Pressure oxidation is a process for extracting

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

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 Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

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 el ...

, 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 steriliza ...

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 caramelizat ...

. The oxidation of the iron sulfide minerals produces

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

, 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 dur ...

. 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 Schw ...

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 Pueblo Viejo mine is an open-pit gold and silver mine in the Sánchez Ramírez Province of the Dominican Republic where mining operations started in 2012 and expect to cease in 2041. It is the largest gold mine in Latin America and 13th largest g ...

in the

Dominican Republic The Dominican Republic is a country located on the island of Hispaniola in the Greater Antilles of the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean. It shares a Maritime boundary, maritime border with Puerto Rico to the east and ...

. 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 oxidizes 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