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Thermal sand reclamation: a strategy for waste minimization.

With the rapid closing of existing landfill sites and the prohibition against opening new ones, the need to limit all types of industrial waste is imperative. The restriction and eventual ban on classified waste dumping has increased the need to find alternatives to waste foundrys and landfills. Based on economics alone, sand reclamation may be the only technology the foundry industry has to meet the requirements of the Resource Conservation and Recovery Act (RCRA).

Waste minimization has become a common term for North American foundrymen as a result of the Environmental Protection Agency's tight regulation on waste sand disposal. In workshops, at conferences and in the workplace, waste minimization is heard more frequently as foundrymen discuss the ever increasing waste sand problem. The term had been associated with efforts to reduce sands classified as hazardous waste, but the May 8, 1990, EPA Land Ban is causing the term to apply to virtually all spent foundrys sands, This is due primarily to the significantly decreasing number of landfills available for waste sand and the resultant cost for spent sand disposal.

Foundries not casting high-lead, copper-based alloys and those using either resin bonded cores in clay-bonded molds or resin-bonded molds and cores have almost never considered their waste sand to be a disposal concern. They either dumped their waste sand on their own property, in local landfills or it was used, untreated, as fill material for construction. The land ban has prohibited or severely regulated these disposal methods.

In some cases, spent foundry sand is classified as a hazardous waste due to the excess quantities of cadmium, chromium, lead, mercury and arsenic, or volatile organic compounds such as phenol, diphenyl, diphenylmethane, diisocyanate, triethylamine and formaldehyde. These hazardous materials enter the sand stream as a result of poured metal laying against bonded sand, or as an ingredient of a bonding mixture itself. They take the form of elemental metals, metallic oxides, cured synthetic resins, degraded resins and clays in various stages of crystalline transformation.

EPA's six preferred hazardous waste management strategies include: [1]

* Reduction: reduce the amount of waste at the source through changes in industrial processes.

* Separation and concentration: isolate wastes from mixtures in which they occur.

* Exchange: transfer wastes through clearinghouses for industrial process recycling.

* Energy/material recovery: reuse and recycle wastes for the original or some other purpose, i.e. materials recovery or energy production.

* Incineration/treatment: destroy, detoxify and neutralize wastes into less harmful substances.

* Secure land disposal: deposit wastes on land using volume reduction, encapsulation, leachate containment, monitoring and controlled air and surface/subsurface waste releases.

Process Change

It is advisable that a foundry investigate each of these strategies for all its wastes due to increasing disposal problems and rising costs. The most logical route is that of waste reduction through changes in industrial processes. This can include energy/material recovery, or, in the case of spent foundry sand, waste separation and recovery strategies. Combining these strategies with the use of thermal sand reclamation equipment, the spent waste sand stream could be reduced by as much as 95%, and associated disposal and handling costs decreases by a like percentage. An additional benefit is that new sand costs normally are reduced by a similar value.

In the case of foundry sand, it makes sense to eliminate or minimize the waste rather than approach the problem with costly development of waste sites or treatment processes requiring permits from local, state and federal agencies. The time it takes to obtain such permits makes these two strategies almost prohibitive in terms of time and money.

With the resurgence of thermal sand reclamation equipment for resin-and clay-bonded sands, waste minimization is now a reality rather than just a possibility, making sand reclamation extremely attractive for several reasons.

First, waste stream volumes made up of spent foundry sand can be reduced by 85-95%. This leaves only 5-15% of the original sand volume still classified as a waste. The associated disposal costs can be reduced nearly as much, even adjusting for the direct reclamation costs.

Second, the process of reclamation of a natural resource is not a treatment process, therefore, special local, state and federal agency permits are not required. Only those permits required for any industrial equipment installations are necessary. These agencies also now endorse this approach as the most viable alternative to landfilling.

Third, new sand costs can be reduced significantly, closely paralleling the waste disposal reduction percentages.

For maximum results in any program designed to minimize waste, a logical and systematic approach is necessary. To meet EPA waste disposal regulations, a comprehensive audit should be taken to characterize waste types by form, volume and weight. This audit will usually require the services of a specialist in the field of management with foundry, metallurgical and chemical experience as these disciplines apply to foundry operations.

