Chromium separation and recovery method for chromium-containing wastewater

Abstract

The invention belongs to the field of resource utilization in textile, leather and printing and dyeing industries, and particularly relates to a chromium separation and recovery method for chromium-containing wastewater. The invention aims to provide a chromium separation and recovery method for chromium-containing wastewater so as to reduce the cost and improve the chromium recovery rate. According to the technical scheme, the chromium separation and recovery method for the chromium-containing wastewater comprises the following steps: step 1, wastewater pretreatment; 2, chromium reduction precipitation; 3, chromium separation and enrichment; 4, chromium purification; and 5, chromium recovery.

Description

Technical Field

[0001] The present invention belongs to the field of resource utilization in the textile, leather, and printing and dyeing industries, and specifically relates to a method for chromium separation and recovery from chromium-containing wastewater. Background Art

[0002] Textile production, leather, printing and dyeing and other industries generate a large amount of chromium-containing wastewater, which is one of the main sources of chromium pollution to the environment. Cr(VI) and Cr(III) are the main chemical forms of chromium in nature. Cr(VI) has characteristics such as significant mobility, bioavailability, toxicity (acute toxicity, carcinogenic, teratogenic, mutagenic), and persistence. It enters the human body through respiration and contact, posing a risk of teratogenicity, carcinogenicity, and mutagenicity. Its toxicity is about 100 times that of Cr(III), posing a serious threat to the ecological system and human health. Therefore, it is necessary to effectively separate chromium in chromium-containing wastewater, so that the wastewater discharge meets the limited requirements of Cr, while realizing the resource recovery and reuse of valuable metal chromium, reducing chromium resource waste, and promoting the green and sustainable development of China's textile, leather, and printing and dyeing industries.

[0003] Patent CN115465968A discloses a method for chromium separation and recovery from chrome tanning waste liquor. By adding wood powder, trivalent chromium ions are in-situ converted into chromium hydroxide on the surface of the wood powder and adsorbed on its surface, accelerating solid-liquid separation, thereby improving the separation purity of trivalent chromium in chrome tanning waste liquor and reducing the consumption of auxiliary coagulant resources. However, this method does not mention whether the chromium residue in the final wastewater meets the discharge standard.

[0004] Patent CN110923477A discloses a method for chromium separation and recovery from chromium-containing leather sludge. This method mainly aims at chromium-containing sludge, adopts concentrated sulfuric acid pretreatment, and then undergoes water leaching. The chromium recovery idea is to separate and extract chromium from the chromium-containing leaching solution, innovatively treating the chromium leaching and recycling of the process. This method obtains good chromium leaching rate and recovery rate for treating chromium sludge, but the sample targeted is solid rather than liquid.

[0005] Patent CN105016387A discloses a method for treating chromium salt sludge. This method is applicable to chromium-containing solids containing a large amount of calcium carbonate, sodium chloride, and a small amount of calcium sulfate, calcium chromate, sodium chromate and other salts. First, reduce and remove chromium, and then leach to perform solid-liquid separation to obtain dechromed liquid and chromium hydroxide; the obtained dechromed liquid is crystallized and purified to realize the resource recovery and utilization of chromium.

[0006] The existing methods for treating chromium-containing wastewater mainly include chemical reduction precipitation, adsorption, membrane filtration, ion exchange, electrochemical reduction, biological reduction and fixation, etc. Many studies have reported the application of single technologies in the separation and recovery of chromium-containing wastewater. Single technologies have disadvantages such as potential secondary pollution, high cost, long time consumption, and low removal efficiency. Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies in the above-mentioned background technology and provide a method for chromium separation and recovery from chromium-containing wastewater, so as to reduce costs and improve the chromium recovery rate.

[0008] The technical solution of the present invention is as follows:

[0009] A method for chromium separation and recovery from chromium-containing wastewater, comprising the following steps:

[0010] Step 1: Pretreatment of wastewater;

[0011] Step 2: Chromium reduction and precipitation;

[0012] Step 3: Chromium separation and enrichment;

[0013] Step 4: Chromium purification;

[0014] Step 5: Chromium recovery.

