Prochazka, J., Landau, J., Nekovar, P., Souhrada, F., Collection Czech. Chem. Commun. 30, 158 (1965). Reman, G. H., Olney, R . B., Chem. Eng. Progr. 51, 141 (1955). Schiebel, E. G., A.I.Ch.E. J . 2, 74 (1956). Scheibel, E . G., Karr, A. E., Ind. Eng. Chem. 42, 1048 (1950). Sherwood, T. K., Evans, J. E., Longcor, J. V. A., Ind. Eng. Chem. 31, 1144 (1939). Szabo, T . T., Lloyd, W . A., Cannon, M . R . , Speaker, S. S., Chem. Eng. Prog. 60,66 (1964). Treybal, R . E., “Liquid Extraction,” 2nd ed., pp. 346, 348, McGraw-Hill, New York, 1963a. Treybal, R. E., “Liquid Extraction,” 2nd ed., p. 487, McGraw-Hill, New York, 196313.
Treybal, R. E., “Liquid Extraction,” 2nd ed., pp. 52330, McGraw-Hill, New York, 1963c. Treybal, R. E., “Liquid Extraction,” 2nd ed., p. 350, McGraw-Hill, New York, 1963d. Treybal, R.E., A. I. Ch. E. National Meeting, Pittsburgh, Pa., May 1964. Tudose, R., International Chem. Eng. 2, 156 (1962). Van Dijck, W. J. D., US.Patent 2,011,186 (1935).
RECEIVED for review October 14, 1968 ACCEPTED April 21, 1969 Division of Industrial and Engineering Chemistry, 156th Meeting,
ACS, Atlantic City. N.J., September 1968.
BENEFlClATlON OF FLORIDA HARD-ROCK PHOSPHATE Selective Flocculation J .
E.
D A V E N P O R T ,
F R A N K
CARROLL,
G .
W .
KIEFFER’,
A N D
S .
C .
W A T K I N S
Tennessee Valley Authority, Muscle Shoals, Ala. 35660 Benefkiation of Florida hard-rock phosphate has been limited to washing a n d screening the ore to recover the plus 20-mesh fraction which contains about 40% of the P?Oj. Attempts w e r e m a d e to recover phosphate from the smaller fractions. Flotation of the sand w a s impractical because the phosphate w a s too soft. Preferential grinding followed by selective flocculation proved to be a technically feasible method for recovering a n additional 40% of the ore PlOi. The product, on a dry basis, contained 32% P?O: a n d 7% R?Oi. This high R?Oi content a n d the cost of dewatering (40% H?O) probably would m a k e recovery under present conditions unfavorable.
THEhard-rock
phosphate field in northwest Florida extends from Suwannee and Columbia Counties southward to Citrus and Hernando Counties, an area approximately 100 miles long and varying from 2 to 30 miles in width (Kibler, 1941, 1944). The phosphate deposits are irregular in shape and scattered. They range up to several acres in area and from a few up to 100 feet in thickness and lie under overburden ranging in thickness from a few to 100 feet. The phosphate occurs as coarse sand, pebbles, and boulders interspersed with quartz sand and clay (Kibler, 1941, 1944). About 4 0 5 of the P20j can be recovered in the plus 20-mesh material as a highgrade concentrate (up to 38% Proj). However, the hard rock is more costly to mine than pebble ore (Ruhlman, 1958) and, as a result. there is no commercial production at present. This paper gives detailed information on the composition and size distribution of ore from a number of hard-rock phosphate deposits and describes tests of the feasibility of recovering the phosphate from the minus 20-mesh fines. One third of the minus 20-mesh phosphate was in the
’ Present
address, 353 Tusculum Road, Nashville, Tenn. 37211
sand fraction (-20 +325 mesh) and two thirds was in the slime. Flotation of the sand was found to be impractical because the phosphate was too soft. Preferential grinding followed by selective flocculation (Haseman, 1951) proved t o be a technically feasible method of recovering an additional 40% of the ore P20,. However, the quality of the concentrate and the cost of dewatering it probably would make recovery under present conditions economically unfavorable. Composition a n d Properties of Ore
Most of the experimental work was done on core samples taken while prospecting various deposits of the hard-rock phosphate in a systematic manner. Core samples from each drill hole were combined in the proper proportions to yield a representative hole sample. Ore from each hole was classified as minable or marginal on the basis of the amount and grade of phosphate in the plus 20-mesh fraction, the ratio of overburden t o ore, and the continuity of the ore body. The hole samples then were combined to yield respresentative samples of the minable and marginal ore occurring in a 40-acre tract of land. Ore from 29 tracts scattered over that portion of the field lying VOL. 8
NO.
