An industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation

Abstract

The present invention discloses an industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation. Through steps such as rapid heating of the secondary aluminum ash in a suspended gas-solid two-phase heat exchange, enrichment of volatile salts and fluorides in the secondary aluminum ash, classified collection of volatile salts and fluorides in the secondary aluminum ash, and high-temperature calcination, the method uses rapid heating by suspended gas-solid two-phase heat exchange and the volatilization-condensation method to enrich the hazardous waste of volatile salts and fluorides in the secondary aluminum ash. By bypass air discharge, non-hazardous volatile salts and hazardous salts are classified and collected and treated, and the fluorides therein are completely volatilized by high-temperature calcination. The present invention has the characteristics of low production cost, less environmental pollution, classified collection of volatile salt substances, thorough treatment of hazardous waste, and high collection efficiency. It greatly reduces the cost compared with the original wet treatment of aluminum ash and is an optimal method for removing and separating volatile salt substances and fluorides and other hazardous waste from industrial aluminum ash.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum ash waste treatment, and in particular to an industrial separation method for removing fluoride hazardous waste in secondary aluminum ash by volatilization and condensation. Background Art

[0002] Aluminum ash is solid waste generated during the electrolytic aluminum process, aluminum profile processing, waste aluminum remelting and reprocessing. It can be divided into primary aluminum ash and secondary aluminum ash according to the different aluminum content and recycling times in the aluminum ash. Primary aluminum ash is produced in the aluminum electrolysis and casting process without adding salt solvent. It mainly contains aluminum and aluminum oxide. The aluminum content can reach 30-70 wt.%. Because it is often white, it can also be called white ash. Secondary aluminum ash is the ash after the primary aluminum ash is physically recovered from aluminum. It mainly contains more aluminum oxide, a small amount of aluminum, and some metal oxides. The color is darker than the primary aluminum ash, gray-black. Because some flux is often added in the recycling and processing stage of recycled aluminum, the chemical and mineral composition of aluminum ash depends on the raw materials and metallurgical processes used in the production process. Therefore, the secondary aluminum ash contains nitrides, carbides, a small amount of graphite and cryolite, etc. According to the latest "National List of Hazardous Wastes" (2021 edition), secondary aluminum ash is classified as toxic and flammable hazardous wastes. Its main pollution sources are ammonia and methane formed by the hydrolysis of nitrides and carbides. Ammonia has a pungent smell, and methane is a flammable and explosive gas. Its accumulation will cause great safety hazards; cryolite in secondary aluminum ash is a fluoride, which is easily soluble in water to form water pollution and easily volatilizes to form air pollution, which brings safety hazards to people's production and life. This is also the main reason why secondary aluminum ash is listed as hazardous waste. Due to the lack of large-scale standardized treatment, about 95% of aluminum ash is stored or landfilled each year, and the historical stockpile has exceeded 10 million tons. Aluminum ash, as a hazardous solid waste in the aluminum production industry, poses a serious challenge to the environment and public health. The industry has called electrolytic aluminum ash "invisible bomb". On the one hand, the high cost of safe disposal of aluminum ash is also a severe challenge faced by the aluminum production industry. On the other hand, insufficient recycling and reuse of aluminum ash is more likely to lead to safety risks from slag toxicity and high flammability. In 2019, China produced about 3.5 million tons of secondary aluminum ash, which seriously restricted the development of the metal aluminum industry. The harmless treatment of secondary aluminum ash is imminent.

[0003] For the treatment of secondary aluminum ash, a large amount of research work has been carried out by predecessors. Among them, the wet treatment process of aluminum ash proposed by Berzelius Umwelt-Service AG is representative: the non-oxides in the aluminum ash are removed by hydrolysis to generate NH3 or CH4, etc., and the gases are collected and used to avoid the adverse effects brought by NH3 or CH4; for water-soluble salts such as KCl, NaCl, and Na3AlF6, etc., they are first dissolved in water and then separated, purified and used to avoid the impact of fluorides on the environment; the remaining substances are alumina or aluminum hydroxide, and high-purity alumina is obtained after dehydration and calcination. However, this method has a large amount of solid waste wet treatment, a large amount of water consumption, complex processes, and serious corrosion of equipment by salt solutions, so the protection of equipment must be strengthened. At present, only Jiaozuo Wanfang Aluminum Industry has designed and put into use a production line. At present, there is an urgent need for an industrial separation method for the harmless treatment of secondary aluminum ash, which can separate volatile salts and fluorides with greater harm through a simple process, with high treatment efficiency and low production costs, promoting environmental protection and improving the economic benefits of enterprises. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides an industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation. The method of high-temperature volatilization - low-temperature condensation is used to enrich volatile salts and fluorides in the secondary aluminum ash in the flue gas, and then classified collection and treatment are carried out through bypass air release. The whole process flow is simple, reducing the production and treatment processes, and effectively solving the problems of many existing wet treatment processes, large treatment volume, and high production costs.

