3 that the volumes of the sodium chromate solutions collected after resin adsorption and elution are different, and the hexavalent chromium ion concentrations in the eluates are also different. Although the hexavalent chromium ion concentration in the eluate of Example 1 is the lowest, the volume of the sodium chromate solution collected is the largest, which means that the eluent used, i.e., the sodium hydroxide solution, is also the largest. Although the volume of the sodium chromate solution collected in Example 2 is less than that in Example 1, the hexavalent chromium ion concentration in the eluate is also higher than that in Example 1. On the contrary, Example 3 has the best comprehensive effect, not only the volume of the eluent used, i.e., the sodium hydroxide solution, is the smallest (the volume of the sodium chromate solution collected is only 13L), but also the hexavalent chromium ion concentration in the eluate is 0.17mg / L. The above results may be related to the different elution process parameters selected in the resin adsorption and elution process, and may also be related to the acid-base concentration and the soaking time selected in the early resin activation process.

[0063] The difference between Example 4 and Example 1, the difference between Example 5 and Example 2, and the difference between Example 6 and Example 3 are all only that the resins used are different. From the result, it can be found that the volume of the sodium chromate solution collected by Example 4 is higher than that of Example 1, and the hexavalent chromium ion concentration in the eluate of Example 4 is also higher than that of Example 1, indicating that not only more eluents, i.e., sodium hydroxide solutions, are used in Example 4, but also the elution effect is even worse. The same situation also occurs in Examples 5 and 6, which may be because the iminodiacetic acid (IDA)-based chelate resin is more suitable for processing chromium-containing wastewater than the thiourea-based chelate resin. However, from the perspective of Examples 1 to 6, whether the iminodiacetic acid (IDA)-based chelate resin or the thiourea-based chelate resin is selected, both can meet the national emission standards while recovering a large amount of chromium-containing compounds.

[0064] The above is only an embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing sodium chromate from chromium-containing wastewater, characterized in that: The following steps are involved: Resin activation: soaking the resin in 15-25wt% hydrochloric acid for 12-24h, then washing the resin with deionized water until the pH value of the eluate is stabilized between 4 and 6, then soaking the resin in 15-25wt% sodium hydroxide solution for 12-24h, then washing the resin with deionized water until the pH value of the eluate is stabilized between 6 and 8, then soaking the resin in 3-7wt% hydrochloric acid for 12-24h, then washing the resin with deionized water until the pH value of the eluate is stabilized between 4 and 6, and then loading the resin into a columnar container to obtain a resin column; Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 2-4 BV / h through a peristaltic pump. After the resin column turns yellow, a 4-12 wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 2-6 BV / h until the resin column resin restores its own color. At this time, the eluate is a sodium chromate solution. The resin column is then washed with deionized water until the pH value of the eluate is stabilized between 6 and 8; Resin regeneration: Use 3-7wt% hydrochloric acid at a flow rate of 2-6BV / h to wash the resin column until the pH value of the eluate is less than 7, then stop washing, then continue soaking with 3-7wt% hydrochloric acid for 12-24h, and finally wash the resin with deionized water until the pH value of the eluate is stabilized between 4 and 6.

2. The method for preparing sodium chromate from chromium-containing wastewater according to claim 1, characterized in that: The following steps are involved: Resin activation: soaking the resin in 15wt% hydrochloric acid for 12h, then washing the resin with deionized water until the pH value of the eluate is equal to 4, then soaking the resin in 15wt% sodium hydroxide solution for 12h, then washing the resin with deionized water until the pH value of the eluate is equal to 6, then soaking the resin in 3wt% hydrochloric acid for 12h, then washing the resin with deionized water until the pH value of the eluate is equal to 4, and then loading the resin into a columnar container to obtain a resin column; Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 2BV / h through a peristaltic pump. After the resin column turns yellow, an 8wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 4BV / h until the resin column resin restores its own color. At this time, the eluate is a sodium chromate solution. The resin column is then washed with deionized water until the pH value of the eluate is equal to 7; Resin regeneration: Use 5wt% hydrochloric acid to wash the resin column at a flow rate of 4BV / h until the pH value of the eluate is less than 7, then stop washing, then continue soaking in 5wt% hydrochloric acid for 12h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 4.

