Method and system for preparing fly ash of white carbon black and aluminum oxide through cooperation of waste heat steam

Abstract

The invention discloses a method and a system for preparing white carbon black aluminum oxide fly ash through waste heat steam cooperation, and relates to the field of waste recycling chemical industry. Fly ash at the present stage is rich in SiO2 / Al2O3, but existing alkaline leaching, carbon content, causticization, evaporation and aluminothermy / alkali / water production loops are dispersed, energy consumption is high, salt discharge is large, and quality fluctuates. According to the invention, 0.6 MPa waste heat steam is used for unified heat supply, and a Na-series closed circuit is constructed to cooperate with CO2; short-time desiliconization, deep cone and leaf filter purification are carried out on alkali slurry at the temperature of not less than 135 DEG C; carrying out continuous carbon separation and acid precipitation to prepare white carbon black; causticizing mother liquor lime milk, and performing multi-effect evaporation and concentration; and performing low A / S sintering, dissolution, aluminum precipitation and roasting on the desilicication slag to obtain aluminum oxide. The method has the practical effects of realizing gradient utilization of steam, reducing alkali consumption, discharging salt, stabilizing quality, synergistically increasing yield, and considering both environmental protection and economical efficiency. The system is composed of a blending / leaching unit, a solid-liquid separation unit, a carbon separation unit, an acid precipitation unit, a drying unit, a causticization unit, an evaporation unit, a back blending unit, an aluminum production unit, a steam main pipe, lime kiln CO2 and a DCS monitor.

Description

Technical Field

[0001] This invention relates to the field of waste resource utilization and chemical industry, specifically to a method and system for co-producing precipitated silica and alumina from fly ash using waste heat steam. Background Technology

[0002] Fly ash is rich in SiO2 and Al2O3. Existing resource utilization pathways involve alkali leaching, separation, carbonization, acid precipitation, and drying to produce precipitated silica (white carbon black), with the desiliconized slag then incorporated into a low-alumina-to-silicon ratio sintering line to produce alumina. A key step is heating the alkali slurry to approximately 135°C with steam for short-term leaching. Subsequent solid-liquid separation is achieved through deep cone settling and pressure filtration, followed by continuous CO2 carbonation, acid precipitation, and spray drying to obtain the precipitated silica product. At the mother liquor end, lime slurry is used for causticization, coupled with multi-effect evaporation and segmented salt removal, concentrating and recycling to create a closed-loop Na-system circulation. Simultaneously, the leaching residue is incorporated into the sintering, leaching, alumina precipitation, and calcination stages to co-produce alumina. If the power plant can obtain approximately 0.6 MPa of waste heat steam and CO2 from lime kiln exhaust gas, the process chain becomes more economical and sustainable.

[0003] However, existing technologies suffer from problems such as dispersed heat sources and insufficient coupling between material circuits. If high-alkali and high-temperature units such as carbonization, causticization, and evaporation lack unified closed-loop control and parameter window management, it can easily lead to increased alkali consumption and salt discharge load, exacerbated scaling and corrosion, and difficulty in graded water reuse. This, in turn, can cause fluctuations in silica performance and instability in alumina line coordination, hindering large-scale production and economic efficiency. In summary, these shortcomings urgently need improvement. Summary of the Invention

[0004] The main objective of this invention is to provide a method and system for the co-production of silica and alumina from fly ash using waste heat steam, which can effectively solve the problems in the background art.

[0005] To achieve the above objectives, the present invention provides a technical solution: a method for co-producing silica and alumina from fly ash using waste heat steam, comprising the following steps: S1: Fly ash is mixed with alkaline solution to form an alkaline slurry, and the alkaline slurry is heated to above 135°C using waste heat steam at 0.53–0.60 MPa and held for no more than 20 minutes to carry out a short-term high-temperature desilication reaction; S2: Low-modulus sodium silicate solution is separated by deep cone sedimentation and pressure filtration; S3: CO2 gas with a volume fraction ≥36% and a temperature ≥80℃ is continuously introduced to carbonate and decompose the low-modulus sodium silicate solution for 90–120 minutes, and crude silica is precipitated. S4: Retain part of the carbonation reaction mother liquor, mix and dissolve the obtained crude silica with the retained liquor to adjust the modulus of sodium silicate solution, form a carbonation reaction section under high modulus conditions, and then introduce CO2 gas again to react and generate crude silica slurry. S5: The crude silica slurry is subjected to acid precipitation treatment, followed by water washing and drying to obtain silica product; S6: The carbonated mother liquor is added to lime milk for causticization treatment, and then concentrated by multi-effect evaporation to a Na2O content of about 346 g / L. The resulting concentrated alkaline solution is recycled for the preparation of the alkaline slurry. S7: The desiliconized leaching residue is fed into the lime-sintering alumina production line to co-produce alumina products.

[0006] Furthermore, in step S2, the obtained sodium silicate leaching solution is divided into two parts, of which approximately 74% of the sodium silicate solution is used for continuous CO2 introduction for carbonation precipitation, and the remaining approximately 26% of the sodium silicate solution is used as a retention solution for re-dissolution to adjust the modulus.

[0007] Furthermore, in step S5, the pH of the crude silica slurry is adjusted from about 9 to about 5.5 using dilute sulfuric acid, and the precipitation reaction is maintained at a temperature not lower than 90°C for 20–30 minutes.

[0008] Further, in step S4, lime milk with a CaO content of about 180 g / L is added to the carbonated mother liquor to make the [CaO] / [Na2O] ratio about 1.20, and the reaction is carried out at 95–105 °C for about 2 hours.

[0009] Furthermore, the S6 process employs a two-stage falling film evaporation process, with a salt removal device installed after each stage of evaporation to remove crystalline salts. The final evaporation is carried out under a vacuum of 0.080–0.088 MPa, and the Na2O content of the concentrated alkaline solution is approximately 346 g / L.

