January 26, 1917.

THE COLLIERY GUARDIAN.

173

leads to the reduction into fine sizes of a certain por-
tion of any particular seam, the percentage depending
on the height of the undercut; that is, on the particular
machine or method used.

The cuttings obtained, called “ bug dust ” by the
miner, are generally assumed to consist only of the finest
sizes of coal, and on account of a high percentage of
dangerous dust contained, are often loaded out before
breaking down the face of the seam undercut. There-
fore, cars of bug dust are generally at the foot of the
shaft when hoisting begins in the morning; conse-
quently, the first railroad cars of screenings loaded may
contain more than the usual percentage of this fine coal,
which is often of lower grade than the regular screenings.

Snubbing and Drilling. — Snubbing is the practice,
before shooting, of cutting a triangular section from
18 in. to 3 ft. high from the lower face of the coal above
the under-cutting. If this is not done, the shooting is
likely to loosen the coal only enough to fill the space
undercut, and, unless excessive powder is used, does
not break it sufficiently to allow easy loading. Proper
snubbing causes the coal to roll and spread out in the
room with a minimum of powder.

The amount of snubbings made is less than 3 per
cent, of the average seam, and since the work is often
done with a hand pick, and there are two free faces to
break to, the relative sizes of the snubbings are large.
No determinations of the sizes have been made. Since
from only two to five 2 in. drill holes are bored to a
depth of from 5 to 8 ft. in a single face, the percentage
of fine coal made in this way is too small to be con-
sidered. These cuttings, however, contain a consider-
able percentage of fine dust. One per cent, or less
screenings probably represents the fine coal made by
snubbing and drilling.

Breaking the Coal from the Face.—By the method of
applying roof pressure or wedging as in longwall mines,
the eoal breaks slowly and into blocks, governed by the
resultant direction of pressure and by the larger inci-
pient cleavage planes, and consequently loosens ready
for loading with a minimum of breakage. At certain
longwall mines as small an amount as 15 per cent,
screenings (1J in. round hole or Jin. bar) has been
reported after loading, hauling, hoisting, and dumping
over the screen. On the whole, the longwall field shows
15 to 20 per cent, more lump coal over 1| in. screen as
compared with the rest of the State. These figures agree
with those gathered by the author at individual mines.
Although the question is somewhat complicated by par-
tial hand pick under-cutting at some of the longwall
mines, the conclusion seems logical that the amount of
soreenings in the coal at the face need not be greater
than from 5 to 10 per cent.

Fig. 4.—Steel Feed Hopper.

Handling the Coal : Loading.—Loading the broken
coal into the mine car in Illinois is done with a hand
shovel, except in two stripping mines, at which the
broken coal is loaded with small steam shovels. At
only one mine, near La Salle, is the seam inclined
enough to necessitate the use of chutes or other re-hand-
ling devices. Breakage from this cause, which is often
a serious question in other States in which the seams
are highly inclined, is therefore at a minimum.
Whether or not the miner loads his coal in the largest
possible lumps or re-breaks it, either by more powder,
by pick, or by the back of a shovel, into' sizes convenient
for shovelling, is a somewhat disputed point. It is not
uncommon, however, to see mine cars containing lumps
possibly as large as 12 in. by 18 in., 2-J-ft. long, and
weighing 3001b.; as large, in fact, as the two men in
the room can lift into the ear.

Haulage.—The coal, having been loaded into mine
oars, is moved to the foot of the shaft by mule or
electric locomotive, or both. At times the writer has
heard complaint about breakage caused by such transit,
especially if in making a trip the locomotive bumps the
loaded cars, jarring the contents considerably, and even
breaking the top load.'

Probably the custom of excessive topping, or loading
the coal to too great a height above the top of the car,
is responsible for most of the breakage in haulage.
Bumping easily knocks off lumps from a poorly-topped
car, which fall on the roadway, and become broken. At
comparatively few mines in the State is the load on the
car limited; these few, however, have a maximum
limit. As an illustration, at one mine the car is sup-
posed to hold 4,0001b., and if any car-load weighs over
4,5001b., the excess weight goes to the checkweighmam

fund. Such a rule is intended to prevent loss Bud
breakage caused by excessive topping of cars. It is
probable that breakage from actual car transportation,
whether below ground or in railroad cars, has been
rather over-estimated.

