GB2147285A - Production of manganese dioxide - Google Patents

Abstract

A process for the production of manganese dioxide comprises:- (a) intimately contacting at least one manganous salt with at least one compound selected from alkali metal permanganates and ammonium permanganate; (b) heating the intimately contacted manganous salt/permanganate material produced in step (a) to between 120 DEG C and 180 DEG C in an oxygen-containing atmosphere until a substantially complete conversion of the manganous salt into a material comprising manganese dioxide has occurred; and (c) heating the material produced in step (b) in an oxygen-containing atmosphere at between 210 DEG C and 300 DEG C until at least 85% of the manganese present as oxide is present as MnO2 having a surface area of no greater than 15 m<2>/gm and an X-ray diffraction pattern indicating relative intensities, when compared with crystalline beta manganese dioxide of no greater than 30% at d value 3.12 ANGSTROM , no greater than 80% at d value 2.41 ANGSTROM , no greater than 65% at d value at 1.63 ANGSTROM and no greater than 65% at d value 1.31 ANGSTROM . c

Description

SPECIFICATION Production of manganese dioxide This invention relates to an improved process for the production of manganese dioxide from manganous (i.e. Mint7) salts. This process comprises intimately contacting at least one manganous salt with at least one compound selected from the group consisting of alkali metal permanganates and ammonium permanganate. The intimately contacted material is subjected to a two-step heating schedule to produce electrochemically active manganese dioxide.

The use of manganese dioxide as an active cathode material (depolarizer) in nonaqueous cells is well known. Among the readily available manganese compounds which have been employed as a source of manganese dioxide are manganous salts such as manganous chloride, manganous sulfate, manganous carbonate and the like. Typically such manganous salts are thermally decomposed to produce manganese dioxide. For example Ad vanced Inorganic Chemistry, F.A. Cotton and G.

Wilkinson, published by Interscience I John Wiley and Sons (3rd Ed. 1972) states, at page 852, that manganese dioxide is normally made by heating manganous nitrate hexahydrate in air at a temperature of about 530 C. However, such procedure leads to the production of a highly crystalline beta form of manganese dioxide, or pyrolusite. This material has been used in aqueous batteries of the Leclanche type but in general does not produce as satisfactory results as do other forms of manganese dioxide.

European Patent Specification No. 27,076 discloses a process for the pyrolysis of manganous nitrate tetrahydrate to form entirely beta type manganese dioxide. More specifically, this process involved heating the Mn(NO3)2.4H20 at 150 C, washing the product so obtained first with warm distilled water and subsequently with 1% ammonium hydroxide solution, and then drying the material at a temperature of the order of 4000C to 450 C. However, when moisture resitant manganese dioxide produced using the process of said European Patent Specification was employed in lithium/nonaqueous cells, such cells did not yield commercially useful efficiencies at temperatures of 21 C and 35 C. We believe that the reason for the poor performance at these temperatures of such thermally decomposed manganous nitrate is that the beta manganese dioxide produced possesses a highly crystalline form.

Other approaches for the production of electrochemically useful manganese dioxide from manganous salts have also been adopted. Among the more useful of these are processes such as that described in U.S. Patent Specification No.

4,048,027 which involves producing amorphous electrolytic manganese dioxide ("EMD") by the electrolysis of manganous nitrate hexahydrate. In general, EMD, which may be heat-treated to reduce its water content as is described in British Patent Specification No. 1,199,426 and U.S. Patent Specification No. 4,133,856, posesses desirable electrochemical properties for non-aqueous cell usage.

However, even heat-treated EMD picks up water so rapidly upon exposure to ambient humidity that, even in a dry room having a relative humidity of from 1-3%, it is difficult to assemble lithium batteries employing EMD as the cathode material which will maintain capacity. As is well known in the art, moisture present in MnOz will react with lithium and/or the nonaqueous electrolyte in a manner which may result in a cell bulging from its initial height.

United States Patent Application Serial No.

