Method for preparing water purifying agent polyaluminum chloride from secondary aluminum ash

Abstract

The application discloses a method for preparing a water purifying agent polyaluminum chloride from secondary aluminum ash, and the method comprises the following steps: S1, mixing the secondary aluminum ash with a composite additive solution, stirring and reacting at 70-90 DEG C, then performing solid-liquid separation and drying the solid; S2, mixing the obtained pretreated aluminum ash with calcium carbonate, pre-baking at 900-1000 DEG C, then melting at 1300-1400 DEG C, and cooling; S3, adding the obtained baked material into 20-25wt% hydrochloric acid, heating the obtained slurry to 80-105 DEG C, stirring and leaching, then diluting the acid leaching solution with water to the final concentration of 5-10wt% hydrochloric acid, then heating to 80-105 DEG C, continuously stirring and leaching, cooling and filtering, concentrating and crystallizing the filtrate, and obtaining polyaluminum chloride. The quality of the polyaluminum chloride is greatly improved through the methods of oxidation strengthening water washing, two-stage dynamic baking, gradient acid leaching and byproduct recovery.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid waste resource utilization, and specifically relates to a method for preparing polyaluminum chloride, a water purifier, from secondary aluminum ash resources. Background Art

[0002] When the traditional landfill method is used to treat aluminum ash, a large amount of ammonia-containing waste gas will be released during the stabilization pretreatment process and cannot be effectively controlled. At the same time, the valuable components in the aluminum ash cannot be recycled, which will cause a large waste of resources.

[0003] In contrast, secondary aluminum ash is typically converted into calcium aluminate for use as a steelmaking additive or as a raw material for polyaluminum chloride water purifiers. While direct use of calcium aluminate is highly effective in producing polyaluminum chloride water purifiers, its high cost has prevented widespread adoption in China. Using secondary aluminum ash, a cost-free raw material, to produce polyaluminum chloride effectively addresses the issues of aluminum ash waste and the high price of calcium aluminate.

[0004] However, in the prior art, the process of preparing polyaluminium chloride from secondary aluminium ash has the following problems:

[0005] 1. The traditional water washing process is prone to particle agglomeration, resulting in a reduction in the reaction interface, which restricts the further improvement of the aluminum nitride hydrolysis efficiency and has low efficiency in removing aluminum nitride and heavy metals, resulting in secondary pollution in the subsequent roasting process. In addition, the ammonia generated by the water washing process is not recycled, resulting in a waste of resources.

[0006] 2. A single calcination temperature can easily result in organic residue or insufficient purity of calcium aluminate (Al2O3 content ≤ 85%). In addition, during the calcination process, the material bulk density is too high, which affects the heat conduction efficiency and easily causes insufficient local reaction, thus limiting the improvement of calcium aluminate purity.

[0007] 3. The acid leaching process experiences large fluctuations in basicity (±15%). Basicity is a key quality indicator for polyaluminium chloride (PAC). Polyaluminium chloride with high basicity generally exhibits improved stability and weather resistance, maintaining stable performance under various environmental conditions. Large fluctuations in basicity mean unstable product quality, potentially impacting its effectiveness and reliability in practical applications. Furthermore, large fluctuations in basicity directly impact the effectiveness of PAC treatment. Low basicity may result in less-than-optimal treatment results, necessitating increased dosage to achieve optimal results. High basicity, while achieving better results, also increases PAC production costs.

[0008] These problems directly affect the quality and stability of polyaluminium chloride. Therefore, a new technology is urgently needed to solve the secondary pollution and quality problems in the preparation process of polyaluminium chloride. Summary of the Invention

[0009] Based on the above-mentioned existing technology, the present invention provides a method for preparing polyaluminum chloride, a water purifier, by resource utilization of secondary aluminum ash. The present invention greatly improves the removal rate of ammonia nitrogen and heavy metals through methods such as oxidation-enhanced water washing, two-stage dynamic roasting, gradient acid leaching and by-product recovery, thereby improving the quality of polyaluminum chloride and achieving the harmless, high-value and closed-loop resource utilization of secondary aluminum ash.

