MATERIAL

FIELD OF THE INVENTION

This invention relates to the cleaning of aggregate, especially rail track ballast material that is contaminated with asbestos fibres, although the invention is not limited to any type of contaminant to be removed from the aggregate. The term "aggregate" is used herein to refer to material also known as "construction aggregate", which refers to a broad category of coarse particulate material that is used in construction and which typically include gravel, crushed stone, slag, recycled concrete, geosynthetic materials, and the like. The invention is described herein largely with reference to aggregate used as rail track ballast material, but it is applicable to various aggregates and is not restricted to rail track ballast material.

BACKGROUND TO THE INVENTION

Material used as rail track ballast often becomes fouled or impaired for a number of reasons to various degrees and increases over time. Causes of ballast fouling or impairment includes, but are not limited to, material/fines rising up from the subgrade, damage attributable to friction processes, spillage during transport and contamination e.g. by chemicals used to treat timber sleepers, welding residues, etc. Fouling or impairment of the track ballast affects its functionality i.e. the material loses its mechanical properties (e.g. becomes worn), reduces the elasticity of the track, impedes water permeability etc., has the potential to degrade the natural environment and poses a significant public health risk. Consequently, in order to sustain its function, conserve the environment and protect the communities directly or indirectly exposed to contaminants it is imperative to maintain, redistribute, screen (remove unsuitable material), clean and/or replace rail track ballast material.

Current practices implemented to counter or react to fouling involves the use of ballast maintenance systems/ballast cleaning, distribution and profiling machines to clean, redistribute or remove, replace and dispose of unsuitable ballast material when required. Unsuitable track ballast material is removed from the track bed and replaced as part of track maintenance activities. However, the possibility of the reintroduction of contaminated material and the dispersion of pollutants to other locations is high. Reason being, the existing ballast maintenance systems/ballast cleaning, distribution and profiling etc. machines are not designed to deal with, remove or treat most contaminants e.g. fuel and lubricants residues, hazardous elements of chemicals used to treat timber sleepers, metals from track corrosion and wear, welding residues, toxic residues of cargo etc. Reprocessed contaminated ballast stone with the potential to contaminate newly installed or previously uncontaminated material therefore remains part of the rail track system. Furthermore, ballast stone discharged as unsuitable material adjacent to rail track systems or carted to various locations has the potential to contaminate the surrounding environment. In addition to the aforesaid, both scenarios have the potential to detrimentally affect the natural environment and to become a public health nuisance. Consequently, there is a need to clean railway ballast material.

Asbestos is another contaminant that has been identified in rail track beds/on track ballast material and is often spread throughout the rail track structure and deposited on adjacent areas during the cleaning, redistribution, profiling, removal, replacement and/or discarding of track ballast material and/or through natural processes. The aforesaid processes increase the extent of pollution by spreading the contaminant throughout the rail track system and to neighbouring areas. Cleaning, redistribution, profiling, removal, replacement and/or discarding of track ballast material therefore exacerbate the current situation regarding asbestos contamination. More importantly, track ballast maintenance etc. exposes rail staff, personnel accompanying ballast maintenance systems/ballast cleaning machines and the general public to the contaminant. Compressed air-atomised water spray nozzles and fine spray nozzles strategically placed near dust sources are in fact used to suppress dust and minimise the exposure factor. However, such a system still does not solve the problem with regards to persistent track ballast contamination and the pollution of surrounding areas. When identified, asbestos contamination usually causes such an environmental/public health concern that track ballast material contaminated with asbestos is typically removed from the area and replaced, irrespective of the structural condition of the ballast material. However, more than often only the track ballast stockpiled adjacent to rail structures and material not affecting the stability/integrity of the rail structure is removed resulting in "inaccessible" asbestos being left behind. The reasoning behind the aforesaid is that the dismantling and the subsequent reassembly of the railway structure to accommodate complete asbestos removal is an expensive activity.

