Rapid Sedimentation Analysis with the Kelly Tube V. V. DESHPANDE
AND
R I . S. TELANG
Laxminarayan Znstitute of Technology, Nagpur University, Nagpur, India
H E Kelly tube ( 4 ) for sedimentation analysis has several Tadvantages. After the experiment is begun, the worker's attention is needed only to take readings from the scale at various intervals; these intervals become long enough in the later part of the experiment to free the worker for other work. This method can be used when a small quantity of sample is available for analysis and when emulsions are to be analyzed. The apparatus is inexpensive and suitable for occasional needs, as it can be assembled readily and does not require specialized knowledge or technique. The Kelly method is not in wide use, however, because of errors resulting from initial sedimentation, surface tension effects in the capillary index arm, inaccuracy in determining the angle of inclination of the index arm, nonuniformity of the capillary bore, continuous recession of the liquid from the side arm which causes progressive altering of the meniscus in the sedimentation tube, evaporation losses, and meniscus lag. Jones and Barlow ( 3 )have rectified the inaccuracies owing to initital sedimentation, determination of the angle of inclination of the index arm, and nonuniformity of the capillary bore. Surface tension effects in the capillary index arm and meniscus lag have been lessened by using nonaqueous index liquids ( 5 , 6, 9). Evaporation losses have been overcome by attaching bulbs containing the medium ( 2 ) and recession of the liquid from the side arm, and inaccuracies in determining the angle of inclination of the index arm have been obviated recently by the present authors (8). It was desirable to examine the reliability of the Kelly method after these various improvements were made. Another objective of this paper was to shorten the length of the experiment, using a technique similar to that adopted by the present authors (1) with the Andreasen pipet method.
OdBn. As the number of manometric indicators was reduced to two, the values of hydrostatic pressure, 7, a t two different depths, x, with variables of t had to be determined. Although the duration of the experiment was longer than OdBn's, it was considerably shorter than Kelly's, whose original apparatus had only one index side arm to obtain nearly the same size distribution analysis.
b
THEORETICAL
Figure 1. Kelly Tube with Manometric Indica tors
Theoretically the size distribution in the 'whole range can be adetermined by noting the variation of the hydrostatic pressure with the distance from the surface a t a definite time, using O d h ' s equation ( 7 ) : 2 x F(r) = uo - .Y1 x;
The next objective was to study critically the modifications of the Kelly tube suggested by Jones and Barlow (5). In citing the advantages of their modification, they did not compare their results with the original Kelly method. The disadvantages of the Jones and Barlow apparatus are that it is complicated and has to be specially fabricated; it requires an air thermostat with its rapid drafts of air causing considerable evaporation losses of the liquid in the apparatus; it cannot be adapted for rapid analysis; and it requires a close attention throughout the progress of the experiment. However, as the Jones and Barlow modification has merits, the authors of this paper simplified the construction of their apparatus and made a comparison of the experiments run with both types of apparatus.
d'r
xd22
making z variable and t constant. F ( r ) = the distribution function, T = the radius of particle, I = the distance from the surface, t = time, T = the hydrostatic pressure a t a depth x and a time t , u = the specific gravity of the suspension, and u1 = the specific gravity of the medium. The experimental arrangement of Od6n sought to make use of Equation 1 by determining d%/dx2 a t distinct times. Attempting to shorten the length of the experiment, Od6n used a sedimentation tube 1.8 meters high and 5 cm. wide connected with a number of capillary tubes as manometric indicators, but because of the experimental difficulties and errors caused by variations in temperature in different parts of the necessarily large instrument, this method was abandoned. For thii investigation, Kelly's dimensions of the apparatus with only two detachable index tubes located a t two different heights in the same sedimentation tube were used. This modification allowed the use of a properly controlled water thermostat that eliminated the variations in temperature encountered by
EXPERIMENTAL
A sample of China clay passing through Tyler screen No. 200 was prepared in bulk for use in all these investigations. The medium of suspension was water. For the rapid analysis, a Kelly tube with two manometric indicators as shown in Figure 1 was used. For this purpose, instead of joining two capillaries to the main settling tube by fusion, two separate capillaries were immersed in the suspension a t two 885
ANALYTICAL CHEMISTRY
886 different heights, as shown in Figure 1. This apparatus could be constructed readily, as it required only elementary lasa blowing. Another advantage of the detachable capillary is &at it can be adjusted to any desirable position. The usual test tube for sedimentation was re laced by a jar, so as to eliminate the use of external supports. T i e capillary side arms needed independent but rigid clamping supports. Thus an all-glass apparatus was obtained without any complicated glass blowing from articles usually available in an ordinary laboratory. The difficulty cited by Jones and Barlow (S), that Kelly’s apparatus should be all glass, is thus overcome by using a detachable capillary side arm. Evidently the index arm had to be bent as shown in Figure 1 in order to keep the index arm immersed in the thermostatic bath. A portion, A , of the bent. arm- necessarily remained exposed above the water level. This was covered by winding a cloth tape around it to prevent heat losses. With the constantlevel device the meniscus can be started quickly a t a definite point. The constantlevel tube should beimmersed u p to a point lower than the tip of the capillary side arm; otherwise an error will be caused by the pressure difference arising fiom the actual column of the suspension disFigure 2. Top of Sedimentation Tube placed by the immersed portion of the constant-level tube. A similar effect is felt on the longer manometric tube because of the displacement of a column of suspension by the shorter manometer; calculations show this effect to be negligible, as the volume displaced by the capillary is small.
