October 20, 1916.

THE COLLIERY GUARDIAN.

751

r STONE DUSTING IN COLLIERIES.

At Cardiff on Monday, 16th inst., the South Wales
Institute again discussed Mr. G. D. Budge’s paper on
“ The Stone Dusting of Steam Coal Collieries.”
Members of the Colliery Managers’ Association, the
South Wales Colliery Officials’ Association, and the
Monmouthshire Colliery Officials’ Association were
present, and Mr. W. Stewart, the president, occupied
the chair.

Dr. W. N. Atkinson said that the production or make
of coal dust was a matter of great importance in South
Wales, as the open type of tram hitherto used caused a
great deal of small coal and dust to be deposited on the
haulage roads. The introduction of dust-tight trams
would greatly reduce this evil in the future. Steps
should also be taken to reduce the dust which went
down the shafts from the screens. Spraying both full
and empty trams was also most beneficial. Whilst it
might not be practicable to apply stone dust at the
working faces, he did not think that was any reason why
the haulage roads should not be dealt with, as it was
through those roads that an explosion almost invariably
passed from one district of the mine to other districts;
and, in his experience, most large explosions did not
originate at working faces. He thought the proportion
of stone dust to coal dust to be aimed at on haulage
roads should be greater than 1 to 1, and he did not con-
sider that in most cases there would be much difficulty
in maintaining a greater proportion of stone dust, whilst,
when all the arrangements for stone dusting were made,
the extra cost of applying more stone dust would be
trifling. The best way of applying stone dust to high
cavities in the roof would no doubt be to arch the road-
way, and fill up the cavities. Unless, however, the
cavities were of considerable length, the dust in them
would probably be innocuous, provided there was suffi-
cient stone dust on the road under them, as in an
explosion all the dust would be raised and mixed
together. More experience was required as to the best
methods of applying the coal dust, and how often fresh
stone dust’ should be added. He had considerable hopes
in the efficiency of stone dusting for preventing wide-
spread explosions; the only fear he had was that the
method might become discredited by being inefficiently
applied. Theoretically speaking, there was no doubt
that it was possible, either by water or by stone dust, to
prevent an explosion spreading from one district of the
mine to another—in the case of water, by keeping the
road continually damp; and in the case of stone dust,
by the application of sufficient dust to render the coal
dust non-explosive. What, however, they had to deal
with was the practicability of making either of these
methods effectual. -A great deal of watering had been
done, but his opinion was that 90 per cent, of it was
utterly useless, simply because it was inefficiently done.
Many of the difficulties which arose in the case of
watering were absent in the case of stone dusting. In
the case of watering, where there was a great current
of air, it was not sufficient to water once or twice a
week, as was very often the case, and it was not suffi-
cient to water the floor only—the dust on the roof and
sides must be treated as well. These practical diffi-
culties were no doubt the chief reason why 90 per cent,
of watering was quite useless. In the case of stone
dusting, there was no such danger of damaging the roads
as there was with watering; and, more important still,
the effects of the stone dust would remain very much
longer than in the case of water. Personally, he thought
if stone dust was sufficiently applied, it would prevent
a repetition of the big explosions of the past. The
Explosions in Mines Committee had fixed a minimum
of what might be useful, but seeing there was no great
difficulty in applying two or three times the amount of
dust as would make a 1 to 1 mixture, he saw no reason
why it should not be.done.

Stone Dusting with Fullers’ Earth.

Mr. J. W. Hutchinson (North's Navigation) said Mr.
Budge had given most useful information as to the
methods of applying stone dust and removing coal dust,
and also as to the quantities periodically required to
keep up the 1 and 1 mixture. This information would
go a long way to assist mining engineers generally in
their work of superintending the application of stone
dust. At the same time, it confirmed his (the speaker’s)
opinion as to the difficulty of keeping roadways, working
under varying conditions, up to the standard laid down
by the recommendations of the Explosions in Mines
Committee. He certainly did not believe that a mix-
ture of 50 per cent, of stone dust with 50 per cent, of
coal dust would prevent the propagation of an explosion.
At the same time, he thought it was their duty, as
mining engineers, to do what they could to experiment
with, and, if possible, adopt as far as practicable, any
method which would further ensure the safety of the
workmen employed at the mines. In his opinion, the
application of stone dust must be done most systematic-
ally, and the dust used must be ground as finely as
possible, to give any definite and satisfactory result.
It might even be necessary to appoint special workmen
and overmen to carry out the work; otherwise the
thorough dusting of the roadways could not be relied
upon, and it would only be done in patches. Stone dust-
ing had been carried out at North’s Collieries for several
months, and various methods of application had been
adopted, such as blowing the dust into the air current
by compressed air, sending trams of dust in with the
journeys, and throwing dust into the roadways (by hand)
from bags. The managers reported that the method
which gave best results was that of throwing the dust
into the cavities and timbers of the roadway by hand.
He was of opinion that finely perforated trams fitted
with slides and sent to and fro with the journeys, or
some automatic distributor, would give the best results.
Experiments were first carried out at North’s by using

