Synthesis, characterization and thermochemistry of synthetic Pb–As, Pb–Cu and Pb–Zn jarosites

The enthalpy of formation from the elements of well characterized Pb-As, Pb-Cu, and Pb-Zn synthetic jarosites, corresponding to chemical formulas (H3O)0.68±0.03Pb0.32±0.002Fe2.86±0.14(SO4)1.69±0.08(AsO4)0.31±0.02(OH)5.59±0.28(H2O)0.41±0.02, (H3O)0.67±0.03Pb0.33±0.02Fe2.71±0.14Cu0.25±0.01(SO4)2±0.00(OH)5.96±0.30(H2O)0.04±0.002 and (H3O)0.57±0.03Pb0.43±0.02Fe2.70±0.14Zn0.21±0.01(SO4)2±0.00(OH)5.95±0.30(H2O)0.05±0.002, was measured by high temperature oxide melt solution calorimetry and gave ΔH°f = -3691.2 ± 8.6 kJ/mol, ΔH°f = -3653.6 ± 8.2 kJ/mol, and ΔH°f = -3669.4 ± 8.4 kJ/mol, respectively. Using estimated entropies, the standard Gibbs free energy of formation from elements at 298 K ΔG°f of the three compounds were calculated to be -3164.8 ± 9.1 kJ/mol, -3131.4 ± 8.7 kJ/mol, and -3153.6 ± 8.9 kJ/mol, respectively. Based on these free energies, their logKsp values are -13.94 ± 1.89, -4.38 ± 1.81 and -3.75 ± 1.80, respectively. For this compounds, a log10{Pb2+} - pH diagram is presented. The diagram shows that the formation of Pb-As jarosite may decrease aqueous arsenic and lead concentrations to meet drinking water standards. The new thermodynamic data confirm that transformation of Pb-As jarosite to plumbojarosite is thermodynamically possible

Incongruent weathering of Cd and Zn from mine tailings: a column leaching study

The weathering of discharged mine tailings can contaminate groundwaters, rivers and floodplains with potentially toxic Cd and Zn, depending on tailings mineralogy, storage, dispersal and climatic conditions. The mechanisms of long-term tailings weathering and its influence on waste piles and floodplain environments were assessed by a column leaching experiment that incorporated tailings and soil from Potosí, Bolivia, and modelled 20 cycles of wet and dry season conditions over three calendar years. Chemical analysis of the leachate and column solids, optical mineralogy, XRD, SEM, EPMA, BCR and water-soluble chemical extractions and speciation modelling were carried out to determine the processes responsible for the leaching of Cd, Fe, S and Zn. Over this period, approximately 50 to 95% of the original Cd and 50 to 60% of the Zn were leached from the columns. Large amounts of leached Cd and Zn at the beginning of the experiment are attributed to the dissolution of soluble sulphate minerals present in the original tailings and formed after the first wetting of the columns. The Zn/Cd mass ratios of the tailings and soil, initially 429 and 400, respectively, vary considerably over the course of the experiment. Low values (between 220 and 300) in the early cycles are attributed to preferential weathering of Cd-rich wurtzite [Zn,Fe)S] and sequestration of Zn in preference to Cd in secondary Fe phases forming in the columns. In the middle cycles, dissolution of secondary Fe(OH)3 under low pH (< 3) conditions, and of ferroan (Cd-poor) sphalerite [Zn,Fe)S], releases Zn and raises the Zn/Cd ratio to 550–600 in the tailings-only columns and up to 1500 in the mixed tailings-soil columns. The very high ratios in the latter are also ascribed to the formation of low molecular weight organic ligands that have high affinity for Zn over Cd. In the later column-cycles, Zn/Cd ratios return to near-initial values, due to the weathering of Fe-poor sphalerite and secondary Fe phases, and the declining preference of Zn over Cd in the soil organic acids under the strongly acidic conditions prevailing in the columns. The formation and dissolution of secondary soluble sulphate minerals also play a role in Cd and Zn cycling, especially at the beginning of the experiment

Computer simulation studies on the mechanisms of toxic element incorporation in Jarosite

Jarosites (KFe3(SO4)2(OH)6) are examples of minerals that are highly effective scavengers of toxic elements, and are abundant in acid rock drainage systems, acid sulfate soils, metallurgical wastes and other contaminated sites. The K, Fe and S sites can be filled by toxic elements such as Cd, Cu and Zn, but the mechanisms of this incorporation are not well understood. We have used atomistic simulation methods to model the jarosite (102) surface and its interactions with Cd, Zn and Cu, in order to understand the mechanisms by which these potentially toxic elements are incorporated into the K-jarosite structure. Our results suggest that incorporation via a defect complex consisting of an impurity cation at the K site charge balanced by a K vacancy stabilises the (102) surface in the order Cu>Zn>Cd

Major and trace metal mobility during weathering of mine tailings: implications for floodplain soils

