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Dry processing ● Dry Jigging of Coal

Summary: The first of the newer generation dry jigs has been commissioned in 2002 in the USA and more than 70 followed in recent years. This paper describes the advantages of the dry jig process. It also presents operating results from several dry coal beneficiation plants in the USA, India, Columbia, Spain and Turkey.

1 Introduction

There are a number of advantages for upgrading coal with dry jigging technology, most notably the lack of process water, which eliminates the need for fines dewatering and slurry confinement. Dry jigging eliminates the product moisture penalty associated with wet processing of thermal coals. Particulate emissions are virtually nonexistent due to inclusions of fabric dust collectors on all modern dry jig plants. Due to the environmentally kind nature of dry jig plants, installation permits are often measured in days, not months. Coupled with all this advantages there has been a fundamental advance in dry jigging efficiency. All of these advantages have renewed the coal industry’s interest in dry jigging.

Air jigs are generally similar to Baum jigs, but they employ air, rather than water to stratify coal and rock and thereby effect a density separation. Pneumatic separation was introduced to the U.S. coal industry prior to 1924 in the form of the air table, a device with many similarities to a Deister table [2]. Commercial use of the air jig commenced in the early 1930’s in the U.S., and became the dominant technology for dry cleaning coal. The original design of the air jig changed only modestly over the decades, and the last commercial plants in the U.S. were decommissioned in the early 1990’s.

In the late 1990’s, a new air jig design was introduced to the coal industry by allmineral. The new air jig employed a different air fluidization system, a different air distribution system, and an instrumentation and control system for controlling the separating gravity. Since 2001, new air jig plants have been installed in the United States, Colombia, Spain, Ukraine and India for both thermal coal and coking coal.

2 Operating principles

The commercial modern air jig operates with a 1.2 m x 2.4 m inclined vibrating deck. The deck vibrators are provided to induce movement of raw coal from the feed end of the deck to the discharge end; the vibration does not have a material effect on the stratification of the coal/reject bed in the machine. Two sources of air provide the energy for stratifying coal and rock: a constant flow to provide general fluidity and the stratifying pulse. The movement of coal and rock particles in the material bed is in many ways similar to the movement of particles in a conventional water-pulsed Baum jig. The steel punch plate deck is designed to support the coal/reject bed and to control and evenly distribute air throughout the bed.

Feed coal is introduced to the deck at a controlled volume using a rotary star gate. As the raw coal feed is transported down the deck, the higher density rock particles settle upon the perforated deck plate and form a thick layer of dense material at the bottom of the material bed. The lower density coal particles migrate to the top of the material bed to form a layer of low-ash coal. A nuclear density gage is installed near the discharge end of the deck to read the density of material across the full width of the deck. As the thickness of the rock layer increases, the density reading increases. When the separation set point is reached the analog control system proportionally increases the speed of the discharge star gate, thereby accommodating changes in feed quality without sacrificing recovery or product quality. The original air jig designs required manual adjustment of discharge gates to deal with changes in feed quality.

The volume of constant air is controlled with the blower damper or variable frequency motor drive. The pulsed air flow is similarly controlled, but also includes pulsations, which are generated with a flutter valve. Dust form the coal is collected in a baghouse, and can report as either product or reject.

3 Applications

The air jig can be used to process -6/+0 mm feeds, -50/+0 mm feeds or sized feeds, such as -50/+6 mm. The coarser-sized feeds can typically be cleaned at higher capacities than finer-sized feeds. Air jigs require discrete particles in order to make an effective separation, the feed to the air jig needs to have a relatively low surface moisture content: 6 % surface moisture or less is generally considered desirable.

It should be noted that inherent moisture is not relevant in the consideration of suitability for cleaning in an air jig. A lignite or subbituminous coal containing 30 % moisture or more would be perfectly acceptable provided the surface moisture content was less than 6 %.

