Modern Portland Cement Production WILLIAM Q. HULL, Associofe Editor in colloborofion with Permanenfe Cement Co., Ookbnd, Calif.
PAUL FRANKLIN Permanente Cemenf Co., Permanente, Coltf.
P
ORTLASD cement plays a leading role as a building material in today’s civilization. I t is considered indispeneahle in side\Talks, highways, bridges, and dams; in the construction of practically all large buildings; and in dry docks, harbors, aiid airports. Both country and city dwellers daily ufe 01’ come in contact with its applications in countless r a y s . The almost universal use of portland cement can be credited to the comparative ease of working it, its st,rength(which increases with age), an 1 its uniformity (which makes it possible t o calculate strengths as reliable as those made for steel). Its permanency and minimum maintenance contribute t o the popularity of its use. Portland cpment’s versiitilitj- as a imterial of cwns(ruction has resulted in a rapid growth of the industr3- in the T:nited St,atrs. I n thP entire decade beginning in 1870,total production amounted t’o only 82,000 barrels in the Cnited States. By the turn of the century, annual production had grown to 8,500,000 barrels. In 1950, over 222,000,000 barrels a-ere produced in 150 j3lants scattered throughout the nation. Table I s h o w t,hat during this period, the dollar value of product,ion jumped from $246,000 t o over S52i,OOO,OOO. 4 breakdown of end uses is shown in Table
11.
830
The history of cement goes hack iiiuch furt,her than its f r e t production in the United State., hoivever. From the time t h a t man first began to build, he sought a material that would bind stones into a solid, formed mass. The Babylonians and Assyrians ptiane found blocks bound used clay for t,his purpose: the \Tit11 lime and gypsum inortar satisfactory in building the Pyramids; the Greeks developed a cement used in structures that s h o ~ e dremarkable durahility. The Romans experienced great success in niakiiig a cement in which slaked lime TTas mixed with volcanic ash from 1 I t . Vesuvius. This material, called pozzolana, produced a cement capable of hardening under water and Kas the first so-called hydraulic cement. \T’ell-known examples of Roninn architecture which have lasted through the ages are the Colosseum, the Basilica of Constantine, and the great Roman haths built around 2 i B.C. The secret of cement t,hat would harden under water was lost during the Dark Ages, and from the period of t,he Romans no advances were made in the technology of building materials until t,he latter part of the eighteenth century. It was then that the modern cement, industq- had its beginning. John Smeaton, an English engineer, was employed t,o huild a lighthouse on the rug-
INDUSTRlAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 5
PLANT PROCESSES-Portland ged coast off Cornwall where repeated failure had been experienced with wooden structures. Smeaton knew that ordinary lime mortar would not harden under water, and this was a requirement if the foundation of the lighthouse was to be firm. I n 1756, he launched a series of investigations which eventually led t o a satisfactory material made from the burning of limestone containing a considerable proportion of clay ( 13 ) . The Eddystone lighthouse, which Smeaton built in 1759, stood for 126 years before it had to be replaced. Not only did Smeaton develop a cementitious material for his purpose, but modern lighthouse construction began with his work. I n making his hydraulic lime, Smeaton used only those layers of the quarry which, after calcining, gave a product that would slake with water. It did not occur to him that the layers which would not slake readily could be burned, and, after grinding, could be converted into a good hydraulic lime. Forty years later, Joseph Parker of England was granted a patent for the manufacture of a hydraulic lime which was made by calcining argillocalcareous nodules (calcareous and clay-bearing material) similar in composition to cement rock, and then grinding the resulting clinker into a powder. I n 1810, Edgar Dobbs obtained an English patent for the manufacture of an artificial Roman cement by mising carbonate of lime and clay in suitable proportions, nioistening, molding into bricks, and burning sufficiently to drive off the carbonic acid without vitrifying the mixture. Other experimenters in the field of cement during the period from 1756 to 1830 were Tiicat and Lesage in France and Parker and Frost in England ( 4 ) . Joseph Aspdin, a bricklayer of Leeds, England, is usually credited with the invention of “portland” cement. I n 1824, he was granted a patent on a hydraulic cement which he called “portland” because it resembled in color the stone from the Isle of Portland off the British coast ( 3 ) . Historians do not give all the credit to Aspdin, however. While he did orginate the name “portland cement,” he did nothim more than make a n artificial Roman cement, which had been done before. He did not carry his burning to the point of incipient vitrification (30 to 35% liquid phase), which is now recognized as an essential step in the manufacture of portland cement as it is known today (5,8, 14). Aspdin’s greatest contribution was his method of carefully proportioning limestone and clay, pulverizing them, and burning the mixture into clinker which was then ground into finished cement. I n contrast to natural cements, produced by burning a naturally occurring mixture of limestone and clay, portland cement is a predetermined and carefully proportioned chemical combination of lime, silica, iron oxide, and alumina. Because the ingredients of natural cement varied widely, the properties of the final product varied just as greatly, and although the natural cement industry was at one time important, over 98% of cement produced in the United States today is portland cement. The origin and development of the cement industry in the United States has been credited to the sudden and large demand for a good hydraulic cement created by the building of canals, beginning with the Erie Canal in 1817 (12). At about this time, large rock deposits were discovered in New York from which natural hydraulic cement could be made with very little processing. By 1890, nearly 10,000,000 barrels of natural cement were being produced annually in the United States and Canada, a large part of which was used in construction of the canal systems. Portland cement began to replace the natural product in England in 1850 and was first manufactured in the United States, in 1870, by the Coplay Cement Co., near Allentown, Pa., an area which has continued to be a n important one in the industry. This and other early plants used vertical stationary kilns. It was necessary to cool each kiln after burning, and there was considerable waste of fuel and time. Development of the rotary kiln, beginning in England in 1885 and further developed by Thomas A. Edison and others in the United States, was one of the greatest factors that May 1954
Table 11.
