126

THE COLLIERY GUARDIAN.

January 18, 1918.

the washed coal, whilst the refuse is confined to the
very dense particles of dirt; whereas if the outlet is
opened too frequently some of the coal escapes with
the dirt and only a very high grade washed coal is
delivered.

With such washers as the trough and the inverted
cone washers, there is only the possibility of varying
the supply of raw coal and water and of regulating
the removal of the refuse. If the supply of raw coal
or water is too great to either the trough or inverted
cone washer, the washed coal contains a large quantity
of dirt, whereas the refuse is confined to the denser
dirt particles. A similar state of affairs is brought
about by releasing too little refuse. • On the con-
trary if the supply of raw coal or water is too small
the usual openings of the refuse valves discharge a
refuse which contains a high percentage of coal. The
washed coal is then of the highest grade. Releasing
the refuse too rapidly produces the same result.

Removal of Impurities.

From the point of view of washing the coal the
impurities may be divided into four classes : (1) Inter-
mixed impurities which are inseparable from the
coal; (2)'extraneous infiltrated impurities which are
sometimes separable from the coal; (3) inter-bedded
impurities which are separable to a large extent, and
(4) intrusive and other impurities from the roof and
floor, usually separable.

Inseparable Matter in the Coal.—In this group is
included the inorganic matter which normally forms
the ash of the coal. If the coal seam has accumulated
under clear water, the ash will be confined to the
mineral matter originally present in the plants, but
where the accumulation of the vegetable remains has
been accompanied by a sedimentary deposit of mineral
matter the resulting coal will contain an ingrained
as well as an inherent ash. As the percentage of
mineral matter in the material increases, the
characteristics of the product change from coal to
“ bone ” coal, an admixture of more than about 50
per cent, of shale with the coal is termed carbonaceous
shale, whilst more rapid deposit of mineral matter
produces in the coal seam an almost pure layer of
shale. To remove the ingrained and inherent ash-
yielding matter from the coal is impossible, but wash-
ing may effect the separation of the coal from the
bone coal and carbonaceous shale.

Imparities Subsequently Deposited in the Coal,
Sometimes Separable.—The impurities most fre-
quently met with in this group are calcite, gypsum
and iron pyrites. Calcite and gypsum are deposited
in the cleavage and bedding planes of the coal, and
therefore occur in the form of thin plates. The
effectual separation of these two minerals from the
coal by washing depends on the thickness of the layers
and the tenacity with which they cling to the coal.
Layers of calcite usually detach easily from the coal,
the thin brittle plates being broken by the subse-
quent handling and tending to enter the washer with
the fine raw coal, from which their removal presents
considerable difficulty. Iron pyrites is precipitated
in the form of thin flakes, or as a nodular or lenti-
cular piece around a nucleus. Usually the plates of
iron pyrites are attached more firmly to the cpal than
is calcite or gypsum. Often the raw coal needs
thorough crushing before washing becomes an effective
means of separation. Perhaps it may be regarded as
legitimate to overlook the discharge, as refuse by the
washing plant, of particles of coal which are ingrained
with iron pyrites, on the ground that if the pyrites
were not removed by some means or other the
percentage of sulphur in the washed coal might be
so high as to render the fuel unfit for foundry or
metallurgical purposes. Incidentally it should be
observed that the appearance of pyrites in the coal
is no indication of the sulphur content of the coal.
Where the pyrites and coal are detached from one
another, no method of separation has given better
results than washing.

Bright and Dull Coal, Bone Coal and Carbonaceous
Shale.—Seams of otherwise bright coal often contain
bands of dull coal. Lomax concludes, from a micro-
scopical examination, that the dull bands are true
coal substance, and has shown that dull coals possess
a higher specific gravity and ash content than bright
coal from the same seam*. Therefore, the dtfll
appearance is attributable partly to the canneloid
form of the coal and partly to a small admixture of
mineral matter. In washing the raw coal the ques-
tion as to what is to be rejected is difficult to answer.
The foreman in charge of the washing plant might be
content to allow dull coal to pass away with the refuse ;
and owing to the higher specific gravity of the dull
coal, the tendency would be for that to take place.
That constitutes a serious loss and suggests that
scientific control of washing plants would yield
improved results. In other cases, particularly where
a dirty coal is being washed, the tendency is for the
light carbonaceous shale to be delivered with the true
coal. The removal of bone coal from the coal that is
required for coking is a matter of importance. Often
in such cases the refuse from the washer is material
with a comparatively high carbon content. At some
collieries that type of refuse is burned under the
boilers without any special arrangements being made
as to the draught or area of grate.

