Light Metals Zinc Part 1 - History, Deposits, and Production
Zinc is as old as the earth's crust and has been used as a component of brass since ancient times. The history of zinc is told from 1374 onwards and is still of great importance today.
The Importance of Zinc in History
The Romans under the rule of Emperor Augustus (20 BC to 14 AD) were probably the first to melt the raw material for brass coins using a mixture of copper and zinc ore without being aware of it.
It was not until 1374 that the Indians realized that zinc was a "new" metal. In ancient Hindu writings from this time, the first descriptions of production processes for zinc metal from ores can be found. In the late 13th century, Marco Polo describes the production of zinc oxide in Persia. There the oxide was used to treat eye infections. At the beginning of the 17th century, European scientists such as Albertus Magnus, Georgius Agricola, and Paracelsus also discovered the value of the new metal. Already in 1720, zinc was extracted on a large scale in Swansea, England.
In 1743 William Champion built the first zinc smelter in Bristol with an annual capacity of about 200 tons. Further smelters were built in Upper Silesia and the Aachen-Lüttich region. After the discovery of its rollability, the first zinc rolling mill was built in Belgium in 1805. The zinc ore mined there was smelted and rolled on site.
The sheets produced by the pack rolling process were mainly used in the building industry and worked well for roofing, roof drainage, building components, and ornamental ironwork. When the material properties no longer met Western European quality requirements, pack rolling that had been well established until then was replaced in Germany at the beginning of the 1980s by new, high-tech processes.
Despite its long tradition, the comparatively young metal is now being discovered for ever new areas of application. It can be found in high-tech products as well as in telecommunications and aerospace. As zinc ores are abundant both geologically and geographically, zinc is considered a "raw material with a future," offering sustainability and recyclability.
Natural Zinc Contents in the Soil - Zinc Ores
Zinc is present in the earth's crust with an average content of 70 mg/kg, which corresponds to a proportion of 0,007 % in the earth's crust. Weathering and erosion of rocks, soils, and sediments by wind and water naturally release small quantities of zinc. Zinc is absorbed, utilized, and excreted by living organisms and plants. In water, air and soil, a natural zinc concentration occurs in this way, which can vary greatly depending on location and conditions.
Typically, the zinc content of the soil varies between 10 and 300 mg/kg. In some areas of our earth, zinc is naturally concentrated up to 5 to 15 % (50,000 to 150,000 mg/kg) through natural geological and geochemical processes. Rock with these high zinc contents is known as ore. The ore contains zinc mostly in the form of sphalerite, i.e., as so-called zinc blende (ZnS), and along with zinc scrap, is the most important raw material for zinc production.
How Does Ore Become a Raw Material for Zinc Production?
Zinc ore can be found in many areas of our earth. It is mined, for example in Canada, South Africa, Thailand, Brazil, Australia, and China. There were also zinc ore deposits in Germany, for instance, near Stolberg in the Rhineland are or at Rammelsberg in the Harz Mountains. Above ground, rare plants can be found in these areas that grow particularly well on zinc-bearing soils - such as the yellow calamine violet, which is named after the old name for zinc ore "calamine".
Zinc ore is no longer mined in Germany today, and the zinc concentrations in the deposits are too low for cost-effective zinc extraction. The raw material for zinc extraction has therefore traveled a long way, for example, from Australia - mostly by ship - before it is processed into zinc metal in Germany. Transportation is expensive and requires a lot of energy. So it goes without saying that the aim is to transport as little material as possible.
Near the mines in which zinc ore is extracted in underground or open-cast operations, some plants enrich the zinc content of the ore (5 to 15 %) to about 55 % in so-called concentrates. The separated rock with low zinc content remains in place. In this way, less material has to be transported, which means that less energy is used for transport and zinc extraction is more environmentally friendly.
How Long Will the Zinc Reserves Last?
Zinc ores are mined in many countries of the world and traded globally. Ore concentrates, fine zinc, commercial zinc, and zinc alloys reach Germany from various regions of the world.
The so-called "range" of zinc reserves is a dynamic quantity. It depends above all on demand for zinc ore. If the demand and thus also the price increases, mining companies increase their exploration activities, i.e., the mining of zinc deposits suitable for extraction. Ore bodies are classified as "mineable" if they can be mined economically with state-of-the-art technology.
Whenever ore bodies are found that are expected to last for a period of about 20 years, exploration activity is curbed until demand and price increase again. As a result, the proven reserves for zinc have been consistently reported to last for about 20 years since the 60s of the previous century. The zinc deposits (resources) that are known today to be presumably mineable will ensure a much longer period of secure zinc supply. An end to zinc ore mining is not foreseeable.
Apart from natural zinc deposits, so-called urban mines are becoming increasingly important. The term refers to zinc, which is used today as a roofing material, facade sheet or galvanized layer on steel and is available for recycling after many decades. The recycling of zinc is an efficient use of resources and thus an important supplement to the extraction of zinc from ores. Over four million tons of zinc are extracted from these so-called secondary raw materials every year.
The Extraction of Zinc from Ores
The most important mineral for zinc production is the so-called zinc blende (ZnS). This means that apart from zinc, the concentrate contains about 20 % sulfur. Additionally, zinc concentrates always contain iron, lead, and silver in varying proportions, because these elements often occur together in the ores.
The challenge is not actually to extract the zinc but to separate the accompanying elements in such a way that they can be used as by-products. First of all, the concentrate is heated to more than 900° C so that ZnS reacts to form ZnO - the expert refers to this as roasting. At the same time, the sulfur combines with oxygen to form gaseous sulfur dioxide (SO2). Sulfuric acid is extracted from the sulfur dioxide in special plants - an important by-product of zinc extraction. Sulfuric acid is needed, for example, for the production of fertilizers.
Hydrometallurgical Zinc Recovery
In Germany, zinc is extracted in the so-called hydrometallurgical process. This involves dissolving the zinc content of the roasted concentrate in sulphuric acid. Iron, lead, and silver remain. These undissolved components can be sold to other companies where lead and silver are recovered. Another by-product of zinc extraction can be indium, for example, which also occurs as a by-element in zinc ores.
The zinc dissolved in acid is transported via pipelines to the electrolysis, where metallic zinc is deposited on sheet metal in high purity due to its electrical properties. Way over 90 % of the zinc produced worldwide is now obtained by hydrometallurgical methods.