Cement
Uses of Portland Cement by Classes of Construction (77) (Continental United States in 1949)
Xonresidential building (commercial, industrial, institutional, social, and recreational) Highways (streets, roads, alleys, bridges, sidewalks, a n d related structures) Residential building (excluding farin dwelling units) Public utilities (railroad, fuel, power, telephone, and telegraph) Military and naval (construction a t military establishments-airfields, cantonments, piers) Conservation and development (all work on r,eclamation, river, harbor, and soil conservation projects) Farm (dwellings and service buildings) Sewer and water (sewage treatment, water s u p p l y , and sewerage systems) All other new construction (including civil airports) RIaintenance and repair (including work on all classes)
Thour. Barrels
%
52,810
22.5
35,397
15.1
32.016
13.6
23.630
10.1
22,734
0.7
17,782
7.6
10,950
4.6
9,048
3.8
4.053
1.7
26.003
11.1
contributed to the growth of the portland cement industry in this country. The Chemistry of Portland Cement Is Not Fully Understood
The definitions of portland cement and the opinions on what shall be included in the term vary considerably in the standard specifications of different countries. I n Great Britain, the official definition is worded: The cement shall be manufactured by intimately mixing together calcareous and argillaceous and/or other silica, alumina, or iron oxide bearing materials, burning them a t a clinkering temperature, and grinding the resulting clinker so as to produce a cement capable of com lying with this specification. S o addition of any material stall be made after burning other than calcium sulfate, or water, or both. No cement to which slag has been added or which is a mixture of poftland cement and slag shall be deemed to comply with this specification (9). I n the United States, the definition is: Portland cement is the product obtained by pulverizing clinker consisting essentially of hydraulic calcium silicates to which no additions have been made subsequent to calcination other than water and/or untreated calcium sulfate, except that additions not to exceed 1.0% of other materials may be interground with the clinker a t the option of the manufacturer, provided such materials in the amounts indicated have been
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
832
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46,No. S
PLANT PROCESSES-Portland showii to be not harmful by test carried out or reviewed by (ASTXIj committee on cement (1j. France briefs the definition even further and calls portland cement an intimate mist'ure of carbonate of lime, silica, alumina, and iron burnt to the point of incipient fusion and ground. Technically portland cement is a greenish-gray, chemically active, impalpable ponder madr by burning t o a high temperature in a rotary kiln a pulverimd mixture containing definite proportions of oxides of calcium, silicon, and aluminum and iron, and grinding t,he resultant clinker. TKOto six per cent of gypsum by weight (based oil masimum limit of 2.0 to 2.57, SO2 in the cement) is added during grinding of clinker t o control setting time. The most commonly used port'land cements contain the following proportions of mineral oxides: Average,
Range,
70
R
Calciuiii oxide Silicon dioxide Aluminum oside Iron oxide iV1agnesiuin oxide Sulfur trioxide
60-66 20-25
G4.0 22.5 6.5 2.5 2.0 1.5
3-8
2-6 0.5-5 1-2,s
Small amounts of sodium, potassium, and traces of other oxides are usually present In t,he cement clinker, the elements are not present as the siniple oxides shown. Silicon dioxide, aluminum oxide, and iron oxide are inactive under ordinary conditions and have no cementitious characteristics. Calcium ox and magnesium oxide absorb moisture and carbon dioxide m the atmosphere to form hydroxides and carbonates and in this form are valueless as cemeritit,iousmaterials. Calcium oxide alone or mixed with magnesium oxide is convert'ed int'o the hydroxide b y proper application of water and forms a cement'itious material but one that does not have the hydraulic characteristics of portland cement (90). The cementitious properties of portland cement are therefore not the result of the presence of t,he simple oxides but of comples hydraulic calcium silicate compounds formed in the burning operation. The actual compounds present did not interest the early investigators. B y trial and error, procedures for selecting raw materials and proportioning and processing them had been developed. Beginning in 1887 with the work of Le Chatelier, who was the first chemist to apply methods employed in petrography to the study of cement, and extending to current work of port'land cement scientists, the knowledge of the chemist,ry of portland cement has been expanded considerably (11). It is now generally agreed that in the calcined clinker the oxides of calcium, silicon, aluminum, and iron are combined to form a mixture of the following compounds: Tricaleiuni silicate Dicalcium silicate Tricalcium aluminate Tetracalcium aluminoferritc
3CaO.SiOZ 2CaO.SiOz 3CaO~&03 4CaO.Al203,Fe~03
For some t,ime, these four compounds have been accepted as the principal compounds of portland cement clinker. (Magnesium oxide is believed to remain in its uncombined state.) More recent work has shown t,he presence of a new ternary compound, GCa0.2Al2O3.Fe2O3 and a series of solid solutions extending from 6Ca0~2A1203~Fe203 to 4CaO.AIZO3.Fe203 (19). The portland cement chemist generally refers to the four important compounds listed as C3S, C2S,C,A, and CIAF. They are generally present in t,he five regular types of portland cement in the ranges shown: 7% CIS
C2S
C8-i CaAF
2,545 7.0-45 2.0-15 6.0-15
While the exact composition of the clinker is not fully understood, increased knowledge is a result of mathematical analysis, physical chemistry, petrography, x-rays, and use of the electron microscope. May 1954
Cement
