Classification and separation are two of the most important processes on a mine site to achieve optimal throughput and material grade and hydrocyclones are a critical part of this process. A source of frustration for many plant operators is roping and incorrect material classification: Weir Minerals explains what roping is and how you can avoid it.

What is hydrocyclone roping?

Hydrochlones operate on a simple principle; a pump transports slurry to a hydrocyclone for separation which is then ejected via two outputs – underflow and overflow. Depending on the application, the slurry contains different particle sizes and varies in density. The overflow stream contains fine particles, which are typically sent to the next stage of processing, while larger particles are sent to the mill for reprocessing.

Normal classification condition: Splash

Operators often try to pump as much material as possible through hydrocyclones, and once full of slurry, the equipment enters one of three operating conditions. The underflow spray angle allows easy identification of the hydrocyclone’s operating condition. The first operating condition is called ‘splash’, where slurry is discharged from the underflow of the hydrocyclone in a wide spray pattern. This is a normal and safe classification condition.

Ideal operating condition two: Semi-roping

With all other conditions unchanged, increasing pressure and pumping more slurry into the hydrocyclone can cause it to move into the second condition, known as ‘semi-roping’. This state is characterised by improved hydrocyclone efficiency and increased density in the underflow. Increasing the cyclone feed density, closing some hydrocyclones within a cluster or reducing the amount of water to the underflow launder through adjustment of the hydrocyclone’s internal fittings can also induce semi-roping. This is the most ideal operating condition for a hydrocyclone, as it improves the bypass of fines, circulating load and separation efficiency.

Inefficient operating condition: roping

Roping is the third and final condition, where pressure is intensified inside the hydrocyclone. As a result, the underflow volume is reduced, causing particles that are larger than the desired product size to travel to the overflow, affecting the following process and damaging metallurgical parameters. The wide spray pattern seen in the splash condition disappears during a roping event, with only a dense discharge line observed.

There is a fine balance between semi-roping and roping. When a hydrocyclone is roping, the slurry is not properly separated. Instead, some of the larger particles will be sent to the overflow and directly to the next stage of processing.

En route, pipe blockages or sanding of underflow tanks can occur. If this arises, the entire process may need to be stopped to clear any blockages, causing downtime.

Key causes of hydrocyclone roping and how to prevent it

Roping can occur when pressure is increased without achieving the required slurry density, which differs according to the application. For example, hydrocyclones processing low density materials such as tailings slurries are less prone to roping. However, as most mill circuits process denser, large particle slurries, roping conditions are more likely to develop.

For circuits containing hydrocyclone clusters, spare hydrocycylones can be utilised to diminish the pressure caused by roping. Alternatively, the pump speed can be reduced to curtail the flow of slurry to the cluster. Water can also be added to the slurry in the underflow tank or other stage of the mill circuit process to reduce cluster feed density, however this is a slower solution.

Monitoring the density and pressure of the slurry pumped within your plant will help prevent hydrocyclone roping and allow your equipment to operate in an optimal condition. To facilitate this, Weir Minerals developed the Synertrex® smart analytics platform and Cavex^® Cyclone Control System (CCCS). This technology uses sensors to capture hydrocyclone vibration data, which is analysed using advanced algorithms and correlated with a hydrocyclone’s three operational discharge conditions. The systems then alert operators when a hydrocyclone is at risk of roping or any blockages are detected, for enhanced reliability, performance and optimum throughput.

“We work with customers to analyse their plant’s configuration and capabilities, and make changes designed to prevent hydrocyclone roping without increasing the site’s footprint. For optimal performance, operators can install Synertrex or Cavex Cyclone Control technology, delivering real-time hydrocyclone data direct to their control room for more informed decision making”, states Eduardo Putz, Product Development Manager for Weir Minerals.

Whether you manually monitor your hydrocyclones or utilise technology to prevent roping and monitor performance, Weir Minerals has the expertise to help operators make better decisions and improve their classification and separation process. To learn more about its capabilities, contact your local representative.