Pressure Oxidation Autoclave: Design & Material Selection

Last updated: 2026-10-24

A pressure oxidation autoclave operating on refractory pyrite or arsenopyrite concentrate typically runs at 190–230 °C (375–445 °F) and 3,000–5,000 kPa (435–725 psi) gauge, converting sulfide sulfur to sulfate so that encapsulated gold becomes amenable to cyanidation. This guide sets out the design envelope, zone-by-zone materials selection, operating parameters, and cost drivers that determine whether that vessel belongs in your flowsheet. Among the pretreatment routes compared in our leaching hub, POX is the highest-temperature, highest-pressure option, and it rewards careful engineering of both the vessel and its metallurgy more than any competing process.

Key Takeaways

• Refractory sulfide-hosted gold typically justifies POX when direct cyanide leaching recovers less than 70–80% (ore-specific).
• Acid autoclaves for pyrite and arsenopyrite concentrates operate at 190–230 °C and 3,000–5,000 kPa gauge (typical).
• Lead-lined acid brick with titanium internals remains the benchmark metallurgy for hot, acidic slurry zones.
• Keep chloride below roughly 100–200 mg/L (typical guideline) wherever titanium wetted parts are specified.
• Operate above about 175 °C (typical) so elemental sulfur oxidizes to sulfate instead of forming viscous molten sulfur.

What Is a Pressure Oxidation Autoclave?

A pressure oxidation autoclave is a horizontal, compartmented pressure vessel that oxidizes sulfide concentrates in an acidic, oxygen-sparged slurry at temperatures and pressures far above atmospheric. Pressure oxidation (POX) is an extractive pretreatment process that decomposes pyrite and arsenopyrite, exposing gold grains locked inside sulfide lattices to downstream cyanidation. In the acid systems discussed here, the reactions are strongly exothermic: sulfide sulfur is converted to sulfate, iron reports to solution as ferric ions, and arsenic precipitates as scorodite (FeAsO₄·2H₂O) at temperatures above roughly 180 °C (typical). Net sulfuric acid generation for a pyritic concentrate is commonly in the range of 100–250 kg H₂SO₄ per tonne of concentrate (illustrative, dry basis, dependent on sulfur grade and carbonate gangue content).

about Leaching

Commercial application to refractory gold began in the late 1980s, when the first refractory-gold POX plants demonstrated that whole-ore and concentrate oxidation could reliably unlock submicroscopic gold (Marsden & House, 2006). Since then the technology has become standard practice wherever flotation produces a sulfide concentrate that cyanide cannot treat directly. The vessel itself resembles the horizontal autoclaves used in nickel laterite high-pressure acid leaching, but the chemistry, materials of construction, and gas duty differ substantially, so cross-borrowing designs between the two services is risky.

Several definitional points clarify the anatomy. A compartment division baffle is an internal wall that forces slurry through submerged underflow ports, turning a single long shell into three to six sequential, well-mixed reactors. This staged arrangement approaches plug-flow behavior, which is essential because oxidation kinetics are concentration-dependent. The letdown system is a train of flash vessels that reduces pressure stepwise, recovering steam from the first flash to preheat incoming feed and minimizing the energy penalty of operating hot. Off-gas equipment strips entrained acid mist and vents the small inert gas load that accumulates from oxygen plant product at roughly 99.5% purity (typical for POX service).

What distinguishes a well-designed POX autoclave from a merely adequate one is not the shell thickness but the integration of feed, oxygen dispersion, agitator duty, and materials selection around a severe corrosion environment: hot sulfuric acid containing ferric sulfate, suspended solids, and dissolved oxygen simultaneously. Every design decision downstream of the flowsheet — brick lining, titanium internals, relief philosophy — follows from that combined attack.

How Is a Pressure Oxidation Autoclave Designed and Operated?

The design of a pressure oxidation autoclave starts from the operating window, which is set by reaction chemistry rather than by mechanical convenience. Sulfide oxidation kinetics accelerate steeply with temperature, but elemental sulfur is liquid between roughly 119 °C and its boiling point and reaches peak viscosity near 160 °C (typical), where it wets sulfide particles and passivates them. Operating above about 175–180 °C keeps sulfur in a regime where it oxidizes further to sulfate, avoiding molten-sulfur handling problems. Total pressure must exceed the water saturation pressure at temperature plus the desired oxygen partial pressure, which places acid autoclaves in the 3,000–5,000 kPa (435–725 psi) gauge band.

