672	THE COLLIERY GUARDIAN.April 5, 1917.

iodide with a specific gravity of anything between 1*5
and 1*9, obviously coal would float in such a solution
and dirt would sink. By that me.ins it was always
easy to separate any given sample of coal into clean
coal and dirt and to find out to what extent one had
reduced one’s ash and how much coal was left in the
dirt. Although that was a good laboratory method,
however, it was obviously impossible in practice on
account of the difficulty and expense of using such
solutions on a large scale, say, by the ton. One had to
adopt some other method, and the other method con-
sisted in taking advantage of the action of gravity. If
a bullet and feather were let fall together in a vacuum,
they would fall at the same rate, for the obvious reason
that the power of gravity was attracting every particle
in the same way, and the unit pull on each particle was
the same. The bullet and the feather fell at different
rates in the air, however, because of the resistance
offered by the air. That phenomenon was still more
marked if a particle were allowed to fall in water.
Instead of falling with a uniformly increasing accelera-
tion, it fed for the first second of time as though it were
falling in vacuo; then the resistance made itself felt,
and increased faster than the pull of gravity. In other
words, they got to an ultimate uniform velocity, and
every particle of given size and given density fell
through the water with a velocity ultimately uniform
which it could not exceed.

It was rather important to arrive at some definite
knowledge of the relation between the ultimate falling
velocity and the size aud specific gravity of the par tildes.
If they took two spheres of any material and called
their diameters D and D', respectively, and their specific
gravities S and S', respectively, and these were allowed
to fall in a medium of density F, then

D S'-F

D' S — F.

For practical purposes, they were dealing only with
air and water. Supposing the specific gravity of water
to be 1, the expression would become

D _ S' - 1

D' S — 1.

To illustrate that, let them take a case of dirt in coal
and suppose that they had dirt of a specific gravity of
2’5 and coal of a specific gravity of 1’5. Supposing
they took two pieces of each. Their diameters were in
the ratio

D = 2’5 - 1 = 1-5 = 3

D' 1’5 — 1	-5	1’

so that, with a heavy coal like that, if the particle of
coal were three times the diameter of the particle of
dirt, both would fall at the same ultimate velocity in
water. If, therefore, they got a mixture of pieces of
coal with pieces of dirt in which the coal was three
times the size of the dirt, they could not separate them.
In order to get an effective separation under those con-
ditions, they were bound to take care, by screening and
by other obvious methods, that no particle in the
mixture could be three times the size of the other. The
formula thus told them what must be the ratio of
screening for practical purposes. For “ diameter of
sphere ” they might read “ size of mesh ” ; it was quite
nearly true enough for practical purposes.

He now desired to point out some exceptions to what
he had been saying. His statements as to the manner
in which particles fell in water were only true when the
particles were of certain definite sizes. When they got
down to very fine sizes, the internal friction of the
water began to play a part, and there was a size below
which these facts no longer held good. They all knew
that very fine coal would float for weeks on the water
that came away from the washeries. In round numbers,
a particle of coal of less than in. diameter no longer
obeyed the rule he had laid down. Very fine particles
like that constituted what they usually called slimes.
Further, those expressions vere only strictly true when
they allowed individual particles to fall into a big
body of water. If the particles were allowed to fall
close together, as was always the case in coal washing,
they got into each other’s way, so to speak, and the true
physical laws could not act. They could get separation
with ordinary coal washing machinery even beyond the
formula he had given, them. The formula could be
applied strictly and safely, natural conditions giving
them a margin of safety. The fundamental principle
upon which all coal washing depended was the way in
which particles of different densities fell in water.
There was a very important application of that ultimate
falling velocity. Supposing that, instead of allowing
the particle to fall in water at rest, one allowed it to fall
into water which was flowing upwards. The particle
would fall with the difference of its ultimate falling
velocity and the rate at which the water was moving,
and, if the water was flowing upwards with a velocity
greater than the ultimate falling velocity, the particle
would not fall at all but would be carried up. They
had there got a definite means of ascertaining at what
point a particle would fall or be carried up. These were
quite plain physical principles, and, whenever they were
given fair play, which was very difficult to do in most
machinery, they were bound to apply. Supposing they
put a lot of particles into a stream of water flowing
downwards, they would find that the small heavy
particles got a start of the others, so that, with a
downward current, the difference in the rate of’ falling
was accentuated in favour of the heavy particles.

Four Definite Principles.

