In order to elucidate the decomposition mechanism of Na2CO3 and Li2CO3 in mold-powder systems employed in the continuous casting of steel, decompositions of Na2CO3 and Li2CO3 were investigated using thermogravimetric (TG) and differential scanning calorimetric (DSC) methods at temperatures up to 1200 °C, under a flow of argon gas. For the case of pure Na2CO3, the thermal decomposition started from its melting point and continued as the temperature was increased, but at a very slow rate. For Li2CO3, however, the decomposition occurred at much faster rates than that for Na2CO3. When carbon black was added to the carbonate particles, the decomposition rates of both Na2CO3 and Li2CO3 were significantly enhanced. From mass-balanced calulations and X-ray diffraction (XRD) analyses of the reaction products, it is concluded that decompositions of Na2CO3 and Li2CO3 with carbon black take place according to the respective reactions of Na2CO3 (1) + 2C (s) = 2Na (g) + 3CO (g) and Li2CO3 (l) + C (s) = Li2O (s) + 2CO (g). It was found that liquid droplets of Na2CO3 were initially isolated due to carbon particles surrounding them, but, as the carbon particles were consumed, the liquid droplets were gradually agglomerated. This effected a reduction of the total surface area of the carbonate, resulting in a dependence of the decomposition rate on the amount of carbon black. For the case of Li2CO3, on the other hand, hardly any agglomeration occurred up to the completion of decomposition, and, hence, the rate was almost independent of the amount of carbon black mixed. The apparent activation energies for the decomposition of Na2CO3 and Li2CO3 with carbon black were found to be similar and were estimated to be 180 to 223 kJ mole−1.

Thermal and carbothermic decomposition of Na 2 CO 3 and Li 2 CO 3

Bubble formation during gas injection into turbulent downward-flowing water is studied using high-speed videos and mathematical models. The bubble size is determined during the initial stages of injection and is very important to turbulent multiphase flow in molten-metal processes. The effects of liquid velocity, gas-injection flow rate, injection hole diameter, and gas composition on the initial bubble-formation behavior have been investigated. Specifically, the bubble-shape evolution, contact angles, size, size range, and formation mode are measured. The bubble size is found to increase with increasing gas-injection flow rate and decreasing liquid velocity and is relatively independent of the gas injection hole size and gas composition. Bubble formation occurs in one of four different modes, depending on the liquid velocity and gas flow rate. Uniform-sized spherical bubbles form and detach from the gas injection hole in mode I for a low liquid speed and small gas flow rate. Modes III and IV occur for high-velocity liquid flows, where the injected gas elongates down along the wall and breaks up into uneven-sized bubbles. An analytical two-stage model is developed to predict the average bubble size, based on realistic force balances, and shows good agreement with measurements. Preliminary results of numerical simulations of bubble formation using a volume-of-fluid (VOF) model qualitatively match experimental observations, but more work is needed to reach a quantitative match. The analytical model is then used to estimate the size of the argon bubbles expected in liquid steel in tundish nozzles for conditions typical of continuous casting with a slide gate. The average argon bubble sizes generated in liquid steel are predicted to be larger than air bubbles in water for the same flow conditions. However, the differences lessen with increasing liquid velocity.

/pdf/bubble-formation-during-horizontal-gas-injection-into-1nmlsrdomu.pdf

Bubble formation during horizontal gas injection into downward flowing liquid

Slag foams have been investigated with smaller bubbles than those used in the previous studies.[5,6,7] The bubbles were generated by argon gas injection with the nozzle of multiple small orifices and by the slag/metal interfacial reaction of FeO in the slag with carbon in the liquid iron. The foam stability in terms of the foam index for a bath-smelting type of slag (CaO-SiO2-Al2O3-FeO) was determined for different bubble sizes. The average diameter of bubbles in the foam was measured by an X-ray video technique. When the foam was generated by the slag/metal interfacial reaction at 1450 °C, it was found that the average bubble diameter varied from less than 1 to more than 5 mm as a function of the sulfur activity in the carbon-saturated liquid iron. The foam index was found to be inversely proportional to the average bubble diameter. A general correlation is obtained by dimensional analysis in order to predict the foam index from the physical properties of the liquid slag and the average size of the gas bubbles in the foam.

Effect of the bubble size and chemical reactions on slag foaming

A one third scale water model experiment was conducted to observe the asymmetric flow and vortexing flow inside a slab continuous-casting mold. Dye-injection experiment was used to show the evolution of the transient flow pattern in the liquid pool without and with gas injection. The spread of the dye was not symmetric about the central plane. The flow pattern inside the mold was not stationary. The black sesames were injected into water to visualize the vortexing flow pattern on the top surface. The changes of shape and location of single vortex and two vortices with time had been observed during experiments. Plant ultrasonic testing (UT) of slabs was used to analyze the slab defects distribution, which indicated that the defects are intermittent and asymmetric. A mathematical model has been developed to analyze the time-dependent flow using the realistic geometries, which includes the submerged entry nozzle (SEN), actual mold, and part of the secondary cooling zone. The transient turbulent flow of molten steel inside the mold has been simulated using the large eddy simulation computational approach. Simulation results agree acceptably well with the water model experimentally observed and plant UT results. The oscillating motions of jet and the turbulence naturally promote the asymmetric flow even without the effects of slide gate nozzle or the existence of clogs inside the SEN. The periodic behavior of transient fluid flow in the mold is identified and characterized. The vortexing flow is resulted from asymmetric flow in the liquid pool. The vortices are located at the low-velocity side adjacent to the SEN, and the positions and sizes are different. Finally, the model is applied to investigate the influence of bubble size and casting speed on the time-dependent bubble distribution and removal fraction from the top surface inside the mold.