The four principal compounds are unstable and when wetted rearrange with different speeds. C3A reacts with great rapidity, generating the greatest amount of heat, and contributes to the initial set of the cement. C3S also reacts rapidly forming gelatinous silica and calcium hydrate, and contributes to the eaily strength of the concrete. The reactions continue until the gelatinous material has bound the grains of sand and crushed stone used in concrete into a hard mass. CZS hydrates slowly over a long period of time and contributes to the later strength of concrete. Hardening continues for years, and the strength of concrete increases with time. -4fter several years, an additional element of strength is a result of crjstallization of the gelatinous calcium hydrate. The hydration of cement is a comples process, however, and the reactions that occur when cement is mixed with water are far from fully understood. Bogue and Lerch have observed the following reactions and hydration products when Forking with each of the compounds separately ( 7 ) :
1. CIS forms crystalline calcium hydroyide and an amorphous calcium silicate of composition approaching 2CaO.SiO2.XH,O. 2. Compounds containing alumina react t o form a fluffy, crystalline, isotropic, hydrated calcium aluminate of a composition 3Ca0~A1z03~6HZ0. 3. CaAF forms an amoiphous hydrate, the composition of which has not definitely been established, and crystalline 3Ca0.A12016Hz0. 4. Gypsum reacts with thc alumina that enters into solution t o form crystalline calcium sulfoaluminate, 3CaO.A1?O33CaS04.31HzO. Generally, it is considered t h a t magnesia does not ieact, and its presence is limited t o 4% by weight in the cement according t o type of cement. When present in amounts greater than this, it contributes t o unsoundness of the concrete. I n practice, the percentage of each of the compounds present is determined generally from calculations based on oxide analysis. The relationships are:
C39 = (4.07 X % CaO) - (7.60 X % SiOz) (6.72 X % AIzO3) - (1.43 X % Fe&) (2.86 X % 900)
-
CzS = (2.87 X % SiOz) - (0.754 X C3A =
% C3S) (2.66 X % A1,03)- (1.69 X % Fez03j CIAF = (3.04 X
% Fez03)
The American Society for Testing Materials has specifications for five types of portland cement. These are: Type I for use in general concrete construction when the special properties specified for Types 11, 111, IV, and V are not required. Type I1 for use in general concrete construction exposed t o moderate sulfate action or where moderate heat of hydration is required as in semimassive concrete projects. Type I11 for use when high early strength is required. Type IV for use n-hen a low heat of hydration is required, as in massive concrete dam construrtion. Type V for use when high sulfate resistance is required, as in sewage disposal plants, or concrete exposed t o severe alkali conditions. The chemical and physical requirements for the five types are shown in Table 111. The fineness of the final cement product is of great importance. Portland cement tests 95 t o 9970 through a 200-mesh screen and 88 to 93% through a 325-mesh screen. The fineness expressed in specific surface area or the surface in square centimeters of one gram of cement has become common in recent years. Lime and silica make up between 70 and 86% of the mass of portland cement. Limestone, shell and chalk or marl combined with shale, clay, slate, or blast furnace slag, are the principal raw materials from which it is made (Table IV). Two processes are used in manufacture, wet process and dry process. When rock is the principal raw material ueed, primary crushing is the first step
INDUSTRIAL AND ENGINEERING CHEMISTRY
833
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table 111.
Portland Cement Specifications (2)
Chemical Requirements Silicon dioxide (Sios), min., Tc Alu_niinum oxide (.\lsOr), max., Yo Ferric oxide (FerOa), max., 5 Magnesium oxide (LIgO), tnas.,
70
Sulfur trioxide ( S O L Inax., 70 Loss on ignition, max., % Insoluble residue, m a r . , Yc Tricalcium silicate (3CaO.Si03 max., Yo Dicalcium silicate (PCaO.SiO&, p i n , , , So Tricalcium aluminate (3CaO.A l d h ) , max., % Physical Require'nients Fineness, specific surface, s q cm./gram Avera'ge value, min. Min. value of any one sample Soundnens Autoclave expansion, m a s . ,
Type
I
Type I1
Type I11
.,.
21 .0
...
.. , , . .
b.0
5.0 2.0' 3.0 0 75
5.0 2.0
.., . . ,
...
1600 1500
^
^
6.0
6'13'5
50
..
...
,
J.
. .
6
6.5
b
5.0 2 0d 3.0 0 75
5,0 2 0 3.0 0.75
4 0 2.0 3.0 0 75
.,.
35
, ,
40
15
...
1700 1600
TsRe
., ...
. . 8
Type IV"
,
..
50 , . .
i
5
1800 1700
18nn 1700
' 10 7 0.50 0,jO 0 . 5 0 0,50 0 , 5 0 Time of setting (alternate methods) Gilmore test A0 60 80 DO m Initial set (min.), minute? 10 Final set (max.), hours 10 10 10 10 Victat test 45 43 45 45 45 Initial set (min.), minutes Final set (inax.), hours 10 10 10 10 10 Compressive strength (min.), l b /sq. inch Test cubes kept 1 d a v in moist air 1250 1 dai, in moist air, 2 days in v a t e r 750 ... ,.. 2500 900 1 day in moist air. 6 days 1800 1500 800 1500 in water ... 1 day in moist air, 27 days 3000 2000 3000 3000 In water Tensile strength (min.), lb./scl. inch Test briquets kept: ... 1 d a y in moist air 276 ... 1 day in moist air, 2 days in water 150 125 ... 375 1 day in moist air. 6 days in water 250 ... 175 230 275 1 day in moist air, 27 days 32: ,.. 325 300 330 in water Types not generally carried in stock. CaA content shall not exceed 5no a n d the C r 4 F plus twice t h e amount of C3A shall not exceed 20%. Maximum limit for SO8 content of Type I cement shall be 2.5'7' when t h e C3.4 is over 8%. d Maximum limit for 301 content of T y p e I11 cement shall be 3.0% when the C I A is over 8 % . .
.
I
in both. I n the wet process, after proper proportioning, raw mnterials are ground, mixed, and charged t o the kiln in a slurry. I n the dry process, no water is added and t,he ground, mixed raw mat,erials are charged t o the kiln in a dry state. The balance of t h e two processes is essentially the same. After calcinstion at approximately 2700" F. in rotary kilns, the clinker is cooled, stored, ground, and shipped.
Table IV.
Materials Used in Portland Cement Manufacture-1 950 ( 7 8 )
Total cement produced. bhl Average ram materials'bbl. cement, lb. Materials Used Cement rock Limestone a n d oyster shells
226,025,849 653
M arl ._I._
Clay a n d shale Blast-furnace slag Gypsum Sand a n d Eandstone Iron materials Other materials (diatomite, fluorspar, pumicite, flue dust, pitch, red m u d a n d rock, hydrated lime, tufa, cinders, calcium chloride, sludge, grinding aids, a n d air-entraining compounds) Total Fuels consumed, 1961 Total Coal 8.324.719 tons 266,797,104 gal. Oil 102,740,236 thous. e n f t . Gar
834
Short Tons 12,981,679 49,035,439 640,462 7 , 1 6 9 ,015 971,125 1 , 6 6 0 461 769 ,806 379,63i
148,290 73,755,919 A v iBbl. Cement 1 1 4 . 6 lb. 8 . 6 8 gal. 1437 CI:. f:.
