Development and Demonstration of an Automation and Control System for Coal Spirals

Abstract

Coal spirals are widely used in coal preparation plants in Illinois and around the world to clean fine coal, typically in the 1 × 0.15 mm size range. Despite their popularity and the trend toward increased automation in modern coal preparation plants, adjustments to the critical process variable for coal spirals, that is, product splitter position, continue to be done manually. Since spiral feed in a plant tends to fluctuate on a regular basis, suitable manual adjustment of splitter position in tens or hundreds of spirals operating in a plant is nearly impossible. As a result, the clean coal yield from a spiral and also the overall plant suffers on a regular basis. This article describes the development of a suitable sensor and control system to adjust the product splitter position of a full-scale spiral. A thorough investigation was made to make use of a basic property of coal slurry that can be measured on-line to correlate it to the constituent solid density and thus to monitor the change in the density gradient across the spiral trough at the discharge end resulting from the usual fluctuation in feed characteristics. An electrical conductivity-based sensing technique was preferred over a capacitive sensor to measure the density gradient using two sensor tubes placed at the critical separation zone of the spiral trough. A PIC24 microcontroller was programmed to send a signal to a DC gear motor based on the new density gradient measured in real time to turn clockwise, counterclockwise or to stay at the same position based on the difference between the conductivity measurement of the present cycle and that of the previous cycle. The automation system was validated by examining the performance of a full-scale spiral while deliberately changing factors like feed solid content, feed washability characteristics, and feed slurry ionic concentration. With a full-scale compound spiral programmed to achieve a specific gravity separation at 1.65, actual D ₅₀ values achieved for two separate tests were 1.64 and 1.73, respectively.

Keywords:

• Automation
• Coal spiral
• Control system
• Density separation
• Sensor

Acknowledgements

The equipment support received from Multotec/PrepTech is greatly appreciated.

Notes

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gcop.