Preparation of Mn.sub.3 O.sub.4

Abstract

A process of preparing Mn.sub.3 O.sub.4, by the reduction of higher manganese oxides, using methane as a reducing gas under controlled temperatures.

Description

FIELD OF THE INVENTION

The invention relates to an improved process for preparing Mn.sub.3 O.sub.4 by the controlled reduction of higher manganese oxides.

BACKGROUND OF THE INVENTION

Mn.sub.3 O.sub.4 has been produced by sintering manganese (II) oxide in the presence of oxygen. Manganese (II) oxide may be produced by heating manganese (II) carbonate, or by heating manganese dioxide.

These prior methods for the preparation of Mn.sub.3 O.sub.4 require high temperatures to sinter the manganese (II) oxide. In addition, it is obviously inefficient to reduce manganese dioxide to manganese (II) oxide, and then re-oxidize manganese (II) oxide to produce Mn.sub.3 O.sub.4.

SUMMARY OF THE INVENTION

The present invention provides an improved process for the preparation of Mn.sub.3 O.sub.4, by the reduction of higher manganese oxides. Unlike prior processes, which were generally conducted at red heat, the present process is characterized by a relatively low temperature in the range from about 250.degree. to about 550.degree. C., depending on the starting material. The starting material is a higher manganese oxide, such as manganite, MnOOH. It has been found that the reduction of a higher manganese oxide may be controlled to recover Mn.sub.3 O.sub.4.

The reduction of a higher manganese oxide, such as manganite, occurs in two steps: a relatively fast step producing Mn.sub.3 O.sub.4, followed by a slower second step producing manganese (II) oxide.

The Mn.sub.3 O.sub.4 produced by the present invention is useful as a source of manganese for the production of ferrites.

It is a feature of the present invention that the two-step reduction of a higher manganese oxide may be stopped after the first step, to recover Mn.sub.3 O.sub.4.

It is an advantage of the present invention that it permits a relatively low temperature reduction of a higher manganese oxide.

It is another advantage of the present invention that it is a dry process, much simpler to carry out than alternate wet processes involving the precipitation of manganese (II) hydroxide.

Still other advantages of the invention will be readily apparent to those of ordinary skill in the art from the detailed description of the invention which follows.

DETAILED DESCRIPTION OF THE INVENTION

The process for the preparation Mn.sub.3 O.sub.4 by the reduction of higher manganese oxides is surprising, since thermodynamic calculations suggest that it is not possible to effect a step-wise reduction of higher manganese oxides, such as MnO.sub.2, Mn.sub.2 O.sub.3, MnOOH, or other intermediate manganese oxides having an oxygen ratio above MnO.sub.1.33 to Mn.sub.3 O.sub.4. Mn.sub.3 O.sub.4 has a manganese to oxygen ratio of MnO.sub.1.33. Surprisingly, it has been found that using methane as a reducing gas permits a practical step-wise reduction of higher manganese oxides to Mn.sub.3 O.sub.4. Other reducing gases such as carbon monoxide or hydrogen do not allow the two-step reduction process to be stopped after the first step, to recover Mn.sub.3 O.sub.4.

The process for the preparation of Mn.sub.3 O.sub.4 by the reduction of Mn.sub.2 O.sub.3 or MnO.sub.2 may be described by the equations:

3Mn.sub.2 O.sub.3 +1/4CH.sub.4 .fwdarw.2Mn.sub.3 O.sub.4 +1/4CO.sub.2 +1/2H.sub.2 O 3MnO.sub.2 +1/2CH.sub.4 .fwdarw.Mn.sub.3 O.sub.4 +1/2CO.sub.2 +H.sub.2 O Similar equations apply to the reduction of MnOOH and other higher manganese oxides in the presence of methane gas to produce Mn.sub.3 O.sub.4. As used herein the term higher manganese oxides refers to manganese oxides having an oxygen ratio above MnO.sub.1.33.

In accordance with the present invention, the higher manganese oxide to be reduced is heated, in a kiln, in the presence of an excess current of methane gas. The temperature to which the higher manganese oxide should be heated ranges from about 250.degree. C. to about 550.degree. C., depending upon the specific higher manganese oxide to be reduced. For example, it has been found that manganese dioxide, MnO.sub.2, should be heated to a temperature from about 250.degree. to about 400.degree. C. In case Mn.sub.2 O.sub.3 or M.sub.n OOH is to be reduced to Mn.sub.3 O.sub.4, it is found that temperatures from about 300.degree. C. to 500.degree. C. should be used. In general, higher temperatures produce larger product particles and/or increased crystallinity. However, it has been found that at temperatures above about 550.degree. C., it is not practical to stop the two-step reduction, to recover Mn.sub.3 O.sub.4. Preferably the reduction of a higher manganese oxide in the presence of an excess of methane gas is conducted at temperatures below about 500.degree. C. to facilitate stopping the two-step reduction after the first step, to recover Mn.sub.3 O.sub.4.

As described above, prior processes required the reoxidation of manganese (II) to produce MN.sub.3 O.sub.4. These prior processes reuired much higher temperatures, up to about 700.degree. C., and therefore resulted in a loss of surface area and activity of the Mn.sub.3 O.sub.4 product because of sintering. The process of the present invention is a significant improvement over the prior process because it permits the reduction of higher oxides of manganese to Mn.sub.3 O.sub.4 at relatively low temperatures, well below sintering temperatures.

EXAMPLE

The reduction of manganite, MnOOH to Mn.sub.3 O.sub.4 was conducted in the presence of a flowing current of excess methane gas, at temperatures ranging from 310.degree. C. to 550.degree. C. The manganese to oxygen ratio of the product produced in each case is set forth below, and illustrated in the accompanying graph.

______________________________________Reduction of MnOOH to Mn.sub.3 O.sub.4FinalTemperature Total Time Product______________________________________450 35 min. MnO.sub.1.38430 15 min. MnO.sub.1.43480 31 min. MnO.sub.1.347510 38 min. MnO.sub.1.334550 39 min. MnO.sub.1.312495 59 min. MnO.sub.1.319310 31 min. MnO.sub.1.42______________________________________

It is understood that various other modifications will be apparent to and can readily be made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Claims

1. A process of perparing Mn.sub.3 O.sub.4 from a higher manganese oxide, comprising the steps of:

immersing a higher manganese oxide in a flow of a reducing gas containing methane,

heating said higher manganese oxide in said flowing reducing gas containing methane to a temperature from about 250.degree. C. to about 550.degree. C., and

recovering Mn.sub.3 O.sub.4.

2. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 1, wherein said reducing gas consists essentially of methane.

3. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 1 or 2, wherein said higher manganese oxide is heated to a temperature from about 250.degree. C. to about 500.degree. C.

4. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 1 or 2, wherein said higher manganese oxide is MnO.sub.2.

5. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 4 wherein said MnO.sub.2 is heated to a temperature from about 250.degree. C. to about 400.degree. C.

6. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 1 or 2, wherein said higher manganese oxide is Mn.sub.2 O.sub.3.

7. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 6, wherein said Mn.sub.2 O.sub.3 is heated to a temperature from about 300.degree. C. to about 500.degree. C.

8. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 1 or 2, wherein said higher manganese oxide is MnOOH.

9. The process of preparing Mn.sub.3 O.sub.4 set forth in claim 8, wherein said MnOOH is heated to a temperature from about 300.degree. C. to about 500.degree. C.