Device and method for recovering heavy metal chromium from tanning wastewater

Abstract

A device and method for recovering heavy metal chromium from tannery wastewater. The outlet of the pretreatment tank of the device is connected to the water inlet of a pretreatment filter press via a pump. The outlet of the pretreatment filter press is connected to the water inlet of a pretreatment wastewater intermediate tank. The outlet of the pretreatment wastewater intermediate tank is connected to the water inlet of an electrolysis device via a pump. The outlet of the electrolysis device is connected to the water inlet of an aging and stirring tank via a pump. The outlet of the aging and stirring tank is connected to the water inlet of a final treatment filter press via a pump. The outlet of the final treatment filter press is connected to the water inlet of a chromium removal wastewater intermediate tank. The present invention also discloses a method for recovering heavy metal chromium from tannery wastewater. The device of the present invention has no secondary pollution, is resource-recyclable, has a simple structure, low energy consumption, and is low in cost. The method of the present invention has a high chromium recovery rate, produces chromium-removed wastewater that meets standards, has low effluent chromaticity, is resource-recyclable, is simple to operate, consumes little reagent, is low in cost, and is suitable for industrial production.

Description

Technical Field

[0001] The present invention relates to a device and method for recovering heavy metals, and in particular to a device and method for recovering heavy metal chromium from tanning wastewater. Background Art

[0002] The amount of wastewater in the leather industry is large. Generally, each production and processing of a pigskin consumes about 0.3 to 0.5 m3 of water. 3 The production and processing of a piece of salt wet cowhide consumes 1.0 to 1.5 m 3 The production and processing of a sheepskin consumes 0.2 to 0.3 m of water. 3 The production and processing of a piece of buffalo leather consumes 1.5 to 2.0 m 3 Depending on the product type and green body type, 60 to 120 m3 of water is required to produce 1 ton of raw leather. 3 As a major wastewater discharger, the tanning industry is also known for its heavy metal chromium pollution. Consequently, it has become a key sector under pollution monitoring. With stricter environmental policies and growing public awareness, the government has implemented a number of mandatory measures. The newly released "Pollutant Emission Standard for Leather and Fur Processing Industries" (GB30486-2013) requires total chromium emissions to be controlled below 1.5 mg / L. This requires companies to strengthen the proper disposal of heavy metal chromium in tanning wastewater and meet environmental standards. Therefore, the effective treatment of heavy metal chromium in chromium-containing wastewater is becoming a growing concern for tanning companies.

[0003] Chromium-containing wastewater generated by tanning plants typically contains 80-100 mg / L of chromium, primarily in the form of inorganic Cr(III). The pH ranges from 2.5 to 4.0, the chloride ion concentration ranges from 2 to 15 g / L, and the chromaticity (dilution method) ranges from 800 to 3500 times. Currently, this low-concentration chromium-containing wastewater is often treated using alkaline coagulation and sedimentation. This method involves adding alkali and coagulants to the wastewater to achieve deep chromium removal. Commonly used coagulants include ferrous sulfate, aluminum sulfate, PAC, and PFC. While this method is simple to operate, it does not completely remove chromium, making it difficult to meet the 1.5 mg / L discharge standard. This method also produces a large amount of chromium-containing sludge, which is considered hazardous waste and is also difficult to treat harmlessly.

[0004] CN102381781A discloses a chromium recovery process for chrome tanning wastewater from leather making. The process effectively diverts and separately collects the chrome tanning wastewater from leather making. The process includes a primary filtration using a 1-3 mm grid, a secondary filtration using a filter press, a reaction under a controlled pH value, and a tertiary filtration using a filter press again. The chromium trioxide content of the obtained filter residue, i.e., the chromium mud, is greater than 8 wt %, and the total chromium content of the filtrate and the discharged wastewater is ≤1.5 mg / L in terms of Cr. However, although the chromium removal effect meets the emission standards, the disadvantage is that the chromium residue obtained has a large amount of impurities and a low chromium trioxide content. It is a solid hazardous waste that is not easy to utilize. Since the chromium mud has a certain solubility when piled in the environment, it is easy to cause secondary pollution. In addition, for chromium contents in the range of 80-100 mg / L, the chromium content in the residue produced is even lower, making recovery more difficult and having less value.

