Charles George Warnford Lock.

Economic mining: a practical handbook for the miner, the metallurgist and ... online

. (page 24 of 76)
Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 24 of 76)
Font size
QR-code for this ebook

finished product after the material has passed through onoe. It
requires very little headroom for tipping and small floor-space. All

Fig. 83. — Humboldt Coal-Breaker.

parts are easily attainable and renewable. The pins are made of th
best tool steel, and last 4 to 5 months without being repointed. Tl
moving parts are well protected from the dust. Usually the«
machines have two divisions for a capacity of 15 tons of ooal pc
hour, and deliver according to the quality of the material : — No-
nuts (3 in. to 2 in. or 80 to 50 mm.), 4 to 6 per cent. ; No. 2 nuts (2 ii
to J in. or 50 to 20 mm.), 60 to 70 per cent. ; fine coal ('under } in. <
20 mm.), 25 to 30 per cent.

As shown in Fig. 83, this machine consists mainly of two stnn
cast-iron side frames, with two or more breaking divisions, which m3
penetrated by the pins fastened on the oscillating arms, at eac
inward stroke of the latter. The swinging arms are driven bv mew
of connecting rods from crank-pins on the bosses of the fly-wbeelB <
both sides of the machine, and the latter are turned on the face

Digitized by



serve as driving pnlleys. Between the frames are the riddles or
shaking soreens, driven by excentrics on the flv- wheel shaft.

So long ago as 1870 dry cleaning of small ooal was saooessfally
accomplished by Hochstrate in Qermany, the object sought being the
production of ** Kndrpel " coal free from dust. The different sizes of
ooal firom sieves were made to fall on an iron plate lying at an angle
of 60*" in such a way that a current of air blown up the plate would
carry off the coal dust into chambers, while tiie heavy ** Kndrpel " coal
fell down the plate with tho heavy pieces of slate, and passed to a
jigger for separation. By this method loss of fine ooal was prevented
(amounting to 8 per cent, by wet concentration), and the fine dust for
coking was delivered to the ovens dry. The economy effected
amounted to 162. 10«. a day on a daily treatment of 470 tons, without
reckoning the prolonged life of the coke ovens.

Figs. 84, 85, illustrate one of the most recent and improved coal-
washing plants erected in this country by the Humboldt Engineering
Works, of Ealk, Germany, for the Fowell-Dufiryn Steam Coal Cb.,
near Aberdare, S. Wales, in which some special difificulties had to be

This washery is intended to wash provisionally 350 tons, and later
500 tons, of nuts and small coal, including dust, per day of 9 hours*
and to reduce the whole or only part to the necessary size for making
first-class coke. The above mentioned quantity of ooal consists of the
mneenings from ordinary bar screens with \\ in. spaces, on which the
pit ooal 18 screened, in previously erected screening plant as found at
most collieries.

The screenings are brought in railway wagons to a self-acting
end-tippler, and tipped on the shoots, of wnich one is for bituminous
and the other for steam coal. By means of sliding shutters these
coals are mixed in fixed proportions in the boot of an elevator, and
lifted by the latter to the large revolving screen 0. In case coals
fibould at some future period arrive on the lower line of rails, on the
oppoflite side of the building, a second elevator has been projected for
a similar purpose.

The revolving screen C consists of 4 concentric cylinders with
perforations of various sizes and pecaliarly shaped spiral divisions
between the screens, which cause the screen to completely empty
its^ at each revolution, and thus avoids all unnecessary breakage of
the nut coals.

The revolving screen delivers 4 sizes of nuts and pea nuts, besides
the small coal and dust. The nuts are washed in the nut washers D,
uid the small coal in the fine washers E.

These latter have felspar beds on the sieves, and the pistons are
worked, as well as in the nut washers, by means of a oifferentiiJ
ijBtem of levers, used in all the Humboldt washers and ore jiggers,
vrhidb causes a quick down-stroke and a slow up-stroke of the pistons,
uiftd which has been found specially efi&cient in practice.

The bodies of the washers, or bashes, are made of cast iron, to
ribiain the greatest possible durability ; and the washers are strongly
Doade in all parts.

