In 2013, the Company began testing a suite of alternative oxidation methods, including chlorination, ambient pressure alkaline oxidation, fine grinding with intense cyanidation, and a procedure similar to the Albion process. The goal was to develop an economically viable process that would be less expensive to build and operate than autoclaves and that would eliminate the need for offsite concentrate sales.
Batch test results were positive and indicated that Hycroft concentrates were amenable to oxidation under atmospheric conditions, using trona to create the appropriate alkaline environment to promote oxidation.
In 2016, the viability of the atmospheric oxidation process using trona was demonstrated in a 10 ton-per-day integrated pilot plant at the mine site. This plant included primary grinding of 3/8” material, followed by flotation, atmospheric oxidation, cyanide leaching, counter-current decantation (CCD) and Merrill-Crowe precipitation.
The objective of the current study is to determine if soda ash, refined from trona, can be used during the oxidation of sulfides in a heap leach operation prior to irrigation with cyanide solution. This process, which is the subject of a pending patent application, will accomplish two goals, namely, the liberation of gold and silver in the sulfides by oxidation, thereby increasing its recovery, and the reduction of the heap’s potential to turn acidic during cyanide leaching.
Over a decade of research into various carbonate oxidation systems has laid the foundation for the pre-oxidation and cyanidation process. A history of processes that have contributed to the development of this technology is included to show the progression of the mechanism used for oxidation as well as the logic that led to current operating procedures.
Process Flow Sheet
The heap leach operation is designed to treat three categories of ore, classified as described below. The process methods applied to Ore Category 3 are covered by a pending patent application.
- Ore Category 1 (ROM ore) – lower grade ore with high cyanide soluble gold is routed directly as run-of-mine to the leach pad and cyanide leached to extract gold and silver. This accounts for 4% of the ore over the life of mine.
- Ore Category 2 (3/4” Crushed ore) – higher grade ore with high cyanide soluble gold is crushed to a P80 of ¾” and cyanide leached to extract gold and silver. This accounts for 2% of the ore over the life of mine. The ore in this category is typically defined as ‘3/4” crushed oxide’ or ‘3/4” crushed transition’.
- Ore Category 3 (1/2” Crushed ore) – low cyanide soluble ratio ores are crushed to a P80 of ½”. The crushed ore is mixed with soda ash to induce an alkaline ‘pre-oxidation’ process. After the desired oxidation level has been achieved, the ore will be rinsed sequentially with water and saturated lime solution, and then leached with cyanide to extract gold and silver. This accounts for 94% of the ore over the life of the mine. The ore in this category is typically defined as ‘1/2” crushed sulfide’ or ‘1/2” crushed transition’.
Pregnant solution from the heap leach will be processed by two existing Merrill-Crowe zinc-cementation facilities.
The Hycroft Mine is projected to begin producing gold and silver from low-grade oxide ore and sulfide ore by cyanide heap leaching in the third quarter of 2019. Compared to a traditional oxide heap leach, cash flow is delayed by a length of time equivalent to the length of the dedicated pre-oxidation process.