4 OCTOBER 1 9 6 9
527
Table 1. Analysis of 40-Acre Tract Samples of Hard-Rock Phosphate Ore
Analysis, 5;
Type
Ore Distribution, F
Grade
P20i
ca0
A1201
FeyOi
SiOn"
F
Ign. loss
Minable
79
High
26.2 19.7
36.7 26.3
3.3 3.7
2.2 1.3
21.6 42.6
2.6 2.2
6.9 4.8
24.0
32.8
3.9
1.9
28.3
2.3
6.0
25.8 17.3
35.9 23.7
4.7 4.9
2.5 2.1
20.6 44.3
2.6 1.6
6.5 6.5
20.2
27.3
4.3
2.1
37.8
1.9
5.8
Low Av. of 29 tracts
Marginal
21
High Low
Av. of 29 tracts a
Insoluble residue from treatment with fuming perchloric acid, termed silica for breuity.
between Dunnellon and Hernando were included in the study. The range in composition of the 29 tract samples is shown in Table 1. Seventy-nine per cent of the ore in the 29 tracts was minable. The P205content of the tract samples of minable ore ranged from about 20 to 26%; the average was 24%. The A1203and Fe203contents averaged 3.9 and 1.9%, respectively. The ore might be used as feed to phosphorus electric furnaces; however, because of the low P20, and high R 2 0 3contents it is not suitable for use in other fertilizer processes. The P20i content of the tract samples of marginal ore ranged from about 17 to 26% and the average was 20%. The average weight ratios of CaO:P205and F:P205 (1.37 and 0.1, respectively) are within the normal range for sedimentary apatites. However, the ratio CaO:P205 for ore from several of the individual tracts (analyses not shown in Table I ) was less than 1.3 and the A1201contents were above normal. Wavellite and crandallite were identified petrographically as minor constituents in these samples. Examination of the individual core samples from these tracts revealed that aluminum phosphates were confined principally to the finer fraction (-100 mesh) of the ore near the surface of the deposits. Evidently, this is an example of alteration of apatite to aluminum phosphates by percolating ground water, similar to that which has occurred to a much greater extent in the pebble phosphate field. Silica occurred principally as quartz grains. The clay was identified by x-ray diffraction as montmorillonite or mixed-layer montmorillonite with minor amounts of kaolinite. T o determine the size distribution of the phosphate, a portion of each tract sample of minable ore was washed and sized. Five hundred grams of ore was blunged with 1 liter of water containing a dispersing agent, sodium hydroxide, in a jar mill with two rubber-covered steel balls for 20 minutes. The solids then were fractionated by screening (1 to 323 mesh), sedimentation (43 to 3 microns), and centrifuging ( < 3 microns), and the fractions were analyzed for P z O ~and Si02. I n the determination of Si02, the sample was dissolved in mixed acids, evaporated to dryness, then taken up in perchloric acid and heated to fuming to ensure dissolution of kaolinite. The washed residue (termed silica in this report for brevity) was quartz, Si02 from clay, and traces of resistant heavy minerals. The distribution of phosphate was similar in all the samples. Typical results are shown in Table 11. The PLODcontent of the plus 20-mesh solids was uniformly high; the average was about 3 7 5 . Below 20 mesh, the P20; decreased with decreasing particle size to 6 5