The two larger sizes of nuts are conveyed in shoots to the screens,



Digitized by


and tlien fall into the boot of an elevator, which lifts them to storage
bunkers, where they can dry further, and whence they are let off by
means of special loading arrangements into railway wagons, to be
sold according to the requirements of the market. In the majority of
cases it is possible to convey the coals away from the washery on a
line of rails at a lower level, in which case the nut coals fall by gravity
from the washers direct to the storage bunkers, and the elevator is

Fig. 84. — Powell-Duffbyn Coal Washer v.

not required. In the case in question the locality would not allow of
this arrangement.

If the nut coals are required for coking purposes, they are con-
veyed with the coals from the other washers to the elevator J, which
lifts them to a creeper to be distributed in the drying towers or
bunkers. From these the coal is conveyed by means of a creeper t*.
the boot of the elevator 0, which lifts them to the disintegrators
N, whence they are dibtributed by means of a creeper to the sturag*'

Digitized by



bonkera for coking coal Q. These have an arrangement for letting
off the ooal as required into hopper wagons, in which it is con-
veyed to the Collin's coke ovens. A start was made with 50 of these
ovens ; this number will be increased to 80, for which the washery is
amply large.

The shales or stones from the washers are lifted by means of the
elevators B to a bin, from which they are run in small tip- wagons to
the waste heap.


No loss of water, except through evaporation, takes place; the
washing water is allowed to clarify itself in the settling pits W, and
is pumped back to the washers by a centrifugal pump. There is no
stream of dirty water running away from the washery to pollute the
neighbouring watercourses, and no settling tanks besides those shown
are required. All the slimes are separated from the water and mixed
with the coking coal by means of suitable mechanical arrange-

p 2

Digitized by



The whole of the machinery is driven hy means of the 160 h.p.
compound condensing engine T, which drives the main shaft and the
pnmp shaft by means of hemp ropes from the fly-wheel pulley.

Steam is supplied by water-tube boilers which are heated by
means of the waste gases from the coke ovens.

Briquettes, — Of late years there has grown up a very important
industry, which aims at utilising coal du8t and inferior coal, such as
lignite, which in their crude states are not adapted for ordinaiy
grates and furnaces.

It is found that when coal dust is heated up to a temperature of
SOO^-GOO" F., it becomes softened, the bituminous portion undergoing
a degree of fusion sufficient to cause the small particles to adhere
together. Apparently, however, this fact is not relied on in the
industrial manufacture of coal-dust briquettes, for recourse is generallj
had to coal-tar pitch as a cementing medium. Scotch pitch answers
well for this purpose, the tar not being distilled to so hard a pitdi u
in the case of English tar, inasmuch as it is rarely considered worth
while to extract from it the small amount of anthracene oil which it
contains. In order to adapt English pitch for patent fuel making, it
is requisite to soften it by the addition of heavy or creosote oil. This
addition is made at the tar-works. Small coal or " slack " received &t
the works is sifted, so as to separate the pieces of coal from the dust
the former being used as fuel for the furnaces. The pitch is crushed
by passing it between a pair of fluted rollers. The coal dust and the
crushed pitch are elevated separately by means of a " Jacob's ladder"
(endless band with little scoops or buckets affixed to it) to a platform
above, where they are fed, in definite proportions, into the top of the
mixing apparatus. The mixer is a vertical iron cylinder about 8 ft
high and 3 ft. wide, having in the centre a revolving shaft or axis
provided with arms and made to revolve by gearii^g above. This
apparatus is open at the top. As the shaft revolves, steam is thrown
into the lower part of the cylinder, and its effect is to soften the
pitch and to damp equably the mixture of pitch and coal dust The
mixture passes out from the lower part of the mixer in the form of a
soft damp powdery material.

At some works this material is received upon an iron plate, from
which by means of appropriate machinery it is swept into moulds
arranged round a circular horizontal revolving table, and kept con-
stantly wetted with water. Each time a mould is filled, a stamper
comes down and compresses the material into a brick. The brick
of patent fuel thus made is lifted out of the mould by machinery as
the table revolves, and is transferred to an endless band, by means of
which it is carried away to be stacked. At other works the moulding
and stamping are effected by an apparatus which works in a hori-
zontal direction. The mixture falls n-om the cylindrical mixer into a
circular pan from which the stamper is fed, and in order to oool the
mixture in this situation a blast of cold air is thrown upon it.

