System and method for disposal of waste acid by limestone-sodium hydroxide two-stage neutralization

Abstract

The application discloses a system and method for disposing waste acid by limestone-liquid alkali two-stage neutralization, and belongs to the technical field of sewage treatment. The system comprises a first-stage limestone coarse neutralization unit, a second-stage liquid alkali fine neutralization unit and a solid-liquid separation unit. The first-stage limestone coarse neutralization unit comprises a limestone neutralization filter kettle and an aeration device. The second-stage liquid alkali fine neutralization unit comprises a pH adjusting pool, a heavy metal removal reaction pool, a coagulation reaction pool, a flocculation reaction pool and an inclined tube sedimentation pool which are sequentially connected. The solid-liquid separation unit comprises a filter press, a screw conveyor and a transfer hopper. The method adopts the above system to treat waste acid, and comprises the following steps: S1, first-stage limestone coarse neutralization; S2, second-stage liquid alkali fine neutralization; and S3, solid-liquid separation. The application can effectively solve the problem that the existing waste acid neutralization disposal process has poor heavy metal removal effect.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a system and method for the two-stage neutralization and treatment of waste acid using limestone and liquid alkali. Background Technology

[0002] With the continuous development of my country's industrialization, industries such as chemical, metallurgical, and electronics generate large amounts of waste liquid containing inorganic acids (such as sulfuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid) during production. These waste acids are highly corrosive, toxic, and cause persistent environmental pollution, and are listed as HW34 hazardous waste (waste acids) in the National Hazardous Waste List (2025 edition). Therefore, how to achieve efficient, safe, and harmless disposal of waste acids is an important issue in the field of environmental protection.

[0003] Currently, the mainstream disposal methods for industrial waste acid hazardous waste are single-stage liquid alkali neutralization technology and single-stage lime neutralization technology. For example, Chinese patent CN 208648852 U proposes a waste acid water filter tank neutralization device that uses limestone to neutralize waste acid. Chinese patent CN 221235373 U proposes a waste acid neutralization device that uses liquid alkali to neutralize waste acid. However, both methods have shortcomings: while the single-stage liquid alkali neutralization process has a fast reaction, high neutralization accuracy, and stable effluent quality, the reagent cost is high, and the reagent consumption is large when disposing of large quantities of high-acidity waste acid, resulting in high production costs for enterprises and poor economic viability for industrial application; the single-stage lime neutralization process has low raw material cost and easy access, but it suffers from slow reaction rate, insufficient neutralization depth, poor pH control accuracy, poor solidification and removal effect of heavy metal ions, and easily generates a large amount of residue, posing a risk of secondary pollution.

[0004] Existing technologies include some two-stage neutralization processes that combine limestone neutralization with liquid alkali neutralization. However, most of these processes only add neutralizing agents and lack the complementary addition of PAC coagulation and PAM flocculation. As a result, tiny metal colloids cannot aggregate and settle, leading to low heavy metal retention rates. Therefore, existing waste acid hazardous waste treatment technologies generally suffer from poor heavy metal removal efficiency. Summary of the Invention

[0005] This invention proposes a system and method for two-stage neutralization of waste acid using limestone and liquid alkali, in order to solve the problem of poor heavy metal removal in existing waste acid neutralization processes.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A system for treating waste acid by limestone-liquid alkali two-stage neutralization includes a first-stage limestone coarse neutralization unit, a second-stage liquid alkali fine neutralization unit, and a solid-liquid separation unit.

[0008] The limestone coarse neutralization unit includes a limestone neutralization filter and an aeration device. The top of the limestone neutralization filter is connected to a waste acid discharge pipe, and the bottom is connected to a neutralization liquid delivery pipe with a control valve. Both the waste acid discharge pipe and the neutralization liquid delivery pipe are connected to the inner cavity of the limestone neutralization filter, which is filled with limestone. The aeration device includes an aeration blower, an air supply pipe, and an aeration pipe. The aeration pipe is buried in the limestone bottom layer and has several aeration ports. One end of the air supply pipe is connected to the aeration pipe, and the other end extends to the outside of the limestone neutralization filter and is connected to the air outlet of the aeration blower.

[0009] The two-stage liquid alkali fine neutralization unit includes a pH adjustment tank, a weight removal reaction tank, a coagulation reaction tank, a flocculation reaction tank, and an inclined tube sedimentation tank connected in sequence. Each of the pH adjustment tank, weight removal reaction tank, coagulation reaction tank, and flocculation reaction tank is equipped with a stirrer and a dosing device. The four dosing devices respectively add liquid alkali, weight removal agent sodium sulfide (Na2S), polyaluminum chloride (PAC) solution, and polyacrylamide (PAM) solution. The pH adjustment tank is connected to the end of the neutralization liquid delivery pipe away from the limestone neutralization filter, and the bottom of the inclined tube sedimentation tank is connected to a sludge delivery pipe with a control valve.

[0010] The solid-liquid separation unit includes a filter press. The feed end of the filter press is connected to the end of the sludge conveying pipe away from the inclined tube sedimentation tank. The effluent end of the filter press and the supernatant effluent end of the inclined tube sedimentation tank are both connected to the inlet end of the wastewater treatment plant through a wastewater conveying pipe.

[0011] Furthermore, the limestone coarse neutralization unit also includes a flushing inlet pipe and a flushing outlet pipe, both of which are connected to the inner cavity of the limestone neutralization filter; the flushing inlet pipe is located at the top of the limestone neutralization filter, and the flushing outlet pipe is located at the bottom of the limestone neutralization filter.

[0012] Furthermore, the pH adjustment tank is connected to the degravation reaction tank through a first overflow hole, the degravation reaction tank is connected to the coagulation reaction tank through a second overflow hole, the coagulation reaction tank is connected to the flocculation reaction tank through a third overflow hole, and the flocculation reaction tank is connected to the inclined tube sedimentation tank through a fourth overflow hole, with the heights of the first, second, third, and fourth overflow holes decreasing sequentially.

[0013] Furthermore, the filter press is a plate and frame filter press.

[0014] Furthermore, the solid-liquid separation unit also includes a screw conveyor and a transfer sludge hopper; the feed inlet of the screw conveyor is connected to the solid discharge outlet of the filter press, and the discharge outlet is located directly above the transfer sludge hopper.

[0015] Furthermore, the wastewater conveying pipe is also connected to the inlet of the limestone neutralization filter and the inlet of the pH adjustment tank via a circulation pipe assembly.

