CALCINATION OR ROASTING OF ORES
The object of calcining or roasting certain ores before treatment is to
dissipate the sulphur or sulphides of arsenic, antimony, lead, &c.,
which are inimical to treatment, whether by ordinary mercuric
amalgamation or lixiviation. The effect of the roasting is first to
sublimate and drive off as fumes the sulphur and a proportion of the
objectionable metals. What is left is either iron oxide, “gossan,” or
the oxides of the other metals. Even lead can thus be oxidised, but
requires more care as it melts nearly as readily as antimony and is much
less volatile. The oxides in the thoroughly roasted ore will not
amalgamate with mercury, and are not acted on by chlorine or cyanogen.
To effect the oxidation of sulphur, it is necessary not only to bring
every particle of sulphur into contact with the oxygen of the air, but
also to provide adequate heat to the particles sufficient to raise them
to the temperature that will induce oxidation. No appreciable effect
follows the mere contact of air with sulphur particles at atmospheric
temperature; but if the particles be raised to a temperature of 500°
Fahr., the sulphur is oxidised to the gaseous sulphur dioxide. The same
action effects the elimination of the arsenic and antimony associated
with gold and silver ores, as when heated to a certain constant
temperature these metals readily oxidise.
The science of calcination consists of the method by which the sulphide
ores, having been crushed to a proper degree of fineness, are raised to
a sufficient temperature and brought into intimate contact with
atmospheric air.
It will be obvious then that the most effective method of roasting will
be one that enables the particles to be thoroughly oxidised at the
lowest cost in fuel and in the most rapid manner.
The roasting processes in practical use may be divided into three
categories:
_First or Process A._--Roasting on a horizontal and stationary hearth,
the flame passing over a mass of ore resting on such hearth. In order to
expose the upper surface of the ore to contact with air the material is
turned over by manual labour. This furnace of the reverberatory type is
provided with side openings by which the turning over of the ore can be
manually effected, and the new ore can be charged and afterwards
withdrawn.
_Second or Process B._--Roasting in a revolving hearth placed at a
slightly inclined angle from the horizontal. The furnace is of
cylindrical form, and is internally lined with refractory material. It
has projections that cause the powdered ore to be lifted above the
flame, and, at a certain height, to fall through the flame and so be
rapidly raised to the temperature required to effect the oxidation of
the oxidisable minerals which it is desired to extract.
The rate, or speed, of revolution of this revolving furnace obviously
depends upon the character of the ore under treatment; it may vary from
two revolutions per minute down to one revolution in thirty minutes. Any
kind of fuel is available, but that of a gaseous character is stated to
be by far the most efficient.
Any ordinary cylinder of a length of 25 ft., and a diameter of 4 ft. 6
in., inclined 1 ft. 6 in. in its length, will calcine from 24 to 48 tons
per diem.
Another form of rotating furnace is one in which the axis is horizontal.
It is much shorter than the inclined type, and the feeding and removal
of the ore is effected by the opening of a retort lid door provided at
the side of the furnace. Openings provided at each end of the furnace
permit the passage of the flame through it, and the revolution of the
furnace turns over the powdered ore and brings it into more or less
sustained contact is continued sufficiently long to ensure the more or
less complete oxidation of the ore particles.
_Third or Process C._--In this process the powdered ore is allowed to
fall in a shower from a considerable height, through the centre of a
vertical shaft up which a flame ascends; the powdered ore in falling
through the flame is heated to an oxidising temperature, and the
sulphides are thus depleted of their sulphur and become oxides.
Another modification of this direct fall or shaft furnace is that in
which the fall of the ore is checked by cross-bars or inclined plates
placed across the shaft; this causes a longer oxidising exposure of the
ore particles.
When the sulphur contents of pyritous ores are sufficiently high, and
after the ore has been initially fired with auxiliary carbonaceous fuel,
it is unnecessary, in a properly designed roasting furnace, to add fuel
to the ore to enable the heat for oxidation to be obtained. The
oxidation or burning of the sulphur will provide all the heat necessary
to maintain the continuity of the process. The temperature necessary for
effecting the elimination of both sulphur and arsenic is not higher than
that equivalent to dull red heat; and provided that there is a
sufficient mass of ore maintained in the furnace, the potential heat
resulting from the oxidation of the sulphur will alone be adequate to
provide all that is necessary to effect the calcination.
TYPES OF FURNACES OF THE DIFFERENT CLASSES THAT ARE IN ACTUAL USE.
“A” OR Reverberatory Class.
The construction of this furnace has already been sufficiently
described. If the roasting is performed in a muffle chamber, the
arrangement employed by Messrs. Leach and Neal, Limited, of Derby, and
designed by Mr. B. H. Thwaite, C.E., can be advantageously employed in
this furnace, which is fired with gaseous fuel. The sensible heat of the
waste gases is utilised to heat the air employed for combustion; and by
a controllable arrangement of combustion, a flame of over 100 feet in
length is obtained, with the result that the furnace from end to end is
maintained at a uniform temperature. By this system, and with gaseous
fuel firing, a very considerable economy in fuel and in repairs to
furnace, and a superior roasting effect, have been obtained.
