It will be at once apparent from the details given above, and from the chemical reaction in the stills—Mn02 411C1 = MnC12 + 2E120 + C12, that the manganese is only an agent in decom posing the hydrochloric acid, issuing from the still, when its work is done, in the form of chloride of manganese, mixed with chloride of iron and free hydrochloric acid, and running off to waste. In addition to the expense entailed by so clumsy a process, the immense volumes of " still liquor," when run from the various works, forms a serious item in river pollution, and a nuisance to the surrounding neighbourhood. Of the processes of " regeneration " from tirne to time devised and carried out, those of Dunlop and Weldon alone deserve special mention. The former was first worked about 1855 at Messrs. Tennant's, St. Rollox Works; it consists in treating the still liquor with carbonate of lime, and decomposing the carbonate of manganese thus obtained by the simple action of heat. The liquors are first neutralized and allowed to settle, clear solution of chloride of manganese remaining supernatant. This is carefully drawn off, and run into a large boiler fitted with a shaft and agitator. Here it is mixed with ground chalk, or milk of chalk ; steam is introduced under a pressure of about three atmospheres, and the whole is kept in constant agitation for three or four hours. The following reaction takes place :— Mn01, Ca003 = MnCO3 CaC12.
When the operation is complete, the contents of the boiler are left to .settle, the clear super natant chloride of calcium is run off, and the precipitated carbonate of manganese is drained and washed to remove all chloride of calcium. The expulsion of the carbonic acid and the oxidation of the manganese are effected in a long oven about 50 ft. in length, 12 ft. wide, and 9 ft. high. Four lines of rails traverse the sole of the chamber ; heat is applied underneath by means of a flue passing down the centre, and returning on both sides. The carbonate of manganese is thrown loosely into sheet-iron wagons, which are slowly drawn backwards and forwards until the four lines of rails are traversed, the contents being subjected for something like forty-eight hours to a constant temperature of about 300° (572°F.). The water and carbonic acid are completely driven off by this process, and the residual manganese is converted by the action of the air—whieh is allowed to circulate through the oven pretty freely, through the loosely closed doors—into a mixture of peroxide and protoxide. Mr. Mactear reports well of this process, but it has not been worked on any large scale except at the St. Rollox Works. The expense of the plant is the chief objection. Some attempt has been made to utilize the chlorine lost in the chloride of calcium by substituting carbonate of magnesia for carbonate of lime, and obtaining hydrochloric acid from the resulting chloride of magnesium ; but this refinement of Dunlop's process has not been found practicable.
The second, and by far the most important, of the processes set on foot for the regeneration of the manganese, is that of Mr. Walter Weldon, which consists in the precipitation of the manganese as
protoxide, and in peroxidation by an injection of air in the presence of an excess of lime. This improved form of an old patent has almost revolutionized the bleach and manganese trades. The old patents that dealt with this simplest of all the methods of regeneration, failed by em ploying only an equivalent proportion of lime, whereby only one-half of the manganese can be obtained as peroxide. Weldon's improvement consists in the addition of a slight excess of lime, which enables, pra.etically, the whole of the manganese to be regenerated. The apparatus necessary for this process, some few minor details being omitted for simplicity's sake, is shown in the drawing, No. 330. A A represent hydrochloric acid cisterns, B B ordinary manganese stills, technioally called " native," but set at such an elevation that their contents can, when required, be run by gravitation (the pipe of communication is omitted in the drawing) into R R, the " mud " stills, entering about half-way up. When the process is first started, the " native " stills are charged and worked precisely in the manner already described, until a good supply of still liquor is secured. This object having been attained, they are only worked to supply the 5 per cent. waste of manganese incurred during the ordinary working of the regenerating process. The still liquors are run off along the spout V into the neutralizing well c. To obviate the inconvenience of stoppage for repairs or cleaning, and to be able to neutralize a large quantity of still liquor rapidly, it is advisable to have two wells, as shown in the drawing, the spindles being thrown in and out of gear by the clutches upon the horizontal shaft. A cheaper form of neutralizer, and one largely employed, is given in Figs. 331 and 332 ; but it is not so satisfactory as the more elaborate apparatus shown in the drawing. In both cases the actual construction of the well is the same, and consists of a circular, or octagonal chamber, about 16 ft. in diameter and 7 ft. deep, built in the ground, with stone or firebrick sides and bottom. Still further to ensure perfect tightness, a good hacking of slag is puddled in behind the stone or brickwork face. The agitating machinery shown in Fig. 330 consists of a cast-iron spindle with stout arms bolted on at right angles, from which project strong metal ploughs or scrapers. These, together with the wrought-iron bolts, require renewal about every two months, but the rest of the apparatus is thoroughly reliable and lasting. The agitators shown in Figs. 331 and 332 are of wood, and are constructed something like a gate, the driving part being set upon a stone centre, and thereby raised above the acid liquors. Occasionally strong chains are hung from the cross beam in place of the vertical arms; but these do not give efficient agitation. The wooden agitators require constant care and repairs, however strongly they may be made.