I/EC
Equipment & Design
An Improved Commercial Fluorine Cell Improvements to the anode connection of commercial fluorine cells results in increasing cell life by 2 to 5 times
by C. A. Powell, R. E. Cable, and W. K. Henderson, Union Carbide Nuclear
Co.
An over-all view of the cell room in a 36-cell fluorine plant a t Union Carbide's Paducah, Ky., location
46 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
H e r e is a typical industrial process improvement study, in which small changes in equipment produce substantial improvement in process operation. Such research in a producing plant is restricted b y existing design which cannot u n d e r g o radical modification without u p setting production schedules or leading to excessive replacement of existing investment. T h e value of the specific improvements described h e r e is probably limited to installations using the subject AEC fluorine cell. However, the cumulative experiences, favorable a n d u n favorable, presented in this a n d in previous reports out of the authors' organization are valuable to all who are concerned with the design a n d operation of commercial fluorine cells.
IN large-scale manufacture of ele mental fluorine, the most signifi cant recent development is the design of two novel fluorine-cell anode con nections. These new connections simplify operations by reducing the number of expensive overhauls needed, and increasing the cell onstream time.
Methods of I m p r o v e m e n t Tried a n d Considered
With the old-type connection, electrolytic corrosion of the metal to carbon anode fastener has limited cell life by causing joint failure. In the first attempt to increase the time before failure, the small anode clamp bolt was replaced with a larger one. This doubled cell life to 32 Χ 106 ampere-hours, but cor rosion products which could cause in ternal electrical shorts were in creased. Occasional desludging was needed to maintain cell-current effi ciency. In a second attempt, the clamp was placed in a recessed hole in the carbon anode and covered with a carbon plug. This increased cell life about five times, 85 X 106 ampere-hours, but because other cell components deteriorated, oc casional desludging was needed here also. Two other approaches were con sidered—use more corrosion-resistant metals and impervious coatings for metal parts of the anode. Of the metals tested bronze was the most corrosion resistant. The coatings would be of plastics, ceramics, or noble metals applied in such a way to prevent electrochemicals corrosion. This was considered the poorest ap proach, because of problems as sociated with producing a uniform coating free from pinholes and not subject to temperature expansion or shrinkage.
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Description of Tests
Test anodes using standard phos phor bronze hangers, clamp plates, and bolts were assembled and operated to failure with no cell-life improvement. Two grades of phos phor bronze were used with different
FILTERS
D I V I S I O N S
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FILTER
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American Machine and Metals, Inc. EAST M O L I N E ,
ILLINOIS
VOL. 52, NO. 7
·
JULY 1960
47 A
EQUIPMENT A N D DESIGN.
specialists in
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Tblhursf CENTRIFUGALS
Failure of the anode assembly with 1 Yi inch cap screw looks like this
results. The Grade A material gave no improvement, and the Grade D, high tin, phosphor bronze failed at one fourth cell life, 4 X 106 ampere-hours. Photo courtesy of Merck & Co., Inc.
Large Bolt Anode
B A T C H - O - M A T I C T PROVIDES EFFICIENCY AND CONSTANT QUALITY CONTROL The completely automated BATCH-O-MATIC reduces exposure time to maintain product stability in centrifugal extractor operations. They can be constructed of stainless steel, "Hastelloy", titanium or other alloys. Corrosion-resistant coatings may be specified. Low, compact design and exclusive CENTERenable BATCH-O-MATIC to handle greater out-of-balance loads with minimum vibration. Low-speed plowing prevents crystal degradation. See complete data in the TOLHURST section in Chemical Engineering Catalog, or write Dept. IET-760 for free bulletins. SLUNG* suspension
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D I V I S I O N
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The next cell selected for test was the standard cell with the large headed bolt. The cell assembly was identical with the standard cell with the exception that the 3 / 4 - X 3 l /V inch anode bolt with a lVs-inch hex head was replaced with a 3 / 4 - X 3 1 /* inch bolt with a 1 W inch head made of the same AISI 4140 material. To produce the same bolt tension on the anode clamp plate with the larger headed bolt, it was necessary to increase bolt torque from 1 50to250 foot pounds. This was determined experimentally by measuring bolt tension with a strain gage. The normal method to ensure metalcarbon joint quality is to tighten bolts with a torque wrench and determine joint resistance at each bolt with a Ductor low resistance ohmmetcr. The assembly of the large bolt anode produced uneven bolt pressures and cracked some carbons. To prevent further trouble it was necessary to specify that the bolt shaft
specialists in
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AJiagara*
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F I L T E R S
by Centrifugal Extrac tion · Pressure Leaf and Continuous Vacuum or Pressure Filtration · Filter Media
This countersunk anode assembly is r e a d y for carbon plug installation
surface be concentric with the bolt thread axis within 0.003 inch total indicated runout for the one-inch section between the thread and the inner finished head surface. Also, the inner finished head surface is to be finished so that when the bolt is revolved about the thread axis the surface; is square within 0.001 inch total indicated runout. The joint produced with the close tolerance bolt offered a resistance of 62 to 82 microhms as compared with a stand ard cell of 80 to 110 microhms. This anode produced a cell life of about 32 Χ 106 ampere-hours. Failure cause was the same with the exception that the corrosion products increased by about 50%. Subsequent test work showed that the sludge proved to be an electrical conductor and would pack in the cathode to screen space in the cell causing the screen to become cathodic with loss of cell current effi ciency. It was necessary to desludge corrosion products from the cell at one-half life. Desludging is accom plished by disassembling the cell, neutralizing the parts with a soda ash solution, and removing corrosion products from the cathode, screen, and cell tank. The cell is then dried and returned to operation. The cell
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Photo courtesy The H u b i n g e r Company, Keokuk, Iowa
The countersunk assembly looks like this when complete
at failure is identical in appearance to previous anode failures.
