Study of the Viscosity of a La2O3-SiO2-FeO Slag System

Abstract The viscosity and break temperature of La2O3–SiO2-FeO slag was investigated to develop low-Al2O3 or Al2O3-free slag for the effective recovery of rare-earth metals. When La2O3 content is fixed (45, 50 and 55 mass%), the viscosity and break temperature of La2O3–SiO2-FeO slag decrease with an increase in FeO content and a decrease in SiO2 content. A higher La2O3 content in the La2O3-SiO2-FeO ternary slag yields a lower slag viscosity but a higher break temperature. Individual minor components of Al2O3, MnO and B2O3 does not affect, or decreases slightly the viscosity of La2O3–SiO2-FeO slag, whereas the slag break temperature is reduced so that the reduction ability order is ranked as B2O3 > Al2O3 > MnO. A small amount of two components Al2O3 + MnO and Al2O3 + B2O3 has little effect on the viscosity of the slag but it has an additive effect on the slag break-temperature reduction.


Introduction
Rare-earth hydrogen-storage alloys and NdFeB magnets are used in various applications. Large amounts of waste from these material production processes and end-of-life rare-earth materials become significant secondary sources of rare-earth and other valuable metal elements. The development of a cost-effective recycling method is required urgently. Hydrometallurgical methods, pyrometallurgical methods and a combination of the two have been developed to recycle rare-earths. In general, industrial hydrometallurgical extraction proceeds by a hydrochloric-acid selective-dissolution method, such as is used in Nd-FeB waste treatment. This method dissolves rare-earth oxides by hydrochloric acid preferentially from the oxidizing roasting NdFeB waste, and then separates different rare earths using solvent extraction [1][2][3]. Despite its high rareearth recovery and the high-purity rare-earth products that form, hydrometallurgy is unable to collect other valuable elements very well and results in excessive acid consumption. Single pyrometallurgical method can recover rare-earth metals together with valuable metal elements and is more environmentally friendly [4][5][6]. The combination of pyrometallurgical and hydrometallurgical advantages could be used to recycle rare-earth and valuable elements simultaneously and reduce environmental pollution. A selected suitable slag system for the former pyrometallurgical step is critical to concentrate rare-earth oxides, for the smooth separation of slag-metal and for the efficiency of subsequent hydrometallurgical process.
Many types of flux have been used to recycle rareearth oxides, such as CaO-CaF 2 [7], CaO-SiO 2 -CaF 2 [8], CaO-SiO 2 [9], Al 2 O 3 -CaO-MgO-SiO 2 -(P 2 O 5 ) [10] and SiO 2 -Al 2 O 3 [11]. However, melting-separation characteristics for slags that are formed by most of the fluxes examined were not good. The purity and added value of the alloy have also been poorly considered. The authors [11] used gas-selective reduction-oxidation to produce highadded-value alloy from the Ni-metal-hydride battery electrode waste and subsequently accomplished excellent slag-metal separation by SiO 2 -Al 2 O 3 flux addition. This method yielded a~50 mass% RexOy-containing slag, which contributed to a saving in hydrochloric-acid consumption in the acid-leaching process.
In addition to the high rare-earth-oxides content and appropriate slag-metal separation characteristics for the formed slag, the selection of flux should be considered to ensure that other slag components do not enter the leaching solution or are separated easily and purified even if they enter the leaching solution. Besides, the latter extraction separation of different rare-earth elements cannot be affected by additive slag components. Some studies [12,13] have indicated that the aluminum ion is an important factor that leads to the emulsification of extractant in the ex-traction process of different types of rare-earth elements. Our previous SiO 2 -Al 2 O 3 flux is effective for single rareearth elements. So, there is no emulsification problem of the leaching agent results. However, different types of rareearth elements usually coexist in rare-earth waste materials, and the application of SiO 2 -Al 2 O 3 flux gets some limitations. It is urgent to develop aluminum-free or low aluminum flux to reduce or prevent emulsification in the latter hydrometallurgical step in addition to separate the slagmetal effectively and recycle other valuable elements in the former pyrometallurgical step.
In this paper, La 2 O 3 was selected as a representative of the rare earth oxides. Acidic SiO 2 does not consume hydrochloric acid in the acid-leaching process. Although FeO can dissolve in the leaching solution, the iron ion is removed easily as a Fe(OH) 3 precipitate by adjusting the pH of the leaching solution to~3-4. SiO 2 and FeO were considered as the main components of slag. The viscosity and break temperature of the La 2 O 3 -SiO 2 -FeO slag system, which are key parameters for effective separation of rare earth oxide-rich slag and valuable metal, were evaluated. The final product was confirmed to be a single FeO phase by X-ray diffraction as shown in Figure 1.

