1902 Encyclopedia > Manganese

Manganese




MANGANESE, a metallic chemical element (symbol Mn; atomic weight 55) widely diffused throughout the mineral kingdom, being an almost constant companion of ferrous oxide, lime, and magnesia in their native carbonates and silicates. Of manganese minerals proper—which are comparatively scarce—the most important is pyrohcsite, the native binoxide, Mn02. This is a black crystalline or crystallized solid with semi-metallic lustre, sufficiently soft to give a (black) streak on paper; hardness, 2 to 2-5; specific gravity, 4'8 to 4-9. It is known in commerce as " black oxide of manganese" or " manganese," and is extensively used for the industrial extraction of chlorine from muriatic acid. Its most extensive beds are found at Ilmenau and Elgersburg, Thuringia; near Giessen, North Hesse; near Mahrisch-Triibau, Moravia; and in Spain. Almost all pyrolusite is contaminated with more or less of the following "manganites"—general formula Mn02.R"0—which besides occur (in the same localities) as independent minerals:—braunite, Mn203 or Mn02MnO; manganite, or grey manganese ore, Mn203.H2O; lumsman-nite, Mn304 or Mn02.2MnO; and psilomelan, a complex mineral, the composition of which generally approximates to 4Mn02.RO + aLI20,—the R being chiefly Ba or K2, but including in general more or less of Ca, Mg, and Mn. These ores are not unlike pyrolusite in their general appearance, but can usually be easily distinguished from it by their greater hardness and other physical pro-perties. Closely allied to psilomelan are those earthy, massive, or reniform mineral mixtures known as " bog-manganese," " cupreous manganese," " earthy cobalt." In the two last-named the RO is chiefly CuO and CoO respec-tively. We must here mention those curious formations known as " manganese nodules " which were so frequently dredged up by the "Challenger" expedition, and with which, it seems, large areas of the ocean's bed are thickly covered. The writer found in one of these, which seemed exceptionally rich in manganese, 20-12 per cent, of bin-oxide of manganese (fully oxidized), 0-4 of oxide of nickel, 0"25 of cobalt, and 0'27 of copper,—a total of 21-04 pet-cent, of the psilomelanic part, not reckoning the CaO, MgO, <fec, belonging to it. All the manganese ores named are available for the manufacture of chlorine, and indeed are so used, as components of what goes in the arts as " man-ganese."

The industrial value of a " manganese " depends, of course, on its actual or virtual percentage of binoxide. A convenient method for its determination was worked out by Fresenius and Will, on the basis of a reaction long before discovered by Turner. When Mn02 is brought in contact with aqueous sulphuric and oxalic acids, it is reduced to MnO (-salt) with formation of carbonic acid; thus : MnO., + C204H2 + H„S04=2H20 + MnOS03 + 2C02. It is easy so to arrange matters that all the C02 leaves the apparatus and nothing else, so that the weight of C02 formed identifies itself with the loss of weight suffered by the co-reagents ; and obviously every one gramme of C02 formed indicates 2CO^ = »^ = 0 '9986 gramme of real Mn02.

A determination of the free water (loss of weight suffered by the powdered ore at 120° C.) must accompany the assay to enable one to compare two analyses made at different times.
For the making of manganese preparations, high class pyrolusite is the most convenient raw material.

Metallic manganese may be prepared by intimately mixing it with lamp-blaek and heating the mixture to whiteness in a blast furnace. But the regulus thus obtained contains a large percentage of combined carbon. A purer metal was obtained by Deville, who started with perfectly pure "red oxide," Mng04, and heated it along with a proportion of sugar-charcoal insufficient for complete reduction in a double crucible made of quicklime. The unreduced oxide (MnO) and part of the lime fuse together into a violet slag, from which the regulus is easily separated. Brunner's manganese (obtained by the reduction of the fluoride with sodium in a clay crucible) is not manganese at all, but a silicide of the metal.

Hugo Tamm, who endeavoured to work out a process for the manu-facture of the metal, gives the following process :—11 parts of good pyrolusite is mixed with 1 part of lamp-black and 6 parte of a flux consisting of 20 parts of lead-free bottle-glass, 7 parts of quick-lime, and 7 of fluor-spar; and the mixture is strongly heated, in a graphite crucible coated over with a mixture of 3 parts of graphite and 1 part of clay, by means of a blast-furnace. There is formed a regulus covered by a green silicious slag containing much protoxide of manganese. The " raw " manganese thus pro-duced is contaminated with about 1 per cent, each of iron, silicon, and carbon, and traces of sulphur, phosphorus, calcium, and aluminium. The green slag in subsequent operations is substituted for part of the white flux. The raw metal, when re-fused with about one-third of its weight of manganous carbonate, yielded a regulus which contained 99'9 per cent, of the metal,—the remaining TV per cent, consisting of carbon, silicon, and iron.

