An Deng Yung Tse Hung and Lawrence K Wang

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CONTENTS

4.1 Industry Description 152

4.1.1 Casting Flow 152

4.1.2 Casting Processes 156

4.1.3 Sand Casting Systems 157

4.1.4 Casting Metals 160

4.2 Characterization of Wastes 161

4.2.1 General 161

4.2.2 Air Emission 162

4.2.3 Wastewater 162

4.2.4 Spent Foundry Sand 163

4.2.5 Baghouse Dust 167

4.2.6 Furnace Slag 167

4.3 Source Reduction 174

4.3.1 Chemical Substitution or Minimization 174

4.3.2 In-Plant Reclamation 175

4.3.3 Waste Segregation 176

4.3.4 Process Modifications to Reduce Emission 176

4.4 Solid Wastes Reuse Technologies 177

4.4.1 General 177

4.4.2 Reuse Evaluation Framework 177

4.4.3 Reuse in Civil Engineering 179

4.4.4 Agricultural Applications 190

4.4.5 General Processes 191

4.4.6 Unresolved Issues 192

4.5 Barriers To Solid Waste Reuse 192

4.5.1 Education 192

4.5.2 Environmental Regulation 192

4.5.3 Guidelines, Procedures, and Specifications 193

4.5.4 Economics 193

4.5.5 Market Potential 193

References 194

4.1 INDUSTRY DESCRIPTION

The metal casting industry, also known as the foundry industry, is one of the largest recyclers in the world. For centuries, this industry has been converting a huge volume (e.g., 15 to 20 million tons in the U.S.) of scrap metal that would otherwise be disposed in landfills, into manufactured useful products. This scrap metal forms the raw material charged into furnaces of the foundry facility and converted into usable castings. The casting categories include many general ferrous and nonferrous metals and their alloys, including iron, steel, aluminum, copper, magnesium, and zinc. Major end-use markets cross all sectors of the global economy, examples being the automotive industry, transportation equipment, construction, mining and oil field machinery, and industrial machinery.

4.1.1 Casting Flow

4.1.1.1 Overview

Metal casting is a process in which molten metal is poured into a mold to produce metal products. In the mold, the molten metal cools and shapes into castings by filling the preset mold internal space. The most common metal casting process is sand casting, which uses sand as the major molding and core-making material. Besides sands, other materials can also be used as molding materials, such as ceramic mold for investment casting and metal mold for die casting.1,2 A general metal casting flow diagram is shown in Figure 4.1. A schematic process is shown in Figure 4.2. The casting begins with the customer demands and material preparations, including metal product specification, sands, binders, and scraps. Manufactured molds and cores are assembled in the assembly area, and made ready for pouring.

As the assembled molds are being placed on the pour-off lines, the scrap metal is melted in the furnace. Molten metal from the furnace is brought to the molds on the pouring lines in a refractory lined pouring ladle. Once poured, the molds are allowed to cool before being sent to the shakeout processes. At the shakeout, the castings are separated from the sand mold. The sand is sent to a reclamation system so that it can be reused in the molding process.

The materials comprising the core and mold in the casting processes have the properties of porosity, cohesion, and refractoriness. Sand has globally been selected as one of the materials meeting the property requirements. Its aggregate porosity, connected as passages, allows air and steam to escape from the mold during casting. The sand particles can adhere together into all kinds of molding shapes. In particular, sands have the ability to withstand severe heat and resist penetration of the molten metal, and impart a smooth and desirable appearance to the casting.

4.1.1.2 Core Making

Cores are separate shapes that are placed in the mold to provide castings with contours, cavities, and passages that are not achievable by the mold alone. A core has to be fixed tightly in place while

Flowchart For Green Sand Mold Process
FIGURE 4.1 General metal casting flow chart. (From U.S. EPA, Summary of Factors Affecting Compliance by Ferrous Foundries, Vol. 1, EPA-340/1-80-020, U.S. EPA, Washington, DC, January 1981.)

Metallics

Fluxes

Metallics

Fluxes

Flowchart Grey Iron Foundry
FIGURE 4.2 Metal casting process.

the metal flows around it. Cores are made by mixing sand with binders and catalysts, which are activated to bond sand into various shapes. Figure 4.3 demonstrates the typical core-making process. The sand and binders are blended uniformly in a mixer. The mixture is then discharged into a core machine, where continuous curing with a catalyst is applied. After the core is cured, it is removed and sent to a molding assembly area.

4.1.1.3 Molding

Molding is the process where a pattern is pressed or embedded into special sand to the desired shape or form. Alternatively, the pattern can be placed on a molding board, and the sand rammed or compressed around the pattern. Figure 4.4 shows the typical molding flow. The sand and binder are first

Brass Foundry Flowchart
FIGURE 4.3 Core making flow chart.
Flowchart For Green Sand Mold Process
FIGURE 4.4 Typical molding flow chart.

mixed in a mixer. Then the mixed sand is discharged onto a pattern face mounted in a mold box. The sand in the box is compacted to its maximum density. Once the sand is set, the pattern is removed from the sand, and cores are placed in the mold. The mold is then closed up and moved to the pouring lines.

