2019 Volume 59 Issue 10 Pages 1801-1805
Nickel-molybdenum alloys can be produced by the carbothermic reduction of spent catalysts containing nickel and molybdenum discarded from the petroleum refinery operations for desulfurization. However, sulfur can be picked up into alloy melts from the sulfur bearing spent catalysts during the smelting process. Thermodynamics of sulfur in this alloy melt is very important for producing low sulfur Ni–Mo alloys to be used in steel industry. In the present study, thermodynamic interaction coefficients of carbon and molybdenum on sulfur in carbon saturated liquid Ni–Mo alloys were measured using the slag/metal equilibration technique at 1773 and 1873 K. The equilibrium sulfur distribution was measured between a slag with a known sulfide capacity and carbon saturated liquid Ni–Mo–S alloys of various compositions. The carbon solubility in liquid Ni–Mo alloy was significantly changed with molybdenum content, and the specific effects of carbon and molybdenum on sulfur was determined using Wagner’s formalism as the first- and second-order interaction parameters.
Solid spent catalysts from the hydro-desulfurization units of petroleum refining industries are alumina based material and contain metallic constituents of nickel and molybdenum up to 15 mass% in the form of oxides and sulfides with a sulfur content up to 3 mass%. These spent catalysts could be used as a cheap source for valuable metals for steel industry via the pyro-metallurgical process such as an electric arc furnace smelting reduction.1) Alumina in the catalysts can be fluxed with lime to form liquid CaO–Al2O3 slags and metallic constituents can be recovered in the form of carbon saturated liquid Ni–Mo alloys by the carbothermic reaction. The slag containing high Al2O3 can be utilized in cement industry and the alloy can be used as a raw material in steelmaking process.
In this process, one of the main issues is how to keep the sulfur in the slag as much as possible and obtain low sulfur Ni–Mo alloys during the smelting reduction process. Therefore, thermodynamic information of sulfur in Ni–Mo alloys is very important to predict the sulfur distribution between a smelting slag and Ni–Mo alloy melt, and also to find out tolerable sulfur content in spent catalysts for producing low sulfur Ni–Mo alloys.
In the present study, the sulfur distribution ratios between CaO–Al2O3 slags and carbon saturated iron, nickel and Ni–Mo alloy melts were measured at 1773 and 1873 K. The carbon solubility in liquid nickel alloys was also determined as a function of alloy composition. Using the Wagner’s interaction parameter formalism (WIPF),2) the specific effects of carbon and molybdenum on sulfur were determined as the first- and second-order interaction parameters. Using thermodynamic parameters determined in the present study, the sulfur distribution between a smelting slag and Ni–Csat.–Mo–S alloy melt of various compositions can be predicted as a function of melt temperature.
The slag-metal equilibration experiments were carried out to measure the sulfur distribution ratio between a CaO–Al2O3 slag and carbon saturated liquid iron, nickel and Ni–Mo alloy melts using an electric resistance furnace heated by MoSi2 heating elements with an alumina reaction tube (OD: 70 mm, ID: 63 mm, H: 1100 mm) at 1773 and 1873 K as shown in Fig. 1. The reaction temperature was measured by a Pt/Pt-13%Rh thermocouple protected with an alumina tube at the bottom of the crucible, and it was controlled within ±1 K using a PID controller. Detailed descriptions of the experimental apparatus and procedure are available in the authors’ recent study.1)
A schematic diagram of the experimental apparatus. (Online version in color.)
Two slag compositions of CaO–Al2O3 (C/A = 50/50 and 40/60 in mass%) slags containing 0 or 0.5 mass%S were prepared by melting the reagent grade CaO, Al2O3 and CaS in a graphite crucible using a high frequency induction furnace. It was cast onto a steel plate, crushed into powder, and stored in a desiccator until use. The master alloys of Fe-2 mass%C, Ni-2 mass%C containing 0 or 0.1 mass%S and Ni-40 mass%Mo-2 mass%C were prepared by melting high purity electrolytic iron, metallic nickel (99.99%purity), metallic molybdenum (99.95%purity), sulfur powder (99%purity) and high purity graphite powder in a high purity alumina crucible in an Ar-10%H2 atmosphere using a high frequency induction furnace. Desired portions of master alloys were charged to make an aimed melt composition for each experiment. Two grams of slag containing 0.3 to 0.5 mass%S and six grams of alloys containing 0 to 0.1 mass%S were placed in the hole of a graphite crucible. The graphite crucible was tied up by molybdenum wire and placed in the hot zone of the furnace for 10 hours. The initial sulfur contents in slag samples were high or low with respect to the sulfur content in equilibrium with metal, and the sulfur was allowed to transfer from slag to metal or vice versa. The oxygen potential was controlled by the C–CO equilibrium, and the CO gas was passed through the silica gel and magnesium perchlorate (Mg(ClO4)2) to remove moisture, ascarite to remove CO2, and then blown into the reaction tube at a flow of 300 mL/min controlled by a mass flow controller.