Viscosity and Break-Temperature Test
The slag viscosity was measured by the internal rotatingcylinder method. The viscosity test principle and instrument were the same as was used in our previous work [14]. According to the designed slag composition,~160 g of prepared reagents were mixed, briquetted and placed into a molybdenum crucible with a 40-mm diameter and a 75-mm height, and the molybdenum crucible was placed inside the constant-temperature zone of the hightemperature resistance furnace. The temperature of the heating furnace was controlled by a Pt-PtRd thermocouple and the temperature-control accuracy was approximately ± 1 K. The sample was heated to 1823 K and held for 1 h in argon at 0.2 NL/min to obtain a homogeneous melt. The molybdenum spindle was immersed into a liquid slag bath and rotated. The viscosity at a constant temperature of 1823 K was obtained. The viscosity was measured continuously while the slag temperature reduction was controlled at 3 K/min. When the viscosity increased to 6.0 Pa·s, the molybdenum spindle stopped rotating and the furnace was heated again to 1823 K so that the molybdenum spindle was removed from the melted slag. The slag break temperature was identified as the temperature at which there is a significant change in viscosity during the cooling cycle.

Viscosity and Break Temperature of La 2 O 3 -SiO 2 -FeO Ternary Slag
The composition, constant-temperature viscosity at 1823 K and break temperature of the La 2 O 3 -SiO 2 -FeO ternary basic slag are given in Table 1. The viscosity curves are shown in Figure 2. For a fixed La 2 O 3 mass content, the viscosity at 1823 K and the break temperature is lowered with a decrease in SiO 2 content from 40 to 30 mass% and an increase in FeO content from 15 to 25 mass%. When the La 2 O 3 content is 50 mass%, the viscosity-temperature curve of the 10 mass% FeO is supplied to observe better the effect of FeO content change on the break temperature of the slag. For a certain FeO content, the viscosity at 1823 K decreases, but the break temperature of the La 2 O 3 -SiO 2 -FeO ternary slag increases as the La 2 O 3 content increases from 45, to 50 to 55 mass% as shown in Table 1.
The decrease in slag viscosity is related closely to a depolymerization of the silicate structure. The free oxygen ions could break the bridged oxygen in the Si-O network structure and simplify the complex network silicate structure to decrease the slag viscosity. The availability of free oxygen ions possibly increases with increasing La 2 O 3 content and thus the viscosity of the La 2 O 3 -SiO 2 -FeO slag decreases. Toop and Samis have reported that FeO supplies free oxygen ions in CaO-SiO 2 -FeO melts [15]. Moreover, FeO contributes to the formation of a low-meltingpoint substance, such as fayalite (Fe 2 SiO 4 ). As a result, increasing FeO content decrease the slag viscosity and melting temperature. The lower viscosity and melting temperature of slag as an FeO additive has been already practiced in steelmaking process. The variation behavior of the vis-

Effect of Single Minor Component on the Based Slag System Viscosity and Break Temperature
In a LaNi 5 -type hydrogen-storage alloy, a small amount of Al and Mn are doped to replace Ni to improve the electrochemical properties [16,17]. In a NdFeB permanent magnet, Co and Al are added to replace Fe to optimize the magnetic performance and corrosion behavior [18,19]. After gas-selective reduction-oxidation, the active elements Mn, Al, B in the hydrogen-storage alloy and in the NdFeB waste are easily oxidized to MnO, Al 2 O 3 and B 2 O 3 [11]. They can remain in the rare-earth-oxide slag and may affect the viscosity and break temperature of the La 2 O 3 -SiO 2 -FeO based slag significantly. The influence of single minor components Al 2 O 3 , MnO and B 2 O 3 on the viscosity and break temperature of the La 2 O 3 -SiO 2 -FeO slag was studied to establish which component has a greater impact on the above values. So that proper viscosities and break temperatures can be obtained by adjusting the slag composition. The high content of rare-earth oxide in the La 2 O 3 -SiO 2 -FeO slag would reduce the acid consumption for the same amount of leaching rare-earth oxides although a high rare-earth-oxide content results in a high break temperature of the slag (shown in Table 1), which is undesirable for slag-metal separation. Therefore, the mass fraction ratio of the main components

