ISIJ International, Vol. 59 (2019),ISIJ International,No. 3 Vol. 59 (2019), No. 3, pp. 583–589 Shear-Force Based Stainless Steel Slag Modification for Chromium Immobilization Qing ZHAO,1)* Chengjun LIU,1) Longhu CAO,1) Maofa JIANG,1) Baokuan LI,2) Henrik SAXÉN3) and Ron ZEVENHOVEN3) 1) Key Laboratory for Ecological Metallurgy of Multimetallic Mineral (Ministry of Education), School of Metallurgy, Northeastern University, Shenyang, 110819 China. 2) School of Metallurgy, Northeastern University, Shenyang, 110819 China. 3) Thermal and Flow Engineering Laboratory, Åbo Akademi University, Åbo/Turku, 20500 Finland. (Received on November 23, 2017; accepted on November 5, 2018; J-STAGE Advance published date: December 20, 2018) Immobilization of chromium in a stable spinel by modification is a powerful way to prevent chromium pollution of stainless steel (SS) slags. The precipitated spinel grain size is usually smaller than 30 μm, however, which limits the effectiveness of the modification. In the literature, very few efforts have been reported on promotion of spinel growth rate by optimizing the dynamic conditions. In this study, the effects of shear force on the spinel grain size evolution under isothermal conditions and during cooling were investigated. The experimental results indicate that the employment of shear force significantly changes the growth behavior of spinel at 1 500°C. The growth mechanism of spinel was studied by using crystal size distribution (CSD) theory, showing different regimes of supply-controlled Ostwald ripening, surface-controlled growth with decaying nucleation rate, and constant rate nucleation and growth at shear rates of 0 s −1, 10.83 s −1, and 21.67 s −1, respectively. On the other hand, at a cooling rate of 5°C·min −1, the shear force was found to have little effect on the crystallization behavior of spinel. The results of static leaching tests indicated that hardly any chromium had leached, which makes the modified SS slag more environmentally friendly when used as a raw material. KEY WORDS: stainless steel slag; spinel; crystal growth; modification; shear force; chromium immobiliza- tion. ration impossible.5) Acid leaching or biological methods that 1. Introduction have often been used to extract chromium from Cr-bearing China has for more than a decade been the main producer residue are not suitable for the SS slag, since most of the of stainless steel and today stands for more than a half of acid is consumed by the Cr-free phases.6,7) Researchers the global production. Approximately one ton of Cr-bearing have reported that chromium can exist in a number of min- stainless steel (SS) slag is generated when three tons of eralogical phases, and the existence of unstable phases like stainless steel are produced.1) Chromium in initial (i.e. dicalcium silicate, merwinite, periclase, and lime enhance before cooling) SS slag is generally bivalent or trivalent. the possible leaching of chromium elution into seawater or Cr2+ is unstable and may be easily oxidized to Cr3+ or Cr6+ acidified water bodies.8,9) However, a spinel is considered during the cooling process or during the storage period. Cr3+ to be a suitable stable phase for controlling the leaching and Cr6+ have different toxicities: Cr3+ is an essential nutri- of chromium from SS slag.20) The results of earlier work ment at small amounts, while Cr6+ is very toxic and may generally agree on that the activity of chromium is strongly be easily released which can lead to severe environmental decreased when bound in a spinel because of the strong pollution or health risk issues.2) Therefore, a treatment of bonding of chromium in that.11–13) Therefore, the leaching the SS slag that converts or stabilizes harmful components of chromium from SS slag is expected to be limited or even is essential prior to the disposal of the slag in a landfill or negligible if all of the chromium would exist in a stable possible application in other processes.3,4) spinel phase. Chromium may be partially recovered from the SS slag Adding spinel-forming agents, such as MgO and Al2O3, by magnetic separation or gravity separation, but the extrac- may promote Cr-containing spinel crystallization.14,15) After tion efficiency is unsatisfactory since some chromium exists such a slag composition adjustment, the grain size of the in the silicate phases making recovery by mechanical sepa- spinel mainly depends on the temperature to which it is exposed and the duration of the exposure.16) Controlling the * Corresponding author: E-mail: [email protected] cooling conditions can affect the mineral transformation of DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2017-678 slag and the growth behavior of spinel, although it is dif- 583 © 2019 ISIJ ISIJ International, Vol. 59 (2019), No. 3 ficult to realize the growth of spinel to a grain