CN111039448A

CN111039448A - Method for removing manganese impurities in acidic solution by ozone

Info

Publication number: CN111039448A
Application number: CN201911276717.8A
Authority: CN
Inventors: 陈燕玉; 石忠洋; 陈永信; 任望保; 李意能
Original assignee: Foshan Dynanonic Technology Co ltd
Current assignee: Foshan Dynanonic Technology Co ltd
Priority date: 2019-12-12
Filing date: 2019-12-12
Publication date: 2020-04-21
Anticipated expiration: 2039-12-12
Also published as: CN111039448B

Abstract

The invention belongs to the technical field of chemical demanganization, and particularly relates to a method for removing manganese impurities in an acidic solution by using ozone. The method comprises the following steps: and introducing ozone into the acidic solution to be subjected to manganese impurity removal for aeration, heating the aerated solution, and then filtering manganese dioxide precipitate to obtain the acidic solution subjected to manganese impurity removal. The method does not need to adjust the pH value of the solution, and Mn is aerated by introducing ozone under acidic conditions²⁺Direct oxidation to MnO₄ ^‑And then heated to accelerate peroxidation of the MnO₄ ^‑With the remainder of Mn²⁺After the neutralization reaction is carried out to generate manganese dioxide precipitate, the manganese impurities can be removed by filtering. The invention can efficiently remove the impurities under the acidic conditionRemoving Mn²⁺The process mainly relates to the energy consumption of the ozone generator, is far lower than the energy consumption of removing manganese impurities by concentration by a recrystallization method, is simple to operate, does not introduce new impurity metal elements, avoids secondary pollution, and is suitable for large-scale industrial production. The invention also relates to a preparation method of manganese dioxide.

Description

Method for removing manganese impurities in acidic solution by ozone

Technical Field

The invention belongs to the technical field of chemical demanganization, and particularly relates to a method for removing manganese impurities in an acidic solution by using ozone and a preparation method of nano manganese dioxide.

Background

Manganese is one of the more abundant elements in nature and widely exists in environmental water, minerals, soil and industrial water. Manganese also often exists in solutions for chemical production, which has negative effects on the purity, function, appearance and the like of the solutions. In recent years, industrial chemical demanganization techniques mainly include contact oxidation, reagent oxidation, and electrochemical oxidation.

The principle of the contact oxidation process is via an oxide such as MnO₂Manganese sand, river sand, modified shale and the like are filled into a filter column, manganese in water is adsorbed and intercepted by an active filter membrane of manganese on the surface of the filter material, and then oxidation is carried out to separate out manganese ions for precipitation^[1]The effective pH for demanganization in this process is generally biased towards neutrality or alkalinity.

The reagent oxidation method is to oxidize soluble Mn (II) in water into insoluble Mn (IV) by adding oxidizing agent with strong oxidizing property, and then separate the particle precipitate by physical method^[2]. Permanganate, ferrate, hydrogen peroxide and the like are generally used, the manganese removal method needs to be carried out under the condition of higher pH, the higher the pH of the solution is, the higher the manganese removal rate is, and therefore, in order to achieve the manganese removal effect, the pH of the solution needs to be increased to be more than 9^[3]. In addition, the addition of strong oxidant easily causes secondary pollution of the reagent, and the reagent cost is high.

The electrochemical oxidation method is a quick, convenient and obvious treatment effect, basically does not need to add chemical substances, and does not cause secondary pollution^[4-5]But are generally not suitable for strong acid systems.

[1] Research progress on the technology and application of removing iron and manganese from groundwater, Tang dynasty spring, Ye Xin, Chen Huimin, et al [ J ] proceedings of university of east China traffic, 2016,33(1): 136-.

[2]TOBIASON J E,BAZILIO A,GOODWILL J,et al.Manganese removal fromdrinking water sources[J].Current Pollution Reports,2016,2(3):168-177.

[3]GOODWILL J E,JIANG Y,RECKHOW D A,et al.Laboratory assessment offerrate for drinking water treatment(in press)[J].Addiction Professional,2016(1):46-46.

[4]BAAJI,YAVUZ Y,KOPARAL A S.Electrocoagulation of heavy metalscontaining model wastewater using monopolar iron electrodes[J].Separation andPurification Technology,2012,86:248-254.

