Paste Thickener: How It Differs and When to Use It

A paste thickener is a high-torque, deep-bed thickener that produces a high-density, non-settling tailings stream. In tailings thickening, the choice between conventional, high-rate, high-compression, and paste units turns on underflow density, water recovery, and downstream handling. You’ll find paste thickeners used where filtered or dry-stack tailings need a low-moisture feed. According to Xinhai’s published equipment specifications, thickeners up to 100 m in diameter are available for these duties. This guide explains the differences and when paste thickening is the right call. For a full range of options, see thickening equipment.

Thickener types and underflow density ranges

Conventional thickeners are low-torque, shallow-bed units that produce underflow solids in the 20–45% w/w range. High-rate thickeners use feedwell dilution and flocculant to lift throughput, producing 30–55% underflow. High-compression thickeners run deeper beds and produce 45–65% solids. A paste thickener extends that logic with a tall sidewall, steep floor, and high-torque rake, reaching 65–75% or more. This is the first hard data point you need: the paste thickener’s job is underflow that won’t segregate. The Society for Mining, Metallurgy & Exploration SME publishes design ranges similar to these in its mineral processing references.

These four categories are not hard-edged. A high-rate unit can be pushed into high-compression territory with more bed depth, and a high-compression unit can approach paste if you add enough torque and sidewall height. The main design levers are feedwell geometry, flocculant contact, rake design, and underflow withdrawal. When you're comparing thickener types, ask for a settling test report that shows the underflow density curve, not just a single point.

Bed depth, rake torque, and the transition to paste

Bed depth is not a passive detail. In a conventional thickener, bed depth might sit under 1 m. Paste thickeners commonly operate with beds 2–4 m deep, and that depth creates a compression zone where yield stress builds. Yield stress is the minimum shear stress needed to make a fluid flow. As yield stress rises, rake torque must climb. CIM guidance Canadian Institute of Mining, Metallurgy and Petroleum describes paste thickener rake torque as often an order of magnitude higher than conventional units. You’re not just adding height; you’re redesigning the drive and rake arms for a dense, non-Newtonian fluid.

The rake isn't just for moving solids to the centre; it also forms channels that let water escape and helps de-aerate the pulp. In paste service, the rake arms must cut through a material that behaves more like a soft solid than a slurry. That's why paste thickeners have much larger drives, reinforced arms, and sometimes a picket fence above the rake. When you size a paste thickener, the torque requirement often controls the design, not the area.

Flocculant dosing strategy

Flocculant makes the difference between a runny underflow and a paste. But dosing is not a set-and-forget number. Start with a jar test at the target feed solids. Then run these steps: (1) dilute the polymer to the supplier’s recommended concentration; (2) dose incrementally, mixing gently for 30–60 seconds; (3) record the interface settling rate and supernatant clarity; (4) vary dose, pH, and mixing energy until the fastest settling plus clearest overflow appears; (5) repeat at plant temperature and feed variability. Because paste thickeners operate at higher bed pressure, overdosing can create a gel that jams the underflow pumps. You’ll need to coordinate flocculant response with the rake drive and the slurry pumps and valves downstream.

Underdosing leaves fine particles in the overflow, hurting water clarity and downstream recovery. Overdosing can produce a sticky, high-viscosity underflow that's hard to pump or mix. A paste thickener usually needs a higher flocculant dose per tonne than a conventional thickener because the target aggregate structure is stronger. But more flocculant does not automatically mean better paste; it can increase operating cost and reduce underflow pumpability. Pilot testing is the only reliable way to lock in a dose-response curve for your ore.

How thickener area is sized from settling tests

Thickener area is not a guess. It comes from batch settling tests, and the standard tools are the Coe-Clevenger method and the Kynch flux approach. You run a cylinder test at several feed concentrations, measure interface height over time, and convert that to settling velocity and solids flux. Then you apply the underflow concentration you need and the rise rate the process allows. The required unit area is expressed in square metres per tonne per day of solids (m²/t/d). The limiting flux point defines the minimum area. Don’t forget dynamic tests for high-rate and paste units: static cylinder tests under-predict compression behaviour. Scale-up from lab to a full-size paste thickener usually includes pilot or dynamic settling columns. This is where a professional sizing review pays off.

The Kynch method assumes a single settling velocity at each concentration and produces a flux curve. You plot solids flux against concentration, draw the underflow line from the desired underflow concentration, and find the limiting point. Coe-Clevenger does the same job with a simpler mass balance but less confidence at high compression. For paste duty, you're operating in the compression zone, so a dynamic test is almost always required. Without dynamic data, you can undersize the thickener and end up with a low-density underflow or overflowing solids.

Water recovery and the tailings thickening case

Water recovery is often the deciding factor. A paste thickener returns more water to the process than a conventional or high-rate unit because the underflow density is higher. In tailings thickening, that recovered water reduces fresh water demand and shrinks the tailings storage footprint. But the trade-off is higher capital cost, deeper foundations, and more power for the rake drive. You’ll need to weigh those costs against the water balance and the final deposition method. For dry stack or filtered tailings, a paste thickener is usually the right upstream step. For a broader comparison of equipment, use the selection guide and talk through your specific duty.

Water recovered from a paste thickener can be returned directly to the mill, reducing the need for fresh makeup water. The exact recovery depends on the feed solids and underflow density, so you need a water balance model built from settling test data. Paste thickening also lowers the transport volume of tailings, which can reduce pipeline sizes and pumping energy over the life of the plant. You don't want to design a tailings system around a number you haven't measured.

When paste thickening is not the right answer

Paste thickening is not a universal fix. If the tailings are already coarse and free-draining, a conventional high-rate thickener may deliver enough water recovery without the capital and operating cost of a paste unit. If your downstream process does not require dry stack or filtered tailings, the extra density may be over-engineering. You should also check whether your site has the geotechnical conditions for a tall, heavy thickener. Deep foundations and high drive power can erase the water-recovery benefit in a small plant.

The decision often comes down to net present value, not just density. Run the numbers for three cases: conventional, high-compression, and paste. Include capital, power, flocculant, water recovery, and tailings storage costs. For a plant that pays a high price for water or faces strict tailings regulations, the paste thickener usually wins. For a low-cost operation with abundant water, a simpler thickener can be the better choice.

Frequently asked questions