June 9, 1916.

THE COLLIERY GUARDIAN.

1103

As a check on the assumptions involved in these calcu-
lations, a direct determination of the heat evolved
during the oxidation has been made. A quantity of
pyrites was placed in a vacuum flask, a measured quan-
tity of oxygen admitted, and the rise in temperature
(usually about 50 degs. Cent.) noted. The experiment
was usually started at 40 degs. Cent. So as. to avoid
loss of heat from the vacuum flask, the bath in which
it was immersed was heated at a rate which kept its
temperature 1 deg. Cent, behind that of the contents
of the flask. The measurement of the quantity of
oxygen absorbed was checked by determining the quan-
tity of sulphate produced by the oxidation. The results
obtained were not very concordant, the mean being 4-5
calories, with an error of ± 0-3 calorie. They, however,
confirm the results. of the calculations, and the best
value for the oxidation under conditions such as exist
in a mine would seem to be 4-3 calories per cubic centi-
metre of oxygen absorbed.

The heat produced by the oxidation of pyrites is,
therefore, twice that produced by the absorption of a
similar quantity of oxygen by coal. Owing to this fact,
and the lower specific heat of pyrites, which is only
about two-thirds that of coal, the same oxygen absorp-
tion in both cases would raise the temperature of pyrites
three times as much as it would that of an equal weight
of coal. Since the heat production is so large, it
becomes of importance to know the rate of oxidation of
pyrites, as if this rate approaches that of coal, the former
will evidently be a possible cause of heating.

Hate of Oxidation of Pyrites.—The rate of oxidation
was determined in the manner adopted for coal oxida-
tions, at 30degs. Cent., with the following results,
which are expressed in cubic centimetres per 100 gramme
hour :—

Sample A (North Staffordshire seam) : Ground to pass
a 60-mesh sieve. The sample contained 81*3 per cent,
of pyrites, the remainder being coaly matter. (Results
corrected to^represent 100 per cent, of pyrites). Oxygen
absorbed in 96 hours, 1,065 c.c.; in 168 hours, 1,670 c.c.
' Sample B (North Staffordshire seam) : Ground to pass
a 10-mesh sieve, but not 30-mesh. Contained 80 per
cent, of pyrites. (Results corrected to represent 100
per cent, of pyrites). Oxygen absorbed in 96 hours,
853 c.c.

Sample C (Barnsley seam) : Ground to pass a 200-mesh
sieve. Contained 98 per cent, of pyrites. Oxygen
absorbed in 96 hours, 913 c.c.; in 168 hours, 1,388 c.c.

Sample D (Barnsley seam) : Part of same sample as

C,	but ground to pass a 10-mesh, and left on 30-mesh
sieve. Oxygen absorbed in 96 hours, 53 c.c.

Samples A and C differed, inasmuch as C showed in
the mass no sign of oxidation when exposed to the air
for a considerable time, whilst A-in similar circumstances
readily became coated with ferrous sulphate. When
reduced to dust, however, the rates of oxidation of the
two samples were very much the same. The reason for
this apparent contradiction is given by samples B and

D,	in which the coarse pyrites from Staffordshire is
shown to have about 17 times as great a rate of oxidation
as the South Yorkshire sample D. Whilst, therefore,-
as might be supposed from the same chemical substance,
the two samples -have much the same rate of oxidation
under identical conditions, in practice these conditions
never become identical. The South Yorkshire pyrites
usually occurs in large masses of a very highly crystalline
form, and in this condition undergoes very little oxida-
tion ii'ideed, partly owing to the relatively small surface
exposed, and partly owing to the nature of that surface,
which consists entirely of unbroken crystal faces. Now,
it is well known that a crystal face may be much more
resistant to chemical action than the interior of the
crystal, and it seems -evident that this resistance to
oxidation its developed to a high degree at a crystal face
in pyrites. When the crystal is broken, the figures for
sample C show that oxidation takes place readily. Some
such hypothesis must be adopted to explain the non-
oxidation of “ brass ” in the mass, and its ready oxida-
tion in the form of dust.

