Using cemented paste backfill to tackle the phosphogypsum stockpile in China:A down-to-earth technology with new vitalities in pollutant retention and CO_(2) abatement

Phosphogypsum(PG),a hard-to-dissipate by-product of the phosphorus fertilizer production industry,places strain on the biogeochemical cycles and ecosystem functions of storage sites.This pervasive problem is already widespread worldwide and requires careful stewardship.In this study,we review the presence of potentially toxic elements(PTEs)in PG and describe their associations with soil properties,anthropogenic activities,and surrounding organisms.Then,we review different ex-/in-situ solutions for promoting the sustainable management of PG,with an emphasis on in-situ cemented paste backfill,which offers a cost-effective and highly scalable opportunity to advance the value-added recovery of PG.However,concerns related to the PTEs'retention capacity and long-term effectiveness limit the implementation of this strategy.Furthermore,given that the large-scale demand for ordinary Portland cement from this conventional option has resulted in significant CO_(2) emissions,the technology has recently undergone additional scrutiny to meet the climate mitigation ambition of the Paris Agreement and China's Carbon Neutrality Economy.Therefore,we discuss the ways by which we can integrate innovative strategies,including supplementary cementitious materials,alternative binder solutions,CO_(2) mineralization,CO_(2) curing,and optimization of the supply chain for the profitability and sustainability of PG remediation.However,to maximize the co-benefits in environmental,social,and economic,future research must bridge the gap between the feasibility of expanding these advanced pathways and the multidisciplinary needs.

Study on synergistic leaching of potassium and phosphorus from potassium feldspar and solid waste phosphogypsum via coupling reactions

To achieve the resource utilization of solid waste phosphogypsum(PG)and tackle the problem of utilizing potassium feldspar(PF),a coupled synergistic process between PG and PF is proposed in this paper.The study investigates the features of P and F in PG,and explores the decomposition of PF using hydrofluoric acid(HF)in the sulfuric acid system for K leaching and leaching of P and F in PG.The impact factors such as sulfuric acid concentration,reaction temperature,reaction time,material ratio(PG/PF),liquid–solid ratio,PF particle size,and PF calcination temperature on the leaching of P and K is systematically investigated in this paper.The results show that under optimal conditions,the leaching rate of K and P reach more than 93%and 96%,respectively.Kinetics study using shrinking core model(SCM)indicates two significant stages with internal diffusion predominantly controlling the leaching of K.The apparent activation energies of these two stages are 11.92 kJ·mol^(-1)and 11.55 kJ·mol^(-1),respectively.

Copper slag assisted coke reduction of phosphogypsum for sulphur dioxide preparation

The reduction of phosphogypsum(PG)to lime slag and SO_(2)using coke can effectively alleviate the environmental problems caused by PG.However,the PG decomposition temperature remains high and the product yield remains poor.By adding additives,the decomposition temperature can be further reduced and PG decomposition rate and product yield can be improved.However,the use of current additives such as Fe_(2)O_(3)and SiO_(2)brings the problem of increasing economic cost.Therefore,it is proposed to use solid waste copper slag(CS)as a new additive to reduce PG to prepare SO2,which can reduce the cost and meet the environmental benefits at the same time.The effects of proportion,temperature and thermostatic time on PG decomposition are investigated by experimental and kinetic analysis combined with FactSage thermodynamic calculations to optimize the roasting conditions.Finally,the reaction mechanism is proposed.It is found that adding CS to the coke and PG system can increase the rate of PG decomposition and SO_(2)yield while lowering the PG decomposition temperature.For example,when the CS/PG mass ratio increases from 0 to 1,PG decomposition rate increases from 83.38%to 99.35%,SO_(2)yield increases from 78.62%to 96.81%,and PG decomposition temperature decreases from 992.4℃to 949.6℃.The optimal reaction parameters are CS/PG mass ratio of 1,Coke/PG mass ratio of 0.06 at 1100℃for 20 min with 99.35%PG decomposition rate and 96.81%SO_(2) yield.The process proceeds according to the following reactions:2CaSO_(4)+ 0.7C + 0.8Fe_(2)SiO_(4)→0.8Ca_(2)SiO_(4)+ 0.2Ca_(2)Fe_(2)O_(5)+ 0.4Fe_(3)O_(4)+2SO_(2)+ 0.7CO_(2)Finally,a process for decomposing PG with coke and CS is proposed.

