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IiYDGSTRIAL A S D ENGIXEERING CHEMISTRY
Vol. 20, KO. 11
surfaces, such as coke, of uniform size will give about 50 per packing material in question (for further discussion of this cent free space. With spheres or any irregular material of point, cf. Blakes). mixed sizes the fractional free space will evidently be deTable of Symbols creased owing to the fine material fitting into the voids beC = Constant of equation (9), c. g. s. units, except p in tween the larger. Generalizations cannot be made for this case, however, it being necessary to evaluate f from weight, centipoises C' = Constant of equation (lo), English units except p in density, and volume for the given case or for a closely analogous one. The estimation of the surface factor S offers no centipoises d = ,Ball diameter, centimeters further difficulties for the case of uniformly sized material, , T is the radius of since it is defined as equal (1-f) ( 3 / ~ )where f = Packing factor or fractional free space I = Length of packing, centimeters the material particles. For mixtures of different sized ma*M = Mass velocity, thousands of pounds per square foot per terial it may be calculated readily (see previous section) if the proportions of the various sizes of material present are hour based on gross column area known. P = Absolute viscosity of the fluid, centipoises, or relative I n any given case where it is thought probable that the viscosity based on water a t 20" C. n = Exponent of general equation (I) fluid flow is in the viscous or stream-line range, it may be more convenient to determine from equation (11) for critical flow P = Pressure drop, cm. of H;O or grams per square centivelocity that the flow is truly in the stream-line range, and meter P = Pressure drop per unit length, English units, inches of then to apply equation (10) directly, without reference to Figure 2. water per foot r = Radius of particle, centimeter These equations have been successfully checked against a 7' = Radius of particle, feet number of cases taken from the practical operation of gas machines, furnaces, or scrubbing systems, although obviously P = Density of fluid, grams per cubic centimeter p' = Density of the fluid, pounds per cubic foot little can be told of the former two cases if considerable coking, coal fusion, or clinkering are taking place. In the case of s = Surface of packing per unit packed volume, square centimeters per cubic centimeter = (1 - f ) ( 3 / r ) scrubbers Zeisberg'O found that, compared with dry lump S' = Same as S, square feet per cubic foot = (1 - j)(3/r') packings, water circulating a t a rate of I1 pounds per square v = Average linear velocity, centimeters per second in foot per minute (3.0 cc. per sq. cm. per minute) gave an increased back pressure of 3 per cent with 6-inch (15.2-cm.) column based on total area of column lumps, and 63 per cent with 0.5- to 1.0-inch (1.3- to 2.5-cm.) vc = Same as V a t the minimum critical point lumps. These data indicate the magnitude of the effect to Acknowledgment be expected in such cases. Application of the general equaThe authors wish to express their thanks to the Combustion tions derived herein to the case of scrubbing systems filled with special-shaped packing material. particularly if these Utilities Corporation for permission to publish the results bodies be hollow as is frequently true, is not possible without herein presented, and to T. E. Schumann for his assistance in empirical compensation for the characteristics of the special connection with the mathematical discussion.
Suspension of Macroscopic Particles in a Turbulent Gas Stream' S. P. Burke and W. B. Plummer COMBUSTION CTILITIES CORPORATION, LINDEN,h-.J.
HE general laws governing the motion of particles
T
through fluids, and more particularly the important rase of the free fall of particles, have been the subject of much theoretical analysis and considerable experimental investigation. The purpose of the present paper is to summarize existing information, present certain additional data, and to discuss practical applications of this information. The experimental studies have been confined to the reactions between macroscopic particles and a turbulent fluid stream. This case is the one usually encountered in practice, but is not necessarily identical with the case most thoroughly studied by previous investigators-via., the free fall of a macroscopic particle through a large quiescent body of fluid, turbulent conditions existing only in the layer immediately surrounding the particle. As derived by application of the principles and methods of dimensional analysis,e the general equation for the resist1
Presented in part before the Division of Gas and Fuel Chemistry a t
the 72nd Meeting of the American Chemical Society, Philadelphia, Pa., September 6 to 11, 1926. 2 "The Mechanical Properties of Fluids," A Collective Work, p. 198, D.Van Nostrand Co , Inc , 1924.
