Classification of Manganese Dioxides PHILIP H. DELANO E . J . Lavino and Company, Plymouth Meeting, Pa. Many manganese dioxide ores of different types and particularly those of interest in the dry cell industry were studied by differential thermal analysis, x-ray diffraction, chemical analysis, and other methods. The x-ray data were most valuable but no substitute was found for battery tests. Only three allotropic varieties of pure manganese dioxide are described-namely, pyrolusite, ramsdellite, and cryptomelane. The so-called y-manganese dioxide
and &manganese dioxide described in the literature are merely impure forms of the different dioxides, or com- ' pounds, and may be found in unlimited variation. A continuous series of compounds was shown to exist intermediate between ramsdellite and pyrolusite. The nature of a cryptomelane series was investigated and by a synthetic method the potassium-bearing end member determined to be potassium octapermanganite, &MnsOie.
P
cal analysis. Its application to manganese dioxide ores was explored as a possible method of securing additional information regarding their nature and composition. Preliminary results were reported by the author in October 1947 ( 4 ) . Briefly, the method depends upon the fact that minerals show breaks in their heating curves at points where changes in composition or physical structure take place. By comparing the temperature of a substance heated a t a definite rate with that of a thermally inert substance such as alumina heated under the same conditions, a differential temperature curve is obtained which is characteristic of the material examined. The temperature at which such transformations occur serves to identify the mineral involved while the magnitude of the transition break indicates the quantity present.
RIOR to World War I the principal manganese dioxide used by the dry battery industry of this country was Caucasian ore assaying 80 to 85% manganese dioxide and composed largely of pyrolusite. Although the performance of dry cells made from this ore was satisfactory according to the standards then prevailing, some manufacturers improved the capacity of certain special cells by addition of artificial manganese dioxides. During World War I imports of manganese ore from the CaucaUS were cut off and it became necessary to find supplies from other parts of the world. Among the most outstanding of the new ores was one from Montana which, even though analyzing only 70 to 7501, manganese dioxide, was found to be far superior to Caucasian ore for battery purposes. Montana ore still gives outstanding performance in many applications. In the early 1920's a new ore from the African Gold Coast was found to have superior battery qualities. Since then it has won nomplete acceptance by the battery industry and today is generally regarded as the most important battery ore. The grade is high, usually 84 to 87% manganese dioxide. Some manganese ores are far superior in dry cell manufacture and others excel in certain chemical processes. For many years E. J. Lavino and Company has been engaged in studying the reasons for these differences but classification is extremely difficult because of poor crystallinity and similarity in appearance, chemical analysis, hardness, color, and other properties. In the course of this investigation many ores have been analyzed and tested. The viewpoint of approach has been that knowledge of the properties of the ores which are good or superior for battery manufacture will ultimately lead to identification of new high-quality natural ores and to development of methods for production of good artificial ores. This has proved to be true. Many methods of examination have been suggested and applied, among which are chemical analysis, various activity tests, differential thermal analysis, and x-ray diffraction. Only with the x-ray has it been possible to classify the minerals on a logical basis and to predict behavior of the ores. Even the x-ray does not indicate the reasons for differences in activity of ores of the same type and the ultimate evaluation of the material for a particular use must still be made by the consumer. In the case of battery ores, this means manufacture into dry cells and test of their discharge characteristics over an extended period of time.
Apparatus for differential thermal analysis has been described by numerous workers ( 1 ) . For the tests on manganese dioxide ores reported here, the Bureau of Mines apparatus was modified slightly but the essential features were retained. A photograph of the furnace arrangement is given in Figure 1. Basically, the apparatus consists of a crucible furnace with controls to heat the sample a t a uniform rate and thermocouples t o measure temperatures. The samples are put into holes drilled in a nickel block supported within a small inside-wound crucible furnace so that each will receive equal heat treatment. A nickel cover over the samples and a metal shield above the block prevent irregularity due to direct rsdiation. The temperature rise is controlled easily to the desired rate of about 12" F. per minute by a Variac voltage regulator which may be driven by a motor either at a
Results of several tests on manganese ores are compared in Figure 3 and the various curves show marked differences. The large peak shown by the African ore and the small peak on Belgian Congo ore curves are in line with their known relative activity but contrariwise, the active Ergogene and Montana ores give small peaks while the relatively inactive Caucasian ore gives a very high double peak. Further tests were made, therefore, t o diecover, if possible, the nature of the reactions responsible for these peaks. In these tests specimens were heated to different temperatures in the critical range and examined to determine the occurrence of any change in compbsition.
