Method for reducing hexavalent chromium in chromium-containing solid waste by using coal direct liquefaction residue

Abstract

The application discloses a method for reducing hexavalent chromium in chromium-containing solid waste by using coal direct liquefaction residue, and belongs to the technical field of solid waste resource utilization and heavy metal pollution control. The system adopted by the method comprises a feeding system, a pyrolysis system, a heating system, an atmosphere control system, a gas circulation and purification system, a gas purging and strengthening system and a product collecting system. Three pyrolysis modes, i.e. in-situ co-pyrolysis mode, non-in-situ pyrolysis mode and in-situ-non-in-situ coupled pyrolysis mode, can be realized by adjusting the feeding system. The coal direct liquefaction residue and the chromium-containing solid waste are co-pyrolyzed at low temperature, so that the efficient reduction and fixation of Cr(VI) and the resource utilization of the residue can be realized simultaneously.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and heavy metal pollution control technology, specifically relating to a method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue. Background Technology

[0002] Chromium-containing solid waste (chromium slag) is a highly hazardous solid waste generated during the production of chromium salts and the smelting of metallic chromium. Its core toxicity stems from highly mobile hexavalent chromium (Cr(VI)) compounds. Cr(VI) is internationally recognized as a potent carcinogen and can migrate into ecosystems through water, soil, and the atmosphere, ultimately accumulating in the human body through the food chain. Existing research confirms that Cr(VI) has a strong oxidative damage effect on living cells, with a toxicity exceeding that of trivalent chromium (Cr(III)) by more than 100 times. Even long-term exposure to low concentrations can induce respiratory cancers, liver and kidney damage, and genetic mutations. China's chromium salt industry sees numerous new additions each year. When these chromium slags are left in the open, the Cr(VI) they contain is easily leached by rainwater, forming chromium-containing leachate that causes severely excessive chromium concentrations in surrounding groundwater. With the National Hazardous Waste List explicitly classifying waste chromium slag as HW21 hazardous waste, and the Chromium Salt Industry Pollutant Emission Standard imposing strict limits on the leaching concentration of Cr(VI) in waste chromium slag (<5 mg / kg), the development of efficient and low-cost waste chromium slag detoxification technologies has become an urgent need for environmental protection and sustainable resource utilization.

[0003] Currently, wet chemical reduction is the mainstream technology for chromium slag treatment. It converts Cr(VI) into less toxic Cr(III) in the liquid phase using reducing agents such as sulfites and ferrous sulfate. However, its dependence on commercial reducing agents leads to high treatment costs. In addition, high-temperature pyrometallurgical and biological methods are also used to varying degrees for Cr(VI) reduction. While high-temperature pyrometallurgical methods (1400~1850℃) can achieve complete detoxification, they are limited by high energy consumption (200~400 kg standard coal / ton of slag). Biological reduction relies on the metabolic reduction of chromium by specific microbial communities and is only suitable for low-chromium concentration systems. Coal direct liquefaction residue, a solid byproduct of coal hydroliquefaction under high temperature and pressure, has an annual production of millions of tons. If the characteristics of coal direct liquefaction residue can be utilized for the treatment of waste chromium slag, it can avoid secondary pollution from hexavalent chromium and provide a potential industrial-scale technological path that aligns with the concept of economic circularity for solving the problem of waste chromium slag treatment. Summary of the Invention

[0004] The purpose of this invention is to provide a method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue. This method involves co-pyrolyzing the coal direct liquefaction residue and the chromium-containing solid waste at low temperature, which can simultaneously achieve efficient reduction and fixation of Cr(VI) and resource utilization of the residue.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue, the system of which includes a feeding system, a pyrolysis system, a heating system, an atmosphere control system, a gas circulation and purification system, a gas purging and enhancement system, and a product collection system; The feeding system includes a feeding device 1 and a feeding device 2 arranged in parallel. A feeding device 3 is provided between the feeding device 1 and the feeding device 2 to keep the feeding device 1 and the feeding device 2 connected or disconnected. The feeding device 1 is used for the entry of chromium-containing solid waste, and the feeding device 2 is used for the entry of coal direct liquefaction residue. The feeding device 1 is also provided with a blocking device, which is used to sequentially pass the chromium-containing solid waste into the feeding device 3 and the feeding device 2. Both the first and second feeding devices are connected to the pyrolysis system. The pyrolysis system includes an inner cylinder arranged coaxially and an outer cylinder sleeved outside the inner cylinder. The inside of the inner cylinder is a waste chromium slag chamber, and the space between the inner and outer cylinders is a dual-material mixing chamber. An inner spiral blade is provided on the inner wall of the inner cylinder, and an outer spiral blade is provided on the outer wall of the inner cylinder. Several permeable micropores are provided on the inner cylinder wall. A reducing gas exhaust device is provided in the axial direction of the inner cylinder. The reducing gas exhaust device includes a hollow rod body with exhaust holes in the circumferential direction. A rotary joint is sleeved on one end of the rod body. The rotary joint is installed at the reducing non-condensable gas inlet of the pyrolysis system. The reducing non-condensable gas inlet is connected to the reducing non-condensable gas outlet of the gas circulation purification system; The method includes in-situ co-pyrolysis mode, non-in-situ pyrolysis mode, and in-situ-non-in-situ coupled pyrolysis mode; Specifically, the following steps are included: a. In-situ co-pyrolysis mode: The chromium-containing solid waste entering the first feeding device is sequentially fed into the third feeding device and the second feeding device through the action of the blocking device. In the second feeding device, the chromium-containing solid waste and the coal direct liquefaction residue are uniformly mixed and enter the dual-material mixing chamber of the pyrolysis system. The heating system is set at a temperature of 400~700℃. The reducing products generated in-situ by the pyrolysis of the coal direct liquefaction residue directly contact the hexavalent chromium-containing solid waste, reducing the hexavalent chromium to trivalent chromium. The non-condensable gas generated is purified by the gas circulation purification system, and the product is collected by the product collection system. b. Non-in-situ pyrolysis mode: Feeding devices one and two are kept separate via feeding device three. Chromium-containing solid waste enters the waste chromium slag chamber of the pyrolysis system through feeding device one, while coal direct liquefaction residue enters the dual-material mixing chamber of the pyrolysis system through feeding device two. The heating system is set at a temperature of 400~700℃. The coal direct liquefaction residue is pyrolyzed separately in the dual-material mixing chamber. The resulting volatile components rich in reducing agents diffuse into the waste chromium slag chamber through permeable micropores, contacting the chromium-containing solid waste and reducing the hexavalent chromium within it. The non-condensable gas produced by pyrolysis is purified by a gas circulation purification system, and the remaining reducing gas is circulated back into the waste chromium slag chamber through a reduction exhaust device to further enhance reduction. The product is collected through a product collection system. c. In-situ-Non-in-situ Coupled Pyrolysis Mode: Feeding device 1 and feeding device 2 are kept connected through feeding device 3. Part of the chromium-containing solid waste enters the waste chromium slag chamber of the pyrolysis system directly through feeding device 1, while the other part is mixed with coal direct liquefaction residue through feeding device 3 and feeding device 2 and enters the dual-material mixing chamber of the pyrolysis system. The mixture in the dual-material mixing chamber undergoes in-situ co-pyrolysis to reduce some of the hexavalent chromium in the waste chromium slag. In the waste chromium slag chamber, a non-in-situ pyrolysis mode is performed to reduce some of the hexavalent chromium in the waste chromium slag.

