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Vol. 18. No. 3
A New Method of Conducting Filtration Tests' By D. R. Sperry 119 MCKEESr., BATAVIA, ILL.
HE usual way of conducting a laboratory filtration test is to feed the sample mixture into a small filter press and note the total filtrate, the total time required, and the amount of cake obtained. This is unquestionably the simplest method, but it does not provide data for the construction of time-discharge and time-pressure curves, which are needed to properly calculate the economical pressure, cake-thickness, filter area, and time per cycle for economical operation. The ordinary way of securing these curves is actually to measure the filtrate as it issues from the filter, by noting the time required to fill (consecutively) receptacles of known capacity and recording the corresponding time and pressure readings; or the amount of filtrate obtained in consecutive known time intervals may be measured and recorded together with the pressure obtaining a t each reading. An actual log of a filtration test of an Epsom salt mixture conducted according to the former of the last two methods follows:
T
Time
-FILTRATELiter Quart
-PRESSURE-Kg./sq. cm. Lbs./sq. in. 0 0
5:301/r 5:35 5:39'/t 5:46'/: 6:55 6:04 6: 15 6 :281/z 6:451/z 7:OO 7 : 17ya 7:45 8: 12
0 0.946 0.946 0.946 0.946 0.946 0.946 0.946 0.946 0.946 0.946 0.946 0.473
9:20
Very slow 4.22 60 Going at rate of 0.946 liter (1 quart) per 20 minutes
Washing 8:20
0 1 1
1 1 1 1
1 1
1 1
1 0.5
2.11 2.82 3.17 3.87 3.87 3.87 3.87 3.87 3.87 4.22 4.22 4.22
(4300) 45 55 55 55 55 55 55 60 60 60
To obtain these data it can be seen that the undivided time of an operator is required for a number of hours (nearly 4 in this case), and that there is abundant opportunity for human error. I n addition time is required to construct the time-discharge and time-pressure curves. The apparatus to be described in this article was devised, not only to reduce the time that the operator must devote to the test, but as well to reduce human error and to obtain the data already worked up and plotted for immediate use and filing. Apparatus
flow directly into the tank. As the liquid level rises owing to the continuous and regular addition of filtrate, the pen also rises, while the cylinder revolves, drawing a time-discharge curve automatically. All that the operator has to do is to see that the mixture is fed into the filter press a t such a rate that the pressure is only gradually built up. He may then use his time otherwise, only occasionally looking over the apparatus to see that the test is progressing properly. On these periodic inspections it is well to note on the curve, just under the pen point, the pressure obtaining a t that instant. I n this way the time-pressure readings are made with very little effort and a record is made automatically of the times of inspection. It is easily possible to operate another pen, which would rise or fall with the pressure, thus drawing a pressure-discharge curve along with the time-discharge curve. Such a device is in the making but it remains to be seen whether the additional complication is warranted. A photograph of this apparatus is shown in Figure 2. The picture was taken during a test of the filtration of thin clay slip. The filtrate enters the tank by means of the short piece of rubber hose, which leads from the gutter outlet on the filter press. The time-discharge curve and the inked base line can be seen encircling the cylinder. Since the MIXTURE
'AiLl
Figure 1 shows a cross section of this apparatus. -4 is an 18.9-liter (5-gallon) container open a t the top. B is a float consisting of a small can sealed air-tight. C is a TANK perpendicular wire soldered to the float. D is a guide block DRA IN which moves freely between guides E. On the guide block Figure I-Diagram of Filtration Apparatus is held a pen, F, the point of which touches a paper held on the surface of wooden cylinder, G, which is slowly rotated cylinder has made at least three revolutions (as indicated by (one revolution per hour) by clock H. the number of times the discharge curve has wound around the cylinder) the apparatus had been running over three Method hours when photographed. A sheet of paper is wound around the cylinder and held Results in place by thumb tacks. Sufficient liquid is placed in the t a n k to cause the float to start to rise. The pen point is This apparatus has been used by the writer for more than then inked and the cylinder is revolved one revolution by a year and has been used in forty-seven tests of various mixthe fingers to draw a base line representing zero quantity tures. The results have been very satisfactory in all cases. of filtrate. The clock is now started, the mixture to be Each filtration curve is numbered and filed away together tested is forced into the filter press, and the filtrate allowed to with data regarding physical properties or other desired information. Long rows of figures are thus eliminated and 1 Received October 14, 1925.
