ORE SEPARATION IN A PACKED= FLUIDIZED BED C. E. C A P E S A N D J . P. S U T H E R L A N D
National Research Council, Ottawa 2, Canada
The segregating tendency of particles of differing densities in a gas-fluidized packed bed has been applied to ore separation. Two columns (5- and 3-cm. i.d.) filled with cylindrical screen packing were used to study the upgrading of a germanium ore and two iron ores. Segregation of the bed components was most efficient with air velocities just above that required for incibient fluidization. Bed height, location of the feed point, and ore feed rate also affected the degree of separation obtained. In the case of the germanium ore fluidized in a 5-cm. diameter bed 108 cm. high with the ore fed two thirds of the way up the bed, the maximum product grades and recovery attainable were achieved with feed rdtes up to 3 0 0 grams per minute.
N FLUID beds
which contain particles of different sizes and/or
I densities, there is a n inherent tendency for the particles to stratify according to their density and size u p the column. The forces which promote segregation have been discussed by Sutherland and Wong (9) and Pruden and Epstein ( 8 ) . Stratification is most readily observed in liquid fluidization, where it has been known for a long time (6, 7 7 ) . During gas fluidization, however, the particle-mixing forces, which are much larger than in the case of liquid fluidization, overcome the particle-segregation forces and an almost uniform solid phase results under normal operating conditions. Hence only under the extreme conditions of incipient fluidization when mixing is a t its lowest value and a t gas velocities approaching the terminal velocities of the particles in the bed are segregation effects noticeable (7, 9). Baffles or packing internals may be added to a gas-fluidized bed to promote segregation. T h e effect of packing (9) is to hinder solids movement and to limit the size of the gas voids moving u p the bed, with an attendant decrease in solids mixing. Slugging is also reduced, so that beds with high aspect ratios may be used, thus reducing solids communication between the top and the bottom of the bed. Hall and Crumley ( 5 ) in studies of the Fischer-Tropsch process found that fluidized particles in narrow baffled beds become distributed a t various levels in the bed such that the height of a particle above the gas inlet is proportional to the product of the particle density and the square of its effective diameter. Botterill and Kunte (7) used baffled gas-fluidized beds to separate continuously particles of the same density according to their sizes. Recently Sutherland and Wong (9) reported on the size segregation of silica sand fluidized with air in tall narrow columns packed with open-ended cylindrical screen packing developed a t the National Research Council. The purpose of the work presented here was to apply the segregation phenomenon to the separation of ores. Because particles will segregate according to size as well as density, the ore particles were closely sized so that they would become stratified u p the column chiefly on the basis of their densities. Fluidized beds of high aspect ratio containing cylindrical screen packing were operated a t air velocities slightly above the minimum fluidization level. Thus separation was accomplished by stratification of the bed particles rather than by blowover of particles from the bed. During most of the work reported separation was carried out on a continuous basis. 330
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Batch equilibrium and rate studies were also made to provide background information for the interpretation of the continuous data. Experimental Details Apparatus. Two fluidization columns were used : a 5-cm. i d . acrylic column 157 cm. in length and a 3-cm. i.d. glass column 100 cm. in length. T h e columns were packed with cylindrical screen packing consisting of hollow open-ended cylinders, ‘/z inch by 1/2 inch, formed from 4-mesh regalvanized steel screen (wire thickness 0.024 inch). T h e packing was dumped in small increments into the column and assumed a random arrangement. The column was fluidized by air which was introduced through a porous steel distributing plate. Figure 1 shows the arrangement of the experimental assembly for a continuous run. The ore to be separated was fed a t a constant rate to an intermediate point in the bed. A portion of the ore was withdrawn at a controlled rate from the bottom of the bed, while the balance overflowed a t the top of the bed. Table I lists the various locations of the feed, top product, and bottom product tubes used. Vibratory feeders regulated by Variacs were used to control the rate a t which ore was supplied to the feed tube and the rate a t which solids discharged from the bottom product tube (3). The feed and take-off tubes on the glass column were 8-mm. i.d. glass tubes, while the take-off tubes on the acrylic column were 13-mm. i.d. acrylic tubes. T h e feed tube on the acrylic column was either a 13-mm. i.d. acrylic tube (for feed rates up to about 300 grams per minute of germanium ore) or a 35-mm. i.d. glass tube (for feed rates in the range of 500 to 1000 grams per minute of germanium ore). I t was necessary to inject air into the feed tube to prevent the ores from bridging across the tube. Ores Studied. The majority of the work reported was done on a germanium ore. Preliminary investigations were also made of the upgrading of two iron ores. The germanium ore consisted chiefly of a coarse sandstone, dispersed with fine “eyelets” of carbonaceous material. The germanium was
Columna No.
