Chemical precipitation within pyritic waste rock

Pyritic waste rock piles generate acid rock drainage (ARD). If the chemical precipitation of metals could be induced within waste rock piles by the weathering products of phosphate and carbonate minerals, seepage metal loads–and treatment costs–could be reduced. Seventeen 70-L drums were filled with refractory ore, supposedly non-reactive waste rock, freshly broken low-pyrite waste rock, ‘old’ low-pyrite rock (weathered four years or longer), freshly broken high-pyrite rock or old high-pyrite rock. Control drums were set aside for each category. Phosphate mining wastes, containing both phosphate and calcium carbonates, were added to the remainder. The drums were weathered for 2.7 years, stored for 4.5 years indoors and re-exposed outdoors for 1.3 years. Discharge was collected and analysed. In the first phase of the experiment, acid generation in the waste rock drums containing natural phosphate rock (NPR) was reduced by 85%. In storage, acidity continued to be generated on the waste rock surfaces but was reduced even further in the second phase of the experiment when the rocks were returned outdoors. Although the mechanisms causing this are not fully understood, carbonate neutralization, along with ferric phosphate and iron hydroxide precipitation on the mineral surfaces, as well as microbial biofilm formation, are likely contributory factors. The results suggest that the standard practice of estimating acid generation potential of base-metal waste rock should be reconsidered.