A method for the reductive preparation of trimanganese tetraoxide

Abstract

This invention belongs to the field of mineral processing technology and provides a method for the reduction preparation of manganese tetroxide. The method involves mixing pyrolusite and ascorbic acid solution, followed by secondary mixing with polyvinyl alcohol solution to obtain a mixed slurry. The mixed slurry is then spray-dried to obtain a precursor. The precursor is then calcined to obtain manganese tetroxide. This invention utilizes the reducing effect of ascorbic acid to convert inert tetravalent manganese in pyrolusite into active divalent manganese ions, which then enter the liquid phase. This allows for efficient separation of manganese from associated insoluble gangue impurities such as silicon and aluminum at the molecular level. Polyvinyl alcohol is introduced as a key structure-directing agent and framework support; the hydroxyl groups on the PVA molecular chain uniformly "anchor" manganese ions to the grid nodes. Through a low-temperature calcination process, a rich mesoporous structure is created in situ, inducing in-situ rearrangement and crystallization of manganese ions at the nanoscale, inhibiting excessive grain growth and preventing the formation of impurity phases at high temperatures.

Description

Technical Field

[0001] This invention relates to the field of mineral processing technology, and in particular to a method for reducing and preparing manganese tetroxide. Background Technology

[0002] In the existing industrial technology system, high-temperature roasting is a classic process for producing manganese tetroxide from pyrolusite. The core idea is to first convert pyrolusite into manganese carbonate intermediates, and then decompose it through high-temperature roasting to obtain manganese tetroxide. Although this process has advantages such as a simple flow, low operating threshold, and relatively low production cost, and can achieve the initial conversion of pyrolusite resources, its limitations are becoming increasingly prominent, and it has become a key bottleneck restricting industrial upgrading.

[0003] Because pyrolusite is a naturally formed complex mineral resource, its mineral structure often contains associated gangue components such as iron, silicon, aluminum, calcium, and magnesium, as well as trace amounts of heavy metal impurities (such as lead, cadmium, and arsenic). Direct calcination is essentially a physicochemical process that only alters the phase composition and lacks separation and purification capabilities. Under high temperatures, the associated impurities in the ore not only cannot be effectively removed but also readily react with manganese oxides to form complex solid solutions or isomorphous structures, causing the impurities to be "locked" within the crystal lattice. This results in the final product often exhibiting a high impurity content, making it difficult to meet "high purity" standards.

[0004] With the rapid development of strategic emerging industries such as new energy batteries, high-end electronic components, and fine catalytic materials, downstream applications have placed extremely stringent demands on the quality of manganese-based raw materials. For example, in lithium-ion battery anode materials, trace amounts of magnetic impurities such as iron and copper can cause micro-short circuits, seriously threatening battery safety. In the production of electronic-grade manganese-zinc ferrite, silicon and aluminum impurities significantly reduce the material's magnetic permeability and sintering density. Products produced by traditional direct calcination methods, due to purity bottlenecks, can only be used as raw materials for low-end metallurgy or crude ceramic colorants, failing to meet the performance requirements of the aforementioned high-end industries for "battery-grade" or "electronic-grade" high-purity materials.

[0005] Therefore, breaking through the limitations of traditional physical calcination in impurity removal and developing a green preparation technology for high-purity manganese tetroxide that can deeply remove impurities and is applicable to low-grade complex mineral sources has become a major technical challenge that urgently needs to be solved in the field of manganese-based new materials. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects in the prior art and provide a method for the reduction preparation of manganese tetroxide.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing manganese tetroxide by reduction, comprising the following steps: (1) Mix pyrolusite and ascorbic acid solution, then add polyvinyl alcohol solution and mix again to obtain a mixed slurry; (2) The mixed slurry was sprayed and then freeze-dried to obtain the precursor; (3) The precursor is calcined to obtain manganese tetroxide.

[0008] Preferably, the particle size of pyrolusite in step (1) is ≤100nm; The concentration of ascorbic acid solution is 0.5~1M. The concentration of the polyvinyl alcohol solution is 5-10%.

