Coal Preparation » Gravity Separation
As the global market shifts toward a low-carbon economy, metallurgical and coking coal producers face stringent requirements to supply higher quality products characterised by lower ash content. Achieving these specifications requires dense medium cyclones (DMCs) to operate at lower density cut points. However, processing suspensions at low densities (typically below 1.40 g/cm3) poses severe inherent challenges due to medium instability. These unstable suspensions exhibit cyclic fluctuations, including slurry surging and air core collapse, which lead to a continuously shifting cut-point, poor separation efficiency, misplaced material, and significant product loss to the discard stream.
Traditional methods used by industry to restore operational stability include adding fine nonmagnetic clays or sourcing ultra-fine magnetite grades. However, these approaches can be operationally undesirable or economically prohibitive. This project investigates a technological alternative: using an applied external magnetic field around a DMC to control slurry rheology and establish operating conditions capable of maintaining high-efficiency separations at low densities.
The primary objective of this project was to evaluate how an applied magnetic field can be used to successfully achieve lower separation cut-points. The project specifically aimed to determine whether the magnetic field directly shifts the physical cut-point for a static feed density (as suggested by historical macroscopic force-gradient theories) or whether it stabilises low-density operations to enable high-efficiency, low-cut-point separations (as suggested by micro-structural interparticle collision models).
The specific technical milestones achieved in this phase were:
- Determine and optimise the operational effects of magnetic field strength and magnet vertical positioning on cyclone performance; and
- Identify the lowest practically attainable stable cut-point and its corresponding probable error (Ep).
The testing was executed on a highly specialised, closed-circuit experimental test rig engineered with a 165mm stainless steel, silicon-carbide-lined dense medium cyclone. The cyclone was operated at a standard operating pressure of 9D via a variable-speed slurry pump. The medium consisted of a commercial ultra-fine grade magnetite suspended in water. Laser diffraction analysis over a two-month period confirmed negligible medium degradation across approximately 50 test runs.
The experimental matrix was divided into two core campaigns:
- Density differential baseline tests; and
- Tracer density partition tests.
The experimental data provided clearly answers to the project's fundamental research questions, establishing that the primary advantage of applying a magnetic field around a DMC is slurry stabilisation rather than direct cut-point manipulation.
The project successfully demonstrated that an applied magnetic field can be used to achieve low separation cut-points in dense medium cyclones. However, it does so by providing a method for stabilising inherently unstable low-density slurry suspensions rather than directly modifying the cut-point. Operating within an optimal magnetic field envelope of 3 to 5 mT effectively minimises underflow fluctuations and density differential instability. This stabilisation enables dense medium circuits to run at ultra-low feed densities, resulting in low cut-points (down to 1.40 g/cm3) with exceptional separation sharpness (Ep = 0.009).