FLOTATION AS APPLIED TO MODERN CEMENT MANUFACTURE G. K. ENGELHART Valley Forge Cement Company, Catasauqua, Penna. Seven cement mills are now using the process, and the preliminary work has been completed for several others. The process was invented by Charles H. Breerwood ( d ) and first applied by him t o the plant of the Valley Forge Cement Company at West Conshohocken, Penna. Since March 15, 1934, all of the cement produced at this plant has been manufactured by this process. Although several descriptions have been published (16, 16, 2U), they relate largely t'o the earlier operations and are incomplete, particularly as to improvements of vital interest to the cement chemist and the cement manufacturer which may tend to broaden the field of minerals separation and offer new means for the recovery or purification of other industrial minerals. The compound chemistry of cement is probably the most complicated subject in inorganic technology, and it is not within the scope of this paper to discuss it farther than to provide a sufficient background for a practical understanding of the Breerwood process. Similarly, froth flotation mechanics and techniques are described principally with relation t o the concentration of calcite and siliceous minerals.
Practical flotation separations of both an oxide mineral (calcite) and siliceous minerals (such as mica) are being made from highly complex argillaceous limestone pulps of extreme fineness. Most of the raw materials must be so finely ground, to free the mineral constituents, that the resulting pulps are slimes which were previously thought to be too fine for flotation concentration methods; one of the limestone pulps successfully processed had a fineness of 325 mesh with 60 per cent of - l o p material. The success of the process in making these differential separations is attributed principally to the use of very small quantities of collecting reagents added in small increments to the pulp in each cell in a stage-oiling circuit. Portland cements of the highest quality and of the various modern types are being produced by this process from materials previously considered both inferior and useless in cement manufacture.
Chemistry of Cement All portland cements are produced by combining, in the order of their proportions, the oxides of calcium, silicon, aluminum, and iron. The relative proportions of their compounds in cement clinker radically affect t'he behavior of cement in concrete. The physical and chemical specifications for the common cement types will be found in the literature cited (1, 9-12). The mineral sources of the essential constituents and their origin were preriously described with relation to the original application of the Breerwood process (15), but reference must again be made to their general composition in view of more recent knowledge of the behavior of certain cement compounds and the more rigid requirements of modern cement specifications. The three important sources of raw materials are :
F
ROTH flotation is now being used as the principal step in a process for deriving cement raw materialmixtures of correct chemical and physical composition, for the manufacture of all known types of cement, from available materials which either may be inferior or actually unusable in conventional manufacture. The complete process involves a combination of grinding, classification, flotation, and thickening methods, each adapted to satisfy the specific requirements of cement chemistry and the peculiar characteristics of calcareous and argillaceous minerals. The purpose is to beneficiate available materials-i. e., to correct the proportions and ratios of the mineral sources of the four oxides essential for the production of any portland cement and, when they are present, to remove proportions of harmful or useless impurities. The corrections are made by removing proportions of one or more minerals occurring in excessive proportions or proportions of undesirable minerals occurring naturally in the available materials; if the available materials are considered to be excessive in certain constituents rather than deficient in others, a description of the process and its objects will be less confusing. It is in the idea of correction that one finds the widest departure from other uses of froth flotation, in which the purpose is invariably to recover or purify one or more minerals or products.
1. The high-grade limestones; they are higher in calcium carbonate content than is desirable for the raw material mixture and are partly corrected by additions of siliceous minerals, usually clays or shales, which serve as the minor component of a two-component mixture. 2. The argillaceous limestones or cement rocks, usually deficient in calcite and containing excessive proportions of total silica and alumina, frequently contaminated by magnesia and alkali compounds; in older methods it has been the practice to make partial corrections by additions of relatively pure limestones, the mixtures frequently being further corrected by additions of iron in the form of ore, blast-furnace flue dust, scale, or the like, and sometimes with sand. 3. The marls or chalks, which are similarly partly corrected by mixture with a clay or shale component. Minor sources include some blast furnace slag, alkali plant waste, and shell deposits.
