Coal Preparation » Dewatering
Paste thickeners produce significantly higher solids underflow concentrations than conventional thickeners through higher beds within the thickeners. They have been introduced into the coal preparation industry for coal tailings dewatering. Control of paste thickeners is critical because the underflow solids have to be within a required viscosity and density window to avoid problems in transportation of the coal tailings by pumps, conveyors or trucks and on the other hand to achieve the required high solids content. Alone paste thickener underflow pumping is proved difficult with variations in thickener underflow density and viscosity resulting in the risks of pump bogging. In particular, due to the variability of paste thickener feed in terms of flow rates and coal types, existing control strategies are generally limited, resulting in large fluctuations in the underflow solids concentration.
The project was conducted to demonstrate the benefit of advanced dynamic control on paste thickeners through the development of dynamical models to describe the mechanism of paste thickening and to relate the underflow solids concentration to a number of key measurements (including bed level, feed solids concentration and feed flow) and operational variables (including underflow rate). Based on the developed dynamic process model, a control algorithm was designed for the paste thickeners.
Plant data from Xstrata's Bulga CHPP was then utilised to validate the process dynamic model. Based on the developed dynamic model, a Model Predictive Control (MPC) algorithm was proposed to control the underflow solids concentration of the paste thickener. The control algorithms were simulated based on the data from Bulga to provide predictions on the potential water saving improvements. The implementation of the MPC was based on a novel approach with three key components:
· A dynamic thickener process model validated by actual plant data;
· An extended Kalman Filter developed using the nonlinear process model to predict unknown model parameters;
· A Model Predictive Control algorithm.
The coal types used in this investigation were found to be difficult to dewater. In order to produce a good paste using these coal types, a high compressive force is required. A bed level of 9 - 10m (thickener height is 11m) is required to produce the paste with 50 wt% underflow solids concentration in all cases.
Analysis on the Bulga CHPP plant data has resulted in the following recommendations. All sensors require regular maintenance and calibration so that their measurements are usable in process monitoring and control, especially the flow meters, bed level and bed mass sensors and density gauge.
Different types of coal tailings require different flocculation dosage to achieve optimal dewatering. However, the effect of flocculation on the paste thickener operation is not measurable by any sensors. The flocculation dosage is controlled manually by the plant operators based on visual observation at the feed-well. Therefore, more investigation on the effect of flocculation dosage to the thickener operation is required if optimal flocculant usage is desired. Additionally, the underflow solids yield stress is also another important variable to control. However, it is not the scope of this project. Additionally, yield stress measurements of the paste thickener product are unavailable. Without such measurement, it is impossible to model the effects of different operating variables to the yield stress and control the yield stress during plant operation.
This project has successfully developed an advanced control approach for the paste thickener underflow solids concentration. Based on the simulation results of the paste thickener using MPC algorithm, the potential improvements to the plant operation are as follows:
· The MPC was able to keep the underflow solids concentration close to 50 wt% more consistently in comparison to the existing plant operation (based on operational data from Bulga June, July, August and October 2012);
· Possible improvement of water savings by the MPC in the range of 6.7 - 30.3 %;
· The ability of MPC to handle physical limitations of the paste thickener (such as maximum sediment level) which translates to avoiding the operation of the paste thickener of solids overflowing.
In addition, the solids inventory inside the paste thickener is always maintained within a safe operating range, which reduces the risks of damaging the rake or stopping the paste thickener operation.
An e-newsletter has also been published for this project, highlighting its significance for the industry.