In England we have practically no lignite, but on the Continent
annually many thousands of tons of otherwise worthless "brown
coal" are converted into excellent fuel as briquettes. Such oosi
requires no binding agent to be added, as it yields on heating

Digitized by



nfficient bitnminoiiB matter to efifeot oohesioo. The whole prooess
Mnnprises three operations : —

1. Crashing and granulating the raw and wet coal ;

2. Drying and heating the crushed material ; and

3. Ck>mpre6fiing the heated coal into briquettes.

The crushing is usually and be>t carried out between two sets of
rteel rollers — one with smooth and the other with closely fluted sur-
bces, and the top set fed with raw and rough brown coal through a
DBcions hopper and shoot. Underneath the lower set a large square
peet-iron sieve is suspended, with a wire bottom compost of two
li&rent meshes, the narrowest above, so as to produce a uniform
BuJl com, w^hilst the coarser part is subjected to re-crushing or used
Oder the boiler. The sieve measures about 6 ft. 9 in. long, 20-26 in.
nde, and the respective meshes are |-i in. and 1-lf in. The sieve
e suspended in an inclined position and moved by a crank, making
tl)OQt 300 to 500 rev. per minute. The finer coal j&om the upper and
unower meshes falls into a shoot, and thence into an elevator, to be
tsised at once on to a higher floor above the adjoining drying apparatus,
(f the coal is of a ligneous texture, a second sifting through a second
md narrower meshed sieve on the upper platform is required, so as tq
^p out all the larger pieces of lignite, which otherwise would greatly
lodanger the subsequent drying and compressing operations by sudden

The drying and heating process aims at evaporation of the greater
put of the water contained in the coal, so as to reduce it to 1 5 per cent.,
\ percentage found most suitable and necessary for compressing the
QDsl into good briquettes. Swelling and sticky coals cause obstructions.
Did require more air and less heat.

Granulated coal can safely be more exposed to hot air than coal of
ft dusty and pyritiferous nature ; the latter would easily ignite when
oposed to a high temperature. A certain class of coal must be even
eooled down before subjected to compression, to prevent stickiness and
Ignition. The real problem for all drying operations is, not only to
eraporate water, but also to induce a uniform liberation of the bitu-
tten; and to these two principal parts of the drying process the
fnatest attention must be paid, in order to obtain well-prepared and
workable coal for the press.

Drying ovens are of very various forms, and may generally be
daiwified according to the heating medium, as follows ; —

1. Heated by fire.

2. Heated by steam.

3. Heated by air.

4. Heated by air and steam.

5. Heated by fire and steam.

Of the first class, a typical example is Jacoby's, which generating
a great heat, is especially adapted for all sorts of bituminous coal,
nqniring a moderate or even high temperature for drying. Feeding,
heat, moisture, and discharge can .be well regulated in it> and it is
built now of such a size that two ovens produce a sufficient quantity

Digitized by



of coal for one pross. They require, however, in general more than
usual constant and careful attention to prevent ignition, and they
cannot be always used with perfect safety for drying very dusty, Bticky^
and swelling coal.

Ovens of the second class, heated by steam passing either throagh
hollow discs or oblong hollow plates, or a perforated shaft surrounded
by coal tubes, are frequently adopted, and at some places with marked
success. Rowold's disc oven is almost of the same construction as the
one first described. Instead of a brick wall the apparatus is housed
in a strong sheet-iron round casing, with four large folding doors, and
the steam is admitted from two hollow columns or standards, opposite
each other, in connection with the hollow discs, through wnidi it
passes around from one side to the other, and thence upwards througb
all the succeeding discs in alternate directions towards the outlet-pipe
into the chimney. The movement of the coal from disc to disc by
scrapers is the same, as also the discharge. Schulz's tubular drum
oven has very simple movements, and is especially suitable and effective
for drying dusty or uniformly small-grained coal, if not sticky or in-
clined to swell.

In the third class the air is heated by waste steam to a temperataie
of about 140°-1 60° F. It is easily accessible, has few moving parts, and
is well adapted for granulated coal.

By the combination of steam with hot air the drying process ii
rendered more uniform, and a higher temperature can be obtained.

Ovens of the first and second classes are complicated and expensiTe
in construction, and not likely to find favour where no facilities exist
for repairs and alterations. The addition of a spacious re-heating
chamber, where the dried coal can be kept at about 160*^-170° F. for
several hours, has been found to produce a much more uniform fuel
Less compression is required when the coal is coarsely granulated
than when it is dusty ; on the average it is 45-60 per cent. The wkter
is generally reduced from 50 per cent, to 16 per cent., and the combus-
tibles are increased from 48 per cent, to 74 per cent, llie aim of the
process is to heat the lignite to the temperature at which tarry matters
are formed, and to pass it to the press immediately before they are
given off. Briquettes made by the combined steam and hot air process
at Zeitz, Prussia, upon analysis gave : —

Moisture 14 percent.

