Kinetic model of desulphurisation by powder injection and blowing in RH refining of molten steel

Abstract

The desulphurisation by powder injection and blowing in the RH refining of molten steel and its mechanism have been considered and analysed. Based on the two-resistance mass transfer theory and the mass balance of sulphur in the system, a kinetic model for the process has been developed. The related parameters of the model, including the mass transfer coefficients and the effective amount of powder in the molten steel being treated for desulphurisation, have been reasonably determined. Modelling and predictions of the process of injecting and blowing the lime based powder flux under assumed operating modes with the different initial contents of sulphur and amounts of powder injected and blown in a RH degasser of 300 t capacity have been carried out using the model. The relevant circulation rate of the liquid steel and the powder injection and blowing rate were taken to be 100 t min^-1 and 150 kg min^-1, respectively. The initial contents of sulphur in the liquid steel to be treated and the amounts of powder injection and blowing were respectively assumed to be 0·007, 0·006, 0·005, 0·004, 0·003, 0·002 wt-%and 10, 8, 6, 5, 4, 3 kg/t steel. The total treatment time for desulphurisation under each mode was set at 24 min, equivalent to eight circulation cycles of the liquid steel to be treated. The results indicated that the predictions made by this model are in good agreement with data from industrial experiments and practice. By injecting and blowing the lime based powder flux with the composition of 85 wt-% lime (CaO) + 15 wt-%fluorspar (CaF₂ ) of 3–5 kg/t steel, it is possible to decrease the sulphur content in the molten steel to an ultralow level below (5–10) × 10^-4 wt-%from (60–80) × 10^-4 wt-%. The total treatment time needed will be 12–20 min. Intensifying the powder injection and blowing operation and increasing the circulation rate of the liquid steel may effectively increase the rate of the process in RH refining. The model may be expected to offer some useful information and a reliable basis for determining the reasonable process parameters and help in optimising the technology of desulphurisation by powder injection and blowing in the RH refining of molten steel.     

钢包浸渍罩钢液喷粉脱硫试验研究

采用２５０ｋｇ感应炉研究了钢包浸渍罩钢液喷粉脱硫工艺，热态模拟试验表明：该工艺可以将钢液硫含量脱除到０．００３０％以下。同时还可降低钢中氮含量。     

Examples of How Fundamental Knowledge can Improve Steelmaking: Desulphurisation Kinetics Calcium and Modification

Sulphur control is an essential part of steel production. This paper summarises two aspects of sulphur in secondary metallurgy. First, it is shown that silicon can contribute to ladle desulphurisation if the ladle slag is low in silica; the effect of silicon is primarily on the equilibrium sulphur level, rather than a specific kinetic effect. Second, sulphur is shown to capture calcium (as calcium sulphide) upon calcium injection to modify inclusions. In steels with less than approximately 100 ppm sulphur, the calcium sulphide subsequently back-reacts with alumina inclusions, to modify the oxide inclusions to calcium aluminates.     

Thermodynamics of Phosphorus and Sulphur Removal during Basic Oxygen Steelmaking

Removal of impurity elements from hot metal is essential in basic oxygen steelmaking. Oxidation of phosphorus from hot metal has been studied by several authors since the early days of steelmaking. Influence of different parameters on the distribution of phosphorus, seen during the recent work of the authors, differs somewhat from that reported earlier. On the other hand, removal of sulphur during steelmaking has drawn much less attention. This may be due to the magnitude of desulphurisation in oxygen steelmaking being relatively low and desulphurisation during hot metal pre‐treatment or in the ladle furnace offering better commercial viability. Further, it is normally accepted that sulphur is removed to steelmaking slag in the form of sulphide only. However, recent investigations have indicated that a significant amount of sulphur removed during basic oxygen steelmaking can exist in the form of sulphate in the slag under oxidising conditions. The distribution of sulphur during steelmaking becomes more important in the event of carry‐over of sulphur‐rich blast‐furnace slag, which increases sulphur load in the BOF. The chemical nature of sulphur in this slag undergoes a gradual transition from sulphide to sulphate as the oxidative refining progresses.     

