Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
COAL PROCESSING METHOD
Document Type and Number:
WIPO Patent Application WO/1998/027186
Kind Code:
A1
Abstract:
A process for the recovery of coal particles from coal tailings comprising the steps of initially treating the coal tailings to separate a proportion of the coal particles having sizes in a specified range, and then subjecting the coal particles to a heat treatment and then an agglomeration process to produce a coal product. The process provides a further means of recovering coal particles of small sizes from slurries.

Inventors:
LEES JEREMEY JAMES (AU)
PRYOR MURRAY HOWARD (AU)
Application Number:
PCT/AU1997/000838
Publication Date:
June 25, 1998
Filing Date:
December 09, 1997
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
MINPRO AUSTRALIA (AU)
LEES JEREMEY JAMES (AU)
PRYOR MURRAY HOWARD (AU)
International Classes:
B01J2/00; B03B9/00; C10L5/08; (IPC1-7): C10L5/00; B01J2/00
Domestic Patent References:
WO1991011502A11991-08-08
Foreign References:
US3073751A1963-01-15
US3401089A1968-09-10
US3562783A1971-02-09
US4133718A1979-01-09
EP0082470A21983-06-29
Attorney, Agent or Firm:
F.B. RICE & CO. (Balmain, NSW 2041, AU)
Download PDF:
Claims:
CLAIMS:
1. A process for the treatment of coal tailings containing coal particles, comprising the steps of: (i) treating the tailings to separate therefrom a proportion of the coal particles; (ii) softening the separated coal particles in a heat treatment process; and (iii) agglomerating the softened coal particles to form a coal product.
2. A process for the treatment of coal tailings containing coal particles, comprising the steps of: (i) forming a slurry containing the coal particles; (ii) treating the slurry to separate therefrom a proportion of the coal particles; (iii) softening the separated coal particles in a heat treatment process; and (iv) agglomerating the softened coal particles to form a coal product.
3. A process for the treatment of coal tailings containing coal particles, comprising the steps of: (i) forming a slurry containing the coal particles; (ii) subjecting the slurry to a first treatment step adapted to recover coal particles having sizes between about 75 microns and about 2 millimetres; (iii) subjecting the slurry to a second treatment step that can recover a proportion of coal particles having sizes less than about 75 microns; (iv) mixing the coal particles recovered by the treatment process in step (ii) with the coal particles recovered by the treatment process in step (iii); (v) softening the separated coal particles in a heat treatment process; and (vi) agglomerating the softened coal particles to form a coal product.
4. The process for the treatment of coal tailings containing coal particles as defined in any one of the preceding claims wherein the heat treatment is undertaken in a furnace or reactor.
5. The process for the treatment of coal tailings containing coal particles as defined in claim 4 wherein the heat treatment is undertaken in a tube, pipe, cyclone, or rotary furnace or reactor.
6. The process for the treatment of coal tailings containing coal particles as defined in claims 4 or 5 wherein the coal particles are heated to a plastic state, with the minimum temperature being about 3500C.
7. The process for the treatment of coal tailings containing coal particles as defined in claim 6 wherein the coal particles prior to undergoing the heat treatment process are dried in a predryer to remove any water present after the earlier processing steps.
8. The process for the treatment of coal tailings containing coal particles as defined in claim 7 wherein the heat treatment vessel is hermetically connected to the predryer to allow the dried coal particles to be moved into the vessel following drying without exposure to water vapour in the atmosphere.
9. The process for the treatment of coal tailings containing coal particles as defined in any one of claims 4 to 8 wherein any volatile liquids or combustible gases liberated from the coal during the heat treatment process are recycled and used to heat the coal particles in the heat treatment process.
10. The process for the treatment of coal tailings containing coal particles as defined in any one of the preceding claims wherein the step of agglomerating is undertaken in an agglomerating means.
11. The process for the treatment of coal tailings containing coal particles as defined in claim 10 wherein the agglomerating means mechanically agglomerates the heat treated coal particles.
12. The process for the treatment of coal tailings containing coal particles as defined in claims 10 or 11 wherein following the agglomerating step, the coal particles are cooled in a cooling device hermetically connected to the agglomerating means.
13. The process for the treatment of coal tailings containing coal particles as defined in claim 12 wherein the cooling device cools the temperature of the agglomerated coal product below the temperature at which the product would ignite on exposure to air.
14. The process for the treatment of coal tailings containing coal particles as defined in any one of the preceding claims wherein the coal product can undergo a further heat treatment.
15. The process for the treatment of coal tailings containing coal particles as defined in claim 14 wherein the further heat treatment includes a multi stage heat treatment process.
16. The process for the treatment of coal tailings containing coal particles as defined in claim 15 wherein the multistage process can include an initial heat treatment at a temperature of at least 2000C to recover semicoke products and still further heat treatments at higher temperatures to recover coke product.
17. The process for the treatment of coal tailings containing coal particles as defined in claim 16 wherein the still further heat treatment steps occur at temperatures up to around 12000C.
18. The process for the treatment of coal tailings containing coal particles as defined in any one of the preceding claims wherein the coal particles are firstly separated from the tailings on the basis of the size, specific gravity, electrical behaviour, magnetic behaviour and/or chemical behaviour of the coal particles in comparison to the remaining constituents of the tailings.
19. The process for the treatment of coal tailings containing coal particles as defined in claim 18 wherein the separation of the coal particles is performed using one or more spiral separators, teeter bed separators, and/or sieve screen decks.
20. The process for the treatment of coal tailings containing coal particles as defined in any one of the preceding claims wherein the step of separating the coal particles from the tailings includes the use of a cyclone separation zone comprising at least two cyclone stages in series.
21. The process for the treatment of coal tailings containing coal particles as defined in any one of the preceding claims wherein the step of separating the coal particles from the tailings includes a froth flotation process wherein the slurry of coal particles is aerated in an aeration vessel to produce a froth product containing coal particles that may overflow the aeration vessel and be recovered or separated by froth scrapers or paddles.
Description:
"Coal processing method" Field of the Invention The present invention relates generally to the recovery of clean coal from coal tailings and in particular to the production of useful coal product from coal fines.

