Reactive Tailings Research Articles

The effects of hardpan layers on the water chemistry from the leaching of pyrrhotite-rich tailings material

Column leaching experiments were used to determine the effects of an iron-rich hardpan layer, on the rate of tailings oxidation and the composition of leachate waters, from the Renison Bell tailings dams in western Tasmania, Australia. One-meter-long PVC columns, filled with tailings, cover material (Cassiterite Flotation Tailings) and hardpan samples from the tailings dams, were leached over a period of 14 weeks. Under dry cover conditions, when hardpan was present, the solute loads peaked at 21–49 days (Fe at 2,294 ppm and SO42− at 4,700 ppm), and stabilised at much lower concentrations after 9 weeks. In contrast, the solute loads steadily increased over time in the column where hardpan was absent (SO42− from 1,800 to 3,100 ppm, and Fe from 407 to 1,692 ppm). Under saturated cover conditions, the solute concentrations in the leachate also increased with time (SO42− from 1,900 to 17,000 ppm, and Fe from 480 to 8,500 ppm). The presence of a hardpan layer between the reactive tailings and cover material has been found to improve leachate water chemistry and lessen the rate of sulphide oxidation.

Neutralisation of acid mine drainage with alkaline industrial residues: laboratory investigation using batch-leaching tests

This study constitutes a first stage of the elaboration of a concept for setting up capillary barriers composed of reactive tailings and alkaline industrial wastes. The aim of this barrier is (1) short-term mitigation by neutralising acid produced by Fe sulfides and (2) long-term mitigation by stabilisation of secondary products. The alkaline industrial wastes, cement kiln dust (CKD) and red mud bauxite (RMB), are used to produce neutral conditions. A series of time-controlled static leaching experiments were performed. Samples of reactive tailings were prepared containing 0, 2, 5, and 10% alkaline material. The water:solid ratios were also varied (3:1, 6:1, and 20:1) in order to characterise the geochemical balance between liquid and solid phases. The data are used to discern whether mineral solubility controls exist for particular ions. The pH results show that the use of 5% CKD, 10% CKD and 10% (CKD+RMB) allows neutral pH conditions to be maintained in the reactive layer. At these percentages, the concentrations of Al, Fe, Cu, Zn and SO 4 in solution are significantly reduced compared with those obtained with the reactive tailings only. Aluminum concentrations are principally controlled by secondary phases like boehmite and gibbsite; Fe, by goethite and ferrihydrite; Cu and Zn, by hydroxides. Calcium and SO 4 concentrations are controlled by precipitation of gypsum. The concentrations of Na and K are found not to be controlled by mineral solubility.

Water Cover Over Mine Tailings and Sludge: Field Studies of Water Quality and Resuspension

Flooding is considered one of the best available technologies for long term storage of acid generating mine waste when suitable site-specific conditions exist. There is, however, a concern that oxidation may still occur. In cases where lime neutralization sludge and reactive sulfide tailings are co-disposed in the tailings pond, wind-induced waves could resuspend the waste and negatively impact the quality of the water cover. Studies were undertaken at the Noranda Inc. Heath Steele Lower Cell tailings impoundment, located in northeastern New Brunswick, Canada, 50 km northwest of city of Miramichi. The stored material in the cell consisted of unoxidized tailings with small amounts of sludge. The 90 ha impoundment acted as a polishing pond, prior to the discharge of final effluent. The pond was keptalkaline (pH of 8.5-10.5) in order to meet regulated discharge limits. On some windy days when the Lower Cell experienced turbulent water conditions, the final effluent exceeded the suspended solids water quality standard of 25 mg/L. The dry mass of suspended sediment measured in 1999 ranged from 1.5 to 434 mg with relatively more material (> 100 mg) being suspended under shallow water cover (= 1 m). Both x-ray diffraction and scanning electron microscopy analyses indicated that the suspended material was mostly lime neutralization sludge and other material composed primarily of calcite and brucite and coatings of aluminum, iron, zinc and manganese hydroxides. Solubility considerations of the carbonate system confirmed that the water cover was supersaturated with respect to calcite. The results suggest that sludge and tailings re-suspension and precipitation of solid phases in the water cover likely combined to produce the observed, occasionally high total suspended solids concentration.

Diffusion of oxygen through a pulp and paper residue barrier: Discussion

The authors have presented an interesting paper on the use of deinking residues to create an oxygen barrier. The paper includes some results from laboratory tests, which can be difficult to perform, and an interpretation based on Fick’s laws solutions. Although the writers, like other researchers active in this field, have expressed some reservations regarding the placement of organic cover materials on reactive (acid producing) tailings (as opposed to inorganic materials such as soils and non-acid generating tailings), it is nevertheless recognized that the proposed approach for controlling the production of acid mine drainage (AMD) is certainly worth investigating. Covers made with deinking residues may eventually be added to the few control methods available for mine operators faced with the prospect of dealing with sulfidic mine and milling wastes. In the following paragraphs, the writers would like to discuss some issues relevant to the problem at hand, including some comments and questions for the authors and interested readers. Air versus equivalent porosity