Volatile matter 44*5 „

Fixed carbon 31*7 „

Ash 9-8 „

These are made of pure lignite, measure 6 in. by 2| in. by 1^ in^
weigh 11 J oz., are not easily broken, and form a good fuel for railway,
manufacturing, smelting, or general purposes. Much smaller bri-
quettes are more convenient for domestic consumption.

Coke, — The term " coke " requires no definition ; the term ** breease "
is equivalent to the term " cinders," it is lighter, looser, and mostly in
smaller pieces than coke. Both are solid residues of the distillaticm
or incomplete combustion of coal, and they vary in appearance and
quality with the mode in which the distillation or combustioQ is
effected, and with the nature of the coal employed.

Digitized by



Coke consists essentially of carbon and fixed inorganic matter of the
Boal from which it has been derived ; but it contains also hydrogen,
oitrogen, oxygen, and snlphur (in the state of iron sulphide). The
blbwing analysis of coke is fairly representative : —

Carbon 85*84

Hydrogen 0*52

Oxygen and nitrogen 1*38

Sulphur 0-86

Ash 11-40

100 00

Half the sulphur in the original coal, or thereabouts, is found in the
nke made from it ; the iron bisulphide in the coal be ng converted
mto protoeulphide by the burning of one equivalent of sulphur and
the production of sulphurous acid. So that, as the sulphur varies in
quantity in the original coal, the quantity of sulphurous acid given ofif
ind of sulphur left in the coke will vary.

Of the various kinds of coal (lignite, cannel, bituminous, and
iQthracite) the bituminous variety is alone applicable to the manu-
Sicture of coke or breeze. Bituminous coal is classified into " caking "
md '^ non-caking." The caking much depends on the manner in
which the coal is heated and the degree of heat to which it is subjected.
Kiiw, the non-caking coal of South Staffordshire, if rapidly exposed
to a high temperature, such as a bright-red heat, in a close vessel,
famisheB a pretty solid hard coke. The caking coals, however, are
tbose which are preferred for coke-makini^. Non-caking coal does
very well for making breeze, such as is employed by the nailers and
^in-makers of South Staffordshire and the adjoining part of Worces-
tershire, and it is largely used for this pui-pose in the neighbourhood
of Dudley.

While there are many purposes for which the coke produced in
gtt-making and in other ways is well suited, such bye-product coke,
M it may be called, is not adapted for iron-smelting, yet the chief
oonsiimption of coke is in this last-named industry. For the pre-
INWition of this special coke, the coal is burned in an insufficiency of
»ir, under control, and by two methods — heaps or •* pile«," and ovens.

The old-fashioned method of burning in piles is not much
i^pted now, as the coke produced is softer than that made in
ovens, and less fitted for use in iron furnaces worked with the hot
l>la8t For this mode of manufacture a flat space of ground is
■elected, and on it are raised many piles, made either circular or in
the form of an elongated bank, usually about 4 ft. high in the middle,
Cttch circular pile being about 8 yd. diam., and coking about 20 tons

"Breeze-making" in South Staffordshire is effected in two ways :
cither the non-caking coal of the district is burned in a heap on the
gnmnd (locally termed " hearth-coking "), or in ovens. By the first
method, a large heap of coal is lighted and allowed to bum away in
great measure, and is then quenched with water. In the second
i&ethod a circular oven standing alone, about 10 ft. diam., is used. It
<WMi8tfl of a floor raised about 2\ ft. from the ground, and enclosed in

Digitized by



a domed chamber, whioli has a chimney opening in the centre and a
square doorway on the level of the floor on one side. The process
differs from coking in the free admission of air (the doorway being
open during the whole time), and in the gradual charging of the
oven. The first layer thrown on the floor having ceased to flaie,
another layer of coal is thrown on it through the doorway, and so on
at intervals, the coal is thrown in layer upon layer, until an accumu-
lation to the height of about 2^ ft. has taken place on the floor. The
whole charge is then drawn into barrows and wheeled away to a
heap on the grotmd, where it is quenched with water, sifted by
hand, and then washed by hand by throwing into a tub of water;
the shaly matter sinks to the bottom and the breeze is skimmed ofll