[0010] The present invention also provides a technical solution: a fly ash system for the co-production of silica and alumina from waste heat steam, comprising the following: The alkaline slurry heating leaching unit consists of a raw ash silo, a mixing tank, a steam heater, and a leaching reactor. A primary solid-liquid separation unit consisting of a deep cone settling device and a filtration device; a silica synthesis unit consisting of a continuous carbonation reaction tank, an acid settling tank, and a spray drying system; and a closed-loop alkali recovery unit consisting of a causticizing device, a causticizing liquid separation equipment, a multi-effect falling film evaporation station, a salt discharge module, and a return mixing tank. Alumina co-production unit consisting of lime, sintering, and calcination; It also includes utilities such as CO2 supply branches, 0.6MPa steam headers, DCS / PLC control units, and online analysis equipment.

[0011] Furthermore, the CO2 supply branch is connected to the flue gas duct of the lime calcining furnace to utilize the tail gas of the lime furnace as a gas source for the carbonation reaction.

[0012] Furthermore, the leaching reactor is a shell-and-tube or tubular structure and is connected to the 0.6MPa steam header via a pipeline to provide heating steam to the leaching reactor.

[0013] Furthermore, the overflow port of the deep cone settling device of the primary solid-liquid separation unit is connected to a leaf filter to purify the obtained sodium silicate solution; the underflow port of the deep cone settling device is connected to a filter press to separate the leaching residue solids.

[0014] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes waste heat steam from power plants as a unified heat source to achieve a cascaded coupling of leaching, evaporation, and drying. Condensate is reused in stages, reducing unit steam consumption and fresh water replenishment. A closed-loop Na-based system is constructed, featuring causticization, multi-effect evaporation, segmented salt discharge, and reprocessing. This system inhibits impurity accumulation, reduces alkali consumption and external salt discharge, and alleviates scaling and corrosion. A dual-modulus route, employing continuous CO2 carbon fractionation combined with remelting and modulus adjustment, stabilizes the specific surface area and structure of silica, reduces batch-to-batch variation, and directly connects desiliconized slag to lime, sintering, leaching, alumina precipitation, and calcination processes. This enables synergistic production of silica and alumina, increases the output value per ton of ash, and improves the plant's operating rate. Closed-loop control of the entire process is implemented through key parameter windows and online monitoring, reducing energy consumption fluctuations and extending the continuous operation cycle. It is highly adaptable to ash source fluctuations and easily adaptable for industrial park transformation and large-scale replication. Simultaneously, it reduces wastewater salt and alkalinity discharge and utilizes tail gas CO2 as a carbon source, achieving significant economic and environmental benefits. Detailed Implementation

[0015] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed merely to enable those skilled in the art to better understand and implement the subject matter described herein, and are not intended to limit the scope, applicability, or examples set forth in the claims. The function and arrangement of the elements discussed may be changed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the various examples. For example, the described methods may be performed in a different order than described, and steps may be added, omitted, or combined. Furthermore, features described in some examples may be combined in other examples.

[0016] As used herein, the term "comprising" and its variations are open terms meaning "including but not limited to". The term "based on" means "at least partially based on". The terms "one embodiment" and "an embodiment" mean "at least one embodiment". The term "another embodiment" means "at least one other embodiment". The terms "first", "second", etc., may refer to different or the same objects. Other definitions, whether explicit or implicit, may be included below. Unless explicitly indicated by the context, the definition of a term shall remain consistent throughout the specification.

[0017] Example 1 This invention provides a technical solution: a method for co-producing silica and alumina from fly ash using waste heat steam, comprising the following steps: S1: Fly ash is mixed with alkaline solution to form an alkaline slurry, and the alkaline slurry is heated to above 135°C using waste heat steam at 0.53–0.60 MPa and held for no more than 20 minutes to carry out a short-term high-temperature desilication reaction; S2: Low-modulus sodium silicate solution is separated by deep cone sedimentation and pressure filtration; S3: CO2 gas with a volume fraction ≥36% and a temperature ≥80℃ is continuously introduced to carbonate and decompose the low-modulus sodium silicate solution for 90–120 minutes, and crude silica is precipitated. S4: Retain part of the carbonation reaction mother liquor, mix and dissolve the obtained crude silica with the retained liquor to adjust the modulus of sodium silicate solution, form a carbonation reaction section under high modulus conditions, and then introduce CO2 gas again to react and generate crude silica slurry. S5: The crude silica slurry is subjected to acid precipitation treatment, followed by water washing and drying to obtain silica product; S6: The carbonated mother liquor is added to lime milk for causticization treatment, and then concentrated by multi-effect evaporation to a Na2O content of about 346 g / L. The resulting concentrated alkaline solution is recycled for the preparation of the alkaline slurry. S7: The desiliconized leaching residue is fed into the lime-sintering alumina production line to co-produce alumina products.

[0018] Furthermore, in step S2, the obtained sodium silicate leaching solution is divided into two parts, of which approximately 74% of the sodium silicate solution is used for continuous CO2 introduction for carbonation precipitation, and the remaining approximately 26% of the sodium silicate solution is used as a retention solution for re-dissolution to adjust the modulus.

[0019] Furthermore, in step S5, the pH of the crude silica slurry is adjusted from about 9 to about 5.5 using dilute sulfuric acid, and the precipitation reaction is maintained at a temperature not lower than 90°C for 20–30 minutes.

[0020] Further, in step S4, lime milk with a CaO content of about 180 g / L is added to the carbonated mother liquor to make the [CaO] / [Na2O] ratio about 1.20, and the reaction is carried out at 95–105 °C for about 2 hours.

