Charles George Warnford Lock.

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

. (page 18 of 76)
Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 18 of 76)
Font size
QR-code for this ebook


bat varying very much with the quality.

* See Spona* ' Enoyclopadia,' pp. 1628, 2023.



Digitized by



Google



156 ECONOMIC MINING.



ARSENIC.

Arsenic is an iron-grey, metallic-looking, brittle substance, sometimes
occurring;; native in veins in the older rocks, but commercially obtained
principally as a sulphide in association with iron and copper pyrites
and tin-stone.

Arsenic itself is a substance of no commercial importance, bnt
some of its compounds, as the oxide, commonly called " white arsenic,"
or '^arsenious acid," and the sulphides, orpiment and realgar, are
largely used for various industrial purposes.

Arsenical pyrites, or " mispickel," is roasted dolely for the arsenic
which it contains; but from the ores of tin and copper arsenic ifi
obtained as a bye-product in the various smelting processes.*

Arsenic sublimes at 424° F., and in order to effect the thorough
roasting of the ore, the temperature must be raised to low redness,
but not beyond, since any increase of temperature above that which
is absolutely necessary for sublimation, must be compensated for by a
greater length of flue, in order that the vapours may be sufficiently
cooled in their progress to be entirely deposited. This, of course,
applies only to the ores (as mispickel) which are roasted solely for
the sake of the arsenic which they contain. When tin and copper
ores are employed, and arsenic is yielded merely as a bye-product, a
much greater heat is required, and consequently the series of flues
and condensing chambers must be longer in proportion, in order that
the requisite space may be afforded for the cooling of the superheated
vapours.

The arsenical tin and copper ores of Cornwall and Devonshire
contain arsenic in the form of arsenical pyrites (Fe As -h Fe Sj), a
compound of arsenide and bisulphide of iron. It is common to see
the manufacture of arsenious acid forming part of the first process to
which the tin ores are subjected in West Cornwall, and the copper
ores in East Cornwall. The quantity of arsenic in the tin ores raised
in Cornwall and dressed there, either at the mines or on the streams
which carry down tin ores from the hills (** stream tin"), varies
greatly, so that at some works the arrangements for collecting
arsenious acid are on a much more extensive scale than they are at
others. The process is similar whether it be a cupreous or a stannous
ore that is dealt with.

The ore, first stamped and then dressed by huddling, is subjected
to a process of calcination, the object of which is to bum off the
arsenic as arsenious acid, and the sulphur as sulphurous acid, and to
peroxidise the iron. Three methods of doing this are commonly in use
in Cornwall. One of these is by burning the ore in a reverberatoiy

* Mnoh useful infonnation on ready means of estimating arsenic will be foond
in a paper by A. Dickinson, ** On the Assaying of Arsenic Ores," in Trana Inst Hin.
and Met., ii. 1 10.



Digitized by



Google



NON-METALLIFEROUS MINERALS. 157

famaoe with a flat bed, on which the ore in the state of powder is
sabjected to a dull red heat for about 24 hours, during which it is
frequently tnmed over. When believed to be sufficiently burned, it
u nked out, either on to a floor or a hearth prepared to receive it.
The material is again huddled, and the product of the washing is
again calcined for about 12 hours, this time with a stronger heat. In
many instances one calcination is found to suffice. This method is
cilled " hand-calcining." If the heat be too great at the first calcina-
tion, or if the material has been introduced not sufficiently dry and
powdery, and if it be not kept well stirred, it is apt to cake, and then
the centre of the caked masses may be insufficiently burned. Hence
the operation requires constant careful watching. The inside
dimensions of these flat furnaces, which are usually built in pairs, are
commonly 20 ft. long, 6^ ft. wide, and 16 in. from bed to roof; a
double fornaoe will treat 8-10 tons per 24 hours, using 3 cwt. coal per
t(m roasted, and employing 2 men on each 8-hour shift, who receive
3-34#. per ton.

