To understand the plume Plume dispersion and the continuous process of settling, deposition and bed formation of mine tailings in this project, first the tailings are characterized. The analyses results enable narrowing down of uncertainties in the critical sediment parameters in the tailings depend on the tailings characteristics. Phase 1 activities were done to understand the sediment characteristics, to narrow down uncertainties in the predicted behaviour of the mine tailings and are the basis of the project to select potential flocculants to be used in Phase 2 and 3. Phase 1 consists of three main activities:
- Collation of typical tailings
- Accurate determination of settling velocities
- Settling and consolidation of the tailings
1) Collation of typical tailing samples for further laboratory analyses
The effect of the floculant is settlement of the tailings will be tested in settling column experiments. The design of the settling column experiments requires information about the water content, particle size distribution and density is needed. The selection of the floculant requires an understanding of the material properties; chemical properties and settling behaviour.
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Based on these conclusions and on the practical limitations to retrieve the sediment from the stock, samples of three locations were used for further analyses: 28, named 28, 46, and 47.After a second round of mineralogical analyses on the fine fraction (< 0.8 mm, see Table 3.1), the conclusions above were confirmed.
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Particle Size Distribution (PSD)
The particle size determines the fall velocity of the particles. In a sample there are multiple grains with different sizes. The particle size distribution (PSD) has been determined with the Malvern device, which is an optical instrument based on the principle of laser diffraction. The pitfall of this optical measuring technique is that the fraction of very fine particles (<10 μm) may be underestimated as they may be overshadowed by larger particles. Therefore, first the sample is diluted in water and measured, then the sample is left to let the larger particles settle for 1 hour. As a second step, the supernatant water with sediment is measured with the Malvern device. The PSD is then computed based on those two measurements. Figure 2 shows the PSD of the 3 selected samples. The variation between the samples is not big, and the median grain size D50 ranges between 4-7 μm (Table 1). The settling velocity of the samples may however still deviated from each other to a larger extent, as the settling velocity is not only determined by the grain size, but also by the mineralogy, influencing floc formation and the amount of water encapsulated in those flocs. However, Table 3.1 shows that also those differences are relatively small.
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Water content and densities
Tailings contain water and sediment particles. The ratio is necessary to design the settling column experiments and has influence on the type and amount of flocculent. The specific water content, dry mass, bulk density and concentration of clay in the samples of the three locations were determined. Of each location subsamples were weighted, dried in an oven at 120 °C (24 hrs) and weighted again to obtain the wet and dry weight. From the water content and bulk density, the clay content can be estimated. The results are presented in Table 3.3. Sample 47 is less consolidated than the other 2 samples, as can be concluded from the bulk density and the high water content. To obtain a certain sediment mass, more tailings of sample 47 are required. Thereafter the compaction of sample 47 will be lower compared to the other two.
Table 2: water content, density and sediment concentrations of the selected samples
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3) Settling and consolidation columns
From left to right: 1SetSet-up consolidation experiments ; After - 2 After 10 minutes of hindered settling phase ; After - 3 After 9 hours: consolidation phase 1
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