Optimisation of ladle slag composition by application of sulphide capacity model

Abstract

The sulphur distribution ratio for ladle slags at Ovako Steel AB has been determined by calculating the sulphide capacity using the Royal Institute of Technology (KTH) model and the alumina activity via an empirical expression suggested by Ohta and Suito, and by using the well established theories for sulphur refining. A parameter study was carried out based on plant data from a previous study on desulphurisation. The effect of carbon and aluminium in the steel, the temperature of the steel, and alumina and lime in the slag on the sulphur distribution ratio was investigated. It was shown that the Al₂ O₃ /CaO ratio had the largest influence on the conditions studied, and that when it increased the sulphur distribution ratio decreased. Based on these results, new plant trials were carried out, in which the alumina content in the slag was changed. The calculated sulphur distribution ratios for these trials were found to be in good agreement with experimentally determined sulphur distribution ratios. It is concluded that the present approach can be used to optimise multicomponent ladle slags with respect to sulphur refining.     

Influence of sulphur addition by cored wire injection on machinability of grey iron

Abstract

Sulphur powder was added to grey iron melts using cored wire injection technology to improve machinability. The influence of different injection speeds into the ladle on sulphur recovery efficiency was studied, followed by an evaluation of the microstructure, mechanical properties and machinability of the cast iron. The peak sulphur recovery efficiency is 82·5%. Machinability increases with sulphur content.     

Slag Emulsification during Liquid Steel Desulphurisation by Gas Injection into the Ladle

According to current practice, the desulphurisation of steel melts is mainly performed in the ladle with lime saturated top slags. The injection of argon gas into liquid steel provides for intensive mixing and accelerated sulphur transfer. During gas stirring treatment, an emulsification of top slag takes place. A numerical model describing the course of the desulphurisation reaction has been developed, which takes all relevant process parameters into account, including those of the emulsification effect. To check the validity of the model, operational tests have been performed on 185‐t‐heats. Results obtained from model calculations and operational tests show an excellent agreement.     

Thermodynamic and kinetic aspects of the desulphurisation reaction in secondary metallurgy

The desulphurisation of liquid steels with a refining ladle top slag is one of the most important processes of secondary metallurgy. But the process control is even now based on empirical rules. Deviations from the sulphur contents aimed at can often be observed in practice. An improvement of the process control supported by knowledge of the thermodynamics and kinetics of the desulphurisation reaction is of great importance, especially for the production of steels with very low sulphur contents. To investigate the removal of sulphur, operational trials were carried out in 185 t steel ladles. In the trials the stirring gas flow rate, the pressure on the bath surface and the steel composition were varied as process parameters. The obtained results show that ladle slags saturated with lime have a high sulphide capacity. The desulphurisation rate of steel melts increases if the specific stirring energy is enhanced by increasing the gas flow rate or reducing the pressure on the bath surface. In the case of production of Al/Si‐killed steel melts, a desulphurisation degree above 90 % can be obtained by a vacuum treatment within 10 minutes.     

Research on slag modifying agents for CaO–Mg based hot metal desulphurisation

Abstract

To improve the slag/iron separation problems in CaO–Mg based hot metal desulphurisation, experiments were carried out to generate a more fluid slag and to reduce the amount of hot iron entrapped within the desulphurising slag. The optimised slag modifying agent effectively decreased the melting temperature and viscosity of CaO–Mg based desulphurising slag. The optimised modifying agent has been successfully applied to industrial production. Industrial tests show that the average desulphurisation exceeded 82% when using the 80CaO–15Mg–5CaF₂ desulphurising agent at 3·5–4·0 kg t^–1, and with an average final S of 0·005%. The skimming time decreased from nearly 12 min to ～6·5 min. The heat loss during desulphurisation decreased to ～19°C which is favourable to the subsequent steelmaking process. Average total Fe content in post-desulphurisation slag is 34% with 0·7–0·8 kg t^–1 modifying agent, a decrease of ～30% in iron loss compared with current status.     

