In refractory gold ores and refractory copper/nickel concentrates, valuable metals are physically encapsulated or chemically bonded inside sulfide crystal lattices (primarily pyrite FeS2, arsenopyrite FeAsS, or chalcopyrite CuFeS2), preventing cyanide leaching or acid extraction. In a POX autoclave operating at 200°C to 230°C and 3,000 to 3,500 kPa under high oxygen overpressure, sulfides are rapidly oxidized into soluble sulfates and iron precipitates (hematite Fe2O3 or basic iron sulfate), breaking down the host mineral matrix and unlocking over 95% of the gold.

The total pressure P_total inside an operating POX autoclave is the sum of the steam saturation pressure P_H2O(T), the oxygen partial pressure P_O2, and non-condensable inert gases (CO2, N2, Ar). At 225°C, pure water saturation pressure is approximately 2,550 kPa absolute. If the total operating pressure is 3,250 kPa_abs and inerts contribute 150 kPa, the effective oxygen partial pressure driving force is P_O2 = 3,250 - 2,550 - 150 = 550 kPa. Maintaining adequate P_O2 (typically 350 to 700 kPa) is critical to prevent oxygen mass transfer starvation.

The oxidation of sulfide sulfur to sulfate is violently exothermic, releasing approximately 11,500 to 13,000 kJ per kilogram of oxidized sulfur. While a fraction of this heat heats the incoming cold pulp from pre-heat temperature (~65-85°C) to operating temperature (220°C), substantial excess heat remains. To maintain constant isothermal conditions in each compartment without boiling dry, high-pressure cold quench water is continuously injected. The quench water rate is directly derived from the net thermal excess divided by the enthalpy rise of the injected water.

Industrial autoclaves use 4 to 6 compartments separated by vertical weir baffles. Hydrodynamically, this creates a continuous stirred-tank reactors in series (CSTR-in-series) flow pattern with minimal axial backmixing. The first compartment experiences the highest reaction rate, largest oxygen consumption, and greatest heat release, while subsequent compartments provide the necessary residence time to achieve deep oxidation conversions (>96%) on slower-reacting composite mineral grains.

At operating temperatures above 180°C, iron sulfates hydrolyze into hematite (Fe2O3), basic ferric sulfate (FeOHSO4), or jarosites (Na/K/H3O-Fe3(SO4)2(OH)6). These compounds precipitate on vessel walls, agitator blades, thermowells, and internal baffles. If operating acidity is too low, heavy jarosite and basic iron sulfate scaling accumulates rapidly, choking weir inter-compartment openings and blinding level instrumentation.