4.1.1.4 Melting and Pouring

Figure 4.5 describes the typical metal melting flow chart. At first, the customer's metal specification for the casting will determine what type of scrap metal will be used to feed the furnace. Once charged, the furnace uses multiple melting powers or burners (that is, electricity, kerosene, gas, coke, charcoal, and used engine oil) to melt the scrap metal. Scrap metals may be fully melted in tens of minutes, depending upon the size of the vessel used. Alloys are added according to the metal specification. The addition is determined by spectro analysis in the melting process. When the melting is complete, the molten metal is placed in a pouring shank and sent to the pouring line.

Furnace types include cupolas, electric arc, induction, hearth or reverberatory, and crucible. Because of the different characteristics of metals, different inputs are required and different pollution is released from each type. Table 4.1 summarizes the types of furnaces depending on the type of metal being used.

The molds are cast and allowed to cool for a suitable time, often 30 to 40min, before shaking out the castings. The shakeout work may be supplied by a vibrating conveyor or a rotating drum, which cause the molds to be broken up by the vibration, exposing the casting for removal. The sand

Flowchart For Green Sand Mold Process
FIGURE 4.5 Typical melting flow chart.

TABLE 4.1

Common Types of Metal-Melting Furnaces

Furnace Type Raw Materials Outputs

Cupola furnace

Iron ore, scrap iron, lime, coke

Electric arc furnace Scrap iron, flux

Molten iron

Molten iron and steel

Induction furnace Scrap iron or non- Molten iron or ferrous metals nonferrous metals

Reverberatory, hearth, Nonferrous metals, Molten nonferrous or crucible furnace flux metals

Process

Alternative layers of metal and coke are fed into the top of the furnace. The metal is melted by hot gasses from the coke combustion. Impurities react with the lime and are separated.

Electric arcs from carbon electrodes melt the scrap metal. The flux reacts with impurities.

Induction furnaces are the most common type used by both ferrous and nonferrous foundries. Copper coils heat the metal using alternating currents. The flux reacts with impurities.

Reverberatory furnaces melt metals in batches using a pot-shaped crucible that holds the metal over an electric heater or fuel-free burner. The flux reacts with impurities.

Source: From WMRC, Primary Metal, Illinois Waste Management and Research Centre, available at http://www.wmrc. uiuc.edu/info/library_docs/manuals/primmetals/chapter3.htm.

from the mold is separated and processed through a reclamation system for reuse in molding and core making.

4.1.1.5 Casting Cleaning and Inspection

The foundry cleaning room is a collection area where castings are finished to meet the casting specifications. A sample flow chart is shown Figure 4.6. When castings are removed from the shake-out, they are run through the shot blast to remove sand and expose the surface for inspection and further work. Castings are inspected for defects such as cracks, flashing, and inclusions. If none are found the castings are sent to the heat-treating department. If defects are present that require welding or grinding the castings are sent to the appropriate area to have the defect corrected. Once rework is completed, the castings are sent to the heat-treating department. After being heat treated, the castings are again sent through the shot blast before being sent to the final inspection area.

Shot blast

Heat treat

Shot blast

Inspection

If fail, rework (weld/grind)

If pass, shipping

FIGURE 4.6 Casting, cleaning and inspection flow chart.

4.1.1.6 Reclamation of Molds and Cores

After shakeout, the return sand is reclaimed by a crushing process and by screening out core lumps, nonmagnetic metallics, and other unwanted material. Burnt binders (such as clay, resin, and other foreign fines) will excessively build up in the matrix of reclaimed molds and cores, and may reduce the gas/heat permeability within the molds and cores. This used molding sand will be put back through the sieve to have the correct amount of water added. Sometimes, the reclaim process may not be sufficient to recondition the technically acceptable or functional refractory materials, which eventually become excess foundry sand and is removed from the system. New sand, additional water, make-up binder, and additional catalysts are added to ensure molding and core-making quality.

4.1.2 Casting Processes

Casting processes can be divided according to the refractory materials used, that is, sand, ceramics, and metals. The principal sand casting processes used in the metal casting industry are sand mold casting, expendable pattern (lost foam) casting, shell mold casting, plaster mold casting, and vacuum (V-process) casting. Processes that use some form of disposable ceramic molds include the ceramic mold process and investment mold casting. Processes that use a reusable metal mold include die casting, permanent mold casting, and centrifugal casting.

4.1.2.1 Sand Casting

This is the earliest and the most commonly used casting process. It has the advantages of wide metal suitability, low cost, and simple operation. It uses sand as a refractory material. Many types of sand are utilized by the foundry industry. However, because of its wide availability and relatively low cost, silica sand is the one that makes most metal castings. Silica sand is composed of the mineral quartz (SiO2), which has a fusion point of approximately 1670°C (3090°F), which is often lowered by the presence of appreciable quantities of minerals with lower fusion points.

4.1.2.2 Shell Casting

In this process, the mold cavity is formed by a shell of resin-bonded sand. The shell is built up in layers, starting with a very fine-grained dip-coat, which is then dusted with a fine powder (molochite or zircon). Once the first coat is set hard, the wax is dipped a second time. The third and successive coats are dusted with coarse stucco. This coarse aggregate builds up the strength of the shell. Shell mold castings surpass ordinary sand castings in surface finish and dimensional accuracy, and cool at slightly higher rates. In addition, equipment costs are higher, and the size and complexity of castings that can be produced are more limited.

