THE POSITION OF THE TUBE MILL
Whether or not we amalgamate after stamping we must fix the position of the tube mill in respect to the stamps, classifiers and plates so that the whole process of extraction may be benefited. The most important modification noticed in stamp milling in consequence of regrinding in tube mills is that the point in the process at which the gold is extracted has changed. In times past the gold was always amalgamated in the mortar of the stamp mill and the plates were immediately in front of the battery. We then saw the plates go to a separate building, still taking the pulp from the stamps: then with the advent of the tube mill in the metallurgical treatment of gold ores the plates were placed in the tube mill circuit and abandoned in front of the battery; then amalgamation was tried in the tube mill and classifier circuits; then we find amalgamation taking place in the tube mill and not in the stamp mortar and then amalgamation was confined to the classifier circuit. Nearly the whole cycle of possible combinations has been tried, and, like many other problems in gold extraction, we must study each particular ore as a distinct problem. The following diagrams, Fig. 10, begin with a free-milling ore and trace the position of the tube mill in various combinations by flowsheets of actual mills now operating and while it does not exhaust the possible combinations, it is sufficiently extensive to show the great variety of ore-reduction processes now used for gold extraction by amalgamation and cyanidation.
A represents an absolutely free-milling process as at the Yellow Aster mill, California, where the whole process of gold extraction is in the battery and on plates.
B shows amalgamating in stamp battery on plates and saving the concentrate, as in the usual California mills.
C notes the first departure from former regular practice as here we have plates in front of the batteries and plates in the tube mill circuit.
D shows the next step where the plates have been taken from the batteries where for so long they occupied an honorable position to be put entirely in the tube mill circuit.
E is a development of the same idea using a Hardinge conical mill to further reduce the stamp mill product, amalgamation taking place in the Hardinge mill and on plates following.
F is a step further showing the plates in the tube mill and classiifier circuits.
G shows the plates entirely in the classifier circuit the tube mill being in a closed circuit with a classifier. This flowsheet is applicable for ores that contain gold in a condition where, even with tube mill grinding, the particles of gold are still too coarse for efficient solution in cyanide of potassium.
H shows amalgamation in pans, and really has nothing to do with the tube mill but shows a variation of gold extraction that will no doubt soon pass into oblivion.
We then have various modifications of the all-sliming process, using cyanide as the sole means of gold extraction with the tube mills in closed circuits with classifiers.
I, the tube mill takes the underflow from cone, the overflow from cone and tube mill product going to the classifier where the slimed material is eliminated.
J shows a simple flowsheet with the tube mill in a closed circuit with a classifier. The elevator may be dispensed with by giving the feed scoop of the tube mill a greater radius. The idea is shown in J2 which also illustrates the same simple combination feeding the dry product of roll crushing to the tube mill at the feed scoop with the coarse material from the classifier.
K shows a distributing cone feeding a classifier, the elimination of slime being from the classifier in closed circuit with tube mill.
L show's two tube mills and two classifiers working in closed circuits, the slime being separated in both classifiers.
M is a complicated flowsheet with cones, classifiers and tube mills, the slime being separated in the last cone.
N is an example of stage reduction with stamps, Hardinge mill and tube mill where each machine is doing the work for which it is best fitted. We note that a shaking screen precedes the stamps, thus bypassing the fine material that does not need crushing in the stamp-mill mortar to the classifying cone, the coarse material from cone going to Hardinge mill. From there on the cylindrical tube mill and classifier are in a closed circuit. This is an ideal arrangement for getting the best results from three types of mills, no machine being compelled to do work that could be done better in another machine.
