How do HPAL and pressure oxidation differ in their autoclave requirements?

Michaël Dreesen · 08 Nov 2026

60+ years of experience
120+ employees
Units up to 10 m diameter
Up to 1,000 tons

HPAL and pressure oxidation (POX) autoclaves differ mainly in the chemistry they contain. HPAL leaches nickel and cobalt from laterite ore in hot sulfuric acid at high pressure, while POX oxidizes sulfide minerals, typically in gold ore, so the metal can be recovered afterward. Those chemistries drive different linings, materials and mechanical demands.

Both processes use large, thick-walled, multi-compartment vessels, so the gap lies in corrosion, erosion and scale rather than in the basic concept. The sections below cover operating conditions, materials, lining options, mechanical design, fabrication and fabricator selection.

Why do HPAL and POX autoclaves operate under different conditions?

HPAL and POX autoclaves operate under different conditions because the reactions differ. HPAL dissolves nickel and cobalt from laterite using sulfuric acid at high temperature and pressure. POX uses oxygen to break down sulfide minerals that lock up gold. Each reaction sets its own corrosion, abrasion and pressure demands.

In HPAL, the slurry is acidic, hot and loaded with fine, abrasive solids. The vessel handles aggressive acid chemistry, and its internals suffer erosion and scaling as well. In POX, the oxidizing environment generates acid in situ, and the exact severity depends on the ore, the oxygen supply and whether the circuit runs under acidic or alkaline conditions.

Pressure oxidation has also been introduced in nickel applications as a greener route to extraction, which blurs the line between the two process families. Exact temperatures, pressures and residence times always come from the process owner or technology licensor, so the mechanical designer works from a stated set of design conditions rather than a generic range.

Which materials of construction suit HPAL versus POX autoclaves?

Titanium suits both HPAL and POX autoclaves, usually as a lining over a carbon steel pressure shell. Grade 17 titanium has been used as lining material in titanium-clad autoclaves for nickel, cobalt and gold. The final choice depends on acid strength, chloride content, temperature and erosion risk.

Titanium earns its place because it resists acidic, oxidizing and chloride-bearing media that would destroy standard stainless steel. Carbon steel supplies the structural strength, and the titanium layer supplies the corrosion barrier. This pairing keeps the vessel mechanically sound without the cost and weight of a solid titanium shell.

Other materials also appear in the wider autoclave system:

Titanium grades for lining the vessel, internals and nozzles
Carbon steel for the pressure-containing shell
Nickel alloys such as Hastelloy or Inconel for selected components in severe service
Zirconium or tantalum for specific high-severity zones, depending on the process

Field experience shows that material selection alone does not prevent damage. Over the years, titanium autoclave designs have been modified in their details to reduce the erosion issues commonly seen in operation, which shows how much design and material work together.

How do clad and brick-lined autoclave constructions compare?

Clad construction bonds a corrosion-resistant metal layer, such as titanium, to a carbon steel shell, while brick-lined construction protects a steel shell with a membrane and acid-resistant bricks. Clad vessels offer a metallic, thin and dimensionally controlled barrier. Brick-lined vessels rely on a layered insulating system with different maintenance needs.

Clad construction

Clad construction suits projects where a thin, continuous metallic barrier is preferred. Explosion cladding allows carbon steel to serve as the structural base while a titanium layer faces the process. Its strengths are a thin lining profile, good thermal behavior and a barrier that can be inspected and repaired by welding. Its challenges are weld quality across the clad layer and the demanding fabrication skills needed.

Brick-lined construction

Acid-resistant brick lining protects the shell with several layers, typically a membrane under refractory brick. It tolerates abrasion and high temperature but adds thickness, weight and installation work. Relining is a significant maintenance task. The shell design must also account for the lining system and its thermal behavior.

Neither route is universally better. The choice depends on the process owner’s experience, the erosion profile, the planned maintenance interval and the project schedule. Buyers should compare lifecycle considerations, not just the initial fabrication scope.

What mechanical design features do large autoclaves need?

Large autoclaves need thick-walled pressure shells, multiple compartments, agitator mounting provisions, erosion-resistant internals and careful nozzle design. The design must follow recognized pressure codes and account for lining behavior, thermal loads and transport limits. Mechanical design turns process requirements into equipment that can be fabricated and operated safely.

Key design considerations usually include:

Code compliance with the applicable pressure equipment code, such as ASME or PED, including the required calculations.
Advanced analysis such as finite element, thermal and fatigue assessment for nozzles, supports and discontinuities.
Compartment and internals layout including baffles, weirs and agitator supports that suit the slurry flow.
Lining integration so that clad or brick systems are considered from the first design step, not added afterward.
Transport and installation planning because very large vessels may weigh several hundred tons and must reach the site in practical sections.

A strong design also considers manufacturability. Details that look simple on a drawing, such as a nozzle in a clad wall or a closely spaced weld, can become costly in the workshop. Engineering for fabrication reduces this risk.

What fabrication challenges come with HPAL and POX autoclaves?

The main fabrication challenges are welding and forming thick plate, bonding and welding titanium cladding without contamination, controlling dimensional accuracy on very large shells, and handling extreme weights. Reference projects include individual autoclaves of roughly 510 to 960 tons, which shows the scale involved.

