B. i.—_Avoiding Leakage of Cold Air._—The admission of cold air was the
cause of much waste in the older processes of working. Each time the
doors were opened, either at the fire-box, or during charging on to the
hearth, large quantities of cold air were admitted; air entered through
the working door whilst slag was skimmed off, whilst matte was being
tapped, and whilst the furnace hearth was being clayed; all of which
operations occupied considerable time. The doors were opened during
the levelling down of the fresh charges, and at later periods when the
charge was stirred and the half-fused masses sticking to the bottom
were worked up.
In modern practice, an essential feature of working is to keep all the
doors closed as much as possible, and, as will be indicated shortly,
every means is taken to eliminate the heat losses from the causes just
referred to. Air leakage is also occasioned by bad grating, which
causes the formation of channels in a few parts of the bed of fuel,
admitting excess of air at these places, instead of causing it to come
regularly through the bed in all parts. Channelling is now checked
by the drop of suction-pressure in the flues, as registered by the
manometer.
B. ii.—_Prevention of Radiation through Walls and Roof._—Such heat
losses are now minimised by thickening these parts, and blanketing the
outside of the roof with sand, keeping the construction together by
very heavy bracing.
B. iii.—_Prevention of Cooling of the Hearth on Withdrawal and on
Charging._—By far the most important cause of heat losses in working
was occasioned by the withdrawal of the whole of the melted products,
the charging of fresh cold ores, and the efficiency of the furnace
was very greatly reduced in consequence. In the older methods, fully
three-quarters of the time and fuel, and almost all the labour, were
spent in manipulating the charges and bringing them up to the point
of fusion, the actual smelting operation being responsible for but a
small proportion. The withdrawal of the hot slag and matte abstracts
much of the heat of the furnace, and the cold charge which is fed
in, not only cools the furnace hearth on which it rests, but being a
poor conductor, prevents the heat from again penetrating through it
to the hearth and to the undermost portion of the charge. It has been
estimated through the use of pyrometers, that the temperature in the
furnace after such withdrawal and recharging may drop to less than 700°
C.—a dull red heat—and there is no way under such circumstances of
heating up the hearth again, except by conduction through the charge.
Some hours’ hard firing were thus required to bring the furnace to the
desired temperature again, after which it was necessary to re-open the
working doors, in order to stir the materials so as to prevent the
half-fused masses, still lying on the hearth, from sticking to it. This
also occasioned delay in the operations, and caused much waste of fuel,
heat, and labour.
B. iv.—_Utilising the Heat of Melted Charges for the Heating of Fresh
Additions._—All the above difficulties, and many others, have been
overcome by maintaining a deep pool of hot molten matte in the furnace,
and by feeding hot charges upon this matte layer. These are two of the
most vital and successful changes introduced into modern reverberatory
practice, and will be reviewed in detail subsequently.
B. v.—_Utilising the Heat of the Escaping Gases as much as
possible._—Improvements in this direction have been brought about—
(_a_) By constructing the furnace of as great a length as
will allow of maintaining the charge in a sufficiently fluid
state to permit of its being tapped from the furthermost end
of the furnace.
(_b_) By using the still hot escaping gases under boilers.
=Modern Reverberatory Practice.=—The requirements for the successful
operation of the reverberatory furnace, and the methods for ensuring
its efficient working which have just been reviewed, involve the
application of the following principles, which are the essential
factors in modern reverberatory smelting practice:—
1. The grade of the furnace products is controlled at the roasters.
2. The melting must be as rapid as possible.
3. The employment of very large furnaces.
4. The use of a heated matte-pool in the furnace.
5. The charging of hot calcines.
6. The regulation of the furnace working by draft pressures.
7. The continuous working of the furnace.
8. Modified constructional details.
1. _Control of Furnace Products at the Roasters._—This feature
has already been indicated in dealing with roasting practice. The
importance of this system in the economy and efficiency of the furnace
working is very marked.
(_a_) The roasting plant affords the most ready means of control over
the desired sulphur elimination, this being its sole function. The
modern roaster is so designed as to allow of almost perfect regulation
in this respect, since amount of feed and rate of passage of the
sulphides through the furnace are under perfect control.
(_b_) The work of the reverberatory is thus confined to one object
only, that of rapid melting down, to which the foreman can give his
sole attention free from the necessity of manipulating the grade of the
matte at the same time.
In modern work it is usual to pass the whole of the charge
(concentrates as well as flux) intended for the reverberatories,
through the roasting plant. The advantages of such procedure are—
(i.) The flux is preheated at little extra expense,
there being usually plenty of heat to spare for
this, and the roaster capacity is not unduly
decreased.
(ii.) Intimate mixing of the charge is assured, and
this greatly facilitates the fusion and reaction.
(iii.) More rapid and thorough roasting is effected,
since the presence of the inert flux prevents
clotting or undue sintering of the sulphides in the
roaster.
(iv.) The charge is found to be in a much better
condition, both physically and chemically, for
successful reverberatory smelting.
Lime in the roaster charge appears to assist the thoroughness of the
roast, whilst an incipient slag formation is commenced owing to the
juxtaposition of basic oxides and silica, in the hotter parts of the
roaster furnace.
2. _Rapidity of melting_ is an indispensable feature of modern work.
The conditions necessary for rapid melting have been reviewed above.
3. _Use of Large Furnaces._—Reverberatory furnaces appear to have
replaced the blast furnace in Great Britain somewhere about 1700,
and by 1854 they were in general use in this country. At this period
the usual dimensions were, for the hearth 13 feet by 9 feet, with
a fire-box 4 feet by 4 feet, the furnace having a capacity of 12
tons per twenty-four hours. In Great Britain the size increased very
slowly, and it was in the United States of America that the important
increase in dimensions and in enormous outputs were developed. The
work was commenced systematically in about 1878 by Richard Pearse (a
Swansea-trained metallurgist) at the Argo Smelter in Colorado. Table
V. indicates the gradual improvements in practice resulting from these
developments (see also Fig. 23, p. 90).