Method for low-alkali-content direct precipitation treatment of manganese-containing wastewater

Abstract

The invention provides a method for low-alkali-content direct precipitation treatment of manganese-containing wastewater, which comprises the following steps: S1, maintaining the manganese-containing wastewater at a first pH value by using an alkaline precipitator, and reacting for at least 100 minutes under the first pH value to obtain a pre-reaction solution; the alkaline precipitant is easily dissolved 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 smaller 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 by adopting the alkaline precipitator, reacting for 20-40 minutes under the second pH value, and carrying out solid-liquid separation to obtain a precipitate and a manganese-removed water body; the pH value of the second pH value is 9.8 to 9.95. According to the method, manganese in the manganese-containing wastewater can be efficiently and selectively removed directly through the alkaline precipitant under the condition of low alkali content.

Description

Technical Field

[0001] The present invention relates to the field of wastewater treatment, and in particular to a method for directly precipitating manganese-containing wastewater with a low alkali amount. Background Art

[0002] The manganese ore drainage water exhibits the characteristics of high-concentration manganese pollution. If the untreated manganese ore drainage water is directly discharged, it will seriously threaten the ecological environment. Its heavy metal pollution penetrates through surface runoff and may even cause cross-regional ecological risks. At the same time, the manganese ore drainage water is rich in calcium ions and magnesium ions, forming a complex multi-ion system, and it is very difficult to selectively precipitate manganese from the manganese ore drainage water. For the removal of manganese in manganese-containing wastewater, the mainstream method is the chemical precipitation method. By using an alkaline precipitant to adjust the pH, the precipitation of manganese in the manganese-containing wastewater can be achieved.

[0003] In the scenario of manganese ore drainage water, since the manganese ore drainage water not only contains manganese ions, but also is rich in calcium ions and magnesium ions, and manganese ions, calcium ions, and magnesium ions can all form precipitates under alkaline conditions. If the traditional chemical precipitation method is used, it is necessary to overdose the alkaline precipitant to maintain the precipitation environment in order to maintain a high removal rate of manganese ions. However, overdosing the alkaline precipitant will not only cause a significant increase in cost, seriously restricting the popularization and use of the technology, but also cause the coprecipitation of calcium and magnesium ions, resulting in the doping of calcium and magnesium in the manganese precipitate. Although the oxidation method also has a certain removal effect on manganese ions, however, the oxidation method also requires the use of a large amount of oxidation agents, which is not only easy to cause new pollution, but also the cost is still high. Although sodium fluoride can pre-remove calcium and magnesium, however, it will cause the residue of fluoride ions and increase the comprehensive cost.

[0004] In view of this, it is necessary to provide a method for directly precipitating manganese-containing wastewater with a low alkali amount to solve or at least alleviate the technical problem of how to efficiently and selectively remove manganese in manganese-containing wastewater with an alkaline precipitant under a low alkali amount. Summary of the Invention

[0005] The main object of the present invention is to provide a method for directly precipitating manganese-containing wastewater with a low alkali amount, aiming to solve the technical problem of how to efficiently and selectively remove manganese in manganese-containing wastewater with an alkaline precipitant under a low alkali amount.

[0006] To achieve the above object, the present invention provides a method for directly precipitating manganese-containing wastewater with a low alkali amount, including the steps of: S1, using an alkaline precipitant to adjust the manganese-containing wastewater to a first pH value, and then using the alkaline precipitant to maintain the first pH value, and reacting for at least 100 min at the first pH value to obtain a pre-reaction solution; the alkaline precipitant is easily soluble in water, and the pH of the first pH value is 9-9.45; The manganese-containing wastewater contains manganese ions, calcium ions, and magnesium ions; in the manganese-containing wastewater, the concentration of the manganese ions 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. Using the alkaline precipitant, adjust the pre-reaction solution to a second pH value, and then use the alkaline precipitant to maintain the second pH value. React for 20-40 min at the second pH value, and perform solid-liquid separation to obtain a precipitate and a manganese-removed water body; the pH of the second pH value is 9.8-9.95.