ance R (grams) acting between a sphere in uniform irrotational rectilinear motion through s fluid possessing viscosity and the fluid is Rg = k p n - l p 2 - n 7 n V n (1) where P is the velocity, T the radius, p and p, respectively, the density and viscosity of the fluid, and g the acceleration due to gravity-all in c. g. s. units. For the important special case of free fall through the fluid this becomes u n =
4T
g(u--p)r3--npL"-*
3k
p"-'
(2)
where C is the terminal velocity of free fall, and u the density of the particle. For the ideal case of slow stream-line motion through a viscous liquid of infinite extent and a t rest a t infinity, R = 6 and n = 1. equation (1) becoming Rg = 6 ~ p 7 V (3) This equation was originally derived by Stokes and is discussed a t length by Lamb.3 Provided the stated conditions a
"Hydrodynamics," p. 338, Cambridge University Press. 1024.
I.YD USTRIAL A-VD ElVGIiVEERING CHEXISTRY
Xovember, 1928
1201
ing importance as regards pneumatic transportation of pulverized materials,lO the combustion of powdered coal, the blow-over of fine fuel from gas producers or water-gas gen2g (u-p)r* u = (4) erators, the design of dust separators, etc., equations (6) and 9P which is the expression commonly known as Stokes' law. In (7) have been further studied as 'applied to comparatively general, this applies only to the fall of extremely small par- large solid particles in turbulent gas streams. Between the two limiting cases of stream-line and turbuticles, being applicable to such cases as the settling of semicolloidal suspensions, or of extremely fine dusts in air. Its lent-flow conditions, the relation between particle ( r , a) and use in such connections has been too widespread for discus- the fluid ( p , p, V ) may be such as to form an intermediate sion here, although an interesting recent publication4 on the case-i. e., one such that in equations (1) and ( 2 ) l < n < 2 . size classification of the extremely fine fractions of powdered Audibert" has determined the velocity necessary to suspend coal by air currents may well be mentioned. Millikan's coal and coke particles of 22 to 147 microns radius in a verwork has, however, shown that Stokes' law has a lower limit tical gas stream. He has expressed his results in the form of r = ku k'u2 for each of applicability w h i c h i s different value of u or p ; reached when the diamethis method of representaters of the particles become The constant of the equation for the force acting betion is a poor one, since the of the order of magnitude of tween a macroscopic spherical particle and a turbulent equation is dimensionally the intermolecular distances gas stream moving past it has been determined as non-homogeneous and since of the fluid. On the other R = 0.00084pV2r2 a separate e q u a t i o n i s hand, by suitable empirical all units being in the c. g. s. system (R = grams). For needed for each gas or mamodification e q u a t i o n (4) irregular coal or coke particles the value of k is slightly terial. BlizardI2 has theremay be applied to the fall higher, t being taken in this case as the radius of the fore recalculated these reof large ( 0 . 5 t o 1.0 cm.) sphere of equal volume. These values have been found sults, which are best repspheres through highly visby determining the loss in weight of various particles resented by the equation cous oils in a finite vessel. hanging in vertical air streams of known velocity, L a d e n b ~ r g for ,~ example, and are confirmed by observation of the velocities re1170 yo.' p0.44 gives quired actually to suspend free particles in a vertical = P0.27 0.17 (8)
are fulfilled, it appiies to spheres of any size. On similar substitution in equation ( 2 ) we obtain
+
(5)
air stream. The available information covering the cases of stream-line or semiturbulent conditions about the particle is discussed and the limits of its applicability shown.
