November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
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indication that the two types of exposure are not exactly equiralmt. I n none oi the samples were there any changes which could be detected visually. Figure 5 is a photograph of Silastic and sj-nthetic rubber samples which have been held looped for a year out of doors. S o n e of the Silastic stocks has an?- tendmcy to crack if held under tension. Figure 6 s h o w some strips t h a t r e r e stretched 20% out of doors for a year and likewise did not check or crack during this period. The n-eights of t h r samples did not vary more than O . l % , and t,heri: were no measurable changes in dimensions. Both the outdoor exposure and the 50-day Weather-Ometer tests indicate that titanium dioxide in t h e filler of Silastic stocks contributes t o the retention of elongation. On the other hand, silicon dioxide contributes t o the retention of tensile strengt,h. Although t,he evidence is not conclusive, it, seems that the tiFigure 6. Stretched Silastic Samples after Outdoor Exposure for tania-filled stocks have slightly better over-all One Year weather stability than the silica-filled stocks. These test,s are being continued at Midland 60” exposure rack facing south at Nidland, X c h . Some other and any iurthrr changes will he recorded everv 6 months. samples were placed in a n Atlas single-arc Weather-Ometer deLITERATURE CITED signed t o intensify the conditions of moisture and ultraviolet light (1) .Im. doc. foi Testing Materials, Standards on Rubber Products, encountered in outdoor weathering. A4.S.T.lI.procedure ( 2 ) Method D412-41, pp. 43-9 (1946). was followed in testing flexed pieces and stretched pieces of (2) Ibid.. Method D51844. D D . 66-9. (3) Ibid., Method D67644T; b ~112-14. . Silastic. Flat moldings-for use in tensile testing were exposed at (4) Ibid.. Method‘D47146T, pp. 146-54. the same time. (5) Ibid., Method D73643T. pp. 158-60. I1 the effects o f Outdoor exposure On (6) Hunter, M.J., Hyde, J . F., Yarrick, E;. L., and Fletcher, H. J., J . Am. Chem SOC..68. 667 (1946). Both tensile strength and elongation are somewhat reduced, (7) Irish. E. M., and Stirrat; J. R:, Product Eng., 18, 146-50 (1947). while durometer values increase- only slightly. Table 11 also (8) Servais, p. c.,I n d i a Rubber World, 114, No. 5, 659-62 (1946). ~ aging on t,ensile shon-s t h a t the effects of 5 0 - d ~\i-eather-Ometcr (9) Servais, P. C., Ibubber Age (N. Y . ) , 58, No. 5 , 579-84 (1946). strength and elongation are of t,he same order of magnitude as RECEITED J u n e 10, 1947. Presented before the Spring Meeting of the Divithose Obtained in year Of outdoor esposure’ Ho”-everl the sion of Rubber Chemistry, AMERIcas CHEMIC.AL SOCIETY,Cleveland, Weather-Ometer aging had little effect. on durometer hardness, a n Ohio, 1947,
Industrial Applications of Diatomite Filters E. G. KO3IINEK I~iJilcoZnc., Chicago, I l l . Diatomite filters are finding wide industrial application because of the adtantages of saving in space and weight and more effectite filtration. Operating rates as high as 8 gallons per minute per square foot can be employed, and, as precoagulation is not required, sparkling-clear filtrate can be obtained in handling many aqueous or nonaqueous solutions. operating costs are higher than the cost of sand filtration, but there are nianj cases where over-all etaluation favors installation of diatomite filters when the installed cost and the space requirements are taken into consideration.
D
I A T O M I T E filters, like many other new or improved types of industrial equipment, n-ere given their first tests on the battle fronts of World K a r 11. It Tvas these tests t h a t brought out the many advantages of diatomite filters xvhich are as important in many respects t o industry as t o the armed forces.