Background Art Solid carbonaceous materials, such as coal, have long been employed as a fuel source whether it be by simple combustion or conversion into a gaseous or liquid fuel. Certain coals when suitably processed into coke also provide an essential raw material in iron making.

All coals contain mineral particulates to some degree. Excessive levels of such mineral particulates are undesirable as they interfere with the combustion of the coal and the formation of coke. The particulates also lead to undesirable increases in ash levels during processing and combustion.

Prior to utilisation, most coals have traditionally undergone a washing treatment. During such a treatment, finely divided coal or coal fines of varying sizes are washed into the waste water together with the mineral particulates and other gangue materials. These coal tailings are typically held in settling ponds on the mine site. In addition to representing a loss of coal, the disposal of the waste water can represent an environmental hazard.

Attempts have been made to recover coal from coal tailings for later use as fuel or, in some instances, for processing into coke, using physical and chemical processing techniques with varying degrees of success. The technique employed will to a significant extent depend on the size range of the coal fines that are to be recovered by the process. For example, it is known that good recovery of coal fines having sizes ranging approximately from 1.7mm to 150-200 microns can be achieved using gravity separation techniques. The present inventors have also found that for the purposes of recovering coal from tailings residues, particles having sizes less than about 150-200 microns are suitable for recovery using froth flotation methods.

It has also long been known to heat and pyrolise coal for the production of petro-chemicals, tars and short residual char. One technique used is that of flash pyrolysis in which the coal is rapidly heated thereby liberating at least some of the volatiles present in the coal such that a combustible carbonaceous residue or char remains.

It would be desirable to provide a new means of processing coal slurries that provided desirable beneficiation of coal fines and the production of a coal product that can be readily handled and, if desired, further processed as required. It would also be desirable but not essential that the new process in providing this improved beneficiation had a cost of production similar to or not significantly higher than presently used processes.