The readily oxidisable North 'Staffordshire pyrites is in
a very different physical state from the South Yorkshire
“ brass,” which is most commonly found in isolated
lumps, whilst the former is usually mixed intimately
with coal. Often this mixture will be quite black, and,
except for its weight, not readily distinguishable from a
“dirty ” coal free from pyrites. When the percentage
of pyrites is very high, it appears to be to- some extent-
crystallised, and the lump presents a dull grey appear-
ance. In the selection of samples A and B all foreign
matter was removed by chipping until the lumps (about
Jin. cubes) had a uniformly grey surface. They were
even then far . from being pure pyrites, and in the
powdered form had a density much lower than the
powdered “ brass,” viz., South Yorkshire “ brass ” (98
per cent. FoS2), density = 4’73 ; North Staffordshire
pyrites (81 per cent. FeS2), density = 3’11.

A mixture of 81 parts of the first sample with 19 of
coal (density, 1’3) to correspond in composition with the
second sample would have a density = 3*15, but the 19
parts of coal would occupy a volume of 14*6, while the
81 parts of pyrites would have a volume of 17*1. In other
words, the pyrites iis distributed through nearly twice
the volume that it would have if pure. There is, then,
a very great physical difference between the compact and
highly crystalline “brass ” and this pyrites disseminated
through coal. Presumably, in the latter state the
pyrites occurs in very fine particles in a porous medium,
and that air will, therefore, be able to reach a far larger
surface than is represented by the nominal size of the
particles. This renders intelligible the results set forth
above for samples A and B, which show that the course
pyrites which will just pass a 10-mesh sieve has a rate of
oxidation of 80 per cent, of that passing a 60-mesh sieve,
although the nominal ratio of the exposed surfaces is at
least 6.

The general shape of the oxidation-time curves for
pyrites is not unlike that of the oxidation-time curves for

coal, but the former show a less marked fall with time.
Whereas in the case of coal the absorption falls to about
7 per cent, of the initial in a w’eek, that of pyrites only
falls to about 25 per cent, in the case of the fine dust.
With the coarser dusts the difference is even more
. marked, for in 96 hours the absorption rate falls to 45 per
cent, for pyrites, as against a fall to 9 per cent, for coal
of similar size.

The decrease in the rate of oxidation with time seems
to be due to the formation of ferrous sulphate on the
surface of the particles of pyrites-, which would, of
course, prevent the ready access of oxygen. After
sample B had been oxidised at 30 degs. Cent., it was
washed very thoroughly with dilute hydrochloric acid in
an apparatus from which air was excluded, dried as com-
pletely as possible on blotting-paper, and the rate of
oxidation re-determined at 30 degs. Cent. It showed a
rate of oxidation of 13'0 and 11'6 respectively at the end
of the second and fourth hours, as compared with a rate
of 15'5 and 13'5 on the- original sample. With the
removal of the products of oxidation, the activity is very
largely restored.

From a comparison of samples C and D, which were
practically pure pyrites, it appears that the rate of oxida-
tion is proportional to the surface exposed, for sample C
has about 20 times the surface of sample I), and the rate
> of oxidation is about 18 times as fast.

Influence of Temperature on Rate of Oxidation.—The
increase of the rate of oxidation with temperature is very
rapid, a rise of TO degs. ■ Cent, in temperature doubling
the rate. The effect of a change in the oxygen per-
centage was ascertained, the results showing that, in
presence of 10-6 per cent, of oxygen, the rate of absorp-
tion was 3-63, but increased progressively with the
amount of oxygen, until with 89-0 per cent, of the latter,
it attained 29-2.

The oxidation of pyrites differs, therefore, in some
respects from the oxidation of coal, because the former
is a simple chemical reaction, whilst the latter is a very
complex one. The rate of oxidation of pyrites, in the
pure state, is affected much more by the size of the
particles than is the oxidation of coal. Whether the
pyrites be free, or mixed with coal, reduction of the
percentage of oxygen in the air reduces the oxidation of
the pyrites much more than it reduces the oxidation of
coal. For example, if the oxygen percentage in the air
is 5, the rate of oxidation of coal is halved, whilst that
of pyrites is reduced to a quarter of the rate in normal
air. An increase in temperature causes a somewhat
greater increase in the oxidation of pyrites than it does
in the case of coal.

