KR0122466B1 - Process for producing beta manganese dioxide - Google Patents

Abstract

No content

Description

Method of manufacturing beta manganese dioxide

1 is the configuration of the temperature for the position in the rotary tube at three different temperatures,

2 is a configuration of the temperature with respect to the position in the rotary tube at the furnace temperature of 475 ° C,

3b is an X-ray diffraction diagram of manganese dioxide according to the present invention,

3C is an X-ray diffraction diagram of manganese dioxide heat treated according to the prior art.

The present invention relates to an improved process for producing beta manganese dioxide. In particular, the present invention relates to a method for converting gamma manganese dioxide into a beta crystal phase in a very short time and to using the beta manganese dioxide as a negative electrode active material in a non-aqueous cell.

Manufacturers of manganese dioxide generally use an electrolytic aqueous process to produce manganese dioxide with a gamma crystal structure. It has long been known that manganese dioxide must be heated in order to convert the crystal structure mainly into beta phase and remove water before gamma manganese dioxide is used as a lithium-third anode material. US Pat. Nos. 4,133,856 and 4,297,231 describe heat treatment of manganese dioxide at 250 ° C. to 430 ° C. for at least 2 hours to convert to beta crystal phase and remove water. Some manufacturers used long heat treatment times, such as 20 hours (8. Lithium-manganese dioxide batteries H. Ikeda, p. 173, Lithium Batteries, J. P. Gabbano, Academic Press (New York 1983)). It was subsequently carried out at a temperature not exceeding 450 ° C., since it was known that manganese dioxide decomposed into lower oxides. In fact, US Pat. No. 4,133,856 shows that heat treatment at temperatures above 430 ° C. produces Mn ₂ O ₃ , which adversely affects the use of manganese dioxide cathodes. 1 and 2 of the patent show a decrease in use due to heat treatment at temperatures higher than 375 ° C.

The problem with this heat treatment is that the time required at a particular temperature is too long for large cell production. Prior art methods required multiple ovens to produce the required amount of material. Therefore, there is a need to shorten the heat treatment process over several hours of the prior art to a shorter time.

Phases used here, such as beta manganese dioxide and beta converted, do not mean that gamma manganese dioxide is converted to 100% beta, but means that the manganese dioxide is heat-treated so that most of the gamma crystal phase is converted to beta crystal phase. In fact, as mentioned below, the beta manganese dioxide referred to herein is, in fact, preferably less than 100% manganese dioxide to be a gamma-beta mixture.

It is therefore an object of the present invention to provide a method of heat treating manganese dioxide for a short time in order to obtain beta manganese dioxide which acts similarly to manganese dioxide heat treated according to the prior art methods in a battery.

An additional object is to provide a process for producing the manganese dioxide continuously.

The present invention is based on the discovery that since the rate at which gamma manganese dioxide is converted to beta is much faster than the rate at which manganese dioxide is decomposed to lower oxides, temperatures above 450 ° C. can be used without adversely affecting battery performance. In accordance with the invention, gamma manganese dioxide particles are heated for only 1 hour to convert gamma manganese dioxide to beta with minimal degradation to lower oxides.

The total heat treatment time consists of three parts:

(1) Rise time for heating MnO ₂ from ambient temperature to heat treatment temperature, t _r ; (2) time at heat treatment temperature, t _h ; (3) the time for cooling MnO ₂ from the heat treatment temperature to ambient temperature, t _c .

In order to minimize the total heat treatment time, it is important to make t _r and t _c very short. Where t _h is the time to control the overall heat treatment process.

Gamma MnO ₂ can be purchased as a fine powder. To obtain a very short t _r , heat must be transferred very effectively from the heating oven to the individual MnO ₂ powder particles. Furthermore, to obtain uniform beta conversion, the total amount of MnO ₂ must be heated uniformly. In accordance with the present invention, a very fast and uniform heating method involves passing MnO ₂ powder through a multi-zoned rotary tube furnace (known as a rotary kiln or rotary dryer).

The features and advantages of the present invention will be described with reference to the following drawings.

The rotary tube includes a long rotatable steel tube with heating elements arranged adjacent to a portion of the outer surface of the tube for heat application. This tube is generally tilted downward from the end of the powder entry to the end of the powder exit. The powder is fed continuously from one end and falls down a slowly rotating tube. As the tube rotates, the powder moves continuously, and because the particles in contact with the heated tube surface change constantly, heat is transferred to the powder very effectively. As a result, t _r is very short. Once the powder has reached the heat treatment temperature, the powder falls through the rest of the heated zone for t _h . The powder then passes through the cooling zone and exits the tube.

