Method for separating and recovering chromium from chromium-containing wastewater

Abstract

The present application belongs to the field of resource utilization of textile, leather and printing and dyeing industry, and particularly relates to a chromium separation and recovery method of chromium-containing wastewater. The purpose is to provide a chromium separation and recovery method of chromium-containing wastewater to reduce cost and improve chromium recovery rate. The technical scheme is a chromium separation and recovery method of chromium-containing wastewater, which comprises the following steps: step 1, wastewater pretreatment; step 2, chromium reduction and precipitation; step 3, chromium separation and enrichment; step 4, chromium purification; and step 5, chromium recovery.

Description

Technical Field

[0001] This invention belongs to the field of resource utilization in the textile, leather, and dyeing industries, and specifically relates to a method for separating and recovering chromium from chromium-containing wastewater. Background Technology

[0002] The textile, leather, and dyeing industries generate large amounts of chromium-containing wastewater, which is one of the main sources of chromium pollution. Cr(VI) and Cr(III) are the main chemical forms of chromium in nature. Cr(VI) exhibits significant mobility, bioavailability, toxicity (acute toxicity, carcinogenicity, teratogenicity, and mutagenicity), and persistence. It enters the human body through inhalation and contact, posing a risk of teratogenicity, carcinogenicity, and mutagenicity, with a toxicity approximately 100 times that of Cr(III), posing a serious threat to ecosystems and human health. Therefore, it is necessary to effectively separate chromium from chromium-containing wastewater to ensure that wastewater discharge meets Cr limit requirements, while simultaneously achieving the resource recovery and reuse of valuable chromium, reducing chromium resource waste, and promoting the green and sustainable development of my country's textile, leather, and dyeing industries.

[0003] Patent CN115465968A discloses a method for separating and recovering chromium from chromium tanning wastewater. This method involves adding wood flour, which causes trivalent chromium ions to be converted into chromium hydroxide in situ on the surface of the wood flour and adsorbed on its surface, thereby accelerating solid-liquid separation, improving the separation purity of trivalent chromium in chromium tanning wastewater, and reducing the consumption of auxiliary coagulant resources. However, this method does not mention whether the residual amount of chromium in the final wastewater meets the discharge standards.

[0004] Patent CN110923477A discloses a method for separating and recovering chromium from chromium-containing sludge in leather tanning. This method mainly targets chromium-containing sludge, adopts concentrated sulfuric acid pretreatment, followed by water leaching, and the chromium recovery idea of ​​separating and extracting chromium from the chromium-containing leachate. This innovative process for chromium leaching and recycling achieves a good chromium leaching rate and recovery rate when treating chromium sludge. However, the sample is a solid, not a liquid.

[0005] Patent CN105016387A discloses a method for treating chromium salt mud. 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, chromium is removed by reduction, and then leaching is carried out to separate liquid and solid to obtain chromium removal solution and chromium hydroxide. The obtained chromium removal solution is crystallized and purified to realize the resource recovery and utilization of chromium.

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

[0007] The purpose of this invention is to overcome the shortcomings in the above-mentioned background technology and provide a method for separating and recovering chromium from chromium-containing wastewater, so as to reduce costs and improve chromium recovery rate.

[0008] The technical solution of this invention is:

[0009] A method for separating and recovering chromium from chromium-containing wastewater includes the following steps:

[0010] Step 1: Wastewater pretreatment;

[0011] Step 2: Chromium reduction precipitation;

[0012] Step 3: Chromium separation and enrichment;

[0013] Step 4: Chromium purification;

[0014] Step 5: Chromium recovery.

[0015] Step 1 includes: filtering or vacuuming chromium-containing wastewater to remove insoluble impurities from the chromium-containing wastewater.

[0016] In step 1, medium-speed filter paper with a pore size of 40 μm is used to filter chromium-containing wastewater.

[0017] 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 generate chromium hydroxide precipitate from Cr(III), and filtering to obtain a chromium solution and chromium hydroxide precipitate.

[0018] Step 3 includes:

[0019] ① Sodium oxalate is added to a chromium solution, and the chromium solution is adsorbed using an anion exchange resin. The chromium desorbed solution is then obtained by desorbing the anion exchange resin.