4.1.2.3 Investment Casting

This process is used to produce intricate, thin-section parts with great dimensional accuracy, fine detail, and very smooth surfaces. All ferrous and nonferrous alloys can be cast in investment molds. Investment casting begins with expendable wax patterns that are assembled into clusters, then coated with a series of successively coarser ceramic slurries. The assembly is then fired in a furnace to dry and harden the ceramic shell and to melt out the wax, leaving a cavity into which molten metal is poured to form the casting.

4.1.2.4 Die Casting

A die is a reusable mold, usually made of steel, for the mass production of small parts in low-melting-point alloys—usually zinc or aluminum alloys. For the mass production of small parts that have no undercuts, the durability and excellent surface quality of the die, in addition to the saving in labor costs, make die casting a competitive and worthwhile process.

4.1.2.5 Permanent Mold Casting

Metal molds and cores are used in permanent mold casting. The process works best in continuous operation so that the mold temperature can be maintained within a fixed operating range. The operating temperature of the mold is one of the most important factors in successful permanent mold casting. Mold cavities are machined from solid blocks of graphite. Mold life is the major cost factor in permanent mold casting.

4.1.2.6 Centrifugal Casting

Centrifugal force is used to introduce molten metal into a mold cavity that is spinning around its axis. Cast iron pipe is produced in centrifugal molds, and copper-base alloy bearings are also commonly produced this way. Permanent metal molds are usually coated or lined to extend operating life.

4.1.3 Sand Casting Systems

Of the many casting processes, sand casting is principally addressed in this section as this process not only prevails in the casting industry, but also generates a vast volume of solid wastes. Sand casting systems are possibly the most versatile foundry method, and are largely divided according to the binder and bonding manners into two categories, green sand and chemically bonded sand systems. The green sand system uses sand, clay, water, and additives as components, and bonds sand particles together by relying upon mechanical forces generated by mixing the clay and water. Chemically bonded sand systems use sand, resins, or inorganic binders, and sometimes water and catalysts, as components. The bonding forces are generated by chemical reactions (polymerization) between the resins/inorganic binders and catalysts.

4.1.3.1 Green sand system

Green sand molding is the most widely used molding process in the world, accounting for up to 90% ferrous sand casting materials.4 It is low in cost, high in performance, and the materials are reusable. Green, in this sense, does not refer to color, but is a technical point, indicating a natural bonding effect (with water, but without artificial binders, additives, or catalysts). Green sand consists of 85 to 95% high-quality silica sand, 4 to 10% bentonite clay (as a binder), 2 to 5% water, and 2 to 10% sea coal (a carbonaceous mold additive to improve casting surface finish). A machine, known as a muller, is used to coat the sand quickly and uniformly with a clay and water mixture (glue). A muller is capable of producing more than several tons of prepared molding sand in a few minutes.

Sand is composed of grains ranging from 0.05 to 2 mm in diameter. The physical properties of sand that can affect green sand system performance include grain shape, grain size distribution, grain fineness, permeability, density, and coefficient of thermal expansion. The chemical properties of sand that can affect green sand system performance include chemical composition, loss on ignition, pH value, and fusion point.

Clays used in foundries include hydrous alumina silicates, known as bentonites. Their properties provide cohesion and plasticity in the green state and also high strength when dried. There are three clays that are commonly used in foundries: western bentonite [sodium bentonite, burnout point 1290°C (2350°F)], southern bentonite [calcium bentonite, burnout point 1065°C (1950°F)], and fire clay [kaolinite, burnout point 1425°C (2600°F)].

The water used for a green sand system should be clean and it should be consistent. It should have a pH that is neutral to alkaline, not acidic, because acids prevent bentonite from swelling. In addition to sand, clay, and water, there are a number of other materials, or additives, that can enhance, control, and optimize the performance of green sand systems.

4.1.3.2 Chemically Bonded Sand Systems

Foundry cores and molds may be made using resin-coated sand prepared by a number of different bonding processes (e.g., no-bake, cold-box, warm-box, hot-box, shell) that use all sorts of different binders (resins) with unique chemistries. These binders can be triggered, based upon the processes, in two ways—self-setting and triggered setting. In the self-setting system (also known as a self-set, cold setting, cold-box, or no-bake process), sand, binder, and a hardening chemical are mixed together; the binder and hardener start to react immediately, but sufficiently slowly to allow the sand to be formed into a mold or core, which continues to harden further until strong enough to allow casting. In triggered setting system, sand and binder are mixed and blown or rammed into a core box. Little or no hardening reaction occurs until triggered by applying heat or a catalyst gas. Setting then takes place in seconds.

Self-setting systems These include the following:

1. Furanes. Furane sands use a furane resin and an acid catalyst. The resins are urea-formaldehyde (UF), phenol-formaldehyde (PF), or UFPF resins with additions of furfuryl alcohol (FA). The speed of setting is controlled by the percentage of acid catalyst used and the strength of the acid. Ratios such as resin at 0.8 to 1.5% of base sand, catalyst at 40 to 60% of resin are normally used, depending on the sand temperature and the speed of setting required. The optimum ambient temperature is 20 to 30°C, resulting in a compressive strength of typically 4000kPa (600psi).