Effect of Al 2 O 3 on La 2 O 3 -SiO 2 -FeO slag viscosity and break temperature
The composition, constant-temperature viscosity at 1823 K and break temperature of the La 2 O 3 -SiO 2 -FeO-Al 2 O 3 slag are listed in Table 2. The viscosity curves are shown in Figure 3. The viscosity change at 1823 K is insignificant but the break temperature is lowered substantially. The break temperature of the La 2 O 3 -SiO 2 -FeO slag that is doped with 0, 2, 4 and 6 mass% Al 2 O 3 is reduced from 1810 to 1800, 1748 and 1719 K, respectively. This result implies that the dissolution of a small amount of Al 2 O 3 into the slag, even from low-cost corundum-crucible corrosion, does not cause melting problems in the slag-metal separa-   [20], in which the slag-composition range and ternary basicity ((W CaO + W MgO )/W SiO2 = 1.5) was approximately same as those in this study (W La2O3 /W SiO2 = 1.83).  Figure 4 shows the curves of viscosity versus temperature for different MnO contents. An increase in MnO addition results in a small decrease in slag viscosity at 1823 K and the break temperature decreases. When the MnO content increases from 0 to 6 and 8 mass%, the slag break temperature is reduced from 1810 to 1772 and 1754 K, respectively. It has been reported that a 0-3 mass% MnO addition lowers the initial and complete melting temperature of the CaO-Al 2 O 3 system mold flux because MnO formed low-melting compounds CaMnSiO 6 and Mn 2 SiO 4 with CaO and SiO 2 [21].

Effect of B 2 O 3 on La 2 O 3 -SiO 2 -FeO slag viscosity and break temperature
The composition, constant-temperature viscosity at 1823 K and break temperature of La 2 O 3 -SiO 2 -FeO-B 2 O 3 slag and the viscosity curves are given in Table 4 and Figure 5. The slag viscosity at 1823 K does not change visibly, but the break temperature of the slag is reduced significantly with an increase in B 2 O 3 content, which is similar to the viscosity variation with the addition of Al 2 O 3 in the slag as shown in Table 2. When B 2 O 3 content changes from 0 to 2, 3 and 4mass%, the break temperature of the slag decreases from 1810 to 1731, 1727 and 1697 K, respectively. This finding agrees with many reports [22,23] although the slag compositions differ. B 2 O 3 is a typical acidic oxide, which behaves as a network former in the current La 2 O 3 -SiO 2 -FeO alkaline slag. Amphoteric oxide Al 2 O 3 also tends to behave as a network former in the alkaline slag. Consequently, the same viscosity-change result in the La 2 O 3 -SiO 2 -FeO slag with    Here, the Al 2 O 3 , MnO and B 2 O 3 compositions were selected to be 6, 6 and 3 mass%, respectively. As shown in Table 5 and Figure 6, the coexistence of 6 mass% Al 2 O 3 + 6 mass% MnO or 6 mass% Al 2 O 3 + 3 mass% B 2 O 3 has no effect on the slag viscosity, whereas they continue to reduce the break temperature to 1662 or 1585 K. For the La 2 O 3 -SiO 2 -FeO-6 mass% Al 2 O 3 -6 mass% For slag-metal separation, the critical point was the selection of a suitable slag system that fulfils demands such as a low viscosity, and a moderate or low break temperature. In general, the break temperature of the slag is lower by 150 K or more than the melting temperature of the metal. The melting temperature of the recycled metal is expected to reach up to 1773 K, so the break temperature of the slag is best below 1623 K. In this experiment, the low viscosity meets the slag-metal separation requirement for all La 2 O 3 -SiO 2 -FeO slag systems. However, the break temperatures of the La 2 O 3 -SiO 2 -FeO based slag and the slag doped with single Al 2 O 3 , MnO and B 2 O 3 are high, although these oxides could reduce the break temperature of the La 2 O 3 -SiO 2 -FeO based slag to different extents. Two residual components 6 mass% Al 2 O 3 +6 mass% MnO decrease the break temperature of the slag to close to 1623 K, and the break temperature decreases to below 1623 K when the Al 2 O 3 and B 2 O 3 contents in the slag are 6 and 3mass% as shown in Table 5. Rare-earth waste often yields two or more minor components in the slag. As a result, the efficient separation of valuable metals and rich rare-earth oxide-containing slag must be achieved using the developed La 2 O 3 -SiO 2 -FeO slag system and simultaneously the slag composition does not affect or affects only slightly the subsequent extraction of rare-earth elements.

Conclusions
In this study, the viscosity and break temperature of La 2 O 3 -SiO 2 -FeO ternary slag and the variation as affected by one or two minor components Al 2 O 3 , MnO and B 2 O 3 were investigated. The following conclusions were made: (1) For 45, 50 or 55 mass% La 2 O 3 , the viscosity at 1823 K and the break temperature of La 2 O 3 -SiO 2 -FeO ternary slag decreased with an increase in FeO content from 15 to 25 mass% and a decrease in SiO 2 content from 40 to 30 mass%. When the FeO content was fixed at 15, 20 or 25 mass%, the slag viscosity decreased, but the break temperature of the La 2 O 3 -SiO 2 -FeO ternary slag increased with an increase in La 2 O 3 content from 45 to 55mass%.