There are several modifications to the two processes. I n a semidry process, only a small amount of water is added, forming pellets or nodules containing approximately 10 to 12% moisture which are charged to the kiln. In a semin-et process, the finished slurry is partly detvatered by filtration, and thc filter cake is fed t o the kiln. X few American plants use a special process for beneficiation of raw materials, an adaptation of the flotation process common in the mining industry. Approximately 60% of all U. S.m d Canadian plants use the n e t process. Permanente Cement Co. Operates Largest Cement Plant in the West
I n 1939, Permanente Cement Co. was awarded a contract to supply cement for construction of Shasta Dam, world's greatest overflow dam and a part of California's Cent,ral Valley Project. At t,hat,t,inie, the coinpany had no existing plant facilit,ies. Const'ruct'ion t ~ a sbegun in June of that' year of a two-kiln plant a t Permanente, in the western foothills of Santa Clara County near 9an Jose, Calif. The plant was in production beforc the end of the year and event'ually supplied nearly 7,000,000 barrels of cement, for Shast,a Dam. Several expansions followed, and in 1951 a fifth kiln was added to the plant increasing the annual production to 7,000,000 barrels (28,000,000 sacks). The plant is the largest in the West and one of the two largest individual plants in the world. I n addition to t,he producing plants at Permanente, the company has distributing facilit,ies in Seattle, %-ash., Portland, Ore., and Anchorage and FairLong Beach, Calif., Honolulu, T. R., banks, Alas. There are three wholly owned subsidiaries: Raiser Gypsum Co., Inc., with plants a t Long Beach and Redwood City, Calif., and a plant under construction in Seattle; Glacier Sand B Gravel Co., in Bteilacooin and Seattle, Kash.; and Permanerite Steamship Co. The lat'ter operat'es two self-unloading bulk cars. Perrnaizente Si/cerbow (58,000 barrels) rier steamships, the and S. S.Permanenteeenlent (40,000 barrels), which supply the distribut'ion points from docks in Redwood City. I n Seattle, cement is loaded into converted, sclf-unloading IST's (19,000 barrels each) which take it to Anchorage; from Portland, converted LSll's (4000 t o 5000 barrels each) take cement t o dam and other construction sites along the Columbia River. Permanente's proved limestone reserves are sufficient for 20 to 25 years of plant operation. Exploratory drilling is proving additional reserves. The quarry is located approximately one and one half miles from the plant. Average year round carbonate content of rock is controlled at 81.5 t o S2.570. This is considerably lower than in most cement plant quarries, where carbonate runs from 85 t o 97%, requiring higher percentages of blending materials. Rock is relatively high in silica and low in iron and aluminum. Average rock analysis (ignited basis) is:
s.
Silica Ferric oxide Alumina Calcium oxidr LIagnesium oside Alkalies Ignition loss
Per Cent 24.60 0.78 1 22 72 74 0 47 0.1'3 30 46
Operations in the quarry proper consist of drilling, blasting, loading, and transporting. T o date, no stripping has been necessary in quarrying limestone. I n some cases, overburden exists as clay deposit,s, but this is generally of such composit'ion that i t can be used in the m i x An electric, rotary drill, which uses compressed air for renioving cuttings, sinks a series(2 t o 14) of i3/B-inch blasting holes ranging in depth from 60 to 125 feet ( 1 0 E ) . Each hole requires approximately 4 hours of drilling time and holds a charge of 800 pounds of 48% bulk strengt,h blasting powder in cartridges. AApproximately7 tons of rock is collapsed per pound of poivder. In many quarries, churn-type drills are used. Thefie are considerably slower, and during their earlier use by Pernmn-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 5
-
~
.
PLANT PROCESSES-Portland
ente, yield per pound of powder averaged only 4 tons of rock. As a result of the chemical composition and geological formation in thin quarry, rock breaks fiuely during primary blasting, and very little secondary shooting is required. A &cubic yard shovel loads the rock into end-dump buggies which hold 23 tons (three dippers) per load ( 4 3 , 11E, 29E). There are five of these buggies, each of which is pulled by tractor power units (6E). Average rock quarry capacity is lo00 tons per hour. The buggies discharge the rock to a scalpinggrieelywith a 6-inch spacing between bars. Formation is such that a high percentage of rock paaseri through the griaely. Crusher facilities include two primary crushers and a group of three secondary crushers. Oversized rock from the scalper flows to a constsnt-speed jaw crusher (56 X 72 inch set a t 10 inches) located below ($E) Discharge loins scalper fines and passes over a 48-inch hy 72-foot conveyor to trausfer station. From this point, it is routed to limestone aurge pile over a 48-inch by 135foot conveyor boom. Equipment to this point o m also be uaed to process low grade rock. In this c m , the t r a d e r station serves 5 8 a means of routing low grade rock to a separate aurge pile. Limestone surge pile bas a normal capacity of 1O,o00 tons. (A second primary jew crusher has just been added simultaneously with the opening of a new quarry face.) Clay is secured either aa overburden from rock quarry or from clay deposits near the quarry. Up to 1000 tone of clay can be removed Der hour. Clay is fairly high in silica content and hae folrpical analyais {unignitedbasis):
,., i
Cement
.
i
. ,,;:
:.'; 2; TO 31/1 inch
TO I
k Inch
I I-."1 MAKE-UP
8
RAW GRINDING 90% -200mssh. 560lb.
HlCKENlNG and BLENDING
1.100 B.t.u./cu.It.
,..
.:,2.,.>,z. ,'
Per Cent
Si02 Fer08 Ah01
42.20
Ignition loea
14.84
e2 Trace oompounda
364 Ib.
8.50
12 Ib.
12.38 14.54 4.18 3.36
(92% -325msshl Under the surge pile is a concrete tunnel where rock or clay is fed through manually operated gates to a conveyor. The 42-inch by 520-foot conveyor tabes the rock (or clay) to a 3-inch scalping screen. Oversized rock goes to a gyratory crusher which operates at 880 revolutions per minute and delivers approximately 250 tons per hour of minus 3'Irinch product ( H E ) . Rock going through the screen by-passes crusher and is fed to crusher discharge. Combined primary crusher plant rock (or clay) paases over a &inch by 43Wfoot conveyor through a mountain side to a second transfer station in close vicinity to the cement plant. Permanente enjoys a distinct advantage in having the quarry located such a short distance from the plant. In many cams, quarry and plant are from several to many miles apart, requiring haulage by truck or rail rather than conveyor. At the tranafer station, rock (or clay) passes by booms to one of three outdoor storage piles: high grade rock (84 to 97% carbonate) goes to a high grade rack storage pile, where normally about 97,200 tons is available; clay goes to a 5000.ton clay storrtge pile; low grade limeatone (79 to 83% carbonate) is conveyed to a 550,ooO-ton storage pile. In addition to limeatone and quarry clay (which represent approximately 92% of raw materiab), Permanente uaes laterite clay and s m a l l amounta of iron in form of pyrites cinders in production of portland cement (Figure 1). These produce an easier burning blended slurry. Iron is purchased in a form sui6 able to introduce in raw grinding department. Laterite, a high aluminum, low d i c a clay, is secured from deposita in Iane, Calif.,and is ala0 introduced in raw grind department. Ita typical composition (unignited basis) is:
810.
