Charles George Warnford Lock.

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

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♦his character. In other words, the sinking of a winze a short



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12 ECONOMIC MINING,

di>tance, or the raising of an upraise for a short distance, will
oftentimes conclusively establish the absence of pay ore bodies
within the region to be explored, without the neceshity of running
frequent drifts through this barren stretch of country.

Of course, no arbitrary lines can be laid down as to the best system
of prospecting, owing to the great differences that prevail in the
occurrence of the ore bodies in various mines ; but there should be
a system. Where the vein is flat and small, and subject to :many
pinches and changes of strike and dip, it sometimes becqmes necessary,
in case the vein is lost, to defer the extension of the drifts until the
stopes have advanced far enough to indicate the direction in which
the extei sion of the vein may be looked for.

Surveys. — Too much care cannot be exercised in laying out first
>]ans with accuracy and precision, and this can only be accomplished
y the aid of levels, dials, and other engineering instruments, which
can always be advantageously bought of W. F. Stanley, Great Turn-
stile, Holbom. Some of his mining specialties are described in his
excellent little work on sui*veying instruments.*

The usual operations of mineral surveying and many hints and
examples are given in the author's * Miners' Pocket Book,' pp. 239-
251. Space can only be found here for a description of an ingenious
way of transferring surface alignments to underground workings
through vertical shafts.^ It is used a great deal in Montana, to
depths of 2000 ft. and more ; the operation monopolises one compart-
ment of the shaft, but the cage may be run meanwhile in the other if
at reduced speed.

The method is simply to hang two plumb-lines in one compart-
ment in line with a determined surface alignment — this line, gene-
rally, being the centre line of the compartment — then range the
instrument in line with the two plumb-lines, at the different levek
where surveys are wanted.

By reference to Fig. 2 the points of the method can be understood.
A is a horizontal cross section of the shaft, at the collar, showing the
plumbing board in place, across the shaft, and the two plumb-Hn^
3 ft. apart, centred in the alignment N. 20° 13' E. The plumbing
board C, D, £ is a 2 in. x 10 in. plank, 8 ft. long, provided with
two movable supports for the wires. The support is a round iron rod
^ in. diam. and 6 in. long, resting in two iron upright pieces D, d.
The rod has a groove across one end of it for the wire to rest in, and
the other end is seated against a set-screw and held against it by a
small coil spring around the rod. The plumbing board is placed
approximately in line and nailed firm ; then centre each of the wires
in line with the instrument, using the screws for setting the wires in
line. The plumb-line should be a No. 22 copper wire; this will
stand a 10-lb. bob, which is sufficiently heavy.

When the wires are centred in the supports, and they are ready
to be let down, a small weight (1 lb.) is attached to the end of the
wire and let down to the lowest level where alignment is wanted, and
there made fast to one comer of the shaft, and pulled taut in the same
corner as at the surface, so there will be no possibility of the other

♦ * Surveying and Levelling Instruments.* t L. Kuhn, En. and Min. Jl.

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PROSPECTING. 13

line oomiog in contact with it while being lowered. When both
wires are down, the bubs are put on and each is placed in a pail of
water ; if the shaft is wet, use about 1 in. of common black oil on the
Burfaoe of the water to prevent rippling by the water dropping down
the shaft into the pails.



The same signals used in hoisting can be used here to advantage,
between the person at the bobs and the one at surface; as, for
mstanoe, three light jerks of the wire to raise it, two to lower, and
one to stop. When the lines are still the instrument is ranged in line
^ih the wires. From experiment, Kuhn found 35 to 40 ft. distant
from the wires to be a good point to place the instrument. It will
facilitate the work to light one of the wires, the one farthest from the



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14 ECONOMIC MINING.

instrument, and have the wire nearest the instrument dark ; in this
way one will be able to distinguish the wires, having one light and
one black. The light should be placed as close as possible to the
wire to be lighted, but itself screened from view at the instrument,
so that only the light reflected from the wire is visible at the instru-
ment.

B is a vertical cross section of the shaft and station, showing bobs
in the pails, the two plumb-lines, and the instrument. F is an
enlarged section of an arrangement which Kuhn used to advantage.
Six common candles are placed close to the wire, and hid from view
by the screen S. The base on which the candles rest should be 2 in.
longer than the screen, then by placing this end of the screen about
1 in. to the side of the line of the wires, the base will be a light
surface fur the dark wire. An incandescent lamp is the best light,
but common candles will answer.

