Charles George Warnford Lock.

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

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^rodnoer to a high state of inCandesoence, then excluding air and
brcing steam through the incandescent fuel, whereby the water is
iroken np into bydro^n and oxygen, the former being liberated and
be latter combining with the carbon of the fuel. Theoretically,

000 cub. ft of water-gas contain 500 cub. ft. or 2 * 635 lb. hydrogen,
nd 500 cub. ft or 36*89 lb. carbon monoxide. The heat developed
Dd absorbed in the operation will be : —

Heat-onlta.
2*635 lb. H absorb in dissociation from water2*635\ .^q q..^

X 62,000 1 163,370

15-81 lb. C burned to CO develop 15-81 x 4400 .. .. 69,564

Excees of heat-absorption over heat-development . . 93 , 806

1 addition to the energy consumed in raising the coal from say 60^ F.
> 1800® F. In practice a further source of loss arises from physical
HWttS. While the generation of water-gas is very rapid and com-
lete for the first few minutes, long before tbe bed of fuel has lost
lOQgh heat to stop the dissociation of water-vapour, the gas will be
nod to contain a very large percentnge of steam, which continues to
tcrease till it is nearly all steam, while the fuel-bed is still at a good
Mt It would seem from this that there is a coating of some kind
on formed on the fuel that prevents the oxygen of the water from
mbining with the carbon, and that does not form at the same tem-
mtnre when both air and steam are used. Some engineers think
is is due to a sort of fusion of the ash, making a thin coating on the
tr£ice of the coal, while others charge it more to a rapid cooling of
le Borface, or both. But W. J. Taylor asserts that when the tem-
aratnre of the fuel-bed of a producer is too low to make water-gas
erne, it is plenty hot enough for making gas with air and steam
gether. From this it is evident that more water-vapour can be
i«ociated in a continuous than in an intermittent process, whence
ises a richer gas and better utilisation of the energy of the fuel
Dsiuned.

So much misapprehension exists among fuel consumers as to the
B of heating power implied by esicaping smoke that the following
•enrations recently published by R. R. Tatlock* deserve to be
delj made known. This well-known authority declares that the
• of any large percentage of combustible matter, and consequently
heating-power, is quite uut of the question. This may be proved in
ways: (1) by calculation of the two sources of heating-power as
}wn by an ansdysis of coal or dross used for steam-raising ; and (2)

actaal analysis of the furnace gases for combustible solids and
bed.

On p. 228 are given the results of these two methods of observation,
g same dross being analysed and also employed as fuel in a works
mace, from which smoky gases were given off which were tested for
ubnstible matters.

* Gliemical News, 1894.

Q 2



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328 ECONOMIC MINING.

1. Analysis of dross employed :—

Per cc p^

Gab, tar, &0. 87*63

Fixed carbon * 49-97

Sulphur 0-4O

Ash 2-72

Water 9 28



lOO-OO

Heating power (practical) due to gaa, tar, &c. . . . . 1:16

„ „ „ fixed carbon .. .. 6*49

7-65

The points to be observed are the relative proportions of heatii^
power represented in the analysis by the number of pounds of wat<
at 212° F. capable of being evaporated to dryness by 1 lb. of the fa<
given out respectively by the combustion of gas, tar. ifea, and by Htx
fixed carbon. These are calculated according to Piayfair's well-know
formula, which was practically tested on coak intended for the Britis
navy, and which snows that while 1 lb. of fixed carbon is capabi
when burned of evaporating 13 lb. of water at 212° F. to dryne
1 lb. of the gas, tar, &c., will only evaporate 3'1 lb. Prom thei
figures it appears that in the coal or dross the gas, tar, &c., only cox
tribute 15 per cent, of the total heat given out during the oombusticB
and that the fixed carbon produces the remainder, or 85 per cent. I
coals with less of the former ingredients and more of the latter, whic
is commonly the case, the proportion given out by the volatile ecu
stituents would be considerably reduced. It is thus perfectly clei
that even though the whole of the volatile matters (which can alon
be accountable for any loss of combustible material) escaped con
bust ion, there could not possibly be a greater loss of heat than 15 pc
cent, of the whole, even in such an extreme case as this represents.

2. Analysis of furnace gases given off during the burning of tli
dross : —

OaMt Teiy Gmm almoet

smoky. free from raioke.

Per cent, by Pw cent by

volume. ▼olame.

