Charles George Warnford Lock.

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

. (page 23 of 76)
Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 23 of 76)
Font size
QR-code for this ebook


coal free from impurities. Coal-washing therefore becomes m iiiiijlljj
being the only meaus by which dirt can be effectively removed. _J

Moreover, in the case of anthracite, the coal as it oomes from. A|

Digitized by VjOOQIC



NON-METALLIFEROUS MINERALS. 201

mine is hardlj marketable. Being very compact and practically free
bom Tolatile oombostible matter, it bums only at the surface ; it is,
therefore, deemed important to have the lumps as nearly of a uniform
size as possible, so that between them a large amount of surface will
remain exposed to the action of the air without checking the draught
too much, or allowing enough air to pass to cool the coal below the
iffnition-point. In other words, if the pieces of coal of the size of a
cneetnot and smaller are mixed with lumps of the size of an egg, they
fill the air-passages and prevent a free draught. Therefore, one of
the most important points in preparation is to have uniform sizing,
and to make as large a number of different sizes as can be produced
without too great expense. It is also essential to remove all dust, which
is of little or no use at present, and depreciates the value of the coal.

Mixed with pure coal, occur varying quantities of slate, "slate-
coal,'' and ^ bony coaL" The term *' slate-coal " designates lumps com-
fKsed partly of coal and partly of slate, in which the pure coal occurs
m such large masses that, by re-breaking, pieces of pure coal of market-
able size can be obtained economically ; and *' bony coal " designates
hmips in which the coal and slate are so interstratified that they can-
not be separated economically by mechanical preparation, or in which
the impurities form such high percentages that the coal cannot be
Kxmomically rendered more pure by mechanical preparation, although
it mar be used for certain purposes in its crude condition.

The problem is, to remove the impurities as completely as possible.
Of course, when slate occurs in separate pieces, it should be eliminated
without further breaking. But slate-coal must be broken into smaller
|»eoes to separate the slaty portions from the coal. It is generally
nnpoesible to sell all the larger lumps as mined, and machinery must
be provided for breaking them up. As the coal comes from the mines
it should be divided into various sizes, and the free slate in each size
ihould be at once removed, either by hand or by mechanical means.
In the first case, the coal is passed along shutes, at the sides of which
itand men and boys who pick out the slate, and in some cases the bony
■nd slate-coal, and allow the pure coal to pass into the pockets.
Kechanical slating of coal depends upon one or more of three physical
duracteristics of the coal and slate : the difference in their specific
xnvity ; the difference of the forms in which they break ; and the
ufference of their angle of friction, or, in other words, the difference
b the angle of a shute, lined with stone or iron, down which the coal
or ^te wiU slide without any increase of velocity. As a rule, slate
trill not slide down a shute which will carry coal.

Machinery for sizing coal may be divided into two classes — ^bar
gratings and screens, the former having openings much longer than
they are wide, while the latter have them practically of uniform
liameter. Gratings may in some special cases be employed to remove
fast or very fine coal, or when only partial sizing is necessary, or for
krge coal, because long flat pieces can fall through with cubical pieces
of mudi smaller dimensions.

In using the old form of continuous bar gratings, part t>f the dirt
and fine cual is often carried over the bar, and is delivered in the
diute at the lower end instead of falling through ; and as the spaces



Digitized by



Google



202 ECONOMIC MINING.

between the bars are parallel and closed at the lower end, long pieces
often wedge and catch, particnlarlj at the bottom, thus necessitating
frequent cleaning.

Movable or oscillating bars are shorter and much flatter than the
fixed bar of the same cleaning capacity. They act as a regulator or
feeder for the breaker, the amount of coal passing over them per
minute being constant, if the supply is sufficient and the number of
revolutions remains the same ; while, by regulating the speed of the
driving shaft, the quantity can be varied at will, within certain
limits. They allow the men upon the platform to get much nearer
their work without danger. With ordinary fixed bars, the pitch
must be sufficient to allow the coal to slide down freely. This it
often does, with ereat velocity, so that the men must remain on the
side. When osciUating bars are used, the coal can be fed upon their
upper end, from which it is gently carried to the platform, allowing
the men to stand safely in front of the bars instead of on the sides,
and bringing the coal cleaner and with less small stuff to the
platform, thus permitting a better separation.

