Pressure leaching dissolves or transforms target minerals inside a sealed vessel held above the normal boiling point of the solution. Raising temperature accelerates reactions that run too slowly at atmospheric conditions. Raising pressure keeps the aqueous phase liquid and forces reactant gas into solution. The vessel that provides those conditions is an autoclave.
Pressure and temperature start the mechanical design envelope. They do not finish it. The same vessel must contain hot acid or caustic, keep an abrasive slurry suspended, disperse oxygen without igniting its own internals, and stay serviceable between shutdowns.
IN BRIEF
-
Pressure leaching exists to run reactions that atmospheric leaching cannot complete economically. The autoclave is the containment that makes those conditions possible.
-
POX, HPAL and alkaline pressure leaching are not variants of one duty. Each attacks different minerals with different chemistry, and each creates a different equipment environment.
-
Pressure and temperature size the pressure boundary. Chemistry, slurry solids, agitation and heat balance decide whether the equipment survives the service.
-
Acid-service autoclaves usually separate the two jobs: carbon steel carries the pressure, and a separate lining or bonded cladding carries the corrosion duty. That choice drives cost, internal volume and outage pattern.
-
Availability is often the binding constraint rather than recovery, and inspection access, lining repair and wear-part replacement are specification decisions rather than maintenance decisions.
On this page: what pressure leaching is · why an autoclave · POX vs. HPAL vs. alkaline · inside the vessel · mechanical envelope · materials and construction · slurry and erosion · heat transfer · maintainability · flowsheet position · specification checklist · FAQs
What pressure leaching is, and where it sits in hydrometallurgy
Hydrometallurgy recovers metals through aqueous chemistry rather than smelting. Leaching is the first step: a lixiviant — acid, caustic, carbonate, or an oxidant-bearing solution — dissolves the target metal from ore or concentrate. Pressure leaching is that operation carried out in a closed vessel above atmospheric pressure, and the change does two things at once.
Temperature raises reaction rate; pressure makes the temperature possible
Slow kinetics is the recurring problem in aqueous processing. Sealing the vessel lets the solution run well above 212°F (100°C) without boiling, which is where the rate gain comes from. Separately, adding reactant gas above the solution’s own vapor pressure raises that gas’s dissolved concentration. Pressure oxidation shows the distinction: oxygen is specified as an overpressure on top of the saturated steam pressure, because the two are controlled independently.
Published equipment literature reports autoclave duties spanning roughly 212°F to 536°F (100–280°C) and 3 to 70 bar (about 44 to 1,015 psi). No single vessel design covers that band, and no ore is a candidate simply because the technology exists. Pressure leaching earns its capital cost only where atmospheric processing recovers too little or consumes too much reagent.
Why the process runs in an autoclave
An industrial mineral-processing autoclave is a large pressure vessel holding reacting slurry at controlled temperature, pressure and chemistry for a defined residence time. It shares only a name with laboratory and medical sterilizers. The vessel must deliver pressure containment, elevated temperature, controlled chemistry, gas addition, continuous slurry handling and adequate residence time — and agitation keeps all of those from varying across it.
Continuous, compartmented and agitated
Many commercial pressure-leach autoclaves operate continuously on a constant feed, with the vessel divided into compartments by internal walls. Each compartment carries its own agitator and baffles.
Compartments are a residence-time device. A single stirred volume behaves like one well-mixed tank, and some feed short-circuits to the outlet. Agitated compartments in series narrow that distribution, so more of the feed sees the full reaction time, with slurry moving between compartments over internal weirs. Batch autoclaves remain common in laboratory and pilot work, where the objective is kinetics data rather than throughput.