High Altitude Exposure Testing of Geotextiles in the Peruvian Andes

The use of geotextiles in the Andean mining industry provides some unique challenges due to climate, altitude, latitude, and the pervasiveness of acid mine drainage (AMD) conditions. Two nonwoven needle-punched polyester geotextiles (420 and 550 g/m2) were deployed at an elevation of 3,675 m in Northern Peru for ultraviolet (UV) deterioration and chemical resistance testing over a 12-month period. The retained grab strength, trapezoidal tear, and index puncture resistance of the UV-exposed specimens ranged from approximately 68 to 124% and 90 to 101% for the 420 and 550 g/m2 specimens, respectively. The coefficient of variation on the measured strengths for the 420 g/m2 specimens showed an increasing trend after 30 days. Immersion in flowing AMD (average pH = 1.82; 5,600 mg/L total metals) did not reduce the retained strengths of the geotextiles below 70% after 180 days. Design challenges associated with reactive tailings particles, and drainage and filter criteria are briefly discussed.

Numerical modelling of the effectiveness of sealants in retarding acid mine drainage from mine waste rock

An extensive study was carried out to determine the effectiveness of various sealants in retarding or preventing the production of acid mine drainage (AMD) from mine waste rock and reactive tailings. In addition to cementitious compounds, the sealants included polymer and organic based solutions. Coated and uncoated (control) samples of mine waste rock and tailings were placed in large leaching columns and the leachate solutions were regularly analyzed for parameters such as pH level, redox potential (Eh), conductivity, sulphate and the dissolved heavy metals, Fe, Ni, and Cu. Concurrent with the laboratory tests, samples were exposed to weathering effects for seven months, which included one complete winter season. Mathematical models were fitted to moving averages of the measured data to establish the relationships between pH and Eh, and conductivity and sulphate concentrations which are necessary for indicating AMD production or lack thereof. These models also enabled the estimation of the terminal pH and Eh values. The results indicate that uncoated samples produce significant AMD and that the polymer and organic based solutions were more effective in retarding AMD than the cementitious coatings.

Application of fly ash in the agglomeration of reactive mine tailings

Agglomeration is one of the most effective methods of dealing with reactive mine tailings in the prevention of acid mine drainage. The agglomeration process, accomplished with the use of a conventional binder such as Portland cement, immobilizes the reactive components of the mine tailings and results in the formation of strong pellets which remain impervious to weathering and leaching. The objective of this study was to evaluate the fly ash as a binder, either by itself or in combination with cement, in the mitigation of acid mine drainage caused by the reactive tailings. Two different acid mine tailings samples differing in their reactivities were chosen. These tailings were obtained from Noranda, Quebec, Canada and Cyprus Thompson Creek, Clayton, Idaho. The fly ash was obtained from Mineral Specialities Inc., Billings, Montana. Agglomeration tests were performed with fly ash and Portland cement. The crushing strength and the leachability of the cured agglomerates were evaluated. Preliminary results indicate that the use of fly ash significantly reduces the requirement of cement without any decrease in strength and immobilization effect. In fact, it was shown that the fly ash improved crushing strength of the agglomerates. This increase was substantial in the case of reactive tailings (Noranda). The leaching (simulated TCLP) tests performed on the Noranda Tailings agglomerates prepared using fly ash/cement resulted in the much lower level of extractable ions concentration and well below the regulatory level than from the agglomerates prepared using either fly ash or cement alone.

Numerical simulations to investigate moisture-retention characteristics in the design of oxygen-limiting covers for reactive mine tailings. Note

Numerical simulations to investigate moisture-retention characteristics in the design of oxygen-limiting covers for reactive mine tailings. Note

Numerical simulations to investigate moisture-retention characteristics in the design of oxygen-limiting covers for reactive mine tailings

Acid generation in reactive mine tailings is an oxidation process that is dependent on availability of molecular oxygen. As a consequence of the diffusion coefficient of oxygen being several orders of magnitude higher in air than in water, influx of atmospheric oxygen into a material at depth can theoretically be minimized by maintaining a protective cover layer at high moisture content. Such oxygen-limiting covers are generally of finer texture than the material being protected. A numerical model was used to investigate the importance of moisture-retention characteristics in the transient drainage of such two-layer systems. The results show that the effectiveness of a material as a moisture-retaining cover is dependent on the magnitude of its air-entry value. The thickness of the cover maintained at full saturation after prolonged drainage also depends on the pressure head at which the underlying material approaches residual saturation. Key words: geologic covers, tailings, numerical simulations, air-entry value, residual saturation, textural layering.

Laboratory study of submerged metal-mine tailings 1: Effect of solid-liquid contact time and aeration on contaminant concentrations

The dissolution of metals from various metal-mine tailings has been studied through a series of submerged tailings batch tests at U.S. Bureau of Mines laboratories. Tailings selected for this study included both reactive (as determined by column leaching studies) and benign tailings. Metal mobilization in aerated (97 pct saturated with oxygen) samples appeared to be consistently greater than metal dissolution from stagnant (79 pct or 70 pct saturated with oxygen) waters. In the early weeks of the study, sulfate concentrations increased with solid-liquid contact time, while calcium dissolution remained nearly constant. Metal concentrations varied in relation to solid-liquid contact time and aeration. Leachate pH appeared to be nearly independent of solid-liquid contact time for basic tailings. Reactive tailings having both a submerged and a surface component demonstrated increased metal mobility.