ON THE DEGASSING KINETICS IN A LADLE EQUIPPED WITH A ROTATING IMPELLER ASSISTED THROUGH PHYSICAL MODELING

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1 ON THE DEGASSING KINETICS IN A LADLE EQUIPPED WITH A ROTATING IMPELLER ASSISTED THROUGH PHYSICAL MODELING Marco RAMÍREZ-ARGÁEZ a, Ivan TORRES a, Eudoxio RAMOS a, Carlos GONZÁLEZ a, José CAMACHO b a Facultad de Química at Universidad Nacional Autónoma de México. Address: Fac. de Química, Edificio D, Cd. Universitaria, C.P , México, D.F., México. b CIATEQ A.C., Address: Av. Manantiales No. 23-A, Parque Industrial Bernardo Quintana, C.P , El Marqués, Qro., México. Abstract A full scale water physical model of an aluminum degassing crucible was used to analyze the kinetics and removal efficiency of dissolved oxygen from water by N 2 injection, similar to hydrogen removal from liquid aluminum. Different process parameters were studied, such as impeller design (two impellers designs were used, named B and C ), rotation velocity, and gas flow rate, to understand the effects of these variables on the degasification kinetics. Oxygen removal from water follows a first order kinetics mathematically represented by an exponential decay. The slope parameter a was used to characterize degassing kinetics, which corresponds to a global mass transfer coefficient used as an operational kinetic parameter. Results show that as rotation velocity, gas flow rate and geometry complexity are increased, oxygen removal kinetics increases. Nevertheless, the main contribution of this work is the introduction of a novel gas injection design, consisting in gas injection through a pipe placed below the impeller instead of the conventional injection through the impeller. This novel technique was tested and compared to the traditional technique, and it was found that under similar operating conditions the novel technique yields an important increase on a values up to 45% for commercial impellers (impellers B and C ). Hydrodynamics of the system was also modified considerably with the introduction of this novel technique: vortex size was decreased and bubble distribution improves. These changes are related to the pump up capacity for each impeller. 1. INTRODUCTION Scientific basis of dissolved hydrogen elimination from molten aluminum alloys by gas injection systems with inert gas, reactive gas or mixture of them are well established [1]. However, degassing remains as a complex operation difficult to study since involves a great number of processes, design and environmental parameters or variables and due to the great complexity of the operation, it results impossible to predict the kinetics of a particular degassing process. Therefore, optimization of the processing operation parameters is empirically achieved by doing very expensive experimental designs in every cast shop [2]. Another major problem is to measure the hydrogen content during the degassing process [3]. Then, physical water modeling is employed as a low cost analysis tool. In the last years this technique has been employed widely for indirect studies on the degasification operation, inclusion elimination and elimination or reduction of some metals such as Na, Mg, Li and Ca in the melt, by inert gas injection or mixing with reactive gases [4-5]. The most studied parameters have been bubble size and its distribution in the bath and the degassing kinetics where it has been concluded that oxygen removal from water by N 2 injection, presents similar kinetic

2 behaviour as hydrogen removal from aluminum through gas injection. Degasification kinetics is expressed frequently as an exponential decay of first order [4-5] according to: a t [% G] [% G] e = 0 Where: [%G] is the dissolved gas concentration in the liquid, a is the slope parameter or global mass transfer coefficient, [%G is initial gas concentrations and t is the treatment time. Results from water physical models have helped to improve impeller designs, comparison among them and process optimization. In this study, performances of two rotor design were characterized (based on the measured oxygen kinetic elimination from water by N 2 injection) under different operating conditions. The impellers present different shapes, being both commercial designs. Notwithstanding, physical model knowledge generated in this work, the most important contribution of this work is the proposal of a novel degasification method, which increases degassing kinetics up to 45% depending on the impeller design and improves hydrodynamics in the system. (1) 2. EXPERIMENTAL PROCEDURE A transparent acrylic cylindrical tank of 32 cm diameter and 45 cm height was built and filled with 30 liters of tap water, which was put inside a cubic acrylic tank. Degassing equipment includes controls of rotation velocity, N, and volumetric gas flow rate, Q. Fig. 1 shows the schematic experimental setup employed for measuring degasification kinetics with the physical model. Two commercial designed impellers with different shapes were made of Nylamid ( B and C respectively). Impeller B is a hollow impeller with lateral nozzles and impeller C has nozzles, a hole and notches. Fig. 2 shows the designs of the impellers employed in this work. Fig. 1. Schematic of the experimental setup. 1 Acrylic tank, 2 Impeller, 3 Exterior acrylic tank, 4 shaft, 5 rotating union, 6 variable velocity motor, 7 degasser, 8 rotameter, 9 valve, 10 pressure regulator, 11 nitrogen tank, 12 Dissolved oxygen meter, 13 Probe. Fig. 2. Impeller B: hollow impeller with lateral windows and Impeller C: hollow impeller with lateral windows and superior notches. Water temperature was kept in the range of 19 to 21 C in order to maintain constant oxygen solubility at local absolute atmospheric pressure of atm. Oxygen saturation of water was carried out by injecting air through a lance by periods from 30 to 60 minutes. Dissolved oxygen (DO) elimination was made by injecting N % at constant flow and the electrochemical equipment HANNA Instruments model HI 9146 was employed to measure DO continuously. Impellers were tested in two ways: a) conventional operation where gas flows through them and b) novel method consisting in

