Tag Archives: oxides

Hycroft Mining continues evaluation of novel sulphide heap oxidation/leach process

After testing out a “novel” oxidation and leaching process at the Hycroft Mine in Nevada, USA, Hycroft Mining Holding Corp is making plans to go back to a conventional oxide leaching setup in 2021.

The company produced 27,392 oz of gold and 178,836 oz of silver in 2020, an almost three-fold increase over 2019. It hit these numbers while operating at a pre-commercial scale using the novel process, which oxidises sulphides ahead of leaching.

It is now planning for run-of-mine production of 45,000-55,000 oz of gold and 400,000-450,000 oz of silver in 2021 using conventional cyanide heap leach.

It is anticipated that mining in the first four months of 2021 will be performed using the existing Hycroft fleet and a rental fleet, moving approximately 1.5 Mt/mth of ore and waste. For the remainder of the year, Hycroft intends to mine some 500,000 t of oxide and transitional ore and waste per month with a more cost-effective mining fleet.

Diane R Garrett, President & Chief Executive Officer, reflected on the results: “2020 was an important year for Hycroft as the company continued to focus on the restart of the Hycroft Mine. Throughout the year, we advanced work on the proprietary two-stage sulphide heap oxidation and leach process and made several important findings that will need to be addressed prior to our implementing the novel technology on a commercial scale.

“In 2021, we expect to mine predominantly oxide and transition material, which are more economic when treated using a conventional run-of-mine heap leaching method, which gives us the opportunity to continue to refine the operating parameters and flowsheet for the new heap leach pad and novel process. While the company continued to make significant progress in better understanding this proprietary process and its application on a commercial scale, the past year also presented some operational challenges, including learning to navigate in a newly emerged COVID-19 world.”

In the last few months, Hycroft says it has worked alongside consultants to identify and investigate opportunities for improvements in operating parameters for the two-stage sulphide heap oxidisation and leach process. The result of the work to date has identified several items that were not considered or included in the original plan and design but are critical to the success of this process. These findings include:

  • Adding a forced air injection system for the leach pad which is a key component of the oxidation process;
  • Developing a system for segregating solution flows to and from the heap leach pad to avoid co-mingling of solutions among heap lifts and ore processing stages that negatively impact recoveries and conditions on the leach pads;
  • Identifying that the finer crushed material requires agglomeration in order to achieve optimal permeability and gold/silver recoveries;
  • Understanding that higher soda ash, caustic soda, and cyanide consumption will be required which Hycroft experienced throughout the 2020 pre-commercial test pad programs and recently confirmed through the review of the test work;
  • Determining that some transitional ores are more economically attractive when processed as direct leach, run-of-mine material; and
  • Concluding that additional variability metallurgical and mineralogy studies will be required to better understand each of the geometallurgical domains in the orebody. While there was some variability work completed in the past, the recent test work has revealed that additional variability test work and compositing is necessary to fully understand the geometallurgy of each domain, and that additional sampling, including sampling below the water table where the predominance of the sulphide resources exist, is required given the complexity and variability of the large orebody.

The additional variability test work will also include detailed mineralogy studies as it is important to understand the role other minerals may play in the overall oxidation process and to enhance Hycroft’s ability to measure oxidation rates accurately and consistently, it said.

The team at Hycroft has developed an approximate $10 million program for drilling and additional metallurgical and mineralogical studies in 2021. This program of work has been approved by the Board of Directors of Hycroft and can be funded from existing cash and Hycroft’s current operating plans.

Hycroft expects to mine and stockpile at least 300,000 tons (272,155 t) of sulphide ore in 2021 that, once sufficient additional work on the novel process has been completed, will be available for testing to further refine operating parameters and measure its performance for large scale application of the oxidation heap leach.

Garrett added: “2021 is a foundational year designed to advance the work necessary in preparation for larger-scale sulphide operations. The team is working diligently to optimise current and future heap leach mine plans and to evaluate all opportunities for more profitable mine plans in the near and medium term. This work involves taking a ‘ground up’ approach working from the orebody out. The company’s prior plan was developed using a $1,200/oz gold price pit shell which leaves profitable ore behind in the current gold and silver price environment. By running pit shells at recent gold and silver prices, we have identified additional areas of oxide mineralisation that can generate cash flows over the next several years and we have already begun to identify areas of higher-grade mineralisation that will become important for mine sequencing and further improving cash flows prior to accessing sulphide material.”

