Long-Term Acid-Generating and Metal Leaching Potential of a Sub-Arctic Oil Shale

Abstract

Shales are increasingly being exploited for oil and unconventional gas. Exploitation of sub-arctic oil shales requires the creation of gravel pads to elevate workings above the heaving effects of ground ice. These gravel pads can potentially generate acidic leachate, which can enhance the mobility of metals from the shale. To examine this potential, pyrite-bearing shale originating from sub-Arctic gravel pad sites were subjected to leaching tests for 600 days at initial pH values ranging from 2 to 5, to simulate potential real world conditions. At set times over the 600 day experiment, pH, oxidation reduction potential (ORP), dissolved oxygen and temperature were recorded and small liquid samples withdrawn and analysed for elemental concentrations using total reflection X-ray fluorescence spectrometry (TRXRF). Six of eight shale samples were found to be acid generating, with pH declining and ORP becoming increasingly positive after 100 days. Two of the eight shale samples produced increasingly alkaline leachate conditions with relatively low ORP after 100 days, indicating an inbuilt buffering capacity. By 600 days the buffering capacity of all samples had been consumed and all leachate samples were acidic. TRXRF analyses demonstrated significant potential for the leaching of S, Fe, Ni, Cu, Zn and Mn with greatest concentrations found in reaction vessels with most acidic pH and highest ORP.

2015 Atomic Spectrometry Update – a review of advances in X-ray fluorescence spectrometry and their applications

“…XRF was one of the techniques used by Tremain et al 213 to analyse char created from coal tailings sourced from two Australian coal mines. Concerning a related topic, Mumford et al 218 were concerned about the potential for acidic leachate from pyrite-bearing gravel pads that are required to elevate workings in the exploration of sub-arctic oil shales above the heaving effects of ground ice. Wang et al 214 investigated the recovery of vanadium by otation from low-grade stone coal, measuring the mineral composition and microstructure of the source material using the XRF, XRD and SEM techniques.…”

“…The conclusions of this investigation were that the clay layer liners containing contaminants had been breached over time due to overloading of the pits and/or structural damage, allowing chemical materials to permeate into the environment and into aquifers. Concerning a related topic, Mumford et al 218 were concerned about the potential for acidic leachate from pyrite-bearing gravel pads that are required to elevate workings in the exploration of sub-arctic oil shales above the heaving effects of ground ice. They used the TXRF technique to analyse small liquid samples withdrawn from the pads and reported that there was signicant potential for the leachate to contain Cu, Fe, Mn, Ni, S and Zn.…”

Mine Waste Characterization, Management and Remediation

“…Dold [5] reviews the complex biogeochemical and mineral dissolution processes in sulfidic mine tailings leading to the formation of acid mine drainage (AMD). Mumford et al [6] and Langman et al [7] describe long-term acid generation from sub-Arctic oil shales and waste rock from the Diavik Diamond Mine in the Northwest Territories of the Canadian Arctic, respectively. In both cases, the acid generation potential is dependent on the concentrations of sulfide, neutralization reactions with carbonates and Al-bearing minerals and temperature.…”