"When the instrument is in line, permanent line points are set
within the caps of the station. When these are set and connected
with the wires by measurement, the transfer of the alignment to this
level is completed, and the other levels are proceeded with in the
same manner, the wires not being molested until all are finished. A
check can be made by recentring the wires 1 in. backward or forward^
which will give a parallel line. However, if the wires are exactly
the same distance apart at the bobs as at the surface, it would li
almost impoh^sible for either of them to touch at any point of the shaft.
One of the wires is the station for the mine and all surveys of the
mine, both surface and underground, begin or are connected with this
station O. Let all angle points be stations; that is, make each
station an angle point running consecutively from O. If traverses
are used in mapping, the O of die surveys is the O of the traverses.



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IS



POWER.

A MATTXB of no small importance is the source from which power is
to be derived for working drills and cutters, hauling and hoisting the
mineral, pumping water out of the workings, and driving the reducing
and dressing machinery.

In general terms, it may be said that

Water power is cheaper, but less dependable owing to frosts and
droughts.

Steam power is dearer, but is more reliable.

Besides these, a third must be mentioned, namely the petroleum
engine, whose great advantage over either is that it can be applied in
positions which almost preclude the other forces.

In addition to these prime sources of power, there are two im-
portant secondary or intermediate motors, which give effect to force
derived from some other source. These are compressed air and
electricity.

Waier Power.— The value of a water power* depends upon a
variety of elements ; numerous conditions may reduce its value. The
esBential points to be considered are as follows: — (a) Quantity of
water during a dry year; (6) uniformity of flow during the year,
considering the storage capacity, natural and artificial ; (c) head of
&11 ; {d) conditions which fix the expense of building dam and canal,
and flowage of land ; («) conditions which affect the cost of founda-
tions for buildings; (/) geological conditions which determine the
permanency of the falls; (g) freight charges for fuel, supplies, raw
materials, and finished product ; (h) how much low-pressure steam can
be used for heating purposes ; and whether exhaust steam can be used
for those purposes ; (t) if water is needed for other purposes than
power, and in what quantities ; {Jc) the greater uniformity of speed
with steam than with water power. The value of a variable power
is usually nothing if its variation is great, unless it is to be supple-
mented by a steam plant. It is of value then only when the cost per
horse-power for the double plant is less than the cbst of steam power
nnder the same conditions for a permanent power. The value of a
deyeloped water power is as follows : If the power can be run cheaper
than steam, the value is that of the power, plus the cost of plant, less
depredation. If it cannot be run as cheaply as steam, considering its
cost, Ac, the value of the power itself is nothing, but the value of
the plant is such a sum as could be paid for it new, which would
hring the total cost of running down to the cost of steam power, less
depreciation. That is, it is worth just what can be got out of the
plant and no more.

• C. T. Main, • Value of Water Power/



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i6 ECONOMIC MINING.

Channels. — The sectioD of the channel in which the water is led
to the water motor depends on the ground in which the channel is
oat. If the channel be made in brickwork or masonry, the angle of
its sides would be 90° ; in stone without mortar, 60° ; in clay, 45° ;
in coarse gravel and stones, 40° ; in finer gravel, 35° ; in sand, 30° ;
in ordinary soil, 25°. Bearding the speed of water in channels, it
will be understood that it runs at its highest rate just below the
water surface, decreasing toward both the bottom and the sides. The
average, speed at which water can be run depends largely upon the
nature of the material in which the channel is cut ; soft material will
not admit of the water running over it so rapidly as hard material.
The nature of the water must aJso be considered. Some water brings
mud, and other carries sand. In some instances, the settling of this
sand in the channel is a great hindrance : but if the water runs at a
speed of 9 in. per second when muddy, and twice as fast when carry-
ing sand, no disadvantage by settling will be felt. The maximum
speed of the stream should be in ordinary soil, 3 in. per second ; sand,
1 ft; fine gravel, 2 ft.; coarse gravel, 3 ft.; stony ground, 4 ft;
rock, 5 ft. ; larger rock, 6 ft ; solid rock, 10 ft.

