How to distinguish pyrite from arsenopyrite?

Researchers from the Ural Federal University have identified the mechanisms and studied the kinetics of the oxidation process of pyrite and arsenopyrite minerals, which are carriers of fine gold particles, with nitric acid. The results obtained are important for improving gold production technologies. The work was published in the journal Hydrometallurgy with the support of the Russian Science Foundation. Gold is a valuable resource that is not only necessary for the manufacture of jewelry, but also indispensable 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 global financial system. Metal extraction processes are one of the main sources of environmental pollution, so the development of low-waste, resource-saving and harmless technologies is extremely important. The existing design of most factories involves the production of environmentally hazardous by-products at almost all stages, including sulfur dioxide, arsenic compounds, cyanides and others. Due to the ongoing depletion of mineral reserves, the use of low-grade raw materials, which often cannot be processed by traditional methods of metallurgical processing, becomes relevant. For example, promising are refractory ore and technogenic materials containing sulfide minerals and serving as gold carriers – pyrite and arsenopyrite. Classical approaches involve the use of high pressure or pyrometallurgical processes, but there is an alternative – exposure to nitric acid, which ensures high environmental safety and a greater degree of gold recovery. Russian researchers studied the kinetics and mechanism of oxidation of arsenopyrite (also called arsenic pyrite) in nitric acid media, including the effect of pyrite on the kinetic characteristics of the dissolution process. Previous studies have been aimed at studying different reaction conditions: the results obtained indicate that in almost all cases the surface of solid particles is “hidden” due to the formation of a film on it (passivation effect). The thermodynamic and kinetic properties of pyrite and arsenopyrite are similar, including their solubility in aqueous media. Pyrite can have a catalytic effect in the dissolution of various sulfide minerals, minimizing the negative phenomenon of passivation, but until now there has been no data on its effect on the dissolution of arsenopyrite. Research on the mutual action of individual minerals on each other in nitric acid environments can be considered pioneering, since these interactions have been practically unstudied. The authors of the article conducted laboratory experiments on leaching (by leaching is meant the transfer of one or more components of a solid material into solution) with nitric acid. To determine the kinetic characteristics of the reactions, samples were taken at regular intervals and analyzed using inductively coupled plasma mass spectrometry (ICP-MS), a highly sensitive method capable of detecting elements in low concentrations. Experiments to study the influence of acid concentration, pyrite and granule size were carried out at different temperatures. Increasing it from 50 to 80 °C increases the efficiency of arsenic extraction from arsenopyrite from 68 to 83% after 1 hour of leaching with a solution of 10% nitric acid. The presence of pyrite and the reduction in particle size have the same positive effect. It is likely that the effect of pyrite on leaching is due to its catalytic effect at an early stage of the process. But the most significant was the increase in the concentration of nitric acid: a change from 10 to 25% allows you to increase the degree of arsenic extraction from 77 to 97%. To compare the effects of pyrite and Fe(III) ions, similar experiments were carried out. The influence of iron, like pyrite, increased the yield of arsenic from 80 to 89%, but proceeded through a different mechanism. The authors also calculated the kinetic parameters of the system, which allowed them to derive a semi-empirical equation for the arsenopyrite leaching process. “The data obtained will allow us to deepen our understanding of the kinetic features of dissolution and the mutual influence of minerals in complex heterogeneous processes, study their mechanisms and reduce possible passivation phenomena. This will help complement the fundamental principles of the oxidation processes of such 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, Candidate of Technical Sciences, Associate Professor, Senior Researcher at the Department of Nonferrous Metallurgy metals UrFU. ARSENOPYRITE – mineral, iron sulfoarsenide. English name: Arsenopyrite (mineral name approved by the IMA) First isolated/described: In 1847 Origin of the name: The name arsenopyrite comes from its composition. “Arsenicum” – in Latin “arsenic”, “pyrite” – the name of iron sulfide Other names (synonyms): Arsenic pyrite, mispickel Types of mineral: Danaite is arsenopyrite with an admixture of cobalt up to 12%.

Sample photos

Arsenopyrite

Aggregate of arsenopyrite, chalcopyrite and quartz Photo upload date: 2012-07-14

Materials

Syngony: Monoclinic
Composition (formula): FeAsS The color of arsenopyrite is steel-gray (at the fracture), tin-white (at the edges of the crystals). Characteristic yellow discoloration. Trait color (color in powder): Trait is grayish-black, sometimes with a brownish tint
Transparency: Opaque
Cleavage: Medium
Fracture: Uneven
Gloss: Metallic
Hardness: 5,5
Specific gravity, g/cm 3 : 5,9-6,2
Special features: Arsenopyrite becomes magnetic when heated. It decomposes in nitric acid, releasing sulfur and arsenic trioxide.

Selection form

Arsenopyrite is often found in the form of well-formed crystals, usually prismatic in appearance, from short-columnar to columnar and needle-shaped. Pseudopyramidal crystals are also common. The faces of the long axis of arsenopyrite crystals are characterized by hatching. Cross-shaped twins and star-shaped tees are often observed. In continuous masses, arsenopyrite forms columnar granular aggregates.

Main diagnostic signs

For arsenopyrite, characteristic diagnostic features are relatively high hardness, tin-white color of crystals, and crystal shapes. When hit with a hammer, it emits a garlicky smell (due to the presence of arsenic in it). It differs from löllingite in its lower specific gravity. From arsenides and sulfoarsenides, nickel and cobalt in granular masses of arsenopyrite can be confidently distinguished only by microscopic or chemical studies.

Related Minerals

Origin

Arsenopyrite is a mineral of hydrothermal origin and is one of the most common arsenic minerals in endogenous deposits. Arsenopyrite is found in significant quantities in gold vein and veinlet-disseminated deposits, where it often contains micro-ingrowths of gold. As a satellite mineral, arsenopyrite is found in a wide variety of hydrothermal deposits of tin, bismuth, tungsten, copper, lead, zinc and others.

Deposits/occurrences

In Russia Dozens of deposits are known in which arsenopyrite is one of the main ore minerals. Significant quantities of arsenopyrite are observed in gold vein deposits: Kochkarskoye (Chelyabinsk region), Darasun (Chita region). At the Darasun deposit, arsenopyrite in association with other sulfides forms druses of columnar crystals. Isometric and flattened arsenopyrite crystals are known at the Zapokrovskoe arsenic deposit (Transbaikalia). In the Dalnegorsk deposit (Primorye) druses of arsenopyrite crystals are also known.
Of foreign fields Noteworthy are the Dzhetygarinskoe gold deposit (North-Western Kazakhstan), where arsenopyrite contains gold, Uch-Imchak (Kyrgyzstan), Boliden (Sweden).

Application

Arsenopyrite ores are the main raw material for the production of various arsenic compounds used in agriculture for pest control, in the paint and varnish industry, and in the leather industry for removing hair from skins. Arsenic is used for alloying lead alloys and for the synthesis of semiconductor materials. Arsenic and all its compounds are poisonous.

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