In Phase 1 the characteristics and behaviour of the tailings is determined. The results of these analysis are the basis of the project to select potential floculants used in Phase 2 and 3.

1) Collation of typical tailing samples for further laboratory analyses

The effect of the floculant is 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. 

Chemical composition

The project area is wide spread, 50 x 30 square kilometers (Figure 1). From this side 50 different samples were available, obtained at different locations within the project zone. This research required a selection of three representative samples of the total batch.  As part of phase 1, the chemical composition was determined based on mineralogical data defined for the coarser grain sizes of each sample (0.8 mm – 80 mm). A Principal Component Analysis (PCA) was done to:

• summarize the chemical composition of the different samples,
• compare the composition of all different size fractions within the samples, and;
• compare the samples to find out the range of composition.

The results showed that:

• the mineralogical composition changes with size fraction, and
• the spatial variation in sediment characteristics, i.e. the mineralogy and granulometry of coarser fractions, is limited. Samples 28 and 32 differ most from the other samples, presumably because of the higher mud content.

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, 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.

Figure 1: Project area Chatham Rise. The sample locations are marked by the red dots.

Particle Size Distribution (PSD)

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.

Table 1: Selected samples with D50, percentage of mud and clay. 

Figure 2: PSD of selected samples

Water content and densities

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.

Table 2: water content, density and sediment concentrations of the selected samples

2) Accurate determination of sediment settling velocities

3) Settling and consolidation columns