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

[0021] In steps 2 and 3, adjusting the pH value to form chromium hydroxide precipitate of Cr(III) includes: adding NaOH to adjust the pH value to 9.5-11.5, stirring evenly, and letting it stand for 30 minutes to form chromium hydroxide precipitate of Cr(III).

[0022] Step 4 includes:

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

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

[0025] Step 5 includes: filtration, water washing, calcination of chromium phosphate precipitate after adding alkali to convert it into hydrated oxide precipitate of chromium, and obtaining chromium trioxide product.

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

[0027] The beneficial effects of this invention are:

[0028] This invention utilizes a chemical reduction precipitation method to reduce highly toxic Cr(VI) to Cr(III). Then, alkali is added to the solution, enriching over 98% of the chromium in the wastewater into a chromium precipitate. Two methods, chemical reduction precipitation and ion exchange resin, are used sequentially to enrich Cr(III), ensuring the treated wastewater meets discharge standards. Incompletely precipitated Cr in the wastewater is adsorbed using anion exchange resin, followed by desorption of chromium from the resin to obtain a chromium-containing solution. This solution is then converted to chromium hydroxide precipitate with sodium hydroxide, achieving over 99.5% chromium separation from the chromium-containing wastewater (meeting wastewater discharge limits). The chromium hydroxide precipitate is purified by dissolving it in sulfuric acid, adding phosphate, and adjusting the pH with sodium carbonate to obtain chromium phosphate precipitate. Alkali is added to convert the chromium phosphate precipitate into hydrated chromium oxide, which is then calcined at 1100-1200℃ to obtain a high-purity chromium product (chromium trioxide). This achieves the recycling and reuse of chromium resources, creating the necessary conditions for the subsequent preparation of high-quality chromium products. This invention overcomes the problems of incomplete chromium separation, difficulty in meeting wastewater discharge standards, and insufficient chromium recovery rate in single-treatment technologies for chromium-containing wastewater. It features a simple chromium separation, enrichment, and recovery process, high comprehensive recovery rate, low cost, and high purity of recovered chromium trioxide products. Attached Figure Description

[0029] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0031] This invention addresses the recovery and utilization of chromium from textile, leather, and dyeing wastewater (the composition of which is completely different from that disclosed in the prior art patent). It comprehensively utilizes reduction precipitation and anion adsorption methods to achieve the separation and enrichment of chromium, and optimizes the separation and enrichment process. The process is simple, has a high recovery rate, and can meet wastewater discharge standards. The entire separation and recovery method is more green and environmentally friendly.

[0032] like Figure 1As shown, a method for separating and recovering chromium from chromium-containing wastewater includes the following steps:

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

[0034] Filtering or vacuum filtration removes insoluble impurities (including suspended solids, grease, etc.) from chromium-containing wastewater to improve subsequent treatment efficiency.

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

[0036] During filtration, medium-speed filter paper with a pore size of 40μm is used to filter chromium-containing wastewater.

[0037] Step 2: Reduction of precipitate

[0038] Add a reducing agent to the chromium-containing wastewater, stir well, and let stand for 30 minutes to convert the highly toxic hexavalent chromium (Cr(VI)) into the less toxic trivalent chromium (Cr(III)). Add NaOH to adjust the pH of the chromium-containing wastewater to 9.5-11.5, stir well, and let stand for 30 minutes to allow Cr(III) to form chromium hydroxide precipitate. Filter to obtain a chromium solution with low content of 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] This step concentrates over 98% of the chromium in the wastewater into the precipitate.

[0041] Step 3: Chromium enrichment

[0042] ① The chromium solution also contains a low content of trivalent chromium (Cr(III)). The chromium (Cr(III)) failed to precipitate due to the influence of organic matter in the chromium solution. Therefore, biodegradable sodium oxalate was added to convert chromium ions into chromium oxalate anions. Then, weakly alkaline anion exchange resin was used for adsorption treatment, so that the remaining chromium in the chromium solution was enriched in the anion exchange resin. The waste liquid after adsorption treatment met the discharge requirements. The anion exchange resin was then desorbed to obtain the chromium desorbed liquid.