Pipes. — Should it be necessary to .lead water through pipes, as is
generally the case for turbines, such pipes should not be longer than
is absolutely necessary, owing to a Iofs by friction in them. From
end to end they should be equal in diameter. Any difference in the
section will cause friction and la«s of efficiency, as every increaae or
decrease in the section alters the speed of the water, and consequently
causes it to whirl at that particular part of the pipe. Sharp bends
should be avoided ; if bends are necessary, they should be arranged
on an easy curve, the radius of which should not be less than double
the diameter of the pipe. It will be understood that sharp bends and
other obstructions in the pipe have a similar effect to that caused by
a decrease in fall.

Many water-power plants otherwise well designed are rendered
inefficient by bringing the water to the turbine in pipes which are
too small for the quantity they have to carry. If much water is to
pass through a small pipe it must of necessity flow fast. Except
under unusual conditions, even for large sized pipes, no higher speed
than 6 ft per second should bo used. High speeds of flow involve a
loss of working head. For example: — If a pipe 1000 ft. long, with
100 ft fall, is 7 in. diam., and the quantity of water flowing tbroueh
it is 100 cub. ft. per minute, the speed is 6 ft. per second. The
pressure at the bottom of the pipe is 43 lb. per sq. in. when no water
is passing, but when 100 cub. ft. per minute is flowing through, the
pressure is reduced to 33j^ lb. per sq. in. ; this is equal to a loss of
22 ft. head. A 9-in. pipe should be used if it is important to make
the most of the water power ; the loss will then be under 7 ft, or only
7 per cent, instead of 22 per cent.

Motors. — Water motors are divided into two classes, vertical water
wheels and turbines, which are mostly horizontal. Vertical water
wheels are classed as undershot wheels, breast wheels, and overshot
wheels. Their efficiency varies very much according to the circum-
stances under which they perform tneir work. Breast and overshot
wheels give up to 75 per cent, of the theoretical power.



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POWER.



17



Ordinary paddle water wheels are not much used in this conntiy,
Irat can be seen in large livers on the Continent, where they float in
the middle of the stream. Their diameter varies from 12 to 20 ft.
The speed on the circumferenoe is abont half the speed of the stream.
Their efficiency ranges from 25 per cent, to 30 per cent.

Undershot wheels are mostly used for small falls, generally less
than 3 ft The diameters var^^ from 10 to 20 ft., liie speed on cir-
camference being about equal to half the speed of the stream, similar
to the paddle water wheel, and the efficiency is also the same, ranging
item 25 to 30 per cent.

The Poncelot wheel ranges in diameter from 10 to 20 ft., the
ipeed on circumference varying from 10 to 12 ft. per second, efficiency
being from 50 to 55 per cent. The fall is about 4 or 5 ft.

There are two kinds of breast wheels, the low and the high. The
diameters of these wheels vary from the fall of the water to double
this measurement. The speed on circumferencte ranges from 5 to 6 ft.,
and the efficiency from 55 to 75 per cent. The water enters the low
breast wheel slightly below its centre, and in the case of the high
breast wheel it enters above the centre.

Overshot wheels are generally used where high falls can be
obtained, and but small water quantums. Their diameters are
generally equal to the fall, or slightly higher, the speed on circum-
ference being from 4 to 5 ft., and the efficiency from Qb to 70 percent.

Below is given a table of water wheels with their usual diameters,
and the head of water at which they work most satisfactorily : —



Fall, QuAKTmr of


Watbb, and Efpicienct op Wateb Wheels.


KindofWbeeL


SuiUble for Falls of


Gallons of Watsr
per second.


Efflclrac7.
percent.


Paddle

rnderebot

Poooelot

Low breast

High breast

PUdi-back

Orenhot


din. to 1ft.

Gin. to 3ft.

9 in. to 5 ft.

20. to 5ft.

5 ft to 10 ft.
10 ft. to 30 ft.
15 ft. to 40 ft.


