COLLABORATIVE
REPORT
LOUIS A. AGNELLO, Associate Editor in collaboration with
HORTON H. MORRIS and FRED A. GUNN, Southern Clays, Inc., Gordon, Ga.
This clay from which crude kaolin is removed is piled on fop of the bank at right with the overburden pile at left. Eventually kaolin, the finished product, is used in papers, paint, plastics, rubber, and ceramic industries
CLAY
means various things to various people. To sculptors and children, it‘s a valued molding material for work or play. But to the average new homeowner, it conjures u p a frustrating mental picture of a barren yard where lawn ought to be. Talk about it to industry, and you find that it ranks highamongcontenders for title of “unsung w-orkhorse.” Observers place total clay sales for consumption by U. S. producers in 1959 a t nearly 47 million short tons-a respectable 7.570gain over 1958. And the immediate outlook is for continued clay growth-keeping pace with the expanding national economy. Long-time clay uses, such as in ceramics and refractories, are fairly common. But clay also plays an unobtrusive, but vital role in a host of widely diverse products which include : slick-page magazines and printing-grade papers, plastics and rubber, catalysts, electrical insulations, insecticides, and medicines.
kaolin, or china clay; ball claJ-, fire clay, bentonite, fuller‘s earth, and miscellaneous clays. The miscellaneous class includes: low grade surface clays and shale for building brick, clay for cement manufacture, clay for light weight-aggregate production, and others. Kaolin is now the leading white pigment-in point of tonnage-consumed by North American industry ( 3 ) . Last year, industry chewed up an estimated 2.5 million short tons of pigment grade
Table I .
Chemical Analyses of Kaolin Clays 41,
Coating Filler
Ceramic Cal(high cined strength)
1gn.Loss
13.73 14.00 0.34 13.60 44.00 44.70 51.03 45.12 38.64 A1203 40.00 39.50 46.25 0.34 0.40 Fen03 0.57 0.34 Ti02 1.36 1.02 1.27 1.00 CaO 0.04 0.05 0.41 - .. ._..0.11 . . ~ . .. 0.28 MgO 0.08 0.07 0.03 0.07 0.59 K 2 0 , Nan0 0 . 0 7 0.05 99.85 99.79 99.39 100.6 Total SiOz
What’s Kaolin?
~~
Clays are hydrous aluminum silicates of a great many mineral species, containing varying proportions of impurities. No two clays are exactly alike. And, despite the wealth of research data available, i1’s rarely possible to substitute one clay for another in industry without considerable experimentation. Classed broadly, principal industrial clays are:
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INDUSTRIAL
AND
Theoretical for 100% kaolin ( A ~ Z O2SiO2, J. ZHzO), % Ignition loss
Si02
13.94 46,55
-41208
39.51
ENGINEERING CHEMISTRY
kaolin. Bulk of it-perhaps better than 7O%-goes to the paper industry as a coating material in high gloss and quality printing paper grades. and as a low cost filler. The paint, plastics, rubber, and specialized ceramics industries account for the remainder. Kaolin is a relatively pure clay material, usually made u p mainly of kaolinite (A1203.2SiOa. 2Hz0) and related hydrous aluminum silicates (2). The name? kaolin, comes from the Chinese “kao ling,” an English interpretation of the word for ”high ridge,’‘ where kaolin deposits were found to occur in China ( 7 ) . Other common kaolin minerals are halloysite, dickite, and anauxiteany of which may predominate in some deposits. Catalytic grade halloysite is a clay with high natural catalytic activity used to manufacture catalysts for hydrogenation or cracking of high sulfur petroleum. I’VhiIe kaolins vary markedly in physical characteristics, their chemical composition fall within pretty tight limits. Good grades contain 37 to 40y0alumina (A1203); 45 to 55yo silica (Sios); u p to about 1% iron reported as ferric oxide (FenOs); 0.5 to 1.570 titanium (reported as TiO*), traces of alkalineearth and alkali metal oxides; and the balance, 8 to 15%, loss on ignition (2). But kaolin’s industrial selling points are by far its physical, rather than chemical, properties. Fresh from the mine, papermaking kaolin is a white,
friable, relatively grit-free material that is slippery to the touch. A pure discreet particle is a thin, flat, hexagonal platelet, which accounts for kaolin’s lubricity and its excellent coating properties. Particle size is the prime difference between coating and filler grades. Coating grades having a particle size below 2 microns range between 70 to 100%. Particles larger than this usually are suitable industrially only as filler. Flow properties, too, determine whether a particular kaolin is suitable as a coating material. But it’s this unique comlow bination of features-whiteness, price, availibility, chemical stability and inertness, and excellent coating and flow properties-that makes kaolin tough to beat as a coating material.