[0005] The technical solution adopted by the present invention is: an industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation, comprising the following steps:

[0006] Step 1: Rapid heating of the suspension of secondary aluminum ash through gas-solid two-phase heat exchange

[0007] The secondary aluminum ash is evenly fed into the suspension preheater system. Under the action of an additional dispersion device and high-speed flowing flue gas, the secondary aluminum ash is evenly dispersed in the high-temperature flue gas. The suspension gas-solid two-phase fluid of the secondary aluminum ash and the high-temperature flue gas sequentially passes through five-stage suspension preheaters, and suspension mixing heating and gas-solid separation are carried out step by step. From the first-stage suspension preheater to the fifth-stage suspension preheater, the secondary aluminum ash is gradually heated from room temperature by suspension; from the fifth-stage suspension preheater to the first-stage suspension preheater, the temperature of the high-temperature flue gas gradually decreases; the entire mixing and heating process takes 5 - 10 minutes to heat the secondary aluminum ash from room temperature to above 950°C; the flue gas from the fifth-stage preheater to the first-stage preheater has a gradually decreasing temperature, and finally the flue gas discharged from the suspension preheater system has a temperature ≤ 200°C. The low-temperature flue gas enters the humidification tower and the bag filter for dust removal treatment and then is discharged up to standard.

[0008] The suspension preheater includes cyclones, inlet air ducts, outlet air ducts and feeding pipes; after the secondary aluminum ash is added, it is evenly dispersed in the high-temperature flue gas under the impact of an additional dispersion device and high-speed air flow, enters the inlet air duct of the cyclone of the first-stage suspension preheater, and performs the first-stage dispersed suspension heating with the high-temperature flue gas rising in the outlet air duct of the cyclone of the second-stage suspension preheater, and is carried into the cyclone of the first-stage suspension preheater for gas-solid separation; the separated secondary aluminum ash enters the inlet air duct of the cyclone of the second-stage suspension preheater through the feeding pipe of the cyclone of the first-stage suspension preheater, performs the second-stage dispersed suspension heating with the high-temperature flue gas rising in the outlet air duct of the cyclone of the third-stage suspension preheater, and is carried into the cyclone of the second-stage suspension preheater for gas-solid separation; and so on, the suspended gas-solid two-phase fluid of the secondary aluminum ash and the high-temperature flue gas sequentially enters the cyclones of the third-stage, fourth-stage and fifth-stage suspension preheaters for step-by-step suspended heating and gas-solid separation. The inlet air pipe of the cyclone of the fifth-stage suspension preheater is connected to the tail gas chamber of the rotary kiln, and its high-temperature flue gas comes from the rotary kiln.

[0009] Step 2: Enrich the volatile salts and fluorides in the secondary aluminum ash

[0010] The high-temperature flue gas from the rotary kiln suspends and mixes with the secondary aluminum ash for step-by-step heating in the suspension preheater system. At the tail gas chamber of the rotary kiln, the volatilized fluorides exist in the flue gas. In the high-temperature part of the fourth and fifth-stage suspension preheater systems, when the flue gas temperature is lower than 1000 °C, the volatilized fluorides in the flue gas condense on the subsequent secondary aluminum ash. The secondary aluminum ash is then separated in the cyclone of the fifth-stage suspension preheater and enters the rotary kiln through its feeding pipe for calcination. When calcined at high temperature, the fluorides volatilize into the flue gas again for enrichment; in the low-temperature parts such as the first to third-stage suspension preheater systems, the volatile salts in the flue gas condense on the subsequent secondary aluminum ash again. As the secondary aluminum ash enters the high-temperature part of the suspension preheater system, they volatilize into the flue gas again for enrichment. This process repeats, and the volatile salts and fluorides in the secondary aluminum ash always exist in the suspension preheater and rotary kiln systems and are continuously enriched by the method of high-temperature volatilization-low-temperature condensation.

[0011] Step 3: Classify and collect the volatile salts and fluorides in the secondary aluminum ash

[0012] 1) Classify and collect the volatile salts and fluorides enriched in the flue gas by setting up the bypass air release device 1 and bypass air release device 2: Set up the bypass air release device 1 in the tail gas chamber of the rotary kiln. The temperature of the gas chamber is 1000°C - 1100°C, and part of the high-temperature flue gas enriched with the fluoride Na3AlF6 is discharged. Set up the bypass air release device 2 in the fifth-stage suspension preheater. The temperature of the flue gas is 800°C - 1000°C, and part of the high-temperature flue gas enriched with volatile salts such as KCl and NaCl is discharged. Determine the time interval for intermittent air release according to the components in the secondary aluminum ash and the concentrations of volatile salts and fluorides in the flue gas; a continuous bypass air release device can also be used, and the air volume of the bypass air release is controlled by a fan and pipeline valves.

[0013] The flue gas discharged at the gas chamber and the outlet duct of the fifth-stage suspension preheater cyclone is determined according to the content of volatile salts and the Na3AlF6 volatile hazardous waste in the secondary aluminum ash, and respectively accounts for 10 - 20% of the system flue gas volume; to maintain the stable working state of the suspension preheater system and avoid uneven dispersion of the secondary aluminum ash in the preheater, at the inlet ducts of the fifth-stage suspension preheater cyclone and the fourth-stage suspension preheater cyclone, hot air tertiary air is respectively blown in three times. The air volume of the blown hot air tertiary air is related to the bypass air release volume and respectively accounts for 10 - 30% of the system flue gas volume.

[0014] 2) Discharge the high-temperature flue gas enriched with volatile salt and fluoride from the bypass air release device 1 and bypass air release device 2 respectively. The flue gas discharged from the bypass air release device 1 mainly consists of the Na3AlF6 volatile hazardous waste and fly ash generated by the calcination of secondary aluminum ash in the rotary kiln. Use the conventional industrial wet treatment of secondary aluminum ash to collect the Na3AlF6 therein and carry out separation and recycling; the flue gas discharged from the bypass air release device 2 mainly consists of volatile salts such as KCl and NaCl and a small amount of fly ash. Use the conventional industrial wet treatment of secondary aluminum ash to collect the KCl and NaCl therein and carry out separation for recycling.

