Solids Mixing

Solids Mixing

0 INTRODUCTION/PURPOSE
1 SCOPE
2 FIELD OF APPLICATION
3 DEFINITIONS
4 BACKGROUND
5 MIXING QUALITY
5.1 Qualitative Mixture Quality
5.2 Quantitative Mixture Quality
5.3 Sampling of Mixtures

6 THE MIXING PROCESS
6.1 Powder Mobility
6.2 Mixing Free-Flowing Powders
6.3 Mixing Cohesive Powders

7 MIXER SELECTION
7.1 Available Equipment
7.2 A Selection Procedure
8 LATERAL THINKING
9 BIBLIOGRAPHY

FIGURES

1 DIVISION BETWEEN FREE FLOWING SOLIDS AND
COHESIVE POWDERS
2 REPRESENTATION OF TYPICAL BINARY POWDER
MIXTURE STATES

3 POSSIBLE STRUCTURES FOR BINARY COHESIVE
POWDERS
4 TUMBLER MIXERS
5 CONVECTIVE MIXERS
6 HIGH SHEAR MIXERS
7 HIGH IMPACTION MIXERS
8 MIXER SELECTION DECISION CHART

1. A,
CPT
CATALYST, PROCESS TECHNOLOGY
CONSULTANCY

Process Engineering Guide:

(GBHE-PEG.-MIX-707

Solids Mixing

supplied to U

2. FIELD OF APPLICATION

DEFINITIONS

BACKGROUND

MIXING QUALITY
altivo Mixture Quality

Quantitative Mixture Quality
Sampling of Mixture

THE MIXING PROCESS

Powder Mobility
Mixing Free-Flowing Powders

3. POWDER:
TUMBLER MIXERS
CONVECTIVE MIXERS
HIGH SHEAR MIXERS
HIGH IMPACTION MIKE!

IXER SELECTION DECISION CHART

IENTS REFERRED TO IN THIS PROCESS
NEERING GUIDE

4. This Guide applies to Process Engineers in GBH Enterprises worldwide.

3 DEFINITIONS
For the purposes of this Guide, the following definitions apply

Scale of The quantity of mixture on which the quality of the mixture is
Scrutiny judged

With the exception of terms used as proper nouns or titles, those terms with initial

capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.

4 BACKGROUND

The mixing of dry particulate solids difers from that of liquid and gaseo.
stems in three important respects:

5. jomized particult
greater content variation or poorer mixture quality than the equivalent
Samples taken from a gaseous or liquid mixture.

differences accentuate the problems of mixing particulate solids, An id
mixing system would have high mobility and fine texture of its ultimate
Particulate solids have poor mobility and coarse texture.

Within the spectrum of industrial solids there are great variations
xture. An initial division can be made between free-flov
powders (see Figure 1)

FIGURE 1 DIVISION BETWEEN FREE FLOWING SOLIDS AND COHESIVE
POWDERS

6. [be higher. This can be a dominant requirement

Itis recommended that the temptation to control particulate solids properties
such that the bulk flow is just free-flowing should be avoided. While potentia
giving the best texture tisfactory flow, itis a dangerou
balancing point as small

5 MIXING QUALITY
51 Qualitive Mixture Quality

When is a mixture well-mixed? This fundamental question has to be asked of all
but itis especially important when coarsely textured powders are

is [Ref. 1), gives some helpful qualitative ideas on mixture quality. A
egregation of a mixture measures the size of regions of unmixed
ial. The Intensity of S res the amount of dilution of regions
of unmixed material.

Evidently, the quality of a mixture is improved by reducing both the scale and

7. it identities the objective of the mixing proces
it enables the mixing problem to be defined statistically;
itis the scale at which the mixture shall ultimately be sampled for quality
I purpo
52 Quantitative Mixture Quality

Having determined the scale of scrutiny for a product itis possible to define three
variance values for a powder mixture:

riance of a completely separated system (2);
the variance of a randomized mixture (Sp)

the variance of an ideal or ordered mixture (SP).

See Figure 2 for pictorial representation of a binary powder

8. 0 Mixture Completely Separa

9. where p and q
in the sample. Lessons from this simple case can be applied to the
more complex multi-component, mult-sized systems. Note that;

Randomization is normally the goal of an industrial mixer so that Sp?
should be as small as possible. This can be done either by having a large
scale of scrutiny, (Le. large A), or for a constant weight of sample to
reduce the parce size. This is the numerical expression for the improved
texture of a cohesive powder.

For scale-up purposes 8; is independent of scale of scrutiny whilst Sp? is
rsely proportional to scale of scrutiny. Betw limiting mixture
lues there is an unknown dependence on scale of scrutiny which make
tial that intermediate mixtures are sampled at the required prc
scale of

With

10. The denominator in equation (4) is the estimate of the mean number of particles
in a sample and is directly comparable with the denominator value A of equation
(2). In order to estimate the limiting variance by equation (4), the size analysis of
the components is required along with a knowledge of particle shape and specific
gravity. This limiting equation for random mixtures has been extended to cover
‘multicomponent mixtures by Stange so that:

In this expression one components regarded as the ey’ component. ithe
variance of more than one component is regarded as critical in a process it could
be necessary to monitor the state of mixedness of these components
independent [Ref 5)

important in that they provide a meth
cally obtainable mixture quality for any particu
ing mixture formulatior
or the scale of scrutiny on attainable mixture quality. In practice this mixture
to Segregation or poor mixer design but
ossibilty in a randor tablished

11. F experimental

‘Samples should be taken of a size equal cale of scrutiny. I is very dificult
to avoid bias in the selection mples from free-flowing soli
ing powders would be:

(@) Avoid sampling from bulk and look for points in the process where the
mixture is flowing: e.g. as a batch mixer is discharging,

ample the entire section of flow.

