November 10, 1916.

THE COLLIERY GUARDIAN.

903

that in different coals a given percentage of water exerts
very different vapour pressures.

An experimental demonstration of this variation in
vapour pressures is described herein, together with the
results of determinations of the heat generated, by
different coals on being wetted; the connection of these
phenomena and certain others with the condition of
the water in coal is shown.

Free Superficial Water in Coal.

Attention may be called first to the matter of the
mechanical draining of water from coal, and the different
amounts of free superficial water retained according to
conditions.

In wet coal the water in excess of a certain percentage,
which depends on the kind of coal, is mechanically held
in the free state, its vapour pressure and other
properties being, for all practical purposes, normal. This
excess moisture may be termed superficial or accidental
moisture. All water in the coal above approximately 3
per cent, in the Appalachian coals, 12 per cent, in the
middle-western, and 22 per cent, in the sub-bituminous
coals may be so classed, although this line of demarca-
tion is more or less variable even among coals of the
same kind.

The percentage of superficial water retained by coal
that has been wetted and the water drained off is greatly
affected by the size of the coal. In screenings or slack,
which contains pieces of many different sizes, the
particles pack closely together and form small inter-
stices which retain water by capillarity. Screened lump,
on the other hand, or run-of-mine with a large propor-
tion of lump, has much larger interstices and relatively
less surface to be wet, and therefore retains less water.

Results of Draining Tests with Coal.

The results in Table I. (which gives the proportion of
water retained by different coals and different sizes of
coal after draining) show that screenings consisting of
all sizes through a J in. mesh screen retain two to seven
times as much water as sized coal ranging from | in. to
1 in. in diameter, which is a large “buckwheat” or
“ pea ” size.

About 21b. of each sample was crushed, screened,
thoroughly mixed, and divided in two equal parte. One
part was taken for immediate analysis, and the other
part was soaked in water for three days. The water
v as then allowed to drain out by gravity for 19 hours,
the coal being held on a 30-mesh screen in a layer about
6 in deep. The samples were then 'transferred entire to
pans, in which they were weighed and air-dried. The
results in the last column in the table refer to the per-
centage of residual water retained after preliminary
drying in a current of air at 30degs. to 35 degs. Cent.

Table I.

Kind of coal.	Total water	Normal water	Water retai ned^.
content.*		content.!	
Bituminous coal from New  River, West Virginia :— Screenings, th ough | in.	P.c.	P.c.	P.c.
mesh screen 	  Sized buckwheat, from I in.	22-27 .	...	252	... 1-03
to 1 in. in diameter 	  B ituminous coal from Pittsburg bed, Pennsylvania:—  Screenings, through I in.	3-17 .	...	2’35	... 1’00
mesh screen 	  Sized buckwheat, from |in.	18/3 .	...	2-68	... 1-15
to I in. in diameter	  Bituminous coal from Illinois  (Macoupin county):— Screenings, through j in.	2’62 .	...	273	... 1’25
mesh screen		  Sized buckwheat, from I in.	28-60 .	..	6-84	... 3-32
to tin. in diameter	  S ub - bituminous coal from Wyoming (Big Horn county) : —  Screenings, through j in.	16-71 .	..	6-63	... 3’40
mesh screen 	  Sized buckwheat, from |in.	32-58 .	.. 14’24	... 6’38
to 1 in. in diameter		18’70 .	... 13-79	... 6’65

* Of coal after shaking and draining, t In commercially
“ dry ” coal. X In coal after “ air-drying’J in laboratory at
3"> degs. Cent.

New River, West Virginia.—This sample was taken from
a barrel containing about 300 lb. of run-of-mine coal from
the Sun mine in Fayette County, that had stood exposed to
the air, under shelter, for five years.

Pittsburg, Pennsylvania.—Sample of coal from the Pitts-
burg bed, recently mined at Herminie in Westmoreland
County.

Illinois.—Sample of run-of-mine coal from Superior No. 1
mine at Gillespie in Macoupin County, that had been
exposed, under shelter, to the air for 11 months.

Wyoming.—Sample of run-of-mine coal, sub-bituminous
in character, from mine of Continental Coal Company at
Kirkby, in Big Horn County, that had stood in a barrel
exposed to the air for four or five months.

Inherent Water.

There is no ready means of separating sharply the
superficial and the inherent water of coal. Unless the
air is saturated, the superficial water can all be removed
by air drying, the rate of evaporation depending on the
temperature, the humidity of the air, and the fineness of
the coal. But at the same time, part of the inherent
water leaves the coal and continues to do so as long as
its vapour pressure is higher than that of the air. In an
atmosphere of very low aqueous tension—one which is
dried, for example, by sulphuric acid or phosphorus
pentoxide—renewed frequently, all of the inherent water
that exerts a vapour pressure greater than that of the
drying agent, in other words, essentially all of the water,
is in time removed from the coal. The process is very
slow in its last stages, equilibrium possibly being fully
attained only after very long periods.

