L. Wallthoff's " mixed method" of extraction is illustrated in Fig. 1302. Its most essential part is the filter-press or swinging vat A, resting by the axles b on cast-iron supports a ; it can be turned round on its axis, and thus completely emptied. One or both of the axles b are hollow, and furnished with a stuffing-box, so that water can circulate in the interior of the axles, whatever the position of the vessel. A tap d regulates the delivery of water from a reservoir tD, which may be 10-30 ft. above the apparatus. The water admitted by the hollow axles b passes by the pipe e into a perforated worm, whence it escapes beneath the double false bottom f. Thus its level is raised slowly and uniformly. At g, is a cover pierced with holes, formiog a diaphragm, provided with a handle, and resting in the interior of the vessel upon circular bearers, where it is held by screws. To prevent the water passing directly along the sides, the double false bottom is fixed to a T-iron rim riveted to the vessel. The large-bore tap n is for letting out the water rapidly when the juice is displaced. At the top of A, is a tap m for outflow of juice.
Once the vessels A are full, the metallic strainer / is placed on the pulp, and the cover g is adjusted. The tap d is then opened, so that the water occupies 12-20 minutes in filling the vessel A. The water enters at the bottom ; as it rises, it displaces the juice in the pulp, mixing more or less with it. The liquid thus approaches the tap m, and escapee at about the normal density of tho juice. The workman soon learns the correct adjustment of the tap d necessary t,o give the proper duration to the operation. The pulp, being lighter than the water, rises as a scum up to the strainer l, but is there retained, so that the liqnid escapes quite clear. The usual length of opera tion required is 20 minutes. The diameter of the vessels A should not exceed about 28 in. With this size, the pulp of about 8 tons of beet can be worked in a day of 24 hours, or say 6 vessels for 50 tons per diem. This system has been very largely adopted in e,ontinental Europe, on account of its good working results. Tho appended table exhibits its capabilities in comparison with other modes :— More recently, Walkhoff bas introduced modifications tending to reduce the labour. His principle is to reduce about 70-80 per cent. of the juice by a preliminary treatment, of the simplest possible character, for which many mechanical appliances already exist. The pulp coming from this treatment is thrown at a into the apparatus shown in Fig.1303. Thence it passes under a great number of blades, which divide it into smell fragments, and thus it reaches the large drum b in a uniform and oontinuous stream, there to be still further comminuted by the edges c, and delivered to the juice-extractor. This latter, called a " revolving filter," is provided with paddles, and resembles a water-wheel. This revolves slowly, and causes the pulp to circulate in opposition to
a current of water entering at e. The completely exhausted pulp is discharged at g, and falls into the gutter A, whence it is conveyed to store. Tbe whole apparatus rests by its axis a on a support m, and is aotuated by the wheels and pulley shown. The tap o serves as an outlet for the water from the apparatus. The water, entering in the desired quantity at e, passes successively into each compartment, and escapes at f as concentrated juice. The apparatus is very siniple, and effects the oompleto extraction of the sugar, without adding more than 5 per cent. of water on the weight of beetroot.
Many other plans, depending more or lees upon maceration, have been proposed, such as Pelletan's, Reichenbach's, Hallette et Boucherie's, Martin et Champonnoie, Schiskoffs, Robert's.
Schutzenbach's, &c., but they do not possess any valuable feature entitling them to notice. The preceding systems are those most generally and successfully applied. A comparison of the results of the foregoing processes, in tabular form, is given on the authority of Walkhoff : for 120 days' work and 6000 tons of beet, the production of juice requires :— Diffusion.—The processes hitherto described for extracting the juice from the beet have depended for success upon the more or less complete rupture of the cells containing the juice. " Diffusion " differs from them essentially, in dispensing with the breaking-up of the cells. The constituents of beet-juke may be classed under two dis tinct groups : (a) " crystal loid," including the sugar and other " salts" capable of assuming a crystalline form ; (b)" colloid " (glue-like), em bracing the gummy or muci laginous matters not capable of crystallization. The two classes are distinguished by a physical fact which forms the basis of all modifications of the diffusion system,—the difference which they mani fest with regard to the power of passing through moist water - tight membranes. Bodies belonging to series (a), dissolved in water, pass through most animal and vegetable membranes (gut, parchment, plant-cells, parchment-paper), when there is water on the other side ; those of series (b) are not possessed of that property. This method of separation is termed " dialysis," " osmosis," or " diffusion," and the membrane is called a " septum " or " dialyzer." The dead cell-walls of the plant itself form an excellent dialyzer ; therefore, by cutting the root into convenient slices, and soaking these in water, the crystalloids (including the sugar) pass through the cells into the sur rounding water, while the colloids mostly remain in the cells. Thus the juice is at once more or -ess purified, and is at the same time less contaminated with vegetable debris resulting from the mechanical breaking-up of the root.