Current asbestos abatement practices involve the removal of contaminated track ballast and disposal of the aforesaid at a licensed landfill site. Track ballast material is typically heavy resulting in its removal for disposal becoming a costly activity - as is the provision of replacement ballast material. In addition to the aforesaid, if the contaminated ballast material is discarded, e.g. on landfill sites, the environmental and public health hazards posed by the contaminants are still not addressed unless the landfill sites and its personnel are equipped to handle such hazardous materials. For example, the contaminants can leach into groundwater or be deposited onto surrounding areas. Discarding hazardous materials in accordance with legislation and at suitably equipped and licensed landfill sites is costly - especially if this is how the bulk of contaminated track ballast material is to be disposed of and if contaminated areas are located far from the nearest licensed landfill site. Furthermore, current licensed landfill sites are running out of airspace and the land available for the establishment of new suitably equipped hazardous waste disposal sites is becoming scarce - which will in due course increase disposal costs even further.

It is possible to wash contaminated track ballast material in order to reduce removal & disposal costs. However water is a scarce/finite natural resource, the processes are costly and the process/contaminants pollute previously unpolluted water used in the system - which then has to be cleaned in costly processes (e.g. to remove heavy metals, hydrocarbons etc.) before returning it to the environment or reusing it in the process. In addition to the aforesaid, these processes would typically use large quantities of fresh water and require suitable water use licenses (for the discharge of water to land/water course). The processes for obtaining such licenses are typically time-consuming and costly. Furthermore, effluent originating from wash plants is often not monitored and analysed adequately or frequently enough which poses significant environmental and public health risks.

The present invention seeks to conserve primary resources, optimise the use of track ballast, provide a cost-effective and environmentally responsible method of treatment for contaminated track ballast material (in situ/in place and/or removed from the rail track bed), remove contaminants from the rail track system and to provide a solution to the persistent reintroduction of contaminants throughout the rail track system.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method for treating contaminated aggregate, said method comprising:

feeding aggregate to a cavity inside a chamber;

passing the aggregate through the cavity;

exposing the aggregate to at least one jet of pressurised air, while passing the aggregate through the cavity, to dislodge contaminants from the aggregate and suspend the dislodged contaminants in air;

withdrawing air from the cavity, with at least some of the dislodged contaminants suspended in the air; and

removing at least some of the suspended contaminants form the air.

The method may include maintaining the cavity at a pressure that is lower than the pressure outside the chamber, i.e. at a negative pressure.

The method may include brushing the aggregate.

The method may include supporting the aggregate on a perforated substrate such as a wire mesh conveyor, a screen, a rotary trammel, or the like, and may include directing at least some of the jets of pressurised air onto the aggregate from below, through the perforated substrate, or from any other angle. The method may include directing at least some of the jets of pressurised air onto the aggregate, from hand operated nozzles.

The method may include separating fine material from the aggregate prior to exposing the aggregate to a jet of pressurised air, e.g. by screening - preferably through multiple sized screens or sieves.

The method may include agitating the aggregate inside the cavity, e.g. by rotating the aggregate in a rotary drum. The aggregate may be fed to the cavity by vacuum suction.

The method may be applied to various aggregates and is particularly suitable for separating contaminants from aggregates in the form of rail track ballast material. BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how it may be put into effect, the invention will now be described by way of non-limiting example, with reference to the accompanying drawings in which: Figure 1 shows a schematic side view of a first embodiment of apparatus, in use, for cleaning aggregate according to the present invention;

Figure 2 shows a schematic plan view of part of the apparatus of Figure 1 ;

Figure 3 shows a schematic side view of a variation on the first embodiment of apparatus, in use, for cleaning aggregate according to the present invention; Figure 4 shows a schematic cross-sectional view of a rotary drum of a second embodiment of apparatus for cleaning aggregate according to the present invention;

Figure 5 shows a schematic side view of the second embodiment of apparatus for cleaning aggregate according to the present invention;

Figure 6 shows a schematic sectional view of part of the apparatus of Figure 5; and Figure 7 shows a schematic side view of a third embodiment of apparatus for cleaning aggregate according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to Figures 1 to 3, plant or apparatus for cleaning aggregate such as rail track ballast material according to the present invention, is generally identified by reference number 10. The same reference numbers are used in Figures 1 to 3 and differences between the drawings (e.g. the omission of the external enclosure in Figure 1 and of the cylindrical brush and blowguns in Figure 3) are mere variations on the process and/or are for clarity of illustration, while the methods illustrated in Figures 1 to 3 are essentially the same and are described below as a single process. These methods / processes are in any event mere examples of the present invention and can be varied, in practice.

