1211

THE COLLIERY GUARDIAN.

June 6, 1913.

A RECORD OF THE ORIGIN OF THE PRINCIPLE
OF STONEDUSTING FOR THE PREVENTION
OF COLLIERY EXPLOSIONS.*

By W. E. Garforth, LL.D., M.Inst.C.E.

The author gives an account of the investigations
that have been made in connection with the danger of
coal du st in mines during the past five years, as well as
an account of the further experiments which have been
conducted underground during the past 12 months in
connection with the application of stonedust.

In the Final Report of the Royal Commission on
Accidents in Mines (1886) certain precautions are
recommended, and the concluding paragraph states
that—

“ The observation recently made in Germany, in
some preliminary expei iments, that a thin layer of
fine angular sand strewn over minedust appeared to
have the effect of preventing the communication of
flame from a blown-out shot to all but the most
inflammable dusts, is worthy of passing notice.”
None of the witnesses who had given evidence before
the 1891 Coaldust Commission tendered any informa-
tion to the Mining Association Committee as to how
stone or inert dust might be utilised as an antidote to
the coaldust danger. Most of the witnesses, including
Dr. W. N. Atkinson and Mr. J. B. Atkinson, continued
to advocate the application of water on coaldust roads.

No action to compel colliery owners to use either
watering or zones was taken by the Home Offi e, even
after serious explosions had occurred, and when it was
known that coaldust had greatly increased the death
roll. Such was the state of affairs in the early part of
1908—a period of 17 years after the publication of the
Royal Commission report in 1891, during which time no
practical antidote for the danger of coaldust had been
offered to the mining world by engineer, chemist or
other scientist.

The author then recalls the experiment on July 18,
1908, in the experimental gallery at Altofts, which was
the first occasion on which the principle was adopted of
presenting specially prepared stonedust to the flame of
a coaldust explosion.

Briefly, the experiments of the Mining Association
were the first to demonstrate—

1.	That coaldust was explosive without the presence
of inflammable gas (May 30, 1908).

2.	That dustless zones were ineffectual (June 15,
1908).

3.	That fine stonedust would extinguish an
explosive flame (July 18,1908).

It may be mentioned that the effect of fine stonedust
on the flame of an explosion is in accordance with the
opinion expressed by the writer at the coroner’s inquest
after the Altofts explosion in 1886, when, for the first
time in the history of mining, the verdict of a jury was
that the men had been killed by an explosion of coal-
dust free from an admixture of explosive gas. The
writer was so convinced, after the experiments in 1908,
of the efficacy of fine stonedust that he has felt justified
in having stonedust applied to more than 20 miles of
underground roadways in collieries with which he is
connected. At the present time those colliery owners
and officials who have witnessed the stonedust demon-
strations, and seen the application of the same inert
dust on the underground roadways during the past four
years, consider the present position of the mine to be
secure from a coaldust explosion.

At the present time there are roads in coalmines
quite as safe as those in metalliferous mines, since 10 lb.
or more of fine stonedust have been distributed for
every pound of coaldust, resting on the sides of the
roadway and on the tops of the timbers—a quantity
much in excess of the mixture of 1 lb. of stonedust with
1 lb. of coaldust which experiments in the gallery proved
to be sufficient to prevent an explosion.

Ooaldust has been successfully dealt with for the past
four years by dislodging it by fine stonedust from the
ledges and pockets, as well as from the surface of the
sides of the roadways, and the tops of timbers. Con-
sequently coaldust roads are no longer dreaded ; they
are simply regarded as a danger which ordinary pre-
cautions may avert or safely deal with, in the same way
that the workings are kept free from water or accumula-
tions of firedamp.

It may be remarked that the ordinary velocity of the
ventilating current does not take away the efficacy of
the stonedust remedy, and if a proper method of stone-
dusting be adopted, there should always be a reserve or
excess of stonedust sufficient to dilute the coaldust.

In his presidential address of 1912 certain additional
facts were given by the author, which further confirmed
previous statements regarding the explosive character
of coaldust and the efficiency of fine stonedust in extin-
guishing flame. The experience of the past 12 months
has confirmed the correctness of the suggestions and
observations stated in that address, especially as regards
the various methods of applying the stonedust that
have been tried during the past four years. The method
most successful is distribution by hand, as by this
means the dust can be better directed, and that with
requisite force to dislodge the coaldust, and replace it
by stonedust.

To make more explicit the further experimental
methods which have been conducted underground
during the last 12 months, the writer submits the
following answers to hypothetical questions:—

Question No. 1: “Is an underground roadway safe
from an explosion if after being stonedusted it becomes
again covered with a layer of coaldust ? ” Experiments
were made on certain underground intake roadways to
obtain increased velocities of air-current from a normal
speed to one of 5,000 ft. per minute. It was found

* From a paper read before the Institution of Mining
Engineers.

stonedust underground would be 35s. in a flat mine.
The total cost, therefore, of keeping such mines
efficiently stonedusted would be less than T^d. per ton
of coal produced, or an annual cost of about £200.
During the past four years the cleaning up of the
stonedust has been done in the ordinary work of keeping
the roads clear for haulage purposes, and the stonedust
has been pulverised in an ordinary pug mill.

