Wet-process Phosphoric Acid Production Research Articles

Enhanced absorption of SO2 from phosphogypsum decomposition by phosphate slurry for phosphoric acid production

Phosphogypsum (PG) is a major industrial by-product of wet process phosphoric acid production, and untreated PG stockpiled on land will cause severe environmental pollution. Thermal treatment of PG is currently the mainstream treatment method PG can be thermally decomposed to produce CaO, and the decomposition process produces large amounts of SO2. In this paper, phosphate slurry was used to absorb SO2 generated during the PG decomposition to produce phosphoric acid. The effects of operating conditions such as pressure, inlet SO2 concentration, and additive content on the desulfurization efficiency, as well as phosphoric acid yield, were investigated. Under the optimal experimental parameters, the desulfurization efficiency was 100% in the first 3 h, and decreased to 67.42% after 5 h, the maximum phosphate concentration in the solution was 1445.92 mg/L. The Density functional theory (DFT) calculations showed that SO2 and O2 adsorbed on the surface of P2O5 underwent to generate SO3, which can react with H2O to produce H2SO4. Moreover, it was found that Fe3+ could enhance the catalytic oxidation process of SO2 and O2 by decreasing the reaction energy barrier. This study should be helpful for the recycling of phosphorus resources.

Role of Additives: Modified Hemihydrate Phosphogypsum Morphology and Enhanced Filtration Performance of Wet-Process Phosphoric Acid.

The morphology of hemihydrate phosphogypsum crystals is of vital importance in the hemihydrate-dihydrate (HH-DH) wet-process phosphoric acid production for high filtration strength. The morphology of hemihydrate phosphogypsum is commonly needlelike due to the strong acidic crystallization environment, which is unfavorable to the following filtration process. In this study, the crystal habit of hemihydrate phosphogypsum with a large aspect ratio was skillfully modified by additives to achieve a higher filtration strength. d-Glucitol (DG) reduces the theoretical aspect ratio of hemihydrate phosphogypsum crystals from 2.076 to 1.583 by interacting with the (002) face of CaSO4·0.5H2O preferentially, and poly(vinyl alcohol) (PVA) facilitates the aggregation of small grains to gather into a clusterlike structure. The modified morphologies of hemihydrate phosphogypsum have a lower bulk density and a larger porosity of the formed filter cake, which increases the filtration strength up to 45.9% when DG is added. Our work provides an in-depth explanation of the evolution mechanism of hemihydrate phosphogypsum morphology with the additives and its influence on the filtration performance. The improved filtration strength would reduce the water content of hemihydrate phosphogypsum and relieve the storage pressure of the phosphogypsum slag dump, which is meaningful to the clean production and process emission reduction of the phosphorus chemical industry.

Mechanical Properties and Optimal Mix Design of Phosphogypsum Cement Mineral Admixture Foam Light Soil

With the rapid growth of road transportation, the increase in road subgrade and pavement diseases has become a pressing issue, requiring the development of cost-effective filling materials that meet both strength and economic requirements. Foam lightweight soil, as a novel construction material, offers excellent characteristics such as adjustability in density and strength, high fluidity, and self-supporting capabilities. It has been widely utilized in various engineering applications, including road subgrade backfilling and retaining wall fillings. However, the conventional application of foam lightweight soil, predominantly cement-based, has raised concerns about pollution and high energy consumption due to large cement dosages. To address this issue, this study proposes the integration of phosphogypsum, a byproduct of wet-process phosphoric acid production, into foam lightweight soil. Phosphogypsum has a significant annual discharge and accumulation, but its comprehensive utilization rate remains relatively low. The research investigates the combination of phosphogypsum and foam lightweight soil by introducing mineral admixtures such as microsilica and slag powder to improve early strength development and reduce the influence of fluoride impurities on early strength. The optimal mix proportions for two types of foam lightweight soil, namely phosphogypsum cement microsilica foam (PGCF) and phosphogypsum cement slag powder foam (PGCS), were determined based on single-factor tests. The key parameters considered for optimization were water–binder ratio, foam content, and phosphogypsum dosage. The findings indicate that both PGCF and PGCS foam lightweight soil possess superior mechanical properties and thermal conductivity. By incorporating phosphogypsum into the mix, the early strength development of foam lightweight soil is effectively improved. Moreover, with suitable mix proportions, the maximum phosphogypsum dosage can be achieved, demonstrating potential economic and environmental benefits. In conclusion, this research provides valuable insights into the effective utilization of phosphogypsum in foam lightweight soil, offering a promising solution for the challenges associated with phosphogypsum disposal and the demand for sustainable construction materials in highway engineering.

