Resource-saving technologies of combustible matter recovery from coal�bearing wastes

Pozdeyev V.N.
OJSC �KENES�, Lyubertsy, Moscow region, Russia

Mikhaltsevich V.V.
OJSC �KENES�, Lyubertsy, Moscow region, Russia

Lyashenko A.P.
OJSC �KENES�, Lyubertsy, Moscow region, Russia

Abstract: In the last decade the quality of saleable coal from coal deposits mined by surface mining method has degraded.

To obtain the required output thin coal seams of complex structure have to be mined, that does not allow to produce coal of desired quality.

One of the directions of resource-saving technology aimed at the improvement of quality of coal from surface mines is coal cleaning at coal preparation plants (CPPs) with the use of steeply inclined separators of KNS type .

The flowsheets including steeply inclined separators KNS are simple. They do not require installation of a number of primary and auxiliary equipment and allow to achieve high performance.

The resource-saving technology of cleaning coal diluted with waste rock in steeply inclined separators of KNS type at the CPPs makes it possible to reduce losses of coal at surface mines and provide maximum recovery of combustible matter and high efficiency of the process at low capital and operating costs.

In Russia some 270.0 million t of coal are mined by surface mining method.

With the existing technology of surface mining and the current level of mechanization no more than 48-50% of coal is extracted from thin coal seams in Kuzbass. Up to now the bulk of coal from the diluted part of workable and adjacent seams is discharged to the dumps together with the overburden. This coal contains up to 30-40% of combustible matter and has ash content of 40-70%. It is characterized by extremely non-uniform composition and it cannot be used as power-plant fuel without cleaning.

The works aimed at improvement of quality of mined coal and reduction of its losses are carried out in two directions: improving of mining technology and using of efficient process of run-of-mine and waste rock diluted coals cleaning.

The analysis of physical and mechanical properties of coals discharged to the dumps shows that , as a rule, their cleaning by conventional gravity methods is not rational. Because of high and varied content of rock fractions which are often represented by easily soaking clay inclusions, the use of dense medium coal preparation processes requires significantly more complex flowsheets at the CPPs, the weight material regeneration becomes particularly difficult and the cost of fuel produced sharply increases.

Jigging can prove not rational, as the efficiency of this process drops when the content of concentrate and middlings fractions is less than 50%. In addition, at higher content of rock in the feed the capacity of jigs substantially decreases.

The Institute of Solid Fossil Fuel Preparation (IOTT) has developed and introduced at a number of plants in Russia, Ukraine and Kazakhstan a technology of cleaning the diluted run-of-mine material using counterflow gravity separators KNS. This technology is also used for mechanical removal of rock from coarse (+25mm) high-ash coal fractions at mines and for cleaning of coarse-grain slurries.

Since 1991 the Research - Innovation and Designing Company "KENES", Russian Federation, Lyubertsy, established on the basis of the IOTT Institute, has been involved in designing and introduction of high-ash coal and slimes cleaning technology.

The shareholders which have the controlling interest in the OJSC �KENES� hold the Patent No 2038854 for invention of the � Device for counter flow gravity separation of minerals� granted on 26 August 1992.

The steeply inclined separator (see Figure 1) consists of a housing of rectangular section inclined at 52-560 to the horizontal. In the middle of the housing a feed pipe with a feed hopper on the top is located. In the upper part of the separator channel section the rock and concentrate decks with partitions are mounted. The position of decks in the channel can be changed using the adjusting devices usually located on the upper cover of the separator.

Each deck is suspended at two points and can be adjusted separately. Using the adjusting devices the decks can be moved parallel to the bottom of the channel, or they can be positioned obliquely when required. The decks are connected to the cover of the separator through rubber collars preventing coarse particles of feed and directed flow of separating medium from entering the nonworking space between the decks and the cover.

