SINTERING AND BRIQUETTING
THE DESULPHURIZATION OF SLIMES BY HEAP ROASTING AT BROKEN HILL[9]
BY E. J. HORWOOD
(August 22, 1903)
It is well known that, owing to the intimate mixture of the
constituents of the Broken Hill sulphide ores, a great deal of crushing
and grinding is required to detach the particles of galena from the
zinc blende and the gangue; and it will be understood, therefore, that
a considerable amount of the material is converted into a slime which
consists of minute but well-defined particles of all the constituents
of the ore, the relative proportions of which depend on the dual
characteristics of hardness and abundance of the various constituents.
An analysis of the slime shows the contents to be as follows;
Galena (PbS) 24.00
Blende (ZnS) 29.00
Pyrite (FeS₂) 3.38
Ferric oxide (Fe₂O₃) 4.17
Ferrous oxide (FeO) contained in garnets 1.03
Oxide of manganese (MnO) contained in rhodonite and garnets 6.66
Alumina (Al₂O₃) contained in kaolin and garnets 5.40
Lime (CaO) contained in garnets, etc. 3.40
Silica (SiO₂) 22.98
Silver (Ag) .06
——————
100.48
Galena, being the softest of these, is found in the slimes to a larger
extent than in the crude ore; it is also, for the same reason, in the
finest state of subdivision, as is well illustrated by the fact that
the last slime to settle in water is invariably much the richest in
lead, while the percentages of the harder constituents, zinc blende and
gangue, show a corresponding reduction in quantity, by reason of their
being generally in larger sized particles and consequently settling
earlier.
The fairly complete liberation of each of the constituent minerals
of the ore that takes place in sliming tends, of course, to help
the production of a high-grade concentrate by the use of tables and
vanners, and undoubtedly a fair recovery of lead is quite possible,
even with existing machines, in the treatment of fine slimes; but,
owing to the great reduction in the capacity of the machines, which
takes place when it is attempted to carry the vanning of the finer
slimes too far, and the consequently greatly increased area of the
machines that would be necessary, the operation, sooner or later,
becomes unprofitable.
The extent to which the vanner treatment of slimes should be carried
is, of course, less in the case of those mines owning smelters than
with those which have to depend on the sale of concentrates as their
sole source of profit. In the case of the Proprietary Company,
all slime produced in crushing is passed over the machines after
classification. A high recovery of lead in the form of concentrates
is, of course, neither expected nor obtained, for reasons already
explained; but the finest lead-bearing slimes are allowed to unite
with the tailings, which are collected from groups of machines, and
are then run into pointed boxes, where, with the aid of hydraulic
classification, the fine rich slimes are washed out and carried to
settling bins and tanks, where the water is stilled and allowed to
deposit its slime, and pass over a wide overflow as clear water. The
slime thus recovered amounts to over 1200 tons weekly, or about 11 per
cent., by weight, of the ore, and assays about 20 per cent. lead, 17
per cent. zinc, and 18 oz. silver, and represents, in lead value, about
11 per cent. of the original lead contents of the crude ore and rather
more than that percentage in silver contents. These slimes are thus a
by-product of the mills, and their production is unavoidable; but as
they are not chargeable with the cost of milling, they are an asset of
considerable value, more especially so since it has been demonstrated
that they can be desulphurized sufficiently for smelting purposes by a
simple operation, and, at the same time, converted into such a physical
condition as renders the material well suited for smelting, owing to
its ability to resist pressure in the furnaces.
The Broken Hill Proprietary Company has many thousands of tons of
these slimes which the smelters have hitherto been unable to cope with,
owing to the roasters being fully occupied with the more valuable
concentrates. Moreover, the desulphurization of slimes in Ropp
mechanical roasters is objectionable for various reasons, namely, owing
to the large amount of dust created with such fine material, resulting
injuriously to the men employed; also on account of the reduction in
the capacity of the roasters, and consequent increase in working cost,
owing to the lightness of the slime, especially when hot, as compared
with concentrates, and the necessity for limiting the thickness of
material on the bed of the roasters to a certain small maximum.
Further, the desulphurization of the slimes is no more complete with
the mechanical roasters than in the case of heap roasting, and the
combined cost of roasting and briquetting being quite three shillings
(or 75c.) per ton in excess of the cost of heap roasting, the
latter possesses many advantages. These heaps are being dealt with,
preparatory to roasting, by picking down the material in lumps of about
5 in. in thickness, while the fine dry smalls, unavoidably produced,
are worked up in a pug mill with water, and dealt with in the same way
as the wet slime produced from current work.
