Charles George Warnford Lock.

Economic mining: a practical handbook for the miner, the metallurgist and ... online

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labour is about two-thirds, (c) there is a certainty that the different
materials have at no time become separated, hence an unvarying
product, and (d) less ground is required for works.

The original cost of a dry plant is greater than for a wet, which,
however, Sa amply compensated for by the rapidity of production, not
having, as in the ** wet process," to wait on a half finished product for
which labour has been paid.

Of other processes for the reduction of the raw material, the only
one worthy of notice is " double-kilning." The hard limestone is
first burned to quicklime, then sufficient water is gently sprinkled
irver it so that it slakes and yet remains a perfectly dry powder ; the
lime powder is mixed with clay or ground shale, and then burned.

The kilns principally used in England ('* dome kilns ") internally
are in the form of an elongated egg, or two frustrums of cones, base
to base ; in height, 30 to 60 ft. ; in widest diameter, 9 to 18 ft. One
40 ft. high would hHve about the following dimensions : At the top,
8 ft. diaui.; from top to 17 ft. down it widens to about 12 ft. ; for the
next 14 ft. (31 ft. from top) it contracts to 8 ft. ; from there to the
grate bars (36 ft. from top) the diameter is 6 ft. The ashpit or draw-
hole is about 4 ft. high. The lining throughout is firebrick.

The mode of burning in this kiln is simple. Wood is placed on
the grate bars, 2 ft. high ; then a layer of ccal, on which alternate
layers of coke or hard coal and dry lumps are arranged. The pro-
portion of coke to material decreases from bottom upward. About
10 per cent, of the weight of the raw material of coke or anthracite
coal is requisite to bum such a kiln.

These kilns are intermittent When once lighted they bum till
all the fuel is exhausted. The reason is that the material being
burned at such a high heat, the cement clinker adheres so strongly to
the sides that it is impossible to draw it all from the bottom without
first lootfiening it from the sides.

Clinker of a brown colour, which when taken from the kiln dusts
exoessiveh', producing a soft, smooth-feeling powder, contains an
excess of clay, is weak in indurating capacity, and will contract after
being u^ed. Much less fuel is required to burn it.

Clinker of a black colour, which does not dust when taken warm
(not hot) from the kiln, and, when powdered, has a bluish cast, con-
tains an excess of lime. If not great, it can be purged of this
dangerous quality by »preading it out on floors till the excess of free
lime has becooje neutralised by the action of the air, and can then be
used without danger. But if the excess of lime is very great, no

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exposure will eradicate it. To manufacture such a cement is expen-
sive : it takes large amounts of fuel to clinker, and time is lost by the
long exposure required.

If there has to be an excess of lime or clay, it would be better ft»r
the manufacturer to have a Blight excess of lime rather than an exoees
of clay.

The powder of a normal or perfectly combined clinker should,
when freshly ground, be of a grey colour, tinged with green, losing
much of its green cast after exposure for a few days ; it should le |
granular in character when finely ground, and should at no time have
a smooth or silky feel.

Many cements contain an excess of lime sulphate, derived eith^
from a contamination of the limestone with gypsum, or the result of
the oxidation of the sulphurets that existed in the clay or shales, or
due to the sulphur that is always present in a greater or less d^ree
in the fuel. This lime sulphate is detrimental to a cement, and
should not be tolerated in appreciable amounts. Over 1^ percent
sulphuric add, or 1^ per cent, carbonic acid, or 1 per cent, magnesia,
is very objectionable.

A convenient source of lime carbonate for cement-making is the
'* lime mud " forming a waste product in alkali manufacture. It is
worked by J. S. Bigby's patent.

The consumption of fuel for cement-burning, per ton of dinker, is
thus stated : —

For open or ohnmber kiln 4} to 6) owt. ooke.

„ Hoffman kiln 3{ „

„ Dietzaoh kiln 2 to 3 owt small coaL

Continuous kilns of the Dietzsch type, in which the materials and
coal are charged in at the top while the burned clinker is withdrawn
at the bottom, are coming into general use, and proving very econo-
mical of fuel. Duryce's revolving furnace using crude petroleum as
fuel, is favoured in America.