a t 100 mesh, increased again to about 33% a t 17 microns, and decreased thereafter. The Si02 content of the size fractions varied inversely with the P?O,. Inspection of the size fractions under the microscope revealed the presence of two distinctly different types of phosphate. Most of the plus 20-mesh phosphate was a hard, glassy mineral that displayed conchoidal fracture, while the minus 20-mesh phosphate was soft, chalky, and often porous. When the P205contents of the size fractions of ore reported in Table I1 were plotted against their Si02 contents (Figure l ) ,several consistent and significant relationships became apparent. All of the points for the fractions between 20 and 325 mesh fell very close t o a straight line which intersected the Pro, axis a t 3i'C and the Si01 axis a t 1005. I t was concluded that these fractions were mixtures of quartz particles with SiO1-free particles of phosphate which contained 37°C P105.Most of the points for the minus 325-mesh fractions fell close to another
Table II. Analysis of Size Fractions of Hard-Rock Phosphate Ore
Size Fraction -1 +2 mesh
-2 +3 -3 +4 -4 +6 -6 +8 -8 +10 -10 +14 -14 +20 -20 +28 -28 +35 -35 +48 -48 +65 -65 +lo0 -100 +l50 -150 +200 -200 +27O -270 +325 -43 +38 micron -38 +17 -17 +3.3 -3.3 +1.6 -1.6 +1.1 -1.1 +0.8 -0.8 +0.7 -0.7
Solids Distribution, c
1.5 2.0 3.8 2.8 2.3 2.6 2.1 2.0 2.0 2.6 5.2 9.7 11.8 5.4 1.9 1.7 2.5 6.2 5.0 3.0 3.0 2.4 2.0 1.7 14.8
Analjscs, P?0, SI fla ( (
35.4 37.5 37.0 36.9 37.3 37.4 36.9 36.3 33.7 25.8 15.0 7.6 6.1 12.1 19.9 22.9 26.0 32.1 32.8 31.6 32.1 30.5 29.0 27.7 12.7
5.2 2.8 3.0 3.0 2.5 2.2 2.4 4.1 10.6 31.6 59.5 79.2 83.3 67.6 46.4 38.0 29.0 9.0 6.3 7.6 6.6 9.0 10.5 12.5 31 9
' Insoluble residue from treatment with fuming perchloric acid (si1ica)
straight line that intersected the P205axis a t 37% and the Si02 axis a t 49%. Since many clays contain about 50% SiO?, it was concluded that these fractions were mixtures of clay with phosphate that contained 3 5 5 P20,. Points that fell between the two lines were assumed to be mixtures of phosphate, quartz, and clay. The points for the plus 20-mesh fractions were too closely grouped to define a curve precisely, but they appeared to fall on a third straight line that intersected the P205axis a t about 39% and the S i 0 2axis a t about 54%. These fractions, therefore, were assumed to consist of particles of phosphate that contained occluded clay. The occluded clay contained about 54‘7 SiO2, and the clay-free phosphate contained 3 9 7 PzOj. I t is evident, therefore, that the hard and soft phosphates differ chemically as well as physically. Assuming the ore to be composed solely of phosphate, quartz, and clay, the approximate percentages of these constituents in each size fraction were calculated from the following equations. Phosphate
+ clay + q u a r t z = 100
(1)
100
P h o s p h a t e = ___ P20iin silica-free phosphate
x P?O, in fraction
(2)
100 - ( p h o s p h a t e + Si04 = X
(3)
Where SiO> is quartz plus silica derived from the clay and X is the nonsiliceous portion.of the clay, principally A 1 0 I, Clay =
100 100 - Si02 in phosphate-free clay x X in fraction
(5)
I n Equation 2, “Proi in silica-free phosphate” was determined by the intercept of the appropriate curve on the P20jaxis (Figure l ) , and “Proj in fraction” was determined by chemical analysis. I n Equation 3, Si02 was determined by chemical analysis. I n Equation 4, “Si01 in phosphate-free clay” was determined by the intercept of the appropriate curve on the Si02 axis (Figure 11, and “X in fraction” was calculated by Equation 3. The calculated mineral content of the fractions is shown in Table 111, and the distribution of minerals in the ore 40
30
I-
W z
I-
50
Size Fraction -1 +2 mesh -2 +3
-3 +4 -4 +6 -6 +8 -8 +10 -10 +14 -14 +20
-20 +28 -28 +35 -35 +48 -48 +65 -65 +IO0 -100 +150 -150 +ZOO -200 +270 -270 +325 -43 +38 micron -38 +17 -17 +3.3 -3.3 +1.6 -1.6 +1.1 -1.1+0.8
-0.8+0.7 -0.i
Approximate Mcnerai Contenf. Phosphate
Cia?
