THE REFRACTORY MATERIALS USED
Of the greatest importance to the glass manufacturer are the refractory
materials upon which the life of his furnace and pots depends. A few
notes giving a description of them and dealing with the manufacture of
the fire-resisting blocks used in building the furnaces will be of
interest.
The chief and most generally used of such materials are the goods. The
best known deposits of fire-clays in this country are those in the
Midlands, Stourbridge, Leeds, and Glasgow districts. In each of these
districts the mining of fire-clays and the manufacture of fire-resisting
goods for furnace work forms an important industry. The theoretical
composition of a true would be a double silicate of alumina, and in this
pure state it would be of a very refractory nature. But, naturally,
fire-clays show the presence of other bases, such as iron, lime,
magnesia, titanium, and alkalies, which, if present to any appreciable
extent, lower the degree of resistance to heat or refractoriness of the
clay. These other bases may be considered as impurities or natural
fluxing agents. The characteristics of a highly refractory clay suitable
for glass manufacturers’ requirements would be: (_a_) that such a clay
should show no signs of softening at the highest heat of the furnace;
(_b_) a squatting point not below Cone 31 or 1690° Centigrade; (_c_) a
high alumina content not below 30 per cent.; (_d_) the greatest freedom
from impurities; (_e_) a fine-grained texture; and (_f_) a high degree
of plasticity. These are the qualities most essential for glass house
work. The figures given by the chemical analyses of good fire-clays
would probably fall within the following limits—
Silica 49% to 65%
Alumina 48% to 31%
Ferric Oxide 0·5% to
1·5%
Titanium Oxide nil to
1·5%
Lime nil to
0·5%
Magnesia nil to ·2%
Total Potash and 0·5% to
Soda 1·8%
Clays of higher silica content than 70 per cent. would not be considered
suitable as pot-clays owing to the case in which glass attacks silicious
clays. It is important that chemical analyses of fire-clays should be
compared with results obtained from the analysis of fired or burnt
samples, or they should be recalculated to allow of such comparison, so
as to exclude the figures for the hygroscopic and chemically combined
water of the clays.
The writer gives the following particulars of a very suitable for glass
house pot-making. It is plastic and highly refractory, and is now being
considerably used by the trade. The clay is supplied by Mansfield Bros.,
Church Gresley. The figures are from a report made by Mr. J. W. Mellor,
D.Sc., of the County Laboratory, Stoke-on-Trent, and are as follows—
Raw Fire-clay Dried at 109° Cent.
Silica 46·45 per
cent.
Titanic Oxide 2·65 per cent.
Alumina 35·32 per
cent.
Ferric Oxide 1·31 per cent.
Manganese Oxide —
Magnesia 0·09 per cent.
Lime 0·41 per cent.
Potash 1·08 per cent.
Soda ·76 per cent.
Loss when 12·14 per
calcined over cent.
109° Cent
The melting point is given as equal to Seger Cone 33
or 1730° Centigrade.
The physical properties of fire-clays vary as well as their chemical
properties. The analysis alone of a is not always sufficient indication
as to its ultimate behavior when in use. Many physical tests have to be
carried out before a clay can be proved satisfactory for a particular
purpose, and much information can be gained by engaging the services of
a specialist upon refractory materials to carry out petrographic,
pyrochemical, and physical tests, and report upon the suitability of the
material for its specific purpose. Fire-clays should be plastic, and
this plasticity should be developed to its utmost to increase the
binding properties of the clay when used. To develop the plasticity,
fire-clays should be weathered or exposed in thin layers to the action
of atmospheric influences. The heat of the sun and the action of frosts
and rain have a direct influence in breaking up the clay and developing
its better properties. The use of new unweathered clay is the cause of
much trouble to the glass manufacturer who makes his own pots and
furnace goods, and on this account he should insist upon having his
clays weathered for some time before use, so as to have them thoroughly
matured. Before fire-clays are weathered or used for important work they
should undergo a process of selection and cleaning. When first raised
from the mines all foreign and inferior portions, carbonaceous matter,
vegetation, iron pyrites, and stones are removed. The best and cleanest
portions of the are sorted out and removed to the weathering beds, where
the lumps are broken down to small pieces about the size of an egg, and
left to mature and season by weathering.
This is then spread out in a layer about 2 ft. deep, and, after a period
of exposure to the action of the weather, the heap is turned by men
shoveling the clay from one side to the other. The clay, under the
continued action of the wind, frost, and rain, disintegrates and slacks
down until it is reduced to a mild, fine-grained mass, which condition
shows it to be well seasoned and ready for use. Fire-clays vary in this
respect: some clays season quickly in the course of a few months, others
take years to develop their proper nature. The former may be classed as
mild fire-clays, the latter as strong fire-clays.