Another apparatus largely used in Cornwall is Brunton's calciner.
It is essentially a circular reverberatory furnace, the floor or bed of
which, made of firebrick laid on a sUghtly curved table of iron,
revolves slowly. Numerous iron " flukes " or ploughs project down
from the roof nearly to the bed, and are so constructed as to turn the
powder over and to move it a little outwards on the floor as it
rerolves. The ore fed in at the middle of the roof is in this way
made to travel slowly towards the edge, and at last to fall into a box
or "wrinkle " prepared for its reception. A portion of ore takes 6 to
8 hours to travel thus to the edge. The calciner is heated by two
fires, placed as near as convenient to each other, opposite the uptake
through which the gases escape. It is said that tins calciner is best
adapted for stamped ores, stream material (ore washed down in the
streams) being too fine to be dealt with in this way. The average
capacity is 4-5 tons per 24 hours, employing 1 man per 12-hour shift,
who receives about 2«. a ton, and consuming lj^2 cwt. coal per ton
ealcmed. This roaster is, perhaps, the most efficient so far as uni-
ft^mity of product goes, but it operates slowly, verifying the general
law that oxidising roasting will not admit of hurry ; it might be much
improved in speed, capacity, and economy of fuel, by furnishing sup-
pWentary supplies of pure hot air.

The third kind of calciner in use in Cornwall is that of Oxland
and Hoddn, shown in Fig. 79. It consists of a wrought-iron
cylinder 20 to 30 ft. long, with an internal diameter of 3 ft. to 4 ft.
( in., and lined with firebrick. It is mounted in an inclined position,
and caused by machinery to revolve slowly ; the ore fed in at the
upper end is turned over and over as it flows slowly towards the
wwzt end, where it is discharged. The turning over of the ore, so as
to expose all parts of it to the heat, is eflected by four longitudinal
lihs of firebrick, which project into the interior. The flame horn a
fcrnaoe enters at the lower end, and the products of combustion and
calcination pass away by a flue from the elevated end. The Oxland
calciner, which is often only an old boiler shell lined with firebrick,
viD least 20^25 tons per 24 hours, consuming only 10-20 lb. coal per



Digitized by



Google



iS8 ECONOMIC MINING.

ton, and employing 1 man and 1 boy per 8-liour shift, who reowTe
10-12(2. a ton. It is thus by far the cheapest in fuel and labour, but
requires 2-3 b.p. motive power, costs more for repairs, and is lea
amenable to control, so that its work is not uniform and sometimM
an excessive draught causes mechanical admixture of solid particle
of ore with the volatilised arsenious acid.

A fourth mode of burning a highly sulphurous arsenical ore, oon«
taining, say, as much as 30 per cent of sulphur — at any rate enongh
to bum by itself without fuel — is in kilns built like the limekilns used
for continuous burning, but covered in at the top, where the ore is
charged in through hoppers, each provided with a damper to olofle the
bottom. The arsenious and sulphurous fumes are carried off from the
upper part of the kiln by flues. The process is very imperfect.

At Deloro, Canada, where large quantities of auriferous mispickel
have been worked, the Oxland calciners first used were modified by
Rothwell, as follows : — In the upper 3-4 ft of the cylinder the usual
brick shelves were retained, but were arranged spirally, so that thej



*' screwed " the charge into the furnace and prevented backing up t
the feed. Beyond that distance, the brick shelves were replaced by ti
diaphragms 12 in. wide, which met in the centre of the cylinder ai
divided it up longitudinally into 4 compartments, so that the ore w
divided into as many equal parts, each revolving in its own oompai
ment. About halfway down the cylinder, the several tiles formii
the diaphragms were set 1 in. apart, so that as the cylinder re vol v
the ore filtered down from one compartment to another, encounterii
the stream of air passing up the furnace, which air was supplied 1
a suction fan near the chimney and was thus always under cx>ntr
With two furnaces thus equipped, one 30 ft. X 5J ft. delivering int<
second 60 ft. x 6^ ft., 48 tons were roasted per 24 hours with 2 m
per 12-hour shift, and a third during the day on the cooling floor, 1
consum'ption of fuel being, it is claimed, reduced by one-half.

With a view to collect the arsenious acid, one or more ohamb
are constructed in the course of the flue (which itself is nsua
capacious) to the chimney shaft. The chamber may vary in hei^
from 7 to 12 ft., and is provided with a number of vertical parti tio



Digitized by



Google



NON-METALLIFEROUS MINERALS.



IS9



springing alternately from opposite sides or ends of the chamber, so
as to oonstitnte the chamber a series of zigzag passages 3 to 4 ft. wide
and 8 or 10 to 40 ft. long. Each passage or section in the zigzag is
prorided with an iron door, by which a workman enters it at dne
intenrals to remove the deposited arsenic, but which at other times is
doeely Inted up. From the chamber the flue usually proceeds either
straight or angularly, and when feasible up a hill-side, to the chimney-
ahaft Fig. 80 is a plan of one of these zigzag chambers. It is 6 ft.
high, but there are other chambers longer and higher. From the
ohamberB at Devon Great Consols a capacious flue passes with an^lar
beodings for 150 yd. up a hill to the chimney stack, 120 ft. high,
the base of which is 80 ft. above the level* of the works. Other works
have chambers much longer. For instance, at the burning-houses in




Fio. 80.— Ansxirio Oon>iN8iKa Flue.



