The sulphuric acid made in the chambers is not strong enough for many of the purposes to which it is applied. The acid can be concentrated by boiling, however, which causes the evaporation of a part of the water with which it is combined. This may be performed in leaden pans up to a strength of 1.750 sp. gr.; but the higher the concentration, the greater the difficulty in disengaging the combined water, so that the temperature at which evaporation takes place rises rapidly and an increasing proportion of acid is distilled over at the same time. The acid cannot be concentrated to monohydrate by simple evaporation of the water, but moderately strong acid will be distilled, and must be afterwards condensed. As acid of more than 1.750 sp. gr, attacks lead very powerfully, and the boiling point of monohydrated acid is very Dearly equivalent to the melting point of lead, the concentration is not carried beyond that point in leaden Teasels, but in retorts of platinum or glut.

Fig. 146.

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When the acid is to be concentrated in platinum vessels, it must first be perfectly purified from nitrogen compounds, as that metal is very rapidly destroyed by them. The most reliable plan of denitrating the sulphuric acid during concentration in leaden pans is by the addition of a small quantity of ammonia sulphate. With tolerably good working, trogen compounds that .1 - .5 per cent. of the ammonia salt will suffice.

Fig. 147 shows the moat recent form of platinum pan and still, manufactured by Johnson, Mat they, and Co., of Hatton Garden. A are platinum pans, with corrugated bottoms and longitudinal or transverse partitions, exposed to the flame in the flue. They can be worked in series, replacing the thick leaden tanks now employed for concentration of the chamber acid. B is a platinum boiler, with corrugated bottom and partitions to receive the add at 142° Tw, (60° B.) or above, from the pans A, completing the concentration to 168 1/2° Tw. (66° B.). C is a head and arm for carrying off the vapour to a leaden condenser (not shown) or direct into the chamber for utilisation, if required. Dia a flask cooler, to receive the concentrated oil of vitriol from the boilers, and to pass it, cooled, into the carboys;

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This is the newest form for the concentration of sulphuric acid, securing great strength, productive power, safety and economy in working, and the highest degree of purity of acid, with a minimum of platinum.

By the corrugated form of bottom (Prentice's patent) the greatest possible amount of strength, surface, and consequent evaporating power is obtained in the boiler or still, and a considerable saving in fuel is effected. By means of the pans the large and costly leaden tanks for the previous concentration of the chamber acid, which require constant repair and renewal and more or less contaminate the acid, can be to a great extent done away with. The setting of these boilers and open pans is of the simplest kind: they are placed upon an iron frame over a straight flue, and they may be multiplied or enlarged to any desired capacity of production, without sacrifice of existing plant. Pans of lead (or any suitable material) of the same form or principle, employed for the first concentration of the chamber acid, are included in this patent. The cooler is of an improved economical and convenient form, easy to clean, and securing great cooling power with a minimum of water and space.

Before the adoption of platinum vessels for completing the concentration of the acid, large glass retorts were used, and these are still employed in works where the glass can be bought at a low figure, or where the quantity of sulphuric acid needed in a concentrated state is small, or where the manufacturer has not sufficient capital to afford a platinum still.

The retorts may be set in two rows in a gallery furnace, and are filled with pure acid at 150° Tw. (62° B.). The number of retorts fed by one fire will depend upon the class of fuel used, as well as the size of the retorts. The retorts are usually protected against sudden changes of temperature by sand baths in iron pots, but other materials have been recently adopted. The jumping of the acid during the boiling is prevented by putting some small pieces of glass, platinum, or gas-retort carbon into the retort. Sometimes a leaden pan is provided underneath the retorts for catching the acid in case of a fracture occurring. The glass of which the retorts are made must be free from alkali, or the acid will attack them rapidly. They are of various dimensions, but those holding about 3 cwt. are most convenient.

The steam evaporated is conducted away by glass arms fitting into the necks of the retorts, and condensed for use. The great drawbacks to glass retorts are that they consume very much more fuel, and that they are constantly liable to accidental breakage, on account of their necessary fragility. The retorts used in this country are cylindrical, about 33 in. high and 20 in. in diameter. Each retort is fired from a separate furnace. The top of the retort is provided with a short wide neck, into which a glass arm is fitted for carrying off the steam. The retorts are filled and emptied by means of leaden siphons, the process being intermittent. It is convenient to allow the acid concentrated during one day to remain in the retorts till the following morning, so that it may cool somewhat during the night.

It will be readily believed that the boiling of such a powerfully corrosive substance as sulphuric acid, requiring a temperature 3 times as great as water, in extremely thin glass flasks holding some 3 cwt. each, is an operation of the greatest delicacy, not to say danger; and it is therefore of primary importance that the "retort house" shall be built of brick or concrete with a most durable roof, and that every crevice shall be securely stopped against the admission of the slightest draught, or drop of fluid, whether rain or acid. The house should also be roomy enough, and especially of a good height, for as ventilation is inadmissible, sufficient space must be left overhead for the accommodation of the extremely pungent and irritating fumes which pour off the retorts during the time that they are being drawn off.

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a, furnace bars; b, bed plate; c, door frame; d, fine holes, one on each side of the fire-place; e, flue holes leading into the main fine f. These holes should not be all of the same site, but should increase in proportion as they are farther removed from the flue hole, so that the amount of draught may be equally distributed among the furnaces. The one nearest to the flue hole may be 3 in. by 3 in., the one farthest from it 3 in. by 6 in., and the others of intermediate sizes, f is a flue leading to a flu hole, about 12 in, by 9 in., fitted with a damper, and conducting to the chimney. In the figure, the flue hole is supposed to be at the end of 6 retort furnaces, but with 12 retorts it may be placed in the middle. A, fire lumps covering in the flue f; i, fire lumps covering the furnace fire; k, thin plates of iron to strengthen the front brickwork, being held in place by m, perpendicular tie bars, and n, horizontal tie rods.

Before commencing to "set" the retorts, it must first be ascertained that the pots are absolutely dry. Some sand must then be thoroughly desiccated and afterwards sifted through fine wire gauze to free it from any small gravel, etc, that may be in it. Put some of this dried sand into the pot and spread it over the bottom with your hand, giving it the hollow shape of the bottom of the pot, until the layer of sand is of a uniform thickness of about 1 or 1 1/4 in., and reaches as far up the pot all round as shown at a in Fig. 149. Take the retort by its neck with both hands, end lower it gently into the pot till it rests on the bed of sand. Place it as nearly as possible in the centre of the pot, and take care that it stands perpendicularly. Should it be found that the sand does not support the retort (when left alone) in an upright position, a little sand must be poured in between the retort and the pot on whichever side support is wanting. When satisfied that the retort is well placed in the pot, take more sand and pour in all around the glass till it rises as high as A. The retort will then be ready for filling with acid to the level d, from which point it will rise during boiling to the level ft.

Fig. 149.

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