Free Moisture and LOI in Hydrated Lime

Importance of the Topic

Hydrated lime is a critical material in water treatment, food processing and construction, where its moisture content and degree of calcination directly impact product quality and performance. Thermogravimetric analysis (TGA) enables precise quantification of free moisture and loss on ignition (LOI), providing a robust quality-control method for assessing hydration and purity of lime samples.

Objectives and Study Overview

This study demonstrates the application of the LECO TGA701 macro-thermogravimetric analyzer for simultaneous measurement of free moisture, LOI at 600 °C and LOI at 1000 °C in hydrated lime. The goal is to establish a reproducible batch method for up to 19 samples, evaluate instrument performance and provide statistical data on repeatability across two instruments and a series of sample batches.

Applied Instrumentation

• Thermogravimetric analyzer: LECO TGA701 macro TGA system
• Sample crucibles: Ceramic (LECO 621-331)
• Atmosphere: High-purity nitrogen, medium flow
• Balance readability: 0.0005 g, operating range 0.8 g–1.2 g

Methodology

Sample preparation involved grinding hydrated lime to < 0.5 mm and placing ~1 g into ceramic crucibles. The TGA method comprised three successive thermal segments:

1. Free moisture determination: 25 °C to 120 °C at 15 °C/min, no hold
2. LOI @ 600 °C: 120 °C to 600 °C at 20 °C/min, 20 min hold
3. LOI @ 1000 °C: 600 °C to 1000 °C at 20 °C/min, 20 min hold

Mass changes at each stage were converted to percentage values using standard TGA equations:

• % Free Moisture = ((Initial mass – Mass at 120 °C) / Initial mass) × 100
• % LOI @ 600 °C = ((Mass at 120 °C – Mass at 600 °C) / Initial mass) × 100
• % LOI @ 1000 °C = ((Mass at 600 °C – Mass at 1000 °C) / Initial mass) × 100

Key Results and Discussion

Analysis of three batches (n = 21 each) on two TGA701 instruments yielded highly consistent results:

• Average free moisture: 0.76–0.77 % (standard deviation 0.01 %)
• Average LOI @ 600 °C: 23.0–23.1 % (standard deviation 0.02–0.03 %)
• Average LOI @ 1000 °C: 1.25–1.30 % (standard deviation 0.02–0.03 %)

Reproducibility between instruments and across batches indicates excellent method precision and suitability for routine quality control.

Benefits and Practical Applications

The described TGA method provides:

• Simultaneous multi-sample analysis, boosting lab throughput
• Rapid, two-hour runtime per batch
• High accuracy and repeatability for moisture and calcination assessment
• Critical data for process control in lime production, wastewater treatment and food industry applications

Future Trends and Potential Applications

Ongoing developments in TGA technology and data integration may include:

• Real-time coupling with evolved gas analysis (e.g., FT-IR, MS) for detailed decomposition profiling
• Automated sample handling and remote monitoring for continuous process control
• Miniaturized and field-deployable TGA systems enabling on-site quality checks

Conclusion

The LECO TGA701 method for free moisture and LOI measurement in hydrated lime demonstrates robust performance, delivering precise, reproducible results essential for quality assurance. Its high throughput and straightforward operation make it an effective tool for industrial and research laboratories.

References

• LECO Corporation. Free Moisture and LOI in Hydrated Lime. Application Note, Saint Joseph, MI, USA.