BLEACHING. BY HARRY ATWATER SMITH, 1887, SOMERVILLE. Bleaching is the chemical art by which textile filaments, &c., used for clothing, are deprived of their natural dark color and rendered nearly or altogether white. The operations which the bleacher has recourse to, differ according to the nature of the bleaching agents and the properties of the stuff to be bleached; and the results are also obtained with more or less rapidity, certainty, economy and perfection. The destruction of the coloring matters attached to the bodies to be bleached is effected in three ways either by the action of air and light, of chlorine, or sulphurous acid. The oldest method of bleaching, which is still used for particular kinds of goods, consists in extending the fabrics on the grass of a meadow so as to expose them to the united action of light, air and water, and then washing them in an alkaline lye. This method of bleaching is effective, but involves a great amount of labor. About 1785, Bertholet proposed the use of chlorine for bleaching vegetable tissues. At first this method met with considerable opposition, but it has been gradually overcome and has now entirely superseded the old method. Chlorine was first used in the form of an aqueous solution; afterward, solutions of chlorine in caustic alkalies, that is to say, solutions of hypochlorite of potassium or sodium. But these have been almost entirely replaced by the hypochlorite of calcium, the so-called chloride of lime or bleaching powder. This well-known body was originally considered to be a compound of chlorine and lime. In 1834, Ballard first gave an explanation of this compound, and his view has been generally adopted. He considers bleaching powder a mixture of calcium hypochlorite and calcium chloride. Ca (OCI)2+Ca Cl2. Another view of the constitution of bleaching powder is taken by Odling. He looks upon this substance as a kind of double salt, Ca{CCI, being at the same time a chloride and a hypochlorite. Chloride of lime is obtained by the action of chlorine gas upon dry-slaked lime. A different action takes place if chlorine is passed through milk of lime: 2 2 Ca(OH)2+2 Cl=Ca Cl2+Ca (O Cl)2 + 2 H2O. If, however, dry-slaked lime is used, a large proportion of the lime remains unaltered, it being only possible to obtain a material containing about 40 per cent. of available chlorine. Hence, the substance would appear to be a mixture of basic salt with chloride of calcium, according to the formula: ОН JOH 3 Ca (OH)2 +2 Cl2=2 Ca OC+Ca Cl2+2 H2O. If water is added to this product the soluble chloride dissolves and the basic hypochlorite decomposes as follows: Bleaching powder is obtained on a large scale in the alkali works as a means of employing the residual hydrochloric acid. For the purpose of evolving the chlorine required for the manufacture, large stills are used. The material now usually employed for the generation of chlorine from hydrochloric acid, is the manganese dioxide, obtained by precipitation in what is known as Weldon's process. The evolved chlorine gas is passed into large chambers, the bottoms of which are covered with dry-slaked lime to a depth of about five inches. The whole is ridged with a rake made for the purpose, so as to enable more surface to be exposed to the action of the chlorine. In these chambers the absorption of chlorine takes about six days. After the lime has been placed upon the floor of the chamber, chlorine is turned on until the chamber is full; the air and any surplus chlorine escaping from the chamber, passes into the next chamber, where there is fresh lime. The air which is displaced from this chamber escapes into the atmosphere, or a third chamber. When the first chamber has been filled with chlorine it is allowed to stand two days, when the gas is nearly absorbed and the lime is found to contain from 25 to 30 per cent. of available chlorine. Any unabsorbed gas is then drawn off, the doors opened and workmen enter and turn the lime. The chamber is now closed again and more chlorine is admitted, enough to raise the amount of chlorine in the lime, after absorption for a second period of two days, to from 36 to 37 per cent. The unabsorbed gas is then drawn off as before and the powder packed and sent to market. As has been seen, bleaching powder is soluble in water, and the solution is used for steeping the material to be bleached. By itself it exerts no bleaching action; but, by exposing the fabrics steeped in it to the action of carbonic acid in air, or by steeping them in a bath of dilute hydrochloric or sulphuric acid, the salt is decomposed and the liberated hypochlorous acid or chlorine exerts its bleaching action. Bleaching powder is largely employed in the bleaching of cotton goods and paper pulp. The operations of bleaching cotton cloth are as follows: 1. Boiling or "bucking" the cloth with milk of lime, 14 lbs. of cloth requiring about 1 lb. of lime, water being added to this in sufficient quantity to cover the cloth. In this process the resinous matters in the fibre are converted into lime soaps. 2. Washing to remove lime and soluble salts. 3. Souring in dilute hydrochloric or sulphuric acid, of specific gravity 1.0075, to decompose lime soaps. 4. Washing to remove acid. 5. Bucking the cloth in dilute caustic soda lye (170 lbs. of soda ash and 80 lbs. of resin soap to 3,500 lbs. of cloth), to remove fatty matters and dirt. 6. Washing and subsequent immersion in a clear dilute solution of bleaching powder of specific gravity 1.0025. This is called "chemicking." 7. Souring in very dilute hydrochloric or sulphuric acid, of specific gravity 1.0075, then washing and drying. The decomposition which takes place when the acid is brought in contact with the bleaching powder is as follows: 2 1. Ca Cl2+Ca (O Cl)2 +4 H Cl=2 Ca Cl2 +2 H2O+2 Cl2. 2. Ca Cl2+Ca (OCl)2+2 H2SO1=2 Ca SO1+2 H2O+2Cl2. 2 Wool and silk are, for the most part, bleached with sulphurous acid. Chlorine is found to exert an injurious action on the fibre. Woolen goods or yarns are bleached by treating them with very mild alkaline liquors, which remove the fatty matters; lant and soap, with soda crystals, being the substances usually employed. Sulphurous acid gas-or, as it is termed in the trade, vapor of burning brimstone-is used to finish wool, giving it whiteness and lustre. The following are the operations for the bleaching of woolen goods, for 40 pieces each 50 yards long : 1. Passed three times through a solution of 25 lbs. of carbonate of soda and 7 lbs. of soap, at a temperature of 100° F.; add lb. soap after every four pieces. 