Charles George Warnford Lock.

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

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use. Plasters which hit off the medium are alone suitable ioit the
work of the potter. The finer varieties of plaster prepared especially
for use in potteries are obtained by a treatment which differs in many
respects from that described above for the commoner kinds. In thi
first place, the direct contact of fuel or even flame is avoided, sinoft
this reduces some of the sulphate to sulphide of calcium, the preseDOi
of which is in many respects objectionable. Secondly, it is necessary
that there should be a better control over the temperature, since if tbo
plaster be not partially dead burnt, it will set too quickly for titf
particular purpose to which it is to be put.

The arrangement employed in France is known as the f<m \
houlanger, or bakers' furnace. The temperature attained in the fnmaci
itself never exceeds low redness. The material preferred is the softeJ
kind of the granular variety of gypsum. This is put in in piece!
about 2J in. thick. After the bakmg, several lumps are broken d
and examined to see that there are no shining crystalline partidei
which would indicate that some of the gypsum had remained imj
changed. Before use, the plaster is ground very fine. This point ij
of considerable practical importanoe. The consistency attained ^ovUi
be such that the material may be rubbed between the finger as^
thumb without any feeling of grittiness. Should there be particles ci
a size to be characterised as ** grit," these will after use appear at tin
surfiice of the mould, with the result that the mould will have to b<
abandoned long before it is really worn out, i. e. before the detail
have lost their sharpness.

The quantity of gypsum mined annually in' the United Kingdffli
is 100,000 to 150,000 tons, worth 7«. to 9«. a ton. In 1889 the Unitai
States produced over 250,000 tons, about half being raised in Michigan
and one-third of the whole product being calcined to make plasty
Analysis of the Michigan rock used as manure gave 78f per cent linx
sulphate, over 19 water, and less than ^ each of magnesia and alumim

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Tkber the names of diatomite, foesil meal,kieselgnhr, <fec., are in-
cluded a number of infusorial earths, in the form of white, grey, or
greenish powder of very low specific gravity, consisting chiefly of the
minnte silicioas shields of diatomaoeaa. Large deposits of fossil dia-
toms have been traced in many parts of the world, and several mines
prodndng kieselgnhr are worked in Europe, but the largest, and those
fielding kieselgnhr of the purest quality and lowest specific gravity,
lie situated near Naterleuss station, on the railway &om Hamburg to
HanoTer. The kieselgnhr is found there from the surface down to a
iepth of about 150 ft, being covered only by thin beds of diluvial and
lUnvial origin. The upper stratum of this large deposit supplies the
vhite kieselgnhr. This quality contains very little organic matter,
tmt some sand ; therefore, after being washed, it gives a very pure
tnd porous product. The second stratum produces grey kieselgnhr,
Kmtaining very little sand, but sufficient organic matter for calcining
t, and the product then is kieselgnhr of the finest quality. If the
sxertionB of colour manufacturers to make a colour which will really
)e imperishable under the influence of strong acids are to be crowned
nth success, this perhaps is the material which might lead to satis-
ictory results.

The lowest and by far the largest stratum, varying from 50 to 100
L in thickness, supplies the green kieselguhr, which contains up to
per cent, of organic matter, showing clear imprints and fragments
f fishes, well-preserved fir-cones, leaves, bark, and twigs of birch, fir,
re In order to utilise the immense deposits of green kieselguhr,
ihis have been erected for burning or calcining it. When dry, this
laterial glows like turf or peat, and this is utilised in the calcining
PX5e«8. The kilns, simple round furnaces, about 15 ft. high by 6 ft.
iam., are filled and lighted at the bottom, no additional fuel being
squired to keep them going. They are continually replenished with
rwn kieselguhr at the top, and the calcined is taken out from the
rates underneath. The product is perfectly free from moisture and
^ganic matter, and has therefore a much higher market value than
reen kieselguhr. Its reddish colour is due to some traces of oxide of

Kieselguhr has many valuable properties. It consists almost ex-
Qsiyely of silica, and is therefore in its pure state as fireproof as any
aterial in the world. It resists the action of the strongest acids,
it it can be easily made to melt after being mixed with an alkali,
▼en by boiling xmder pressure, combination may be effected. Sili-
ite of sodium or water glass has been made of it in this way, but
lute sand is now generally used for this article, because kieselguhr
w risen considerably in price, and owing to its low specific gravity,

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very large meltiDg pots had to be employed. Its great porosity,
although a drawback for this particular industry, makes it yery
yaluable for the manufacture of dynamite.

