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Coal Preparation

Turboflotation

Coal Preparation » Fine Coal

Published: August 96Project Number: C4045-C5445

Get ReportAuthor: P Ofori, Bruce Firth | CSIRO Energy Technology

Froth flotation is currently the most widely used method for fine particle processing but the capital and operating costs are high. This is mainly due to the use of the force of gravity to drive the separation process and the resulting long residence times for the material in the cells.

This project is aimed at developing an inexpensive compact process based on centrifugally enhanced flotation, which could reduce the material residence time by an order of magnitude. A key feature of this technology is the separation and individual optimisation of the sub-processes of flotation; bubble generation, particle/bubble contacting and froth-pulp separation.

In the Turbo-Flotation process, air is added directly to the feed slurry via a vertical jet mixer eductor, which promotes the formation of air bubbles. The feed then passes through an in-line mixer that promotes particle/bubble contact and attachment. Finally, the mixture is injected tangentially into a cycloid separator, imposing a centrifugal force to enhance the separation process.

Initial proof of concept testing in a 90 mm separator showed acceptable metallurgical results at a volumetric capacity 20 times that of a conventional flotation system of similar size. Scale up is being approached in two stages, each involving a 40-fold increase in capacity. This report covers the first scale up step, to a development scale separator (300mm-diameter). Funding is being sought for the next stage, which will be a 1 m diameter commercial unit.

The scale up is based on:

  • constant residence time of material flowing through the system
  • constant Froude number (the centrifugal force on the slurry as it enters the separator), and
  • geometric similitude.

Testing was conducted at South Blackwater and Catherine Hill Bay plants.

South Blackwater
The process stream treated was a portion of the thickener feed, which at the time of the tests was rejected after thickening. Product ash levels achieved ranged from 4% to 13% with combustibles recovery ranging from 40 to 85% from feed ash values varying from 11% to 32%. The best result achieved with the unit operating in single stage mode was 72% combustibles recovery at 7.2% ash from a feed ash of 19%. With recycle of about 30% of the tailings to feed, the combustibles recovery was raised to 85% at 10.6% ash.

The results were comparable to those achieved using the Australian Standard Flotation {basic) test. The feed to the Development cell was about 20 tons per hour of slurry with about 4% solids content, which was in line with the original estimates.

Catherine Hill Bay

A portion of the feed to the full scale Jameson cell treating the classifying cyclone overflow and other streams in the fines circuit was continuously admitted into the feed sump of the Turbo-Flotation unit for processing.

  • In single stage operation without recycle, yields of 54-68% were achieved with product ash values between 9 and 11% from feed with ash values ranging between 16 and 20%.
  • With recycle of about 30% of the tailings back to the feed, yield of 60-80% were achieved with 10% ash.
  • In two-stage operation, yields of 80-88% were achieved at product ash of 11-13%.

These results were comparable to that obtained in the Australian Standard flotation tests. The ash values were usually 1-2% higher than samples taken from the Jameson cell (with washwater) during the experiments but the yields were 5-8% higher.

Froth washing was used to reduce the Turbo-Flotation product ash levels by 1-2 percentage points with some loss in yield.

Conclusions

It has been successfully demonstrated on a 300-mm diameter Development unit that metallurgical performance equivalent to that achieved by conventional flotation systems may be achieved using this technology. This performance is attained at the original design capacity, which is an order of magnitude higher than conventional technology.

The scale-up concepts have been confirmed on this development scale, and design and operating data acquired to form the basis of scale-up to the commercial demonstration scale Turbo-Flotation unit.

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