This article presents the importance of a holistic approach to improve the performance of mine waste disposal systems treating ores with a variable mineralogy. Mine tailings that contain clay minerals such as iIlite, kaolinite and/or smectites are particularly difficult to dewater using thickeners. Small variations in mineralogy result in significant variability in thickener underflow densities and delivered flow rates that must be transported to the tailings storage facility. A brownfields upgrade recently completed by Paterson & Cooke demonstrated the importance of considering both the variable thickening and pumping requirements simultaneously, and not treating each as independent systems.
In this case, the thickening circuit was historically unable to consistently achieve the required underflow density and produced a considerably more dilute underflow density as well as occasionally ‘sliming’ where solids report to the overflow weir (see Figure 1). The limitations of the thickening circuit affected the downstream tailings disposal pumping system as the viscosity and volume of tailings varied significantly, resulting in blockages and operational challenges.
Figure 1: Example of a Sliming Thickener
A comprehensive test campaign was conducted, both on-site and at P&C’s laboratories to better understand the material properties and dewatering requirements as mineralogy varied. The test work showed that the material can be thickened to the required density for the majority of the time and the variation in mineralogy has the following effects on the system:
- High clay content material is more viscous and limits underflow densities that can be achieved at a particular slurry yield stress. This requires the system to operate at higher flowrates to achieve the required tonnage throughput
- Lower clay content decreases viscosity and can be thickened to higher densities at relatively low yield stresses and requires the system to operate at lower flowrates
Figure 2 shows the envelope of the various samples that were tested. The required underflow density is achievable for low viscosity material but challenging for high viscosity material where the yield stress increases quickly at low densities. To deliver the required tonnages when the material is viscous and can only be thickened to low densities results in a high flow rate, and as viscosity decreases and underflow density increases, the required flow rates that must be pumped to the tailings facility decrease.
Figure 2: Rheology Envelope
Thickener and Pumping System Operating Range
To achieve the necessary underflow densities, the thickening circuit required a number of upgrades, however the constraints of the pumping system needed to be understood to determine a suitable operating range for both the thickening circuit and the pumping system. Through a series of trade-off studies it was decided to limit the yield stress to 30 Pa for the following reasons:
- a limited number of upgrades to the thickening circuit were required
- the thickened underflow could still be delivered to the underflow pumps without blocking the suction inlet while still having sufficient Net Positive Suction Head (NPSH); and;
- the existing tailing pumps could be used, however sections of the pipeline would need to be upgraded to accommodate the higher pressures needed to pump to the furthest point of the extended tailings facility
As this was a brownfields site it was decided to upgrade the complete thickener circuit in stages, starting with one thickener to determine the incremental improvements of each change before upgrading all the thickeners on site. The following stepwise changes were done, evaluating the improvements in performance each time:
- There were regular blockages of the underflow pump suction inlet and piping when the material was very viscous. The underflow discharge piping diameter was increased, and large suction diameter froth pumps were installed. These pumps have a specially designed impeller with inlet vanes that introduce viscous material into the impeller
- The test work showed that pickets contributed to the effective dewatering of the floccules and pickets were fitted to the thickener rake by the thickener supplier. The rake speed was reduced to allow the fine particles to consolidate, and together with the pickets, resulted in a significant improvement in underflow density
- The thickener instrumentation, such as mud bed pressure and mud bed level transmitters were refurbished and the thickener control system was updated based on P&C’s control philosophy to run autonomously. This included reducing the step change of the flocculant control system, introduction of a re-circulation loop and controlling underflow pumping rate based on correct density and flow measurements
The thickener responded well with the upgrades as shown in the graph below.
The fully upgraded thickener produced consistently higher underflow densities, matching, and in some cases, exceeding the plant’s target density. The increase in density and the system’s capability of accommodating the higher density and yield stress material had the following positive impacts on the overall tailings disposal system:
- Consistent operation of thickener and less underflow density variability
- Lower volumetric throughput reporting to the fines tailings pump station. The pump station that previously consisted of four pump trains could be reduced to only three trains whilst still delivering the required tonnage throughput
- Considerable savings in flocculant
- Reduced downtime of thickeners due to blockages
- Improved water management of the mine due to lower volume of water reporting to the tailings storage facilities
About The Author
BEng (Mechanical), Pr.Eng
Kobus is a senior mechanical engineer based at Paterson & Cooke's Cape Town office. Since joining the company in 2011, he has been managing and executing a wide range of pump and pipeline projects from conceptual level through to construction and commissioning. Kobus has extensive experience in pump station and plant layout design as well as plant and overland piping route selection based on thermal expansion and pipe stress limitations.