Mercury: Extraction, Production, and Practical Applications
Mercury is a highly toxic metal that, unlike other metals, exists as a liquid at room temperature. It is fairly uncommon in that it accounts for about 0. 08 parts per million (ppm) of Earth’s crust. Most of the world’s supply was produced in Spain and Italy, but production was either stopped of the mines were hollowed out. Now most mercury is found and produced in China. Typically, mercury is not found in nature as a pure metal, it is found bonded to other elements within ores near hot spring or volcanic regions. The most common ore in which mercury is found is a red sulfide called cinnabar (HgS).
Major Extraction Processes and Refining
Mercury is extracted from its ores one of two ways. It can be done through pyrometallurgy, which applies heat and is referred to as a distillation process, or it can be done though hydrometallurgy, which utilizes an aqueous solution of sodium hydroxide and sodium sulfide. Hydrometallurgy is more expensive than pyrometallurgy, and it is not effective when dealing with ores of irregular composition. The pyrometallurgy process for mercury is achieved first by mixing the cinnabar ore with charcoal or coke, which is then placed at the top of the furnace. Then, by heating the air in the furnace to approximately 300°C, the sulfide ore reacts with the Oxygen in the air and causes the formation of Sulfur dioxide (SO2). Due to the low boiling point of mercury, the metal is able to be liberated from the sulfide in a gas form. After the mercury gases escape the sulfide, they are condensed into liquid mercury by being forced through a series of U-shaped tubes. The oxidation-reduction reaction that occurs within the furnace creates thermal energy, which further raises the temperature of the furnace to approximately 700°C, and dries the remaining coke. For the pyrometallugy process, there are different furnace types used depending on where the extraction process is being done. In European countries, the most common type of furnace used is the Cermak-Spirek shaft furnace. This furnace can reduce both coarse and fine materials, and can accept multiple grades of mercury ore. In the United States the most typical furnaces used are rotary furnaces and multiple hearth furnaces, which have higher capacities and allow for continuous operations. The United States also takes advantage of mechanical feeding of ores and discharge, which is safer for workers within the facilities because it reduces the risk of exposure to mercury vapors, sulfur dioxide fumes, dust, and debris. This mechanical method also reduces the cost of labor needed to run the processing facilities.
World Production
For most of the 1900's, the United States produced the most mercury at 1,000 tons per year. The United States also was the largest mercury exporter exporting nearly 390 tons a year. During the 1970's, many regulations were passed that reduced human contact with mercury due to its toxic nature. Mercury was last produced as a primary commodity in the United States in 1992 and by 2000's there were no longer any mercury producing mines and production was limited to only metal recovered from various scrap materials including batteries, fluorescent lamps, medical devices, thermostats, and mercury-contaminated soils. In 2012, only two mercury electrolysis plants were in operation in the United States and as of January 1st, 2013, the export of elemental mercury was banned in the United States under the Mercury Export Ban Act of 2008. Major countries including China, Mexico, Kyrgyzstan, Russia, Peru, Spain, Slovenia, and Ukraine have most of the world's mercury resources. It is estimated that approximately 2,000 tons of mercury is produced annually. However, most countries do not directly report their mercury production values and values that are reported have a high degree of uncertainty.
The amount of mercury reported above fluctuates dramatically year to year because mercury is frequently stored in the production country until a large amount of material is available to be exported. With the consumption of mercury declining from year to year, the current production and present resources will be adequate for another century of use. Mercury consumption will likely decline over the next few years due to the increased use of fluorescent light bulbs, which us minimal mercury, and digital thermometers instead of mercury containing thermometers. Also, as stricter environmental regulations and new technologies are developed, mercury supply and demand can change drastically in the near future.
Applications
Mercury has many interesting applications due to the fact that it is the only metal that is liquid at room temperature. One of the most common uses is as a thermometer. Mercury has a very high coefficient of thermal expansion compared to other metals with a value of 182 10^-6/K, for example versus values of 69, 87, and 54 for aluminum, lead, and silver, respectively. Because it is so sensitive, even a small change in temperature results in significant volume expansion that is readable on a marked scale. It also does not stick to glass. Another property of mercury is that is has a very high density. This made it very useful as a barometer to measure pressure. The pressure is measure in the millimeters of mercury. However, its use in in decline due to its toxic properties.
Another common use of mercury was in the manufacturing of sodium hydroxide and chlorine by the electrolysis of brine. This process of producing sodium, called the Castner-Kellner process, was perhaps the largest use of mercury. The metallic sodium is formed as a mixture at a cathode that is made from mercury. Water is then reacted with the sodium to make sodium hydroxide. Nowadays, most production facilities are using membrane cell or diaphragm cell technologies instead of the mercury cell process due to safety concerns.
Mercury was also commonly used in medical applications, specifically in dental amalgams. It was used in combination with gold, silver, and tin, which is a very pliable mixture and can expand to fill the entire cavity when inserted into the tooth. One of its organic compounds called thimerosal was used to help preserve vaccines. However, this is not really in use anymore due to speculations of links between the mercury compound and triggering autism in young children. There have been no scientific studies proving this but it has still been removed from or reduced to negligible amounts in all US vaccines. Another mercury compound, merbromin, was also a common ingredient in antiseptic for minor scrapes and cuts. However, again it is not still in popular use in the US.
Mercury is being phased out (or is already phased out) of most applications due to the fact that exposure to it can cause serious health problems and is especially a threat to the development of a child. Depending on the type of mercury, duration and route of exposure, and the age of the person the severity of health effects may vary. It primarily affects the neurologic, GI, and renal organ systems. Mercury is now only mainly used in the chemical industry as a catalyst for reactions or in electrical switches and rectifiers.