Transient Asymmetric Flow and Bubble Transport Inside a Slab Continuous-Casting Mold

The thermodynamics of phosphate and phosphide in CaO-CaF2 melts was studied by equilibrating the melts in a graphite boat with CO-Ar mixtures and Ag-P alloys between 1400 and 1550 ‡C. The dependence of phosphate and phosphide concentrations on the partial pressure of phosphorus, the partial pressure of oxygen, and slag composition confirmed the behavior of P3 and PO 4 3 in the melts as thermodynamically expected, giving 0.52 and 8.5 x 10−4 as the values for the activity coefficients of Ca3/2PO4 and Ca3/2P, respectively.

Thermodynamics of phosphate and phosphide in CaO-CaF2 melts

Physical model of slag foaming was developed by using results of cold and hot model experiments. The bubble size at the slag/metal interface, the void fraction of foam and the film life of a bubble at the top surface of slag were calculated. The governing factors of slag foaming have been clarified with this model. It was confirmed that the bubble size at the slag/metal interface is determined basically by the static balance between the buoyancy force and the adhesive force to the slag/metal interface. It was resulted that the slag/metal interfacial tension and the surface tension of metal also affect the foam height besides the surface tension and the viscosity of slag because they change the bubble size.

Physical Model of Slag Foaming.

The formation of bubbles at a nozzle and subsequent rising behavior of them in a molten iron bath at 1250°C were observed using a high-voltage X-ray fluoroscope and a high-speed video camera. The frequency of bubble formation at the nozzle exit, the mean bubble diameter and the mean bubble rising velocity were obtained for a side range of injected argon gas flow rate. Empirical correlations of these quantities were proposed and compared with previously published experimental data and empirical correlations.It has been commonly believed that the frequency of bubble formation at a nozzle placed in a molten metal bath depends solely on the gas flow rate and the outer diameter of the nozzle when the gas flow rate is relatively high and the wettability between the nozzle material and the molten metal is bad. The present experimental results, however, revealed that the frequency of bubble formation has a close relationship not with the outer diameter but with the inner diameter of the nozzle for a higher gas flow rate. Furthermore, the bubble frequency depended on the gas flow rate and the physical properties of gas and molten metal. The critical gas flow rate for the initiation of small bubbles due to disintegration of large bubbles coming from the nozzle exit was approximately 60 cm3/s under the present experimental conditions. The bubble behavior near the bath surface also was made clear.

X-ray fluoroscopic observation of bubble characteristics in a molten iron bath

In the smelting reduction process of the thick slag layer type, it is important to keep slag height stably without abnormal slag foaming. This study investigated the mechanism of slag foaming control with carbonaceous materials by experiments using a 1-ton smelting reduction furnace and by X-ray fluoroscopic observation of phenomena occurring in a small graphite crucible. As the carbonaceous material was not wettable with slag, small bubbles coalesced on its surface. Because the large bubbles due to the coalescence of small bubbles rise through the foaming slag layer at a relatively high speed, the foaming of slag may be restrained. It was found that the smaller the size of the carbonaceous material was, the more effective was the carbonaceous material in controlling the foaming of slag. A certain extent of relative motion between carbonaceous material and slag was necessary to promote the coalescence of small bubbles. It was experimentally confirmed that the smelting reduction in 100-ton scale furnace can stably operate under the condition that the carbonaceous material is present in a sufficient amount in the slag.

/pdf/slag-foaming-in-smelting-reduction-and-its-control-with-v3o8dtnfxv.pdf

Slag Foaming in Smelting Reduction and Its Control with Carbonaceous Materials

Refining of 18%Cr-8%Ni Steel with Ca-CaF 2 Solution

The invention relates to an electroslag surfacing of cylindrical components, such as a rolling-mill roll for iron production. The invention seeks to solve the problems of the electrodes for supplying a high current to the slag, and; how to prevent excess fusion-penetration of the base metal from occurring during the thick surfacing which results in an increase in heat input and a decrease in welding speed or the speed of rising the metal pool. In the present invention, a multi-phase alternating current is supplied to the slag via a plurality of consumable electrodes each in the form of a bar having a large cross section, and further the surfacing is conducted while rotating the workpiece and continuously adding molten metal to the surface of the slag bath. The invention can be mainly utilized for the production and repairing of cylindrical composite components.

Naoki Tokumitsu

Papers

Physical Model of Slag Foaming.

X-ray fluoroscopic observation of bubble characteristics in a molten iron bath

Slag Foaming in Smelting Reduction and Its Control with Carbonaceous Materials

Refining of 18%Cr-8%Ni Steel with Ca-CaF 2 Solution

Method of electroslag surfacing of components having a cylindrical surface