25 c.c. of pulp was removed from the cylinder and replaced by 25 c.c. of discharge pulp. After thoroughly mixing, allowed to settle for an interval of 2 min. (c) 30 c.c. of pulp removed from (b) and replaced by 30 c.c. of discharge pulp, settling interval 2 min. (d) 45 c.c. of pulp removed from (c) and replaced by 45 c.c. of discharge pulp, settling interval 3 min. (e) 75 c.c. of pulp removed from (d) and replaced by 75 c.c. of discharge pulp, settling interval 3 min. 30 Computed Capacity Rate of Settling in Feet per Hour Pounds per hour per square foot. 3 2 2001 uenbs 20 5 Parts of water to one part ore in sample rated Feed 17.5:1 CAPACITY Reported Actual capacity, 8.55 pounds per square foot per hour. FIG. 12. SLIME-SETTLING DATA, HOMESTAKE MILL, LEAD, S. D. (f) 100 c.c. of pulp removed from (e) and replaced by 100 c.c. of discharge pulp, settling interval 4 min. (g) 160 c.c. of pulp removed from (f) and replaced by 160 c.c. of discharge pulp, settling interval 4 min. (h) 260 c.c. of pulp removed from (g) and replaced by 260 c.c. of discharge pulp, settling interval 6 min. Throughout a series of tests the precaution must be taken of thoroughly mixing the contents of the cylinder both before removing pulp and before starting the test on the pulp of altered consistency. The curves in Fig. 11, showing the settling behavior of Liberty Bell pulp, illustrate the method of plotting used in presenting the results of the tests which have been taken as a basis of comparison with the performance of commercial thickeners. In the lower curve the settling rate in feet per hour is taken as the ordinate, while the ratio of water to dry slime is taken as the abscissa. Each point shown on the curve represents an individual test made upon slime of the dilution noted. The tests appear upon the curve from left to right in the order performed, which places the highest dilution at the origin. FIG. 13. SLIME-SETTLING DATA, GOLDEN CYCLE MILL, COLORADO SPRINGS, Col. In the upper curve, the computed capacity in pounds per hour of dry slime per square foot of settling area is taken as the ordinate, while the dilution is taken as the abscissa, as in the lower curve. The following table gives the actual capacities, together with the computed capacities, of thickeners used at the various mills enumerated, as shown in curves, Figs. 4 to 13, inclusive. In each case the tests were made at the mill by the local staff, according to directions supplied by us. From the data received the capacities were computed, employing the method described in the paper. Special Apparatus for Small-Scale Continuous Settling Tests Although, as demonstrated, reasonably accurate results may be obtained by settling tests made in small glass cylinders (provided the results are properly interpreted), we recommend for testing a special device in which tests may be carried out under the same conditions of depth as would obtain in practice. Since the diameter of the cylinder, if over 2 in. does not in any way affect the settling rate, it is possible to construct from standard iron pipe and fittings, available in most localities, a device which will give results exactly parallel to those to be expected in practice. Fig. 14 shows in detail such a device, which in our experimental work has given very satisfactory results. The body or container is a piece of 4-in. black, iron gas-pipe, threaded at both ends (galvanized iron pipe or fittings should be avoided )on one end of which is screwed a 4-in. pipe flange, while on the other end is screwed a special cast-iron cone. Into the bottom of this cone is screwed a 2-in. nipple and connected to this is a 2-in. iron plug cock, into which is screwed another 12-in. nipple to prevent spattering. If it is not possible to procure the cone-shaped casting for the bottom, the same effect may be obtained by plugging the bottom of the pipe and screwing the discharge valve into a hole tapped at the center of the plug. A thin sheet-iron cone is * Portland.—The bulk of this slime settles readily but there is a small amount of colloidal material which has a tendency to remain in suspension after the bulk of the pulp has settled. In order to obtain as clear solution as possible for precipitation, the thickener was operated below capacity, as indicated by the fact that there was 6 ft. of clear solution. † Presidio and West End.-These thickeners were being operated very much below capacity, as indicated by the fact that there was 5 to 6 ft. of clear solution. made of such a size that the top just fits the pipe, while the apex is of the same size as the hole in the plug. This cone is slipped inside of the pipe and downward until it rests upon the bottom. Along the side of the 4-in. pipe, at regular intervals of 12 in., are bored holes tapped for a 12-in. thread. Into each of these is screwed a 2-in. nipple, to which is attached a 12-in. plug cock; to this, in each case, is attached an elbow and nipple to divert the outflow downward. The 4-in. pipe is arranged with a tripod or other suitable means of support so that it is maintained in a vertical position at a sufficient distance from the floor for the thickened pulp to be withdrawn readily from the plug cock at the bottom. Resting on top of the main pipe or container is a funnel for introducing the pulp. Attached to this funnel by means of iron sleeves are 11-in. lengths of black iron pipe, the idea being to make each section, with the sleeve when screwed into position, 12 in. in length and so arranged that the outlet from the stem of the funnel is somewhat below the particular cock on the side of the apparatus in use. Any depth of feed in multiples of 12 in. may be obtained by screwing on to the funnel tube the necessary number of 12-in. units. To the lowermost unit in use is attached a cupshaped iron casting for the purpose of distributing the flow of pulp. This may be an ordinary 21⁄2-in. cast-iron drip cup, such as is frequently used beneath bearings, attached in such a manner as not to obstruct the outlet of the pipe. The apparatus shown in Fig. 14 has a maximum available depth of 9 ft. for the thickening zone. Therefore, settling tests may be performed in steps of 1 ft., representing what may be expected in tanks where the thickening zone varies from 2 to 9 ft. in depth. If a study of the behavior of deeper tanks is desired, a longer containing pipe may be employed. To illustrate the use of this piece of apparatus: Assume that the settling behavior of a given pulp and the capacity per square foot of settling area of a tank having a thickening zone 5 ft. in depth are to be determined. First make a preliminary settling test in a glass cylinder according to the directions previously given. Let it be assumed that the results are as follows: The critical point in the glass cylinder is ascertained to occur at a dilution of 3 parts fluid to 1 part solid and the mean settling rate down to this point is 1 ft. per hour. After these data are obtained, proceed with the large test as follows: Through the feed funnel introduce pulp of a consistency of 31⁄2 parts fluid to 1 part solids, until the large containing tube is filled to 5 ft. Since in a 5-ft. column of 31⁄2 to 1 pulp, 1⁄2 part water is equivalent to 7.65 in., it follows that this particular pulp must settle a distance greater than 7.65 in. before it reaches the critical point. Therefore, 1 ft. of feed with a consistency of say 7 parts fluid to 1 part solids, may be added to bring the pulp surface up to cock D6. After 1 hr., test the settling by opening cock D5 to ascertain if clear fluid can be withdrawn. Close cock D5, open cock D6 and pour feed in through the funnel until a murky overflow |