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oxide, a compound of these two, MnO, Mn 2 O 3 , is quite indifferent
to the action of acids; so also is the deutoxide or black oxide,
MnO 2 : but the two higher oxides have well marked acid charac-
ters. The formula for manganic acid is MnO 3 , and that for per-
manganic acid Mn 2 O 7 . Neither of these acids, however, can be
obtained except in combination with water or a metallic oxide.

The protoxide, (MnO, Eg. 35*5), may be obtained easily by
igniting carbonate of manganese in a current of hydrogen ; it is of
an olive-green colour, and unless it has been strongly heated,
absorbs oxygen from the air ; if ignited in the air it burns, and is
converted into a brown superior oxide. It may be obtained as a
white hydrate by decomposing a salt of manganese by any alkali,
but it immediately begins to absorb oxygen from the air, and turns
brown. It is soluble in ammonia, especially if the solution contain
an ammoniacal salt. The protoxide unites with acids, forming
well-characterized salts.

Sesquioxide, (Mn 2 O 3 ), Eg. 79, Sp. Gr. 4-82. This oxide is
found in its anhydrous form in acute octohedra, constituting
braunite ; it occurs naturally in a hydrated state in manganite, which
is of a blackish brown colour, and forms brilliant prismatic crys-
tals, (Mn 2 O 3 , aq). The sesquioxide of manganese may be obtained
as a brown hydrate, by passing chlorine through the carbonate of
the protoxide suspended in water, and afterwards removing the
excess of the carbonate by dilute nitric acid. Sulphuric acid dis-
solves it slowly, and forms a deep red solution with it, and hydro-
chloric acid in the cold also forms a soluble compound with it ; if
these solutions be heated they are decomposed, and a salt of the
protoxide is formed. When ignited, the sesquioxide loses one-eighth
of its oxygen, and leaves the red oxide.

Binoxide or peroxide, (MnO 2 ), Eg. 43*5, Sp. Gr. 4*94. This
oxide is the black manganese of commerce and the pyrolusite of
mineralogists. It is found in steel-grey rhombic prisms. Psilomelane



COMMERCIAL ASSAY OF OXIDE OF MANGANESE. 919

is a black stalactitic or amorphous variety frequently mixed with one
of the lower oxides of the metal. Wad is also a hydrated peroxide
of manganese, with a variable amount of water ; it is in a
less compact form than psilomelane, and is of a brown colour.
Small quantities of cobalt and of the carbonates of the earths are
frequent constituents of these ores. Binoxide of manganese is a
good conductor of electricity, and is strongly electro-negative in the
voltaic circuit. When ignited it gives off one-third of its oxygen,
and the red oxide is left; 3 MnO 2 = (MnO, Mn 2 O 3 ) +Q 2 : if heated
with concentrated sulphuric acid, half its oxygen escapes, and a pro-
tosulphate is formed; MnO 2 +HO, SO 3 = MnO, SO 3 -fHO-fO.
With hydrochloric acid chlorine is abundantly evolved, and a
protochloride of manganese is left. Nitric acid has but little effect
upon it. Peroxide of manganese is procured in a hydrated form as
a reddish brown powder, when manganate or permanganate of
potash is decomposed by an acid. When the red oxide is treated with
nitric acid, a hydrate of the peroxide is left, containing 4MnO 2 + aq.

(664) Commercial Assay of Oxide of Manganese. The com-
mercial value of black oxide of manganese depends upon the
proportion of chlorine which a given weight will liberate when it
is heated with hydrochloric acid. This quantity of chlorine varies
much in different samples, and is dependent upon the proportion of
oxygen which the oxide of manganese contains in excess of that
which is necessary to its existence as protoxide. A convenient
method of estimating this excess of oxygen is founded upon the
circumstance that the black oxide of manganese is decomposed in
the presence of oxalic acid and free sulphuric acid ; protosulphate
of manganese is formed, and all the excess of oxygen reacts upon
the oxalic acid and converts it into carbonic acid, which passes off
with effervescence. If the mixture be weighed before the decom-
position is effected, and again after it has been completed, the loss
will indicate the amount of carbonic acid ; and from this the
available amount of oxygen is readily calculated. The reaction
may be traced thus : MiiO 2 + SO 3 + C 2 O 3 yields MnO, SO 3 + 2 CO 2 .
Each equivalent of peroxide of manganese gives 2 equivalents, or
exactly its own weight of carbonic acid.

