A device and method for recycling wastewater from stainless steel industry and crystallizing concentrated water

Abstract

The present invention discloses a device and method for recycling medium and crystallizing concentrated water from stainless steel industrial wastewater, comprising the following steps: stainless steel pickling wastewater and oily wastewater are respectively pre-treated and enter a water distribution tank, and then enter a two-stage denitrification reactor, an aerobic tank, and a secondary sedimentation tank for treatment, and then enter a reclaimed water recycling system, and enter a reclaimed water recycling tank after being treated with UF+RO+ROR+MVR. The present invention adopts wastewater source diversion, segmented treatment and sludge classification and recovery processes, designs different pretreatment processes respectively, and adds chemicals in a targeted manner, which can achieve a simultaneous reduction in the amount of chemicals and sludge production. Combined with biochemical treatment and the reclaimed water recycling system, the reclaimed water recycling rate is increased to 95%-98%, and the system sludge is classified, collected and dehydrated. It can not only completely and comprehensively remove various pollutants in the wastewater and ensure its stable treatment effect, but also the high and medium water recycling rates can greatly reduce daily operating costs and realize the resource utilization of wastewater / sludge.

Description

Technical Field

[0001] The present invention relates to the technical field of wastewater treatment, and in particular to a device and method for recycling wastewater from stainless steel industry and crystallizing concentrated water. Background Art

[0002] Stainless steel offers excellent corrosion resistance, durability, high and low-temperature performance, excellent processability, and impact resistance. Its ease of recycling has led to a steady increase in stainless steel production, and its widespread use in industrial and civilian applications such as construction, petrochemicals, and automotive, with promising applications. However, the disposal of pickling wastewater and oily wastewater generated during the stainless steel production process remains a major challenge.

[0003] After various mechanical processes, a thin and dense oxide film will form on the surface of stainless steel. Its main components contain a large amount of chromium oxide (Cr2O3), nickel oxide (NiO), and insoluble iron chromium oxide (FeO·Cr2O3). This oxide film needs to be completely removed by surface treatment such as pickling. Pickling 1 ton of stainless steel will produce 1 to 3 m 3 Stainless steel pickling wastewater, a large amount of which is discharged into water bodies, contains components that can be harmful to the environment and human health and must be treated harmlessly. Stainless steel surface treatment is mainly based on pickling. Since Cr2O3 is difficult to dissolve in a single acid, pickling usually uses a mixture of nitric acid, sulfuric acid, and hydrofluoric acid.

[0004] Pickling is divided into pre-pickling and final pickling. Pre-pickling is to remove or partially remove the oxide scale on the surface of stainless steel according to the degree of oxidation, and final pickling is to remove the residual oxide and chromium deposits. Wastewater mainly comes from processes such as stainless steel pickling and post-degreasing rinsing. It contains a large amount of iron ions, chromium ions, nickel ions, fluorides and nitrides, etc., and its composition is very complex. At present, the main bottleneck restricting the upgrading and continued growth of the stainless steel industry is the wastewater generated in its production process. The wastewater is highly acidic (pH value is 1-2), high in total nitrogen (more than 3000 mg / L), highly toxic (containing toxic pollutants such as nickel, chromium, and fluorine), and has a large output (about 1-3m 3 / t), difficult to handle, etc.

[0005] Stainless steel pickling wastewater contains Cr 6+ Cr 3+ The acidic wastewater produced by the stainless steel industry contains a large amount of COD, TN, petroleum and other pollutants. Therefore, it is of great significance to research and develop a reclaimed water reuse device for wastewater treatment of stainless steel industry wastewater. Summary of the Invention

[0006] In response to the deficiencies in the prior art, the present invention provides a device and method for recycling recycled water and crystallizing concentrated water from stainless steel industrial wastewater. Pickling wastewater and oily wastewater in the stainless steel industrial wastewater are classified, collected, and treated, and a step-by-step sedimentation method is adopted for both. The wastewater is treated in steps according to the different precipitation conditions of various metal and other pollutants, thereby improving treatment efficiency, reducing sludge production, and reducing treatment costs. A two-stage denitrification reactor and an aerobic tank are combined in the biochemical treatment stage to reduce the addition of carbon sources, improve denitrification efficiency, and improve the efficiency of biochemical treatment of organic matter. The recycled water recycling system has a high utilization rate and less crystals and waste residues. Various pollutants in the wastewater are completely and comprehensively removed with stable treatment effects. The high recycled water recycling rate can greatly reduce daily operating costs and realize resource utilization of wastewater / sludge.

