Pyrite Particles Research Articles

Dissolution kinetics of natural FeS 2 in carbon tetrachloride and water-carbon tetrachloride media saturated by chlorine

The dissolution kinetics of natural pyrite samples in Cl 2-saturated carbon tetrachloride and water-carbon tetrachloride media have been investigated. The effects of pyrite particle size, reaction time, temperature, stirring speed and water-to-carbon tetrachloride ratio were studied. The dissolution rate of pyrite in these media increased with increase in temperature and decrease in particle size. In water-carbon tetrachloride medium, the rate was independent of the stirring speed. However, in carbon tetrachloride medium alone, the increase in the stirring speed significantly increased the dissolution rate. The experimental data were analyzed on the basis of unreacted shrinking core model. It has been found that the dissolution is controlled by diffusion through the ash layer in carbon tetrachloride and that it is chemically-controlled in water-carbon tetrachloride medium. The activation energies for the diffusion- and the chemically-controlled steps were 32.3 and 22.3 kJ mol −1, respectively.

A fundamental investigation of the flame kinetics of coal pyrite

The kinetics of the transformations which pyrite (FeS 2) particles undergo in coal flames were studied in a laboratory burner. The size of the particles leaving the burner was measured with an optical sizer and the composition of the sampled particles was analysed by computer-controlled scanning electron microscopy (CCSEM). The pyrite particles decomposed into pyrrhotite (FeS) in a time corresponding to that required to transfer the necessary heat from the gas to the particles to sustain the endothermic decomposition reaction. There were no further reactions in a reducing flame, only rapid melting. In an oxidizing flame, decomposition and melting occurred prior to oxidation. The pyrrhotite droplets oxidized to solid iron at a rate corresponding to the rate of transport of oxygen from the gas to the droplets. The gas-side transport of oxygen is thus the rate-limiting step for the oxidation; the liquid-side transport of oxygen in the droplets is apparently faster. This surprisingly rapid transport is explained by internal convective flows induced by the friction of the droplets with the surrounding gas. This friction in turn is induced by the gravitational motion of the droplets relative to the gas. In the final stage of oxidation, cenospheres were produced. The standard CCSEM method classified these cenospheres as small particles, which led to an initial erroneous conclusion that the particles had fragmented.

Short-pulse microwave treatment of disseminated sulfide ores

Theoretical and experimental analyses are used to study the feasibility of using short-pulse microwave energy as a pretreatment step in comminution. Ores of interest contain microwave-absorbing mineral in a low-absorbing gangue. A finite element numerical model is used to predict the thermomechanical response of a single pyrite particle in a calcite host rock during microwave heating. The effects of power density and mineral particle size are presented. Using microwave power densities within those expected in currently available equipment, significant temperature differences are predicted between the pyrite particle and the calcite matrix. Resulting thermomechanical stress exceeds typical strengths of these minerals. Laboratory tests verify rapid, selective heating of crushed sulfide minerals. Tests with mixtures of pure minerals show heating rates directly proportional to the mass of high-loss mineral; thus, shielding of mineral grains by other grains does not appear to be significant. A cursory energy-use analysis suggests that short pulse microwave treatment can be accomplished with relatively small energy input compared with current energy consumption in comminution. Although improved comminution has yet to be experimentally demonstrated, initial results are promising from both a technical and an economic standpoint.

PYRITE MICROENCAPSULATION: POTENTIAL FOR ABATEMENT OF ACID MINE DRAINAGE1

Oxidation of pyrite in mining waste or overburden is the main source of acid mine drainage (AMD) production which causes major environmental pollution. Presently, the most common method of controlling AMD problems is through the mixing alkaline substances, such as limestone, with the AMD producing materials. However, the effectiveness of this method is still questionable. The main reason for this is that the surface of pyrite particles in mining waste are still exposed to the atmospheric O{sub 2} after treatment. Experimental evidence on novel pyrite microencapsulation technologies currently under development in our laboratory are presented. It was demonstrated that these technologies, which include ferric hydroxide-phosphate-coatings and ferric-hydroxide-silica coatings, could effectively protect pyrite from oxidation.

