All-element high-value resource recycling process for red mud

Abstract

The invention discloses a red mud all-element high-value resource recycling process, and relates to the technical field of red mud. Sodium silicate and sodium alkali are efficiently separated through washing of the double-helix mud washing machine, concentration of the thickener, dealkalization leaching of ammonium chloride, classification of the hydrocyclone, targeted treatment of coarse and fine particles, high-value utilization of silicon alkali overflow liquid, two-stage membrane electrodialysis and the like; the problems of recovery of sodium silicate and electronic-grade silica sol in the red mud, removal and conversion of sodium hydroxide into high-purity sodium hydroxide and full-component decomposition, separation and enrichment of valuable elements (silicon, aluminum, iron, titanium, vanadium, heavy metals and three-rare elements) are completely solved, and finally full-element utilization of the red mud, especially enrichment of alkali-soluble three-rare elements, is realized. The commercial value of the red mud is far higher than that of bauxite, and a feasible industrial path is provided for harmless, recycling and high-value utilization of the red mud.

Description

Technical Field

[0001] This invention relates to the field of red mud technology, specifically to a high-value resource recovery process for all elements of red mud. Background Technology

[0002] Red mud, a highly alkaline and complex solid waste inevitably produced during the Bayer process for alumina production, has a fixed production ratio with alumina output—1.0-2.5 tons of red mud are generated for every ton of alumina produced (the specific output varies depending on the grade of bauxite, with lower-grade bauxite corresponding to higher red mud production). In terms of composition, red mud is not a single substance but a mixed system containing various oxides, salts, and trace elements: besides silicon (existing as sodium silicate and silicon dioxide, content 6-25%), which constitutes the largest proportion, and incompletely extracted aluminum (existing as alumina or aluminates, content 16-28%), it also contains metal oxides such as calcium (calcium oxide content 5-15%), iron (iron oxide content 10-30%, which is the main reason for the reddish-brown color of red mud), and titanium (titanium dioxide content 2-8%), as well as high-concentration sodium alkali introduced by the process (sodium oxide content 2-8%, causing the pH value of red mud to remain in a strongly alkaline range of 10-13 year-round).

[0003] Currently, global red mud emissions exceed 150 million tons annually, with China, as the world's largest alumina producer (accounting for over 60% of global production), exceeding 100 million tons annually. However, the industry has yet to develop a red mud treatment technology that combines economic feasibility with efficient resource recovery. Existing technologies are either prohibitively expensive (e.g., high-temperature roasting costs over 500 yuan per ton of red mud), making them unaffordable for companies, or have low resource recovery rates, preventing large-scale profitability. Ultimately, the global comprehensive utilization rate of red mud is less than 10%, with over 90% of red mud requiring damming or deep-sea landfill for disposal. This not only occupies significant land resources (approximately one acre of land is needed for every 10,000 tons of red mud stored) but also poses environmental risks of highly alkaline leachate polluting soil and groundwater, becoming a key bottleneck restricting the green development of the alumina industry. Behind this predicament, the core difficulty lies in the separation and extraction of sodium silicate and sodium alkali, with technical obstacles directly impacting the economic benefits of red mud resource utilization.

[0004] I. Core Challenges: High-purity and economical separation and extraction of sodium silicate and sodium alkali. Among the many components of red mud, sodium silicate and sodium alkali (mainly NaOH) are not only the main cause of its strong alkalinity, but also potential recyclable resources. Sodium silicate is an important raw material for industries such as glass, ceramics, and detergents, while sodium alkali is a core auxiliary material in the Bayer process for alumina production. Achieving high-purity, low-cost recovery of both could not only reduce the alkalinity hazards of red mud but also generate additional revenue for enterprises. However, in actual separation, two major technical obstacles make it difficult to achieve: 1. The strong binding between sodium silicate and the red mud matrix makes separation difficult. In the Bayer process, silicon in bauxite reacts with NaOH in a high-temperature, high-pressure (typically 140-260℃, 0.3-3.0MPa) sodium hydroxide solution to form sodium silicate. However, the formed sodium silicate does not exist in a free state; instead, it reacts rapidly with sodium aluminate in the solution to form a stable sodium aluminum silicate complex (commonly known as "sodium silicate slag"). This complex has a compact structure and extremely strong chemical stability, making it difficult to completely decompose into individual sodium silicate and sodium aluminate even under strong acid (such as 20% hydrochloric acid) or high-temperature (such as calcination at 800℃) conditions. Approximately 80% of the sodium silicate in red mud exists in this complex form within the red mud matrix, tightly bound to components such as alumina and iron oxide, forming a mixed system where each component is intertwined with the other. Traditional separation methods (such as room temperature leaching) can only extract 10-15% of free sodium silicate from red mud, and the extraction rate of complexed sodium silicate is less than 30%. In addition, the extracted sodium silicate solution will be mixed with a large amount of aluminum and iron impurities (impurity content exceeds 10%). Subsequent purification requires multiple precipitation and filtration processes, and the purification cost of each ton of sodium silicate exceeds 800 yuan, far exceeding the market price (about 600 yuan / ton), making the separation and extraction economically worthless.

