Preparation of TiC ceramics from hot Ti-bearing blast furnace slag: Carbothermal reduction,supergravity separation and spark plasma sintering

TiC ceramics, a kind of ultra-high temperature ceramics, are primarily prepared by the synthesized TiC powders from high purity metallic titanium or titania. In this work, a novel method for sustainable utilization of hot Ti-bearing blast furnace slag to prepare TiC ceramics was developed. Firstly, the Ti was efficiently transformed into the TiC in hot Ti-bearing blast furnace slag through carbothermal reduction. The high-purity TiC powders were fully recovered from molten carbonized Ti-bearing slag through supergravity separation, where the mass fraction and recovery ratio of Ti in TiC powders were up to 77.89 wt.% and 95.58 %. The TiC ceramics with a relative density of 98.23 % were prepared from the recovered TiC powders via spark plasma sintering method, which possess the favourable mechanical properties including Vickers hardness (Hv) of 23.3 ± 0.4 GPa, fracture toughness (K_IC) of 3.96 ± 0.24 MPa m^1/2 and flexural strength of 371.9 ± 15.4 MPa.     

An overview on oxyfuel coal combustion—State of the art research and technology development

Oxyfuel combustion is seen as one of the major options for CO₂ capture for future clean coal technologies. The paper provides an overview on research activities and technology development through a fundamental research underpinning the Australia/Japan Oxyfuel Feasibility Project. Studies on oxyfuel combustion on a pilot-scale furnace and a laboratory scale drop tube furnace are presented and compared with computational fluid dynamics (CFD) predictions. The research has made several contributions to current knowledge, including; comprehensive assessment on oxyfuel combustion in a pilot-scale oxyfuel furnace, modifying the design criterion for an oxy retrofit by matching heat transfer, a new 4-grey gas model which accurately predicts emissivity of the gases in oxy-fired furnaces has been developed for furnace modelling, the first measurements of coal reactivity comparisons in air and oxyfuel at laboratory and pilot-scale; and predictions of observed delays in flame ignition in oxy-firing.     

Structural characteristics and hydration kinetics of modified steel slag

This study investigates the structural characteristics and hydration kinetics of modified basic oxygen furnace steel slag. The basic oxygen furnace steel slag (BOFS) was mixed with electric arc furnace steel slag (EAFS) in appropriate ratios and heated again at high temperature in the laboratory. The mineralogical and structural characteristics of both BOFS and modified steel slag (MSS) were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy, Raman and Fourier transform infrared spectroscopies. The results show that modification increases alite content in MSS and decreases alite crystal size with the formation of C₆AF₂. One more obvious heat evolution peak appears in MSS's heat-flow rate curves in comparison to BOFS, becoming similar to that of typical Portland cement paste. As a result, its cementitious activity is much improved.     

Corrosion of refractory alumina plates used in the sliding gate system of steelmaking ladle: Chemical experiment

The aim of this study is to investigate the corrosion process of the refractory Al₂O₃-ZrO_2-C slide gate plate caused by chemical interactions with secondary refining slag. The plate is part of the steelmaking ladle slide gate system, therefore its lifespan extension and integrity are of concern. Post mortem plates with slag adhered on their channel surface were analysed and static cup corrosion test experiments were performed in a controlled atmosphere furnace for 1 h at 1600 °C. The corrosion product phases Gehlenite (Al₂Ca₂O₇Si) and Spinel (MgAl₂O₄) identified by DRX were formed after the experiment, showing that the slag is able to chemically corrode the Al₂O₃-ZrO₂-C plates. In addition, a comparison of the interface region between the slag and the post mortem plate and the interface region between the slag and the static cup corrosion test specimens showed that the corrosion products are swept away by the fluid flow. Even though chemical corrosion by slag has been identified as one of the causes of the plate channel degradation, the study suggests that it acts synergistically with other mechanisms of degradation.     