Several issues deserve attention:

Cladding integrity. Titanium is reactive when hot, so welding needs strict cleanliness and shielding to avoid brittle weld zones.
Thick-wall welding. Heavy sections demand qualified procedures, controlled heat input and thorough examination.
Dimensional control. Large diameters and long shells must stay within tolerance so that internals, heads and agitator mounts fit.
Heavy handling. Rolling, lifting, turning and shipping equipment of this size requires dedicated workshop capacity and logistics planning.
Inspection and testing. Non-destructive examination and pressure testing must cover both the base shell and the lining.

Fabricators with proven experience in titanium-clad construction and heavy plate work reduce the risk of rework, delay and early failure in service.

How should buyers choose an autoclave fabricator for either process?

Buyers should choose an autoclave fabricator on demonstrated capability, not price alone. The key criteria are proven experience with similar size and materials, in-house mechanical engineering, recognized certifications, workshop capacity for heavy equipment and a transparent approach to quality control. These factors determine whether a project meets its schedule and performance targets.

A practical checklist covers:

Verifiable references for large autoclaves in comparable materials and weights
Experience with titanium or other clad constructions in thick sections
Certifications such as ISO 9001, PED and ASME
Lifting, rolling and handling capacity for the required dimensions and tonnage
Engineering support that can review process requirements for manufacturability early
Clear inspection plans, documentation and project management structure

Early involvement matters. Bringing the fabricator into the design phase lets the team resolve manufacturability questions before steel is cut, which usually protects both cost and schedule across the project’s life.

How Coek helps with HPAL and POX autoclaves

Coek Engineering designs the mechanical side of custom autoclaves and fabricates them in titanium-clad construction, working from process requirements supplied by customers, EPC contractors and technology owners. Coek has delivered autoclaves for nickel, cobalt and gold projects, including units for Goro Nickel and Ambatovy.

Titanium-clad autoclave fabrication using Ti Grade 17 lining, with design details refined over time to address erosion
Reference vessels with total weights of roughly 510 to 960 tons
In-house mechanical engineering, including finite element and thermal analysis, to codes such as ASME
ISO 9001, PED and ASME certifications
Manufacturing capability for equipment up to about 10 m in diameter and 1,000 tons

To see the full range of capabilities, explore our industrial autoclaves. If you are scoping an HPAL or POX project, contact our team to discuss your requirements.

Frequently Asked Questions

How do I get started specifying an autoclave for an HPAL or POX project?

Start by collecting the design conditions from your process owner or technology licensor: temperature, pressure, slurry chemistry (acid strength, chloride content), solids loading and required residence time. Then share these with an experienced fabricator and mechanical designer early, ideally before the basic design is frozen, so they can review lining options, compartment layout, code requirements (ASME or PED) and transport limits. A short feasibility review at this stage usually prevents costly redesign later.

What are the most common causes of premature failure in titanium-lined autoclaves?

The most frequent causes are erosion of the lining and internals by abrasive slurry, weld defects or contamination in the titanium layer, and poor detailing around nozzles, agitator mounts and compartment weirs. Operating outside the stated design conditions, such as higher chloride levels or temperature excursions, can also accelerate damage. Good design detailing, qualified welding procedures and strict control of operating parameters address most of these risks.

How often does an autoclave lining need inspection or repair?

There is no universal interval, since it depends on ore abrasiveness, slurry chemistry, lining type and the operator’s maintenance plan. Most operators schedule inspections during planned shutdowns, using visual checks, thickness measurements and non-destructive examination of the lining and welds. Titanium cladding can often be repaired by welding, while brick lining typically requires partial or full relining, so build the expected maintenance interval into your lifecycle cost comparison.

What should I do if my existing autoclave shows erosion in the compartments or internals?

First, document the damage pattern with a detailed inspection and thickness mapping, and compare it against the slurry flow, agitator position and baffle or weir layout. Then work with the designer or a fabricator to decide between local repair, replacement of internals or a modified design that reduces flow velocities at the affected areas. Titanium autoclave designs have been refined over the years specifically to limit erosion, so upgrading details during a repair campaign can extend service life.

Can one autoclave design be reused for both HPAL and POX duties?

Not without a careful review. Although the basic vessel concept is similar, differences in acid strength, chloride levels, oxygen supply and erosion profile can change the required lining, materials and internals. If you are considering switching duty or running a new process in an existing vessel, have the mechanical designer re-evaluate the design conditions, lining compatibility and code calculations against the new process data.

How do I plan transport and installation for a vessel weighing several hundred tons?

Decide early whether the autoclave will be shipped as a complete vessel or in sections, since this affects shell design, welding at site and lining installation. Check the fabricator’s proximity to a port or waterway, the route restrictions from port to site, and the lifting equipment available at the site. Involving a logistics specialist during the design phase helps align vessel dimensions and weight with practical transport limits.

What documentation and testing should I expect from the fabricator?

Expect design calculations to the applicable code, welding procedure and welder qualifications, material certificates, a fabrication and inspection plan, and records of non-destructive examination covering both the carbon steel shell and the lining. A final pressure test and dimensional report are standard. Agreeing on the inspection and test plan, and on any third-party inspection requirements, before fabrication starts avoids disputes and delays later.