MINERAL PROCESSING KNOWLEDGE · TIN ORE PROCESSING
Pyrite, pyrrhotite, arsenopyrite and chalcopyrite are dense enough to follow cassiterite into a table concentrate. Flotation desulfurisation, placed ahead of gravity, removes them first.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
A tin gravity circuit is designed to keep heavy minerals and reject light ones. Sulfide minerals do not cooperate with that logic. They are heavy enough to be retained with cassiterite, yet they carry sulfur and arsenic that a smelter will not accept. The standard solution is to remove them by flotation before the ore ever reaches a table.
Technical note: The density ranges and reagent classes quoted here describe common sulfide-bearing tin ores and are given for orientation only. Flowsheet placement, reagent selection and target concentrate specifications must be confirmed by mineralogical examination, flotation testwork and site-specific requirements.
1. Dense Sulfides That Behave Like Heavy Minerals
Tin ores are frequently associated with pyrite, pyrrhotite, arsenopyrite and chalcopyrite. These sulfides have specific gravities in the range of 4.9–5.2 g/cm³, lower than cassiterite at 6.8–7.1 g/cm³ but far above the silicate gangue that a gravity circuit is designed to reject. Within the hydraulic conditions of a shaking table or a jig, a dense sulfide grain settles much as a cassiterite grain of similar size does.
The consequence is that a conventional gravity circuit cannot distinguish between them. Both report to the concentrate, and the tin concentrate arrives at the smelter carrying sulfur and arsenic above permitted limits. At that point the material is not a low-grade concentrate that can be blended away; it may be unsaleable altogether, and the tin it contains is effectively lost with it.
2. What Sulfide Contamination Costs
Contamination is expensive in three ways. First, the tin grade of the concentrate is diluted by material that carries no tin value, so every tonne shipped contains less payable metal. Second, sulfur and arsenic attract penalties or outright rejection, and arsenic limits in particular are strict because of handling and emission controls at the smelter. Third, and most importantly, the loss is often discovered late. Once sulfides are mixed into a table concentrate, removing them requires fine grinding and additional flotation on a small, high-value stream, which is far more difficult than treating the whole ore before gravity.
The general principle is therefore to remove sulfides at the coarsest practical size, while the tonnage to be floated is still a small fraction of the plant feed and while the sulfide grains are still well liberated. A desulfurisation stage in the right place is inexpensive; a cleaning stage bolted onto a finished concentrate rarely is.
3. Desulfurisation Flotation Ahead of the Tables
In a sulfide-bearing tin ore, the pulp is routed to a flotation desulfurisation circuit before it reaches the shaking tables. Xanthate-type collectors and a frother are added, and all sulfide minerals are floated preferentially. The flotation cell underflow, a sulfide-free pulp, is then sent to gravity concentration on the shaking tables.
- The duty is bulk, not selective. Every sulfide must leave, so pyrite, pyrrhotite, arsenopyrite and chalcopyrite are floated together rather than separated from one another.
- The valuable mineral stays in the underflow. Cassiterite is not the flotation target; it is deliberately kept in the cell underflow and passed to gravity.
- Placement matters. Desulfurisation sits ahead of the tables, so the gravity concentrate is clean when it is produced and no re-treatment of a finished concentrate is needed.
Because cassiterite is brittle, the flotation stage must be gentle. Agitation and residence time should be sufficient to float the sulfides without over-grinding or over-agitating the pulp, and the feed should already be classified by a classifier so that slimes neither consume reagents nor report to the froth by entrainment. The flotation machine itself should be sized for a stable froth and consistent mass pull rather than for maximum air.
4. Control Points and Circuit Placement
A desulfurisation circuit is simple in principle and easy to run badly. The points below most often determine whether the tin concentrate leaves the plant on specification.
| Control point | What to watch | Why it matters |
|---|---|---|
| Liberation of sulfide grains | Whether sulfides are exposed at the grinding size | Locked sulfide particles float poorly and report to the tables |
| Collector and frother addition | Stability of reagent dosing, not only total consumption | Under-dosing leaves sulfides in the underflow; over-dosing carries gangue into the froth |
| Froth removal and mass pull | Consistency of froth depth and removal rate | A poorly removed froth returns sulfides to the circuit |
| Flotation underflow quality | Residual sulfide content before gravity | The underflow is the table feed, so it sets final concentrate quality |
| Final concentrate | Sulfur and arsenic against specification | These are the acceptance criteria that define a saleable tin concentrate |
Placement relative to classification also deserves attention. Flotation works best on a pulp of stable density and controlled size distribution, so the desulfurisation stage normally sits after grinding and classification but before the gravity concentrators. Where a plant treats several ore types, each should be tested separately, because sulfide content and mineralogy can vary enough to change the required flotation response. The floated stream leaves the gravity circuit at low mass; whether it is discarded or retained for its own metal content depends on its analysis and on project economics.
Conclusion
Sulfides are the heavy-mineral exception that tin gravity circuits cannot handle. At 4.9–5.2 g/cm³ they settle alongside cassiterite and contaminate the concentrate with sulfur and arsenic, and by then separation is far more costly than it would have been at the head of the circuit. Flotation desulfurisation ahead of the tables, using xanthate collectors and a frother to float all sulfides and leave a clean underflow for gravity, is the standard and effective answer. Removed early, sulfides are a small nuisance stream; left to the tables, they can turn a saleable concentrate into waste.
How LiCheng Tech can help
LiCheng Tech supplies flotation machines, shaking tables, jigs and classifiers for sulfide-bearing tin ores. We can help you decide where desulfurisation belongs in the flowsheet, review reagent and froth control practice, and specify equipment for the flotation and gravity stages so the final tin concentrate meets sulfur and arsenic limits. Our tin ore processing overview covers the wider circuit context.
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