Summary of the Invention According to a first aspect, the present invention consists in a process for the treatment of coal tailings containing coal particles, comprising the steps of: (i) treating the tailings to separate therefrom a proportion of the coal particles; (ii) softening the separated coal particles in a heat treatment process; and (iii) agglomerating the softened coal particles to form a coal product.

According to a second aspect, the present invention consists in a process for the treatment of coal tailings containing coal particles, comprising the steps of: (i) forming a slurry containing the coal particles; (ii) treating the slurry to separate therefrom a proportion of the coal particles; (iii) softening the separated coal particles in a heat treatment process; and (iv) agglomerating the softened coal particles to form a coal product.

According to a third aspect, the present invention consists in a process for the treatment of coal tailings containing coal particles, comprising the steps of: (i) forming a slurry containing the coal particles; (ii) subjecting the slurry to a first treatment step that can recover coal particles having sizes between about 75 microns and 2 mm; (iii) subjecting the slurry to a second treatment step that can recover a proportion of coal particles having sizes less than about 75 microns; (iv) mixing the coal particles recovered by the treatment process in step (ii) with the coal particles recovered by the treatment process in step (iii);

(v) softening the separated coal particles in a heat treatment process; and (vi) agglomerating the softened coal particles to form a coal product.

In the above aspects, the coal particles undergo a heat treatment process. The heat treatment may be undertaken in any suitable vessel or retort adapted for the purpose including a tube, pipe. cyclone, or rotary furnace or reactor. Prior to undergoing the heat treatment, the coal particles are preferably dried in a predryer to remove any water present after the earlier processing steps. The heat treatment vessel is preferably hermetically connected to the predryer to allow the dried coal particles to be moved into the vessel following drying without exposure to water vapour in the atmosphere. The coal particles are preferably heated to a plastic state with 0<BR> <BR> the minimum temperature being about 350 C. Any volatile liquids or combustible gases liberated from the coal during the heat treatment process can be recycled and used to heat the coal particles in the heat treatment process.

The agglomerating step preferably comprises a mechanical means of agglomerating the plasticised coal particles. The agglomeration can comprise extruding the plasticised coal particles, for example. by using a pellet mill.

Other possible agglomerating techniques include forming agglomerates by tumbling, vibrating, shaking or paddle mixing the plasticised coal particles.

A pelletising pan, rotary drum agglomerator, or fluidised bed can also be utilised in the agglomerating step.

Following the agglomerating step, the coal particles are preferably cooled in a cooling device that would be typically hermetically connected to the agglomerating means. The cooler would preferably bring the temperature of the agglomerated coal product below the temperature at which the product would ignite on exposure to air, and recover waste heat.

If desired, further processing of the product can also be undertaken including further heat treatments. In one embodiment, the further heat treatments can include a multi-stage heat treatment process. This multi- stage process can include an initial heat treatment at a temperature of at least 2000C to recover semi-coke products and further heat treatments at higher temperatures to recover coke product. In this embodiment, the further heat treatment steps could occur at temperatures up to around 1200PC.

In the above aspects, the step of treating the coal slurry to obtain the coal particles that will then undergo the heat treatment step can be undertaken by any suitable means. For example, the coal particles can be separated and recovered from gangue materials on the basis of the size, specific gravity, electrical behaviour, magnetic behaviour or chemical behaviour of the coal particles in comparison to the remaining constituents of the tailings. The separation of the coal particles that will undergo the heat treatment step from the remaining coal particles can also be undertaken by any suitable means including separation on the basis of size and specific gravity of the particles.

In one preferred embodiment, the treatment step can include a specific gravity separation step. This step is preferably adapted to recover coal particles having dimensions between around 1. 7mm to around 75 microns. This step may be performed by one or more spiral separators or classifiers. The spiral separators may be replaced by teeter bed separators or similar suitable gravity separation devices. In a spiral separator, a number of helical sluices are mounted about a single vertical column below a slurry feed box. The slurry in its descent on each sluice tends to stratify with the denser fraction of the minerals moving towards the axis of the separator and the less dense materials being carried to the outer part of the sluice. The separated fractions are recovered in separate outlets at the lower end of the separator.

