Method for extracting aluminum oxide by treating fly ash through nitric acid method

Abstract

The invention provides a method for extracting aluminum oxide by treating fly ash through a nitric acid method, and belongs to the technical field of non-ferrous metallurgy. According to the method for extracting the aluminum oxide by treating the fly ash through the nitric acid method, external voltage is applied during acid leaching, ions in nitric acid are promoted to migrate, the electron obtaining capacity of hydrogen ions is improved, the reaction with the aluminum oxide can be accelerated, the dissolution process is promoted, and therefore leaching of the aluminum oxide at the temperature of 60-100 DEG C under the normal pressure is achieved; the production safety is improved; by adopting fluidized roasting, the mineral activity is improved, the roasting time is shortened to the second level, the energy consumption is reduced, and the production efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of nonferrous metal metallurgy, in particular to a method for extracting aluminum oxide by treating fly ash with a nitric acid method. Background Art

[0002] Fly ash is a byproduct of coal burning in thermal power plants and is the main source of solid waste pollution in my country. Fly ash is rich in alumina and is a potential substitute for bauxite. Using fly ash in the production of alumina can not only solve the problem of alumina resource shortage, but also achieve high value-added waste recycling and effectively prevent fly ash stacking from polluting the environment, which has great practical significance.

[0003] At present, there are two main methods for extracting alumina from high-alumina fly ash: acid method and alkaline method. The alkaline process, based on the Bayer process, has the disadvantages of high energy consumption and the production of a large amount of tailings. The acid process mainly includes hydrochloric acid leaching, concentrated sulfuric acid leaching and nitric acid leaching; during the production process of hydrochloric acid leaching, the acidic gas of hydrogen chloride is easy to volatilize and the equipment is seriously corroded; the sulfuric acid leaching method has the disadvantages of high leaching temperature and high pressure. Compared with hydrochloric acid and sulfuric acid leaching methods, nitric acid leaching has better selectivity and produces less wastewater, which can be recycled through simple treatment.

[0004] The factors that affect the alumina leaching rate of fly ash treated with nitric acid include roasting time, acid leaching solid-liquid ratio, acid leaching time and temperature. In order to improve the extraction rate, the traditional method of extracting alumina with nitric acid is usually carried out under high temperature and pressure conditions, with a temperature of 130-180°C and a pressure of 0.2-1.0MPa. This not only increases equipment investment and energy consumption, but also accelerates nitric acid decomposition in a high temperature environment, generating more heat and gas, posing a greater safety hazard. In addition, the traditional treatment method is to carry out minute-to-hour roasting in a kiln, which has a long reaction time and high energy consumption. Summary of the invention

[0005] The purpose of the present invention is to provide a method for extracting aluminum oxide from fly ash by treating fly ash with nitric acid. The method provided by the present invention does not require high temperature and high pressure acid leaching and long time roasting, improves production safety, and has a higher aluminum oxide extraction rate.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for extracting aluminum oxide by treating fly ash with a nitric acid method, comprising the following steps:

[0008] 1) acid leaching fly ash in a nitric acid solution to obtain an acid leaching solution; applying a voltage during the acid leaching, the voltage being 1 to 5 V; the temperature of the acid leaching being 60 to 100° C., and the pressure of the acid leaching being normal pressure;

[0009] 2) crystallizing the acid leaching solution obtained in step 1) to obtain Al(NO3)3·9H2O crystals;

[0010] 3) Fluidizing and calcining the Al(NO3)3·9H2O crystals obtained in step 2) to obtain aluminum oxide.

[0011] Preferably, the concentration of the nitric acid solution in step 1) is 4-8 mol / L.

[0012] Preferably, the molar ratio of nitric acid in the nitric acid solution in step 1) to alumina in fly ash is (6-8):1.

[0013] Preferably, the acid leaching time in step 1) is 3 to 5 hours.

