Method for preparing aluminum-magnesium layered double hydroxides by utilizing secondary aluminum ash

Abstract

The invention discloses a method for preparing aluminum-magnesium layered double hydroxides by utilizing secondary aluminum ash, and belongs to the field of recycling of solid wastes in the aluminum industry. The method comprises the following steps: hydrolyzing secondary aluminum ash, and filtering to remove water-soluble impurities to obtain a solid mixture containing aluminum hydroxide; the method comprises the following steps: selectively dissolving aluminum in a solid mixture containing aluminum hydroxide by adopting alkali liquor with the pH value of more than or equal to 14, adding a magnesium salt solution into an obtained leaching solution, and carrying out precipitation reaction, so as to obtain the aluminum hydroxide, and stirring in the selective dissolving process to enable acid gas to escape. According to the method, the technological process is simple, the condition is mild, the aluminum recovery efficiency reaches 94% or above, and green recovery and high-value utilization of aluminum resources in the secondary aluminum ash in the aluminum industry can be achieved.

Description

Technical Field

[0001] The present invention relates to the field of resource utilization of solid wastes in the aluminum industry, and particularly to a high-value conversion process for reactive hazardous waste aluminum ash. Background Art

[0002] Secondary aluminum ash (SAD) refers to a typical and inevitable waste in the aluminum industry. Generally, about 110 kg of secondary aluminum ash slag is generated per ton of aluminum produced. Its main components are Al, Al 2 O 3 , Fe, SiO 2 , nitrides, metal oxides, etc. Among them, the most abundant Al and Al 2 O 3 can reach 10 - 80 wt%.

[0003] Under normal circumstances, 95% of the solid waste is directly landfilled. It has currently been included in the "National List of Hazardous Wastes (2025 Edition)", and its hazardous characteristic is reactivity, which may lead to dangerous situations such as explosion or fire; at the same time, SAD is rich in aluminum resources and has a high recovery rate, which is 5 - 30% higher than that of bauxite. Various treatment technologies such as sintering, acid leaching, and alkali leaching have become the focus of alumina and aluminum resource recovery. However, the high-value utilization efficiency of SAD is extremely low, resulting in huge resource waste. To solve the environmental pollution risk and waste problems of aluminum resources, it is highly necessary to develop a high-value-added aluminum resource recovery process.

[0004] Most of the resource treatment processes for SAD are mainly used to prepare various building materials, but the market is tending to saturation and the economic benefits are low. Chinese Patent CN119076590A discloses a method for the resource utilization of nitrogen and aluminum in secondary aluminum ash. The secondary aluminum ash is dried, pulverized, and mixed with a buffer solution containing specific bioenzymes. Through the catalytic hydrolysis of the bioenzymes and microorganisms capable of converting nitrogen and aluminum ions, alumina for aluminum production or industrial applications and nitrogen fertilizers for agricultural production are obtained. This method involves the use of various bacterial agents, with high costs, long time, and low economic benefits. Chinese Patent CN119059542A discloses a hazardous waste aluminum ash treatment process. After mixing the aluminum ash with a calcium-containing raw material, calcium aluminate is obtained through ball milling and high-temperature sintering. However, the high-temperature sintering in the treatment process has disadvantages such as high energy consumption and high carbon emissions. Chinese Patent CN118598154A discloses a method for preparing zeolite molecular sieves using industrial solid waste. The secondary aluminum ash is subjected to high-temperature roasting, water immersion, drying, and then alkali addition for high-temperature roasting again to obtain sodium aluminate clinker. After dissolution, a sodium metaaluminate solution can be obtained. At the same time, silica is obtained by high-temperature roasting and acid leaching of fly ash and sodium carbonate. Subsequently, the silica is alkali-dissolved and mixed with the sodium metaaluminate solution to prepare zeolite molecular sieves. The zeolite molecular sieves obtained by this method can be used to treat fluoride-containing wastewater with a fluoride content lower than 25 mg / L, and the material has extremely strong selective adsorption for fluoride ions and can be widely used in the deep treatment of fluoride-containing wastewater. However, the above steps are too complex and have high energy consumption and poor economic benefits.

