A Guide to Mercury Applications in Metalurgy

Mercury, or 'quicksilver' as it is otherwise known, is a dense, toxic metal element that exists in liquid form at room temperature. Produced and studied for millennia, mercury's use has steadily declined since the 1980s as a result of greater attention to the negative health impacts that it has on humans and the environment.

Properties

• Atomic symbol: Hg
• Atomic number: 80
• Element Category: Transition metal
• Density: 15.534g/cm³
• Melting Point: -38.9°C (102°F)
• Boiling Point: 356.9°C (674.4°F)
• Electrical Resistivity: 95.8 microhm/cm (20°C)

Characteristics

At room temperature, mercury is a thick, silvery liquid with a very high density and low heat conductivity. It has a relatively high electrical conductivity and readily forms amalgams (alloys) with gold and silver.

One of mercury's most valued characteristics is its ability to uniformly expand and contract over its entire liquid range, in response to changes in pressure and temperature. Mercury is also highly toxic to both humans and the environment, which has resulted in drastic reductions to its production and use over the last several decades.

History

Mercury’s earliest use can be traced back to 1500 BC when it was used to adorn tombs in ancient Egypt. Likely due to its unique properties, mercury was used, studied and prized by numerous civilizations, including the ancient Greeks, Romans, Chinese and Mayans.

For centuries, people believed that mercury possessed special healing properties and, consequently, used it as a diuretic and painkiller, as well as in medicines to treat various ailments from depression to syphilis. It has been used in cosmetics and as a decorative material. Alchemists in the Middle Ages were particularly interested in mercury's ability to extract gold from ore.

Early on, it became clear that the mysterious liquid metal was toxic to humans because of the high instance of insanity and death in mercury mines. It, however, did not prevent experimentation. The use of mercury nitrate to convert fur to felt, often employed by 18th and 19th-century hat makers, resulted in the expression 'mad as a hatter.'

Between 1554 and 1558, Bartolome de Medina developed the patio process for extracting silver from ores using mercury. The patio process relies on mercury's ability to amalgam with silver. Supported by large mercury mines in Almaden, Spain, and Huancavelica, Peru, the patio process was critical to the rapid expansion of Spanish silver production during the 17th and 18th centuries. Later, during the California gold rush, variations of the patio process were used to extract gold.

By the second half of the 20th century, increasing amounts of research began to prove a correlation between chemical waste run-off and methyl-mercury content in seafood. Attention was put on the metal's health effects on humans. In recent years, the United States and the European Union have put in place strict regulations on the production, use, and disposal of mercury.

Production

Mercury is a very rare metal and is most often found in the ores cinnabar and livingstonite. It is produced as a primary product and as a by-product of gold, zinc, and copper.

Mercury can be produced from cinnabar, a sulfide ore (HgS), by burning off the sulfide content in a rotary kiln or multiple hearth furnaces. Crushed mercury ore is mixed with charcoal or coking coal and burned at temperatures above 300°C (570°F). Oxygen is pumped into the furnace, which combines with the sulfur, releasing sulfur dioxide and creating a mercury vapor that can be collected and cooled for further refinement as a pure metal.

By passing the mercury vapor through a water-cooled condenser, the mercury, which has a high boiling point, is the first to condense into its liquid metal form and collected. About 95% of the mercury content of cinnabar ore can be recovered using this process.

Mercury can also be leached from ores using sodium hydroxide and sodium sulfide. Recovery of the mercury is done by precipitation using aluminum or electrolysis. Through distillation, mercury can be purified to greater than 99.999%.

Commercial-grade, 99.99% mercury is sold in 76lb (34.5kg) wrought iron or steel flasks.

Worldwide mercury production was estimated by the US Geological Survey (USGS) to be 2,250 tons in 2010. China currently supplies about 70% of global production, followed by Kyrgyzstan (11.1%), Chile (7.8%) and Peru (4.5%).

The largest producers and suppliers of mercury include the Khaidarkan Mercury Plant in Kyrgyzstan, producers in the Tongren-Fenghuang mercury belt of China and Minas de Almadén y Arrayanes, S.A., which formerly operated the historic Almaden mercury mine in Spain and is now responsible for the recycling and management of a large percentage of European mercury.

Applications

Production of, and demand for, mercury has steadily declined since its peak in the early 1980s.

The primary application for mercury metal in North America and Europe is in cathode cells, which are used for the production of caustic soda. In the US, this accounts for 75% of mercury demand, although the demand for such cells has decreased 97% since 1995, as modern Chlor-alkali plants have adopted membrane cell or diaphragm cell technologies.

In China, the polyvinylchloride (PVC) industry is the largest consumer of mercury. Production of coal-based PVC, like that produced in China, requires the use of mercury as a catalyst. According to the USGS, mercury used in the production of plastics like PVC may account for as much as 50% of global demand.

Perhaps the most well-known use of mercury is in thermometers and barometers. However, this use is also steadily declining. Galinstan (an alloy of gallium, indium, and tin) has mostly replaced mercury in thermometers because of the alloy's lower toxicity.

Mercury's ability to amalgamate with precious metals, aiding in their recovery, has resulted in its continued use in many developing countries with alluvial gold mines.

While contentious, the use of mercury in dental amalgams continues and, despite the development of alternatives, is still a major industry for the metal.

One of the few uses for mercury that has been growing in recent years is in compact fluorescent light bulbs (CFLs). Government programs encouraging the elimination of less energy efficient incandescent bulbs have supported demand for CFLs, which require gaseous mercury.

Mercury compounds are also used in batteries, drugs, industrial chemicals, paints and mercury-fulminate, a detonator for explosives.

Trade Regulations

Recent efforts have been made by the US and EU to regulate the trade of mercury. Under the Mercury Export Ban Act of 2008, the export of mercury from the US will be prohibited beginning January 1, 2013. Exports of mercury from all EU member states were banned as of March 2011. Norway has already put in place a ban on the production, import, and export of mercury.

Sources:

An Introduction to Metallurgy. Joseph Newton, Second Edition. New York, John Wiley & Sons, Inc. 1947.

Mercury: Element of the Ancients.

Source: http://www.dartmouth.edu/~toxmetal/toxic-metals/mercury/

Encyclopædia Britannica. Mercury Processing (2011).

Retrieved from http://www.britannica.com/EBchecked/topic/375927/mercury-processing