Landfill mining
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Landfill mining and reclamation (LFMR) is a process whereby solid wastes which have previously been landfilled are excavated and processed.[1] The function of landfill mining is to reduce the amount of landfill mass encapsulated within the closed landfill and/or temporarily remove hazardous material to allow protective measures to be taken before the landfill mass is replaced. In the process mining recovers valuable recyclable materials, a combustible fraction, soil, and landfill space. The aeration of the landfill soil is a secondary benefit regrading the landfills future use. The overall appearance of the landfill mining procedure is a sequence of processing machines laid out in a functional conveyor system. The operating principle is to excavate, sieve and sort the landfill material.
The concept of landfill mining was introduced as early as 1953 at the Hiriya landfill operated by the Dan Region Authority next to the City of Tel Aviv, Israel.[2] Waste contains many resources with high value. The most notable of which are non-ferrous metals such as aluminium cans and metal scrap. The concentration of aluminium in many landfills is higher than the concentration of aluminum in bauxite from which the metal is derived.
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[edit] Practical Applications
Landfill mining is also possible in countries where land is not available for new landfill sites. In this instance landfill space can be reclaimed by the extraction of biodegradable waste and other substances then refilled with wastes requiring disposal.
Mining construction landfill sites is the simplest form of landfill mining. Construction landfills contain three basic components, wood, scrap metal and gypsum, or drywall, along with a minimal amount of other construction materials. The wood collected can be used as fuel in coal burning power plants and the scrap metal reprocessed.
Mining of municipal landfills is more complicated and has to be based on the expected content of the landfill. Older landfills, in the United States before 1994, were often capped and closed, essentially entombing the waste. This can be beneficial for waste recovery. It can also create a higher risk for toxic waste and leachate exposure as the landfill has not fully processed the stewing wastes. Mining of bioreactor landfills and properly stabilized modern sanitary landfills provides its own benefits. The biodegradable wastes are more easily sieved out, leaving the non biodegradable materials readily accessible. The quality of these materials for recycling and reprocessing purposes is not as high as initially recycled materials, however materials such as aluminum and steel are usually excluded from this.
Landfill mining is most useful as a method to remediate hazardous landfills. Landfills that were established before landfill liner technology was well established often leak their unprocessed leachate into underlying aquifers. This is both an environmental hazard and also a legal liability. In the US, Environmental Protection Agency fines can tax the local economy up to 30 years after the site has closed. Mining the landfill simply to lay a safe liner is a last, but sometimes necessary resort.
[edit] Tools and Machinery
The parts of the mining process are the different mining machines. Depending on the complexity of the process more or fewer machines can be used. Machinery is easily transported on trucks from site to site, mounted on trailers. The following machines are added in order in increase of mining complexity.
- Excavators
- Moving floor and elevator conveyor belts
- A coarse rotating trommel screen
- A fine rotating trommel screen
- A magnet
- Front end loader
- Odour control sprayer
[edit] The Mechanics of Mining
An excavator is also called a 'digger', in reference to its function. The excavator is a vehicle with a backhoe and an operator’s cab mounted on a rotating platform. The vehicle runs on two heavy tracks similar to a tank. The excavator uncovers the landfilled materials and places them on the conveyor belt to be taken to the sorting machinery.
Moving floor conveyor belts use reciprocating slats to move material. The connected slats, the conveyor belt, are wrapped in a continuous loop around two rotating pulleys. Elevator belts have regularly spaced partitions with adjustable connections. The most visible example is a mall elevator. The elevator belt is used to transport loose material up steep inclines. The moving floor and elevator belts move the excavated landfill mass from the excavation site to the sorting site, outside of the landfill mass.
A trommel is a screened cylinder that rotates to separate materials by size. A trommel is made of steel and rotates at approximately 3 rpms. A trommel is approximately 20 feet long and 6 feet in diameter. A large trommel has holes 6 inches to 2 feet wide. A small trommel has holes one inch wide. The large trommel separates materials like appliances and fabrics. The smaller, second trommel allows the biodegraded soil fraction to pass through leaving non-biodegradable, recyclable materials on the screen to be collected.
An electromagnet is used to remove the metal material, save aluminum which is not Ferro-magnetic, from the waste mass as it passes along the conveyor belt.
A front end loader is a wheeled tractor with a wide square tilting bucket to lift and move material. Front end loaders can also have the bucket replaced with a hydraulic powered “clamshell” bucket that allows the loader to act like a miniature dozer. As the name loader implies the most efficient use of the machine is to move large amounts of material over short distances. Digging is possible, but the short range of the movable arms does not allow a front end loader to dig very deep. Front end loaders move landfill mass onto the conveyor belts and move sorted materials to trucks for further processing.
Odour control sprayers are wheeled tractors with a cab and movable spray arm mounted on a rotating platform. A large reservoir tank mounted behind the cab holds neutralising agents, usually in liquid form, to reduce the smell of exposed wastes.
[edit] Operational flow
Excavators dig up waste mass and transport it, with the help of front end loaders, onto elevator and moving floor conveyor belts. The conveyor belts empty into a coarse, rotating trommel. The large holes in the screen allow most wastes to pass through, leaving behind the over-sized, non-processable materials. The over-sized wastes are removed from inside the screen. The coarse trommel empties into the fine rotating trommel. The fine rotating trommel allows the soil fraction to pass through, leaving mid-sized, non-biodegradable, mostly recyclable materials. The materials are removed from the screen. These materials are put on a second conveyor belt where an electromagnet removes any metal debris. Depending on the level of resource recovery material can be put through an air classifier which separates light organic material from heavy organic material. The separate streams are then loaded, by front end loaders, onto trucks either for further processing or for sale. Further manual processing can be done on site if processing facilities are too far away to justify the transportation costs.
[edit] See also
[edit] References
- ^ Landfill Mining Landfill Mining, Preserving Resources through Integrated Sustainable Management of Waste, Technical Brief from the World Resource Foundation
- ^ Landfill Mining Paper D.J. van der Zee, et al (2003?) Assessing the Opportunities of Landfill Mining