Two steps leaching process for recovery of rare earths from moroccan phosphogypsum

Phosphogypsum (PG), a primary by-product of the wet process phosphoric acid production, poses significant challenges to the fertilizer industry due to its voluminous generation; for each ton of phosphoric acid (P2O5), 5 tons of phosphogypsum are generated. Traditional management practices involving landfill storage or sea discharge are associated with environmental and health risks. As a result, there is an urgent need for innovative management strategies. One promising avenue is the valorization of valuable elements present in PG, notably rare earths elements (REEs). Although previous research has delved into REEs leaching from PG, fully understanding the mechanisms remains a complex endeavor due to the intricacy of the matrix and the variability in impurities concentrations, including that of rare earths. To address this knowledge gap, this study presents a robust two-step hydrometallurgical process that explores the relationship between REEs concentration and their leaching efficiency. Initial experiments focused on optimizing REEs leaching from Moroccan PG, using three agents: nitric acid, hydrochloric acid, and sulfuric acid, under systematic conditions. Subsequent experiments investigated the correlation between REEs concentrations and their leaching efficiency from PG. Three PG samples with deliberate REEs concentration were prepared and subjected to leaching under previously determined optimal conditions. Both nitric and hydrochloric acids demonstrated high leaching efficiencies with results reaching up to 60%. Intriguingly, when the concentration of REEs in PG was adjusted, the leaching efficiency surged to 76% under analogous conditions. This underlines a strong correlation between REEs leaching efficiency and their concentration in PG, with calcium sulfate solubility, emerging as a pivotal factor influencing both variables. Data from these systematic leaching experiments were further utilized for kinetics investigations, revealing that the diffusion of REEs through the product layer dominates as the rate-controlling step. In conclusion, this research bridges critical gaps in understanding the correlation between the concentration of REEs and their leaching from PG and offers insights propelling REEs recovery for potential widespread applications.

Simultaneous removal of sulfur dioxide and nitrogen oxide from flue gas by phosphorus sludge: The performance and absorption mechanism

Developing low-cost and green simultaneous desulfurization and denitrification technologies is of great significance for sulfur dioxide (SO2) and nitrogen oxide (NOx) emission control at low temperatures, especially for small and medium-sized coal-fired boilers and furnaces. Herein, phosphorus sludge, an industrial waste from the production process of yellow phosphorus, has been developed to simultaneously eliminate SO2 and NOx from coal-fired flue gas. The key factors affecting the experimental results indicate that desulfurization and denitrification efficiency of over 95% can be achieved at a low temperature of 55 °C. Further, the absorption mechanism was investigated by characterizing the solid and liquid phases of the phosphorus sludge during the absorption process. The efficient removal of SO2 is attributed to the abundance of iron (Fe3+) and manganese (Mn2+) in the absorbent. SO2 can be rapidly catalyzed and converted to SO42− by them. The key to NOx removal is the oxidation of NO toward water-soluble high-valent nitrogen oxides by oxidizing reactive substances induced via yellow phosphorus, which are then absorbed by water and converted to NO3−. Meanwhile, yellow phosphorus is oxidized to phosphoric acid (H3PO4). The spent absorption slurry can be reused through wet process phosphoric acid production, as it contains sulfuric acid (H2SO4), nitric acid (HNO3), and H3PO4. Accordingly, this is a technology with broad application prospects.

An Evaluation of Phosphogypsum (PG) -Derived Nanohydroxyapatite (HAP) Synthesis Methods and Waste Management as a Phosphorus Source in the Agricultural Industry

As a raw material, phosphorus availability in agriculture has been a critical concern in terms of phosphate rock consumption related to a price-dependent supply-demand chain. Novel approaches have been proposed for alternative phosphorus sources in agricultural applications. Although nano-hydroxyapatite (nHA) is a commonly used bioceramic material, its utilization as an alternative phosphorus fertilizer is an emerging field. Phosphogypsum (PG), a by-product of wet process phosphoric acid production, is a promising raw material for nHA production. With an approximate 300 Mton annual accumulation rate of PG puts pressure on the research for introducing alternative strategies for the resource utilization of legacy PG piles in an environment-friendly and cost-efficient manner. Valorization of PG as the calcium and phosphorus precursor in nHA production would both provide an efficient waste management strategy and low-cost raw material. This study gives a brief review of the various synthesis routes on PG-derived nHA and criticizes nHA utilization as a phosphorus fertilizer.

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.

