Production of pyrrhotites by pyrite reduction

Research in coal conversion, aimed at gaining an understanding of the catalytic and magnetic properties of iron sulphides found in coal, could benefit from studying well-defined materials. A method is given for the production of pyrrhotites of known stoichiometry by the reduction of pyrite. Theoretical calculations are provided for this reduction by H₂ and CO. Experimental results, confirmed by X-ray analysis, are given for the H₂ reduction of pyrite to form three different pyrrhotites in the temperature range 400–500 °C.     

Effect of pyrite and pyrrhotite on free radical formation in coal

The effects of pyrite (FeS₂) and pyrrhotite (Fe₇S₈) on free radical formation in a coal sample (81% carbon content) have been investigated by electron spin resonance (e.s.r.) spectroscopy. Changes in the e.s.r. parameters (spin concentration g^-1, n, linewidth ΔH and g-value) were monitored in samples of coal, coal+8% FeS₂ and coal+8% Fe₇S₈, as these samples were heated in vacuum or in hydrogen from room temperature to 500 °C, in steps of 50 °C for a residence time of 30 min at each temperature. In vacuum heating, changes in n begin to occur at 400 °C, 350 °C and 300 °C respectively for coal, coal+8%Fe₇S₈ and coal+8% FeS₂ samples whereas in H₂, the corresponding temperatures are 250 °C, 200 °C and 150 °C. Changes in ΔH and g were also observed at these temperatures. The maximum increase in n occured for coal+8% FeS₂ samples whereas the minimum increase was observed for the pure coal sample. It is argued that enhancement in n is due to two mechanisms: the pyrite to pyrrhotite conversion and the presence of pyrrhotite itself. The detailed nature of the catalytic activity of pyrrhotite is not known.     

Mössbauer study of the thermal decomposition of FeS2 in coal

The Mössbauer effect has been used to study the transformations of FeS₂ in four different coals: IL No. 6, Ky $\text{9}\text{14}$, Blacksville No. 2, and Powhatan No. 5. The transformations of FeS₂ in the coals were studied in an inert atmosphere. It was observed that the pyrrhotites formed from FeS₂ have a considerable reduction in the isomer shift at 440 °C as compared to the values obtained in the absence of coal. This effect is associated with the interaction of the pyrrhotites with the coal constituents at high temperatures. There is also a significant line-broadening at 440 °C. This broadening is due either to vacancy motion in the iron sulphides and/or to motional broadening due to particle motion in the coal-derived liquids. The percentage conversion of pyrite to pyrrhotite depends markedly on time as well as type of coal. The weathering of the coal has a detrimental effect on the rate of conversion of pyrite to pyrrhotite. The ferrous sulphate layers covering the pyrite particles hinder the removal of sulphur from that surface. The major factor affecting the $\text{Fe}\text{S}$ ratio is the total amount of sulphur available for H₂S formation. Partial H₂S pressure is the crucial quantity controlling the stoichiometry of the pyrrhotites. Hence, a high percentage of H₂S in the reactor at high temperature will assure the formation of pyrrhotites with a high number of metal vacancies.     

Coal gasification studies: Part II. Reduction in the presence of I2 with H2, and H2O + metal,at pressures up to 3500 p.s.i. and temperatures of 600°C in all quartz reactors

An experimental method is described which allows coal reduction experiments to be made in all quartz reactors at temperatures up to 600°C and pressures of 3500 p.s.i. The method is applied to the study of the reactions: Coal + H₂O + I₂ + Metal(Metal = Fe, Cr, Mo, Ni, Sn, Zn); H₂O + I₂ + Metal, and Coal + H₂ + I₂. It is shown that iodine is a catalyst for the water + metal hydrogen-producing reactions, and that the composite systems with coal are capable with Cr and Zn of completely reducing coal to gaseous hydrocarbons at 600°C. Thermodynamic and kinetic data are used to show that iodine functions as a coal reduction catalyst in the presence of molecular hydrogen at 600°C by permitting a finite steady state hydrogen atom concentration to be attained in the system.     

