Compounds of Copper — Copper Mattes — The Varieties of
Commercial Copper — Ores of Copper — Preliminary
Treatment of Ores, Sampling.
=Compounds of Copper.=—From the smelting point of view, the three most
important classes of copper compounds are the oxides, the sulphides,
and the silicates.
_Copper Oxides._—Of the oxides, two are of importance—cuprous oxide,
Cu_{2}O, and cupric oxide, CuO—the first-named particularly, having
extensive connection with smelting practice.
Cuprous oxide is black when in the massive form, and has a red hematite
colour when powdered. It is readily formed by the oxidation of copper,
and melts at a red heat without decomposition; further heating in the
presence of air produces the cupric oxide which is less fusible. As has
been already indicated, cuprous oxide dissolves in the molten metal. It
is easily reduced to metallic copper by heating with carbon, the metal
being also obtained if the oxide be heated in the presence of reducing
gases; it combines readily with silica when heated, yielding fusible
silicates.
When cuprous oxide is heated with sulphide of iron, the copper, having
a greater affinity for the sulphur than iron possesses, enters into
combination with it, forming copper sulphide and iron oxide, and if
sufficient silica be present, a silicate of iron slag is produced. When
melted with copper sulphide, cuprous oxide yields metallic copper with
liberation of sulphur dioxide according to the equation—
Cu_{2}S + 2Cu_{2}O ➡ 6Cu + SO_{2}.
This reaction is a quantitative one, and takes place in the Direct
Process of Nicholl and James as operated at Swansea. The excess
of either constituent remains unchanged. The reaction is of great
importance in the processes of copper extraction, since upon it depends
the liberation of metallic copper from the sulphide, both in the old
roaster process and in the modern converter operation.
_Sulphides of Copper._—Of the sulphides Cu_{2}S and CuS, the former
only is of metallurgical importance. It is grey black, brittle, and
crystalline, its melting point is about 1,135° C., and its specific
gravity, when cold, about 5·5. Owing to the great affinity of sulphur
for copper, this element acts as practically the universal carrier of
the metal in smelting work, detaching the copper from all other forms
of combination, and collecting it as sulphide, mixed with the sulphides
of other metals, particularly that of iron—copper sulphide and iron
sulphide alloying in all proportions.
When copper sulphide is melted with an excess of sulphur, it remains
unchanged; when melted with copper and subsequently cooled, the
sulphide and metal separate as such, although it is believed that
small amounts of copper are present in solid solution in the sulphide
on solidification, but that they separate from it during a dimorphic
change in the material, which occurs at about 103° C. The sulphide
reacts with iron with liberation of some metallic copper, and the
formation of some iron sulphide which associates itself with the rest
of the copper sulphide, forming a matte. This matte is not further
affected by iron, so that it is not possible to completely decompose
copper sulphide by this means.
When heated in a powdered condition in excess of air, copper sulphide
is oxidised, oxides of copper and sulphur being produced. There occur
probably several intermediate reactions, and several intermediate
products are formed, but the main effect is represented by the
equations—
┌─
│ Cu_{2}S + 2O ➡ 2Cu + SO_{2}
─┤
│ 2Cu + O ➡ Cu_{2}O
└─
which take place simultaneously, the copper represented in the first
equation being oxidised spontaneously according to the second, and the
resultant is the reaction Cu_{2}S + 3O ➡ Cu_{2}O + SO_{2}.
In the furnace operations, some of the SO_{2} in the presence of air
and oxidisable material, and in contact with the heated brickwork
becomes oxidised to SO_{3}, which, interacting with the oxides and
sulphides present, combines to form copper sulphate and cupric oxide.
At a higher temperature the sulphate is again decomposed to CuO and
SO_{3}, some of which passes off and is free to oxidise more sulphide;
the rest is decomposed to SO_{2} and oxygen. These reactions occur
during the roasting of charges containing copper sulphides.
