3. Results and discussion

Figure 2 shows the relationship of the magnesium content measured by AES with respect to the treatment time from 1 to 60 minutes, at 750, 800, and 850°C and


Table 1.

Chemical composition of aluminum scrap and ZnO used by aluminothermic reduction process.

Application of the Aluminothermic Reduction Process for Magnesium Removal in Aluminum… DOI: http://dx.doi.org/10.5772/intechopen.102407

Figure 1. (a) Top view photography of furnace, (b) scheme of the interior furnace, and (c) the impeller used during the experimentation in aluminothermic reduction process.

stirring velocity of the impeller was 200, 250, and 300 rpm. The results show a significant decrease in magnesium content for all conditions of work. Figure 3a–c shows the increase of zinc content in the experiments. Therefore, it can be deduced that the reaction of Eq. (1) governed the process of magnesium removal and zinc added in melted aluminum. The temperature is the variable with the strongest influence on the process, no matter the agitation speed, observing better results at 800 and 850°C, while at 750°C the contents of magnesium and Zn were out the specification for to be used in the automotive industry.

$$\mathbf{1}/2\ \mathrm{Al}\_{\mathrm{(l)}} + \mathbf{1}/4\ \mathrm{Mg}\_{\mathrm{(l)}} + \mathrm{ZnO}\_{\mathrm{(s)}} \bullet \mathbf{1}/4\ \mathrm{MgAl}\_2\mathrm{O}\_{4(s)} + \mathrm{Zn(l)}\ \Delta\mathrm{G}^0 = -223.99\ \mathrm{kJ} \tag{1}$$

Graphics of Figures 2 and 3 indicate that the processes have an exponential reaction after 20 minutes because this is the time necessary for wettability of the ZnO solid particle with molten aluminum, which allows the reaction to be carried out, where magnesium has an important effect [13, 14]. Figure 3d shows the ratio in wt% of magnesium and zinc content at 800°C and 250 rpm, this experiment was more favorable in the present study, because both contents are in the specification for automotive application, giving values <0.06 wt% of Mg and 5.52 wt% of zinc. Other zinc contents obtained were 1.66, 2.3, 2.3, 3.2, and 4 wt%, values used in the aluminum industry in the 700 series alloys. Thus, the reduction process of ZnO powder can remove magnesium and incorporate zinc in molten aluminum.

According to Hashiguchi [15], the change in the magnesium content can also be to the reaction between dissolved magnesium and ambient oxygen gas too, the process is carried out at ambient conditions can be explained by the reaction of

#### Figure 2.

Relationship between magnesium content and time with respect to temperature and stirring velocity, (a) 200, (b) 250, and (c) 300 rpm.

#### Figure 3.

(a–c) Relationship between zinc content and time with respect to temperature and stirring velocity, and (d) relationship between magnesium and zinc content in molten aluminum at 800°C and 250 rpm.

Application of the Aluminothermic Reduction Process for Magnesium Removal in Aluminum… DOI: http://dx.doi.org/10.5772/intechopen.102407

Figure 4. XRD pattern of the slags produced at 250 rpm and (a) 750°C, (b) 800°C, and (c) 850°C during aluminothermic reduction of ZnO.

Eq. (2). This can be demonstrated by the XRD analysis in Figure 4, where the slags obtained in the experiments at 250 rpm and at 750, 800, 850°C are presented, the compounds of these results are MgAl2O4, MgO, and Al2O3. Furthermore, it has been reported that the reactions involved between solid ZnO and molten Al-Mg can be carried out by the Eqs. (1)–(9), values of free energy calculated at 850°C.