[0005] 2. The dispersibility and loss of sodium alkali increase recovery costs. In addition to existing sodium alkali in red mud in the form of compounds such as sodium silicate and sodium aluminate, about 10-20% of the sodium alkali is adsorbed on the surface of red mud particles or encapsulated in the tiny pores of red mud, forming "adsorbed sodium" and "encapsulated sodium". Existing recovery technologies (such as washing) use a large amount of water to wash the red mud, which can dissolve some of the adsorbed sodium, but it will dilute the sodium alkali concentration (the NaOH concentration in the solution after washing is usually less than 5%). Subsequent evaporation and concentration require a large amount of heat energy (concentrating 1 ton of 5% NaOH solution to 30% requires about 80 kg of standard coal, costing more than 60 yuan); while encapsulated sodium is tightly encapsulated by red mud particles, making it difficult for water to penetrate, resulting in a recovery rate of less than 50%. More importantly, sodium silicate and sodium alkali interfere with each other during the separation process. If sodium alkali is extracted first, strong acid or high temperature conditions will cause sodium silicate to decompose simultaneously, generating silica precipitate that clogs equipment pipes. If sodium silicate is extracted first, excess acid (such as sulfuric acid) needs to be added to convert sodium silicate into silica gel, which will react with sodium alkali to generate byproducts such as sodium sulfate, making it impossible to recycle sodium alkali. This "mutual constraint" characteristic puts the separation and extraction of sodium silicate and sodium alkali in a dilemma of "either low purity or high cost," making it impossible to achieve an economic breakthrough.

[0006] II. Root Cause of the Problem: The inherent characteristics of the Bayer process result in an "inherently complex" composition of red mud. A deeper exploration of the root causes of these core difficulties reveals that their essence lies in the inherent characteristic of the Bayer process: "sacrificing impurity separation for efficient alumina extraction." The Bayer process, currently the most widely used alumina production process globally (accounting for over 90% of global alumina production), operates on the principle of selectively dissolving alumina by utilizing the difference in solubility between alumina and other components in strongly alkaline solutions. In high-temperature, high-pressure sodium hydroxide solutions, alumina in bauxite reacts with NaOH to form soluble sodium aluminate, while components such as iron and titanium, which do not react with NaOH, precipitate directly, forming the "skeleton components" of red mud. However, the presence of silicon disrupts this balance of selective dissolution—silicon reacts with NaOH to form soluble sodium silicate, which in turn reacts rapidly with sodium aluminate in the solution to form a sodium polyaluminate complex.

[0007] The formation of this complex leads to two main problems. First, it results in the loss of sodium aluminate—some of it precipitates into the red mud along with the complex, resulting in an alumina content of 16-28% in the red mud (equivalent to a loss of approximately 50-100 kg of alumina per ton of alumina produced due to silicon interference). Second, it "fixes" silicon in the red mud, forming stable complexed silicon, which greatly complicates the subsequent separation and extraction of sodium silicate. Furthermore, this process characteristic causes sodium alkali to exist in the red mud in a multi-stage form—compound, adsorbed, and encapsulated—further increasing the complexity of sodium alkali recovery.