After each experiment, the crucible was pulled out of the furnace and quenched rapidly by a helium gas blowing. The slag and metal samples were analyzed for Ca, Al, Fe, Ni and Mo by the ICP-AES analyzer (ARCOS, SPECTRO). Sulfur and carbon contents were analyzed by the carbon/sulfur analyzer (CS-800, ELTRA). The sulfur content in metal samples was measured with an accuracy of ± 1 mass ppm using the standard sample of steel containing 7 ± 1 mass ppm sulfur.
Due to large discrepancies and lack of data on temperature dependency in earlier investigations,4,5,6) Venal and Geiger3) restudied the Ni–S system for dilute sulfur concentration range up to 0.7 mass% using the improved gas-metal equilibration technique in the temperature range from 1773 to 1848 K. In order to overcome the inherent thermal diffusion errors arising from big difference in molecular weight of gaseous species, they optimized the gas flow rate of argon added H2S-H2 gas mixtures. Also, they approached the equilibrium for reaction (1) from both directions, i.e., from initially low and high sulfur contents in liquid nickel with respect to the equilibrium values for a given PH2S/PH2 ratio. They determined the standard Gibbs free energy for sulfur dissolution in liquid nickel in the temperature range of 1773–1848 K as
Venal and Geiger reported the value of in Ni–S melt as −1453/T + 0.748 at 1773–1848 K.3) In the present study, the standard Gibbs free energy for sulfur dissolution in liquid nickel and the value reported by Venal and Geiger were used.
Thermodynamics of sulfur in liquid nickel alloys can be also determined using the slag-metal equilibration technique. The sulfur distribution ratio between slag and metal at equilibrium is defined as
The sulfide capacity of a slag is defined by:7)
The sulfide capacity of a slag can be obtained from the measured LS between a slag and liquid iron at a controlled PO2 since the thermodynamic behaviour of sulfur, fS in liquid iron is well known.9,10,11) Conversely, using a slag of the known sulfide capacity value, the fS(Ni) values in liquid nickel alloys can be obtained from the measured LS at a controlled PO2 using the following relation obtained from Eqs. (3), (5) and (6).
For a carbon saturated Ni–Mo–S alloy, the activity coefficient of sulfur can be expressed as the following relation:
The equilibrium sulfur distributions, LS between two composition CaO–Al2O3 slags (C/A = 50/50 and 40/60 in mass%) and carbon saturated liquid iron and nickel were measured under CO atmosphere at 1773 and 1873 K as summarized in Table 1. Figure 2 compares LS between CaO–Al2O3 slags and carbon saturated liquid metals. The LS values were obtained by approaching the sulfur equilibrium from both directions by transfer of sulfur from slag to metal (downward triangles) and metal to slag (upward triangles), and they agree well with each other. It is apparent that the equilibrium sulfur distribution is lower for carbon saturated liquid nickel as compared with that for carbon saturated liquid iron.
Sulfur distribution between carbon saturated Fe/Ni melts and CaO–Al2O3 slags.
Therefore, the sulfide capacity of a slag can be calculated from Eq. (6) using the LS values for carbon saturated liquid iron assuming that the C–CO reaction (aC =1, PCO = 1 atm) controls the oxygen partial pressure in the system.
The partial pressure of sulfur, PS2 can be calculated from the thermochemical data for sulfur in the liquid iron by:
The value of fS(Fe) in Fe–Csat.–S melt can be expressed as the following equation using the interaction parameters.
The log CS values of CaO–Al2O3 slags (C/A = 50/50 and 40/60 in mass%) determined in the present study are −3.11 and −3.23 at 1773 and 1873 K, respectively. Figure 3 compares the log CS values of CaO–Al2O3 slags determined in the present study together with the data reported by other workers as a function of slag composition in CaO–Al2O3 system. The CS values determined by the slag/metal equilibration method using Fe–Csat.–S melt in the present study are in excellent agreement with those data determined by different experimental techniques.12,13,14,15) This also confirms that thermodynamics of sulfur in Fe–Csat.–S melt9,10) used in the present study is valid.
Sulfide capacity of CaO–Al2O3 slags.