2. Wash twice in warm water. 3. Passed three times through a solution of 25 lbs. of carbonate of soda at 120° F., and add lb. of soap again after every four pieces. 4. Sulphured in a room for twelve hours, using 25 lbs. of sulphur for 40 pieces. 5. Passed three times through a solution of soda as in No. 3. 6. Sulphured again as in No. 4. 7. Soda liquor again as in No. 3. 8. Washed twice through warm water. 9. Sulphured a third time as in No. 4. 10. Washed twice in warm water and then in cold water. 11. Blued with extract of indigo (indigo-carmine) according to taste. The operation of bleaching silk is always preceded by removing the gummy substance attached to and externally covering the fibre. This is effected by boiling the raw silk in soap and water. For the purpose of bleaching silk, nothing but water, soap and sulphur (for making SO) are used. Alkalies are rarely used; they are injurious to the fibre. The process is terminated by passing in an extremely diluted sour (solution of sulphuric acid in water), so weak as to be scarcely acid to the taste. Sulphuring is only used for silks intended to be left white or to be dyed or printed with bright and light colors. This operation requires great care and should be seldom resorted to. Coming back to the bleaching of cotton goods again. We found that there were seven main operations in the bleaching of cotton goods; but in all there are some sixteen distinct operations, occupying usually from five to eight days. This has been shortened down to hours by an ingenious process invented by Mr. Thompson. The principal feature of his process consists in the peculiar and original method of chemicking and souring, these operations being performed at the same time. The bleaching liquid used is a very dilute solution of chloride of lime, of about specific gravity 1.0006. The decomposing or souring agent is carbonic anhydride, commonly known as carbonic acid gas. Another original feature in this process is the use of a solution of triethylrosaniline and oxalic acid, through which the goods are passed to discharge the faint natural yellow tinge of the cotton. In carrying out the Thompson process, the goods are first boiled in an alkaline solution and washed. They are then placed in an airtight keir, connected on the one hand with a vessel containing the bleaching solution, and on the other with a gas-holder containing the carbonic acid gas. After the goods have been placed in the bleaching keir, precisely as is done in the chemicking vat in the ordinary process, the keir is made air-tight. An air tap in the cover is then opened, and the bleaching solution forced into the keir until quite full, thus expelling the air. Then the air-tap is closed and the carbonic acid gas connection opened. After the bleaching solution has remained long enough in the keir in order to thoroughly soak the material, which is about five minutes, the liquor is drawn off into the vessel containing the bleaching solution. The gas connection being opened, the carbonic acid gas, under pressure of water in the holder, follows the retreating liquor. The gas remains in contact with the moistened material forty-five minutes, by which time it will have decomposed the whole of the chloride of lime in the bleaching solution on the fibre of the cloth. The bleaching liquor is again run in upon the material in the keir, forcing the carbonic acid gas in the holder again, but returns when the liquor is run out. This alternating is repeated till the material is of sufficient whiteness. For a medium grade of cloth, that has been boiled three hours, will take about eight hours to bleach. When the bleaching in the keir is finished the material is thoroughly washed and slowly passed through the solution of triethylrosaniline and again washed, when the process is complete. The total time occupied ranges from eight to twelve hours, according to the nature of the fabric. From these results we see that carbonic acid gas is a valuable bleaching medium, inasmuch as it is just strong enough to break up hypochlorites forming free hypochlorous acid, and not free chlorine, as is the case when hydrochloric acid is used. Now, if we can perform the bleaching operation by making a solution containing free hypochlorous acid, in which to immerse the materials to be bleached, we will have a much better and shorter method. Experimenting, I found such a liquor by precipitating a solution of bleaching powder with a solution of bicarbonate of soda. This solution contains free hypochlorous acid in an alkaline solution as represented by the following formula : Ca (O Cl)2+NaHCO3-Ca CO2+Na O Cl+HO CI. = Ca (O Cl)2+2 Na HCO3=Ca CO3+2 HO Cl+Na2 CO3. When a solution containing free hypochlorous acid is heated it breaks up into oxygen and hydrochloric acid, as follows: 2 HOCI+^=02+2 HCl. This was the case where the solution resulting from the precipitation of chlorine-water with bicarbonate of soda was heated. The evolved gas was collected over caustic alkali, and upon testing it, was found to be oxygen; thus proving, without a doubt, that the solution contained free hypochlorous acid. By precipitating chlorine-water with a solution of sodium carbonate, we obtain a solution commonly known as "Labaraque solution," a hypochlorite of soda, which ought not to contain any free hypochlorous acid, as by the following formula: Ca (OCI)2+Na2CO3=Ca CO2+2 Na O Cl. Upon heating this solution, I did not obtain any oxygen over caustic alkali, proving the absence of free hypochlorous acid. I then transmitted a stream of washed carbonic acid gas through the solution, and found that it had decomposed the solution into free hypochlorous acid and sodium carbonate, which is evident from the formula: H2CO3+2 Na O Cl=2 H O Cl+Na2CO3. This is what ought to take place, as we learned from the Thompson process. It proves my point that the first solution I made contains |