SPIRAL CLASSIFICATORS

Spiral Classifiers: Key Systems for Efficient Material Processing

Spiral classifiers – sometimes referred to as spiral concentrators or spiral classificators – are machines used primarily in the mining and mineral processing industry to separate particles by size and density in a liquid slurry. While they share a similar screw mechanism with fine material washers, spiral classifiers are typically employed in ore beneficiation circuits rather than aggregate production. Their main function is to classify and segregate fine from coarse particles in a mixture, often to prepare a finely ground ore stream for the next stage of processing. In essence, a spiral classifier continuously sorts a flowing wet pulp of ground ore into two streams: (1) settled coarse solids (sand fraction) that are pushed along by the spiral and discharged at one end, and (2) overflow of finer solids (slime fraction) that are carried away by the water at the opposite end​.

Working Principle:

Spiral classifiers operate on the fundamental principle that solid particles will settle at different speeds in a fluid based on their size, shape, and density​. The classifier consists of a large open-topped inclined trough through which a slurry (mixture of water and finely crushed ore) flows. A rotating helix (spiral blade) stirs the slurry and conveys settled particles. Here’s how it works step by step:

• Slurry from a mill (for example, the output of a ball mill in a grinding circuit) enters the classifier near the lower end of its trough​. The trough is inclined so that one end is higher (the “discharge” end for coarse material) and the other is lower (the “overflow” end for fines).

• As the slurry fills the classifier, the coarser or heavier particles begin to settle toward the bottom of the trough (they have a higher settling velocity). Finer or lighter particles remain suspended in the water longer.

• The spiral is continuously turning in the slurry, driven by an electric motor and gearbox. Unlike some types of classifiers, the spiral doesn’t exit the slurry; it stays submerged and rotating, which gently pushes the settled particles “up” the incline toward the higher end, against the flow of water​. This stirring action also prevents the settled solids from packing tightly, while conveying them.

• As the spiral moves the settled coarse particles (sometimes called “sand” in mining terminology) up the slope, these particles eventually reach the top end of the trough where they are lifted out of the liquid and discharged – often into the feed inlet of the grinding mill again (hence achieving a closed-loop circuit)​. In other cases, the coarse discharge might go to further crushing or reject if it’s truly waste.

• Meanwhile, the finer particles that remain suspended are carried by the flowing water to the lower end of the trough. The classifier is designed with an overflow weir at the lower end, which allows the water and fine slurry to exit once it reaches a certain level​. This overflow, containing fine material, is typically the desired product (e.g., ready for the next process like flotation or as final concentrate, depending on the setup).

• The result is a separation: “underflow” (coarse, often fed back to a mill for further grinding) and “overflow” (fine, passed on or collected). The separation size (cut point) can be adjusted by changing factors like the slurry flow rate, slurry density, spiral speed, and the height of the overflow weir. Steeper inclines or faster spiral speeds tend to pull more material out (even fines), whereas a shallow incline or slower speed allows more fines to remain and overflow​. Operators balance these variables to achieve the target cut size without overloading the classifier.

Spiral classifiers are often categorized by the height of the weir and submergence of the spiral: the two common types are high-weir classifiers and submerged classifiers​. In a high-weir classifier, the spiral’s rotating top is generally above the slurry level, and these units are used for coarser separations (typically producing an overflow with particle size >0.15 mm)​. In a submerged spiral classifier, the spiral is almost entirely submerged along its length and can achieve finer separations (overflow particle sizes <0.15 mm)​. The level of submergence (e.g. 100%, 125%, 150% of spiral diameter) and the pitch of the spiral flights can be varied to suit the material characteristics​. Some classifiers even have dual or triple spiral configurations (two or three side-by-side spirals in one trough) for higher capacity​.

Design & Components:

Although similar in concept to sand screws, spiral classifiers have design features tailored to ore processing use cases:

• Spiral and Trough: Typically made of wear-resistant steel, since they handle abrasive ores. The trough (tank) can have straight or flared sides; flared designs give more settling area for improved separation​. Sizes can be quite large – industrial spiral classifiers can reach spiral diameters up to 120 inches (3 m) and lengths stretching several meters​. The slope of the trough is fixed (around 12–18° incline)​.

• Drive and Lifting Mechanism:A heavy-duty electric motor and gearbox drive the spiral rotation. Importantly, spiral classifiers usually feature a spiral lifting device (manual or motorized). This mechanism can raise the spiral assembly out of the slurry when the classifier is stopped or during maintenance​. Lifting the spiral prevents it from being buried in sand (avoiding a start-up jam condition known as “sanding up”) and allows inspection of the spiral blade condition. It also permits clearing out of the settled sand if needed.

• Overflow and Underflow Outlets:The classifier has an overflow lip or weir at the low end for fines, and an underflow/spigot or discharge at the high end for coarse material. Often, a valve or gate is included at the coarse discharge to control the flow of sand back to the process​.

