Several types of dewatering equipment are available to separate solids and liquid in slurries to reduce or eliminate settling ponds, but one is better for overall tailings and water management. McLanahan Corp. maintains that filter presses are the best choice for settling pond elimination.
The aggregate and mineral industries have historically used the simplest and cheapest dewatering technology to manage waste slurry tailings. These old methods include dumping directly into streams, rivers and lakes or using mining surfaces or underground quarries/pits for disposal.
For a number of decades these industries have been pressured to eliminate older ways of waste dumping and look for more efficient waste management technologies. Initial improvement in tailings management technology involved creating ponds or large impoundment dams to hold the waste slurry for the life of the quarry or mine.
Reducing/Eliminating Settling Ponds – Over the past 10 years or more, pressures to improve tailings management practices have increased further due to a number of factors, including:
• Stricter environmental regulations.
• Rising operational costs.
• Higher waste handling costs.
• Increased water conservation requirements.
• Catastrophic impoundment failures.
• The need for better risk mitigation.
• Physical space limitations for operational expansion.
Leaders in the aggregate and mineral industries have started evaluating and investing in more efficient, cost-effective systems to dewater tailings. The systems include: ultra-fines separation, slurry thickening and slurry dewatering equipment technologies.
The UFR and Thickener equipment technologies separate and concentrate the waste materials, while the dewatering equipment recovers additional water from the waste stream and creates nearly dry solids.
Types of dewatering equipment – Plate filter presses, belt filter presses and centrifuges are competing dewatering equipment used in the aggregate and mineral industries today.
Centrifuges – Centrifuge dewatering is a high-speed process that uses the force from the rapid rotation of a cylindrical bowl to separate wastewater solids from liquid.
These dewatering devices work best with material that is more easily dewatered, has a larger particle size distribution and/or a low clay content, though they generally are not going to be as efficient as a filter press since they cannot generate the high-pressure dewatering forces of the latter.
Centrifuges operate as continuous feed units that remove solids by a scroll conveyor and discharge liquid over the weir. The bowl is conical-shaped to help lift solids out of the liquid, allowing them to dry on an inclined surface before being discharged.
Centrifuge equipment has high up-front investment cost and high operating cost. It is built to tight mechanical design specification to operate efficiently at very high rotational speeds.
Operating costs are high due to high electrical power requirements, expensive replacement parts and the need to use dewatering chemicals.
In addition, centrifuge operation requires skilled, experienced operators. Centrifuges have a reputation of high noise, high speed vibrations and are subject to frequent wear issues.
Because centrifuges are smaller than filter presses, the footprint could be less than that of a large plate press depending on how many units are required.
Belt Filter Presses – Belt filter presses are another type of equipment used to remove water from waste slurries to produce a non-liquid material referred to as a “cake”.
In a belt filter press, the waste slurry is forced between two tensioned porous belts. As the belts pass over and under rollers of various diameters, the liquid is squeezed out of slurry while the solid cake material is retained between the belts. Increased pressure is created as the belt passes over rollers that decrease in diameter.
There are many designs of belt filtration processes, but all incorporate the following basic features:
• Polymer conditioning zone.
• Gravity drainage zones.
• Low-pressure squeezing zone.
• High pressure squeezing zones.
Advanced designs provide a large filtration area, additional rollers, and variable belt speeds that can increase cake solids by 5%.
The general mechanical components of a belt filter press include dewatering belts, rollers and bearings, belt tracking and tensioning system, controls and drives, and a belt washing system.
Belt filter presses have low to medium initial capital cost, with a smaller footprint for installation. Operating costs tend to be high due the need for a full-time operator and the use of chemical dewatering aids.
Maintenance costs are also expensive due to belt and roller wear as well as failure from continuous movement of the tensioned belts between the rollers. Belts and roller bearings require frequent replacement, creating lower overall availability.
Belt presses have a reputation of requiring frequent belt washing and are very sensitive to process variations.
Recessed Plate Filter Presses – Recessed plate filter presses are used create a dewatered cake material by separating the liquid and solid materials in waste slurries.
These filter presses are among the oldest types of dewatering devices and are commonly used in aggregate and mineral processing waste slurry applications.
Typical recessed plate filter presses are composed of a heavy-duty support framework holding a series of polypropylene plates with concave surfaces on each side of the plate. Each plate is lined with a synthetic filter cloth. The plates are tightly clamped together with hydraulic pressure, creating a void space between the plate surfaces.
A high-pressure slurry feed pump is used to force slurry material into the void spaces between the plates. The slurry solids are captured between the plates, while the liquid passes through the filter cloths mounted on the plate surfaces.
When no additional slurry can be pumped into the filter press, the slurry feed pump is stopped and the plates are separated, allowing the cakes to fall by gravity out of the press.
The up-front investment for a plate filter press can be high due the size of the equipment and required support structure.
Depending on the type and amount of material being dewatered, modern filter press equipment can be very big and require a large space for installation and operation. Common polypropylene plate sizes today can be up to 8-ft. wide by 8-ft. tall, with large filter presses holding up to 200 plates.
Plate filter press operating costs are low due to the requirement of only a part-time operator, lower wear parts cost and typically no need for chemical dewatering aids.
Because the slurry material is captured completely between the filter plates, very high dewatering pressures (up to 225 psi or higher) can be applied to the slurry. The high dewatering pressures produce the highest cake solids concentration possible of any mechanical dewatering equipment.
Membrane Plates – Plate filter presses can also employ a second type of plate called a membrane plate. In this case, the plate has a flexible membrane surface made from polypropylene or other synthetic flexible material such as hard rubber.
The membrane can be thermally fused to the concave plate core, or the synthetic membrane’s edge can be embedded into the concave plate core in a small channel machined in the plate surfaces.
The same initial process is used in a membrane plate press, with a high-pressure slurry pump feeding the press to fill and dewater as much slurry as possible.
After the slurry feed pump is stopped and while the filter plates are still clamped together, the membrane surfaces are expanded using compressed air or water. This inflated membrane compresses the cake material, removing additional moisture to the lowest possible levels.
Plate Filter Press Advantages – No other dewatering process has shown to be more efficient at removing liquid from slurries as the plate filter press. Additional benefits of plate filter presses include:
• Highest amount of recovered water.
• Driest cakes.
• Lowest operating cost.
• Lower waste handling cost, haul less water.
• Most stable solid material generated.
Because of these advantages and the high efficiency of the plate filter press, it is the best choice of equipment to eliminate settling ponds.
McLanahan Corp., www.mclanahan.com