2. Phenolic-isocyanates (phenolic-urethanes). The binder is supplied in three parts: a phenolic resin in an organic solvent (0.8%), methylene diphenyl diisocyanate (MDI) (0.5%), and a liquid amine catalyst. When mixed with sand, the amine causes a reaction between the resin and the MDI, forming urethane bonds, which rapidly set the mixture. The speed of setting is controlled by the type of catalyst. The optimum cure temperature is 25 to 30°C. Compression strength is typically over 4000 kPa (600 psi).

3. Alkaline phenolic resin, ester hardened. The binder is a low viscosity, highly alkaline phenolic resole resin (1.2 to 1.7%). The hardener is a liquid organic ester (18 to 25%). Sand is mixed with hardener and resin, usually in a continuous mixer. The speed of setting is controlled by the type of ester used. Low sand temperature slows the cure rate, but special hardeners are available for cold and warm sand. In 24 h compression strength can reach 4000 kPa (600 psi).

4. Ester silicate process. Sand is mixed with a suitable grade of sodium silicate (2.2 to 2.8% of sand weight), often incorporating a breakdown agent, together with 10 to 12% (based on silicate) of liquid organic ester hardener. The acid ester reacts with the sodium silicate, hardening the sand. The speed of hardening is controlled by the type of ester used. The final compression and tensile strength achieved are 2000 to 5000 kPa (300 to 700 psi) and 700 kPa (100 psi), respectively.

5. Portland cement process. Sand is mixed with portland cement (10% of sand weight) and water (5% of sand weight). Molds are air dried for 24 h and may then be dried out more rapidly.

Triggered setting systems

Triggered setting systems are used to make cores for repetition foundries. After the mixed sand is blown into the core box, the cores must be cured in the box until sufficient strength has been achieved to allow stripping without damage or distortion. Usually the core continues to harden after stripping.

Tensile strengths of 1000 to 2000 kPa (150 to 300 psi) are typical, equating roughly to transverse strengths of 1500 to 3000 kPa. Final strengths may be higher. Triggered setting systems are categorized into two groups, that is, heat triggered processes and gas triggered processes.

Heat triggered processes include the following bonding systems:

1. Shell process. Sand is precoated with a solid phenolic novolak resin and a catalyst to form a dry, free-flowing material. The coated sand is blown into a heated core box or dumped onto a heated pattern plate, causing the resin to melt and then harden. Shell molds are normally 20 to 25 mm thick. Resin additions are 2.5 to 4.5% of sand weight, and the catalyst hexamine is added at 11 to 14% of the resin content. The minimum curing time is 90 s but 2 min is common. A 3.5% resin content will give a tensile strength of 1400 kPa (200 psi).

2. Hot-box process. The binder is an aqueous PFUF or UFFA resin, and the catalyst is an aqueous solution of ammonium salts, usually chloride and bromide. Sand is mixed with the liquid resin (2.0 to 2.5% of sand weight) and catalyst (20 to 25% of resin weight) and blown into a heated core box. The heat liberates acid vapor from the catalyst, which triggers the hardening reaction. Hardening continues after removal of the core from the box. Thin section cores cure in 5 to 10 s. As cores increase in section size, curing time must be extended up to about 1 min for a 50-mm section. The final tensile strength is 1400 to 2800 kPa (200 to 400 psi).

3. Warm-box process. The binder, 1.3 to 1.5% of sand weight, is a reactive, high FA binder. The catalyst, 20% of sand weight, is usually a copper salt of sulfonic acid. Sand, binder, and catalyst are mixed and blown into a heated core box. The heat activates the catalyst, which causes the binder to cure. Curing time is 10 to 30 s depending on thickness. The final tensile strength can be 3000 to 4000kPa (400 to 600psi).

4. Oil sand. Certain natural oils, such as linseed oil, known as "drying oils," polymerize and harden when exposed to air and heat. Silica sand is mixed with the drying oil (1 to 2% of sand weight), a cereal binder (1 to 2% of sand weight), and water (2 to 2.5% of sand weight). The resulting mixture is either manually packed or blown into a cold core box. Applied backing will harden the oil and the core becomes rigid. A recirculation air oven is needed because oxygen is necessary to harden the oil. The temperature is normally 230°C, allowing 1 h for each 25-mm section thickness. Correctly baked cores develop a tensile strength of 1340 kPa (200 psi).

Gas triggered processes include the following bonding systems:

1. Phenolic-urethane-amine gassed (cold-box) process. The binder is supplied in two parts: a solvent-based phenolic resin (0.8 to 1.5% of sand weight), a polyisocyanate (0.8 to 1.5% of sand weight), MDI (methylene diphenyl diisocyanate) in a solvent. The resins are mixed with sand and the mixture blown into a core box. An amine gas [TEA (triethylamine) or DMEA (dimethyl ethyl amine)] is blown into the core, catalyzing the reaction and causing almost instant hardening. The tensile strength immediately after curing is high at 2000 kPa (300 psi), and the transverse strength is 2700 kPa (400 psi).

2. Alkaline phenolic resin gassed process. Alkaline phenol-formaldehyde resin containing a coupling agent is used. The resin (2.0 to 2.5% of sand weight) is mixed with sand, and the mixture is blown into a core box. Carbon dioxide is passed through the mixture, lowering the pH and activating the coupling agent, which causes crosslinking and hardening of the resin. Strength continues to develop after the core is ejected as further crosslinking occurs and moisture dries out. The compression strength is 2000 to 3000 kPa (300 to 400 psi), and the tensile strength is 500 to 800kPa (70 to 110psi).