[0007] Furthermore, the alkaline precipitant includes strong alkaline hydroxides; the strong alkaline hydroxides include one or more of sodium hydroxide and potassium hydroxide.

[0008] Furthermore, in step S1, the molar ratio of the addition amount of the alkaline precipitant to the manganese ions is 2.3-3.3:1.

[0009] Furthermore, in step S1, the molar ratio of the addition amount of the alkaline precipitant to the manganese ions is 2.3-2.7:1 or 3.2-3.3:1.

[0010] Furthermore, in step S1 and step S2, the total addition amount of the alkaline precipitant and the molar ratio of the manganese ions is 2.9-3.8:1.

[0011] Furthermore, the total addition amount of the alkaline precipitant and the molar ratio of the manganese ions is 2.9-3.1:1 or 3.7-3.8:1.

[0012] Furthermore, the pH of the first pH value is 9.15-9.25 or 9.35-9.45; the pH of the second pH value is 9.85-9.95.

[0013] Furthermore, the mass concentration ratio of the calcium ions and the magnesium ions in the manganese-containing wastewater is 1-2:1.

[0014] Furthermore, the reaction time at the first pH value is 110-130 min.

[0015] Furthermore, in the process of converting the manganese-containing wastewater into the manganese-removed water body, only the alkaline precipitant is added, or only the alkaline precipitant and a flocculant are added.

[0016] Compared with the prior art, the present invention has at least the following advantages: The present invention can directly and efficiently remove manganese in manganese-containing wastewater through an alkaline precipitant at a low alkali dosage, with a high removal efficiency of manganese and no massive precipitation of calcium and magnesium. Moreover, the present invention can not only reduce the dosage of the alkaline precipitant but also does not require chemicals such as oxidants and sodium fluoride. The present invention directly uses an alkaline precipitant to deeply and selectively remove manganese by chemical precipitation, overcoming the technical problems that chemical precipitation will increase the alkali dosage and cause massive precipitation of calcium and magnesium.

[0017] Based on the solubility product difference, the present invention preferentially removes manganese in the low pH range to avoid premature precipitation and encapsulation of manganese ions by calcium and magnesium in traditional methods, reducing alkali waste and the amount of coprecipitation sludge. 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 short-term increasing the pH in the second step, the end point is precisely controlled to avoid excessive alkali addition. By precisely controlling the duration of the high pH range, the nucleation kinetics of magnesium is inhibited, and 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 inhibited. The present invention is based on the enhancement of the seed effect for precipitation. The manganese precipitate formed in the first step can serve as a "seed" to adsorb residual manganese ions in the second step, accelerating their removal and further reducing the dependence on high pH.

[0018] The present invention uses "phased matching of solubility product thresholds" + "precise control of reaction time", which not only utilizes the thermodynamic precipitation difference between manganese and calcium and magnesium but also kinetically inhibits the risk of coprecipitation of calcium and magnesium, avoiding the defect of "forcing excessive precipitation of calcium and magnesium to remove trace manganese" in traditional precipitation processes. The present invention also has dual economic and environmental benefits and can directly reduce the treatment cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0020] Figure 1 It is a schematic flow diagram of directly precipitating and treating manganese-containing wastewater with a low alkali dosage in the present invention.

[0021] The realization, functional features, and advantages of the object of the present invention will be further described in conjunction with the embodiments with reference to the drawings. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Next, in combination with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0023] Moreover, the technical solutions between various embodiments of the present invention can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0024] When the embodiments give a numerical range, it should be understood that unless otherwise specified in the present invention, both endpoints of each numerical range and any value between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present invention, based on the understanding of those of ordinary skill in the art of the prior art and the description of the present invention, any method, device, and material similar or equivalent to the methods, devices, and materials described in the embodiments of the present invention can also be used to implement the present invention.

