Carletonville-based ZebraTube was created in 2016 by its two pre-existing sister companies, Spilo and StitchWise, which have both been active in the local design, manufacture and use of geotextile bags for over 20 years.
“Spilo is our textile manufacturer, while StitchWise is our bag manufacturer and an established specialist in the manufacture, supply and deployment of backfill bags for the mining industry. Like our ZebraTube dewatering bags, backfill bags also use woven geotextile fabrics made in Paarl by Spilo,” says Spagnuolo.
“While there are other geotextiles on the market, what we have is total control and flexibility of the end product, starting right at the beginning with the extrusion of polymer fibres. Polypropylene, additives and other chemicals needed for the polymer extrusion process are sourced from our local petrochemical industry. We extrude polypropylene fibres from this process and weave them into geotextiles with different strengths, capacities, permeability and dewatering characteristics. The geotextiles are then sent up to us in Carletonville in rolls, where they are stitched into bags,” she explains.
“This is our strength. We have control of the whole manufacturing process and each stage goes through an extensive range of tests to determine suitability for purpose: we test the tensile strength of strands from every batch of an extrusion before it is woven. We then retest every roll of woven fabric for strength and permeability after weaving.
“If we need a UV-rated bag for a sun-exposed application, for example, we can include an additive during the extrusion process that will impart the required UV performance to the geotextile fabric we use for the finished bag,” Spagnuolo explains. “We have the flexibility to develop customised textiles to cater for very specific applications and that is carried over to the weave of the fabric and to the final design of the bag,” she adds.
“We at ZebraTube deal with dewatering applications, specifying and producing dewatering tubes for slurry and sludge removal applications and sediment separation projects,” she continues, while pointing to the black and white signature weave of the ZebraTube geotextile, and a lighter, finer blue weave fabric. “The black and white weave is a stronger, low-flow geotextile with lower permeability and higher mass per m2, while the lighter weave is recommended for higher flow applications,” she says.
While the finished bags can be used in a number of applications, their principal use is for the dredging of ponds at minerals processing or wastewater treatment plants. “Wastewater treatment involves removing the sludge from catchment ponds at some stage once the ponds reach capacity. To re-establish the capacity of the ponds, they routinely need to be cleared of accumulated settled solids,” says Spagnuolo.
“A dredging contractor will come in to dredge the dam and remove slurry, but the slurry generally needs to be transported away to a safe disposal site. First the slurry needs to be dewatered, though, to minimise water loss and to make transportation easier and more efficient – you need to dry it so that you can move it,” she tells MechChem Africa.
Spagnuolo says that, while dewatering using a bag is a relatively simple process, there is an active side to it. The slurry is pumped into the bag via a hose fed into a 110 mm bag inlet and secured. As the bag begins to fill, the slurry in contact with the surface fabric dewaters quickly through the gaps in the weave. A thin ‘caked’ layer of relatively dry solids forms on the fabric surface, closing these gaps and creating a very efficient filter for the fines.
“Initially, some the fines pass though, but once the caking effect kicks in, solids retention and dewatering efficiency improves. We can retain particulate down to 10 µm because of this filter caking effect,” she assures.
While dredging a large pond, pumping into ZebraTube bags may need to continue for several weeks. “Once each bag is filled, the top surface will sometimes be flattened or evened out, which pushes more water through and creates a little more space for additional slurry. Then, when full capacity has been reached, the hose is removed and the bag is simply left to passively drain, which can also take several weeks, depending on how dry the solids needs to be,” Spagnuolo says.
Given sufficient space, some operators simply leave the bag in place or, if removal is required, the bag is broken open and the dry solids loaded into trucks for safe disposal elsewhere.
How much can each bag contain? “We work on a 9.0 m or 10 m tube circumference and we make tubes of up to 60 m long which in terms of volume can theoretically contain about 370 m3 of material,” she responds, adding that, due to the displacement effect, the water storage capacity of the pond increases by a similar amount.
Multiple bags are often used, however, and bags are also stackable to save space.
With respect to implementation, Spagnuolo says some civil work is required. “Typically, a slab or liner needs to put down along with some form of sump and channelling to allow the recovered water to flow back into the process. But this is a quick process, easy to deploy and very effective.
“We have recently done some work at a colliery to recover coal fines, for example. This is usually done using expensive capital equipment such as belt filters and filter presses, but if operating under capital constraints, the use of our ZebraTube geobags is a very cost effective interim alternative.
“This is a new miner that intends to install a belt filter when capital is made available. The use of our bags, however, is a very cost effective solution that enables the fines to be recovered while complying with environmental norms. It is also an ideal solution for emergencies, when installed dewatering equipment breaks down or when flooding temporarily increases dewatering demand,” she says.
Spagnuolo emphasises that ZebraTube is not simply a supplier of geobags. The company has a testing laboratory in Carletonville that is used for tests such as moisture and particles size analysis so that it can offer the best dewatering solutions possible for each specific dewatering application.
“We also have test units that we can take to site to do hanging bag trials with a clients product. These units are on skids and allow us to determine the drainage rate required, the filtrate quality, the moisture content of dewatered solids and other factors so as to determine which of our geotextiles is most suitable and which bag sizes will best suit the site and the application.
“We also have a pressurised bag unit, which mimics a big bag under pumping pressure. We call this the pressurised pillow test, and it uses a 10×1.0 m bag to establish the appropriate pumping rate for the application,” she adds.
“We believe our solution is ideal for junior and small scale miners and for cash strapped municipalities looking to improve their wastewater treatment efficiency. It is an easy to implement low cost dewatering alternative that enables operators to raise the efficiency of their water use while fully complying with environmental containment requirements,” Spagnuolo concludes.