Feed preparation usually targets a flotation concentrate ground to a P80 of 45–75 µm (typical), pumped to the autoclave through interlock-protected isolation valves. Heat-up on startup uses direct steam injection or, more economically, recycle of the first-flash slurry, whose recovered steam preheats incoming feed; on many plants the sulfide oxidation exotherm is self-sustaining after startup, and the design problem becomes surplus heat removal by quench water at the final compartment. Oxygen is injected through sparger lances beneath each agitator, and oxygen utilization of 80–90% (typical) is expected from well-designed gas dispersion systems. Agitators are dual-impeller machines combining axial flow for solids suspension with radial or hydrofoil action for fine gas dispersion, and each compartment carries its own drive. Oxygen supply comes from on-site vacuum swing adsorption (VSA) or cryogenic plants; because every percentage point of inert gas in the oxygen product must be vented through the off-gas system, POX service generally specifies purity of 99.5% or better (typical). Oxygen piping is designed to ASME B31.3 with strict oxygen-cleaning procedures.

Acid and water balances close the design loop. Sulfide oxidation generates sulfuric acid, carbonate gangue consumes it, and the discharge free acid concentration must stay high enough to keep ferric iron dissolved. Estimate the full balance early with our acid consumption calculation tool, because acid strength drives both the materials selection described below and the neutralization cost after oxidation.

Which Materials of Construction Suit Each Zone of a Pressure Oxidation Autoclave?

Materials selection for a pressure oxidation autoclave is a zone-by-zone exercise, because the slurry zone and the vapor space present different chemistries. The hot slurry zone combines 10–50 g/L free sulfuric acid, ferric sulfate, dissolved oxygen at elevated partial pressure, and moving abrasive solids at 190–230 °C — one of the most aggressive environments in extractive metallurgy. The upper shell and vapor space, by contrast, see condensing acid mist at somewhat cooler temperatures, while the letdown system adds erosion to corrosion. The conventional answer for the slurry zone is a lead membrane covered by two layers of silicate acid brick: the brick provides thermal insulation and erosion protection, while the lead sheet behind it is the true acid barrier protecting the carbon steel shell. Titanium Grade 7 or Grade 2 internals — division baffle wear plates, thermowells, dip tubes, spargers, and agitator blades or full shafts — complete the classic package.

Titanium dominates autoclave hydrometallurgy internals because its passive TiO₂ film is stable in hot, strongly oxidizing acid — exactly the ferric-sulfate/oxygen chemistry of POX. Its limits are specific and must be respected in specification: chloride above roughly 100–200 mg/L in the liquor (typical guideline) promotes pitting and, more insidiously, hydriding embrittlement; fluoride is intolerable at any concentration; and galvanic coupling or overaggressive impressed-current protection can generate hydrogen that titanium absorbs. Where crevices exist — behind wear plates, at gasketed joints — titanium Grade 7 with its palladium addition resists crevice corrosion better than Grade 2.

Three scenarios cover most projects. Scenario one, chloride-free fresh process water and a pyrite or arsenopyrite concentrate: specify lead-lined acid brick in the slurry zone, brick or super austenitic stainless in the vapor space, and titanium Grade 7 internals; this is the benchmark and rarely wrong. Scenario two, seawater or high-chloride process water: eliminate titanium wetted parts entirely and select internals by coupon testing at actual liquor chemistry — nickel alloys in the C-276 or alloy 59 families are the usual candidates — with acid brick and lead retained on the shell; do not specify titanium on the assumption that trace chloride is harmless, because concentration occurs in evaporating films under brickwork. Scenario three, a moderate-temperature duty (below roughly 200 °C) with acid below about 30 g/L and low chloride: an unlined super austenitic vessel or 2205 duplex in the vapor zone can cut cost, but keep brick in the slurry zone. The recommended sequence for any project is therefore: define liquor chemistry from testwork, set chloride and fluoride limits first, then fix the metallurgy zone by zone.

What Operational Problems Commonly Arise, and How Are They Fixed?

1. Acid brick lining failure. Shell hot spots, discoloration of paint, or moisture at leak-detection ports → thermal shock from heat-up or cool-down faster than about 20–40 °C/h (illustrative limit), mortar voids, or mechanical impact → enforce controlled temperature ramps in operating procedures, repair voids during scheduled outages, and monitor shell thermography routinely.
2. Titanium hydriding. Rising hydrogen content in coupon or scrap analysis, brittle fracture of an internal fitting → chloride above the specified limit, fluoride contamination, or hydrogen pickup from galvanic couples and overprotective cathodic systems → requalify the water chemistry, remove titanium from affected circuits, isolate galvanic couples with insulators, and audit any cathodic protection design.
3. Elemental sulfur accumulation. Rising agitator current, viscous slurry, foaming, and falling conversion → operating temperature below the sulfur passivation band or localized oxygen starvation near spargers → raise temperature above ~180 °C (typical), verify oxygen distribution and utilization, and wash out accumulated sulfur during the next shutdown.
4. Oxygen sparger blockage. Falling off-gas oxygen, declining utilization despite unchanged feed → scale or coarse solids plugging sparger holes → fit dual spargers per compartment so one can be isolated and backflushed, strain the feed, and inspect spargers at every outage.
5. Compartment short-circuiting. Sulfide-sulfur conversion consistently below locked-cycle testwork predictions at design residence time → eroded underflow ports or damaged division baffles letting slurry bypass stages → survey internals during shutdown, rebuild baffle wear plates, and confirm agitator gas-holdup performance.
6. Foaming and level instability. Erratic level transmitter readings, slurry carryover into the vent system → residual flotation reagents and fine gas bubbles stabilizing foam → strip reagents in the concentrate handling circuit, dose antifoam under automatic control, and review level control tuning.
7. Jarosite and basic ferric sulfate scale. Loss of working volume, nozzle pluggage, falling heat transfer → free acid below the control band with high ferric concentration → restore acid control, review the acid balance, and plan mechanical descaling; scaling behavior and its diagnostics are covered in detail in our autoclave scaling resource.