Carbonic acid 5-0 3*5

„ oxide none none

Hydrocarbons trace none

Nitrogen 79*9 79-9

Oxygen 151 16-6



10000 100-00

It has been asserted that carbonic oxide is given off in considerab]
quantity when much smoke is being produced, but it does not app^
in this case ; and Herapel, in his work on * Gas Analysis,' comes to tii
conclusion that little or no combustible gases are present in fama<
gases. He says, " Furnace gases usually contain only carbon dioxidj
oxygen, and nitrogen. All other gases are present in but very smal
amounts. In oft-repeated analyses the author has always found onl
traces of carbon monoxide, methane, and the heavy hydrocarbons.
This is in complete accord with the analyses given above, and it m^



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NON-METALLIFEROUS MINERALS. 229

be taken for granted that the presence of carbonic oxide or other
combngtible gases in furnace jcas'^s is a most unusual occurrence.
This is quite conclusive evidence that no appreciable loss of heat, even
when the iuraace gases are smoky, can be attributed to the passing
away of the products of impeifect combustion in the gaseous form at
ieast

That there is loss of combustible matter in the smoke is an un-
dooUed fact, but the quantity seems also to be greatly magnified in
certain random statements. In the experiment referred to above, the
loot was also collected during 1 J hr., with the following results : —

Grains per 100 cub. ft. of
fumaoe gases.

Oftrbnnaceons matter 30*81

Ash or mineral matter 20*65

Total soot 51*46

It will be observed that the soot collected consisted largely of
nineral or incombustible matter. In several experiments to estimate
he soot in furnace gases similar results were obtained, and the
iTerage would come very close to the quoted results of this special
est.

To find how much carbonaceous matter is actually lost as smoke,
twill be necessary to know the number of cubic feet of furnace gases
;iven off by the combustion of, say, 1 ton of the dross. If the per-
entage of carbonic acid in the furnace gases is taken at 5 per cent.,
lie total volume of these given off from 1 ton of dross would be about
40,000 cub. ft. measured at the ordinary temperature and pressure,
Qd this would contain 41 lb. of carbonaceous matter and 27 lb. of
mineral matter. This would represent 1 • 8 per cent, of the volatile
letters (gas, tar, &c.) given in the analysis of the dross ; and if from
^is is now calculated the heating power according to Play fair's
'nnala,it will only come to 0'067. This figure, compared with the
rsctical heating power (7 • 65) of the dross, goes to show that the
^d combustible matter of the smoke can only account for the very
iiall percentage of 0*74 of the total heating power which can be
)tained from the coal.

From the results of these experiments it is evident that the loss of
*abnstible matters in smoke is very small indeed, and that the belief
immense loss by this cause is simply a &llacy, and not corroborated
^ experiment. In adopting methods of removing the smoke nui-
Dce, it must therefore be borne in mind that there is little or no
^ in burning smoke, and that other methods of dealing with the
oblem, such as Dulier's smoke absorption process, ought also to
teire consideration.



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230 ECONOMIC MINING.



FULLERS' EABTH. *

This material cannot be said to possess a definite chemical oomp
sition, but in general terms it may be described as an unctuous so
silicate of alumina. Two varieties are distinguished in the trad
one hayinff a blue cast of colour, the other a yellow. Analyses sho
the following ayerage percentages : —

Blue. Yellow.

Alnminoiu earth 18 11

SilioiooB earth 42 44

Lime 4 5

Maiprnena 2 2

Oxide of iron 6 10

Soda 5 5

Water 23 28

The fullers* earth beds which are worked in Bedfordshire ai
Surrey occur in the Lower Qreensand formation, whereas those ei
countered near Bath belone to the Oolitic age. There are numeroi
other occurrences of fullers earth in various formations in other par
of England, but they have not yet attained any degree of industri
importance, nor does their quality in most instances indicate tk
such will be likely in the future.

Usually, fullers' earth deposits are worked by stripping the ovc
burden and quarrying the mineral by open cuts, but in the Wobm
Sands, properly timbered underground galleries are now driW
replacing the series of little independent pits or '* earth wells ** wh«
were fietvoured when the industry was in the hands of numerous snu
operators.

