(1) A marked saving is effected in operating the
multiple system (this has been estimated by Barnett,
at as much as $2 per ton of refined metal—8s. 4d.);
(2) A greater efficiency is obtained, the tank
efficiency of the multiple system being 95 per
cent. compared with 90 per cent. for the series-system;
(3) Less copper is held up in the multiple system,
since less anode copper is required, under like
conditions as regards cathode surface and current
density.
OUTLINE OF THE PROCESS.—The electro-refining industry is a highly
specialised one, and the methods of putting the comparatively simple
underlying principles into practical operation have assumed great
complexity and diversity in detail, concerning which Ulke has collected
and published much valuable information.
The following details have more particular reference to the
multiple system of working, as being the most representative of the
electro-refining methods in general use.
The outlines of the process constitute the passing of direct electric
current through tanks containing acidified solutions of copper
sulphate, employing plates of crude copper as anodes, and depositing
pure metal upon cathode-plates of specially-refined thin sheets of pure
copper. The precious metals and most of the impurities of the anode
metal are liberated as small insoluble particles which gradually settle
to the bottom of the tanks in the form of mud, soluble constituents,
such as iron and zinc, first passing into solution.
_General Conditions—Anodes._—The usual dimensions of the anode-plates
are 3 feet high by 2 feet 6 inches wide and about 2 inches thick; they
are generally cast with lugs, so as to allow of suspension in the
tanks. The anode metal is usually brought by a preparatory operation,
to as high a state of purity as is economically practicable—
(_a_) In order to obtain smooth and sound electrodes.
(_b_) To ensure better working in the tanks.
(_a_) The necessity for the employment of solid and even anodes
has already been indicated; it allows of closer suspension of the
electrodes, lessens the liability of sprouting and unevenness on the
deposits and the irregular wear and breaking up of the anode-plates
before they are sufficiently worn away.
(_b_) The more free the metal is from iron, sulphur, zinc, nickel,
etc., the purer remains the electrolyte, since these elements pass into
solution at a greater speed than does the copper itself, and, gradually
concentrating in the tank liquors, render them more and more impure—the
purity of the metal deposited at the cathode being in consequence
decreased. The preliminary refining and bringing up to pitch of the
metal before casting into anodes, as already described, thus has for
its object the preparation of electrodes in a suitable mechanical as
well as chemical condition. The copper content is rarely less than
98 per cent. and is often more than 99 per cent. The gold and silver
contents are not affected by this preliminary treatment, nor are, to
any great extent, the proportions of arsenic, antimony, bismuth, etc.
The size of the anode-plate varies somewhat at different refineries,
the usual standard dimensions being indicated above; the size depends
to a large extent upon the facilities for handling the electrodes and
on the circuit system operated. There is a tendency at several works
possessing suitable facilities, to increase the size of the electrodes.
_The Cathodes._—The copper is deposited from the electrolyte upon
cathode sheets, which are usually thin plates of pure copper
corresponding in size to the anodes. As these sheets cannot be
conveniently provided with suitable lugs for suspension, they are
usually made of somewhat greater length than the anodes, so as to allow
of bending over the cross-conductors; otherwise they are furnished with
metallic clips for attachment to these bars.
These cathode sheets are prepared by depositing layers of pure metal
upon plates of refined copper of suitable surface dimensions and of
about ¼ inch thickness. Each side of these plates, which are specially
smoothed, is first slightly oiled so as to allow of the subsequent
convenient stripping of the sheet when made, and it is then well
coated with graphite in order to present a conducting surface on
which deposition can proceed. The cathode-sheets are deposited either
in the regular tanks of the refinery or in vats specially devoted to
the purpose, using in that case, pure electrolyte, and working in the
usual manner. On attaining a thickness of about 1/25 inch, the sheet is
stripped off, cleaned and clipped ready for use as a cathode.
