|Helium is extracted by from natural combustible gas. Dried gas, purified from CO2, is supplied to heat exchanger and separator system. Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane) resulting in crude helium gas purified by successive exposures to lowering temperatures. Throttling to a pressure of 2 MPa and distillation under temperature -28°C, -41°C and -110°C makes all hydrocarbons separated away; under pressure of 1.2 MPa, nitrogen and other gases are precipitated out, which results the gaseous mixture enriched by 3% helium. The last throttling to the pressure of 1.0 MPa makes the helium concentration increased to 30-50%, and then, cooling by boiling nitrogen at -203°C and 0.04 MPa raises it to 90%. Crude Helium (70 - 90 vol. %) is rectified of hydrogen (4 - 5%) at 650-800°K and dried by silica gel. The fine rectification is completed by cooling crude helium boiling under vacuum. Adsorption on activated charcoal in adsorbers is used as a final purification step, usually resulting in 99.995% purity of Grade A helium. The purity of Grade B helium purification is about 99.80%|
|The chief available sources of helium are the air, certain minerals, and a few mineral springs. Originally the cheapest way of preparing helium was, undoubtedly, by heating a suitable mineral, e.g. monazite sand or cleveite, either alone or with dilute sulphuric acid. |
The apparatus used for the preparation of helium according to this method may be of the form depicted in fig. The finely powdered mineral is placed in the iron tube T, which is heated to redness in a suitable furnace. The open end of this tube is fitted with a rubber stopper carrying a single delivery tube, and the small water-jacket W cools the end of the tube that projects from the furnace and protects the rubber connection from injury. The evolved gas is freed from water and carbon dioxide by passage over solid potash in the vessel D and is finally collected in a reservoir R filled with mercury or strong potash solution according as the amount of gas dealt with is small or large. The tube P connects with a Topler pump, and the open manometer M indicates the pressure within the apparatus.
|Helium preparation |
In carrying out an experiment the mineral is introduced into the iron tube and the whole apparatus is evacuated. On heating the tube a slow evolution of gas commences and continues for many hours. When the pressure within the apparatus becomes equal to the atmospheric pressure the gas is collected in the reservoir R, until the evolution practically ceases. R is then shut off and the residual gas removed from the other part of the apparatus and transferred either to R or to another reservoir.
A modification of this method consists in heating the mineral in an atmosphere of carbon dioxide (prepared from magnesite by heating) and collecting the gas over potash. It is stated that the best results are obtained by heating the mineral to 1000°-1200° C. in a porcelain tube.
Another method which is more expeditious and gives a better yield is to heat the mineral with about its own weight of acid potassium sulphate in a hard glass tube. The mixture is very liable to froth, and the tube should not be more than half full.
Most minerals give the largest yield of helium when boiled with dilate sulphuric acid. This operation is best carried out in a round flask of hard glass, fitted as in fig. The rubber stopper 11 fits some way into the conical neck so that a layer of mercury on the top makes all joints tight. The upper end of the condenser C can be connected at will either with a reservoir for the evolved gas or with a Tcipler pump.
|Helium effective preparation |
In performing an experiment the flask and the contained mineral (which must be finely powdered) are freed from air by introducing successive small quantities of water through the funnel F and pumping away the water-vapour. Dilute sulphuric acid (1:8), which has been boiled just previously to expel air, etc., is then run in and boiled with the mineral for about 30 minutes. When the gas evolved at atmospheric pressure has been collected, the reservoir is shut off and the residual gas removed from the rest of the apparatus through the pump and transferred to the reservoir.
As 100 grams of cleveite will give over 500 c.c. of gas and can be obtained for about 10s., the cost of preparation of crude helium by this method works out at about £1 per litre. Other minerals available for the preparation of helium by this-method (i.e. which yield from 1.0 to 1.5 c.c. or more of gas per gram) are fergusonite, samarskite, and monazite - monazite sand was used by Onnes as the source of the large quantities of helium required for his researches upon its liquefaction.