fullers’ earth, and a main haulage was chosen of fairly
even dimensions throughout, the coal dust being cleaned
from the sides and floor, by hand, as far as possible.
The roadway was the return airway, and the inclination
from the main engine parting to the pit was an average
rise of 1 in 6. Fullers’ earth was distributed into the
roadway at a point 1,400 yds. from the pit bottom by
compressed air, and it could be easily seen in suspension
in the air current at the pit bottom. Further quantities
were distributed at points 300 yds. apart and nearer to -
the pit bottom, until the whole roadway had the appear-
ance of being whitewashed; and although this had been
done nine months ago, the effect of the fullers’ earth was
still plainly to be seen in places. Since that time the
roadway had been subjected to stone dusting periodic-
ally, and the layers of fullers’ earth, coal dust, and
stone dust showed quite plainly in crevices. He (Mr.
Hutchinson) would be pleased to know if Mr. Budge had
made any experiments as to the amount of coal dust
deposited in main haulage roads on return airways,
where the journeys were travelling with the air current,
as compared with main intakes, where the journeys were
travelling against the air current at a similar speed and
velocity of air current, either on main-and-tail or end-
less rope haulages. With regard to trams, he was of
opinion that a requisite number of trams, with open
ends or doors, could be kept in each district of a mine,
to be used for rubbish only. Special roadways or part-
ings could be used in which to store them. But trams
with doors were essential to the South Wales coal field.
On the general problem, he believed it would be very
unwise to trust to stone dusting alone to prevent the
propagation of an explosion, and agreed with Mr. W. W.
Hood that safety zones of arched roadways, kept well
watered, were advisable, and would prove very effec-
tive. He was, however, doubtful that a zone of
100 yds., or even 200 yds., in length would prevent pro-
pagation. It would depend on circumstances and the
relative point of ignition. The question of coal dust at
the face appeared to be the most difficult problem to
deal with. From actual observations and analyses, he
had found that the stall roads and headings near the
coal face in the steam coal pits did not, as a rule, con-
tain a large amount of coal dust. This was due to the
heavy squeeze, and the fact that heavy repairs were
being'constantly carried out. Under certain conditions,
and with certain types of machines, he considered
machine coal-cutting in the longwall faces of the steam
coal pits of South Wales positively dangerous, especially
when cutting in coal with a hard and uneven floor con-
taining ironstone balls or nodules. The fine dust pro-
duced was most excessive, and the sparks given off by
the cutter did not tend to safety, even if compressed
air was the motive power. Mention had been made of
natural stone dust roadways. He took it for granted
that what was meant was a roadway which had had
coal dust in it, and which had been ripped and cleaned
so that there was no coal dust left. Reference had
been made to the explosion at Senghenydd being held
up in one part of the mine by the accidental presence
of stone dust. If he (Mr. Hutchinson) remembered
rightly, the roadway referred to had just been thoroughly
cleaned throughout and repaired, and all coal dust taken
away. Therefore, the reason the explosion did not
carry further was because there was no coal dust to
carry it. Stone dusting, watering, and safety zones
should make a most useful combination. He had tried
experiments on a main road, which was thoroughly
watered, sprays being fixed at about 200 yds. apart.
They then commenced blowing the coal dust out of the
timbers. He was at the other side of the sprays, but
as soon as he walked out of the sprays he could scarcely
open his eyes owing to the large quantity of dust which
had been deposited above the timbers and in crevices
which the water had not been able to reach, even after
the water had been dashed up amongst the timbers. It
was impossible to get at the dust in the big cavities and
other holes above the timbers.

Blowers and Falls.

Mr. George Hann (Aberdare) said, after reading the
previous discussion, he found his views more nearly
coincided with those of Mr. David Evans (Oakdale)
than with any other speaker. A great deal had been
made of the danger of dust at the coal face, and the
difficulty of effectively dealing with it. Mr. Budge had
partly met this by stone dusting where shots were fired.
The chief risk of an explosion at the face—assuming
that shot-firing was not done during the day, and it was
not usually done in steam coal collieries—arose from an
ignition of gas, which could be largely obviated by
better ventilation. But supposing the gas became
ignited, there was quite a fair chance of its being
stopped if the stone dusting had been done properly.
One of the principal benefits of stone dusting—especi-
ally where there was a limited supply of stone dust—
was in its application to the neighbourhood of blowers
of gas, or accumulations of gas, near the face, and there
should be no difficulty in keeping 20 or 30 yds. around
a blower up to 75 per cent, of stone dust. As to the
deposition of coal dust on the stone dust, this could be
remedied by frequent dressings of stone dust. Where
a colliery was stone dusted throughout, z.e., every part
except the face, the risk of shot-firing was greatly
reduced. If a fall occurred—one of the dangers—and
a cloud of dust should come the moment the shot was
being fired, the dust ought to be harmless. Water
zones, hundreds of yards in length, were used for some
years at Powell Duffryn collieries, but they were dis-
continued, not being considered effective in stopping the
spread of an explosion. He should like Mr. Hood to
tell them what length of zone he considered would stop
an explosion in ordinary mining conditions. It was
difficult to get, say, 400 yds. of ferro-concrete arching
put in in deep steam coal seams. If they were going
to be any good, the arches must have inverts; and the
difficulty of putting an invert in a road over which coal

