548

THE COLLIERY GUARDIAN.

September 22, 1916.

STONE DUSTING IN STEAM COAL
COLLIERIES*

By G. D. Budge.

Absolute compliance with the recommendations of the
Explosions in Mines Committee on the Stone-dusting of
Mines is a matter of no small difficulty, and is almost
impassible in all parts of a colliery, unless persistent
efforts are directed towards reducing the “ make ” of
coal dust. The writer therefore wishes in the first place
to refer to some of the sources of the finest coal dust.

Colliery Screens.—It has been proved -that 11b. of
coal dust to every 100 cu. ft. of air space gives on
ignition a spreading flame in a gallery; and if a gallery
is at all representative of an underground roadway, it
would seem that every hundredweight of screen dust
which enters the downcast in the air current is suffi-
cient to give 75 yds. of roadway of average section, the
minimum mixture for propagation. The comparison is
made merely to emphasise the necessity for satisfactory
dust extraction plant on colliery screens, which are
situated close to the downcast shaft.

Trams.—It is unfortunate that the only really dust-
proof tram, namely, the box tram, cannot be used gene-
rally. Trams with doors will always liberate coal dust,
or even coal, and in collieries, where so much rubbish
has to be unloaded underground, it is unlikely that any
extended use of doorless trams will be practicable. At
some collieries 15 per cent, of the trams are of this type,
and little inconvenience is experienced.

Dust in Empty Trams.—A considerable quantity of
dust is carried down the pit in the empty trams, and it
is an excellent plan to fix a spray on the empty road
leading from the screens to the shaft, in order to
thoroughly drench the trams.

Dust made in Shaft.—If the shaft is wet, it is some-
times worth while to allow a slight overflow from the
garlands. It is possible in this way to deal with some
of the finest dust which would otherwise be deposited
on the roadways.

Haulage.—It is difficult to estimate the weight of
dust raised by journeys, but approximately, the dust
cloud raised by a journey travelling at six miles per
hour is only one-third of that raised when the speed is
12 miles per hour. High-speed haulage should cer-
tainly be avoided. A badly laid road has also a detri-
mental effect.

Filling of Trams at the Coal Face.—Much depends
on the method of filling - a tram. A recent return of
results from a haulage road showed a reduction of 20
per cent, in ash, although the usual rate of stone-
dusting had not been altered. The explanation was that
the velocity of the air had been largely increased owing
to the speed of the fan having been raised; and, further,
that a considerable quantity of coal was coming from
some solid headings,-where all the small coal was filled
out, and the tops of the trams were so heavily bedded
with this small coal that a dense dust cloud was raised
from each journey.

Ventilation.—Where it is possible to keep down the
velocity of the air on haulage roads, the quantity of
dust to be dealt with will be reduced.

Watering.

Where conditions will admit of it, water can be used
to such good purpose that it ought still to play a con-
siderable part in dealing with coal dust. If a road has
settled to such an extent that the use of water does not
seriously affect the repairs, it is certainly worth while
to keep the floor of an engine plane in a moist condition.
The dust, made by the grinding action of trams, and
traffic of all descriptions, is thus prevented from being
raised into the air by passing journeys. A combination
of the two methods, that is, stone-dusting the roof and
sides and watering. the floor, gives quite the most
effective system that can be adopted. In one respect
water is absolutely essential—namely, in the form of a
spray directed on to journeys of coal, before or immedi-
ately after leaving double partings. It is important to
emphasise the necessity for using water in this way
where conditions will admit of it, and, further, of
accepting consequences up to a reasonable limit where
conditions will not admit of it. It is the best method
of reducing the film of coal dust which forms on the
top of the stone dust.

Stone Dusting.

In approaching the subject of the application of stone
dust, it is well to deal in the first place with the material
which is available at collieries for grinding purposes.
It is necessary to determine by analysis the percentage
of crystalline silica and other hard substances in the
material. In many collieries there is an ample supply
containing no harmful proportion of these.

At some collieries whe-pe coal-cutters are at work in
stone, stone dust of a kind is produced; but it is not of
the required fineness, and there is always a certain
amount of coal ground up, owing to pieces falling on the
machine. The stone may also contain harmful quan-
tities of hard material. It is therefore generally
impossible to obtain a standard quality of dust from
this source.