$$\text{Mg}\_{\text{(l)}} + \text{O}\_{2(g)} \rightarrow \text{MgO}\_{(s)}\ \Delta \text{G}^0 = -\text{970.09 kJ} \tag{2}$$

$$\text{4/3 Al}\_{2(l)} + \text{O}\_{2(g)} \xrightarrow{} \text{2/3Al}\_2\text{O}\_{3(s)}\text{ }\Delta\text{G}^0 = -882.46\text{ kJ} \tag{3}$$

$$\text{Al}\_2\text{O}\_{3(s)} + \text{Mg}\_{(l)} + 1/2\text{O}\_{2(g)} = \text{MgAl}\_2\text{O}\_{4(s)}\text{ }\Delta\text{G}^0 = -10\text{36.56 kJ} \tag{4}$$

$$\text{(2/3 Al}\_{\text{(l)}} + \text{1/3 MgO}\_{\text{(s)}} + \text{ZnO}\_{\text{(s)}} \bullet \text{1/3 MgAl}\_2\text{O}\_{4(s)} + \text{Zn(l)}\text{ }\Delta\text{G}^0 = -220.23 \text{ kJ} \tag{5}$$

$$\text{4/3 Al}\_{\text{(l)}} + 2\text{ZnO}\_{\text{(s)}} = \text{2/3 Al}\_2\text{O}\_{\text{3(s)}} + 2\text{Zn}\_{\text{(l)}}\text{ }\Delta\text{G}^0 = -205.82\text{ kJ} \tag{6}$$

#### Figure 5.

SEM images of Al-Zn alloys from aluminothermic reduction process at 250 rpm and (a) 750°C, (b) 800°C, and (c) 850°C. (d) Al6(Fe,Mn) and Al12(Fe,Mn)3Si phases on Al-Zn matrix.

$$\text{Mg}\_{\text{(l)}} + \text{ZnO}\_{\text{(s)}} \bullet \text{MgO}\_{\text{(s)}} + \text{Zn}\_{\text{(l)}} \Delta \text{G}^0 = -244.91 \text{ kJ} \tag{7}$$

$$\text{Al}\_2\text{O}\_{3(s)} + \text{3Mg}\_{(l)} \rightarrow \text{3MgO}\_{(s)} + 2\text{Al}\_{(l)}\text{ }\Delta\text{G}^0 = -\text{117.69 kJ} \tag{8}$$

$$\text{Al}\_2\text{O}\_{3(s)} + \text{MgO}\_{(s)} \rightarrow \text{MgAl}\_2\text{O}\_{4(s)}\text{ }\Delta\text{G}^0 = -\text{92.02 kJ} \tag{9}$$

According to the free energy values of each reaction calculated at 850°C, the decrease of magnesium and the increase of Zn in the aluminothermic reduction process can be carried out mainly by reactions of the Eqs. (1), (2), and (4)–(7) while Eqs. (3), (8), and (9) are reactions for slag formation.

The beverage cans used in this work were without any type of refining treatment, therefore, the alloy presented higher contents of Fe, Si, Mn. Figure 5 shows microstructure after the aluminothermic reduction process of secondary aluminum with a matrix rich in zinc and second phases of Al6(Fe, Mn) and Al12(Fe, Mn)3Si, the first phase with massive blocky morphology due to the content of the Mn in the alloy, according to Liu [16], this element is the one that determines the morphology of this type of intermetallic, while the phase Al12(Fe, Mn)3Si was presented less quantity, this phase is formed by the cooling rate and diffusion of Si to the Al6(Fe, Mn). Previous studies [17, 18] indicate that this transformation can also be carried out in processes where there is a loss of magnesium and Si was effective nucleating substrates for the transformation of Al6(Fe, Mn) to Al12(Fe, Mn)3Si.

### 4. Conclusion

Magnesium was removed from the aluminum scrap using the aluminothermic reduction process of ZnO powder. The temperatures favored the decrease of

Application of the Aluminothermic Reduction Process for Magnesium Removal in Aluminum… DOI: http://dx.doi.org/10.5772/intechopen.102407

magnesium in the metal bath and generated reaction products as MgAl2O4, MgO, and Al2O3 in solid-state, so that this process can help to reduce the emission of polluting gas. Also, the ZnO powder was reduced, and the zinc incorporated into the melted aluminum microstructure of the alloys has an Al-Zn matrix and second phases of Al6(Fe, Mn) and Al12(Fe, Mn)3Si in the cast state. In this case, this process can be favorable for the manufacture of alloys that can be applied in the automotive sector.