[0005] CN101549925A discloses a method for electrolytically recovering hexavalent chromium from electrochromium plating wastewater, comprising the following steps: sequentially adding a sulfuric acid solution, sodium metabisulfite, and a sodium carbonate solution to the electrochromium plating wastewater; the electrochromium plating wastewater after reaction flows into a precipitator for precipitation; after precipitation, the supernatant flows into a neutralization tank for neutralization, the electrochromium plating wastewater is input into a filter press for filter pressing, and the obtained trivalent chromium salt is washed with clean water; the electrochromium plating wastewater, trivalent chromium salt, and sulfuric acid are added to an electrolytic cell of a concentrator and sulfuric acid is added to a porous tank of the concentrator; and when the hexavalent chromium concentration in the electrochromium plating wastewater in the electrolytic cell of the concentrator reaches greater than 20 g / L and the trivalent chromium concentration is less than 1 g / L, the electrochromium plating wastewater can be used as a replenisher for electrochromium plating. Although this method can recover hexavalent chromium from electroplating chromium wastewater through electrolysis, its shortcomings are: the process is complex and long, and the types and quantities of reagents added are large. More importantly, since trivalent chromium is a cation, when direct current is applied for electrolysis, under normal circumstances, trivalent chromium ions (cations) move toward the cathode and have a very low probability of moving toward the anode. Only trivalent chromium ions near the anode can lose three electrons to become hexavalent chromium ions. Therefore, the efficiency of converting trivalent chromium into hexavalent chromium through electrolysis is very low. Therefore, in actual use, the electrolysis time is too long and the cost is too high, which limits its large-scale industrial use.

[0006] CN103922521A discloses a method for treating chrome tanning wastewater and recovering chromium. This method treats chromium-containing wastewater and recovers the chromium therein through pretreatment, pH adjustment, electrocoagulation, aeration and sedimentation, sludge reaction tank treatment, and electrocatalytic oxidation. However, this method has disadvantages such as a long process, a large amount of reagents added, and low electrocatalytic oxidation efficiency. In particular, this method is more suitable for chromium-containing wastewater with a high chromium content (for example, the chromium content in the embodiment is as high as 2500 mg / L), but is less effective for wastewater with a low chromium content.

[0007] CN103695970A discloses a method for treating chromium-containing tanning wastewater and recovering metallic chromium. The method comprises performing simple pretreatment on the tanning wastewater to remove solid impurities; adding urea, boric acid, and methanol to the wastewater after removing impurities, and adjusting the pH value; and then applying electricity for electrolysis to obtain metallic chromium. However, this method is only suitable for recovering chromium from wastewater with high chromium content. Under normal circumstances, Cr 3+ Electrolysis conditions generally require that Cr in the electrolyte 3+ The concentration is 95-105 g / L. Under this condition, the DC power consumption is 11,000-18,500 kW·h / tCr. However, the chromium content in chrome-containing tanning wastewater is generally not high, especially wastewater with a chromium content below 100 mg / L. It is difficult to achieve the conditions for normal electrolysis of trivalent chromium. If low-content chromium wastewater is treated by electrolysis to produce metallic chromium, the DC power consumption will be even higher, which is uneconomical.

[0008] In summary, it is urgent to find a device and method for recovering heavy metal chromium from leather wastewater with high chromium recovery rate, chromium removal wastewater that meets standards, low effluent chroma, no secondary pollution, recyclable resources, simple operation, low chemical consumption, low energy consumption, low cost, and suitable for industrial production. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a device for recovering heavy metal chromium from tanning wastewater which has no secondary pollution, is resource-recyclable, has a simple structure, low energy consumption and low cost.

[0010] The technical problem further to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a method for recovering heavy metal chromium from tanning wastewater with high chromium recovery rate, chromium wastewater meeting standards, low effluent chroma, no secondary pollution, recyclable resources, simple operation, low chemical consumption, low cost, and suitable for industrial production.

[0011] The technical solution adopted by the present invention to solve its technical problems is as follows: a device for recovering heavy metal chromium from tanning wastewater, wherein the water outlet of a pretreatment tank is connected to the water inlet of a pretreatment filter press through a pump, the water outlet of the pretreatment filter press is connected to the water inlet of a pretreatment wastewater intermediate tank, the water outlet of the pretreatment wastewater intermediate tank is connected to the water inlet of an electrolysis device through a pump, the water outlet of the electrolysis device is connected to the water inlet of an aging and stirring tank through a pump, the water outlet of the aging and stirring tank is connected to the water inlet of a final treatment filter press through a pump, and the water outlet of the final treatment filter press is connected to the water inlet of the chromium removal wastewater intermediate tank.

[0012] The working process of the device of the present invention is as follows: first, the tanning wastewater is pretreated in a pretreatment tank, filtered by a pretreatment filter press, mechanical debris is discharged, the pretreated wastewater is stored in a pretreatment wastewater intermediate tank, the pretreated wastewater enters an electrolysis device for electrolysis, the electrolyzed wastewater is aged in an aging stirring tank, and finally, filtered by a final treatment filter press, chromium-containing ferrite is discharged, and the chromium-removed wastewater is stored in a chromium-removed wastewater intermediate tank.

[0013] Preferably, the electrolysis device is a tank electrolysis device or a cyclone electrowinning device.

[0014] Preferably, the anode and cathode of the electrolysis device are replaceable with an inert electrode or an iron plate, or the electrolysis device is composed of three electrolytic cells connected in series, wherein the anode is, in turn, an inert electrode, an iron plate, and an inert electrode. When both the anode and cathode of the electrolysis device are replaceable, the anode can be replaced with, for example, an inert electrode, an iron plate, and an inert electrode, and the cathode can be replaced with, in turn, an iron plate, an inert electrode, and an iron plate, depending on the needs of the electrolysis operation. When the electrolysis device consists of three electrolytic cells connected in series, wastewater enters the three electrolytic cells in sequence for electrolysis.