size above was injected into the furnace at a rate of 500 mL·min −1 to 30 μm.17,18) Until today, very few efforts have been made prevent Cr6+ from forming during the modification process. to optimize the dynamic conditions of spinel growth via a The temperature was first elevated to 1 600°C for sample modification process. pre-melting, and then decreased to 1 500°C for shear-force Chromite with a common formula of (Mg,Fe2 + ) based modification. When the temperature had decreased to (Cr,Al,Fe3+) is the most important Cr-rich mineral world- the set point, the molybdenum agitator was moved down wide, and has a crystal structure and chemical composition until it was fully immersed in the molten slag, followed by similar to the spinel in SS slag.19) Furthermore, chromite stirring and timing. Different stirring rates of 0 rpm (refer- normally crystallizes as an early phase from mafic magmas, ence experiment), 25 rpm, 50 rpm, 75 rpm and 100 rpm which is similar to the crystallization process of spinel from were investigated in this study. The mean shear rate (γ, s −1) molten SS slag.20) Single ore bodies of chromite may hold is used to characterize the shear force exerted on the sample, a few million tons, and the geological shearing force has which is defined as been reported to play a significant role in the chromite min- KN .................................... (1) eralization and growth processes by improving the kinetic conditions of crystallization.21,22) Inspired by these research where K is a constant which should be K =13 r −1 for the findings in chromite mineralogy, it is innovatively proposed impeller,23) N is the stirring rate as revolutions per second. in this paper that exerting a shear force during melting mod- Thus, the corresponding shear rates studied were 0 s −1, ification of SS slag may improve the chromium stabilization 5.42 s −1, 10.83 s −1, 16.25 s −1 and 21.67 s −1, respectively. and promote the spinel growth process. Cooling treatment was carried out at a rate of 5°C·min −1 In the present study, a shear force was applied to tradi- after 60 min of isothermal treatment at 1 500°C. The agita- tional modification process of SS slag, and the effects of tion was stopped and the molybdenum agitator was taken the shearing rate on the grain size evolution of spinel under out of the sample when the temperature had decreased to isothermal as well as cooling process conditions were inves- 1 300°C. Sampling was performed after 0 min, 15 min, tigated. Furthermore, the growth mechanism of spinel under 30 min, 45 min and 60 min during the isothermal process various shear force conditions was studied using crystal size and at 1 400°C, 1 350°C, 1 300°C, 1 250°C, 1 200°C and distribution (CSD) theory. A static leaching test at labora- 1 150°C during the cooling process using a quartz tube, and tory scale was carried out to evaluate the leaching property the obtained samples were immediately quenched in water. of chromium from such treated SS slag. Below 1 150°C, SS slag rapidly solidified and was cooled to room temperature in a furnace. The experimental apparatus used in this study is shown in Fig. 1. 2. Material and Methods CSD theory for particulate processes in chemical engi- 2.1. Materials neering24) is widely used in investigations on the growth Reagents of calcium oxide (CaO ≥ 98.0 wt.%), magne- sium oxide (MgO ≥ 99.9 wt.%), silicon oxide (SiO2 ≥ 99.0 wt.%), ferrous oxalate (FeC2O4·2H2O ≥ 99.9 wt.%), alumi- num oxide (Al2O3 ≥ 99.0 wt.%), chromium oxide (Cr2O3 ≥ 99.0 wt.%), and calcium fluoride (CaF2 ≥ 98.5 wt.%) were used to prepare a synthetic SS slag, in which some alumi- num oxide was employed as a spinel-forming agent based on earlier findings.13) The content of aluminum oxide is normally below 10 wt.%, but it was increased to 12 wt.% in this study. The raw materials were dried at 110°C for 10 h before weighting and mixing. The chemical composition of the target SS slag studied in this work is shown in Table 1. All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Methods A tube furnace was modified by equipping it with a shear force input device (i.e., a system containing a molybde- num agitator and control components), and employed for modification of SS slag. A sample of 140 g was poured into a molybdenum crucible, which was then placed into a graphite crucible in the modified tube furnace. Argon Table 1. Chemical composition of SS slag studied in this work, Fig. 1. Illustration of the experimental apparatus used for modifi- wt.%. cation process. 1-cap; 2-furnace cover; 3-corundum tube; 4-insulation layer; 5-thermocouple A; 6-molybdenum agi- CaO SiO2 MgO FeO Al2O3 Cr2O3 CaF2 tator; 7-stainless steel slag; 8-refractory; 9-gas inlet; 10-thermocouple B; 11-graphite crucible; 12-molybdenum 40.8 27.2 9.0 3.0 12.0 5.0 3.0 crucible; 13-MoSi2 heating elements; 14-gas outlet.