A resource treatment method for COD detection waste liquid

Abstract

A method for resource-recycling COD test wastewater utilizes the properties of AgCl and HgS, which have very low solubility products in water and are insoluble in sulfuric acid. NaCl solution is sequentially added to the wastewater. H2S gas separates silver from the wastewater in the form of AgCl and mercury in the form of HgS. Chromium is then separated as Cr(OH)3 through sulfur dioxide reduction and pH adjustment. A simple process is used to sequentially recover silver, mercury, and chromium from the COD test wastewater. The recovered products are of high purity, achieving complete separation of the heavy metals silver, mercury, and chromium. The treated wastewater then undergoes simple crystallization to precipitate sodium sulfate, allowing it to meet discharge standards. This allows the wastewater to be recycled, transforming waste into treasure and harm into benefit.

Description

Technical Field

[0001] The invention relates to a waste liquid treatment method, in particular to a waste liquid treatment method generated by COD testing using a potassium dichromate method. Background Art

[0002] Chemical oxygen demand (COD) is an important comprehensive indicator for evaluating the relative content of organic matter in water bodies and a key indicator for controlling the operation of wastewater treatment plants. Methods for determining COD include spectrophotometry, kinetics, potentiometrics, and the classical reflux method. Currently, the potassium dichromate method is the universally recognized standard method for determining COD due to its high oxidation rate, good reproducibility, and accuracy and reliability. Specifically, it measures the amount of oxidant consumed when treating a water sample with potassium dichromate under strong acid and heating conditions, expressed in mg / L of oxygen. Chemical oxygen demand refers to the amount of oxidant consumed when a water sample is treated with a certain strong oxidant under certain conditions. It is an indicator of the amount of reducing substances in the water and an important indicator of water quality. Reducing substances in water include organic matter, nitrite, ferrous salt, sulfide, etc. In the process of using this method to determine COD, in order to accelerate the oxidation of straight-chain hydrocarbon organic matter in the wastewater, a certain amount of sulfuric acid and silver sulfate solution must be added during reflux, in which silver sulfate only plays a catalytic role and is not consumed. Sometimes, reagents such as mercuric sulfate are added to eliminate the interference of chloride ions. It can be seen that COD waste liquid is highly acidic and contains high concentrations of toxic indicators and harmful substances, such as Ag. + Cr 3+ Cr 6+ 、Hg 2+ Among them, chromium and mercury are harmful to both humans and the environment, especially hexavalent chromium in COD wastewater, which is more toxic. The current maximum allowable discharge concentrations set by the "Integrated Wastewater Discharge Standard" (GB8978-1996) are: hexavalent chromium 0.5mg / L, total mercury 0.05mg / L, total silver 0.5mg / L, and total chromium 1.5mg / L. If discharged directly, it can pollute water bodies about 20,000 times its discharge volume. The typical content of various metal elements in COD test wastewater is: total mercury 9.7g / L, total silver 1.21g / L, and total chromium 3-4g / L. If COD test wastewater is discharged into water bodies without treatment, it can be enriched in organisms, posing a great threat to the environment and human health. Therefore, the harmless, reduced, and resource-based treatment of COD test wastewater is an urgent problem that needs to be solved.

[0003] Currently, there is little research on the recovery of COD wastewater. The conventional methods used are to convert heavy metal ions in the wastewater into precipitates, or to replace and separate some of the metals with active metals, or to use electrochemical methods to directly recover silver from the wastewater. In addition, in the process of recovering silver or precipitating heavy metals, the scheme of introducing anions to precipitate and separate multiple metal ions simultaneously is often used. There is little research on the direct separation and recovery of heavy metals from the solution. Although the above methods can separate and recover some heavy metals in COD wastewater, the purity of the recovered products is not high. The obtained products often require repeated purification and the preparation methods are cumbersome and complex. The heavy metal recovery rate is not high, and it is impossible to achieve a high degree of separation and recovery of Hg and Ag respectively.

[0004] CN102583815A discloses a method for treating waste liquid generated by online determination of COD using potassium dichromate method, comprising adding an interference-removing solvent to the waste liquid; adding a reducing salt to the mixed solution after the reaction; and adding an alkali to the mixed solution to make Cr 3+ 、Hg 2+ 、Ag + The ions are converted into a combined state and precipitated and collected. CN105906105B also discloses a method for treating COD determination wastewater, comprising adding ferrous sulfide to the COD determination wastewater to precipitate mercury and silver, and reducing Cr(6+) to Cr(3+), followed by a first filtration process to obtain a first filtrate; adding sodium hydroxide to the first filtrate to precipitate chromium and iron, and performing a second filtration process to obtain a second filtrate; and passing the second filtrate through a coconut shell adsorption column to collect a third filtrate.

