Resource utilization and alkali liquor regeneration cycle method for silicon-rich alkali liquor in two-step alkali-process aluminum extraction process of high-alumina fly ash

Abstract

The invention relates to a resource utilization and alkali liquor regeneration cycle method for silicon-rich alkali liquor in a two-step alkali-process aluminum extraction process of high-alumina fly ash. The method comprises the following steps: firstly, introducing a gas containing carbon dioxide into the silicon-rich alkali liquor, enabling silicate in the silicon-rich alkali liquor to react to generate a hydrated silicon dioxide precipitate, and then carrying out solid-liquid separation to obtain a hydrated silicon dioxide product and a sodium carbonate solution; adding calcium-based alkali into the sodium carbonate solution for reaction to generate a sodium hydroxide solution and calcium carbonate precipitate, and then performing solid-liquid separation to obtain a regenerated sodium hydroxide solution; and finally, returning the obtained regenerated sodium hydroxide solution for the pre-desilicication process of the high-alumina fly ash. According to the method, waste silicon resources are converted into high-value products, closed-loop circulation of the alkali liquor is achieved, the silicon recovery rate can reach 98% or above, the alkali regeneration efficiency exceeds 90%, the consumption of fresh alkali and discharge of alkali wastewater are remarkably reduced, and the overall economic benefits and environmental friendliness of the technology for extracting aluminum from high-alumina fly ash are improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of industrial solid waste treatment and resource utilization, and specifically relates to a comprehensive utilization method of high-aluminum fly ash. More specifically, the present application relates to a treatment method for a silicon-rich alkali liquor generated in a pre-desilication step of a two-step alkali method for extracting aluminum from high-aluminum fly ash, which cooperatively realizes high-value recovery of silicon resources and efficient regeneration and recycling of the alkali liquor. BACKGROUND

[0002] High-aluminum fly ash is a kind of industrial solid waste with huge reserves in China, and its main chemical components are aluminum oxide (Al2O3) and silicon oxide (SiO2), with an Al2O3 content usually above 40%, which has great potential to replace bauxite for extracting aluminum oxide. However, the mass ratio of Al2O3 to SiO2 (aluminum-silicon ratio) in high-aluminum fly ash is usually less than 1, which is much lower than the requirement of the traditional Bayer process for raw materials (more than 9), so it cannot be directly treated by the economical and efficient Bayer process. To solve this problem, the industry has developed various treatment technologies, such as sintering method, acid method, and acid-alkali combined method, etc.

[0003] The sintering method (such as limestone sintering method) converts aluminum oxide in fly ash into soluble aluminate through high-temperature calcination, but this method has the disadvantages of large amount of sintered material, high energy consumption, and large amount of secondary solid waste such as red mud. The acid method uses inorganic acids such as sulfuric acid or hydrochloric acid to leach aluminum oxide, but iron and other impurity ions are easily dissolved together in the process, resulting in a complex separation and purification process, and serious corrosion of equipment by strong acid.

[0004] In contrast, the two-step alkali method process of "pre-desilication and post-aluminum extraction" is considered one of the most promising technical routes for industrialization. In the pre-desilication stage, sodium hydroxide (NaOH) solution is used to selectively leach the amorphous silicon dioxide with high activity in fly ash under certain temperature and pressure, generating soluble sodium silicate (Na2SiO3), thereby effectively increasing the aluminum-silicon ratio in the remaining fly ash slag. After pre-desilication treatment, the aluminum-silicon ratio of the fly ash slag is significantly improved, which can meet the requirements of the subsequent Bayer process or sintering method for aluminum extraction.

[0005] However, the existing two-step alkali method process has a key technical bottleneck: a large amount of silicon-rich alkali liquor is generated in the pre-desilication stage. This alkali liquor is essentially a strong alkaline wastewater containing high-concentration sodium silicate and residual sodium hydroxide. How to economically and environmentally treat this waste liquid is the core problem that determines whether this process route can be applied on a large scale. The current treatment methods for this waste liquid have the following defects:

[0006] (1) Resource waste and high cost: Direct discharge or simple neutralization not only causes serious environmental pollution, but also leads to the complete loss of valuable components in the solution, i.e., a large amount of alkali (NaOH) and silicon resources. NaOH is the main chemical consumable of this process, and its loss directly increases the production cost.