[0008] It can be said that the design goal of the Bayer process is to "maximize alumina extraction" rather than "optimize impurity separation." This inherently means that red mud, from its formation, possesses characteristics of "high alkalinity, high complexity, and the coexistence of resources and impurities." This inherent deficiency makes it difficult for subsequent red mud treatment technologies to circumvent the mutual interference of silicon, aluminum, and sodium, ultimately leading to high costs in the sodium silicate and sodium alkali recovery stages and failing to achieve economic benefits. Therefore, to overcome the bottleneck in the resource utilization of red mud, it is necessary not only to innovate in separation and extraction technologies but also to start from the source of the process and explore a new model of "integrated Bayer process optimization and red mud pretreatment" to fundamentally reduce the difficulty of red mud treatment. Summary of the Invention

[0009] The technical problem to be solved by this invention is to overcome the shortcomings of the existing technology and provide a high-value resource recovery process for all elements of red mud, which constitutes a complete red mud dealkalization and all-element recovery system. The goal is to achieve, through reasonable process design, the comprehensive recovery of valuable components, effective connection with market demand, sharing of huge investment risks, maximization of economic benefits and complete eradication of environmental pollution, thereby ensuring the economic rationality of project investment.

[0010] The technical solution of this invention is as follows: The process for high-value resource recovery of all elements from red mud includes the following steps: After the S1 red mud slurry is washed by a double spiral washing machine, it is concentrated by a thickener. The bottom slurry enters the leaching equipment and reacts with the ammonium chloride saturated solution to generate ammonia and chloride salt solution, thus completing the dealkalization treatment of the red mud slurry. After S2 dealkali treatment, the red mud slurry is classified by hydrocyclone to separate materials with a mesh size of 600 or larger and materials with a mesh size of 600 or smaller. The material with a mesh size of 600 or larger separated by S3 is subjected to flotation to extract silicon components. After flotation, it undergoes solid-liquid separation. The separated solid material is heated and modified to recover iron elements, resulting in aluminum-rich material. The separated liquid material is filtered and adsorbed by resin to obtain divalent salt water, sodium chloride, and water. The vanadium, scandium, and gallium elements adsorbed by the resin are desorbed. The material separated by S4 with a mesh size of less than 600 is then reselected to obtain material enriched with silicon, iron, aluminum, titanium, vanadium, heavy metals and rare elements. In step S5, the overflow liquid from the thickener in S1 undergoes solid-liquid separation. The separated liquid material is then filtered and adsorbed by resin to obtain divalent salt water and sodium chloride. The adsorbed rare elements such as vanadium, scandium, and gallium are desorbed after the resin adsorption. The material filtered in step S5 (S6) enters a primary dual-stage membrane electrodialysis unit to decompose and produce electronic-grade silica sol and high-purity sodium hydroxide solution.

[0011] Preferably, in step S1, the thickener is a three-layer thickener.

[0012] Preferably, in step S1, the generated ammonia gas enters a phosphoric acid absorption tower for treatment.

[0013] Preferably, in step S1, the leaching equipment operates under negative pressure.

[0014] Preferably, in step S3, the temperature for heat modification is 250-400℃.

[0015] Preferably, in step S3, the separated liquid material is subjected to the following filtration processes in sequence: precision filtration, microfiltration, ultrafiltration, sodium filtration, and reverse osmosis.

[0016] Preferably, in step S5, the separated liquid material undergoes the following filtration processes in sequence: precision filtration, microfiltration, ultrafiltration, and sodium filtration, with resin adsorption occurring after precision filtration and before microfiltration.

[0017] Preferably, the sodium chloride obtained in steps S3 and S5 is concentrated and evaporated before entering a two-stage double-membrane electrodialysis device to decompose and produce high-purity hydrochloric acid and high-purity sodium hydroxide solution.

[0018] Preferably, the calcium chloride in the divalent salt obtained in steps S3 and S5 is reacted with ammonium carbonate to produce calcium carbonate, which is then calcined to obtain calcium oxide. The remaining ammonium chloride in the divalent salt is regenerated and reused in step S1.