POX, HPAL and alkaline pressure leaching are different duties
These terms are related but not interchangeable. They describe different chemistries applied to different minerals, and confusing them leads straight to the wrong materials specification. The clean distinction is what each process attacks. POX oxidizes sulfide sulfur. HPAL dissolves oxide and hydroxide minerals with sulfuric acid. Alkaline pressure leaching uses caustic or carbonate chemistry, usually because the ore contains acid-consuming minerals or the target forms a soluble alkaline salt.
| Process | Typical feed context | General chemistry | Dominant equipment challenge |
|---|---|---|---|
| Pressure oxidation (POX) | Refractory gold in pyrite/arsenopyrite; copper and zinc sulfide concentrates | Oxygen oxidizes sulfides; the reaction generates sulfuric acid | Exothermic heat removal; oxygen handling and titanium ignition risk; hot acid with abrasive residue |
| High-pressure acid leach (HPAL) | Nickel–cobalt laterite, including iron-rich limonitic ores | Sulfuric acid attack at high temperature; iron rejected as hematite | Heat input and steam recovery; hot sulfuric acid; scale that reduces availability |
| Alkaline pressure leaching | Scheelite and wolframite; alkaline POX on carbonaceous gold ores; bauxite digestion | Sodium hydroxide or carbonate dissolves the target as a soluble salt | Caustic-side corrosion and scaling; a different alloy logic entirely |
Pressure oxidation
POX exists because some gold is not accessible to cyanide. Where an appreciable portion of the gold is locked inside pyrite or arsenopyrite, conventional cyanidation cannot reach it, and oxidizing the host sulfide exposes it for a downstream leach. Published SGS data for refractory-gold POX gives representative conditions of 190 to 230°C (about 374°F to 446°F) with an oxygen overpressure of 350 to 700 kPa (50 to 100 psi). Those are reported figures for that duty, not a universal setting.
Sulfide oxidation generates its own sulfuric acid and releases heat, so a POX autoclave operates in an acid environment it creates internally, and it often needs to reject heat rather than add it.
High-pressure acid leach
HPAL is the nickel and cobalt laterite route, and it attacks oxide and hydroxide minerals rather than oxidizing sulfides. Published results report nickel and cobalt recoveries above 95% in 60 to 90 minutes at 300 to 400 kg of sulfuric acid per ton of ore, which is low consumption relative to other laterite routes, and describe the process as effective across a wide range of ore samples including refractory iron-rich limonitic material.
Operating conditions vary by orebody and by licensor, and should be taken from the specific project rather than from a general figure. Equipment supplier literature for HPAL service cites roughly 480°F (250°C), 40–60 bar (580–850 psi) and 25–35% solids as representative of the duty.
The equipment consequence is heat and scale. The reaction needs steam, so heat recovery drives circuit design. And the liquor leaves the autoclave supersaturated with aluminum, iron and silica species, which allows scale to form on interior surfaces and reduce equipment availability.
Alkaline pressure leaching
Alkaline chemistry is selected when acid is the wrong reagent. Tungsten is the clearest case: scheelite and wolframite are digested with sodium hydroxide or sodium carbonate to yield soluble sodium tungstate, at reported conditions around 160–200°C and 8–12 bar for the caustic route. Alkaline POX is also applied to carbonaceous and high-carbonate gold ores, where carbonate would consume acid before it reached the sulfide. Bauxite digestion is the same operation under another name, with reported digestion temperatures of about 140–150°C for gibbsitic bauxite against 240–270°C for boehmitic and diasporic material. Mineral form sets the operating regime, not the metal recovered.
What happens inside the vessel
Slurry enters at temperature, chemistry is established, and gas is introduced where the reaction requires it. Agitation keeps solids suspended and evens out temperature and concentration while the reaction proceeds and heat is added or removed. Product slurry leaves through a pressure letdown system. None of those steps is independent. Poor suspension creates a settled bed, reducing reacting surface area. Poor gas dispersion starves the reaction by limiting dissolved oxygen at the particle surface. Both produce local gradients, which change the corrosion environment the wetted metal actually sees. The vessel does not experience an average condition; it experiences the worst local one.
Agitation
Agitation is a process function, not a mixing convenience. It suspends solids, disperses gas, distributes heat, and suppresses the dead zones where deposits and localized corrosion start. Baffles do the geometric half of that work, turning horizontal flow into vertical movement, suppressing vortex formation and strengthening axial circulation.