After weathering, the clay is carted or conveyed to the clay-grinding
plant, where it is stored under cover until it is dry enough to be
ground on the clay-mill. Here the clay is fed into a revolving pan, and
crushed under heavy iron runners, and, after passing through
perforations in the bottom of the pan, it is elevated on to screens
which sieve the clay to a requisite degree of fineness. It is then
admixed with a large proportion of ground-burnt and the mixture is
tempered with water until it forms a plastic mass of dough, which is
conveyed to the workshops where the furnace blocks or pots are to be
made. These making and drying shops have false or double floors, under
which steam or heated air is passed using pipes or flues below the
floors, giving the steady and uniform heat which is necessary to dry the
goods as they are made. Heavy goods should on no account be hurried in
drying, lest trouble should occur through the goods cracking or warping.
In making the blocks for the furnaces the workman takes a portion of the
prepared clay and tramps the plastic mass into a wooden frame, or mold,
the shape and size of the block required, with due allowance made for
shrinkage. The blocks are made on the warm floor, which is of cement or
overlaid with quarries. When the mold is filled the surplus clay is cut
off and the wooden frame is lifted up, leaving the clay block on the
floor. The empty mold is then cleaned and refilled. The blocks are left
until they attain considerable stiffness from the evaporation of the
water present by the heat of the room. They are then dressed and cut to
the final shape desired, after which they are further dried until they
become quite hard and white. When thoroughly dry the blocks are removed
from the drying sheds to the kiln for burning.
In burning thick and heavy blocks much care and vigilance is required in
expelling the chemically combined water present in the clay, and, as the
temperature rises and approaches red heat, the rate of heating should be
retarded to allow proper oxidation to take place throughout the
structure of the blocks, and prevent black cores from being formed. In
all fire-clays, besides the mechanically admixed water used in preparing
the clay to a plastic mass, which is mostly driven off whilst in the
drying shed, there exists water in a chemically combined state. This the
combined water is not expelled below 250° Centigrade, and is tenaciously
held by many varieties of mild fire-clays. Due care has to be exercised
in dehydrating goods made from such clays; therefore the man in charge
of the burning regulates his fires, keeping the kiln at a moderate heat
for some time to allow this chemically combined water to be properly and
completely expelled. This dehydration stage in burning clay goods occurs
between the temperatures of 300° and 650° Centigrade.
After the dehydration stage of burning is completed, the fireman raises
the temperature within the kiln to a dull red heat, when the next stage
in the process of burning begins. This is the oxidation period, during
which any organic carbonaceous matter present in the clay is expelled.
During this stage in burning, goods require extended time, to allow for
the heated air to permeate and get to the interior portions of the
blocks and oxidize the cores; otherwise the blocks are badly burnt.
After the oxidation stage is completed, the fireman raises the heat
quickly until he obtains a high temperature, sufficient to eliminate and
complete the shrinkage of the goods. When this heat is sufficient to
complete the fire-shrinkage, the kiln is finished and is allowed to cool
down. The blocks, when cold, are then withdrawn and delivered to the
furnace builder.
For the erection of the furnaces several grades of blocks are used,
according to the conditions and nature of the heat they have to resist.
In the presence of reducing agents, fuel ash, or glass, goods vary
greatly as to their suitability. So the local conditions to which they
are to be subjected whilst under heat should be first ascertained, and
the mixtures for the blocks adapted accordingly. So many differences
exist in the pyrochemical and physical properties of clays that their
misuse is often apt to occur if the conditions under which they are to
be used are not properly understood and allowed for. A may show a the
high degree of refractoriness under a fusion test, and yet be less
suitable for a specific purpose than one of less refractoriness showing
better physical properties and of the more suitable chemical
constitution. The size of grain in both the burnt clay and raw clay used
in the mixtures for making glasshouse furnace blocks is of the greatest
importance. In many cases it is necessary to grade the ground-burnt
material used, so that the proportion of coarse grains to the fine flour
can be regulated to suit requirements. The burnt clay used in making the
furnace blocks should be hard and well burnt, to prevent any
after-shrinkage of the goods when they are used in the furnace.
Fire-clay goods for glass house furnaces should not be burnt at a lower
temperature than Cone 12, and in the construction of gas-fired furnaces
and tanks, burning the blocks at a higher temperature, Cone 14 would
give much better results.
On the Continent the glass manufacturers usually grind and mix their
fire-clays, with the result that they know exactly what they are using
in making their pots and furnace goods, and they are not then dependent
upon outside firms to carry out their wishes. English glass
manufacturers usually buy their clays ready mixed, and as often as not
have perforce to take the mixtures offered by the clay firms.
Unfortunately, in Great Britain many of the firms who supply the
refractory requirements of the glass trade are exceptionally backward in
applying technical knowledge to their trade; consequently, progress is
somewhat retarded in the glass trade as far as the refractory materials
are concerned. So obstinate is this ignorance of science that quite
recently one well-known firm replied to an inquiry for samples of
fire-clays to be sent for important research work then being undertaken
upon the resources of the country, stating “that, as their clay product
was perfect, and research work was quite unnecessary.” It often turns
out that their conservatism is simply a cloak to hide ignorance, as it
is quite evident to any technicist that there is ample scope for
improvement in the present goods on the market, and such an open
opportunity for a scientific investigation into the nature of their
fire-clays, however well known they may be, should be welcomed with
delight, and every facility and assistance offered for research chemists
to improve their material, and apply tests with the object of developing
the best properties of such refractories for special purposes.
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