What Does a POX Autoclave Cost to Build and Run?

Cost estimating for a pressure oxidation autoclave circuit is dominated by the vessel itself, the oxygen plant, and the letdown and thickening equipment around it. The figures below are illustrative 2026 US dollars for a refractory-gold concentrate duty, based on ASME BPVC Section VIII, Division 2 construction (Div. 2 permits thinner shells than Div. 1 at these pressures, which matters when the shell must also resist brick-lining loads), and they exclude owner’s costs, acid plants, and neutralization facilities. Fabricated weight is the primary driver: a brick-lined Div. 2 shell for a 400 m³ working volume commonly exceeds 300 t of plate (illustrative), and brickwork installation is a specialist trade that can add months to the schedule.

Tank Leaching vs Heap Leaching, step by step

Operating cost beyond oxygen includes neutralization of the acid autoclave discharge, grinding, and labor; oxygen and neutralization typically together account for the largest share of variable cost (illustrative). The decision framework against competing routes follows directly from these numbers. Choose POX when sulfide sulfur grades are high enough to be autogenous (commonly above roughly 20% S, illustrative), when arsenic is present in quantities that make roasting off-gas handling expensive, and when recovery uplift over direct cyanidation exceeds the oxygen and capital burden. Roasting competes on energy recovery and mature sulfur dioxide handling but carries emission-control capex and arsenic trioxide liabilities. Atmospheric leaching routes are cheaper but rarely achieve the sulfide-sulfur oxidation that arsenopyrite-hosted gold requires. Biooxidation is capital-light and robust but slower, with a larger footprint and climate sensitivity — see our POX vs biooxidation comparison and the broader bioleaching overview for that trade-off in full.

Illustrative Composite Case: 1,500 t/d Refractory Gold Concentrate

The following is an illustrative composite case assembled from typical design values; it does not describe any specific client or operation. A project treats 1,500 t/d (dry basis) of a flotation concentrate assaying 31% sulfide sulfur, 7.5% arsenic, and 42 g/t gold. Direct cyanidation of the concentrate recovers only 58% of the gold because most of it is locked in arsenopyrite, so the flowsheet adds POX followed by carbon-in-leach on the washed residue. The selected vessel is a horizontal, brick-lined pressure oxidation autoclave of 4.9 m internal diameter and 32 m length with four compartments, providing roughly 420 m³ of working volume for a 90-minute residence time at 35 wt% feed solids (all figures illustrative).

Leaching Reactor Volume Calculator from Residence Time

Design operating conditions are 210 °C and 3,800 kPa gauge with an oxygen partial pressure of about 600 kPa. Oxygen demand is 130 kg per tonne of concentrate (illustrative), supplied by a VSA plant at 99.5% purity. At design throughput the circuit achieves 97% sulfide-sulfur oxidation and fixes 99.5% of arsenic as scorodite in the autoclave solids, keeping dissolved arsenic in the neutralization feed low. Gold recovery across the POX–carbon-in-leach circuit is 93.5%, against 58% without pretreatment — an uplift of roughly 35 percentage points that pays for the entire autoclave circuit in this illustrative model. Net acid generation of 180 kg H₂SO₄ per tonne of concentrate is absorbed by downstream neutralization with limestone. This composite shows the pattern process engineers should expect: oxidation performance, not vessel mechanics, is what the business case turns on.

Frequently Asked Questions

How long does it take to design, fabricate, and commission a POX autoclave??

Typical schedules run 24–36 months from flowsheet lock to first hot concentrate. Testwork and piloting occupy 3–6 months, vessel fabrication 8–12 months for a large Division 2 shell, site brickwork 2–4 months, and construction plus commissioning the balance. Long-lead items — plate, forgings, agitator drives, and the oxygen plant — should be ordered immediately after design freeze, because brickwork can only begin once code inspections close.

What is the difference between acid POX and alkaline POX??

