Efficiency of processing operations can be calculated by comparing the theoretical throughput of the machine against the actual throughput within a given time period and stating this in percentage terms. This simple measurement takes into consideration all of the factors impacting the machine operation including downtime for non-routine maintenance and operator efficiency. Typically within an industrial environment, process efficiencies of 95% are normally targeted, with 85% being an acceptable efficiency level. Within sorting installations, experience has shown that these efficiency levels are very difficult to reach and the machine efficiency of the Sortbat equipment is in the order of 70%. Two factors are very influential in determining efficiency of battery sorting. The quality of the batteries to be sorted, and the balance of the machine throughput in terms of individual cell sizes, versus the actual mix of cell sizes found within the collected cells.
It is important to recognise that sorting of collected batteries is a waste handling operation. Rarely are batteries received into the installation in the same condition in which they were put onto the market. The batteries casings can be damaged by bulk handling or corroded due to storage in outdoor, humid conditions. Furthermore they will also contain a degree of non-battery waste, including packaging materials, nails, film canisters and a whole host of other foreign objects, which the equipment was not designed to handle. Consequently downtime due to blockages within the equipment can be frequent. This problem can be tackled in one of two ways. Firstly the machine can be modified, based on experience, to be more tolerant to frequently occurring problems resulting from battery condition. Secondly the problem of waste and condition of collected batteries can be addressed at source. For any sorting process to be operated efficiently, a combination of both of these approaches is necessary.
The second and equally important factor is the balance of the design throughput of the machine to the actual market mix of batteries. Within the Sortbat machine design, each of the standard consumer cell sizes, AAA, AA, C and D, are either processed separately, or together with only one other size (i.e. AAA and AA together, C and D together). This choice is made in order to best fit the capability of the machine with the mix of batteries being collected from the market. Today the machine operated in The Netherlands has four separate sorting lines: one for a combined C and D stream, two for AA size only and one for a combined AA, AAA stream. Although this is a close approximation for the consumer battery market today, it is not perfect, and efficiencies can be seriously reduced if batches of single size only batteries are received for processing, during which time the lines for the other cell sizes lay redundant.
Other successful battery sorting plants operate in Europe today based on similar principles to the Sortbat model. These include a 1,500 tonne per shift facility owned and operated by Trienekens, Germany, and a 1,000 tonne per year facility at Euro-Bat-Tri, in Lyon France.
The Trienekens operation uses two unique and patented steps during the size separation phase. This includes a rotating, angled disc to separate prismatic batteries
Figure 4. Sensor used to identify zero-mercury batteries from cylindrical ones. Following this, the batteries are loaded into elevators and fed to a polycord based electronic separation stage almost identical to stage IV of the Sortbat process. This section is in fact a refurbished, redundant line from the Sortbat machine, although the central processing unit, as well as certain decision-making criteria, has been changed.
The Euro-Bat-Tri facility was initially designed to pass batteries through the Tri-Mag sensor simply by dropping them under gravity down an inclined tube. This method however, imposed speed restrictions on the operation due to the down-stream mechanical handling involved in separating the selected batteries into their appropriate chemical streams. The facility is currently undergoing a complete refurbishment and the machine is being upgraded to a 10,000 tonne per year operation in line with the anticipated demand from the french market. This high capacity machine will also operate using the polycord transport principle.
Other sorting principles are currently under development, including a vision based system and an X-ray system. The X-ray system is in the late stages of development
Figure 4. Sensor used to identify zero-mercury batteries and should prove to be a very accurate and reliable sorting method. Currently the designers aim is to own and operate the system in-house and not to make the equipment commercially available. The problems with the commercialising such a system are likely to be the capital investment required in the X-ray sensor and the safety aspects of operating such equipment within a waste-handling environment.
The principle of using vision for sorting is not new and is used very successfully in other fields. Cameras take images of the product as it passes and these are compared to a database of images stored within a central processing unit. Due to speed and memory constraints, the processor will not consider the entire image when making its decision on product category, but will concentrate on one or two unique identifiers only.
When applying this technology to batteries, it is important to appreciate that many thousands of unique battery labels are marketed today. These include main manufacturer brands such as Duracell, Energizer, Varta etc., as well as private labelled batteries for retail chains. The database needs to allow for subtle differences in the battery label, such as a change in position of a visible date code, as well as more obvious ones such as a Duracell or Energizer label with or without an on-cell power meter. Historical label changes need to be included too. Each manufacturer has potentially scores of changes to his main brand product label over the past ten years.
For batteries, the unique identifiers could include, for example, the position of the '+' sign on a Duracell label compared to, say, the front edge of the letter 'D'. Each of these particular labels will need to be stored twice within the data base to allow for the battery being presented to the camera either top-up or bottom-up as this will alter the position and orientation of the unique identifier. These identifiers will be different for each individual battery label held within the processors memory.
