materials Handling ~~
~~
Hydraulic conveying may solve many solids handling problems T. W. Rodes
s
I K C E 1936 slurry and cement have been pumped through pipes a t a mill of the National Portland Cement Co. ( 2 ) through a system briefly described as follows: Water is added to quarry rock as it goes into the grinders. ~h~ slurry discharged from the tube mills is delivered to a serving two pumps. The two pumps discharge therr contents into a common pipeline 8 inches in diameter, which delivers the slurry into slurry silos. Lubricated plug valves are used on both the suction and delivery pump lines. There are six slurry silos with conical bottoms, arranged in two rows. The bottoms of the silos are connected by means of duplicate sets of piping. One system leads t o a slurry Pump ahead of a 6-inch pump driven by a 100-hp. motor. If the slurry in any silo must be blended with that in another silo, the requisite amount of slurry is drawn off from each and delivered to the slurry pump. This slurry is then pumped into a third silo.
There is a large slurry basin holding slurrythousand sufficientbarrels for manufacturing several of cement, ~h~ pumps for handling the slurry leaving the basin in a pump pit at One end Of the basin. There are three 3-inch pumps for delivering the slurry to the kilns, each coupled direct to a 40-hp. motor.
the huge number pf "lids that are conveyed In the foregoing system' it be said 'Onsidering
lions
Of
that hydraulic conveying is a major means for handling bulk solids. We might also say it is not a particularly new technique for handling materials. Earlier, experiments on conveying crushed ores suspended in the material ore slimes in pipelines failed because of the high velocity of flow required to convey the ore and because of difficulties due to clogging when shutdowns became necessary. Certain magnetites, artificially prepared magnetic substances, and attrition iron from ball mill circuits, when magnetized, form suspensions that do not pack. Wuensch (3) has indicated the practicability of pipeline transportion of ores. viscous media are used to f a d tate suspending the heavier ore partides at low velocities, to minimize abrasion, and to avoid segregation of November 1955
ore particles in the line during a shutdown. According to a proposal by Wuensch, the ore is crushed to - 3 inch or finer and introduced into a conical-shaped hopper, in which a part of the medium is recirculated to facilitate feeding the ore-medium mixture into the piston-diaphragm pumping unit. It would be necessary
pressure liquid dishcarges fromthe nozzle, the suction substance is entrained. A diffuser of the eductor causes the two materials to be mixed and they are discharged against a counter pressure. Unlike other types of pumps, the eductors mill handle solids and can be used intermittently, a s they are self-priming. Uses include
to pump 160 gallons of the oremedium mixture to transport 1 ton of average-type ore per minute. When the mixture reaches its destination, i t is demagnetized by passing i t through a n alternating current field. This deflccculates the magnetic component, and the medium is separated from the ore over a vibrating screen fitted with '/*-inch lower-deck cloth. The l/s-inch ore-medium mixture is then further treated in other cones, magnetized by a direct current field, and flocculated. The slimes are recovered and pumped to a medium storage tank for re-use. Three sizes of ore particles are collected, - 3 to +'/a inch (coarse), - l/s t o +SO mesh (fine), and -80 mesh (very fine). The return medium can be recirculated intermittently by reversing the flow in the pipeline, or returned to the initial pumping unit in a separate pipeline.
pumping sand, cleaning sludges out of pits, and conveying granular solids through pipes. A very common and most useful service is conveying foam powder in fire-fighting equipment. Pumping sand-water slurries
Experiments on the pumping of sand-mater slurries through 2- and 3inch pipes were described by R. A. Smith, a t a meeting of The Institution of Chemical Engineers in London, March 15, 1955. Smith indicated that many problems are faced by the designer of an installation for pumping slurries. The pressure drop in the system must be estimated in order to choose a suitable pump for the service. All precautions must be taken to avoid piping arrangements that clog or choke and the system designed so i t nil1 be
Water jet eductor
The water-jet eductor is use"lids and Slurful for
ries. There are 'pplications in the materials handling field and these devices may serve a mixing function as well as to convey solids in suspension. Water-jet eductors operate on the jet principle, utilizing the kinetic energy of one liquid to cause the flow of another. Through the use of a conveying nozzle, the velocity of the pressure liquid is increased to the maximum possible with the pressure to which it is subjected. As the
Figures 1 and 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Materials Handling possible to restart after a power failure or other cause necessitates a shutdown. The investigation made by Smith was undertaken as an introductory study. An experimental installation was set up for studying the pumping of slurries t h o u h 2 inch- and 3-inch-diameter horiaontafpi ks. Many of the slurry-pumping probLms met with in the chemical industry are concerned with difficult fluids, which are liable to choke the pipe by the deposition of scale on the wall of the pipe, or by the growth of crystals, or by the coagulation of colloidal material. The problems of handling such slurries are obviously outside the scope of an introductory investigation and must be dealt with by experiments on a sample of the slurry concerned; the present investigation was, therefore, confined to slurries of sand and water as typical of a simple slurry free from any of these complications.