[5]AKBAL F,CAMC S.Copper,chromium and nickel removal from metalplating wastewater by electrocoagulation[J].Separation and PurificationTechnology,2011,248-253.

Disclosure of Invention

The invention aims to provide a method for removing manganese impurities in an acidic solution by ozone, and aims to solve the technical problems that the existing reagent oxidation method cannot effectively remove manganese from the acidic solution, secondary pollution is easily caused, the production cost of concentration manganese removal by a recrystallization method is high, and the like.

The invention also aims to provide a preparation method of nano manganese dioxide.

In order to achieve the purpose of the invention, the technical scheme adopted by the invention is as follows:

a method for removing manganese impurities in an acidic solution by ozone comprises the following steps:

and introducing ozone into the acidic solution to be subjected to manganese impurity removal for aeration, heating and stirring the aerated solution, and then filtering manganese dioxide precipitate to obtain the acidic solution subjected to manganese impurity removal.

As a preferable technical scheme of the invention, the mass ratio of the ozone to the manganese impurities in the acidic solution to be subjected to manganese impurity removal is more than or equal to 0.87: 1.

As a preferable technical scheme of the invention, the heating temperature is 50-95 ℃.

As a preferable technical scheme of the invention, the heating time is 0.5-2 h.

In a preferred embodiment of the present invention, the heating is performed while stirring.

As a preferable technical scheme of the invention, the pH value of the acidic solution to be subjected to manganese impurity removal is 0-7.

As a preferable technical scheme of the invention, in the acidic solution to be subjected to manganese impurity removal, the main solute is soluble nitrate and/or sulfate.

As a preferable technical scheme of the invention, in the acidic solution to be subjected to manganese impurity removal, the mass of the main solute accounts for less than or equal to 60% of the mass of the acidic solution to be subjected to manganese impurity removal.

As a preferable technical scheme of the invention, in the acidic solution to be subjected to manganese impurity removal, the mass of the manganese impurity accounts for 0-10% of the mass of the acidic solution to be subjected to manganese impurity removal.

As a further preferable technical scheme of the invention, the content of the manganese impurities in the acidic solution to be subjected to manganese impurity removal is 1-1000 ppm.

In order to achieve the above object, the present invention provides a method for preparing manganese dioxide, comprising the steps of:

introducing ozone into the acidic solution to be subjected to manganese impurity removal for aeration, heating and stirring the aerated solution, filtering manganese dioxide precipitate, performing acid washing filtration and water washing filtration on the obtained manganese dioxide precipitate until the pH value of a filtrate is 6-7, and drying and crushing the filtrate to obtain manganese dioxide.

The invention introduces ozone under acidic condition for aeration to remove Mn²⁺Direct oxidation to MnO₄ ^-And then heated to accelerate peroxidation of the MnO₄ ^-With the remainder of Mn²⁺After the neutralization reaction is carried out to generate manganese dioxide precipitate, the manganese impurities can be removed by filtering. By the method, the effect of removing the manganese impurities in the acidic solution can be realized without adjusting the pH of the solution, the removal rate is up to more than 97%, the removal of the manganese impurities is more thorough and far higher than 87% removal rate of a conventional recrystallization method, and the problem of removing the manganese impurities in the acidic solution is solvedDifficult technical difficulty of manganese removal, and is particularly suitable for preparing high-quality acid salt products with low manganese impurities. In addition, the method for removing the manganese impurities in the acidic solution by using the ozone is simple to operate, does not introduce new impurity metal elements, avoids secondary pollution, mainly relates to the energy consumption of an ozone generator in the process, is far lower than the energy consumption in the concentration process by a recrystallization method, and is suitable for large-scale industrial production.

The invention also utilizes the byproduct manganese dioxide precipitate in the method for removing the manganese impurities in the acidic solution by using the ozone as the raw material for preparing the manganese dioxide, the quality of the obtained manganese dioxide is changed according to the difference of main solutes in the acidic solution to be subjected to manganese removal, and the submicron-grade manganese dioxide and the nanoscale manganese dioxide can be prepared specifically, so that the method has the advantages of saving the production cost, recycling chemical products and the like.

Drawings

FIG. 1 is an SEM photograph of manganese dioxide produced as a by-product in example 1.

