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Hòa tan vàng từ quặng bằng hóa học được khám phá năm 1887
Tổng quan
Gold cyanidation dissolves gold from crushed ore using dilute sodium cyanide solution, a process patented by MacArthur and Forrest in 1887. Despite gold's famous chemical inertness, it dissolves readily in aerated cyanide solution due to the extraordinary stability of the gold-cyanide complex [Au(CN)₂]⁻. This process recovers over 90% of gold from ore and accounts for approximately 90% of all gold produced worldwide. Gold is recovered from the cyanide solution by adsorption onto activated carbon (CIP/CIL) or by zinc cementation (Merrill-Crowe process).
Quy trình hóa học
Crushed and milled gold ore (typically 1-10 g Au/ton) is leached in dilute NaCN solution (0.02-0.05%) at pH 10-11 with air sparging for 24-72 hours. Gold dissolves as [Au(CN)₂]⁻. In the CIL (carbon-in-leach) process, activated carbon is added to the leach tanks to adsorb the gold complex. Loaded carbon is stripped with hot caustic cyanide, and gold is recovered by electrowinning onto steel wool cathodes, followed by smelting to dore bullion.
Au(CN)₂⁻ + C(activated) → Au(CN)₂⁻·C (carbon adsorption)
2Au(CN)₂⁻ + Zn → Zn(CN)₄²⁻ + 2Au (zinc cementation, Merrill-Crowe)
Nguyên liệu thô
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Sodium cyanide (NaCN) — Andrussow process (HCN from CH₄ + NH₃ + O₂) + NaOH (Gold complexing agent (lixiviant))
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Crushed gold ore — Open pit or underground gold mining (Gold source (1-10 g/ton typical))
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Calcium oxide (CaO, lime) — Calcination of limestone (pH control (maintains pH >10 to prevent HCN))
Sản phẩm cuối
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Gold dore bullion — Refined to 99.99% gold for investment, jewelry, electronics (85-95% Au, remainder Ag and base metals)
Environmental Impact
Cyanide is extremely toxic to all forms of life. Tailings dam failures (Baia Mare 2000, Samarco 2015) have caused devastating environmental disasters. Cyanide-bearing tailings require detoxification (typically with H₂O₂ or SO₂/air). The International Cyanide Management Code provides voluntary standards. Mercury amalgamation by artisanal miners causes severe mercury pollution.
Lưu ý an toàn
- ⚠ Sodium cyanide is acutely lethal — LD₅₀ 6 mg/kg (oral)
- ⚠ HCN gas forms if pH drops below 9.5 — continuous pH monitoring critical
- ⚠ Tailings storage facilities pose long-term geotechnical and environmental risks
- ⚠ Personal protective equipment and cyanide antidote kits mandatory on-site
Đổi mới gần đây
Thiosulfate leaching (non-toxic alternative) is commercially applied at Barrick's Goldstrike mine (Nevada).
Glycine-based gold leaching is being developed as a milder alternative.
Cyanide destruction using INCO SO₂/air process reduces WAD cyanide to <1 ppm in tailings.
Gold recovery from electronic waste (urban mining) uses selective leaching.
Quy mô sản xuất
3500
tấn/năm
$220 billion
giá trị thị trường
Xem thêm Metallurgy & Metal Processing
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