Raw Phosphogypsum Research Articles

Exploring the role of calcium sulfate in supersulfated cement: A study of rheological, mechanical, and microstructural properties

In order to investigate the impact of different calcium sulfate (CS) sources and forms on the rheology, hydration, and mechanical properties of supersulfated cement (SSC), SSC pastes were produced with anhydrite from gypsum calcined at 650ºC and raw and calcined phosphogypsum (PG) at 150, 350, and 650ºC. Isothermal calorimetry, rheometry using parallel-plate geometry, compressive strength testing, and microstructural analysis through X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG) were performed. Results showed that SSC containing gypsum and PG calcined at 650ºC exhibited similar rheological behavior. Pastes containing PG calcined at 150ºC and raw PG respectively, exhibited the highest and lowest shear stress at a given shear rate. The calcination temperature and CS source did not significantly impact SSC compressive strength. Rietveld XRD analysis revealed that pastes with PG calcined at 350ºC and gypsum calcined at 650ºC had the lowest and highest ettringite development, respectively, from 7 to 28 days of curing.

Utilizing raw phosphogypsum to prepare one-part geopolymer: Mechanical properties, optimization, and hydration mechanisms

The accumulation of phosphogypsum (PG) poses environmental risks. This study investigates the feasibility and performance of one-part geopolymers derived from a ternary system, where raw phosphogypsum (PG) partially replaces ground blast furnace slag (GBFS) in combination with fly ash (FA). The effects of raw PG on geopolymerization kinetics, mechanical properties, and efflorescence were examined. PG addition affected compressive strength, gel formation, and efflorescence. An optimal PG replacement rate of 10–20 % for GBFS was identified to balance the positive effect on ettringite formation and the negative impact on gel phase formation. Using a composite alkali activator, combining NaOH and Na2SiO3, promotes strength development and enhances the anti-efflorescence properties of one-part geopolymers synthesized with PG. One-part PG-incorporated geopolymers demonstrate excellent performance in the fixation of heavy metal contaminants in raw PG, offering a more convenient and safer alternative to conventional two-part synthesis methods.

Influences of pretreatment methods on the mechanical and environmental behaviors of PG-GGBS-LM ternary stabilizer.

Phosphogypsum is a kind of acidic industrial byproducts with high content of soluble phosphorus and fluorine pollutants, which requires to be pretreated when used as cementitious material to (partial) replace traditional Portland cement. In this study, five different pretreatment methods were proposed for comparative analysis to examine the pretreatment effect on the mechanical and environmental behaviors of ternary phosphogypsum (PG), ground granulated blast-furnace slag (GGBS), and lime (LM) mixed stabilizer. Series laboratory tests, including unconfined compressive strength (UCS), pH, phosphorus (P)/fluorine (F) leaching, scanning electron microscopy (SEM), and X-ray diffraction (XRD) tests, were conducted to comprehend the macro- and microscopic mechanism. The results show that it is essential to grind raw PG to finer powdered state, so that it reacts more easily and quickly with LM and water. In addition, it was noticed that the UCS and P/F leaching concentration are not only affected by the mixing proportion of the PG-GGBS-LM ternary stabilizer, but also by the curing duration. The UCS increases rapidly from initial curing period and then grows slowly after 28days of curing. From the perspective of strength evolution, mixing proportion of PG: GGBS: LM = 15:80:5 is optimal, but considering the economy and environmental related issues, PG: GGBS: LM = 30:65:5 was regarded as a more attractive choice. The findings can provide a reference for the selection of pretreatment methods and design of PG-based cementitious materials suited for stabilized soils.