For example, rotary tube furnaces purchased from New York, Lancaster, Harper Electric Furnace Co., Ltd. are suitable for heat treatment mainly to convert gamma manganese dioxide to beta crystal phase. The rotary tube furnace has a stainless steel tube approximately 7,5 inches long and approximately 3.387 m (11.5 ft) long. This tube essentially has a smooth inner surface. The slope of the tube can vary from 0-5 °. The tube can be spun at 0-18 RPM. The passage time of manganese dioxide through the tube is determined by the RPM and the slope used. The furnace has three independent and continuous reversible zones, each 0.6096 m (2 ft) in length.

The first zone begins below 0.6096m (2ft) of the coffin and the last zone ends at 3.3284m (3ft) of the coffin. Thus, the last 0.945 m (3.5 ft) of the tube is the cooling zone.

The temperature profile of manganese dioxide through the tube will be explained with reference to the following experiment. Three separate heat treatments were performed at 400 ° C, 450 ° C and 500 ° C. The following procedure was used for each treatment. All three heating zones were placed at the same temperature. Manganese dioxide was fed through the tube at a rate of about 0.272 kg (0.6 Lb) / minute. The angle of inclination was 1 ° and the tube was rotated at 3 RPM to allow powder to pass through the tube at a rate of about 0.061 m (about 0.2 ft) / minute. Typically, when the furnace temperature starts at ambient temperature, it takes about 2-3 hours to reach steady state conditions. This condition is achieved when thermal equilibrium is reached between the heating element, the tube, and manganese dioxide. Thermocouples are placed every 0.6096 m (2 ft) of the tube, so that they are embedded in the manganese dioxide powder flowing through the tube. Steady state occurs when each thermocouple exhibits a non-wave temperature. 1 shows the steady state temperature profile along the tube length for three heat treatment temperatures. In order to collect enough data for analysis, each heat treatment was performed for about 2 hours after reaching steady state. As shown in FIG. 1, the heat treatment temperature is reached in a third region located between 1.83 m (6 ft) and 2.44 m (8 ft) of the tube. Thus, manganese dioxide is heated for about 10 minutes at the maximum heat treatment temperature of the particular operation. The conversion from gamma to beta crystalline phase begins around 350 ° C. and FIG. 1 shows that the time above this temperature is 10 minutes or more. Table 1 shows the various time intervals for the three heat treatment temperatures.

Table 1 shows that the higher the temperature in the heated zone, the faster the rise time to the beta conversion temperature (350 ° C.) and the longer it will stay for that same fraction of the material, longer than that at lower temperatures. Shows. Therefore, the total heat treatment time can be minimized by keeping the heating zone at a temperature higher than about 450 ° C. If high temperatures such as 500 ° C. are used, care must be taken to avoid heat treatment times long enough to form lower oxides of manganese. Operation at 500 ° C. for 60 minutes did not show any lower oxide formation as measured by X-ray analysis, but there were some lower oxides when operated similarly for 105 minutes. Figure 1 also shows that manganese dioxide does not reach the temperature of the heating zone. Therefore, the heating zone must be fixed at a temperature slightly above the desired heat treatment temperature in order to reach this temperature.

The% beta conversion of the material obtained from each of the three heat treatments described above was analyzed. This was done using X-ray diffraction in the following manner. Each powder sample was taken and the X-ray diffractogram was measured using CuKα irradiation. To remove the fluorescence, a monochrometer is placed between the scintillation detector and the sample. By comparing the counts at 110 reflections to the external standard known as 100% beta and the counts at this reflection to the sample, 100 reflections can be used to measure beta%. The ratio of counts is given in beta% of the sample.

In order for manganese dioxide to have sufficient performance as a negative electrode active material in a lithium battery, beta% should be at least 60% or more but less than 90%. Outside this range, the use of cathodes is worse than the use of materials within this range. Beta% is preferably about 65% -85%. Table 1 shows the% beta conversion at three temperatures. Each value given represents the average of three different samples taken from the same operation. The data show that th 19 minutes is sufficient to convert gamma manganese dioxide to approximately 60% beta when the heating zone is set at 450 ° C. This is an electrode sufficient for use as negative electrode active manganese dioxide in non-aqueous cells. On the other hand, 400 ° C. is a sufficient heat treatment temperature for using longer prior art time intervals, but as indicated by 40% beta conversion, it is not sufficient for the short time of the present invention.