[0019] Compared with the prior art, the present invention has the following advantages: 1. Precisely solve the core problems of red mud treatment, and achieve high-purity and economical separation of sodium silicate and sodium alkali. Addressing the core pain points of existing technologies, namely the strong bond between sodium silicate and red mud matrix and the large loss due to sodium alkali dispersion, this invention achieves a breakthrough through multi-stage process synergy: It employs countercurrent water washing combined with thickener pretreatment to efficiently dissolve adsorbed and free sodium silicate and sodium alkali from the red mud, with the washing concentration controlled at 2-5% and the concentrated pH > 9, avoiding the low extraction rate problem of traditional room-temperature leaching methods; subsequently, leaching is performed using a saturated ammonium chloride solution under negative pressure, utilizing a chemical reaction to convert the complexed sodium silicate and sodium alkali into a separable chloride solution and ammonia gas, completely breaking the stable structure of the sodium aluminum silicate complex. This significantly improves the sodium silicate extraction rate and reduces the content of aluminum and iron impurities in the leachate, meeting subsequent processing requirements without the need for multiple purification steps. This solves the economic dilemma of traditional processes where "purification costs exceed 800 yuan / ton, higher than the market price," achieving the targeted separation and high-value utilization of silicon and sodium elements.

[0020] 2. Construct a comprehensive recycling system to significantly improve resource utilization and reduce the pollution risk of solid waste storage. Compared to the current situation where "the global utilization rate of red mud is less than 10%, and more than 90% relies on stockpiling," this invention achieves the resource utilization of all components of red mud through "graded treatment + targeted recycling": A hydrocyclone separates the dealkalized red mud into materials of 600 mesh or smaller. Materials larger than 600 mesh undergo flotation for silicon extraction and heating modification for iron extraction; the resulting aluminum-rich material can be returned to the Bayer process system, solving the problem of resource waste. Materials smaller than 600 mesh are enriched for silicon, iron, aluminum, titanium, and rare earth elements through gravity separation, avoiding the loss of valuable elements caused by traditional stockpiling. Simultaneously, the thickener overflow liquid is filtered and adsorbed with resin to recover vanadium, scandium, gallium, and other rare earth elements. Ultimately, the utilization rate of red mud solid waste is increased, significantly reducing the land occupied by stockpiling, and strongly alkaline substances (sodium alkali, sodium silicate) are converted into harmless products, completely eliminating the environmental risk of strongly alkaline leachate polluting soil and groundwater, realizing the transformation of red mud from "harmful waste residue" to "artificial composite mineral."

[0021] 3. Innovative process pathways enhance product added value, significantly improve economic efficiency, and reduce investment risks. Addressing the issue of existing technologies having processing costs exceeding 500 RMB / ton and difficulty in achieving large-scale profitability, this invention achieves a breakthrough in economic benefits through process upgrades and high-value product design: A single-stage, two-layer membrane electrodialysis device directly converts the purified sodium silicate solution into electronic-grade silica sol, a product with a market value exceeding 20,000 RMB / ton, far surpassing the commercial value of traditional sodium silicate (approximately 600 RMB / ton). Simultaneously, the ammonia gas generated during leaching is used in a phosphoric acid absorption tower to produce ammonium phosphate fertilizer (priced at over 3,000 RMB / ton), and the recovered high-purity sodium hydroxide can be directly reused in Bayer process production or sold externally, forming a "high-value-added product matrix." Furthermore, the hot water countercurrent washing and negative pressure leaching technologies used in the process have lower energy consumption compared to traditional high-temperature roasting methods (requiring temperatures above 800℃); and the ammonium chloride solution can be partially regenerated through subsequent processes, further controlling reagent costs. Therefore, this invention transforms red mud treatment from a "high-cost burden" into a "profitable process," effectively mitigating project investment risks and shortening the expected investment payback period to 2-3 years, providing alumina companies with an economically feasible red mud treatment solution. Attached Figure Description

[0022] Figure 1 This is a flowchart of the high-value resource recovery process for all elements of red mud according to the present invention. Detailed Implementation

[0023] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention.

