Experimental study for enhanced drainage from sand stockpiles

In Bacchus March Boral Quarry the sand is excavated, washed, graded to sizes and then stockpiled on the site to dry. In the quarry plant the whole process of producing a 10m to 12m high sand stockpile takes about 7 days and then another 7 to 8 days before the sand dries and reaches the saleable 5-6% moisture content (volumetric).

The drying period has been the concern of the quarry. A method to enhance the drying process would improve the quarry productivity. The aim of this research is to evaluate the viable options for an enhanced drainage of the sand piles in the quarry. Three methods have been proposed for this research is: i) the use of Surfactants, ii) the use of Drainage System (Spiral Drain) and iii) the use of surfactant and Spiral drain.

Several sample piles of fine and medium sands of 60cm heights were constructed in the RMIT Hydraulics Lab for subsequent testing. Two moisture probes and two tensiometers were placed at two different levels inside the sand pile to monitor the changes of moisture and suction pressure, respectively. A weighing scale was also place at the bottom of the pile setup to monitor the changes of weight due to drainage and evaporation. All these sensors recorded the data at five minutes intervals for several days.

Two surfactants, namely Naphthalene Sulphonate Formaldehyde Condensate (NSFC) and Calcium Lignosulphonate (CL) were used to the sample piles in different doses and the moisture changes were monitored. These two surfactants are in common use in the concrete industry.
In the second method a porous spiral drain was procured to place under the sample sand stockpiles at RMIT Lab. The suction system consisted of a small vacuum pump attached to a small container to separate water from incoming air-water mixture entered into the porous spiral pipe at the base of the sand stockpile.

The surfactants it did not improve the drainage considerably as assumed initially. The saving of time was not significant in the process. The results from medium sand showed that the moisture have reduced to 14% and 13% for CL (525 ml) and NSFC (100 g) application respectively, in 12 days from the initial 32% at the base of the pile. It is to note that the free draining (i.e., without the use of spiral drain and surfactants) process for the same medium sand took 14 days to reduce the moisture to 16% level.
In contrast, the results from the spiral drainage system showed a substantial improvement on the moisture reduction. The drying time has reduced to 3 to 4 days to bring the moisture to 8% level for both fine and medium sands. The spiral drain found to be the optimum methods for drying the sand pile.

A test was also run with the use of spiral drainage and surfactant combined. The result was close to the spiral drainage system. Adding surfactant did not show significant improvement over the suction system.