Researchers from the Ural Federal University have identified the mechanisms and studied the kinetics of nitric acid oxidation of pyrite and arsenopyrite minerals, which are carriers of fine gold particles. The results obtained are important for the improvement of gold extraction technologies. The work was published in the journal Hydrometallurgy with the support of the Russian Science Foundation.
Gold is a valuable resource, which is not only necessary for jewelry manufacturing, but also irreplaceable in microelectronics, scientific research, including nuclear research, in dentistry and pharmacology, in food and other industries. Gold is also the most important element of the world financial system.
Metal extraction processes are one of the main sources of environmental pollution, so it is extremely important to develop low-waste, resource-saving and harmless technologies. The existing scheme of most factories implies at almost all stages the production of environmentally hazardous by-products, including sulphur dioxide, arsenic compounds, cyanides and others. Due to the continuing depletion of mineral reserves, the use of low-grade raw materials, which often do not lend themselves to traditional metallurgical processing methods, is becoming relevant. For example, stubborn ore and technogenic materials containing sulfide minerals and serving as gold carriers - pyrite and arsenopyrite - are promising. Classical approaches involve the use of high pressure or pyrometallurgical processes, but there is an alternative - exposure to nitric acid, which provides high environmental safety and a greater degree of gold recovery.
The Russian researchers studied the kinetics and mechanism of oxidation of arsenopyrite (also known as arsenic cupric) in nitric acid media, including the influence of pyrite on the kinetic characteristics of the dissolution process. Previous studies were aimed at investigating different reaction conditions: the results obtained indicate that in almost all cases the surface of solid particles "hides" due to the formation of a film on it (passivation effect). Thermodynamic and kinetic properties of pyrite and arsenopyrite are similar, including close values of solubility in aqueous media. Pyrite can have a catalytic effect in the dissolution of various sulfide minerals, minimizing the negative phenomenon of passivation, but so far there has been no data on its effect on the dissolution of arsenopyrite. Studies on the mutual action of individual minerals on each other in nitric acid media can be considered pioneering, since these interactions are practically unexplored.
The authors of the article conducted laboratory experiments on leaching (leaching is understood as the transfer of one or more components of solid material into solution) with nitric acid. To establish the kinetic characteristics of the reactions, samples were taken at regular intervals and analyzed by inductively coupled plasma mass spectrometry (ICP-MS), a highly sensitive method capable of determining the content of elements at low concentrations.
Experiments on the effect of acid concentration, pyrite concentration and pellet size were carried out at different temperatures. Its increase from 50 to 80 °C increases the efficiency of arsenic extraction from arsenopyrite from 68 to 83% after 1 hour of leaching with 10% nitric acid solution. The presence of pyrite and particle size reduction has the same positive effect. Probably, the effect of pyrite on leaching is due to its catalytic action at the early stage of the process. But the increase in nitric acid concentration had the greatest significance: a change from 10 to 25% increases arsenic recovery from 77 to 97%. To compare the influence of pyrite and Fe (III) ions, similar experiments were carried out. The effect of iron, like pyrite, increased the arsenic yield from 80 to 89%, but by a different mechanism. The authors also calculated the kinetic parameters of the system, which allowed them to derive a semi-empirical equation for the leaching process of arsenopyrite.
"The findings will enhance our understanding of the kinetic features of dissolution and mutual influence of minerals in complex heterogeneous processes, explore their mechanisms and reduce likely passivation phenomena. This will help to supplement the fundamental basis of oxidation processes of similar materials contained in the processed raw materials of the mining and metallurgical complex of Russia and the world, taking into account environmental factors to minimize environmental pollution," says Denis Rogozhnikov, PhD in Technical Sciences, Associate Professor, Senior Researcher at the Department of Non-Ferrous Metals Metallurgy of UrFU.