[0016] This invention also provides a method for treating waste acid using a two-stage neutralization process with limestone and liquid alkali. The method comprises the following steps:

[0017] S1, First stage limestone coarse neutralization: Waste acid is transported to the limestone neutralization filter to undergo a neutralization reaction with limestone, and continuous aeration is carried out in the limestone neutralization filter by an aeration blower. After the neutralization reaction is completed, a first stage neutralized liquid is obtained.

[0018] S2, Second-stage liquid alkali fine neutralization: The first-stage neutralized liquid obtained in step S1 is transported to the pH adjustment tank, and after pH adjustment, it enters the coagulation reaction tank, and after coagulation reaction, it enters the flocculation reaction tank, and after flocculation reaction, it finally enters the inclined tube sedimentation tank for sedimentation, producing sedimentation tank bottom sludge and inclined tube supernatant.

[0019] S3. Solid-liquid separation: The sediment from the sedimentation tank generated in step S2 is transported to a filter press for dewatering and pressing to produce filter cake and filtrate. The filtrate is combined with the supernatant from the inclined tube generated in step S2 and transported to the wastewater treatment plant through the wastewater conveying pipe. The filter cake is transferred to a special hazardous waste temporary storage bin.

[0020] Furthermore, in step S1, the neutralization reaction time is 40-100 min, the air-to-water ratio during aeration is 6:1-12:1, and the pH of the neutralized solution is 3.2-4.5.

[0021] Furthermore, in step S2, the pH of the waste liquid after pH adjustment is 8.0~9.5; the dosage of polyaluminum chloride (PAC) solution is 100~220 mg / L, the stirring speed of the agitator in the coagulation reaction tank is 100~150 r / min, and the coagulation reaction time is 10~25 min; the dosage of polyacrylamide (PAM) solution is 3~8 mg / L, the stirring speed of the agitator in the flocculation reaction tank is 20~40 r / min, and the flocculation reaction time is 10~30 min; the sedimentation time of the waste liquid in the inclined tube sedimentation tank is 60~120 min.

[0022] Furthermore, in step S3, the moisture content of the filter cake is 55%~60%, and the pH of the filtrate is 7~8; after the filter cake is discharged from the filter press, it enters the screw conveyor and is transported to the transfer sludge hopper by the screw conveyor, and then the transfer sludge hopper loaded with the filter cake is transferred to the special hazardous waste temporary storage bin by a forklift.

[0023] By adopting the above technical solution, the present invention has the following beneficial effects:

[0024] This invention employs a two-stage limestone-liquid alkali neutralization process for waste acid. It fully leverages the advantages of limestone (primarily CaCO3) being inexpensive and widely available. In the first stage of neutralization, most of the free acid in the waste acid is consumed, significantly reducing acidity. The second stage uses liquid alkali for fine adjustment, precisely controlling the pH within the target range. Following this, three further steps—sodium sulfide for weight removal, PAC coagulation, PAM flocculation, and inclined tube sedimentation—promote the agglomeration and sedimentation of tiny metal colloids, ensuring sufficient precipitation of heavy metal ions and significantly improving the removal efficiency of heavy metals from wastewater. This two-stage synergistic treatment not only significantly reduces the amount of liquid alkali used and operating costs but also reduces the amount of neutralization residue generated, combining economic efficiency with environmental friendliness. It represents a highly efficient and feasible technical approach for the harmless disposal of hazardous acid waste. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the system composition proposed in this invention;

[0026] Figure 2 This is a schematic diagram of the method flow proposed in this invention;

[0027] The components in the attached diagram are labeled as follows: 1-Limestone neutralization filter, 2-Limestone, 3-Waste acid discharge pipe, 4-Neutralization liquid conveying pipe, 5-Aeration blower, 6-Air supply pipe, 7-Aeration pipe, 8-Flushing inlet pipe, 9-Flushing outlet pipe, 10-pH adjustment tank, 11-Coagulation reaction tank, 12-Flocculation reaction tank, 13-Inclined tube sedimentation tank, 14-Agitator, 15-Dosing device, 16-Sludge conveying pipe, 17-Filter press, 18-Wastewater conveying pipe, 19-Sewage treatment plant, 20-Screw conveyor, 21-Transfer sludge hopper, 22-Circulation pipe assembly, 23-Degravity removal reaction tank. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] like Figure 1 As shown, the present invention proposes a system for the two-stage neutralization and treatment of waste acid using limestone and liquid alkali, comprising a first-stage limestone coarse neutralization unit, a second-stage liquid alkali fine neutralization unit, and a solid-liquid separation unit.

[0030] The first-stage limestone coarse neutralization unit includes a limestone neutralization filter 1, an aeration device, a flushing inlet pipe 8, and a flushing outlet pipe 9. A waste acid discharge pipe 3 is connected to the top of the limestone neutralization filter 1, and a neutralization liquid delivery pipe 4 with a control valve is connected to the bottom. Both the waste acid discharge pipe 3 and the neutralization liquid delivery pipe 4 are connected to the inner cavity of the limestone neutralization filter 1, which is filled with limestone 2. The acidic effluent after limestone coarse neutralization is quantitatively delivered to the second-stage liquid alkali fine neutralization unit through the neutralization liquid delivery pipe 4. A water pump is installed on the neutralization liquid delivery pipe 4; two pumps are connected in parallel, one for standby and one for use.

[0031] The aeration device includes an aeration blower 5, an air supply pipe 6, and an aeration pipe 7. The aeration pipe 7 is buried in the bottom layer of limestone 2 and has several aeration ports. One end of the air supply pipe 6 is connected to the aeration pipe 7, and the other end extends to the outside of the limestone neutralization filter tank 1 and is connected to the air outlet of the aeration blower 5. After waste acid is introduced into the limestone neutralization filter tank 1, continuous aeration disturbs the limestone packing, enhances the neutralization reaction between waste acid and calcium carbonate, consumes a large amount of free acid in the waste acid, and significantly reduces the acidity of the waste liquid. At the same time, heavy metal ions in the wastewater react with carbonate and hydroxide ions to initially form hydroxide and carbonate precipitates.