[0021] Furthermore, the S6 process employs a two-stage falling film evaporation process, with a salt removal device installed after each stage of evaporation to remove crystalline salts. The final evaporation is carried out under a vacuum of 0.080–0.088 MPa, and the Na2O content of the concentrated alkaline solution is approximately 346 g / L.

[0022] An example of a method for simultaneously preparing silica and alumina from fly ash, the specific steps of which are as follows: Raw material preparation: Dry power plant fly ash is preheated to approximately 100°C and mixed with recycled alkaline mother liquor to prepare an alkaline slurry. The mother liquor used is the concentrated alkaline solution obtained from the previous recycling step. The temperature of the prepared alkaline slurry is approximately 70°C, and the alkaline concentration (Na₂O) is approximately 346 g / L. The mixing ratio is controlled at approximately 1.3 mg / L per ton of dry fly ash. 3 Mother liquor.

[0023] Alkali leaching for desilication: The above-mentioned alkaline slurry is heated to approximately 135°C in a closed reaction vessel using waste heat steam from the plant. The waste heat steam pressure is approximately 0.6 MPa, and the slurry is heated through a shell-and-tube heat exchanger. The reaction is maintained at this temperature for approximately 20 minutes, during which SiO2 in the fly ash is dissolved by a high-concentration NaOH solution, generating a silicon-containing alkaline solution. After the desilication reaction is complete, the SiO2 leaching rate in the slurry is high, and the active silicon in the fly ash is essentially absorbed into the liquid phase.

[0024] Solid-liquid separation: The slurry after the desilication reaction is separated into solids and liquids through sedimentation and filtration. First, the slurry enters a deep cone settling tank for clarification, where solids settle and separate. The supernatant obtained from sedimentation is further filtered and purified by a vertical leaf filter to remove suspended solids. The purified sodium silicate solution is clear and transparent, separating into two streams: a 74% sodium silicate solution is used as raw material for subsequent low-modulus carbonation, while the 26% sodium silicate solution is temporarily reserved for later use. The dense slurry at the bottom of the settling tank is dewatered using a filter press, and the filter cake is the desilication ash, with a moisture content of approximately 40%, in which aluminum is mainly retained in the solid phase in the form of aluminosilicates. This desilication ash filter cake is washed and sent to the alumina production system for later use.

[0025] Continuous Carbonation: The 74% low-modulus sodium silicate solution is fed into a continuous carbonation reactor, where carbon is decomposed using CO2 gas. The carbonation reaction is carried out in a series of mechanically stirred reactors. CO2 gas with a volume fraction of approximately ≥36% is introduced at the inlet, and the reaction temperature is controlled at ≥80℃. The carbonation time is approximately 90–120 minutes to ensure that SiO2 in the sodium silicate precipitates as much as possible. Through the reaction of CO2 with sodium silicate, silica slurry and sodium carbonate solution are generated, and the residual SiO2 concentration in the reaction solution can be reduced to ≤2g / L. The carbonated slurry is subjected to solid-liquid separation by a filter press. The filter residue is the crude silica wet filter cake; the filtrate is the low-modulus carbon mother liquor, rich in Na2CO3 and a small amount of NaOH, which is subsequently used for causticization to recover alkali. The crude silica filter cake is washed once with water to reduce the alkali content, and then transferred to the next step of processing.

[0026] Secondary dissolution: The wet filter cake of crude silica obtained from the first carbon fractionation is immediately added to the reconstitution reaction tank, along with the previously retained 26% sodium silicate solution. The mixture is stirred and mixed at approximately 90°C to allow the crude silica to redissolve or fully react. This process generates a sodium silicate solution with a higher modulus. By adjusting the ratio of crude silica to the desilication solution, the modulus of the high-modulus sodium silicate solution is controlled to be approximately 2.0, which is beneficial for the subsequent preparation of high-quality silica. The high-modulus sodium silicate solution obtained from the reconstitution is clarified and separated into solid and liquid components using a leaf filter to remove undissolved matter, yielding a clarified sodium silicate concentrate for later use.

[0027] Secondary Carbonation and Precipitation of Silica: The high-modulus sodium silicate concentrate is fed into another set of continuous carbonation reaction tanks, and CO2 is introduced for a second carbonation decomposition. The carbonation reaction temperature is controlled at no less than 80℃, and the time is about 90–120 minutes, generating a slurry containing colloidal silica. Subsequently, the carbonation slurry is immediately separated into solid and liquid components by a filter press, and the filter residue is a fine silica wet cake containing alkali. To improve the purity and performance of silica, the wet filter cake needs to be acid-precipitated: the wet filter cake is put into an acid precipitation reaction tank, an appropriate amount of dilute sulfuric acid is added, and the mixture is stirred at ≥90℃ for 20–30 minutes to lower the pH from about 9 to 5.5. During the acid precipitation process, the silica is further purified, and impurity metal ions are removed. After acid precipitation, the silica wet filter cake is separated again by filtration and repeatedly washed with hot water until the filtrate is nearly neutral.

[0028] Preparation of silica product by drying: The wet silica cake after acid precipitation and washing is sent to a spray dryer for drying, and the outlet temperature of the drying medium is controlled at 105±2℃. After drying, the finished silica product is obtained as a loose white powder, the main component of which is SiO2, with a loss on ignition ≤7%, and the quality meets the requirements of industrial grade silica.

