Tin (Sn, atomic number 50) is a soft, silvery-white metal valued for its low melting point and excellent resistance to corrosion. This versatility makes it useful in solder for electronics, as a protective coating in tinplate for food packaging, and in numerous alloys. Extracting this metal from its raw state is a multi-step industrial process that transforms the mined ore into a commercially usable form.
The Primary Source of Tin
The vast majority of the world’s tin supply comes from a single mineral: Cassiterite (tin dioxide, \(\text{SnO}_2\)). Cassiterite is the sole tin mineral considered economically viable for large-scale extraction due to its high tin content, which can reach 78.77% by weight in its pure form. Its stability also contributes to its concentration in secondary deposits.
Cassiterite is found in two main geological settings, which dictate the initial mining approach. Primary lode deposits occur as veins within hard rock, often granite, requiring conventional underground or open-pit mining techniques. Secondary alluvial, or placer, deposits form when weathering breaks down the primary rock, concentrating the heavy Cassiterite particles in riverbeds or floodplains. These secondary deposits often account for the bulk of accessible reserves and are typically mined using less intensive methods like dredging or hydraulic mining.
Preparing the Ore for Processing
Before chemical extraction, the raw ore must undergo beneficiation, or concentration, to significantly increase the tin content. Mined ore typically contains less than 1% tin, which must be raised to a concentrate grade of 60% to 70% \(\text{SnO}_2\) for efficient smelting. This process involves crushing and grinding the rock to liberate the fine Cassiterite particles from the surrounding waste rock, known as gangue.
Gravity separation is the most common and effective concentration method due to Cassiterite’s physical properties. Cassiterite has a high specific gravity (typically \(6.8\) to \(7.1 \text{ g/cm}^3\)), making it significantly denser than most associated gangue minerals like quartz. Equipment such as shaking tables, jigs, and spirals use this density difference to separate the heavy tin oxide from the lighter waste material. The heavy minerals settle faster in a fluid medium, effectively concentrating the Cassiterite.
For fine-grained material, which gravity methods separate less effectively, auxiliary techniques are employed. Flotation may recover fine particles by exploiting differences in mineral surface properties using chemical reagents. Magnetic separation is used when the ore contains iron-bearing impurities like magnetite or ilmenite, which are magnetic and can be pulled out. This prevents them from interfering with the subsequent smelting process. The goal of this preparation stage is to produce a clean, high-grade concentrate ready for the furnace.
Smelting and Purification
The concentrated tin oxide is subjected to smelting, the core chemical process, which uses carbothermic reduction to liberate the metal. The concentrate is mixed with a carbon source (typically anthracite coal or coke) and a flux, such as limestone, which removes impurities by forming a molten slag. This mixture is charged into a reduction furnace (reverberatory or electric arc) and heated to temperatures between \(1,300^\circ\text{C}\) and \(1,400^\circ\text{C}\).
At these high temperatures, the carbon reacts with the tin oxide, reducing it to molten crude tin metal and carbon monoxide or carbon dioxide gas. The simplified reaction is \(\text{SnO}_2 + \text{C} \rightarrow \text{Sn} + \text{CO}_2\). The molten tin, being denser than the slag, pools at the bottom of the furnace and is periodically tapped off. This initial stage produces crude tin (90% to 99% purity) and a tin-rich slag layer that still holds valuable metal.
Because of the tin content remaining in the slag, a two-stage smelting process is often necessary. The initial slag is re-treated under harsher reducing conditions to recover the remaining tin metal. The crude metal from both stages then moves on to the purification, or refining, stage to remove residual metals like iron, copper, lead, and arsenic.
The most common method for purification is fire refining, which often includes liquation. Since tin has a low melting point (\(232^\circ\text{C}\)), the crude tin is slowly heated on a sloping hearth, allowing the pure tin to melt and flow away from impurities with higher melting points. Further refining steps, like “boiling” (stirring the molten tin with air or steam to oxidize and skim off impurities), bring the metal to commercial purity, typically around 99.85%. For high-purity applications, electrolytic refining is used, where the crude tin is cast into anodes and dissolved in an acidic electrolyte, resulting in a deposit of tin on the cathode that can reach purity levels of 99.99% or higher.