Influence of temperature on dynamic leaching characteristics of coal gangue

Long-term stockpiling of coal gangue would cause acidic pollution at surrounding area and heavy metal con- tent to exceed the standard. The coal gangue was obtained from Yangquan coalmine in Shanxi to study the mechanism of the influence that coal gangue’s self-heating oxidation has on the leaching and release characteristics of acidic pol- lutants. The dynamic leaching experiments were carried out in an automatic temperature-controlled leaching system.The leaching rules of leaching solutions’ pH values,electrical conductivities ( ECs),oxidation-reduction potentials (ORPs) and sulfate contents (SO24- ) were investigated at different temperatures. Also,the respective mineral compo- nents of coal gangue samples before and after the leaching operations were analyzed semi-quantitatively. The result shows that weathered coal gangue contains a great quantity of soluble sulfates,and the content of acidic components is greater than the sum of alkaline and argillaceous mineral components. The sulfate content of wetting solution exceeds the subsurface class Ⅲ water index at the beginning of the experiment,so it can be regarded as highly mineralized wa- ter. Besides,the wetting solution assumes weak acidity and oxidation state,but the oxidizing property does not exceed the value of the general defined oxidizing soil (400 mV). As time goes on,however,the pH value will increase at first, then decrease and tend to be stable eventually. At t = 3 h,the pH value reaches a maximum of about 7. 34-8. 03. The sulfate content and EC value will decrease sharply at first and then stabilize at around 2 500 mg / L and 2 mS / cm re- spectively,while the ORP will be fluctuating within the range of 325. 9 mV and 435. 2 mV. When temperature rises from 50 to 200 ℃ ,the steady state pH value (pHs ) of solution and the residual pyrite content in coal gangue will de- crease at first and then increase,and pHs values are maintained less than 5. 79 at 150 ℃ to 200 ℃ . The sulfate and gypsum contents will increase at first and then decrease while the volatility of ORP will increase gradually as tempera- ture increases,and the relative standard deviation (RSD) increases from 9. 54% to 10. 06% . The EC value is not sig- nificantly affected by temperature. All these indexes will obtain their extreme values at 150 ℃ ,the extreme values of pHs ,min and ORPmax are 4. 98 and 435. 2 mV,respectively,which present a strong acidity and strong oxidation state. Therefore,the oxidation of acid-generating substances in coal gangue can be divided into weathering oxidation and au- tothermal oxidation,which are mainly affected by the combination of ambient temperature,water content and degree of air contact. The acid pollution evolution process caused by the autothermal oxidation of coal gangue is determined by alkaline components,nitrous substances and acidic components produced by oxidation,also,the leaching release rate of three components gradually tend to be stable after long-term storage. This paper explains the mechanism of tempera- ture’s influence on the acid production of pyrite oxidation in coal gangue,which could provide a research basis for forecasting the dynamic evolution of self-heated coal gangue hills’ acidic pollution caused by rainfall leaching and in- filtrating.