A method for recovering boron carbide abrasive from SIC double-disc lapping slurry waste

Abstract

The present application relates to the field of environmental protection and resource technology, and in particular, relates to a method for recycling boron carbide abrasive in SIC double-sided grinding waste liquid, and discloses a method for recycling boron carbide abrasive in SIC double-sided grinding waste liquid. The method aims to solve the problems of low recovery rate and poor purity caused by organic suspending agent in the waste liquid in traditional recycling. The method is characterized in that it comprises: waste liquid cooperates with oxidation and gel breaking, and solid-liquid concentration; concentrated slurry is high-gradient magnetic separated to remove iron; precise acidolysis purification; multi-stage cyclone separation of boron carbide and silicon carbide is strengthened by ultrasonic depolymerization; and the abrasive is dehydrated, dried and inspected. The present application realizes efficient and high-purity recovery of boron carbide abrasive by completely disintegrating the gel system of the waste liquid, the purity can reach more than 98.5%, the recovery rate is more than 95%, and a technical path is provided for the recycling of high-value abrasives.

Description

Technical Field

[0001] This invention belongs to the field of environmental protection and resource utilization technology. Specifically, it relates to a method for recovering boron carbide abrasive from the grinding waste liquid of a SiC double-sided mill. Background Technology

[0002] This invention belongs to the field of semiconductor material processing technology, and more specifically, relates to a method for recovering boron carbide (B4C) abrasive from silicon carbide (SiC) wafer grinding waste liquid.

[0003] Against the backdrop of the rapid development of third-generation semiconductor materials technology, represented by silicon carbide, its excellent high-temperature, high-frequency, high-power, and high-voltage performance makes it play an indispensable and increasingly important role in cutting-edge industries such as new energy vehicles, rail transportation, photovoltaic power generation, and 5G communications. To obtain silicon carbide wafers that meet stringent electronic performance requirements, the manufacturing process involves a series of precise mechanical and chemical processing steps. Among these, double-sided grinding is a core process to ensure the flatness and thickness uniformity of the wafer surface. In this process, boron carbide, due to its extremely high hardness and excellent chemical stability, is widely used as the main grinding medium. However, this process inevitably generates a large amount of complex grinding waste liquid. This waste liquid not only contains high-value boron carbide abrasive but also contains silicon carbide micropowder generated during the grinding process, iron-based impurities worn off from the equipment, and organic suspending agents such as polyethylene glycol added to maintain the suspension of the abrasive. If this waste liquid is discharged directly, it will not only pose a serious threat to the water and soil environment, but also mean a huge waste of expensive boron carbide resources, significantly increasing production costs, which is contrary to the current industrial orientation of sustainable development and green manufacturing.

[0004] To address this challenge, those skilled in the art have undertaken numerous explorations. For example, Chinese patent CN107902659B discloses a method for recovering and purifying boron carbide from sapphire grinding waste. This method focuses on processing solid waste, employing a series of physicochemical processes including sieving, magnetic separation, acid dissolution, and hydrothermal reaction to remove impurities from the waste, achieving efficient recovery of ultrafine boron carbide powder. From a technological contribution perspective, this patent successfully constructs a purification pathway for boron carbide in solid-phase mixtures, verifying the feasibility of removing impurities through strong chemical methods (such as strong acid dissolution and hydrothermal reaction) in specific application scenarios. Another representative technology, such as the automatic sorting system for ultrafine boron carbide powder disclosed in Chinese patent CN104511367B, focuses on solving the problem of particle size classification. This system, through a cleverly designed settling tank, stirring system, and siphon control system, utilizes the different settling velocities of particles in a liquid medium to achieve precise particle size sorting of newly prepared, relatively homogeneous boron carbide micropowder. The value of this approach lies in providing an automated and high-precision physical separation method for obtaining qualified abrasives with a specific particle size distribution.

[0005] However, with the continuous refinement of silicon carbide wafer processing technology and increasingly stringent environmental requirements, the inherent limitations of the aforementioned technical solutions in their design concepts and technical approaches have gradually become apparent when applied to treating a specific source—namely, silicon carbide double-sided milling waste liquid. These limitations collectively point to a deep-seated technical contradiction. The core of this contradiction does not simply stem from differences in the liquid properties or impurity types of the waste liquid, but rather from the stable colloidal structure formed by the complex interactions between the components within the waste liquid system, and the systematic antagonistic effect of this structure on existing separation technologies. Specifically, the fundamental function of the organic suspending agent in the silicon carbide milling waste liquid is to maintain the stable suspension of micron-sized particles such as boron carbide and silicon carbide during the milling process, preventing their sedimentation. This deliberately constructed stable colloidal system, designed to ensure milling effectiveness, becomes the biggest obstacle to solid-liquid separation and particle sorting in the recycling stage. The effectiveness of any physical separation method based on gravity sedimentation, centrifugation, or simple filtration is significantly weakened by the stabilizing effect of the suspending agent, resulting in low separation efficiency and difficulty in guaranteeing recovery rates. Furthermore, these organic molecules can coat the surface of boron carbide, silicon carbide, and even iron impurity particles, forming an "organic protective film." This film not only hinders effective separation between particles but also severely affects the efficiency of subsequent chemical treatment. For example, during magnetic separation to remove iron, the magnetic responsiveness of iron particles coated with organic matter decreases significantly; during acid washing to remove ionic or non-magnetic iron compounds, this organic film hinders sufficient contact between the acid and impurities, leading to incomplete chemical reactions and insufficient impurity removal rates. Therefore, existing technologies generally neglect the priority treatment of "organic suspending agents"—a key interfering factor—in wastewater, attempting to directly apply separation logic designed for inorganic solid waste or idealized suspensions, inevitably resulting in inefficient and ineffective treatment. While the strong acid and hydrothermal reaction in CN107902659B is powerful, its direct application to liquid systems containing large amounts of organic matter not only triggers uncontrollable side reactions and generates secondary pollutants but also imposes high corrosiveness requirements on the equipment, making it uneconomical for treating large-scale liquid wastewater. The physical sorting system of CN104511367B is designed under the premise of an ideal environment with few impurities and no organic stabilizers. When faced with the complex colloidal system of real grinding waste liquid, its sorting accuracy and efficiency will be out of the question.

[0006] Therefore, the technological challenge in this field is no longer simply how to recover boron carbide, but how to systematically and strategically break down the specific artificially stabilized colloidal system of grinding waste liquid. This requires the recovery process to have a completely new pre-process, the purpose of which is to first break the stable structure constructed by the organic suspending agent, "liberating" all solid particles from the colloidal binding, before subsequent impurity removal and classification operations can be effectively implemented. Therefore, how to develop a method that can first effectively break down the stable colloidal system formed by the organic suspending agent in the waste liquid, then efficiently and gently remove complex iron impurities step by step, and finally achieve precise separation of boron carbide and silicon carbide micropowder without introducing secondary pollution has become a key technical problem that urgently needs to be solved by those skilled in the art in promoting the green manufacturing and resource recycling of silicon carbide wafers. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies, specifically: when treating silicon carbide grinding waste liquid containing organic suspending agents, traditional recovery methods fail to preferentially break down the stable colloidal system constructed by the suspending agent, resulting in low efficiency and unsatisfactory recovery rates in subsequent physical and chemical purification and classification steps, and posing a risk of introducing secondary pollution. To solve this technical problem, this invention provides a method for recovering boron carbide abrasive from SiC double-sided mill grinding waste liquid. This method introduces a series of interconnected technical steps, including synergistic oxidation and colloidal breaking pretreatment, high-gradient magnetic separation, precision acid hydrolysis, and ultrasonic depolymerization-enhanced multi-stage cyclone separation. The aim is to fundamentally dismantle the stable colloidal structure of the waste liquid, achieving thorough cleaning of the solid particles' surface, and on this basis, efficiently and precisely separating and recovering high-purity boron carbide abrasive.

