A Simple Pilot-Plant Electrolytic Cell for
.....
Producing Dialdehyde Starch Demand for large quantities of dialdehyde starch has prompted the design - and fabrication of this new cell. It may also be useful for similar processes on materials other than siarch H. F. CONWAY and V.
E. SOHNS
Northern Utilization Research and Development Division, U. S. Department of Agriculture, Peoria, 111.
the center of the cathode support. This wooden support rests on the anode frame, and has holes through which the cathode compartments are inserted. The cathode compartments are arranged in seven pairs so that there is a pair on either side of each anode. These compartments are thimbles of porous ceramic material (Alundum, grade RA 84), 10.5 inches long, 1.5 inches in diameter, and with the upper 3.9 inches of surface glazed to prevent diffusion of hydrogen into the anolyte. In each thimble is a cathode rod and glass tubing (Figure 1). The cathodesfabricated from 1l 1 / 2 inches of 5/16-inch iron rod and immersed in the catholyte to a depth of 7l/4 inches are connected to the bus bar, and the inlet and outlet tubes are, respectively, manifolded together. Pinch clamps on the water inlets permit flushing each compartment individually. T o provide agitation of the starchanolyte slurry, two air-driven agitators with stainless steel blades and shafts are mounted on supports near the cell and positioned near the diagonal and at such depth that the blades are bdow the bottom of the anodes. Cooling is provided by a coil of 3/s-inch inside diameter lead tubing. The power source is a three-phase selenium rectifier having an output of 150
amperes and 12 volts maximum; output voltage is continuously variable to permit changes in current.
Dialdehyde Starch Production
Cell Description
Materials and Methods of Analysis. The starch used was commercial grade pearl cornstarch. All solid reagents were reagent grade, powdered or granular. The sulfuric acid was usually technical grade. Dialdehyde starch was estimated tluring the runs by an alkali consumption method ( 3 ) . Samples of the starch slurry were filtered in a small sintered-glass funnel using vacuum. They were then washed a t least four times with distilled water and twice with acetone by first turning off the vacuum, introducing the washing agent into the funnel, stirring, and finally by reapplying the vacuum. The wet starch was then transferred to a watch glass and dried in an air oven at 110" C. for at least 15 minutes. A sample of dried solids was used in the alkali method. The combined filtrate and water washes from this procedure were transferred to a volumetric flask from which, after dilution to volume, were taken aliquots for estimating iodic and periodic acids. These analyses. and analyses for dialdehyde content of the finished dried product were carried out by the methods of Mehltretter and others (5).
The cell consists of: multiple anodes in a brass holding frame, cathode assemblies in a wooden support, and a cooling coil fitted together in a lead-lined wooden box. This pilot-plant cell was patterned after the laboratory model used by Mehltretter and coworkers for this reaction. By multiunit construction the new cell was designed to increase production about fortyfold. The internal box dimensions, 8 X 18 X 11 inches in depth, provide a starch-anolyte slurry working volume of about 5.5 gallons. The six anodes are cut from '/s-inch lead alloy containing 14% silver so that a total area of 437 square inches is available to the anolyte. The anodes are electrically connected by short wires to a brass frame that is connected to the positive terminal of the power rectifier. The negative terminal of the rectifier is connected to a bus bar mounted along
The process is a combination of electrolytic oxidation of iodic acid to periodic acid and of chemical oxidation of starch to dialdehyde starch by the periodic acid with the entire operation being carried out in an electrolytic cell. Iodic acid is oxidized to periodic acid at the anode, and the periodic acid then oxidizes the starch in the anolvte to dialdehyde starch. During starch oxidation the periodic acid is reduced to iodic acid. Thus, oxidized starch is prepared in several stages-namely, setting u p the cell, electrolysis, and recovery. Each stage contributes to the quality of thp finished product. Prior to insertion, the anodes are coated with reactive lead dioxide. This is accomplished in a separate cell, u hich contains 5% sulfuric acid and lead cathodes, at a current density of 0.18 to 0.23 ampere per square inch for about 1 to 1.5
DmmC
the past few years, interest has developed in the dialdehyde starches as potentially useful industrial commodities. The unique properties of these products and their derivatives for possible applications in sizings, adhesives, thickening agents, and tanning have been shown (2, 6), and the methods ( 5 ) have indicated that these specially oxidized starches could be made at a reasonable cost. Patents pertaining to the electrolytic periodate oxidation of cereal starches to produce dialdehyde starches have also been issued ( 7 , 4 ) . Demand for relatively large quantities has prompted the design, fabrication, and operation of a simple electrolytic cell, details of which are described. This cell may also be used for similar processes on materials other than starch. The preparation of the dialdehyde derivative of corn starch is given as an example of how the cell is used.
Preparation of Dialdehyde Starch.
