Wet-process Phosphoric Acid Research Articles

Preparation of polyphosphoric acid and recovery of valuable fluorine resources though a microwave intensification flash evaporation process

Traditional evaporators for the concentration of wet process phosphoric acid (WPA) suffer from concentration, corrosion, and low fluorine yield (less than 40 wt.%). Based on the excellent heating and permittivity properties of phosphoric acid in microwave fields, a microwave intensification flash evaporation (MFE) system is proposed to concentrate highly corrosive WPA for preparing polyphosphoric acid (PPA) and efficiently recovering valuable fluoride resources. A polytetrafluoroethylene tank in a microwave resonant cavity coupled with a vacuum pump is used to facilitate boiling and evaporation. This study reports the evolution of the PPA preparation process by flash evaporation, defluorination, and the boiling phenomenon. In addition, the influencing mechanism of pressure and temperature on PPA preparation and fluorine recovery are investigated. The experimental results indicate the successful preparation of PPA with a P2O5 content higher than 75 wt.% and the cost is considerably much lower than the traditional method. Meanwhile, the recovery yield of fluorine exceeds 90 wt.%, alleviating the depletion of strategic natural resources and potential risk of environmental pollution. This study provides a basis for the efficient preparation of PPA and recovery of fluorine resource from WPA. It is also potentially significant to industries such as metallurgy and sewage treatment.

Identification and Quantification of the Organic Matter in the Fouling Industrial of Wet Phosphoric Acid Process

The organic matter of rock phosphate has been the subject of numerous studies. The valuation and resolu-tion of some problems, such as fouling, encountered in the industrial production of phosphoric acid are of great interest, and enable better production yields of phosphoric acid. The solid deposits formed in the production of wet process phosphoric acid were characterized and revealed the malladrite, gypsum, trace metals, and the presence of organic matter. The identification of the organic matter was carried out in the fouling samples using FT-IR and Raman spectroscopies. The quantitative determination of the total organic matter content of the fouling samples was performed using the Walkley-Black and calcination methods. The knowledge of the organic matter in the fouling layers allows a better understanding of phenomena in processes.

Waste Treatment and Resource Utilization: Removal and recovery of soluble impurities from nitric acid leaching residue of phosphate rock by electrokinetic

Electrodynamic cleaning techniques for removing and recycling soluble impurities (phosphorus, fluorine, Sr, Pb, Cr, Cu, and Mn) in nitric acid leaching residues of phosphate rock (NG) were investigated. The PO43− concentration of the anode chamber was increased to 207.94 mg/L, while the F− concentration of the cathode chamber rose to 28.33 mg/L. Besides, the soluble phosphorus levels in the cathode regions (R1 and R2) were decreased by 98.02% and 90.16%, respectively. Moreover, the dissolved heavy metals, namely Mn, Cr, Cu and Pb in NG were enriched in the anode chamber, and Sr was enriched in the cathode chamber. The synergistic effects of electrocoagulation and electrodeposition lowered the levels of phosphate, fluoride, and heavy metals in the cathode, thus reducing the decreasing electrical conductivity. The migration and accumulation behaviors of soluble impurities in an electric field were regulated by electromigration, electro-osmotic flow, and electrolysis reaction. Considering that, this paper provides support for the harmless treatment of NG and the recovery of valuable elements, which is conducive to developing the wet-process phosphoric acid process and reducing the large-scale storage of phosphogypsum.

Preparation of gypsum with high purity and whiteness from phosphogypsum for CO2 mineral sequestration

Phosphogypsum (PG) is a solid waste product generated during wet-process phosphoric acid production. Various impurities considerably reduce the purity, whiteness, and application range of PG. This article analyzes the physical properties of PG in detail and systematically examines the content and distribution of impurities. Based on the obtained results, a simple process for the efficient removal of almost all impurities in the PG is proposed. The purity and whiteness of the purified gypsum (CaSO4) significantly increased to 99% and 92%, respectively. The migration of impurities and the material balance of this process were then analyzed. Most importantly, the purified gypsum showed high CO2 sequestration efficiency for CO2 mineral sequestration, through which a high value-added CaCO3 product was obtained.