[0010] The technical solution adopted to achieve the above-mentioned purpose of the present invention is:

[0011] A method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash comprises the following steps:

[0012] S1. Mixing the secondary aluminum ash with the composite additive solution, heating to 70-90° C., stirring and reacting at 70-90° C. for 3-7 hours, and after the reaction is completed, performing solid-liquid separation, and drying the obtained solid to obtain pretreated aluminum ash;

[0013] The composite additive solution is an aqueous solution of hydrogen peroxide and polyvinyl alcohol;

[0014] Wet aluminum nitride removal is based on the principle that aluminum nitride reacts with water to generate aluminum oxide and ammonia. The reaction equation is shown in formula (1):

[0015] AlN+2H2O→AlO(OH)+NH3↑(1),

[0016] During water washing, 0.1-0.5% polyvinyl alcohol (PVA) is introduced as a dispersing medium, and its colloidal properties are used to form a steric hindrance effect. At the same time, H2O2 is added, and PVA and hydrogen peroxide form a composite oxidation system. The aluminum ash particles are fully dispersed through colloidal adsorption, which can effectively accelerate the hydrolysis of aluminum nitride and simultaneously complex heavy metals to improve the impurity removal efficiency.

[0017] S2. Mix the pretreated aluminum ash and calcium carbonate, heat them to 900-1000°C, pre-calcine them at 900-1000°C for 0.5-1.5 h, then heat them to 1300-1400°C, melt them at 1300-1400°C for 1-3 h, and cool them to obtain a calcined material;

[0018] A single roasting temperature can easily result in residual organic matter or insufficient calcium aluminate purity. However, a two-stage dynamic roasting technology can improve the purity of the calcium aluminate product. During the high-temperature roasting stage of pre-treated aluminum ash, the carbonization and decomposition of PVA agglomerates adsorbed on the aluminum ash create a microporous structure, which increases the reactivity of the aluminum ash-calcium carbonate mixture without introducing impurities.

[0019] S3. First, add the roasted material to 20-25wt% hydrochloric acid, heat the resulting slurry to 80-105°C, stir and leach at 80-105°C for 0.5-2h, then dilute the obtained acid leachate with water to make the final concentration of hydrochloric acid in the acid leachate be 5-10wt%, heat the diluted acid leachate to 80-105°C, continue stirring and leaching at 80-105°C for 1-3h, cool and filter to obtain a filtrate, concentrate and crystallize to obtain a solid water purifier polyaluminum chloride.

[0020] The filter residue produced in this step can be used as building material raw material for secondary utilization to avoid waste.

[0021] The calcium aluminate acid leaching method uses calcium aluminate as a raw material, reacting it with hydrochloric acid at a certain temperature to produce liquid polyaluminium chloride. This method has the advantages of simple process, low production cost, and high basicity and aluminum content in polyaluminium chloride. However, its disadvantage is that the acid leaching process causes large fluctuations in the basicity of polyaluminium chloride (±15%). Therefore, a gradient acid leaching method can achieve precise control of the basicity of polyaluminium chloride.

[0022] Furthermore, the solid-liquid ratio of the secondary aluminum ash to the composite additive solution is 1 g:2-8 mL.

[0023] Furthermore, in the composite additive solution, the concentration of hydrogen peroxide is 0.5-2 wt %, and the concentration of polyvinyl alcohol is 0.1-0.5 wt %.

[0024] Furthermore, in step S1, sulfuric acid is used to absorb ammonia generated during the reaction while stirring the reaction, and the generated ammonium sulfate solution is collected and concentrated and crystallized. The obtained ammonium sulfate can be used as a nitrogen fertilizer product.

[0025] Furthermore, in step S1, when drying the solid material, the residual heat from the pre-baking and melting in step S2 can be utilized for drying. Utilizing the residual heat from the high-temperature calcination furnace to dry the washed aluminum ash can significantly reduce energy consumption and thus production costs.

[0026] Furthermore, in the mixture of the pretreated aluminum ash and calcium carbonate, the proportion of secondary aluminum ash is 50-75wt%, and the proportion of calcium carbonate is 25-50wt%.

[0027] Furthermore, in step S3, the solid-liquid ratio of the calcined material to the hydrochloric acid is 1 g:1-4 mL.

