Charles George Warnford Lock.

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

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be first process it is held in solution, though, from the presence of
be two copper chlorides, it cannot be readOy separated either as
)dide or in the metallic state. It should be farther said that by the
ae of an acid solvent for the copper oxide in the second process there
t incurred the risk of brining arsenic into solution should this be
iresent in large quantities m uie ore.

To meet uiese difiSculties a third form has been patented. By
ttacking the oxidised copper ores with a solution of common salt and
errons chloride, the latter is decomposed, and the resulting clear and
leufcral' solution, separated from ferric oxide, but containing cupric
Ad cuprous chlorides, is next treated with sulphurous acid gas to
covert the former into cuprous chloride with separation of free acid,
from such an acid cuprous solution any silver present is readily and
CHupletely separated by metallic copper, after which the whole or
nearly the whole of the dissolved copper may be precipitated by
aetallic iron, care being taken to arrest the process before the free
icid begins to attack the iron. In this way a solution is obtained
ontainiug, besides the regenerated ferrous chloride, a considerable
ODOunt of acid. This solution is used to attack a fresh portion of
oddised copper ore, which first neutralises the acid, and then decom-
>oees the ferrous chloride, the ferric oxide from which carries down
aiy arsenic that may have been previously dissolved by the acid,
rhe neutral solution of the copper salts thus obtained is then again
;reated with sulphurous acid, and the steps, as above described, are
repeated indefinitely. A necessary condition is a cheap source of bul-
phurous acid. The copper produced is very pure and the consump-
tioD of iron is small.

When oxidation is accomplished by burning, the next step is to

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434 ECONOMIC MINING.

wash out the soluble copper salts. This is often done in a nmgli wi|
by watering the ore piles, but mucli more efficiently and satisfBictanlj
by proper washing in tanks. At Bio Tinto the ore as fast as ~

and while still warm, is conveyed by tip wagons to rongh i

tanks, cemented and asphalted inside, measuring 30 ft. long, 8
wide, and 3 ft. deep, and provided with a false bottom of rougl
planks, between which the liquors can escape to small partitions ii
the comers, famished with pegged holes at various levels. The ~
wash water is left on for 24 hours, and is followed by others, up to
or 10 sometimes, at 12-hour intervals, the whole operation oooup;
7 or 8 days. The liquors vary in strength and may contain 1
250 gr. copper per gal. Weak liquors are used instead of clean wai
for washing new charges. The extraction is about 60 per ceni
Spent solutions (after precipitation of the copper) are sometimes
for further washings ; at Maidenpec they are occasionally
rated " by blowing in air, which precipitates enough iron pei
(ochre for paint-making) to cover the cost. The consumption
water is in any case large — often 10 tons per ton of ore, or 600 toi
per ton of copper produced — but far less than in the Lake mills f<
mechanical concentration. I

Precipitation. — ^The precipitation (cementation) of the dissolve
copper is almost everywhere effected by iron. In the earliest tim^
old scrap iron was used, because it was cheap and produced a dean pre
cipitate ; but this has been superseded by pig iron, especially on aooomi
of the rapid wasting of scrap by rust when exposed to the atmoapben
The metallic iron being alone useful (all the impurities in the pi
lowering the standard of the copper produced) onlv good qualities (
pig iron should be employed, graphite, silica (sand), arsenic, phosph(
rus, d^c, should in no case exceed together 6 to 8 per oent. Li Spania
practice it has been found that open-grained No. 3 grey develan
pig gives very satisfactor^r results, better even than iron made i
BUbao from ores of a superior quality ; pigs should be free fVom saa
and either be cast in half-size or be broken in two before use, so as '
facilitate handling by a single man, and increase the exposed surft
for the deposition of the copper. A very large proportion of the in
used for precipitation is wasted. Theoretically, 100 tons of cop]
should not consume more than 88 tons of iron ; while in the I
practice each ton of copper consumes for its precipitation 1^1|
iron, and in most mines quantities varying from 2 to 3 tons of ir
per ton of ea%cara or precipitate are consumed. This loss is due
the excess of ferric salts or acids in the liquors when they reaoh
iron in the tanks or canals ; it can be remedied to great extent
application of sulphurous acid vapours to the liquors of lixiviati<
or by the action of lime or limestone previous to their entrance u
the precipitating plant, always remembering that the liquors mi
be auowed a slight acidity, without which precipitation and loss
copper would take plaoe.

