A separation system and method for recycling leaf filter waste residue in an alumina production process

Abstract

The application discloses a separation system for recycling leaf filter waste residue in an alumina production process, comprising a pressure separation device, a feed pump connected between a leaf filter waste residue slurry source and a feed inlet of the pressure separation device, an external tank dilution circuit, an overflow outlet of the pressure separation device connected to an inlet front end of the feed pump through a shunt pipeline, a underflow pump connected to an underflow outlet of the pressure separation device, a vacuum belt filter connected to an outlet of the underflow pump, a pulping tank arranged below a filter cake discharge outlet of the vacuum belt filter, and a conveying pump connected between an outlet of the pulping tank and a pre-desilication process of an alumina production system. Through the synergistic effect of high solid content mud layer sedimentation technology and external tank dilution technology, the industry problem that leaf filter waste residue is difficult to settle, concentrate and filter and separate due to easy hydrolysis under the condition of low caustic alkali concentration and low molecular ratio and a large amount of fine planktonic matter is effectively solved.

Description

Technical Field

[0001] This invention relates to the field of alumina production technology, and specifically to a separation system and method for recovering leaf filter waste residue from the alumina production process. Background Technology

[0002] The Bayer process is the mainstream technology for alumina production worldwide. In this process, bauxite is leached under high temperature, high pressure, and high alkali conditions, and the alumina enters the liquid phase, forming a sodium aluminate solution (crude liquid). To ensure the product quality of subsequent decomposition processes, the crude liquid must be filtered by a leaf filter to remove fine suspended solids (solid impurities). To improve filtration efficiency, lime slurry is usually added as a filter aid in leaf filtration. The lime slurry reacts with the sodium aluminate solution to produce tricalcium aluminate hexahydrate (hydrated calcium aluminate). This compound has high hardness, large particles, and is loose and porous, which increases the looseness and permeability of the filter cake, reduces filtration resistance, and thus increases the filter's capacity. After each filtration cycle, the cake is backflushed with a fine liquid, and the filter cake and the backflushing liquid are discharged together into the filter cake tank to form leaf filter waste slurry.

[0003] However, due to the low caustic alkali concentration (150–160 g / L as sodium oxide) and low molecular weight ratio (ak 1.38–1.45) of sodium aluminate solution, it is highly susceptible to hydrolysis. Combined with the large amount of fine suspended matter generated by leaf filters, the settling and filtration performance of the waste slurry deteriorates drastically, making effective settling, concentration, and filtration difficult using conventional methods. Because of this technical bottleneck, existing alumina production enterprises generally discard leaf filter waste along with red mud, causing a serious environmental burden and significant resource waste. Compositional analysis shows that leaf filter waste is rich in recyclable components: tricalcium aluminate hexahydrate content exceeds 90%, with calcium oxide (CaO) content of approximately 35%–45%, alumina (Al2O3) content as high as 24%–30%, and an aluminum-silicon ratio (A / S) of approximately 10. This indicates that the slag is not ordinary waste; its alumina content is considerable, and its aluminum-silicon ratio is even higher than that of some low-grade bauxite. In other words, every ton of waste leaf filter residue is equivalent to the loss of a large amount of alumina and calcium oxide that could have been returned to the production process, directly increasing bauxite consumption and lime consumption, and increasing production costs.

[0004] Several technical solutions have been proposed for the recycling of leaf filter residue slurry. For example, Chinese patent CN105565349B proposes that after sedimentation and separation, the leaf filter residue slurry, after dealuminization and pressure filtration, be returned to the lime batching silo to recover alumina and calcium oxide. However, this technical solution faces significant problems in practical application: returning it to the raw material mill batching will affect the feed rate and batching stability of the slurry, and the sedimentation and separation process itself is difficult to achieve efficient solid-liquid separation, which has prevented the technology from being industrialized to date.

[0005] For example, Chinese patent CN105152195B proposes mixing the controlled filter cake with the mother liquor before feeding it into the raw material mill for ore blending, and then having it enter the leaching system along with the raw ore slurry. However, this method does not solve the problems of filter cake slurry concentration and dehydration. Directly returning the low-concentration slurry to the system will introduce a large amount of water, disrupting the water balance of the leaching system and increasing steam consumption. Furthermore, research has shown that after the leaf filter cake is returned to the post-leaching tank, the mixed materials have relatively poor settling performance, which will adversely affect subsequent settling processes.

