Novel route for recycling of steelmaking slag by means of the hydrothermal hot-pressing method

Abstract

In the last decade, much efforts have been directed toward the recycling of the steelmaking slags, due to their high production volume. A large amount of the slags produced at the steelmaking plants has been mainly used for building materials such as raw material for cement, roadbed material, and for fertilizer [1]. There are, however, some difficulties in recycling the slags, because they usually consist of various compounds with a large variation of the compositions depending on the processes of the steelmaking. The hydrothermal hot pressing (hereafter abbreviated as HHP) technique is an attractive method to transform these slags into useful building materials. Hitherto, the HHP technique has been used to obtain densified materials, including low sinterable materials such as carbonates, hydrates and hydroxides [2–4], under hydrothermal conditions at low temperatures below 300 ◦C with uniaxial compression of the raw powders. Glass powders were also densified by the addition of a small amount of pure water [5, 6]. The characteristics of this technique are the low temperature processing using a simple apparatus and the possibility of the densification of various compounds. The HHP technique has been applied to densify waste glass [7], sludge ash [8], radioactive wastes [9], concrete wastes [10, 11], and so on. The present work showed the feasibility of densifying the basic oxygen furnace (BOF) slag produced at NKK by means of the HHP method. The wet chemical analysis shows that the slag consists of CaO 41.8 wt%, Fe2O3 30.17 wt%, SiO2 10.28 wt%, Al2O3 3.94 wt%, Mn2O3 4.11 wt%, P2O5 3.11 wt% and other minor constituents. The large content of Ca is one of the characteristics of this slag. The slag was previously milled for 12 h and sieved to get a powder with a particle size between 46–53 μm for the HHP treatments. The slag powder (5 g) was well kneaded with different contents of water (0–15 wt%), and the mixture was then placed into the cylindrical chamber of the autoclave for HHP treatments, with inner diameter of 20 mm [12]. The slag powder was uniaxially loaded at various pressures (25– 45 MPa) and simultaneously heated at constant rate of 10 ◦C/min up to a desired temperature (150 and 250 ◦C). The densification of the slag powder was conducted by holding at the desired temperature for a fixed time in the range of 15–120 min. After the HHP treatments, the autoclave was cooled down to room temperature and the solid compact was taken out from the autoclave. The tensile strength of the compact was measured by means of the Brazilian test [13], and the bulk density was estimated by the volume and weight of the compacted specimens. The effects of reaction time, loading pressure, and water content on tensile strength and bulk density of the compacts were evaluated at 250 ◦C. The results are shown in Fig. 1. The influence of the reaction time with constant water content of 10 wt% and uniaxial load of 25 MPa is shown in Fig. 1a. The tensile strength considerably increased with the increase in reaction time up to 30 min, and its increase was reduced with the further increase in reaction time. The bulk density linearly increased with the increase in reaction time. It should be noted that the scales in Fig. 1a are smaller than those in Fig.1b and c. As a result, the reaction time had smaller effects on the properties of the compacts than the loading pressure and water content, which suggested that the densification of the slag powder proceeded in a short time. On the other hand, the increase in loading pressure and water content significantly improved the properties of the compacts (Fig. 1b and c). The high loading pressure accelerated the densification of the slag powder and increased the density and mechanical strength of the compacts as well. A considerable increase of both tensile strength and bulk density was observed with increasing the water content up to 10 wt%. The densification of the slag powder seems to be independent of high water content over 10 wt%, because the bulk density