as representing this case, where R' and L are the radius and length of the vertical cylindrical container. 9 case -of more importance to the engineering field is that involving turbulent conditions surrounding the particles. This is represented by relatively high rates of flow of fluids of relatively low viscosity (gases, water, etc.) past the particle, or by the free fall of relatively large and dense particles through such fluids. The exponent n now becomes 2 and equation (1) becomes
Rg = kpr2V2 Jvhile from equation ( 2 ) we obtain
(6)
(7)
I
the constant Iz being indeterminate from theoretical considerations alone. Allen6 has investigated the case by determining the terminal velocity of steel balls (up to 6 mm. diameter) falling through water, finding for l o 4 k / g of equation (6) the value 5.5 * 0.2. Available data covering gases as the given fluid are not', however, concordant. Richardson7 has investigated the case of spheres shot vertically into the air, obtaining values ranging from 5.5 to 8.4. LunnonJ8 observing the fall of steel balls through deep mine shafts, obtains 5.39, while numerous other workers9 report values from 5 to 10. Since this case of turbulent fluid conditions is of govern4
Bouton and Pratt, Carnegie Inst. Tech., Bull. Coal Mining Invesfiga-
fions 12 (1924). Ann. P h y s i k , 23, 447 (1907). Phil. Mag., [51 60, 323, 610 (1900). 7 Proc. Phys. SOC.London, 36, 67 (1924). P r w . ROY.SOL.( L o n d o n ) , llOA, 302 (1926); Phil. M a g . 161 47, 173 (1924). 9 Schmidt, A n n . P h y s i k , 61, 633 (1920); Shakespear, Phil. M a g . , 28, 728 (1914); Pennell, Adwisory Comm. Aeronautics ( E n g l a n d ) , 190 (1914); Cook, Phil. Mag., [6] 39, 360 (1920). 5
P
This equation is dimensionally homogeneous, the constant having the dimensions of g , and may be directly derived from (a), n being eaual to 1.58. Since the completion of the tvorgreported herein, hlartinI3 has described extended experiments on the air velocities required to support various sizes of crushed quartz grains. He assumes a region between viscous and turbulent flow conditions-i. e., between equations (4)and (7)-in which the relation between U and r is a straight-line function. However, there is every reason to believe, by anslogy from the known laws governing the flow of fluids through pipes or through granular packings, that the transition from equation (4) to (7) must take place as a smooth and continuous function of n in the general expression ( 2 ) . This will be discussed later. Experimental Procedure and Results
The method employed for determining the force acting between the particle and the gas stream was as simple and direct as possible, as shown in Figure 1. The sphere or other particle was supported from t'he pan of an ordinary analytical balance, by means of a fine silk thread or human hair, so as to hang about 15 cm. down into the center of a vertical pipe (8.0 or 16.0 cm. diameter). Air a t constant humidity and temperature was measured by a calibrated 1.58-cm. Venturi meter, and passed up through the vertical pipe, the decrease in weight of the sphere and supporting filament being determined for various air velocities. The smaller (lead) spheres up t'o 6.4 mm. diameter were fastened to the supporting filament by pricking a small needle hole in one side and closing it down over the filament, while with the larger (glass) spheres and the coal and coke fragments a minute lump of sealing wax was used. Of the three verticdly superposed Cramp, J . SOL.Chem. I n d . , 44, 207T (192.5). A n n . M i n e s , 1, 153 (1922). 12 1. Franklin Inst., 197, 199 ( 1 9 2 4 ) . l a Trans. Cer. SOL.( E n g l n n d ) ,26, 21 (1927).