_The Electrolyte._—The electrolyte is essentially an acid solution of
copper sulphate. The average proportions are from 15 to 20 per cent.
of copper sulphate crystals, and from 5 to 10 per cent. of sulphuric
acid—the usual density of the solution ranging from 1·12 to 1·25. The
liquid under ordinary conditions of working, remains reasonably pure
for a considerable time. It tends, however, to decrease in acidity and
to increase in copper contents, partly owing to the presence of cuprous
oxide in the metal, which passes into solution independently of the
indirect transference of metallic copper from anode to cathode. The
composition of the tank liquors must, therefore, be frequently checked.
The gold and silver values do not pass into solution under ordinary
working conditions, and the addition of a small quantity of common-salt
or of hydrochloric acid to the vat effectually prevents any silver from
remaining in solution in the liquors.
A considerable proportion of the arsenic in the tough-pitch
anode-copper, existing as arsenate, is deposited with the mud residues,
it being insoluble and non-conducting. Arsenic in a reduced condition
is, however, soluble, and may gradually concentrate in the liquors
and contaminate the cathode copper, unless suitable precautions are
taken. Some of the reduced arsenic, moreover, tends to produce a slimy
arsenite of copper, which, though insoluble, exists in a colloidal
non-settling form. Addition of ammonium sulphate to the electrolyte
prevents this formation, whilst combined aëration and heating promote
the precipitation of arsenic as insoluble arsenates which settle with
the tank slimes.
Some of the antimony and bismuth tend to first pass into solution, but
for the most part they are precipitated as insoluble basic salts. Under
suitable conditions with respect to the acidity and copper contents
of the electrolyte, there is little tendency for deposition of these
impurities with the copper, but deviation from the correct composition
is liable to cause contamination of the deposited metal. These
impurities, when in solution, tend to be oxidised by aëration, and this
operation greatly encourages their precipitation with the mud.
Iron readily dissolves in the electrolyte, forming soluble ferrous
sulphate which tends to gradually accumulate in the solution. This
contamination spoils the quality of the deposited metal, and interferes
with the process of deposition, decreasing the conductivity of the
bath and thus necessitating higher voltage. Aëration of the solution,
especially when warmed, leads to the formation of basic ferric
sulphates which are insoluble, and which therefore accumulate at the
bottom of the tank.
Selenium and tellurium, which when present most probably exist as
insoluble selenides and tellurides of copper or silver, are also
precipitated, and thus do not find their way into the deposited metal.
The copper itself is deposited from the electrolyte on to the
cathode-sheets by the action of the current, whilst at the anodes,
the metal passes into solution, and the other constituents are
either dissolved or precipitated. It follows that in an undisturbed
solution, the liquid near to the cathode becomes gradually impoverished
in copper, resulting in a decrease in the rate of deposition and
necessitating greater electrical pressure, whilst in the neighbourhood
of the anode, the liquor is proportionately stronger in copper and
less acid in character. Should these conditions continue to any great
extent, the working of the bath is seriously interfered with, since
diffusion proceeds too slowly for uniformity to be restored, and
in order to secure uniform composition of the electrolyte, it must
be maintained in gentle motion by some system of circulation. This
agitation and mixing is assisted by the aëration of the bath for the
purpose of hastening the oxidation and precipitation of several of the
impurities—this being effected by blowing through the liquid a gentle
supply of air.
_Temperature._—The electrical resistance of the solution decreases as
the temperature rises, and in practice the bath is maintained at a
uniform temperature of 45° to 50° C. By this means a useful increase in
conductivity is obtained, the strength of the deposited copper being at
the same time greatly augmented.
_Electrical Conditions._—The electrical factors which mainly control
the working of the electrolytic process are those of—
(_a_) Current density.
(_b_) Voltage.
(_a_) _Current Density._—The quantity of metal deposited from an
electrolyte is proportional to the current which passes, and to the
electro-chemical equivalent of the particular metal. Thus a current of
one ampere will deposit from copper sulphate solution, 1·1832 grammes
of copper per hour, and the total quantity deposited in any given
time is determined by the product of the current, the time, and this
electro-chemical equivalent (which is determined experimentally).