Screens proper may be either fixed or movable. The former
consists simply of an inclined plane, formed either of woven wire
screens, or punched or cast plates with round, square, elliptical, &c^
holes. The coal is allowed to slide or roll by gravity, not too rapidly,
down the plane; the larger pieces pass over and the smaller fall
through. By placing several screens with openings of decreasing
size underneath one another, or a series with openings of increasing
size, in the same shute below one another, any desired number of
sizes can be made. The objection to these is that their capacity is
limited, the sizing is imperfect, and the screens clog more or less.

Movable screens are of two types. In the first type the screening
surface forms a cylinder and revolves about its axis. This form of
screen has been often described, and is used in almost all the
anthracite collieries. In the other type the screening surface ifl
approximately horizontal, and the motion and action are very simihi
to those of an ordinary hand-sieve. In many cases the screen o
moved backward and forward in an approximately horizontal plane
This motion, combined with the inclination of the sieve, causes thi
coal which is fed on the higher part of the screen to travel graduall]
acTOBS it, allowing the smaller particles to fall through. This fom
has in many cases been adopted for small coals, and has lonfr beei
used in metal mining. In other cases the approximately borizonti
screen receives a gyratory motion, like the motion a moulder gives t
his sieve when screening his sand. Its great advantage is that th
whole surface of the screen is constantly in action, while, in tb
revolving screen of say 6 ft. diam., only about 8 in. of the 16 f
circumference is at any one time in action, unless the screen i
overcrowded, and the revolving of the screen acts like an elevate
and tends to throw the coal back into the screen.

The problem in constructing a gyrating screen, when the Bore«
is to be large and must make a great number of sizes, is to support :
in such a manner that it will gyrate easily and safely, and at tl
same time that it will be self-contained, so that the centrifugal fon



Digitized by



Google



NON-METALLIFEROUS MINERALS. 203

will be oounterbalanoed and will not shake the building. A
saooessfiil method conBists eesentially in supporting one horizontal
plane upon another by means of three or more double cones, while
the motion of syration is given to the upper plate by a orank upon a
ihaft passing trough and joumalled in the lower plates.

The screening surfaces have always circular holes, varying from
5^ to ^ in. diam. Cast iron is sometimes used when the holes are
bu^ but punched steel is preferred, being much lighter. Copper is
used for snudl sizes when the water is very acid.

For breaking coal two methods are used. When the lumps are
large and the pieces of slate attached to them are of such a character
IS to render it economical, the larger lumps are broken by hand, the
men using picks made for that purpose. In this way large pieces of
pure coal or pure slate can often be obtained ; but by far the larger
portion of the breaking is done bv corrugated rolls.

The principle upon which rolls act may be explained as follows.
[o the operation of rolls as ordinarily constructed, i.e. with pointed
^th, the point of one of the teeth inserts itself into a lump of coal
nasing through, and breaks it much as the stroke of a pick would
lo; that is, the lines of fracture radiate approximately from the
>oiDt where the tooth strikes the lump of coal. If two pieces of
ound iron are placed parallel to one another, and at such a distance
rart that a piece of coal will just be supported by them, and if a
oird piece of round iron, placed midway between and in a direction
wallel to and above the other two, is then brought down upon the
oal, the piece of coal will break near the middle like a piece of wood
ubjected to a load in the middle too great for it to bear. The result
f this action is generally to break the lump into two pieces of nearly
be same size. This is the result sought to be attained with the
ormgated rolls, and it is for this reason that the plan of breaking
rom one size as far as possible into the next size oelow, has been
dopted, ujBing separate rolls each time. Experiment has taught
bat, although all sizes below the size which is being broken are
Iways made, yet the most economical method is to break any size as
bc^ly as possible into the size immediately below it, of course at
adi time eliminating all the coal below the size that you wish to
reak, before passing that size through the rolls. If a piece of any
Lze is simply broken as nearly as possible in two, for the next size,
to amount of small coal made is very much less than if the same
ieoe were struck near the centre with a pick and broken into a
amber of fragments. The old practice, which has not entirely
laappeared, was to arrange the rolls in such a manner that putting
kem farther apart or closer together would increase or decrease the
nantity of the lar^r sizes of coal. But where arrangements are
lade to break the sizes successively this is not necessary.

