How does lignite removal work and what equipment is needed?

Lignite is one of the most stubborn contaminants a sand producer can face. It hides in otherwise good deposits, slips through standard washing circuits and only reveals itself once the sand has gone into concrete. By then, the damage is done. This article explains what lignite is, why it causes so much trouble, how the removal process works and the equipment needed to deal with it properly.

What is lignite and why does it cause problems?

Lignite is a soft, low grade brown coal found in many sand and gravel deposits, particularly those laid down in old river systems and estuaries. The particles are often a similar size to the sand grains around them, which is exactly why they cause such a headache. They cannot be sieved out, because they pass through the same apertures as the sand worth keeping.

The real trouble starts in concrete. Lignite particles absorb water, swell and then break down near the surface of a slab or precast unit. The result is pop outs, pitting and brown staining that no contractor wants to see. British and European standards, including BS EN 12620, set strict limits on lightweight organic contaminants in concrete sand. Fail the spec and the load gets rejected, along with the supplier’s reputation.

How the removal process actually works

The good news is that lignite has one weakness. It is much lighter than sand. Silica sand has a specific gravity of around 2.65, while lignite typically sits between 1.1 and 1.3. Effective lignite removal exploits this density difference using a controlled upward flow of water, often called a rising current.

Inside the separation vessel, sand slurry is fed into a chamber where water flows steadily upwards. The denser sand grains are heavy enough to settle against the current and sink to the bottom, where they are drawn off for dewatering. The lightweight lignite cannot fight the flow. It is lifted over a weir and carried away with the overflow water, where it can be captured and removed from the circuit.

Getting the water velocity right is everything. Too slow and the lignite settles with the sand. Too fast and fine sand is washed over the weir with the contaminants, losing saleable product. Modern systems allow the rising current to be adjusted to suit the feed material, which matters when deposits vary from one face of the quarry to the next.

The equipment needed

A complete lignite removal circuit is more than a single vessel. Each stage prepares the material for the next, and skipping one usually shows up in the final product.

Feed preparation and screening

Before any density separation takes place, the raw feed needs sizing. An inclined vibrating screen at the front of the plant removes oversize material and splits the feed into workable fractions. Clean sizing at this stage protects the pumps and keeps the separation vessel running on a consistent feed.

Hydrocyclones and pumps

Silts and clays interfere with density separation, so the slurry is usually deslimed through a hydrocyclone before it reaches the lignite unit. Slurry pumps move material between stages and keep the feed pressure steady, which helps the rising current do its job.

The separation vessel

This is the heart of the system. The vessel houses the rising current chamber, the weir and the controls that let operators fine tune the water flow. Well designed units also include a means of capturing the lignite laden overflow, typically a fine sieve bend or trash screen, so the contaminant leaves the circuit rather than recirculating.

Dewatering screens

Once the sand is clean, it still needs to be drip free and ready to sell. Dewatering screens bring the moisture content down to a level suitable for stockpiling and haulage, usually somewhere around 10 to 12 per cent.

A typical circuit from start to finish

Raw material is screened, slurried and pumped to a cyclone for desliming. The underflow passes into the separation vessel, where the rising current lifts the lignite over the weir and the clean sand sinks. The overflow runs across a fine screen to trap the lignite, while the sand is drawn off and dewatered ready for the stockpile.

Is lignite removal worth the investment?

For producers sitting on contaminated reserves, the answer is often yes. A well specified removal circuit can turn marginal deposits into consistent, in spec concrete sand. That can mean fewer rejected loads, access to higher value markets and a longer working life for the quarry. When set against the cost of sterilised reserves or lost contracts, the equipment can often justify its cost.

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