Where the ordinary reverberatory hearth is fired with solid coal from an
end grate, the temperature is at its maximum near the firing end, and
tails off at the extreme gas outlet end. The ores in this furnace should
therefore be fed in at the colder end of the hearth and be gradually
worked or “rabbled” forward to the firing end.
One disadvantage of the reverberatory furnace is the fact that it is
impossible to avoid the incursion of air during the manual rabbling
action, and this tends to cool the furnace.
The cost of roasting, to obtain the more or less complete oxidation, or
what is known in mining parlance as a “sweet roast” (because a perfectly
roasted ore is nearly odourless) varies considerably, the variation
depending of course upon the character of the ore and the cost of labour
and fuel.
There are several modifications of the reverberatory furnace in use,
designed mechanically to effect the rabbling. One of the most successful
is that known as the Horse-shoe furnace. In plan the hearth of the
furnace resembles a horse-shoe.
The stirring of the ore over the hearth is effected by means of
carriages fixed in the centre of the furnace and having laterally
projecting arms, carrying stirrers, that move along the hearth and turn
over the pulverised ore.
In operation, half the carriages are traversing the furnace, and half
are resting in the cooling space, so that a control over the temperature
of the stirrers is established.
This furnace is stated to be more economical in labour than other
mechanically stirred reverberatory furnaces, and there is also said to
be an economy in fuel.
Usually the mechanical stirring furnaces give trouble and should be
avoided, but the horse-shoe type possesses qualifications worthy of
consideration.
“B.” The Revolving Cylinder Furnace.
Of these some of the best known to me are: The Howell-White, The
Brückner, The Thwaite-Denny, and the Molesworth.
The Brückner is a cylinder, turning on its horizontal axis and carried
by four rollers.
The batch of ore usually charged into the two charging hoppers weighs
about four tons. When the two charging doors are brought under the
hopper mouth, the contents of the hopper fall directly into the
cylinder.
The ends or throats of the furnace are reduced just sufficiently to
allow the flame evolved from a grated furnace to pass completely through
the cylinder.
A characteristic size for this Brückner furnace is one having a length
of 12 feet and a diameter of 6 feet. A furnace of this capacity will
have an inclusive weight (iron and brickwork) of 15 tons.
The time of operation, with the Brückner, will vary with the character
of the ore under treatment and the nature of the fuel employed. Four
hours is the minimum and twelve hours should be the maximum time of
operation.
By the addition of common salt with the batch of ore, such of its
constituents as are amenable to the action of chlorine are chlorinated
as well as freed from sulphur.
Where the ore contains any considerable quantity of silver which should
be saved, the addition of the salt is necessary as the silver is very
liable to become so oxidised in the process of roasting as to render its
after treatment almost impossible. I know a case in point where an
average of nearly five ounces of silver to the ton, at that time worth
30_s_., was lost owing to ignorance on this subject. Had the ore been
calcined with salt, NaCl, the bulk of this silver would have been
amalgamated and thus saved. It was the extraordinary fineness of the
gold saved by amalgamation as against my tests of the ore by fire assay
that put me on the track of a most indefensible loss.
_The Howell-White Furnace._--This furnace consists of a cast iron
revolving cylinder, averaging 25 feet in length and 4 ft. 4 in. in
diameter, which revolves on four friction rollers, resting on truck
wheels, rotated by ordinary gearing.
The power required for effecting the revolution should not exceed four
indicated horse-power.
The cylinder is internally lined with firebrick, projecting pieces
causing the powdered ore to be raised over the flame through which it
showers, and is thereby subjected to the influence of heat and to direct
contact oxidation.
The inclination of the cylinder, which is variable, promotes the gradual
descension of the ore from the higher to the lower end. It is fed into
the upper end, by a special form of feed hopper, and is discharged into
a pit at the lower end, from which the ore can be withdrawn at any time.
The gross weight of this furnace, which is, however, made in segments to
be afterwards bolted together, is some ninety to one hundred tons.
The furnace is fired with coal on a grated hearth, built at the lower
end; it is more economical both in fuel and in labour than an ordinary
reverberatory furnace.
_The Thwaite-Denny Revolving Furnace._--This new type of furnace, which
is fired with gaseous fuel, is stated to combine the advantages of the
Stetefeldt, the Howell-White, and the Brückner.
It is constructed as follows:--Three short cylinders, conical in shape
and of graduated dimensions, are superposed one over the other, their
ends terminating in two vertical shafts of brickwork, by which the three
cylinders are connected. The powdered ore is fed into the uppermost
cylinder and gravitates through the series. The highest cylinder is the
largest in diameter, the lowest the smallest.
The gas flame, burnt in a Bunsen arrangement, enters the smallest end of
the lowest cylinder and passes through it; then returns through the
intermediate cylinder above it, being directed by the brickwork
shaft--from one cylinder to the other--till finally the gases flow
through the topmost cylinder and enter into a dust depositing chamber.
