Fluidised Limestone Reactors for the Remediation of Acidic Drainage Waters

The remediation of acidic water resulting from open cut mining in geologies rich in iron pyrites is one of the major environmental issues facing the domestic and international mining industry today. In 2002 the Western Australian State Government joined with coal and tin mining companies based in South West WA and the Australian Coal Association Research Program to fund a five year study on issues relating to acidic waters in final mining voids. The overall goal of the study was to develop technologies for modeling water quality and reducing acidity and metal contamination in acidic mining lakes, or water pumped from these lakes, and to investigate the application of the remediated lakes or mine lake water for commercial or community based activities.  The study was coordinated by the Centre for Sustainable Mine Lakes (CSML), a collaboration of research groups from WA's four public universities and sponsoring mining companies and included an investigation on the development and application of Fluidised Limestone Reactors (FLRs) in the remediation of acidic mine lake water.

Innovative, low cost cylindrical and conical FLRs were developed, installed and trialed at Wesfarmers Premier Coal and a proposal was developed for the construction and installation of a commercial sized reactor, similar in size to reactors proposed for treatment of acidic water arising from acid sulphate soils (~5L/sec flow rates). The research proved the FLR to be a cost effective system that could raise mine lake water pH and remove iron and aluminium through the use of conical shaped reactors. However, it was also found that fluidisation properties and neutralising efficiency varied with change in vessel dimensions and limestone aggregate properties. Further investigations to assess alterations in fluidisation dynamics that occur with scaling-up the present design were required to inform the commercial potential for large scale FLR treatment systems (>20L/sec) suitable for ADM and other large scale water treatment operations.

The potential benefit of design refinement for optimum performance of the conical shaped FLR led to a Curtin University based research group being awarded an ACARP grant in 2006 of $121,430 for a 14-month project titled 'Fluidised Limestone Reactors for the Remediation of Acidic Drainage Waters'. The Project aimed to construct, install and evaluate the 5L/sec FLR design produced by the CSML research group and to apply computer modeling along with laboratory and field investigations to the development of design guidelines for FLRs that efficiently utilise limestone and are capable of raising the pH above 6 in waters rich in iron and aluminium.

A one month trial of the up-scaled FLR was conducted using acidic water (approx pH 3.0) at the Chicken Creek site. The Chick Creek FLR was shown to raise pH to over pH 6 and significantly reduce iron, aluminium and other dissolved metals in outflow water. No accumulation of Al in the FLR sediment and insignificant  accumulations of Fe, Zn, Cu and Pb were observed. The finding of no accumulation of aluminium in the sediment of FLR was found to be in conflict with the results of a previous study on the Collie Aquafarm FLR in which about 60% or more of the aluminium removed from the water was retained in the cone sediment. Experimental conditions and design parameters that might account for these differing observations were studied through a desk top review of publications on fluidisation within conical reactors and through a second trial conducted at the Collie Aquafarm. The results of these studies confirmed that the design of the conical shaped reactor, in particular the slope of the conical reactor, was the major determinant of the retention of metal particulates within the reactor.