[0014] Preferably, the fly ash in step 1) is pretreated before use, and the pretreatment includes mixing slurry, grinding, magnetic separation and filtering performed in sequence.

[0015] Preferably, in the step 2), the acid leaching solution is firstly subjected to impurity removal before crystallization, and the impurity removal includes iron precipitation, calcium removal and solid-liquid separation in sequence; the iron precipitation method is: adjusting the pH value of the acid leaching solution to 3-5 to precipitate iron ions.

[0016] Preferably, the temperature of the fluidized calcination in step 3) is 700-900°C.

[0017] Preferably, the fluidized calcination time in step 3) is 8 to 10 seconds.

[0018] Preferably, the flue gas generated by the fluidized roasting in step 3) is recovered and reused.

[0019] The present invention provides a method for extracting aluminum oxide from fly ash by treating fly ash with nitric acid, comprising the following steps: 1) acid leaching fly ash in a nitric acid solution to obtain an acid leaching solution; applying a voltage during the acid leaching, the voltage is 1-5V; the temperature of the acid leaching is 60-100°C, and the pressure of the acid leaching is normal pressure; 2) crystallizing the acid leaching solution obtained in the step 1) to obtain Al(NO3)3·9H2O crystals; 3) fluidizing and roasting the Al(NO3)3·9H2O crystals obtained in the step 2) to obtain aluminum oxide. The present invention applies an external voltage during the acid leaching to promote the migration of ions in nitric acid, improve the ability of hydrogen ions to obtain electrons, accelerate the reaction with aluminum oxide, and promote the dissolution process, thereby realizing the leaching of aluminum oxide at 60-100°C and normal pressure, and improving production safety; by adopting fluidized roasting, the mineral activity is improved, the roasting time is shortened to seconds, the energy consumption is reduced, and the production efficiency is improved. The results of the embodiment show that the aluminum oxide extraction rate of the method of the present invention reaches 91.6-96%. DETAILED DESCRIPTION

[0020] All raw materials of the present invention have no particular limitation on their sources and can be purchased from the market or prepared according to conventional methods known to those skilled in the art.

[0021] The present invention provides a method for extracting aluminum oxide by treating fly ash with a nitric acid method, comprising the following steps:

[0022] 1) acid leaching fly ash in a nitric acid solution to obtain an acid leaching solution; applying a voltage during the acid leaching, the voltage being 1 to 5 V; the temperature of the acid leaching being 60 to 100° C., and the pressure of the acid leaching being normal pressure;

[0023] 2) crystallizing the acid leaching solution obtained in step 1) to obtain Al(NO3)3·9H2O crystals;

[0024] 3) Fluidizing and calcining the Al(NO3)3·9H2O crystals obtained in step 2) to obtain aluminum oxide.

[0025] The invention carries out acid leaching of fly ash in a nitric acid solution to obtain an acid leaching solution.

[0026] The present invention preferably performs a component test on the fly ash to be treated. The present invention determines the content of alumina by testing the components of the fly ash, which is beneficial to controlling the use of various materials and can also calculate the extraction rate of alumina. The present invention has no particular limitation on the method of testing the components of the fly ash, as long as the content of the main components in the fly ash can be determined.

[0027] In the present invention, the fly ash is preferably pretreated before use, and the pretreatment preferably includes mixing slurry, grinding, magnetic separation and filtering in sequence. The present invention refines the fly ash and preliminarily removes impurities through pretreatment, thereby increasing the content of alumina in the fly ash, which is beneficial to further improve the extraction rate of alumina and reduce the consumption of nitric acid.

[0028] In the present invention, the mixed slurry is preferably dispersed in a solvent. The present invention has no special requirements for the solvent, as long as the fly ash can be dispersed. As an embodiment of the present invention, the solvent can be water, an alcohol solvent or a mixed solvent thereof. The present invention has no special requirements for the amount of the solvent, as long as the fly ash can be evenly dispersed. In an embodiment of the present invention, the amount of fly ash and solvent is 1000g: (3~5)L.

