Effect of Physical Weathering on Tailings

Abstract: The mining activity produces minerals to supply to the modern society with commodities. During the ore refining process large amounts of tailings is generated as waste. Tailings are the result of crushing and wet milling, typically in the size range of sand to silt and angular in shape. Tailings are in general deposited in tailings dams for permanent storage. Tailings dams are usually considered as walk-away solutions and needs to be designed and constructed to be safe in a long time perspective. Several incidents around the world has occurred in tailings dams both during construction and operation and after closure of the activities. The consequences of the failures may be fatal to the local society and harm the surrounding environment. Considering the consequences of failures and relatively few studies on tailings properties in a long term perspective there is a need for research. As a consequence of the operation and raising procedures of tailings dams, the conditions in the tailings dams could be considered to be dynamic in a longer perspective. Grain size distribution, formation of layers, pore pressures and stress states are continuously changing during the operation. Tailings may be susceptible to weathering in the deposit environment. The change of these factors needs to be addressed in the design of walk-away solutions. In this work image analysis, oedometer-, triaxial-, direct shear- and attrition tests has been carried out to study the tailings particle influence on fundamental geotechnical properties in a case study. The parameters have been compared to similar-sized natural materials. The effect of loading and physical weathering has been studied and evaluated by image analysis and sieving. The comparative test by natural materials shows that tailings, probably due to the irregular shape, initially arranges in looser interparticle structures. The looser arrangement makes tailings fills more susceptible to settlement compared to natural deposits in the same size range. The two most identified factors affecting the tailings on a particle level was the type of physical weathering and grain size. Static load, shearing and milling decreases the grain size but the effect on the individual particles were different. Large grains tend to maintain the shape or get rounded by physical degradation and finer grains get more angular by milling but rounded by shearing. An attempt has been done to predict the effect on friction angle as a function of angularity, grain size and initial void ratio. The increase in angularity in the model suggests an increase in the friction angle and so the increase of regularity (decrees of elongation relation). However the reduction of size will either reduces or compensates this effect. More studies are needed to verify this.