Sustainability is today?s buzz word in design, development, manufacturing and processing. In other words, it is imperative to control the environmental impact of any project or initiative. In aggregate washing, that means being water wise. Obviously, sound water efficiency practices protect surface and ground water resources – and ultimately keep producers far more profitable.
Water shortage in quarries is common, especially in Australia, where rainfall is the second lowest of all continents. Producers may wish to gain the benefits of washing, but have very little water on-site, or they may not have the appropriate footprint to properly maintain a settling pond. Or, if the operation is portable, it will face a variety of water use challenges from site to site.
In any event, the washing of sand and aggregate results in the discharge of dirty water from wet screening decks, sand screws, or sand classifiers. The waste water typically carries fines out to a series of settling ponds. Although the common method of treating wash water fines, it is not the most sustainable. As such, water wise washing operations are employing hydrocyclones, water clarifiers, flocculant systems and belt presses to efficiently reclaim and reuse wash water – in some cases, even eliminating the need for a settling pond.
TRADITIONAL SETTLING PONDS
As they require no additional capital equipment, settling ponds may seem to be the most cost-efficient choice. However, the method can be problematic, said Bob Gralton, president of Wisconsin-based Clearwater Industries, which provides custom-designed water treatment equipment for the aggregate industry. Mr Gralton cited a number of settling pond disadvantages:
? Water recovered for washing may be lost to evaporation or percolation in the pond, which requires the operator to provide ?make-up water? to the plant.
? The real estate for large enough settling ponds may not exist at the plant site.
? The cost and time of cleaning the ponds with a dragline or excavator may be excessive and inefficient – especially the fine material that flows downstream and settles very slowly.
? Dirty water may get back to the plant, limiting production, producing washed material out of spec, or even shutting down production.
? Ponds may present hazards to nearby equipment and to workers.
However, settling ponds suit some sites, said Mr Gralton, particularly for ?the producer who has a large area where the dirty water can be deposited and the fines need never be recovered?. He added that the perfect example is the exhausted portion of a large quarry which acts as a settling pond. ?Clean water is recovered from the opposite end of the pond and the fines settle down in the deep quarry bottom never to be dealt with again,? he said.
WATER CLARIFIERS
Water clarifiers are valuable to the producer keen to eliminate settling ponds altogether, and reclaim 90 per cent of the water immediately. ?For producers who have an area that has already been mined and will be reclaimed, a good clarifier can pump the solids to that area, and it never needs to be conveyed, loaded or hauled again,? said Mr Gralton. He stressed that while clarifiers require a significant initial capital outlay, they have minimal maintenance and operating costs.
Portable clarifiers are an addition to the market. Clearwater Industries recently introduced its new Model 2000 portable water clarifier for use in mining and sand and gravel applications. It allows operators to take a dirty water stream, and produce clean water immediately, concentrating the fines or solids to a thick state. Via this streamlined ability to recycle and reuse valuable water resources, users can meet water conservation targets, ensure an uninterrupted quality water supply, avoid the risks of plant shutdowns due to water shortage, minimise the costs of water consumption and waste water management, and reduce environmental impact.
The benefits of a portable, closed-circuit clarifier are well illustrated by examining its use in a rural-based sand and gravel processing application. An Idaho-based sand and gravel operation constantly works seven local sites, and completes custom crushing and washing projects for regional customers. The operation is as efficient as its water resource management – and when water is tight – production may come to a halt. In past years, its portable washing projects had been limited by water availability at certain sites. Most jobs had to be scheduled in the northern spring as the highest water table was available then. Ultimately, the operation would always run out of water and by June it would have to stop as there wasn?t enough recharge water to fill a well back up.
Consequently, the operation upgraded its processes with the purchase of the Model 2000 portable, closed-circuit water clarifier. As its name indicates, the unit delivers up to 7.5 kilolitres per minute, at 20 per cent solids by volume. The system is complete with an automated flocculant system, a 30kW drive, and a hydraulically-driven mud pump that will move mud up to 243 metres. The Model 2000 typically requires three hours for setup with features such as fold-up catwalk handrails.
The new clarifier allows the portable operation to recycle and reuse water, while conserving its makeup water. As a result, this opens up new opportunities for them – sites with little or even no water, or no place for a settling pond.
?Traditional closed-loop systems typically require ponds for water discharge and for feed water to the plant,? explained Mr Gralton. ?Alternatively, the unit used by this portable operation has a clean water tank placed right on the clarifier. From this tank, the water is pumped directly to the wash plant. The totally self-contained unit includes a dry polymer feed system, hydraulic package, control panels, a mud discharge pump, and a clean water return pump to supply the wash plant. With this closed-circuit operation, at least 90 per cent of the water is reclaimed, and the need for a pond is eliminated,? he said.
