Charles George Warnford Lock.

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

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draught of which draws off the vapours from the kiln. . In the oooise
of the pipe is placed a continuous condenser similar to that in use at
oil- works, and this succeeds in condensing some at least of the offen-
sive mattei-s ; the condensed oils in this case have a marketable valne.

Where ordinary close kilns, snch as the Newcastle kiln, are used,
the smoke and vapours may be conducted to a chimney which
will discharge them at such an elevation as to prevent their beooming
a nuisance.

Following is a description of a modem Califomian lime-kiln which
is supplied with limestone by trucks worked by a gravity pnUey from
the quarries above. The kiln is an upright circular furnace aboot
60 ft. high, tapering from a circumference of about 100 ft. at the base,
to about 40 at the top ; it is surmounted by a smokestack 60 ft. high.
This kiln is connected with an outer wall of ordinary brick, and ao
inner one of firebrick, the space intervening being filled with concrete,
altogether forming a wall about 6 ft. thick.

The trucks from the quarry are lowered to the charging platform
which leads to the door of the furnace. About 20 ft. beneath thd
charging platform, the kiln is surrounded by a firing door. On a
level with this floor are 3 fireplaces, placed at equal distances in tiw
main body of the kiln. The ashpits beneath the grates of the fire-
places extend directly down through the wall of the kiln, leading
separately and directly to the ground or drawing floor, about 20 fi
below. The space in the kiln l^tween the level of the fireplaoes and
the ground floor constitutes a cooling chamber, the lime being drawn
from 3 openings at the bottom of the kiln. This kiln is charged bv
filling the cooling chamber wijth waste rock up to the level of tl^
fireplaces, above which 18 truckloads, equal to about 30 tons of lime-
stone, are dumped. The fires are then lighted, being fed with 4 ft
sticks of redwood.

After burning 3 days, a charge equal to 24 barrels of waste rock
is drawn from the draw-holes at the bottom of the kiln, just as thougb
it were burnt lime, and fresh limestone is added at the charging door
to keep the charge at the proper height in the kiln. After 6 draw-
ings, which now take place in 24 hours, the waste rock is all draws
out, and the lime begins to make its appearance. The lime is sorted
and shovelled into raw-hide baskets, and hauled to the depot where it
is shipped in bulk. This kiln consumes 4J-6 cords of wood in 24
hours, producing 160 barrels of lime in that time.



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NON-METALLIFEROUS MINERALS. 265



MICA.

The minerals known as mica are oomplex silicates of alumina, with
other bases (iron, soda, potash, magnesia, and lithia). They occur in
fimall scales, as a constituent of common granite, or in distinct crystals
fiometimes 3 or 4 ft. diam. The mica minerals are : —

(1) Muscovite, or potash mica, the most common variety, is known
u "white mica," and furnishes the transparent sheets for stove-doors,
hz. In thin sheets it is colourless; in thick blocks, white, grey,
brown, or wine-coloured. Muscovite is much harder than any of the
other mica minerals.

(2) Biotite, or magnesia-iron mica, is very common in Canada, and
fonishes what is knovm as '* amber mica." Even in thin sheets it is
<flily partially transparent, being highly coloured with iron oxide.
Biotite is often found in small, very dark, even black, scales and
crystals.

Mu80ovite and biotite are the only varieties of commercial impor-
tance. Besides these are phlogopite, or magnesia mica, found mostly
in ciystalline limestone, and lepidolite, or lithia mica.

Mica (muscovite) in crystals large enough to make merchantable
itieets occurs in veins or dikes of very coarse granite, usually in
gnnitio country-rock, gneiss, mica-schist, porphyritic granite, &c.
The quartz, mica, and felspar of these veins of granite appear in large
crystals or masses. The mica often occurs in regular strings of
Tystttls parallel and near to the hanging or foot- wall. More fre-
quently, however, the mica crystals are found in irregular bunches
iny where in the vein— especially noticeable where a vein ** bellies **
-or in ofinhoots.

Mica veins often contain such minerals as beryl, tourmaline, garnet,
>)lambite, samarstite, and cassiterite. In the phosphate districts of
•fitem Ontario and Quebec, junber mica is found associated with
thosphate veins, at times occurring on the walls and at times forming
b© whole vein filling.

