INDUSTRIAL ELECTROLYSIS
The metal copper, as obtained from copper ore, contains many impurities
of various kinds. For most purposes these impurities greatly affect the
value of the copper, and before the metal can be of much commercial
use they must be got rid of in some way. In the previous chapter, in
describing how to copperplate an old spoon, we saw that the anode need
not consist of pure copper, because in any case nothing but the pure
metal would be deposited upon the spoon. This fact forms the basis of
the important industry of electrolytic copper refining. The process is
exactly the same as ordinary copperplating, except that the cathode
always consists of absolutely pure copper. This is generally in the
form of a sheet no thicker than thin paper, but sometimes a number of
suspended wires are used instead. A solution of copper sulphate is used
as usual for the electrolyte, and the anode is a thick cast plate of
the impure copper. The result of passing a current through the solution
is that copper is taken from the anode and carried to the cathode, the
impurities falling to the bottom of the vat and accumulating as a sort
of slime. In this way thick slabs of pure copper are obtained, ready to
be melted down and cast into ingots.
The impurities in the raw copper vary according to the ore from which
it is obtained, and sometimes gold and silver are found amongst them.
When the copper is known to contain these metals the deposit at the
bottom of the refining vats is carefully collected, and from it a
considerable quantity of gold and silver is recovered. It is estimated
that about half a million tons of copper are refined every year. An
immense amount of this pure copper is used for electrical purposes, for
making conducting wires and cables, and innumerable parts of electric
appliances and machinery of all kinds; in fact it is calculated that
more than half of the copper produced all over the world is used in
this way.
A similar method is employed to obtain the precious metals in a pure
state, from the substance known as “bullion”; which consists usually of
an intermingling of gold, silver, and copper, with perhaps also lead.
Just as in copper refining, the raw material is used as the anode, and
a strip of pure gold or silver, according to which metal is required,
as the cathode. A silver solution is used if silver is wanted, and a
gold solution if gold is to be deposited.
The metal aluminium has come into general use with surprising rapidity,
and during the last twenty-five or thirty years the amount of this
metal produced annually has increased from two or three tons to many
thousands of tons. Aluminium occurs naturally in large quantities,
in the form of alumina, or oxide of aluminium, but for a long time
experimenters despaired of ever obtaining the pure metal cheaply
on a commercial scale. The oxides of most metals can be reduced,
that is deprived of their oxygen, by heating them with carbon; but
aluminium oxide holds on to its oxygen with extraordinary tenacity,
and absolutely refuses to be parted from it in this way. One process
after another was tried, without success, and cheap aluminium seemed
to be an impossibility until about 1887, when two chemists, Hall, an
American, and Héroult, a Frenchman, discovered a satisfactory solution
of the problem. These chemists, who were then scarcely out of their
student days, worked quite independently of one another, and it is a
remarkable fact that their methods, which are practically alike, were
discovered at almost the same time. The process is an interesting
mixture of electrolysis and electric heating. An iron crucible
containing a mixture of alumina, fluorspar, and cryolite is heated. The
two last-named substances are quickly fused, and the alumina dissolves
in the resulting fluid. When the mixture has reached the fluid state,
electrodes made of carbon are dipped into it, and a current is passed
through; with the result that oxygen is given off at the anode, and
metallic aluminium is produced at the cathode, in molten drops. This
molten metal is heavier than the rest of the fluid, and so it falls
to the bottom. From here it is drawn off at intervals, while fresh
alumina is added as required, so that the process goes on without
interruption. After the first fusing of the mixture no further outside
heat is required, for the heat produced by the passage of the current
is sufficient to keep the materials in a fluid state. Vast quantities
of aluminium are produced in this way at Niagara Falls, and in Scotland
and Switzerland.
Most of us are familiar with the substance known as caustic soda.
The chemical name for this is sodium hydrate, and its preparation by
electrolysis is interesting. Common salt is a chemical compound of the
metal sodium and the greenish coloured, evil smelling gas chlorine,
its proper name being sodium chloride. A solution of this in water is
placed in a vat or cell, and a current is sent through it. The solution
is then split up into chlorine, at the anode, and sodium at the
cathode. Sodium has a remarkably strong liking for water, and as soon
as it is set free from the chlorine it combines with the water of the
solution, and a new solution of sodium hydrate is formed. The water in
this is then got rid of, and solid caustic soda remains.
Amongst the many purposes for which caustic soda is used is the
preparation of oxygen and hydrogen. Water, to which a little sulphuric
acid has been added, is split up by a current into oxygen and hydrogen,
as we saw in Chapter V. This method may be used for the preparation of
these two gases on a commercial scale, but more usually a solution of
caustic soda is used as the electrolyte. If the oxygen and hydrogen
are not to be used at the place where they are produced, they are
forced under tremendous pressure into steel cylinders, and at a
lantern lecture these cylinders may be seen supplying the gas for the
lime-light. Although the cylinders are specially made and tested for
strength, they are covered with a sort of rope netting; so that if by
any chance one happened to burst, the shattered fragments of metal
would be caught by the netting, instead of flying all over the room and
possibly injuring a number of people. A large quantity of hydrogen is
prepared by this process for filling balloons and military airships.