Study of optimal Ca-Si injection position in gas stirred ladle based on water model experiment and flow simulation

Abstract

A mathematical model based on a computational fluid dynamics technique named SOLA-SURF and the k-? two equation turbulence model, and a water model that is one-quarter the scale of an actual ladle system, have been developed in the present study to find the optimal Ca-Si injection position for the ladle secondary refining process. Based on experimental measurements and observations from the water model and simulated results of the mathematical model, which has been verified by the experimental measurements, fluid flow phenomena and corresponding diffusion of the injected Ca-Si under various design and operating conditions have been investigated. The water model experiments and simulated results from the mathematical model for actual ladle operation show that, with an injection depth of 0·8 m and argon gas flowrate of 100, 200, and 300 L min^-1, the optimal position for injection is located on the tuyere-circle centre plane, opposite side to the tuyere, and 0·5-0·8R away from the circle centre. For this injection position, the mixing time is shorter and the flow pattern favours transport of the additive to the bottom of the ladle.     

Desulphurisation of high sulphur coal to improve industrial utilisation

Abstract

Coal from northeastern India is considered to be a very good quality coking coal owing to its low ash content (<8%). It has an excellent caking property, and after carbonisation it can provide a very good CSR (coke strength, after reaction with CO₂). On the other hand, its metallurgical application is not so encouraging as a result of its high sulphur content (3-7%). Normally the acceptable sulphur content in metallurgical coke is 0·7% maximum. About 90% of the sulphur is present in organic form, and the remaining 10% as pyrites and sulphates. Removal of sulphur from the organic types is somewhat complicated as it cannot be done by any physical process such as benificiation or flotation, whereas sulphur in pyrites and sulphates can be removed by physical means. With a view to lowering the sulphur content of the coal, desulphurisation studies were carried out at elevated temperatures ranging from 400 to 950°C under the flow of various reducing gases such as coke oven gas, water gas, steam and pure hydrogen. The retention time at each temperature of experimentation was usually varied at 15, 30 and 45 min. It was observed that the sulphur content in the coal was decreased in every experiment with the various reducing gases. The extent of sulphur removal in treatment of the coal with coke oven gas was up to 78·1%, that with steam 83·1% and that with water gas 85·7%. The minimum sulphur content found in desulphurised coal was 0·86%, which could not be used for metallurgical purposes unblended. In the case of experiments with steam, a severe loss of carbon (15-20%) was observed because of reaction with the steam, whereas no such losses were observed in experiments with the other two reducing gases. Hydrogen lowered the sulphur content at a faster rate in comparison with the other gases, but could not increase the degree of reduction. The present study may help to establish the technoeconomic feasibility of the processes; however, large scale experimental studies are required for confirmation. I&S/1721     

Smelting vanadium steel by direct alloying with self-reduction briquette of V2O5

Abstract

Experimental research on smelting vanadium steel by direct alloying with self-reduction briquettes of V₂O₅ has been conducted in a medium frequency induction furnace of the laboratory and in a 60 t ladle of Shijiazhuang Iron and Steel Co. Ltd. The result indicates that the amount of the reductant (FeSi) and the flux (CaO) inside the self-reduction briquette of V₂O₅ have effects on the yield of vanadium during the direct alloying, and the optimum composition of the self-reduction briquette of V₂O₅ is 23·88V₂O₅–29·75FeSi–15·93CaO–30·44CaF₂. When smelting vanadium steel by direct alloying in a 60t ladle, the average yield of V is 96·54%. Meanwhile, the steel product could achieve the required quality level.     

Sulphur removal in ironmaking and oxygen steelmaking

Sulphur removal in the ironmaking and oxygen steelmaking process is reviewed. A sulphur balance is made for the steelmaking process of Tata Steel IJmuiden, the Netherlands. There are four stages where sulphur can be removed: in the blast furnace (BF), during hot metal (HM) pretreatment, in the converter and during the secondary metallurgy (SM) treatment. For sulphur removal a low oxygen activity and a basic slag are required. In the BF typically 90% of the sulphur is removed; still, the HM contains about 0.03% of sulphur. Different HM desulphurisation processes are used worldwide. With co-injection or the Kanbara reactor, sulphur concentrations below 0.001% are reached. Basic slag helps desulphurisation in the converter. However, sulphur increase is not uncommon in the converter due to high oxygen activity and sulphur input via scrap and additions. For low sulphur concentrations SM desulphurisation, with a decreased oxygen activity and a basic slag, is always required.     

