Grain Size Effects on Mine Water Quality and Acid/Neutral Rock Drainage Production in Kinetic Testing Using Recsk Porphyry Skarn Cu–Zn Deposit Rocks

The quality of mine drainage from sulphide-containing waste rock dumps is influenced by various factors, notably grain size. This study aimed to investigate the effects of grain size on acid/neutral rock drainage production through kinetic tests performed on samples from the Recsk porphyry skarn Cu–Zn deposit in Hungary, an area known for generating acid rock drainage (ARD). Five columns with distinct grain size ranges (1–2 mm, 2–4 mm, 4–8 mm, 8–16 mm, and 16–32 mm) were utilized. Initially, static tests determined the total and available neutralizing potential (NP) and acid potential (AP) for each size range. While total NP and AP values were consistent across sizes, available NP increased as grain size decreased. The neutralization potential ratio was consistently less than 1, indicating a tendency for acid production. Kinetic tests revealed that grain sizes of 1–4 mm maintained a circumneutral pH, whereas larger grain sizes produced lower pH levels. Notably, higher alkalinity was associated with finer grains. Sulphate release rates decreased with larger grain sizes; however, the highest sulphide oxidation occurred in the intermediate size range. This study reinforces the concept that metal mobility increases with decreasing pH irrespective of grain size.