[0032] Both the flushing inlet pipe 8 and the flushing outlet pipe 9 are connected to the inner cavity of the limestone neutralization filter 1. The flushing inlet pipe 8 is located at the top of the limestone neutralization filter 1, and the flushing outlet pipe 9 is located at the bottom of the limestone neutralization filter 1. The flushing inlet pipe 8 is used to input flushing water to flush the limestone 2. Limestone packing is prone to caking and clogging due to long-term reaction; therefore, flushing water is periodically introduced to flush the limestone packing, reducing the probability of clogging. The high-turbidity flushing wastewater generated is transported to a sedimentation tank for pretreatment via a separate pipeline, and then transported to the plant's wastewater treatment plant 19 for unified treatment.

[0033] The two-stage liquid alkali fine neutralization unit includes a pH adjustment tank 10, a weight removal reaction tank 23, a coagulation reaction tank 11, a flocculation reaction tank 12, and an inclined tube sedimentation tank 13 connected in sequence. Each of the pH adjustment tank 10, weight removal reaction tank 23, coagulation reaction tank 11, and flocculation reaction tank 12 is equipped with a stirrer 14 and a dosing device 15. The four dosing devices 15 respectively add liquid alkali, the weight removal agent sodium sulfide (Na2S), polyaluminum chloride (PAC) solution, and polyacrylamide (PAM) solution. The stirrer 14 and dosing devices 15 ensure continuous and quantitative dosing, and ensure that the added reagents are thoroughly mixed with the waste liquid.

[0034] Specifically, the pH adjustment tank 10 is connected to the degravation reaction tank 23 through a first overflow hole. The degravation reaction tank 23 is connected to the coagulation reaction tank 11 through a second overflow hole. The coagulation reaction tank 11 is connected to the flocculation reaction tank 12 through a third overflow hole. The flocculation reaction tank 12 is connected to the inclined tube sedimentation tank 13 through a fourth overflow hole. The heights of the first, second, third, and fourth overflow holes decrease sequentially. The waste liquid overflowing from the pH adjustment tank 10 enters the coagulation reaction tank 11, the waste liquid overflowing from the coagulation reaction tank 11 enters the flocculation reaction tank 12, and the waste liquid overflowing from the flocculation reaction tank 12 enters the inclined tube sedimentation tank 13.

[0035] The pH adjustment tank 10 is connected to the end of the neutralization liquid delivery pipe 4 away from the limestone neutralization filter 1. The bottom of the inclined tube sedimentation tank 13 is connected to a sludge delivery pipe 16 with a control valve. A sludge pump is installed on the sludge delivery pipe 16, and two sludge pumps are connected in parallel, one for standby and one for use.

[0036] pH fine adjustment: The waste liquid enters the pH adjustment tank 10, and sodium hydroxide (liquid alkali) is added. The pH of the water body is precisely adjusted to the alkaline range where heavy metals are completely precipitated through online pH monitoring, thus completing fine neutralization and making up for the shortcoming of limestone 2 in not being able to precisely control the pH. Heavy metal ions are fully generated into insoluble hydroxide precipitates.

[0037] Heavy metal removal reaction: After pH fine adjustment, the alkaline wastewater flows by gravity into the heavy metal removal reaction tank. Sodium sulfide (Na2S) is added as a heavy metal removal agent and the mixture is continuously stirred. In the system, hydroxide ions and sulfur ions work synergistically: free copper, nickel, zinc, lead, cadmium, and other heavy metal ions in the water react with OH- ions... - On the one hand, it generates heavy metal hydroxides, and on the other hand, it reacts with S. 2- Combined to form metal sulfide precipitates with a solubility product much lower than that of hydroxides; for complexed heavy metals, S 2- By leveraging its strong coordination competition ability, it captures heavy metal ions from complexes and transforms them into less soluble sulfide precipitates, significantly improving the depth of heavy metal removal. It completely converts residual trace heavy metals into suspended solid particles, making up for the deficiency of single alkali neutralization in removing complexed heavy metals and creating conditions for subsequent coagulation and flocculation formation.

[0038] Coagulation reaction: The waste liquid flows into the coagulation reaction tank 11, and polyaluminum chloride (PAC) solution is added. Stirring causes the tiny metal hydroxide colloids to destabilize and aggregate to form fine flocs.

[0039] Flocculation reaction: The waste liquid enters the flocculation reaction tank 12, polyacrylamide (PAM) is added, and the mixture is slowly stirred to allow the small flocs to clump together to form large, dense flocs, which facilitates subsequent sedimentation and separation.

[0040] Inclined tube sedimentation: The mixed liquor enters the inclined tube sedimentation tank 13. The flocs settle rapidly to the bottom sludge hopper by gravity. The clear liquid overflows and is discharged. The bottom sedimented sludge is transported to the solid-liquid separation unit by a sludge pump.

[0041] The solid-liquid separation unit includes a filter press 17, a screw conveyor 20, and a sludge transfer hopper 21. The feed end of the filter press 17 is connected to the end of the sludge conveying pipe 16 away from the inclined tube sedimentation tank 13. The effluent end of the filter press 17 and the supernatant effluent end of the inclined tube sedimentation tank 13 are both connected to the inlet end of the wastewater treatment plant 19 via a wastewater conveying pipe 18. The filter press 17 is a plate and frame filter press. The feed inlet of the screw conveyor 20 is connected to the solid discharge outlet of the filter press 17, and the discharge outlet is located directly above the sludge transfer hopper 21.

[0042] Solid-liquid separation: ① The concentrated sludge at the bottom of the inclined tube sedimentation tank 13 is pressurized by a sludge pump and sent to a plate and frame filter press, where high-pressure extrusion achieves sludge-water separation; ② Solid products: The metal hydroxide filter cake retained in the filter plate is a heavy metal hazardous waste, which is transferred to the transfer sludge hopper 21 by a screw conveyor 20, and then transferred to the hazardous waste temporary storage room by the transfer sludge hopper 21; ③ Liquid water production: The clear water passing through the filter cloth of the plate and frame filter press is collected and transported to the plant's integrated wastewater treatment plant 19 for further deep biological / physicochemical treatment, and discharged after meeting the standards.

[0043] Wastewater conveying pipe 18 is also connected to the inlet of limestone neutralization filter 1 and the inlet of pH adjustment tank 10 through circulation pipe assembly 22. The permeate of plate and frame filter press and the supernatant of inclined tube sedimentation tank 13 can be returned to the first stage limestone coarse neutralization unit and the second stage liquid alkali fine neutralization unit through circulation pipe assembly 22 to further improve the removal effect of heavy metals.

[0044] In addition, electric valves, flow meters, etc. can be installed on each conveying pipeline as needed to adjust the flow rate of waste acid feed, reagent addition, sludge conveying, etc.