[0009] As a preferred option, the mass ratio of pyrolusite, ascorbic acid, and polyvinyl alcohol is 1:2.2~2.5:5~10.

[0010] Preferably, the mixing speed in step (1) is 300~500 rpm and the time is 30~60 min; The secondary mixing speed is 500~800 rpm, and the time is 30~45 min.

[0011] Preferably, the spraying medium in step (2) is liquid nitrogen; The spray distance is 10-20cm, and the soaking time is 10-15min.

[0012] Preferably, the freeze-drying temperature in step (2) is -30~-50℃, the vacuum degree is 10~20Pa, and the time is 24~30h.

[0013] Preferably, the heating rate of the calcination treatment in step (3) is 2~5℃ / min, the target temperature is 300~350℃, and the holding time is 4~6h.

[0014] This invention provides a method for the reduction preparation of manganese tetroxide, comprising the following steps: (1) mixing pyrolusite and ascorbic acid solution, then adding polyvinyl alcohol solution for secondary mixing to obtain a mixed slurry; (2) spraying the mixed slurry and freeze-drying to obtain a precursor; (3) calcining the precursor to obtain manganese tetroxide. This invention introduces a chemical selective leaching mechanism. Through the reduction effect of ascorbic acid, the inert tetravalent manganese in pyrolusite is converted into active divalent manganese ions that enter the liquid phase. This process breaks the constraints of traditional solid-phase reactions, enabling efficient separation of manganese elements from associated insoluble gangue impurities such as silicon and aluminum at the molecular level. This "liquid-phase purification" strategy eliminates inherent impurities in the raw materials from the source, overcoming the defect of impurities being "locked" into the crystal lattice in the direct calcination process, and laying the material foundation for the preparation of high-purity manganese tetroxide.

[0015] Building upon this foundation, polyvinyl alcohol (PVA) was introduced as a key structure-directing agent and framework support, playing an irreplaceable role in "confining and pore-forming." The hydroxyl groups on the PVA molecular chain not only coordinate with manganese ions, preventing their aggregation and sedimentation in solution, but also construct a stable three-dimensional network framework during freezing, uniformly "anchoring" manganese ions to the network nodes. In subsequent freeze-drying, the PVA framework effectively supports the pore structure, preventing pore collapse after ice crystal sublimation, successfully replicating the ion dispersion state in the liquid phase to the solid-phase aerogel, thus avoiding the particle densification problem caused by traditional thermal drying.

[0016] Finally, through a low-temperature calcination process, the PVA framework, acting as a sacrificial template, is oxidized and removed, creating a rich mesoporous structure in situ. This induces in-situ rearrangement and crystallization of manganese ions at the nanoscale, inhibiting excessive grain growth and preventing the formation of impurity phases at high temperatures. This process achieves both the clean removal of organic matter and the preservation of nanoscale grain size and high specific surface area. This process route, through the bridging role of PVA, organically combines "chemical purification" with "physical structure and properties," completely solving the problems of low activity and poor purity in traditional products, and achieving a qualitative leap from low-end metallurgical raw materials to high-value-added functional materials.

[0017] This invention innovates the traditional physical co-firing process into a refined preparation process of "chemical purification - structural regulation - directional crystallization", which successfully produces high-purity manganese tetroxide and has practical significance. Detailed Implementation

[0018] This invention provides a method for preparing manganese tetroxide by reduction, comprising the following steps: (1) Mix pyrolusite and ascorbic acid solution, then add polyvinyl alcohol solution and mix again to obtain a mixed slurry; (2) The mixed slurry was sprayed and then freeze-dried to obtain the precursor; (3) The precursor is calcined to obtain manganese tetroxide.

[0019] In this invention, the particle size of pyrolusite in step (1) is preferably ≤100nm, more preferably ≤80nm, and even more preferably ≤60nm.

[0020] In this invention, the concentration of the ascorbic acid solution is preferably 0.5~1M, more preferably 0.6~0.9M, and even more preferably 0.7~0.8M.

