Objective
The recovery of post-consumer scrap from domestic refuse and other secondary sources poses a number of difficulties which hitherto have proven insurmountable.
Post consumer scrap typically contains varying amounts of paper, plastics, organics, putrescibles and aluminium which must be removed almost entirely before the tin and steel can be removed effectively.
The primary objective of the Llanelli project was to scale-up from a pilot plant to a plant processing and producing:
- a ferrous, shredded product of exceptional cleanliness and minimal aluminium content prior to detinning.
- a detinning process to recover tin and produce a pure ferrous product of proven quality for subsequent use in high grade steel making.
During the year 1,486 tonnes of raw material was processed, yielding 0.78 tonnes of tin scraped off cathodes and 1,313 tonnes of detinned product.
FERROUS
By varying the shredder impeller/cage differential speeds, the density of the ferrous pellets could be varied. The preferred density to ensure adequate detinning and production throughputs was 1,120 - 1,200 kg./cubic metre. The tin content of the ferrous product from the Preparation Plant had a tin content varying between 0.16 and 0.25% tin; the weighted average was 0.21%.
The detinned production varied in tin content from 0.09 to 0.06% tin; the weighted average was 0.072%
TIN
Tin content of deposit from cathodes averaged 88.3%.
MATERIALS BALANCE
An attempt at Materials Balance was conducted based on bulk weights processed and produced.
PROBLEMS
Problems encountered in processing were directly attributable to the higher than anticipated aluminium content of the raw material. In the short term, these were overcome by employing aliquor bleed-off to maintain the caustic soda concentration/balance.
Whereas this has proved successful, it does result in a loss of tin from the system due to dumping liquors containing 5 gpl tin (equivalent to 800 kg. tin in the system). Tin losses could be reduced by prolonged electrolysis, i.e. reducing electrolytic detinning efficiency.
1. The main source of post-consumer cans can be classified as follows:
- Cans magnetically extracted from raw or processed refuse.
- Cans from voluntary collection centres (can banks).
- Cans from kerbside collection ventures which collect and separate tinplate/aluminium containers. 2. Cans are delivered to the plant in loose, flattened, crushed or baled form. Baled cans are broken apart in an impact mill before processing.
3. Mixed can materials are fed by crane onto a belt conveyor and pass under an overband magnet to remove aluminium cans, plastic bottles and other non-ferrous contamination. This stream then passes through an eddy current magnet to produce clean aluminium.
The tinplate cans are then conveyed to a primary shredder for first stage reduction followed by air classification to remove coarse dirt. The resultant shred is then conveyed to a secondary shredder for final reduction to liberate aluminium from bi-metallic cans and is followed by additional air classification. The final shredded product is then passed over a double drain magnetic separator to remove secondary aluminium and then conveyed to storage bunkers.
4. The shred is conveyed from the bunkers to the loading station on the detinned section which comprises 4 electrolytic strimming tanks containing 4 baskets and 5 cathodes each. The electrolyte is a dilute caustic soda solution heated to 70 deg. C and current is applied at 5,000 amps. After the tin has been stripped from the shredded material the ferrous material is discharged from the baskets with water and conveyed to a bunker to await despatch. At regular intervals, tin deposited on the cathodes is stripped, washed and compressed into briquettes for subsequent sale.
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Coordinator
B17 9NJ Birmingham
United Kingdom
The total costs incurred by this organisation to participate in the project, including direct and indirect costs. This amount is a subset of the overall project budget.