[0015] The working principle of the electrolysis device is: first, electrolysis is performed using an inert anode to produce a certain amount of oxidant in the wastewater, then, electrolysis is performed using an iron plate anode to produce a certain amount of divalent iron ions in the wastewater, and finally, electrolysis is performed again using an inert anode to further produce oxidant in the wastewater, allowing the iron ions in the wastewater to react with the chromium ions in the wastewater to generate ferrite.

[0016] Preferably, the cathode of the electrolysis device is an iron plate or an inert electrode.

[0017] Preferably, the material of the inert electrode is graphite, iridium tantalum or ruthenium iridium.

[0018] Preferably, an aeration device is provided at the bottom of the electrolysis device, which is mainly used to ensure that ferrite is generated more quickly.

[0019] Preferably, the aeration device is a hot air aeration device.

[0020] Preferably, the pretreatment tank and the aging stirring tank are both provided with a stirring device. The purpose of the pretreatment tank is to adjust the pH value of the wastewater and the chloride ion content in the wastewater. The purpose of the aging stirring tank is to further stabilize the formation of ferrite.

[0021] Preferably, the pretreatment tank and the aging stirring tank are made of plastic or corrosion-resistant stainless steel.

[0022] Preferably, the pretreatment tank is further provided with a water inlet.

[0023] Preferably, the pre-treatment filter press and the final treatment filter press are plate filter presses or diaphragm filter presses.

[0024] Preferably, both the pre-treatment filter press and the final treatment filter press are provided with a discharge port.

[0025] Preferably, the chromium removal wastewater intermediate tank is also provided with a water outlet.

[0026] The present invention further solves the technical problem by adopting the following technical solution: a method for recovering heavy metal chromium from tanning wastewater, comprising the following steps:

[0027] (1) Pretreatment: Add alkali to the tanning wastewater, stir and react, filter, and obtain pretreated wastewater;

[0028] (2) Electrolysis: The pretreated wastewater obtained in step (1) is subjected to three aeration electrolysis steps to obtain electrolyzed wastewater;

[0029] (3) Aging of ferrite: adding alkali to the electrolytic wastewater obtained in step (2), stirring and aging, filtering, and obtaining chromium-containing ferrite and chromium-removed wastewater.

[0030] Preferably, in step (1), the main components of the tanning wastewater are: Cr 80-100 mg / L, Cl - 2-15 g / L, pH value 2.5-4.1, chromaticity 800-3500 times. The tanning wastewater used in the present invention comes from a tannery. The chromaticity in the method of the present invention is obtained by dilution multiple method.

[0031] Preferably, in step (1), the alkali is one or more of sodium hydroxide, calcium hydroxide or calcium oxide. Sodium hydroxide is more preferred because it can reduce the amount of slag.

[0032] Preferably, in step (1), the amount of the base used is such that the pH value after the stirring reaction is between 5 and 8. Under the aforementioned pH conditions, the electrolysis is facilitated to proceed smoothly.

[0033] Preferably, in step (1), the stirring reaction temperature is room temperature, the stirring speed is 60-100 rpm, and the time is 30-60 min. Acid-base neutralization reaction occurs during the stirring reaction, which is more conducive to quickly reaching the pH value under the above conditions.

[0034] Preferably, in step (1), the chloride ion concentration in the pretreated wastewater is controlled at 5 to 15 g / L. If the concentration is insufficient, chloride ions are supplemented. At this chloride ion concentration, it is easier to obtain a certain amount of chlorine by electrolysis, which is used to oxidize divalent iron to trivalent iron as a component of ferrite. If the chloride ion content is too low, the electrolysis time is long and energy consumption increases. If the chloride ion content is too high, the cost increases and the salt content in the wastewater increases, which is not conducive to wastewater treatment.

[0035] Step (1) is filtered after pretreatment to remove mechanical impurities in the wastewater, thereby reducing the impact of the impurities on subsequent electrolysis and ferrite quality.

[0036] Preferably, in step (2), the anodes used in the three-step aeration electrolysis are, in order, an inert electrode, an iron plate, and an inert electrode, and the cathode is an iron plate or an inert electrode. The purpose of electrolysis is to provide the wastewater with the divalent iron ions and trivalent iron ions required for the formation of ferrite. At the same time, during the process of the divalent iron ions and trivalent iron ions forming ferrite, the chromium ions in the wastewater enter the lattice of the ferrite, forming a stable component. Aeration can not only maintain a certain temperature, but also reduce the concentration difference of the wastewater during the electrolysis process.

[0037] Preferably, in step (2), the material of the inert electrode is graphite, iridium tantalum or ruthenium iridium, etc.

