A polyaluminum chloride flocculant for softening hard water, a preparation method and application thereof

Abstract

The application provides a softened hard water polyaluminum chloride flocculant, a preparation method and application thereof, and comprises the following steps: respectively taking sodium iron ethylenediaminetetraacetate, sodium alginate and sodium aluminate, mixing to obtain a complexed aluminum alginate mixed salt; mixing water and the complexed aluminum alginate mixed salt, stirring to obtain a complexed aluminum alginate mixed glue solution; dropping a hydrochloric acid solution into the complexed aluminum alginate mixed glue solution to adjust pH, then aging the complexed aluminum alginate mixed glue solution after pH adjustment, and spray drying to obtain the softened hard water polyaluminum chloride flocculant.

Description

Technical Field

[0001] This invention belongs to the field of environmental engineering and wastewater treatment technology, and particularly relates to a polyaluminum chloride flocculant for softening hard water, its preparation method and application. Background Technology

[0002] Water hardness refers to the content of dissolved salts in water, specifically calcium and magnesium salts. High-hardness water refers to water with a high content of calcium and magnesium ions. The human body has a certain tolerance to water hardness; switching to water of different hardness (especially high-hardness water) can cause temporary gastrointestinal disturbances. High-hardness water poses certain health risks. The high calcium and magnesium ions it contains can combine with fatty acids and phosphates in food to form insoluble precipitates, affecting digestion and absorption. Long-term consumption of high-hardness water may lead to indigestion, stomach pain, and constipation. Furthermore, the calcium and magnesium ions in high-hardness water can combine with sulfate ions to form insoluble calcium sulfate and magnesium sulfate precipitates, increasing the risk of arteriosclerosis. The high calcium and magnesium ions can also combine with carbon dioxide in the air on the skin and scalp to form insoluble calcium carbonate and magnesium carbonate precipitates, clogging pores and hair follicles, leading to dry, rough, and dull skin, and dry, brittle, and easily broken hair. Meanwhile, in industrial applications, the calcium and magnesium ions in hard water easily combine with bicarbonate ions during heating to form insoluble calcium carbonate and magnesium carbonate precipitates. These precipitates adhere to equipment such as water heaters and boilers, leading to problems such as scaling, reduced efficiency, and shortened service life.

[0003] Polyaluminum chloride (PAC) flocculant is an inorganic polymeric coagulant with strong charge neutralization, excellent bridging adsorption, and selective adsorption of dissolved substances. While PAC is a highly efficient water purifier, it has some limitations in treating hard water and heavy metal-contaminated wastewater. In hard water, the high calcium and magnesium ion content significantly affects its coagulation effect. For wastewater containing heavy metal pollutants, PAC cannot effectively capture heavy metal ions or form insoluble precipitates. Furthermore, excessive addition of PAC not only fails to purify the water but may also cause turbidity and the formation of flocs. Therefore, developing a novel PAC flocculant with specific functionalities is crucial for softening hard water and purifying heavy metal-contaminated water. Summary of the Invention

[0004] To address the aforementioned technical problems, one objective of this invention is to provide a method for preparing a polyaluminum chloride flocculant for softening hard water. The preparation process is simple and can simultaneously and efficiently soften water and remove heavy metal pollutants.

[0005] One objective of this invention is to provide a polyaluminum chloride flocculant for softening hard water, prepared using the aforementioned method for preparing polyaluminum chloride flocculant for softening hard water.

[0006] One objective of this invention is to provide an application of a polyaluminum chloride flocculant for softening hard water in the field of wastewater treatment.

[0007] Note that the description of these objectives does not preclude the existence of other objectives. One aspect of the invention does not require achieving all of the above objectives. Objectives other than those described above can be extracted from the specification, drawings, and claims.

[0008] The present invention achieves the above-mentioned technical objectives through the following technical means.

[0009] A method for preparing a polyaluminum chloride flocculant for softening hard water includes the following steps:

[0010] Preparation of complexed aluminum alginate mixed salt: Weigh out sodium iron ethylenediaminetetraacetate, sodium alginate and sodium aluminate respectively, mix them to obtain complexed aluminum alginate mixed salt;

[0011] Preparation of complexed aluminum alginate mixed solution: Water is mixed with the complexed aluminum alginate mixed salt and stirred to obtain complexed aluminum alginate mixed solution;

[0012] To prepare a polyaluminum chloride flocculant for softening hard water: hydrochloric acid solution was added dropwise to the complexed aluminum alginate mixture to adjust the pH, and then the pH-adjusted complexed aluminum alginate mixture was aged and spray-dried to obtain the polyaluminum chloride flocculant for softening hard water.