FerOs
&OS
cao %?ion1-
Per cent 30 BO 20.87 24.17 8.32 0.71 16.01
BARRELOF PORTLAND CEMEN
Figure 1.
Raw Materials Used in the Productiw of One Barrel of Portland Cement
Pozzolan cement, used for applications alternately subjected
to wetting and drying and to control reactive aggxgates, contains approximately 15% of reactive siliceous material added juat before milling of clinker. Pomolan material in itself hae no hydraulic properties b* reacts rapidly with lime releaeed during cement hydration to give a more durable product when used m concrete. A reactive shale is used commonly for the additive. Pozmlan cement represents approximately 5% of Permanente's annual production. Rock Is Further Reduced in Sire in Secondary Crushing Plant
Crushed rock from primary cruahing plant has seen only the beginning of ita reduction in sbe-eventually 90% of it must pass through a 2oO-meah scmn. Next &e reduction occur8 in secondary crushing plant. Rock from high and low grade storage pilea is blended in ratio of 1 to 3 in secondary c n d i n g plant to maintain average of 82 to 82.575 carbonate content. h m either of these piles it is drawn down to conveyors which feed to a 5 X 8 foot double-deck vibrating sereen (I-inch opening in top, "Anch slotted bottom). Fines paas directly to cement rock storage pile. Oversbed rock pasees to @ crusher set a t 1'/2 inches (capacity of,lZ7 Discharge passes to one of two 6 'X &lfrfeot by alrinch dotted screens. Fines pass to storage and m d s e d rock to one of two
I N B U S T R I A L A N D EN G INE E R I N G CHEMISTRY
. ,
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table V. ~ Circuit
Ref.
1
(1E)
~
Ball Mills ~Diameter i ~ Length , Feet Inches Feet Inches
Primary Raw Grinding Circuits Rake Classifiers Width, Feet Hp.
Hp.
Equip. Ref.
Tube Nills -_.___I Length Feet Inches Ilp.
--___
Diameter ____ reet
Inches
(B4i')
.. '?
..
(1 YE)
8
Closed 2 3
{%;
9 9 9
6 0 6
1 1 1
0 1 0
0 6 0
500 500 600
14
40
12
30 30
12
, . .
(IYE)
7
..
I
0
0
.. ..
.
ZG 2
6
0 0
500 500
Open 4
9
6
13
21/2
600
short-head cone crushers set a t 3j8 inches. Each has a capacity of 100 tons per hour and the two are close-circuited on themselve8 (1323). Combined product from the secondary inch and 90% minus inch crushing plant is 100% minus '/z at a rate of 310 tons per hour. It is conveyed t o a 5000-toii surge pile from which it is conveyed to one of three raw storage bins v i t h capacities of 300 and 400 tons each. 5l/?-fOOt
Permanente Uses the Wet-Process in Production of Cement
Before calcination, the raw material must be furthei reduced in size. This is accomplished in the raw grinding department, R hich is further divided into primary and secondary operations (Figure 2). Equipment in the primary grinding department is shown in Table V. Three of the four primary raw grinding circuits are operated in closed and one in open circuit operation The open circuit (KO.4) was added during the last expansion program and was a result of insufficient thickening capacity. In this ciicuit, raw rock is introduced t o a ball mill with sufficient water t o make a 30 t o 35% slurry (by weight). The ball mill is operated in open circuit with a 39-foot tube mill. Discharge (90% minus 200 mesh, 68 t o 70% solids) i s eufficientlg- fine to bypass thickening equipment used in processing discharge from other circuits and is pumped directly t o the slurry blending department. Two of the remaining three ciicuits (Nos. 1 and 2) are used entirely for rock grinding. Rock is fed to ball mills and mater added at same rate. Discharge passes t o a 16-foot rake classifier ( 6 E ) . Fines ( 3 5 t o 40% minus 200 mesh) pass directly t o launders or troughs and are pumped t o bowl classifiers, and tailings (plus 8 mesh) are recirculated In grinding clay and iron, material is first passed through a hammer mill which reduces the particle size t o a maximum of 11j2inches. It then passes to the third of the three closed circuit grinding systems ( S o . 3). Fines from the rake classifier pass t o three hummer screens operated in parallel and in closed circuit Rith the ball mill. Fines from these screens (minus 14 mesh) are pumped to two 7 X 26 foot tube mills. Discharge from the mills goes t o blending tanks. There are four secondary hall mills. Each is 9 feet 6 inches in diameter and 10 feet long and is operated by a 500-hp. motor (fE,16E, 293). These are operated in closed circuit v.ith 28foot bowl classificrs and 16 X 42 foot rakes. I n the secondary grinding system, there is a 150 to 250% circulating load. Overflow from the bowl classifiers (14 to 18% solids, 88 t o 92% minus 200 mesh) is pumped to one of three outside thickeners. Two are 150 feet in diameter and 10 feet deep (at wall) and the third is 125 feet in diameter and 11 feet deep. Freeboard ranges from 10 t o 20 inches. Overflow, which is fairly clear water (less than 1% solids) is pumped t o a settling pond. Water from the settling pond is re-used with fresh water. A solids content of 62 to 64% is desirable in thickener underflow. When this is maintained, underflow is pumped directly t o slurry blend tanks. When solids content of underflow falls below 61%, a portion of it is pumped t o a battery of thiee 8 X 30 foot rotary filters ( f 4 E ) .
836
..
..