[0025] See Figure 1 for understanding. The present invention provides a method for directly precipitating and treating manganese-containing wastewater with a low alkali content, including the steps: S1, using an alkaline precipitant to adjust the manganese-containing wastewater to a first pH value, and then using the alkaline precipitant to maintain the first pH value. The manganese-containing wastewater reacts at the first pH value for at least 100 min to obtain a pre-reaction solution. During the reaction process of the present invention, stirring is maintained.

[0026] In the step S1, the time for maintaining the first pH value is equal to the reaction time; further, the reaction time of the manganese-containing wastewater at the first pH value is 100 - 140 min, and further 110 - 130 min.

[0027] 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 further is sodium hydroxide.

[0028] In the present invention, the pH of the first pH value is 9 - 9.45; further, the pH of the first pH value is 9.15 - 9.25 or 9.35 - 9.45 or 9 - 9.4 or 9.2 - 9.4 or 9.15 - 9.45.

[0029] As an explanation of the dosage of the alkaline precipitant in step S1 (the amount used to adjust the manganese-containing wastewater to the first pH value and maintain the first pH value), the molar ratio of the added amount of the alkaline precipitant to the manganese ions is 2.3-3.3:1; further, the molar ratio of the added amount of the alkaline precipitant 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.

[0030] As an explanation of the manganese-containing wastewater, the manganese-containing wastewater is manganese ore drainage 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 the manganese ions is 100-500 mg / L, further 200-300 mg / L; the concentration of the calcium ions is 400-600 mg / L, further 400-500 mg / L; the concentration of the magnesium ions is 300-500 mg / L, further 300-400 mg / L.

[0031] In the present invention, the concentration of the manganese ions is less than the sum of the concentrations of the calcium ions and the magnesium ions; specifically, the mass concentration of the manganese ions is less than the sum of the mass concentrations of the calcium ions and the magnesium ions. The mass concentration of the manganese ions in the manganese-containing wastewater is denoted as the manganese mass concentration, the sum of the mass concentrations of the calcium ions and the magnesium ions in the manganese-containing wastewater is denoted as the calcium-magnesium mass concentration, and the ratio of the manganese mass concentration to the calcium-magnesium mass concentration is 1:2-5, further 1:3-4; the concentration ratio of the calcium ions to the magnesium ions in the manganese-containing wastewater is 1-2:1, and the concentration ratio is the mass concentration ratio.

[0032] S2, using the alkaline precipitant, adjusting the pre-reaction solution to a second pH value, and then using the alkaline precipitant to maintain the second pH value. The pre-reaction solution reacts at the second pH value for 20-40 min. After the reaction ends, solid-liquid separation is performed to obtain a precipitate and a manganese-removed water body (effluent).

[0033] Further, the reaction time of the pre-reaction solution at the second pH value is 25-35 min, further 28-32 min, specifically 30 min.

[0034] In step S2, the time for maintaining the second pH value is equal to the reaction time, or the time for adjusting to the second pH value and maintaining the second pH value is equal to the reaction time; in the reaction process of the present invention, stirring is maintained. In the present invention, for the convenience of analysis, in actual cases, the reaction time of step S2 includes the time for adjusting the pre-reaction solution to the second pH value; since the time for adjusting the pre-reaction solution to the second pH value is short, it will not affect the relevance of the reaction time and the technical effect.

[0035] In the present invention, after reaching the reaction time, the reaction is terminated; for example, after the reaction ends, solid-liquid separation is immediately carried out, 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.

[0036] In the present invention, the precipitate is a manganese-containing precipitate; in the manganese-removed water body, the concentration of manganese ions is less than 2 mg / L, further less than 0.5 mg / L or is 0.4 - 1.25 mg / L.

[0037] In the present invention, the pH of the second pH value is 9.8 - 9.95; further, the pH of the second pH value is 9.85 - 9.95.