Wetting-drying effect on the strength and microstructure of cementphosphogypsum stabilized soils

Phosphogypsum has often been used as an effective and environmentally friendly binder for partial replacement of cement,improving the engineering properties of slurries with high water content.However,the influence of phosphogypsum on the physicomechnical properties of stabilized soil subjected to wettingedrying cycles is not well understood to date.In this study,the effect of phosphogypsum on the durability of stabilized soil was studied by conducting a series of laboratory experiments,illustrating the changes in mass loss,pH value and unconfined compressive strength(qu)with wettingdrying cycles.The test results showed that the presence of phosphogypsum significantly restrained the mass loss in the early stage(lower than the 4th cycle),which in turn led to a higher qu of stabilized soil than that without phosphogypsum.After the 4th cycle,a sudden increase in mass loss was observed for stabilized soil with phosphogypsum,resulting in a significant drop in qu to a value lower than those without phosphogypsum at the 6th cycle.In addition,the qu of stabilized soils correlated well with the measured soil pH irrespective of phosphogypsum content for all wettingedrying tests.According to the microstructure observation via scanning electron microscope(SEM)and X-ray diffraction(XRD)tests,the mechanisms relating the sudden loss of qu for the stabilized soils with phosphogypsum after the 4th wetting-drying cycle are summarized as follows:(i)the disappearance of ettringite weakening the cementation bonding effect,(ii)the generation of a larger extent of microcrack,and(iii)a lower pH value,in comparison with the stabilized soil without phosphogypsum.

Effect of Lightweight Aggregates Incorporation on the Mechanical Properties and Shrinkage Compensation of a Cement-Ground Granulated Blast Furnace Slag-Phosphogypsum Ternary System

Shrinkage-induced cracking is a common issue in concrete structures,where the formation of cracks not only affects the aesthetic appearance of concrete but also potentially reduces its durability and strength.In this study,the effect of ceramsite sand addition on the properties of a ternary system of cement-ground granulated blast furnace slag(GGBFS)-phosphogypsum(PG)is investigated.In particular,the fluidity,rheology,hydration heat,compressive strength,autogenous shrinkage,and drying shrinkage of the considered mortar specimens are analyzed.The results indicate that an increase in PG content leads to a decrease in fluidity,higher viscosity,lower exothermic peak,and lower compressive strength.However,the shrinkage of the mortar specimens is effectively compensated.The incorporation of internal curing water from ceramsite sand improves fluidity,decreases both yield stress and viscosity,enhances the degree of hydration,and induces mortar expansion.However,the inferior mechanical properties of the ceramsite sand generally produce a decrease in the compressive strength.

Upcycling of phosphogypsum waste for efficient zinc-ion batteries

Zinc metal is a promising anode material for next-generation aqueous batteries,but its practical application is limited by the formation of zinc dendrite.To prevent zinc dendrite growth,various Zn^(2+)-conducting but water-isolating solid-electrolyte interphase(SEI)films have been developed,however,the required high-purity chemical materials are extremely expensive.In this work,phosphogypsum(PG),an industrial byproduct produced from the phosphoric acid industry,is employed as a multifunctional protective layer to navigate uniform zinc deposition.Theoretical and experimental results demonstrate that PG-derived CaSO_(4)2H_(2)O can act as an artificial SEI layer to provide fast channels for Zn^(2+)transport.Moreover,CaSO_(4)2H_(2)O could release calcium ions(Ca^(2+))due to its relatively high Kspvalue,which have a higher binding energy than that of Zn^(2+)on the Zn surface,thus preferentially adsorbing to the tips of the protuberances to force zinc ions to nucleate at inert region.As a result,the Zn@PG anode achieves a high Coulombic efficiency of 99.5%during 500 cycles and long-time stability over 1000 hours at 1 m A cm^(-2).Our findings will not only construct a low-cost artificial SEI film for practical metal batteries,but also achieve a high-value utilization of phosphogypsum waste.

