Low Capital Investment Options to Support Mining Activities
Paste backfill has become known as a superior ground support tool and mine waste disposal method, albeit usually one with a higher initial price tag when compared to hydraulic or rock fill. The capital costs associated with the implementation of a permanent paste backfill plant can put a strain on mining project budgets where early, positive cash flow from mining activities is required.
The capital costs of a permanent paste backfill plant, as well as the current market conditions around the world may therefore result in mines requiring paste backfill solutions where operating expenditure is preferred over capital investment.
In order to address these capital cost challenges a mobile paste plant (MPP) consisting of customized concrete equipment was developed that could process dry source material into paste backfill while being compact enough to move around. These MPP’s have successfully been implemented by Paterson & Cooke (P&C) on several projects.
Typical MPP Set-up – Material Loading Bins, Cement Silo and Mixing Equipment
Based on Paterson & Cooke’s experience, the production rates from an MPP can be between 20 and 100 m³/h. Where larger flow rates are required, multiple MPP’s can be implemented to operate in conjunction with each other or customized designs can be provided for unique applications.
Feed Material Sources to the MPP
The feed material to the MPP is sourced from readily available materials on the mine and may consist of either (or a combination of):
- Existing tailings dams as semi-dry material
- Tailings in slurry form from the mill – making up a portion of the recipe; the balance from dry sands, silts or other aggregates
- Pit or river sands
Depending on the source of feed material, the moisture content and particle size distribution may vary on a daily basis. It is therefore vital that test work on the available sources is completed in order for the designer to develop suitable mix designs before implementation of the MPP. Trained plant operators and suitable QCPs are also required to ensure that in-time adjustments are made for material variations during operation.
Sample PSDs and Operating Envelope from Available Sources to MPP
The mix design from the MPP requires a proper understanding of the required strength gain and rheology (flow behaviour) of the final product to be delivered underground. Binder types and concentrations must be understood and backed up through test work. The mixture should ensure that a suitable paste backfill product is produced that can be delivered underground using a gravity or pumped system while developing sufficient strength once placed.
Depending on the duration of the project, the paste system can be fully mobile, or semi-portable; capable of moving to a new hole within hours or a couple of days, depending on the requirements. The MPP utilises modified concrete equipment for processing of the feed material, binder and trim water into a paste backfill mixture and, depending on the process designed, a homogenous mix of paste is controlled by the on-board logic controller, supplemented by regular quality testing.
The product from the paste system discharges into a hopper for gravity flow underground or into a paste pump, depending on the underground hydraulics. The paste pump is generally trailer or truck mounted and forms part of the overall paste production and delivery unit.
Typical MPP Paste Pour (left), Remote Borehole (middle) and Paste Filled Stope (right)
Pros and Cons
The suitability of a mobile custom paste plant will depend on the application, however the following pros and cons are listed for these types of solutions:
- The operation can be mobilized/demobilized quickly
- Little to no capital is required; note that for operating mines, the underground distribution system remains common to a contracted MPP or a permanent facility
- The truck mounted set-up can be moved quickly for multiple filling areas or ‘pods’
- A skid-mounted set-up is available for semi-permanent applications; these can be diesel or electrically driven
- The system lends itself well to a contracted approach from P&C for one-off projects or medium-term service contracts (where operations pay by the m³ or by the hour)
- Quality control of the paste process has been upgraded to greater than that required in the concrete industry
Truck Mounted MPP Set Up for Filling of Multiple Areas
- The raw materials must be nearby and suitable for the application
- Upfront screening and blending of material is required
- Equipment normally used in the concrete industry requires modification for continuous, reliable service
- The throughput for MPP is somewhat limited (80 to 100 m³/h) although multiple MPP’s could be hired
- Unit cost per ton of paste is higher than a permanent facility owing to the cost of shifting capital to equipment rental
- Specialized knowledge is required for the application and mix design. P&C’s capabilities include the test work, engineering and site supervision to ensure the mix suits the application
The customized mobile paste system provides a solution where little to no capital investment is required to implement a paste backfill system and it lends itself well to contracted project delivery. Mines struggling to raise capital can start backfilling on an operating contract. When done with experience and the right equipment, the paste backfill product is of the same quality as a permanent paste plant while simultaneously allowing variability in feed materials available on the mine.
Paterson & Cooke have been involved in many paste backfill plant design and supply projects throughout the world for many years. We have extensive experience in delivering permanent and mobile paste plants, each designed to suit the specific needs of our clients.
For additional information and to determine if the MPP application will suit your operations needs, please contact any member of P&C’s global backfill group.
About The Author
Bernardt van der Spuy
BTech Eng (Mechanical), Pr.Tech.Eng
Bernardt is an experienced mechanical engineer with a strong exposure to backfill systems design and operations. He has extensive, international backfill experience, which includes plant design, reticulation systems design, underground placement and backfill operations from conceptual phase to construction and commissioning.