[0015] The dust concentration in the flue gas discharged from the bypass air release device is small, only 1 - 5% of the total secondary aluminum ash treatment volume, but the content of the enriched volatile salts is high, about 20 times more concentrated than the initial value, greatly reducing the treatment volume of solid waste during the new process wet treatment; the solid fly ash without salts and hazardous waste after wet treatment accounts for about 5 - 10% of the total secondary aluminum ash volume and can re-enter the suspension preheater and enter the rotary kiln together with the subsequent secondary aluminum ash for dense sintering.

[0016] Step Four: High-temperature Calcination

[0017] In Step 1, the secondary aluminum ash separated through multi-stage suspension preheating and gas-solid separation enters a rotary kiln for high-temperature calcination. The firing temperature in the rotary kiln is not lower than 1100°C, the calcination temperature is 1100°C - 1400°C, and the calcination time is 30 - 50 min. The secondary aluminum ash after high-temperature calcination is cooled and the heat is recovered and utilized.

[0018] Only KCl and NaCl can volatilize at 800°C - 1000°C, while Na6AlF3 starts to volatilize only when the temperature exceeds 1000°C. The volatilization of Na6AlF3 is difficult to carry out in the suspension preheater system. The secondary aluminum ash containing agglomerated Na6AlF3 must be calcined at high temperature in a rotary kiln to promote the volatilization of Na6AlF3. Through high-temperature and long-time calcination, the volatile salts and fluorides in the secondary aluminum ash are completely volatilized without remaining in the calcined product. The waste heat of the calcined aluminum ash is recovered as combustion-supporting air, etc., to reduce the energy consumption of the calcination system; the recovered high-temperature hot air enters the suspension preheater system through the tertiary air duct, providing the oxygen and oxidation atmosphere required for the oxidation of non-oxides in the secondary aluminum ash.

[0019] Cooling the secondary aluminum ash after high-temperature calcination, recovering low-temperature hot air and high-temperature air: Part of the high-temperature air enters the inlet ducts of the cyclones of the fifth-stage suspension preheater and the fourth-stage suspension preheater as tertiary air respectively, making the high-temperature flue gas in the suspension preheater system in an oxidation atmosphere, fully oxidizing the graphite and other non-oxides in the secondary aluminum ash, which is beneficial to the complete oxidation of combustibles and non-oxides in the secondary aluminum ash; the low-temperature hot air enters the multi-channel burner at the kiln head of the rotary kiln as the combustion-supporting air for the primary air: To lengthen the flame length, make the material heated evenly, and extend the residence time of the material in the high-temperature area, a reducing flame combustion method with insufficient combustion-supporting air is adopted in the multi-channel burner; Another part of the high-temperature air enters the rotary kiln as secondary air to supplement the insufficient combustion-supporting air, making the fuel burn completely, and finally the high-temperature flue gas is in a weak oxidation atmosphere. Because hot air is used as the combustion-supporting air, the fuel consumption of the aluminum ash calcination system can be effectively reduced.

[0020] The suspension preheater, additional dispersion device, bypass air release device, multi-channel burner, etc. described in this method are all publicly available process equipment in the cement industry.

[0021] The mechanism of the present invention is as follows: According to the thermal analysis test results of the secondary aluminum ash sample and relevant literature, it is proved that 800°C to 1000°C is the volatilization range of salts such as NaCl and KCl. Thermal analysis shows that the secondary aluminum ash has a large mass loss in this temperature range, corresponding to it; when the temperature exceeds 1000°C, the secondary aluminum ash still has a mass loss, while the melting point of sodium hexafluoroaluminate (Na3AlF6) is 1009°C, and its vapor pressure in the molten state is 253.2 Pa, which is easy to volatilize, corresponding to each other. Therefore, Na3AlF6 can volatilize into the flue gas only when the temperature exceeds 1000°C. Thus, it can be seen that volatile chlorides and fluorides have obvious volatilization temperature boundaries, providing the possibility for high-temperature volatilization and classified harmless treatment of hazardous wastes such as volatile salts and fluorides in secondary aluminum ash.

[0022] In step one, the secondary aluminum ash is added through the inlet of the suspension preheater. Relying on the high-speed turbulent flue gas and the additional dispersion device, the secondary aluminum ash is evenly dispersed in the high-temperature flue gas under the impact of the high-speed airflow, exchanges heat with the high-temperature flue gas in the suspended state, the secondary aluminum ash heats up while the flue gas cools down, and the mixed secondary aluminum ash and flue gas, the gas-solid two-phase fluid, enter the cyclones of each stage of the suspension preheater in turn for gas-solid separation; by utilizing the characteristics of fast heat transfer and mass transfer rates between the gas-solid two-phase in the suspended state, while rapidly heating up by heat transfer, the mass transfer is accelerated, the heating rate of the secondary aluminum ash is fast, the heat transfer efficiency is high, and the heating rate of the secondary aluminum ash and the thermal utilization efficiency of the high-temperature calcination system are improved.

[0023] In step one, the hot flue gas has a good mixing effect with the secondary aluminum ash in the high-speed turbulent state. While having efficient heat transfer, it has a very good mass transfer effect, can meet the oxygen and oxidation atmosphere required for the oxidation of non-oxides, enable the volatilized salts to diffuse in time, effectively reduce their vapor pressure, and promote the volatilization of volatile salts and hazardous wastes such as Na3AlF6 at high temperature, reducing the content of volatile salts and hazardous wastes such as Na3AlF6 in the calcined aluminum ash; the volatile salts and hazardous waste salts of Na3AlF6 volatilized into the flue gas will condense onto the subsequent aluminum ash as the temperature of the flue gas decreases; the secondary aluminum ash containing volatile salts and hazardous fluoride salts will have the salts in it volatilize into the flue gas again in the high-temperature environment. By such cycling, the enrichment of volatile salts and hazardous wastes such as Na3AlF6 in the flue gas is thus formed.

