The main purpose of a Coal Handling and Preparation Plant(CHPP) is to remove impurities from raw coal, enhancing its quality and value for various uses, such as power generation or steelmaking. Here’s a step-by-step look at how it works:
Receiving and Handling
Raw coal arrives at the CHPP from the mine, via conveyor belts or trucks. This raw coal, often called run-of-mine (ROM) coal, contains various impurities, including rocks, dirt, and other debris. The first step is screening, where the coal is passed over screens or through vibrating screens to separate and remove large rocks and debris. This initial screening helps in reducing the size of the coal and prepares it for further processing.
Crushing
The screened coal is then subjected to crushing, where it is broken down into smaller pieces. Crushing makes the coal easier to handle and process. The coal is usually crushed using crushers such as jaw crushers, impact crushers, or hammer mills. This process reduces the coal to a more uniform size, which is essential for the subsequent steps.
Screening
After crushing, the coal undergoes further screening to sort it by size. This involves passing the crushed coal through a series of screens with different mesh sizes. Each screen captures a specific size range of coal particles. The sorted coal is then directed to different processing streams based on its size. Smaller particles may go directly to washing, while larger particles may require additional crushing.
Washing and Cleaning
Dense Medium Separation: The coal is mixed with a dense medium, which is a liquid with a specific gravity between coal and impurities. This medium can be magnetite or ferrosilicon. Since coal is lighter than the dense medium, it floats, while heavier impurities sink. This separation process effectively removes a significant portion of the impurities.
Water Washing: The floating coal is then washed with water to remove remaining soil, rock, ash, sulfur, and other contaminants. Water washing involves passing the coal through various water-based processes, such as jigs, classifiers, and froth flotation cells.
Cyclones and Spirals: Cyclones use centrifugal forces to further clean the coal. The mixture of coal and water is fed into a cyclone, where the centrifugal force causes the heavier impurities to be thrown to the outer edges and collected as underflow, while the lighter coal particles are carried upward as overflow. Spirals work similarly, using gravity and centrifugal forces to separate impurities from the coal as it flows down a spiral-shaped channel.
Dewatering
Excess water is removed from the coal using screens, centrifuges, or filters. Dewatering ensures that the final product has the appropriate moisture content for storage and transport. This step is crucial because too much moisture can lead to handling issues and degrade the quality of the coal.
Blending and Mixing
Cleaned coal of different qualities and sizes may be blended to achieve a specific product quality. Blending ensures consistency in the quality of the coal, meeting the specifications required by different customers or applications. This step may involve mixing coal from different sources or adjusting the proportions of various coal types.
Storage
The processed coal is stored in stockpiles or silos until it is ready for transport. Proper storage conditions are maintained to prevent spontaneous combustion and degradation of coal quality.
Transport
The cleaned coal is loaded onto conveyors, trains, trucks, or pipelines for delivery to customers or further processing facilities. Transport systems are designed to minimise handling losses and ensure that the coal reaches its destination in good condition.