Two conditions specially favour precipitation : (a) the action
heat, which increases production and hastens the operation ; (6) ''
the copper liquors be run with speed over the iron, which is



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METALLIFEROUS MINERALS. 435

ected by giving a steep inoline to the canals in which the iron is
arged. The actinic action of the sun is also beneficial. The length
the precipitating canals mnst be such that the liquors escaping
«n the lower end shall be practically free from copper. The pre-
)itate obtained from the upper portion of the canals, near their
let, is much cleaner and purer than what follows, while as the out-
is neared the cascara is found to be much mixed, and coated with
dc salts of iron, arsenic, &c. It is advantageous to collect the
alities separately. The precipil|ate should be frequently and regu-
ly collected, so as to offer fresh $urfaces of iron to the action of the
pper liquors. All the precipitate must be fully and carefully
kshed in special apparatus as soon as gathered ; and as rapidly as
Bsible afterwards it must be carried to artificially heated drying
018, so as to avoid the alteration of the metallic copper into various
ts of copper by the action of air and moisture. When dry and
>], the granular copper is put into bags, while the flake copper is
M)ed in separate sacks, all kept under shelter till marketed.
The quality of the precipitate produced at Bio Tinto is : 1st class,
-95 per cent, copper ; 2na, 74-78 ; 3rd, over 60.
The whole cost of the cementation processes in Spain, including
I wasteful consumption of 1 ' 78 of iron for 1 of copper, is : —



Gonsamptionof iroD, 1*78 675*48

Wages 104-84

Storas 9-99

LooomotiTe serrioe 8*27

Maintenance of railroads 1*73

Bepairs 63-63

Add : for salt for lixiTiatlon and various other items 60 * 00



918-94



as is equal to 9Z. 2s. 7(2. per ton of fine copper. If to this sum we

i hi 3s. for the expenses of lixiviation, we have for the total cost in

ain of a ton of fine copper, by treating pyritous ores holding 0*6 to

Sper cent, copper (exclusive of the cost of mining), 142. 6s. 7d, The

t total cost has to be increased by the amount of the cost of the ore

ivered to the lixiviation groxmd, administration, freights, insurance,

• These items vary greatly, but Deby states that one of the prin-

«1 mines in the province of Huelva makes copper at the mines at a

t of 212. 7s. per ton of fine metal. The proauction of pig-iron in

i midst of the oopper region firom native ores will lead to a further

rering of cost.

it Maidenpec special precautions are taken to obtain a good pre-

ite (92 per cent, copper) by working the solutions at a tempera-

below 66^ F., keeping tiie tanks covered, and stopping the

pitation before the s<Sutions are fully exhausted, the spent

ions being used over and over again for washing the *' roast."

cost of plant and erections for treating 10,000 tons of ore per

m is given * at 40002., the wood, stone, and lime being obtained

* Brenton Symona

2 F 2

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436 ECONOMIC MINING.

on the oompany's property. The coet of treating the ore is givoi
follows : —

Mining cost per ton 2 10

Transport 1 10

Bednction chftrges 1 II

Admiuifltration 2

Total 8 7

Beckoning on obtaining 1^ per oent. oopper (from ore averagi]
about 2 per cent.), the cost of the oopper is about 852. per ton ; 7\ \
cent, ore yielding about Ij^ per cent, direct, and the remainder at t
rate of - 1 per cent, per annum, can be worked to yield copper at abo
the same cost.

