224

THE COLLIERY GUARDIAN.

February 2, 1917.

temperature rise was comparatively regular until 800
degs. Cent, was passed, at which temperature' there was
an exothermic reaction, and the temperature rose very
rapidly.’ If the heating were continued beyond 1,200
degs. Cent., ;a new chemical re-arrangement of the mole-
cules took place, and what Mellor regarded as a mixture
of Al203 4- 2SiO2 re-combined as A12O3, SiO2 4- SiA2, and
the mineral silimanite began to crystallise with evolution
of heat. According to Mellor—and the experiments had
recently been repeated with some suoceiss in Sheffield—
the heating curves of fireclays were very similar to those
of kaolin, with an additional. “ terrace ” in the neigh-
bourhood of 200 degs. Cent, which seemed to belong to
the decomposition of those colloidal hydrates of silica
and silicate of alumina, and perhaps also some hydrates
of iron and aluminium, to the presence of which the,
fireclays were said to owe their plasticity.

When air-dried fireclay was gradually heated it
expanded a little in the neighbourhood of the tempera-
tures where its water was given off, but on the average
it shrank or contracted continuously right up to those
high temperatures at which the rigidity of the mass
began to fail. At the lower temperatures the contrac-
tion might fairly be correlated with the loss of water
from the solid substance, which, unless it were attended
with some almost unbelievable reduction of density due
to polymeric dissociation in the Al203, 2SiO2 (which was
no longer hydrous), must bring about a corresponding
increase of porosity m the brick. When, for the pur-
poses of brick formation, raw clay was mixed with water,
the volume of the clay increased, and when the water in
the moulded brick was dried out in passing through the
drying shed, it was the surface tension of the water films
which pulled the clay particles together, and caused the
brick to shrink7. When in the course of the burning of
the bricks the water had been driven off, and the tem-
perature was not yet high enough for the silicate fluxes
to be at work, the causes of the continued shrinkage were
somewhat obscure, and though Mellor’s observations had
persuaded him that, within what were the kaolin par-
ticles, a process analogous to the polymerisation of the
alumina and perhaps the silica was in progress, this
explanation could hardly account for the drawing
together of the various particles, which was the only
process by which a brick could continue to contract.
Above 900 degs. Cent, the rate of increase of density of
pure fireclay materials proceeded more rapidly, probably
because the fluxing temperature of the felspathic
minerals with the silica, which at lower temperatures
was colloidal, had been passed, and the eutectic fluids
which had been produced lost their viscosity and spread
from the centres at which they were first formed over
the surface and into the pores of the dehydrated kaolin
grains, and—again by surface tension—pulled the grains
together. As the temperature continued to rise, more
and more of the fluxing materials melted, came into
action as viscous liquids, and spread by surface tension
and diffused into the mass of the particles, which, 'as
they became so permeated by fluid, gradually lost their
individuality, and merged 'with the fluid; so that, in
mass, the aggregate took on the aspect described by
the term vitrification.

Action of the Fluxes.

Up to 900 degs. Cent, the clay particles which had
dried out and contracted, touched only at a few points,
so that the brick body was in that friable condition
alluded to by the brick maker when he said that the
bricks were biscuiting, and had not yet begun to
“ make.” Above 900 degs. Gent, the fluxes spread with
moderate rapidity, and the chips of felspar which
appeared scattered through a mass of good class fire-
clay before firing to this temperature, were, quickly
absorbed and disappeared. The process of vitrification
by contained fluxes was one which could never be com-
pleted instantaneously, and though if, in a specified
material, the fixing of the maximum temperature of
fining would determine how many and which of the
fluxes present should be brought into operation, it was
only by further fixing the rate of rise of temperature
above the lower limit of their fluxing temperatures that
one could control the contraction and other vitrification
effects which the “ burning off ” would produce.

Partly by the spreading of the fluxes over the surfaces
of the still solid particles, and partly by the reactions
between the fluxes and the particles over which they
spread, the concentration of those fluxes at particular
points was gradually reduced, and unless the further
rise of temperature brought with it the liquefication of
other materials, it might be that by the continued firing
of the brick at a particular temperature, the supply of
fluxes became “ dried up.” In practice, when a parti-
cular fireclay was of such high quality that the bricks
became dried up or “ short ” in course of manufacture,
the fire was taken off before such action went too far.
Bricks which were “ short ” by reason of this lack of
fluxes, always became “ leathery ” at the high tempera-
ture which prevailed in steel melting furnaces, and as
they were unsuitable, for coke oven construction, and
commanded prices which were high as compared with
normal good class firebricks, which could be made strong
by partial vitrification, in the burning, they should be
reserved for the steel makers’ use. When, from bricks
which consisted essentially of clay or kaolinite burnt
to the point of vitrification, and afterwards allowed to
cool, thin slices were cut and examined under the micro-
scope, unless they had been held for so long a time
above 1,200 degs. Cent, that the crystals of silimanite
formed about that temperature had grown. to finite
dimensions, there was little to observe except the tex-
ture and the circumstance that the bonding material
appeared to consist of glass. When, after cooling, such
bricks were re-heated, they appeared to expand with a
regular coefficient of expansion not very different from
that of hard glass or porcelain, and even at temperatures
far above those at which, in the first heating of the clay,
the action of the fluxes began to produce pronounced

contraction effects, the normal rate of expansion was
maintained. -Only when, by diminution of its viscosity
with rising temperature, the bonding glass became once
more effectively a liquid, did the vitrification process
begin again, and then, of course, the brick ceased to
behave as a refractory solid.

