Rake thickener systems are critical solids-liquid separation units widely used in mineral processing and industrial wastewater treatment. These large, circular tanks employ rotating rake arms that gently agitate and compact the settling solids, allowing clarified water to overflow for reuse. By efficiently concentrating slurry solids into a compressible bed, rake thickeners enable significant water recovery and dramatically reduced tailings volumes. In mining applications, for example, effective thickening enables producers to shrink or eliminate costly settling ponds and tailings dams​. As a result, limited freshwater supplies can be conserved and the environmental footprint of tailings disposal minimized.

Technical Design and Specifications

 Rake thickeners consist of a large-diameter circular tank with a central feedwell, a drive mechanism, and radial rake arms. Slurry and flocculant are introduced into the feedwell, which is often optimized via computational fluid dynamics (CFD) to evenly distribute the flow and minimize polymer usage​. Settling particles accumulate on the floor in a mud bed, and the rake arms continuously move the solids toward a central discharge. Many designs include dewatering wedges or pickets on the rake to improve water release. The drive mechanism—usually a central column or perimetric gearbox—rotates the rake slowly (often only 0.1–0.5 rpm) through the mud bed. Some systems also feature a hydraulic “rake-lift” function that raises the rake to protect the drive if the torque becomes excessive.

Manufacturers may offer flat-bottom (on-grade) or raised (elevated) thickener tanks. On-ground tanks minimize the vertical suction lift, while elevated tanks allow shorter underflow suction lines​. In all cases, the thickener floor is sloped (typically ~1–2%) toward a central launder or sump. High-density thickener variants build on this with steeper floors and taller sidewalls to further compress the mud bed under gravity​. For example, a high-density thickener can discharge a pumpable slurry (50–60% solids) that still has only about 50–100 Pa yield stress​; paste-thickener variants push solids even higher (toward ~65–70% by weight). Thickener sizes vary: small clarifiers may be under 10 m in diameter, but major tailings thickeners can exceed 50–60 m. Many plants use parallel trains of multiple thickeners to reach required capacity.

Operational Benefits in Mining

Rake thickeners provide substantial operational advantages, especially in mining. The high underflow density and water recovery they achieve reduce fresh water intake and effluent volume. For instance, high-rate rake thickeners can recover roughly 85% of the feed water for reuse​. Paste thickeners reach up to ~90% water recovery, discharging very dense slurry​. These high-solids outputs translate to a much smaller volume of slurry being sent to tailings storage. In practical terms, this results in smaller tailings impoundments and a lower environmental footprint. In some cases, sites have demonstrated 90% or greater reuse of process water via thickening.

Greater throughput per unit area is another benefit. The large surface area of shallow high-rate thickeners, combined with effective flocculation, allows more tonnes per hour to be processed. For example, installing a single 53 m-diameter high-rate thickener consolidated what was formerly a two-stage circuit into one unit​. This unit treated over 5.2 Mtpa of iron-ore tailings and produced 53–60% solids underflow (compared to ~40% previously), boosting pumping efficiency by about 10%​. The same installation reduced overflow solids from ~8% to under 150 ppm​, making virtually all overflow water reusable. In practical terms, this meant roughly RMB 1.5 million per year in saved electricity (fewer pumps and drives)​ and improved downstream recovery due to cleaner recycle. These gains illustrate how an optimized rake thickener can transform tailings management.

To summarize, modern rake thickening technology can significantly increase plant capacity and stability. A single advanced thickener can replace multiple older units, reducing operational complexity and energy draw​. The improved overflow clarity and higher-density underflow both emerged as direct outcomes of the upgrade, validating the value of current thickener designs.

The table contrasts the three basic thickening approaches. Deep-cone thickeners (right column) achieve the highest underflow density and water recovery​, but require deeper construction and higher initial cost. High-rate thickeners (center) maximize throughput by using enhanced flocculation and very large settling area​. Conventional rake thickeners (left) are robust and flexible for general-purpose use, albeit with somewhat lower solids capture than the other types.

In high-density or deep-cone thickening applications, the settling tank height can exceed 8 meters to allow greater compression of the mud bed. The illustrated deep-cone thickener has a tall conical vessel with no rotating mechanism, making it inherently simpler and low-maintenance. Such units can achieve up to ~95% water recovery​ while delivering a very high-solids underflow.

Role in Water Recovery and Tailings Management

Recovering process water and managing tailings are core objectives of rake thickeners. By recycling overflow liquid, plants can maintain a largely closed water circuit and dramatically lower freshwater consumption. Thickener overflow typically becomes high-quality process water, while the concentrated tailings slurry (with most of the water removed) is sent for disposal or further dewatering. For example, the above case study achieved overflow solids under 150 ppm​, meaning that essentially all water could be recirculated in the plant. This closed-loop operation can approach 90–95% recovery of process water in practice.

The denser tailings slurry has additional benefits. A higher-solids discharge occupies less volume in ponds or impoundments for a given ore throughput, reducing dam footprint and seepage. Many mines combine rake thickeners with pump-to-paste or filtered tailings systems to further enhance stability and water conservation. Overall, effective thickening can greatly shrink the size of tailings facilities, lower seepage losses, and simplify environmental permitting.

Industrial and Secondary Applications

Rake thickeners are also used extensively outside mining. Market analyses report key sectors include chemical production, metallurgical processing, and water/wastewater treatment​. In municipal water plants, large gravity thickeners concentrate sewage sludge before digestion or drying. In pulp and paper mills, thickeners recover cellulose fibers and clarify whitewater. Chemical plants and refineries use thickeners to separate fine precipitates (e.g. catalysts or hydroxides) from liquids. In all these cases the common theme is treating a slurry so that clean water is recycled and only a compact underflow is handled as waste.

Polygonmachine Rake Thickener Systems

Polygonmachine is a specialized equipment manufacturer with expertise in thickening solutions, including rake thickeners. Their product lineup covers conventional, high-density, and high-rate thickeners for mining, metallurgical, and wastewater applications. Polygonmachine emphasizes sturdy steel construction and process-optimized designs. For example, their thickener feedwells are custom-engineered (via CFD) for each application to ensure even flow and minimal polymer use​. Their thickeners use robust drive assemblies (gearbox or hydraulic) and incorporate protective features (e.g. automatic rake-lift and torque cutoff) similar to industry best practices​.

The company also offers full-service support. This typically begins with laboratory or pilot testing of the slurry to determine optimal flocculant type and dose and to measure key rheology (e.g. yield stress). Polygonmachine’s engineers then handle mechanical and civil design of the tank and supporting structures. During commissioning, they assist with flocculant dosing calibration, rake speed tuning, and operator training. Beyond startup, Polygonmachine provides ongoing monitoring and maintenance assistance; many clients report that performance guarantees and on-site support from Polygonmachine help sustain target outputs. In effect, their approach combines design innovation with hands-on service to ensure reliable, high-performance thickener operation.