IN-PLANT DEMONSTRATION OF A LOW COST AUTOMATION SYSTEM FOR COAL SPIRALS
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Spirals concentrators typically clean 70% of the feed to the fine circuit of a coal preparation plant, which generally amounts to 6 to 8% of the entire run-of-mine coal. They are widely used due to the ease of their operation, their low cost of installation and operation, and their ability to achieve high clean coal yield. However, unlike most other density-based coal cleaning circuits, spiral circuits have no means of being controlled automatically or from a plant control room through a PLC. In fact, the spiral circuit is the only link in the chain of parallel cleaning circuits in a modern day preparation plant which has yet to be automated.
Through two previous ICCI-funded projects, the coal preparation research group at Southern Illinois University (SIU) has successfully developed a proprietary automation system to adjust the splitter position on a full-scale spiral so that a desired density cut point can be achieved by the spiral concentrator even with a fluctuating feed environment. Fluctuating feed conditions commonly encountered in a plant may include changes in feed quality (i.e., coal washability characteristics) or changes in the mass flow of solids to spiral cleaning circuit (i.e., altering the feed solid content). These changes affect spiral cleaning performance and can negatively impact the spiral product if the splitter position remains unaltered. Adjustment of the splitter position is required to maintain a constant density cut point when dealing with these fluctuations. Past studies (Abbott, 1982; Luttrell et al., 2003) indicate that it is essential to maintain the same density cut point for each spiral in a spiral bank to achieve the maximum yield from a spiral circuit. In addition, it is well known that preparation plants operating with multiple parallel cleaning circuits for different size fractions of coal can provide the best possible plant yield for a given product quality only if the incremental product quality achieved in each cleaning circuit can be maintained at the same level, a condition that requires the density cut point for each cleaning circuit to be the same.
The spiral automation system, named SIU\'s Smart Spiral Component (SSSC), operates on the principle that electrical conductivity of solid particles varies with different types of solid materials. Clean coal particles are known to be generally less conductive than inorganic mineral particles. In fact, a 2nd order polynomial relationship between electrical conductivity and solid density was successfully fitted to experimental data meaning higher density materials resulted in higher electrical conductivity or, in other words, lower electrical resistivity. The SSSC includes two conductivity-based sensors, conductivity measurement circuitry, a PIC microcontroller, two tabular solenoids, a DC motor, and a newly designed splitter box. Each of the two sensors consists of two stainless steel rings placed inside a sampling tube, equipped with a bottom plug controlled by a solenoid. The two sensors are used to establish the density gradient in the critical region (about 7 inch long) across the spiral trough at the discharge end. These sensors measure the conductivity of a settled pack of solids, which is created during the short time period (a couple of minutes) when the bottom of sampling tube is plugged by turning off the solenoid. After the conductivity of the solid pack is measured in each sampling tube, the density gradient between the two measurement points across the spiral trough is established. Based on that measurement and the difference between it and the previous measurement, a PIC24 microcontroller actuates the DC gear motor to turn clock-wise or counter-clock-wise or to stay at the same position. Both solenoids are then turned on to empty the sampling tubes and complete one control cycle. The control cycle time for the spiral control system in a plant environment is 30 minutes, which can be varied as needed.
The aim of this in-plant study was to demonstrate the functionality of the automation system by operating it on a continuous basis for several months in a real plant environment. The Creek Paum plant, one of Knight Hawk Coal Company\'s preparation plants operating with two cleaning circuits (i.e., heavy medium cyclone and spiral) served as the host site for this project. One of the four triple-start spirals from the spiral bank was replaced by a new set of smart spirals (triple-start spiral fitted with a SSSC). A new splitter box was designed and fabricated to fit to the discharge end of the triple-start spiral. The electronic printed circuit board (PCB) was designed at SIU, but fabricated and assembled by commercial vendors (Sunstone and Streaming Circuits) to insure structural integrity and reliability in the plant environment. The Creek Paum plant treats coal originating from three different sources with the Hawk Eye Mine being the predominant source. Although the control system was developed with optional switches for three coal types, it was only used for Hawk Eye coal because of the significant number of tests required for calibration and the infrequency with which the other two coals were processed. After the system was calibrated and optimized, a long-term test was conducted over a period of nearly three months to compare the performance of the smart spiral with that of conventional spirals operating in the plant. A total of thirty sets of samples were collected on different operating days for the Hawk Eye coal. An open demonstration of the smart spiral operation, conducted during this long-term test period, was well attended by more than two dozen professionals representing various coal-related organizations in Illinois and beyond.
Comparative clean coal recovery values obtained from smart spirals and conventional spirals, respectively, at a variety of levels of ash rejection were as follow: 91.8% vs. 91.3% at an ash rejection level of 65.7%; 92.3% vs. 89.8% at an ash rejection level of 67.3%; 93.1% vs. 89.9%, at an ash rejection level of 69.7%, and 92.8% vs. 92.7% at an ash rejection level of 71.2%. The improvement in clean coal yield obtained from the smart spiral varied over a wide range, but averaged around 2%. For a triple-start spiral capable of treating nearly 10 tons per hour (tph) of raw coal, this translates to a gain of 0.2 tph of clean coal. Over the course of a year, 800 to 1500 additional tons of clean coal can be sold generating $40,000 to $75,000 of added revenue for each triple-start spiral. Clearly, this strong financial gain far outweighs the added cost of about $450 for the automation system component, justifying quick commercialization of the spiral automation system demonstrated in this project.