[0029] Causticization alkali recovery: The low-modulus carbon mother liquor obtained in step 4 is fed into a causticization reaction tank and reacted with slaked lime slurry to recover and regenerate NaOH. The lime slurry is made from lime powder and water at a CaO concentration of approximately 180 g / L. The causticization reaction is carried out at ≥90℃ for about 2 hours, with the amount of lime slurry added controlled according to a CaO:Na2O ratio of 1.20:1. In the causticization tank, Na2CO3 reacts with Ca(OH)2 to generate NaOH and CaCO3 precipitates. After causticization, the slurry undergoes solid-liquid separation using a filter press: the filter residue is CaCO3 produced by causticization, containing a small amount of unreacted lime and impurities, and is immediately sent to the alumina production system after filtration as part of the sintering raw materials; the filtrate is a high-concentration NaOH solution. Calculations show that the causticization reaction efficiency can reach over 85%. The causticization liquid has essentially regenerated NaOH, but it still contains certain impurities and excess Na2CO3, requiring further concentration and impurity removal before recycling.

[0030] Multi-effect evaporation and salt separation: The causticized alkali solution is fed into a multi-effect evaporation system for concentration and purification. The first stage of evaporation takes place in a four-effect falling film evaporator, utilizing the waste heat of multi-stage steam to gradually concentrate the causticized solution, and discharging some of the separated impurities through a settling tank. After the first stage of evaporation, the Na₂O concentration of the alkali solution increases from approximately 56 g / L to ~300 g / L. It then enters the second stage, a double-effect forced circulation evaporator for further concentration, ultimately concentrating the causticized mother liquor to approximately Na₂O ≈ 346 g / L, comparable to the initial leaching alkali concentration. The final effect of the second stage evaporation also includes a crystallization tank and centrifuge / filtration equipment to force the precipitation of the enriched impurities. The entire evaporation process is carried out under reduced pressure, with a final vacuum of approximately 0.08–0.09 MPa to reduce evaporation temperature and save energy. The separated crystalline salts, after washing, can be properly disposed of as by-product soda ash or waste. The condensate obtained from evaporation is collected in stages. A portion is reused as qualified process water for preparing the bottom liquor for silica leaching and product washing, while the other portion, containing excessive impurities, is used as unqualified water for processes such as red mud washing, thus achieving water recycling. Finally, the concentrated caustic alkali solution obtained from the evaporation system is returned to the raw material slurry mixing tank and mixed with fresh fly ash, completing the alkali solution cycle. In addition, by externally supplementing a small amount of commercial liquid alkali to compensate for system alkali consumption losses, the required amount of caustic alkali for the entire silica leaching-precipitation cycle can be maintained.

[0031] Aluminum production by sintering desiliconized ash: The desiliconized ash filter cake obtained in step 3 is used for alumina extraction. The typical chemical composition of desiliconized ash is approximately 48.0% Al2O3, 30.7% SiO2, 3.2% Fe2O3, and 4.4% CaO, with an aluminum-silicon ratio (A / S) of approximately 1.56. Since the A / S ratio for alumina extraction solely from desiliconized ash is too low, this embodiment employs a low A / S ratio sintering process to co-process desiliconized ash and other auxiliary materials. Specifically, desiliconized ash is mixed with limestone powder and high-iron bauxite in a certain proportion, and a small amount of bituminous coal is added as an internal heat source to form a uniform raw meal. Limestone provides CaO additives, and high-iron bauxite supplements and increases the A / S ratio of the mixture, thereby improving the sintering reaction conditions. In addition, the aforementioned causticized slag is also mixed into the raw meal, serving both as a calcareous raw material and realizing the utilization of waste residue. After molding, the prepared raw meal is sent to a sintering furnace for calcination. Sintering employs a rotary kiln high-temperature calcination process, with the furnace temperature controlled within the range of 1250–1350℃. This allows Al₂O₃ in the material to react with added Na₂O to form soluble sodium aluminate, while the corresponding SiO₂ reacts with CaO to form a stable calcium silicate mineral phase. In this embodiment, the clinker calcination utilizes a low aluminum-to-silicon ratio sintering technology and is equipped with an automatic kiln-front fire monitoring control system to stabilize the high-temperature calcination conditions. After approximately one hour of high-temperature sintering, fly ash and auxiliary materials form sintered clinker. After being discharged from the kiln, the sintered clinker is rapidly cooled to ≤70℃, then pulverized and ground for use in aluminum leaching.

[0032] Clinker leaching and red mud separation: Sintered clinker powder is added to a leaching tank and leached with hot water or dilute alkaline solution to extract sodium aluminate from the clinker. The first leaching is carried out at near-boiling point temperature under normal pressure, allowing most of the sodium aluminate to dissolve in the liquid phase, while insoluble substances such as calcium silicate remain in the solid phase. The leaching slurry is then separated by solid-liquid separation equipment: a deep cone settling tank is used for liquid-solid separation of the red mud, and a polymeric flocculant is added to improve the settling performance of the red mud. The clarified sodium aluminate solution is separated, and the concentrated red mud slurry at the bottom of the settling tank is further filtered and washed; the filter cake is the red mud waste. The washing water is preferably recycled process water. The washed red mud has a low alkali content and can be discarded or reused. This red mud separation process adopts a continuous deep cone settling + pressure filtration combined process, which can achieve a solid content of 300–400 g / L in the red mud slurry and a solid content of ≤250 mg / L in the overflow clarified liquid.

[0033] Sodium aluminate solution desilication purification: The crude sodium aluminate solution obtained from sintering and leaching often contains a certain amount of silicon impurities. Direct decomposition would affect product quality, so pre-desilication purification is necessary before decomposition. First, the crude sodium aluminate solution undergoes pre-desilication at atmospheric pressure, followed by standing at a relatively high temperature for a certain time or adding a small amount of seed crystals to precipitate some SiO2. Then, the solution is sent to a pressure desilication reactor for further removal of silicon impurities under high pressure. Pressure desilication is usually carried out at a temperature ≥240℃ and a certain pressure, which generates gibbsite and desilication products. Finally, an appropriate amount of lime slurry is added to the desilicationd sodium aluminate solution for a second-stage deep desilication treatment. The lime slurry reacts with the SiO2 in the solution to generate stable solid phases such as hydrated calcium silicate and hydrated garnet, thereby reducing residual SiO2 to a very low level. Through the above two-stage desilication process (at atmospheric pressure and pressure), the SiO2 content in the refined sodium aluminate solution is significantly reduced, ensuring high-quality aluminum hydroxide is obtained from subsequent decomposition.