10
11
INDUSTRIAL AND ENGINEERING CHEiWISTRY
1202
1.0-cm. holes bored in the bottom of the balance case and in the shelf supporting it to permit passage of the supporting filament, two were covered by movable brass plates in each of which a 2.0-mm. hole was drilled. Although the lowest of these openings was only 15 cm. above the top of the vertical pipe, the air was so well baffled that in a blank run at a velocity of 206 cm. per second in the pipe no effect on the balance pan could be detected, while at 824 cm. per second the loss in weight was only 0.0008 gram,
Vol. 20, No. 11
of, , , ’T /L’%> for various conditions), and 104 k/g the constant of equation (6). Since in this equation for spheres r 2 expresses. or is proportional to, the cross-sectional area of the sphere, when dealing with non-spherical particles i t is most logical similarly to relate the resistance to A , where A represents the actual maximum cross-sectional area in square centimeters as projected on a section perpendicular t o the line of air flow; for spheres of radius r it equals ar2; for a cube of side “a” suspended by the middle of a face it equals a2, if suspended by the middle of an edge it equals a* fi if suspended by a corner it equals a2 for irregular particles it was determined by repeatedly tracing and planimetering the shadow cast by the particle as actually suspended. using an approximately parallel beam of light. I n the calculation of k for ‘cubes or irregular particles A / a has been used in place of rz of equation (4), this term being equal to the square of the radius of the sphere of equal projected crosssectional area. The density of air, p, has in all cases been taken as 0.00117 gram per cubic centimeter, this being the value for air saturated with water a t 27” C., the temperature prevailing during these runs.
4%
A - Std. f o r 4”ripe.
Table I-Spheres
D
I l l
I
I
A Wcor. 10‘k/g Hair suspqnsion, air flow 1500 cubic feet (42.5 cu. m.) per hour in 16-cm. pipe. Vmax. = 187 cm. per sec., AWfil. = 0.0022 gram 1.530 0.0193 0.0171 7.9 0.64p 0.0056 0.0034 9.0 0.305 0.0029 .... 0.150 0.0024 .... ... Thread suspension, air flow 200.0 cubic feet (56.6 cu. m.) per hour in ?6-cm. pipe. Vn.ax. = 249 cm. per sec., Awfil. = 0.0055 gram 2.515 0,0930 0.0875 8.4 1,530 0.0370 0.0315 8.3 0.635 0.0112 0.0057 8.7 0.305 0.0064 .... 0.150 0,0059 .... ... Awobsd.
...
...
Figure 1-Apparatus for D e t e r m i n i n g Force Acting between Particle a n d Gas S t r e a m
this correction being only a small percentage of the total force acting a t this velocity in actual runs, but being applied where necessary. That no “end effects’’ were being introduced by hanging the particles only 15 cm. down into the top of the pipe was shown by duplicate runs on a 6.35-mm. ball hung, respectively, 15.0 and 7.0 cm. from the top, the difference between the runs being within the error of readingi. e., 0.1 mg. Even a t high velocities the particles hung with remarkable steadiness and the balance could be read with satisfactory precision, the sensitivity being in most cases 0.1 mg. The correction required due to the action of the air stream on the suspension filament was determined by plotting for convenient constant velocities the force acting on ball plus filament for each size of ball, and extrapolating these curves to zero ball diameter. The data for the 1.50- and 3.05-mm. balls were used merely for the determination of these curves, since in these cases the force acting on the ball itself is not large in comparison with that acting on the supporting filament, so that the probable error in the calculation of the force on the ball would be over 10 per cent. The original data of the various runs, when plotted on logarithmic coordinates, all fell accurately on straight lines having slopes of 2/1, or in other words, 4 W = +V2 for all runs. These data are therefore not presented herein, save those used in calculating k of equation (4), which are given in the following Tables I to 111. Here D is the ball diameter in cm., 4 W o b s d . the observed loss in weight in grams, AWm. that due t o the action on the suspension filament, 4WOor. = AWobd. - AWtil., vnm,the actual h e a r velocity a t the center of the pipe in cm. per second (determined from plotI4 14 Walker, Lewis, and McAdams, ‘‘Principles of Chemical Engineering,” p. 68, McGraw-Hill Book Co., Inc., 1923.
