Manufacture Of Phosphoric Acid Research Articles

Utilization of waste phosphogypsum in high-strength geopolymer concrete: Performance optimization and mechanistic exploration

Phosphogypsum (PG) is an industrial waste produced during the manufacture of phosphoric acid. In this study, dihydrate phosphogypsum (DPG), ground granulated blast furnace slag (GGBFS), fly ash (FA), Portland cement clinker (PCC), sulfate alumina cement clinker (SACC), water reducer, sand, and stone were combined to create a phosphogypsum-based geopolymer (PBG) concrete. Concrete mix design was carried out based on the principle of maximum packing density, and the effects of water reducer type, sand ratio, water-to-binder ratio, and DPG content on PBG concrete properties were investigated. Results indicated that compared to SiKa ViscoCrete 540P (SVC 540P) water reducer, Melflux PLUS 1087L (MP1087L) significantly enhanced the performance of PBG concrete. With a water-to-binder ratio of 0.34, a sand ratio of 32.2 %, and a PG:GGBFS:FA ratio of 5:4:1, the 28-day unconfined compressive strength (UCS) of the concrete exceeded 60 MPa. The concrete primarily consisted of phases such as dihydrate gypsum (CaSO4·2H2O), ettringite (Ca6(Al(OH)6)2(SO4)3(H2O)26), polymer (-Si-O-Al-), quartz (SiO2), albite (Na(AlSi3O8)) and Muscovite ((K,Na)Al2(Si,Al)4O10(OH)2). Using two mild alkalis as activators, PBG concrete can avoid the efflorescence often associated with geopolymer preparation and can cure at 20 °C. PBG concrete offers high strength, ease of preparation, and environmental benefits, providing a new avenue for the reuse of PG.

Simultaneous recycling of both desalination reject brine water and phosphogypsum waste in manufacturing K2Mg(SO4)2·6H2O fertilizer

The purpose of this paper is twofold. The first is to convert phosphogypsum (PG) waste, mainly composed by gypsum (CaSO4·2H2O), into potassium sulfate (K2SO4) compound, which help to reduce the environmental impact and open new ways and valuable chain for the industrial manufacturing of phosphoric acid. The second is to recover MgSO4·7H2O from desalination reject brine water (RBW), thereby minimizing its associated environmental impact. The novelty of the current study is to simultaneously use the recovered salts K2SO4 and MgSO4·7H2O to produce K2Mg(SO4)2.6H2O (K2MgS6), considered as a double fertilizer. Additionally, Solid–Liquid Equilibria (SLE) are frequently applied to several industry domains. SLE are an interesting outline to visualize the precipitation, separation, and purification of a solid phase and the pathways by which crystallization can occur. The SLE of the ternary phase diagrams K2SO4-MgSO4-H2O at 25 °C and 0 °C were especially used to successfully determine the operating conditions and the design of a crystallization process during the PG/RBW conversion into K2MgS6. Several characterization techniques (i.e., XRD, DTA/DTG, SEM/EDS, FTIR) were employed to identify the solids formed during this process. An analysis on the distribution of natural radionuclides and heavy metals was carried out to confirm the effectiveness of the developed process. The main conclusion of this study was that K2MgS6 fertilizer can be manufactured by combining PG and RBW from desalination plants. Furthermore, the formed fertilizer, K2MgS6, is highly recommended for many applications in the agriculture sector.

Enhancing compressive strength of phosphogypsum-based geopolymer cement with five alkaline activator combinations

Phosphogypsum (PG) is a bulk industrial solid waste generated during phosphoric acid manufacturing. This study investigated a novel phosphogypsum-based geopolymer (PBG) cement through L16(45) orthogonal experiments. The impact of five alkaline activators, including Portland cement clinker (PCC), sulfate alumina cement clinker (SACC), aluminate cement (AC), magnesium oxide (MgO), and calcium oxide (CaO), on the performance of PBG cement was analyzed, and the optimal mix ratio was determined. Results indicate that all four alkaline activators, excluding CaO, can enhance unconfined compressive strength (UCS) of PBG cement. Factor index analysis revealed that an optimal composition comprising 4 % PCC, 6 % SACC, 2 % AC, and 4 % MgO yielded the highest 28-day UCS of 45.63 MPa. PBG cement primarily consists of dihydrate gypsum (CaSO4·2H2O), ettringite (Ca6(Al(OH)6)2(SO4)3(H2O)25.7), and polymers. Of these constituents, the polymers accounted for 61.9 wt% of the total composition and provided most of the strength. This study provided a new approach for making better use of waste PG.