Manganese metal is grey, like cast iron (Deville's had a reddish hue like bismuth); its specific gravity is about 8 ; it is hard and brittle, and about as difficult to fuse as wrought iron. Tt readily tarnishes in ordinary air; even pure water, and much more dilute acid, attack it with evolution of hydrogen and formation of manganous (MnO) hydrate or salt. It is worth stating that neither MnO nor MnCl2 is reducible at a red heat by hydrogen gas; yet Bunsen succeeded in obtaining metallic manganese by the electrolysis of a concentrated solution of the chloride, using a strong current and a negative electrode of very small area.
Oxides.—Pure peroxide can be obtained artificially by keeping the pure nitrate at 200* C. But really pure nitrate is hard to procure. Perhaps the only method for obtain-ing a really pure preparation is Volhard's : 10 grammes of " pure " (iron- and cobalt-free) manganous sulphate is dissolved in half a litre of water and 100 cm. of nitric acid of 1 '2 specific gravity; the solution is heated to boiling, and strong solution of permanganate of potash added until the MnO is nearly but not quite down, and the mixture kept for a while on a water-bath. The precipitate of binoxide formed (according to equation Mn207 + 3MnO = 5Mn02) is washed, first with dilute nitric acid, then with water, and dried (when it retains some combined water).
When binoxide of manganese is heated to redness—in vacuum, air, oxygen, nitrogen—it loses oxygen with formation of lower oxides. This phenomenon was investigated by W. Dittmar, who found that, when the binoxide is heated in a constantly renewed atmosphere, the result, for a given temperature, depends only on the partial tension of the oxygen in that atmosphere. Pure (brown-red) Mn304 remains when this tension is less, while (black) Mn„0:! remains when the tension is greater than a certain limit value;). In Dittmar's experiments (which were all made at a temperature somewhat above the melting point of sterling silver), the value p was found to lie close to 26 cm. of mercury. An exact determina-tion of this critical point w-as not possible, because the temperature was not perfectly constant, and an increase in temperature is equi-valent to a diminution in the partial tension of the oxygen. Hence, supposing the oxide Mn203 to be heated, say in vacuum and within a close apparatus, it will give off oxygen at any temperature greater than a certain minimum t0, and at any temperature t0 + At the gas-evolution will come to a stop as soon as the tension of the gas has come up to the critical value p corresponding to this t0 + At, —p increasing with At.

The protoxide, MnO, is most readily obtained by heating any higher oxide to redness in a current of hydrogen gas, as a dull green powder which gets readily discoloured by oxidation in ordinary air. It is not acted on by water, but readily dissolves in aqueous acids, with formation of manganous salts.

The sulphate, MnS04, is prepared by making pyrolusite into a paste with concentrated sulphuric acid and then heating in a crucible to dull redness until vapours of the acid cease to come off. The ferric and aluminic sulphates (originally present) are now, at least mostly, decomposed and reduced to insoluble basic salts, so that the residue when treated with water and filtered may yield a solution free of these impurities, and, of course, of baryta.
I Should any iron or alumina be left it is easily eliminated by digestion with a little carbonate of manganese (prepared from a small portion of the solution by precipitation with carbonate of soda) and filtration. Cobalt and nickel, if present, can be removed by fractional precipitation with sulphide of sodium (or H2S in the presence of MnC03 ?) ; the black sulphide of Co or Ni comes down first, the (flesh-coloured) MnS afterwards. But lime, magnesia, and alkalies (which are frequently present) are very difficult to get rid of. Compare the section on binoxide. The salt, according to the temperature at which it crystallizes, takes up 7 or 5 or 4 or even 3 or 1 H20. Crystallized sulphate of manganese generally exhibits a rose-red tint ; but this is owing to the presence of a trace of manganic salt (if not to cobalt salt). The pure salt is colourless.





The chloride, MnCl„.—The crude chloride contained in the preparation of chlorine from the binoxide and muri-atic acid is purified by methods analogous to those explained for the sulphate. This (very soluble) salt crystallizes at 15°-20° C, with 4H20. To obtain real MnCl2, the salt must be dehydrated in a current of dry hydrochloric acid gas.

The carbonate, MnCO,, is obtained by precipitating the solution of the sulphate or chloride by excess of carbonate of soda on boiling. It is a white precipitate, soluble in 8000 parts of water, which, when dried in the air, gets slightly oxidized with discoloration.