Quartz
(100P,O 389”)
(100 X 46 1
(by difference)
91 96 95 95 96 96 94 93
2 4 a 3 4 6 6
(100 P,O 37‘)
(100 x 51’)
91
0 0 0 0 0 0 0 0
70 40 21 17 33 54 62 70 87 89 85 87 82 78 75 34
8
2
2 1
0 1 0 0 1 9 30 60 79 83 67 46 38 28
8
5 1
14 13 17 22 25 66
3 0
io
1 0 0 0
Value at appropriate intercept on P,Oi a x i s , Figure 1 . Calculated b j Equation 3. ‘ Value at appropriate intercept on SiO?axis, Figure I.
(4)
Q u a r t z = 100 - ( p h o s p h a t e + clay)
s
Table 111. Calculated Mineral Content of Size Fractions of Hard-Rock Phosphate Ore
20
“ 0
is shown graphically in Figure 2. The plus 20-mesh solids consisted of about 95‘; phosphate, j C - c clay, and a trace of quartz. The minus 20- plus 325-mesh solids consisted of about two thirds quartz and one third phosphate. The minus 325-mesh solids consisted of about two thirds phosphate, one third clay, and a very small amount of quartz. These three size fractions of the ore are called rock (+20 mesh), sand (-20 +325 mesh), and slime (-325 mesh) in subsequent discussion. Table IV shows that the average distribution of P?Ojamong the rock, sand, and slime fractions of the minable ore from 29 tracts was 39, 23, and 3 8 5 , respectively. The average for the marginal ore was 29, 23, and 48‘2. The foregoing data indicate that virtually all of the hard rock was recovered from the ore by the earlier beneficiation methods (screening a t 20 mesh) and that additional recovery of P 2 0 ~would require separation of the soft, friable phosphate from the quartz in the sand fraction and separation of finely divided phosphate from more finely divided clay in the slime. Flotation of Sand Fraction (-20 $325 Mesh)
2
10
0
20
40
60
80
100
SI02 CONTENT.%
Figure 1. Relation between P2Oi a n d S i 0 2 contents o f size fractions of h a r d - r o c k p h o s p h a t e o r e
Laboratory sink-float tests with a heavy liquid (specific gravity, 2.82) showed that the sand fraction of the ore was a mixture of quartz grains and particles of highgrade phosphate. I n tests with a composite representing the 29 tracts, about 9 0 ‘ ~of the P20, was separated as a concentrate containing 3 5 5 P20,. However, when frothflotation tests were made with deslimed minus 35-mesh sand and anionic collectors, selectivity was very poor under VOL. 8 NO. 4 OCTOBER 1 9 6 9
529
PARTICLE
MICRONS N
loof
3
N
CLAY
I'
V
for successful flotation of the sand on a plant scale. Therefore, recovery of the phosphate by selective flocculation was tested.
SIZE
-
m
'
I
'
I
'
Selective Flocculation of Slime (-325 Mesh)
,
PHOSPHATE
CLAY
6 80-
w
?
t-1 2 3
60-
0
1 "0
20 40 60 80 ORE S O L I D S , CUMULATIVE WT.
Figure 2. Distribution of phate ore
mineyak
IO0
in hard-rock phos-
Table IV. Distribution of P& in Fractions of Hard-Rock Phosphate Ore
(Weighted auerage of 29 tracts)
P?O,, 5;
Solids
Size,
Distribution, 5
Fraction
mesh
Rock
-1 +20 -20 +325 -325
Content
Distribution
35 15 26 24
39 23 38 100
31 11 25 20
29 23 48 100
(MinableOre) Sand Slime Head
...
21 38 35 100
(Marginal Ore) Rock
Sand Slime Head
-1 +20 -20 +325 -325
...