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.

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.

Moderately efficient leaching of rare earth elements from phosphogypsum via crystal regulation with EDTA-2Na during gypsum phase transformation and recovery by precipitation

Phosphogypsum (PG) is a bulk solid waste generated during the production of wet-process phosphoric acid, which is considered an important secondary resource for rare earth elements (REEs) due to a large amount of REEs locked in them. However, a large part of REEs is encapsulated in the gypsum crystal lattices, and it is difficult to recover this fraction of REEs. In this study, REEs were effectively leached from PG through synergistic control of the phase transformation of PG into α-hemihydrate gypsum (α-HH) and the crystal morphology and particle size of α-HH in sulfuric acid solutions under atmospheric conditions. The dissolution process of the phase transition could effectively release REEs trapped in the gypsum lattice. Meanwhile, during the recrystallization process, thick and short columnar α-HH crystals with a low specific surface area were formed through the regulation of EDTA-2Na, which weakened the adsorption of the released REEs on recrystallized crystal surfaces and further improved the REE leaching efficiency. When EDTA-2Na was added at 0.5%, coarse columnar α-HH crystals with a specific surface area of only 0.57 m2/g could be prepared, and the REE leaching efficiency could reach 59.3% after 2 h of leaching. Notably, the coarse columnar α-HH is a high-strength gypsum product with high value-added. Additionally, the REEs in the leachate were enriched using the sequential precipitation method. Therefore, this study initially achieved the combination of the efficient leaching of REEs and the preparation of high-value-added gypsum products from PG in sulfuric acid solutions and provided a new idea for the comprehensive utilization of PG.

Simultaneous removal of phosphorus and organic contaminants from phosphogypsum using hydrothermal method for gypsum resource regeneration

Phosphogypsum (PG) is a common chemical by-product from wet process phosphoric acid production, accumulated stockpiles of 300 million tons in China. The challenge for the comprehensive utilization of PG is thorough removal of phosphorus and organic contaminants from PG, which inevitably entails the loss of overall whiteness and limits its utilization in building materials and other fields. This work investigated the speciation of phosphorus and organic contaminants in PG by FTIR spectroscopy, Raman spectroscopy, XRD, SEM-EDS and GC-MS. On this basis, by using K2S2O8 as a mineralizer, PG was transformed into anhydrous calcium sulfate powder and the organic contaminants were fully decomposed (removal rate >80%, with residual TOC of 0.078%) under hydrothermal treatment. Under optimal hydrothermal conditions (0.3 M K2S2O8, heated at 120 °C for 12 h), the whiteness of product significantly improved from 40.9 to 97 and effective removal of phosphorus was achieved (more than 76%). The residual phosphorus in treated PG was 0.108%, which met the standard of PG as a building material in China GB/T-23456 2018 (P content < 0.2%). The mechanism study revealed that the phase transformation of calcium sulfate from dihydrate to anhydrous played a dominant role in the removal of phosphorus, H+ and SO42- from K2S2O8 decomposition facilitated the dissolution-recrystallization process of calcium sulfate, allowing the phosphorus in the lattice to escape. In addition, by thermal activation, K2S2O8 generated free radicals of SO4•- and OH• dissipated the organic contaminants in PG. This work provides theoretical and practical bases for the harmless disposal and resource recycling of industrial by-product gypsum, and it is of great significance to build the "zero-waste city".

Deep removal of phosphorus and synchronous preparation of high-strength gypsum from phosphogypsum by crystal modification in NaCl-HCl solutions

Phosphogypsum (PG) is a bulk solid waste that is generated during the production of wet-process phosphoric acid. Presently, the impurities (especially co-crystalline phosphorus impurities) and low value-added recycling products restrict the utilization of PG. In this study, the complete deconstruction of co-crystalline phosphorus encapsulated in gypsum lattices was realized through phase transformation from PG to α-hemihydrate gypsum (α-HH) in NaCl solutions, which was the key to efficiently eliminating phosphorus impurities from PG. Simultaneously, the addition of HCl into NaCl solutions could fully dissolve the phosphates released from gypsum lattices and distinctly increase the phosphorus leaching efficiency. Moreover, synergistic regulation of crystal morphology and particle size of α-HH was obtained in NaCl-HCl solutions by crystal seed induction, and regular large-grained α-HH with small specific surface areas was prepared, which weakened the phosphorus adsorption on the product surfaces and further improved the leaching efficiency. Importantly, the prepared α-HH were high-strength gypsum with high added value. Consequently, deep phosphorus removal (>99%) and synchronous preparation of high-strength gypsum (>30 MPa) with high added value from PG was achieved by regulating the phase transition from PG to α-HH and controlling the α-HH morphology and size by crystal seed induction in mixed solutions of NaCl and HCl under mild conditions. This work provides instructive guidance for efficient impurity removal and high value-added utilization of industrial byproduct gypsum at low cost.

