Nitrogen Injection in Molten Aluminum in a Tank Degasser

The injection of nitrogen in molten aluminum through a static impeller in a tank degassing unit is studied. Using basic principles of fluid mechanics, it is analyzed the influence of the nozzle diameter on the bubble diameter and the mean residence time of the bubbles in the molten aluminum. By means of transient isothermal 2D Computational Fluid Dynamics (CFD) simulations, the influence of the nitrogen volumetric flow rate on the phase distributions and the tank degasser dynamics is studied. Finally, an adiabatic CFD simulation is carried out in order to elucidate the changes in the molten aluminum temperature due to the injection of nitrogen at ambient temperature. This simulation shows that molten aluminum does not suffer drastic temperature reductions given that, in spite that the nitrogen is fed at ambient temperature, the mass of nitrogen is relatively small compared with the mass of aluminum.


Introduction
Molten aluminum is very reactive chemically, and promptly reacts with water forming atomic hydrogen which is easily dissolved in it. Water comes from the atmospheric water vapor, wet refractory, wet raw materials, combustion products, and so on. Unfortunately, when molten aluminum is cast in semi-finished forms and solidifies, atomic hydrogen is no longer dissolved in solid aluminum and forms bubbles of molecular hydrogen which in its turn gives raise to small but harmful pores. Porosity is one of the worst defects in aluminum castings given that it causes a significant decrease of the mechanical properties and the corrosion resistance [1]. molten aluminum. For example, gaseous and solid fluxes, which are blends of chloride and fluoride salts, may be used to remove dissolved hydrogen in aluminum and its alloys by chemical reaction [2] [3]. The removal of hydrogen from molten aluminum with vacuum is based on the equilibrium relationship between the dissolved hydrogen and the hydrogen partial pressure in ambient atmosphere [4] [5] [6]. Ultrasonic degassing of aluminum rests on the diffusion of dissolved hydrogen to cavitation bubbles assisted by their pulsation and their evacuation from the molten metal [7] [8] [9]. However, the worldwide most employed process for hydrogen removal from molten aluminum is the inert gas injection by rotating impeller (IGIRI). In this process an inert gas (argon or nitrogen) enters into the molten aluminum through a rotating impeller with several nozzles. The gas bubbles are generated by shear and collision with the vanes of the rotor [4].
The IGIRI process has been extensively analyzed by plant trials, physical modeling and computer simulations in order to elucidate the most important parameters and variables which determine the efficiency of hydrogen removal from molten aluminum. In [10] it is analyzed the influence of four major parameters (gas flow rate, rotational speed, refining time, and stewing time) on the degassing rate of an aluminum alloy with the IGIRI process by using an orthogonal experiment methodology. Authors report that the rotational speed has the greatest impact on degassing. Recently, the hydrodynamic performance of a stirred ladle for an aluminum degassing system was studied by means of water model experiments in [11]. The authors explore the dynamic flow features using diverse impellers with geometrical differences, and the performance is evaluated at two gas flow rates. The Particle Image Velocimetry (PIV) technique was applied to obtain the velocity fields during the degassing process. In [12] [13], numerical simulations with Computational Fluid Dynamics (CFD) software were used to simulate the fluid flow in a water physical model of a molten aluminum ladle with IGIRI process for degassing. The effect of critical process parameters such as rotor speed, gas flow rate, and the point of gas injection on the fluid flow and vortex formation were analyzed. The effect of the impeller design on degassing kinetics using the IGIRI process is studied too. The effect of the IGIRI degassing process variables on the casting quality of an aluminum alloy was investigated in [14]. The authors report that the effectiveness of the rotary degassing process is highly dependent on the combination of rotational speed and the gas flow rate, and that a wrong combination of these factors may result in no improvement or even degradation in the quality of castings.
In this work, the injection of nitrogen in molten aluminum through a submerged static impeller in a tank degassing unit is considered. The impeller has six nozzles located at its submerged end. Unlike the above works, here the bubble size, the mean residence time, the stirring power, and the mixing time are determined for the degasser tank. The influence of the nozzle diameter on the bubble diameter and the mean residence time of the bubbles in the molten aluminum were mathematically analyzed. Besides, using transient 2D Computational Fluid Dynamics (CFD) simulations, the influence of the nitrogen volumetric flow rate on the phase distributions, the system dynamics and the temperature distribution is studied.

Degasser Description
The considered tank degasser is depicted in Figure 1. It resembles a degasser commonly employed at industry nowadays. For the sake of simplicity, the impeller is here considered static, i.e. the impeller is not rotating. Parameters of the degasser and physical properties of the involved phases are shown in Table 1 and Table 2, respectively.

Basic Fluid Mechanics
Obtaining the proper diameter of nitrogen bubbles, their number and rise velocity is essential to get an efficient degassing of molten aluminum. The diameter of gas bubbles in the molten metal increases from an initial detachment value to a final value when the bubble leaves the metal bath [17]. The growth of the bubble diameter is explained by the thermal expansion of the gas in the molten aluminum, the decrease in the metalostatic pressure as the bubble ascends, and the coalescence phenomenon between bubbles. Of course, some breakage phenomenon may occur which contributes to reduce in some degree the bubble diameter. Buoyancy and drag forces govern the rise velocity of bubbles in a liquid column. These forces are strongly dependent on the fluid properties and gravity [18]. Rise velocity determines the mean residence time of the bubbles inside the molten metal, and in turn, mean residence time affects the degassing rate. For injection of gases in molten metals at low gas flow rates, a balance between surface tension and buoyancy forces gives the following relationship to determine the bubble diameter [19]: where d b is the bubble diameter (m), d n is the nozzle diameter (m), σ is the surface tension of the molten metal (N/m), g is the gravity acceleration (m/s 2 ), ρ m is the density of the molten metal (kg/m 3 ), and ρ g is the density of the gas (kg/m 3 ).
For a single bubble, which almost spherical shape due to surface tension, Stokes's law is employed to determine the rise velocity [18]: where v b is the rise velocity of the bubble (m/s), and μ m is the viscosity of the molten metal (kg/m·s).
In absence of rotation of the impeller and for slender degassing tanks, the bubbles travel an approximate linear path from the point of detachment to the molten metal surface. Then, the mean residence time can be estimated from the relationship [20] where ε is the stirring power of the melt (W/ton), Q is the gas flowrate (m 3 /h), T is the melt temperature (˚K), W is the melt weight (kg), and P 0 is the gas pressure at the melt surface (atm).
In [21], the mixing time is defined as the time required for achieving 95% of

Conclusions
The injection of nitrogen in molten aluminum in an industrial-like tank degasser with a static impeller was studied. From a basic fluid mechanics analysis and World Journal of Engineering and Technology   1) In absence of rotation of the impeller, the value of the diameter of the impeller nozzles is very important given that it determines the nitrogen bubble diameter, and hence the rise velocity and the mean residence time of this bubble.
2) The nitrogen volumetric flow rate affects in a non-linear decreasing pattern the mixing time of molten aluminum.
3) As the nitrogen volumetric flow rate is increased, the dynamics of the tank degasser becomes faster. This means that less time is required to achieve the desired degree of stirring.
4) Temperature of molten aluminum is slightly affected by the presence of nitrogen in the melt, in spite that the nitrogen is injected at ambient temperature.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.