The authors have studied integrated steelworks that are able to achieve carbon neutrality. The process used in the study was a combination of a blast furnace and a pre-reducing furnace. The cases where hydrogen and ethylene were used as fuel were compared based on numerical simulations. As a result, it was found that when hydrogen was used, the reduction in CO₂ emissions from the entire steelworks was only 45%, whereas when ethylene was used, CO₂ emissions from the entire steelworks could be reduced by 86%. Ethylene can be obtained by the dehydration reaction of bioethanol, and can also be synthesized using surplus renewable electricity, which is expected to increase in the future, and CO₂ emitted from steelworks.

The coke-oven wastewater contains high concentrations of ammonia, COD (chemical oxygen demand) and toxic compounds such as phenols, cyanides, and thiocyanate. This wastewater is one of the most toxic industrial wastewater. Although the activated sludge process has been applied to treatment of the coke-oven wastewater, the treatment was occasionally deteriorated by inhibition of toxic compounds. Especially, the nitrification step was sensitive to toxic compounds, nitrite (NO₂⁻) was often accumulated due to incomplete nitrification. Nitrite is toxic and has a negative effect on COD degrading bacteria. The complete nitrification was required for stable treatment.

In this study, aiming at simultaneous removal of ammonia and COD from the coke-oven wastewater, a bench-scale nitrification/denitrification plant was operated. By long-term acclimation and the addition of an iron source, the complete nitrification was stably occurred for about 8 months, and a nitrogen removal efficiency of nearly 70% was achieved. In addition, COD in the influent was consumed as an organic source for denitrification, and the addition of an organic source was able to be minimized.

Used plastic waste flowing into ocean has become a worldwide problem. In recent years international trade in used waste plastics has been regulated. Therefore, a large amount of used plastic should be disposed domestically. On the other hand, used waste plastics with high calorific value could be used as an energy source. Then a gasification process of used plastics using fluidized bed has been newly developed. In this process, used plastics were decomposed in a fluidized bed reactor at around 600°C which was lower temperature than that used in current commercial processes. Higher calorific value gas could be attained by a gasification reaction control at the lower temperature. Hydrogen enriched gas generated from the water gas shift reaction of the basic oxygen furnace gas was used as fluidizing, or gasifying agent since hydrogen was considered to have an effect for promoting the decomposition reaction of hydrocarbon in used plastics. As fluid medium in the reactor, catalysts were used to improve gasification efficiency. In this study, the effect of gasification temperature and the type of the catalyst on a calorific value of produced gas and a gasification efficiency were investigated. High calorific value gas (LHV: 5000 kcal/Nm³) could be successfully produced from pyrolysis of used plastics by appropriate gasification temperature and catalyst.

Steel slag, a by-product of the steelmaking process, contains free-lime which can expand upon hydration. This expansion characteristic necessitates pre-aging when used as a subbase material. Various countries regulate the expansion characteristics of steel slag through different test methods and standards. However, the temperature conditions of these tests and their correlation to actual environments are not clearly defined. This study aimed to evaluate the temperature dependence of the immersion expansion ratio of steel slag for road construction through expansion tests. The results identified the rate-limiting step in the hydration process and calculated the reaction rate constant. The study compared the immersion expansion test method with actual environments to quantitatively assess the degree of acceleration in the tests. It was found that the hydration of steel slag follows a pseudo-first-order surface reaction. By analyzing the temperature changes in the water tank, the study estimated the continuous immersion expansion test period corresponding to the JIS method. The 80°C continuous immersion expansion test period of 4.1–4.6 days was equivalent to the 10-day JIS immersion expansion test. The JIS immersion expansion standard was found to correspond to a legal service life of 10 years or more.

