NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 114 MARCH 2017 Technical Report UDC 621 . 78 . 066 . 6 - 936 . 7 Measurement Technique of Solubility of Metal Oxides in Melts for Controlling the Solid-liquid Interface Reactions at High Temperatures Ryutaro TOKI* Takashi DOI Nobuo OTSUKA Abstract To control the solubility behavior of metal oxides in melts which is one of the solid-liquid interface reactions at high temperatures, this study is aimed at establishing the measure- ment technique of the solubility of metal oxides to binary Na2B4O7-B2O3 melt focused on the basicity. The basicity sensor using the solid electrolytes was employed. The correlation be- tween the basicity and the solubility of the metal oxides in the melt was revealed by the measurement technique. From the solubility dependence on the melt basicity, both the quantitative assessment of the solubility of the metal oxides and the estimate of the dissolu- tion reactions of the metal oxides were allowed. 1. Introduction the O2− activity represents basicity.3) Therefore, in order to explain Processes involved in steel products frequently cause the steel the solid-liquid interface reactions at high temperatures, the basicity materials, refractories, and scales to contact melts (slag, antioxi- measurement technology such as a pH meter is thought to be useful. dants, etc.) under high temperatures. Various reactions occur at the However, it is very difficult to actually measure the O2− activity. solid-liquid interface formed when there is such contact at high tem- Consequently, basicity that can be used as the indexes to be meas- peratures. For example, reactions in the steelmaking process include ured or calculated has been suggested. Such indexes include the ba- 1) the metal oxide dissolution reaction , which occurs at the interface sicity that is defined by the mass ratio of CaO/SiO2 well-known in between slag and an oxide refractory, and electrochemical reac- the field of slag 2), the optical basicity proposed by Duffy, et al. 4), and tions 2) via an antioxidant between molten steel and an immersion the basicity that is defined by Na O activity (a ) 2). In this study, 2 Na2O nozzle refractory. It is important to control the solid-liquid interface the use of basicity B that is defined by Na2O activity shown in equa- reactions at high temperatures, which are considered to precipitate tion (1) is considered. The reason for this includes the fact that the the erosion of a refractory or nozzle clogging. However, since it is set measurement of basicity B under high temperatures can be per- difficult to actually measure a reaction that is occurring at the sol- formed relatively easily using a basicity sensor 5–13) that uses a solid id-liquid interface under high temperatures, there are several issues electrolyte. to be resolved. To control the solid-liquid interface reactions occur- B = − log a (1) ring at high temperatures, it is important to identify the factors that Na2O control each phenomenon while clarifying it. Therefore, in this The solubility behavior of metal oxides in melts based on the study, we establish measurement technology under a condition at basicity B described above has already been investigated, in which high temperatures, which will be instrumental in clarifying the sol- the fluxing model is known as the melting reaction mechanism. id-liquid interface reactions at high temperatures. With this model, the basicity of a melt is acid (low Na2O activity), One of the dominant factors of the solid-liquid interface reac- and the dissolution reaction of a metal oxide accords with the basic- 2− tions at high temperatures is O activity. Similar to a solution sys- ity solubility; when the basicity of a melt is basic (high Na2O activi- tem in which the H+ activity represents pH, in molten oxide systems, ty), it accords with acid solubility.5–7) To clarify the solubility behav- * Researcher, Fundamental Metallurgy Research Lab., Advanced Technology Research Laboratories 1-8 Fuso-cho, Amagasaki, Hyogo Pref. 660-0891 - 96 - NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 114 MARCH 2017 ior of protective oxide films in melts of sulfuric acid considered to are indicated as follows. involve the abnormal corrosion of gas turbines used for airplanes/ + − Electrode [A] Ag + e = Ag (2) power generation, the metal oxide solubility of Fe2O3, Cr2O3, Al2O3, 1 − 2− Electrode [B] O2 + 2e = O (3) SiO2, etc. depending on the basicity of Na2SO4 melts has been re- 2 ported.5–7) Since the measurement technology used for this molten The reaction at each electrode is indicated as Na2SO4 allows for estimation of the oxide solubility and the dissolu- − 2− Electrode [A] Ag2SO4 + 2e = 2Ag + SO4 (4) tion reaction formula, we examined the application of the fluxing 1 Electrode [B] 2Na+ + O + 2e− = Na O (5) model to the molten metal oxide system used in the processes in- 2 2 2 volved in steel products. and thereby equation (6) for the overall cell reaction can be ob- The binary Na2O-B2O3 melt has long been used for various fields tained. 