Basic Guidelines for Design of Slurry Mixing Agitators

Hi Friends,

The major technological facilities for Alumina Refinery comprises of various types of stationery and rotating process equipment like crushers, conveyors, ball mills, tanks, pumps, piping, agitators, evaporators, heat exchangers, causticizers, filters and calciners.  Majority of the process equipment in the Alumina refinery are tailor-made suiting the plant capacity requirement.

In the opinion of equipment designers and process experts, the design of agitators for mixing the slurry with the desired degree of agitation is considered to be complicated and tricky issue. In previous post, we have explained the associated terminology used in design of slurry mixing agitators.

In view of requirement of suitable agitators for Alumina refinery, it was felt essential to briefly outline the mixing process and elaborate the basic guidelines with associated parameters required for drive rating calculations for slurry mixing agitators suiting various process conditions in  Alumina refinery.

MIXING PROCESSES

The mixing process in hydrometallurgical industries cam be categorized for easy understanding as tabulated below-

Physical	Class	Chemical
Suspension	Liquid – Solid	Dissolving / homogeneity
Dispersion	Liquid – Gas	Absorption
Emulsification	Immiscible Liquids	Extraction
Blending	Miscible Liquids	Reactions
Pumping	Fluid Motion	Heat Transfer/material transfer

Alumina Refinery can broadly be divided into two main process areas –

(i)   Red Area and

(ii)  White Area

In the red area of the Alumina Refinery, the main purposes of agitators for such mixed tanks are the following –

• Keeping solids in suspension
• Homogeneity of slurry

Agitator components mainly include impeller, shaft, gear box and drive motor suitably designed for meeting the process requirement.  The mixing process for keeping the solids in suspension is known as off-bottom suspension.  This process keeps the solid particles moving just above the bottom of the vessel preventing the settling of solids.

The other process used for homogeneity of slurry is popularly known as Full tank suspension.  In case of full tank suspension, the degree of agitation is close to 10.  Hence the agitator always moves the tank bottom solids to the top level allowing its transfer to next tank connected in series.

  

Due to the actual characteristics of alumina hydrate or sizes of tanks with very large useful capacity, following are additional challenges for the designer while designing the agitators to be used in the white area of the Alumina Refinery -

a)    Low shearing rate of selected impellors to avoid attrition of solids crystals

b)   Low power consumption

Typical physical characteristics of slurry components are given below -

-    Specific gravity of aluminate liquor: 1.23 –1.29 (liquid)

-          Specific gravity of alumina hydrate: 2.42 (solids)

-          Bauxite                                              : 2.58 (solids)

-          Bauxite residue (Red mud)             : 2.3 (solids)

TYPES OF AGITATORS / MIXERS

There are three types of agitators used in Alumina refineries –

a)   Top Entry

b)    Side Entry

In the Alumina refinery, top entry agitators are the preferred choice over other options.

ELEMENTS OF MIXER DESIGN

Following are the key elements of mixer design –

Impellers

Shafts

Drive Assembly

Process conditions

Impeller Power Characteristics

Mechanical Design

FACTORS AFFECTING POWER CONSUPMTION

Power consumption for agitation / mixing is affected by following main factors:

-          Specific Gravity

-          Viscosity

-          Impeller type

-          Impeller diameter

-          Impeller location

-          Tip speed

-          Baffles in tank

Impeller power and pumping capacity of agitator are calculated using following mathematical relations-

Impeller Power, P = N_p x N³ x D⁵

Impeller Pumping Capacity, Q = N_q x N x D³

  

MECHANICAL DESIGN OF IMPELLERS

The Impeller is the most important element of the mixer. Only the impeller produces the required process result by converting the rotational energy of the shaft into the necessary proportions of Flow and Shear. Process efficiency requires the matching of impeller characteristics to Process requirements. Ideal Impeller for Flow controlled applications are expected to posses following basic features -

-          Low Power  - N_p

-          High Flow   - N_q

-          Weightless

-          Low Fluid Forces

-          Low Cost

Hence adequate precautions must be taken by Agitator manufacturers while selecting / designing the impellers considering above critical aspects as power number and flow number depend on the type of impeller.

  

DIMENSIONLESS NUMBERS USED FOR DESIGN OF AGITATORS

Dimensionless numbers often correlate with some performance parameter and greatly aid engineering analysis and design. Main dimensionless numbers used for arriving at drive rating for agitators are listed below and detailed in subsequent paragraphs –

• N_Re   :  Reynolds Number
• N_p   :  Power Number
• N_q   :  Flow Number

 Reynolds number N_Re is defined as below –

D*V*r

N_Re =  ---------

                µ

Where, N_Re = Reynolds Number,

              D   = Pipe diameter,

             V    = Fluid velocity and

              r    = Fluid density.

The above formula is generally used for fluid flow through pipes.  Reynolds number has been so valuable for dealing with flow in pipes that analogous numbers are desirable for other flow situations such as mixing in tanks and transfer from gas bubbles. The same properties used in pipes will give a dimensionless number in another system if the units are consistent. The problem is assigning the property and the decisions can seem arbitrary. The logical choices for a Reynolds number for a rising bubble are bubble diameter, relative velocity of the bubble versus the fluid density and fluid viscosity.

Another Reynolds number that for a stirred tank is less obvious. The fluid properties seem to be good choices for r and µ, but there are problems in selecting D and V. The impeller diameter is used for D even though an impeller also has height and usually several blades that must affect mixing or else process engineers would not experiment with impeller designs. In the quest for something to use for V, the revolutions per second of the impeller is chosen even though it is not a velocity. There is no length dimension in this term, so the dimensionless number is created by using the impeller diameter and viscosity of slurry.
The mixing Reynolds number is:

D²*N*r

N_Re =  ---------

                µ

where D = Impeller diameter in m

N = Revolutions per second

r        =  density in kg/m³ 

µ   = viscosity in kg / m.sec

This illogical Reynolds number is highly useful and it correlates quite well when comparing systems that employ roughly similar impellers. Scale up of mixing uses a correlation of Reynolds number with Power number defined as:

P * g

N_p  = ---------

           N³ D⁵

Where,

N_p  = Power Number

P    = Applied power for rotation of agitator

g    = Gravitational constant


Dimensionless power number and flow number represent the characteristics of impellers fitted in agitator shaft.  For turbulent regime, these numbers are found to be constant. Hope, all issues pertaining to basis for design of slurry mixing agitators have been covered. Please put your views / suggestions / comments, if any.

If you like this article, then press your rating as  +1  .

Thanks and regards.

Kunwar Rajendra