A method for treating manganese-containing wastewater by direct precipitation with low alkali content

Abstract

The present invention provides a method for treating manganese-containing wastewater by direct precipitation with low alkali content, comprising the steps of: S1, maintaining the manganese-containing wastewater at a first pH value using an alkaline precipitant, reacting at least 100 minutes at the first pH value to obtain a pre-reaction liquid; the alkaline precipitant is easily soluble in water, and the pH value of the first pH value is 9-9.45; the manganese-containing wastewater contains manganese ions, calcium ions, and magnesium ions; the mass concentration of the manganese ions is less than the sum of the mass concentrations of the calcium ions and the magnesium ions; S2, maintaining the pre-reaction liquid at a second pH value using the alkaline precipitant, reacting at the second pH value for 20-40 minutes, and solid-liquid separation to obtain a precipitate and manganese removal water body; the pH value of the second pH value is 9.8-9.95. The present invention can efficiently and selectively remove manganese from manganese-containing wastewater directly by an alkaline precipitant under low alkali content.

Description

Technical Field

[0001] The present invention relates to the field of wastewater treatment, in particular to a method for treating manganese-containing wastewater through direct precipitation with low alkali content. Background Art

[0002] Manganese mine water is characterized by high concentrations of manganese pollution. Direct discharge of untreated manganese mine water poses a serious threat to the ecological environment. Heavy metal pollution can penetrate through surface runoff and even pose cross-regional ecological risks. Furthermore, manganese mine water is rich in calcium and magnesium ions, forming a complex multi-ion system that makes selective precipitation of manganese difficult. Chemical precipitation is the mainstream method for removing manganese from manganese-containing wastewater. This can be achieved by adjusting the pH with an alkaline precipitant.

[0003] In the case of manganese mine water, since it contains not only manganese ions but also calcium and magnesium ions, and manganese, calcium, and magnesium ions can all precipitate under alkaline conditions, traditional chemical precipitation methods require excessive addition of alkaline precipitants to maintain a high manganese ion removal rate. However, excessive addition of alkaline precipitants not only significantly increases costs, severely restricting the widespread use of the technology, but also leads to the co-precipitation of calcium and magnesium ions, resulting in the inclusion of calcium and magnesium in the manganese precipitate. Although oxidation methods can also have a certain removal effect on manganese ions, they also require a large amount of oxidizing agents, which is not only prone to causing new pollution but also remains costly. Although sodium fluoride can pre-remove calcium and magnesium, it will cause residual fluoride ions and increase overall costs.

[0004] In view of this, it is necessary to provide a method for treating manganese-containing wastewater by direct precipitation with low alkali content to solve or at least alleviate the technical problem of how to efficiently and selectively remove manganese from manganese-containing wastewater by alkaline precipitants under low alkali content. Summary of the Invention

[0005] The main purpose of the present invention is to provide a method for treating manganese-containing wastewater by direct precipitation with low alkali content, aiming to solve the technical problem of how to efficiently and selectively remove manganese from manganese-containing wastewater by an alkaline precipitant under low alkali content.

[0006] To achieve the above object, the present invention provides a method for treating manganese-containing wastewater by direct precipitation with low alkali content, comprising the steps of:

[0007] S1, using an alkaline precipitant to adjust the manganese-containing wastewater to a first pH, then using the alkaline precipitant to maintain the first pH, reacting at the first pH for at least 100 minutes to obtain a pre-reaction solution; the alkaline precipitant is easily soluble in water, and the pH of the first pH is 9-9.45;

[0008] The manganese-containing wastewater contains manganese ions, calcium ions, and magnesium ions; the concentration of the manganese ions in the manganese-containing wastewater is 100-500 mg / L; the mass concentration of the manganese ions is less than the sum of the mass concentrations of the calcium ions and the magnesium ions;

[0009] S2, using the alkaline precipitant, adjusting the pre-reaction liquid to a second pH, then using the alkaline precipitant to maintain the second pH, reacting at the second pH for 20-40 minutes, and separating the solid and liquid to obtain a precipitated and manganese-removed water body; the pH of the second pH is 9.8-9.95.

[0010] Furthermore, the alkaline precipitant includes a strong alkaline hydroxide; the strong alkaline hydroxide includes one or more of sodium hydroxide and potassium hydroxide.

[0011] Furthermore, in step S1, the molar ratio of the added amount of the alkaline precipitant to the manganese ions is 2.3-3.3:1.