The slime, as produced by the mills, is run from bins into railway
trucks in a semi-fluid condition, and shortly after being tipped
alongside one of the various sidings on the mine is in a fit condition
to be cut with shovels into rough bricks, which dry with fair rapidity,
and when required for roasting are easily reloaded into railway trucks.
As each man can cut about 20 tons of bricks per day, the cost is small.
Various other methods of lumping the slime were tried, including
trucking the semi-fluid material on movable trams, alongside which were
set laths, about 9 in. apart, which enabled long slabs to be formed
9 in. wide and 5 in. thick, which were, after drying, picked up in
suitable lumps and loaded in platform trucks, thence on railway trucks.
Owing to the inferior roasting that takes place with bricks having flat
sides, which are liable to come into close contact in roasting, and
to the rather high labor cost, this method was discontinued. Another
method was to allow the slime to dry partially after being emptied
from railway trucks, and to break it into lumps by means of picks;
but this method entailed the making of an increased amount of smalls,
besides taking up more siding room, owing to the extra time required
for drying, as compared with the method now in use. Ordinary bricking
machines could, of course, be used, but when the cost of handling the
slime before and after bricking is counted, the cost would be greater
than with the simple method now in use; the material being in too
fluid a condition for making into bricks until some time elapses for
drying, a double handling would be necessitated before sending it to
the bricking machine. If, however, the slime could be allowed time to
dry sufficiently in the trucks, bricking by machinery would probably be
preferable. Rather more than 10 per cent. of smalls is made in handling
the lumps in and out of the railway trucks, and this is, as already
noted, worked up with water in a pug mill at the sintering works, and
used partly for covering the heaps with slime to exclude an excessive
amount of air. The balance is thrown out and cut into bricks, as
already described.
At the heaps the lumps are at present being thrown from one man to
another to reach their destination in the heap, but the sidings have
been laid out in duplicate with a view to enabling traveling cranes to
be used on the line next the heap, the lumps to be loaded primarily
into wooden skips fitting the trucks. It is probable, however, that
the lumps will require to be handled out of the skips into their place
in the heap, as the brittle nature of the material may be found to
render automatic tipping impracticable. A considerable saving in labor
would nevertheless accompany the use of cranes, which would likewise be
advantageous in loading the sintered material.
In order to reduce the inconvenience arising from fumes, length is very
desirable in siding accommodation, so that heap building may be carried
on at a sufficient distance from the burning kilns. It is for the same
reason preferable to build in a large tonnage at one time, lighting
the heaps altogether. As the heaps burn about two weeks only, long
intervals intervene, during which the fumes are absent.
In the experimental stages of slime roasting, fuel, chiefly wood, was
used in quantities up to 5 per cent., and was placed on the ground at
the bottom of the heap, where also a number of flues, loosely built
bricks, were placed for the circulation of air. The amount of fuel
used has, however, been gradually reduced, until the present practice
of placing no fuel whatever in the bottom was arrived at; but instead
less than 1 per cent. of wood is now burned in small enlargements of
the flues, under the outer portion of the pile, and placed about 12
ft. apart at the centers. This is found to be sufficient to start the
roasting operation within 24 hours of lighting, after which no further
fuel is necessary.
As regards the dimensions of the heaps, the width found most suitable
is 22 ft. at the base, the sides sloping up rather flatter than one to
one, with a flat section on top reaching about 7 ft. in hight. As there
is always about 6 in. of the outer crust imperfectly roasted, it is
advisable to make the length as great as possible, thus minimizing the
surface exposed. The company is building heaps up to 2000 ft. long.
During roasting care is required to regulate the air supply, the object
being to avoid too fierce a roast, which tends to sinter and partially
fuse the material on the outer portions of the lumps, while inside
there is raw slime. By extending the roast over a longer period this is
avoided, and a more complete desulphurization is effected. Experiments
conducted by Mr. Bradford, the chief assayer, demonstrated that, at a
temperature of 400 deg. C., the sulphide slime is converted into basic
sulphate, while at a temperature of 800 deg. C. the material becomes
sintered owing to the decomposition of the basic sulphate and the
formation of fusible silicate of lead.