Boosting synergistic recovery of ammonia nitrogen and phosphate from phosphorus chemical wastewater by co-pyrolyzing the biomass and magnesite

In this study, biomass was co-pyrolyzed with the low-cost magnesite to prepare nano-MgO loaded biochar (n-MgO/biochar) composites for the simultaneous recovery of NH4+-N and PO43--P consisting in wastewater. It was found from the outcomes that the addition of biomass effectively reduced the pyrolysis temperature of magnesite, and the dispersion of biochar promoted the reactive activation of nano-scale MgO. The nano-MgO could provide Mg2+ and OH− simultaneously for the efficient recovery of NH4+-N and PO43--P. At a pH of approximately 9 and an N/P/Mg molar ratio of 1:2:1.5, the NH4+-N and PO43--P in the experimental wastewater were recovered by 93.7 % and 84.8 %, respectively. Mechanism analysis revealed that chemical adsorption, with the main product being MgNH4PO4·6H2O, predominantly controlled the recovery process. Furthermore, n-MgO/biochar demonstrated a successful recovery of NH4+-N and PO43--P consisting in the complex phosphogypsum leachate. What’s more, NaH2PO4, n-MgO/biochar/P, and raw phosphogypsum leachate could all serve as effective phosphorus sources for recovering NH4+-N consisting in nitrogenous wastewater with phosphorus deficiency. Conclusively, this study provides an alternative method for the simultaneous recovery of NH4+-N and PO43--P consisting in the wastewater.

Thermal and Mechanical Properties Enhancement of Cement Mortar using Phosphogypsum Waste: Experimental and Modeling Study

This research presents an in-depth investigation into the application of phosphogypsum (PG), a by-product of phosphate fertilizer plants and chemical industries, as a replacement material for cement in mortar, with a focus on enhancing its thermal and mechanical properties. The influence of PG as a partial replacement for cement on the compressive strength of mortar after 3, 7, and 28 days is investigated. Utilizing the Box-Behnken design within Response Surface Methodology, this study analyzed factors, such as sulfuric acid concentration, washing time, calcination temperature, and PG to cement ratio. Results indicate that optimal PG levels enhance mortar strength, particularly at 28 days, through sustained ettringite formation and microstructure optimization. Sulfuric acid concentration and calcination temperature were identified as the most significant elements influencing compressive strength, with the latter improving PG quality and reactivity. A PG to cement ratio up to 10% was found beneficial, while washing time had a negligible effect. The research highlights a critical synergy between the sulfuric acid concentration applied during the purification of PG and the calcination temperature. A significant improvement of 21% in compressive strength was achieved, underscoring the combined effect of chemical and thermal treatment on PG's efficacy in mortar. The increased sulfuric acid concentration is presumed to purify the PG by removing impurities, thus improving its reactivity. Concurrently, calcination alters the PG's crystalline structure and diminishes its organic composition. This interdependent optimization is instrumental in enhancing the structural integrity of PG-modified mortar. The potential for raw PG to be used as an insulating material is more pronounced at higher replacement rates (10%), while sulphuric acid treated PG (SCPG) and heat treated PG (HTPG) seem to be unable to provide a clear insulative advantage.

The effects of admixtures on the durability properties of phosphogypsum-based cementitious materials

This study involved proportionally blending beta-hemihydrate phosphogypsum (β-HPG) with raw phosphogypsum to prepare a phosphogypsum-based composite cementitious material (PGCM). The admixtures of quicklime (1–3%), cement (12–18%), and silica fume (5–9%) were selected to fabricate the PGCM mixture. The effect of each admixture on the durability performance of PGCM was evaluated using a single-factor experimental approach. The results showed that cement reduced significantly the dissolution rate and increased the strength coefficient, exhibiting the best effect on the antifreeze performance of PGCM. Quicklime and silica fume had no significant effect on the early antifreeze performance of PGCM but improved the later antifreeze performance. Moreover, the quicklime improved the weathering resistance of PGCM, whereas a high content of quicklime reduced the PGCM’s strength coefficient. Silica fume improved the weathering resistance of PGCM in later phases, and cement had little effect on the weathering resistance of PGCM. Additionally, an increasing amount of cement significantly improved the water resistance of PGCM, and quicklime and cement significantly increased the length of PGCM specimens. Microscopic analysis showed that the generation hydration products mainly filled aggregate pores of raw phosphogypsum and enhanced raw phosphogypsum aggregates’ interfacial adhesion, thereby improving the macroscopic mechanical and water resistance properties of PGCM. The findings on PGCM’s durability properties can provide more theoretical foundations and technical supports for advancing phosphogypsum recycling.

