‫ﻓﺮﻉ ﺍﻟﺴﻴﺮﺍﻣﻴﻚ ﻭﻣﻮﺍﺩ ﺍﻟﺒﻨﺎﺀ‬/‫ﺍﻟﻤﺮﺣﻠﺔ ﺍﻟﺮﺍﺑﻌﺔ‬
Advanced Ceramic
2-2.2.2 Evaporation of the Liquid
Evaporation of the liquid provides another method for bringing a solution
to super saturation, there by causing the nucleation and growth of particles. The
simplest case is a solution of a single salt. For the production of fine particles,
nucleation must be fast and growth slow. This requires that the solution be
brought to a state of super saturation very rapidly so that a large number of
nuclei are formed in a short time.
One way of doing this is to break the solution up into very small droplets so
that the surface area over which evaporation takes place increases enormously.
For a solution of two or more salts, a further problem must be considered.
Normally the salts will be in different concentrations and will have different
solubilities. Evaporation of the liquid will cause different rates of precipitation,
leading to segregation of the solids. Here again, the formation of very small
droplets will limit the segregation to the droplets since no mass is transferred
between individual droplets. Furthermore, for a given droplet size, the size of
the particle becomes smaller for more dilute solutions. This means that we can
further reduce the scale of segregation by the use of dilute solutions. We now
consider some of the practical ways of producing powders by the evaporation of
liquid solutions.
Spray Drying
In spray drying, a solution is broken up into fine droplets by a fluid
atomizer and sprayed into a drying chamber (Fig. 2.10) Contact between the
spray and drying medium (commonly hot air) leads to evaporation of moisture.
The product, consisting of dry particles of the metal salt, is carried out in the air
stream leaving the chamber and collected using a bag collector or a cyclone.
Spray drying principles, equipment, and applications are described in detail by
Masters. A variety of atomizers are available and these are usually categorized
according to the manner in which energy is supplied to produce the droplets.
1
‫ﻓﺮﻉ ﺍﻟﺴﻴﺮﺍﻣﻴﻚ ﻭﻣﻮﺍﺩ ﺍﻟﺒﻨﺎﺀ‬/‫ﺍﻟﻤﺮﺣﻠﺔ ﺍﻟﺮﺍﺑﻌﺔ‬
Advanced Ceramic
In rotary atomization (often referred to as centrifugal atomization), the liquid is
centrifugally accelerated to high velocity by a spinning disk located at the top of
the drying chamber before being discharged into the chamber. In pressure
atomization, pressure nozzles atomize the solution by accelerating it through a
large pressure difference and injecting it into the chamber. Pneumatic
atomization occurs when the solution is impacted by a stream of high-speed gas
from a nozzle.
Ultrasonic atomization involves passing the solution over a piezoelectric
device that is vibrating rapidly. Droplet sizes ranging from less than 10 µm to
over 100 µm can be produced by these atomizers.
The solutions in spray drying are commonly aqueous solutions of metal salts.
Sulfates and chlorides are often used because of their high solubility. In the
drying chamber, the temperature and flow pattern of the hot air as well as the
design of the chamber determine the rate of moisture removal from the droplet
and the maximum temperature (typically less than ~300°C) that the particles
will experience. The key solution parameters are the size of the droplet and the
concentration and composition of the metal salt. These parameters control the
primary particle size and the size and morphology of the agglomerate. The
morphology of the agglomerate is not very critical in spray drying of solutions
because the particle characteristics are largely determined by subsequent
calcination and milling steps. Under suitable conditions, spherical agglomerates
with a primary particle size of ~0.1 µm or less can be obtained. Because the
temperature in the drying chamber is commonly insufficient to cause
decomposition or solid-state reaction, the spray-dried salt must be subjected to
additional processing steps such as calcination and milling to achieve suitable
characteristics for processing.
Spray drying of solutions has been found to be useful for the preparation of
ferrite powders. For Ni-Zn ferrite, the solution of sulfates was broken up into
droplets (10–20 µm) by a rotary atomizer. The powder obtained by spray drying
was in the form of hollow spheres having the same size as the original droplets.
2
‫ﻓﺮﻉ ﺍﻟﺴﻴﺮﺍﻣﻴﻚ ﻭﻣﻮﺍﺩ ﺍﻟﺒﻨﺎﺀ‬/‫ﺍﻟﻤﺮﺣﻠﺔ ﺍﻟﺮﺍﺑﻌﺔ‬
Advanced Ceramic
Calcination at 800–1000°C produced a fully reacted powder consisting of
agglomerates with a primary particle size of ~0.2 µm. The ground powder
(Particle size < 1 µm) was compacted and sintered to almost theoretical density.
FIGURE 2.10 Schematic of a spray dryer for the production of powders.
3