1166

THE COLLIERY GUARDIAN.

June 22, 1917.

it by a moist and warm air current, must be evap-
orated before the temperature can rise above boiling
point; and the large amount of heat required to
evaporate moisture greatly retards the heating of coa]
until the boiling point has been passed, as pointed out
in detail by Mr. Winmill. But coal which has been
considerably raised in temperature by deposit of
moisture will usually combine with a far greater
amount of oxygen than at the lower temperature, and
will therefore continue to heat much more rapidly
than would otherwise be the case. Thus the Barnsley
Hard coal at Doncaster takes up seven or eight time >
as much oxygen, at boiling point as at 86 degs. Fahr.,
although the same rule certainly does not apply to
other varieties of coal—for instance, the Welsh steam
coal examined by Mr. Winmill.

In a thick seam of coal, or in a correspondingly dee])
stack of coal on the surface, it appears that there is
sufficient dep'th for up and down convection currents
to cause cumulative heating. Many of the fires occur-
ring in breaks of the coal along the sides of main roads
at Hamstead Colliery, as recorded by Mr. Meacheni
and the writer, appear to have originated in this way,
although perhaps still more were due to a constant
slow current of air through breaks connected at one
end *with the intake and at the other with the return
by the side of a stopping or air bridge.

* Considerable fresh light has been thrown by the
Doncaster investigations on the nature of the chemical
change in the oxidation of coal. Mr. Winmill showed
quite conclusively that in. the Barnsley and many other
seams it is not pyrites to any appreciable extent, but
other substances in the coal, that oxidise. The total
oxidation at any temperature up to 160 degs. Cent, is
limited, but increases with the temperature, so that at
160 degs. Cent, about 50 times as much oxygen is taken
up as at 30 degs. Cent. In this oxidation hardly any
carbon dioxide is formed at low temperatures; but
Mr. Graham has found that at low temperatures about
30 to 40 per cent, of the oxygen taken up is given off
again as water, as compared with only about 5 per
cent, as carbon dioxide. As the temperature rises,
however, the proportion given off as carbon dioxide
increases, until at 160 degs. Cent, about half the
oxygen taken is given off as CO2. By observing the
relation between disappearance of oxygen and increase
of carbon dioxide in air coming from a heated area, it
is thus possible, in the case of the Barnsley seam at
Doncaster, to form some idea of the temperature of
the heated coal. From this altered relation between
oxygen absorption and carbon dioxide production, it
seems probable that the heat liberated per unit of
oxygen absorption is considerably greater at high than
at low temperatures.

Influence of Pyrites.

In some other kinds of coal, the oxidation of pyrites
was found to play a considerable part in the heat pro-
duction, and probably a predominating part in certain
cases. The behaviour of various kinds of pyrites as
regard oxidation was carefully investigated by Mr.
Winmill. The lumps of pyrites in the Barnsley seam
hardly oxidise at all, and remain bright almost indefi-
nitely. In some other seams the more impure and
disseminated pyrites oxidises very readily, as is easily
proved from the sulphuric acid formed. But when the
crystals of the stable pyrites were ground up into a
powder, very rapid oxidation occurred. The cubical
crystals of pyrites found in some forms of slate are
well known for their absolute resistance to oxidation,
since they remain bright and show no signs of oxida-
tion even after exposure to the weather for an indefi-
nite time on the roof of a house. Mr. Graham found
that even these crystals oxidise very readily when
powdered. The crystalline form appears to protect
the pyrites in some way from oxidation. Mr. Winmill
found that the heat formed per litre of oxygen
absorbed by pyrites at ordinary temperatures is about
4*5 calories, or fully twice as much as in the oxidation
of coal at the same temperature. Powdered pyrites
will fire very rapidly in an air current if loss of heat
is prevented.