These materials must be ground to extreme fineness, with intimate contact between the particles of the several constitu643
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ents because the reactions, in burning the mixture to cement clinker, cannot be permitted to take place in complete solution if the desired compounds are to be formed. I n burning the ordinary mixture for standard cement, the liquid phase will usually not exceed about 25 per cent of the total weight of the material. This liquid comprises the iron and alumina, a small proportion of the lime, and relatively little of the silica. Although i t is usual practice to grind the mixture t o a fineness ranging between 90 and 95 per cent of -200mesh matcrial, coarsely crystalliiie minerals, particularly quartz, in the coarscr fractions do not react favorably, if at all, under the burning conditions described. The presence of coarse quartz, for example, presents the problem that if attempts are made to causc it to react by increasing the time or temperature of burning, the finer minerals become “overburned”. Substantial proportions of ovcrhurned compounds in cenient adversely affect the early strength of concrete. The prilicipal clinker compounds are tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. The usual impurities are magnesia and alkalies. Only a negligible proportion of magnesia combines in the range of portland cement clinker. If the liquid phase is allovrcd to crystallize, magnesia crystallizes from solution in the mineral form, periclase, and becomes a principal contributor to delayed expansion in concrete; if a substantial proportion exists, disintegration of the concrete may occur in the form of “map cracking”. If the clinker is abruptly chilled to solidify the liquid phase as an undercooled liquid or “glass”, proportions of magnesia within the limits allowed by specification are innocuous. However, one purpose of the Breerwood process is to eliminate proportions of this adulterant, particularly to bring available rosemes of raw materials within the limits allowed. The alkalies usually occur in combined form in the silicates, principally in the micas, which are also the principal sources of alumina. The compounds present in cement clinker and their proportions largely determine the behavior of cement in concrete. Comparatively small variations in these relations radically change the hehavior and the classification of tbe cement, particularly with respect t o the type of concrete structure for which it. is adapted. Any reference to the behavior of cement
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must be qualified with relation t o particle size and particle size distribution and to the physical character of the compounds which iormcd the liquid phase, each of which prcEoundly influences the characteristics of the concrete. The conventional mcthods of cement manufacture are commonly called the “wet” and “dry” processes. The adoption of either process depends largely upon the nat.ure of the available materials, particularly tho moisture content. Highly developed mcthods exist for making the partial corrections of the raw material compositions described, but in any case these methods are additive in that one or more natural materials are added to anotlicr and blended to form the ultimate mixture. They are usually directed tonrard a dcsired rclation between lime and total silica; although additions of iron oxide may also be used, with the principal object of decreasing the proportion of alumina available to form tricalcium aluminate, these corrections rarely result in the essential correction of the unfortunately termed “minor constituents” iron and alumina, which are “minor” only in the sense that their percentage proportions arc comparatively low. I n the sources of raw materials available for cement manufacture, alumina usually occurs in the form of silicates, principally the micas, kaolin, and the feldspars, and almost always in too great abundance t o permit correction of the ultimate mixture by substitution or additive methods. This is particularly true with respect to the manufacture of cements of low heat of hydration (If) and those resistant to sulfate and chloride solutions (fa),such as are utilized in sea water, aggressive ground waters, and concrete roads when calcium chloride is used for ice removal. A proportion of alumina is essential t o the ultimate mixture, because it serves as a flux to promote the desired reactions between lime and silica; i t also prevents the formation of undesired iron compounds when present in proper ratio t o iron, and its principal compound contributes to the early strength of concrete. 1t.s proportion, however, must be carefully limited for the manufacture of modern cements and particularly the special cements described (if,I $ ) . I n concrete i t has the disadvantages of liberating a disproportionate quantity of heat of hydration, its hydrated compounds are of limited resistance t o sulfates, and i t increases the degree of delayed expansion. Iron oxide