FIG. 2 is an SEM photograph of manganese dioxide produced as a by-product in example 2.

Detailed Description

In order to make the objects, technical solutions and technical effects of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described, and the embodiments described below are a part of the embodiments of the present invention, but not all of the embodiments. All other embodiments obtained by a person of ordinary skill in the art without any inventive step in connection with the embodiments of the present invention shall fall within the scope of protection of the present invention.

In the description of the present invention, it should be understood that the weight of the related components mentioned in the embodiments of the present invention may not only refer to the specific content of each component, but also represent the proportional relationship of the weight among the components, and therefore, it is within the scope of the disclosure that the content of the related components is scaled up or down according to the embodiments of the present invention. Specifically, the weight in the embodiment of the present invention may be a unit of mass known in the chemical field such as μ g, mg, g, kg, etc.

The embodiment of the invention provides a method for removing manganese impurities in an acidic solution by using ozone, which comprises the following steps:

Ozone and Mn²⁺The oxidation reaction of (a) mainly includes the following two types:

first, ozone reacts Mn²⁺Oxidation to MnO₄ ^-The reaction formula is as follows: mn²⁺+O₃+H₂O→MnO₄ ^-+2H⁺；

Second, ozone converts Mn²⁺Oxidation to MnO₂The reaction formula is as follows: mn²⁺+O₃+H₂O→MnO₂+2H⁺+O₂。

Since ozone has strong oxidizing properties, it is more prone to Mn in acidic or neutral systems²⁺Over-oxidation to MnO₄ ^-Mn is difficult to be incorporated²⁺Oxidation to MnO₂Precipitation and thus effective removal of manganese in the more acidic systems presents certain difficulties. The invention overcomes the technical difficulties, and in order to achieve the purpose of removing manganese in an acid system, ozone is introduced into an acid solution to be subjected to manganese impurity removal for a period of time for aeration under the condition that the pH value of the acid solution is not required to be adjusted, so that Mn is obtained²⁺Is oxidized to MnO₄ ^-MnO accelerated by heating₄ ^-With the remainder of Mn²⁺The neutralization reaction is carried out to generate manganese dioxide precipitate and the manganese dioxide precipitate is filtered to remove the manganese impurities in the acid solution. In addition, the demanganization method of the invention selects ozone as the oxidant, does not introduce new impurity metal elements, and avoids secondary pollution.

The mass of ozone introduced has a direct relationship with the content of manganese impurities, and therefore, in some embodiments, to ensure that the introduced ozone removes as much manganese impurities as possible, the mass ratio between ozone and manganese impurities in the acidic solution from which the manganese impurities are to be removed should be controlled to be greater than or equal to 0.87: 1.

In some embodiments, the temperature of the heating may be controlled at50-95 ℃. MnO capable of accelerating peroxidation by controlling heating temperature in the temperature range in the embodiment of the invention₄ ^-With the remainder of Mn²⁺The neutralization reaction is carried out to generate manganese dioxide precipitate, and the removal efficiency of manganese impurities is improved.

In some embodiments, the heating time may be controlled to be 0.5-2 h. The embodiment of the invention controls the heating time within the time range, thereby accelerating the peroxide MnO₄ ^-With the remainder of Mn²⁺The neutralization reaction is carried out to generate manganese dioxide precipitate, so that the removal efficiency of manganese impurities is improved, the heating time is not excessively prolonged, and the increase of energy consumption is avoided.

In some embodiments, stirring is also performed during heating, and the stirring speed is less than or equal to 2000 rpm. MnO contributing to accelerated peroxidation by stirring in the heating process in the embodiment of the invention₄ ^-With the remainder of Mn²⁺The neutralization reaction is carried out to generate manganese dioxide precipitate, thereby improving the removal efficiency of manganese impurities.

In some embodiments, the acidic solution to be freed of manganese impurities has a pH of 0 to 7. The method can lead the Mn to be led in by ozone under the acidic environment or the neutral environment with the pH value of 0-7²⁺Is oxidized to MnO₄ ^-MnO accelerated by heating₄ ^-With the remainder of Mn²⁺The manganese dioxide precipitate is generated by the centering reaction and is filtered, thereby realizing the removal of manganese impurities and solving the problem of difficult demanganization of acidic solution.

