Are you planning to install a new paste fill system at your mine? Or maybe your ore reserves have increased and mining production is scheduled to be ramped up going forward. It is vital that any mine considering implementing a new paste plant or increasing mining production rates should be comfortable that the capacity of their system will not leave them in a bind and become a bottleneck for mining operations. By correctly sizing the plant for your mining needs, you will always have the fill operations waiting on the mining and not the other way around.
The capacity of a paste ‘system’ is generally determined as the instantaneous paste fill flow rate that can be delivered to the underground operations. This instantaneous design flow rate directly determines the required size of the paste fill production equipment within the plant as well as the reticulation system to the underground workings.
Utilisation Rate of a Paste Plant
Determining the required paste plant capacity is generally dependent on:
- The mining plan over the life of mine (LOM)
- The availability of the mill to supply tailings
- The availability of the underground operations to receive backfill
- The expected utilisation rate of the paste fill system
Paste plants run intermittently and have a lower utilisation rate than the mill which runs continually at a typical availability of between 90 to 98%. The paste plant therefore buffers the link between the ‘continuous’ mill operation and the ‘intermittent’ underground paste filling operation. There can be significant downtime associated with backfilling when the availability of the paste plant, the underground distribution, the mill and the mining activities are combined. Not to mention the lost time if a borehole or pipeline is ever plugged and requires extended clearing by a drill or contractor. Extra capacity in the paste plant is therefore vital for catch-up and maintenance. A typical paste fill system utilisation breakdown is illustrated in Table 1.
Table 1: Typical Paste Plant Utilisation Breakdown
Every mine is different and will have local factors that affect the utilisation rate agreed upon during the design process. A typical paste plant utilisation rate is illustrated below. It is however important to apply the local mine’s input as, for example, some may choose to pour backfill through shift-change using remote cameras.
Example of Paste Plant Utilisation
24 h/d x 365 d/y = 8760 hrs total available per year
- 3 weeks/year shutdown for maintenance/holidays
- 3 weeks/year borehole/pipeline plugged
- 4 hrs/day blasting/shift/pipe change
- 2 hrs/shift start-up, shutdown, pre and post-flushing
- 1 shift/month material/labour shortage
- 1 shift/month breakdown
- 1 shift/month power outage/failure
- 1 shift/month barricade delays
- 1 shift/month no place to pour (stope not ready for filling)
- 1 shift/month no tailings available
- 0.5 shift/month instrument delay
The above results in a total of 4232 hrs available for achievable backfill placement, or an annual utilisation rate of ~50%.
Utilisation Rate Survey Existing Paste Fill Systems in Operation
A survey of several operating paste fill systems around the world was conducted by Paterson & Cooke (P&C) personnel to determine the typical utilisation rate achieved. The design capacities of the systems ranged from 40 to 275 m³/h and included gravity as well as pumped systems. Figure 1 presents the outcome of the survey. It was found that the majority (~45%) of the systems included in the survey operated at a utilisation rate of 40% to 60%. Some of the systems provided utilisation rates as low as 10-20% while others provided rates of 80-100% which shows the effect of the local influences of each unique operation.
Figure 1: Survey of the Utilisation Rate of Several Existing Paste Fill Operations around the World
The downtime of one of the mines was broken down into surface, underground and operational downtime as presented in Figure 2.
Figure 2: Downtime Breakdown of Existing Mine
Plant Sizing Example 1 – New Plant
Gold Mine X is considering implementing a paste fill system to support mining activities. The current mining schedule with paste fill is presented as follows:
Table II: Mine X Mining Schedule
The peak paste fill requirements occur in 2025 with a paste volume of ~528 300 m³/y required to sustain the mining production rate of 1 400 000 t/y. It should be noted that mine planners will typically prepare a separate paste fill schedule which will likely not correspond directly to the ore tonnes mined for the year. However, for this example, it is assumed that all stopes are filled in the year that they are mined.
The instantaneous paste fill requirements (plant capacity) at the peak mining production rate is determined as follows, taking cognisance of the various utilisation rates within the mining cycle.
Figure 3: Plant Sizing Example 1 – New Plant
The above shows that a nominal design flow rate of ~100 m³/h will be able to sustain the maximum mining production rate in 2025 at a paste plant utilisation rate of 60%. Furthermore, the nominal design flow range can be expanded to a design range of between 90 m³/h (-10%) and 110 m³/h (+10%) which will ensure that the maximum required paste fill volumes can still be produced if the paste plant utilisation was to drop to 55% due to unforeseen circumstances. It should be noted that in some cases an operation will elect to design the paste plant to accept the full tailings production rate and will live with a lower paste plant utilisation in order to simplify the tailings management pumping system. This all-or-nothing design philosophy eliminates the wide swings in tailings pumping duties to the TSF.
Plant Sizing Example 2 – Existing Plant Upgrade
Further exploration drilling at Mine X increases the reserves and an upgrade to the mill is scheduled to increase the mill capacity from 1 500 000 t/y to 2 000 000 t/y. The mining plan is updated with the mining requirements peaking in 2027 at 1 900 000 t/y with a required paste volume of 716 980 m³/y. The instantaneous design flow rate requirements at 60% utilisation increase to 136 m³/h as presented in Figure 3.
Figure 5: Plant Sizing Example 2 – Existing Plant Upgrade
Figure 5 indicates that the original design flow rate range (90 to 110 m³/h) of the paste plant will not be able to satisfy the backfill requirements in year 2027 of the revised mining schedule at a utilisation rate of 60%. Mine X has the following options, or combination of options available to ensure the paste fill requirements are met for the revised mining plan, starting with the lowest cost but most effective approach. P&C provides all of these services and have historically assisted several mines to implement these measures.
- Increase the utilisation rate of the paste fill system by adopting a comprehensive backfill management plan to provide structure and order to the paste fill operations. In addition, train operators on the paste fill system to react efficiently and accordingly during upset conditions.
- Audit the existing system and determine possible upgrades to the paste plant that can increase capacity. In most cases the diameter of the underground reticulation system may be the bottleneck and upgrades or twinning may be required.
- If options 1 and 2 do not provide a successful solution, the third option is to construct another paste plant – this could be in another location more favourable to the future ore bodies, or an entirely larger system to phase out the original plant. This will be the most capital-intensive option and would need to be constructed in parallel so that backfilling operations were not interrupted from the original system.
It is vital that any mine considering implementing a new paste plant, or increasing production should be comfortable that the capacity of their backfill system will not apply pressure to their cycle times and become a bottleneck for mining operations.
P&C specialise in the design of fit for purpose backfill systems that meet the client’s production and capital needs. We understand that each mining operation is unique and therefore we do not sell “off-the-shelf” type plants that can compromise mining production when things go wrong. We ensure that we always incorporate all known current and future needs of the mine and thereby provide an end product that we are happy to provide a performance guarantee on.
P&C have also developed a comprehensive backfill management plan (tailored to each client’s specific needs) which provides structure and order to the paste fill operations. P&C’s international backfill team can also provide on-site training to the mine’s paste fill staff to increase utilisation rates by reducing downtime.
Finally, we also have vast experience on conducting audits and designing upgrades to existing plants as the mine’s production needs change over time.
For additional information, please contact us via the Contact Form on this website.
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.