The writer was formerly inclined to' attribute to
pyrites the main part in the spontaneous heating of
coal, and in the oxidation processes which occur gener-
ally in underground workings. The oxidation of
pyrites seemed to account for the very limited amount
of oxygen absorption which occurs, since the propor-
tion of pyrites is very limited. The liberation, by the
sulphuric acid formed, of carbon dioxide from the car-
bonate of lime or magnesia present in or near the coal,
seemed to account for the carbon dioxide found in
•ordinary blackdamp, which, as Dr. Atkinson and the
writer pointed out, is nothing but the residual gas
from an oxidation process. The oxidation of pyrites
does account satisfactorily for the formation of black-
damp in some cases ; but it is now clear that in the
heating of coal another type of oxidation is usually
involved. There remains, however, a puzzle. In the
oxidation of coal at low temperatures, as studied
hitherto in the Doncaster experiments, oxygen dis-
appears, but hardly any carbon dioxide is formed.
But if the return air of an average mine or a sample
of average blackdamp is analysed, it is found that
usually at least half as much carbon dioxide has been
given off in the mine as oxygen has disappeared.
Where does this carbon dioxide come from? The
writer never doubted that a study of the oxidation of
coal would reveal its source; but hitherto this expec-
tation has not been realised, as (except in the case of
lignite) every sample of coal studied in the laboratory
has hitherto steadfastly refused to give off enough of
carbon dioxide to account for the proportion present
in return air. This is one of the questions on which
Mr. Graham is engaged at present; and its solution
may throw much further light on the whole subject.

Influence of Oxygen Percentage.

Mr. Winmill studied the relation between the per-
centage of oxygen present in tfie air and the rate of
oxidation, other conditions being equal. He found
that the rate of oxidation varies, not directly, but
about as the square root of the oxygen percentage. In

order to diminish the rate of oxidation to half, it
would thus be necessary to diminish the oxygen per-
centage to about a fourth. If the oxygen percentage
be diminished by about a sixth, a candle or lamp will
cease to burn, but the rate of spontaneous oxidation
of coal will only be diminished by about a twelfth. It
is therefore practically useless to endeavour to stop
spontaneous heating by merely making so slight a
reduction in the oxygen percentage of the air. Spon-
taneous heating will go on even if the oxygen per-
centage is so far reduced that a man or animal would
rapidly die in convulsions if exposed to the air.

An interesting point, confirmed by Messrs. Winmill
and Graham, is that a little carbon monoxide was
always formed in the laboratory experiments on spon-
taneous oxidation of coal at ordinary temperatures.
Even when a large excess of oxygen was present, this
carbon monoxide was formed. Much to his surprise,
the writer had discovered the formation of carbon
monoxide in 1898,* but failed to discover it in ordi-
nary samples of blackdamp collected underground.
Here there seems to be another puzzle with regard to
the changes in mine air. Mysterious cases of carbon
monoxide poisoning where no appreciable heating of
coal was present are occasionally reported—for
instance, among men working in coal bunkers; but
usually the occurrence of carbon monoxide poisoning
is taken as evidence of heating in coal. As a matter
of fact, Mr. Graham found that the higher the tem-
perature at which coal is being oxidised, the greater is
the proportion of the oxygen absorbed that is given off
again as carbon monoxide. As regards both the pre-
sence of so much carbon dioxide and the absence of
carbon monoxide, the blackdamp ordinarily met with
in mines differs from the laboratory blackdamp of
oxidation experiments with coal, and the reason of
this is still an unsolved mystery pointing to the need
of further investigation.

Practical Applications.

The practical application of the knowledge gained
by laboratory investigation of spontaneous heating is
’a matter for. mining engineers rather than for scien-
tific investigators. A mining engineer has to con-
sider many other things than the mere prevention of
spontaneous fires. In particular, he has to work a
mine so that it shall pay, and that the method of
working shall be as safe as possible all round. He
may thus have to leave coal underground at the risk
of spontaneous fires, or in other ways to adopt methods
of working which entail risks of heating. Neverthe-
less, it may be of service if, by way of initiating dis-
cussion, some provisional practical applications are
referred to at the close of this paper. It must be
understood, however, that most of these conclusions
have already been arrived at in other ways than as
deductions from scientific experiments.