[0024] Example 1 like Figure 1 As shown, the red mud full-element high-value resource recovery process of this embodiment includes the following steps: S1 Hot water countercurrent washing, ammonium chloride dealkali removal and ammonia recovery After being washed by a double-spiral mud washing machine (with the concentration controlled at 2-5% during washing), the red mud slurry is concentrated by a three-layer thickener, with the pH controlled at >9. After countercurrent washing and concentration using hot water below 85℃, the underflow slurry enters the leaching equipment, where it reacts with a saturated ammonium chloride solution to generate ammonia and chloride salt solution, completing the dealkalization treatment of the red mud slurry. The leaching equipment operates under negative pressure, promoting the reaction and efficiently releasing ammonia. The ammonia then enters a phosphate absorption tower for further treatment, directly producing high-value ammonium phosphate fertilizer.

[0025] This step aims to dissolve and separate soluble sodium silicate and sodium hydroxide from the red mud.

[0026] S2 hydrocyclone classification After dealkalization, the red mud slurry is classified by hydrocyclone to separate materials with a mesh size of 600 or larger (fine particles, mainly fine mud and hydroxides) and materials with a mesh size of less than 600 (coarse particles, mainly large mineral particles).

[0027] This stage, by first dissolving and recovering soluble silicate and then classifying the solid phase, fundamentally avoids the problem of incomplete dealkali removal caused by sodium silicate gelation, achieves economical and efficient separation of sodium silicate and sodium hydroxide, and solves the problem of bidirectional removal of alkali metals and alkaline earth metals.

[0028] S3 Fine Particulate Material Processing The separated material with a mesh size of 600 or larger is subjected to flotation to extract the silicon component. After flotation, it undergoes solid-liquid separation. The separated solid material is heated to 300°C for modification, followed by air and magnetic separation to extract the iron element, resulting in an aluminum-rich material. Approximately 1 ton of alumina can be extracted from 4.5-5 tons of aluminum-rich material, which is then returned to the Bayer process system for aluminum recovery. The separated liquid material undergoes sequential processing including precision filtration, resin adsorption, microfiltration, ultrafiltration, sodium filtration, and reverse osmosis to obtain divalent brine, sodium chloride, and water. The resin adsorption process desorbs adsorbed vanadium, scandium, and gallium elements.

[0029] S4 Coarse Particle Material Processing The separated materials below 600 mesh are then subjected to gravity separation to effectively separate and enrich materials containing silicon, iron, aluminum, titanium, vanadium, heavy metals, and rare elements.

[0030] High-value utilization of S5 silica-alkali overflow liquid In step S1, the overflow liquid from the thickener undergoes solid-liquid separation. The separated liquid material is then subjected to precision filtration, resin adsorption, microfiltration, ultrafiltration, and sodium filtration to obtain divalent brine and sodium chloride. The resin adsorption process desorbs adsorbed rare elements such as vanadium, scandium, and gallium. After this purification step, the raw material meets the standards for producing silica sol.

[0031] S6 Bipolar Membrane Electrodialysis The purified material from step S5 is passed into a single-stage double-membrane electrodialysis device to decompose and produce two high-purity products: high-purity silica sol and high-purity sodium hydroxide solution.

[0032] Among them, silica sol can be concentrated through membrane filtration and low-temperature evaporation at temperatures below 40°C to prepare a series of high-purity silica sol products with concentrations of 20%, 30%, 40%, and 50%. Electronic-grade silica sol can fetch over 20,000 yuan per ton. High-purity sodium hydroxide can be directly reused or sold as a product.

[0033] This stage upgrades the traditional red mud dealkali removal process into a high-value-added industry that recovers high-purity electronic-grade silica sol and high-purity sodium hydroxide, which is the key to opening the door to red mud resource utilization.

[0034] Example 2 Based on Example 1, such as Figure 1 As shown, in this embodiment, the sodium chloride obtained in steps S3 and S5 is concentrated and evaporated before being fed into a two-stage double-membrane electrodialysis device to decompose and produce high-purity hydrochloric acid and high-purity sodium hydroxide solutions. Simultaneously, the calcium chloride in the divalent salts obtained in steps S3 and S5 reacts with ammonium carbonate to produce calcium carbonate, which is then calcined to obtain calcium oxide. The remaining ammonium chloride in the divalent salts is regenerated and reused in step S1.