The apparatus of Will and Fresenius, fig. 291, p. 740, is well
adapted to the performance of this experiment : 100 grains of the
manganese to be tested is reduced to an extremely fine powder,
and mixed with 150 grains of oxalic acid ; the mixture is placed in
the flask, a, and about i J ounce of water is added : the experiment
is then proceeded with exactly as in the method already described for
estimating carbonic acid in a carbonate (481). The decomposition



9.20 MANGANIC ACID.

of the ore is known to be complete as soon as all the black par-
ticles have disappeared.

If the sample of manganese contain a carbonate of any of the
earths, as may readily be ascertained by the effervescence which
will be occasioned on moistening a portion of the oxide with dilute
nitric acid, it will be necessary to remove this carbonate. This is
easily done by washing the weighed portion with nitric acid diluted
with from 16 to 20 parts of water in the flask itself; as soon as
the effervescence has ceased, the acid liquid must be carefully
poured off, and the flask filled up once or twice with distilled
water ; the oxide must be allowed to subside ; in order to retain
any suspended particles, the washings may be thrown upon a small
filter, which is afterwards introduced into the flask, and the expe-
riment is then proceeded with as usual.

The Red Oxide, (MnO, Mn 2 O 3 , Eq. ii4'5), corresponds to the
black oxide of iron ; it is formed by igniting any of the oxides of
manganese in the open air : it occurs native in hausmannite,
either massive or in four-sided pyramidal crystals of a black
colour. The oxide is soluble in phosphoric and in sulphuric acid.

(665) Manganic Acid, (MnO 3 ), Eq. 51*5. When equal weights
of caustic potash and peroxide of manganese are fused together, a
substance is formed which, when dissolved in a small quantity of
water, has a green colour, but which, when largely diluted, becomes
purple, and ultimately claret-coloured. This substance has been
long known, owing to these changes of colour, under the name of
mineral chameleon. The colouring material is manganic acid,
which, when in combination with potash, has a green colour ; it is
an unstable compound, and readily either parts with oxygen, or
absorbs a larger amount of it, in the latter case forming a red
compound ; and hence these changes of colour are produced.

Manganic acid has never been obtained except in combination
with some powerful base. It may be procured by heating finely
powdered peroxide of manganese with its own weight of hydrate of
potash, of soda, or baryta ; if the green mass thus obtained be heated
with a small quantity of cold water, it is partially dissolved, forming
a green solution, from which the manganate of the base may be
obtained in crystals by evaporating it in vacuo over sulphuric acid.
These crystals are isomorphous with the corresponding sulphates
and chrornates. Manganate of potash, (KO, MnO 3 ), is anhydrous,
and readily soluble in water. It has a very intense colouring
power ; this fact enables manganese in very minute quantity to be
detected before the blowpipe ; the material supposed to contain it
is fused upon platinum foil with a little carbonate of potash or



PERMANGANIC ACID. 921

of soda ; if any trace of manganese be present, a green colour is
imparted to the fused mass. The m?.nganates are very unstable;
they are decomposed by boiling their solutions.

In the solid form they are readily decomposed by elevation of
temperature, and oxygen is evolved ; an excess of potash renders the
salt more stable : organic matter also readily abstracts oxygen from
them. Their solutions must not even be filtered through paper.
Sulphurous and phosphorous acids readily reduce the manganates
to a salt of protoxide of manganese ; sulphurous acid and manga-
nate of potash, for example, produce the following result :

KO, MnO 3 + 2 SO 2 =MnO, SO 3 + KO, SO 3 .