19 43 38 100
all conditions. I n many tests, more quartz than phosphate was floated. Flotation of quartz with amine collectors effectively concentrated the phosphate when necessary precautions were taken to minimize the adverse effects of slime. Best results were obtained when a water-sand suspension was agitated vigorously a t high pulp density to grind the softer phosphate, the sand was deslimed thoroughly by repeated decantation, and conditioning and flotation were carried out in a way to avoid production of more slime. Under these conditions, flotation with 0.25 pound of amine per ton of dry sand yielded a concentrate that contained 35'; P 2 0 jwith a recovery of 90%. However, the presence of about 0.4% of slime in the flotation feed increased the collector consumption by as much as sixfold and decreased the grade of concentrate to less than 30% Pd&. Because of the softness of the phosphate, it would be difficult, if not impossible, to meet the requirements 530
I & E C PROCESS D E S I G N A N D DEVELOPMENT
Selective fluocculation may be accomplished by dispersing a dilute suspension of phosphate slime with sodium hydroxide, partially reflocculating the solids with starch, and separating the flocs by sedimentation and decantation. Recovery of solids in the flocs can be controlled by varying the proportion of dispersant and starch (Haseman, 1951). Selective flocculation of the hard-rock phosphate slime was investigated first by means of small-scale batch tests. In these tests, 500 ml. of a 5% suspension of slime was dispersed by stirring with a given amount of sodium hydroxide, and 100-ml. aliquots were reflocculated by shaking with different amounts of potato starch. The flocs were allowed to settle for 15 minutes and the supernatant suspension was siphoned off. The effect of dosage of reagents on the content and recovery of Pz05 in the flocs is illustrated in Table V. Reagent Dosage. When the dosage of sodium hydroxide was less than 2 pounds per ton of dry slime solids, 1 pound of starch reflocculated nearly all the solids with little consequent enrichment in PzOS. As the sodium hydroxide was increased to 3 pounds, the grade of the concentrate increased and more starch was required to recover a given amount of P205.Further increase in the sodium hydroxide decreased recovery of P205 without improving grade. A plot of the data in Table V, as shown in Figure 3, reveals a continuous and orderly relation between the grade and recovery of P205in the selectively flocculated solids. The grade of the concentrate was nearly constant until about 65% of the phosphate in the slime had been reflocculated, but decreased progressively as additional solids were flocculated. This decrease in the grade of the floc concentrate is similar to the decrease in grade of the size fractions of the slime shown in Table I1 and suggests that the selective action of the starch is based on the size of the particles rather than their mineral composition. From the curve in Figure 3, it appears that recovery of 75% of the P205 in this slime a t a grade of 30.770 offers about the best compromise between increasing recovery and decreasing grade. The reagent dosage needed for this separation was 3 pounds of sodium hydroxide and about 2.5 pounds of starch (by interpolation) per ton of slime solids.
Table V. Effect of Dosage of Sodium Hydroxide and Starch on Floc Concentrate Obtained by Selective Flocculation of Hard-Rock Phosphate Slime
Starch, Lb Ton Solids 2
1
NaOH, Lb. Ton Content 8 4 3 2 1.5 0
31.5 31.6 31.5 29.5 26.5 26.2
3
P a , cc ReRe(:ouep (~ ~ u e r y Content 43 45 56 83 97 100
31.5 31.4 31.1 29.1
... ...
51 64 71 86
...
...
ReContent
couey
31.6 31.6 30.0 28.2
54 61 82 92
... ...
...
...
32
M I L D ATTRITION
30-
43
17
1.6
_ VIGOROUS ATTRITION
PARTICLE SIZE, 0.7 43
MICRONS 17 3.3 1.6 0.7
SLIME I
28
-
26 40
50
60 70 80 P205 RECOVERY, Y.