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I'hk cKaracteristio property possessed by the mineral substances
whioh are classified under the term clay is plasticity, due to hydrous
alumina silicate. The purest clay is known as kaolin, or china clay ;
it is composed of minute particles loosely aggregated, resembling
(when magnified) fish-roe in appearance, and starch globules in
physical qualities. The crystalline components of clay are generally
deognated as sand, and consist of fragments of ditferent mineral
Bpeciee, sucli as quartz, felspar, mica, hornblende, tourmaline, magnetic
Iron, &C. These crystalline substances may be found in particles not
mudi lar^r than the globules of kaolin proper, and thus remain
M2«pended ij?ith it in water, while the coarser sand settles quickly, and
[Day be separated from the kaolin and the finer crystalline materials.
Ihe contamination of kaolin with these fine crystalline particles
[^Te rise to the erroneous deduction that kaolin itself had a crystal-
line Btmcture.

Other compounds present in clays, which have an important
bearing in determining their economic value, are iron pyrites, lime
Rilphate, lime carbonate, dolomite, carbonaceous and bituminous
Eoatter, iron oxide, <S:c. To these are due the various colours which
characterise ordinary clays, and which vary in their efifect upon the
naterial i^hen it is applied for technical purposes.

Disintegration of the primitive rocks, and the rocks that were
igain formed from them, going on through millions of years, has
-csolted in a deposition of vast amounts of cEiys of more or less purity,
rhey occur in beds of varying thickness, and follow the stratification
n dip and strike of the underlying rock. These primary deposits of
slay have been rearranged many times by subsequent geological
ih^ges, iivhich have sometimes resulted in the purification of the
ilay mineral proper, and at other times in its degeneration.

The power to pass into a plastic state decreases in proportion as a
and J element is mixed with the clay. It is strongest in the ^' fat "
ind weakest in the " lean " clays. A " fat " clay dries very slowly
aid unevenly, and warps and cracks in drying. Equivalent quanti-
i£s of different fluxing oxides produce the same efiect upon the
osibilitj of a clay ; for example, if analysis shows that a certain clay
ontains magnesia *3 per cent, and that another contains potash
7 per cent., we may expect with certainty that both will have the
ame resisting power to the action of fire. The presence of these
everal substances in the clay does not influence the effect produced
J each singly, their fusibility increasing only with the higher sum
f their combining weights.

Briek Clay. — Ordinary yellow brick clay contains iron as oxide
nd as carbonate chemically combined with water in the form of
3 drates. The expulsion of this water in the process of burning

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imparts a red colour, due to the convereion of the hydrated oxidee c
iron into the anhydrous form. The principal constituent of brie
clay, and that upon which its plasticity depends, is silica. Thi
constituent used alone shrinks and cracks in diying, war}« an
becomes very hard when baked. Silica is also present in near!
all clays in an uncombined state, such as sand. A proper pr
portion of sand prevents cracking, shrinkage, and warping, an
famishes silica necessary for a partial fusion of the materials, whic
increases the strength of the brick. l*he sand also makes the brie
more shapely and equable in texture ; but an excess of sand in cla
renders the brick made from it too brittle. A small quantity of lin
carbonate has a beneficial effect upon brick clay in two ways-^
lessens the contraction of the newly made bricks in drying, and aci
as a flux in the kiln by the formation of lime silicate, which bini
the particles together. It is evident from this that exoesB of lin
carbonate in the day would cause the brick to melt and lose its ahap
Iron pyrites in a brick clay is objectionable ; cdso the presence <
carbonaceous matter to any extent, as a black discoloration ia likd
to occur. Common salt is nearly always present in minute qxuuititi<
in clay. In that near the seashore the amount is apt to be so grei
that bricks made from it are of poor quality. Salt melts readily sa
glazes the outside of the bricks, and the heat cannot be raided c
maintained sufficiently long to bum them to the core, or into goo*
hard brick ; as a consequence, they are soft and, from the presence i
the decomposed salts of magnesia and soda, are always damp, owing t
the tendency of these salts to absorb moisture from the atmospheii
The presence of the alkaline carbonates in clay, to any notable ext^ii
prevents its being used as a brick clay, the alkali causing the materii
to melt readily.

Brickmakers divide clays into three classes : —

Plastic or strong clays, which are chiefly^ alumina silicate ; thel
are called by the workmen " fat " clays.

Loams or mild clays are those in which a considerable proportic
of sand is intermixed.

