CHLORIDE OF LIME 785 



It consists in transforming the chloride of manganese, first into carbonate and then 

 into oxide, by the action of heat. Whatever impurity the chloride of manganese may 

 contain as chloride of iron, for instance is first separated, either by calcination or 

 by the agency of a suitable precipitant. Practical working has shown that the 

 carbonate of manganese thus treated yields an oxide of a richness equivalent to that 

 of 80 per cent, of peroxide. The carbonate of manganese may be obtained by 

 precipitation from the chloride by carbonate of ammonia. The chloride of ammonium 

 resulting from this treatment may either be employed as such, or it may be re- 

 transformed in the usual way into carbonate for the precipitation of fresh chloride of 

 manganese. Hydrate of lime is also used as a precipitant, the resulting hydrated 

 oxide of manganese being subsequently converted into carbonate by the transmission 

 through it of a stream of carbonic acid. 



By another process, carbonate of manganese is obtained by passing carbonic acid 

 through the solution of chloride of manganese which has been previously mixed with 

 a quantity of carbonate of soda. The carbonate of soda, under the influence of car- 

 bonic acid, decomposes the chloride of manganese into carbonate, from which the 

 oxide can be obtained. The essential feature of this invention is the production of 

 artificial oxide of manganese, by first converting the chloride into carbonate, and 

 afterwards this latter into oxide, by the joint agencies of heat and atmospheric air. 



In a process suggested by Binks and Gatty, the chloride of manganese in the still- 

 liquor is heated with nitrate of soda, whereby a mixture of nitrous acid and oxygen is 

 evolved, while a residue of chloride of sodium and oxide of manganese is obtained. 



But the most valuable process which has yet been proposed for regenerating the 

 manganese from the spent liquors is that which has been lately introduced by Mr. 

 Walter Weldon, and is now largely employed in this country. To understand the 

 working of this process it should be remembered that the acid liquor left in the 

 chlorine-stills, by the action of hydrochloric acid on peroxide of manganese, consists 

 mainly of a solution of protochloride of manganese (manganous chloride), with the 

 sesquichlorides of iron and aluminium, chloride of calcium, and such excess of hydro- 

 chloric acid as may have been employed. This liquid is run from the generators into 

 a well, where it is neutralised by addition of finely-divided chalk, the neutralisation 

 being facilitated by keeping the liquid stirred up by a mechanical agitator. Not 

 only is the free acid thus neutralised, but the chlorides of iron and aluminium are 

 decomposed, and the oxides precipitated. The neutral liquor is then pumped up into 

 tanks, called ' chloride of manganese settlers,' where the sulphate of lime, the oxide 

 of iron, and the alumina held in suspension are rapidly deposited, and a clear solution 

 of manganous chloride, of a pale pink colour, is obtained. This clear liquor is run 

 off into a tank below called the ' oxidiser,' in which it is treated with an excess of 

 milk of lime to precipitate the manganese as hydrated protoxide, which is converted 

 into a higher oxide by the injection of air. The oxidiser is generally a cylindrical 

 iron vessel, about 12 feet in diameter and 22 feet deep. The reactions in this vessel 

 are best effected at a temperature of from 130 to 170 F., and if the charge of 

 manganous liquor is not sufficiently hot, its temperature is raised by injection of 

 steam. When sufficiently heated, the liquor is subjected to a blast of atmospheric air 

 introduced through a large pipe reaching nearly to the bottom of the oxidiser. The 

 milk of lime is then rapidly run in from a reservoir above, and the blowing continued 

 until peroxidation ceases a point usually attained when from 80 to 85 per cent, of 

 the manganese has been converted into peroxide. The thin black sludge resulting 

 from this action contains chloride of calcium and peroxide of manganese associated 

 with a variable proportion of protoxide of manganese and lime. Each cubic foot of 

 mud contains about 2 Ibs. of peroxide of manganese. The product is run off from 

 the oxidiser to the ' mud settlers,' where a thick black deposit, containing about four 

 Ibs. of the peroxide to the cubic foot, gradually subsides. This deposit, separated 

 from the supernatant liquid, is transferred, while still moist, to the chlorine-stills, 

 where it is treated with hydrochloric acid for production of chlorine, and the cycle ot 

 operations thus begins afresh, and may be continued indefinitely the manganese 

 merely playing the part of a carrier or medium of transference between the oxygen 

 of the atmosphere and the hydrochloric acid. In an experiment continued for five 

 hours, during which time 1 75,000 cubic feet of air was blown through the sludge in 

 the oxidiser, it was found that more than 4 cwte. or 14 - 8 per cent, of the oxygen was 

 absorbed, serving to reproduce 24 cwts. of peroxide of manganese. The cost of 

 recovering the manganese by this process is said to be less than one-fourth the average 

 cost of an equivalent amount of native manganese. 



Nature of Bleaching Powder. An elaborate series of experiments on the manufac- 

 ture of chloride of lime was made in 1822 by Dr. Ure, the most important results of 

 which are embodied in the subjoined Table : 



VOL. I. 33 