He would lay down four definite principles upon
which coal-washing machines were made. The first was
the treatment of a dirty coal in an ascending or moving
current of water that would move or lift the light
particles and allow the heavy particles to drop. (By
“light’’ and “heavy” he referred to the specific
gravities). The next principle was that of exposing the

mixture of coal and dirt to alternate upward and down’
ward currents, as was done in the familiar jigs or
bashes. Another set of conditions came into play if
they allowed particles to be sluiced down a surface.
The larger particle would be washed down more quickly
than the smaller, because it exposed a larger surface to
a quicker flow—the flow of water being quicker at its
surface than below its surface. The fourth principle
to operate which had been attempted was based on the
idea of the particles being exposed not in a fixed trough
but in a trough or tray or table which was being shaken
to and fro. The small heavy particles would be jerked
forward more rapidly than the light particles and by
that means would become separated. In the application
of any or several of these principles, it was always
possible to separate a mixture of particles in accordance
with their specific gravities, provided that they had
been properly sized and that the disparity of size
between the b’ggest and smallest was not too great, as
indicated by the formula.

All these considerations would be fairly easy if they
had nothing but clean cod and clean dirt to deal with.
The great difficulties came in because they had particles
consisting partly of coal and partly of dirt. However
they dealt with their coal, however they crushed it,
unless they crushed it excessively fine they were bound
to be faced with that difficulty. It was a purely com-
mercial question as to how they were going to handle it.
It was obviously no good crushing the coal so small
and making it exceptionally clean unless they could get
a better price for the coke made by it. The price they
got for their best quality of coke regulated the clean-
ness to which they got their coal. By crushing it fine
enough, they could ultimately separate it all into pure
coal and pure dirt—but that costs money and, for
practical purposes, the coal was free from dirt when it
contained so little that it no longer paid to deal with it.

Prof. Louis proceeded to describe, with the aid of
lantern illustrations, machines exemplifying the four
principles he had enunciated. Continuing, he said that
in a complete washery, the coal was brought from the
colliery screens in various ways—conveyors, railway
wagons, tubs and so forth. The first thing to be done
was to size it. There were various methods of sizing it,
but the most usual sizers were drum screens. If they
had got to design a complete washery, the best plan in
commencing was to establish a flow sheet. That done,
they were then in a position to build their washery on
right lines. In all coal washeries one was troubled with
sludge. There was always a lot of coal in the water
flowing from the washery with which they could not
deal satisfactorily. So far, he did not know of any
satisfactory way of dealing with that sludge. It
occasionally happened that the sludge was the most
valuable part of the coal from the coke maker’s point of
view, containing the most highly bituminous parts of
the coal. There had been cases in Germany where it
had been found that the washed coal would not coke
without the sludge, but that if the sludge wTas collected
and mixed with the coal it did coke, the difference was
sometimes so great as that. The most satisfactory way
of which the speaker knew of dealing with the sludge
was by air-blowing the coal before one attempted to
wash it. Several appliances for that purpose had been
devised and used on the Continent. So far as he knew
they had not been used in this country, and he par-
ticularly desired to draw the attention of his audience
to that method of dealing with sludge, because he
thought there was a great deal of important work to
be done in connection with it. The method appeared
to him to contain the germ of a very important
principle indeed, and it was one he recommended to the
serious consideration of all who had coal washeries and
were troubled with sludge.

As to the economic side, he could only say that he
was, necessarily, quite alive to the importance of the
question of cost, and was not suggesting that coal
washing should be applied, except in cases where the
enhanced value of the product and the material saved
would pay for the cost of washing, including, of course,
interest and redemption of plant and so on. It was
astounding, however, how many instances one met with
in which, put up in the first instance, perhaps, to save a
market, a coal washery had turned out to be a thorough
commercial success. His own opinion was that,
wherever people had any difficulty at all in coke making
or in utilising the colliery products, they would probably
find that a properly and scientifically designed washery
would be a solution of their difficulty.

The Discussion.

The Chairman remarked that the separation of the
dust was very important. He had had considerable
trouble with dust, small coal and sludge at one or two
collieries, and that was the first time he had ever heard
of the separation of dust from small coal by air blowing.

Mr. G. A. Phillipson said it would be rather
interesting to hear Prof. Louis’ opinion as to the advan-
tages of washing after classification or before classifica-
tion. It seemed to the speaker that to separate the
coal into its different sizes and then wash would introduce
a very much larger plant. As to the dust treatment, it
had been tried in this country, but he could not say
whether it was being continued. In the salt area of South
Yorkshire the fine dust was removed by blowing and was
carried forward into the washed coal, the object being
to dry the washed coal. The dry dust, not having been
in contact with the water, contained a higher percentage
of salt (sodium chloride) than it would otherwise have
done. In consequence, when this was put into the oven,
a larger proportion of fixed ammonia, developed as
ammonium chloride, was found in the ammoniacal
liquor, and difficulty in dealing with it caused the
abandonment of the process.

A member asked whether, in screening the coal, after
getting down to an eighth or sixteenth of an inch
screen, there was not a great difficulty in getting the

coal to screen at that size without adding water or some
spray to keep the screen clear.