Structural and Distribution of Impurities in the Fouling in Wet-Process Phosphoric Acid at T = 80°C

In this investigation, the mineral deposits formed in the digestion step in wet phosphoric acid production were studied. The mineralogical characterization of the fouling precipitates obtained at the temperature 80°C from the digestion step in 16-monthly production cycle was carried out. Observing successive layers, based on their colors, allowed us to distinguish fouling layers. Detailed structural aspects, and subsequently, chemical and physical properties of the fouling were determined using complementary analytical and characterization techniques. Particle size fractionation was then used to gain each fouling layer sample homogenized on the micro-scale (63μm). The major elements in the fouling were determined to be silica (Si₂O) and sodium (Na). Some chemical elements of major impurities, as well as traces, K, Al, Mg, Fe, Cu, and Zn, were detected using XRF which cannot appear in their proper solid phases. The obtained solid phases in different fouling layers are principally composed of the malladrite and gypsum minerals. The distribution of impurities into the present structures was investigated. Indeed, the mesh parameters of malladrite in the fouling layers were calculated and compared with those of the pure phase, and a good concordance is obtained. The FT-IR spectra confirm the presence of mallardite and gypsum phases and reveal a supplementary band that has been assigned to C-O vibrations in acids, esters, or ethers. The appearance of this band indicates the possible presence of organic matter within the fouling layer.

Utilization of phosphogypsum to synthesize α-hemihydrate gypsum in H3PO4–H2O solution

Phosphogypsum (PG) is a kind of industrial solid waste with high impurity content and low utilization rate. In this study, α-hemihydrate gypsum (α-HH) was synthesized from PG in H3PO4–H2O solution (wet-process phosphoric acid solution and phosphoric acid solution) under atmospheric pressure. The crystal morphology of α-HH was controlled by adding F and Al compound regulators, and the mechanical properties of α-HH were improved. The difference of α-HH synthesized by PG with different qualities was analyzed, and the mechanism of α-HH morphology was studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy-dispersive spectroscopy (EDS). Results showed that in the wet-process phosphoric acid solution, α-HH synthesized from white PG (by H3PO4–H2SO4 two-step method) had better whiteness, purity, and mechanical properties compared with that from industrial PG, and the performance indexes of the α-HH products met the requirements of α50 grade in JC/T 2038–2010 α-high-strength gypsum in China and have a good commercial application prospect. Moreover, in the phosphoric acid solution, the average diameter of the α-HH crystal increased from 8.1 µm to 54.8 µm, the average aspect (L/D) ratio decreased from 32.65 to 0.52, and the induction period and growth period of the α-HH crystal were prolonged with the increase in regulator dosage. F− and Al3+ ions were preferred to adsorb on the crystal faces. The growth of the crystal parallel to c-axis (200), (020), and (400) was inhibited, and the morphology of the α-HH crystal changed from whisker to short column. When the dosage of the regulator was 1.1 %, the 2 h bending strength of the α-HH product was 6.56 MPa, and the drying compressive strength was 52.4 MPa. This study provides a new technique and method for the synthesis of high-strength α-HH products from PG.

Treatment of distiller grain with wet-process phosphoric acid leads to biochar for the sustained release of nutrients and adsorption of Cr(VI).

Soil amendment products, such as biochar, with both sustained nutrient release and heavy metal retention properties are of great need in agricultural and environmental industries. Herein, we successfully prepared a new biochar material with multinutrient sustained-release characteristics and chromium removal potential derived from distiller grain by wet-process phosphoric acid (WPPA) modification without washing. SEM, TEM TG-IR, in situ DRIFTS and XRD characterization indicated that biochar and polyphosphate formed simultaneously and were tightly intertwined by one-step pyrolysis. The optimal product (PKBC-400) had the most stable carbon structure and an adequate P-O-P structure with less P loss. Batch experiments illustrated that 92.83% P (ortho-P), 85.94% K, 41.49% Fe, 78.42% Al and 65.60% Mg were continuously released in water from PKBC-400 within 63 days, and the maximum Cr removal rate reached 83.57% (50mg/L K2Cr2O7, pH=3.0) with an increased BET surface area (304.0557m2/g) after nutrient release. SEM, IC and 31P NMR analyses revealed that the dissolution and hydrolysis of polyphosphates not only realized the sustained release of multiple nutrients but also significantly improved the sustained release performance. The proposed resource utilization strategy provided new ideas for Cr hazard control, biomass waste utilization and fertilizer development.