Table I . Columns Used in Study Height abooe Air Distributor, Cm. Bottom TOP Feed product product take-off point take-off
108 75 9 59 42 9 3 34 26 9 4 108 42 9 5 75 50 2 a Columns 1 to 4. 157 cm. long, acrylic plastic. cm. long, glass. 1 2
Column Diameter, Cm .
5 5
5 5 3 Column 5 .
700
2 50 c7 0
X
200
Y
I 2
5
150
3 +
z,U
100
er n Y
50
0 0
10
20
30
40
50
60
PERCENT WEIGHT LOSS AT 90O0-10OO0 C. (PERCENT IGNITABLES)
Figure 2. Correlation between germanium content and weight loss on ignition for ore studied Hence the crushing operation was relatively inefficient in liberating the carbonaceous material from the siliceous gangue.
Figure 1 .
Experimental assembly
A. Pressure-regulated air supply E. Feed hopper and vibratory feeder 6. Rotameters F. Feed inlet C. Air humidifier D. Manometer
G. Top product overflow
H.
Bottom product take-off
associated with the carbonaceous material, and was present in a concentration of 0.006% [see (70) for the spectrographic method of analysis used for germanium content]. T h e ore was crushed in a Wiley inill to liberate the germanium-containing coal from the siliceous gangue, and a -20 +35 Tyler mesh sample of the crushed material was prepared by screening for use in the experiments. As shown by Figure 2, a correlation was found between the germanium content of the ore and its percentage weight loss when ignited a t 900" to 1000° C. (or per cent ignitables). Subsequent analyses were done by this ignition method, and results are reported as per cent ignitables, with the understanding that this analysis may be related to germanium content.
A density analysis (using mixtures of methylene iodide and carbon tetrachloride and a sink-float technique) was performed on the -20 +35-mesh sample used in the experiments (Table 11). The maximum ignitable concentration in any particles in this ore is 94%. Only 42% of the ignitable material in the ore is present in the lightest particles in the ore, while the remainder is contained in the denser particles.
Table II.
Density Analysis of Germanium Ore (-20 4-35 Mesh) Fraction Density, of Total G .lCc. Weight % Ignitables, % Ignitables 1.2-1.6 6.5 94 0.42 1.6-2.0 0.2 55 0.01 2.0-2.4 12.9 21 0.18 2.4-2.5 19.0 11 0.14 2.5-2.7 61.3 6 0.25
T h e iron ores studied were a hematite ore containing 40,8y0 Fe and 30.5% insolubles, and a high grade magnetite ore containing 67% Fe and 4.7% insolubles, the major insoluble in each case being silica. T h e method of analysis used for Fe and insolubles content is detailed (70). T h e hematite ore had been screened to -20 +35 Tyler mesh and the magnetite ore to -35 +I00 Tyler mesh. Procedure. CONTIWOERUNS. The column was filled with sufficient ore to give the desired bed height at the required fluidizing gas velocity. The bed was kept fluidizrd for 10 minutes before the feed, which had been preset to the desired rate, was started. Simultaneously with the start of the feed, bottom withdrawal of solids began a t the desired rate. Since a limited amount of ore was available, it was necessary to recycle the products from the column, after being intimately mixed, as feed. Initially, * / 3 of the available ore was fed to the column and the products were discarded to allow the column inventory to reach its steady-state composition for the operating conditions used. The remaining third of the ore was then processed and the products were mixed and recycled as feed for the remainder of the run. During the latter part of the run several samples were taken from the top and bottom discharges and were analyzed. Normally a run lasted long enough to ensure about ten complete changes of the bed. (The runs a t high feed rates with the germanium ore and the runs with the iron ores were shorter. However, the mass balances over the column were comparable to those found during the longer runs.) The feed was the remixed products during the last seven to eight of these changes. T h e composition of the feed a t the start of the run agreed to within about 6% with its composition a t the end of the run. In addition, the total quantity of ignitable material in the germanium ore (or iron in the iron ore) entering the column per minute agreed to within about 57, with the total quantity of ignitable material leaving the column per minute in the two discharges. I t may be concluded that the procedure outlined ensured that the bed was a t steady state while samples were being taken.
