With drought in South Australia’s northwest region threatening the viability of a producer’s sand washing operation, a supplier has come to the rescue with a series of advanced water management solutions.
Clinton Sands is a repeat customer for Terex Washing Systems (TWS) Australia. Back in 2017, the company purchased a bespoke Finesmaster Hydro wet classification system for high efficiency separation of fine and coarse sands.
In July 2019, TWS Australia installed a TWS Aquaclear water management system to process the tailings from the Finesmaster Hydro.
The drought conditions experienced on the Yorke Peninsula, South Australia in 2018 brought the issue of water security firmly into focus for the managers of Clinton Sands. Without water, the business of washing sand would not be possible and on a few occasions, they were close to running out completely.
Clinton Sands director Frank Femia wanted the business to become more efficient in its use of water. Effective water management was suddenly a high priority in order to remain operational. Femia contacted James Murphy of TWS Australia late in 2018 to discuss options within the company’s new and expanding range of water management solutions.
TWS launched Aquaclear prior to Hillhead, the UK’s outdoor quarrying plant and equipment expo, in 2017. The various pieces of equipment offered under the Aquaclear banner are designed to remove solids and contaminants from process water or tailings and allow the clarified water to be recycled immediately.
The TWS Aquaclear range includes submersible and vertical shaft pumps, centrifugal pumps, flocculant dosing systems, deep cone thickeners, rake thickeners, filter presses and centrifuges.
Buffer tanks and clarified water storage tanks are also supplied for a closed loop system of water management. Other more specialised equipment such as the patented hydro-flotation system and electro-coagulation system offer advanced technology for the removal of heavy metals and hydrocarbons from process water. This specialist equipment is more frequently used in wash recycling applications.
Maintaining low costs
Clinton Sands’ requirements of a water management system were very clear: produce high density sludge, recycle the maximum amount of water possible, reduce water loss by evaporation and maintain low operational cost.
With this set of guidelines, TWS proposed a system that would tick all the boxes, a solution that offered the highest rates of water recovery on the market and technology and smarts that ensured the operational costs would be much lower than the customers’ expectations.
The entire 290m3 per hour of process water from the TWS Finesmaster Hydro was directed to an in-ground sump tank. An automated submersible pump with low and high level sensors draws the process water out of the sump and discharges it into a central mixing column within a 150,000-litre deep cone thickener silo.
As the slurry passes through a turbulence-inducing static mixing chamber, a chemical flocculant is injected to encourage rapid settlement of the suspended solids. The TWS FDU2200 (FDU – Flocculant Dosing Unit) is an automated control system that continually measures the settlement rate of the solids and live adjusts the flocculant dose accordingly.
If the solids settlement rate is deemed faster than needed, the system will reduce the volume of chemicals being injected per cubic metre of slurry. In contrast, if the settlement rate is deemed not fast enough, the system will automatically increase the volume of chemicals used. The benefit of using these “smarts” is an overall reduction in the use of flocculant and therefore operational cost.
As the treated slurry enters the mixing column of the deep cone thickener, the process of flocculation is already well in progress. The solid particles become atomically attracted to each other and as they bind together, they mimic larger particles, thus sinking to the bottom of the thickener.
Clarified water, which is now free of silts and clay particles, rises up and overflows the weir at the top of the deep cone thickener. Clinton Sands has chosen to store the clarified water in sealed poly storage tanks to prevent further losses by evaporation.
The small diameter of the TWS Aquaclear deep cone thickener has less than half the surface area exposed to evaporation by the sun, compared to an adequately sized rake thickener. Most operators Australia wide will understand this is a very important factor to consider.
The geometric design of the thickener is such that the sludge is encouraged, without any moving parts, to collect at the base. As the thickener narrows to the base of the cone section, the weight of the water above creates a head pressure that encourages density separation and due to a basic principle of fluid dynamics, additional water is forced upwards. The result is a thickened slurry discharging at 45 per cent solids content.
This high density sludge is the thickest consistency that can be achieved without further dewatering. For example, a rake thickener with its moving parts, high power consumption, high rates of wear and large surface area exposed to the sun will only produce a sludge of a maximum 30 to 35 per cent solids.
There are important benefits of having a thickened sludge with higher solids content. First, more water is recovered and recycled back into the system. This was an important requirement of the client.
The next benefit is realised downstream. The thickened sludge would usually be either discharged to evaporation ponds or stored for further dewatering in the likes of a filter press or centrifuge. As the underflow from the Aquaclear deep cone thickener is of higher percentage solids and therefore higher density, its volume in cubic metres per tonne of sand washed will be much lower.
When discharging to evaporation ponds, as is the case at Clinton Sands, the capacity of the ponds is increased when measured in the number of days of sand washing.
Filter press selection
The same principle applies when choosing a suitably sized centrifuge or filter press. The two important criteria for selection are the volume of sludge to be processed and the percentage solids of the sludge.
By having 45 per cent solids upon entering the filter press, we are asking the filter press to remove less water than if we delivered sludge with 30 per cent solids. By reducing the volume of water needed to pass through the filter cloth, the cycle times are reduced dramatically. Reductions in cycle time of more than 20 minutes are not uncommon when converting from a rake thickener to a deep cone thickener.
With these improvements in performance, the selection of a more compact filter press can save the customer quite a lot in capital expense.
The TWS water management system at Clinton Sands was installed in June 2019 and commissioned in July and the benefits were realised immediately.
The closed loop system recovers 95 per cent of the water used in the system and has reduced the footprint of Clinton Sands’ tailings network allowing the old ponds to be serviced into rejuvenated water storage dams.
In Australia, the distributors of TWS products are Terex Jaques (Queensland, New South Wales, ACT, Victoria, South Australia, Tasmania) and OPS Screening & Crushing Equipment (Western Australia, NT).
Source: Terex Washing Systems Australasia/Terex Jaques