High phosphate slag fertilizer (SHP) contains phosphorous, calcium, magnesium and silicon, expected to be effective in supplying plant nutrients and improving soil acidity. We clarified the characteristics and effects of the SHP as a fertilizer. Nippon Steel Corporation prepared the SHP prototypes by concentrating phosphorus through high-temperature processing. Five types (SHP12, SHP15, SHP18-1, SHP18-2, SHP27-2) were examined through dissolution test, pot cultivation experiment, and evaluation of improving effect on soil acidity. The SHP12 was insoluble in water but soluble in various acids and most SHP contained more than 95% of acid-soluble phosphate and silicate. Above-ground growth and phosphate uptake of spinach was higher in the SHP12 plot than in no-phosphate, ammonium phosphate and fused phosphate plots, indicating that phosphorus in the SHP12 could be absorbed more easily than the common phosphate fertilizers. Above-ground growth and phosphate uptake of wheat was also high in the SHP12 and SHP15 plots, which was comparable to ammonium phosphate plot. However, the SHP27-2 had little effect as a phosphate fertilizer. The SHP12 and SHP15 could be expected to supply calcium and silicon as well as phosphorous. Although most SHP showed the same ability to improve soil acidity as calcium carbonate, it might be necessary to avoid excessive increase in soil pH depending on the soil condition and the ability of SHP. We can conclude that if the composition of the SHP is optimized, it can be used as a fertilizer with multiple effects depending on soil types, nutrient status, and crop types.

Since 10 million tons of steelmaking slag, which contains a few percent of phosphorus, are annually produced, the phosphorus amount in the slag is equivalent to the annual import volume of phosphorus rock in Japan. Therefore, the steelmaking slag is attracting attention as a potential phosphorus resources. Phosphorus-concentrated slag obtained by the dephosphorization reaction between high phosphorus hot metal and oxidizing slag at high temperature contains phosphorous comparable to that of phosphorus rock. However, because of high FeO concentration, it is difficult to use for phosphorus resources directly. In this work, the effects of pH, acid type and leaching method on the dissolution behavior of phosphorus from P-concentrated slag were investigated. As a result, phosphorus dissolution progressed at lower pH, and was promoted by the addition of citric acid, which is known as a chelate former. When nylon mill pot stirring with citric acid and alumina mill pot stirring with nitric acid were compared to impeller stirring, respectively. By combining nylon mill pot stirring and citrate leachate, phosphorus dissolution was accelerated, because the slag was pulverized during stirring and a formation of insoluble metal-phosphate was inhibited by the formation of complex ion between leached metal cation and citrate. When the slag was leached with alumina mill pot while controlling pH by nitric acid, the phosphorus dissolution ratio lowered since phosphorus ion and aluminum ion, which is supplied by the dissolution of pot and crushing ball during leaching, constructed secondary products with low solubility along with other dissolved ions.

Since it has been urgently demanded to secure the secondary phosphorus resources domestically in addition to the establishment of a stable supplying path of phosphate ore from overseas, the recovery of phosphorus from steelmaking slags is attracting attention. The phosphorus-enriched slag obtained by dephosphorizing the high phosphorus hot metal, which was prepared by the reduction of conventional steelmaking slag, is thought to be useful as a raw phosphorus resource. It was reported in our previous paper that phosphorus can be effectively separated from P-enriched slag by the citric acid leaching method. In this study, the effects of citric acid concentration in solution, solution temperature, slag/solution mass ratio, and P-enriched slag composition on phosphorus dissolution behavior were additively investigated. A phosphorus dissolution from the P-enriched slag was significantly promoted by increasing the citric acid concentration in solution. However, even if the solution temperature was increased during leaching, the speed and ratio of phosphorus dissolution from the P-enriched slag did not change. With increasing the slag/solution mass ratio, the concentration of each element in solution increased, while their dissolution ratios decreased. Furthermore, the phosphorus dissolution ratio was suppressed in the case of the P-enriched slag with low CaO and high P₂O₅ concentrations. Appropriate conditions for various factors affecting phosphorus dissolution behavior from P-enriched slag were discussed.