[0005] Chinese Patent CN111268709A discloses a method for the resource utilization of secondary aluminum ash. After mixing secondary aluminum ash, fly ash, and calcium oxide, the pH value of the solution is adjusted using an acidic gas to obtain calcium-aluminum layered double metal hydroxide. However, this method does not effectively separate impurities in the mixture; the acidic gases (carbon dioxide or sulfur dioxide) used are extremely likely to react with calcium ions to produce calcium carbonate or calcium sulfite precipitates, resulting in the failure of the synthesis of calcium-aluminum layered double metal hydroxide. Therefore, it is difficult to realize the resource utilization of secondary aluminum ash by the above method. Summary of the Invention

[0006] Aiming at the above problems existing in the prior art and realizing the high-value utilization of secondary aluminum ash (SAD) resources, the purpose of the present invention is to provide a method for preparing aluminum-magnesium layered double metal hydroxide using secondary aluminum ash. Through a two-step method of water immersion-alkali immersion, the aluminum-magnesium layered double metal hydroxide prepared using secondary aluminum ash effectively avoids the hindrance and interference of various impurity ions during the synthesis of aluminum-magnesium layered double metal hydroxide, and realizes the "treatment of pollution with waste" and high-value utilization of secondary aluminum ash.

[0007] In order to achieve the above technical objectives, the present invention provides a method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash. The method involves hydrolyzing the secondary aluminum ash, filtering to remove water-soluble impurities, and obtaining a solid mixture containing aluminum hydroxide; selectively dissolving aluminum in the solid mixture containing aluminum hydroxide using an alkali solution with pH≥14, adding a magnesium salt solution to the obtained leaching solution for precipitation reaction to obtain the product; during the selective dissolution process, stirring is required to assist the escape of acidic gases.

[0008] Secondary aluminum ash contains a relatively high content of aluminum ions. Using secondary aluminum ash to prepare aluminum-magnesium layered double hydroxide can achieve the effective utilization of aluminum ions. However, at the same time, secondary aluminum ash also contains a large amount of impurity ions. The presence of impurity ions will not only significantly affect the purity of aluminum-magnesium layered double hydroxide but also have an adverse effect on the adsorption performance of aluminum-magnesium layered double hydroxide. Especially the presence of CaO, if not removed, will react with AlO 2 - 、Mg 2+ competitively to form Ca-Al-LDH under alkaline conditions. Therefore, the key to obtaining aluminum-magnesium layered double hydroxide lies in achieving the selective separation of aluminum ions and magnesium ions from other impurities in secondary aluminum ash.

[0009] The key to the technical solution of the present invention is to uniformly convert various aluminum sources in secondary aluminum ash into soluble AlO 2 - through two steps of hydrolysis and alkali leaching, maximizing the aluminum utilization rate, and at the same time using the solubility differences of Mg(OH) 2 、Ca(OH) 2 and Al(OH) 3 to achieve deep separation of impurities. Specifically, the mechanism of the aluminum-magnesium layered double hydroxide prepared by the present invention is as follows: First, metallic aluminum, aluminum nitride, and aluminum carbide in secondary aluminum ash are converted into aluminum hydroxide precipitate through hydrolysis. At the same time, magnesium oxide and calcium oxide are respectively converted into magnesium hydroxide precipitate and calcium hydroxide precipitate. After filtration, water-soluble impurities in secondary aluminum ash can be removed (chemical reaction formulas are as shown in (1)~(5)); then, in an alkali solution with pH≥14, Al(OH) 3 and unreacted Al 2 O 3 in the solid mixture containing aluminum hydroxide can be further dissolved to form AlO 2 - and enter the solution, thereby achieving the separation from solid impurities such as Mg(OH) 2 、Ca(OH) 2 、Fe 2 O 3 . However, at the same time, SiO 2It will also be converted into soluble silicate (chemical reaction formulas are as shown in (6) - (8)). The leaching solution only contains aluminate and silicate, and other impurity ions have been separated. The presence of silicate does not affect the purity of the aluminum-magnesium layered double hydroxide. Finally, by directly adding magnesium salt to the alkaline leaching solution, it promotes the co-precipitation of AlO 2 - and Mg 2+ to form Mg 2 Al(OH) 7 to achieve separation from silicate (chemical reaction formula is as shown in (9)).