Certain of the natural sources of helium mentioned above yield the gas in sufficient quantity to afford a useful supply of it. The chief of these are the springs of Bath and Mazieres, the Tini soffioni at Larderello, and the natural gas wells of Dexter. The device illustrated in fig. has been used by Ramsay in collecting the gas from mineral springs. The tin vessel V is provided with taps above and below (B, C), and both it and the tube A are first completely filled with water. On bringing the funnel attached to the tube A over the stream of gas rising through the water, and opening the taps B and C, the gas passes into V, while the displaced water flows back into the well. When gas is seen to issue from the lower end of C the vessel is known to be full, and the taps are closed.
|Helium collection |
An excellent method of obtaining helium, which would probably prove comparatively inexpensive where the necessary plant is available, is that devised by Claude. The apparatus used, consists essentially of a modification of the column used for the isolation of pure oxygen and nitrogen by the fractionation of liquid air, whereby the most volatile gases are collected apart.
|On account of its great volatility at very low tempera tures, helium is more easily purified than any other member of the group. The usual procedure is to remove nitrogen and hydrogen, if present, by passing the crude helium over a heated mixture of quicklime and magnesium filings, and then over red-hot copper oxide. In the case of gas from cleveite or monazite, which contains no appreciable amount of other inert gases, the residue from this operation is already fairly pure helium. Should the gas contain argon, as, e.g., when obtained from mineral springs, it is necessary to cool it to a low temperature by means of liquid air boiling under reduced pressure - any nitrogen or argon present is liquefied, and helium can be pumped off. Neon, if present, can be removed by cooling the gas with liquid hydrogen. At this temperature all gases are liquefied except helium. |
The best method for the purification of helium, however, depends on the fact, discovered by Dewar, that cocoanut charcoal at the temperature of liquid air completely absorbs all gases except helium. A suitable form of apparatus is indicated in fig. The mixed gases are introduced into the apparatus from a gas-holder, allowed to remain in contact with the cold charcoal for half an hour, and the pure helium is finally removed through a Topler pump and collected in another gas-holder.
|Helium purification |
An investigation of the relative degrees of absorption of helium, neon, hydrogen, and nitrogen by cocoa-nut charcoal
|Nitrogen at -182.5° C||Hydrogen at -195.5° C||Neon at -195.5° C||Helium at -195.5° C|
at low temperatures has been made by Claude. The results are given in the preceding table, where the columns A give the volume in c.c. of gas absorbed by 100 gm. of charcoal, while the columns B give the corresponding gas pressure in mm. of mercury.
Leduc states that the absorption of helium in charcoal follows Henry's law, and in this respect, therefore, it differs from other gases.
From a consideration of Claude's figures it will be evident that while the method can give a sharp separation of helium from hydrogen and nitrogen and is, consequently, excellent for the purification of helium from minerals, it can only separate helium and neon if used as a method of fractionation.
Small amounts of helium in a vacuum tube may be purified by taking advantage of the fact that the finely divided platinum deposited upon the walls of the tube by the prolonged passage of a discharge can absorb helium in considerable amount. Nitrogen, argon, etc., remain unabsorbed, and may be pumped out of the tube, while the helium can then be driven out of the platinum deposit by heating the tube with a free flame.
Jaquerod and Perrot noticed that at a temperature of 1100° C. fused quartz is permeable to helium and hydrogen, but not to other gases, and upon this fact they based a method for the purification of helium. Fig. is a diagram of their apparatus. The quartz bulb B is enclosed within a wide platinum tube A into which the impure helium, mixed with 5 per cent, of oxygen, is introduced at a pressure slightly over 1 atmosphere. The interior of the bulb having been evacuated, the central part of the platinum tube is heated to the proper temperature, and the helium which diffuses into the bulb is pumped away into a reservoir. The method is slow, but is stated to give a very pure product. Apparently the success of the experiment depends on the selection of a suitable sample of quartz for the bulb, as Watson attempted to use the method for the purification of helium, but found that the quartz of his apparatus would not allow the passage of more than traces of helium at the temperatures employed.
|Apparatus for the purification of helium |