[0007] In order to solve the above technical problems, the present invention provides a method for recycling water and crystallizing concentrated water from stainless steel industrial wastewater, wherein the stainless steel industrial wastewater includes stainless steel pickling wastewater and oily wastewater, comprising the following steps:

[0008] S1. The stainless steel pickling wastewater is collected in the pickling wastewater regulating tank and then enters the two-stage coagulation and sedimentation pretreatment system to remove organic matter, SS (suspended solids) and heavy metal ions in the wastewater. Specifically:

[0009] a1. The stainless steel pickling wastewater enters the primary reaction tank, where ammonium bisulfite is first added to reduce the highly toxic hexavalent chromium to trivalent chromium. Then, lime milk is added to precipitate hydroxides of metal ions including trivalent chromium, trivalent iron, and divalent nickel, and to precipitate calcium fluoride of fluoride. Finally, PAC (polyaluminum chloride) and PAM (polyacrylamide) are added for flocculation.

[0010] a2. The effluent from the primary reaction tank enters the primary sedimentation tank for sedimentation and water separation;

[0011] a3. The effluent from the primary sedimentation tank enters the secondary reaction tank, where heavy precipitant, lime milk, PAC and PAM are added for further flocculation;

[0012] a4. The effluent from the secondary reaction tank enters the secondary sedimentation tank for further sedimentation and separation of mud and water;

[0013] The oily wastewater is collected in the oily wastewater regulating tank and then enters the coagulation sedimentation + flotation pretreatment system to remove grease and SS in the wastewater. Specifically:

[0014] b1. The oily wastewater enters the pretreatment reaction tank I, where lime milk, demulsifier, PAC and PAM are added to flocculate organic matter, colloidal matter and suspended solids;

[0015] b2. The effluent from the pretreatment reaction tank I enters the pretreatment sedimentation tank I for sedimentation and water separation;

[0016] b3. The effluent from the pretreatment sedimentation tank I enters the pretreatment reaction tank II, where PAC and PAM are added for further flocculation;

[0017] b4. The effluent from the pretreatment reaction tank II enters the flotation tank. Air is introduced into the wastewater and precipitated from the water in the form of tiny bubbles, which serve as carriers. Pollutants such as emulsified oil and tiny suspended particles in the wastewater adhere to the bubbles and float to the surface of the water along with the bubbles, forming foam (a three-phase mixture of air, water, and particles (oil)). The foam or scum is collected to separate impurities and purify the wastewater, further reducing the concentration of emulsified oil and suspended matter.

[0018] S2. The effluent from the secondary sedimentation tank and the flotation tank is mixed in the water distribution tank and then enters the biochemical treatment stage, specifically:

[0019] S21, the effluent from the water distribution tank enters a two-stage denitrification reactor, where anaerobic microorganisms are used to hydrolyze, acidify, and denitrify the refractory organic matter into easily degradable organic matter, and decompose the macromolecular organic matter into small molecular organic matter, thereby increasing the BOD (biochemical oxygen demand) / COD (chemical oxygen demand) ratio and reducing nitrates into nitrogen gas.

[0020] S22, the effluent from the two-stage denitrification reactor enters the aerobic tank to remove NH3-N and BOD5 (biochemical oxygen demand per day) in the wastewater;

[0021] S23, the effluent from the aerobic tank enters the secondary sedimentation tank for mud-water separation and then enters the biochemical effluent tank;

[0022] S3, the effluent from the biochemical effluent pool enters the reclaimed water reuse system, specifically:

[0023] The effluent from the biochemical effluent pool passes through a filtration device, an ultrafiltration (UF) device, and a reverse osmosis device in sequence, intercepting inorganic ions, colloidal substances, and macromolecular solutes in the water, and then enters a reclaimed water reuse pool.

[0024] Furthermore, the reverse osmosis device includes RO (reverse osmosis module) and ROR (concentrated water reverse osmosis module). RO produces 50%-60% fresh water which enters the reclaimed water reuse pool, and the concentrated water produced enters the ROR for further concentration treatment. The fresh water produced by the ROR enters the reclaimed water reuse pool, thereby improving the water reuse rate.