The usual method of coking is in specially constructed ovens.
The simplest form of coke-oven is a flat-bottomed chamber, ardi-
roofed, made of firebrick or other refractory material, provided wi^
two openings, one in the roof to serve as an outlet for the volatile
products of carbonisation and as an inlet for introducing the coal,
and the other in the circumference or wall to serve as a doorway for
withdrawing the coke. Such ovens pass under distinctive names in
accordance with various minor modifications of form and arrange-
ment. The ordinary " bee-hive " oven is of circular form, covered
with a domed roof; the diameter of the floor varies from 8 to 11 ft., and
the height of the dome in the centre from 5 to about 8 ft. The
''rectangular" oven has an oblong floor, say 14 ft. long and 6 or 7 ft
wide (the dimensions varying however), and arched roof. Sometimes
the oven is less than 5 ft. high, and the floor is raised above the level
of the ground outside.

Coke ovens may stand separately or in pairs, side by side or back
to back ; but where coking is carried on to a large extent they are
erected many together, side by side in a row, or two rows may be
placed back to back, so as to form a block of 30-100 ovens, the door-
ways of one row being on one side of the block and those of the other
row on the other side. On the summit of each row in these cases
there commonly runs a tramway to carry slack in wagons for
charging the ovens through their roofs. When it is intended to
draw the coke in one block by machinery, as is done in the case of
some rectangular ovens, the floor rises a little, and narrows a little
also, towards the back of the oven. The doorway of a coke oven
runs from the floor about half-way up the front of the oven, and its
width varies, chiefly to correspond with the mode of drawing the
coke adopted. When the coke is to be raked out, the width of the
doorway is only 2-^ ft. ; but when it is to be discharged in one bloc^
by a mechanical arrangement, it extends the whole width of the
firont of the oven. During the process of coking, this front opening
is either bricked up or closed by an iron plate or door ; or the door
consists of a frame of iron in which bricks are laid, and is raised
when requisite by means of a pulley and counterpoise. Beside the
door there may be only the central aperture or chimney in the roof;
but commonly there is a flue proceeding from the back part of the
oven, by which the products of combustion are carried off^ the oentzal

Digitized by



opening in the roof beine only used for oheurging the oven. Where
he oven Ib oharged by the door there may be no central opening in

Coke oveDB, onoe set to work, are kept at work until it is necessary
stop them for repairs. As soon as posttible after a charge is drawn,
nd before the walls have had time to cool, the oven is re-charged

It is customary to nse for coking in ovens what is technically^
enned ** slack/' uiat is to say, the dusty coal for which a sale is not
readOy fonnd as for lumps of coaL It is also customary, when the
K^ is to be used for iron furnaces, to wash the slack so as to remove
ran it shaly matter and some of the pyrites.

It is ckdmed that ^ bee-hive " coke is so much better for blast-
^tmaoe nae than coke from recuperative ovens, that it pays to throw
kway 40 per cent, of the coal, or whatever else may be the difference
)etwe6n the two systems. This claim was substantiated in a measure,
lySir L Lowthian Bell, who experimented with several thousand
ODS of bee-hive and Simon-Carv^ coke made in a recuperative oven,
i&d fonnd that the bee-hive coke was about 10 per cent, better than
he Simon-Carv^ Sir Bernard Samuelson, however, in repeating
he experiment, did not altogether confirm this result. But the
(xperienoe in France, covering a number of years, is clearly in favour
A the reoaperative oven ; and certainly the loss of 10 per cent, in
Bductng power, as reported by Bell, is not to be taken as condem-
latory of a system that is highly prized elsewhere. In the manu-
icture of ooke there has not been the same scientific and economic
ffogrees as in the mining of coal and in the production of pig-iron

Nearly all the coke in the United States is made in bee-hive ovens,
the attempts to introduce other systems of coking not having been
nooessful commercially. As a rule the gases are not consumed ;
one establishments, however, utilise them for heating purposes,
r&r and ammoniacal liauors are not saved, so that the 40 per cent.
»r 50 per cent, of volatile matter passes off into the air and is wasted*

Diuing the twelve years ending with 1891 the coke production of
the United States was 82,806,438 tons, representing 137,458,687 tons

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 24 of 76)