[0029] The present invention has no special requirements for the grinding equipment, as long as the fly ash can be crushed and refined. As an embodiment of the present invention, the grinding equipment can be a wet ultra-high speed grinder, a tubular disperser or a magnetic grinder.

[0030] In the present invention, the fly ash is preferably ground to a particle size of 74 μm or less with a particle mass ratio of not less than 60%. The fly ash is ground into fine particles, which is beneficial to the subsequent impurity removal and extraction of alumina.

[0031] The present invention has no special requirements for the magnetic separation equipment, as long as it can remove iron from fly ash. As an embodiment of the present invention, the magnetic separation equipment can be a permanent magnetic separator or an electromagnetic magnetic separator.

[0032] The present invention has no special requirements for the filtering equipment, as long as it can separate the slurry into solid and liquid. As an embodiment of the present invention, the filtering equipment can be a vacuum filter, a pressure filter or a centrifugal filter; in an embodiment of the present invention, the filtering equipment is a vacuum filter.

[0033] As an embodiment of the present invention, fly ash is pretreated to obtain a filter cake, and the filter cake is acid-leached in a nitric acid solution to obtain an acid leaching solution.

[0034] In the present invention, the concentration of the nitric acid solution is preferably 4-8 mol / L. As an embodiment of the present invention, the concentration of the nitric acid solution can be 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L or 8 mol / L. In the present invention, the concentration of the nitric acid solution within the above range is conducive to further improving the extraction rate and extraction speed of aluminum oxide.

[0035] In the present invention, the molar ratio of nitric acid in the nitric acid solution to alumina in fly ash is preferably (6-8):1. As an embodiment of the present invention, the molar ratio of nitric acid in the nitric acid solution to alumina in fly ash can be 6:1, 7:1 or 8:1. In the present invention, the molar ratio of nitric acid in the nitric acid solution to alumina in fly ash is within the above range, which is conducive to further improving the extraction rate of alumina.

[0036] As an embodiment of the present invention, the liquid-solid ratio of the nitric acid solution to the fly ash can be (3-10) L:1 kg, (4-9) L:1 kg, or (5-8) L:1 kg. When the liquid-solid ratio of nitric acid to fly ash is within the above range, the fly ash and nitric acid can be fully contacted, which is beneficial to further improve the extraction rate of aluminum oxide.

[0037] In the present invention, a voltage is applied during the acid leaching, and the voltage is 1 to 5V. As an embodiment of the present invention, the voltage may be 2V, 3V, 4V or 5V. The magnitude of the voltage affects the effect of the acid leaching. If the voltage is too small, the effect of improving the acid leaching is not obvious. If the voltage is too high, side reactions may occur, which is not conducive to the extraction of aluminum oxide. The present invention limits the voltage within the above range, which can improve the effect of the acid leaching, reduce the temperature of the acid leaching, and realize the acid leaching under normal pressure.

[0038] The present invention has no particular requirements for the manner of applying voltage, as long as the acid leaching can be performed under voltage conditions. As an embodiment of the present invention, the acid leaching can be performed in an electrolytic cell, and the voltage can be applied through the electrodes of the electrolytic cell.

[0039] In the present invention, the acid leaching temperature is 60-100°C, preferably 80-100°C, and more preferably 90-100°C. The higher the acid leaching temperature, the more conducive it is to the extraction of alumina, but too high a temperature will accelerate the decomposition of nitric acid and increase the potential safety hazards of production. In the present invention, the acid leaching pressure is normal pressure. The present invention reduces the acid leaching temperature to below 100°C by applying a voltage during acid leaching, and performs the acid leaching under normal pressure, which greatly improves the safety of production.

