146 



in some cases, bj^ the addition of organic colloidal matter, such as 

 humic acid prior to the use of alkali. 



The apparatus for treating clays consists of a wooden trough 

 with one electrode of wire gauze, and the other a metal drum rotating 

 in the trough containing the alkaline clay slip. The electric current 

 is then passed through the fluid, using the gauze and drum as poles. 

 In this apparatus, a further slight purification is effected owing to the 

 tendency of the impurities to travel to one electrode whilst the clay 

 (with some silica) travels to the other. The clay and finest silica 

 particles assume a negative charge, but that of the silica is so slight 

 that the silica remains almost stationary in the fluid, whilst the clay 

 travels to the anode. Pyrite, alumina and ferric oxides are positively 

 charged and travel with the water towards the cathode. The 

 dominant feature in the speed of migration is the valency of the 

 material attracted to the diaphragm. 



The chief use of the electro-osmosis process is for producing a 

 clay paste sufficiently dry for commercial purposes, as the greater 

 part of the purification is effected before the current is applied. 



The electro-osmose process whilst theoretically interesting, is not, 

 at present, regarded as of much practical importance, as the use of 

 electricity to separate clay and water is more costly than other equally 

 efficacious methods. Moreover, the finest particles of free silica 

 migrate simultaneously with the " true clay " so that only a very 

 limited purification by the electric current is possible, the greater 

 part having been done by the electrolyte added, which is not an 

 essential part of the osmosis process. 



In East Germany, it has been used for some time on a commercial 

 scale, and about two years ago an English sjoidicate was formed to 

 exploit the Schwerin patents, and to suj)ply certain requirements 

 of the Optical Department of the Ministry of Munitions. For further 

 information on the electro-osmose process for the treatment of clays 

 see " The British Association Report on Colloids, II, 1918," pp. 42-4, 

 47-52. 



A possible method of purifying clay and separating the colloidal 

 silica is due to Billitzer (1905), who found that in N/2 to N/10 

 solutions of hydrochloric acid, the charge of coUoidal sUica is changed 

 from electro-positive to electro-negative so that the careful addition 

 of acid should enable a practical separation of colloidal silica from 

 colloidal clay to be made. The importance of this suggestion appears 

 to have been overlooked. 



In separating clay and* wafer, as when it is desired to dry a clay 

 slip or suspension, use may be made of the electric and colloidal 

 properties of the clay. This is an essential feature of the Schwerin 

 electro -osmosis process^^ previously described. On passing a current 

 of electricity through the clay slip, the clay migrates to, and is retained 

 by the rotary electrode (anode) and the water tends to travel to the 

 cathode, so that the clay removed from the drum is drier than from 

 a filter press worked at a pressure of 150 lb. per sq. inch. Thus, 

 some ball clays may be obtained with only 17 or 18 per cent, of 

 water, and so dry that it cannot be pugged, whilst the same clay. 