Dumping into Skips at the Shaft Bottom.—At nearly
all the mines in the State coal is hoisted to the surface
and into the tipple in the mine ears. Attempts have
been made to dump the coal from the mine car at the
bottom of the shaft into skips, which are hoisted into
the tipple, and there dumped automatically. When
one mine abandoned this system for the ordinary self-
dumping cage, am estimated gain of 10 per cent, in the
coarser sizes resulted. A mine in the longwall field
using skips produces 15 per cent, more screenings over
the same size of screen than the average of six other
mines in the district using cages. In Franklin county
a mine operating 7-ton skips reports 6 per cent, more
1| in. screenings than the average of 13 other mines in

Fig. 5.—Lip Screen.

the district, and only 12 per cent, of 6 in. lump against
an average of 22 per cent, for the other mines men-
tioned which hoist the mine car in self-dumping cages.

Breakage in Surface Plants.—Final preparation and
sizing in the tipple are carried on under the advantages
of daylight and easy access to, all parts of the plant;
arid the success attained depends largely upon the
design and erection of the tipple building, ohuteis,
screens, conveyors, loading devices, ete. The general
types of Illinois coal mine tipples or surface plants have
already been described.*

Breakage in Self-Dumping Cages.—In older
designs, the cage, after being brought to a
quick stop, is tipped suddenly with consider-
able throw and jerk, throwing the coal out of
the tipped car. At the required angle for
dumping, the coal at the top end of the car
is frequently from 6 to 8 ft. above the dump
shoe, thus giving a considerable momentum
to the outgoing load. If the shape of the
dump shoe and connecting chutes is care-
fully designed, the coal will slide out with a
minimum of breakage and danger.

In the interests of safety, as well as break-
age, more care should be taken, to prevent
the coal being dumped from missing its
proper chute, and falling either down the
shaft or to the surface around the head
frame. It is not unusual to see considerable
ooal falling when dumping takes place.

Dumping into Weigh Box and on to the
Screen.—The coal on being dumped from
ths car, slides down a short dump chute into
the weigh box. In one of the older tipples,
a dump chute, 6 ft. long, and set at an angle
of 75degs., gives an additional falling height
to the coal of about 5 ft. At this mine 4
per cent, more screenings are produced
than at a neighbouring one with a modern
tipple. The increase in screenings can be attributed to
such defects. The tipple breakage is even apparent to
the eye.

The, slope, of the bottom of the type of weigh box, pain,
hopper, or basket, illustrated in fig. 3, is usually 35 to
40degs., and since the upper end may be from 10 to
12 ft. above the lower end, the distance moved, and the
speed acquired by the coal may cause considerable
breakage before it comes to rest, and is ready to be
weighed. This is evident from the flying bits of coal
noticeable in the usual type. The weigh box or basket
is usually made large enough to hold two or even three
loads from the mine car, and although this means waste
space, lost headroom, and chance for extra breakage
if used for single weighings only, the large capacity
allows the hoisting to proceed continuously, even
though the screens below may be stopped.

Weigh boxes with a more gentle Slope, or at least a
changing and decreasing slope, which would bring the
coal to rest without a shock, and then would deliver it
on the screens below in a more gradual manner, would
lie an improvement. Weigh boxes of this typo have
been designed, but complaint is made that they tend
to decrease the speed of operation. A weigh box having
a bottom sloping at a low enough angle to allow a car
of dry lump to slide gently may not be steep enough to
handle a car of moist or of fine coal, and if the angle is
great enough to allow wet coal or slack to run, lump
coal usually acquires considerable velocity in passing
down the incline.

Use of Screen Feed Hoppers.—The custom of allow-
ing coal from the weigh box to discharge, either at the

* Colliery Guardian, January 19, 1917, p. 121.

end or bottom, direct to the shaking screen is meeting
with disfavour for the following reasons :—

(1)	A drop between the weigh box and screen is a
factor in the amount of breakage; in certain, tipples a
drop of 3| ft. has been noted.

(2)	Coal is weighed in from 2- to 5-ton lots, and when
this amount is dumped suddenly on the upper screen,
it throws an extra weight upon it, which seriously affects
the balance of weight that should be maintained between
the upper and lower screens, and gives additional vibra-
tion to the screen structure.

(3)	The sudden rush of material from the weigh box
frequently chokes the fine screens (which usually come
first), and allows considerable fine coal to be carried into
the coarser sizes, damaging their appearance.