The mineral as taken from the workings is always mixed
more or less foreign substances, and needs preparation. This coi
in crushing and levigating, by which the finest particles (or val
portion) are carried ofi* in suspension by a stream of water to seti
tanks, while the coarser (chiefly impurities) are arrested, quite a
length of shallow trough with slight ribs across the bottom
necessary to effect complete separation. The apparatus is call
*' maggie " in Devonshire and Somersetshire. Finally the impalp
mud of fullers' earth is dried, first by standing for a long time
tanks, from which the water is drawn off by degrees from
surface downwards by means of little holes in the sides, stopped
pegs ; and lastly by applying fire beneath a very large shallow ta
with a floor of porous tiles, lying over a series of wide flat flues,
heat from the fire passing through aU the flues to a chimney,
thus sucking much of the moisture through the porous floor.

Fullers' earth possesses great detergent power, and is much \
for cleaning woollen goods froiii grease, and in refining oils and Is



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NON-METALLIFEROUS MINERALS. 231

quantities being sent from England to Chicago for the latter
porpose.

There seems to be much difference of opinion * among oonsumers
«8 to the respective merits of yellow and blue earth. Probably there
18 ]ittie to choose between them, except that the greater prevalence of
iron salts in the bine kinds may be detrimental in some applications.
The Wobnm earth is less calcareous than that of Bath ; and there is
more iron in those of Bedford and Surrey than in that of Gloucester.
Jktensive beda of earth occurring at Bhiwlas, N. Wales^f while
^nneising almost exactly the same composition as some earth from
Kntfield, Surrey, present a ^reat contrast in physical characters, being
10ft, earthy, friable and dai^ grey, iuBtead of hard and greasy feeling.

^ A. G. G. Cameron. '* Geology, Mining, and Economic Uses of Fallen' Earth,"
Tnas. Fed. Inst Min. Engs., vi. 204.
t P. G. Sanfoid, GeoL Mag., x. 160.



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232 ECONOMIC MINING.



GEMS.

Under this heading are included the few precions mineral prodncti
used for personal adornment in the form of jewellery, and comprising
chiefly the diamond, emerald, lapis lazuli, opal, ruhy, sapphire, topaz,
and turquoise. Semi-preoious stones such as agate, calcedony, (hijx,
&c., used in architectural display are embraced under Stone.

Dtamond. — In composition the diamond is pure carbon ; it has \
sp. gr. of 3*515-3-525, its hardness is 10, and its hue Taries &om
colourless transparency to white, grey, brown, red, yellow, green,
blue, and even black.

The by far most important diamond fields of the present day lie
in Griqualand West, a portion of Cape Colony, chiefly in a radius of
1^ miles around the De Beers mine, in a blue conglomerate.

The diamantiferous or ''blue" ground is a hard, dark greenish*
blue cement, which requires to be Inasted with dynamite. It might
properly be called a brecciated rock or cement, since the mass is
composed of angular pieces of black shale, irregular pieoes of mica
and several more or less decomposed minerals, all imbedded in a miM
of indurated talc, or rook of that nature.

Large uiasses of ** floating " shale or reef are found imbedded ia
the *'blue," also basaltic boulders from one foot to many feet in
diameter. These are, without doubt, portions of the rook whiok
surrounded the opening or crater before the deposit of the **blne"
was made.

Gardner Williams is of opinion that the formation of the diamond-
bearing deposits was due to aqueous rather than igneous agencies—
possibly to something in the nature of mud-volcanoes. His reasom
for this belief are, first, the physical appearance of the mass of
diamond-liearing ground, and, second, the existence of the diamonds
themselves, the presence of which seems more natural in an aqueous
than an igneous deposit

That the masses of diamantiferous material occupy the craters oC
former volcanoes, there is but little doubt.

In whatever manner the diamonds may have been formed, thej
must have been crystallised before they were deposited in the mass is
which they now occur. No diamonds have been found either in the
shale or in the hard rock surrounding the ''blue,'* as might bi
expected if the diamonds had crystallised where they are found
Again, a great many fragments of broken diamonds are found, and
the corresponding pieces are not found near them, showing that they
are not in their onginal position.

One fact deserves special mention, namely, the variety d
diamonds not only in difi'erent mines, but in different parts of thi
same mine. It requires no great skill to determine from which pboi

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NON-METALLIFEROUS MINERALS. 233

\ lot of stoDes haTo been taken. The peculiarities consist in colour,
size, oryttallisation, black spots in the crystals, and the amount of
broken or irregularly shaped nieces, called ** bort."

In one part of the mine tne stones are perfect octahedrons, while
in another part the crystallisation is more imperfect; in one part
the stones will be white while in another the majority of stones will
be yellow.