In the first place, it would seem well’ worth while to
have a chemical examination made of the coal worked
at any mine with regard to its tendency to spontaneous
heating. The results of such an examination might
be of great importance towards coming to a decision
as to the best method of laying out and working, par-
. ticularly in the case of deep seams where special
trouble in preventing crushing may be anticipated.

If the coal is liable to heating, and particularly if
it crushes easily, the method of working should be such
as to reduce to a minimum the occurrence of crushing.
This can be done by avoiding narrow pillars or ribs,
and protecting edges of unworked coal by stone sup-
ports to take off pressure ; also, by carefully avoiding
falls of roof coal if any such coal has to be left. In
the case of a very thick seam, where the sides of a
road in solid coal are liable to breaks, it may be advis-
able to have the main roads above or below the coal.

In order to avoid as far as possible percolation of
air through goaves containing coal, it seems desirable
to keep the ventilating pressure as low as possible.
The flow of air through broken coal or other porous
material varies directly as the pressure, and not as the
square root of the pressure. Hence a low ventilating
pressure seems speciallv important. In order to work
with a low ventilating pressure, it is, of course, neces-
sary to have roomy roads. As a further safeguard, it
seems desirable to avoid as far as possible air crossings,
etc., where air might be slowly sucked through broken
coal, and to have the cross gates and gate roads ripped,
so that they are well above the goaf. If by these or
other means the goaf can be kent so airtight that onlv
firedamp or blackdamp is contained in its interstices,
it cannot fire. Any disused road which might serve as
a channel of access of air to the goaf should be stowed
as airtight as possible. The roof should be allowed to
settle down evenlv behind an advancing face, with a
minimum of breakage of anv roof coal left. To this
end all prons should be drawn. Gob-fires have often
been found to stnH near a prop which has been left in.

When a seam is not flat, or where faults are met
with, the control of spontaneous combustion becomes
more complex, since convection currents due to differ-
ence of temperature must tend to cause air currents
in the goaf, and coal mw have to be left near a fault.
Much study of the conditions will be required in order
to find means of meeting these conditions by suitable
methods of working.

Gob-fires could, of course, be entirely avoided if it
were possible to remove the whole of the coal, good or
bad. including slack. Considering the potential value
M even the most inferior coal, it is a national reproach
that a,nv coal should be wasted by being left undor-
o-round or in waste beans on the surface. But for this
it is not merelv the mining engineer wlio is to blame.
The nroner utilisation of coal of every sort, and speci-
ally inferior coal which is at present wasted instead
of beino- made to vield up its potential energy and
valuable bv-nroducts, is a national problem which is
now claiming more and more attention. With the
gradual solution of this problem we may hope that the
necessitv for troubling about gob-fires will to a, large
extent disappear.

Practical experience has shown that when from any

* Trans. Inst. M. E., 1898, vol. xvi., p. 457.

cause the temperature of broken coal has been raised,
even after long previous exposure to air, spontaneous
firing is apt to occur. This is now much more intelli-
gible ; for although previous exposure to air may have
exhausted the substances oxidisable at an ordinary
temperature, the rise or temperature renders further
substances liable to oxidation, so that oxidation is
again started. Coal that has been warmed by the sun.
or by proximity to some source of heat, may thus easily
fire under favourable conditions. In warm weather
special care is needed not to stack coal in too thick a
layer, although coal which has once been well exposed
to air in a not too thick layer will be all the safer after-
wards, so that the height of the stack can safely be
added to. The reasons for avoiding heating by the
sun, or breakage of coal, when a cargo is shipped are
now evident. The reason why the height of a large
stack of coal can be safely increased if efficient means
are taken for the ventilation of the stack are equally
evident. Prof. Porter, in the report already referred
to, describes, with diagrams and photographs, the very
simple and efficient ventilation method adopted by the
Canadian Pacific Railway Company for preventing
spontaneous fires in their large stacks of coal.

