172

THE COLLIERY GUARDIAN.

January 26, 1917.

the picker cannot readily detect the different sizes of
impurities mixed with the different sizes of coal. The
more uniform the size of the coal that passes the picker,
the better the impurities can be removed. A recently-
built dock cleaning plant sizes the coal for picking pur-
poses, and then re-unites the sizes to meet the demands
of the market.

In many places in Illinois hand-picking is practised
on the lump size only, in others the 3 in. egg is the lower
limit. It is probable that unless the refuse in Such sizes
had some special value, it would not pay to mine and
hand pick coal in Illinois containing more than from 3 to
5 per cent, of refuse in the large sizes. Fortunately, in
most districts the bulk of refuse passes into the finer
sizes.

At present washing is practised on 3 or 3-1 in. coal as
a maximum size. This is partly on account of the
mechanical difficulties encountered in building jigs for
washing larger sizes. The average amount of impurities
removed from the washed coals in 35 plants examined
was 11 per cent., with a maximum of 36 per cent., and a
minimum of 5 per cent. It is difficult to estimate an
exact limit for commercially profitable washing, because
of the varying character of the refuse and its degree of
freedom from contained coal. On the whole, 31 in. is
the maximum size, and about 7 per cent, refuse, con-
taining at least 60 per cent, of ash or its equivalent,
represents a possible minimum of removable impurities.
(Under European conditionis, this limit has been put at-
from 4 to 6 per cent, impurities.) As a lower limit,
coal under | in. in size is benefited little by washing,
and under from 20- to 50-mesh probably not at all.
Considerable doubt exists as to the proper method of
handling the small sizes of Illinois coals, on account of
the frequent occurrence of a. high percentage of ash.

At a considerable number of mines, especially in the
southern part of the State, where re-screening plants are
used for the separation of nut coal and screenings, all
sizes below from 2 to 3 in. are divided into as many as
five distinct sizes. Most of the impurities generally
stay with the finest of these sizes, leaving two or three
of the largest sizes of nut coal practically as clean as
the lump. At several mines one or two men were noted
in these re-screeners picking the largest sizes of nut,
perhaps from 3 to 1| in. in size. Although no figures
of amounts picked were obtainable, it is evident that
the large number of pieces of this size that must be
picked out of the coal to produce a ton of refuse makes
the problem of doubtful economic value. If two men
are picking 100 tons per day of such nut, the extra cost
is roughly 5c. per ton. From the ash standpoint, one
man must pick one ton per day of this fine material to
reduce the ash content 1 per cent. The question, does
the comparatively small amount picked pay for the
better appearance, must be solved for each coal and each
market condition.

Impurity and Inspection Standards.—Coal inspection
is becoming more rigid, and large users, such as rail-
roads, frequently have their own inspectors, even at the
mines of the producer. Much coal is inspected by the
buyer by a hasty examination of the top of the railroad
car; but to secure the best results, more strict exami-
nation, such as inspecting the inside of the load while
it is being loaded, is necessary. If hopper-bottom rail-
road cars are used, and delivery is made through these
to a bin or a stock pile, individual car inspection, is
difficult.

A set of impurity and inspection standards has been
proposed for railroads using bituminous coal, as follow :—

The seller further agrees that all coal delivered under this
contract shall not contain more than....per cent, removable
non-combustible or nearly non-oombustible impurities. The
quality of coal furnished....shall be subject to the inspection
of the buyer, and the buyer’s inspector....has the right to
reject any of said coal which____does not conform to speci-
fications, at whatever point the same may be found. The
buyer’s inspectors shall have access to tire seller's tipple
screens, scales, washer, and yards while the coal is being
hoisted and prepared.

All coal delivered....may be inspected at the mines by
the....(representative) of the buyer....such inspection and

refusal to be final and conclusive.

A further paragraph covers the discovery of inferior
coni after shipment when the same was not inspected
at the mines.

Besides individual contracts, the most specific stan-
dardisation of Illinois and other Middle-West bituminous
coals in regard to the percentage of non-combustible or
nearly non-combustible impurities allowable has been
used by several railroads drawing their supply from
mines in the territory mentioned.

Breakage.

