Phosphogypsum pretreatment with CaCO3 to improve the backfill performance

Abstract

When acid phosphogypsum (APG) is recycled as aggregate of cemented backfill in mines, pretreatment is usually needed to reduce its adverse impact on the backfill. In this study, a new approach of using a neutral salt (CaCO3) to pretreat APG has been proposed to enhance the strength development of the cemented PG backfill. The role of CaCO3 in cemented PG backfill acting as a reactant is explored. In the pretreatment stage, CaCO3 reacted with the residual acid in PG, and the generated calcium ions could solidify the impurities, such as soluble phosphate and fluoride, with solidification rates reaching 90% and 99%, respectively. This neutral modifier buffered pH values of PG aggregate from 1.8 to about 5.0, and excess dosage of CaCO3 did not adjust the pH value of aggregate to an excessively high level. Hence, there was no interference with the hydration reaction of the binder (cement). Using PG pretreated with CaCO3 as aggregate for cemented backfill effectively accelerated the setting process of backfill and increased the unconfined compressive strength (UCS) of the backfill matrix from 0.14 MPa to 2.27 MPa at 28-day age. The present results suggest that CaCO3 pretreatment is an effective way to improve the accessibility of PG as aggregate, thus significantly increasing the utilization rate of PG and enhancing the backfill performance.