Preparation method of polyaluminum chloride flocculant for urban domestic sewage treatment

Abstract

The present invention relates to the technical field of flocculants, and specifically discloses a preparation method of a polyaluminum chloride flocculant for urban domestic sewage treatment, which includes adding water to dilute and heating a polyaluminum chloride stock solution with a pH < 3 and an Al2O3 solid content ≥ 10%, and then adding a strong alkali solution to gradually adjust the pH of the polyaluminum chloride stock solution to 3.0 - 3.6 to obtain a polyaluminum chloride solution, that is, a polyaluminum chloride flocculant, and further adding nano magnetic powder to form a magnetic coagulation polyaluminum chloride solution. The polyaluminum chloride flocculant obtained by the preparation method of the present invention has a higher Al c content, taking into account a reasonable Al c and Al b distribution, and has a good treatment effect on urban domestic sewage, with a high COD removal rate. In particular, the magnetic coagulation polyaluminum chloride solution is more likely to form dense flocs, and has a lower usage amount under the same treatment effect compared with commercially available polyaluminum chloride with a similar basicity.

Description

Technical Field

[0001] The present invention relates to the technical field of flocculants, and particularly to a preparation method of a polyaluminum chloride flocculant for treating urban domestic sewage. Background Art

[0002] Polyaluminum chloride coagulation is the most widely used technology for treating urban domestic sewage at present. Research shows that the morphological distribution of aluminum in the hydrolysis products of polyaluminum chloride is the key to affecting its coagulation effect. The aluminum morphological distribution of polyaluminum chloride mainly includes low-polymeric Al a , medium-polymeric Al b , and high-polymeric Al c .

[0003] At present, the polyaluminum chloride products on the market are mainly medium-polymeric Al b , because quite a number of researchers believe that Al b is a measure of the quality of coagulation effect. Therefore, it is necessary to ensure that the content of Al b in polyaluminum chloride reaches the maximum. However, the characteristics of urban domestic sewage are that the content of negatively charged metabolites such as dissolved organic matter, such as proteins, humic substances, and microorganisms, in the sewage is very high. This makes the charge adsorption and chelation of the existing single-form polyaluminum chloride products on dissolved organic matter limited. On the one hand, it affects the sewage treatment effect, and on the other hand, it is easy to form loose flocs. Under the operating conditions such as the hydraulic force of the mixer, the loose flocs are easy to break. After the flocs break, the sedimentation performance decreases significantly, and the flocculation effect is further reduced. Therefore, it is necessary to develop a polyaluminum chloride flocculant suitable for the water quality characteristics of urban domestic sewage.

[0004] Some studies show that high-polymeric Al c may play an important role in the flocculation effect of polyaluminum chloride. This is because high-polymeric Al c is a polynuclear cation complex, which has more abundant binding sites than low- and medium-polymeric aluminum, is more likely to be destabilized to form flocs to adsorb impurities, and has a better adsorption effect on macromolecular negatively charged highly soluble organic matter. For example, Xia Deqiang, Suo Longning, etc. (Study on the aluminum morphological distribution and flocculation performance of composite flocculant APAC [J], Chinese Journal of Environmental Engineering, 2011, 5(9): 2039-2042) prepared a composite polyaluminum chloride flocculant containing modified attapulgite and applied it to domestic sewage treatment. The treatment effect showed a positive correlation with the content of Al c . However, this study did not disclose what kind of modified attapulgite was specifically used, and the modified attapulgite may affect the aluminum morphological distribution in polyaluminum chloride. Moreover, high-polymeric aluminum chloride is easy to nucleate and crystallize and precipitate, and its storage stability is generally not high. Summary of the Invention

[0005] Aiming at the problem that the treatment effect of existing polyaluminum chloride products on urban domestic sewage with characteristics such as high protein and high humus needs to be improved, the purpose of the present invention is to provide a preparation method of a polyaluminum chloride flocculant. By post-treating the polyaluminum chloride stock solution, the content of high-polymeric Al c in the polyaluminum chloride flocculant is increased, the distribution of high-polymeric Al c and medium-polymeric Al b is balanced, and the treatment capacity of the polyaluminum chloride flocculant for urban domestic sewage is improved. Compared with polyaluminum chloride products on the market, the same treatment effect can be achieved with a smaller amount of the polyaluminum chloride flocculant.

