Ore straight from the pit, known as run-of-mine (ROM), is a mixture of boulders, lumps and fines with chemistry that varies from bench to bench. No buyer wants it in that form. The steps between the pit and the truck are what turn ore into a product.
Step one: crushing
Crushing reduces ROM ore to a size the buyer's process can take.
- Primary crushing, often with a jaw crusher, breaks the largest boulders down.
- Secondary crushing, where needed, brings material down further and makes the size more uniform.
The target depends on the customer. A refinery's grinding circuit, a refractory maker's calciner and a cement plant's raw mill each have their own preferred feed size. Crushing at the mine means customers receive ore ready for their grinding, digestion, calcining or kiln feed systems.
Crushing also breaks apart lumps of ore from attached clay or cap material, which makes the next step more effective.
Step two: screening
Screens are vibrating decks with openings of set sizes. Material passes over them and splits into size fractions, for example oversize, lumps and fines.
Screening does more than control size. In lateritic bauxite, clay minerals tend to report to the finest fraction. Separating fines can therefore lower the silica in the lump product, upgrading it for buyers who are sensitive to silica. See reactive silica in bauxite.
Step three: grading
Grading sorts lots by chemistry. Using face samples and stockpile assays, material is routed into separate grade stockpiles, so that each one meets a defined specification.
| Grade stockpile | What it is sorted for |
|---|---|
| Metallurgical | High available alumina, low reactive silica |
| Refractory & abrasive | Very high alumina, low iron and titania |
| Cement | Predictable iron and silica |
| Custom | A buyer's own specification, agreed per contract |
This is how one ore body supplies several bauxite grades.
Step four: testing before dispatch
Each graded lot is then sampled at the minehead stockpile and again at loading. Read why we test twice.
Why do it at the mine?
- Lower freight cost per useful tonne. Oversize and off-spec material is not shipped.
- Fewer surprises at the customer's plant, because size and chemistry are already within limits.
- Better use of the deposit. Material that does not suit one grade often suits another.
At Minequor's Raigad site, crushing, screening and grading are all done on site. The full sequence is on our operations page.