From the above explanation of the effect of the various constituents of ores on smelter practice, one can readily see that the maximum return is obtained when lead, copper or zinc concentrates are shipped to the respective plants which specialize in their treatment. For example, if you ship a lead-copper concentrate to a lead smelter, you will receive about 30 cents a pound less for the copper, while if this same lead-copper concentrate is shipped to a typical copper plant, you will be paid for only half the lead and will get about 40 cents a pound less for that lead. If the lead and copper minerals are separated into two products by selective flotation and each shipped to the proper smelter, you will receive the maximum return on both.
This difference in net returns is still more apparent when zinc is considered, for zinc in a bulk concentrate is very undesirable at lead or copper smelters. You usually not only get nothing from these plants for the zinc, but are penalized for it above a certain minimum, and the amount admitted without penalty is as low as 5% in some instances. On the other hand, if the zinc is separated and shipped to a zinc plant, it becomes an asset instead of a liability.
It is, therefore, evident that milling is the solution of the problem of separating the various minerals into concentrates from which the maximum returns will be secured from the various treatment plants. In our general metallurgical and milling practice we have tested many different types of ores and have built mills that have successfully carried out the procedure indicated in the laboratory. The above table clearly shows how a complex ore can be separated into its various constituents to secure the maximum commercial results. It is doubtful whether the ore could be mined at a profit without using these standard modern milling methods.
Also, by the use of the Mineral Jig we are now able to remove the free gold from these complex ores, and even from flotation concentrates, and this gold is then recovered by amalgamation as bullion, whereas formerly it was sent to the smelter in a form difficult to sample and was paid for at a reduced price.
This discussion does not pretend to be a treatise on smelting. We have merely attempted to explain briefly the principal factors which affect the schedules. We hope that our suggestions in regard to the separation of various metals and the amalgamation of metallics, both gold and silver, will give ideas to many of our friends, and that we may be able to render further assistance in the future.
We have attempted to show how every producer of concentrates should study his own individual problem to be able to net the greatest commercial return, how possible changes in processes should be investigated if they indicate an opportunity to produce material for which there is a better market, and how changing conditions in metal prices and in the requirements of smelters affect the problem. Perhaps you can increase your profits by amalgamating the metallics recovered by a Mineral Jig prior to flotation; possibly you can do better by making a cleaner concentrate; it may be that separating the lead from the copper will mean more profits; you may need another flotation circuit to recover zinc now going into the tailings; you might be able to recover more gold by making a separate iron flotation concentrate, or by cyanidation of your flotation tailing. Such changes in mill flowsheets are not visionary, but practical. All these things can be done, have been done, and are being constantly studied. If you lack the equipment, the staff or the time, or desire to check your results, the metallurgical laboratory is available for such investigations.