For the vessel fabricator, agitation appears as interface engineering: nozzle location and reinforcement, agitator support loads, shaft and seal penetrations, baffle and compartment wall attachment, and the tolerances those internals demand. One documented titanium-internals project held vessel roundness to 0.4% so the internals would fit. The agitation system is typically supplied by a specialist mixing vendor; the vessel has to accept it.
Oxygen injection and a real design tension
Where oxygen is the reactant, injection and dispersion set the achievable reaction rate, and gas-phase oxygen concentration is a controlled variable rather than a maximized one. That control has a second purpose. Titanium is commonly used for wetted components in POX service, and it burns in oxygen-enriched atmospheres once its oxide film is breached. Published engineering guidance identifies limiting oxygen concentration and preventing high-velocity gas impingement on surfaces as the controls that reduce that risk. Because oxygen concentration depends on autoclave pressure and temperature, ignition risk becomes a function of operating procedures and control interlocks.
The same variable that drives oxygen mass transfer therefore drives ignition risk. This is one reason oxygen spargers are frequently specified in super duplex stainless or a nickel alloy rather than titanium, even inside a titanium-lined vessel.
Pressure and temperature set only part of the mechanical envelope
Design pressure and temperature drive the conventional work: shell and head thickness, allowable stress, flange ratings, nozzle reinforcement, supports and thermal expansion. Cyclic duty from startup, shutdown and upset belongs there, as does the reline outage — a thermal cycle whether or not it appears on the data sheet. Two further points deserve care.
The construction code is set by the project and the jurisdiction. ASME Boiler and Pressure Vessel Code Section VIII, Division 1 covers "the design, fabrication, inspection, testing, and certification of pressure vessels operating at either internal or external pressures exceeding 15 psig." It is a consensus standard, carrying legal force where a jurisdictional authority adopts it. Adopted editions differ, and the National Board’s NB-370 synopsis compiles what each jurisdiction requires.
Code requirements and owner requirements are different layers. ASME Section VIII, Division 1 Part UCL addresses vessels with corrosion-resistant integral cladding, weld metal overlay cladding, or applied linings. Owner and EPC specifications may impose requirements beyond the construction code. The Code sets a floor; the project specification sets the requirement. The Harris overview of pressure vessel design codes covers how those layers stack.
Chemistry decides the wetted materials, and construction separates the two jobs
Acid-service autoclaves often separate the pressure-retaining and corrosion-resistance functions rather than relying on a single solid-alloy construction. It is built so two different jobs are done by two different materials.
Pressure boundary and corrosion barrier are separate systems
The typical arrangement puts a carbon steel pressure vessel behind an acid-resistant membrane and refractory lining system, with titanium or other corrosion-resistant metals used for nozzle liners, compartment walls and baffles. The result is a composite of ceramics, metals and polymers.
Within that composite the layers do different work, and a specification that treats the lining as one item usually gets this wrong. The membrane is the chemical barrier, protecting the steel from acid attack. The brick over it is the mechanical shield, protecting the membrane from abrasion and impact.
Membrane-and-brick against bonded titanium
The alternative bonds a corrosion-resistant metal directly to the pressure boundary, most often explosion-bonded titanium clad plate under ASTM B898, which removes the thick refractory wall and recovers internal volume. As process severity and operating temperature rise, bonded clad construction generally becomes the more practical choice, though neither approach is universally correct.
| Membrane and acid brick | Explosion-bonded titanium clad | |
|---|---|---|
| Internal volume | Lining consumes significant diameter | Recovers volume for the same shell |
| Repair | Repairable in place; specialist masonry trades | Welding repair on a thin barrier |
| Principal risk | Membrane damage under a failed brick course | Cladding thinned by repeated repair |
| Outage pattern | Recurring multi-week reline campaigns | Fewer planned interventions, higher consequence per repair |
Wetted materials beyond the shell
Titanium carries a service-specific caution beyond ignition. Hydriding requires three conditions present together: low or high pH, or a surface damaged by abrasion; temperature above about 70°C (158°F); and a mechanism generating hydrogen, such as a galvanic couple or dynamic abrasion. A POX autoclave can satisfy all three at once, which makes it a design consideration rather than a corrosion allowance. The exotic alloy selection guide covers the wider titanium envelope.
