Patent GB-428375-A (GB)
Title: Improvements in or relating to the preparation of chromic acid and sparingly soluble chromates
IPC: C01G37/14
Filed: 1933-11-09
Abstract: 428,375. Electrolytic production of chromic acid and chromates. PEARSON, R. E., 267, High Holborn, London, and GILBERT, W. V., 336, Abbey House, Westminster. Nov. 9, 1933, Nos. 31289/33 and 12075/34. [Class 41] A solution of an alkali metal chromate is electrolyzed without the addition of salts of other acids using a soluble anode, the electrolysis being continued until chromic acid is produced either in the free state or in combination with the metal of the anode. In an example, a 50 per cent sodium bichromate solution is electrolyzed with a chromium anode and a stainless steel cathode both enclosed in porous pots. Chromic acid is formed in the anode pot and sodium chromate in the cathode pot. In a modification a solution of lime in chromic acid is filled into the cathode pot ; chromic acid is formed in the anode pot and a precipitate of calcium chromate in the cathode pot. In another example a zinc anode is employed, a solution of zinc chromate in chromic acid being formed in the anode pot. The anode solution is then transferred to the cathode pot and the anode pot filled with fresh bichromate solution ; zinc chromate is precipitated in the cathode pot and a further solution of zinc chromate is formed in the anode pot. A similar result may be obtained by using an anode and cathode of the same anodically soluble metal and periodically reversing the current. Alternatively the chromate may be precipitated from the solution obtained at the anode by making the solution neutral or alkaline. In the electrolysis of sodium bichromate solution with a zinc anode without a diaphragm, zinc chromate is precipitated by reaction of the anode and cathode products. Basic chromates of varying constitution may be separated as electrolysis proceeds. Alkaline earth metal chromates or lead chromate may be obtained by suspending or dissolving the oxide, hydroxide or carbonate of the metal in a sodium bichromate solution in the anode pot using an anode of chromium or ferrochrome. In applications in which a porous pot is not employed, some restriction of diffusion, e.g. by means of a loosely woven asbestos cloth, is desirable, and the anode product should be separated before substantial diffusion has taken place. The solution of chromic acid obtained by anodic solution of chromium or ferrochrome in sodium bichromate solution without a diaphragm may be further treated in a diaphragm cell employing an anode of pure chromium or an inert anode of carbon or lead. Chromic acid may be separated from the anode product by crystallization. Alkaline sodium chromate solution obtained at the cathode in any of the processes according to the invention may be reconverted to sodium bichromate by auxiliary electrolysis with an anode of low grade (high carbon) ferrochrome and without a diaphragm ; the anode may be in the form of lumps or granules supported on a lead plate. Iron is precipitated as hydrated oxide and chromate. According to the first Provisional Specification, a solution of sodium bichromate may be electrolyzed without a diaphragm with a chromium anode arranged at the bottom of the vessel and the cathode at the top, the chromic acid and caustic soda formed at the anode and cathode respectively being maintained separate by gravity.