Figure 1. Installation diagram of KNS separator
1 - housing; 2 � rock deck; 3 - concentrate deck; 4 - deck adjustment devices; 5 � feed pipe; 6 � feed hopper; 7 - dehydrating elevator.

The bottom part of the separator is connected by means of a flange and an adapter tray to the dehydrating elevator 7 for rejects removal, and its upper part is terminated with a tray for concentrate discharge.

Water is supplied to the separator at two points. Generally water is supplied into the elevator shoe and passes through it into the rock channel of the separator. As a rule, water is additionally supplied to the concentrate channel together with the initial feed providing for required separation density.

The raw material to be cleaned is supplied continuously through the feed pipe to the central part of the separator channel. At the same time, water is supplied at a given flowrate through the elevator shoe to the bottom part of the channel. Heavy fractions precipitate to the bottom layer which moves in the opposite direction to the water flow. Light fractions are carried by the flow upward through the overflow lip of the separator.

The areas of partitions, which are the zones of local increase of flow rate, are at the same time the multiple zones of increased loosening of material. The down flow carrying heavy particles of material is regularly loosened and compacted and releases light particles in the upward current area.

Thus, along the full length of the working area of separator, a counter flow cleaning process primarily by gravity separation takes place.

The main design parameters of separator adjustment are flow areas of its rock and concentrate channels.

The range of these parameters for each type of the KNS separators is known. At the precommissioning stage optimal values of flow areas of rock and concentrate channels of the KNS separator for each type of material to be treated are selected based on the experience.

The main operating parameters to be adjusted at the KNS separators are the following:

  • capacity;
  • water consumption in rock section;
  • water consumption in concentrate section.

The total consumption of water supplied to the KNS separator is adjusted by a level control device. This device controls water level in the elevator, which determines water flow velocity in the KNS separator , and thus, the separation density.

The higher is water level in the elevator, the greater is water flow and velocity with which it passes through the KNS separator, and consequently, the separation density.

On condition that the flow areas of rock and concentrate channels of separator are well selected for the given material, the water level is set by a level control device and the throughput is stable, the operating efficiency of the KNS separators in one-stage cleaning of high-ash coal is ��m=120-140 kg/m3 at separation density of 1700-1800 kg/m3. The separation density in the KNS separators varies within 1400-2000 kg/m3.

The purpose of the KNS separators determines their design philosophy.

At present the KNS separators are used for cleaning of classified and unclassified coal of 0-150mm size with a unit capacity of up to 400 t/h. The channel width in the separators comes up to 1600 mm.

For cleaning of 0-13(25) mm coal a standard size of separators with a capacity up to 150 t/h is produced.

The standard sizes of the KNS separators intended for cleaning of slimes include separators with a capacity up to 100t/h.

Separators for cleaning of fine coal and slimes have specific design features, such as 1.2-1.5 times greater width of concentrate channel as compared with the rock channel, reduced distance between deck plates and reduced height of flow areas of separator. The reason for it is the necessity to reduce the turbulence in the separation area during density separation of fine material.

From 1974 to 1998 twelve (12) single-stage plants equipped with the KNS separators for coal recovery from coal-bearing overburden removed during surface mining operated in Russia. These plants provided cleaning depth of 0.5(1)mm and operated from May to October.

They produced clean coal with 17-19% ash content. The slimes of 0-0.5(1)mm were not cleaned and were discharged to sludge ponds of surface mines or external settling ponds.

In 1991 �KENES� Company developed single-stage seasonal modular units, and over the period of seven years 5 units of this type with a capacity of 150 - 220 t/h were put into operation. They comprise:

  • a traveling bin with a feeder and a grate of 300x300 mm. The bin can move along the conveyor;
  • a unit for pretreatment of 0-300 mm coal with waste rock before cleaning, consisting of 3 modules : a bin with a feeder, a grate with slits of 100-150 mm and a jaw-crusher for crushing of over-sized material to less than 100-150 mm;
  • a preparation unit for coal with rock consisting of 11 modules equipped with a dehydrating elevator, a screen, a KNS 108L or KNS88A steeply inclined separator depending on unit capacity, a sump for slurry water, a pressure tank, a pump and classifying hydrocyclones.

