80% of zinc mines are underground, 8% are of the open pit type and the remainder is a combination of both. However, in terms of production volume, open pit mines account for as much as 15%, underground mines produce 64% and 21% of mine production comes from the combined underground and open pit mining.
Rarely is the ore, as mined, rich enough to be used directly by smelters; it needs to be concentrated. Zinc ores contain 5-15% zinc. To concentrate the ore it is first crushed and then ground to enable optimal separation from the other minerals. Typically, a zinc concentrate contains about 55% of zinc with some copper, lead and iron. Zinc concentration is usually done at the mine site to keep transport costs to smelters as low as possible.
Roasting & Sintering
Over 95% of the world’s zinc is produced from zinc blende (ZnS). Apart from zinc the concentrate contains some 25-30% or more sulphur as well as different amounts of iron, lead and silver and other minerals. Before metallic zinc can be recovered, by using either hydrometallurgical or pyrometallurgical techniques, sulphur in the concentrate must be removed. This is done by roasting or sintering. The concentrate is brought to a temperature of more than 900°C where zinc sulphide (ZnS) converts into the more active zinc oxide (ZnO). At the same time sulphur reacts with oxygen giving out sulphur dioxide which subsequently is converted to sulphuric acid – an important commercial by-product.
The Hydrometallurgical Process
In a leaching stage the zinc oxide is separated from the other calcines. Sulphuric acid is used to do this. The zinc content dissolves whereas iron precipitates and lead and silver remain undissolved. However, the dissolved solution contains some impurities which need to be eliminated in order to obtain a high-purity zinc product at the end of the production process. Purification is mainly done by adding zinc dust to the solution. As all the elements to be removed lie below zinc in the electrochemical series they can be precipitated by cementation. The thus obtained purified solution passes an electrolytic process where the purified solution is electrolyzed between lead alloy anodes and aluminium cathodes. An electrical current is circulated through the electrolyte by applying an electrical difference of 3.3-3.5 volts between the anode and cathode causing the zinc to deposit on the aluminium cathodes in high purity. The deposited zinc is stripped off, dried, melted and cast into ingots. The zinc ingots may have different grades: High Grade (HG) 99.95% and Special High Grade (SHG) 99.99% of zinc. Today over 90% zinc is produced hydrometallurgically in electrolytic plants.
The Pyrometallurgical Process
The Imperial Smelting process is based on the reduction of zinc and lead into metal with carbon in a specially designed Imperial Smelting furnace. The IS process is an energy-intensive process and thus became very expensive following the rise of energy prices. Today, Imperial Smelting furnaces are only in operation in China, India, Japan and Poland.