I nterbedded Impurities, Separable.—This group
includes the impurities which have been freed from
their attachment to the coal during the process of
mining. As hand picking is applicable only to lump
coal, in which the impurities are more dispersed, it is
cleaned, as a rule, by washing. The efficiency in
separating the impurities from the coal depends on
their physical properties. Where the impurities
possess a high specific gravity, and naturally break
into pieces which are not unduly different in shape
from the coal particles, a good separation is possible;
but in cases where the material, such as clay, disin-
tegrates, an exceptionally large proportion gets into
the fine coal, from which the removal is a matter of

great difficulty. Indeed, for fine sized dirty coal it
would appear that re-washing is a matter worthy of
careful consideration, especially where the coal is
required for the production of a high-dlass coke. At
several collieries the practice of picking the lumps of
coal ingrained with impurity, for subsequent crushing
and washing, is in active operation and appears to be
the only method of dealing efficaceously with particles
in which the impurity is incorporated with the texture
of the raw coal.

Extraneous Matter from the Boof and Floor.
Occasionally in a seam of coal, material from the roof
and floor has been forced into fissures in the coal bed
by the pressure of the superincumbent strata. That
largely depends on the nature of the roof and floor,
and if those are soft, such intrusive impurities are
common. In mining, such intrusive impurities are
detached to a great extent from the coal and there-
fore are removed fairly efficiently by washing.
Frequently the soft impurities disintegrate in water
to form slimes. Naturally the quality of the roof
and floor has a large influence on the quantity of dirt
in the coal. Where the floor and roof are soft and
crumbly a large percentage is mixed unavoidably with
the coal. Further impurities are introduced from the
floor by hasty shovelling, especially when the floor
has been disturbed by shot-firing. Extraneous matter
from the floor also appears to owe its origin to under-
cutting in the top layer of the floor instead of in the
lower layer of the coal. Such impurities are not
attached to the coal and are separated readily by
washing.

As regards the specific gravity of coal, Nebel has
shown that the apparent specific gravity of coal
diminishes as the coal dries. A freshly mined sample
may have an apparent specific gravity of about 1-35
which on air drying diminishes to 1-20, and after more
complete drying to 1*15. As the specific gravity of
the coal diminishes, the possibility of effective wash-
ing obviously increases, on the implied condition that
the specific gravities of the impurities are unchanged
in drying. To a considerable extent that is true, in
so far as such no.n-porous impurities as pyrites and
shale retained their original specific gravities.
Fireclay, however, may be more porous than coal, but
it will regain its initial specific gravity more readily
on immersion in water. Nebel has shown that the
specific gravity of a sample of dried coal half-an-hour
after immersion in water changed only from 115 to
1-25. At the end of one hour the specific gravity
was 1-30, in two hours 1-32, and in ten hours 1-35.
Therefore it would appear that, where the impurities
in the coal are either more porous than the coal, or
non-porous, it will be beneficial to wash the raw coal
some time after it has been mined. It has been
shown that, under settling conditions where the
specific gravity of the quick sand medium is 1-15, the
ratio of the sizes of coal and shale was 8-33. After
the coal had been dried, the specific gravity would
have diminished from 1*30 to 1-20, the ratio then
becoming:

A.	2-40 - 115

A., 1*20 - 115
r* = 25

That is to say, the hindered settling ratio is three
times as great as with moist materials. Hence it is
possible to wash the dried material with a range in
size three times as large, or the same sizes will be
washed far more efficiently. With fine dry coal it
is necessary to feed the raw coal into the washer in
a wet condition; otherwise the coal floats on the water
in clotted masses without being moistened. In that
case the wetting should be delayed as long as possible.
It might be suggested that it would be economical to
build washers away from the pit and near the market,
so that, during transportation, the coal would be air-
dried and ready for washing. A point in favour of
that is that the freightage on the water held by the
washed coal would be saved, but against that must be
reckoned the counterbalancing cost of transport on
about 10 per cent, of refuse.

Behaviour of Flat Pieces of Impurity During
Washing.—An erroneous idea seems to be prevalent
with regard to the removal from raw coal of
detached flat, plate - like shaped impurities. It is
usually stated that, where the impurities existed in
the form of laminae, they cannot be separated from
the coal by washing. From very careful observation,
contrary conclusions, however, have been reached.
When a piece of flat shale of specific gravity exceed-
ing about 2-30 is placed on the mineral bed of a jig
washer it is found that after a few pulsations of the
water the shale has turned on edge * and has com-
mencent its descent to the screen of the washer. In a
short time it reaches the screen and passes out with
the refuse. On the other hand the frequency with
which flat pieces of carbonaceous shale of specific
gravity less than 2-30 are found amongst the washed
coal suggest that if the specific gravity of the flat
impurity is less than 2*30 it does not behave in the
manner stated above. The piece retains its position
near the surface of the bed and is removed in the
washed coal. ’ In trough washers the flat pieces of
shale are seen to lie eventually along the bed of the
trough and are therefore removed easily as refuse.
With a specific gravity of less than 2-30 the flat pieces
of carbonaceous shale seem to be caught in currents
of the water and be conveyed over the dams in the
trough. In fact, such pieces are observed to turn
completely over in the trough, with the result that
they are only partly submerged in the water. With
the Robinson washer it is equally true to state that,
although the flat pieces of shale, on the average, take
longer to be discharged, they are released eventually
with the refuse. On the other hand, in dealing with
flat pieces of carbonaceous shale, it is doubtful
whether the particles will appear in the washed coal
or in the refuse.