Figure 1 . ponents
Exposed view of cell com-
A. The three separate assemblies (top to bottom) cathode diophrogms, anodes, lead-lined wooden box B. Diaphragm sectioned to expose cathode rod os well os inlet and outlet glass tubes
VOL. 51, NO. 5
MAY 1 9 5 9
637
O X I D A T I O N 1iME HOURS
Figure 2. Oxidation of starch and conversion of iodic acid to periodic acid during electrolysis follow a definite pattern
hours. The coated anodes are then rinsed with water and placed in position in the cell in which the starch is to be oxidized. The cathode-diaphragm unit is placed and the agitators positioned. Immediately before adding the starch-anolyte suspension to the cell, the diaphragms are filled with 570 caustic solution through the water inlet manifold. The anolyte is prepared at a concentration of 0.1 mole of equivalent iodic acid and 0.38 mole total sodium sulfate per mole of starch. For each batch, 12.15 pounds of starch, dry basis, are used, which results in 36 square inches of effective anode area per pound of starch. The actual anolyte solution is made by adding to 3.2 gallons of distilled water, 0.38 pound of concentrated sulfuric acid, and then 1.48 pounds of sodium iodate slowly and with agitation. After the iodate dissolves, 3.53 pounds of sodium sulfate are added; when it is in solution, the volume is increased to 4.0 gallons. Before charging the cell, 12.15 pounds of starch on a moisture-free basis are suspended in the anolyte solution. The resulting starch-anolyte suspension is then poured into the cell. The agitators are started, and the current adjusted a t 68 to 72 amperes. Electrolysis is carried out for at least 48 hours for high oxidation levels. The currenL is held constant by periodic adjustment of the voltage, and the temperature held close to 77’ F. by adjusting the flow rate of the cooling water. About 1 pound of sodium bicarbonate is added to the anolyte at least once during an extended run to provide sodium ions to maintain conductivity. Octyl alcohol is used during the oxidation to control foaming of the anolyte. The catholyte concentration is reduced 2 or 3 times per run by displacing about two thirds of the catholyte
638
with distilled water. Samples are taken a t convenient intervals to determine dialdehyde content and conversion of iodic to periodic acid. When electrolysis is completed, the starch-anolyte reaction mixture is removed from the cell for recovery. The cell components are usually cleaned a t this time for re-use or they may be stored temporarily in the cell filled with water. Normally, before re-use the anodes are anodically polarized ; but, they may require complete removal of the accumulated lead dioxide coating periodically to prevent undue contamination of the product. The dialdehyde starch product is recovered by filtration in a pair of stainless steel Buchner funnels with 5-gallon receivers. After the anolyte solution is removed, the cake is reslurried with about 3 gallons of distilled water, and the slurry refiltered. Most of the iodic acid is removed in about five washings. The wash water is tested for iodate ions by adding a few drops of sulfuric acid to a small sample and then adding a few drops of potassium iodide solution. A yellow color indicates the presence of iodate. The initial moisture content of a wellfiltered, washed dialdehyde starch is around 4070, The product is usually dried for 24 hours at 110’ F., which reduces the moisture content to about 12%. It is then ground in a hammer mill with a ‘/az-inch screen to a powder, 90% of which passes an 80-mesh screen. The ground product is stored in polyethylene bags. Oxidations to date follow a pattern (Figure 2). The straight line represents the cumulative production of oxygen at the anode in moles per mole of total starch present. The curved line shows how the oxygen is used by the starch to produce dialdehyde starch, the reaction becoming less efficient at the higher degrees of oxidation. Bars represent the range over many cells and indicate the total reproducibility of operation, sampling, and analysis. The lower curve in this figure indicates the moles of periodate present in the anolyte per mole of total starch. Washing the product proceeds, Figure 3, in geometric diminution of concentration in the wash liquor after the first separation of anolyte solution. From the standpoint of iodic acid recovery, five washes are sufficient. After this, however, the removal proceeds more slowly, so that elimination of the final traces becomes rather difficult. After the seventh wash, the concentration in the wash water becomes almost constant. About 10/0 of the iodic acid is not accounted for. Discussion
The construction of the cell is simple, so that assembly, cleaning, and replacement of parts are not difficult. Oxidation of the starch is satisfactory. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
TOTAL G A L L O N S OF FILTRATE AND W A S H LIQUORS
Figure 3. Removal o f iodic acid during filtration and washing proceeds more slowly after the third wash
cell, as described, has been very useful in preparing samples of dialdehyde starch of several levels of oxidation for use-tests. Although the cell operates well, it is not represented to be commercially applicable in its present form. A number of problems are involved in attaining an economical process for producing dialdehyde starch. Studies on recycling of filtered anolyte liquors, on the recovery of iodate and periodate from various process liquors, and on other factors are now in progress. Acknowledgment
The advice given by C. L. Mehltretter of the Cereal Crops Laboratory of this Division is gratefully acknowledged. literature Cited
(1) Dvonch, W., Mehltretter, C. L. (to U. S. of America, represented by Secy. of -4gr.), U. S. Patent 2,648,629 (Aug. 11, 1953’1. (2?-Fiachione, E. M., Harris, E. H., Fein, M. L., Korn, A. H., Naghski, J., Wells, P. A., J . Am. Leather Chemists’ Assoc. LIII, 77 (1958). (3) Hofreiter, B. T., Alexander, B. H., Wolff. Wolff, I. A.. A., Anal. Chem. 27, 1930 (1955). rnerica, (4) Mehltretter, C. L. (to U. S. of America, represented by Secy. of .4gr.), U. S. Patent 2,713,553 (July 19, 1955). (5) Mehltretter, C. L., Rankin, J. C., Watson. P. R.. IND. ENG. CHEM.49, 350 (1957). ’ ( 6 ) Sloan, J. W., Hofreiter, B. T., Mellies, R. L., Wolff, I. A., Ibid., 48, 1165 (1956). RECEIVED for review October 1, 1958 ACCEPTED January 8, 195 9
Mention of firm names of commercial products does not constitute an endorsement by the U. s. Department of Agriculture.
