The mystery of loss on ignition: How does it affect your product quality and process?
What is Loss on Ignition (LOI)?
LOI, often abbreviated as LOI, refers to the percentage of mass lost by a material after it has been burned for a sufficient period of time at a specific high temperature, relative to the original sample mass.
Simply put, it's the portion of a material that "burns away" at high temperatures.
The basic formula is: Loss on Ignition (%) = [(Mass before burning - Mass after burning) / Mass before burning] × 100% During testing, the heating program, final temperature, and holding time must be strictly controlled because different conditions lead to different degrees of decomposition or volatilization, thus affecting the results. For example, the temperatures used for testing calcium carbonate and kaolin are different.
What does it represent in inorganic powder materials?
LOI is not a single indicator, but a comprehensive one. It represents the sum of all components in the material that can volatilize, decompose, oxidize, or burn at the given burning temperature. What it specifically represents highly depends on the type of material and the burning temperature.
The following are their main significance and representative components in inorganic powder materials:
1. Volatile substances and organic matter
Physically adsorbed water: Atmospheric moisture adsorbed onto the material surface.
Water of crystallization or structural water: Water existing in the mineral crystal structure in the form of H₂O molecules (e.g., gypsum CaSO₄·2H₂O, kaolinite Al₂O₃·2SiO₂·2H₂O), which will be removed at specific temperatures.
Organic matter/organic compounds: Such as dispersants, surface modifiers (e.g., coupling agents), organic impurities from natural minerals, etc., which will be volatilized as CO₂ and H₂O at high temperatures.
2. Carbonate decomposition
This is one of the most common and important sources. Many inorganic powders are carbonate minerals or contain carbonate impurities.
Calcium carbonate: CaCO₃ → CaO + CO₂↑ (typically 850°C - 1000°C)
Magnesium carbonate: MgCO₃ → MgO + CO₂↑
Loss on ignition directly reflects the carbonate content. For example, the theoretical loss on ignition of pure calcium carbonate is 44%.
3. Decomposition or oxidation of other compounds
Sulfate decomposition: For example, gypsum loses its water of crystallization and may partially decompose at high temperatures.
Sulfide oxidation: For example, pyrite (FeS₂) is oxidized to Fe₂O₃, releasing SO₂.
Ferrous oxidation: The oxidation of FeO to Fe₂O₃ increases weight, but in some complex systems containing reducing agents, it may also manifest as part of the weight loss.
Ammonium salt decomposition: If ammonium salt auxiliaries are used.
Nitrate decomposition.
4. Phase transition weight loss at high temperatures (relatively rare)
Some minerals undergo structural collapse and release gases at specific high temperatures.
Practical Applications of Loss on Ignition (LOI):
1. Rapid Determination of Purity and Composition:
For calcium carbonate powder, an LOI close to 44% indicates high purity; a lower LOI may indicate the presence of inert impurities such as silica sand and clay.
For kaolin, its LOI mainly originates from structural water (approximately 14%) and organic matter, and can be used to assess its purity and calcination characteristics.
For aluminum hydroxide/magnesium hydroxide, LOI originates from the dehydroxylation reaction and is a key indicator of its purity and thermal stability.
2. Process Control Indicator:
In the cement industry, the LOI of raw materials (mixtures of limestone, clay, etc.) is a critical kiln feed control parameter. It reflects the accuracy of batching and the degree of carbonate decomposition, directly affecting clinker quality and energy consumption.
In the ceramics and refractory materials industries, LOI determines the shrinkage, porosity, and deformation risk of the green body during firing, and is an important basis for formula design.
3. Differentiating Material Types:
Calcined kaolin has an extremely low loss on ignition (LOI) (typically <1%), while hydrated kaolin has an LOI of around 13-15%. The two can be easily distinguished using LOI.
4. Impact on Downstream Application Performance:
In polymer composites such as plastics, rubber, and coatings, fillers with high LOI may generate gas at high processing temperatures, leading to blistering, bulging, surface defects, or decreased mechanical properties in the finished product.
Therefore, in these applications, inorganic fillers with low and stable LOI (e.g., calcined fillers) are typically required.
