Bleaching by electricity.

The Textile Mercury, 13.12.1890

(Continued from page 374.)

It was in 1883, after the failures of M. Naudin, that M. Hermite was led to take up the subject of the electrolysis of the chlorides. After prolonged experiments on sodium chloride, he relinquished this substance, firstly for calcium chloride, and then for magnesium chloride, the latter salt in his opinion lending itself best to transformation into a bleaching agent. In the Hermite process the "electrolyser" is of galvanised cast-iron. Along the bottom rune a pipe, pierced with numerous holes, by means of which the solution enters the vat. A channel runs along the upper edge of the "electrolyser," into which the electrolysed solution overflows and finds its way to the bleaching vats. The cathodes consist of a number of discs of zinc mounted on two spindles, which slowly rotate. Between each pair of discs is an anode plate, consisting of platinum gauze fixed in an ebonite frame and communicating by a leader lug with a copper bar. In order to keep the zinc discs perfectly clean, flexible ebonite scrapers are fixed to the anodes, and as the zinc discs revolve these scrapers brush their surfaces clear of any deposit. In carrying out operations on a large scale several "electrolveers" are connected in series. Generally speaking a current of from 1,000 to 1,200 amperes at a pressure of five volts is employed.

From the point of view of economy the anode should he a good conductor, cheap, and unattacked by the products of electrolysis. Carbon seems therefore to be the most suitable substance for this purpose, and in many processes it has been and is employed. M. Hermite, however, after lengthy experiments failed to obtain a grade of carbon proving sufficiently resistant to the action of electrolysed chlorides, finding that carbon oxidised and disintegrated. The classical researches of Bartoli and Pasogli in 1882 showed that whenever oxygen wax liberated at the anode, carbon is attacked, and we have as a result carbonic oxide, carbonic acid. and a black substance, "mellogen," etc: and also in the case of graphite, graphitic acid. According to the same investigators, when chloride solutions are electrolyse the disintegration of the retort carbon employed is the more rapid the more dilute the solutions. Since, therefore, in the Hermite process the magnesium chloride solution is used very weak, platinum anodes become absolutely necessary, and this renders the "electrolysers" costly, and tends to prevent the general introduction of the process, especially on a small scale. M. Hermite has endeavoured to make use of platinised copper and porcelain, but the platinum was rapidly attacked by the electrolysed solution. Up to the present Hermite has employed a 5 per cent. solution of magnesium chloride, but he now proposes to use a more economical bath. After having tried with some success a solution of Starsfurt "Carnallite," which is a double chloride of magnesiumand potassium, he now makes use of a solution containing z; per cent. of magnesium chloride and 5 per cent. of sea salt, a small quantity of recently precipitated magnesia being added to the bath. When this solution is electrolysed only the water and the magnesium chloride appear to be decomposed; the solution remains perfectly clear, and if the sodium chloride were electrolysed, soda would be formed and magnesia precipitated, the liquid being clouded.

(To be continued.)