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Wilhelm, Wroughton, and Loeffel ( 2 ) have examined the problem of the flow of suspensions through pipes and an internal document ( 1 ) gave some measurements of pressure drop in the pumping of boiler-ash slurry through an experimental installation. Both these references showed t h a t a t high velocities the correlation between friction factor and Reynolds number was the same for a slurry as for a homogeneous liquid (the density being taken as that of the slurry and the viscosity as that of water); as the velocity was reduced, a critical velocity was reached, below which the friction factor increased rapidly with decreasing velocity, ultimately reaching a value many times greater than that for the homogeneous liquid. When pumping a slurry it is usually thought necessary to use a velocity of 6 or 7 feet per second, which is appreciably greater than the economic velocity of 3.5 to 4 feet per second for water. A saving could be made if the velocity of a slurry could be reduced to 4 or 5 feet per second without choking the pipes. It was decided that in the initial program further information on the pressure drop in horkontal pipes should be obtained and the conditions under which solids begin to settle on the bottom of a horizontal pipe should be studied (the velocity below which solids settle on the bottom of a pipe is termed the “settling velocity”), using sand slurries of different concentrations and particle sizes. Information was also sought on the efficiency of the centrifugal pump for slurries of different concentrations.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 11
Figure 1 shows the diagrammatic arrangement of the experimental installation and Figure 2 is the arrangement devised for periodically flushing the pressure connections with clean water to prevent them from clogging. Sixty-six tests were carried out and several measurements were made during each test t o determine pressure drop, first a t the maximum rate of flow and then at reduced rates. The concentrations of slurry were varied from clean water to about 700 kg. of solid per cubic inch of slurry. The following is Smith's summary. -t
Experiments on the pumping of sandwater slurries through 2- and 3-inch-bore horizontal pipes are described. Sands of several size gradings were used at concentrations up to 27y0 by volume. The pressure drop was measured a t velocities from 3 to 8 feet per second and the velocity a t which sand first began t o settle on the bottom of the pipe was noted. The pressure drop was found to be always greater than that calculated on the assumption that the slurry behaved as a homogeneous liquid of the same density. For all sizes of particles in both sizes of pipe and at all concentrations greater than 10% by volume, solids first began to settle on the bottom of the pipe when the velocity of t h e slurry was about 5 feet per second. The results are compared with a published correlation based on experiments with closely graded materials. To apply this correlation to materials of mixed sizes it is necessary to calculate the equivalent diameter as the diameter of a particle whose surface area-volume ratio is equal to that of all the particles. It is shown that the correlation can be used to predict pressure drop approximately, but for more accurate information it would be necessary to carry out a small-scale experiment on the pumping of a sample of the actual slurry. The characteristic curves of the centrifugal slurry pump used in the experiments were determined. It was found that the efficiency of the pump fell only slightly with increasing concentration of slurry up to 2401, by volume. We believe there are numerous applications for hydraulic conveying and with recent developments in pumps, the principle will be found both efficient and economical where applicable.
Large, denre floc formed with alum plus activated d i r a sol
Strong, bulky floc formed in raw water by activated silica sol and alum traps suspended matter in water and then settles rapidly. The result is sparkling water, clear as crystal. Activated silica sol refers to negatively charged colloidal particles produced from our N sodium silicate (41' Re., ratio 1:3.22) and a reacting chemical like sodium bicarbonate, chlorine, sulfuric acid, ammonium sulphate. (Our N-Sol Processes for preparing sols are licensed without royalty.) Silica sol also is used successfully in clarifying waste waters, such as paper mill, refinery, textile, canning, plating. Available on request instruction sheet and samples of N silicate with reacting solution for jar tests.
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literature cited (1) Butz, R. H., Heating, Piping, Air Conditioning,8, 442 (August 1936). (2) Wilhelm, R. H., Wroughton, D. M., and Loeffel, W. F., IXD. ENG. CHEM.,31,622 (1939). (3) Wuensch, C. E., Eng. Mining J.,145, No. 4, 91 (1944).
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November 1955
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