Experimental investigations on recycling the BOF-slag by reductive heating and as a medium for CO2 phosphorus and arsenic removal

Abstract

Basic-oxygen furnace (BOF) slag is the second abundant by-product from the steel-making process It is not readily recycled for high phosphorus content and volume instability This study performed reductive heating experiments to modify the composition of BOF-slag and the resulted products can satisfy the requirements for recycling in steel plants or constructions Mineral CO2 sequestrations and P-As adsorptions by BOF-slag were also investigated to broaden the possible applications of BOF-slag recycling In an attempt to eliminate the phosphorus-hosting dicalcium silicate (C2S) and volumetrically unstable free-lime experiments of reductively heating the BOF-slag with quartz-sand or serpentine as a basicity modifier were carried out at 1300–1600°C The C2S and free-lime were reacted out in the products The mineral reactions involved reduction of Ca-ferrite and Mg-w?stite to form metal iron together with CaO and MgO respectively Then these oxides re-equilibrated with C2S and SiO2 to form merwinite and akermanite The phosphorus in C2S reacted with Fe to form Fe2P The products were characterized by silicate–metal segregation Textural and compositional variations indicated that the extent of such segregation increased with increasing temperature and decreasing grain size of the starting mixtures Mass balance calculations showed that phosphorus was concentrated into the metal domain instead of being removed from the initial mixtures Compared to the products from heating the slag–serpentine mixture those from heating the slag–quartz mixture were characterized by higher extents of silicate–metal segregation with lower phosphorus contents of ~0 1% in the silicate domain which therefore is recyclable To optimize silicate–metal segregation for recycling the BOF-slag it is suggested (1) bringing the BOF-slag composition within the low temperature side of the C2S–merwinite–akermanite Alkemade triangle by adding quartz-sand (2) reductively heating to 1500–1600°C and (3) cooling slowly then quenching to suppress C2S crystallization from melts BOF-slag contains > 35% CaO a potential component for CO2 sequestration Slag–water–CO2 reaction experiments were conducted with the longest reaction duration extending to 96 hours under high CO2 pressures reaching of 100–300 kg/cm2 to optimize BOF-slag carbonation conditions to address carbonation mechanisms and to evaluate V and Cr release from slag carbonation The slag carbonation degree generally reached the maximum values after 24 hours’ slag–water–CO2 reaction and was controlled by slag particle size and reaction temperature The maximum carbonation degree of 71% was produced from the experiment using fine slag of ? 0 5 mm under 100°C and a CO2 pressure of 250 kg/cm2 with a water/slag ratio of 5 V release from the slag to water was significantly enhanced (generally > 2 orders) by slag carbonation In contrast slag carbonation did not promote Cr release until the reaction duration exceeded 24 hours However the water Cr content was generally at least an order lower than the V concentration which decreased when the reaction duration exceeded 24 hours Therefore long reaction durations of 48–96 hours are proposed to reduce environmental impacts while keeping high carbonation degrees Mineral textures and water compositions indicated that all the CaO- and MgO-containing minerals contributed to slag carbonation Since Mg-w?stite can also be carbonated the conventional expression that only considered carbonation of the CaO-containing minerals undervalued the CO2 sequestration capability of the BOF-slag by ~20% Therefore the BOF-slag is a better CO2 storage medium than that previously recognized The adsorption and precipitation of P and As from aqueous solutions by BOF-slag were also investigated Experiments and modeling results showed that the pseudo-second order rate equation provided the best correlation for the adsorption kinetics of P and As Coexistence of P and As in the solution exhibited obvious competitions for adsorption sites and the efficiency of As removal was significantly suppressed indicating the preferential selectivity of P prior to As by BOF-slag According to the kinetic experiments the reaction durations were 5 and 20 days respectively for P and As to reach equilibrium when adsorbed by BOF-slag For the experimental results of adsorption isotherms both P and As can be well described by the Langmuir equations at low initial concentrations Calculated from the linear regression the maximum adsorption weights were 12 9 mg/g-slag for P and 3 03 mg/g-slag for As However precipitates occurred when initial concentration increased to > 400 ppm P and > 75 ppm As and the highest P and As removal can increase to 17 9 mg/g-slag for P and 8 48 mg/g-slag for As Trace amounts of secondary phases of Fe-P compounds were identified by XRD EDS analyses demonstrated that the precipitations of P and As from aqueous solution occurred via the decomposition of C2S and followed by the formation of Fe-P and Fe-As compounds The increasing P and As contents in the C2S grains from the interiors to the edges of liquid-solid interfaces indicated that C2S was the main phase dominated the P and As precipitations in BOF-slag Consequently it can be concluded that BOF-slag is qualified as adsorbents for P and As removal from aqueous solution

Date of Award	2016 Jul 27
Original language	English
Supervisor	Huai-Jen Yang (Supervisor)