[0013] In the above scheme, the specific steps for preparing the complexed aluminum alginate mixed salt are as follows:

[0014] Weigh out ethylenediaminetetraacetic acid ferric sodium salt, sodium alginate, and sodium aluminate respectively according to the mass ratio of (0.5-5.5):(2.5-17.5):100, mix them, and obtain a complex aluminum alginate mixed salt.

[0015] In the above scheme, the specific steps for preparing the complexed aluminum alginate mixed adhesive are as follows:

[0016] Mix water with complexed aluminum alginate at a water-to-solid ratio of (0.5–2.5): 1 mL: mg and stir for 10–30 minutes to obtain a complexed aluminum alginate mixture.

[0017] In the above scheme, the specific steps for preparing the polyaluminum chloride flocculant for softening hard water are as follows:

[0018] Prepare a 5%–30% hydrochloric acid solution, add the hydrochloric acid solution dropwise to the complexed aluminum alginate mixture to adjust the pH to 2–6; then age the pH-adjusted complexed aluminum alginate mixture at 50–150℃ for 6–36 hours, and spray dry to obtain a polyaluminum chloride flocculant for softening hard water.

[0019] A polyaluminum chloride flocculant for softening hard water is prepared according to the preparation method of the polyaluminum chloride flocculant for softening hard water.

[0020] An application of the softening hard water polyaluminum chloride flocculant, specifically its application in the field of wastewater treatment.

[0021] Preferably, in this invention, sodium ferric ethylenediaminetetraacetate, sodium alginate, and sodium aluminate are weighed out in a mass ratio of (0.5–5.5):(2.5–17.5):100, respectively, and mixed to obtain a complexed aluminum alginate mixed salt. Water is mixed with the complexed aluminum alginate mixed salt at a water-to-solid ratio of (0.5–2.5):1 mL:mg, and stirred for 10–30 minutes to obtain a complexed aluminum alginate mixed colloid. A 5%–30% hydrochloric acid solution is prepared, and then the hydrochloric acid solution is added dropwise to the complexed aluminum alginate mixed colloid to adjust the pH to 2–6. The pH-adjusted complexed aluminum alginate mixed colloid is then aged at 50–150°C for 6–36 hours, and spray-dried to obtain a polyaluminum chloride flocculant for softening hard water.

[0022] Reaction mechanism:

[0023] When the complexed aluminum alginate mixture is dissolved in water, the sodium ferric ethylenediaminetetraacetate, sodium alginate, and sodium aluminate, being of the same charge, repel each other and disperse thoroughly. After adjusting the pH by adding hydrochloric acid solution to the complexed aluminum alginate mixture, the aluminate ions undergo hydrolysis and polymerization during aging to form polyaluminum chloride (PAC). During the aluminate hydrolysis and polymerization process, the sodium ferric ethylenediaminetetraacetate complexes with iron ions, grafting them onto the PAC structure, thus achieving carboxyl complexation of PAC. Meanwhile, the alginate ions combine with hydrogen ions to form alginic acid, which is then electrostatically adsorbed and filled into the PAC molecular structure, resulting in complete integration of alginic acid and PAC.

[0024] When polyaluminum chloride flocculant for softening hard water is added to wastewater with high hardness and high concentration of heavy metals, the flocculant effectively flocculates suspended solid particles through netting, sweeping, and hydroxyl bridging, and efficiently captures calcium and magnesium ions through fused alginic acid, thereby softening the wastewater. Furthermore, the iron sodium salt of diaminetetraacetic acid grafted through ion exchange releases iron ions and complexes with heavy metal and calcium ions, further softening the wastewater and efficiently removing heavy metal ions.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] The preparation process of this invention is simple. During the hydrolysis and polymerization of aluminate, sodium ferric ethylenediaminetetraacetate is grafted onto the polyaluminum chloride structure through complexed iron ions, thereby achieving carboxyl complexation of polyaluminum chloride. Alginic acid is adsorbed and filled into the polyaluminum chloride molecular structure through electrostatic adsorption. The prepared polyaluminum chloride flocculant for softening hard water not only has the function of traditional polyaluminum chloride mesh sweeping, but also can simultaneously and efficiently soften water and remove heavy metal pollutants. The maximum suspended solids removal capacity is 3078 mg / g, the maximum total hardness removal capacity is 2765 mg / g, the maximum COD removal capacity is 5142 mg / g, and the maximum mercury removal capacity is 78 mg / g.