The main practical outcome, so far, of the Doncaster
investigations has been to supply much more definitely
the reasons for measures which practical experience,
often very dearly won, has shown to be necessary in
individual cases. The fresh knowledge acquired will,
however, the writer trusts, make it much more easy
to predict what practical measures are, or are not,
really needed, and to devise new and improved
methods of preventing spontaneous fires in coal.

The writer cannot conclude this paper without some
reference to the two men, now gone, who initiated the
Doncaster laboratory, and whose help and advice were
invaluable at every step. The original. Doncaster Coal
Owners’ Research Committee consisted of Sir Arthur
Markham, as chairman, representing the Brodsworth
and Bullcroft colliery companies, and Mr. J. W.
Fryar, as secretary, representing Messrs. Barber,
Walker and Company. Both have fallen in the war—
not on the battlefield, but as a result of their whole-
hearted devotion to the greatly increased work and
responsibility thrown on them. Nothing which the
writer could say can adequately express his sense of
the personal loss which their deaths have been to him.
Both were men of great practical ability, wide out-
look, and strong human sympathies; and both were
always ready, at whatever personal cost, to fight for
what they considered right and in the public interest.
To their inspiration is mainly due such success as has
attended the work of the Doncaster laboratory.

ASBESTOS ROOFING.

We had an opportunity recently of inspecting the
new works erected by Messrs, the British Everite and
Asbestolite Works Limited at Farnworth and Bold,
near Warrington, Lancashire, for the manufacture of
asbestos-cement corrugated and flat roofing sheets and
tiles.

These sheets were introduced some years ago by
Messrs. Felber, Jucker and Company, of Manchester,
and Mr. Jucker, the managing director, of the new
company, specially interested himself in this particular
building material. It was originally made in Canada
and Switzerland, and there being a demand for a
material to take the place of galvanised iron corru-
gated sheeting, the life of which in some parts of
industrial Lancashire is not more than from three to
five years, Mr. Jucker entered into negotiations with
the Societe Suisse des Usines Eternit for the manufac-
ture of asbestos-cement corrugated sheeting, with the
corrugations made exactly to • the same dimensions as
the British standard for corrugated iron sheeting. The
result is that as replacements and repairs to a
building made of iron sheeting becomes necessary,
these may be now made with asbestos-cement sheets
of exactly the same size.

The value of this material is, of course, its long life
compared to the life of galvanised iron, as it is not
acted upon by any deleterious atmospheric fumes or
corrosion from ordinary weather conditions. It is
composed of a mixture of finely-ground asbestos and
cement, made in a machine which gives it a fibrous
texture, and builds up the plate in layers until the
requisite thickness is reached, whereupon it is passed
to the drying room, where it remains for about 36
hours, after which it is matured for some weeks in the
open air before being sold. It is not affected by any
fumes from chemical works, and only improves with
age and ordinary weather conditions.

It is adapted for and used in exactly the same way
as corrugated iron sheeting, being nailed on to wood
purlins or standards, and fastened by hook bolts on
angle iron purlins. It is strong and reliable, and can
be used with great advantage instead of corrugated
iron sheets, which it seems capable of displacing to a
large extent for building purposes, in course of time.

Saskatchewan Lignite Deposits.—According to a report
in the Gazette, (Montreal), the Federal Government is to
expend 300,000 dots, (about £61,000) in developing the
lignite deposits at Estevan, in Saskatchewan. Prelimi-
nary investigations are to be carried out by the Saskatche-
wan University. The development of these deposits, it
is believed, will solve the fuel problem of this province.

Coal for Steam Boiler Purposes.-—The Controller of Coal
Mines recently circulated a large number of forms asking
for information regarding coal used by steam boiler owners
throughout the country. Many of these have not yet been
returned, and firms who have received such forms are
therefore requested to forward the particulars asked for
to the Controller at once. Otherwise it may not be pos-
sible for their requirements to receive consideration in
connection with a scheme which is being prepared to
enable a suitable supply to be maintained for all concerned.
Any firms or persons using coal for land boilers who have
not received a copy of the form should apply to the Con-
troller of Coal Mines, Box D.S.R., 8, Richmond-terrace,
Whitehall, S.W. 1.