In one embodiment, the treatment step further includes at least one sieve screen deck over which is passed the slurry of coal particles. The sieve screen deck can be rapped or vibrated as needs dictate. The slot aperture of the deck will be set to a size as required by the application and could vary between individual sieve screen decks in the treatment process. In another embodiment, the treatment step can include a screening drum mounted substantially vertically as described in International Patent Application No PCT/AU97/00003, the contents of which are incorporated herein by reference.

Another means of treating the coal tailings could comprise or include a cyclone separation zone comprising, preferably, at least two cyclone stages in series. The cyclone separation zone would preferably be used as a treatment step of the coal slurry prior to it entering a froth flotation process described below.

In another embodiment, the treatment step could include a froth flotation process where a coal slurry is aerated in an aeration vessel in which the coal particles attach to the air bubbles and rise while the gangue particles fall towards the bottom of the vessel. The rising air bubbles with the coal particles attached produce a froth product that may overflow the aeration vessel and be recovered or may be separated by conventional means such as froth scrapers or paddles.

The froth flotation step, when combined with a cyclone separation zone having two cyclone separation zones in series as described above, would preferably substantially remove from the slurry coal particles having dimensions in the range less than about 150-200 microns to 75-100 microns.

In a further embodiment, two or more aeration vessels may be utilised in series to ensure good recovery of the coal particles from the slurry.

One desired result of the above process, apart from the production of a coal product of a size that can be readily further processed or utilised, is a change in the chemistry of the coal, in particular, its specific heat value in comparison to the coal tailings fed into the process defined herein.

Brief Description of the Drawings By way of example only, preferred embodiments of the invention are now described with reference to the accompanying drawings, in which: Fig. 1 is a flow chart of one embodiment of the invention; and Fig. 2 is a flow diagram of one preferred embodiment of the tailings treatment step in the process depicted in Fig. 1.

Preferred Mode of CarrvinR out the Invention A flow chart of one embodiment of the process according to the present invention is generally depicted as 10 in Figure 1.

In this embodiment, a coal slurry, which may have been dredged from a coal tailings pond, is fed through an initial treatment step 11 to remove coal particles of sizes greater than around 75 microns from the slurry.

One possible embodiment of the processes that may be performed at step 11 is depicted in Fig. 2.

Referring to Fig. 2, the coal slurry in treatment step 11 is firstly fed over a screen 100 to remove all particles greater than 1. 7mm that are recovered and processed as required. The water and particles of less than 1.7nun are fed into a sump 101 where the slurry pulp density is adjusted to between 10-30% by weight solids, preferably 24%, and pumped by pump 102

into a hydrocyclone 103 that has an included cone angle of 150. Oversized particles and smaller dense particles form the underflow having a pulp density of from 40-60% while the generally smaller particles and larger less dense particles form the. overflow which is fed to a sump 104 forming part of a secondary treatment circuit. The underflow from the hydrocyclone 103 is repulped to a density of 20-50%, preferably 25%, and fed over a rapped or vibrated sieve screen deck 105 that has a radius of about 1.9 metres, an arc angle of about 350 and a slot aperture of about 380 microns. The water and fines tend to flow through the screen 105 to produce an underflow that flows into sump 104. The particles having a size above about 250 microns will form an overflow from the sieve screen 105 having a pulp density of about 40-60%.

While the underflow from the hydrocyclone 103 is generally of larger particles, it is contaminated with a proportion of smaller and denser particles that would normally end up iI1 the final coal product stream and raise its gangue content. By following the hydrocyclone 103 with a sieve screen deck 105 the proportion of smaller particles can be reduced as the sieve screen deck 105 classifies solely on size with no allowance for density difference.

The overflow stream from the sieve screen deck 105 thus comprises coal particles and gangue having a particle size from approximately 250 microns to 1.7mm.