[0027] Note that the description of these effects does not preclude the existence of other effects. One aspect of the invention does not necessarily have all the aforementioned effects. Effects other than those described above can be readily observed and extracted from the description, drawings, claims, etc. Attached Figure Description

[0028] Figure 1 This is a flowchart of the processing method of the present invention. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto. Unless otherwise specified, the raw materials, reagents, materials, etc. used in the following embodiments can be obtained commercially or prepared by conventional methods in the art.

[0030] Example 1: Effect of the mass ratio of sodium iron ethylenediaminetetraacetate, sodium alginate, and sodium aluminate on the performance of the prepared polyaluminum chloride flocculant for softening hard water.

[0031] Weigh out sodium ferric ethylenediaminetetraacetate, sodium alginate, and sodium aluminate according to the following mass ratios: 0.5:2.5:100, 0.5:10:100, 0.5:17.5:100, 3:2.5:100, 3:10:100, 3:17.5:100, 5.5:2.5:100, 5.5:10:100, and 5.5:17.5:100, respectively, and mix them to obtain a complexed aluminum alginate mixed salt. Mix water with the complexed aluminum alginate mixed salt at a water-to-solid ratio of 0.5:1 mL:mg and stir for 10 minutes to obtain a complexed aluminum alginate mixed gel solution. Prepare a 5% hydrochloric acid solution, and then add the hydrochloric acid solution dropwise to the complexed aluminum alginate mixed gel solution to adjust the pH to 2. Subsequently, the pH-adjusted complexed aluminum alginate mixture was aged at 50°C for 6 hours and then spray-dried to obtain a polyaluminum chloride flocculant for softening hard water.

[0032] Sampling and basic properties of municipal solid waste leachate: The leachate used in the experiment was taken from a waste-to-energy incineration plant in Suzhou. The suspended solids concentration of this batch of leachate was 3567 mg / L, the COD concentration was 45782 mg / L, and the total hardness was 18740 mg / L.

[0033] Preparation of waste liquid for the experiment: Take 1L of the above landfill leachate, mix in 0.2g of mercuric chloride, and stir until the mercuric chloride is completely dissolved to obtain the waste liquid for the experiment.

[0034] Purification test: Add 1g of softening hard water polyaluminum chloride flocculant to the waste liquid used in the test, mix, stir for 5 minutes, centrifuge at 5000rpm for 5 minutes to complete the purification test.

[0035] COD Concentration Detection and COD Removal Capacity Calculation: The chemical oxygen demand (COD) concentration in the solution was determined according to the national standard "Determination of Chemical Oxygen Demand in Water - Dichromate Method" (GB 11914-1989). The COD removal capacity was calculated according to formula (1), where Q... COD COD removal capacity (mg / g), c0 and c t The values ​​represent the COD concentration (mg / L) of the landfill leachate before and after treatment, respectively; m represents the mass of the added polyaluminum chloride flocculant for softening hard water (1g); and V represents the volume of the waste liquid used in the experiment (1L).

[0036]

[0037] Suspended solids detection and suspended solids removal capacity calculation: Suspended solids were determined according to the "Determination of Suspended Solids in Water - Gravimetric Method" (GB / T11901-1989). The suspended solids removal capacity was calculated using formula (2), where Q... SS For suspended solids removal capacity (mg / g), c SS0 and c SSt The values ​​represent the suspended solids concentrations (mg / L) of the landfill leachate before and after treatment, respectively; m represents the mass of the added polyaluminum chloride flocculant for softening hard water (1g); and V represents the volume of the waste liquid used in the experiment (1L).

[0038]

[0039] Total hardness testing and total hardness removal capacity calculation: The total hardness of the liquid was determined according to the "Determination of Total Calcium and Magnesium in Water" (GB7477-87). The ammonia nitrogen removal capacity was calculated according to formula (3), where Q... TH c represents the ammonia nitrogen removal capacity (mg / g). TH0 To determine the initial concentration (mg / L) of ammonia nitrogen in the leachate before treatment, c THtThe residual ammonia nitrogen concentration in the treated leachate is (mg / L), m is the mass of the added polyaluminum chloride flocculant for softening hard water (1g), and V is the volume of waste liquid used in the experiment (1L).