=Copper Mattes.=—On smelting a furnace charge which contains both
copper and sulphur, the sulphur appears to have a stronger attraction
for the copper than for any of the other metals usually present, and
only when this affinity has been satisfied is the excess sulphur free
to combine with other constituents of the charge. The fusible copper
sulphide which is thus produced, has the power of mixing completely
with any more sulphides which may be present, especially with sulphide
of iron.
The fused sulphides resulting from such furnace operations are termed
_copper-mattes_. They may contain from a mere trace to upwards of 80
per cent. of copper, and in ordinary work, sulphide of iron is the
other constituent present in the greatest proportion, but sulphides
of nickel, silver, zinc, or lead, etc., may also be found, as well as
arsenides and antimonides.
These facts relating to the collection of the copper as a constituent
of a fused sulphide product, form the basis of modern copper-smelting
work. In view of the practical importance of the mixed sulphides,
the diagram representing their equilibrium requires notice. A number
of workers have studied the question with widely differing results.
Röntgen made an exhaustive investigation of the system FeS—Cu_{2}S, and
published a very complete diagram of the series, working with FeS and
Cu_{2}S in the pure state.
The sulphides as commonly met with, especially in smelting practice,
do not however, occur as materials of the composition denoted by
the formulæ Cu_{2}S and FeS. The ordinary commercial sulphide of iron
corresponds more closely to the impure eutectic of the iron-FeS
system, containing about 85 per cent. of FeS and 15 per cent. of
iron, and melts at about 970° C., whereas the pure FeS has a melting
point of upwards of 1,180° C. At the elevated temperatures of the
copper-smelting furnace, pure FeS tends to lose sulphur and to assume
the composition of the eutectic. There are, further, good reasons for
believing that copper sulphide behaves in a somewhat similar manner, so
that the series of sulphides constituting the mattes of practice are
not represented by pure materials so well as by a series composed of
mixtures of the respective eutectics.
The diagram of this series of industrial sulphides was worked out by
Hofman, Caypless, and Harrington, and gives a fair summary of the
melting points of the series of mattes. It is reproduced in fig. 8. The
temperatures may be supplemented by Gibb’s determinations of 1,121° C.
for the 71·7 per cent. copper matte, and 1,098° C. for the 80 per cent.
matte.
The problem of the constitution of mattes is, however, a very complex
one, and is not yet satisfactory settled. An interesting view was put
forward by Gibb and Philp. Mattes corresponding to the formula 5Cu_{2}S
. FeS (copper 71·7 per cent.), when examined microscopically, appeared
to be homogeneous, and indicated some form of combination between
the sulphides in these proportions. Lower-grade mattes were assumed
to consist of this compound substance and excess FeS. Iron sulphide
was held to be capable of carrying a certain quantity of copper in
solution, and mattes might, therefore, carry this copper, according to
the amount of excess FeS which they contained. Within certain limits
the lower the grade of the matte—_i.e._, the more FeS present—the
more copper was held in solution, and with a fall of temperature this
solubility was lessened, and moss copper was set free in the solid
matte.
[Illustration: Fig. 8.—Freezing-Point Curve of Iron-Copper Sulphides
(Mattes).]
Deposition of copper may also be accounted for by a variation in the
solubility for copper, accompanying the well-marked dimorphic change
occurring in FeS at 130° C. whilst another possible cause of the
separation of moss copper is the partial decomposition of Cu_{2}S,
being effected, as previously indicated, by the free iron of the
iron-FeS eutectic which constitutes the iron sulphide component of
copper mattes. The whole subject is thus of considerable complexity,
and involves questions of thermal and chemical equilibrium.
The appearance, chemical constitution, and physical properties
of mattes vary according to the rate of cooling, and are further
influenced by the nature and amount of the impurities they contain,
and the following statement must be understood to be more or less
general:—Usually low-grade mattes (up to 20 per cent. or so of copper)
are more or less stony in fracture, with a bluish-purple colour; as the
copper contents increase, a reddening of the colour occurs, and also
an increase in the crystalline character and brittleness. Considerable
quantities of moss copper are present in these mattes. Beyond 30 per
cent. of copper, increased softness and brittleness result, with a
darkening towards blue-black in the colour, whilst with the 60 to 70
per cent. mattes the colour becomes in general of a steel-grey hue.