In practical operation, a factor which is amongst the most important
of those governing the working of a plant, is the current density,
or current per unit of area of depositing surface, since from this
factor the rate of deposition upon the cathode plates is determined,
and from it the power requirements, accommodation, etc., for the
plant are fixed. The current density is subject to wide variation,
but, as a general rule, it ranges from 8 to 18 amperes per square
foot of plate-area. Its value is largely dependent upon the speed of
working, the cost of power, and the composition of the anode metal, the
electrolyte and the desired product, etc.
In general, high current-density possesses many advantages, resulting
from the fact that it occasions a more rapid deposition. It causes a
proportionately greater output, consequently the stock of metal held
back in the tanks is reduced, and hence there is less capital locked
up in the form of metal undergoing treatment, and less plant and
accommodation are required for the same output.
The current density permissible is, however, limited by the composition
of the electrodes and the solution. High current-density causes rapid
dissolution of the anodes, and if the plates are not particularly
free from impurity, the electrolyte rapidly becomes contaminated,
since its dissolving power on the impurities becomes greater with
increased electrolytic action, and this affords less opportunity for
the precipitation and settling of the injurious constituents. In
consequence, the cathode copper is contaminated through this mechanical
inclusion of impurities, whilst electro-deposition of some of these
materials may also be encouraged. The presence of much silver in the
anodes causes the rapid breaking-up of the plates, especially if the
current density be high, and thus the separation of the values in the
slimes is not so efficiently managed. With high values in the anode
copper, it is necessary to reduce the current density to 8 or 10
amperes per square foot, whereas with purer metal a density of as much
as 16 to 20 may be conveniently employed.
(_b_) _Voltage._—Electrical pressure is required in order to force the
depositing current through the electrolyte against the resistances in
the circuit. The voltage required depends upon the current density,
the composition and temperature of the electrolyte, the composition
of the anodes, and also upon the general conditions of working. These
being constant, the voltage necessary is largely a factor of the number
of electrodes in series in the tank and of the distance apart of the
plates. Under ordinary circumstances this voltage varies from 0·1 to
about 0·3 volt. High voltage is to be avoided, owing to the danger of
short-circuiting, especially in cases where the accumulation of mud in
the tanks, or the impregnation of metallic salts in the tank walls, or
the growth of excrescences upon the plates, lead to the passage of the
current through these conductors rather than through the electrolyte
solution itself. Short circuiting naturally diminishes the output of
the plant.
These electrical factors which form the basis of the power requirements
of the refinery, call for careful observation during the progress
of the operations in order to ensure successful working and a high
efficiency of the plant.
The current from the dynamos is brought by heavy leads and is
distributed through the sets of tanks in the manner best suited to the
installation. At one of the newer works, dynamos producing about 6,000
amperes at 120 to 150 volts, supply sufficient current to operate a set
of 400 vats working on the lines just indicated.
[Illustration: Fig. 73.—Indicating Methods of Suspending and Connecting
Electrodes (Perth Amboy, N.J.).]
_The Depositing Tanks._—The tanks are usually constructed of wood,
such as strong pitch pine, and they are lead-lined. The cross-section
is usually such as will allow a space of about 3 inches between the
edges of the electrode and the wall, and a 6-inch space from the
tank-bottom to the lower edge of the plate. The length of the vats
varies considerably, according to the desired output and to convenience
of working—10 to 15 feet being average dimensions. This size of vat
will hold 15 to 25 anodes together with a corresponding number of
cathodes (16 to 26). The tanks are arranged across the building in a
number of rows which are usually stepped down in stages of about 2 to
3 inches each, so as to assist as much as possible the circulation of
the electrolyte through the system by gravity, and the vats are set in
pairs with aisle-ways between (see Fig. 72).
Leads run along each side of the tank, the current being conveyed to
all the anodes at once by resting one lug of each plate upon the lead
which runs along one side of the tank, carefully insulating the other
lug from the conductor situated at the opposite side, this being used
for connecting up the cathodes. The cathode-sheets are suspended from
metallic cross-bars, which rest upon their own conducting lead, and
are carefully insulated from the anode lead at the other side. The
solutions are heated by means of steam coils.