[0006] The above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue includes feeding device one, feeding device three, feeding device two, and blocking device, all equipped with screws for material conveying; a blocking plate is provided at the end of the screw of the blocking device to prevent material in feeding device one from directly entering the pyrolysis system.

[0007] The above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue has an outer spiral blade pitch of 50-200 mm, used to transport coal direct liquefaction residue or a mixture of coal direct liquefaction residue and chromium-containing waste into a dual-material mixing chamber; and an inner spiral blade pitch of 30-150 mm, used to transport chromium-containing solid waste into a waste chromium slag chamber.

[0008] The above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue includes an external heating system comprising an electromagnetic induction coil, a resistance heater, or a gas nozzle heater.

[0009] The above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue includes an atmosphere control system used to maintain an inert atmosphere within the pyrolysis system. The inert gas is nitrogen, argon, or helium, or a vacuum micro-negative pressure is used to maintain the inert environment.

[0010] The above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue includes a gas circulation purification system comprising a non-condensable gas inlet, a dehumidification device, a dust removal device, a non-condensable gas grading-purification device, and a reducing non-condensable gas outlet. The non-condensable gas grading-purification device is used to separate the reducing gas and the tail gas to be treated.

[0011] The above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue includes a gas purging enhancement system comprising a reduction exhaust device. The reduction exhaust device can dynamically rotate under the action of a rotary joint, and the blown airflow is used to disturb the material in the waste chromium slag chamber and increase its contact area with the reducing gas.

[0012] The above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue has a permeable micropore diameter of 0.001~20mm.

[0013] In the above-mentioned method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue, in step a, the reducing products generated by pyrolysis mainly include pyrolytic carbon, olefins, alkanes, aromatic hydrocarbons, hydrogen, and carbon monoxide; the non-condensable gases generated mainly include hydrogen, carbon monoxide, and low-carbon hydrocarbon reducing non-condensable gases.

[0014] Compared with the prior art, the present invention brings the following beneficial technical effects: (1) Synergistic effect of low cost and high efficiency in detoxification and stabilization. This invention innovatively utilizes widely available and low-cost coal direct liquefaction residue as a reducing agent and stabilizing matrix raw material. Under relatively low coal direct liquefaction residue dosage (wide mixing ratio 1:0~100) and pyrolysis temperature (300~1200℃), efficient reduction of hexavalent chromium and stabilization and fixation of trivalent chromium in hexavalent chromium-containing solid waste can be achieved. This method avoids dependence on purchased industrial reducing agents and significantly reduces the overall cost of chromium-containing solid waste treatment (including material costs and energy consumption costs). The detoxified and stabilized products (chromium-containing residue and coal direct liquefaction residue) have good resource utilization potential and can be directly applied to the production of basic building materials.

[0015] (2) Synergistic Resource Utilization Value Enhancement and Process Optimization. During pyrolysis, the metal oxide components (such as Fe2O3, CaO, etc.) contained in hexavalent chromium solid waste exhibit catalytic activity on the pyrolysis behavior of coal direct liquefaction residue, effectively increasing the yield of its liquid phase products and significantly promoting the lightening of products, thus significantly enhancing the high-value utilization prospects of the liquid phase products from the pyrolysis of coal direct liquefaction residue. Simultaneously, the reducing non-condensable gases generated during pyrolysis can be recycled after deep purification, improving the utilization efficiency of the reducing agent and helping to enhance the reduction effect of hexavalent chromium. This method has a clear process route and reliable equipment, providing an effective technical approach for the harmless and resource-based treatment of chromium-containing solid waste. Attached Figure Description

[0016] The present invention will be further described below with reference to the accompanying drawings: Figure 1 This is a schematic diagram of a process for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to the present invention. Figure 2 A simplified structural diagram of the hexavalent chromium reduction apparatus of the present invention is shown; Figure 3 A simplified structural diagram of the feeding system of the present invention is shown; Figure 4 A structural diagram of the pyrolysis reactor of the present invention is shown; Figure 5 A diagram of the gas circulation and purification system of the present invention is shown; Figure 6 A structural diagram of the reducing gas exhaust device of the present invention is shown; Figure 7 The TG-DTG curve of the coal direct liquefaction residue pyrolysis of the present invention is shown; Figure 8 (a) shows the FTIR diagram of the remaining solid products after pyrolysis of the coal direct liquefaction residue of the present invention, and (b) shows the FTIR diagram of the remaining solid products after pyrolysis of the coal direct liquefaction residue / waste chromium slag. Figure 9 Graph showing the variation of carbon chain length in pyrolysis oil.