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also serves as a flux. Its compound apparently forms first, limiting the quantity of alumina remaining t o form tricalcium aluminate. It liberates the least quantity of heat, during hydration of the cement, of any of the clinker compounds; but it has no hydraulic value and in large proportions may decrease the resistance of concrete to aggressive solutions. Its ratio to alumina must be controlled to prevent the formation of dicalcium aluminoferrite, an undesirable clinker compound. The Breerwood process permits accurate control of the proportion of alumina, and the removal of its mineral sources has the unexpected advantage of decreasing the free alkalies in the cement, the sources of which are in the silicates. Mechanics of Flotation There is an extensive literature in technical publications and patents on the agitation froth process; some of it is accompanied by excellent bibliographies (21,24),but it is largely limited to the concentration of metallics, particularly the sulfides. Weinig and Carpenter (24), in common with other authorities, generally characterize the flotation of oxidized ores as being not so important as that of the sulfides with relation to tonnage, t o the low unit value of the contained minerals in some of them, and to the fineness to which they must be ground, which “makes the fine grinding necessary for flotation economically impossible”. The flotation of siliceous or acid minerals is of more recent origin and probably would be considered even less important than that of the oxides. However, the description of the raw materials of the Valley Forge plant and its flow sheet show that separations can be made by froth flotation well within the range of practical economics, even though they are from materials ground to a fineness far beyond anything previously contemplated by the flotation chemist and from the material of the lowest commercial value of any subjected to mineral separation methods. Flotation Reagents
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fish oil fatty acids, their emulsions and soaps, and refined talloel (IS) which is a mixture of fatty and resin acids derived from the black liquor soap of the sulfate process for the manufacture of paper. Talloel soap (25) and emulsions (18, 19), are effective collectors of calcite. Some soaps which are useful for certain argillaceous limestone pulps have definite frothing characteristics; these collectors are usually saponified fatty and resin acids, a t least one of which (25) requires no separate frother under appropriate conditions. I n general, the emulsions are more satisfactory than the soaps in the flotation of calcite, the common mineral form of calcium carbonate, since, in the differential separation of this mineral, soaps sometimes create an excessive and comparatively barren froth. Unmodified fatty acids of high titer are generally unsatisfactory for use in the usual argillaceous limestone pulps, in view of the difficulty in feeding them in small increments to each cell of a stage-oiling circuit and dispersing them rapidly throughout the flotation pulp. The successful concentration of calcite from the siliceous minerals in these finely divided pulps is principally the result of adding the collecting reagent in extremely small quantities to each cell of the circuit. The total quantity of collecting reagent used a t the Valley Forge plant is about 0.5 pound per ton of feed; this quantity is only about one fifth to one tenth of that used in the flotation of the much coarser phosphate pulps. In the flotation of phosphate, for example, the usual practice is to add fatty acid and a mineral oil, together with an excess of sodium hydroxide beyond that necessary to saponify the fatty acid, and to mix it with a dense pulp prior to dilution and concentration by flotation. If mineral oils are used with fatty acids, they are generally undesirable in the flotation of limestone pulps since they tend to promote heavy flocculation; but when dispersed in an emulsion of a soap, they tend to reduce excessive frothing (19). The collectors for the siliceous minerals are described by Lenher (14) as those wThich give in solution the long-chain surface-active group in the positive ion and in which the negative ion is usually a halogen. They are variously called “aliphatic amines”, “positive ion”, or “cationic” reagents.
A wide variety of flotation agents and reagents has been found to be useful in the beneficiation of argillaceous limestone pulps, but the quantities used and the methods of control are unusual. The successful differential separations are largely due to the use of limited quantities of collecting reagents introduced in small increments in stage-oiling circuits. The typical frothers-mixtures of monohydric alcohols, dilute resinates, and cresylic acid-are satisfactory. The quantities required are low and are introduced a t various points in the circuit to maintain the desired froth balance. At Valley Forge a total of 0.04 pound of the alcohol frother per ton of rock, suspended in about 4 tons of water, is sufficient for the entire circuit. The major proportion of the water of the pulp is returned t o the flotation circuit, which accounts in part for the economy in frothing agent consumption. The usual collecting reagents or promoters for oxide ore minerals may be employednamely, the unsaturated fatty acids, resin acids, their soaps and emulsions (i. e., substances which in solution release a fatty or resin acid radical). Resin acids are generC o u r t e s y , American C y a n a m i d Company ally inferior collectors, but a t least one mixture of resin and fatty acids appears to FLOW DIAGRAM OF A FAGERGREN SUBAERATION FLOTATION MACHINE(CELL) have unusual efficiency (IS). The common The flow of the mineral pulp is indicated by plain arrows. The flow of air oxide mineral collectors are oleic acid, induced by the rotor is shown by arrows terminating in circles.