Steelmaking slag contains a considerable amount of phosphorus, which is widely used in human society. Phosphorus recovered from steelmaking slag will provide a new resource, and reusing the steelmaking slag, from which phosphorus has been removed, in the steel manufacturing process reduces the total slag volume. In this study, phosphorus was separated from phosphorus-concentrated slag, which was produced through the oxidation of high-phosphorus hot metal using a small amount of steelmaking slag. The leachate of the phosphorus-concentrated slag was obtained by agitating the slag in citric acid at pH=3 using a mill pot containing mill balls. To separate phosphorus from the leachate by the precipitation of calcium phosphate, the pH of the leachate was increased by adding NaOH solution to increase the pH from 3 to 11, and by adding Ca(OH)₂ solution or Ca(OH)₂ powder to increase the pH from 3 to 11. The recovery ratio of phosphorus was over 75% for these methods. The P₂O₅ content in the recovered precipitates was over 30 mass%, which is higher than that in natural phosphorus ores. It is calculated that 50% of phosphorus in hot metal could be recovered as precipitates, and when the leaching residue was recycled for hot metal dephosphorization, the amount of slag emissions was reduced by 34% compared to the current hot metal dephosphorization operation. It is suggested that phosphorus can be recovered from steelmaking slag and slag emissions can be decreased through slag treatment processes such as slag reduction, dephosphorization, acid leaching, and precipitation.

Ionic liquids are salts, and their melting points were less than 100 ºC under atmospheric pressure. In particular, ionic liquids, which melting point of them were less than room temperature under atmospheric pressure, have been researched in recent decades. Ionic liquids are well-known to designer’s solvents since configuration of cation and anion are easily to change then specificity is exhibited. In this work, novel protic ionic liquids (PILs) which are applied for CO₂ sorption materials have been synthesized using aromatic amine such as alkylpyridine derivatives and N,N-dimethylaniline and bis (trifluoromethanesulfonyl) amide due to co-existing proton conductivity and hydrophobicity. Synthesized PILs exhibited hydrophobicity since 1-octanol–water partition coefficients were more than 0. Also, their ionic conductivities were approximately 10⁻³ S cm⁻¹ at room temperature, and their mechanism of ionic conduction were different one compared with general electrolyte solution due to high viscosities. In addition, several PILs were high selectivity for CO₂ sorption compared with N₂. Furthermore, PILs gelatinized with bis[4-(1H,1H,2H,2H-tridecafluorooctyl)phenyl] terephthalates due to improvement of mobility. Some of gel–sol transition temperatures of 5 wt% gels were approximately 100 ºC. The driving force of gelation with ionic liquids might be an entropy driven. Sorption selectivity for CO₂ and N₂ were similar one before and after gelation.

It is known as Al₂O₃ dispersed with Ni particles composite (Ni/Al₂O₃) has self-healing function which recovers mechanical properties by the high-temperature oxidation of dispersoids. Ni/Al₂O₃ appear self-healing function not only by the contribution of high-temperature oxidation of Ni particles exposed on sample surface, but also contribution of grain boundary diffusion of Ni ions through Al₂O₃ matrix. The objective of this study is to investigate the influence of Ni ion diffusion on the growth of NiAl₂O₄ self-healing layers with different Ni volume fractions.

Dence Ni/Al₂O₃ were prepared and etched nitric acid to remove surface Ni particles. Samples were heat treated at 1100–1355°C for 6–48 h in air.

NiAl₂O₄ self-healing layer is observed on the sample surfaces with and without etching. The NiAl₂O₄ self-healing layer grows with parabolic manner with respect to the heat treatment time, indicating that the diffusion of Ni ion through the Al₂O₃ grain boundary is the rate-controlling. The contribution of surface Ni to the growth of NiAl₂O₄ layers is independent of heat treatment conditions due to the surface Ni particles are defined by the Ni dispersion ratio. The contribution of Ni ion diffusion increases linearly with increasing Ni volume fraction. It indicates that the amount of Ni ions diffused through the grain boundary is proportional to the Ni volume fraction.