Variation of Extraction Yield and Droplet Size during Emulsification Solvent Extraction of Wet-Process Phosphoric Acid

This research focuses on emulsification solvent extraction for purifying wet-process phosphoric acid. The effects of various parameters, including stirring speed, phase ratio and emulsification time on the extraction yield and droplet size distribution were investigated. The optimal conditions were determined. The influence of time and physical properties of solvent on average droplet diameter was fitted by dimensional analysis. The results showed that the mathematical model has a strong agreement with the experimental data. This model was reliable to predicting the droplet size in different conditions of manufacturing emulsion.

Clean and Efficient Utilization of Symbiotic Elements in Wet-Process Phosphoric Acid: Study on High-Value Process of Aluminum

Clean and Efficient Utilization of Symbiotic Elements in Wet-Process Phosphoric Acid: Study on High-Value Process of Aluminum

Effect of phosphogypsum use as a waste recycling on GHG emissions by mineral carbonisation method

The release of greenhouse gas emissions into the atmosphere as a result of anthropogenic sources and industrial applications has triggered the increase in global temperatures called global warming and related climate change. Phosphogypsum (PG) is a by-product of the wet process phosphoric acid (H3PO4) production process, which chemically consists of calcium sulfate dihydrate (CaSO4·2H2O) with some impurities. Annual PG accumulation has reached 300 Mtons and a strategy is needed to ensure efficient, continuous and bulk consumption. Due to the high amount of calcium it contains, PG is a material suitable for use in CO2 capture and storage processes to form stable solid carbonate compounds. This process, called mineral carbonisation of PG, contributes to sustainable development goals by providing the multiple benefits of both the utilisation of an industrial by-product and the realisation of CO2 capture and storage technology.

Recovery and recycling core of phosphogypsum: Characteristic hazardous elements risk assessment and analysis

Phosphogypsum(PG) is a solid waste produced by wet-process phosphoric acid. However, the heavy metals and other pollutants contained in PG have not been controlled in the process of resource utilization, and there are potential risks to the ecological environment and human health. In this paper, the basic physical and chemical properties (XRD, XRF, FTIR,TGA and XPS) and heavy metal pollution of four samples from four PG accumulation areas in China were studied. The study found that the microstructure of different samples of PG was not much different, but there were some differences in heavy metal content. The characteristic heavy metals in PG were determined as “As, Cd, Cr and Hg” by comprehensive evaluation methods of different characteristic heavy metals. At the same time, it was found that the valence of heavy metals in PG was related to the process of wet-process phosphoric acid. The harm of gypsum to the environment was mainly caused by heavy metal elements of As, Cd, Cr, Hg. Using the risk exposure model recommended by the United States Environmental Protection Agency, it was found that there were carcinogenic risks for adults and children in soil intake due to As and Cr pollution. Also, the characteristic heavy metals in PG were determined as “As(As3+, As5+), Cd, Cr(Cr3+, Cr6+) and Hg(Hg2+)”. Carcinogenic and non-carcinogenic risks caused by oral intake. Therefore, considering that this was closely related to the long-term accumulation of PG, heavy metals in PG enter the human body through the soil, which causes serious harm to human health. Combined with the above results, the hazard prevention and control model of PG comprehensive utilization was scientifically established from three aspects of hazard identification, risk analysis, and hazard characterization, and the risk control value of PG was determined according to the use scenario of PG. Therefore, this study can provide adequate support for PG's sustainable, efficient, safe, and environmentally friendly utilization.

