The quantity of hydrochloric acid must be carefully gauged—continued till just a faint smell of sulphurous acid is apparent—and the contents of the " tub " must he well agitated from time to time. The liquors are next run into tanks where the freed sulphur settles out, and is washed, dried, 2,nd melted—usually by Schaffner's process, of which some further notice will be given. Ingenious though this extraction method is, and successful in so far as it deprives tbe tank waste of those constituents which make it an absolute nuisance, it must be confessed that it does not repay the expenditure upon plant, and the _trouble of carrying out a delicate operation. It is, however, carried on to some considerable extent by manufacturers who, from the situation of their works, are specially compelled to guard against nuisance. TJnless carefully watched, both in the oxidizing and decomposing operations, an appreciable amount of sulphuretted hydrogen is given off. The average amount of sulphur recovered is two-fifths of the total amount in the waste.
Schaffner's process is, in the matter of oxidation, very similar to Mond's, except that the waste is first separated into small heaps in the open air and left to natural oxidation for some weeks. After lixiviation of the heaps the residue is again oxidized by naechs,nical means, and this time in the tanks in which it has been lixiviated. After a second lixiviation, the operation of blowing and washing is u third time performed. The liquors, containing polysulphides and hyposulphites are run into a kuir of stone or cast-iron cisterns, s,nd treated with hydrochloric acid. The polysulphides ore first decomposed and converted into chlorides, with liberation of sulphur and sulphureth d hydrogen. The hydrochloric acid so far is run only iuto one of the cisterns. After decomposing the polysulphides the acid acts upon the hyposulphites, freeing sulphur and, now, sulphurous acid. The latter passes over into the second cistern and converts all the sulphur compounds of the liquor contained therein into hyposulphites. The contents of the first cistern are freed from sulphurous acid by blowing in steam, and are then run off from the bottom and a fresh charge of yellow liquor is introduced. The hydroohlorie acid is now run into the second cistern. Sulphur is freed and generated, the latter returning sulphurous acid to No. 1 cistern, and converting the polysulphides of the fresh liquor into hyposulphites. So, after the first operation, the process is continuous, sulphur and sulphurous acid being only freed, the former draws off with the liquor and the latter returning to the other cistern to act upon the sulphides. The liquor drawn from the cisterns contains besides the freed sulphur, calcium chloride, sodium chloride, and calcium sulphate. The sulphur is freed by filtration and washed. The purification of the product has already been mentioned when speaking of Mood's process. A cylinder of east-iron is enclosed within an outer cylinder of wrought iron, with a small space between, the whole being inclined at an angle of some 15°. The
washed sulphur, mixed with a little water and milk of lime, is introduced into the inner cylinder, and steam at a pressure of 1 to 2 atmospheres blown into the space between the cylinders. Finally entering the inner cylinder, through the charging door, it melts the sulphur and passes off through un exit pipe loaded with a suitable safety valve. A mechanical stirrer, revolving with the axis of the cylinder, materially assists in the melting operation. The fused sulphur collects in the lower part of the cylinder and is run off into moulds. The supernatant, lighter liquor is then run to waste. Tho addition of lime is made to neutralize any acid there may he in the mixture of sulphur and water. This process is exceedingly ingenious, and doubtless Mond has drawn a great deal upon it. Owing, however, to imperfect oxidation, only about one-half the total sulphur contained in the waste is regenerated.
Hoffman's process is, so far ns the oxidation of the tank waste is concerned, exactly similar to Schnffner's. The yellow liquors, however, are treated with waste still liquor (chloride of iron and manganese), the free acid and chloride of iron decomposing the sulphides and hyposulphites. The first resulting sulphuretted hydrogen is burned, while the sulphurous acid converts the poly sulphides into hyposulphito. Since the adoption of Weldon's process for the recovery of manganese from still liquors, Hoffman's process has been abandoned.
The last process to be mentioned is that of Mactear, of the St. Rollox Works. It depends upon the decomposition of the sulphides of calcium of the tank waste by hydrochloric acid in the presence of sulphurous aeid. The drainings from the tank heaps, or the waste itself (old or new) ground into a milk with water, are treated with a solution of sulphurous acid in water, and hydrochloric acid. The sulphurous acid solution is obtained from the combustion of pyrites, or refuse sulphur, the product being passed up wooden towers packed with coke, down which a stream of water is run. The solution mixes with the yellow liquors on its way to the decomposing vessel, the polysulphides aro converted into hyposulphites, and the whole is treated with hydroelllorie acid with the result already described when describing other processes for the regeneration of sulphur. During decomposition the temperature is kept at 63° (145 F.) by passing in steam. Mactear's process certainly has the advantages of a more perfect purifying of the waste, and a lower cost of sulphur over its rivals. The detailed cost of one ton of sulphur is as follows :— A plant to produce 30 tons of sulphur per week costs about 20001. Mactear states that he obtains about 90 per cent. of the sulphur contained in the waste, or drainiugs, using the ono or the other as may best suit—when the drainage is plentiful old waste, when small the fresher material.