Charles George Warnford Lock.

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

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temperature at which the bricks have been burned greatly influences
their durability. The higher it has been, the better the bricks ia
this respect. In practice, bricks from the same kiln will show marked
differences in regard to resistance to weathering, a fact attributed to
differences in the temperature of burning. It is important, therefore,
in designing kilns, to have the flues so arranged that the tempeiatore
is uniform throughout

(3) Dolomite and lime brick, cooled with water when red-^iot,
fall to pieces very rapidly ; but this disintegrating process is much
retarded by adding clay in their manufacture, in direct pro^rtion to
the percentage added. When the bricks are, after cooling wiUi water,
reheated to redness, they do not entirely recover their resistance to
weathering, but it takes a few days longer for them to disintegrate.
Cooling with water has little effect on magnesia and magnesite brick.
They had not fallen to pieces after they had been kept a year. More or
less all basic brick crack by cooling in water when red-hot, but tLei«
cracks are rarely so large that they break at once. Whon, howcTer,
disintegration sets in, the bricks split in the direction of these cracks^
generally, in conformity with their form, at right angles to their two
axes. In the case of magnesia and magnesite bricks, also, a slight
disintegration is noticeable, it being possible, after a few months, to
break them by strong pressure of the hand in the direction of the*
cracks. The surface of^these cracks is dull, while the fracture of tin
brick is otherwise brightly crystalline.

(4) Dolomite, lime, and magnesite bricks, unless made of impnre
material, shrink about 24 per cent, when exposed to the highest
white heat. Bricks made of strongly calcined magnesia shrink (mly
4 per cent. All substances that tend to decrease Sie refractory chi
racter of basic bricks increase their shrinkage.

(6) Lime and dolomite bricks are equafiy attacked by the cindd
formed in metallurgical processes, while magnesia bricks show mnci



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NON-METALLIFEROUS MINERALS. 329

Dore resiBtanoe. The oxides of iron are the worst enemies of basic
tnicks, and therefore particular pains must be taken in choosing raw
naterials, with the view of having them as free as possible from
nddes of iron, which make the bricks less refractory without at the
tune time increasing their durabilitj in dry air. Silica, phosphoric
idd, and the oxides of manganese are not so destmctiye to basic
ffick.

Simimarising, Wasmn states that undoubtedly the best material
or basic bricks is magnesia preheated at the highest white heat.
Phe bricks made from this material are remarkaUe for their dura-
bility in dry as well as in moist air, for their power of resistance to
be action of cinder at high temperatures, and for the small amount
I shrinkage.

One great practical drawback to the lime and dolomite bricks is,
hat they disintegrate in so comparatively short a time, so that it
I impossible to manufacture a laree stock of them. The heavy
hrinkage, too, is disagreeable, leadmg to the production of very
uny irr^ularly shaped bricks, and causing large joints in the
uaoniy, which in turn lead to its rapid destruction.

All these drawbacks disappear with the magnesia brick. But
beir cost excludes them, and they would be available only, if, at
resent prices, they would last 3 to 4 times longer than lime or
olomite brick. Practical experience has shown, however, that their
SQBtanoe to the action of cinder is not much greater.

The compounding of a basic brick from several constituents
eoessitates the complete pulverising and mixing of the materials,
Aking them up into bricks, and then burning and remnding them.
he material must be calcined so long and at such a hi^ tem-
»rature that it will not afterward either slack, except upon long
cposure, or contract at any heat to which it may be put in the con-
irter. The calcination must be done with care, since it is desirable
tat the pieces burned should not split under the influence of
iat, and that there should not be a large amount of fine material
x)duced. This consideration is important and affects the cost
ifavourably.

Magnesia prepared artificially was used at Horde for several years,
tt was found too expensive.

At Horde magnesia was extracted from dolomite on a large
de, and had the advantage of being perfectly homogeneous and
nost pure; it could be stored much longer than dolomite bricks,
^o processes were used, equally simple and good. Dr. Scheibler's
ocess consists in burning the dolomite to £ive off the carbonic
id, and then making a thick milk of it with water. Into this is
ored water containmg 10 to 15 per cent, by volume of molasses.
ie mixture is carefully stirred witn a mechanical stirrer. In a few
iments saocharate of lime is formed, and remains in solution while
e magnesia is precipitated. Put through a filter-press, the m^nesia
uains behind and the saocharate of. lime passes through. Tliis is
?n treated with carbonic acid (which precipitates the lime as car-
oate) and put through a filter-press; the lime after washing is



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330 ECONOMIC MINING.

used or thrown away; and the molasses is used over again. The
composition of the magnesia so obtained was :

Per ccdC

Silica, iron oxide and alumina 1*47

Lime , 2*18

Magnesia .' .. .. 95 99

99-64

The loss in molasses was 5 to 10 per cent., and too large for
economical \aantifiEtcture, thongh the qualities of the bricks were
imexceptionable.