[0038] As an illustration of the total addition amount of the basic precipitating agent (the sum of the addition amounts of the basic precipitating agent in step S1 and step S2), in step S1 and step S2, the molar ratio of the total addition amount of the basic precipitating agent to the manganese ions is 2.9 - 3.8:1; further, the molar ratio of the total addition amount of the basic precipitating agent to the manganese ions is 3 - 3.75:1 or 2.9 - 3.1:1 or 3.7 - 3.8:1.

[0039] In the present invention, as an optional method in the industrial application process, before solid-liquid separation, the reaction product can be flocculated, and then the solid is separated to obtain the precipitate and the manganese-removed water body. Flocculation can be carried out using a flocculant.

[0040] The method of the present invention belongs to directly precipitating and treating manganese-containing wastewater at a low alkali amount, and only directly chemically precipitates manganese ions; the direct precipitation in the present invention means that the present invention can directly carry out deep selective precipitation of manganese ions only through chemical precipitation with a basic precipitating agent.

[0041] The present invention does not need to add other agents such as oxidants and sodium fluoride, nor does it need to add agents other than the basic precipitating agent, or does not need to add agents other than the basic precipitating agent and the flocculant; therefore, in the process of converting the manganese-containing wastewater into the manganese-removed water body, the present invention only adds the basic precipitating agent, or only adds the basic precipitating agent and the flocculant; and the present invention only adjusts and maintains the first alkalinity and the second alkalinity.

[0042] The present invention adopts a step-by-step pH adjustment method to directly precipitate manganese ions; first, an appropriate amount of alkali is added to avoid excessive pH, thereby achieving selective precipitation of manganese ions, and then a small amount of alkali is continued to be added to maintain a high pH for a short time to precipitate the residual manganese ions. The present invention greatly reduces the dosage of the reagent, has low cost, and can simultaneously achieve resource utilization and deep purification of manganese. In the present invention, the manganese ion recovery rate is large, manganese-containing wastewater meets the discharge standards (manganese ions <2 mg / L), the recovered manganese has a high purity, and calcium and magnesium doping is reduced, which has a good industrial application prospect.

[0043] The following is a partial principle analysis of the present invention: 1. The first step (maintain at pH 9-9.45 for a longer period of time): (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 in an unsaturated state.

[0044] (2) Kinetic control: Maintaining a stable pH for a long time provides sufficient precipitation time for manganese, allowing it to form stable crystal nuclei and grow into larger particles. During 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.

[0045] 2. Step 2 (pH 9.8-9.95, maintained for a shorter period of time): (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 completely; magnesium is in a metastable state (insufficient supersaturation); and calcium is still in an unsaturated state.

[0046] (2) Key to dynamics: Deep purification of manganese: The residual trace manganese has extremely low solubility and quickly reaches supersaturation after a slight increase in pH. It is preferentially adsorbed or co-precipitated on the surface of existing manganese precipitates (seed effect) without the need for long-term reaction.

[0047] Calcium and magnesium inhibition mechanism: Magnesium: Although the increase in pH makes 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.

[0048] Calcium: pH 9.8-9.95 is still far below the pH required for its large-scale precipitation.

[0049] (3) Advantage of low manganese concentration: The manganese concentration has been greatly reduced after the first step. When the pH 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 conditions for large-scale precipitation.

[0050] (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-time operation avoids a large amount of precipitation by "truncating the time window", preventing magnesium from completing the nucleation-growth process.

[0051] The following are specific examples of the present invention: Example 1 At room temperature, sodium hydroxide was added to manganese ore drainage water (pH = 7.3) to raise the pH to 9.2 ± 0.05 and maintain it. After reacting for 2 h (the molar ratio of the cumulative added sodium hydroxide to manganese ions in the manganese ore drainage water was 2.5:1), a pre-reaction solution was obtained; after sampling and separating the solid matter for detection, the calcium, magnesium, and manganese concentrations were 477.75, 378.50, and 21.5 mg / L, respectively.

