technical case study
Impurities and Chloride Effects In real-world scenarios, sulfuric acid is rarely chemically pure. Halides, especially chlorides, severely impact stainless steel performance. Chloride presence combined with higher temperatures worsens corrosion resistance. At 200 ppm chloride, even 904L and 254SMO lose protection across much of the concentration spectrum, particularly between 40–60%. Material selection must then prioritise lower temperatures. With over 2000 ppm of chlorides, protection at high acid concentrations diminishes significantly, although performance improves at lower concentrations and higher temperatures. Metal Ions and Localised Corrosion Metal ions present in the solution also influence corrosion behaviour. Pure sulphuric acid typically causes uniform corrosion, but metal ions act as oxidisers, strengthening the passive chromium oxide layer and improving corrosion resistance. However, high metal ion concentrations and elevated temperatures increase the risk of localised corrosion such as pitting. Alloy additions like molybdenum and nitrogen can mitigate this risk. Environmental Scenarios A summary of typical plant conditions is shown in the following scenarios: • Scenarios A - C: Sulphuric acid, varying metal ion and
During dewatering, separation, and washing, thickeners are employed. Grade 2404 performs well in uranium extraction, while grade 2101 is effective for copper and cobalt. Leaching is not always acidic. For lithium hydroxide production, the environment is alkaline, using sodium carbonate at 20 bar and 200°C. Material Selection Factors Key considerations for material selection include:
• Internal environment • Service temperature • External environment • Structural requirements
• Fabrication processes • Life Cycle Costing (LCC) Sulphuric Acid Resistance
Sulphuric acid is the most commonly used chemical in hydrometallurgical applications. Iso-corrosion diagrams are the best analytical tool for material suitability. In these diagrams, areas below the line indicate corrosion rates under 0.1 mm/year, considered acceptable. For example, grade 904L, developed specifically for sulfuric acid, covers the full concentration range if kept below certain temperatures. Grade 316L is only suitable at extremely low and high concentrations. Titanium and carbon steel can also perform well at these extremes. Although not shown, grade 254SMO performs effectively across the full range.
chloride concentrations at 98°C • Scenario D: Conditions at 150°C
23
Issue 3 – 2025
Made with FlippingBook - professional solution for displaying marketing and sales documents online