Page 8 - Curriculum Visions Dynamic Book

Extracting zinc from its ore
Zinc is a very reactive element and so
is not found as a native metal. Because it is reactive, zinc is difficult to separate from its compounds, and as a result little use was made of the element before the 19th and 20th centuries.
The main use of zinc was as an accidental constituent of copper by
the ancient Egyptians. It was only in 1721 that German metallurgist Johann Henckel managed to extract zinc as
a metal. Nevertheless, it was a century before it came into more widespread
use because a commercial method of smelting the ore could not be found.
In fact, in many cases even today, the cost of smelting zinc alone from the low grade ore in which it is
found would make it uneconomic. However, it is usually associated with other elements such as lead, copper, gold and silver. Thus, by extracting
a number of elements, zinc can be recovered economically.
As with other low-grade ores, the metal has to be extracted in a number
of steps, designed to concentrate the
ore and remove as much waste rock
as possible before the expensive smelting process begins.
When the ore has been enriched,
it can be smelted; but to achieve the highest quality, a further stage of refining is necessary using an electrolytic cell.
Processing zinc ore
The first stage of zinc processing
is called froth flotation. The ore is crushed to a fine powder and placed into a vat with water, and wetting and frothing chemicals.
Jets of air are introduced into
the vat, causing the frothing agents to make bubbles. The chemicals
in the vat prevent the metals from becoming wetted by the water. Because they remain dry they can be caught up by air bubbles and lifted to the surface
of the vat, where they can be floated off and collected. The wet particles of rock, by contrast, sink to the bottom.
After flotation, the ore may be concentrated to produce up to one- half metal. At this concentration it can be roasted, which changes
the zinc sulphide into zinc oxide. A byproduct of the roasting operation, sulphur dioxide, is usually converted to sulphuric acid and sold.
Zinc oxide is then smelted with
a supply of coke in a furnace. The coke, being almost pure carbon, reduces the oxide to metal and produces carbon monoxide gas. The zinc metal, which has a low boiling point, forms a vapour in the smelter, and this is distilled and collected
in order to make it into ingots. The overall reaction is shown opposite.
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