|Thermal Reclamation of Chemically Bonded Foundry Sand||United Kingdom||1994||Full scale|
MANUFACTURE OF FABRICATED METAL PRODUCTS # 45
The United Kingdom Environmental Technology Best Practice Program is a government initiative. It promotes the use of better environmental practices that reduce business costs for United Kingdom industry and commerce. It has initiated over 60 Good Practice Guides, case studies and research and development projects. Publications have so far been produced for around half of these projects covering a wide range of topics and industry sectors. The following case study is part of this initiative.
Triplex Alloys Ltd, part of the Triplex Lloyd group, is a major supplier of aluminum castings to the automotive industry with annual sales of £7 million and a production capacity of 1,500 tons/year.
The company produces castings by two principal routes - gravity die-casting and chemically bonded, boxless sand molding. The latter accounts for about 8 tons/week of castings, about 33% of the foundry's output. An additional small output results from low-pressure die-casting and greensand sections.
Cleaner Production Principle
Recovery, Reuse and Recycle; New technology
Cleaner Production Application
Triplex Alloys Ltd at Darlaston in the West Midlands produces aluminum castings which can weigh up to 30 kilos. Castings are primarily produced by gravity die-casting and chemically bonded, boxless sand molding. The company uses phenolic urethane binder systems on its two molding lines and in the core shop.
In common with many United Kingdom foundries, Triplex Alloys uses mechanical attrition (particulation) to reclaim its spent chemically bonded sand. However, binder residues remaining on the particulated sand grains affect the amount of particulated sand that can be used to make molds. New sand must be mixed with the particulated sand to ensure adequate mold strength and casting quality. At Triplex, the requirement for new sand was 75 tons/week costing £58,000/year. Disposal costs for the same quantity of used sand at landfill sites was £24,500/year.
Thermal reclamation is a technique which removes the binder residues left on the sand grains after particulation. Prior to thermal reclamation, extraneous material must be removed from the particulated sand by using screens and, where necessary, separators. Thermally reclaimed sand is technically acceptable for molding and coremaking and can be used instead of new sand. This means that the costs for new sand and disposal of used sand are reduced or eliminated.
In the past, Triplex decided against investing in a thermal reclamation unit because both purchase and disposal costs for sand were low. However, in recent years these costs have increased substantially and disposal sites are now subject to tighter control. This makes the economics of thermal reclamation more attractive. Therefore, in 1994, the company decided to invest in a thermal reclamation unit. The company subsequently applied successfully for the project to be independently monitored under the Environmental Technology Best Practice Program.
Triplex Alloys bought a low-cost, prototype, thermal reclamation unit which is designed to treat 0.5 tons/hour of particulated sand. The unit comprises two "fluidized-bed" chambers located side by side in a steel casing. particulated sand is fed to the unit by an above-bed screw feeder. The sand falls into the first chamber and settles as a bed around a gas-fired burner. Compressed air fluidized the bed, and the burner, which is immersed horizontally in the bed itself, heats the sand to 720oC. At this temperature the binder residues are completely combusted. The temperature is monitored by a probe at the top of the chamber.
Thermally reclaimed sand leaves the first chamber over a regulating weir and enters the second chamber where it forms another bed. This bed is also fluidized and a bank of water tubes immersed in the bed cools the sand to 35oC. The cooled sand is then blown into a hopper for use in the molding and coremaking process.
Emissions from the unit are ducted to a dry-bag filter which is fitted with an automatic shaking mechanism for bag dedusting.
The unit is now working at full capacity. However, some modifications and adjustments were carried out by the manufacturer during the first few months of operation.
Monitoring of the maximum and average levels of particulates, volatile organic compounds (VOCs), combustion gases, formaldehyde and isocyanate emissions was undertaken to: assess compliance with regulatory requirements evaluate the effects of different operating conditions on environmental emissions.
