BLEACHING POWDER, a combination of chlorine and dry slaked lime (see BLEACH Ise.), was first manufactured on a large scale in Glasgow by Mr. Charles Tennant, who obtained a patent for its preparation in 1799. The substances employed in pre preparing the chlorine are common salt (chloride of sodium), black oxide of manganese, and sulphuric acid. The operation may be conducted in one or in two stages. The vessel used is a still. The whole apparatus is made of strong sheet lead, or of cast iron, or of grooved stones fitting closely. The more general plan is to have the upper part of lead, and the under part of cast iron. The lower third of the still has usually a double jacket, or double walls, between which steam is admitted through a pipe for heating the contents of the still. In using the apparatus, 100 parts of black oxide of manganese (MnO,) and 150 parts of common salt (NaCI) are introduced by an opening in the top, which is closed by a water-joint; 185 parts of sulphuric acid (S0,), of specific gravity 1000, are then poured in by a funnel, and on the admission of steam into the jacket, chlorine is evolved, and issues by a tube at the head of the still. The theory of the changes that occur in the still is represented in the following table, there being two equivalents or atomic weights of sulphmic acid for one equivalent of each of the other ingredients: Chloride of sodium (NaCI) C1, Chlorine escapes as gas.
Na 0 Oxide of manganese 0E102) lfn ( 0 Sulphuric acid (SO,) SO, Sulphuric acid (SO,) SO, Mn0S02, Ne0S03, Sulphate of Sulphate manganese, of soda, Itft in still.
The pipe which carries away the chlorine gas is connected with a stone or leaden chamber, iuto whielt it enters at ono or more points, and the chlorine entering, conies in contact with dry slaked lime in fine powder, with which the floor of the chamber is covered to the depth of some inches. The chlorine is rapidly absorbed by the lime,
which, when the absorption flags, is stirred from time to time by wooden rakes. The process must not be allowed to proceed too quickly, as much heat is evolved during the combination of the chlorine with the lime; and if the temperature of the chamber rises beyond 110° F.. the power of combination is very much lessened.
The material which is left in the still as a residuum, consisting of the sulphate of manganese and the sulphate of soda mixed together, is comparatively worthless, and accordingly it is found more economical in large chemical works to divide the process of the manufacture of B. P. into two stages, at each of which the residuum is of commer cial use, and can be worked up into marketable products. The first stage is to heat the common salt and sulphuric acid together, when hydrochloric acid (q:v.) is disengaged in the gaseous state, and is received in proper vessels, and the sulphate of soda—from which common soda (q.v.)is prepared—is left in the retort or still. The hydrochloric acid thus obtained is then placed along with the black oxide of manganese in the still, and on the steam-heat being applied, chlorine is evolved and is conducted to the lime chamber, while chloride of manganese and water are left in the still. The decomposition is stated thus: Hydrochloric acid j 01, Chlorine escapes as gas.
H 0 Oxide of manganese DIn 0 Hydrochloric acid II