A quantity of syrup having been made, the amount of sulphuric acid and earthy salts present is ascertained, the latter by means of the soap test (see Analysis). It is now heated to boiling, and a solution of barium hydrate in hot water is added in trifling excess beyond what is required to combine with and throw down all the sulphuric acid ; this is immediately followed by an addition of ammonium triphospbate, equivalent to or slightly in excess of the total earthy metals. These will consist of any excess of barium hydrate that may have been added, together with any calcium or magnesium originally present. A mixture of barium sulphate, barium triphosphate, and calcium and magnesium triphosphates, goes down, sweeping with it from the syrup much of the glutinous and colouring matters. The syrup is next passed through a Taylor filter, to separate the pre cipitate ; if the operation has been properly conducted, tbe syrup should oontain some free ammonia, and just a trifling excess of the ammonium triphospliate, but no earthy bases nor sulphuric acid. The syrup is now fit to be boiled and crystallized.
For some timo after this process had been devised by Lagrange, it was impossible to procure ammonium triphosphate at reasonable prices, the only mode of manufacture being the production (1) of neutral ammonium phosphate, by saturating pure syrupy phosphoric acid with ammonia, so as to form a highly saturated solution of the salt, and (2) then adding one more equivalent of ammonia, so as to throw down ammonium triphosphate, which latter salt is only soluble in weak aqueoua ammonia to the extent of about 6 per cent. F. Maxwell-Lyte, however, introduced a method of producing pure ammonium triphospbate from the acid cakium phosphate afforded by natural phosphates, which at once reduced the price of ammonium triphosphate from 2s. 6d. to 8d. a lb., and thus placed the salt within easy reach of the sugar-makers. Guions, who employed the process in their refinery, state that, besides affording an additional 5-10 per cent. of crystallized sugar, they are enabled to work with far less anitnal-black (char). The process is equally adapted to the defecation of raw beet-syrups, and was thus worked for some titue by Daniel, near Compiegne.
Filtration through Anerta/-black.—The defecated and carbonated juice has in a great measure lost its alkaline character, having been deprived of the greater part of the dissolved lime by means of the carbonic acid. There is, however, still sorne lime to be removed, as well as a considerable quantity of gummy and albuminous substances. These, and the colouring matter which gives a brown tint to tile juice, are in a large degree eliminated by passing the juice through animal-black (char, animal charcoal). This is done by taking the juice from the carbonating-pan into an iron
cistern, and them heating it nearly to the boiling-point, afterwards passing it through vessels filled with granulated animal charcoal. The juice finds its way through this gradually to the bottom ; and runs out while a fresh supply is poured iu tbe top. The charcoal has a considerable power of absorbing bodies such as dextrine, and with long time and hot liquor, the notion is intensified, and the purification is materially great. This juice to be sent through the charcoal filters is a turbid sticky mass ; it is el,vatc(1 either by a pump or a monte-jus into a cistern situated above the series of filtera. After tho filtration, the juice is in the condition lirn,w1) as " thin "; it is nearly colourless, and is largely freed from lime, and from gummy and albuminoua bodies which escaped the action of the lime. In this state, it passes to the concen trating system.
The filters used are of two kicels. The older sort, known as " Dumont filters," consist of cast iron cylinders, 6-12 ft. high, and 3 ft. or more diam., open at top, furnished with a false bottom covered with cloth, as well as a man-hole at the level of the false bottom. The cylinder is filled with black, and the juice is run in at top st such a speed that the black remains always covered with a thin layer of liquid. A pipe, leading frotn the-bottom, curves up, in the form of a swan neck, to half-way up the cylinder.
The other kind, termed "closed filters,'' are shown in section in Fig. 1316, and as a battery in Fig. 1317. They arc 32 in. diam. and 12-16 ft. high; the juice enters by the pipe a b, coming from a eistern placed at a higher level, and escapes by a pipe c leaving the bottom, and bent up to the summit (t of the cylinder. This modification possesses the advantage of effecting the filtration out of reach of air and chills, and enables several filters to be in communication, so as to multiply the height of charcoal through which the juice passes. When a filter is judged to be no longer effective, as seen by the questionable colour of the liquor, the supply of juice is stopped and replaced by boiling water, and when the water has driven out the saccharine fluid, the tap e is opened at the bottom, the liquor is run out, the black is withdrawn at the man-hole f, and the filter is washed, and re charged with new black over which a current of boiling water is passed. The filter is then ready for use again.