In practice, the sulphur contents of the material, which originally
are about 14 per cent., become reduced to from 6.5 to 8.5 per cent.,
half in the form of basic sulphate and half as sulphides; much of the
material sinters and becomes matted together in a fairly solid mass.
The heaps are built without chimneys of any kind; a strip about 5
ft. wide along the crest of the pile is left uncovered by plastered
slime, and this, together with the open way in which the lumps are
built in, allows a natural draft to be set up, which can be regulated
by partly closing the open ends of the flues at the base of the pile.
Masonry kilns were used in the earlier stages with good results, which,
however, were not so much better than those obtained by the heap method
as to justify the expense of building, taking into consideration, too,
the extra cost of handling the roasted material in the necessarily more
confined space.
Much interest has been taken in the chemical reactions which take
place in the operation of desulphurization of these slimes, it being
contended, on the one hand, that the unexpectedly rapid roast which
takes place may be due to the sulphide being in a very fine state of
subdivision, and more or less porous, thus allowing the air ready
access to the sulphur, producing sulphurous acid gas (SO₂). On the
other hand, others, of whom Mr. Carmichael is the chief exponent, claim
that several reactions take place during the operation, connected
with the rhodonite and lime compounds present in the slimes, which he
describes as follows:
“The temperature of the kilns having reached a dull red heat, the
rhodonite (silicate of manganese) is converted into manganous oxide
and silica; at a rather higher temperature the calcium compounds are
also split up, with formation of calcium sulphide, the sulphur being
provided by the slimes. The air permeating the mass oxidizes the
manganese oxide and calcium sulphide into manganese tetroxide and
calcium sulphate respectively, as shown as follows;
3MnO + O = Mn₃O₄
CaS + 4O = CaSO₄,
and, as such, are carriers of a form of concentrated oxygen to the
sulphide slimes, with a corresponding reduction to manganous oxide and
calcium sulphide, as shown by the following equation, in the case of
lead:
PbS + 4Mn₃O₄ = PbSO₄ + 12MnO
PbS + CaSO₄ = PbSO₄ + CaS.
The oxidation of the manganous oxide and calcium sulphide is repeated,
and these alternate reactions recur until the desulphurization ceases,
or the kiln cools down to a temperature below which oxidation cannot
occur. These reactions, being heat-producing, provide part of the heat
necessary for desulphurization, which is brought about by certain
concurrent reactions between metallic sulphates and sulphide.
“The first that probably occurs is that in which two equivalents of the
metallic sulphide react on one of the metallic sulphate with reduction
to the metal, metallic sulphide, and sulphurous acid, as shown by the
following equation in the case of lead:
2PbS + PbSO₄ = 2Pb + PbS + 2SO₂.
“The metal so formed, in the presence of air, is oxidized, and in this
state reacts on a further portion of the metallic sulphide produced,
with an increased formation of metal and evolution of sulphurous acid,
according to the following equation, in the case of lead:
2PbO + PbS = Pb + SO₂.
“The metal so produced in this reaction is wholly reoxidized by the
oxygen of the air current, and being free to react on still further
portions of the metallic sulphide, repeats the reaction, and becomes
an important factor in the desulphurizing of the undecomposed portion
of the material. As the desulphurization proceeds, and the sulphate of
metal accumulates, reactions are set up between the metallic sulphide
and different multiple proportions of the metallic sulphate, with the
formation of metal, metallic oxide, and evolution of sulphurous acid,
as follows:
“With two equivalents of metallic sulphate to one equivalent of
metallic sulphide, in the case of lead, according to the following
equation:
PbS + 2PbSO₄ = 2PbO + Pb + 3SO₂.
“With three equivalents of metallic sulphate to one of metallic
sulphide, in the case of lead, according to the following equation:
PbS + 3PbSO₄ = 4PbO + 4SO₂.”