[0008] To achieve the above-mentioned objectives, the technical solution adopted by this invention specifically includes the following steps:

[0009] First, the waste liquid undergoes synergistic oxidation and gel breaking, along with solid-liquid concentration. This step aims to thoroughly decompose the organic suspending agents in the waste liquid using advanced oxidation processes, disrupting its colloidal stability and initially enriching the dispersed solid particles. Specifically, the collected silicon carbide double-sided milling waste liquid is pumped into a synergistic oxidation and gel breaking reaction system at a set flow rate via a peristaltic feed pump. The core of this system is a microwave-enhanced heterogeneous Fenton reactor. The reactor's main structure is a high-temperature resistant quartz glass tube with an inner diameter of 50 mm and a wall thickness of 3 mm, spirally wound within a multimode microwave resonant cavity. This microwave resonant cavity is made of SUS316L stainless steel and internally coupled with an industrial-grade magnetron array with a frequency of 2450 MHz and an output power continuously adjustable from 1.0 to 5.0 kW. The reactor system further includes a fixed-bed structure for catalyst loading, located at the inlet section of a quartz glass tube. This fixed-bed structure is filled with γ-alumina spheres with a particle size of 3 to 5 mm. The surface of these spheres is loaded with 3% to 5% (w / w) of iron oxide nanoparticles as a heterogeneous Fenton-like catalyst via an impregnation-calcination method. Before the waste liquid enters the reactor, a metering injection system connected to the main feed line precisely injects a 30% (w / w) hydrogen peroxide solution into the waste liquid at a ratio of 0.1% to 0.5% relative to the waste liquid's volumetric flow rate, and thoroughly mixes it with the waste liquid using a static mixer. When the waste liquid containing hydrogen peroxide and solid particles flows through the supported catalyst fixed bed, the iron oxide catalyst reacts with the hydrogen peroxide, continuously generating highly oxidizing hydroxyl radicals in situ. Simultaneously, the magnetron array is activated, and the generated microwave energy is absorbed by the waste liquid within the reaction chamber, selectively heating the polar molecules (water molecules and functional groups on the long chains of organic molecules such as polyethylene glycol) in the waste liquid. This creates a localized high-temperature region at the microscopic scale, which significantly accelerates the generation rate and mass transfer efficiency of hydroxyl radicals and significantly improves their chain-breaking and mineralization efficiency for large organic molecules such as polyethylene glycol. The hydraulic residence time of the waste liquid in the reactor is precisely controlled to be between 10 and 15 minutes. After this treatment, the total organic carbon (TOC) content in the waste liquid decreases from the initial 3000-5000 mg / L to below 50 mg / L, and its stable colloidal structure is completely destroyed. The liquid-solid mixture flowing out of the synergistic oxidation and decolloidalization reaction system enters a pH adjustment and flocculation sedimentation unit. This unit includes a 5-cubic-meter fiberglass (FRP) sedimentation tank with a conical bottom. First, a 10% sodium hydroxide solution is added dropwise to the mixture using a diaphragm pump linked to an online pH monitor, precisely adjusting the pH of the system to 8.5. Under this pH condition, the residual iron ions in the waste liquid and those dissolved during the reaction will be completely converted into ferric hydroxide precipitate.Subsequently, anionic polyacrylamide (APAM) was added to the settling tank as a flocculant at a concentration of 2 mg / L. Under the slow agitation of a low-speed paddle agitator (30 rpm) inside the tank, the ferric hydroxide precipitate acted as flocculation nuclei, bridging and sweeping away the boron carbide, silicon carbide micropowder, and other solid impurities in the waste liquid that had lost their organic coating, forming large, dense flocs that rapidly settled to the conical bottom of the tank under gravity. After 2 hours of settling, the supernatant was discharged through the top overflow outlet and entered the subsequent wastewater treatment system; the lower layer of concentrated solid material (containing approximately 20% to 30% solids) was discharged through the bottom spiral discharge valve, forming a concentrated slurry to be treated.

[0010] Second, a high-gradient magnetic separation process is performed on the concentrated slurry to remove iron. The purpose of this step is to efficiently remove micron- and submicron-sized magnetic impurity particles in the concentrated slurry, existing as elemental iron and iron oxides, using a high-intensity, high-gradient magnetic field. Specifically, the concentrated slurry produced in the previous step is pumped by a screw pump into the separation chamber of a vertical annular high-gradient magnetic separator (HGMS). The core component of the HGMS is an electromagnetic system consisting of an excitation coil and a pure iron yoke, capable of generating a background magnetic field strength of up to 1.5 Tesla. Inside its separation chamber, a magnetic media mesh is filled with 430-type ferritic stainless steel wires with a diameter of 50 micrometers and a filling rate of 8%. When the concentrated slurry flows upward through the strongly magnetized magnetic media mesh, an extremely high magnetic field gradient is generated on and around the wire surface. All magnetically responsive iron-based impurity particles in the slurry, regardless of their size, are captured and adsorbed onto the wire surface by the powerful magnetic force. Non-magnetic boron carbide and silicon carbide particles pass through the magnetic separator along with the slurry and exit from the top outlet of the magnetic separator, forming a slurry after iron removal. This magnetic separation process continues until the online inductively coupled plasma optical emission spectrometer (ICP-OES) on the outlet pipeline detects an increasing trend in the iron concentration in the slurry, indicating that the magnetic separator is nearing saturation. At this point, the control system automatically stops the feed and cuts off the current to the excitation coil, eliminating the background magnetic field. Subsequently, a high-pressure backwashing system is activated, using deionized water at a flow rate three times the operating flow rate to flush the magnetic separator from top to bottom, washing off all adsorbed iron impurities, which are collected as iron-containing waste. After backwashing, the system re-establishes the magnetic field and begins the next working cycle. After this high-gradient magnetic separation step, the total iron content in the slurry is reduced from approximately 500-1000 ppm (based on solids) to below 20 ppm.

[0011] Third, a precise acid hydrolysis purification treatment is performed on the iron removal slurry. This step aims to completely remove residual iron impurities in the slurry, which exist in the form of non-magnetic or weakly magnetic compounds (such as partial iron hydroxides, carbonates, or complex salts formed with silicon), through chemical dissolution. Specifically, the iron removal slurry, after high-gradient magnetic separation, is pumped into a 3-cubic-meter jacketed glass reactor lined with Hastelloy C-276. This reactor is equipped with a temperature control system, a dual-parameter online monitoring system for pH and oxidation-reduction potential (ORP), and a frequency-controlled PTFE anchor stirrer. First, circulating water is introduced into the reactor through the jacket to precisely control the slurry temperature at 50 degrees Celsius. Then, the stirrer is started, and a 2 mol / L hydrochloric acid solution is slowly added dropwise into the reactor through a corrosion-resistant metering pump. The pH / ORP dual-parameter online monitoring system monitors the chemical state of the slurry in the reactor in real time. The hydrochloric acid addition process is managed by a closed-loop control logic: the control system uses pH as the primary control parameter and ORP as a secondary verification parameter. The hydrochloric acid dropping rate is dynamically adjusted to ensure that the pH of the slurry decreases smoothly over 30 minutes and eventually stabilizes at 1.5. Simultaneously, the ORP value increases with acid addition, and its final stable value (approximately +600 mV relative to a standard hydrogen electrode) serves as one of the criteria for determining the reaction endpoint, ensuring the system has sufficient oxidizing power to dissolve all forms of iron compounds. The reaction is continuously stirred for 60 minutes at pH 1.5 and temperature 50°C. During this period, all residual iron compounds are converted into soluble ferrous chloride or ferric chloride. After the reaction, heating and stirring are stopped, and a large amount of ultrapure water with a resistivity greater than 18 MΩ·cm is injected into the reactor to dilute and wash the slurry. The washing process employs a decantation method, where the mixture is allowed to stand for one hour after each water injection to allow the solid particles to settle completely. Then, a vacuum pump is used to extract the acidic supernatant containing ferric chloride and residual hydrochloric acid. This washing-settling-extraction process is repeated five times until the pH of the slurry in the reactor rises above 6.5, and no ferric ions are detected in the supernatant using potassium thiocyanate reagent. After this precise acidolysis purification step, the total iron content in the slurry, based on solids, is further reduced to below 1 ppm. At this point, the slurry mainly consists of high-purity boron carbide and silicon carbide micropowders, as well as pure water.