[0017] In the diagram: 1. Feeding device one, 2. Feeding device two, 3. Feeding device three, 4. Blocking device, 5. Screw, 6. Inner spiral blade, 7. Outer spiral blade, 8. Reducing non-condensable gas inlet, 9. Feed port one, 10. Feed port two, 11. Waste chromium slag chamber, 12. Dual material mixing chamber, 13. Non-condensable gas outlet, 14. Gas purging enhancement system, 15. Dehumidification device, 16. Dust removal device, 17. Reducing non-condensable gas outlet, 18. Non-condensable gas grading-purification device, 19. Rotary joint, 20. Reducing gas exhaust device, 21. Exhaust port, 22. Pyrolysis oil collection port, 23. Discharge port one, 24. Discharge port two. Detailed Implementation

[0018] This invention proposes a method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue. To make the advantages and technical solutions of this invention clearer and more explicit, the invention will be further described below with reference to specific embodiments.

[0019] It is understood that the connection relationships described in this application refer to direct or indirect connections. For example, the connection between A and B can be a direct connection between A and B, or an indirect connection between A and B through one or more other electrical components. For example, A can be directly connected to C, and C can be directly connected to B, thus achieving a connection between A and B through C. It is also understood that the "A connects to B" described in this application can be a direct connection between A and B, or an indirect connection between A and B through one or more other electrical components.

[0020] In the description of this application, unless otherwise stated, " / " means "or". For example, A / B can mean A or B. The "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A exists alone, A and B exist simultaneously, and B exists alone.

[0021] In the description of this application, the words "first," "second," etc., are used only to distinguish different objects and do not limit the quantity or order of execution, nor do they imply that they must be different. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0022] This invention aims to overcome the shortcomings of existing technologies for treating chromium-containing solid waste (especially highly toxic hexavalent chromium (Cr(VI))), such as high cost, low reduction efficiency, risk of secondary pollution, and insufficient added value from resource recovery. Specifically, existing wet chemical reduction relies on expensive commercial reducing agents; high-temperature pyrolysis can completely detoxify the waste, but its energy consumption is extremely high (200-400 kg standard coal / ton of slag); biological methods are only applicable to low-concentration systems and their efficiency is limited.

[0023] The main technical concept of this invention is that the core of the method lies in the synergistic use of various reducing substances produced by the pyrolysis of coal direct liquefaction residue, and by optimizing the pyrolysis process mode, achieving efficient valence state conversion and stabilization of hexavalent chromium at a lower temperature.

[0024] Coal direct liquefaction residue, a solid byproduct of the high-temperature, high-pressure hydrogenation liquefaction process, has an annual production of millions of tons. Its rich content of various reducing components exhibits a strong reducing ability for Cr(VI), primarily through the following mechanisms: (1) Volatile matter releases reducing gases: Organic matter in the residue of direct coal liquefaction (such as incompletely converted coal tar, asphaltene, semi-coke precursors, etc.) releases a large amount of reducing gases during pyrolysis, such as hydrogen (H2), carbon monoxide (CO), methane (CH4) and low molecular weight hydrocarbons. These gases, as effective electron donors, can directly react with Cr(VI) at suitable temperatures to undergo redox reactions (Cr(VI) + H2 → Cr(III) + H2O, Cr(VI) + CO → Cr(III) + CO2).

[0025] (2) Reduction of solid carbonaceous matter: The residue of direct coal liquefaction contains abundant fixed carbon components (such as semi-coke and coke), which are strong reducing agents. Under high temperature conditions, carbon can react directly with Cr(VI) oxides (CrO3, etc.) to reduce Cr(VI) to Cr(III) (3C+4CrO3→2Cr2O3+3CO2↑).

[0026] (3) Electron transfer of active free radicals: The highly active organic free radicals (such as alkyl free radicals and aromatic free radicals) generated during the pyrolysis of organic matter have strong reducing properties and can reduce Cr(VI) through electron transfer.

[0027] (4) Ash solid solution and physical barrier: The metal oxides (Fe2O3, Al2O3 and CaO, etc.) contained in the ash of direct coal liquefaction residue can participate in mineral phase reconstruction (such as forming calcium aluminoferrite minerals) at 600-900℃, and stably dissolve the generated Cr(III) in the mineral lattice. At the same time, the porous matrix formed by carbonization of the residue can physically block the re-oxidation of chromium.

[0028] (5) Contribution of sulfur-containing components to reduction: FeS2 contained in the coal direct liquefaction residue and the active sulfur species (hydrogen sulfide and elemental sulfur, etc.) produced by its thermal decomposition can also reduce Cr(VI) to Cr(III) through electron transfer, while being oxidized to sulfur dioxide or sulfate (3FeS2+8CrO3→4Cr2O3+Fe3O4+6SO2↑, 3H2S+2CrO4). 2- +10H + →2Cr 3+ +3S↓+8H2O, 3S+4CrO3→2Cr2O3+3SO2↑). Based on the above characteristics of coal direct liquefaction residue, low-temperature co-pyrolysis treatment with hexavalent chromium-containing solid waste can simultaneously achieve efficient reduction and fixation of Cr(VI) and resource utilization of the residue (the detoxified products can be used as building material raw materials, etc.).

[0029] Combination Figures 1 to 6 As shown, the system used in this invention includes a feeding system, a pyrolysis system, a heating system, an atmosphere control system, a gas circulation and purification system, a gas purging and enhancement system, and a product collection system.