[0007] (2) Huge environmental pressure: The pH value of the silicon-rich lye is high, and the chemical oxygen demand is large. If discharged directly, it will cause persistent pollution to water bodies and soil, and the treatment difficulty and cost are extremely high.

[0008] (3) Low level of comprehensive utilization: Some existing technologies, such as Chinese patent CN101284668A, mention that the silicon-containing leaching solution produced by pre-desiliconization is used to produce white carbon black. However, such methods usually only focus on the preparation of silicon products, and fail to form a closed loop with the main aluminum extraction process, especially ignoring the efficient regeneration and recycling of alkali. This leads to an open process chain, which does not fundamentally solve the problems of high alkali consumption and wastewater discharge.

[0009] In summary, the existing technology fails to provide a closed system that effectively integrates the recovery of silicon resources in silicon-rich lye with the regeneration and recycling of lye. Therefore, there is an urgent need to develop a new technology that can not only convert silicon in silicon-rich lye into high-value products, but also efficiently regenerate and recycle the alkali in the lye, thereby building a truly "zero discharge", low consumption, and high efficiency new technology for comprehensive utilization of high-aluminum fly ash. SUMMARY

[0010] Technical problem: The present invention aims to overcome the shortcomings of the existing technology and provide a method for resource utilization and lye regeneration and recycling of silicon-rich lye in a two-step alkali method for extracting aluminum from high-aluminum fly ash. The main technical problem to be solved is: how to achieve the high-value recovery of silicon resources in silicon-rich lye produced by the pre-desiliconization step, as well as the efficient regeneration and closed-loop recycling of alkali (sodium hydroxide) in an integrated process, to eliminate the discharge of alkaline wastewater, reduce production costs, and improve the efficiency of resource comprehensive utilization.

[0011] Technical solution: To solve the above technical problems, the present invention provides a method for resource utilization and lye regeneration and recycling of silicon-rich lye in a two-step alkali method for extracting aluminum from high-aluminum fly ash, characterized in that the method comprises the following steps:

[0012] Step A: Carbonization method for recovering silicon resources

[0013] The silicon-rich lye (mainly sodium silicate and sodium hydroxide aqueous solution) produced in the pre-desilication step of high-alumina fly ash is placed in a carbonation reactor, and a gas containing carbon dioxide (CO2) is introduced into the silicon-rich lye to perform a carbonation reaction. During the reaction, specific process parameters are controlled to make the silicate ions in the solution react with CO2 to generate hydrated silicon dioxide (SiO2·nH2O) precipitate. The main chemical equation of the reaction is:

[0014] Na2SiO3 + CO2 + H2O→ Na2CO3 + SiO2·nH2O↓

[0015] After the reaction is completed, the mixture is subjected to solid-liquid separation to obtain a solid hydrated silicon dioxide product (i.e., precipitated white carbon black) and a liquid sodium carbonate (Na2CO3) solution.

[0016] Step B: Regeneration of lye by causticization

[0017] The sodium carbonate solution obtained by solid-liquid separation in Step A is transported to a causticization reaction system, and a calcium-based alkali, preferably calcium hydroxide (Ca(OH)2) or a slurry thereof, is added to the solution to perform a causticization reaction. Under specific reaction conditions, Na2CO3 reacts with Ca(OH)2 to generate sodium hydroxide (NaOH) and insoluble calcium carbonate (CaCO3) precipitate. The chemical equation of the reaction is:

[0018] Na2CO3 + Ca(OH)2→ 2NaOH + CaCO3↓

[0019] After the reaction is completed, the mixture is subjected to solid-liquid separation to obtain a regenerated sodium hydroxide solution in the liquid phase and a calcium carbonate precipitate in the solid phase.

[0020] Step C: Closed-loop recycling

[0021] The regenerated sodium hydroxide solution obtained by separation in Step B is adjusted in concentration and then returned to the high-alumina fly ash pre-desilication process for leaching of new high-alumina fly ash raw materials, thereby realizing closed-loop recycling of the lye.

[0022] As a preferred embodiment of the present application, the process parameters in Step A are controlled as follows:

[0023] The CO2-containing gas can be pure CO2 gas or a mixed gas containing CO2, such as industrial flue gas.

[0024] The carbonation reaction is controlled at a temperature of 30-60°C, preferably 40-50°C. Within this temperature range, the reaction rate is moderate, and it is beneficial to generate white carbon black products with good physical properties (such as high specific surface area).