Tht, advantages can ht, suniniarized a!: savings in rpace and w i g h t and more effective filtration. I n some instances, as cited in test wsults which are later discussed, the diatomite filters afforded a solution to filtration problems which could not he satisfactorily handlrd by conventional methods. In others the considerably lower installed cost of a diatomite filter has led to its selection itistrad of that of sand filters for thc filtration of preclarified \vat csr . The f o h n - i n g tabulation compares the United States ;irniy’s pack and mobile units with pressure sand filters, and illustrates the saving in spacc and wc4ght rvhich resulted from their use: Pack Unit Filter area, sq. f t . 3.6 ~ ~ ~ e g d h t c , a P b a c i t ~ , g , p . n l .l 5 30 Diameter, in. 8 Over-all height, in. 2% ~ l ~ ~ ~ 10 ~ X 10 ~
Pressure Mobile Sand FiIter Unit 7.1 10 15 50 2860 350 18 36 30 73 ~37 X ~49 ~ 21 , X 24i
Pressure Sand Filter 19.6 50 8050 60 82 ~8 1 X .78
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INDUSTRIAL AND ENGINEERING CHEMISTRY
I n addition to the fact that the saving in space and weight alone would justify the use of diatomite filters, an equally important benefit is derived from their use in that chlorine-resistant organisms are removed, bs are all suspended solids, by means of filtration through diatomaceous earth. A sand filter is virtually a atrainer which will catch only particles of silt and other suspended matter unless the water is properly coagulated prior to filtration The accumulation of coagulum or floc particles resulting from precoagulation forms a gelatinous m a t which retains the mud, bacteria, etc., removed by the filter. T h e proper pretreatment of the water required considerable technique on the part of the operator, as an excessive amount of coagulum n ill clog a filter; whereas the formation of an exceedingly fine floc will cause pasgage of some of the floc and the contaminating substances present in the raw water through the filter (5). Whereas a sand filter reguires an operator n-ith experience and knowledge of filter operations, excellent filtration is obtained through the use of diatomite filters even when no refinements of coagulation technique are employed in the pretreatment, because the diatomaceous earth forms a porous cake Kith evceedingly fine openings. It thereby filters out considerably smaller inipriiities than could be removed tyith a sand filter. Black and Spaulding (1) reported that, in a series of sixty-three Blter cycles. the diatomite filtered water after coagulation and settling could be expected t o approximate the United States Public Health Service standard, even though chlorination was not employed. It was further found that, while rapid sand filtration failed t o remove cysts of Endamoeba histolytica, diatomite Blters effected virtually complete removal of cysts under most severe tests. United States Brmy Engineers reported that the diatomaceous earth filters removed not only cysts of amoebic dysentery but also the cercariae of sehistosomia (blood flukes) which TTere prevalent in Philippine n-aters. These advantages led to the adoption of the diatomite filter as the standard for the pack and mobile purification units of the United States Army. TYPES OF DIATOMITE FILTERS
Several types of diatomite filters are being manufactured, the primary difference among them being in the construction of the filter elements. Diatomaceous earth requires a permeable base to support the filter cake. Although many materials are suitable for this purpose, including porous paper filter pads. 6lter cloths, x-ire screens, wire-wound tubes, porous stones, and rubber tubes, the material differ with respect to durability, response to backwash, and liability to plugging T h e helically n ire-wound elements and the porouc refractory elements m r e found to be the most satisfactory for the mobile units. T h e former Tvere used in the puiification units of the United States and British Armies and the latter in I'nited State, . Navy units. T h e n-ire-wound elements used b ~the British Army consisted of a wire winding over a fluted hollow metal core, whereas the units used by the Xmeiican Army utilized plastic for the core of the elements to reduce LTeight further. T h e mirewound elements had a spacing of 0.0027 inch between the wires, and the fact that the space opened rapidly from this minimum in both diiections made it practically impossible for anything to lodge between the wires. A further advantage of this construction is the fact that no backwash is necessary n-ith xirewound elements except to flush out the spent diatomite. Moreover, as the elements have smooth surfaces with relatively wide spacing betrveen the wires, plugging of the elements is a remote possibility.
