In the neutralizing well, the still liquors are mixed with roughly ground chalk, and agitation is kept up until the acid is thoroughly neutralized. This part of the operation requires care, that no injury may be done afterwards by the presence of an acid. Decomposition of the chlorides of iron and alumina is also effected by the chalk. The final liquors consist of chlorides of manganese and calcium, with exeess of chalk and small quantities of oxide of iron and alumina, and of sulphate of lime. It may be stated in passing, that the liquor from the native stills, is mucli more aeid than that from the mud stills, as it is far easier to regulate the amount of acid in working the latter than the former. After being allowed to settle for a short time, the liquors are pumped, by the engine E, into the cisterns F, F. Here they are allowed to settle, until the supernatant liquor is perfectly bright and clear, of a sliglit pink colour. By means of any convenient form of siphon—indisrubber tubes, which float always upon the surfaee of the liquor, being perhaps the best, the clear liquors are next transferred to the " oxidizers," shown at G. These are of strong sheet iron, well riveted and transversely stayed ; they should be not less than 10 or 12 ft. in diameter, and 30 to 35 ft. in depth. A large 10-in. iron pipe N passes from the blowing engines (whieh should be in dupli cate) down the centre of the oxidizer, and terminates in a crosshead pipe, pierced with holes to effeet perfect distribution of the air. Sometimes the air distributor is proteeted by a false bottom ; but this is not neeessary, as the blast of air keeps the holes well open. A smaller pipe is run down the centre of the oxidizer, for the injeetion of steam. The manganese liquor is run into the oxidizer to a depth of about 15 ft., and is raised by steam to a temperature of 55° (130°F.) or there abouts. In the meantime, a carefully prepared milk of lime has been eolleeted in the vessel J. The lime apparatus will be readily understood from the drawing. The milk is made in the upper of the two sheot-iron vessels, the lumps of calcined lime being placed in a wire basket to separate out all hard pieces and stones. Not less than 151b. of hydrate should go to the cubic foot of cream, and to ensure perfect fineness the liquor must be carefully sieved on its passage from the upper to the lower vessel. A revolving sieve turned by the small engine which agitates the liquor is the best form of apparatus. In many cases the milk of lime is pumped from the second vessel into a third, or " batch," cistern, placed above the oxidizers, to obviate the injury caused by any possible breakdovvn of the lime pump during the administering of the charge to the loan_ ganese liquors. Whichever syetem is adopted, thorough ag,itation of the prepared cream before using must be effected. The manganese liquor in the oxidizer having been brought up to a proper temperature, the milk of lime is pumped (see pipe K, Fig. 330), or run, in, and the blowing engine slowly set away to effect perfect admixture. The addition of lime is continued until the filtrate from a sample of liquor drawn from a small tap near the bottom of the oxidizer ceases to give a precipitate of purple permanganate of calcium with a solution of bleaching powder, showing that the whole of the manganese has been precipitated as oxide. A further quantity of milk of lime is then added, and the blowing engine set away rapidly, the injection of air being continued until peroxidation is complete. It may be mentioned here, that it is of the utmost importance to have oxidizers and blowing engine of ample capacity and first-rate quality, any serious hitch in the process spoiling tho batch in course of operation. Several forms of blowing engine are employed, that shown in Fig. 333 being the best in the writer's opinion. It is the design of Messrs. Daglish, of St. Helen's, and is used now very extensively with the best results. Next to this the best form is a horizontal engine with wooden flaps, faced with indiarubber, in the covers of the air cylinder instead of the older form of valve. Although only ono oxidizer is shown in the draw ing, the smallest plant should have two, a batch of manganese liquor being prepared in one, while the charge in the other is being blown. Not only is a saving of time effected, but the risk of stoppage is guarded against by this plan.