[0034] Aluminum hydroxide precipitation decomposition: The purified sodium aluminate solution is decomposed in batches or in separate streams to precipitate solid aluminum hydroxide. One portion of the concentrate is decomposed using a seed decomposition method: A suitable amount of aluminum hydroxide seed crystals is added to multiple decomposition tanks with gradually decreasing temperatures, promoting the precipitation of aluminum oxide ions into Al(OH)3 crystals. Another portion of the concentrate is decomposed using carbonation decomposition technology: CO2 gas is introduced to convert sodium aluminate into aluminum hydroxide and Na2CO3. Carbonation decomposition is carried out in a mechanically stirred tank; the CO2 can be obtained from the CO2-rich tail gas of a lime kiln, which is purified and directly bubbled into the solution for reaction. The aluminum hydroxide slurries produced by the two decomposition methods are combined and then separated into solid and liquid components using a filter press to obtain a wet aluminum hydroxide filter cake. The filter cake is washed several times with hot water to reduce the Na2O content to ≤0.5%. The mother liquors are collected separately: the seed mother liquor mainly contains NaOH / NaAlO2 and can be recycled; the carbonation mother liquor is mainly a Na2CO3 solution and can be incorporated into the causticizing process to recover caustic soda. The introduction of carbon separation technology in this step improves the aluminum precipitation rate and makes full use of the CO2 gas produced as a by-product of the process, thus achieving carbon recycling.

[0035] Alumina calcination: The obtained aluminum hydroxide filter cake is dried and then calcined in a calcination furnace to obtain alumina product. The calcination equipment adopts a suspension calcination furnace, which rapidly decomposes and dehydrates aluminum hydroxide to produce Al2O3 at a high temperature of approximately 1100℃. Suspension calcination has the characteristics of rapid heat transfer and short residence time, and can complete the dehydration and decomposition of aluminum hydroxide within a few seconds to tens of seconds, resulting in a white alumina powder. Ultimately, this embodiment can co-produce approximately 42,600 tons of silica and 98,800 tons of alumina annually from 200,000 tons of fly ash, realizing the comprehensive and efficient utilization of silicon and aluminum resources in fly ash.

[0036] Example 2 This invention also provides a technical solution: a fly ash system for the co-production of silica and alumina from waste heat steam, comprising the following: The alkaline slurry heating leaching unit consists of a raw ash silo, a mixing tank, a steam heater, and a leaching reactor. A primary solid-liquid separation unit consisting of a deep cone settling device and a filtration device; a silica synthesis unit consisting of a continuous carbonation reaction tank, an acid settling tank, and a spray drying system; and a closed-loop alkali recovery unit consisting of a causticizing device, a causticizing liquid separation equipment, a multi-effect falling film evaporation station, a salt discharge module, and a return mixing tank. Alumina co-production unit consisting of lime, sintering, and calcination; It also includes utilities such as CO2 supply branches, 0.6MPa steam headers, DCS / PLC control units, and online analysis equipment.

[0037] Furthermore, the CO2 supply branch is connected to the flue gas duct of the lime calcining furnace to utilize the tail gas of the lime furnace as a gas source for the carbonation reaction.

[0038] Furthermore, the leaching reactor is a shell-and-tube or tubular structure and is connected to the 0.6MPa steam header via a pipeline to provide heating steam to the leaching reactor.

[0039] Furthermore, the overflow port of the deep cone settling device of the primary solid-liquid separation unit is connected to a leaf filter to purify the obtained sodium silicate solution; the underflow port of the deep cone settling device is connected to a filter press to separate the leaching residue solids.

[0040] Combining the above methods and processes, a complete process flow system is constructed. This system mainly consists of the following unit equipment; the specifications, models, and configurations of each unit are selected based on the original process requirements: Raw material pretreatment unit: Includes fly ash storage and conveying equipment, as well as mother liquor recycling and slurry preparation devices. After preheating, the fly ash enters the slurry preparation tank and is mixed with circulating concentrated alkali mother liquor at a ratio of approximately 1.3 m³ / h. 3 The pulp is prepared by mixing alkali slurry at a ratio of 1 ton / ton. The pulp preparation tank is equipped with stirring and heat preservation functions to maintain the pulp temperature at approximately 70°C. Several alkali-resistant pumps and pipelines are provided to transport the prepared alkali slurry to the leaching reaction unit.

[0041] The leaching heating unit consists of a single-tube indirect heater and a high-pressure reaction tank. The tube heat exchanger uses a set of coaxial tube heaters with an outer tube diameter of Φ219×6mm and an inner tube diameter of Φ108×8–10mm, for a total heat exchange length of approximately 400m. Inside the heat exchanger, superheated steam at 0.6MPa releases heat in the outer tube, heating the slurry to the required 135℃ for the leaching reaction. The slurry then enters the high-pressure, sealed leaching reaction tank and is held at this temperature for 20 minutes to complete the leaching process. This unit is equipped with a safety valve and an automatic temperature control device to ensure strict control over the reaction temperature and time.