[0043] ② Add NaOH to adjust the pH of the chromium desorption solution to 9.5-11.5, stir evenly and let stand for 30 minutes to allow Cr(III) to form chromium hydroxide precipitate. Filter to obtain chromium hydroxide precipitate and waste liquid that meets the discharge requirements. The chromium precipitate obtained from desorption precipitation is used for the next step of treatment to achieve the separation and enrichment of chromium.

[0044] This step uses salt as a medium. Sodium oxalate is added to the chromium solution, and more than 90% of the remaining chromium is enriched by the anion exchange resin and desorbed. The residual total Cr concentration in the waste liquid from adsorption treatment and the waste liquid from chromium desorption precipitation is less than 1.50 mg / L, thus meeting the emission standards or reuse standards. No additional acid or alkali is needed to adjust the pH value, reducing recycling costs and reagent pollution.

[0045] Step 4: Chromium purification

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

[0047] Chromium hydroxide precipitate was dissolved using sulfuric acid with a concentration of 6 mol / L. The stoichiometric ratio of chromium hydroxide precipitate to H2SO4 was 1:1.8-2.2.

[0048] ② Add phosphate and sodium carbonate to control the pH of the solution to 1.5-2.5. Stir at 40-105℃ for 0.5-3 hours to allow chromium to precipitate in the form of CrPO4. Filter to obtain high-purity chromium phosphate precipitate and waste liquid to remove interference from other metal ions. The chromium in the waste liquid meets the wastewater discharge standards.

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

[0050] Step 5: Chromium Recovery

[0051] The chromium phosphate precipitate was filtered, washed with water, and then added to NaOH solution. The mixture was stirred for 0.5-2 hours to convert the chromium phosphate precipitate into a hydrated oxide precipitate of chromium. The precipitate was then calcined to obtain chromium trioxide (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℃, and the calcination time is 1.5-3h.

[0054] Example 1

[0055] Step 1: Take 1000ml of chromium-containing wastewater with a total Cr concentration of 500mg / L and perform pretreatment by vacuum filtration to remove oil 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, let stand for 30 minutes, then add NaOH to adjust the pH value of the solution, stir evenly, let stand for 30 minutes, and filter to obtain chromium solution and chromium hydroxide precipitate.

[0057] Step 3: The chromium solution is passed through an anion exchange resin to adsorb Cr from the solution. After adsorption equilibrium is reached, 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 after desorption and precipitation is 0.823 mg / L, which meets the emission standards.

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

[0059] Sodium phosphate (Na3PO4 to Cr(III) stoichiometric ratio of 2:1) was added to the solution, and the pH value of the solution was controlled by adding sodium carbonate. The mixture was stirred at 105℃ for 0.5 h, and chromium precipitated as chromium phosphate.

[0060] Step 5: Filter, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:4), stir at 30℃ for 2 h, and filter to obtain the hydrated oxide precipitate of chromium. Calcine the hydrated oxide of chromium at 1100℃ for 1.5 h to obtain chromium trioxide product with a purity of 99.2%.

[0061] The chromium recovery rate was 99.5% throughout the process.

[0062] Example 2

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

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

[0065] Step 3:

[0066] A chromium solution is passed through an anion exchange resin, where Cr in the solution is adsorbed. After adsorption equilibrium is reached, 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 after desorption and precipitation is 0.902 mg / L, which meets the emission standards.

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

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

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

[0070] Step 5: Filter, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:5), stir at 30℃ for 0.5 h, and filter to separate the hydrated oxide precipitate of chromium. Calcine the hydrated oxide of chromium at 1200℃ for 3 h to obtain chromium trioxide product with a purity of 98.9%.

[0071] The chromium recovery rate was 99.1% throughout the process.

[0072] Example 3

[0073] Step 1: Take 1000ml of chromium-containing wastewater with a total Cr concentration of 100mg / L and perform pretreatment by vacuum filtration 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, let stand for 30 minutes, then add NaOH to adjust the pH value of the solution, stir evenly, let stand for 30 minutes, and filter to obtain chromium solution and chromium hydroxide precipitate.

[0075] Step 3:

[0076] A chromium solution is passed through an anion exchange resin, where Cr in the solution is adsorbed. After adsorption equilibrium is reached, 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 after desorption and precipitation is 1.023 mg / L, which meets the emission standards.