Early attempts to use this technology as a stand-alone battery sorting method have proved unsuccessful due to the vast processor power and memory required to operate at speed. Having said this, advances in computer processing power and memory seem to be made almost daily and the cost of these powerful new tools are decreasing too. This could therefore prove to be a valuable technology for future sorting operations. In the meantime, such systems are proving useful as an enhancement to the current sensor technology already in use today. Sortbat have recently developed a vision system to complement the selection of mercury free cells for low cost recycling. Due to the restrictions discussed above, this system will not be capable of replacing existing sensors or detectors, but will concentrate on selecting certain label types which are known to be mercury free, yet pre-date the introduction of the ultra violet marker. These could include Philips 'Powercheck', Energizer and Duracell with on-cell testers and Varta batteries which have a '0% Hg, 0% Cd' ring around the cell.
With the elimination of mercury from all primary consumer batteries, a less complex recycling route became available for them. Today alkaline and zinc carbon batteries can be successfully recycled within the existing metals industry. A number of options are available within this sector which can be successfully demonstrated to recycle batteries.
The main area of interest for recycling spent primary batteries is the steelmaking Electric Arc Furnace (EAF). The melting of steel scrap within electric arc furnaces by the metals industry is an example of one of the world's largest and most successful recycling operations. This process is the starting point in recycling steel and zinc from the domestic, commercial and industrial sectors. These include scrap from the construction and automotive industries as well as white goods, food and beverage containers and numerous other end-of-life consumer products.
Electric arc furnaces are operated worldwide at a large scale, typically 250,000 to 750,000 tonnes per year. An electric arc furnace consists of a dished hearth surmounted by a vertical cylinder (the side walls) and capped by a domed roof. The hearth and lower side walls are refractory lined to protect them from the liquid steel and slag. Generally the upper walls and outer roof are constructed of water-cooled steel panels. Three electrodes which carry the electrical power to the furnace enter via the roof. The central roof area is consequently also made of refractory to prevent arcing between the electrodes. Each furnace is also equipped with a door to view the operation and take samples, and a taphole through which the liquid steel is to be poured. Electric arc furnace fume and gases are extracted through a duct in the roof. The dust is extracted from the gas stream using a bag filtration system before the cleaned gases are vented via the stack. Power to the electrodes is supplied from a transformer operating at a secondary voltage between about 200 and 1,000 volts. Supplementary energy can be supplied from a combination of oxygen or carbon lances and oxygen fuel burners.
Scrap metal is charged batch-wise to the furnace from a scrap bucket by removing the roof and electrode structure and dropping the scrap in. Carbon, to aid the steelmaking operation and for extra energy, and lime, to form the basis of the slag, are also added at this time. The roof and electrodes are then brought back over the furnace shell and the charge is melted by energy supplied from the electrodes and supplementary devices. The steel begins to melt at temperatures around 1500°C and when most of it has been melted, a sample is taken to check the chemical analysis and determine alloy requirements. Two or more scrap charges are normally required to provide a full batch. During and after melting, chemical reactions occur between the contaminants and alloying elements in the scrap, the molten slag and the oxygen. This leads to a separation of the unwanted constituents from the liquid steel bath. These elements either dissolve in the slag or form gases which are extracted. The slag floats on the liquid steel and is usually removed by pouring off through the door. The steel is usually ready to 'tap' at between 1600 and 1700°C. At this stage the tap-hole is opened, the furnace tilted and the steel poured into a refractory lined ladle where alloying additions or any further processing is undertaken before the product is finally cast.
The cycle time of the EAF will depend upon the size of the furnace and the power input rate. Electric arc furnaces vary considerably in size from a one tonne total charge weight up to 300 tonnes. The fastest furnaces have 'tap-to-tap' times of around 30 to 40 minutes and power inputs of greater than 1 MW per tonne; slower furnaces may take 2 or 3 hours. This 'tap-to-tap' time is made up of charging, 'power-on time', refining, sampling, and tapping.
One common misconception of the steel industry is that it produces steel products, together with a number of waste streams including slag, dust and millscale, which require disposal. But the truth is very different. Environmental awareness and economic necessity have prompted the steel industry to seek alternative solutions to disposal.
Today the metals industry has evolved to the point where these traditional waste streams have become useful and valuable by-products. Furnace slag, for example, can be crushed, screened and sold as high-grade construction aggregate. Clean millscale is used in several applications, including the cement and ferro-alloy industries. The furnace dust, which is rich in zinc, is commonly sent to the zinc industry where it is processed within a Waelz kiln as part of the zinc metal recovery process. If the level of zinc in the dust is high enough, it can be treated directly within an Imperial Smelting
Furnace (ISF), thereby avoiding an entire processing phase and all of the associated environmental impacts.
At most facilities, external companies are based permanently on the steel-making site to operate by-product recovery processes.
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