0
39
4112
1000
Filter cake, containing 82% solids, is remixed with balance of slurry t,o raise per cent solids to dceired levels. Filtrate is rcturned to t,he thickeners. Proper Proportioning of Raw Materials
Is Made in Slurry Blending Department I n thc slurry blending department there are eight tanks of 16,000 cubic feet each and one of 30,000 cubic feet capacity. These are used for unblended slurries of primary clay, iron, and laterite and blends of these TT-ith limestone slurry. Proportioning of the raw materials occum at this point. From chemical analysis and calculations, correct volumes from each tank are pumped t o a Blurry blend tBnB. The flurry blcnd is thoroughly mixed and pumped t o a kiln feed tank. A typical blending procedure is as follon-s:
-4 blend tank is filled n-ith at leayt 12,500 cubic feet oE raw limestone slurry. To this is added approximately 250 cubic feet of quarry clay, 260 cubic feet of laterite c!ay, and 100 cubic feet' of iron slurry. The t'ank is agitated v i t h air (30 pounds per square inch) for 10 to 30 minnres. I t is analyzcd and, if satisfactory, is pumped t o kiln feed tank. Here both air and mechanical agitation are provided. -\ir is int,roduced through nozzles mounted on the rotating agitat,or. Ilolrs are protected x i t h a sleeve of artificial rubber which permits compi pass through but prevent's slurry from clogging holes. The blended slurry is now ready t o be "burned." During clinkering, the series of chemical compounds making up portland cement are synthesized. Clinkering l a k e s placc in a rotary kiln when sufficient heat is supplied t o properly proportioned blend of raw material;. The cement industry has been responsible for the growth in size of the rotary ltilri fmm an average common diameter of 5 t o 12 feet and from lengths of 25 to over 400 feet. Some of the largest kilns in the cement industry arc the five at Permanentc's plant, which are approxiniately 450 feet long with diameters of 12 feet (Table VI). The kilns arc inclined at, inch per foot of length. Burning zones are lined with a 9inch layer of periclase-chrome (basic magnesite) huriicd refractory brick from Kaiser's plant a t hIoss Landing, Calif. Balance of the kiln is lined with 9-inch and 6-inch high alumina refractory brick (alumina content ranging from 70 to 40%). Kormally, natural gas purchased from the local utility (948,000 cubic feet per hour) is used. During extremely cold Tveather, Bunker C fuel oil (6600 gallons per hour) is used (1 to 2 months a year).
Table VI.
Kiln Data
Diinensiona CapacCenter Xormal ity, Ends sections Length, Speed. Bbl./ Feet Inches Feet Inches feet R.P.hI. Day 12 2 11 2 454 67 3850 57 2 11 2 464 12 3860 60 0 12 0 444 3850 12 64 2 11 2 454 12 3880 70 2 12 2 448 4600 12
-
Kiln 1
2 3
4 5
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
*
Rquip.
Ref.
(19E)
(19E)
(PBE)
(1YE) (19E)
Vol. 46, No. 5
PLANT PROCESSE+Pol?land
May 1954
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Cement.
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Raw Grinding Department in Mill Building Ball mills in closed circuit with rake and bowl classifiers permit accurate control of fineness of r a w slurry
The slurry is fed to the kilns through a gravity flox feed pipe b y a rotating disk-type feeder which is e)-nchronized iyith speed of kiln rotation. Overflow from the feeder ( 5 to IO'%) is delivered back to the kiln feed t a n l r by gravity floiv. Fuel is admitted by the burner which extends 8 feet from hood into discharge end of kiln. I3urning zone begins approximately 15 feet from hood and is approximately 50 feet long. I n this zone, the temperature reaches 2900" F. and the majority of the kiln reactions occur. Primary air is provided by 75-hp. fans. A chain system, consisting of 50 foot sections of 6/8-inch low carbon steel chain (made u p of segments from 3 t'o 3 1 / 2 feet in length) is suspended in the upper section of the kiln. These become coated with slurry and increase heat transfer, facilitating evaporation of \yater. Back-end (feed-end) temperature is approximately 70O'F. Permanente Developed Unique Process for Utilization of Dust from Kilns
I n all portland cement plants, large quantities of finely- divided ground solids enter the kiln, and the kiln gases are heavily laden with dust particles extremely small in size (85% minus 326 mesh). I n many plants, the dust is recovered, and in some i t is returned to the kiln systems. I n early operations, Permanente pugmilled the recovered dust with kiln feed slurry and reintroduced it t o t,he t,hen esisting four kilns. This process is generally used in wet process plants. The system had several disadvant,ages. Permanente's plant, was const,antly expanding, and the rated capacity of the kilns was constantly being exceeded. Introduction of the dust vias an additional load. Secondly, introduction of dust resulted in changes in chemical balance of the mix (t,he dust containing more lime and less silica than the mix from which it came). Following experimental work over an extended period of time, it n-as decided to correct the romposition of the dust by addition of clay, 838
nodulize the mixture, and reintroduce the resulting nodules to the kiln feed (Figure 3). Addition of t>hefifth kiln provided an opportunity t o develop the process as it is now used (10). Dust is recovered in a vertical electrical precipitation plant containing eight units of five sections each (87E). Operating potential is 40,000 to 60,000 volts, and the plant treats 1,000,000 cubic feed of gas (400" F.) per minut'e. It is normal t o rap the collecting units once or twice during each 8-hour shift,. The plant operates at 99% collection efficiency. Collected dust d r o p t o hoppers from which it is carried by a screv and airpad ronveyor to a, duat surge tank of 120 tans capacit>y. This tank is air agit,ated through ceramic pads, and a screw con+eyor takes the dust t o a bucket e1evat)or which discharges into a dust constant head box. Overflow from the hcad box returns t o the dupt tank, ScreFv conveyors feed the dust from the head box to one of two nodulizing drums. Speed of the conveyors is synchronized with the alternator on t'he kiln motor drive shaft. Dust is introduced to the nodulizing drum with 3 to 5% clay, introduced as a thin slurry containing 18 to 257, solids. This is prepared from clay blending slurry containing 40 t o 507, solids. Clay from the blending tank is pumped t o a 7000-gallon storage tank equipped with air agitation. A rubber-linpd pump conveys thc slurry to a control tank which overflow hack t o the storage tank. h density controller-recorder maintains flow of a slurry of the predetermined and constant per cent solids to the nodulizing drums. The nodu!izer usually used is an 8 X 17 foot drum equipprd with beater-type agitator and scraper and operated at 8.5 revolutions per minute. Formerly the clay slurry was introduced through special alloJ- nozzlcs; these were found unnecessary, and it is now fed t,hrough a perforatcd steel pipe. The beater rods, running one third the length of the drum and rotating in the opposite direction provide complete mixing of the dust and clay slurry. Kodules, averaging about 1 inch in
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46,No. 5
PLANT PROCESSES-Portland Cement r..r
.
