Using modified quartz sand for phosphate pollution control in cemented phosphogypsum (PG) backfill

Cemented phosphogypsum (PG) backfill technology is a new alternative for massive disposal of PG as well as to provide a safe underground mining workplace. However, phosphate in bleeding water and leachates of backfill might be discharged into its surrounding environment. To reduce phosphate pollution with source control concept, modified quartz sand (MQS) was used during the mixing of backfill slurry. Experimental results showed that when the MQS was added in the backfill slurry, phosphate concentrations in the bleeding water decreased significantly with mixing time, which was attributed to a synergetic effect of MQS adsorption and binder hydration. As a result, phosphate concentration in the MQS-added backfill could be reduced to 0.3 mg/L in 20 min, therefore ensuring the phosphate concentration in bleeding water below the first grade discharge standard (0.5 mg/L, GB 8978 -1996) in a short mixing time. In comparison, the time needed for control group to meet this discharge standard was over 60 min. Then, through phosphate adsorption static experiments, it is found that the MQS adsorption process was in accordance with the Langmuir model and pseudo-second-order kinetic model, illustrating that the MQS was a monolayer chemical adsorption for phosphate in the cemented PG backfill system. Furthermore, by monitoring the leachates of backfill with and without MQS, the phosphate concentrations were always less than 0.05 mg/L, indicating that the addition of MQS had no adverse effect on the phosphate release in the long run. Besides, the addition of MQS can promote hydration process and optimize aggregate gradation to enhance strength development of backfill. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

The addition of modified quartz sand (MQS) can significantly reduce phosphate concentration in backfill, meeting discharge standards in a short time without adverse effects on long-term phosphate release. Furthermore, MQS addition promotes hydration processes, optimizes aggregate gradation, and enhances strength development of backfill.