Process Overflow by Manhattan: Scrubber Series

Aspect Ratios

Rotary drum scrubbers are a cornerstone of many mineral processing circuits, especially where sticky ores and clay-rich materials complicate downstream operations. While factors like feed size, throughput, and liner design receive much attention, one of the most influential parameters is the aspect ratio of the scrubber. Aspect ratio simply refers to the length of the drum divided by its diameter (L/D). This ratio influences residence time, scrubbing intensity, and overall circuit performance.

Typical Aspect Ratios

The aspect ratio of a rotary scrubber determines its geometry. A general rule of thumb outside of specialised applications calls for an aspect ratio between 1 and 2.5:

• Low aspect ratio (1:1 to 1.5:1)

• Moderate aspect ratio (1.5:1 to 2:1)

• High aspect ratio (2:1 to 2.5:1)

Residence Time

The most direct impact of aspect ratio is on residence time:

• Shorter drums discharge material quickly, reducing the chance of over-scrubbing and maintaining high throughput.

• Longer drums hold material longer, giving clays and weathered rock more time to receive kinetic energy.

Scrubbing Energy Delivery Mechanisms

Aspect ratio influences the primary mechanism that delivers energy to the ore:

• Low aspect ratios rely more on the energy imparted by lifting and dropping motion; suitable for less cohesive ores.

• High aspect ratios promote repeated cascading and abrasion, ideal for sticky materials that need more aggressive, shear-like energy to be broken down.

  
  

The choice here determines whether the scrubber acts as a rapid washing unit or as a deep-cleaning, liberation tool, along with many other factors.

Throughput vs. Liberation

There is always a trade-off between capacity and scrubbing effectiveness:

• Low L/D scrubbers: higher throughput, lower energy use per ton, but less complete removal of clays.

• High L/D scrubbers: lower throughput per installed diameter, but cleaner product.

Power and Wear Considerations

• Longer scrubbers typically draw more power because of the extended retention and increased cascading of material.

• Short

  r scrubbers are more energy efficient but may see higher localized wear patterns, especially at the feed end.

Practical Selection Guidelines

• Coarse, free-digging ores with minimal clay: Favor lower aspect ratios (1.3–1.6:1).

• Moderately sticky ores: Standard designs in the 1.8–2.2:1 range balance throughput and scrubbing.

• Highly cohesive clays (laterites, bauxites, certain gold ores): Higher ratios (2.5:1) provide the residence and abrasion needed to achieve acceptable discharge quality.

  
  

Final Thoughts

Aspect ratio is not a one-size-fits-all parameter. Choosing the correct geometry requires understanding the ore’s clay content, stickiness, and the downstream process requirements.

Contact Manhattan’s engineering team today to discuss the right scrubber solution for your project and ensure your plant runs at peak performance.

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