Fluid flow and mixing in secondary metallurgy

Mixing in metallurgical vessels is determined by bulk flow, turbulence and diffusion. In this presentation, the main features of mixing liquid steel by gas stirring are discussed. The criteria for bubbling and jetting during gas injection are considered including interaction of powder load on flow regimes. It is of interest to know the domains of bubbling and jetting behaviour in order to minimize the effect of back attack which is responsible for additional wear of bottom linings and tuyere pipes. Quantitative information is given on velocity, the radius and the pumping capacity of the buoyant plume. The relations lead to the conclusion that the gas flow should be apportioned to two or more plugs in order to increase the efficiency of the pumping effect. Furthermore, the macroscopic flow pattern is substantially influenced by the ratio of height to diameter of the ladle. Especially at high ratios dead volumes near the ladle bottom are observed. Further examples for the influence of fluid flow on reactions, dispersing and separating in the ladle and tundish are given. At first approach, mixing time depends on the gas flow rate or on the specific power input. But the details are much more complicated. It is shown that the mixing process can be described by two types of models among others: the turbulent recirculation model and the model of partial volumes. The comparison of calculated concentration-time-dependencies with plant results confirms the practicability and usefulness of the model of partial volumes. It is also concluded that the water model results for ladles may be transferred to plant conditions. In total all examples that are discussed emphasize the great significance of model simulations and model calculations as instruments for process improvement and process development.     

Phasengleichgewichte des Schnittes FeOn–CaO im Dreistoffsystem FeO–Fe2O3–CaO bei Eisensättigung

Überprüfung der Schmelzgleichgewichte des Dreistoffsystems FeO–Fe₂O₃–CaO auf der wüstitreichen, eisengesättigten Seite des Systems. Angaben über die Ergebnisse der thermischen Analysen, isothermen Halteversuche und mikroskopischen Untersuchungen. Aussagen über den quasibinären Schnitt Eisen-Kalkferrit bei Temperaturen unter 1150 °C sowie über den quasibinären Schnitt Calciowüstit-Kalk über 1150 °C.     

Development of a reduction process and test results of the reduction equilibria of wustite as a function of oxygen content and temperature

A new measurement technique allows the continuous measurement of the oxygen contents of the wustite as a function of temperature without having to pass through the disintegration of the wustite to iron and magnetite and have the accompanying problems during the chemical analysis by cooling the sample. Measurements are carried out in form of isothermal oxygen reduction tests, starting with pure Fe₂O₃ in a thermogravimetric balance with an upstream gas mixer which, operating with CO₂/CO gas mixtures, ensures a graduation of the input reduction potential CO'₂ in steps of 1%. Simultaneous weighing of the test material sample permits, by means of the measured weight loss and relation to the charged amount of Fe₂O₃, a very accurate measurement of the oxygen concentration within the test material at any point in time during testing. Testing was carried out along these lines in the range between 700 and 1300°C. The results show that, at constant temperatures, the wustite oxygen contents decrease linearly from the right- to the left-hand boundary of the wustite field, with the oxygen pressure of the gas phase (log p_CO2/p_CO or log pO₂). When comparing these results with the findings of Darken and Gurry, a difference becomes apparent in the curves of the log pO₂ values as a function of the temperature. While our own curves are substantiated by very many measured values, the curves of Darken and Gurry are based on only twenty-three values at four different temperatures. Based on this result, systems of equations are developed which describe the homogeneous field of the wustite in the Fe-O-C reduction system without any gaps. An equivalent system of equations can also be developed from the values of a literature evaluation carried out by Giddings and Gordon. The curves resulting from the two equations do not differ very greatly. But the differences are large enough for corresponding variances to be expected when establishing the wustite boundaries and, thus, the position of the wustite field in the phase diagram.     

Stoffumsatz bei diskontinuierlichen und kontinuierlichen Stahlerzeugungsverfahren

Bedeutung kontinuierlicher Verfahren. Stoffumsatz im diskontinuierlich betriebenen Reaktor. Grenzflächenvergrößerung durch Emulgierung der Phasen. Kontinuierlicher Gleichstromreaktor. Verweilzeitspektrum von einem Reaktor und von Reaktorkaskaden. Stoffumsatz in der Reaktorkaskade. Stoffumsatz im Gegenstrom ohne und mit Rückvermischung. Übergangsfunktion einer 6 m langen elektromagnetischen Gegenstromrinne. Abschätzung des Grenzflächenvergrößerungsfaktors. Einfluß der Tropfenverteilungen zwischen Schlacke und Metall.     

Einfluß des Gefüges auf die mechanischen Eigenschaften von Gußeisen mit rd. 3% C und 14% Cr für Walzen

Ermittlung der mechanischen Eigenschaften unterschiedlicher Bereiche gegossener Walzen mit 360 mm Ballendurchmesser und 200 mm Zapfendurchmesser aus drei Schmelzen eines Gußeisens mit rd. 3% C, 0,8% Si, 0,8% Mn, 0,14% P, 0,014% S, 14,2% Cr, 0,35% Mo und 0,5% Ni in zwei Wärmebehandlungszuständen. Zahlenmäßiger Zusammenhang zwischen der Biegefließgrenze und geeigneten Kennwerten für den Gefügezustand unter Bezugnahme auf die Beanspruchungsrichtung. Einfluß des Gefügezustandes auf das Formänderungsverhalten im statischen Biegeversuch.     

High pressure effect on polyethylene crosslinking initiated by benzoyl peroxide

High pressure effects (300, 600, and 800 MPa) on polyethylene crosslinking initiated by benzoyl peroxide have been studied at 120°C. An increase in pressure leads to a decrease in yield of the insoluble gel; the yields at 300, 600, and 800 MPa were respectively 96, 86, and 79%. With increasing pressure the rate of polyethylene crosslinking decreases and the rate of benzoyl peroxide decomposition at 300, 600, 800 MPa can be anticipated from the time of crosslinking. The activation volume $\text{ΔV}{}^3\text{=9.1}\text{cm}{}^3\text{mol}{}^{\mathrm{?1}}$ was determined for benzoyl peroxide decomposition on polyethylene at 120°C.     

A double-blind multicenter trial comparing sertraline and citalopram in patients with major depression treated in general practice

The purpose of this double-blind, multicenter trial was to compare the efficacy and safety of sertraline (50-150 mg/day) with those of citalopram (20-60 mg/day) in patients with major depression in general practice during 24 weeks of treatment. The patients were assessed using the Montgomery-Asberg Depression Rating Scale and the Clinical Global Impressions of severity and improvement scales. Observed and spontaneously reported adverse events were recorded and side-effects were assessed by means of the UKU Side-Effect Scale. Altogether 400 patients were randomized into the study. A total of 308 patients completed the 24-week study in accordance with the protocol. A significant reduction in the total Montgomery-Asberg Depression Rating Scale scores was observed in both treatment groups as early as 2 weeks, with no statistically significant differences between the drugs. In the intention to treat-last observation carried forward analysis 76% responded to treatment in the sertraline and 81% in the citalopram group. The final mean doses were 82 mg/day (64% higher than baseline) in the sertraline group and 34 mg/day (70% higher than baseline) in the citalopram group. The response rate in completers in accordance with protocol was 90% in the sertraline group and 93% in the citalopram group. The side-effects were those usually seen, and both sertraline and citalopram were considered to be well tolerated. It was concluded that patients with major depression in general practice respond well to 24 weeks of treatment with sertraline or citalopram. With regard to efficacy, no statistically significant differences were found between the drugs.