Amalgamating gold ores on plates in the tube mill or classifier circuit has, in cyanide plants, succeeded the time-honored position of the plates in front of the battery, especially in large mills where the amalgamating may be carried on in a separate department. If the process involves the use of plates after the classifiers, the pulp will be dilute and the plates will have a low inclination but if the plates are placed immediately at the tube mill discharge before dilution, the plates must be steep to take off the thick pulp.

The effect of taking plates away from the battery and putting them in the tube mill circuit is indicated by the following experience in South Africa:

The net result was that the amount of gold amalgamated remained the same, the area of the plates was greatly reduced and the total extraction slightly increased. The pulp in the tube mill circuit contained 55 per cent, moisture and the plates have a grade of 18 per cent, or 2.16 in. to the foot.

At the Robinson Deep mill, South Africa, where amalgamation is practised in both circuits of stamps and tube mills, the percentage of total recovery is as follows: From battery plates, 47 per cent.; from tube mill plates, 23 per cent.; total extraction, 70 per cent. ; and recovery by cyaniding, 30 per cent. From these figures it must not be assumed that the recovery from amalgamation has been increased 23 per cent, from what could be obtained from battery plates alone, for by the use of the tube mill the ore is crushed to a coarser mesh in the stamps, and the tube mill continuing the process releases gold that could be less advantageously recovered by the stamps alone.