AOD/CLU process modelling: optimum mixed reductants addition

Abstract

To increase the productivity of the AOD/CLU converter process in stainless steelmaking, single slag practice, i.e. simultaneous top slag reduction and desulphurisation, is an attractive alternative. Furthermore, if the combined addition of reductants FeSi and Al is used, desulphurisation can be optimised. A model approach to calculating the additions of FeSi, Al, and fluxes to recover the valuable elements of the slag and to reach the aim sulphur level is presented. The paper also describes an online verification of the model, where model calculations recommended additions to a 75 t AOD converter. Finally, some aspects of model prediction are given. It was found that it is possible to obtain a sulphur content lower than 20 ppm by utilising single slag practice based on a mixed reductant addition and on the observation that the AOD converter appears to be close to thermodynamic equilibrium from a sulphur point of view after the refining operations. However, a successful operation requires strict control with respect to electric arc furnace slag carryover and crude steel silicon content.     

Numerical modelling of fluid flow and desulphurisation kinetics in an argon-stirred ladle furnace

A full-scale, three-dimensional, transient CFD modelling approach capable of predicting the three-phase fluid-flow characteristics and desulphurisation behaviour in an argon-stirred ladle was developed. The model can accurately predict the molten steel flow and slag eye behaviour. The predicted sulphur content in ladle as a function of time agrees well with the experimental data. The effects of the initial sulphur content, the gas flow rate and the slag layer thickness on the desulphurisation efficiency were also investigated. The predicted results show that the desulphurisation efficiency improves with the increase of the initial sulphur content, the gas flow rate and the slag layer thickness. Higher gas flow rate can improve the slag–steel interaction, which, in turn, helps improving the desulphurisation rate. The thinner the slag layer, the larger the slag eyes and the smaller the interfacial area between the slag and steel phases. The consequence is the decrease in the desulphurisation rate.     

Minimisation of calcium additions to low carbon steel grades

Abstract

This investigation aimed to determine the in plant feasibility of decreasing, to an amount close to the minimum literature value, the calcium addition to liquid steel for prevention of alumina buildup during continuous casting. Six plant trials were carried out at calcium additions of 0·14 kg/t of steel (reduced from the original 0·19 kg/t), added to the second ladle of a two or three ladle sequence. Total oxygen samples were taken at the ladle furnace and tundish to determine total oxygen and nitrogen contents of the steel. The total oxygen content at the ladle furnace varied between 19 and 26 ppm, with a slight degree of reoxidation between the ladle furnace and the caster. Alumina clogging was successfully prevented by the addition of 0·14 kg calcium/t of steel during the first five trials. During the sixth trial the submerged entry nozzle (SEN) failed and, although the stopper behaved as if clogging occurred, this behaviour was caused by the poor perfomance of the SEN rather than actual clogging. Microanalysis of inclusions in steel samples revealed a distribution in degrees of modification between different inclusions, and the formation of a substantial amount of CaS (which is taken to indicate overmodification, based on equilibrium calculations). However, the CaS is mostly associated with at least partly liquefied oxide inclusions, which is likely to reduce the potential clogging effect of solid CaS.     

Kinetics of desulphurisation of molten pig iron

The analysis of the experimental data obtained in the present work and also of some classical data reported in literature on the basis of simple rate theory, surface renewal theory and the multicomponent mixed transport control theory has shown that in molten pig iron containing silicon the sulphur transfer to slags is controlled by mass transport in the metal phase. In the initial stages the desulphurisation rate is fast but after a few minutes it slows down. In the case of silicon containing iron this second slow stage, as also observed by other workers in the past, is due perhaps to the formation of a micro layer of silica enriched reaction product at the slag-metal interface. On addition of aluminium the silica layer gets reduced and sulphur transfer is enhanced; once again the rate controlling step becomes mass transport in the metal phase. An important conclusion regarding the timing of aluminium is that maximum benefit of aluminium addition can be realised after about 4–5 min of adding the desulphurisation flux.     

Inclusion composition control in tyre cord steel by top slag refining

Abstract

In order to improve the inclusion type and composition in tyre cord steel, ladle furnace refining has been simulated by laboratory experiments and thermodynamic calculation. It was found that slag metal reaction time and top slag composition have an important influence on the inclusion compositions in the final steel. To produce the desired low melting point ductile inclusions the optimum conditions were: reaction time 60 min, basicity (CaO/SiO₂) of top slag in the range of 1·0–1·2 and Al₂O₃ content of slag in the range of 3–9 mass-%. These were then confirmed in industrial trials.