Utilization of phosphogypsum in phenol removal from coking wastewater

The paper presents the results of the application of raw phosphogypsum as an adsorbent for the preliminary treatment of coke-chemical wastewater with an initial concentration of phenol 395 mg/L. Studies in batch mode have proved that phenol removal is promoted by increasing the adsorbent dose and effluent temperature, the optimal phosphogypsum dose was found to be 5 g/L. The adsorption kinetics follows a pseudo-first-order model, the maximum adsorption capacity of phosphogypsum reaches 85 mg/g, which provides purification from phenol at the level of 85–90%. The process is spontaneous and endothermic. In column mode, at an effluent flow rate of 3 mL/min and the height of the fixed bed of 15 mm adsorbent dynamic capacity reached 124 mg/g. The obtained data indicate that raw phosphogypsum, available in huge quantities in the dumps of phosphoric acid plants, can be effectively used for the pretreatment of phenolic wastewater before biological treatment.

Preparation of phosphogypsum-based cemented paste backfill and its environmental impact based on multi-source industrial solid waste

Cemented Paste Backfill (CPB) technology is considered as an effective method to solve the serious problems caused by the solid wastes’ accumulation, and to mitigate the risk of mined-out areas. However, the serious influence on the environment and higher pro-treating costs of solid wastes limits their application in CPB. Meanwhile, due to the high carbon emissions and high costs of Portland cement, its use as a binder in CPB is also limited. This paper used raw phosphogypsum (PG) as filler. Yellow phosphorus slag (YPS), fly ash (FA), steel slag (SS), and Portland cement were mixed to make a composite binder. Solid wastes addition was ranged from 80% to 98%. The feasibility of CPB co-preparation with a high percentage of multi-source industrial solid waste was verified by analyzing the performance and environmental impact of CPB produced with different percentages of solid wastes. The results showed that a higher PG addition was harmful to the property of CPB, leading to a lower fluidity, longer setting time, and weaken strength. However, the development and use of composite binder offset those negative aspects effectively. Composite binder is a CaO-rich binder that provides an alkaline environment for hydration reactions which enhanced CPB strength. Because its unconfined compressive strength reached to 3.1 MPa ∼ 19.5 MPa after 60 d’s curing. This will provide a highly strengthen support to the mine to be filled with CPB, thus hindering its re-collapse. The gradually increased hydration products observed from the microstructural characterization further corroborated the strength development path of the CPB. Moreover, the adsorption and precipitation process in the alkaline system contributed to the stabilization of the contaminants from those solid wastes. Therefore, the CPB technology based on the multi-source industrial solid waste is considered as a green, promising, and cost-saving choice on supporting safe mining and mitigating environmental pollution.

A flotation combined extraction process for improving the whiteness of phosphogypsum

Every year, the production of industrial phosphoric acid generates more than 100 Tg of phosphogypsum (PG), leading to significant environmental damage and the occupation of a vast amount of land space. The urgent need to explore applications for PG has become increasingly apparent. However, impurities such as organic substances, slime, phosphorite, and SiO2 reduce the whiteness of PG, making it difficult to utilize for high-value applications. To address this issue, this study employed a two-stage flotation process to remove the majority of impurities, including SiO2, organic substances, and fine slime adhered to the surface of PG particles. The raw PG sample was first sieved to remove some SiO2 particles. After flotation, sulfuric acid and tributyl phosphate were introduced to decompose the PG particles and remove the impurities wrapped inside. Following this flotation combined extraction process, the whiteness of the PG sample improved from 54.1% to 92.9%, meeting the requirements for building walls and filters.