[0006] Other literature reports a recovery process using "plate and frame filter press filtration - circulating mother liquor blending - raw material grinding and ore blending". However, plate and frame filter press is an intermittent operation, with low processing efficiency, easy clogging of filter cloth, high labor intensity, and also faces the problem of affecting the stability of ore blending after being returned to the raw material grinding. Summary of the Invention

[0007] This application provides a separation system and method for recycling leaf filter waste residue from the alumina production process, achieving efficient liquid-solid separation of leaf filter waste residue and effective recovery of usable resources, ultimately achieving the goals of improving resource utilization, reducing production costs, and reducing solid waste emissions.

[0008] To achieve the above objectives, the first aspect of this application provides the following technical solution: A separation system for recycling leaf filter waste residue from alumina production process includes: a pressurized separation device, which is a pressure vessel with an overflow port at the top and a bottom flow port at the bottom, and an inlet center cylinder and a diffuser bell mouth inside. A feed pump is connected between the source of the leaf filter waste slurry and the inlet of the pressurized separation device, and is used to transport the leaf filter waste slurry into the pressurized separation device for sedimentation and separation. The external dilution circuit, wherein the overflow port of the pressurized separation device is connected to the inlet of the feed pump through a diversion pipe, is used to guide part of the clarified overflow to be mixed and diluted with the original feed slurry; An underflow pump, connected to the underflow port of the pressurized separation device, is used to deliver the high-concentration underflow obtained by sedimentation separation. A vacuum belt filter, connected to the outlet of the underflow pump, is used to perform solid-liquid separation on the high-concentration underflow to obtain filter cake and filtrate; A slurry preparation tank is located below the filter cake outlet of the vacuum belt filter and is used to mix the filter cake with the evaporation mother liquor to prepare a slurry. A delivery pump is connected between the outlet of the slurry tank and the pre-desiliconization process of the alumina production system to deliver the prepared slurry to the pre-desiliconization process.

[0009] Furthermore, the feed center cylinder is located at the upper 1 / 3 of the straight cylinder section, and the diffuser flare is located below the feed center cylinder.

[0010] In the optimized configuration, the feed pipe of the pressurized separation device enters the feed center cylinder tangentially at a downward inclined angle, and the feed pipe has a sleeve structure.

[0011] Furthermore, the top of the pressurized separation device is provided with several overflow pipes evenly distributed in the circumference, and each overflow pipe has multiple small holes evenly opened on its wall.

[0012] Furthermore, the bottom of the pressurized separation device is provided with at least two underflow ports at different heights.

[0013] In some embodiments, the straight section of the pressurized separation device is provided with several mud layer detection ports at different heights along the vertical direction.

[0014] A second aspect of this application provides a separation method using the separation system described in the first aspect, comprising the following steps: S1. External dilution and sedimentation separation: The leaf filter waste slurry at a temperature of 90-100℃ is pumped to the pressurized separation device, where a high solids mud layer is formed and maintained. A portion of the clarified overflow from the pressurized separation device is directed to be forcibly diluted with the original feed slurry. The diluted slurry is then pumped back to the inlet of the pressurized separation device by the feed pump, where it undergoes sedimentation separation under pressure to obtain clarified overflow and high-concentration underflow, respectively. S2. Solid-liquid separation and resource recovery: The high-concentration underflow is pumped to a vacuum belt filter for solid-liquid separation to obtain filter cake and filtrate; the filtrate is sent to the coarse liquid tank of the alumina production system; the filter cake enters the slurry tank and is mixed with the evaporation mother liquor, and then pumped to the pre-desiliconization process, where it is mixed with the bauxite slurry in the pre-desiliconization tank and sent to the leaching unit for high-temperature leaching to recover the calcium aluminate.

[0015] Furthermore, in step S1, the volume ratio of the forced dilution reflux overflow to the original feed slurry is 0.1:1-0.4:1.

