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happens to be low, the crystals are sometimes precipitated and remain for a considerable time without decomposition, forming a crust of half an inch or more in thickness on the sides of the chamber. As the atmosphere of the chamber is thus deprived of its nitric oxide or peroxide of nitrogen, unless the temperature is raised or more steam introduced to cause the decomposition, the formation of more sulphuric acid is prevented. A convenient temperature at which to maintain the chamber is 130° or 140° Fahr.

On considering the chemical changes which occur in this process, it will be manifest that if the supply of sulphurous acid and air is properly apportioned, the only refuse is the nitrogen gas of the air, together with some nitric oxide gas, means for the exit of which should be provided by a waste-pipe. In fig. 37, this part of the apparatus is represented at n. The pipe may be prolonged to eighty or a hundred feet to ensure the complete condensation of the acid. Nitrogen being a comparatively light gas is displaced by the influx of heavier gases at e.

Iron pyrites was first extensively employed in the manufacture of sulphuric acid by Mr. Hill, proprietor

* The influence of temperature on the reactions which take place in the leaden chamber has been demonstrated in a decisive manner by an experiment of MM. Clement and Desormes. Into a glass globe containing a small piece of ice they introduced simultaneously sulphurous acid, oxygen, and nitric oxide, and plunged the globe in ice-cold water. The consequence was, that no sulphuric acid was formed, though all the materials necessary to its production were present; but on heating the globe to the temperature of 100° Fahr., immediate action ensued, and hydrated sulphuric acid was condensed on the sides of the globe in visible striæ.

of chemical works at Deptford and at Battersea, who obtained a patent for the application of this and other native sulphurets in 1818. Since the expiration of Mr. Hill's patent right, and especially since the attempt to establish a monopoly in the sulphur trade, many other manufacturers have availed themselves of this material, so that, at present, pyrites is much more largely employed than sulphur. When this mineral is calcined in such a manner as to have free access of air, the whole of its sulphur burns off as sulphurous acid, leaving a residue of red oxide of iron.

The presence of arsenic in larger or smaller proportions in all the ordinary varieties of this mineral constitutes one capital objection to its application in the manufacture of sulphuric acid, the acid it affords being always slightly contaminated with arsenic (arsenious acid). The quantity of this substance, however, which finds access to the chamber is seldom more than a mere trace, for nearly the whole is condensed in the flue of the pyrites burner; the flue being prolonged, in some works, to the length of thirty or forty feet, to expose it to the cooling influence of the atmosphere. The pyrites met with at the Isle of Sheppy is almost entirely free from arsenic.*

The construction of the ordinary pyrites burner

* In the specification of a patent for some improvements in the manufacture of sulphuric acid (obtained in 1840), Messrs. Blair and Watson claim the use of sulphuretted hydrogen as a means of separating arsenic from sulphuric acid. The acid is submitted to the action of sulphuretted hydrogen in the state in which it is withdrawn from the chamber, when the arsenic is precipitated from the dilute acid in the state of orpiment, or King's yellow. Sulphuretted hydrogen occasions no precipitate unless the acid is dilute.

[blocks in formation]

Its

may be conceived by reference to figs. 39 and 40. Fig. 39 represents the front, and fig. 40 a perpendicular section from the front to the back. The burner somewhat resembles a lime-kiln, its interior space being in the form of an inverted pyramid. entire height from the basement is from eight to ten feet; its external width at bottom from seven to nine feet, and at top about six or seven feet. Its widest part in the interior is at seven or eight feet from the base, where the width is five or six feet; from this it contracts gradually to the bottom, where it is about two feet wide. The mineral is broken into small pieces about the size of a hen's egg, and thrown into the burner through the top or feeding door a. Across the interior, on a level with this door, are placed a few iron bars to support the nitre pot b, fig. 40. From the bottom of the burner to the door a are five or six smaller doors for stoking the ore in its descent. All the doors except the lowest are made tight with luting during the combustion of the

pyrites, the air having access through the bottom door only. The gases are conducted from the burners by the flue c, and the calcined ore is withdrawn at the bottom door. In a burner of this construction, and of the largest size above mentioned, about a ton of pyrites may be burned in twelve hours, the ore being introduced every hour.

In the form of small masses pyrites is sufficiently combustible to continue burning of itself, when once ignited with a proper supply of air; but if finely divided, its combustion generally requires to be assisted by heat derived from another source. One form of burner for small pyrites comprises two rectangular earthenware retorts, each about ten feet long, three feet deep, and two feet broad, with an earthen shelf extending its whole length, placed horizontally. The retorts are supported by several rows of fire-brick. Between the two retorts is the fire-place, about two feet in width and five feet in length; the fire plays around each retort, passing through the spaces between the rows of bricks which support the retorts. Air enters the retort through a small aperture in the door at one end, and the gases pass out at the other end and are conveyed into the flue of the large burner. About a ton and a half of small pyrites may be burned in the two retorts in twenty-four hours.*

A patent has been lately obtained by Mr. Farmer for a furnace of particular construction for the com

* The burner for small pyrites at some works consists of a series of several shelves of fire-tiles placed at four or five inches apart. The breadth of the burner is about six feet; its height three feet six inches, and its depth eight feet. Each shelf is furnished with openings through the iron door in front, to introduce the pyrites and withdraw the calcined ore. The shelves do not reach quite to the back

bustion of pyrites and other sulphurets, with a view of obtaining either sulphurous acid or sulphur without the aid of fuel. A perpendicular section of the furnace is represented in fig. 41. The furnace is first heated with coals or burning pyrites from a previous operation, and then fed with pyrites in small lumps through the door-way a; the amount of air

Fig. 41.

f

necessary for the combustion is regulated by that opening, and also by the doorway b on the other side at bottom. The small fragments which fall through the grating c are received in a second grating d, the bars of which are placed closer together

than those of c. The sulphurous acid gas produced by the combustion of the pyrites escapes by the chimney e, and is conducted into the leaden chamber. To obtain sulphur from pyrites by this furnace, the doorway a is kept entirely closed except when more pyrites is to be introduced, just sufficient air being admitted through the opening b to maintain the pyrites in a state of low combustion. A little unburned sulphur is then given off, and may be collected in a leaden chamber or receiver communicating with the furnace by the chimney e. The occasional admission

of the burner, so that a clear space exists there through which the sulphurous acid from all the shelves rises and escapes at the flue. The nitre pot is introduced through an opening at the back of the burner, and the extraneous heat necessary to assist the combustion of the small pyrites is derived from a furnace situated on one side of the burner.

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