[0030] like Figure 3 As shown, the feeding system includes two parallel feeding devices: Feeding Device 1 and Feeding Device 2. A Feeding Device 3 is positioned between Feeding Device 1 and Feeding Device 2 to maintain / disconnect them. Essentially, Feeding Device 1, Feeding Device 3, and Feeding Device 2 are arranged in an I-shape. Specifically, Feeding Device 1 includes a screw barrel and a screw 5 located inside the screw barrel. Under the action of the screw 5, chromium-containing solid waste is transported to the feed inlet 9 of the pyrolysis system. Similarly, Feeding Device 2 has a similar structure to Feeding Device 1, also including a screw barrel and a screw. Under the action of the screw, coal direct liquefaction residue is transported to the feed inlet 10 of the pyrolysis system.

[0031] To achieve the three operating modes of this invention, the first feeding device is also equipped with a blocking device. This blocking device prevents chromium-containing solid waste from directly entering the first feeding port, instead guiding it sequentially into the third feeding device and the second feeding device. Specifically, the structure of the blocking device matches the screw barrel of the first feeding device. After removing the screw from the screw barrel, the blocking device can be quickly installed to prevent the raw material from directly entering the first feeding port.

[0032] Combination Figure 2 and Figure 4 As shown, the pyrolysis system includes a pyrolysis reactor, which comprises an inner cylinder coaxially arranged and an outer cylinder sleeved outside the inner cylinder. The inner cylinder contains a waste chromium slag chamber 11, and a dual-material mixing chamber 12 is located between the inner and outer cylinders. An inner helical blade 6 with a preferred pitch of 30-150 mm is provided on the inner wall of the inner cylinder, and an outer helical blade 7 with a preferred pitch of 50-200 mm is provided on the outer wall of the inner cylinder. Several permeable micropores with a diameter of 0.001-20 mm are provided on the inner cylinder wall to allow the transfer of volatile components generated during pyrolysis between the chambers (dual-material mixing chamber → waste chromium slag chamber). The inner helical blade 6 and the outer helical blade 7 typically rotate synchronously with adjustable speed.

[0033] A reducing gas exhaust device 20 is provided in the axial direction of the inner cylinder. The reducing gas exhaust device includes a hollow rod body with an exhaust hole 21 in the circumferential direction. A rotary joint 19 is sleeved on one end of the rod body. The rotary joint is installed in the reducing non-condensable gas inlet 8 of the pyrolysis system. The reducing non-condensable gas inlet is connected to the reducing non-condensable gas outlet 17 of the gas circulation purification system.

[0034] The heating system utilizes an external heat source, such as an electromagnetic induction coil, a resistance heater, or a gas nozzle surrounding the reactor structure, to provide a controllable pyrolysis temperature in the range of 300 to 1200°C.

[0035] An atmosphere control system is equipped with an inert gas (such as nitrogen, argon, or helium) injection port or a vacuum pump port to establish and maintain an inert atmosphere within the reactor, or to maintain an inert environment using a vacuum micro-negative pressure.

[0036] like Figure 5 As shown, the gas circulation purification system includes: a non-condensable gas outlet 13, a dehumidifier 15, a dust removal device 16, a non-condensable gas grading and purification device 18, and a reducing non-condensable gas outlet 17. The non-condensable gas grading and purification device is used to separate reducing gases (hydrogen, carbon monoxide, and low-carbon hydrocarbons, etc.) and tail gas to be treated. The reducing non-condensable gas outlet is connected to the reducing non-condensable gas inlet, and the reducing non-condensable gas inlet is connected back to the pyrolysis reactor through a rotary joint.

[0037] The gas purging enhancement system 14 includes a reducing gas exhaust device 20, which can rotate dynamically when reducing gas is introduced. The blown airflow can disturb the material in the waste chromium slag chamber and increase its contact area with the reducing gas.

[0038] Product collection system: The system includes pyrolysis oil collection port 22, discharge port one 23, and discharge port two 24.

[0039] The pyrolysis oil collection port collects liquid phase products, discharge port 1 discharges residue from the dual-material mixing chamber, discharge port 2 discharges residue from the waste chromium slag chamber, and the exhaust gas outlet discharges purified non-reducing exhaust gas.

[0040] Using the above system, the present invention can realize three pyrolysis modes, namely in-situ co-pyrolysis mode, non-in-situ pyrolysis mode, and in-situ-non-in-situ coupled pyrolysis mode.

[0041] Specifically, the following steps are included: (1) In-situ co-pyrolysis mode: The chromium-containing solid waste entering the first feeding device is sequentially fed into the third feeding device and the second feeding device through the action of the blocking device. In the second feeding device, the chromium-containing solid waste and the coal direct liquefaction residue are uniformly mixed and enter the dual material mixing chamber of the pyrolysis system. The heating system is set at a temperature of 400~700℃. The reducing products generated in-situ by the pyrolysis of the coal direct liquefaction residue directly contact the hexavalent chromium-containing solid waste, reducing the hexavalent chromium to trivalent chromium. The non-condensable gas generated is purified by the gas circulation purification system, and the product is collected by the product collection system.

[0042] Further, the specific steps are as follows: Feeding system: The screw of feeding device one is replaced by the screw of the blocking device, and feeding devices two and three operate; Material path: Chromium-containing solid waste enters feeding device three through feeding device one, blocked by the raw material blocking device at the end of the screw. The screw of feeding device three conveys it to feeding device two, where it is uniformly mixed with the coal direct liquefaction residue from feeding device two in the downstream mixing zone of the feeding device. The mixture is then conveyed by the screw of feeding device two into the dual-material mixing chamber of the pyrolysis reactor. Pyrolysis and reduction: The mixture is heated in the dual-material mixing chamber (set temperature, such as 400~700℃). The reducing products (solid-gas, liquid-gas, and gas phase) generated in situ by the pyrolysis of the coal direct liquefaction residue directly contact the hexavalent chromium-containing solid waste, reducing hexavalent chromium to trivalent chromium. The generated non-condensable gas is purified and then discharged or partially utilized. Products: Pyrolysis oil is discharged from the pyrolysis oil collection port; residue (pyrolysis products of mixed materials) is discharged from discharge port one; waste chromium slag chamber is empty, discharge port two is closed.