3. The method for preparing sodium chromate from chromium-containing wastewater according to claim 1, characterized in that: The following steps are involved: Resin activation: soaking the resin in 20wt% hydrochloric acid for 18h, then washing the resin with deionized water until the pH value of the eluate is equal to 5, then soaking the resin in 20wt% sodium hydroxide solution for 18h, then washing the resin with deionized water until the pH value of the eluate is equal to 7, then soaking the resin in 5wt% hydrochloric acid for 18h, then washing the resin with deionized water until the pH value of the eluate is equal to 5, and then loading the resin into a columnar container to obtain a resin column; Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 3BV / h through a peristaltic pump. After the resin column turns yellow, a 4wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 2BV / h for 2 bed volumes (BV), and then an 8wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 4BV / h for 3 bed volumes (BV). Finally, a 12wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 6BV / h until the resin column restores its own color. At this time, the eluate is a sodium chromate solution, and the resin column is washed with deionized water until the pH value of the eluate is equal to 7; Resin regeneration: Use 5wt% hydrochloric acid to wash the resin column at a flow rate of 4BV / h until the pH value of the eluate is less than 7, then stop washing, then continue soaking in 5wt% hydrochloric acid for 18h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 5.

4. The method for preparing sodium chromate from chromium-containing wastewater according to claim 1, characterized in that: The following steps are involved: Resin activation: soaking the resin in 25wt% hydrochloric acid for 24h, then washing the resin with deionized water until the pH value of the eluate is equal to 6, then soaking the resin in 25wt% sodium hydroxide solution for 24h, then washing the resin with deionized water until the pH value of the eluate is equal to 7, then soaking the resin in 7wt% hydrochloric acid for 24h, then washing the resin with deionized water until the pH value of the eluate is equal to 6, and then loading the resin into a columnar container to obtain a resin column; Resin adsorption elution: The chromium-containing wastewater is pumped into the resin column at a flow rate of 4BV / h through a peristaltic pump. After the resin column turns yellow, a 4wt% sodium hydroxide solution is used to backwash the resin column at a flow rate of 4BV / h for 2 bed volumes (BV), followed by a 10wt% sodium hydroxide solution at a flow rate of 4BV / h to continue backwashing the resin column for 3 bed volumes (BV), and finally a 12wt% sodium hydroxide solution is used to continue backwashing the resin column at a flow rate of 6BV / h until the resin column restores its own color. At this time, the eluate is a sodium chromate solution, and the resin column is washed with deionized water until the pH value of the eluate is equal to 7; Resin regeneration: Use 7wt% hydrochloric acid to wash the resin column at a flow rate of 6BV / h until the pH value of the eluate is less than 7, then stop washing, then continue soaking in 7wt% hydrochloric acid for 24h, and finally wash the resin with deionized water until the pH value of the eluate is equal to 6.

5. The method for preparing sodium chromate from chromium-containing wastewater according to any one of claims 1 to 4, characterized in that: The resin is any one of an iminodiacetic acid (IDA)-based chelating resin or a thiourea-based chelating resin.

6. The method for preparing sodium chromate from chromium-containing wastewater according to claim 5, characterized in that: The diameter of the iminodiacetic acid (IDA)-based chelating resin particles is 0.5-0.6 mm, and the molecular formula is R-CH2N(COOH)2.

7. The method for preparing sodium chromate from chromium-containing wastewater according to claim 5, characterized in that: The thiourea chelating resin has a diameter of 0.5-0.6 mm and a molecular formula of R-SCNH (NH2).