Page 28 - Curriculum Visions Dynamic Book

Refining
From the chemist’s point of view,
ores can be refined in two ways.
The simplest is to reduce or oxidise
the ore (depending on the nature of
the ore). This is what happens in an iron blast furnace (shown on the previous page) and in copper refining, shown on this page. But even then the reaction can only happen at high temperatures, showing that a large input of energy
is needed.
Even the best of chemical reactions
cannot completely remove all of the impurities in a metal, so ores refined in
a furnace do not produce pure metals. Indeed, some metals are so reactive that they cannot be separated from their ores by chemical reactions at all. Aluminium and zinc are important examples. This
is why many metals are refined to their final stage of purity by electrical means (a process called electrolysis).
 The last stage of refining copper is done by electrolysis. The impure copper from the furnace is made into one electrode of an electrolysis cell. The other electrode is made from a thin sheet of pure copper. The copper is refined by placing the two electrodes in a copper sulphate bath and passing a current
between them. The impure copper on the anode corrodes and pure copper collects on the cathode, When the cathode has acquired sufficient copper, it will be melted down and made into wires and sheet metal.
The giant industrial electrolysis and the laboratory equivalent are shown here.
The reactivity of metals
Metals can be arranged in a list, called a reactivity series, with the most reactive
at the top and the least reactive at the bottom. Those with low reactivity are
the easiest to refine; those with high reactivity are far more difficult. Thus aluminium, which is a reactive metal, can only be separated from its ore, bauxite (aluminium oxide), by the use of electrical energy, a process called electrolysis. On the other hand, iron and copper oxides are much easier to refine (at least until the highest purity is required) because copper and iron are less reactive metals. Here a high temperature is sufficient to ensure that the ore reacts.
REACTIVITY SERIES
Element
Reactivity
potassium sodium calcium magnesium aluminium manganese chromium zinc
iron cadmium tin
lead copper mercury silver gold platinum
most reactive
least reactive
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 In this simple laboratory demonstration of the purification of copper a current is passed through two impure copper strips suspended in copper sulphate solution (the electrolyte). Pure copper is deposited on the cathode of the cell.