A mica vein is only a vein of very coarse granite, in which the
eispar, quartz, and mica have crystallised on a large scale. It differs
rum ordinary granite chiefly in this respect, that while in granite
be crystallising forces have, in a measure, interfered with each other,
u a mica vein each has had, so to speak, free play. The crystals of
uca in granite seldom attain a greater size than ^V ^ i ^* across ; a
ingle mica " block " from Mitchell County, Carolina, made two two-
onw wagon loads, and could not have weighed less than 2000 lb.
L single block of " A " mica from the Mart Wiseman mine in Mitchell
'ounty, was 6 ft. long and 3 ft. wide. The crystals of felspar in
Tanite are seldom larger than ^ to ^ in. across. A single felspar
rjstal from the Balsam Gap mica mine, Buncombe County, weighs
*X) lb. Although no large quartz crystals have been obtained from



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266 ECONOMIC MINING.

these mines, large masses of crystallised quartz (generally the darker
coloured sorts) are constantly met with. The €MXX)mpanying small
red garnets are generally spriukled through the quartz, and not
through the mica or felspar.

The mica veins in North Carolina are true fissure veins, differing
in this respect from the mica veins of New Hampshire, which, accord-
ing to Shaler, appear to be obscure beds closely following the general
run of the apparent bedding that characterises the granites in this
part of the country.

Hitchcock ranks the Grafton (New Hampshire) mica veins in tlie
gneissic senes, and says that valuable deposits are found only within
the fibrolite area (mica schist with fibrolite, one of the supposed
divisions of the Montalban Qroup). Thi>4 fibrolite area lies between
the two great areas of porphyritio gneiss, very well developed between
Bumney and Hebron.

Of the influence of the walling on the quantity and quality of the
mica, but little is known. Some of the more experienced miners in
Mitchell County say that both the quantity and the quality of ^
mica depend upon the character of the walling and of the vein ; bnt
so many accessory circumstances influence the quality of the mica, such,
for instance, as the width of the vein, the presence of flat and curved
mica, of crystallised felspar, &c., that the time has not yet oome for
expressing an opinion. These circumstances may depend more or less
upon the character of the walling ; but if so, it is not known just what
the connection is. The same may be said as to the influence of width, |
depth, dip, strike, and accompanying minerals.

Below the zone of atmospheric influences, rarely extending below
20 ft., and sometimes not below 10 ft., the vein becomes more solid,
and the quality of the mica improves. The width of the veins varies
much, from 3 ft. to 40 ft., sometimes in the same mine varying from
3 ft. to 20 ft. Nipping of the vein is a common occurrence, occasion-
ally to almost entire obliteration. The "stringers'* that make off
from the main vein penetrate into the wall rock at various angles, tnd
though narrow sometimes yield fine mica.

The rough mica is hoisted from the mine in blocks of considerable
size, weighing 50 to 250 lb., tabular in shape, and more or lees con-
taminated with fragments of felspar, quartz, waste mica, &c. It is
the purpose of the dressing to free the blocks fro ji all materials not
made use of in preparing cut mica. This is all done by hand, and
consists in cleaving a block with thin steel wedges along the planes of
lamination, separating it into a number of tabular pieces about \ in.
thick, and as large as the stock will allow. These pieces are thea
further cleaved until the proper thickness for cut mica is attained, thil
being, according to the use it is to be put to, | to yV w^-* ^^ even thiih
ner. The workman doing this also frees the sheets from adhering
quartz, fragments of mica, &c., and passes them to the ** scriber."

Scribing is an operation demanding a considerable degree of skill
and experience. Upon it depends the yield of cut from block mica.
It is performed by laying upon the sheet the pattern by which it is t»
be cut, and marking or scribing around it with a knife or simiii
instrument. The patterns are pieces of tin, sheet iron, &c., with '



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NON METALLIFEROUS MINERALS. 267

shape and size determined by the order from the mica brokers or
dealers in large cities, or by the stove-maker himself. In Mitchell
Ck>unty, Carolina, alone are about 100 different patterns, and their
shape and size are constantly var3ring according to the fashion for stove
wixidows. The size of cut mica was formerly of much greater con-
sequence than at present Several years ago there was a regular and
systematic increase in value with the increase in size, the quality of
course remaining the same. This is true to some extent now, though
there appears to be a decided tendency towards smaller patterns. The
first noticeable change in that respect was perhaps in 1883-84, when
the stove manufacturers were compelled by the scarcity of large mica
to U86 smaller sheets. They found the change so advantageous to
their pockets that they persevered in it. Not that small mica is as
valuable as large mica, but large sheets are not as valuable as they
were ten years ago. There is a limit below which it is not safe to go,
probably 3 by 6 in. The patterns range in size from 1 by 1 in. up to
8 by 10 in., or as large as the stock will permit, increasing \ in. each
time. As the value of the mica increases at the same time, it becomes
necessary to cut from a given rough sheet the largest number of patterns
of the highest market value. The price of mica depends not only upon
the size, but also upon its freedom from specks, stains, cloudiness, and
striations^ these governing its quality. Of late, too, a certain '* amber ''
or rum-coloured mica has become fashionable, and fancy prices are
■ometimes paid for a good lot of extra *' rum " mica. The regular
etiourless or ** white " mica, however, commands the bulk of the trade.
Certain mines are famous for ** rum " mica.