[0045] like Figure 2 As shown, the present invention also provides a method for treating waste acid using a two-stage neutralization process with limestone and liquid alkali. The method for treating waste acid using the aforementioned two-stage neutralization process with limestone and liquid alkali includes the following steps:

[0046] S1, First stage limestone coarse neutralization: Waste acid is transported to a limestone neutralization filter to undergo a neutralization reaction with limestone, and continuous aeration is carried out in the limestone neutralization filter by an aeration blower. After the neutralization reaction is completed, a first stage neutralized liquid is obtained.

[0047] The neutralization reaction time is 40-100 min, the air-to-water ratio during aeration is 6:1-12:1, and the pH of the first neutralization solution is 3.2-4.5.

[0048] S2, Second-stage liquid alkali fine neutralization: The first-stage neutralized liquid obtained in step S1 is transported to the pH adjustment tank, and after pH adjustment, it enters the coagulation reaction tank, and after coagulation reaction, it enters the flocculation reaction tank, and after flocculation reaction, it finally enters the inclined tube sedimentation tank for sedimentation, producing sediment bottom sludge and inclined tube supernatant.

[0049] The pH of the waste liquid after pH adjustment is 8.0~9.5; the dosage of polyaluminum chloride (PAC) solution is 100~220 mg / L, the stirring speed of the agitator in the coagulation reaction tank is 100~150 r / min, and the coagulation reaction time is 10~25 min; the dosage of polyacrylamide (PAM) solution is 3~8 mg / L, the stirring speed of the agitator in the flocculation reaction tank is 20~40 r / min, and the flocculation reaction time is 10~30 min; the sedimentation time of the waste liquid in the inclined tube sedimentation tank is 60~120 min.

[0050] S3. Solid-liquid separation: The sediment from the sedimentation tank generated in step S2 is transported to a filter press for dewatering, producing filter cake and filtrate. The filtrate is combined with the supernatant from the inclined tube generated in step S2 and transported to the wastewater treatment plant through a wastewater pipeline. The filter cake is transferred to a special hazardous waste temporary storage bin.

[0051] The filter cake has a moisture content of 55% to 60% and a pH of 7 to 8 in the filtrate. After being discharged from the filter press, the filter cake enters a screw conveyor and is transported to a transfer sludge hopper. A forklift then transfers the sludge hopper containing the filter cake to a dedicated hazardous waste temporary storage bin.

[0052] In this invention, limestone is used as a cheap and readily available neutralizing agent, which significantly reduces the consumption of liquid alkali and lowers operating costs. The two-stage neutralization and graded acid control are achieved: the first stage reduces acid at low cost, and the second stage precisely controls the precipitation conditions for heavy metals, resulting in stable treatment effects. The aerated filter simultaneously completes neutralization and filtration, and the inclined tube sedimentation + plate and frame filter press achieves dual solid-liquid separation, ensuring thorough removal of heavy metals. The generated sludge is separately filtered and managed as hazardous waste, which significantly reduces the risk of secondary pollution from fluoride and heavy metals.

[0053] The present invention will be further illustrated below through specific embodiments. These embodiments are adapted to four types of HW34 waste acid hazardous waste working conditions: PCB electronics, metallurgical pickling, fine chemical waste acid, and large-scale centralized disposal in industrial parks, and are matched with a limestone-liquid alkali two-stage neutralization process.

[0054] Example 1: Hazardous waste disposal of mixed sulfuric acid / hydrochloric acid containing copper and nickel in the PCB electronics industry

[0055] 1. Waste acid raw material parameters

[0056] HW34 waste acid, a hazardous waste from a circuit board etching process, has a daily processing capacity of 100 tons. The original solution has a pH of 1.0, a total acidity of 120 g / L from free sulfuric acid and hydrochloric acid, and contains Cu. 2+95mg / L, Ni 2+ 42 mg / L, which is classified as a hazardous waste liquid with high acidity and high heavy metal content.

[0057] 2. Process Operation Steps

[0058] 1) A section of limestone coarse and medium

[0059] Waste acid is quantitatively fed into a bottom-aerated limestone neutralization filter, with limestone blocks containing 80% CaCO3 as the packing material. Aeration is continuously maintained by a blower at an air-to-water ratio of 8:1. The reaction proceeds for 60 minutes at room temperature, allowing calcium carbonate and free acid to fully react, resulting in a stable effluent pH of 4.0. Every 8 hours, the limestone neutralization filter is flushed through a flushing pipeline to rinse the packing material. High-turbidity flushing wastewater is collected separately, precipitated, and transported to the wastewater treatment plant, without mixing with the main product water.

[0060] 2) Two-stage liquid alkali fine neutralization

[0061] The effluent with pH=4.0 automatically overflows to the second-stage pH adjustment tank. With online pH linkage, 32% liquid alkali is added via the corresponding dosing device to precisely adjust the pH to 8.5. It then enters the gravity removal reaction tank with a Na2S dosage of 200 mg / L. Next, it is transferred to the coagulation reaction tank where a 10% PAC solution is added at a dosage of 150 mg / L, and the mixture is rapidly stirred at 120 rpm for 10 minutes. Finally, it is transferred to the flocculation reaction tank where a 0.1% anionic PAM solution is added at a dosage of 4 mg / L, and the mixture is slowly stirred at 30 rpm for 15 minutes to form dense metal hydroxide flocs. The mixed solution then enters the inclined tube sedimentation tank for settling for 60 minutes.

[0062] 3) Solid-liquid separation

[0063] The sludge at the bottom of the inclined tube sedimentation tank is pumped into a plate and frame filter press for dewatering. The filter cake has a moisture content of 58% and is transferred to a special hazardous waste temporary storage bin. The pH of the filter filtrate is 7.2. The filter filtrate is combined with the supernatant from the inclined tube and transported to the plant's integrated wastewater treatment plant for further treatment.

[0064] 3. Comparison of treatment effects and costs

[0065] The total copper and total nickel in the effluent are both <0.1mg / L, meeting the influent standards of the wastewater treatment plant; the comprehensive treatment cost of the reagents is 0.64 yuan / t of waste acid; the same amount of waste acid is neutralized directly with single-stage liquid alkali at a cost of 12.25 yuan / t of waste acid, reducing the reagent cost by 94.8%; there is no violent heat release and no large amount of acid mist emission, greatly eliminating the risk of equipment corrosion and personnel burns.