[0012] Furthermore, in step S1, the molar ratio of the added amount of the alkaline precipitant to the manganese ions is 2.3-2.7:1 or 3.2-3.3:1.

[0013] Furthermore, in step S1 and step S2, the molar ratio of the total amount of the alkaline precipitant added to the manganese ion is 2.9-3.8:1.

[0014] Furthermore, the molar ratio of the total amount of the alkaline precipitant added to the manganese ion is 2.9-3.1:1 or 3.7-3.8:1.

[0015] Furthermore, the pH of the first acidity is 9.15-9.25 or 9.35-9.45; the pH of the second acidity is 9.85-9.95.

[0016] Furthermore, the mass concentration ratio of the calcium ions to the magnesium ions in the manganese-containing wastewater is 1-2:1.

[0017] Furthermore, the reaction time at the first pH is 110-130 min.

[0018] Furthermore, in the process of converting the manganese-containing wastewater into the manganese-removing water body, only the alkaline precipitant is added, or only the alkaline precipitant and flocculant are added.

[0019] Compared with the prior art, the present invention has at least the following advantages:

[0020] The present invention can directly and selectively remove manganese from manganese-containing wastewater using an alkaline precipitant under low alkali conditions, with high manganese removal efficiency and without causing large amounts of calcium and magnesium precipitation. Furthermore, the present invention can not only reduce the amount of alkaline precipitant used, but also eliminate the need for oxidants, sodium fluoride and other agents. The present invention directly uses an alkaline precipitant to perform deep selective removal of manganese in a chemical precipitation manner, thus overcoming the technical difficulties of chemical precipitation in increasing the alkali content and causing large amounts of calcium and magnesium precipitation.

[0021] The present invention is based on the difference in solubility product and preferentially removes manganese in the low pH range, avoiding the premature precipitation of calcium and magnesium that encapsulates manganese ions in traditional methods, reducing alkali waste and the amount of co-precipitated sludge. The present invention is based on kinetic control enhancement. After adjusting the pH in the first step, the pH is maintained stable for a long time to promote manganese precipitation and form stable crystals; by raising the pH for a short time in the second step, the end point is accurately controlled to avoid excessive alkali addition, and the nucleation kinetics of magnesium are suppressed by accurately controlling the duration of the high pH range. At the same time, the extremely low solubility product of manganese is utilized to achieve rapid and deep purification of manganese, and the synchronous precipitation of calcium and magnesium is suppressed. The present invention is based on the seed effect to enhance precipitation. The manganese precipitate formed in the first step can be used as a "seed" to adsorb residual manganese ions in the second step, accelerate their removal, and further reduce dependence on high pH.

[0022] By combining "staged matching of solubility product thresholds" with "precise reaction time control," this method leverages the thermodynamic precipitation differences between manganese and calcium and magnesium, while also kinetically suppressing the risk of calcium and magnesium co-precipitation. This avoids the drawback of traditional precipitation processes, where excessive precipitation of calcium and magnesium is required to remove trace amounts of manganese. This method also offers both economic and environmental benefits, directly reducing processing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0024] Figure 1 This is a schematic diagram of the process for treating manganese-containing wastewater by direct precipitation with low alkali content in the present invention.

[0025] The realization of the objectives, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0026] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0027] Moreover, the technical solutions between the various embodiments of the present invention may be combined with each other, but this must be based on the fact that ordinary technicians in this field can implement them. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0028] When numerical ranges are given in the examples, it should be understood that unless otherwise specified herein, both endpoints of each numerical range and any value between the two endpoints may be used. Unless otherwise defined, all technical and scientific terms used herein are consistent with the prior art as understood by those skilled in the art and the description of the present invention. Any prior art methods, devices, and materials similar or equivalent to those described in the examples of the present invention may also be used to implement the present invention.

[0029] See also Figure 1 It is understood that the present invention provides a method for treating manganese-containing wastewater by direct precipitation with low alkali content, comprising the steps of:

[0030] S1, using an alkaline precipitant to adjust the manganese-containing wastewater to a first pH, then using the alkaline precipitant to maintain the first pH, and the manganese-containing wastewater reacts at the first pH for at least 100 minutes to obtain a pre-reaction liquid; during the reaction process of the present invention, stirring is maintained.