Pyrolysis of coal and iron oxides mixtures. 2. Reduction of iron oxides

The reduction of iron oxides during the pyrolysis of blends of coal and iron oxides on a laboratory scale, has been studied. The pyrolysis of blends of bituminous coal and 30 wt% of magnetite or hematite has been studied by thermogravimetry and analysis of gases, using a heating rate of 3.2 K min^?1. The state of iron in ferrocoke has been established by X-ray diffraction. A primary reduction by hydrogen and carbon monoxide of the hematite has been observed at between 400 °C and 500 °C, but hidden in thermogravimetric measurements by primary volatilization of the coal. At ≈600 °C magnetite is progressively reduced to wustite and then to iron. This reduction starts a little earlier if the heating rate is slow and the coal rank is low and progresses more rapidly when using hematite. Except for higher heating rates in the coal-magnetite blends, the reduction is complete at 1000 °C. The reductants are H₂ and CO, with production of H₂O and CO₂. When the temperature is increased the reduction by CO becomes of increasing importance, being mainly produced from the coke by the Boudouard reaction. The consumption of coke for the reduction of iron oxides is therefore more important at higher temperatures. Lignite is clearly a better reducing agent than the other coals, because of larger quantities of CO produced from the start of its pyrolysis, and the good reactivity of its char towards CO₂ and H₂O.     

Reaction of pyritic sulphur in coal with lime and calcined dolomite during the coking process

Various reactions occur between pyrite (FeS₂) in coal and CaO to form CaS when a finely pulverized intimate mixture of coal and CaO is coked at 900 °C in an inert atmosphere. The effectiveness of lime and calcined dolomite (CaO.MgO) in promoting this reaction has been evaluated; calcined dolomite is somewhat more effective than lime over coal/ oxide weight ratios from 2.8 to 14. The degree of conversion of pyrite to calcium sulphide at a coal/calcined dolomite weight ratio of 7 has been determined as a function of time at 900 °C; coking time in excess of 2.5 h does not have a significant effect. It has been shown that the total sulphur lost on coking coal/lime mixtures decreases and the percentage of FeS, originating from the dissociation of pyrite, converted to CaS increases as the amount of lime added increases. But although the total sulphur content of coke produced in the presence of CaO is then higher than when the coal is coked without lime, the pyritic sulphur has been converted to CaS which is more amenable to chemical conversion to H₂S. A method is outlined for determining CaS in the coked mixture in the presence of FeS.     

Decomposition of pyrite and trapping of sulphur in a coal matrix during pyrolysis of coal

The thermal decomposition of pyrite crystals in coal from the Prince Colliery, Cape Breton, Nova Scotia, has been studied to determine both the temperature of decomposition and the distribution of the sulphur liberated into the coal matrix. Under the experimental conditions used, pyrite decomposes to pyrrhotite between 500 and 550 °C. For small (20 μm) crystals embedded in the coal matrix, essentially all of the sulphur liberated by the decomposition of FeS₂ becomes trapped in the matrix within a distance of 15 μm.     

Electrochemical reduction of pyrite in aqueous NaCl solution

Optimum conditions for pyrite removal from a high-sulfur coal by electrochemical reduction during flotation are determined by orthogonal experiments. The electrochemical reduction process of pure pyrite is examined with XRD, electrochemical and chemical analysis. The results show that the electrochemical reduction products of pyrite are FeS and S²⁻. During this process, the reactions at cathode are: FeS₂+2e→FeS+S²⁻ and 2H⁺+2e→H₂. The corresponding electrode potentials and kinetic equation are determined. The conversion of hydrophobic pyrite to hydrophilic FeS and S²⁻ by electrochemical reduction is beneficial to desulfurization from coal in floatation process.     

