ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT work on aerosol collection was a part of a contract, No. AT (30-3)-28, with the United States Atomic Energy Commission in the Engineering Experiment Station of the University of Illinois, on which R. B. Feild was a paretime research assistant. The authors are indebted to L. Bryce Andersen for the measurements on rates of oxygen absorption during atomization and to William E. Ran2 for many helpful suggestions concerning the work. Nomenclafure
L-L
Lb.)
\--1
.
kL
= gas film coefficient, 1b.-moles/(hr.)(sq. ft.)(atm.) = liquid film coefficient, 1b.-moles per (hr.)(sq. ft.)(lb.-
KE
=
KI
=
L
= =
ICG
n
hi, Nu’
= = =
pa,
= = =
QP
=
t
=
TO
=
S T u0 zit
V z eo
mole/cu. ft.) dimensionlem parameter for aerosol collection by electrostatic attraction dimensionless parameter for aerosol collection by induced charges liquid to gas ratio, ga1./1000 cu. ft. of gas number of aerosol particles; subscript o refers to initial number rate of absorption or desorption, gram-moles/min. h-usselt number for particle collection partial pressure of solute gas, atm. rate of sample collection, ml./min. rate of liquid injection, ml. ,/min. electric charge per unit area of collector surface, COUlombs/sq. em. electric charge on particle, coulombs radial distance, inches radius of spray zone, inches
=
= = = = =
= =
E?
= =
e
= =
p p
gas constant
= radius of duct, inches = cross-section area of spray zone, sq. inches
el 7
Consistent units are used in equations: final values are expressed in the following units. = specific drop surface, sq. ft./cu. f t . of gas a = empirical correction factor for reeistance of gas t o moveC ment of small particles, dimensionless (11) c = concentration, gram-moles/l. Ds.71 = diffusivity, sq. cm./sec. D, = diameter of droplet or spherical collector, microns D p = diameter of aerosol particle, microns D,, = volume-surface mean diameter of droplets or aerosol particles, microns n = over-all collection efficiency = 1 - E no = fraction of gas desorbed = proportionality constant in Equation 11 (ga1./1000 cu.
;rk
R Rd
= =
absolute temperature, O K. velocity of aerosol stream, cm./sec. gas velocity relative t o the liquid a t Venturi throat, ft.! sec. volume of spray zone, cu. inches axial distance along Venturi throat, or duct, inches permittivity of free space dielectric constant of particle dielectric constant of gas efficiency of collection, dimensionless dimensipnless parameter for collection by Brownian diffusion viscosity of gas, poises gas density, gramslml.; subscript p indicates particle density dimensionless parameter for collection by inertial impaction
literature Cited (1) Comings, E. W., Adams, C. H., and Shippee, E. D., ISD. ENQ. CHEY., 40, 74 (1948). (2) Ekman, F. O., and Johnstone, H. F., Ibid., 43, 1358 (1951). (.3,) Feild. R. R., M.S. thesis in chemical engineering. University of
Illinois, 1950. Feild, R. B., Ph.D. thesis in chemical engineering, Unirersity of Illinois, 1952. ( 5 ) Johnstone, H. F., and Roberts, 11. H., IND.ENG.CHEM.,41, 2417 (1949). (6) Johnstone, H. F., and Williams, G. C., Ibid., 31, 993 (1939). ( i )Jones, W. P., and Anthony, A. IT., Jr., “Proc. U. B. Technical Conference on Air Pollution, May 1960,” p. 318, Kew York, McGraw-Hill Book Co., 1952. ( 8 ) LaRler, V. K., and Hochberg, S., Chem. Recs., 44, 341 (1944). (9) Nukiyama, B., and Tanasawa, Y., Trans. Soc. M e e k . EngT8. (4)
(Jaman). 5 . S o . 18. 68 (1939). (10) Ranz,’W.’E.,’andWong, J: R . , Arch. I n d . Hug. and Occupational M e d . , 5 , 464 (1952). (11) Ranz, W. E., and Wong. J. R..IND.ENG.CHZM.,44, 1371 (1952). (12) Tassler, &I.C., Ph.D. thesis in chemical engineering, University of Illinois, 1952. RECEIVED for review October 13, 1953. ACCEPTED February 18, 1954. Presented a t a conference on Atomization, Sprays, and Droplets sgonsored by Project SQUID, a t the Technological Institute, Northwestern University, Evanston. Ill., September 24 and 2 6 , 1953.
