In this two-part series, Fadila Khelfaoui and Luc Vernhes explore how valve coatings perform under the extreme conditions of hydrometallurgical processes like HPAL and POx. Part one will examine the limitations of conventional coatings and the challenges they face in severe service. Part two will introduce VEL-8 – a breakthrough ceramic coating engineered to deliver superior durability and reliability in the harshest environments.
By Fadila Khelfaoui and Luc Vernhes
In hydrometallurgical operations, valves are exposed to some of the most extreme service conditions in the industry. High-Pressure Acid Leaching (HPAL) and Pressure Oxidation (POx) expose equipment to elevated temperatures, high pressures, and highly corrosive slurries. These conditions strain not only valve bodies and trim but also the integrity of protective coatings, especially on sealing surfaces and moving components. Understanding the limitations of current coating technologies is essential to advancing reliability and unlocking next-generation solutions.
Understanding the process environment
HPAL and POx are both used to extract metals from ores that are difficult to treat through conventional means:
- HPAL targets nickel and cobalt recovery from laterites, operating at 250–300°C and pressures up to 65 bars, using concentrated sulfuric acid slurries.
- POx treats ores containing gold, copper, molybdenum and uranium at 200–230°C and pressures up to 40 bars, using oxygen and acid in a slurry phase.
Despite their chemical differences, both processes rely on critical equipment such as valves, pumps and autoclaves as shown in Figure 1. These components must endure highly aggressive conditions caused by hot sulfuric acid corrosion, erosion and scale buildup, particularly in feed and discharge lines. Failures in these areas are typically attributed to coating degradation rather than mechanical breakdown. As a result, coating performance is a key factor in ensuring sustained operational reliability.
The global footprint of HPAL and POx plants (Figure 2) shows widespread adoption of Velan severe-service ball valves. According to field data and customer feedback, over 95% of these valves meet or exceed operational expectations, confirming the strength of the platform and underscoring the need for continued coating innovation.
Table 1: Field performance benchmarking of legacy coating technologies.
| QtyX, Appl. (NPS,Cl) | Time in service (months) | number of cycles | common failure mode |
| 3X heater feed pump (10, 150) | 30-36 | > 500 | Some components exhibited coating delamination, while others were cleaned and returned to service |
| 12X feed pump suction (10, 300) | 30-36 | > 500 | |
| 12X feed pump discharge (10, 300) | 12-24 | >500 | |
| 6X autoclave discharge (14, 600) | ≤6 | ~ 50 | Coating delamination and erosion on the ball and seat surfaces |
In-service coating limitations
Over the last two decades, improvements in valve design and metallurgy have advanced equipment performance in hydrometallurgy. However, coating durability remains a key limitation, particularly in the most aggressive service zones. While some valves last up to 36 months, those in discharge lines often fail in 6 months or less due to:
- Cracking and spallation, especially in high impact, thermally cycled zones
- Delamination, exposing base materials to direct slurry attack
- Erosion and thinning, leading to leakage and eventual valve failure
These degradation modes highlight the need for a more advanced coating solution that delivers not only corrosion and wear resistance, but also mechanical toughness.
Evolution of coating technology in hydrometallurgy
Coating development has evolved to meet the increasing demands of HPAL and POx operations (see Table 2). Initial efforts in the 1990s relied on chromium oxide (Cr2O3) coatings. While effective in POx, they lacked corrosion resistance for HPAL and degraded prematurely. By 1998, titanium dioxide (TiO2) coatings emerged as a more corrosion-resistant alternative. However, they fell short in abrasive slurry environments due to limited wear resistance. In 2006, Velan introduced a Cr2O3–TiO2 composite, designed to balance wear and corrosion protection. While more robust, the system still faced issues with crack propagation and long-term durability. Recognising this gap, Velan launched a dedicated R&D programme to develop a next-generation coating — VEL-8.
Table 2: Historical progression of coating solutions in hydrometallurgy.
| 1995 | 1998 | 2006 | 2018 | 2019-2020 | 2021 |
| Cr2O3 | TiO2 Ta bond | Proprietary blend Cr2O3-TiO2 |
VEL-8 | VEL-8 field trial | VEL-8 industrialization |
| Used in the earliest HPAL installations – Failure by corrosion |
Cr2O3 replaced with TiO2 to improve corrosion – Insufficient wear |
Cr2O3-TiO2 replaced TiO2 to balance corrosion & wear validated by autoclave test and field performance – Limited failures due to crack propagation |
VEL-8 replaced Cr2O3-TiO2 to enhance toughness and crack resistance – Patented technology |
Field trials in HAPL & POx on valve and non-valve | Mass production
Global deployment |
Conclusion
Despite notable advancements over the past two decades, legacy coatings continue to fall short when exposed to the most aggressive service zones in hydrometallurgical operations. Challenges such as delamination, erosion and cracking persist, particularly in high-impact, high-temperature, and corrosive environments like autoclave discharge lines. These recurring limitations have highlighted the need for a more robust and reliable surface protection solution that can withstand the combined demands of corrosion, wear and mechanical stress.
Recognising this critical gap, Velan launched a focused research and development initiative aimed at redefining coating performance under extreme process conditions. This effort led to the creation of VEL-8, a next-generation coating engineered to deliver enhanced durability, crack resistance, and long-term reliability. The development journey and performance highlights of VEL-8 will be detailed in Part 2 of this article.
What will follow in part two
In part two, Fadila Khelfaoui and Luc Vernhes will introduce VEL-8, a next-generation ceramic coating engineered for the harshest hydrometallurgical environments. They will explore its unique formulation, advanced application process and superior lab and field performance. From extended wear life to enhanced crack resistance, VEL-8 sets a new benchmark for reliability in HPAL and POx operations.
About this Technical Story
This Technical Story is an article from our Valve World Magazine, August 2025 issue. To read other featured stories and many more articles, subscribe to our print magazine. Available in both print and digital formats. DIGITAL MAGAZINE SUBSCRIPTIONS ARE NOW FREE.
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