[0021] In this invention, the concentration of the polyvinyl alcohol solution is preferably 5-10%, more preferably 6-9%, and even more preferably 7-8%.

[0022] In this invention, the mass ratio of pyrolusite, ascorbic acid and polyvinyl alcohol is preferably 1:2.2~2.5:5~10, more preferably 1:2.25~2.45:6~9, and even more preferably 1:2.3~2.4:7~8.

[0023] In this invention, the mixing speed in step (1) is preferably 300~500 rpm, more preferably 350~450 rpm, and even more preferably 380~420 rpm; the mixing time is preferably 30~60 min, more preferably 35~55 min, and even more preferably 40~50 min.

[0024] In this invention, the rotation speed of the secondary mixing is preferably 500~800 rpm, more preferably 550~750 rpm, and even more preferably 600~700 rpm; the time is preferably 30~45 min, more preferably 35~40 min, and even more preferably 36~38 min.

[0025] In this invention, the spraying medium in step (2) is liquid nitrogen, and the mixed slurry is sprayed into liquid nitrogen for shaping.

[0026] In this invention, the spraying distance is preferably 10-20cm, more preferably 12-18cm, and even more preferably 14-16cm; the soaking time is preferably 10-15min, more preferably 11-14min, and even more preferably 12-13min.

[0027] In this invention, the freeze-drying temperature in step (2) is preferably -30~-50℃, more preferably -35~-45℃, and even more preferably -38~-42℃; the vacuum degree is preferably 10~20Pa, more preferably 12~18Pa, and even more preferably 14~16Pa; the time is preferably 24~30h, more preferably 26~28h, and even more preferably 26.5~27h.

[0028] In this invention, the heating rate of the calcination treatment in step (3) is preferably 2~5℃ / min, more preferably 2.5~4.5℃ / min, and even more preferably 3~4℃ / min; the target temperature is preferably 300~350℃, more preferably 310~340℃, and even more preferably 320~330℃; the holding time is preferably 4~6h, more preferably 4.5~5.5h, and even more preferably 4.8~5.2h.

[0029] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0030] Example 1

[0031] Pyrolusite was wet-milled to a particle size of 100 nm. A 0.8 M ascorbic acid solution and an 8% polyvinyl alcohol solution were prepared, with the mass ratio of pyrolusite, ascorbic acid and polyvinyl alcohol controlled at 1:2.5:10. The pyrolusite and ascorbic acid solution were mixed and stirred at 400 rpm for 50 min. Then the polyvinyl alcohol solution was added and stirred at 600 rpm for 30 min to obtain a mixed slurry.

[0032] The mixed slurry was spray-set, with the spray distance from the nozzle to the liquid nitrogen surface controlled at 15 cm, and set in liquid nitrogen for 12 min; then dried at -40℃ and 15 Pa for 24 h to obtain the precursor.

[0033] The precursor was calcined at a controlled heating rate of 3℃ / min until it reached 330℃, and then held at that temperature for 5 hours to obtain manganese tetroxide. The manganese recovery rate was 98.1%, and the purity of manganese tetroxide was 99.5%.

[0034] Example 2

[0035] Pyrolusite was wet-milled to a particle size of 80 nm. A 0.6 M ascorbic acid solution and a 5% polyvinyl alcohol solution were prepared, with the mass ratio of pyrolusite, ascorbic acid and polyvinyl alcohol controlled at 1:2.2:6. The pyrolusite and ascorbic acid solution were mixed and stirred at 300 rpm for 30 min. Then the polyvinyl alcohol solution was added and stirred at 600 rpm for 40 min to obtain a mixed slurry.

[0036] The mixed slurry was spray-set, with the spray distance from the nozzle to the liquid nitrogen surface controlled at 10 cm, and set in liquid nitrogen for 15 min; then dried at -30℃ and 10 Pa for 26 h to obtain the precursor.