[0012] Fourth, a multi-stage hydrocyclone classification and ultrasonic depolymerization synergistic separation process is implemented. The core of this step is to utilize the significant density difference between boron carbide (density approximately 2.52 g / cm³) and silicon carbide (density approximately 3.21 g / cm³) to achieve efficient separation through multi-stage hydrocyclones, supplemented by online ultrasonic depolymerization technology to ensure separation accuracy. Specifically, the slurry, after precision acid hydrolysis purification and adjustment of the solid content to 10%, is pumped through a high-pressure diaphragm pump into a three-stage countercurrent hydrocyclone separation system. This system consists of three identical hydrocyclones (designated C1, C2, and C3) connected in series, made of high-wear-resistant alumina ceramic and with a classification particle size of 15 micrometers. Before the slurry enters the first-stage hydrocyclone C1, a ring-shaped online ultrasonic depolymerization device is installed on its delivery pipeline. The device includes a 2 kW, 20 kHz ultrasonic transducer, whose strong cavitation effect instantly breaks up any particle agglomerates in the slurry caused by van der Waals forces or electrostatic attraction, ensuring that each particle enters the hydrocyclone as a single unit. This is the physical prerequisite for achieving high-precision density classification. After entering the first-stage hydrocyclone C1, under the strong centrifugal force generated by the high-speed rotating flow field, the denser silicon carbide particles tend to move towards the wall and eventually exit from the underflow outlet, forming a silicon carbide-rich underflow slurry; while the less dense boron carbide particles concentrate in the central region and are discharged from the overflow outlet with most of the liquid, forming a boron carbide-rich overflow slurry. To improve recovery rate and purity, the system employs a countercurrent washing configuration: the overflow of C1 (rich in B4C) serves as the feed to the second-stage hydrocyclone C2; ​​the underflow of C2 (containing a small amount of SiC) returns to the feed line of C1 to mix with the original slurry; and the overflow of C2 flows out as the final boron carbide product. Simultaneously, the underflow of C1 (rich in SiC) serves as the feed to the third-stage hydrocyclone C3; the overflow of C3 (containing a small amount of B4C) returns to the feed line of C1; and the underflow of C3 flows out as the final silicon carbide byproduct. Fresh makeup water is added from the feed end of C3 to compensate for the moisture carried away by the overflow of C2 and the underflow of C3, and to wash the silicon carbide in C3. Through this three-stage countercurrent synergistic separation operation, the boron carbide suspension obtained from the C2 overflow port can stably achieve a boron carbide mass purity of over 98.5% in its solids, while the silicon carbide content is controlled below 1.0%, and the total boron carbide recovery rate reaches over 95%.

[0013] Fifth, the boron carbide abrasive is dehydrated, dried, and the finished product is inspected. This step aims to convert the separated high-purity boron carbide suspension into dry powder that meets the requirements of the grinding process. Specifically, the boron carbide suspension from the multi-stage cyclone separation system is first fed into a horizontal screw discharge sedimentation centrifuge for preliminary dehydration. Inside the centrifuge, under the action of a centrifugal force field with an acceleration of up to 5000 times the force of gravity, the solid and liquid phases are rapidly separated, and the wet filter cake with a water content reduced to below 20% is continuously discharged. The separated clean water is recycled to the preceding acid washing process. Subsequently, the wet filter cake discharged from the centrifuge is sent to a vibrating fluidized bed dryer for final drying through a closed screw conveyor. Inside the dryer, hot air (temperature controlled at 90 degrees Celsius) is blown in evenly from below the air distribution plate, causing the boron carbide filter cake particles to be fluidized under the dual action of vibration and hot air flow, achieving full and uniform contact between the particles and the hot air, and the moisture is rapidly vaporized and carried away. The material resides in the dryer for approximately 30 minutes, resulting in a final dried boron carbide powder with a moisture content of less than 0.1%. The dried powder is then fed into a sieving unit equipped with an electromagnetic vibrating screen via a fully enclosed negative pressure pneumatic conveying system. This sieving unit uses a 200-mesh standard sieve to remove any small aggregates of large particles or foreign impurities that may be generated during processing, ensuring the upper limit of the final product particle size. Finally, samples of the sieved boron carbide powder are taken, and a particle size analyzer based on laser diffraction is used to comprehensively test the particle size distribution according to the national standard GB / T 19077.1-2016. X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) are used for final confirmation of phase purity and impurity element content, ensuring that the recovered boron carbide abrasive meets or exceeds the original purchase specifications in terms of purity, particle size distribution, and impurity content, allowing it to be directly returned to the double-sided grinding process of silicon carbide wafers for reuse.

[0014] The beneficial effects of this invention: The core innovation of the method disclosed in this invention lies in its departure from the traditional approach of directly separating solids from waste liquid. Instead, it first utilizes microwave-enhanced advanced oxidation technology to thoroughly degrade the organic suspending agent, which is the core obstacle to separation, at the molecular level, achieving "chemical cleaning" of the particle surface and "colloidal breaking" of the system. Based on this, a series of highly synergistic unit operations are coupled, including pH-controlled flocculation concentration, high-gradient magnetic separation, precisely controlled acid hydrolysis purification, and ultrasonically depolymerized multi-stage countercurrent cyclone separation. Each step is precisely designed to target specific impurities or specific separation tasks, creating the most favorable conditions for subsequent steps, thus forming a complete, logically rigorous, and highly efficient closed-loop recovery system. This system not only solves the fundamental problem of existing technologies being unable to effectively treat stable colloidal waste liquids, but also achieves significant technological breakthroughs in key performance indicators such as impurity removal depth, product purity, and recovery rate, providing a practical and feasible technical path for the silicon carbide processing industry to achieve the recycling and clean production of high-value abrasives. Attached Figure Description

[0015] Figure 1 This is a flowchart illustrating the method of the present invention.

[0016] Figure 2 This is a schematic diagram of the synergistic oxidation depolymerization reaction system described in this invention.

[0017] Figure 3 This is a schematic diagram of the pH adjustment and flocculation sedimentation unit described in this invention.

[0018] Figure 4 This is a schematic diagram of the process flow of the multi-stage countercurrent cyclone separation system described in this invention.

[0019] The attached figures are labeled as follows: 10. Synergistic oxidation depolymerization reaction system; 11. Microwave resonant cavity; 12. Quartz glass pipe; 13. Magnetron array; 14. Fixed bed structure; 20. pH adjustment and flocculation sedimentation unit; 21. Sedimentation tank; 22. Low-speed paddle agitator; 23. Spiral discharge valve; 24. Overflow port; 30. Hydrocyclone separation system; 31. Annular online ultrasonic depolymerization device; 32. First-stage hydrocyclone; 33. Second-stage hydrocyclone; 34. Third-stage hydrocyclone. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the following will describe in detail, with reference to the accompanying drawings and specific embodiments, a method for recovering boron carbide abrasive from SiC double-sided mill grinding waste liquid disclosed in this invention. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute any limitation thereof. Those skilled in the art, guided by the technical solutions of this invention, can easily implement various equivalent substitutions or modifications to the method described herein, and all such modifications should fall within the protection scope of this invention.