As, after the scribing, the sheets are cut with heavy shears along
the lines marked down, it will at once appear that much skill and
experience are required of a good scriber. He must be constantly on
the alert to fumiuh from every piece the largest number of valuable
eut sheets. With the diversity in patterns and prices, and the vari-
ation in the mica itself, this becomes no easy task. A good scriber
always commands good wages, for upon his skill depends the yield of
cut from block mica. No matter how much block mica is brought to
bank, nor how good the quality of it, if the sheets be not properly
scribed the yield of cut mica diminishes, and with it the profit. A
really skilful scriber will get from a given block twice as much cut
mica as a beginner. He sees at a glance just what patterns a certain
»heet should yield, he instantly detects flaws, stains, &c., and with a
few rapid movements of his marking implement he '* scribes " the
sheet and passes it to the " cutter," who merely cuts the sheet through
along the lines marked. The dififerent sizes are then cleaned of the
fine filaments of mica with a stiff brush, wrapped in strong paper,
generally in 1 lb. packages, boxed and shipped.

Cut mica is the only product of a mica mine that is sold on a com-
mercial scale. It determines the value of the mine. So much depends
on the quality of the blocks and of the rough sheets, whether they are
stained, or cloudy, or flawy, or striated ; so much depends on the skill
of the scriber, and other local conditions, that what is here said is to
be taken as applicable to average conditions.

On the average, therefore, 100 lb. of block mica should yield 10 to



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268 ECONOMIC MINING.

12 lb. of cut mica. Instances are not unknown where the yield has
fallen to 5 per cent. ; it has risen at some mines to 33 per cent., and
once to 75 per cent. With the general average of block mica a 12 per
cent, ^ield in cut mica is considered a fair return. These 12 lb. will
Tary in value according to the quality and size of the patterns, tke
highest price being 16<. a lb., and the average price not far from 7i.

A 12 per cent, yield with these figures will give an average value
of 4Z. per 100 lb, of block mica.

Mica is now being extensively and very cheaply mined in India,
and even a 35 per cent, (id-^nilorem duty has not sufficed to prevent
Indian mica from successfuUv competing with the indigenous article
in the United States, especially in the best qualities of cut mica.

The Canadian deposits of amber mica are so large and cheaply
worked that they practically supply all demands which do not require
transparency. The production varies from 40,000 lb. upward per
annum. It is well adapted for electrical applications, in which flexi-
bility and perfect cleavage are essential, while colour is of no moment

The United States afford some 60,000 lb. annually, but import even
more.

In recent years the preparation of ground mica has become an
industry of itself. Waste or scrap mica is generally used. The
difficulties of grinding are great, owing to the tough and scaly natore
of the materisd. Mills which work weU on almost everything else fail
utterly on mica. Becently there has been a return to old-fashioned
burr-stones, though most of the manufacturers keep their process a
secret. The grinding is usually wet.

Ground mica is now largely used for purposes of decoration, as in
the manufacture of wall-paper, where the coarsest grades are used to
give a frosted and spangled efifect, and the finest grades to form a
metallic white surface. It is also used in making a lustrous hair
powder, &c. Medium sizes of ground mica are used in the manu&ctue
of lubricants for journals and axle-bearings. Some manufacturers
grind mica to a very fine powder for '* specialties," but the sijses of
ground mica usually made are 24, 40, 60, 70, 80, 100, 140, 160, and 200
meshes to the inch, and the prioes range from 2^. to bd, per lb.
Scrap mica for grinding is bought for about 50<. per ton at the mine.
It must be free from rust or specks, which would affect the colour and
lustre of the product.



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NON METALLIFEROUS MINERALS, 269



PEARLS.