Characteristics and cementitious properties of ladle slag fines from steel production

Ladle slag is a by-product from further refining molten steel after coming out of a basic oxygen furnace (BOF) or an electric arc furnace (EAF). Air-cooled ladle slag has a very large portion of fine particles due to the conversion of β-C₂S to γ-C₂S during the cooling process. X-ray diffraction (XRD) analysis of three ladle slag fine samples passing 100, 200 and 325 mesh indicates that the major mineral in ladle slag fines is γ-C₂S, which does not show cementitious property in water. Experimental results have indicated that ladle slag fines show significant cementitious property in the presence of an alkaline activator. The finer the ladle slag is, the better the cementitious property of the slag is.     

A study on the relationship between porosity of the cement paste with mineral additives and compressive strength of mortar based on this paste

Mercury intrusion porosimetry study was carried out on ordinary Portland cement (OPC) pastes with 10% to 40% mineral additives, such as steel-making slag, granulated blast furnace slag and fly ash. For all samples, the porosity of paste and compressive strength of mortar based on this paste were determined at 3, 7, 28, 90 and 180 days. Relationship between the porosity and strength was investigated and some equations for the strength-porosity relationship were presented according to Balshin multiplicative model. Results show that mineral additives delayed process that micropore structure of OPC paste developed and strength development of sample with mineral additives was faster than that of OPC sample. Balshin equation fits the results of strength and porosity of all samples and there is a strongly quantitative relationship between strength and porosity. After being mixed with mineral additives, the intrinsic strength σ₀ and power n both increased and the sequence of σ₀ and n for different mineral additives was fly ash>steel-making slag>blast furnace slag.     

Interaction of Al2O3-rich slag with MgO–C refractories during VOD refining—MgO and spinel layer formation at the slag/refractory interface

The interaction mechanisms between a pitch-bonded MgO–C refractory and an Al₂O₃ rich (～15 wt%) stainless steelmaking slag were investigated by rotating finger tests in a vacuum induction furnace. A porous MgO layer (instead of a dense MgO layer) was observed at the hot face of the MgO–C bricks. This implies that under the present low oxygen pressure conditions, the oxygen supply from the slag is insufficient to meet the demand of reoxidising the entire amount of Mg vapor generated from the MgO–C reaction to form a fully dense MgO layer. A Mg(Al,Cr)₂O₄ spinel layer with zoning was found at the slag/brick interface in the top slag zone specimen of Test 3 (CHS3). Based on the thermodynamic analyses with and experimental data, a mechanism of Mg(Al,Cr)₂O₄ spinel formation is proposed. Initially, hot face periclase grains take up Cr₂O₃, and to a much lesser extent, Al₂O₃ from the slag. The further diffusion of Cr₂O₃ and Al₂O₃ from the slag establishes a spinel layer of three distinct compositions of the type MgAl_2(1?x)Cr_2xO₄, with x decreasing when moving from the interior to the exterior spinel layer. Due to the low oxygen pressures, the thermodynamically less stable, dissolved Cr₂O₃ in the hot face periclase decomposes and forms chromium-rich metal droplets.     

A basic gunning material interfaces study

Corrosion mechanisms of various basic gunning materials were investigated using crucible tests. The low corrosion resistance of olivine-containing materials was shown to be related to melted phases formation which occurs, at the gunning material/slag interface, at high temperature. MgO-rich materials are more resistant to basic slag attack. Moreover, increasing the refractory material CaO content by means of dolomite addition rises the slag penetration resistance and lowers the bulk modification resulting from slag interaction. In these materials, the corrosion mechanism involves a slag reaction with the starting binder, (NaPO₃)_n, and dolomite initially contained in the gunning material. A dolomite-containing material was projected on slag-covered MgO-C bricks, using a laboratory test equipment. Investigations carried out at the gunned material/slag/bricks interface show that the suitable silicophosphate bond still forms, at interface, despite slag interaction.     