was withdrawn quickly out of the sus ension before the start of taking readings. I n all experiments fumner’s procedure of correctin for meniscus lag was used. The medium was siphoned from &e sedimentation tube and the value of the lag subtracted from all time readings. Evaporation losses were minimized by filling bulbs with the medium and submerging them in the thermostatic bath (8). The mouth of the sedimentation tube was covered with a metallic lid made in two cut parts, so that it could be fitted after the experiment was started. The gaps in the lid were closed with wax. It was necessary to anticipate where the meniscus in the index arm would reach after pouring the suspension, so that the index arm could be kept filled with the medium up to that point. Otherwise, a part of the suspension would enter the vertical portion of the side arm. It is preferable to siphon out the medium from the sedimentation tube in the early stages of the experiment, as pouring the medium out would disturb the vertical position of the apparatus and thus affect the position of the meniscus in the index arms. 401
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DIAMETER, MICRONS 101
Figure 4.
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Readings of Kelly and Jones and Barlow Manometers 0 Barlow modification experiment A Double manometer experiment DISCUSSION
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Figure 3.
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Sedimentation Results for Two Manometers A
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To employ the technique of Jones and Barlow, Kelly’s apparatus of glass was used with the folloning alterations. Figure 2 shows the top of the sedimentation tube.
As it was difficult to secure a glass rod with a uniform diameter in this laboratory, a brass rod, R, o i t h a uniform diameter was fabricated on a precision grinding machine in the institute workshop. Instead of using a micrometer for the movement of R, its movement was operated manually using a guide sleeve of a close-fitting glass tube, G , to ensure a true vertical movement of the rod which was checked by two plumb lines, and S,, adopting the technique used previously by the authors (8). For this purpose a pin, P , was fixed with wax in a small hole drilled into the center of the top face of R for observation through a vertical traveling microscope (or cathetometer), X. The microscope recorded the vertical movement of E . A clamp supported R in its various positions. Initial stirring of the suspension was done by means of a stirrer operated by hand. The stirrer
The results for the two manometers plotted in Figure 3 are mutually consistent; the points lie fairly even on either side of a common curve drawn through them. The double manometer system therefore appears feasible for rapid sedimentation analysis. The calculations were made using Kelly’s formula ( 4 )without any alterations, since a constant-level device was used. The calculations were based on the consideration that the two manometers function independently of each other and the whole experiment is as good as two separate experiments performed simultaneously with a common sedimentation tube. The volumes of the suspensions for the two manometers were taken into account only up to their respective depths. The per cent undersize for each of the two manometers was calculated separately and then correlated graphically in Figure 3. The longer manometer is useful in the region of coarse particles, \\ hile the shorter manometer reduces the duration of the experiment. To determine the per cent undersize for a particle size of 2.68 microns, 99 minutes were required with the shorter manometer, whereas to determine a similar value for a particle of 2.66 microns with the longer manometer, 204 minutes were required. For obtaining the values of the per cent undersize corresponding to a size of about 2.68 microns, the whole experiment could be finished in 99 minutes by using this double manometer system. As the longer manometer readings would be as good as a fully independent Kelly experiment (K. readings), its readings have been compared graphically in Figure 4 with the readings obtained n i t h the Jones and Barlow modification (J. & B. readings). The two curves almost coincide in the region of the fine particle size, But in the region of the coarse particle size, K. readings are somewhat lower than J. & B. readings, which can be theoretically