Mine tailings discharged to river systems have the potential to release significant quantities of major and trace metals to waters and soils when weathered. To provide data on the mechanisms and magnitudes of short- and long-term tailings weathering and its influence on floodplain environments, three calendar year-long column leaching experiments that incorporated tailings from Potosí, Bolivia, and soil from affected downstream floodplains, were carried out. These experiments were designed to model 20 cycles of wet and dry season conditions. Two duplicate columns modeled sub-aerial tailings weathering alone, a third modeled the effects of long-term floodplain tailings contamination and a fourth modeled that of a tailings dam spill on a previously contaminated floodplain. As far as was practical local climatic conditions were modeled. Chemical analysis of the leachate and column solids, optical mineralogy, XRD, SEM, EPMA, BCR and water-soluble chemical extractions and speciation modeling were carried out to determine the processes responsible for the leaching of Al, Ca, Cu, K, Na, Mg, Mn, Sn, Sr and Ti. Over the 20 cycles, the pH declined to a floor of ca. 2 in all columns. Calcium, Cu, Mg, Mn and Na showed significant cumulative losses of up to 100%, 60%, 30%, 95% and 40%, respectively, compared to those of Al, K, Sr, Sn and Ti, which were up to 3%, 1.5%, 5%, 1% and 0.05%, respectively. The high losses are attributed to the dissolution of relatively soluble minerals such as biotite, and oxidation of chalcopyrite and Cu-sulfosalts, while low losses are attributed to the presence of sparingly soluble minerals such as svanbergite, cassiterite and rutile. These results strongly suggest that the release of tailings to floodplains should be limited or prohibited, and that all tailings should be removed from floodplains following dam spills

Dissolution of jarosite [KFe3(SO4)2(OH)6] at pH 2 and 8: insights from batch experiments and computational modelling

Jarosite [KFe3(SO4)2(OH)6] is a mineral that is common in acidic, sulphate-rich environments, such as acid sulphate soils derived from pyrite-bearing sediments, weathering zones of sulphide ore deposits and acid mine or acid rock drainage (ARD/AMD) sites. The structure of jarosite is based on linear tetrahedral–octahedral–tetrahedral (T–O–T) sheets, made up from slightly distorted FeO6 octahedra and SO4 tetrahedra. Batch dissolution experiments carried out on synthetic jarosite at pH 2, to mimic environments affected by ARD/AMD, and at pH 8, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)2), suggest first order dissolution kinetics. Both dissolution reactions are incongruent, as revealed by non-ideal dissolution of the parent solids and, in the case of the pH 8 dissolution, because a secondary goethite precipitate forms on the surface of the dissolving jarosite grains. The pH 2 dissolution yields only aqueous K, Fe, and SO4. Aqueous, residual solid, and computational modelling of the jarosite structure and surfaces using the GULP and MARVIN codes, respectively, show for the first time that there is selective dissolution of the A- and T-sites, which contain K and SO4, respectively, relative to Fe, which is located deep within the T–O–T jarosite structure. These results have implications for the chemistry of ARD/AMD waters, and for understanding reaction pathways of ARD/AMD mineral dissolution

Raman and IR spectroscopic studies of alunite-supergroup compounds containing AI, Cr3+, Fe3+ and V3+ at the B site

Raman and IR spectroscopies, X-ray diffraction and chemical analysis have been used to characterize synthetic compounds of the alunite supergroup of formula AB3(SO4)2(OH)6, with A = Na+, K+, H3O+ and B = Al, Cr3+, Fe3+, V3+. For each A-site cation, the proportion of B-site vacancies decreases in the same order as the effective ionic radius of the B-site cations (Al > Cr > Fe > V). Raman and IR spectra of the compounds with B = Al and Fe3+ (alunite- and jarosite-group phases) show very close agreement with previously published spectra. The spectra for Cr- and V-based analogues show similarities to the alunite- and jarosite-group phases, and particularly the latter. The Raman and IR spectra are similar in the high-wavenumber ranges (around 3400–3500 cm–1), but are different in the intermediate and, particularly, the lower ranges, <700 cm–1, probably owing to the different responses of Raman and IR to SO4 and metal–O (M–O) group symmetries. The a parameter correlates well with {nu}1SO4, {nu}2SO4 and {nu}4SO4 Raman and {nu}4SO4 IR wavenumbers. There are differences in the OH-stretching regions, and shifts in the main {nu}SO4 and M–O peaks of the alunite-group phases, and Cr-, Fe- and V-based analogues with substitution of K, Na or H3O at the A site in the Raman and IR spectra. Raman and IR spectroscopies are therefore useful in distinguishing these compounds

Mine tailings dams: Characteristics, failure, environmental impacts, and remediation

On a global scale demand for the products of the extractive industries is ever increasing. Extraction of the targeted resource results in the concurrent production of a significant volume of waste material, including tailings, which are mixtures of crushed rock and processing fluids from mills, washeries or concentrators that remain after the extraction of economic metals, minerals, mineral fuels or coal. The volume of tailings is normally far in excess of the liberated resource, and the tailings often contain potentially hazardous contaminants. A priority for a reasonable and responsible mining organization must be to proactively isolate the tailings so as to forestall them from entering groundwaters, rivers, lakes and the wind. There is ample evidence that, should such tailings enter these environments they may contaminate food chains and drinking water. Furthermore, the tailings undergo physical and chemical change after they have been deposited. The chemical changes are most often a function of exposure to atmospheric oxidation and tends to make previously, perhaps safely held contaminants mobile and available. If the tailings are stored under water, contact with the atmosphere is substantially reduced, thereby forestalling oxygen-mediated chemical change. It is therefore accepted practice for tailings to be stored in isolated impoundments under water and behind dams. However, these dams frequently fail, releasing enormous quantities of tailings into river catchments. These accidents pose a serious threat to animal and human health and are of concern for extractive industries and the wider community. It is therefore of importance to understand the nature of the material held within these dams, what best safety practice is for these structures and, should the worst happen, what adverse effects such accidents might have on the wider environment and how these might be mitigated. This paper reviews these factors, covering the characteristics, types and magnitudes, environmental impacts, and remediation of mine tailings dam failures. © 2014 Elsevier Ltd