[0024] In step one, in the preheating and heating-up stage of the secondary aluminum ash, the flue gas with an oxidation atmosphere causes the non-oxides and combustibles in the aluminum ash to undergo oxidation reactions, releasing heat and further reducing the heat consumption of the system.

[0025] In Step 3, volatile salts are enriched by volatilization and condensation, and then a bypass air discharge system is used to separately collect and treat the volatile salts and the Na3AlF6 volatile hazardous waste enriched in the high-temperature flue gas, reducing the wet treatment burden of volatile hazardous waste such as Na3AlF6 and improving its recovery efficiency, avoiding the impact of hazardous waste on the environment. Moreover, through high-temperature treatment, the obtained calcined product has few impurities and can be used in refractory materials, abrasive tools, ceramics, and composite materials, etc. It can be used to replace high-aluminum bauxite in the production of brown fused alumina, spinel, and aluminate cement, replace alumina micropowder in the ceramic industry, and also replace alumina in the electrolytic aluminum industry.

[0026] In Step 3, bypass air discharge devices are respectively set at the flue gas chamber and the outlet air duct of the fifth-stage suspension preheater cyclone because the melting points and volatilization temperatures of volatile salts and the fluoride Na3AlF6 are different: at the flue gas chamber at the kiln tail, the temperature of the flue gas is higher than 1000 °C, and the discharged flue gas mainly contains the hazardous substance of the fluoride Na3AlF6; after passing through the five-stage preheater cyclone, the temperature of the flue gas decreases, and the discharged flue gas mainly contains volatile salts such as KCl and NaCl. Separately collecting them is also convenient for the separation and reuse of different salts.

[0027] In Step 4, the secondary aluminum ash after high-temperature calcination is cooled and the heat is recycled. Under the condition of meeting the heating and temperature-raising requirements of the rotary kiln, the amount of combustion-supporting air is reduced, and partially preheated air is used for combustion support, reducing fuel consumption and meeting the requirements of energy conservation and emission reduction.

[0028] In Step 4, high-temperature air is respectively blown into the inlet air ducts of the fifth-stage suspension preheater cyclone and the fourth-stage suspension preheater cyclone as the tertiary air. The functions are as follows: to avoid the phenomenon that the working state of the suspension preheater system becomes unstable due to bypass air discharge, resulting in poor dispersibility of the secondary aluminum ash; the hot air tertiary air blown in at these two places simultaneously meets the oxidation atmosphere and concentration requirements of the flue gas in the suspension preheater system, strengthening the oxidation of the secondary aluminum ash, accelerating the oxidation rate of non-oxides (such as AlN, Al4C, metallic Al, and a small amount of graphite electrode inclusions, etc.) in the secondary aluminum ash, and having little impact on the working temperature of the preheater system; and because the gas-solid mass transfer is fast in the suspended state, the partial pressure of volatile salts (such as KCl, NaCl, Na3AlF6, etc.) in the secondary aluminum ash can be reduced, accelerating their volatilization.

[0029] The beneficial effects of the present invention are as follows:

[0030] The present invention has the characteristics of low treatment production cost, less environmental pollution, high separation efficiency of volatile salts and fluorides, thorough treatment of hazardous waste, and high collection efficiency, reducing the energy consumption of the aluminum ash calcination system, and is an optimal method for treating industrial hazardous waste such as aluminum ash.

[0031] 1) By adopting the methods of multi-stage suspension preheating, high-temperature volatilization - low-temperature condensation, the volatile salts and fluorides in secondary aluminum ash are enriched in the flue gas, and the volatile salts and fluorides are separated respectively, greatly reducing the wet starting treatment amount of secondary aluminum ash, improving the collection and treatment efficiency of waste, avoiding environmental pollution to water bodies, etc., and reducing the production cost.

[0032] 2) By adopting the treatment method of the present invention, non-oxides are oxidized at high temperature, avoiding the generation of NH3 or CH4 and its adverse effects on production safety.

[0033] 3) The whole process is simple, reducing the production treatment process, rationally utilizing waste heat, having no solid waste discharge, and calcining secondary aluminum ash to meet the requirements of subsequent production and use.

[0034] 4) Small and new cement calcining systems or newly built four-stage and five-stage suspension preheaters can be utilized to make full use of the existing equipment process system, improve the treatment and recycling efficiency of secondary aluminum ash, and reduce the production treatment cost. Description of the Drawings

[0035] Figure 1 is a process flow schematic diagram of the industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation of the present invention;

[0036] Note: The dotted line with an arrow is the upward air flow of the flue gas, and the solid line with an arrow is the material flow direction of secondary aluminum ash;

[0037] 1. First-stage suspension preheater; 2. Second-stage suspension preheater; 3. Third-stage suspension preheater; 4. Fourth-stage suspension preheater; 5. Fifth-stage suspension preheater; 6. Air inlet pipe; 7. Air outlet pipe; 8. Feed pipe; 9. Rotary kiln; 10. Smoke chamber; 11. First bypass air release device; 12. Second bypass air release device; 13. Primary air; 14. Secondary air; 15. Tertiary air; 16. Humidifying tower; 17. Bag filter; 18. Cooling equipment; 19. Secondary aluminum ash inlet.