Effects of steam on the sulfation of limestone and NOx formation in an air- and oxy-fired pilot-scale circulating fluidized bed combustor

The existing fluidized bed combustion literature on sulfation shows that above 30% conversion, direct sulfation via reaction with CaCO₃ is faster than indirect sulfation with CaO. However, while this is true for dry flue gases, it is not the case if steam (H₂O_(g)) is present at realistic levels for coal combustion, and it has been confirmed by experiments employing thermogravimetric analysis (TGA) and tube furnace (TF) testing that direct sulfation is in fact slower than indirect sulfation for nearly all levels of conversion if steam (H₂O_(g)) is present. In this work we have also examined the effects of H₂O_(g) on SO₂ capture and NH₃ oxidation to NO_x over calcium-containing compounds under air- and oxy-fired conditions in a pilot-scale circulating fluidized bed combustor (CFBC) utilizing limestone addition. The results of the pilot-scale tests confirm suggestions from our previous work that sulfur capture from the air firing of low-moisture fuels benefits from steam-sulfation. For petroleum coke, the addition of 8%vol H₂O_(g) resulted in increased SO₂ retention and Ca utilization, as well as decreased NO_x emissions by up to 44%. The simultaneous reduction of SO₂ and NO_x was attributed to enhanced solid-state diffusion (sintering) by H₂O_(g). Under oxy-fuel-firing conditions, H₂O_(g) addition also resulted in decreased NO_x emissions, but the pilot-scale tests showed poorer sulfur capture performance and calcium utilization as compared to air firing when H₂O_(g) was present, thereby reconfirming the TGA/TF results. It appears that most bench-scale work on sulfation to date has underestimated the true rate of reaction for sulfation in the presence of H₂O_(g). This conclusion explains at least in part why indirect sulfation is often faster than direct sulfation in pilot plant studies on oxy-fuel circulating fluidized bed combustion. Moreover, this work stresses the importance of including H₂O_(g) in bench-scale experiments that attempt to simulate real combustion environments.     

Research on slag-resistance of ZrN-SiAlON-SiC-C composite refractory in different atmospheres

The ZrN-SiAlON composite powder was synthesized using low-grade zircon and bauxite by carbothermal reduction nitridation at first and then ZrN-SiAlON-SiC-C composite refractory were fabricated with the ZrN-SiAlON powder, SiC particles, and a small amount of Si powder as raw materials and sucrose as the binder. The slag resistance of these composites in O₂, N₂ and Ar atmosphere was investigated by X-ray diffraction, scanning electron microscopy, and energy dispersion spectra. The results show that the pores in the inside of ZrN-SiAlON-SiC-C composite refractory were enlarged in oxygen atmosphere due to oxidation, which leads to the decrease in slag resistance. In argon atmosphere, blast furnace slag destroyed the sintered body of zircon, corundum, and cristobalite with the formation of CaZrO₃, then infiltrated into and filled the pores inside the refractory to form a dense layer, which hindered the further erosion of the blast furnace slag. In the reducing atmosphere, the interfacial energy of the gas-liquid phases became larger due to the reactions between blast furnace slag liquid and the gas, resulting in a larger wetting angle which prevented the erosion.     

Degradation mechanisms of magnesia-carbon refractories by high-alumina stainless steel slags under vacuum

The corrosion behaviour of a pitch-bonded magnesia-carbon refractory by an Al₂O₃ rich (∼15 wt.%) stainless steelmaking slag was investigated by rotating finger tests in a vacuum induction furnace at high temperature (>1650 °C) and low oxygen partial pressure (1.5–4.3 × 10⁻¹⁰ atm). This study confirms the poor slagline behaviour of MgO-C bricks industrially observed in VOD ladles. Higher temperatures and longer exposure times lead to more severe slag infiltration and direct MgO dissolution. The intrinsic MgO-C reaction is the major decarburisation mechanism, while extrinsic decarburisation by oxygen from the atmosphere and/or reducible slag components (CrO_x, FeO_x) was limited. Three kinds of metallic particles with different size, shape, location, composition and origin were observed in the refractory specimens. Concurrently, the thermodynamic conditions for the formation of a protective Mg(Al,Cr)₂O₄ spinel layer at the slag/refractory interface are investigated. The industrial relevance of this spinel layer formation is discussed with respect to the chosen Al₂O₃ level. Guidelines are proposed to minimise MgO refractory dissolution in VOD slaglines.     

