Method and system for recovering chromium element in leaching waste liquid

Abstract

The invention relates to the field of chromium-containing waste liquid recovery, in particular to a method and system for recovering chromium in leaching waste liquid, and the method comprises the following steps: S1, adding alkali into the chromium-containing leaching waste liquid to adjust the pH value to 8 or above, and then adding ozone for oxidation to oxidize trivalent chromium into hexavalent chromium to obtain alkali-adjusted oxidized waste liquid; s2, adding a flocculation precipitator into the alkali-adjusting oxidation waste liquid, and removing impurities in a precipitation form to obtain a flocculation precipitation waste liquid; s3, acid is added into the flocculent precipitation waste liquid to adjust the pH value to 6.5 or below, then a reducing agent is added, hexavalent chromium is reduced into trivalent chromium, and acid-adjusted reduction waste liquid is obtained; s4, alkali is added into the acid-adjusted reduction waste liquid, the pH is adjusted to 8.5-9.5, trivalent chromium is precipitated in the form of chromic hydroxide, and chromic hydroxide precipitate and supernate are obtained. The method can enrich and recover the chromium element in the leaching waste liquid, has a high recovery rate, and has the advantages of high treatment efficiency, strong treatment capability, simple operation, stable recovery and the like.

Description

Technical Field

[0001] The invention relates to the field of chromium-containing waste liquid recovery, in particular to a method and a system for recovering chromium elements in leaching waste liquid. Background Art

[0002] According to the "National Soil Pollution Survey Bulletin", the total amount of soil pollutants in the country exceeded the standard by 16.1%, of which chromium is the main pollutant. The pollutants in chromium-contaminated soil are hexavalent chromium, and there is also a large amount of trivalent chromium with lower toxicity. Soil chromium pollution significantly reduces soil quality and leads to a decline in food quality; it is also enriched in the human body through the food chain, endangering human health.

[0003] The leaching remediation method uses chemical reagents to transfer chromium from the soil to the liquid phase, thereby reducing the chromium in the soil and achieving the purpose of soil remediation. The selection of an eluent that will not cause secondary pollution and has a good removal effect is the most important. Eluents include water, inorganic acids, artificial chelating agents, surfactants, etc. Each type of eluent has certain shortcomings. For cost considerations, water is generally used as an eluent for direct leaching. The chromium-contaminated soil leaching remediation system uses an acidic leaching agent to transfer hexavalent chromium in the soil to the eluent, and at the same time, the hexavalent chromium is reduced to trivalent chromium through a chemical reaction. At the same time, the trivalent chromium in the soil will also be transferred to the eluent during the above process.

[0004] According to "Chen Songmin. Research on the leaching and remediation technology of hexavalent chromium in contaminated soil [J]. Guangdong Chemical Industry, 2023, 50(04): 173-175+172." Using water as an eluent can achieve a removal rate of about 30%. It can be seen that after the leaching and remediation of chromium-contaminated soil is completed, the leaching waste liquid will contain a large amount of chromium. As an important metal resource, chromium can be used in industrial production such as stainless steel, auto parts, aerospace, and refractory materials. However, the domestic chromium-contaminated soil leaching and remediation system focuses on the remediation of contaminated soil and the recovery of leaching waste liquid, ignoring the recovery of chromium in the leaching waste liquid. At present, the leaching and remediation system is equipped with a leaching waste liquid disposal and reuse system. The chromium in the leaching waste liquid is reduced and precipitated in the wastewater treatment system and disposed of as hazardous waste in the form of water treatment sludge, resulting in the loss of chromium resources in the soil.

[0005] Therefore, there is an urgent need for a method for enriching and recovering chromium in leaching waste liquid to reduce the waste of chromium resources and save disposal costs. Summary of the invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a method for recovering chromium in leaching waste liquid, so as to at least achieve the effects of high treatment efficiency, strong treatment capacity, simple operation and stable recovery.