[0031] In step S1, the time for maintaining the first pH is equal to the reaction time; further, the reaction time of the manganese-containing wastewater at the first pH is 100-140 minutes, further 110-130 minutes.

[0032] In the present invention, the alkaline precipitant is easily soluble in water; the alkaline precipitant includes or is a strong alkaline hydroxide; the strong alkaline hydroxide includes or is one or more of sodium hydroxide and potassium hydroxide, and is further sodium hydroxide.

[0033] In the present invention, the pH of the first acidity is 9-9.45; further, the pH of the first acidity is 9.15-9.25 or 9.35-9.45 or 9-9.4 or 9.2-9.4 or 9.15-9.45.

[0034] As an illustration of the amount of the alkaline precipitant added in step S1 (the amount used to adjust the manganese-containing wastewater to the first pH and maintain the first pH), the molar ratio of the amount of the alkaline precipitant added to the manganese ions is 2.3-3.3:1; further, the molar ratio of the amount of the alkaline precipitant added to the manganese ions is 2.5-3.25 or 2.3-2.7:1 or 2.4-2.6:1 or 3.2-3.3:1.

[0035] As an illustration of the manganese-containing wastewater, the manganese-containing wastewater is manganese mine water; the pH of the manganese-containing wastewater is 7.2-7.4; the manganese-containing wastewater contains manganese ions, calcium ions, and magnesium ions; in the manganese-containing wastewater, the concentration of manganese ions is 100-500 mg / L, further 200-300 mg / L; the concentration of calcium ions is 400-600 mg / L, further 400-500 mg / L; the concentration of magnesium ions is 300-500 mg / L, further 300-400 mg / L.

[0036] In the present invention, the concentration of the manganese ion is less than the sum of the concentrations of the calcium ion and the magnesium ion; specifically, the mass concentration of the manganese ion is less than the sum of the mass concentrations of the calcium ion and the magnesium ion. The mass concentration of the manganese ion in the manganese-containing wastewater is recorded as the manganese mass concentration, and the sum of the mass concentrations of the calcium ion and the magnesium ion in the manganese-containing wastewater is recorded as the calcium-magnesium mass concentration. The ratio of the manganese mass concentration to the calcium-magnesium mass concentration is 1:2-5, and further 1:3-4. The concentration ratio of the calcium ion to the magnesium ion in the manganese-containing wastewater is 1-2:1, and the concentration ratio is the mass concentration ratio.

[0037] S2, using the alkaline precipitant to adjust the pre-reaction liquid to a second pH, then using the alkaline precipitant to maintain the second pH, the pre-reaction liquid reacts at the second pH for 20-40 minutes, and after the reaction is completed, solid-liquid separation is performed to obtain a precipitate and manganese-removed water (effluent).

[0038] Furthermore, the reaction time of the pre-reaction liquid at the second pH is 25-35 min, further 28-32 min, specifically 30 min.

[0039] In step S2, the time for maintaining the second pH is equal to the reaction time, or the time for adjusting to the second pH and maintaining the second pH is equal to the reaction time; in the present invention, stirring is maintained during the reaction process. In the present invention, for ease of analysis, in actual cases, the reaction time of step S2 includes the time for adjusting the pre-reaction liquid to the second pH; since the time for adjusting the pre-reaction liquid to the second pH is relatively short, it does not affect the correlation between the reaction time and the technical effect.

[0040] In the present invention, the reaction is terminated after the reaction time is reached; for example, solid-liquid separation is performed immediately after the reaction is completed, thereby preventing calcium and magnesium from entering the manganese-containing precipitate due to long-term reaction and affecting the purity of manganese in the manganese-containing precipitate.

[0041] In the present invention, the precipitate is a manganese-containing precipitate; in the manganese-removing water, the concentration of manganese ions is less than 2 mg / L, further less than 0.5 mg / L or 0.4-1.25 mg / L.

[0042] In the present invention, the pH of the second acidity is 9.8-9.95; further, the pH of the second acidity is 9.85-9.95.

[0043] As an illustration of the total amount of the alkaline precipitant added (the sum of the amounts of the alkaline precipitant added in step S1 and step S2), in step S1 and step S2, the molar ratio of the total amount of the alkaline precipitant added to the manganese ion is 2.9-3.8:1; further, the molar ratio of the total amount of the alkaline precipitant added to the manganese ion is 3-3.75:1 or 2.9-3.1:1 or 3.7-3.8:1.