It is used to a considerable extent in the manufacture of Tsrioiu
cleansing preparations, either in the form of powder or so-called soap.
There is but a step between the crude minend and the mercbantablfi
articles used for cleansing purposes. To manufacture a polishing
powder it is necessary only to clean and grind the crude mineral, tho
particles of which are loosely adherent, while in making soap tin
pulverised mineral is mixed with the other ingredients of soap naaDO-
facture. The greater portion of the American product is dried is
furnaces at the pits, and used for making protective coating foi
boilers. As an absorbent in the manufacture of dynamite from nitro-
glycerine, the American product does not possess sufficient absorbent
properties ; and even the German product has been largely supplanted
by wood pulp, which answers the purpose excellently and is mndi

The American output in 1889 was over 3000 tons, nearly all from
Dankirk, Maryland, and valued at about 28«. a ton. The yield feU
to 1700 tons in 1893. Various assays gave the following results :—




Abi n^JT^ft

Iron protoxide


Ferric oxide

Magnesia, soda, potash, 8alphnr,i

and organic matter. j

Loss on ignition ..'

Water at red heat


From Pope's





FVoni Morris

New Jersey.





From Dear












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Apart from its wide distribution in the organic kingdoms, iodine is
of common occnrrenoo in the mineral kingdom, notably as sodinm
iodide in many kinds of rock salt, as sodinm iodate in the mother-
liqoor from nitrate of soda works, as calcinm iodide in the oqean, and
in combination with potassium, sodium, magnesium, and calcium in
oumj springs.

Li ^e ccliehe or raw sodinm nitrate (Chili saltpetre) deposits of
South America, iodine is encountered in the form of sodium iodate, in
qtttntities varying from mere traces to 50 per cent., and its recovery
b conducted on an industrial scale, as described by R. Harvey.*

The aqwi viejd^ or mother-liquor, of these works contains about —


Sodimn nitrate, NaNO, 28

„ chloride, NaCl 11

M sulphate, Na,804 3

Ma^eeinm sulphate, MgS04 3

Sodium iodate, NalO, 22

Water, H,0 83

The mother-liquor is conducted through the pipe for mother- water
lo the precipitators, which are constructed of 2-in. tongued and
;iooved timber, lined with sheet lead, to prevent leakage by warping
md shrinking ; they are stayed transversely by f-in. bolts. The re-
igent for precipitating the iodine is the run-off from the tanks for
cid deposits in sufficient quantity to precipitate the iodine held in
olntion, which is determined by measuring previous to precipitation.
file wings, or fans, which are also of wood, are then turned by hand
mtil the liquor becomes thoroughly mixed with the acid.

This causes most of the iodine to fall to the bottom of the precipi-
^rs in slimes and flakes, and some to rise to the surface as a black
roth. The iodine on the surface is skimmed off by large wooden
poons and placed in clarifying tanks, and the mother-liquor is then
mwn off to the tank for mother-water after precipitation. Thence
' is returned to the sodium nitrate department, where it is again
nd, again becomes impregnated with iodine, and again goes through
eimilar procesa

The deposit of iodine left in the bottom of the precipitators is
iken out and placed in the clarifying tanks, where it undergoes a
tries of washings with pure water. It is then filtered and partially
tied in a filter press, whence it is taken and pressed in the forming
[ess, and is removed from the movable bottom of the press in blocks
' cheese form, 8 in. diam. by 6 in. thick. 4'he blocks are next placed
I a cast-iron retort, to which are attached 8 earthenware receivers,
ich 3 ft. long by 2 ft. 6 in. diam. The last or end receiver is -stopped

♦ Min. Proc. Inst. C.E.. paper No. 1850.


