PASTE & FILTERED TAILINGS
A tailored approach to backfill pump selection
Piston pumps , like the GEHO ® DH Series , can handle large particle sizes provided the particles are kept in suspension
Job Kruyswijk , Manager Integrated Slurry Solutions , Weir Minerals , tackles the subject of getting pump choice right for backfilling projects
Backfilling mined stopes is an essential part of many underground mining operations and , in the future , it is likely to become more common for various factors relating to safety , stability and sustainability . There are also economic benefits ; ore that was previously left in place as support pillars can , if backfilling provides an alternative form of stability , then be mined . With high commodity prices , this is an increasingly attractive option for operators .
Many miners commonly use gravity-fed systems to deliver the backfill to the stopes ; however , pumping backfill has several advantages . Pumping increases the reticulation envelope of the backfill system because paste can be transported over a longer distance ; this provides operators with greater flexibility , allowing them to situate the batch plant closer to the thickener . Moreover , higher density pastes can improve water conservation and reduce the impact backfilling has on the mine dewatering system . Pumping also enhances safety by eliminating the need for trucking and facilitating the use of denser – and therefore more stable – backfill mixes .
Pump selection
Typically , backfill is pumped using piston pumps , like the GEHO ® DH Series , because they deliver high pressure output . However , Weir Minerals takes a more nuanced , site-specific approach . Its specialists analyse the backfill mix ’ s particle size distribution ( PSD ) and settling behaviour and select the most suitable pump .
Particle size distribution
Piston pumps , like the GEHO ® DH Series , can handle large particle sizes provided the particles are kept in suspension . These backfill mixes are thickened and have a paste-like viscosity – cemented aggregate fill and aggregate paste fill are examples of these kinds of mixes . It ’ s vital to maintain between 15 % -20% fines in the backfill mix design .
If the backfill contains too many coarse particles and not enough fines it can settle in the slurry cylinder , causing significant wear to the piston each time it impacts it and , ultimately , leading to pump failure . In contrast , if the backfill mix contains a lot of fines , like a hydraulic fill or a cemented hydraulic fill , then it may be more efficient to use a piston diaphragm pump , like the GEHO ® T / ZPM Series , which has a lower Capex and higher capacity – up to 1,600 m 3 / h .
The GEHO ® piston diaphragm pump is a crankshaft driven pump , usually driven by a gearbox , motor and variable speed drive . The pump diaphragms protect the moving parts from the abrasive slurry , and the pump non-return valves are the only wear parts .
Settling behaviour
The stress-induced transition from solid to liquid state is known as ‘ yielding ’. Pastes , which can be made to yield and flow under sufficient supplied stress , have solid-like properties at rest . Backfill mixes that have a high yield stress – up to approximately 250 Pa – can be pumped by piston diaphragm pumps , like the GEHO ® T / ZPM Series . If the backfill mix is stiffer than this , then piston pumps , like the GEHO ® DH Series , are ideally suited .
The GEHO ® DH series has two models of hydraulic driven piston pumps : the GEHO ® DHC , which features cone valves and is suitable for pumping paste with a maximum solids diameter of 30mm and the GEHO ® DHT , which uses a transfer tube and can work with larger particles , up to 100mm .
Within the pump , slurry is confined to the slurry cylinder and piston , which uses a precision machined 0.40mm wear resistance layer to ensure the pump can operate for long periods of time without extensive maintenance . GEHO ® hydraulic driven piston pumps can also be equipped with the GEHO ® VZ-system , which prevents pulsation – ensuring more consistent duty and thus achieving higher performance while generating less noise and wear than a pulsating pump .
The advantage of any paste backfill over hydraulic backfill – which is usually just untreated tailings – is that it has yield , which means operators can pump it slower , thereby reducing wear on the pipeline and components . It is also easier to convert it to a controlled flow system , which ensures the properties of the backfill are more controlled and more reliable .
Contrastingly , an uncontrolled flow system , which is what occurs in gravity-fed system where the material is allowed to reach terminal velocity , generally causes greater wear and potentially results in segregation in the material .
Mine backfilling in the future
Growing demand for the minerals that will be essential for the transition to a low carbon economy , combined with lower grades of ore , mean that , in the future , the amount of tailings will continue to rise . This poses significant environmental , safety , social and economic challenges .
The Global Industry Standard on Tailings Management is explicit that operators have to take responsibility and prioritise the safety of tailings facilities , through all phases of a facility ’ s lifecycle , including closure and postclosure . Backfilling closed mines with tailings is an effective means of reducing the material stored on the surface , which , in turn , reduces the risk of the tailings storage facility ( TSF ) failing . If miners are going to maintain their social licence to operate , it means protecting and doing what ’ s best for the environment and communities living around the site . Backfilling closed mines with tailings will , in the future , play an important role is realising those ends .
IM