2. Objectives, Scopes & Significance
• To evaluate the nature of flow inside the mixing tank with controlled inflow
of AMD
• To investigate the impact of mechanical mixing in fluid flow pattern and
fluid flow rate.
• Facilitates a better management of the environmental problem in regards to
pollution of streams caused due to AMD.
• Recover water for beneficial reuse and opportunity of metal extraction.
3. Acid Mine Drainage (AMD)-Overview
• AMD is the acidic water produced as a result of a mining operation when
the weathering processes oxidise sulphide rich minerals.
• AMD is a major environmental and economic challenge for mining
industries.
• Environmental issue: AMD pollute the nearby stream and has detrimental
effects on aquatic plants and animals.
• Source of AMD: Coal and metal mines
4. Treatment Method
• Lime neutralization is one of the widely used chemical method for AMD treatment.
• This treatment process involves neutralization and precipitation of minuscule particles by
controlling pH with lime.
• Basic chemistry of neutralization: The hydrated (slaked) lime is fed as slurry, which dissolves
to increase pH,
𝐶𝑎𝑂 + 𝐻2 𝑂 → 𝐶𝑎(𝑂𝐻)2
𝐶𝑎(𝑂𝐻)2 → 𝐶𝑎2+
+ 2𝑂𝐻−
• The increased pH then provides hydroxide ions which combine with the dissolved metals
𝐴𝑙3+
+ 3𝑂𝐻−
→ 𝐴𝑙(𝑂𝐻)3
𝐶𝑢3+
+ 2𝑂𝐻−
→ 𝐶𝑢(𝑂𝐻)2 and many more
5. CFD Evaluation
• Computational Fluid Dynamics (CFD) can be advantageous to evaluate the
path of residence through tank.
• CFD provides insights to the influence factors such as fluid velocity and
dead zones with in the fluid flow zone of the mixing (neutralization) tank.
• Modelling and simulation tool such as ANSYS FLUENT in particular can
be nominated due to its control-volume based approach for high accuracy.
7. CFD Model Description
• Using ANSYS Meshing, mesh for the geometry previously modelled was
automatically generated using CFD physics and Fluent solver preferences.
• A relatively fine mesh was generated by modifying the element size and maximum
(face) sizes to approximately 10-3 and 0.09 respectively.
• This fine mesh was acceptable to use since the computers used were powerful
enough to process it with reasonable computing time.
• A significant amount of nodes was created by doing this, and therefore by using
Fluent to solve the model, a more precise solution would be output by the software.
9. Simplified 2-D model
• The actual three-dimensional
structure was then reduced to
2-D to save time and
computational power needed.
• It can be considered
equivalent since the flow
modelled in 2-D will be
exactly the same throughout
the depth of the 3-D
structure.
11. Solution Setup (Continued)
• Side walls
• Wall Motion: Stationary Wall
• Shear Condition: No Slip
• Others: Default
• Inlets
• Mass flow Specification Method: Components
• Reference Frame: Absolute
• Supersonic/Initial Gauge Pressure (pascal):
0 (constant)
• Mass flow rate (kg/s): 53.6 l/s (constant)
• Turbulence: K and Epsilon (Default)
o Propeller
Wall: Moving wall
Motion: Rotational
Speed: 72 rpm (7.54 rad/sec)
Shear Condition: No Slip
o Outlet
Gauge Pressure (pascal): 0
(constant)
Other: Default
12. Solution Method
• Pressure Velocity Coupling
• Scheme: SIMPLE
• Spatial Discretization
• Gradient: Least Squares Cell Based
• Pressure: Second Order
• Momentum: Second Order Upwind
• Transient Formulation: Second Order
Implicit
Solution Controls
o Under-Relaxation Factors: Default
Solution Initialization
o Initialization Methods: Hybrid Initialization
o Compute from: inlet
o Reference Frame: Relative to Cell Zone
o Initial Values: Will be automatically added in
when inlet is selected to compute from
17. Analysis of the Computational Data
• From the velocity profiles displayed earlier, it is evident that in each case, as
the fluid moved from the rotating blade of the propeller to the pressure
outlet, flow velocity along the centre has increased.
• The centreline velocity of the fluid was maximum for the rotating blade
positioned at centre which is 1.47 m/s and higher for any case.
• Positioning the propeller near the inlets reduced the dead zones.
• Number of blades increased the hydrodynamic efficiency, but in diminishing
manner.
18. Analysis(Continued)
• The total pressure experienced by the fluid in the flow zone inside the tank was
evident due to the mechanical mixing and was observed to be higher for the longer
propeller positioned at the centre.
• The flow experienced an adverse pressure gradient on either side of the propeller
such that two maximum regions were located along its top and bottom.
• Continuing along the sides of and before the propeller, the flow experienced a
favourable pressure gradient.
• From the turbulent kinetic energy plot, the trailing behind the propeller existed a
region of high turbulence which peaked and followed as oscillating pattern towards
the outlet.
19. Limitation of the Study
• This study is limited to the investigation of fluid flow in the mixing tank and does
not extend further to highlights the overall treatment process in the conventional
treatment plant.
• The input data during CFD simulation comprises guessing or imprecision which
could lead to a less reliable result.
• Since the simulation of the reactive flow is not presented in the research, this
chemically inert system simulation could be less reliable.
• Errors could occur during modelling, discretization, iteration and implementation.
20. Conclusion
• Through the application of computational modelling and simulation techniques, an
investigation of the behaviour of flow inside the mixing tank was carried out.
• It was found that flow velocity generally increased along the centreline of the channel due to
the rotational propeller blades.
• Rotation of the blades in each case resulted in greater magnitudes of flow velocity at the
outlet.
• The position of the stirrer at centre of the tank was deemed to be more effective than any
other positions.
• For the given flow rates of AMD and slurry, two to three blades are suitable, however, this
could be different for different flow rates.