Countersunk Anode Cell
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PRODUCTIVITY
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T h e F E I N C STRING DISCHARGE filter eliminates blow-back a n d produces a cake of low moisture content, PRECOAT filters give high clarity of effluent a n d flow rates. A complete line of SCRAPER a n d ROLLER DISCHARGE filters is avail able. T h e HORIZONTAL filter offers extremely high hourly o u t p u t . See complete d a t a in t h e F E I N C section in Chemical Engineering Catalog, or write Dept. I E F E - 7 6 0 for free bulletins.
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The next anode tested was the cor rosion protected bolt in a hole drilled in the carbon. The 2-inch National Carbon Grade YBD carbon anode is drilled completely through with a 13 /i6-inch diameter hole for the bolt; this hole is counterbored to a depth of 3 /4-inch by a hole 1.384 inches in diameter which furnishes a shoulder to hold the bolt head. The 3 / 4 - X 2 7 / 8 -inch AISI 4140 slot-head bolt is inserted with a Vie-inch thick X 3 /i- X 1 Vs-inch copper washer under the head for carbon protection. The bolt is torqued to 120 foot pounds having an anode joint resistance of 49 to 65 microhms on the Ductor tester. The bolt head is protected Irom corrosion by a 1.384-inch di ameter by •/Vinch thick tapered carbon plug. This cell failed at 85 Χ 106 ampere-hours due to ex cessive corrosion of the metal anode bars. Corrosion of the bars is at tributed to the corrosive action of elemental fluorine and electrolysis, particularly at sections of the bar ex periencing high current densities. The countersunk cell has to be desludged at half life due to the cor-
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NATURAL AND MAN-MADE FILTER MEDIA
by Centrifugal Extraction · Pressure Leaf and Continuous Vacuum or Pressure Filtration · Filter Media
N o w compare the failure of the countersunk assembly with the conventional c a p screw installation shown, p a g e 4 8 A
rosion of the metal hanger and the cell tank.
Future Work
The future work indicated by the described development is the ultimate removal of the last metallic cell anode parts and replacement with a carbon hanger. A strong carbon hanger to carbon joint with low electrical resistance must be developed as well as a practical way to conduct electricity into the carbon hanger. The use of carbon as an electrical conductor seems impractical due to the large conducting area necessary and due to the fact that carbon deteriorates in fluorine at temperatures above 300° F. The most practical arrangement would seem to be a vertical copper conductor bolted into the carbon hanger and the bolt sealed from electrolyte.
FABRIC CONSTRUCTION DETAILS ARE NOT ENOUGH! CORRECT DESIGN AND FIT AFFECT FILTER MEDIA LIFE AND PERFORMANCE FEON manufactures correctly designed •
Anode
bags and
diaphragms
•
Centrifugal
•
Dust
tubes
•
Plate
and frame
•
Pressure leaf
discs
•
Pressure leaf
sacks
•
Vacuum disc sector
•
Vacuum drum
•
Yardage
liners
filter
covers
bags
covers, belts,
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or slit fo
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See complete data in the FEON section in Chemical Engineering Catalog, or write Dept. IEF-760 for Bulletin A180. Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 115516th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly.
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·
JULY
1960
51 A