The plant 10 includes a chamber housing 12 that partly encloses a chamber cavity 14 and the chamber housing 12 is, in turn, housed inside an external enclosure 16, which could be a cargo container, or the like. The air pressure inside the external enclosure 16 is maintained at a moderate vacuum, by withdrawing air through an extractor 18 and controlling flow of air into the enclosure via an inlet 20. The purpose of maintaining this vacuum is to avoid contaminants from escaping from the enclosure 16 and air samples are taken from time to time, as necessary, to ensure that this is effective. Typical flow of air inside the external enclosure 16 is shown by arrows in Figure 3.

A supply conveyor 22 supplies contaminated aggregate, which could be particulate rail track railway ballast material, or other construction aggregate - and which is referred to as "stone" 24 herein, for brevity. The supply conveyor 22 passes through the external enclosure 16 with a narrow fit and with suitable arrangements such as flexible curtains, to minimise air ingress (if desired) and the stone 24 is deposited from the end of the supply conveyor onto an inclined grid 26 of wire mesh, or the like, which allows fines to pass through the grid under gravity and to be collected in a fines container 28.

The stone 24 is discharged from the end of the inclined grid 26 onto a perforated substrate in the form of a wire mesh conveyor 30, on which the stone is supported while the conveyor 30 passes it through the chamber cavity 14. Air pressure inside the chamber cavity 14 is also maintained at a vacuum (preferably even lower than the external enclosure 16) and the feed of stone 24 into and out of the chamber cavity by the wire mesh conveyor also takes place with suitable arrangements to minimise air passing between the enclosure 16 and the chamber cavity 14. The purpose of maintaining the vacuum in the chamber cavity 14 is to avoid contaminants from escaping from the cavity and air samples are taken as necessary, to ensure that this is effective. Typical flow of air inside the chamber cavity 14 is shown by arrows in Figure 3.

Inside the chamber cavity 14, the stone 24 passes underneath a self-adjusting, rotating cylindrical brush 32, which loosens contaminants such as asbestos fibres from the surfaces of the stone. Compressed air is supplied via a manifold 34 to an array of upwardly directed fixed nozzles 36 that are positioned below the wire mesh conveyor 30 and jets of compressed air blow from the nozzles, through the wire mesh conveyor onto the stone. Further nozzles (not shown in the drawings) can be provided to direct jets of compressed air onto the stone 24 from the sides and/or from above. Hand-operated nozzles or blowguns 38 are supplied with compressed air from the manifold 34 or elsewhere and can be operated by hand to direct jets of compressed air onto the stone. The supply of compressed air to the manifold 34 and blowguns 38 is not shown in the drawings in detail, but these supplies are conventional in design, using compressors, piping and the like, to supply compressed air from outside the chamber housing 12 in an airtight manner. Exposing the stone 24 to these various jets of pressurised air dislodges contaminants such as asbestos fibres from the stone surfaces and suspends the dislodged contaminants in the air inside the chamber cavity 14.

While the stone 24 passes through the chamber cavity 14, it can be spread evenly on the wire mesh conveyor 30, preferably by a suitable spreader (not shown) as the stone enters the chamber cavity 14, but also by the cylindrical brush 32 and/or by hand, via glove ports 40 - which also allow operators access to the blowguns 38. A laminated window 42 is shown in Figure 1 , to allow operators to view the inside of the chamber cavity 14. Air inside the chamber cavity 14, with suspended contaminants, is withdrawn from the chamber cavity via an extraction point 44 and this is preferably done by means of a vacuum that is strong enough to maintain the negative pressure (vacuum) inside the chamber cavity mentioned above, to prevent contaminants from escaping from the chamber cavity, other than via the extraction point 44.

The air with suspended contaminants withdrawn from the chamber cavity 14 via the extraction point 44, is fed to a filter unit (not shown in Figures 1 to 3) where the contaminants are removed from the air. The filter unit preferably includes an industrial centrifugal (cyclone) filter for initial recovery of contaminants, a bag filter unit for further recovery of contaminants and lastly, a high-efficiency particulate air filter (HEPA filter). The filtered air is vented to atmosphere. The filter arrangement will depend on the nature and quantities of the contaminants to be removed from the stone.

