A Complete Mercury-
Purification System FALTER A . CARLSON ANI L. F. BORCH.4KDT General >liIls, Inc., Research Labnratnric*. RIinneapnlis, nlinn.
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S YA\RIOUS i ecoinirientled laboratory nietliotk of purifying mercury mine or all of the following unit operations are used: washing and diying, oxidation of iiiipurities 1)y aeration, distillation, and elect1olysis. The rriethod of AIeyer, 1863 ( 7 ) ,of washing by passing the mercury in a fine stream through a long column of dilute nitric acid is well known. Better washing through a gireii height of column is obtained by using the “zigzag” column of Friedrichs ( 4 ) . Diying has uqually been done in a n open dish. Alewijn ( 1 ) and Bur-tyn (3) passed air through niercury as a separate operation to remove some of the riictallic impurities as oxides. following this step hy filtration. More often air is bubbled through the mercury during distillation a5 described by Hulett ( 6 ) . A number of papers have been publiihed describing glass or metal stills, some automatic, opeyating under wrious degiees of reduced piessui e. The1 e are several descriptions of electrolytic methods (2+5 , 8 ) .
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FIGCRE 1. GENERAL VIEWOF COMPLETED APPARATUS
FIGURE 2. WASHINQCOLUMN AND STILL 94
FEBRL-.4RY 1.5. 1938
AN.kL1 TIC.kI, EDITION
There was needed in this laboratory a complete mercurypurification system t h a t was portable and a t least, semicontinuow, and would furnish mercury of a purity satisfactory for use in diffusion pumps, gages, thermoregulators, etc. It was decided to proTide for washing, drying, and distillatioil with optional aeration. The final assembly contains little that is new, either as t o methods of purification or apparatus ; howe\.er, the assembly has proved so useful and comenient iii this laboratory t h a t it is felt a description of a complete unit .;hoiiltl l w of int,erest to a number of readers.
Apparatus and Operation
A general view of the assembled apparatus is show11 in Figure 1. Figures 2, 3, and 4 illustrate the design of t h e conipoileiit parts, described below. WASHER. The mercury is introduced into the washing colunin, a), through funnel B which has the delivery end bent ton-ard the side wall of the column and pulled out to a capillary, C. The stream of mercury projected against the wall of the column breaks up into very fine droplets. This is most easily accomplished when the jet of the funnel projects below the surface of the washing liquid. The droplets zigzag t,hrough the solution by bouncing stepnise un the indentations formed in t h r side wall of the column as shown in the drawing. The washed mercury collected at the bottom is delivered to a receptacle through the capillary column, D,which is so proportioned as t o support the wash solution in the usual manner.
.i (Figure
93
DRYER.A top and a front view of the dryer are sliobvn in Figure 3 . The washed mercury is transferred to funnel E of the dryer and admitted in a 2.25-kg. (5-pound) charge to F , which is then somewhat lese than half full. The mercury is agitated and dried by pulling warm air (approximately 100" C.) through it. This is accomplished by evacuating the chamber, F , by means of a water pump connected to t'he outlet of trap G. This vacuuni maintained above the mercury causes air to enter at GG below the surface of the mercury and proceed through it in successive bursts v-hich give very effective agitation. Preliminary to paseing through the mercury the air is heated by contact with heating coil J , which is made of 5.18 meters (17 feet) of KO.28 Nichrome wire random-wound to promote more efficient heat transfer. The incoming air first passes through a filter plug of glass wool held in place by disks of fine-mesh screen cut t o fit expanded sections in the glass tuhe as shown at H . The air enters at H , is heated in passing through J , bubbles through the mercury starting at GG, picks up xater from it, and goes through the remainder of chamher F , through the trap, and out at G to the water pump. The dry mercury is drained out through stopcock K . S o lubricant is used on either of the stopcocks shown. The minimum drying time per charge is 30 minutes. STILL. The dry mercury is put into reservoir L (Figure 2) where it is supported by the float valve assemhly, M. A float valve is used since it automatically maintains B constant, level, and can be made entirely of glass. In making it, a piece of snialldiameter glass rod, M c , is sealed to bulb M a . The opposite end of M c is ground to a narron- seat at M d with the capillary tubing delivering the mercury from the reservoir. The final step of the grinding is done n-ith F F F Carborundum. Six glass knobs, M b , on the surface of the bulb furnish a minimum of frictional rontart with the surrounding tuhe, and thus permit ease of
FIGURE 3
necessary the capillary can be removed and replaced easily and without danger of breaking the flask. A carefully ground glass plug, Xc, is provided for use when distillation without air is desired. Rather than pass air over the surface of the mercury, ionie workers prefer to draw air through it. This tends to decrease bumping of the boiling mercury. The connection, I’,to the condenser, Z, is made long enough so that no impure mercury can he carried over mechanically in a moment of unexpectedly vigorous boiling. The condensed mercury drops down into another barometric leg, d A , which feeds delivery tube CC through trap BB. Sufficient mercury is held in BB to fill tube A A t o barometric height, when evacuating the still in starting a new run. During distillation pure mercury is delivered continuously t o a clean dry bottle, DD. Connection to the vacuum pump is through a trap, EE, which is surrounded by a dry ice-acetone mixture. Tube F F is turned u p m r d to allow the mercury collected in it to drain back.