[0035] This invention achieves outstanding economic efficiency through product value enhancement and material recycling: High-value products, such as electronic-grade silica sol (priced at over 20,000 yuan per ton), ammonium phosphate fertilizer (priced at over 3,000 yuan per ton), high-purity sodium hydroxide, and enriched rare elements, constitute diversified profit points.

[0036] Low-cost core reagent: The core chemical raw material ammonium chloride (priced below 200 yuan / ton) can be recycled within the system, and the consumption of red mud is only 0.1-0.25 tons per ton, making the operating cost controllable.

[0037] Although the initial investment is substantial, thanks to comprehensive element recovery and a high-value-added product matrix, the investment payback period is expected to be controllable within 2-3 years, making it economically feasible. The process of this invention fully demonstrates that the commercial value of red mud far exceeds that of primary bauxite, representing a "free" artificial composite mineral resource that urgently needs development.

[0038] In summary, this invention completely solves the problems of sodium silicate recovery, sodium hydroxide removal and conversion into high-purity sodium hydroxide, and the full-component decomposition, separation, and enrichment of valuable elements (silicon, aluminum, iron, titanium, vanadium, heavy metals, and rare earth elements) in red mud slurry. Ultimately, it achieves the full-element utilization of red mud, especially the enrichment of alkali-soluble rare earth elements, making its commercial value far exceed that of bauxite, and providing a practical and feasible industrial path for the harmless, resource-based, and high-value utilization of red mud.

Claims

1. A high-value resource recovery process for all elements of red mud, characterized in that, Includes the following steps: After the S1 red mud slurry is washed by a double spiral washing machine, it is concentrated by a thickener. The bottom slurry enters the leaching equipment and reacts with the ammonium chloride saturated solution to generate ammonia and chloride salt solution, thus completing the dealkalization treatment of the red mud slurry. After S2 dealkali treatment, the red mud slurry is classified by hydrocyclone to separate materials with a mesh size of 600 or larger and materials with a mesh size of 600 or smaller. The material with a mesh size of 600 or larger separated by S3 is subjected to flotation to extract silicon components. After flotation, it undergoes solid-liquid separation. The separated solid material is heated and modified to recover iron elements, resulting in aluminum-rich material. The separated liquid material is filtered and adsorbed by resin to obtain divalent salt water, sodium chloride, and water. The vanadium, scandium, and gallium elements adsorbed by the resin are desorbed. The material separated by S4 with a mesh size of less than 600 is then reselected to obtain material enriched with silicon, iron, aluminum, titanium, vanadium, heavy metals and rare elements. In step S5, the overflow liquid from the thickener in S1 undergoes solid-liquid separation. The separated liquid material is then filtered and adsorbed by resin to obtain divalent salt water and sodium chloride. The vanadium, scandium, and gallium elements adsorbed by the resin are desorbed after adsorption. The material filtered in step S5 (S6) enters a primary dual-stage membrane electrodialysis unit to decompose and produce silica sol and sodium hydroxide solution.

2. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, In step S1, a three-layer thickener is used.

3. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, In step S1, the generated ammonia gas enters the phosphoric acid absorption tower for treatment.

4. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, In step S1, the leaching equipment operates under negative pressure.

5. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, In step S3, the temperature for heat modification is 250-400℃.

6. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, In step S3, the separated liquid material undergoes the following filtration processes in sequence: precision filtration, microfiltration, ultrafiltration, sodium filtration, and reverse osmosis.

7. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, In step S5, the separated liquid material undergoes the following filtration processes in sequence: precision filtration, microfiltration, ultrafiltration, and sodium filtration. Resin adsorption is performed after precision filtration and before microfiltration.

8. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, The sodium chloride obtained in steps S3 and S5 is concentrated and evaporated before entering a two-stage double-membrane electrodialysis device to decompose and produce hydrochloric acid and sodium hydroxide solutions.

9. The red mud full-element high-value resource recovery process as described in claim 1, characterized in that, The calcium chloride in the divalent salt obtained in steps S3 and S5 reacts with ammonium carbonate to produce calcium carbonate, which is then calcined to obtain calcium oxide. The remaining ammonium chloride in the divalent salt is regenerated and reused in step S1.

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