[0038] Figure 2 is the DSC diagram of secondary aluminum ash in the embodiment. Detailed Embodiments

[0039] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, any method for treating secondary aluminum ash, such as heating and raising the temperature of secondary aluminum ash by suspending preheating gas-solid two-phase heat transfer, enriching volatile salts and fluoride hazardous wastes in aluminum ash by volatilization and condensation methods, removing volatile salts and fluoride hazardous wastes in secondary aluminum ash by high-temperature volatilization methods, classifying and collecting volatile salts and fluoride hazardous wastes in aluminum ash by a bypass air release device, accelerating the oxidation of non-oxides in aluminum ash by gas-solid two-phase mass transfer of flue gas in an oxidizing atmosphere in a suspended state to avoid the impact of CH4 and NH3 generated by wet treatment on safe production, and all other embodiments obtained by those of ordinary skill in the art without creative efforts, fall within the scope of protection of the present invention.

[0040] Using the method of the present invention, hazardous wastes such as volatile salts and fluorides in secondary aluminum ash can be industrially separated from secondary aluminum ash, and the secondary aluminum ash waste can be treated separately by classification. The following further details the specific implementation manners of the present invention with reference to examples.

[0041] An industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation according to the present invention includes the following steps:

[0042] Step 1: Rapid heating of secondary aluminum ash by suspending gas-solid two-phase heat transfer

[0043] The secondary aluminum ash is discharged from the aluminum ash silo, transported to the tower of the suspension preheater system by a bucket elevator, evenly fed by a screw conveyor, and added to the suspension preheater system through the secondary aluminum ash inlet 19. Relying on the high-speed turbulent flue gas and an additional dispersion device, the secondary aluminum ash is evenly dispersed in the high-temperature flue gas. The secondary aluminum ash is in a suspended state and undergoes gas-solid two-phase heat transfer with the high-temperature flue gas. The secondary aluminum ash is heated up while the flue gas is cooled down. The suspended mixed gas-solid two-phase fluid of the secondary aluminum ash and the high-temperature flue gas successively passes through the first to fifth-stage suspension preheaters for multi-stage suspension heating and gas-solid separation, heating the secondary aluminum ash from room temperature to above 950 °C. The separation efficiency of the cyclone of the first-stage preheater reaches 92% or more, avoiding more dust from entering the waste gas treatment system; the separation efficiency of the cyclone of the fifth-stage preheater reaches 89% or more, increasing the amount of secondary aluminum ash entering the kiln; the separation efficiency of the cyclones of the intermediate-stage preheaters is 80 - 85%, reducing the operating resistance of the entire preheater system. The entire mixing and heating process takes 5 - 10 minutes, and the finally discharged flue gas temperature is lower than 200 °C. The discharged flue gas is discharged from the suspension preheater system through the air outlet pipe 7 of the cyclone of the first-stage suspension preheater 1 and enters the humidifying tower 16 and the bag filter 17 for dust removal treatment and then is discharged up to standard.

[0044] The suspension preheater system is a five-stage suspension preheater connected in sequence. Each stage of the suspension preheater includes a cyclone, an air inlet pipe 6, an air outlet pipe 7, and a feeding pipe 8. After the secondary aluminum ash is added, it is evenly dispersed in the high-temperature flue gas under the impact of the high-speed airflow of the high-temperature flue gas rising in the additional dispersion device and the air outlet pipe 7 of the cyclone of the second-stage suspension preheater 2, and enters the air inlet pipe 6 of the cyclone of the first-stage suspension preheater 1. It is heated by the first dispersion and suspension with the high-temperature flue gas rising in the air outlet pipe 7 of the cyclone of the second-stage suspension preheater 2 and is carried into the cyclone of the first-stage suspension preheater 1 for gas-solid separation. The separated secondary aluminum ash enters the air inlet pipe 6 of the cyclone of the second-stage suspension preheater 2 through the feeding pipe 8 of the cyclone of the first-stage suspension preheater 1, and is heated by the second dispersion and suspension with the high-temperature flue gas rising in the air outlet pipe 7 of the cyclone of the third-stage suspension preheater 3 and is carried into the cyclone of the second-stage suspension preheater 2 for gas-solid separation; and so on. The suspension mixed gas-solid two-phase airflow of the secondary aluminum ash and the high-temperature flue gas sequentially enters the cyclones of the third-stage, fourth-stage, and fifth-stage suspension preheaters (3, 4, 5) for step-by-step suspension heating and gas-solid separation. The air inlet pipe 6 of the cyclone of the fifth-stage suspension preheater 5 is connected to the tail gas chamber 10 of the rotary kiln 9, and its high-temperature flue gas comes from the rotary kiln 9.

[0045] Step 2: Enrich the volatile salts and fluorides in the secondary aluminum ash

[0046] Enrich the volatile salts and fluorides in the secondary aluminum ash by the way of volatilization-condensation; the suspension mixed gas-solid two-phase fluid of the secondary aluminum ash and the high-temperature flue gas sequentially passes through the five-stage suspension preheater, from the first-stage suspension preheater 1 to the fifth-stage suspension preheater 5, and the secondary aluminum ash is gradually heated from room temperature by suspension; from the fifth-stage suspension preheater 5 to the first-stage suspension preheater 1, the temperature of the high-temperature flue gas gradually decreases; when the high-temperature flue gas coming from the rotary kiln 9 enters the fifth-stage suspension preheater 5 and is dispersed in contact with the secondary aluminum ash, the temperature of the flue gas decreases, and the fluorides in the flue gas are immediately condensed in the secondary aluminum ash; when the flue gas meets the secondary aluminum ash in the low-temperature part of the suspension preheater system (the first-stage to the third-stage suspension preheaters), the volatile salts are condensed in the secondary aluminum ash, and as the secondary aluminum ash enters the high-temperature part of the suspension preheater system, the volatile salts therein volatilize again into the flue gas and are enriched; the secondary aluminum ash enters the rotary kiln 9 for high-temperature calcination, and the fluorides therein volatilize again into the flue gas. This process repeats, and the volatile salts and fluorides in the secondary aluminum ash always exist in the suspension preheater and the rotary kiln 9 system, and are continuously enriched in the flue gas by the method of high-temperature volatilization-low-temperature condensation.