Study on the Recycling of P in Phosphogypsum Leachate

After washing and curing, P is transported from the phosphogypsum to the leachate during the phosphogypsum detoxification process, providing two ideas for phosphorus recovery from phosphogypsum leachate: 1) preparation of calcium hydrogen phosphate for feed;2) preparation of calcium phosphate. A ready-to-use calcium oxide slurry was used to recover P from phosphogypsum leachate at a slurry concentration of 20% and a quantitative link between calcium to phosphorus ratio and fixation rate was fitted by mixed use batch experiments, reaction kinetics and thermodynamics, and theoretical calculations were used to demonstrate that phosphorus cannot be completely reused in the preparation of calcium hydrogen phosphate. The findings demonstrated that: a) the residual phosphorus concentration was in the range of 1300 - 1500 mg/L for the preparation of type I feed grade calcium hydrogen phosphate from phosphogypsum leachate;b) the P removal effect could reach 99.99% for the preparation of calcium phosphate from phosphogypsum using the theoretical equation: fixation rate = 87.91 - 10.96(Ca/P) + 3.22(Ca/P)2 (R2 = 0.9954);c) The procedure follows the suggested secondary kinetics, and according to the Freundlich isothermal model, the reaction process is under the control of the chemical reaction, with a reaction index of 0.7605. This study can be used as a theoretical guide for the recovery of P from phosphogypsum leachate, the preparation of products to bring about economic by-products, and the purification of wastewater for reuse.

Investigation of Al2O3 and Fe2O3 transmission and transformation during the decomposition of phosphogypsum

Phosphogypsum(PG) is a solid waste produced in the phosphate fertilizer industry and is environmentally harmful.The decomposition of PG to recycle calcium and sulfur is a proper way to reutilize PG. Current work aims at enriching the basic theory of coal decomposition process of PG. The emphasis was laid on the exploration of impact of main impurities on the process. On the other hand, according to Reaction Module, Equilib Module, and Phase Diagram Module of FactS age, the simulation computation was done on the systems of pure gypsum mixed with coal,with or without impurities for avoiding other impurities interference. Later, possible reactions in the process were deduced. Additionally, experiments were conducted in a TG-DTA integrated thermal gravimetric analyzer and a tube furnace. The products from the experiments were characterized and analyzed to verify the accuracy of theoretical calculations. The results showed that these impurities can change the decomposition process of PG. For example, aluminum oxide was transformed to calcium sulfoaluminate, while iron oxide was transformed to dicalcium ferrite. Furthermore, the results help to further improve the basic theory of phosphogypsum decomposition.

Variation of alkaline characteristics in bauxite residue under phosphogypsum amendment

Aiming at alkaline problem of bauxite residue,this work focused variation of alkaline characteristics in bauxite residue through phosphogypsum treatment.The results demonstrated that the pH of bauxite residue reduced from initial 10.83 to 8.70 when 1.50 wt%phosphogypsum was added for 91 d.The removal rates of free alkali and exchangeable sodium were 97.94%and 75.87%,respectively.Meanwhile,significant positive correlations(P<0.05)existed between pH and free alkali,exchangeable sodium.The effect of free alkali composition was CO3^2–>OH^–>AlO2^–>HCO3^–.In addition,alkaline phase decreased from 52.81%to 48.58%and gypsum stably presented in bauxite residue which continuously provided Ca^2+to inhibit dissolution of combined alkali.Furthermore,phosphogypsum promoted formation of macroaggregate structure,increased Ca^2+,decreased Na+and Al^3+on the surface of bauxite residue significantly,ultimately promoting soil formation in bauxite residue.