DIFFERENTIAL THERMAL ANALYSIS
The method of differential thermal analysis is well known, particularly in application to clays and clay minerals which are very fine or amorphous and cannot be distinguished by x-ray or chemi-
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Figure 1. Differential Thermal Conductivity Furnace
The product of decomposition of Caucasian manganese dioxide is first a-manganic oxide which is transformed by continued heating or higher temperature to bixbyite and hausmannite. This was not recognized a t first because of the similarity between the patterns of a-manganic oxide and hausmannite. The double peak in the curve and also differences in color of various reduced oxides are thus explained.
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camera 14.32 cm. in diameter. Some of the results from this work are presented below. The lit'erature on t'he nature of manganese dioxide is critically summarized in the recent article by Cole, Wadsley, and Walkley (a). Their conclusion that y-manganese dioxide is derived from ramsdellite rather than pyrolusite is confirmed by the x-ray patterns given in Figure 4. The origin of the samples is shown in Table 11. 4 whole series of compounds has been found to exist intermediate between the end members ramsdellite and pyrolusite. Those given are t,ypical of many available pat,terns and were chosen to shoiv the gradual change from well-crystallized ramsdellite through poorly crystallized y-manganese dioxide to well-crystallized pyrolusite. The number of intermediate pattern types is unlimited and any distinction in their designation must necessarily be arbitrary. Furthermore, sharp distinction cannot always be drawn with assurance because many ores, the Montana, for example, contain manganese dioxides in such a complex mixture that the relative strengths of t,he different component patterns are difficult to determine. The close relationship between the African Gold Coast mineral and ramsdellite is quit'e clear. The mineral can be considered poorly crystallized and impure ramsdellite, or designated a~ y-manganese dioxide; both terms have been used in the literature. Pattern G-286 is typical of the mineral first described by Dubois ( 5 ) in 1936 and named y-manganese dioxide by Glemser (8) in 1939. The term is generally ext'ended to samples G-598 and G-596 with weak 4.0 lines which are typical of electrolytic products. bIcMurdie apparently derived his pattern for y-manganese dioxide (9, 1 0 ) from an electrolytic product with weak 4.0 line and a carbon irnpurky line at 3.3. y-Manganese dioxide possesses excellent battery properties.
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o----INDICATOR BECiiIAN CONGO
AC
Figure 2.
MONTANA
Wiring Diagram for Thermal Analysis Apparatus
A similar study of pure synthetic pyrolusite showed a continuous transition of pyrolusite to bixbyite without a break according to both x-ray and chemical analysis. The double peak in this case is still unexplained. y-Manganese dioxide starts transforming to pyrolusite at 450" F. and is completely transformed below 750" F. Thermal analysis shows little heat effect and no definite temperature for this reaction in the case of African ore as well as the artificial materials. The x-ray shows no distinction between pyrolusite from African dioxide and that occurring in Caucasian ore but the former produces only a single peak a t about 1200O F. on the curves. Cryptomelane is more stable than either y-manganese dioxide or pyrolusite, the temperature of decomposition being increased by presence of alkalies. The thermal break about 1800" F. accompanies formation of hausmannite and braunite.
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CAUCASIAN A
AFRICAN
050
7
0
200
400
600
800
TWPERATURE
Figure 3.
1000 OF
IMO SAMPLE
1400
1600
BOO
'F
Differential Thermal Analysis of Manganese Dioxide Ores
X-RAY ANALYSIS
Main reliance in the research investigation has been placed on x-ray diffraction and chemical analysis. More than 1500 samples have been x-rayed, many samples have been prepared by different methods, and most of them analyzed chemically. Specimens of the principal types have been analyzed in detail and tested for battery performance. Patterns were made on a General Electric XRD machine with an iron target using unfiltered radiation and a
Present terminology of the manganese dioxides is confused and disputed partly because of their complex nature and partly because of looseness that has developed in their nomenclature. It is desirable, therefore, to redefine terms. Pure anhydrous manganese dioxide has been demonstrated in only two forms-namely, ramsdellite and pyrolusite-which occur naturally and are limited to well-crystallized materials
