Dr. Luis Sobral

Abstract:

The presence of mineral sulfides such as pyrite, chalcopyrite etc., can pose significant challenges in gold extraction processes, regardless of the extraction route used, including the use of cyanide and thiosulfate [1]. These sulfides consume the reagents used in the aforementioned extraction processes, leading to operational problems due to the increasing ionic strength during such processes, which impacts the solubility of the oxidizing agent, particularly dissolved oxygen, which is of utmost importance for extracting gold [2]. The presence of these sulfides in gold ores represents a major challenge in gold extraction [3], as they react with leaching agents with their consequent consumption, leading to a reduction in the efficacy of gold extraction processes causing operational problems. 

Therefore, this study aimed to carry out comparative tests to point to the effectiveness of the gold extraction process, from samples of gold ore subjected to three distinct forms of mineral processing: conventional crushing, in jaw crusher, crushing in high pressure grinding rolls (i.e., HPGR - High Pressure Grinding Rolls) and electrodynamic fragmentation (i.e., HVPF - High Voltage Pulse Fragmentation). 

In this study, different grinding operations were used, such as the use of a jaw crusher, high pressure grinding rolls, and high voltage pulse fragmentation, aiming to determine the influence of these unit operations on the bio-oxidative process (pre-treatment) of the aforementioned mineral sulfides, as preliminary steps to the extraction of gold particles trapped in the matrices of the aforementioned sulfides, substantially reducing the consumption of gold leaching agents, thus avoiding the grinding operation, which is the most expensive unit operation of extractive metallurgy.

In the bio-oxidation process, the tests were conducted in acrylic columns, filled with 3.5 kg of ore and subjected to different mineral processing for 30 days. The columns were fed from the top with a solution of salts from the MKM medium (i.e., Modified Kelly Medium - with the following composition: (NH₄)₂SO₄: 0.08 g.L^-1; MgSO₄.7H₂O: 0.08 g.L^-1; K₂HPO₄: 0.008 g.L^-1), at a flow rate of 10 L/h/m², and this solution was recirculated throughout the experiment. The mineral bed was aerated with an upward flow of humidified air at a flow rate of 0.5 L/min, 3 liters of leaching solution from the MKM culture medium were used, adjusted to pH 1.8 with 5M sulfuric acid when necessary, and maintained at 30°C. The culture medium included a mixed culture of Acidithiobacillus ferrooxidans(LR lineage), Acidithiobacillus thiooxidans (FG-01), and Leptospirillum ferrooxidans (ATCC 53992), each with a population density of 10⁷ cells/mL. For the gold extraction process, the material, after the bio-oxidation tests as bio-oxidative pre-treatment, duly exempted from acid solution residue, by successive aqueous washes, was placed in a glass column through which a descending flow of aerated cyanide solution passed, in distinct concentrations of free cyanide (usually varying from 3 to 10 g.L^-1), coming from a glass reactor, with a useful volume of 5 liters. Once the cyanidation was stopped, the leachate was analyzed by atomic absorption spectrometry to measure the extraction of gold. 

As initial results, the 30 day bio-oxidation experiments showed that high voltage pulse fragmentation (HVPF) was more effective in extracting copper and nickel from gold ore, compared to other crushing operations. HVPF generated microfissures that exposed the mineral sulfides to the microorganisms in the acid solution, increasing the concentrations of copper and nickel in the leachate. Crushing via HPGR showed less nickel solubilization and did not surpass the other operations at any time. These results underscore the need for continuous optimization of crushing operations for each type of ore. The cyanidation tests indicate that HVPF is the most effective operation, allowing a more efficient exposure of the gold particles to the joint action of the cyanide, complexing agent, with oxygen, oxidizing agent. The cyanidation tests revealed that processing via HVPF was the most efficient unit operation in the release of gold particles, followed by HPGR and, lastly, by the jaw crusher. 

It is concluded that the operation of high voltage pulse fragmentation (HVPF) stands out as the most efficient both in the bio-oxidation process and in the cyanidation of gold ore, providing a greater extraction of gold.