[0021] Reference Figure 1 This invention demonstrates the complete process flow of a method for recovering boron carbide abrasive from SiC double-sided mill grinding waste liquid. This method does not involve simple physical separation of the waste liquid, but rather constructs a series of interconnected and progressively deepening chemical and physical synergistic treatment systems. Its core lies in fundamentally disrupting the colloidal stability of the waste liquid, laying the foundation for subsequent efficient separation.

[0022] In one specific embodiment, the technical solution of the present invention begins with the synergistic oxidation degelatinization and solid-liquid concentration treatment of waste liquid. The purpose of this step is to completely decompose the organic suspending agent, mainly polyethylene glycol (PEG), in the waste liquid. This suspending agent is the root cause of the difficulty in settling solid particles, their tendency to agglomerate, and the difficulty in cleaning the surface. Specifically, the grinding waste liquid collected from the double-sided grinding process of silicon carbide wafers (whose initial total organic carbon (TOC) content is typically 3000 to 5000 mg / L, solid content is approximately 3% to 5%, viscosity is high, and its appearance is a uniform and stable gray-black suspension) is temporarily stored in a buffer tank with a stirring function. Subsequently, the waste liquid is pumped into the synergistic oxidation degelatinization reaction system 10 at a precisely calibrated constant flow rate ranging from 5 to 8 L / min using a peristaltic feed pump (e.g., LongerPump WT600-4F).

[0023] Reference Figure 2The diagram details the internal structure of the synergistic oxidation depolymerization reaction system 10. At its core is a specially designed microwave-enhanced heterogeneous Fenton-like reactor. The main body of the reactor is a microwave resonant cavity 11, specifically 600 mm × 600 mm × 800 mm, welded from 5 mm thick SUS316L stainless steel sheet, with a mirror-polished inner surface to maximize microwave reflection efficiency. A magnetron array 13 is coupled inside the cavity, consisting of four 1.5 kW, 2450 MHz industrial-grade water-cooled magnetrons symmetrically distributed on the cavity sidewalls. The microwave energy from each magnetron is fed into the cavity through a standard WR340 waveguide and homogenized by a stepper motor-driven mode stirrer mounted on the top of the cavity to eliminate standing wave effects and ensure uniform heating of the reactants. Inside the reaction cavity, a continuous high-temperature resistant quartz glass tube 12 is spirally coiled. The pipe is 15 meters long, 50 millimeters in inner diameter, and 3 millimeters thick. Its spiral pitch and diameter have been optimized to ensure that the hydraulic residence time of the waste liquid in the pipe is precisely controlled between 10 and 15 minutes at the set flow rate.

[0024] Before the waste liquid enters the quartz glass pipe 12, a static mixing unit and a catalyst injection point are installed on its delivery line. A precision metering pump (e.g., Prominent-Gala1601) connected in a T-shape to the main feed line continuously injects a 30% (w / w) hydrogen peroxide (H2O2) solution at a ratio of 0.1% to 0.5% relative to the waste liquid volume flow rate (the specific ratio is adjusted online based on the TOC concentration of the feed waste liquid). Immediately after the injection point, a PTFE static mixer containing 12 mixing units is connected to ensure that the hydrogen peroxide is uniformly mixed with the waste liquid at the molecular level before entering the reactor.

[0025] The mixed waste liquid then enters a fixed-bed structure 14 located at the inlet section of the quartz glass pipe 12. This fixed-bed structure 14 is 500 mm long and is filled with spherical γ-alumina (γ-Al₂O₃) supports with a particle size of 3 to 5 mm. The supports were pre-loaded with catalyst using an impregnation-calcination method. The preparation process is as follows: first, the γ-Al₂O₃ spheres are vacuum impregnated in a 1 mol / L ferric nitrate solution for 2 hours; then, they are dried at 120°C for 4 hours; finally, they are calcined in a muffle furnace at 500°C in air for 3 hours, causing the ferric nitrate to completely decompose into uniformly attached iron oxide (Fe₂O₃) nanoparticles on the surface of the supports. The final loading was determined to be 4% of the total mass.

[0026] When waste liquid containing hydrogen peroxide flows through the fixed-bed structure 14 loaded with iron oxide, a heterogeneous Fenton-like reaction is triggered. The iron oxide surface, acting as a catalytic active center, reacts with hydrogen peroxide to continuously generate hydroxyl radicals (·OH) with extremely high oxidation potentials (E0 = 2.80V) in situ. Simultaneously, the magnetron array 13 within the microwave resonant cavity 11 is activated, and its output power is controlled by a PID closed-loop system based on feedback from the temperature sensor at the waste liquid outlet, typically set between 3.0 and 5.0 kW. Microwave energy is efficiently absorbed by the waste liquid within the reaction cavity. Because water molecules and organic molecules such as polyethylene glycol have highly polar groups such as ether bonds and hydroxyl groups, the selective heating effect of microwaves causes them to rapidly heat up at the microscale, forming numerous localized "hot spots." This non-equilibrium high temperature at the microscale greatly accelerates the generation rate and mass transfer process of hydroxyl radicals. It also directly acts on macromolecular organic compounds such as polyethylene glycol, causing their molecular chains to vibrate and twist, exposing more reaction sites. This significantly improves the efficiency of hydroxyl radicals in breaking chains, opening rings, and ultimately mineralizing them into carbon dioxide and water. The entire synergistic oxidation process maintains the temperature of the waste liquid at 85 to 95 degrees Celsius without requiring macroscopic and costly overall heating of the entire system. After this step, the removal rate of total organic carbon (TOC) in the waste liquid can reach over 99%, with its content dropping dramatically from several thousand mg / L initially to below 50 mg / L. More importantly, the organic protective layer coating the surface of boron carbide and silicon carbide particles is completely stripped away, the original stable colloidal system of the waste liquid is completely destroyed, and solid particles are released, creating extremely favorable conditions for subsequent sedimentation and separation.

[0027] The liquid-solid mixture flowing out of the synergistic oxidation depolymerization reaction system 10, utilizing its own residual heat, directly enters a pH adjustment and flocculation sedimentation unit 20. (Refer to...) Figure 3The main body of the unit is a 5-cubic-meter fiberglass (FRP) settling tank 21 with a 60-degree conical bottom. First, a 10% (w / w) sodium hydroxide (NaOH) solution is precisely added to the mixture via a corrosion-resistant diaphragm pump linked to an online pH meter (e.g., E+HOrbisint CPS11D) installed inside the tank. The control system steadily raises the pH of the system within 10 minutes and eventually stabilizes it at 8.5 ± 0.1. Under this pH condition, trace amounts of iron ions (Fe2+ / Fe3+) that may dissolve in the waste liquid during oxidation are quantitatively converted into hydrophobic ferric hydroxide (Fe(OH)3) precipitate. Subsequently, anionic polyacrylamide (APAM) with a molecular weight of 12 million is added to the settling tank 21 at a concentration of 2 mg / L. The APAM solution is pre-prepared as a 0.1% stock solution in a dedicated preparation device and matured for 2 hours before use. Inside the tank, a low-speed paddle agitator 22 (rotation speed set at 30 rpm, paddles designed with wide anchors to generate a gentle overall circulation flow and avoid breaking the existing flocs) slowly agitates the newly formed ferric hydroxide precipitate, which acts as an ideal coagulation nucleus. Together with the long molecular chains of APAM, it bridges and sweeps, efficiently capturing and encapsulating boron carbide, silicon carbide micropowder, and other solid impurities in the waste liquid that have lost their organic coating, forming large, dense flocs. These flocs quickly settle to the conical bottom of the tank under gravity. After two hours of sufficient settling, the upper clear liquid becomes clear and transparent, and is discharged through the top overflow port 24, flowing into the plant's wastewater treatment system for final treatment. The lower layer of concentrated solid material, with its solid content increased from the initial 3%-5% to 20%-30%, is intermittently discharged through the bottom pneumatic screw discharge valve 23, forming concentrated slurry to enter the next processing stage.

