The amount of fuel needed will be about 11 cwt. of good steam coal.
The following illustration, Fig. 80, shows the most recent form of platinum pan and still, manufactured by Messrs. Johnson, Matthey, and Co., of Hatton Garden. A 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 acid, at 60° Baume, or above, from the pans A A, completing the concentration to 66° Bauma (1.845 sp. gr.). C is es head and arm for carrying off the vapour to a leaden condenser (not shown) or direct into the chamber for utilization, if required. D is a flask cooler, to receive the concentrated oil of vitriol from the boilers, and to pass it, cooled, into the carboys ; the leaden water jacket should be so arranged that the cold water may have the greatest possible amount of cooling power.
This is the newest form for the concentration of sulphuric acid, securing great strength, productive power, safety and economy in working, and 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 apace.
Briefly, the chief advantages of this construction are :—Great economy in Brat outlay and in daily expense of working : this may be estimated at about 50 per cent. each in the first weight of platinum and in the consumption of fuel. Great intrinsic value in a realizable form in proportion to the cost. Purity of acid. And freedom from danger.
In making the necessary joints, this firm has long since abandoned the use of gold ; nothing but pure platinum being employed throughout the apparatus.
Fig. 81 represents the most recent Continental disposition of leaden and platinum concentrating apparatus, the novelties of which consist of compactness of arrangement, and especially the adaptation of gas as a, heating medium throughout. The platinum retort or boiler is shown at A the platinum pans at B and C and the leaden concentrating pans at D E F Gr H and K The chamber acid enters the leaden concentrating pan D at a strength of 91° Tw., leaves
the last leaden pan K to enter the first platinum pan C at 113° Tw., leaves C to enter B at 146° Tw., and leaves this last platinum pan at 161° Tw. to enter the retort, which it leaves con centrated to 170° Tw.
The part lettered A' to K.' shows the arrangement of the flues whereby the beat is led under the platinum retort and pans, and leaden pans, until it escapes into the opening M M leading to the chimney.
The acid vapours, escaping from the platinum retort or boiler A are condensed in the horizontal leaden worm lying in the trough P which is filled with water ; this worm is about 4 in. internal diameter, and 45 ft. long.
Concentration in Leaden Pans and Glass Retorts.—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. The number of retorts fed by one fire will depend upon the class of fuel used, as well as the size of the retorts. Formerly the retorts were protected against sudden changes of temperature by sand baths in iron pots, but more recently other materials have been adopted without any great benefit that we know of. The jumping of the acid during the boiling is prevented by putting some small pieces of glass or platinum 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 in the chambers in lieu of water or steam. 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.