[0047] Step 3: Classify and collect the volatile salts and fluorides in the secondary aluminum ash

[0048] By setting up the bypass air release device I 11 and the bypass air release device II 12, the enriched volatile salts and fluorides are collected separately. The bypass air release device I 11 is set at the position of the kiln tail smoke chamber 10 of the rotary kiln 9. The temperature of the smoke chamber 10 is about 1100 °C, and part of the high-temperature flue gas enriched with fluoride Na3AlF6 is discharged. The bypass air release device II 12 is set at the position of the outlet air pipe of the cyclone of the fifth-stage suspension preheater 5. The temperature of the flue gas is 950 °C, and part of the high-temperature flue gas enriched with volatile salts such as KCl and NaCl is discharged. The bypass air release is carried out once every 1 - 3 minutes. The hot flue gas containing volatile salt-containing substances and fluoride-containing dangerous waste discharged by the bypass air release is respectively treated by the conventional industrial secondary aluminum ash wet treatment method to collect the salts therein for recycling. The discharged gas has a low dust concentration but a high content of volatile salt-containing substances, reducing the amount of solid waste in the wet treatment of aluminum ash. The salt-free solid fly ash after treatment re-enters the suspension preheater and enters the rotary kiln 9 together with the subsequent secondary aluminum ash for dense sintering.

[0049] When treating the waste gas discharged by the bypass air release, the treatment amount of the solid waste in the wet treatment is only the soot in the waste gas, which is much smaller than the treatment amount of the solid waste in the existing wet treatment of secondary aluminum ash, about 5 - 10% of its total amount. Accordingly, the scale of the corresponding treatment equipment is small, the floor area is small, and the treatment cost is greatly reduced. Moreover, by using the two-stage bypass air release treatment, the chlorides and fluorides are directly separated without the need for separate separation.

[0050] While carrying out the bypass air release, hot air tertiary air 15 collected after the strengthened cooling of the calcined aluminum ash is respectively blown into the inlet air pipes 6 of the cyclones of the fifth-stage suspension preheater 5 and the fourth-stage suspension preheater 4. The temperature of the hot air tertiary air 15 is about 700 °C. The air volume of the hot air tertiary air 15 blown into the inlet air pipe 6 of the cyclone of the fifth-stage suspension preheater 5 accounts for 20% of the flue gas volume of the system. The air volume of the hot air tertiary air 15 blown into the inlet air pipe 6 of the cyclone of the fourth-stage suspension preheater 4 accounts for 20% of the flue gas volume of the system.

[0051] Step Four: High-temperature Calcination:

[0052] In Step 1, the secondary aluminum ash separated through multi-stage suspension preheating enters the rotary kiln 9 for high-temperature calcination. The rotary kiln 9 uses natural gas as fuel and is heated by a multi-channel burner at the kiln head. The combustion flame is in a reducing atmosphere, which can lengthen the flame length, better uniformly heat the materials, and extend the residence time of the materials in the high-temperature zone. When calcined in the rotary kiln 9, the calcination temperature is 1300 °C and the calcination time is 50 min. The calcined secondary aluminum ash is cooled by the cooling equipment 18: At the end of the cooling of the calcined secondary aluminum ash, the low-temperature hot air is collected as the primary air 13 and enters the multi-channel burner at the kiln head of the rotary kiln 9 as the combustion-supporting air. The temperature of the combustion-supporting air is between 250 °C and 300 °C, reducing the fuel consumption during the calcination of the rotary kiln 9; at the same time, high-temperature secondary air 14 is blown into the kiln as the combustion-supporting air to make the fuel burn fully. The flue gas in the kiln is in a weakly oxidizing atmosphere. Under the condition of meeting the heating and temperature-rising requirements of the rotary kiln, the use amount of the combustion-supporting air can be reduced, and the fuel consumption can be reduced; at the air inlets 6 of the cyclones of the fifth-stage suspension preheater 5 and the fourth-stage suspension preheater 4, hot air at 700 °C collected during the enhanced cooling of the calcined aluminum ash is respectively blown in as the tertiary air 15 to make the suspension preheating system in an oxidizing atmosphere. The amount of hot air blown into each part is 20% of the total flue gas volume.

[0053] Only KCl and NaCl in the secondary aluminum ash volatilize at 800 °C - 1000 °C, while Na6AlF3 starts to volatilize only when the temperature exceeds 1000 °C. It is difficult for fluorides to volatilize in the suspension preheater system. Only high-temperature calcination in the rotary kiln can promote their volatilization. Through high-temperature and long-time calcination, the volatile salts and fluorides in the secondary aluminum ash are completely volatilized without remaining in the calcination product.

[0054] A thermal analyzer is used to detect the weight change of the secondary aluminum ash sample during the heating process, analyze the thermal change characteristics of the main components in the secondary aluminum ash sample, and thus inversely infer the approximate content of the main phases and their reaction temperature ranges, providing basic data for the high-temperature volatilization and harmless treatment of hazardous waste such as volatile salts and fluorides in the secondary aluminum ash.