Environmental investigation on the use of a phosphogypsum-based road material: Radiological and leaching assessment

The transformation of phosphate ore into phosphoric acid results in the generation of high volumes of phosphogypsum (PG), an industrial by-product largely stockpiled worldwide. This solution, considered as the least damaging to the environment, constitutes a risk for the receiving environment due to the presence of harmful impurities such as heavy metals and radionuclides which hinder its large-scale valorization. This paper presents an environmental characterization of Moroccan phosphogypsum and an investigation on the environmental performance of a new lime (L) - fly ash (FA) treated phosphogypsum based road material. The concentration of metallic trace elements (Cr, Pb, Ni, Zn, Cu) in raw phosphogypsum ranged between 0.2 and 243 ppm, while its radioactivity reached 970 Bq/kg for Ra-226. The environmental performance of the proposed new road material (40% PG, 42% FA, 18% L) was evaluated using radiological risk indices besides static and dynamic leaching tests. The results showed a radioactivity reduction up to 82%, and an immobilization of metallic trace elements ranging from 25 to 100%. The stabilization/solidification mechanisms involved in the lime - fly ash treatment would be responsible for the fixation of these contaminants within the newly formed matrix.

Utilization of Industrial Waste Phosphogypsum as Geomaterial: A Review

Phosphogypsum (PG) is a waste product of phosphate fertilizer and phosphoric acid industries. Vast quantities of the waste product are generally left stockpiled on the ground or discharged into rivers or ponds, and pose serious environmental concerns. Several countries are facing problems with the handling, storage, and utilization of PG. This article is a comprehensive review of the physicochemical, microstructural, geotechnical, radiation, and leaching characteristics of raw PG. The performance of PG with other admixtures (cement, fly ash, lime, ground granulated blast furnace slag) and soils (clays, loess, sands) has also been reviewed. In addition, the factors, such as pH, calcination, forms of PG, and curing methods, that affect the geotechnical, durability, and leaching characteristics of PG mixtures are also discussed. Based on the review, it has been observed that raw PG has high sulfate content, high water absorption capacity, and high variability in strength, leaching, and radioactivity properties; thus, using raw PG alone as a backfill or embankment material is not suitable. However, the geotechnical, durability, and leaching properties of PG may be enhanced by using it in combination with other admixtures. The excessive usage of PG should also be undertaken with caution, as the higher amount of PG may lead to a decrease in the strength and the formation of cracks. There are studies that have reported the optimum content of PG; however, the range is quite varied and can differ for soil types. Therefore, it is recommended to investigate the optimum content of PG before the start of any road or ground improvement project. The compaction characteristics of PG-admixed soils are highly contrasting and hence need more comprehensive studies. The strength characteristics and resilient modulus are also highly variable and hence need more exhaustive research for actual usage of PG in road construction. Other critical issues that require attention have been highlighted, and associated research gaps that could be explored in future research works have been identified.

The behaviour of selected rare-earth elements during the conversion of phosphogypsum to calcium sulphide and residue

Phosphogypsum (PG) is a large hazardous waste from fertiliser and phosphoric acid industries from which useful products including rare-earth elements (REEs) can be recovered depending on the treatment process. Its conversion to calcium sulphide (CaS) which was achieved at 95% followed by the formation of S, CaCO3 and residue is one of the plausible treatment processes leading to economic and environmental benefits. This study aimed at monitoring selected REEs behaviour during the conversion of (PG) to (CaS). The concentrations of REEs in the raw PG, the produced CaS and the obtained residue were determined after digestion (microwave and traditional acid leaching) using ICP-OES. The effect of CO2 and H2S used in the process of forming CaCO3 and S from PG on the concentrations of REEs was also investigated. Microwave digestion proved to be more effective than traditional acid leaching in the recovery of REEs. Microwave digestion using 3 mL HNO3 + 1 mL HCl was more effective than 1 mL HNO3 + 3 mL in REEs recovery. CaS contained the highest amount of Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, La and Y with values of 2646, 476, 2255, 320, 60.5, 376, 79.8, 1.24, 476, 1185 and 318 µg/g respectively. Based on these findings, the residue could be further processed to recover REEs despite less than 40% decrease in concentration for the majority of REEs observed due to the use of H2S and CO2. CO2 was found to be more suitable as fewer REEs were leached as compared to H2S. All things considered, the obtained residue could be a good secondary source of REEs as it is easier to leach, retained good amount of REEs and lesser impurities.