[0043] (2) Non-in-situ pyrolysis mode: Feeding device 1 and feeding device 2 are kept out of contact through feeding device 3. Chromium-containing solid waste enters the waste chromium slag chamber of the pyrolysis system through feeding device 1, and coal direct liquefaction residue enters the dual-material mixing chamber of the pyrolysis system through feeding device 2. The heating system is set at a temperature of 400~700℃. Coal direct liquefaction residue is pyrolyzed separately in the dual-material mixing chamber. The volatile components rich in reducing properties are diffused into the waste chromium slag chamber through the permeable micropores, contact the chromium-containing solid waste and reduce the hexavalent chromium in it. The non-condensable gas generated by pyrolysis is purified by the gas circulation purification system. The remaining reducing gas is circulated into the waste chromium slag chamber through the reduction exhaust device to further enhance the reduction. The product is collected through the product collection system.

[0044] Further, the specific steps are as follows: Feeding System: Feeding devices one and two operate normally, while feeding device three is not in operation. Chromium-containing solid waste enters the waste chromium slag chamber of the pyrolysis system through feeding device one, while coal direct liquefaction residue enters the dual-material mixing chamber of the pyrolysis system through feeding device two. Pyrolysis and Reduction: The coal direct liquefaction residue is pyrolyzed separately in the dual-material mixing chamber. The resulting volatile components, rich in reducing substances (hydrogen, carbon monoxide, hydrocarbon free radicals, and oxygen-containing compounds, etc.), diffuse through permeable micropores into the adjacent waste chromium slag chamber, contacting and reducing the hexavalent chromium in the solid waste containing hexavalent chromium within that chamber. The non-condensable gas (containing a large amount of reducing components) generated by pyrolysis is purified, and its reducing gases (hydrogen, carbon monoxide, and low-carbon hydrocarbons, etc.) are circulated into the waste chromium slag chamber through a reducing gas exhaust device to further enhance reduction. The remaining tail gas is safely discharged after purification. Products: Pyrolysis oil is discharged from the pyrolysis oil collection port; pyrolysis residue from direct coal liquefaction is discharged from discharge port one; chromium-containing waste residue after reduction treatment is discharged from discharge port two.

[0045] (3) In-situ-non-in-situ coupled pyrolysis mode: Feeding device 1 and feeding device 2 are kept connected through feeding device 3. Part of the chromium-containing solid waste enters the waste chromium slag chamber of the pyrolysis system directly through feeding device 1, and the other part enters the dual material mixing chamber of the pyrolysis system through feeding device 3 and feeding device 2 and mixed with coal direct liquefaction residue. The mixture in the dual material mixing chamber undergoes in-situ co-pyrolysis mode to reduce some of the hexavalent chromium in the waste chromium slag; and non-in-situ pyrolysis mode is carried out in the waste chromium slag chamber to reduce some of the hexavalent chromium in the waste chromium slag.

[0046] Further, the specific steps are as follows: Feeding System: Feeding Device 1, Feeding Device 2, and Feeding Device 3 are maintained in operation; Material Path: Solid waste containing hexavalent chromium enters Feeding Device 1, and a portion of the solid waste containing hexavalent chromium enters Feeding Device 2 via Feeding Device 3, where it mixes with the coal direct liquefaction residue from Feeding Device 2. The mixture is then conveyed by a screw conveyor into the dual-material mixing chamber (in-situ portion). Another portion of the chromium-containing solid waste enters the waste chromium slag chamber (non-in-situ portion) directly via Feeding Device 1 and the screw conveyor. Pyrolysis and Reduction: The mixture in the dual-material mixing chamber undergoes in-situ co-pyrolysis to reduce the hexavalent chromium in this portion. The volatile reducing components generated in this process enter the waste chromium slag chamber through permeable micropores. The solid waste containing hexavalent chromium entering the waste chromium slag chamber receives both reducing volatiles diffused from the dual-material mixing chamber and purified reducing non-condensable gas circulated in, which work together to reduce the hexavalent chromium in the chamber. Products: Pyrolysis oil is discharged from the pyrolysis oil collection port; residue from the dual-material mixing chamber is discharged from discharge port one; residue from the waste chromium slag chamber is discharged from discharge port two.

[0047] Furthermore, the preferred range of the mass mixing ratio of coal direct liquefaction residue and hexavalent chromium-containing solid waste is determined according to the target mode and processing volume (e.g., 1:1 in the example).

[0048] Pyrolysis temperature: The reactor operating temperature is set between 300 and 1200℃. Based on the TG-DTG curve of the coal direct liquefaction residue (… Figure 7 To obtain its corresponding pyrolysis characteristics (main pyrolysis range 300~500℃, maximum weight loss about 425℃), preferably 400~700℃.

[0049] Residence time: The residence time of the material in the pyrolysis reactor is controlled between 1 and 100 min by adjusting the speed of the screw conveyor. The optimal time should ensure that pyrolysis is complete and reduction is finished (e.g., 5 to 30 min).

[0050] Inert atmosphere: Continuously introduce high-purity nitrogen (or other inert gases such as argon and helium) to maintain a slight positive pressure, or use a vacuum pump to maintain a slight negative pressure (such as -5~-1kPa) to ensure an oxygen-deficient pyrolysis environment.

[0051] Gas circulation: The circulating gas flow rate is adjusted according to the pyrolysis gas production and reduction requirements to ensure sufficient reducing atmosphere in the waste chromium slag chamber. The purging device speed is matched with the gas flow rate to optimize the disturbance effect.

[0052] The basic electron reduction reaction involved in this invention is shown below: H2→2H·→2H + +2e - R-CH3→R-CH2·+H·→H + +e - CO→CO2+2e - H2S→2H + +S 0 +2e - R-CH2-C=O→R=CH-C=O+H·→H + +e - The present invention will be further described below with reference to specific embodiments.

