Screens in the Process Industries ALBERT E. REED The W. S. Tyler Company, Cleveland, Ohio
T
HE proper applica-
tion and use of woven wire screen and screening machinery is of growing importance in every industry where niaterials are separated accordingto particle size, but particularly in the process and chemical industries because of the extreme accuracy required. To illustrate, consider the requirements in the manufacture of acid phosphate for fertilizer. The phosphate rock is usually ground to 100 per cent -65 mesh and sometimes to 100 per cent -80 mesh with from 65 to 85 per cent -100 mesh. If the phosphate rock is not ground this fine, the time required Fiomrr; 1. W I R E SCREENS for the acidulation procAXTER $:XFOSURE IN 1 H E SAXE ess is too great. If the Soi,u,rIoN F O R T H E S A U E LENGTH OF TIME material is finer ahead High durability w m shown only by of the acidulating procthe aoieen st the bottom. ess, t h e results m a y again be unsatisfactory. Therefore the grinding of fertilizer of this kind must be continuously controlled by means of screens. Another example is in the manufacture of cement where the fineness of the particles and the proportioning of the various sizes has an all-important effect on both the grinding efficiency and the strength of the finished product. Before
A.
Drawn wire
B.
screens were used in closed circuit with preliminary grinders, material as coarse as 8 and 10 mesh was delivered to finishing mills, which greatly reduced their efficiency. By the use of screens a feed of 100 per cent -20 mesh or finer up to -35 mesh is now delivered to the finishing milts. Since the various grinding operations in the manufacture of cement require about 40 per cent of the total labor, 80 per cent of the total power, and 75 per cent of the total repairs, and since increases of 50 to 100 per cent in the capacity of grinders is achieved by closing the circuit with screens which can be operated on less than one horsepower per unit, the advantage of closing the grinder circuit with screens is evident. Compared with most other processes, screening is remarkarkably low in cost. In screening salt, for instance, as inany as five separations have been made by one manufacturer at a screening cost of only $0.00227 per ton.
Special Problems The use of screening machinery and screen cloth in the process industries gives rise to a number of special problems. For example, the selection of the metal is of primary consideration both in order to extend the life of the screen and to avoid contamination of the prorluct with oxides and other compounds formed by the action of the chemicals on certain metals. The use of soreen cloth in the process industries also brings up the problem of screening materials in the presence of water and other liquids. Materials screened range all the way from slightly damp solids to liquids containing only a small percentage of solid material. The presence of liquids not only involves the selection of clotli which can withstand the action of various chemicals, but brings up the problem of corrosion and electrolytic action caused by the presence of t v o or more different metals. The screening of liquids differs considerably from the screening of solids because of the surface tension of the liquids and
Wire suitsble for w e ~ ~ l n g
c. wire tW soft for use
IN GRAIXSTRUCT~LE OF THREE TYPES OF WIREC w r (X200) ~ FIGURE 2. DIFWERENCE
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INDUSTRIAL AND ENGINEERING CHEMISTRY
VOI,. 30, NO. 12
DECEMBER, 1938
INDUSTRIAL AND ENGINEERING CI1EMISTRY
It is important to note that there is no specific rule for the selection of wire cloth for special uses; the only dependahle selection is by the trial-and-error method. By testing a number of samples of different metals in a quantity of the solution employed in the process, i t is possible to select the one which will give most satisfactory all-around service. The screens in Figure 1 were all exposed to the same solution for the same length of time, and all but one of them were badly damaged or totally destroyed hy the action of the chemicals. The one which stood up in this test was selected for use. I n making tests to determine the proper wire cloth to be used, the coaperation of the wire cloth manufacturer is extremely helpful. By taking advantage of the previous experience of the manufacturer, it is often possible to eliminate many choices which he indicates would be unsuitable for the purpose. I n adapting and selecting various types of wire cloth for use in the process industries, it has been found that considerable help may he derived from microscopic examinations of the metal. It has been proved that every operation in the drawing and annealing processes has a definite effect on the grain structure of the wire. It is necessary to he sure only that the grain structure is similar in order to obtain similar results in service from two different pieces of wire cloth. Photographs showing the difference in grain structure of three different samples of wire cloth are shown in Figure 2. A and C show wire unsuitable for the purpose, and B represents wire which has been treated to give it the necessary properties for the work to be done. Woven-wire screen now promises to play an important part in the mining industry for use in place of claasi6ers. Screening offers the advantage of accurate separation according to exact size of the particles, regardless of specific gravity, whereas classification depends on a difference in the weight of the particles in order to separate them. Classification caunot promise a defioite division according to size. Recent experiments a t the Copper C I S concentrator plant of the International Nickel Company have proved the practicability of screening a t 65 and 100 mesh in closed circuit with grinders.
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Screening is likewise proving its efficiencyin the production of color, coating, and filler solutions for paper; highly profitable volumes are being screened with a product which is 99.25 per cent -325 mesh.
EmCTaIC SCREEN E q m P P e n WITH A SPECIAL TYPE OF FEEDER FOR SCREEXINO SUOARJUICE
Another recent application of wire screens in the process industries has been in the developnient of a 230-mesh bolting cloth. This cloth has an opening equivalent t o a 200-mesh testing sieve with a 46 per cent discharge area, giving it remarkable bolting capacity as compared with silk and gits gauze. It is also far more durable. In the fertilizer industry, as well as in other applications where damp, sticky, or fibrous materials are handled, many advantages are secured by the use of a long slot opening screen This type of screen has a greatly increascd capacity and avoids blinding. Another recent deyeIopment which promises to add to the usefulness of woven-wire screen are two types known as flat top and rolled top. These screens present a smooth upper surfaoe without projecting knuckles of wire. This reduces the friction of material passing over the screen and FIGURI~ 3. ROLLEE-TOP BACKINO SCREEN FOR USE IN SUPPORT- facilitates scraping or shoveling the screening surface. The IN0 F I N S %REENS, ESPEOULLY FOR FILTEaINO OPERAT~ONS rolled-top screen (Figure 3) is used as a supporting surface under fine wire cloth; the smooth level surface provides maximum support with a minimum of friction so as to reduce The use of corrosion and abrasion-resisting wire screen has the wear of the fine clotb greatly. aided greatly in securing efficient, economical operation. The rapidly expanding process industries will find hundreds High-speed electrically vibrated screens are employed for of new applications for the use of woven-wire screen snd this purpose; the action of these screens is especially suited screening procesw within the next few years. The accurate, for wet screening of fine solids suspended in water. Electric positive selection of materials according to particle siae has vibration is especially effective in putting the material not been achieved by any other method, and screening meets through the screen without blinding; the impact of the all the requirements of a high-capacity, continuous, low-cost vibration is sufficient ta prevent the material from wedging operation. between the wires. This type of mechanism is not damaged by water or other liquids. RSCZSWO J U ~ Ya t , 1a3s.