Valorisation diagnosis of waste from the decontamination of phosphogypsum leachates through a combined calcium carbonate/hydroxide process

Phosphogypsum is an industrial waste considered as naturally occurring radioactive material. Stack disposal and exposure to the environmental condition involve the production of acid leachates with high potential pollutant loads as heavy metals and radionuclides. In this study, a sequential neutralisation process was applied for cleaning the generated releases, and the two obtained residues were characterised from the physical-chemical and radiological point of view before their valorisation. The cleaning process was made up of two steps: the first one using calcium carbonate until pH = 3.5, and the second one using calcium hydroxide until pH = 12. The residue obtained in the first step was mostly calcium fluoride, while in the second step most phosphates were precipitated, mainly as hydroxyapatite. The final liquid was treated to reduce pH lower than 9, which is the limit included in the current directive for discharges of liquid effluents into coastal waters. The main conclusion was that the solids from the first step could be valorised as an additive in the manufacture of commercial Portland cements and ceramics, while the solids from the second step could be used as raw material for the phosphoric acid manufacture.

Effect of HPO42− and brushite on gypsum reactivity and implications for utilization of phosphogypsum in plaster production

The implementation of a sustainable development strategy holds the potential to enhance productivity, profitability, and overall efficiency by leveraging by-products from other industries. In plaster manufacturing the substitution of natural gypsum with synthetic gypsum is worth exploring. Phosphogypsum (PG) represents a significant fraction of synthetic gypsum production, arising as a by-product of phosphoric acid manufacturing. However, the presence of impurities in PG, particularly phosphorus, poses challenges to its use as a plaster material. To identify the phases present in PG, quantify their occurrences and understand their effects on dehydration and hydration processes, our study aimed to examine simplified models. These models are specifically designed to provide insights into the underlying mechanisms governing the hydration reactivity of hemihydrate. Pure gypsum and brushite were synthesized, and mechanical mixing was carried out to explore how brushite affects the characteristics of gypsum. Additionally, a solid solution of Ca(SO4)1-x(HPO4)x·2H2O, where 0 < x < 1, was prepared to characterize and quantify the syncrystallized HPO42− as well as the effects of this impurity on typical gypsum use cases. The synthesized materials underwent physical and chemical characterization using SEM, XRD, IR spectroscopy, DSC, pH and conductivity measurements, and ion chromatography. The results demonstrated that the HPO42− ions can substitute for sulfate ions in gypsum to form solid solutions. The maximum quantity of HPO42− ions in the gypsum lattice is approximately 10%, leading to the crystallization of gypsum into sand rose shape rather than needle crystals. As the concentration of HPO42− ions increases, a new phase known as ardealite (Ca(SO4)1-x(HPO4)x·2H2O) with 0.42 < x < 0.54 emerges, followed by brushite (CaHPO4·2H2O). During the calcination of brushite at 160 °C, a mixture of brushite and monetite formed that exhibits different properties compared to its natural counterpart. In the presence of syncrystallized HPO42− ions, the transformation from anhydrite III to anhydrite II occurred at higher temperatures than for pure gypsum. The reactivity of calcined samples indicate that HPO42− ions from the dehydration products of brushite caused a delay in hydration, with the maximum delay observed at a calcination temperature of 160 °C. At this temperature, monetite begins to crystallize, and it is characterized by its inert nature, exhibiting no reactivity with water. This explains the observed decrease in setting times as the calcination temperature rises. Moreover, syncrystallized HPO42− ions induced a significant delay in the hydration process. Our research revealed that adjusting the calcination temperature can mitigate the retarding effect associated with this impurity, suggesting an industrial tendency to use the lowest possible temperature during the calcination process of PG containing syncrystallized phosphate impurity. In addition, we have observed that adjusting the pH of the mixing water to lower values can significantly accelerate setting times.

