798

THE COLLIERY GUARDIAN.

April 19, 1918.

draining the latter, along with the washed fine coal,
and automatic clarification of the washing water. In
that paper replies would be found to some of the
conclusions arrived at by Mr. Drakeley. With
regard to the efficiency of the washing process, he
did not consider Mr. Drakeley’s system at all satis-
factory. He appeared to have used a medium of
1-4 specific gravity, for floatings and sinkings, to
arrive at an efficiency of about 50 per cent. The usual
guarantee for washeries was that the washed coal
should contain not more than 2 per cent, of free dirt,
and the washed dirt not more than 2 per cent, of
free coal. In the majority of cases the free coal was
assumed as floatings in 1*35 specific gravity solution,
and free dirt, sinkings in 1’5 specific gravity solution.
Assuming, therefore, that the medium for arriving
at the efficiency of the washing process had been taken
at 1*5, the results of the coal mentioned by Mr.
Drakeley would be approximately as follow: -r-

Raw coal. Washed coal. Refuse.

Percent. Percent. Percent.

Floatings ........... 90	  98	  20

Sinkings ............ 10	  2	  80

Output (as before)..	100	  90	  10

In accordance with Mr. Drakeley’s formula, the coal
would have an 80 per cent, qualitative efficiency, and
a quantitative efficiency of 97*7 per cent., or a general
efficiency of 78-4 per cent. Supposing the medium
to have been 1-35 specific gravity, the following results
would be likely: —

Raw Coal. Washed coal. Refuse.
Per cent. Per cent. Per cent.

Floatings............ 75	  85	  2

Sinkings ............ 25	   15	  98

Output (as before)..	100	  90	  10

resulting in a qualitative efficiency of 40 per cent.,
and a quantitative efficiency of 99*7, or a general
efficiency of 39 88 per cent. The higher the medium
was taken for the floating and sinking test, the
higher would be the efficiency of the washery. At
1*8 specific gravity there would practically be 100 per
cent, efficiency, with the result that, if such a formula
were adopted as a scientific basis, all washery men
would be anxious to have the medium at as high a
point as possible, so as to claim up to 100 per cent,
efficiency for their plant. The method advocated by
the writer was to obtain the actual yield of washed
coal and the amount of dirt extracted, with the ash
contents of both, take an average sample of the washed
coal and test in the laboratory for the theoretical
ash contents in the coal and dirt at the yield obtained
in the washery. If there was not much intermediate
coal between 1-35 and 1*5 specific gravity, the ash in
washed coal and dirt should be fairly near the
theoretical results. Otherwise, allowances had to be
made in accordance with the amount of intermediate
coal between 1*35 and 1*5, and the proportionate
increase in ash of the washed coal, and decrease in ash
for dirt, allowed for. It did not appear feasible to
fix upon a definite formula for the efficiency of the
washery, as coals varied so very much in their consti-
tution, ash contents, etc. He quite agreed with Mr.
Drakeley that the methods of treating the coal in
washing plants must be carefully considered by
experts, in accordance with the nature and constitu-
tion of the coal, and the plant specially designed to
suit specific purposes. Also the results of the washery
should bo constantly tested to maintain the efficiency
of the plant. His firm had for a number of years
supplied an outfit for that purpose—namely, to test
the percentages of floatings in the dirt in 1’35, and
sinkings in the coal in a 1*5 solution. A plant having
originally been set to wash the coal at the highest
possible efficiency, it was a simple matter for anyone
to test whether the original results were maintained,
a trained chemist not being necessary for that
purpose. A chemist was certainly required in setting
the plant at the start, which, however, was part of
the washery constructor’s business. Nebel’s sugges-
tion of allowing coal to dry previous to washing had
not proved satisfactory in practice, any washery man
knowing that the washing results were not so good
for stocked coal as for coal coming direct out of the
pit. That was due to the deterioration of the coal
through weathering, the coal breaking up and the
soft shales becoming clay dust, which dissolved in
water and was retained among the coal by filtration
unless the water was continuously changed. In a
large capacity washery the changing of the water was
not feasible, and as the cost of laying up the coal and
reloading would be fairly high, that suggestion was
not likely to be followed. The above-mentioned soft
shales, in their original condition as mined, had very
little time to dissolve. They were immediately
separated from the coal in the washing box and dealt
with along with the dirt.