Beneficiation of Florida inum Phosphate Ore J. E. DAVENPORT, FRANK CARROLL,
AND
GRADY TARBUTTON
Division o f Chemical Developmenf, Tennessee Valley Aufhorify, Wilson Dam, Ala.
E
XTEKSIVE deposits of a soft aluminiferous phosphate occur in Florida ( 2 ) . The aluminum phosphate apparently is a product of the leaching of pebble phosphate. It frequently overlies beds of pebble phosphate. For lack of a practicable method of using the leached-zone ore, it is vasted with the overburden when the pebble is mined. An estimated 14,000,000 tons were discarded in 1952 ( 1 ) . The ore is low in grade and does not lend itself readily to beneficiation. It cannot yield a phosphate concentrate equivalent t o that from the pebble, because the niaximum theoretical phosphorus pentoxide content of the aluininum phosphate minerals is only about 33yo,as compared with 42% for fluorapatite. The ore is a potential source of uranium as well as phosphorus, however, and TV.4 studied its beneficiation at the request of the Atomic Energy Commission.
1608
Three proce3ses for beneficiating the ore were developed. The detailed study of the variables in the processes is beyond the scope of this paper. The processes are presented with little amplification beyond the results that they consistently yielded under optimum conditions. Quartz le Chief Gangue Cons+ituent of Aggrega,ed Ore
The aluminuni phosphate is a buff earthy ore that is characterized by the prc-ence of reakly consolidated aggregates having an extremely porous structure. The principal mineral components are wavellite, pseudon-avelhte, fluorapatite, quartz sand, and clay The relative amounts of the different minerals vary greatly, but quartz generally is the major gangur constituent. A
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
Vol. 46, No. 8
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Distribution of PZOs in Size Fractions of Aluminum Phosphate Ore
Table 1.
@lid? Distribution,
% 8.7 8.7 4.0 3.8 3.4 1.9 9.5 10.0 9.5 9.6 6.1 6.1 1.7 17.0 100
Size Fraction +O. 74 inch -0.74 0.37inch -0.37 inch 4 mesh -4 +8 mesh -8 4-14 mesh -14 +28 mesh -28 +48 mesh -48 +65 mesh -65 +lo0 mesh -100 +150 mesh -150 +200 mesh -200 f325 mesh -326 mesh +15 p
+ +
-15
p
Cumulative head Combined f14 and -325 mesh
47.3
PPOS
Content,
% 17.1 18.1 18.0 18.6 20.3 11.3 6.2 4.6 5.9 6.6 5.3 7.6 14.1 21.8 12.4
19.2
PzOs,
Distribution,
% 12.0 12.8 5.8 5.7 5.1 1.7 4.7 3.8 4.5 5.2 2.6 3.7 1.9 30.1
AcidInsol. Content,
% 46.0 42.6 40.0 36.2 31.1 69.7 79.3 84.6 80.8 79.5 83.4 75.7 52.5 21.0
... ...
ILOO
73.7
sample representing 70 tons of the ore had the following chemical composition : Composition, Wt. % Ignition P z O ~ CaO A1203 Fe203 Si02 F Loss 11.8 2.6 16.2 1.6 57.6 0.4 9.2
A 15-pound sample of ore-hand-blunged with water-was sized by screening and sedimentation. The distribution of phosphorus pentoxide in the separates is shown in Table I. The approximate mineralogical composition of the size fractions of the ore was calculated from the data in Table I. The method of calculation was based on the following considerations. The average grain size of the phosphate minerals was about 5 microns (estimated by microscopic examination), and the grains were so intimately intergrown with each other and with kaolinite as to be inseparable by physical methods. This impure phosphatic material behaved as an entity in mineral-dressing operations. The ore also contained quartz and clay in forms t h a t were separable from the phosphate.
100
2
14
SIZE., MESH ~48 I00
,
I 1
t - !