[0042] Sedimentation and Filtration Separation Unit: Used for solid-liquid separation of slurry and purification of filtrate, equipped with multi-stage separation equipment. First, a deep conical sedimentation tank (Φ10.5m × 10m) is installed for gravity sedimentation separation of the slurry. An underflow pump is installed at the bottom of the sedimentation tank to send the high-concentration underflow slurry to the filter press process; the overflow from the top is sent to the clarification filter via an overflow weir. To improve separation efficiency, a flocculant dosing system is added to the sedimentation tank to improve solid sedimentation performance. The underflow slurry obtained from sedimentation enters the filter press for dewatering: a filter with a filtration area of ​​200m² is selected. 2 Four quick-opening plate and frame filter presses, each with a processing capacity of approximately 20 t / h, are used for filter cake dewatering and washing. The filter cake produced by these filter presses is desilicon ash, with a moisture content reduced to below 45%. The sediment overflow undergoes two stages of fine filtration: the first stage uses a vertical leaf filter to clarify the sodium silicate solution at a maximum flow rate of 97.5 m³ / h. 3 The required effective filtration area for / h calculation is approximately 81m². 2 The actual selected filter area is 130m². 2 Two stainless steel vertical leaf filters are installed to ensure that the filtered sodium silicate concentrate contains ≤15mg / L of suspended solids. Secondary filtration also utilizes leaf filters or filter presses in some processes to separately treat the re-dissolved high-modulus sodium silicate solution and the acid-precipitated silica sol slurry, using a 90m³ / min filter press. 2 Two vertical leaf filters are used for filtration of the returned solution. Redundant equipment is installed at each critical filtration node in the system to improve operational reliability.

[0043] The carbonation reactor consists of two carbon separation systems: a low-modulus system and a high-modulus system. The low-modulus carbon separation system is used for the initial carbonation of sodium silicate solution, employing a series of mechanically stirred reaction tanks. Two sets of fully carbonized tanks are designed, each set containing four Φ4m×8m stirred tanks connected in series, for a total of eight tanks operating in parallel. Each set of carbon separation tanks provides approximately 2 hours of total residence time to ensure complete reaction. The high-modulus carbon separation system is used for secondary carbonation, equipped with slightly smaller stirred tanks: four Φ3m×7m carbon separation tanks are used as a group, with two groups connected in parallel, also providing a total residence time of 2 hours. Each reaction tank is equipped with an independent stirring drive and a CO2 distribution pipe. The CO2 gas source is provided by the lime furnace (described later), and the gas is distributed to the inlet of each carbon separation tank via pipelines, with an inlet gas CO2 concentration of not less than 36%. The carbonation reactor also includes a tail gas emission and absorption system; unreacted CO2 and small amounts of NOx generated are washed with alkaline solution to meet emission standards before being discharged.

[0044] Acid Precipitation Tank: Used for the acid purification and precipitation of silica. The acid precipitation reactor is an acid-resistant stirred tank, approximately Φ3m×4m in size, equipped with mechanical stirring and a heating jacket. Two units are selected for series or parallel operation depending on the silica slurry processing volume. Each acid precipitation tank is equipped with an acid metering and feeding system, adding 98% concentrated sulfuric acid diluted at a 1:4 ratio. The precipitation tank is equipped with an online pH monitor, automatically stopping acid feeding when the pH drops to around 5.5. The outlet of the acid precipitation tank is connected to a plate and frame filter press for solid-liquid separation to obtain a wet silica cake. The filter press is selected to be the same as or slightly larger than the one mentioned above to ensure that the filter cake moisture content is ≤80% and the residual alkali is ≤1%. The waste liquid from the acid precipitation process is rich in Na2SO4 and can be incorporated into the evaporation and crystallization system for unified treatment.

[0045] Drying Equipment: Spray dryers are used for drying silica. Based on the material throughput, three centrifugal spray drying towers are selected, each with an evaporation capacity of approximately 3.0 t / h. During normal operation, two towers are in operation and one is on standby to accommodate maintenance and load fluctuations. The drying tower inlet is equipped with a steam-heated air heater to increase the hot air temperature and quickly remove moisture from the wet material; the outlet is equipped with a cyclone separator and bag filter to capture fine silica particles after drying, preventing excessive dust emissions. The drying system has automatic temperature control to regulate the inlet / outlet temperatures within the process requirements. The dried silica product is pneumatically conveyed to a finished product storage silo.

[0046] The causticization system comprises two parts: lime slurry preparation and causticization reaction, used to convert sodium silicate carbon mother liquor into NaOH. The lime slurry preparation section combines a digester and a storage tank. Two Φ1.8m×17.2m rotary lime digesters are selected, each processing 12t / h of quicklime. The digesters operate intermittently for 16 hours / day to fully digest the daily required amount of lime. The digested lime slurry enters a lime slurry storage tank for buffering. Two Φ6m×6m mechanically stirred tanks are used for the storage tank, with an effective volume of 144m³. 3 / unit, based on a maximum lime slurry flow rate of 70.9m³ 3 The buffer time is approximately 3 hours based on the calculated rate per hour. The causticizing reaction section utilizes continuous stirred tanks connected in series. Six Φ5m×6m stirred causticizing tanks are configured, arranged in two groups connected in series. Each causticizing tank has an effective volume of approximately 100m³. 3 The total residence time for the six tanks is approximately 1.5 hours, meeting the 2-hour reaction requirement. The causticizing tanks are equipped with steam heating and insulation to maintain a temperature of 95–105℃. Carbonate mother liquor and lime slurry are continuously added to the first tank at a set ratio, and the slurry flows sequentially through each tank to complete the reaction. The slurry from the last tank flows by gravity to the solid-liquid separation equipment: two horizontal vacuum belt filters are designed and installed to filter and separate the causticized calcium carbonate solid residue. The unit processing capacity of the vacuum belt filters is approximately 0.4 t / m³. 2 Two units are sufficient to handle the separation of caustic slag. The filter cake is discharged via a discharger and sent to the sintering system; the filtrate, which is the regenerated caustic soda solution, is collected and then enters the subsequent evaporation process.

