Could cryogenics crystallize as an option for sand reclamation?

This cold process, currently under development, may offer foundries another way to reclaim both green sand and chemically bonded systems.

Sand reclamation may be one of the rare opportunities in metalcasting in which capital investment reaps both sound environmental and financial rewards - a winning combination for good business. However, very few environmental projects have provided an opportunity to contribute to the profitability of an operation while also improving the environment.

Over the years, many different methods of sand reclamation have been tried, and, to some degree, they have been successful. However, none have been capable of recovering the total invested value of the discarded sand.

The organic materials within green sand systems and bentonite clay can be difficult to effectively separate from sand grains. In addition, some systems may produce waste, which then requires further treatment before it can be discarded. Overprocessing also can result in high sand losses and low core strengths. Other reclamation systems can potentially alter the sand surface chemistry, change the pH and destroy both the bentonite and seacoal.

Another possible avenue for foundries could be cryogenic sand processing. This novel reclamation process is currently being tested for use with both green sand and resin-bonded systems. In this process, liquid nitrogen (LIN) is used to reduce sand temperature as low as-112F (-80C). The sub-zero temperature provides a mechanism to separate sand from other usable and costly constituents within a foundry's sand system.

The Idea Behind the Process

The concept of cold reclamation is based upon a number of observations and theories. First, green sand contains water, both free and chemically bonded. When frozen under the correct conditions, the water in the bentonite gel that binds the sand particles together becomes brittle. Second, when ice crystals form, they create stress points that can be fractured. Third, as temperatures are lowered, adhesive forces fail, and the sand can be liberated from the bentonite and other sand mixture constituents.

When resin-bonded systems are treated with very low temperatures, the brittleness of the resin increases, adhesion fails and the sand can be separated from the binders.

Cryogenics in Action

The generation of cold conditions for process application uses naturally occurring and abundant raw materials such as nitrogen (N) and carbon dioxide (C[O.sub.2]). Conversion of the gases to their liquid phases provides the cold temperatures required for the process. After the sand is exposed to the cold liquid or cold gas streams, the sand components are separated, and the gas returns unchanged to the atmosphere.

Equipment for the entire process includes heat and cold recovery design considerations to optimize temperatures and minimize operating expenses. A cryogenic system would involve the installation of duct work (-square footage and configuration dependent on the design of the system), a magnetic separator to remove metallics from system sand, a muller, a screen system and a lumpbreaker, meant to break sand down to the smallest size possible.

The process of cryogenic treatment includes subjecting the system or molding sand to increasingly lower temperatures without allowing the sand to lump together [ILLUSTRATION FOR FIGURE 1 OMITTED]. One method of cryogenic recovery uses cold N gas and LIN treatment through a rotary tunnel similar to those used to freeze food. After freezing, the bentonite-bonded sand particles are subjected to additional attrition, the sand is separated from the other components, and it is returned to room temperature in the coreroom or storage silo. The bentonite/seacoal mixture is returned for use in the green sand system. Since low temperatures are essential for efficiency, the system is carefully designed to include heat exchangers in the LIN system.

Putting Cryogenics to the Test

Researchers are currently trying to determine the feasibility of this new process. Cryogenic reclamation has been tested in the U.S. and Europe, and the results show that sand, bentonite and seacoal can successfully be separated in this way. Laboratory trials have successfully reduced the amount of bentonite and seacoal present in clay-bonded molding sand by as much as 80%. In addition, analysis of the bentonite and seacoal mixture separated from the sand confirms that the cold temperature does not affect the bonding mechanics of Table clay. The pH, acid demand value (ADV) and the sand chemistry also do not appear to be adversely affected.

Similar tests conducted on resin-bonded systems have resulted in reducing the loss on ignition (LOI) without affecting the sand chemistry. Evaluations of alkaline phenolic systems that have been cryogenically treated have seen a significant reduction in pH, ADV and potassium and sodium hydroxide levels as well. The reduction of these chemical properties offers alternatives to mechanical reclamation since additional sand for dilution purposes are not necessary.

Since sand is not chemically altered in the cryogenic process, rebonding of recovered sand is not affected and environmental considerations are minimized. When analyzing the cryogenically treated sand from a U.S. farm implement foundry, an independent laboratory found very little in the way of dead, burned clay. It determined that the treated sand, clay and seacoal would be "highly reusable and would therefore have significant value."

Costs in Perspective

Spent foundry sand accounted for 37% of the waste generated by the ferrous foundry industry, according to the Cast Metal Coalition's 1998 Technology Roadmap. The relative size of the sand wastestream in the U.S. is quite large, amounting to more than 8 million tons of sand/year. At a price of $30/ton for sand, the foundry industry forgoes a $240 million/year opportunity. When the cost of transportation to and from the foundry and the value of bentonite and seacoal are added, the opportunity can easily approach $500 million/year.

Table 1. Energy Savings

Unit = 5 ton/hr reclamation capacity operating 8400 hr/year

Total Energy Use                        Proposed Technology

                                        Btu millions/year/unit

Thermal Requirement
(Btu/metric ton)                        0

Electrical Motors                       100 (@68,241 Btu/hr)

Transport
(68,241 Btu/metric ton)                 26

Sand Additions                          310 (@5%)

LIN Production                          4100 (@1.1 ton/hr and
                                        1.7 million Btu/metric ton)

Total per Unit                          4536
Table 2. Waste Savings

Waste Savings 1 Unit = 5 ton/hr reclamation plant operating 8400
hr/year

Waste Generated                        Waste       Cryogenics
                                     Category      Technology

ton/yr/unit
Waste Sand                             Solid           382
Waste Bentonite                        Solid            76
Particulate Emissions                  Solid            76
NOx Emissions (e.g. [N.sub.2]O)        Gas               2
SOx Emissions                          Gas               3
Total of Waste (tons)                                  539

The major source of the problem within the metalcasting industry is the need for clean, washed and dried sand that is compatible with core production. Currently, economics is a consideration in separating usable core sand from the green sand systems employed within most high-production foundries. Today's practice is simply to order new sand and discard the diluted system sand that contains usable bentonite and seacoal.