The three main forms of waste materials generated by sand systems are;

* degraded and undegraded organic hydrocarbons;

* calcined and uncalcined inorganic compounds;

* elemental metals and their oxides.

Clay-bonded sand systems generate all three forms of waste, whereas, chemically-bonded sands form only the first and last of the above. Generally, the organic hydrocarbons and elemental metals with their oxides are mechanically bonded to the inorganic compounds and undegraded synthetic resins on the sand grains. The elemental ferrous metal particles must by separated out magnetically and fluidized screening used for nonferrous materials.

In the last decade, all waste disposal costs have increased dramatically. The following is a list of costs associated with landfill disposal on foundry property, exclusive of other possible charges not covered here:

* special permits,

* construction of an approved disposal site and monitoring system,

* maintaining site in accordance with federal regulations,

* hauling spent sand from foundry to disposal site.

In the case of an off-site landfill, costs would include freight, hauling permits and related insurance, tipping fees, fees for classified wastes and landfill liability insurance. [2] Once these per ton costs are calculated, the sum can be added to the sand system waste stream from the foundry.

The following is a case study of a Midwestern foundry waste sand stream that was being landfilled in 1989. A typical unreclaimed system sand from a clay-bonded molding line with resin-bonded cores, it was comprised of:
 Total % [Total Weight
Base silica
 (AFS #60-70) 1900 lb 85.43%
Clay-bond
 (total clay) 160 lb 7.19%
Additives
 (carbonaceous) 54 lb 2.43%
Resin binders
 (core residue) 6 lb 0.27%
Silica fines
 (-140mesh) 1001b 4.50%
Oxide 4 lb 0.18%
 Total 2224 lb 100.0%


In this case, there was no silica available for reuse; it produced 2224 lb of waste material for disposal, 11.2% more waste than is made up by the sand in this waste stream. A thermal sand reclaimer with a post-pneumatic scrubber was proposed for this foundry. It was designed to process the same amount of spent sand to produce 1800 lb of silica for reuse in the core room or molding line as new sand addition, a yield of 90% on the base silica. (Fig. 1). This means only 1 0% of new sand (200 lb) will have to be purchased to replace the fines removed by the process, leaving just 364 lb of disposable waste material and reducing disposal costs to 16.4% of the original calculation. The amount of waste sand will be reduced by about 84%.

Using this example, the monetary benefits resulting from the operation of thermal sand reclamation equipment in a clay-bonded system with resin-bonded cores is evident.

In a foundry using 6000 tons of new sand and assuming a $50 per ton direct disposal cost, the annual bill is $333,600. Using a thermal sand reclaimer, the annual disposal cost is reduced to $54,600, a savings of $279,000.

A second foundry, utilizing a totally resin-bonded system and a mechanical reclaimer, (Fig. 2) has a typical make up of mechanically reclaimed sand as follows:
 Total Wgt % of Total Weight
Base silica
 (AFS #50-60) 1330 lb 65.5%
New sand
 addition
 (each cycle) 609 lb 30.0%
Resin binder +
 catalyst 26 lb 1.3%
Silica fines
 (- 140 mesh) 61 lb 3.0%
Oxide 41b 0.2%
 Total 2030 lb 100.0%


Note that the values from the table used for waste generated without a reclamation system include the weight added by the resin binders, metallic oxides and carbonaceous additives.

The mechanical reclaimer produces approximately 1330 lb of sand suitable for reuse in the molding line leaving approximately 700 lb of waste. If that same system sand is processed through only a thermal sand reclaimer, there can be 1900 lbs of sand returned to the system or reuse in the molding line or as new sand in the coreroom. This is a yield of 95% on the base silica, meaning only 100 lb of new sand must be purchased to replace the fines removed by the process. The amount of waste is reduced from 700 to 104 lb, a reduction of 85%.

Again, the case of a foundry purchasing 6000 tons of new sand annually will be the basis for a quick cost review. The amount of new sand required for this foundry means that there is a like amount being disposed of each year. if the direct disposal costs are estimated at $50 per ton, the disposal costs will be $304,500 annually. A thermal sand reclaimer can reduce this cost by as much as 95%, saving about $289,275 a year.

Special Considerations

Finally, there are two special cases to be considered, both of which will increase in importance in the immediate future.