Bituminous coals vary from 0'5 to 2’0 on Mohr’s scale
of hardness. They ere, as a rule, decidedly brittle and
friable, tending to break into more or less cubical blocks,
depending on physical structure, such as frequency of
bedding laminae or planes of stratification, and develop-
ment of cleat or vertical cleavage. Cleat may be
defined as the tendency for the coal to break into more
or loss cubical blocks along vertical or highly-inclined
planes either parallel or normal to the face of the coal.

Natural Physical Factors Causing Breakage.-—Illinois
coal may be considered comparatively hard, rather
porous, more or less brittle, and friable; tending to break
or split easily along definite major bedding lamin® 5 to
12 in. apart, and somewhat less easily along small or
minor bedding planes. Vertical cleavage, prominent
in many bituminous coals, is generally lacking, or is
developed only incipiently. This incipient cleavage is
more marked in one direction than in the other, the
lump coal usually being rather smooth and regular on
the breakage faces in two directions, but tending to be
irregular on the third face. This incipient cleavage
<letermines the lines on which the coal frequently breaks
into small blocks on first being heated. This cracking,
which is probably due to shrinkage caused by the heat

expelling the water, is also often more strongly
developed in one vertical plane than in the other.

There is considerable difference in the natural ten-
dency towards breakage in the different seams and in
the different districts. The coal of seams No. 1 and
No. 2 in the northern field are of average hardness
(firmness or cohesion); an example appears in the coal
from seam No. 2 in the La Salle district, which breaks
into rectanigulair lumps capable of withstanding much
handling before breakage. The coal from seam No. 5
in the Springfield and Peoria districts is harder and
firmer than the average. Seam No. 6, west of the
DuQuoin anticline, although covering a large area, is
generally .regarded as somewhat friable, especially in the
north-east and south-west portions, and also in those

Fig. 2.—Hand-picking Egg Coal whilst Loading.

mines just west of the anticline, in which it is
close to the surface. Seam No. 6, east of the
DuQuoin anticline, and in general wherever it is
situated under several hundred feet of cover,
consists of a hard firm coal, especially
in the smaller sizes. This coal • with-
stands without breakage handling 'and ship-
ment, and therefore forms a good domestic fuel
in markets in which clean coal and close sizing
are pre-requisites. The coal of seam No. 5 in
Saline county is also of a firm texture, and well
adapted for re-screening. Seam No. 6 in the
Danville district contains rather friable coal.
Seam No. 2 (Big Muddy), partly by reason of
the cleavage noted, produces blocky lumps of a
bright lustre. The coal of seam No. 7, Dan-
ville district, on bhe whole, is softer and more
brittle than the others, perhaps on account of
its nearness to the surface. It cannot undergo
transportation as well as the average. It is
interesting to note that in the deepest mine in
the State, at Assumption, a cotal is produced
from seams Nos. 1 and 2, which commands a
premium, partly on account of its blocky firm
character.

Importance of the Breakage Problem.—Aside
from the inherent friability or tendency of the
coals to break, the mining and prepar-
ations, or, in general, any moving, dropping, or
handling of a brittle substance like Illinois coal,
results in more, or less breakage with each operation.
Breakage, therefore, commences with the first opera-
tions of mining, continues with each succeeding opera-
tion, and ceases only when the coal is consumed. This
gradual breaking of such a material into smaller sizes due
to handling, exposure, etc., is called degradation.

None of the problems confronting the Illinois mining
industry to-day is more important commercially than
that of breakage. Although this problem, as far as sur-
face preparation is concerned, could be solved so that a
considerable increase in value per ton of the whole pro-
duct would result, yet it has been neglected in all except
a few plants recently erected. Breakage constantly
reduces the larger sizes to form the smaller. The effect
of this sizing on the prices received for the coal by the
operator has been calculated to reduce the value (differ-
ence between lump coal and screenings) by over 50 per
cent. Thus it is advantageous to produce the free burn-
ing Illinois coal in as coarse lumps as possible, each per
cent, of lump gained (or each per cent, of breakage
reduced) being worth O'77c. per ton to the operator. At
a mine at which 3,000 tons per day are produced, this
means 23-10 dols. per day, or nearly 600 dots, per month
saved.