Closson's process is based on the use of magnesium-chloride, a
by-product of the manufacture carried on at Stassfurt. A quantity (rf
the Stassfurt magnesium chloride is added to dolomite which has be^
burned and made into a milk, both being mixed together with sufficient
water in an agitator. The reaction takes place rapidly, and when quite
complete the tub is tapped from the bottom and its contents are nm
through an ordinary sugar filter-press. The lime chloride runs out
and leaves the pure magnesia hydrate in the filter. This is carefiilly
washed with water, and the lime chloride is collected in a basin. To
utilise this material it is carried to a receptacle like that in which
blast-furnace gases are washed, except that revolving wheels etii
the chloride, making a thorough mixture of the gases and liqnid.
Two of these receptacles are placed together back to back. A \TdT«
which can be reversed sends the gases to either side, and thus kee]
up a continuous working. Into this box a quantity of freshly bum
dolomite is put, together with the lime chloride. The blast-^
gases passing through precipitate the lime, have their carbonic aci<
or a part of it removed, and are thus rendered more oombustibL
They deposit, besides, a considerable quantity of the solid materi ^
carried off mechanically with them and are thus cleansed. Magnesi
chloride is re-formed and remains in solution. The liquor drawn ol
is filtered ; the mud and lime carbonate are thrown away ; and tlij
magnesium chloride is used over again. There is a loss of 5 to 6 p«
cent, of chloride of magnesium. The magnesia obtained by ^
method is made into bricks and burned. It is then reduced t
powder, mixed with a little water, and formed into bricks of anj
shape in a hydraulic press, which, with 4 men, makes 4000 smaJ
bricks a day, two at a time. The same press is used for the Ui
bricks. After a few hours' standing in a dry place the bricks aa
quite hard. The filter-press makes one filtering and washing i|
45 minutes, and contains 25 moulds, turning out about a ton of
nesia a day. To produce the ton of magnesia, about 3000 lb. <
dolomite and 20,000 lb. of magnesium chloride are required. Tli
total cost is about 49. per ton. The analysis of the magnesia pr«
duced is :

PeroenL

Silica, iron oxide, alumina 1*05

Lime 1-94

Magnesia 96*90

98-99



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NON-METALLIFEROUS MINERALS. 331

The difference between tbe products of the two processes is liardly
)pfeciable, and they are nearly alike in cost. Olosson's process is
ily applicable in the vicinity of works like those of Stassfurt.
sheibler's'can be nsed wherever molasses can be had at a moderate

306.

Magnesia is made out of sea-water (which contains about 4 lb. as
doride or sulphate in 1 <mb. yd. of water) on a large scale on the
Mediterranean coast of France, at Aigues Mortes, with milk of lime,
he sea-water is pumped into a tank made of masonry, and milk of
me in the proportion of 1 • 5 per cent, of lime for every 1 per oient.
' magnesia is pumped into it at the same time. From here it flows
to two similar tanks, where the two liquids are mixed mechanically,
id then filtered into shallow excavations about 1000 ft. long and
> ft wide, on the bottom of which is a bed of clean beach-«and.
lien enough has collected, the stream is turned off, and the pre-
pitate is allowed to dry in the sun, taking 20 to 30 days ; in winter
niTwt be artificially dried. The dried material is calcined at white
eat, ground, and made into bricks.

Magnesia has great advantages in simplicity of operation and
liformity of product ; the amount necessary for a ton of steel is only
le-third as much as is required of dolomite, while it lasts much
Dger in the converter. It can be made into bricks by mixing with
ster only ; and the bricks can be kept much longer, and resist
^esbnre at least as well as the others.

All substances .used for the manufacture of basic refractory
ftterials must be calcined at a very high temperature, in order to
ile it certain that no subsequent change of form by contraction
ill take place. Of the two general methods of calcination, the first,
hich is very generally employed, is carried out in the shaft fomace ;
ie Becond, in some kind of gas famace, the object in this case being
5 only to have a very high temperature, but also to avoid the
trodaction of silica or other impurities which come from the ashes
the coke. It has been found expedient, in most works in England,
bum in cupolas, raising the temperature to a white heat, and then
lecting from the product those pieces only which are of a certain
lour, rejecting every piece not sufficiently burned, or which shows
tj kind of agglomeration, and breaking off any adhering silicious
ftterial formed by the ashes of the fuel.

