1168

THE COLLIERY GUARDIAN.

June 4, 1915.

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Notes on Sampling.*

By T. W. D. GREGORY, F.C.S.t

Deals both with the sampling of minerals and mine gas;
the author pays especial attention to the subject of
slack sampling, pointing out the desirability of pro-
ducing an even fuel with a consistent ash percentage.
_______________________________

The following notes on sampling deal with the taking
and preparation of such samples as are from time to
time required in connection with colliery work. They
do not attempt to touch on any work which may be
necessary afterwards in the laboratory. It is difficult
to exaggerate the importance of the careful sampling of
any mass of material which is not entirely homogeneous
in character.	!

The object is so to extract from a large mass of
material a small quantity that it may be truly repre-
sentative of the whole. Vast quantities of materials are
bought and sold on the analysis of a sample weighing a
few grammes, but the few grammes of sample have
been so patiently prepared that they may represent, and
correctly represent, the constitution of many tons.

To deal first with such material as coal, there are, as
may easily be imagined, various methods suggested for
reducing the bulk of material in such a way that, say,
even a day’s output could be represented by a bottle of
coal of such size that it could easily be carried away in
one’s pocket. The method chosen will depend largely
on the material to be sampled, the bulk and position.
A generally accepted method is to withdraw in a
suitable way, say, one-twentieth to one-fiftieth of
material; for.example, say, in transferring a heap of
coal to wagons, every twentieth barrow, smaller amounts
every twentieth shovel, elevators every twentieth bucket.
Samples collected in this way to be further reduced
must have their size of particle reduced to, say, one-
twentieth of its previous size, mixed, and again the
twentieth part taken. This process continued gives a
workable sample. The “ coning ” or “ quartering ”
method can now be adopted, the material being piled in
cone form by shovels, each shovelful of material being
thrown directly over the centre of the growing cone in
such a way that the material spreads evenly down to
the sides. This can be done with a little practice in
the f- flowing manner:—

“ Coning ” or “ Quartering.”

The shovel containing the material is jerked upwards
over the centre of the cone and quickly withdrawn to
allow the material to drop clear. It will be noticed
during the process of cone building that the coarser
particles tend to run to a greater extent than the fine,
which cling more to the centre. This being so, it is
essential that the cone should have a truly vertical
centre; otherwise, as will be seen later, trouble will
arise. After all the sample has been coned, the heap is
levelled by carefully working the material from the cone
apex downwards and outwards, round and round, until
the whole assumes the shape of a circular disc,
technically designated as a “ pancake.” Having arrived
at this stage, the sample has now to be quartered.
This process consists in dividing the disc carefully into
four equal sectors by means of boards or metal sheets.
Two opposite sectors are taken, the remaining two being
discarded. The sample, before re-coning and quartering,
usually requires to have the size of its particles reduced
by crushing, and this process is continued until a
suitable amount of sample has been arrived at. One
ton of material would require to be coned five times in
order to obtain a sample of 701b. The potential errors
arising from a badly-built cone will now be apparent.
Unless the centre of the cone containing the greater
proportion of fine is kept truly vertical, the greater
portion of the fine will find its way into one or other of
the sectors, and so introduce in most cases a serious
error.

From the above it will be seen that to procure a
sample two distinct processes are entailed : (a) Reducing
bulk of sample, (&) reducing size of particles of material.

Bearing on the chances of success in sampling,
E. G-. Bailey has done some interesting work, and, in
order to appreciate the importance of care in various
directions, I put before you some of his more important
figures:—

Given equal care in all cases, the following table shows
that the larger the proportion of sample the better
chance has it of representing the whole :—

Percentage error.

Bulk ratio.	Sample and

bulk.

1	:	100	................ 1-4

2	:	100	................ 0-8

5	:	100	................ 0*6

pieces.

Lb.

6*7

2’5

0'75

0*38	.......................

0*21

....

.................

0*046	.......................

0 018	.......................

Again, on the fineness of the particle compared with
bulk of material, taking a coal containing 5 per cent, of
ash, in order that the error should be less than 1 per-
cent., the weight of sample compared with the weight of
the largest piece or particle should not be less than
shown in the following table :—

Weight of the largest	Amount of sample

not less than

Lb.

... 39.000

... 12'500

...	3,800

1,900

600

230

90

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* A paper read before the North Staffordshire Institute
of Mining and Mechanical Engineers.

f Lecturer in mining chemistry and experimental science,
Stoke-on-Trent Mining School.

