Process Design Assignment On Analysing A Steel Furnace

Question

Task: Provide a detailed process description to understand a process design for steel furnace.

Answer

Introduction
To understand the processing design in a steel furnace discussed in this Process design assignment, we need to have an idea about the steel furnace. A steel furnace is similarly like a blast furnace that is used for smelting to produce industry-related metals like lead, steel, copper, or mainly pig iron (Shin et al., 2016). Though in blast furnace all types of metals are smelted in a steel furnace, only the steel is melted. In these types of furnaces, ores, fuel, and flux are continually provided via the top surface of the stove, meanwhile, in the lower section of the furnace, a continuous blow of hot air is given through serially connected pipes commonly known as tuyeres. This process of blowing hot air down the lower section of the furnace is to continue the flow of chemical reactions throughout the blast furnace as the metals fall in a downward direction (Liu et al., 2016). Generally, this process is the same for each of the furnaces only the materials that are used for making them differ.

Overall Process Description
In the process of steelmaking, various aspects such as silicon, sulphur, nitrogen, phosphorous and excessive of carbon (considered as one of the most essential contamination) are firstly removed from the sourced iron and elements of alloy such as carbon, nickel, chromium, manganese and vanadium are added to produce distinguished grades of the steel (Feng et al., 2017). There are mainly 2 essential processes for making up of the steel. They are: -

1. Basic oxygen steelmaking – This process of steel making generally involves blast furnace’s liquid pig iron, and for the material's main feed it uses scrap steel.
2. Electric Arc Furnace (EAF) steelmaking – Unlike the raw materials used in the process of basic oxygen steelmaking, EAF uses Direct Reduced Iron (DRI) or sometimes even scrap steels for the material’s main feed (Ariyama et al., 2016).

However, there are modern processes of steelmaking which can be easily divided into categories like Primary steelmaking and Secondary steelmaking. They are discussed in this Process design assignment below:

1. Primary Steelmaking – This process involves the conversion of liquid iron from that blast furnace and a scrap of steel into the steel through the process basic oxygen steelmaking or the melting up of DRI i.e., direct reduced iron or scrap steel in the furnace of electric arc (Zhou et al., 2016). The method basic oxygen steelmaking is one of the methods of primary steelmaking which involves the conversion of molten pig- iron (considered highly rich in carbon) into steel. Oxygen is blown through the molten pig iron that lowers the alloy's carbon content and changes or converts it into steel. This whole process is also called essential because of the refractories chemical nature – magnesium oxide & calcium oxide (Yang et al., 2017). This lines the vessel to be able to cope up with the corrosive environment and high temp of that molten metal and vessel's slag.
2. Secondary Steelmaking – This process is majorly performed in Ladles. Few of the operations that have been performed in ladles include vacuum degassing, inclusion removal, homogenization, and inclusion chemistry modification, de-oxidation, desulphurization, and alloy addition. Nowadays, it is widespread to perform metallurgical operations in gas stirred in ladles, with the heating of electric arc on the furnace’s lid (Zhao, Wang & Yan, 2017).   

Block diagram for the entire process of steelmaking

Figure 1: Process flow diagram of the plant

Process Flow Diagram (PFD) for a small section of the plant

Figure 2: Process flow diagram for the steel furnace

Detailed Process description for blast furnace within this process design assignment
Blast furnace forms the core of all the process in the procedure of steel making. The heaters are usually tall sometimes even more than the height of a ten-story building. In terms of the structure of the furnace, its surface is lined with refractory crucibles, and then this layer is superimposed with crucibles made up of earthen materials. The output of the furnace is the pig iron in the molten state and the input- commonly termed as charge includes flux, coke (a form of carbon) and other iron-bearing materials.