pared for market. T h e more experienced English producers, on the other hand, already had developed their processes to a high state. Actually, there’s little to choose between the two on a quality basis. English clays generally gain the nod slightly in whiteness but usually are a little more coarse and abrasive than domestic kaolin ( 3 ) . With the emphasis on improved clay technology and processing, domestic kaolin began to win over papermakers, and close the gap on imports. T h e balance shifted in favor of domestic kaolin about 1930, and U. S. producers began to pull ahead rapidly. By 1953, imported kaolin accounted for only about 6% of the total U. S. consumption, against the 88% it represented in 1910 (2).
History and Development
Domestic Deposits
Prior to World W a r I, imports of china clay-chiefly from the vast primary kaolin deposits in the Cornwall district of England-held the lion’s share of our domestic paper market ( 3 ) . The war cut off this supply and gave American kaolin producers the opportunity to refine their processes and upgrade their product. But, even u p until about 1930, U. S. papermakers considered English imports superior to domestic kaolin, particularly for paper coating ( 3 ) . This feeling stemmed from the fact that a t first the American clays were very poorly pre-
Bulk of industrial kaolin mined in this country comes from an irregular strip extending generally northeasterly across Georgia from a point near Columbus, Ga., on the west to Aiken, S. C., on the east (4). Four of the five top kaolin producers are located with a 50-mile radius in East Central Georgia. T h e big five producers turn out an estimated 1.2 million short tons, or about 50% of domestic industry’s annual consumption of pigment-grade kaolin. Experts peg known reserves of very high grade crude clays in the Southeast a t better than 100 million tons, not to mention the enormous
supply of lower quality deposits chat could be upgraded for industrial markets if the need arose ( 3 ) . These deposits are sedimentary, or secondary, kaolins. Millions of years ago, their parent rocks-feldspar and mica schists, and probably volcanic ash, and a host of other aluminous materialslikely were found along the Piedmont Plateau. Slowly, by the disintegrating action of the elements (both chemical and physical) these rocks rotted and the fine particle debris was washed generally southward into coastal lagoons which formed the shores of the Atlantic Ocean (located a t that time about where the city of Macon, Ga. now stands) (4). One of the top k a d i n producers is Southern Clays, 1nc.-a progressive, research-oriented company with executive headquarters in hTew York, and huge mining and processing operations in Gordon, Ga. Southern Clays’ Lustra, Columbia, Whitetex, and Al-Sil-Ates lines are familiar sights to the paper, plastics, paint, rubber, and ceramics industries. T h e company got its start in 1927 under the name P. W. Martin, Gordon Clays, Inc. After a reorganization in 1947, the company became Southern Clays, Inc. Massive Processing Equipment Southern Clays’ huge Gordon plant covers a 50-acre site in this small town located in East Central Georgia, about 20 miles east of Macon. A trip through the VOL. 52, NO. 5
M A Y 1960
371
\
w SOTARY CRYER
\ /
PLANT PROCESS SERIES
LOADING AND BAGGING i
'
9
1
L DRUM D R Y E R
I
v
I v
A Y
Flowsheet for the manufacture o f kaolin, Southern Clays, Inc., Gordon, Ga.
372
INDUSTRIAL AND ENGINEERING CHEMISTRY
mine and plant is a study in huge-volume materials handling and processing techniques. Items include :
b Massive earth moving equipment, capable of gouging out mountains of earth to get a t the clay deposits b Its own internal railroad, with large diesel locomotives to haul mine cars over eight miles from the mine to the plant Giant process equipment-particlesize classifiers, filters, dryers, and calciners.