476,639 filed on March 24, 1983, which Application is a continuation-in- part of United States Patent Application Serial No. 451,877 filed December 31, 1981, which Applications are herein incorporated by reference, discloses a novel form of manganese dioxide and a process for the production thereof.

This novel manganese dioxide possesses a low surface area coupled with a high degree of amorphousness. More specifically, such manganese dioxide has a surface area of no greater than about 15 m2/gm and an X-ray diffraction pattern indicating relative intensities, when compared with crystalline beta manganese dioxide (I.C. Sample No. 6), of no greater than about 30% at d value 3.12 A, no greater than about 80% at d value 2.41 A, no greater than about 65% at d value 1.63 A and no greater than about 65% at d value 1.31 . It is believed that these properties are the reason why this form of manganese dioxide possesses a high resistance to water pickup and a high degree of electrochemical activity. Crystalline beta manganese dioxide (I.C. No. 6) refers to l.C.Sample No. 6 described by A. Kozawa and R.A. Powers in Proceedings of the First Manganese Dioxide Symposium, pg. 4, Volume 1, Cleveland, 1975, published by the electrochemical Society, Inc. This form of manganese dioxide will produce essentially the same X-ray diffraction pattern as that reported as ASTM Card No. 24-735.

However, although the manganese dioxide of this application possesses desirable properties, the process taught therein for producing such manganese dioxide is restricted to using manganese nitrate hexahydrate-containing starting material.

Although manganous nitrate hexahydrate may be prepared from commercially available materials by means well known to one skilled in the art (see, for example, "Nitrate hydrates des metaux bivalens", D. Weigel, B. Imelik and M. Prettre, Bull. Soc.

Chem. Fr. 836 (1964)), it would nevertheless be desirable to possess a process which could produce this form of manganese dioxide and/or manganese dioxide having properties similar to this form of manganese dioxide from a wide variety of starting materials. In addition, unless carefully monitored, the process taught in Application No. 476,639 tends to produce manganese dioxide having a very low surface area of about 1.2 square meters per gram or less. This very low surface area appears to adversely affect the pulse performance of such manganese dioxide at room and lower temperature, i.e. 21 C and below.It is believed that by slightly increasing the surface area of the manganese dioxide produced, i.e. to above about 3.0 square meters per gram, pulse performance could be improved to an acceptable level wihout materially adversely affecting the manganese dioxide's resistance to water pickup.

It has now been found possible to provide a process for the production of manganese dioxide having a relatively low surface area coupled with a high degree of amorphousness which process can employ a wide range of starting materials, and also to provide a process for the production of manganese dioxide having a relatively low surface area coupled with a high degree of amorphousness which manganese dioxide exhibits desirable pulse properties in nonaqueous cells at room temperature.

According to the present invention there is provided a process for the production of manganese dioxide which comprises: (a) intimately contacting at least one manganous salt with at least one compound selected from ammonium permanganate and alkali metal permanganates; (b) heating the intimately contacted manganous saltipermanganate produced in step (a) to between about 120 C and about 180 C, preferably to about 150 C, in an oxygen-containing atmosphere until a substantially complete conversion of the manganous salt into a material comprising manganese dioxide has occurred; and (c) heating the material produced in step (b) in an oxygen-containing atmosphere at between about 210 C until at least about 85- of the manganese present as an oxide is present as MnO2 having a surface area of no greater than about 15 m2/gm and an X-ray diffraction pattern indicating relative intensities, when compared with crystalline beta manganese dioxide of no greater than about 30% at d value 3.12 A, no greater than about 80% at d value 2.41 A, no greater than about 65% at d value 1.63 A and no greater than about 65% at d value 1.31 A.