Mr. J. M. Draper, of Cardiff, wrote that it was a
commonplace that the present methods of the produc-
tion and consumption of coal were in many respects
extremely wasteful, but there appeared every proba-
bility that, after the war, some of these methods would
be improved. The stress of necessity had compelled
them to produce a far greater quantity of by-products;
greater energy had been given to the task of discover-
ing commercial uses for coals which had hitherto been
literally thrown away; a more enlightened opinion
had been created on the need for closer co-operation
between the man of science and the man of business.
In the South Wales coal field Lord Rhondda had shown
himself a pioneer of this new movement. He had
established an up-to-date laboratory at the Glamorgan
collieries, and appointed a chemist to engage in
research work. Because he was able to realise the
commercial possibilities of certain forms of waste coal
there had recently been installed at the Glamorgan
collieries a plant which might save the country
millions of tons. One of the most common forms of
waste was where it had been found impracticable to
separate fine coal from dirt. It was impossible to
ascertain the actual amount of coal which went to the
tip, or went to waste in the form of “ fines,” but in
the aggregate it must represent several millions of
tons per annum. Where washing was practised, the

“fines” came off in the form of slurry with, of
course, a very large amount of ash. At not a few
collieries, where the conditions were favourable, this
slurry was turned into a watercourse, and so went
to waste, but at some few others it was recovered and
burnt, more or less inefficiently, under the boilers.
Possibly it was more generally delivered to the dump.
Where coke was made the method most generally
adopted was to distribute the drained slurry fairly
evenly over the crushed coal passing to the storage
bunker at the ovens. The most ardent advocate of
this method would not claim a low ash content for
the slurry, whilst the dirt often present in the form
of fireclay had an undesirable affinity for moisture.
The alternative method was to take out the fines by
a preliminary screening before the coal passed to the
washery, and afterwards to sprinkle the dry dust on
to the washed fuel. The disadvantages of this were
obvious. But in many instances the slurry or dust
was tipped. Again, the most economical way of
disposing of colliery “cleanings” was by delivering
them to the tip or stowing in the goaf. Although
this material contained a certain percentage of pick-
able fuel, it had not hitherto been commercially prac-
ticable to recover it, owing to the large tonnage of
fines which had to be passed over the belt to recover
the relatively small percentage of sizable coal. In
view of these facts it was evident that the spoil
banks contained much valuable material, and in the
not very distant future a considerable percentage
would be recovered. Each year this became more and
more a matter of national importance. The fines to
be dealt with consisted of particles capable of passing
through extremely fine meshes, the greater part of
them being from J in. to 1 / 10Q in., or, say, from 64
to 10,000 to the sq. in. The writer had for some years
devoted attention to the problem of the separation
of fine coal from the dirt. Success could not be
expected from any form of mechanically moved
apparatus, whether plunger, reciprocating, or bash
type, since none of these designs would permit the
finer particles to settle in the permissible time, even
if the grains of higher gravity could be removed after
separation. Therefore, it was necessary to consider
some form of apparatus which, while free from the
agitation of moving parts, would permit the operation
of the mathematical laws governing the fall of bodies
in liquid media in as perfect a manner as possible,
whilst at the same time enabling the heavier particles
to be removed after separation without disturbance
of either. The apparatus evolved, after numerous
experiments, consisted of a simple tubular vessel into
which the raw fuel was passed at the upper end,
into the washer or separator, where it met an ascend-
ing stream of water, delivered through an hydraulic
main. The flow was regulated by a valve which, when
once set for a particular class of material, was locked
in position. There was a second water connection
from the tank, in which the water was maintained
by ball and cock at the same level as the overflow over
the sill of the discharge. The tank being open at
the top acted as a balancing column to the separating
chamber, ensuring efficient separation. The opera-