[0047] Multi-effect evaporation system: Used to concentrate the causticized NaOH solution and crystallize out impurity salts. Based on material balance, this system has a large evaporation capacity and is designed with a six-effect forced circulation evaporation process. The specific configuration includes: a set of six-effect tubular falling film evaporators, combined with a three-stage flash evaporator, and a two-effect forced circulation evaporation crystallizer. The six-effect falling film evaporator utilizes a progressively decreasing pressure gradient to continuously concentrate the causticized liquid. A flash tank is installed after each evaporator to recover the waste heat of the secondary steam, improving heat utilization efficiency. When the evaporation concentration reaches a certain level, the solution enters the forced circulation evaporation crystallizer for further evaporation of water and precipitation of impurity crystals such as Na2CO3. The crystallizer uses a stirred forced circulation system, which can handle high-concentration, high-viscosity solutions and prevent scaling and clogging. The entire evaporation system is designed with a maximum evaporation capacity of approximately 200 t / h to meet the needs of the entire process mother liquor treatment. The condensate generated by evaporation is collected separately: qualified condensate is sent to carbon bottom liquid preparation and white carbon black filter cake washing, while unqualified condensate is sent to red mud washing, etc., to achieve water recycling.

[0048] Reconstituted and Na2CO3 crystallization equipment: The circulating mother liquor after evaporation and concentration is temporarily stored in a mother liquor tank for reuse in leaching during slurry preparation. Two Φ6m×8m storage tanks are selected for the mother liquor tank, each with a volume of approximately 200m³. 3 This is used to store concentrated alkali mother liquor. Simultaneously, a crystallization settling tank and separation equipment are installed in the crystallization section of the evaporator to remove accumulated salt impurities from the system. Two crystallization settling tanks (Φ10.5m × 3m each) are designed and operate in parallel. The supernatant from the settling tanks is returned to the system for circulation, while the bottom slurry is sent to a crystallization salt filter for dewatering. A 40m² crystallization salt filter is selected. 2 Two vertical disc filters are used to separate Na2CO3 crystals. The filtered crystals have a moisture content of ≤35% and can be periodically discharged from the system. Crystallization and salt removal prevents impurities from accumulating in the circulating alkaline solution, thus maintaining stable system operation.

[0049] Lime kiln and sintering section: The lime kiln is the key equipment for providing quicklime and CO2 gas, and a vertical rotary lime kiln is selected. Based on the maximum limestone feeding rate of 35.86 t / h, at least two Φ4.5m×26m lime kilns with a single capacity of 20 t / h are required. In this embodiment, three Φ4.5×26m coal-fired lime kilns are selected, using anthracite as fuel, with air supplied by a blower for combustion assistance. The calcination temperature of the lime kiln is approximately 1100℃, and the limestone decomposition rate is controlled at ≥90%. The quicklime discharged from the kiln is cooled, screened for particle size, and sent to the lime slurry machine for pulping. The furnace gas is rapidly cooled and scrubbed to remove dust, and then used as a CO2 source. The sintering section is used to process raw materials such as desiliconized ash to extract alumina, and includes sub-units such as raw material batching, sintering kiln, clinker cooling, and grinding. The raw material batching system consists of metering and conveying equipment, which feeds desiliconized ash, causticizing slag, limestone, high-iron bauxite, etc., into the mixer according to the set ratio, and adds an appropriate amount of pulverized coal for internal combustion. The mixture can be pelletized using a pelletizing machine to improve the sintering effect. The sintering furnace adopts a rotary kiln process; considering the processing capacity, one to two rotary kilns with a diameter of approximately Φ3.6m and a length of approximately 80m can be selected. The maximum temperature inside the kiln is approximately 1300℃, and the pellets undergo a solid-phase reaction to generate sintered clinker after remaining in the kiln for 1-2 hours. A burner is installed at the kiln head to supplement the heat from bituminous coal combustion, while a waste heat recovery device is installed at the kiln tail to preheat the combustion air and materials. The sintered clinker is rapidly cooled to below 70℃ by a grate cooler, and then crushed to a suitable leaching particle size by a crusher and ball mill. The sintering section also includes a dust removal system and clinker silos. This low-alumina-silicon ratio sintering system is the core of alumina extraction from fly ash, employing automatic combustion control and high-temperature monitoring technology to ensure clinker quality.

[0050] CO2 Gas Source System: This system utilizes the self-supplied lime kiln gas as the primary source of CO2 required for carbonation. The high-temperature exhaust gas from the lime kiln contains approximately 38% or more CO2. Waste heat is recovered and purified by a waste heat boiler and a Venturi scrubber, removing dust to achieve a dust content ≤10mg / Nm³. 3 The purified CO2-rich gas is piped into the carbonation unit. In actual operation, to ensure continuous and stable carbonation, a CO2 buffer tank and a blower may be configured for pressure stabilization. When the CO2 output from the lime furnace is insufficient, external pure CO2 cylinders or other exhaust gas sources can be used as supplementary sources. The entire CO2 supply system is equipped with an online analyzer to monitor gas composition and ensure that the CO2 concentration at the inlet of the carbonation tank meets process requirements.

[0051] DCS Control Unit: All key equipment and parameters in the plant are centrally monitored by the distributed control system. The DCS system is equipped with tiered control stations to automatically adjust and interlock parameters such as leaching temperature, carbonation pH, acid addition, sintering kiln temperature, and evaporation vacuum. Operators monitor the process parameter curves of each unit in real time through the central control room to ensure stable and coordinated operation of all processes. Parameters such as leaching reaction temperature and time can be set and recorded on the DCS interface. In case of abnormalities, the DCS system can alarm and interlock to shut down the system, preventing accidents. Through comprehensive automated control, the various units of this system are organically connected, stably producing qualified silica and alumina products.