[0055] Perform thermal analysis on the original secondary aluminum ash sample. From Figure 2 the DSC graph of the secondary aluminum ash, it can be seen that:

[0056] 1) Between 800 °C and 1000 °C, there is a rapid weight loss interval. From the DTG curve, the maximum weight loss rate is at 1000 °C. Above 1000 °C, the weight loss rate slows down, and the weight loss rate remains constant at 1200 °C. The corresponding DSC curve shows an endothermic phenomenon. The data shows that the weight loss below 1000 °C is related to the volatilization of salt fluxes such as NaCl and KCl in the aluminum ash. Differential thermal analysis shows that NaCl has a strong endothermic peak at 800 °C and the maximum weight loss rate at 900 °C. Pure KCl starts to volatilize at 770 °C, and the volatilization end temperature is 950 °C. When other substances are present, its volatilization rate will be reduced.

[0057] 2) The melting point of sodium hexafluoroaluminate (Na3AlF6) is 1009 °C, and its vapor pressure in the molten state is 253.2 Pa. It is volatile. Therefore, the weight loss when the temperature exceeds 1000 °C is related to the volatilization of Na3AlF6.

[0058] Based on the thermal analysis test results of the secondary aluminum ash sample and relevant data, it is proved that 800 °C - 1000 °C is the volatilization interval of salts such as NaCl and KCl in the secondary aluminum ash. Thermal analysis shows that there is a large mass loss corresponding to it. The melting point of sodium hexafluoroaluminate (Na3AlF6) is 1009 °C, and its vapor pressure in the molten state is 253.2 Pa. It is volatile. Therefore, Na3AlF6 can volatilize into the flue gas only when the temperature exceeds 1000 °C. There are obvious temperature boundaries for the volatilization of volatile chlorides and fluorides, which provides the possibility for high-temperature volatilization and classified harmless treatment of volatile salts and fluorides and other hazardous wastes in the secondary aluminum ash.

[0059] The chemical composition contents of the secondary aluminum ash before and after calcination are shown in Table 1:

[0060] Table 1 Chemical composition in secondary aluminum ash before and after calcination (wt%)

[0061]

[0062] The analysis of the chemical composition in the secondary aluminum ash before and after calcination shows that the contents of F and Cl in the secondary aluminum ash after calcination decrease sharply, indicating that high-temperature calcination of secondary aluminum ash is a reliable method to thoroughly remove chloride and fluoride hazardous wastes in secondary aluminum ash.

[0063] According to the alumina content of the original secondary aluminum ash, the alumina content of the calcined aluminum ash is determined, and the use of the calcined aluminum ash is determined: The calcined aluminum ash with a high magnesia content can be used for the production of calcined or electrofused magnesia-aluminum spinel; The calcined aluminum ash with a high calcium oxide content can be used for the production of aluminate cement; The calcined aluminum ash with a high alumina content or a high sodium oxide and potassium oxide content can be used to replace alumina micropowder in the opacifying glaze of sanitary ceramics after iron removal; The calcined aluminum ash with a high alumina content and low other impurity contents can be used for the production of electrofused brown fused alumina, and can also replace industrial alumina for electrolytic production of metallic aluminum.

Claims

1. An industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation, characterized in that, It includes the following steps: Step 1: Rapid temperature rise of secondary aluminum ash through suspension gas-solid two-phase heat exchange The secondary aluminum ash is evenly fed into the suspension preheater system. Under the action of the additional dispersion device and the high-speed flowing flue gas, the secondary aluminum ash is evenly dispersed in the high-temperature flue gas. The suspension gas-solid two-phase fluid of the secondary aluminum ash and the high-temperature flue gas successively passes through five-stage suspension preheaters, and is gradually subjected to suspension mixing heating and gas-solid separation. From the first-stage suspension preheater to the fifth-stage suspension preheater, the secondary aluminum ash is gradually heated from room temperature by suspension; from the fifth-stage suspension preheater to the first-stage suspension preheater, the temperature of the high-temperature flue gas gradually decreases; the entire mixing and heating process takes 5 to 10 minutes, heating the secondary aluminum ash from room temperature to above 950°C, and finally the flue gas discharged from the suspension preheater system has a temperature ≤ 200°C. The low-temperature flue gas enters the humidifying tower and the bag filter, and is discharged up to standard after dust removal treatment; Step 2: Enrich the volatile salts and fluorides in the secondary aluminum ash The high-temperature flue gas from the rotary kiln gradually heats the secondary aluminum ash through suspension mixing in the suspension preheater system. At the tail gas chamber of the rotary kiln, part of the fluorides volatilized and enriched in the flue gas are discharged here; in the high-temperature part of the fourth and fifth-stage suspension preheater systems, when the flue gas temperature is lower than 1000°C, the fluorides volatilized in the flue gas condense in the subsequent secondary aluminum ash, and are separated from the gas and solid in the fifth-stage suspension preheater, and enter the rotary kiln with the secondary aluminum ash for calcination. The fluorides volatilize into the flue gas again, and so on and are enriched in the high-temperature flue gas; in the low-temperature part of the preheater of the first to third-stage suspension preheater systems, the volatile salts condense in the subsequent secondary aluminum ash, and enter the high-temperature part of the suspension preheater system with the secondary aluminum ash, and volatilize into the flue gas again and are enriched; again and again, the volatile salts and fluorides in the secondary aluminum ash always circulate in the suspension preheater and the rotary kiln system, and are continuously enriched by the method of high-temperature volatilization-low-temperature condensation; Step 3: Classify and collect the volatile salts and fluorides in the secondary aluminum ash 1) The enriched volatile salts and fluorides are classified and collected by setting up bypass air release device 1 and bypass air release device 2: Bypass air release device 1 is set at the tail gas chamber of the rotary kiln, and the temperature of the gas chamber is 1000°C to 1100°C, and part of the high-temperature flue gas discharging the enriched fluoride Na3AlF6 is discharged; Bypass air release device 2 is set at the outlet air pipe of the cyclone of the fifth-stage suspension preheater, and the temperature of the flue gas is 800°C to 1000°C, and part of the high-temperature flue gas discharging the enriched volatile salts of KCl and NaCl is discharged; The intermittent air release is carried out at a time interval determined according to the components in the secondary aluminum ash and the concentration of volatile salts and fluorides in the flue gas; A continuous bypass air release device can also be used, and the bypass air release air volume is controlled through a fan and a pipeline valve; 2) High-temperature flue gas enriched with volatile salts and fluorides is discharged from the bypass air release device 1 and the bypass air release device 2 respectively. The flue gas components of the bypass air release device 1 are mainly Na3AlF6 volatile hazardous waste and fly ash generated from the calcination of secondary aluminum ash in the rotary kiln. Conventional industrial wet treatment of secondary aluminum ash is adopted to collect Na3AlF6 therein and conduct separation and recycling; the flue gas components of the bypass air release device 2 are mainly KCl and NaCl volatile salts and a small amount of fly ash. Conventional industrial wet treatment of secondary aluminum ash is adopted to collect KCl and NaCl therein and conduct separation and recycling; Step Four: High-temperature Calcination The secondary aluminum ash separated by multi-stage suspension preheating and gas-solid separation in Step One enters the rotary kiln for high-temperature calcination. The calcination temperature is 1100°C to 1400°C, and the calcination time is 30 to 50 minutes; the secondary aluminum ash after high-temperature calcination is cooled and the heat is recycled.

