Plant Design And Construction

Coal preparation plant

Process Design There is no consistent layout for new coal preparation plant currently being installed, nor is there any universally-accepted design that covers all operations. As discussed earlier, particular technologies are favoured for a number of reasons. These include the physical or chemical properties of the coal to be treated, the product specification, the range of products required, the supporting infrastructure, the perceived cost, the complexity of operation and, not least local preference for familiar or indigenous technologies. Consequently, coal preparation plant designs are many and varied. However, two major categories of plant have emerged: total-washing systems and partial-washing systems. Figure 17 compares flowsheet examples of both types of washing system.

Total-washing systems meet the markets for high-quality premium products (eg metallurgical coal), requiring a relatively complex flowsheet design. These systems are common in Australia, the RSA and the USA.

Partial-washing systems are aimed at the preparation of PSF, using a less complex flowsheet design, where untreated fines are extracted by dry screening before the washery and then blended back with the washed coal to meet the PSF specification. These systems are common in the UK and the RSA and are also considered suitable for the preparation of PSF in India. The example shown in Figure 17 is for a simple de-stoning flowsheet without the beneficiation of fine coal.

Coal preparation plant can also be classified according to the main separating process employed, ie either jig washing or DM separation. Coal washability can vary considerably throughout the world and this variation is reflected in the common choice of process technologies in each region. Figure 18 shows the densimetric profiles of a number of coals.

In general, because of the potential for higher accuracy of separation than jig washing, a coal preparation plant using DM technology is a more common choice for difficult-to-wash coals. For coals that are easier to wash, the benefits of increased accuracy of separation are less clear and the preferred choice may be either DM separation or jig washing.

A significant proportion of the world’s coal production, eg coals found in the RSA, are Gondwana in origin and are typified by high levels of ash, finely disseminated throughout the coal matrix. These coals are difficult to wash and DM systems are favoured for separation of the plus 0.5mm fractions. One exception is India, another Gondwana coal region, where jig washing has been favoured in the past, possibly because the relatively poor yield of clean coal for metallurgical use produced by jig washing has been compensated for by the use of jig discard as a saleable PSF.

For the easier-to-clean coals, typically found in the Northern hemisphere, jig-washing processes can often achieve efficient separations, depending on the quality of the product required. However, even for these coals, there is now a general trend towards the increased use of DM systems due to significant tightening of product specifications and increased price penalties for variations from these specifications. This trend has also been aided by the change in relative economics of the two competing processes.

Isaac Plains Coal Mine, Australia

Traditionally, it has been perceived that the capital and operating costs of DM plant are higher than those of jig washing. This perception has changed gradually over the past decade, largely through the introduction of large-diameter DM cyclones. This has allowed the use of a single DM cyclone process instead of the traditional use of DM static baths and cyclones in combination. This has greatly simplified circuit design and reduced the capital and operating costs of DM plant.

For fine coal cleaning, froth flotation has been the traditional choice. However, in the past decade, the widespread adoption of spiral concentrators and teeter-bed separators has redefined the scope of fine coal cleaning. In effect, these techniques, together with the use of scroll centrifuges to dewater the clean coal, have introduced a low-cost, intermediate processing stage that can be used in combination with other coarse and fine coal cleaning processes to allow a wide range of plant configuration to meet individual requirements. As an added benefit, the introduction of this intermediate stage has allowed the desCiros of the DM feed to be carried out at a coarser size (eg 1-1.5mm), rather than the traditional 0.5mm. This had made desCiros easier and has reduced the carryover of slimes to the DM plant.

Another significant influence on the design of fines treatment plant has been the introduction of selective flotation processes, such as the flotation column. These processes are now used, often in combination with the fine-coal density separation techniques, to increase process flexibility and improve product quality.

With increasing concerns over discharge of effluents from processing plant, tailings treatment has continued to be a major issue in coal preparation plant design. In this area, a major influence in recent years has been the introduction of MRFs for final tailings dewatering as a lower-cost alternative to the traditional plate and frame filter presses. However, final tailings dewatering remains a high-cost activity and is still avoided where possible, with thickened tailings being discharged to lagoons for further settlement and dewatering.

Plant Construction

Traditionally, coal preparation plant have been tall structures with multiple levels to maximise the use of gravity flow. The supporting steelwork and building cover has also tended to be an integral structure. More recently, particularly in the western industrialised countries, there has been a trend towards construction of plant with single-level operating floors, with much of the material transfer being pumped between processes. Despite the larger footprint of low-level structures, there are advantages in costs of construction and accessibility for maintenance. A single-level structure also allows much more flexibility in terms of modification of plant equipment and the introduction of new systems.

Another well-established trend is towards modular plant designs. These allow the flexibility to incorporate new processes within the flowsheet or to increase throughput by the addition of new modules in parallel. Modular plant can also be readily dismantled for transfer to another site. Characteristically, modular plant are constructed in relatively simple, singlelevel, steel box structures for fast construction and portability.

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One Response to “ Coal preparation plant ”

  1. Demand:coal preparation and beneficiation plant | Materials: Coal Mine | Customer from South Africa Says:

    Please contact me regarding our inquiry for coal preparation and beneficiation plant required for South Africa

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