The volatility of sulphide of lead—especially in the presence of an
inert gas such as sulphurous acid—being greater than that of the
sulphate, oxide, or the metal itself, it might be thought that the
conditions are conducive to a serious loss of lead. This, however, is
reduced to a minimum, owing to the easily volatilized sulphide being
trapped, as non-volatile sulphate, by small portions of sulphuric
anhydride (SO₃), which is formed by a catalytic reaction set up
between the hot ore, sulphurous acid, and the air passing through
the mass. Owing to the non-volatility of the silver compounds in the
slimes, the loss of this metal has been found to be inappreciable. The
zinc contents of the slime are reduced appreciably, thus rendering the
material more suitable for smelting. After desulphurization ceases,
a few days are allowed for cooling off. On the breaking up of the
mass for despatch to the smelters, as much of the lower portion of
the walls is left intact as possible, so that it can be utilized for
the next roast, thus avoiding the re-building of the whole of the
walls.[10]
THE PREPARATION OF FINE MATERIAL FOR SMELTING
BY T. J. GREENWAY
(January 12, 1905)
In the course of smelting, at the works of the company known as the
Broken Hill Proprietary Block 14, material which consisted chiefly of
silver-lead concentrate and slime, resulting from the concentration
of the Broken Hill complex sulphide ore, I had to contend with all
the troubles which attend the treatment of large quantities of finely
divided material in blast furnaces. With the view of avoiding these
troubles, I experimented with various briquetting processes; and,
after a number of more or less unsatisfactory experiences, I adopted a
procedure similar to that followed in manufacturing ordinary bricks by
what is known as the semi-dry brick-pressing process. This method of
briquetting not only converts the finely divided material cheaply and
effectively into hard semi-fused lumps, which are especially suitable
for the heavy furnace burdens required by modern smelting practice, but
also eliminates sulphur, arsenic, etc., to a great extent; therefore,
it is capable of wide application in dealing with concentrate, slime,
and other finely divided material containing lead, copper and the
precious metals.
This briquetting process comprises the following series of operations:
1. Mixing the finely divided material with water and newly slaked lime.
2. Pressing the mixture into blocks of the size and shape of ordinary
bricks.
3. Stacking the briquettes in suitably covered kilns.
4. Burning the briquettes, so as to harden them, without melting, at
the same time eliminating sulphur, arsenic, etc.
1. The material is dumped into a mixing plant, together with such
proportions of screened slaked lime (usually from three to five per
cent.) and water as shall produce a powdery mixture which will, on
being squeezed in the hand, cohere into dry lumps. In preparing the
mixture, it is well to mix sandy material with suitable proportions
of fine, such as slime, in order that the finer material may act as a
binding agent.
The mixer used by me consists of an iron trough, about 8 ft. long,
traversed by a pair of revolving shafts, carrying a series of knives
arranged screw-fashion; and so placed that the knives on one shaft
travel through the spaces between the knives on the other shaft.
The various materials are dumped into one end of the mixing trough,
from barrows or trucks, and are delivered continuously at the other
end of the trough, into an elevator which conveys the mixture to the
brick-pressing plant.
2. The plant employed was the semi-dry brick-press. This machine
receives the mixture from the elevators, and delivers it in the form
of briquettes, which can at once be stacked in the kilns. It was found
that such material as concentrate and slime has comparatively little
mobility in the dies during the pressing operation; this necessitates
the use of a device which provides for the accurate filling of the
dies. It was also found that the materials treated by smelters vary
in compressibility, and this renders necessary the adoption of a
brick-pressing plant having plungers which are forced into the dies by
means of adjustable springs, brick-presses having plungers actuated by
rigid mechanism being extremely liable to jam and break.
3. Briquettes made from such material as concentrate and slime vary
in fusibility; they are also combustible, and while being burned they
produce large quantities of smoke containing sulphurous acid and other
objectionable fumes. It is therefore necessary that such briquettes be
burned in kilns provided with arrangements for accurately controlling
the burning operations, and for conveniently disposing of the smoke.
Suitable kilns, which will contain from 30 to 50 tons of briquettes
per setting, are employed for this purpose. Regenerative kilns of the
Hoffman type might be used for dealing with some classes of material,
but, for general purposes, the kilns as designed here will be found
more convenient.
The briquettes are stacked according to the character of the material
and the object to be obtained. The various methods of stacking, and the
reasons for adopting them, can be readily learned by studying ordinary
brick-burning operations in any large brick-yard. After the stacking
is complete the kiln-fronts are built up with burnt briquettes produced
in conducting previous operations, and all the joints are well luted.