Table 11-Cubes Thread suspension, air flow 1000 cubic feet (28.3 cu. pipe. Vrnax. = 500 cm. per sec., AW 111. = A Aw’obnd. Sphere (D = 0,635) 0.317 0,0285 0.0774 Cube hung b y face 0.617 0.1050 Cube hung by edge 0.868 0.1160 Cube hung by corner 1.092
m.) per hour in 8-cm. 0.0080 gram AWcor. 104k/g 0.0205 8.5 0.0694 14.5 0.0970 14.5 0.1080 12.8
Table 111-Irregular Particles Hair suspension, air flow 1500 cubic feet (42.5 cu. m.) per hour in 16-cm. pipe. VrnaX. = 187 cm. per sec., AWfil. = 0.0020 gram NO. MATERIAL A AWobsd. AWcor. 1o‘k/g 11.6 0.0510 0.0490 1 Coal 3.6 0.0760 0.0740 9.7 2 Coke 5.9 8.3 0.0450 0.0430 3 Coke 4.0 9.5 Oa0510 0.0490 4 Coke 4.4 6.6 0,0370 0.0350 5 Coal 4.1 10.2 0,0690 0.0670 6 Coal 5.2
Experiments were then carried out on the size classification of coarse coke particles in vertical air currents, as a check on the value of k as above determined. Again the apparatus was extremely simple, consisting of a 1.58-cm. Venturi meter and suitable connections to a vertical 1.5meter length of ordinary 5.08 cm. inside diameter glass tubing. A second 1.5-meter length of glass tubing was vertically superposed on the upper end of the first one, the two being separated by a hand-operated slide valve. I n operation approximately 150 grams of crushed and roughly sized coke were placed upon a bottom screen, and the air velocity was increased until a reasonable number of coke particles were suspended in the upper tube. The plate valve was then shut so as to trap these particles, the air velocity through the lower tube reduced, and the upper tube removed and emptied. On replacing the upper tube the procedure was repeated, the Venturi meter being read just before closing the slide valve. The data of these runs are graphically presented in Figure 2. As there noted, three different mixtures of crushed coke,
Xovember, 1928
I N D L;STRId L A N D ENGINEERING CHENISTRY
of progressively increasing average size, were used as starting material. The points for each of these mixtures are shown separately. The average particle weight was determined from the total weight of 50 to 150 of the suspended particles. The theoretical curve was derived by calculation from equation (7), using lo4 k / g = 9.3. Discussion of Results
The average value of lo4 k / g in equation (6) for spheres is determined from the values shown in Tables I and the first value of Table 11. The second value of Table I (hair support. 0.640 ball) has been omitted from the average, since tlie deviation a.\s three IO times the average deviation. The final ave r a g e v a l u e thus be05 comes 8.4 * 0.2. The values of Table I11 for irregular coal and coke p a r t i c l e s give as an average 9.3 * 1.2. 01 The d i f f e r e n c e betJveen this value of k f o r s p h e r e s and the 005 values of 5.5 (Allen) or 5.4 (Lunnon) is attributable to a basic difference in the experimental procedures. As is pointed out by GibOM s0n.l5 there is little or no direct experimental OM5 evidence to show that t h e r e s i s t a n c e is the s a m e r e g a r d l e s s of whether t h e fluid moves past the particle or the particle moves OOOl 200 so0 lW0 PWO through the fluid. UnFigure 2-Carry-Over Velocity for Coke der conditions of pure Fines viscous flow the cases are probably identical. I n the latter case, at low velocities, the rate a t which the solid moves through the liquid is the actual velocity with which the solid meets the particles or filaments of the liquid. I n the case where the entire liquid is in turbulent flow, the velocity of many particles or filaments of the liquid is considerably greater than the observed mean translational velocity of the fluid. Since the square of the statistical sum of such velocity terms is less than the statistical sum of the squares of these terms, it is evident that the kinetic energy of the fluid, and therefore the resistance to the flow about the solid, may be higher than that indicated by its mass times the square of its mean translational velocity. Another point of difference in the systems presented by the present experiments and those of Allen lies in the fact that a sphere freely falling under non-idealized conditions will inevitably develop rotation and that when we determine k from the weight and terminal velocity of the particle in equation (6)) we neglect a term +TB(W)~ on the left-hand side of the equation (where w represents the rotational velocity), and hence obtain lower values of k than would be the case were rotation absent or allowed for in the calculation. All things considered, the agreement of the average constant determined for irregular coal and coke particles with that determined for spheres is good. The agreement of the 15 "The Mechanical Properties of Fluids," A Collective Work, p. 185, D Van Nostrand Co , Inc , 1924
1203
