|Air is the only real source of neon. In the process of separation by low-temperature rectification most volatile components move into the first isolated part. It is collected from under the cover of the air-fractionating apparatus. This primary gas yields only 3 - 10 % of neon-helium mixture, and the rest of the fracture is nitrogen. It is natural because the volume concentration of neon in 1000 litres of air is 18.2 sm3 and the concentration of helium is 5 sm3. The mixture is then sent to the dephlegmator, that is a distillation tube, in which the most part of nitrogen is condensed. As a result the concentration of neon-helium binary mixture grows up to 35-40%. Another apparatus called dephlegmator-adsorber almost completes the nitrogen separation process. Depending on the degree of purification the final mixture contains 30-75% of neon and 10-25% of helium.|
Original Neon purification
- The earlier methods used for the isolation of pure neon all depended on the fractional distillation of liquefied mixtures of neon, argon, krypton, and xenon. That originally used by Ramsay and Travers has been outlined above.
- Another method used by these investigators was to take the gas escaping from the Hampson liquefier (which consists chiefly of nitrogen, together with all the more volatile constituents of air) and return it to the intake of the compression pump. It was thus again partially liquefied, and deprived of a further proportion of its less volatile constituents, and a gas was ultimately obtained comparatively rich in helium and neon. This concentration of the lighter gases could also be brought about by liquefying the whole of the gas escaping from the air liquefier and blowing a current of air through the liquid; the portion that first evaporated contained most of the helium and neon present.
The enriched gas was next freed from oxygen and nitrogen by the usual chemical methods, and the residue was again liquefied and fractionated to remove argon. Finally, neon was separated from helium by cooling the mixture to the temperature of liquid hydrogen and pumping off the still gaseous helium from the solid neon.
- A simpler method for separating neon from the mixture of inert gases obtained from air depends on the use of cooled charcoal in the manner devised by Dewar. It is found that when the mixture is brought in contact with charcoal cooled to about - 100° C., the argon, krypton, and xenon are completely absorbed, while the greater part of the helium and neon can be pumped away in the gaseous state. The neon can then be separated from helium by bringing the gas in contact with charcoal cooled to the temperature of liquid air (- 180° to - 190° C.); the neon is largely absorbed, while all the helium with a little of the neon can be pumped off. When the charcoal is allowed to warm up to the ordinary temperature, the neon is evolved in a fairly pure state.
From the gas retained in the first lot of charcoal, krypton and xenon can be obtained.
|Claudes apparatus for liquid air distilation |
- The most convenient method of obtaining neon from the air is by means of a modification of Claude's apparatus for the fractional condensation and distillation of liquid air, which is depicted diagrammatically in fig. Cooled air under pressure is introduced into the vessel A and ascends the tubes BB', which are cooled in a bath of liquid oxygen. Here the greater part of the oxygen and some nitrogen are condensed, and the liquid falls back into the reservoir A. The gas next passes down other tubes DD', which are also surrounded by the liquid oxygen, and here the nitrogen is condensed almost completely, and collects in the reservoir E.
The upper part of the apparatus consists of a rectifying column, similar in construction to that of Coffey's still. By the pressure of gas in the tubes the liquid in the reservoir A is forced up the tube t1 and delivered into the column at a point a little below the top, while the liquid in E is similarly forced to the top of the column through the tube t2.
The oxygen in C is at its boiling-point under atmospheric pressure, and the highest temperature is found in this liquid and at the bottom of the rectifying column. On the other hand, the liquid in E when suddenly placed under atmospheric pressure at the top of the column, boils rapidly, losing the greater part of its nitrogen and producing the lowest temperature found in any part of the apparatus. As the liquid passes down the column it becomes steadily warmer, and meets a current of gas which becomes steadily cooler as it ascends. The net result of these heat interchanges is that practically all the oxygen in the ascending gas is liquefied and drops back into C, while all the nitrogen in the descending liquid is distilled off and passes out into the atmosphere at F.
This description of the apparatus and its working is necessary in order to make clear the nature of the modification by which the helium and neon present in the air can be obtained. It will be evident that, as these gases have extremely low boiling-points, they will remain as gas in the space above the liquid in E. A narrow tube X is therefore provided, by which these uncondensed gases are led up to the top of the column, where they pass through the spirals Y, situated at the point where the temperature is lowest (vide supra). Here a great part of the residual nitrogen is condensed and falls back into the small reservoir Z, from which it is blown off from time to time through the cock W.
When the flow of gas and liquid in the various parts of the apparatus is suitably adjusted, the residual gas escaping from the top of the small spirals contains all the helium, neon, and hydrogen present in the air delivered into the apparatus, together with about 50 per cent, of nitrogen.
From the crude light gas obtained by this method the nitrogen may be at once removed by the ordinary chemical methods (see Argon), but a simpler plan is to fractionate the whole over cooled charcoal repeatedly. The nitrogen is absorbed most readily, the neon less readily, and the helium hardly at all at the temperatures used. A saving of time, however, may be effected if after several fractionations have been made, the nitrogen is absorbed by chemical methods. The progress of the purification is best followed by means of determinations of density - spectroscopic examination is almost useless, as large quantities of helium may escape detection.It will be evident from the above descriptions that neon is the most difficult of the inert gases to isolate. This is due partly to the extremely minute proportion present in the air, the only available source, and also to the fact that it has to be isolated from the middle fractions of the inert gas, in which purity is more difficult to attain than in end fractions.
Evidence of the homogeneity of neon has been obtained by the observation that its vapour pressure is unchanged during the fractional distillation of the liquefied gas. Quartz at 1000° C. is permeable to neon, but much less so than to helium.