4. In burning briquettes made from pyrite or other self-burning
material, it is simply necessary to maintain a fire in the kiln
fireplaces for a period of from 10 to 20 hours. When it is judged that
this firing has been continued long enough, the fire-bars are drawn
and the fronts are luted with burnt briquettes in the same manner as
the kiln-fronts. Holes about two inches square are then made in these
lutings, through which the air required for the further burning of the
briquettes is allowed to enter the kilns under proper control. After
the fireplaces are thus closed the progress of the burning, which
continues for periods of from three to six days, is watched through
small inspection holes made in the kiln-fronts; and when it is seen
that the burning is complete the fronts are partially torn away,
in order to accelerate the cooling of the burnt briquettes, which
are broken down and conveyed to the smelters as soon as they can be
conveniently handled.
When briquettes made from pyrite concentrate, or of other free-burning
material, are thus treated, they are not only sintered but they are
also more or less effectively roasted, and it may be taken for granted
that any ore which can be effectively roasted in the lump form in kilns
or stalls will form briquettes that will both sinter and roast well;
indeed, one may say more than this, for briquettes which will sinter
and roast well can be made from many classes of ore that cannot be
effectively treated by ordinary kiln-and stall-roasting operations;
and, moreover, good-burning briquettes may be made from mixtures of
free-burning and poor-burning material. Briquettes containing large
proportions of pyrite or other free-burning material will, unless the
air-supply is properly controlled, often heat up to such an extent as
to fuse into solid masses, much in the same manner as matte of pyritic
ore will melt when it is unskilfully handled in roasting. In dealing
with material which will not burn freely, such as roasted concentrate,
the briquetting is conducted with the intention of sintering the
material; and in this case the firing of the kilns is continued for
periods of from three to four days, the procedure being similar in
every way to that followed in burning ordinary bricks.
When conducting my earlier briquetting operations I made the
briquettes by simply pugging the finely divided material, following
a practice similar to that adopted in producing “slop-made” bricks
by hand. This method of making the briquettes was attended with a
number of obvious disadvantages, and was abandoned as soon as the
semi-dry brick-pressing plant became available. The extent to which
this process, or modifications of it, may be applied is shown by the
fact that, following upon information given by me, the Broken Hill
Proprietary Company adopted a similar method of sintering and roasting
slime, consisting of about 20 per cent. galena, 20 per cent. blende,
and 60 per cent. silicious gangue. The procedure followed in this
case consisted of simply pugging the slime, and running the pug upon
a floor to dry; afterward cutting the dried material into lumps by
means of suitable cutting tools, and then piling the lumps over firing
foundations, following a practice similar to that pursued in conducting
ordinary heap-roasting. This company is now treating from 500 to 1000
tons of slime weekly in this manner. It is, however, certain that
better results would attend the treatment of this material by making
this slime into briquettes and burning them in kilns.
The cost of briquetting and burning material in the manner first
described, with labor at 25c. per hour, and wood or coal at $4 per ton,
amounts to from $1 to $1.50 per ton of material.
THE BRIQUETTING OF MINERALS
BY ROBERT SCHORR
(November 22, 1902)
The value of briquetting in connection with metallurgical processes and
the manufacture of artificial stone is well understood and appreciated.
In smelting plants there is always more or less flue dust, fine ores,
and sometimes fine concentrates to be treated, but the charging
of such fine material directly into a furnace would cause trouble
and irregularities, and would lessen its capacity also. As mineral
briquetting cannot be effected without considerable wear upon the
machinery and without quite appreciable expense in binder, labor, and
handling, many smelters try to avoid it.
The financial question, however, is not as serious as it may at first
appear, and taking the large output of modern briquetting machines in
consideration, the cost for repairs amounts only to a few cents per ton
of briquetted material. The total cost depends in the first place on
the cost of labor, power and the binder, and in most American smelters
it varies between $0.65 and $1.25 per ton of briquettes.
Ordinary brick presses, with clay as a binder, were used in Europe as
well as in this country, but they are too slow and expensive for large
propositions and the presence of clay is usually undesirable.
The English Yeadon (fuel) press has also been used for some years at
the Carlton Iron Company’s Works at Ferryhill in England, and at the
Ore and Fuel Company’s plant at Coatbridge in the same country; also by
some Continental firms. Dupuis & Sons, Paris, furnished a few presses
which are mostly used for manganese and iron ores and pyrites. In
some localities coke dust is added. The making of clay briquettes or
mud-cakes is the crudest form of briquetting; but while heat has to
be expended to evaporate the 40 to 50 per cent. of moisture in them,
and while considerable flue dust is made, this method is better than
feeding fine ore or flue dust directly into the furnace.