At Gopperopolis, California, the saturated leach waters are a
veyed to tanks, from which they are charg^ed into a large reTolTX
drum, holding 1 6,000 gaL at a cnarge, making about 6 rev. a mimr
With the liquor a certain amount of scrap sheet iron is charged ; tJ
scrap is obtained in San Francisco and laid down at the works i
about 20 doL (4Z.) per ton. The drum is supplied with automatio vali
in the end, to allow of the escape of the hydro^n gas formed. Af)
a sufficient length of time, which depends on tbe degree of satarsti
of the copper solution, the manholes of the drum are opened, and tl
mass is discharged through wire screens placed over them into tan!
immediately bdow, the scrap iron being detained on the screei
From the tanks, the cement copper that had formed in the drum, aft
settling, is shovelled on to heated drying floors, and stirred till t|
moisture is evaporated, when it is sacked and shipped. The sm^
are double for this purpose, the inner sack being made of bm^
muslin.

Sulphuretted hydrogen has been used to precipitate oopper
sulphide, but has many disadvantages. Lime (as a cream) has I
employed where the precipitate is required for fluxing, aacid lim
cheap ; but this precipitate is rendered very low in copper by li
admixtures of lime and iron salts. Electrolytic deposition offers m
advantages, and is applied in refining and on ores which have 1]
first smelted to a matte ; but the cost of installation and neeJ
enormous power preclude at present its direct application to any 1
rich ores.

Smelting. — ^In some cases both ooncentration of the oopper
recovery of bye-products are better accomplished by raw smel
than by wet treatment. With sulphides a preliminary desulphurii
operation is introduced, indiscriminately termed ''^cinatKHi"
** roasting.*'

Heap-roastine is the cheapest way of dealing with pyritous <
and can be used for ores carrying onlv 15 per cent, sulphur by pla(
abundance of wood under the pile, and even 8 per cent, by mixing \k
sawdust, &c., with the ore. The chief drawbacks are (a) the slowi
of the process where the value of the copper must be tied up for 1
months ; (h) owing to the great size of the heaps, it must be oal
on out of doors, whereby much copper is lost by leaching, by blot
away in fine dust, and trampling under foot while loading and

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METALLIFEROUS MINERALS, 437

oading; (e) the escape of the sulpharons fames into the atmo-
pbere at a low elevation and in a concentrated form ; (<2) waste of
nlphnr.

In calcining ores containing 65 per cent, iron pyrites, a convenient
ixe for the heaps is 20 by 50 ft. and 8 to 5 ft. high. A pile of this
ize, containing perhaps 250 tons, lighted on the leeward side, well
OTered with fines, and carefully watched, will bum about 9 weeks
nd will furnish a product containing 6 to 10 percent, sulphur. Some
reeks before the burning is completed in the very heart of the pile,
vo-thirds of its contents can be removed without interfering with
be process. As the whole advantage of heap-roasting lies in its
xtreme cheapness (l9. 6c{. to 3«. 6(2. a ton), every precaution should
e taken to- save expense in handling the ore, e. g. the ore-car from
be mine or dressing-house can be brought on a trestle over a long
ine of roast heaps, while the track which leads to the furnaces runs
trallel to these and on a level below the ground. Shaping and
ring the pile need experience. The ore should be broken to suitable
ize, hand labour, when not exceeding 2«. per ton, beiug preferable to
jaw-crusher, owing to the smaller proportion of fines produced — say
2 per cent, as against 20. It is advantageous to pass the broken ore
ver 2 riddles, thus making 3 sizes : the largest,. constituting perhaps
per cent, of the whole, should be dumped directly upon the wood,
)nning the main body of the heap, which should be rectangular^ with
traight edges and square comers ; the next size, called *' ragging,"
bout 20 per cent., is that portion which has been separated by passing
biOQgh a |-in. riddle, and should be built up on the outside of the
cap with great care, forming a layer 8 in. thick at bottom, and
tpering up to about 2 in. at top ; the remaining 10 per cent, coubists
f fines, separated from the *' ragging " by a 6-mesh screen, and should
e transported to the heap and arranged in piles at a convenient
boveUing distance, none being placed upon the heap until the wood
A8 been kindled, and the strong fumes of sulphurous acid show that
be ore is thoroughly ignited. Then the lower portion of the pile is
Apidly and evenly covered with the fines, leaving the top still exposed
mtil the heat becomes so great as to warn the attendant that the
entral portion of the ore is in danger of melting. The top is then
OTered thinly, and for the first few days careful watching is required
keep the combustion regular and gradually increase the protecting
fcjer uf fines. By the end of the fourtii day the pile should be burning
lowly and evenly, smoking slightly from its entire surface, and no
totter in one part than another. From this time until the process is
■omplete, only careful watching is required ; a few shovels of fines
cattered in one place or another, as the draught may indicate, com-
ttises all the labour necessary for roasting 200-300 tons of ore.
^bont ^j^ cord of wood and \ day's labour per ton of ore will build
uid bum the heap and load the roasted ore into cars ready for the
melter.

