The Effect of CaO-MgO Mixture on Desulfurization of Molten Ni-Base Superalloy

Abstract

Desulfurization ability and hydration resistance of CaO-MgO mixture were studied by comparing CaO-MgO, CaO and MgO rods. In desulfurization experiment, Ni-base superalloy and NiS powder were melted together, and each rod was held in the melt for a prescribed time under 1600 °C. S content in the alloy was lower in CaO-MgO desulfurization than that in CaO desulfurization in every holding time up to 300 seconds. Almost no desulfurization effect was confirmed in MgO. From EPMA and XRD analyses, it is considered that CaO-Al₂O₃-MgO ternary compound and CaS were generated and coexisted in the CaO-MgO rod during the desulfurization process. In the CaO-MgO desulfurization, the compound contained MgO as its component, and that decreased the liquidus temperature of the compound. Therefore, it is considered that the fraction of liquid increased, and the effective diffusion coefficient of S in the compound was larger than in the CaO desulfurization. In hydration experiment, each rod was maintained in an enclosed vessel with weighing paper and a cup with water for a certain period. The mass increasing ratio of the CaO-MgO was almost equal to that of the CaO. From the above, it is clear that CaO-MgO is a better desulfurization agent than CaO.

Appendix

Derivation of ρ_CaO in the CaO-MgO Rod in the Desulfurization Rate Formula

When the CaO-MgO rod is used, ρ_CaO in the desulfurization rate formula[20] is the density obtained by considering only CaO particles in the rod expect MgO particles. The method of calculating its value used in this study is explained below. The explanation assumes the case in which the nominal composition of the rod is used.

At first, the volume ratio of CaO and MgO was calculated without considering the porosity of the rod. The density of CaO and MgO is 3.34 × 10⁶ g/m³ and 3.58 × 10⁶ g/m³ respectively. Moreover, the nominal composition of the rod shows that the mass ratio of CaO and MgO becomes 7:3. Thus, the volume ratio becomes 5:2. Next, the ratio of the volume of the rod without the pores to the whole rod was calculated. Figure A1 shows the EPMA analysis result of O in the rod before the desulfurization experiment.

Fig. A1

EPMA analysis result of O in the CaO-MgO rod before the desulfurization experiment. The area surrounded by the yellow frame was considered, and the ratio of the particles which exist there to the whole surrounded area was calculated

In this figure, CaO and MgO particles exist in the regions where O is observed. So the area surrounded by the yellow frame was considered, and the ratio of the particles which exist there to the whole surrounded area was obtained using ImageJ. As a result, it was calculated to be 74.2 pct, and it was regarded as the ratio of the volume without the pores to the whole rod. According to the result shown above, by defining V m³ as the volume of the whole rod including the pores, the volume which the particles occupies becomes 0.742V m³. Using the volume ratio of CaO and MgO and the density of CaO, the mass of CaO in the rod is 1.77V × 10⁶ g. Dividing this by V, ρ_CaO in the CaO-MgO rod is calculated to be 1.77 × 10⁶ g/m³.

When using the results of the quantitative analysis about the composition of the CaO-MgO rod by powder X-ray diffractometry, the only difference from the explanation above is that the ratio of CaO and MgO was 61.2 and 38.8 in weight percent, respectively. Thus, it is possible to calculate ρ_CaO based on the method mentioned above.

In addition to ρ_CaO, the porosity of the CaO-MgO rod can also be estimated to be 25.8 pct because the ratio of the volume without the pores to the whole rod is predicted to be 74.2 pct.

Calculation of the Composition of the Compound Generated in the Rod

The following explains the calculation method for the ratio of CaO, Al₂O₃, and MgO in calcium aluminate (CaO rod) and in CaO-Al₂O₃-MgO ternary compound (CaO-MgO rod) during the desulfurization.