[0040]

[0041] Mercury ion concentration detection and removal rate calculation: The mercury ion concentration in the leachate was determined according to the standard "Water Quality - Determination of Mercury, Arsenic, Selenium, Bismuth and Antimony - Atomic Fluorescence Method" (HJ694-2014). The mercury ion removal capacity was calculated according to formula (4), where Q... Hg The mercury ion removal capacity (mg / g), c Hg0 To determine the initial concentration (mg / L) of mercury ions in the pretreatment leachate, c Hgt The concentration of mercury ions in the treated leachate is (mg / L), m is the mass of the added polyaluminum chloride flocculant for softening hard water (1g), and V is the volume of waste liquid used in the experiment (1L).

[0042]

[0043] The results for suspended solids, total hardness, COD, and mercury removal capacity are shown in Table 1.

[0044] Table 1. Effect of mass ratio of sodium ferric ethylenediaminetetraacetate, sodium alginate, and sodium aluminate on the performance of the prepared polyaluminum chloride flocculant for softening hard water.

[0045]

[0046] Weigh out sodium ferric ethylenediaminetetraacetate (EDTA), sodium alginate, and sodium aluminate respectively in a mass ratio of 0.5–5.5:2.5–17.5:100, and mix them to obtain a complexed aluminum alginate mixed salt. After dissolving the complexed aluminum alginate mixed salt in water, the EDTA, sodium alginate, and sodium aluminate, being of the same charge, repel each other and disperse fully. After adjusting the pH by adding hydrochloric acid solution to the complexed aluminum alginate mixed solution, the aluminate ions undergo hydrolysis and polymerization during aging to form polyaluminum chloride (PAC). During the hydrolysis and polymerization of aluminate ions, EDTA is grafted onto the PAC structure through the complexed iron ions, thereby achieving carboxyl complexation of PAC. Meanwhile, the alginate ions combine with hydrogen ions to form alginic acid, which is then electrostatically adsorbed and filled into the PAC molecular structure, allowing for complete integration of alginic acid and PAC. The prepared polyaluminum chloride flocculant for softening hard water has a suspended solids removal capacity of ≥2872 mg / g, a total hardness removal capacity of ≥2496 mg / g, a COD removal capacity of ≥4821 mg / g, and a mercury removal capacity of ≥56 mg / g.

[0047] Example 2: Effect of pH on the performance of the prepared polyaluminum chloride flocculant for softening hard water

[0048] Weigh out sodium ferric ethylenediaminetetraacetate, sodium alginate, and sodium aluminate respectively according to a mass ratio of 5.5:17.5:100, and mix them to obtain a complexed aluminum alginate mixed salt. Mix water with the complexed aluminum alginate mixed salt according to a water-to-solid ratio of 1.5:1 mL:mg, and stir for 20 minutes to obtain a complexed aluminum alginate mixed gel. Prepare a 17.5% hydrochloric acid solution, and then add the hydrochloric acid solution dropwise to adjust the pH to 2, 3, 4, 5, and 6. Then, age the pH-adjusted complexed aluminum alginate mixed gel at 100℃ for 21 hours, and spray dry to obtain a polyaluminum chloride flocculant for softening hard water.

[0049] The sampling and basic properties description of leachate from municipal solid waste, preparation of waste liquid for testing, purification test, COD concentration detection and COD removal capacity calculation, suspended solids detection and suspended solids removal capacity calculation, total hardness detection and total hardness removal capacity calculation, and mercury ion concentration detection and removal rate calculation are all the same as in Example 1.

[0050] The results for suspended solids, total hardness, COD, and mercury removal capacity are shown in Table 2.

[0051] Table 2. Effect of pH on the performance of the prepared polyaluminum chloride flocculant for softening hard water.