[0044] In the present invention, as an optional method in the industrial application process, the reaction product can be flocculated before solid-liquid separation, and then the solid is separated to obtain the precipitate and the manganese removal water. The flocculation can use a flocculant.

[0045] The method of the present invention is a method for treating manganese-containing wastewater by direct precipitation under low alkali conditions, and only requires direct chemical precipitation of manganese ions. The direct precipitation in the present invention means that the present invention can directly perform deep selective precipitation of manganese ions only through chemical precipitation with an alkaline precipitant.

[0046] The present invention does not require the addition of other agents such as oxidants and sodium fluoride, nor does it require the addition of agents other than the alkaline precipitant, or does not require the addition of agents other than the alkaline precipitant and flocculant; therefore, in the process of converting the manganese-containing wastewater into the manganese-removing water body, the present invention only adds the alkaline precipitant, or only adds the alkaline precipitant and flocculant; and the present invention only adjusts and maintains the first alkalinity and the second alkalinity.

[0047] The present invention uses a step-by-step pH adjustment method to directly precipitate manganese ions. Initially, an appropriate amount of alkali is added to prevent excessively high pH, achieving selective precipitation of manganese ions. Subsequently, a small amount of alkali is added to maintain a high pH for a short period of time to precipitate residual manganese ions. This method significantly reduces the amount of reagents added, is cost-effective, and simultaneously achieves resource utilization and deep purification of manganese. The present invention achieves a high manganese ion recovery rate, ensures that manganese-containing wastewater meets discharge standards (manganese ions <2 mg / L), and has high recovered manganese purity with reduced calcium and magnesium doping, demonstrating promising industrial application prospects.

[0048] The following is a partial principle analysis of the present invention:

[0049] 1. The first step (maintaining pH 9-9.45 for a longer period of time):

[0050] (1) Thermodynamic advantage: The solubility product of Mn(OH)2 is much smaller than that of Mg(OH)2 and Ca(OH)2. At a pH of 9-9.45, only manganese hydroxide is supersaturated, while calcium and magnesium are still unsaturated.

[0051] (2) Kinetic control: Maintaining pH stability for a long time provides sufficient precipitation time for manganese, allowing it to form stable crystal nuclei and grow into larger particles; at this stage, the precipitation rate of manganese is much higher than that of calcium and magnesium, and it preferentially consumes manganese ions in the solution.

[0052] 2. Second step (pH 9.8-9.95, maintained for a shorter period of time):

[0053] (1) Thermodynamic changes: At this time, the pH is close to the solubility product threshold of Mg(OH)2, but has not yet reached it; magnesium is in a metastable state (insufficient supersaturation); calcium is still in an unsaturated state.

[0054] (2) Dynamics key:

[0055] Deep purification of manganese: Due to its extremely low solubility, the residual trace manganese quickly reaches supersaturation after a slight increase in pH, and is preferentially adsorbed or co-precipitated on the surface of existing manganese precipitates (seed effect), without the need for long-term reaction.

[0056] Calcium and magnesium inhibition mechanism:

[0057] Magnesium: Although the increase in pH brings it close to the solubility product, the supersaturation is insufficient and nucleation takes time (magnesium has a high nucleation energy); short-term operation does not allow magnesium to nucleate and precipitate in large quantities.

[0058] Calcium: pH 9.8-9.95 is still far below the pH required for its large-scale precipitation.

[0059] (3) Advantage of low concentration of manganese: The concentration of manganese has been greatly reduced after the first step. When the pH value is 9.8-9.95 in the second step, manganese is still in a supersaturated state and will continue to precipitate; however, the supersaturation of magnesium is insufficient, and calcium has not reached the pH condition for large-scale precipitation.

[0060] (4) Competition between time and supersaturation: The precipitation of magnesium requires sufficient supersaturation to drive nucleation, and after nucleation, it takes time to grow into sedimentable particles; short-term operation prevents magnesium from completing the nucleation-growth process through "time window truncation", thereby avoiding large-scale precipitation.

[0061] The following are specific examples of the present invention:

[0062] Example 1

[0063] At room temperature, sodium hydroxide was added to the manganese ore gushing water (pH 7.3) to raise the pH to 9.2±0.05 and maintain the pH, and the reaction was carried out for 2 h (the molar ratio of the cumulative added sodium hydroxide to the manganese ions in the manganese ore gushing water was 2.5:1) to obtain a pre-reaction liquid; after sampling and separating the solid matter, the calcium, magnesium, and manganese concentrations were 477.75, 378.50, and 21.5 mg / L, respectively.