Marls or calcareous clays contain a notable quantity of lime cft]

*^ Malm " is a name applied to an artificial marl, made by addio
to and interujixing with the clay a proper proportion of lime cal

As a general rule, a clay fit for the manufacture of a firsi-cU
quality of brick is not met with in nature, being deficient either |
sand or lime. A good brick clay is one that contains sufficient fu^il
elements to bind the mass together, but not so much as to make fl
bricks adhere to each other or become vitrified. Such days contd
20 to 30 per cent, alumina, and 50 to 60 per cent silica, the remaini
consisting principally of lime and magnesia carbonates and iA

Pure or " fat " clays are sometimes used without any admixtia
Bricks thus made are generally deficient in weathering qualities. J

The following may be taken as the ordinary me&od of makl
what are known as "clamp-bricks" (i.e. bricks which are euJ

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aentlj to be burned in clamps) about London. The preparation
nnmenoes in the winter, during which season the brick-earth is dug
ad mixed with chalk and fine ashes. These ashes are the siftings
rom the contents of the dust-bins of London houses, and consist of
ie finer particles of coal and breeze (cindern), mixed inevitably with
iline matters from the burned coal and with organic matter both of
egetable and animal origin. For the ordinary kinds of brick, 20
tuddrons of ashes and 15 tons of chalk are mixed with every
700 cub. ft. of brick earth. At the Burham works, near Maidstone,
rhere the basis of the ordinary clamp bricks is a red clay which lies
slow the ohalk and above the blue clay (gault), out of which the
brtUmd cement is manufiictured, the preparation of the " turf," as it
\ Uiere teriued, is as follows : — A heap is made of successive layers of
lay (which contains chalk enough for the brickmaker's purposes),
iikd, and ashes in the following proportions, viz. 1 ft. deep of clay,
in. of sand, and f in. of ashes, and these layers are repeated until a
np about 10 ft. high is formed. The heap is left to become
weathered '' until the spring, when it is dug down and well mixed.

China Clay, — The whole of the china-clay used in the United
ongdom, and most of that used on the Continent of Europe, amount-
ig in the aggregate to upwards of 3,000,000 tons yearly, is produced
& Cornwall and the western part of Devonshire. The greater pro-
ioftiun of that used in the United States is mined in Pennsylvania
ud Delaware.

Beanlting from the decomposition of a rock composed of felspar
ad quartz, it is found in pockets or beds in low and occasionally
vampy ground.

Kaolin is generally proved by boring, or by sinking small shafts.
¥hen the position of a deposit is determined, the overlying soil is
emoved and the clay is uncovered. The clay is toilsome to excavate ;
ke strongest steel-pointed shovels are required for the work. It is
emoved by means of carts, cars, or derricks, and the bed is drained
gr pumping. The clay, as extracted, is treated in a washing-machine,
n which revolves a horizontal shaft, 3 or 4 in. diam., carrying kidves
l2 in. long, at 4 in. pitch. A stream of water is turned on, and the
ilay is charged at the top or hopper entrance. It is divided as it
Mtfses through the machine, and the sand or quartz delivered with
he clay and water settles in a box or sump, whence it is continually
bevelled out. The clay combined with water, to the consistency of
■earn, runa slowly off" into a number of troughs, where impurities
l»ttJe to the bottom, and whence it is turned into large vats, where it
■mains until it becomes quite thick. From tht se it is pumped into
Eiter-preflses, which consist of wooden panels or diaphragms, each of
which, contains a canvas bag. The water escapes througti the
Rtersticee of the canvas, and the clay is of such consistency that
t can be handled and placed on shelves in open air to dry ready for

Kaolin ia improved by exposure. If piled and allowed to freeze
tnd thaw in winter, it is found to be the tougher for it in the spring.
Krom 30 to 50 per cent, of washed kaolin is obtained from the crude
imy. The quartz washed from the clay is pulverised and sold to the

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1 88


potters, who use it in the body of the ware, and also with felspar ai
a glaze.

Coi' POSITION OP China Clays.

China. StydeuxJ Aue.



New Jeney.


Silica .. ..


48-37 1 46-00







Alumina ..


84-95 139-00







Iron oxide ..









Lime ..










liagnesia ..







, ,


Alkalies ..



, ,







Water .. ..