(666) Permanganic Acid, (Mn 2 O 7 ), Eg. in. If a solution of
manganate of potash be largely diluted with water, the colour
changes from green to violet ; the manganic acid passes to a higher
state of oxidation, and permanganate of potash is formed.

This salt is best prepared by mixing intimately 4 parts of finely
powdered peroxide of manganese with 3^ parts of chlorate of
potash ; 5 parts of hydrate of potash are dissolved in a small
quantity of water, and added to the mixture, which is dried and
reduced to powder, and then heated to dull redness for an hour in
an earthen crucible. When cold, the mass is treated with a large
quantity of water ; the solution on evaporation yields beautiful red
acicular crystals of permanganate of potash, (KO, Mn 2 O 7 ) . These
crystals are isomorphous with those of perchlorate of potash ; they
require about 1 6 parts of cold water for solution. Permanganate
of potash is in certain cases a useful oxidizing agent : it may be
employed to detect the occurrence of sulphurous acid in solution
in sulphuric or hydrochloric acid ; sulphurous acid quickly deoxi-
dizes it and destroys its colour if present.

The permanganates are much more stable than the manganates;
their solutions may be boiled without undergoing decomposition.
Organic matter, however, combines with part of the oxygen con-
tained in the acid, and reduces it first to manganic acid and then
to the binoxide of the metal, which is precipitated as a hydrate in
flocculi: their solutions, therefore, must not be filtered through paper,
but through a funnel loosely plugged with asbestos. When ignited,
oxygen is given out, and a manganate is reproduced, which, if the
heat be too great, is in turn decomposed with a further extrication
of oxygen. Most of the permanganates are freely soluble in water ;
the permanganate of silver is the least soluble of these salts. If
concentrated solutions of permanganate of potash and of nitrate of
silver be mixed together, a red crystalline permanganate of silver



922 SULPHIDE, AND CHLORIDES OF MANGANESE.

separates. It may be employed for tlie preparation of the other per-
manganates ; if it be levigated with water, and mixed wdth a solution
of the chloride of the metal, of which the permanganate is required,
double decomposition occurs, and chloride of silver is formed, whilst
the desired permanganate is obtained in solution. In this way the
permanganate of baryta may be procured, and from it the per-
manganic acid may be obtained in solution, by the cautious ^addition
of dilute sulphuric acid, as long as any precipitate is produced . On
evaporation it may be obtained as a brown partially crystalline
mass, which is very soluble in water ; its solution is decomposed by
mere elevation of the temperature ; a little beyond 100 F. hydrated
peroxide of manganese is deposited, and oxygen gas escapes.

(667) Protosulphide of Manganese is obtained as a yellowish
red hydrate, by precipitating a salt of the protoxide by hydrosul-
phate of ammonia. It speedily becomes oxidized by exposure to the
air. A native sulphide of manganese is occasionally met with of a
brownish black or steel-grey colour and feeble metallic lustre. The
other sulphides of manganese have not been accurately examined.

(668) Chlorides of Manganese. Three chlorides of this metal
may be obtained : a protochloride, a sesquichloride, and a per-
chloride (Mn 2 CI 7 ) : but there is some doubt as to the composition
of the latter compound.

Protochloride of Manganese, (MnCl -}- 4 aq) ; Eg. 634-36.
This substance is obtained abundantly as a waste product in the
preparation of chlorine by acting on the black oxide of the metal :
the chlorine escapes, and the chloride of manganese is dissolved. If
this solution be evaporated to dryness, re- dissolved in water, and
subjected to a current of sulphuretted hydrogen to reduce the
iron to the state of protoxide, the manganese may be obtained
free from iron, nickel, and cobalt by suspending freshly precipi-
tated sulphide of manganese in water, and adding it to the liquid
as long as the fresh portions of sulphide become blackened ; the
manganese displaces the other metals from their solution, and
they are precipitated as black hydrated sulphides : for example,
FeCl4-HO,MnS=MnCl + HO, FeS.