[0066] Example 2: Hazardous waste disposal of hydrofluoric acid and sulfuric acid waste from pickling of metallurgical steel

[0067] 1. Waste acid raw material parameters

[0068] HW34 waste acid from pickling cold-rolled steel, daily processing capacity 180t, raw solution pH=0.9, containing 85g / L sulfuric acid, 18g / L hydrofluoric acid, and Fe. 3+ 1200mg / L, trace amounts of lead and cadmium heavy metals, extremely corrosive, easily produces calcium fluoride fine suspensions.

[0069] 2. Process Operation Steps

[0070] 1) A section of limestone coarse and medium

[0071] Waste acid is fed into a limestone neutralization filter kettle with a limestone packing height of 2.2m. It is aerated by an aeration device with an air-to-water ratio of 10:1 and a reaction time of 75 minutes. Calcium carbonate preferentially neutralizes sulfuric acid and hydrofluoric acid, producing coarse precipitates of calcium sulfate and calcium fluoride. The pH of the effluent stabilizes at 3.8. The packing is rinsed every 6 hours, and the rinsing wastewater is pretreated separately before being incorporated into the sewage treatment plant.

[0072] 2) Two-stage liquid alkali fine neutralization

[0073] The effluent from the first stage enters the pH adjustment tank of the second stage, where liquid alkali is automatically added to adjust the pH to 9.2, ensuring complete precipitation of lead, cadmium, and iron. Na₂S, PAC, and PAM are then added sequentially: Na₂S at 100 mg / L, PAC at 200 mg / L, with a coagulation stirring rate of 100 rpm and a coagulation reaction time of 20 min; PAM at 5 mg / L, with a flocculation stirring rate of 25 rpm and a flocculation reaction time of 20 min. The total retention time for coagulation and flocculation is 40 min, and the surface loading of the inclined tube sedimentation tank is 1.0 m³. 3 / (m 2 ·h), efficiently retaining fluorine-containing and heavy metal flocs.

[0074] 3) Solid-liquid separation

[0075] The concentrated sludge from the inclined tube sedimentation tank is continuously conveyed to a plate and frame filter press for filtration. The filter cake is hazardous waste containing fluorine and heavy metals, with a moisture content of 55%, and the pH of the filtered liquid is 7.5.

[0076] 3. Implementation Results

[0077] Limestone pre-neutralization consumes 88% of the free acid, and the liquid alkali consumption is only 1 / 18 of that in a single-stage alkali neutralization scheme; limestone generates coarse calcium fluoride crystals in advance, increasing the floc settling speed by 60% and reducing the civil engineering volume of the sedimentation tank by 40%; it avoids the direct and violent reaction of high-concentration HF with concentrated alkali and eliminates the corrosion of workshop equipment by hydrofluoric acid mist.

[0078] Example 3: Small-batch intermittent treatment of multi-component mixed waste acid (sulfuric acid + nitric acid + heavy metals) from fine chemical industry

[0079] 1. Waste acid raw material parameters

[0080] The laboratory and fine chemical plant intermittently generate HW34 waste acid, with a single batch processing 15t. The original solution has a pH of 0.8 and a total concentration of mixed inorganic acids of 150g / L. It contains multiple heavy metals such as zinc, chromium, and manganese. The waste acid generation is highly intermittent and the water quality fluctuates drastically.

[0081] 2. Process Operation Steps

[0082] 1) Intermittent coarse and fine treatment of a section of limestone

[0083] The batch of waste acid is fed into the limestone neutralization filter in one go. The limestone packing undergoes a static reaction combined with bottom aeration, with an air-to-water ratio of 6:1 and a residence time of 90 minutes. The final pH is controlled at 4.2. After the batch treatment is completed, the packing is immediately rinsed to prevent nitrate and sulfate scale from forming and encapsulating on the limestone.

[0084] 2) Two-stage liquid alkali fine neutralization

[0085] An online pH sensor provides real-time feedback, allowing for the gradual, step-by-step addition of liquid alkali to slowly raise the pH to 9.0, preventing sudden pH increases that could cause colloids to float. Na₂S is added for weight removal at a dosage of 300 mg / L. PAC and PAM are added in stages for coagulation and flocculation, with gradient stirring speed control to ensure complete agglomeration of minute heavy metal colloids. The PAC dosage is 180 mg / L, the coagulation stirring rate is 100–150 r / min, and the coagulation reaction time is 25 min. The PAM dosage is 6 mg / L, the flocculation stirring rate is 20–40 r / min, and the flocculation reaction time is 30 min. Inclined tube sedimentation takes 120 min.

[0086] 3) Solid-liquid separation

[0087] After 2 hours of sedimentation in the inclined tube, the sludge was conveyed to a plate and frame filter press for filtration. The filter cake had a moisture content of 60%, and the pH of the filtrate was 8.

[0088] 3. Comparison of process advantages with existing single-stage processes

[0089] Traditional single-stage intermittent alkali neutralization: pH fluctuates drastically during feeding, heavy metal colloids float to the surface, and heavy metals in the effluent often exceed the standard; This embodiment uses a two-stage process: limestone provides a buffer system, pH is adjusted stably, the heavy metal removal rate is stable at ≥99.5%, and batch water quality fluctuations are eliminated.

[0090] Example 4: Centralized, large-scale, continuous treatment of waste acid in an industrial park (industrial-scale example)

[0091] 1. Waste acid raw material parameters

[0092] The industrial park centrally collects and stores various types of waste acid and hazardous waste from enterprises, with a continuous treatment capacity of 500t / d. The pH range of the waste acid is 0.7~1.5, and the types of acids are mixed (sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid). There are many types of heavy metals and their concentrations fluctuate greatly, which is a high-load, large-scale treatment operation.

[0093] 2. Process Operation Steps

[0094] 1) A section of limestone continuously coarse and medium-sized

[0095] Two limestone neutralization filter kettles are set up in parallel, one for use and one for standby. Waste acid is fed continuously and evenly, the aeration gas-to-water ratio is 7:1, the neutralization reaction is carried out for 50 minutes, and the pH of the effluent is stably controlled at 3.5~4.0. An automatic timed flushing system is set up, with a separate pipeline for flushing wastewater and a matching small pre-sedimentation tank for pretreatment to avoid impacting the downstream biological system.

