AU749085B2 - Combined tailings disposal system - Google Patents

Description

P/01011Oi 28dB/I Regulation 3.2(2)

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT 4* 4 4 9@ .4 4* *4 4 4 4 .444 Application Number: Lodged: 4.

4 4* 4 4 4 *4*4 4. 4 4* 4* Invention Title: COMBINED TAILINGS DISPOSAL SYSTEM The following statement is a full description of this Invention, Including the best method of performing It known to us COMBINED TAILINGS DISPOSAL SYSTEM Technical Field The patent invention relates to the transport and disposal of coal mine washery waste.

Background Art This invention relates to the disposal of coarse rejects (less than 100 mm particle size) and fine tailings (less than 0.5 mm particle size) from coal mine washeries. Current practice at most coal mines is to truck the dry coarse rejects to a disposal site and pump the fine tailings in slurry form to a tailings dam.

Because of the fine particle size of the tailings, settling and consolidation in the tailings dam is very slow. Often a dry crust forms on top of the dam deposit with the material below remaining fluid for many years. With increasing *environmental pressures the tailings dam approach is becoming unacceptable.

Some mines have recently begun pumping the coarse rejects and tailings together to the disposal area. To achieve reliable operation and avoid the possibility of plugging, the volume concentration of coarse rejects in the mixture must be kept low, generally around 12 percent. This means large quantities of water must be added, with the result that the tailings concentration in the mixture is very low, typically less than 10 percent by weight concentration.

The low tailings concentration means that the tailings in the slurry provide little support to the coarse rejects during pipeline transport. Consequently, high velocities are required to transport the slurry, with attendant high pumping power requirements (known as "brute force" pumping) and very high pipe wear rates. As a result costs are high particularly for long pumping distances.

Ideally, the aim of combined disposal is to achieve a near homogeneous deposit with the fine tailings interspersed amongst the coarse rejects. With a "brute force" system the low tailings concentration means most of the coarse rejects deposit near the discharge and much of the fine tailings wash to the extremities of the disposal area. In this area the deposit is therefore similar to a conventional fine tailings dam, which is not ideal.

A number of attempts have also been made to combine high concentration tailings direct from a thickener (a device which increases the concentration of particles in a liquid flow by removing some of the moisture), to give the tailings a concentration of typically 35%, with the coarse rejects (known as "high density combined pumping"). This would be ideal since tailings would be more likely to stay interspersed with the rejects upon disposal due to the viscous nature of the tailings. However the ratio of coarse rejects to fine tailings is generally such that the volume concentration of coarse rejects in the combined mixture is very high (typically around This means the system is prone to blockage and reliable operation is generally not achieved.

It is an object of the present invention to provide a viable method for transport and disposal of combined coarse rejects and fine tailings, where the ratio of coarse rejects to fine tailings processed through the system is relatively high.

Summary of Invention ~According to one aspect the present invention provides an apparatus for 15 transport and disposal of coal wastes from a coal washery, including: sump means for receiving and mixing coarse rejects, high concentration fine tailings and entrainment fluid so as to form a slurry; transport means for transporting said slurry via a conduit to a disposal site; 20 separating means at said disposal site for removing substantially all of 000said coarse rejects and a portion of said fine tailings from said slurry; and recycle transport means for returning the remaining slurry after separation lO i to said sump means so as to form at least part of said entrainment fluid.

Preferably, means for adding water to the sump means and associated control means for monitoring the properties of the slurry are also provided.

According to another aspect the present invention provides a method for transport and disposal of coal waste from a coal washery, including the steps of: i) at a first site, mixing coarse rejects, fine tailings and entrainment fluid to form a slurry; ii) transporting said slurry via a conduit to a disposal area; iii) separating substantially all of said coarse rejects and a portion of _aa:i fine tailings from said slurry; iv) transporting the remaining slurry to said first site so as to provide at least part of said entrainment fluid.

The present invention overcomes the disadvantages of both the "brute force" and high density pumping systems by pumping the coarse rejects at relatively low volume concentration with high concentration tailings direct from the thickener. This is achieved by recycling some of the fine tailings in a separate recycle pipeline.

Some of the advantages of the system are as follows: 1. Tailings and rejects are disposed at maximum concentration with the resulting benefits of a more homogeneous and environmentally attractive deposit.

2. Tailings and rejects are disposed at similar concentrations as might be achieved with high density pumping but in a reliable manner without the dangers of plugging associated with high density pumping.

3. The rejects are pumped in high concentration tailings rather than the dilute tailings associated with the "brute force" system. The support offered by the high concentration viscous tailings allows lower velocities to be used with resulting lower pumping power requirements and lower pipe wear rates. Both these mean significantly lower costs than the "brute force" system.

