(_a_) The clotting of the sulphides is prevented, since
the first part of the roasting proceeds at a
comparatively very low temperature.
(_b_) The sulphur in the ore often provides sufficient
heat to maintain the roasting in progress during
the early stages.
(_c_) The hottest parts of the furnace are where the
roasted infusible oxides arrive, so but little
clotting or sintering occurs here.
The capacity of the four-bedded hand-roaster is 7 to 15 tons per
twenty-four hours, depending on the sulphur proportion in the charge
and in the roasted product.
It is a very useful form of furnace when labour is cheap. The furnace
works very efficiently, but in the New World, where manual labour was
dear, labour costs became prohibitive, and in order to economise in
this direction, mechanical rabbling was introduced.
_The O’Harra Calciner_ (1885) was essentially the old type of furnace,
double hearthed and mechanically rabbled. It consisted of long straight
furnace hearths. The rabbles were ploughs dragged through the furnace
by means of endless chains which were carried over grooved pulleys,
situated outside the furnace, at the ends. This was an important
invention, giving a continuous feed and discharge, a much larger
output, and efficient and regular stirring without much hand labour.
The rabbles became cooled on issuing from the hearth. The capacity was
50 tons per day from furnaces of 90 feet × 9 feet hearths, giving a
roasting capacity of 61 lbs. of ore per square foot of hearth area,
compared with about 33 lbs. per square foot with the old hand calciner.
In working the early forms of this furnace there were many mechanical
troubles and breakdowns, and the subsequent modifications of this
form consisted largely of devices for the purpose of overcoming such
difficulties.
_Modifications and Improvements._—_Allen_, instead of a rope to carry
the ploughs, used small wheeled carriages, running on a track which was
laid along the floor.
_Brown_ (important) ran the carriages along narrow corridors at either
side of the hearth, so as to protect the ropes and carriages from the
very corrosive action of the furnace gases. A continuous narrow slit
along the inner wall of the corridors allowed the arm carrying the
plough to travel forward.
_Wethey_; _Keller_; worked on very similar principles. The chief
improvements were in details, and had for their object the prevention
of wear and tear, and of the break-down of parts.
_Prosser._—Very similar; used at Swansea Works.
_Ropp._—The carriage runs underneath the bed, and supports a vertical
shaft which passes through a slot along the furnace hearth and carries
the arms furnished with ploughs.
[Illustration: Fig. 17.—O’Harra Furnace (Fraser-Chalmers), illustrating
Principle of Mechanical Rabbling by Travelling Ploughs.]
The Ropp and Prosser calciners work very successfully. The hearth is
about 105 feet long × 11 feet wide, with a capacity of about 36 tons
per day.
[Illustration: Fig. 18.—Section through Mechanically Rabbled Roaster
Furnace (illustrating Improvements for Protecting Driving Mechanism).]
_Brown Horse-Shoe Furnace_ operates on the same principle as the above,
except that the hearth is bent round in order to save space.
_Pearse-Turrett_ (1892 at Argo).—In this type of furnace the bed is
curved round in the form of a circle. The rabbling ploughs are carried
at the ends of arms which are attached to an upright rotating spindle.
The spindle is set in the centre of the space enclosed by the circular
hearth.
In all the above classes of furnace, the firing is done, when
necessary, from fireplaces built at intervals along the sides of the
furnace; either coal or gas being employed as fuel.
B. ROTATING HEARTHS.—This type of furnace is still reverberatory, but
instead of making use of mechanical rabbling, the hearth rotates, in
order to give agitation to the materials and assist their discharge.
(_a_) _Intermittent Working—The Brückner Roaster._—The details and
working of this roaster are familiar. The furnace was invented in 1864
for gold and silver ore-roasting in Colorado, and was later introduced
for the roasting of copper ores, being at one time the furnace most
commonly used for the purpose. It was employed all over the Western
States, and at one works alone, 56 were at one time in use.