To roast matte in heaps calls for some slight modifications. Owing
^ tbe extreme fusibility of this material, and the large amount of
^pper tied up, the heaps contain only 30 to 40 tons. As matte con-
^^ much less sulphur than ore, it is necessary to use a thicker bed



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438 ECONOMIC MINING.

of wood, and after the first burning, which should be completed i
5 days, the pile should be turned on to a new layer of wood, breakin
all the clinkers to egg size again, and piling the ontside of the hea]
which will be found but slightly affected by the first burning, directl
upon the new bed of wood; tne second burning lasts about 5 day
and a third is usually completed in 4. Matte containing 30 per o»i
copper, after 3 thorough burnings, should yield white metal (70 pt
oenO when smelted rapidly through a cupola furnace.

Eeruel-roastine is a peculiar concentration of copper which alwaj
takes place when low-grade ores are calcined without &ee aooess <
air, even balls of fines yielding kernels in some instances. It i
occasionally ayailed of. Thus at F5ldal, Norway, a pyritea ooi
taining 45 to 48 per cent, sulphur, with 2 to 2^ per cent, copper, is
3 to 6 per cent, insoluble matter, is burned in heaps in the open a
as slowly as possible, 3 to 4 months beine usually required. Afh
roasting, the kernels, containing, unwashed, only about 3f per cen
copper, are first washed so as to remoye the soluble sulphates, whe
the kernels are easily picked out firom the dried ore, and, as they can
much less adherinj^ oxide scale, assay 12 and even 15^ per cent. Tl
nett proportion of copper obtained in the kernel is only |— | of ti
total copper contents of the ore ; about \ the remainder is recoyerc
by washing, and the rest is lost in the burnt residues. The tot]
cost of roasting and hand-picking per ton of crude p^tes is aboi
1«. Zd. At Agordo, Italy, kernel-roasting on chalcopynte mixed wi^
iron bisulphide, carrying 1^2 per cent, copper, gaye kernels asaayiij
33 per cent. ; women and children remoye the crusts and wash ^
kemelft, producing a good liquor for precipitation, while the dea
kernels go to the blast furnace. Peters tried a similar method d
1^ per cent, pyritous ore at Strafford, Vermont, but failed becaui
only a small proportion of the kernels were so roasted as to admit \
ready separation of the iron oxide crust, for which purpose breaka
and jigs haye preyed quite unsuitable.