Description An illustrative embodiment of the present invention is shown schematically in Figure 1. The major components are: a collection sump centrifugal slurry pump(s) main pipeline separation device separation sump recycle pump and recycle pipeline High concentration tailings from the thickener are pumped into sump from pipe (14) and coarse rejects are supplied via conveyor The amount of moisture present with the rejects is minimised by first allowing the rejects to drain in a hopper, or by some other means. The resulting mixture is then pumped by the main slurry pump(s) through pipeline to the separation device The separation device separates the coarse rejects (typically plus 1 or plus 2 mm) from the majority of the tailings which report to sump This device could typically consist of a sieve bend screen mounted over separation sump The coarse rejects flow over the top of the screen (as indicated by arrow with most of the fine tailings falling through the screen into separation sump thereby being separated from the course tailings. The recycle pump returns a proportion of the fine tailings through recycle pipeline to sump The remaining portion overflows (as indicated by arrow from the separation sump to flow with the coarse rejects from the top of the screen into the disposal area.

Solids concentrations in the main and recycle pipelines will vary depending on the relative flow rates of rejects and tailings, the tailings concentration supplied by the thickener, and the amount of moisture in the rejects. Typical concentrations are as follows: Main Pipelines Total solids concentration 45% to Volume concentration of rejects 10% to Solids concentration of fine tailings portion 30% to Recycle Pipelines Tailings concentration 30% to Flow Stream into Disposal Site Total solids concentration 65% to Volume concentration of rejects 35% to Solids concentration of fine tailings portion 30% to Total flow rate in the main pipeline is typically about twice that in the recycle pipeline with about two thirds of the tailings recycled.

System control can be by a number of methods. A possible method of control, which however does not exclude other methods, is shown in Figure 2. One or more of the main slurry pumps is driven by a variable speed motor the speed of which is controlled by a controller (10) linked to a magnetic flow meter By this means the flow rate in the main pipeline is maintained constant with variations in reject concentration in the main pipeline flow. The recycle pump is fixed speed with flow rate in the recycle pipeline controlled by a flow control valve (12) controlled by a level transmitter (13) on the main sump Level transmitter (13) interfaces with level control circuitry By this means the level in the sump is maintained at a constant level. The flow control valve (12) controls the flow down to a certain preset minimum so as to maintain a minimum safe velocity in the recycle pipeline With the above control system, operation is as follows: With the main pipeline operating at a fixed flow rate if the flow of rejects via conveyor (8) increases, the concentration of rejects in the sump rises. The extra volume occupied by the rejects requires a reduction in the recycle flow which is achieved by partial closing of control valve Partial closing of this valve causes more tailings to overflow sump and flow into the disposal area. The converse occurs if rejects flow rate decreases. Control valve (12) opens to .oocompensate for the reduced volume of rejects in sump oooeo The entrainment fluid may also consist of water introduced to sump via inlet pipe The flow of water through pipe (17) is also controlled by level control circuitry (18) via control valve (19).

It will be appreciated that alternate means and additional features may be added within the general scope of the present invention.

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Claims (7)

1. An apparatus for transport and disposal of coal wastes from a coal washery, said apparatus including; sump means for receiving and mixing coarse rejects, high concentration fine tailings and entrainment fluid so as to form a slurry; transport means for transporting said slurry via a conduit to a disposal site; separating means at said disposal site for removing substantially all of said course rejects and a portion of said fine tailings from said slurry; and recycle transport means for returning the remaining slurry after separation to said sump means to form at least part of said entrainment fluid.

2. An apparatus according to claim 1 further including adding means for oo••o adding water to said sump means for forming at least a part of said entrainment o• fluid. loll

3. An apparatus according to anyone of claims 1 or 2 including; flow rate control means for maintaining constant flow rate in said transport means; and recycle flow rate control means for controlling flow rate in said recycle transport means.

4. A method for transport and disposal of coal wastes from a coal washery, said method including; at a first site, mixing coarse rejects, fine tailings and entrainment fluid to form a slurry; transporting said slurry via a conduit to a disposal area; separating substantially all of said coarse rejects and a portion of said fine tailings from said slurry; and returning the remaining slurry to said first site to provide at least a part of said entrainment fluid. A method according to claim 4 including adding water at said first site for forming at least a part of said entrainment fluid.

6. A method according to anyone of claims 4 or 5 including; controlling the rate at which said slurry is transported through said conduit; and controlling the rate at which said remaining slurry is returned to said first site. i 7. An apparatus for transport and disposal of coal wastes substantially as herein described with reference to the accompanying figures.

8. A method of transport and disposal of coal wastes substantially as herein described with reference to the accompanying figures. DATED this 26th day of June 1998. SLURRY SYSTEMS PTY LTD WATERMARK PATENT TRADEMARK ATTORNEYS UNIT 1, THE VILLAGE, RIVERSIDE CORPORATE PARK

39-117 DELHI ROAD, NORTH RYDE NSW 2113 AUSTRALIA PNF:MP:RM DOC014 AU002316.WPC