[0064] Sodium hydroxide was further added to the pre-reaction liquid to raise the pH to 9.9±0.05 and maintain the pH. After 30 minutes of reaction (at this time, the total reaction time was 150 minutes, and the molar ratio of the cumulative added sodium hydroxide to the manganese ions in the manganese ore gushing water was 3:1), manganese-containing precipitate and manganese-removed water (effluent) were separated; in the manganese-removed water, the concentrations of calcium, magnesium, and manganese were 474.5, 376.00, and 1.25 mg / L, respectively, achieving deep purification of manganese ions in the manganese ore gushing water.

[0065] In this embodiment, the main ions and concentrations in the manganese ore water are shown in Table 1.

[0066] Table 1 Main ions and concentrations in mine water from a mining area (mg / L)

[0067]

[0068] During the reaction process of this example, the detection results of calcium, magnesium, and manganese are shown in Table 2; during the detection, the solid matter was separated.

[0069] Table 2 Concentration detection during the reaction process

[0070]

[0071] Example 2

[0072] At room temperature, sodium hydroxide was added to the manganese ore gushing water (same as in Example 1) to raise the pH to 9.4±0.05 and maintain the pH, and the reaction was carried out for 2 h (the molar ratio of the cumulative added sodium hydroxide to the manganese ions in the manganese ore gushing water was 3.25:1) to obtain a pre-reaction liquid; after sampling and separating the solid matter, the calcium, magnesium, and manganese concentrations were 453.82, 369.67, and 3.75 mg / L, respectively.

[0073] Sodium hydroxide was further added to the pre-reaction liquid to raise the pH to 9.9±0.05 and maintain the pH. After 30 minutes of reaction (at this time, the total reaction time was 150 minutes, and the molar ratio of the cumulative added sodium hydroxide to the manganese ions in the manganese ore water was 3.75:1), manganese-containing precipitate and manganese-removed water (effluent) were separated; in the manganese-removed water, the concentrations of calcium, magnesium, and manganese were 425.47, 363.60, and 0.4 mg / L, respectively, achieving deep purification of manganese ions in the manganese ore water.

[0074] During the reaction process of this example, the detection results of calcium, magnesium, and manganese are shown in Table 3; during the detection, the solid matter was separated.

[0075] Table 3 Concentration detection during the reaction process

[0076]

[0077] Comparative Example 1

[0078] At room temperature, sodium hydroxide was added to the manganese ore gushing water (same as in Example 1) to raise the pH to a preset alkalinity and maintain it. After reacting for 150 minutes, the precipitate and the effluent were separated.

[0079] In this comparative example, the pH values of the preset alkalinity are 9.9±0.05 and 10.4±0.05, respectively. The alkali addition ratios and effluent manganese concentrations corresponding to the different preset alkalinity values are shown in Table 4.

[0080] Table 4 Alkali dosage ratio and effluent manganese concentration corresponding to different preset alkalinity

[0081]

[0082] As shown in Table 4, it can be seen that when the alkali addition ratio is 8:1, the manganese ion concentration in the effluent can be lower than 2 mg / L.

[0083] Further testing showed that when the alkali addition ratio was 6.8:1, the calcium ion concentration in the effluent was 387.75 mg / L and the magnesium ion concentration was 201.75 mg / L; when the alkali addition ratio was 8:1, the calcium ion concentration in the effluent was 299.5 mg / L and the magnesium ion concentration was 75.5 mg / L.

[0084] Comparative Example 2

[0085] At room temperature, sodium hydroxide was added to the manganese ore gushing water (same as in Example 1) to raise the pH to 9.3±0.05 and maintain the pH, and the reaction was carried out for 2 h to obtain a pre-reaction liquid. After sampling and separation of the solid matter, the concentrations of calcium, magnesium, and manganese were 468.61, 374.28, and 12.63 mg / L, respectively.

[0086] Sodium hydroxide was further added to the pre-reaction liquid to raise the pH to 9.7±0.05 and maintain the pH. After 30 minutes of reaction, the precipitate and manganese-removed water (effluent) were separated. The concentrations of calcium, magnesium, and manganese in the manganese-removed water were 439.06, 364.68, and 2.87 mg / L, respectively.

