MINERAL PROCESSING KNOWLEDGE · TIN ORE PROCESSING
How staged grinding, rod mills and high-frequency screening keep brittle cassiterite out of the slime fraction in tin gravity circuits.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Cassiterite (SnO2) is one of the most rewarding minerals to treat by gravity and one of the easiest to lose. At 6.8–7.1 g/cm³ it contrasts strongly with ordinary silicate gangue, so jigs and shaking tables recover it at comparatively coarse sizes without reagents. The same crystal is extremely brittle. Under the impact of a grinding mill it fractures along its cleavage planes, reports to the finest fractions as slime, and is rarely recovered at all. Most underperforming tin gravity plants have a slime problem created upstream of the separators, not a separator problem.
Technical note: The density and size ranges quoted here describe the typical behaviour of cassiterite-bearing ores and are given for orientation only. Particle-size splits, equipment selection and achievable grades must be confirmed by representative sampling, mineralogical examination and pilot or locked-cycle testwork on your own deposit.
1. Why Cassiterite Is So Easy to Lose
Gravity concentration depends on a density difference large enough to be exploited within a realistic residence time. Grinding energy needed to liberate cassiterite from quartz or from a sulfide matrix is also energy that will break the cassiterite itself once it is free.
That creates a narrow operating window. Grind too coarse and cassiterite stays locked in composite particles no jig or table can upgrade. Grind too fine and the liberated grains become slime, where film flow, pulp viscosity and particle collisions overwhelm the settling differences gravity relies on. The objective is therefore not simply to liberate, but to liberate at the coarsest possible size and remove each liberated particle from the mill circuit as soon as it appears.
2. The Four Rules of Feed Preparation
A stable tin gravity flowsheet rests on four rules that are simpler to state than to hold in daily operation: multi-stage grinding, stage liberation, stage screening and early recovery of coarse particles. Together they define how comminution is arranged long before any separator is chosen.
- Multi-stage grinding. Total size reduction is divided into several smaller steps, so material already liberated is not exposed to further grinding energy.
- Stage liberation. Each grinding step stops once the target mineral is sufficiently liberated, judged on the actual ore rather than a single plant-wide target.
- Stage screening. Each grinding product is classified immediately, so the finished size fraction leaves the circuit before the next stage begins.
- Early recovery of coarse particles. Coarse, well-liberated cassiterite is the cheapest tin in the plant and should be captured while a jig can still treat it efficiently.
The rules reinforce one another: staged grinding needs screening that reliably removes finished material, and screening needs gravity equipment sized to take it immediately.
3. Rod Mills and High-Frequency Screens in Practice
The grinding equipment matters as much as the grind size. A ball mill is a poor choice for first-stage coarse grinding of a tin ore: its point-contact impact produces a wide spread of sizes and a significant fines tail, precisely the fraction in which cassiterite is lost. A rod mill is preferred. Its rods grind by line contact along the charge, producing a narrower size distribution and far less shattering of cassiterite that has already been liberated.
Discharge should not be recycled as a single stream. It passes immediately to a high-frequency vibrating screen, which classifies at a fine cut with the acceleration needed to keep damp, dense material moving. Screen undersize goes directly to gravity: a jig machine for the coarser sizes and a shaking table for the finer sizes. Only the oversize returns to the mill.
The critical rule is that a particle already liberated must never make a second pass through the mill. Every unnecessary pass adds fines, adds grinding energy and adds a small, permanent loss of tin to the slime. Screen efficiency is a recovery parameter, not a materials-handling detail, and worn panels or an overloaded deck should be treated as production losses.
4. Diagnosing Slime Loss in a Running Circuit
Slime loss is rarely announced by a single alarm. It appears as a slow drift: tailings that still carry visible heavy mineral, a concentrate grade that creeps down at constant throughput, or a slime overflow that assays far higher than expected. The table below lists symptoms that point back to feed preparation rather than to separator settings.
| Symptom | Likely cause | Practical response |
|---|---|---|
| Fine heavy mineral visible in jig or table tailings | Liberated cassiterite has been ground into the slime range | Review grinding stage splits and screen cut sizes before adjusting separator settings |
| Recirculating load rises while concentrate grade falls | Liberated cassiterite is returning to the mill with the oversize | Check screen performance and the oversize return path |
| Table feed is noticeably finer than the design curve | Screen bypass, worn panels or excess water in classification | Inspect screening and classification ahead of the gravity stage |
| Tin reports strongly in the slime overflow | The desliming cut is too coarse for the ore | Reassess the desliming cut and the route for fines treatment |
A size-by-size assay of table feed, concentrate and tailings will usually show within a few shifts whether the loss sits in the separators or upstream, and that answer decides the response.
Conclusion
Cassiterite rewards gravity concentration generously, but only when the circuit respects how fragile it is. A density of 6.8–7.1 g/cm³ makes coarse tin easy to recover; brittleness makes fine tin almost impossible. The answer is not a more elaborate separator but feed preparation built on multi-stage grinding, stage liberation, stage screening and early recovery of coarse particles, with rod mills and high-frequency screens in place of ball mills and blind recycling. Held consistently, these rules protect the one thing a tin plant cannot manufacture: particle size.
How LiCheng Tech can help
LiCheng Tech supplies gravity and auxiliary equipment for tin ore processing, including jigs, shaking tables, classifiers and screening units, and works with project teams on flowsheet review and equipment selection. If your tin recovery is drifting, we can examine grinding and screening practice, size-by-size data and separator duty together to find where fines are being created and lost. Our approach is described further on the tin ore processing page.
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