Indirect mineral carbonation of titanium-bearing blast furnace slag coupled with recovery of TiO2 and Al2O3

Large quantities of CO_2 and blast furnace slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO_2 emission reduction and comprehensive utilization of the solid waste.This paper describes a novel route for indirect mineral carbonation of titanium-bearing blast furnace(TBBF) slag,in which the TBBF slag is roasted with recyclable(NH_4)_2SO_4(AS) at low temperatures and converted into the sulphates of various valuable metals, including calcium, magnesium, aluminium and titanium. High value added Ti-and Al-rich products can be obtained through stepwise precipitation of the leaching solution from the roasted slag. The NH_3 produced during the roasting is used to capture CO_2 from flue gases. The NH_4HCO_3 and(NH_4)_2CO_3 thus obtained are used to carbonate the CaSO_4-containing leaching residue and MgSO_4-rich leaching solution, respectively. In this study, the process parameters and efficiency for the roasting, carbonation and Ti and Al recovery were investigated in detail. The results showed that the sulfation ratios of calcium,magnesium, titanium and aluminium reached 92.6%, 87% and 84.4%, respectively, after roasting at an AS-to-TBBF slag mass ratio of 2:1 and 350 °C for 2 h. The leaching solution was subjected to hydrolysis at 102 °C for 4 h with a Ti hydrolysis ratio of 95.7% and the purity of TiO_2 in the calcined hydrolysate reached 98 wt%.99.7% of aluminium in the Ti-depleted leaching solution was precipitated by using NH_3. The carbonation products of Ca and Mg were CaCO_3 and(NH_4)_2 Mg(CO_3)_2·4H_2O, respectively. The latter can be decomposed into MgCO_3 at 100–200 °C with simultaneous recovery of the NH_3 for reuse. In this process, approximately 82.1% of Ca and 84.2%of Mg in the TBBF slag were transformed into stable carbonates and the total CO_2 sequestration capacity per ton of TBBF slag reached up to 239.7 kg. The TiO_2 obtained can be used directly as an end product, while the Al-rich precipitate and the two carbonation products can act, respectively, as raw materials for electrolytic aluminium,cement and light magnesium carbonate production for the replacement of natural resources.     

Erosion mechanism of tabular alumina of various microstructures under different basicity of blast furnace slag

In this study, three types of commercial tabular alumina aggregates with different microstructural characteristics were selected for corrosion tests on blast furnace slags with varying basicities. A corrosion mechanism with different microstructural characteristics is proposed, and the dominant microstructures in high- and low-basicity environments are evaluated. The pore structure was found to have a crucial influence on the dissolution of tabular alumina aggregates in the blast furnace slag. Under low-basicity (0.5–0.7) conditions, a small amount of clustered pore structure was beneficial for obtaining the maximum thickness (40–60 μm) of the isolation layer between the tabular alumina aggregate and slag. Under high-basicity (0.9–1.1) conditions, the circular pore structure tended to form a uniform and stable double-layer isolation structure of calcium hexaaluminate and MgAl₂O₄ spinel.     

Mechanical properties of lightweight aggregates

Lightweight aggregates (LWAs) were successfully produced both in a pilot-scale rotary kiln and in a laboratory chamber furnace. The mechanical properties of LWA were investigated in detail applying the European standard crushing resistance test (CR-test) as well as the single pellet compression test (spc-test). The spc-test showed that LWA pellets with porosities <82% behave similar to solid brittle spheres under compression when considering only the solid fraction of the pellet and the strength may be calculated according to σ_crit = F_crit/d² where σ_crit is a porosity independent strength, F_crit is the measured load at failure and d the solid diameter (assuming zero porosity). It was reasoned that catastrophic failure was due to tensile stresses in the centre of the pellet and the strength was observed to increase exponentially with decreasing sample size. The relationship between the CR- and spc-test has been established facilitating “translation” of strength data between the two different test methods.     