[0010] 2Al + 6H 2 O = 2Al(OH) 3 (s) + 3H 2 (g) (1)

[0011] AlN + 3H 2 O = Al(OH) 3 (s) + NH 3 (g) (2)

[0012] Al 4 C 3 + 12H 2 O = 4Al(OH) 3 (s) + 3CH 4 (g) (3)

[0013] MgO + H 2 O = Mg(OH) 2 (s) (4)

[0014] CaO + H 2 O = Ca(OH) 2 (s) (5)

[0015] Al 2 O 3 + 2OH - = 2AlO 2 - + H 2 O (6)

[0016] Al(OH) 3 + OH - = AlO 2 - + 2H 2 O (7)

[0017] SiO 2 + 2OH - = SiO 3 2- + H 2 O (8)

[0018] AlO 2 - + 2Mg 2+ + 2H 2 O + 3OH - =Mg 2 Al(OH) 7 (s)(9)

[0019] During the coprecipitation process, if carbon dioxide is present in the leaching solution, it will form magnesium carbonate precipitation with magnesium ions, resulting in the inability to prepare aluminum-magnesium layered double hydroxide; however, the inventors found that during the selective dissolution process, various metal impurity ions will form hydroxide precipitation and release a large amount of heat. At this time, stirring can promote the escape of carbon dioxide from the leaching solution, avoiding the formation of magnesium carbonate precipitation under alkaline conditions and leading to the failure of synthesis.

[0020] As a preferred embodiment, the gas generated by the reactions of the reaction formulas (1) to (3) can be used as fuel gas for heating after being washed and dried with acid solution.

[0021] As a preferred embodiment, the stirring speed of the selective dissolution is 200 - 500 rpm.

[0022] As a preferred embodiment, the secondary aluminum ash is derived from recycled aluminum and the aluminum processing process, and its components include SiO 2 、Al 2 O 3 、Fe 2 O 3 、CaO, MgO, AlN, Al 4 C 3 and Al.

[0023] As a preferred embodiment, the secondary aluminum ash is crushed and sieved to a particle size less than 0.15 mm before hydrolysis. The crushing and sieving treatment is beneficial for the full reaction of each component in the secondary aluminum ash, realizing the utilization of aluminum ions and the selective separation from other impurities.

[0024] As a preferred embodiment, the conditions of the hydrolysis are: temperature is 80 - 100 °C, liquid-solid ratio is 2 - 5 mL / g, and time is 1 - 6 h. In the present invention, increasing the hydrolysis temperature will significantly accelerate the hydrolysis reaction rate of the aluminum source (metallic Al, AlN, Al 4 C 3 ), shortening the reaction time. High temperature promotes the more complete conversion of the aluminum source into Al(OH) 3 precipitation, reducing the unreacted residue, thereby improving the utilization rate of aluminum. And at high temperature, the solubility of Ca(OH) 2 decreases with the increase of temperature, promoting its precipitation in solid form, and reducing Ca in the subsequent alkali leaching step2+ Interference (to avoid the formation of Ca-Al-LDH heterophase. Within the liquid-solid ratio range selected in the present invention, on the one hand, it can prevent the solution from being evaporated to dryness during heating, causing soluble salts to crystallize, and on the other hand, it is to prevent acidic gases such as carbon dioxide in the solution from escaping and then being redissolved. When the liquid-solid ratio is too low, the reaction will be uneven, leaving unhydrolyzed aluminum source residues and reducing the aluminum recovery rate.)