3 injecting gas underneath the impeller by a lance. Arrows in Fig. 1 show gas paths in each technique. Table 1 shows the complete set of experiments. Table 1. Experimental process parameters for DO elimination in water by N 2 injection Exp. No. N (rpm) Impeller Q(l/min) Exp. No. N (rpm) Impeller Q(l/min) B C B C 7 To calculate the degassing efficiency, in ppm of oxygen eliminated per volume of injected nitrogen, eq. (2), the nitrogen volume used to remove 90% of the original oxygen content, VN 2, was measured. e = [ O] [ O] 0 V N 2 f Degassing efficiency change ( e) from conventional (e c ) to novel gas injection (e u ) was computed as: eu e Δe = e c c (2) *100 (3) Global mass transfer coefficient variation from conventional to the novel N 2 injection point, a, is: au ac Δa = a c *100 (4) Subscripts u and c in eqs. (3) and (4) indicate underneath and conventional gas injection respectively. 3. RESULTS AND DISCUSSION Fig. 3 and Fig. 4 show the effect of Q and N on the DO elimination kinetics using impellers B and C respectively. Both effects, increasing N and Q, enhance removal rate kinetics. Increasing purge gas flow rate causes a higher volume fraction of purge gas in the melt and smaller stable bubble radius, increasing the total surface area of bubbles available for trapping dissolved gas. The higher rotation speed of the impeller produces smaller bubble sizes, more turbulence and bath circulation (The fluid is pumped up from the bottom of the impeller and ejected through the lateral nozzles) so a better mixed bath is developed with higher liquid-bubble interfacial area. Regarding the effect of impeller design on DO elimination kinetics it can be seen that the faster gas removal is obtained by impeller C followed by impeller B, since there is an additional momentum provided by the notches present in impeller C.

4 Fig. 3. Effect of Q and N on DO elimination kinetics for impeller B. Fig. 4. Effect of Q and N on DO elimination kinetics for impeller C. 1.0 Underneath 1.0 Underneath Conventional Conventional Fig. 5. DO elimination kinetics. Comparing conventional and underneath impeller injections for impeller B at N=. Fig. 6. DO elimination kinetics. Comparing conventional and underneath impeller injections for impeller C at N=. Fig. 5 and Fig. 6 show a comparison between conventional and the novel method of gas injection underneath the impeller on the DO elimination kinetics for impellers B and C respectively. In these figures it is shown that, for a rotation speed of and two purge gas flow rates, the novel method of gas injection increases notoriously the oxygen removal efficiency when compared against the conventional method. Gas injection below the impeller improves mass transfer of oxygen at the water/bubble interfaces. Additionally, it has been found in previous studies [6] that gas injection through the impeller avoids an intimate and efficient momentum transfer between the impeller and the liquid which reduces convection and turbulence in the system. When gas is injected underneath the impeller there is no loss of momentum transfer from the impeller to the liquid. On the contrary, gas injection underneath the impeller improves circulation of liquid in the ladle and increases the sucking pressure below the impeller. This higher momentum produced in the novel technique, in comparison with the conventional technique, help gas bubbles to be dispersed uniformly along the ladle even far away from the impeller (see Fig. 7), increasing also the residence time of bubbles into the melt.

