PASTE & TAILINGS 2021
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 .
Plate filter presses & eliminating settling ponds
Dennis Zink , Global Product Manager at McLanahan Corporation , argues that no other dewatering process has shown to be more efficient at removing liquid from slurries as the plate filter press
Several types of dewatering equipment are available to separate solids and liquid in slurries to reduce or eliminate settling ponds , but using one is better for overall tailings and water management . 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 and catastrophic impoundment failures . There is also a need for better risk mitigation . But there are often 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 .
Types of dewatering equipment – centrifuges & filter presses
Plate filter presses , belt filter presses and centrifuges are competing dewatering equipment used in the aggregate and mineral industries today . 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 conicalshaped 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 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 : n Polymer conditioning zone n Gravity drainage zones n Low-pressure squeezing zone n 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 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
P4 International Mining SUPPLEMENT | JULY 2021