This is the first in a trilogy of short articles on zinc sheeting, presented by Rheinzink. This private company was founded in 1966 and today is recognised as a world leader in the manufacturing of titanium zinc roofing, cladding and roof drainage systems.
The history, production and usage of zinc
Before it was discovered that iron ore could be processed to form a metal, zinc was used to produce brass as well as salts for medical purposes – two applications that were in common use before 3000 BC – and it wasn’t until 1200 AD in India that metallic zinc was produced, although the name “zinc” only began to appear in the 17th century.
Metallic zinc was formed by heating the zinc ore indirectly with charcoal in a covered crucible, which produced zinc vapour that was cooled by the ambient air in a condensation receptacle underneath the crucible – the forerunner to today’s hi-tech processes.
During the 18th century the extraction of zinc from the ore took place all over Europe and England, with smelters in horizontal retort ovens leading the emergence of new zinc extraction processes that first had to overcome the release of sulphur dioxide into the environment, which was achieved by converting it into sulphuric acid.
In 1805 a pack-rolling process was developed to roll zinc into smooth sheets of metal at between 100-150ºC, which gave rise to the material being used successfully in the building industry to make roof coverings, gutters and downpipes, with sheets in standard dimensions of 1m x 2m. (it wasn’t until the mid-1960s that modern technology superseded this process).
The galvanising of steel sheeting and large structural steel components then became a key application for zinc, thanks to its anti-corrosion properties, and from the 19th century the production of zinc increased, together with the increasing use of steel.
Modern production processes
In nature zinc is found in the form of compounds involving oxygen or sulphur. The most important zinc mineral is zinc blende (ZnS), which is the chief source of zinc, consisting largely of zinc sulphide in crystalline form.
The weathering of zinc blende gives rise to carbonatic and siliceous zinc minerals, known as oxidic zinc ore or calamine. In addition, zinc deposits generally contain other valuable metals on a scale that is of economic interest, with lead occurring most frequently. Other important metals are copper, silver, iron, manganese and cadmium, as well as low levels of other metals.
Approximately 90% of zinc ore is mined by underground working. Normally the ore is processed into concentrates near the deposit site in several stages via flotation and then used as the base material for subsequent smelting.
The imperial smelting process and New Jersey zinc distillation (for refining crude zinc) and zinc electrolysis are the commonly available processes for the production of zinc from these concentrates. There are also plants for resmelting and liquation, i.e. for the thermic separation of zinc scrap.
High-grade zinc is produced during zinc electrolysis, using a hydrometallurgical process which involves the following steps:
• Roasting – the roasted blende is oxidised from the concentrates, producing SO2 containing off-gas, which undergoes further processing into H2S04 in a separate system.
• Leaching – where the roasted blende is dissolved in sulphuric acid.
• Purification of leach – here accompanying elements such as copper and cadmium are separated from the leach, resulting in the so-called neutral leach.
• Electrolysis – the zinc is deposited electrolytically from the neutral leach on the cathode.
• Resmelting – in a final step the cathodes are resmelted into blocks of zinc.
Zinc electrolysis results in a high utilisation level of the concentrates, and increasing use is also being made of secondary raw materials rich in zinc in this hydrometallurgical process for the extraction of zinc.
Part 2 of this trilogy will appear in the next issue of Walls & Roofs in Africa, covering imperial smelting, processing zinc scrap and the production of sheets and strips.