[0025] As a preferred embodiment, the alkali solution includes one or two of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate, and the concentration is 1-5 mol / L. When the concentration of the alkali solution is too low, aluminum hydroxide cannot react to form soluble meta-aluminate / salt and cannot be separated from other impurities.)

[0026] As a preferred embodiment, the conditions for selective dissolution are: the temperature is 25-100 °C, the time is 1-6 h, and the liquid-solid ratio of the alkali solution to the solid mixture containing aluminum hydroxide is 2-5 mL / g.)

[0027] As a preferred embodiment, the conditions for the precipitation reaction are: the temperature is 80-100 °C, after adjusting the pH to 9.0-12.0, stirring and reacting for 1-6 h. The pH control of the precipitation reaction in the present invention is crucial for whether aluminum-magnesium layered double metal hydroxide can be formed. When the pH is lower than 9, precipitates such as aluminum hydroxide will be produced instead of the gel of hydrotalcite (aluminum-magnesium layered double metal hydroxide), and when the pH is higher than 12, the meta-aluminate form will not change and it cannot be prepared. At the same time, stirring during the precipitation reaction can ensure the full progress of the reaction and avoid the formation of aluminum hydroxide and magnesium hydroxide impurities. Further preferably, the pH is 10-10.5.)

[0028] As a preferred embodiment, the concentration of the magnesium salt solution is 0.04-0.06 mol / L, and the volume ratio of the magnesium salt solution to the leaching solution is (1-4):1. The size of the volume ratio of the magnesium salt solution to the leaching solution has a direct impact on the purity and adsorption performance of the synthesized aluminum-magnesium layered double metal hydroxide. As the amount of magnesium salt used increases, the purity and adsorption performance of the aluminum-magnesium layered double metal hydroxide first increase and then decrease. This is mainly because when the magnesium content is too low, the purity of the synthesized aluminum-magnesium layered double metal hydroxide is too low, affecting the adsorption effect; while when the magnesium content is too high, the aluminum content in the same mass of the material decreases, resulting in a significant decrease in the surface charge of the material, making the adsorption effect of the material decline. Further preferably, the volume ratio is (2-3):1.)

[0029] As a preferred embodiment, the filtrate after the hydrolysis and filtration of secondary aluminum ash is used to wash the filter residue after selective dissolution to obtain filter residue 1 and filtrate 2; the filter residue 1 is used as a raw material for building materials.)

[0030] As a preferred embodiment, the purity of the aluminum-magnesium layered double hydroxide is greater than 95%. The aluminum-magnesium layered double hydroxide prepared from secondary aluminum ash by the present invention has high purity, comparable to commercially available hydrotalcite of the same purity, greatly reducing the production cost and realizing the reuse of solid waste at the same time.

[0031] As a preferred embodiment, the aluminum-magnesium layered double hydroxide can be used for the adsorption of heavy metal ion-containing solutions or wastewaters with a concentration range of 5-300 mg / L; wherein, the heavy metal ions include at least one of cadmium, copper, lead, zinc, mercury, chromium, arsenic, antimony, cobalt, nickel, vanadium and manganese. The aluminum-magnesium layered double hydroxide prepared by the present invention has good adsorption performance, and the adsorption amount that can be removed simultaneously can reach 200 mg / g (including cations and oxygen anions).

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) Compared with other synthesis processes of aluminum-magnesium layered double hydroxides, the raw material cost of the present invention is low and the source is wide. The prepared aluminum-magnesium layered double hydroxide can efficiently remove heavy metal cations such as Cd 2+ etc. and oxygen anions such as CrO 4 2- etc. at the same time, has higher application value, and the adsorption performance of heavy metal ions is comparable to that of aluminum-magnesium layered double hydroxides produced using pure chemical reagents.

