Characteristics and treatment of mine water from three historical coal workings in Yorkshire, UK: interrelationships between rates in geochemical, environmental and operational processes

摘要

Oxidation processes in three ferruginous coal mine water systems in Yorkshire, UK - studied over a period of some 20 years - are summarised. Filtered mine water samples from the Sheephouse Wood (SHW) system gave rates of iron(II) oxidation in homogeneous medium (determined in the laboratory in terms of the thermodynamic rate constant k) consistent with authoritative long-established literature reported for other aquatic systems. The calculated mean value of k was 2.35 x 10~(13)M~(-2)atm~(-1)min~(-1) (pH 6.4-7.0; relative standard deviation ±12.5%). For similar measurements in heterogeneous medium - in the presence of a set large excess of pre-precipitated hydrated iron(III) oxide, ochre (360 times by mass of initial dissolved ferrous iron) - a (conditional) rate constant k_c was found to be some two orders of ten greater. This represented an example of rate enhancement by autocatalytic reaction. Field measurements in a well-defined mine water channel at SHW provided semi-quantitative evidence that practical rates of mine water oxidation are also relatively greatly affected by seasonal environmental factors.Observations of a pilot plant at the Woolley (W) system confirmed that net alkaline mine water was rapidly oxidised via autocatalytic reaction when passed through a tank containing a thick slurry of ochre. Carry-over of fine oxidised particulates tended to occur. Work at the full scale Bullhouse (BH) system, constructed with an unusually large lagoon, directed inlet pipework under water through settling ochre to optimise autocatalytic enhancement. Despite net acidity at this site, treatment met discharge objectives on oxidation and settlement for some 12 years, based on a long residence time (initially some 17 days). Progressive ochre build-up indicated a need thereafter for desludging. The results are discussed in terms of variable autocatalytic reaction, which is evidently normal in such systems, and, together with other parameters, relevance to the perceived state-of-the-art of treatment in coal mine water systems.