The usual length was 18 feet 6 inches and the diameter, 8 feet 6
inches; giving an output of about 12 tons per twenty-four hours. It was
furnished with a removable fireplace, used to start the roasting. The
operation could then be allowed to proceed by itself, the fireplace
being wheeled away to another hearth, and being eventually brought
back to the first hearth for about three hours, in order to give the
required higher finishing temperature. Several dust chambers were
attached to this, as to all forms of roasting furnaces, which by their
nature and manner of work are apt to produce considerable quantities of
dust.
The advantages of the Brückner cylinder lay largely in the fact that
it afforded good control of the sulphur contents in the charge,
since the ore could be retained in the furnace until the sulphur was
sufficiently low. The furnace is simple to work, and not so liable to
get out of order as many other forms. It possesses however, distinct
disadvantages in that its working is intermittent, its use involves
comparatively high fuel costs, whilst the discharging presents
considerable difficulty and trouble to the labour employed, on account
of the awkwardness and the high temperature of the discharge, and the
sulphurous gases evolved.
Its use has now been very largely discontinued.
_Improvements_—(_b_) _Continuous Working._—The continuous type of
roasting furnace of this class involves the use of sloping cylindrical
hearths which rotate, and so agitate and help to discharge the
materials.
_Oxland_ (1868) first introduced this type in Cornwall, for the
roasting of tin ores.
The Oxland furnace was an inclined cylinder, the material was fed in
at the top, and by the rotation of the cylinder the charge gradually
travelled downwards, approaching nearer and nearer to the fire, and
being discharged close to the fire-box.
_White_ (1872) improved this furnace, and the White cylinder is largely
used in South Wales. The cylinder revolves slowly by friction gearing;
inside are four lines of projecting brick-work which form a shelf, thus
assisting the agitation of the charge.
_The White-Howell Furnace_ is somewhat similar to the White, but is
unlined for the greater part of its length, except at the lower end
near the fire-box, where it is much wider and is bricked. It is stated
to work more satisfactorily than the older form, having a larger
capacity and using but little fuel.
The furnace is employed at the Cape Copper Works, South Wales, for
matte-roasting. It is here 60 feet long, 7 feet diameter, inclined 6
inches in 60 feet, makes 8 revolutions per hour, and has a capacity of
10 tons of charge per day.
_Argall Furnace._—Consists essentially of four narrow tubes bound
together, each 28 feet long, 2 feet diameter, and lined. It works
rapidly, having a capacity of 40 to 50 tons per day, but is used more
for the roasting of cupriferous gold ores than at the copper smelters.
C. THE MACDOUGAL TYPE.—The most important form of modern roaster
furnace, and that most generally employed, is the MacDougal type. The
first furnace on this principle was invented by Parkes in 1860. The
design embodied two hearths, one above the other. Vertically down the
centre of these passed a spindle, supporting arms from which were
suspended the ploughs, and the rotation of this spindle carried the
arms over the beds.
As devised by Parkes, various mechanical difficulties were found,
and the working was intermittent, but the principle was recognised
as important. MacDougal in 1873 introduced his modification of the
furnace, primarily for the roasting of pyrites, at a Liverpool works,
and this form has now supplanted many of the older types for copper ore
roasting, and is in operation at most of the new smelting works.
_Principles of the MacDougal Type._—The furnace consists of an iron
cylinder lined with brick. Six circular hearths are constructed inside,
one above the other, and the vertical spindle carrying the arms and
ploughs for each hearth passes through the centre of the furnace. The
ore is ploughed towards openings on each hearth, which communicate
with the hearth below; the charge thus travelling from the outer edge
towards the centre, through the central opening to the middle of the
next floor, then outwards to the openings at the edge, and so on. The
original MacDougal furnace was 12 feet high and 6 feet in diameter. It
was improved by Herreshof in the direction of better rabbling mechanism
and greater ease of repair. The central spindle was an air-cooled
shaft, the supporting arms were made so as to be easily removable
from the shaft to facilitate repairs, and the furnace was enlarged.