Study on the mechanical properties, microstructure and hydration mechanism of phosphogypsum-based geopolymer cement with added micron-sized silica particles

Phosphogypsum (PG) is a solid waste product that is generated during the manufacture of phosphoric acid. In this study, beta-hemihydrate phosphogypsum (β-HPG), slag, sodium metasilicate (SM), citric acid (CA) and micron-sized silica particles (MSSP) were mixed to form a phosphogypsum-based geopolymer (PBG) cement. The results revealed that by adding 3 % MSSP, the 28-day unconfined compressive strength (UCS) reached its highest level of 59.2 MPa. Furthermore, adding 2 % MSSP resulted in the greatest water resistance, with a softening coefficient of 0.92. Additionally, X-ray diffractometry (XRD) Rietveld analysis, scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) imaging, thermogravimetry-differential scanning calorimetry (TG-DSC) analysis, high-temperature XRD analysis, and mercury intrusion porosimetry (MIP) testing were employed to analyse the PBG cement. Results indicated that dihydrate gypsum (CaSO4·2H2O), ettringite (Ca6(Al(OH)6)2(SO4)3(H2O)26), and polymers (Nan{−(SiO2)zAlO2}n·ωH2O) were the principal hydration products in the PBG cement system. Also, the addition of MSSP led to reduced porosity, lower pore-specific surface area, and fewer cracks in the cement structure. After exceeding temperatures of 990.9 °C, these polymers gradually decomposed into Na6(AlSiO4)6, Al2O3, and SiO2. PBG cement is characterized by its easy processing, high strength, eco-friendly nature, and high PG incorporation. It shows promise as a potential substitute for traditional Portland cement in certain sectors, thereby facilitating the consumption of waste gypsum.

Fired brick production using phosphogypsum and phosphate mining waste

The main objective of this study is to propose advanced solutions for recycling phosphogypsum (PG) in the construction industry. In Morocco, PG has been produced in large quantities (around 20 million tons) by a series of phosphoric acid manufacturing plants for many years. To address this issue, fired bricks were created using PG and red clay (RC), a waste material from Moroccan phosphate mines. The pressed brick samples were dried at 105 °C and then fired at temperatures between 900 and 1100 °C for 2–5 h. The PG, RC, and sintered briquettes were characterized and analyzed through thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), physical properties and response surface methodology (RSM). The Toxicity characteristic leaching procedure test (TCLP) was also conducted on the raw materials and the fired brick samples to assess contamination risk. At all temperatures, the addition of PG results in a microstructure with high porosity that significantly impacts the mechanical characteristics. Despite this, the mechanical and physical performance of the fired brick samples meet the required standards. The TCLP test results did not detect any release of contaminants. The samples fired at 1056 °C for 4.5 h with a PG content of 40% demonstrate optimal conditions, exhibiting a notable bending strength of 13.15 MPa, a moderate density, and minimal water absorption. These findings indicate a promising and sustainable future trend in the construction industry by incorporating PG in fired brick production, providing a viable solution for waste management. Furthermore, comparing the results with previous works highlights the significant improvements achieved in terms of meeting industry standards and minimizing contamination risks associated with the utilization of mine wastes in fired bricks.

Chemical characterization of phosphogypsum produced from raw phosphate rock from the phosphoric acid manufacturing process

Chemical characterization of phosphogypsum produced from raw phosphate rock from the phosphoric acid manufacturing process

Investigating the novel process for thorough removal of eutectic phosphate impurities from phosphogypsum

Phosphogypsum is the major component in solid waste of phosphoric acid manufacturing. After purification, phosphogypsum can be used as an alternative material to natural gypsum. Phosphate impurities, especially eutectic phosphorus, are challenging to be removed thoroughly from phosphogypsum. In this work, we reported a novel phosphogypsum purification process, in which the phase transition activation by calcination was combined with sulfuric acid dissolution during hydration. The two critical parameters, activation temperature and H2SO4 concentration were optimized for this process. Moreover, powder X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectrometer (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to determine the purification efficiency for removing eutectic phosphate impurity from phosphogypsum. The comprehensive characterization results suggested that this novel process can remove eutectic phosphate impurities through the mutual transformation of different crystalline forms of calcium sulfate combined with sulfuric acid dissolution. Under the optimal purification condition (165 °C activation and 1.0 wt% H2SO4), the removal rate of eutectic phosphorus reached up to 98.3%, corresponding to overall eutectic phosphorus content decreasing from 0.58% to 0.01%. Meanwhile, the water-soluble phosphorus and insoluble phosphorus were also completely removed. Thus, 99.4% of phosphate impurity (as P2O5) was removed from phosphogypsum after the two-step treatment. The findings of this paper indicated that this novel process could separate phosphate impurity in an efficient and thorough manner, enabling sustainable utilization of phosphogypsum.