[0037] The precursor was calcined at a controlled heating rate of 2℃ / min until it reached 300℃, and then held at that temperature for 4 hours to obtain manganese tetroxide. The manganese recovery rate was 98.3%, and the purity of manganese tetroxide was 99.3%.

[0038] Example 3

[0039] Pyrolusite was wet-milled to a particle size of 100 nm. A 1 M ascorbic acid solution and a 9% polyvinyl alcohol solution were prepared, with the mass ratio of pyrolusite, ascorbic acid and polyvinyl alcohol controlled at 1:2.4:7. The pyrolusite and ascorbic acid solution were mixed and stirred at 500 rpm for 30 min. Then the polyvinyl alcohol solution was added and stirred at 500 rpm for 40 min to obtain a mixed slurry.

[0040] The mixed slurry was spray-set, with the spray distance from the nozzle to the liquid nitrogen surface controlled at 20 cm, and set in liquid nitrogen for 15 min; then dried at -45℃ and 18 Pa for 30 h to obtain the precursor.

[0041] The precursor was calcined at a controlled heating rate of 5℃ / min until it reached 340℃, and then held at that temperature for 4 hours to obtain manganese tetroxide. The manganese recovery rate was 98.6%, and the purity of manganese tetroxide was 99.4%.

[0042] Example 4

[0043] Pyrolusite was wet-milled to a particle size of 90 nm. A 0.6 M ascorbic acid solution and a 7% polyvinyl alcohol solution were prepared, with the mass ratio of pyrolusite, ascorbic acid and polyvinyl alcohol controlled at 1:2.2:10. The pyrolusite and ascorbic acid solution were mixed and stirred at 350 rpm for 60 min. Then the polyvinyl alcohol solution was added and stirred at 600 rpm for 45 min to obtain a mixed slurry.

[0044] The mixed slurry was spray-set, with the spray distance from the nozzle to the liquid nitrogen surface controlled at 15 cm, and set in liquid nitrogen for 10 min; then dried at -30℃ and 10 Pa for 28 h to obtain the precursor.

[0045] The precursor was calcined at a controlled heating rate of 3℃ / min until it reached 300℃, and then held at that temperature for 6 hours to obtain manganese tetroxide. The manganese recovery rate was 98.1%, and the purity of manganese tetroxide was 99.3%.

[0046] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing manganese tetroxide by reduction, characterized in that, Includes the following steps: (1) Mix pyrolusite and ascorbic acid solution, then add polyvinyl alcohol solution and mix again to obtain a mixed slurry; (2) The mixed slurry was sprayed and then freeze-dried to obtain the precursor; (3) The precursor is calcined to obtain manganese tetroxide.

2. The method for preparing manganese tetroxide by reduction as described in claim 1, characterized in that, In step (1), the particle size of pyrolusite is ≤100nm; The concentration of ascorbic acid solution is 0.5~1M. The concentration of the polyvinyl alcohol solution is 5-10%.

3. The method for preparing manganese tetroxide by reduction as described in claim 2, characterized in that, The mass ratio of pyrolusite, ascorbic acid, and polyvinyl alcohol is 1:2.2~2.5:5~10.

4. The method for preparing manganese tetroxide by reduction as described in claim 3, characterized in that, The mixing speed in step (1) is 300~500 rpm, and the time is 30~60 min; The secondary mixing speed is 500~800 rpm, and the time is 30~45 min.

5. The method for preparing manganese tetroxide by reduction as described in claim 4, characterized in that, The spraying medium in step (2) is liquid nitrogen; The spray distance is 10-20cm, and the soaking time is 10-15min.

6. The method for preparing manganese tetroxide by reduction as described in claim 5, characterized in that, In step (2), the freeze-drying temperature is -30~-50℃, the vacuum degree is 10~20Pa, and the time is 24~30h.

7. The method for preparing manganese tetroxide by reduction as described in claim 6, characterized in that, In step (3), the heating rate of the calcination treatment is 2~5℃ / min, the target temperature is 300~350℃, and the holding time is 4~6h.