42 SECTIONAL ADDRESSES. 



Low Temperature Tar. 



There is at the present time in this country no process of chemical 

 industry which is more in the public eye than low temperature carbonisa- 

 tion of coal. The matter is of supreme national importance, for the larger 

 problems facing this mode of utilising coal are both economic and 

 technical and turn on the exploitation to the best advantage of the 

 resulting products : smokeless fuel, gas, aqueous liquor and tar. Now 

 since any marked appreciation can be expected only in the case of the 

 last of these products, it follows that processes tending to an increase in 

 the value of the tar are of fundamental interest. 



During the last five years a systematic study of the chemical con- 

 stituents of low temperature tar has been in progress in the Teddington 

 laboratory and, in our experiments on this material, quantities of the 

 order of 40 gallons have been handled in the semi-scale plant. The 

 starting materials, supplied by H.M. Fuel Research Station as part of 

 the Government's scheme of scientific investigation into the utilisation of 

 our national resources of coal, consist of pedigree tars derived from coals 

 of definite origin carbonised under carefully controlled and reproducible 

 conditions. 



It was soon found that although low temperature tar had been pro- 

 duced at carbonising temperatures of about 600°, yet it could not again 

 be heated even to comparatively low temperatures — round about 150° — ■ 

 without undergoing considerable alterations of a chemical nature. Accord- 

 ingly, distillation processes were replaced by milder methods of extraction, 

 and the tar was not heated above 120° until its more decomposable 

 constituents had been removed. 



A representative tar from a typical bituminous coal (Kinneil coal) was 

 heated to 120° to remove light oils and adhering aqueous liquor, and the 

 residue extracted by systematic use of solvents to separate it into its 

 major constituents : neutral oils and waxes, aromatic hydrocarbons, 

 bases, phenols and carboxylic acids. It was then noticed that each of 

 these five main groups of products could be separated into two fractions, 

 one portion consisting of crystallisable substances conveniently termed 

 ' crystalloids,' the other portion composed of amorphous resinous materials 

 to which the name ' resinoids ' was applied. 



The Crystalloids of Tar. 



Waxes and Neutral Oils. — From the least volatile fractions of neutral 

 oils, waxes are obtained melting over a considerable range of temperature, 

 and X-ray analysis of the less fusible of these waxes has revealed the 

 presence of hydrocarbon chains containing 26, 27 and 29 carbon atoms. 



The neutral oils contain both saturated and unsaturated hydrocarbons 

 and also oxygenated substances reacting with ferrichloric acid, HFeCl.,. 



Aromatic Hydrocarbons. — Naphthalene, a characteristic major con- 

 stituent of high temperature tar, is present in low temperature tar, 

 together with p-methylnaphthalene, but only in such small proportions 

 that they have to be separated through their picrates. 



The least volatile tar oils after removal of waxes and resins deposit 

 on cooling a material analogous to the green grease of high temperature 

 tar. This product consists principally of the methyl derivatives of 



