Hydrogen is one of the elements and is the lightest substance known. It is obtained by the decomposition of water into oxygen and hydrogen, both gases being collected and used. Hydrogen is also prepared by passing steam over coke heated to a dull red. If the temperature is not too high, carbon dioxide and hydrogen will be formed (C+2H20 = 2H2+C02) but the carbon dioxide may be removed by passing the gas through a vessel of slaked lime. Hydrogen may be liquefied and, when mixed with air or oxygen, is explosive. It is not poisonous but may cause death if inhaled because it will exclude oxygen from the lungs. When hydrogen and oxygen are mixed to form a gas in welding and cutting, they produce a temperature of about 2500 degrees centigrade.
Oxygen is the most important of all of the elements and is used as one of the gases in nearly all welding processes. It was discovered in 1774 by Priestley and Scheele, both working independently, and in 1789 Lavoisier proved that its presence was necessary for combustion in the air. Oxygen was liquefied in 1877 by Pictet at a pressure of 320 atmospheres.
Oxygen is produced commercially by three methods: from the air by liquefaction and fractional distillation; from water by electrolytic action; and from potassium chlorate. The production of oxygen from air by liquefaction is by far the greatest source of this gas today for welding purposes, though electrolytic apparatus has recently been developed which is making a strong competitor where power is cheap. The oxygen used for welding must be free from chlorine, although the usual mixture of 5 per cent of nitrogen and from 2 to 3 per cent of hydrogen is no disadvantage. Its production is not a very complicated process, but the apparatus is rather expensive, and only those plants requiring 1000 feet or more per week can afford to make their own gas.
The principal process for producing oxygen from the air is that developed by Linde and consists in liquefying the air and separating the nitrogen and oxygen by fractional distillation, similar to rectifying spirits. The air is first compressed to 1800 pounds per square-inch pressure, and cooled by ice and salt, or ammonia. When the air is compressed, as stated, its temperature rises because of the compression and must be cooled before the compression is continued. The air is, therefore, allowed to expand, thereby cooling itself; then it flows back over the pipes containing the oncoming air, thus cooling the whole body of air. This cool air is again compressed and expanded, growing colder with each expansion, until it is sufficiently near 350 degrees Fahrenheit so that the final expansion to atmospheric pressure liquefies it. Liquid air is 80 per cent nitrogen and 20 per cent oxygen, and commercial oxygen is 95 per cent pure. The balance is nitrogen and is not harmful for welding purposes.
Oxygen is sold in tanks containing 5, 25, 50, or 100 cubic feet, as desired, and the tanks themselves may be either bought or rented. They can be recharged when empty, and each tank is equipped with a reducing valve to regulate the pressure when using. A pressure gage must also be used, when using the oxygen, and leakage must be looked out for because of the high pressure.
The production of oxygen by the electrolytic decomposition of water is the method used most in Europe and gives two volumes of hydrogen to one of oxygen. The process consists in passing an electric current at a pressure of 2 or 3 volts through an electrolyte or solution of sodium or of potassium hydroxide. Direct current is used and oxygen rises from the water around the positive terminal plate and hydrogen from around the negative plate, each gas being conducted through separate pipes to compressors for storage. The tank, in which the electrolytic action takes place, is usually of cast iron and is divided into two sections by a partition running part way down. One terminal is placed in each section, and the temperature is maintained at about 165 degrees Fahrenheit because the action requires a lower voltage at this temperature than at any other. From 240 to 325 amperes are used and the gases are about 99 per cent pure. Purity is important because foreign elements may burn into the metal when welding.
The apparatus for making oxygen from potassium chlorate is comparatively simple and low priced and is especially suitable for use in out-of-the-way plants doing welding. The process is based on the fact that, when potassium chlorate is heated, it produces a somewhat large percentage of oxygen of 97 to 98 per cent purity. In order to prevent the chlorate from melting and flashing under heat, about 13 parts of manganese dioxide are mixed with each 100 parts of the potassium chlorate, and the gas is given three scrubbings before storing. The mixture is first packed tightly into a retort, heated slowly with a gas flame, and the oxygen is carried through three washer tanks, filled with a solution of sodium hydroxide, and then into a gasometer. From here it is compressed into steel cylinders at 300 pounds per square inch for service. The makers of this apparatus claim that there is but little oxygen lost through leakage because of the low pressures used; that but little oxygen is lost in recharging the retort for the same reason; and that the gas is thoroughly washed in the scrubbers because the bubbles are so large. One pound of chemicals costing about 8 cents will produce about 4½ cubic feet of oxygen at a total cost for everything of about 2¼ cents per cubic foot of gas.
Pintsch gas was originally developed for lighting purposes and is used for lighting steam railway cars. It is an oil gas made from crude petroleum or similar oils and will safely stand a high degree of compression, it being used at various pressures for different purposes. Works for the supply of the gas are now established in nearly all of the large cities in the United States, Canada, and Mexico and gas can be obtained in pressures up to 100 atmospheres (1500 pounds pressure per square inch). It can also be obtained in flasks at 12 atmospheres (180 pounds pressure), but it is used at about 25 pounds pressure for cutting and welding. On account of its high heating value and its stability, or resistance to pre-ignition, it is coming into use for high temperature work in conjunction with oxygen and bids fair to become a serious competitor of acetylene. Water Gas. Water gas is a mixture of carbon monoxide and hydrogen and is formed by passing steam over or through incandescent coke, thus causing the steam to decompose into oxygen and hydrogen. The oxygen combines with carbon from the coke and forms carbon monoxide, with a little carbon dioxide, and a slight impurity in the form of hydrogen sulphide from the sulphur in the coke. The impurity can be removed with lime or iron oxide, as when making coal gas. Thirty-five pounds of coke are used for each 1000 cubic feet of gas, on an average, and the composition of the water gas is approximately as follows:
Hydrocarbons and vapors ............................
Carbonic oxides .........................................
The apparatus used for generating water gas is comparatively simple and consists mainly of a generator and a superheater, with connections for taking off the gas and for the supply of air and steam. Water gas gives an extremely high temperature when burned and is used a great deal in Europe for heating metals preparatory to welding by hammering as well as for fusing, as in some other processes. Owing to its lack of odor when pure, it is dangerous if it escapes.