[0052] Those skilled in the art will understand that the various embodiments disclosed above can be modified and altered in various ways without departing from the spirit of the invention. Therefore, the scope of protection of this invention should be defined by the appended claims.

[0053] It should be noted that not all steps and units in the above processes are necessary; some steps or units can be omitted as needed. The execution order of each step is not fixed and can be determined as required. The device structure described in the above embodiments can be a physical structure or a logical structure. That is, some units may be implemented by the same physical entity, or some units may be implemented by multiple physical entities, or they may be jointly implemented by certain components in multiple independent devices.

[0054] The specific embodiments described above are exemplary embodiments, but do not represent all embodiments that can be implemented or fall within the scope of the claims. The term "exemplary" as used throughout this specification means "serving as an example, instance, or illustration" and does not imply that it is "preferred" or "advantageous" compared to other embodiments. Specific details are included to provide an understanding of the described techniques. However, these techniques can be practiced without these specific details. In some instances, well-known structures and apparatuses are shown in block diagram form to avoid obscuring the concepts of the described embodiments.

[0055] The foregoing description of this disclosure is provided to enable any person skilled in the art to implement or use this disclosure. Various modifications to this disclosure will be apparent to those skilled in the art, and the general principles defined herein can be applied to other variations without departing from the scope of this disclosure. Therefore, this disclosure is not limited to the examples and designs described herein, but is consistent with the widest scope of the principles and novel features disclosed herein.

Claims

1. A method for producing silica and alumina from fly ash using waste heat steam, characterized in that: Includes the following steps: S1: Fly ash is mixed with alkaline solution to form an alkaline slurry, and the alkaline slurry is heated to above 135°C using waste heat steam at 0.53–0.60 MPa and held for no more than 20 minutes to carry out a short-term high-temperature desilication reaction; S2: Low-modulus sodium silicate solution is separated by deep cone sedimentation and pressure filtration; S3: CO2 gas with a volume fraction ≥36% and a temperature ≥80℃ is continuously introduced to carbonate and decompose the low-modulus sodium silicate solution for 90–120 minutes, and crude silica is precipitated. S4: Retain part of the carbonation reaction mother liquor, mix and dissolve the obtained crude silica with the retained liquor to adjust the modulus of sodium silicate solution, form a carbonation reaction section under high modulus conditions, and then introduce CO2 gas again to react and generate crude silica slurry. S5: The crude silica slurry is subjected to acid precipitation treatment, followed by water washing and drying to obtain silica product; S6: The carbonated mother liquor is added to lime milk for causticization treatment, and then concentrated by multi-effect evaporation to a Na2O content of about 346 g / L. The resulting concentrated alkaline solution is recycled for the preparation of the alkaline slurry. S7: The desiliconized leaching residue is fed into the lime-sintering alumina production line to co-produce alumina products.

2. The method for producing silica and alumina from fly ash using waste heat steam according to claim 1, characterized in that: In step S2, the obtained sodium silicate leaching solution is divided into two parts, of which about 74% of the sodium silicate solution is used for continuous CO2 introduction for carbonation precipitation, and the remaining about 26% of the sodium silicate solution is used as a retention solution for re-dissolution to adjust the modulus.

3. The method for co-producing silica and alumina from fly ash using waste heat steam according to claim 1, characterized in that: In step S5, the pH of the crude silica slurry is adjusted from about 9 to about 5.5 using dilute sulfuric acid, and the precipitation reaction is maintained at a temperature not lower than 90°C for 20–30 minutes.

4. The method for co-producing silica and alumina from fly ash using waste heat steam according to claim 1, characterized in that: In step S4, lime milk with a CaO content of approximately 180 g / L is added to the carbonated mother liquor to make the [CaO] / [Na2O] ratio approximately 1.20, and the reaction is carried out at 95–105 °C for approximately 2 hours.

5. The method for co-producing silica and alumina from fly ash using waste heat steam according to claim 1, characterized in that: The S6 process employs a two-stage falling film evaporation process. After each stage of evaporation, a salt removal device is installed to remove crystalline salts. The final evaporation is carried out under a vacuum of 0.080–0.088 MPa, and the Na2O content of the concentrated alkaline solution is approximately 346 g / L.

6. A fly ash system for the co-production of silica and alumina from waste heat steam, characterized by comprising the following: The alkaline slurry heating leaching unit consists of a raw ash silo, a mixing tank, a steam heater, and a leaching reactor. A primary solid-liquid separation unit consisting of a deep cone settling device and a filtration device; a silica synthesis unit consisting of a continuous carbonation reaction tank, an acid settling tank, and a spray drying system; and a closed-loop alkali recovery unit consisting of a causticizing device, a causticizing liquid separation equipment, a multi-effect falling film evaporation station, a salt discharge module, and a return mixing tank. Alumina co-production unit consisting of lime, sintering, and calcination; It also includes utilities such as CO2 supply branches, 0.6MPa steam headers, DCS / PLC control units, and online analysis equipment.

7. The fly ash system for the co-production of silica and alumina from waste heat steam according to claim 6, characterized in that: The CO2 supply branch is connected to the flue gas duct of the lime calcination furnace to utilize the tail gas of the lime furnace as a gas source for the carbonation reaction.

8. The fly ash system for the co-production of silica and alumina from waste heat steam according to claim 6, characterized in that: The leaching reactor is a shell-and-tube or tubular structure and is connected to the 0.6MPa steam header via pipeline to provide heating steam to the leaching reactor.

9. A fly ash system for the co-production of silica and alumina from waste heat steam according to claim 6, characterized in that: The overflow port of the deep cone settling device of the primary solid-liquid separation unit is connected to the leaf filter to purify the obtained sodium silicate solution; the underflow port of the deep cone settling device is connected to the filter press to separate the leaching residue solids.