3. The SO2process. Sand is mixed with a furane polymer resin (1.2 to 1.4% of sand weight) and an organic peroxide (such as methyl ethyl ketone peroxide at 25 to 60% of resin weight). The mixture is blown into the core box and hardened by passing sulfur dioxide gas through the compacted sand. The gas reacts with the peroxide-forming carbon trioxide and then H2SO4, which hardens the resin binder. The tensile strength is 1250 kPa (180 psi) after 6 h.

4. SO2-cured epoxy resin. Modified epoxy/acrylic resins (1.2 to 1.4% of sand weight) are mixed with organic peroxide (26 to 60% of resin weight), the mixture is blown into the core box and a hardening mechanism similar to the SO2 process takes place.

5. Ester-cured alkaline phenolic system. The resin is an alkaline phenolic resin (essentially the same as the self-hardening resins of this type). Sand is mixed with the resin and blown or manually packed into a core box. A vaporized ester, methyl formate, is passed through the sand, hardening the binder. The total resin and peroxide addition is 1.5%. Compression strengths of 5000 kPa (700 psi) are possible.

6. Carbon dioxide-silica process. Sand is mixed with sodium silicate (3.0 to 3.5% of sand volume), and the mixture is blown or hand-rammed into a core box or around a pattern. Carbon dioxide gas is passed through the compacted sand to harden the binder. The bonding strength eliminates the need for drying or baking the mold and metal can be poured into the mold immediately. Over-gassing should be avoided because it makes the mixture friable.

4.1.4 Casting Metals

The metal casting industry conventionally divides casting products into ferrous and nonferrous metals, in particular, iron-based, steel-based, aluminum-based, and copper-based castings. The other castings of low fractions include magnesium, lead, zinc, and their alloys. In the U.S., the foundry industry currently produces 11 million tons of metal product per year, with a shipment value of $19 billion. Of them, iron and steel accounted for 84% of metals cast.5 The remaining 15% of foundry operations are concerned with aluminum, copper, zinc, and lead production. Table 4.2 summarizes critical physical and thermal properties of aluminum, iron/steel, and cast iron.

4.1.4.1 Iron Castings

Iron is the world's most widely used metal. Iron castings encompass a family of ferrous alloys: gray iron, alloy iron, white iron, malleable iron, ductile iron, and compacted graphite iron. Wide variations in properties can be achieved by varying the balance between carbon and silicon, by alloying, and by applying various types of heat treatment. Iron castings have good fluidity and mold filling during the casting process, with low shrinkage on cooling.

TABLE 4.2

Physical and Mechanical Properties of Aluminum, Iron/Steel, and Cast Iron

TABLE 4.2

Physical and Mechanical Properties of Aluminum, Iron/Steel, and Cast Iron

Properties

Al

Fe

Gray Iron

Ductile Iron

Content (%)

Pure

Pure

10-40

40-70

Atomic weight (g/cm3)

26.98

55.85

48.44

48.44

Density, solid (g/cm3)

2.70

7.86

7.1-7.35

7.06-7.44

Density, liquid (g/cm3)

2.38

7.01

7.1

7.1

Melting point (°C)

660.37

1536

1120-1180

1120-1180

Thermal expansion coefficient, 0°C (K-1 10-6)

23.9

11.7

11.7

11.4-12.8

Heat conductivity, 25°C (W/m K)

237

74.4

45-52

25-42

4.1.4.2 Steel Castings

The most common types of steels used in castings are carbon steels, which contain only carbon as the major alloying element. Carbon steels are classified by their carbon content into three groups: low-carbon steel (C < 0.20%), medium-carbon steel (C = 0.20 to 0.50%), and high-carbon steel (C > 0.50%). Steel's hardness also depends upon the carbon content.

4.1.4.3 Aluminum Castings

Aluminum is a light metal with good tensile strength. It is easily cast, extruded, or pressed. At present, aluminum is the second most widely used metal after iron. Aluminum castings can be cast by virtually all of the common casting processes. It is common to add the alloying constituents as solids to molten aluminum: Al-Cu, Al-Mg, Al-Zn, Al-Sn. The potential for the use of aluminum in automotive applications is considerable, including engine blocks, heads, pistons, rocker covers, inlet manifolds, differential casings, steering boxes, brackets, wheels, and so on. Castings may also be used for household and hospital utensils, and machinery.

4.1.4.4 Copper Castings

Copper is a soft metal that is resistant to corrosion and is a good conductor of heat and electricity. It is most commonly used for electrical wiring and hot water pipes. Copper is second only to aluminum in importance among the nonferrous metals. Products include bushings and bearings, propellers, and other cast products. Copper-base alloys are grouped according to composition: pure copper, high-copper alloys, brasses, leaded brasses, bronzes, aluminum bronzes, silicon bronzes, copper-nickel alloys, and copper-nickel-zinc alloys. In brasses, zinc is the principal alloying element. Tin is the principal alloying element in cast bronze alloys. Copper castings are produced by several methods, including centrifugal molds, green sand molds, and die casting.