Development and optimization of phosphogypsum-based geopolymer cement

Phosphogypsum (PG) is a solid waste product that is generated in the phosphoric acid manufacturing process, most of which is currently sent to landfills rather than reused. In this study, beta-hemihydrate phosphogypsum (β-HPG), slag, fly ash (FA) and alkali activator were mixed to form a phosphogypsum-based geopolymer (PBG) cement. The effects of the slag and FA content, citric acid (CA) dosage, sodium metasilicate (SM) dosage, and water addition process were investigated. The results indicated that the 28-day unconfined compressive strength (UCS) of the cement can exceed 60 MPa when a reasonable mix proportion is adopted. The hydration products and strength formation mechanism of the PBG cement were assessed using XRD-Rietveld and SEM-EDS. The micro-analysis results indicated that the main hydration products were dihydrate phosphogypsum, ettringite, and CSH gel, which jointly provided the strength for the PBG cement. The PBG cement may improve the utilisation prospects of PG waste.

Geochemical and mineralogical characterization of phosphogypsum and leaching tests for the prediction of the mobility of trace elements.

Phosphoric acid manufacturing generates large amounts of phosphogypsum (PG); a by-product generally disposed in the surface or evacuated in the seawater without any pretreatment. Phosphogypsum may host non-negligible amounts of valuable elements such as rare earth elements (REEs), which are critical elements on the global market. Surface disposal of PG may be a sustainable option to allow further processing in order to recover valuable elements. However, surface disposal exposes PG to atmospheric conditions (e.g., water, oxygen) which may increase their reactivity and accelerate the release rate of chemical species. This study aims to evaluate the trace element release rate from PG at atmospheric conditions. The studied PG samples were collected from a Moroccan phosphate treatment plant. The samples were characterized for their (i) chemical composition using inductively coupled plasma optical emission spectrometry (ICP-OES) for major elements and inductively coupled plasma mass spectrometry (ICP-MS) for trace elements; (ii) mineralogical composition by X-ray diffraction (XRD), scanning electron microscope equipped with energy-dispersive spectrometer (SEM-EDS), laser-induced breakdown spectroscopy (LIBS), and the mineral chemical composition was analyzed by electron probe microanalyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS); and (iii) chemical species release rate using leaching tests over 24h at 25 and 60°C. Chemically, the PG samples were mainly composed of Ca (23.03-23.35 wt.%), S (17.65-17.71 wt.%), and Si (0.75-0.82 wt.%), and non-negligible amounts of trace elements: REE (344-349ppm), Cd (3.5-7.4ppm), U (9.3-27.4ppm). Mineralogically, the PGs are mainly formed by gypsum (94.2-95.9 wt.%) and quartz (1.67-1.76 wt.%). In terms of chemical species release, the PGs showed a higher reactivity at 60°C compared to room temperature with a higher release rate at the beginning of the leaching tests. Quantitatively, the PG samples released 3.57-4.11µg/L/day of REE, 3.18-17.29µg/L/day of U, and 1.67-5.49µg/L/day of Cd. Based on the leaching results, we concluded that the trace elements (e.g., U, Cd, REE) are incorporated in PG crystal lattice, which may explain their low concentrations in the leachates. Consequently, total digestion of PG matrix is required to solubilize REE.

