Determining the Enthalpy of an Fe-Ni Alloy at Various Temperatures Using the ‘STA’ PT 1600 Equipment

Production of Fe-Ni alloy is a difficult process, beginning with the treatment of Fe-Ni ore and involving use of a rotary kiln, an electrical furnace, and finally a converter for the refinement of the metal [1, 5]. The costs of fuel for the rotary kiln and the electricity for melting the calcine are high.

The TGA/DSC equipment measures the heat as well as the change in weight of a material as a function of temperature or time in a controlled environment. The simultaneous measurements of these characteristics not only improve productivity, but also simplify the interpretation of results. The data obtained make it possible to differentiate between endothermic and exothermic processes that do not experience weight loss (melting and crystal-lisation) and those that do experience weight loss (reduction). The simultaneous thermic analyser LINSEIS STA PT 1600 can be used for determining coinciding differences in mass (Tg) and caloric reactions (DSC) of a sample in a 25°C to 1650° temperature range. This simultaneous thermic analyser delivers high accuracy and high resolution. The equipment used for this research is located at the Department of Materials and Metallurgy of the University of Mitrovica ‘Isa Boletini’ (UMIB) and has the following three sensors: TG, TG-DTA, and TG-DSC.

All of the LINSEIS thermo-analytic instruments are controlled from a personal computer (PC). The individual software modules function exclusively using the Microsoft operating system Windows. The software is made up of three modules: temperature control, data acquisition, and data evaluation. The 32-bit program encompasses all fundamental properties for preparing, executing, and evaluating an STA measurement. Thanks to our application experts and specialists, LINSEIS was able to develop a user-friendly, all-inclusive software.

The standard enthalpy of forming a compound is defined as the energy associated with the reaction to form the compound from its constituent elements [3, 4]. The standard enthalpy of formation is a basic thermodynamic property that determines its phase stability, which can be coupled with other thermodynamic data to calculate phase diagrams.

We carried out the analysis of the Fe-Ni alloy at the Ferronikel plant's laboratory in Drenas and obtained the following composition expressed as the percentage of each element [2] (Table 1):

Figure 1

Figure 2

Figure 3

The Fe-Ni alloy sample with the composition found in (Table 1) was prepared at the laboratory of UMIB Department of Materials and Metallurgy. For the preparation of the Fe-Ni alloy sample, we relied on the available parameters of the LINSEIS ‘PT STA 1600’.

The sample has the following parameters:

Length: 4 mm

The sample was weighed on an analytical scale (Figure 4) at the laboratory of the Faculty of Food Technology (UMIB).

Figure 4

The Fe-Ni alloy sample, after polishing, was placed on the ‘alumina’ plate, and the conditions of the analysis were as follows:

Maximum temperature = 800 °C, for a dura­tion of 45 minutes

The initial temperature is the initial extrapolated temperature of the reaction; the final temperature is the final extrapolated temperature of the reaction. In an exothermic reaction, energy is released because the total energy of the products is lower than the total energy of the reactants. As a result, the change in enthalpy, ΔH, for an exothermic reaction will always be negative. The heat variations in chemical reactions are often measured at the laboratory under conditions in which the reaction system is open to the ambient temperatures. In that case, the system is exposed to constant pressure. Chemists routinely evaluate changes in the enthalpy of chemical systems as the reactants are converted into products. The heat absorbed or released by a reaction at constant pressure is the same as the change in enthalpy and is identified by the symbol ΔH.

Figure 5

Figure 6

Using the LINSEIS ‘PT STA 1600’ to analyse the Fe-Ni alloy, we have determined the enthalpy, which decreases as the temperature rises (ΔH<0), leading to the conclusion that these reactions are exothermic [3, 4]. Energy must be inserted into the system to break the chemical bonds, as they frequently do not break away spontaneously. The formation of bonds includes the release of energy; therefore, if more energy is produced in the formation of the bond than is necessary to break the bond, the enthalpy is negative—as we have found in the Fe-Ni alloy [3]. In this example, the change in enthalpy is negative because heat is released from the system, causing the overall enthalpy of the system to decrease [3, 4].

The heat of the reaction is the change of enthalpy for a chemical reaction [3]. The determination of the enthalpy of the Fe-Ni alloy supports our conclusion that the Fe-Ni alloy is an appropriate product for the acquisition of special steels and other stable alloys [3, 4].