[0038] Preferably, in step (2), the current density of the first aeration electrolysis is 300 to 1000 A / m 2 The current consumption is 0.6-1.6A·h / L, the temperature is 10-40°C, and the duration is 10-20 minutes. The purpose of the first aeration electrolysis is to produce a certain amount of oxidant, such as chlorine, in the wastewater. Under the described conditions, the amount of chlorine produced is more easily controlled, oxidizing some organic and inorganic matter in the wastewater to reduce color while retaining a sufficient amount of chlorine for the subsequent oxidation of divalent iron ions to trivalent iron ions, preparing the conditions for ferrite formation. Because the wastewater contains some reducing substances that consume some chlorine, the electrolysis time using an inert electrode as the anode is longer than when using an iron plate as the anode.

[0039] Preferably, in step (2), the current density of the second aeration electrolysis is 300 to 1000 A / m 2 The current consumption is 1.0-2.1A·h / L, the temperature is 20-50°C, and the time is 10-20 minutes. The amount of iron plate dissolved is controlled to be 0.95-2.0g / L of wastewater. The purpose of the second aeration electrolysis is to dissolve a certain amount of iron through electrolysis. Then, the chlorine generated by the first aeration electrolysis oxidizes some of the divalent iron to produce trivalent iron, so that the molar ratio of divalent iron to trivalent iron in the wastewater reaches 1:2, which forms the ferrite. The wastewater contains a certain amount of trivalent chromium ions, which also form a component of the ferrite along with the trivalent iron ions. Due to the low chromium content in the wastewater, the trivalent chromium ions integrate into the ferrite lattice formed by the divalent iron and trivalent iron ions, forming a strong structure. Under these conditions, the molar ratio of divalent iron to trivalent iron in the wastewater is more conducive to forming a stable ferrite, and at the specified temperature, the formation of ferrite is more favorable.

[0040] Preferably, in step (2), the current density of the third aeration electrolysis is 300 to 1000 A / m 2 , current consumption is 1.0~1.6A·h / L, temperature is 30~80℃, and time is 10~20min, until the color of wastewater changes from dark green to brown-black.

[0041] Preferably, in step (2), the aeration rate of the three aeration electrolysis steps is 0.1 to 0.2 L / min·L of wastewater, and the aeration pressure is 0.01 to 0.05 MPa. Aeration during the electrolysis process helps to reduce concentration differences by fully stirring the wastewater, which is more conducive to the smooth progress of electrolysis.

[0042] Preferably, in step (3), the base is sodium hydroxide or the like.

[0043] Preferably, in step (3), the amount of the base used is to adjust the pH value to 7 to 10. Within the pH value range, the formed ferrite is more stable, insoluble in common acids and bases, and even more insoluble in water, ensuring that the chromium therein does not cause secondary pollution.

[0044] Preferably, in step (3), the stirring and aging temperature is room temperature, the stirring speed is 60-100 rpm, and the time is 10-20 minutes. During the electrolysis process, most of the ferrite is formed, but it is not complete. It is necessary to continue stirring and aging to further age and stabilize the ferrite generated by the electrolyzed wastewater.

[0045] The beneficial effects of the present invention are as follows:

[0046] (1) The device of the present invention has a high chromium recovery rate, no secondary pollution, recyclable resources, simple structure, low energy consumption and low cost;

[0047] (2) The method of the present invention organically combines electrolysis and ferrite generation technology, with a chromium recovery rate of up to 99.94%. The chromium content in the chromium-removed wastewater is ≤0.10 mg / L, which is far lower than the GB30486-2013 emission standard (total chromium emission concentration below 1.5 mg / L). At the same time, it can remove odor and suspended solids in the wastewater, reduce the chroma to 45 times, and the effluent is clear; there is no difficult-to-treat chromium-containing sludge, no chromium-containing hazardous waste, and the wet slag production rate of ferrite slag is low, generally 10 kg / m 3 The process has no secondary pollution and the produced ferrite slag can be used as raw material for ferrochrome alloy, realizing the recycling of chromium resources. The process has simple operation, low reagent consumption, low cost and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Schematic diagram of the device for recovering heavy metal chromium from tanning wastewater according to Examples 1 and 2 of the present invention. DETAILED DESCRIPTION

[0049] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0050] The tanning wastewater used in the present invention is from a large leather manufacturing industrial park in Hebei Province. The main components of the tanning wastewater 1 are: Cr 80.00 mg / L, Cl - 5.16g / L, pH value is 2.50, chromaticity is 800 times, the main components of tanning wastewater 2 are: Cr 92.00mg / L, Cl - 2.58g / L, pH value is 3.56, chromaticity is 2500 times, the main components of tanning wastewater 3 are: Cr 96.00mg / L, Cl - 6.89g / L, pH value is 3.69, chromaticity is 3000 times, the main components of tanning wastewater 4 are: Cr 100.00mg / L, Cl - The raw materials and chemical reagents used in the examples of the present invention were obtained through conventional commercial channels unless otherwise specified.

