Despite the best intentions during the engineering and design phase, things can go amiss in a pastefill system and the pipeline ends up becoming plugged. The first step is to safely empty the boreholes as quickly as possible and attempt to clear the line with water. After ‘saving’ as much of the system as possible, the remaining level piping and boreholes should be cleaned with high pressure water or other manual methods.
While the cleaning process is underway, a parallel investigation is often undertaken to understand the cause of the blockage with the aim of preventing similar instances in the future.
What tools do Pastefill Engineers have at their disposal to perform this forensic analysis?
- The key tool is the SCADA system which captures the operating data before and during the time of the incident.
- Key instrument outputs that should be recorded include pressures from throughout the underground pipeline, throughputs for each component of the paste (water, binder, tailings and other materials, if added), mixer power draw, and pump speed.
- Visual observations from the pour point and punctual QA/QC data including paste slump, % solids, and particle size also provide information for the investigation.
The main question is: What caused the blockage? From experience, the blockage can be a result of:
- A physical restriction in the line
- A change in paste material properties which influences the rheology and prevents paste from flowing through the system
- A rupture which results in the downstream column of paste being difficult to remobilize.
The analysis to determine the cause of the blockage can have the following general format:
- Examine the plant infrastructure: Did a foreign object enter the paste reticulation system? Is the hopper screen in place? Any cement build-up on the hopper edges? Missing teeth from a loader? Any broken paddles in the mixer?
- Examine the paste ingredients’ properties: Including % solids, particle size, % binder, water to cement ratio and tailing to aggregate ratios, if applicable. Was the paste quality acceptable for successful transportation to the stope? Did the paste have properties that accelerated the setting of the paste in the line after blockage? Did the mixer power draw spike, indicating that an off-spec, thicker paste was produced?
- Check the hydraulic model to compare predicted pressures to the recorded pipeline pressures: Identify abnormalities in the pressure trends that indicate the problem area. Did a spike or drop in pressure occur on certain levels? Did the startup of the pour and the pre-flush of the pour occur normally? Is there an indication that the line was not sufficiently flushed or that the material was thinner/thicker than expected? Did the pipeline filling occur from the bottom levels up (instead of from the top – down), which can indicate that high forces were exerted on the pipeline?
- If a pipeline over-pressurization occurred, examine the pressure relief area: Was material found that indicates that an obstruction caused the pipe to over pressurize or was there evidence of a worn pipe that gradually failed? Examine the coupling / flange for wear pattern or indication of installation error. Was there hammering, or other changes in sound noted in the area of the rupture?
- Examine the pipeline history: Was this pipeline worn or new? Have there been frequent change-outs of this pipe section, indicating a recurring problem? Were any camera surveys performed recently to indicate the borehole condition?
With this information, it is possible to piece together the series of events that led to the pipeline blockage.
Interpretation of underground pipeline pressure readings is a valuable skill. It can be promoted on site by reviewing the findings of the blockage incident with the operations personnel, including the signs leading up to the event, so that they can read the signs given by the system data and possibly avoid similar events in the future.
Look out for more of our Mine Backfill Forensics series in the near future
About The Author
P.Eng. BSc Eng (Metallurgical), MASc Eng (Mining & Materials)
Maureen is a Senior Process Engineer in Paterson & Cooke’s Sudbury, Ontario practice. She has worked in mining and milling for over 18 years and specifically in paste backfill design and operations for over 12 years. Her expertise includes backfill management planning, hydraulic modeling of distribution systems and start-up/commissioning of backfill operations.
Maureen is bilingual (English/French), a licensed Professional Engineer in Ontario and Quebec, and has authored several papers involving paste plant operation, troubleshooting and pipe wear during the transport of paste tailings. She has recently completed her Masters in paste system wear and is beginning her PhD.