Bioleaching of pyrite by acidophilic thermophile Acidianus brierleyi

The kinetics of bioleaching of pyrite (FeS(2)) by the acidophilic thermophilic bacterium Acidianus brierleyi was studied in a well-mixed batch reactor. Experiments were done at 65 degrees C and pH 1.5 on adsorption of A. brierleyi onto pyrite particles, liquid-phase oxidation of ferrous iron by A. brierleyi, and microbial leaching of pyrite. The adsorption of A. brierleyi was a fast process; equilibrium was attained within the first 30 min of exposure to pyrite. The adsorption equilibrium data were well correlated with the Langmuir isotherm. The oxidation of ferrous iron was markedly accelerated in the presence of A. brierleyi, and the growth yield on ferrous iron was determined. The bioleaching of pyrite by A. brierleyi was found to take place with a direct attack by adsorbed cells on the surface of pyrite, the chemical leaching of pyrite by ferric iron being insignificant. Rate data collected under a wide variety of operating variables were analyzed to determine kinetic and stoichiometric parameters for the microbial pyrite leaching. The specific growth rate on pyrite for A. brierleyi was about four times that for the mesophilic bacterium, Thiobacillus ferrooxidans, whereas the growth yields on pyrite for the two microbes were approximately equal to one another in magnitude. A comparison of A. brierleyi with T. ferrooxidans for pyrite leachability demonstrated the thermophile to be much more effective. (c) 1995 John Wiley & Sons, Inc.

Two-wavelength pyrometry study of the combustion of sulfide minerals: Part I. Apparatus and general observations

The temperatures of individual sulfide particles combusting in air have been measured with a two-wavelength radiation pyrometer. The pyrometer design allowed the temperature of the combusting particles to be measured for 15 to 30 ms; and the response of the system was sufficient to determine the heating rates of particles during reaction. The combustion of galena, pyrite, and pyrrhotite, in air, at 1130 K was investigated. The different minerals produced distinct combustion pulses which were recorded repeatedly. Galena particles reacted in air at a relatively constant apparent temperature of between 1400 and 1700 K. Reaction product fume was observed trailing behind the combusting particles. The pyrometer pulses recorded during the combustion of pyrite and pyrrhotite particles were similar; however, pyrite tended to ignite more rapidly than pyrrhotite. During combustion in air, the temperature of the iron sulfide particles increased at a constant rate up to a maximum of 2200 to 2600 K after which rapid cooling occurred; a sudden rise in combustion energy at peak temperature was likely due to an merease in the particle size resulting from the formation of cenospheres.

Two-wavelength pyrometry study of the combustion of sulfide minerals: Part III. The influence of oxygen concentration on pyrite combustion

The strong influence of oxygen concentration on the combustion of pyrite at a furnace temperature of 1130 K has been investigated by pyrometer measurements of the temperature of individual particles and by morphological examination of the reaction products. Three distinct types of behavior were identified depending on the oxygen concentration. With oxygen levels between 10 and 40 pct, the maximum temperature of the pyrite particles increased linearly from about 2000 to over 3000 K while heating rates also climbed. The characteristics of the combustion pulses from the pyrometer revealed that cenosphere inflation occurred at the maximum particle temperature and that the freezing of a magnetite shell is subsequent to, not associated with, sudden expansion of the particles. For oxygen concentrations between 40 and 80 pct, the maximum particle temperature was independent of oxygen concentration, remaining constant at 3000 to 3400 K; however, heating rates were some-what variable. The combustion pulse characteristics again suggested that particles inflate into cenospheres but there was an increased tendency for them to rupture. At oxygen concentrations of 80 pct and greater, higher heating rates were measured but the maximum particle temperatures were limited to 3200 to 3500 K. The combustion pulse characteristics in oxygen suggested that the termination of reactions is consistent with material ejection or particle disintegration as opposed to cenosphere inflation. The results generally were consistent with the reaction rate being governed by oxygen transport in the gas phase, although at higher oxygen concentrations evaporation of the iron species and other physical phenomena limited the particle temperature.

Removal of hexavalent chromium anions from solutions by pyrite fines

Bench scale experiments were conducted in the laboratory, aiming to the removal of hexavalent chromium from aqueous solutions, by dispersed pyrite fine particles. Pyrite (iron sulphide mineral), was found to act as an efficient Cr(VI) reducing agent. The resulted hydroxo-Cr(III) species were found to be precipitated (and removed) onto the pyrite particles. The process mechanism was also examined by ς-potential measurements and speciation studies. Promising results were obtained at highly acidic pH values (around 1–2), showing an interesting aspect of fine (solid waste) mineral particles utilization, in the removal of a priority pollutant.