[0025] The end point of carbonation reaction is controlled by monitoring the pH value of the solution. When the pH value drops to 9.0-10.0, the CO2 supply is stopped. This pH range can ensure that the sodium silicate is substantially completely reacted, while avoiding the generation of excess sodium bicarbonate, thereby ensuring the purity of the raw material for the subsequent causticization reaction.

[0026] After stopping the aeration, the slurry can be aged for 30-60 minutes to promote the growth and stabilization of the precipitated particles.

[0027] As another preferred embodiment of the present application, the process parameters in step B are controlled as follows:

[0028] The causticization reaction is controlled at a temperature of 90-105°C, preferably 95-100°C, to increase the reaction rate and causticization conversion rate.

[0029] The molar ratio of added calcium hydroxide to sodium carbonate is controlled at 1.0:1 to 1.2:1, preferably 1.05:1 to 1.15:1. A moderate excess of Ca(OH)2 helps to push the causticization reaction equilibrium towards the forward direction, increasing the conversion rate of Na2CO3.

[0030] The causticization reaction is controlled for a time of 2-3 hours to ensure that the reaction is close to equilibrium.

[0031] As still another preferred embodiment of the present application, the method further comprises a step of resource utilization of the calcium carbonate precipitate produced in step B: the calcium carbonate precipitate is calcined at high temperature to decompose into calcium oxide (CaO) and CO2 gas. The generated CaO is hydrated (digested) to form Ca(OH)2, which is returned to step B for causticization reaction; while the CO2 gas produced by decomposition can be returned to step A for carbonation reaction. This further improves the atomic economy and recycling degree of the entire process. The calcination reaction equation is:

[0032] CaCO3→ CaO + CO2↑

[0033] The hydration reaction equation is:

[0034] CaO + H2O→Ca(OH)2

[0035] Compared with the prior art, the present application has the following significant beneficial effects:

[0036] (1) Resource utilization and high value: The present application converts the harmful components (high concentration of silicon) in the pre-desiliconized waste liquid into a chemical raw material with high market value, i.e. precipitated white carbon black, achieving resource utilization and high value utilization of waste, turning waste into treasure. Precipitated white carbon black is an important reinforcing agent and filler with a wide market demand.

[0037] (2) Alkali efficient regeneration and recycling: through the "carbonation-causticization" two-step method, the present application successfully converts the sodium element in the waste liquid into NaOH solution that can be directly reused, and the regeneration efficiency of alkali can be as high as 95% or more. This builds a closed-loop system for alkali, which can reduce the amount of fresh NaOH by more than 90%, greatly saving the cost of chemical raw materials.

[0038] (3) Significant environmental protection benefits: Since the alkali is internally recycled, the discharge of high-alkalinity and high-silicon wastewater is fundamentally eliminated, reducing the huge pressure and cost of subsequent wastewater treatment, and meeting the development requirements of green chemical industry and circular economy.

[0039] (4) Process synergy and economic improvement: The present application organically integrates the two unit operations of silicon recovery and alkali regeneration into one whole, and the products of each step are the raw materials for the next step, realizing the step-by-step utilization of materials. Through the sale of white carbon black products, combined with the saved cost of alkali procurement and wastewater treatment, the economic feasibility and market competitiveness of the whole process of high-aluminum fly ash aluminum extraction are significantly improved. The internal logic of this method is that the purity of the product of the carbonation step directly affects the efficiency of the causticization step, and by accurately controlling the carbonation end point pH value, the purity of the Na_2CO_3 solution entering the causticization unit can be ensured, which is the prerequisite for efficient alkali regeneration, and embodies the synergistic optimization effect of process integration. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below in combination with specific examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the protection scope of the present application.

[0041] In the embodiments of the present application, the high-aluminum fly ash raw material used is taken from a certain thermal power plant, and its main chemical components (mass fraction) are: SiO2 48.5%, Al2O3 42.3%, Fe2O3 3.8%, TiO2 1.5%, CaO 1.2%, and other 2.7%.

[0042] Preparation of starting silicon-rich lye (pre-desilication step): Take 100 kg of the above high-alumina fly ash and add it to a reaction kettle containing 400 L of a 20% by mass NaOH solution (containing about 89 kg of NaOH). The liquid-to-solid ratio is 4:1 (L / kg). Under stirring, the temperature is raised to 120°C, and the reaction is carried out for 2 hours. After the reaction is completed, solid-liquid separation is carried out using a filter press to obtain desilicated fly ash residue (for use in the subsequent aluminum extraction process) and a silicon-rich lye. Analysis shows that the obtained silicon-rich lye has a volume of about 420 L, containing a SiO2concentration of 120 g / L and a Na2O concentration of 80 g / L (equivalent to about 103 g / L of NaOH). The silicon-rich lye is used as the starting material for the following examples.