[0040] In the present invention, the acid leaching time is preferably 3 to 5 hours, more preferably 3.5 to 4.5 hours. When the acid leaching time is within the above range, the aluminum oxide in the fly ash can fully react with the nitric acid, which is beneficial to further improve the extraction rate of aluminum oxide.

[0041] In the present invention, the acid leaching is preferably carried out under stirring conditions, and the stirring speed is preferably 80 to 100 r / min, more preferably 85 to 95 r / min. When the stirring speed is within the above range, the fly ash and nitric acid can be fully contacted, which is beneficial to further improve the extraction rate of alumina.

[0042] After obtaining the acid leaching solution, the present invention crystallizes the acid leaching solution to obtain Al(NO3)3·9H2O crystals.

[0043] In the present invention, the acid leaching solution is preferably subjected to impurity removal before crystallization; the impurity removal preferably includes iron precipitation, calcium removal and solid-liquid separation performed in sequence.

[0044] In the present invention, the method of precipitation of iron is preferably: adjusting the pH value of the acid leaching solution to precipitate iron ions; the pH value is preferably 3-5; in an embodiment of the present invention, the pH value may be 3.5, 3.9, 4.0, 4.2, 4.5 or 4.8. In the present invention, it is preferred to use an alkali solution to adjust the pH value of the acid leaching solution. The present invention has no requirements on the type and concentration of the alkali solution, as long as the pH value of the acid leaching solution can be adjusted to 3-5. In an embodiment of the present invention, the alkali solution is a sodium hydroxide solution. By adjusting the pH value of the acid leaching solution to 3-5, the present invention can remove iron ions in the form of precipitation, while aluminum ions will not precipitate in large quantities at this pH value.

[0045] The present invention has no particular requirements for the specific method of removing calcium, as long as the calcium in the acid leaching solution can be removed. In the implementation of the present invention, Tulsimer CH-93 resin is used to remove calcium in the acid leaching solution, and the calcium removal rate can reach 99.2%.

[0046] The present invention has no special requirements for the solid-liquid separation method, as long as the insoluble matter in the acid leaching solution can be removed. In the embodiment of the present invention, the solid-liquid separation is performed by sedimentation.

[0047] In the present invention, the crystallization method is preferably evaporative crystallization. The temperature of the evaporative crystallization is preferably 20 to 80°C, more preferably 40 to 75°C, and further preferably 60 to 70°C. As an embodiment of the present invention, the temperature of the evaporative crystallization may be 45°C, 50°C, 55°C, 65°C or 72°C. The higher the temperature of the evaporative crystallization, the faster the crystallization rate, but too fast a crystallization rate is not conducive to improving the purity of the crystal, and too high a temperature will cause the crystal to decompose, reduce the amount of crystallization, and also waste energy.

[0048] In the present invention, the crystallization is preferably carried out under stirring conditions, and the stirring rate is preferably 300-500 r / min. As an embodiment of the present invention, the stirring rate can be 350 r / min, 380 r / min, 400 r / min, 430 r / min, 450 r / min or 470 r / min. The stirring rate within the above range is conducive to the precipitation of crystals.

[0049] After the crystallization is completed, the present invention preferably separates the crystals obtained by the crystallization from the mother liquor, washes and dries them, and obtains Al(NO3)3·9H2O crystals. The present invention has no particular requirements for the specific method of the separation, as long as the crystals can be separated from the mother liquor. As an embodiment of the present invention, the separation can be centrifugation, filtration, sedimentation flotation or super gravity separation.

[0050] The present invention has no particular requirements for the specific method of washing, as long as the impurities on the surface of the crystal can be removed. As an embodiment of the present invention, deionized water or purified water can be used for washing. The present invention has no particular requirements for the specific method of drying, as long as the moisture on the surface of the crystal can be removed. As an embodiment of the present invention, natural drying or hot air drying can be used.

[0051] After obtaining Al(NO3)3·9H2O crystals, the present invention fluidizes and roasts the Al(NO3)3·9H2O crystals to obtain aluminum oxide.