• Support Structure and Bearings:Given their size, classifiers have large support structures and one or more intermediate bearings to support the spiral shaft. The lower end of the shaft often runs in a submerged bearing (usually a wear-resistant bushing) which is lubricated and designed to operate under water​. The design must keep this lower bearing protected from grit to avoid frequent replacement.

Applications in Mineral Processing:

Spiral classifiers have been staple equipment in mineral beneficiation for decades​. A primary use is in closed-circuit grinding: after a mill grinds the ore, the classifier separates out the fine fraction for the next step (e.g. flotation or leaching) while returning the coarse fraction to the mill for further grinding​. This prevents over-grinding of the valuable minerals and improves the efficiency of the mill by not overloading it with fines​. The quality of classification directly affects downstream processes – well-classified material leads to better recovery rates and product grades in separation steps​. Spiral classifiers are also used in gravity concentration plants to split slurry streams; for example, to separate (de-slime) the feed to a shaking table or cyclone, ensuring ultra-fines (slimes) don’t upset those processes. They can be found in iron ore, gold, copper, zinc, lead processing – essentially any operation where a silt/slime-free product and a controlled grain size distribution is required. Additionally, some older or simpler washing operations (like washing of phosphate or coal fines) have employed spiral classifiers to remove clay if other methods are not available.

Advantages and Characteristics:

Although in modern large-scale plants hydrocyclones have partly taken over the role of classification due to their compact size and high throughput, spiral classifiers offer distinct benefits that keep them relevant in many installations:

• Better Cut-Size Control: Spiral classifiers can deliver high classification accuracy for a certain particle size range. They allow a calmer separation environment (just gravity and gentle swirling), which can result in more precise cut separation compared to the turbulence inside a cyclone. In fact, some experts note that spiral classifiers can be superior to hydrocyclones in classification efficiency and product accuracy when dealing with certain size ranges, especially when one wants to minimize fines in the underflow.

• Simplicity and Low Maintenance: These machines have a simple, stable operation and are relatively easy to operate and maintain​. There are no high-pressure pumps or complex controls required (unlike hydrocyclones that need consistent pressure). Operators can visually observe the separation in the trough and make adjustments. Maintenance tends to involve periodic wear part replacement (the spiral blade edges, lower bearing, etc.) and lubrication of the drive – tasks that can be planned during shutdowns. The robust construction means they tolerate upsets (like a short surge of heavy solids) better without immediately failing.

• Dewatering Capability: Spiral classifiers also dewater the coarse solids as they are lifted out of the pool. The coarse discharge emerges relatively dry (typically with 20-30% moisture), which can reduce the load on subsequent dewatering equipment. Cyclones, by contrast, eject both streams as liquids requiring extra dewatering steps.

• Visual Feedback and Adjustability: An operator can directly see the sand bed in the classifier and the clarity of overflow, allowing for intuitive control. Adjusting the overflow weir height (some classifiers have adjustable weir gates) and the slurry density gives a straightforward way to fine-tune performance. This hands-on adjustability can be advantageous in operations where feed material characteristics fluctuate and need frequent response.

Maintenance Considerations:

Proper operation and care will prevent common issues with spiral classifiers:

• One notorious problem is a classifier becoming “sanded up”, meaning so much sand has settled that the spiral is bogged down and cannot turn​. This can happen if the slurry flow is too low (allowing excessive settling) or if the spiral speed is too slow for a high solids throughput. To avoid this, the incline and spiral speed must be balanced with the feed rate. Also, always raise the spiral out of the slurry before shutting off the motor to avoid it being buried.

• Keep the spray water (if any) and overflow launder clear. In some designs, water sprays are used to clean the spiral or help move fines. Blockages can upset the separation.

• Monitor the lower bearing (the one submerged in slurry). This part takes a lot of abuse – ensure it’s getting proper lubrication (many use grease or oil bath systems) and replace the bushing or seals at recommended intervals. Modern designs often use improved materials like polyurethane or composite sleeves for better wear and corrosion resistance.

• Regularly check the spiral blades (flights) for wear. Over time, the metal (or rubber lining on some) will wear thin, altering the separation efficiency. Replace or rebuild flights when they become too worn. Also ensure the spiral remains straight and true; misalignment can cause rubbing on the trough.

• Keep an eye on slurry density: If the slurry is too dilute, separation efficiency drops (fines might settle with coarse); if too dense, everything may settle and overload the spiral. Operators often aim for an optimal pulp density and may install density gauges to control water addition as feed density changes.

• As with fine washers, gearbox and bearing maintenance (oil changes, greasing, seal checks) is vital due to the heavy loads and continuous operation.

Spiral classifiers, though somewhat large and old-fashioned in appearance, continue to be a reliable solution, especially in operations where the advantages of easy operation and the ability to handle large volumes gently are valued. They remain an important piece of the mineral processing puzzle, working alongside newer technologies.