[0052]

[0053] A hydrochloric acid solution was prepared and then added dropwise to the complexed aluminum alginate mixture to adjust the pH to 2-6. During aging, aluminate ions undergo hydrolysis and polymerization to form polyaluminum chloride (PAC). During the hydrolysis and polymerization of aluminate ions, sodium iron ethylenediaminetetraacetate (EDTA) is grafted onto the PAC structure through complexed iron ions, thereby achieving carboxyl complexation of PAC. Meanwhile, alginate ions combine with hydrogen ions to form alginic acid, which is then electrostatically adsorbed and filled into the PAC molecular structure, allowing for complete integration of alginic acid and PAC. The prepared PAC flocculant for softening hard water exhibits suspended solids removal capacities greater than or equal to 2975 mg / g, total hardness removal capacities greater than or equal to 2538 mg / g, COD removal capacities greater than or equal to 4956 mg / g, and mercury removal capacities greater than or equal to 69 mg / g.

[0054] Example 3: Effect of aging temperature on the performance of the prepared polyaluminum chloride flocculant for softening hard water

[0055] Weigh out sodium ferric ethylenediaminetetraacetate, sodium alginate, and sodium aluminate respectively in a mass ratio of 5.5:17.5:100, and mix them to obtain a complexed aluminum alginate mixed salt. Mix water with the complexed aluminum alginate mixed salt at a water-to-solid ratio of 2.5:1 mL:mg and stir for 30 minutes to obtain a complexed aluminum alginate mixed colloid. Prepare a 30% hydrochloric acid solution, and then add the hydrochloric acid solution dropwise to adjust the pH to 6 to the complexed aluminum alginate mixed colloid. Subsequently, age the pH-adjusted complexed aluminum alginate mixed colloid at temperatures of 50℃, 75℃, 100℃, 125℃, and 150℃ for 36 hours, and then spray dry to obtain a polyaluminum chloride flocculant for softening hard water.

[0056] The sampling and basic properties description of leachate from municipal solid waste, preparation of waste liquid for testing, purification test, COD concentration detection and COD removal capacity calculation, suspended solids detection and suspended solids removal capacity calculation, total hardness detection and total hardness removal capacity calculation, and mercury ion concentration detection and removal rate calculation are all the same as in Example 1.

[0057] The results for suspended solids, total hardness, COD, and mercury removal capacity are shown in Table 3.

[0058] Table 3. Effect of aging temperature on the performance of the prepared polyaluminum chloride flocculant for softening hard water.

[0059]

[0060] When the aging temperature is 50–150℃, aluminate undergoes hydrolysis and polymerization during the aging process to form polyaluminum chloride (PAC). During the hydrolysis and polymerization of aluminate, sodium ferric ethylenediaminetetraacetate (EDTA) is grafted onto the PAC structure through complexed iron ions, thereby achieving carboxyl complexation of PAC. Meanwhile, alginate combines with hydrogen ions to form alginic acid, which is then electrostatically adsorbed and filled into the PAC molecular structure, allowing for complete integration between alginic acid and PAC. The prepared PAC flocculant for softening hard water exhibits suspended solids removal capacities greater than or equal to 2992 mg / g, total hardness removal capacities greater than or equal to 2613 mg / g, COD removal capacities greater than or equal to 4985 mg / g, and mercury removal capacities greater than or equal to 72 mg / g.

[0061] The effect of different comparative processes on the performance of the prepared polyaluminum chloride flocculant for softening hard water

[0062] The process of this invention is as follows: Sodium ferric ethylenediaminetetraacetate (EDTA), sodium alginate, and sodium aluminate are weighed out separately according to a mass ratio of 5.5:17.5:100, and mixed to obtain a complexed aluminum alginate mixed salt. Water is mixed with the complexed aluminum alginate mixed salt at a water-to-solid ratio of 2.5:1 mL:mg, and stirred for 30 minutes to obtain a complexed aluminum alginate mixed colloid. A 30% hydrochloric acid solution is prepared, and then the hydrochloric acid solution is added dropwise to the complexed aluminum alginate mixed colloid to adjust the pH to 6. The pH-adjusted complexed aluminum alginate mixed colloid is then aged at 150℃ for 36 hours, and spray-dried to obtain a polyaluminum chloride flocculant for softening hard water.

[0063] Comparative Process 1: Sodium alginate and sodium aluminate were weighed separately according to a mass ratio of 17.5:100 and mixed to obtain a mixed salt of aluminum alginate. Water was mixed with the mixed salt of aluminum alginate at a water-to-solid ratio of 2.5:1 mL:mg, and stirred for 30 minutes to obtain a mixed alginate gel. A 30% hydrochloric acid solution was prepared, and then the hydrochloric acid solution was added dropwise to the mixed alginate gel to adjust the pH to 6. The pH-adjusted mixed alginate gel was then aged at 150℃ for 36 hours and spray-dried to obtain a polyaluminum chloride flocculant for softening hard water.