The first one is the introduction of the alkaline, ester-cured, water-base phenolic resins. This system has many environmental advantages, but reclamation using conventional equipment is difficult. [2] The most effective reclamation in terms of rebond tensile strength at normal binder levels, has been wet scrubbing, but this creates the problem of the disposal of effluent water. A leachate test will indicate a phenol and formaldehyde level in excess of the allowable Threshold Limit Value (TLV). To solve this problem, a chemical neutralizer has been developed to enhance thermal reclamation by eliminating the production of contaminated waste water.

The second case is the growing use of the expendable pattern casting (EPC) process. initially, it was assumed that reclamation of the binderless sand used in EPC would create no problems, appearing to be a matter of cooling the dump-out sand to maintain its temperature below 150F (65C) to prevent heat distortion of the polystyrene patterns and to reclassify the grain distribution to maintain base permeability of the system sand.

Though the two steps are still essential, thermal destruction of the hydrocarbon residuals from the polystyrene patterns and coating materials must also be carried out under closely controlled conditions. As the sand is repeatedly cycled through the casting process, the vaporized styrene patterns condense on the sand grains during each pouring cycle. When the quantity of condensed styrene on the sand grains reaches 0.35-0.36% by dry weight of the sand, defective castings result from the shifts in the base sand permeability. Current practice is to replace 50% of the sand system with new sand, discarding the contaminated sand in a local landfill, but now this will be prohibited by federal regulations.

An alternative procedure is to thermally reclaim the sand on a continuous basis by side-streaming a fixed percentage and feeding the regenerated sand back into the system. To arrive at the most economically sized thermal reclaimer, the dilution rate of reclaimed sand must be matched to the maximum rate of styrene contamination accumulation. The build-up rate depends on the geometric share of the largest configuration of patterns the flasks will accept and the rate of pour-off.

Environmental Concerns

The two major concerns when considering thermal reclamation are the possibility of unburned hydrocarbons in the stack gasses and the presence of untreated hazardous elements in the solid wastes collected in the baghouse hopper.

To ensure a clean stack, it is absolutely essential to provide adequate air at the calcining stage of thermal reclamation and introduce a dwell time in the exhaust system prior to releasing the offgas to the environmental control system. The patented fluid-bed reactor unit incorporates these features and meets the requirements of state and federal regulations defining particulate loading, neutralization of acid gasses and unburned hydrocarbons.

Untreated hazardous elements in the solid waste stream collected in the baghouse hopper are the result of manufacturing processes taking place at exhaust points connected to the main baghouse duct. If there are no hazardous elements in the particulate from the exhaust flues of the thermal reclaimer, coupling it to its own dedicated baghouse can be justified. The dusts from such a baghouse can be transported to a local landfill. Coupling the thermal reclaimer to a baghouse already serving the melt or cleaning department may result in an increase in hazardous waste to be transported to an approved dump site, increasing disposal costs.

An Alternative

The magnitude of potential savings available with a thermal sand reclaimer system and the relief from those pressures associated with sand disposal create a very attractive alternative for the foundry.

The waste disposal savings noted above consider only the direct disposal costs and do not include the costs related to procuring the equipment required to reclaim the sand, nor do they take into consideration those savings related to the reduction in new sand requirements which would apply to all systems.

Waste minimization and the resultant cost reductions are the obvious driving forces behind the utilization of thermal sand reclamation. The elimination of landfill sites, a situation that can only get worse, will dictate that decisions be made with regard to all waste streams, one of which is sand system waste.

In brief, the key benefits from the operation of a thermal sand reclaimer for the disposal of waste sand material generated by a foundry are:

* recovering a natural resource to lower foundry costs (reclaiming sand for reuse);

* reducing waste problems and related costs;

* reducing new sand costs;

* confining landfill disposal to contaminated baghouse dusts and fines.

References

1. H. M. Freeman, "Hazardous Waste Minimization: A Strategy For Environmental Improvements," Journal of the Air Pollution Control Assn (Jan 1988).

2. G. J. Reier and R. S. L. Andrews, "Process Parameters for Thermal Sand Reclamation," AFS Transactions #89-145, (1989).

(Figures omitted)
COPYRIGHT 1990 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Reier, Gerald J.
Publication:Modern Casting
Date:May 1, 1990
Words:2385
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