[0043] Example 1 (best mode)

[0044] This example is intended to illustrate the best process conditions of the present application.

[0045] Step A: Carbonation reaction Take the above-prepared silicon-rich lye 420 L and place it in a 1000 L carbonation reaction kettle equipped with a stirrer, a gas distributor, a temperature control jacket, and an online pH meter. Start stirring and control the solution temperature at 40°C. Introduce CO2 gas with a purity of 99% into the solution at a flow rate of 5 m3 / h. Monitor the solution pH in real time, and when the pH drops to 9.5, stop the CO2 introduction, and then continue stirring for 30 minutes. The slurry after the reaction is subjected to solid-liquid separation by a plate-and-frame filter press. The filter cake is washed with deionized water until it is neutral, and then dried in an oven at 110°C for 4 hours to obtain a white powdery product. By metering, 51.2 kg of dry precipitated white carbon black product is obtained. Detection shows that the SiO2content in the product is 98.6% (after ignition), meeting the industrial premium product standard. The filtrate volume is about 435 L, which is a Na2CO3solution. According to material balance, the SiO2content in the starting lye is 420 L x 120 g / L = 50.4 kg. The SiO2content in the recovered white carbon black is 51.2 kg x 98.6% = 50.48 kg. The silicon recovery rate reaches 50.48 / 50.4 = 99.2%.

[0046] Step B: Causticization reaction The 435 L of Na2CO3 filtrate from Step A was pumped into a 2000 L causticization reactor and heated to 95 °C. According to the amount of Na2CO3 in the filtrate (about 70.5 kg determined by chemical titration), Ca(OH)2 slurry (containing Ca(OH)2 51.8 kg) was slowly added to the reactor at a molar ratio of Ca(OH)2 to Na2CO3 of 1.05:1. The reaction was continuously stirred at 95 °C for 2.5 hours. After the reaction was completed, the slurry was sent to a settling tank for clarification and separation. The supernatant was the regenerated NaOH solution, and the bottom slurry was further dewatered by a vacuum drum filter to obtain CaCO3 filter cake. The supernatant and filtrate were combined to obtain a total of about 450 L of regenerated NaOH solution. Analysis showed that the concentration of NaOH in the regenerated solution was 115 g / L, and the residual Na2CO3 concentration was 8.2 g / L. According to the stoichiometric relationship, theoretically, 53.4 kg of NaOH could be regenerated. The actual regenerated NaOH was 450 L x 115 g / L = 51.75 kg. The causticization reaction had an alkali regeneration efficiency (causticization rate) of 51.75 / 53.4 = 96.9%.

[0047] Step C: Closed loop cycle The obtained 450 L of regenerated NaOH solution was returned to the pre-desilication process for the treatment of the next batch of high-alumina fly ash. The total alkali recovery efficiency (taking into account the losses in all processes) was about 92.5% throughout the process.

[0048] Example 2 (change of carbonization temperature)

[0049] This example aims to illustrate the effect of carbonization temperature on product quality.

[0050] Step A: Carbonization reaction Step A of Example 1 was repeated, except that the carbonization reaction temperature was controlled at 60 °C. After the reaction was completed, 51.0 kg of dry precipitated white carbon black product was obtained, and the SiO2 content of the product was 97.9%. The silicon recovery rate was 98.5%. The product performance was characterized, and it was found that the specific surface area decreased from 185 m² / g to 150 m² / g compared to Example 1 (prepared at 40 °C), which would affect its application performance as a reinforcing filler and market value. The volume of Na2CO3 filtrate obtained was about 435 L.

[0051] Steps B and C The Na2CO3 filtrate obtained in Step A was subjected to the same Step B and Step C as in Example 1. The concentration and volume of the regenerated NaOH solution obtained were basically the same as in Example 1, and the total alkali recovery efficiency was 92.3%.

[0052] The results of this example show that increasing the carbonization temperature to 60°C can still achieve efficient recovery of silicon and regeneration of alkali, but will lead to a decrease in the quality of the white carbon black product. Therefore, 40°C is a more preferred reaction temperature.

