JOHANNESBURG (miningweekly.com) – Initial testing indicated that gold and silver recoveries in the mid-80% range from rougher sulphide concentrate could be achieved at US miner Allied Nevada’s Hycroft operation through the use of an ambient pressure alkaline oxidation process, the company stated on Tuesday.

Allied Nevada provided preliminary results for ongoing oxidation testwork completed on Hycroft sulphide concentrate, stating that it undertook the testing to identify the viability of an on-site alternative for concentrate oxidation, allowing for processing of 100% of the rougher concentrate and production of doré on site.

“Preliminary economics indicate operating costs for an on-site ambient pressure alkaline oxidation process will be less than $4/t of whole ore, which includes operation of an oxygen plant. If air is used instead of oxygen, we expect operating costs to be lower. These preliminary operating costs compare favourably with operating costs of an autoclave of approximately $9.6/t of whole ore,” Allied Nevada stated.

Allied Nevada has been examining options for treating Hycroft sulphidic ores since 2007.

The company’s original focus was on traditional oxidation methods currently employed in the industry, including pressure oxidation (POX), roasting and direct cyanidation. Testwork on these processes concluded that each of these options would be feasible, with varying degrees of economic recovery.

A mine plan was developed using on-site POX to treat one-third of the rougher concentrate and sell the remaining concentrate for processing at third-party facilities.

In late 2012, Allied Nevada began a review of other oxidation processes, starting with bio-oxidation (BIOX), with the goal of determining an economically viable on-site process instead of building and operating an autoclave and relying on off-site sales.

The BIOX testing was completed by SGS Canada, in Lakefield, Ontario and SGS South Africa, in Johannesburg, in collaboration with Biomin South Africa.

Final results from the BIOX testing indicated that the process was viable, and also identified a number of important characteristics regarding the oxidation of Hycroft concentrates. Most importantly, the ore appeared to require less than complete levels of sulphide oxidation to achieve acceptable gold and silver recoveries, providing potentially significant capital and operating cost savings.

Using this information, a first phase test programme was conducted on a suite of commonly used oxidation methods, including chlorination, ambient pressure alkaline oxidation, fine-grind with intense cyanidation, and the Albion oxidation process. Initial results using each of these methods have been positive, the company said.

“Testing indicates that processing rougher concentrate may be optimal as cleaner concentrates contain on average approximately 10% less metal owing to losses in the flotation cleaning process. Thus, a focus on treating rougher concentrate, to increase overall gold and silver recovery and project economics, will continue.”

Ambient pressure alkaline oxidation testing of rougher concentrate at an oxidation time of 24 hours, a reaction temperature of 60 °C and a grind size of 44 microns, resulted in a sulphide oxidation percentage of 57% and recoveries of 85% gold and 82% silver.

Altering the testing parameters for rougher concentrate by increasing the temperature to 75 °C and significantly decreasing retention time to eight hours resulted in recoveries of 82% for gold and silver from 44-micron material.

Continuing to refine the optimal operating parameters could translate to a reduction in capital and operating costs without significantly impacting recoveries.

The results were achieved using oxygen as the oxidant; however, preliminary testwork using air instead of oxygen had similar results. Factors being considered in ongoing testwork included reagent volumes, the possible use of air in place of oxygen, and the identification of the minimum temperature and reaction time for effective sulphide oxidation at various grind sizes. Further optimisation was required to determine the best economics for the project.

Engineering and development consultancy Hatch had been contracted by Allied Nevada to conduct the next phase of testing, which comprised an engineering study and a review of recent metallurgical testing.

“Our goals in the next phase are to further refine the flow sheet, define optimal parameters, and develop capital and operating costs, as well as a preliminary economic analysis,” Allied Nevada stated.