SODA.—Utilization

suspended, floats on the top. The sulphur is runoff through the opening, l, and cast into moulds,whilst the liquor runs to waste. The object of theaddition of lime is both to saturate any excess ofacid and to form a little calcium sulphide, whichcombines with any arsenic (from the raw materials)to calcium sulpharseniate, soluble in water, andtherefore removed along with the supernatant liquor.It will be observed that the contents of the innervessel cannot be forced back into the steam pipe,as would happen if the latter entered direct intothe mass; and, on the other hand, the agitationby the stirrer causes a much more direct heatingand action on the sulphur than a mere jacketingwith steam could produce.

Schaffner’s method is practised in many workson the Continent with perfect success, more par-ticularly since he has adopted (from Mond’s process)the oxidation in the tanks themselves for the secondand third time ; but it must be stated that his modeof decomposing the yellow liquors, ingenious as itis, has been objected to as yielding too little sulphurand too much calcium sulphate. This will be spokenof further on.

Mond’s Process .—(Discovered about 1860, patented1868). The first experiments were suggested byGuckelberger, who, however, did not pursue it toa practical issue, so that Mond must be fully creditedwith the merit of having made the process a success.Mond tried from the first to hasten the oxidation ofthe waste by an artificial current of air, and this isthe special feature' of his process. At first he ex-posed it to the current of air on shelves; but thishad to be abandoned on account of the large amountof time, and space, and labour required. He thenfound that the waste obtained by the method oflixiviation (Shanks’) now universal in this countrycan be oxidised in a much simpler manner, viz., byblowing air direct under the false bottom of thelixiviating tanks. The number of the latter must betrebled, and each of them is, apart from its otherfittings, provided with a 3 - inch branch from an8-inch air main, in which a Schiele’s 30-inch fanblast maintains a constant pressure of 3 or 4 inches(water). As soon as the last soda liquor has beenrun off any of the tanks, the blast is put on by takingout a small damper in the air-pipe branch. This is,however, not done by haphazard; even the indica-tion of the ordinary air-gauge in the main is notsufficient, as with equal pressure in the main theeffect on the waste might vary very much accordingto its density, and consequently its permeability forair. Mond tries, therefore, by means of a differen-tial ether anemometer, what real quantity of airpasses through the waste, and this is done for everytank. The operator very soon knows for each sizeof tank how many degrees of the anemometer arerequired to oxidise the waste, so that on subsequentlixiviation it may yield a liquid of the proper composi-tion. It is found in practice that the false bottom isquite sufficient to cause an equal distribution of theair through the waste, even if (as necessary for con-veniently cleaning out the tanks) the air-pipe is fixed

of Black-asii Waste. 845

in one of the corners. The waste soon begins toheat, rising up to 200° Fahr.; it gives off quantitiesof steam, and becomes gradually covered by spotsof a bright yellow colour, which, together with thetemperature and the quantity of steam passing off,enable the workman to judge very well when he isto stop blowing. Usually twelve to sixteen hoursare required for this. During the blowing mostly nosulphuretted hydrogen whatever is given off; butoccasionally this is the case to a very slight extent.The oxidised waste is then lixiviated, first with weakliquor from a previous operation, of about 6° to 8°Tw.; what runs off is strong liquor of 18° Tw., andis run as such to the decomposing tubs till it hasgone down to 10° Tw. Water is now run on the top,and what runs off at the bottom is collected in acistern as weak liquor, to be used in the next opera-tion ; this is continued till the strength of the run-ning-off liquor is only 2° or 3° Tw. The water isnow stopped, and the first lixiviation thus at an end;it takes from eight to nine hours. Now the air-blastis put on for the second time, followed by a secondlixiviation, a third blowing, and a third lixiviation.Where there is not sufficient room the waste is onlyblown twice, but this is not advisable. It is, indeed,best to blow it four or even five times, in order tomake the ultimate residue more harmless; but thisis only practicable where the operation is not carriedout in the soda tanks, but in tanks specially con-structed for the purpose. After the waste has beencast out of the tanks, they must be cleaned andwashed out most carefully, in order to remove everyparticle of sulphur liquor, which would of course bemost injurious in the next lixiviation of black-ashballs, now immediately following. For the samepurpose it is advisable to put a false bottom in thetank, made of bricks and cement, sloping towardsthe run-off cock, so that no liquor can lodge anywhereunderneath the perforated bottom. It cannot bedenied that the work with Mond’s tanks, in com-bination with the ordinary ball lixiviation, is some-what complicated ; that by the least carelessness ofthe workmen the soda liquors are liable to becomecontaminated with more or less sulphur liquor; andthat the whole arrangement of trebling the tanksand providing them with a large number of addi-tional fittings is very cumbersome. It would there-fore seem advisable not to interfere with the ordinarysoda tanks at all, but to cast the waste from them inthe usual manner, and subject it to the blowing,&c., in a separate set of tanks altogether; and insome factories this is now usually done.

Mond’s great merit is to have drawn specialattention to the necessity of directing the oxidationmethodically, so as to obtain liquors of a specialcomposition ; of having devised the means for doingso; and of having prescribed easy and expeditiousplans for testing the liquors for the same object.It is very well known that in the course, of theoxidation, be it the slow one by spontaneous actionof the air, or the quick one by Mond’s blast, whichacts in as many hours as the other takes days orweeks, in the first instance principally polysulphides