Natural Isotopes Separated by New Methods C O U N T E R C U R R E N T techniques, featuring simplicity of apparatus and operation, have been established by t h e National Bureau of Standards as practical means for obtaining natural isotopes. T h u s far a countercurrent electromigration method for the concentration of the natural isotopes of potassium and chlorine and a countercurrent reflux still for the separation of the natural isotopes of mercury have been developed. Further research is expected to lead t o similar techniques for the concentration of the natural isotopes of other elements, t h e separation of like elements such as t h e rare earths, separation of pure hydrocarbons from petroleum, and the isolation of vitamins from animal and vegetable products. Countercurrent electromigration makes use of the difference in the ionic mobilities of the isotopes of an element in a singlestep method without the need of a vacuum system or other elaborate accessories. The basic principle of operation of the method rests in an imposed flow of electrolyte through the solution i n a direction opposite to the migration of ions to be separated. If the rate of flow of the electrolytic countercurrent is made equal and opposite to t h e average rate of ion transport, then only the lighter isotopic ions will make headway against the current while the ions of the heavier isotope are carried in the opposite direction.
In the laboratory setup a s diagrammed below, the electrodes are platinum gauze, and the packing material between cathode and anode is 100-mesh sand which provides a much higher reflux efficiency per unit length than m a y be achieved by other fractionation methods of isotope separation. When t h e current is turned on, potassium ions migrate toward the cathode while sulfate ions migrate toward the anode. An electrocute stream flow is induced in the cathode-to-anode direction by adding sulfuric acid t o the cathode compartment and potassium hydroxide to the anode compartment. T h e flow of electrolyte through the packing is just equal to the average rate of progress of potassium ions toward t h e cathode when the concentrations and rates of addition of the restituent solutions, so called since they act t o restore the original chemical compoposition of the electrolyte in the cell, are such that the concentration and p H of the electrolyte throughout the cell remain constant during the run. T h e net transport of potassium ion i s thus reduced to zero as the lighter K 89 ions make headway against the electrolytic stream while the heavier K 41 ions are washed back. The packing, where the point of balance between the ion migration and electrolyte stream flow is confined, thus behaves as a fractionating column under total re vox and K 39 is concentrated i n the cath%** compartment while the concentration oi K 39 in the anode compartment remains normal since it is constantly supplied with new potassium hydroxide. The system requires very little attention, and with the use of automatic controls for the addi-
Schematic diagram of equipment for the separation of natural isotopes of potassium by countercurrent electromigration. Numbers in parentheses express solution concentrations as equivalent ratios of solute to tcater
tibn o f the restituent liquids i t becomes entirely self-rcgtUating. In trie work o n concentration of t h e light potassium isotoj>e, t h e ratio K 3 2 / K 4 1 was increased from 1 4 . 2 in ordinary potassium to> 24 i n the coaoentrate after a run of 449 hours. Ttie simultaneous concentration of botli K3* and K 41 in cathode and anode compartments, respectively, has also been found practical by an extension of the above method. By a similar procedure, necessarily modified, the chlorine isotopes, Ol 36 a n d CI37, have been concentrated. Coun*ercxirrert,t
Reflux
Still
The countercxirrent reflux still is really a series of îmolecular stills s o connected that recoml>ina"tion o f fractions takes place automatically by g r a v i t y feed. A separation that would have required 55 individual arid recombination distillations was obtained in one s t e p with a 10-compartment still. The countercrurrent reflux still consists of a series o f evaporating surfaces, or pools, s e t adjaceznt to each other but at slightly different levels. A cooled roof directly above each pool serves to condense the vapor. Tîie roofs are s o sloped t h a t the condensate mil run along the surface to fall hrto the adjacent cell higher up. Each I>ool i s eqo* peel with a spillover which allows liquid «o run back in an amount ec[ual to tîie condensed vapor carried forward. The ovear-all operation is such that tfce light fraction increases toward the upper end, while the heavy fraction increases in concentration toward the lower end o f the still. T h e apparatms has been used successfully for t h e separation of the isotopes of naercixry, and i s expected t o prove extremely -valuable in t h e separation of hydrocarbons "that do n o t differ appreciably in boiling point or are unable t o vçriths-tand the liigh temperatures of ordinary «distillation.
KOH(|:30) H 2 S0 4 (l 30)
Diagram oflO-cell cascade systemfor separation of m&rmry isotopes by countercurrent reflux molecular distillation
K2SO4(I:6O:
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CHEMICAL
As Ν D E N G Î N E E R I N'G iN>EW S