Vol. 39, No. 11
A4nyporous and rigid base or septum satisfactory for the support of the diatomite mill be effective for the filtration operation, and a clear filtrate will usually be obtained if a sufficiently fine grade of diatomite is used. However, if the septum is susceptible to clogging because of the small diameter of the pores and the inability to clean the septum adequately by backmashing, periodic cleaning of the filter elements n-ill be necessary. According to Hollberg ( 4 ) , tests Txith porous stone elements indicated that clogging would take place in a relatively short time when the elements nere precoated using raw mater. Hollberg further discussed cleaning of the porous stone elements by soaking them in a concentrated solution of sodium acid sulfate and then immersing them in a 10% caustic solution. Although this cleansing operation could be applied to portable units, such periodic cleaning might be an inconvenient and expensive operation in a large filter plant. Moreover, the septum construction is even more important when the filtration of an unstable solution is considered. For instance, if a supersaturated solution were being filtered, a considerably greater tendency would exist for deposition in the fine pores of a porous stone element than in the wire-wound dement. I n any event, unless it Aould be possible to run long term pilot tests n i t h a liquid which was considered as being unqtahle, it ~ o u l dbe advisahle to make provisions for a periodic cleaning of the filter elements. TYPICAL FILTER.Figure 1illustrates a tlpical diatomite filter T h e filter shell n hich houses the elements is usually of plastic or glass in the smaller sizes. I n larger sizes the shell is of stccl or other metal, depending on the character of the liquid to be filtered, and an observation port iq provided so that formation of the filter cake can br observed. The elements are mounted verticallv on a removable plate and are inserted as a unit in the filter shell. The usual inlet, outlet, drain, and rewash connections are provided
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947
1415
ture a variety of grades of diatomite, and i t is generally a relatively simple matter to select the filter aid best suited t o a particular filtration problem. I n some instances, where a liquor having a high p H is to be filtered, and IT-here silica contamination is t o be avoided, a purified wood cellulvse can be used t o advantage as a filter aid. The Bronn Company offers a series of grades of a purified wood cellulohe from which all lignin, ash, and chemical salts have been removed. This has been used to advantage in the filtration of dilute caustic soda solutions. Asbestos powder is also used to advantage in some applications. In filtration of pretreated water, a precoat of 0.5 to 1 ounce per square foot of filter area is usually used. As the water is ielatively clear prior to filtration, and as the suspended particles are flocculated, the accumulated suspended matter is sufficiently porous t o permit long filter runs without a n excessively rapid increase in pressure drop through the filter. On the other hand, if the suspended solids are evceedingly fine, or if they tend to form a n impervious mat over the precoat, i t is necessary to feed diatomite continuously. By this means a porous cake is deposited on the precoat; this results in a less rapid increase in pressure drop and, consequently, in longer filter runs. I n cases where continuous diatomite feed is employed, the dosage will approximate a total of 3 ounces per square foot; but this will vary, de >ending upon the characteristics of the suspended matter in the water. COMPARISON OF COSTS
Figure 1.
Large Filter with Area of 80 Square Feet
with a backwash release valve in place of a x a s h valve on n sand filter. The filter is backwashed by releasing the pressure in the shell by opening this valve; this pressure release reverses the flow for about 30 seconds and cleans the elements. The spent diatomite settles in the conical bottom of the tank, from 1%-hichi t can be conveniently drained. The diatomite for the new precoat is added as slurry through the diatomite charger attached to the side of the tank. The ra\v liquor is Then added through the inlet at the bottom of the tank, the liquid and filter aid being forced upward around the elements. The precoat develops within a few seconds, and, after a short period of filtering to waste or recirculating the filtrate. a sparkling clear filtrate is obtained.
The amount of filtrate obtained per filter run and the amount of filter aid required depend upon the nature and the amount of solids in the water to be filtered: Influent Turbidity, P.P,hl. 2-3
0.04
Approx. Cost/ 1000 Gal., Cents 0.2
0.1-0.2 0.25-0.5
1.25-2.5
Diatomite, Lb./1000 Gal
10 60
0.5-1.0
SELECTION OF FILTER AID
Elsenbast arid 3lorris ( 2 ) tabulated the following particle sizes for Celite 503 and Filter-Cel: Particle Size, Microns
> 40
40-20 20-10 10-6 6-2