It is an advantage of the present invention to provide a process for the continuous conversion of large amounts of gamma manganese dioxide to the beta phase for a shorter time than the processes according to the prior art without forming harmful lower oxides. Below we discuss a 48-hour long production run that illustrates the preferred mode of operation. The heating zones were each placed at 475 ° C. The angle of the tube was 1 degree and the rotation speed was 3 RPM. The feed rate was about 0.272 kg (0.6 lb) / minute and the passage time through the entire length of the tube was 55 minutes. It took about 2 hours to reach steady state conditions. Manganese dioxide collected during this period was scraped. Thereafter, the samples collected regularly through the operation were continuously heat treated for 46 hours. Thermocouples are located every 0.6096 m (2 ft) below the tube and the temperature profile of manganese dioxide is shown in FIG. The maximum temperature reached was about 470 ° C. and manganese dioxide was heated to 450-470 ° C. for about 10 minutes.

About 456.314 kg (1600 Lb) of beta manganese dioxide was collected at the end of 46 hours. The average beta% of the sample obtained through the manipulation was about 80% beta. X in MnOx represents the oxidation state of manganese. Gamma manganese dioxide was x = 1.95 before heat treatment due to the presence of other manganese species. The x value was determined by chemical analysis of the material prepared according to the present invention and found to be 1.95, indicating that no degradation occurred. In comparison, the batch method of prior art heating at 350 ° C. produced 49.44 kg (190 Lb) of about 75% of beta manganese dioxide after 24 hours heat treatment. As described above, the present invention produces 394.6 kg (864 lb) at the same time. Thus, under the above conditions, the present invention increased beta manganese dioxide production of cathode quality by more than four times. At temperatures above about 450 [deg.] C., t may be reduced, resulting in higher production rates.

3b and 3c show X-ray diffraction diagrams of manganese dioxide heat treated according to the method of the present invention and the prior method, respectively. The diffractogram was obtained by irradiating 2θ angle of 15-40 ° C. using CuKα irradiation. The location of the 110 reflections is due to the lattice spacing of the beta phase. Prior art material was 81% beta and material produced according to the present disease was 82% beta. The diffractograms are substantially identical, which confirms that the present invention provides a material similar to that produced by the prior method. No reflection at about 33 ° indicates that no expected lower oxide is present.

The% beta conversion can be increased by maintaining manganese dioxide for at least 10 minutes at the heat treatment temperature. However, at a given heat treatment temperature, most beta conversions occur during the first 10 minutes. For example, in the 450 ° C. heat treatment described above, the beta conversion is about 60% in the first 10 minutes and only increases to 65% when heated for a total of 90 minutes. Thus, beta% is increased more rapidly at high temperatures than at long temperatures. According to the present invention, manganese dioxide can be heat-treated to about 450 ° C. or higher for up to 1 hour, but heating at temperatures above 450 ° C. for longer than 30 minutes is not desirable.

As described above, in order to minimize the total heat treatment time, it is preferable to minimize t. In the heat treatment, t was generally less than 1 hour. Since lower oxides are not formed at temperatures reached during this period, even if this period is longer than one hour, it is not harmful to the finally formed manganese dioxide. However, in order to maximize the production of manganese dioxide, t is preferably not more than 1 hour.

After gamma manganese dioxide is heat-treated in beta phase, a negative electrode can be prepared by a conventional method. For example, heat treated manganese dioxide is combined with a conductive agent and a binder. This mixture is then shaped into the desired cathode structure. In a button cell, the mixture is compressed into a disc shape, and the disc is compressed into a negative electrode can of the button cell to form a negative electrode. In a spiral wound cell, the mixture is applied between both sides of the conductive metal grit and compressed between rollers to obtain proper adhesion. A long thin cathode is obtained which can be wound spirally with the anode and separator.

Suitable conductors for use are any of the conductors commonly used in the art for producing positive electrodes for nonaqueous batteries, including acetylene black and carbon black. Binders for use in this process are materials that can provide sufficient bonding properties known in the art to produce manganese dioxide anodes for non-aqueous batteries. Suitable binders include polytetrafluoroethylene, copolymers of tetrafluoroethylene and hexafluoroprylene, copolymers of tetrafluoroethylene and ethylene, and fluoro resins including chlorotrifluoroethylene.

After the cathode is formed, it is heat treated to remove the absorbed water. This heat treatment is generally carried out at 150-300 ° C. and can be carried out in a drying oven or in a vacuum. After the heat treatment, the dried negative electrodes are collected in a cell.

A mixture of 91% beta manganese dioxide, 6% carbon, and 3% polytetrafluoroethylene was prepared. The mixture is coated on both sides of the expanded stainless steel grit and the coated grit is compressed between the rollers. The cathodes thus prepared are heat treated at 200 ° C. under vacuum. The dried negative electrodes are spirally wound together with a positive electrode composed of lithium foil and a separator composed of microporous polypropylene. The spirally wound electrode pile is placed in a cylindrical barrel. Dispense through conventional non-aqueous electrolyte and crimp the cover. Each electrode has a tab connected to it. One tab is connected to the canister and the other tab is connected to the cover, which is applied as an electrode end.