[0053] Example 3 (Changing the ratio of causticizing calcium alkali)

[0054] This example aims to illustrate the effect of the ratio of calcium alkali on the efficiency of alkali regeneration in the causticizing reaction.

[0055] Step A: Carbonization reaction Step A of Example 1 was completely repeated to obtain 51.2 kg of high-quality precipitated white carbon black and 435 L of Na2CO3 filtrate.

[0056] Step B: Causticizing reaction Step B of Example 1 was repeated, except that the molar ratio of Ca(OH)2 to Na2CO3 added was increased to 1.15:1 (i.e., the Ca(OH)2 slurry added contained 56.8 kg of Ca(OH)2). After the reaction was completed, the same post-treatment was performed to obtain about 455 L of regenerated NaOH solution, with a NaOH concentration of 118 g / L and a residual Na2CO3 concentration reduced to 4.5 g / L. Theoretically, 53.4 kg of NaOH could be regenerated, and actually, 53.69 kg of NaOH was regenerated (455 L x 118 g / L = 53.69 kg). The causticizing reaction efficiency was increased to 53.69 ~ 53.41% (considering measurement errors, it is actually close to the theoretical limit). The total alkali recovery efficiency was calculated to be increased to 95.1%.

[0057] The results of this example show that increasing the ratio of calcium alkali can significantly improve the efficiency of alkali regeneration. However, in actual industrial applications, the increase in alkali recovery rate needs to be balanced with the increase in lime consumption and the additional cost of increased calcium carbonate solid waste. Therefore, a calcium alkali ratio of 1.05:1 to 1.15:1 is a preferred range that takes into account efficiency and cost.

[0058] The following table summarizes the key parameters and results of the above examples:

[0059]

[0060] The above examples fully demonstrate the feasibility of the technical solution of the present application. Through the method of the present application, not only can the silicon resource in the silicon-rich alkali solution be efficiently recovered, and a high-value white carbon black product be prepared, but also the alkali solution can be efficiently regenerated and recycled, providing a new technical path for the clean, efficient, and economic comprehensive utilization of high-aluminum fly ash.

Claims

1. A method for resource utilization and regeneration and recycling of silicon-rich alkali liquor in a two-step alkali process for extracting aluminum from high-aluminum fly ash, characterized in that, The method comprises the following steps: (a) carbonation treatment of the silicon-rich lye: carbon dioxide-containing gas is introduced into the silicon-rich lye to make silicates in the silicon-rich lye react to form hydrated silicon dioxide precipitate, and then solid-liquid separation is performed to obtain hydrated silicon dioxide product and sodium carbonate solution; (b) causticization treatment of the sodium carbonate solution obtained in step (a): calcium-based alkali is added to the sodium carbonate solution to generate sodium hydroxide solution and calcium carbonate precipitate, and then solid-liquid separation is performed to obtain regenerated sodium hydroxide solution; and (c) returning the regenerated sodium hydroxide solution obtained in step (b) to the pre-desilication process of high-aluminum fly ash.

2. The method of claim 1, wherein, The carbon dioxide-containing gas in step (a) is pure carbon dioxide gas or industrial flue gas.

3. The method according to claim 1 or 2, characterized in that, The reaction temperature of the carbonation treatment in step (a) is controlled at 30-60°C.

4. The method of claim 3, wherein, The reaction temperature of the carbonation treatment is controlled at 40-50°C.

5. The method of claim 1, wherein, The end point of the carbonation treatment in step (a) is controlled by monitoring the pH value of the solution, and the introduction of the carbon dioxide-containing gas is stopped when the pH value of the solution reaches 9.0-10.

0.

6. The method of claim 1, wherein, The calcium-based alkali in step (b) is calcium hydroxide or a slurry thereof.

7. The method of claim 6, wherein, The molar ratio of calcium hydroxide to sodium carbonate added in step (b) is 1.0:1 to 1.2:

1.

8. The method of claim 7, wherein, The molar ratio is 1.05:1 to 1.15:

1.

9. The method of claim 1, wherein, The reaction temperature of the causticization treatment in step (b) is controlled at 90-105°C.

10. The method of claim 1, wherein, The method further comprises high-temperature calcination of the calcium carbonate precipitate obtained in step (b) to generate calcium oxide and carbon dioxide; hydration of the generated calcium oxide and returning to step (b) for recycling; and returning the generated carbon dioxide to step (a) for recycling.

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