Pressure oxidation is a process for extracting

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...

from refractory ore. The most common refractory ores are

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

and

arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases eleme ...

, which are sulfide ores that trap the gold within them. Refractory ores require pre-treatment before the gold can be adequately extracted. The pressure oxidation process is used to prepare such ores for conventional gold extraction processes such as cyanidation. It is performed in an

autoclave An autoclave is a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure in relation to ambient pressure and/or temperature. Autoclaves are used before surgical procedures to perform sterilizat ...

at high pressure and temperature, where high-purity oxygen mixes with a slurry of ore. When the original sulfide minerals are oxidized at high temperature and pressure, it completely releases the trapped gold. Pressure oxidation has a very high gold recovery rate, normally at least 10% higher than

roasting Roasting is a cooking method that uses dry heat where hot air covers the food, cooking it evenly on all sides with temperatures of at least from an open flame, oven, or other heat source. Roasting can enhance the flavor through caramelization ...

. The oxidation of the iron sulfide minerals produces

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

, soluble compounds such as ferric sulfate, and solids such as iron sulfate or

jarosite Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct duri ...

. The iron-based solids produced pose an environmental challenge, as they can release acid and heavy metals to the environment. They can also make later precious metal recovery more difficult. Arsenic in the ore is converted to solid

scorodite Scorodite is a common hydrated iron arsenate mineral, with the chemical formula FeAsO4·2H2O. It is found in hydrothermal deposits and as a secondary mineral in gossans worldwide. Scorodite weathers to limonite. Scorodite was discovered in the S ...

inside the autoclave, allowing it to be easily disposed of. This is an advantage over processes such as roasting where these toxic products are released as gases. A disadvantage of pressure oxidation is that any silver in the feed material will often react to form silver jarosite inside the autoclave, making it difficult and expensive to recover the silver. An example of a mine utilizing this technology is the

Pueblo Viejo mine The Pueblo Viejo mine is a gold mine located in the north-central region of the Dominican Republic in the Sánchez Ramírez Province. It is the largest gold mine in Latin America and fifth largest in the world. It is also the first exploited mine ...

in the

Dominican Republic The Dominican Republic ( ; es, República Dominicana, ) is a country located on the island of Hispaniola in the Greater Antilles archipelago of the Caribbean region. It occupies the eastern five-eighths of the island, which it shares wit ...

. At Pueblo Viejo, the process is performed by injecting high-purity oxygen into autoclaves operating at 230 degrees C and 40 bar of pressure. The resulting chemical reactions oxides the sulfide minerals the gold is trapped within. The oxidation of pyrite is highly exothermic, allowing the autoclave to operate at this temperature without an external heat source.

References

Metallurgy Metallurgical processes {{Industry-stub