constant 47.8 in equation (9) following, with that of 50.9 cited by ?\lartin13is also satisfactory. As shown by Table 11, the constant k for cubes hanging in various positions is appreciably higher than for spheres or irregular particles, here also r2 being taken as A / T . This being a special case of little practical significance, it has not been investigated in any detail. Consideration of the data obtained by suspending mixtures of coke particles in a vertical air current, as shown by Figure 3, leads to confirmation, for the type of system under consideration, of the values of k herein determined. The observed "suspension velocities," C, for the smaller particles agree well with those calculated from equation (7) using lo4 k / g = 9.3. The low support velocity values found for the largest sized particles are due to their effect in reducing the free area of the tube and thus increasing the effective linear velocity over that actually observed. For convenience in application we may rewrite equation 171, as follow, using lo4k / g = 9.3 for irregular particles: U = 4 7 . 8 ( (~ p)d/p)'.S (9) it being again noted for emphasis that U is the velocity of the turbulent fluid medium of density p required t o suspend a particle of diameter d and density 6 ,all units in the c. g. s. system. The lower limit of applicability of this equation is discussed in the following section. General Discussion
For the sake of clarity in the application and use of the present and similar data, it seems desirable to develop the general expression ( 2 ) into a comprehensive form applicable to all types of flow. In so doing we are merely following the procedure which has become standard for the treatment of data on fluid flow through pipes, etc. Rlartin13 visualizes a region between stream-line and turbulent conditions about the falling particle, in which the terminal velocity is a straight-line function of the particle size. This. however, is an undesirable method of treatment, since it violates the principle of dimensional homogeneity of the expressions involved.
t 6 IO'
'i /' ~
1
I
I 1
'1 I i
,
'
I
I
L _ _ ~ -L - I- _ _ v
I
t
+--*r
1
il , I
1
i .
~
t
In accordance with the procedure for pipe-flow expressions, this may be expressed
I N D U S T R I A L AND ENGINEERING CHEMISTRY
1204
u s c.14 Pd
(11)
where C is an exponential function of the expression ((u-p)d3p/pZ) and may therefore be conveniently represented by plotting on a logarithmic scale, against ((a- p ) d 3 p / p z ) as abscissas. C obviously is numerically defined, in any given set of data, as equal to U M p . I n Figure 3 the data of hlartin13 for crushed quartz grains in turbulent air streams have been plotted in this manner save that the curve has been made smooth and continuous in the transition range. Points G and C'represent the critical points between which he postulates a straight-line law connecting velocity and particle diameter, while C" evidently represents the hypothetical critical point between stream-line and turbulent conditions. Points A-A ' represent the limit cases of the present work, and show its excellent agreement with that cited by Martin. Points B-B' represent approximately the limits and equation for Audibert's experiments, which evidently are in poor agreement. Points D-D' represent the limits of tests performed by hlartin using a vertical current of air in stream-line flow, and indicate that the state of flow of the fluid stream has little effect on the size of particles supported thereby, although this, if generally true, is a rather .surprising conclusion. In actual use, Figure 3 is directly applicable to determining the fluid velocity required to support a given particle or the terminal velocity of the particle falling through the fluid. , The procedure is to evaluate the term ( ( u-p ) d 3 p / p z ) ) determine C from the curve, and solve equation (11). Unfortu-
Vol. 20, No. 11
nately, one type of problem important in practice-viz. calculation of the largest particle suspended in a fluid stream of given velocity-must be solved by indirect approximations unless it is known in advance that the case in question must lie above or below the critical region C-C'. Above this region equation (9) applies, while below point C the curve of Figure 3 is represented by the expression
u =
35.2 (u-
p)d2
P
(12)
The constant of this expression is somewhat lower than that (54.5) calculated from Stokes' law (equation 4). It must, however, be realized that there is still considerable uncertainty attached to any general expressions for the fall of particles through fluids or their support in fluid streams. As discussed in an earlier part of this paper, even for large particles falling under turbulent conditions the turbulence of the fluid may still be a factor of importance. Similarly, the factors governing the suspension in a turbulent fluid stream of small particles, which would fall through a quiescent body of the fluid under stream-line conditions, are still more uncertain. Moreover, the accurate location of the critical region (C-C', Figure 3) and the shape of the curve in this region cannot be considered to be satisfactorily established. It is hoped that these points will be clarified in time. Acknowledgment
The authors wish to express their appreciation to the Combustion Utilities Corporation for permission to publish the results of these tests.