[0015] The said Step 1 includes: filtering or suction filtering the chromium-containing wastewater to remove insoluble impurities in the chromium-containing wastewater.

[0016] In the said Step 1, suction filter the chromium-containing wastewater with a medium-speed filter paper with a pore size of 40 μm.

[0017] The said Step 2 includes: adding a reducing agent to reduce Cr(VI) in the chromium-containing wastewater to Cr(III), adjusting the pH value of the chromium-containing wastewater to make Cr(III) form chromium hydroxide precipitate, and filtering to obtain a chromium solution and chromium hydroxide precipitate.

[0018] The said Step 3 includes:

[0019] ① Add sodium oxalate to the chromium solution, adsorb the chromium solution with an anion exchange resin, and perform desorption treatment on the anion exchange resin to obtain a chromium desorption solution.

[0020] ② Adjust the pH value of the chromium desorption solution to make Cr(III) form chromium hydroxide precipitate, and filter to obtain chromium hydroxide precipitate.

[0021] In the said Step 2 and Step 3, adjusting the pH value to make Cr(III) form chromium hydroxide precipitate includes: adding NaOH to adjust the pH value to 9.5 - 11.5, stirring evenly and then standing for 30 min to make Cr(III) form chromium hydroxide precipitate.

[0022] The said Step 4 includes:

[0023] ① Combine the chromium hydroxide precipitates obtained in Step 2 and Step 3, dissolve the chromium hydroxide precipitate with sulfuric acid to obtain a chromium solution;

[0024] ② Add phosphate, add sodium carbonate to adjust the pH value of the solution to 1.5 - 2.5, and filter to obtain chromium phosphate precipitate.

[0025] Step 5 includes: suction filtration, washing with water, converting the chromium phosphate precipitate into a hydrated chromium oxide precipitate by adding an alkali and then calcining to obtain a chromium sesquioxide product.

[0026] In step 5, the calcination temperature is 1100 - 1200 °C and the calcination time is 1.5 - 3 h.

[0027] The beneficial effects of the present invention are:

[0028] By means of the chemical reduction precipitation method, the highly toxic Cr(VI) is reduced to Cr(III), and then an alkali is added to the solution to enrich more than 98% of the chromium in the wastewater into the chromium precipitate; two methods of chemical reduction precipitation and ion exchange resin are selected to enrich Cr(III) successively, and the treated wastewater can meet the discharge standard; the uncompletely precipitated Cr in the wastewater is adsorbed by an anion exchange resin, and then the chromium in the exchange resin is desorbed to obtain a chromium-containing solution, and then the chromium solution is converted into chromium hydroxide precipitate by adding sodium hydroxide to achieve the separation of more than 99.5% of the chromium in the chromium-containing wastewater (meeting the limit requirements for wastewater discharge); the chromium hydroxide precipitate is purified, dissolved with sulfuric acid, phosphate is added, and the pH value is adjusted with sodium carbonate to obtain a chromium phosphate precipitate; an alkali is added to convert the chromium phosphate precipitate into a chromium-containing hydrated oxide, and then the precipitate is calcined at 1100 - 1200 °C to finally obtain a high-purity chromium product (chromium sesquioxide), realizing the recycling of chromium resources and creating necessary conditions for the preparation of subsequent high-quality chromium products. The present invention overcomes the problems of incomplete chromium separation, difficulty in meeting the sewage discharge standard, and low chromium recovery rate existing in the single treatment technology of chromium-containing wastewater, and has the characteristics of simple chromium separation, enrichment and recovery process, high comprehensive recovery rate, low cost, and high purity of the recovered chromium sesquioxide product. Description of the Drawings

[0029] Figure 1 is a flow chart of the present invention. Detailed Embodiments

[0030] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] The present invention aims at the chromium recovery and utilization of textile, leather and printing and dyeing wastewater (the wastewater components are completely different from those of the patented technology in the background art), comprehensively uses the reduction precipitation method and the anion adsorption method to realize the separation and enrichment of chromium, and optimizes the separation and enrichment process. The process method is simple, the recovery rate is high, the wastewater discharge standard can be met, and the whole separation and recovery method is more environmentally friendly.

