Coal Preparation

Options For the Addition and Control of Non-Magnetic Material in the Correct Medium

Coal Preparation » Gravity Separation

Published: June 20Project Number: C24050

Get ReportAuthor: Michael O’Brien | CSIRO

The non-magnetic concentration in the correct medium is important, particularly at correct medium densities below RD 1.4 where the Dense Medium Cyclone (DMC) differential - which is the difference between the density of the underflow medium and the overflow medium - may be too high. This can produce instability in the cyclone operation, and the loss of valuable coal.

The addition of non-magnetics to the correct medium, increases the volume concentration of fine particles, allowing particle inter-particle interactions and hindered settling which stabilises the medium, and decreases the differential. Nominally a differential of between 0.2 and 0.4 (Crowden et al. 2013) should be maintained, although this is not a hard figure, as some cyclones operate outside of this range without issues. Generally if the differential is above 0.4, excessive separation of medium can take place in the cyclone, and the cyclone  can become unstable. Below 0.2, viscosity issues in the correct medium can prevent good separation, particularly of the finer particles which will result in loss of coal to the reject, and/or reject material in the product.

This project examined methods that can be used to maintain the concentration of non-magnetics in the correct medium at a level that promotes medium stability, approximately 20% by weight in the correct medium solids, (O'Brien et al. 2014). The most prospective method found  was to control the addition of the non-magnetic material from the thickener underflow using a control valve with inputs from the nucleonic density gauge anda magnetite monitor which provides the concentration of magnetite in the correct medium and allows the calculation of the percent non-magnetics in the medium.  The cyclone differential which is available online from electrical impedance spectrometers in the drain sections of the product and reject screens  is also monitored and used as an input to the control system  

A system was installed at a coal preparation plant and operated over a 12 hour period with regular controled  on and off periods. Despite the influence of feed changes and bleed valve changes which will effect the non-magnetic concentration in the correct medium the system performed as expected with the amount of non-magnetics material increasing when the system was on and decreasing when the system was off.  

It is recommended that this type of system be employed where low densities of separation are common and medium instability is an issue. By controlling the bleed valve from this system highly viscous mediums with differentials below 0.2 could also be controlled to maintain a “healthy” differential without loss of coal.


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