Reduction of Tricalcium Phosphate by Carbon' K. D. Jacob and D. S. Reynolds FERTILIZER A N D FIXEDNITROGEN INVESTIGATIONS, BUREAU OF CHEMISTRY AND SOILS,WASHINGTOS, D. C
for the manufacture N T H E volatilization of phosphoric acid, the primary reaction occurring in the furnace results in the formation of elemental phosphorus and is customarily represented by the equation :
then calcium phosphide is formed instead of free phosphorus. Xelsen16 obtained partial reduction of tricalcium phosphate by heating with carbon a t temperatures above 1400' C. On the other hand, ROSS,Mehring, and Jones'g volatilized 90 per cent of the phosphorus present in small mixtures of equal Ca3(PO& 3Si02 5C = 3CaSiOa 5CO P2 weights of phosphate rock and coke by heating for 1 hour a t The phosphorus is oxidized either within or outside the fur- 1300" C. under close temperature control. However, connace proper and is finally recovered as orthophosphoric acid.18 clusions as to the completeness of the reaction between triFrom the standpoint of commercial operation of the process, calcium phosphate and carbon alone cannot be drawn from one of the primary functions of the silica is to combine with the results of these experiments, because of the presence of the lime to form a liquid slag which can be tapped from the appreciable quantities of silica in the phosphate rock and coke. furnace. The investigations of Berthier, ' Hempel,'ONielsen, The present paper gives the results of an investigation of the Mehring, and Joneslg show, however, that silica factors affecting the reduction of tricalcium phosphate by and ROSS, also plays a definite part in accelerating the reaction and re- carbon under closely controlled conditions in the absence of ducing the temperature a t which reduction begins. The silica. last-named investigators have further shown that it is not Materials necessary to form a liquid slag in order to obtain practically complete volatilization of phosphorus from small mixtures TRICALCIUM PHOSPHATE-SWeral samples O f supposedly of phosphate rock, silica, and coke. c. P. tricalcium phosphate were analyzed, but none of them Definite information on the factors affecting the reduction contained the proper Pa05--Ca0 ratio and all were contamby carbon, in the absence of silica, of tricalcium phosphate, inated with varying quantities of impurities. One of these which for most purposes may be considered as the primary samples, designated as tricalcium phosphate KO.1, was used constituent of phosphate rock, is essential in order to be in some of the experiments after it was first heated at 900able to draw reliable conclusions regarding the effect of 950' C. for 3 hours to remove free and combined water and silica on the reaction. Berthierl was unable to reduce phos- then passed through a 200-mesh sieve. Practically pure triphate of lime with carbon alone, and according to ThorpeZ2 calcium phosphate was prepared by slowly adding, with contricalcium phosphate is not reduced by carbon in the absence stant stirring, a dilute solution of pure phosphoric acid to a of silica except a t excessively high temperatures, and even water suspension of pure lime and evaporating to dryness on the steam bath. The product, which contained some free 1 Received M a y 19,1928. lime and dicalcium phosphate, was finely ground and mixed * Numbers in text refer t o bibliography at end of paper.
I
+
+
+
+
