The Release of Pollutants through the Bleeding of Cemented Phosphogypsum Backfill: Link to Protocols for Slurry Preparation.

The present study investigated the effects of protocols for slurry preparation on the release of pollutants into bleeding water from cemented phosphogypsum (PG) backfill. Backfill slurry was prepared using four different protocols in which different parameters varied, including binder/PG ratio, solid concentration, binder type and mixing procedure. The concentrations of phosphate, fluoride and sulfate and the pH values of the obtained bleeding water were measured. The results demonstrated that the slurry preparation protocols affected the quantities of pollutants through the concentrations of pollutants in bleeding water and the bleeding rate. On the one hand, the binder/PG ratio was the key factor influencing the concentrations of all pollutants in bleeding water. Comparatively speaking, the binder type and mixing procedure had an obvious influence on the fluoride concentration but had little influence on the phosphate and sulfate concentrations in the bleeding water. On the other hand, the protocols for slurry preparation affected the bleeding rate by determining the water retention and water content of the backfill slurry. The most effective protocol for slurry preparation for cemented PG backfill could reduce the bleeding rate and enhance the immobilization of pollutants, minimizing the phosphate concentration in bleeding water to below 0.2 mg/L. However, it appeared that the fluoride concentration was still tens of milligrams per liter (over the limit of 10 ten milligrams per liter), to which attention should be paid.

Compressive Strength Prediction of Cemented Backfill Containing Phosphate Tailings Using Extreme Gradient Boosting Optimized by Whale Optimization Algorithm.

Unconfined compressive strength (UCS) is the most significant mechanical index for cemented backfill, and it is mainly determined by traditional mechanical tests. This study optimized the extreme gradient boosting (XGBoost) model by utilizing the whale optimization algorithm (WOA) to construct a hybrid model for the UCS prediction of cemented backfill. The PT proportion, the OPC proportion, the FA proportion, the solid concentration, and the curing age were selected as input variables, and the UCS of the cemented PT backfill was selected as the output variable. The original XGBoost model, the XGBoost model optimized by particle swarm optimization (PSO-XGBoost), and the decision tree (DT) model were also constructed for comparison with the WOA-XGBoost model. The results showed that the values of the root mean square error (RMSE), coefficient of determination (R2), and mean absolute error (MAE) obtained from the WOA-XGBoost model, XGBoost model, PSO-XGBoost model, and DT model were equal to (0.241, 0.967, 0.184), (0.426, 0.917, 0.336), (0.316, 0.943, 0.258), and (0.464, 0.852, 0.357), respectively. The results show that the proposed WOA-XGBoost has better prediction accuracy than the other machine learning models, confirming the ability of the WOA to enhance XGBoost in cemented PT backfill strength prediction. The WOA-XGBoost model could be a fast and accurate method for the UCS prediction of cemented PT backfill.

Effect of phosphorus on the properties of phosphogypsum-based cemented backfill.

When phosphogypsum (PG) is used as an aggregate for backfill, phosphate in the PG might influence the hydration process and escape into the environment. The current study aimed to understand phosphate dynamics during the PG-based backfilling process by adding different amounts and types of phosphates (H3PO4, KH2PO4, K3PO4, and Ca3(PO4)2). The results indicate that the majority of the phosphate was first immobilized by PG depending on the types, and the residual dissolved phosphate (RDP) could be further stabilized/solidified (S/S) in the backfill via the hydration process. However, increasing RDP content lowered unconfined compressive strength of the backfill, attributing to the suppression of the hydration process and a loosened backfill structure. Furthermore, the environmental behavior of phosphate was studied by measuring dissolved phosphate in bleeding water and leachate. For bleeding water, a high RDP content might lead to the phosphate concentration exceeding the national standard limit (GB 8978-1996) depending on the phosphate types, and it was recommended that the RDP content should be controlled or converted to Ca3(PO4)2 or K3PO4 before PG inclusion into in the backfill. For leachate, the phosphate concentration was always below the standard limit, indicating that the cemented backfill ensured long-term S/S of the phosphate.