Added: Kandace Deckert - Date: 02.09.2021 10:54 - Views: 33119 - Clicks: 2193
Steel slag is a byproduct of the steelmaking and steel refining processes. This paper provides an overview of the different types of steel slag that are free slags from basic-oxygen-furnace BOF steelmaking, electric-arc-furnace EAF steelmaking, and ladle-furnace steel refining processes.
SEM micrographs showed that the majority of the sand-size steel slag particles had subangular to angular shapes. The characteristics of the steel slag samples considered in this study are discussed in the context of a detailed review of steel slag properties. The steelmaking industries free slags the US generate 10—15 million tons of steel slag every year. Utilization of steel slag in civil engineering applications can alleviate the need for their disposal and reduce the use of natural resources. A better understanding of the properties of steel slag is required for large volumes of this material to be utilized in a technically sound manner in civil engineering applications.
Knowledge of the chemical, mineralogical, and morphological properties of steel slags is essential because their cementitious and mechanical properties, which play a key role in their utilization, are closely linked to these properties. As an example, the frictional properties of steel slag are influenced by its morphology and mineralogy. Similarly, the volumetric stability of steel slag is a function of its chemistry and mineralogy.
The chemical, mineralogical, and morphological characteristics of steel slag are determined by the processes that generate this material. Therefore, knowledge of the different types of steelmaking and refining operations that produce steel slag as a byproduct is also required. This paper provides an overview of steel slag generation and a literature review on free slags chemical and mineralogical properties of steel slags. Moreover, the mineralogical and morphological characteristics of steel slag samples generated from two steel plants in Indiana were evaluated through XRD analyses and SEM studies.
Slags are named based on the furnaces from which they are generated. Figure 1 shows a flow chart for the iron and steelmaking processes and the types of slag generated from each process [ 12 ]. The main types of slags that are generated from the iron and steelmaking industries are classified as follow: i blast-furnace slag ironmaking slagii steel-furnace slag, a basic-oxygen-furnace BOF slag, b electric-arc-furnace EAF slag, c ladle slag.
Basic-oxygen furnaces, which are located at integrated steel mills in association with a blast furnace, are charged with the molten iron produced in the blast furnace and steel scraps.
Figure 2 shows a schematic representation of a basic-oxygen furnace [ 14 ]. During the blowing cycle, which lasts approximately 20—25 minutes, intense oxidation reactions remove the impurities of the charge. The scrap is thereby melted, and the carbon content of the molten iron is lowered [ 13 ]. In order to remove the unwanted chemical elements of the melt, the furnace is also charged with fluxing agents, such as lime CaO or dolomite MgCa CO 3 2during the oxygen blowing cycles.
The impurities combine with the burnt lime or dolomite forming slag and reducing the amount of undesirable substances in the melt. Samples of the molten metal are collected near the end of the blowing cycle and tested for their chemical composition. Once the desired chemical composition is achieved, the oxygen lance is pulled up from the furnace. Slag resulting from the steelmaking process floats on top of the molten steel.
The basic-oxygen furnace is tilted in one direction in order to tap the steel into ladles. The steel produced in the basic-oxygen furnace can either undergo further refining in a secondary refining unit or be sent directly to a continuous caster where semifinished shapes blooms, billets, or slabs are solidified in integrated steel mills.
After all the steel is removed from the basic-oxygen furnace, it is tilted again in the opposite direction to pour the liquid slag into ladles. The slag generated from a steelmaking cycle is later processed, and the final product after processing is referred to as basic-oxygen-furnace slag BOF slag.
The chemical reactions occurring during the removal of impurities determine the chemical composition of the basic-oxygen-furnace slag [ 135 ]. Electric-arc furnaces mini mills use high-power electric arcs, instead of gaseous fuels, to produce the heat necessary to melt recycled steel scrap and to convert it into high quality steel.
The electric-arc furnace steelmaking process is not dependent on the production from a blast furnace since the main feed for it is steel scrap with some pig iron. Electric-arc furnaces are equipped with graphite electrodes and resemble giant kettles with a spout or an eccentric notch on one side. The roof of the electric-arc furnaces can pivot and swing to facilitate the loading of raw materials.
Free slags scraps, either as heavy melt large slabs and beams or in shredded form are separated, graded, and sorted into different classes of steel in scrap yards. Scrap baskets are loaded carefully with different types of scrap according to their size and density to ensure free slags both the melting conditions in the furnace and the chemistry of the finished steel are within the targeted range [ 1 — 3 ]. The electric-arc furnace steelmaking process starts with the charging of various types of steel scrap to the furnace using steel scrap baskets. Next, graphite electrodes are lowered into the furnace.
Then, an arc is struck, which causes electricity to travel through the electrodes and the metal itself. The electric arc and the resistance of the metal to this flow of electricity generate the heat. As the scrap melts, the electrodes are driven deeper through the layers of scrap. In some steel plants, during this process, oxygen is also injected through a lance to cut the scrap into smaller sizes.
As the melting process progresses, a pool of liquid steel is generated at the bottom of the furnace. CaO, in the form of burnt lime or dolomite, is either introduced to the furnace together with the scrap or is blown into the furnace during melting. After several baskets of scraps have melted, the refining metallurgical operations e. During the steel refining period, oxygen is injected into the molten steel through an oxygen lance. Some iron, together with other impurities in the hot metal, including aluminum, silicon, manganese, phosphorus, and carbon, are oxidized during the oxygen injections.
