Tin is a highly workable metal that was once as valuable as silver for jewelry, coins, and special dishware. Today it is used as sheets in the construction of buildings and roofs, for soldering or joining metal parts, for storage containers, and in alloys like bronze and Babbitt metal
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What is Tin?
Tin (Sn) is a relatively soft and ductile metal with a silvery white colour. It has a density of 7.29 grams per cubic centimetre, a low melting point of 231.88 °C (449.38 °F), and a high boiling point of 2,625 °C (4,757 °F). Tin is allotropic; that is, it takes on more than one form. The normal form is white tin, or beta tin, which has a body-centred tetragonal crystal structure. The second allotrope, gray or alpha tin, has a face-centred cubic structure. Gray tin is theoretically stable below 13 °C (55 °F), but in practice it is readily formed only at about −40 °C (−40 °F). This transformation is difficult to initiate and is severely retarded by the presence of alloying elements or trace impurities. Nonetheless, it has given rise to the extremely rare laboratory curiosity known as tin pest.
Tin finds industrial application both as a metal and in chemical compounds. As a metal it is used in a very wide variety of industrial applications—but almost always in combination with other elements as an alloy or coating, since its intrinsic softness precludes its use as a structural material. Although tin is usually a minor constituent in alloys, it is an essential one on account of the way in which its special properties confer improvements to the matrix metal.
The major commercial applications of tin are in tinplate, solder alloys, bearing metals, tin and alloy coatings (both plated and hot-coated), pewter, bronzes, and fusible alloys. In its chemical reactions, tin exists in two valence states (II and IV) and is amphoteric (able to react as both an acid and a base). In addition, it can link directly with carbon to form organometallic compounds. These properties have given rise to many important uses for tin chemicals—for example, in electroplating, agricultural and pharmaceutical products, and plastics and ceramics.
When and where tin was discovered?
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Tin, its alloys, and its compounds have been known to humans for thousands of years. A number of references to the element can be found in the Bible. Tin was apparently known to other civilizations also. For example, the sacred Hindu book Rig Veda, written in about 1000 B.C., mentions tin among other metals known to the Hindus.
The alloy of tin known as bronze was probably produced even earlier than the pure metal. An alloy is made by melting and mixing two or more metals. The mixture has properties that are different than any of the metals alone. The Egyptians, Mesopotamians, Babylonians, and Peruvians were producing bronze as far back as 2000 B.C. The alloy was probably discovered accidentally when copper and tin compounds were heated together. Over time, a method for producing consistent bronze was developed.
Bronze became popular among ancient peoples because it was harder and tougher than copper. Before the discovery of bronze, many metal items were made out of copper. But copper is soft and bends easily. Bronze is a much better replacement for copper in tools, eating utensils, and weapons. Bronze marked a significant advance in human civilization. This strong alloy improved transportation methods, food preparation, and quality of life during a period now known as the Bronze Age (4000—3000 B.C. ).
The origin of the name tin is lost in history. Some scholars believe it is named for the Etruscan god Tinia. During the Middle Ages, the metal was known by its Latin name, stannum. It is from this name that the element’s symbol, Sn, is derived.
Beyond bronze, tin’s greatest contribution to humankind was probably the humble tin can. The can had its origins in the perennial problem of how to feed an on-the-move army. Napoleon Bonaparte offered a reward in 1795 to anyone who could come up with a way to preserve food for military use. In 1810, French chef Nicolas Appert won the 12,000-franc prize by inventing canning — the process of sealing food or drink in a jar or bottle with the use of boiling water.
This discovery cleared the way for the invention of the tin can only a year later. In 1810, British merchant Peter Durand got a patent for using tinplated steel to can food. Tin resists corrosion, making it an ideal covering for relatively cheap steel.
The tin can arrived on American shores in 1818, and Thomas Kensett & Co, a manufacturing company, patented the tin can in America in 1825. The Civil War prompted the increased popularity of the tin can, as generals once again searched for a way to feed their soldiers.
Tin’s heyday ended in the mid-20th century, however, when Coors Brewery introduced the first aluminum can. Cheaper, lighter and recyclable, aluminum rapidly overtook tin and steel.
What are its characteristics?
- Strength: Tin is one of the weakest metals. You can, for example, bend or crush a tin can with your bare hands. This property does not allow tin to be used on its own as a structural metal.