Using the sulfide capacities of CaO–Al2O3 slags determined in the present study, the effect of carbon and molybdenum on sulfur in liquid nickel can be determined from the sulfur distribution ratios between those slags and Ni–Csat.–Mo–S melts. The nickel and molybdenum contents in slag were negligible, therefore the effect of their oxides on sulfide capacity of slag was ignored. Figure 4 shows the LS values between CaO–Al2O3 slags and Ni–Csat.–Mo–S melts containing Mo up to 33.6 mass% at 1773 and 1873 K under CO atmosphere. The effect of molybdenum on the sulfur distribution was not linear over all concentration range studied. Figure 5 shows the variation of carbon solubility as a function of molybdenum content in alloy melts. The carbon solubility in liquid nickel was determined as 2.23 and 2.38 mass% at 1773 and 1873 K, respectively. These values are in excellent agreement with Lucas et al.’s carbon solubility data of [%C]sat. = 0.07 + 1.44 × 10−3 T(°C) in liquid nickel at 1327–1700°C.16) Molybdenum addition increases the carbon solubility significantly as shown in Fig. 5.
Sulfur distribution between CaO–Al2O3 slags and Ni–Csat.–Mo melts.
Carbon solubility in Ni–Mo melts.
Therefore, using the relations of Eqs. (7) and (8), the specific effect of molybdenum and carbon on sulfur in liquid nickel can be obtained simultaneously from the LS values measured for Ni–Csat.–S and Ni–Csat.–Mo–S melts with CaO–Al2O3 slags of the known sulfide capacity values determined in the present study. Figure 6 shows the interaction coefficient of molybdenum on sulfur in liquid nickel, plotted vs. molybdenum content in Ni–Csat.–Mo–S system at 1773 and 1873 K. The relations were not affected by the temperature. As shown in Fig. 6, the effect of molybdenum on sulfur is not linear so that the first- and second-order interaction parameters, and can be determined as 0.047 and −0.00045, respectively, in the temperature range of 1773–1873 K. As discussed earlier, Venal and Geiger3) studied the Ni–S system using the gas-metal equilibration technique. They determined the effect of molybdenum on sulfur in Ni–Mo–S melt containing molybdenum up to 20.54 mass% at 1848 K. However, they reported only the first-order interaction parameter value of 0.053 in the limited range of molybdenum content below 10 mass% where a linear relationship of vs. mass%Mo was obtained. However, as shown in Fig. 6, Venal and Geiger’s data for their whole molybdenum range up to 20.54 mass% are in excellent agreement with the relation determined in the present study.
The relation of vs. [%Mo] in liquid Ni–Csat.–Mo–S.
Using the same method, the interaction coefficient of carbon on sulfur in liquid nickel, was determined from the LS values and plotted vs. carbon content in Ni–Csat.–S and Ni–Csat.–Mo–S alloy melts as shown in Fig. 7. The relations were not affected by the temperature at 1773 and 1873 K. The first-order interaction parameter of carbon on sulfur in liquid nickel, , can be determined as 0.06 by the regression analysis of data determined at carbon solubility range of 2.23–3.4 mass% in Ni–Csat.–S and Ni–Csat.–Mo–S alloy melts.
The relation of vs. [%C] in liquid Ni–Csat.–Mo–S.
Using thermodynamic parameters determined in the present study, the sulfur distribution between a smelting slag and carbon saturated Ni–Mo alloy melt of various compositions can be predicted. The LS values were calculated for CaO–Al2O3 slags of the predetermined sulfide capacities and Ni–Csat.–Mo–S melts of various compositions at 1773 and 1873 K. They are in excellent agreement with the experimental results as shown in Table 1.
As shown in Fig. 2, the desulfurization efficiency in carbon saturated liquid nickel was poor as compared with that in carbon saturated liquid iron because of lower value and lower carbon solubility in liquid nickel. However, the LS can be significantly increased by the addition of molybdenum in liquid nickel as shown in Table 1 because of much higher value of 0.047 in liquid nickel than in liquid iron which is known as 0.0064 at 1873 K.1) One can expect that the LS value between 40%CaO-60%Al2O3 slag and Ni–Csat.-30%Mo melt is 1.5 times higher than the value for Fe–Csat.-30%Mo melt at 1873 K.
The effect of carbon and molybdenum on sulfur in liquid Ni–Csat.–Mo–S alloys have been determined from the equilibrium sulfur distribution between a CaO–Al2O3 slag and liquid nickel alloys of carbon saturated nickel and Ni–Mo melts under the oxygen potentials controlled by C–CO equilibrium at 1773 and 1873 K. Carbon and molybdenum both increase the activity coefficient of sulfur in nickel base melt significantly. The first and second-order interaction parameters of carbon and molybdenum on sulfur in liquid Ni–Csat.–Mo–S alloys at 1773–1873 K can be expressed as:
(2.23 ≤ C ≤ 3.4 mass%)
(Mo ≤ 33.6 mass%)
This paper was supported by Korea Institute for Advancement of Technology(KIAT) grant funded by the Korea Government(MOTIE) (P0002019, The Competency Development Program for Industry Specialist).