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They modify the surfaces of the siliceous mineral grains with limited filming of the oxides. Many of the amine reagents have a critical p H range and are ineffective in alkaline pulps. However, the principal members of the class described by Lenher are highly efficient in pulps in which the alkalinity ranges from pH 7.4 to 8, the normal limits of alkalinity of cement raw material pulps. Breerwood and Williams (7) discovered that the reagents which they preferred in this class, when used in normal minimum quantities of about 0.4 pound per ton of feed and sometimes considerably less, would float all or substantially all of the mineral constituents, including the acidic, oxide, and sulfide minerals present in argillaceous limestone pulps; but they also found that by carefully limiting the quantities and by introducing very small increments a t each step in a stage-oiling circuit, differential separations can be made, silicates being readily separated from silica and calcite. For example, these carefully limited quantities may be as low as 0.05 pound per ton of feed, the total quantity of cationic reagent used at Valley Forge for the concentration and removal of the micas. Further, the micas and talc may be separated as a concentrate to be discarded, to recover the calcite, quartz, and composite rock particles as a tailing of flotation. A sulfide such as pyrite, commonly believed to be floated only by reagents of a different class, can be removed with the silicates without substantial loss of quartz. Under careful control, silicate minerals, such as the micas and talc, can be separated from feldspars, quartz, and calcite. It is believed that the operations a t the Valley Forge plant are the first commercial use of reagents of this general class; the preferred reagent is a mixture of similar compounds in which dodecyl amine hydrochloride is the principal constituent. In addition to frothers and collectors, reagents with various functions are frequently useful. Dispersers are especially useful in finely divided argillaceous limestone pulps to reduce natural flocculation and colloidal filming and thereby improve both flotation and classification. I n these complex pulps, calcium lignin sulfonate has been the most effective, either
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alone or with additions of sodium silicate or soda ash (6, 6 ) . Depressors are used to decrease floatability of an undesired mineral; e. g., calcium lignin sulfonate, effective as a dispenser, also depresses carbon in various mineral pulps and overcomes its tendency to cause flocculation and to consume disproportionate quantities of both fatty acid and cationic reagents (4). There are two theories as to the action of collecting reagents on the surfaces of the mineral grains which they concentrate. The subject is highly controversial (22) and is referred to only with relation to the practical effects that take place in a flotation cell. The chemical theory assumes a chemical reaction (a metathesis) between the anion of a fatty acid, for example, and the hydrolyzed surface of the oxide ore mineral to produce an oriented monomolecular film of insoluble metallic soape. g., calcium oleate. The adsorption theory supposes a surface tension difference a t the solid-liquid interface, in which a surface-active part of the collector unites with the hydrolyzed surface of the mineral grain. I n any case, the ore mineral particle is a t least partially filmed, which makes the surface water repellent and accordingly subject to air-bubble attachment. The mechanics of froth flotation comprise air-bubble attachment to the water-repellent surfaces of the ore minerals. These bubbles are considered to be re-formed from solution a t the water-solid interfaces. I n sulfide flotation, and possibly in the flotation of coarse oxide ore mineral aggregates such as Florida phosphate, the air bubbles are said to be relatively large, the ore particles adhering to them and being floated to the surface of the pulp like a basket attached to a balloon. At the surface the particles and bubbles attach themselves to the froth bubbles, are stabilized by the frothing agent, and overflow as the concentrate. I n the concentration of calcite from argillaceous limestones, the mechanics are quite different, particularly because of the extreme fineness of the particles and the nature of air-bubble attachment. The air bubbles attached to the calcite grains are so fine that they cannot be seen at magnifications as great as 150 diameters (the limit of practical magnification) owing to movements attributable to surface tension. A number of grains of calcite may be seen bound together in groups in the form of air bubblemineral floccules, in which the particles are each surrounded and bonded by air bubbles in an increasing order of sizes from the grain surfaces; the air entrained in each floccule causes it to rise and attach itself to a large froth bubble on the surface of the cell.
