The settlement and so the dispersion of the tailings can be influenced by the use of flocculants. It binds the fine sediments resulting in larger particles with an higher fall velocity. In example of flocculant is starch. The type of flocculant and the dosage varies per sediment sample. In Phase 2 the optimum flocculant dosage was determined by conducting
- Zeta potential measurements, for the bounding with flocs
- Settling-jar tests, gives the hindered settling velocity.
Zeta-potential
The zeta potential test reveals the surface charge of the sediment particles. For a zeta potential close to zero flocculation is most efficient, so the optimal flocculant dosage is reached at zero zeta potential, Although this principle applies to fresh water conditions at low sediment concentrations and low shear rates, the results from the zeta potential measurements can be scaled to sea water and higher sediment concentrations.
Settling-jar tests
To confirm the optimum flocculant dosage, the settling-jar tests with sea water and a high sediment concentration is also done for a small variety of flocculant dosages (Figure 1). This revealed an optimum flocculant dosage of 0.8 mg of flocculant per gram of sediment. The hindered settling velocity in absence of flocculant is approximately 0.7 mm/s, depending on the actual sediment concentration. This increases to 1.7 mm/s, i.e. a factor 2.5, again depending on the sediment concentration. This result is valid for a high turbulent regime applied to the sediment-water-flocculant mixture, before settling, equivalent to the turbulence in the operational circumstances (discharge pipe). The turbulence has a large impact on the efficiency of the flocculant, as the hindered settling velocity increases even further to 6.5 mm/s when a low turbulence regime preceeds the settling phase.
Figure 1: Set-up Jar test
Figure 2: Effect of different concentrations of flocculant on the settling velocity.
