Paul H. Simms

Soil Covers for Controlling Acid Generation in Mine Tailings: a Laboratory Evaluation of the Physics and Geochemistry

To evaluate the effectiveness of soil covers, column experiments were conducted on tailings protected by a three-layer soil cover and tailings directly exposed in the open laboratory for a period of 760 days. Periodic rain application was performed to simulate field conditions, and at four times during the experiments the pore water was completely flushed out of each column for analysis. Profiles of oxygen, temperature, and volumetric water content were measured throughout the experiment, and the post-testing pore water quality was also characterized. A one-dimensional semi-analytic diffusion model was used to simulate oxygen profiles in the uncovered tailings. Modelling performed using the geochemical code MINTEQ showed that in the laboratory, aluminium concentrations in the tailings pore water were controlled by Al(OH)SO4, sulphate by gypsum and Al(OH)SO4and iron by lepidocrocite in the upper half and by ferrihydrite in the lower half. In the field, however, the iron oxyhydroxide minerals formed in the oxidized zone appear to be dissolving. It was found that the cover was effective in preventing significant desaturation of the clay, even over a 150-day drying period. The covered tailings did not oxidize much during the experiments. In the uncovered tailings, oxygen modelling and examination of the geochemistry show that the rate of gross oxidation and the advancement of the oxidation front decreases with time. However, pore water quality is controlled by geochemical processes other than oxidation, as reported in the field.

Effectiveness of Protective Layers in Reducing Metal Fluxes from Flooded Pre-Oxidized Mine Tailings

A problem in implementing water covers over existing tailingimpoundments is the dissolution of minerals produced throughoxidation and the subsequent flux of metals into the water cover.One possible solution is to place a protective layer of non-reactive soil at the tailings/water interface to inhibit metaltransport. A laboratory evaluation of different water coversystems was performed employing columns packed with tailingssubmerged beneath 1 m of water. A ten-centimeter layer of sand orpeat was placed at the tailings/water interface. The experimentswere kept stagnant for 183 days, and then flushed with water at asteady rate for 468 days. Both protective covers prevented degradation of water cover qualityfor the duration of the experiments, as pH exceeded 5.5; however,the quality of the tailings pore water remained poor and evendeclined slightly from pH > 4 to pH < 3. Leaching of iron andsulphate from the tailings with a sand protective layer ceasedduring the experiments. Conversely, in the columns with a peatlayer, substantial leaching of metals and sulphate from thetailings continued to the end of the experiments. It ispostulated that the peat is a source of chelating agents, such asorganic acids, which are known to accelerate the dissolution ofcertain minerals formed through weathering. The sensitivity ofmetal transfer rates to the thickness and type of protectivecover above the tailings were modelled. A 10-cm peat layer waspredicted to prevent substantial degradation of the water coverfor at least ten years.

A Comparison of Different Laboratory Techniques to Simulate Stress and Moisture History of Hard Rock Mine Tailings

In thickened tailings technology, tailings are depo sited at solids concentrations sufficient to preven t segregation and allow for formation of gently sloped stacks. Po st-deposition, thickened tailings are known to gain strength through a combination of hindered settling, desicca tion, and consolidation. Recently, it is understood that the shear strength and geotechnical stability of the st ack is dependent on the degree of desiccation or dr ying time for a given layer, as well as consolidation history dur ing subsequent deposition. This paper presents some preliminary investigations into how best to reprodu ce this stress history for element testing. Three l aboratory methods for simulating stress history of thickened tailings layers are introduced for preparing sample s for testing in an NGI type simple shear apparatus. In the first method, tailings are reconstituted in the simple s hear mold, desiccated to different degrees, and re-wetted befo re shearing. The second method consists of simulati ng the thickened tailings deposition in a column, followed by sample extraction using a shearing thin-wall sa mpler. In the third method, a flume is employed to simulate t he movement of thickened tailings layers from the d eposition point down the beach. The third method is intended to assess whether the movement of tailings down the beach constitutes an important part of stress history, an d influences the evolution of the fabric. This pape r recommends the most appropriate method for preparing samples t o assess the geotechnical behaviour of thickened ta ilings. It is concluded that desiccation to the shrinkage limi t could significantly increase the monotonic shear strength of hard rock mine tailings; however, with continuing d esiccation beyond the shrinkage limit, additional monotonic strength gain is minimal.