(4)	The sudden load thrown on the screein puts an
undue strain on the driving belt, causing slipping unless
it is laced to a considerable and often damaging tension.
At one mine two engines are used to drive the screens,
thus avoiding the dead centre troubles, and lessening the
strains just mentioned. This scheme has not been
adopted generally.

(5)	Any stoppage of the screens for the purposes of
repair or to shift railroad cars causes the cessation of
hoisting after one or two dumps have filled the weigh
box; that is, there is no storage capacity between weigh \
box and screen.

For these reasons, steel feed hoppers (fig. 4), with
sloping sides, and holding from two to four mine cars
of coal, are, in many of the newer installations, placed
under the weigh box and at the head of the screen.
The bottoms of these hoppers have a reciprocating
motion, due to an adjustable crank arm, the full stroke
being from 8 to 12 in., and the speed from 40 to 60
revolutions per minute. The lower part of the end of
the box next to the screen is cut away, thus allowing
coal to fall on the screen gently and regularly with each
back stroke of the bottom. The partial load of coal
usually in these hoppers reduces the breakage, since a
new load from the weigh basket falls gently on to coal
instead of striking steel. If such a feed hopper cannot
be used on account of a lack of sufficient head room,
some form of conveyor with sides may accomplish the
same purpose.

. The most serious objection to the use of a feed hopper
is that when two or three mine cars of coal are in it nt
the same time, and the coal is being mixed and fed
coaiibinuously on to the screen, it is impossible for a coa}
inspector to property single out individual cars of dirty
coal. Some companies claim that by good engineering
design of the various tipple appliances these feed hoppers
may be dispensed with.

Screen Breakage : Gravity Bar Screens. — Usually
there is a dead or blank plate at the top and bottom of
the bar screen, the former to spread out the coal before
screening, the latter to collect it into the loading chute
to the railroad car. Since at certain times the coal may
be damp or extra fine, or weather conditions unfavour-
able, a considerably greater slope is given the screens
than is necessary to allow dry lump to screen by gravity.
The result is that a lump “attains considerable velocity
on the soreens, and suffers breakage when allowed to
shoot directly into a railroad car. It is difficult to See
haw a car of commercial lump of good size and appear-
ance can be made under such conditions, unless a simple
steel adjustable chute is installed between bar screen
and railroad car to slow up the coal, and allow it to fall
more gently into the car. The breakage is frequently
increased by the chains, car wheels, or short logs of
wood which are hung above and about half-way down
the screen, for the purpose of checking the flight of the
coal, or of turning the lumps in order to free them from
adhering fines. Also lumps frequently become' wedged
in the bars, and upon being struck by succeeding lumps
are broken, and drop into the soreenings. At many
tipples at which bar screens are used to prepare coal for
special purposes not in commercial competition with
lump coal from adjoining mines, breakage is of secondary
importance; the; bar screen being used to free the lump
from the impurities in the slack rather than from the
slack coal itself.

Shaking Screens.—The great variance in speed, in
slope, and in size of holes, found in the different designs
of shaker screens, cause differences in breakage and
freedom of one size from another. In most designs the
small holes come first, and often become clogged by
sudden rushes of coal. The best results are obtained
at the mines at which this fine screen is protected by
a false one of larger opening placed about 1 ft. above it,
for the purpose of providing against overloading and
crushing by the bulk of the lump coal. Where drops
of more than 1 ft. are allowed in passing from upper to
lower screen, some splintering of large lumps can gener-
ally be detected.

On a number of screens, lumps of coal fall into the
larger screen holes, and are not dislodged until worn or
broken enough to pass into the undersize. This fault-
may be remedied by increasing the slope, speed, or
length of throw of the screen, or by a combination of
these points.

Revolving, Trommel, or Roller Screen Breakage.—On
account of the breakage caused by roller screens, they
are not used in Illinois tipples for the screening of lump
coal. They are,, however, commonly used, especially in
the southern part of the State, for producing the smaller
sizes of coal, and, indeed, they were the only kind of
screens used in re-screening plants till about 1910. Now,
they are being replaced in the newest re-screeners by
the Parrish screen or other approved shakers designed
especially for small coal. Roller screened coal can often
be distinguished by the rounded corners of the indi-
vidual pieces, showing that attrition has taken place in
the screen, with a consequent production of the' dust
sizes.

The severest criticism of the roller screen is that in
shape and in general principle of action it is similar