In the first, a mixture of the ore with charcoal is exposed to heat on
the hearth of a reverberatory furnace fired with coal.
In the second, the tin ore is fused in a blast furnace, called a
blowing-house, supplied with wood charcoal. This method is practised in
only a few works, in order to obtain a very pure quality of tin, called
_grain tin_ in England, and _étain en larmes_ in France; a metal
required for certain arts, as dyeing, &c. This method is applied merely
to stream tin.
In the _smelting-houses_, where the tin is worked in reverberatories,
two kinds of furnaces are employed; the reduction and the refining
furnaces.
[Illustration: 1151 1152]
_Figs._ 1151, 1152. represent the furnaces for smelting tin at St.
Austle, in Cornwall; the former being a longitudinal section, the latter
a ground plan, _a_ is the fire-door, through which pitcoal is laid upon
the grate _b_; _c_ is the fire-bridge; _d_, the door for introducing the
ore; _e_, the door through which the ore is worked upon the hearth _f_;
_g_, the stoke-hole; _h_, an aperture in the vault or roof, which is
opened at the discharge of the waste schlich, to secure the free escape
of the fumes up the chimney; _i_, _i_, air channels, for admitting cold
air under the fire-bridge and the sole of the hearth, with the view of
protecting them from injury by the intensity of the heat above. _k_,
_k_, are basins into which the melted tin is drawn off; _l_, the flue;
_m_, the chimney, from 35 to 50 feet high. The roasted and washed
schlich is mixed with small coal or culm, along with a little slaked
lime, or fluor spar, as a flux; each charge of ore amounts to from 15 to
24 cwt., and contains from 60 to 70 per cent. of metal.
[Illustration: 1153 1154]
_Fig._ 1153. represents in a vertical section through the tuyère, and
_fig._ 1154. in a horizontal section, in the dotted line _x_, _x_, of
_fig._ 1153., the furnace employed for smelting tin at the Erzgebirge
mines, in Saxony. _a_, are the furnace pillars, of gneiss; _b_, _b_, are
shrouding or casing walls; _c_, the tuyère wall; _d_, front wall, both
of granite; as also the tuyère _e_. _f_, the sole-stone, of granite,
hewn out basin-shaped; _g_, the _eye_, through which the tin and slag
are drawn off into the fore-hearth _h_; _i_, the stoke-hearth; _k_, _k_,
the light ash chambers; _l_, the arch of the tuyère; _m_, _m_, the
common flue, which is placed under the furnace and the hearths, and has
its outlet under the vault of the tuyère.
In the smelting furnaces at Geyer the following dimensions are
preferred:--Length of the tuyère wall, 11 inches; of the breast wall, 11
inches; depth of the furnace, 17 inches. High chimney-stalks are
advantageous where a great quantity of ores is to be reduced, but not
otherwise.
The _refining furnaces_ are similar to those which serve for reducing
the ore; only, instead of a basin of reception, they have a refining
basin placed alongside, into which the tin is run. This basin is about 4
feet in diameter, and 32 inches deep; it consists of an iron pan, placed
over a grate, in which a fire may be kindled. Above this pan there is a
turning gib, by means of which a billet of wood may be thrust down into
the bath of metal, and kept there by wheeling the gibbet over it,
lowering a rod, and fixing it in that position.
The works in which the blast furnaces are employed, are called
_blowing-houses_. The smelting furnaces are 6 feet high, from the bottom
of the crucible (concave hearth) to the throat, which is placed at the
origin of a long and narrow chimney, interrupted by a chamber, where the
metallic dust, carried off by the blast, is deposited. This chamber is
not placed vertically over the furnace; but the lower portion of the
chimney has an oblique direction from it. The furnace is lined with an
upright cylinder of cast iron, coated internally with loam, with an
opening in it for the blast. This opening, which corresponds to the
lateral face opposite to the charging side, receives a _tuyère_, in
which the nozzles of two cylinder single bellows, driven by a
water-wheel, are planted. The _tuyère_ opens at a small height above the
sole of the furnace. On a level with the sole, the iron cylinder
presents a slope, below which is the hemispherical basin of reception,
set partly beneath the interior space of the furnace, and partly
without. Near the corner of the building there is a second basin of
reception, larger than the first, which can discharge itself into the
former by a sloping gutter. Near this basin there is another, for the
refining operation. These are all made either of brick or cast iron.
The quality of the average ground-tin ore prepared for smelting is such,
that 20 parts of it yield from 12-1/2 to 13 of metallic tin, (62-1/2 to
65 per cent.) The treatment consists of two operations, _smelting_ and
_refining_.