tion of a unit was as follows. The valve from the
water tank was opened till the water rose very nearly
to the outflow governed by the sill, which was
adjustable. The hydraulic valve was then opened
till the water was just flowing through the opening-
above the sill. The material was now fed into the
chamber, the heavies passing downwards through the
uprising current of water, and out through the dis-
charge, whilst the fines were carried upwards with the
stream and over the discharge sill. Any degree of
separation could be obtained by regulating the flow
of water through the hydraulic valve. The operation
was simple, positive and continuous. So long as the
upward flow of water was maintained no change was
possible in the separation. Once adjusted, no skilled
attention was required, and the ash results were
invariably the same. There were no moving parts to
get out of order, and the cost of operation was
extremely low. The plant installed at the Llwynypia
Colliery consisted of four units, each capable of treat-
ing one ton of material per hour. The whole, together
with the settling tank, occupied a floor space of 6ft.
by 5 ft. 6 in. A single unit required about 1 sq. ft.
of floor space. The slurries from the Elliott washers
were delivered to a concrete settling tank, a per-
centage of unwashed fines was also delivered to this
tank. From the tank it was dredged by scraper con-
veyor. The slurry averaged 27 to 30 per cent, ash,
of which about 5 per cent, was mixed. A sizing test
gave the following:—Over | in. square wire mesh,
2-90 per cent. ; over J in. square wire mesh, 5’44 per
cent.; over 1/40 in. square wire mesh, 55*64 per cent.,
and through in. square wire mesh, 35*55 per cent.
Of the last product, over 50 per cent, would pass
through a 100 ton mesh screen. The slurry was first
delivered into a revolving screen, where it was
separated, in water, into two sizes—through 1/30 in.,
and over V30 in. This was not theoretically good,
but was adopted as an expedient for demonstration
purposes. The sized coal was raised by bucket
elevators to the hoppers which fed the washing or
separating units—three for the over size and one for
the through fines. The washed fines passed over a
100’ mesh brass wire screen, for the purpose of taking
out as much water as possible, and thence direct to
the trucks, where the drainage was completed. The
water draining from the trucks was quite clear, so
that it was evident no fine washed fuel was lost.
With regard to the moisture content, the material
could be charged into the ovens after standing for
8 hours; in • practice the time allowed was 24 hours,
after which the content of moisture did not exceed
8 per cent. The dirt passed downwards into a collect-
ing chamber. A careful inspection showed that the
waste was free from usable coal; an average analysis
gave 68 per cent, to 72 per cent, of ash. The plant
dealt with about 50 tons of material per day, reducing
the ash from 27 or 30 per cent, to 6*5 or 7 per cent.
As the fixed ash in this coal was 5 per cent., the
excellence of the separation was apparent, whilst the

percentage recovered was within 1 per cent, to 2 per
cent of the theoretical coal contained in the original
unwashed material. The use of the plant rendered
the recovery of coal from colliery cleanings a
thoroughly practical and profitable proposition. The
cleanings so treated yielded approximately 70 per
cent, of saleable coal at a cost not exceeding 2s. 6d.
per ton of material handled. Prior to the instal-
lation of the separators the whole of the cleanings
were delivered to the tip at a cost of 6d. to 9d. per
ton. The apparatus was likely to be extremely useful
for preparing fuels for coal briquettes, but to meet
the requirements on this account it would be necessary
to evolve an efficient and economical method of drying
the treated material before passing it to the briquette
machinery. Its scope was not confined to the recovery
of “fines” coal; it could effect economically the
separation of metallic fines from ores, silica from pure
iron ore, concentrates from sands, lead from calcite,
zinc from tailings or sauds, and notably metallic fines
from ground slags. All products previously con-
sidered too low in grade value could be treated, so
long as a little difference in the relative specific
gravities existed. Mr. Draper added that a plant was
in operation at Llwynypia Colliery capable of deal-
ing with 200 tons in 10 hours. The whole of the
material from 3/16 of an inch down to 0 was passed
through the apparatus and entered the coke oven in
two hours with a moisture content averaging 8 per
cent, only, and invariably with only about 1 per cent,
over the fixed ash of the coal.