[0028] Next, the method proceeds to the second crucial step: high-gradient magnetic separation to remove iron from the concentrated slurry. This step is specifically designed to efficiently remove micron- and submicron-sized strongly magnetic impurity particles in the concentrated slurry, existing as elemental iron (primarily from equipment wear) and iron oxides. Specifically, the concentrated slurry from the previous step is pumped at a flow rate of 1.5 cubic meters per hour into the separation chamber of a vertical annular high-gradient magnetic separator (HGMS) using a wear-resistant screw pump (e.g., the NEMO® BY series). At its core, this magnetic separator (e.g., the LGS-1000 model) is an electromagnetic system consisting of multi-turn copper excitation coils and a pure iron yoke, capable of generating a background magnetic field strength of up to 1.5 Tesla in the central region of the separation chamber. The separation chamber is filled with a magnetic medium mesh woven from 430-type ferritic stainless steel wire. This wire, with a diameter of 50 micrometers, undergoes a special corrugated pressing process and is loosely packed into the separation chamber, with a filling rate precisely controlled at 8% (by volume). As the concentrated slurry flows upward through a strongly magnetized magnetic media grid, an extremely high magnetic field gradient (up to 10^4 T / m) is generated in the tiny space on and around the steel wire due to the high convergence of magnetic field lines. All magnetically responsive iron-based impurity particles in the slurry, regardless of their size or morphology, are captured by this powerful magnetic gradient and firmly adsorbed onto the steel wire surface. Non-magnetic boron carbide and silicon carbide particles are almost unaffected, passing through the magnetic media grid with the slurry and exiting from the top outlet of the magnetic separator, forming the iron-removed slurry. This magnetic separation process continues. To achieve precise process control, an online inductively coupled plasma optical emission spectrometer (ICP-OES) with an automatic sampling interface is installed on the discharge pipe of the magnetic separator. This interface automatically collects samples every 5 minutes to quickly determine the iron concentration in the slurry. When the iron concentration begins to show a significant upward trend from the baseline value (usually below 20 ppm), it indicates that the adsorption capacity of the magnetic media grid is approaching saturation. At this point, the PLC control system immediately and automatically executes the unloading procedure: first, it stops the feed pump and cuts off the DC power supply to the excitation coil, causing the background magnetic field to disappear within seconds. Then, a high-pressure backwashing system is activated, using deionized water at a flow rate of 4.5 cubic meters per hour (three times the working flow rate) to forcefully flush the magnetic media mesh from top to bottom, stripping and washing off all adsorbed iron impurities from the steel wire and collecting and treating them as iron-containing waste. A complete backwashing process lasts approximately 3 minutes. After backwashing, the system re-establishes the magnetic field, restarts the feed pump, and begins the next working cycle. After this high-gradient magnetic separation step, the total iron content in the slurry (based on dry solids) is steadily reduced from approximately 500-1000 ppm (partly from wear and partly from ferric hydroxide acting as coagulation nuclei in the previous step) to below 20 ppm, creating favorable conditions for subsequent precision chemical purification.

[0029] The third step, following this, involves precise acid leaching and purification of the iron-removing slurry. The goal of this step is to completely remove trace iron impurities from the slurry through a precisely controlled chemical dissolution process, in the form of non-magnetic or weakly magnetic compounds (such as partially uncaptured ferric hydroxide, possible carbonates, or complex salts formed with silicon). Specifically, the iron-removing slurry, after high-gradient magnetic separation, is pumped into a 3-cubic-meter jacketed glass reactor lined with 3 mm thick Hastelloy C-276. The reactor's materials ensure excellent resistance to hot hydrochloric acid environments. The reactor is equipped with a jacketed temperature control system, which, through an external hot and cold oil temperature control unit, circulates heat transfer oil into the jacket to precisely control the temperature of the slurry inside the reactor at 50 ± 1 degrees Celsius. Simultaneously, a composite probe (e.g., Mettler Toledo InPro 3250i) integrating a pH electrode, oxidation-reduction potential (ORP) electrode, and temperature sensor is installed inside the reactor for real-time, continuous monitoring of the chemical state of the slurry. An anchor-type stirrer, manufactured from a single piece of PTFE material and controlled by a frequency converter, slowly agitates at 60 rpm to ensure uniform suspension of the slurry and mass transfer of reactants. Once everything is ready, a 2 mol / L analytical grade hydrochloric acid solution is slowly added dropwise to the reactor via a corrosion-resistant fluoroplastic magnetically driven metering pump. The hydrochloric acid addition process is finely managed by a PLC-based closed-loop control logic: the control system uses pH as the primary control parameter and ORP as an auxiliary verification and endpoint determination parameter. Initially, the dropping rate can be slightly faster; when the pH drops below 3.0, the system switches to PID control mode, dynamically adjusting the dropping rate to ensure that the pH of the slurry decreases smoothly and without overshoot within 30 minutes, eventually stabilizing at 1.5. Meanwhile, the ORP value will continue to rise with the addition of acid and the oxidative dissolution of iron compounds. Its final stable value (approximately +600 mV relative to the standard hydrogen electrode) serves as another key indicator of the reaction reaching equilibrium, ensuring that the system has sufficient acidity and oxidizing power to dissolve all possible forms of residual iron compounds. The reaction was carried out with continuous stirring for 60 minutes at pH 1.5 and temperature 50°C. During this period, all residual ferric hydroxide, ferric oxide, etc., were converted into soluble ferrous chloride (FeCl2) or ferric chloride (FeCl3). After the reaction was completed, heating and stirring were stopped. Subsequently, a large amount of ultrapure water (prepared by a two-stage reverse osmosis plus EDI system) with a resistivity greater than 18 MΩ·cm was injected into the reactor to dilute and wash the slurry, thoroughly removing dissolved iron salts and residual hydrochloric acid.The washing process employs a decantation method. Specifically, after each filling with ultrapure water, the mixture is allowed to stand for one hour until the solid boron carbide and silicon carbide particles have completely settled. Then, using a height-adjustable PTFE siphon equipped with a vacuum suction device, the upper layer of acidic supernatant containing ferric chloride and residual hydrochloric acid is carefully extracted until the liquid level is close to the surface of the precipitate. This washing-settling-extraction process is repeated five times. After the final wash, a sample of the supernatant is tested with potassium thiocyanate (KSCN) reagent with a sensitivity of 0.1 ppm. The absence of the characteristic red color of ferric ions is confirmed, and the pH of the slurry in the vessel has risen to above 6.5, indicating complete washing. Through this precise acid hydrolysis purification step, the total iron content in the slurry, based on solids, is further reduced to an extremely low level below 1 ppm. At this point, the slurry mainly consists of high-purity boron carbide and silicon carbide micropowder and pure water, providing ideal pure material for the final density classification.