[0028] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0029] 1. When washing the secondary aluminum ash with water, the present invention introduces polyvinyl alcohol (PVA) as a dispersion medium, utilizes its colloidal properties to form a steric hindrance effect, and simultaneously adds H2O2. PVA and hydrogen peroxide form a composite oxidation system. The aluminum ash particles are fully dispersed through colloidal adsorption, which can effectively accelerate the hydrolysis of aluminum nitride and simultaneously complex heavy metals, thereby improving the impurity removal efficiency.

[0030] 2. This invention addresses the problem of residual organic matter and insufficient calcium aluminate purity often caused by a single calcination temperature. Instead, it utilizes a two-stage dynamic calcination process to effectively improve the purity of the calcium aluminate product. Furthermore, during the calcination process, the polyvinyl alcohol adsorbed on the pretreated aluminum ash decomposes to produce CO2 and H2O, which facilitates the formation of a porous structure in the calcined raw material. This significantly increases the calcination reaction interface, thereby improving the purity of the resulting calcium aluminate while preventing the introduction of new impurities.

[0031] 3. In order to solve the problem that the basicity of polyaluminium chloride obtained during the acid leaching process fluctuates greatly, the present invention adopts a gradient acid leaching method to achieve precise control of the basicity of polyaluminium chloride. DETAILED DESCRIPTION

[0032] In order to facilitate those skilled in the art to understand and implement the present invention, the present invention is further described in detail below in conjunction with embodiments. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0033] The main components of the secondary aluminum ash used in the following examples and comparative examples are tested as follows:

[0034] Representative samples were first extracted from the secondary aluminum ash using the cone quartering method. The samples were then oven-dried at 100°C and then crushed and ground to 100 μm for chemical composition and phase analysis. To investigate the chemical composition of the secondary aluminum ash, the multi-element composition of the samples was first determined using XRF. The primary chemical components of the secondary aluminum ash were then analyzed using chemical dissolution and ICP methods. The results are shown in Table 1.

[0035] Table 1 Analysis of main elements of secondary aluminum ash (wt%)

[0036]

[0037] Example 1

[0038] 1. Add hydrogen peroxide and polyvinyl alcohol to water respectively to prepare a composite additive solution. In the composite additive solution, the mass concentration of H2O2 is 1.0%, and the mass concentration of polyvinyl alcohol is 0.3%.

[0039] 2. Mix the secondary aluminum ash and the composite additive solution at a solid-liquid ratio of 1g:4mL, heat to 80°C, and stir at 80°C for 5 hours. During the heating and stirring process, absorb the generated ammonia with sulfuric acid and collect the generated ammonium sulfate solution. After the reaction is complete, filter and dry the filter cake using the residual heat of the calciner to obtain a pretreated aluminum ash with a smooth surface, uniform distribution, and minimal agglomeration.

[0040] 3. Stir and mix the pretreated aluminum ash and calcium carbonate in a mass ratio of 3:1, add the evenly mixed mixture into a roasting furnace, heat it to 950°C, pre-roast it at 950°C for 1 hour, then heat it to 1350°C, melt it at 1350°C for 2 hours, and cool it to obtain a roasted material.

[0041] 4. The roasted material was first added to 20 wt% hydrochloric acid according to the solid-liquid ratio of 1 g: 2 mL, and the resulting slurry was stirred and leached at 95 ° C for 1.5 h, and then diluted to the obtained acid leachate. The final concentration of hydrochloric acid in the acid leachate was 5 wt%. The diluted acid leachate was continued to be stirred and leached at 85 ° C for 2 h, cooled and filtered to obtain a filtrate, concentrated and crystallized to obtain a solid water purifier polyaluminum chloride.

[0042] 5. Repeat steps 1-4 twice and measure the basicity of the prepared polyaluminium chloride three times.

[0043] Example 2

[0044] 1. Add hydrogen peroxide and polyvinyl alcohol to water respectively to prepare a composite additive solution. In the composite additive solution, the mass concentration of H2O2 is 2.0%, and the mass concentration of polyvinyl alcohol is 0.5%.

[0045] 2. Mix the secondary aluminum ash with the composite additive solution at a solid-liquid ratio of 1g:8mL, heat to 90°C, and stir at 90°C for 5 hours. During the heating and stirring process, sulfuric acid is used to absorb the generated ammonia gas, and the resulting ammonium sulfate solution is collected. After the reaction is complete, filter the filter cake and dry it using the residual heat of the calciner to obtain a pretreated aluminum ash with a smooth surface, uniform distribution, and minimal agglomeration.

