When magnesite is bought and sold, the shared language is the chemical analysis report. It usually gives percentages as oxides. Reading the values correctly is essential both to judge a product and to compare suppliers.
The report, line by line
| Component | What it tells you |
|---|---|
| MgO (magnesium oxide) | The valuable part of the product. The higher, the better. |
| SiO₂ (silica) | One of the key impurities; in refractories it forms phases that melt at low temperature. |
| CaO (lime) | Judged together with its ratio to silica, not on its own. |
| Fe₂O₃ (iron oxide) | Darkens the colour and lowers high-temperature performance. |
| Al₂O₃ (alumina) | Usually present in small amounts, but monitored. |
| Loss on ignition (LOI) | The weight lost when the sample is heated to high temperature; in raw magnesite it is mostly CO₂. |
Why is the loss on ignition so high?
About 52% of pure magnesite by weight is carbon dioxide. So the loss on ignition of raw magnesite is around 50%, and its MgO can be at most 47.8%. That is not a low value; the material simply has not been fired yet.
Comparing raw and calcined values
To see what product a raw magnesite will give after firing, the values are converted to a loss-free, or calcined, basis: MgO (calcined basis) = raw MgO × 100 / (100 − LOI).
For example, magnesite measured at 46.5% MgO and 51.0% loss on ignition in the raw analysis contains about 94.9% MgO on a calcined basis. Silica, lime and iron oxide are converted with the same factor, here roughly 2: silica of 0.5% in the raw analysis becomes about 1% in the fired product.
The CaO/SiO₂ ratio
In sintered magnesia for refractories, the ratio of lime to silica matters too. When it is high enough (around 2 or more by weight), silica is bound as dicalcium silicate, which has a high melting point. When it is low, calcium magnesium silicates that melt at lower temperatures form, and the refractory loses high-temperature strength.
Good sample, good analysis
An analysis is only as good as the sample. It should be based not on a single lump but on a representative sample taken from different points of the stockpile, then mixed and reduced. Contact our sales team for the current analyses of our products and to request samples.