[0032] Such as Figure 1As shown in the figure, a method for chromium separation and recovery from chromium-containing wastewater includes the following steps:

[0033] Step 1: Pretreatment of chromium-containing wastewater

[0034] Filter the chromium-containing wastewater by suction filtration or filtration to remove insoluble impurities (including suspended solids, grease, etc.) in the chromium-containing wastewater, so as to improve the subsequent treatment effect.

[0035] The chromium content of the chromium-containing wastewater is 100 - 500 mg / L.

[0036] During suction filtration, use medium-speed filter paper with a pore size of 40 μm to filter the chromium-containing wastewater by suction.

[0037] Step 2: Reduction and precipitation

[0038] Add a reducing agent to the chromium-containing wastewater, stir evenly and then let it stand for 30 min to convert highly toxic hexavalent chromium (Cr(VI)) into less toxic trivalent chromium (Cr(Ⅲ)). Add NaOH to adjust the pH value of the chromium-containing wastewater to 9.5 - 11.5, stir evenly and then let it stand for 30 min to form chromium hydroxide precipitate. Filter to obtain a chromium solution with low-content trivalent chromium (Cr(Ⅲ)) and chromium hydroxide precipitate.

[0039] The stoichiometric ratio of the reducing agent to Cr(VI) in the chromium-containing wastewater is 4 - 4.5:1. The reducing agent is Na2SO3 or FeSO4.

[0040] In this step, more than 98% of the chromium in the wastewater is enriched in the precipitate.

[0041] Step 3: Chromium enrichment

[0042] ① There is still low-content trivalent chromium (Cr(Ⅲ)) in the chromium solution. Due to the influence of organic matter in the chromium solution, chromium (Cr(Ⅲ)) fails to precipitate. Therefore, biodegradable sodium oxalate is added to convert chromium ions into chromium oxalate anions, and then adsorbed with a weakly basic anion exchange resin, so that the remaining chromium in the chromium solution is enriched in the anion exchange resin. The waste liquid after adsorption treatment meets the discharge requirements, and the anion exchange resin is desorbed to obtain a chromium desorption solution.

[0043] ② Add NaOH to adjust the pH value of the chromium desorption solution to 9.5 - 11.5, stir evenly and then let it stand for 30 min to form chromium hydroxide precipitate. Filter to obtain chromium hydroxide precipitate and waste liquid that meets the discharge requirements. The chromium precipitate obtained by desorbing the precipitate is used for the next step to achieve the separation and enrichment of chromium.

[0044] In this step, salt is used as a medium, sodium oxalate is added to the chromium solution, more than 90% of the remaining chromium is enriched by anion exchange resin, and the chromium is desorbed. The total residual Cr concentration of the waste liquid from the adsorption treatment and the waste liquid from the chromium desorption precipitation is less than 1.50 mg / L, thus meeting the discharge standard or reuse standard. There is no need to add extra acid or alkali to adjust the pH value, reducing the recovery cost and reagent pollution.

[0045] Step 4: Chromium purification

[0046] ① Combine the chromium hydroxide precipitates obtained in Step 2 and Step 3, and dissolve the combined chromium hydroxide precipitate with acid to obtain a chromium solution.

[0047] Sulfuric acid is used to dissolve the chromium hydroxide precipitate, the concentration of sulfuric acid is 6 mol / L, and the stoichiometric ratio of chromium hydroxide precipitate to H2SO4 is 1:1.8 - 2.2.

[0048] ② Add phosphate, add sodium carbonate to control the pH value of the solution to 1.5 - 2.5, stir at 40 - 105 °C for 0.5 - 3 h, and let the chromium precipitate in the form of CrPO4. Filter to obtain high-purity chromium phosphate precipitate and waste liquid to remove the interference of other metal ions, and the chromium in the waste liquid meets the wastewater discharge standard.