[0007] The objective of the present invention is achieved through the following technical solutions:

[0008] A method for recovering chromium in leaching waste liquid comprises the following steps:

[0009] S1: adding alkali to the chromium-containing leaching waste liquid to adjust the pH to above 8, and then adding ozone for oxidation to oxidize the trivalent chromium to hexavalent chromium to obtain an alkali-adjusted oxidation waste liquid;

[0010] S2: adding a flocculation precipitant to the alkali-adjusted oxidation waste liquid to remove impurities in the form of precipitation to obtain a flocculation precipitation waste liquid;

[0011] S3: adding acid to the flocculation precipitation waste liquid to adjust the pH to below 6.5, and then adding a reducing agent to reduce the hexavalent chromium to trivalent chromium to obtain an acid-adjusted reduced waste liquid;

[0012] S4: adding alkali to the acid-adjusted reduction waste liquid to adjust the pH to 8.5-9.5, precipitating the trivalent chromium in the form of chromium hydroxide to obtain chromium hydroxide sludge and supernatant.

[0013] Furthermore, in step S1, the chromium-containing leaching waste liquid is the leaching waste liquid obtained by eluting chromium-contaminated soil with water.

[0014] Furthermore, in step S1, the base includes potassium hydroxide or sodium hydroxide.

[0015] It is worth noting that the amount of ozone added is controlled according to the total chromium concentration in the elution liquid to ensure that the molar concentration ratio of ozone to total chromium is 1.5: 1. However, in actual engineering operations, due to the long total chromium concentration detection cycle, it is impossible to provide timely feedback to guide the amount of ozone generated. Therefore, the amount of ozone added is calculated according to the average removal rate of eluted chromium to calculate the chromium content in the elution liquid, and the amount of ozone generated is calculated as 1.5 times the theoretical reaction amount.

[0016] Furthermore, in step S2, the flocculation precipitant includes polyaluminium chloride and polyacrylamide.

[0017] Furthermore, in step S2, the impurities include particulate matter, suspended matter, colloids and metal cations.

[0018] Furthermore, in step S3, the acid is an inorganic acid, including hydrochloric acid or nitric acid.

[0019] Furthermore, in step S3, the reducing agent is sodium sulfite or sodium pyrosulfite.

[0020] It is worth noting that the amount of the reducing agent added is controlled according to the total chromium concentration in the leaching solution, and the addition ratio is to completely reduce the hexavalent chromium to twice the amount of trivalent chromium (for example, the molar amount of sodium sulfite to hexavalent chromium is 3:1). However, in actual engineering operations, since the leaching equipment is in continuous operation and the total chromium concentration detection cycle in the leaching solution is long, it is difficult to provide timely feedback to guide the reduction implementation. The total chromium concentration in the soil will have detection data before construction. Therefore, the actual operation is to calculate the chromium content in the leaching solution according to the average removal rate of the leached chromium, and assuming that it has been completely oxidized to hexavalent chromium in the previous step, the addition ratio of the reducing agent is twice the theoretical reaction amount of the complete reaction.

[0021] Furthermore, in step S4, the base includes potassium hydroxide or sodium hydroxide.

[0022] Furthermore, the method further comprises: filtering the chromium hydroxide sludge in step S4, and then heating and drying it to obtain a chromium oxide powder product.

[0023] Furthermore, the method further comprises: adjusting the pH of the supernatant in step S4 to neutral, and then recovering it as elution water.

[0024] Furthermore, the method further comprises: adjusting the pH of the filtrate obtained after the filtration to neutral, and then recovering it as rinsing water.

[0025] Another object of the present invention is to provide a system for recovering chromium in leaching waste liquid, which is used to perform the method for recovering chromium in leaching waste liquid, and the system comprises:

[0026] The alkali-adjusting oxidation system is used to adjust the pH of the chromium-containing leaching waste liquid to above 8 by adding alkali, and then add ozone for oxidation to oxidize the trivalent chromium into hexavalent chromium to obtain the alkali-adjusting oxidation waste liquid;

[0027] A flocculation and precipitation system is used to add a flocculation and precipitation agent to the alkali-adjusting oxidation waste liquid to remove impurities in the form of precipitation to obtain a flocculation and precipitation waste liquid;

[0028] An acid adjustment and reduction system is used to add acid to the flocculation and precipitation waste liquid to adjust the pH to below 6.5, and then add a reducing agent to reduce hexavalent chromium to trivalent chromium to obtain an acid adjustment and reduction waste liquid;

[0029] The alkali adjustment precipitation system is used to add alkali to the acid adjustment reduction waste liquid to adjust the pH to 8.5-9.5, precipitate the trivalent chromium in the form of chromium hydroxide, and obtain chromium hydroxide sludge and supernatant.

[0030] Furthermore, the system further comprises:

[0031] Chromium hydroxide concentration system: used to collect the chromium hydroxide sludge.