[0006] The present invention provides the following technical solutions:

[0007] A preparation method of a polyaluminum chloride flocculant, comprising:

[0008] Adding water to dilute and heating the polyaluminum chloride stock solution with pH < 3 and Al2O3 solid content ≥ 10%, and then adding a strong base solution to gradually adjust the pH of the polyaluminum chloride stock solution to 3.0 - 3.6 to obtain a polyaluminum chloride solution, that is, the polyaluminum chloride flocculant.

[0009] The polyaluminum chloride solution of the present invention, that is, the polyaluminum chloride flocculant, adds water to dilute the polyaluminum chloride stock solution to adjust the ionic form in the solution, and then slowly drops a strong base solution into the polyaluminum chloride stock solution with a high solid content under heating conditions to slowly increase the pH of the solution. Compared with weak bases, it is more conducive to the full reaction of aluminum ions in the solution to generate high-polymeric aluminum Al c , improve the content of Al c , and finally control the pH to 3.0 - 3.6 to maintain a reasonable ratio distribution of Al c and Al b . The prepared polyaluminum chloride solution has a basicity of 60 - 80% and has a higher urban sewage treatment capacity.

[0010] As a preference of the method of the present invention, the pH of the polyaluminum chloride stock solution is 0.5 - 1.0. Within this range, aluminum ions are mainly distributed in the form of low-polymeric monomers (30 - 40% low basicity), which helps the polymerization reaction during the continuous dropping process of the solution to directly form high-polymeric Al c .

[0011] As a preference of the method of the present invention, the mass ratio of the polyaluminum chloride stock solution to water is 6 - 9:1. Reasonably control the amount of water added to maintain a suitable solid content and at the same time maintain a sensitive change in pH during the dropping of the strong base.

[0012] As a preference of the method of the present invention, the change rate of pH ≤ 0.1 / min. Too fast a change in pH will affect the formation of Al c .

[0013] Preferably in the method of the present invention, the rate of change of pH is 0.05 - 0.07 / min. Within this range, the reaction can proceed fully, taking into account the ionic form distribution, improving the thermal stability of the polyaluminum chloride solution, and avoiding thermal de - stabilization during storage.

[0014] Preferably in the method of the present invention, the strong base solution is sodium hydroxide solution. The preferred concentration of sodium hydroxide solution is 1 - 2 mol / L. If it is too low, the change of pH is too slow, and the amount of sodium hydroxide solution added increases, affecting the ionic form distribution. If it is too high, the change of pH is too fast and it is not easy to operate.

[0015] Preferably in the method of the present invention, the heating temperature is 45 - 60°C.

[0016] Preferably in the method of the present invention, it further includes adding nano - magnetic powder to the polyaluminum chloride solution after adjusting the pH. The magnetic powder used is such as magnetic nano - iron oxide (Fe₃O₄), magnetic magnesium oxide, etc. Magnetic particles help to form denser flocs during the flocculation process, improving the flocculation effect.

[0017] Preferably in the method of the present invention,

[0018] the particle size of the nano - magnetic powder is 200 - 350 nm;

[0019] and / or, the addition amount of the nano - magnetic powder is 0.3 - 1 wt% of the polyaluminum chloride solution.

[0020] Preferably in the method of the present invention, the nano - magnetic powder is used after being processed through the following process:

[0021] The nano - magnetic powder is dispersed in a sodium stearate solution, and the mass ratio of the nano - magnetic powder to stearic acid is 100:1 - 5, and then spray - dried to obtain modified nano - magnetic powder. After being treated with sodium stearate, the surface charge of the magnetic powder can be passivated, reducing the influence on the polyaluminum chloride solution during the addition of the magnetic powder and avoiding the formation of precipitation.