[0051] A device for recovering heavy metal chromium from tanning wastewater Example 1

[0052] like Figure 1 As shown, the water outlet 1-2 of the pretreatment tank 1 is connected to the water inlet 3-1 of the pretreatment membrane filter press 3 through a pump 2, the water outlet 3-2 of the pretreatment membrane filter press 3 is connected to the water inlet 4-1 of the pretreatment wastewater intermediate tank 4, the water outlet 4-2 of the pretreatment wastewater intermediate tank 4 is connected to the water inlet 5-1 of the tank electrolysis device 5 through a pump 2, the water outlet 5-2 of the tank electrolysis device 5 is connected to the water inlet 6-1 of the aging stirring tank 6 through a pump 2, the water outlet 6-2 of the aging stirring tank 6 is connected to the water inlet 7-1 of the final treatment membrane filter press 7 through a pump 2, and the water outlet 7-2 of the final treatment membrane filter press 7 is connected to the water inlet 7-1 of the final treatment membrane filter press 7. It is connected to the water inlet 8-1 of the chromium removal wastewater intermediate tank 8; the anode of the tank electrolysis device 5 can be replaced with a graphite electrode or an iron plate, and the cathode can be replaced with an iron plate or a graphite electrode; a hot air aeration device 5-3 is provided at the bottom of the tank electrolysis device 5; stirring devices 1-3 and 6-3 are provided in the pretreatment tank 1 and the aging stirring tank 6; the material of the pretreatment tank 1 and the aging stirring tank 6 is corrosion-resistant stainless steel; a water inlet 1-1 is also provided on the pretreatment tank 1; discharge ports 3-3 and 7-3 are both provided on the pretreatment membrane filter press 3 and the final treatment membrane filter press 7; a water outlet 8-2 is also provided on the chromium removal wastewater intermediate tank 8.

[0053] The working process of the device of the present invention is as follows: first, the tanning wastewater is pretreated in a pretreatment tank 1, filtered through a pretreatment membrane filter press 3, and mechanical debris is discharged. The pretreated wastewater is stored in a pretreatment wastewater intermediate tank 4. The pretreated wastewater then enters a tank electrolysis device 5. According to the needs of the electrolysis operation, the anode is replaced with a graphite electrode, an iron plate, and a graphite electrode in sequence, and the cathode is replaced with an iron plate, a graphite electrode, and an iron plate in sequence. After electrolysis, the electrolyzed wastewater is aged in an aging stirring tank 6. Finally, after filtering through a final treatment membrane filter press 7, chromium-containing ferrite is discharged, and the chromium-removed wastewater is stored in a chromium-removed wastewater intermediate tank 8.

[0054] Example 2 of a device for recovering heavy metal chromium from tanning wastewater

[0055] The only difference between the device of the present invention and Example 1 is that the anode of the tank electrolysis device 5 can be replaced with an iridium-tantalum electrode, a graphite electrode or an iron plate, and the cathode can be replaced with an iron plate or a graphite electrode.

[0056] The operating process of the device of the present invention differs from that of Example 1 only in that, according to the needs of the electrolysis operation, the anode is replaced with an iridium-tantalum electrode, an iron plate, and a graphite electrode, and the cathode is replaced with an iron plate, a graphite electrode, and an iron plate. The rest is the same as in Example 1.

[0057] Example 3 of a device for recovering heavy metal chromium from tanning wastewater

[0058] The only difference between the device of the present invention and Example 1 is that the tank electrolysis device is composed of three electrolytic cells connected in series, wherein the anode is an iridium-tantalum electrode, an iron plate, and a graphite electrode, and the cathode is an iron plate, an iridium-tantalum electrode, and an iron plate. The rest is the same as Example 1.

[0059] The working process of the device of the present invention is different from that of Example 1 only in that the pretreated wastewater then enters the tank electrolysis device composed of three electrolytic cells connected in series for electrolysis. The rest is the same as Example 1.

[0060] Example 4 of a device for recovering heavy metal chromium from tanning wastewater

[0061] The only difference between the present invention and Example 3 is that the tank electrolysis device is composed of three electrolytic cells connected in series, wherein the anode is a ruthenium-iridium electrode, an iron plate, and a ruthenium-iridium electrode, and the cathode is an iron plate, a graphite electrode, and an iron plate. The rest is the same as Example 3.

[0062] The working process of the device of the present invention is the same as that of Example 3.