These tests showed that, provided the unit was correctly set up and controlled, emission levels were reasonably constant and process emissions were considerably less than current limits in Process Guidance note 2/4 (1991).
It is essential that emissions are monitored, particularly after installation, to ensure that the combustion conditions are adequate to destroy the binder residues. In addition, good process control is vital in maintaining low emission levels and compliance with environmental regulations. The critical parameters for control are: regular maintenance; an adequate bed and above-bed temperature; control of air supply to the fluidized beds; and, control of the sand feed-rate to within 20% of the rated capacity.
Samples of new sand, particulated sand and thermally reclaimed sand were compared in the laboratory by loss on ignition (LOI) testing, granulometry and photomicrography. Various sand mixes were also assessed for bench life and strength development.
The thermally reclaimed sand had LOI values of less than 0.1%, well below the LOI levels of the new sand (0.29%). Sieve analysis of the various sands revealed no significant differences in average grain sizes. However, the particulated and thermally reclaimed sands were slightly coarser than the new sand due to the presence of agglomerated grains and fines removal during reclamation.
Photomicrographs show that particulated sand grains are coated in binder residues whereas thermally reclaimed sand is as "clean" as new sand.
The shorter bench life of thermally reclaimed sand compared with new sand is probably due to the retention of some contaminants from the casting process, even though it appears "clean". Although there is a slight difference in the bench life of the sand blends, there is no significant difference in their sand molding performance.
One-hour curing strength data indicate that there is little difference between new and thermally reclaimed sands or between sand mixes containing the reclaimed sand.
The foundry normally operates with a basic blend of 70% particulated sand and 30% new sand. For those jobs where surface condition is important, the mix is changed to 50:50.
Castings Technology International's metal-penetration test-piece was used to investigate the effects of replacing new sand with thermally reclaimed sand and of increasing the percentage of new/thermally reclaimed sand in the mix. Examination of the surface condition of the test-piece faces revealed that neither the use of thermally reclaimed sand in the sand mix nor a change from 70:30 to 50:50 had any significant effect,; the surface finish was similar in all cases.
The foundry's scrap records for the six months before and after installation of the thermal reclaimer show that scrap levels for mold production remained consistently less than 2% throughout this period. Scrap records for castings indicate some improvement in both total and sand-related scrap levels, although there is no direct evidence linking the reclaimer to these changes.
Environmental and Economic Benefits
At Triplex, thermally reclaimed sand is now used instead of new sand. Therefore, costs associated with new sand purchase and used sand disposal are virtually eliminated.
The operating costs for the thermal reclamation unit at 0.5 tons/hour throughput, which produces the foundry's requirement of 75 tons/week of thermally reclaimed sand are summarized below.
Thermal reclamation operating costs (1994 prices):
The prototype thermal reclamation unit installed at Triplex cost £30,000. However, it is estimated that, at 1994 prices, a production unit installed at a site similar to Triplex would cost about £48,000.
The significant economic benefits of thermal reclamation are summarized below.
|Purchase cost of new sand (Congleton 60)||£16.80/ton(£58,000/year*)|
|Disposal cost of used sand||£7.10/ton(£24,500/year*)|
|Total cost without reclamation||£23.90/ton(£82,500/year*)|
|Cost of thermal reclamation||£3.96/ton(£13,700/year*)|
|Payback - prototype unit costing £30,000||6 months|
|Payback - production unit costing £48,000||9 months|
* Foundry operating for 46 weeks/year
Installation of a thermal reclamation unit at Triplex Alloys' foundry has eliminated the need to both purchase new sand and dispose of used sand.
Particulate and volatile organic compound emissions from the thermal reclamation unit are well below the prescribed limits, while emissions of formaldehyde, isocyanates and combustion gases are also at low levels.
This case study was submitted to UNEP IE by the ETBP Program (address cited above). It was edited for the ICPIC diskette in July 1997. It has not undergone a UNEP IE formal technical review.
Subsequently the case study has undergone a technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.