Page 26 - Chemical Technology

24 Chemical Technology • January 2013
Increased hardness of metals will generally provide
an increased life. Miller and Schmidt12 compared the ero-
sion rates of 16 metals in a recycled slurry test system
using 2% by weight silica sand in water. The impeller tip
velocity was 15,7 m/s and the temperature was 16°C. In
addition to the erosion rate for each metal, they included
the metal’s hardness. The best fit for their data was
logarithmic. However, probably due to the synergistic cor-
rosion effects, the data was fairly dispersed. A plot of this
data is provided in Figure 3. The effect of particle hard-
ness depends on whether erosion is ductile or brittle. For
brittle erosion, the effect of particle hardness is much
more pronounced than for ductile erosion.
solids handling
Changes in particle size can change the erosion mecha-
nism. Stachowiak and Batchelor13 reported that as the par-
ticle size was increased from 8,75 μm to 127 μm, the mode
of erosion changed from ductile to brittle. The erosion study
was for silicon carbide particles impinging on glass, steel,
graphite and ceramics. The particle velocity was 152 m/s.
Design for erosion minimization
Because maximum velocities in mixing processes seldom
exceed 6 m/s, erosion and corrosion-erosion of materials
are fatigue processes for most mixing processes. There’s
generally not enough particle kinetic energy to cause
ductile erosion where there is some plastic flow of mate-
rial. The fatigue process occurs on a micro or localized
scale, and, as with macro-scale fatigue, two stages of the
erosion process have been observed. There is an incuba-
tion period followed by the formation and growth of pits
involving the removal of the metal or material. Refer to a
materials' behaviour text such as that by Hertzberg14 for a
more in-depth discussion on material behaviour.
Due to the vast number of parameters that can affect
erosion or erosion-corrosion processes, and the fact that
this area of mixer service has not been widely studied, it is
very difficult to predict a priori what the rate of erosion will
be for any given liquid-solid application. However, there are
certain factors within the control of the designer that can
be used to optimize the life of the mixer’s wetted parts.
Figure 3: Wear rate data of Miller and Schmidt
Most mixer designers will not have control over the
type of slurry, the percent solids, the hardness of the
solids, the shape of the solids, the liquid, the pH, etc.
However designers will generally have control over:
• The mixer wetted parts' materials, coating or lining
• The impeller style
• The impeller horsepower and speed combination.
Material selection
The choice in selecting a material is to either go hard or
soft and elastic. All else being equal, when selecting a
metal alloy, a higher hardness will lead to a longer life.
Thus when selecting a metal alloy material, select a hard
material which will also provide good corrosion resistance.
There are a number of hard surface ceramic coatings
such as tungsten carbide or silicon carbide, which could
be applied to the high-wear areas such as impeller blades.
Ceramics are the most wear resistant but are low in tough-
ness and impact strength. Ceramic coatings as well must
be corrosion resistant to the liquid medium. Ceramics also
do not have the ability to absorb much strain. These high
strains on flexing blades may allow cracking of the ceramic
coating. Ceramic coating applicators should be able to
provide the maximum allowable strain for the ceramic
coating under consideration. Coatings of the more com-
mon ceramic materials tend to be more costly than high
hardness metals, or elastomeric coverings.15
Glass-lined equipment has a glass hardness of 5 to
6 on the moh scale. For the great majority of solids, this
hardness would be very acceptable. However there are
numerous materials and minerals including: Al2O3, SiO 2,
WC, SiC, and ZiO2 that have higher harnesses and would
tend to wear away the glass lining. Glass linings have very
many of the same limitations as ceramic coatings. They
tend to be brittle and cannot tolerate much strain.
Elastomeric coverings in the order of 3/8” thick for
industrial scale impellers have a long history of providing
longer life in slurry applications. Instead of having to ab-
sorb most of the particle’s impact energy, an elastomer
releases most of the energy back to the particle after
impact. Elastomeric lining manufacturers and applicators
will generally recommend an elastomeric hardness of 40-
60 Durometer A for optimum life. As with metals or hard
surface coatings, the lining must also be compatible with
the fluid medium. An elastomer’s hardness is directly
related to its corrosion resistance. However, as an elas-
tomer’s hardness increases above a 40A Durometer, its
erosion resistance decreases. A Durometer selection of
40-60 A is somewhat of a compromise between erosion
and corrosion resistance. Elastomers should not be used
when large particles are present. The term “large parti-
cles” is relative to the impinging velocity and mass of the
particle, as well as the thickness of the elastomeric cov-
ering. If the impinging particle can bottom out against the
metallic substrate, elastomeric coverings should not be
used. Even if most of the slurry might be suitable, a small
percentage of tramp particles can do significant damage
to the elastomeric covering. Since impact energy is a
function of the impingement angle, the leading edges of