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V O L U M E 2 4 , N O . 5, M A Y 1 9 5 2 esplained. The drawback of the I- are added for the purpose of endowing certain minerals, particularly sulfides such as galena or pyrite, with the property of selectmively adhering to gas bubbles. The gas-adhering minerals are rafted to the top of the flotation suspension and removed as a froth. The quantities of santhates used are minute, on the order of a fen- parts per million parts of water. Xs a result the yuantitative distribution of the xanthate radical, as among minerals, solution, and gases, is known only in a very general way. Accordingly, it was thought that sulfur-labeled xanthates might prove useful in providing sensitive and precise analyses. A method \vas worked out for the analysis of aqueous santhat,e solutions, involving quantitative oxidation of xanthate sulfur to sulfate ion and (IT-iththe addition of inactive sulfate) quantitative precipitation of this labeled sulfate as barium sulfate of infinite radiochemical thickness. In this respect the procedure differs from that of Henriques et al. ( 7 ) . The new method gives accurate solution analyses. It mas established that the adsorbate could be quantitatively leached from the mineral and the leach liquor analyzed by oxidation and precipitation, as above. Various aspects of mineral-xanthate systems cnn.be studied to advantage with radioactive xanthate. These include: (1) a determination of the partition of xanthate between solution and mineral; and ascertainment of (2) the reversibilitj-, or otherwise, of xanthate-mineral action; (3) the factors that determine rate a t which xanthat'e is absorbed by the mineral; and (4) the effect of other ions in solution on xanthate adsorption. The experimental adsorption studies could be conducted by abstraction tests or by passing solution through a column of the mineral particles ( 3 ) . In all cases, quantitative measurements are required of the radioactive xanthate concentration in solution and of the two-dimensional concentration a t the mineral surface. The xanthate collect'or used in this investigation ( 1 ) was potassium ethyl xanthate. The compound was synthesized in the L..
1 Present address, Atomic Energy Research Establishment, Harwell. Berkshire, England.
usual manner from carbon disulfide containing the radioisotope, S", in the radiochemical laboratory of Tracerlab, Inc., Boston, Mass. The nonradioactive compound was supplied by the Great' Kestern Division, Dow Chemical Co., Pittsburg, Calif. PROPERTIES AND DETECTIOS O F SULFUR33
Sulfur3jis a IOK energy beta emitter (maximum energy 0.169 m.e.v.) with a half-life of 87.1 day,?. The tieta-spectrum has been st'udied (6). The extrapolated range obtaind from n Feather plot ( 5 )is 31.7 + 0.5 mg. per sq. rm. JTith the stainless-steel precipitation apparatus (Tracerlab, Inc.) used for collecting barium sulfate precipitates ''infinite thickness" for the precipitates is 80.5 nig. The iwight of barium sulfate precipitate varied soniexhat from test to test, even when the tests m r e duplicates. Xccordingly, and in view of the low range of the heta-rays, the effect of sample thickness on activity was studied. Figure I shows that the obberved activity from a precipitate made 11-ith a standard amount of S35 is inversely proportional t o the w i g h t of precipitatc. This inverse proportionality should hold for infinite thickness. I t holds even for precipitates weighing as little as 70 mg. because of the small contribution from the lon-eut layers of the precipitate sample. Table I s h o w how the observed activity varies depending on the detector used (gas-flow counter or end-window type Geiger-XIuller
Table I.
Relatire Counting Efficiency of Yarious Devices with Same Sample of Barium Sulfate Background,
C./X
Observed, c., bl.
Counting Effectiveness*
G.11. end-window counter 1.49 rng./sq. cm. 26.6 247 8 8.31 3.1 mg./sq. ern., first shelf (nearest tube) 22.9 68 6 1.993 3.1 mg./sq. em., second shelf 22.9 37 0 0 615 781 7 Gas-flow counter 21.1 46 0 Counting effectiveness m a y he defined as the ratio of the net count due t o the sample (observed minus background) t o the background count. It gives a relative measure of the value of various devices a n d setups.