[0030] The fourth core step of the method of this invention is to perform a multi-stage hydrocyclone separation process combined with ultrasonic depolymerization. The physical basis of this step lies in the significant density difference of approximately 27% between boron carbide (density approximately 2.52 g / cm³) and silicon carbide (density approximately 3.21 g / cm³). Utilizing this difference, a specially designed multi-stage hydrocyclone system is used to achieve efficient separation of the two. (Refer to...) Figure 4 The diagram clearly illustrates the process flow of the multi-stage countercurrent hydrocyclone separation system 30. First, the slurry, after precision acid hydrolysis purification and washing, is placed in a stirred buffer tank. The solids content is precisely adjusted to 10% (mass fraction) by adding or removing the supernatant. Excessively high or low solids content will affect the efficiency of the hydrocyclone separation. Subsequently, the slurry is pumped into a three-stage countercurrent hydrocyclone separation system at a constant pressure of 0.3 MPa using a high-pressure diaphragm pump (whose pump head and valve ball are made of highly wear-resistant zirconia ceramic).

[0031] Before the slurry enters the first-stage hydrocyclone 32 (C1), a ring-shaped online ultrasonic deagglomeration device 31 is installed in series on its delivery pipeline. The device has a stainless steel casing and contains multiple sets of 2 kW, 20 kHz piezoelectric ceramic ultrasonic transducers arranged in a ring inside. As the slurry flows through this ring space, the powerful ultrasonic energy generates a violent cavitation effect in the liquid, resulting in the instantaneous formation, oscillation, and collapse of countless tiny bubbles. The microjets and shock waves generated in this process instantly disperse any particle agglomerates in the slurry caused by van der Waals forces or residual electrostatic attraction, ensuring that each boron carbide and silicon carbide particle enters the hydrocyclone as an independent monomer. This is a crucial physical prerequisite for achieving high-precision classification based on density differences; otherwise, the behavior of agglomerates (e.g., a large SiC particle adhering to several small B4C particles) will be unpredictable, severely compromising the separation effect.

[0032] This hydrocyclone separation system consists of three small hydrocyclones with identical structural parameters, integrally sintered from high-purity (>99.5%), high-wear-resistant alumina ceramic. The designed classification particle size (for quartz) is 15 micrometers. The three hydrocyclones are designated as first-stage hydrocyclone 32 (C1), second-stage hydrocyclone 33 (C2), and third-stage hydrocyclone 34 (C3). The system employs a classic countercurrent washing configuration to simultaneously achieve high purity and high recovery rate. The specific process is as follows: The raw slurry, after ultrasonic deagglomeration, is fed into the first-stage hydrocyclone C1. Inside C1, the slurry forms a high-speed rotating flow field under the action of the tangential inlet, generating centrifugal force up to several thousand times that of gravity. The denser silicon carbide particles are subjected to even greater centrifugal force, are thrown against the vessel wall, and spiral downwards along the wall, eventually exiting from the underflow outlet, forming a silicon carbide-rich underflow slurry. The less dense boron carbide particles are subject to less centrifugal force and are more easily carried to the low-pressure core in the central region by the radially inward liquid flow. They are then discharged from the overflow port above along with most of the liquid, forming an overflow slurry rich in boron carbide.

[0033] To purify boron carbide and recover its silicon carbide, the overflow from C1 (rich in B4C but still containing a small amount of fine SiC particles) is directly fed into the second-stage hydrocyclone C2 as its feed. In C2, density separation is performed again, and its overflow becomes the final high-purity boron carbide product stream, containing extremely high purity boron carbide particles. The underflow from C2 (containing a small amount of SiC separated from the C1 overflow and a small amount of unseparated B4C) is recycled back to the feed line of C1, mixed with the original slurry, and then re-enter C1 for separation. This configuration is equivalent to a secondary refining of the boron carbide product.

[0034] Simultaneously, to purify silicon carbide and recover boron carbide, the underflow from C1 (rich in SiC but still containing a small amount of B4C) is fed into the third-stage hydrocyclone C3 as its feed. Fresh replenishment process water is added from the feed line of C3 to wash the SiC in the C1 underflow. In C3, density separation occurs again, and its underflow becomes the final silicon carbide byproduct stream, containing high-purity silicon carbide; while the overflow from C3 (containing a small amount of B4C separated from the C1 underflow and a small amount of unseparated SiC) is also recycled back to the feed line of C1. This configuration is equivalent to a re-recovery of the material discarded by C1, improving the overall boron carbide recovery rate. Through this three-stage countercurrent synergistic separation operation, the various streams achieve dynamic equilibrium under the precise control of PLC-controlled pressure and flow regulating valves. The boron carbide suspension obtained from the overflow port of the second-stage hydrocyclone C2 has a boron carbide purity that can be stably maintained at over 98.5%, while the silicon carbide content is strictly controlled to below 1.0%. At the same time, the total boron carbide recovery rate (based on the total amount of B4C entering the unit) can reach over 95%.

[0035] The final step of the method of this invention is to perform dehydration, drying, and finished product inspection of the recovered boron carbide abrasive. This step aims to convert the separated high-purity boron carbide suspension (solid content of approximately 8%) into a dry powder with good flowability that meets the requirements of subsequent grinding processes. Specifically, the boron carbide suspension from the overflow port of the C2 multi-stage hydrocyclone separation system is first fed into a horizontal screw discharge sedimentation centrifuge (e.g., Alfa Laval P2 type) for efficient preliminary dehydration. Inside the centrifuge, under the action of a centrifugal force field of up to 5000g, the denser solid boron carbide particles are rapidly thrown against the inner wall of the drum and form a compact filter cake, which is continuously scraped out of the machine by an internal screw conveyor at a slight differential speed. The separated water is discharged through the overflow port and can be collected and recycled to the preceding acid washing process to save water. After centrifugal dehydration, the moisture content of the material is reduced from approximately 92% to below 20%, forming a wet filter cake with a certain degree of plasticity.

[0036] Subsequently, the wet filter cake discharged from the centrifuge is evenly fed into a continuous vibrating fluidized bed dryer for final drying via a fully enclosed screw conveyor. This dryer (e.g., the Changzhou Keda ZLG series) has a bed size of 0.6m × 5m, and its air distribution plate is made of SUS316L stainless steel with a special pore size and opening ratio. Clean hot air (temperature precisely controlled at 90 degrees Celsius by a PID system), after being treated by an electric heater and a high-efficiency filter, is evenly blown in from below the air distribution plate. Simultaneously, a pair of vibrating motors installed on both sides of the dryer generate excitation forces with specific frequency and amplitude, causing the bed to vibrate in a complex manner. Under the dual action of vibration and hot air flow, the boron carbide wet filter cake is broken up and fluidized, "boiling" and being conveyed forward on the bed surface. This fluidized state greatly increases the contact area and relative motion between the particles and the hot air, resulting in extremely high heat and mass transfer efficiency, and the moisture is rapidly and gently vaporized and carried away. The residence time of the material in the dryer is controlled by adjusting the vibration intensity and the airflow of the induced draft fan, and is usually about 30 minutes. The moisture content of the dried boron carbide powder discharged from the end of the dryer is consistently below 0.1%, as measured by an online infrared moisture analyzer.

[0037] The dried powder is directly fed into a finished product screening unit equipped with an electromagnetic vibrating screen via a fully enclosed negative pressure pneumatic conveying system. The core of this screening unit is a circular vibrating screen fitted with a 200-mesh (74-micron) standard sieve. Its purpose is to remove any small amounts of hard, large particle aggregates or any foreign impurities that may be introduced during processing and drying, ensuring that the final product particle size meets specifications and preventing scratches during subsequent semiconductor wafer fabrication.