In the tipple, a long, high angle dump chute, a poorly
designed weigh box, or a drop of several feet on to the
screens, may cause the breaking of several per cent, of
lump coal into the finer sizes. At some mines the sound
of the coal when dumjied from the car is audible above
the rest of the surface noises at a distance of several
hundred yards from the. tipple. The louder the noise,
the greater the breakage. In some cases the dumping

takes place with ®uch force that the coal actually seems
to be thrown from the oar through a distance of from
12 to 15 ft. into the bottom of the weigh box, in which
it is more or less broken by the rest of the load falling
upon it. While the great speed of such apparatus, in
some cases three dumps per minute, is a notable achieve-
ment, good engineering demands a closer study of the
problem of decreasing the breakage during preparation.

Theoretical Considerations of Breakape. — Considered
from a theoretical standpoint breakage of coal takes place
by splitting along planes of minimum strength, rather
than by actual .shearing or crushing of the coal substance
itself. Thus the actual measure of the force in pounds
per square inch required to crush or break coal is
extremely variable. The application of Rittinger s
theory that the work necessary to break or crush
minerals is proportional to the reduction in diameter,
shows, for example, that three times as much power
must be expended in breaking 6 in. cubes into 1-1-in.
cubes, as in breaking 6 in. cubes into 3 in. cubes.

The kinetic energy developed in any lump of coal
being moved—in other words, the force with which one
piece strikes another and causes breakage—-is directly
proportional to its weight and to the square of its velo-
city, or if dropping, directly proportional to the distance
through which it is dropped. In simpler language, a
piece thrown or moved with twice the speed of another,
has four times as much j>ower to break or to shatter.
The practical effects of these larwis will become evident
when breakage in bins, in chutes, and in other surface
plant devices is discussed. Since force is proportional
to weight, the general effect of size or weight is that
barge pieces striking on their own ragged edges sustain
breakage from a drop that will not affect the smaller
sizes.

If coal is dropped piece by piece from a height of from
10 to 15 ft. upon iron or steel, it will show from three
to four times as much breakage as if dropped on to
wood. It is claimed, in unloading a bin or a car, that
the first drop only causes breakage, and that after the
pile below has started there is little breakage. Pro-
bably the breakage is greater in the dumping of large
volumes than in the dumping of small volumes of coal.
Still another point is the effect of squeezing and rubbing
of the particles against one another in a bin from which
coal is being drawn from the bottom. If other condi-
tions are equal, the actual breakage is greater in hand-
ling or transporting sized coals than in handling or trans-
porting run-of-mine coal, because the fines in run-of-
mine coal form a bed or nest upon which the large sharp
pieces of lump may ride without breakage.

General Analysis of the Breakage Problem. — In

Fig. 3.—Weigh Box.

general, during mining and preparation of a coal,
breakage may occur in any of the following necessary
operations :—

A. Breakage
in the mine
incident to

f	fl.	In longwall

mines.

a.	Undercutting.	j	2.	Shooting after

b.	Snubbing and drilling.	J	undercutting

c.	Breaking the coal from	i	and solid

J the face.	I shooting.

I 3. Permissible ex-

l plosives.

(1. I oading.

2. Haulage.

d.	Handling ..........3. Dumping into

' a skip at shaft
L bottom.

' e. Use of self-dumping

B. Breakage
in surface)
plants inci-|
dent to I

j. Washing.

cage.

f.	Dumping into weigh

box and on to the
screen.

g.	Screening.

h.	1 oading ............

i.	Kescreeners and bins.

("1. Passage over
loading chutes,
aprons, or
■( booms.

I 2. Loading and
trimming rail-
L road cars.

C. Breakage in transportation andre-handling.

Breakage in the Mine : Under-cutting.—About 52 per-
cent. of the tonnage in Illinois is produced in mines in
which under-cutting is practised either by hand picks,
by machines of the puncher type, or by electric chain
machines. The last-named type is the most common,
and its use is increasing. Contrary to the practice in
many bituminous fields, under-cutting in Illinois takes
place almost entirely in the coal; the exceptions being
at several mines in the longwall field, in which hand
picks are used to undercut in the clay bottom. This