GB2213497A - Preparation of coal fines for pelletisation - Google Patents

Abstract

A portion of the coal fines in a moist mass thereof is dried so as to remove only substantially all of the surface moisture on the coal fine particles. The surface dried fines which retain inherent moisture within the particles are then able to be mixed with a dry binding material to produce a mixture which is then mixed with the remaining portion of the coal fines. The resulting mixture is pelletised. Also apparatus for carrying out the process as shown in the figure. <IMAGE>

Description

TITLE: Preparation of Coal Fines for Pelletisation The present invention relates to a method of and apparatus for preparing coal fines for pelletisation.

In a coal fed slagging gasifier, such as a British Gas/Lurgi ash slagging gasifier, only a limited proportion of coal fines may be incorporated in the top feed to the gasifier. It has therefore been proposed that coal supplied to a gasification site is screened to separate out fines which in themselves are not suitable as top feed. A range of screening techniques exists but for this particular purpose a wet screening process is very attractive so as to provide efficient separation with coal feeds of high moisture content.

A previously proposed screening process for use at the front end of a slagging gasifier plant is shown in accompanying Figure 1. The process equipment incorporates two wet screens in series, a primary screen 1 and a secondary screen 2. Part of the overflow 3 from the secondary screen is backblended into the overflow 4 from the primary screen to form the top feed for a coal gasifier. The remaining process equipment is required to dewater the underflow 5 from the secondary screen which is fed to a desliming screen 6 from which the fraction of fines greater than say 0.5mm passes to a centrifuge 7 for dewatering. From the desliming screen the fraction of fines of say 0.5mm or less passes for dewatering to a thickener 8 and vacuum filter 9.The dewatered secondary screen underflow products, from the centrifuge and vacuum filter, together with the remaining part of the overflow 8 from the secondary screen form the feed to the fines handling plant.

In a known fines handling plant for preparing fines for pelletisation coal fines from the screening plant are fed to a ball mill in which the fines are not only pulverised but also dried to almost or substantially zero moisture content. The drying is effected by passage through the ball mill of hot exhaust gases from a hot gas generator, such as a combustor. The fines fed into the ball mill may have a total moisture content of, for example, 17% by weight and the dried pulverised coal fines may have a total moisture content of, for example, about 1%. The dried pulverised coal fines are then separated from the hot gases, moistened, if required, by a small quantity of water, and subsequently mixed with dry powdered bentonite binder.Water is added to the mixture of pulverised fines and bentonite so that after a sufficient intermediate storage period the bentonite has swelled to maximise its binding properties. The resulting mixture is homogenised and charged, by means of a weigh feeder and fluffer, at a constant rate to a pelletiser. Water is then introduced to agglomerate the coal, for example, by spraying water on to a pelletising disc. The formed pellets are screened and undersize material may be recycled.

It will be apparent from the above that water forming both the surface or free moisture on the coal fine particles and the inherent moisture content within the coal fine particles is first substantially completely removed from the fines fed to the handling plant and subsequently at different stages water is added to the previously dried fines, or a mixture including the fines, prior to the pelletising stage.

A method of preparing coal fines for pelletising is now being proposed which omits the step of substantially completely drying the fines and the subsequent one or more steps of adding water to the previously dried fines prior to the pelletisation stage.

According to one apsect of the invention a method of preparing coal fines for pelletising from a mass or cake of coal fines containing free moisture or surface moisture, comprises drying a portion of the moist cake so as to remove substantially all of the surface moisture only, mixing the surface dried portion with a binding material and mixing together the surface dried portion/binder mixture with at least a part of the remaining portion of the moist cake to produce the resulting mixture suitable for pelletising.

The coal fines may be wet milled, pulverised or ground to provide a slurry which is thickened and filtered to produce the moist filter cake. The wet milled coal fines may be supplied directly from one or more coal fines separating means.

The invention will now be described, by way of example only, with reference to Figure 2 of the accompanying drawings, which shows in schematic form one embodiment of apparatus for performing a method according to the invention.

r Figure 2 shows the integration of wet screening of coal fines with pelletisation. The important features include the use of wet milling to achieve the coal size range for pelletising, mixing with a binder, eg bentonite-, and a mixing procedure which minimises the amount of coal drying.

