Case Study
Cleanout of Coal Mine Settling Ponds
A large Indonesian mining corporation operating an-open-pit coal mine in a remote part of South Kalimantan, Indonesia, faced a problem on how to dewater silt sediments from settling ponds that had reached their capacity.
As part of the settling ponds cleanout efforts, a TenCate Geotube® dewatering facility was constructed to contain and dewater sediments extracted from the ponds. In total, more than 300 Geotube® dewatering tubes have been used to dewater more than 1,000,000 m3 of slurry containing 170,000 m³ of dry solids annually.
To optimize land space the Geotube® dewatering tubes are stacked up to seven layers high based on standardized units with a circumference of 36.6m and lengths between 17.4m to 61.4m to create large solid landfill structures
THE CHALLENGE
The problem facing the mine was how to deal with high volumes of silt in evaporation ponds. The silt, originating from a mix of wastewater from the mines open pits as well as eroded silty rainwater runoff from within the mine catchment area, discharged into the ponds rapidly filling the header ponds before overflowing and clogging adjacent ponds.
Initially, the problem was solved by constructing new ponds until there was a shortage of free land space. Cleaning the ponds required a constant combination of silt dredging and truck relocation in a constant cycle to prevent runoff with high levels of suspended solids (TSS) discharging into and polluting adjacent river channels.
As silt levels in the ponds increased, the cost of dewatering ponds became prohibitive, requiring a more proactive and cost effective permanent Geotube® dewatering solution.
THE DESIGN
To prove the effectiveness and viability of TenCate Geotube® dewatering tubes, a series of on-site evaluation tests was undertaken. Firstly, a series of Rapid Dewatering Tests (RDT) was conducted to determine the most suitable polymer, the optimum polymer dosage and estimate the final end-of-dewatering solids concentration. Figure 1 and 2 shows the RDT test being undertaken and the effluent and dewatered solids captured.
Secondly, a pilot tube trial was conducted using a few scaled-down Geotube® units that would simulate actual process conditions. The pilot trial validated the results of the RDT evaluation and was used to quantify information and operational implications that was then used in the final project design. Figure 3 shows the pilot trial set-up which utilized a Geotube® GT 500D fabric with a circumference of 13.7m and length of 30.6m.
The pilot trial Geotube® units was laid out over a membrane lined platform to reduce ground infiltration of the effluent. The slurry pumped into the Geotube® units was dosed with polymer at the rate determined earlier by the RDT test. Figure 4 shows the final dewatered cake achieved in the pilot trial Geotube® units.
THE CONSTRUCTION
A Geotube® dewatering facility was constructed to contain and dewater slurry extracted from the ponds. Construction of the dewatering pad started in August 2015.
Prior to placement of the Geotube® dewatering tubes, the surface area of the land was levelled and cleared of sharp objects such as rocks, roots etc as shown in Figure 5a. The pad preparation included the placement of a LDPE liner, drainage trenches, HDPE piping, manifolds and flexible hoses. A working space of about 5m was also allocated between the edge of the tubes and drainage for the operators as shown in Figure 5b.
The first layer of Geotube® GT500D with a circumference of 36.6m and length of 61.4m was installed. Slurry was then pumped into the Geotube® dewatering tubes from a floating dredger in the ponds at a flow rate of 350-500 m³/hour in a 10-hour operation daily. The polymer used was a cationic flocculant injected at a dosage of 1.5 to 3.0 kg/MTds that was into the slurry prior to delivery into the Geotube® dewatering tubes.
The maximum filled height was 2.65m and the final height after consolidation was 1.8m. After the initial Geotube® dewatering tubes have been consolidated, the following layer was then installed (Figure 5c). This process continued until seven layers of Geotube® dewatering tubes was installed. Effluent discharged from the Geotube® dewatering tubes was pumped back into the settling ponds.
THE PERFORMANCE
The Geotube® dewatering technology installed in this largescaled project has proven to be a highly effective and efficient solution with significant benefits in cost savings and environmental sustainability. The design of stacking seven layers of Geotube® dewatering tubes enabled land space to be well optimized while harnessing the performance of Geotube® technology.
Geotube® dewatering technology improved the water discharge quality by substantially reducing the total suspended solids (TSS) being discharged back into the ponds with its ability of achieving high rates of solids retention and effluent discharge as compared to the previous dewatering methods used.
The solids contained in the Geotube® dewatering tubes also eliminated the risk of rainfall erosion and silt runoff. Presently, the solids in the Geotube® dewatering tubes are being reclaimed as infrastructure earthworks fill, reducing carbon footprint while returning a revenue contribution to the mine operations.
This project has the distinction of being one of the biggest Geotube® dewatering projects ever undertaken in the world providing a wealth of knowledge on the economics of extreme large scale Geotube® dewatering
