Determination of Free Acid in Hydrometallurgical Leach Liquors

Importance of the topic

Accurate measurement of free acid in hydrometallurgical leach liquors is essential for optimizing metal recovery processes, ensuring consistent product quality and minimizing chemical consumption. Monitoring acid levels supports efficient downstream processing and environmental compliance.

Objectives and overview

This application note describes a titrimetric procedure to quantify free acid in acidic leach solutions. The method employs potassium oxalate to mask Fe(III) and other metal ions, followed by NaOH titration to determine the acid concentration expressed as H2SO4 equivalent.

Methodology

• Sample preparation: Pipette 10 mL of acidic leach liquor into a polypropylene titration tube.
• Masking step: Add 10 mL of 30% w/w potassium oxalate solution and 10 mL deionized water; mix thoroughly.
• Titration: Deliver 1.0 M NaOH at 4 mL/min, single exothermic endpoint, stirring at speed 8, 10 s delay before start, data smoothing factor 50.
• Endpoint detection: Monitor temperature rise and calculate the second derivative (ERC) to identify the endpoint precisely.

Instrumentation used

• Automated titrator with 802 stirrer and temperature sensor capable of exothermic endpoint detection.

Key results and discussion

• Leach liquor containing Fe(II), Fe(III), Mg, Al, Mn, Cr, Cu, Co and Ca yielded free acid of 15.7 ± 0.06 g/L (H2SO4 equivalent, n=5).
• Potassium oxalate effectively masked metal interferences, producing a single, sharp endpoint.
• Titration curves showed clear temperature increases aligned with NaOH addition; ERC curves enabled reproducible endpoint determination.

Benefits and practical applications of the method

• Fast and accurate free acid determination supports process control in metallurgical operations.
• Effective masking reduces errors from metal cations, improving accuracy.
• Automation enhances throughput and repeatability for quality assurance and control labs.

Future trends and potential applications

• Incorporation of advanced algorithms and chemometric tools to refine endpoint detection.
• Adaptation to flow injection or in-line titration systems for real-time monitoring.
• Extension to alternative acid systems and broader leach chemistries beyond sulfuric acid equivalents.

Conclusion

The described titrimetric approach delivers a reliable, efficient solution for free acid quantification in hydrometallurgical leach liquors, supporting improved process efficiency, product quality and environmental compliance.

References

• Thermo Fisher Scientific Application Note No. H-104. Determination of Free Acid in Hydrometallurgical Leach Liquors.