*
to 170' F. to a rubber conveyor belt. This portion of the cooler diameter, are di8oharged from the noduliser to a conveyor system discharge ia conveyed to one of two %foot cone crushera which tbat goes to a point in the kiln below the chain section of kiln reduce the size tp, m j m s . llh incb, (.%E). finer clinker, which No. 5. This kiln ia equipped at this point with a nodule scoop passes through the&$+, .by-passes.the crusher.. The combined feeder c o n & t i i of six scoops, each wit& a 1-foot 9-inch by 1-foot cooler dischmge is-conveyed t o one'of five large concrete clinker 5'/rincb open area (%JE). The scoops feed the nodules through storage bins (four of 47,650 cubic feet and one of 52,540 cubic the w a b of the kiln at a point where the temperature is 1200' feet). These bins are equipped witb feeders, and conveyors to 1400' F. At present, the feed to kiln No. 5 consists of a p below them go @ clinker bins in the finishgrinding department. proximately 50% nodules (82% solids) and 50% slurry (66% solids) for an aver-all average of approximately 73 to 74% solids. Kiln No. 3 is also equipped for dust feed and is used when kiln No. 5 is out of operation. The s y & m haa proved very satisfactory, and several advantages' have been experienced (U. S. patent pending). The per cent solids in the feed to the remaining four kilns was boosted from 61 to 65'% when dust feed to them wss discontinued. Fuel coneumption was reduced by lil/r%. The increase in clinker capacity from the addition of tbe 6fth kiln wa8 10 to 11% greater t b m could have been expected without the nodulising system, More uniform and smooth burning operation has ala0 been Figure 3. Dust Recovery and Nodulizing System experienced, and the complicated pugmill system, with high maintenance Dust is recovered from the air discharge from the clinker costs, has also been eliminated along witb an accompanying coolers in mbcbanical dust colle$ors with eight I(rincb dust horsepower reduction of 40 to 45%. valves (%E). Approximately 1.5 tons of dust are removed each Under proper operating conditions, clinker emerges from the hour. From the collectors, it is taken by belt conveyors to the kiln in the form of rounded, irregular b d s ranging in ske from clinker bins in the finish grinding department. T h e air heated */i to l'/% inches in diameter. They are red hot at kiln discharge during cooling of the clinker is used as secondary combustion sir (2400' F.). When cooled, they are velvety &ck or gray black, in the rotary kilns. and many tiny sparkling facets can be observed. In early plants, the hot clinker dropped or was conveyed directly to the storage pile, where a spray of water was directed Camen, ,r ,,Flnirhu upon it. It was sometimes ground when still hot, but it wss beof in lieved bv some that urindabilitv and soundness were imnroved by stor& the clinker-f; days or even weeks (6). Finish grinding @-.the, ivnker &'&e ,&,&e m9t important Simple rotary-type coolers, usually installed below the kilns unit processes i 4 , o e m e n t , ~ ~ ~ u ~and : ' t- u~b~&.in l : vmfus types both open and,clased circbita and 8c&l@&id.bY are wed in many plants. The integral cooler, ewntially an of air se~wat& are used in the finaf&ceaSinS h p . elannation of the kiln, was develomd more recently. Inasmuch grind department at Permanente's plant, there aa the beat content of clinker is high-2Q0,OOO B.t.;. per barrelare 10 binssthat feed the merent grinding circuits. These inproper considemtion to its recovery and utilization is an imclude five fonalinker, three for gypsum, and two for admixture portant factor in plant design and operation. of psealan qd rook used in pozzolan and plastic cement. There At Permanente's plant, the clinker i, cooled and heat ie reare six separate closed grinding circuits (Table VII). covered in 7 x 44 foot air-quenobing coolera of the inclined grate Circuit No. 1 is typical of the six circuits. Clinker and g y p type (7E). A cooler is located below and in line with each kiln. sum (3% by weight) are fed to the mill preliminator (a 91/r X 10 The clinker a t 2400° F. drops through a chute upon iylined foot ball mill). A grinding aid, consisting of triethanohnhe alternately stationary and reciprocating grates, the lat& proand highly puriIied soluble calcium salts of modified lignin pelline, the clinker to the cooler discharge. Air is drawn into sulfonic acids, is introduced in liquid form at the rate of 160D.c~. the cooler by 77,000 cubic feet per minute fans from below the per minute (maximum of 0.043% by weight of cement). T h grates and up through the clinker. Clinker discharges at 100'
Ci,cuiy
-
Table VII. cirwit 1 2
Diampter Feed Clinker
gypsum
Ciinker
KYPB,"rn
a
May 1954
adyrture
Ft. Q
In.
Q
Finish Grinding Circuits (Closed)
Preliminatom Equip.
'&be
Length In.
e
Ref. Ft. (2s~) io
o
e
(im
e
io
-D=
Ft.
In.
MNI
8
5w .
8
o o
HP.
Mills Length
(am
w
(sm
40
o o
separators
Hp.
iwo
iwo
7 E uip No.
Ft.
a a
ie ie
8d.'