Data-driven modeling and optimization of an industrial phosphoric acid production unit

In this work, a supervised machine learning (ML) multi-output regression approach is investigated to build predictive models for an industrial unit of phosphoric acid production. More specifically, multioutput data-driven regression is applied to simultaneously estimate nine outputs (Reactor temperature, chemical yield (RC), P2O5 concentration in the phosphoric acid, and chemical losses in gypsum) under different operating conditions. The presented methods are linear regression and decision tree regression models. The use of decision tree regression provides high accuracy compared to linear regression. The decision tree model leads to a high value of the coefficient of determination (R2 = 0.994, on the testing set not used for the modeling), and to low values of the mean squared error (MSE) and mean absolute error (MAE). The best parameters of the decision tree provide higher fitness values than other depth levels. The optimal values in the training stage are 0.002, 0.007, and 0.994 for MSE, MAE, and R2, respectively. Applying decision tree regression can correctly model the data of the phosphoric acid manufacturing unit with satisfying fitness criterion and important conclusions on the process coherent with phenomenological models, as well as supplementary and novel insights.

Modelling and Optimisation of Copper Adsorption in Solution by the Response Surface Method

Copper is considered a heavy metal that can be toxic at certain concentrations and its presence in water is a potential threat to public health. These heavy metals also contribute to a remarkable degradation of the environment, hence the need for effective treatment methods to remove them. In this study, a mixture of titaniferous sand and calcium silicate was used as adsorbent material to eliminate copper in solution. The calcium silicate was synthesised from fluosilicic acid, which is a by-product of phosphoric acid manufacture. The titaniferous sand is a residue from a mining industry. Both adsorbents were characterised by infrared spectroscopy and X-ray fluorescence to determine their compositions and physicochemical properties. The response surfaces, through the Box-Behnken model, were used to model and optimise various adsorption parameters, namely initial copper concentration (A: 60 - 200 mg/L), adsorbent dose (B: 0.1 - 0.6 g) and pH (C: 4 - 10). The copper removal efficiency (98.92%), after statistical analysis, was obtained under the following optimal conditions: an adsorbent dose of 0.55 g, an initial copper concentration of 197.25 mg/L and a pH of 9.85. The study of the effects of the operating parameters showed that they had a positive effect on the copper removal efficiency.

Separation of calcium chloride from waste acidic raffinate in HCl wet process for phosphoric acid manufacture: Simulated and experimental study

Phosphoric acid manufacture by HCl wet process is a phosphogypsum-free route, which is promising for replacing traditional H2SO4 wet process. However, large amount of CaCl2-containing acidic raffinate was discharged after solvent extraction of H3PO4. In this study, pure CaCl2·2H2O was recovered from the raffinate. The ionic forms and residual amount changes with the pH rise in the raffinate was first simulated by Visual MINTEQ software, and found the precipitation for the components was in the following sequential order of Si>F>P>Al>Cr>Pb≈Mg. Based on the calculation, one-step and multi-steps neutralization processes were designed for separation of impurities. Feed grade calcium hydrophosphate can be separated from the raffinate by multi-steps neutralization. A well-crystallized CaCl2·2H2O assaying 98.9 wt% was obtained after evaporative crystallization of the fluorine-depleted solution. Combined with our previous studies on the phosphate ore digestion and H3PO4 solvent extraction, a complete HCl wet process for H3PO4 manufacture was established.

МІНЕРАЛЬНИЙ СКЛАД, ГЕОХІМІЧНІ ОСОБЛИВОСТІ І ВПЛИВ НА ДОВКІЛЛЯ ВІДВАЛУ ФОСФОГІПСУ НОВОРОЗДІЛЬСЬКОГО ЗАВОДУ СКЛАДНИХ МІНЕРАЛЬНИХ ДОБРИВ (ЛЬВІВСЬКА ОБЛ.)