On evaporation the chloride of manganese crystallizes in a
tabular form with 4 equivalents of water. It is of a delicate pink
colour and slightly deliquescent ; by heat an anhydrous chloride
may be obtained, which is soluble in alcohol, and crystallizes with
two equivalents of alcohol.

A sesquichloride may be obtained in solution by acting on the
sesquioxide of manganese with cold hydrochloric acid : it is of a
dark brown colour : it must be concentrated by evaporation in vacuo.



OTHER SALTS OF MANGANESE. 923

The Perchloride Mn 2 Cl 7 ?, (Dumas,) is obtained as a greenish-
yellow gas, which condenses at o F. to a greenish-brown liquid,
by dissolving permanganate of potash in oil of vitriol and adding
fused chloride of sodium in small portions at a time. The fumes
in a moist air assume a purple colour from the formation of per-
manganic acid : water decomposes it instantly, forming a red
solution of permanganic and hydrochloric acids. It is probable
that this compound is an oxychloride of the metal, somewhat
analogous to chlorochromic acid (657).

Fluorides of manganese, corresponding to each of these chlo-
rides, have been formed.

(669) Sulphate of Manganese (MnO, SO 3 , 5 aq, Eg. 75*5 + 45),
is obtained by dissolving the binoxide in sulphuric acid ; it crys-
tallizes below 42 with 7 equivalents of water ; between 45 and
68 with 5 equivalents ; between 68 and 86 with 4 equivalents of
water (Brandes). It forms a double salt with sulphate of potash
(MnO, SO 3 4-KO, SO 3 + 6 aq), which is isomorphous with the
corresponding double sulphate of magnesia. The sesquisulphate
of manganese forms a double salt with sulphate of potash, which
crystallizes in octohedra (KO, SO 3 + Mn 2 O 3 , 3 SO 3 -f 24 aq), and
corresponds in form and composition with common alum.

Carbonate of Manganese (MnO, CO 2 ) ; Eq. 57*5. The anhy-
drous carbonate forms the native manganese spar, and frequently
accompanies spathic iron : the artificial carbonate may be ob-
tained as a white hydrate 2 (MnO, CO 2 ) + HO, on precipitating the
chloride by an alkaline carbonate : it becomes brownish by drying.

(670) CHARACTERS OF THE SALTS OF MANGANESE. The salts of
this metal are of a delicate rose colour ; they have an astringent
taste. With potash and soda they yield a white precipitate of
hydrated protoxide, which rapidly absorbs oxygen, and becomes
brown by exposure to the air. Ammonia gives a similar precipi-
tate, which is soluble in excess of the ammoniacal liquid, especially
when it contains chloride of ammonium ; the solution quickly
absorbs oxygen, and deposits a brown .hydrated peroxide of man-
ganese. The alkaline carbonates give a white precipitate of car-
bonate of manganese, soluble in chloride of ammonium. With
hydrosulphate of ammonia a characteristic flesh-coloured hydrated
sulphide of manganese is formed, which becomes brown by expo-
sure to the air. Sulphuretted hydrogen gives no precipitate in
the solutions of manganese. Ftrrocyanide of potassium gives a
white precipitate ; in neutral solutions ferridcyanide of potassium
produces a brown precipitate. Mr. Cruni has pointed out an



924 SEPARATION OF MANGANESE FROM THE FOREGOING METALS.

extremely delicate test of the presence of manganese in the re-
action which occurs if a compound of manganese be mixed with
dilute nitric acid, and a little peroxide of lead be added : on
boiling the mixture, the red colour of permanganic acid is pro-
duced by a trace of manganese which is too small to be otherwise
recognised. Before the blowpipe, when fused on platinum wire or
foil, with a little carbonate of soda, the compounds of manganese
give a very characteristic bluish-green opaque bead : a bead of
borax becomes violet in the oxidizing flame, if manganese be
present ; the colour disappears in the reducing flame.