Conclusions.—From the results described, it is pos-
sible to calculate roughly the rate of -spontaneous heat-
ing of a specimen of finely divided pyrites, on the
assumption that all the heat developed is used in raising
the temperature of the pyrites, and that none is lost.
Very approximately the rate of increase works out as
follows :—With an initial temperature of 30 degs. Cent.,
after one hour, the. temperature rises to 40 degs.; after
two hours, to 55 degs.; and after three hours, to- 90 degs.
Cent. With the same initial temperature, the same rise
in temperature with an average oxidisable coal would
take 48 hours. It is clear, therefore, that where finely
divided pyrites occurs in considerable proportion among
coal, it may be the chief factor in originating combus-
tion. It has been stated that small heaps of coal left at
the face have been known to fire-in 24 to 48 hours in North
Staffordshire, a statement which is quite intelligible if
pyrites originated the heating, but not if coal alone is
held responsible.

In considering the heating effect of pyrites, a careful
distinction must be drawn between the rate of heating
of pure finely divided pyrites and the effect of a moderate
quantity of pyrites mixed intimately with coal. The
rapid increase in temperature of pure pyrites is due
partly to its low specific heat, and partly to the rapid
increase of oxidation with rising temperature. If suffi-
cient foreign matter be mixed with the pyrites to mini-
mise the initial rise in temperature, the rate of increase
is very considerably decreased. Thus, if 10 per cent,
of finely divided pyrites be added to Barnsley Hard coal,
and the mixture carefully heat insulated and allowed to
absorb oxygen, the temperature could rise 10-6 degs.
Cent, in seven hours, whilst the Barnsley Hard coal by
itself would under the same conditions rise 10 degs.
Cent. The effect of adding 10 per. cent, of pyrites is
very small indeed. Pyrites, therefore, can only be the
cause of a heating when it is present in considerable
proportion, and in a very finely divided state. In order
to equal the effect of a normal oxidisable coal, about 30
per cent, of pyrites must be present if the coal with
which it is mixed has so small a rate of oxidation that
by itself it is not liable to fire. If the pyrites occurs in
lumps, or veins, in the form of the highly crystalline
“ brass,” it contributes nothing to the initial heating.

Export Prohibitions.—The Board of Trade Journal notifies
some additions to the list of articles which, in various
countries abroad, are subject to import or export restrictions.
East Africa prohibits the exportation of magnesite to all
destinations other than the United Kingdom, British posses-
sions and protectorates; also silica bricks to all foreign
countries in . Europe and on the Mediterranean and Black
Seas other than France, Russia (except through Baltic
ports), Italy, Spain, and Portugal. The Federated Malay
States prohibits (in lieu of “ haematite pig iron ”) the expor-
tation of pig iron containing less than 0-1 per cent, of phos-
phorus, including haematite pig iron, and all other pig iron
containing more than 0-1 per cent, of phosphorus, but less
than l-5 pcr cent, of silicon, together with less than 0-09
per cent, of sulphur, to all destinations other than the
United Kingdom, and British possessions and protectorates.
New Zealand prohibits the importation of all explosives con-
taining more than 50 per cent, of nitroglycerine. Japan
notifies the temporary duty-free importation of iron cylinders
for compressed gas and iron drums for oils, acids, or
molasses.

LETTERS TO THE EDITORS.

The Editors are not responsible either for the statements
made, or the opinions expressed by correspondents.

All communications must be authenticated by the name and
address of the sender, whether for publication or not.
No'notice can be taken of anonymous communications.

As replies to questions are only given by way of published
answers to correspondents, and not by letter, stamped
addressed envelopes are not required to be sent.

MIMING SURVEYORS’ EXAMINATIONS, &c.

Sins,—No one seemed to answer “ Meridian’s ”
enquiry in your issue of the 26th ult. The fact is that
mine surveyors are not recognised as they should be.
But no intended head surveyor should be allowed to sit
for the certificate of the Coal . Mines Regulation Act
without he has been articled to a qualified surveyor or
a mining engineer in practice as a surveyor of mines.
A young man might be able to conduct a survey or a
levelling, but, to anyone who knows, much more than
that is required. Imagine a young man being proficient
with only two years’ experience. Ridiculous in itself!

However, there is not much encouragement for most
surveyors, the salaries being low for the important, and
at times arduous, work they have to perform.

June 6, 1916.	North Point.

MINING INDUSTRY AND MILITARY
SERVICE.

