These liquors, therefore, leave behind practically all the calcium compounds, the alumina, silica, magnesia, carbon, and some little soda.

The most objectionable, perhaps the only reasonably objectionable, constituent, is the calcium sulphide. In the immediate neighbourhood of Widnes, St Helens, Accrington, and Liverpool, and indeed anywhere within reasonable distance where tipping spaces for it could be procured, there are thousands upon thousands of tons of this alkali waste.

Messrs Muspratt & Dawson give the following as its composition, fresh from the vats, and six weeks old :

The story of the successful recovery of the sulphur from the waste, which is now in operation at practically all works using the Leblanc process, would require volumes. But after the cessation of the unproductial work of investigators who spent many years upon it, Mr A. M. Chance perfected the well known Chance process of recovery, which consists, briefly, in treating the waste with carbonic acid gas, so liberating the sulphuretted hydrogen, which is burnt with just and only just sufficient air in a Claus kiln, wherein the sulphur is deposited and recovered.

The diagrams 108, 109, taken from a paper read by Mr Chance before the Society of Chemical Industry, March 1888, make the operation very plain. In fig. 81, 1, 2, 3, 4, 5, 6, 7, are waste cylinders or carbonators, L the CO, main, C connecting main, E exit main, R return main.

The action of carbonic acid on calcium sulphide (in the presence of water) is to form calcium carbonate and sulphuretted hydrogen. But if the sulphuretted hydrogen be passed on to the further vat waste containing calcium sulphide, the latter absorbs it to form calcium sulphydrate, and this again gives up the whole of the sulphuretted hydrogen when subject to the action of carbonic acid. The process is therefore threefold:

(1) CaS+ H2O + CO2 = H2S + CaCO3.

(2) CaS+H2S= CaS2H2.

(3) CaS2H2+ CO2 + H2O = CaCO3 + 2H2S.

The process is now best described in Mr Chance's own words, fig. 108 being referred to :

"At 7.0 a.m. Nos. 7 and 1 vessels were charged with fresh waste. The lime kiln gases were pumped direct into No. 3 vessel, and then through vessels Nos. 4, 5, 6, 7, and 1, by opening all the intermediate taps; the useless gases escaped from No. 1 vessel into the exit main, and thence through a purifier into the air for a period of 1h. 40m.

"At 8.40 the sulphuretted hydrogen gases from No. 5 were sufficiently strong for use, the calcium sulphide having mostly been converted into calcium sulphydrate; the sulphuretted hydrogen gases taken from No. 5 tested over 30 per cent. of sulphuretted hydrogen, whilst the useless gases from No. 1 tested only 1 per cent., thus showing that vat waste is a very efficient absorbent of sulphuretted hydrogen.

"The connections were then altered so that the sulphuretted hydrogen gases could be taken from No. 5 to the gas holder; No. 2 vessel, containing the partially carbonated waste, was put into series, and the lime-kiln gases were now pumped through Nos. 2, 3, 4 and 5, the sulphuretted hydrogen gases from No. 5 continuing to be taken to the gasholder for a period of 2h. 25m. till 11.5. It was then found that the sulphuretted hydrogen gases were under 30 per cent., and that Nos. 2 and 3 were carbonated so thoroughly that water filtered from the mud did not tinge lead paper; Nos. 2 and 3 were therefore emptied and re-charged.

"At 12.25 p.m. the lime-kiln gases were pumped into No. 4, containing partially carbonated waste, and thence through Nos. 5, 6, and 7; the sulphuretted hydrogen gases from No. 7 passing to the holder from 12.25 to 3.30-i.e., during 3h. 5m.

CO2

"At 11.5 a.m. the lime-kiln gases were pumped into No. 5, and thence through Nos. 6, 7, 1, 2, and 3; the useless gases issuing from No. 3 passed into the purifiers, and thence escaped until 12.25-i.e., during 1h. 20m.

оо

"At 3.30. Nos. 4 and 5 were re-charged with fresh waste, the lime-kiln gases were pumped into No. 7, and thence through Nos. 1, 2, 3, 4, and 5; the useless gases from No. 5 passed away until 4.35-i.e. during 1h. 5m.

"From 4.35 till 8.20 the lime-kiln gases were pumped through Nos. 6, 7, 1 and 2, the sulphuretted hydrogen gases from No. 2 being simultaneously taken to the gasholder-i.e., during 3h. 45m.

"At 8.20 Nos. 6 and 7 vessels having been re-charged, the lime-kiln gases were pumped through Nos. 2, 3, 4, 5, 6, and 7, the useless gases from No. 7 passing away until 9.20-i.e., during one hour.

"At 9.20 the lime-kiln gases were pumped through Nos. 1, 2, 3, and 4 till 1.5, the sulphuretted hydrogen gases being meanwhile taken to the gasholder until 1.5-i.e. during 3h. 45m.

"At 1.5 p.m. Nos. 1 and 2 were re-charged, and the series of operations recommenced.

"It will thus be seen that during 1h. 40m., 1h. 20m., 1h. 5m., 1h., 5h. 5m., the useless gases were escaping, whilst during 2h. 25m., 3h. 5m., 3h. 45m., 3h. 45m., 13 hours, the valuable sulphuretted hydrogen gases were being used; but about forty-five minutes must be allowed for and deducted while the vessels were discharged and re-charged and the various taps altered."

The mixture of sulphuretted hydrogen and air in the proportion 2H,S: O2 is passed at A into the Claus kiln (fig. 109A) through the perforated hearth. P, on which there is a layer of iron oxide O, superimposed on broken bricks. Combustion takes place here, the oxide supplying heat to start the reaction, and the sulphur and steam pass on through D, C, and S. From the chamber