[0032] Furthermore, the system also includes:

[0033] Filter press system: used for filtering the chromium oxide sludge to remove excess water, and obtain chromium hydroxide sludge and filtrate after filtering;

[0034] Drying system: used for heating and drying the chromium hydroxide sludge after the filter pressing to obtain the chromium oxide powder product.

[0035] Furthermore, the system also includes:

[0036] pH adjustment system: used to adjust the pH of the supernatant to neutral.

[0037] Furthermore, the pH adjustment system is also used to adjust the pH of the filtrate to neutral.

[0038] Another object of the present invention is to provide a device for recovering chromium in leaching waste liquid, which is used to implement the above method or the above system.

[0039] The beneficial effects of the present invention are:

[0040] The method and system for recovering chromium in leaching waste liquid provided by the present invention can enrich and recover chromium in leaching waste liquid, with a high recovery rate, reducing waste of chromium resources and saving disposal costs. It also has the advantages of high processing efficiency, strong processing capacity, simple operation, stable recovery, etc.

[0041] The method and system for recovering chromium in leaching waste liquid provided by the present invention can also simultaneously recover water in the leaching waste liquid and reuse it as leaching liquid for leaching contaminated soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The schematic diagram of the system for recovering chromium element in the leaching waste liquid of the present invention;

[0043] Figure 2 The first half structure of the device for recovering chromium from leaching waste liquid in Example 2 (including an alkali adjustment oxidation system and a flocculation precipitation system);

[0044] Figure 3 The second half structure of the equipment for recovering chromium from leaching waste liquid in Example 2 (including an acid adjustment reduction system, an alkali adjustment precipitation system, a chromium oxide concentration system, a filter press system and a drying system);

[0045] In the figure, 1, first alkali solution feed port; 2, first alkali adjustment system; 3, first pH electrode; 4, alkali adjustment oxidation system; 5, ozone generator; 6, first stirring motor; 7, first stirring shaft; 8, first stirring blade; 9, ozone collection system; 10, sludge pump; 11, flocculation sedimentation system; 12, PAC feed port; 13, PAM feed port; 14, first sedimentation inclined plate; 15, conveying pipeline; 16, water treatment sludge disposal system; 17, acid adjustment system; 18, acid solution feed port; 19, reducing agent feed port; 20. Clean water recycling tank; 21. Chromium hydroxide concentration tank; 22. Filter press system; 23. Drying system; 63. Second stirring motor; 73. Second stirring shaft; 83. Second stirring blade; 31. Second pH electrode; 32. Third pH electrode; 61. Third stirring motor; 71. Third stirring shaft; 81. Third stirring blade; 62. Fourth stirring motor; 72. Fourth stirring shaft; 82. Fourth stirring blade; 101. Second alkali solution feed port; 202. Second alkali adjustment system; 141. Second precipitation inclined plate. DETAILED DESCRIPTION

[0046] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings, but the protection scope of the present invention is not limited to the following.

[0047] Example 1

[0048] The chromium-contaminated soil was eluted and chromium was removed by elution remediation method (using water as elution agent), and 10m3 of chromium-containing elution wastewater was obtained. 3 (After testing, hexavalent chromium was not detected in the wastewater, and the concentration of trivalent chromium was about 14070 mg / L). The obtained chromium-containing leaching wastewater was treated as follows:

[0049] 1) The chromium-containing leaching wastewater enters the alkali adjustment oxidation system, and the reaction pH is controlled above 8 by the alkali adjustment system (potassium hydroxide is used for alkali adjustment). At the same time, the ozone generator generates ozone and enters the wastewater (the amount of ozone generated is calculated based on the total chromium in the soil being dissolved into the leaching wastewater, ensuring that the molar concentration ratio of ozone to total chromium is 1.5:1), and all trivalent chromium in the solution is oxidized to hexavalent chromium. The excess ozone enters the ozone collection system to obtain the alkali adjustment oxidation wastewater.

[0050] 2) The alkali-adjusted oxidation wastewater enters the flocculation and precipitation system. By adding PAC and PAM, the particulate matter, suspended matter, colloids, metal cations, etc. in the leaching wastewater are eliminated in the form of precipitation to obtain flocculation and precipitation wastewater.