[0063] A method for recovering heavy metal chromium from tanning wastewater Example 1

[0064] (1) Pretreatment: Add NaOH to 1 L of tanning wastewater 1, stir at room temperature and 60 rpm, and react for 30 min until the pH value reaches 5.00. Filter to obtain pretreated wastewater;

[0065] (2) Electrolysis: The pretreated wastewater obtained in step (1) was subjected to three aeration electrolysis steps to obtain electrolyzed wastewater (end point pH value was 6.5); wherein, the current density of the first aeration electrolysis (anode was graphite electrode, cathode was iron plate) was 300A / m 2 , the current consumption was 0.61A·h / L, the temperature was 15℃, and the time was 10 min; the current density of the second aeration electrolysis (the anode was an iron plate and the cathode was a graphite electrode) was 300A / m 2 , the current consumption is 1.02A·h / L, the temperature is 25℃, the time is 10min, and the amount of iron plate dissolved is controlled to be 0.97g / L wastewater; the current density of the third aeration electrolysis (anode is graphite electrode, cathode is iron plate) is 300A / m 2 , the current consumption is 1.02A·h / L, the temperature is 30°C, and the time is 10min, until the color of the wastewater changes from dark green to brown-black; the aeration volume of the three electrolysis is 0.1L / min·L wastewater, and the aeration pressure is 0.01MPa;

[0066] (3) Aging of ferrite: NaOH was added to the electrolytic wastewater obtained in step (2) until the pH value was 7.00, and the mixture was stirred and aged for 10 min at room temperature and a stirring speed of 60 rpm. The mixture was filtered to obtain 2.20 g of chromium-containing ferrite (dry basis, including 43.18% Fe and 3.632% Cr) and 0.98 L of chromium-removed wastewater (0.08 mg / L Cr, 46 times the chroma, and a pH value of 7.01).

[0067] After testing, the recovery rate of Cr was 99.90%, which can be used as the raw material of ferrochrome alloy; the chromium content in the chromium removal wastewater was far lower than the GB30486-2013 emission standard. At the same time, the odor and suspended matter in the wastewater were removed, the color was reduced, and the effluent was clear.

[0068] A method for recovering heavy metal chromium from tanning wastewater Example 2

[0069] (1) Pretreatment: Add NaOH to 4 L of tanning wastewater 2, stir at room temperature and 70 rpm, and react for 35 min until the pH value reaches 5.51. Filter and add chloride ions to 9.36 g / L to obtain pretreated wastewater.

[0070] (2) Electrolysis: The pretreated wastewater obtained in step (1) was subjected to three aeration electrolysis steps to obtain electrolyzed wastewater (end point pH value was 8.04); wherein, the current density of the first aeration electrolysis (the anode was an iridium tantalum electrode and the cathode was an iron plate) was 1000 A / m 2 , the current consumption was 1.30A·h / L, the temperature was 25℃, and the time was 12 min; the current density of the second aeration electrolysis (the anode was an iron plate and the cathode was a graphite electrode) was 1000A / m 2 The current consumption is 1.81A·h / L, the temperature is 35℃, the time is 14min, and the amount of iron plate dissolved is controlled to be 1.72g / L wastewater; the current density of the third aeration electrolysis (anode is graphite electrode, cathode is iron plate) is 1000A / m 2 , the current consumption is 1.38A·h / L, the temperature is 40°C, and the time is 12min, until the color of the wastewater changes from dark green to brown-black; the aeration volume of the three electrolysis is 0.12L / min·L wastewater, and the aeration pressure is 0.02MPa;

[0071] (3) Aging of ferrite: NaOH was added to the electrolytic wastewater obtained in step (2) until the pH value was 8.09. The mixture was stirred and aged for 15 min at room temperature with a stirring speed of 85 rpm. The mixture was filtered to obtain 12.15 g of chromium-containing ferrite (dry basis, including 54.73% Fe and 3.027% Cr) and 3.86 L of chromium-removed wastewater (0.07 mg / L Cr, 49 times the chroma, and a pH of 8.10).

[0072] After testing, the recovery rate of Cr was 99.94%, which can be used as the raw material of ferrochrome alloy; the chromium content in the chromium removal wastewater was far lower than the GB30486-2013 emission standard. At the same time, the odor and suspended matter in the wastewater were removed, the color was reduced, and the effluent was clear.

[0073] A method for recovering heavy metal chromium from tanning wastewater Example 3

[0074] (1) Preprocessing: within 10m 3 NaOH was added to the tanning wastewater 3, and the mixture was stirred at room temperature and a stirring speed of 85 rpm for 40 min until the pH value reached 7.12, and then filtered to obtain pretreated wastewater;

[0075] (2) Electrolysis: The pretreated wastewater obtained in step (1) was subjected to three aeration electrolysis steps to obtain electrolyzed wastewater (end point pH value was 8.01); wherein, the current density of the first aeration electrolysis (the anode was an iridium tantalum electrode and the cathode was an iron plate) was 800 A / m 2 , the current consumption is 1.52A·h / L, the temperature is 35℃, and the time is 15min; the current density of the second aeration electrolysis (anode is iron plate, cathode is iridium tantalum electrode) is 800A / m 2, the current consumption is 2.04A·h / L, the temperature is 45℃, the time is 20min, and the amount of iron plate dissolved is controlled to be 1.94g / L wastewater; the current density of the third aeration electrolysis (anode is graphite electrode, cathode is iron plate) is 800A / m 2 , the current consumption is 1.53A·h / L, the temperature is 58°C, and the time is 14 minutes, until the color of the wastewater changes from dark green to brown-black; the aeration volume of the three electrolysis is 0.18L / min·L wastewater, and the aeration pressure is 0.04MPa;