In some embodiments, the acidic solution to be depleted of manganese impurities has a bulk solute including, but not limited to, nitrate, sulfate, or a mixture thereof. Nitrate and sulfate are selected as representative acid solutions because most of the solutions are strong acid and weak base solutions, which meet the acid environment required by the demanganization method of the invention.

Further, in the acidic solution to be removed of the manganese impurities, the main solute is selected from at least one of iron nitrate, aluminum nitrate, magnesium nitrate, chromium nitrate, ferric sulfate, aluminum sulfate, chromium sulfate and zinc sulfate. The main solutes are relatively stable in an acidic environment, do not undergo an oxidation-reduction reaction with ozone, and are beneficial to the efficiency of removing manganese impurities by ozone.

In some embodiments, the mass of the main solute in the acidic solution to be removed of the manganese impurities is less than or equal to 60% of the mass of the acidic solution to be removed of the manganese impurities. This is because the solution viscosity is related to the formation of particles of manganese dioxide precipitate. When the content of the main solute is low, the viscosity of the solution system is low, the generated manganese dioxide precipitate particles are large, and although the manganese dioxide precipitate particles are easy to filter, the energy consumption of the subsequent concentration step is increased; when the content of the main solute exceeds 60%, the solute is easily precipitated during the heating and stirring process (for example, when the content of the iron nitrate exceeds 60%, the iron nitrate is easily precipitated during the heating and stirring process of the present invention), and the yield is further affected.

In some embodiments, the content of the manganese impurities can be controlled to be 0-10% of the mass of the acidic solution from which the manganese impurities are to be removed. This is because the manganese impurities are in this content range, and a better demanganization effect can be achieved by using the method of the invention.

Furthermore, the content of the manganese impurities can be controlled to be 1-1000ppm so as to further improve the manganese removal effect.

In some embodiments, the acidic solution to be freed of Mn impurities contains no Mn available²⁺Substances which undergo competing reactions, e.g. Fe²⁺、Cu⁺、Cl^-Aldehyde groups, and the like. Since such materials can react with Mn²⁺Compete for ozone, cause the consumption of ozone to increase, increase the cost and simultaneously do not benefit Mn²⁺And (4) removing. When the acidic solution is strongly acidic, since the solution can dissolve most of the metal species, it can react with Mn²⁺The substances competing for ozone are very few, and ozone has a strong oxidation rate, so that the competing substances can be simultaneously oxidized, and although the influence on the manganese removal efficiency is not great, a certain amount of ozone is additionally consumed, so that the situation is avoided as much as possible.

The ozone generation may be performed in a manner commonly used in the art. In some embodiments, the ozone is generated by using an ozone generator, and the specification of the ozone generator can be adjusted according to the acid solution of the manganese impurities to be removed, the pH value, the content of the manganese impurities and the like, and can specifically adopt, but is not limited to 100g/h, 5kg/h, 10kg/h and 30 kg/h.

The time of ozone aeration is directly related to the used equipment and the model thereof and the manganese impurity content, and it can be understood that the more the manganese impurity content to be treated is, the longer the time of ozone aeration is correspondingly. In some embodiments, the time of aeration may be controlled to be 5-60 min.

The higher the solution temperature, the shorter the aeration time required. Thus, in some embodiments, the temperature of the acidic solution during aeration may be adjusted to 0-100 ℃. It should be noted that, in the process of producing some acidic solutions, the reaction process generates more heat, so that the temperature of the obtained acidic solution is increased, and in this case, additional temperature adjustment may not be needed.

In some embodiments, the ozone aeration device used may be determined based on the scale of aeration and the efficiency of aeration, including but not limited to aeration discs, venturis, and the like.

The filtering mode of the invention is any filtering mode which can be adopted in the technical field. However, because the ionic strength of the main solute in the acidic solution of the manganese impurities to be removed is different, the particle size of the generated manganese dioxide precipitate is different, and in order to achieve higher filtration efficiency, different filtration methods can be adopted according to actual conditions, including but not limited to ceramic membrane filtration, high-pressure filtration, suction filtration, and normal-pressure filtration.

In some embodiments, the acidic solution from which the manganese impurities are to be removed has a bulk solute ion concentration greater than 9%, which tends to form submicron manganese dioxide precipitates, and so filtration may be carried out using conventional filtration methods, such as filter pressing.

