Steel slag: Applications for AMD control (original) (raw)

1998, Proceedings of the 1998 conference on …

In both laboratory and field studies, steel slags were found to generate exceptionally high levels of alkalinity over extended periods. Steel slags also have high neutralization potentials and can be used as alkaline amendments to acid-producing materials. This paper discusses the alkalinity-generating capacity of steel slags, their metal leaching potential and applications for acid mine drainage control. Results indicate that steel slags can provide highly concentrated alkaline recharge to acid mine wastes. Proper design and sizing offers the potential for a low-to zero-maintenance method for treating acid mine drainage (AMD) within the spoil pile itself. Since slags form around the melting point of iron, >2,700 o F, most compounds which have a low boiling point have been driven off. Most of the residuals are encased as oxides or in a calcium-alumino-silicate glassy matrix. Fortunately, the matrix is soluble and releases calcium and manganese oxides which drive the pH above 10. Since slag is a glass in its coarser form (e.g.-1/8 in.) it will, unless compacted, maintain high permeability regardless of how much water has passed through it. Unlike lime, steel slags do not absorb CO 2 from the air and convert back to relatively insoluble calcite: Ca (OH) 2 + CO 2 ®Ca CO 3 +H 2 O. This is an extremely important property, since it means slag will generate high levels of alkalinity even after years of open storage. STEEL SLAG: WHAT IS IT? Technically, slag is nearly any solid which melts and forms a silicate glass during a metal refining process. In the power industry, slag is ash which melts and sticks to the walls or pipes of the boiler. In the base-metal industry, slags result from the smelting of various ores of copper, zinc, lead, etc. These slags can have high concentrations of heavy metals. In this paper, we are only discussing slags from the steel-making process. In making steel, iron ore or scrap metal is melted in combination with limestone, dolomite or lime. Pure iron is soft, bends easily under loads, and has limited uses. Small amounts of carbon, nickel, manganese, and other elements turn iron into various alloys of steel. There are hundreds of grades of steel ranging from basic carbon steel to high grade stainless. Steel making begins by reducing any metal oxides in the melt to pure iron metal, while scavenging ions such as aluminum, silicon, and phosphorous. The later three elements are bad news for steel as they cause it to become weak, brittle, or otherwise difficult to roll into sheet in a predictable way. For that matter, they make it nearly impossible to make anything useful out of iron. (Even though iron is much more readily available, its impurities caused bronze to become the metal of choice for tools after stone became obsolete.) Fortunately, our ancestors discovered that irons imperfections could be controlled by adding limestone or dolomite. These calcium compounds mix with aluminum, silicon and phosphorous to form slag. Slag then floats to the top of the melt, is poured off, and sent to disposal. Slag starts its life at about 2,700 o F and cools almost immediately.