Hant molltiscs line tlie interior of their shells wltli a coating formed
of flltemate layers of animal membrane and carbonate of lime ; this,
in some species, assumes a nacreous or pearly lustre, and forms the
rabstanoe known as "mother-of-pearl." A superabundance of this
eecretiosi is often produced in drops or tuberosities, adhering to the
interior of the shell, or lodged in the fleshy part of the occupant ; these
fonn the " pearls " of commerce. The formation of mother-of-pearl is
evidently a natural and unvarying process with certain species of
moUusc, though little research has been made as to the conditions
which favour or retard it. The production of pearls, on the other
hand, at least in the case of the true pearl mussel, is accounted accidental
(possibly on insnfSoient grounds), and is generally attributed to disease
or injury suffered by the occupant of the shell.

During the summer months, the Arabs prosecute a small pearl-
fishery along the coasts of the Bed Sea. The captured molluscs are
taken ashore and exposed to the sun, when they quickly open ; they
are then examined for pearls, and thrown away. The headquarters of
this fishery is Jedda. The pearl-mussel fishery in the Persian Gulf,
principally on the banks of tne island of Bahrein, is also in the hands
of the Arabs. The best beds are said to be level, and formed of fine
whitish sand, overlying the coral, in dear water. About 4000 to 5000
boats are engaged, and the annual value of the harvest may be placed
at 600,0002. The beds occur at all depths down to 18 fitthoms, and
probably lower ; the chief diving is in 4 to 7 fathoms.

The Ceylon or Tinnevelly fishery is situated on the west coast of
Ceylon, in the Gulf of Manaar, southwards of the island of that name,
and along the opposite coast of the Indian continent, near Tuticorin.
The banks lie in eroups : the first, opposite the village of Arippu,
comprises the so-caUed reria-Par, Peria-Par Earai, Cheval-Par, Eallu«
tidel-Par, and Modaragam-Par ; &cingthe village of Earaitivu, is the
bank of that name ; and off the village of Chilaw, are Earakupanai-
Par and Jekenpedai-Par. These banks are 6 to 8 miles from the shore,
and dj to 8i fathoms below the surface. They consist of masses of
rocky ground, rising from the sandy bottom, and are probably exposed
to ocean cvirrents.

After the pearls are collected, they are classified, sized, and valued.
The classification is as follows : - -(l) Ame^ pearls of perfect sphericity
and lustre ; (2) anoAorUy failing in one of these points ; (3) moBengoBj
faHmg slightly in both points; (4) kaJippo, fauine still more; (5)
torowd^ double; (6) f>e«fa(, misshapen ; (7) oodwoe^he&uty ; (8^ man-
d^Mgoe^hentorioldea; (9) AuroZ, very small and miashapen ; (10) ihool^
** seed." The sizing is effected by passing them through a succession
of brass cullenders, called '* baskets," having the size and shape of large
«auioers. There are 10 to 12 of these. The first is perforated with



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270 ECONOMIC MINING.

20 holes, and the pearls which do not pass through it by shaking are
called " of the 20th basket" The succeeding baskets have 30, 50, 80,
100, 200, 400, 600, 800, 1000, each giving the name corresponding
with its number of holes to the pearls that do not pass through.
After sizing, the pearls are weighed, and their value is then expressed
at a rate " per chow*' which term embraces all the qualities which have
been estimated.

The great Queensland pearl-fishery in Torres Straits is carried on
by boats, with Malay divers, in water of 4 to 6 fathoms. The peail
mussels of Torres Straits have a weight of 3 to 6 lb., and even 10 lb.

Diving for pearl is one of the chief occupations of both sexes of
natives in the islands of the South Pacific. The mollusc here sought
is the mother-of-pearl-yielding mussel, which inhabits the interior
lagoons of the great coral atoUs, It frequents the clean growing coral
where it can attach itself free from sand or drift, and where there is
considerable influx and efflux of tide. It is also to be found in great
numbers under the breakers that beat upon the outer reefs, and
probably at greater depths in the sea beyond.

Pearls of consideraole value are sometimes found in fresh-water
moUusca, so much so that the search for them is quite a domestic
industry in some localities, especially during extra dry seasons. It
would seem that seeking for pearls may be rewarded in any creek or
river where limestone is the country rock, since the unios haye a
tendency to secrete nacreous matter wherever carbonate of lime is
provided.

For further information the reader is referred to Spons' Ency-
clopsddia.



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NON-METALLIFEROUS MINERALS. 271



PEAT.

