The principle involved is the oxidation of methyl alcohol vapour mixed with air, by means of heated "contact" substances. On passing the resulting mixture of vapours through a suitable rectifying apparatus, the water and formaldehyde are condensed, whilst unchanged methyl alcohol vapour passes on, to be itself condensed and used again.

A useful preliminary study of the process will be found in a series of papers by Orloff,2 a short account of which may be given here. A measured quantity of air was passed through methyl alcohol of various strengths and at various temperatures, and the mixture of alcohol vapour and air was led over the various heated catalytic agents. These were copper turnings, fireclay, and coke impregnated with finely-divided copper, asbestos impregnated with freshly-reduced copper, ceric sulphate, thorium oxide, " molecular ' platinum, and metallic platinum. Of these, the coppered coke at 380-420°, and the metallic platinum at 330-400°, gave the best yields of formaldehyde. With the former 39.8 per cent., and with the latter 41 per cent. of the methyl alcohol could be converted into formaldehyde. The oxidation of methyl alcohol is exothermic, and under suitable conditions will proceed without the aid of external heat when once started. In other experiments, a current of air at a velocity of 2 5 litres per second was passed through methyl alcohol warmed to 53.5-55°. The air and vapour issuing were led over a layer of reduced copper gauze, 10 to 15 cm. long, in a tube of hard glass contained in a jacket of iron and asbestos, which at the beginning was heated to about 300°. No further heating was required when the reaction was once started. Under these conditions, about 49 per cent. of the methyl alcohol was converted into formaldehyde. After condensing this, the gases remaining contained up to 20 per cent. of hydrogen, together with carbon dioxide, carbon monoxide, oxygen, and nitrogen. Automatic igniters were devised, to do away with the need for heating the tube containing the catalyser at the beginning of the process. These igniters consisted of pumice impregnated with a solution of chloro-platinic acid or ammonium palladium chloride, and heated to redness, whereby finely-divided platinum or palladium is left in the pores of the pumice. In contact with this material the mixture of methyl alcohol vapour and air, previously heated to 100°, ignites, and itself heats up the copper gauze catalyser to incandescence.

1 J. Amer, Chem. Soc., 1885, 7, 42.

2 Abstracted in J. Soc. Chem. Ind., 1908, 27, 139, 419, and 1170.

As applied industrially, one form of this process is as follows. Compressed air passes at constant pressure to a " carburetting " vessel in which it is heated, and where it meets a fine rain of methyl alcohol, so that a warm mixture of air and alcohol vapour emerges. This mixture is led into the oxidising vessel containing the catalyst. Copper gauze is usually employed, sometimes coated with silver. In Calvert's patent1 the catalysts may be copper wool, coppered asbestos, or silvered or platinised pumice, arranged between partitions of copper gauze. The methyl alcohol in this process (Calvert's) is passed with hot air on to a hot rotary fan, and the air thus "carburetted" is then led into the reaction-chamber containing the catalysts. Orloff's oxidiser consists of a large number of glass tubes containing copper gauze, each glass tube being placed in an external copper tube.

The mixed vapours issuing from the oxidising vessel pass into a rectifying or washing tower, where the water and formaldehyde are separated from the unchanged methyl alcohol, hydrogen, and other gases, and collected as a strong solution (38 to 39 per cent.) of formaldehyde, constituting the commercial "formalin." Meanwhile, the remaining gases and vapours pass to a condenser, where most of the methyl alcohol is recovered. The small quantity still remaining is washed out of the issuing gases by passing them through water, and the solution distilled to obtain the methyl alcohol.

1 No. 814 of 1915.

Fairly pure methyl alcohol of 90 per cent. strength, and containing not more than 1 per cent. of acetone, is used for this process.

According to Birsten and others,1 working with a vacuum distillation apparatus, even dilute solutions of methyl alcohol (16 per cent.) will with a low vacuum (690 mm.) give off vapours to an air current sufficiently rich in methyl alcohol to be used for producing formaldehyde.