New process aids sulfur oxidation at Hycroft Mine near Winnemucca
WINNEMUCCA, Nev. — Hycroft Mining Corporation is currently in the final phases of proving a large-scale innovative two-stage sulfur ore oxidation process without the traditional method of using a mill, thus eliminating the need for hundreds of millions of dollars in capital to sustain the production of mining gold and silver at the mine near Winnemucca.
Hycroft began a mill demonstration plant in 2016, which successfully demonstrated a new Alkaline Atmospheric Oxidation (AAO) mill process (tank oxidation with sulfides) and built two five-thousand ton demonstration leach test pads for the newly developed two-stage sulfide oxidation and leach process.
Since the demonstration leach test pads were successful, Hycroft General Manager Nigel Bain said the goal now is to control the chemistry and refine the oxidation timing and cutoff versus recovery.
Bain said that one of the things learned throughout the development of the sulfur oxidation process is that in order for oxidation to occur, the sample of ore cannot be too saturated. Oxygen levels are measured in the oxidation heap leach throughout the process along with the pH and alkalinity. Irrigation with soda ash aids in the oxidation process of sulfur ore and increases pH.
The mine has a lab in which test columns contain a small scale process of the heap leach to mirror and manipulate the process that occurs in the large-scale heap leach. Hycroft also built seven demonstration pads for the new leach process with success.
“Economically, the quicker we can put it (ore) under cyanide leach, the quicker we get cash flow from that work and that’s probably the biggest question; whether we can control it enough to know whether we need to oxidize it further,” said Bain.
Once the sulfur ore is oxidized, Bain said it’s critical to gain control of the pH following the soda ash rinse which oxidizes the ore. The original process involved using Trona mined from Green River, Wyoming and transitioned into using a more refined version of Trona, known as soda ash or sodium carbonate which produces a higher pH due to being more concentrated.
Once the sulfur ore is oxidized and rinsed, the pH is recovered and it is processed through the cyanide leach process that the mine has historically used for all oxide ore mined. Oxide ore is easier to process as it doesn’t have to go through the oxidation process prior to cyanide leaching.
The cyanide leach process pulls the precious minerals from the ore body, generating a “pregnant solution” to be processed further in a Merrill Crowe plant.
Due to the different characteristics of the sulfide and oxide ore, each type is processed through the crusher individually rather than mixed, so at any given time there is only one type of ore being processed through the crusher and then it is cleaned out.
The oxidation heap leaching process involves ore being left out in the elements to oxidize for three to six months, making rock porous so cyanide can be leached through the ore and pull the minerals being mined.
Currently, there are three heap leach oxidation pads containing ore from three different parts of the mine oxidizing and are critical to proving up the process that the mine has been developing for three years. The reason for the three pads is that each type of ore has a unique oxidation time.
Bain said the site is a series of north-south poles between open pits, including the Brimstone, Central and Camel, with crosscutting faults containing most of the mineralization of low-grade gold and silver.
The site has two Merrill Crowe processing plants which process the pregnant cyanide solution to eliminate unnecessary minerals, solids and liquids; leaving the precious metals to be refined. The smaller Brimstone plant can process approximately 5,000 gallons per minute and was built in 2009 and the larger plant on the north side of the mine can process 21,500 gallons per minute and was built between 2012 and 2013.
Hycroft’s gold and silver mine is located in the Sulfur Mining district, 54 miles west of Winnemucca. In 2015 the company filed a chapter 11 reorganization bankruptcy and subsequently laid off over 200 employees. The mine is now back in production following the reorganization and completion of the successful feasibility study.
Thom said the mine contains a 30 million ounce gold equivalent reserve, with approximately 9 million ounces of gold and close to a billion ounces of silver. Bain said this translates to a 34-35 year mine life with the current annual revenue split nearly in half between gold and silver.
Bain also said there is a higher concentration of gold in the north area of the mine, which is refractory encapsulated. The silver is located primarily in the southern area of the mine and is highly cyanide soluble. Thom said it is believed that two different events deposited the gold and silver, based upon the way they are deposited.
“I have been with the company for 10 years so I’ve watched it come from nothing to blow all the way up to almost 400 people and sadly back down again, so I’m really happy to be able to say that we’re working towards building back up,” said Thom. “This time it’s going to be a very measured growth cycle for us.”
Go to alliednevada.com to learn more about Hycroft.
This article first published in the 2019 edition of Winnemucca Publishing’s Mining magazine.
The cuts would come as a direct result of reduced tax collections caused by business closures across the Silver State due to the COVID-19 pandemic.