Advanced oxidation processes for wet-process phosphoric acid: Enhanced phosphorus recovery and removal of organic matters

Green and efficient phosphorus leaching from phosphate rock (PR) is the key to producing high-quality products and reducing energy consumption. Organic matters in the PR affect the efficiency of wet-process phosphoric acid production. In this study, advanced oxidation processes (AOPs) were employed to remove organic matters, thereby intensifying the acidolysis reaction of the PR. The characteristics, effects of reaction conditions on the leaching efficiency, and leaching reaction were systematically studied. Under the optimal reaction conditions with the H2O2 concentration of 0.04 mL/mL, reaction temperature of 70 °C, reaction time of 120 min, and particle size of −150 μm, a high leaching efficiency of 88.4% was achieved in the O3-H2O2 reaction (OHR) case. In addition, the total organic carbon content in the leach liquor is 12.2 mg/L, which is significantly decreased by 79.9%. This demonstrated the advantages of AOP systems in strengthening the efficient leaching of phosphorus as well as beneficial environmental impact.

Processing and analysis of phosphoric acid production data using machine learning methods

The attack and filtration unit is the main section of the wet process phosphoric acid production. It must ensure maximum production of phosphoric acid while reducing P2O5 losses. The main purpose of this work is to search for a machine learning model able to predict the production of phosphoric acid.Industrial data collected during the operation of the process was used to find the appropriate machine learning model for the prediction of phosphoric acid production. The collected data are first prepared and cleaned, then we elaborated the correlation matrix for all the variables Xj and the output of filter A and filter B and look for the relations between the inputs and outputs. Then we identified the variation ranges of the parameters of each filter for a best production of phosphoric acid at the two filters. Finally, three models were analyzed and tested. These are the "K Neighbors", "Random Forest", and "Decision Tree" models. We conclude that the last two models present the best results with an MSE of 4%.

The Influence of Impurities on the Dehydration and Conversion Process of Calcium Sulfate Dihydrate to α-Calcium Sulfate Hemihydrate in the Two-Step Wet-Process Phosphoric Acid Production

The dehydration process and kinetics of calcium sulfate dihydrate to α-calcium sulfate hemihydrate were studied under simulated dihydrate–hemihydrate two-step wet-process phosphoric acid, and Fe3+, Al3+, Mg2+, SiF62–, and mixed impurities were introduced to explore their influence on the values of kinetic parameters. The properties of the dehydration conversion product α-calcium sulfate hemihydrate were characterized and analyzed by using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric and differential scanning calorimetry to reveal the interaction mechanism between impurities and α-calcium sulfate hemihydrate crystals. Results showed that the dehydration process of calcium sulfate dihydrate to α-calcium sulfate hemihydrate follows the dispersive kinetic model in the wet-process phosphoric acid solution. After Fe3+, Al3+, Mg2+, SiF62–, and mixed impurities were introduced into the system, the activation enthalpy increased, and the activation entropy decreased due to the reduction of the kinetic parameters α and β, causing the nucleation barrier of α-calcium sulfate hemihydrate crystals to increase, thereby inhibiting the dehydration and transformation of calcium sulfate dihydrate to α-calcium sulfate hemihydrate. The order of inhibition is mixed impurities > Al3+ > SiF62– > Fe3+ > Mg2+. Fe3+, Al3+, and Mg2+ impurities were combined with phosphate ions to form phosphate precipitates, which were selectively covered on different crystal surfaces of α-calcium sulfate hemihydrate, thereby hindering the growth of α-calcium sulfate hemihydrate crystals. Several Ca2+ ions existed on the (111) crystal plane in the c-axial direction, and SiF62– ions were combined with Ca2+ ions, thereby hindering the one-dimensional growth in the c-axial direction. Consequently, the aspect ratio of α-calcium sulfate hemihydrate crystal decreased, and the morphology gradually evolved to a short columnar structure.

