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12th International Coal Preparation Congress, May 23-27, 1994, Cracow, POLAND

 C-1                                      

THE STATE OF HARD COAL GRAVITY CLEANING PROCESS

David W. Horsfall (1), Stanistew Btaszczyriski (2), Witold Z. Drogori (3)
(1) Department of Metallurgy
University of the Witwatersrand
Johannesburg, South Africa
(2) Department of Mineral Processing and Waste Utilization
Silesian Technical University
Gliwice, Poland
(3) SEPARATOR
Main Study and Design Office for Coal Processing Katowice, Poland

1. INTRODUCTION

     Gravity cleaning processes are closely associated with operations of feed coal pretreatment and dewatering of products.
     Due to the variety of types of machines and equipment used for feed coal pretreatment, cleaning and dewatering of products, the gravity cleaning process may be systematized into 3 groups:
1 group - cleaning of coarse and medium coal sizes above 20(10) mm,
2 group - cleaning of small sizes within the range of (20)10+2(0.5) mm,
3 group - cleaning of fine coal sizes below (2)0.5 mm.
The subject of this elaboration are the basic types of machines being applied in foreign and Polish coal preparation plants with particular consideration given to cleaning of fine and ultra fine coal sizes.

2. CLEANING OF COARSE AND MEDIUM COAL SIZES

 The cleaning of coarse and medium coal sizes is carried out mainly by means of dense magnetite medium separators of DREWBOY type for coal sizes above 10 mm, WEMCO, TESKA and DISA for the range of 200+10 mm coal as well as in jigs employed for 150+10 mm
coal sizes, LARCODEMS separators for cleaning of 100+0.5 mm feed coal should also be mentioned here.
Separators have the highest cleaning efficiency. Imperfection index of separators and jigs amount to about 0.05 and 0.1 respectively.
    In Polish coal preparation plants processing of coarse and medium coal sizes is done mainly in DISA baths for the range of 200+20(10) mm size of up to 400 tph capacity.
The feed coal pretreatment for cleaning in separators and jigs as well as dcwatering of products is done on screens.
In Polish coal preparation plants, for dry screening of 200+0 mm coal into 200+20(10) mm and (20)10+0 mm coal sizes curvilinear screens of PZ type and capacity up to 700 tph are preferred.

3. CLEANING OF SMALL COAL SIZES

The beneficiation of small coal sizes is performed mainly in dense magnetite medium cyclones for coal size range 30+0.5(0.16) mm or in jigs of HUMBOLDT WED AG, В АТАС, ALLMINERAL and KOMAG types for size range 20+2 mm, 20+0.5 mm or 20+0.16 mm.
In Polish coking coal preparation plants, cleaning of small coal sizes is done by means of KOMAG type jigs for the size ranges 20+0.5 mm or 50+0.5 mm of capacities up to 500 tph and 800 tph respectively. The cleaning of small coal sizes in new fine steam coal preparation plants is performed in jigs of KOMAG or ALLMINERAL types.
Cyclones are characterized by the highest cleaning efficiency. Imperfection index of cyclones and jigs for 20+2 mm and 20+0.5 mm coal sizes is about 0.08, 0.12 and 0.15 respectively.
Cleaning of coal in cyclones require more of both investment and operating costs in comparison with separation in jigs.
   The final dewatering of clean coals and intermediate products from jigs and cyclones is realized in dewatering machines SIEBTECHNIK, HUMBOLDT WEDAG and WEMCO, while final dewatering of rejects is done on screens.
Dewatering of clean coals and intermediate products in Polish coal preparation plants is carried out mainly in vibrating centrifuges of WOW-1.3 type with capacity approx. 200 tph.

4. CLEANING OF ULTRA FINE COAL SIZES

 The cleaning of ultrafine coal sizes is conducted first of all by means of spiral separators of REICHERT type for coal sized 2+0.5 mm and 0.5+0.04 mm or 2+0.1 mm, hydrocyclones - for cleaning of 0.5+0.1 mm coal size and with flotation machines of WEMCO, HUMBOLDT WEDAG, DENWER and ALLFLOT types for 0.5+0 mm or 0.2+0 mm sizes of coal.
    In Polish plants for coking coal processing, flotation machines of YL-YL type and capacity approx. 700 m/h are applied mainly as well as vacuum filters FTC-150 of up to 40 tph capacity.
The spiral separators are characterized with the highest cleaning efficiency, while the hydrocyclones have the lowest - imperfection index 0.35.
    The operation of feed coal pretreatment for spiral separators, hydrocyclones and flotation machines is executed in classifying cyclones.
    Dewatering of clean coals of 2+0.5 mm size from spiral separators is performed in screw CONTURBEX dewatering machines, and rejects are dewatered on screens.
    Dewatering of clean coals from 0.5+0.04 spiral separators, hydrocyclones and flotation machines is done by means of vacuum filters or screen-bowl centrifuges, while rejects are dewatered in chamber presses, belt presses or screenless centrifuges.
     In new Polish fine steam coal preparation plants the following machines have been employed for dewatering of fine coal sizes:
— ALLFLOT flotation machines of ALLMINERAL make - Jankowice colliery,
— HBF-96 hyperbaric filters from ANDRITZ AG - Siersza and Jaworzno collieries as well as FTC-150 vacuum filters - Jankowice colliery,
— belt presses of CPF-2200 type - Siersza colliery,
— chamber presses of ROW-570 type - Jan Kanty colliery,
— CONTURBEX screw type dewatering machines of SIEBTECHNIK make - Janina colliery.
  Conceptual process flowsheet of water-slurry circuit with cleaning of slurry in cyclones or flotation machines for home steam coal preparation plants is shown in Fig.1.

Fig. 1. Process flowsheet of a water-slurry circuit with flotation of slurry.

    Fig. 1 presents dewatering of clean coal on vacuum or pressure filters as well as dewatering of rejects in chamber or belt presses. The selection between vacuum filters and pressure ones depends on required free moisture content in the blend of 20+0.5 mm and 0.5+0 mm clean coals. The selection of press type depends on the efficiency of dewatering operation, number of presses employed as well as the amount of investment and operating costs. Dewatering done by means of belt presses involves lower investment costs and higher operating costs in comparison with chamber presses.

5. TECHNOLOGICAL MODELS OF DESIGNED PLANTS FOR 20+0 mm COAL

The technological models of Polish preparation plants for medium- and high-calorific fine steam coals worked out by SEPARATOR, Katowice are specified in Table 1.
    Table 1. Plant models.

The machines used for dewatering of products in plant models are listed in Table 2. 
    Table 2. Machines for dewatering of products.

Fig. 2. Process flowsheet of 20+0 mm fine coal preparation plant.

The process flowsheet of the I plant model is shown in Fig. 2. The characteristic feature of plant models is dry screening of the part of small coal sizes of 5+0 mm and leaving them uncleaned. The advantage of the models is reduction of capital-consuming water-slurry circuits as well as a possibility of 3 sorts of saleable coals to be produced, i.e. raw fine coal, cleaned fine coal and a blend made of raw and cleaned fine coal. The fault of the plant models is uncleaning of the part of fine coal. The I and П models present economical solution of a plant due to limited investment means involved.

6. CONCLUSIONS

1.The machines used for pretreatment, cleaning and dewatering of coarse and medium coal sizes are characterized by high capacity and technological efficiency as well as good material-consumption and energy-consumption index.
2.The machines employed for cleaning of small and ultra fine coal sizes have sufficient both capacity and technological efficiency.
3.The development should be oriented especially for machines for dewatering of small and ultra fine coal sizes.

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