The raw coal with rock is loaded by an excavator or a flight conveyor into the traveling bin for run-of-mine material. Lumps of more than 300 mm size are separated by grates located on the top of the bin. 0-300 mm fraction is supplied by the feeder to the belt conveyor and then is transported to the pretreatment unit , where 100(150)-300 mm fraction is separated by grates and then is crushed to less than 150(100) mm and combined with the undersize. The run-of-mine material of 0-150(100) mm is supplied by conveyor to the steeply inclined separator where rock and concentrate are separated. The rock is dewatered by the elevator and the concentrate is dewatered by the screen.

The dewatered rock and concentrate are transported to corresponding storage facilities. Slurry water is thickened in cyclones and is rejected into external slurry ponds.

The water-slime circuit is closed through external slurry ponds (sludge tanks). The unit is made of dismountable modules which can be manufactured and equipped at a factory. The modules can be assembled in a very short time. The adaptation to local conditions is specific for each unit.

Due to toughening of market requirements to saleable coal quality, since 1995 Russian coal companies have started ordering the facilities for production the end product with ash content of 6 - 12(14)% depending on the coal rank.

The �KENES� Company has developed and the �Kuzbassrazrezugol� coal company (Kemerovo city) has realized the projects and modified the existing single-stage units (to allow for the reduction of ash content of fine coal of 0-25 mm size and cleaning of 0-1mm slimes which had been previously discharged into sludge ponds), as well as double-stage units with cleaning depth up to 0.2 mm.

A typical double-stage unit of this type is a seasonal preparation plant used for coal recovery from coal-bearing overburden rock at Krasny Brod surface mine of �Kuzbassrazrezugol� Coal Company.

Figure 2 shows the circuit diagram of the preparation plant equipment.

The plant was built in 2006. Its capacity is 280 t/h. The coals processed are a mixture of �� (low-caking) and � (lean) ranks with average ash content of 54%.

The unit produces saleable coal of +25-150mm size with ash content up to 8%, moisture content of 7% and 0-25mm size with ash content of 11 - 14% and moisture content of 9%.

The ash content of saleable coal of 0-25mm size varies according to market requirements.

The total yield of saleable coal is 30-32%. The average ash content of rejects including slimes is 74%.

Figures 3 and 4 are the pictures of the coal preparation plant in Erunakovskay and a CPP with a steeply inclined separator at Taldinsky surface mine of �Kuzbassrazrezugol� Coal Company.

The preparation process of run-of-mine material before its cleaning in double-stage units is the same as for single-stage units. The difference appears after the first stage of cleaning (rough cleaning) of 0-150mm size run-of-mine material in the steeply inclined separator KNS-138.

The rough cleaning concentrate is dehydrated on a double-deck screen where two coal fractions of 25-50mm size and 1-25mm size are separated.

Coarse coal of 25-150mm is transported directly to the storage facility as it is free from gangue constituent.

1-25mm coal has ash content of up to 22% and is supplied to the steeply inclined separator KNS 80/120 for the second stage of cleaning.

Figure 2. Circuit diagram of preparation plant equipment
Legend: 1- Steeply inclined separator KNS 138L 2- Dehydrating elevator EO-10� 3- Screen GISL 62 4- Steeply inclined separator KNS 80/100 5- Dehydrating elevator EO-4S 6- Screen GISL-62 7- Centrifuge FVSh-100 8- Pump 9- Battery of hydrocyclones GTsCh 360.� 10- Steeply inclined separator KNS 40/60 11,12- Vibrating screen SV-1 13- Clarification tank 14- Battery of hydrocyclones GTsCh 360 15- Screen GISL-62 16-Pump

Figure 3. Coal preparation plant with a steeply inclined separator in Erunakovskay

Figure 4. Coal preparation plant with a steeply inclined separator at Taldinskiy surface mine

The concentrate is dehydrated by a double-deck screen with openings of 8 mm and 0.5mm. The 8 mm fraction leaving the screen is then dehydrated in a screw centrifuge.

