Tin, a metallic element with the symbol Sn and atomic number 50, is a soft, silvery-white metal known for its low melting point of \(231.88^\circ\text{C}\). Its historical use dates back to the Bronze Age, around 3000 BCE, when it was alloyed with copper to create bronze. Today, tin is widely used in solder, tinplate for food containers, and various alloys because of its corrosion resistance and non-toxic nature. Production involves a multi-stage process to transform raw, low-grade ore into a commercially viable, high-purity metal.
Primary Sources of Tin Ore
The vast majority of the world’s tin is sourced from a single mineral, tin dioxide, commonly known as Cassiterite. This dense, heavy mineral is the only tin mineral of commercial significance and is found in two main types of geological deposits. Primary deposits, known as lode deposits, occur as hydrothermal veins within granite formations, often containing up to four percent tin content. These hard-rock sources are found in regions like Bolivia and parts of China. Secondary deposits, also called alluvial or placer deposits, result from the weathering and erosion of primary lodes. Because Cassiterite is highly resistant to weathering, it accumulates as fine-grained material in ancient riverbeds and offshore areas. Approximately 80 percent of the world’s identified tin resources are found in these secondary deposits, which typically average a much lower tin content, often around \(0.01\) percent. Major global production is concentrated in Southeast Asia, with China and Indonesia being among the largest producers.
Preparing the Ore for Extraction
The initial stage of processing focuses on increasing the concentration of Cassiterite within the mined ore. Since the Cassiterite is embedded within hard rock, the first mechanical steps involve crushing and grinding the ore to liberate the mineral from the surrounding gangue. Care must be taken during this size reduction process to prevent “over-crushing” the brittle Cassiterite, which creates excessively fine particles difficult to separate later. The ore is typically passed through jaw and cone crushers before being finely ground in rod mills. Following crushing, the process relies heavily on gravity separation techniques, which exploit the significant density difference between the heavy tin dioxide and lighter waste rock. Jigs, spirals, and shaking tables are used to separate the heavier Cassiterite from the lighter minerals, often after washing and desliming steps to remove fine clay and silt. For complex ores, additional separation steps are necessary to further purify the concentrate. These steps include froth flotation, which removes sulfide minerals, and magnetic separation, which eliminates iron oxides. The resulting high-grade tin concentrate typically assays around 70 percent tin and is ready for the high-temperature chemical extraction process.
The Smelting Process
Smelting is the chemical reduction process where concentrated tin oxide is transformed into crude metallic tin. This extraction occurs in a high-temperature environment, typically utilizing reverberatory or electric furnaces. The prepared tin concentrate is mixed with a reducing agent, usually carbon (anthracite coal or coke), along with a flux such as limestone. The carbon reduces the tin oxide to molten tin metal, releasing carbon monoxide gas. The high temperatures ensure that the tin melts and separates from the other materials. The added flux reacts with non-tin impurities, primarily silicates and iron oxides, to form a molten layer of slag. Since the molten tin is denser, it settles at the bottom of the furnace, while the lighter slag floats on top. This density difference allows the two layers to be tapped off separately. The product of this initial smelting is crude tin, which may contain about 80 percent tin, and a tin-rich slag that requires further processing to recover its remaining metal content.
Refining and Purification
The crude tin produced from the smelting furnace is not pure enough for most commercial applications and must undergo a final purification stage. The crude metal contains various impurities, including iron, lead, copper, bismuth, and antimony. Refining aims to achieve a commercial-grade product, generally 99.8 percent tin or higher. Fire refining is a series of pyrometallurgical steps that rely on the different melting points and chemical behaviors of the metals. Liquation is often the first step, where the crude tin is heated just above its melting point, allowing the pure tin to melt and run off, leaving behind solid impurities with higher melting points, such as iron compounds. Subsequent steps involve oxidation, where compressed air or steam is blown through the molten tin. This process oxidizes remaining impurities like zinc and aluminum, which rise to the surface as dross to be skimmed off. For complex purification, particularly the removal of lead and bismuth, vacuum distillation may be employed to selectively vaporize these impurities at high temperatures. When the highest purity is required, electrolytic refining is used as a final step. In this process, an electric current is passed through an electrolyte solution, causing the pure tin to dissolve from the impure anode and deposit onto a cathode, capable of producing tin with a purity up to 99.999 percent.