E
k
-
4
4
I n summary, the kaolin cycle is: exploration, mining, dispersing, cleaning, particle-size separation, flocculation, bleaching, and drying (optional). Another optional final step is heat treating to drive off the chemically combined water which improves certain clay properties (primarily whiteness). Chemistry Chemistry plays leading roles in the dispersing, flocculating, and bleaching stages. T h e addition of certain agents (such as condensed sodium phosphates or silicates) to a kaolin suspension causes a change in the charge distribution a t the surface of the clay particles. As a result, repulsive forces between particles become greater than the attractive forces and the clay is said to be dispersed. This is largely due to highly charged segments of the absorbed phosphate or silicate chain protruding from the kaolin plate. As the particles repel each other and the liquid acts as a lubricant, thick, mudlike kaolin suspensions change into thin, free-flowing dispersions. Lowering the p H with acid or alum, reverses the situation and the clay particles floc, or agglomerate. Bleaching is essentially an iron-reduction operation-reducing ferric, to ferrous iron with a resulting improvement in visual whiteness. Exploration Kaolin occurs in lenselike deposits from 20 to 100 feet below the surface of the ground. Southern Clays’ mining engineers, in a never-ending process of prospecting, drilling, and laboratory analysis, seek out these deposits and rate them according to purity. The company has three full-time core-drilling teams looking for new kaolin reservesoperating primarily within a 50 to 80 mile radius of the Gordon plant. First step in exploration is to lay out new property on a geometric grid pattern. T h e crew then clears the land, and drills test holes on grid points with a wet-core drill. Two strong indications of usable clay deposits are : Drilling becomes harder Cooling wash-water from the drill turns milky-colored. VOL. 52, NO. 5
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moisture. As the mine cars dump the crude a t the plant, bulldozers push it into screw-type crushers and mill feeders (72E, 73E) that break the lumps down into a uniform feed of approximately one ton per minute. The clay moves along to log washer blungers (74E) where water and chemical dispersants (phosphates and/or silicates) are added. The blungers beat this mixture into a mildly alkaline (pH 7 to 7.5), claywater slip, or suspension. From the blungers, the clay slip is fed by centrifugal pumps through a network of wet cyclones-first stage of particle size classification ( 4 E ) . Underflow rejects from the wet cyclones-a 4570 solids mixture of low grade clay, quartz sand, coarse mica, and general consolidated agglomerates-is watered down to about 20 to 3070 solids and forced through a second battery of wet cyclones (4E). This second pass boosts the initial recover of usable fine clay particles by about 207,. Waste underflow here funnels off to holding ponds for subsequent clarification of the water. Overflow from the secondary wet cyclones is recycled into the mill stream. First Stage Fines T h e fine particle size overflow (about
607, below 2 microns) from the first
When these happen, the crew removes the drill, puts on a core barrel, re-enters the hole, and drills through into the clay strata. T h e core barrel picks u p a 10foot sample. I t is pulled out, indexed as to location, and put into the core box for analysis. Based on this analysis, mining engineers calculate total clay yield for the property, as well as estimates of production cost for this particular crude. Mining methods and schedules are drawn up, and the property is logged into the long range program, to be picked u p whenever desired.
railway system. Locomotives-25and 4.5-ton diesels-bring in strings of sixyard, side-dump mine cars, which are loaded by diesel-powered shovels and then hauled some eight miles to the plant for stockpiling and subsequent processing. Processing Crude kaolin, as it comes from the mine, is in lump form, mildly acidic (about p H 5), and contains some 14%
battery of wet cyclones passes through two stages of smaller wet cyclones. This step removes about 97% of all material coarser than 200 mesh. T h e clay slip then flows on through 250-mesh vibrating screens, which filter out virtually all fine mica and other residue to 250 mesh. The bulk of the slip is pumped into a vast outdoor open tank farm for further particle size classification. I n these 18,000-gallon, batch-classifica tion tanks, the clay slip is allowed to fractionate, or differentially settle, to the desired particle size range. O n the average, 14 hours of settling gives a slip containing clay in suspension with an average
Open-Pit Mining Kaolin mining is essentially open-pit parallel-strip process. In Southern Clays’ mining operation, the overburden-mantle of nonusable material such as topsoil, red clay, and sand-is removed in two successive steps. First, two giant twinengine Euclid scrapers (each with a 20yard capacity) strip off the top 60 feet of overburden ( 8 E ) . T h e remaining overburden and clay are handled by a massive walking-type dragline which has a sevenyard bucket and a 160-foot boom (ZE). After stripping off the rest of the overburden, the dragline scoops out the kaolin crude and stockpiles it along side the cut. h-ext, workers lay down railroad tracks parallel to the stockpile and link it u p to the company’s internal
374
After initial cleaning steps, bulk of the clay suspension is pumped to a vast outdoor open tank farm (partial view here) for further particle size classification. In these 18,000-gallon tanks, clay suspension i s allowed to fractionate to the desired particale size range
INDUSTRIAL AND ENGINEERING CHEMISTRY
ir
particle size of 84% finer than 2 microns. T h e slip is drawn off the top of these tanks and treated immediately with sulfuric acid. T h e acid drops the p H to about 4, flocculating the slip, and preventing further classification. T h e flow sheet illustrates the parallel classification systems alternatively used. Meanwhile, the remaining slip moves from the vibrating screens through small wet cyclones for additional classification (423). I n addition to batch tanks and wet cyclones, Southern Clays is now installing two giant 125-hp. centrifuges which will supplement and replace existing classification equipment ( 7 7E). These units, which develop 2000 times the force of gravity (g’s), can turn out 7 to 15 tons of coating grade clay per hour, depending on grade or fineness. T h e coarse clay fraction from the particle size separation is reslurried, re(250 mesh), flocculated, screened bleached, vacuum filtered, and rotary dried. T h e product-low priced, coarse material-is shipped in bulk %ton railroad cars. I t goes primarily to paper, plastics, rubber, and paint markets as a filler or pigment extender.