To reduce ash content of 0-25 mm size concentrate the recirculation of the second stage rejects to the first cleaning stage is provided. In this case the ash content of the second stage concentrate will not exceed 8%.

In this coal-cleaning flowsheet the first stage will yield clean rock and the second stage will yield clean concentrate.

In this case the efficiency of density separation of material in two stages can be calculated as for a single apparatus which yields two end products.

According to calculations, the efficiency of cleaning of 1-150mm coal in two stages in the steeply inclined separators is ��m=100 kg/m3.

The 0-1mm slime is cleaned in two stages. At the first stage the slime is treated in hydrocyclones.

Coal of +0.15-0.8 mm is efficiently cleaned in hydrocyclones, therefore the concentrate 1 (overflow of hydrocyclones) is directed for thickening in hydrocyclones with a diameter of 360mm, where the main part of 0-0.15mm fraction is separated, partly returned to the first stage of the process and sent to clarifying tanks or press-filters, and the concentrate of +0.15-0.8mm size is dehydrated on the bended sieve, the screen, in the screw centrifuge and is fed to the fine coal conveyor.

The underflow of hydrocyclones is supplied to the bended sieve where high-ash 0-0.5mm coal fraction is separated and discharged to wastes and 0.15-1mm slime is recleaned in a steeply inclined separator KNS for slimes. After having left the steeply inclined separator the concentrate 2 is sent to the bended sieve with the openings of 0.5mm, where 0-0.5mm coal fraction is rejected to waste, and +0.5mm fraction is dehydrated on the screen, in the screw centrifuge and sent to the fine coal conveyor. This coal-cleaning flowsheet is resistant to variations of load volume, solid content in the slurry, different screen and fractional composition of the slime.

The ash-content of cleaned slime is within 14-15%, the ash content of rejects is 60-65%.

The total amount of circulating process water at all three stages of preparation in the steeply inclined separators is 3.5m3/h, of which 1.5m3/h is clarified water.

Circulating water is supplied to the first stage of cleaning. For the second and the third stages of cleaning in the steeply inclined separators clarified water is usually supplied, mainly to wash off the coal from clay rock and soaking rock.

Circulating water is also supplied to the second stage, and clarified water is fed to sprinklers for coal washing off on screens and coal dehydration in centrifuges. As a result, the consumption of clarified water is significantly reduced. For slurry water clarification sludge ponds are used.

This flowsheet was implemented at a Ukrainian preparation plant for anthracite recovery from mine waste.

Seasonal preparation plants are located close to mining sites at surface coal mines, their construction does not require high capital expenditures, the water-slime process circuits are closed through sludge ponds, mine workings of surface mines or settling ponds in waste dumps where water is filtered through the waste dump and slime settles in the ponds. The slime is removed from settling ponds to waste dumps in winter.

Water supply of the plant is assured by two pumps for circulation and clarified water.

As a rule, the equipment in the preparation plant building is placed to a cascade scheme. The separation medium (water) is pumped upward to the process hydrocyclones, and when moving downward, it makes all the operations of separation, such as cleaning in steeply inclined separators KNS, thickening in hydrocyclones, dehydration on screens and in centrifuges.

The �KENES� Company produces all standard sizes of steeply inclined separators KNS, hydrocyclones, bended screens , special equipment for single-stage and double-stage units.

Conclusions: The resource - saving technology of diluted coals cleaning in steeply inclined separators of KNS type at preparation plants provides for the reduction of coal losses at surface mines with maximum extraction of combustible matter and high efficiency of the process at low capital and operating costs.