By the expression “ inherent water ” in coal or in any
material therefore is meant the water which exists as
such, but which has a vapour pressure less than the
normal.

Methods of Separation a<nd Analysis.

The superficial and the inherent water together may
thus be determined by drying over an efficient desiccant,
if proper precautions are taken and sufficient time
allowed. The process is hastened, but not made more
thorough, by carrying it on in a vacuum. The tempera-
ture required for practically complete drying in a reason-
able time depends on the nature of the material—that is,
on the vapour pressure which a small but appreciable
amount of water in the material exerts. The analytical
method in common use for determination of moisture in
coal consists in drying 1 grm. of the powdered and pre-
viously air-dried coal in a current of dry air for one hour
at 105 degs. Cent. This method determines the inherent
water not removed by air-drying. It involves slight
errors due to oxidation, and possibly also, in sub-
bituminous coals and ligpites, to decomposition. Also
a more serious error results in determinations on these
coals when the atmosphere is not thoroughly dry, as
quite an appreciable percentage of inherent water
remains in the coal at very low vapour pressure and
leaves the coal, in the last stages of drying, very slowly.
It was noted, for example, that Wyoming sub-bitumin-
ous coal at 20 degs. Cent, in an atmosphere of 0-8 mm.
aqueous tension retained 2-7 per cent, water after ten
days’ drying at ordinary temperature.

d

Fig. 2.—Apparatus for Measuring Heat
GIVEN OFF BY COAL WHEN WETTED.

When a number of coal samples of high water content
are being dried together in the same space, as is fre-
quent practice in a laboratory drying oven, the aqueous
tension of the atmosphere surrounding the samples in the
oven may become sufficiently high to cause serious error
on this account.

Choice of Desiccants.

In connection with the choice of methods of drying
coal and the desiccating material to be used, it is impor-
tant to bear in mind that the completeness of drying
depends on the relation of the aqueous vapour pressure
in the coal to that in the atmosphere surrounding it. As
long as a greater vapour pressure is exerted by the
moisture in the coal than exists in the atmosphere, water
will pass from the coal to the atmosphere. Sulphuric
acid containing less than 5 per cent, water (specific
gravity 1-84) and phosphorus pentoxide reduce the
aqueous vapour pressure of the surrounding atmosphere
at 25 degs. Cent, to less than 0-01 mm. of mercury, and
are therefore excellent desiccating agents. Calcium
chloride, on the other hand, in the presence of its
di-hydrate (CaCl2.2H2O) exerts an aqueous tension at
24 degs. Cent, of 2-17 mm. As with this aqueous pres-
sure at 24 degs. Cent, some kinds of coal may contain as
much as 5 per cent, water, it is evident that calcium
chloride leaves a considerable amount of water in some
coals at ordinary temperatures. By heating the coal, the
vapour pressure of its water content is much increased,
and the coal can thus be more thoroughly dried, but only
to the point of equilibrium with the vapour pressure of
the calcium chloride or other drying agent used.

Determining the Vapour Pressure of the Inherent Water.

By keeping samples of different coals over mixtures
of sulphuric acid and water of various known vapour
pressures so as to bring them as nearly as practicable
into moisture equilibrium, there have been determined
the aqueous vapour pressures corresponding to* different
percentages of water in the coal. The results, which
are given in fig. 1, show that for any given pressure
the four types of coal examined contained widely
different percentages of water; or, conversely, equal
percentages of water in the various coals had very
different vapour pressures. For example, a content of
about 3 per cent, of water seemed in New River (West
Virginia) coal to have the normal vapour pressure of
water, in coal from the Pittsburg bed to have a vapour

pressure of about 83 per cent, of the normal, in Illinois
coal about 18 per cent., and -in Wyoming sub-bitu-
minous coal about 4 per cent, of the normal. The
remarkably low aqueous vapour pressure of sub-bitu-
minous (Wyoming) coal containing water is of import-
ance in choosing drying agents, as already mentioned.
It can readily be foreseen that, with an equal content
of water, these different coals would behave very
differently in an atmosphere of given humidity, with
respect to giving up or taking up moisture.

The method used in the experiments consisted in
bringing the coal as nearly as possible into moisture-
vapour equilibrium with diluted sulphuric acid of
known aqueous vapour pressure. The specific gravity
of the sulphuric acid and water mixture was measured
by weighing, and its vapour pressure established from
the determined specific gravity by reference to
Regnault’s values.

The coal samples were pulverised and sized between
80- and 100-mesh screens. They were contained in
weighing bottles having well-fitting glass stoppers, and
these bottles were permitted to stand with the stoppers
removed in desiccators over the sulphuric acid mix-
ture, the air being exhausted from the vessel until
the pressure was less than 5 mm. above the aqueous
tension.