In conclusion, the writer wishes to repeat what he
stated at Nottingham in January last, that the time has
now arrived to apply in practice what has been success-
fully proved by experiment. Experience has shown
that fine stonedust possesses those qualities which are
essential in a preventive for a coaldust danger—namely,
that it does not introduce any new form of danger,
is universally adaptable, can be supplied at a low cost,
and has on all occasions proved to be effective. It
should be looked upon as a n?cessary precaution in the
same way that ventilation is adequately maintained to
dilute and carry off noxious gases, that pumps are
maintained to prevent the flooding of the mine, and
that timbering is carried out to guard against falls of
roof. In the process of time precautions against the
danger of coaldust will be looked upon as part of the
ordinary routine of the mine. If these precautions are
carried out, then the writer believes that coalmines will
be made safer than ever before.

that at a velocity of less than 1,500 ft. per minute,
ordinary dry coaldust was not disturbed, but that |
at a greater rate the coaldust was raised and carried ’
away as a cloud in air. Stonedust, however, was not
disturbed, until the velocity reached 2,600 ft. per minute,
above which speed it was carried away by the air-
current as a cloud. A velocity of 2,600 ft. per
minute is equal to 29’5 miles per hour. In
the Altofts experimental gallery one of the
lowest velocities recorded as the speed at which
an explosion travelled was 120 miles per hour, or four
times faster than the air-current which has been found
necessary to raise stonedust and coaldust as a cloud.
The recorded rate of travel of some of the more violent
explosions in the gallery was 300 miles, and in one
particular explosion 1,300 miles per hour. Hence it
will be understood that even if there were a layer of
coaldust overlying the stonedust on an underground
roadway, the whole of the dust present would be raised
by the “pioneering wave” of an explosion. Judging
from the records obtained at the experimental gallery,
the explosion would be stopped and the further produc-
tion of poisonous gases in the roadways prevented,
provided that the percentage of stonedust was not less
than that suggested.

Question No. 2: “ Can stonedust be applied to all
places where the coaldust is liable to be deposited ? ”
During these experiments stonedust has been thrown
by hand to dislodge the accumulations of coaldust
resting on the roof timbers and to replace them by
stonedust. The stonedust-laden air current distributes
the dust on the roof ti mbers and in the interstices of
the mine. When settled the bulk of the stonedust will
remain in position harmlessly resting on the strata
ready for use. As an example of how stonedust may be
carried on an air current, 2| tons of stonedust were
placed on a main intake road along which 90,000 cubic
feet of air were passing per minute. A jet of compressed
air was introduced into the heap of stonedust to raise
the dust in the air current. The heavier dust settled in
the first 600 ft., but the lighter dust was carried for a
distance of two miles, a larger proportion settling on
the roof than on the floor.

Question No. 3: “ What quantity of stonedust is it
necessary to use on a roadway in which there is a thick
deposit of fine coaldust ? ” As already suggested, first
dislodge the coaldust and replace it with stonedust. The
road is now treated with a heavy dressing of stonedust,
the quantity varying with the size of the road. As any
excess of stonedust increases the margin or factor of
safety, it is a cheap form of insurance against explo-
sions. An average first dressing at Alrofts is 2 cwt. per
lineal yard, as it has been found in practice more
economical to apply heavy dressings at longer intervals
than light ones at shorter intervals. The principle
which has been constantly kept in view has been to
preserve the same grey appearance on the roadways as
the return airways of the Silkstone pit were known to
have had before and after the explosion in 1886. The
first dressing, when the proportion of coaldust present
was large, was a quantity of stonedust in the proportion
of, say, 4 to 1; but, as the additional dressings of stone-
dust have been added to the earlier dressings, the
percentage of coaldust has in consequence been reduced
until there are on some roads as much as 20 of stone-
dust to 1 of coaldust. No regular practice is made of
analysing the proportion of dust present, but when it is
thought necessary analyses are made.

Question No. 4 : “ What is the best kind of inert dust
to use ? ” The opininion expressed by Dr, Haldane, the
investigations by Dr. Beattie, and the microscopical
examinations show that the best material, from the
physiological standpoint, is soft shale as free as possible
from crystalline silica. Neither coaldust nor stonedust
affect the general health of the miner, and each may be
regarded as equally harmless.

In addition to the opinions expressed by scientists, it
may be stated that the men working in the stone for
the past 30 years are equal in health and robustness to
the men who have been working in the coal.

Stonedust by reason of its light-grey colour readily
shows any fresh deposit of coaldust. Its greater weight
and its binding qualities allow it to be thrown in such a
way as to displace coaldust from the ledges on which it
rests, which lighter inert dusts are incapable of doing.