[0016] In some embodiments, the operating pressure in the pressurized separation device in step S1 is 0.2-0.5 MPa; the operating vacuum degree of the vacuum belt filter in step S2 is -0.03 to -0.06 MPa, and the high-temperature leaching temperature is 250-280°C.

[0017] In the optimized form, the solids content of the high-solids mud layer in step S1 is stabilized at 25%-35%, the solids content of the high-concentration underflow is not less than 30%, and the suspended solids content of the clarified overflow is less than 300 mg / L; the moisture content of the filter cake in step S2 is less than 45%.

[0018] Working principle and beneficial effects of the present invention: (1) By combining high-solids-content mud layer settling technology with off-tank dilution technology, highly efficient solid-liquid separation of extremely difficult-to-settle leaf filter residue was achieved. The high-solids-content mud layer, as a dynamic filter medium, can effectively capture fine particles in the slurry and enhance the settling process; off-tank dilution optimizes the initial settling conditions by adjusting the solids content and viscosity of the feed slurry. The synergistic effect of the two technologies enables the efficient separation of leaf filter residue that could not be settled by conventional methods, resulting in a clarified overflow with a suspended solids content of less than 300 mg / L and a high-concentration underflow with a solids content of not less than 30%, thus overcoming a long-standing technical challenge in this field.

[0019] (2) The leaf filter cake (containing over 90% tricalcium aluminate hexahydrate, with approximately 35-45% calcium oxide and 24-30% alumina) after being processed by this invention is sent to the pre-desiliconization process after being mixed with the evaporation mother liquor. It is then mixed with bauxite slurry and sent to the leaching unit for leaching at a high temperature of 250-280℃, where the calcium aluminate decomposes into calcium oxide and alumina. Calcium oxide promotes the leaching of alumina from the bauxite, while alumina enters the solution and becomes part of the product. Through this process, the leaf filter residue is transformed from "waste" into a secondary "resource" that can be returned to the main process, realizing the recycling of valuable components. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of a leaf filter waste separation system for recovering alumina production process according to this application; Figure 2 for Figure 1 A schematic diagram of the structure of a medium-pressure separation device.

[0021] The reference numerals in the accompanying drawings include: feed pump 1, pressurized separation device 2, underflow pump 3, vacuum belt filter 4, slurry tank 5, conveying pump 6, feed pipe a, overflow pipes b1~b4, underflow port c1~c2, mud layer detection port d1~d3, safety valve port e, non-condensable gas vent f, instrument port g1~g3, maintenance port h, feed center cylinder i, and diffuser bell port j. Detailed Implementation

[0022] The following detailed description illustrates the specific implementation method: Example 1: As Figure 1 As shown, the separation system in this embodiment 1 includes: a pressurized separation device 2, a feed pump 1, an external dilution circuit, an underflow pump 3, a vacuum belt filter 4, a pulping tank 5, and a transfer pump 6.

[0023] The pressurized separation device 2 is a vertical pressure vessel, and its specific structure is as follows: Figure 2As shown: It consists of one feed pipe a, four overflow pipes b1~b4, two underflow ports c1 and c2, three mud layer detection ports d1~d3, one safety valve port e, one non-condensable gas vent f, three instrument ports g1~g3 and maintenance port h; The equipment center is equipped with a central feed cylinder i and a diffuser flare j, as well as a mixer and a rake. The feed pipe a enters the central feed cylinder tangentially downwards at a 45° angle. The slurry enters the central feed cylinder at an average flow rate of 2.2 m / s. The 45° angle is the most suitable angle at this velocity, providing sufficient downward force to maintain the flow pattern while retaining a sufficient tangential component to form effective swirling mixing. The feed pipe uses a sleeve design, facilitating the replacement of worn or clogged feed pipes. Four overflow pipes, b1~b4, are evenly distributed in four directions of the overflow level, with each overflow pipe uniformly covered with orifices, forming a four-way honeycomb overflow pipe combination design. The total opening area of ​​all the orifices is greater than or equal to the cross-sectional area of ​​the main pipe connecting the overflow pipes, achieving uniform flow distribution within the settling equipment, balancing the internal flow field, eliminating dead zones, and simultaneously improving processing stability and anti-interference performance.