[0025] Furthermore, in a1, the primary reaction tank includes a first cell, a second cell, a third cell and a fourth cell connected in sequence, the first cell is added with alkali to adjust the pH to 4.3-5.6, and 0.2 kg / m 3 ~0.28kg / m 3 The second compartment adds lime milk until the pH is 7.2-9.0, and the third compartment adds 0.15kg / m 3~0.2kg / m 3 PAC, the fourth grid is added with 0.005kg / m 3 ~0.008kg / m 3 For PAM, the stirring intensity from the first to the third grid is 68r / min, and the stirring intensity of the fourth grid is 40r / min.

[0026] Wherein, the chemical reaction formula of the first cell of the primary reaction tank is:

[0027] Cr2O7 2- +3HSO3 - +5H +→ 2Cr 3+ +3SO4 2- +4H2O;

[0028] The chemical reaction formula of the second cell of the primary reaction tank includes:

[0029] Ca 2+ +2F - =CaF2↓;

[0030] Ni 2+ +2OH - =Ni(OH)2↓;

[0031] Fe 3+ +3OH - =Fe(OH)3↓;

[0032] Cr 3+ +3OH - =Cr(OH)3↓;

[0033] Mn + +nOH - =Mn(OH)n↓.

[0034] Furthermore, in a3, the secondary reaction tank includes a first cell, a second cell, a third cell and a fourth cell connected in sequence, and the first cell is dosed with 0.1 kg / m 3 ~0.15kg / m 3 The second compartment adds lime milk to a pH of 6.5 to 8.0, and the third compartment adds 0.1 kg / m 3 ~0.15kg / m 3 PAC, the fourth grid is added with 0.002kg / m 3 ~0.005kg / m 3 For PAM, the stirring intensity from the first to the third grid is 68r / min, and the stirring intensity of the fourth grid is 40r / min.

[0035] Furthermore, the surface load of the primary sedimentation tank and the secondary sedimentation tank is 0.7 to 1.0 m 3 / ㎡·h, the sludge from the primary sedimentation tank and the secondary sedimentation tank enters the physicochemical sludge tank, is filtered by the plate and frame filter press, and then transported out for disposal, and the filtrate is returned to the water distribution tank.

[0036] Furthermore, in b1, the pretreatment reaction tank I includes the first, second, third and fourth compartments connected in sequence, the first compartment is added with lime milk to adjust the pH to 10-11, the second compartment is added with 0.2 kg / m 3 ~0.3kg / m 3 The third grid is added with 0.15kg / m 3 ~0.2kg / m 3 PAC, the fourth grid is added with 0.005kg / m 3 ~0.008kg / m 3 PAM.

[0037] Furthermore, in b3, the pretreatment reaction tank II includes a first grid and a second grid connected in sequence, and the first grid is added with 0.1 kg / m 3 ~0.15kg / m 3 PAC, the second grid is added with 0.002kg / m 3 ~0.005kg / m 3 PAM.

[0038] Furthermore, the sludge from the pre-treatment sedimentation tank I and the flotation tank enters the oily sludge tank, is dehydrated by the screw press dehydrator and then transported out for treatment, and the filtrate is returned to the water distribution tank.

[0039] Furthermore, corresponding accident pools are set up for the pickling wastewater and oily wastewater. When an abnormal situation occurs, the wastewater enters the accident pool for buffering and is then pumped into the normal treatment process for processing.

[0040] Furthermore, in S2, the temperature of the two-stage denitrification reactor is 35±2°C, and the residence time is 2 to 2.5 days; a biological composite filler is configured inside the two-stage denitrification reactor to fix microorganisms, thereby increasing the solid residence time and making the reaction more thorough;

[0041] And / or, the two-stage denitrification reactor is provided with a circulation pump to return the wastewater from the upper part of the reactor to the bottom of the reactor, thereby reducing the shock load of the water inlet, uniforming the water quality and quantity, and increasing the contact time between the wastewater and the sludge.