Experiments have been carried out with flue dust,
and also with dust taken from economisers, but the
writer has not found any material of this description
which is satisfactory. Apart from the carbon present in
flue dust (which may even amount to 50 per cent.),
there is generally present a varying quantity of fused
silicates, which under microscopic examination prove
to be in the form of sharp-edged and needle-like
particles. These silicates, if inhaled, might bring
about the fibroid condition of the lung, which is specifi-
cally mentioned in the First Report of the Explosives in
Mines Committee as being so dangerous.

* From Proceedings of the South Wales Institute of Engi-
neers (by permission),

It is therefore necessary to giind the dust from care-
fully selected material, and a good result is obtained
from a Griffin mill, which receives the material from a
stonebreaker in inch cube size. The principle on which
this mill works is right, the action being purely centri-
fugal. It has also sufficient flexibility to deal with hard
materials which may inadvertently be put into it.

The finished product can best be taken underground
in ordinary trams, without covers. Any dust that is
blown off the top of the tram will do more effectual
work than any dust which is applied by artificial means,
for the simple reason that, given the correct degree of
fineness, the deposition will follow similar lines to the
coal dust deposition. A brattice cloth lining is necessary
in the tram, but there is no waste in this, as the brattice
cloth can be used underground. If stone dust is con-
veyed to a colliery in bags, the bags ought not to be
sent down the pit, as, regularity of supply being all
important, the best way of ensuring this is to make use
only of such appliances as are essential to the output of
coal being maintained. There is little likelihood of a
tram of stone dust not being sent promptly to its desti-
nation. It is advisable to provide a number of storages
in certain places, and also in each fireman's district.
Many firemen prefer to keep a cask full of stone dust in
each heading.

The question of the degree of fineness to which stone
dust should be ground is one on which there is con-
siderable difference of opinion and can only be decided
after careful examination of dust samples taken from
all parts of a colliery.

In order to arrive at the degree of fineness of repre-
sentative accumulations of coal dust, a few samples
were taken from places where the process of deposition
was not disturbed by journeys of coal passing, or by
men working. It was found that, while a proportion of
the dust at the coal face is of the coarser description,
there is generally present a dangerous quantity of dust
sufficiently fine to pass through a 180 sieve. Secondly,
that accumulations present in sheltered places and in
return airways are composed mainly of the finest
particles; and, thirdly, that the dust found in suspen-
sion in the air on main haulage roads is the finest of

3'aoiti Tee

Fig. 1.—Ejector for Road Cleaning and for Applying Stone Dust.

When Used for Cleaning.—The suction hose (a) is not used for road cleaning, and is disconnected at (d). The
compressed-air jet (6) induces a strong air current through the pipe (d), and this air current is used to blow the
dust out of all the crevices.

When Used for Stone-dusting.—Suction hose (a) dips into a tram full of stone dust. The compressed air jet (6)
induces a strong air current through the suction hose (a), and stone dust is drawn through and projected with
considerable force at the outlet (c).

all. This forms the film which overlies the stone dust,
and which, under exceptional circumstance^, has been
proved to be capable of carrying on an explosion of mild
intensity without disturbing the stone dust. However
finely the stone dust is ground, it can never rise in
suspension in the atmosphere with the facility of coal
dust.

The writer is therefore of the opinion that, although
the disturbance produced by an explosion would cause a
coarse stone dust to be displaced, it is better to err on
the safe side, and, to use incombustible dust, 70 per
cent, of which will pass through a 180 sieve. It will be
found impossible in practice actually to pass this pro-
portion through the 180 sieve, as the sieve clogs, but if
a 50 sieve is fixed in the grinding mill the product will
generally be found to be of the degree of fineness recom-
mended. If the whole of the dust has to pass through
a 50 sieve, at least 70 per cent, of it will pass through a
180 sieve.

Applying Stone Dust.

After a road has been cleaned by hand, and after the
sides and roof have been thoroughly brushed, there will
certainly remain coal dust in dangerous quantity.
Various mechanical appliances are in use for cleaning
roads, also for applying stone dust. One of such appli-
ances is shown in fig. 1; but until some arrangement is
devised which will catch the dust and prevent it from
escaping into the workings and return airways, the
excellent work done by the ejector illustrated is of little
avail. By this method a road can be made practically
free of dust, but a cloud of dust can be observed
1,000 yds. distant from the point at which the ejector is
at work, in spite of the use of fine sprays through which
the dust must pass. Still the principle is right, and all
that is required is a satisfactory collector.