[0053] To verify the feasibility and effectiveness of this invention, the following experimental studies were conducted. The main components of the coal liquefaction residue and hexavalent chromium-containing solid waste used are shown in Table 1. Key testing methods included: three-phase product yield (tubular furnace pyrolysis); gas phase product composition (gas chromatography); liquid phase product composition (gas chromatography-mass spectrometry); solid phase product functional groups (Fourier transform infrared spectroscopy); and Cr(VI) leaching concentration (diphenylcarbazide spectrophotometry, GB / T 15555.4-1995).

[0054] Table 1. Main components of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0055] The presence of high levels of metallic and sulfur elements in the residue from direct coal liquefaction indicates that the catalyst FeS2 may have been used in the coal-to-oil process. The solid waste containing hexavalent chromium also contains a large amount of metallic elements, with a relative chromium content of 4.13% and a leaching concentration of 254.65 mg / kg.

[0056] Example 1: Pyrolysis characteristics and product analysis of coal liquefaction residues.

[0057] Experimental objective: To obtain basic data on coal direct liquefaction residue.

[0058] Method: The direct coal liquefaction residue was pyrolyzed separately using a tubular furnace under a nitrogen atmosphere, with the temperature increased to the set temperature (400℃, 500℃, 600℃ and 700℃) at a rate of 10℃ / min and held at that temperature for 30 min.

[0059] Results: The three-phase distribution is shown in Table 2. With increasing temperature, the gas phase yield showed an increasing trend, while the liquid phase yield first increased and then decreased, reaching its maximum at 600℃ (19.98%). The solid phase yield, on the other hand, showed the opposite trend, first decreasing and then increasing, reaching its minimum at 600℃ (78.77%).

[0060] Table 2 Distribution of three-phase products from coal direct liquefaction residue

[0061] The composition of the gaseous products is shown in Table 3. The main components of the gaseous products are low-carbon alkanes (C1-C4, >82%). As the temperature increases, the yield of low-carbon alkanes gradually decreases, while the yields of low-carbon olefins, hydrogen, and carbon dioxide show an increasing trend.

[0062] Table 3 Distribution of gaseous products from coal direct liquefaction residue

[0063] The composition of the liquid phase products is shown in Table 4. The main component of the liquid phase products is aromatic hydrocarbons (>62%). As the temperature increases, the yield of alkane gradually decreases, while the yields of olefins, aromatic hydrocarbons and oxides show an increasing trend.

[0064] Table 4 Distribution of liquid phase products from coal direct liquefaction residue

[0065] Example 2: Effect of in-situ co-pyrolysis mode

[0066] Objective: To verify the effectiveness of Mode 1.

[0067] Method: Coal direct liquefaction residue and hexavalent chromium-containing solid waste were mixed at a mass ratio of 1:1 and co-pyrolyzed in a tubular furnace under the same conditions as in Example 1 (simulating in-situ mode).

[0068] Results: The three-phase distribution is shown in Table 5. Compared with Table 2, the addition of hexavalent chromium-containing solid waste significantly increased the gas phase (5.70% vs 1.55% at 700℃) and liquid phase (26.82% vs 19.98% at 600℃), indicating that it has a catalytic effect on coal direct liquefaction residue.

[0069] Table 5. Distribution of three-phase products from in-situ co-pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0070] The composition of the gaseous products is shown in Table 6. Compared with Table 3, the yields of low-carbon alkanes and hydrogen decreased more significantly, while the yields of low-carbon olefins and carbon dioxide increased more significantly. This phenomenon indicates that hydrogen and low-carbon alkanes participate in the reduction of hexavalent chromium through an electron transfer mechanism, which leads to an increase in the yield of low-carbon olefins. The increase in the yield of carbon dioxide may be related to the release of oxygen during the reduction reaction of hexavalent chromium.

[0071] Table 6 Distribution of gaseous products from in-situ co-pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0072] The composition of the liquid-phase products is shown in Table 7. Compared with Table 4, the yields of alkanes and aromatics decreased, while the yields of olefins and oxides increased. This phenomenon suggests that the alkanes and aromatics produced during pyrolysis may participate in the reduction reaction of hexavalent chromium, or that the presence of hexavalent chromium-containing solid waste inhibited the aromatization reaction and promoted the reaction of oxygen with free radicals, thus increasing the oxide content.

[0073] Table 7 Distribution of liquid phase products from in-situ co-pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0074] The reduction rate of hexavalent chromium is shown in Table 8. During the in-situ co-pyrolysis process, the reduction rate of hexavalent chromium by the coal direct liquefaction residue is extremely high, exceeding 98.60% under the condition of a 1:1 mixing ratio.

[0075] Table 8. Reduction rate of hexavalent chromium by in-situ co-pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0076] Figure 8 The image shows the pyrolysis of coal direct liquefaction residue alone. Figure 8 a) and co-pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste ( Figure 8 b) FTIR plot. Figure 8 The results show that co-pyrolysis weakens the stretching vibration intensity of CO and C=O bonds, confirming that oxygen-containing functional groups participate in the reduction reaction of hexavalent chromium.

[0077] Example 3: Effects of the non-in-situ pyrolysis mode

[0078] Objective: To verify the effectiveness of Mode 2.

[0079] Method: Coal direct liquefaction residue was pyrolyzed in the upstream isothermal zone of a tubular furnace, and the resulting volatiles were directly fed into the downstream isothermal zone's bed of hexavalent chromium-containing solid waste (simulating an in-situ model). The material-to-material ratio was 1:1, and the temperature was the same as before.

[0080] Results: The three-phase distribution is shown in Table 9. The yields of both liquid and solid phases were lower than those of coal direct liquefaction residue pyrolysis alone (Table 2) and in-situ co-pyrolysis mode (Table 5), but the gas phase yield was higher than that in Tables 2 and 5, indicating that non-in-situ pyrolysis can promote the conversion of pyrolysis volatiles into small molecule gases.