The gases evolved increase as the flame flows through the series and the
ore is reduced by the expulsion of its sulphur, arsenic, &c., as it
descends from the top to the bottom. The top cylinder is made larger
than the one below it and the middle cylinder is made larger than the
lowest one in proportion to the increased bulk of gases and ore.
The powdered ore in descending through the cylinders is lifted up and
showers through the flame, falling in its descent a distance of over
1000 feet. By the time it reaches the bottom the ore is thoroughly
roasted.
Provision is made for the introduction of separate supplies of air and
gas into each cylinder; this enables the oxidising treatment to be
controlled exactly as desired so as to effect the best results with all
kinds of ore. Each cylinder is driven from its own independent gearing,
and the speed of each cylinder can be varied at will.
The output of this type of furnace, the operations of which appear to be
more controllable than those of similar appliances, depends, of course,
upon the nature of the ore, but may be considered to range within the
limits of twelve to fifty tons in twenty-four hours, and the cost of
roasting will vary from 2_s._ 6_d._ to 4_s._ per ton, depending upon the
quality of ore and of fuel.
The gaseous fuel generating system permits not only the absolute control
over the temperature in the furnace, but the use of the commonest kinds
of coal, and even charcoal is available.
The power required to drive the Thwaite-Denny furnace is four indicated
horse-power.
_The Molesworth Furnace_ also is a revolving cylindrical appliance,
which, to say the least of it, is in many respects novel and ingenious.
It consists of a slightly cone-shaped, cast-iron cylinder about fourteen
feet long, the outlet end being the larger to allow for the expansion of
the gases. Internal studs are so arranged as to keep the ore agitated;
and spiral flanges convey it to the outlet end continually, shooting it
across the cylinder. The cylinder is encased in a brick furnace. The
firing is provided from _outside_, the inventor maintaining that the
products of combustion are inimical to rapid oxidisation, to specially
promote which he introduces an excess of oxygen produced in a small
retort set in the roof of the furnace and fed from time to time with
small quantities of nitrate of soda and sulphuric acid. Ores containing
much sulphur virtually calcine themselves. I have seen this appliance
doing good work. The difficulties appeared to be principally mechanical.
There are other furnaces which work with outside heat, but I have not
seen them in action.
“C.”--Shaft Type of Furnace.
In one form of this furnace, instead of allowing the ore to descend in a
direct clear fall the descent is impeded by inclined planes placed at
different levels in the height of the shaft, the ore descending from one
plane to the other.
_The Stetefeldt Shaft Furnace._--Although very expensive in first cost,
has many advantages. No motive power is required and the structure of
the furnace is of a durable character. Its disadvantages are:--Want
of control, and the occasionally imperfect character of the roasting
originating therefrom.
Three sizes of Stetefeldt’s furnaces are constructed:
The largest will roast from 40 to 80 tons per diem.
The intermediate ” ” 20 ” 40 ” ” ”
The smallest ” ” 10 ” 20 ” ” ”
A good furnace should bring down the sulphur contents even of
concentrates so as to be innocuous to mercuric amalgamation. The sulphur
left in the ore should never be allowed to exceed two per cent.
A forty per cent. pyritous or other sulphide ore should be roasted in a
revolving furnace in thirty to forty minutes, and without any auxiliary
fuel.
For ordinary purposes a 40-foot chimney is adequate for furnace work;
such a chimney four feet square inside at the base, tapering to 2’ 6” at
the summit, will require 12,000 red bricks, and 1500 fire-bricks for an
internal lining to a height of 12 feet from the base of the chimney
shaft.
When second-hand Lancashire or Cornish boiler flues are available, they
make admirable and inexpensive chimneys. The advantage of wrought-iron
or steel chimneys lies in the convenience of removal and erection. They
should be made in sections of 20 feet long, three steel wire guy-ropes
attached to a ring, riveted to a ring two-thirds of the height of the
chimney, and attached to holdfasts driven into the ground; tightening
couplings should be provided for each wire.
Flue dust depositing chambers should be built in the line of the flues
between the furnace and the chimney; they consist simply of carefully
built brick chambers, with openings to enable workmen to enter and
rapidly clear away the deposited matters. The chambers, three or four
times the cross sectional area of the chimney flue, and ten to twenty
feet long, can be built of brickwork, set in cement; the walls are
provided with a cavity, filled with sand or Portland cement, so that
there will be no danger of the incursion of air. In all furnace work the
greatest possible precautions should be taken to prevent the least
cracking of either joints or bricks. It is surprising how much the
inadequate draft of a good chimney is due to cracks or orifices in the
flues; and therefore a competent furnace-man should see to it that his
flues are thoroughly sound, and free from openings through which the air
can enter.[2]
[2] For full details of the most recent improvements in the
cyanide process and in other methods of extraction, the reader is
referred to Dr. T. K. Rose’s “Metallurgy of Gold,” third edition.