EFFECT OF ZnO/PbO AND FeOx/SiO2 RATIO ON THE VISCOSITY OF LEAD

The effect of ZnO/PbO and FeOX/SiO2 on the viscosity of the PbO-ZnO-Fe3O4-SiO2-CaO slag was measured using the rotating spindle method. The slag viscosity decreased with decreasing ZnO/PbO mass ratio because of the depolymerization of the silicate structures. The viscosity decreased with increasing FeOX/SiO2 mass ratio as the experimental temperature was above 1623 K, while the viscosity increased significantly with the increase of FeOX/SiO2 as the experimental temperature below 1623 K because of the phase transition and the change of slag melting point. According to XRD analysis of as-quenched slag, the spinel and the zincite phase increased with increasing FeOX/SiO2. Increasing ZnO/PbO and FeOX/SiO2 could enhance the crystallization capacity of the slag. FTIR analysis revealed that the degree of polymerization of the as-quenched slags decreased with decreasing ZnO/PbO and increasing FeOX/SiO2. The temperature dependencies of the viscosity on ZnO/PbO and FeOX/SiO2 were investigated, and the apparent activation energies of each system were found to be between 169.5 to 227.4 KJ/mol, and 151.1 to 676.4 KJ/mol, respectively.


Introduction
Lead sulfide concentrate is the main raw material for lead smelting. The traditional routes of extracting lead from lead sulfide concentrate include a desulfurization process in a bath smelting process, followed by a reduction process to obtain metal lead. The oxygen-enriched bottom blown smelting in a desulfurization process is widely used in lead and copper smelting processes in China due to low energy consumption and due to it being environmentally friendly [1][2]. The limited resources of high grade lead concentrate have aroused interest in the use of alternative lead bearing material which is seldom used in the bottom blown furnace due to the restrictions on the composition of raw materials in the existing lead smelting processes. The high Zn content raw materials such as lead and zinc mixed concentrate are especially abundant in China. Due to the high zinc content these raw materials will lead to the reduction of PbO content in the oxidation slag, the variation in the mass transfer in desulfurization process, the increase of slag melting point, and the change of slag viscosity. Viscosity is one of the important physiochemical properties of slag, and low viscosity could promote the efficiency of the desulfurization reaction of slag. Considering the effect of slag composition on its physical properties, the slag composition must be optimized to accommodate changes to its viscosity. FeO X /SiO 2 is an effective means to adjust slag physical property in the desulfurization process of lead smelting [3]; it is important to study the effect of FeO X /SiO 2 on the viscosity of slag with high ZnO content. Therefore, in order to make the oxidation furnace operate effectively, it is necessary to understand the dependence of slag viscosity on temperature and composition. Gupta [4] studied the viscosity of PbO-SiO 2 melts and found that the viscosity of slag decreased with the increasing content of PbO because of the depolymerization of the silicate network structure. Battle and Hager [5] found that the addition of SiO 2 increased the viscosity of lead smelting slag, while the additions of ZnO decreased the viscosity at high temperatures, but raised the slag liquidus temperature. Lv et al. [6] investigated the effect of FeO content on the viscosity and structure in the SiO 2 -MgO-FeO-CaO-Al 2 O 3 slag system. The results showed that the viscosity decreased and the mass fraction of the suspended particles increased with the increase of FeO content.
The final desulfurization slag can usually be approximately considered a mixture of oxides including PbO, ZnO, FeO X , SiO 2 , and CaO from the oxidation of lead raw material and the flux of oxygen-J. Min. Metall. Sect. B-Metall., 56 (1) (2020) 27 -33 enriched bottom blown furnace. However, there is a lack of the available data with respect to the viscosity of PbO-ZnO-Fe 3 O 4 -SiO 2 -CaO system slag. Therefore, the effect of ZnO/PbO and FeO X /SiO 2 on the viscosity in PbO-ZnO-Fe 3 O 4 -SiO 2 -CaO system was initially investigated in this article, and the effect of ZnO/PbO and FeO X /SiO 2 on the activation energy (E η ) for the viscous flow was estimated as well. The phase relations of the slag were studied by XRD. Furthermore, Fourier transform infra-red (FTIR) techniques were used to obtain the structural information of the molten slag.

