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During the mining process a plume of material will be placed released above the bed. In time the particles within the plume will settle. This hindered settling process is related to time. The higher the settling velocity, the faster the particles reach the bed and chances of spreading of the plume leading to environmental issues . The flocculant will influence this hindered settling velocityis smaller. When using flocculant, the settling time and dispersion can be influenced. To understand the influence effect of the flocculant, first the hindered settling of the material without flocculant is tested in 1 m tall settling columns. For each tailings type, two different concentrations are used to resolve the two unknowns; gelling concentration en the maximum settling velocity. These two material properties determine the formation of a bed and settling of an individual particle in still water. The material is placed into the columns (well mixed). Thereafter the hindered settling process is recorded with camera's. From this data set the hindered settling velocity can be determined as shown in the graph below. The higher the concentration, the lower the settling velocity because more particles hinder each other. For concentrations larger than 150 g/l, the settling velocity reaches zero. The particles are close enough to each other to form a (gelled) bed. The maximum fall velocity is determined by the weight of the particle and the surrounding concentration. However it should be taken into account that these are controlled laboratory conditions. In the field the plume be exposed to currents or storm conditions enforcing uplift conditions.
Figure 3: hindered settling regime of the three samples for given concentration.
3) Settling and consolidation columns
To (partly) mimic the conditions at sea, the tailings are placed in 1m high columns. For every sample an expected minimum and maximum concentration is used. The material is mixed and for 3 weeks the experiment was recorded by an automated camera (Figure 4).
From Figure 4: from left to right : 1Set-up consolidation experiments - 2 After 10 minutes of hindered settling phase - 3 After 9 hours: consolidation phase 1
After settling the material will start to consolidate and the strength of the bed will increase. The strength of the bed determines the sensitivity to re-suspension and possible dispersion of the tailings . At three moments in time the strength of the bed is measured by vane tests. A vane is a rotating rod with 4 blades at the end. The soil resistance to rotation is a measure for the strength. Figure 5 shows an illustration of the device and an results of sample 47.
Figure 5: Left: Vane test, Right: Result of measures peak shear strength
The strength of the bed was measured after 20 hrs, 1 week and 3 weeks of settlement. The results are shown in Figure 7. As can be seen, there is an rapid increase in strength in the first week. After this week the rate of increase decreases. This is according to theory. Thereafter there are some samples with a lower strength in week 3 compared to week 1. Probably this is due to artifacts.