The original system of mining the Eimberley ground, namely by
open quarry, was the best for a depth of say 2^ ft., because the
mine could have been worked in no other way while the claims
were operated by indiyidual owners. But as soon as a greater
depth was attained, the removal of the surrounding ** reef" and the
tlmost constant covering up of some part of the mine with fallen
leeC rendered the cost of mining very great and also prevented the
mining of the " blue " for monws at a time in the covered parts of
the mine.

Two methods are now advocated for working the mine. Many
ding with pertinacity to the old open-mine method, and advocate
the removal of the shale to an angle of safety, so that it would be
impoKible for it to cave into the mine. The friends of this method
«re relying upon the hard rock remaining in place, when exposed
ibr several hundred feet in depth. The enormous quantity of shale
(4,679,000 cub. yd.) to be removed makes this method too expensive ;
and besides, sooner or later the hard rock would give way and
the mine be filled with huge masses of exceedingly hard rock. The
ooet of removing this, should it fall into the mine, would be very
peat

The other method of working the mine is by sinking shafts, in
the solid ground outside of the mine, and drifdng to uie **blue'*
ground. The *'blue" ground can then be mined by drifting and
sloping. The greatest difficulty in the way of close mining is the
Kardtv of timber. All timbers and other lumber have to be brought
&om ^e Baltic or from America. The ground must therefore be
ttined with the use of as few timbers as possible.

The depth of the diamond-bearing deposit is wholly unknown,
Old has not been tested by boring. The blue ground is richer below
ittn it was near surface. The water pumped out yearly is 11-13
luUion gaL, at a cost of 6(2. per 100 gal.

The diamantiferous '* blue " ground is blasted out in the mines,
i&d hauled to surface by large skips working in inclined shafts,
rbe De Beers No. 2 shaft (Fig. 86) is capable of landing 3000 tons a
lay.

The unit of measurement locally employed is the ** load,'' 1 load,
r truckful, weighing 22-25 cwt. Each skip of the pair at this shaft
^gB up* ai^d automatically dumps into a hopper, sufficient material
make 8 or 4 loads per ascent. A train of empty detached trucks is
vought underneath tiie shoot by which the hopper is discharged, and
< truck after truck is filled, it is attached, bv means of a Y fork on
he top of the truck, to a slow-travelling endless wire cable, which
onveys the train along a tramway to the weathering floors. A
ontinuous train of loaded trucks may be thus seen in procession.



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NON METALLIFEROUS MINERALS, 235

from the ooonter which projeots over the tram line, engaging each
truck as it passes. The mining and hauling capabilities of the plant
are in excess of the facilities for the subsequent treatment of the
" blue ground " ; thus, in the first year of the Consolidated Company's
operations, while some 950,000 loads were mined and hauled, only
some 720,000 loads were washed for diamonds. A reserve of '' blue
ground" is thus being accumulated, and a longer period can be
allowed for its more complete disintegration on the weathering floors
before it is washed for diamonds.

The weathering floors are widely scattered over the company's
property, and together cover an area which may be estimated in
square miles. XJpon these floors — which are simply areas of hard
rolled ground, moderately level, and free from vegetation — the lumps
of " blue ground," in stones measuring from about 9 in. diam, down-
wards, are spread out in a shallow layer not more than 12 in. deep.
The length of time for which the diamantiferous earth is thus
expof^ed to atmospheric influence is generally about one year, but by
turning over the lumps, harrowing tiie stuff, and artificially watering
it» its disintegration can be greatly hastened, and the whole made
ready for washing after an exposure of 6 months, or even less.

The first treatment of the weathered '* blue ground " is conducted
in a number of small isolated works, situated for convenience in
proximity to the different weathering floors. To all intents and
purposes this first treatment is the same as that pursued by those
who are engaged in rewashing old heaps, only the plant of the
Consolidated Company is in all cases larger, and is actuated by steam
power instead of by manual labour. The disintegrated mass, when
agitated with an excess of water, yields a fine clay mud, which over-
flows bj a lip on the edge of the pan, to be raised by bucket elevators
to the summit of the debris heap, over which it is poured, while the
granular residue remaining in the pan is reserved for the second
treatment. Owing to the scarcity of good building stone, the
retaining walls, to support the debris heaps as they grow higher and
higher hj the continual addition of waste mud, are constructed of
large and heav^ sand-bags, flattened out and piled one on top of
another. An illustration of one of these establishments is given
in Fig. 87.