Separation of phosphoric acid and magnesium from wet process phosphoric acid by solvent extraction

ABSTRACT Wet process phosphoric acid (WPA) contains a high concentration of metal cations. Our research group developed a P-15 extractant that can effectively remove metal cations in WPA. Different metal cations supported by the extractant are stripped and recycled by different stripping agents. In this study, ammonium sulfate is used as a stripping agent to separate magnesium ions from the extractant. So that phosphoric acid and magnesium ions can be separated and separately prepared into their own products. To determine the optimal operation parameters of the magnesium ion stripping process, single factor experiments are conducted. Under the reaction temperature of 333.15 K, stirring speed of 300 rpm, phase ratio (A/O) of 1/2, ammonium sulfate concentration of 3.305 mol/L, and reaction time of 30 min, the stripping efficiency reaches 75%. A stripping equilibrium curve at 333.15 K is plotted to determine the optimal reaction order for countercurrent stripping. Through two-stage countercurrent stripping, the magnesium ion concentration in the extractant could be reduced to approximately 0.0018 mol/L when A/O = 1/2. Magnesium ion concentration in extraction can be reduced from 0.0419 mol/L to 0.0026 mol/L through two-stage stripping countercurrent cascade. The concentration of each kind of impurity ion in ammonium magnesium sulfate crystals is all below 500 ppm.

Experimental investigation of iron removal from wet phosphoric acid through chemical precipitation process

In the present work, the combination of experimental investigation and statistical analysis has been used for the first time to study iron precipitation from wet-process phosphoric acid. Firstly, eight types of additives including different carbonate and hydroxide salts were studied in the chemical precipitation reaction in order to decrease the iron content in wet-process phosphoric acid. From different studied reagents, calcium carbonate and calcium hydroxide showed the best to remove ferric ions from crude wet-phosphoric acid. Under the optimum conditions, about 80% and 83% of the iron can be removed at 70 ℃ after the addition of calcium carbonate and calcium hydroxide, respectively. The maximum P2O5 losses from crude phosphoric acid was less than 4% after the addition of calcium hydroxide. Based on the Arrhenius behavior, the activation energy of iron precipitation was calculated about 47.43 and 51.55 kJ/mol for calcium carbonate and calcium hydroxide addition, respectively. The iron state studies showed that the pretreatment process was unnecessary to reduce iron state from ferric to ferrous form before precipitation. Furthermore, the kinetics parameters showed that the iron precipitation process was an endothermic reaction. The XRD analysis of the precipitates produced confirmed the formation of Fe(OH)3, α-Fe2O3, α-FeOOH and FeCO3 through the reaction of iron with carbonate and hydroxide ions. Possible pathways of iron precipitation were proposed. Both calcium carbonate and calcium hydroxide showed good efficiency to remove iron from wet-process phosphoric acid accompanied by moderate P2O5 losses. The Taguchi technique was applied to optimize the parameters using signal-to-noise ratio analysis as well as ANOVA. The analysis showed a greater susceptibility of the response against changes in the value of calcium carbonate, more than calcium hydroxide.

Extraction of H3PO4 from low-grade phosphate rocks leachate of HCl-route by the mixture of TBP and IPE: Optimization, mass transfer and mechanism

The mixed solvent of tributylphosphate and isopropyl ether was used to extract H3PO4 and separate Ca2+ and Cl− from low concentration wet process phosphoric acid prepared by low-grade collophane and 30 wt% hydrochloric acid solution. Then, the effects of extraction temperature, composition of solvent mixture, aqueous/organic phase ratio, extraction time and extraction stage were investigated, respectively. The result shows that the extraction process of H3PO4 is an exothermic process and the enthalpy changes (ΔH) is −12.50 kJ mol−1. The McCabe–Thiele diagram shows two counter-current stages achieves about 76.9 wt% H3PO4 extraction at 293 K. Based on the mass transfer theory of liquid–liquid extraction, extraction yields for all tested component were summarized as an exponential function of cross-flow extraction stages. Meanwhie, extraction mechanism studies show that the comlex of αH3PO4 βTBP was formed in extraction process and higher Ca2+ concentration was beneficial for H3PO4 extraction. Meanwhile, the effects of stripping temperature, aqueous/organic phase ratio and stripping time on the re-extraction performance of H3PO4, Ca2+ and Cl− were studied, respectively. The result of experiment will provide some theoretical support for the industrial application of HCl-route wet process phosphoric acid on low-grade phosphate rock and phosphate tailings.