_Distribution of the Electrolyte._—The necessary circulation of the
electrolyte is effected as much as possible by the natural action of
gravity. The tanks of the top row in the depositing-house receive a
constant supply of fresh solution from upper distributing vats, whilst
old electrolyte is drawn off from near the bottom of the tanks, and
flows over to those on the next and lower level. Fresh solution thus
enters at the top of the tank, old solution is drawn off from below,
and thus a uniform density and composition is maintained. From the
tanks situated at the lowest level, the solution passes to a well, and
from there is pumped up to the store-tanks or, when necessary, to the
purifying tanks; air-pressure pumps being often employed for this work.
[Illustration: Fig. 74.—Indicating Connections for Circulation of
Electrolyte (Barnett).]
In course of time—and under the modern system of working with
moderately pure anodes, this period is of considerable duration—the
gradual accumulation in the electrolyte, of the small quantities of
impurity which are dissolved from the anode, may render the liquid so
impure, that a danger arises of contamination of the cathode copper
to such a degree that it becomes unfit for conductivity work. It then
becomes necessary to purify the solution. In present-day practice, this
continued accumulation of impurity in the electrolyte is prevented
by continuously withdrawing, for separate purification, a certain
proportion of the electrolyte from the circuit—replacing it by a fresh
supply of pure solution from the store-tanks. Constant regeneration,
purification and circulation are thus effected, whilst uniform
composition is maintained.
After considerable use, the electrolyte solution gradually tends to
increase in copper contents, and the first stages in the scheme of
treatment for the old solution is to recover this excess of copper,
which is effected in tanks known as “liberating tanks.” These are
similar in general features to the refining vats, except that lead
plates are employed instead of the copper anodes, so that the excess
metal is deposited without any addition of copper being made to the
solution, from the anodes. In due course, the desired composition in
the electrolyte is once more attained.
When the solutions have become too impure for further use in the tanks,
the bulk of the copper sulphate is recovered by evaporation in large
pans, followed by crystallisation in somewhat shallow vats of large
dimensions. The crude blue vitriol is further purified by repeated
crystallisation, and any copper which still remains in the solution is
then precipitated on scrap iron, the cement copper being worked through
the furnaces again. Excess acid is also often recovered on further
evaporation of the liquors, and is employed in the subsequent treatment
of the slimes.
_Working._—In the large modern refineries, the anodes are carried to
and from the tank-house by cars, and at the tank-room are suspended
from frames which are conveyed over the baths by means of overhead
electrical cranes of about 10 tons lifting capacity. These rectangular
frames correspond in size to the dimensions of the tanks, and are
constructed of steel girders. Under the longer sides of this frame
a series of hooks project, upon which the lugs of the anodes rest,
and the hooks are placed at distances corresponding to the eventual
position of the plates in the tank, so that the whole series of anodes
can be dumped into position at one operation.
The cathodes are placed in a second rack, and likewise brought into
position, between the anodes. The solution is then turned into the
tank, the current started, and the refining proceeds, with a steady
flow of liquid circulating through the system. The operations of
changing electrodes, cleaning and reloading occupy about one hour,
and, but for this manipulation, the process under normal working is
continuous. In ordinary practice, about 20 to 25 lbs. of copper are
deposited daily on each cathode. Constant examination is made as to the
electrical conditions, and the composition, temperature, and density of
the solutions.
The anodes usually remain in the bath for a period of about six weeks,
and they are then removed from the tank, scrubbed, and sent back to the
furnaces to be remelted and re-cast into fresh anodes, the quantity of
such anode scrap under good working conditions amounting to about 9 or
10 per cent. of the original metal.
[Illustration: Fig. 75.—Tank-house, showing Anode Crane (Ulke).]
The cathodes remain in the tanks for about one week, by which time a
deposit of from 150 to 170 lbs. of pure metal has been obtained upon
each. The practice of frequently replacing the cathodes possesses,
among other advantages, those of maintaining a more even current
density over the plates, of preventing the growth of excrescences and
the irregular dissolution of the anodes, and of lessening the danger
of breakdown of the somewhat slenderly suspended cathodes, by putting
less weight on the supports. The removal in one operation of the entire
batch of cathodes from the bath is effected by means of the suspended
hook-frame, as employed in charging. The plates are rinsed, the top
edges are cut off and returned with the anode scrap, whilst the pure
electrolytic copper passes to the refining and casting furnaces, where
it is prepared for the market.
