Thermally reclaiming furan-bonded sands.
Purchasing new foundry sand and properly disposing used sand have become ever-increasing expenses for today's foundries. However, using the latest technology, they can be minimized.
Thermal reclamation has now improved to the point that 100% sand reclamation can be seriously considered--and applied to coremaking as well.
Two U.K. foundries recently took a major step forward in reducing new sand and sand disposal costs by installing gas-fired, thermal sand reclamation systems for both mold- and coremaking operations.
Factors of Reclamation
Thermal sand reclamation has been used in the U.K. for the last 20 years. Early units, however, were limited to shell molding foundries that used high-priced zircon sand systems. In recent years, several new thermal systems have been installed, mainly by automotive foundries.
These foundries operate on phenolic urethane (Isocure) resin systems, where mechanical reclamation is insufficient to allow reuse in high percentages due to residual reactive binder levels.
Foundries operating furan resin systems can mechanically reclaim 60-95% of sand, depending on the quality of the TABULAR DATA OMITTED material being cast. The average level of mechanical reclamation has been 75-85%. The operation of these systems is governed by residual resin and catalyst buildup on the sand grains, which may be measured in terms of loss on ignition (LOI), acid demand value (ADV), residual sulfur, nitrogen and dust level.
Variation of these characteristics can create casting defects as well as environmental problems because of emissions during pouring. For example, calculations show that a mold made from 100% mechanically reclaimed sand could yield an S|O.sub.2~ emission up to four times greater than the same mold made in new sand, even though the reclaimed sand requires less acid catalyst for bonding than the new sand.
Transportation costs are a major factor in the price of new sand and are unlikely to be reduced. Disposal costs are increasing dramatically due to legislation on dumping. In fact, total disposal costs higher than $100/ton have recently been reported.
These factors significantly altered the economic case for thermal sand reclamation. Payback periods of one to two years are now common for complete installations. An even shorter payback return can occur when existing elements of a foundry can be used or where no sand reclamation is practiced.
The thermal reclamation feed system holds up to 24 hours worth of sand. The sand is fed through the furnace section, which is a multi-zone fluidized bed. Each zone has a bank of burners and its own temperature control system. Fluidizing keeps the sand well-mixed to prevent cold spots and incomplete thermal cleansing. The sand is retained in the furnace to ensure that all resins are burned off.
After leaving the furnace, the hot sand passes through an indirect heat exchanger that transfers heat from the sand back to the furnace's fluidizing air. This is one of the features that contributes to the system's high thermal efficiency.
Next, the cooler-classifier--an indirect, water-cooled, fin-tubed heat exchanger--cools the sand down to ambient temperature. The sand then flows into the classifying section, where higher velocity air removes unwanted fines.
Thermal reclamation systems are totally self-supervising. They are sized on a 24-hr use basis, normally five days a week. Thus, they considerably reduce the required hourly throughput rate as indicated by two examples.
Case A represents a situation in which a foundry decides to reduce new sand purchasing and disposal costs, while Case B illustrates a foundry that is committed to improving its environment while trying to keep new sand and disposal costs to a minimum.
Both U.K. foundries installed Simplicity/Richards PX2500G gas-fired thermal reclamation systems with a nominal capacity of 2.5 tons/hour.
Wilsons Foundry, Bishop Auckland, England, produces carbon, low-alloy, and stainless steel castings such as pump bodies, impellers, valve parts and turbine parts. For 15 years, the firm used a mechanical reclamation system providing an average of 70% reuse, and disposed about 150 tons of sand per week. Molds are bonded with Perstorp Fergusson P252 furan resin catalyzed by paratoluenesulphonic acid (PTSA).
Test results on the thermally reclaimed sand compared favorably with those of new sand. The reclaimed sand had a slightly slower setting time and a higher final strength, which allowed the company to reduce resin addition levels.
Although there was no initial intention to use the thermally reclaimed sand for coremaking, excellent sand test results convinced officials to perform trials on cores. Following successful trial tests, they are now using the sand to produce pepset-bonded cores on a production basis.
The second foundry, Rockwell PMC Ltd., Peterborough, England, created the provision that each and every change made must maintain the company's first-class environmental standards.
The firm installed a thermal reclamation system to treat all of the sand used in the foundry. The foundry produces castings in gray and alloy irons, and a wide range of carbon, low-alloy, stainless and heat-resistant steels weighing up to 10 tons. Molds and cores are bonded with furan resins catalyzed by PTSA.
The strength characteristics of the thermally reclaimed sand are virtually identical to those of Wilsons Foundry, although the higher iron oxide content of the base sand creates a reddish appearance after thermal treatment. Rockwell PMC also reduced the percentage of resin addition.
Both foundries employ the latest gas-fired, fluidized bed thermal reclaimers--featuring zero-expansion linings to reduce maintenance to annual inspection only. Heat is recovered from TABULAR DATA OMITTED TABULAR DATA OMITTED the discharged sand to provide preheating of the combustion air up to 900F.
The measured gas usage on a furan system with a 1.55 LOI is just 10 therms/ton and the ancillary electrical requirement for the complete system is about 40 kW per ton at 100% installed power.
The computerized design of the combustion system provides emissions well below U.K. and European regulations.
The feed material injection system is totally enclosed, avoiding any problems of interfacing a cold feed system with a hot furnace. Feed height is substantially reduced. In addition, dust from a mechanical reclamation system can be fed into the unit to supplement fuel requirements and eliminate problems of disposal.
Besides the two systems described above, other thermal reclamation systems are installed in foundries around the world and are operating on binder systems such as phosphoric acid cured furan, pepset, Isocure, S|O.sub.2~ cured furan and epoxy furan.
This type of thermal equipment will rapidly supplement existing mechanical reclaimers and form the basis for foundries looking to recycle green sand tailings for core shop reuse--already proven to be feasible.
Thermal sand reclamation systems can provide important advantages to the user, even where mechanical reclamation is already in place. Sand can be 100% reused for cores as well as molds to save on new sand costs and greatly minimize disposal costs and concerns. Even more importantly, because of its cost benefits, the equipment can pay for itself rapidly.
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|Date:||Jan 1, 1993|
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