[0064] Comparative Process 2: Sodium ferric ethylenediaminetetraacetate (EDTA) and sodium aluminate were weighed and mixed at a mass ratio of 5.5:100 to obtain a complexed aluminum mixed salt. Water was mixed with the complexed aluminum mixed salt at a water-to-solid ratio of 2.5:1 mL:mg, and stirred for 30 minutes to obtain a complexed aluminum mixed colloid. A 30% hydrochloric acid solution was prepared, and then the hydrochloric acid solution was added dropwise to the complexed aluminum mixed colloid to adjust the pH to 6. The pH-adjusted complexed aluminum mixed colloid was then aged at 150℃ for 36 hours and spray-dried to obtain a polyaluminum chloride flocculant for softening hard water.

[0065] The sampling and basic properties description of leachate from municipal solid waste, preparation of waste liquid for testing, purification test, COD concentration detection and COD removal capacity calculation, suspended solids detection and suspended solids removal capacity calculation, total hardness detection and total hardness removal capacity calculation, and mercury ion concentration detection and removal rate calculation are all the same as in Example 1.

[0066] The results for suspended solids, total hardness, COD, and mercury removal capacity are shown in Table 4.

[0067] Table 4. Effects of different comparative processes on the performance of the prepared polyaluminum chloride flocculant for softening hard water.

[0068]

[0069] The polyaluminum chloride flocculant for softening hard water prepared by the process of this invention achieves significantly higher levels of suspended solids, total hardness, COD, and mercury removal capacity than the flocculants prepared by comparative processes 1 and 2.

[0070] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a polyaluminum chloride flocculant for softening hard water, characterized in that, Includes the following steps: Preparation of complexed aluminum alginate mixed salt: Weigh out sodium iron ethylenediaminetetraacetate, sodium alginate and sodium aluminate respectively, mix them to obtain complexed aluminum alginate mixed salt; Preparation of complexed aluminum alginate mixed solution: Water is mixed with the complexed aluminum alginate mixed salt and stirred to obtain complexed aluminum alginate mixed solution; To prepare a polyaluminum chloride flocculant for softening hard water: hydrochloric acid solution was added dropwise to the complexed aluminum alginate mixture to adjust the pH, and then the pH-adjusted complexed aluminum alginate mixture was aged and spray-dried to obtain the polyaluminum chloride flocculant for softening hard water.

2. The preparation method of the polyaluminum chloride flocculant for softening hard water according to claim 1, characterized in that, The specific steps for preparing the complexed aluminum alginate mixed salt are as follows: Weigh out sodium ferric ethylenediaminetetraacetate, sodium alginate, and sodium aluminate respectively according to the mass ratio of (0.5~5.5):(2.5~17.5):100, mix them, and obtain a complex aluminum alginate mixed salt.

3. The preparation method of the polyaluminum chloride flocculant for softening hard water according to claim 1, characterized in that, The specific steps for preparing the complexed aluminum alginate mixed solution are as follows: Mix water and complexed aluminum alginate mixed salt at a water-to-solid ratio of (0.5~2.5) mL: 1 mg and stir for 10~30 minutes to obtain complexed aluminum alginate mixed gel solution.

4. The preparation method of the polyaluminum chloride flocculant for softening hard water according to claim 1, characterized in that, The specific steps for preparing the polyaluminum chloride flocculant for softening hard water are as follows: Prepare a 5%~30% hydrochloric acid solution, add the hydrochloric acid solution dropwise to the complexed aluminum alginate mixed solution to adjust the pH to 2~6; then age the pH-adjusted complexed aluminum alginate mixed solution at a temperature of 50~150℃ for 6~36 hours, and spray dry to obtain a polyaluminum chloride flocculant for softening hard water.

5. A polyaluminum chloride flocculant for softening hard water, characterized in that, The polyaluminum chloride flocculant for softening hard water is prepared according to any one of claims 1-4.

6. An application of the polyaluminum chloride flocculant for softening hard water according to claim 5, characterized in that, Application of the polyaluminum chloride flocculant for softening hard water in the field of wastewater treatment.

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