Peat consists of the cnmulatively resolved fibrous parts of certain
mosses and graminaceaB. It gradually darkens from brown to black
iritli increasing age. Judging from Dr. Angus Smith's residts, it
grows at the rate of about 1 in. a year. A pectinous substance has
been found amongst its constituents. As a fuel, it is most economi-
cally used at the spot where it is grown. It has been, however,
dcfitructively distilled at a low temperature for tar, a branch of
industry now scarcely profitable. The process gives a very porous,
friable charcoal, possessed of great decolorising power; gas rich in
ORrbonic dioxide is also given off. A ton of good peat may yield more
than 5600 cub. ft. of gas. The purified gas contains about 11 per
cent of vaporised hvdrocarbides, 37 of marsh gas, 31 of hydrogen,
tnd 19 of carbonic oxide ; it is thus (as its mode of formation suggests)
less oxygenated than wood gas, but more oxygenated than coal gas.
The liquor is rich in hydric acetate, which amounts to about * 2 per
cent on the x>eat ; ammonic sulphate, taken similarly, exceeds 1 per
cent Qood peat yields about 3 to 6 per cent, of tar proper, which is
oomrokFatively easy to purify by the usual method.

The method of cutting peat in the Highlands of Scotland is very
difTerent from that adopted for cutting peat from bogs. In the first
place trenches are opened at distances of about 10 yd. apart ; and,
scoording to the nature of the ground, these trenches are made 50 to
500 yd. long. After removing the surface sod at the places where
the trenches are to be cut, for a width of 3 ft. along the whole line of
the trench, the peat-cutter digs out the peat with a peculiar-shaped
tool, in slices of about 1 ft. square and 3 or 4 in. thick. As fast as
these slices are cut, another man takes them off the peat iron and
throws them on the surface, so as to spread them out as much as
possible. In this waj prisms of peat, measuring 3 ft in width and
depth, are cut out at intervals of 10 yd., and the number of slices cut
in each trench are just as many as a man can throw on both sides of
the trench without shifting his position except from one end of the
trench to the other as the cutting advances.

In succeeding years the peat is cut from the two banks thus
formed in each trench, to a width of only 18 in. and a deptii of 3 ft.
The advantage of this system of cutting is that there is no necessity
for removing the peat by barrows to the spreading-ground, a pro-
ceeding which is attended with considerable expense for labour.
Whm the peat is cut in this way from a bank 150 yd. long, it will
give 75 cub. yd. of wet peat, and the number of slices into which this
is divided will be about 8000. Then, as the banks are 10 yd. apart,
there are 5 yd. width of drying ground to each bank, or a superficial
area of 6750 sq. ft to each bank of 150 yd. long. Cutting it in this
^ay every year, it would take 10 years to remove the whole of the
peat to a depth of 3 ft As the banks are cut away in successive
years the area of spreading-ground on the surface is reduced, and some

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272 ECONOMIC MINING.

of the peat has to be spread at the bottom of the trench, the area of
which increases as that of the banks' surface is reduced by the
cutting.

The peat cut to a width of 18 in. and a depth of 3 ft. from a bank
of 150 yd. lonK9 is what is called an iron's work, and the 75 yd. of
peat 80 cut yields about 10 tons of dry peat, so that to cut 7000 or
8000 tons of dry peat would require 750 irons' work, or banks about
64 miles in lengtn, and extending OTor an area of about \ sq. mile.
This area of ground would supply 7000-8000 tons every year for
10 years.

The cutting and spreading of peat in this way forms but a pro-
portion of the cost of the dry peat. A far more considerable portion
of its cost results from the labour of collecting the dry peat and
bringing it to the place where it is to be used. Herein lies one of the
greatest difficulties of employing peat on any very extensive soak
Whatever mode may be adopted for collecting the dried peat to one
spot for use, the cost of carriage will increase in proportion to tlie
increase in tiie quantity of peat consumed at that spot.

Another prominent difficulty attending the use of peat oonsLsts in
obtaining it in a dry state, fit for use as fuel or otherwise. Mountain
peat, as it occurs naturally, contains as much as 80 per cent, of water,
even when it has been well drained, and bog peat often contains veir
much more. Consequently, to obtain 1 ton of dry peat, 5 tons of
material have to be dug and spread, and 4 tons of water have to be
got rid of by evaporation. When mountain peat is cut in slices and
spread out on the ground during dry weather, the drying eoes on
rapidly, the surface of the pieces acquires a kind of skin, which is not
wettea again by rain, and the peat, in the course of a week, is snffi-
ciently hardened to be handled ; the pieces are then set up on edge,
so that the air may play on both sides, and in the course of 6 to 8



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 30 of 76)