Batteries having negative electrode active beta manganese dioxide prepared according to the present invention have similar discharge characteristics as cells having manganese dioxide heat treated according to the methods of the prior art. Therefore, the rapid heating of the present invention does not adversely affect the electrochemical properties of manganese dioxide.

Modifications within the scope of the present invention may be made to the heat treatment process. For example, gamma manganese dioxide can be preheated separately to about 300 ° C. and then introduced into a rotating tube. This shortens the overall residence time of manganese dioxide in the heating zone and increases the production rate. The heat released by manganese dioxide in the cooling zone of the tube can be moved and used for preheating for more efficient use of energy. Effective removal of heat in the cooling zone can also contribute to shortening the overall process time.

Rotary kiln technology is well known, and the equation representing the relationship between the angle of inclination, rotational speed and residence time in the tube is as follows:

T = (KL) / (DRtan (A))

Where T is the transit time, K is a constant, L is the length of the tube, D is the diameter of the tube, R is RPM, and A is the angle of the tube. Therefore, while an angle of 1 ° and a rotation of 3 RPM are used above, other angles and rotational speeds may be used to provide the same or different residence time. Moreover, the feed rate of manganese dioxide is preferably 0.045359 to 0.45359 kg (0.1 to 1.0 lb) / minute for the rotary tube, and can be operated at a speed of about 1.36 kg (3 Lb) / minute. Larger speeds may be used for larger furnaces than those mentioned here.

One rotary tube furnace consists essentially of a tube having a smooth inner surface. The inner surface can be modified to better mix the manganese dioxide powder. For example, elongated fins can be provided 90 ° below the tube, which lifts manganese dioxide higher than would occur with a smooth surface and as a lifter to pour the powder towards the center of the tube. Apply. Another embodiment may have an inner surface with a recessed bone that draws material upon rotation and pours it into the center of the tube.

Gamma manganese dioxide contains up to 5% water when the manufacturer recovers it. Most of this water is removed during the heat treatment and means must be provided to remove it from the furnace. An additional action of the swivel tube tilt provides a chimney effect, allowing the removal of water vapor from the upper end of the tube. To aid in water vapor removal, an inert gas can be passed from the lower end of the tube to the upper end. However, the gas flow rate is so great that manganese dioxide particles should not move together in the tube.

There may be other means besides a rotary tube furnace which can quickly heat manganese dioxide. For example, a fluidized bed through which heated air passes can provide rapid heating. In this process, gamma manganese dioxide powder is transferred to a fluidized bed chamber. The rapid upward flow of hot air disperses the powder into the fluidized bed and heats it to the heat treatment temperature in a short time. The powder is left in the chamber for a time sufficient to beta conversion, and then the converted powder is collected and transferred from the chamber to cool. Such heating methods are suitable for continuous or batch operation.

Other changes can be made in the process without departing from the scope of the present invention. The foregoing specific descriptions of the method of the present invention are intended to illustrate, but not to limit the scope of the claimed invention.

Claims (7)

Gamma manganese dioxide particles are fed to the chamber, and the gamma manganese dioxide is continuously heated in the chamber to a temperature above 450 ° C. to continuously change the relative position of the MnO ₂ particles and to form most of the manganese dioxide without forming harmful amounts of lower oxides. Maintaining the temperature of manganese dioxide above 450 ° C. during the time of conversion to the crystalline phase, and recovering the heat treated manganese dioxide from the chamber. The method of claim 1, wherein the manganese dioxide is heated to a temperature of 450 ° C. or higher within 1 hour, and the average temperature of the manganese dioxide particles is maintained at 450 ° C. or higher within 1 hour. The method of claim 1, wherein the gamma manganese dioxide is heated to a temperature of at least 450 ° C. within 1 hour and the temperature of the manganese dioxide is maintained at at least 465 ° C. within 30 minutes to convert most of the manganese dioxide into the beta phase. The method of claim 2, wherein manganese dioxide is continuously supplied to the chamber. The method of claim 4, wherein gamma manganese dioxide is supplied to the chamber at a rate of 0.045359 to 1.36 kg (0.1-3 Lb) / minute. The method of claim 3, wherein the gamma manganese dioxide is preheated to 300 ° C. and then fed to the chamber. The method of claim 3, wherein 60 to 90% of the heat treated manganese dioxide is in beta crystal phase.

Images