Geochemistry Of Tailings Research Articles

S and O isotope ratios of sulfate suggest enhanced monosulfides oxidation in a historical Pb–Zn mine site

Aqueous and solid tailings geochemistry of two contrasting depositional sites (water-saturated and subaerial) in a historical Pb–Zn mine tailings in Balya, Turkey were chemically and isotopically (δ34S and δ18O) characterized to trace source of sulfate and elucidate oxidation pathways of sulfide mixtures. Aqueous geochemistry data of water-soluble fractions (n = 15) obtained from the batch leaching tests applied on the subaerial mine tailings (SA-MT) revealed three geochemically distinct zones: i) oxidized zone (OXZ, 10–75 cm), ii) transition zone (TZ, 75–125 cm) and iii) unoxidized zone (U-OXZ, 125–350 cm). The OXZ is characterized by a sharp decrease in sulfur and metals contents of the tailing rocks while high sulfate and metals concentrations in their water-soluble fractions. In contrast, the TZ and U-OXZ zones are mainly characterized by low sulfate but relatively high metal concentrations with slightly acidic pH values (4.9–6.2). Compared to the SA-MT tailings geochemical profiles of the pore waters of the water-saturated tailings (S-MT) showed remarkably high sulfate, Fetot and metals concentrations with strong acidic pH values (1.9–3.8). The δ34S and δ18O values of sulfate suggest that galena and/or sphalerite are the main source of sulfate and metals at the tailings site. The differences in sulfur isotope values (Δ34SSO4-sulfide) appear to be reflective of the extend of oxidation of monosulfides (e.g. galena) under two contrasting conditions of the tailings. The large sulfur isotope differences (Δ34SSO4-sulfides ~ -4.5 ± 0.2‰) imply incomplete oxidation of monosulfides to sulfate with aqueous geochemistry record of low sulfate, high metal and slightly acidic pH values. In contrast, the small Δ34SSO4-sulfide (<1 ± 0.2‰) values indicate the tailings site where complete oxidation of monosulfides to sulfate occurred resulting in acidic condition having with high sulfate, metal concentrations. The δ18OSO4 values were enriched than those of background water in tailings sites with the similar Δ18OSO4–H2O values (+8.9 to +11‰). Based on field data revealing enhanced galena and/or sphalerite oxidation relative to pyrite, the observed Δ18OSO4–H2O (avg., 9‰) values coupled with the relationship between δ34SSO4 values and sulfate concentration can be reflective of oxidation of monosulfides in contrast to disulfide minerals (e.g, pyrite) within polymetallic rich AMD systems.

Geochemistry of uranium mill tailings in the Athabasca Basin, Saskatchewan, Canada: A review

The Athabasca Basin, located in northern Saskatchewan, Canada, is a major source of global U and an important economic driver for the province and country. Athabasca Basin U deposits consist of uraninite and pitchblende dominated by quartz and aluminosilicates and varying amounts of sulfide and arsenide minerals associated with varying concentrations of As, Se, Mo, Ni, and 226Ra (elements of concern; EOCs). Processing these U ores results in tailings that are often enriched in EOCs. Mill treatment processes are designed to generate tailings that minimize the long-term environmental impact of U tailings, although many challenges exist in reaching this goal. Many studies have contributed to our understanding of the geochemistry of these tailings and EOCs and their potential impact on the surrounding hydrosphere. Using nearly two decades of data from tailings samples, mill sampling campaigns, and laboratory experiments, this review provides a comprehensive analysis of the geochemistry and long-term behavior of U tailings in the Athabasca Basin and develops a geochemical model of the tailings. Results of this review are applicable to tailings generated from other milling operations with comparable acid leaching hydrometallurgical processes. This holistic review also highlights the limitations to our current understanding of U tailings geochemistry.

Mineralogy and geochemistry of tailings from a gold mine in northeastern Thailand

ABSTRACTMineralogical and geochemical characteristics of tailings from a gold mine in northeastern Thailand were investigated in relation to acid mine drainage (AMD) and the release potentials of toxic elements. The tailings can be divided into upper tailings and lower tailings. The upper tailings usually contain pyrrhotite, pyrite ± chalcopyrite, calcite, quartz, andradite and diopside. The lower tailings mainly contain goethite, quartz, chlorite, muscovite, calcite and hematite ± pyrrhotite. These assemblages clearly relate to the original types of gold deposit prior to mining and mineral processing. The upper tailings are defined as potential acid forming (PAF), whereas the lower tailings are classified as non-acid forming (NAF). Regarding heavy metals, apart from high Mn level, the other heavy metals appear to have low concentrations in the upper tailings. On the other hand, the lower tailings contain high contents of As, Cu and Pb, which appear to be higher than the National Total Threshold Limit Concentrations. Goethite, the main mineral assemblage in the lower tailings, reveals characteristic of arsenic adsorbent. As a result, the tailing pond is recommended to be covered to prevent the oxidizing processes of the upper tailings; otherwise, AMD generation may take place soon after the mine closure. Land reclamation and monitoring plans must be planned very well and carried out with great care since arsenic contamination has been reported in steam water close to the tailing dam.