In green sand foundries, each new ton received and used in the coreroom ends up in the system sand after pouring. Since the molding system sand has been diluted with this new sand, bentonite and seacoal must be added to maintain the desired molding properties. Now that there is an extra ton of sand in the system, 1 ton of sand and 60-200 lb each of bentonite and seacoal must be removed from the system and discarded [ILLUSTRATION FOR FIGURE 2 OMITTED].

Each ton of sand thrown away represents a loss of valuable materials. Typically, the core sand is rarely separated from the system sand. Since this shakeout sand contains the same levels of bentonite and seacoal as the rest of the sand system, each ton thrown away has an increased value over the raw sand purchase This value is equal to the cost of sand, bentonite, seacoal, freight and the mulling cost. In other words, the $15/ton paid for sand and the $5 freight is now worth nearly $30 since bentonite and seacoal have been added to each ton going to disposal. Cryogenic treatment and separation isolates the bentonite/seacoal mixture from the sand that permits returning the bentonite/seacoal mixture to the sand system [ILLUSTRATION FOR FIGURE 3 OMITTED].

Economics and the Environment

The price of new sand and additives for foundry applications continues to escalate for a variety of reasons. They include increased production costs associated with quality control, environmental legislation and transportation of sand, bentonite and seacoal, and supply and demand forces, since they are non-renewable resources. In addition, legislation governing production and disposal of nonrenewable resources and foundry waste also contributes to increased operating expenses for both the supplier and the user. Unfortunately, as production increases, demand for additional sand follows and the problems continue to grow. Cost will continue to escalate for foundries as the legislation increases and these resources are consumed. Recycling and reuse of these foundry by-products in other industries also may alleviate some of the economic and environmental issues associated with disposal.

In addition to finding alternative uses for foundry sand, current and future environmental pressures upon the industry's use of non-renewable resources also must be considered. Recent interest in the conservation of sand, bentonite and coal, along with possible environmental concerns resulting from mining operations, places an additional burden on the foundry industry. The industry must develop methods of conservation and optimize the use and reuse of resources in foundry practice.

Some sand recovery systems do not produce sand suitable for coreroom consumption, and they do not allow for the recovery of bentonite and seacoal. In addition, because a large volume of sand must then be purchased to dilute the system sand.

Using a cold separation process, the sand, bentonite and seacoal components of spent sand can be separated for reuse in the foundry and applied to nobake or resin-bonded systems.

Cryogenic sand reclamation of foundry sand has the potential to provide major improvements in energy consumption (Table 1). Operating cost is significantly less than thermal units.

Table 3. Greenhouse Gas Emission Savings

Unit = 5 ton/hr reclamation plant operating 8400 hr/year

Type of Greenhouse Gas                Cryogenics
                                      Technology
                                     ton/year/unit

C[O.sub.2]                               638
C[H.sub.4]                                1
l-butane                                  0
Chloroform                                0
Total                                    639
Table 4. Economic Breakdown

Unit = 5 ton/hr reclamation plant operating 8400 hr/year

                                       Cryogenics
                                       Technology

Capital Cost of Unit                    $410,000
Depreciation (years)                          10
Depreciation Cost(*)                       4,530
Maintenance Cost per Year(**)             14,438
Operating Cost                           693,000
Wearing Cost                              31,500
Operating Labor Costs                    210,000
Make-up Sand Cost                         63,000
Bentonite Loss                                1%
Bentonite Cost                            54,600
Disposal Cost                             54,600
Total Annualized Cost                  1,175,668

* Annual fixed (capital) cost based on 13.3% of installed cost
(10% depreciation and 6% interest on capital over 10 years)

** Maintenance labor at (1) 1 hr/8 hr operation and (2) 0.25 hr/8
hr operation

Cost and Waste Reduction

In addition to increasing profits by reducing the volume of sand, bentonite and seacoal purchased and currently discarded by the foundry, the process reduces environmental related concerns by minimizing the wastestreams significantly (Table 2). Additional benefits include:

* sand loss due to fines, spill, transport, etc. is estimated at less than 5% for cryogenic reclamation;

* bentonite clay loss is estimated at 1% for cryogenic reclamation.

Cryogenics also offers the potential to reduce emissions (Table 3). The primary C[O.sub.2] benefits are due to:

* no combustion required of the coal portion (5%) of green sand, keeping 192.6 lb C[O.sub.2]/unit ton/hr from being released;

* no heat required, so 117 lb of C[O.sub.2]/unit ton/hr do not enter the atmosphere.

Overall, the economics of cryogenic reclamation appear promising (Table 4).

Cryogenics' Potential

Plans for development of this process for sand reclamation include construction, installation and operation of pilot facilities at 1-2 tons/hr for reclamation of both green sand and chemically bonded systems. More tests could begin at these pilot facilities as early as October. Since 1% clay is probably the maximum for reuse based on previous sand tests in which clay was added to raw sand, researchers have not spent a great deal of effort on binder strength development tests. Those tests are next on the agenda, after the separation of sand is maximized.

The success of the process will be measured by the value of the products generated during the separation of spent sand components. Objectives include the requirement that sand processed must be capable of reuse in, both the coreroom and the molding system. Seacoal and bentonite must be usable in the green sand system.

Obviously, the process must be cost-effective, reducing the total cost of operations below the current levels, even when capital expenditures are included.