[0052] In the present invention, the temperature of the fluidized calcination is preferably 700-900° C., more preferably 750-850° C. Al(NO3)3·9H2O crystals will decompose into aluminum oxide and nitrogen oxides during the calcination process. The present invention controls the calcination temperature within the above range, which is conducive to controlling the progress of the reaction, avoiding too low a temperature and too slow a decomposition rate, and avoiding too high a temperature and causing a large amount of nitrogen oxides to be produced in a short time, which is highly dangerous.

[0053] In the present invention, the fluidized calcination time is preferably 8 to 10 seconds. As an embodiment of the present invention, the calcination time can be 8 seconds, 8.5 seconds, 9 seconds, 9.5 seconds or 10 seconds. The present invention shortens the calcination time to seconds by adopting fluidized calcination, greatly shortens the calcination time and saves energy.

[0054] As an embodiment of the present invention, the fluidized calcination device adopts a vertical tube furnace complete set, the calcination furnace adopts a temperature controller to control the calcination temperature, a quartz tube reactor is arranged near the electric heating element, and a porous quartz sand core is built in, which is used as a gasification bed, and the air permeability is 0.03-0.06m 3 / m 2 ·s.

[0055] In the present invention, it is preferred to recycle the flue gas generated by the fluidized roasting. The present invention can prepare nitric acid again by recycling the roasting flue gas, thereby reducing the cost of nitric acid in the entire process, and there is no waste gas emission, which is environmentally friendly. As an embodiment of the present invention, the flue gas recycling and reuse can adopt a flue gas circulation and SNCR combined denitrification process system, or a system that adopts nitrate gas circulation pyrolysis to produce nitric acid.

[0056] The present invention applies an external voltage during acid leaching to promote the migration of ions in nitric acid, thereby improving the ability of hydrogen ions to obtain electrons, accelerating the reaction with aluminum oxide, and promoting the dissolution process, thereby achieving the leaching of aluminum oxide at 60-100° C. and normal pressure, thereby improving production safety; by adopting fluidized roasting, the activity of the mineral is increased, the roasting time is shortened to seconds, the energy consumption is reduced, and the production efficiency is improved.

[0057] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0058] Example 1

[0059] A method for extracting aluminum oxide by treating fly ash with nitric acid, the specific steps are:

[0060] The fly ash from a power plant was analyzed for its composition, and the main components are shown in Table 1.

[0061] Table 1 Main chemical components of fly ash

[0062] composition <![CDATA[Al2O3]]> <![CDATA[SiO2]]> CaO <![CDATA[Fe2O3]]> <![CDATA[Na2O]]> <![CDATA[TiO2]]> SUM content(%) 51.86 40.30 0.13 0.44 0.87 1.49 95.09

[0063] 1000g of fly ash from the power plant was mixed with 3L of water to form an original slurry; the original slurry was ground using a wet ultra-high-speed grinder until the particles below 74μm accounted for 60% (mass ratio), and then magnetically separated and vacuum filtered using a permanent magnetic separator. The filter cake obtained was acid-leached with nitric acid in an electrolytic cell, and an electrode was inserted into the electrolytic cell to ensure that the electrode was in full contact with the solution. An external voltage was applied, and the voltage was 2V; the concentration of nitric acid was 5mol / L, the liquid-to-solid ratio was 6:1, the molar ratio of nitric acid to alumina in fly ash was 6:1, the stirring speed was 80r / min, the dissolution reaction was carried out at 60°C, and the dissolution time was 3h.

[0064] An appropriate amount of sodium hydroxide is added to the acid leaching solution to adjust the pH value to 4, so that the iron ions are precipitated in the form of iron hydroxide, and Tulsimer CH-93 resin is used to remove calcium impurities; the slurry is separated into solid and liquid by sedimentation, and the obtained aluminum nitrate solution is evaporated and concentrated; the crystallization temperature is 50°C and the stirring speed is 300r / min, so that the aluminum salt is gradually crystallized and precipitated, and the crystals are separated from the mother liquor by filtration to obtain Al(NO3)3·9H2O crystals.