- Ductility: Tin is a very ductile metal at room temperature, and is also quite malleable. When chilled below 55 degrees Fahrenheit, tin slowly changes from a form known as “beta tin” to “alpha tin,” which is much less ductile. Tin is also much less ductile above roughly 392 degrees Fahrenheit.
- Conductivity: Tin and some of its alloys are excellent electrical conductors. Over half of the tin used industrially ends up in solder for making electrical connections.
One of tin’s most interesting properties is its tendency to give off a strange screeching sound when it is bent. This sound is sometimes known as “tin cry.”
Tin is relatively unaffected by both water and oxygen at room temperatures. It does not rust, corrode, or react in any other way. This explains one of its major uses: as a coating to protect other metals. At higher temperatures, however, the metal reacts with both water (as steam) and oxygen to form tin oxide.
Similarly, tin is attacked only slowly by dilute acids such as hydrochloric acid (HCl) and sulfuric acid (H 2 SO 4 ). Dilute acids are mixtures that contain small amounts of acid dissolved in large amounts of water. This property also makes tin a good protective covering. It does not react with acids as rapidly as do many other kinds of metals, such as iron, and can be used, therefore, as a covering for those metals.
Tin dissolves easily in concentrated acids, however, and in hot alkaline solutions, such as hot, concentrated potassium hydroxide (KOH). The metal also reacts with the halogens to form compounds such as tin chloride and tin bromide. It also forms compounds with sulfur, selenium, and tellurium.
What are the various stages of metal processing?
The process of extracting tin from tin ore varies according to the source of the ore deposit and the amount of impurities found in the ore. The tin deposits in Bolivia and England are located deep underground and require the use of tunnels to reach the ore. The ore in these deposits may contain about 0.8-1.0% tin by weight. Tin deposits in Malaysia, Indonesia, and Thailand are located in the gravel along streambeds and require the use of dredges or pumps to reach the ore. The ore in these deposits may contain as little as 0.015% tin by weight. Over 80% of the world’s tin is found in these low-grade gravel deposits.
Regardless of the source, each process consists of several steps in which the unwanted materials are physically or chemically removed, and the concentration of tin is progressively increased. Some of these steps are conducted at the mine site, while others may be conducted at separate facilities.
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- When the gravel deposits are located at or below the water level in the stream, they are brought up by a floating dredge, operating in an artificial pond created along the streambed. The dredge excavates the gravel using a long boom fitted either with chain-driven buckets or with a submersed rotating cutter head and suction pipe. The gravel passes through a series of revolving screens and shaker tables onboard the dredge to separate the soil, sand, and stones from the tin ore. The remaining ore is then collected and transferred ashore for further processing. When the gravel deposits are located in dry areas at or above the water level in the stream, they are first broken up with jets of water pumped through large nozzles. The resulting muddy slurry is trapped in an artificial pond. A pump located at the lowest point in the pond pumps the slurry up into a wooden trough, called a palong, which has a gentle downward slope along its length. The tin ore, which is heavier than the sand and soil in the mud, tends to sink and is trapped behind a series of wooden slats, called riffles. Periodically the trapped ore is dumped from the palong and is collected for further processing.
- The ore enters the cleaning or dressing shed adjacent to the mining operation. First, it passes through several vibrating screens to separate out coarser foreign materials. It may then pass through a classifying tank filled with water, where the ore sinks to the bottom while the very small silt particles are carried away. It may also pass through a floatation tank, where certain chemicals are added to make the tin particles rise to the surface and overflow into troughs.
- Finally the ore is dried, screened again, and passed through a magnetic separator to remove any iron particles. The resulting tin concentrate is now about 70-77% tin by weight and consists of almost pure cassiterite.
- The tin concentrate is placed in a furnace along with carbon in the form of either coal or fuel oil. If a tin concentrate with excess impurities is used, limestone and sand may also be added to react with the impurities. As the materials are heated to about carbon, the carbon reacts with the carbon dioxide in the furnace atmosphere to form carbon monoxide. In turn the carbon monoxide reacts with the cassiterite in the tin concentrate to form crude tin and carbon dioxide. If limestone and sand are used, they react with any silica or iron present in the concentrate to form a slag.
- Because tin readily forms compounds with many materials, it often reacts with the slag. As a result, the slag from the first furnace contains an appreciable amount of tin and must be processed further before it is discarded. The slag is heated in a second furnace along with additional carbon, scrap iron, and limestone. As before, crude tin is formed and recovered along with a certain amount of residual slag.