Fot" slate picking usually the ooal coming from the bcreens passes
arwn a simple rirate or trough, and the men or boys are placed
itber above or alongside of it, picking out the slate as tiie coal passes
f * There are three objections to this : — (a) When a laree quantity
r ooal 18 passing, the men can really only pick out the siftte on top,
[Vcli slate being hidden. (6) One and the same piece of coal, having

Digitized by VjOOQIC



204 ECONOMIC MINING.



n



a slaty appearance, may be picked up by each slate-picker, in sneoes-
sion and returned to the shute, thus wasting labour, (c) The work
done by each picker cannot be judged.

For these reasons, a different type of picking shute has been
adopted, consisting essentially (1) of a supply shute which reodve*
the slaty coal ; (2) of intermediate shutes where the picking is done ;
and (3) of delivery-shutes which carry off the coal picked over. The
coal from the screen or jig slides down the supply-shute, on each sid»
of which the intermediates are placed, as close to each other ai
possible, there being room enough between each two picking-shut*
for a man or boy. At the other end of the intermediate is ^
deliverynshute. The supply and delivery-shutes have the eaitf
inclination, but the former is a little the higher, so as to give a sligU
inclination to the intermediate, the axis of which is placed at H
angle of about 8° to 10** with the horizontal, and 25° to 28° with tin
supply-shute. The slate-picker, who sits with his face towards th
upper end of the shute, causes a thin stream of coal to pass in iron
of him, cleaning it thoroughly as it passes. The same coal is handle
by one man only, with this exception, that one or two men are place
at the end of the delivery- shute to inspect the coal, and take out ai
pieces of slate which may have escaped the regular pickox
Immediately over the supply-shute, and supported on iron rods,
the half-round slate-shute, into which the pickers throw their slati
slate-coal, &c. This continues to the bottom, where it is examinei
and the slate-coal is picked out and taken to the rolls to be broksi
up and prepared. ,'

The jigs used for the larger coal consist essentially of a woodi
box, lined with iron plates where the plunger works, and where i
coal is put. In order to guide the water from the plunger to the j
a semicircular row of planks is put in. The coal, rising to the to
is skimmed off by a series of flat strips of iron, carried on two t
of link-belt chain. As the coal is scraped up the inclined plane,
water drains back. At the top of this inclined plane is a small fl
covered with iron, which is nearly hoiizontal, but inclines alight
towards the jig. The coal forms here a pile, and the water dral
from it back to the ji^. As each successive quantity of ooal
brought up by the flights on the chain it pushes a correspond!
quantity, which has been drained, off the other side down the shfl
where it goes either to the picking-shutes to be picked, or directly-
the pocket, if it is (as in the case of the small sizes) already cU
enough. The opening through which the slate passes, is regulated'
elevating or depressing the plate so as to allow the largest pieo«
slate to pass under it

Automatic slate-pickers depend upon the fact that -while
generally breaks into cubical masses, pieces of slate of the 8
length and width are of very much less thickness. Henoe, <
quantity of slate and coal which has been passed through a screen
properly sized, the slate, if placed edge¥ase, will drop through a
over which the coal will pass. There are two types of automatic d
pickers, one intended to be placed in a shute and to be fixed, and
other to be plaoed in the discharge b'p of a gyrating screen and



Digitized by



Google



NON-METALLIFEROUS MINERALS, 205

The fixed slate-picker is of iron cast in one piece, and oonsists
asaenti&lly of a series of V-tronghs, one side of the V being shorter
ind at right angles to the other. The lower half of the casting has a
taper slit in the short side, so arranged that anything lying on the
Long side of the trough and of not too great height can slide out
Lhrongh it. Any lump which is thicker than the height of the slit
prill of course be retained in the trough. The slits widen as they
approach the lower end. The apparatus is placed in an ordinaiy
trough or shute down which the coal slides, receiving pitch enough
fco allow the coal to slide over freely, but with not too great velocity.
\b the coal and slate come down the shutes, each lump places itself
in one or other of the grooves or troughs, which are made a little
wider than the largest lump of the size for which the slate-picker is
to be employed. As the lumps slide down, all the flatter pieces tend
to paas oat through the slit on the side, while the cubical lumps go
[>ver. Should a piece catch in the slit in consequence of the increase
in height towards the end, some one of the pieces which follow will
genendly knock it loose, so that it does not remain and block the
slits. This is an important point. The slits if made parallel would
loon clog. The flat pieces, which are mostly slate, and which fall
throngh the taper slit, pass over a shute or picking-table, where they
ftre examined by a boy, who takes out any flat coal that may have
uume through with the slate.

