Part 1 of this article covered the most common problems in thickener operations caused by thickener design errors and changes in ore properties.
Part 2 will discuss in detail the problems caused by mechanical and maintenance issues (including issues with thickener instrumentation and control) and what can be done to solve them. Conclusions and recommendations are provided.
3. Mechanical and maintenance issues
Sometimes problems that originate from failures in thickener components can occur. The most common are described below.
Due to specific events of high torque as well as corrosion of the rake mechanism, the thickener gradually loses its torque capacity. In this scenario, Operators tend to run the system with a lower torque set point than that considered in the design to avoid a serious failure (e.g breakage of the mechanism). This affects the thickening capacity by working with lower % solids (refer to figure 3). Regular rake maintenance and sometimes mechanism overhaul will be necessary to keep the system working properly.
Another common issue is the wear of the discharge pump that loses its flow capacity. This occurs mainly in centrifugal pumps that are very sensitive to increased rheology, which not only affects the thickener stability, but also the discharge pressure of the pump.
Instrumentation and control issues
Errors in the choice of instrumentation technology and their installation can result in inefficient thickener operations.
Another common issue reported is the inaccuracy of measurements mainly due to issues related to calibration not properly completed. These issues can be seen more frequently in density and flow meters. Inaccuracy in these measurements results in errors in the % solids calculated and operational decisions made.
Also common is the failure of external pulp level sensors becoming fouled with dust or mud which impact the measurements.
Problems also occur when the control strategy is misconceived. This results in unstable thickener phases (clear, interface and mud layers, see figure 4). As a rule of thumb, in conventional thickeners, the interface level should be kept between 0.5 to 1.0 m below the surface. On the other hand, the mud level should be kept between 2.0 to 3.0 m over the thickener bottom.
Figure 4. Thickener settling layers
An example of a suitable control strategy is presented in figure 5. Here, both interface level control and mud level control are used.
Figure 5. Control loop schematic using interface and mud level
- Interface level control
Interface level control is used to maintain an acceptable %solids in the overflow while minimizing the flocculant usage.
Interface level is controlled by manipulating flocculant concentration or flow rate. When the interface level starts raising, the suspended solids increase and contaminate the clear layer. Then, the control loop increases the flocculant dose in turn lowering the interface layer to the predetermined setting.
- Mud level control
Mud level control is used to maintain an optimum underflow density as high as possible to reduce the quantity of water discharge with the slurry while avoiding the bogging of discharge pumps.
Mud level is controlled by manipulating the underflow pumping rate. When the mud level decreases there is less compression in the mud layer. This results in less % solids in the thickener discharge. In this case, the control loop decreases the pump speed in order to increase the mud level, consequently increasing the %solids in the discharge.
Table 1 summarizes the most common problems in thickeners, explaining the cause and the mitigating measures.
Table 1. Typical thickening operation issues
4. Conclusions and recommendations
The main issues that mining plants face in thickener operations have their origin in three main sources: design, mineral variations and mechanical or instrumentation and control problems.
1.- The design stage is the most relevant aspect to determine the diameter (area) and the torque of the thickener. For a fit for purpose design, the industry and tank manufacturers recommend a) to carry out laboratory and pilot tests with representative samples along the mining plan, b) to perform sensitivity analysis versus mineral particle size distribution (granulometry) and c) to apply safety factors according to the type of thickening technology to be used.
2.- The variation of mineral properties should be approached as an additional issue to geometallurgy. In the past, geometallurgy studies only focused on hardness-treatment and metallurgical recovery. Now, they include the sedimentation rate and rheology as control parameters for planning the production by limiting the content of problematic minerals such as clays or micas.
3.- The mechanical problems of the thickener drive mechanism should be controlled through a preventive maintenance program. Periodic reviews of corrosion damage, especially in plants using salty water, should be implemented. High torque safety limits should also be set. Sanding problems should be controlled upstream in the grinding stage with safety screens or avoiding the reverse grinding circuit (a source of coarse particles fed directly to the hydrocyclones).
4.- Regarding instrumentation and control problems, the periodic calibration of density meters, flow meters and level sensors is a mandatory task in order to have high accuracy and availability of measurements that will feed the control loops.
5.- To improve the control strategies, consider technologies such as SmartDiver that deliver a greater number of continuous real time measurements (layers in the thickener and density profile) to implement more efficient control philosophies.
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