Elaboration of compressed earth blocks based on phosphogypsum and phosphate mining by-products

The present paper aims to propose new recycling solutions to maximize the use of phosphogypsum (PG) in construction applications. The performance of several formulations for the manufacture of cement-stabilized compressed earth bricks (CEBs) made of raw PG mixed with phosphate mining waste rocks (Calcareous material (CM) and red clay (RC)) was investigated. The main roles of CM and RC are respectively to neutralize the acidity generated by the PG and improve the consistency of CEBs. This research work started at a laboratory scale to optimize and find the best formulation that will satisfy national and international standards. The optimized mixtures were scaled up through pilot experiments using two brick shapes (solid and hollowed). The impact of the brick shape elaborated in the pilot scale was also examined. The mineralogical, environmental, and geotechnical characteristics of the raw materials were also studied. The toxicity characteristic leaching test (TCLP) was performed on the raw PG and also on the ground bricks to investigate the risk of contamination. The most optimized laboratory mixture involved 40% PG and only 8% of ordinary Portland cement. This mixture allowed to reach an unconfined compressive strength after 28 days around 8.1 and 7.7 MPa at laboratory and pilot scale respectively. The thermal conductivity of the developed CEBs was interesting; between 0.484 and 0.506 W m−1 K−1. The values of bulk density and water absorption coefficient by capillarity are in accordance with the requirements indicated by the standards. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses highlighted the formation of tobermorite (5CaO·6SiO2,2H2O) and ettringite (Ca6Al2(SO4)3(OH)12,26H2O). The TCLP test of PG used and optimized bricks in this study showed no contaminant release. The obtained results are very promising; they showed that the elaborated CEBs can be adapted to compressed earth blocks applications.

Influence mechanism of alkaline environment on the hydration of hemihydrate phosphogypsum: A comparative study of various alkali

• The influence of various alkali on hydration and microstructure of HPGP was investigated. • Two opposite effects of alkali resulted by neutralization effect and ion strength effect on hydration of HPGP were observed. • Gypsum crystal morphology was significantly affected in alkaline environment except of excessive lime. • The influence effect of alkaline environment is related to its alkalinity and the impurity content in HPG. Although lime neutralization is widely recommended for raw phosphogypsum pretreatment, the influence mechanism of alkaline environment on hydration of hemihydrate phosphogypsum (HPG) is not clear. In the present paper, effect of alkaline environment created by different kinds of alkali including Mg(OH) 2 , NH 3 ·H 2 O, NaOH and Ca(OH) 2 with variable contents on hydration and microstructure of HPG were investigated. Abnormal setting and strength degradation were observed in the low-alkaline environment, while the setting time was normalized and crystal size developed in the high-alkaline environment regardless of the alkali category. Interestingly, the strength of HPG in the alkali environment only increased with excessive Ca(OH) 2 . It is verified that the formation of Ca 3 (PO 4 ) 2 from neutralization reaction in the low-alkaline environment served as retarder, and the excessive alkali beyond the threshold value of neutralization reaction played as accelerator in the high-alkaline environment, while only excessive lime particles promoted the development of gypsum crystal at c axis probably due to its absorption of Ca 3 (PO 4 ) 2 . This finding would promote the application of HPG in an alkaline environment and thus improve the utilization rate of phosphogypsum.

