Mineralogical characteristics of sulfuric acid formation during bacterial oxidation of flotation concentrate

. The article presents the results of a study on the formation of sulfuric acid in the process of bacterial oxidation of sulfide ore flotation concentrate. The results are estimated from the point of view of the thermodynamics of the course of some bacterial oxidation reactions, calculating the standard enthalpies of formation - (ΔН 0298 ), entropy (ΔS 0298 ) and Gibbs energy of formation (ΔG 0298 ) of some reactions and substances at 298 K. Conclusions about the formation of sulfuric acid based on X-ray phase and electron scanning microscope results of changes in mineralogical and elemental composition in the processes of bacterial oxidation of flotation concentrate are substantiated.


Introduction
In present time the world's reserves of deposits with high initial content and easily recoverable ores are practically depleted and at the same time the share of gold is increasing in primaryh rudah, related to the category of persistent gold-bearing mineral raw materials. These minerals consist ofIt mainly from pyrite, arsenopyrite, chalcopyrite, hydroarsenatebut iron, oxidebut manganese and othersh [1][2].
Microorganisms are an important evolutionary link that catalyzes the appearance of new mineral compounds and accelerates their formation. As is known, biochemical reactions occurring during the biological oxidation of sulfide minerals can occur either by a direct or indirect mechanism [3]. The direct mechanism requires close physical contact between the bacteria and the mineral surface to ensure that the bacteria attach to the mineral, while the indirect mechanism involves the action of bacterial-generated iron sulfate (III). Iron ions are mainly found in the composition of ore minerals-pyrite (FeS2) and arsenopyrite (FeAsS), also as part of the fayalita (Fe 2 [SiCl 4 ]) [1,4].
Iron atoms in the composition of sulfide minerals are in divalent form [5]. During bacterial leaching in sulfuric acid oxidation reactors, they pass into solution in the form of ferrous and trivalent iron sulfates. In the process of bacterial leaching, pyrite sulfide sulfur is oxidized to sulfate, and nitrous ironabout up to the oxide level and two products are formed; iron sulfate and sulfuric acid.
Iron sulphate chemically oxidizes pyrite and arsenopyrite, having formed nitrous iron sulfate and elementaryuyu serat. Nitrous iron is again oxidized by bacteria to oxide, which can be consumed as an oxidizer of pyrite and arsenopyrite.
The leading role in the implementation of these transformations belongs to the thion bacteria T. ferrooxidans, which oxidizes sulfides and nitrous iron, T. thiooxidanya, which oxidizes elemental sulfur.
Bacterial oxidation of persistent gold-bearing ores mainly occurs intensively at pH=1.2-1.4 and at a temperature of 303-316 K.
This raises a question. Due to the dissolution of what ore mineral is a highly acidic environment formed?
To clarify this issue, please contact us The mineralogical composition of the solid part of the bacterial oxidation process was studied, including changes in the pyrite concentration (FeS2), arsenopyrite (FeAsS), stibnite (Sb 2 S 3 ), jarosite (MFe 3 (SO 4 ) 2 (OH) 6 , by modern physico-chemical methods of analysis.

Materials and methods
Samples were taken from an experimental reactor with a volume of 50 liters. 34 liters of inoculum from the BIOX module No. 1 with a pH of 1.32 and a density of 1100 g/l were used to start the reactor.
Flotation concentrate was brought from the UPS (raw material preparation unit) in the amount of 20 liters, with a density of 1335 g / l, the content of total sulfur-17.8 %, sulfide sulfur-14.13 %, total carbon-2.47 %, organic carbon-1.34 (sample No. 793).
The inoculum was poured into the reactor and 1 liter of flotation concentrate was supplied every 2 hours. After starting the reactor, samples were taken based on the pH of the medium:flotation concentrate pH-8. Henceforth, all tabular and graphic materials will refer to samples by their names. The diffractograms were obtained on a powder X-ray diffractometer of the brand "Shimadzu XRD-6100", equipped with a copper (Cu) by phone (K ∞1=1,5406Å, To ∞2 = 1,5443, To∞2 / K ∞1=0,5. The detector is a scintillation detector. A scanning electron microscope was used (Carl Zeiss, Germany) with an energy-dispersion element analyzer (Oxford Instruments, United Kingdom).