These oxidized components combine with lime CaO to form slag. As the steel is refined, carbon powder is also injected through the slag phase floating on the surface of the molten steel, leading to the formation of carbon monoxide. The carbon monoxide gas formed causes the slag to foam, thereby increasing the efficiency of the thermal energy transfer. Once the desired chemical composition of the steel is achieved, the electric-arc furnace is tilted, and the slag and steel are tapped out of the furnace into separate ladles. Steel is poured into a ladle and transferred to a secondary steelmaking station for further refining.
The molten slag is carried to a slag-processing unit with ladles or slag pot free slags [ 1 — 35 ]. In electric-arc furnaces, up to tons of steel can be manufactured per cycle a cycle takes one to three hours to complete. Initially, the EAF steelmaking process was more expensive than the BOF process and, hence, it was only used for production of high quality steels.
After completion of the primary steelmaking operations, steel produced by the BOF or EAF processes can be further refined to obtain the desired chemical composition. These refining processes are called secondary steelmaking operations. Refining processes are common in the production of high-grade steels.
The most important functions of secondary refining processes are final desulfurization, degassing of oxygen, nitrogen, and hydrogen, removal of impurities, free slags final decarburization done for ultralow carbon steels. Depending on the quality of the desired steel, molten free slags produced in the EAF and BOF process goes through some or all of the above mentioned refining processes [ 12 ].
Most of the mini mills and integrated steel mills have ladle-furnace refining stations for secondary metallurgical processes. Figure 3 shows a schematic representation of an electric-arc-furnace and a ladle-refining unit associated with it [ 24 ]. Ladle furnaces, which look like smaller versions of EAF furnaces, also have three graphite electrodes connected to an arc transformer used to heat the steel.
Typically, the bottom of the ladle furnace has a pipeline through which argon gas is free slags for stirring and homogenization of the liquid steel in the furnace. By injecting desulfurizing agents such as Ca, Mg, CaSi, CaC 2 through a lance, the sulfur concentration in the steel can be lowered to 0. The addition of silicon and aluminum during deoxidation forms silica SiO 2 and alumina Al 2 O 3 ; these oxides are later absorbed by the slag generated by the refining process.
In addition, in order to adjust precisely the chemical composition of the steel to produce different grades of steel, the desired alloys are added to the molten steel through an alloy hopper that is connected to the ladle furnace. Ladle furnaces also function as a storage unit for the steel before the initiation of casting operations.
Therefore, ladle furnaces reduce the cost of high-grade steel production and allow flexibility in the steelmaking operations [ 12 ]. Ladle slag is generated during the steel refining processes in which several alloys are added to the ladle furnace to produce different grades of steel.
Table 1 provides the chemical composition of basic-oxygen-furnace BOFfree slags EAFand ladle slags from various sources [ 7 — 22 ]. During the conversion of molten iron into steel, a percentage of the iron Fe in the hot metal cannot be recovered into the steel produced. This oxidized iron is observed in the chemical composition of the BOF slag. The Al 2 O 3 and MgO contents are in the 0.
The EAF steelmaking process is essentially a steel scrap recycling process. Therefore, the chemical composition of EAF slag depends ificantly on the properties of the recycled steel. Information on the chemical composition of ladle slags LS is limited in the literature. During the steel refining process, different alloys are fed into the ladle furnace in order to obtain the desired steel grade.
Hence, the chemical composition of ladle slag is highly dependent on the grade of steel produced. On the other hand, the Al 2 O 3 and CaO contents are typically higher for ladle slags refer to Table 1.
Crystal formation is a function of both the chemical composition of the melt and its cooling rate. Silica rich blast-furnace slag vitrifies forms a glassy phase easily when it is rapidly cooled. Steel slag has a lower silica content than blast-furnace slag and, hence, steel slag seldom vitrifies even when rapidly cooled. Tossavainen et al. Reddy et al. These studies indicate that even when rapidly cooled, in general, steel slag tends to crystallize due to its chemical free slags.
Several researchers studied the mineralogical composition of steel slags. X-ray diffraction analysis of steel slag samples shows a complex structure with many overlapping peaks reflecting the crystalline phases present in steel slag.
These crystalline phases free slags to be mainly due to the chemical composition of steel slag and the slow cooling rate applied during processing [ 126 — 28 ]. The feed charge into the furnaces vary from one steelmaking plant to another, so variations in the chemical constituents of steel slags produced at different steelmaking plants are expected. Table 2 presents the minerals identified in steel slags, as reported in the literature [ 813161720212528 — 30 ]. Ladle slag has a lower FeO content, and polymorphs of C 2 S are therefore frequently observed as the main phase [ 19242729 ].
Due to the presence of unstable phases in its mineralogy, steel slags can show volumetric instability, caused mainly by the presence of free CaO. In the presence of water, free lime hydrates and forms portlandite Ca OH 2. Portlandite has a lower density than CaO and, hence, hydration of free CaO in volume increase.
Ramachandran et al. Their study also demonstrated that hydration of lime by exposure to water vapor causes more expansion than hydration caused by exposure to water due to the effect of temperature. The fact that limes hydrates quickly suggests that the majority of the free lime in steel slag will hydrate in a few days if it is given access to water.
However, residual lime can be embedded in small pockets in gravel-size steel slag particles.Free slags
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DETERMINATION OF ORIGINAL FREE LIME CONTENT OF WEATHERED IRON AND STEEL SLAGS BY THERMOGRAVIMETRIC ANALYSIS