[0005] CN103555956A discloses a method for recovering silver from COD Cr detection waste liquid. This method utilizes the fact that chlorides such as Cr and Hg dissolve in water, while AgCl has a very small solubility product in water and is insoluble in sulfuric acid. A saturated NaCl solution is added to the waste liquid to separate the silver from the waste liquid in the form of AgCl. The silver is then dissolved in a Na2SO3 solution, reduced with formaldehyde, and smelted to obtain metallic silver. This method only recovers silver and does not involve the recovery of other heavy metals in the COD waste liquid.

[0006] CN104211243B discloses a method for removing silver, mercury, and chromium from COD Cr online monitoring wastewater. Sodium chloride is added to the wastewater to react, and the AgCl generated by the reaction precipitates naturally; the supernatant is passed into a microwave chemical reaction device, cooled after microwave radiation, and then passed into a pH adjustment tank; calcium hydroxide is added to the pH adjustment tank to adjust the pH value, and the solution is allowed to settle; the supernatant is passed into a mercury removal device, and sodium sulfide is added to react, and the HgS generated by the reaction precipitates naturally; the supernatant is passed into a chromium removal device, and sodium hydroxide is added to the chromium removal device to adjust the pH value of the supernatant, and then passed into a solid-liquid separation device for solid-liquid separation. The separated liquid is filtered through an activated carbon filter and discharged. However, the invention recovers silver by adding excess sodium chloride to the wastewater for reaction. This method will simultaneously produce Hg2Cl2 precipitation, which is not conducive to the recovery and separation of Ag, and is costly and has certain requirements for the device.

[0007] CN114262100A discloses a method and device for treating COD detection waste liquid, comprising the following steps: filtering the COD detection waste liquid through a filtering unit; removing Ag+ from the waste liquid through a silver removal unit that adds sodium chloride solution, collecting the precipitate, and recovering silver; and reducing Cr in the waste liquid through a chemical reaction reduction unit that adds sodium bisulfite solution. 6+ Converted to Cr 3+ Step 4: Remove Cr from the wastewater by adding sodium hydroxide neutralization reagent to the neutralization reaction unit 3+ and Hg 2+ Step 5: Reduce the metal ions in the waste liquid through the electrodeposition unit; Step 6: Adsorb the residual metal ions in the waste liquid of step 5 through the ion exchange resin device, so that the heavy metal ions are adsorbed and meet the national emission standards. The COD detection waste liquid is treated by a combination of physical and chemical methods, which greatly reduces the Cr in the waste liquid. 6+ 、Hg 2+ 、Ag + The concentration is reduced so that it can reach the standard for direct discharge, but the separation and recovery of chromium and mercury is not achieved.

[0008] It can be seen that most of the current research on COD wastewater focuses on the recovery of precious metal silver and the discharge of liquid after precipitation of heavy metals. There is no disclosure of a technology that can sequentially recover metallic silver, mercury, and chromium in the wastewater with high purity while treating COD detection wastewater. This type of solution can effectively ensure the safe discharge of COD wastewater while also bringing certain economic benefits. At the same time, the existing disclosed silver recovery technology mostly adds excessive hydrochloric acid or sodium chloride to precipitate silver ions, and then performs impurity removal and purification. It is not noted that when excessive chloride ions are introduced into the acidic system, Hg2Cl2 is formed and precipitated together with AgCl, which will also lead to the complication of the entire silver recovery process and reduce the purity of the recovered silver. It can be seen that it is urgent to develop a resource-based treatment method for COD detection wastewater that simultaneously recovers silver, mercury, and chromium. Summary of the Invention

[0009] In order to solve the defects in the prior art, the present invention provides a resource-based treatment method for COD detection waste liquid. The method can be used for the recycling and treatment of COD detection waste liquid, adopts a simple process to realize the sequential recovery of silver, mercury and chromium in COD detection waste liquid, and the recovered products have high purity and can be directly used as raw materials for subsequent use; the recovery method is simple, and the heavy metals silver, mercury and chromium are completely separated by simple precipitation and filtration, without using other salts, and the salt concentration of the solution is not high during the entire treatment process; the heavy metal content of the COD detection waste liquid treated by this method meets the requirements of the "Integrated Sewage Discharge Standard", and the treated liquid can be directly discharged.