The samples in most of the tests stood in the desic-
cators for 10 days between weighings, the temperature
being that of the room, 20 to 25 degs. Cent. The four
coals investigated were all placed in the same desic-
cator, and weighed at the same intervals. The final
weight after drying the sample over sulphuric acid of
increasing strength up to the pure acid (1*84 specific
gravity) was used as the basis of calculation. From
this dry condition the coals were progressively hydrated
and dehydrated, the water content being computed from
the weight of the sample when a condition of equili-
brium had been reached.

Coal Samples Used in Tests.

Essential data on the coal samples used in the deter-
minations of vapour pressure are given below". They
also include descriptions of the coal samples used in the
tests to determine the thermal effect of wetting dry
coal.

Wyoming (Lab. No. 43). — Sub-bituminous lump (| in.)
from No. 2 mine at Dietz, Sheridan County, Wyoming;
kept in stoppered glas bottle for four years.

Illinois (Lab. No. 48). — Bituminous lump (J in.) from
Dering No. 11 mine, at West Frankfort, Franklin
County, Illinois; kept in stoppered glass bottle for five
years.

Pittsburg (Lab. No. 395).—Bituminous coal from the Pitts-
burg bed in the Schoenberger mine at Baird, Washing-
ton County, Pennsylvania; kept in closed can for three
years.

Pittsburg (Lab. No. 650).—Bituminous coal from the Pitts-
burg bed in the Keystone shaft mine, at Madison, West-
moreland County, Pennsylvania; kept in closed can for
one year.

New River (Lab. No. 601).—Semi-bituminous lump from
the Sewell bed in the Sun mine, at Sun, Fayette County,
West Virginia; sample submerged under "water four
years.

Anthracite (Lab. No. 663).—Anthracite lump from Lykens
No. 5 bed at Brookside, Schuylkill County, Pennsyl-
vania ; kept for six months in stoppered bottle.

Lignite (Lab. No. 14). — Brown lignite from Williston,
Williams County, North Dakota, United States Reclam-
ation Service mine; kept in closed vessel six years.

Peat (Lab. No. 27).—Air-dried peat blocks from Orlando,
Florida; kept 5£ years in closed vessel.

Results of the Tests.

The results of the tests to determine the vapour
pressure of water in coals are summarised in Table II.

		Table II.				
		Water in coal,			grammes per	
			100 grammes of dry coal.			
		Aqueous			if?	
		vapour		00  o 6	bee?	® S
Condition of sample, pressure (at20°C.)					P X	> c3
		Mm. of mercury			K'S	J
Coal rehydrated from						
the dry condition .		... 0’15 ...	1-52	... 0 73 ...	0 36 .	.. 0-25
Do.	do	... 0-82 ...	, 2 82	... 1-46 ...	0’62 ?	.. 0-34
Do.	do.	... 1’73 ..	401	... 2 06 ...	0-83 .	.. 0’44
Do.	do.	... 3’40 ...	. 5-45	... 3’24 ..	1-20 .	.. 0 65
Do.	do.	.. 6 32 ...	8 77	... 4’12 ...	1-54 .	.. 0’88
Do.	do.	.. 10-83 ...	1386	... 635 ...	211 .	.. 1’30
Do.	do.	.. 14-48 ...	19’41	... 9’68 ...	274 .	.. 2 01
Do.	do.	.. 17'36 ...	24’88	...11-76 ...	4 75 .	.. 3’19
Coal dehydrated again						
after hydrating ....		.. 14’45 ...	22’41	...10-15 ...	306 .	.. 2 34
Do.	do.	.. 10-83 ...	18-69	... 9’04 ...	2 53 ..	.. 1-68
Do.	do.	.. 6'32 ...	1281	... 5’41 ...	1’90 ..	.. 1-13
Do.	do.	.. 340 ...	D--44	... 4’62 ...	1-66 ..	.. 0’97
Do.	do.	.. 173 ...	7-82	... 3’84 ...	1-31 ..	. 0 79
Do.	do.	.. 0-82 ...	6-45	... 3’13 ...	0’93 ..	. 0'57
	Discussion of		the ;	Results.		

The results of the experiments show an apparent dis-
crepancy between the vapour pressure of samples pre-
pared by hydrating and those of samples prepared by
dehydrating; it seems that for a given wrater content
the vapour pressure depends to some extent on whether
the coal is brought to this water content from a lower
or a higher content. This variation is partly and
perhaps largely due to the very slow rate at wrhich the
moisture exchange takes place when the vapour pres-
sure of the moisture in the coal and the vapour pressure
of the atmosphere are near equilibrium, making it pro-
bable that in the experiments complete equilibrium was
not attained. The true moisture content for any given
vapour pressure, although probably somewhat different
in samples prepared in different -ways, lies therefore
within a smaller range than was indicated by the experi-
ment herein described. A possible explanation of the
apparatus hysteresis in the curves in fig. 1, w"hich show