[0022] The beneficial effects of the present invention are as follows:

[0023] The polyaluminum chloride flocculant obtained by the preparation method of the present invention has a higher Al c content, taking into account a reasonable Al c and Al b distribution, has a good treatment effect on urban domestic sewage, has a high COD removal rate, and compared with commercially available polyaluminum chloride with a similar basicity, the usage amount is lower under the same treatment effect; especially when further introducing nano - magnetic powder to form magnetic coagulation polyaluminum chloride, denser flocs can be formed during the flocculation process, the flocs are not easy to break and disperse, and the flocculation effect is further improved. Detailed Embodiments

[0024] The following further describes the detailed embodiments of the present invention.

[0025] Unless otherwise specified, the raw materials used in the present invention can be obtained from the market or are commonly used in the art; unless otherwise specified, the methods in the following embodiments are all conventional methods in the art.

[0026] Part 1: Preparation of polyaluminum chloride flocculant.

[0027] Example 1

[0028] A method for preparing a polyaluminum chloride flocculant is as follows:

[0029] (1) Add 30 g of water to 220 g of polyaluminum chloride stock solution (Xiaoshan Sanjiang water purification agent product, Al2O3 solid content 10% - 12%) with a pH of 0.8, stir and mix, and then heat to 50 °C;

[0030] (2) Slowly dropwise add sodium hydroxide solution (1 M) to the hot polyaluminum chloride stock solution, and control the pH of the polyaluminum chloride stock solution to increase to 3.5 at a rate of +0.05 / min to obtain a polyaluminum chloride solution.

[0031] Example 2

[0032] A method for preparing a polyaluminum chloride flocculant is as follows:

[0033] (1) Add 27.5 g of water to 220 g of polyaluminum chloride stock solution (Xiaoshan Sanjiang water purification agent product, Al2O3 solid content 10% - 12%) with a pH of 0.5, stir and mix, and then heat to 45 °C;

[0034] (2) Slowly dropwise add sodium hydroxide solution (2 M) to the hot polyaluminum chloride stock solution, and control the pH of the polyaluminum chloride stock solution to increase to 3.0 at a rate of +0.05 / min to obtain a polyaluminum chloride solution.

[0035] Example 3

[0036] A method for preparing a polyaluminum chloride flocculant is as follows:

[0037] (1) Add 36.6 g of water to 220 g of polyaluminum chloride stock solution (Xiaoshan Sanjiang water purification agent product, Al2O3 solid content 10% - 12%) with a pH of 1.0, stir and mix, and then heat to 60 °C;

[0038] (2) Slowly dropwise add sodium hydroxide solution (1 M) to the hot polyaluminum chloride stock solution, and control the pH of the polyaluminum chloride stock solution to increase to 3.6 at a rate of +0.05 / min to obtain a polyaluminum chloride solution.

[0039] Comparative Example 1

[0040] The difference from Example 1 is that the pH change rate is controlled at 0.2 / min.

[0041] Comparative Example 2

[0042] The difference from Example 1 is that the polyaluminum chloride solution is not diluted with water.

[0043] Comparative Example 3

[0044] The difference from Example 1 is that sodium hydroxide solution is added dropwise until the pH end point is 4.5.

[0045] Performance Test

[0046] 1. The polyaluminum chloride solution prepared in Example 1 was tested, and the results are shown in Table 1 below.

[0047] Table 1 Performance Indexes of Flocculants

[0048] Test Items Test Standards Test Results Appearance Colorless to light yellow transparent liquid Light yellow and relatively clear liquid Al2O3% Content ≥10.0％ 10.8 Basicity% 60-80％ 69.9 pH (10g / L aqueous solution) 3.5-5.0 4.43 Mass fraction of arsenic (As) / % ≤0.0002 0.00011 Mass fraction of lead (Pb) / % ≤0.001 0.00012 Mass fraction of cadmium (Cd) / % ≤0.0002 ≤0.00001 Mass fraction of mercury (Hg) / % ≤0.00001 0.000001 Mass fraction of chromium (Cr) / % ≤0.0005 0.00030 Mass fraction of iron (Fe) / % ≤0.2 0.055

[0049] As can be seen from the above table, the basicity of the prepared polyaluminum chloride flocculant reaches 69.9%, and it has a relatively high Al c content. All indexes meet the requirements of the liquid in GB / T 22627-2014 "Water Treatment Agent - Polyaluminum Chloride", and the quality of the flocculant meets the enterprise standard Q / SJB 003—2022 "Magnetic Coagulation Polyaluminum Chloride for Urban Domestic Sewage Treatment Plants".