After the stone 24 has been treated in the chamber cavity 14, the stone is guided by guides 46 to narrow the process stream of stone and the wire mesh conveyor 30 feeds it through exit openings in the chamber housing 12 and external enclosure 16 (with suitable arrangements to minimise air ingress into the chamber housing and external enclosure) and the treated stone is stockpiled for reuse, after suitable testing to ensure adequate decontamination of the stone.

Filter elements in which the contaminants have been recovered, as well as organic and inorganic contaminants collected in dust collectors or bag filters, are removed from the plant 10 time to time and are double bagged, labelled and discarded as hazardous waste. Instead, or in addition, these waste materials can undergo thermal treatment such as incineration, to reduce the volume and/or mass or material that requires disposing, even further. The volume and mass of this hazardous waste are very small fractions of the volume and mass of the contaminated stone that has been treated in the plant 10 - and that would otherwise have needed disposal as hazardous waste.

Referring to Figures 4 to 6, another embodiment of plant for cleaning contaminated stone 24 is shown. This plant is identified, generally, by reference number 60 and features that are common between the plant 60 and the plant 10, are identified by the same reference numbers - even if the configurations of these features vary between the plants 10 and 60. The plant 60 includes a chamber housing 12 that encloses a chamber cavity 14 that is preferably maintained under vacuum. Inside the chamber cavity 14, a robust rotary drum 62 or trommel is provided, which is supported and driven to rotate about an inclined rotational axis 64. The drum 62 has a generally cylindrical shape with an inlet opening 66 at its upper end and a discharge opening 68 at its lower end. Along an upper portion of the drum 62, circumferential flanges 70 extend inwards from the cylindrical wall of the drum and longitudinal ribs 72 extend between the flanges, to form a rib cage-like structure, with multiple box-shaped open recesses 74 along the inside of the drum 62. A lower part of the inlet opening 66 is in register with an opening 76 in the chamber housing 12 and on the outside of the chamber, a hopper or chute 78 feeds the contaminated stone 24 to the opening 76. A manifold 34 extends into the inside of the drum 62 through the inlet opening 66 and is connected to a supply of compressed air (not shown), to blow jets of the compressed air through multiple nozzles 36 on the inside of the drum. An extraction inlet 80 extends into the chamber cavity 14 through the discharge opening 68 and is configured to draw air from the chamber cavity and feed it to a filter unit 82. (An alternative position for the filter unit 82 is shown in broken lines in Figure 5.) The operation of the plant 60 is similar to the plant 10 described above, in that contaminated stone 24 is fed into the chute 78 and is fed via the openings 76 and 66 to the inside of the chamber cavity 14, under gravity, while avoiding excessive air ingress with the infeed of stone 24. Inside the drum 62, rotation of the drum agitates the stone 24 by causing the stone to be lifted up the ascending side of the rotating drum, inside the recesses 74, but to drop from the recesses, under gravity, into other recesses below, as the recesses approach the top of their rotation. The impact of this dropping action assists in dislodging asbestos fibres and other contaminants from the surfaces of the stone 24. In an alternative embodiment, a drum 62 with an internal spiral formation can be used (nor shown), but the ribcage-like structure described above is easier and less costly to manufacture and its operation is preferred.

As the stone 24 is repeatedly dropped inside the rotating drum 62, it is also exposed to jets of compressed air from the nozzles 36 - which can be aimed at the stone in any direction, but the manifold should preferably be clear of the path of dropping stone. The jets of compressed air also assists in dislodging asbestos fibres and other contaminants from the stone 24 and causes the contaminants to be suspended in the air inside the chamber cavity 14. The repeated dropping action of the stone, imparted by the rotating drum 62 causes the individual stones to be "flipped over" and to have different sides/aspects of the stones exposed to the jets of compressed air.

The contaminants preferably remain trapped inside the chamber cavity 14 (owing to the moderate vacuum maintained in the chamber cavity) and the air, with suspended contaminants is constantly withdrawn through the extraction inlet and the air is filtered in the filter unit 80, much as described above with reference to Figures 1 to 3. The rotation of the drum 62 at its inclined orientation, causes the stone 24 to migrate gradually towards the bottom of the drum and clean stone is discharged from the bottom of the drum through a suitable discharge arrangement. As the stone 24 migrates along the drum 62, it is also screened by screens provided at intervals in the ribcage structure, in the form of perforated sections within the drum.