Another member asked if there was any practice
of bringing all coal down to the same size, instead of
separating the sizes—crushing all the coal down.

It was remarked also, with reference to Prof. Louis’
allusion to air blowing, that there was an arrangement
of hoods and a fan for drawing away the fine coal from
the picking belts and screens at a colliery, the object
being to improve the conditions under which the men
worked. Such an arrangement in coking collieries
would add to the make of that very fine- coal of which
Prof. Louis spoke.

■Replying, Prof. Louis said the question of washing
before or after sizing was very debatable, and he had
known as many good washers take one view as the other.
Personally he was in favour of classifying first and
washing afterwards. Theoretically it would not matter
much which one did, but practically he found that, if
one washed first and sized afterwards, one got an
amount of breakage in one’s secondary sizing that
rather interfered with it. That was the only technical
objection of which he was aware, otherwise he really
did not think that there was very much in the question.
It was quite true, he thought, that one could get a -
rather more compact plant if one washed first and
sized afterwards, but, as a general rule, washeries were
built in places where land was not particularly valuable,
and he was not certain that trying to save building
space to the utmost was sound business. He considered
that most washeries he had seen erred on the side of
being too cramped. He had heard of the difficulty of
the elimination of chlorides and salt. It was put before
him some years ago, and there, as he understood, the
blowing was not with the object of separating out the
fine dust, but with the hope of drying the coal, and the
lines of the work were very different. As to the method
mentioned of drawing dust from the screens, he saw no
reason at all why the fine coal drawn off in that way
should not be collected and saved. Many of the coal
plants, especially picking belts, were, he was afraid,
rather ghastly examples of how much fine coal could be
wasted. He bad been in plenty of heapsteads, especially
in South Wales, where he had waded in fine coal over
his boot-tops. Fine coal was excellent material for
firing boilers, and there was no reason why it could not
be utilised in that way, even where they were not dealing
with a coking coal, where they were satisfied that it was
worth collecting. As to screening very small coal, he
agreed that it was not practicable commercially.
With very small coal, it was very much better
to separate by means of a set of V-boxes. As to
crushing all coal approximately to the same size,
before washing, he had only known that done on the
Continent, where he had known them to crush every-
thing down to J in. or f in. before starting to wash.
The idea in these cases, he believed, was that they could
wash out a good deal of clean coal. He could see
nothing to be gained by it in ordinary practice.
Provided that one’s inch nuts washed satisfactorily to
begin with, he saw no reason why they should be crushed
smaller. Replying to a question as to how the labora-
tory tests compared with practical results, the lecturer
said the advantage of the laboratory tests was that they
showed them the theoretical best attainable by working
in the ways described. The laboratory results repre-
sented the ideal towards which the mechanical methods
must strive..

Moving a vote to Prof. Louis, the Chairman remarked
that one Continental firm which put its coal washer on
the market about 15 years ago said, at that time,
“Classify first and then wash.” Now, however, the
firm said, “Wash first and then classify.” The advice
had reference to the same washer. As to the dust
drawn from the picking belts, he knew of one place
where it was being recovered and was fetching a very
good price for certain uses.

The expression of thanks concluded the meeting.

London Coal Prices.—The Controller of Coal Mines is
investigating the position with respect to retail coal prices
in the London district under the voluntary arrangement
made between the President of the Board of Trade and
the London merchants in May 1915, and he has arranged
that, for the present, pending a detailed investigation, a
reduction of Is. per ton in the public advertised prices,
and Id. per cwt. in the prices of small quantities sold
from trolleys and shops, shall come into operation as from
April 1. He states cases have occurred where prices
have been charged by colliery companies to London coal
merchants which exceed the limits prescribed by the Price
of Coal (Limitation) Act, and also cases in which the inter-
vention of factors has operated in the direction of an undue
inflation of prices charged to such merchants, the result
being that a number of smaller merchants have difficulty in
selling to the public at the agreed prices. The Controller
has issued a notice to all colliery companies affected requir-
ing them to revise forthwith the whole of their prices
charged for coal despatched for the London trade, so that
they do not exceed in any case the maximum prices per-
mitted by the Price of Coal (Limitation) Act. Instructions
have also been issued to factors that, pending a complete
survey of the position, their charges must be revised so
that the colliery prices comply with the above instruction,
and that the total factoring charges in no case exceed the
provisional maximum sum of Is. 6d. per ton between the
colliery and the distributor, no matter through how many
factors’ hands the coal passes. The attention of both the
collieries and the factors has been called to the necessity of
forwarding the same tonnage of coal to London and the
southern and eastern counties month by month in future
as was supplied last year, in order that adequate stocks
may be built up during the summer for next winter. The
Controller has particularly warned merchants buying coal,
as well as sellers of coal, that all measures will be taken
which may be found necessary to enforce their compliance
with these instructions.