Usage of biowashing to remove impurities and heavy metals in raw phosphogypsum and calcined phosphogypsum for cement paste preparation

In the previous studies, chemical, physical, and heat treatment methods are commonly used to treat waste phosphogypsum (PG). This study reports a new method that uses biowashing to remove phosphorus, fluorine impurities, and 12 kinds of heavy metals in PG. Raw phosphogypsum (RPG) and calcined phosphogypsum (CPG) were treated by biowashing, and the potential of treated PG used for cement paste preparation was investigated. The effects of biosolution on the stability of PG and the growth of bacteria in RPG and CPG were analyzed by zeta potential and optical density. A series of characterization tests were used to analyze the removal of impurity in PG after biowashing. Additionally, the removal ratio for the impurities, compressive strength, and leaching behavior of PG-incorporated cement paste (PGC) were discussed, and the mechanism was revealed according to the test results. The results showed that, compared with RPG, the growth of bacteria was better in CPG, and the phase of CPG after biowashing was more stable. Biomineralization could effectively remove 74–77% of phosphorus and fluorine impurities in PG, but the removal ratio in CPG was 45–55% higher than that in RPG. The PG after biowashing contained a large amount of calcium carbonate, which improved the early strength of the PGC by 15–20%. Compared with PGC, the leaching concentration of heavy metals in biowashed-PG incorporated cement (APGC) decreased by 50–90%. Biowahing removes the impurities mainly by turning the insoluble impurities into soluble H3PO4 and HF, which are filtered out with bacterial solutions. This study provides a new idea for the management of PG and expands the application field of microbially induced carbonate precipitation (MICP).

The study on the effect of flotation purification on the performance of \u03b1-hemihydrate gypsum prepared from phosphogypsum

Phosphogypsum (PG) is a massive industrial solid waste. In this paper, PG was purified by flotation method, and α-hemihydrate gypsum (α-HH) was prepared by the autoclaving method. The morphology of α-HH was adjusted by adding different doses of Maleic acid and Aluminium sulfate. The results showed that after flotation purification, the impurity content in PG was significantly reduced, the soluble phosphorus content decreased from 0.48 to 0.07%, the PG purity increased from 73.12 to 94.37%, and the PG whiteness risen from 19.4 to 40.5. Then the performance of α-HH prepared from PG before and after purification was compared. Fixing the amount of aluminium sulfate at 0.2 wt%, the reaction temperature at 140 °C, and the reaction time at 120 min, the average length/diameter ratio of α-HH crystals decreased from 7.2 to 0.6 as the amount of Maleic acid increased from 0 to 0.17 wt%. When the amount of Maleic acid was 0.13 wt%, the α-hemihydrate gypsum reached the best mechanical properties. The mechanical strength of high strength gypsum prepared from PG concentrate was significantly better than that of raw PG, indicating that flotation purification can effectively improve the performance of PG. In this study, a new method of PG purification and resource utilization was proposed.

Preparation of anhydrite from phosphogypsum: Influence of phosphorus and fluorine impurities on the performances

Phosphogypsum (PG), as a bulk industrial solid waste with high and complicated impurities content, makes it difficult to be utilized. In this paper, raw PG by adding the soluble phosphorus (P2O5) and fluorine (F) content of 0–1.5 wt% was used to prepare anhydrite with calcination treatment method, and the influence of P2O5 and F impurities on the hydration and hardening properties of anhydrite was investigated. Results show that obvious defects appeared in anhydrite facets due to the formation of calcium pyrophosphate (Ca2P2O7), calcium fluoride (CaF2). Besides, sodium sulfate (Na2SO4) was formed, resulting in the accelerated hydration process. Moreover, Na2SO4 presents a better improvement, which made a higher hydration degree at an early age (3 days). However, the dihydrate crystal size was refined due to the formation of CaF2, resulting in reduced early strength. Remarkably, the later strength (28 days) of all the samples shows similar values, mainly due to the similar hydration degree and microstructure. Besides, the later strength and microstructure of anhydrite prepared from PG2 with different impurities and morphology are similar to PG1. The work shows a potential application for PG utilization and lays a foundation for preparing anhydrite from PG.