Results
Bacterial attack of arsenopyrite and pyrite occurs mainly through a direct contact mechanism. The equations for the oxidation reactions of pyrite and arsenopyrite are given below (see reactions 1-9 and 12-20):  (4) and (7) are repeated, forming Fe 2 (SO 4 ) 3 and H 2 SO 4 . Some bacterial oxidation reactions can be characterized in terms of thermodynamics. The enthalpy of formation, entropy of formation, and Gibbs energy of the reaction were calculated using the following formulas: Standardth enthalpy of formation ∆H 0 298 , entropy ∆S 0 298 and Gibbs energy ∆G 0 298 a certain personsubstances at 298 K (Table 1).
∆ reac � 0 -the reaction under these conditions proceeds spontaneously ∆∆ � 0 � the reaction does not occur spontaneously ( Table 2) [9]. Analysis of the study results ( Table 2) shows that the numerical value of the Gibbs energy of the formation ∆G 0 298 arsenopyrite dissolution reactions (No. 9) are less than pyrite dissolution reactions (No. 5). This indicates that the arsenopyrite dissolution reaction proceeds more easily than the pyrite dissolution reaction. These results are also consistent with the results of X-ray phase and scanning electron microscopy.
According to some data, intermediate products Iron ions (III) can break the chemical bond in the lattice between iron and disulfide after the disulfide group is oxidized to thiosulfate. According to the "thiosulfate" mechanism, pyrite and arsenopyrite are exposed to an iron hexahydrate ion (III), as a result, the first intermediate product of oxidation is the thiosulfate ion, in the next step, the thiosulfate is oxidized Fe 3+ -ions to tetrathionate, which hydrolyzes to form a sulfate ion and disulfate monosulfonic acid (R-SO 3 RH), and it is oxidized to threetionata, threetionate is hydrolyzed to sulfate and thiosulfate ions.
As by-products during the oxidation process, the formation of pentathionate and the elementapnoi sera. Chemical or bacterial oxidation of intermediates occurs with the participation of Fe 3+ -ions or molecular oxygen (as an electron acceptor). Since thiosulfate is a key compound in the process of biooxidation of the sulfuric part of pyrite, this mechanism is interpreted as thiosulfate.
The reaction equation is given by below.
Samples from the experimental reactor were studied using the following methods: electron microscopy c energy-dispersion detector for semi-quantitative elemental analysis and x-ray phase analysis. Samples from the process chain were studied by X-ray phase analysis, the results of which are shown in Figure 1. X-ray phase analysis by the Rietveld method was performed using the Profex-Open sourse XRD and Reitveld Refinement software [9], the results of which are presented in Table 3. Analysis of the study results (Table 3) shows that mine-ralogical composition of sample No. 793, mainly composed of pyrite (FeS 2 ) -13.7%, quartz (SiO 2 ) -16.9 %, muscovite (KAl 2 (AlSi 3 O 10 )(OH) 2 ) -30.2%, anorthite (CaOꞏAl 2 O 3 ꞏ2SiO 2 ) -11.4%, arsenopyrite (FeAsS) -3.27% and other components.
Diffractograms of the above samples processed also by the Rietveld method, generalized values the results of which are presented in Table 4.  Table 4 presents the results of mineralogical analysis of solid samples numbered in the laboratory of GTI hydrometallurgical plant. The composition of minerals is quite diverse, represented mainly by transformations of rock-forming minerals. Indeed, the majority of microorganisms interacted with rock-forming, accessory, and ore minerals. While pyrite, arsenopyrite, muscovite, rutile, jarosite, and molybdite are ore minerals, the vast majority of minerals were rock -forming-quartz, feldspar, dickite, calcite, gypsum, ankerite, and others. In quantitative terms it is the rock-forming minerals that have been exposed to microorganisms. The most interesting aspect of the research was the maximum change in aluminosilicate minerals.
Analysis of the study results shows that the amount of pyrite varies in the samples from 13.7% to 1.79%, and arsenopyrite from 3.27% before 0.37%, and in samples No. 870 and No. 917 are missing.ov arsenopyrite. These results show that the formation of sulfuric acid, mostly, to the imageeIt is produced by arsenopyrite and pyrite.

Conclusion
Comparison of the elemental composition of selected solid samples №793, №842, №843, №869, №870 and №917, obtained with a scanning electron microscope, also confirms the formation of sulfuric acid, mainly from arsenopyrite and pyrite. For example, the content ofe elementapThe sulfur content decreases from 8.61% to 1.90%, and the arsenic content decreases from 0.64% to 0.16%. Thus, the formation of sulfuric acid in the process of bacterial oxidation of the flotation concentrate is justified by the fact that the mineral arsenopyrite undergoes oxidation at the beginning, and then pyrite and other sulfur-containing minerals are oxidized.