[0050] 2. Urban Domestic Sewage Treatment Test

[0051] 1 L of sewage from an urban sewage treatment plant with a COD value of 150 mg / L, containing impurities such as protein and humic acid, was taken respectively. The polyaluminum chloride flocculants prepared in Examples 1 - 3 and Comparative Examples 1 - 3 were added to the sewage for treatment, and commercially available polyaluminum chloride (basicity 70%, solid content 12.0%) was used as a reference. The COD removal rate was calculated: the percentage of the COD removal amount to the initial COD value. The results are shown in Table 2 below.

[0052] Table 2 COD Removal Rate of Sewage Treatment

[0053]

[0054] As can be seen from the above table, at the same dosage, the polyaluminum chloride flocculant prepared by the scheme of the present application has a higher biochemical sewage treatment effect and a higher COD removal rate. The polyaluminum chloride flocculants prepared in Comparative Example 1 and Comparative Example 2 have no obvious advantage over the commercially available polyaluminum chloride. This may be because the pH change rate is too fast or the ion concentration in the polyaluminum chloride stock solution is relatively high, which affects the Alc Sufficient formation. The application effect of the polyaluminum chloride flocculant in Comparative Example 3 was lower than that of the commercially available product within the test range, which may be because the pH value at the titration end point was too high. Although it was beneficial to the formation of high Al c , it disrupted the suitable coexistence range of Al c and Alb, affecting the performance of the flocculant.

[0055] Part Two: Influence of the pH change rate on the storage stability of the flocculant.

[0056] Example 4

[0057] The difference from Example 1 was that the pH change rate was controlled at 0.02 / min.

[0058] Example 5

[0059] The difference from Example 1 was that the pH change rate was controlled at 0.07 / min.

[0060] Example 6 (0.1)

[0061] The difference from Example 1 was that the pH change rate was controlled at 0.1 / min.

[0062] Stability test:

[0063] The polyaluminum chloride solutions prepared in the above examples were used for COD removal test of urban sewage, showing similar COD removal effects. However, the medium and high polymeric polyaluminum chloride solutions were more likely to be destabilized and precipitated under the influence of the environment, especially the influence of temperature, affecting the storage stability. Therefore, a heat resistance test was further carried out:

[0064] 100 mL of each of the polyaluminum chloride solutions prepared in Example 1 and Examples 4 - 6 was sealed and placed at 50 °C, and the precipitation time of each polyaluminum chloride solution was recorded. The results are shown in Table 3.

[0065] Table 3 Stability test

[0066] Item Example 4 Example 1 Example 5 Example 6 pH change rate / min 0.02 0.05 0.07 0.1 Precipitation appearance time / h 56 69 66 51

[0067] It can be seen from the above table that when the pH change rate was 0.05 - 0.07, the precipitation time of the polyaluminum chloride solution was later, with higher thermal stability and being beneficial for storage. This may be due to the influence of the pH change rate on the production and distribution of polyvalent ions during the preparation process. Generally, it is considered that the slower the pH change rate, the more complete the reaction, and the higher the performance of the polyaluminum chloride may be. However, in terms of thermal stability, there is still an optimal pH change rate.

[0068] Part Three: Adding magnetic powder for coagulation.

[0069] Example 7

[0070] The difference from Example 1 is that magnetic nano-ferroferric oxide powder is added to the polyaluminum chloride solution prepared in step (2), and the addition amount is 0.5 wt% of the mass of polyaluminum chloride. The average particle size of the used magnetic nano-ferroferric oxide is 300 nm.

[0071] Example 8

[0072] The difference from Example 1 is that magnetic nano-ferroferric oxide powder is added to the polyaluminum chloride solution prepared in step (2), and the addition amount is 0.3 wt% of the mass of polyaluminum chloride. The average particle size of the used magnetic nano-ferroferric oxide is 350 nm.

