Sand reuse: user requirements.

Asphaltic concrete is yet another attractive market for spent foundry sand--yet foundries must understand how to approach these users.

In 1992, Ontario produced about 11 million tons of asphaltic hot mix (for black-top). During the same time, the Ontario foundry industry used about 600,000 tons of sand.

Although General Motors Foundry of Canada, St. Catharines, Ontario, has provided spend foundry sand to an asphalt supplier for more than three years, the market has been left generally untouched.

Assuming certain quality criteria is maintained, 15% of the total mix can be replaced by spend foundry sand. This means a potential 1.65 million tons per year in Ontario alone--or close to three times the total sand consumption of the foundry industry. Why, then, is this use of the secondary raw material "spent foundry sand" not more widely practiced?

Market Demand

As in any market-based society, several rules must be followed to make such a project come to life.

It is the vendor's responsibility to find or create a market for its product, and the customer's responsibility to find new and more economical sources for materials and services. The asphalt industry is not in the business of waste disposal. Thus, it will not accept "waste" sand but is quite prepared to consider spend sand--a secondary raw material--as a substitute for the presently used virgin sand.

Vendors must supply a quality product at an acceptable price at required quantities and times. The asphalt industry does not feel obligated to do market and applications research for a new vendor.

Since the available product "spent sand" is contaminated to various degrees and because such contamination may vary significantly from foundry to foundry, the asphalt industry is in no position to judge what degree of beneficiation would be needed to meet its quality criteria.

The industry is quite prepared, however, to give interested vendors the necessary specifications that must be maintained. This would enable individual foundries to judge what degree of processing needs to be done to its effluent stream "spent sand" to meet asphalt companies' requirements. The asphalt company would then expect to receive the raw material sand within specified quality parameters, and in quantities and at times needed for its operation. The firm would pay for the material at a reasonable price--which would be measured against the present source of supply.

Or, it would equally be prepared to consider beneficiation of the spent sand at its own facility. To judge the cost of such beneficiation, however, the condition of the input material would have to be defined in detail and in a reliable manner. The bottom line is that the total cost of the raw material sand, when using recycled foundry sand, must not be higher than the cost for virgin material.

Language Problem

The asphalt industry defines "sand" as a granular material with given physical and chemical characteristics. Large lumps, dust, sludge and other foundry "by-products" aren't included in this definition. This means processing has to take place either at the foundry or at the asphalt plant--with the cost for processing considered part of the purchase price.

Presenting material property data in a form commonly used by the asphalt industry is another requirement. The foundry uses a representation "retained on each screen," while the concrete industry uses a representation "percentage passing each screen" as a summary curve. If foundries expect to successfully market spent sand, they must use the language generally accepted in the industry, since it is also practiced by present suppliers.

Quality Consideration

Although foundry sand generally has a sieve analysis that doesn't match asphalt industry requirements, this problem can be corrected. Blending with a coarser component is possible and will certainly be considered by the asphalt industry. Using crushed and screened slag for the coarse component is a possibility. However, while slag also is a solid by-product of the foundry process, it involves a variety of considerations. Blending is standard practice now.

One critical parameter is the amount of "fines" (defined as material minus 75 microns, which is about 200 mesh U.S. sieve). It may be difficult for spent foundry sands originating from green sand systems to meet this requirement without beneficiation.

The asphalt industry prefers angular aggregates over the rounded form frequently used in foundries. While this is a detrimental characteristic, it is not judged so severe as to eliminate spend foundry sand as a usable aggregate.

Chemical analysis specifications must also be met. A main concern is metal, and sand should be metal-free. A minor amount (1-1.5%) may be tolerable, if this metal is equal to or smaller than the sand grains in size and does not lead to a hazardous leaching condition.

Foundry Options

A critical requirement is not to exceed approximately 3% fines minus 75 microns. This means that virtually all chemically bonded sands need only complete particulation with dust and metal removal during this process.

In green sand, clay removal is required in most cases. Even with 18 minutes processing on a pneumatic reclaimer to remove clay, it is difficult to reach the limit of 3% for AFS clay.

Different equipment is available, however, that permits foundries to achieve a 3% AFS clay level within a processing time of 3.5 minutes or less. The reason for this success is attrition action and drying of the sand occur simultaneously. The cost for this approach is about $4-6 per ton of sand processed.

Sand cooling after processing isn't required (as would be if the sand were to be reused in the foundry), since the aggregate will be heated in the production of the "hot mix" asphalt concrete anyway. Because the equipment/process used for fine removal is grinding equipment that also dries the process material, it is exempted from opacity monitoring and semi-annual reporting as legislated by EPA.

Cost Consideration

Cost for virgin aggregate in Ontario is about $7.20 (U.S.) per ton delivered to the user's plant. If the above processing cost is considered, plus transportation, handling and storage, it becomes quickly obvious that in most cases, some form of subsidy is required if the total cost for the aggregate shouldn't be higher than the presently used material.

Disposal costs for the effluent stream from the beneficiation of the spent sand must also be considered. This means that if the foundry does all the beneficiation and delivers a certified quality product to the asphalt plant, it could expect to receive payment. If, on the other hand, the asphalt plant handles all beneficiation, the foundry would likely need to pay a processing fee in addition to delivering the material free of charge to the asphalt plant.

Environment/Liability

Raw materials in an asphalt plant are stored in piles outside. Since there is a phase shift with respect to supply and demand (the foundry supplying 12 months a year but asphalt only being laid down during construction periods), stockpiling would at any rate be necessary. Clearly, the foundry can supply only materials that would not lead to liability problems at the asphalt plant with respect to leaching, stormwater legislation and other considerations. This leads to these consequences:

* no material that is leachate-toxic at time of shipping is acceptable for the asphalt producer.

* if the foundry does the beneficiation, the material must be certified and guaranteed to meet all legal environmental requirements for open stockpiling.

* if the asphalt plant performs the beneficiation, the "contaminated" spent sand must already meet storage requirements previously described. Or, storage has to be under roof or with a leachate collection system--all of which again increases the subsidy to be paid by the foundry.

Know Your Competition

The foundry industry is not the only industry that has approached asphalt producers for the use of their process effluent. Glass, rubber and roofing shingles have all been evaluated as a substitute material and offer as much promise in asphalt production as foundry sand. It is obvious that it can't be the asphalt industry's objective to conduct research for any or all of these interested groups.

Rather, the asphalt industry is carefully listening to all proposals and will eventually cooperate with the partner who offers the most advantageous solution. And it should be understood that this shouldn't be interpreted to mean "the highest subsidy." While the cost is as important as in any industry, the consideration of assured quality (with no negative environmental impacts) rates equally high.

This information was excerpted from a recent CMI course on beneficial reuse and sand reclamation.