50 to 200
20 to 1000
20 to 800
20 to 600
20 to 600
15 to 200
10 to 100


25 to 30
25 to .SO
50 to 60
70 to 75
70 to 75
70 to 75
65 to 70



With paddle and undershot wheels, the quantity of water is of
leas importance, as in cases where such wheels are used there is
generally more water available than is necessary to drive them. The
pitch-hack water wheel mentioned- in the table is similar to an over-
shot wheel, but turns in the same direction as the breast wheel, that
is, in the opposite direction to which the water is running. All
paddle, undershot, and Poncelot wheels work in the same direction as
the stream is flowing.

The only merit possessed by the preceding forms of water motor
is their simplicity, making them available where a more efficient but
more complex form would be undesirable, owing to inability to
execute necessary repairs. Whenever possible, they are now replaced
I7 tnrbioes, whose great advantage is that they utilise the vis viva



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i8 ECONOMIC MINING,

possessed by the water in virtue of the velocity with which it arrives
npOD the wheel, this velocity being due to height of fall. The water
is brought upon the buckets or blades of the turning portion of the
wheel, or turbine proper, by channels distributed over the whole, or
sometimes over a portion only of the circumference of the turbine ;
these, with their various parts, constitute the fixed part of the wheel,
sometimes called the distributor. Turbines may be ejected upon
either vertical or horizontal shafts. There are two classes of turbines
with a vertical shaft In those of the first class the water arrives
horizontally upon the blades of the revolving part of the wheel through
the interior of the latter, and issues horizontally, thus flowing away
from the axis. The revolving blades form thus a series of verticid
cylindrical channels included between two horizontal walls. In those
of the second class, the water enters the wheel from above and issues
from below, remaining thus at a constant distance from the axis.

In any application of water power, or indeed any other form of
power, cej-tain losses of efiect are unavoidable ; but the turbines made
on the " vortex " pattern, as designed by the late Prof. J. Thomson,
and manufactured by Oilbert Gilkes & Co., Kendal, materially reduce
these losses. Id them the power is obtained with slower velocity of
water than in ordinary turbines. This is effected by balancing the
centrifugal force of the water in the revolving wheel against the
pressure due to half the head, so that only fme-hcdf the fall or head is
employed in giving velocity to the water, the other half acting simply
in the condition of fluid pressure. Hence the velocity of the water
in no part of its course exceeds that due to one-half of the fall, and
the loss from fluid friction and agitation of the water is thus
materially lessened. The principle of injection of the water from
without towards the centre produces another saving of effect, since
it admits of the use of long and well-formed channels, by which the
water is made gradually and regularly to converge in passing from
the outer chamber (where it is comparatively at rest) to the point of
entrance to the wheel chamber, where its velocity should be greatest
Further, from the same principle of injection towards the centre,
there is an accordance between the velocities of all parts of the
moving wheel and the proper velocities of the water in its passage
between the points of entrance and discharge. The water when it
has its greatest velocity is admitted to the circumference of the wheel,
which 18 the most rapidly moving part, and when it has, as far as
possible, imparted its power to the wheel, leaves at the central portion,
which has the least motion. The water enters from the guide
passages, with the velocity at which the outer circumference of the
wheel is moving and without change of direction, so that there is no
loss from impact. The steadiness and regularity of motion of vortex
turbines are remarkable, consequent upon the action of the centrifugal
force of the water, which on any increase in the velocity of tha,
revolving wheel augments, and so checks the supply entering from
the guide-passages ; and on any diminution of the velocity of th0
wheel, decreases and admits the water more ftieely; thus counter-
acting, in degree, the irregularities of speed arising from variationl
in the work to be performed.



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POWER. 19

THe double vortex with movable guide-blades is the best means
of applying water power in many situations, and should be adopted
on 2kll medium and high falls in cases in which the amount of power
employed varies considerably at different times, and the saving of
water is important, so that it is necessary to use as small a quantity
as possible to do the work required ; or when the available supply of
water is at times less than the full amount for which the turbine is
designed. The consumption of water can tben be economised to the
utmost, as the passages can be regulated to admit only the exact
quantity needed to do the work, or to suit the available supply. If
the power required and the quantity of water available be very con-
stant, the guide-blades may be fixed and the apparatus simplified, a



Fig. 3 —Turbine House, Heltelltk.

considerable saving in first cost being effected. The orifices through
which the water is directed on to the revolving wheel are made of
each a size as is necessary for the passage of the quantity intended to
be consumed when the turbine is in full work.