[0046] 3. Stir and mix the pretreated aluminum ash and calcium carbonate in a mass ratio of 4:1, add the evenly mixed mixture into the roasting furnace, heat it to 1000℃, pre-roast it at 1000℃ for 1 hour, then heat it to 1400℃, melt it at 1400℃ for 2 hours, and cool it to obtain the roasting material.

[0047] 4. The roasted material was first added to 25 wt% hydrochloric acid according to the solid-liquid ratio of 1 g: 2 mL, and the resulting slurry was stirred and leached at 95 ° C for 1.5 h. Then, water was added to the obtained acid leachate to dilute the final concentration of hydrochloric acid in the acid leachate to 10 wt%. The diluted acid leachate was further stirred and leached at 85 ° C for 2 h, cooled and filtered to obtain a filtrate, concentrated and crystallized to obtain a solid water purifier polyaluminum chloride.

[0048] 5. Repeat steps 1-4 twice and measure the basicity of the prepared polyaluminium chloride three times.

[0049] Comparative Example 1

[0050] 1. Mix secondary aluminum ash with a 1.0wt% H2O2 aqueous solution at a solid-liquid ratio of 1g:4mL, heat to 80°C, and stir at 80°C for 5 hours. During the heating and stirring process, sulfuric acid is used to absorb the generated ammonia gas, and the generated ammonium sulfate solution is collected. After the reaction is complete, filter the filter cake and dry it using the residual heat of the calciner to obtain pretreated aluminum ash with poor dispersion and partial agglomeration.

[0051] 2. Stir and mix the pretreated aluminum ash and calcium carbonate in a mass ratio of 3:1, add the evenly mixed mixture into a roasting furnace, heat it to 950°C, pre-roast it at 950°C for 1 hour, then heat it to 1350°C, melt it at 1350°C for 2 hours, and cool it to obtain a roasted material.

[0052] 3. The roasted material was first added to 20 wt% hydrochloric acid according to the solid-liquid ratio of 1 g: 2 mL, and the resulting slurry was stirred and leached at 95 ° C for 1.5 h. Then, water was added to the obtained acid leachate to dilute it so that the final concentration of hydrochloric acid in the acid leachate was 5 wt%. The diluted acid leachate was continued to be stirred and leached at 85 ° C for 2 h, cooled and filtered to obtain a filtrate, concentrated and crystallized to obtain a solid water purifier polyaluminum chloride.

[0053] Comparative Example 2

[0054] 1. Mix secondary aluminum ash with a 0.3wt% PVA aqueous solution at a solid-liquid ratio of 1 g:4 mL, heat to 80°C, and stir at 80°C for 5 hours. During the heating and stirring process, sulfuric acid is used to absorb the generated ammonia gas, and the generated ammonium sulfate solution is collected. After the reaction is complete, filter the filter cake and dry it using the residual heat of the calciner to obtain pretreated aluminum ash with good dispersion and minimal agglomeration.

[0055] 2. Stir and mix the pretreated aluminum ash and calcium carbonate in a mass ratio of 3:1, add the evenly mixed mixture into a roasting furnace, heat it to 950°C, pre-roast it at 950°C for 1 hour, then heat it to 1350°C, melt it at 1350°C for 2 hours, and cool it to obtain a roasted material.

[0056] 3. The roasted material was first added to 20 wt% hydrochloric acid according to the solid-liquid ratio of 1 g: 2 mL, and the resulting slurry was stirred and leached at 95 ° C for 1.5 h. Then, water was added to the obtained acid leachate to dilute it so that the final concentration of hydrochloric acid in the acid leachate was 5 wt%. The diluted acid leachate was continued to be stirred and leached at 85 ° C for 2 h, cooled and filtered to obtain a filtrate, concentrated and crystallized to obtain a solid water purifier polyaluminum chloride.

[0057] Comparative Example 3

[0058] 1. Mix secondary aluminum ash with water at a solid-liquid ratio of 1g:4mL, heat to 80°C, and stir at 80°C for 5 hours. During the heating and stirring process, sulfuric acid is used to absorb the generated ammonia gas, and the generated ammonium sulfate solution is collected. After the reaction is complete, filter the filter cake and dry it using the residual heat of the roaster to obtain pretreated aluminum ash with a rough surface, poor dispersibility, and obvious agglomeration.