Potential Microencapsulation of Pyrite by Artificial Inducement of Ferric Phosphate Coatings

AbstractPyrite in mining waste is initially oxidized by the atmospheric O2, releasing acidity and Fe2+. At a pH below 3.5, Fe2+ is rapidly reoxidixed by Thiobacillus ferrooxidans to Fe3+, which oxidizes pyrite at a much faster rate than O2. Commonly, the approach used in preventing pyrite oxidation in the field involves the use of limestone. This approach, however, has a short span of effectiveness because the surfaces of pyrite particles in mining waste are still exposed to the atmospheric O2 after treatment. The objective of this study was to examine the feasibility in controlling pyrite oxidation by creating an FePO4 coating on pyrite surfaces to block access of the atmospheric O2 to pyritic surfaces. The coating methodology involved leaching pyrite using a chromatographic column with a 1 cm i.d. and a flow rate of 0.5 mL min−1 at 40°C with a PO4 solution containing hydrogen peroxide (H2O2); when this solution reached pyrite surfaces, H2O2 oxidized the surface portion of pyrite and released Fe3+ so that Fe phosphate precipitated and formed a passive coating on pyritic surfaces. This laboratory‐scale study demonstrated that Fe phosphate coatings on pyritic surfaces could be established with a solution containing as low as 10−4 tool L−1 phosphate and 0.03 mol L−1 H2O2 and that the FePO4 phosphate coating could effectively protect pyrite from oxidizing further.

Kinetics of pyrite oxidation in sodium carbonate solutions

The kinetics of pyrite oxidation in sodium carbonate solutions were investigated in a stirred vessel, under temperatures ranging from 50 °C to 85 °C, oxygen partial pressures from 0 to 1 atm, particle size fractions from −150 + 106 to −38 + 10 µm (−100 + 150 Mesh to −400 Mesh + 10 µm) and pH values of up to 12.5. The rate of the oxidation reaction is described by the following expression:−dN/dt = SbkpO 2 0.5 [OH−]0.1 whereN represents moles of pyrite,S is the surface area of the solid particles,b is a stoichiometric factor,k is an apparent rate constant, pO```2`` is the oxygen partial pressure, and [OH−] is the hydroxyl ion concentration. The experimental data were fitted by a stochastic model for chemically controlled reactions, represented by the following fractional conversion(X) vs time (t) equation: (1−X)−2/3−1 =k STt The assumption behind this model,i.e., surface heterogeneity leading to preferential dissolution, is supported by the micrographs of reacted pyrite particles, showing pits created by localized dissolution beneath an oxide layer. In addition to the surface texture, the magnitude of the activation energy (60.9 kJ/mol or 14.6 ± 2.7 kcal/mol), the independence of rate on the stirring speed, the inverse relationship between the rate constant and the initial particle diameter, and the fractional reaction orders are also in agreement with a mechanism controlled by chemical reaction.

Kinetics of pyrite oxidation in aqueous suspension by nitric acid

The kinetics of pyrite oxidation in aqueous suspension by nitric acid has been investigated. The effects of pyrite particle size (75–90, 106–125, 180–250, 250–425, 425–850 μ), reaction time (0–12 600 s), nitric acid concentration (5–30° by weight), temperature (13–80°C), and speed of stirring (5–20 s −1), were studied. The sulfur in the FeS 2 was oxidized to sulfate and elemental sulfur during the leaching. Most of the oxidized pyritic sulfur was found in the solution as sulfate. All of the elemental sulfur was oxidized to sulfate as the reaction progressed to completion. The leaching rate was independent of stirring speed. The kinetic parameters were determined based on the shrinking core model. The apparent activation energies for the leaching of pyrite were found to be 52 kJ mol −1 in the 25° HNO 3 and 25 kJ mol −1 for the temperature range 13–40°C and 28 kJ mol −1 for temperature range 50–80°C in the 10% HNO 3.

Microwave Pretreatment of Coal Prior to Magnetic Separation

New methods to reduce the total sulphur content of coals are being developed since the introduction of legislation requiring cuts in sulphur emissions to the atmosphere. It is known that weakly paramagnetic pyrite (FeS2) particles are difficult to remove from clean coal by standard magnetic separation techniques. Data is presented here comparing standard magnetic separation tests to results achieved when the coal is subjected to a caustic microwave leach pretreatment, prior to conventional dry magnetic separation. A significant increase in the removal of total sulphur is observed when this pretreatment is applied to the coal.