_First operation; deoxidization of the ore, and fusion of the
tin._--Before throwing the ore into the smelting furnace, it is mixed
with from one-fifth to one-eighth of its weight of _blind coal_, in
powder, called _culm_; and a little slaked lime is sometimes added, to
render the ore more fusible. These matters are carefully blended, and
damped with water, to render the charging easier, and to prevent the
blast from sweeping any of it away at the commencement. From 12 to 16
cwt. are introduced at a charge; and the doors are immediately closed
and luted, while the heat is progressively raised. Were the fire too
strong at first, the tin oxide would unite with the quartz of the
gangue, and form an enamel. The heat is applied for 6 or 8 hours, during
which the doors are not opened; of course the materials are not stirred.
By this time the reduction is, in general, finished; the door of the
furnace is removed, and the melted mass is worked up to complete the
separation of the tin from the scoriæ, and to ascertain if the operation
be in sufficient forwardness. When the reduction seems to be finished,
the scoriæ are taken out at the same door, with an iron rake, and
divided into three sorts; those of the first class A, which constitute
at least three-fourths of the whole, are as poor as possible, and may be
thrown away; the scoriæ of the second class B, which contain some small
grains of tin, are sent to the stamps; those of the third class C, which
are last removed from the surface of the bath of tin, are set apart, and
re-smelted, as containing a considerable quantity of metal in the form
of grain tin. These scoriæ are in small quantity. The stamp slag
contains fully 5 per cent. of metallic tin.
As soon as the scoriæ are cleared away, the channel is opened which
leads to the basin of reception, into which the tin consequently flows
out. Here it is left for some time, that the scoriæ which may be still
mixed with the metal, may separate, in virtue of the difference of their
specific gravities. When the tin has sufficiently settled, it is lifted
out with ladles, and poured into cast-iron moulds, in each of which a
bit of wood is fixed, to form a hole in the ingot, for the purpose of
drawing it out when it becomes cold.
_Refining of tin._--The object of this operation is to separate from the
tin, as completely as possible, the metals reduced and alloyed along
with it. These are, principally, iron, copper, arsenic, and tungsten; to
which are joined, in small quantities, some sulphurets and arseniurets
that have escaped decomposition, a little unreduced oxide of tin, and
also some earthy matters which have not passed off with the scoriæ.
_Liquation._--The refining of tin consists of two operations; the first
being a liquation, which, in the interior, is effected in a
reverberatory furnace, similar to that employed in smelting the ore.
(_figs._ 1151, 1152.) The blocks being arranged on the hearth of the
furnace, near the bridge, are moderately heated; the tin melts, and
flows away into the refining-basin; but, after a certain time, the
blocks cease to afford tin, and leave on the hearth a residuum,
consisting of a very ferruginous alloy.
Fresh tin blocks are now arranged on the remains of the first; and thus
the liquation is continued till the refining-basin be sufficiently full,
when it contains about 5 tons. The residuums are set aside, to be
treated as shall be presently pointed out.
_Refining proper._--Now begins the second part of the process. Into the
tin-bath, billets of green wood are plunged, by aid of the gibbet above
described. The disengagement of gas from the green wood produces a
constant ebullition in the tin; bringing up to its surface a species of
froth, and causing the impurest and densest parts to fall to the bottom.
That froth, composed almost wholly of the oxides of tin and foreign
metals, is successively skimmed off, and thrown back into the furnace.
When it is judged that the tin has boiled long enough, the green wood is
lifted out, and the bath is allowed to settle. It separates into
different zones, the upper being the purest; those of the middle are
charged with a little of the foreign metals; and the lower are much
contaminated with them. When the tin begins to cool, and when a more
complete separation of its different qualities cannot be looked for, it
is lifted out in ladles, and poured into cast-iron moulds. It is
obvious, that the order in which the successive blocks are obtained, is
that of their purity; those formed from the bottom of the basin being
usually so impure, that they must be subjected anew to the refining
process, as if they had been directly smelted from the ore.
The refining operation takes 5 or 6 hours; namely, an hour to fill the
basin, three hours to boil the tin with the green wood, and from one to
two hours for the subsidence.
Sometimes a simpler operation, called _tossing_, is substituted for the
above artificial ebullition. To effect it, a workman lifts some tin in a
ladle, and lets it fall back into the boiler, from a considerable
height, so as to agitate the whole mass. He continues this manipulation
for a certain time; after which, he skims with care the surface of the
bath. The tin is afterwards poured into moulds, unless it be still
impure. In this case, the separation of the metals is completed by
keeping the tin in a fused state in the boiler for a certain period,
without agitation; whereby the upper portion of the bath (at least
one-half) is pure enough for the market.
The moulds into which the tin blocks are cast, are usually made of
granite. Their capacity is such, that each block shall weigh a little
more than three hundred weight. This metal is called block tin. The law
requires them to be stamped or _coined_ by public officers, before being
exposed to sale. The purest block tin is called refined tin.
The treatment just detailed gives rise to two stanniferous residuums,
which have to be smelted again. These are--