[0087] During the reaction process of this comparative example, the detection results of calcium, magnesium and manganese are shown in Table 5; during the detection, the solid matter was separated.

[0088] Table 5 Concentration detection during the reaction process

[0089]

[0090] Comparative Example 3

[0091] At room temperature, sodium hydroxide was added to the manganese ore gushing water (same as in Example 1) to raise the pH to 9.2±0.05 and maintain the pH, and the reaction was carried out for 2 h to obtain a pre-reaction liquid; after sampling and separation of the solid matter, the concentrations of calcium, magnesium, and manganese were 477.75, 378.50, and 21.5 mg / L, respectively.

[0092] Sodium hydroxide was further added to the pre-reaction solution to raise the pH to 9.6±0.05 and maintain the pH. After 30 minutes of reaction, the precipitate and manganese-removed water (effluent) were separated. The concentrations of calcium, magnesium, and manganese in the manganese-removed water were 476.12, 377.24, and 4.96 mg / L, respectively.

[0093] During the reaction process of this comparative example, the detection results of calcium, magnesium and manganese are shown in Table 6; during the detection, the solid matter was separated.

[0094] Table 6 Concentration detection during the reaction process

[0095]

[0096] The above technical solutions of the present invention are only preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made by using the contents of the present invention's description and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for treating manganese-containing wastewater by direct precipitation with low alkali content, characterized in that: Including steps: S1, using an alkaline precipitant to adjust the manganese-containing wastewater to a first pH, then using the alkaline precipitant to maintain the first pH, reacting at the first pH for 100-140 minutes to obtain a pre-reaction solution; The alkaline precipitant is easily soluble in water and includes a strong alkaline hydroxide, which includes one or more of sodium hydroxide and potassium hydroxide; the pH of the first acidity is 9-9.45; The manganese-containing wastewater contains manganese ions, calcium ions, and magnesium ions; the concentration of the manganese ions in the manganese-containing wastewater is 100-500 mg / L; the mass concentration of the manganese ions is less than the sum of the mass concentrations of the calcium ions and the magnesium ions; S2, adjusting the pre-reaction liquid to a second pH using the alkaline precipitant, then maintaining the second pH using the alkaline precipitant, reacting at the second pH for 20-40 minutes, and performing solid-liquid separation to obtain a precipitated and manganese-removed water body; wherein the pH of the second pH is 9.8-9.95; During the process of converting the manganese-containing wastewater into the manganese-removing water body, only the alkaline precipitant is added; or, before solid-liquid separation, the reaction product is flocculated using a flocculant, and during the process of converting the manganese-containing wastewater into the manganese-removing water body, only the alkaline precipitant and the flocculant are added.

2. The method for treating manganese-containing wastewater by low-alkali direct precipitation according to claim 1, characterized in that: In the step S1, the molar ratio of the added amount of the alkaline precipitant to the manganese ions is 2.3-3.3:

1.

3. The method for treating manganese-containing wastewater by low-alkali direct precipitation according to claim 2, characterized in that: In the step S1, the molar ratio of the added amount of the alkaline precipitant to the manganese ions is 2.3-2.7:1 or 3.2-3.3:

1.

4. The method for treating manganese-containing wastewater by low-alkali direct precipitation according to claim 1, characterized in that: In the step S1 and the step S2, the molar ratio of the total amount of the alkaline precipitant added to the manganese ion is 2.9-3.8:

1.

5. The method for treating manganese-containing wastewater by low-alkali direct precipitation according to claim 4, characterized in that: The molar ratio of the total amount of the alkaline precipitant added to the manganese ion is 2.9-3.1:1 or 3.7-3.8:

1.

6. The method for treating manganese-containing wastewater by low-alkali direct precipitation according to claim 1, characterized in that: The pH value of the first acidity and alkalinity is 9.15-9.25 or 9.35-9.45; the pH value of the second acidity and alkalinity is 9.85-9.

95.

7. The method for treating manganese-containing wastewater by low-alkali direct precipitation according to claim 1, characterized in that: The mass concentration ratio of the calcium ions to the magnesium ions in the manganese-containing wastewater is 1-2:

1.

8. The method for treating manganese-containing wastewater by low-alkali direct precipitation according to claim 1, characterized in that: The reaction time at the first pH is 110-130 min.

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