Coal Preparation

Froth Washing in Mechanical Cells

Coal Preparation » Fine Coal

Published: May 01Project Number: C7041

Get ReportAuthor: Bruce Atkinson, Noel Lambert, Graeme Jameson | University of Newcastle, OJK Technology, TUNRA

The objectives of this project were:

  • To demonstrate the effectiveness of froth washing on full-scale conventional flotation cells.
  • To develop design parameters for installation to any mechanical flotation bank.

Froth washing is used in mineral flotation to reduce the amount of entrained gangue material that reports with the valuable mineral in the froth concentrate. Clean water is introduced into or on top of the froth, and percolates downwards through it, replacing the water that has been entrained by bubbles. Since this water initially contains ash particles in the same concentration as in the tailings from the flotation cell, entrainment can lead to high-ash flotation product. If clean water is applied, it can replace most or all of the entrained water, leading to low-ash flotation product.

Froth washing was carried out on full-scale operating flotation cells at two coal preparation plants - Norwich Park CHPP, Queensland; and Macquarie CHPP, Hunter Valley. The campaign at Norwich Park gave inconclusive results, because of the inability to obtain sufficient wash water.

A successful campaign was conducted at the Macquarie CHPP. On average, there was a decrease in product ash from 10.6 to 9.1 percent. There was a reduction in the mass yield from 79 to 75 percent, and in the combustibles yield from 93 to 90 percent. Typical wash water ratios ranged from 1.1 to 4.9. The yield-ash data fitted well on an ultimate flotation response curve for the coal. The effect of the wash water was to move the point down the curve.

A design method applicable to any mechanical flotation cell has been developed. A tray distributor is used, constructed from HDPE or other corrosion-resistant material, with 5 mm diameter holes drilled on a triangular pattern. It is recommended that the installation should allow for wash water application rates of 0.13 to 0.3 cm/s on a superficial basis, corresponding to water rates of 4.7 to 10.8 m3/m2/hr.


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