[0049] The phosphate used is Na3PO4, and the stoichiometric ratio of Na3PO4 to Cr(Ⅲ) of the chromium hydroxide precipitate is 1.5 - 2.5:1.

[0050] Step 5: Chromium recovery

[0051] Perform suction filtration and wash with water. Add the chromium phosphate precipitate to the NaOH solution, stir and react for 0.5 - 2 h to convert the chromium phosphate precipitate into a hydrated chromium oxide precipitate, and then calcine to obtain a chromium sesquioxide product (purity greater than 99.2%).

[0052] The stoichiometric ratio of CrPO4 to NaOH is 1:4 - 5, and the concentration of the NaOH solution is 10 mol / L.

[0053] The calcination temperature is 1100 - 1200 °C, and the calcination time is 1.5 - 3 h.

[0054] Example 1

[0055] Step 1: Take 1000 ml of chromium-containing wastewater with a total Cr concentration of 500 mg / L, perform suction filtration pretreatment to remove oil stains and suspended solids.

[0056] Step 2: Add Na2SO3 to the chromium-containing wastewater (the stoichiometric ratio of Na2SO3 to Cr(VI) in the chromium-containing wastewater is 4:1), stir evenly and let stand for 30 min, then add NaOH to adjust the pH value of the solution, stir evenly and let stand for 30 min, and filter to obtain a chromium solution and a chromium hydroxide precipitate.

[0057] Step 3: The chromium solution passes through an anion exchange resin to adsorb Cr in the solution. After reaching the adsorption equilibrium, the resin is desorbed to obtain chromium hydroxide precipitate. The residual Cr concentration in the waste liquid treated by the anion exchange resin and the waste liquid of desorption precipitation is 0.823 mg / L, meeting the discharge standard.

[0058] Step 4: Purify the enriched chromium precipitate. Combine the chromium precipitates and add H2SO4 to dissolve the chromium precipitate (the stoichiometric ratio of Cr to H2SO4 is 1:2.0);

[0059] Add sodium phosphate to the solution (the stoichiometric ratio of Na3PO4 to Cr(III) is 2:1), control the pH value of the solution by adding sodium carbonate, stir at 105 °C for 0.5 h, and the chromium precipitates in the form of chromium phosphate.

[0060] Step 5: Perform suction filtration, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:4), stir at 30 °C for 2 h, and perform suction filtration to separate and obtain the hydrated chromium oxide precipitate. The hydrated chromium oxide is calcined at 1100 °C for 1.5 h to obtain a chromium trioxide product with a purity of 99.2%.

[0061] The recovery rate of chromium in the whole process is 99.5%.

[0062] Example 2

[0063] Step 1: Take 1000 ml of chromium-containing wastewater with a total Cr concentration of 300 mg / L, perform suction filtration pretreatment to remove oil and suspended solids.

[0064] Step 2: Add FeSO4 to the chromium-containing wastewater (the stoichiometric ratio of FeSO4 to Cr(VI) in the chromium-containing wastewater is 4.5:1), stir evenly and let stand for 30 min, then add NaOH to adjust the pH value of the solution, stir evenly and let stand for 30 min, and filter to obtain a chromium solution and chromium hydroxide precipitate.

[0065] Step 3:

[0066] The chromium solution passes through an anion exchange resin to adsorb Cr in the solution. After reaching the adsorption equilibrium, the resin is desorbed to obtain chromium hydroxide precipitate. The residual Cr concentration in the waste liquid treated by the anion exchange resin and the waste liquid of desorption precipitation is 0.902 mg / L, meeting the discharge standard.

[0067] Step 4: Purify the enriched chromium precipitate.

[0068] Combine the chromium precipitates and add H2SO4 to dissolve the chromium precipitate (the stoichiometric ratio of Cr to H2SO4 is 1:1.8);

[0069] Sodium phosphate (the stoichiometric ratio of Na3PO4 to Cr(Ⅲ) is 2.5:1) was added to the solution, and it was continuously stirred. The pH value of the solution was controlled by adding sodium carbonate, and it was stirred at 80 °C for 1.5 h. Chromium precipitated in the form of chromium phosphate.