12-62 12-74




12^10: 11-62



Fireclay. — Fireclays are almost exclusively obtained from th^
coal measures, where they often form the bed on which the coal liet
They are mined simultaneously with the coal. They are remarkabl;
pure, sometimes consisting virtually of silica. As they are reqairet
to withstand high temperatures, objectionable impurities woiUd b
iron oxides (above 6 per cent.), and magnesia, lime, soda or potasl
(if exceeding a total of 3 per cent.). One of the most renown^ fire
clays contains 97 per cent, silica, 1*39 alumina, '5 water, '48 ferrou
oxide, *2 potash and soda, and *019 lime. Weathering improve

The clay from which the well-known Mt. Savage firebrick is mad|
is found as a very hard rock-like mass, only to be obtained by blasting
in veins 7 to 14 ft. thick, which appear to belong to the Carboniferoa
period. As first taken from the mine, it is in the form of large block]
of a rich grey or a dark-brown colour, with highly polished surfacd
which are often beautifully mottled. After exposure to the air fo'
3 or 4 months, these blocks can be easily broken, by aid of a sledge
hammer, into small ones which still have highly polished sur&oeti
1 he clay is almost infusible before the blow-pipe, and on this aoooonl
as well as on account of its peculiar formation and structure, a fall ani
complete analysis is not devoid of interest.

It has a sp. gr. of 2*54 and a hardness of 3*5. An ultimat
analysis gave the following result : —

Water 9-88 per cent.

Silica 60-19 „

Alumina 29* 10 „

Iron oxide 0*89 „

Lime none

Magnesia ' tnice |

Alkalies 0-03 „

Carbon 002 „

Total 100-10 „

This analysis shows that the clay is free from any admixtures i
felspar or other double silicate, and only contains 0*92 per oent
fluxing substances. The value of a fireclay, however, depends not oi
on the absence of double silicates and fluxing substances, but also

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}k)w much of the silica exists united to the aluminium and how much

in the free state as sand.

The detennination of the free and combined silica made according

to the method proposed by Forchhammer gave the following

results: —

Combined silioa 2809 per cent.

Free Bllica or sand 31*84 „

Hydrated Bilica 0-23 „

There are two methods of determining the fusibility or refract
toriness of fireclay — theoretical and experimental.

In the former, conclusions are drawn from the chemical composi-
tion ; in the latter, from the changes which the clay undergoes when
subjected to intense heat.

In the theoretical method, leaving out the hygroscopic and com-
b'ned water and organic matter as not affecting the fire-resisting
property, the refractory constituents are alumina and silica, and the
kxinz constituents are magnesia, lime, soda, potash, and ferric oxide,
^cooroing to Bischef, alumina is the least ^sible component, and
dlica follows close upon it, though a mixture of the two has a much
iower melting-point. The manner in which silica affects the refrac-
toriness of alumina is variable.

The least fusible mixture consists of one molecule of alumina and
two of silica, and melts at a temperature indicated by Soger cone
So. 35. The fusibility increases with the amount of silica up to the
pportion lAl^Oj: ITSiO,, and then decreases (on account of the
preponderance of silica, which cannot combine to form a silicate),
mtil, finally, the alumina disappears and the melting-point of silica,
iqnal to that of Soger cone No. 35, is reached. The practical deduc-
»n is that the refractoriness of a fireclay increases with the amount
ff almnina it contains.

The effects of the fluxing constituents have been studied by
Jischof and Bichters, who propound the law that the fluxing property
I inversely proportional to the molecular weights ; thus 40 magnesia
ronld have a slagging effect equivalent to 56 lime, 62 soda, 94 pot-
ih or 160 ferric oxide. Soger maintains that ferric oxide has a
tponger slagging effect than any other of the four bases. Whatever
oay be-the precise order of these bases, it is a settled fact that if they
xceed 6 per cent, of the ignited clay it cannot be classed as refractory.

To sum up, the fire-resisting power of a clay,* considered from a
hemical point of view, depends first upon the character of the
^ing-constituents and their relation to alumina and silica, and
Mindly, on the relation of alumina and silica to one another.
Sachof arrives at the refractory character of a clay by what he calls
be refractory quotient, which he obtains by dividing the quotient of
be oxygen of the fluxes into that of the alumina, by the quotient of
be oxygen of the alumina into that of the silica, thus :

Qin AI2O3 ^ in SiO^
OinKO • OinAljOa*

Bischof s method has found pretty general acceptance within the
eceasary limitations. It is, however, questioned to some extent by

Digitized by VjOOQIC


Seger, who recommends adding the ratio of the fluxing oonstitnent to
the alumina with that to the silica, and multiplying this sum by the
quotient obtained from dividing the latter into the former, thus :

O in Al ^Oa 9_i^SiOa\ / O in Al ^ ^ O in SiOA
O in RO "^ O in RO / V O in RO " O in RO /

No deduction, however, made from chemical analyses can have the
force of a positive determination, because analysis necessarily i^oree
the physical constitution, whereas a coarse-grained clay is less losible
than a fine-grained, and a compact than a loose one.