Table 111. Kaolin Sold or Used by Producers in the United States Thousands of Short Thousands of Year Tons Year Short Tons 1900 1
2 3 4
5 6
7 8 9 1910 1
2 3 4
5 6
7 8 9 1920 1 2 3 4
5 6
7 8 9
Bleaching Following flocculation, the fine clay slip is pumped to 35,000-gallon agitated batch tanks. Here, operators adjust the p H to 3.5 with sulfuric acid, then add bleach (zinc or sodium hydrosulfite). As ferric iron is reduced to ferrous, the clay slip changes from a creamy to a bluewhite color. Recently, the company constructed its own bleach plant, and now makes some 60 to 7oy0of its annual 1 million-pound demand for bleach. Here, zinc metal reacts with sulfur dioxide to give zinc hydrosulfite. Treatment with caustic then converts zinc hydrosulfite to sodium hydrosulfite :
Spray
dryer control panel
has a
60 97 134 66 123 138 149 132 108 131
1930
148 151 170 181
1940
1
2 3 4
5 6 7 8 9 1
175
2 3 4
167 236 246 208 184
6 7 8 9
268 163 276 337 327 367 432 454 496 518
5
1950 1
2
3 4
5 6 7 8 9
534 443 345 41 1 426 524 639 732 595 781 834 1088 947 929 873 940 1322 1425 1569 1416 1751 1866 1829 1884 1873 2166 2250 2184 2222 2464 (estd.)
Source, Bureau of Mines, U. S. Department of the Interior.
Zn
+ 2S02
-f
Zn S p 0 4
After bleaching, the clay slip is run into another group of large agitated holding tanks. From these tanks it is circulated continuously through a system of high-speed (6000 r.p.m.), nozzletype centrifuges rated a t 12,000 times the force of (g’s) (3E). These give a mechanical decanting action and partially
diagram showing
dewater the slip-boosting solids from 18 to 20% to 28 to 30%. T h e thickened slip passes on to continuousrotaryvacuum filters where the solids are boosted to 60 to 65% (5E-7E). Drying At this point, the process splits off in two streams. Part of the acid clay cake goes to rotary dryers (9E) where water content is reduced to less than 370, then either bagged or loaded bulk in 50-ton railroad cars for shipment. Product is Southern Clays’ acid line. T h e rest of the acid cake is reslurried in high speed mechanical blungers by the carefully controlled addition of dispersants, such as tetrasodium pyrophosphate tripolyphosphate, and Calgon (sodium hexametaphosphate)-about 0.3To dispersant on a dry clay basis. Part of this high-solids, low-viscosity clay dispersion (63y0 solids) is pumped to 10,000-gallon cypress storage tanks. I t moves next to atmospheric, doubledrum, steam dryers (7E) which bring slurry to the desired moisture content for shipping. Customer moisture specifications vary from bone dry to 5% water. T h e product is one of Southern Clays’ predispersed line of clays. T h e customer simply adds water to get a dispersion of fine particle size clay. Another portion of the blunger slurry goes directly into tank cars for delivery. T h e customer can pump this product to his use point. Still another portion of the blunger slurry goes to 75,000-gallon agitated steel holding tanks. I t feeds continuously from these tanks to a 12ton-per-hour spray dryer (70E). Product, containing about 1% moisture, is loaded into cars for shipment or stored in silos. Heat Treating Clays from the various drying operation can be heat treated to drive off the
Rotary dryer reduces acid clay cake to less than 3% moisture in the preshipment stage of acid clay production
materials flow VOL. 52,
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Thickened clay slip ( 2 8 to 3070) passes onto continuous rotary vacuum filters (above) where clay solids are increased between 60 to 65%. Note filtered clay cake dropping onto conveyor belt