[0065] The crystals were washed with deionized water to remove impurities and then entered the drying process. Natural air-drying was used to dry the obtained aluminum nitrate crystals. The roasting device was a fluidized bed roasting device (Luoyang Bolaimant Experimental Electric Furnace Co., Ltd., BLMT-GB-12-8-80). The roasting temperature was 800°C and the roasting time was 8s. 476.95g of aluminum oxide was obtained with a purity of 99.6%, which met the YS / T274-1998 standard. The aluminum oxide extraction rate was 91.6%.

[0066] The flue gas produced by roasting is recovered through flue gas recirculation and SNCR combined denitrification process, which converts nitric acid gas into liquid nitric acid and is sent back to the dissolution unit for recycling.

[0067] Example 2

[0068] A method for extracting aluminum oxide by treating fly ash with nitric acid, the specific steps are:

[0069] 1000 g of fly ash (same as in Example 1) from a power plant was mixed with 4 L of methanol to form an original slurry; the original slurry was ground using a tubular disperser until particles below 74 μm accounted for 65% (mass ratio), and then magnetically separated and vacuum filtered using an electromagnetic separator. The obtained filter cake was acid-leached with nitric acid in an electrolytic cell, and an electrode was inserted into the electrolytic cell to ensure that the electrode was in full contact with the solution. An external voltage was applied, and the voltage was 3 V; the concentration of nitric acid was 6 mol / L, the liquid-to-solid ratio was 5:1, the molar ratio of nitric acid to alumina in fly ash was 6:1, the stirring speed was 88 r / min, and the slurry was subjected to dissolution reaction at 80°C for 3.5 h.

[0070] An appropriate amount of sodium hydroxide is added to the acid leaching solution to adjust the pH value to 3, so that the iron ions are precipitated in the form of iron hydroxide, and Tulsimer CH-93 resin is used to remove calcium impurities; the slurry is separated into solid and liquid by sedimentation, and the obtained aluminum nitrate solution is evaporated and concentrated; the crystallization temperature is 40°C and the stirring speed is 450r / min, so that the aluminum salt is gradually crystallized and precipitated, and then the crystals are separated from the mother liquor by precipitation flotation method to obtain Al(NO3)3·9H2O crystals.

[0071] The crystals are washed with deionized water to remove impurities and then enter a drying process. Natural drying is selected to dry the crystals. The obtained aluminum nitrate crystals are roasted in a fluidized bed roasting device at a temperature of 750°C for 9 seconds to obtain 482.08 g of aluminum oxide with a purity of 99.4%, which meets the YS / T274-1998 standard. The aluminum oxide extraction rate is 92.4%.

[0072] The flue gas generated by roasting is converted into liquid nitric acid through the system and method for producing nitric acid by circulating pyrolysis of nitrate gas, and then recovered and sent back to the dissolution unit for recycling.

[0073] Example 3

[0074] A method for extracting aluminum oxide by treating fly ash with nitric acid, the specific steps are:

[0075] 1000 g of fly ash (same as in Example 1) from a power plant was mixed with 3 L of water to form an original slurry; the original slurry was ground using a magnetic grinder until particles below 74 μm accounted for 70% (mass ratio), and then magnetically separated and vacuum filtered using a permanent magnetic separator. The obtained filter cake was acid-leached with nitric acid in an electrolytic cell, and an electrode was inserted into the electrolytic cell to ensure that the electrode was in full contact with the solution. An external voltage was applied, and the voltage was 4 V; the concentration of nitric acid was 7 mol / L, the liquid-to-solid ratio was 5:1, the molar ratio of nitric acid to alumina in fly ash was 7:1, and the stirring speed was 100 r / min; the slurry was subjected to a dissolution reaction at 90°C for 4.5 h.

