Two bauxites can show the same alumina on paper and still behave like different ores in a refinery. The difference is the mineral that carries the aluminium. There are three that matter: gibbsite, boehmite and diaspore.
The three aluminium minerals
| Mineral | Formula | Al₂O₃ in pure mineral | Where it is common |
|---|---|---|---|
| Gibbsite | Al(OH)₃ | About 65% | Young lateritic bauxites in the tropics |
| Boehmite | γ-AlO(OH) | About 85% | Older or more deeply buried deposits |
| Diaspore | α-AlO(OH) | About 85% | Ancient or metamorphosed deposits |
Gibbsite holds three hydroxyl groups and so carries more water; boehmite and diaspore are the denser "monohydrates". That is why gibbsitic ore shows a higher LOI on a lab report.
Why mineral form changes refining cost
In the Bayer process, caustic soda must dissolve the aluminium mineral. The three minerals dissolve under very different conditions.
- Gibbsite dissolves at around 140–150 °C. This "low-temperature digestion" uses less energy and simpler equipment.
- Boehmite needs much higher temperatures, typically well above 200 °C, with higher pressure.
- Diaspore is the hardest of all, needing high temperatures and additives such as lime.
A refinery is designed around the mineral it expects. A low-temperature plant cannot recover alumina locked in boehmite; it ends up in the residue. So even a small boehmite content in a gibbsitic ore reduces the alumina that a low-temperature refinery can actually use.
There is a second effect. At higher digestion temperatures, more of the quartz in the ore becomes reactive, adding to caustic soda losses. See reactive silica in bauxite.
Why Konkan bauxite is gibbsitic
Gibbsite forms during tropical weathering at surface temperatures. Over geological time, burial, heat or pressure can convert it to boehmite and then diaspore. The lateritic bauxites of Maharashtra's Konkan coast formed on the Deccan basalt and have never been deeply buried, so their aluminium remains largely gibbsite. We explain this history in how lateritic bauxite forms on the Deccan basalt.
What it means for non-refinery buyers
Mineral form matters beyond the refinery too.
- Refractory makers calcine bauxite to drive off water. Gibbsitic ore loses more weight on calcination, which affects yield per tonne of raw ore bought.
- Cement plants mainly care about oxide chemistry, but the water released from gibbsite in the kiln is part of the heat balance.
- Freight: chemically bound water is carried in every tonne shipped, so it is worth comparing offers on a dry, calcined or available-alumina basis.
How mineralogy is measured
Oxide analysis cannot tell gibbsite from boehmite. Laboratories use X-ray diffraction (XRD) to identify minerals, thermal analysis to see at which temperature water is released, and digestion tests to measure available alumina directly. Ask for the method when it is not stated.