(671) Estimation of Manganese, and Separation from the
Alkalies. Manganese is generally estimated in analysis in the
form of the red oxide of manganese, which contains 72*7 per cent,
of the metal. For this purpose it is precipitated from a boiling
solution of its salts by carbonate of potash or soda, and the pre-
cipitated carbonate is well washed and then heated to redness, by
which carbonic acid is expelled, and the red oxide is produced by
absorption of oxygen from the air.

Separation of Manganese from the Alkaline Earths. The solu-
tion must be rendered nearly neutral, and hydrosulphate of ammonia
must be added, which precipitates the manganese as sulphide : the
sulphide must then be re-dissolved in hydrochloric acid, precipitated
by carbonate of potash, and the manganese is estimated, after igni-
tion, as red oxide. It is however apt thus to retain some portions
of the earths. The oxide must therefore be again re-dissolved in
hydrochloric acid ; chloride of ammonium must be added, and then
a mixture of ammonia and carbonate of ammonia in excess ; the
manganese will be held in solution ; but if lime, baryta, or strontia
be present, it will remain undissolved in the form of carbonate, which
must be collected on a filter, weighed, and deducted from the result.

Separation from Zinc and Cadmium. The solution is mixed with
acetate of potash and free acetic acid, then sulphuretted hydrogen is
transmitted ; the zinc and cadmium precipitate as sulphides, and
the manganese remains in solution. If zinc be absent, the addi-
tion of acetic acid and the acetate is unnecessary.

The separation of manganese from cobalt and nickel is attended
with considerable difficulty. Ebelmen recommends that the
mixed oxides, after they have been reduced by ignition, be heated
to dull redness in a current of sulphuretted hydrogen, the current of
gas being continued as long as any water is formed, and until the
mixture has been allowed to cool completely ; the metals are thus
converted into sulphides. Dilute hydrochloric acid in the cold



SOURCES OF TIN. 925

dissolves the sulphide of manganese, but leaves the sulphides of
cobalt and nickel unacted upon.

Separation from Iron, Chromium, Uranium, Alumina, and
Glucina. This is readily effected by converting the iron into a
persalt, and after largely diluting the solution with water, digesting
it upon finely levigated carbonate of baryta. Manganese alone
remains in the liquid, the other oxides being displaced by baryta.
The excess of baryta is removed by sulphuric acid, and the man-
ganese precipitated by carbonate of soda.



CHAPTER XV.

GROUP V. CERTAIN METALS WHICH FORM ACIDS WITH OXYGEN.

I. TIN.

Symbol, Sn; Equivalent, 58-82 ; Specific Gravity, 7*292.

(672) THIS beautiful metal is one of those which have
been longest known to man, as it is mentioned in the Books of
Moses. Tin, however, is met with in but few localities. Its only
ore of importance is the deutoxide, or tin-stone, which occurs crys-
tallized in prisms, isomorphous with those of titanic acid. It is
usually found in veins, running through primitive rocks of por-
phyry, granite, or clay-slate, and is generally mingled with the sul-
phides and arsenides of copper and iron, and frequently also with
wolfram. The most celebrated tin mines are those of Cornwall,
which were worked previous to the Roman invasion ; they furnish
annually upwards of 3000 tons of the metal. The mines of
Malacca also yield a very pure tin : the metal is likewise obtained
to a smaller extent from Mexico. The tin veins in Cornwall are
frequently associated with those of copper, and they run almost
invariably east and west. The tin ore is often met with in alluvial
soils, whither it has been carried from its original position by the
action of water. In this case the ore occurs in detached, rounded
masses, and is very pure, constituting what is termed stream tin.
The position of the veins is frequently traced by following the
stream towards its source, up to the point where the ore ceases to
be found ; a careful examination of the vicinity generally leads to
the discovery of the vein.