In the gyrating slate-picker the upper part is done away with,
uid only the part with the slit is used. This is placed on the
liJHcharge-sbnte attached to the gyrating screen, so arranged that the
gyrating motion of the screen h£M a tendency to throw the coal and
Mate against the short high side. In this way the latter is thrown
Dut and passes to a jig or picking-table.

A method of mechanically removing slate used in Wyoming
Donflists essentially of an inclined plane, down which the lumps of
Doal and slate are allowed to slide freely. The plane may be covered
irith iron, stone, or slate. The angle is such that the slate will slide
io^n uniformly while the velocity of the coal increases. There is a
gap at the end of the inclined plane, over which the coal jumps by
firtue of the greater velocity acquired in sliding down the plane,
•rhiltt the slate, moving slowly, drops into it. There are a number
A devices for changing the pitch of the shute, the form of the
>p^fting, <fec.

A recent report by the Mining Institute of Scotland concludes by

winting out that the methods and appliances in use in any one

lic^rict can seldum be adopted as a whole in a similar form in

pother. This applies in many instances to collieries in the same

»t2ict, and even to dififerent seams worked by the same shaft. The

tore of the coal, the associated and interbedded strata, the skill,

BtomSy and prejudices of workmen, the markets to be supplied, the

jying requirements of competition, and the caprice of the public,

,ve all to be taken into account when designing plant for classifying

A deaoing coaL

While coal with marked characteristics can with care be selected
idergronnd so as to be filled separately, no process can be profitably

Digitized by LjOOQIC



2o6 ECONOMIC MINING.

applied UDdergronnd for effeotaally remoying refuse, especially the
smaller particles. To dean coal properly, it must be treated on the
surface.

As a considerable percentage of droRs is made in transit from the
cage to the railway wagon, it is evident that the best resnlts are
got where attention is paid to the form of hntch and tumbler, the
inclination of screens, and the drop into wagons; and this is
specially important in the case of soft coals. The careful hand-
packing of large coal into the wagons, as practised in the Nottingham
district, has advantages.

For effective screening, especially when a large output has to be
dealt with, there appears to be no better contrivance than the aingk
or double jigger, or shaking screen, going at 90-100 strokes p^
minute, and having an inclination suited to the class of coal to be
dealt with. There is a preference for wire-meshing for such
screens at some collieries, and at others bars or perforated platea
are preferred.

For picking, the shaking screen just referred to, or the travelling
band, or both combined, is the most effective and economical — the
band being about 4 ft. wide, 40-60 ft. long, and moving at a speed
of 80-60 ft. per minute, according to the quantity of coal to be passed
Ample length of band allows large coal to be sized and loaded into
separate wagons by hand with despatch and economy.

In every case it is necessary that the coal be delivered regularly
from the tip hopper to the jigger or travelling band. This can be
accomplished by regulating sluices worked by an attendant of
automatically by the intervention of a slow-motion band.
Good light is essential to efficient picking.

A rough rule for deciding l^e number and length of pi<ddng
tables may be stated as follows: — One picking table for every dd
tons per hour of triping output, travelling at uie rate of 40 ft pei
minute, and having a length of 10 ft. for every 3 per cent, of matend
to be picked off, plus 15 ft.

The cost for htbour of this system may be taken at about l^«L-2i
per ton of round coal for every 5 per cent, of material picked out o
that coal.

