Tin Ore Gravity Separation: The Flowsheet Explained

The main tin mineral is cassiterite (SnO₂). At about 6.8–7.1 g/cm³ it is far denser than quartz at about 2.65 g/cm³, so gravity separation is the backbone of tin processing. Cassiterite is also brittle: it breaks into slimes easily during crushing and grinding, and very fine cassiterite is hard to recover by gravity. Hence the old rule of tin dressing — recover it as early as you can, reject waste as early as you can — taking liberated cassiterite out early, discarding liberated gangue early and avoiding overgrinding.

Tin deposits fall into two broad groups. In alluvial tin the cassiterite is largely liberated already, so the flowsheet is mostly washing, desliming and gravity separation, much like placer gold — the Bangka-Belitung area of Indonesia is a well-known example. Hard-rock (lode) tin has to be crushed and ground, and the cassiterite often occurs with sulfides such as pyrite, arsenopyrite and chalcopyrite, and with wolframite or magnetite, which makes the flowsheet more involved. The sections below describe the full hard-rock route; for alluvial tin, the crushing and grinding stages can be left out.

Typical flowsheet

1. Crushing and stage grinding

Crushing is normally done in two or three stages to a size suitable for the mill. Unlike many flotation plants that grind everything fine in one go, tin plants usually grind and separate in stages: the first stage grinds only to a fairly coarse size, jigs and tables take out the liberated coarse cassiterite, and the unliberated composite particles are reground as middlings and treated again. This noticeably reduces overgrinding of the cassiterite.

A rod mill breaks ore by line contact, giving a more even product with less overgrinding, so it is often used for primary grinding; ball mills are typically used for regrinding middlings. Each mill runs in closed circuit with a classifier so that particles leave the mill as soon as they are fine enough.

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Ball millSpiral classifier

2. Classification: a narrow size range for every machine

Shaking tables and other gravity machines are sensitive to the feed size range: the narrower the range, the sharper the separation. Tin gravity plants therefore usually remove part of the slimes with a desliming cone or cyclones first, then split the material into several size fractions in a multi-spigot hydraulic classifier. Each fraction goes to its own machines and decks: the coarser ones to jigs and coarse-sand tables, the medium and fine ones to spirals and fine-sand tables, and the finest to the slime circuit.

3. Roughing with jigs and spirals

Jigs handle the coarser fractions and take out liberated coarse cassiterite at the front of the plant, which reduces the tonnage that has to be ground further. Spiral chutes need no power, take little floor space and handle large tonnages, making them a good choice for roughing and rejecting tailings in the medium and fine fractions — they greatly cut the load going to the tables. The rougher concentrates from both then go to the tables for cleaning.

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Spiral chute separation circuit

4. Cleaning on shaking tables

The shaking table is the heart of a tin gravity plant. Each size fraction should go to the matching deck — coarse-sand, fine-sand and slime decks differ in riffle pattern, stroke and speed. A table produces concentrate, middlings and tailings at the same time: the concentrate goes on to upgrading, the middlings go back for regrinding or retreatment, and the tailings are checked and discarded. Tin plants often run a fairly large number of tables in groups by size fraction, so that every unit works under the right conditions. For setting tables up, see How a Shaking Table Works and How to Adjust It.

5. Treating the slimes

However carefully grinding is controlled, some cassiterite always ends up in the slimes. The slime circuit usually thickens and classifies this material first, then recovers it on centrifugal concentrators or slime tables. For even finer cassiterite, dedicated cassiterite flotation is an option, but the reagent scheme is complex and must be worked out by testwork. Tin lost in the slimes is often the biggest weakness in a plant's overall recovery, so it deserves its own attention at the design stage.

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6. Upgrading: removing sulfides and iron minerals

Gravity tin concentrate often carries sulfides such as pyrite and arsenopyrite. With densities of roughly 4.1–6.1 g/cm³ they are close enough to cassiterite that tables alone cannot remove them completely, and sulfur and arsenic hurt concentrate quality and saleability. The usual remedies are:

• Sulfide flotation — sulfides are floated off with xanthate-type collectors while the cassiterite stays in the cell product.
• Magnetic separation — magnetite is removed with low-intensity magnets; weakly magnetic minerals such as wolframite and ilmenite can be separated with high-intensity magnets and, where worthwhile, sold as by-products.
• Other methods — alluvial tin concentrates often contain zircon and monazite, which may call for electrostatic separation or similar steps.

The final upgrading circuit depends on what the rougher concentrate actually contains, so a mineralogical analysis of the concentrate at the design stage is well worth doing.

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