To return to the process. The amount of lime required for perfect precipitation varies consider ably—varies with the quality of the milk itself, the presence of foreign matters and coarser par ticles, and the action of the liquor. Lime dis solves slightly in chloride of calcium, as also does protoxide of mangauese, and these reactions render a varying ainount of excess of lime neces sary. Usually it is found necessary to add about
1 .2 equivalents : the further arnount introduced in excess, after precipitation has been ascertained to be complete, raises the quantity to 1.5 or 1.6 equivalents. On the other hand, too great excess of lime causes loss through the formation of com pounds which neutralize the acid in the stills without liberating chlorine. The compound of lime and manganese thus formed is usually termed " base," and every finished charge is finally tested for " bases," with a view to keep them as low as possible. The amount should not exceed •75 per cent. About two-thirds of the way through the operation of blowing, it is very usual to run into the oxidizer a portion of fresh man ganese liquor. The lime of the " base" decom poses the chloride of manganese, forming a fresh portion of protoxide to be operated upon, and freeing a certain amount of peroxide before com bined with it.
The length of time required for complete peroxidation varies with the size of engine, number of revolutions, and depth of charge. Not altogether, however, because if the air be injected at too great a speed, it simply passes through the liquor and ie wasted. It is important to have as great a depth of charge as Possible, as the surface presented to the action of the air is thereby increased and the absorption of oxygen more complete. The liquor is at fir,t, after the addition of the lime, yellowieh-white, but rapidly turns darker ae the operation proceeds, and should finally be converted into a very thin black mud, consisting of solution of chloride of calcium, holding in suspension various compounds of manganese and lime—or " manganites,"—manganese and manganese—Mn0 and Mn02—, and the peroxide. From the oxidizer, the whole mass is run into settlers, P, P,P, P in the drawing, and left to Bettie. Upon an average, about three and a half or four hours are required to complete the blowing operation.
After remaining in the mud settlers for four or five hours—or until a charge of mud is wanted r—the chloride of calcium which floats clear on the top is drawn off by means of any convenient form of siphon, and run to waste. At the bottom of the eettler will be found a layer of thick black mud to a depth of about 10 in.—a mixture of various rnanganese compounds and lime—which repre sents the finished and regenerated article. It is usual, after removing the chloride of calcium from the mud, to run in another charge from the oxidizers, and eo collect a large quantity of the final mud. This is done, of course, to economize settler room, and also to prevent the drying and caking of the mud probably attendant upon a thin layer being left. From tirne to time the stock of mud in the settlers should be thoroughly agitated to ensure an even consistency. The settlers themselves are built preferably of stout sheet iron, well riveted, and supported by cross stays. Occasionally they are constructed of wood, like huge tubs, but any such form of apparatus is faul ty .
From the settlers the mud is run, as may be required, into the stills R, R, Fig. 330, through the luted funnels Q. A section of Weldon still is given in Fig. 334, which will better show its con struction. It is usually octagonal in shape—sometimes equare—about 9 ft. deep, and each eide 4 ft.
wide. If square, the still may be 10 ft. wide, but stones of this size are very apt to cmek. The octagonal form is both stronger and cheaper. Each side may bo made of two slabs, to save expense ; the lower piece in this case should be 5 ft. 6 in. high, and not less than 7 in. thick, and the upper portion 4 ft. high and 5 or 6 in. thick. The sides of the atill are strongly bound up with lf-in. iron girders, and are jointed together with the indiarubber rode before described ; or the flat aurfaces are carefully dressed and bedded with china clay and tar, and a stemmed diamond run down the centre. The bottom may be of one solid piece, 12 in. in thicknesa, or formed in sections. The former is the better but more expensive plan. The sides should sit 11 in. into the bottom stone—or stones. The cover is usually formed of segments, and is let and stemmed into u ledge cut in the sides. These segments rest in the centre upon a strong stone shaft or pillar, 10 in. square, through the middle of which is drilled a groove for the injection of steam, which passes out near the bottom of the still at right angles to the axis of the steam shaft. Finally, a lute is set in the cover for the introduction of acid, a large 4-in. earthenware tap is let into the front, close to the bottom, to let off the spent liquors to the neutralizing well, and a small sample tap is stemmed into one side, rather more than half-way down. In the cover are also set a large pipe to convey the chlorine gas to the chambers, and a manhole.