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Phosphogypsum and aluminum ash are typical bulk industrial solid wastes produced by phosphorus chemical and aluminum electrolysis industries respectively, both of which are characterized by large yield, low utilization rate and high environmental risk. In this article, high-density blocks were synthesized for the first time by densification treatment with phosphogypsum and aluminum ash. The mechanical and waterproof properties of the blocks formed after densification treatment under different ratios, densification pressures and curing time were also investigated. The experimental results indicate that the blocks treated by densification demonstrate a remarkable comprehensive performance under the optimal conditions of a ratio between phosphogypsum and aluminum ash of 1:9, densification pressure of 300 MPa and curing time of 5 d. Under such conditions, the compressive and flexural strength of the blocks are 58 MPa and 1.7 MPa respectively, with water absorption rate of about 5.3% and softening coefficient of 0.99. The leachable efficiency of toxic and harmful elements in the block materials is low, meeting the requirements for environmental protection, and thus densification treatment can be used for the harmless treatment of solid waste. The method provides a feasible research idea for simulating the change of underground minerals under the action of surface pressure and exploring the harmless and resourceful treatment of bulk solid waste. The technology is expected to be applied in the field of high-density filling and building materials prepared from solid waste. [ABSTRACT FROM AUTHOR]

Phosphogypsum (PG), as a typical industrial solid waste produced in the wet production process of phosphoric acid, is harmful to the environment and difficult to utilize. In order to explore the migration behavior of impurities in PG under electrokinetic treatment and provide a new treatment technology for the purification and harmless treatment, this paper studied the distribution of soluble impurities (PO 4 3−, F−, Sr, Pb, Cr, Cu, and Mn) in PG under different electrokinetic treatment conditions. The results indicated that most of water-soluble impurities demonstrated obvious migration distribution laws at different voltages. Under the influence of electroosmotic flow, F− accumulated continually at the cathode. Positive MF+ was produced in acidic circumstances by combining F− and metal ions and travelled to the cathode under the combined action of electric field force and electroosmotic flow. Under electrokinetic treatment, the concentration of PO 4 3− in the anode chamber increased while that of in the cathode chamber decreased. A large number of anions in PG were attached to the anode by coulomb force, reducing the potential. PO 4 3− was s largely enriched in the anode chamber, which is controlled by the free diffusion effect of ions. Heavy metals exhibited higher leaching efficiency under anodic acid conditions. With the increase of applied voltage, the concentration of heavy metals in the anode chamber gradually increased, which was related to the activity of solution, pH and other factors. The above results indicate that electrokinetic treatment is a novel and efficient PG treatment technology, which has a broad application prospect in impurity removal and PG resource recovery. [Display omitted] [ABSTRACT FROM AUTHOR]

• Solid state reaction was used for solidification and stabilization of phosphogypsum for the first time. • The two-stage solidification/stabilization method of phosphogypsum was adopted. • This study achieved the effect of long-term solidification and stabilization of phosphogypsum. In this paper, an innovative method for highly targeted solidification/stabilization (S/S) of harmful substances in phosphogypsum (PG) was proposed. The release behavior of hazardous substances in PG after mixed reaction was explored using lime or carbide slag (CS) as alkaline regulator and then polyaluminium chloride (PAC), polyferric sulfate (PFS) and polyacrylamide (PAM) as highly targeted S/S agents. The experimental results show that lime has a long-term buffering effect on the pH of PG and a significant stabilizing effect on P, F and heavy metals. When more than 5 % lime was added, the leaching concentration of heavy metals was less than 0.1 mg/L. Moreover, PAM, PAC and PFS have remarkable S/S effects on P, F and heavy metals in the order of PAM > PFS > PAC. When the dosage of PAM was more than 5 %, the leaching concentration of P and F was less than 0.2 mg/L and 1 mg/L, respectively, and that of heavy metals tends to be 0 mg/L, indicating that PAM demonstrates a significant continuous S/S effect and thus provides a feasible pretreatment method for harmless filling, soil utilization and high value-added resource utilization of PG. The high targeted S/S technology of hazardous substances studied in this paper directly provides theoretical and data support for the harmless and resource-based treatment of large-scale solid waste PG produced, alleviates the pressure of solid waste environmental protection in the phosphorus chemical industry, and provides a new research idea for the harmless treatment of other toxic and harmful large-scale solid wastes, reducing the cost of solid waste treatment and simplifying the treatment process. [ABSTRACT FROM AUTHOR]