[0081] Table 9 Distribution of three-phase products from non-in-situ pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0082] The composition of the gaseous products is shown in Table 10. Compared with Table 6, the yields of low-carbon alkanes and hydrogen decrease more significantly, while the yields of low-carbon olefins and carbon dioxide increase more significantly. This trend indicates that the catalytic effect of the coal direct liquefaction residue still exists.

[0083] Table 10 Distribution of non-in-situ pyrolysis gaseous products from direct coal liquefaction residue and hexavalent chromium-containing solid waste

[0084] The composition of the liquid phase products is shown in Table 11. Compared with Table 7, the yields of alkanes and aromatics decreased more significantly, while the yields of olefins and oxides increased further.

[0085] Table 11 Distribution of non-in-situ pyrolysis liquid phase products of coal direct liquefaction residue and waste chromium slag

[0086] The reduction rate of hexavalent chromium is shown in Table 12. During the non-in-situ pyrolysis process, the reduction rate of hexavalent chromium by the coal direct liquefaction residue is also extremely high, exceeding 98.66% under the condition of a 1:1 mixing ratio.

[0087] Table 12 Reduction rate of hexavalent chromium by in-situ co-pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0088] Example 4: Effect of in-situ-non-in-situ coupled pyrolysis mode

[0089] Objective: To verify the effectiveness of Mode 3.

[0090] Method: Two portions of hexavalent chromium-containing solid waste are simultaneously placed in a tubular furnace. One portion is mixed with coal direct liquefaction residue at a 1:1 ratio (in-situ portion), while the other portion is placed separately (out-of-situ portion). The total material ratio is coal direct liquefaction residue : in-situ hexavalent chromium-containing solid waste : out-of-situ hexavalent chromium-containing solid waste = 1:1:1. The temperature is the same as before. Pyrolysis volatiles and gases can act on the hexavalent chromium in the out-of-situ portion for reduction.

[0091] Results: The three-phase distribution is shown in Table 13. The gas-phase yield of coupled pyrolysis was higher than that of the previous three examples, indicating that the volatiles generated during pyrolysis underwent a secondary catalytic reaction.

[0092] Table 13 Distribution of three-phase products from in-situ and non-in-situ coupled pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0093] The composition of the gaseous products is shown in Table 14. Comparing Tables 3, 6, and 10, the yields of low-carbon alkanes and hydrogen show the most significant decreases, while the yields of low-carbon olefins and carbon dioxide also show the most significant increases. This trend indicates that the volatile matter undergoing two catalytic reactions can significantly alter the product distribution.

[0094] Table 14 Distribution of gaseous products from in-situ and non-in-situ coupled pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0095] The composition of the liquid phase products is shown in Table 15. Compared with Tables 4, 7 and 11, the yields of alkanes and aromatics decreased most significantly, while the yields of olefins and oxides increased most significantly.

[0096] Table 15 Distribution of liquid phase products from in-situ and non-in-situ coupled pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste

[0097] The reduction rates of hexavalent chromium are shown in Table 16. The reduction rates of hexavalent chromium in the in-situ co-pyrolysis portion all exceeded 98.70%, while the reduction rates in the off-situ pyrolysis portion all exceeded 98.16%.

[0098] Table 16. Reduction rate of hexavalent chromium by in-situ and non-in-situ coupled pyrolysis of coal direct liquefaction residue and hexavalent chromium-containing solid waste.

[0099] Note: 1 and 2 represent in-situ co-pyrolysis and non-in-situ co-pyrolysis, respectively.

[0100] Furthermore, this invention also investigated the catalytic effect of hexavalent chromium-containing solid waste on the lightening of liquid-phase products from the pyrolysis of coal direct liquefaction residues. For example... Figure 9 As shown, the classification analysis of products based on carbon chain length revealed that, regardless of in-situ, out-of-situ, or coupled pyrolysis modes, metal oxides in hexavalent chromium-containing solid waste can significantly promote the lightening of liquid phase products. This is manifested in a significant increase in the proportion of C5-C9 light components in the products, which is much higher than the results of pyrolysis of coal direct liquefaction residue alone. Even in the coupled pyrolysis mode, this catalytic effect still exists.

[0101] The above embodiments fully demonstrate that: High efficiency reduction: The three pyrolysis modes all achieved a reduction rate of over 98% for hexavalent chromium in solid waste containing hexavalent chromium within a wide temperature range (400~700℃), meeting strict environmental protection standards (<5mg / kg leaching concentration).

[0102] Resource synergy: The active metal sites (Mg, Al, and Cr, etc.) in hexavalent chromium-containing solid waste effectively catalyze the pyrolysis of coal direct liquefaction residue, which can improve the yield of liquid-phase products and significantly promote the lightening of the material. The detoxified solid residue (rich in stabilized trivalent chromium and carbon / mineral components) has the potential for resource utilization as a building material raw material, enabling the secondary use of the residue.

[0103] Process optimization: The non-in-situ and coupled modes fully utilize the volatiles and non-condensable reducing gases formed during pyrolysis, improving the reduction efficiency of hexavalent chromium. Equipment design (double helix, micropores, purging device) effectively ensures material transport, heat transfer, and reaction contact.

[0104] Flexible and economical: The three modes can be flexibly selected or combined according to the characteristics of raw materials, processing scale and target products, and the "waste-to-waste" approach can be achieved by using coal direct liquefaction residue, which greatly reduces the dependence on commercial reducing agents and processing costs.

[0105] In summary, this invention, through innovative pyrolysis technology and equipment design, achieves efficient and low-cost detoxification and resource recovery of hexavalent chromium-containing solid waste, demonstrating significant industrial application prospects. Those skilled in the art can make adaptive adjustments and modifications to the equipment dimensions, specific parameters, operating conditions, etc., without departing from the core principles of this invention.

[0106] Any parts not mentioned in this invention can be achieved by referring to existing technologies.

[0107] Those skilled in the art should recognize that the above embodiments are only used to illustrate this application and are not intended to limit this application. Any appropriate changes and variations made to the above embodiments within the essential spirit and scope of this application fall within the scope of protection claimed in this application.