Stall-roasting being a little more costly than heap-roas^g i
generally appli^ only to matte, but is equally suited to raw ^
and reduces the risk of loss by storms, Ac, besides working well ool
low percentage of sulphur. It is always best to build stalls in blod
or rows of 4 to 12 or more, as a large saying in both brick and ina
work is effected, and the heat retained in so large a mass of maaoni^
and communicated to continuous stalls, is highly adyantageous \
the roasting process. It is a great mistake to buUd stalls without '
brick arch oyer them, as the arch assists in retaining the heat and i
forcing the smoke to ascend to the flue. The point of greatest '
portance in construction, howeyer, is to secure a proper foundal
for the brickwork, and to tie the furnaces with strong bucksta'
and |-in. iron rods, although when the arches are built as sei
circles, which is usually the case, there is do lateral thrust. Still
expansion from heat would soon destroy the brickwork if it
not properly tied, and any undue saying in this direction is al
mistaken economy. Ordinary dimensions of stalls are : — Width, 5
depth, 6 ft. ; depth of ashpit, 1 ft. 6 in. ; height from grate to spri
of arch, 4 ft. 8 in. ; thickness of main walls, 1 ft. 6 in.; thicknee*!

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METALLIFEROUS MINERALS, 439

liviBion walls, 1 ft. A stall of this size will hold 5 tons of white
netal; an increase of capacity within reasonable limits would lead
4) a decided saving in ^el and labonr, and not be detrimental to the
)rooe88.

After thoroughly clearing the grate-bars from any fused matte
rom a previous burning, and plastering the walls carefully, wherever
they show signs of wear, with a thick mortar of burned and raw
^lay in equal parts, about 10 cub. ft of bard wood, in ordinary 4-ft.
migths, is arranged upon the grate-bars as evenly as possible, and
Jl spaces between the sticks are chinked with small stuff and split
ogs. Hard wood is much preferable to soft, and a thicker layer of
irood should be placed at the front and sides than in the middle. As
h& stall is gradually filled, pack with chips and brush or small coal
x>th on the front and sides, nearly to the top. The front is built up
ooeely with fire-brick placed on edge. The matte should be broken
o the size of the fist or smaller ; and to facilitate this operation the
nmaoe from which it is produced should be tapped on to thick iron
))ate8 in such a manner that the matte forms a layer not more than
\ in. thick, which will be brittle, and can be spalled by boys at small
txpense.

In Japan, stalls for calcination are built of rough stonework,
HrithoQt any chimney, and are usually protected from the weather.

The cost of stall-roasting at Boston is given * at 9{i. per ton of
ire, or 2«. 6<2. per ton of matte, labour accounting for f .

Eiln-roasting is of several kinds. When the sulphurous acid
l^nerated is to be utilised for making sulphuric acid, special forms
)i kiln are employed, which will be found described in detail in the
author's work on sulphuric acid (1872). When the ore is in a dusty
condition, choice lies between the old-fashioned brick reverberatory,
vertical sbelf furnaces, automatic revolving roasters, or hearths with
automatic rabblers.

Beverberatories should not be less than 50-60 ft. long, and the
nrorking doors should be provided with rollers for carrying the
Rabbling paddle, and not be situated opposite each other. The bed,
but not tiie arched roof, should rise toward the charging end. A
Furnace 75 ft. long and 17 ft. outside diam. will require 40,000 common
brick (in addition to stone foundation), 8000 fire-brick for lining,
ind 5 casks fire-clay, besides 5-10 tons iron tie-bars, &c. They are
being replaced by mechanical furnaces where labour is dear, but they
remain unequalled ibr producing a really *' sweet *' roast

Of vertical shelf furnaces the best is Fauvers.t

Revolving furnaces bear a general resemblance to the Ozland cal-
inner (p. 157) ; they are chieflv the Howell White, in which the period
of roasting is controlled by the speed of rotation ; and the Brtickner,
in which the ends are contracted, so that the charge can be retained
as long as debired. A medium-sized Bruckner measures 18 ft. long
by 7 ft. diam., takes 6-8 tons at a charge, and weighs about 14 tons.
The Anaconda Work^t use 136 of them ; some are gas-fired, 22 ft. lung,
and roast a 16-ton charge (from 35 down to 3]^ per cent, sulphur) in

• T. Egleuton, ** Point Shirley Copper Works," School Mines Qly.
t C. G. W. Lock, * Miners' Pocket-Book,' p. 369.