Objections to the cupola are, that, as the temperature cannot be
plated, there is danger that the dolomite will not be heated
fficiently to prevent further shrinkage in the converter; that it
ikes a considerable quantity of fine dust by abrasion ; and that the
h of the fuel is likely to form more or less fusible silicious attach-
ents to the pieces. As it in quite impossible to always have a
Tfectly pure coal, the material will frit and form engorgements in
fi furnace which after a time force it out of blast. This is partly
oiedied by having the centre on wheels. But the labour entailed is
eat

Continuous regenerative calcining-fumaces use less fuel than
« cupola, and are free from the objections just named ; so that, by



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ECONOMIC MINING.

ittle of the material is damaged. The heartihs onl;
oes are made of basic material, the sides and roof booni
!io inconvenience results &om this, since the mateiu
d does not touch the sides of the f amaoe at alL Con
ttle fuel is burned in order to produce the temperatoi



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NON-METALLIFEROUS MINERALS. 333



SALT.

DoiQfON salt, sodium chloride, is one of the most widely distributed
mineials, ocourrin^ in varying proportions in all sea-water, as well as
in enormous beds in strata of certain geological ages.

To oommence at home, the features of uie remarkable salt beds df
Cheshire are of special interest, commercial, domestic, and historical.
Fhe formation of the ^' meres," so peculiar to Cheshire, is believed to
laye been due to the solution in past ages of beds of rock-salt lying
it great depths below the surface. The depth of the water in one of
iiQse meres, at Bostheme, has never been accurately ascertained.

The area of the district in which the saliferous marls have been
lepoflited is computed at 700-800 square miles. These marls rest
^efly upon red and variegated sandstones, the exceptions beine
Mefly south of Macclesfield and beyond the east of Congleton to Odd
{ode; and also in part of Lancashire, where they rest upon the
^boniferous formation. Beyond Frodsham, extending in a westerly
nd then in a northerly direction, the salt deposits in the geological
pochs seem to have been formed in what is now occupied by part of
06 estuary of the Mersey. Li the salt districts of Cheshire, outcrops
f the saliferous marls and marlstones exist in several localities, as at
Lcton and Winsford.

No outcrop of rock-salt now remains in the salt districts of the
Jnited Kingdom. The depth to the top of the salt-rock, called the
ock-head, is 132 ft. at Nortiiwich, and 195 ft. at Winsford, whilst at
Gddlewioh no rooknBalt has yet been discovered.

Geologists are not agreed as to the manner in which these enor-
lOQB beds of rock-salt were formed. There is, however, a leaning by
nnpetent observers to the theory that during the Permian age
Dooessive subsidences and upheavals of the land took place ; that at
kch depression of the surface the sea overflowed an extensive low-
ring area, where . eventually a deposit of salt was formed by
raporation ; and that by repetitions of this process the vast beds
iiich we now find were accumulated. Li a rough way, the thickness
f the beds has been averaged at 150 ft., and the extent 20 miles by
2 or 15 miles ; but this extent has not been actually proved.

Practically, the deposits are considered inexhaustible. The upper
irfaoe of the rock-salt appears to undulate similar to the undulations
r the surface of the ground. It is upon the top bed of the rock-salt
lat the brine called rock-head brine ordinarily lies.

The raising of rock-salt is not now carried on to the same extent
s formerly. Brine is abundant, and is more readily converted into
to salt of commerce, the '' rock " produced by the mines being used
Imost exclusively in the alkali manufacture. The stores of brine
ill undoubtedly exist so long as there is plenty of rock-salt to which
irfaoe-water, percolating through the superincumbent earth and

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334 ECONOMIC MINING,

marl, can have aooess. The rock is very readily converted into brine
by tiie simple process of solution, which is ceaselessly carried on
under ground by the silent operations of Nature. As soon as it
reaches a certain depth, the water finds its way into some one of the
innumerable streams which are for ever eating away the rock-bead,
and flowing, in the form of brine, towards the works where pumping
is carried on. These streams are locally called " brine-runs," and
have been proved to extend for miles. The law by which they are
guided seems only the requirement of a supply of water, and descent
Some of them, as the suWdences show, take nearly straight courts,
whilst others twist about in various directions. The course taken
will probably be where the resistance is least, or where the rock-
salt is softest, or in hollows on its surface.

