Method for preparing polyaluminum chloride from waste aluminum-containing sludge

Abstract

The invention belongs to the technical field of waste treatment, and particularly relates to a method for preparing polyaluminum chloride from waste aluminum-containing sludge. The method comprises the following steps: (1) adding ammonium nitrate into the waste aluminum-containing sludge, uniformly mixing, and carrying out a stirring reaction to obtain pretreated sludge; (2) adding ammonia water into the pretreated sludge, uniformly stirring to enable the pH value of the pretreated sludge to be 9-10, and then settling through natural gravity to obtain upper-layer liquid and lower-layer calcium-removed precipitate; (3) adding hydrochloric acid into the calcium-removed precipitate, uniformly mixing to enable the pH value of the calcium-removed precipitate to be 1.5-2.0, and then reacting to obtain a reaction solution; (4) standing the reaction liquid to obtain liquid polyaluminum chloride; adjusting the pH value of the liquid polyaluminum chloride, and performing spray drying to obtain solid polyaluminum chloride. According to the method, the influence of calcium ions on sewage can be reduced, and the content of effective components in the prepared polyaluminum chloride is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of waste treatment, and particularly relates to a method for preparing polyaluminum chloride by using waste aluminum-containing sludge. Background Art

[0002] During the production process of aluminum alloys, aluminum-containing sludge will be generated. These sludges mainly come from the precipitates, filter residues and other by-products in the wastewater treatment process. The aluminum-containing sludge is mainly composed of colloidal aluminum hydroxide and has a relatively high aluminum content. After the revision of the National List of Hazardous Wastes (2021 Edition), the sludge from the acid (alkali) washing, roughening, sulfuric acid anodic treatment, and phosphoric acid chemical polishing wastewater treatment of aluminum profiles (plates) is clearly defined as general solid waste. Traditional treatment methods for aluminum-containing sludge include ball milling, screening, melting processes, etc., which are used to extract metallic aluminum and make refractory materials. Although these methods are effective, they may cause secondary pollution. In the future, the treatment and disposal of sludge should develop in the direction of green and low-carbon, with the goal of resource recovery. In many industrial sewage treatment projects, considering the cost of sewage treatment agents, lime is used to replace NaOH for pH adjustment, which will result in a large amount of calcium ions in the sludge.

[0003] Chinese Patent No. CN103833060A, a production method of pure polyaluminum chloride, uses aluminum-containing sludge of grade HW17 and waste hydrochloric acid of grade HW34 as raw materials to produce pure polyaluminum chloride. This method directly uses acid washing and fails to separate calcium ions, which will increase the hardness of sewage and easily lead to problems such as scaling during long-term operation.

[0004] Chinese Patent No. CN 107434255A discloses a method for producing polyaluminum chloride and gypsum series products by using waste hydrochloric acid containing aluminum and aluminum-containing sludge. Although the final product contains gypsum, a large amount of calcium ions will also be washed down during the acid washing process. If a large amount of calcium ions are used in sewage treatment for a long time, it will increase the hardness of sewage, affect the subsequent process, and at the same time, the formation of calcium carbonate precipitation will cause scaling inside the pipeline, which is not conducive to the stable operation of the sewage treatment plant.

[0005] Chinese Patent No. CN 106277085 A discloses a method for preparing polyaluminum chloride by using the sludge of aluminum profile factories. Fenton's reagent is used to decompose the soluble organic matter in the sludge of aluminum profile factories, and heavy metal ion capturers are used to precipitate and remove heavy metal ions, and alumina is converted into polyaluminum chloride, reducing the solid waste discharge and saving the landfill cost. However, the sludge of the aluminum profile industry has very little organic matter content, the Fenton effect is relatively limited, and the reduction is not obvious; the cost of Fenton's reagent is increased.

[0006] Therefore, there is an urgent need for a method for preparing polyaluminum chloride by using waste aluminum-containing sludge that can reduce the impact of calcium ions on sewage and at the same time improve the effective components of polyaluminum. Summary of the Invention

[0007] The object of the present invention is to provide a method for preparing polyaluminum chloride using waste aluminum-containing sludge.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] A method for preparing polyaluminum chloride using waste aluminum-containing sludge, comprising the following steps:

[0010] (1) Ammonium nitrate is added to the waste aluminum-containing sludge, mixed evenly, and subjected to stirring reaction to obtain pretreated sludge;

[0011] (2) Ammonia water is added to the pretreated sludge, stirred evenly to make the pH of the pretreated sludge between 9 and 10, and then settled by natural gravity to obtain the upper liquid and the lower calcium-removed precipitate;

[0012] (3) Hydrochloric acid is added to the calcium-removed precipitate, mixed evenly to make the pH value of the calcium-removed precipitate 1.5 - 2.0, and then reacted to obtain a reaction solution;

[0013] (4) The reaction solution is allowed to stand to obtain liquid polyaluminum chloride; the pH of the liquid polyaluminum chloride is adjusted and spray-dried to obtain solid polyaluminum chloride.