Phosphogypsum of Novorozdilsky Plant of Complex Mineral Fertilizers (Lviv region) is a typical artificial neogenic mineral formation, solid waste by-product generated when sulphuric acid reacts with the phosphorus-containing ores (apatite or phosphorite concentrates) during manufacturing of phosphoric acid. The raw materials for fertilizer production at the plant were sulphuric acid, which was obtained from the sulphur ores of Rozdilske SMCE “Sirka”, apatite concentrate from the Khibiny deposit (Kola Peninsula) and, to a lesser extent, phosphorite concentrate from sedimentary phosphorites. The plant has not been operating since 1995, but 4.5 million tons of phosphogypsum and also acidic mineralized water in the settling lake have accumulated on its territory since then. Gypsum (85–95 %) and bassanite (5–10 %) dominate in the mineral composition of phosphogypsum, there are also undissolved apatite (up to 2–3 %) and trace amounts of minerals that were originally in apatite concentrate (quartz, nepheline, pyroxene, aegirine, feldspar, biotite, etc.). The surface of the phosphogypsum heap is a phytophilous man-made substrate that is actively overgrown with woody vegetation. This is due to the filtration properties of phosphogypsum Василь Дяків, Ельвіра Джумеля, Мирослав Ковальчук та ін. 106 ISSN 2078-6220. Мінералогічний збірник. 2022. № 72 (they promote water accumulation and leaching of phytotoxic compounds from the zone of active water exchange) and the presence of a significant amount of phosphates. Currently, a dense aspen-sea buckthorn forest grows on the slopes of the dump. Acidic infiltrates are formed in the process of infiltration of precipitation through the thickness of the phosphogypsum dump, they are accumulated in the settling lake (Central/Acid Lake). In terms of chemical composition, it is sulphate-phosphate sodium-magnesium-calcium water with high fluorine content and mineralization of 2931.6 mg/dm3. The cessation of the use of the lake (as a settling tank for the plant's circulating waters) and the constant accumulation of infiltrate have led to a number of environmental problems. Acidic infiltrates flow into the mine canal and significantly impair the quality of water in it. Only due to the dilution of infiltrates with clean water of the drainage channel (with a flow rate of three to four times higher) before flowing into the transboundary Dniester River hydrochemical composition of water meets the normative values of salt composition and content of major pollutants. Deterioration of the water quality of the mine canal and, as a consequence, pollution of the Dniester in case of prolonged drought or reduction of the debit of the drainage canal are envisaged. To minimize the negative impact of the phosphogypsum dump on the environment, a project to build a cascade of filter dams from local carbonate materials has been proposed. Limestones have the ability to neutralize acidic waters; they are highly permeable and contain sulphur, which causes the binding of heavy metals to insoluble compounds. Due to the interaction of carbonates with acidic infiltrates, water-insoluble calcium phosphates and calcium fluoride (fluorite) are formed, which helps to purify water from F and P, and newly formed hydrates reduce the moisture content of the phosphogypsum mixture with flotation tails. The proposed approach will make it possible to control a constant source of pollution and restore the ecological balance disturbed by economic activities

CADMIUM IN PHOSPHOROUS FERTILIZERS: BALANCE AND TRENDS

The fertilizer industry is developing its activities by processing and upgrading natural phosphates, for several years that it has been in a conformist dynamic to face environmental, economic, financial, and political constraints. These constraints have incubated a significant industrial evolution in terms of technological progress, product diversification, knowledge, and innovation. Specifically, in wet phosphoric acid manufacturing, the 1980s brought a huge upheaval and changes in terms of strategies and practices following the development of environmental regulations. The environmentalist awareness, linked to the political and community trends, has resulted in the development of regulatory norms, as well as strict market demand for high-quality phosphoric acid and fertilizers products. Thus, cadmium has become an increasing concern to the phosphate industry and is considered one of the most important challenges currently facing the fertilizer industry for continuous market supply and growth. The present article gives an overview of cadmium in terms of market, applications, exposure, mobility, and bioavailability, and discusses balance and figures regarding cadmium in the phosphate and fertilizer industry. The article also outlines the technological aspects of cadmium removal from wet phosphoric acid, considering recent developments and conclusions that still confirm a challenge to find out an economic and available technology. Keywords: Cadmium, Phosphoric Acid, Market, Balance, Phosphate Fertilizers.

