Loss on Ignition (LOI): Concept and Significance
LOI, also known as loss on ignition, refers to the percentage of mass lost by a material after it has been ignited under specified temperature and time conditions, relative to the original sample mass. This indicator reflects the total amount of components in the material that can volatilize, decompose, burn, or undergo other chemical reactions at high temperatures, leading to mass loss.
Basic Definition and Measurement LOI is calculated from the mass change of the sample before and after ignition, using the following formula: LOI (%) = [(Mass before ignition – Mass after ignition) / Mass before ignition] × 100% Strict control of the temperature program, final ignition temperature, and holding time is crucial during measurement. Because the thermal behavior of different materials varies significantly, the conditions directly affect the accuracy and comparability of the results.
Significance of LOI in Inorganic Powder Materials LOI is a comprehensive indicator; its specific meaning depends on the chemical composition of the material and the selected ignition conditions. It primarily reflects the content of the following components:
1. Moisture and Volatile Substances
Adsorbed Water: Environmental moisture adsorbed on the powder surface.
Crystallization Water/Structural Water: Water molecules present in the mineral lattice (such as gypsum, kaolinite, etc.), which are removed at specific temperatures.
Organic Substances: Including residual dispersants, surface modifiers, natural organic impurities, etc., which are released as gases upon combustion at high temperatures.
2. Carbonate Decomposition
Carbonates are a major component or common impurity in many inorganic powders, decomposing at high temperatures to release carbon dioxide.
For example, calcium carbonate (CaCO₃) decomposes at approximately 850–1000°C, with a theoretical loss on ignition of 44%. Therefore, loss on ignition can be directly used to assess carbonate content and raw material purity.
3. Other Chemical Reactions
Sulfates, Sulfides: Such as gypsum dehydration and pyrite oxidation releasing SO₂.
Ammonium Salts, Nitrates: Thermal decomposition of processing aids or impurities.
Redox Reactions: Oxidation of certain iron-containing minerals may lead to weight gain, but this may still be accounted for in the overall weight loss.
4. High-Temperature Phase Transformation Weight Loss
A few minerals experience structural collapse and gas release at high temperatures, resulting in a decrease in mass.
Practical Applications of Loss on Ignition
1. Rapid Assessment of Purity and Composition
For calcium carbonate, a loss on ignition close to 44% indicates high purity; significantly lower values suggest the presence of inert impurities such as silicon and aluminum.
For kaolin, the loss on ignition mainly comes from structural water (approximately 14%) and organic matter, and can be used to determine its type (e.g., significant differences between hydrated and calcined types) and purity.
For aluminum hydroxide, magnesium hydroxide, etc., the loss on ignition corresponds to the dehydroxylation process and is an important basis for measuring thermal stability and purity.
2. Production Process Control
In the cement industry, the loss on ignition of raw materials is a key control parameter, reflecting the carbonate content and proportion accuracy, directly affecting clinker quality and energy consumption.
In the ceramics and refractory materials industries, loss on ignition (LOI) is related to the shrinkage, porosity formation, and deformation tendency of the green body during sintering, and is an important reference for formulation design.
3. Material Type Identification
The LOI of calcined kaolin is typically below 1%, while that of hydrated kaolin can reach 13–15%. This can be easily distinguished using LOI.
4. Impact on Downstream Applications
In composite materials such as plastics, rubber, and coatings, fillers with high LOI are prone to generating gas at processing temperatures, which may lead to blistering, surface defects, or a decrease in mechanical properties.
Therefore, high-end applications typically require fillers with low and stable LOI, and if necessary, reduce their value through processes such as calcination.
