Publisher: Marco Cassens, Förder- und Anlagentechnik (FAT), Germany
Is used sand a questionable hazardous waste or a usable raw material? How do new sand and landfill costs develop? Are there ways to regenerate used sand and how much waste can be expected? Is there a gentle regeneration process that hardly changes the moulding material properties? And can such a process become a worthwhile investment for foundries through good results and moderate energy consumption?
Foundry operators will have to deal with these questions more intensively in the future.
FAT’s thermal reclamation plant : A solution approach
After mechanical reclamation, the moulding material must be recycled with as consistent a quality as possible. For this purpose, it is often necessary to dispose of a part of the sand, at a high price and enrich the resulting “gap” with new sand. If the internal sand circulation is now supplemented by the thermal regeneration plants “own new sand”, the disposal costs for used sand, as well as procurement costs for new sand, can be reduced by up to 95%. Due to its compact and modular design, the thermal regeneration plant can be integrated into any existing plant concept and is designed for continuous operation without additional operating personnel. In recent years, concepts for used sand based on organic binding agents such as furan, cold box, pep set, croning or alpha set have been implemented.
For test purposes, FAT has installed a thermal regeneration plant with a capacity of 0,5 t/h at their headquarters in Niederfischbach. Here, cold resin sands can be thermally regenerated by the customer on a trial basis and then analysed.
Focal Point Furnace Bottom
The quality of the sand is heavily reliant on the air-gas mixture. The pore burner system developed by FAT, ensures an even and continuous thin layer of sand to flow on the fluidization bottom of the combustion chamber.
In the furnace, each individual grain of sand is continually in the flame and therefore treated optimally. A “flame carpet” on the furnace floor ensures particularly good combustion of the binding agents and constant quality of the sand batches. This means that even finer grains can be regenerated very well. In a following unit, the hot sand is simultaneously cooled and dedusted. In order to reduce gas consumption, the thermal energy of the hot sand is partially recovered during cooling and returned to the process.
Thermal Reclaimed Sand
After thermal treatment, the organic binder shells are nearly completely burnt. In addition, there is the geometric change of the individual grains after the thermal treatment. Above a temperature of 573°C, the quartz leap ensures a reversible change in volume. Not only residual binders but also corners and edges flake off the grain. As a result, thermal reclaimed quartz grains have a specifically smaller surface area than many new sands and thus have a reducing effect on binder consumption.
The thermal reclamation plant leads to effective waste reduction with moderate energy consumption and can therefore be regarded as a contribution to environmental protection and resource conservation. Especially viewed against the background of economic considerations, this solution is becoming more and more interesting for operators. Payback times of 1.5 to 2.5 years are realistic. With increasing plant size and sand throughput, this value decreases accordingly.
An Outlook on Future Application Possibilities
In addition to the recycling of used sand, the disposal of filter dusts is also becoming more and more important in many companies. The reasons are the same as for the problem of used sand.
FAT’s thermal reclamation plant is also a solution here. Filter dust from no- bake plants can be used as an energy source for combustion in the furnace. To analyse the behaviour of the dust during operation of the reclamation plant, the FAT test plant was extended by a dosing unit. A defined amount of dust was added to the used sand in the combustion chamber.
Results are promising, because during the thermal treatment in the furnace, the loss on ignition of the dust could be reduced by ~50% and the gas consumption could also be reduced because of the energy of the residual binder in the dust. Due to the fine sand content in the dust, which leaves the plant with the thermal reclaim, the amount of dust could also be halved.
Advantage Over Mechanical Regeneration
The organic load in the resulting filter dust of the thermal regeneration plant is as follows: ~1 to 2% of the sand circulation volume and therefore explicitly below the high loaded dust content of approx. 5 to 15%, which occurs during purely mechanical regeneration.