Herreshof used air-cooling for the spindle and arms, as shown in Fig.
20.
[Illustration: Fig. 19.—MacDougal Roaster—Vertical Section.]
[Illustration: Fig. 20.—Herreshof Furnace—Section indicating
Connections for cooling Rabbles and Spindles.]
Evans, and subsequently Klepetko, in working the furnaces in Montana,
introduced, in about 1892, various marked improvements. The dimensions
were increased, enlarging the output. The spindle and arms were
water-cooled, which improvement removed much of the great difficulty
in working the MacDougal furnace, where the rapid wearing out of
working parts, and the difficulty of their removal, repair, and renewal
interfered greatly with efficient working.
Many of these troubles have now been overcome in the Evans-Klepetko
type, and in the still further improvements since made at Anaconda.
The general arrangement of the floors, spindles, arms and other
details shown in the Herreshof furnace (Fig. 20) are preserved in the
Evans-Klepetko and similar types of roaster; the chief alterations are
in matters of detail, the results of which have however, been important.
Furnaces of this improved kind are now used all over the West; there
are 64 at Anaconda, Mont.; 32 at the International Smelter, Tooele,
Utah; 24 at Garfield, Utah; 16 at Steptoe, Nevada; and also at
Balakala, Cal., Cerro de Pasco, Peru, and other large smelting centres.
_Important Advantages._—Of the marked advantages of this type of
furnace, the following are perhaps the most striking and important:—
(1) There is a great saving of floor space by having the six
hearths one above the other.
(2) The use of a central common spindle carrying the arms
and ploughs simplifies the mechanism.
(3) The form is convenient for the compact arrangement of
a roasting plant of many units for feeding, discharge,
and supervision.
(4) Very little heat is lost by radiation, as the heat
passes mostly from one hearth to another.
(5) Very little fuel is required, none with heavy sulphides
(except for starting), as the heat of oxidation of the iron
and sulphur usually yields a high enough temperature to keep
the operation going. The fuel costs are lower than in other
types of roaster.
(6) Thorough rabbling, greater uniformity and better mixing
of product, continuous and regular feed and discharge are
obtained.
(7) The roasting is thorough, and perfect control of the
degree of oxidation is ensured by adjusting the rate of
passage of the ore through the furnace, which is regulated
by varying the ore feed and the speed of rotation of the
rabbles.
(8) Great saving in labour costs and difficulties. The
labour in roasting plants is extremely arduous, on account
of the high temperature of the material, and is dangerous on
account of the atmosphere.
=The Evans-Klepetko-MacDougal Roasting Furnace Plant at Anaconda.=—The
roasting plant at Anaconda formerly consisted of 56 Brückner cylinders,
which were eventually all scrapped and replaced by new plant of the
MacDougal type, subsequently greatly modified and improved as one
difficulty after another had to be overcome.
The saving in working costs resulting from this replacement of the
Brückners by MacDougal roasters is reckoned at about 5 cents (2½d.) on
every ton of calcines treated.
The roasters are arranged in four rows of 16 each, running east and
west. The charge cars travel along tracks at a height of 20 feet above,
discharging into rows of bins, one situated over each calciner.
[Illustration: Fig. 21.—Spindle Connections and Guide Shields of
Evans-Klepetko Roasters.]
_Details of Furnace._—Height, 18 feet 3½ inches; diameter, 16 feet. Six
hearths. The spindle is made in three lengths, each to carry the arms
for two hearths; it is 18 inches in diameter and is water-cooled. The
rabble arms are 6 feet long, half round, and flanged on the lower side;
they too, are hollow and water-cooled. The rabble-blades were formerly
cast in one piece with base plate, so as to slide on to the arms, but
are made now with detachable blades, which slide into grooves on the
base plate, so as to facilitate removal for repairs; the blades are 6
inches square and 1½ inches thick (Fig. 32).
