The whole is then agitated with air, when a sample taken from the tank and filtered should give a clear filtrate.
Having obtained this clear solution, agitation is stopped, and[Pg 112] the contents of the tank passed through a filter press. The scum, which accumulates on the treatment tank, may be transferred to a perforated box suspended over the tank, and the liquor allowed to drain from it. The filtered liquor is now rendered slightly alkaline by the addition of caustic soda or carbonate, and, after filtering, is ready for evaporation.
The acid and alum salt used in the above treatment must be carefully examined for the presence of arsenic, and any deliveries of either article, which contain that impurity, rejected.
Lime, bog ore, and various metallic salts, such as ferric chloride, barium chloride, and copper sulphate have been suggested, and in some instances are used instead of aluminium sulphate, but the latter is generally employed.