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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.

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 [

Many methods and technologies are employed at industry for degassing of 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 [

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 [

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.

The considered tank degasser is depicted in

PARAMETER | VALUE |
---|---|

Molten aluminum volume | 0.43 m^{3} ^{ } |

Molten aluminum weight | 989 kg |

Diameter | 0.937 m |

Nitrogen flow rate | 1, 1.50, 2, 3 m^{3}/h |

Pressure at the molten metal surface | 1 atm |

Impeller immersion depth | 0.55 m |

Molten aluminum height | 0.60 m |

Impeller height | 0.74 m |

Impeller diameter | 0.05 m |

Impeller head diameter | 0.18 m |

Number of nozzles | 6 |

Nozzle diameter | 0.01 m |

PROPERTY | MOLTEN ALUMINUM | NITROGEN |
---|---|---|

Density, kg/m^{3 } | 2300 | 1.138 |

Viscosity, kg/(m・s) | 0.0029 | 1.663 × 10^{−}^{5 } |

Surface tension, N/m | 0.90 | --- |

Temperature, ˚K | 973 | 298 |

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 [

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 [

d b = [ 6 d n σ g ( ρ m − ρ g ) ] 1 / 3 (1)

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 [

v b = g ( ρ m − ρ g ) d b 2 18 μ m (2)

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 [

τ r = H v b (3)

where τ_{r} is the mean residence time (s), and H is the impeller immersion depth (m).

Degassing and homogenization of molten metal temperature and composition by gas bubbling is primarily caused by the dissipation of the buoyant energy of the injected gas. The stirring power of the molten metal is estimated here from a modified version of the Pluschkell’s relationship [

ε ˙ = 14.23 ( Q T 60 W ) ln ( 1 + H 1.48 P 0 ) (4)

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 [

τ m = 116 ( ε ˙ ) − 1 / 3 D 5 / 3 H − 1 (5)

where τ_{m} is the mixing time (s), and D is the diameter of the degasser tank (m).

On the other hand, transient 2D CFD simulations were carried out to elucidate the influence of the nitrogen volumetric flow rate on the phase distributions and the tank degasser dynamics. The conservation of momentum and mass of the molten aluminum and nitrogen was modeled using the Navier-Stokes and the continuity equations [

and therefore underestimate their mean residence time. However, at least qualitatively,

^{3}/h, the decreasing of the mixing time is slower.

Results of the CFD transient isothermal 2D simulations show the evolution of the phase distributions in the tank degasser, and these results are depicted in Figures 5-7 for values of the nitrogen flow rate of 1, 1.5 and 2 m^{3}/h, respectively. A 0.0001 s of time step was employed to guarantee numerical stability, and just two nozzles were considered in the computer experiments. Bubble penetration and rise in the molten aluminum increases as time elapses, as is appreciated in these figures. Besides, it can be noted that the nitrogen flow rates accelerates the tank degasser dynamics, i.e. the time required to achieve the same level of stirring in the melt decreases as the nitrogen flow rate is increased. In ^{3}/h. Flow lines of

A non-isothermal CFD simulation was carried out under adiabatic conditions for a nitrogen flow rate of 3 m^{3}/h. Simulation was stopped once a global mass balance detected spitting of molten aluminum.

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

Computational Fluid Dynamics simulations, the following conclusions are established:

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.

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

Maldonado, L.A., Barron, M.A. and Miranda, D.Y. (2018) Nitrogen Injection in Molten Aluminum in a Tank Degasser. World Journal of Engineering and Technology, 6, 685-695. https://doi.org/10.4236/wjet.2018.64044