It is believed that the rate at which a particular form of manganese dioxide will pick up water is related to the surface area of such manganese dioxide. However, it is also believed that a certain minimum surface area is desirable in order to achieve desirable pulse performance at room temperature (i.e. about 21 C) and below. Although certain embodiments of this invention may produce manganese dioxide having somewhat higher surface areas (e.g. up to about 50 m2/gm) the preferred embodiments of the process of this invention will produce a manganese dioxide having a surface area of between about 3 m2/gm and about 15 m2/gm. Consequently, it is believed that manganese dioxide produced in accordance with the process of this invention will possess desirable resistance to water pickup coupled with acceptable pulse performance.

In addition, it is theorized that manganese dioxide which possesses a less ordered form will exhibit superior electrochemical properties relative to manganese dioxide which possesses a highly crystalline structure. It is thus believed that because the manganese dioxide produced by the process of this invention possesses a relatively amorphous structure it is admirably suited for use in electrochemical cells.

As is employed herein, the term "manganous salt" refers to a compound containing manganese having a +2 valence state. Illustrative of the man, ganous salts which may be employed in the process of this invention are manganous nitrate, manganous chloride, manganous sufalte, manganous carbonate, as well as mixtures thereof. However, care should be taken when manganous chloride is utilized as chlorine gas will be liberated during the manganese dioxide formation. The manganous salt starting material may be either hydrated or anhydrous. Because of its commercial availability and because of the desirability of the product produced therefrom, the preferred manganous salt starting material is a 50% aqueous solution of manganous nitrate.

Illustrative of the alkali metal permanganates which may be employed are potassium permanganate, lithium permanganate and sodium permanganate. Although ammonium permanganate may be employed the use of this compound is discouraged due to its explosive properties.

The alkali metal permanganate and/or ammonium permanganate may be added to the manganous salt starting material in an amount of between about 1 and about 20 weight percent, preferably between about 5 and about 10 weight percent, based on the ratio of the weight of anhydrous alkali metal permanganate andlor ammonium permanganate to anhydrous manganous salt.

The process of this invention is typically performed as follows. The manganous salt starting material is brought into intimate contact with the alkali metal permanganate by any means well known to one skilled in the art such as grinding, rolling, blending and the like. (Because of the explosive nature of ammonium permanganate this compound is not included in this description of a typical practice of the invention.However, if one desires to employ such compound it would be used exactly as are the alkali metal permanganates.) A preferred method for achieving such intimate contact when manganous nitrate hexahydrate is employed as the starting material is to heat a mixture of the manganous nitrate hexahydrate and the alkali metal permanganate at a temperature sufficiently high to melt the manganous nitrate hexahydrate, i.e. of about 100 C. The mixture is then allowed to cool to form a solidified mass, which is ground to produce an intimately contacted manganous nitratelalkali metal permanganate material. This melting should be accomplished in an oxygen-containing atmosphere.

The heating steps of the process of this invention are typically carried out in a closed reactor so that the humidity in the reactor may be controlled.

The reactor should be equipped with an inlet so that oxygen or an oxygen-containing gas may be pumped in. The reactor should also possess a vent through which the gas liberated by the reaction may exit. An oil-filled trap may be employed in conjunction with such venting means in order to prevent the back diffusion of air or water vapor.

The intimately contacted manganous salt/alkali metal permanganate material is placed in the reactor and a continuous flow of dry or wet oxygencontaining gas is begun. As used herein, the term "dry oxygen-containing gas" refers to a gas which has not been bubbled through water prior to its entry into the reactor. Conversely, the term "wet oxygen-containing gas" refers to a gas which has been bubbled through water prior to its entry into the reactor. The reactor is then preferably slowly heated to between about 120 C and about 180 C, preferably to about 150 C. Such heating step should take preferably from about 15 to about 30 minutes, although longer or shorter time periods may be employed.

Once the desired temperature has been reached, the system is maintained in such temperature range until a substantially complete conversion of the manganous salt into a compound comprising manganese dioxide has occurred. This conversion is typically indicated by the formation of a black mass. Typically, such temperature is maintained for a period of between about 1 and about 4 hours, preferably for about 2 hours. As will be recognized by one skilled in the art, this period will depend upon the temperature selected, reaction batch size, the composition of the starting material, as well as upon other similar factors. The oxygen-containing gas which is fed into the reactor during this constant temperature period may be wet or dry.