14) 1 related to the surface cleaner to remove oxides on metal surfaces, Na SO + 2Ag + O = Na O + Ag SO (6) boronizing of steel surfaces 15) using salt baths, and slag components, 2 4 2 2 2 2 4 3, 12, 13) etc. For the melt systems, whereas the basicity measurement Assuming that Na2SO4, Ag2SO4, and Na2O are components 1, 2 and has been reported, the detailed influence of the basicity on the metal 3, the Gibbs free energy change of equation (6), ΔG, is written as oxide solubility has yet to be elucidated. ΔG = [ΔG O + ΔG O − ΔG O − RT ln (a /a )] Therefore, this study investigates a Na O-B O melt, which 3 2 1 1 2 2 2 3 1/2 + RT ln (a /P [B] ) (7) melts at low temperature and is easily handled, with the aim of es- 3 O2 tablishing the measurement technology of metal oxide solubility for Where R is the gas constant, T is the absolute temperature, ai is the O the basicity in order to clarify the metal oxide dissolution reactions. activity of the i component, ΔGi is Gibbs standard free energy of the i component, and P [B] is the oxygen potential on the side of O2 2. Experiment Method 16) electrode [B]. 2.1 Basicity sensor When assuming regular solution approximation ai = γi Ni (γi : i 5–7) In this study, the application of the basicity sensor used for component activity factor, Ni : i component molar fraction), a1/a2 is the measurement of the basicity of a Na2SO4 melt to a Na2B4O7-B2O3 indicated as melt was examined. Figure 1 shows a schematic view of the basici- a /a = γ N /γ N ty sensor prepared. The basicity sensor consists of two sensors: an 1 2 1 1 2 2 = (N1 / N2) (γ1 / γ2) oxide ion sensor and oxygen sensor. Each sensor is described below. = (N /N ) exp [α (N 2 − N 2) / (RT)] (8) 2.1.1 Oxide ion sensor 1 2 2 1 2 2 5–7) 3, 12, 13) γ / γ = exp [α (N − N ) / (RT)] (9) Within an oxide ion sensor, mullite , β-alumina , or 1 2 2 1 17, 18) quartz glass is mainly used as a sodium ion solid electrolyte un- Then the ratio of γ1/γ2 is indicated as equation (9) using the interac- 17) der high temperatures. In this study, the sensor was prepared using tion constant α of a Na2SO4-Ag2SO4 mixed melt at temperature T. quartz glass, which is difficult to dissolve in a Na2B4O7-B2O3 melt. Equation (8) is substituted into equation (7) using the value of As shown in Fig. 1, the electrochemical equilibrium reactions at Gibbs standard free energy for Na2SO4, Ag2SO4 and Na2O at 1 173 19) electrode [A] and electrode [B] in the following cell, in which a K , and electromotive force V1 is obtained from equation (10), quartz glass bulkhead is placed between electrode [A] and electrode V = − ΔG / (2F) 1 a [B], = −1.448 − 0.116 log Na 2 O (at 1173 K, V in V) (10) (P [B])1/2 1 (−)Ag | Na SO -10 mol%Ag SO | | Na B O -B O | Pt (+), Air O2 2 4 2 4 2 4 7 2 3 where F represents the Faraday constant. Here, a can be obtained ↑ ↑ ↑ Na2O from P [B] measured using the oxygen probe described later and Electrode Quart z Electrode O2 electromotive force V . [A] ← Na+ → [B] 1 2.1.2 Oxygen sensor Stabilized zirconia (Y2O3-ZrO2, MgO-ZrO2, CaO-ZrO2, etc.), which is an oxide ion solid electrolyte has often been employed as an oxygen sensor. As shown in Fig. 1, after fixing the platinum wire with platinum paste to the internal surface at the bottom of the stabi- lized zirconia tube (Y2O3-ZrO2), the tube was heated thereby adher- ing the platinum wire. Electromotive force V2 between the electrodes of the following cell, in which a zirconia bulkhead is placed between electrode [A] and electrode [B], (−) Pt | air (P = 0.21 atm) | | Na B O -B O | Pt (+) O2 2 4 7 2 3 ↑ ↑ ↑ Electrode ZrO2 Electrode [A] ← O2− → [B] depends on the oxygen potential on both sides of the zirconia bulk- head. When dried air is used as the standard gas on the electrode [A] side, P [B] can be obtained from equation (11). O2 0.21 V = RT ln (11) 2 4F ( P [B] ) O2 Fig. 1 Schematic representation of the basicity sensor - 97 - NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT No. 114 MARCH 2017 2.2 Basicity measurement of Na2B4O7-B2O3 melt tion of the melt used for the measurement.