Over 7 million tons of zinc are produced annually worldwide. Nearly 50% of the amount is used for galvanizing to protect steel from corrosion. Approximately 19% are used to produce brass and 16% go into the production of zinc base alloys to supply e.g. the die casting industry. Significant amounts are also utilized for compounds such as zinc oxide and zinc sulfate and semi-manufactures including roofing, gutters and down-pipes.
These firt use suppliers then convert zinc into in a broad range of products. Main application areas are: construction (45%) followed by transport (25%), consumer goods & electrical appliances (23%) and general engineering (7%).
Saturday, March 15, 2008
Zinc Recycling
At present, approximately 70% of the zinc produced worldwide originates from mined ores and 30% from recycled or secondary zinc. The level of recycling is increasing each year, in step with progress in the technology of zinc production and zinc recycling. Today, over 80% of the zinc available for recycling is indeed recycled.
Zinc is recycled at all stages of production and use – for example, from scrap that arises during the production of galvanized steel sheet, from scrap generated during manufacturing and installation processes, and from end-of-life products.
Zinc-coated steel and other zinc containing products are slow to enter the recycling circuit due to the very nature of their durability. The life of zinc-containing products is variable and can range from 10-15 years for cars or household appliances, to over 100 years for zinc sheet used for roofing. Street lighting columns made of zinc-coated steel can remain in service for 40 years or much longer, and transmission towers for over 70 years. All these products tend to be replaced due to obsolescence, not because the zinc has ceased to protect the underlying steel. For example, zinc coated steel poles placed in the Australian outback a hundred years ago are still in excellent condition(3).
The presence of zinc coating on steel does not restrict steel's recyclability and all types of zinc-coated products are recyclable(4). Zinc coated steel is recycled along with other steel scrap during the steel production process - the zinc volatilises and is then recovered.
Zinc coated steels are easily collected and treated in existing process streams. The Electric Arc Furnace (EAF) is the most widely used process for recycling zinc-coated steel. The high temperatures cause zinc - which is volatile at high temperatures - to leave the furnace along with other gases. The gas stream is treated and the zinc collected in the dust, of which zinc (18-35%) and iron are the main constituents. These dusts undergo an enrichment process in a rotary kiln, known as a Waelz kiln. This leads to the production of zinc oxide, which in turn becomes a raw material for the production of zinc metal. Several new technologies are in use or under development for processing EAF dusts and the valuable metals they contain.
Zinc is recycled at all stages of production and use – for example, from scrap that arises during the production of galvanized steel sheet, from scrap generated during manufacturing and installation processes, and from end-of-life products.
Zinc-coated steel and other zinc containing products are slow to enter the recycling circuit due to the very nature of their durability. The life of zinc-containing products is variable and can range from 10-15 years for cars or household appliances, to over 100 years for zinc sheet used for roofing. Street lighting columns made of zinc-coated steel can remain in service for 40 years or much longer, and transmission towers for over 70 years. All these products tend to be replaced due to obsolescence, not because the zinc has ceased to protect the underlying steel. For example, zinc coated steel poles placed in the Australian outback a hundred years ago are still in excellent condition(3).
The presence of zinc coating on steel does not restrict steel's recyclability and all types of zinc-coated products are recyclable(4). Zinc coated steel is recycled along with other steel scrap during the steel production process - the zinc volatilises and is then recovered.
Zinc coated steels are easily collected and treated in existing process streams. The Electric Arc Furnace (EAF) is the most widely used process for recycling zinc-coated steel. The high temperatures cause zinc - which is volatile at high temperatures - to leave the furnace along with other gases. The gas stream is treated and the zinc collected in the dust, of which zinc (18-35%) and iron are the main constituents. These dusts undergo an enrichment process in a rotary kiln, known as a Waelz kiln. This leads to the production of zinc oxide, which in turn becomes a raw material for the production of zinc metal. Several new technologies are in use or under development for processing EAF dusts and the valuable metals they contain.
The Production of Zinc
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 history of zinc
Centuries before zinc was discovered in the metallic form, its ores were used for making brass and zinc compounds, its ores were used for healing wounds and sore eyes. It is believed that the Romans first made brass in the time of Augustus (20 B.C. – 14 A.D.). In the 13th century Marco Polo described the manufacture of zinc oxide in Persia.