Impact of elevated phosphogypsum on soil fertility and its aerobic biotransformation through indigenous microorganisms (IMO's) based technology

Phosphogypsum (PG) is a waste by-product of phosphate fertilizer industry, produced in huge amount during the manufacture of phosphoric acid by economic wet process. Assessment of PG toxicity on soil has been poorly emphasized, therefore an efficient methods needs to be adopted to assess its toxic effect on soil fertility. We also need an effective eco-technological strategies for better waste PG management in order to improve the environmental health. The present study aimed to investigate the impact of PG toxicity on fertile soil and utilization of indigenous microorganisms for aerobic detoxification of PG contaminated soil to evaluate the scope for biostimulation based in situ bioremediation. In this study it is evident that application of PG to fertile soil in certain concentration results highly acidic, sulfate rich, aerobic environment, thus severely weakens the metabolic activity of the indigenous microorganisms. This investigation via microcosm based study further evaluated the intrinsic biotransformation ability of these microorganisms and found that was enhanced significantly (>95% reduction in sulfate concentration in 180 days) with carbon, nitrogen and phosphate amendments. Community level physiological profiling analyses indicated distinct shift in metabolic abilities following carbon amendments. Our study for the first time may help to formulate a strategy in aerobic biotransformation of PG contaminated soil environment, yet appreciable rate by supplying adequate nutrients.

Efficient removal of phosphate impurities in waste phosphogypsum for the production of cement

Phosphogypsum (PG) is an industry solid waste produced from phosphoric acid manufacture. To reduce environmental pollution of the PG, H2C2O4 was employed to purify it, which then can be used for cement production. The optimal concentration of H2C2O4 for PG purification was determined. In addition, differential thermal analysis (DTA), X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) were used to determine the removal of phosphate impurity in PG. The effects of purified PG on cement hydration and the environmental implications were also investigated. The results demonstrate that H2C2O4 can remove the intercrystalline phosphate impurities by destroying the part of the crystal structure of gypsum. With the best treatment concentration of 1% H2C2O4, 77.7% of phosphate impurity (as P2O5) was removed from PG, which subsequently shortened the final setting time down to 220min and successfully met the national standard (GB 175-1999). Portland cement prepared by the 1% H2C2O4 treated PG possessed a comparable 3d compressive strength of 20.8MPa and a 28d compressive strength of 44.6MPa. It is concluded that PG purified by 1% H2C2O4 treatment can be used for cement production. Meanwhile, this H2C2O4 treatment can effectively reduce the environmental pollution from PG and offer a sustainable method for the utilization of PG.

Development of in-House Industrial Fluosilicic Acid Certified Reference Material: Certification of H2SiF6 Mass Fraction

Fluosilicic acid is a by-product of the chemical phosphate industry, mainly during the manufacture of phosphoric acid and triple super phosphate (TSP). To ensure the accurate measurement of the H2SiF6 mass fraction in this by-product, method validation is required, which needs a certified reference material (CRM) with its traceability to the International System of Units (SI). This work describes the development of a certified reference material of fluosilicic acid, which is commercially unavailable. Details of all steps, such as sample preparation, homogeneity and stability studies, value assignment, establishment of metrological traceability, and uncertainty estimation of the certified reference material, are fully described. The H2SiF6 mass fraction in the CRM was quantified by two analytical methods, i.e., UV-VIS as a primary method of analysis and flame mode atomic absorption spectroscopy (AAS) as a second method. It is worth noting that the results obtained from each method were in good agreement. The CRM certified value and corresponding expanded uncertainty, obtained from the combined standard uncertainty multiplied by the coverage factor (k = 2), for a confidence interval of 95%, was (91.5 ± 11.7) g·kg−1. The shelf life of the developed CRM is determined to be 1 year, provided that storage conditions are ensured. The developed CRM can be applied to validate analytical methods, improve the accuracy of measurement data as well as to establish the meteorological traceability of analytical results.

Effect of phosphate quality on foam generation during the phosphoric acid production process

The presence of organics materials in phosphate ores generates foam during the manufacture of phosphoric acid, thus affecting the production performance and the quality of the products. Today, in the phosphate industry defoamers are used to reduce the negative impact of foam phosphoric production. In the present work, we focused on root cause evaluation for foam generation during the phosphoric acid production process, we evaluated the effect of phosphate rock quality in terms of organic carbon (OC) and carbonate content, on foam generation using a laboratory protocol for foaming ability evaluation. The results show that there is a relationship between the volume of the foam generated and the values of those impurities, while the volume of foam generated is higher when the concentration of OC and Carbonate is high. In our work, we confirm that the foaming ability of the phosphate rock can be avoided if the concentration of organic carbon less than 0.1% and less than 3% for carbonate.