The arms on separate floors are set alternately at right angles. Of the
two arms for each floor, one carries six blades, the other seven, so
that the furrows resulting from one set of blades are turned over by
the other. The blades are set so as to direct the ore from the outer to
the inner edge or _vice versâ_, according to the particular hearth. The
spindle and connections are protected from falling ore by shields which
are bolted on. The rabbles move slowly, making a 2½-inch furrow in a
5-inch layer of material.
_Capacity._—40 to 45 tons per day each, reducing the sulphur in the
charge from 30 per cent. to about 8·0 per cent. The output of the plant
is about 3,000 tons of calcines daily.
_System of Working._—Since reverberatory furnaces are used essentially
as remelting furnaces only, the roasting plant is operated so as to
yield a product of such composition as will directly produce a suitable
matte and slag on melting in the reverberatories. The fluxes required
for the calculated reverberatory charge are, therefore, sent through
the roasters mixed with the fine concentrate; such practice possessing
many advantages. The charge thus consists of fine concentrate from the
concentrator settling tanks, and screened lime-rock flux (too fine to
be used in the blast furnaces). The limestone lightens the charge,
decreases the tendency to clotting of the pure sulphides, chemically
assists oxidation, preheats and thoroughly mixes the flux, and ensures
a uniformly mixed charge for the reverberatory furnaces; whilst the
extra cost involved is but very small.
Three per cent. of lime is used; 40 tons of concentrates, 1¼ tons of
lime-rock, and 1¼ tons of flue-dust being charged per twenty-four hours
per furnace, through an automatic gravity feed, the opening of which is
closed and opened by an eccentric. The speed of the eccentric and the
extent of the opening are adjustable.
_Working._—Charge contains 25 to 35 per cent. sulphur.
[Illustration: Fig. 22.—Rabble-blades and Bases.]
_1st Hearth._—Temperature about 230° C. (black heat). This is
practically a drying floor, and the wet ore wears the rabbles away
rather quickly. Special forms of plough are being introduced. About 4
per cent. of sulphur is driven off from the pyrites.
_2nd Hearth._—Hotter; not quite red, except near outer edge. About 5
per cent. of sulphur burnt off.
_3rd Hearth._—Bright red heat (about 700° C.). Sulphur can be seen
burning off the ridges of calcines, with a blue flame. 5 per cent. of
sulphur eliminated. There is some clotting, and the sinter sticks to
the rabble-blades, and has to be barred off occasionally.
_4th Hearth._—Bright red heat (about 750° C.), uniformly bright, but
the flame has ceased. Sulphur loss, 4 per cent.
_5th Hearth._—The hottest (800° C.). Bright red.
_Bottom Hearth._—Cooler, dark red (about 650° C.). The doors on this
floor are left open. The charge is guided towards openings at the outer
edge to discharge chutes whilst still red hot, and it is fed from here
whilst hot into the reverberatory furnace-bins.
Efficient dust catchers and settlers are essential on the roasting
plant. The gases escaping at a temperature of about 315° C. contain 2
per cent. of SO_{2} by volume, 5 per cent. by weight. The ore takes
2¼ hours to pass through the furnace. Practically no fuel is required
except to warm up the roaster on commencing work.
_Labour._—The requirements are small. There is one general foreman
for the plant, and two helpers for each set of four furnaces. The
conditions are rather trying, especially during the discharge of the
calcines into the reverberatory charge cars.
ROASTING ORES POORER IN SULPHUR, IN MACDOUGAL ROASTERS.—The Anaconda
concentrates carry sufficient sulphur (33 per cent.) to supply all
the heat necessary for carrying out the roasting operations. When the
sulphur is below this requisite quantity, some extra heating may be
required, though, on the other hand, the reduction which is necessary
in the sulphur contents is lessened, depending, of course, on the
proportions of copper and iron in the charge. At Garfield, Utah,
where the concentrate only contains 20 per cent. of sulphur, the fuel
required for all roaster purposes is equivalent to 0·2 per cent. of the
charge, one of the calcines’ outlets being converted into a fireplace.