90
100
QUARTZ
2o
Figure 3. Relation between grade and recovery of phosphate in selectively flocculated solids
0
Agitation. The intensity and duration of agitation in the dispersion step affected the P& content of the reflocculated solids. T o investigate the effect of mechanical agitation, one portion of a slime was stirred for 5 minutes a t 1500 r.p.m. with a laboratory mixer, and another portion of the same slime was stirred for 30 minutes a t 3600 r.p.m. on a soil-dispersing machine. Aliquots of each portion of slime then were dispersed with sodium hydroxide with gentle stirring for 1 minute and reflocculated with starch. The appropriate dosages of reagents were determined as illustrated in Table V. T o compare the effect of agitation on the composition of size fractions of the slime, other aliquots of each of the two portions of slime were thoroughly dispersed with 5 pounds each of sodium hydroxide and sodium tripolyphosphate, and fractionated a t different particle sizes by centrifuging. Results of the tests are shown in Figure 4. Increased intensity and duration of agitation improved the grade of the concentrates recovered by centrifuging and by selective flocculation by about 1.5 units. T h e marked similarity between the two pairs of grade-recovery curves strongly supports the conclusion that, with controlled dispersion, the effectiveness of starch depends on the size of the particle rather than its chemical composi-
0
20
,
40 60 80 100 0 20 4 0 60 SLIME S O L I D S , CUMULATIVE WT. %
80
100
Figure 5. Effect of attrition grinding on size distribution of minerals in hard-rock phosphate slime PAR TIC L E S I Z E , MICRONS
loo4i
SLIME 16
s w
501
PHOSPHATE
= I t
43
1.6
0.7
SLIME 8
43
=' 100
17
17 3.3 1.6 0.7
U L
OO
50
100
0
50
100
SLIME SOLIDS, C U M U L A T I V E WT. %
Figure 6. Mineral distribution in representative samples of hard-rock phosphate slime
34
32
1500 RW
CENTRIFUGED
30
40
50
60
70
80
90
100
PeOe RECOVERY,Y.
Figure 4. Effect of agitating phosphate slime on grade and recovery of phosphate in solids separated by centrifuging and selective flocculation (Numerals on curves indicate separate size, microns)
tion. The grades of the centrifuge concentrates were 2.5 to 3 units higher than the grades of corresponding floc concentrates. Presumably, this difference resulted from mechanical entrapment of fine clay in the flocs and the presence of more of the mother suspension in the interstices of the loosely settled flocs than in the tightly packed centrifuge sediment. Apparently, the strong shear developed in the soildispersing machine broke down clay aggregates without grinding phosphate appreciably. T o test this conclusion, the mineral composition of the fractions of slime separated by centrifuging were calculated by the method illustrated in Table 111. The results (Figure 5 ) show that more vigorous attrition caused a significant reduction in the amount of plus 0.7-micron clay in the slime. Figure 6 shows the mineral distribution in four other samples that are fairly representative of the range in composition of the hard-rock slime. I n all the slimes, most of the phosphate was coarser and most of the clay finer than about 1 micron. Quartz was a minor constituent. The grade of the potential slime concentrate is limited, therefore, by the amount of coarse clay ( > 1 micron) in the slime, and the P205 recovery by the amount of fine phosphate ( < 1 micron). VOL. 8 N O . 4 OCTOBER 1969
531
Table VI. Chemical Analysis of Water Samples Available for Use in Selective Flocculation Tests
Analysis, P . P . M .
Hardness" Water
Fla. lake Fla. river Fla. well TVA tap Tuscumbia spring Tuscumbia spring' a
Total 37 99 233 118
Bicarbonate 36 66 228 81
Noncarbonate
Ca
Na
C1
SO,
1 33 5 37
12 33 70 41
Mg 2 4 14 5
Specific Conductance, Micmmhos
5 4 2 23
7 6 4 23
2 26 2 22
100 220 450 330
132
123
9
49
2
4
6
4
262
31
...
...
...
...
...
...
...
84
Expressed as CaCO,. 'Softened by addition of Ca(OHir equivalent to initial hardness.