_Collection of the Slimes._—Depending upon the working conditions
of the refinery, but usually at intervals of three months, the
precipitated slimes are collected and the tanks are cleaned out. The
quantity of slime deposited is generally not very large, from 15 to 25
lbs. per tank being a not unusual yield. The current and the supply
of solution are cut off, the plates removed, the contents of the tank
allowed to settle, the liquid siphoned off to within about 6 inches
of the bottom, and the residues are swilled out through a trap at the
bottom of the tank. The sludge passes through a sieve that separates
the lumps of anode copper which have broken off and fallen to the
bottom of the tank, the slime then passes to the special refinery for
treatment. The processes adopted for recovering the gold and silver
from this residue are highly specialised, and belong properly to the
technology of refining of the precious metals.
_Modifications of Electrolytic Refining._—Great success has not yet
attended the attempts which have been made to employ copper matte
in the form of anodes in electrolytic refining processes, and the
method is not in operation at any of the great modern works. Marchésé,
Hoepfner, Siemens-Halske, Keith, and others have introduced processes,
but their practical operation is attended with very great difficulty
and but little commercial success. Matte is exceedingly brittle and it
readily breaks up, it is a bad conductor and necessitates the use of
high voltage, the solutions become very foul, and the processes require
very special apparatus and equipment.
Methods for the production, by electrolytic processes, of pure
copper in forms ready for service, such as wires or tubes, have been
introduced successfully by a number of workers, including Elmore,
Thomerson, and Cowper-Coles. Several of these methods are now in
apparently successful commercial operation, and the published results
of the working of the processes and of tests on the deposited materials
offer considerable promise for their future industrial application
for special purposes, if not for general use. The attaining of the
necessary compactness, toughness, and strength of the metallic product
is aided by the employment of pressure during deposition, as by
burnishers, or by very rapid rotation of the depositing surfaces in the
solutions. Details of these processes and products may be found from
the references subsequently given.
=Bringing up to Pitch and Casting the Merchant Copper.=—The final
stages in the smelting process from ore to market-metal are those of
“fining,” toughening, and casting the cathode copper, the object of
these operations being to impart to the metal the chemical composition
and mechanical and physical properties which are required in order to
fit it for the market, and also to prepare it into a suitable form for
service. In addition to cathode copper, other forms of the metal, if of
suitable composition, are also treated with this object.
For conductivity copper, however, these final operations are conducted
on metal from which practically all the impurities have been removed,
but which is not sufficiently tough and homogeneous or which is not in
a suitable shape for immediate industrial use. The toughening operation
consists almost entirely of adjusting the percentage of oxides in
the metal, partly in order to overcome the influence of any traces
of injurious impurity that might remain, but mainly to exercise the
functions previously indicated, of imparting by its more or less direct
action upon the metal, a definite toughening and strengthening effect.
The mechanism of the action is not perfectly understood, but the recent
work referred to in Lecture II., p. 28, affords useful evidence as to
its possible mode of action.
The actual refining operation and the furnace employed for the process
are exactly similar to those used in preparing the metal to ensure the
casting of sound ingots, as already described. The operations consist
of a preliminary aëration, by means of which any oxidisable impurity
still remaining in the metal is oxidised out, mainly through the action
of copper oxide which is formed during the process in some considerable
excess.
After the copper has become “dry” or over-oxidised, which condition
is characterised by brittleness, depressed surface, and brick-like
purple-red fracture of the metal, it is reduced by poling and timbering
operations to a definite point, viz.: until a sample ingot of the
metal indicates a maximum of toughness, accompanied by level surface
and bright salmon-coloured silky fracture—it is then of “tough-pitch”
quality.
_The furnace_ employed for the refining has already been described.
One of the main features in which it differs from the ordinary modern
reverberatory smelting-furnace is that owing to the exceedingly high
heat-conducting power of metallic copper, and to the absence of an
insulating layer of non-conducting slag, there is little danger of
much chilling action occurring on the hearth of the furnace; the
temperature may, indeed, often become too high rather than too low.