Mounting
A11 the apparatus is mounted on a specially constructed table as shown in Figure 1. The dimensions of the table are: over-all height, including casters, 88 cm.; length, 88 cm.; width, 51 em.; width of open section in top, 21 cm.; height
FIGURE 4
of bottom shelf, 2 i em. The two shelves are plywood, suitably braced, and placed, as shown in the photograph, to act’ as trays to catch the small amount of mercury t h a t is inevitably spilled. A small hole placed in a corner of each shelf provides a convenient means for draining. The interior corners of the shelves are rounded t o a fillet with pIastic wood. The support for the dryer is made of 17-mm. outside diameter pipe (standard 0.375-inch iron pipe) ; the remaining supports are made largely of 21-mm. pipe (standard 0.5inch pipe). Sliding tees fastened b y setscrews are used where convenience dictates. The entire layout is so arranged that all the leads for power, water, drain, and vacuum are a t the same end of the table. Separate switches with pilot light’s are provided for the two heating circuits.
Operating Notes movement. The actual dimensions of the float are not critical; with the proportions used here the bulb floats only about 33 per cent submerged in operation. The buoyancy of the remaining 67 per cent acts as a reserve to exert more pressure on the valve in case there is some tendency to stick. The copstant mercury l e d in both legs of the E-tube, N , in turn maintains a constant level in the boiler, 0. Adjustment for variation in baromet,ric pressure and for pressure changes due t,o the air leak is made by raising or lowering the assembly of L, M , and N as a unit. This ordinarily needs checking only once a day. Figure 1shows how these parts are supported and adjusted. Pa and Pb are pipe tees bored out to make a smooth sliding fit on the supporting standard Q, made of 21-mm. outside diameter pipe (standard 0.5-inch iron pipe). 9 screw eye, R, is soldered into the side of the sliding tee, Pa; the eye of the screw makes a smooth sliding fit over the threaded support and guide rod, S. Adjustable nut T is used to hold the assembly a t the desired point, while stop U limit’s the upward movement and thus prevents the barometric leg, W (Figure 2), from breakingthroughthe bottom of tube h’. The remaining clamps are made to fit the parts as shown. Heat is supplied to the boiler by the electric heater, V , made by winding 6.1 meters (20 feet) of No. 22 Nichrome wire into a coil on a 4.7-mm. (0.1875-inch) mandrel. The element is coiled into a pancake spiral conforming to the shape of t’he flask and embedded in Sauereisen electric heater cement (made by the Sauereisen Cements Co., Pittsburgh, Pa.) in a shallow can of convenient size, using a cork as a core to provide a hole in the center for the supply tube, mi. I t is important t o oxidize the element before this step, since adjacent turns of a bright wire embedded in cement may short-circuit and burn out the heater. The oxidizing can be done by stretching the coil in the air and running it a t full line voltage (115 volts) for about 30 minutes. Using an external resistance of 8 ohms in series with the heater across the 115-volt line, t’he effective input to the heater is 230 uratts. X (Figure 2) is the arrangement for pulling (blowing) air across the surface of the heated mercury when desired. The inner tube is pulled down to a capillary tip, Xu. The ring-seal, X b , is made at a distance of about 4 cm. from the flask, so that whenever
Very oily and dirty mercury is given a preliminary batch washing with acetone and then t a p water and strained through a cloth. A pin-holed filter paper is used in funnel B. Nitric acid (25 per cent) is used in the column for the first washes, The final washes are with distilled water. No attempt is made here to state the criteria by which the purity of the mercury can be determined. T h e treatment prior t o distillation is naturally governed by the contemplated use of the final product. It is sufficient for the purposes of this laboratory t o be guided by the appearance of the mercury in the intermediate steps and by the behavior of the final product when used. Any glass blowing t h a t is done on the system after it has had mercury in it should be done with a dry-ice trap in the line to the blowing tube tmoprotect the worker from mercury vapors. T h e rate of distillation a t 230 watts input to the heater is 400 t o 450 grams per hour. T h e reservoir is large enough so t,hat the distillation can easily continue 10 hours without &ten tion.
Literature Cited (1) Alewijn, W. F., ChenL. m’eekblad, 30, 687 (1933). (2) Brummer, E., and Narag-Szabo, St. V., Z. Elektrocherri., 31,95-7 (1925). (3) Burstyn, W., Z . tech. Pliysik, 13, 505 (1932). (4) Friedrichs, F.. Z . angew. Chenz., 27, 24 (1914). ( 5 ) Hanke, M. E., and Johnson, M.,Science, 78, 414-16 (1933). (6) Hulett, Phys. Rev., 33, 312 (1911). (7) Meyer, Lothar, 2. anal. Chem., 2, 241 (1863). (8) Patten. H. E., and Mains, G . H., J. ISD. ENG.CHEbf., 9, 600-3 (1917). RECEIVED August 24, 1937. Paper S o . 12. Journal Series, General Mills, I n r , , Research Laboratories.