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440 ECONOMIC MINING.

36 hours ; they are arranged in sets of 6, and are charged from \
trayelling hopper. Ordinarily oxidation is far from being completely
attained in a Briickner owiog to insufficient air-snpply, and to over-
come this E. M. Clark has added a water-jacketed air pipe fed bj %
blast, which hastens the roasting, but is very difficult to keep in
repair. The best results obtained with Brfickners appear to be:—
12 tons of ore to each man employed, consuming 167 lb. coal per t(m,
reducing the sulphur from 40 per cent to 7 per cent. ; ore at least
as fine as \ in. The Omaha and Oi'ant Company use 3 famioa,
requiring 4 men per 24 hours, to roast 20 tons from 40 per cent down
to 2 per cent, sulphur, consuming 1^ tons coal per charge of 12 toiu:
they estimate the cost at 90 a (3«. 9c2.) per ton, as against 2^ duL
(10«.) in reverberatories.

Of hearths with mechanical rabblers the 0*Hara is the earlicft
type, succeeded by the Brown-Allen, the Brown, and the Pearce.
The Brown-Allen is made with stirrer carriages and a mechanism
which moves the ploughs or scrapers outside the roasting hearth. A
furnace of average size has a roasting hearth 8 ft. wide in the detr
and about 90 It. long, or, including the lower stage, a condniurag
hearth 8 ft wide by 180 ft. long. To take care of a furnace, 1 man
only on a shift is required, the whole action being automatia The
cost of roasting, while varying somewhat, according to the cost of
material and labour, will average between Z». and 4«. per ton, indudisg
repairs. The capacity of such a furnace is about 35 tons per day, and
the results are substantially as follows : When the ore is cnubed
reasonably fine, and 20 tons per day are treated, the calcined ore
averages 1 to 2 per cent sulphur; 25 to 28 tons a day, 3 to 3*5 per
cent; 30 to 35 tons, 3*5 to 5 per cent; and if the fumaoe is pushed
to its maximum capacity, 85 to 40 tons of roasted ore, they carry m
average of about 6 per cent. The power required is about 2^ h.p. for
an average of 35 tons per day. The cost of construction varies some-
what with local conditions, but runs from 1600i. to 2400i. per fumaoe,
including the necessary stacks. The Brown difiers chieflv in having
the hearth horseshoe-shaped, with an interval for cooling the rabblers,
thus lengthening their lives. The cost of construction is much kw
than that of the preceding.

The Pearce turret furnace (Fig. 118) consists of an ordio&ry
reverberatorjr hearth a built in a circular form, the centre of the circle
being occupied by the central column 6 supporting the radiating ani»
c, which carry the rabble blades d. Ore is fed mechanically at e, «ri
after traversing the whole circle of the hearth, at any desired speed,
falls by gravity into the pit/. Air is forced through the pipe arms c
and discharged against the rabble blades d, performing the d<»il^
duty of cooling the ironwork and furnishing neated air to the rotst
Two or more automatically fed stepgrate &*eplaoes g supply foel
The space beneath the hearth is utilised as a dust flue leading tc
chamber A. The cost of a 36 ft Pearce furnace is 4000 del. (800i.) fe
ironwork and erection (Denver figures), with 1500 dol. (3001.) royiltr.
Repairs are confined to renewal of rabble blades every 3-4 weeii
One man per shiit can feed fuel and ore, remove roasted ore,
attend to machinery of one furnace. Air supply can be controlled



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METALLIFEROUS MINERALS,



441



only throngh arms c but also at inlets t and mouths ib. The degree of
roasting can be adjusted to any desired point, from a dead roast to



Online LibraryCharles George Warnford LockEconomic mining: a practical handbook for the miner, the metallurgist and ... → online text (page 49 of 76)