In underground mining, two systems are vital to ensure operational safety and efficiency: mine ventilation and mine dewatering, also known as mine effluent handling.
Mine dewatering systems must be reliable and robust in order to provide a dry, safe underground work environment. The earliest underground mines used Cornish beam engines to pump water to the surface, but, since then, the systems have become much more complicated, comprising an array of pumps, sumps, and pipes, often in complex layouts.
Significantly, the complexity of these systems increases as the mine continues to develop and expand. As the mine goes deeper, pumping stations and piping need to be added, which increases the maintenance and servicing requirements. In many instances, it may be more cost effective to replace these complex systems with a single lift system, whereby a single PD pump can replace many centrifugal pumps. This provides a more sustainable alternative, helping the miner to reduce its CO2 emissions.
However, it is a capital-intensive solution, which means that there are several important factors to consider when determining whether it makes operational and economic sense.
With typical mine dewatering systems, pumping stations are progressively added as the mine goes deeper. This means that the capital costs increase incrementally, while the operational costs accumulate over the life of mine (LOM).
With this in mind, the question is: at what depth does a single lift system become more feasible than a multistage system?
A single lift system basically replaces the core of the multistage system; water is then collected in a central sump, where a single, high-head PD pump pumps it out of the mine in a single lift.
These systems are driven by the head requirements, which is why Weir Minerals utilises its GEHO® PD pumps in these applications.
The single lift system can use the same decline as the multistage system; however, the best results are achieved if existing vertical access to the surface is available as is the case with – blind shafts, ventilation shafts, hoisting or manway shafts or boreholes – Compared with decline access, which is essentially a system of ramps and crosscuts that connect the access points, shaft access means less piping length is needed to reach the surface, reducing costs and potentially determining the project’s feasibility.
Moreover, if the effluent can be pumped out of the mine vertically it lowers the head requirements significantly. Power consumption and carbon emissions are around 30% lower for a single lift system; the higher the flow and the deeper the mine goes, the greater these savings become.
Repair and maintenance costs are also another important consideration when assessing the feasibility of the competing systems. As the mine gets deeper, the operational costs associated with maintaining, repairing, and servicing more pump stations increases. Moreover, as the mine goes deeper, there is an increase in solids entering the system, which, depending on the pumps being used, can increase wear, often leading to premature failure.
GEHO® PD pumps, in contrast, can handle significantly higher solid concentrates than it encounters in dewatering applications, ensuring greater reliability and pump efficiency.
Weir Minerals has observed that cost overruns are typical when expanding multistage pumping systems, particularly when operating at significant depths. It is not uncommon to see the actual costs overrun the original planned costs by 30-45%. In most instances, this is the result of higher repair and maintenance costs, which may seem incidental – compared with replacing the entire system with a single life system, that is – but undermine the economic viability of the system.
Put simply, increased depths mean more pumping stations, greater complexity, a higher influx of coarse particles and, as a result, a higher likelihood that something may go wrong, leading to unplanned downtime and expenses.
Taking all these factors into account and assuming the LOM is more than, say, five years, based on the work Weir Minerals has done in the past, as the depth increases it begins to make economic sense to consider replacing multistage centrifugal dewatering systems with single lift GEHO® PD pumping system.
Mine dewatering systems are typically designed during the pre-feasibility study phase and, because they are built up out of many low-cost items, it is rare that the operator reviews the system and looks to optimise it. However, these low-cost items and service orders – routine maintenance and inspections, seals that need to be replaced, etc. – continue to accumulate to the point at which they exceed the cost of installing a new, single lift system.
There is always room to improve these legacy systems and Weir Minerals has the engineering expertise and the product portfolio to assist miners in carrying out the detailed analysis required to determine whether implementing a single lift system would not only improve their dewatering operations, but also reduce their operating costs.
PD system versus multistage centrifugal system
Firstly, centrifugal dewatering pumps are typically designed to handle clean water, which means that, as the mine gets deeper and there’s an influx of coarse particles, the pumps wear life is radically reduced and there’s an increased likelihood of pump failure. In contrast, GEHO® PD pumps can handle slurry concentrations up to 85%, which far exceeds the requirements for dewatering applications.
Centrifugal pumps operate efficiently at their best efficiency point (BEP) – typically defined as 70% (or 75%) to 110% (or 115%) of flow at BEP – but if that’s deviated from, then the efficiency drops. This can become a challenge in mines that have large operational windows. For instance, mines located in regions that experience significant fluctuations in seasonal water ingress typically have a large operating envelope. In these environments, multistage centrifugal pumping systems are often less efficient.
GEHO® PD pumps, on the other hand, remain efficient throughout the entire operating envelope. From a sustainability perspective, the required head, combined with the efficiency of the pumping system, dictates the power consumption. While the exact savings will vary from site to site, installing PD pumps typically results in around 20% instant energy cost reductions at the nominal flowrate. And during seasonal ingress, these savings can increase to more than 30%.
Despite this, mine dewatering is often an overlooked opportunity to reduce OPEX and improve the sustainability of a mine’s operations.
Weir Minerals’ proposal is a modest one: simply, if the LOM is more than five years and the depth of the mine increases, mine operators should review their dewatering system and conduct a detailed analysis comparing the operational and economic benefits of a multistage centrifugal pumping system versus a single lift GEHO® PD pumping system.
AUTHOR: Job Kruyswijk, Weir Minerals, Manager Integrated Slurry Solutions