The Borax Smelting Step for Mercury-Free Gold: An Honest Energy, Loss, Throughput and Grade Budget for Direct Smelting of Gravity Concentrate

Process design and first-principles budget for borax-flux direct smelting of gravity concentrate as a mercury-free alternative to amalgamation. SIMULATED, NOT MEASURED; nothing was built, operated or field-tested. For a 1.00 kg concentrate batch at 1150 degrees Celsius, useful heat is 4.509 MJ and fuel energy is 20.50 MJ (0.446 kg LPG), the batch costs $5.80, and one furnace completes 5 batches per 8-hour day. At 2000 g/t concentrate, borax recovers 94.5 percent of the gold at $3.07 per gram with 0.11 g lost to slag, versus 82.0 percent for amalgamation at $3.05 per gram with 3.0 g of mercury lost per batch. The hard boundary is 714.3 g/t: below that concentrate grade recovery falls below 90 percent, so the gravity circuit must deliver at least 714.3 g/t for the method to close.