[0034] (2) By the two-step method of water leaching-alkali leaching, the aluminum-magnesium layered double hydroxide prepared from secondary aluminum ash by the present invention effectively avoids the hindrance and interference of various impurity ions during the synthesis process of the aluminum-magnesium layered double hydroxide, realizes the "treatment of pollution with waste" and high-value utilization of secondary aluminum ash, and at the same time the purity of the prepared aluminum-magnesium layered double hydroxide can reach more than 95%.

[0035] (3) The present invention cleverly utilizes that various metal impurity ions will form hydroxide precipitates and release a large amount of heat during the alkali leaching process, and stirs to promote the escape of carbon dioxide from the leaching solution, avoiding the formation of magnesium carbonate precipitate under alkaline conditions.

[0036] (4) The process flow of the present invention is simple, the conditions are mild, and the recovery efficiency is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is an XRD pattern comparison diagram of the aluminum-magnesium layered double hydroxide (SAD-LDH, where the volume ratio of the alkali leaching solution to the 0.06 mol / L magnesium chloride solution is 1:2) prepared from secondary aluminum ash in Example 1 of the present invention and the aluminum-magnesium layered double hydroxide (LDH) synthesized using pure chemical reagents in Comparative Example 1.

[0038] As can be seen from the figure, SAD-LDH and LDH have the same characteristic peaks and no impurity peaks, indicating that the aluminum-magnesium layered double hydroxide can be obtained by treating secondary aluminum ash according to the present invention.

[0039] Figure 2 Adsorption effect comparison of SAD-LDH prepared in Example 1 and LDH prepared in Comparative Example 1 under the same conditions (4 g / L, 25 °C, 2 h, 300 rpm) for Cd 2+ and CrO 4 2-

[0040] As can be seen from the figure, the aluminum-magnesium layered double hydroxide synthesized in the present invention can efficiently remove Cd in the form of cations 2+ and CrO in the form of oxygen anions 4 2- simultaneously, and the adsorption effect on heavy metal ions is comparable to that of the aluminum-magnesium layered double hydroxide produced using pure chemical reagents.

[0041] Figure 3 XRD pattern comparison of the aluminum-magnesium layered double hydroxide (SAD-LDH) prepared from secondary aluminum ash in Comparative Example 2.

[0042] As can be seen from the figure, the synthesized material is actually a mixture of aluminum hydroxide and magnesium hydroxide.

[0043] Figure 4 XRD pattern comparison of the aluminum-magnesium layered double hydroxide (SAD-LDH) prepared from secondary aluminum ash in Comparative Example 3.

[0044] As can be seen from the figure, the synthesized material contains aluminum-magnesium layered double hydroxide but also contains a large amount of impurities such as aluminum hydroxide and magnesium carbonate.

[0045] Figure 5 XRD pattern comparison of the aluminum-magnesium layered double hydroxide (SAD-LDH) prepared from secondary aluminum ash in Comparative Example 4.

[0046] As can be seen from the figure, the synthesized material contains aluminum-magnesium layered double hydroxide but also contains a large amount of impurities such as aluminum hydroxide and magnesium hydroxide. Specific Embodiments

[0047] The following examples are intended to further illustrate the present invention rather than limit the present invention.

[0048] Taking secondary aluminum ash (SAD) generated by an aluminum enterprise as an example. The main element contents were measured (wt.%) as: SiO 2 14.0%, Al 2 O​3 25.0%, Fe 2 O 3 4.5%, CaO 5.37%, MgO 9.83%, AlN 23.6%, Al 4 C 3 6.7%, Al 6.68%, etc.