^ Tuckingmill Valley there is a zigzag chamber containing 18
pvagea, each 33 ft. long and 7 ft. high by 4 ft. wide, and at the
paat rool ** burning-house," close to it, there are 400 yd. and more,
|K«t1y of similar ziezag, 10 or 12 ft. high, between the oalciners
|Bd the chimney. The flues are also in all cases provided with doors
ll convenient distances when they are above ground, as they usually
pe ; but sometimes they are constructed under ground, and then they
pre to be opened above to gain access to the interior of them.

If the flues be duly tight, all the sulphurous acid from the com-
■stion of the ore not condensed with the watery vapour in the flues
Uid this is probably only a small proportion of the whole) escapes
Ito the air by the chimney, and with it so much of the arsenious
lid passes off as the flues have failed to arrest. In works where the
li is more sulphurous than arsenical, but little pains are sometimes
iIeoi to collect the arsenic, and then the larger proportion of it goes



Digitized by



Google



i6o



ECONOMIC MINING.



off by the chimney with the snlphnrous acid. By bnilding the firs^
portion of the condenser as a spacious chamber, and making the eiii
for the vaporous part of the fnmes some 10-12 ft high above the
entrance, considerable mechanical cleansing can be secured, as i^
remains hot enongh to avoid condensation in the chamber. The sides
and bottom of this chamber are made steeply sloping, and the feed
of the raw ore is carried through it. By building a lip of boiler iroii
into the base of the chamber it will deliver ore and dust within the
grasp of the spiral shelves described above.

The arsenious acid taken from the flues and zigzags of tL^
burning-houses is more or less crystalline, and mixed with sootj
matters and also with some* moisture which is acid from the oxidatio^
of the sulphur of the ores ; it has therefore to be refined. For thifl







Figs. 81, 82.— Befookg Absbnio.

purpose a reverberatory furnace is again used, with a flat, square, en
oblong floor, and smaller than the furnace used for hand-calcining
The fuel used is mostly coke, and sometimes a second fire is used, th«
flue from which is made to pass beneath the floor of the oven. TIm
volatilised arsenic is collected in zigzags as before. Such refined
arsenic is powdery : when lump or vitreous arsenious acid is to b<
made the arrangement adopted is of a different character. There an
two forms of apparatus in use for this purpose. In the one form
Fig. 81, a strong circular cast-iron dish a, about 2 ft. wide, is prcv
vided, which is supported by the flange over a fire ; on this is placed
a conical iron cover &, termed the '* kettle," and the flange of tlM
kettle is closely fitted to that of the dish by wedges. At the summii
of the cone is a hole about 2 in. wide, which, during the sublimatidi
of the arsenic, is closed by a stopper e. A pair of these dishes aM
placed so as to be heated by one fire, in a sort of closet terminstb^



Digitized by



Google



NON-METALLIFEROUS MINERALS, i6i

aboTo in a low cbinmey and enoloeed in front by an iron door, so that
anj arseniona acid that may escape shall not pass into the room, and
80 injure the workmen. The charging is effected in portions thus :
the kettle being wedged on, a quantity of crude arsenious acid is
introduced by a funnel into the hole at the top, and falling down
fonns a little conical heap in the centre of the dish. A similar firesh
charge is introduced about eyety 2 hours, and at the end of about
24 hours the fire is put out and the kettle is removed, when the
ritieous arsenic is found coating its interior, and is chipped out.

The other form of apparatus is indicated by Fig. 79. It oonsierte
of four flanged pieces bolted together by their flanges. The lower-
most is a strong cast-iron pot a, about 2 ft. deep and 18 in. wide,
heated by a fire beneath and all round. Above this are two cylin-
drical pieces &, rising above the pot to a height of about 3 ft., and
surmounting these is a conical piece e, from the summit of which a
3-in. pipe conveys any uncondensed matters into a zigzag chamber.
The charge is all introduced at once, and the process of volatilisation
18 completed in about 4 hours. The fire is then put out, and the
apparatus is allowed to cool; the vitreous arsenious acid is found
Iming the three top pieces. VN hatever escapes condensation in the
apparatus is passed into the zigzag, and not as in the other form of
apparatus into the air.

