CASE STUDY

Dewatering of ultra-fine CaCO3 & Na2CO3

ABOUT ZEBRATUBE®

ZebraTube® has its origins in the manufacturing of large dewatering bags for the deepest mines in the world. We have more than two decades of experience in retaining solids and percolating liquids through our woven geotextile bags. 

ZebraTube® products are designed and manufactured in South Africa. Our geotextiles are produced and woven locally by experienced weavers and thereafter converted to dewatering bags. Our complete control over the manufacturing process allows for design flexibility and speed of delivery. We cater for both large and small projects and our ease of deployment means ZebraTube is ideal for emergency dewatering projects.

Each roll of geotextile goes through a comprehensive set of tests at our factory to ensure absolute quality and traceability. Extruded strands are tested before weaving for tensile strength and once woven, each roll of geotextile is tested for tensile strength and permeability.

 

Background

A water treatment company based in Gauteng operates and maintains a 22 MLD water treatment plant, producing potable water which supplies the mining community around the mining complex. Their plant consists of an ion exchange process which requires regeneration of the resin once it is loaded with water hardness causing components. This regeneration is done by making up fresh brine, a combination of chemicals used during this process produces a slurry that needs to be dewatered. The filtrate in this scenario is of high value and the solid material is disposed of.

ZebraTube® was approached by the water treatment company seeking a solution for their slurry dewatering. This was after experiencing several failures with their centrifuge. Although the feed solids concentration was as low as 2 %, no filter aid was used in the process. The feed slurry comprised of mostly sodium carbonate as well as calcium and magnesium carbonate.

To contain and dewater the fine slurry with a density of 1.04 t/m3, ZebraTube® supplied 1-ton woven low flow geotextile bags lined with a non-woven layer on the inside. This combination has resulted in very good filtrate quality, with the total suspended solids in the filtrate measuring values as low as 4 mg/l.

A trial was performed on this bag to evaluate parameters such as the dewatering rate, filtrate quality in terms of turbidity and total suspended solids as well as the moisture content. These were dependent on the slurry specific gravity, the slurry’s particle size distribution and the feed rate, as this influences the quality of the filtrate produced.

 

Figure 1: Feed to dewatering bags
Figure 2: Filtrate from dewatering bags

 

Trial results

The slurry was being fed from a 20 000 Litre reactor into the geotextile bags using a submersible pump as can be seen in Figure 1. Due to the volume of the slurry, four bags were used for containment and dewatering.

The filling of one lined bag only took 4 minutes. At the initial filling stages, the filtrate total suspended solids were quite high as the slurry hadn’t formed a thin layer of filter cake on the surface in the geotextile, which results in some of the slurry passing through the filtration medium. This explains the high dewatering rate that is experienced at the initial filling stages.

Figure 2 shows the clear filtrate collected from the dewatering bags. The change in total suspended solids with time can also be seen in Figure 4. It can be seen that at initial filling, the total suspended solids were a bit high, which is to be expected, however this decreased with time.

At only 10 minutes, during passive drainage, the total suspended solids were already at 4 mg/l.

A comparison of the feed slurry versus the filtrate can be seen in Figure 3.

Two samples were submitted for moisture content analysis. The first sample submitted had a standing period of only 15 hours and it had achieved a moisture content of 60 %. A standing period of 3 days for the sample, achieved 57 % moisture content. Please note that these samples were kept indoors and had not yet been taken out to be dried by full atmospheric conditions. Please see in Table 1 below.

 

Table 1: Moisture content analysis

Sample ID

Standing time

Moisture content analysis %

Sample 1

15 hours

60%

Sample 2

3 days

57%

Figure 3: Feed slurry vs Filtrate
Figure 4: Filtrate total suspended solids
Figure 5: Particle size distribution feed slurry

Figure 5 shows the particle size distribution of the feed slurry. It can be depicted that 90 % of the sample recorded a particle size smaller than 39.9 µm and 10% of the sample recorded a particle size smaller than 5.61 µm.

 

Conclusion

The lined bags provided a consistently clear filtrate quality for the client. In comparison with the centrifuge, they were much more user-friendly and required no specialized knowledge.