[0073] Example 9

[0074] The difference from Example 1 is that magnetic nano-ferroferric oxide powder is added to the polyaluminum chloride solution prepared in step (2), and the addition amount is 1 wt% of the mass of polyaluminum chloride. The average particle size of the used magnetic nano-ferroferric oxide is 200 nm.

[0075] Example 10

[0076] The difference from Example 8 is that the used magnetic nano-ferroferric oxide powder is treated through the following process before use:

[0077] Disperse the nano-magnetic powder into a sodium stearate solution (5 wt%), and the mass ratio of the nano-magnetic powder to sodium stearate is 100:5, and then spray drying to obtain modified nano-magnetic powder.

[0078] Example 11

[0079] The difference from Example 8 is that the used magnetic nano-ferroferric oxide powder is treated through the following process before use:

[0080] Disperse the nano-magnetic powder into a sodium stearate solution (1 wt%), and the mass ratio of the nano-magnetic powder to sodium stearate is 100:1, and then spray drying to obtain modified nano-magnetic powder.

[0081] Comparative Example 4

[0082] The difference from Example 9 is that sodium dodecyl sulfate is used to replace sodium stearate.

[0083] Urban domestic sewage treatment test

[0084] Carry out the urban domestic sewage treatment test according to the above method, the initial COD value is 150 mg / L, and the results are shown in Table 4 below.

[0085] Table 4 COD removal rate of sewage treatment

[0086]

[0087] As can be seen from the above table, after adding magnetic nanoparticles, the magnetic coagulation polyaluminum chloride solution as shown in Examples 7 to 11 can be obtained, which can further improve the COD removal effect. This is because the magnetic particles contribute to the formation of magnetic coagulants and improve the compactness of the flocs formed by polyaluminum chloride. After treating the magnetic nanoparticles with sodium stearate or sodium dodecyl sulfate, as shown in Examples 10, 11 and Comparative Example 4, the removal effect is slightly improved, and sodium stearate or sodium dodecyl sulfate helps the dispersion of the nanoparticles. More importantly, when repeating the method of Example 7 to prepare the magnetic coagulation polyaluminum chloride solution, there will be slight precipitation 1-2 times / 10 times during the addition of magnetic nanoparticles. However, after treatment with sodium stearate, as in Example 10, no precipitation will occur, but sodium dodecyl sulfate cannot achieve the corresponding effect. This may be because sodium stearate has a longer molecular chain coating on the surface of the magnetic nanoparticles, which can better ease the process of dispersing the magnetic nanoparticles into the polyaluminum chloride solution.

Claims

1. A preparation method of a polyaluminum chloride flocculant, characterized in that, Including: Dilute the polyaluminum chloride stock solution with a pH < 3 and an Al2O3 solid content ≥ 10% by adding water, heat it and add a strong base solution. The change rate of pH is 0.05 - 0.07 / min, and gradually adjust the pH of the polyaluminum chloride stock solution to 3.0 - 3.6 to obtain a polyaluminum chloride solution.

2. The preparation method according to claim 1, characterized in that The pH of the polyaluminum chloride stock solution is 0.5 - 1.

0.

3. The preparation method according to claim 1 or 2, characterized in that The mass ratio of the polyaluminum chloride stock solution to water is 6 - 9:

1.

4. The preparation method according to claim 1, wherein, The strong base solution is sodium hydroxide solution or potassium hydroxide solution.

5. The preparation method according to claim 1, characterized in that, The heating temperature is 45 - 60 °C.

6. The preparation method according to claim 1, wherein It also includes adding nano magnetic powder to the polyaluminum chloride solution after adjusting the pH.

7. The preparation method according to claim 6, characterized in that, The particle size of the nano magnetic powder is 200 - 350 nm.

8. The preparation method according to claim 6, characterized in that, The addition amount of the nano magnetic powder is 0.3 - 1 wt% of the polyaluminum chloride solution.

9. The preparation method according to claim 6, characterized in that, The nano magnetic powder is used after being processed through the following process: Disperse the nano magnetic powder into a sodium stearate solution. The mass ratio of the nano magnetic powder to stearic acid is 100:1 - 5, and then spray dry to obtain modified nano magnetic powder.

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