Issues in coated ferrous scrap.

Most iron castings receive a coating before entering service. This coating remains intact even when the casting is eventually scrapped, recycled and sold to the foundry as a melting stock. Coating removal at the foundry isn't feasible because of its direct costs and cost of disposing coating debris, which may be classified as hazardous. Therefore, most iron melters must melt the cast iron scrap, coatings and all.

Once in the furnace, the coatings will decompose or be altered with the residue, ending up distributed among the three phases present - the metal itself, the slag phase always present with liquid metal or the gas phase such as the blast air in a cupola or the atmosphere over the electric furnace bath. Any of these phases may contain some or nearly all of the coating residue.

In cast iron, coating remnants may exert deleterious metallurgical effects. For example, zinc (Zn) vapor can migrate through furnace refractories to condense on and short-out power coils. On the other hand, the presence of certain carbide stabilizers may only become apparent long afterward. In slag, there may be an unwelcome attack or buildup on refractories, and coating remnants may make proper slag disposal an expensive chore. Finally, if the coating residue passes into the gas phase, it may present a workplace hazard and/or environmental problem.

A variety of coatings are available and more are being developed every year. Most, however, fall into one of the following three categories. Each presents different problems and hazards to the iron caster.

Metallic Plating - This type (electrolytic or otherwise) consists of a "witches brew" of metallic elements, most of which are on the EPA list of hazardous materials - Zn, lead (Pb), chromium (Cr), nickel, cadmium (Cd), etc. There are strict limits on how much of these materials can be emitted to the environment - either as airborne particles or fumes or when collected in liquid or solid wastes. The high costs of disposal are apparent.

Also, if sufficiently concentrated in the iron, platings may have a profound influence on properties. Examples are Cr and Pb, which can result in white iron structures or Widmanstatten (mesh) graphite, respectively.

Nonmetallic Overlays - This category consists of the nonpaint and nonmetal coatings principally made up of porcelain and similar materials, such as the porcelain glaze on white plumbing. They present fewer environmental concerns, but sometimes greater metallurgical/operational concerns. An example is boron (B), which is present in many ceramic coatings. When B is dissolved in cast iron, it can drastically alter critical properties - even at low concentrations. If B finds its way into the slag phase, it can seriously damage the refractories (especially silica-based).

Also likely to be present in porcelain-like coatings are sodium, potassium and barium (Ba) oxides that - as strong fluxes - can be nearly as harmful to refractories as B. Moreover, Ba is on the EPA list of hazardous materials and disposing slag or refractories containing Ba may present special difficulties.

Paints - Finally, the iron melter must deal with the paint-like class (including powder coatings) that smokily decomposes on heating, giving off copious hydrocarbons (VOCs) and other nasties, making life in the foundry unpleasant. While mostly "out of sight" in a cupola shaft or in an electric furnace preheater, decomposition still occurs and the breakdown products are emissions that must be dealt with properly.

There are two major concerns with painted scrap. First are the VOCs given off on decomposition. VOC emissions to the atmosphere are closely regulated and if they're collected, special precautions may be needed for final disposal.

The second problem are the pigments present in coatings. These often are Pb, Cd and Zn oxides. In this form, these elements are even more difficult to deal with than metallics. In fact, studies have established that the principal source of Pb and Cd in iron foundry solid waste aren't metallic solids or coatings, but paints. Also consider that, as pigments, the compounds are finely ground powders and almost none enters the iron or slag phases. It all goes into the air.

As for general remedies - the list is short. The obvious one is to restrict receipts of recycled scrap iron and steel to only the uncoated (black) grades. There isn't much of this scrap around, however, and what is available is expensive and escalating in cost.

Another solution might be to selectively remove only the worst offender coatings, or have the scrap processor do it. The steel industry is attempting this with tin plate scrap. Undoubtedly, this practice will raise prices, perhaps above the price of uncoated grades.

Also, pig iron may be substituted for purchased scrap, as is widely practiced at relatively low percentages. Due to cost, however, pig iron is mostly used only to supplement scrap, not replace it. Availability and use is limited, but hopefully, other clean, nontraditional sources of iron units may become available in the mid- to long-term.

There is one solution that may be within the foundry's area of control - its own returned castings that, for whatever reason, have been coated by the customer. It seems possible and desirable to leave that problem entirely in the customer's lap. Issue the credit, if that's appropriate, but insist on disposal elsewhere.