Increase in the copper contents leads to an increase in the density—a
matter which has important applications in connection with the
economical separation of matte from slag, and the slag-losses in
smelting practice.
The specific gravity of the 13 per cent. copper matte is about 4·80.
" 43 " " " 5·18.
" 60 " " " 5·42.
" 80 " " " 5·55.
(_Gibb_ and _Philp_.)
The density in the fluid state, which is the important condition in
smelting, is less than this, and may indeed be somewhat different,
owing to changes in the constitution of the material.
_Copper Silicate_ is formed by the action of copper oxide and silica
on heating. The silicate is decomposed when heated in the presence
of sulphides, resulting in the formation of sulphide of copper and
silicate of the second metal, in consequence of the great affinity of
copper and sulphur. Upon this fact depends the extraction of copper
from various silicate ores, as well as the cleaning of slags high in
copper, which are often added to the sulphide charges in the furnace
with this object. When heated with iron, the silicate is reduced to
metallic copper with the production of silicate of iron; it is also
reduced by carbon in the presence of metallic oxides capable of uniting
with the silica which is liberated.
=The Varieties of Commercial Copper.=—The copper employed industrially
comes into the market in widely differing forms. Different varieties
are named according to the method of manufacture, the uses for which
they are intended, the locality in which they are produced, or by
special trade names. The most important variety is:—
_Electrolytically-refined High-conductivity Copper_, which is largely
used for electrical work. The methods by which it is produced ensure
that most of the impurities inimical to high conductivity have been
removed, and the metal is specially free from arsenic, antimony, and
bismuth, as well as from silver and gold. As ordinarily produced
at the electrolytic refinery, it is in the form of cathode plates,
often about 3 feet × 2 feet 6 inches by ¾ inch thick, weighing 150
to 170 lbs. It is then remelted in order to bring it “up to pitch,”
and to give it the necessary mechanical properties, so that it may
be transformed at once into the particular form suitable for the
electrical purposes intended. Such metal often comes into the market in
the form of wire-bar ingots, cakes, or billets, weighing from 70 to 500
lbs. when in bar form, and from 100 to 400 lbs. when in other shapes.
Electrolytic copper is also suitable for the manufacture of alloys.
_Lake Copper._—The copper ores of the Lake Superior district are
particularly pure, and on smelting and furnace-refining yield a
metallic product of great purity which also possesses good mechanical
properties. It is, therefore, particularly suitable for electrical
work. By reason of its satisfactory properties, Lake copper realises
prices which usually rule somewhat higher than those of ordinary
electrolytic copper as quoted on the New York market.
_Best Select Copper._—For the production of copper alloys, such as
best brass, etc., it is essential that the copper should be pure.
The impurities which are present in ordinary tough copper, and which
may be valuable for imparting strength to the material, have a very
harmful effect when present in alloys. In the older Welsh process of
manufacturing copper, a special method was employed for obtaining metal
free from these impurities, especially arsenic and antimony. This was
known as the “best selecting” process.
The principle underlying the method was to conduct the furnace
operations to the stage at which a small quantity of copper, known as
“copper bottoms,” was obtained. The metal so produced has the property
of collecting from the rest of the matte-charge in the furnace, the
gold, the silver, and the great bulk of the other impurities, owing
to its greater solvent power for them. As a result, the greater part
of the matte (“white metal”) was left pure, and from this material
the copper was extracted by continuing the furnace operations in the
usual manner, the resulting product being known as “best select” (B.S.)
copper.
The process was later used principally for the extraction of the gold
in the charge, rather than for obtaining specially pure copper. The
product is essentially a British one, and was largely used for the
manufacture of high quality alloys.
“_Tough Pitch Copper._”—The operation of “bringing copper up to pitch”
has for its object the imparting to the metal of the toughness and
mechanical strength required for industrial service. The process
resolves itself into the adjustment of the correct proportion of
oxygen, the function of which is largely to eliminate the gases from
the copper, or to overcome their deleterious effects, as well as to
convert the otherwise more injurious metalloid impurities into a less
harmful form.