2. The industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation according to claim 1, characterized in that: The suspension preheater in Step One includes a cyclone, an air inlet pipe, an air outlet pipe, and a feeding pipe; after the secondary aluminum ash is added, it is evenly dispersed in the high-temperature flue gas under the impact of the additional dispersion device and high-speed air flow, enters the air inlet pipe of the cyclone of the first-stage suspension preheater, and conducts the first dispersion suspension heating with the rising high-temperature flue gas in the air outlet pipe of the cyclone of the second-stage suspension preheater, and is carried into the cyclone of the first-stage suspension preheater for gas-solid separation. The separated secondary aluminum ash enters the air inlet pipe of the cyclone of the second-stage suspension preheater through the feeding pipe of the cyclone of the first-stage suspension preheater, and conducts the second dispersion suspension heating with the rising high-temperature flue gas in the air outlet pipe of the cyclone of the third-stage suspension preheater, and is carried into the cyclone of the second-stage suspension preheater for gas-solid separation; and so on. The suspension mixed gas-solid two-phase fluid of the secondary aluminum ash and the high-temperature flue gas sequentially enters the cyclones of the third-stage, fourth-stage, and fifth-stage suspension preheaters for step-by-step suspension heating and gas-solid separation. The air inlet pipe of the fifth-stage suspension preheater is connected to the tail gas chamber of the rotary kiln, and its high-temperature flue gas comes from the rotary kiln.

3. The industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation according to claim 1, characterized in that: In Step Four, the secondary aluminum ash after high-temperature calcination is cooled to recover low-temperature hot air and high-temperature hot air. Among them, part of the high-temperature hot air is used as the tertiary air and enters the air inlets of the cyclones of the fifth-stage suspension preheater and the fourth-stage suspension preheater respectively, so that the high-temperature flue gas in the suspension preheater system is in an oxidizing atmosphere, and the graphite and other non-oxides in the secondary aluminum ash are fully oxidized; another part of the high-temperature air is used as the secondary air and enters the rotary kiln to supplement the insufficient combustion-supporting air, reducing the fuel consumption of the aluminum ash calcination system; the low-temperature hot air is used as the primary air combustion-supporting air and enters the multi-channel burner at the head of the rotary kiln: in order to lengthen the flame length, make the calcination temperature in the rotary kiln uniform, extend the high-temperature zone, and extend the high-temperature residence time of the material, the multi-channel burner flame is a reducing flame; under the action of the high-temperature secondary air entering the kiln in the later stage, the flue gas generated by combustion is finally in a weakly oxidizing atmosphere; using hot air for combustion support can reduce the fuel consumption during the calcination of the rotary kiln.

4. The industrial separation method for removing fluoride hazardous waste from secondary aluminum ash by volatilization and condensation according to claim 1, characterized in that: In Step 3, the flue gas volume discharged from the smoke chamber and the outlet air duct of the fifth-stage suspension preheater cyclone is determined according to the contents of volatile salts and Na3AlF6 volatile hazardous waste in the secondary aluminum ash, accounting for 10-20% of the system flue gas volume respectively; to maintain the stable working state of the suspension preheater system, hot air (tertiary air) is respectively blown into the inlet air ducts of the fifth-stage suspension preheater cyclone and the fourth-stage suspension preheater cyclone. This hot air is sourced from the heat recovery during the cooling of the calcined aluminum ash; the air volume of the blown hot air (tertiary air) is related to the bypass air discharge volume, accounting for 10-30% of the system flue gas volume respectively.

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