• The reaction mechanism between solid-solid phase was simulated by the PHREEQC. • The application potential of carbide slag in the safe disposal of PG was clarified. • The behavior of soluble harmful impurities in PG were studied at different pH. This study explored the potential applications of quicklime and carbide slag in the preparation of phosphogypsum (PG)-based ecological restoration materials. The migration and transformation behavior of large amounts of soluble impurities (PO 4 3−, F−, NH 3 -N, Cr6+, Mn, and Cu) in PG were studied. A solid–solid phase reaction was innovatively employed, and PHREEQC was used as the simulation software to investigate the solidification/leaching mechanism of pollutants in PG treated with different amounts of alkaline materials. The results showed that acid–base condition is a dominant factor controlling the migration and transformation of P, F, NH 3 -N, and heavy metals. The addition of a small amount of quicklime or carbide slag to PG considerably improved its pH. Quicklime and carbide slag exhibited remarkable stabilization effects on PO 4 3−, F−, Cr6+, and Mn, and the leaching concentration of water-soluble fluorine and NH 3 -N in PG added with quicklime decreased substantially. In general, when preparing PG-based ecological restoration materials, the addition of carbide slag exhibited a stronger acid buffering capacity, pollutant stabilization capacity, and potential nutrition than that of quicklime. In particular, the use of carbide slag instead of quicklime in PG treatment can reduce the cost of PG treatment by approximately $1.3–1.5 per ton. [Display omitted] [ABSTRACT FROM AUTHOR]

Phosphogypsum is one of the hottest issues in the field of environmental solid waste treatment, with complex and changeable composition. Meanwhile, phosphogypsum contains a large number of impurities, thus leading to the low resource utilization rate, and it can only be stockpiled in large quantities. Phosphogypsum occupies a lot of land and poses a serious pollution threat to the ecological environment. This paper mainly summarizes the existing pretreatment and resource utilization technology of phosphogypsum. The pretreatment mainly includes dry method and wet method. The resource utilization technology mainly includes building materials, chemical raw materials, agriculture, environmental functional materials, filling materials, carbon sequestration and rare and precious extraction. Although there are many aspects of resource utilization of phosphogypsum, the existing technology is far from being able to consume a large amount of accumulated and generated phosphogypsum. Through the analysis, the comparison and mechanism analysis of the existing multifaceted and multi-level resource treatment technologies of phosphogypsum, the four promising resource utilization directions of phosphogypsum are put forward, mainly including prefabricated building materials, eco-friendly materials and soil materials, and new green functional materials and chemical fillers. Moreover, this paper summarizes the research basis of multi field and all-round treatment and disposal of phosphogypsum, which reduces repeated researches and development, as well as the treatment cost of phosphogypsum. This paper could provide a feasible research direction for the resource treatment technology of phosphogypsum in the future, so as to improve the consumption of phosphogypsum and reduce environmental risks. [Display omitted] • The main resource utilization technologies and application directions of phosphogypsum are comprehensively combed. • The research basis of multi field and all-round treatment and disposal of phosphogypsum is summarized, which reduces the repeated research and development of phosphogypsum. • The technical prospect of phosphogypsum resource utilization and large-scale consumption is put forward. [ABSTRACT FROM AUTHOR]

Phosphogypsum is one of the typical by-products of phosphorus chemical industry. As a strategic industry related to the national livelihood of China, phosphorus chemical industry has accumulated and produced a significant amount of phosphogypsum. In general, phosphogypsum contains approximately 80%–95% calcium sulfate dihydrate, and less than 5% toxic and harmful elements. In this paper, toxic and hazardous components in phosphogypsum were efficiently solidified and stabilized by highly targeted solidification and stabilization technology. Calcium carbide slag or lime was used as an alkali-base neutralizer of phosphogypsum, and polymeric ferric sulfate or polymeric aluminum chloride as a directional solidification stabilizer to analyze the leaching toxicity of the mixed powder in 1, 3, 5 and 15 days. The experimental results demonstrate excellent solidification and stabilization effect with the leaching pH of 6–9, the leaching concentration of P, F and heavy metals of less than 0.5 mg/L, 10 mg/L and 0.1 mg/L, respectively, which meets the requirements of relevant international standards. Mechanistic analysis indicates that the solidification and stabilization of toxic and hazardous substances in phosphogypsum is perfectly achieved owing to the generation, adsorption and encapsulation of insoluble substances. This technology can reduce the costs and difficulty in the phosphogypsum treatment, and has extensive application potentials. • A high targeted solidification/stabilization method for toxic and harmful substances in phosphogypsum was proposed. • The powder reaction is adopted for the first time in the harmless treatment of phosphogypsum. • The addition of phosphogypsum is more than 90%, which reduces the cost of phosphogypsum treatment and simplifies the treatment technology. [ABSTRACT FROM AUTHOR]

[Display omitted] • 100% phosphogypsum without any pretreatment and external additives. • The pure pressed finished product has superior mechanical and waterproof properties. • Meet the relevant regulations for construction materials and environmental requirements. • The process is cost effective and relieves environmental stress. The resourceful use of solid waste has continued. The research produced a new eco-friendly, high-strength, water-resistant building material by using the simplest mechanical method of pressing 100% phosphogypsum. The compressive strength of the blocks was higher than 80 MPa and the flexural strength was 5.5 MPa when the pressure was 300 MPa and the placing time was 5d. SEM, FTIR, TG-DTG and XRD of the products showed the blocks pressed from phosphogypsum didn't change the chemical composition of the original phosphogypsum and produce new substances at different pressures, but the increase in pressure changed the crystal structure of CaSO 4 ·2H 2 O. [ABSTRACT FROM AUTHOR]