[0059] 2. Stir and mix the pretreated aluminum ash and calcium carbonate in a mass ratio of 3:1, add the evenly mixed mixture into a roasting furnace, heat it to 950°C, pre-roast it at 950°C for 1 hour, then heat it to 1350°C, melt it at 1350°C for 2 hours, and cool it to obtain a roasted material.

[0060] 3. The roasted material was first added to 20 wt% hydrochloric acid according to the solid-liquid ratio of 1 g: 2 mL, and the resulting slurry was stirred and leached at 95 ° C for 1.5 h. Then, water was added to the obtained acid leachate to dilute it so that the final concentration of hydrochloric acid in the acid leachate was 5 wt%. The diluted acid leachate was continued to be stirred and leached at 85 ° C for 2 h, cooled and filtered to obtain a filtrate, concentrated and crystallized to obtain a solid water purifier polyaluminum chloride.

[0061] Comparative Example 4

[0062] 1. Add hydrogen peroxide and polyvinyl alcohol to water respectively to prepare a composite additive solution. In the composite additive solution, the mass concentration of H2O2 is 1.0% and the concentration of polyvinyl alcohol is 0.3%.

[0063] 2. Mix the secondary aluminum ash and the composite additive solution at a solid-liquid ratio of 1g:4mL, heat to 80°C, and stir at 80°C for 5 hours. During the heating and stirring process, absorb the generated ammonia with sulfuric acid and collect the generated ammonium sulfate solution. After the reaction is complete, filter and dry the filter cake using the residual heat of the calciner to obtain a pretreated aluminum ash with a smooth surface, uniform distribution, and minimal agglomeration.

[0064] 3. The pretreated aluminum ash and calcium carbonate were stirred and mixed in a mass ratio of 3:1, and the uniformly mixed mixture was added into a roasting furnace, heated to 1350°C, melted at 1350°C for 2 h, and cooled to obtain a roasted material.

[0065] 4. The roasted material was first added to 20 wt% hydrochloric acid according to the solid-liquid ratio of 1 g: 2 mL, and the resulting slurry was stirred and leached at 95 ° C for 1.5 h. Water was then added to the obtained acid leachate to gradually reduce the acid concentration so that the final concentration of hydrochloric acid in the acid leachate was 5 wt%. The diluted acid leachate was continued to be stirred and leached at 85 ° C for 2 h, cooled and filtered to obtain a filtrate, concentrated and crystallized to obtain a solid water purifier polyaluminum chloride.

[0066] Comparative Example 5

[0067] 1. Add hydrogen peroxide and polyvinyl alcohol to water respectively to prepare a composite additive solution. In the composite additive solution, the mass concentration of H2O2 is 1.0%, and the mass concentration of polyvinyl alcohol is 0.3%.

[0068] 2. Mix the secondary aluminum ash and the composite additive solution at a solid-liquid ratio of 1g:4mL, heat to 80°C, and stir at 80°C for 5 hours. During the heating and stirring process, absorb the generated ammonia with sulfuric acid and collect the generated ammonium sulfate solution. After the reaction is complete, filter and dry the filter cake using the residual heat of the calciner to obtain a pretreated aluminum ash with a smooth surface, uniform distribution, and minimal agglomeration.

[0069] 3. Stir and mix the pretreated aluminum ash and calcium carbonate in a mass ratio of 3:1, add the evenly mixed mixture into the roasting furnace, heat it to 950℃, pre-roast it at 950℃ for 1 hour, then heat it to 1350℃, melt it at 1350℃ for 2 hours, and cool it to obtain the roasting material.

[0070] 4. Add the roasted material to 20 wt% hydrochloric acid at a solid-liquid ratio of 1 g:2 mL, heat the resulting slurry to 95 ° C, stir and leach at 95 ° C for 3 hours, cool and filter to obtain a filtrate, concentrate and crystallize the filtrate to obtain a solid water purifier polyaluminum chloride.

[0071] 5. Repeat steps 1-4 twice and measure the basicity of the prepared polyaluminium chloride three times.