Sample preparation
The slag used in this experiment was synthesized with analytical pure grade PbO, ZnO, Fe 3 O 4 , SiO 2 , and CaO provided by Sinopharm Chemical Reagent co., Ltd. The slag composition (FeO X /SiO 2 =1.08, CaO/SiO 2 =0.5) was based on the chemical composition of oxidation slag of typical oxygenenriched bottom blown furnaces in the lead smelting industry in China.
The reagent was weighted according to the required composition given in Table 1 and mixed to become homogeneous. The loss caused by PbO volatilization [7] at high temperature was compensated by the excess addition of PbO (~1.09 times the theoretical quantity).
The composition of the slag after the experiment was presented in Table 2; it can be seen that there were some changes in the composition of the slag after the viscosity measurement. Besides, the test results showed that Al 2 O 3 concentration (less than 2.5 wt. %) was dissolved in the slag from the corrosion of the crucible in viscosity measurement. In addition, the Al 2 O 3 content in the slag before and after the viscosity measurement was measured. The results illustrated that the Al 2 O 3 content varied in the range from 2.32 wt. % to 2.45 wt. %, and the change of Al 2 O 3 content before and after the viscosity measurement was less than 0.15 wt. %, which indicated that the enrichment of Al 2 O 3 in the slag from the corrosion of the crucible was mainly in the process of heating and equilibrium (1623 K). Therefore, the influence of the change of Al 2 O 3 content in the slag on viscosity measurement could be neglected. The same method was used to measure the viscosity of copper smelting slag [8][9], and the results demonstrated that the solubility of Al 2 O 3 was negligible in the copper smelting or "Cu 2 O"-SiO 2 -Al 2 O 3 melt slag under experimental conditions.

Experimental apparatus and procedures
In this research, the viscosity of molten slag was determined by the rotating cylinder method. Fig. 1 shows the schematic diagram of the viscosity experimental device. An alumina crucible (inner It should be pointed out that the bob which was used to measure the viscosity had almost no change in the diameter before and after the experiment, which indicated that the influence of the diameter variation on the viscosity measurement could be neglected. About 250 g of mixture was heated to 1623 K and equilibrated for 1h to obtain the homogeneous slag. The bob was inserted into the molten slag and maintained at 10 mm above the bottom of the crucible, and then the viscometer (model: RTW-10) started measuring with the spindle at a fixed rate of 200 rpm. The value of slag viscosity was calculated and recorded by the computer. The Ar was introduced into the furnace at a flow rate of 1.2 L/min to prevent the oxidation of slag. The errors of viscosity measurement were less than 0.002 Pa•S. When the temperature reached at each target temperature, the slag equilibrated at each experimental temperature for 30 minutes.
Kim and Sohn [10] used the breaking temperature to describe the property of slag. The breaking temperature is the critical temperature at which the viscosity varied greatly during the cooling cycle. The molten slag could be assumed to be full liquid above the breaking temperature [11].
After the viscosity measurement, the slag was reheated to the target temperature. Subsequently, parts of the slag were removed from the slag by the iron rod and quenched in water for the structure analysis. The slag phase and the structure were analyzed by X-ray diffractometer (Bruker, D8 ADVANCE, Germany) and Fourier Transform Infrared Spectrometer (Nicolet, iS50, USA), respectively. According to work done by Gupta [4] and Jin [12] et al., both PbO and ZnO suggested to be a weak basic oxide in a silicate melts, which could have decreased the slag viscosity by depolymerizing the slag network structure. It seems that PbO is better than ZnO in According to the study of Jin et al. [12], the increasing ZnO content in slag would lead to the increase of crystallization capacity of the slag. In this study, the breaking temperature increased rapidly with the increase of PbO/ZnO, which further confirmed this point. Fig. 3 shows the XRD analysis of the as-quenched slags with various ZnO/PbO in the PbO-ZnO-Fe 3 O 4 -SiO 2 -CaO slag system. It can be seen that the main crystalline phase for the ZnO/PbO = 0.33 (ZnO=15 wt. %, PbO=45 wt. %) was identified as the spinel phase (Zn x Fe 3-x O 4+y ). The peaks intensity of the spinel phase was gradually increased as the increase of ZnO/PbO. When the ZnO/PbO increased to 0.714 (ZnO=25 wt. %, PbO=35 wt. %), the diffraction peaks for the zincite phase (ZnO) emerged. As the ZnO/PbO continued to increase, the diffraction peaks for ZnO and Fe 3 O 4 became more pronounced. It is reported that the increase of the spinel and the zincite phase in the slag is the main reason for the increase of viscosity in the lead bath smelting process [13]. Thus, a subsequent decrease of ZnO/PbO ratio showed an increase effect in lowering the slag viscosity.

