Purification of the Rare Gases D. S. GIBBS, H. J. SVEC, AND R. E. HARRINGTON Institute f o r Atomic Research and Department of Chemistry, Iowa State College, Ames, Iowa
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methods employed for the purification of the rare gases by means of hot metals can be classified according So the physical state of the metals during the purification process: finely divided metals (4, 5, 7 , 8, 10, 16, 13, 16), liquid metals, amalgams and alloys (3,Q), and metal vapors ( 1 , 2, 6 , 11, 14, 16). The method employing finely divided metals is most commonly used, as i t requires only the controlled heating of a tube packed with metal particles. Although purification efficacy is greatly reduced as the metal particles become coated with reaction products, methods employing finely divided metals are used in many instances because of ease of construction and maintenance of apparatus, and because they are cheaper and safer for purification of rare gases having a high oxygen content. A distinction should be made here between static and dynamic purification methods. I n the static methods, impure gas is introduced into a purification chamber and the chamber is sealed. A small amount of reactive metal in the chamber is then heated and the impurities are slowly removed from the gas as they are brought in contact with the metal by convection currents which are set up in the chamber. I n the dynamic method, a continuous flow of impure gas is directed through a chamber containing hot metal. The dynamic method is especially useful when a continuous flow of an inert atmosphere is required. A continuous flow of pas may be purified by the static method, if a large enough volume of compressed impure gas is treated with a hot metal. The main emphasis of this study was fixed on the removal of oxygen and nitrogen impurities from rare gases and, where feasible, oxygen from nitrogen. Carbon dioxide, water vapor, and hydrocarbons were not included because, generally, they are readily removed by other methods. All gas analyses were made using a mass spectrometer. Particular attention was given to methods employing a dynamic procedure with finely divided metal. I n every case, the impure gas was allowed to pass only once through the purification chamber, The purpose of this work was to compare the efficacies of several metals for the removal of impurities from inert gases under controlled physical conditions: ( 1 ) particle size of metals, (2) temperatures of the purification tube, and (3) flow rate of the impure feed gas through the purification tube. Because of the difficulties associated with determining the surface area of irregularly shaped particles, surface area was assumed t o be relatively constant for all the metals. Except for a few cases, in which the metals were too pyrophoric to grind and screen, all the metals employed had identical screen analyses. Equal volumes of metal particles ground under similar conditions and having identical screen analyses were assumed to have equivalent surface areas. The purification tube was filled with metal particles for a standard length arbitrarily fixed by the length of the heating element of the furnace used. The effects of flow rate and temperature on purification efficacy of the metals were studied between room temperature and temperatures approaching the melting points of the metals. Work with metals that demoxistrated getting properties only after melting was discontinued until they could be further evaluated in liquid-metal methods, outside the realm of this study. The metals employed in this study are listed in Table I. They were selected on the basis of previously reported use, chemical reactivity with the impurities in question, and availability and coat. Flow rates and temperatures used in the purification
studies are reported for each metal. The lineal rate of gas flow is also reported to minimize the dependence of these data on the specific apparatus used in this study, as the volume flow rate tells little about the velocity a t which the gas passes through the purification tube unless the geometry of the tube is defined,
Table I.
Source and Pwrity of Metals
Metal Aluminum Barium Brass Calcium
Purity, % Reagent grade 99 67 Cu, 38 Zn 99.5+
Calcium-magnesium alloy Cast iron
10 Mg, 90 Ca
Source
J. T. Baker Chemical Co. A. D. MacKay
+
Redistilled in Ames Laboratory, AEC Prepared in Ames Laboratory, AEC
94-Fe, 3.5 C , 2.5 Y1
Copper Cerium Lanthanum Magnesium lhorium Titanium Uranium
Reagent grade
99.8
99 8 99.8 QQ 8 DeBoer process Standard Mallinokrodt 99.9 99.7
Zinc Zirconium
J. T. Baker Chemical Co. Prepared in Ames Laboratory AEC Prepared in Ames Laboratory: AEC
__ - -.
.T. . T _ . .linker .-_. -.Chpmi?nl - .__. .__ - Cn
Prepai"ed in Ames Laboratory, AEC Bureau of Mines Mallirickrodt Chemical Works
J. T. Baker Chemical Co. Prepared in Ames Laboratory, AEC
APPARATUS
The apparatus designed and constructed for the gas purification studies is shown diagrammatically in Figure 1. It consisted of three sections: ( I ) a controlled gas feed system, (11) apurification tube, and (111) a gas-sampling system, The apparatus was assembled of borosilicate glass (Corning7740) tubing and high vacuum grade stopcocks. A cylinder of compressed impure gas, A , was used for the feed gas to the purification train. Gas flow was controlled by means of the regulator, B , and the flow rate was measured with the flowmeter, D (FishSchurmann Rotometer). The gas was dried using magnesium perchlorate and phosphorus pentoxide in the towers a t C .
I
I I
II
I
m
Figure 1. Experimental purification train I. Gas feed control components 11. Purification components 111. Gas-sampling components
Vycor tubing, 27 mm. in inside diameter, was used for the purification chamber, F . The tube was rovided with ball and socket and standard-taper joints t o faciitate easy removal for recharging with metal particles. It was heated by means of an electric furnace, which was provided with a simple on-off tem289
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Here are gas f l ~ wrates and reaction temperatures for a wide variety of metals used in removing O2 and Nz from the rare gases and in removing 0, from nitrogen
. . .Ca-lOyo Mg alloy and barium look
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especially good for purifying the rare gases a%low temperatures
perature controller (Model J Gardsman, Taco West Corp.). A purification-tube bypass, E to G, made it possible t o flush the train and make an approximate adjustment of the flow rate before starting gas flow through the purification tube. The vacuum line, V , was used to evacuate the purification tube when not in use. A manometer, H , indicated the pressure a t which the gas was fed into the purification tube. Samples of gas were collected in 50-ml. sample bulbs through st,andard-taper joints fitted t o the stopcock a t J . The sample bulbs, L,were made x i t h stopcocks and standard-taper joint's a t both ends t o facilitate transfer of gas from the train to the bulbs without contaminat'ion. The bulbs were evacuated before collection of a gas sample by means of the vacuum line connection a t M . The sample system was provided with a bubbler, I