Removal of cadmium from phosphoric acid in the presence of chloride ions using commercially available anion exchange resins

The production of cadmium-free phosphoric acid is crucial for the manufacturing of environmentally friendly fertilizers. One of the most promising methods for purification of phosphoric acid is ion exchange/adsorption. The article presents studies on the adsorption of cadmium from a 40 % phosphoric acid solution in the presence of chloride ions with the use of commercially available anion exchange resins. Five borderline cases were examined in a batch system, as well as kinetic studies were performed. The effect of chloride (0.2–2.2 wt%) and cadmium (5–2500 mg/kg) concentration on adsorption with the use of two best resins was studied in depth. The Amberlite PWA5 resin was selected for column tests and for experiments with wet process phosphoric acid simulated solution (WPPA-SS) to prove the concept of practical application of the resin. The obtained results indicate that PWA5 has a number of advantages over the resins tested so far, e.g., the resin is characterized by high cadmium adsorption efficiency (89 % in batch experiments with WPPA-SS and 95.5 % in column tests), the resin effectively adsorbs cadmium in the presence of low chloride concentrations (0.2–0.5 wt%), and the column with this resin can work properly even at high flow rates (40 BV/h).

Purification of industrial wet phosphoric acid solution by sulfide precipitation in batch and continuous modes: Performance analysis, kinetic modeling, and precipitate characterization

The purification of wet-process phosphoric acid has always been a challenge for both academic and industrial researchers. A treatment unit was developed to mitigate metallic impurities, such as cadmium, from industrial dilute wet phosphoric acid solution (25% P2O5) by NaHS precipitation in batch and continuous modes. The effects of operating parameters on the purification performance were evaluated: namely Sulfur/Cd2+ molar ratio, the presence of other metallic ions besides Cd2+, temperature, and hydraulic residence time. In the batch mode, equilibrium was attained in less than 20 min; augmenting Sulfur/Cd2+ molar ratio increased the CdS precipitation effectiveness up to 86%, but temperature increment had an adverse effect. In the continuous mode, a hydraulic residence time of 40 min was necessary to reach the same yield as in the batch mode at 25 °C. Sulfidation was found to eliminate simultaneously other metallic impurities (especially Cu and As) originally present in the wet-produced phosphoric acid. Finally, NaHS precipitation was able to reduce the concentration of cadmium from 65.95 mg Cd/kg P2O5 to 11.15 mg Cd/kg P2O5 from untreated industrial WPA solution (25% P2O5) at 25 °C, which outperforms most of the published data based on alternative treatment processes.

Competitive extraction and stripping behaviors of rare earth elements from industrial wet-process phosphoric acid using di-(2-ethyl-hexyl) phosphoric acid solvent: Optimization and thermodynamic studies

Rare earth elements (REEs) have gained a great amount of attention in the last decades, as they become indispensable and ubiquitous owing to their versatile applications in modern world and high technologies. The main aim of this work is to study the extraction behavior of REEs from the wet process phosphoric acid (WPPA) using Di-(2-ethylhexyl) phosphoric acid (D2EHPA) with a focus on yttrium (Y) and scandium (Sc) as the major REEs. The extraction and stripping operating conditions of REEs from WPPA were optimized, including the extractant concentration, the O:A ratio (organic phase / aqueous phase), the contact time, the residence time and stirring speed. Single-stage extraction results revealed an extraction efficiency of 58 and 56 % for ΣREEs and Sc, respectively.Besides, it was demonstrated that D2EHPA shows a high affinity towards yttrium (Y) with high extraction of 91 % under the optimized conditions of 2 M of D2EHPA, O:A = 1, 600 rpm during 20 min. The McCabe–Thiele diagrams were established to specify the required theoretical number of extraction stages of ΣREEs. It was revealed that four stages are required for ΣREEs extraction, two stages for Sc and a single extraction stage for Y at an O:A ratio of 1. The best extraction efficiencies were obtained for a total percentage of 97, 87 and 90 % for ΣREEs, Sc and Y, respectively. The stripping was successfully achieved using H2SO4 solution with efficiencies of 73 and 90 % for ΣREEs and Y, respectively. Finally, an integrated process for the recovery of REEs from WPPA was proposed.

