The ASTM slump test has been the backbone of the pastefill plant quality control program since it was adopted from the concrete industry in the early days of pastefill system development. It is a simple test that provides a quick evaluation of the consistency of the paste. This consistency is related to the ability of the paste to flow, or its rheology. Understanding the paste’s flow characteristics is critical to stable pipeline operation. Thicker paste mixes have higher friction and corresponding pressure losses in the pipeline. For each paste and pipeline configuration, there is a unique paste rheology that will best balance the system.
In the slump test, the distance that the paste sloughs out of its original cone shape provides an indication of the paste’s consistency, impacting its flow properties. In general, paste slump ranges from 6 to 8 inches. This test is highly dependent on the operator’s technique and repeatability. It is a relative measurement that is not readily converted to the paste’s rheological properties and so cannot be incorporated into pressure loss models. More recently, the Boger slump technique was developed to provide a quantitative estimate of the paste’s yield stress but it too is an estimate and is susceptible to similar measurement variability as the ASTM slump test.
With the modern mine becoming connected, increasing amounts of real time data are available to the paste plant operator. Pressure loss per meter in the pipeline can be calculated in real time using two pressure sensors installed on a straight run of pipe a known distance apart. This is a quantitative measurement that instantaneously reflects the changes in the viscous properties of the paste (i.e., it is an indirect measure of the rheology). The pressure losses at corresponding flow rates can be tracked and trended to provide the operator with greater understanding of how the flow behavior in the pipeline is evolving. Such online measurements provide finer resolution on the pressure losses in the pipeline than would be possible with a slump test.
With this trend, the question arises whether the slump test is relevant anymore. In general, more data is advantageous but there is still value in checking in on the paste consistency the low-tech way.
One reason is that at some point instruments fail. Pipeline pressure readings are the most important feedback an operator can have to control and troubleshoot the pipeline operation, and yet there is an industry wide delay of response time in repairing broken instrumentation, such as pressure gauges. The pressure loss and flow rate readings must be reliable and accurate for an operator to gain confidence in the feedback and use it for control.
The other reason is that the slump test provides the operator with the opportunity to see the final paste product in person while the pressure measurements cannot. By doing a slump test, the operator is also observing the paste quality prepared by the plant on surface before it is sent underground. It is during these brief interactions that the operator can observe changes in the paste such as paste consistency / stickiness, lumps or clay balls, tendency to bleed water and presence of unmixed binder. These observations are clues to changes in the circuit which could affect the paste quality or flow in the pipeline. In the same way a geologist examines the core in person after reviewing the core log photos, or the metallurgist observes the flotation froth after checking the on-line analyzer results, there is value in physically seeing the paste. The value of the hands-on feedback will not diminish with improvement in data collection.
As more on-line control is included in pastefill systems, the slump test will transform to a quality assurance role, rather than quality control. Instead of taking a slump test routinely on the hour, the slump test can be used punctually to visually confirm the paste consistency and aid the operator in setting up the circuit. After that, a well-tuned pair of pressure gauges relatively close to the paste plant discharge can be successfully used to control the paste consistency throughout the pour. But if the gauges are not working or the control loops are lagging, it is always good to have the slump test to fall back on to check the consistency of the paste and confirm changes being made on the operator interface screen are being realized in the field.
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.