[0042] Furthermore, in S2, the volumetric load of the aerobic tank is 0.3-0.5 kg COD / m 3 d. The nitrified liquid in the aerobic tank is refluxed to the two-stage denitrification reactor, and the reflux rate is 0.5-1.0 times the influent rate;

[0043] And / or, the surface load of the secondary sedimentation tank is 0.6 to 0.7 m 3 / ㎡·h, the residence time is 2 to 3 hours, which should not be too long to avoid nitrification and affect the effluent water quality. Part of the sludge settled in the secondary sedimentation tank is returned to the two-stage denitrification reactor and the aerobic tank, and part enters the biochemical sludge tank. After being treated by the plate and frame filter press, it is transported out for treatment, and the filtrate is returned to the water distribution tank.

[0044] The two-stage denitrification reactor uses the nitrite and nitrate carried by the wastewater itself and the nitrification liquid refluxed from the aerobic tank. In an anaerobic environment (DO<0.5mg / L), denitrification reduces nitrate to nitrogen gas and decomposes large molecular organic matter into small molecular substances, thereby improving the subsequent aerobic removal efficiency. This process is completed by denitrifying bacteria. The reaction formula includes:

[0045] NO3 - →NO2 - →NO→N2O→N2;

[0046] NO3 - +5[H](organic electron donor)→1 / 2N2+2H2O+0H;

[0047] NO2 - +3[H](organic electron donor)→1 / 2N2+H2O+0H;

[0048] [H] is a substance that can provide electrons and reduce NOx-N to nitrogen, including organic matter and sulfides. Bacteria that carry out this reaction primarily include facultative bacteria such as Proteus, Micrococcus, Pseudomonas, Bacillus, Alcaligenes, and Flavobacterium, which are widely found in nature. In the presence of molecular oxygen, O2 acts as the final electron acceptor, oxidizing organic matter for respiration. In the absence of molecular oxygen, NOx-N is used for respiration. This conversion between molecular oxygen and NOx-N is easy, and even frequent exchange does not inhibit denitrification. Nitrogen in wastewater is primarily introduced as nitrate nitrogen from the nitric acid in stainless steel pickling solutions. Therefore, denitrification can be directly carried out using an external carbon source.

[0049] Furthermore, in S3, the reverse osmosis (ROR) module further concentrates the water to produce a 30%-40% concentrate, which then enters the MVR unit for evaporation. The resulting condensate enters the reclaimed water reuse tank. The MVR unit utilizes a centrally-circulating MVR evaporator to directly solidify the resulting concentrate, eliminating the system load caused by mother liquor reflux. All MVR evaporators come standard with CIP automatic cleaning, effectively preventing scaling. The scraper dryer, with its spring-loaded scraper, continuously cleans the inner surface of the cylinder, preventing scaling.

[0050] In the method of the present invention, the RO module, the ROR module and the MVR device output fresh water to the reclaimed water reuse pool, and the reclaimed water utilization rate is increased from the traditional 60-70% to 95-98%.

[0051] A second aspect of the present invention provides a device for reuse of water and crystallization of concentrated water from stainless steel industrial wastewater, for implementing the method described in the first aspect, comprising: a pickling wastewater regulating tank and an oily wastewater regulating tank, wherein the pickling wastewater regulating tank is connected to a two-stage coagulation and sedimentation pretreatment system, and the oily wastewater regulating tank is connected to a coagulation and sedimentation + flotation pretreatment system, and both the two-stage coagulation and sedimentation pretreatment system and the coagulation and sedimentation + flotation pretreatment system are connected to a water distribution tank;

[0052] The outlet direction of the water distribution tank also includes a two-stage denitrification reactor, an aerobic tank, a secondary sedimentation tank, a filtration device, an ultrafiltration device, a reverse osmosis device and a reclaimed water reuse tank connected in sequence;

[0053] The concentrated water outlet of the reverse osmosis device is connected to the MVR device, and the condensed water outlet of the MVR device is connected to the reclaimed water reuse pool.

[0054] Furthermore, the filtering device includes a sand filter tank, an activated carbon filter tank and a resin softening tank connected in sequence.

[0055] Beneficial effects of the present invention:

[0056] In view of the different types and amounts of pollutants in the pickling wastewater and oily wastewater from the stainless steel industry, the present invention adopts wastewater source diversion, segmented treatment and sludge classification and recovery processes, designs different pretreatment processes respectively, and adds chemicals in a targeted manner. The pretreatment is designed as a multi-stage process, which can achieve a simultaneous reduction in the amount of chemicals and sludge production.

[0057] The biochemical treatment process of the present invention adopts a two-stage denitrification reactor with composite fillers combined with an aerobic tank to reduce the addition of carbon source, improve the denitrification efficiency and the aerobic biochemical treatment efficiency of organic matter, and reduce the output of biochemical sludge.