Air jigs are less efficient than comparable water-based cleaning processes, so the ideal applications for air jigs are in those instances where water is unavailable or problematic. They have been employed to clean coals to improve market value and in some instances reduce tax on coal products [3]. A small mining operation that must truck high-ash ROM coal a long distance to a central washery is another suitable (destining) application. An air jig can remove much of the rock in the raw coal with little product loss, thereby reducing haulage cost, processing costs, and the cost of reject disposal at the central washery.

Low rank coals are ideal applications for dry cleaning with air jigs. In many instances, a low rank coal can be contaminated with enough dilution or parting to make blending with inspec product impractical. If only a 5 % reduction in ash is required, say from 15 % ash to 10 % ash (dry basis), the energy gain from ash reduction in wet cleaning will be substantially offset by a 5 % gain in surface moisture. Cleaning the same feed with an air jig provides the same benefit in ash reduction without the moisture gain from wet processing [3].

Air jigs are well suited for separation of low near gravity feed types and for separation of material with 1.8  relative densitiy (RD) and higher. Although the air jig has been used for separation of coal at gravities as low as 1.50 RD in lignite, there is a sacrifice of recovery and quality compared to wet processing. If the percentage of near gravity material (NGM) exceeds 10 %, wet processing will usually prove to be the best option.

Specific performance results from processing coals of different countries are shown in Table 1. Dependent on the raw coal, the results can show substantial variations in product quality. The Turkish and Indian raw coals with up to 55 % ash (dry basis) produce clean coal products with ash varying from 15-35 % on dry basis.

4 Comparison to wet processing for high and low near gravity separations

In Fig. 2, the near gravity material curves of two coal samples from Mongolia are plotted. Wherein the sample 1 contains a high amount (15-52 %) NGM for a separation density range of 1.5 to 1.6 RD, the quantity of NGM for sample 2 (3-11 %) indicates an easier separation of the coal. This is also reflected in the comparison between the wet (side pulse jig and dry process results shown for one coarse and one fine coal test in Tables 2 and 3.

For a target ash value of approximately 16 % on dry basis (sample 1) the yield is about 67 % for a wet (side pulsed jig) process, whereas only 37 % (dry basis) can be achieved in a dry process.

In sample 2 with a low content of near gravity material a yield of almost 51 % (dry basis) can be achieved, which is approximately 10 % (dry basis) below the reachable yield in a wet process (side pulsed jig) process for a <12 % (dry basis) target ash value. This indicates air jigs are more likely suited to low NGM separations in applications where lower grade specifications are produced for blending purposes.

5 Utilisation of air jig rejects

Applications exist where both air jig product and reject can be used for power generation, which is common practice in India where coal processing with high percentage of near gravity material is frequent [4].

The concept applied there is to have air jigs to clean the raw coal to 35 % ash (dry basis) for use in pulverized coal boilers and to blend air jig reject with the dust collector fines, which will reduce the reject ash below 60 % for use in fluidized bed boilers. This concept can maximize the use of coal resources and minimize environmental impact [4].

6 Performance

A single 1,2 m x 2.4 m air jig has a nominal capacity of 30 to 60 t/h, depending upon the mean particle size. Table 4 shows general guidelines for air jig cleaning capacity.

The capacities per jig can be doubled by using the recently developed type “allair MP” with two jigging beds in one unit giving capacities of up to 120 t/h. The combination of residual energy (air pressure and volume) from the jig bottom layer surface with an additional added airflow from the main blower (via bypass) is used inside the same jig plant footprint [4]. An example for a 100 t/h dry beneficiation plant with two jigging machines including ancillary equipment is show in the lead picture.

7 Conclusion

Although wet processes have higher efficiencies and achieve higher product grades at higher yields, the dry jigging process is an alternative especially in areas with lack of water.

Wet processes require large water reclamation installations including thickeners, filtration equipment and pumps, which is not needed for the dry jigging process. No slimes are generated and the fine product of the air filter can at times be included in the final product. The handling of the dry product is simpler and compared to a wet/dewatered product with surface moisture the carbon content can be lower for an equal calorific value.