When the fall of water is very high, the periphery of a turbine
wheel must move at a very high speed, and if the revolving wheel is
submerged, as in the case of Vortex or Lunedale turbines, there is
some loss of power in the friction of the wheel-covers against the
water. Again, if the wheel be of so small a diameter as to admit of
an arrangement by which it rc^'*"*" *^'> water all round, the speed
of the axis must be very h' inconveniently so. It is

therefore, in the case of a hif 7 to make a wheel of such

diameter as will suit the sp id to construct it in such

a manner that it need not i all round, and need not

c 2

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20 ECONOMIC MINING.

nm snbmerged. Such are known as ** Impulse Turbines.*' There is
no pressure between the guide-blades and the wheel, and as the water



§

T



enters\the buckets with no pressure it is freely deviated by them, an.
takes avcourse quite independent of their shape. The action of tb

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\



POWER.



2t



water on the wheel depends on the angle through which each particle
is deviated whilst freely flowing over the buckets, and as these latter
are not inll there is no disturbance of the action as they pass in front of,
or away from, the jets. The well-known Girard turbine is of this type.

The transmission of power obtained from water, to a considerable
distance, for use underground, is very well illustrated at the lead
mines of the Greenside Mining Co., near the village of Patterdale, at
the head of Ulleswater. The mines are on the slopes of Helvellyn.
The Bed Tarn and the Keppel Cove Tarn form the natural reservoirs
in which the water is stored for use in these mines. Although this
water is stored many hundred feet above the place where the power
is required, it has until recent years been allowed to flow down the
stream bed until it reached the mines, where it was made use of in
Vortex turbines and water wheels ; but since it has become easy to
transmit the power electrically, the water is made to do work on its
way downhill from the reservoirs. A channel has been cut from the
Tarns, nearly following the contour lines for about a mile, where the
water passes into a timber pen trough at the head of 15 in. pipes,
which, descending veiy rapidly, bring the water into the turbine
house (Fig. 3). This house is 400 ft. below the pentrough, and
contains the turbine (Fig. 4), which is capable of giving 10(7 h.p.
This turbine drives a aynamo, and the current, at 500 volts, is con-
veyed to the mines as diown in Fig. 3.

The Pelton wheel possesses undoubted advantages over some other
turbines on very high falls. In common with the Girard, its efficiency
is unaffected by the diameter of the wheel, and therefore the nimiber
of revolutions may be made small or great as reqtiired. It is, more-
over, cheap, is easily kept in repair, and its efficiency under high falls
is good, lliere are some cases in which it is the best form of turbine
that can be used. In remote mining districts, duplicate buckets or
nozzles can be fixed by any intelligent labourer. In mining work,
when the water, before it reaches the turbine, has been used fur sort-
ing or milling, it contains sand, which in time cuts both the nozzles
and the buckets, and it is a great advantage to be able to replace
these without more than a few minutes' stoppage. The annexed table
gives approximate costs of Pelton wheels to develop various powers
with certain heads of water : —



IIP.


100 ft.


200 ft.


300 ft.


400 ft.


600 ft.


600 ft.


700 ft.




£


£


£


£


£


£


£


10


86


22


16


8


10


12


14


20


42


30


23


25


18


19


20


30


56


40


81


26


26


20


21


40


70


46


38


32


27


27


24


50


71


46


89


83


83


28


29


75


90


55


45


89


40


34


35


100


92


66


52


46


40


41


36


150


118


87


62


53


47


48


41


200


120


100


63


54


54


49


50


900


120


102


109


65


66


54


52


400


135


110


111


86


67


68


55


500


150


130


113


117


89


72


69



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22 ECONOMIC MINING.

Calculating H.P. given. — The driving power of water is obtained
by its weight, and not by its velocity.

The power developed by a certain weight of water falling a
certain height is equal to the product of the water in lb. and the fiill
in ft. The theoretical power in a fall of the water is consequently
equal to

62*4 l b. X cub, f t. pe reeoond x fa ll in ft.
550

62 '4 lb. being the weight of 1 cub. ft. of water, and 550 foot
pounds per second being equal to 1 h.p. Some wheels, however, are



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 3 of 76)