[0049] Example 1

[0050] After crushing and screening the SAD to obtain a 100-mesh sample, it was placed in a vacuum drying oven at 100 °C and dried for 6 h. 10 g of the sample was mixed with deionized water in a PP beaker at a solid-liquid ratio of 5 mL / g, the temperature was controlled at 80 °C, and after reacting for 2 h at a rotation speed of 300 rpm, solid-liquid separation was carried out to obtain a solid mixture containing aluminum hydroxide. The gas generated during the reaction was washed with acid solution, dried with a desiccant, and collected with an air bag. The above solid mixture containing aluminum hydroxide was rinsed with deionized water until the pH of the washing solution was neutral and then dried. Then, it was mixed with a 4 mol / L sodium hydroxide solution at a liquid-solid ratio of 5 mL / g and the pH was adjusted to 14. The temperature was controlled at 25 °C, and after reacting for 2 h at a rotation speed of 450 rpm, solid-liquid separation was carried out to obtain an alkali leaching solution, and the aluminum recovery rate was 94.91%. The alkali leaching solution and a 0.06 mol / L magnesium chloride solution were mixed at volume ratios of 1:1, 1:2, 1:3, and 1:4 respectively, and then 0.1 mol / L HCl solution was added to adjust the pH of the secondary filtrate to 10.0, and after stirring and reacting for 2 h, aluminum-magnesium layered double metal hydroxide was precipitated, and the purities were 62.10%, 99.21%, 95.56%, and 90.61% respectively.

[0051] Heavy metal ion adsorption experiment: Weigh 1 g of aluminum-magnesium layered double metal hydroxide and add it to 300 mL of wastewater containing heavy metals Cd 2+ and CrO 4 2- (initial concentration is about 50 mg / L, pH = 5). Under the condition that the adsorption temperature is 25 °C, continuously stir for 120 min, and then centrifuge to obtain the treated wastewater.

[0052] The adsorption effect of aluminum-magnesium layered double metal hydroxide on heavy metals Cd 2+ and CrO 4 2- is shown in Table 1.

[0053] Table 1 Influence of the volume ratio of alkali leaching solution and magnesium chloride solution on the performance of the prepared aluminum-magnesium layered double metal hydroxide

[0054]

[0055] Example 2

[0056] The 100-mesh sample obtained by crushing and screening SAD was placed in a vacuum drying oven at 100 °C and dried for 6 h. 10 g of the sample was mixed with deionized water in a PP beaker at a solid-liquid ratio of 5 mL / g, the temperature was controlled at 80 °C, and after reacting for 2 h at a rotation speed of 300 rpm, solid-liquid separation was carried out to obtain a solid mixture containing aluminum hydroxide. The gas generated during the reaction was washed with acid solution, dried with a desiccant and collected with an air bag. The above solid mixture containing aluminum hydroxide was rinsed with deionized water until the pH of the washing liquid was neutral and then dried. Then, it was mixed with 4 mol / L sodium hydroxide solution at a solid-liquid ratio of 5 mL / g and the pH was adjusted to 14. The temperature was controlled at 25 °C, and after reacting for 2 h at a rotation speed of 450 rpm, solid-liquid separation was carried out to obtain an alkali leaching solution. The alkali leaching solution was mixed with 0.06 mol / L magnesium chloride solution according to a volume ratio of 1:2, and 0.1 mol / L HCl solution was added to adjust the pH of the secondary filtrate to 9.0, 10.0, 11.0, and 12.0 respectively, and then stirred and reacted for 2 h to precipitate aluminum-magnesium layered double metal hydroxides with purities of 82.33%, 99.21%, 81.36%, and 68.96% respectively.

[0057] The heavy metal ion adsorption experiment was the same as in Example 1. The aluminum-magnesium layered double metal hydroxide's adsorption of heavy metals Cd 2+ and CrO 4 2- is shown in Table 2.