The only other method of avoiding briquetting is by fusing ore fines in
slagging reverberatory furnaces and by adding flue dust in the slagging
pit, thus incorporating it with the slagging ore. This is practised
sometimes in silver-lead smelters, but in connection with copper or
iron smelters it is not practicable.
In briquetting minerals a thorough mixing and kneading is of the first
importance. If this is done properly a comparatively low pressure will
suffice to create a good and solid briquette, which after six to eight
hours of air-drying, or after a speedier elimination of the surplus of
moisture in hot-air chambers, will be ready for the furnace charge. A
good briquette should permit transportation without excessive breakage
or dust a few hours after being made, and it should retain its shape in
the furnace until completely fused, so as to create as little flue dust
as possible. The briquette should be dense, otherwise it will crumble
under the influence of bad weather.
The two presses on the American machinery market are the type built by
the Chisholm, Boyd & White Company, of Chicago, and the briquetting
machine manufactured by the H. S. Mould Company, of Pittsburg. Both are
extensively used, and in many metallurgical plants it will pay well to
adopt them.
From 4 to 6 per cent. of milk of lime is generally used as binder,
and this has a desirable fluxing influence also. A complete outfit
comprises, besides the press, a mixer for slacking the lime, and a
feed-pump which discharges the liquid in proportion into the main mixer
wherein the ore fines, flue dust, or concentrates are shoveled.
The Chisholm, Boyd & White Company’s press makes 80 briquettes per
minute, which, with a new disk, are of 4 in. diameter and 2½ in. hight,
thus giving about 872 cu. ft. of briquette volume per 10 hours, or 50
to 80 tons, depending on the weight of the material. With the wear of
the disk the hight of the briquettes is reduced and consequently the
capacity of the machine also. The disk weighs about 1600 lb., and as
most large smelters have their own foundries it can be replaced with
little expense. About 30 effective horse-power is usually provided for
driving the apparatus. The machine is too well known to metallurgists
and engineers to require further comment or description.
The H. S. Mould Company has also succeeded in making its machine a
thorough practical success. This machine is a plunger-type press. The
largest press built employs six plungers, and at 25 revolutions it
makes 150 briquettes of 3 in. diameter and 3 in. hight, or 1080 cu. ft.
per 10 hours. Its rated capacity is 100 tons per 10 hours.
In using a plunger-type press the material should not contain more
than 7 per cent. mechanical moisture. If wet concentrates have to
be briquetted it is necessary to add dry ore fines or flue dust to
arrive at a proper consistency. The briquettes are very solid and only
air-drying for a few hours is necessary.
The cylindrical shape of briquettes is very good, as it insures
a proper air circulation in the furnace and consequently a rapid
oxidation and fusion.
The wear of the Mould Company’s press is mostly confined to the chilled
iron bushings and to the pistons. Auxiliary machinery consists of
the slacker, the feeder and the main mixer. The press is of a very
substantial design, and it is claimed that the cost of repairs does not
amount to more than 3c. per ton of briquettes.
Wear and tear is unavoidable in a crude operation like briquetting; to
treat flue dust, ore fines, and fine concentrates successfully, it is
almost absolutely necessary to resort to it.
Edison used a number of intermittent-acting presses at his magnetic
iron-separation works in New Jersey, but this plant shut down some time
ago.
A BRICKING PLANT FOR FLUE DUST AND FINE ORES
BY JAMES C. BENNETT
(September 15, 1904)
The plant, which is here described, for bricking fine ores and flue
dust, was designed and the plans produced in the engineering department
of the Selby smelter. The machinery contained in the plant consists of
a Boyd four-mold brick press, a 7 ft. wet pan or Chile mill, a 50 h.p.
induction motor, and a conveyor-elevator, together with the necessary
pulleys and shafting.
The press, Chile mill, and motor need no special mention, as they all
are from standard patterns and bought, without alterations, from the
respective builders. The Chile mill was purchased from the builders
of the brick press. The conveyor-elevator was built on the premises
and consists of a 14 in. eight-ply rubber belt, with buckets of sheet
steel placed at intervals of 6 in., running over flanged pulleys. The
buckets, or more properly speaking the flights, are made from No.