In consequence, it is not so usual to construct the furnace with a
very massive hearth foundation as for smelting, but to build it upon
a vault or upon a series of piers. With this type of foundation, the
very considerable, but practically unavoidable, absorption of metal
in the hearth-material is reduced to a minimum. It is usual to work
a charge consisting of scrap and oxide in the furnace before the
regular smelting campaign begins in order to “season” the hearth.
This procedure allows the primary absorption of copper by the
hearth-material, and assists its consolidation, whilst the action of
the oxide promotes a surface glazing which lessens the tendency for
further absorption of copper, and gives a good surface to the working
bed. As has been already stated, the hearth is generally built of
brickwork rather than of sand. The furnace is constructed to hold from
80 up to 200 tons of metal. The method of working differs mainly from
that previously described, in that instead of pouring molten copper
into the furnace, as is usual with converter-metal, the cathode plates
must be charged in a different manner.
In order to deal with such a large quantity of charge in this bulky
form, without occupying so much time as to make the whole operation
too protracted, it is usual to employ some form of charging machine
rather than to use hand labour for the operation. In some cases a small
melting furnace is employed solely for the purpose of preparing the
metal in a molten form for feeding into the refining furnace. The type
of cathode-charger most used is very similar in operation to the Welman
charger for steel furnaces, and by its means, 100 tons of material can
be charged per hour.
_Operation._—The refining and toughening process is conducted in the
six stages of:—
(_a_) Charging.
(_b_) Melting.
(_c_) Skimming.
(_d_) Oxidation, by aëration.
(_e_) Reduction, by poling.
(_f_) Casting.
(_a_) The charging is sometimes conducted in stages, this being indeed
unavoidable when very large quantities of material are worked with, the
bulk of which, when solid, would more than fill the furnace. Two-thirds
or three-quarters of the material may be put in at first and just
melted down slowly, after which the remainder is added.
Owing to the not infrequent presence of sulphur in the furnace coals,
and to its ready affinity for copper, resulting in undesirable
consequences for the commercial metal, contamination by this element
is usually prevented, as much as possible, by giving to the cathodes a
wash of lime previous to charging.
(_b_) The melting is generally conducted somewhat slowly, so as to
allow some oxidation of the metal during this stage, which may occupy
some twelve hours. Skimming of slag as it forms, and subsequent blowing
of the copper towards the end of the melting stage are frequently
resorted to.
(_c_) The slag which accumulates, sometimes in considerable quantity,
is skimmed off as occasion requires. When converter metal of such
purity as not to need electrolytic refining is treated directly in
the furnace, much of this slag is converter-slag introduced from the
ladle, and requires to be skimmed off at an early stage. In the usual
process of melting cathode-copper, slag is produced from the last
traces of iron which may have remained in the metal. In order to render
it sufficiently viscid to be pulled out by the skimmer, ashes from the
fire-grate are thrown upon and rabbled into the slag. This skimming may
continue for some time, and a very rich coppery slag is pulled off,
from which the metal values are subsequently recovered.
(_d_) The oxidation of the small quantities of impurity still remaining
in the metal is completed by the operation of airing, as already
described, and the action is continued in order to produce a small
excess of oxide until the copper is “dry.” The time occupied for this
airing is now not very protracted, since most of the impurities have
been previously removed from the metal.