[0010] The object of the present invention is to provide a method for resource recovery of COD detection wastewater, comprising the following steps:

[0011] (1) Allow the recovered COD test waste liquid to stand naturally for 1 to 20 hours, so that the color of the waste liquid changes from light red to light blue. This step is to reduce the impact of the indicator on subsequent recovery and prevent the indicator from being adsorbed in the silver chloride precipitate, resulting in a decrease in the purity of silver chloride. The standing time can be set as needed, and can be 3 to 10 hours, preferably until the waste liquid color changes to light blue.

[0012] (2) Heat the COD detection waste liquid after the treatment in step (1), keep it warm at 40-70℃ for 1-5min, and then cool it to room temperature to obtain a reaction solution. This step is quite different from the conventional COD waste liquid treatment and recovery process. The inventors found that in the process of COD detection of products, the mercury present will inevitably be partially reduced, so that a small amount of mercury in the solution exists as mercurous ions. This part of mercurous ions cannot be completely removed by simple stirring. The presence of this part of mercurous particles will affect the purity of silver chloride and the recovery rate of mercury to a certain extent. The inventors found through a large number of experiments that if the waste liquid is heated for a period of time, this part of mercurous ions can be reduced by Cr 6+Complete oxidation. After repeated experiments, the inventors finally found that a treatment process of keeping the temperature at 40-70°C for 1-5 minutes can achieve complete removal of mercurous ions in the waste liquid.

[0013] (3) Measure the silver content in the reaction solution, add saturated sodium chloride solution according to the Cl:Ag molar ratio of 1 to 1.5:1, and a white AgCl precipitate will be generated in the reaction solution. After the reaction is complete, let it stand for a certain period of time, filter it, wash the precipitate repeatedly with distilled water, and dry it to obtain a powdery AgCl precipitate. The inventors found that under acidic conditions, in the presence of Cr 6+ In the system, excess Cl - When precipitating silver ions in COD wastewater, first excess Cl - It will complex with AgCl precipitation, causing AgCl to dissolve in the form of complex anions, affecting the recovery of Ag. In addition, excess Cl - It will also cause a small amount of mercurous chloride precipitation in the system. In order to improve the recovery rate of silver and avoid the influence of mercurous chloride on the recovery of silver chloride, the addition of Cl - Before introducing the chlorine-containing compound, measure the Ag in the system. + The silver content is measured by slowly adding a saturated sodium chloride solution at a Cl:Ag molar ratio of 1 to 1.5:1, which effectively prevents the presence of mercurous chloride. Preferably, a saturated sodium chloride solution is added to the reaction solution at a Cl:Ag molar ratio of 1.05 to 1.2:1. The silver chloride is dried in a vacuum drying oven at 50°C for 1 hour. The present invention measures silver content using any of atomic absorption spectrometry, X-ray fluorescence spectrometry, and titration. The titration can be performed using a KSCN standard solution and trivalent iron ions as an indicator.

[0014] (4) Add SO2 gas to the filtrate of step (3) under stirring at a flow rate of 2 to 5 L / min to remove Cr in the filtrate. 6+ All reduced to Cr 3+ ; Using gas SO2 can achieve Cr 6+ It can not only reduce the filtration efficiency but also avoid the excessive salt content in the system caused by the use of reducing salts, and will not introduce other interfering metal ions.

[0015] (5) H2S gas is introduced into the filtrate in step (4) under stirring at a flow rate of 0.5 to 1 L / min, preferably a hydrogen sulfide flow rate of 0.7 L / min, and a black HgS precipitate slowly appears in the filtrate. After the precipitation is complete, the filtrate and the black solid precipitate are filtered to obtain the filtrate and the black solid precipitate. The black solid precipitate is dried in an oven at 70°C for 1 hour to obtain HgS;