[0070] Step 5: Perform suction filtration, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:5), stir at 30 °C for 0.5 h, and perform suction filtration to separate and obtain the hydrated chromium oxide precipitate. The hydrated chromium oxide was calcined at 1200 °C for 3 h to obtain a chromium sesquioxide product with a purity of 98.9%.

[0071] The recovery rate of chromium in the whole process was 99.1%.

[0072] Example 3

[0073] Step 1: Take 1000 ml of chromium-containing wastewater with a total Cr concentration of 100 mg / L, perform suction filtration pretreatment to remove oil and suspended solids.

[0074] Step 2: Add Na2SO3 to the chromium-containing wastewater (the stoichiometric ratio of Na2SO3 to Cr(VI) in the chromium-containing wastewater is 4.2:1), stir evenly and let it stand for 30 min, then add NaOH to adjust the pH value of the solution, stir evenly and let it stand for 30 min, and filter to obtain the chromium solution and chromium hydroxide precipitate.

[0075] Step 3:

[0076] The chromium solution passed through an anion exchange resin to adsorb Cr in the solution. After reaching the adsorption equilibrium, the resin was desorbed to obtain a chromium hydroxide precipitate. The residual Cr concentration in the waste liquid treated by the anion exchange resin and the waste liquid from desorption precipitation was 1.023 mg / L, meeting the discharge standard.

[0077] Step 4: Purify the enriched chromium precipitate.

[0078] The chromium precipitates were combined, and H2SO4 was added to dissolve the chromium precipitate (the stoichiometric ratio of Cr to H2SO4 is 1:2.2);

[0079] Sodium phosphate (the stoichiometric ratio of Na3PO4 to Cr(Ⅲ) is 1.5:1) was added to the solution, and it was continuously stirred. The pH value of the solution was controlled by adding sodium carbonate, and it was stirred at 45 °C for 3 h. Chromium precipitated in the form of chromium phosphate.

[0080] Step 5: Perform suction filtration, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:4.5), stir at 30 °C for 1.5 h, and perform suction filtration to separate and obtain the hydrated chromium oxide precipitate. The hydrated chromium oxide is calcined at 1150 °C for 2 h to obtain a chromium sesquioxide product with a purity of 98.0%.

[0081] The chromium recovery rate for the entire process is 98.1%.

[0082] Comparative Example 1

[0083] Compared with Example 3, the difference is that after chromium reduction and precipitation, the filtrate is not subjected to ion exchange treatment, and the chromium precipitate is directly purified. The specific steps are as follows:

[0084] Step 1: 1000 ml of chromium-containing wastewater with a Cr concentration of 500 mg / L is subjected to suction filtration pretreatment to remove oil and suspended matter.

[0085] Step 2: Add FeSO4 to the chromium-containing wastewater (the stoichiometric ratio of FeSO4 to Cr(VI) in the chromium-containing wastewater is 5:1), stir evenly and let stand for 15 min, then add NaOH to adjust the pH value of the solution, stir evenly and let stand for 15 min, and filter to obtain a chromium solution and a chromium hydroxide precipitate.

[0086] The chromium content in the chromium solution is 10 mg / L, which does not meet the discharge standard.

[0087] Step 4: Purify the enriched chromium precipitate.

[0088] Dissolve the chromium precipitate in sulfuric acid;

[0089] Add sodium phosphate to the solution (the stoichiometric ratio of Na3PO4 to Cr(III) is 2.5:1), control the pH value of the solution by adding sodium carbonate, stir at 105 °C for 0.5 h, and the chromium precipitates in the form of chromium phosphate;

[0090] Step 5: Perform suction filtration, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:4), stir at 30 °C for 2 h, and perform suction filtration to separate and obtain the hydrated chromium oxide precipitate. The hydrated chromium oxide precipitate is calcined at 1200 °C for 2 h to obtain a chromium sesquioxide product with a purity of 98.0%.