[0058] The present invention combines the UF+RO+ROR+MVR processes in the reclaimed water reuse system, greatly improving the reclaimed water reuse rate to 95%-98%, compensating for the daily operating costs of the wastewater station and reducing the cost of transporting and disposing of crystals / waste residues.

[0059] This patented device and method can not only completely and comprehensively remove various pollutants in wastewater and ensure its stable treatment effect, but also the high water reuse rate can greatly reduce daily operating costs and realize the resource utilization of wastewater / sludge. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0061] Figure 1 The invention relates to a device and method for recycling wastewater from stainless steel industry and crystallizing concentrated water. DETAILED DESCRIPTION

[0062] The following will provide a clear and complete description of the technical solutions of the present invention in conjunction with specific embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0063] Reference Figure 1 As shown, this embodiment relates to a method for recycling wastewater from stainless steel industry and crystallizing concentrated water. The process flow chart is as follows Figure 1 As shown, the specific processing process is:

[0064] (1) The stainless steel pickling wastewater is collected in the pickling wastewater regulating tank and then enters the two-stage coagulation and sedimentation pretreatment system to remove organic matter, SS (suspended solids) and heavy metal ions in the wastewater, specifically:

[0065] a1. The stainless steel pickling wastewater enters the primary reaction tank, which includes the first, second, third and fourth compartments connected in sequence. Alkali is added to the first compartment to adjust the pH to 5, and 0.25 kg / m 3 The second compartment adds lime milk until the pH is 8, and the third compartment adds 0.15kg / m 3 PAC, the fourth grid is added with 0.008kg / m 3 For PAM, the stirring intensity from the first to the third grid is 68r / min, and the stirring intensity of the fourth grid is 40r / min;

[0066] a2. The effluent from the primary reaction tank enters the primary sedimentation tank for sedimentation and water separation;

[0067] a3. The effluent from the primary sedimentation tank enters the secondary reaction tank, which includes the first, second, third and fourth cells connected in sequence. The first cell is dosed with 0.12 kg / m 3 The second compartment adds lime milk to a pH of 7.0, and the third compartment adds 0.12 kg / m 3PAC, the fourth grid is added with 0.003kg / m 3 For PAM, the stirring intensity from the first to the third grid is 68r / min, and the stirring intensity of the fourth grid is 40r / min;

[0068] a4. The effluent from the secondary reaction tank enters the secondary sedimentation tank for further sedimentation and separation of mud and water;

[0069] The oily wastewater is collected in the oily wastewater regulating tank and then enters the coagulation sedimentation + flotation pretreatment system to remove grease and SS in the wastewater. Specifically:

[0070] b1. The oily wastewater enters the pretreatment reaction tank I, which includes the first, second, third and fourth compartments connected in sequence. Lime milk is added to the first compartment to adjust the pH to 10.5, and 0.25 kg / m 3 The third grid is added with 0.18 kg / m 3 PAC, the fourth grid is added with 0.008kg / m 3 PAM;

[0071] b2. The effluent from the pretreatment reaction tank I enters the pretreatment sedimentation tank I for sedimentation and separation of sludge and water;

[0072] b3, the effluent from the pretreatment sedimentation tank I enters the pretreatment reaction tank II, and 0.12kg / m 3 PAC and 0.003kg / m 3 PAM further flocculates;

[0073] b4. The effluent from the pretreatment reaction tank II enters the flotation tank. Air is introduced into the wastewater and precipitated from the water in the form of tiny bubbles, which serve as carriers. Pollutants such as emulsified oil and tiny suspended particles in the wastewater adhere to the bubbles and float to the surface of the water along with the bubbles, forming foam (a three-phase mixture of air, water, and particles (oil)). The foam or scum is collected to separate impurities and purify the wastewater, further reducing the concentration of emulsified oil and suspended matter.

[0074] (2) The effluent from the secondary sedimentation tank and the flotation tank is mixed in the water distribution tank and then enters the biochemical treatment stage, specifically:

[0075] c1. The effluent from the water distribution tank enters a two-stage denitrification reactor, where anaerobic microorganisms hydrolyze, acidify, and denitrify to decompose refractory organic matter into easily degradable organic matter and macromolecular organic matter into small molecular organic matter, thereby increasing the BOD (biochemical oxygen demand) / COD (chemical oxygen demand) ratio and reducing nitrate to nitrogen gas. The temperature of the two-stage denitrification reactor is 35° C., and the residence time is 2 days.