[0016] (6) Add sodium hydroxide solution to the filtrate obtained in step (5), adjust the pH of the filtrate to 6-7, continue stirring to produce Cr(OH)3 precipitation, filter and separate, and dry the precipitation to obtain Cr(OH)3. The NaOH solution used to adjust the pH in this step is a sodium hydroxide solution with a concentration of 2-5M. The pH of the system is adjusted to 6.5 to achieve the Cr in the system. 3+ All of it is converted into Cr(OH)3 and precipitated. The filtrate obtained by filtration has sodium sulfate as its main component. In order to meet the requirements for waste liquid discharge, the filtrate needs to be separated and recovered for sodium sulfate. The recovery method adopted is sodium sulfate crystallization. The specific steps of the crystallization are: evaporating and concentrating the filtrate at a concentration temperature of 50-150°C. When the filtrate is concentrated to crystallize and the solid content of the remaining liquid is 10-15%, solid-liquid separation is performed at 50-100°C to obtain sodium sulfate crystals. According to actual requirements, the crystallization process can be repeated many times. The liquid after crystallization can be directly discharged. At this time, the solution contains only a small amount of sodium chloride, which fully meets the requirements of the "Integrated Sewage Discharge Standard".

[0017] Compared with the prior art, the technical solution of this application has the following beneficial effects:

[0018] The present invention uses displacement, precipitation, filtration, and other methods to recover heavy metals from wastewater. This process can rapidly and comprehensively treat wastewater generated by COD determination using the potassium dichromate method, converting a highly toxic solution that is difficult to store and transport into a solid that is easy to store and transport and has a lower toxicity. This process has the following advantages:

[0019] 1. The present invention converts heavy metal ions into chemically precipitable solids and uses a simple process to sequentially recover silver, mercury, and chromium from COD detection wastewater. The recovered products have high purity and achieve complete separation of heavy metals silver, mercury, and chromium.

[0020] 2. The treated waste liquid can meet the discharge standards after simple crystallization to precipitate sodium sulfate, achieving rapid and effective treatment of the waste liquid and high recovery rate of COD waste liquid products;

[0021] 3. By heating the reaction before and accurately adjusting the chloride ion content, the generation of mercurous chloride is effectively eliminated, and the recovery rate of silver and the purity of silver chloride are improved. Conventional raw materials are used to reduce the generation of salts such as sodium sulfate in the system, and the salt concentration in the system does not increase significantly during the recovery process.

[0022] 4. The recycling process is simple and does not require special instruments and equipment. The reaction effect is easy to identify by color and the recycling cost is low, so that the COD test waste liquid can be recycled, turning waste into treasure and harm into benefit. DETAILED DESCRIPTION

[0023] To make the technical problems solved by the present invention, the technical solutions adopted, and the technical effects achieved more clearly, the technical solutions of the embodiments of the present invention will be further described in detail below. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making any creative efforts shall fall within the scope of protection of the present invention.

[0024] It should be noted that references to "embodiments" herein mean that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of such phrases in various locations in the specification does not necessarily refer to the same embodiment, nor does it necessarily refer to independent or alternative embodiments that are mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0025] The experimental methods in the following examples are conventional methods unless otherwise specified. The materials and reagents used in the following examples are commercially available unless otherwise specified.

[0026] Example 1

[0027] The COD test waste liquid was recovered and allowed to stand naturally for 3 hours until the color of the waste liquid changed from light red to light blue. The COD test waste liquid after the natural color change was heated to 40°C, kept warm for 5 minutes, and then cooled to room temperature to obtain a reaction solution. The Ag content in the reaction solution was measured using KSCN standard solution and trivalent iron ions as indicators. + The content of the reaction mixture is measured by slowly adding a saturated sodium chloride solution dropwise according to a Cl:Ag molar ratio of 1.05 to 1.5:1, and a white AgCl precipitate is generated in the reaction solution. After the reaction is complete, the mixture is allowed to stand for a certain period of time, filtered, and the precipitate is repeatedly washed with distilled water for multiple times to remove other adsorbed ions, and then dried to obtain a powdery AgCl precipitate. SO2 gas is introduced into the obtained filtrate under stirring at a flow rate of 2 L / min, and the reaction is continued for 10 minutes. H2S gas is continued to be introduced under stirring at a flow rate of 0.5 L / min, and a black HgS precipitate slowly appears in the filtrate. After the precipitation is complete after continuous introduction of H2S gas, the mixture is filtered to obtain a filtrate and a black solid precipitate. The black solid precipitate is dried in an oven at 70°C for 1 hour to obtain HgS by drying. A 2M sodium hydroxide solution is added to the filtered filtrate, and the pH of the filtrate is adjusted to 7. The mixture is continuously stirred to generate a Cr(OH)3 precipitate, which is separated by filtration and dried to obtain Cr(OH)3. The obtained product was evaporated and concentrated at a concentration temperature of 50° C. When crystals were precipitated and the solid content of the remaining liquid was 10%, solid-liquid separation was performed once at 50° C. to obtain sodium sulfate crystals. The liquid after crystallization was analyzed for heavy metal content.