[0091] The chromium recovery rate for the entire process is 96%. It is lower than the above 3 examples.

[0092] The advantages of the present invention are as follows:

[0093] 1. Efficient separation: Through two treatment processes of first reduction precipitation and then anion exchange resin adsorption, deep separation of chromium in chromium-containing wastewater is achieved, with more than 99.5% chromium separation and meeting the discharge standard of chromium in wastewater.

[0094] 2. Low-cost enrichment: Using a reducing agent to reduce Cr(VI) and adding a precipitant, the acidic anion exchange resin adsorbent can be reused, with low treatment costs.

[0095] 3. Environmentally friendly: The whole process uses few chemical reagents and does not produce secondary pollution problems, being environmentally friendly.

[0096] 4. Resource reuse: Through recovery treatment, resource reuse of valuable metal chromium is achieved.

[0097] Preferred embodiments of the present invention are shown in the accompanying drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described in this specification. On the contrary, these embodiments are provided to make the understanding of the disclosure of the present invention more thorough and comprehensive.

Claims

1. A method for separating and recovering chromium from chromium-containing wastewater, comprising the following steps: Step 1: wastewater pretreatment; Step 2: chromium reduction precipitation; Step 3: separation and enrichment of chromium; Step 4: Chromium purification; Step 5: Chromium recovery.

2. The method for separating and recovering chromium from chromium-containing wastewater according to claim 1, characterized in that: The step 1 comprises: filtering or suction filtering the chromium-containing wastewater to remove insoluble impurities in the chromium-containing wastewater.

3. The method for separating and recovering chromium from chromium-containing wastewater according to claim 2, characterized in that: In the step 1, the chromium-containing wastewater is filtered using medium-speed filter paper with a pore size of 40 μm.

4. The method for separating and recovering chromium from chromium-containing wastewater according to claim 1 or 3, characterized in that: The step 2 comprises adding a reducing agent to reduce Cr(VI) in the chromium-containing wastewater to Cr(III), adjusting the pH value of the chromium-containing wastewater to generate chromium hydroxide precipitate from Cr(III), and filtering to obtain a chromium solution and a chromium hydroxide precipitate.

5. The method for separating and recovering chromium from chromium-containing wastewater according to claim 4, characterized in that: The step 3 comprises: ① adding sodium oxalate to the chromium solution, adsorbing the chromium solution with an anion exchange resin, and desorbing the anion exchange resin to obtain a chromium desorption solution; ② Adjust the pH value of the chromium desorption solution to make Cr(III) generate chromium hydroxide precipitate, and filter to obtain the chromium hydroxide precipitate.

6. The method for separating and recovering chromium from chromium-containing wastewater according to claim 5, characterized in that: In step 2 and step 3, adjusting the pH value to make Cr(III) generate chromium hydroxide precipitate includes: adding NaOH to adjust the pH value to 9.5-11.5, stirring evenly and then standing for 30 minutes to make Cr(III) generate chromium hydroxide precipitate.

7. The method for separating and recovering chromium from chromium-containing wastewater according to claim 6, characterized in that: The step 4 comprises: ① combining the chromium hydroxide precipitates obtained in step 2 and step 3, and dissolving the chromium hydroxide precipitates with acid to obtain a chromium solution; ② Add phosphate and sodium carbonate to adjust the pH value of the solution to 1.5-2.5, and filter to obtain chromium phosphate precipitate.

8. The method for separating and recovering chromium from chromium-containing wastewater according to claim 7, characterized in that: The step 5 comprises: suction filtration, water washing, adding alkali to convert the chromium phosphate precipitate into a chromium hydrate oxide precipitate and then calcining to obtain a chromium trioxide product.

9. The method for separating and recovering chromium from chromium-containing wastewater according to claim 8, characterized in that: In the step 5, the calcination temperature is 1100-1200° C., and the calcination time is 1.5-3 hours.

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