844 SODA.— Utilization

Besides, the very great complication and largenumber of operations, causing a correspondingamount of manual labour and skilled supervision,would seem to make the process not easily workablein this country. So far as the recovery of manganeseis concerned, it has been superseded by Weldon’sprocess even at Dieuze itself.

Schqff tier's Process .—(Used since 1862 on a largescale, patented 1865).—The tank waste is allowed toweather in heaps for several weeks, till its colour hasbecome greenish-yellow; the oxidation going on in themeantime is witnessed to by a considerable develop-ment of heat. The mass is broken up, and left againfor twenty-four hours, and then lixiviated; theyellow liquor resulting contains polysulphides andhyposulphites. The residue is again oxidised, butthis time in the tanks themselves in which it hasbeen lixiviated, and which are connected and fittedup like the ordinary (Shanks’) soda tanks. This isdone by blowing the warm gas from a neighbouringchimney underneath the false bottoms of the tanks,and thus through the mass, by means of a fan blast.Six to eight hours suffice for this; the mass is thenlixiviated, oxidised again, and lixiviated a third time.The mixture of liquors ought to contain a suitableproportion between polysulphide and hyposulphide,similar to what Mond prescribes (see below). Thespecial peculiarity of Sciiaffner’s process, and thatwhich seems to be its greatest recommendation, isthe ingenious way in which he treats the mixtureof yellow liquors. It is based upon the following-facts :—In a liquor containing at the same timehyposulphites and polysuiphides, hydrochloric acid,in the first instance, decomposes the polysuiphides,and that with precipitation of sulphur and evolutionof sulphuretted hydrogen. The hyposulphites insuch liquor are decomposed later on by hydrochloricacid, yielding gaseous sulphurous acid, and alsoprecipitated sulphur. The polysuiphides in suchliquors, if sulphurous acid is passed through them,are converted into hyposulphites, again with pre-cipitation of sulphur.

These reactions are practically carried out in apair of vessels made of stone or (preferably) of castiron, furnished with steam pipes, trial cocks, gas-delivery pipes, &c., which work in this way, thatin each of them, turn by turn, sulphurous acid isevolved and conveyed into the other one. Onlyonce— i.e., in the very beginning of the work—is sul-phuretted hydrogen evolved. When both vesselshave been filled with yellow liquor, hydrochloricacid is run into one of them up to the point at which allpolysuiphides are decomposed. These are convertedinto chlorides, sulphur is precipitated, and sul-phuretted hydrogen is given off, which escapesthrough the second vessel into the open air, for thisone time. Now everything is ready for constantwork. Henceforth the acid only acts upon hypo-sulphites ; it only evolves sulphurous acid, whichis always used for converting the polysuiphides ofthe next following portion of liquor into hyposul-phite, always with precipitation of sulphur. Thishappens at once by running more hydrochloric acid

of Black-ash Waste.

into the first vessel. When no more sulphurousacid is evolved from it, steam is blown into theliquid to drive off that which is absorbed in it, andthe whole, liquid and sulphur, are let out by openinga screw valve at the bottom. The vessel is re-charged with yellow liquor, and now hydrochloricacid is run into the second vessel, whose contentsare now all converted into hyposulphite, so thatonly sulphurous acid is evolved, which is carriedback into the first vessel, and thus the play of theapparatus continues constantly, turn by turn foreach vessel. The decomposed liquid containsessentially calcium chloride, a little sodium chloride,calcium sulphate (partly from the oxidation, partlyfrom the sulphuric acid contained in the hydrochloricacid employed, partly from secondary decomposi-tions), along with the precipitated sulphur. Themixture is run on to filters, and the sulphur washedout to a certain extent. The way in which it isfurther purified and converted into a saleable stateis another important feature of Schaffner’s process.He mixes the sulphur with sufficient water to forma thin paste and some milk of lime, and runs it intoa peculiarly-constructed melting apparatus, whoseconstruction will be made clear by the subjoinedwoodcut (Fig. 84). Two cylinders are firmly joinedtogether—an outer one of wrought iron, pp, and

■yy/tty/X.

w/z/A <m;.

sna

an inner one of cast iron, q q. The ends are commonto both, and a mechanical stirring apparatus, r, passesthrough them, through stuffing boxes, s s. The wholeis mounted in an inclined position. The paste of sul-phur, water, and a little lime (along with impurities)is charged through the man-hole, m, and the corre-sponding opening, o, in the inner cylinder, into thelatter. When the man-hole has been closed again,steam of If atmospheres’ pressure is blown in throughthe pipe, b. It first circulates round the innercylinder, then enters the latter through o, is broughtinto intimate contact with the mass in the same bymeans of the stirrer, and ultimately finds its wayinto the dome, d, which is provided with a valve, a,an exit pipe, h. and a safety valve, x. The sulphuris melted by the heat of the steam, and when thestirrer is stopped, it gathers in the lowest part ofthe inner cylinder ; whilst the specifically higherliquor, containing the calcium sulphate mechanically