Effect of phosphogypsum on saline-alkalinity and aggregate stability of bauxite residue

A column experiment was conducted to investigate the effect of phosphogypsum(PG)on the salinealkalinity,and aggregate stability of bauxite residue.Results showed that:with increasing leaching time,the concentrations of saline−alkali ions decreased while the SO_(4)^(2-)concentration increased in bauxite residue leachate;compared with CK(control group)treatment,pH,electric conductivity(EC),exchangeable sodium percentage(ESP),sodium absorption ratio(SAR),and exchangeable Na+content of bauxite residue were reduced following PG treatment;average particle sizes in aggregates following CK and PG treatments were determined to be 155 and 193 nm,respectively.SR-μCT test results also confirmed that bauxite residue following PG treatment acquired larger aggregates and larger pore diameter.These results indicate that the PG treatment could significantly modulate the saline-alkalinity,and simultaneously enhance aggregate stability of bauxite residue,which provides a facile approach to reclaim bauxite residue disposal areas.

Activation of Anhydrate Phosphogypsum by K_2SO_4 and Hemihydrate Gypsum

Lime pretreated phosphogypsum（PG） was calcined at 500 ℃ to produce anhydrate gypsum cement. Due to the slow hydration of anhydrate gypsum, additives, K2SO4 and hemihydrate gypsum were selected to accelerate the hydration of anhydrate. The hydration characteristics, the resistance to hydrodynamic water, and the mineralogical studies were investigated. The experimental results suggest that activated by K2SO4 and hemihydrate, anhydrate PG hydrates much more rapidly than that in the presence of only K2SO4 or in the absence of additives. The binder has proper setting time, good strength development, and relatively better resistance to water. The hardened binder has hydrated products of rod or stick like shaped dihydrate gypsum crystals.

Development of a Soil Stabilizer for Road Subgrade Based on Original Phosphogypsum

The research used industrial by-products original phosphogypsum(PG)as the main raw material,slag(SG)and Portland cement(PC)as auxiliary materials,and the optimal additive amount was determined according to the compressive strength value of the sample.Comprehensively evaluate the water resistance and volume stability of the samples,and determine the best formula for new roadbed stabilized materials.The results showed that when the weight ratio of PG,slag and cement was OPG:SG:PC=6:3:1,and mixed with 5%micro silica fume(MSF)and 3‰hydroxypropyl methyl cellulose(HPMC),the sample’s comprehensive performance was the best,specifically,the sample’s compressive strength in 60 days reached 28.8 MPa,the softening coefficient reached 0.9,and the expansion rate was stable at about−0.2%.In addition,the mechanism of action of enhancers MSF and HPMC was analyzed according to use Vicat device,X-ray diffractometer and scanning electron microscope.The best formula SP3GH3 has the best curing effect on soil.The 28-day unconfined compressive strength(UCS)of the sample reached 2.4 MPa,the expansion rate was less than 0.09%,and the water stability coefficient was above 0.79,which was higher than that of the samples cured by traditional cement and lime during the same period.

Hydration of Activated Anhydrate Phosphogypsum

Phosphogypsum（PG） calcined at 500 ℃ was activated by K2SO4 and salt lime with loose structure as seed crystal. To determine the effect of activation, hydration of the anhydrate PG activated was investigated by examination of the setting time, the proportion of hydrated anhydrate PG, the microstructure of the hardenite and the resistance to water. Results show that activated anhydrate PG hydrates much more rapidly than that in the absence of activators. The activated anhydrate PG has proper setting time and hydrated proportion. The resistance to water was greatly improved. SEM photos show that the set activated PG has hydrated products of rod-like and closely connected crystals. The different addition of activator leads to different compact structure.