[0076] (3) Aging of ferrite: NaOH was added to the electrolytic wastewater obtained in step (2) until the pH value reached 9.00, and the mixture was stirred and aged for 20 min at room temperature and a stirring speed of 90 rpm. The mixture was filtered to obtain 30.45 kg of chromium-containing ferrite (dry basis, of which Fe 61.98%, Cr 3.150%) and 9.8 m 3 Chromium removal wastewater (Cr 0.06 mg / L, chroma 45 times, pH 9.00).

[0077] After testing, the recovery rate of Cr was 99.91%, which can be used as raw material for ferrochrome alloy; the chromium content in the chromium removal wastewater was far lower than the GB30486-2013 emission standard. At the same time, the odor and suspended matter in the wastewater were removed, the color was reduced, and the effluent was clear.

[0078] A method for recovering heavy metal chromium from tanning wastewater Example 4

[0079] (1) Pretreatment: within 15m 3 NaOH was added to the tanning wastewater 4, and the mixture was stirred at room temperature and a stirring speed of 100 rpm for 60 minutes until the pH value reached 8.00, and then filtered to obtain pretreated wastewater;

[0080] (2) Electrolysis: The pretreated wastewater obtained in step (1) was subjected to three aeration electrolysis steps to obtain electrolyzed wastewater (end point pH value was 9.45); wherein, the current density of the first aeration electrolysis (anode was ruthenium iridium electrode, cathode was iron plate) was 500A / m 2 , the current consumption was 1.01A·h / L, the temperature was 35℃, and the time was 20 min; the current density of the second aeration electrolysis (the anode was an iron plate and the cathode was a graphite electrode) was 500A / m 2 The current consumption is 1.27A·h / L, the temperature is 48℃, the time is 18min, and the amount of iron plate dissolved is controlled to be 1.21g / L wastewater; the current density of the third aeration electrolysis (anode is ruthenium iridium electrode, cathode is iron plate) is 500A / m 2, the current consumption is 1.24A·h / L, the temperature is 58°C, and the time is 20min, until the color of the wastewater changes from dark green to brown-black; the aeration volume of the three electrolysis is 0.2L / min·L wastewater, and the aeration pressure is 0.05MPa;

[0081] (3) Aging of ferrite: NaOH was added to the electrolytic wastewater obtained in step (2) until the pH value was 10.00, and the mixture was stirred and aged for 20 min at room temperature and a stirring speed of 100 rpm. The mixture was filtered to obtain 29.48 kg of chromium-containing ferrite (dry basis, of which Fe 60.42%, Cr 5.083%) and 14.8 m 3 Chromium removal wastewater (Cr 0.10 mg / L, chroma 48 times, pH 10.00).

[0082] After testing, the recovery rate of Cr was 99.90%, which can be used as the raw material of ferrochrome alloy; the chromium content in the chromium removal wastewater was far lower than the GB30486-2013 emission standard. At the same time, the odor and suspended matter in the wastewater were removed, the color was reduced, and the effluent was clear.

Claims

1. A method for recovering heavy metal chromium from tanning wastewater, characterized in that: The following steps are involved: (1) Pretreatment: adding alkali to the tanning wastewater, stirring and reacting, filtering, and obtaining pretreated wastewater; the amount of the alkali is such that the pH value after the stirring reaction is 5 to 8; the chloride ion concentration in the pretreated wastewater needs to be controlled at 5 to 15 g / L, and if it is insufficient, the chloride ion is supplemented; (2) Electrolysis: The pretreated wastewater obtained in step (1) is subjected to three aeration electrolysis cycles to obtain electrolyzed wastewater; the anodes used in the three aeration electrolysis cycles are, in sequence, an inert electrode, an iron plate, and an inert electrode; the cathodes are, in sequence, an iron plate, an inert electrode, and an iron plate; the current density of the first aeration electrolysis cycle is 300 to 1000 A / m 2 The current consumption is 0.6~1.6A·h / L, the temperature is 10~40℃, and the time is 10~20min; the current density of the second aeration electrolysis is 300~1000A / m 2 The current consumption is 1.0~2.1A·h / L, the temperature is 20~50℃, the time is 10~20min, and the amount of iron plate dissolved is controlled to be 0.95~2.0g / L wastewater; the current density of the third aeration electrolysis is 300~1000A / m 2 , the current consumption is 1.0~1.6A·h / L, the temperature is 30~80℃, and the time is 10~20min, until the color of the wastewater changes from dark green to brown-black; (3) Aging of ferrite: adding alkali to the electrolytic wastewater obtained in step (2), stirring and aging, filtering, and obtaining chromium-containing ferrite and chromium-removed wastewater; The device used in the method is as follows: the water outlet of the pretreatment tank is connected to the water inlet of the pretreatment filter press through a pump, the water outlet of the pretreatment filter press is connected to the water inlet of the pretreatment wastewater intermediate tank, the water outlet of the pretreatment wastewater intermediate tank is connected to the water inlet of the electrolysis device through a pump, the water outlet of the electrolysis device is connected to the water inlet of the aging and stirring tank through a pump, the water outlet of the aging and stirring tank is connected to the water inlet of the final treatment filter press through a pump, and the water outlet of the final treatment filter press is connected to the water inlet of the chromium removal wastewater intermediate tank; the electrolysis device is composed of three electrolysis cells connected in series, whose anodes are, in sequence, an inert electrode, an iron plate, and an inert electrode; the cathode of the electrolysis device is, in sequence, an iron plate, an inert electrode, and an iron plate; an aeration device is provided at the bottom of the electrolysis device; and the aeration device is a hot air aeration device.