Mössbauer study of transformations of pyrite under conditions of coal liquefaction

The Mössbauer effect is used to study in-situ transformations of pyrite under conditions of coal liquefaction based on Illinois No. 6 coal from St. Clair County. A marked reduction is observed at high temperatures in the isomer shift of the iron sulphides during coal liquefactions. By contrast the pure sulphides do not show such a strong effect in the presence of solvent and hydrogen. This reduction in the isomer shift may result from covalent bonding between the iron on the pyrrhotite surfaces and the coalderived liquid and gases. Marked broadening of the linewidth of Fe_{1 ? x}S occurs above 300 °C in the presence of solvent and hydrogen. The stoichiometries of the pyrrhotites formed in the different runs were determined and a correlation was observed between the total amount of sulphur in the coal and the iron deficiency in Fe_{1 ? x}S. Coal-derived liquids are more active in enhancing pyrite decomposition than tetralin. Both H₂S and Fe_{1 ? x}S seem to be actively involved in the liquefaction process.     

Two-stage coal liquefaction using sequential mineral and hydrotreating catalysts

Two-stage coal liquefaction offers significant improvements over single-stage processing in terms of product yields. The proposed two-stage operation utilizes an inexpensive and readily available mineral or disposable catalyst in the first stage followed by a commercial hydrotreating catalyst in the second stage. Single stage processing at 450°C and 425°C both show metal sulfides, i.e. pyrite, to be effective in increasing oil yields. In two-stage processing at 450°/410°C the sequence of pyrite followed by NiMo/Al₂O₃ is the most effective combination for producing oil (pentane soluble materials). Two stage processing at 425°/425°C utilizing sulfided liquefaction residue ash or pyrite as first-stage catalysts yields the highest percentage of oil. The improvements shown by solubility product distributions are verified by distillation curves of the reaction product. Evidence of pore diffusion limitation is apparent in the pelletized NiMo/Al₂O₃. Changes in catalyst morphology may be necessary to achieve maximum yields.     

Mössbäuer study of decomposition of pyrite in hydrogen

A well-characterized Illinois No. 6 coal and mineral pyrite of different particle sizes have been studied by ⁵⁷Fe Mössbauer spectroscopy at various temperatures between 25 and 400 °C in a hydrogen atmosphere. Particle sizes of the pyrites were measured by SEM. The transformation of pyrite to pyrrhotites commences at 300 °C in all the samples and is almost complete at ≈400 °C for the Illinois No. 6 coal. The decomposition of coal and mineral pyrite in a hydrogen atmosphere is dependent on particle size. Breakage of larger particles occurs during the decomposition. A significant reduction in the activation energy as a function of the particle size was also observed.     

Coal liquefaction by colloidal iron sulphide catalyst

Taiheiyo coal, which was treated with an aqueous solution of dodecyltrimethylammonium chloride, adsorbed colloidal iron sulphide prepared from FeS0₄ · 7H₂O and Na₂S · 9H₂O in aqueous media. The adsorbed colloidal iron sulphide was much more effective as a catalyst for the liquefaction of the coal itself than the usual powder-type iron sulphide. Thus in differential thermal analysis under hydrogen, the coal with 0.35wt% adsorbed colloidal iron sulphide gave an exothermic peak at 401 °C, which was ≈20 °C lower than when using the powder-type iron sulphide. The coal was smoothly hydrogenated at 450 °C to give a yield of ≈60% liquid products.     

Oxidation of pyrite in coal to magnetite

When bituminous coal is heated in an inert atmosphere (He) containing small amounts of oxygen at 393–455 °C, pyrite (FeS₂) in coal is partially converted to magnetite (Fe₃0₄). The maximum amount of Fe₃0₄ formed during the time of heating corresponds to 5–20% of the total pyrite present, depending on the coal sample. The magnetite forms as an outer crust on the pyrite grains. The fact that the magnetic properties of the pyrite grains are substantially increased by the magnetite crust suggests that pyrite can be separated from coal by use of a low magnetic field. In a laboratory test, 75% removal is obtained by means of a 500 Oe magnet on three samples, and 60% on a fourth sample.     

