The tailing from a stamp mill or other intermediate crusher often contains considerable oversize, which holds enough metal to pay the cost of regrinding to a finer mesh and extracting the metal therefrom by amalgamation. For this purpose we may use Chilian mills, pans or tube mills. The tube mill requires less attention, can be adjusted to produce a granulated or slimed product by altering the feed of ore and water, and has a greater capacity for fine grinding than other machines, and has therefore been chosen to follow stamps with the plates taken away from the battery and put in the tube mill circuit. The plates then receive no coarse material, and the maximum amount of metal is recovered by amalgamation without requiring the stamps to break the ore to a fineness not warranted by their weight. As this regrinding is not often required finer than 100-mesh the Hardinge conical mill is equal to other grinding machines on the market for this class of work. As a grinder to 200, I have disqualified the Hardinge mill but for this particular purpose it may be equal to the cylindrical tube mill. What follows in respect to amalgamating with tube mills must therefore pertain particularly to the Hardinge conical mill.

Ores may be reground in water or in cyanide solution and amalgamated inside the tube mill or after the pulp has been discharged. For convenience the subject will be treated as follows:

I. General hints on amalgamation.
II. Amalgamating in cyanide solution.
III. Amalgamating inside the tube mill.
IV. Amalgamating in the tube mill circuit.

I. GENERAL HINTS ON AMALGAMATION
The first requisite of the successful amalgamator is clean mercury for that which has been fouled by grease or impurities will not combine readily with metallic particles. If the mercury is newly bought, or has previously been used, it should be digested with weak nitric acid and stirred frequently to dissolve the impurities. As mercury likewise dissolves in nitric acid an iron nail will cause its precipitation, the iron taking the place of the mercury. The iron salts may be washed off with water and should grease still appear on the surface a little metallic sodium will get rid of this, but do not add enough sodium to cause an iron nail to amalgamate.

If the mercury must be retorted, conduct the process at the lowest heat possible and keep the mercury coming off last for future distillation, as it contains most of the volatile impurities. If the distilled mercury is condensed in a vessel with dilute sulphuric acid, the impurities coming over with the mercury will be dissolved and the latter left bright and clean.