The Minnesota Department of Natural Resources has been conducting laboratory and field rock weathering studies since the mid-1970s. These studies include weathering tests on rocks and tailings associated with nonferrous deposits in Minnesota and methods of mitigation applicable to both nonferrous and ferrous mine wastes. The following eight research reports were completed during fiscal years 2012 thru 2014.
Project Summary: The Minnesota Department of Natural Resources has studied Duluth Complex mine waste drainage quality and mitigation of problematic drainage since the mid-1970s. A report was written to summarize research conducted on waste characterization techniques, laboratory and field dissolution of Duluth Complex rock and tailings, and mitigation of problematic mine waste drainage. Reports issued on the various projects were identified for those seeking additional detail.
Project Application: This report is a directory for a large body of information that provides a technical foundation for developing and evaluating submittals required for environmental review and permitting of proposed nonferrous mining of Duluth Complex rock (Minnesota Rules Ch. 6132.000).
Project Summary: Coarse and fine fractions of Duluth Complex waste rock from the Partridge River Intrusion (PRI) were characterized (particle size distribution, chemistry, mineralogy) and subjected to laboratory dissolution testing for eight years (ongoing). Drainage pH values reflected the balance between rates of acid production by sulfide mineral oxidation and acid neutralization by silicate mineral dissolution and tended to decrease (become more acidic) with increasing sulfur content. Drainage pH was highest at the beginning of the experiment. Minimum values were reached within two years for the fine particles, but drainage pH from many of the coarse particle samples continued to decrease over the entire eight years. Minimum pH values for a specific sulfur content were not substantially different between the two size fractions. Minimum drainage pH values from both the fine PRI and South Kawishiwi Intrusion (SKI) rocks tested previously were near 6 for sulfur contents less than or equal to roughly 0.3 percent, but for higher sulfur contents PRI values were about a unit higher than SKI values of similar sulfur content. This indicated that the PRI rocks could neutralize acid more rapidly than the SKI rocks. More generally, the dependence of drainage pH on sulfur content exhibited similar trends for the two Duluth Complex intrusions, but the dependences differed quantitatively.
Project Application: These results are empirical benchmarks for drainage pH and solute release rates from Partridge River Intrusion rock that provide the scientific background for developing characterization requirements (Minnesota Rules Ch. 6132.1000) for proposals submitted for environmental review. The results indicate that, whereas dissolution of rock from different intrusions is dependent on sulfur content and dissolution time, the quantitative dependence on these factors varies between intrusions. This information will aid in determining testing requirements for environmental review of mining proposals (Minnesota Rules Ch. 6132.1000) and interpretation of results generated. The study results were applied during environmental review for the PolyMet project to evaluate data submitted and extrapolate laboratory waste rock dissolution data to field conditions.
Project Summary: Laboratory dissolution tests were conducted on well-characterized Duluth Complex tailings (0.2 %S) for 10 years (ongoing) to examine 1) the long-term dissolution behavior of the tailings; 2) the influence of moisture content on reactions; and 3) the influence of tailings depth (1.7 vs 7.5 cm) on reactions. Drainage pH values decreased within the circumneutral range for 100 to more than 400 weeks, depending on reaction conditions. Sulfide oxidation in saturated (non-evaporating but not submerged) reactors was more rapid than that in unsaturated (evaporating) reactors and yielded slightly lower drainage pH values than those from the saturated reactors. These lower pH values (6.4 to 6.6) produced nickel concentrations in drainage from the saturated reactors that were roughly 25 times those in drainage from the unsaturated reactors. Sulfide mineral oxidation for deeper tailings beds was limited at depth by oxygen diffusion, but acid neutralization reactions were not. Consequently, the deeper beds yielded higher drainage pH values and lower trace metal concentrations than corresponding values from shallower beds.