12 steel plate, flanged to produce the back and ends, with the ends
secured to the flanged bottom by one rivet in each. The plant has been
in operation for sixteen months and there have been few or no repairs
to the elevator, except to renew the belt, which is attacked by the
acid contained in the charges. This first belt was in continuous use
for nine months. As originally designed, the capacity was 100 tons per
day of 12 hours, but this was found to require a speed so high that
the workmen were unable to handle the output of the press. The speed
was, consequently, reduced about 25 per cent., which brings the output
down to about 75 tons per day. This output, as expressed in weight,
naturally varies somewhat owing to the variation in the weight of the
material handled.
It is probable that the capacity could be increased to about 90 tons
by enlarging the bricks, which could be done, but would require a
considerable amount of alteration in the machine, as it is designed to
produce a standard sized building brick. By this method of increase,
however, the work of handling would not be materially increased,
because the number of bricks would be the same as with the present
output of 75 tons; there would be about 16 per cent. more to handle,
by weight. Working on the basis of 100 tons capacity, the bins were
designed to afford storage room for about three days’ run, or a little
over 300 tons. The bins are made entirely of steel, in order that
the hot material may be dumped into them directly from the roasting
furnaces, thus saving one handling. In order that there may be room
for several kinds of material, the bins are divided into seven
compartments, three on one side and four on the other. The lower part
is of ⅜ in. steel plate, and the upper, about one-half the hight, of
5/16 in. plate.
It may be well to call attention to the method of handling the
material, preparatory to its delivery to the brick press. The bins are
constructed, as will be seen by the drawing, with their floor set 2.5
ft. above the working floor, which enables the workmen to reach the
material with a minimum effort. The floor of the bins project 2.5 ft.
in front of the face, thus forming a platform on which the shoveling
may be done without the necessity of bending over. In this projecting
platform are cut rectangular holes 12 × 18 in., which are placed
midway between the openings in the front of the bins and furnished
with screens to stop any stray bolts or other coarse material that
might injure the press. This position of the holes through the platform
was adopted so that, in the event of the material running out beyond
the opening in the face, it would not fall directly upon the floor.
Two buckets are provided, with a capacity of 7 cu. ft. each, which is
the size of a single charge of the Chile mill. These buckets have a
hopper-shaped bottom fixed with a swinging gate which is operated by
the foot; thus the bucket can be run over the pan of the Chile mill and
the charge dumped directly into it. The buckets run on an overhead iron
track (1 in. by 3 in.) hung 7 ft. in the clear, above the floor.
The method of making up the charge is as follows: The bucket is
run under the hole in the platform nearest to the compartment
containing the material of which the charge is partly composed, and
a predetermined number of shovelfuls is drawn out and put into the
bucket, which is then pushed on to the next compartment from which
material is wanted, where the operation is repeated. After charging
into the bucket the requisite amount of ore or flue dust, the bucket
is run to the back of the building, where the necessary amount of lime
(slaked) is added. By putting the lime in last, it is so surrounded by
the dust or ore that it has not the opportunity to stick to the sides
of the bucket in discharging, as it otherwise would.
[Illustration: FIG. 1 (_a_).—Plant for Bricking Ores, Selby Smelter.
(Plan.)]
The number of men required to operate the entire plant, exclusive
of those employed in bringing the material to the bins and emptying
the cars into them, is 12, placed as follows; One preparing the lime
for use, one removing the charge from the mill and supplying the
elevator-conveyor, which is accomplished by means of a specially
shaped, long-handled shovel; one keeping the supply spout of the press
clear (an attempt was made to do this mechanically, but was found to be
unsuccessful, owing to the extremely sticky nature of the material, and
so was discarded in favor of manual labor); one to control the press in
case of mishap and to keep the dies clean; one oiler; three receiving
the bricks from the press and taking the brick-loaded cars from the
press to the drying-house, and two placing the bricks on the shelves.
[Illustration: FIG. 1 (_b_).—Plant for Bricking Ores, Selby Smelter.
(Elevation.)]
The drying-house scarcely requires description; it is but a roofed
shed, without sides, fitted with stalls into which the bricks are set
on portable shelves, as close as working conditions will permit. The
means of drying, at the present time, is by the natural circulation
of air, but a mechanical system is in contemplation, by which the
air will be drawn into the building from the outside and forced to
find its way out through the bricks. The drying-house is adjacent to
the pressing plant, in fact forms the back of it, so that there is a
minimum distance to haul the product. The time required for drying the
bricks sufficiently for them to withstand the necessary handling is,
depending on the weather, from two to eight days, the usual time being
about three days.