TECHNOLOGY
Felvation speeds powder fractionation Method of fractionating powders by particle size combines advantages, avoids drawbacks of elutriation and sieving
Scientists at Chicago's IIT Research Institute are developing a novel approach to fractionating powders. According to its inventors—senior physicist Brian H. Kaye and associate chemist Meryl R. Jackson—the method, dubbed felvation, combines the most advantageous aspects of both elutriation and sieving. But it avoids the drawbacks inherent in these, the most commonly used ways of dividing powders into particle-size fractions. Dr. Kaye says that the new approach has promise of being faster and more accurate than most of today's methods—especially for fractionating very fine powders of 10 microns or less. A fractionation can be completed in about 10 minutes with the new technique, compared
Combination of fluidization, elutriation, and sieving fractionates powders by particle size
To filter system to remove fine particles from stream
Sieving — Finer particles pass through
n
Flow from reservoir
^ Micromesh ' sieve Elutriation Larger particles tend to settle
J^iL
Fluidization
n\U
j/
Powderfluidizedby flow
50 C&EN MARCH 6, 1967
Powder to be fractionated Coarse wire screen
with the several hours needed with many present elutriation methods. Also, the IITRI method avoids the binding and clogging problems common to many sieving operations. The method is being developed as an analytical tool. But Dr. Kaye says that there is no reason why it could not be applied to particle-size control in manufacturing powdered products. Control of the particle size of powders is critical in many industries. For instance, many drugs have to be of an optimum particle size or they are absorbed either too quickly or too slowly by the body. Particle size is also important to the performance of paint fillers and pigments. The strength of sintered metal parts depends on the particle size distribution of the powders used in making them. Catalysts in fluidized-bed reactors used by the petroleum industry must have the right particle size distribution if they are to work properly. If the particles are too big they remain inert at the bottom of the reactor. If too small they are swept away with the product stream. Also, the reinforcing effect of pigments used in the rubber and plastics industries is related to particle size. Even in the food industry, particle size is critical. For instance, if crystalline sugar contains discrete particles larger than 20 microns, it feels gritty. Particle size distribution can be determined by two basic approaches: directly and by fractionation. The direct method involves actually measuring individual particles—for instance, visually with a microscope. Other direct methods include sedimentation and stream methods by which particles in a fluid are detected and measured as they pass through a sensing zone monitored with light beams, ultrasonic waves, or electrical resistance measurements. In fractionation, the powder is separated into fractions of different particle size. Elutriation and sieving are common methods. The basic apparatus for the IITRI method consists of a column 4 inches in diameter and 24 inches high. It has a tapered conical section at the bottom. A carrier fluid such as water is flowed up the column. The powder is first fluidized at slow flow rates in a predispersion unit and at the bottom of the column. Increasing flow rates then give a crude separation by elutriating the dispersed powder up
FELVAT10N. Meryl R. Jackson (left) and Dr. Brian H. Kaye prepare the felvation column for an experiment. With the column, they can fractionate mixtures of very fine particles—10 microns and less—quickly and accurately
the column. The final separation comes as the finest particles pass up through a micromesh sieve placed across the column. These particles then pass from the top of the column and are filtered from the fluid stream. The system can be used with either one or several sieves arranged across the column. When several are used, they are in order of decreasing aperture size up the column. Elutriation is a process of separating powders into fractions by suspending them in a moving fluid, either liquid or gas. In the usual vertical elutriator, particles smaller than a certain size are carried up the column with the fluid. Larger particles settle to the bottom. The main problem with the method is that flow velocity is not uniform across the column. It is faster in the center than near the sides. This difference disturbs the flow and prevents a sharp separation of the two fractions. Also, the flow rate needed to fractionate very fine powders can be so slow that up to several days are required for one test, Dr. Kaye says. An example of a commercially available air elutriator is a particle size analyzer offered by American Instrument Co., Inc. It can divide a powder into any number of fractions from 5 to 75 microns. It can take as little as two hours for an analysis, but it can take much longer. Even the basically simple sieving
process has its troubles when it comes to very fine powders, Dr. Kaye points out. For instance, a sieve usually has to act as a "go, no-go" gage and as a support for the powder. This puts a severe strain on the easily damaged and expensive sieves used with lowmicron particle sizes. Moreover, fine sieves are easy to clog and hard to clean. Another drawback is that mechanical shaking of the sieves can cause attrition of the powder and change its particle size analysis. Several methods are used to overcome these problems in sieving methods currently used. For instance, one system uses a vacuum below the sieve to speed passage of the fine particles and an air jet system to clean the sieve apertures. In the past year, AllenBradley Co. has introduced a new sonic sifter. This unit uses an air column sonically pulsed to lift the powder in the sieve, disturb it, and then drop it back on the sieve again 50 times every second. This method can complete a dry sifting in a few minutes. The action of the air column tends to keep the sieves clean. With the IITRI method, the powder to be fractionated is placed in a narrow feed tube at the base of the felvation column. A slow rate of fluid flow is started and the first stage of the process is reached when the powder becomes a fluidized bed in the conical base of the column. This highly turbulent state gives excellent
dispersion of the particles, according to Dr. Kaye. Increasing the fluid flow gives a primary separation as the finer particles start to move up the tube. As the fluid flow rate is slowly increased the smaller particles reach and pass through the fine-mesh sieve placed across the column. The flow rate is gradually increased further so that larger particles are elutriated up through the sieve. Because the sieve acts as a throttle to fluid flow, there is a locally increased flow rate through it. This prevents particles that have passed through from falling back on the sieve and blocking it. The final stage of felvation occurs when particles just larger than the sieve apertures are elutriated up to the sieve surface. These large particles do not immediately block the sieve unless there is a further increase in flow rate. As these particles might start to block the apertures, the local flow rate around them drops and they tend to fall from the sieve. With this system the sieve acts only as a gage. It does not have to support the powder. Because elutriation is used only as a crude primary separation and as a way to transport particles to the sieve, there is no need to design the system to give exact laminar flow. Dr. Kaye points out that another advantage of felvation is economy. The fine-mesh electroformed sieves used in the column are only about 1 inch across. They would cost only a few dollars each if produced commercially, he says. The 3-inchdiameter electroformed screen used in mechanically shaken sieving operations costs up to $90. In tests at IITRI, Dr. Kaye and Mr. Jackson have fractionated 1-gram samples of chromium powder through a 25-micron screen in 10 minutes using 1 gallon of water as the fluid. They recovered 97% of the powder and the sieve suffered no damage and needed no cleaning between runs. The test could probably be run in less than 10 minutes once optimum conditions are determined, Dr. Kaye notes. In other experiments, 2-gram samples were fractionated through a column with both 10-micron and 18micron sieves in one hour with 9 8 % recovery. Again, the fractionation could probably be speeded with more experimentation. MARCH 6, 1967 C&EN
51