Near the central works of the Consolidated Company, at which
the diamonds are finally separated, there is a washing installation of
oonnderably larger size, but it is worked on precisely the same
principle as the smaller ones. It consists of a series of some 10 pans,
at different levels, and the waste mud from all of them is run by
gravitation to depositing tanks beneath the lowest of the series.
IVom these the mud is discharged by sluioe doors into iron box-
f^arnages of about 80 gal. capacity, and these are transported by an
iierial wire-rope railway to tne summit of a debris heap, 200 or 300
yd. distant, where they are automatically tipped and emptied.

The granular residue from the various washing establishments is
Domr conveyed, for the second treatment, to the central works, known
locally as ih& '' pulsators." These pulsators consist simply of a small
ryrdinary ore-dressing plant, such as is universally employed for the



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236



ECONOMIC MINING.



purpose of Beparating and concentrating poor ores of copper, lead, and
other minerafs. It comprises two sets of jigs (right-hand and left-




hand) having 4 compartments each, and fitted with the neoenaiyl
sizing and classifying trommels. The alternating ebh and fiovj



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NON'METALLIFEROUS MINERALS. 237

movement of the water iu the jigs frees the gravel from all adhering
mud, and allows the diamonds to be readily detected afterwards. This
plant is capable of treating 5000 loads of diamantiferous oonoentrates
a day, this being the product from the washing of 50,000 loads of
'* blae ground."

From the pulsators, the clean, and uniformly-sized gravel and
mud of four grades of fineness, is removed to the sorting tables, to be
carefdlly looked over and picked by hand, in the same way as the
sands from the re-washed old heaps. The sorting tables are placed
underneath a long shed, open in front to a commodious yard, the
whole of this department being enclosed by a substantial wall and
guarded by sentries. The examination of the coarser gravel is
entmsted to white men, either English or Africander. They are
paid iw'ell, and are not subjected to the ignominy of being searched
before leaving work as are the Kaffirs. The finer gravel or sand is
scmtiiiiaed entirely by Kaffirs, most of them being convicts. Tin
ressels, with inverted conical lids having a small hole at the apex and
closed by locks, the \ejs of which are kept by the superintendent of
the department, are piaced on each sorting table, and every diamond,
as Boon as it is discovered, must be dropped into this receptacle.
Bouf^hly speaking, the average yield of diamonds per load of *' blue
ground " is 1 to 1^ carat, but the value of the carat is liable to con-
siderable fluctuations. During the first year of the Consolidated
Company's existence, thet realised value per carat was just under 20«.,
while during the second year nearly d3«. per carat was realised, the
total output being 1,608,880 carats, valued at 2,641,558/. Very many
of the stones are ** off-coloured, and do not realise the price per carat
of stones from river workings.

A large amount of manual labour is required on the floors in
picking and spalling the lumps. A new method of rolling and
harrowing the blue ground was introduced by A. W. Davis, the general
manager of the Bultfontein Company. Thiis company formerly used
rollers and harrows drawn bv cattle in the usual manner adopted by
other mines. The new method is the adaptation of a 22-ton steam
roller, the hind rolls of which are 7 ft. and the front rolls 4 ft. 7 in.
diam. The roller covers a 9-ft. track. Steel ribs are bolted dia-
gonally to each roller, and a harrow is attached to the back of the
tender. After the blue ground is crushed by the roller teeth, it is
turned up for a second crushing by the harrow. This process is con-
tinned nntil the desired degree of fineness is obtained. It dispenses
with the costly and tedious method formerly employed, and renders
the mine independent of ^he natives on the crushing floors. Further,
it tends to check diamon . stealing, as more diamonds have been stolen
Ax>m the floors than from elsewhere. Although coal costs 82. per ton,
the coBtof running the ruxler, inclusive of everything, does not exceed
50L a -week.

The washing machine or pan is made of steel, 10 to 15 ft. diam.
and 1^ to 2 ft. deep. This is fixed, and from the central shaft the
arms, 10 in number, are revolved. On these are fixed several knives
ijt teeth, the object of which is to agitate the material under treat-
iDent« These reach to within \ in. of the bottom of the pan. The



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238 ECONOMIC MINING.

stufif, mixed with water, enters at the outer rim of the pan, and tlie
light waste is taken away at the oentre. The tailings are lifted by
means of an elevator and banked, the bank in some cases reaching a
height of 60 ft. At the top of the elevator the buckets deliver 2ie
tailings on a suitable screen on which most of the solid matter nms
to waste, while the thick water is led back by a launder to the



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 26 of 76)