Again, in order to get the percentage of the ash of the
coal within the bounds already mentioned, it was neces-
sary to bring the particles of the coal to the size shown
in the following table before quartering :— '■

Weight of fraction.	Size to be crushed.
Lb.	Diam. in inches.
7,500				 2
3,800				 1*5
1,200				 1*0
460		 0	75
180				 0*5
40				 0*425
5		'.		 0-2
0’5					 0*1
0*25 			 0*076

As a minimum, therefore, he recommends the
following:—

Maximum size of particle 2 in. 4mesh. 8mesh. 20 mesh.
Minimum amount of frac-

tion in pounds ....... 8,300 ... 1,100 .. 120 ...‘ 3

Impurities and Moisture.

These figures are interesting, because they were
obtained from expeiiments made on coal. I do not
know to what degree of accuracy they take one, but,
from my own experience, good results can be obtained
by quartering fractions considerably less in weight for
size of particle. Bearing further on this point, I notice
a report of the Proceedings of the American Society for
Testing Materials, Part 1., Committee Reports, vol. xiv.,
1914, dealing with the preparation of samples, states
that the accuracy of the sample is largely dependent
on the thorough mixing and crushing of the impurities,
such as shale, pyrites, &c., to such a degree of fineness
before each quartering or dividing that the inclusion of
a few pieces more or less in the various quarters or
divisions cannot materially change the character of the
final sample. Further, that a nearly constant ratio
between the largest particle of impurity and the weight
of sample is essential in each stage of the reduction;
otherwise, excessive errors are introduced at the
particular point of the sampling where this ratio is
exceeded.

The amount of sample, therefore, which can be
despatched to the laboratory will depend on the relative
proportion of free impurity and the practical limits of
fineness to which these impurities can be crushed at the
place of sampling.

The following table is given:—

Maximum weight
Size of	of sample

largest	which can be

impurities. '	regarded as

satisfactory.

Lb.

..........................	75

....................................................	30

..........................	9

________............................ 5

......................................... 3-5

In the above table, the ash formed by the largest
lump of pyrites is approximately 0*02 per cent, of weight
of sample. These quantities are very different from
those recommended by Bailey, but, according to results
given of duplicate samples, fairly accurate figures are
obtained. Of course, a lot depends on the nature of the
coal. Some coals naturally contain little free dirt, and
I know from experience it is much easier to get a satis-
factory' sample in such cases than where one has to
contend with quantities of free dirt.

In general, m samples of coal taken for commercial
purposes, the moisture is not required to be determined
accurately. Consequently, for consignment the sample
can be placed in a well-made box, completeiy filled and
fitted with a good lid. If the estimation of moisture is
important, the whole, or. a portion of the sample
submitted, may be placed in a tin box or boxes, with
well-fitting lid, and the lid sealed with electrician’s
adhesive tape or rubber solutioned band, to prevent loss
of moisture. Samples for research work on gases,
oxidation, &c., require more careful treatment. The
above general remarks apply to the sampling of coal,
coke, ironstone, &c., in bulk. I wish now to deal with
particular cases of sampling.

Samples of a Seam of Coal.

A sample representative of a seam is frequently
required, and I suggest the following method as one
giving satisfactory results:—One, two, or even more
representative portions of the face should be chosen,
depending on the character of the seam ; and. when
possible, a face freshly exposed is best. The face is then
prepared for a width of from 4 to 5 feet from roof to
floor by removing dust and loose fragments. This can
be done by means of a brush. In the centre nf the
cleaned portion the face of the coal is removed to a
depth of about ^in., and about 9 in. wide from roof to
floor. The floor surrounding is then cleaned of debris
and a sheet of stout canvas, or other suitable material,
carefully spread with one edge close up to the coal. If
the floor is wet a sheet of brattice cloth may be laid
down first. The sample is now carefully taken by means
of a pick cutting in the centre of the prepared surface.
The cut should be from 3 to 4 inches square, and should
pass from roof to floor. Bands of dirt, &c., which in the
ordinary'working of the seam are rejected, must not be
included in the sample, and great care must be exercised
in removing the same. In this manner a sample is
obtained of from 5 to 6 pounds of coal per foot of seam
thickness. If the sample obtained is too bulky to box,
it may be dealt with on a clean iron plate, crushing by
means of a flat hammer or improvised pestle until the
whole sample can be. passed through at least % in. sieve.
Pieces of dirt, when seen, should be broken as small as

possible. It is then well mixed and coned and quartered,
as described in the earlier part of my paper.

If a reliable moisture test is required, it is always
advisable to sample as quickly as possible, and seal in
the pit where humidity conditions are normal for the
seam.

Sampling of Slack.