[0014] Preferably, in step (1), the mass ratio of the waste aluminum-containing sludge to ammonium nitrate is 1:(0.3 - 0.5).

[0015] Preferably, in step (1), the reaction temperature is 60 - 80 °C, and the stirring reaction is carried out for 1 - 2 h.

[0016] By adding ammonium nitrate to the waste aluminum-containing sludge, the metal oxides (including CaO, MgO, Fe2O3, A12O3) in the waste aluminum-containing sludge can be converted into nitrates soluble in water.

[0017] Preferably, in step (2), the residual amount of calcium ions in the lower calcium-removed precipitate ≤ 0.6 wt%.

[0018] Preferably, in step (3), the mass fraction of hydrochloric acid is 20% - 30%.

[0019] By adding ammonia water to the pretreated sludge to make the pH between 9 and 10, Mg 2+ , Al 3+ and Fe 3+ can be precipitated in the form of hydroxides respectively, while Ca 2+ will not precipitate, and is settled by natural gravity, so as to achieve effective separation. Calcium exists in the upper liquid in the form of ions. It is measured that the residual amount of calcium ions in the lower precipitate after the above treatment ≤ 0.6%.

[0020] Preferably, the reaction conditions in step (3) are as follows: temperature 80 - 90°C, stirring reaction for 2 - 3 h.

[0021] Industrial hydrochloric acid is added to the precipitate after calcium removal, and the reaction is carried out at a specific pH and a specific temperature, with high dissolution efficiency of the aluminum salt in the sludge, improving the release rate of the aluminum salt.

[0022] Preferably, the reaction solution is allowed to stand for 24 - 48 h in step (4).

[0023] Preferably, sodium hydroxide is used to adjust the pH of the liquid polyaluminum chloride to 7 in step (4).

[0024] Preferably, the basicity of the liquid polyaluminum chloride is 75 - 90% in step (4), the content of Al2O3 is 10 - 15%, and the total heavy metal content ≤ 40 mg / L.

[0025] Preferably, the content of Al2O3 in the solid polyaluminum chloride is 25 - 30 wt% in step (4).

[0026] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0027] 1. The present invention provides a method for preparing polyaluminum chloride using waste aluminum-containing sludge, which can realize the resource utilization of waste aluminum sludge, replace production raw materials such as bauxite, alumina, and aluminum hydroxide with aluminum-containing sludge, and also achieve sludge reduction. The method of the present invention has low treatment cost, and after productization, the economic benefit is more obvious.

[0028] 2. The present invention removes a large amount of calcium ions in the waste aluminum-containing sludge, which can reduce the hardness of sewage, is beneficial to the subsequent process, and can avoid scaling inside the pipeline. It not only effectively solves the problems of increased hardness and scaling caused by the increase of calcium ions in the traditional sewage treatment process, but also shows significant environmental and social economic benefits in many aspects, thereby extending the service life of sewage treatment facilities. The content of the effective components in the polyaluminum prepared by the present invention is high, which means that under the premise of achieving the same water purification effect, the dosage of chemical agents required is reduced, further reducing the risk of secondary pollution.

[0029] In summary, the present invention is conducive to promoting green development and achieving the goal of energy conservation and emission reduction, and has far-reaching significance for promoting environmental protection and improving resource utilization efficiency. Specific embodiments

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are only a part rather than all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0031] Example 1

[0032] This embodiment provides a method for preparing polyaluminum chloride from waste aluminum-containing sludge, which includes the following steps:

[0033] (1) Ammonium nitrate is added to the waste aluminum-containing sludge, and the waste aluminum-containing sludge and ammonium nitrate are mixed at a mass ratio of 1:0.4, the reaction temperature is 70 °C, and stirring is carried out for 1.5 hours to obtain pretreated sludge;

[0034] (2) Ammonia water is added to the pretreated sludge, stirred evenly, the pH is measured to be 9.5, and then sedimentation is carried out by natural gravity to obtain the upper liquid and the calcium-removed precipitate in the lower layer; the calcium-removed precipitate includes aluminum hydroxide and a small amount of iron hydroxide.

[0035] (3) Hydrochloric acid with a concentration of 25 wt% is added to the calcium-removed precipitate, mixed evenly to make the pH value 1.8, and the reaction is carried out at a temperature of 85 °C for 2.6 h to obtain a reaction solution;

[0036] (4) The reaction solution is allowed to stand for 48 h to obtain liquid polyaluminum chloride; sodium hydroxide is added to the liquid polyaluminum to make the pH 7, and spray drying is carried out to obtain solid polyaluminum chloride.

[0037] The differences between Examples 2-10 and Example 1 are shown in Table 2 specifically.

[0038] Table 2 Reaction condition settings of the methods in Examples 1-10

[0039]

[0040] Performance test

[0041] Polyaluminum chloride is prepared according to the methods in Examples 1-10. The waste aluminum-containing sludge comes from an aluminum profile processing factory in Sihui, Zhaoqing. The composition of the waste aluminum-containing sludge is shown in Table 2.

