The preparation plant, at the old Glenlee site near Narellan in New South Wales, was originally established in the mid-1950s and was, at one stage, considered the largest such facility in the southern hemisphere.

In 2002 Sada Coal Preparation Services restarted operations at the site with a two-stage program involving processing feed from a nearby coal mine and later reprocessing the slurry in the tailings dams at Glenlee.

The operation was successful until the tailings dam material was introduced into the float circuit. Sada could no longer achieve the target sub-12% ash content, due in part to the high-ash, ultrafine particle size of the product.

“Processing the old Glenlee tailings not only got off to a rocky start but became a testing time for all concerned,” said Sada managing director Peter Dunbier.

“What should have been a relatively smooth transition – we did expect some compatibility problems in the early stages – became a period of protracted frustration as we experimented with collector and frother combinations as well as changes to cell settings and some plant modifications to achieve target ash and yield.”

Nalco Australia, a supplier of water treatment and process improvement services, chemicals and equipment programs, trialled its flotation program at the site late last year using highly selective reagents developed to maximise the separation of coal from the associated ash constituents.

A series of optimisation trials were run initially to determine reference levels and then progressed to establishing the reagent levels required to produce a flotation product at the target ash level.

The trial made a difference within hours of initiation, Dunbier said. “The ash content of the feed from the tailings dams can vary from 20 percent to 50 percent and change almost minute by minute but with the Nalco program in place we consistently achieve our sub-12 percent ash content target.

“And the reagents also successfully remove the ultra-slimes from the feed to eliminate any impact on the processing circuit.”

Latest production figures show that Sada is processing some 1200 tonnes per day, yielding 35% fine coal at 11% ash from a two-stage flotation process, an upgrade of the single-stage processing circuit initially used.

Dunbier said the change to the rougher and cleaner system was based on test results that showed refloating the primary froth product would produce a consistent sub-12% ash product with little impact on yield – test results borne out at production levels.