Acid POX oxidizes sulfide concentrates in sulfuric acid medium, which the reactions themselves generate, and suits pyrite and arsenopyrite feeds. Alkaline POX uses sodium hydroxide or carbonate and is limited to low-sulfide, arsenical feeds, because caustic consumption scales directly with sulfur content and quickly becomes uneconomic. Most refractory-gold and base-metal sulfide applications worldwide are acid autoclaves; alkaline systems remain a niche for specific arsenic-fixation duties.

Can POX treat whole ore instead of flotation concentrate??

Technically yes — the first refractory-gold plants did exactly that — but whole-ore duty multiplies vessel volume and oxygen demand by the mass pull of flotation, often 5–20% of the ore. For low-grade oxide or shallowly sulfide ores, cyanide leaching on a heap leaching system is far cheaper per tonne. Concentrating to a small, high-sulfide mass first is almost always the economic route.

Why is titanium so common inside acid autoclaves??

Titanium’s passive oxide film is stable in hot, oxidizing sulfuric acid containing ferric sulfate and dissolved oxygen — precisely the POX environment — and it tolerates wet oxidized species that destroy stainless steels. Grade 2 serves most duties; Grade 7 with palladium resists crevice corrosion behind wear plates. Its weaknesses are chloride hydriding, fluoride attack, and hydrogen pickup, which is why water chemistry limits belong in the specification, not the operating manual.

What happens to arsenic during POX??

Arsenopyrite and arsenian pyrite oxidize to ferric arsenate species, and above roughly 180 °C (typical) iron and arsenic coprecipitate as crystalline scorodite, FeAsO₄·2H₂O. Scorodite settles well, contains about 30% arsenic by mass, and is accepted as a stable disposal form in many jurisdictions. Below that temperature band arsenic remains soluble as arsenite or arsenate, which multiplies the cost and complexity of effluent neutralization and long-term stewardship.

How is overpressure protection designed for an autoclave??

ASME BPVC Section VIII requires relieving capacity for every credible overpressure scenario: blocked outlet, oxygen plant upset, fire, and control failure. The engineering difficulty is two-phase relief — flashing slurry, not just steam — so relief devices and their inlet piping are sized with two-phase flow methods, often using rupture pins alongside safety valves. All connected piping is designed to ASME B31.3, with oxygen-service cleaning and material compatibility rules applied to every oxygen line.

Is POX compatible with carbon-in-leach gold recovery afterward??

Yes, and it is the standard pairing: the autoclave discharge is washed in counter-current decantation thickeners, partially neutralized, and fed to carbon-in-leach where the now-exposed gold adsorbs onto carbon. Residual elemental sulfur and soluble species from incomplete oxidation are the usual complications, which is why sulfide-sulfur oxidation targets of 95–99% (typical) protect both gold recovery and downstream reagent consumption. Silver recovery is often proportionally lower than gold and should be testwork-confirmed.

Conclusion

For refractory sulfide-hosted gold and arsenical concentrates, a well-engineered pressure oxidation autoclave is the highest-certainty pretreatment route available, provided the design respects three disciplines: an operating window held above the sulfur passivation band, a materials philosophy that matches metallurgy to each corrosion zone with explicit chloride and fluoride limits, and a code-compliant relief and oxygen-handling system designed in from the first heat balance rather than bolted on at the end. Getting these right at the desktop stage costs engineering hours; discovering them at commissioning costs years of production. Most of the uncertainty that remains — kinetics at your liquor chemistry, acid balance, materials coupons in your water — is resolvable with modest testing before any plate is cut. Lexin-tech supports that step directly: our pilot testing program generates the kinetic, acid-balance, and corrosion data your pressure oxidation autoclave design needs, and the calculators in our resources library let your team screen the economics before committing to a pilot campaign.

References

1. Habashi, F. (1993). A Textbook of Hydrometallurgy. Métallurgie Extractive Québec, Québec City.
2. Marsden, J. and House, I. (2006). The Chemistry of Gold Extraction, 2nd ed. SME, Littleton, Colorado.
3. Free, M. L. (2013). Hydrometallurgy: Fundamentals and Applications. Wiley, Hoboken, New Jersey.
4. Perry, R. H. and Green, D. W. (2008). Perry’s Chemical Engineers’ Handbook, 8th ed. McGraw-Hill, New York.
5. U.S. Geological Survey (2024). Mineral Commodity Summaries 2024 — Gold. Reston, Virginia. https://www.usgs.gov/centers/national-minerals-information-center/gold-statistics-and-information
6. ASME. Boiler and Pressure Vessel Code, Section VIII, Divisions 1 and 2: Rules for Construction of Pressure Vessels. American Society of Mechanical Engineers, New York.
7. ASME. B31.3, Process Piping. American Society of Mechanical Engineers, New York.

Changelog

2026-10-24 — First published.