ALUM 113 







Here tho sulphur has evidently existed in combination with iron, which has boon 

 united to oxygen by tho analysts. The amount of sulphate shows a partial disinte- 

 gration 'and other changes. 



Lampadius gives another analysis with much more sulphur : 



Alum- Shale from Siettda* 



Sulphate of alumina 2-68 



Potash-alum .......... 0-47 



Sulphate of iron ...... 0-95 



Sulphate of lime 1-70 



Silica \ jo-32 



Alumina . . . .. . . . . . . 9-21 



Magnesia traces 



Oxide of iron . ......... 2-30 



Oxide of manganese ........ 0-31 



Sulphur 7-13 



Water ........... 33-90 



Carbon ......... . 31-03 



100-00 



When alum is made of such shale, the object is first of all to oxidise th sulphur, 

 forming sulphuric acid. This acid then dissolves the alumina. The result may be 

 accomplished by allowing the shale to disintegrate spontaneously in the air, tho 

 sulphur oxidising and dissolving the alumina. But in general, as at Whitby and 

 Campsie, combustion must be resorted to. This can be accomplished without the use 

 of coal, further than is needful simply to set fire to that portion which exists in tho 

 ehale itself. Indeed, the Campsie shale, having more coal than is desirable for slow 

 combustion, is mixed with some spent material, in order to diminish the force of the 

 heat. 



The sulphur is united with the iron, forming a bisulphide, each atom of which 

 must, therefore, take up seven atoms of oxygen, FeS' J 4-70=FeO. S0* + S0*. When 

 combustion takes place, the sulphur oxidises : if rapid combustion is permitted, sul- 

 phurous acid gas escapes ; if slow combustion, the sulphurous acid penetrates the 

 mass slowly, receives another atom of oxygen, unites to a base, and a sulphate is the 

 consequence. Sulphate of iron is formed, and free sulphuric acid. In the process it 

 is probable that the oxidation is completed by means of the iron. Protoxide of iron 

 readily becomes peroxide ; the sulphurous acid- readily decomposes peroxide, forming 

 sulphuric acid and protoxide of iron. This protoxide of iron is again converted into 

 peroxide, and if not dissolved is rendered, to a great extent, difficult to dissolve, by 

 reason of the heat of the mass. For this reason partly, there is less sulphate of iron 

 in the alum than might be expected. To- effect these changes it is desirable to burn 

 very slowly, so as to allow no- loss of sulphurous acid, and, in washing, to allow 

 the water to stand a long time on -the burnt- ore. Another method, by which the 

 sulphuric acid is transferred to the alumina, is the peroxidation of the protoxide 

 in the sulphate of iron; -acid 'is by this- means set free and begins to act on the 

 alumina. 



The protosulphate of iron being formed, it is removed by boiling down the liquor 

 until the protosulphate of iron crystallises out, at the same time the solution becom- 

 ing saturated with the aluminous salt. The sulphate of iron is soluble in 0-3 of hot 

 water, the alum in 0-06. The liquid around the crystals on the remaining mother- 

 liquor contains iron also ; this is washed off by adding pure liquors. 



The presence of lime or magnesia in the ores is, of course, a means of abstract- 

 ing acid, preventing the alumina being dissolved, and even precipitating it when 

 dissolved. 



Knapp says that at Salzweiler, near Duttweiler, in Rhenish Prussia, the roasting 

 of the ore takes place in the pit or mine. The stratum of brown coal which lies 

 under it, having been accidentally set fire to in 1660, has smouldered till tho present 

 time without intermission. 



When the ores are roasted, one-half of the sulphur is freed and sent into the mass, 

 or escapes as sulphurous acid; and the remaining protosulphide of iron is after- 

 wards converted into green vitriol. 



When the calcined mineral becomes thoroughly cold, we may proceed to the 

 lixiviation. But as, from the first construction of the piles or beds till their com- 

 plete calcination, many weeks, or even months, may elapse, care ought to be taken 

 to provide a sufficient number or extent of them, so as to have an adequate supply 

 of material for carrying on the lixiviating and crystallising processes during tho 



VOL. I. I 