Project Application: The results provided scientific and technical information necessary to address mine waste characterization questions inherent to environmental review and permitting of nonferrous metal mines (Minnesota Rules Ch. 6132.1000). More specifically, the 10 years of data are a scientific foundation that yields a benchmark describing dissolution of low-sulfur Duluth Complex tailings, a description that includes drainage pH and rates of solute release. Analysis of the data generated yielded quantification of the dependence of dissolution behavior on water content and tailings depth, factors that must be considered for predicting solute release from tailings during environmental review. Thus, the experimental results will help determine testing required for mine waste characterization during environmental review and permitting (Minnesota Rules Ch. 6132.000). The study results also provide information necessary to extrapolate laboratory test data to field conditions, and were used to evaluate data submitted during environmental review for the PolyMet project.
Project Summary: Twenty-five rock Dunka mine blast hole samples from the Duluth Complex and underlying Virginia Formation were characterized (physical, chemical, mineralogical) and subjected to laboratory dissolution testing for 24 years (ongoing). Drainage pH and solute release rates were a function of sulfur content and time of dissolution. Samples with S ≤ 0.22% (Group I) maintained drainage pH ≥ 6; samples with 0.4% ≤ S ≤ 0.92% (Group II) produced drainage pH ≥ 4, and samples 1.12% ≤ S ≤ 1.64% (Group III) produced minimum drainage pH values in the range of 3.2 to 3.5. As sulfur content increased, rates of sulfate and trace metal release increased as well. Based on drainage pH alone, disposal of waste rock in Group I would require no rigorous reclamation. In order to meet water quality standards, waste rock producing drainage pH values similar to Group III samples would require the most rigorous control measures of the samples tested. Drainage pH typically decreased for 300 to 400 weeks, but this trend was observed for as long as 800 weeks. Ultimately drainage pH increased and sulfate release rates decreased. At the end of the experiment, there was typically 50% of sulfide minerals left unoxidized. Rates of sulfide and silicate mineral dissolution were estimated based on observed release of sulfate and major cations.
Project Application: The intent of this testing was to generate data that would assist in the environmentally sound management of Duluth Complex waste rock generated during mining. To attain this objective the dependences of drainage quality on solid-phase composition of troctolitic/gabbroic Duluth Complex rocks and dissolution time were established. By establishing such relationships, solid-phase composition of these rocks would provide beneficial insight on their dissolution behavior in the field. Dissolution rates of major minerals present were also calculated to aid in extrapolation of results to operational scale and to other deposits within the Duluth Complex.
Project Summary: Placing mine wastes containing sulfide minerals under water (subaqueous disposal) might provide a method of secure, permanent disposal by reducing the rate of sulfide mineral oxidation and the consequent solute release. For sulfide-bearing mine wastes from Minnesota, experiments on thin rock layers were conducted for four to six years to examine the mitigative potential of 1) subaqueous disposal; 2) subaqueous disposal augmented by addition of alkalinity to the water cover; and 3) incorporation of an oxygen consuming compost layer at the water/rock interface of mine waste disposed under water. Under subaerial (simulating disposal on the land’s surface) conditions, rates of sulfide mineral oxidation increased by a factor of five after 120 weeks of reaction. Associated with this increase, pH decreased from the upper threes to 2.5 and trace metals concentrations increased by as much as a factor of 30. Sulfide oxidation rates for the thin rock layers under all subaqueous disposal conditions were initially similar to the early subaerial rates but declined after two to six years of reaction. The minimum pH for subaqueous disposal with no modification was 3.3. Small additions of alkalinity maintained pH in the circumneutral range, but yielded sulfide mineral oxidation rates similar to subaqueous disposal without modification. Maximum release rates for copper, nickel, cobalt, and zinc from the subaerial rock were 10 to 100 times those from the subaqueous rock.
Project Application: This survey experiment indicated subaqueous disposal presents a potentially attractive option for limiting acid and trace metal release from sulfide-bearing mine wastes in Minnesota. It further indicated that this method of disposal requires consideration of site specific conditions because it can yield acidic water in the absence of some addition of alkalinity, perhaps from surrounding rock or groundwater. The results indicated more detailed experimentation examining subaqueous disposal was warranted.