Bacterial pyrite oxidation: Influence of morphological, physical and chemical properties

Physical and chemical characteristics of six types of pyrite isolated from ores and coals as well as the mechanism of their oxidation by Thiobacillus ferrooxidans and Sulfobacillus thermosulfidooxidans were investigated. The effect of monocrystallinity, orientation of crystals, and microaggregation on the oxidation degree of polish rock section was clearly revealed. The investigation of the surface of powdered pyrite particles showed the ratio S 2−/Fe 2+ of pyrite to be equal to 2.7. This indicates that the surface of the pyrite particles was deficient in metal or redundant in sulfur. This ratio holds in the coarse of pyrite oxidation by T. ferrooxidans, indicating that Fe 2+ and S 2− are oxidized simultaneously. In the presence of S. thermosulfidooxidans the ratio S 2−/Fe 2+ is reduced to 1.0 for some types of pyrite indicating preferential oxidation of S 2−. The kinetics and mechanism of bacterial oxidation of pyrites are discussed.

Flame transformations and burner slagging in a 2.5 MW furnace firing pulverized coal: 2. Slagging

Results are reported of a 3 day slagging trial with a 2.5 MW semi-industrial-scale furnace in which the near-burner conditions of a wall-fired boiler were simulated. The fuel was a pyrite-rich pulverized coal. Slag deposit probes were placed near the burner wall, simulating the refractory burner quarl with surrounding wall tubing. The side of the probe facing the incoming fuel stream was covered with a molten deposit similar to that on the burner quarl of the furnace and to deposits in the near-burner region of full-sized furnaces. Slag was formed on the refractory material of the probes and not on the water-cooled tubes surrounding it, suggesting that slagging in the near-burner region in real boilers may also be initiated on the refractory quarl rather than on the surrounding water-wall. The ash deposits were analysed in-depth by SEM and EPMA. The nature of the deposits was related to the changes undergone by the minerals in the flame before deposition. It was deduced that deposition of molten pyrrhotite (the first flame product of pyrite) together with silicates resulted in an iron silicate deposit with a very low melting point, accounting for the molten character of the slag. The deposit adhered less strongly to silicon carbide as the probe material than to alumina. The reverse side of the probe, facing downstream, was covered with a loose powder of discrete iron oxide and silicate particles, consistent with the presence of fully oxidized solid pyrite particles in the furnace at this position.

The behaviour of bed material during fluidized bed gasification: The effects of mineral matter interactions

In the British Coal Topping cycle coal is partially gasified in a pressurized fluidized bed gasifier to give a char and ash. A study of eight UK coals has shown that under these conditions each coal produced an ash which has a much finer size distribution than the parent coal. Changes in ash composition with size fraction have been related to the mineral matter in each coal. Each ash showed a decrease in SiO 2 and Al 2O 3 with decreasing size and an increase in Fe 2O 3 content. Fine pyrite particles have been observed to coat the surfaces of many of the larger clay particles. In instances where agglomeration between clay particles had occurred bonds of either FeSO or an iron aluminosilicate glass were observed. Part washing one of the coals preferentially removed quartz, kaolinite and illite, thus enriching the remaining mineral matter in pyrite. The run-of-mine coal, although with a higher ash content, would be expected to show a lower propensity towards the formation of agglomerates.

Evidence of reciprocal organic matter-pyrite interactions affecting sulfur removal during coal pyrolysis

The mutual influences of the organic matter and pyrite on the removal of sulfur during coal pyrolysis were studied. Several high-sulfur, low-rank Spanish coals with different organic and pyritic sulfur contents were pyrolysed in a swept fixed-bed reactor to temperatures up to 800 °C at 5 K min −1. The coals and chars were demineralized with HF-HCl and examined by SEM-EDX and X-ray diffraction. A calibration curve of organic sulfur concentration vs. peak/background ratio was established for demineralized coals. The results indicated that organic sulfur tends to decrease with increasing pyrolysis temperature for coal samples containing very little or no pyrite. However, starting from 500 °C the organic sulfur content of chars from pyrite-containing coals increased, and the degree of dispersion of organic sulfur also increased with temperature. These findings indicated that H 2S released from pyrite decomposition can be reincorporated in char as organic sulfur. The influence of the organic matter on pyrite decomposition was investigated using float-sink fractions with pyrite contents of 0.4–33 wt% and organic matter contents of 20–80 wt%. Samples with higher pyrite contents released very little sulfur as H 2S at >630 °C, whereas samples with lower pyrite contents and hence higher organic matter contents evolved more H 2S. SEM-EDX and XRD showed that at 600 °C and with low organic matter contents, pyrite remained practically unaltered and pyrrhotite was very scarce. At 800 °C the transformation was incomplete, resulting in a pyrite core surrounded by a pyrrhotite shell. On the other hand, with high organic matter contents, pyrite particles showed evident signs of decomposition and were almost entirely transformed into pyrrhotite at 600 °C. These results show that only partial desulfurization of high-sulfur coals can be achieved by pyrolysis, owing to organic matter-pyrite interactions.