A novel hierarchical structured calcium magnesium ammonium polyphosphate for high-performance slow-release fertilizer

In this work, a new slow-release fertilizer (SRF) of calcium magnesium ammonium polyphosphate (CMAPP) is constructed using unpurified wet-process phosphoric acid (WPA), discarded phosphorus tailings and urea. The molecular weight distribution (MWD), polymerization degree (PD), morphology, crystal phases and chemical environment of phosphorus (P) of CMAPP are characterized systematcially to identify its structure, speculate the polymerization mechanism and release characteristics. The resulting MWD and hierarchical PD of CMAPP are sensitive to calcination temperature and time. The optimal polymerization process is calcination at 225 °C for 10 min. During the polymerization, pyrophosphate is formed by the condensation of urea-phosphate intermediate and HRPO4−, then the -OH in pyrophosphate further condense with other -OH to form longer chain polyphosphates. The optimal CMAPP is an amorphous porous-structured aggregates with PD¯ of 2.87. The leaching and release experiment show that CMAPP not only ensure the continuous supply of orthophosphate, but also alleviate the nutrients leaching by introducing of calcium (Ca) and magnesium (Mg) for anchoring. This SRF for Ca, Mg and P nutrients derived from discarded phosphorus tailings and unpurified WPA might be a cost-effective candidate to improve agronomic benefits.

Research on formulation optimization and hydration mechanism of phosphogypsum-based filling cementitious materials

Phosphogypsum is a solid waste produced in the production of wet-process phosphoric acid. The main component is calcium sulfate dihydrate, and its constant calcium activity is very low. It is a very difficult waste to handle. The slag produced in the ironmaking processes of the Jiuquan Iron and Steel Company has high activity and can be mixed with phosphogypsum to prepare backfilling cementitious materials for the backfilling of goaf (see Introduction below) to prevent its subsidence and cracking and subsequent subsidence of the land surface above the goaf. In this paper, an orthogonal test of material strength under different ratios and finenesses was carried out, and the optimal formula of phosphogypsum-based cementitious material was obtained. On this basis, SEM (Scanning Electron Microscope) and XRD (X-ray Diffraction) were used to observe the internal structure of phosphogypsum-based cementitious materials. The results show that: when the content of quicklime is 5.5%–6.0%, the content of phosphogypsum is 30%–33%, the content of NaOH is 1.5%–2.0%, the content of Glauber’s salt is 2.5%–3%, the average particle size is 15.88 μm, and grinding aids and an early strengthening agent were added, and in the case of the strengthening agent, the 3-days and 7-days strengths of the filling reached the highest values of 1.58 MPa (mean flexural strength) and 4.1 MPa, respectively. The formation of calcium silicate hydrates gel (C-S-H) and ettringite (AFt) gradually increased with age, and the structure became denser. The ettringite plays the role of interspersed support in the filling body, thereby improving the overall strength. This research provides a theoretical basis and practical reference for the treatment of goaf.

Release and recovery of fluorine and iodine in the production and concentration of wet-process phosphoric acid from phosphate rock

The recovery of fluorine and iodine in the production and concentration of wet-process phosphoric acid (WPA) can not only reduce environmental pollution, but also increase the added value of production. In this work, the phosphate rock was digested with H2SO4, and the remaining H2SO4 in the WPA was removed with Ca(H2PO4)2, then the WPA was concentrated under vacuum and different temperatures. Fluorine and iodine in the absorption solution were recovered in the forms of CaF2 and I2, respectively. The results showed, under the optimal digestion condition of phosphate rock, the release rate of phosphorus was 97.21 %, with 25.14 % of P2O5 in WPA; in gas and liquid phase, the mass percentages were 1.35 % and 94.87 % for fluorine, and 65.21 % and 29.89 % for iodine, respectively. In the concentration of WPA under 15–17 kPa, the release rates were 89.79 % for fluorine at 80–130 °C and 62.44 % for iodine at 80 °C, with 52.34 % of P2O5 in WPA at 130 °C. Recovery rates and purity were 93.26 % and 97.24 % for fluorine, 85.63 % and 99.47 % for iodine, respectively. Therefore, this method was simple, low consumption, high recovery and purity for fluorine and iodine.