[0024] Two bottom discharge ports, c1 and c2, are located at different heights at the bottom of the cone and the bottom of the straight section of the equipment, respectively. When the bottom discharge is obstructed, the system can perform online emergency handling without stopping the machine. Three mud layer detection ports, d1~d3, are set at different heights along the vertical direction in the straight section, located in the expected settling zone and mud layer zone, respectively. The mud layer detection ports are set with sleeves for easy replacement.

[0025] The feed center cylinder i has a diameter of 780mm, effectively creating a stable and moderate swirling field. This allows denser solid particles to be rapidly thrown towards the cylinder wall, ensuring the slurry maintains a low and uniform axial descent velocity within the cylinder and preventing high-speed jet impact on the sedimentation layer at the bottom of the equipment. The diffuser nozzle j has an opening of 15°, guiding fluid expansion and deceleration to achieve a stable and uniform flow field. The height of the center cylinder i is approximately one-third of the way up the entire straight cylinder section. The core principle of treating difficult-to-settle materials is "contact flocculation, synchronous separation and compression," and this center cylinder height enables operation in high-solids mud-containing layers. The diffuser nozzle j makes the feed gentler, ensuring the slurry spreads smoothly onto the mud surface, avoiding jet impact.

[0026] Feed pump 1 is connected between the source of the leaf filter waste slurry and the inlet of the pressurized separator 2. The external dilution loop is configured as follows: a branch pipe is led out from the main pipe of the four overflow pipes b1~b4 at the top of the pressurized separator 2. This branch pipe is connected to the inlet pipe of feed pump 1, thus forming an external forced dilution circulation loop between the pressurized separator 2 and the inlet of feed pump 1. A flow regulating valve is installed on this branch pipe to precisely regulate the return overflow flow rate. Part of the clarified overflow is led to the inlet of feed pump 1 through this branch pipe, where it is forcibly mixed and diluted with the original leaf filter waste slurry in the inlet pipe of feed pump 1. The diluted mixed slurry is then pumped back to the inlet of the pressurized separator 2 by feed pump 1.

[0027] The underflow pump 3 is connected to the combined outlet of the two underflow ports c1 and c2 of the pressurized separation device 2. The vacuum belt filter 4 is connected to the outlet of the underflow pump 3. The pulping tank 5 is located below the filter cake discharge outlet of the vacuum belt filter 4. The conveying pump 6 is connected between the outlet of the pulping tank 5 and the pre-desiliconization process of the alumina production system.

[0028] A method for separating leaf filter waste residue from alumina production processes includes the following steps: S1. External dilution and sedimentation separation: Leaf filter waste slurry at 95℃ is pumped to pressurized separation unit 2 via feed pump 1 at a flow rate of 150 cubic meters per hour. The operating pressure of pressurized separation unit 2 is maintained at 0.30 MPa (G). By controlling the underflow discharge rate, a high-solids-content mud layer with a stable solids content (weight percentage) of 30% ± 5% is established and maintained inside the unit.

[0029] Through an external dilution loop, approximately 30-40% of the clarified overflow generated by the pressurized separator 2 is diverted back to the inlet of the feed pump 1 via a diversion pipe. There, it is forcibly mixed and diluted with the original leaf filter waste slurry. The dilution ratio (volume ratio) is controlled by a flow regulating valve to be 0.2:1 (return overflow: original slurry). The diluted slurry is then pumped back to the inlet of the pressurized separator 2 via the feed pump 1. Stable sedimentation separation is performed under pressure, yielding clarified overflow and high-concentration underflow. The final product is clarified overflow with a suspended solids content below 200 mg / L and high-concentration underflow with a solids content of not less than 36%. A portion of the clarified overflow is used for dilution, while the remainder is guided to the coarse liquid tank of the alumina production system for the next production step.