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

[0078] Combine the chromium precipitates and add H2SO4 to dissolve them (the stoichiometric ratio of Cr to H2SO4 is 1:2.2).

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

[0080] Step 5: Filter, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:4.5), stir at 30℃ for 1.5 h, and filter to separate the hydrated oxide precipitate of chromium. Calcine the hydrated oxide of chromium at 1150℃ for 2 h to obtain chromium trioxide product with a purity of 98.0%.

[0081] The chromium recovery rate was 98.1% throughout the process.

[0082] Comparative Example 1

[0083] Compared with Example 3, the difference is that after chromium reduction 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: 1000ml of chromium-containing wastewater with a Cr concentration of 500mg / L is pretreated by vacuum filtration to remove oil and suspended solids.

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

[0086] Step 3: The chromium content of the chromium solution is 10 mg / L, which does not meet the emission standards.

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

[0088] Chromium precipitate is dissolved in sulfuric acid;

[0089] Sodium phosphate (Na3PO4 to Cr(III) stoichiometric ratio of 2.5:1) was added to the solution, and the pH value of the solution was controlled by adding sodium carbonate. The mixture was stirred at 105℃ for 0.5 h, and chromium precipitated as chromium phosphate.

[0090] Step 5: Filter, wash with water, then add 10 mol / L NaOH (the stoichiometric ratio of CrPO4 to NaOH is 1:4), stir at 30℃ for 2 h, and filter to separate the hydrated oxide precipitate of chromium. Calcine the hydrated oxide precipitate of chromium at 1200℃ for 2 h to obtain chromium trioxide product with a purity of 98.0%.

[0091] The chromium recovery rate throughout the process was 96%, lower than the three examples mentioned above.

[0092] The advantages of this invention are:

[0093] 1. High-efficiency separation: Through two treatment processes, namely reduction precipitation followed by anion exchange resin adsorption, deep separation of chromium in chromium wastewater is achieved, with a chromium separation rate of over 99.5%, meeting the chromium discharge standards for wastewater.

[0094] 2. Low-cost enrichment: Cr(VI) is reduced using a reducing agent and a precipitant is added. The acidic anion exchange resin adsorbent can be reused, resulting in low treatment costs.

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

[0096] 4. Resource reuse: Through recycling, the valuable metal chromium is reused.

[0097] The accompanying drawings illustrate preferred embodiments of the invention. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

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: Chromium separation and enrichment; Step 4: Chromium purification; Step 5: Chromium recovery; 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 generate chromium hydroxide precipitate from Cr(III), and filtering to obtain a chromium solution and chromium hydroxide precipitate. Step 3 includes: ① Sodium oxalate is added to a chromium solution, and the chromium solution is adsorbed using an anion exchange resin. The anion exchange resin is then desorbed to obtain a chromium desorbed solution. ② Adjust the pH value of the chromium desorption solution to generate chromium hydroxide precipitate from Cr(III), and filter to obtain chromium hydroxide precipitate; Step 4 includes: ① Combine the chromium hydroxide precipitates obtained in steps 2 and 3, and dissolve the chromium hydroxide precipitates with acid to obtain a chromium solution; ② Add phosphate, adjust the pH of the solution to 1.5-2.5 with sodium carbonate, and filter to obtain chromium phosphate precipitate; Step 5 includes: filtration, water washing, adding alkali to convert the chromium phosphate precipitate into a hydrated oxide precipitate of chromium, followed by calcination to obtain the chromium trioxide product.

2. The method for chromium separation and recovery from chromium-containing wastewater according to claim 1, characterized in that: Step 1 includes: filtering or vacuuming chromium-containing wastewater to remove insoluble impurities from the chromium-containing wastewater.

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

4. The method for chromium separation and recovery from chromium-containing wastewater according to claim 3, characterized in that: In steps 2 and 3, adjusting the pH value to form chromium hydroxide precipitate of Cr(III) includes: adding NaOH to adjust the pH value to 9.5-11.5, stirring evenly, and letting it stand for 30 min to form chromium hydroxide precipitate of Cr(III).

5. The method for chromium separation and recovery from chromium-containing wastewater according to claim 4, characterized in that: In step 5, the calcination temperature is 1100-1200℃ and the calcination time is 1.5-3h.

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