Experimental evidence of extremally acidic environment due to self-heating of coal-mining wastes

Coal exploitation has resulted in vast amounts of waste rocks gathered in dumps of different sizes, shapes and volumes. Changes due to their storage can lead to self-heating and self-ignition, which generate the acidification of coal wastes. Long-term burning leads to the sulphates and chlorides crust formation and sometimes hematite of spherulitic shape. A series of simple laboratory experiments were conducted to replicate the conditions leading to the formation of hematite spherules from goethite.

The first experiment involved heating at a low temperature. The starting materials were goethite α-FeOOH alone or goethite mixed with five reactants known from the coal-waste dumps (salammoniac NH₄Cl, sulphur S₈, phtalimide C₈H₅NO₂, FeSO₄·7H₂O, naphthalene C₁₀H₈)  mixed in fifteen combinations and held at 150^oC for 90 hours in a heating chamber. Hematite formed but particle shapes remained unchanged.

The second experiment was conducted at a higher temperature to verify if temperature alone is crucial to spherule formation. Temperatures measured at the burning dumps reach 1000^oC. Goethite samples were held in a tubular furnace at 450-900^oC in an open system for 90 minutes. Hematite formed but with irregular shapes, suggesting that other factors could be more critical.

Ubiquitous in dumps, water was added to the third experiment involving six combinations of reactants and a temperature of 150^oC held for 78 h, pH = 5-7. Partially rounded hematite resulted from that bore little comparison with that seen in the dump.

The fourth experiment involved ten different mixtures of reactants (goethite, salammoniac, FeSO₄·7H₂O, naphthalene, NaCl, and powdered coal waste from the Radlin dump) heated in a tubular furnace at 700^oC for 5 hours in an open system. The high temperature combined with unrestricted oxygen access resulted in the rapid crystallization of octahedral magnetite and platy hematite.

In the fifth experiment goethite in distilled water was mixed with NaCl and AlCl₃, the temperature was relatively low (200^oC), the duration was extended to 7 days, oxygen access was limited, and pH = 5-7. Cubic magnetite crystals and small hematite spherules with Ni or Cr impurities formed.

The sixth experiment conducted two mixtures: goethite + NaCl + AlCl₃ + distilled water + MnSO₄ and goethite + NaCl + AlCl₃ + distilled water + FeSO₄ in a semi-closed system at 200^oC and lasted 16 days. The pH was lowered to 3. In addition to octahedral crystals of magnetite and shapeless hematite, spherules of Al- and (Al, Si) glass formed.

The seventh experiment was conducted in a semi-closed system with pH = 0, a low temperature of 200^oC and a relatively long duration of 7 days. Under these conditions, and with goethite, AlCl₃, MnSO₄ and HCl as reactants, hematite spherules finally formed.

The laboratory experiments took into account the following factors noticed at burning coal waste dumps: temperature, fire duration, oxygen access, water presence, pH, reactants, their amount, combinations, state of matter, grinding, and mixing. Of these, pH stands out as crucial. It proves that hematitic spherules can be an indicator of an extremely acidic environment during the recent and earlier dump fires.