A lab can measure alumina to a fraction of a percent. But the sample it measures is a few hundred grams taken from thousands of tonnes. If that sample is not representative, the most precise analysis in the world is precisely wrong. Quality control in bauxite therefore begins long before the lab, with how samples are taken.
Why bauxite is hard to sample
Bauxite is a natural, mixed material. A single stockpile can contain lumps and fines, harder and softer ore, and pockets with slightly different chemistry. Fines can carry more clay, and so more silica, than lumps. Grab a handful from the surface and you may get a very different answer from a sample taken through the whole pile.
The principles of good sampling
- Many increments. A sample should be built from many small portions taken across the lot, not a few large ones.
- Every part has a chance. The method must reach the inside and bottom of a pile, not just the easy surface.
- Sample moving material when possible. Taking increments from a moving stream, such as during loading, is generally more representative than sampling a static heap.
- Reduce carefully. The combined sample is crushed, mixed and divided down to lab size by riffling or a similar method, so nothing is lost or biased along the way.
- Record everything. Lot, location, time, method and who took the sample.
Why we test twice
At Minequor every lot is sampled at two points.
| Checkpoint | What it confirms |
|---|---|
| Minehead stockpile | The lot matches the grade it was graded into, before it is committed to an order |
| Loading | The material actually leaving site still matches, after handling, weather and time in stockpile |
Bauxite can change between those points. Rain adds moisture. Handling breaks lumps and creates fines. Stockpiles can segregate by size. Testing twice means drift is caught at the mine, not discovered at the customer's gate. Final specifications are agreed per contract and confirmed by lab analysis.
What the lab measures
A typical bauxite analysis covers:
- Chemistry: Al₂O₃, SiO₂, Fe₂O₃, TiO₂, LOI and any minor elements in the contract.
- Available alumina and reactive silica for refinery buyers, by a digestion test.
- Free moisture, measured on an as-received sample.
- Size distribution, by screening.
Our guide to reading a bauxite specification sheet explains each of these.
Independent inspection
For larger contracts and exports, buyers often appoint an independent inspection agency to witness or carry out sampling at loading and issue its own certificate. Good suppliers welcome it: a third-party result that matches the mine's own record builds trust faster than any brochure.
Consistency is the product
For a refinery, a cement kiln or a refractory plant, the biggest risk is not average chemistry but variation. A plant tuned to one ore struggles when the next lot is different. Sampling and testing are how a mine turns a natural, variable ore into a consistent product.