After the first heating step discussed above is complete, the reaction product of such step is further heated to between about 210 C and about 300 C, preferably to about 250 C. When empoyed as a single process, this second temperature increase is preferably carried out over a period of between about 15 and about 60 minutes, most preferably over a period of about 20 minutes. This second heating step may take place in the presence of a wet or dry oxygen-containing atmosphere.

Once the desired temperature of this second heating step has been reached, the reaction is maintained at such temperature for a period sufficient to alter the characteristics of the manganese dioxide-containing material produced in the first heating step such that at least about 85% of the manganese present as oxide is present as MnO2.

Typically, this period is of a duration of between about 1 and about 4 hours, preferably for about 2 hours. Care should be taken to avoid maintaining this higher temperature for too long a period or the product will become too crystalline and will thus exhibit less desirable electrochemical properties. The crystallinity of the manganese dioxide during this second heating step may be monitored by periodically subjecting a sample of the manganese dioxide produced to X-ray diffraction analysis. The product may then be allowed to cool to room temperature, such cooling preferably being done in a dry oxygen-containing atmosphere.

The raw manganese dioxide produced should preferably be washed with an acidic solution in order to remove the alkali metal ions. Most preferably, for many of the manganous salts of this invention, the acid used in such wash will correspond to the anion of the manganous salt employed. Thus hydrochloric acid is preferably employed when manganous chloride is used as the starting material, sulfuric acid is preferred when manganous sulfate is employed, and nitric acid is preferred when manganous nitrate is employed. For manganous carbonate, hydrochloric, nitric or sulfuric acid may be utilized. This acid wash is preferably performed at an elevated temperature of between about 60 C and about 70 C.

The acid washed manganese dioxide may then be washed with water and subsequently dried by heating at a temperature of between about 125 C and about iSOC.

The manganese dioxide produced in accordance with the process of this invention exhibits desirable electrochemical properties, including desirable pulse capabilities at room temperature. Moreover, many of the embodiments of this invention will produce manganese dioxide having a surface area of less than 15 m2/gm. Thus this manganese dioxide will be able to be handled in a dry room (having a relative humidity of between about 1% and about 3%) rather than having to be handled in a dry box (having a water content of between about 50 ppm and about 100 ppm) prior to its insertion into nonaqueous cells having lithium anodes.

Examples The following Examples are intended to further illustrate the invention and are not intended to limit the scope of the invention in any manner.

Example 1 A quantity of intimately associated manganous nitrate hexahydrate and potasisum permanganate was prepared as follows. Eighty weight percent (19.24 grams) of Mn(NO3)2.6H20 and twenty weight percent (3 grams) KMnO4 were placed in a 500 ml conical flask equipped with a two hole rubber stopper. The flask was connected to an oxygen tank through one of the holes, while the other provided a gas outlet. (These weight percentages are based on the ratio of anhydrous Mn(NO3)2:KMnO4.) The mixture was heated at 100 C until the manganous nitrate hexahydrate melted. There was little dissolution of the KMnO4 in the molten manganous nitrate hexahydrate, so the solidified mass was cooled and removed from the flask and ground.

This ground mixed material was placed into the same flask assembly described above. The flask was placed in a furnace and the temperature was raised from room temperature to 150 C over a 20 minute period. During this period dry oxygen was fed into the flask directly from the tank at the rate of about 1 cubic ft/hour.

When the temperature reached 150 C, the system was maintained at this temperature for a period of about two hours. The temperature was then raised to about 250 C over a half-hour period. Throughout this entire period (i.e. the 2-hour hold at 1500C and the subsequent raising of the temperature to 250 C) dry oxygen continued to flow into the flask directly from the tank at the rate of about 1 1 cubic ft/hour.

The temperature of the flask was maintained at 250on for two hours and was then allowed to cool to room temperature. Dry oxygen was continually fed into the reactor during this period.