[0030] S2, Separation of leaf filter residue and resource recovery: the high-concentration underflow from the pressurized separation device 2 is delivered via the汇总 outlet of the two underflow outlets c1 and c2 to a vacuum belt filter 4 through an underflow pump 3 for solid-liquid separation. The operating vacuum of the vacuum belt filter 4 is maintained at -0.035 to -0.053 MPa, and a filter cake with a moisture content of less than 40% by weight is obtained under this condition. The filtrate (overflow) separated by the vacuum belt filter 4 enters the crude liquor tank of the alumina production system for the next process. The filter cake separated by the vacuum belt filter 4 is discharged into a pulping tank 5 arranged below the discharge port of the filter, and is blended with the evaporation mother liquid from the evaporation process, the blended solid content is controlled at 300 g / L, a uniform slurry is formed by stirring, and the slurry is delivered to a pre-desilication tank through a delivery pump 6, mixed with the bauxite slurry in the pre-desilication tank and then fed into a digestion unit, and the calcium aluminate therein is dissolved, recovered and utilized under the high-temperature, high-pressure and high-alkali condition of 260~270°C. Phase analysis and chemical composition analysis are performed on the filter cake obtained in this Example 1, and the results are shown in Tables 1 to 3.

[0031] Table 1 - Sample No.: 20260515-Zhong-Yi-1#

[0032] Table 2 - Sample No.: 20260517-8-2-2#

[0033] Table 3 - Sample No.: 20260515-Zhong-Yi-3#

[0034] The present application addresses the industrial problems that in the leaf filtration process, sodium aluminate solution is prone to hydrolysis due to low caustic concentration and low molecular ratio, and is difficult to separate and filter; and a large amount of fine floating matter discharged from leaf filtration causes the leaf filter residue to be difficult to comprehensively utilize, and high-value components are wasted along with red mud. Through the synergistic effect of the "high solid content mud layer" sedimentation technology and the "outside-tank dilution" technology, the separation efficiency is significantly improved, and effective recovery of available resources in the leaf filter residue is achieved.

[0035] By recovering available resources in the leaf filter residue, the demand of the production system for external raw materials is directly reduced, thereby cutting the upstream carbon emissions accompanying raw material production (such as limestone calcination and bauxite digestion), achieving process carbon reduction driven by resource recycling; and by realizing effective recovery and utilization of the residue, the emission of red mud is greatly reduced. This not only reduces the environmental pressure on storage yards, but also directly reduces carbon sink losses caused by land occupation and ecosystem damage, which is in line with the low-carbon development path of solid waste source reduction.

[0036] This separation system optimizes the process, improves the recovery rate of leaf filter residue, and indirectly reduces energy consumption in slurry treatment, transportation, and waste disposal, thereby reducing indirect carbon emissions during production. This invention transforms leaf filter residue from "waste" into a high-value "resource," achieving efficient resource utilization while providing a practical carbon reduction path for the alumina industry. It highly aligns with the national "dual-carbon" strategic goals and has significant economic, environmental, and climate benefits.

[0037] In this invention, the height of the central cylinder of the pressurized separation device 2 is maintained at the upper 1 / 3 of the straight cylinder section. By integrating and innovatively employing two core technologies—"high-solids mud-containing layer" and "forced dilution outside the tank"—a highly efficient and stable leaf filter waste separation and resource recovery system is constructed. For the first time, this system systematically solves the long-standing technical bottleneck in the industry of leaf filter waste's difficulty in settling and filtering, successfully achieving effective recycling of leaf filter waste. Its core value lies in transforming leaf filter waste (containing over 90% tricalcium aluminate hexahydrate, with approximately 35-45% calcium oxide (CaO) and 24-30% alumina (Al2O3), and an aluminum-silicon ratio (A / S) of approximately 10)—which is traditionally discarded as waste—into a secondary resource that can be returned to the main process, achieving efficient recycling of usable resources.

[0038] The implementation of this invention not only brings direct economic benefits to production enterprises (reducing lime and bauxite consumption), but also makes significant contributions to environmental protection and carbon emission control: through waste reduction at the source and resource recycling, it effectively reduces carbon emissions from upstream raw material production and carbon sink losses at end-of-pipe storage sites. Therefore, this invention is not only a breakthrough in process technology, but also a key practical technology for promoting the transformation and upgrading of the alumina industry towards green, low-carbon, and circular development, with broad prospects for promotion and application and important strategic significance.