[0096] 2) Two-stage liquid alkali fine neutralization

[0097] The effluent enters the pH adjustment tank, where the PLC-linked online pH monitoring and liquid alkali dosing device maintain a pH range of 8.3 to 9.0. Na2S is added to the degravity reaction tank at a dosage of 200 to 300 mg / L. PAC is continuously added to the coagulation reaction tank at a dosage of 120 to 180 mg / L, and PAM is added to the flocculation reaction tank at a dosage of 3 to 6 mg / L. Sludge is continuously discharged from the ultra-large inclined tube sedimentation tank.

[0098] 3) Solid-liquid separation

[0099] The sludge pump continuously transports sludge to two plate and frame filter presses for alternating dewatering. The screw conveyor and transfer sludge hopper transfer the hazardous waste filter cake to a dedicated hazardous waste temporary storage bin. The filter cake has a moisture content of 56%. The filter liquid (pH=7.8) and the supernatant from the inclined tube sedimentation tank are combined and continuously sent to the integrated wastewater treatment plant.

[0100] 3. Industrialization operation indicators

[0101] The operating cost of the reagents is stable at 0.6~0.7 yuan / t of waste acid. Compared with the original single-stage liquid alkali neutralization system in the park, the annual reagent procurement cost is reduced by more than 94%. The dual-layer solid-liquid separation (inclined tube sedimentation + plate and frame filter press) reduces the sludge moisture content from 86% in the traditional process to 55%~60%, and reduces the amount of hazardous waste transfer and disposal by 30%. The fully automated linkage control reduces the amount of manual operation by 70% and eliminates the risk of neutralization failure and excessive wastewater discharge caused by manual adjustment.

[0102] Comparative Example 1: Single-stage direct neutralization process with liquid alkali (current mainstream soda ash neutralization technology)

[0103] 1. Processing Object

[0104] The same HW34 waste acid hazardous waste from the PCB industry as in Example 1 was selected, with a daily processing capacity of 100t, a raw solution pH of 1.0, a total free acid of 120g / L, and Cu. 2+ 95mg / L, Ni 2+ 42 mg / L, treatment target: effluent pH 7-9, heavy metals meet standards.

[0105] 2. Operating procedures

[0106] Waste acid is directly fed into a single-stage equalization tank without a limestone pretreatment unit. Only 98% flake sodium hydroxide is added to prepare 32% liquid alkali for one-time neutralization. There is no staged reaction, only simple stirring is set up, and PAC and PAM are added simultaneously. The sludge at the bottom of the sedimentation tank is directly discharged. Only simple sedimentation is performed, without plate and frame deep filtration or independent flushing pipeline.

[0107] 3. Results

[0108] 1) Reagent consumption: It relies entirely on high-priced sodium hydroxide to neutralize high-concentration free waste acid, resulting in large acid and alkali consumption. The reagent cost is much higher than that of the limestone + liquid alkali combination process. The comprehensive reagent cost is 12.25 yuan / t waste acid, which is 19 times that of the process of this invention. The annual reagent procurement cost is extremely high, and the economic efficiency is extremely poor when the treatment scale is large.

[0109] 2) Safety conditions: The instantaneous mixing of concentrated alkali and high acid generates intense heat, and the workshop continuously produces a large amount of irritating acid mist. The inner walls of the equipment are severely corroded, and there were 3 incidents of acid and alkali splash burns to operators throughout the year.

[0110] 3) Water quality stability: There is no buffer system. Even a small amount of liquid alkali can cause large pH fluctuations. The pH often jumps between 3 and 11. Online pH adjustment is lagging, and the effluent contains excessive levels of fluoride and some heavy metals.

[0111] 4) Solid-liquid separation: The metal hydroxide colloidal particles generated by directly adding liquid alkali are extremely fine. Without the pre-precipitation effect of limestone, the flocs are loose and the settling speed is slow. The sedimentation tank volume is large and the sludge moisture content is as high as 87%. The sludge production by filter press is greatly increased. The sludge production is 32% higher than that of this invention, and the cost of hazardous waste transfer and disposal is greatly increased.

[0112] 4. Summary of Existing Defects

[0113] The chemical costs are extremely high, the exothermic acid mist poses a significant safety risk, pH control is difficult, sludge production is large, and fluoride and heavy metals in the effluent frequently exceed the standards.

[0114] Comparative Example 2: Single-stage limestone filter neutralization process (traditional single limestone pretreatment technology)

[0115] 1. Processing Object

[0116] Same as Example 2, metallurgical pickling waste acid containing fluorine HW34, daily processing capacity 180t, original solution pH=0.9, containing sulfuric acid, hydrofluoric acid, Fe, lead, cadmium heavy metals.

[0117] 2. Operating procedures

[0118] It only has a limestone neutralization filter and aeration device, without a second-stage liquid alkali fine neutralization unit; it relies on limestone as a single agent for neutralization, without adding additional liquid alkali, and relies on limestone itself to buffer and adjust the pH; it is equipped with a common sedimentation tank, without segmented coagulation and flocculation processes.

[0119] 3. Results

[0120] 1) Insufficient neutralization depth: The pH of the limestone effluent is only 3.6 to 5.2, which cannot reach the pH of 8.5 or above required for complete precipitation of heavy metals. The total lead, total cadmium and total iron in the effluent continue to exceed the standards and cannot be sent to the plant's wastewater treatment plant.

[0121] 2) Severe clogging of the packing material: The neutralization produces calcium sulfate and calcium fluoride crystals that coat the surface of the limestone packing material. After 3 days of operation, the water flow becomes unbalanced and the neutralization efficiency decreases. The packing material needs to be manually disassembled and cleaned every day, resulting in high labor costs for operation and maintenance.

[0122] 3) Poor solid-liquid separation effect: Only a small amount of precipitate is generated by limestone itself, and tiny heavy metal colloids cannot be aggregated. The turbidity of the effluent from the sedimentation tank exceeds the standard for a long time, and the downstream pipeline is prone to scaling and blockage.

[0123] 4) No deep dewatering: The sludge in the sedimentation tank has a water content of 88%, resulting in a large amount of neutralization residue and a high risk of secondary heavy metal pollution.

[0124] 4. Summary of Existing Defects

[0125] The pH adjustment limit is low, the heavy metal removal is not up to standard, the limestone packing is prone to caking and clogging, the sludge production is large, and the effluent control requirements for hazardous waste treatment cannot be met.

[0126] Comparative Example 3: Simplified two-stage neutralization (limestone + liquid alkali, without bottom aeration enhancement)

[0127] 1. Processing Object

[0128] The same as in Example 3, fine chemical intermittent mixed waste acid, 15t per batch, pH=0.8, containing sulfuric acid, nitric acid, and heavy metals such as zinc, chromium, and manganese.