Slack cannot, of course, be dealt with until it has
been separated from the other coals. This separation
takes place at the screens, and at the screens, I consider,
is the best place to get a really representative sample.
Even in places where only one seam is worked the slack
will vary, particularly with regard to its. ash content.
Carelessness in loading out will be shown in the quality
of the slack. Where two or more seams are con-
tributing to the slack Output, more variation will be
expected in the quality, depending on the varying
amounts drawn from the respective seams. It is an
advantage from time to time to sample at the screens
the slack made from each of the respective seams con-
tributing to the slack output. It is my intention fitter
to refer to the commercial value of an even slack.

Slack already in wagons can be satisfactorily sampled
by holing from top to floor in at least three places,
taking a centre line down the length of the wagon.
Samples should be taken in the centre and half way
between the centre and either end of the wagon. ■ I
have frequently noticed that the slack in the wagon
shows that it has been, loaded at the screens in three
positions, mounds being noticeable at the centre and at
either end. In such cases a sample tak<-n as suggested
would be a truly representative sample of the whole of the
slack filling the wa gon. Samples taken without due regard
to the possibility of their being representative are worse
than u-eless, for they are misleading. Consequently,
samples taken from the top of. a wagon only obviously
can have little value. In ordinary cases one might ask
why three holes, and not, say. six ? My reply is that
the line must and has to be drawn somewhere, and I
have found from experience that the above method is
satisfactory.

The best result I have had where a duplicate sample
was taken was a case of six wagons, the two ash tests
giving 13 40 and 13*38 per cent, respectively. In this
case the slack was wet and in consequence easier to
sample than dry slack. My worst case was one of six
wagons of dry slack, the. respective ash tests showing a
difference of 0*5 per cent. The dirt pieces, however,
were large, and in such exceptional circumstances more
sample holes might with advantage have been put
down.

After all, if the sampler is out for. a really conscien-
tious sample, given a knowledge of the principle of
sampling, commonsense is an important factor.

Before leaving this part of my subject, I would like
in a few words to refer to a point I have previously
mentioned, namely, the value to firms who are
endeavouring to adopt more scientific methods of using
their fuel of a fuel the ash content of which is uniform.
I have heard of slack users who have stated that they
preferred a supply of slack, uniformly bad within
reason, to a supply which might on the average be
better, but which showed a considerable variation in the
ash content. May I take a simple example ? A man
is receiving a slack, the average ash content of which is,
say, 10 per cent.; he is using automatic stokers ; he is
regulating his air supply scientifically to get the best
results. With a variable slack he may at one time be
firing with slack containing 5 per cent, ash, at another
time with 15 per cent. ash. On the one hand, his ah-
is insufficient to burn the extra 5 per cent, combustible
matter, and consequently he loses the value of the good
fuel. On the other hand, his air supply is too much for
the combustible matter present, which is' 5 per cent,
below the average. Again, he is losing value, not only
through having to heat up extra inert matter, but also
to heat up air which is not required for combustion. * I
need not point out the advantage he would obtain if
only the ash content could be kept more regular, even
though the average were higher than 10 per cent. To
take an actual case, I sampled some time ago 157
wagons of slack in consecutive order as they left the
screens. The average ash content of the whole was
11*1 per cent. The individual tests of the wagons,
however, showed the following variations :—

Percentage of ash higher than average.

2% 3% 4% 5% 6% 7% 8% 9% 10% 11%
No. of wagons_____42...24...15...11.,.6... 5... 4...—... —... 1

Percentage of ash lower than the average.

2%"”	3%	4%	5%	6%	7%

No. of wagons ....	47 ... 24 ...	14	... 10	... 3	,.v 1

In the above	table, the wagons	within	2 per	cent, of

the average ash	content are	not	dealt with.	Now I

know that there are great difficulties, perhaps, so great
that they cannot well be overcome, but I may be
pardoned if I express a hope that a discussion may be
raised on the possibility of turning out a more even fuel.

Sampling Mine Air.

Now I am going to give you a few notes on the
sampling of mine air. Dr. Haldane has covered, and
well covered, this ground, but perhaps some hints based
on my own personal experience may be of service.
Sampling air is not like sampling coal or other material
of like character. Air, mine air, as we know it, is a
mixture of gases, sometimes an uneven mixture, but if
left alone and together, they will in time “cone” and
“ quarter ” themselves; in other words, they diffuse
and become a perfect mixture. It, therefore, follows
that if it is desirable to know the composition of the
air at. say, the bottom of the upcast shaft, we are
perfectly justified in taking a sample bottle, 3 or 4
ounces capacity, filling it by the right means with
return air, stoppering it, and bringing - it out-of the