It is a recognized fact that chemical bodies in a nascent state are characterized by peculiarly energetic affinities, and the results of numerous experiments permit us to affirm that animal and vegetable fibers are rapidly bleached when they are placed in contact with oxides and chlorides which, when submitted to electrolysis, permit oxygen and chlorine to disengage themselves in the nascent state.
The coloring matter that impregnates the majority of vegetable textile substances, such as cotton, flax, and hemp, to cite only those most generally known, is in fact completely destroyed only by the combined action of oxygen and chlorine, which always act in the same manner, whether the fibers be in a raw or woven state.
In the application of electrolysis to the bleaching of textile materials, it is only necessary to have the electrodes of any sufficiently powerful generator of electricity end in a vessel containing in aqueous solution such decolorizing agents as the hypochlorites in general, and chlorides, bromides, and iodides that are capable of disengaging chlorine, and iodine or an iodide in a nascent state. These gases perform the role of oxidizing or decolorizing agents.
The fibers that are immersed in the solution during the passage of the electric current must necessarily remain therein for a greater or less length of time, according to the nature of the material to be bleached, and must, after this first operation, be washed, rinsed, and dried.
The use of an electric current for decomposing the metallic chlorides and disengaging their elements is not new, and there have been specially utilized for this purpose, up to the present time, the alkaline hypochlorites that are obtained by well known processes.
In the latter case the metal is brought to the state of oxide in presence of the water that is necessary for the reaction. But the results obtained in practicing this method are deceiving, as far as bleaching is concerned, and it is evidently more rational and economical to endeavor to compound the hypochlorite directly by borrowing all its elements from the metallic chloride itself, and from the water by means of which such transformation is to be effected. This is a reversal of the problem, and, à propos thereof, we would call the attention of the reader to an apparatus invented by Messrs. Naudin & Schneider for effecting such synthesis in a simple and practical manner.
If a solution of chloride of sodium or kitchen salt, NaCl, be submitted to electrolysis in a hermetically closed vessel containing the material to be bleached, a formation of hypochlorite of soda is produced in the following way:
2NaCl + 2 HO = NaCl + NaO, ClO + 4H.
In operating in this manner we shall have the advantage that results from the nascent body through the electrical double decomposition of the chloride of sodium and water, which puts the chlorine, the metal, the hydrogen, and the oxygen simultaneously in presence. The chlorine and oxygen will combine their action to decolorize the textile material.
While starting from this idea, it will nevertheless be preferable to adopt Naudin & Schneider's arrangement.
The apparatus consists of a hermetically closed electrolyzer, A, into the lower part of which enters the electrodes, E and F, of any electrical machine whatever. The receptacle, A, is provided with a safety-tube, T, that issues from its upper part and communicates with a reservoir, B. A second tube, D, forms a communication between the electrolyzer and the vessel, C. The liquid contained in this latter is sucked up by a pump, P, and forced to the lower part of the vessel, A, by means of the tubes, G and H.
The apparatus operates as follows:
The closed vessel, C, in which the material to be bleached is put, is filled, as is also the electrolyzer, with a solution of chloride of sodium. This solution is then submitted to the action of an electric current, when, as a consequence of the chemical decomposition of the chloride and the water, the elements in a nascent state form hypochlorite of soda. When the partial or total conversion of the liquid has been effected (this being ascertained by chlorometric tests), the pump, P, is set rapidly in operation, and, as a consequence, draws up the chloride of sodium from the bottom of the vessel, C, to the lower part of the electrolyzer, A. The hypochlorite that has formed passes through the tube, D (as a natural consequence of the elevation of the level of the liquid in A brought about by the entrance of a new supply of chloride), and distributes itself throughout the vessel, C, where it acts upon the textile material.
APPARATUS FOR BLEACHING TEXTILE FIBERS
The safety-tube, T, which is attached to the electrolyzer, permits of the escape of the hydrogen which is produced during the chemical reaction, and fixes, through an alkaline solution contained in the reservoir, B, the chloride whose escape might discommode the operator.
As may be conceived, the slow transfer of the saline solution from the receptacle, C, to the electrolyzer, and its rapid conversion into decolorizing chloride, as well as its prompt application upon the materials to be bleached, presents important advantages.
While, in the present state of the industries that make use of bleaching chlorides, the chloride of sodium is converted into hydrochloric acid, which, in order to disengage chlorine, must in its turn react upon binoxide of manganese, we shall be able, with this new method, to utilize the chloride of sodium, which is derived from ordinary salt works, and extract from it the constituent elements of the hypochlorite by a simple displacement of molecules produced under the influence of an electric current.
Another and very serious advantage of electric bleaching is that of having constantly at hand a fresh solution of hypochlorite possessing a uniform decolorizing power, which may be regulated by the always known intensity of the current.
We must remark that the hypochlorites require a certain length of time to permit the chlorine to become disengaged, and that, besides, all chlorides, bromides, and iodides that are isomorphous are capable of undergoing an analogous chemical transformation and of being employed for the same purpose. This is especially the case with the chlorides of potassium or barium, the bromides of strontium or calcium, and the iodides of aluminum or magnesium. On another hand, as sea water contains different chlorides, it results that it might serve directly as a raw material for bleaching textile fibers. Then, when the solution of chloride of sodium has been deprived of its chlorine by electrolysis, there remains a solution of caustic soda which may be utilized for scouring fibers.--H. Danzer, in Le Génie Civil.