(_e_) The copper is then brought up to pitch by “poling” in the
manner previously indicated, except that at this final stage, the
testing of the metal and the adjusting of the oxygen proportion are
conducted with much greater precision than was necessary for the simple
production of the sound anode plates. In the present instance, the
character of the metal and its value as a commercial article largely
depend upon the care and accuracy with which the correct “pitch” is
reached and is maintained in the bath during the entire period of
casting of the metal. The poling for the “shaking out” of the gases is
rarely necessary with cathode metal, and the addition of the cover of
carbonaceous material for the purpose of effecting the reduction of the
oxides to the desired extent, is made either at the commencement of
poling or else shortly afterwards. After some time, a series of small
samples is taken at intervals, by means of ladles, and the surface of
the ingot is examined. The depression characteristic of dry copper
gradually becomes less marked, the brick-like fracture appears finer
and finer until it becomes silky, whilst the colour eventually turns to
a very delicate salmon-pink. Meanwhile the mechanical properties have
gradually improved, signs of brittleness disappear, and somewhat larger
samples of the metal, which are now taken and tested, are characterised
by a very marked toughness and strength. This is the moment at which
the poling must cease. The residual copper-oxide has now reached the
proportion which was necessary for the imparting of the best mechanical
properties, and the metal is tough-pitch. The skill of the workman is
now exercised to the highest degree, in maintaining the metal in this
condition during the whole of the subsequent casting period. Oxidation
must be avoided in order to prevent a reversion to dry copper, whilst
any further reducing action removes some of the necessary oxide, and
results in “over-poling.” The metal would then become brittle again,
coarsely fibrous and possibly somewhat spongy in fracture and very pale
in colour, whilst in setting it would show a ridge upon the surface.
In that case it would be necessary to “air” the metal again until it
became dry, and then to pole it back to the “tough-pitch” stage.
The copper, when of correct pitch, is therefore removed from the
furnace and cast at once; this being readily conducted through the
tapping slot, the level of which is gradually lowered. The metal then
flows down the spout to the ladle, and is poured into the moulds
attached to some form of mechanical casting machine; the ingots being
finally dropped into a water-bosh, weighed, sampled, and stacked, and
are then in a condition ready for the market.
Phosphorus is sometimes employed for giving soundness to the
castings, being added to the bath in small quantities in the form
of phosphor-copper containing about 10 per cent. of the non-metal.
Although very little of this phosphorus is retained by the metal, being
mostly eliminated as oxide, special caution is required in employing
it for high-grade conductivity copper, since the effect of very small
quantities has a deleterious influence upon the conducting properties.
Silicon also is used for a similar purpose, and causes a considerable
increase in toughness.
(_f_) When intended for conductivity work, the metal is cast into
the form of “wire-bars” of very varied shape and size, according to
requirements; thus the 100-lb. bars are about 3 feet long by about 3
inches square section, the 500-lb. bars 7 feet long by about 4½ inches
square. Furnace samples weighing about 1 lb. are drawn down gradually
to about ⅛-inch wire, and are tested for conductivity, as well as for
strength and toughness, occasional analysis being also undertaken,
whilst samples of the wire-bars in market form are similarly examined.
[Illustration: _a b_ Fig. 76.—Microstructure of Commercial Copper
containing Oxygen (Hofman).
_a._ Calumet and Hecla copper after 60 minutes’ poling.
0·22 per cent. oxygen = 1·98 per cent. Cu_{2}O.
_b._ Calumet and Hecla “dry” copper before poling.
0·64 per cent. oxygen = 5·76 per cent. Cu_{2}O.
Compare with Fig. 6, p. 28.
(_By permission of American Institution of Mining Engineers._)]
References.
Ulke, T., “Modern Electrolytic Copper Refining.”
(With complete Bibliography).
Peters, E. D., “Modern Copper Smelting” (1905).
(Chapter xviii. by M. Barnett.).
Mineral Industry. Annual Review.
Cowper Coles, S., “An Electrolytic Process for the Production
of Copper Wire.”
_Proc. B’ham. Met. Soc._, 1908–9, p. 5.
Schnabel and Louis, “Handbook of Metallurgy,” pp. 327–369.
Wraith, W., “Sampling Copper Anodes at Anaconda.”
_Trans. Amer. Inst. Min. Eng._, March, 1910.
“The De Lamar Electro-Refinery at Chrome, New Jersey.”
_Eng. and Min. Journ._, Jan. 13, 1906, p. 73.
Flinn, F. B., “Electrolytic Copper.”
_The Metal Industry_, April, 1910, p. 112, vol. ii., No. 3.
Greenawalt, W. E., “The Greenawalt Electrolytic Process.”
_Eng. and Min. Journ._, Nov. 26, 1910, p. 1062.
See also References to Johnson (p. 34), Hofman and others (p. 50),
Keller (pp. 50, 215), and Peters (p. 80).