Next after kaolinite, the mineral which was most
abundant in the fireclays mined in this country was
quartz, which in the form of comminuted fragments had
survived unchanged all those processes of weathering and
continued soaking in water and the various dilute solu-
tions which abounded in soils and in the pores of rocks,
the action of which had secured the chemical breakdown
of most of the other constituents of igneous rocks.
Unlike the silicate of alumina, which combined with
water and formed kaolinite or clay, quartz behaved as
if quite immune from the attack of water at ordinary
temperatures, 'and though in plastic clays there was
often plenty of colloidal silicic acid, it would appear
that this hydrate of silica had been produced by the
weathering of minerals other than quartz.

Effect of Heat on Quartz.

When quartz was heated it expanded, and even in a
mass of aggregated parti dies it never showed any signs
of shrinkage due either to the action of water films, or,
at higher temperatures, by the pulling of the fluxes.
From ordinary 'temperatures up to about 575 degs. Cent,
it maintained its state unchanged, but 'at that tempera-
ture it absorbed some latent heat without changing its
density, 'and inverted from a to /3 quartz. Above
575 degs. Cent, it expanded regularly up to 870 degs.
Cent. Above 870degs. Cent., by reason of pome pro-
found change in the constitution of the silica molecule,
quartz as a form of silica wais metastable, and, if time
were allowed, and if the environment were suitable, it
would break down, and, expanding abruptly and con-
siderably, passed over, into a substance indistinguishable
from tridymite, which mineral at ordinary temperatures
had a density less than 2-33, as compared with the 2’66
of quartz. In the neighbourhood of 870 degs. Cent.,
which was the lowest temperature at which the'change
from quartz to tridymite could occur, the energy change
involved in the change of phase was slight, and the rate
of the inversion comparatively sluggish. When the
temperature rose above 900degs. Cent., the energy
change involved became more considerable, but even
up to 1,300degs. Cent., unless there was some catalytic
solvent present to help the migration of the molecules
(in quartz particles not more than 1 mm. in diameter)
many hours were required to complete the conversion;
and when the temperature had not exceeded 1,100 degs.
Cent., but had remained above 1,000 degs. Cent, for
hours together each day for several years, silica particles
in which kernels of quartz were surrounded with a zone
of tridymite had been known to survive. Above 1,300
degs. Cent., unless by reason of very rapid heating, the
change from a to ft quartz at 575 degs. Cent,
had failed to complete itself, the' change from quartz to
tridymite became comparatively rapid, but with very
rapid heating it was quite possible to raise the tempera-
ture of metastable quartz, until at 1,470 degs. Cent, it
quite suddenly lost its crystalline characters, and,
expanding enormously, passed into a thoroughly vitreous
mass.

At this same temperature of 1,470 degs. Cent, the
tridymite, which should always form if the heating had
been done slowly, began to change over to crystobalite,
which was an apparently cubic mineral possessing the
very desirable property that it maintained its rigidity
up to the temperature of its true melting point, at 1,625
degs. Cent. Since the density of crystobalite (measured
at ordinary temperatures) was 2*27, compared with 2*28
to 2-33 of tridymite, this last inversion of silica was not
accompanied by any very serious volume change. .

Behaviour of Mixed Constituents.

Having considered the physical behaviour of the two
main constituent minerals of fireclay when they were
segregated and fired separately, it was interesting to
enquire how they behaved when mixed and fired together.
Almost any pair of chemically-related substances, mixed
together and brought to the temperature region of the
melting point of one of them, either reacted to form new
compounds, or to some extent dissolved each in the other
and formed a eutectic which had a lower melting point
than either one of them. Pure kaolin, heated alone,
did not melt as a single substance, but when heated to
500 degs. Cent, gave up its water, and at about 1,200
degs. Cent, the dehydrated residue became a mixture of
silimanite with a form of silica (probably tridymite),
which on heating passed into crystobalite', and this mix-
bure itself formed a eutectic point which, as determined
by Rankin, was at 1,600 degs. Cent., or only some
25 degs. below the melting point of. crystobalite. From
this it appeared quite evident that kaolin and quartz
could not flux each other, and that therefore they might
be mixed in any proportion without loss of refractory
properties by reason of the- admixture.