Here the output per furnace per day approaches 55 tons, roasting the
sulphur from 20 per cent. down to 10 to 11 per cent. The flue-dust
losses at this plant are 6 per cent., so efficient dust catching
appliances are essential.
=The Costs of Roasting in the MacDougal Furnace.=—Ricketts has recently
published a valuable analysis of the costs of the roasting operations
at the Cananea Smelter. The figures must, however, be understood to
apply strictly to the conditions prevailing at this particular camp.
The roaster plant consists of 32 improved MacDougal furnaces. The
charge supplied to the roasters assays—
Copper, 5·2 per cent.
Iron, 28·4 "
Sulphur, 29·9 "
Silica, 23·6 "
Alumina, 3·7 "
whilst the product (“calcines”) has an average composition of
Copper, 6·3 per cent.
Iron, 34·5 "
Sulphur, 7·7 "
Silica, 28·6 "
Alumina, 4·4 "
The plant operated on the following quantities of material, from
February to July, 1911, inclusive:—
Concentrates, 32,929 short tons = 76·08 per cent. of charge.
Fine sulphide ores, 9,590 " = 22·16 " "
Limestone, 762 " = 1·76 " "
—————— ——————
Total charge, 43,281 " = 100·00 " "
Weight of “calcines”
produced, 35,533 " = 82·10 " "
—————— ——————
Shrinkage, 7,748 " = 17·90 " "
====== ======
The total costs of roasting (from roaster charge-bins to reverberatory
furnace) worked out at 38·45 cents per ton, the distribution of these
costs being as follows:—
Total Costs. Cost per Dry Ton.
Sampling, $222·62 $0·0051
Bedding, 2,016·27 0·0466
Reclaiming, 3,072·71 0·0710
Operating furnaces, 7,680·58 0·1775
Hauling calcines, 911·01 0·0210
General expenses, 1,639·99 0·0379
———————— ——————-
Total direct costs, $15,543·38 $0·3591
Cost of flux, 1,097·85 0·0254
———————— ——————-
Total costs, $16,641·23 $0·3845
========== =======
ANALYSIS OF COST—
(1) _Operating_—
Labour, $7,398·56 $0·1709
Power, 1,577·64 0·0365
Fuel, 773·08 0·0179
Water, 78·71 0·0018
Sundries, 10·30 0·0002
Flux, 1,097·85 0·0254
—————————— ——————-——
$10,936·14 $0·2527
(2) _Repairs_—
Labour, $2,507·35 $0·0579
Shop expense, 361·00 0·0083
Supplies, 2,836·74 0·0656
———————-—— ————-——
Total costs, $16,641·23 $0·3845
========== =======
References.
Peters, E. D., “Principles” and “Practice of Modern Copper Smelting.”
Cloud, T. C., “The M‘Murty-Rogers Process for Desulphurising Copper
Ores.” _Trans. Inst. Min. and Met._, vol. xvi., 1906–7, p. 311.
Hofman, H. O., “Recent Progress in Blast Roasting.”
_Bulletin Amer. Inst. Min. Eng._, No. 42, June, 1910.
Austin, L. S., “The Washoe Plant of the Anaconda Copper Mining
Company.” _Trans. Amer. Inst. Min. Eng._, vol. xxxviii.,
1906, p. 560.
Rickets, L. D., “Developments in Cananea Practice.”
_Engineering and Mining Journal_, Oct. 7th, 1911, p. 693.
Redick F. Moore, “Recent Reverberatory Smelting Practice.”
_Engineering and Mining Journal_, May 14th, 1910, p. 1021.