traffic had to be worked was pretty obvious. The roof
had to be supported, and it interfered with the iron-
work in the ferro-concrete. As to arching high holes,
there were conditions in which this could not be done
without risking a heavy fall and a stoppage of the road-
way. With regard to trams, the covering—he assumed
Mr. Halbaum suggested they should be roughly covered
—would reduce the load carried and the winding capa-
city of the colliery generally. It seemed to be agreed
that large trams were the most economical; and if they
were to compete in the market with other nations, these
points must have some little weight in considering
methods. The problem before them was not only how
to make a colliery safe, but also how to achieve this
highly desirable end in the most economical way. If,
for example, the retention of trams loaded above the
top could be made possible by the application of a few
extra tons of stone dust, there was no reason why they
should cease to build up the coal above the top. The
abandonment of the present practice of loading trams
would reduce the average weight by at least 30 per
cent., whereas this could be obviated by extra stone
dusting to dilute the extra dust made by the built-up
trams. The experiments of Mr. Budge at Aberaman,
and the results at Oakdale given by Mr. Evans, proved
that the coal dust could be dealt with at a reasonable
cost, without doing away with open and built-up trams.
Assuming the output to be about 8,000 tons, the cost
of stone dusting, according to Mr. Evans’s figures,
would be rather under Id. per ton. He (Mr. Hann) was
sure it was not above that figure. That being so, he
could not see there was any cause for reviving the
canvas-covered tram, especially when the same effect
could be more easily obtained by watering the journey
before it came out.

Mr. John Kane (United National, Watts town) said
he always understood that the part of Senghenydd mine
which escaped the devastating effects of the explosion,
missed the severity of the blast because the explosion
found vent up the shaft. Turning to Mr. Budge’s paper,
he would ask the author to say exactly what he meant
by a 1 to 1 mixture. If he meant 1 lb. of coal dust
and 11b. of inert dust, they were up against a problem
that might lead to false security, arising from the
differences in specific gravity. Another point was with
regard to the nature of the stone dust used. Some
dusts had a greater affinity for water than others. The
Explosions in Mines Committee reported that a 1 to 1
mixture would prevent an ignition, but would not pre-
vent propagation. This being the case, it appeared to
him they had overcome 90 per cent, of the trouble,
because, speaking from memory, he believed 90 per
cent, of big explosions had originated not in the face,
but back on the roads. He was not saying this mix-
ture of 1 to 1 was the right thing. What he hoped for
was a dust which distilled carbon dioxide at a low
temperature. They needed to experiment with different
kinds of dust.

Size and Shape of Dust Particles.

Principal Griffiths, D.Sc., F.R.S. (University
College, Cardiff), said he assumed that what was aimed
at was to get such a dust that the proportions of inert
dust to coal dust should remain constant in all parts of
the coiiiery in a current of air. Nowt, that was not a
question of density, but of size. If they made their
dust sufficiently fine, they could disregard its density
altogether. That was to say, the question of the
carriage of dust in a draught of air had really little
concern with its density, but was almost wholly con-
cerned with its size. This was a point upon which
experiments were required. Another important matter
was the shape of the inert dust particles. It was a
question of getting the molecular sizes, rather than the
grain sizes, between which, of course, there was a great
gap. He thought he could undertake to procure an
explosion of inert dust in the laboratory. If they made
inert dust sufficiently fine, it would itself help to propa-
gate an explosion. There was the question of the
period of vibration of air.

Mr. A. S. Tallis (Tredegar Coal Company) said he
was a thorough believer in stone dusting the roadways,
and if they could not make their mines absolutely safe,
they were going a long way to make them safer than
hitherto. He did not think any mining engineer could
see a colliery that had been properly stone dusted with-
out confessing that it was a great deal safer than in the
old days, when no stone dusting was done. Of
course, for certain evils in a colliery there were things
to be done that were even more efficacious than stone
dusting. One of these was to do away with cavities
as far as possible.

Mr. J. Dyer Lewis, H.M. inspector of mines, said
he did not think there was the same care exercised
to-day in the filling of trams as was 'the case some years
ago. With regard to the Senghenydd disaster, there
was .no reason for believing that one part of the mine
was any cleaner than the rest. The velocity throughout
the pit was not very high, and the shaft itself formed
a safety valve. With regard to the seats of explosions,
it should not be forgotten, in view of what had been
said that day, that within the last 15 years in South
Wales at least three explosions had started at the face.
For his own part, he believed in a combination of
stone dusting and watering as contributories to the
greater safety of the mine. He had observed in some
pits that stone dust was distributed about haulage roads
which had not been cleaned at all. This was a waste
of money and effort, and throwing dust into people’s
eyes.

Mr. W. W. Hood said although he was a strong
advocate of watering, he did not see why stone dusting
and watering should not be carried out.

Mr. Jacob Ray (Treharris) was inclined to think that
greater safety lay in a higher proportion of stone dust
than 1 to 1.