Assessing metal transfer to vegetation and grazers on reclaimed pyritic Zn and Pb tailings.

A study of the concentrations of zinc and lead in an engineered soil capping system overlying sulphide mine tailings was undertaken. Tailings geochemistry, soil cover and vegetation were monitored over a 4-year period, and a cattle grazing demonstration exercise was conducted over a 1-year period. Whilst the tailings had a relatively high pyrite content and demonstrated oxidation, a circum neutral pH was observed for the duration of the study period due to the high dolomitic content. No evidence of metal mobility into the soil cover and vegetation was observed over the monitoring period. Relatively high Zn herbage content is attributed to the glacial till component of the soil cover. Similarly, no evidence of metal transfer to grazing cattle was observed through blood and tissue analysis with Zn content not significantly different from control animals. Pb tissue content was below limit of detection.

Microbial communities in low permeability, high pH uranium mine tailings: characterization and potential effects

To describe the diversity and metabolic potential of microbial communities in uranium mine tailings characterized by high pH, high metal concentration and low permeability. To assess microbial diversity and their potential to influence the geochemistry of uranium mine tailings using aerobic and anaerobic culture-based methods, in conjunction with next generation sequencing and clone library sequencing targeting two universal bacterial markers (the 16S rRNA and cpn60 genes). Growth assays revealed that 69% of the 59 distinct culturable isolates evaluated were multiple-metal resistant, with 15% exhibiting dual-metal hypertolerance. There was a moderately positive correlation coefficient (R=0·43, P<0·05) between multiple-metal resistance of the isolates and their enzyme expression profile. Of the isolates tested, 17 reduced amorphous iron, 22 reduced molybdate and seven oxidized arsenite. Based on next generation sequencing, tailings depth was shown to influence bacterial community composition, with the difference in the microbial diversity of the upper (0-20m) and middle (20-40m) tailings zones being highly significant (P<0·01) from the lower zone (40-60m) and the difference in diversity of the upper and middle tailings zone being significant (P<0·05). Phylotypes closely related to well-known sulfate-reducing and iron-reducing bacteria were identified with low abundance, yet relatively high diversity. The presence of a population of metabolically-diverse, metal-resistant micro-organisms within the tailings environment, along with their demonstrated capacity for transforming metal elements, suggests that these organisms have the potential to influence the long-term geochemistry of the tailings. This study is the first investigation of the diversity and functional potential of micro-organisms present in low permeability, high pH uranium mine tailings.

English

Over the last 15-20 years, there has been a large emphasis on tailings geochemistry and hydrogeology. Waste rocks piles, on the other hand, have received much less attention. Especially in the porphyry copper mining industry, waste rock piles can represent a larger potential for groundwater and surface water contamination than tailings since the waste rocks are commonly unsaturated and coarse. This setting allows for greater oxygen availablility throughout the pile, thus increasing the oxidation potential. In general, there are three flow systems within waste rocks: macro flow (high flow); matrix flow (low flow) and micro flow (within particles) that together with waste rock composition effect seepage water quality. These systems have been investigated at the Tyrone Mine. Continuous seepage water quality monitoring was performed at one of the stockpiles for approximately one year. The selected stockpile consisted primarily of oxide material with low sulfide content, and had never been leached. Seepage was observed throughout the year and seepage monitoring consisted of flow rate, pH and electric conductivity measurements. Waste rock samples were analyzed for acid base accounting and mineralogy, and kinetic tests were performed. The flow rate together with rainfall, hydrogeological data from the stockpile, stockpile infiltration and evaporation rates, and groundwater recharge rates were used to calculate macro/matrix components in the system using the kinematic wave theory. The results of this modeling indicated that only approximately 10 % of the infiltrating rainwater flowed through the macro pores all the way to the bottom. The majority of the water flowed through the matrix. Water quality monitoring showed a pH increase during low flow periods and a rapid drop during heavy rainstorms. Electric conductivity had a slow decrease during the dry periods and a drastic increase shortly after heavy rainstorms. The geochemical data indicated a buildup of easily leachable secondary acid generating minerals during the dry periods that were being leached during heavy rain storms. The matrix also possessed a neutralizing effect from silicate mineral dissolution that was working effectively only in the slow matrix flow zones.