Breaking and Crushing in Relation to Coal Washing.

In view of the fact that the efficient washing of the
coal rapidly increases in difficulty as the size of the
material diminishes, it would be imagined that the
question of the production of fine material would be
ever to the fore in the colliery officials’ thoughts. In the
majority of cases, however, it seems to have escaped
entirely the consideration which is its due. The
maximum reduction in ash content is obtained in
washing material of about in. diameter. Any
reduction in the diameter beyond the limit results in
a rapid decrease in the efficiency of the separation.
Therefore, as far as washing is concerned, the pro-
duction of fine raw coal should be restricted to the
utmost minimum. • Hence,»any influence tending to
reduce the size of the raw coal previous to its delivery
into the washer is of great importance. For example,
the breakage produced by dropping coal on to an
iron plate from a height of 10 to 15 ft. is three or
four times as much as the breakage when dropped on
a wooden board. Hence wooden linings might effect
a great saving in the larger sizes and thereby increase
the efficiency of the washing. If the coal descends
gently upon an accumulated bed of other coal, the
breakage is not abnormal. . In the handling of a raw
coal which needs crushing before it can be washed
effectively, the question of breakage is relatively
unimportant, but where the coal and impurity are,
well detached, breakage should be reduced to the
utmost in order to restrict the production of fine
stuff which cannot be washed satisfactorily. It would
he interesting to estimate the relative breakage of
coal in the surface plants at a number of collieries.
In the author’s opinion many startling values would
be obtained, owing to the fact that the building of
the surface plant is of a standard form in which
sufficient variation cannot be made to provide for
the handling of coals of widely differing friability.

For a washing plant to be used advantageously the
materials sent to the washer must consist of a merely
mechanical admixture of impurities with coal. A
washer obviously fails entirely in dealing with coal
which is intergrown with impurities. Such raw coals
need judicious crushing so that the impurities are
freed from their attachment to the coal without
undue production of fine material. Washing then
may be an effectual means of purification, but crushing
must not be indulged in to an unlimited extent.
Jiingst concludes, from his investigations that the
limit of fineness, according to quality, is to in.,
and below that size the fines cannot be dealt with
profitably in a washer. Indeed, there appears to be
no known process of separating the shale dust from
the finest raw coal dust. At certain collieries it is
contended that the production of small coal is a
matter of little importance, since the whole of the
material is sent to the coke ovens. Although in such
cases there is no waste, it should be borne in mind
that the fine washed coal is always of an inferior
quality, and that a coal which had been washed more
successfully in a larger size might have yielded a
better coke. From these investigations it would seem
that for the preparation of a clean fine coal it would
be an advantage to wash the optium sized raw coal
(1J in.) and crush it finely after washing. The usual
practice in washing non-coking coals is to recover
from the water all the saleable coal, after which the
water is led to a settling pond in which the slime
settles. Then the water is pumped back to the
washer. Owing to the fact that no practicable and
efficient method has been devised for removing the
high percentage of moisture from the slime, the dis-
posal of it is a difficult matter. At some collieries
ti is consumed under boilers generating steam at a
low pressure. At others huge mounds of the slime
have been accumulated in the hope that further
investigation and progress might render them profit-
ably available. Frequently the accumulations assume
such large proportions as to make it necessary to
commence throwing the slime on the waste heap.
Although at the present time there is no sale for the
slime it cannot be regarded as a true waste product,
despite its high ash content. The question therefore
arises as to whether it is possible to reduce that loss
of slime which, in the wet state, amounts to 3 to 5
per cent, of the total washed products.

Removal of Dust Before Washing.

Several tests have been made to determine whether
any advantage would accrue from a removal of the
dust from the raw coal previous to washing. The
sample of raw coal was agitated gently in a series
of sieves of mesh 30, 60, 90 and, 120 to the inch
respectively. The ash content of the powders was
determined and compared with the ash content of the
corresponding dried slime. In all cases, the fine coal
removed from the raw coal yielded less ash than the
settlings. The following is an example: —

Slime from coal “A.”

The whole of the slime
passed through a
sieve of mesh 120
to the inch.

Percentage of ash.

26*35

It is obvious, therefore, that during the washing
process some of the impurities disintegrate and pass
away with the finest coal. Hence it would appear to
be economical to remove the dust previous to washing,
and that dust could be mixed with the washed coal
without unduly diminishing its value. The settlings
would be composed of a larger proportion of ash-
yielding constituents and could be regarded justly as
refuse. The author has not visited a colliery at
which the dust was separated from the raw coal and
so cannot state how far such a process would solve the
slime problem; but the adoption of a dust extractor
seems worthy of consideration.

Experimental.—The samples of the coal and refuse
were collected in tins which held about 600 grams of
material. In the laboratory the samples were exposed

Raw coal t( A.”

Powder sieved.'

(----------a----------Percentage

Throuffh. Over. of ash.

30 mesh 60 mesh ... 23*09

60	,,	.. 90.	„	...	22*48

90	„	... 120	„	...	20*50

120 .............. —	... 17'71