From the results of powder X-ray diffractometry, it is considered that Al existed only as a constituent of Al₂O₃ component in the compound during the desulfurization. So the weight percent concentration of Al₂O₃ ${\text{W}}_{\text{Al}_{2}{\text{O}_3}}$ (wt pct) in the compound can be calculated from that of Al W_Al (wt pct) obtained by EPMA analysis. Using 27 g/mol of the atomic weight of Al and 16 g/mol of that of O, ${\text{W}}_{\text{Al}_{2}{\text{O}_3}}$ is expressed as follows.

$$ {\text{W}}_{{{\text{Al}}_{2} {\text{O}}_{3} }} = 102{\text{W}}_{{{\text{Al}}}} /54 $$

(A1)

In the CaO-MgO desulfurization, as the result of powder X-ray diffractometry shows, it can be said that Mg existed only as a constituent of MgO component in the compound in the analysis point. Thus, using the weight percent concentration of Mg W_Mg (wt pct) obtained by EPMA analysis, the weight percent concentration of MgO W_MgO can be calculated as follows.

$$ {\text{W}}_{{{\text{MgO}}}} = 40{\text{W}}_{{{\text{Mg}}}} /24 $$

(A2)

In Eq. [A2], the atomic weight of Mg is 24 g/mol. It should be noted that the value of W_MgO is 0 in the CaO desulfurization.

The calculation method of CaO component can be considered as mentioned below. According to the results of EPMA analysis, there is a possibility that CaS coexisted in the calcium aluminate or CaO-Al₂O₃-MgO ternary compound during the desulfurization. This means that Ca can be contained in both CaO and CaS. Therefore, Ca in CaO component can be estimated by subtracting Ca contained in CaS from all the Ca which exists in the analysis point. By obtaining the weight percent concentration of Ca W_Ca (wt pct) and that of S W_S (wt pct) in EPMA analysis, the weight percent concentration of W_CaO (wt pct) is shown as follows.

$$ {\text{W}}_{{{\text{CaO}}}} = \frac{{56({\text{W}}_{{{\text{Ca}}}} - 40{\text{W}}_{{\text{S}}} /32)}}{{40}} $$

(A3)

In Eq. [A3], the atomic weight of Ca and S is 40 and 32 g/mol, respectively. When the weight percent concentrations of Al₂O₃, MgO, and CaO are calculated as explained above, the existence of CaS is also taken into consideration. In other words, the values obtained are different from the ones in the calcium aluminate or CaO-Al₂O₃-MgO ternary compound generated in the rod. Therefore, defining ${\text{W'}}_{\text{Al}_{2}{\text{O}_3}}$, W’_MgO, and W’_CaO (all in wt pct) as the weight present concentration of Al₂O₃, MgO, and CaO in the generated compound respectively, these values are expressed as follows.

$$ {\text{W'}}_{{{\text{Al}}_{2} {\text{O}}_{3} }} = 100{\text{W}}_{{{\text{Al}}_{2} {\text{O}}_{3} }} /({\text{W}}_{{{\text{Al}}_{2} {\text{O}}_{3} }} + {\text{W}}_{{{\text{MgO}}}} + {\text{W}}_{{{\text{CaO}}}} ) $$

(A4)

$$ {\text{W'}}_{{{\text{MgO}}}} = 100{\text{W}}_{{{\text{MgO}}}} /({\text{W}}_{{{\text{Al}}_{2} {\text{O}}_{3} }} + {\text{W}}_{{{\text{MgO}}}} + {\text{W}}_{{{\text{CaO}}}} ) $$

(A5)

$$ {\text{W'}}_{{{\text{CaO}}}} = 100{\text{W}}_{{{\text{CaO}}}} /({\text{W}}_{{{\text{Al}}_{2} {\text{O}}_{3} }} + {\text{W}}_{{{\text{MgO}}}} + {\text{W}}_{{{\text{CaO}}}} ) $$

(A6)

The values of W’_Al2O3, W’_MgO, and W’_CaO obtained according to these equations are plotted in Figure 14.