In some embodiments, the acidic solution to be purified of manganese impurities has a bulk solute ion concentration of less than 8% and tends to form nanoscale manganese dioxide precipitates, which are less effective than conventional filtration methods, and thus, for improved filtration efficiency, filtration can be accomplished using special filtration methods such as ceramic membrane cross-flow filtration.

The preparation method of manganese dioxide provided by the invention comprises the following steps:

The method takes the byproduct manganese dioxide precipitate in the method for removing manganese in the manganese-containing acidic solution as a raw material, processes the raw material to remove impurities, can obtain submicron manganese dioxide and nanoscale manganese dioxide with higher purity, and has the advantages of saving production cost, secondary utilization of chemical products and the like.

In some embodiments, the number of acidic filtrations is 2-3. In the embodiment of the present invention, the manganese dioxide precipitate is acid-washed, so that the manganese dioxide precipitate can react with the impurity metal oxide (such as ferric oxide, ferrous oxide, etc.) contained in the manganese dioxide precipitate to form salts, and the salts are dissolved in the acid solution to be removed, which is helpful for improving the purity of the manganese dioxide product.

In some embodiments, the number of water wash filtrations is 2-3. In the embodiment of the invention, the acid-washed filtrate manganese dioxide is washed by water, so that the manganese dioxide is favorably converted from acidity to neutrality.

In some embodiments, the drying temperature is 80-100 ℃ to achieve a faster drying effect.

In some embodiments, the drying time is 10-24 hours to substantially dry the manganese dioxide.

In order to make the above implementation details and operations of the present invention clearly understood by those skilled in the art, and to make the progress of the method for removing manganese from a manganese-containing acidic solution obvious in the embodiments of the present invention, the above technical solutions are illustrated by a plurality of examples below.

Example 1

(1) preparing ferric nitrate Fe (NO) with Fe content of 6.09 wt%₃)₃5L of solution, the temperature of the solution is 30 ℃, the pH value is 0, and the content of manganese impurities is 327.90 ppm;

(2) an aeration disc is used as an aeration device, and an ozone generator with the specification of 100g/h is adopted for aeration for 10 min;

(3) heating the solution after the ozone aeration treatment to 80 ℃, and preserving the heat for 1 h;

(4) filtering, removing filter residue, and collecting filtrate to obtain liquid product.

ICP detection of the finished product shows that the content of the residual manganese impurities is 18.20ppm, and the impurity removal rate is 94.45%.

Acid washing the filter residue, filtering, washing with water, filtering to pH 6-7, oven drying at 80 deg.C for 24 hr, and pulverizing to obtain manganese dioxide (shown in figure 1).

Example 2

(1) preparing Fe (NO) nitrate with Fe content of 10.05 wt%₃)₃5L of solution, the temperature of the solution is 30 ℃, the pH value is 0, and the content of manganese impurities is 322.00 ppm;

ICP detection of the finished product shows that the content of the residual manganese impurities is 8.80ppm, and the impurity removal rate is 97.27%.

And (3) carrying out acid washing and filtering on the filter residue, washing with water and filtering until the pH value is 6-7, drying the filter residue at 100 ℃ for 10 hours, and crushing to obtain manganese dioxide (shown in figure 2).

Example 3

(1) preparing Fe (NO) ferric nitrate with Fe content of 10.10 wt%₃)₃5L of solution, the temperature of the solution is 30 ℃, the pH value is 0, and the content of manganese impurities is 623.30 ppm;

(2) an aeration disc is used as an aeration device, and an ozone generator with the specification of 100g/h is adopted for aeration for 30 min;

(3) heating the solution after the ozone aeration treatment to 50 ℃, and preserving the heat for 2 hours;

ICP detects the finished product, the content of the residual manganese impurities is 12.70ppm, and the impurity removal rate is 97.96%.

Example 4

(1) preparing Fe (NO) nitrate with Fe content of 10.06 wt%₃)₃5L of solution, the temperature of the solution is 30 ℃, the pH value is 0, and the content of manganese impurities is 989.50 ppm;

(2) aerating for 60min by using an ozone generator with the specification of 100g/h and taking an aeration disc as an aeration device;

(3) heating the solution after the ozone aeration treatment to 95 ℃, and preserving the heat for 0.5 h;

ICP detects the finished product, the content of the residual manganese impurities is 10.80ppm, and the impurity removal rate is 98.91%.