[0072] The calcined materials prepared in Examples 1-2 and Comparative Examples 1-5 were tested for Al2O3 content, CaO content, and aluminum dissolution rate. The results are shown in Table 2 below (the values ​​for Example 1 and Comparative Example 5 are the average values ​​of three measurements):

[0073] Table 2

[0074]

[0075] As shown in Table 2, the addition of polyvinyl alcohol and two-stage dynamic calcination can significantly improve the quality of the calcined material, making the Al2O3 content significantly higher than 55% and the CaO content lower than 25%, which can significantly increase the aluminum dissolution rate.

[0076] The polyaluminium chloride prepared in Examples 1-2 and Comparative Examples 1-5 was tested for ammonia nitrogen removal rate, heavy metal content, Al2O3 content and basicity. The results are shown in Table 3 below (both Example 1 and Comparative Example 5 are average values ​​of 3 measurements):

[0077] Table 3

[0078]

[0079] Table 3 shows that the addition of the polyvinyl alcohol and hydrogen peroxide composite additive produced a synergistic effect, significantly improving the removal rates of ammonia nitrogen and heavy metals. After adding the polyvinyl alcohol and hydrogen peroxide composite additive, the heavy metal content of polyaluminum chloride met national standards. The highest standard for drinking water-grade polyaluminum chloride contains the following heavy metals: As ≤ 0.0001, Pb ≤ 0.0005, Cr ≤ 0.0005, and Cd ≤ 0.0001.

[0080] The basicity of the polyaluminium chloride prepared three times in Example 1-2 and Comparative Example 5 was tested, and the results are shown in Table 4 below:

[0081] Table 4

[0082]

[0083] As shown in Table 4, the basicity of the polyaluminium chloride prepared three times in Examples 1-2 is basically unchanged, while the basicity of the polyaluminium chloride prepared three times in Comparative Example 5 fluctuates greatly and is relatively low. This shows that the gradient acid leaching method can achieve precise control of the basicity of polyaluminium chloride.

Claims

1. A method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash, characterized in that The steps include: S1. Mixing the secondary aluminum ash with the composite additive solution, heating to 70-90° C., stirring and reacting at 70-90° C. for 3-7 hours, and after the reaction is completed, performing solid-liquid separation, and drying the obtained solid to obtain pretreated aluminum ash; The composite additive solution is an aqueous solution of hydrogen peroxide and polyvinyl alcohol; S2. Mix the pretreated aluminum ash and calcium carbonate, heat them to 900-1000°C, pre-calcine them at 900-1000°C for 0.5-1.5 h, then heat them to 1300-1400°C, melt them at 1300-1400°C for 1-3 h, and cool them to obtain a calcined material; S3. First, add the roasted material to 20-25wt% hydrochloric acid, heat the resulting slurry to 80-105°C, stir and leach at 80-105°C for 0.5-2h, then dilute the obtained acid leachate with water to make the final concentration of hydrochloric acid in the acid leachate be 5-10wt%, heat the diluted acid leachate to 80-105°C, continue stirring and leaching at 80-105°C for 1-3h, cool and filter to obtain a filtrate, concentrate and crystallize to obtain a solid water purifier polyaluminum chloride.

2. The method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash according to claim 1, characterized in that: The solid-liquid ratio of the secondary aluminum ash to the composite additive solution is 1 g:2-8 mL.

3. The method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash according to claim 1, characterized in that: In the composite additive solution, the concentration of hydrogen peroxide is 0.5-2wt%, and the concentration of polyvinyl alcohol is 0.1-0.5wt%.

4. The method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash according to claim 1, characterized in that: In step S1, sulfuric acid is used to absorb ammonia generated during the reaction while stirring the reaction, and the generated ammonium sulfate solution is collected.

5. The method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash according to claim 1, characterized in that: In step S1, when the solid material is dried, the residual heat from the pre-calcination and melting in step S2 can be used for drying.

6. The method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash according to claim 1, characterized in that: In the mixture of pretreated aluminum ash and calcium carbonate, the mass proportion of secondary aluminum ash is 50-75%, and the mass proportion of calcium carbonate is 25-50%.

7. The method for preparing polyaluminium chloride as a water purifier by recycling secondary aluminium ash according to claim 1, characterized in that: In step S3, the solid-liquid ratio of the calcined material to the hydrochloric acid is 1 g:1-4 mL.

Images