[0028] The obtained product was analyzed and the purity of AgCl was 99.8%, the purity of mercury sulfide was 99.9%, the recovery rate of silver was 100%, and the heavy metal Ag in the final liquid was 100%. + 、Hg 2+ Cr 3+ Cr 6+ The content is almost zero, far below the national standard requirements.

[0029] Example 2

[0030] The COD test waste liquid was recovered and allowed to stand naturally for 10 hours until the color of the waste liquid changed from light red to light blue. The COD test waste liquid after the natural color change was heated at 70°C for 1 minute and then cooled to room temperature to obtain a reaction solution. The Ag in the reaction solution was measured by atomic absorption spectrometry. + The content of the reaction mixture was measured by slowly adding a saturated sodium chloride solution dropwise according to a Cl:Ag molar ratio of 1.2:1, and a white AgCl precipitate was generated in the reaction solution. After the reaction was complete, the mixture was allowed to stand for a certain period of time, filtered, and the precipitate was repeatedly washed with distilled water for several times to remove other adsorbed ions, and dried to obtain a powdery AgCl precipitate. SO2 gas was introduced into the obtained filtrate under stirring at a flow rate of 5 L / min, and the reaction was continued for 2 minutes. H2S gas was continued to be introduced under stirring at a flow rate of 0.7 L / min, and a black HgS precipitate slowly appeared in the filtrate. After the precipitation was complete after continuous introduction of H2S gas, the mixture was filtered to obtain a filtrate and a black solid precipitate. The black solid precipitate was dried in an oven at 70°C for 1 hour to obtain HgS by drying. 5M sodium hydroxide solution was added to the filtered filtrate, and the pH of the filtrate was adjusted to 6. The mixture was continuously stirred to generate a Cr(OH)3 precipitate, which was separated by filtration and dried to obtain Cr(OH)3. The obtained product was evaporated and concentrated at a concentration temperature of 150° C. until crystals precipitated and the solid content of the remaining liquid was 15%. Solid-liquid separation was then performed at 100° C. The above evaporation, concentration and solid-liquid separation steps were repeated three times to obtain sodium sulfate crystals. The liquid after the three crystallizations was analyzed for heavy metal content.

[0031] The obtained product was analyzed and the purity of AgCl was 99.7%, the purity of mercury sulfide was 99.9%, the recovery rate of silver was 100%, and the heavy metal Ag in the final liquid was 100%. + 、Hg 2+ Cr 3+ Cr 6+ The content is almost zero, far below the national standard requirements.

[0032] Example 3

[0033] The COD test waste liquid was recovered and allowed to stand naturally for 5 hours until the color of the waste liquid changed from light red to light blue. The COD test waste liquid after the natural color change was heated at 50°C for 3 minutes and then cooled to room temperature to obtain a reaction solution. The Ag content in the reaction solution was measured by X-ray fluorescence spectrometry. + The content of the reaction mixture was measured by slowly adding a saturated sodium chloride solution dropwise according to a Cl:Ag molar ratio of 1.5:1, and a white AgCl precipitate was generated in the reaction solution. After the reaction was complete, the mixture was allowed to stand for a certain period of time, filtered, and the precipitate was repeatedly washed with distilled water for several times to remove other adsorbed ions, and dried to obtain a powdery AgCl precipitate. SO2 gas was introduced into the obtained filtrate under stirring at a flow rate of 3 L / min, and the reaction was continued for 6 minutes. H2S gas was continued to be introduced under stirring at a flow rate of 1 L / min, and a black HgS precipitate slowly appeared in the filtrate. After the precipitation was complete after continuous introduction of H2S gas, the mixture was filtered to obtain a filtrate and a black solid precipitate. The black solid precipitate was dried in an oven at 70°C for 1 hour to obtain HgS by drying. 3M sodium hydroxide solution was added to the filtered filtrate, and the pH of the filtrate was adjusted to 6.5. The mixture was continuously stirred to generate a Cr(OH)3 precipitate, which was separated by filtration and dried to obtain Cr(OH)3. The obtained product was evaporated and concentrated at a concentration temperature of 70° C., and when crystals were precipitated and the solid content of the remaining liquid was 15%, solid-liquid separation was performed once at 80° C. to obtain sodium sulfate crystals. The liquid after crystallization was analyzed for heavy metal content.