With this ejector -a stretch of main haulage road,
610yds. long, was cleaned, the work being commenced
at the pit bottom, and the dust driven inbye with the
aid of fine sprays, each accounting for about 1 cwt. of
dust. The strong air current, induced by the com-
pressed air jet, was directed into all holes above timber.
Careful examination after cleaning showed no visible
sign of coal dust. Stone dust was then applied with
the same ejector, beginning 50 yds. from the inbye end
of the selected stretch of roadway. The quantity used

wa*s 10,2001b.—equivalent to 16-7 lb. per yard of road-
way. Samples were taken after three days had elapsed
and after 300 tons of coal had passed over the road, and
were found to contain 86-8 to 89*9 per cent, of ash. A
deposit of | in. in thickness of stone dust could be
observed on timber 15 ft. above rail level.

Another useful appliance for distributing stone dust
is illustrated in fig. 2, designed by Mr. George Hann.
The results obtained after continued use of this distri-
butor have been very satisfactory. The idea is to raise
in suspension in the atmosphere a quantity of incom-
bustible dust equal to or in excess of the quantity of coal
dust raised by passing journeys. The dust distributed in
this way does really effective work, being in suspension
in the air when it is actually required, and meeting
with coal dust under conditions much more favourable
to intimate intermixture than is possible by any other
means. The adjustment can be 'arranged to release
varying quantities, and 0-72 oz. of dust per tram i»s the
quantity fixed for a road on which 800 trams per day are
travelling.

No rule can be laid down to specify the quantity of
stone dust to be applied or the frequency of stone dust-
ing, and everything depends on the conditions existing
at the different parts of a colliery. The writer proposes
to discuss the results obtained, commencing at the coal
face, and continuing from stall roads back to the down-
cast pit.

At the Coal Face.

In the first place, the area in the vicinity of a blower
of gas ought to be thoroughly stone dusted: in this way
a real precaution is taken where the material for
ignition is known to exist. Firemen should be bound
to’ stone dust a blower in the same way as they are
bound to try to clear it.

A sample of dust taken at the coal face •will vary in
ash according to the amount of stone present in the coal
seam, the friability of the stone and of the coal, and to
the thickness and nature of the rippings required on
the road. The variation is generally within the limits
of 3-5 per cent, and 30 per cent.

In one case 10 lb. of dust which would pass through
a safety lamp gauze were taken from an area of

22-5 sq. ft. representing about 58 cu. ft. of air space. It
may therefore be said that there is normally present at
the coal face 15 times as much dust as is necessary to
give the conditions suitable for -the lower limit of propa-
gation.

When the area in the vicinity of a blower of gas is
thoroughly stone dusted, the well-defined danger is
dealt with, but it is important to realise -that there may
be present in the coal face the material necessary to
cause inflammation of such violence that a 50 per cent,
mixture throughout the roadways would have merely a
retarding but not a “ stopping ” effect.

Roads in the Proximity of the Coal Face.

Stall roads and headings, cutting faces, contain dust
which varies between the limits of 3 per cent, and 50
per cent., the general variation being from 10 per cent,
to 50 per cent.

A seam in which there are beds of friable stone, and
where the roof is soft, causing pieces to powder when
ripped down, will yield a dust fairly high in ash; more
particularly is this the case if the roads require second
or third rippings. The most dangerous stall road, as
far as coal dust is concerned, is the road driven in a
clean seam of coal under an excellent roof.

In order to obtain reliable information, tests were
made on a large number of roads, and gave results show-
ing that an application of 11*8 lb. per yard (along with
subsequent shot-firing stone dustings) is sufficient to
maintain an average percentage of a^sh throughout these
roads of 65-9, after a lapse of 100 days from the date of
the last stone dusting.

With regard to stone dust applied before shot-firing,
in one instance 401b. of stone dust were applied, which
is sufficient to raise a cloud so dense that the shotman
on it finds it necessary to stand on the intake side when
firing. The results showed that little of the stone dust
remained on the stall road, and although in this parti-
cular case 130 yds. of roadway bad actually received an
application amounting to 301b. per yard, no permanent
result was obtained.

Where the conditions permit of shot-firing, it will
generally be found that the air current in the coal face
is stronger than that on the road, and undoubtedly the
stone dust i.s carried along the coal face; it is surprising