Courtesy, Lone Star Cement Corporation
SEPARATION PLANT OF CIA ARGENTINA DE CEMENTO PORTLAND AT PAR AN^ Hydroseparators appear in the foreground,the cell house and thickener in the background, and the kiln at the extreme right.
Valley Forge Materials At the Valley Forge plant, compositions for all known types of portland cement can be produced solely from the argillaceous limestone quarried on the property, but the flexibility of the Breernood process makes it possible to substitute a small quantity of additional silica for the manufacture of special high silica-low alumina cements, without going to extremes in mineral separation. With relation to any modern type of cement, the rock is deficient in calcite and uncombined silica,
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SEPARATION OPERATIONS
$.
GROUND
PRODUCTr3
FINE CALCITE FLOTA-
TION
W
I
4,
THICKENER LOW ALUMINA L O W MAGNESIA
FLOTATlON
-
FIGURE 1. FLOWSHEET
O F THE MINERALS s E P A R A T I O S PL.4ST VALLEYFORGE CEMENTCOMPASY Recovered materials are indicated as products 1 t o 4. Rejects are indicated as wastes 1 to 3.
OF THE
excessive in total silica and alumina, and satisfactory as to iron, although part of the iron occurs as pyrite which is not wholly desirable in view of its sulfur content. The principal minerals are calcite, quartz, muscovite mica, phlogopite mica, feldspar, limonite, pyrite, pyrrhotite, and dolomite; all of them which are not magnesian compounds are useful in cement manufacture. The removal of a micaceous concentrate, comprising a relatively negligible proportion of the total tonnage, is sufficient to correct the deficiency in calcite, to correct the deficiency in uncombined silica for ordinary types and to permit its substitution in part for special cements, to remove the excess of alumina, to decrease satisfactorily the proportion of magnesia, and similarly to reduce the proportion of the alkalies, which in these materials are combined with the micas. Since these materials are typical of complex argillaceous limestones and because the flow sheet employed is more complete than those in use in other plants, a detailed description of the flow sheet will be relied upon to illustrate what has been accomplished and what significance the process may have with relation to other minerals. Valley Forge Flow Sheet
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sition for standard cement, but principally to provide two fractions of limited particle-size ranges to be concentrated in separate flotation circuits. By limiting the particle size ranges, cleaner concentrations can be made in differential separations from mineral pulps of this general character. The coarser fractions are subjected to rougher flotation in the presence of a frothing agent and a fatty-acid-type collecting rclagent; the concentration is made in a controlled stage-oiling circuit in the presence of a total of 0.5 pound of the fatty acid per ton of feed. This concentrate is the first completed product of the process. The tailings of this operation are hydraulically classified to take advantage of the slow sedimentation rate of mica particles, which is about half that of other mineral particles of the same maximuni diameter, to remove the finer suspended micaceous matter a5 an overflow (5, 6) which forms the first waste product. The underflom- of the tailings classifier is subjected to flotation in a stage-oiling circuit in the presence of a total of 0.05 pound per t o n of feect of a^cationic flotation reagent, which concentrates the remainder of the free micas and the pyrite in the froth. This froth concentrate is the second waste product; the tailings of this operation are the second recovered product of the process, but they require further grincling to make them suitable for burning. Except when compositions for sulfate-resisting cements are being prepared, when all of the finer fractions are subjected to flotation, a part of the first classifier overflow is required as a source of argillaceous minerals and is delivered to t’he thickener as the third recovered product of the process. The remainder of the finer fraction is subjected to flotation in a rougher circuit in the presence of a fatty-acid-type collector to produce a calcite concentrate which forms the fourth recovered product. The tailings of this operation are highly micaceous and so lorn in calcite that they are discarded as the third waste product. All of the recovered products are continuously delivered to the thickener and recombined to form the ult