Project Summary: Preliminary experiments indicated that placing mine wastes containing sulfide minerals under water (subaqueous disposal) might provide a method of secure, permanent disposal by reducing the rate of sulfide mineral oxidation and the consequent solute release. Column experiments were conducted for six years to evaluate subaqueous disposal without augmentation and subaqueous disposal with barriers of taconite tailings and mixtures of tailings and yard waste compost and tailings and limestone at the water-rock interface. The pH from the subaerial controls decreased below 6 after 134 weeks of reaction. With the exception of surface water from the subaqueous controls (pH in upper sixes), pH from all subaqueous columns ranged from 7 to 8. Removal of soluble components present on the rock at the beginning of the experiment (analogous to products accumulated on waste rock prior to disposal under water) required three pore volumes of flow. Subsequently, sulfate release rates decreased with increasing depth of tailings barrier, irrespective of the presence of compost or limestone. Rates with a 10 cm barrier depth were about 15% of those without a barrier layer.
Project Application: The results support placing mine wastes under water as a method of secure, permanent disposal by reducing the rate of sulfide mineral oxidation and the consequent solute release. They further indicate that a layer of fine-grained material above the rock will further reduce oxidation rates. PolyMet has proposed disposal of their higher sulfur waste rock under water.
Project Summary: The particle size experiment was designed to 1) determine the variation of waste rock composition with particle size and 2) determine the dependence of leachate pH and solute release rates (e.g., sulfate) on particle size. The study included over 18 years of laboratory weathering data for six different particle size fractions of rock from the Duluth Complex. The study found 1) crushing of Duluth Complex rock resulted in enrichment of sulfide minerals in the finer particle size fractions (< 0.5 mm); 2) minimum leachate pH from the three finest particle fractions (<0.053 to 0.5 mm) was about 4 whereas the three largest particle size fractions (0.5-19 mm) had a minimum leachate pH between about 5 and 6; and 3) sulfate release rates from the finer particle fractions were up to about 100 times greater than the largest size fraction (6.35-19 mm). The relationship between particle size and sulfate release rate was expressed mathematically and used to accurately predict the leachate sulfate concentration from a large (~1,000 tonnes) sulfide-bearing Duluth Complex rock pile.
Project Application: The particle size experiment provides insight on the relationship between rock particle size and waste rock leachate quality and provided a method to extrapolate laboratory test results to operational scale predictions. Mining companies can implement this methodology in mine waste characterization studies to evaluate water quality impact predictions required for environmental impact statements and mine permits (required by MN Rules Ch. 6130.2100 and 6132.1000). Data from the experiment can also be used to inform strategies to mitigate waste rock leachate.
Project Summary: Rock samples from the Ely Greenstone were tested for 12 years in the laboratory (14 samples) and field (4 samples) to evaluate the relationship between rock sulfur concentration and waste rock leachate pH and sulfate release rates. The laboratory tests indicated 1) greenstone samples with a sulfur concentration greater than about 0.1 wt% could produce a leachate pH less than 6, a common water quality standard, and 2) sulfate release rates increase systematically with increasing rock sulfur concentration. Leachate pH from a field rock pile with a 0.67 wt% sulfur concentration decreased to less than 6 after nine years. Rock piles with sulfur concentrations of 0.02, 0.2 and 0.39 wt% produced a leachate pH greater than 6, although pH values from the 0.2 and 0.39 wt% sulfur piles are declining. Field rock pile sulfate release rates increased with increasing rock sulfur content. Comparison of the laboratory and field sulfate release rates indicated the laboratory rates were approximately seven times greater than the field rates.
Project Application: The Ely Greenstone experiment provided a foundation for determining the potential for the development of acidic leachate from rock that could be mined from the Ely Greenstone. Data from this study can be used to 1) guide waste characterization studies (required by MN Rules Ch. 6132.1000) for mining in the Ely Greenstone, 2) inform extrapolation from laboratory tests to operation conditions as is required for environmental review and permitting, and 3) provide a benchmark data set for comparison to test results submitted during environmental review and mine permitting.