2. The method for recovering heavy metal chromium from tanning wastewater according to claim 1, characterized in that: The electrolysis device is a tank electrolysis device or a cyclone electrowinning device; the material of the inert electrode is graphite, iridium tantalum or ruthenium iridium.

3. The method for recovering heavy metal chromium from tanning wastewater according to claim 1 or 2, characterized in that: The pretreatment tank and the aging stirring tank are both provided with a stirring device; the materials of the pretreatment tank and the aging stirring tank are plastic or corrosion-resistant stainless steel; the pretreatment tank is also provided with a water inlet; the pretreatment filter press and the final treatment filter press are plate filter presses or diaphragm filter presses; the pretreatment filter press and the final treatment filter press are both provided with a discharge port; the chromium removal wastewater intermediate tank is also provided with a water outlet.

4. The method for recovering heavy metal chromium from tanning wastewater according to claim 1 or 2, characterized in that: In step (1), the main components of the tanning wastewater are: Cr 80-100 mg / L, Cl - 2 to 15 g / L; the pH value of the tanning wastewater is 2.5 to 4.1, and the chromaticity is 800 to 3500 times; the alkali is one or more of sodium hydroxide, calcium hydroxide, and calcium oxide; the stirring reaction temperature is room temperature, the stirring speed is 60 to 100 rpm, and the time is 30 to 60 min.

5. The method for recovering heavy metal chromium from tanning wastewater according to claim 3, characterized in that: In step (1), the main components of the tanning wastewater are: Cr 80-100 mg / L, Cl - 2 to 15 g / L; the pH value of the tanning wastewater is 2.5 to 4.1, and the chromaticity is 800 to 3500 times; the alkali is one or more of sodium hydroxide, calcium hydroxide, and calcium oxide; the stirring reaction temperature is room temperature, the stirring speed is 60 to 100 rpm, and the time is 30 to 60 min.

6. The method for recovering heavy metal chromium from tanning wastewater according to claim 1 or 2, characterized in that: In step (2), the aeration volume of the three-step aeration electrolysis is 0.1 to 0.2 L / min·L wastewater, and the aeration pressure is 0.01 to 0.05 MPa.

7. The method for recovering heavy metal chromium from tanning wastewater according to claim 3, characterized in that: In step (2), the aeration volume of the three-step aeration electrolysis is 0.1 to 0.2 L / min·L wastewater, and the aeration pressure is 0.01 to 0.05 MPa.

8. The method for recovering heavy metal chromium from tanning wastewater according to claim 4, characterized in that: In step (2), the aeration volume of the three-step aeration electrolysis is 0.1 to 0.2 L / min·L wastewater, and the aeration pressure is 0.01 to 0.05 MPa.

9. The method for recovering heavy metal chromium from tanning wastewater according to claim 1 or 2, characterized in that: In step (3), the base is sodium hydroxide; the amount of the base is used to adjust the pH value to 7-10; the stirring and aging temperature is room temperature, the stirring speed is 60-100 rpm, and the time is 10-20 min.

10. The method for recovering heavy metal chromium from tanning wastewater according to claim 3, characterized in that: In step (3), the base is sodium hydroxide; the amount of the base is used to adjust the pH value to 7-10; the stirring and aging temperature is room temperature, the stirring speed is 60-100 rpm, and the time is 10-20 min.

11. The method for recovering heavy metal chromium from tanning wastewater according to claim 4, characterized in that: In step (3), the base is sodium hydroxide; the amount of the base is used to adjust the pH value to 7-10; the stirring and aging temperature is room temperature, the stirring speed is 60-100 rpm, and the time is 10-20 min.

12. The method for recovering heavy metal chromium from tanning wastewater according to claim 6, characterized in that: In step (3), the base is sodium hydroxide; the amount of the base is used to adjust the pH value to 7-10; the stirring and aging temperature is room temperature, the stirring speed is 60-100 rpm, and the time is 10-20 min.

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