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by a wooden end and clay joint. The joints of the receivers are also
made of clay. When the retort is charged, the crude iodine is sub-
limed by a slow fire. After sublimation, the retort is allowed to cool,
the joints of the receivers are broken, the receivers are taken down
and emptied, and the contents are placed in taired kegs for exporta-

The crude iodine, previous to sublimation, contains : iodine, 80 to
85 per cent. ; non-volatile matter, 6 to 10 per cent. ; the remaindei
being water. The reagent for the precipitation of iodine is the acid
sodium sulphite, NaHSOg, formed by saturating the aqueous solution
of **salnatron" (impure sodium carbonate, NajCOa) with fumes d
burning sulphur. " Salnatri>n " is formed by burning coal-dust witi
sodium nitrate, thus : 2NaN03 -f C = NajCOa + NjOj. Its imporitie
consist of sodium chloride and sulphate, earthy matters, and unbum
coal ; the latter are eliminated by dissolving the salnatron in watei
and settling.

The fumes from the burning sulphur are generated in a firebrid
oven, and are drawn by an ejector from the oven to the drainer, whid
catches the particles of partly burned sulphur, and from the draine
to the cylindrical fume receivers, which are charged with " salnatron^
solution, and are traversed with perforated pipes for the passage c
the fumes.

The steam for the ejector is taken from a small horizontal boili
at the extremity of the building. The building is well ventilate!
and is made of wood and corrugated iron. The apparatus employe
cost 23,000 dollars Chilian currency. During the months of Octobi
and November 1881, there were exported from Iquique 7560 lb. \
sublimed iodine, manufactured by this plant.

A method proposed by Thiercelin for use in Chili and Peru is i
follows: — The mother-liquors resulting from the manufacture ^
sodium nitrate a^ treated with a mixture of sulphurous acid :
soda sulphite, in due proportions, and the iodine is precipitated \
black powder. The precipitated iodine is put into earthen jars, «
the bottom of which are layers of quartz sand, fine at top and ooaii
below ; from this it is removed by earthen spoons into boxee lixK
with gypsum, and a great part of the water is thus removed. It
sometimes sold in this impure state, or is further purified by snblifl

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This mineral is nothing more nor less than a species of pitch coal
fmrnd in detached masses, grained like wood, splitting horizontally,
light, and moderately hard. It is often confounded with " cannel "
coal, but it is quite distinct. Cannel coal is much harder than jet,
h&8 DO grain, and splits in any direction. Jet is not easily fused, and
requires a moderately strong heat, burning with a fine, greenish-
white flame, and emitting a bituminous smell.

In England it is found in greatest quantities in the neighbour-
Wod of Whitby, in Yorkshire. There it is mixed with bitumenised
rood and coniferous trees in the Upper Lias or alum shale of
he district. In Prussia it occurs in association with amber, and is
itmed by the amber-diggers "black amber." In France large
pantities are found in the department of the Aude, where a great
iTunber of artisans find steady employment in fashioning it into
nariee, religious beads, and ornamental trinkets when fashion
bnands them. In Spain, jet is found at Yillaviciosa, in the province
f Astnrias, and is manufactured principally at Oviedo.

Jet is of two distinct species, bard and soft ; the latter is of very
linor importance.

Hard jet is found in strata known as jet rock which occur in the
ias formation, some 90 ft. above the main band of Cleveland iron-
tone; it is discovered in compressed masses in layers of very
iferent sizes, being generally ^-2^ in. thick, 4-30 in. wide and 4
r 5 fL long. It invariably tapers away, running, as the miners say,
) a "feather edge."

These jet layers are always protected b^ a skin, the colour making
tiother division ; for that found in the cliflGs by the sea has always a
loe skin, while that discovered in the inland hills has a yellow
ttting. The jet found in the same mine varies very much in quality;
B worst specimens, those which are quite brown and will not take a
)lish, are termed " dazed " jet.

Soft jet is confined to the Lower Oolite — ^in the sandstone and
»le— some 480 ft. higher than the hard jet, and is undoubtedly of
irely ligneous origin, the fibre and the branches of trees being more
1^8 distinctly marked.

The most valuable finds of jet have been washed down by the
a's action, where the jet rock crops out in the cliffis, and on the
iffs, where the seams are exposed. Nearly all the jet now obtained

found inland, and cliff jet is worked with the same mining
erations as that lying under the inland hills.