4.2 CHARACTERIZATION OF WASTES

Three major solid wastes—spent foundry sand, furnace slag, and baghouse dust—are discharged from metal casting facilities. In the U.S., the annual generation of foundry solid waste is believed to range from 9 to 13.6 million metric tons (10 to 15 million tons).6 Of them, spent foundry sand can account for nearly 70% of a foundry's total wastestream.1 In addition to solid waste, wastewater and air emissions are also discharged from a metal casting facility. Reliable quantification of physical properties and chemical characterization of the byproduct is important for the marketability of the materials. This section focuses on the characterization of the solid wastes of the metal casting industry. Characterization of air emission and wastewater shall also be addressed according to some limited documental data. Mainly four aspects of characterization for a solid wastestream are included: origin, physical properties, chemical properties, and mechanical properties.

4.2.1 General

Prior to their acceptance for beneficial treatment or reuse, foundry wastes discharged from casting processes are characterized and must comply with environmental protection laws and regulations. Countries vary significantly in constituting environmental protection laws. In the U.S., numerous federal environmental laws (or acts) and regulations have been promulgated to protect human health and the environment. Table 4.3 lists most of the federal laws or regulations involved in managing wastes of the metal casting industry. These acts are the unique measures assessing the environmental impact and reuse acceptance of foundry solid waste. Thus, detailed physical and chemical characterization of foundry waste materials is necessary in order to obtain permits for reusing foundry byproducts.

TABLE 4.3

Federal Legislation Related to Solid Waste Management

Title

Year of Promulgation or Amendment

Solid Waste Disposal Act (SWDA)

National Environmental Policy Act (NEPA)

Occupational Safety and Health Act (OSHA)

Clean Air Act (CAA)

Clean Water Act (CWA)

Safe Drinking Water Act (SDWA)

Toxic Substances Control Act (TSCA)

Resource Conservation and Recovery Act (RCRA)

Comprehensive Environmental Response, Compensation and Liabilities Act (CERCLA or Superfund) Hazardous and Solid Waste Amendments (HSWA) Superfund Amendments Pollution Prevention Act (PPA)

1965

1969

1970

1970, 1977, 1990 1977, 1981, 1987 1974, 1977, 1986 1976

1976, 1980 1980

1984 1986 1990

4.2.2 Air Emission

4.2.2.1 Origin

Air emission, known as a gaseous waste, is the largest waste source from foundries.2 Emission sources include the binder systems used in mold making, vapors from metal melting, and airborne sand used in the pouring and shakeout steps. Very limited quantified data are available about the characterization of air emissions. They are thought containing metals dust, semivolatile and volatile organic compounds. They mainly come from the melting procedures. Pouring and cooling steps contribute about 16% of the total organic and semivolatile wastes from foundries.7

4.2.2.2 Characterization

Air emission composition is closely related to its form of generation or collection. Cupola furnaces produce more metallic air emissions than other furnace types. Lower metal emissions are released from induction furnaces and core- and mold-making processes. Emissions from the pouring process depend on the metal temperature. The hotter the metals, the higher the metal emissions.7 Organic air emissions arise largely from vaporized resins, solvents, and catalysts, which are used extensively in core- and mold-making steps. With the promulgation of the Clean Air Act and its amendments, as well as increasingly stringent regulations from U.S. EPA, more air emissions studies are being conducted.

The principal gases produced were found to be hydrogen, carbon monoxide, carbon dioxide, methane, nitrogen, oxygen, and water vapor. Volatile hydrocarbons, including ethane, ethylene, propane, propylene, acetylene, FA, methanol, and ethanol, constitute up to 5% of the gas volume. Benzene, toluene, nitrous oxide, and hydrogen cyanide were identified in the atmosphere near a pouring line in a foundry using alkyd isocyanate resin bonded molds. Concentrations detected in the foundry atmosphere were generally low.

4.2.3 Wastewater 4.2.3.1 Origin

Wastewater discharge, known as liquid pollution in a facility, makes up a small portion of the total wastestream from foundries.2 Wastewater mainly comes from the noncontact cooling water used to cool metal and other work pieces or from wet scrubber air emission systems. Water runoff from floor cleaning and other maintenance procedures may also contribute to wastewater. However, the volumes of liquid waste are relatively small and do not pose a large pollution problem for foundries. Some plants have water treatment facilities to remove contaminants for water reuse.

U.S. EPA promulgated wastewater discharge regulations for the foundry industry in October 1985, which are published in the Code of Federal Regulations at 40 CFR Part 464.8 The regulations cover 28 process segments (processes such as casting quench, grinding scrubber, mold cooling) in four subcategories: aluminum casting, copper casting, ferrous casting, and zinc casting. It is noted that the cast metals have unique properties that influence the way they are melted and processed and, thus, affect the process wastewater characteristics.

4.2.3.2 Characterization

Table 4.4 presents wastewater flow characterization for the foundry industry by casting metals. Also presented in this table is the level of process water recycle, and the number of plants surveyed with central wastewater treatment facilities for all of the processes at that plant. The discharge flow represents all processes within the specific metal casting facilities.