Effect of ZnO/PbO on slag viscosity
The temperature dependence of viscosity can be represented by the Arrhenius equation [8]. (1) where η is the viscosity, Pa•S; A is a constant, Pa•S; R is the gas constant, 8.314 J/mol -1 •K -1 ; T is the absolute temperature, K; E η is the apparent activation energy stand for the sensibility of viscosity to temperature, and the change of E η can illustrate the variation in the frictional resistance of viscous flow and the change of the slag structure [14], J/mol; Fig. 4 shows a linear fit between the natural logarithm of the viscosity (Ln η ) and the reciprocal temperature (1/T). Clearly, the viscosity decreased with increasing temperature. The value of E η could be obtained from the linear fitting results of Ln η with 1/T. The corresponding E η with variation slag composition was calculated to be between 169.5 and 227.4 KJ/mol, and the results were listed in Table 3. It can be seen that the increase of ZnO/PbO led to the increase of apparent activation energy of the viscosity. Moreover, the decrease of E η illustrated the formation of small structural units in the slag and the decrease of resistance for viscous flow [12], which was similar to the change of measured viscosity.   [15]; it was confirmed by the FTIR results as well. The same result was reported by other investigators [6]. However, when experimental temperature was below 1598 K, the viscosity increased significantly with the increase of FeO X /SiO 2 . Note that the increase of Fe 3 O 4 in lead smelting slag can increase the melting point of the slag, thus causing various viscosity [13]. With the increase of FeO X /SiO 2 , the gradient of the viscosity curve got sharper below the breaking temperature, which further confirmed that the breaking temperature of the slag increased with the increase of Fe/SiO 2 .

Effect of FeOx/SiO 2 on slag viscosity
In order to know the relationship between the viscosity and the slag composition with the increase of FeO X /SiO 2 , the XRD analysis of the as-quenched slags with various FeO X /SiO 2 in the PbO-ZnO-Fe 3 O 4 -SiO 2 -CaO slag system was carried out. As shown in Fig. 6, the main crystalline phase of the slag was identified as zincite when FeO X /SiO 2 was at 0.65. With FeO X /SiO 2 increasing to 0.86, the diffraction peaks for the spinel phase became observable. Moreover, the spinel phase increased as FeO X /SiO 2 continued to increase. This illustrated that the addition of silica could inhibit the formation of the spinel phase.
Based on the above study results, the increase of FeO X /SiO 2 would not only increase the slag melting point, but also led to the phase transformation of the slag. This could be the reason that led to the change of the slag with various FeO X /SiO 2 ratio as experimental temperature was below 1598 K.
The viscosities of various FeO X /SiO 2 slag as a function of reciprocal of the temperature were shown in Fig. 7. E η in this system was estimated to be between 151.1 and 676.4 KJ/mol, and the corresponding E η with various FeO X /SiO 2 was listed in Table 4.

FTIR Analysis
It is generally considered that the viscosity is related to the degree of polymerization of silicate slag [16][17]. The FTIR transmittance of the as-quenched  slag as a function of wavenumbers at different ZnO/PbO and FeO X /SiO 2 are exhibited in Figure 10 and 11. The infrared spectra of the silicate slag is mostly concentrated in the wavenumber range of 1200 to 400 cm -1 [18][19][20]. This wavenumber region includes the stretching vibration bands of the [SiO 4 ]-tetrahedra (wavenumber: 1100-850) and [AlO 4 ]-tetrahedra (wavenumber: 750-590), and the Si-O bending vibration bands (wavenumber:~500), respectively [6].
In the study of silicate melt structure, the average number of non-bridge oxygen (NBO/Si) is often used to illustrate the polymerization of silicate slag structures [21], where the lower NBO/Si indicates the higher degree of polymerization of the molten slag [11]. The wavenumber region between 1200 and 800 cm -1 is widely known to be the [SiO 4 ]-tetrahedra peaks [11] with various NBO/Si (NBO/Si=1, 2, 3, 4). According to the study of B. O. Mysen et al [22], the wavenumber region of 1100-1050 cm -1 , 980-950 cm -1 , 920-900 cm -1 , 880-850 cm -1 represent the band groups of [SiO 4 ]-tetrahedral with NBO/Si = 1 (sheets), 2 (chains), 3 (dimers) and 4 (monomers), respectively. Clearly, as shown in Fig. 8, the [SiO 4 ]-tetrahedra peaks at about 750 to 1150 cm -1 got weaker when the ZnO/PbO was decreased, which demonstrated that the structure of the slag could be depolymerized by the decrease of ZnO/PbO. The results observed in Fig. 9 show that the characteristic stretching vibration band for the large silicate network structures became less pronounced and the depth of the convoluted band became shallower, which illustrated that the increase of FeO X /SiO 2 was likely to depolymerize the silicate network structure into simpler polymer type units. Therefore, the depolymerization of the silicate network structure could reduce the polymerization degree of complex viscous units, thus decreasing the slag viscosity.