Shell metallurgy does not finish the materials problem in any case. Agitator shafts and impellers, compartment walls, baffles, spargers, dip pipes, nozzle liners, thermowells and fasteners all face the same environment with less metal and more stress. Galvanic pairing is the detail most often missed: dissimilar wetted metals in a conductive acidic liquor form couples, which is why insulating details appear between components of different alloys.
Slurry and solids: "abrasive" is a measurement, not an adjective
Adding solids changes the problem in ways clean-liquid experience does not predict. Solids settle, so the vessel needs continuous suspension. They abrade, so wear becomes a design variable. They deposit in low-velocity zones, where corrosion under deposits behaves differently from general corrosion. The common error runs one way: assuming every fine mineral slurry is severely abrasive. Particle size, hardness, solids concentration, velocity and flow regime all contribute, and a soft fine-ground residue at low velocity can be far less damaging than a coarse hard fraction through a nozzle.
That judgment can be measured. ASTM G75 establishes the Miller Number as an index of the relative abrasivity of slurries, and the SAR Number as an index of the relative abrasion response of materials. The standard treats slurries at approximately 50 or lower as pumpable with minor abrasive damage, and expects greater damage above that. The method is framed around pump liquid-ends, so it does not transfer directly to autoclave internals — but it turns an adjective into a number.
Where erosion and corrosion act together they are not additive. Erosion removes the passive film, corrosion attacks the fresh surface, and the combined rate can exceed the sum of the separate mechanisms; ASTM G119 addresses that synergism.
Heat transfer runs in both directions
Thermal design here is process-specific in a way that catches engineers who have seen only one type of autoclave. Sulfide oxidation is exothermic, so where sulfide content is high enough a POX circuit runs autogenously and must reject heat, typically through quench water or cooling coils. HPAL is the opposite case: the reaction requires heat, so slurry preheating and steam addition drive the design.
Both lean on flash steam. Preheaters use vent steam from the flash vessels to raise feed slurry temperature, and the flash vessels themselves return slurry to atmospheric conditions in one or more stages. Staged letdown of this kind behaves much like regenerative feedwater heating in a power plant, and it carries a consequence that is easy to miss: recovering flash steam avoids diluting the leach liquor as well as saving fuel.
The surrounding equipment inherits three complications: supersaturated liquors scale transfer surfaces as readily as vessel walls, slurry-side duties constrain tube diameter and velocity, and the exchanger sees the same liquor as the vessel. Feed preheaters, flash letdown, condensers and solution coolers are conventional fabricated equipment in unconventional service, and shell-and-tube heat exchangers in high-alloy or clad construction are among the configurations applied to these duties.
Maintainability decides the campaign, and it is specified up front
Practical output is set by availability, and availability is set by how quickly the vessel can be inspected, cleaned, repaired and returned to service. That is decided during specification, not during the first outage. Scale deposited from supersaturated liquors is an access problem before it is a chemistry problem, and so is the recurring reline. The items worth settling are consistent across duties:
-
Access for internal inspection, lining survey and internals removal, including manway size and position
-
Inspection methods for the corrosion barrier — liner and membrane survey, and thickness verification on clad construction
-
Nozzle and penetration inspection, since nozzle liners are usually the most complex detail in the vessel
-
Agitator provisions: shaft removal, seal service, and the lifting access those require
-
Wear components made replaceable, with monitoring locations agreed before fabrication
-
A repair philosophy, including how much cladding thickness may be lost before replacement rather than rework
Where the autoclave sits in the flowsheet
The autoclave is one unit in a train, and the equipment around it is often where the fabrication scope lands: feed tanks, slurry preheaters, blowback vessels, flash vessels, off-gas scrubbers, condensers and neutralization vessels. Pressure leaching is also an intermediate step — the autoclave produces a leached slurry, not a product. Downstream operations may include solid–liquid separation, precipitation, solvent extraction, crystallization and electrowinning, and raffinate or bleed streams frequently reach evaporation and crystallization equipment.