[0058] Table 2 Influence of solution pH on the performance of the prepared aluminum-magnesium layered double metal hydroxide

[0059]

[0060] Comparative Example 1

[0061] Analytical pure aluminum nitrate and magnesium chloride were respectively prepared into 0.06 mol / L aluminum ion solution and magnesium ion solution, and after mixing according to a volume ratio of 1:2, 0.1 mol / L HCl solution was added to adjust the pH of the secondary filtrate to 10.0, and then stirred and reacted for 2 h to precipitate aluminum-magnesium layered double metal hydroxide with a purity of 99.61%. The material structure is shown in the appendix Figure 1 .

[0062] Heavy metal ion adsorption experiment: Weigh 1 g of aluminum-magnesium layered double metal hydroxide and add it to 300 mL of wastewater containing heavy metals Cd 2+ and CrO 4 2- (initial concentration is about 50 mg / L, pH = 5), continuously stir for 120 min at an adsorption temperature of 25 °C, and centrifuge to obtain the treated wastewater.

[0063] The aluminum-magnesium layered double metal hydroxide's adsorption of heavy metals Cd 2+ and CrO 42- The adsorption effect is shown in the appendix Figure 2 .

[0064] Comparative Example 2

[0065] The SAD was crushed and screened to obtain a 100-mesh sample, which was placed in a vacuum drying oven at 100 °C for 6 h. 10 g of the sample was mixed with deionized water in a PP beaker at a solid-liquid ratio of 5 mL / g, the temperature was controlled at 80 °C, and after reacting for 2 h at a rotation speed of 300 rpm, solid-liquid separation was carried out to obtain harmless aluminum ash. The gas generated during the reaction was washed with acid solution, dried with a desiccant, and then collected with an air bag. The above-mentioned harmless aluminum ash was rinsed with deionized water until the pH of the washing solution was neutral and then dried. Then, it was mixed with 4 mol / L sodium hydroxide solution at a liquid-solid ratio of 5 mL / g, and the pH was adjusted to 13. The temperature was controlled at 25 °C, and after reacting for 2 h at a rotation speed of 450 rpm, solid-liquid separation was carried out to obtain an alkali leaching solution. The alkali leaching solution was mixed with 0.06 mol / L magnesium chloride solution at a volume ratio of 1:2, and then 0.1 mol / L HCl solution was added to adjust the pH of the secondary filtrate to 10.0, and then stirred and reacted for 2 h to precipitate. After detection, the obtained material was a mixture of aluminum hydroxide and magnesium hydroxide, and the structure is shown in the appendix Figure 3 . It shows that when the pH of the alkali solution is less than 14, aluminum hydroxide cannot react to form soluble meta-aluminate / salt and cannot be separated from other impurities.

[0066] Comparative Example 3

[0067] The SAD was crushed and screened to obtain a 100-mesh sample, which was placed in a vacuum drying oven at 100 °C for 6 h. 10 g of the sample was mixed with deionized water in a PP beaker at a solid-liquid ratio of 5 mL / g, the temperature was controlled at 80 °C, and after reacting for 2 h at a rotation speed of 300 rpm, solid-liquid separation was carried out to obtain harmless aluminum ash. The gas generated during the reaction was washed with acid solution, dried with a desiccant, and then collected with an air bag. The above-mentioned harmless aluminum ash was rinsed with deionized water until the pH of the washing solution was neutral and then dried. Then, it was mixed with 4 mol / L sodium hydroxide solution at a liquid-solid ratio of 5 mL / g, and the pH was adjusted to 14. The temperature was controlled at 25 °C, and after reacting for 2 h (without stirring during the reaction), solid-liquid separation was carried out to obtain an alkali leaching solution. The alkali leaching solution was mixed with 0.06 mol / L magnesium chloride solution at a volume ratio of 1:2, and then 0.1 mol / L HCl solution was added to adjust the pH of the secondary filtrate to 10.0, and then stirred and reacted for 2 h to precipitate. After detection, the obtained material contained impurities such as aluminum hydroxide and magnesium carbonate, and the purity was about 30%, and the structure is shown in the appendix Figure 4 .

