Pyrites, decomposition

Individial filter cake compositions vary widely. As conversion increases, sulfur and ash increase while oxygen and hydrogen and possibly nitrogen concentrations in the filter cakes decrease. The average filter cake yield is 30 weight percent of the as-fed coal. The sulfur in the filter cake averaged 49 percent of the sulfur in the coal feed and is made up of the sulfur remaining after partial pyrite decomposition and sulfate sulfur. 

The most obvious change in the clays themselves during these experiments was a darkening beginning with pyrite decomposition. 

The extent of pyrite decomposition was followed by obtaining S/Fe atomic ratios at various points across selected pyrite inclusions, the latter in the size range 10-20 pm. Figure 1 is a plot of this ratio versus "distance from interface within pyrite for each of the coals at different temperatures. The results indicate that, for pyrite of this size, decomposition to pyrrhotite occurs in the temperature range 450-500°C, for all the coals studied. The reaction essentially was complete at 500°C. 

The incorporation of the sulphur from the decomposition of pyrite, into the surrounding coal matrix, was also followed by scanning electron microscopy. This involved spot analysis for sulphur out from the pyrite/coal interface into the surrounding matrix. These results follow the trend presented above for pyrite decomposition. At 400 and 450°C, little sulphur transfer was observed at 500however, there was an observed increase in the J sulphur (Fig. 5), in the region around the decomposed particle. 

Pyrite Decomposition. This study shows a slightly lower... 

DiffMod - Modelling Oxygen Diffusion and Pyrite Decomposition in the Unsaturated Zone Based on Ground Air Oxygen Distribution... 

DiffMod - Modelling Oxygen Diffusion and Pyrite Decomposition... 

Figure 4.1 Pyrite decomposition in soil column experiment development of oxygen depth distribution and cumulative amount of iron release over time initial pyrite content.

A pyrite decomposition process is an instationary process since the solid phase reactant is being consumed under pyrite-oxidizing conditions causing changes in both reactive surface and mineral mass. Only in a fictitious system, in which the velocity of the depyritization front penetration by chance equals the surface erosion stationary conditions may be found. Diffusion under instationary conditions may be described by numerical solutions of the transport reaction equation considering Picks second law. Reactions can be integrated quite simply, if transport and reaction are decoupled in a... 

Convective gas phase transport in natural unsaturated soil systems may be induced by variations in temperature, air pressure and groundwater, by wind pressure or by seepage water movement after rain events. All these processes are of minor importance in lab column experiments and have not been implemented in DiffModV. The only convective movement in the gaseous phase considered in DiffMod is the ground air recharge due to gas volume decrease caused by oxygen consumption. The effects of this type of convection have to be taken into consideration as they result in an increase of the initial oxygen penetration depth or the thickness of the pyrite oxidation zone and thus induce an increase of the total pyrite decomposition rate on the column by 10 to 15 %. 

Figure 4.4 Modelled oxygen time-depth distribution and cumulative iron release during pyrite decomposition in column experiment (compare experimental data in Figure 4.1).

Figure 4.5 Modelled time-depth distribution of pyrite decomposition rates, pyrite contents and reaction product concentrations in the seepage water.

Figure 4.6 Modelled time-depth distribution of oxygen concentrations, pyrite decomposition rates and pyrite contents cumulative iron release bisected diffusion-effective porosity with regard to modelled column experiment (same effect as duplication of tortuosity factor).

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