[0039] The above description is merely a preferred embodiment of this application and does not limit the patent scope of this application. Any equivalent structural transformations made based on the inventive concept of this application and the contents of the specification and drawings of this application, or direct / indirect applications in other related technical fields, are included within the patent protection scope of this application.

Claims

1. A separation system for recovering leaf filter waste residue from alumina production, characterized in that, include: A pressurized separation device, which is a pressure vessel, has an overflow port at the top and a bottom flow port at the bottom, and has a feed center cylinder and a diffuser bell inside; A feed pump is connected between the source of the leaf filter waste slurry and the inlet of the pressurized separation device, and is used to transport the leaf filter waste slurry into the pressurized separation device for sedimentation and separation. The external dilution circuit, wherein the overflow port of the pressurized separation device is connected to the inlet of the feed pump through a diversion pipe, is used to guide part of the clarified overflow to be mixed and diluted with the original feed slurry; An underflow pump, connected to the underflow port of the pressurized separation device, is used to deliver the high-concentration underflow obtained by sedimentation separation. A vacuum belt filter, connected to the outlet of the underflow pump, is used to perform solid-liquid separation on the high-concentration underflow to obtain filter cake and filtrate; A slurry preparation tank is located below the filter cake outlet of the vacuum belt filter and is used to mix the filter cake with the evaporation mother liquor to prepare a slurry. A delivery pump is connected between the outlet of the slurry tank and the pre-desiliconization process of the alumina production system. It is used to deliver the prepared slurry to the pre-desiliconization process, mix it with the bauxite slurry, and then send it to the leaching unit.

2. The separation system according to claim 1, characterized in that: The feed center cylinder is located at the upper 1 / 3 of the straight cylinder section, and the diffuser flare is located below the feed center cylinder.

3. The separation system according to claim 2, characterized in that, The feed pipe of the pressurized separation device enters the feed center cylinder tangentially at a downward inclined angle, and the feed pipe has a sleeve structure.

4. The separation system according to claim 3, characterized in that, The top of the pressurized separation device is provided with several overflow pipes evenly distributed in the circumference, and each overflow pipe has multiple small holes evenly opened on its wall.

5. The separation system according to claim 4, characterized in that, The bottom of the pressurized separation device is provided with at least two underflow ports at different heights.

6. The separation system according to claim 5, characterized in that, The straight section of the pressurized separation device is provided with several mud layer detection ports at different heights along the vertical direction.

7. A separation method, employing the separation system as described in any one of claims 1 to 6, characterized in that, The steps include the following: S1. External dilution and sedimentation separation: The leaf filter waste slurry at a temperature of 90-100℃ is pumped to the pressurized separation device, where a high solids mud layer is formed and maintained. A portion of the clarified overflow from the pressurized separation device is directed to be forcibly diluted with the original feed slurry. The diluted slurry is then pumped back to the inlet of the pressurized separation device by the feed pump, where it undergoes sedimentation separation under pressure to obtain clarified overflow and high-concentration underflow, respectively. S2. Solid-liquid separation and resource recovery: The high-concentration underflow is pumped to a vacuum belt filter for solid-liquid separation to obtain filter cake and filtrate; the filtrate is sent to the coarse liquid tank of the alumina production system; the filter cake enters the slurry tank and is mixed with the evaporation mother liquor, and then pumped to the pre-desiliconization process, where it is mixed with the bauxite slurry in the pre-desiliconization tank and sent to the leaching unit for high-temperature leaching to recover the calcium aluminate.

8. The separation method according to claim 7, characterized in that, In step S1, the volume ratio of the forced dilution reflux overflow to the original feed slurry is 0.1:1-0.4:

1.

9. The separation method according to claim 8, characterized in that, The operating pressure in the pressurized separation device in step S1 is 0.2-0.5 MPa; the operating vacuum degree of the vacuum belt filter in step S2 is -0.03 to -0.06 MPa, and the high-temperature leaching temperature is 250-280℃.

10. The separation method according to claim 9, characterized in that, In step S1, the solids content of the high-solids mud layer is stable at 25%-35%, the solids content of the high-concentration underflow is not less than 30%, and the suspended solids content of the clarified overflow is less than 300 mg / L; in step S2, the moisture content of the filter cake is less than 45%.

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