[0052] Sodium hydroxide was continuously added to the pre-reaction solution to raise the pH to 9.9 ± 0.05 and maintain it. After reacting for 30 min (at this time, the total reaction time was 150 min, and the molar ratio of the cumulative added sodium hydroxide to manganese ions in the manganese ore drainage water was 3:1), a manganese-containing precipitate and manganese-removed water body (effluent) were separated; in the manganese-removed water body, the calcium, magnesium, and manganese concentrations were 474.5, 376.00, and 1.25 mg / L, respectively, achieving deep purification of manganese ions in the manganese ore drainage water.

[0053] In this example, the main ions and their concentrations in the manganese ore drainage water are shown in Table 1.

[0054] Table 1 Main ions and their concentrations (mg / L) in the ore drainage water of a certain mining area

[0055] During the reaction process of this example, the detection conditions of calcium, magnesium, and manganese are shown in Table 2; when detecting, the solid matter was separated.

[0056] Table 2 Concentration detection conditions during the reaction process

[0057] Example 2 At room temperature, sodium hydroxide was added to manganese ore drainage water (the same as in Example 1) to raise the pH to 9.4 ± 0.05 and maintain it. After reacting for 2 h (the molar ratio of the cumulative added sodium hydroxide to manganese ions in the manganese ore drainage water was 3.25:1), a pre-reaction solution was obtained; after sampling and separating the solid matter for detection, the calcium, magnesium, and manganese concentrations were 453.82, 369.67, and 3.75 mg / L, respectively.

[0058] Continue to add sodium hydroxide to the pre-reaction solution, raise the pH to 9.9 ± 0.05 and maintain it. After reacting for 30 min (at this time, the total reaction time is 150 min, and the molar ratio of the cumulative added sodium hydroxide to manganese ions in the manganese ore effluent is 3.75:1), a manganese-containing precipitate and manganese-removed water body (effluent) are separated; in the manganese-removed water body, the concentrations of calcium, magnesium, and manganese are 425.47, 363.60, and 0.4 mg / L respectively, achieving deep purification of manganese ions in the manganese ore effluent.

[0059] During the reaction process of this example, the detection conditions of calcium, magnesium, and manganese are shown in Table 3; during the detection, the solid matter was separated.

[0060] Table 3 Concentration detection conditions during the reaction process

[0061] Comparative Example 1 At room temperature, add sodium hydroxide to the manganese ore effluent (the same as in Example 1), raise the pH to the preset alkalinity and maintain it. After reacting for 150 min, a precipitate and effluent are separated.

[0062] In this comparative example, the pH values of the preset alkalinity are 9.9 ± 0.05 and 10.4 ± 0.05 respectively; the alkali dosing ratios and effluent manganese concentrations corresponding to different preset alkalinities are shown in Table 4.

[0063] Table 4 Alkali dosing ratios and effluent manganese concentrations corresponding to different preset alkalinities

[0064] As can be seen from Table 4, when the alkali dosing ratio is 8:1, the manganese ion concentration in the effluent can be lower than 2 mg / L.

[0065] After further detection, when the alkali dosing ratio is 6.8:1, the calcium ion concentration in the effluent is 387.75 mg / L, and the magnesium ion concentration is 201.75 mg / L; when the alkali dosing ratio is 8:1, the calcium ion concentration in the effluent is 299.5 mg / L, and the magnesium ion concentration is 75.5 mg / L.

[0066] Comparative Example 2 At room temperature, add sodium hydroxide to the manganese ore effluent (the same as in Example 1), raise the pH to 9.3 ± 0.05 and maintain it. React for 2 h to obtain a pre-reaction solution; after sampling and separating the solid matter for detection, the calcium, magnesium, and manganese concentrations are 468.61, 374.28, and 12.63 mg / L respectively.

[0067] Sodium hydroxide was continuously added to the pre-reaction solution to raise the pH to 9.7 ± 0.05 and maintained. After reacting for 30 min, the precipitate and manganese-removed water body (effluent) were separated; in the manganese-removed water body, the concentrations of calcium, magnesium, and manganese were 439.06, 364.68, and 2.87 mg / L, respectively.