The effect of the addition of alumina on the viscosity,surface tension,and foaming efficiency of 2.5(CaO/SiO2)-xAl2O3-yFeO-MgO melts

Viscosity and surface tension strongly influence the efficiency of slag foam in metallurgical processes. An excellent foaming slag preserves heat and lowers the cost of smelting in an electric furnace. In this study, we investigated the viscosity, surface tension, and foaming efficiency of a 2.5CaO/SiO₂-xAl₂O₃-yFeO-MgO slag. We also investigated the different valence oxygen ions by X-ray photoelectron spectroscopy (XPS). The results showed that with a gradual increase in the content of Al₂O₃, the viscosity initially increased and then decreased, and the changes in surface tension followed a similar pattern. The change in viscosity was caused by the increase in the degree of polymerization of the slag, which was determined by the competitive relationship between polymerization and the reduction in the stability of the overall network structure. Adding a small amount of Al₂O₃ to the slag slightly increased the number of Al–O–Al structures, whereas adding a large amount of the Al₂O₃ led to the formation of low-strength Al–O–Si structures, which reduced the stability of the network structure, thus reducing the viscosity. Because the surface tension is related to the concentration of non-bridging oxygens (NBOs), when the NBO content increased, the instability of the surface structure caused an increase in energy, thus increasing the surface tension. In addition, the CaO–SiO₂–5MgO-xAl₂O₃-yFeO five-element oxide in this study had the lowest surface tension at the same NBO concentration, which positively contributed to slag foaming. Finally, When the Al₂O₃ content in the system increased from 5.1 to 15.7 wt%, the foaming efficiency increased from 24.2 to 69.2 (minute‧centimeters), an increase of 286%.     

酸度及酸介质对硫酸盐掺杂的含钛高炉渣光催化还原Cr (Ⅵ)的影响

利用高能球磨法制备了钙钛矿型硫酸盐掺杂的含钛高炉渣光催化剂,探讨了该催化剂的光催化原理,并系统地研究了不同酸度及酸介质对钙钛矿型硫酸盐掺杂的含钛高炉渣光催化还原Cr (Ⅵ)效率的影响。结果表明,酸度以及酸介质对六价铬的光催化还原效率都有十分重要的影响。在强酸性条件下（pH=1.5）,六价铬的光催化还原效率最高;且随着pH值的增大,六价铬的光催化还原效率显著降低。不同酸根离子对STBBFS催化剂光催化还原六价铬的促进作用按PO₄^3-＜NO₃^-＜Cl^-＜SO₄^2-逐渐增强;在光催化反应8 h后,六价铬的还原率差别显著,分别为9.32％、48.73%、100％、100％。因此,为了获得较高的光催化效果,必须控制六价铬废水的pH值以及用硫酸调节溶液的酸度。     

Replacement behavior of Ti and Mg and selective separation of MgxTi3-xO5 (x=1, 0.9, 0.75, 0.6) from Ti-bearing electric furnace smelting slag via super-gravity

Ti-bearing electric furnace smelting slag produced from direct reduction and electric furnace smelting process of vanadium titanomagnetite ore, contains a high TiO₂ content of 40–55 wt%. While a mass of Mg, Al, Ca and Si are closely mixed with Ti in the slag, which greatly limits the recovery of Ti resource from the slag. In this work, the replacement behavior of Ti and Mg in Mg_xTi_3-xO₅ was studied, and selective separation of various Mg_xTi_3-xO₅ phases from Ti-bearing electric furnace smelting slag was conducted via super-gravity. It was found that Mg could replace Ti in Ti₃O₅ and form the Mg_xTi_3-xO₅, and increasing of cooling rate greatly limited the doping of Mg into Mg_xTi_3-xO₅, the Mg_xTi_3-xO₅ was transformed from (MgTi₂)O₅ to (Mg_0.9Ti_2.1)O₅, (Mg_0.75Ti_2.25)O₅ and (Mg_0.6Ti_2.4)O₅ respectively. On this basis, various Mg_xTi_3-xO_{5 (x = 1, 0.9, 0.75, 0.6)} phases were selectively separated from the smelting slag via super-gravity, where the mass fraction of TiO₂ was increased from 78.69 to 90.32 wt% while that of MgO was decreased from 13.56 to 6.98 wt% with the increase of Ti/Mg ratio in Mg_xTi_3-xO₅. Moreover, the replacement mechanism of Mg and Ti was confirmed from characterization of crystal structure and lattice parameter of various separated high-purity Mg_xTi_3-xO₅ crystals.     