What engineers should define before requesting equipment
An inquiry carrying pressure, temperature and volume alone cannot be quoted accurately, because those numbers leave out most of what determines the design. Five groups of information make it answerable.
-
Process — feed mineralogy; solids concentration; particle size and hardness; lixiviant chemistry and expected free acid or alkali; gas requirement; heat release; residence time
-
Thermal — operating and design temperature; heating and cooling strategy; startup and shutdown cycles; available utilities
-
Mechanical — operating and design pressure; working volume and fill level; orientation; compartment arrangement; cyclic conditions; agitator loads; transport limits
-
Materials — corrosion data for the actual liquor, including trace oxidizing species and halides; erosion expectations, measured where possible; required wetted materials; the corrosion-barrier approach and its repair philosophy
-
Integration — agitator supplier and interface; instrumentation and nozzle schedule; supporting equipment tie-ins; maintenance access; governing code and jurisdiction
Harris Thermal fabricates ASME Section VIII, Division 1 pressure vessels, shell-and-tube heat exchangers and custom process equipment for demanding mining and mineral-processing applications, including evaporators, condensers and crystallizers. Our high-alloy fabrication capabilities — titanium, zirconium, tantalum, nickel alloys, duplex and super duplex stainless, 6-moly grades and clad construction, with nondestructive examination, PMI and hydrostatic testing in house — can support pressure-leach and hydrometallurgical equipment requirements where the application fits our design and fabrication capabilities.
The most useful thing an engineer brings to an equipment inquiry is not a vessel sketch. It is a complete description of the environment the equipment has to survive.
FAQs about pressure leaching and mineral-processing autoclaves
What is pressure leaching in mineral processing?
Pressure leaching dissolves or chemically transforms target minerals inside a sealed vessel held above the solution’s normal boiling point. The elevated temperature accelerates reaction kinetics; the elevated pressure keeps the solution liquid and raises the dissolved concentration of any reactant gas. It is used where atmospheric leaching recovers too little or consumes too much reagent.
What is a mining autoclave?
A mining autoclave is a large pressure vessel that holds reacting slurry at controlled temperature, pressure and chemistry for a defined residence time. Commercial units commonly operate continuously and are divided into agitated compartments in series, which narrows the residence-time distribution. It has nothing in common with a laboratory sterilizer beyond the name.
What is the difference between pressure leaching and pressure oxidation?
Pressure leaching is the general category: any leach carried out in an autoclave above atmospheric pressure. Pressure oxidation (POX) is the specific type in which oxygen oxidizes sulfide minerals, most often to liberate refractory gold or treat copper and zinc concentrates. All POX is pressure leaching; not all pressure leaching is POX.
What is HPAL?
High-pressure acid leaching is the sulfuric acid pressure leach applied to nickel–cobalt laterite ore, including iron-rich limonitic material. Unlike POX it attacks oxide and hydroxide minerals rather than oxidizing sulfides, so it requires heat input rather than heat removal. Published results report nickel and cobalt recoveries above 95% with residence times of roughly 60 to 90 minutes.
What materials are used for pressure-leach autoclaves?
Acid-service autoclaves usually separate the pressure boundary from the corrosion barrier. A carbon steel shell carries the pressure, protected either by an acid-resistant membrane with brick over it, or by bonded titanium cladding in direct process contact. Internals use titanium, nickel alloys, super duplex stainless or non-metallics.
Why is agitation important in a pressure-leach autoclave?
Agitation keeps solids suspended so reacting surface stays available, disperses reactant gas, evens out temperature and concentration, and suppresses dead zones where deposits and localized corrosion begin. Because it is a process function, it drives vessel geometry, nozzle arrangement and dimensional tolerances.