- The residual slag from the second furnace is heated one more time to recover any tin that has formed compounds with iron. This material is known as the hard head. The remaining slag is discarded.
- The crude tin from the first furnace is placed in a low-temperature furnace along with the crude tin recovered from the slag plus the hard head. Because tin has a melting temperature much lower than most metals, it is possible to carefully raise the temperature of the furnace so that only the tin melts, leaving any other metals as solids. The melted tin runs down an inclined surface and is collected in a poling kettle, while the other materials remain behind. This process is called liquidation and it effectively removes much of the iron, arsenic, copper, and antimony that may be present.
- The molten tin in the poling kettle is agitated with steam, compressed air, or poles of green wood. This process is called boiling. The green wood, being moist, produces steam along with the mechanical stirring of the poles. It was from this crude, but effective use of wood poles that the poling kettle got its name. Most of the remaining impurities rise to the surface to form a scum, which is removed. The refined tin is now about 99.8% pure.
- For applications requiring an even higher purity, the tin may be processed further in an electrolytic refining plant. The tin is poured into molds to form large electrical anodes, which act as the positive terminals for the electrorefining process. Each anode is placed in an individual tank, and a sheet of tin is placed at the opposite end of the tank to act as the cathode, or negative terminal. The tanks are filled with an electrically conducting solution. When an electrical current is passed through each tank, the tin is stripped off the anode and is deposited on the cathode. The remaining impurities, which are generally bismuth and lead, fall out of the solution and form a slime at the bottom of the tank.
- The cathodes are remelted, and the refined tin is cast in iron molds to form ingots or bars, which are then shipped to the various end users. Lower purity tin is usually cast into ingots weighing 25-100 lb (11-45 kg). Higher purity tin is cast into smaller bars weighing about 2 lb (1 kg).
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How is it used today?
Tin is also used in the manufacture of other alloys. Bronze, for example, is an alloy of tin and copper. It is used in a wide variety of industrial products, such as spark-resistant tools, springs, wire, electrical devices, water gauges, and valves.
One application of tin that was once important is in the manufacture of “tin foil.” Tin foil is a very thin sheet of tin used to wrap candies, tobacco, and other products. The tin protected the products from spoiling by exposure to air. Today, most tin foil is actually thin sheets of aluminum because aluminum is less expensive.
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A very important application of tin is tinplating. Tinplating is the process by which a thin coat of tin is placed on the surface of steel, iron, or another metal. Tin is not affected by air, oxygen, water, acids, and bases to the extent that steel, iron, and other metals are. So the tin coating acts as a protective layer. Perhaps the best known example of tin plating is in the production of food cans. Tin cans are made of steel and are covered with a thin layer of tin. Most food and drink cans today are made out of aluminum because it is cheaper.
Another tin alloy is Babbitt metal. Babbitt metal is a soft alloy made of any number of metals, including arsenic, cadmium, lead, or tin. Babbitt metal is used to make ball bearings for large industrial machinery. The Babbitt metal is laid down as a thin coating on heavier metal, such as iron or steel. The Babbitt metal retains a thin layer of lubricating oil more efficiently than iron or steel.
The largest amount of tin used in the United States goes to the production of solder. Solder is an alloy, usually made of tin and lead, with a low melting point. It is used to join two metals to each other. For example, metal wires are attached to electrical devices by means of solder. Solder is also used by plumbers to seal the joint between two metal pipes.
Other uses are:
- tin chloride (SnCl 2 ): used in the manufacture of dyes, polymers, and textiles; in the silvering of mirrors; as a food preservative; as an additive in perfumes used in soaps; and as an anti-gumming agent in lubricating oils.
- tin oxide (SnO 2 ): used in the manufacture of special kinds of glass, ceramic glazes and colors, perfumes and cosmetics, and textiles; and as a polishing material for steel, glass, and other materials
- tin chromate (SnCrO 4 or Sn(CrO 4 ) 2 ): brown or yellowish-brown compounds used as a coloring agent for porcelain and china
- tin fluoride (SnF 2 ) and tin pyrophosphate (Sne 2 P 2 O 7 ): used as toothpaste additives to help protect against cavities
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info sources: https://www.britannica.com/technology/tin-processing