The overflow stream from the sieve screen deck 105 is repulped in sump 106 to a pulp density of 20% and pumped by pump 107 to a bank of spiral separators 108 where the particles are separated by density into a product stream and a reject stream. The product stream is fed to a further bank of spiral separators 109 to clean the product stream and to produce a final product stream and a reject stream. The reject streams from spiral separators 108 and 109 are combined and then conveyed by sump 110 and pump 111 to the froth flotation vessel 12 (see Fig. 1). The final product stream from spiral separators 109 is dewatered on a sieve screen deck 112 similar to sieve screen deck 105 and the overflow is fed either directly or indirectly to product storage for later processing. The underflows from sieve screen deck 112 are directed to sump 104 where they join the underflows from the hydrocyclone 103 and the sieve screen deck 105.

The sump 104 feeds a fine particle separation circuit through pump 113. The pump 113 feeds a slurry of pulp density of about 25% to a

hydrocyclone 114 of a smaller included cone angle than hydrocyclone 103.

The overflow from hydrocyclone 114 is fed to sump 110 while the underflow having a pulp density of about 50% is repulped to 20% and fed over sieve screen deck 114a similar to sieve screen deck 105 except that the gap width is about 100 microns. The underflow from sieve screen deck 114a is fed to sump 110 while the overflow containing particles of greater than about 75 microns is fed to a sump 115 where it is repulped and fed by pump 116 to a bank of spiral separators 117. The product stream from spiral separators 117 is fed to cleaning spiral separator 118. The reject stream from each of these separators is fed to sump 110 while the final product stream from spiral separator 118, which has a majority of coal particles having sizes between about 75 microns and 250 microns, is fed to a dewatering sieve screen deck similar to deck 114 where the coal particles of greater than about 75 microns are recovered and further processed as required. If desired, a hydrocyclone similar to the hydrocyclones 103 and 114 may precede each of the dewatering sieve screen decks.

In the process depicted in Fig. 1, the slurry from sump 110 which predominantly should contain particles less than 75 microns in size is then fed via a feed pipe into a second treatment step 12. The treatment step 12 comprises a froth flotation vessel in which air and coal particles are fed into the vessel. The coal particles preferentially attach to the air bubbles that rise upwardly in the flotation agent in the vessel, normally an aqueous solution, to form a froth product that is scaped from the top of the vessel using a scraper blade. The coal particles are then washed and filtered from the froth product ready for further processing. The gangue particles introduced into the vessel settle downwardly in the vessel and can be recovered from the bottom of the vessel as desired.

The slurry of coal particles recovered from the treatment step 12 are then mixed in a mixer, generally depicted as 13, with the coal particles recovered from the final product stream of spiral separator 118 in treatment step 11.

The mixture of particles is then fed into a predryer unit 14 which sufficiently heats the unit to vaporise any water. Hermetically connected to the predryer unit 14 is a heat treatment vessel 15 into which the coal particles are moved following drying in the predryer unit 14. In the heat treatment vessel 15, the coal particles are heated to a temperature sufficient

to plasticise the coal, namely at least 3500C. Any volatiles liberated from the coal during this heat treatment process can be recycled and used to provide energy for the heating of the coal made in vessel 15.

The plasticised coal is then processed in a pellet mill 16 that agglomerates the plasticised coal and extrudes it into agglomerates conveniently sized for further handling. Other agglomerators could be employed including a pug mill, a pelletising pan, a rotary drum agglomerator or a fluidised bed.

The agglomerates are then passed through a cooler unit 17 that is also hermetically connected to the pellet mill 16. The cooler unit 17 brings the temperature of the coal product below a temperature at which it would ignite if exposed to air.

The cooled coal agglomerates 18 produced by the process 10 can be further processed as required, the further processing including further heat treatment steps.

The process 10 provides a means of removing coal particles having a range of dimensions in an efficient manner from a slurry of coal tailings and converting the recovered coal particles into a commercially useful and more easily handled product. The heat treatment step also desirably produces a chemical change in the coal, with the coal product 18 having a lower level of volatiles, an increase in fixed carbon, and an increase in specific heat value in comparison to the coal fines present in the tailings dam.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.