Experimental methods may be classed as direct and indirect
Until lately all the direct methods have given only what may \k
called qualitative results, that is small samples of clay were exposed
to an elevated temperature and the effect was noted. Bischof coats t
piece of oiled paper with a clay paste, and gets thin tablets off wher
the clay dries. Another test is to place a sample of the dry pul-
verulent clay in a crucible and heat in a furnace. His effective testi
to distinguish fritting from fusing are to draw a line with pen anc
ink over the fracture of the sample or touch it with the tongue. I
it is fritted it will adhere, and the ink will spread as it would oi
blotting-paper ; if it is fused it will not stick to the tongue, and thi
pen and ink line will be sharp and clear. The transition froii
qualitative to quantitative work is made by Otto, who forma tw<
small test-bricks (4J by 2f by 1} in.) from a uniform mixture of hal
raw and half burnt clay, places them alternately with two othei
bricks of the same size and of known properties to form an oblong oi
a refractory pedestal in a crucible furnace, and then heats them witi
charcoal, coke, and forced draught for about 2 hours.

The only quantitative direct method is that by Seger and Cramer
who form from the sample of clay to be tested a number of cones
inclose a test-cone with two different numbers of the standard conei
in a magnesia crucible, and heat with gas-carbon in a Devitle fomaa
lined with chromite.

In these experiments the sample has necessarily been excdudet
from view and the temperature of the furnace could not be controUec
with any degree of nicety. To overcome these difficulties an Attemp
has been made by H. 0. Hofman and C. D. Demond* to oonstmct i
furnace in which the temperature could be easily measured and th
samples watched, and to devise a method of testing which did no
require temperatures near that of the melting-point of platinxui]
This furnace employs ordinary illuminating gas, and is full;
described by the authors in their paper.

For additional information refer to article Clay, in Spons' ' Enej
clopsedia,' and article Brick-making in Spons' * Dictionary o

♦ TranB. Amer. Inst. Min. Engs., 1S94.

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jr domestic and industrial importance, coal may be placed foremost
mong mineral products. G^logically it is most prominent in strata
f the Primary System, conferring a special name upon the beds lying
etween the Permian and Devonian formations ; but large quantities
f mineral fuel are also derived from other formations.

Though all coals may be attributed to a like source, viz. accu-
inktions of Tegetable matters under certain conditions of pressure
nd exclusion of air, no mineral shows a greater inconstancy of com>
oBi'tion. A few examples of British coals will suflRce to illustrate

(aj Specially good steam coal, burning freely and yielding very
[ttle ash, shows on analysis : —

CSarbon 76-94

Hydrogen 5-20

Nitrogen trace

Solphor 0-38

Oxygen 14*37

Ash 311

(&) Good domestic coal, and valuable for gas making, steam
ifing, and iron smelting, affording nearly 60 per cent, of coke :

Carbon 81-36

Hydrogen 6-28

Nitrogen 1-53

Sulphur 1-57

Oxygen 637

Ash 2-89

(c) Strong durable coal, yielding much ash :

Carbon 73-52

Hydn^gen 5-69

Nitrogen .. 2*04

Solphnr 2-27

Oxygen 6-48

Ash 10-00

(i) Kotten coal, coking very slightly, weak, and smelling
ipleasantly in domestic grates from excess of animal remains :

Carbon 66314

Hydrogen 5-627

Oxygen 22-861

Nitrogen 565

Solphnr 2-364

Ash 2-269

Water 34-660

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(c) Coal rich in oil and parafiSn, of high illnminating power, an
therefore valuable for gas making :

Volatile matter 68-40

Coke 31-60

Ash 22-80

Sulphur -53

Sulphur in volatile matter *45

(f) Coals of moderate quality, used for steam raising and iro
smelting :

Carbon 64*9 .. 63-8

VolatUe matter .. .. 346 .. 34*8

Ash -5 .. 1-4

From the researches of Mahler, Johnson, and Gruner, it appear
that the economic value of a coal, which may be said to depend on ii
steam raising qualities, may be deduced from its chemical con
position, and the outcome of their investigations is condensed in tl
subjoined table :

Appboxihate Heating Value of Coals.

Percental^ of Fixed

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 21 of 76)