Electrolyte. The electrolyte content of the process water also proved of primary importance in selective flocculation. If the electrolyte content of the slime suspension was too high, the suspension could not be dispersed sufficiently for the flocs to form or else they failed to settle. The laboratory tap water used in the initial tests was objectionable because of its high electrolyte content; also, the electrolyte content varied with time of year. Distilled water was satisfactory, but its use in evaluating the process was considered unrealistic. T o determine the quality of the water that would be available to a beneficiation plant located in the southern part of the hard-rock field, samples of water were collected from nearby sources: the Withlacoochee River, Lake Tsala Apopka, and a well on one of the tracts that was prospected. Analyses of these samples with those of a sample of laboratory tap water and a sample taken from a spring located near the laboratory are shown in Table VI. With the exception of the lake water, the electrolyte content of the Florida water was high. However, the major part of the electrolyte in all three Florida samples was calcium bicarbonate, which can be precipitated economically with lime. The local spring water was similar to the Florida water and of intermediate hardness. When softened with lime, the spring water had a residual hardness of 31 p.p.m. and a specific conductance of 84 micromhos. The softened spring water was used in subsequent flocculation tests. T o determine the effectiveness of beneficiating the hardrock phosphate slime by selective flocculation, batch tests were made on slime from each of the 29 tract-samples of ore. Optimum beneficiation was determined by the procedure illustrated in Table V and Figure 3. The results of the tests are summarized in Table VII. The slime samples fell into two groups. Twenty-one of the samples could be dispersed with 1 to 4 pounds (average 2.3) of sodium hydroxide per ton of slime solids, whereas the other eight samples required 5 to 14 pounds (average 8.1). The average specific conductance of the difficultly dispersible suspensions was nearly double that of the readily dispersible group (299 us. 153 micromhos). About three quarters of the phosphate in the slimes was recovered in the floc concentrate, but the P 2 0 5content of the concentrate from the readily dispersible group was about two units higher than that from the difficultly dispersible group (31.3 us. 29.2%). Continuous Selective Flocculation. Since selective flocculation offered promise in beneficiating the hard-rock 532
I & E C PROCESS D E S I G N A N D DEVELOPMENT
Table VII. Selective Flocculation of Slime Suspensions Prepared by Washing Hard-Rock Phosphate Ore in Softened Spring Water"
Specific Conductance o j P20,in Suspension, Lb Ton Solids Slime, i Mccmmhos NaOH Starch
P20, cn Flocs, c; Content Recowry
Normally Dispersible (21 Samples) 24-30 Av. 27.0
85-238 153
1-4 2.3
0.5-3.0 1.5
30-34 31.3
67-88 76
DifficultlyDispersible (8 Samples) 22.5-28.5 Av. 25.1
180-415 299
5-14 8.1
0.5-2.0 1.1
27-32 29.2
72-86 78
' p H , 9; specific conductance, 84 micromhos
phosphate slime, the process was tested in a continuous system. In these tests, the ore was washed by tumbling it in a ball mill with water containing sodium hydroxide. The plus 20-mesh rock was screened from the ball mill discharge and the minus 20-mesh slurry was passed through two vertical centrifugal sand pumps connected in series. The pumps served a double purpose: to grind soft phosphate to a finer size than the quartz, and to grind the clay to a finer size than the phosphate. The overflow from the second pump was passed through a hydraulic cyclone to separate the sand from the slime. The dispersed slime from the cyclone was reflocculated by the addition of starch with slow stirring in a small mixer. The flocs were separated from the supernatant suspension in a bowl classifier. The underflow from the classifier was a rough concentrate-the flocs contained mechanically entrapped clay and the interstitial liquor was mother suspension. T o eliminate clay further, the rough concentrate was repulped with water, stirred vigorously to destroy the flocs, and treated with a small amount (0.2 to 0.5 pound per ton) of starch, and the resulting flocs were recovered in a second bowl classifier. Best results were obtained when the dosage of sodium hydroxide to the ball mill was low enough to permit high recovery of phosphate ( >80%) in the rough concentrate. However, this practice often resulted in the formation of flocs that settled poorly. This condition was corrected by adding supplemental sodium hydroxide to the flocculated feed to the first classifier. Addition of an appropriate amount of sodium hydroxide a t this point
flocculation was particularly effective in improving the grade of the concentrates from the dificultly dispersible slimes. Elimination in the first stage of part of the watersoluble electrolyte extracted from the ore improved dispersion in the second stage. The average grade of the concentrates from the continuous tests was 32.4% P?O,, but the AlGI was 5.15 and Fe2O.{was 2.3%. The high iron and aluminum contents probably were derived principally from clay, but the low CaO:P20j ratio (
