Abstract
The environmental pollution caused by the discharge of phosphogypsum (PG) and phosphorous slag (PS) is a common issue for all countries. In order to fully utilize hemihydrate PG (HPG) and PS and treat goafs in mines, the HPG and PS were used as cementitious materials for cemented paste backfill (CPB). The physical and chemical properties of HPG and PS were first analyzed, and then, the characteristics of CPB were evaluated through fluidity tests, gas detection, uniaxial compressive strength (UCS) tests, bleeding tests, and scanning electron microscopy (SEM). After this, the underground environmental impact of CPB-based HPG and PS was investigated through a dynamic leachability experiment. The results show that (1) the UCS of CPB increases with the increase of the HPG content and mass fraction, and the addition of 3% quicklime can eliminate CO2, H2S, and SO2 generated from the slurry of CPB-based HPG-PS; (2) the addition of 3% quicklime and 5% cement to the HPG-PS mixtures can offset the strength loss of CPB in the late curing stage; (3) the UCS of the recommended specimen reaches 1.15–3.32 MPa after curing from 7 to 28 days, with their slump values varying from 15 mm to 26 mm, and the bleeding rates between 0.87% and 1.15%, which can meet the technical requirements of mining methods; (4) the UCS of CPB is the result of the cohydration reaction of hemihydrate gypsum (HG) in HPG and active Al2O3 and SiO2 in PS; and (5) the leaching indexes meet Category IV of the Chinese Groundwater Quality Standard (DZ/T 0290-2015). The results of this investigation provide a cost-efficient way for the efficient mining of phosphate resources and the comprehensive utilization of HPG and PS.
Highlights
Phosphogypsum (PG) is a main by-product of wet-process phosphoric acid
(1) e hemihydrate phosphogypsum (HPG) had a moisture content and pH value of 12% and 3.1, respectively. e main chemical components of HPG are shown in Table 1. e X-ray diffraction (XRD) of HPG is shown in Figure 1(a). e main phase of HPG was CaSO4 · 0.5H2O and contained a small amount of CaSO4 · 2H2O and quartz (SiO2), which presented better self-gelling properties [33]. e surface of HPG particles was coarse, and the deposits were impurities such as phosphide and fluoride [34], as shown in Figure 1(b). e particle-size distribution of HPG is shown in Figure 2(a), for which the particle size of less than 50 μm accounts for 45.34%
We investigated the application characteristics of recycling HPG and Phosphorus slag (PS) as cemented backfill materials in cemented paste backfill (CPB) under the modification of quicklime and cement by fluidity tests, gas detection, uniaxial compressive strength (UCS) tests, and scanning electron microscopy (SEM)
Summary
Phosphogypsum (PG) is a main by-product of wet-process phosphoric acid. PG can be classified into dihydrate phosphogypsum (DPG) and hemihydrate phosphogypsum (HPG) according to different processing methods [1, 2]. DPG, the main phase of which is CaSO4 · 2H2O, cannot form a solidified body without heat treatment. HPG is mainly composed of CaSO4 · 0.5H2O, which presents better self-gelling properties during CaSO4 · 0.5H2O conversion to CaSO4 · 2H2O [3]. Phosphorus slag (PS) is a by-product of yellow phosphor production via the electric furnace method and is mainly composed of CaSiO3, which contains active SiO2 and Al2O3 and has weak gelling properties [4, 5]. PG and PS contain a small amount of harmful impurities such as phosphorus, fluorine, and heavy metals, which limit its comprehensive utilization [1, 6, 7]
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