Referring to Figure 7, another embodiment of plant for cleaning contaminated stone 24 is shown. This plant is identified, generally, by reference number 90 and features that are common between the plant 60 and the plant 90, are identified by the same reference numbers - even if the configurations of these features vary between the plants 60 and 90.

Like the plant 16 shown in Figures 4 to 6, the plant 90 includes a chamber housing 12 enclosing a chamber cavity 14 that is preferably maintained under vacuum, a perforated rotary drum 62 or trommel that rotates about an inclined rotational axis 64 in the chamber, with an extraction inlet 80 extending into the lower end of the drum, for withdrawing air with suspended contaminants from the inside of the drum and feeding it to an external filter unit.

At the higher end of the drum 62, the chamber housing 12 is connected to a feed hopper 92. Stone for decontamination can be fed to the hopper 92 in various ways, but in a preferred embodiment of the invention, the stone is lifted from a rest position, such as a position forming part of a rail track ballast, by means of vacuum suction and the vacuum suction is used to convey the stone to the hopper 92. Some contaminants and/or fines that are conveyed along with the stone, but that does not adhere to the stone, are separated from the stone by suitable mechanical separation means, such as centrifugal separation, screening, or the like and only the stone is fed to the hopper 92, while the fines/contaminants are collected separately.

Another separation of contaminants from the stone takes place inside the cavity 14. The stone fed via the hopper 92 is received in a separation mechanism 94 which could take various forms, such as an inclined screen; fixed, rotating and/or reciprocal brushes; an array of rollers; or the like - and preferably includes any apparatus that can dislodge contaminants from the stone mechanically, with friction, agitation, or the like. The separation mechanism 94 is preferably inclined to allow stone to travel under gravity along the separation mechanism and to be deposited inside the drum 62 at a position intermediate the ends of the drum, where fines and/or contaminants dislodged from the stone in the separation mechanism 94, passes through the perforated wall of the drum 62 and is collected in a waste bag 96, or the like.

Compressed air is blown via a manifold and nozzles (not shown) in a plurality of jets from the outside of the drum 62, through the perforated cylindrical wall of the drum, onto the stone that is agitated by rotation of the drum and that migrates gradually longitudinally along the drum under gravity. Instead, or in addition, compressed air may be blown onto the stone via jets directed to the stone from other angles, while the stone is inside the cavity 14.

Apart from the differences discussed above, between the plant 90 and the plant 60 shown in Figures 4 to 6, the plant 90 operates substantially the same as the plant 60, with contaminants being lifted from the stone in the drum by the air jets impinging on the stone, the dislodged contaminants being suspended in the air inside the cavity 14, from where it is withdrawn through the extraction inlet 80, to be filtered.

Referring to all the drawings, the plant 10, 60 and/or 90 can be a permanent/fixed plant or it can be mobile, which could allowing it to be self-propelled or it could be towed, e.g. it can be towed on tracks. The plant can be used independently, but in embodiments of the invention where the plant is intended for removing contaminants from rail track ballast material, the plant can be incorporated into, or used in combination with rail structure maintenance systems, ballast cleaning or maintenance machines, or the like.

Different features of the different embodiments of the invention described above, are interchangeable with one another and/or can be used in different combinations. In particular, the different perforated substrates in the forms of the wire mesh conveyor 30 and the rotary screen of the drum 62 can be interchanged, combined and/or substituted with other perforated supports (e.g. a vibrating inclined screen). Similarly, various feed mechanisms such as the vacuum supply described with reference to Figure 7, the conveyor 22 shown in Figure 3, and/or other feed structures such as the chute 78 of Figures 5 and 6, the inclined screen of Figure 1 and the hopper 92 of Figure 7; can be interchanged, combined and or substituted. Also, the various nozzle arrangements shown in the different embodiments can be interchanged, combined and/or substituted.

Further to the advantages of the present invention mentioned above, which includes removing asbestos fibres from the stone 24, the invention also removes other contaminants such as sand, grit, silica dust, lead concentrate, manganese and other heavy metals.

Further, the invention reduces the carbon footprint linked to transportation of the stone 24, reduces the exploitation of natural resources by recovering and re-using the stone, reduces the need for new "tipping" sites for dumping discarded material, which reduces the need to use productive land for such tipping sites, and decreases the load on landfill sites that are designated to accept hazardous materials.