Control of Acid Generation from Pyrite Oxidation in a Highly Reactive Natural Waste: A Laboratory Case Study

Yan Zhou,Jun Li,Russell C Schumann,Andrea R Gerson,Michael D Short,Roger St C Smart,Gujie Qian

doi:10.3390/min7060089

Abstract

Laboratory kinetic leach column (KLC) tests were carried out to define the conditions required to control acid generation from a highly reactive, potentially acid-forming (PAF) iron ore waste rock. It was found that lime addition (0.1 wt % blended) plus either blending of silicates (25 wt % K-feldspar and 25 wt % chlorite), or addition of a non-acid forming (NAF) top cover containing about 10% dolomite (PAF:NAF = 5:1 wt %), when watered/flushed with lime-saturated water, greatly reduced acid generation as compared to the control KLC (PAF alone, watered/flushed with Milli-Q water), but did not result in circum-neutral pH as required for pyrite surface passivation and effective acid and metalliferous drainage (AMD) mitigation. In contrast, the combined use of these treatments—blended lime and silicates with an NAF cover and watering/flushing with lime-saturated water—resulted in leachate pH of 12 (up to 24 weeks). Mass balance calculations for Ca2+ and scanning electron microscopy (SEM) analyses suggest that calcite or gypsum may have formed in the NAF-amended KLCs and lime with added silicate KLC. Although the combined approach in the form trialled here may not be practical or cost-effective, control of a highly reactive natural PAF waste by pyrite surface passivation appears to be possible, and an improved treatment methodology (e.g., slightly increased lime blending without the need for further lime watering/flushing) could usefully be examined in the future.

Highlights

Acid and metalliferous drainage (AMD) from both operating and inactive/abandoned mines is a global environmental problem
The results clearly indicate that both silicates and non-acid forming (NAF) play positive roles in reducing primarily the metal acidity generated from potentially acid-forming (PAF), and that only a combined use of lime addition, NAF covers and silicates blending can maintain circum-neutral/basic pH as required for the establishment and maintenance of pyrite surface passivation
Using laboratory-scale kinetic leach column (KLC) tests, combinations of PAF waste blended with lime and silicates and the application of natural NAF materials as top covers, in conjunction with saturated lime-water watering/flushing, were employed to determine the conditions required to control acid generation from a highly reactive natural PAF waste material

Summary

Introduction

Acid and metalliferous drainage (AMD) from both operating and inactive/abandoned mines is a global environmental problem. AMD is triggered naturally by the oxidation of sulfide minerals, in particular pyrite—the most abundant sulfide mineral on Earth—through chemical and microbially-mediated reactions with both surface water and oxygen [1]. Acidic leachates containing toxic metals/metalloids, such as Mn, Zn, Cd, Pb, As and Se, can be released. AMD is a complex environmental issue, due to both the severity and long-lasting nature of its impacts. For the mining industry and governments, AMD remediation is expensive, with costs in the US and Canada alone estimated to be in the tens of billions of dollars [2]. Cost-effective and sustainable strategies for AMD prevention and remediation are required

Objectives

Results

Conclusion