Extraction of impurities from phosphorite ore in concentration and processing

The article presents a review of the reports on the composition and properties of phosphorite ore from the Polpino deposit and on the problems connected with the ore processing by dry fractional crushing–sifting separation. The content of impurities in this ore exceeds the standard, which eliminates technological capability of straight sulfuric acid treatment of the ore for wet-process phosphoric acid production. The attempted wet processing of the ore by solutions of weak organic acids, selected because of their availability, regenerative capacity and environmental safety, is described. It is found that the organic acids are lo w-effective in extraction of alkaline-earth metal impurities: extraction is not higher than 3.5 and 9.5 % with acetic and formic acids, respectively; total recovery of Al2O3 and Fe2O3 is not higher than 2.0 and 8.0 %, and is factually independent of concentration of the acids. The authors prove straight processability of phosphorite ore by sulfuric acid treatment with production of compound fertilizers, including selective removal of impurities and carbonate minerals, and controlled recovery of iron and aluminium. The optimization parameters of the process are concentration of nitric acid, the acid–ore interaction time and temperature. These parameters should ensure complete extraction of the target component to solution (99.0 %), as well as controlled removal of iron impurities (18.0–20.0 %) and aluminium impurities (31–32 %), which have an adverse influence on rheological properties of suspensions. Thus, the article proves technological capability of straight processing of Polpino phosphorite resources containing Р2О5–15.3 %, CaO–27.42 %, MgO–0.95 %, Fe2O3–2.58 % and Al2O3–3.65 % without pretreatment.

Adsorption of Rare Earth Elements onto the Phosphogypsum a Waste Byproduct

Phosphogypsum (PG), the waste byproduct resulting from wet process phosphoric acid production, is employed as a selective and effective adsorbent for total rare earth elements (REEs) from aqueous solution and leach liquor. The elaboration of PG adsorbent complemented after some physical treatments. Adsorption and elution studies carried out in experimental batches, including the effect of pH, adsorbent dose, initial REE concentration, and equilibrium time. Adsorption of REEs onto PG fitted well with Langmuir isotherm with a theoretical capacity surpassed 357 mg/g. REEs were effectively eluted from loaded PG with 2 mol L−1 HCl acid with an efficiency of 94%. PG showed an outstanding selectivity towards REEs in the presence of many cations and anions, for instance (Fe3+, UO22+, Ca2+, SO42−, NO3−). Different qualitative techniques such as EDS, SEM, and FTIR used to emphasize the adsorption of REEs onto PG. The film diffusion model was the preponderant adsorption mechanism for REEs; also, the adsorption process has a good accordance with pseudo-second-order kinetic model.

Water–Phosphorus Nexus for Wet-Process Phosphoric Acid Production

The water–phosphorus nexus problem for wet-process phosphoric acid production is first addressed in this article. A systematic methodology for water system optimization and water–phosphorus nexus analysis is proposed. On the basis of the preliminary process flowsheet and water flow rate balance, the potential water sources and sinks as well as the key component can be extracted. The mathematical model for the water system optimization integrated with water flow rate balance is presented. The process flowsheet can be improved according to the optimized water system. The flow rate of fresh water is reduced from 1803.98 t/h (preliminary design) to 160.98 t/h (improved design). The utilization efficiencies of the phosphorus element (calculated as P2O5) are calculated for the preliminary and improved designs. Because the process for wet-process phosphoric acid production is mostly in the aqueous phase, the utilization efficiency of the phosphorus element is increased from 94.22% (preliminary design) to 98.76% ...

Effect of Mg2+, Al3+, and Fe3+ ions on crystallization of type α hemi-hydrated calcium sulfate under simulated conditions of hemi-hydrate process of phosphoric acid

The present study investigated the effects of Mg2+, Al3+, and Fe3+ ions, as additives, on crystallization of type α hemi-hydrated calcium sulfate (α-HH) under simulated conditions of hemi-hydrate wet-process phosphoric acid production. Results showed that induction time of α-HH without additives reduces with increasing supersaturated ratios, but increases in the presence of Mg2+, Al3+, and Fe3+ ions. Under the same concentration of impurity ions, the order of extending induction time of α-HH is as follows: Fe3+＞Mg2+＞Al3+, and surface energy and critical nucleus radius of α-HH increase in the presence of Mg2+, Al3+, and Fe3+ ions. Accordingly, nucleation rate and growth efficiency of α-HH crystals reduce. Length of α-HH crystals decreases and its diameter thicken in the presence of Mg2+, Al3+, and Fe3+ ions. The crystals of α-HH easily grow in the direction of [0 1 0], and feature long-needles. On the other hand, Mg2+, Al3+, and Fe3+ ions are mainly adsorbed on the exposed surface {1 −1 0} of α-HH crystals. Thus, crystal shapes of α-HH eventually show wedge or short columns.