BATCHRIJNS.Studies were made of the equilibrium distribution of the ore components in the column. Sufficient ore was added to the column so that the desired bed height was achieved when the fluidizing gas velocity was set a t the required level. T h e bed was fluidized for 1 hour and samples VOL. 5
NO. 3
JULY 1966
331
(approximately 20 grams total mass) were taken through tubes in the side of the column a t the top and bottom of the bed and a t a n intermediate point. T h e samples taken were assumed representative of the ore a t that level. T h e bed was recharged for each equilibrium run under different combinations of bed height and air velocity. T h e rate a t which segregation was occurring was also investigated. I n this case the required amount of ore was added in small well mixed increments to ensure initial uniformity along the length of the bed. Samples were taken from the top and bottom of the bed by means of a sample thief a t intervals after the air had been turned on. The time required for sampling each time was about 5 seconds and less than 2% of the bed had been removed when the run ended.
HEIGHT ABOVE AIR DISTRIBUTOR
75 cm.
o'O-O-O~O")-O-O
a
IO
I
0
3
2
4
AIR VELOCITY Results and Discussion
T h e amount of separation achieved with the germanium ore in batch fluidization experiments is shown in Figure 3, in which the concentration of ignitable material a t the bottom, two thirds of the way up, and a t the top of a bed of the ore 108 cm. high is shown as a function of air flow rate. A plot of the pressure drop across the bed as a function of air velocity, shown in Figure 4, indicates a minimum fluidizing velocity of about 1.4 feet per second for the germanium ore. Referring again to Figure 3, it is seen that the degree of separation is high a t gas velocities slightly greater than that required for incipient fluidization, decreases to a minimum, and then increases again as the air velocity is raised. This variation, which has also been observed in the segregation of beds of particles of mixed size but constant density, was reasoned to arise from the fact that the net segregation observed represents a n equilibrium between mixing and segregating processes. Since the rates of these processes vary differently with gas flow, a minimum in the degree of separation is observed (9). Visual observations indicated that a t the higher gas velocities reported in Figure 3, some of the ore particles in the bed were on the verge of being carried out of the bed. This is in keeping with the fact that the calculated value of the terminal velocity [by Leva's (6) method] of the smallest lightest particle in the system (0.028-cm. diameter, 1.6 grams per cc. density) is 4 to 5 feet per second. The maximum degree of separation achieved in the batch experiments reported in Figure 3 gave a concentration of about 50% ignitables a t the top of the bed and 6 to 7% a t the bottom of the bed. The lightest fraction of the germanium ore contained 94y0 ignitable material, while the densest fraction contained 6y0 ignitable material. Similarly, analyses of the various sizes of particles in the ore (20 to 35 mesh) indicated a continuous increase in ignitable content with decreasing particle size, ranging from 12% ignitables in the largest particles to 19% ignitables in the smallest particles. Hence it is evident that density segregation of the ore particles is much more important than size segregation in accounting for the gradient of ignitable concentration observed in these experiments along the length of the fluidized bed. The data given in Figure 3 indicate that density segregation is more complete a t the bottom of the bed than a t the top. This is due, in part, to the fact that there is a spread of particle sizes as well as in densities in the ore. Consequently there will be overlapping, with some of the denser, smaller particles becoming stratified to the same position in the bed as less dense, larger particles. Since the lighter component is present in much lower concentration than the heavier component, this overlapping will cause a greater reduction in the grade of the top product than in that of the bottom. In addition, some of the difference in segregation efficiency along the length of the column is probably due to the fact that segregation is a function of the level of fluidization. T h e heavier components 332
l&EC
PROCESS DESIGN A N D DEVELOPMENT
5
6
ft./sec.
Figure 3. Effect of air velocity on batch segregation of germanium ore 200 I50 m
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