Laboratory Evaluation for Utilization of Phosphogypsum through Carbide Slag Highly-Effective Activating Anhydrous Phosphogypsum

Carbide slag was used as an activator to improve the activity of anhydrous phosphogypsum.Carbide slag could greatly improve the mechanical strength of anhydrous phosphogypsum than K_(2)SO_(4).The compressive strength of 11 wt%carbide slag and 1 wt%K_(2)SO_(4)activated anhydrous phosphogypsum increased greatly to 8.6 MPa at 3 d,and 11.9 MPa at 7 d,and 16.0 MPa at 28 d,respectively.The rate of hydration heat was accelerated and the total hydration heat was increased,and more calcium sulfate dihydrate was formed and cross-linked with other parts which improved the compressive strength of anhydrous phosphogypsum under the effects of different activators.It was indicated that carbide slag was a highly effective and cost-efficient activator.The result provides a highly effective and low-cost method which results in a novel and high value-added method for the utilization of phosphogypsum in the future.

Maximizing the exploitation of phosphogypsum wastes using soaking technique with citric acid, recovering rare-earth and residual phosphate contents

Phosphogypsum(PG), the main by-product of phosphoric acid production industries, is considered one of the most important secondary sources of rare earth elements(REEs). The current study focuses on the recovery of REEs content and the residual phosphate content existing in the PG with preserving on the CaSO_(4)skeleton to be used in other various applications. These attainments are carried out using citric acid leaching process via soaking technique. Several dissolution parameters for REEs using citric acid were studied, including soaking time, soaking temperature, citric acid concentration, solid-to-liquid ratio, and recycling of the citrate leaching solutions in the further REEs dissolution experiments. The best-operating conditions were 14 d of soaking time, 7.5% citric acid concentration, and the solid-toliquid ratio of 1/5 at ambient temperature. About 79.57% dissolution efficiency of REEs was achieved using the optimal conditions. Applying four soaking stages by mixing different fresh PG samples with the same citrate solution sequentially, cumulative dissolution efficiency for REEs was found to be 64.7% under optimal soaking conditions. REEs were recovered using Dowex 50X8 resin from citrate solutions with 96% extraction efficiency. Dissolution kinetics proved the pseudo-first-order nature, reversible reactions, and two activation energies for all REEs.

Immobilization and leaching characteristics of fluoride from phosphogypsumbased cemented paste backfill

Phosphogypsum(PG)is a typical by-product of phosphoric acid and phosphate fertilizers during acid digestion.The application of cemented paste backfill(CPB)has been feasibly investigated for the remediation of PG.The present study evaluated fluorine immobilization mechanisms and attempted to construct a related thermodynamic and geochemical modeling to describe the related stabilization performance.Physico-chemical and mineralogical analyses were performed on PG and hardened PG-based CPB(PCPB).The correlated macro-and micro-structural properties were obtained from the analysis of the combination of unconfined compressive strength and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy imaging.Acid/base-dependent leaching tests were performed to ascertain fluoride leachab-ility.In addition,Gibbs Energy Minimization Software and PHREEQC were applied as tools to characterize the PCPB hydration and deduce its geochemical characteristics.The results proved that multiple factors are involved in fluorine stabilization,among which the calcium silicate hy-drate gel was found to be associated with retention.Although the quantitative comparison with the experimental data shows that further modi-fication should be introduced into the simulation before being used as a predictive implement to determine PG management options,the im-portance of acid/base concentration in regulating the leaching behavior was confirmed.Moreover,the modeling enabled the identification of the impurity phases controlling the stability and leachability.