Amplifying his remarks on' the question of the change
from quartz to. tridymite, the lecturer said the Germans
had taken great advantage of it. The success of the
German coke oven brick was due to the extent to which
this change, when the brick got into' the oven, was able
to, and did, exactly counter-balance the shrinking which
was taking place- after the kaolin—the part which had
lost its water, and which had gone up to the tempera-
ture at which these low melting point eutectics had
come into work—was beginning to pull them together.
Just as fast as these fluxes pulled the coke oven brick
together, just so fast came the change from the meta-
stable quartz into the tridymite. If, by burning the
bricks, all the stuff had changed into tridymite, the
other process continued to go on, and they shrank. The
English people, in so far as they had a specification for
coke oven bricks, had that specification determining the
silica percentage. They said, We have only got the
two minerals, the kaolin and the quartz, and if we use

86 of 84 per cent, of silica we shall know that the
•expansion due to this tridymite will be just right to
counter-balance the shrinkage due to the other stuff.”
That was not correct. Because they had a positive and
negative, it did not follow that they were equal and
opposite. They could, and they should, so arrange
matters that, for the particular temperature they were
going to give the material, the expansion and contrac-
tion did keep equal and opposite through periods
running into years. But they would have to know the
temperature at which the bricks were expected to be
used, and to find the state of manufacture that they
had to go to in order to balance that. With regard to
the bricks that were going to be used in the hottest part
of the oven, they would want to know that they had had
a lot of silica put into them, and the expansion taken
out, or they would have to use silica by itself. If they
could make a tridymite brick by itself, and get the
expansion out, knowing that it was never going to pass
1,470degs. Cent., they could trust that material, and
use it; but if they were going to work at temperatures
—say, 1,000 degs.—where the rate of tridymitisation
was SO' slow that little grains a millimetre in diameter
would tolerate the temperature for a year or 18 months
—unless there was some catalytic agent to tickle them
off—they could have that expansion exactly balancing
the slow dragging together of the< highly refractory
kaolin which had been properly fired. He had no doubt
that, when we knew enough about it, by balancing the
expansion of the quartz against the contraction of the
burnt kaolin, we should eventually be able to get a
brick that, we could make hot and cold, use in an oven,
and heat and cool repeatedly up to a certain tempera-
ture, without its changing its shape; and if the brick
did not change its shape, the oven would remain the
same diameter and the same size from one year’s end
to the next. .

Porosity.

Referring to the question of porosity, he said that if
they had a kaolin in which these liquids had developed
to such an extent that they were slightly sticky, and
had a texture like nitch, and if in that state expansion
took place, that expansion---which was a. solid expansion,
without any liquids at all—would be able to elbow the
other particles together, and drive the fluxes into the
places where they were wanted, and porosity would be
obtained. He did not know to what extent porosity in
the bricks was a matter of importance, but he believed
it was of great importance. He believed that a lot of
the nitrogen which they found in their gases had come
in through the coke oven bricks in many cases, and he
believed that if they had ovens now, where in the process
of manufacture the expansion had been just recently
got down, and the kaolin had not contracted and was not
contracting during the years of working, then they kept
their gas and did not lose it. If they were not properly
balanced, little cracks, smaller than hair cracks, gradu-
ally developed, and the gases outside* and the gases inside
mixed in any odd way that happened. Then, he thought,
they had ceased to have anything analogous to a flask,
and had got something much more analogous to a piece
of porous biscuit ware..

CANADIAN COAL IM 1916.

Tbe Dominion Department of Mines has received
from the principal coal operators in Canada returns of
their production for 10 months, supplemented in most
cases with estimates for November and December.

On the basis of the record available, it is estimated
that the total production of coal in Canada during the
calendar year 1916 will approximate 14,365,000 short
tons (equivalent to 12,825,892 gross tons). The estimate
is believed to be fairly close for Novia Scotia and British
Columbia. In Alberta, however, there are so many
small operators that final returns may show a wider
variation from the estimates now made. >

By provinces the estimate is as follows, the figures
for 1915 being included for comparison.

Estimated Coal Production in Canada, 1916.

Production of coal. Inc. ( + ) •
i---------k---------or

1915.	1916. d e. (-)

Short tons. Short tons. Short tons.

Novia Scotia........ 7,463,370	...	6,950,000	...-	513,370

New Brunswick ....	127,391	...	135,000	... +	7,609

Saskatchewan.......	240,107	..?	260,000	...+	19,893

Alberta ........... 3,360,818	... 4,F‘0,000 ...+1,039,182-

British Columbia ...	2,065,613	...	2,620,000	... +	554,387

Yukon.................  9,724	...	—	...	—

13,267,023 ... 14,365,000 ...+1,007,977

The 1916 production exceeded that of the two previous
years, the increase over 1915 being about 8 per cent.
Novia Scotia is apparently the only province that has
not made an increased production, the falling off in
this province being a little less than 8 per cent. The
increase in Alberta is nearly 32 per cent, and in British
Columbia nearly 27 per cent. The production in New
Brunswick, Saskatchewan, and British Columbia is the
highest on record. ‘ No estimates are available yet as
to the Yukon output.

* Can. Min. Inst. Bull.

The London County Council have to a large extent .aban-
doned the ordinary contract arrangements, and are now
buying month by month the necessary coal supplies for the
drainage works, and for offices, schools, etc.

The French Minister of Public Works has issued a Decree
ordering all the goods depots on the. Paris Grand Circle
Railway and inside that circle to be kept open until 8 p.m.
(Sundays included) for the delivery of fuel, in order to facili-
tate distribution.