NWT RESEARCH DATABASE

Regions: North Slave Region

Tags: contaminants, arsenic, Giant Mine, waste rock

Objective(s): To provide an enhanced understanding of the physical and geochemical properties of the roaster waste; and, to screen for potentially viable remediation alternatives that may warrant additional research (e.g., long-term laboratory experiments, pilot-scale trials).

Project Description: This licence has been issued for the scientific research application No.4862. The overall goals of the four projects proposed within Phase 1 are to provide an enhanced understanding of the physical and geochemical properties of the roaster waste; and, to screen for potentially viable remediation alternatives that may warrant additional research (e.g., long-term laboratory experiments, pilot-scale trials). The research includes four projects as follows: Project 1: Examination of arsenic trioxide dust composition and solubility The research team will collect complementary chemical and physical data to assess the heterogeneity of arsenic trioxide (As2O3) dust samples. The team will conduct screening tests to constrain the variability in bulk characteristics of all samples: 1) determine the bulk chemical composition of all samples by low-temperature digestion methods (e.g., aqua regia) and detection by either inductively coupled plasma – optical emission spectroscopy (ICP–OES) or inductively coupled plasma – mass spectrometry (ICP–MS); and 2) characterize the bulk mineralogy of the samples with powder X-ray diffraction (XRD). Based on these results, split samples into batches based on their composition and mineralogical properties (e.g., Sb concentrations). The research team will conduct solubility experiments, fine-grained As2O3 will be synthesized in the presence of different antimony (Sb) concentrations. The synthesized solids will cover a range of Sb concentrations observed in the initial As2O3 characterization work. The final chemical composition of these solids will be determined by measuring residual aqueous arsenic (As) and Sb concentrations in supernatant solutions. Low-temperature digestion and ICP-MS detection will provide confirmation of As2O3 compositions. Particle size and crystal morphology of the synthetic As2O3 solids will be examined by a transmission electron microscopy (TEM). Laboratory-based powder XRD will provide information on crystal structure, whereas As and Sb coordination will be examined using synchrotron-based methods. Project 2: Sulfidation of As2O3 to low-solubility arsenic sulfide (As2S3) The rate of sulfidation reactions will be assessed as a function of the initial potential of hydrogen (pH) and the initial ratio of sulfide to As. The sulfide ion (S2-) is the reaction-progress variable and concentrations will be determined using either a sulfide-specific ion selective electrode in the high range or colourimetry in the low concentration range. The solid product from sulfidation will be dried and the mineralogical composition characterized using XRD, scanning electron microscopy (SEM), and TEM. A subsample of the solids will be digested and analyzed by ICP-OES and ICP-MS. The stability of the As2S3 reaction product will be studied using batch kinetic reactions. The experiments will be conducted with simulated mine-water compositions. The principal experimental variables will be pH and dissolved oxygen concentration, and the rate of oxidative dissolution will be monitored with dissolved As and sulfate (SO42-) as the reaction progress variables. Major, minor, and trace-element geochemistry of the experimental solutions will be monitored. Following the oxidative dissolution experiments, the solid-phase residue will be characterized by XRD and SEM-EDS. Project 3: Incorporation of As2O3 into cemented-paste backfill Representative arsenic trioxide dust samples from the Giant Mine and a homogenized sample of tailings from the site will be used in the paste backfill recipes used to immobilize the arsenic trioxide dust. A variety of tests will be used to characterize and constrain the arsenic chemistry of the tailings and dust samples. Paste backfill samples will be prepared and mixed to reach targeted consistency and will be poured into molds and allowed to cure in a humidity-controlled room to replicate underground conditions. Mechanical strength will be evaluated to determine the strength acquisition of cemented paste backfill (CPB). Uniaxial compression strength tests will be performed on paste samples. Rheology of the paste mixtures will be evaluated to determine how the dust content impacts hydraulic transport properties of the paste. Leaching tests will be conducted to evaluate the extent of As stabilization in the paste-backfill matrix, and to evaluate the long-term durability of the optimal CPB formulation. Monolithic leaching tests will be performed on paste-hardened samples to evaluate As leaching rates with time. Humidity cells of disaggregated CPB samples will be also performed on the optimal recipes to simulate behaviour of paste backfill when it loses structural integrity. Project 4: Leaching behaviour and geochemical stability of vitrified arsenical glass Samples of vitrified slag will be obtained from Dundee Sustainable Technologies through Giant Mine Oversite Board (GMOB). These vitrified materials will be characterized to determine the composition and mineralogical properties. A series of leach tests and geochemical extractions will be conducted to provide insights into the leaching properties and the potential for As remobilization under differing geochemical conditions. All leach tests and extractions will be performed on samples of the as-received particles and on pulverized samples. Optical microscopy and X-ray diffraction will be used to assess the amorphous nature of the vitrified product, and to identify potential crystalline constituents. Scanning electron microscopy – energy dispersive X-ray spectroscopy will be used to provide solid-phase stoichiometric associations. Synchrotron-based X-ray absorption spectroscopy will be used to elucidate As bonding, speciation, and coordination in the vitrified product. The research team will provide Giant Mine Oversight Board with annual progress reports, which will include detailed research progress updates and data summaries. In addition, the results of this research will be described in academic theses, conference presentations, conference proceedings papers, and refereed journal articles. The research team will interact directly with personnel involved in site remediation at Giant Mine. Research results will be disseminated to the public via community-engagement activities; graduate students and other research personnel are willing to provide presentations to local school groups and to other members of the community through knowledge-mobilization workshops as organized by GMOB. The fieldwork for this study will be conducted from April 1, 2021 to December 31, 2021.