Effect of the Basicity on Mineralogical Phases and Micro-Structure of Dephosphorization Slag in the New Double Slag Converter Steelmaking Process

In the present work, the effect of the basicity at the lower range from 0.98 to 2.13 on dephosphorization of hot metal at 1623 K was studied through high-temperature laboratorial experiments. With the increase of the basicity from 0.98 to 2.13, the P and C contents in hot metal rapidly decrease and increase at first, and then gradually decrease and increase, respectively. From the scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD) results, with the increase of the basicity, the phase containing the high P content changes from the matrix phase into the phosphorus (P)-rich phase. Under the present experimental conditions, the P-rich phase can only be precipitated from the liquid slag when the basicity is higher than 1.55, which is a benefit to the dephosphorization. As the Raman intensity of the P-O-Ca structure unit in the P-rich phase is significantly higher than that of the P-O-Si structure unit, most of the phosphorus in the P-rich phase exists in the P-O-Ca structure unit and a small amount of phosphorus exists in the P-O-Si structure unit. With the increase of the basicity of the dephosphorization slag, the activity coefficient of P2O5, γ(P2O5) , in the liquid phase decreases, while the basicity in the liquid phase increases.

Effect of Fe2O3 Content and Acid on the Leaching Behavior of Phosphorus from Dephosphorization Slag

Dephosphorization slag contains considerable quantities of valuable components, such as P2O5 and FeOx. To recover P from dephosphorization slag, selective leaching has been adopted to separate the P-concentrating mineral phase. In this study, the effect of Fe2O3 content in slag and acid on the leaching behavior of P from dephosphorization slag was investigated. It was found that a higher Fe2O3 content in slag resulted in a higher P2O5 content in the C2S–C3P solid solution. Increasing the Fe2O3 content in slag promoted the dissolution of P and simultaneously suppressed the dissolution of other elements, facilitating the selective leaching of P. In the hydrochloric acid solution, more than 81% of P could be dissolved from dephosphorization slag at pH 4, and the dissolution ratio of Fe was nearly zero, achieving excellent selective leaching. Although better selective leaching was also realized in the citric acid solution at pH 5, hydrochloric acid was considered the appropriate leaching agent from the perspective of leaching cost. Through selective leaching, almost all the C2S–C3P solid solution was dissolved from dephosphorization slag, and the Fe-bearing matrix phase and magnesioferrite remained in the residue. The residue with low P2O5 content can be reutilized in ironmaking or steelmaking processes.

Study on Slag Forming Route of Dephosphorization in Combined Blown Converter

In order to achieve the purpose of high-efficiency dephosphorization by single-slag method during the combined blown converter steelmaking process, the CaO-SiO2-FetO-MgO-MnO-P2O5 slag system was taken as the research object, and the slag-forming route of dephosphorization was studied. The effects of slag compositions on the liquidus and the contour map of phosphorus distribution ratios (recorded as Lp) were calculated by thermodynamics software FactSage, and then the theoretic slag-forming route of dephosphorization was obtained. The effects of slag compositions on dephosphorization rate and Lp were studied by a high-temperature experiment. Based on the results of the theoretic calculation and high-temperature experiment, the actual slag-forming route of dephosphorization by the single-slag process in the combined blown converter was obtained: The initial slag composition should be around 15.0%CaO-44.0%SiO2-41.0%FetO. The composition of high-efficiency dephosphorization slag should be around 50.8%CaO-24.2%SiO2-25%FetO. The final slag composition should be around 65.6%CaO-28.3%SiO2-6.1%FetO. After using the actual slag-forming route in the production, the dephosphorization rate was increased by 3.6%, and the consumption of slagging materials was reduced by 3.78 kg/t.

Dephosphorization in Double Slag Converter Steelmaking Process at Different Temperatures by Industrial Experiments

In the present work, the effect of dephosphorization endpoint temperature on the dephosphorization of hot metal was studied for the double slag converter steelmaking process under the conditions of low temperature and low basicity by the industrial experiments. In the temperature range of 1350–1450 °C, with an increasing dephosphorization endpoint temperature, the dephosphorization ratio and phosphorus distribution ratio first increase and then decrease. The phosphorus content in hot metal first decreases and then increases at the end of dephosphorization. At the dephosphorization temperature range of 1385–1410 °C, the dephosphorization ratio is higher than 55%, the P2O5 content in the dephosphorization slag is 3.93–4.17%, the logLP value is 1.76–2.09, the value of PCOP-C of the selective oxidation reaction of carbon and phosphorus is 53–80 Pa, and the aFeO value is 0.284–0.312. The path of phosphorus in hot metal entering the P-rich phase of dephosphorization slag can be reasonably inferred as: hot metal → Fe-rich phase → P-rich phase. Under the present industrial experimental conditions, the dephosphorization and rephosphorization reactions are in dynamic equilibrium at 1413 °C. Considering the experimental results and thermodynamic calculation results of industrial experiments by the double slag dephosphorization process, the optimal temperature range for intermediate deslagging is about 1400–1420 °C.

Mechanism of Phosphorus Enrichment in Dephosphorization Slag Produced Using the Technology of Integrating Dephosphorization and Decarburization

In order to better understand and develop the technology of integrating dephosphorization and decarburization in a single converter (abbreviated as IDDSC), the relevant thermodynamic issues were discussed by calculation. Based on the thermodynamic calculation, the bridges between the phosphorus distribution ratio, temperature, and slag composition were constructed. Besides, the connections between the dephosphorization behavior and the microstructure of slag were also established by investigating four heats of hot metal smelt using IDDSC technology. As a result, the mechanism of phosphorus enrichment in the dephosphorization slag was revealed. Also, the results show that the dephosphorization efficiency increases gradually with increasing slag basicity. While the dephosphorization efficiency increases first and then decreases with the increase of FeO content in slag. There is a competition relationship between P2O5 and FeO in reacting with CaO and SiO2. When CaO/FeO is relatively high, not enough FeO is provided. Thus P2O5 is in priority to react with CaO and SiO2 through [3n + 2](CaO) + 2SiO2 + n(P2O5) = n(3CaO·P2O5)-2CaO·SiO2(s), generating P2O5-rich nC2S-C3P solid solution which promotes the removal of [P] from the hot metal. When CaO/FeO is relatively low, FeO competes over P2O5 in reacting with CaO and SiO2 through a(CaO) + b(SiO2) + c(FeO) = aCaO·bSiO2·cFeO(s), generating CaFeSiO4 instead of P2O5-rich solid solution. As a consequence, the slag with low CaO/FeO shows a poor dephosphorization ability.

Dephosphorization mechanism and phase change in the reduction of converter slag

In the steelmaking process, the remained converter slag with P-containing materials is harmful to the molten steel in the next furnace. By the slag-splashing process with air blowing, P in the molten slag can be reduced and separated from the slag in the form of gas. In this paper, the reduction experiments of converter slag were carried out by flowing N2 at high temperatures, and the P-containing gas formed in this process was cooled and collected in water. It is found that the P gas was mainly composed of P2, which matched well with the results predicted by the FactSage7.2 calculation. The reaction mechanism of gasification dephosphorization in molten slag was analyzed, which was mainly proceeded on the surface of the slag. SEM-EDS and XRD were used to detect and characterize the samples. The ore phase of the converter slag and the gasification dephosphorization slag was determined, and the phase change was analyzed and compared.