[0076] An appropriate amount of sodium hydroxide is added to the acid leaching solution to adjust the pH value to 5, so that the iron ions are precipitated in the form of iron hydroxide, and Tulsimer CH-93 resin is used to remove calcium impurities; the slurry is separated into solid and liquid by sedimentation, and the obtained aluminum nitrate solution is evaporated and concentrated; the crystallization temperature is 45°C and the stirring speed is 400r / min, so that the aluminum salt is gradually crystallized and precipitated, and then the crystals are separated from the mother liquor by an ultra-gravity separation method to obtain Al(NO3)3·9H2O crystals.

[0077] The crystals are washed with deionized water to remove impurities and then enter a drying process. Natural drying is selected to dry the crystals. The obtained aluminum nitrate crystals are roasted in a fluidized bed roasting device at a temperature of 780°C for 8.5 seconds to obtain 487.33 g of aluminum oxide with a purity of 99.5%, which meets the YS / T274-1998 standard. The aluminum oxide extraction rate is 93.5%.

[0078] The flue gas generated by roasting is converted into liquid nitric acid through the system and method for producing nitric acid by circulating pyrolysis of nitrate gas, and then recovered and sent back to the dissolution unit for recycling.

[0079] Example 4

[0080] A method for extracting aluminum oxide by treating fly ash with nitric acid, the specific steps are:

[0081] 1000 g of fly ash (same as in Example 1) from a power plant was mixed with 3.5 L of a mixed solvent (methanol and water mixed in equal volumes) to form an original slurry; the original slurry was ground using a magnetic grinder until particles below 74 μm accounted for 70% (mass ratio), and then magnetically separated and vacuum filtered using an electromagnetic separator. The obtained filter cake was acid-leached with nitric acid in an electrolytic cell, and an electrode was inserted into the electrolytic cell to ensure that the electrode was in full contact with the solution, and an external voltage was applied, the voltage of which was 2 V; the concentration of nitric acid was 5 mol / L, the liquid-to-solid ratio was 8:1, the molar ratio of nitric acid to alumina in fly ash was 8:1, the stirring speed was 90 r / min, and the slurry was subjected to a dissolution reaction at 95°C for 5 h.

[0082] An appropriate amount of sodium hydroxide is added to the acid leaching solution to adjust the pH value to 4.6, so that the iron ions are precipitated in the form of iron hydroxide, and Tulsimer CH-93 resin is used to remove calcium impurities; the slurry is separated into solid and liquid by sedimentation, and the obtained aluminum nitrate solution is evaporated and concentrated; the crystallization temperature is 55°C and the stirring speed is 500r / min, so that the aluminum salt is gradually crystallized and precipitated, and then the crystals are separated from the mother liquor by centrifugation to obtain Al(NO3)3·9H2O crystals.

[0083] The crystals are washed with deionized water to remove impurities and then enter a drying process. Natural air drying is selected to dry the obtained aluminum nitrate crystals. The roasting device is a fluidized roasting device. The roasting temperature is 895°C and the roasting time is 9s. 498.23g of aluminum oxide is obtained with a purity of 99.3%, which meets the YS / T274-1998 standard. The aluminum oxide extraction rate is 95.4%.

[0084] The flue gas produced by roasting is recovered through flue gas recirculation and SNCR combined denitrification process, which converts nitric acid gas into liquid nitric acid and is sent back to the dissolution unit for recycling.

