Chiết xuất Thi錫từ Cassiterite — Sàng lọc và Nóng chảy Quặng Thi錫

Cassiterit (SnO₂) là tin dioxide, quặng chính của thiếc. Nó có độ cứng Mohs từ 6-7, trọng lượng riêng rất cao từ 6,8-7,1 (nặng hơn nhiều so với cát thông thường là 2,6), và có màu từ đen đến nâu đỏ. Mật độ cao giúp dễ dàng tách biệt bằng trọng lực (chọn vàng), tương tự như việc chọn vàng.

Cassiterite is found as heavy black or dark brown grains in alluvial (stream) deposits downstream from tin-bearing granite. Look in stream gravels, inside bends of rivers where heavy minerals concentrate, and behind large boulders. Historical tin regions include Cornwall (UK), Southeast Asia, Bolivia, and central Africa.

Shovel gravel and sand from locations where heavy minerals accumulate: bedrock crevices, behind boulders, and the inside of river bends. Fill a bucket with raw gravel. The cassiterite grains are small (1-5 mm) and mixed with ordinary sand, quartz, and other minerals.

Use a gold pan or wide shallow bowl. Add a scoop of gravel and fill halfway with water. Swirl and tilt to wash away light material. Heavy cassiterite grains sink to the bottom center. Continue washing until only black heavy grains remain. This is identical to gold panning technique.

Cassiterite grains are: black to dark reddish-brown, very heavy (noticeably heavier than quartz), have a brilliant adamantine (diamond-like) luster when scratched, and a white to pale brown streak. Magnetite (another heavy black mineral) is magnetic — test with a magnet to separate it from cassiterite.

Repeat panning until you have a concentrate of mostly cassiterite. Use the magnet to remove any magnetite grains. You need at least 500 g of clean cassiterite concentrate for a worthwhile smelt. From 20 kg of gravel in a good tin-bearing stream, expect 100-500 g of concentrate.

Spread the cassiterite concentrate on a flat surface in the sun to dry completely. Wet ore in a hot crucible causes dangerous steam explosions. Drying takes 2-4 hours in direct sunlight. The dry concentrate should feel gritty and pour freely.

Use a clay or graphite crucible that can withstand 1200°C. Mix the dry cassiterite with an equal volume of powdered charcoal. The charcoal provides carbon for reduction: SnO₂ + 2C → Sn + 2CO₂. Tin melts at only 232°C, far below the smelting temperature, so it collects as liquid at the bottom.

Place the loaded crucible in a charcoal forge with bellows. Tin smelting requires lower temperature than copper or iron — about 1000-1200°C is sufficient for reduction. Pack charcoal around and above the crucible. Put on safety equipment: leather apron, face shield, leather gloves.

Pump the bellows to bring the forge to operating temperature. Maintain heat for 1-2 hours. The cassiterite reduces to metallic tin which, being liquid above 232°C, pools at the bottom of the crucible below a layer of slag and ash. Tin smelting is one of the easiest metal reductions.

Using crucible tongs, lift the crucible from the forge. Carefully pour the contents into a preheated sand or stone mold. The heavy molten tin pours first (bottom of crucible), followed by lighter slag. Tin is silvery-white when molten and solidifies quickly due to its low melting point.

Tin solidifies within minutes. Once cool, remove the ingot from the mold. The tin ingot will be covered with a thin layer of slag — break this off by tapping. Clean tin is silvery-white and very soft — you can scratch it with a fingernail (Mohs hardness 1.5).

Tin has distinctive properties: bending a tin bar produces an audible crackling sound called 'tin cry' caused by crystal twinning. It is very soft (scratched by a coin), silvery-white, and does not rust. When heated, it melts easily at 232°C — far below a candle flame temperature.

Weigh the clean tin ingot. From 500 g of good cassiterite concentrate, expect 300-350 g of metallic tin (60-70% yield). Record the source location, concentrate weight, smelt time, and tin yield. This tin is ready for alloying with copper to make bronze, or for tinplating, soldering, and pewterwork.