[0068] During the reaction process of this comparative example, the detection conditions of calcium, magnesium, and manganese are shown in Table 5; during the detection, the solid matter was separated.

[0069] Table 5 Concentration detection conditions during the reaction process

[0070] Comparative Example 3 At room temperature, sodium hydroxide was added to the manganese ore water inrush (the same as in Example 1) to raise the pH to 9.2 ± 0.05 and maintained. After reacting for 2 h, a pre-reaction solution was obtained; after sampling and separating the solid matter for detection, the concentrations of calcium, magnesium, and manganese were 477.75, 378.50, and 21.5 mg / L, respectively.

[0071] Sodium hydroxide was continuously added to the pre-reaction solution to raise the pH to 9.6 ± 0.05 and maintained. After reacting for 30 min, the precipitate and manganese-removed water body (effluent) were separated; in the manganese-removed water body, the concentrations of calcium, magnesium, and manganese were 476.12, 377.24, and 4.96 mg / L, respectively.

[0072] During the reaction process of this comparative example, the detection conditions of calcium, magnesium, and manganese are shown in Table 6; during the detection, the solid matter was separated.

[0073] Table 6 Concentration detection conditions during the reaction process

[0074] In the above technical solutions of the present invention, the above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structural transformation made under the technical concept of the present invention by using the content of the specification and drawings of the present invention, or directly / indirectly applied in other related technical fields, is included in the patent protection scope of the present invention.

Claims

1. A method for directly precipitating and treating manganese-containing wastewater with a low alkali content, characterized in that, Including the steps: S1, using an alkaline precipitant, adjusting the manganese-containing wastewater to a first pH value, and then using the alkaline precipitant to maintain the first pH value, reacting for at least 100 min at the first pH value to obtain a pre-reaction solution; The alkaline precipitant is easily soluble in water, and the pH of the first pH value is 9 - 9.45; The manganese-containing wastewater contains manganese ions, calcium ions, and magnesium ions; in the manganese-containing wastewater, the concentration of the manganese ions 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, using the alkaline precipitant, adjusting the pre-reaction solution to a second pH value, and then using the alkaline precipitant to maintain the second pH value, reacting for 20 - 40 min at the second pH value, and performing solid-liquid separation to obtain a precipitate and a manganese-removed water body; the pH of the second pH value is 9.8 - 9.

95.

2. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 1, characterized in that, The alkaline precipitant includes strong alkaline hydroxides; the strong alkaline hydroxides include one or more of sodium hydroxide and potassium hydroxide.

3. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 2, characterized in that, In the step S1, the molar ratio of the addition amount of the alkaline precipitant to the manganese ions is 2.3 - 3.3:

1.

4. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 3, characterized in that, In the step S1, the molar ratio of the addition amount of the alkaline precipitant to the manganese ions is 2.3 - 2.7:1 or 3.2 - 3.3:

1.

5. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 2, characterized in that, In the step S1 and the step S2, the total molar ratio of the addition amount of the alkaline precipitant to the manganese ions is 2.9 - 3.8:

1.

6. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 5, characterized in that, The total molar ratio of the addition amount of the alkaline precipitant to the manganese ions is 2.9 - 3.1:1 or 3.7 - 3.8:

1.

7. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 1, characterized in that, The pH of the first pH value is 9.15 - 9.25 or 9.35 - 9.45; the pH of the second pH value is 9.85 - 9.

95.

8. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 1, characterized in that, The mass concentration ratio of the calcium ions and the magnesium ions in the manganese-containing wastewater is 1 - 2:

1.

9. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to claim 1, characterized in that, The reaction time at the first pH value is 110 - 130 min.

10. The method for directly precipitating and treating manganese-containing wastewater with low alkali content according to any one of claims 1-9, characterized in that, In the process of converting the manganese-containing wastewater into the manganese-removed water body, only the alkaline precipitant is added, or only the alkaline precipitant and a flocculant are added.

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