Sample frequently and avoid a sampling frequency which coincides
process frequenc

mpling of cohesive powders is less of a problem because of the reduction
gregation.

A sample withdrawn from a mixture is a 'point sample at a carefully chosen sc

f scrutiny. When this sample is analyzed for quality itis unlikely that the qual
required for analysis i me as of scrutiny for the process and itis
tal that sub-sampling of the point sample is done in an efficient spinning rifle.

Thatis, for analytical purposes a bulk sample has tobe taken from the point

12. a

free-flowing mixtures, and
cohesive or structured mixtur

The division between the two regimes is not easily defined. The bulk effect can
be seen by rolling the mixture in a bottle. The free-flowing powder solids move
‘smoothly with well defined planes of movement whilst the cohesive powder
exhibits stick-slip' motion with irregular surface characteristics.

Particle size is probably the dominant influence on the type of flow regime. The
gravitational force associated with a large particle is much larger than any
restraining interpanticulate forces with the result that individual particles retain
their freedom of movement. A cle size decreases various
interparticulate forces can potentially dominate and the particles attempt to
retain a structured arrangement.

For particulate solids on the boundary between free-tlow and cohesivity the
been found to be a
he powder [Ref. 8)

The identification of he flo ids is important as it determines the

13. gross
of the

continuous mixing is an ex y of avoiding gross
ontinuous mixer the quantities of mixture at the point of

mixing are minimized al scale of segregation is considerably
reduced.

Even if a large mass of free-flowing solids is ‘satisfactorily’ mixed great

processes can destroy the mixture quality that has been so carefully created
Only when the mixture has been ‘frozen’ at its final point of usage can the mixture
be regarded as safe. This is the point at which the particles lose their mobility or
at which segregation is no longer important and could be the formation of a
tablet, the filing of a pas

14. highlighted the statis ive particles. Another
major bonus of mixing cohesive powders structural nature of the

der prevents the mobilty of individual particles and the gi regation
encountered with free-flowing powders is seldom met.

The nature and the strength ofthe interparticulate forces acting within a cohesive

powder structure is receiving a great deal of attention. The reason for this is that

ifthe structure of the powder can be manipulated by the careful choice of particle
‘apes and sizes then an approach can be made to the ideal or ordered mixture

with zero variance, S?. Figure 2 illustrates the basic mixture conditions for

Unstructured, free-flowing particles for a binary system. If the panicles are
uctured then some interesting possibilties arise.

If there is an equal structural strength between black and white partic
black and black particles then a randomly structured powder will resul.

If one of the ingredients is ‘self loving’ e. black particles bond by

black particles then the structure of Figure!3 (a) is obtained. If one of the

ingredients prefers to bond to a dissimilar particle then the preferred structure 3
s an approach to ordered mixing,

The self loving preference of Figure 3 (a) is a common problem in the fine

16. Poor mixture quality for ntle shearing action useful for
flowing powders. some cohesive and lubrication
processes

A high speed impactor or Good for product integrity
intensifier can be adde

give greater process

flexibility

See video [Ref. 12] for more detail of mechanis

Double Co

19. rs with a variety
of mixing mechanisms.

he mixer perlorm 1
the flow characteristics of the
powder changed?

For example an edge runner mill
with a fitted ribbon blender,

a tumbler mixer with a high speed
impactor or a high speed impactor
which also permits gentle
circulation of the powder

In-ine Impact

20. im conditions [see Ref. 6, Chapter 3]. Producing a mixture of the
+ quality is more problematical but a decision sequence would be:

Is the process statistically possible (see 5.1 and 5.2)?
What is the flow nature of the powder to be mixed (see!6.1)?

What mechanism of mixing should be used (see 5.3 and 6.4)? and if
possible view video [Ref. 11] ‘Mechanisms of Mixing Powder

Choose a mixer. It is often desirable to choose a mixer with more then on
mixing mechanism as the process and role of the mixer might well chan
(see 7.1).

I possible test the mixer before installation at the correct scale of scrutiny
(seel5.1) using non-biased sampling techniques (see 5.3) at a mixer
capacity as close as possible to the required duty. Check performance
quality and mixing time (see 5.2).

Figure 8 gives a parallel selection procedure based on a decision chart.

21. 8 LATERAL THINKING

Powder mixing can be a difficult process and for free-flowing powders it can be
dificult task t

22. me evident on close examination of how
the customer actually uses the process mixture.

23. Stange, K., Chem. Ing. Tech, 1963, 35, 580.

Hamby, N., Edwards, M.F. and Nienow, A.W., Mixing in the Pı
Industries, 1985, Pub. Butterworth

Allen, T. and Khan, AAA, The Chem. Eng., 1970, 109.

Harnby, N., Hawkins, A.E., and Vandame, D., Chem. Eng. Sci

Hamby, N., Video ‘Filing a Box with Powder

Williams, JC. and Knan, H.l., The Chem. Eng. 193

Hamby, N., Video ‘Mechanisms of Powder Mixing!

24. [10

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