Pyrite caustic

Kostina, G. M. and Chernyak, A. S., 1979. Investigation of the mechanism of electrochemical oxidation of arsenopyrite and pyrite in caustic soda solution. Zhumal Prikladnoi Khimii, 52 1532- 1535... 

XRF data on coals treated with molten caustic at fitnes Laboratory showed that levels of Fe, which is predominantly associated with pyrite in coal, were reduced by about 90% for each of the two runs. Reductions in concentrations of other elements which form abundant minerals in coal, including Al, K, and Si, were also substantial. Because roughly 90% of the ash was removed, substantial concentration reductions in the major mineral-forming elements are expected. 

Chemical treatment of coal has also been used to further reduce the mineral matter content. Usually the physical techniques mentioned earlier are used to reduce the inorganic sulfur species (mostly pyrite) to a very low level. Since there is a great concern about the sulfur contents of coal, and the organic sulfur content exceeds the permissible standards for many high-sulfur coals, non-physical or chemical techniques have been tried to further reduce the sulfur content. Strong caustic reagents are used most often to remove the more refractory sulfur forms. 

Previous work has shown that ash-forming mineral matter, including iron pyrites, can be removed from coal by leaching the finely divided material with a hot caustic solution under pressure followed by washing with a dilute mineral acid (1-6). Recently, similar results have been achieved in leaching fine-size coal with hot sodium carbonate solutions (J7). In both cases, quartz appears to dissolve in the hot alkaline solution, while clay minerals and iron pyrite are converted into acid-soluble compounds which are removed in the acid washing step. Acid also removes any carbonate minerals which are present in the coal. 

Al-Razi described much other equipment that would be familiar to any student of chemistry beakers, flasks, crystallizing dishes, spatulas, funnels, filters made of cloth, and pestles and mortars. His shelves contained not only all the known metals, but also many other substances such as pyrites, malachite, lapis lazuli, gypsum, hematite, galena, turquoise, stibnite, alum, green vitriol, natron, borax, salt, lime, potash, cinnabar, white and red lead, iron oxide, copper oxide, vinegar, and probably caustic soda, glycerol, and sulfuric and nitric acids. 

Unless the coal being washed contains appreciable quantities of soluble salts, cleaning processes do not materially alter water composition. However, difhculties may be encountered when iron carbonates and pyrite and pyrite are present, particularly if the coal passes through stockpiles that allow some oxidation of the iron salts. The contamination is usually indicated by a substantial lowering of pH, from a normal range of 6-7.5 to values of 3 or even less, and may necessitate addition of alkali (caustic soda or lime) to restore near-neutral conditions. This type of problem is not often encountered and the nsnal type of water treatment involves clarification to remove suspended salts (slime) after which the water may be recirculated. 

Ordinary p57rites (fool s gold, marcasite) has the formula FeSg. This material is not affected when warmed with 3 N caustic alkali, whereas arseno-pyrites (FeAsSg) reacts with the alkali and yields S ions that can be detected by the black precipitate which is produced on the addition of a lead salt. The following partial reaction, which involves the participation of the atmospheric oxygen, probably occurs. 

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