When the fumes are, as is the case at some works, carried in tight
flues up a hill-side, and discharged from the top of a tall chimney, or
where the works are situated in a wild uninhabited part of the
coimtry, no offence or damage is occasioned by the fumes so discharged ;
Imt in some cases, where carelessness or bad arrangements of the flues
and chimney prevail, the nuisance from the fumes is considerable,
espedally when the burning-houses are situated at the bottom of a
narrow valley, and the chimney tops are not higher than the level of
the head of the valley. Nuisance may also arise from drawing the
ore before it is thoroughly calcined, and from leaky flues. The un-
condensed fumes should be discharged at such an elevation and at
such a distance from dwellings, crops, and water supplies that they
•hall be thoroughly dispersed before they have time to fall. Where
this is not practicable, other remedies are not easily to be found, as
the add fumes given off by the calciners are largely diluted with air,
and most difficult of condensation. Any additional condensing appli-
ance, such as a water spray or dip tower, greatly impedes the dmught,
and renders a blower or fan almost a necessity. Where there is a
demand for impure sulphuric acid, the fumes may be conduct^
through leaden chambers afier precipitation of as much arsenic as can
be thrown down; but where the ore is rich in arsenic so much air has
to be admitted for its oxidation that this plan is not feasible.*

The quality of commercial arsenic depends upon its purity. The
best quality is perfectly white, but the presence of sulphur, due to its
deposition with the arsenic in the condensing flues or chambers,
imparts to the latter a highly objectionable yellow tinge. This is
especially characteristic of the German article, which is often called
"yellow arsenic." The arsenic from Swansea is also yellowish in

* For farther details of manofiacture, see Spons' * Encyclopndia,' p. 336.

M

Digitized by VjOOQIC



1 62 ECONOMIC MINING.

colour, while that from the works in Cornwall and Devon is entiioly
free from stilphur. That obtained from treating tin whits is often so
soiled by smoke as to be called *' arsenical soot," and requires to be
re-snblimed before putting on the market. Befined arsenic should be
in compact, Titreous lumps, and perfectly free from particles of
metallic arsenic ; when of bad quality, it is in loose masses, more or
less soft and friable, due to re-subliming the crude acid under too
feeble a temperature.

The production of arsenic in Great Britain fluctuates between
about 4000 and 8000 tons annually, with a value of about 6 to 92. a too.

Considerable quantities of white arsenic have of late years been
produced at the (Catling mines near Marmora (Deloro), Ontario,
Canada, as a bye-product of the treatment of auriferous mispickel for
recovery of the precious metal, the mispickel oocurring abundantly
in quartz veins in granite.



Digitized by



Google



NON-METALLIFEROUS MINERALS. 163



ASBESTOS.

Thb ^miliar fibrous Bubetanoe oommonlj knovm as asbestos embraces
at least two distinct minerals — asbestos proper, which is a variety of
hornblende; and cbrysotile, which is a variety of terpentine, and
readily distingnishable by its yielding water when heated in a closed
tnbe. Thongh both minerals are found in the altered crystalline
rocks, each has its particular associates, and while asbestos proper
occnn in metamorphics rich in hornblende, chrysotile is encountered
b distinct veins penetrating masses of serpentine, which have gene-
rally resulted from the alteration of eruptive rocks rich in olivine.
Commercially, the two minerals are indiscriminately known as asbes-
tos, but they possess marked features governing their mercantile
▼aloe, despite close resemblances in cliemical composition. The
Italian product is true asbestos ; the Canadian is chrysotile. These
two countries practically supply the whole output.

Although asbestos mining and prospecting in Canada has con-
tinned for years, and the areas of serpentine are very extensive, the
portions in which asbestos of good quality or in paying quantity is
found, are in comparison so small that mining operations are practi-
cally confined to two centres only a short distance apart These
places may be reached in a few hours from the city of Quebec by
train on the Quebec Central Railway, which runs through the heart
of the asbestos mining district at Thetford and Black Lake Stations,
about midway between Quebec and Sherbrook. The formation is
Betamorphio, and the asbestos is found in a belt of serpentine which
eitends from the township of Broughton on the north-east, to that of
Ham on the south-west, and includes the townships of Thetford,
Coleraine, Ireland, and Wolfreston. The whole district in which
asbestos is thus mined has a radius of about 10 English miles.

