COAL TAR 405 



hot envelope of coke the gas gets poorer and poorer as the carbonisation proceeds, so that for 

 the last hour during which gas is coming off it is practically non-luminous. 



In the vertical retort and full horizontal the space above the coal is done away with, and 

 as the heat penetrates the charge, gas, tar, vapour and steam distil inwards from the exterior 

 portions, and the vapours condensing in front of the zone of heat form a pasty tube through 

 the cool coal core of which the gases escape without undue degradation to the mouthpiece. 

 In this way about two thirds of the charge is distilled under conditions favourable to the 

 formation of good gas and tar, whilst a larger proportion of ammonia escapes destruction, 

 but as soon as the central core gets tar-logged the remaining gas and vapour are forced back 

 through the red hot column of coke and undergo complete degradation. 



During the last few years no very marked improvements have taken place in the purifica- 

 tion of coal gas, but a very interesting revival of an old process under new conditions has 

 taken place at the Cheltenham gasworks, which may prove of great value. 

 Coal liming. About thirty years ago the Cooper process of mixing lime with the coal before 

 carbonisation, in order to increase the yield of ammonia, was extensively tried, 

 but abandoned on account of the increase in ash it gave in the coke. J. Paterson and 

 Twycross have now tried moistening the surface of the coal after crushing with a small steam 

 jet, and dusting 2 per cent of lime over it, which adheres uniformly to the moistened surface, 

 and shows a marked result not only in the amount of ammonia obtained, but in the purifica- 

 tion of the gas from bisulphide of carbon, the sulphur compounds other than sulphuretted 

 hydrogen being reduced to 20 grains per 100 cubic feet, instead of being double that amount, 

 whilst the increase in the ash of the coke is practically negligible. 



Attempts are also being made to introduce direct processes for the recovery of the ammo- 

 nia as sulphate, by washing the gas after removal of the tar with dilute sulphuric acid, as is 

 done in some coke oven plants, whilst a very interesting system of recovering 

 Burknelsers tne ammonia as sulphate and at the same time utilising the sulphur of the 

 sulphuretted hydrogen in the crude gas is also under trial. This is known as 

 W. Burkheiser's process, and consists of removing the tar and passing the gas with the ammo- 

 nia and sulphuretted hydrogen in it through porous hydrated ferric oxide warmed to 60 C., 

 which removes the whole of the sulphuretted hydrogen, but not ammonia; when the purifier 

 is saturated with sulphide, a current of air and steam is led through it, the sulphur burns, 

 forming sulphur di- and tri-oxide, mixed with the residual nitrogen of the air: these gases 

 are then passed in small quantities into the gas containing ammonia flowing from another 

 purifier of the same kind, and the sulphate and sulphite of ammonia so formed is washed 

 out of the gas in a scrubber with a dilute solution of ammonium sulphate, which is used until 

 concentrated, when a mixture of two thirds ammonium sulphate and one third ammonium 

 sulphite crystallises out. (VIVIAN B. LEWES.) 



THE COAL-TAR INDUSTRY 1 



The history of coal-tar and its utilisation has undergone an interesting phase during 

 the past few years. It is well known that the quality and value of coal-tar is as depend- 

 ent upon the temperature at which the eoal is carbonised as upon the character of the 

 coal, whilst the amount of heating and contact with red hot surfaces that the tar vapours 

 are subjected to after their formation also influences its composition; as a result the tar 

 has shown distinct changes with every alteration in gas manufacture, these changes 

 affecting the amounts of the products that can be obtained from it on distillation. 



These changes are best seen by the percentage fractions obtained from the tar: 



Period. . . . . T;;. 1820 to 1850. 1850101898. 1898101911. 



Process of carbonisation Iron retorts. Fireclay Fireclay 



retorts. retorts. 



Temperature of carbonisa- 

 tion 8ooC. i,oooC. i,o50C. 



Yield of gas, cubic feet per 



ton 9,000 10,500 11,500 



Composition of tar by weight 



Ammoniacal liquor ... 1 . 04 1 . 02 o . 383 



Crude naptha . . .... 3.73 3.45 0.995 



Light oils . ,. ., . ..--,. 4.47 2.59 0.567 



Creosote oil . ,: . . . 27.29 27.33 I 9-44 



Anthracene oil . ,. . . . 18.13 13-77 12.280 



Pitch :-.:. 41.80 47-67 64.080 



Specific gravity of tar . . . 1.140 I-I54 1.206 



1 See E. B. vi, 595 el seq. 