INDUSTRIAL A X D ESGINEERING CHEMISTRY
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simple curves substituted, which have been made mechanically without human error. Figure 3 shows the filtration curve of 2s’ BQ. cherry flavored sirup. It will be noted that the pressure did not exceed 1.97 kg. per sq. cm. (28 lbs. per sq. in.), that the duration of the test was less than 1 hour, and that the maximum amount of filtrate produced was about 3.325 kg. (3.5 quarts). The filter area used was 464.5 sq. cm. (0.5 sq. ft.). Nofe-In all of the time-discharge curves in this paper, the pressure is given in kilograms per square centimeter followed, in parenthesis, by the pressure expressed in pounds per square inch.
FiQure %Filtration Apparatus
The curve in Figure 4, for casein, shows the run to be in excess of 2.5 hours, and since the last part of the curve is nearly parallel with the base line (indicating zero filtrate flow) the filter press chambers were evidently filled. The pressure reached a maximum of 4.57 kg. per sq. cm. (65 lbs. per sq. in.) and the total filtrate was a little over 4.21 liters (1 gallon). The filter area was 9.29 sq. cm. (1 sq. ft.). It should be noted that the start of the curve is a t A . This is continued to A ’ , a t which point the pen left the edge of the paper which lay a t point B on the cylinder. The curve, therefore, continued from B to B’ and from C’ to the end.
Figure 3-Filtration
Curve for Cherry Sirup
Figure 5, the filtration curve for fire clay slip containing about 9 per cent solids, illustrates how the case is taken care of where the quantity of filtrate delivered to the recorder tank is in excess of the tank capacity. The initial part of the curve is from A to A’. From A’ it passes over the
e
I
c
D h W R 5
Figure 4-Filtration
c
’
A
Curve for Casein
discharged. The flat part of the curve represents the time used to open the pressure vessel and put in the main batch of varnish and close again. After that the pressure reached a maximum of 3.52 kg. per sq. cm. (50 lbs. per sq. in.) and the total filtrate was about 7.32 liters (73/4 quarts). The filter area was 929 sq. cm. (I sq. ft.). Figure 7 is a good example of a rapid filtering materialnickel sulfate and chloride plating liquor. Although only 464.5 sq. cm. ( l / 2 sq. ft.) of filter area were used, over 26.5 liters (7 gallons) of filtrate were obtained in less than 1 hour. The maximum pressure was 2.11 kg. per sq. cm. (30 Ibs. per sq. in.). Since the end part of the curve is far from parallel to the base line, the chambers in the filter were not filled a t the end of the run. Figure 8 is a record of the filtering of a sodium tungstate mixture and the subsequent washing of the cake using 929 sq. cm. (1 sq. ft.) of filter area. The filtration commenced at A and finished at B. The total discharge was about 19.85 liters (21 quarts). The total time was slightly less than 0.4 hour. The maximum pressure reached was 0.775 kg. per sq. cm. (11 lbs. per sq. in.), The chamber was not
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Figure 6-Filtration Figure %Filtration
$HOURS
Curve for a Varnish
Curve for Fire Clay Slip
t'l N I C K E L CkLORlDE
AND S U L P H I T E PLATING LiQUOR.
Figure 7-Filtration
Curve for Nickel Sulfate and Chloride Plating Liquor
Figure 8-Filtration Curve of Sodium Tungstate Mlxture a n d Subsequent Washing of Cake
filled, as the upward slope of the curve at B indicates. Filtration was stopped at that point in order to permit simple washing. Washing commenced at C and was continued at a constant pressure of 1.41 kilos per sq. cm. (20 lbs. per sq. in.) for nearly 0.4 of an hour, or until D. The total wash water used was about 26.5 liters (28 quarts). Samples of wash water were taken a t points 1, 2, 3, and 4, and tested for presence of tungsten trioxide with cinchonine in presence of a weak acid. Samples 3 and 4 showed practically no precipitate. Analysis of Curves
Plain typewriter-size paper with no rulings is used on the cylinder. When analyzing a curve, there is placed over it a piece of clear celluloid (Figure 9) on which there is a straight line marked off in divisions corresponding to 0.1 hour. At each division point there is a small hole about the size of a pencil point. This line is placed coincident with the base line with the first point a t the zero time position. The 0.1-hour points are then marked on the base line by marking on the paper through the holes. On the same piece of celluloid are also marked two intersecting lines representing the coordinates of the curve, the vertical line being marked off in divisions corresponding to liters and quarts of filtrate. The celluloid is now placed on the curve with the horizontal line coincident with the base line (which has already been marked off on the paper in 0.1-hour divisions). By sliding the horizontal line along the base line the total discharge of filtrate at any time instant can be read with ease, the same as though the curve were laid off on cross-section paper.
Figure 9-Celluloid
Triangle for Analyzing Curve