For round coal, say above 1^ in. cube, the dry process is universaU]
employed, and this process can be successfully applied to nuts iron
say I in. upwards, where the refose does not exceed 2-3, or even 4 pe
cent. ; and the table capacity required, judging from the examples ii
the report, is about one table for every 20 tons per hour, travelling a
the rate of 30 ft. per minute, and having an effective length of 15 fi
for every 1^ per cent, of material picked off. The cost for laboa
will probably be }d.-l^. for every 1 per cent, picked off. BcJan<»
screens, on which the coal is picked, are available only when tfa
amount of material to be picked off is very small, say 1-1^ per cent
For all small under f in., and for dross from 1^ in. downwainda^ witi
more refuse than 2-4 per cent., the wet process is most applicable.

In the wet process it is desirable to have the arrangement bo tha
the small coal can be delivered direct from the screens into A
washing tanks without the intervention of wagons. In all ti



Digitized by



Google



NON-METALLIFEROUS MINERALS, 207

systems of washing, the best fesults are obtained by sizing the small
ooal before it reaches the machine. This can most conveniently be
done by passing it through revolving screens with meshes of varying
size. The snpply and degree of pulsation or agitation of the water
require careful adjustment to suit the various sizes of ooal to be treated,
and the relative specific gravity of coal and impurities.

To remove the refuse from the smaller sizes, say under f in., the
felspar washer is the most effective. The felspar system is the most
valuable where the ooal is crushed before washing and is to be used
for ooke-making.

Where the coal and the refuse approach one another in specific
gravity, it appears that in some cases the trough washer gives the
best results. It is applicable for small quantities only, and requires a
large flow of water and extra labour, but it has the recommendation
of simplicity and small capital cost. It may also be sometimes utilised
as a means of transport where the distance from the pit to the wagons
or coke ovens is considerable.

The Bobinson washer is cheap as regards first cost and upkeep,
and requires little water. It largelv depends for its efficiency on the
attention and skill of the man in cnarge, who may often be tempted
to pass more through it than it can effectually clean.

Speaking generally, more elaborate machinery is effective in avoid-
ing waste in proportion to its cost ; but the capital charges and upkeep
ire alao high in proportion.

Other things being equal, ooal will be washed best with an
ibondant supply of clean water ; but the more water used, the greater
:he risk of fine coal being lost, and the greater the difficulty of filtra-
ion. Water to wash coal for coking shoidd not be often used over
igain, as dirty water dulls the coke.

The particulars furnished as to settling ponds do not give sufficient
lata to justify any definite^ conclusion as to their capacity in relation
o the quantity of coal washed. In most cases no record was kept of
iie quantity of water used ; but settling ponds are a necessity, and
Jxeir capacity will depend on the special circumstances of each case.

There seems no better way of filtering the foul water, after it has
I'AfiBed through the settling ponds, than pumping it on to the rubbish
leap, and allowing it to percolate through.

The washed gum of coal not suited for coking is meantime used
Lmost entirely for firing cx>lliery boilers. Briquettes are made of it
a a small extent, but new outlets are required for this product.

The large quantity to be treated daily, and the varviug nature
od proportions of the coal and dirt to be separated, render washing,
% most collieries, a troublesome process ; and unqualified satisfaction
\ seldom expressed as regards any machine in use. In some cases
be machine may not be quite adapted to the peculiarities of the coal
neated, or it may be over-driven, or not have a sufficiency of water,
r be allowed to get out of repair, all or any of these causes leading
> disappointment as to results.

An example of a coal-breaker is shown in Fig. 83. It is made by
30 Humboldt Engineeriug Works, at Kalk, near Cologne, and is
ptecially suitable for breaking hard coal or anthracite, and coke, and

Digitized by VjOOQIC



2o8 ECONOMIC MINING,

has the advantage that it makes very little small coal and dust, or
nuts larger than required. These advantages are obtained firstly by
splitting the coal by means of pointed pins, instead of crushing it m
usual between rollers or jaws, thus naturally causing much less dust
and slack. The process of breaking is further prevented from going
too far by an arrangement of screens, which separates the nuts which !
have attained the size required in the first division, and prevents them
entering the following divisions, and so on, thus avoiding all nn-j
necessary breaking. The capacity of the machine is also considerably
increased, as the lower divisions have comparatively light work to da
All unnecessary breaking of the finished nuts by intermediate appa-
ratus and repeated tipping is avoided, as the machine delivers the



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 23 of 76)