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Coal Preparation

Investigation of Moisture Reduction Methods for Horizontal Belt Filters with the Single Leaf Filter Test

Coal Preparation » Dewatering

Published: April 99Project Number: C6049

Get ReportAuthor: Florent Bourgeois, Tofazzel Hague, Chris Clarkson | University of Queensland, C Clarkson & Associates

This project builds on earlier work, that concluded that controlling cake microstructure was a potential means of reducing equilibrium cake moisture. This result led to the hypothesis that by modifying cake microstructure during the drying phase of the filtration cycle, cake moisture could possibly be lowered. Secondly, the project established the potential of the Single Leaf Filter Test (SLFT) apparatus as a possible tool for sizing horizontal vacuum belt filters.

The objectives of this project were:

  • to trial microstructure alteration methods for reducing cake moisture on horizontal vacuum belt filters, and
  • to derive and test a scale-up procedure for sizing full-scale horizontal belt filters based on data obtained with the Single Leaf Filter Test (SLFT) apparatus.

Moisture reduction by mechanical cake mixing

Initial attempts to alter cake microstructure using vibration and compaction, conducted on pilot-scale and full-scale horizontal belt filters, concluded that neither altered cake microstructure sufficiently to reduce cake moisture.

Continuous cake mixing trials were conducted under controlled laboratory conditions, using a double-helix ribbon blade mixer to agitate filter cakes formed in top-fed laboratory leaf filtration tests during the drying phase. Such mixing reduced cake moisture by up to 4%. Faster dewatering kinetics were also observed. It is estimated that belt length could be reduced by 20% to 30% while maintaining cake moistures.

Several factors were identified as having an impact on moisture reduction by cake mixing:

  • Moisture reduction was found to be very sensitive to the time at which mixing is initiated during the drying phase.
  • Moderate air purging was found to marginally improve moisture reduction.
  • Mechanical mixing was most effective with finer size fractions.
  • The mixing technique was very effective in coping with the extremely cohesive nature of moist fine coal particles

Projected values for discharge moisture and belt size were obtained from a comprehensive series of laboratory mixing tests with a fine coal flotation concentrate from Central Queensland. The values are summarised in the table below.

With Mechanical mixing time at which mixing starts (s) *Predicted belt area (m2) Cake discharge moisture (%)
No - 100 25.8
Yes 30 67 25.8
Yes 40 80 23.2
* Fixed belt width assumed

Vacuum belt filter sizing using the Single Leaf Filter Test apparatus

The project confirmed the possibility of using the SLFT apparatus to size horizontal vacuum belt filters. Firstly, site tests confirmed the agreement between SLFT and belt filter performance under the same operating conditions. A method for scaling up SLFT test results was derived. It permits calculation of the minimum effective belt filter area required to meet a number of user specified constraints, including feed rate and cake discharge moisture. The technique was compared to actual plant performance data and good agreement resulted. It also proved useful as a means of deriving information about the behaviour of operating belt filters.

In spite of adverse experimental conditions that prevented conclusive comparison of HBF and SLFT form times, excellent agreement was obtained between their respective dewatering kinetics, confirming the potential of the SLFT as an HBF sizing tool.

A sizing scheme was developed from which belt length, width, linear speed, operating cake height and minimum vacuum pump capacity, necessary to treat a specified tonnage and yield a set product moisture, can be determined from SLFT measurements alone. The results were found to compare well with the average HBF performance at BHP Coal Riverside mine.

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