I
I
20 40 60 80 MATRIX SOLIDS, CUMULATIVE %
100
Figure 1. Weight Distribution of Mineral Components in Size Fractions of Aluminum Phosphate Ore The most nearly clay-free fractions that were separated from the ore by any method contained about 33% phosphorus pentoxide. This material was assumed to be representative of the phosphate complex-that is, the phosphate minerals and the inseparable impurities. The amount of the complex in the separates in Table I was calculated by multiplying phosphorus pentoxide content by a factor of 3.
August 1954
+
*
Process I Includes Rejection of Minus 12-Mesh Plus 30-Micron Portion of Ore
A 1000-pound batch of the ore was treated in standard mineraldressing equipment. The ore was tumbled with water in a revolving drum t o disintegrate weak sandy aggregates. The drum discharged onto a 12-mesh screen. T o grind the phosphate preferentially t o the quartz, the suspension passing the screen waa fed through a lightly loaded rod mill and then was recirculated through a centrifugal pump. Finally, the suspension was classified a t 30 microns with a hydraulic cyclone. The reeults of the test are shown in Table 11.
Table II. Beneficiation of Aluminum Phosphate Ore by Sizing and Grinding, Process I Nominal Size Fraction -3 inches +12 mesh -12 mesh +30 p p
Cumulative head Combined +12 mesh and -30 p
200325
QUARTZ
U; 4 0 1
+
-30
1
0
The acid-insoluble material contained both free quartz and residual silica from leaching of the kaolinite. The difference, 2, between 100 and the sum of the phosphate and acid-insoluble material represented alumina and water from the clay. Whereas the weight ratio SiOz:(AlzOs HzO) for kaolinite is about 0.8, a value of 0.6 was used in the calculations, because x also included some free ferric oxide and small amounts of other minerals. Thus, clay silica = 0.62; quartz = (acid-insoluble material) - 0.62; and clay = z 0.62. The weight distribution of the mineral components, as calculated by this empirical method, is shown in Figure 1. Several conclusions about beneficiation of the ore were drawn from Table I and Figure 1. Most of the clay and nearly a third of the phosphate were disaggregated to slime by mild blunging. The slime could not be discarded, because it contained too much phosphate; neither could it be enriched by conventional mineraldressing techniques, because the minerals were too finely divided. Since the principal gangue mineral-quartz-occurred largely in the minus 14- plus 325-mesh fraction, the rejection of approximately this fraction in a uFashing operation was investigated.
Solids Distribution,
% 29.8 46.5 23.7 100 53.5
PzO6 Content,
% 18.2 4.3 24.7 13.3
21.0
PZOS,
Distribution,
Enrichment Ratio
41 15 44 100
1.37
85
1.58
%
... ...
1.86
By this process, 85% of the phosphate in the ore was recovered in a concentrate containing 21% phosphorus pentoxide. Although nearly equal amounts of phosphate were recovered in the fine and coarse fractions of the ore, the fine fraction was enriched more than twice as much as the coarse fraction. A portion of the untreated ore was leached with acid. A screen analysis of the plus 400-mesh (37 microns) fraction of the residue (Figure 2) showed that practically all the acid-insoluble material was finer than 20 mesh. The quartz in the plus 14mesh fraction of the ore evidently was present as inclusions in agglomerates of phosphate and clay. Process I I Is Based on Preferential Grinding of Phosphate in Whole Ore
In an effort t o grind as much of the phosphate as possible to slime size, a 550-pound portion of the ore was ground to minus 28 mesh in a rod mill. The 18 X 30 inch mill contained 150 pounds of 1-inch rods. The rate of grinding was 380 pounds per hour. The discharge was ground by attrition in a centrifugal pump for 30 minutes, which gave an average of 50 to 60 passes through the pump. The suspension then was classified a t 40 microns with a hydraulic cyclone. The results of this experiment are shown in Table 111.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1609
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table 111.
Beneficiation of Aluminum Phosphate Ore b y Grinding Whole Ore, Process II
Kominal Size Fraction - 2 8 mesh +40 ---1Op
Solids Distribution,
Content,
65 35 100
6.3 22 8 12.1
PzO~
7%
%
ji
,
Cumulative head
r
Paos,
Distribution,
r
Enrichment Ratio
70
. .
33 67 100
1 88
..