Fresh water is pumped to the wash plant from a 56 kilolitre supply tank parked next to the clarifier. After the material is processed, the dirty water is discharged into a sump that sits remotely from the clarifier. From there, the discharge water is pumped into a flocculant tank atop the clarifier where the polymers are added. Next, the flow separates into twin clarifying tanks where the suspended solids settle out of the waste water. Then the clear water overflows back into a clean water tank, which is mounted on the clarifier. This tank has a ?keep-full? system on it. If its float drops, water is added from the makeup tank to keep the system in balance. The thick mud is pumped from the bottom of the clarifiers, usually into an excavated area for drying. ?The big thing is that the unit produces a drier mud and that saves big on water,? said Mr Gralton.
Doing portable washing the old way has always involved trucking in large volumes of water. Ponds are filled and then they evaporate or leak into the ground – even in instances where ponds are shotcreted or paved. Portable clarifiers allow the operation to bypass the clean water pond altogether.
HYDROCYCLONES
Hydrocyclones are tapered cones that receive the dirty wash water at a high velocity. The water travels in a tight circle within the cone and centrifugal force throws the largest particles to the outside of the cone where they slide down the cone and out the bottom. Rather than discarding all the particles into a settling pond, the cones can recover the #200-mesh and larger material, which can be sold as aglime, mineral filler, lining for utility trenches, mortar, grout additives and more. This means that the hydrocyclone can deliver a welcome return on investment.
Mr Gralton explained that while the hydrocyclone will effectively remove coarse material from the dirty water, it will pass the midsize and very fine particles out to the ponds. The resulting fine material in the pond is extremely ?slurpy,? which hampers any periodic cleaning of the pond, as the material must be dipped from the pond, cast to the side, and allowed to dry. This dried material is hard to handle, and equally hard to load into trucks. For this reason, Mr Gralton suggested that producers send the very fine overflow from the hydrocyclone to a clarifier, or to a pond that is not intended for cleaning.
FLOCCULANTS
Flocculants rapidly settle out virtually all of the suspended solids in a dirty water stream. They are required when operating a water clarifier or belt press, but are optional when using a settling pond. They cannot be used with hydrocyclones. Liquid flocculants require very little hardware and can be introduced via a small chemical metering pump with some dilution water added. However, liquid flocculants are not environmentally friendly and often separate or stratify in the container before use. Dry flocculants are more environmentally friendly but require more sophisticated equipment to get them into the solution properly. ?It is imperative to find a vendor with the expertise, products, and resources to select the right flocculant for a given application,? said Mr Gralton, who added that their use does not eliminate the settling pond, but does eliminate some problems associated with ponds.
Mr Gralton suggested that flocculants are a good solution for the producer who wants to guarantee that only clean water gets recirculated back into the plant, for those who want to reduce the number of settling ponds on the site, and for those who don?t mind ?dipping out? a pond, but want to do it far more efficiently.
BELT PRESSES
Belt presses have been considered the ultimate in fines recovery and dewatering – but Mr Gralton said that they are complex and costly to run as they must be accompanied by a ?thickener? tank, a flocculant system and an extensive foundation with sumps. The operating cost is further escalated by the need for a full-time operator.
The belt press is preceded by the thickener tank which acts like a low performing clarifier. Dirty water is fed into the large-diameter thickener where flocculants are added. Clean water runs off the top of the thickener and back to the plant. Mud is pumped from the thickener at 25 per cent solids by weight, and is sent to the belt press. En route to the press, more flocculants and coagulants are added to stiffen and condition the mud prior to dewatering in the press. The mud is laid upon a synthetic belt where gravity drains off the loose water. Another belt comes down and ?sandwiches? the mud layer. Then the belts run around rollers and mechanically compress the trapped mud, squeezing out the water. The resulting fines (70 per cent solids) are discharged from the machine in a ?hamburger patty-like? consistency and can be stacked prior to loadout.
Mr Gralton explained that prior to the the new ?plate and frame presses,? the belt press was the only solution for the producer who had no available property for a settling pond or an area to deposit fines for reclamation. ?If the producer wants to immediately stack and haul the fines, the belt press had been, until recently, the only solution,? he said. Mr Gralton suggested that the plate and frame press is now the new trend. Plate and frame presses are dewatering machines that utilise pressure to remove the liquid from liquid-solid slurry. ?Although the plate and frame press requires a significant capital investment, and must also be preceded by a clarifier, their use does not require the addition of costly flocculants and does not need to be manned by a full-time operator. That cuts much of the operating costs,? he said.
SUSTAINABLE SOLUTIONS
It?s been said that water is ?the oil market of the future?. With that in mind it is important to perform a water audit to determine where and how you use water at your facility, not only in the aggregate washing process, but in every facet of your operation. Develop a maintenance program that routinely inspects all plumbing equipment and fixtures, water lines, spray systems, valves and pumps. Metering at strategic points in the facility helps to detect leaks and maintains minimum flow rates. But above all, employ the optimal reuse and recycling systems for aggregate washing. It pays to be water wise.