Dispersing Powders in Liquids, Part 1, by Ralph D. Nelson, Jr.
The world is full of slurries. We encounter systems of solids
dispersed in liquids every day -- starting with the pulp in our
orange juice and continuing through our evening toothpaste.
Many food products go through a slurry stage -- in baking, both
careful control of ingredients and an experienced eye are needed
to make the final adjustments required to make successful pancake
batter, cookie dough, pudding, or sauce. Our homes are built
using plaster, linoleum tile, filled plastics, paint, pastes, and
grouting material, all of which are either formed from or applied
as slurries. The roads we drive on are made from slurries of
concrete or asphalt. The books we read are printed with slurries
of ink on paper made from wood fiber slurry and coated with clay
slurry.
Many translucent textile fibers are spun from slurries containing
clay or titanium dioxide, and fabrics are often colored blue by
applying a slurry of copper phthalocyanine pigment. Ceramic
slurries are used to manufacture everything from flower pots to
electronic insulators. Silver dispersions are used to print
electronic circuit boards, and silver bromide dispersions are
used to coat photographic backing films, which are themselves
made from dispersions of carbon black in plastic. Slurries of
clay are used as ``drilling mud'' for oil wells, and slurries of
coal in water and oil are finding increasing use as fuel for
industrial burners. The suitability of rural roads for transport
and of river bottoms for bridge piers depends on moisture
content, mineralogical composition, and what is adsorbed on the
particles.
Nearer to our hearts are those lotions and potions that make us
more attractive by coloring our nails, covering up our pallid
cheeks, softening our hands, or polishing our shoes. As you read
through this series, keep in mind the many slurry systems about
you, and see how their behavior illustrates the principles
discussed in this series.
Goals of This Endeavour
This series of articles covers a topic that is so broad and complex
that either a list of major texts or a table of commercial
surfactants would exceed the pages available for this effort.
This series is thus selective rather than comprehensive. It is
designed to be used by either novices (after taking introductory
college-level courses in physics and chemistry) or more
experienced technologists (after graduate study and several years
of industrial experience).
Novices will see what problems can be expected in industrial
systems that involve dispersions and can gain an understanding of
the structure of the solid clumps being dispersed, particle
physics, surfactant phenomena, dispersion nomenclature, solution
and surface chemistry, and thermodynamics. The novice who
wishes to gain further competence in dispersion technology can
follow the detailed suggestions at the end of this chapter.
More experienced technologists will find a discussion of the
chemical classes of surfactants, commments on the process for
selecting a dispersant, suggestions on how to utilize solution
and surface chemistry to best disperse a particular solid in a
particular liquid, references to more comprehensive treatments of
the models, equations and data to illustrate dispersion behavior,
tests (for both the plant floor and the research lab) for
evaluating the quality of a dispersion and suggestions of
alternate ways to deal with common dispersion problems.
This series does NOT attempt to give the linguistic sources for
dispersion terminology, discuss the history of why the specified
models are thought to be best, show extensions of those models,
derive equations, include data beyond the most common
industrial solids and liquids and surfactants, or provide
illustrative problems that might serve as homework assignments.
In compensation, it refers the reader to many readily available
textbooks, advanced treatises, symposia reprints, encyclopedic
collections of data, technical organizations, and manufacturers
of surfactants where such supplementary material may be found.
When you have completed your study of this series,
- You should have a heightened awareness of the
complexities of solid-liquid interactions and the scientific
principles that govern them.
- You should be able to use the principles,
equations, and data from this series (and other sources) to select
several surfactants that will successfully disperse a given
powder in a given liquid, and you should be able to make
comparisons and optimizations to arrive at the best concentration
of the best surfactant from that group.
- You should be able to understand the
terminology of technical articles and advertisements relating to
the dispersion of powders in liquids.
- You should have some fresh ideas about how to
approach and how to solve dispersion problems.
- You should know how to locate consultants,
vendors, articles, and books that deal with dispersion problems.