A facile approach for large-scale recovery of phosphogypsum: An insight from its performance

The calcination treatment for recycling phosphogypsum (PG) is very promising, which can transform PG into a binder material and remove harmful impurities. To effectively recycle and reutilize PG, the influence of calcination temperatures on the properties of calcined and hydrated PG with and without pretreatment should be investigated. This paper systematically studied the effect of calcination temperature on the properties of water-washed and untreated plaster. The calcined untreated (UPG) and water-washed PG (WPG) were characterized by an X-ray fluorescence spectrometer (XRF), X-ray diffraction analyzer (XRD), and isothermal calorimetry, as well as by determining water requirement for normal consistency, setting time, whereas hydrated PG products were characterized by XRD, differential scanning calorimetry (DSC), thermogravimetry (TG), derivative thermogravimetry (DTG), and scanning electron microscope (SEM), as well as by measuring the mechanical properties. The results show that the chemical and mineralogical composition of calcined untreated PG (CUG) is similar to that of calcined water-washed PG (CWG) at the same calcination temperatures, and higher temperatures resulted in lower F content but no change in P2O5 content. At low calcination temperatures (150 °C–400 °C), the cumulative heat of CUG was lower than that of CWG, and the hydration rate is similar; however, the cumulative heat and hydration rate of CUG at 700 °C or 800 °C were much higher than that of CWG, resulting in the significant reduction in setting time. Hydrated CUG (HUG) showed tiny crystals with lower interlocking at low calcination temperatures, but a more compact microstructure with defined crystals was observed in the HUG-800 due to the high formation of dihydrate gypsum; thus, HUG calcined at low temperature showed a lower strength but at 800 °C showed the maximum strength. This work indicates that calcination at 800 °C showed a potential application for the recovery and utilization of raw PG because of its excellent performance.

Characterisation of Purified Gypsum and Insoluble Impurities Obtained from Phosphogypsum Waste

In this study, the chemical and phase composition of two samples of phosphogypsum from the waste dumps of the Industry of ChemicalProducts “Elixir – Prahovo” (Serbia) were examined, as well as the possibility of recrystallization of gypsum from an aqueoussuspension of phosphogypsum. The negative effect of higher temperatures on the solubility of calcium sulfate (13.08 mmol/dm3 at95°C vs. 15.43 mmol/dm3 at 40°C) was utilized. In several repeated cycles, calcium sulfate component was progressively dissolvedin water at room temperature and then precipitated at 100°C, using the same liquid phase throughout the experiment. Therefore,phosphogypsum was separated into recrystallized (purified) gypsum, insoluble residue and supernatant, and the mass balance forthe experiment was calculated. Elemental, XRD and SEM-EDS analyses were performed on raw phosphogypsum, purified gypsumand insoluble residue. The whiteness of raw phosphogypsum and purified gypsum were determined and compared. The main objectiveof the study was to investigate the nature of insoluble impurities, in order to define and optimize the methods for their removalduring a potential industrial processing of phosphogypsum.

Beneficiation of Waste Fly Ash and Phosphogypsum— The Development of a New Material

Waste phosphogypsum (PG) was treated with citric acid, oxalic acid, sodium carbonate and sodium bicarbonate to reduce the contaminants in the material and render the material applicable for other applications. The chemical composition revealed that the material was laden with contaminants such as fluorides and phosphorous which have a detrimental effect on the development of material strength. Citric acid was the best leaching reagent to reduce the radionuclides in PG and it was selected as the leaching reagent to treat PG. The chemical composition of both the raw PG and treated PG showed that there was insufficient pozzolans in the materials to trigger the pozzolanic reaction for strength development. Therefore the PG had to be stabilized with fly ash and lime. The optimum mix ratio of the raw PG composite that yielded the highest UCS was made up of 50% raw PG and 30% FA, while 30% treated PG and 50% FA yielded the highest strength. The variation in strengths between the raw and treated PG was due to differences in the microstructure of the materials and the particle size distribution. The strength obtained met the minimum requirements for the material to be used in bulk as building construction elements.