[0051] 3) The flocculation and precipitation waste liquid enters the acid adjustment and reduction system, and the reaction pH is controlled below 6.5 (using nitric acid) by the acid adjustment system. At the same time, a reducing agent (sodium metabisulfite) is added to the waste water to reduce all hexavalent chromium in the solution to trivalent chromium (the addition ratio of sodium metabisulfite is twice the consumption of the complete reaction of the two), and an acid adjustment and reduction waste liquid is obtained.

[0052] 4) The acid reduction waste liquid enters the alkali adjustment precipitation system, and the pH of the solution is controlled at 9±0.5 by adding alkaline substances (potassium hydroxide) through the alkali adjustment system, and the trivalent chromium in the solution is precipitated in the form of chromium hydroxide. The reaction supernatant enters the clear water reuse tank, and the precipitate is pumped into the chromium hydroxide concentration tank through the sludge pump.

[0053] 5) The chromium hydroxide sludge in the chromium hydroxide concentration tank is pumped into a filter press to form a chromium hydroxide filter cake, which is then dried in a drying device to obtain chromium oxide powder.

[0054] About 17.24 kg of chromium oxide powder was successfully collected, of which the purity of chromium oxide was 96.7%. The recovery rate of chromium element from the eluent was 72.12%, and no chromium element was detected in the supernatant.

[0055] Example 2

[0056] The chromium-contaminated soil was eluted and chromium was removed by elution remediation method (using water as elution agent), and 10m3 of chromium-containing elution wastewater was obtained. 3 (After testing, hexavalent chromium was not detected in the wastewater, and the concentration of trivalent chromium was about 2940 mg / L). Figure 2-3 As shown), the obtained chromium-containing leaching waste liquid is treated, and the specific method is as follows:

[0057] 1) The chromium leaching wastewater enters the alkali adjustment oxidation system 4 from the alkali solution feed port, and alkali solution (sodium hydroxide) is added from the first alkali solution feed port 1 through the first alkali adjustment system 2 to adjust the alkali, and the reaction pH is controlled to be above 8 (detected by the first pH electrode 3). At the same time, the ozone generator 5 generates ozone and enters the wastewater (the amount of ozone generated is calculated based on the total chromium in the soil being completely dissolved into the leaching wastewater, ensuring that the molar concentration ratio of ozone to total chromium is 1.5:1), and all trivalent chromium in the solution is oxidized to hexavalent chromium to obtain alkali adjustment oxidation wastewater, and the excess ozone enters the ozone collection system 9.

[0058] 2) The alkali-adjusted oxidation wastewater is introduced into the flocculation precipitation system 11, and PAC and PAM are added through the PAC feeding port 12 and the PAM feeding port 13 to eliminate the particulate matter, suspended matter, colloid, metal cations, etc. in the leaching wastewater in the form of precipitation to obtain flocculation precipitation wastewater.

[0059] 3) The flocculation precipitation waste liquid is introduced into the acid adjustment reduction system. First, the acid adjustment system 17 is used to add acid (hydrochloric acid) from the acid feed port 18 to control the pH below 6.5 (detected by the second pH electrode 31). At the same time, a reducing agent (sodium sulfite) is added to the waste water from the reducing agent feed port 19 to reduce all hexavalent chromium in the solution to trivalent chromium (the sodium sulfite addition ratio is twice the amount consumed by the complete reaction of the two), thereby obtaining an acid adjustment reduction waste liquid.

[0060] 4) The acid reduction waste liquid is passed into the alkali adjustment precipitation system, first through the alkali adjustment system 202, add alkaline substances (sodium hydroxide) from the second alkali solution feed port 101 to control the solution pH at 9±0.5 (detected by the third pH electrode 32), and precipitate the trivalent chromium in the solution in the form of chromium hydroxide. The reaction supernatant enters the clean water reuse tank, and the precipitate is pumped into the chromium hydroxide concentration tank 21 through the sludge pump 10.

[0061] 5) The chromium hydroxide sludge in the chromium hydroxide concentration tank 21 is pumped into the filter press system 22, and filtered by a filter press to obtain a chromium hydroxide filter cake, which then enters the drying system 23 and is dried by a drying device to obtain chromium oxide powder.

