Page 24 - Chemical Technology

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Chemical Technology • January 2013

solids handling

Introduction

There are numerous applications of mixers that deal with

erosive solids, especially in the minerals processing and

power industries. In many of these applications, there

is an erosion-corrosion synergistic effect on the wear

of a mixer’s wetted parts, particularly the impeller. This

article pulls together the authors’ research with numer-

ous articles on erosion and erosion corrosion to permit a

designer to optimize the cost-based life of eroding mixer

parts before replacement is required.

There are a large number of factors that can affect the

rate of erosion. Many of these factors have been known

and studied to some extent:

• Chemical environment

• Hardness of solids

• Density of solids

• Difference in liquid and solid density

• Percent solids

• Shape of solids

• Fluid regime (turbulent, transitional or laminar)

• Fluid rheology (eg, pseudoplasticity)

• Hardness of the mixer’s wetted parts

• Young’s modulus of the mixer’s wetted parts

• Hydraulic efficiency of the impeller (kinetic energy

dissipation rates near the impeller blades)

• Impact velocity

• Impact frequency

• Angle of impact.

Theoretically the rate of volume loss of material is due to

the kinetic energy lost when a particle impacts a materi-

al¹. This would suggest a velocity exponent of 2. However,

presented below, experimental velocity exponents have

ranged from 1,5 to 4,0. The general form of the equation

relating erosion rate to velocity is given by:

E = K · V

n

f

(



)

where

E

=

volumetric

erosion rate

Design mixers to minimize effects

of erosion and corrosion erosion

by Julian Fasano, Mixer Engineering Co, Troy, Ohio, USA, Eric E Janz, Chemineer, Inc, Dayton, Ohio, USA and Kevin Myers,

University of Dayton, Department of Chemical & Materials Engineering, Ohio, USA

This article was

originally published

by Hindawi Publish-

ing Corporation, in

the International

Journal of Chemical

Engineering, Volume

2012, Article ID

171838, 8 pages,

doi:10.1155/2012/

171838 and is copy-

Fasano

et al

.

Abstract

A thorough review of the major parameters that affect solid-liquid slurry wear on impellers and

techniques for minimizing wear is presented. These major parameters include (i) chemical

environment, (ii) hardness of solids, (iii) density of solids, (iv) percent solids, (v) shape of solids,

(vi) fluid regime (turbulent, transitional, or laminar), (vii) hardness of the mixer’s wetted parts, (viii)

hydraulic efficiency of the impeller (kinetic energy dissipation rates near the impeller blades), (ix)

impact velocity, and (x) impact frequency. Techniques for minimizing the wear on impellers cover

the choice of impeller, size and speed of the impeller, alloy selection, and surface coating or

coverings. An example is provided as well as an assessment of the approximate life improvement.

K

=

constant (function of all parameters other than

V

or



)

V

=

particle velocity or relative velocity for rotating

systems (impellers)

n

=

velocity exponent

(can also be a function of other parameters)



=

impingement angle

Most investigators have used this general equation form.

Sapate and RamaRao

2

used a power law correlation

between volumetric erosion rate and impingement veloc-

ity in a non-rotating system. They observed exponents on

velocity of 1,91 to 2,52. The velocity exponent showed an

increasing trend with increasing hardness of the alloys

irrespective of the hardness of the erodent particles and

the impingement angle of the alloys investigated.

Stack

3

and others investigated the effect that corrosion

plays in an erosion environment. These investigators studied

various parameters of the corrosion-erosion environment

in a non-rotating system. They observed velocity exponents

that ranged from 1,4 to 3,5. They concluded that exponents

derived for erosion of alloys under erosion-dominated

conditions can be correlated to those derived for the strictly

ductile erosion process. These are typically very near the

theoretical “2” for the strictly ductile erosion process.

However, those for the erosion-corrosion dominated regime

are higher than for the erosion dominated regime and were

in the 2,5 to 3,5 range. A publication by the Hydraulics Insti-

tute

4

suggests that the erosion velocity exponent for pumps

in slurry transport is in the order of 2,5-3,0

4

.

Fort

5

and others studied pitched blade impellers 100 mm

in diameter with a blade width of 20 mm in water-solid

slurries under turbulent conditions. These impellers were

studied at pitch angles of 20°, 35° and 45°. These

studies included a slurry of 18,3% by volume of gypsum

having a mean particle diameter of 0,1 mm and a 10% by

volume slurry of 0,4 mm mean diameter sand particles.

From their studies they concluded the following:

• Particles of the lower hardness gypsum generated

uniform sheet erosion over the entire surface of the

impeller while the particles of sand, having a higher