Sulphur removal from coal by electrolysis

Anodic oxidation of acidic coal slurries at potentials of 0.8–1.0 V, on the NHE scale, causes pyrite present in coal to react and dissolve while H₂ is produced simultaneously at the cathode. If coal is filtered from unelectrolysed coal slurries, the filtrates show similar electrochemical reactivity. Cyclic voltammetry applied to either slurry orfiltrate indicates that the reversible, one-electron oxidation of Fe(II) to Fe(III) is a major reaction, but chemical assays reveal that sulphide also is oxidized, to S ° and SO₄^2?.The electrochemical reactions may involve direct oxidation of pyrite when coal particles strike the anode; it is likely, in addition, that dissolved ionic Fe may act as an electron shuttle between solid pyrite and the anode. Other experiments without coal also indicate that Fe(III) (which can be produced from Fe(II) at the anode) can oxidize pyrite and thiophene. Preliminary experiments with lignite and a bituminous coal suggest that up to 40% of the total sulphur can be removed from coal by slurry electrolysis in acidic electrolyte with either platinum or graphite electrodes.     

Superacid coal chemistry: 1. HF:BF3 catalysed depolymerization—ionic hydroliquefaction of coals under mild conditions

HF:BF₃:H₂ catalysed depolymerization and hydroliquefaction of coal was studied. This superacidic system was found to be extremely effective for low temperature liquefaction of coal. Illinois No. 6 coal could be solubilized in pyridine to the extent of > 90% by treating it at ≈ 105 °C for 4 h. Under somewhat more elevated temperature (150–170 °C) cyclohexane extractabilities of up to 22% and distillability of up to 28% is achieved. A hydrogen donor solvent such as isopentane is shown to improve the efficiency of the superacidic catalyst for the conversion of coal to cyclohexane soluble products.     

Reactions of coal with the chlorides of iron,chromium and copper

The study of X-ray diffraction patterns indicates the possibility of the formation of intercalation compounds in several coals after reactions with FeCl₃, FeCl₃·6H₂O, CuCl₂, CuCl₂·2H₂O, and CrCl₂· 6H₂O. These reactions were carried out without solvent at temperatures ranging from 215 to 250 °C. X-ray evidence suggests that washing of the products, obtained from these reactions, with dilute acid returns the coal starting material substantially unchanged. X-ray and chemical evidence shows that reaction of FeCl₃ and FeCl₃·6H₂O with coal results in the reduction of some Fe(III) to Fe(II).     

Differential scanning calorimetry studies on coal. 2. Hydrogenation of coals

Results of exothermic heats involved during hydrogenation of twenty U.S. raw coals of varying rank at 5 · 6 MPa (gauge) and temperatures up to 570 °C are reported. The heat evolved during hydrogenation up to 570 °C decreases with increase in coal rank. A part of the total heat released during hydrogenation of coals appears to be due to the exothermic reaction between H₂ and surface carbon-oxygen complexes removed during the reaction. The transition temperature, that is the temperature corresponding to the onset of exotherms, is markedly dependent on coal rank. A sharp increase in the transition temperature occurs for coals having a carbon content, on a dry-ash-free basis, in the 75–80% range. Demineralization of coals lower in rank than HVA bituminous decreases the heat of hydrogenation; in the case of higher-rank coals, exothermic heats increase upon demineralization. The presence of pyrite has a beneficial catalytic effect on coal hydrogenation.     

Coal gasification studies: Part I. Single stage complete gasification of coal using water as the hydrogen source

The complete gasification of coal to low molecular weight hydrocarbons has been achieved in a single stage process using water as the source of hydrogen. Reaction times of one hour, and a temperature of 600°C were required. The reactions were carried out in a stainless steel reactor with iodine or FeI₂ as a catalyst. It is shown that FeI₂ is a catalyst for the reaction Stainless Steel + H₂O → H₂ + Metal Oxide and also for the coal hydrogenation reaction. The apparent excellent reduction efficiency is probably a consequence of the good contact between the coal sample and the catalyst, which at the reaction temperature has a significant vapor pressure.