[0062] It should be noted that the water treatment sludge disposal system 16 in the figure is mainly used for the concentration and dehydration of water treatment sludge after PAC and PAM flocculation and precipitation. In actual projects, the sludge generated in this link is dehydrated and temporarily stored as hazardous waste. The stirring mechanism (including the first / second / third / fourth stirring motor, stirring shaft, stirring blades 6, 7, 8 / 63, 73, 83 / 61, 71, 81 / 62, 72, 82) is used for wastewater stirring, accelerating the mixing and reaction of the reagents, and avoiding floc precipitation. The first / second precipitation inclined plate 14 / 141 is mainly used to slow down the flow rate and promote floc precipitation and collection.

[0063] Finally, about 39.82 kg of chromium oxide powder was successfully collected, of which the purity of chromium oxide was 97.66%, the recovery rate of chromium from the elution waste liquid was 90.51%, and no chromium was detected in the recycled supernatant.

[0064] The above is only a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, and should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be modified within the scope of the concept described herein through the above teachings or the technology or knowledge of the relevant field. The changes and modifications made by those skilled in the art shall not deviate from the spirit and scope of the present invention, and shall be within the scope of protection of the claims attached to the present invention.

Claims

1. A method for recovering chromium from leaching waste liquid, characterized in that: The following steps are involved: S1: adding alkali to the chromium-containing leaching waste liquid to adjust the pH to above 8, and then adding ozone for oxidation to oxidize the trivalent chromium to hexavalent chromium to obtain an alkali-adjusted oxidation waste liquid; S2: adding a flocculation precipitant to the alkali-adjusted oxidation waste liquid to remove impurities in the form of precipitation to obtain a flocculation precipitation waste liquid; S3: adding acid to the flocculation precipitation waste liquid to adjust the pH to below 6.5, and then adding a reducing agent to reduce the hexavalent chromium to trivalent chromium to obtain an acid-adjusted reduced waste liquid; S4: adding alkali to the acid-adjusted reduction waste liquid to adjust the pH to 8.5-9.5, precipitating the trivalent chromium in the form of chromium hydroxide to obtain chromium hydroxide sludge and supernatant.

2. The method according to claim 1, characterized in that: In step S1, the chromium-containing leaching waste liquid is the leaching waste liquid obtained by eluting chromium-contaminated soil with water.

3. The method according to claim 1, characterized in that: In step S2, the flocculation precipitant includes polyaluminium chloride and polyacrylamide.

4. The method according to claim 1, characterized in that: In step S3, the reducing agent is sodium sulfite or sodium pyrosulfite.

5. The method according to claim 1, characterized in that Also includes: The chromium hydroxide sludge in step S4 is filtered by pressure, and then heated and dried to obtain a chromium oxide powder product.

6. The method according to claim 1, characterized in that Also includes: The pH of the supernatant in step S4 is adjusted to neutral, and then recovered as elution water.

7. A system for recovering chromium from leaching waste liquid, characterized in that: For executing the method according to any one of claims 1 to 6, the system comprises: The alkali-adjusting oxidation system is used to adjust the pH of the chromium-containing leaching waste liquid to above 8 by adding alkali, and then add ozone for oxidation to oxidize the trivalent chromium into hexavalent chromium to obtain the alkali-adjusting oxidation waste liquid; A flocculation and precipitation system is used to add a flocculation and precipitation agent to the alkali-adjusting oxidation waste liquid to remove impurities in the form of precipitation to obtain a flocculation and precipitation waste liquid; An acid adjustment and reduction system is used to add acid to the flocculation and precipitation waste liquid to adjust the pH to below 6.5, and then add a reducing agent to reduce hexavalent chromium to trivalent chromium to obtain an acid adjustment and reduction waste liquid; The alkali adjustment precipitation system is used to add alkali to the acid adjustment reduction waste liquid to adjust the pH to 8.5-9.5, precipitate the trivalent chromium in the form of chromium hydroxide, and obtain chromium hydroxide sludge and supernatant.

8. The system according to claim 7, characterized in that Also includes: Chromium hydroxide concentration system: used to collect the chromium hydroxide sludge; pH adjustment system: used to adjust the pH of the supernatant to neutral.

9. The system according to claim 7, characterized in that Also includes: Filter press system: used to filter the chromium hydroxide sludge to remove excess water; Drying system: used for heating and drying the chromium hydroxide sludge after the filter pressing to obtain the chromium oxide powder product.

10. A device for recovering chromium from leaching waste liquid, characterized in that: Used to execute the method according to any one of claims 1-6 or the system according to any one of claims 7-9.

Images