Factors that influence the formation and stability of hydrated ferrous sulfate in coal dusts. Possible relation to the emphysema of coal miners.

Epidemiological studies have shown that a causal relationship may exist between coal dust exposure and emphysema in coal miners. Emphysema can be considered as one of the human pathologies associated with oxidative stress, resulting from oxidant-induced alpha 1-antitrypsin (alpha 1-AT) inactivation and uncontrolled proteolysis of lung tissue. We have previously reported that certain coal dusts contained hydrated ferrous sulfate (FeSO4) that inactivated alpha 1-AT [Huang et al. (1993) Chem. Res. Toxicol. 6, 452-458]. In the present study, we have shown that the FeSO4 originated from oxidation of pyrite (FeS2), which is a typical contaminant of coal dusts. The relative humidity and microenvironment around individual pyrite particles influence the formation of FeSO4 in the coal. However, the subsequent human exposure to coal dust containing FeSO4 depends on the stability of the formed FeSO4. We found that pH played the most important role in stabilizing the FeSO4, such that a final pH < 4.5 after oxidation of pyrite stabilized FeSO4, whereas at high pH the conversion of reactive Fe2+ to Fe3+ was immediate. Sulfuric acid (H2SO4), which is also produced by the oxidation of pyrite, can lower the pH, but it can also be neutralized by other minerals in coal dusts, such as calcite (CaCO3). The stability of FeSO4 in coal dust can also be influenced by the length of exposure to air. Our studies demonstrated that coal samples differed in their capacity to stabilize FeSO4. This current study strengthens our previous reported hypothesis that emphysema, which occurs irregularly in coal miners, could be directly related to exposure to coal dust containing FeSO4.

Effect of Oxidation and Thioglycolic Acid on Separation of Coal and Pyrite by Selective Oil Agglomeration

The separation of coal and pyrite particles by selective agglomeration with oil can be troublesome for coals which are weakly hydrophobic or hydrophilic and, therefore, are not strongly agglomerable. The hydrophobicity of lower rank coals and other coals which have become oxidized is generally low. On the other hand, the hydrophobicity of oxidized pyrite can be appreciable. The problem of separating such coals from pyrite was studied by oxidizing a moderately hydrophobic coal with air at 150°C for either 8, 20, or 72 hr to systematically reduce its hydrophobicity. Samples of the raw and treated coal were subsequently mixed with aged mineral pyrite from Peru and agglomerated with heptane. Although a mixture of raw, unoxidized coal and pyrite was largely separated by this method, it proved increasingly difficult to separate the materials as the coal became increasingly oxidized. While raw coal was agglomerated in preference to pyrite, this preference changed as the coal was oxidized. After 20 hr. of oxi...

Modeling the Thermomechanical Response of Ore Materials During Microwave Processing

ABSTRACTA finite element numerical model is used to predict the thermal and mechanical response of mineral-bearing ores irradiated by microwave energy. The model considers a small, spherical, pyrite particle surrounded by a matrix of calcite. Power density data are determined from the dielectric properties of the mineral and host rock materials at typical microwave frequencies and power capabilities. The effects of varying power density and mineral particle diameter are studied. Using power densities within the expected achievable range for pyrite, significant temperature differences are predicted between the mineral particle and host rock. These temperature gradients lead to circumferential tensile stresses in the host rock well in excess of the reported uniaxial tensile strength of common rock materials. It is shown that, for a fixed microwave energy source, both the temperature difference between the mineral and host rock, and the peak tensile stress in the host rock are reduced as the mineral particle size is reduced. Recent experimental efforts to corroborate this numerical study are briefly described.

A kinetic assessment of substantial oxidation by Sulfolobus acidocaldarius in pyrite dissolution

The relative contributions of biological and chemical reactions to the total rate of pyrite oxidation in the presence of Sulfolobus acidocaldarius were studied on the basis of experimental work coupled with mathematical modeling. Kinetic constants for the individual reactions were determined in independent experiments. The specific growth rate of cells on the pyrite surface, which is the only unknown parameter, was assumed to be ( μ s = 0.1 h −1) and justified by the agreement of the simulated results of a proposed model and the experimental results. The model includes: reversible adsorption, biological dissolution of pyrite by the adsorbed cells, chemical dissolution of pyrite accompanied by the reduction of ferric ions to ferrous ions, biochemical oxidation of ferrous ions to ferric ions by free cells, and change of the surface area of pyrite particles. It is suggested that the contributions of direct (biological) and indirect (chemical) reactions to the total rate of pyrite oxidation were in a ratio of 2:1.