Chemically, the reduction process takes in the core of the blast furnace. The charge, as described earlier, is put into the furnace from the top, which is supplemented by a blast of hot air to force the commands into the container. Large stoves are used to create pressure from hot-air. The fuel required to combust the charge in the furnace is injected through the openings called tuyeres in the furnace.  The fuel could either in the liquid or gaseous state. The tuyeres are placed at the bottom of the furnace, which just above the base. The base is made up of crucibles. The fuel ignites the coke with the help of oxygen present in the furnace and then begins the reduction process of iron materials. The oxygen present in the iron ore is removed during this process, and the iron melts and flows down the crucible.

 The products coming out of the furnace include both the molten iron and the slag (impurities developed during the chemical process). The slag is lighter than the liquid iron and hence floats on the surface, which enables easy removal of the contaminants from the liquid iron. The gases that develop come out of the top exits (Feng et al., 2017). All the gases mentioned in this process design assignment which can be seen coming out of the steel industries are these coming out of the furnace. The gas can also be cleaned for re-use into the initial blasting of the charge. The continuous burning life of a steel furnace could reach up to 6 years in general.

Design criteria for blast furnace
Below table describes the design criteria of the blast furnace and the composition of materials required:

Mass balance for blast furnace
Stream Tables

To ascertain the mass balance for blast furnace, the following assumptions are made for the charge:

• Iron is in the form of hot metal when it enters the furnace
• Oxygen entry is in the form of air blast
• Carbon entry is in the form of coke

At first, we measure the calorific value of charge, which is a sum of the heat generated from the combustion of coke, nit coke, and coal, which gives the total energy output. Then the combustion calculation of hot air is done, which considers the water in moist gas as well as the dry gas (Shin et al., 2016). This enables us to calculate the oxygen enrichment present in the air.

Next step is to analyze the composition, which includes iron ore, sinter, dunite, dolomite, quartzite, coke, coal, and nut coke and then followed by an analysis of hot metal which includes silicon, manganese, phosphorus, titanium and carbon and volatile matter.

The following step is analyzing the dust in the mass balance steps. First, the heat generated from dust catcher is calculated, followed by the energy spent in gas cleaning and the energy dissipated through the cooling from water.

Slag analysis mentioned in this Process design assignment is used to determine the mass of impurities coming out of the furnace. The weight can directly be measured as it is a part of the continuous process. From all the analysis above, it can be estimated that about 70% of the mass is retained from the charges. This percentage is the sum of the molten pig iron and slag coming out of the furnace to the total mass of charge initially put in. The remaining is lost in the form of water vapors and gases.

Reference List
Ariyama, T., Sato, M., Nouchi, T., & Takahashi, K. (2016). Evolution of the blast furnace process toward reductant flexibility and carbon dioxide mitigation in steel works. ISIJ International, ISIJINT-2016.

Feng, H., Chen, L., Liu, X., & Xie, Z. (2017). Constructal design for an iron and steel production process based on the objectives of steel yield and useful energy. International Journal of Heat and Mass Transfer, 111, 1192-1205.

Liu, X., Chen, L., Feng, H., Qin, X., & Sun, F. (2016). Process design assignment. Constructal design of a blast furnace iron-making process based on multi-objective optimization. Energy, 109, 137-151.

Shin, H. O., Yang, J. M., Yoon, Y. S., & Mitchell, D. (2016). Mix design of concrete for prestressed concrete sleepers using blast furnace slag and steel fibers. Cement and Concrete Composites, 74, 39-53.

Yang, L., Jiang, G., Chen, X., Li, G., Li, T., & Chen, X. (2017). Design of integrated steel production scheduling knowledge network system. Cluster Computing, 1-10.

Zhao, J., Wang, D., & Yan, P. (2017). Design and experimental study of a ternary blended cement containing high volume steel slag and blast-furnace slag based on Fuller distribution model. Construction and Building Materials, 140, 248-256.

Zhou, C., Tang, G., Wang, J., Fu, D., Okosun, T., Silaen, A., & Wu, B. (2016). Comprehensive numerical modeling of the blast furnace ironmaking process. JOM, 68(5), 1353-1362.