The resultant raw manganese dioxide was washed at 70 C in a 10 percent aqueous nitric acid solution, filtered and washed several times with water. The final product was dried by heating for 16 hours at a temperature of between about 125 C and about 150QC.

Elemental analysis revealed that such product was 98.63 weight percent manganese dioxide based upon the total weight of the product, said manganese dioxide having the formula MnOzoo. In the formula MnOx, "x" is defined as [1 + P peroxidation "peroxidation" being defined 100 as the total manganese content in tetravalent form.

The surface area of such product was determined to be 10.7 m2/gm as measured by the BET method.

This method is described in detail by S. Brunauer, P. Emmet and E. Teller in J. Am. Chem. Soc., Vol.

60, pp. 309-316, (1938).

Example 2 Using the apparatus employed in Example 1, several grams of manganese dioxide were produced utilizing manganous nitrate hexahydrate as the manganous salt and employing a process similar to that employed in Example 1, except that the manganous nitrate to potassium permanganate ratio was 90:10 by weight. The resulting product was determined to be 94.92 weight percent manganese dioxide based on the total weight of the product, said manganese dioxide having the formula MnOi.s#. The surface area of the product was deter- mined to be 6.70 square meters per gram.

Example 3 Using the apparatus employed in Example 1, several grams of manganese dioxide were produced by a process similar to that employed in Example 2 except that lithium permanganate trihydrate was used in place of potassium permanganate. The ratio of manganous nitrate hexahydrate to lithium permanganate trihydrate was 90:10, based upon the ratio of Mn)NO3)2:LiMnO4.

The product was determined to contain 88.85 weight percent manganese dioxide based on the total weight of the product, said manganese dioxide having the formula MnO.ss. The surface area was determined to be 3.15 square meters per gram.

Example 4 Using the apparatus employed in Example 1, several grams of manganese nitrate hexahydrate were treated using the procedure employed in Example 3 except that the ratio of manganous nitrate hexahydrate to lithium permanganate trihydrate was 95:5 (based on the ratio of Mn(NO3)2:LiMnO4).

The product was determined to contain 93.36 weight percent manganese dioxide based on the total weight of the product, said manganese dioxide having the formula MnOr.ss.

Example 5 8.25 gm KMnO4 and 150 ml of 50% Mn(NO3)2 were heated together in air in an evaporating dish until oxidation occurred at about 130 C. The temperature was then raised to 150 C for 2 hours, followed by 250 C for 2 hours in air. MnO# was obtained containing 3.06% potassium, which was then treated with 10% nitric acid solution, washed with distilled water and dried overnight at 110 C.

Analysis showed the product comprised 97.38% manganese dioxide based on the total weight of the product, said manganese dioxide having the formula MnO1 ss. The surface area was 5.1 square meters per gram. X-ray analysis showed a pyrolusite pattern having a degree of amorphousness as defined in United States Patent Application Serial No. 476,639.

Example 6 The material of Example 1 was run in a flooded cell with a nonaqueous electrolyte comprised of about 40 volume percent 1,3-dioxolane, about 30 volume percent 1,2-dimethoxyethane, about 30 volume percent 3-methyloxazolidone plus about 0.2 percent dimethylisoxazole and containing 1 mole per liter of solvent of LiCF3SO3. The contents of the cell comprised 40 milligrams of lithium as the anode; about 1.5 ml of the above-described electrolyte; a nonwoven polypropylene separator which adsorbed some of the electrolyte and 0.35 gm of a cathode mix which consisted of 86.2 wt.% MnO# (produced as described in Example 1), 8.5 wt.% graphite, 2.1 wt.% acetylene black and 3.2 wt.% polytetrafluoroethylene binder.