[0129] 2. Operating procedures

[0130] The system consists of a limestone neutralization tank and a liquid alkali regulating tank. The two tanks have the same structure as the present invention, but the bottom aeration system is eliminated, and the limestone is mixed only by mechanical stirring. There is no timed independent flushing pipeline, and the flushing wastewater is directly mixed into the main effluent. There is only simple stirring, without gradient coagulation and flocculation processes.

[0131] 3. Results

[0132] 1) Incomplete reaction of limestone: Without aeration and disturbance, limestone packing is prone to sedimentation at the bottom of the tank, resulting in insufficient solid-liquid contact. Under the same residence time, only 62% of free acid can be neutralized, leading to a significant increase in the amount of liquid alkali added in the second stage and a 7-fold increase in reagent costs.

[0133] 2) Rapid compaction of packing material: Sediment adheres to the limestone surface and cannot be peeled off, requiring manual cleaning of the packing material every 2 days, resulting in a 40% decrease in treatment efficiency;

[0134] 3) Large fluctuations in water quality: The flushing wastewater with high turbidity directly enters the secondary equalization tank, which destroys the flocs and causes the effluent to intermittently exceed the standard for heavy metals;

[0135] 4) Small flocs: The lack of PAC and PAM fractionation prevents the colloids from agglomerating, resulting in turbid supernatant in the sedimentation tank.

[0136] 4. Summary of Existing Defects

[0137] The neutralization efficiency of limestone is low, the cost of reagents is increasing, the packing material needs to be cleaned frequently, the flushing wastewater impacts water quality, and the removal of heavy metals is unstable.

[0138] Comparative Example 4: Two-stage neutralization process (limestone + liquid alkali, no coagulation and flocculation + plate and frame deep pressure filtration)

[0139] 1. Processing Object

[0140] Similar to Example 4, the industrial park can process 500 tons of mixed HW34 waste acid per day.

[0141] 2. Operating procedures

[0142] The limestone aeration filter and the second-stage liquid alkali pH adjustment process are retained, but the independent coagulation reaction tank and flocculation reaction tank are cancelled, and PAC and PAM are not added; only ordinary inclined tube sedimentation tanks are used, and the sludge does not go through plate and frame filter press, but is directly transferred and disposed of with a moisture content of more than 85%.

[0143] 3. Results

[0144] 1) Decreased heavy metal retention rate: Due to the lack of coagulation and flocculation, tiny hydroxide colloids cannot aggregate and settle, and about 12% of heavy metals are lost with the effluent. The heavy metal index of the effluent is close to the emission standard limit, which is extremely risky.

[0145] 2) Sludge disposal costs surge: The sludge has a moisture content of 86%, compared to the 55%–60% moisture content of this invention, which nearly doubles the volume of hazardous waste transport and increases the annual hazardous waste disposal cost by 35%.

[0146] 3) High load on sedimentation tank: Fine suspended solids continue to float to the surface, and flocs float on the effluent weir of the sedimentation tank all year round, which can easily clog the downstream conveying pipeline and increase the frequency of equipment maintenance by 2 times.

[0147] 4) The turbidity of the supernatant is too high, which increases the treatment load of the plant's integrated wastewater treatment station and makes the biological system susceptible to impact from suspended solids.

[0148] 4. Summary of Existing Defects

[0149] Incomplete removal of heavy metals, high sludge moisture content, high hazardous waste disposal costs, and heavy operational pressure on downstream wastewater treatment plants.

[0150] Overall comparative conclusion:

[0151] 1. Pure single-stage liquid alkali process: It achieves the treatment standard, but the problems of reagent cost, safety risk and sludge volume are prominent.

[0152] 2. Pure single-stage limestone process: The raw materials are cheap, but the neutralization depth is insufficient, and heavy metals cannot meet the emission standards;

[0153] 3. The simplified two-stage non-aeration and non-coagulation filter press process, although it has a two-stage neutralization basic structure, lacks key supporting units, and has obvious shortcomings in neutralization efficiency, water quality stability, operating costs and hazardous waste management.

[0154] 4. This invention adopts a complete synergistic solution of limestone aeration for coarse neutralization, liquid alkali for precise pH adjustment, graded coagulation and flocculation, inclined tube sedimentation, plate and frame deep pressure filtration, and independent flushing and diversion. It overcomes all the defects of the four comparative methods and takes into account economy, safety, effluent stability and hazardous waste harmless management.

[0155] Compared with the comparative example, the embodiments of the present invention have the following advantages:

[0156] 1. Compared with the existing single-stage liquid alkali neutralization: the cost of reagents in the examples is reduced to about 1 / 19 of the original process, and defects such as neutralization exotherm, acid mist, drastic pH fluctuations, and difficulty in floc settling are eliminated;

[0157] 2. Compared with the existing single-stage limestone neutralization: the two-stage liquid alkali precisely controls the pH to the range where heavy metals are completely precipitated, and the heavy metals in the effluent are stably up to standard, solving the pain points of insufficient limestone neutralization depth and poor heavy metal removal.

[0158] 3. The examples cover mainstream HW34 waste acid hazardous waste treatment conditions such as small-batch intermittent, continuous large-scale, fluorine-containing, and containing multiple heavy metals. The process is highly versatile, stable in operation, and combines economy, safety, and environmental protection.

[0159] This invention employs a two-stage limestone-liquid alkali neutralization process for waste acid. It fully leverages the advantages of limestone (primarily CaCO3) being inexpensive and widely available. In the first stage of neutralization, most of the free acid in the waste acid is consumed, significantly reducing acidity. The second stage uses liquid alkali for fine adjustment, precisely controlling the pH within the target range. Following this, three further steps—sodium sulfide for weight removal, PAC coagulation, PAM flocculation, and inclined tube sedimentation—promote the agglomeration and sedimentation of tiny metal colloids, ensuring sufficient precipitation of heavy metal ions and significantly improving the removal efficiency of heavy metals from wastewater. This two-stage synergistic treatment not only significantly reduces the amount of liquid alkali used and operating costs but also reduces the amount of neutralization residue generated, combining economic efficiency with environmental friendliness. It represents a highly efficient and feasible technical approach for the harmless disposal of hazardous acid waste.

[0160] The above description is a detailed description of the preferred embodiments of the present invention. However, the embodiments are not intended to limit the scope of the patent application of the present invention. All equivalent changes or modifications made under the technical spirit of the present invention should fall within the patent scope covered by the present invention.