[0042] Table 2 Composition analysis data of aluminum-containing sludge (dry basis)

[0043] Element Al Ca Mg Fe Mass fraction % 25.16 10.63 0.10 1.68

[0044] Refer to the national standard GB / T 22627-2014 to detect the insoluble matter in solid polyaluminum chloride (the requirement is ≤0.6%).

[0045] Aluminum salt release rate (%) = (Free aluminum ions (Al3+) and partially unpolymerized aluminum salts in the solution after the sludge in step (3) is dissolved by hydrochloric acid acidification) ÷ Total mass of aluminum in the waste aluminum-containing sludge × 100%.

[0046] The test results are shown in Table 3.

[0047] Table 3 Performance test results of polyaluminum chloride

[0048]

[0049] It can be seen from Table 3 that the polyaluminum chloride products prepared in Examples 1-3 have excellent comprehensive performance and can be directly used for sewage purification.

[0050] In the method of the present invention, when ammonia water is added to make pH = 9-10, the precipitation rate of Al 3+ is 92%-95%, and the concentration of Ca 2+ in the residual liquid > 500 mg / L (not precipitated). In Example 4, when the pH is 10.5 after adding ammonia water, the aluminum salt dissolves to form AlO2 - , resulting in aluminum loss, and the precipitation rate of Al 3+ drops to 80%-85%, and a small amount of Ca(OH)2 precipitation begins to form for Ca 2+ , and the calcium ion removal effect is not ideal. Conclusion: pH > 10 will reduce the aluminum recovery rate and introduce calcium impurities, so the pH needs to be strictly controlled at 9-10.

[0051] In the method of the present invention, when hydrochloric acid is added to make pH = 1.5-2.0, the aluminum salt release rate reaches 85%-90%. In Example 5, the acidification pH is 3.0, significantly reducing the aluminum salt release rate. Conclusion: The pH needs to be maintained at 1.5-2.0 during the acidification stage.

[0052] In Example 6, when the addition amount of ammonium nitrate is less than 0.3, the conversion rate of metal oxides is less than 70%. In Example 7, when the addition amount of ammonium nitrate is higher than 0.5, the volatilization of NH3 intensifies, the viscosity of the reaction solution increases, affecting subsequent separation, the viscosity of the reaction solution increases, and the insolubles increase. At the same time, it is found that when the concentration of ammonium nitrate is insufficient or excessive, the heavy metal content becomes high.

[0053] In Example 8, when the hydrochloric acid concentration < 20%, the dissolution rate of Al(OH)3 is slow, and the reaction takes more than 4 hours, affecting the production capacity, and at the same time the release rate decreases.

[0054] In Example 9, when the hydrochloric acid concentration > 30%, Cl2 gas is generated, there is a risk of side reactions. At the same time, the heavy metal content exceeds the standard.

[0055] In Example 10, when the aging time < 24 h, the degree of polymerization is insufficient (basicity < 60%).

[0056] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing polyaluminum chloride using waste aluminum-containing sludge, characterized in that, It includes the following steps: (1) Add ammonium nitrate to the waste aluminum-containing sludge, mix evenly, and carry out a stirring reaction to obtain pretreated sludge; (2) Add ammonia water to the pretreated sludge, stir evenly to make the pH of the pretreated sludge between 9 and 10, and then settle by natural gravity to obtain the upper liquid and the calcium-removed precipitate in the lower layer; (3) Add hydrochloric acid to the calcium-removed precipitate in the lower layer, mix evenly to make the pH value of the calcium-removed precipitate 1.5 - 2.0, and then carry out a reaction to obtain a reaction solution; (4) Let the reaction solution stand to obtain liquid polyaluminum chloride; adjust the pH of the liquid polyaluminum chloride and carry out spray drying to obtain solid polyaluminum chloride.

2. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, characterized in that, In step (1), the mass ratio of the waste aluminum-containing sludge to ammonium nitrate is 1:(0.3 - 0.5).

3. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, wherein In step (1), the reaction temperature is 60 - 80°C, and the stirring reaction is carried out for 1 - 2 h.

4. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, characterized in that, In step (2), the residual amount of calcium ions in the calcium-removed precipitate in the lower layer is ≤0.6 wt%.

5. The method for preparing polyaluminum chloride using waste aluminum-containing sludge according to claim 1, characterized in that, In step (3), the mass fraction of hydrochloric acid is 20% - 30%.

6. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, characterized in that, In step (3), the reaction conditions are: temperature 80 - 90°C, and the stirring reaction is carried out for 2 - 3 h.

7. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, characterized in that, In step (4), let the reaction solution stand for 24 - 48 h.

8. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, wherein, In step (4), use sodium hydroxide to adjust the pH of the liquid polyaluminum to 7 - 9.

9. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, characterized in that, In step (4), the basicity of the liquid polyaluminum chloride is 75 - 90%, the content of Al2O3 is 10 - 15%, and the total heavy metal content is ≤40 mg / L.

10. The method for preparing polyaluminum chloride by using waste aluminum-containing sludge according to claim 1, characterized in that, In step (4), the content of Al2O3 in the solid polyaluminum chloride is 25 - 30 wt%.

Images