7-
1
r
5t
L
L
IO
14
20
65
28
100
nentially with time, hoyever, and prolonged grinding by attrition probably would not be economically feasible. The beneficiation shown in Table 111, therefore, is assumed t o represent a practical limit for preferential grinding of the phosphate. Beneficiafion by Dry Methods Is a5 Effective as Wet-Process Treatment
The phosphatic slime produced in the &et treatments is difficult to d e m t e r . The suspensions settle t o a maximum of 20 t o 25% solids on standing. They were dewatered t o 40% solids in a specially designed thickener and to 60% solids by filtration. The rest of the w-ater had t o be evaporated. With the objective of avoiding a costly dewatering step, processes I and I1 were tried in a dry system. I n piocess I the dried ore (crushed to minus 3 inches) was tumbled in a revolving drum. The drum discharged onto a 12-mesh screen. The minus 12-mesh fraction was ground in the rod mill a t a rate of 300 pounds per hour and then classified a t 55 microns in 4 passes through a Raymond whiazer air classifier. T o compensate for the lack of an attrition grinding step, thc time of grinding in the rod mill was increased, and the fines were separated a t a coarser particle size than in the y e t process. The results of the experiment are shom-n in Table IT,
150
S I Z E , MESH
Figure 2. Screen Analysis of Acid-Insoluble Residue from Aluminum Phosphate Ore Two thirds of the phosphate u-as recovered as slime, and the ratio of enrichment was 1.88. A portion of the ground ore was sized to determine the size distribution of the remaining third of the phosphate. The results (Figure 3) indicated t h a t disaggregation of more of the phosphate t o slime in the rod mill probably would entail grinding of the quartz as 11-ell.
Table IV.
Dry Beneficiation of Aluminum Phosphate Ore by Process I
Sominal Size Fraction - 3 inches t 1 2 iuesh - 1 2 mesh +55 p -55 & Cumillative head Combined +12 mesh and --35 si
Solids Distribution,
P?Oi Distribution,
21.8 53.6 24.6 100
17.5
33
Enrichment Ratio 1.51
4,3 22.2
47
100
1.92
1I.A
40 4
20 0
81
1 72
%
PzOa Content,
%
76
20
...
...
I n process I1 the ore u-as crushed to minus 4 mesh in a hammer mill, ground to minus 28 mesh in the rod mill, and classified at 55 microns with the air separator. The results are shown in Table T'.
Table V.
QUARTZ
Dry Beneficiation of Aluminum Phosphate Ore b y Process I1
h-orninal Size Fraction -?? mesh + 5 5 p --3J
G-i 0
40
60
so
DistriPzOa
Eniicli-
bution,
mont Ratio
%
7%
F o!,
&
~
100
MATRiX SOLIDS, CUMULATIVE %
Figure 3. Weight Distribution of Mineral Cornponents in Size Fractions of Aluminum Phosphate Ore after Grinding to Minus 28 Mesh To determine the feasibility of grinding more of the phosphate to slime by attrition, a portion of the product from the rod mill was screened on a 325-mesh screen, and aliquots of the minus 28- plus 325-mesh sand mere agitated a t high pulp density for various intervals mith a high speed soil disperser. The rate of attrition grinding y a s measured by the amount of minus 323mesh phosphate produced. The results are shown in Figure 4. The curve in Figure 4 indicates that 50% of the p h o q h a t e in the minus 28- plus 325-mesh sand from the rod mill can be ground to elime by attrition. The rate of grinding decreased expo-
1610
P2Or Content,
Cuinulative head
PHOSPHATE
20
DistriSolids bution,
The dry treatments gave about the same beneficiation as the corresponding wet treatments (Tables I1 and 111). Grinding and Flokalion Are Used Is Separate Phosphate and Quartz Process 11-grinding all the ore and separating the phosphatic hies-gave a richer concentrate than process I Some 30 t o 40% of the phosphate escaped preferential grinding in process TI, however, and the flotation of this reeidual granular Phosphate from the quartz sand was studied. The phosphate proved unamenable to flotation with a fattv acid collector. An exorbitant amount of fatty acid was required, and both the grade of the concentrate and the recovery of phosphate were low. Better separation of the t n o minerals was achieved by floating
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 8
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table VI.
