History of Thermal Sand Reclamation
Competitive Systems
The first known account of commercial thermal sand reclamation in the Unites States is believed to have occurred in the 1940’s. www.afsbirmingham.com/timeline-of-casting A foundry producing cast iron pipe converted a rabble tooth ore roaster to thermally reclaim shell sand used for the bell end of centrifugally cast pipe. This was rather an oddity in that the cost of mined silica sand, along with transport costs was very low for most foundries.
The Arab oil embargo began in 1973. This, combined with mounting EPA regulations caused a significant increase in the delivered cost and the cost of disposal of sand for many foundries. Hence, solutions were researched to alleviate the cost increases. Equipment specifically designed for thermal sand reclamation was then introduced to the industry as a replacement for new sand.
In the 1970’s, the earliest thermal reclamation systems were developed using high temperature rotary kilns. Kilns have historically been used to create a chemical change of the material being processed, where the kiln’s atmosphere was not part of the reaction. In the case of thermal sand reclamation, reaction with the oxygen bearing atmosphere inside the kiln is required. The problem is that in conventional kilns, sand has limited contact with the oxygen bearing atmosphere. At any point in time, a high percentage of the sand is below its own surface in the kiln, effectively smothering itself. Because of the poor exposure, the conventional kiln requires a long retention time which makes the equipment rather large. They also typically use a tire and trunion arrangement for rotation. This requires precision alignment and fully machined bearing surfaces. The overall result is a large, expensive piece of equipment that provides marginal results. The use of rotary kilns eventually gave way to fluid bed systems for thermal sand reclamation.Fluid bed reclaimers did solve the problem of sand exposure to the atmosphere. However the fluidizing gas must serve two masters: 1) the volume required for fluidization. 2) the volume required to provide oxygen for combusting the carbon and hydrocarbons in the sand. The air required for fluidization is usually much greater than that required for the combustion of the material in the sand. The result is lower efficiencies caused by heating excessive amounts of fluidizing air. Also, beginning at the colder inlet end, and continuing until the sand is heated to a temperature where combustion can occur; much of the VOC’s are released into the atmosphere above the sand surface where it ignites and heats the atmosphere, but not the sand. In order to help the efficiency, heat exchangers may be added to the system. This helps the energy consumption, but drives up the complexity and the cost.