[0034] The obtained product was analyzed and the purity of AgCl was 99.9%, the purity of mercury sulfide was 99.9%, the recovery rate of silver was 100%, and the heavy metal Ag in the final liquid was 99.9%. + 、Hg 2+ Cr 3+ Cr 6+ The content is almost zero, far below the national standard requirements.

[0035] Example 4

[0036] The COD test waste liquid was recovered and allowed to stand naturally for 5 hours until the color of the waste liquid changed from light red to light blue. The COD test waste liquid after the natural color change was heated at 60 ° C for 2 minutes and then cooled to room temperature to obtain a reaction solution. The Ag content in the reaction solution was measured using KSCN standard solution and trivalent iron ions as indicators. +content, slowly add saturated sodium chloride solution dropwise according to the Cl:Ag molar ratio of 1:1, and a white AgCl precipitate is generated in the reaction solution. After the reaction is complete, let it stand for a certain time, filter, and repeatedly wash the precipitate with distilled water for several times to remove other adsorbed ions, and dry to obtain a powdery AgCl precipitate; SO2 gas is introduced into the obtained filtrate under stirring conditions at a flow rate of 2 L / min, and the reaction is continued for 10 minutes. H2S gas is continued to be introduced under stirring conditions at a flow rate of 1 L / min, and a black HgS precipitate will slowly appear in the filtrate. After the precipitation is complete after continuous introduction of H2S gas, filter to obtain a filtrate and a black solid precipitate, and dry the black solid precipitate in an oven at 70°C for 1 hour to dry to obtain HgS; 5M sodium hydroxide solution is added to the filtered filtrate, the pH of the filtrate is adjusted to 7, and continuous stirring is continued to generate Cr(OH)3 precipitate, which is filtered and separated, and the precipitate is dried to obtain Cr(OH)3. The obtained product was evaporated and concentrated at a concentration temperature of 50° C. until crystals precipitated and the solid content of the remaining liquid was 10%. Then, solid-liquid separation was performed at 100° C. The above evaporation, concentration and solid-liquid separation steps were repeated five times to obtain sodium sulfate crystals. The liquid after the five crystallizations was analyzed for heavy metal content.

[0037] The obtained product was analyzed and the purity of AgCl was 99.6%, the purity of mercury sulfide was 99.9%, the recovery rate of silver was 99.8%, and the heavy metal Ag in the final liquid was 99.8%. + 、Hg 2+ Cr 3+ Cr 6+ The content is almost zero, far below the national standard. Sodium sulfate crystals are obtained, and the liquid after crystallization is analyzed for heavy metal content.

[0038] Comparative Example 1

[0039] The COD test waste liquid was recovered and placed naturally for 3 hours to change the color of the waste liquid from light red to light blue; KSCN standard solution and trivalent iron ions were used as indicators to measure the Ag content in the COD test waste liquid after the natural color change. +The content of the reaction mixture is measured by slowly adding a saturated sodium chloride solution dropwise according to a Cl:Ag molar ratio of 1.05 to 1.5:1, and a white AgCl precipitate is generated in the reaction solution. After the reaction is complete, the mixture is allowed to stand for a certain period of time, filtered, and the precipitate is repeatedly washed with distilled water for multiple times to remove other adsorbed ions, and then dried to obtain a powdery AgCl precipitate. SO2 gas is introduced into the obtained filtrate under stirring at a flow rate of 2 L / min, and the reaction is continued for 10 minutes. H2S gas is continued to be introduced under stirring at a flow rate of 0.5 L / min, and a black HgS precipitate slowly appears in the filtrate. After the precipitation is complete after continuous introduction of H2S gas, the mixture is filtered to obtain a filtrate and a black solid precipitate. The black solid precipitate is dried in an oven at 70°C for 1 hour to obtain HgS by drying. A 2M sodium hydroxide solution is added to the filtered filtrate, and the pH of the filtrate is adjusted to 7. The mixture is continuously stirred to generate a Cr(OH)3 precipitate, which is separated by filtration and dried to obtain Cr(OH)3. The obtained product was evaporated and concentrated at a concentration temperature of 50° C. When crystals were precipitated and the solid content of the remaining liquid was 10%, solid-liquid separation was performed once at 50° C. to obtain sodium sulfate crystals. The liquid after crystallization was analyzed for heavy metal content.

