In the melting process, metal scraps and fluxes (limestone or dolomite) are charged into a furnace, sometimes along with coke for fuel if a blast furnace is used. Upon heating using electricity (arc furnace) or burning (blast furnace), scraps are melted into a molten phase. The metal is subsequently gravimetrically separated from the composite flux, leaving the residual slag. Flux is used to adequately render the slag fluid so that it can be separated from the molten iron, and it then flows freely from the cupola.
Furnace slag is a nonmetallic byproduct produced in the melting process. It consists primarily of silicates, alumina silicates, and calcium-alumina-silicates. The molten slag, which absorbs much of the sulfur from the charge, comprises ~20% by mass of iron production. As a byproduct of the melting process, furnace slags vary considerably in form depending on the melted metals used, furnace types, and slag cooling method. Figure 4.7 demonstrates the major types of furnace slags.
For iron slag, subcategories include air-cooled blast furnace slag, expanded or foamed slag, pelletized slag, and granulated blast furnace slag. The generation of each slag is described below:
1. Air-cooled blast furnace slag. If the liquid slag is poured into beds and slowly cooled under ambient conditions, a crystalline structure is formed, and a hard, lump slag is generated, which can subsequently be crushed and screened.
2. Expanded or foamed blast furnace slag. If the molten slag is cooled and solidified by adding a controlled volume of water, air, or steam, the process of cooling and solidification can be accelerated, increasing the cellular nature of the slag and generating a lightweight expanded or foamed product. Foamed slag is distinguishable from air-cooled blast furnace slag by its relatively high porosity and low bulk density.
3. Pelletized blast furnace slag. If the molten slag is cooled and solidified with water and air quenched in a spinning drum, pellets, rather than a solid mass, can be produced. By controlling the process, the pellets can be made more crystalline, which is beneficial for aggregate use, or more vitrified (glassy), which is more desirable in cementitious applications. More rapid quenching results in greater vitrification and less crystallization.
4. Granulated blast furnace slag. If the molten slag is cooled and solidified by rapid water quenching to a glassy state, little or no crystallization occurs. This process results in the formation of sand-sized (or frit-like) fragments, usually with some friable clinker-like material. The physical structure and gradation of granulated slag depend on the chemical composition of the slag, its temperature at the time of water quenching, and the method of production. When crushed or milled to very fine cement-sized particles, ground granulated blast furnace slag has cementitious properties, which makes it a suitable partial replacement for or additive to portland cement.
Steel slag, a byproduct of steel making, is a complex solution of silicates and oxides that solidifies upon cooling. The main components are consist of carbon, silicon, manganese, phosphorus, some iron as liquid oxides, lime, and dolime. There are several different types of steel slag produced during the steel-making process. These different types are referred to as furnace or tap slag, synthetic or ladle slags, and pit or cleanout slag.
Nonferrous slag is mostly formed by dumping it into a pit and simply allowing it to air cool, solidifying under ambient conditions. A small proportion is granulated, and by using rapid water and air quenching results in the production of a vitrified product. Similar to the generation of iron slag, the cooling rate has a strong influence on the mineralogy and, consequently, the physical and cementitious properties of the nonferrous slag. Slag generation is highly dependent on specific processes and sources. Consequently, slag properties can vary between plants and different ore sources, and must be investigated on a case-by-case basis.
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