Organization of the Contents
This series is designed to help you rapidly find, understand, and
apply the concepts, equations, data, and suppliers that you need
to solve a specific problem. It isn't necessary to read it from
beginning to end; the sections contain some redundancy to help
those who use the Index or the Table of Contents to locate a
topic for immediate study.
Part 1a defines many of the terms used in industrial slurry
operations and the important features relating to dispersion
technology. Part 1b illustrates the terms used to
describe clumps of particles and the concepts involved in
characterizing particle volume distribution. Part 2 reviews
the fundamental forces between particles as modified by the
suspension liquid and surface hydrolysis. Part 3 surveys the
chemical classes of particles and liquids and surfactants,
illustrating the chemical structures of many classes of
surfactants.
Following this discussion of the system using models that focus
on the forces between individual particles and molecules, we
examine the system using models that focus on the free energy
of a two-dimensional interfacial phase separating two two bulk
phases. Part 4 reviews thermodynamic concepts for bulk phases
and explains the new terms required when an interface is present.
It includes a discussion of adsorption on the solid. Part 5a
discusses the formation of flocs, micelles, and surface coatings.
Finally we get to the practical considerations of selecting a
dispersant and quantifying the behavior of a slurry. Part 5b
provides a procedure for selecting a dispersant for a particular
solid in a particular liquid and then optimizing that
formulation. PArt 6 describes tests to characterize the
quality of a dispersion and outlines the principles behind the
advanced instrumental techniques available for monitoring the
type, quantity, and location of the chemicals present in a slurry.
Tables provide data for use in determining how typical
solids (Part 2b), liquids (Part 2c), and surfactants (Part 3b)
interact in slurries. Part 3b provides lists manufacturers
from whom you may learn about surfactants beyond those noted
in this series.
Part 4b defines the symbols and units used for the more common
variables; the less common ones are defined near the equations in
which they are used. SI units have been used throughout; the
factors for conversion from other units are given in Part 4b.
The units associated with a variable are given is square
brackets in the text -- for example, the sedimentation velocity
is vsed [m/s].
Definitions of technical terms are included at the most
appropriate point in the text.
The Annotated Bibliography describes numerous major texts
and data compilations that may satisfy more advanced or more specific needs.
Difficulties with the Subject
The solid state is the most complex state of matter because the
molecules within a solid cannot readily move to new positions.
This has several major consequences:
- It prevents the surface from responding to the
surface tension, which would otherwise pull it into a simple
spherical shape.
- It prevents solid particles which come into
contact from merging into a single piece of matter.
- It limits thermal diffusion, which would
otherwise allow heterogeneities in surface or bulk composition to
become distributed evenly.
Thus, while only a few parameters are required to describe a
bottle of an impure liquid such as oil, many parameters are
required to characterize a bucket of an impure powder such as
coal, which is comprised of particles of various sizes, shapes,
and compositions. The dispersion of a commercial powder in a
commercial liquid using a commercial surfactant (with its own
impurities) introduces further complexities due to the
interactions between the major components of the solid, liquid,
and surfactant and all the impurities.
The purist might just as well stop reading right here and avoid
surface science altogether. There will never be models,
equations, and data that can describe a slurry as well as we can
describe gas and liquid systems. The best we can do is to gather
information describing the range of materials expected to be
present, choose data and equations for a model which we believe
will be illustrative of the system, make some tests to
characterize the prospective mixtures, and hope that the results
will lead us in a fruitful direction.
The process of choosing surfactants has been called a "black
art", implying that practitioners must use nonscientific (and
thus disreputable) means to solve problems. While it is true
that the addition of dried snake egg yolks and the use of a few
vigorous curses will help to disperse a powder in a liquid,
responsible slurry technologists will make better progress by
following the sound technical principles of surface science. So,
look through the extensive range of topics presented here, learn
some the nomenclature required to understand the literature of
surface science, and then enjoy the benefits that flow from
applying the concepts of surface science to both industrial
problems and everyday life.