Evaluation of long-term sulfide oxidation processes within pyrrhotite-rich tailings, Lynn Lake, Manitoba

Oxidation reactions have depleted sulfide minerals in the shallow tailings and have generated sulfate- and metal-rich pore water throughout the East Tailings Management Area (ETMA) at Lynn Lake, Manitoba, Canada. Information concerning the tailings geochemistry and mineralogy suggest the sulfide oxidation processes have reached an advanced stage in the area proximal to the point of tailings discharge. In contrast, the distal tailings, or slimes area, have a higher moisture content close to the impoundment surface, thereby impeding the ingress of oxygen and limiting sulfide oxidation. Numerical modelling of sulfide oxidation indicates the maximum rate of release for sulfate, Fe, and Ni occurred shortly after tailings deposition ceased. Although the sulfide minerals have been depleted in the very shallow tailings, the modelling suggests that sulfide oxidation will continue for hundreds and possibly thousands of years. The combination of sulfide minerals, principally pyrrhotite, that is susceptible to weathering processes and the relatively dry, coarse-grained nature of the tailings have resulted in the formation of a massive-hardpan layer in the proximal area of the ETMA. Because extensive accumulations of secondary oxyhydroxides of ferric iron are already present, remediation strategies for the ETMA should focus on mitigating the release of sulfide oxidation products rather than on preventing further oxidation.

A fly ash/biosludge dry cover for the mitigation of AMD at the falun mine

The sulphide mine tailings in Falun, Sweden is covered with a mixture of fly ash and biosludge as a measure against acid mine drainage. This is a study of the tailings geochemistry by means of pH, Eh, iron, sulphate, calcium and organic carbon concentrations from a field control site and field studies of the covered deposit. The sulphate/iron mole ratios of the interstitial solutions can be used as a proxy to indicate dominating chemical processes in the tailings. In the Falun tailings no ferric iron oxidation of the mining waste seems to take place. Thermodynamic calculation of the redox potential demonstrates which half-cell reaction is governing the redox potential variations. The organic carbon concentrations are very high in the covered Galgberget tailings deposit compared to the control site. This carbon originates from the cover material and is used as a measure against oxygen penetration to the mine tailings. The drawback of the dissolved organic material is that it can act as complexing agents for heavy metals. Also, the dissolved organic material is according to an empiric model decreasing rapidly and of negligible importance after a short time, approximately 30 years. The Galgberget tailings deposit is greatly oversaturated with respect to gypsum making the formation of hardpans possible. Compared to the uncovered control site the field studies of the covered Galgberget tailings deposit show a decrease in the oxidation of iron sulphide minerals and an increase in pH of the leachate with time.

The geochemistry of mine-waste pore water affected by the combined disposal of natrojarosite and base-metal sulphide tailings at Kidd Creek, Timmins, Ontario

Since 1985 natrojarosite residue (NaFe3(SO4)2(OH)6) from the zinc refinery at Kidd Creek near Timmins, Ontario, has been disposed of with mine tailings containing up to 25 wt.% pyrite. Pore-water geochemical data have been collected from depth profiles through the vadose and saturated zones in the tailings. Three zones are defined by the concentrations of metals, SO4, and HCO3. In the deepest zone the concentrations of most metals and anions are below detection and the concentrations of Fe, Mg, Na, K, Zn, Mn, HCO3, and SO4 are low, similar to those in the mill discharge water. Higher concentrations of Na, K, Fe, Mg, Mn, Zn, Pb, As, SO4, and HCO3 occur within an intermediate zone. These increased concentrations are attributed to natrojarosite dissolution. A surficial zone affected by sulphide oxidation contains high concentrations of metals and SO4. High concentrations of Cd, Co, Cu, Ni, Cr, and Al in this zone characterize the effects of sulphide oxidation on pore-water composition as distinct from the effects of natrojarosite dissolution. The H+ released by Fe3+ hydrolysis following natrojarosite dissolution is neutralized by carbonate-mineral dissolution that reduces the acid-neutralization capacity of the tailings and releases HCO3. Natrojarosite codisposal with tailings also causes deterioration of the pore-water quality because the dissolution of natrojarosite releases Na, K, Fe, Pb, and SO4 to the pore water. Key words : mine tailings, natrojarosite, jarosite, tailings geochemistry, sulphide oxidation.