Example 5

(1) preparing zinc sulfate ZnSO with the Zn content of 9.97 wt%₄5L of solution, the temperature of the solution is 30 ℃, the pH value is 0, and the content of manganese impurities is 605.40 ppm;

ICP detects the finished product, the content of the residual manganese impurities is 17.13ppm, and the impurity removal rate is 97.17%.

Comparative example 1

The method for removing the manganese impurities by an evaporation concentration crystallization method comprises the following steps:

(1) preparing Fe (NO) ferric nitrate with Fe content of 9.95 wt%₃)₃5L of solution, the temperature of the solution is 30 ℃, the pH value is 0, and the content of manganese impurities is 318.90 ppm;

(2) heating the solution to 50-100 ℃, vacuumizing to provide a negative pressure environment, keeping the relative vacuum degree of-0.01 MPa to-0.1 MPa, and keeping the temperature and pressure for 1-10 hours to evaporate, concentrate and crystallize the solution, wherein stirring is carried out in the process;

(3) ferric nitrate Fe (NO) in nonahydrate₃)₃·9H₂A large amount of O crystals are separated out, the solution is filtered when being concentrated to 1 to 5 percent, and filter cakes are collected to obtain ferric nitrate nonahydrate Fe (NO)₃)₃·9H₂And (4) O, preparing a solid finished product. The ferric nitrate nonahydrate can be prepared into ferric nitrate Fe (NO) with Fe content of 10 wt% by adding water₃)₃And (5) solution finishing.

ICP detects the finished product, the content of the residual manganese impurities is 38.90ppm, and the impurity removal rate is 87.80 percent.

The ICP measurements for the finished products of examples 1-5 and comparative example 1 are shown in table 1.

TABLE 1 ICP measurement of finished products

From the above detection results, it can be seen that the manganese removal effect in the acidic solution can be significantly improved in examples 1 to 5 compared with the method for removing manganese impurities by the evaporative concentration crystallization method in comparative example 1. Under the same conditions, the removal rate of the manganese impurities is more than 97 percent, which is much higher than that of the conventional recrystallization method, and the manganese removal effect is more thorough.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims

1. The method for removing the manganese impurities in the acidic solution by using the ozone is characterized by comprising the following steps of:

and introducing ozone into the acidic solution to be subjected to manganese impurity removal for aeration, heating the aerated solution, and then filtering manganese dioxide precipitate to obtain the acidic solution subjected to manganese impurity removal.

2. The method for removing the manganese impurities in the acidic solution by using the ozone as claimed in claim 1, wherein the mass ratio of the ozone to the manganese impurities in the acidic solution to be removed is not less than 0.87: 1.

3. The method for removing manganese impurities in acidic solution by ozone as claimed in claim 1, wherein the temperature of heating is 50-95 ℃; and/or the presence of a gas in the gas,

the heating time is 0.5-2 h.

4. The method of claim 1, wherein the heating is performed while stirring.

5. The method for removing manganese impurities in acidic solution by using ozone as claimed in any one of claims 1 to 4, wherein the pH value of the acidic solution to be removed is 0-7.

6. The method for removing the manganese impurities in the acidic solution by using the ozone as claimed in any one of claims 1 to 4, wherein the main solute in the acidic solution to be removed of the manganese impurities is soluble nitrate and/or sulfate.

7. The method for removing manganese impurities in acidic solution by using ozone as claimed in any one of claims 1-4, wherein the mass of the main solute is less than or equal to 60% of the mass of the acidic solution to be removed of manganese impurities.

8. The method for removing the manganese impurities in the acidic solution by using the ozone as claimed in any one of claims 1 to 4, wherein the mass of the manganese impurities in the acidic solution to be removed accounts for 0 to 10 percent of the mass of the acidic solution to be removed.

9. The method for removing manganese impurities from an acidic solution with ozone as claimed in claim 8, wherein the content of said manganese impurities is 1-1000 ppm.

10. The preparation method of manganese dioxide is characterized by comprising the following steps: