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F E B R U A R Y / M A R C H 2 0 1 7 www.resourceworld.com 71 grind the ore to fine powder (micron size) using rods, balls, with the ore in the SAG mill to assist with the grinding, while the mills rotate. Once the ore is ground to the prescribed size for optimum recovery of the economic minerals, then these metals are recovered by a number of processes. Gold and sil - ver can be recovered by gravity for the free gold/silver followed by cyanidation of the lower grade ore to make doré bars on site from both products. Alternatively, after gravity, a flotation step can be used to produce a lower grade concentrate to be shipped to smelters worldwide for final processing. Base metal and polymetallic ores usually use differential flotation to produce one or more concentrates to be sent to smelters worldwide. Heap leaching has been used for some time for lower grade, precious metal and copper deposits, whereby the ore only has to be crushed to a size that liberates most of the economic metals and does not have to be ground, a quarter inch to a few inches in size is sufficient to achieve metal recoveries in the order of 60% to 80%. Heap leaching eliminates the need for a grinding circuit, flotation and/or large cyanide tanks, thus reducing the initial capital and operating costs. The crushed ore is trucked or con - veyed to a leach pad where an impervious liner collects the leach solution from the heap, which has been sprayed with cyanide as each layer of ore is placed. Carbon-in-Pulp, Carbon-in-Leach, Merrill Crowe, Solution-Extraction-Electro-Winning are a few of the processes to recover pre - cious metals from the cyanide solution. Gold and silver doré that is produced on site from conventional milling and heap leaching will be sent to refineries includ- ing the Canadian mint for final refining to 99.999% purity from a doré bar that con- tains between 80% and 99% gold or silver. Doré bars provide quick cash flow from production with low shipment and refin- ing costs compared to concentrates of large volumes that may require shipment halfway around the world with payments for the metals staggered over months. Tailings from all of the processes must be deposited in a conventional tailings storage facility, near the plant, with a dam and often lined, or dried and stacked, or deposited underground after thickening, or after it has been made into a paste. If cyanide is used, then residual cyanide in the tailings stream must be destroyed prior to leaving the plant. In summary, building a mine takes a long time from discovery to mining the deposit, usually 10 years or more with a huge amount of capital invested by many investors from the high-risk exploration stage through feasibility to building the mine. The skills and expertise of people from very diverse backgrounds are used in the conception and planning regarding the technical, environmental, social and eco - nomic impacts of the project on the local habitat, communities and the country. n