Far more oxidizable is the hydrate, Mn(OH)2, as ob-tained by precipitation of manganous solution by caustic alkalies. In the presence of an excess of alkali or other strong soluble base (such as lime, for instance) the oxidation progresses very rapidly, with formation, ultimately, of a black manganite MnO„.BO (e.g., Mn02CaO). This is the rationale of the famous " Weldon Process " for the recovery of the " manganese " from chlorine liquors (see BLEACH-ING POWDEK). A mixed solution of chloride of manganese and sal-ammoniac, when mixed with ammonia, gives no pre-cipitate ; but the alkaline liquor readily absorbs oxygen from the atmosphere with formation of a brown precipitate of a higher oxide. If, immediately after addition of the ammonia, the excess of volatile alkali is chased away by boiling, the resulting (neutral or slightly acid) liquor remains clear, even in air. Hereupon is founded a method for the separation of ferric iron and alumina from man-ganese.

Manganic salts, i.e., salts of Mn203, are produced only under very special conditions. Solutions containing the sulphate and a phosphate respectively are obtained by heating finely divided pyro-lusite with strong sulphuric or phosphoric acid. Both products dissolve in water with formation of intensely purple solutions, which, however, are very unstable, showing a great tendency to pass into the manganous condition. Any manganic salt when boiled with hydrochloric acid gives manganous salt with evolution of chlorine. This tendency separates them sharply from the corre-sponding compounds of iron.

Manganates and permanganates (compounds with bases of the hypothetical oxides MnOs and Mn207).-—The most important of these is the manganate of potash, K2Mn04. Four parts of very finely divided binoxide of manganese and 3|f parts of chlorate of potash are evaporated to dry-ness with the solution of 5 parts of caustic potash, and the residue ignited (not fused) in platinum crucibles until all the chlorate is decomposed. The intensely green mass, containing large excess of caustic alkali, dissolves in water into an intensely green solution from which crystals of the salt K2Mn04 canbeobtained ; but when the alkali is neutral-ized by an acid, the liquor turns intensely purple with for-mation of permanganate and a precipitate of alkaliferous binoxide : 2K2Mn04+ 2H00 = 2KMn04+ 2KHO + 2H and 2H + K2Mn04 = H20 + K2Mn03. The purple solution when alkalinized with potash contaminated with organic matter reassumes its original green colour (whence its old name of " chamaeleon minérale"). For the preparation of permanganate of potash it is best to pass chlorine into the green solution, when the whole of the manganese assumes the permanganate form : K2Mn04 + CI = KQ + KMnG4.

From the purple liquid crystals of permanganate are easily obtained by evaporation. The crystals (long prisms) are isomorphous with perchlorate of potash, KC104. They are soluble in 16 parts of cold and far less of hot water. They are almost black, and endowed with a peculiar greenish or bluish metallic lustre. Their powder is red. Their aqueous solution is most intensely purple, one milligramme of the salt giving a perceptible colour to a whole litre and more of water. On addition of acid the solution, through libera-tion of Mn207, becomes pink.

Of all wet-way reagents, manganates and permanganates are the most powerful oxidizing agents, especially when they are employed in conjunction with free alkali, or (the permanganates) along with free mineral acid. By one or the other of the two combinations most oxidizable inorganic and almost all organic substances are promptly oxidized. Hence both manganates and permanganates are extensively employed as disinfectants, and, in chemical laboratories, as oxidizing agents. For the former purpose impure forms of the soda salts are generally used, while pure per-manganate of potash, nowadays, is exclusively employed for scientific or analytical laboratory work.

The ultimate fate of the reagent depends on whether alkali or acid was used as an auxiliary agent. In the former case the salt passes successively into (green) manganate and (insoluble brown) hydrated binoxide of manganese,—three-sevenths of the oxygen in the Mn207 being utilized. In the presence of free acid (sulphuric works best) the Mn207 loses five-sevenths of its oxygen with formation of a colourless solution of manganous (MnO) salt. Hence, supposing such a change to take place promptly, and the reagent to be added gradually, the exact point of completed oxidation is reached when the liquid, by the addition of another drop of the permanganate, assumes a permanent pink colour. This is the principle of a num-ber of processes for the determination of certain reducing agents by means of a standard solution of permanganate.

Analysis.—A manganiferous substance when fused up with carbonate of soda on platinum in the presence of air yields a green mass (manganate), the colour being more distinct after cooling. Manganese oxides, when fused up with a borax bead in the oxidizing flame, impart to it an intense amethyst colour, which disappears in the reducing flame. To detect manganese in a solution of mineral salts, we first eliminate wdiat can be precipitated by sulphuretted hydrogen in the presence of mineral acid. In the filtrate the iron (if present) is oxidized by boiling with a granule of chlorate of potash, and the ferric oxide precipitated along with the alumina by addition of sal-ammoniac and excess of ammonia, and boiling off
the free volatile alkali. From the filtrate the manganese is precipitated by sulphide of ammonium, as a sulphide which, when pure, exhibits a delicate flesh red colour but is readily discoloured, by oxidation, when in contact with air. Cobalt, nickel, and zinc, if present, go down with the manganese, but can be eliminated by treatment of the washed sulphides with acetic acid, which dissolves the manganese only. (W. D.)







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