(673) Extraction of Metallic Tin. In order to extract the
metal from the ore, it is subjected to a series of operations,

3 o



926 EXTRACTION OF METALLIC TIN.

some of which are of a mechanical, and others of a chemical
character. They may be classified thus :

1. Stamping and washing to remove the earthy and lighter
portions.

2. Boasting, to decompose the pyrites and get rid of the arsenic
and sulphur.

3. Washing, to dissolve out sulphate of copper and carry off
the oxide of iron.

4. Reduction, by which the tin is separated from the oxygen
and the gangue or earthy matter.

5. Refining, or liquation, and boiling with green wood.

1. The purer portions of the ore are first picked out by hand; the
residue, consisting chiefly of tin-stone, with the earthy impurities of
the matrix, mixed with arsenical copper and iron pyrites, passes to
the stamping mill, where it is reduced to a coarse powder. This
powder is then huddled and washed to remove the lighter impurities.

2. The heavier portion, however, still retains a considerable
quantity of arsenical iron and copper pyrites. The next operation
is intended to get rid of these substances; with this view the
washed ore is roasted in a reverberatory furnace until the arsenic
and a good deal of the sulphur are expelled, and the ore becomes
converted into yellowish-brown powder ; this process usually lasts
about twelve hours. During this roasting, frequent stirring is
necessary in order to expose fresh surfaces freely to the air. By
this means the iron pyrites is decomposed, and is converted
into sulphurous acid and peroxide of iron ; the arsenic is expelled
as arsenious acid, and the greater part of the sulphide of copper is
converted into sulphate of copper ; this conversion is completed by
exposing the mass in a moistened state to the air for some days.

3. The sulphate of copper is then dissolved out by lixiviation;
after which the principal part of the peroxide of iron, as it is much
lighter than the oxide of tin, is got rid of by washing.

4. The washed ore is now ready for reduction. In order to
attain this object it is mixed with from one-fifth to one-eighth of
its weight of powdered anthracite or of charcoal, and with a small
proportion of lime to facilitate the fusion of the siliceous gangue,
which still remains mingled with the ore. The mixture having
been rendered damp, for the purpose of preventing the finer particles
from being carried away by the current of air, is introduced
into the reducing furnace. This is a reverberatory furnace with
a low arch or crown. The charge having been placed upon the
hearth, the doors are closed up and the heat is gradually raised for



EXTRACTION OF METALLIC TIN. 927

five or six hours ; the oxide of tin is thus reduced by the carbon,
before the temperature rises high enough to cause the oxide to
fuse with the silica, with which it would form an enamel difficult
of reduction. Towards the end of the operation the heat is raised
until it becomes very intense, the slags are thus rendered fluid,
and the reduced metal subsides to the bottom and is allowed to
run off into cast-iron pans, from which it is ladled off into
moulds ; but the ingots thus obtained are by no means pure.

5. They are therefore next submitted to a process of liquation,
which consists in heating the ingots to incipient fusion, upon the
bed of a reverberatory furnace : the purer tin being the more fusible
portion, gradually melts out and leaves an alloy which has a higher
melting point. This less fusible portion when remelted forms the
inferior variety called block tin. The tin which has run out of the
ingots is drawn off into a second pan in which the metal is gently
heated, being kept in a state of fusion by a fire underneath ;
here it is agitated briskly by thrusting into the mass stakes soaked
in water; the steam thus produced, as it bubbles up through the
molten metal, carries the dust, slag, and other mechanical impurities
to the surface After this treatment has been continued for
about three hours, the metal is allowed to remain undisturbed



Online LibraryWilliam Allen MillerElements of chemistry: (Volume 2) → online text (page 52 of 79)