The Effect of Ozonation and Filtration on AOC (Assimilable Organic Carbon) Value of Water from Eutrophic Lake

The effects of applying ozone into the source water of Cheng-Ching Lake Water Works (CCLWW) on the analysis of AOC (assimilable organic carbon) were compared in the laboratory and pilot-scale tests. CCLWW takes its raw water from an eutrophic lake. A pilot plant, established in CCLWW in southern Taiwan, was performed to improve the quality of water obtained by the former treatment processes. The direct application of ozone to the source water of CCLWW is called the pre-O₃ process. The post-O₃ process involves the treatment of effluent with ozone through a sand filter, following other treatments, including pre-O₃, coagulation and sedimentation. In a laboratory test, a 0.45 μm membrane filter was used to replace the facility of filtration for a sand filter. AOC_Total comprises AOC_P17 and AOC_NOX, which were determined using the P. fluorescens strain P17 and the Spirillum species strain NOX, respectively. During over 2 years' sampling in eutrophic lake, it revealed that AOC_P17 contributed substantially to AOC_Total. However, the filtrate from the source water obtained by filtering through a 0.45 μm membrane filter had an AOC_Total much lower than that of the source water, especially for the considerable decrease of AOC_P17. Also, the AOC value in source water is increased with algae number but not with NPDOC (non-purgeable dissolved organic carbon). This result indicated that algae numbers existing in the eutrophic lake might affect the analysis of AOC. Following the pre-O₃ process at the pilot-scale plant, the AOC_P17 was markedly lower than that of the source water, and AOC_NOX was slightly higher than that of the source water. However, when post-O₃ was added to the effluent from a sand filter at the pilot-scale plant, AOC_NOX exceeded that before post-O₃, while AOCP17 differed slightly from that before post-O₃. Apparently, this difference may be due to the algae number existing in the water samples. These results were verified by applying ozone to the source water, and to filtrate obtained by filtering through a 0.45 μm membrane filter in a lab-scale test, respectively.     

Comparative kinetic study of coal gasification with steam and CO<Subscript>2</Subscript> in molten blast furnace slags

To make a comparison between coal gasification in molten blast furnace slag (MBFS) in different ambience and choose an appropriate agent to recover BF slag’s waste heat entirely, coal gasification with steam and CO₂ in molten blast furnace slags was studied by isothermal thermo-gravimetric analysis. The effects of temperature and addition of MBFS were studied. Carbon conversion and reaction rate increased with increasing temperature and MBFS. Volumetric model (VM), shrinking core model (SCM), and diffusion model (DM) were applied to describe the coal gasification behavior of FX coal. The most appropriate model describing the coal gasification was SCM in steam ambience and VM in CO₂ ambience, respectively. The reaction rate constant k(T) in CO₂ ambience is greater than that in steam ambience, which means the gasification reactivity of coal in CO₂ ambience is better than that in steam ambience. BF slag can effectively reduce the activation energy E_A of coal gasification reaction in different ambiences. But, the difference of activation energies is not large in different ambiences. Based on the results of kinetic analysis including k(T) and E_A calculated by the established model, CO₂ was chosen to be the most appropriate agent.     

Optimization of coal reburning in a 1 MW tangentially fired furnace

This paper deals with an experimental study on the influence of coal reburn on NO_x reduction efficiency, unburned carbon in fly ash and the furnace temperature distribution along the height in a 1 MW (heat input power) tangentially firing furnace with multiple low NO_x control technologies. Several variables associated with the reburn system have been investigated in the experiment which includes the air stoichiometry in reburn zone, the location of reburn burner and reburn coal fineness. The optimum location of reburn nozzles has been found where NO_x reduction efficiency is highest. With the decrease of reburn coal size (average diameter from 53.69 μm to 11.47 μm), NO_x reduction efficiency increases slightly, but the burnout performance of coal is improved noticeably. In the process of coal reburning, the temperature of flue gas is 70–90 °C lower in primary combustion, but 130–150 °C higher at the top of furnace as compared to baseline.