As the company considers life-of-mine development and planning for the Hycroft deposit, particularly in the current gold and silver price environment, Hycroft says it is prudent to evaluate proven processing technologies for treating some ore types that may be more profitable than only using the two-stage sulphide heap oxidation and leaching process.

Potential opportunities being examined by the company in 2021 include: developing an understanding of the grade range distribution of the sulphide material; completing on-going work on the higher-grade areas of Hycroft; and following up on historical high-grade intercepts.

In order to capitalise on these potential opportunities, which take advantage of the current commodity price environment, Hycroft believes that it should also evaluate the benefits of a multi-process operation. Long-term operating scenarios may include conventional run-of-mine cyanide heap leaching for the oxide and transitional material, sulphide heap oxidation and leaching using the novel process, and an appropriately sized milling and flotation plant for processing the higher-grade ranges of sulphide material.

“The company believes that the plan it has put in place for 2021 will provide the new team the time to fully consider and evaluate these opportunities and make any necessary changes to improve the leach pads, process plants and process flowsheet, maintain and develop its workforce, and advance the project, in order to further enhance the value of the project,” it said. “As the test work advances and alternative processes are considered, the company expects to perform technical studies and trade-off evaluations which may result in an updated feasibility study.”

Corvus Gold weighs heap leach, BIOX route at Mother Lode project

Corvus Gold has announced the results of a preliminary economic assessment (PEA) for its Mother Lode project (MLP) in southwest Nevada, USA.

The study outlines a project able to produce 170,980 oz/y of gold and 79,600 oz/y of silver through a combination of heap leaching and biological oxidation (BIOX).

Based off measured and indicated resources of 1.55 Moz of gold and 1.51 Moz of silver, and inferred resources of 170,000 oz of gold and 400,000 oz of silver, the study estimated an operation with an eight-year mine life and an initial price tag of $406 million. Using a $1,500/oz gold price, the payback period was estimated at 2.7 years.

Jeffrey Pontius, President and CEO of Corvus, said: “The results from our initial PEA study of the Mother Lode standalone project are encouraging. The potential projected annual gold production of over 200,000 oz/y during the first three years is significant, especially as a new project, which could provide an increase to the Corvus Gold future production profile. The PEA demonstrates the preliminary potential for the project, on its own, to produce significant free cash flow and after-tax NPV.

“Corvus now has a number of strategic alternatives available to it to drive shareholder value. Given the recent interest in the Bullfrog Gold District from multiple producing companies with adjacent land packages, Corvus’ compelling and accretive projects and its commanding land position with what we believe is demonstrated high exploration potential, has positioned the company to deliver increased shareholder value.”

The Mother Lode project is modelled as a large, open pit, with a biological oxidation mill to treat the higher-grade sulphide mineralisation and a heap leach pad for treatment of oxide mineralisation. The mining plan uses standard mining practices and has a production scale that is currently being used by many operating mines, the company said.

The new PEA study, which assumes a standalone project, also includes a number of other changes to the previously assumed concept for the combined Mother Lode-North Bullfrog project configuration, which enhance the project. This includes:

  • Employing BIOX to treat the MLP sulphide mineralisation would increase gold recovery by 11%. Metallurgical testing of a stirred BIOX process on the MLP whole sulphide mineralisation from Mother Lode demonstrated gold recoveries of 91-92% as compared with the previous testing on MLP flotation concentrate;
  • Eliminating the need for the concentrator, oxygen plant and autoclave components of the previous plant concept simplified the plant conceptual design and would reduce the process plant capital cost;
  • Ongoing work expanding Mother Lode resources in the CIZ area and the main Zone with ongoing work at improving the open-pit mining and development plans will enhance project economics; and
  • New experimental work on conducting the BIOX process in a heap is being tested as follow-up on past successful studies and could substantially cut operation and capital costs with the elimination of the mill circuit.

The PEA only includes drill results completed up until September 2020. In addition, Corvus will continue follow-up mineral resource expansion work and new discovery drilling on priority targets at both the North Bullfrog and Mother Lode properties through 2021, it said.