Our other important salt-field is near Middlesbrough.* For
something like 20 miles the river Tees flows through a country,
before it reaches the North Sea, belonging, geologically, to the Ken
Bed Sandstone formation. Over the salt area this rook is rarelj
visible, owing to a covering of aUuvial matter of great thickneM.
On the right bank of the river the New Bed Sandstone underlies the
Lower Lias beds, which form the Cleveland Hills at this point; and
on the left bank it gradually rises until it reaches the surface neai
HartlepooL After this the magnesian limestone, upon which the Bed
Sandstone rests, becomes the surface-rock over the eastern portion d
the county of Durham. I

In 1859, in the hope of obtaining water, a well was sunk to a dep^
of 1200 ft., during the whole of wmch the water was found to con-
tain so much sulphate of lime as to be useless. At the depth just
mentioned, a bed of rock-salt was struck, which proved to haTe i
thickness of about 100 ft.

The method of raising the salt to the surface is by Bolution, in
such a way that the column of descending water is made to' raise tiM
brine nearly as high as the difierences of specific gravity between th«
two liquids will permit. Saturated brine contams 26^ per cent ol
its weight of salt, and has a sp. gr. of 1 * 204 ; hence a column of stk^
a solution of 997 ft will support one of pure water having a beigW
of 1200 ft. In other words, a column of fresh water of 1200 ft will
bring the brine within 203 ft. of the surface.

A hole, say, 12 in. diam. at the surface is commenced, and retaind
of this size as long as it is safe, on accouut of its weight, to let dowtl
a wrought-iron tube something under 12 in. diam. This tube basr
thickness of J-in., and is used for the purpose of supporting the sides
of the hole. The boring is thus continued, and a second lengtb ^
tube is lowered down the inside of the first, and so on until ^
bottom of the salt-bed is reached. The portion of this outer or retain
ing tube, where it passes through the bed of salt, is pierced with tw^
sets of apertures, the upper edge of the higher set coinciding with t'
top of the seam, and the other set occupying the lower portion of
tube (Fig. 96). Within this tube so placed and secured at its Ic
extremity, by means of a cavity sunk in the limestone, a seoond

* L. Bell, ♦• Manufacture of Stdt near Middlesbrough," Proc. Inst Cir. Eu^
paper No. 2260.



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NON'METALLIFEROUS MINERALS,

!fl lowered, having an outer diameter 2-4 in. less
than the interior diameter of the first tube. This
Utter serves for pumping the brine. It is secured
at its lower end in the same way as the other,
and is then provided with snore holes, by which
the brine is admitted. The pump itself (Fig. 97)
is an ordinary one fitted with a bucket and clack,
bat, in addition, at the surface is a plunger,
which serves to force the brine into an air-vessel
for pnrposes of distribution. The bucket and
:kck are placed some feet below the point at
Mrhich the brine is raised by the column of
fresh water descending in the annulus between
the two tubes.

The rate at which the salt is dissolved de-
pends on the extent of surface exposed to the
Ktion of the water. This at first is very slow
uid the quantity of salt for some months fur-
lished by a well is inconsiderable, and the brine
lifled is very weak.

When the cavity formed by the water is suflfi-
iently large to admit of a few hours* pumping of .
aturated brine, the machinery is put in motion, ^
(rawing at first the stronger brine, which, from ^
ts greater specific gravity, occupies the lower 5:
K)rtion of the cavity. As it is raised, fresh >
rater flows in through the holes in the outer ^
wbe. The solvent power of the newly admitted J
rater is greater than that of water partially satu- ^
ated, and being also lighter it occupies the ^
ipper stratum of the excavated space. The effect ^
f these two circumstances is, the removal of a
anch larger quantity of the salt on the upper
wface of the seam than at the lower, giving the
avity the form of an inverted cone.

It is obvious that a mode of extraction, which
emoves the greatest quantity of the mineral
rhere it is most wanted for supporting the roof,
inst always be accompanied by considerable
anger from falls of the superincumbent rock,
^his danger is intensified in the Middlesbrough
iBtrict by a layer of 40 ft. of sandstone, lying
inmediately above the salt, being interstratified
rith marls, which, being affected by the inflow-
tig water, cause large masses to be detached,
8 is evidenced by occasional discoloratious of the
irine.