[0038] Finally, the sieved boron carbide powder undergoes rigorous quality inspection. Samples from each batch are taken and analyzed using a particle size analyzer based on laser diffraction (e.g., Malvern Mastersizer 3000) according to national standard GB / T 19077.1-2016, comprehensively examining key particle size distribution parameters such as D10, D50, and D90. Simultaneously, samples are sent to a laboratory for X-ray diffraction (XRD, e.g., Bruker D8 Advance) to analyze phase purity, confirming boron carbide as the main crystalline phase and quantitatively analyzing the content of impurity phases such as silicon carbide. Furthermore, inductively coupled plasma mass spectrometry (ICP-MS, e.g., Agilent 7900) is used for final confirmation of the content of trace metal impurity elements (especially Fe, Al, Ca, Mg, etc.). Only when the recycled boron carbide abrasive meets or exceeds the original purchase specifications in terms of key indicators such as purity, particle size distribution, and impurity content can it be judged as a qualified product, packaged and stored, and directly returned to the double-sided grinding process of silicon carbide wafers for reuse, thereby achieving a closed-loop cycle of high-value materials and a significant reduction in production costs.

[0039] Example 1

[0040] This embodiment aims to specifically demonstrate the practical application effect of the method of the present invention. Ten cubic meters of SIC double-sided milling waste liquid were taken from a semiconductor material processing plant. Upon testing, its initial state parameters were as follows: solid content 4.2% (mass fraction), total organic carbon (TOC) 4250 mg / L, the main organic compound being polyethylene glycol (PEG-400), total iron content (on a dry basis) 870 ppm, and the original average particle size D50 of the solid particles in the waste liquid was 9.5 micrometers. The mass ratio of boron carbide to silicon carbide was approximately 1:1.2.

[0041] (1) Synergistic oxidative degelatination and solid-liquid concentration: The above-mentioned waste liquid was pumped into the synergistic oxidative degelatination reaction system described in the embodiment at a flow rate of 6 L / min. The injection ratio of hydrogen peroxide (30%) was set to 0.4% of the waste liquid volume flow rate. The total power of the microwave system was set to 4.5 kW, and the reactor outlet temperature was stabilized at 90±2 degrees Celsius. The residence time of the waste liquid in the reactor was 12.5 minutes. The treated liquid entered the flocculation sedimentation unit, the pH was adjusted to 8.5 with NaOH solution, and APAM with a concentration of 2 mg / L was added. After standing sedimentation for 2 hours, the supernatant was discharged, and about 1.8 cubic meters of concentrated slurry with a solid content of 23.5% was obtained. Sampling and testing showed that the TOC content of the supernatant was reduced to 35 mg / L, and the TOC removal rate reached 99.2%.

[0042] (2) High gradient magnetic separation: The concentrated slurry was pumped into a high gradient magnetic separator with a background magnetic field strength of 1.5 Tesla at a flow rate of 1.5 cubic meters per hour. After one complete working cycle (approximately 45 minutes of feeding and 3 minutes of backwashing), the iron-removed slurry was collected. Sampling analysis showed that the total iron content in the slurry (on a dry basis of solids) was reduced to 18 ppm.

[0043] (3) Precision acid hydrolysis purification: The iron-removed slurry was pumped into the reactor, heated to 50 degrees Celsius, and 2 mol / L hydrochloric acid was added dropwise to adjust the pH to 1.5. The reaction was carried out under these conditions for 60 minutes. After the reaction, the slurry was washed five times with ultrapure water until neutral. The final slurry sample was analyzed by ICP-MS, and the total iron content (on a dry basis of solids) was reduced to 0.8 ppm.

[0044] (44) Multi-stage cyclone separation: Adjust the solid content of the purified slurry to 10%, pump it into the three-stage countercurrent cyclone separation system described in the embodiment at a pressure of 0.3 MPa, and turn on the online ultrasonic depolymerization device. After stable operation, collect the boron carbide product suspension from the overflow port of the second-stage hydrocyclone (C2).

[0045] (5) Dehydration, drying, and testing: The collected boron carbide suspension was centrifuged for dehydration, dried in a vibrating fluidized bed (90°C), and sieved through a 200-mesh sieve to obtain the final boron carbide powder product. A comprehensive test was performed on the product, and the results are as follows: the boron carbide phase purity (XRD quantitative analysis) was 98.8%, the silicon carbide content was 0.9%, and the content of other impurities was 0.3%. The total iron content (ICP-MS) was 0.9 ppm. Particle size distribution: D10 = 4.5 μm, D50 = 8.8 μm, D90 = 15.2 μm. The total recovery rate of boron carbide (relative to the total amount entering the system) was calculated to be 95.6%.

[0046] Comparative Example 1

[0047] This comparative example aims to illustrate the necessity of the key technical steps of the present invention. The same 10 cubic meters of initial waste liquid as in Example 1 was used. The treatment method employed a simplified conventional process, skipping the synergistic oxidation and gel breaking pretreatment step.

[0048] (1) Direct flocculation and sedimentation: NaOH solution was directly added to the original waste liquid to adjust the pH to 8.5, and then APAM with a concentration of 5 mg / L was added (the dosage needs to be increased due to interference from organic matter). Under stirring, the flocs formed slowly and were small, resulting in poor sedimentation. After standing for 4 hours, the solid content of the lower layer of concentrated slurry was only 10.5%, and the supernatant was still turbid, with a TOC content as high as 3100 mg / L.

[0049] (2) High-gradient magnetic separation: The slurry with low concentration and high organic content was pumped into the same magnetic separator. Due to the PEG coating on the particle surface, its magnetic responsiveness was weakened, and the slurry viscosity was high, which seriously affected the separation efficiency. After treatment, the total iron content in the slurry was still as high as 320 ppm.

[0050] (3) Acid hydrolysis purification: The slurry after magnetic separation is acid hydrolyzed. Due to the presence of a large amount of organic matter, a large amount of hydrochloric acid is consumed, and the reaction is incomplete. After washing, the total iron content is still 95 ppm.

[0051] (4) Cyclone Classification: The acid-hydrolyzed slurry was classified by cyclone classification. Due to the poor surface cleanliness of the particles and the possibility of repolymerization of some organic matter under acidic conditions, a large amount of agglomeration occurred between the particles, which could not be effectively dispersed by ultrasound. The classification effect was extremely poor. The product obtained from the C2 overflow port was tested and found to have a boron carbide purity of only 75.3%, containing as much as 22.5% silicon carbide. The total recovery rate of boron carbide was less than 60%.

[0052] Results Comparison

[0053] The table below clearly compares the key performance indicators of Example 1 and Comparative Example 1:

[0054] performance indicators Example 1 (using the method of the present invention) Comparative Example 1 (Traditional Simplification Method) Final boron carbide product purity (%) 98.8 75.3 Total boron carbide recovery rate (%) 95.6 < 60 Total iron content of the final product (ppm) 0.9 95 Final product SiC content (%) 0.9 22.5 TOC removal rate (%) 99.2 Ineffective processing Process stability and controllability High efficiency, with each parameter precisely controllable. Low, severely affected by organic matter

[0055] Through the detailed description and data comparison of the above embodiments and comparative examples, it is clear that the method disclosed in this invention, especially its original synergistic oxidation and gel breaking pretreatment step, is the key to successfully achieving high-purity and high-recovery-rate recovery of boron carbide abrasives. This method fundamentally solves the technical bottleneck of existing technologies when dealing with stable colloidal waste liquids. Each subsequent step is designed with high specificity and synergy, together forming a complete, efficient, and industrially feasible technical solution.