[0076] c2. The effluent from the two-stage denitrification reactor enters the aerobic tank to remove NH3-N and BOD5 from the wastewater;

[0077] c3. The effluent from the aerobic tank enters the secondary sedimentation tank for mud-water separation and then enters the biochemical effluent tank;

[0078] (3) The effluent from the biochemical effluent pool enters the reclaimed water reuse system, specifically: the effluent from the biochemical effluent pool passes through the sand filter tank, activated carbon filter tank and resin softening tank, ultrafiltration device, RO in sequence, and the generated fresh water enters the reclaimed water reuse pool; the concentrated water generated by RO enters the ROR for further concentration treatment, the fresh water generated by the ROR enters the reclaimed water reuse pool, the concentrated water enters the MVR device for further concentration, and the condensed water enters the reclaimed water reuse pool.

[0079] During the process, sludge from the primary and secondary sedimentation tanks enters the physicochemical sludge tank, where it is filtered through a plate and frame filter press and then transported for disposal. The filtrate is returned to the water distribution tank. Sludge from the pretreatment sedimentation tank I and the flotation tank enters the oily sludge tank, where it is dehydrated through a screw press and then transported for disposal. The filtrate is returned to the water distribution tank. Part of the sludge from the secondary sedimentation tank is returned to the two-stage denitrification reactor and aerobic tank, while part enters the biochemical sludge tank, where it is treated through a plate and frame filter press and then transported for disposal. The filtrate is returned to the water distribution tank. A high-pressure diaphragm filter press is used to dehydrate the physicochemical sludge to a moisture content of approximately 60%, while a high-pressure diaphragm filter press is used to dehydrate the biochemical sludge to a moisture content of approximately 80%.

[0080] The removal of various substances by each processing unit in this embodiment is shown in Table 1:

[0081] Table 1 Removal of various substances by each treatment unit

[0082]

[0083]

[0084] In summary, the present invention adopts wastewater source diversion, segmented treatment and sludge classification recovery processes in view of the different types and water volumes of pollutants in the pickling wastewater and oily wastewater of the stainless steel industry, designs different pretreatment processes respectively, and adds chemicals in a targeted manner. The pretreatment is designed as a multi-stage segmentation, which can achieve the simultaneous reduction of the amount of chemicals and the sludge production. Various indicators of the effluent meet the standards of the reclaimed water reuse system, and after passing through the reclaimed water reuse system, the effluent meets the reuse standards, and the reuse rate reaches 98%.