Effect of Sodium Hydroxide on the Properties of Phosphogypsum based Cement

The effect of sodium hydroxide （NaOH） amount on phosphogypsum based cement was investigated. The mechanical performances and hydration mechanism of the phosphogypsum-based cement with different proportions of NaOH and steel slag were analyzed based on setting time, volume stability, strength test, XRD and SEM analyses. The experimental results show that, NaOH as an alkali activator significantly reduces the cement setting time and improves the cement early strength. But the acceleration of hydration proces produces coarse crystalline hydration products and the osteoporosis structure of hardened paste, which has a negative effect on later age strength. The combination of 1% NaOH and 5% steel slag as alkali activating agents is optimal with respect to early and later age strengths. Overdose of NaOH not only decreases the cement strength at later age, but also may cause problem of volume stability.

Performace of a Kind of Organic Emulsion Coated Phosphogypsum Particles

The phosphogypsum particles coated with organic emulsion were prepared by coating three kinds of organic emulsion of silicone-acrylic,styrene-acrylic and acrylic.The structure and mineralogical characteristics,acid and alkali resistance,water resistance and dissociation degree of coated phosphogypsum were studied by means of SEM-EDAX,optical microscope,gravimetric analysis and chemical detection.The results showed that the initial emulsion can be used directly for coating and granulation of phosphogypsum.The spray re-coating was carried out by using diluted emulsion with the water-to-emulsion mass ratio of 1∶1.The organic emulsion coated phosphogypsum crystal can be clearly observed,and the mixing zone at internal boundary between the organic emulsion and phosphogypsum crystal was distinguishable under the optical microscope.SEM photos showed that the surface of coated phosphogypsum particles was smooth.And the basic elements of C,O,S and Ca can be detected by EDAX.Organic emulsion can be solidified into film-network between the phosphogypsum crystals inside of the coated particles,and it can play a protective role in improving the acid resistance,alkali resistance and water resistance of phosphogypsum,and reduce the degree of dissociation of phosphogypsum in water.

Kinetics of nitric acid leaching of low-grade rare earth elements from phosphogypsum

Phosphogypsum(PG)is a potential resource for rare earth elements(REEs).Several studies have been carried out on REE leaching from PG.However,few in-depth studies have investigated the kinetics of this leaching process.In this study,the leaching kinetics of REEs from PG in nitric acid at different temperatures were explored in depth.The experiments show that the maximum leaching recovery for ΣREE was 58.5%,75.9%and 83.4%at 30,60 and 80℃,respectively.Additionally,among La,Ce,Y and Nd,Y had the highest leaching rate.A new shrinking core model(SCM)based on the dissolution reaction of a cylindrical solid particle with interfacial transfer and diffusion across the product layer as the rate-controlling step was deduced and could well fit the leaching process of REEs from PG.The activation energies for the leaching of La,Ce,Y and Nd were determined on the basis of the new cylindrical SCM.In summary,the cylindrical SCM was a more suitable fitting model than the spherical SCM,and the interfacial transfer and diffusion across the product layer were the rate-controlling step for REE leaching from the PG sample.

Thermochemical decomposition of phosphogypsum with Fe-P slag via a solid-state reaction

Phosphogypsum is a solid waste sourced from the wet-process phosphoric acid industry,which causes severe environmental damages.Its utilization was limited by its high decomposition temperature and high energy consumption.Herein,an Fe-P slag,which is a solid waste that mainly comprises iron phosphide(FeP)and diiron phosphide(Fe_(2)P),can dramatically decrease the decomposition temperature of phosphogypsum.It was found that the Fe-P slag and CaSO_(4) can react as shown in the following reaction equation:2Fe_(1.5)P+3CaSO_(4)+6CO_(2)→Ca_(3)(PO_(4))_(2)+Fe_(3)O_(4)+3SO_(2)+6CO.Its reaction mechanism was further determined using the thermodynamic method.It was found that CaS was the key intermediate for this reaction.The CaSO_(4) conversion for this method can reach approximately 97%under the optimized roasting conditions:the molar ratio between Fe_(1.5) P and CaSO_(4) of 2:3,roasting temperature of 900℃,a roasting time of 8 h.