[0068] Comparative Example 4

[0069] The 100-mesh sample obtained by crushing and screening SAD is placed in a vacuum drying oven at 100 °C and dried for 6 h. Take 10 g of the sample and mix it with deionized water in a PP beaker at a solid-liquid ratio of 5 mL / g. Control the temperature at 80 °C and react for 2 h at a rotation speed of 300 rpm, then perform solid-liquid separation to obtain harmless aluminum ash. The gas generated during the reaction is washed with acid solution, dried with a desiccant, and collected in an air bag. The above-mentioned harmless aluminum ash is rinsed with deionized water until the pH of the washing liquid is neutral, then dried, and then mixed with 4 mol / L sodium hydroxide solution at a liquid-solid ratio of 5 mL / g and the pH is adjusted to 14. Control the temperature at 25 °C and react for 2 h at a rotation speed of 450 rpm, then perform solid-liquid separation to obtain an alkali leaching solution. The alkali leaching solution is mixed with 0.06 mol / L magnesium chloride solution at a volume ratio of 1:2, and then 0.1 mol / L HCl solution is added to adjust the pH of the secondary filtrate to 10.0, and then react for 2 h to precipitate (without stirring during the reaction). After detection, the obtained material contains impurities such as aluminum hydroxide and magnesium hydroxide, and the purity is about 45%. The structure is shown in the appendix Figure 5 .

[0070] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention.

Claims

1. A method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash, characterized in that: The secondary aluminum ash is hydrolyzed and filtered to remove water-soluble impurities to obtain a solid mixture containing aluminum hydroxide; the solid mixture containing aluminum hydroxide is selectively dissolved by an alkali solution with a pH of ≥14, and a magnesium salt solution is added to the obtained leachate for precipitation reaction to obtain; The selective dissolution process needs to be assisted by stirring to allow the acidic gas to escape.

2. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 1, characterized in that: The secondary aluminum ash is derived from recycled aluminum and aluminum material processing, and its main components include SiO2, Al2O3, Fe2O3, CaO, MgO, AlN, Al4C3 and Al.

3. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 2, characterized in that: The secondary aluminum ash is crushed and sieved before being hydrolyzed to a particle size of less than 0.15 mm.

4. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 3, characterized in that: The hydrolysis conditions are: temperature of 50-100° C., liquid-to-solid ratio of 2-5 mL / g, and time of 1-6 h.

5. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 1, characterized in that: The alkali solution includes one or two of sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate, and the concentration is 1-5 mol / L.

6. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 5, characterized in that: The selective dissolution conditions are: temperature of 25-100° C., time of 1-6 hours, and liquid-to-solid ratio of the alkali solution and the solid mixture containing aluminum hydroxide of 2-5 mL / g.

7. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 6, characterized in that: The conditions of the precipitation reaction are: the temperature is 50-100° C., the pH is adjusted to 9.0-12.0, and the reaction is stirred for 1-6 hours; The concentration of the magnesium salt solution is 0.04-0.06 mol / L, and the volume ratio of the magnesium salt solution to the leaching solution is (1-4):

1.

8. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to any one of claims 1 to 7, characterized in that: The filtrate after the secondary aluminum ash is hydrolyzed and filtered is used to wash the selectively dissolved filter residue to obtain filter residue 1 and filtrate 2; the filter residue 1 is used as a raw material for building materials.

9. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 8, characterized in that: The purity of the aluminum-magnesium layered double metal hydroxide is greater than 95%.

10. The method for preparing aluminum-magnesium layered double hydroxide using secondary aluminum ash according to claim 9, characterized in that: The aluminum-magnesium layered double metal hydroxide can be used to treat solutions or wastewater containing at least one of cadmium, copper, lead, zinc, mercury, chromium, arsenic, antimony, cobalt, nickel, vanadium and manganese, and its adsorption capacity can reach 200 mg / g.

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