Modern Sand Reclamation Technologies for Economy, Environment Friendliness & Energy Efficiency

Unlike green sand, chemically bonded sand cannot be used again and again without reclamation because in this system, the mixed sand gets it`s strength through chemical reaction which is irreversible in nature. Each sand particle is coated with this reacted chemical, which behaves like an inert element after usage once. This inert coating also called “Dead” binder needs to be removed from the used sand because it being brittle in nature gives rise to increased fines in the sand. “Dead” binder is present in the form of layers adhering to the surface of the sand grains. These layers if not removed changes the property of the sand and makes it totally unsuitable for further usage because proper strength would not be achieved even with higher chemical percentage. Sand grain modification is another important aspect of reclamation. During reclamation rubbing take place between sand grains and also against surfaces of the various equipment of the reclamation system at different stages. As a result sharp corners of the original sand (which is available from the nature) get rounded causing reduced surface to volume ratio, which ultimately reduce binder demand. This obviously reduces the chance of getting defective casting due to formation of gas. Main methods of Sand Reclamation are – (1) Mechanical Attrition Reclamation and (2) Thermal Sand Reclamation. In Mechanical Attrition Reclamation rubbing of sand grains against each other take place by mechanical means like vibration, fluidization etc. This process cannot remove all the binder coatings. As a result about 10 to 20% fresh sand needs to be added with the sand to keep LOI value within limit. Generally such sand is reused for the same binder system. Thermal Reclamation is actually a combination of Mechanical Attrition Reclamation and Thermal Reclamation. In this process, mechanically reclaimed sand is heated to a temperature of about 800 degrees C. Heating takes place in a specially designed furnace where the sand is both fluidized as well as heated. Thus rubbing of sand against sand takes place here, too. Various equipment involved in a Thermal Reclamation System are – Lump Reducer, Pneumatic Transporter, Screw Feeder, Combustor with Sand Preheater and Air Preheater, Fluidized Bed Cooler and Dust Extraction System. Of these equipments Combustor is the special furnace in which the mechanically reclaimed sand gets fluidized as well as heated. To utilize the waste heat both incoming sand as well as air is preheated with the help of waste flue gas coming out of the system. From the Screw Feeder mentioned above, mechanically reclaimed sand is fed to the Sand Preheater of the Combustor whereby the incoming sand to the furnace gets preheated. Afterwards, the sand is both fluidized as well as heated in the furnace itself. As a result rubbing of sand against sand as well as burning of dead binders, both are accomplished at the same place. Thermally reclaimed sand is better than mechanically reclaimed sand as well as fresh sand from various aspects as follows : (1) Thermally reclaimed sand undergoes lower thermal expansion causing better mould stability. (2) Thermally reclaimed sand is better than fresh sand because it is more rounded in shape causing lesser binder demand. (3) Irrespective of the binder system in the previous cycle, thermally reclaimed sand can be used with any chemical binder system in the subsequent cycle. (4) As most of the sand is reused, almost no dumping is necessary resulting in safer environment. (5) Conserves natural resources by eliminating requirement of new sand. (6) This is a highly energy efficient process.