The apparatus comprises a primary screen 20 and a secondary screen 21.

Coal is fed into the primary screen and the overflow 22 from this screen provides the top feed for a slagging gasifier (not shown). The underflow 23 from the primary screen together with any required make-up water is fed to the secondary screen 21. Part of the overflow 24a from the secondary screen is fed into or backblended with the overflow from the primary screen. The remaining part of the overflow 24b from the secondary screen is combined with the underflow 25 from that screen and fed to a wet ball mill 26 to which water may be added if required. In the wet ball mill 26 the coal is crushed or pulverised to the size range required for pelletisation. The crushed coal is then dewatered in a thickener 27 and pressure filter 28 so as to produce a very moist filter cake of coal fines.The moist filter cake of coal fines may contain, for example, say 22% free moisture. At this stage intimate mixing of the very moist filter cake with a small quantity of dry binder, such as bentonite binder, would be difficult if the binder was added directly to the moist filter cake. However, this problem is overcome in the present embodiment by the use of two mixers 29, 30.

One portion of the moist filter cake is first surface dried in drier 31 so that the coal fines in this portion have zero or substantially zero surface or free moisture content. The surface dried coal does however still contain inherent moisture inside the coal particles up to a content in the region of 5%. Thus, the coal fines are not dried so as to approach substantially nil total water content. A binder, such as dry bentonite, is then added to and mixed with the surface dried coal fines in mixer 29. The blend of surface dried coal fines and bentonite from mixer 29 is added to mixer 30 together with the remaining portion of the moist filter cake.

The resulting mixture from mixer 30 will have a moisture content dependent on what proportion of the moist filter cake was surface dried so as to have substantially zero surface moisture. If the free moisture content of the moist filter cake is 22% with a total moisture content of about 27% because of an inherent non-free moisture content of about 5%, in the present embodiment it can be arranged that such a proportion is surface dried so as to produce a resulting mixture (formed from the 22% free moisture filter cake, the surface dried material and the dry bentonite) having a total moisture content of about 18%.

This resulting mixture is substantially homogenised and may have a similar moisture content and pelletising characteristics to material from which pellets for a British Gas/Lurgi gasifier may be made. The resulting homogenised mixture can then be charged, by means of a weigh feeder 32 and fluffer 33, at a constant rate to a pelletiser 34, as previously described above in relation to the known fines handling plant. Thus, water is sprayed onto a pelletising disc 34a to agglomerate the coal fines and the formed pellets pass to a roller screen 35 where they are screened.

Undersized material may be recycled to mixer 30.

Having regard to the invention as described in the above embodiment, it will be appreciated that it has a number of advantages over the previously proposed procedure briefly described earlier. The present scheme is simple and involves lower capital costs owing to the reduction in process equipment, for example, the loss of the desliming screen, the centrifuge, the hot gas swept ball mill and associated equipment such as the hot gas generator. The present scheme also involves lower running costs because of the reduction in process equipment, particularly because of the reduced coal drying requirement.

Also, the power requirements for wet milling are lower than that for a similar dry milling duty.

Whilst a particular embodiment has been described above it will be appreciated that various modifications may be made without departing from the scope of the invention.

For example, the relative proportion of the coal fines in the moist filter cake subjected to surface moisture drying can be altered so as to produce resulting mixtures for pelletising having moisture contents other than 18% total moisture. Also different binders may be used.

Claims (5)

1. A method of preparing coal fines for pelletising from a mass or cake of coal fines containing free moisture or surface moisture, comprising drying a portion of the moist cake so as to remove substantially all of the surface moisture only, mixing the surface dried portion with a binding material and mixing together the surface dried portion/binder mixture with at least a part of the remaining portion of the moist cake to produce a resulting mixture suitable for pelletising.

2. A method as claimed in Claim 1, wherein coal fines are wet milled, pulverised or ground to provide a slurry which is thickened and filtered to produce the moist filter cake.

3. A method as claimed in Claim 2, wherein the coal fines which are wet milled have been supplied directly from one or more coal fines separating means.

4. A method as claimed in Claim 1 substantially as hereinbefore described with reference to figure 2 of the accompanying drawings.

5. Apparatus for preparing coal fines for pelletising, substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.