So long as a large proportion of the dust is sufficiently
finely pulverised, as already shown, the slightly greater
weight is not detrimental, as both stone and coal dusts
are easily raised by air currents of velocities far lower
than those of the slowest explosive blasts. Stonedust
has a steep angle of repose, and owing to its clinging
qualities it adheres to vertical surfaces. Fluedust does
not possess these advantages; owing to its lightness
and its shape it does not bind so well in the hand,
it does not displace coaldust from the sides and ledges,
and in consequence of its darker colour it is difficult to
tell what proportion of coaldust is mixed with it, and
there have been strong complaints, from the men at
Altofts who have had to deal with it, of its effects on the
eyes, nose and throat. As stated last year, it is not a
healthy dust to breathe, and if it were introduced in the
underground roadways before it has been tested by
Dr. Beattie (who the writer suggests should make
similar experiments to those he made with stone-
dust), and before further experiments as to its suita-
bility have been made, it might cause considerable
inconvenience.

Question No. 5: “ What would be the cost of pre-
paring the stonedust ? ” Including depreciation, repairs
of plant, power and labour, the cost of pulverising the
dust has been ascertained to be about 2s. per ton.
Twenty tons of dust per week are sufficient for a large
colliery raising over half-a-million tons of coal a year,
working four seams—some of the coal faces 2§ miles
distant from the winding shaft—or say £2 per week for
preparing the dust. The cost of applying 20 tons of

THE SLOW COMBUSTION OF COALDUST ANO
ITS THERMAL VALUE *

By F. E. E. Lamplough, M.A., and A. Muriel
Hill, B.Sc.

The investigations described were undertaken, in
connection with the experimental work of the Doncaster
Coalowners’ Committee, to elucidate to some extent the
nature of the chemical changes involved in the slow
combustion of coaldust, and to determine if possible the
value of the heat changes accompanying the oxidation
of different varieties of coal.

Work on the subject was commenced in July 1912,
and attempts were first made to determine quantitatively
the course of the oxidation which occurred in coaldust
maintained at a temperature of 110 degs. Cent, in
electric heaters. In September, Dr. Haldane suggested
that if the difficulties involved in the determination
of the thermal effect accompanying oxidation could
be overcome successfully, the investigation would be of
considerable practical value, and at the same time
would throw important light on the problem under
consideration. Financial assistance was promised by
the Doncaster Coalowners’ Committee, and work in
this direction was at once commenced.

Experimental Method.—The evolution of heat was
measured by carrying out the oxidation of coaldust in a
vacuum flask immersed in a water bath of regulated
temperature. A sensitive thermo-electric method was
used to record and measure continuously the tempera-
ture of the coaldust. The absorption of oxygen was
determined by recording the fall in pressure continuously
by specially-devised apparatus. Analyses were made at
the beginning and end of the experiment to correct for
the carbon dioxide developed and for the evolution of
methane, &c. The experimental details and diagrams
will be included in the paper as finally published.

Experimental Results.—The coals experimented with
were obtained from different localities. Some samples
from the Bullhurst seam were kindly sent by Mr. A. M.
Henshaw. Other samples of coal were supplied by the-
Doncaster Coalowners’ Committee. Numerous experi-
ments were carried out before it was possible to overcome-
all the difficulties of working with the elaborate
apparatus, and to make the method suitable for a
reliable quantitative measurement of the heat produced
during the absorption of a known volume of oxygen ;
but in all these preliminary experiments the results were
qualitatively the same, and it was obvious early in the
work that there was a close connection between the
evolution of heat and the absorption of oxygen.

Table I. is a summary of the results of reliable experi-
ments obtained with the final form of the apparatus. In
no case does heating take place without absorption of
oxygen, and, conversely, absorption of oxygen is always-
attended by heating of the coal. These two processes
are obviously very intimately connected, and, taking
into account the very different qualities of coal examined,,
the numbers in the last column, which give the amounts
of heat evolved during the absorption of 1 cubic centi-
metre of oxygen, are surprisingly concordant. The
mean value obtained is 3’4 calories per cubic centimetre
of dry oxygen measured at 0 deg. Cent. When carbon
is burnt to carbon monoxide and to carbon dioxide, the
respective amounts of heat liberated are 2’6 and 4’4
calories during the consumption of 1 cubic centimetre
of oxygen. All the samples, with the exception of
anthracite, Bullhurst, and iron pyrites, were from the
Barnsley seam.

In the case of “ top softs” a sample of gas was taken
during the progress of the experiment, and using the
analysis of this sample, calculations were made of the
thermal value of the absorption which occurred during
two periods—the first six hours and the following
17 hours of the experiment. It will be seen that the
values in the two parts of the experiment were nearly
the same.

An experiment was made with a sample of pyrites
occurring in coal. The material was collected from the
coal thrown away during unloading in a large coalyardr
and the sample was apparently free from carbonaceous
matter. The thermal value of the total chemical
changes taking place during the oxidation of pyrites
proved to be very dose to the value given for the oxida-

* From a paper read before the Institution of Mining
Engineers.