5 a) B Conventional b) C Conventional d) B underneath e) C underneath Fig. 7. Water physical model pictures for B and C impellers rotating at N= and with a gas flow rate of Q= of N 2. ((a) and (b) are Conventional gas injection through the shaft, while (c) and (d) are underneath injections). Vortex sizes and bubble distributions into the tank can be evaluated qualitatively from Fig. 7 for rotors B and C at, with a N 2 gas flow rate of, for conventional injection and underneath impeller injection, respectively. When gas is injected underneath impeller, the vortex size decreases considerably and a better bubble distribution is observed in the ladle. Hydrogen removal kinetics has been proposed as a first order reaction kinetics characterized by the a value from equation 1. This a value represents global mass transfer and it was used to compare DO rates. Table 2 shows a values, changes in the global mass transfer coefficient from conventional to the novel N 2 injection point, a, degassing efficiency, e (expressed as ppm of O 2 eliminated from the water divided over the liters of N 2 consumed) and the change in degassing efficiency from conventional to novel gas injection, Δe for all experiments. Table 2. Values of a, Δa, e and Δe obtained in all experiments with both conventional and underneath the impeller gas injection. Exp. A *1000 (s -1 ) Conven. (a c ) Under (a u ) a E (ppmo 2 /lt N 2 )* e Exp. a *1000 (s -1 ) Conven. (a c ) Under (a u ) a e (ppmo 2 /lt N 2 )* (0.18) (0.13) (0.31) (5) e From Table 2, it is confirmed that as N, Q and the complexity of the impeller increase values of a also increase. As expected, an increase in N increases the kinetic parameter a and the efficiency e due to the more turbulent flow associated. Also, an increment in Q (under similar conditions of bath agitation) promotes a decrease in degassing efficiency, since a higher gas flow rate injected into the bath implies bubbles leaving the liquid with lower oxygen content and therefore saturation of oxygen in the bubbles is more difficult to reach. Table 2 also shows that underneath impeller injection increases the mass transfer coefficient, a, and the efficiency, e, for impellers B and C. Biggest improvements from traditional to the novel injection technique in terms of degassing kinetics were obtained for impeller C at 536rpm and 7l/min (Δa of 45.5%), and for impeller B at the same conditions (Δa of 38.2%). The

6 biggest improvements from traditional to the novel injection technique in terms of gas consumption for impeller C are found at 536rpm and 7l/min (Δe of 47%), while for impeller B at 536rpm and 7l/min (Δe of 44.4%). Increment of a is moderated when N was at for impeller C, less than 25%. As N is increased, inertial forces and meanly velocity tangential component is increased and gas injection effect is decreased so the point of injection turned out to be less important. From a practical point of view, changing gas injection point by employing a lance as injector simplifies considerably the degasser design and reduces its cost and it reduces the risk of gas leaking from any degasser zone including the shaft as many are made of porous graphite. Also, this work shows that the impeller design plays an important role in the degasification efficiency. 4. CONCLUSIONS The main conclusions of this work are: Dissolved oxygen gas elimination kinetics by inert gas injection, N 2, follows a first order exponential decay, with the a parameter employed to characterize degasification kinetics. Gas injection underneath impeller enhances impeller degassing performance, improving hydrodynamics and thus kinetics degassing. Additional advantages of this novel technique for gas injection are vortex size reduction, better bubble distribution and reduction in bubble size. The novel gas injection design increased degassing kinetics and its efficiency up to 45 % and 47 % respectively in comparison with conventional gas injection systems. Dissolved gas elimination rate is increased as geometry complexity, rotation velocity of impeller and gas flow rate are increased. ACKNOWLEDGEMENTS Authors acknowledge CONACYT for financial support (Project 60033) and A. Amaro for his valuable technical assistance. LITERATURE REFERENCES [1] SIGWORTH, G. K. A scientific basis for degassing aluminum. AFS transactions, 1987, year 95, page [2] NEFF, D. V. Understanding aluminum degassing. Modern Casting, 2002, year 92, nr. 5, page [3] SHIVKUMAR, S., WANG, L., APELIAN, D. Molten metal processing of advanced cast aluminum alloys. JOM, 1991, year 43, nr. 1, page [4] BOEUF, F., REY, M., WUILLOUD, Y E. Metal Batch Treatment Optimization of Rotor Running Conditions. From Conference Proceedings Light Metals, TMS, Warrendale, PA, USA, 1993, s [5] NILMANI, M., WILLIAMS, G.K. A low cost solution to gas lancing problems. Materials Australia, 1995, year 27, nr. 5, page 120. [6] CAMACHO-MARTÍNEZ, J., RAMÍREZ-ARGÁEZ, M. A., ZENIT, R., JUÁREZ-HERNÁNDEZ, A., BARCEINAS- SÁNCHEZ, J.D. O., TRÁPAGA-MARTÍNEZ, G. Physical modeling of aluminum degassing operation with rotating impellers -Hydrodynamic analysis-. Accepted in Materials and Manufacturing Processes, 2010.