554 GAS, COAL 



impurities ifl not difficult. Slaked lime, either in the form of moist powder, or sus- 

 pended in water as milk of lime, absorbs the whole of them ; whilst it has no per- 

 ceptible effect upon the other constituents of the gas. By this process of purification 

 the sulphuretted hydrogen and caustic lime are converted into sulphide of calcium 

 and water ; the former being non-volatile, does not mix with the gas. Hydrosulphato 

 of sulphide of ammonium is in like manner converted into sulphide of calcium, water, 

 and ammonia: j>art of the latter is retained by the moisture present in the purifying 

 material, but the remainder mixes with the gas, from which, however, it can be 

 removed by contact with a large surface of water. Carbonic acid unites with caustic 

 lime with great energy, forming carbonate of lime, a perfectly non-volatile material ; 

 and thus the acid gas is effectually retained. Carbonate of ammonia is under similar 

 circumstances decomposed, carbonate of lime being formed and ammonia liberated ; 

 the last, as before, being only partially retained by the moisture present, and requiring, 

 when ' dry lime ' is used, a subsequent application of water for its complete removal. 

 Although in the wet-lime purifying process a given weight of Jimo can remove a 

 much larger volume of impurities, yet the dry-lime process possesses so many mani- 

 pulatory advantages that it is now all but universally employed where lime is used as 

 the purifying agent. The maximum amount of sulphuretted hydrogen or of carbonic 

 acid which can be absorbed by 1 Ib. of quick-lime, in the so-called dry and wet states 

 respectively, is seen from the following table : 



Cubic feet of Cubic feet of 



Sulphuretted hydrogen Carbonic acid 



1 Ib. of quick-lime used as dry lime absorbs . 6'78 . . 3-39 



1 Ib. of quick-lime used as wet lime absorbs 678 . 078 



In practice, however, the absorption actually effected is, even under the most 

 favourable circumstances, considerably less than here indicated. As a substitute for 

 lime in the purification of gas a mixture of hydrated peroxide of iron and sulphate of 

 lime has lately come into extensive use. This material is prepared in the first place 

 by mixing slaked lime with hydrated peroxide of iron, the composition being rendered 

 more porous by the addition of a certain proportion of sawdust. This mixture is 

 now in a condition to remove those impurities from coal-gas which are abstracted by 

 lime. The peroxide of iron absorbs sulphuretted hydrogen and sulphide of ammonium, 

 and becomes converted into sulphide of iron. The slaked lime absorbs carbonic acid 

 and carbonate of ammonia until it is converted into subcarbonato of lime. When the 

 absorbing powers of the mixture are nearly exhausted, the covers of the purifiers are 

 removed and the mixture is exposed to the air. The following change is then said to 

 take place. The sulphide of iron rapidly absorbs oxygen and becomes converted first 

 into sulphate of protoxide of iron and finally into sulphate of peroxide, which latter is 

 decomposed by the carbonate of lime, carbonic acid being evolved as gas, whilst sul- 

 phate of lime and peroxide of iron are produced; the mixture is thus again rendered 

 available for the process of purification ; the peroxide of iron acts as before, but in 

 the place of quick-lime we have now sulphate of lime, which is quite effectual for the 

 removal of carbonate of ammonia, with which it forms carbonate of lime and sulphate 

 of ammonia ; but the mixture is incapable of removing free carbonic acid, and it is 

 therefore necessary to provide a separate dry-lime purifier for the removal of this pis. 

 When the purifying material is again saturated with the noxious gases, another 

 exposure to atmospheric oxygen restores it again to its active condition, the only 

 permanent effect upon it being the accumulation of sulphate of ammonia within its 

 pores. If this latter salt be occasionally dissolved out with water, the mixture- may 

 be used over and over again to an almost unlimited extent. It has been found that 

 this process can be much simplified ; and Mr. Hills, who has brought gas-purification 

 to great perfection, recommends that hydrated peroxide of iron should bo merely 

 mixed with a considerable bulk of sawdust and placed in the purifiers. After the gas 

 has passed through this mixture for 18 hours, it is shut off and replaced by a current 

 of air forced through by a fanner for 3 hours. The sulphide of iron is thus oxidised, 

 sulphur being separated and hydrated peroxide of iron regenerated ; and the purifying 

 material being now revivified, the gas may be passed through it again as before. In 

 this way it is only found necessary to remove the material once a month in order in 

 separate the lowest stratum of about an inch in thickness, which has become d'^c d 

 up with tar. A proportional quantity of fresh mixture of hytlratod peroxide of im-i 

 and sawdust having been added, the whole is again returned to the purifier. It, is 

 difficult to conceive a more simple and inexpensive process of purification than this. 

 It does not, however, remove carbonic acid. Several other materials have been pro- 

 posed for the separation of sulphuretted hydrogen from coal-gas, such as sulphate .f 

 lead, and chloride of manganese, but they possess no peculiar advantages, and have 

 never boon extensively adopted, 