The process is very simple. A mine is commenced by drifting
to the face a passage of 7 fU by 5 ft. A tramway is then laid
wn, and the shale is tilted from the mouth of the mine ; the drift is
ntinued for about 120 ft. at the rate of 2-4 ft. a day ; then cross




drifts are started in a yariety of directions. As soon as the rock
becomes too hard, the miners retire, palling in the roofe as they
recede, for the bulk of the jet is found generally in the falling top

Rough hard jet varies in value from 4«. to 21<. per lb., according
to its closeness of texture, direction of grain, freedom from flaws, and .
breadth for working. Soft jet varies from 5«. 6(1. to 30«. per stone. ;
The price of Spanish is about the same as that of English soft jet {
The Whitby hard jet is the best, not only for working, but it wOl
take a fine polish, which it will retain for years, and it can 1»
worked up into finer designs on account of its greater tenacity and

Great Britain produced 618 lb. of jet, value 124Z., in 1889 ; and
1228 lb., value 245/., in 1890. Spain turned out 55 tons of jet in
1890. f



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LofE is one of the oommoneBt and most widely distributed minerals,
n at the same time one of the most nsefol. As chalk, as limestone,
ud as dolomite or magnesian limestone, it ocours in enormous beds,
belonging notably to the Cretaceous, the Carboniferous, and the
Oolitic systems, and easily worked by open quarrying. The com-
position of some representative limestones is given below.


OoUtle. 1 Magnealan.

Lime earbonate .. .. .. ..

Ka^Deda earbonate


)rgiiiio matter


86i to 974

i ,. 3|

„ 2i



26i to 45}
18 „ 44
n 51
„ IJ

« 2f

The method of working deposits of limestone is influenced chiefly
17 the relative thickness of the limestone and the overburden, or
'tirring," and partly also by the character of ilie roof formed by the
iTerbnrden. In some of the underground workings in Scotland * the
nt-tum of stone per man per day at the working face is 4 tons, which
ronld hardly be exceeded in quarrpng. At tne open works in N.
Vales, after the overburden has been removed, the men are paid at
be rate of TcL-Sd. a ton for getting and loading the rock, finding their
wn powder, but not tools or sharpening.

For agricultural purposes these minerals undergo no preparation ;
at for industrial application in the form of quicklime for mortar-
aking, &a, the chalk and limestone which contain no or very little
lagnesia, are burned in kilns to a caustic condition. To make 1 ton
' quicklime requires about If ton of limestone, and consumes 4-8 owt.

Lime is burned, similarly to bricks, in either open or dosed kilns,
id either coal or coke is generally in use as the fuel. The feeding

of material and fuel may be intermittent, each charge being burned
id drawn before a new charge is introduced ; or the feeding and
-awing may be continuous, in which case the material and fuel are
<J in at the top, as space is afforded by tihe drawing of burned lime

the bottom of the kiln. The fortner method of burning is most in
le with closed kilns, and the latter where the kilns are open.

The ordinary form of an open lime-kiln is that of a deep pit,
krru%ring towards the bottom, and lined with refractory stone or

* J. Horition, ''Lime«tune Mining in Scotland," Trans. Fed. Inat. Min. Enge.,

Digitized by



firebrick ; and at the bottom on one side is a shoot or opening thioog
which the burned lime is discharged. The exterior is built aqua;
with stone, and where a pair of such pits are placed side by side t]
discharge is effected into an arched passage constructed betwerai the
below. Such a kiln is worked on the continuous system, alteroft
layers of limestone or chalk and fuel being charged in. Another kii
of open kiln used for burning Lias limestone is a shallow pit ; after tl
pit has been filled, more limestone and fuel are laid on the top so
to form a conical heap, which is finally covered with a layer of eaii
patted down upon its surface. This VjIti is worked on the inte
mittent system.

The closed kilus in common use are similar to some of the doa
kilns uned for brick-burning. One of these forms, favoared in tJ
glass-making districts of the north of England, is the *' Newcast
kiln." It consists of an arched chamber, having comnmnicatioi
behind at the lower part with a flue, and having an opening or doa
way in front, which during burning is bricked up. The front pa
is marked off from the rest of the floor by a low brick ridge, behir
which the limestone is. built up, yet so as to leave near the fioi
passages about 18 in. high from the ridge to within about 8 ft. of tli
back wall of the kiln. The fire is sometimes placed on the floor i
front of the ridge, and small air openings are left in the bricked-u
doorway; but in other cases fires are kindled over fire-bars wit
ashpits beneath : in either case there are openings provided for th
feeding in of fuel. The smoke passes into the flue, and is conduct^
by it from all the kilns in a series to a chimney. This kiln is worke
intermittently. Another form of close kiln is essentially an ordinar
open kiln covered or domed in at the top, having an opening near tq
top which conducts smoke, &c., into a flue. At alkali works, wher
coke is used as fuel, the carbonic acid generated from the combustiof
is carried into the works by a pipe, and used in the manufacture i
carbonate of soda.