Claims

1. A method for recovering boron carbide abrasive from SiC double-sided mill grinding waste liquid, characterized in that, Includes the following steps: The grinding waste liquid is subjected to synergistic oxidation degelatination and solid-liquid concentration treatment. The synergistic oxidation degelatination treatment is carried out through a microwave-enhanced heterogeneous Fenton reactor to generate hydroxyl radicals in situ and use microwave energy to accelerate the degradation of organic suspending agents, thereby destroying the colloidal stability of the waste liquid and obtaining concentrated slurry. The concentrated slurry is subjected to high-gradient magnetic separation to remove magnetic impurity particles, thereby obtaining an iron-free slurry. The iron removal slurry is subjected to precision acid hydrolysis purification treatment to remove residual non-magnetic or weakly magnetic iron impurities in the slurry by chemical dissolution, thereby obtaining purified slurry. The purified slurry is subjected to multi-stage cyclone classification and synergistic separation treatment. The steps include ultrasonic depolymerization of the purified slurry, followed by multi-stage cyclone classification treatment using a three-stage countercurrent connection method. The density difference between boron carbide and silicon carbide is utilized to separate and obtain a suspension rich in boron carbide.

2. The method according to claim 1, characterized in that, The steps of performing synergistic oxidative degelatinization and solid-liquid concentration treatment specifically include: The grinding waste liquid is pumped into a synergistic oxidation debonding reaction system (10), which is equipped with a microwave-enhanced heterogeneous Fenton reactor. Before the waste liquid enters the reactor, hydrogen peroxide solution is injected into the waste liquid and mixed with it. Waste liquid containing hydrogen peroxide is passed through a fixed bed structure (14) in the reactor containing iron oxide nanoparticles loaded as heterogeneous Fenton reaction catalysts to generate hydroxyl radicals in situ. Simultaneously, the magnetron array (13) coupled to the reactor is activated to accelerate the generation of hydroxyl radicals and the degradation of organic suspending agents through microwave energy, thereby destroying the colloidal structure of the waste liquid and obtaining a liquid-solid mixture; The reactor body is a quartz glass tube (12) spirally wound inside a multimode microwave resonant cavity (11).

3. The method according to claim 2, characterized in that, The steps of performing synergistic oxidative degelatinization and solid-liquid concentration treatment also include: The liquid-solid mixture flowing out of the synergistic oxidative depolymerization reaction system (10) is fed into a pH adjustment and flocculation sedimentation unit (20). In the pH adjustment and flocculation sedimentation unit (20), sodium hydroxide solution is added dropwise to the liquid-solid mixture to adjust the pH value of the system to 8.5 so that the iron ions remaining in the waste liquid and dissolved during the reaction are converted into iron hydroxide precipitate. Subsequently, anionic polyacrylamide was added to the unit as a flocculant. Under the stirring of a low-speed paddle mixer (22), the ferric hydroxide precipitate and the flocculant bridged and swept the solid impurities in the waste liquid, forming flocs and settling. After standing in the settling tank (21) of the flocculation settling unit (20), the upper clear liquid is discharged, and the lower concentrated solid material is discharged from the spiral discharge valve (23) at the bottom of the settling tank (21) to form the concentrated slurry.

4. The method according to claim 1, characterized in that, The steps for performing high-gradient magnetic separation to remove iron specifically include: The concentrated slurry is pumped into the separation chamber of a vertical annular high gradient magnetic separator. The separation chamber is filled with a magnetic medium mesh made of ferritic stainless steel wire, and a magnetic field with a background magnetic field strength of 1.5 Tesla is generated by an electromagnetic system. The concentrated slurry is passed through the magnetic medium mesh in a magnetized state. The high magnetic field gradient generated on the surface of the steel wire captures and adsorbs iron-based impurity particles in the slurry, while non-magnetic boron carbide and silicon carbide particles pass through the magnetic medium mesh and are discharged, forming the iron removal slurry. When the iron concentration in the iron removal slurry is detected to be rising, the feeding is stopped and the background magnetic field is eliminated. Then, the high-pressure backwashing system is started to backwash the magnetic medium mesh with deionized water to wash away and collect the adsorbed iron impurities.

5. The method according to claim 1, characterized in that, The steps for performing the precision acid hydrolysis purification process specifically include: The iron removal slurry is pumped into a jacketed glass reactor lined with Hastelloy alloy, and the slurry temperature is controlled at 50 degrees Celsius. Start the stirrer and slowly add a 2 mol / L hydrochloric acid solution to the vessel. At the same time, use a dual-parameter online monitoring system for pH and redox potential to control the droplet process in a closed loop, so that the pH value of the slurry decreases steadily within 30 minutes and eventually stabilizes at 1.

5. The reaction was stirred continuously for 60 minutes at a pH of 1.5 and a temperature of 50 degrees Celsius to convert all residual iron compounds into soluble ferric chloride.

6. The method according to claim 5, characterized in that, The step of performing the precision acid hydrolysis purification process also includes: After the reaction is completed, ultrapure water is injected into the reaction vessel to dilute and wash the slurry; Washing was performed using a decantation method, which included repeatedly injecting water, allowing the mixture to settle, and using a vacuum pump to extract the acidic supernatant containing ferric chloride and residual hydrochloric acid. The process of water injection-sedimentation-extraction was repeated five times until the pH value of the slurry in the reactor rose to above 6.5, and no ferric ions were detected in the supernatant using potassium thiocyanate reagent, thus obtaining the purified slurry.

7. The method according to claim 1, characterized in that, The steps for performing multi-stage cyclone classification and synergistic separation processing specifically include: After adjusting the solid content of the purified slurry to 10%, it is first passed through an annular online ultrasonic depolymerization device (31) before entering a hydrocyclone separation system (30). The cavitation effect generated by the ultrasonic deagglomeration device (31) is used to break up any particle agglomerates that may exist in the slurry due to van der Waals forces or electrostatic attraction, ensuring that each particle enters the hydrocyclone separation system (30) in the form of a single particle, providing a physical basis for high-precision density classification.

8. The method according to claim 7, characterized in that, The hydrocyclone separation system (30) is constructed using a three-stage countercurrent connection, including a first-stage hydrocyclone (32), a second-stage hydrocyclone (33), and a third-stage hydrocyclone (34). Its separation process is as follows: The purified slurry after ultrasonic depolymerization is used as the feed for the first-stage hydrocyclone (32), its overflow is used as the feed for the second-stage hydrocyclone (33), and its underflow is used as the feed for the third-stage hydrocyclone (34). The overflow of the second-stage hydrocyclone (33) flows out as the final boron carbide-rich suspension product, and its underflow returns to the feed line of the first-stage hydrocyclone (32); The underflow of the third-stage hydrocyclone (34) flows out as a silicon carbide by-product, and its overflow returns to the feed line of the first-stage hydrocyclone (32). To achieve countercurrent washing, fresh replenishment water is mixed with the underflow from the first-stage hydrocyclone (32) before entering the third-stage hydrocyclone (34) as the feed for the third-stage hydrocyclone (34).

9. The method according to claim 1, characterized in that, The method further includes, after the multi-stage cyclone classification and synergistic separation treatment, a step of dehydration and drying of the obtained boron carbide-rich suspension, specifically: The boron carbide-rich suspension was fed into a horizontal screw discharge sedimentation centrifuge for preliminary dehydration to obtain a wet filter cake with a water content of less than 20%. The wet filter cake is fed into a vibrating fluidized bed dryer for final drying. Under the action of hot air at 90 degrees Celsius, the boron carbide filter cake particles are fluidized, and finally dry boron carbide powder with a moisture content of less than 0.1% is obtained.

10. The method according to claim 9, characterized in that, The method further includes a step of performing a finished product inspection on the obtained dried boron carbide powder after the drying process, specifically: The dried boron carbide powder is fed into a sieving unit equipped with a 200-mesh standard sieve through a fully enclosed negative pressure pneumatic conveying system to remove large particle aggregates. The sieved boron carbide powder was sampled, and its particle size distribution was detected using a particle size analyzer based on the principle of laser diffraction. The phase purity and impurity element content were confirmed using X-ray diffraction and inductively coupled plasma mass spectrometry to ensure that the recovered boron carbide abrasive meets the specifications for reuse.

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