Unexpected Non-acid Drainage from Sulfidic Rock Waste

Andrea R Gerson,Catherine Davis,Roger St C Smart,Peter J Rolley,Stephen Doyle,Sandrin T Feig

doi:10.1038/s41598-019-40357-4

Andrea R Gerson, Catherine Davis + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-40357-4

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Abstract

Most rock extraction sites, including mine sites and building construction sites, require a plan to assess, and mitigate if present, the risk of acid mine drainage (AMD). AMD is typically the major environmental concern where sulfide minerals are present in the excavated material and AMD prediction and remediation is based on internationally-accepted acid-base accounting (ABA) tests of representative field samples. This paper demonstrates that standardized ABA tests may not always be provide the correct AMD classification for commonly occurring waste rocks containing low-pyrite and -carbonate due to mineralogic assumptions inherent in their design. The application of these standard ABA tests at a copper mine site in South Australia resulted in the classification of a portion of its waste material as potentially acid forming in apparent contradiction to long term field measurements. Full definition of the sulfide and silicate minerals enabled re-evaluation of the weathering reactions occurring. The overall rate of neutralisation due to silicate dissolution was found to always exceed the rate of acid generation, in agreement with field observations. Consequently, the waste rock was redefined as non-acid forming. The methods developed represent a significant advance in AMD prediction and more strategic, cost-effective environmental planning, with potential for reclassification of wastes with similar characteristics.

Highlights

Executing the tasks to remove the risk of acid mine drainage (AMD) is the greatest environmentally-related cost for many sulfide-containing mines and construction sites around the world
From 2011 fresh waste rock was defined as potentially acid forming (PAF) if the total sulfur content was >0.3 wt% and requiring, by statutory regulation, encapsulation at end of mine life to remove the risk of AMD
This paper presents the results of a re-examination of the assumptions in the standard acid-base accounting (ABA) tests and the reasons why they fail in this instance, and a description of an alternative method, using readily accessible laboratory facilities to improve the prediction of AMD for materials characterised by low-pyrite and low-carbonate content

Summary

Introduction

Executing the tasks to remove the risk of acid mine drainage (AMD) is the greatest environmentally-related cost for many sulfide-containing mines and construction sites around the world. These results are not consistent with the predictions from the standard acid-base accounting (ABA) tests that characterise rocks with >0.3 wt% S as having potential to generate AMD.

Results

Conclusion