[0040] The obtained product was analyzed and the purity of AgCl was 99.5%, the purity of mercury sulfide was 99.9%, the recovery rate of silver was 99.7%, and the heavy metal Ag in the final liquid was 99.7%. + 、Hg 2+ Cr 3+ Cr 6+ The content is almost zero, far below the national standard requirements.

[0041] Comparative Example 2

[0042] The COD test waste liquid was recovered and allowed to stand naturally for 5 hours until the color of the waste liquid changed from light red to light blue. The COD test waste liquid after the natural color change was heated at 60 ° C for 2 minutes and then cooled to room temperature to obtain a reaction solution. The Ag content in the reaction solution was measured using KSCN standard solution and trivalent iron ions as indicators. +content, slowly add saturated sodium chloride solution dropwise according to the Cl:Ag molar ratio of 2:1, and a white AgCl precipitate is generated in the reaction solution. After the reaction is complete, let it stand for a certain time, filter, and repeatedly wash the precipitate with distilled water for several times to remove other adsorbed ions, and dry to obtain a powdery AgCl precipitate; SO2 gas is introduced into the obtained filtrate under stirring conditions at a flow rate of 2 L / min, and the reaction is continued for 10 minutes. H2S gas is continued to be introduced under stirring conditions at a flow rate of 0.5 L / min, and a black HgS precipitate will slowly appear in the filtrate. After the precipitation is complete after continuous introduction of H2S gas, filter to obtain a filtrate and a black solid precipitate, and dry the black solid precipitate in an oven at 70°C for 1 hour to dry to obtain HgS; 2M sodium hydroxide solution is added to the filtered filtrate, the pH of the filtrate is adjusted to 7, and continuous stirring is continued to generate Cr(OH)3 precipitate, which is filtered and separated, and the precipitate is dried to obtain Cr(OH)3. The obtained product was evaporated and concentrated at a concentration temperature of 50° C. When crystals were precipitated and the solid content of the remaining liquid was 10%, solid-liquid separation was performed once at 50° C. to obtain sodium sulfate crystals. The liquid after crystallization was analyzed for heavy metal content.

[0043] The obtained product was analyzed and the purity of AgCl was 99.7%, the purity of mercury sulfide was 99.5%, the recovery rate of silver was 99.1%, and the heavy metal Ag in the final liquid was 99.1%. + 、Hg 2+ Cr 3+ Cr 6+ The content is almost zero, far below the national standard requirements.

[0044] Comparative Example 3

[0045] The COD test waste liquid was recovered and allowed to stand naturally for 5 hours until the color of the waste liquid changed from light red to light blue. The COD test waste liquid after the natural color change was heated at 60 ° C for 2 minutes and then cooled to room temperature to obtain a reaction solution. The Ag content in the reaction solution was measured using KSCN standard solution and trivalent iron ions as indicators. +The content of nitric acid was measured by slowly adding saturated sodium chloride solution dropwise according to the Cl:Ag molar ratio of 5:1, and a white AgCl precipitate was generated in the reaction solution. After the reaction was complete, the mixture was allowed to stand for a certain period of time, filtered, and the precipitate was repeatedly washed with distilled water for several times to remove other adsorbed ions, and dried to obtain a powdery AgCl precipitate. SO2 gas was introduced into the obtained filtrate under stirring at a flow rate of 2 L / min, and the reaction was continued for 10 minutes. H2S gas was continued to be introduced under stirring at a flow rate of 0.5 L / min, and a black HgS precipitate slowly appeared in the filtrate. After the precipitation was complete after continuous introduction of H2S gas, the mixture was filtered to obtain a filtrate and a black solid precipitate. The black solid precipitate was dried in an oven at 70°C for 1 hour to obtain HgS by drying. 2M sodium hydroxide solution was added to the filtered filtrate, and the pH of the filtrate was adjusted to 7. The mixture was continuously stirred to generate a Cr(OH)3 precipitate, which was separated by filtration and dried to obtain Cr(OH)3. The obtained product was evaporated and concentrated at a concentration temperature of 50° C. When crystals were precipitated and the solid content of the remaining liquid was 10%, solid-liquid separation was performed once at 50° C. to obtain sodium sulfate crystals. The liquid after crystallization was analyzed for heavy metal content.

[0046] The obtained product was analyzed and the purity of AgCl was 99.4%, the purity of mercury sulfide was 98.9%, the recovery rate of silver was 98.4%, and the heavy metal Ag in the final liquid was 98.9%. + 、Hg 2+ Cr 3+ Cr 6+ The content is almost zero, far below the national standard requirements.