Claims

1. A system for the two-stage neutralization and treatment of waste acid using limestone and liquid alkali, characterized in that, It includes a limestone coarse neutralization unit, a liquid alkali fine neutralization unit, and a solid-liquid separation unit; The limestone coarse neutralization unit includes a limestone neutralization filter and an aeration device. The top of the limestone neutralization filter is connected to a waste acid discharge pipe, and the bottom is connected to a neutralization liquid delivery pipe with a control valve. Both the waste acid discharge pipe and the neutralization liquid delivery pipe are connected to the inner cavity of the limestone neutralization filter, which is filled with limestone. The aeration device includes an aeration blower, an air supply pipe, and an aeration pipe. The aeration pipe is buried in the limestone bottom layer and has several aeration ports. One end of the air supply pipe is connected to the aeration pipe, and the other end extends to the outside of the limestone neutralization filter and is connected to the air outlet of the aeration blower. The two-stage liquid alkali fine neutralization unit includes a pH adjustment tank, a weight removal reaction tank, a coagulation reaction tank, a flocculation reaction tank, and an inclined tube sedimentation tank connected in sequence. Each of the pH adjustment tank, weight removal reaction tank, coagulation reaction tank, and flocculation reaction tank is equipped with a stirrer and a dosing device. The four dosing devices respectively add liquid alkali, weight removal agent sodium sulfide (Na2S), polyaluminum chloride (PAC) solution, and polyacrylamide (PAM) solution. The pH adjustment tank is connected to the end of the neutralization liquid delivery pipe away from the limestone neutralization filter, and the bottom of the inclined tube sedimentation tank is connected to a sludge delivery pipe with a control valve. The solid-liquid separation unit includes a filter press. The feed end of the filter press is connected to the end of the sludge conveying pipe away from the inclined tube sedimentation tank. The effluent end of the filter press and the supernatant effluent end of the inclined tube sedimentation tank are both connected to the inlet end of the wastewater treatment plant through a wastewater conveying pipe.

2. The system for two-stage neutralization and treatment of waste acid using limestone and liquid alkali according to claim 1, characterized in that, The limestone coarse neutralization unit also includes a flushing inlet pipe and a flushing outlet pipe, both of which are connected to the inner cavity of the limestone neutralization filter. The flushing inlet pipe is located at the top of the limestone neutralization filter, and the flushing outlet pipe is located at the bottom of the limestone neutralization filter.

3. The system for two-stage neutralization and treatment of waste acid using limestone and liquid alkali according to claim 1, characterized in that, The pH adjustment tank is connected to the degravation reaction tank through the first overflow hole, the degravation reaction tank is connected to the coagulation reaction tank through the second overflow hole, the coagulation reaction tank is connected to the flocculation reaction tank through the third overflow hole, and the flocculation reaction tank is connected to the inclined tube sedimentation tank through the fourth overflow hole. The heights of the first, second, third, and fourth overflow holes decrease sequentially.

4. The system for two-stage neutralization and treatment of waste acid using limestone and liquid alkali according to claim 1, characterized in that, The filter press is a plate and frame filter press.

5. The system for two-stage neutralization and treatment of waste acid using limestone and liquid alkali according to claim 1, characterized in that, The solid-liquid separation unit also includes a screw conveyor and a transfer sludge hopper; the feed inlet of the screw conveyor is connected to the solid discharge outlet of the filter press, and the discharge outlet is located directly above the transfer sludge hopper.

6. The system for two-stage neutralization and treatment of waste acid using limestone and liquid alkali according to claim 1, characterized in that, The wastewater conveying pipe is also connected to the inlet of the limestone neutralization filter and the inlet of the pH adjustment tank via a circulation pipe assembly.

7. A method for two-stage neutralization and treatment of waste acid using limestone and liquid alkali, characterized in that, The system for treating waste acid using a two-stage neutralization process of limestone and liquid alkali as described in any one of claims 1 to 6 includes the following steps: S1, First stage limestone coarse neutralization: Waste acid is transported to the limestone neutralization filter to undergo a neutralization reaction with limestone, and continuous aeration is carried out in the limestone neutralization filter by an aeration blower. After the neutralization reaction is completed, a first stage neutralized liquid is obtained. S2, Second-stage liquid alkali fine neutralization: The first-stage neutralized liquid obtained in step S1 is transported to the pH adjustment tank, and after pH adjustment, it enters the coagulation reaction tank, and after coagulation reaction, it enters the flocculation reaction tank, and after flocculation reaction, it finally enters the inclined tube sedimentation tank for sedimentation, producing sedimentation tank bottom sludge and inclined tube supernatant. S3. Solid-liquid separation: The sediment from the sedimentation tank generated in step S2 is transported to a filter press for dewatering and pressing to produce filter cake and filtrate. The filtrate is combined with the supernatant from the inclined tube generated in step S2 and transported to the wastewater treatment plant through the wastewater conveying pipe. The filter cake is transferred to a special hazardous waste temporary storage bin.

8. The method for two-stage neutralization and disposal of waste acid using limestone and liquid alkali according to claim 7, characterized in that, In step S1, the neutralization reaction time is 40-100 min, the air-to-water ratio during aeration is 6:1-12:1, and the pH of the neutralized solution is 3.2-4.

5.

9. The method for two-stage neutralization and disposal of waste acid using limestone and liquid alkali according to claim 7, characterized in that, In step S2, the pH of the waste liquid after pH adjustment is 8.0~9.5; the dosage of polyaluminum chloride (PAC) solution is 100~220 mg / L, the stirring rate of the agitator in the coagulation reaction tank is 100~150 r / min, and the coagulation reaction time is 10~25 min; the dosage of polyacrylamide (PAM) solution is 3~8 mg / L, the stirring rate of the agitator in the flocculation reaction tank is 20~40 r / min, and the flocculation reaction time is 10~30 min; the sedimentation time of the waste liquid in the inclined tube sedimentation tank is 60~120 min.

10. The method for two-stage neutralization and disposal of waste acid using limestone and liquid alkali according to claim 7, characterized in that, In step S3, the moisture content of the filter cake is 55%~60%, and the pH of the filtrate is 7~8. After the filter cake is discharged from the filter press, it enters the screw conveyor and is transported to the transfer sludge hopper by the screw conveyor. Then, the transfer sludge hopper loaded with the filter cake is transferred to the special hazardous waste temporary storage bin by a forklift.

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