Tests were run at 210C and 35tC until a 2-volt cutoff was reached. Load resistors of 30,000 ohms (0.1 mA/cm2 cathode current density) were used for continuous discharge and a superimposed load of 250 ohms for 2 seconds once every three days for the pulse discharge. The MONO2 delivered 83.7% efficiency (based on a one electron reduction) to a 2-volt cutoff at 21 C and 94.0% at 35 C. The pulse behaviour of the manganese dioxide produced by the process of this invention was approximately equal to that exhibited by heat-treated Tekkosha EMD at these temperatures.

Claims (16)

1. A process for the production of manganese dioxide which comprises: (a) intimately contacting at least one manganous salt with at least one compound selected from alkali metal permanganate and ammonium permanganate; (b) heating the intimately contacted manganous salt/permanganate material produced in step (a) to between about 120 C and about 180 C in an oxygen-containing atmosphere until a substantially complete conversion of the manganous salt into a material comprising manganese dioxide has occurred; and (c) heating the material produced in step (b) in an oxygen-containing atmosphere at between about 210 C and about 300 C until at least about 85% of the manganese present as oxide is present as MnO# having a surface area of no greater than about 15 m2/gm and an X-ray diffraction pattern indicating relative intensities, when compared with crystalline beta manganese dioxide of no greater than about 30% at d value 3.12 , no greater than about 80% at d value 2.41 A, no greater than 65% at d value 1.63 A and no greater than about 65% at d value 1.31 A.

2. A process as claimed in claim 1 in which the alkali metal permanganate and/or ammonium permanganate is contacted with the manganous salt in an amount comprising between about 1 and about 20 weight percent, based on the weight ratio of anhydrous alkali metal permanganate and/or ammonium permanganate to anhydrous manganous salt.

3. A process as claimed in claim 2 in which the alkali metal permanganate and/or ammonium permanganate is contacted with the manganous salt in an amount comrising between about 5 and about 10 weight percent, based on the weight ratio of anhydrous alkali metal permanganate and/or ammonium permanganate to anhydrous manganous salt.

4. A process as claimed in any of claims 1 to 3 in which the temperature in step (b) is about 150 C.

5. A process as claimed in any of claims 1 to 3 in which the temperature in step (b) is maintained for a period of between about 1 and about 4 hours.

6. A process as claimed in claim 5 in which the temperature in step (b) is maintained for a period of about 2 hours.

7. A process as claimed in any of claims 1 to 6 in which the temperature in step (c) is elevated to 250 C.

8. A process as claimed in any of claims 1 to 7 in which the temperature in step (c) is maintained for a period of between about 1 hour and about 4 hours.

9. A process as claimed in claim 8 in which the temperature in step (c) is maintained for about 2 hours.

10. A process as claimed in any of claims 1 to 9 in which the manganous salt is selected from manganous nitrate, manganous chloride, manganous sulfate, manganous carbonate and mixtures thereof.

11. A process as claimed in claim 10 in which the manganous salt is a 50% aqueous solution of manganous nitrate.

12. A process as claimed in any of claims 1 to 11 in which the alkali metal permanganate is selected from sodium permanganate, lithium permanganate, potassium permanganate and mixtures thereof.

13. A process as claimed in any of claims 1 to 9 in which the manganous salt is manganous nitrate hexahydrate which is intimately contacted with the alkali metal permanganate and/or ammonium permanganate in step (a) by heating in an oxygencontaining atmosphere said manganous nitrate hexahydrate and said permanganate at a temperature sufficient to melt manganese nitrate hexahydrate, allowing the melted manganese material to cool into a mass, and grinding said mass.

14. A process as claimed in any of claims 1 to 13 in which the manganese dioxide produced in step (c) is washed with an acidic solution to remove alkali metal ions.

15. A process as claimed in claim 14 in which the manganese dioxide is washed with an acid corresponding to the anion of the manganous salt, except that hydrochloric, nitric or sulfuric acid is used when the manganous salt is manganous carbonate.

16. A process as claimed in claim 14 or 15 in which the manganese dioxide is washed with the acidic solution at between about 60 C and about 70 C, the acid washed manganese dioxide is then washed with water and subsequently dried at between about 125 C and 150 C.