The adoption of the over-bead charging system, which has been described when speaking of hand furnaces, is of course imperative in the case of revolvers. This part of the operation differs materially from the mixing process already set forth. A usual charge fur an ordinarily sized revolver consists of 30 cwt. of sulphate, 33 to 35 cwt. of chalk—or 27 cwt. of good limestone, and 17 to 19 cwt. of small coal. What is termed the " liming " process is still followed by many manu facturers. The chalk and two-thirds of the mixing coal are first tipped into the revolver. A slow motion is given to the cylinder, and after about three-quarters of an hour, a blue flame appearing at the pan end of the furnace denotes that a portion of the lime is causticized. The revolver is then brought with its charging hole under the hopper, or waggon, and the sulphate and remainder of the coal tipped in. While this is being done the pan damper is nearly closed, so as to prevent a rapid draught carrying the finely divided sulphate and coal away. For about twenty minutes the cylinder is turned with a slow motion until the workman judges that the sulphate is melted. A quicker motion, of about two revolutions per minute, is then given, and continued for half an hour or so, until the workman judges, from the appearance of the bright flame, the state of consistency of the charge, and the before-mentioned " pipes," that the decomposition is complete. The bogies have been in the meantime drawn up underneath the cylinder. A rapid motion is given for a few minutes, so as to collect the fused ball well down to the centre of the cylinder side, and as the speed is slackened the doer is unhooked, and the charge falls out into the waggons. As each one fills, it is dragged forward, an empty one immediately taking its place. So rapidly is this discharging now accomplished, that eight or nine bogies are filled before the slowly revolving cylinder carries the discharging bole upwards. One revolution then suffices to collect the remaining pettions of ball and discharge them into a couple of empty bogies.
James Mactear, of Glasgow, to whom great credit is due for bringing the revolvers to their present perfection, has adopted a method of charging which obviates the necessity for this tedious "liming " operation. It should be premised that the greatest difficulty met with in the mechanical process has been the fact that the balls produced were so close and hard that no amount of steaming in the tanks could properly lixiviate them. By adding an excess of lime, and causticizing it, it was supposed that the balls would burst in the tanks and fall easily, and a very fair success has attended the operation. Mactear, however, has established the fact that little more than an equivalent quantity of limestone need be added at first, and that the whole charge may be tipped in at once. After decomposing the sulphate thoroughly, the cylinder meanwhile revolving slowly, a small quantity of caustic lirne in lumps is dropped in. The cylinder is then made to revolve rapidly, the whole charge mixed up, and drawn without loss of time. Besides keeping the ball open, this additiou of the caustic lime cools the cylinder to some extent, and prevents any burning of the ball while the last stages are being gone through. The addition of a small quantity of cinders, or coal, along with the lime tends to keep the ball still more porous and readily lixiviated. This simplification of the original process has done away with several of the
evils attending mechanical balling. The output of the furnace is greatly increased, by shortening the timo required to work off a ball ; tho quantity of alkali waste is reduced, and a proportional saving of lime and fuel effected.
Tho size of revolvers, and, consequently, their output, is being constantly increased. Mactear states that his last erected furnace is capable of decomposing 330 tons of sulphate per week. The ordinary revolver, however, with the usual method of working will not decompose above 150 tons per week. In this enormous capability lies one of the evils of the system. A small works, manufacturing only its 150 tons of sulphate per week, caunot afford to come to an absolute standstill, while repairs are being made in its one revolver. And the old evil of close hard balls is still not altogether cured. It requires a higher temperature to dissolve a revolver ball than one made in a hand furnace. Tho question of temperature of tank water will come up again when treating of the lixiviation of the balls.
Until a few years ago the length of time required for discharging presented an almost insuperable difficulty, the first portions of the ball being " green," while the last were burned. With better mechanical contrivances and more thorough experience in regulating the speed of the oylinder this evil has been practically done away with.
Lixiviation of the Black Ash.—The next process is to extract the sodium compounds from the black ash by dissolving the balls in warm water. It will be uotieed by referring to the analysis given that about one half of a ball is soluble, and the remainder insoluble—the latter consisting of various impurities, but chiefly a mixture of various sulphides, sulphites, and oxide of lime. For the purpose of lixiviation, the balls iire broken into pieces, and thrown into the series of tanks, shown in Figs. 249, 250, and 251. Water at about 33° (100° F.) to 43° (110° F.), and the second liquors, are thou run upon them, the soluble compounds drawn off to the settlers, and the insoluble residue thrown out. During the process of breaking up, the quality of the balls may be judged by an experienced eye almost as correctly as by complete analysis, and the careful attention of the manufacturer should be specially and unremittingly devoted to this point of review. The interior of a ball should present a clear, steel grey appearance, well honeycombed. It should break readily with a sharp ring, preferably splitting right down the centre. The outer crust should not be loose, or too well defined, and lumps of undecomposed sulphate should be conspicuous by their absence A pinkish shade shows a green ball, a dull red a burned one. Irregularity of appearance, with white lumps and dark patches, shows want of work, a general soft " mushy " character, an ill judged mixture, or too long exposure to the air. The exact amount of harm a ball receives by lying too long before lixiviation is a matter of doubt. If put into the tanks too hot, the temperature of the water is raised too high, if left upon the ground more than forty-eight hours or so, a certain amount of decomposition, with oxidation of the lime compounds, takes place. As a general rule, twelve hours may be taken as the best time for a ball to lie before being tanked.