[0085] Example 5

[0086] A method for extracting aluminum oxide by treating fly ash with nitric acid, the specific steps are:

[0087] 1000 g of fly ash (same as in Example 1) from a power plant was mixed with 5 L of methanol to form an original slurry; the original slurry was ground using a tubular disperser until particles below 74 μm accounted for 75% (mass ratio), and then magnetically separated and vacuum filtered using a permanent magnetic separator. The obtained filter cake was acid-leached with nitric acid in an electrolytic cell, and an electrode was inserted into the electrolytic cell to ensure that the electrode was in full contact with the solution. An external voltage was applied, and the voltage was 5 V; the concentration of nitric acid was 4 mol / L, the liquid-to-solid ratio was 10:1, the molar ratio of nitric acid to alumina in fly ash was 8:1, the stirring speed was 95 r / min, and the slurry was subjected to a dissolution reaction at a temperature of 100°C for 3.5 h.

[0088] An appropriate amount of sodium hydroxide is added to the acid leaching solution to adjust the pH value to 3.8, so that the iron ions are precipitated in the form of iron hydroxide, and Tulsimer CH-93 resin is used to remove calcium impurities; the slurry is separated into solid and liquid by sedimentation, and the obtained aluminum nitrate solution is evaporated and concentrated; the crystallization temperature is 75°C and the stirring speed is 420r / min, so that the aluminum salt is gradually crystallized and precipitated, and then the crystals are separated from the mother liquor by precipitation flotation method to obtain Al(NO3)3·9H2O crystals.

[0089] The crystals are washed with deionized water to remove impurities and then enter a drying process. Natural drying is selected to dry the crystals. The obtained aluminum nitrate crystals are roasted in a fluidized bed roasting device at a temperature of 900°C for 10 seconds to obtain 500.86 g of aluminum oxide with a purity of 99.4%, which meets the YS / T274-1998 standard. The aluminum oxide extraction rate is 96%.

[0090] The flue gas produced by roasting is recovered through flue gas recirculation and SNCR combined denitrification process, which converts nitric acid gas into liquid nitric acid and is sent back to the dissolution unit for recycling.

[0091] It can be seen from the above embodiments that the method provided by the present invention can extract alumina from fly ash at a temperature of 60-100°C and normal pressure, with an extraction rate of 91.6-96% and a purity of alumina of 99.3-99.6%, which meets the standard of YS / T274-1998.

[0092] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for extracting aluminum oxide by treating fly ash with nitric acid, characterized in that: The following steps are involved: 1) acid leaching fly ash in a nitric acid solution to obtain an acid leaching solution; applying a voltage during the acid leaching, the voltage being 1 to 5 V; the temperature of the acid leaching being 60 to 100° C., and the pressure of the acid leaching being normal pressure; 2) crystallizing the acid leaching solution obtained in step 1) to obtain Al(NO3)3·9H2O crystals; 3) Fluidizing and calcining the Al(NO3)3·9H2O crystals obtained in step 2) to obtain aluminum oxide.

2. The method according to claim 1, characterized in that The concentration of the nitric acid solution in step 1) is 4-8 mol / L.

3. The method according to claim 1 or 2, characterized in that: In the step 1), the molar ratio of nitric acid in the nitric acid solution to aluminum oxide in the fly ash is (6-8):

1.

4. The method according to claim 3, characterized in that The acid leaching time in step 1) is 3 to 5 hours.

5. The method according to claim 1, characterized in that The temperature of fluidized calcination in step 3) is 700-900°C.

6. The method according to claim 5, characterized in that The time of fluidized calcination in step 3) is 8 to 10 seconds.

7. The method according to claim 1, characterized in that The fly ash in step 1) is pretreated before use, and the pretreatment includes mixing slurry, grinding, magnetic separation and filtering in sequence.

8. The method according to claim 1, characterized in that: In the step 2), the acid leaching solution is firstly subjected to impurity removal before crystallization; the impurity removal includes iron precipitation, calcium removal and solid-liquid separation performed in sequence.

9. The method according to claim 8, characterized in that The method for precipitating iron is as follows: adjusting the pH value of the acid leaching solution to 3-5 to precipitate iron ions.

10. The method according to claim 1, characterized in that The flue gas generated by the fluidized roasting in step 3) is recovered and reused.