Many toxic pollutants were detected in the process wastewaters from metal molding and casting processes. The toxic pollutants detected most frequently in concentrations at or above 0.1 mg/L were phenolic compounds and heavy metals. The pollutants include 2,4,6-trichlorophenol, 2,4-dimethyl-phenol, phenol, 2-ethylhexyl, cadmium, chromium, copper, lead, nickel, and zinc. Each type of operation in the foundry industry can produce different types of pollutants in the wastewater stream. Also, because each subcategory operation often involves different processes, pollutant concentrations per casting metals may vary.

4.2.4 Spent Foundry Sand 4.2.4.1 Origin

Foundries purchase new, virgin sand to make casting molds, and the sand is reused numerous times within the foundry. However, heat and mechanical abrasion eventually render the sand unsuitable for use in casting molds, and a portion of the sand is continuously removed and replaced with virgin sand. The removed sand becomes spent foundry sand, which is discarded from the foundry facility.

In the U.S., the foundry industry produces roughly seven to eight million tons of spent sand each year,1 which are available to be recycled into nonfoundry applications. However, less than 15% of

TABLE 4.4

Wastewater Flow Characterization by Casting Metals

Iron/Steel Aluminum Copper and Magnesium Casting Casting Alloy Casting Casting Zinc Casting

Applied flow (ML/yr)

Recycle flow (ML/yr)

Direct discharge flow (ML/yr)

Indirect discharge flow (ML/yr)

100% recycle flow (ML/yr)

Central treatment facilities (no. of plants)

Operation treatment facilities (no. of plants)

397,GGG 317,000 69,3GG 11,600 189,GGG

14,5GG 753G 57GG 126G 4G8

34,9GG 25,3GG 961G

48 3340 1G 14

4G4G 343G 5G5G 100 1G1G

1G9 2G5

12 2G

13 12

Note: ML/yr, million liters per year.

the spent foundry sand is recycled. It is believed that a greater percentage of spent foundry sand can be safely and economically recycled, as has been encouraged by many successful case studies. Concentrating energies on the largest volume stream first will have the greatest economic impact for the industry as a whole.

Spent material often contains casting residues, such as degraded binders, metals, and oversized mold/core materials. Spent foundry sand may also contain some leachable contaminants, including heavy metals and phenols that are absorbed by the sand during the molding process and casting operations.9 The detection of heavy metals is of greater concern in nonferrous foundry sands generated from nonferrous foundries.10 Spent foundry sand from the brass or bronze foundries, in particular, may contain high concentrations of cadmium, lead, copper, nickel, and zinc.11

4.2.4.2 Physical Properties

Spent foundry sand can be divided, based upon bonding processes, into two categories—spent green sand and spent chemically bonded sand. Spent green sand is black in color due to its carbon content, and has clay contents that result in a fraction of the material passing a No. 200 sieve (0.075 mm). Chemically bonded sands are generally yellowish in color and coarser in texture than clay bonded sands.

Physical properties involve tests of the physical index parameters of the materials. For spent foundry sand, these parameters include particle gradation, unit weight, specific density, moisture content, adsorption, hydraulic conductivity, clay content, plastic limit, and plastic index. These parameters determine the suitability of spent foundry sand for uses in potential applications. Typical physical properties of spent green foundry sand are listed in Table 4.5.

The grain size distribution of spent foundry sand is very uniform, with approximately 85 to 95% of the material between 0.6 mm and 0.15 mm (No. 30 and No. 100) sieve sizes. Five to twelve percent of foundry sand can be expected to be smaller than 0.075 mm (No. 200 sieve). The particle shape is typically subangular to round. Spent foundry sand gradations are too fine to satisfy the fine aggregate standard specified in specification ASTM C33 Standard Specification for Concrete Aggregates.

Spent foundry sand has low absorption and is nonplastic. Reported values of absorption were found to vary widely (0 to 5%), which can also be attributed to the presence of binders and

TABLE 4.5

Typical Physical Properties of Spent Green Foundry Sand

TABLE 4.5

Typical Physical Properties of Spent Green Foundry Sand

Property

Results

Test Methods

Specific gravity

2.39-2.55

ASTM D854

Bulk relative density, kg/m3 (lb/ft3)

2590 (160)

ASTM C48/AASHTO T84

Absorption (%)

0.45

ASTM C128

Moisture content (%)

0.1-10.1

ASTM D2216

Clay lumps and friable particles

1-44

ASTM C142/AASHTO T112

Coefficient of permeability (cm/s)

10-3-10-6

AASHTO T215/ASTM D2434

Plastic limit/plastic index

Nonplastic

AASHTO T90/ASTM D4318

Source: From AFS, Alternative Utilization of Foundry Waste Sand, final report (Phase I) for Illinois Department of Commerce and Community Affairs, American Foundrymen's Society, Des Plaines, IL, July 1991. Javed, S. and Lovell, C.W., Use of Foundry Sand in Highway Construction, Joint Highway Research Project No. C-36-50 N, Purdue University, West Lafayette, IN, July 1994. Javed, S., Lovell, C. W., and Wood, L.E., Waste Foundry Sand in Asphalt Concrete, in Transportation Research Record, No 1437, Transportation Research Board, Washington, DC, 1994.