Mr. Noah Williams (hon. secretary) remarked that
the float and sink test was now recognised as a
standard method for determining the feasibility and
efficiency of washing various kinds of coal. It could
be’used as a check on the work done by a plant, and
in comparing the efficiency of one plant with another.
It would be useful if the author would give a
description of the apparatus he used for making the
float and sink tests, and the number of tests that could
be made in a day. A description and illustration
of the machine used for making float and sink tests
at the Government station at Denver was given in
Bulletin 5 of the Bureau of Mines, United States.
The machine took about three times the quantity Mr.
Drakeley used, namely, 2 kilogs., as against 60d grms.,
and from .30 to 35 tests were made per day. The
screen was of the same size as that used by Mr.
Drakeley, namely, 60 meshes to the inch. The author
gave the impression that the Germans had been the
pioneers in coal washing, but the credit should be
given to the Belgians, who were the first to erect
jigger machines in this country about 40 or 50 years
ago. He believed it was at the Ebbw Vale Colliery,
South Wales.

Mr. Gerrard objected to credit for the introduction
of coal washing into England being given to either
Germans or Belgians. It was adopted at a colliery
near Wigan more than 50 years ago. The plant was
put down about 1864.

Mr. Drummond Paton asked Mr. Drakeley for the
actual dimensions at which coal washing became
useful. His experience had been that the best method
was to clear out everything below 1/16 in. In South
Wales one did not get the same sort of dust as in
Lancashire; the slack was pure coal. Ordinary slack
could not be cleaned by washing. Therefore the best
plan was to take the dirt out. The coal could then
be heated successfully, and if necessary they could
deal with the very fine dust and control the ash content
afterwards. This system was adopted in Belgium and
the North of France. The fine floury dust should
be taken clear away because they could not improve
it, so why turn it into the washery?

Mr. Siddall agreed. This very fine floury dust
spoiled the commercial aspect of the washed coal by
pasting it over with dirt, and it also had a distinct
effect upon the washer.

Mr. Gerrard stated that he had visited a colliery
where they'worked with a washery, and he produced
specimens of the dirt taken out. .There were
three very important points in favour of the system.
Instead of accumulating large unsightly heaps around
the coal pits, the dirt could be taken into the mines.
There would be an end of the danger from under-
ground fires, an end of the leaving of coal for the
support of buildings, and water would be kept out.

Mr. Williams explained that he referred to jigger
washers. Trough washers were previously in use in
Lancashire, and those were what Mr. Gerrard had in
mind.

Mr. Thompson said that the difficulty was that the
dirt was sometimes of a very friable nature, in fact,
it almost dissolved, and he did not quite see how it
could be dealt with as Mr. Paton advised.

Mr. Drakeley said he did not deal with the paper
which Mr. Hunter mentioned, because he had no
opportunity of examining the plant, and he based his
own paper entirely on practical work carried on in
Lancashire. It was impossible for him at the moment
to answer fully Mr. Hunter’s contribution to the dis-
cussion. The main point seemed to be whether the
specific gravity of the solution should be 1*35, 1*4,
or 1*5. He was in a difficulty about that when he
started, because at every colliery he visited there was
a new class of coal, which needed a new solution, but
their people would have said, “You have used a
different solution, and that is why the results are not
as good.” He took it as near the average as he
possibly could, and all the tests were made at 1*35
specific gravity. He did not see any advantage in
using the solution. In reply to Mr. Hunter, the ash
content had absolutely nothing to do with the washing,
it was an accidental thing which came about from the
fact that the material which floated contained a small
quantity of ash. One could not base any work on the
ash content ; one was bound to come back to specific
gravity for determination. With regard to dried
coal, if the coal was dry it did actually wash better,
having a lower specific gravity. He could quite
understand that stacked coal (which was not quite