Flotation of Quartz from Phosphate Sand, Process 111
Flotation Product Float Nonfloat Cumulative bead
Solids Distribution,
Content,
PzOq Distribution,
%
%
82.6 17.4 100
2.0 27.4 6.4
25 75 100
PzOa
%
Enrichment Ratio . I .
4.28
...
the quartz with an amine collector, To minimize the requirement of collector, the flotation feed was deslimed thoroughly, and precautions were taken t o avoid the production of slime during flotation. Table VI gives the results of floating the minus 28-mesh plus 40-micron sand from process I1 (Table 111)
Applicability of Process Depends on Constituents and Physical Characteristics of Deposits The results of beneficiating the ore by the three processes are summarized in Table VII. Beneficiation data on another s h i p ment of ore that contained 16% phosphorus pentoxide are included for comparison. Process I, with its lower grinding and dewatering costs, probably is preferable to processes I1 and I11 in treatment of the 12% ore. The greater enrichment attainable with process I1 offsets the lower recovery of phosphate only when the grade of the concentrate is a major consideration. Process I11 gives the richest concentrate with a very high recovery.
Table VII. Relative Effectiveness of Three Methods in Beneficiation of Two Lots of Aluminum Phosphate Ore P20a in Concentrate, Process
I (net) I1 (wet) I11 I
(wet) I1 (wet) 111
I1 = o
,
,
,
,
10 20 30 40 ATTRITION GRINDING TIME, MIN
, I 50
Figure 4. Rate of Grinding Aluminum Phosphate b y Attrition Since coarse quartz does not float readily, the plus 48-mesh fraction of the sand, which contained little phosphate (Figure 3) mas screened out and later combined with the quartzitic flotation product. The flotation feed was mixed with water in a batch flotation cell to make a 15% suspension, the reagents were added, and the quartz was floated without conditioning. The following reagents were used: Pound/Ton Dry Solids in Feed Sodium hydroxide 0.1 Primary amine acetate (Armour, Armac T ) 0.3 Mixture of higher alcohols (Du Pont, Frother B-23) 0.1
By process 111,75% of the phosphate in the sand was recovered in a flotation product that contained 27.4% phosphorus pentoxide. Combination of this product with the fine concentrate listed in Table I11 yielded an over-all recovery of 92% of the phosphate in a concentrate containing 24% phosphorus pentoxide.
END
August 1954
OF ENGINEERING,
Grade
%
Recovery
Enrichment Ratio
Ore Y o 1. PzOs Content 12% 21 0 85 22.8 67 24 0 92
1.58 1.88 1 98
Ore KO.2, PtOs Content 16% 22.6 66 25.2 50 27.6 95
1.43 1.57 1.72
The 16% ore differs from the 12% ore in two important respects. The coarse concentrate is relatively lower in grade, and more of the phosphate is resistant to grinding. As a result, the 16% ore is beneficiated less than the 12% ore in either process I or 11. Process I11 is particularly effective in concentrating the 16% ore. The success of the relatively inexpensive process I hinges upon the presence of fairly high grade agglomerates that can be separated as a coarse concentrate. Process I1 is particularly suited t o ore in which the agglomerates are low in grade and most of the phosphate is soft. The more expensive process I11 has wider application and excels the other processes in both enrichment and recovery of phosphate. The applicability of the respective processes to the aluminum phosphate deposits as a whole can be generalized only when more is learned about the make-up of the deposits. Utilization of the concentrate in the production of phosphate fertilizer is currently being investigated.
Literature Cited (1) Barr, J. A., Jr., Katl. Fertilizer Assoc., Fertilizer Process Progr., 2, No. 10, 4 (1953). (2) Hill, W. L., Armiger, W. H., and Gooch, S. D., Trans. A m . Inst. Mining Met. Engrs., 187, Tech. Publ. 2860-€I(1950). RECEIVED for review January 11, 1954. ACCEPTED March 12, 1954. Presented a t the joint Southwest and Southeast Regional Conclave of the .4MERICAx ~ m E ; \ r r c A LS O C I E T Y , New Orleans, La., December 1953.
DESIGN, AND PROCESS DEVELOPMENT SECTION
INDUSTRIAL AND ENGINEERING CHEMISTRY
1611