Source: From AFS, Alternative Utilization of Foundry Waste Sand, final report (Phase I) for Illinois Department of Commerce and Community Affairs, American Foundrymen's Society, Des Plaines, IL, July 1991. Javed, S. and Lovell, C.W., Use of Foundry Sand in Highway Construction, Joint Highway Research Project No. C-36-50 N, Purdue University, West Lafayette, IN, July 1994. Javed, S., Lovell, C. W., and Wood, L.E., Waste Foundry Sand in Asphalt Concrete, in Transportation Research Record, No 1437, Transportation Research Board, Washington, DC, 1994.

additives.11,12 The content of organic impurities (particularly from sea coal binder systems) can vary widely and can be quite high. This may preclude its use in applications where organic impurities could be important (e.g., portland cement concrete aggregate).9 The specific gravity of foundry sand has been found to vary from 2.39 to 2.55. This variability has been attributed to the variability in fines and additive contents in different samples.11 In general, foundry sands are dry, with moisture contents less than 2%. A large fraction of clay lumps and friable particles have been reported, which are attributed to the lumps associated with the molded sand, which are easily disintegrated in the test procedure.11 The variation in permeability listed in Table 4.5 is a direct result of the fraction of fines in the samples collected.

4.2.4.3 Chemical Compositions

The chemical compositions of materials are usually expressed in terms of simple oxides calculated from elemental analysis determined by x-ray fluorescence. For spent foundry sand, the chemical parameters include bulk oxides mass composition, loss on ignition, and total oxygen demand. Table 4.6 lists the general chemical properties of spend foundry sand. It is shown that spent foundry sand consists primarily of silica dioxide.

Depending on the binder and type of metal cast, the pH of spent foundry sand12 can vary from approximately 4 to 8. As such, it has been reported that some spent foundry sands can be corrosive to metals.14 Spent foundry sand must be monitored to assess the need to establish controls for potential phenol discharges.9,15,16

4.2.4.4 Trace Element Characterization

Trace element characterization represents concentrations of elements that are contaminated in materials or their leachates in a trace level, generally reported in units of mg/kg or mg/L. Although in minimum quantities, trace elements need to be characterized to assess the hazardous impact of the

TABLE 4.6

Foundry Sand Sample Chemical Oxide Composition

TABLE 4.6

Foundry Sand Sample Chemical Oxide Composition

Constituent

Value (%)

SiO2

87.91

Al2Ö3

4.70

Fe2O3

0.94

CaO

0.14

MgO

0.30

SO3

0.09

Na2O

0.19

K2O

0.25

TiO2

0.15

P2O5

0.00

Mn2O3

0.02

SrO

0.03

Loss on ignition

5.15, on average

Total

99.87

Source: From AFS, Alternative Utilization of Foundry Waste Sand, final report (Phase I) for Illinois Department of Commerce and Community Affairs, American Foundrymen's Society, Des Plaines, IL, July 1991.

Source: From AFS, Alternative Utilization of Foundry Waste Sand, final report (Phase I) for Illinois Department of Commerce and Community Affairs, American Foundrymen's Society, Des Plaines, IL, July 1991.

materials and their compliance with environmental protection laws. Total analyses and leaching analyses are generally used. The former quantifies as a dry-basis the mass percentage of trace elements by following U.S. EPA standard environmental analytical methods or approved analytical chemistry methods, relying upon techniques of inductively coupled plasma atomic emission mass spectrometry (ICP-AES, ICP-MS) and gas chromatography interfaced with a mass spectrometer (GC-MS). Leaching analysis is often run as a simulation of the field extraction effect, in which materials are extracted into aqueous media by leachate fluid, groundwater, rainfall, or other fluids. Currently, three leaching protocols are frequently documented to simulate field extraction variation: the Toxicity Characteristic Leaching Procedure (TCLP, U.S. EPA Method 1311), the Synthetic Precipitation Leaching Procedure (SPLP, U.S. EPA Method 1312), and the Standard Test Method for Shake Extraction of Solid Waste with Water (ASTM D3987). TCLP and SPLP are acidic toxicity tests, whereas ASTM D3987 is a neutral leaching procedure.

Many studies have been conducted on metal contaminants in foundry sands. Spent foundry sand segregated from the other wastestreams leaches regulated metals well below the toxicity characteristic levels.12 It is also found that spent foundry sands produced by iron, steel, and aluminum foundries are rarely hazardous, whereas spent foundry sand collected from copper-based facilities may render leachate with regulated elements exceeding regulation threshold values.12,14 Only iron and manganese, which are not regulated under RCRA, were recorded at increased leaching potentials on a number of occasions. Lead, chromium, copper, and zinc are reported to be of concern for mixed foundry wastes. There is no direct correlation between the total metal content and the leachability under TCLP. Quantities of total metal content in spent and virgin sands and in sandy soils are typically of the same order of magnitude, which suggests an opportunity for spent foundry sand replacing conventional sand and natural soil in many applications, without posing environmental threats.

Few studies have been conducted to determine organic residues in spent foundry sand and leach-ates from disposal sites. It is reported that several organic compounds are present in the spent foundry sand but have concentrations below the regulated toxicity characteristic limits. Organic compounds of concern include benzoic acid, naphthalene, methylnaphthalenes, phenol, methylenebisphenol, diethylphenol, and 3-methylbutanoic acids.12 These compounds are thought to be derived from the decomposition of organic binders such as phenolic urethane, furan, and alkyd isocyanate.

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