It has been found that some wells either afford
^eak brine, or the quantity of strong brine is
onsiderably below the average of other stations.
^e first source of loss is, in some cases, probably



335



I



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i




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536 ECONOMIC MINING. ,

dae to the screwed joints of the pump not being tight, or the metal itedi
of the pipe containing a flaw. Either of these defects permits wata
from the annnlos to enter the pnmping-tabe, thus diluting the brine
which of course entails extra expense in evaporation. Thediminntici



Fig. 97.— Bbikb Well Pump.



in the yield of strong brine is possibly also due to another caua
The rock-salt contains a good deal of earthy matter, distribated i
layers throughout its mass, as exhibited by the following analysee :-h



Salt. Earthy mAtt«r, &c
a. 98-42 1-58

6. 87-90 62-10

0. 45-80 54-70



Sftlt. Earthy] ^

d. 45*00 55*00

«. 92*60 7-50

/. 78-40 21 60



With so much sterile matter to deal with, it is not improbable th|
the sloping sides of the cavity may get covered with a coating of mu
of greater or less thickness, which probably interferes with the acti^i
of the water on the salt. i

A pump is considered to do good work if it is raisini^ brit
8 hours in the 24, and producing enough to give 250 tons of salt p^
week. I

The Middlesbrough salt field has been proved to cover an area {
1\ sq. miles, and each square mile is reckoned to contain oM
100,000,000 tons of salt. But beds of salt are not always wh^
geologists call "conformable," but rather what en^eers call "pockotyj
and uncertain. IW instance the rock-salt mines in the ijdand i



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NON-METALLIFEROUS MINERALS. 337

Sidly, which ooonr in a district consiisting entirely of gypsnm (lime
salphate) mixed with great masses of clay, are very uncertain, and a
linking for rock-salt is quite as likely to find a pocket of sulphur as a
pocket of salt.

The consequences of removing 100 ft. of salt over an area of 1\
niles, even ^m a depth of 1000 or 1500 ft, must necessarily be
terions. Another aspect is the injustice of one person extracting salt
lom another's property by pumping water out saturated with brine,
he water finding its way into neighbours* territory, and thus robbing
he ground of its solid contents.

Again, the question arises whether economy is obtained by this
Dethod of extracting salt as compared with mining salt in blocks,
f the sinking of the shafts were a less difficult matter than it has
EToved to be, and than it appears to be from the nature of the strata
D be pierced, the salt ought to be raised by some other process ; but if
bere is a bed of salt 100 ft. thick, of which only a very flattened cone
I to be ^t out round the bore-hole, and then fresh bore-holes are
eeded at abort intervals, obviously a great deal is desirable in point
r economy of utilising the deposit. No doubt this method, taken
x»m the district of Lorraine, has certain advantages, and, so far as it
fts gone, it appears to be very satisfactory; but in Lorraine, the
rstem has been sometimes superseded, by afterwards sinking shafts
id worldng the ground in a miner-like fashion, on finding that by
le old system there has been a vast amount of waste of useful mineittl.
L Middlesbrough, the deposit is so great that it pays better to
crifioe some of the mineral than to incur the increased cost neces-
ry to recover it alL The estimate for sinking a shaft down to the
It has been variously put at 100,000/. to 200,000/., while the value
the prodact is only 5«. or 6«. a ton.

The G^erman Government saltworks at Schdnebeck produce
GDually about 70,000 tous. The borings extend in a north-west to
Dth-east direction for about 6 miles in tiie alluvial plain of the Elbe,
be salt deposit, which is in two beds between the upper and lower
anbers of the Bunter Sandstone series, is overlaid by the limestone
the Mnschelkalk and the New Bed Marl, the whole having an
egulax dip to the south-west. None of these formations, however,
pears at the surface, which is entirely covered by lignite-bearing
rtiary strata, forming the flat ground by the river; so that the
oovery bas been entirely made by borings extending over a period
EDoie than 30 years.

The arrangements adopted for drawing brine from the borings
Dpiise (1} protection of the purer portions of the bed of salt firom
«e containing magnesian salts, by a coating of sand or concrete ;

ooatixi^ the strata immediately above the salt-bed with a casing of
terete, for the double purpose of providing a bearing for the lower
I of the wooden lining, and for ehutting off all chance of the in-
ration of land water from above and the consequent introduction of
d and mnd into the brine; (3) protection of the bore-hole by



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 37 of 76)