[0047] By comparing the above embodiments and comparative examples, the resource recovery method of the COD detection waste liquid specific to the present application can effectively solve the current problem of COD detection waste liquid discharge. The present invention uses methods such as replacement, precipitation, and filtration to recover heavy metals from the waste liquid, and can quickly and comprehensively treat the waste liquid generated by the potassium dichromate method for determining COD, thereby realizing the conversion of a solution that is not easy to store and transport and has high toxicity into a solid that is easy to store and transport and has low toxicity. The present invention uses simple means to convert heavy metal ions into a precipitable solid in a chemical state, thereby realizing the sequential recovery of silver, mercury, and chromium in the COD detection waste liquid, and the recovered product has a high purity, thereby realizing the complete separation of heavy metals silver, mercury, and chromium, and the treated waste liquid can be discharged after simple crystallization to precipitate sodium sulfate, thus turning waste into treasure and harm into benefit.

[0048] The above is a detailed introduction to a resource-based treatment method for COD detection wastewater. The above content is a further detailed description of the present invention in combination with a specific preferred embodiment, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, without departing from the concept of the present invention, its architectural form can be flexible and changeable, and a series of products can be derived. Just making a few simple deductions or replacements should be regarded as belonging to the scope of patent protection of the present invention as determined by the submitted claims.

Claims

1. A method for recycling COD detection waste liquid, characterized in that: It consists of the following steps: (1) Place the recovered COD test waste liquid naturally for 1 to 20 hours, so that the color of the waste liquid changes from light red to light blue; (2) heating the COD detection waste liquid after treatment in step (1) and keeping it at 40-70°C for 1-5 minutes to completely remove the mercurous ions in the waste liquid, and then cooling it to room temperature to obtain a reaction solution; (3) Measure the silver content in the reaction solution and add saturated sodium chloride solution according to the Cl:Ag molar ratio of 1~1.5:

1. At this time, a white AgCl precipitate is generated in the reaction solution. After the reaction is complete, let it stand for a certain period of time, filter it, and repeatedly wash the precipitate with distilled water for several times. Dry it to obtain a powdery AgCl precipitate; (4) Add SO2 gas to the filtrate of step (3) under stirring conditions at a flow rate of 2~5L / min to remove Cr in the filtrate. 6+ All reduced to Cr 3+ ; (5) Under stirring conditions, H2S gas is introduced into the filtrate in step (4) at a flow rate of 0.5~1L / min, and a black HgS precipitate slowly appears. After the precipitation is complete, the H2S gas is continuously introduced, and the filtrate and the black solid precipitate are obtained by filtering. The black solid precipitate is dried to obtain HgS; (6) Add sodium hydroxide solution to the filtrate obtained in step (5), adjust the pH of the filtrate to 6-7, continue stirring to produce Cr(OH)3 precipitate, filter and separate, and dry the precipitate to obtain Cr(OH)3.

2. The method for recycling COD detection waste liquid according to claim 1, characterized in that: In step (3), the silver content is measured by any one of atomic absorption spectrometry, X-ray fluorescence spectrometry, and titration.

3. The resource recovery treatment method for COD detection waste liquid according to claim 1, characterized in that: In step (3), a saturated sodium chloride solution is added to the reaction solution at a Cl:Ag molar ratio of 1.05 to 1.2:1, and the silver chloride is dried in a vacuum drying oven at 50° C. for 1 h.

4. The method for recycling COD detection waste liquid according to claim 1, characterized in that: In step (5), the H2S gas flow rate is 0.7 L / min, and the HgS is dried in an oven at 70°C for 1 h.

5. The method for recycling COD detection waste liquid according to claim 1, characterized in that: The concentration of the NaOH solution added in step (6) is 2~5M, and the pH of the filtrate is adjusted to 6.

5.

6. The method for recycling COD detection waste liquid according to claim 1, characterized in that: Step (6) also includes crystallizing the filtrate obtained by filtration and separation to collect sodium sulfate.

7. The method for recycling COD detection waste liquid according to claim 6, characterized in that: The specific steps of the crystallization are as follows: evaporating and concentrating the filtrate obtained in step (6) at a concentration temperature of 50 to 150° C., concentrating until crystals are precipitated and the solid content of the remaining liquid is 10 to 15%, and then performing solid-liquid separation at 50 to 100° C. to obtain sodium sulfate crystals.

8. The method for recycling COD detection waste liquid according to claim 6, characterized in that: The crystallization can be repeated multiple times.