Dr. W. N. Atkinson presided at the meeting of Glamorgan
colliery tribunal on May 26, and the revision of-the lists of
single men who had obtained employment since August 15
was proceeded with, but not many were released for military
service. From the Cambrian Colliery a few were dispensed
with, it being considered that these could be replaced; but
the colliery representatives generally were able to demon-
strate in the majority of cases that the men were engaged
in necessary work, and therefore could not be spared. A
tew men were released from the Ocean Collieries, one of them
being a surface labourer who was engaged in October, and
another, having been a grocer’s- assistant, also working now
as a surfaceman. But a careful revision was made in view
of the fact that the companies cannot spare oven surface
labourers.

From the Cymmer Colliery as many as 1,100 men have
joined the Colours out of 3,000 employees;

Enlistment for the tunnelling companies, which has been
in progress for the past two or three weeks, has been very
satisfactory. Lieut. Gill, formerly a miners’ leader in
South Wales, stated at Maesteg that the call for volunteers
from skilled underground workers had met with such a
response that the lauthorities had decided to take no more
skilled men. Among the batches of recruits iare no fewer
tnan 25 from Lewis-Merthyr pits.—Capt. Lewis informed an
Ebbw Vale meeting that the corps which blew up Hill 60
was composed largely of Monmouthshire men.

The Warrington Rural tribunal recently heard a state-
ment by the secretary of the Collins Green Colliery Company,
who appealed for exemption for six men employed as stores
and pay clerks: Nine men out of 19 in the offices- had joined
the Forces, and about 1,300 men had left the colliery. Over
75 per cent, of the coal was supplied to munition and utility
works. The Rev. A. M. Mitchell asked if girls could not be
employed in the offices, and the secretary of the company
replied that neither a male nor a female could be trained to
take charge of the wages sheets under six months. All the
mon were granted exemption except one, aged 18 years.

At the Wrexham tribunal recently, a hooker-on applied for
exemption on the grounds that he was the sole support of his
mother, aged 64, to whom he paid T1 per week. There was
also a brother who earned 12s. per week. Applicant has
only been working at the colliery since 'September last year,
prior to which he was a gardener. The application was
refused.

At the Wrexham tribunal recently, a local colliery company
appealed for exemption for their chartered secretary in
charge of the commercial work .ait a new colliery which is
now being sunk in the neighbourhood of Wrexham. Three
months’ exemption was granted, but the representative of
the colliery company said he could not submit to the decision,
-and that he would have to carry the case to appeal, as he
understood that under the new7 regulations he would not be
able to appeal iagain at the end of the three months. The
president of the tribunal said that he had no knowledge of
any such new regulations, and that applicant might appeal
again at the end of the three months. The representative
said he would appeal at the end of that period.

At the Stourbridge tribunal on Saturday, conditional exemp-
tion was granted a manager of a local coal wharf belonging to
Messrs. D. and F. Fellows. It was stated the firm was
supplying munition works, and that, in addition, they had
700 to 800 customers. The Mayor said the man was engaged
in a certified occupation.—At the same court, three months’
exemption was granted a Stourbridge coal merchant, who was
engaged completing orders for private customers.—Three
months’ exemption was granted to a coal clerk, who was
said to be responsible for 158 railway wagons and the
accounts. The man possessed technical knowledge of the
business. The Mayor said that such men were essential in
civil occupations.

The Walker Coal Company Limited, appealing at New-
castle for the exemption of its acting secretary and fitter,
stated that the man had taken the place of the fitter, who
had now joined the Army. Only one other man of military
age remained in the offices. The colliery would have to
close down if the man appealed for had to go. He was
granted conditional exemption, provided that he joined the
Volunteer Training Corps.

The Wigan tribunal has decided to grant temporary
exemption to the end of August to a mining lecturer at the
local technical college.

At Maryport tribunal, Mr. John Wharton, engineer and
ironfounder, was granted total exemption as head of the
firm of Joseph Wharton and Sons, of the foundry, Maryport.
At the same tribunal, Mr. F. E. Mandle, head of Mandie and
Sons, applied on business grounds, .and because his butler
had gone and .a proportion of his men. He had contracts
From Gretna, and was developing two limestone quarries.
Total exemption w-as granted.