[0085] The present invention has been described in detail above with reference to specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art will appreciate that various equivalent substitutions, modifications, or improvements may be made to the technical solutions and implementations of the present invention without departing from the spirit and scope of the present invention, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for recycling wastewater from stainless steel industry and crystallizing concentrated water, wherein the stainless steel industry wastewater includes stainless steel pickling wastewater and oily wastewater, characterized in that: The steps include: S1. The stainless steel pickling wastewater is collected in the pickling wastewater regulating tank and then enters the two-stage coagulation and sedimentation pretreatment system, specifically: a1. The stainless steel pickling wastewater enters the primary reaction tank, where ammonium bisulfite is first added to reduce the highly toxic hexavalent chromium to trivalent chromium, and then lime milk is added to precipitate trivalent chromium, trivalent iron and divalent nickel into hydroxides, and fluoride into calcium fluoride, and finally PAC and PAM are added for flocculation. The primary reaction tank comprises a first cell, a second cell, a third cell and a fourth cell connected in sequence. Alkali is added to the first cell to adjust the pH to 4.3-5.6, and 0.2 kg / m 3 ~0.28kg / m 3 The second compartment adds lime milk until the pH is 7.2-9.0, and the third compartment adds 0.15kg / m 3 ~0.2kg / m 3 PAC, the fourth grid is added with 0.005kg / m 3 ~0.008kg / m 3 For PAM, the stirring intensity from the first to the third grid is 68r / min, and the stirring intensity of the fourth grid is 40r / min; a2. The effluent from the primary reaction tank enters the primary sedimentation tank for sludge-water separation; a3. The effluent from the primary sedimentation tank enters the secondary reaction tank, which includes the first, second, third and fourth cells connected in sequence. The first cell is dosed with 0.1 kg / m 3 ~0.15kg / m 3 The second compartment adds lime milk to a pH of 6.5 to 8.0, and the third compartment adds 0.1 kg / m 3 ~0.15kg / m 3 PAC, the fourth grid is added with 0.002kg / m 3 ~0.005kg / m 3 For PAM, the stirring intensity from the first to the third grid is 68r / min, and the stirring intensity of the fourth grid is 40r / min; a4. The effluent from the secondary reaction tank enters the secondary sedimentation tank for further sedimentation and separation of mud and water; The oily wastewater is collected in the oily wastewater regulating tank and then enters the coagulation sedimentation + flotation pretreatment system, specifically: b1. The oily wastewater enters the pretreatment reaction tank I, where lime milk, demulsifier, PAC and PAM are added to flocculate organic matter, colloidal matter and suspended solids; b2. The effluent from the pretreatment reaction tank I enters the pretreatment sedimentation tank I for sedimentation and water separation; b3. The effluent from the pretreatment sedimentation tank I enters the pretreatment reaction tank II, where PAC and PAM are added for further flocculation; b4. The effluent from the pretreatment reaction tank II enters the flotation tank to further reduce the concentration of emulsified oil and suspended solids; S2. The effluent from the secondary sedimentation tank and the flotation tank is mixed in the water distribution tank and then enters the biochemical treatment stage, specifically: S21, the effluent from the water distribution tank enters a two-stage denitrification reactor, where anaerobic microorganisms are used to hydrolyze, acidify, and denitrify the refractory organic matter into easily degradable organic matter, decompose the macromolecular organic matter into small molecular organic matter, and simultaneously reduce nitrate to nitrogen gas; S22, the effluent from the two-stage denitrification reactor enters the aerobic tank to remove NH3-N and BOD5 in the wastewater; S23, the effluent from the aerobic tank enters the secondary sedimentation tank for mud-water separation and then enters the biochemical effluent tank; S3, the effluent from the biochemical effluent pool enters the reclaimed water reuse system, specifically: The effluent from the biochemical effluent pool passes through a filtration device, an ultrafiltration device, and a reverse osmosis device in sequence, intercepting inorganic ions, colloidal substances, and macromolecular solutes in the water, and then enters the reclaimed water reuse pool; the concentrated water produced by the reverse osmosis device enters the MVR device for evaporation and concentration, and the condensed water produced enters the reclaimed water reuse pool.

2. The method for recycling wastewater from stainless steel industry and crystallizing concentrated water according to claim 1, characterized in that: In b1, the pretreatment reaction tank I includes the first, second, third and fourth compartments connected in sequence. The first compartment is added with lime milk to adjust the pH to 10-11, and the second compartment is added with 0.2 kg / m 3 ~0.3kg / m 3 The third grid is added with 0.15kg / m 3 ~0.2kg / m 3 PAC, the fourth grid is added with 0.005kg / m 3 ~0.008kg / m 3 PAM.

3. The method for recycling wastewater from stainless steel industry and crystallizing concentrated water according to claim 1, characterized in that: In b3, the pretreatment reaction tank II includes a first grid and a second grid connected in sequence, and the first grid is dosed with 0.1 kg / m 3 ~0.15kg / m 3 PAC, the second grid is added with 0.002kg / m 3 ~0.005kg / m 3 PAM.

4. The method for recycling wastewater from stainless steel industry and crystallizing concentrated water according to claim 1, characterized in that: In S2, the temperature of the two-stage denitrification reactor is 35±2°C, and the residence time is 2 to 2.5 days; And / or, the two-stage denitrification reactor is provided with a circulation pump to reflux the wastewater from the upper part of the reactor to the bottom of the reactor.

5. The method for recycling wastewater from stainless steel industry and crystallizing concentrated water according to claim 1, characterized in that: In S2, the volume load of the aerobic tank is 0.3-0.5 kg COD / m 3 d. The nitrified liquid in the aerobic tank is refluxed to the two-stage denitrification reactor, and the reflux rate is 0.5-1.0 times the influent rate; And / or, the surface load of the secondary sedimentation tank is 0.6 to 0.7 m 3 / ㎡·h, stay time 2 to 3h.

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