Claims

1. A method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue, the system comprising a feeding system, a pyrolysis system, a heating system, an atmosphere control system, a gas circulation and purification system, a gas purging enhancement system, and a product collection system, characterized in that: The feeding system includes a feeding device 1 and a feeding device 2 arranged in parallel. A feeding device 3 is provided between the feeding device 1 and the feeding device 2 to keep the feeding device 1 and the feeding device 2 connected or disconnected. The feeding device 1 is used for the entry of chromium-containing solid waste, and the feeding device 2 is used for the entry of coal direct liquefaction residue. The feeding device 1 is also provided with a blocking device, which is used to sequentially pass the chromium-containing solid waste into the feeding device 3 and the feeding device 2. Both the first and second feeding devices are connected to the pyrolysis system. The pyrolysis system includes an inner cylinder arranged coaxially and an outer cylinder sleeved outside the inner cylinder. The inside of the inner cylinder is a waste chromium slag chamber, and the space between the inner and outer cylinders is a dual-material mixing chamber. An inner spiral blade is provided on the inner wall of the inner cylinder, and an outer spiral blade is provided on the outer wall of the inner cylinder. Several permeable micropores are provided on the inner cylinder wall. A reducing gas exhaust device is provided in the axial direction of the inner cylinder. The reducing gas exhaust device includes a hollow rod body with exhaust holes in the circumferential direction. A rotary joint is sleeved on one end of the rod body. The rotary joint is installed at the reducing non-condensable gas inlet of the pyrolysis system. The reducing non-condensable gas inlet is connected to the reducing non-condensable gas outlet of the gas circulation purification system; The method includes in-situ co-pyrolysis mode, non-in-situ pyrolysis mode, and in-situ-non-in-situ coupled pyrolysis mode; Specifically, the following steps are included: a. In-situ co-pyrolysis mode: The chromium-containing solid waste entering the first feeding device is sequentially fed into the third feeding device and the second feeding device through the blocking device. In the second feeding device, the chromium-containing solid waste and the coal direct liquefaction residue are uniformly mixed and enter the dual-material mixing chamber of the pyrolysis system. The heating system is set at a temperature of 400~700℃. The reducing products generated in-situ by the pyrolysis of the coal direct liquefaction residue directly contact the hexavalent chromium-containing solid waste, reducing the hexavalent chromium to trivalent chromium. The generated non-condensable gas is purified by the gas circulation purification system, and the product is collected by the product collection system. b. Non-in-situ pyrolysis mode: Feeding devices one and two are kept separate via feeding device three. Chromium-containing solid waste enters the waste chromium slag chamber of the pyrolysis system through feeding device one, while coal direct liquefaction residue enters the dual-material mixing chamber of the pyrolysis system through feeding device two. The heating system is set at a temperature of 400~700℃. The coal direct liquefaction residue is pyrolyzed separately in the dual-material mixing chamber. The resulting volatile components rich in reducing agents diffuse into the waste chromium slag chamber through permeable micropores, contacting the chromium-containing solid waste and reducing the hexavalent chromium within it. The non-condensable gas produced by pyrolysis is purified by a gas circulation purification system, and the remaining reducing gas is circulated back into the waste chromium slag chamber through a reduction exhaust device to further enhance reduction. The product is collected through a product collection system. c. In-situ-Non-in-situ Coupled Pyrolysis Mode: Feeding device 1 and feeding device 2 are kept connected through feeding device 3. Part of the chromium-containing solid waste enters the waste chromium slag chamber of the pyrolysis system directly through feeding device 1, while the other part is mixed with coal direct liquefaction residue through feeding device 3 and feeding device 2 and enters the dual-material mixing chamber of the pyrolysis system. The mixture in the dual-material mixing chamber undergoes in-situ co-pyrolysis to reduce some of the hexavalent chromium in the waste chromium slag. In the waste chromium slag chamber, a non-in-situ pyrolysis mode is performed to reduce some of the hexavalent chromium in the waste chromium slag.

2. The method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: The feeding device 1, feeding device 3, feeding device 2, and blocking device are all equipped with screws, which convey materials. A blocking plate is provided at the end of the screw of the blocking device to prevent the material in feeding device 1 from directly entering the pyrolysis system.

3. The method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: The pitch of the outer spiral blades is 50~200mm, used to transport coal direct liquefaction residue or a mixture of coal direct liquefaction residue and chromium-containing waste into the dual-material mixing chamber; the pitch of the inner spiral blades is 30~150mm, used to transport chromium-containing solid waste into the waste chromium slag chamber.

4. The method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: The heating system is an external heating method, including electromagnetic induction coils, resistance heaters, or gas nozzle heaters.

5. The method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: The atmosphere control system is used to maintain an inert atmosphere within the pyrolysis system. The inert gas is nitrogen, argon, or helium, or a vacuum micro-negative pressure is used to maintain the inert environment.

6. The method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: The gas circulation purification system includes a non-condensable gas inlet, a dehumidification device, a dust removal device, a non-condensable gas grading and purification device, and a reducing non-condensable gas outlet. The non-condensable gas grading and purification device is used to separate the reducing gas and the exhaust gas to be treated.

7. The method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: The gas purging enhancement system includes a reduction exhaust device, which can dynamically rotate under the action of a rotary joint. The blown airflow is used to disturb the material in the waste chromium slag chamber and increase its contact area with the reducing gas.

8. The method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: The diameter of the breathable micropores is 0.001~20mm.

9. A method for reducing hexavalent chromium in chromium-containing solid waste using coal direct liquefaction residue according to claim 1, characterized in that: In step a, the reducing products generated by pyrolysis mainly include pyrolytic carbon, alkenes, alkanes, aromatic hydrocarbons, hydrogen, and carbon monoxide; the non-condensable gases generated mainly include hydrogen, carbon monoxide, and low-carbon hydrocarbon reducing non-condensable gases.

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