SODA.— Utilization of Tank AVaste. 843
The wages (including repairs) should be within40.9. per ton, and the following general calculation isgiven by Morison, to whom the above statementsof yield and costs are due as well:—
S. D. £ S. D
Fuel, . 6£ tons at 7 0 .... 2 5 6
Saltcake, . 37 cwts. at 3 3 .... 6 0 3
Limestone, . 22 “ 0 4 .... 0 7 .4
Lime, . 11 “ 1 0 .... 0 11 0
Salt. 3 “ 0 8 .... 0 2 0
Nitre,. 40 lbs. at 0 .... 0 5 0
Wages,. 2 0 0
Water,. 0 1 6
Drums,. 0 16 0
Charges and shipment,. 0 15 0
Oflice expenses and management, . 0 5 0
Interest on capital, depreciation, and stores, . 0 16 0
Total cost f.o.b., Liverpool, . 14 4 7
This was the cost of 60 per cent, white caustic in1874; but evidently it must be made more cheaplyin years of cheaper coal, &c., as it is often soldbelow the above price.
Utilization of Tank Waste. —Only those attemptsin this direction which have been crowned withsome measure of success will be mentioned in thefollowing:—
Hyposulphite of Soda (or Lime) from TankWaste. —W. S. Losii in 1852 patented the followingprocess:—The waste is exposed to the air for sometime (a week will do). The mass is then lixiviated,and the solution contains partly sodium, partlycalcium hyposulphite. By adding some sodiumcarbonate all hyposulphite is converted into thesodium salt which is separated from the precipitateof calcium carbonate, and the solution is concen-trated by evaporation. The crystals of sodium hypo-sulphite are purified by redissolving. This processhas ever since been carried out successfully at theWalker Alkali Works, near Newcastle-on-Tyne .
E. Kopp has modified the process by mixing thewaste with 10 to 15 per cent, sulphur and 12 to 15times its weight of water, and boiling it for an hour.The solution of calcium polysulphide thus obtainedis put into a closed vessel furnished with agitatinggear, and treated with sulphurous acid, drawn offand settled. The clear liquor is now a solution ofcalcium hyposulphite, which can be recovered byslow evaporation and crystallization. Stevensonhas shown that this process can be carried out safely,without decomposing the salt, by using a vacuumboiler. If sodium hyposulphite is wanted, thecalcium salt is decomposed by sodium sulphate, thesolution separated from the gypsum, concentrated,and allowed to crystallize.
It must be borne in mind that the demand forhyposulphites is a very limited one, and that eventhe most successful process for their manufacturecan only apply to a comparatively trifling portionof the tank waste produced in alkali works. Themethods for recovering the sulphur from the wholeof the waste are, therefore, of much greaterimportance.
Recovery of Sulphur from Tank Waste. —Threeprocesses have been carried out in the large practicefor this object, and a combination of the last two of
them is now frequently employed. In order of timeit seems as if Sciiaffner had found out his methodfirst, followed by Mond shortly after, and quiteindependently; Hofmann’s process was inventedlast of all, though described here first.
Process of P. W. Hofmann (1866). — The tankwaste is oxidised in heaps, and it is mixed at thesame time with ferric oxide obtained in its workingup, as will be seen below; this greatly assists theoxidation. After six or seven days the waste islixiviated, and yields a liquor containing mostlypolysulphides. The residue is oxidised again, lixivi-ated, and the same process repeated a third time ;the last liquor contains more hyposulphites. Thelixiviation takes place in methodically arranged vats.
The liquors are all united, and decomposed by meansof the acid “ still liquor ” (from the manufacture ofchlorine by native manganese). The free acid andchlorine, and. the ferric chloride, are all active in ■decomposing the sulphide and hyposulphites; and ifthe proportions of the liquors were right, onlysulphur is precipitated, but no sulphuretted hydrogenis evolved. As this is not always avoidable, theliquors are first mixed in a closed apparatus andthe gas is conducted through a small wood fire,which inflames the sulphuretted hydrogen ; the sul-phurous acid is conducted into a tank containing“yellow liquors” from the lixiviation of waste, andthis converts its polysulphides into hyposulphites.
The separated sulphur is washed and dried; theneutral manganese chloride is treated with freshtank waste and a little lime, till all iron is pre-cipitated and the ferric oxide mixed with the bulkof the waste for oxidation, as mentioned above.
The manganese chloride, now quite free from iron,is treated with “yellow liquor,” which results in aprecipitate of manganese sulphide containing freesulphur. After drying and calcination (during whichthe sulphur is turned into sulphurous acid, and thelatter conveyed into a sulphuric acid chamber) aresidue is obtained consisting of a mixture of Mn g 0 4and MnS0 4 ; the latter is removed by lixiviationand the residue sold to glass works, for which it isvaluable on account of its freedom from iron. Themanganese sulphate is recovered in a pure state byevaporation; it is mixed with sodium nitrate andcalcined in the sulphur furnace. The result is sodiumsulphate and manganese nitrate; the latter, how-ever, is at once decomposed into a mixture ofdifferent oxides of manganese and nitrogen tetroxide(N 2 0 4 ), which gas enters the lead chambers andthere acts in the well-known manner. The residueis lixiviated. The solution of sodium sulphate isused for manufacturing this salt, or for making“ pearl hardening ” (sulphate of lime); the residuecorresponds to a manganese ore containing 55 percent, of peroxide, and is used for manufacturingchlorine in the usual stills.
Hofmann’s process has been used for a number ofyears in the large works at Dieuze , in Lorraine ;but it has not spread any further, and it is notlikely to do so, since Weldon’s manganese recoveryprocess is much simpler and more economical.