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giving sharp patterns such as G-532 and G-412, respectively. Pyrolusite was designated as p-manganese dioxide by Dubois (6). Ramsdellite may be considered as Glemser’s mineral in pure, well-crystallized form and, therefore to be truly y-manganese dioxide. There is at least the possibility of a-manganese dioxide as a third form with the crystal structure of cryptomelane. This was claimed by Dubois in 1936 but has not been confirmed as pure 100% manganese dioxide. All other materials which are being discussed currently and considered as manganese dioxides are either impure or manganese compounds. This is especially true of cryptomelane and McMurdie’s &manganese dioxide, which are definitely compounds containing essential alkali. Glemser’s designation of an impure material aa ymanganese dioxide is unfortunate although the name has conveniently served to designate a certain class of manganese oxides having good battery properties. There is no sharp line of demarcation between the impure forms of manganese dioxide related to ramsdellite or the poorly crystallized varieties of pyrolusite. Between the end members a continuous series exists of active, impure, but homogeneous substances with distorted space lattices. To name and distinguish each member of the series would be most difficult. As y-manganese dioxide is now used, it is an omnibus term referring to any poorly crystallized manganese dioxide mineral based on the ramsdellite pattern and usually characterized by the presence of the x-ray diffraction line at d,,, = 4.0. In recognition of this difficulty, practice within this laboratory is to use the name “dellite” for materials giving a pattern typical of poorly crystallized rams(del1ite) such as G-341 and G-818. The sharp lines in the patterns of the pure manganese dioxides indicate well-formed, orderly crystals while the broad lines and blackened backgrounds in the intermediate patterns show that primary crystal size is small and the lattices strongly disturbed. Cryptomelane is frequently encountered in nature and has been prepared in several ways. Although different samples vary widely in composition, the x-ray pattern is essentially constant and G-740 in Figure 5 is typical of well-crystallized material. The chemical formula KMnaOle was tentatively assigned by Fleischer and Richmond ( 7 ) but many natural and artificial samples have shown insignificant amounts of potassium. Furthermore, Dubois prepared a mineral which he designated as a-manganese dioxide by decomposition of permanganic acid which waa necessarily potassium-free but gave the typical cryptomelane x-ray pattern.
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TABLE I. THERMAL DECOMPOSITION OF CAUCASIAN MANGANESE DIOXIDE
Temp., Time, F. Min. 1200 15 30 1200 1200 60 1250 15 1250 30 1250 60 15 1300
MnOz,
Mn,
59.37 55.20 54.68 54.85 53.20 51.81 49.03
61.74 61.91 62.57 62.24 61.19 61.74 63.50
MnO/ Mn 0 392 0.437 0.448 0.443 0 . 451 0,470 0.512
1300 1300 1400 1400 1400 Original ore
45.99 44.51 42.60 43.03 43.81 86.32
62.84 62.26 62.72 63.44 63.77 56.20
0.538 0.548 0.571 0.571 0.566 0.029
30 60 15 30 60
..
%
%
X-Ray Analysis a-MnnOa or-Mn2Oa a-MnzOs a-MnzOs, bixbyite trace a-MnzOs, bixhyite small a-MnzOa, bixbyite large Bixbyite, a-Mn,Oa, hausmannite Bixbyite, hausmannite Bixbyite, hausmannite Bixbyite, hausmannite Bixbyite, hausmannite Bixbyite, hausmannite Pyrolusite, manganite trace, quartz trace
TABLE 11. SELECTED MANGANESE DIOXIDESFROM RAMSDELLITEPYROLUSITE SERIES G-532, ramsdellite, well-crystallized specimen G-341, principal MnOz mineral occurring in African Gold Coast battery ore, pure selected specimen G-818, imperfectly crystallized Mn02, artificial product G-2S6, y-MnOa, artificial G-598, y-MnOz, artificial, typical of electrolytic products G-596, y-MnOz, similar t o G-598, weak 4.0 line G-379, pyrolusite, poorly crystallized artificial material G-412, pyrolusite, well-crystallized material prepared from manganous
nitrate
Figure 5. X-Ray Diffraction Patterns of Cryptomelane-Permanganite Series
A compound of manganese dioxide with prominent lines around = 7.3, 3.6, 2.45, and 1.41 frequently has been obtained by precipitation under oxidizing conditions from strongly alkaline solutions. Feitknecht and Marti ( 6 ) called it manganous manganite. Pattern G-790 in Figure 5 is typical and was prepared according to the method of Copeland, Griffith, and Schertzinger ( 3 ) by air oxidation of manganous hydroxide. McMurdie proposed (9, 10) the name &manganese dioxide for a material giving an x-ray pattern with only two diffuse lines a t 2.4 and 1.4, respectively. A typical pattern is G-517 in Figure 6. To clarify the nature of these compounds, mixtures of manganese dioxide (pyrolusite) in definite molecular proportion with potassium carbonate were heated at 700” C. until the products d,,,
Figure 4. X-Ray Diffraction Patterns of Ramsdellite-Pyrolusite Series
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the so-called manganous manganite or &manganese dioxide are deficient in alkali content compared to the saturated form, I