chemically combined water. This product, featuring higher brightness and bener opacity is used extensively in the paint industry as a partial replacement for the more expensive pigment, titanium dioxide. Too, heat treated clays show excellent electrical properties, and find their way into coverings for insulating wires and cables. Heat treated clays are somewhat more abrasive-an added selling point to certain industries. But this property somewhat buffers the product’s added whiteness appeal to papermakers. Future Outlook for Kaolin Predicted growth in the paper, plastics, rubber, and ceramic industries indicate a comfortable growth rate for the fine kaolin supplier. Nonetheless, large amounts of time and money are being expended on research as the Space Age unfolds heretofore undreamed- of applications for kaolin. Southern Clays. for instance, presently has under construction a new research laboratory and pilot plant facilities of some 22,000 sq. feet area. Researchers in the field are fascinated by the combination of properties offered by kaolin-high purity, chemical inert-
Belt carries the clay cake to a small blunger where dispersant (right) i s added, converting the mudlike acid cake into free-flowing high solids clay dispersion
ness, large and active surface. ciystal shape factors. variations on firing. and the like. T h e rcpiacement of metals by ceramics for many applications, especially for very high temperature uses, has already occurred-the development of some ductility in ceramic bodies could open a host of new fields. The high purity of the deposits makes possible certain chemical reactions of potential interest and the large surface area, properly activated, offers catalytic materials for numerous uses (automobile exhausts, alkylation reaction, and nuclear waste adsorption-to mention some). The shape factor is of controlling importance to much of the coating industry a t the present time. But the modification of flow properties by rrl-stal shape could offer interesting possibilities to manufacturers of shock absorbers. Combination of the inorganic clay structure with organic materials gives hybrid systems having properties of interest to many branches of industry. Regardless of future possibilities existing markets for kaolin are keeping producers hopping. T h e present demands of the paper industry for new and better coating and filling pigments, the increasing demand of the burgeoning plastics industry for suitable fillers, as
\vel1 as the revolution in the ceramics industry are causing the “unsung workhorse” to move at an ever increasing pace. They have changed the mining of chalk from a sleepy Southern family occupation into an industrial giant. Literature Cited (1) Georgia Kaolin Handbook, Bull. 1, Georgia Kaolin Co.: Elizabeth. N. J., 1956. (2) Gunsallus, B. L., “Mineral Facts and Problems,” Bull. 5 5 6 , Bureau of Mines, U. S. Department of Interior, Washington 25, D. C., 1955. ( 3 ) Kirk, R. E., Otlimer, D. F., “Encyclopedia of Chemical Technology,” Vol. 4, pp. 57-71, Interscience, New York, 1949.
yo;;,
(4) S. C. “Paper Coating Pigments, Tu$@ Monograph Series No. 20, Chap. 5, 1958. Process Equipment (1E) Blaw-Knox Co., Buffalo, Ti. Y . , Buflovak twin drum atmospheric steam driers. (2E) Bucyrus-Erie Co., South Milwaukee, M‘is., dragline excavator, Model 7 W. (3E) DeLaval Co., Poughkeepsie, hT.Y . , centrifuges, Model ACVO. (4E) Dorr-Oliver Inc., Stamford, Conn.. wet cyclones. (5E) Ibid., rotary drum vacuum filters, 11.6 X 16feet. (6E) Eimco Corp., Salt Lake, Utah, special rotary drum vacuum filters, 11.6 X 24feet.
Gordon Plant Vital Statistics Annual clay shipments, tons 275,000-300,000 125,000-1 50,000 Coating clays Filler clay 100,000 50,000 Miscellaneous clays Chemicals used per year, million pounds Dispersing agents (phosphates, silicates, and special additives) 1.7-2 750,000-1 Bleaching agents (hydrosulfites, zinc, and sodium) 1-1.25 Flocculating agents (sulfuric acid, alum) 74-86 Fuel consumption (virtually all gas), million cubic feet/month Power consumption, million kw.-hr./month 1.5 W a t e r consumption, gaIIons/minute 1,800
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(1OEj Nichols Engineering & Research Corp., New York, N. Y., spray dryer, 30 fhot S. S. (11E) Sharples Corp., Philadelphia, Pa., Super-D-Canter, P-7000. (12E) Southern Clays, Inc., Gordon, Ga., screw-type crusher, Southern Clays’ design. (13E) Zbid., mill feeder, Southern Clays’ design. (14E) Ibid.,long washer blunger, Southern Clays’ design.
