The calcined sludge is mixed with lime and soda in the following proportions and fused: After fusing the mass, three samples examined were found to contain equal to The mass is then extracted with water, the lixiviation liquors oxidised by means of sulphuric acid, and the bichromates caused to crystallise out. Regelsberger describes what is said to be a more economical method of recovery by means of an electric current. The chromium hydrates to be oxidised are mixed with alkaline chlorides and submitted to the action of the current; oxidising hypochlorites are immediately formed. A nearly saturated solution of alkaline chloride is used and a quantity of chromium oxide added equivalent to the current used in a given time. The following equation represents the action where the oxide is used, though either the chloride or sulphate may be substituted : Cr2(OH)+2NaCl[+6NaCl] + 2H2O= Na2Cr2O7+8H+2C][6NaCl]. When potassium chloride is used as the electrolyte, bichromate crystals fall out from the warm solution, but in the case of sodium chloride being used, separation must be effected. The method is evidently too precise and complicated for any bye-process. CHAPTER VIII. PAPER MILL REFUSE. THE basis of all papers is vegetable fibre or cellulose, the crude material, so far as the papermaker is concerned, being wood-pulp, esparto grass, linen cuttings, cotton cuttings, rag, bagging, straw, tarpaulin, ropes, peat, etc. His object is to divest the raw material of the matters associated with the cellulose-resins, gums, silica, fats or oils, in the case of grasses; grease, colouring matter and adventitious dirt, in the case of other material. The general process may be summarised as (1) Dusting and picking. (2) Boiling the raw stuff with alkali-to saponify fats, form resinates and soften the fibres. (3) Washing the raw stuff after boiling, now termed half-stuff. (4) Breaking up half-stuff to pulp and washing in breaking engine. (5) Bleaching the washed and pulped half-stuff. (6) Further pulping and washing the bleached stuff. (7) Passing pulp (after sizing, colouring, loading and mixing) through paper machine, in which it is strained, layered, rolled, calendered, and cut, emerging as paper. The solid waste from process No. 1 may be taken away as manure or incinerated. That from No. 2 (the soda liquors) is grossly polluting, while on the other hand the alkali therein may be profitably recovered. The strength of the fresh soda liquor added varies with the nature of the raw stuff. For rags, 5 per cent. to 10 per cent. of the weight in caustic soda is added for high pressure boilers. For esparto grass about 20 per cent. of the weight in caustic soda, according to the nature of the grass, the actual concentration of the liquor varying between 6° and 16° Twaddell. The strength is reduced in boiling to about 4° to 6° Twaddell, and is further reduced by the wash-water from the boiled grass to between 2° and 4° Twaddell. The soda can be recovered from these liquors by evaporating off the water and incinerating the residual thick liquors. The commonest and probably the most economical form of evaporator is the multiple effect vacuum evaporator, the principle of which may be here described, as it is referred to in the consideration of both wool fats and glycerine. A pound of water requires, at atmospheric pressure and 32° F., (212-32) = 180 units of heat (B.T.U.) to bring it to the boil. To completely convert it into steam 966 further units are required. Total, 1146 units. This steam formed will give out the 966 units again on being condensed to water. That is to say, if 966 pounds of water are used for condensing 1 pound of steam at 212° F. and atmospheric pressure, these 966 pounds will be raised 1°, 483 lbs. 2°, and so on, the condensed steam remaining at 212. One pound of water at 32° F. would be raised to 212°-boiling point, and (966-180)=786 units would remain in the steam, of which 180 or 186 lb. only would be condensed. 966 Now the boiling point of water varies with the pressure, being lower at lower pressures, and vice versa. At a vacuum of 5 inches of mercury or -12.2 lbs. per square inch, the boiling point is 134°; 10 inches mercury or -9.8 lbs. per square inch, 162°; and, at 15 inches mercury or -7.3 lbs., 180°. Moreover, as the heat-absorbing or conducting power of water varies with the temperature, the total number of units required to vaporize it or convert it into steam varies also-the latent heat of steam. But this quantity of heat for a pound of water is 1115.2-(708 x t), where t is the boiling point in degrees F. Taking the three temperatures above, therefore, the latent heat, or units required for steam conversion, would be- Now let it be assumed that there are three closed vessels, A, B, and C, containing water at the temperatures and pressures as indicated in the above table. If a pound of steam be delivered from D at the higher temperature into A, out at the lower, it will be condensed to a pound of water at 180°, but in the act will yield 966+ (212 −180) = 998 units of heat, more than sufficient to convert one pound of the water in A into steam; and if this steam be mechanically drawn away so as to constantly maintain the pressure in A at 15 inches, one pound of it delivered into the vessel B would condense to a pound of water at 162°, yielding 987 +(180 – 162) = 1005 units, sufficient to convert one pound of water in B into steam; and if again a pound of this steam were conducted into the vessel C, it would be condensed to a pound of water at 134°, yielding in the act 100+ (162 – 134) = 1028 units of heat, more than sufficient to convert a pound of the water in C into steam. Thus we get three separate pounds of water converted into steam, and the final conditions are pretty much as at the start. But the arrangement of the triple effect evaporator is such that the steam condensed is not allowed to mix with the water already existing in the vessels; it is conducted away separately, and thus for the trouble or expense of maintaining the vacuum and the difference in temperature, three pounds of water are evaporated at the expense of the one pound of steam injected, or as many pounds as there are pans. The liquors for concentration are conducted from the one vessel and through the other, enclosed in circulating tubes, and the steam played and condensed on the outside of the tubes, a vacuum being maintained within. A complete arrangement is shown in fig. 90-that of Messrs Fawcett, Preston & Co., Ltd., Liverpool. The liquors are concentrated first, to some extent, in the steam generator A, and afterwards delivered by means of the feed engine E into the first pan C', inside vertical tubes, the steam from the generator A surrounding these, in jackets. What condenses in these jackets is conveyed by gravitation to a reservoir F, while the steam from the liquor itself is drawn over through the jackets of the next pan C" via the tube H, being condensed to some extent by the cooler liquors therein, but evaporating water from these, by raising the temperature. This operation is repeated in all pans up to last, Civ, for which a special condenser, D, of cold water is provided. B is the feed engine for crude liquor to the steam generator, and E the air pump or vacuum engine. The liquors are kept on the move from the first to the last pan, entering the first at from 2° to 4° and leaving the last at about 46° Twaddell, being reduced to about one-sixth in bulk. The weight of water evaporated per pound of coal with a triple effect is from 30 to 40 lbs., according to the mechanical efficiency of the engines and pumps and the calorific efficiency of the primary generator A. At the Darwen Mill of Messrs Dimmock in 1893 one week's working averaged Another interesting form of evaporator is the Yaryan' multiple effect (figs. 91 and 92), very suitable for weak liquors. The crude liquors are received in tank B, fig. 91, diagram 22, and are fed upwards by delivery pipe m from the feed pump C through a series of feed water heaters p. Each of these is connected with the contiguous pan or 'effect,' the evaporated steam from which is circulated through it for the purpose of both raising the temperature of feed and increasing the vacuum in pan-in short, the feed is used to some extent instead of cooling water, with the double advantage of power saved and the heat usually carried away by cooling water utilised. The crude liquor then enters the highest 'effect' at D. From thence it passes through the interior of the horizontal tubes H, emerging at X into a separator or steam trap I, and after the separation of steam, via Z, into the next pan. The first live steam charge is delivered at F, in chamber G, outside the tubes H, the water of condensation being received at H', and the generated steam from liquors under concentration |