By 1374, zinc was recognized in India as a new metal – the 8th metal known to man at that time. At Zawar, India, both zinc metal and zinc oxide were produced from the 12th to the 16th century. Zinc metal was used to make brass and zinc oxide served medical purposes.
From India, zinc manufacturing moved to China in the 17th century where it developed as an industry to supply the needs of the brass industry.
Zinc was recognized in Europe as a separate metal in the 16th century when Agricola (1490 – 1555) observed that a metal called “zincum” was produced in Slesia and Paracelsus (1493 – 1541) stated clearly that “zincum” was a new metal. In 1743, the first European zinc smelter was established in Bristol in the United Kingdom using a vertical retort procedure. A major technological improvement was achieved with the development of the horizontal retort process in Germany which led to the erection of smelting works in Slesia, Liege, Belgium and Aachen, the Rhineland and the Ruhr areas in Germany. In 1836 hot-dip galvanizing, the oldest anti-corrosion process, was introduced in France. Zinc production in the United States started in 1850.
For about 500 years zinc was produced from its oxide ores before the more abundant sulfides became the major source of supply. On the technological side there was a drastic change in 1916 when the electrolytic process was introduced on a large scale replacing the pyrometallurgical process as the dominating production method.
By 1374, zinc was recognized in India as a new metal – the 8th metal known to man at that time. At Zawar, India, both zinc metal and zinc oxide were produced from the 12th to the 16th century. Zinc metal was used to make brass and zinc oxide served medical purposes.
From India, zinc manufacturing moved to China in the 17th century where it developed as an industry to supply the needs of the brass industry.
Zinc was recognized in Europe as a separate metal in the 16th century when Agricola (1490 – 1555) observed that a metal called “zincum” was produced in Slesia and Paracelsus (1493 – 1541) stated clearly that “zincum” was a new metal. In 1743, the first European zinc smelter was established in Bristol in the United Kingdom using a vertical retort procedure. A major technological improvement was achieved with the development of the horizontal retort process in Germany which led to the erection of smelting works in Slesia, Liege, Belgium and Aachen, the Rhineland and the Ruhr areas in Germany. In 1836 hot-dip galvanizing, the oldest anti-corrosion process, was introduced in France. Zinc production in the United States started in 1850.
For about 500 years zinc was produced from its oxide ores before the more abundant sulfides became the major source of supply. On the technological side there was a drastic change in 1916 when the electrolytic process was introduced on a large scale replacing the pyrometallurgical process as the dominating production method.
Monday, March 10, 2008
Brass sheet, strip and wire
The 70/30 brass to be made into sheet, strip or wire form must be significantly free of harmful impurities in order to retain ductility when cold. It can then be rolled, drawn, deep drawn, swaged, riveted, spun or cold formed. It is normal therefore to make it substantially from virgin copper and zinc, together with process scrap arising from processing that has been kept clean, carefully segregated and identified.
Special purpose alloys
Brass swarf arising from machining operations can be economically remelted but it should be substantially free from excess lubricant, especially those including organic compounds that cause unacceptable fumes during remelting. The presence in brass of some other elements such as lead is often required to improve machinability so such scrap is frequently acceptable. Besides the common free-machining brasses, there are many others made for special purposes with properties modified to give extra strength, hardness, corrosion resistance or other attributes, so strict segregation of scrap is essential.
When brass is remelted, there is usually some evolution of the more volatile zinc. This is made up in the melt to bring it back within specification. The zinc is evolved as oxide that is drawn off and trapped in a baghouse and recycled for the manufacture of other products.
When brass is remelted, there is usually some evolution of the more volatile zinc. This is made up in the melt to bring it back within specification. The zinc is evolved as oxide that is drawn off and trapped in a baghouse and recycled for the manufacture of other products.
Brass machined parts and stampings
Free machining 60/40 brass with small additions of lead is particularly cost-effective and environmentally friendly when used for the manufacture of machined parts and stampings. Firstly, the swarf produced as a result of high speed machining (typically metal removal rates are two to three times those of mild steel) can be sold as scrap. Secondly, when brass components reach the end of their long and useful life they too can be readily recycled. The stamping temperatures for brasses are lower than those required for ferrous alloys making it an energy efficient process; the low stamping temperature also gives an increased die life. Stamping is a near-net-shape process but the small amount of swarf produced by machining is again recyclable. Brass is also cost-effective and kind to the environment because it does not normally need plating or painting to prevent corrosion.
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