In modern practice, practically all copper is brought up to pitch, but
it is useful to distinguish between tough-pitch furnace-refined copper
and tough-pitch electrolytic copper.
The former is the brand to which the general term “tough pitch
copper” is best applied, this name having been given to the product
from the refining furnaces of the old Welsh and similar processes.
Before the converter method was introduced into copper practice,
the furnace processes for extracting copper from the ores resulted
in the production of a crude “blister” copper, into which several
injurious constituents, if originally present in the ore, found their
way. The principal impurity was usually arsenic. Although this was
also removable by special refining methods, and with some difficulty,
it was known, as has been indicated, that when arsenic is present
under suitable conditions and in proper proportions, it is capable of
imparting considerable strength and rigidity to the metal. Such copper
being particularly suited for various engineering and mechanical uses,
the arsenic being sometimes even specified for and purposely added—as
in fire-box plates and stay bolts, though it is never employed for
conductivity work or for the manufacture of alloys if any considerable
proportion be present—the metal found a ready market when brought to
pitch.
Tough pitch copper may thus vary largely in composition, especially
in arsenical contents, up to about the 0·5 per cent. already
indicated as being mechanically very useful. The actual process, as
used for bringing all classes of metal to pitch, will be described
in detail later, it being practically the same whether conducted on
furnace-refined metal, converter metal, or on electrolytic copper, as
a necessary preliminary to casting into the various forms of ingot in
which it is to be marketed.
In preparing the tough metal from crude copper, the more oxidisable
impurities (iron, sulphur, etc.) are first removed by a thorough
oxidation during or after melting down, this being known as “airing.”
The operation oxidises some of the copper, and it is probable that the
copper oxide thus formed plays an important part in getting rid of
impurities. By the time they have been thoroughly expelled, the metal
is considerably over-oxidised. Samples taken at this stage exhibit
the following characteristics:—The ingot has a depression down the
centre line, the material is very brittle, the fracture is brick-like
in texture and purple-red in colour, whilst much copper oxide and
oxidule-eutectic are seen on examination under the microscope. This
material is known as _Dry Copper_; it is merely an intermediate
product, and is commercially useless. The excess of oxygen is removed
by “poling”—that is, reduction, effected largely by charcoal, as well
as by reducing gases—successive samples showing less and less of the
characteristics of dry copper. The surface becomes level, the metal
exceedingly tough, the fracture fine-grained to silky in texture, and
a fine salmon-pink in colour. With satisfactory mechanical properties,
the metal has now become =tough pitch copper=.
If the poling—that is, the reduction of the oxidised constituents of
the tough pitch copper—be carried too far, the metal becomes brittle
again, being known as _over-poled copper_. The fracture then tends to
become coarse and fibrous, the colour lighter, and the upper surface
of the ingot exhibits a ridge. The reasons for these effects have not
yet been quite fully explained, but there is no doubt that they arise
from the removal of oxygen from the oxygenated constituents, and the
withdrawal from the metal of the protecting influence of the cuprous
oxide. Such influences are to some extent physical, since they prevent
the retention of the reducing gases; partly mechanical, in their
effects on the properties of the metal _per se_, and partly chemical,
as the oxide had probably entered into chemical combination with some
of the objectionable impurities, producing compounds, in which form
they were much less harmful. The removal of this oxygen from the metal
breaks down such combinations, leaving the reduced impurity again to
exercise its destructive effects on the properties. Over-poled copper,
like dry copper, being brittle, is commercially useless as such, and is
really an intermediate product, the metal being brought to pitch again
by further aëration to make it “dry,” after which it may be poled back
to correct pitch. As already stated, the over-poling effects are not
due to any intrinsic action of carbon directly on the copper itself.
Summarising, it may be stated that the most important commercial
varieties of copper are:—
=Electrolytically-refined metal=, employed for electrical work (also
for alloy-making).
=Tough Pitch Copper= for engineering uses.
=Best Select Copper= for alloy manufacture.
And, in addition, Lake Copper and some Converter Bars.