(ism
CYdOWll
Diam., No. Ft. i** 8
(16s) i
i
8
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
840
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
PLANT PROCESSES-Portland keeps the balls free of coating thereby increasing grinding efficiency. Discharge from the preliminator passes directly t o the 8 x 40 foot cement tube mill. Discharge from the mill is pumped by a cement pump to a collecting cyclone. In cement pumps, a screw feeds the cement ponder into an aeration chamber where air jets admit streams of compressed air which mix with the cement to form a fluid mass ( 9 E ) . The discharge from the cyclone goes to one of two mechanical air separatois (15E). Coarse particles are recycled to the tube mill. These mills carry a circulating load of 200 to 250%. Fines go by means of a 12-inch airpad conveyor (8E) to one of two cement coolers. The air pad conveyor consihts of an inclined porous medium (canvass) which supports the cement in the upper poition of an enclosed system. Low pres3ure air is drawn from below and up through the canvas and cement, reducing the angle of repose to a point (below 4') where cement is semifluidized and flows. The coolers, designed by Permanente, are chambers containing a bottom constructed of 12 X 12 inch square porous tiles through which air is drawn continuously. Cement enters at 230" to 250' F. and is discharged a t 180' to 200" F. From the coolers, the cement goes by means of a 12 inch airpad conveyor to the cement storage silos. Cement storage includes 27-30 ioot diameter by 90 feet high concrete silos, each with a capacity of 15,904 barrels. Star bin space between each four adjoining silos (3'780 cubic feet each) is also utilized. Total storage capacity is 490,000 barrcls. A411 silos and star bins are filled through airpad conveyors. From storage silos, cement can be pumped directly t o bulk loading cars and trucks or to packing department. I n packing department, it can be bulk loaded to trucks or packed by 4-spout baggers into multiwall paper bags holding 94 pounds of cement (Z%E). Permanente Produces Numerous Kinds of Cement for Specific Applications
make a waterproof joint between bricks.
Cement
Exterior cement stucco
is a mixture of a special cement (composed of standard portland interground with plasticizing agents) with sand and water. Four types of oil well cements, which have properties for widely spread demands of the oil industry, are produced through careful control of their chemical composition. Factors of Cost in CementMaking Vary Widely
Plant location, proximity to raw materials, quality of raw materials, labor supply, and utility rates all materially affect the cost of production of portland cement. However, in contrast to many industries, fuel is the single most expensive cost component in a barrel of cement. Over the country, this constitutes from one quarter to one third of the cost of manufacture. Raw materials, on the other hand, represent a comparatively smaller part of the cost, ranging from a very minor portion to a maximum of one half the over-all cost. The portland cement industry is highly mechanized. As outlined in this report, there are many extremely large pieces of machinery and equipment. Maintenance, therefore, is a major item of operating Copts. Labor costs vary widely-production per man-hour ranges from 2l/2 to 8 barrels of cement. Power consumption is from 18 t o 25 kw.-hr. per barrel. Though processing water is extensively re-used, consumption is still high. (Permanente discharges approximately 400 gallons per minute to atmosphere with kiln exit gases.) Other important factors are grinding balls in the ball-and-tube mills, mill liners, and kiln brick. When bagged, the cost of bags is a significant factor. Additional charge per barrel (four sacks) is from 30 to 40 cents as compared with bulk shipment. Even this does not in general represent the total cost of the bags themselves plus packing expenses. In the past 5 t o 6 years st Permanente, the trend has been heavily in the direction of bulk shipment. There has also been a marked increase in bulk s h i p ping by truck in comparison with rail shipment. Improved, lightweight bulk trucks have been introduced in the past year or two. Permanente has added a fleet of these carriers to its formerly much smaller number of bulk trucks.
Weather conditions in the West range from semitropical heat t o subzero cold; arid desert land is separated is somr cases by only a few miles horn regions of heavy rain. Too, irrigation canals in many areas pass through regions where soil is impregnated with alkaline salts, which are destructive to ordinary concrrte. As a result, many types of cenicnt arc required to meet all construction requirement s. Permanente manufactures several kinds of cement. Standard portland cement (Type I ) constitutes hy far the major part of output. T j p r s I1 (modified portland), I11 (high early s t r e n g t h ) , IV (loa-heat), and V (sulfatc rrsistant) are produced in smaller proportions for construction piojects requiring their particular eharactrristics. As previously ment i o n r d , Permanente also produces pozzolan cement, which resists sulfate and salt- or fresh-water action better than standard portland cement. T h e c o m p a n y also makes a modified high early strength cement, ground finer than standard portland cement to produce a plastic concrete which sets rapidly. This product is used in making cement pipes and makes possible the daily production of hundreds of pipes from a single mold machine. A cement mortar, composed of portland cement with Four-Spout Packing Machines Fill 28 Sacks (94 Pounds Each) per Minute additives and plasticizers, is used to
May 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
841
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Portland Cement Industry looks to Continued Growth
The gron.th of Permanente’s cement plant from an original two kilns t o five in only 13 years points up in a dramatic way the future of the western portland cement industry. Some of the espansion was simply the result of 1oc.ation in the fast gron.ing West and increased requirements for building material? during t h e period of Korld R a r I1 and since. Pcrnianente’e unique transportation system for lo^ cost bulk cement, vi.hich encompasses a distribution range of more than 2000 miles versus the 200 to 300 miles for the average cement company, has contributed t o the company’s growth. But, on a national scale, the phenomenal grovth of the industry and the opt,imist,icoutlook for its future are results of it planned and energetic program directed tau-ard the development of nexr uses of portland cement and improved techniques of application. It has been said that more progress was made in developments affecting portland cement in the first 2 years of World War I1 construction than any previous 10-year period (16). Many war-time developments-Tr~iich expedited construction and saved labor, time, and money-are being applied t o advantage in civilian projects. .I feir of the technical advances bear mentioning. Much work has been done relating t o fire resistance, insulation, and acoustics. S s a result! concrete walls can no\r be insulated for almost any range of temperature changes; concrete mixtures ran be designed to have either low or high conductivity properties; soundproofness and desired acoust,ical characteristics can lie incorporated. Techniques have been developed LThich have increased the yater tightness of concrete. At present, any good contractor can make concrete mixtures with any degree of water tightncss; a t the samc time, mistures range from densitiee as lon- as 30 pounds per cubic foot to 250 pounds per cubic foot. These mixtures can be made in an almost unlimited number of colors and color combinations. In construction, there have been many advances in prefsbrication of concrete buildings, which have and will further increase markets for portland cement in house construction. There has also been marked iniprovement in c,oncrete floor and roof design. Preetreesed concrete pipe, tanks, and bridges have received much study and are becoming commonplace. While there is some shift’ing in the order of portland cement uses (Table 11)) all continue t o increase, and the fut,ure growth of the industry seems ensured. Acknowledgment
The authors wish to acknowledge the cooperation of I