At the Harper Works of the Buxton Lime Co., a Hofmann kiln i
used. It is of similar construction to that in use for brick-buminj
except that there are openings to the flues both on the outer to
inner side of each compartment. It has 28 chambers, which ai
worked, in sets of 14, round and round the kiln. The time oocupie
in working round the kiln in this way (and therefore the tin
between charging and discharging each compartment) is 14->2
days, equally good lime being made in any case, whether the kiln \
worked quickly or slowly.

The nuisance proceeding from lime-burning is chiefly that of th
smoke which issues from the kiln at a low level. The ooal bums t
a smouldering way, and gives rise to more offensive products th*
when it burns freely. That which comes off from a row of larg
kilns is sometimes very abundant, and clouds the air for a very loni
distance as it floats away near the ground. No smoke and very littJ
obvious vapour, beside watery vapour, arise from kilns in whidi cok
is used, unless the coke has a little small coal mixed with it, or wbti
some coal is used towards evening to keep the kiln going during tb
night. If cinders are used, with which organic (chiefly vegetable
e is mixed, the eflBuvia are very offensive, the odour being simik

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to that arisiiig from the clamp burning of bricks. The burning of
Oarboniferons limestone gives rise to a particularly ofifensive fume,
Sue to the evolution of similar products to those which are obtained
bj the distillation of oil-shales. The gases given off from a burning
luDe-kiln are poisonous, and may injure the health of persons
redding near, if they chance to enter the house in any considerable

Since the greater part of the nuisance of lime-burning is due to
the products of the imperfect or slow combustion of the fuel used, one
obviouB mode of lessening the nuisance is so to burn as to use as small
\ quantity of the fuel as is practicable for the attainment of a good
nralt, or to use such fuel as does not emit offensive smoke. At most
of the large lime works in Derbyshire, the
Ibel used is the commonest and cheapest
ooal obtainable. Some is very largely
mixed with shaly matter (" bass '*), and
the quantity therefore thrown into the
kiln with the stone is enormous. The
bad quality of the coal necessitates this
extravagant use of it, and the result of
the burning is commensurably bad. A
good deal of the stone introduced is dis-
diarged imperfectly burned, and mixed
with the lime drawn are clinkers and
stony matters, and the lime has to be
picked over by hand. Thus two pro-
ducts are obtained, namely, lime and re-
Ibse matter, both of which have to be
carted away separately. Such a mode of
burning is not only a source of nuisance,
but is obviously wasteful of fuel, lime,
and labour.

Precisely the same kind of stone
which is burned at the Grin works at
Bnrbage, is burned in the Hofmann kiln
at Harper, with the evolution of scarcely
any perceptible smoke, and with the use
of no more than 3 or 4 cwt. of coal to
each ton of lime made, while the propor-
tion of refose picked from the lime is
most insignificant. Good Bakewell slack
is used at the Hofmann kiln.

A form of kiln invented by Spencer,
and which is in use at his works at
Lnthersdale, does good work with but
little evolution of smoke, and with great pjQ
economy of coal. Fig. 88 shows the prin-
ciple on which the kiln is constructed. The kiln is made in two cham-
berea 6, one above the other, with a sufi&ciently wide communication c
between them. The limestone is charged in at the top of the upper
chamber, and good slack is introduced at small openings or channels
e, nmnd the top of the lower chamber, access to these apertures being

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—Improved Lime Kiln.


had by passages /oonstructed in this situation and between adjoining
kilns. The feeding of limestone is continuous, and the lime is drawn
as usual below at a. In this way the combustion of the fuel is caused
to take place where it will produce the maximum effect, and the waste
heat warms up the stone in the upper chamber before it falls into the
lower chamber, where it is burned. Such a kiln is best constructed
on the side of a hill for convenience of supplying the limestone at the
top and drawing the lime at the bottom.

At the Loan Head works, one of the kilns, which is partiallj
closed at the top, has an arrangement for collecting and condensing
the oils, &c., which came off during the burning of the carboniferota
limestone. It consists of a pipe or flue leading to a chimney, the

Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 29 of 76)