The asbestos-bearing lode of serpentine varies in width from 10 to
100 fL, and in many parts contains innumerable veins of asbestos
▼arying from \ in. up to 4 in. thick, which cross and re-cross each
other in every direction and at every angle. Frequently several veins
ire found parallel to each other, being only separated by thin layers
of chrome ore. The outcrop of the veins is generally impure, contain-
ing oxide of iron, and being leathery instead of fibrous ; but a few
feet in, this disappears, and the veins are of a beautiful greenish-
vhite colonr.

llie mining of asbestos is wholly conducted by opencast or
quarrying, the drilling, blasting, and removing of the broken
rock oat of the pits to the dumps going hand in hand with the
^thering of the asbestos mineral and transport of the same to the
iiesnng establishment or cobbing shed. The average cost of this in
Quebec may be set at about l|d. per ton of rock handled for drilling,
1^ for blasting, and 12^. for removing of rock and gathering asbes-
tca in the pits. The cost of production of 1 ton of asbestos will

M 2



Digitized by VjOOQIC



1 64 ECONOMIC MINING.

naturally depend npon the nnmber of tons of rock to be removed
per ton of asbestos, or npon the richness of each indiyidnal mine.
Few places show gronnd as rich as 50 tons of rock per ton of asbestos,
while others run as high as 150 and more. One per cent, of asbestos
in the works, with a fair proportion of high grade (that is, long fibre),
is considered a profitable mine at the present price of the product
Dressing or cobbing consists in separating the asbestos fibre m>m ike
adhering rock, and grading it in cufferent qualities, followed bj pack-
ing, transport to railroad, loading, shipping, and marketing. 0^
course, the cost of cobbing varies considerably, according to the
quality of the material. While some stuff will break from the stone
very easily, other kinds require considerable labour. Some occasion-
ally may be contracted for at 12«. per ton (this, however, never in-
cludes the manipulation of cobbing stones), while other material may
cost as high as 32. and more. Including the breal^g of the cobbing
stones, 28«. may be estimated as the average cost of cobbing for a ton
of marketable matoriaL Bags and bagging, with transport to railway
and loading, may be set at an average cost of 8«. per ton ; supplies,
such as fael, steel, iron, timber, and other material and repairs, 22«.
per ton; general business expenses, management, insurance, offices,
and marketing product, 24». ^r ton : to which is to be added weai
and tear of machinery, depreciation, and interest on capital.

The grading is done in three qualities. ^' No. 1 " represents the
longest and finest fibre, from about f in. up, without discoloration
and is used chiefly for spinning purposes. ^' No. 2 " is composed of an
inferior grade of lone fibre, being either harsh and less flexible oi
discoloured (princip^ly from iron), and short but clear, — Le. un-
broken fibre from about \ in. up—and is principally used for inferioi
qualities of wick-packings and for fillers. '^ No. 3 " consists of pieces
containing still shorter fibre, to a large extent intermixed with aer
pontine and iron, and is, when cleaned from foreign substances, prin
cipally xu9ed in the manufacture of paper, cardboard, &c.

It has been noticed by Dr. Ells that the serpentine carrying tbt
marketable asbestos is generally of some shade of green on fresh frac
ture, usually a greyish green, and in which are contained numeronj
small particles of iron, both magnetic and chromic, more usually Uh
former. Serpentines that have a black, hard, chippy aspect do no
promise welL In the asbestos-bearing rock proper the veins of asbestoi
are seen, without any special arrangement, intersecting the mass of ^<
rock generally in every direction. In size they range from mere thread
sometimes close together, to a thickness of 1 to 2 in. and, very oc
casionally, 3 to 4 in. The asbestos from these larger sized veini
provided it contains no serious impurities, is classed as "No. 1," aiu
is used for spinning and weaving.

Hand labour, which is much employed in Italy, owing to the diffi
cult nature of the ground, has been largely superseded in Canada 1/
the use of steam derricks, drills worked by compressed air, and oihe
appliances.

Of Italian asbestos there are three distinct varieties, viz. the grej^
which has a long strong fibre, and is saponaceous to tiie touch ; th



Digitized by



Google



NON-METALLIFEROUS MINERALS. 165

fiony, which has a smooth, Bilky appearance, but is dry to the touch ;
and asbestos powder, which, while posseesiDg all the heat-resisting
properties of the preceding, orambles into powder when crushed.

When asbestos is first found in any new place, generally the ooly
soperficial indication is that the cracks in the rocks are filled with a
white powdery substance. When the surface is broken up, this
nsoally changes into a leathery looking material, and entering still
fiirther, the true asbestos fibre may be found.

The Italian mineral lies in beds and pockets which are mostly
reached by open quarrying, dynamite being largely used. Transport
of the crude mineral down the mountain side is often effected by
loading it in a sort of sled, about 8 cwt. at a time, which two men can
guide over the ground, whether it be snow-covered or not.



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 18 of 76)