Elsevier

Chemosphere

Volume 248, June 2020, 125979
Chemosphere

Chemical behavior of fluorine and phosphorus in chemical looping gasification using phosphogypsum as an oxygen carrier

https://doi.org/10.1016/j.chemosphere.2020.125979Get rights and content

Highlights

  • Evaluate the gasification performance and fluorine and phosphorus chemical behavior in CLG using PG as an oxygen carrier.

  • The main pollutant was gaseous HF in CLG, which was transformed from NaF.

  • The Ca3(PO4)2 particle layer existed in the middle of the solid particles, acting as a glue between the particles.

Abstract

In China, the amount of phosphogypsum (PG) has exceeded 250 million tons with more than 55 million tons of growth rates each year. As the micro constituent, fluorine and phosphorus restrict the resourceful disposal of PG. This paper focused on chemical looping gasification (CLG) which used PG as an oxygen carrier, systematically investigated the gasification performance and chemical behavior of fluorine and phosphorus contained in PG during CLG process. Main conclusions are as follows. The main pollutant of chemical looping gasification process was HF, which was transformed from NaF. Phosphorus transformed from water-soluble phosphorus (Ca(H2PO4)2, Ca(HPO4)) into insoluble Ca3(PO4)2.20 reducing-oxidizing cycles were investigated, and a less and less fluorine content in oxygen carrier was found because its phase transformation from solid NaF to gaseous HF, and the phosphorus content in oxygen carrier changed slightly under the current conditions. The Ca3(PO4)2 particle layers existed in both the middle of the reduced solid particles and the middle of the cycled oxygen carrier particles, confirmed to actually act as a glue between the particles. Furthermore, transformation routes of fluorine and phosphorus during the CLG process were discussed and the generation of syngas in CLG process needed to be purified.

Introduction

Phosphoric acid is an intermediate material for the production of phosphate fertilizer and phosphate (Mohammed et al., 2018), playing an important role in the national economy. The “wet method” accounts for 85%–90% (Macías et al., 2017; Gu et al., 2018) of the total world phosphoric acid production, considering as a milestone in the development of the phosphoric acid industry. However, this wet process of phosphoric acid production produces a large amount of by-product PG (Hentati et al., 2015a). It estimated that current world production of PG is about 100–280 million tons per year (Pérez-López et al., 2010; Yang et al., 2013a), while only 15% is recycled as building materials, agricultural fertilizers, soil stabilization amendments and set controllers in the manufacture of cement (El-Didamony et al., 2013; Jalali et al., 2016; Macias et al., 2017) and most of them are deposited as wastes (Ercikdi et al., 2015; ). In Korea, about 30 million tons of PG is deposit (Yang et al., 2009). In Algeria, there are two major ways to dispose PG: pouring it into the sea or river and burying it (Kacimi et al., 2006). In China, the amount of PG has exceeded 250 million tons with more than 55 million tons of growth rates each year (Yang et al., 2013b; Tian et al., 2016), and the deposition of those PG not only occupy large area of land but also bring many environmental problems, such as dust, groundwater and soil pollution (Ying, 2007). Therefore, PG management is one of the most serious problems currently facing the phosphate industry.

PG is mainly composed of CaSO4·2H2O but also contains some impurities, as well as many trace elements such as Al, Na et al. (Cuadri et al., 2014; Macias et al., 2017). The concentration of impurities element silicon is highest and then are fluorine and phosphorus (Kandil et al., 2019). SiO2 in PG mainly exists as quartz, playing a role of inert support and no bad effect on performance properties of PG (Yang et al., 2007). However, fluorine and phosphorus restricts the resourceful disposal of PG Yang et al., 2007). The fluorine in PG exists as water-soluble fluorine (NaF) and insoluble fluoride (CaF2). Among the different forms of water-soluble fluorine and insoluble fluoride, the effect of water-soluble fluorine (NaF) are worse (Zhao et al., 2015). The insoluble fluoride (CaF2) only plays a role of inert supports and has no effect on performance properties of PG (Xu et al., 2010). The phosphorus in PG exists as water-soluble phosphorus (Ca(H2PO4)2, Ca(HPO4)), co-crystallized phosphorus (CaSO4·2H2O, CaHPO4·2H2O) and insoluble phosphorus (Ca3(PO4)2) (Xu et al., 2010). Among the different forms of phosphorus phase, the effects of water-soluble phosphorus (Ca(H2PO4)2、Ca(HPO4)) are worse and are adsorbed on the surface of gypsum crystal (Yang et al., 2007). Co-crystallized phosphorus exists as co-crystals CaSO4·2H2O and CaHPO4·2H2O by some HPO42− instead of SO42− entering the CaSO4·2H2O crystal lattice to form (Xu et al., 2010). Insoluble phosphorus (Ca3(PO4)2) plays a role of inert supports and exists in coarse particle of PG (Xu et al., 2010).

Water-soluble fluorine and water-soluble phosphorus in PG are major pollutant and have the worse effects on performance properties of PG and its products (Yang et al., 2007). When PG is used to fix CO2, water-soluble fluorine has worse effects on the physical properties and filtration properties of calcium carbonate products (Lee et al., 2012; Song et al., 2012). Water-soluble phosphorus in PG delays the setting and hardening of cement and building gypsum to cause the low performance products. In addition, fire resistance performance, the mechanical properties and the surface roughness of PG products are effected by impurity elements fluorine and phosphorus in PG (Macias et al., 2017). The existing technologies are either complicated or expensive (Cánovas et al., 2017; Rychkov et al., 2018). It recommended that PG should be treated by a new technology to minimize or immobilize its harmful components.

Recently, Our group has paid great interest on using PG as an oxygen carrier in CLG duo to its mainly composed of CaSO4·2H2O and inert supports SiO2, Al2O3. The component CaSO4·2H2O is considered to be an oxygen carrier instead of native gypsum and the role of inert supports SiO2, Al2O3 is significant for carrying the oxides and heat. In addition, the established pollution abatement requirements increased the necessity to restrict the output of fluorine and phosphorus to the environment during the utilization of PG. Therefore, it is important to investigate the chemical behavior of fluorine and phosphorus in PG during CLG process. This study evaluates the chemical behavior of fluorine and phosphorus systematically to provide a theoretical guide for control fluorine and phosphorus during CLG process, which is very important for the new methodology of PG used as an oxygen carrier, since it has the potential to handle management of hazardous industrial waste PG simultaneously. Therefore, exploring the chemical behavior of fluorine and phosphorus in CLG of PG as an oxygen carrier has the great significance to promote the comprehensive utilization of PG.

Section snippets

Materials

In this work, a PG from Yunnan, China was used as an oxygen carrier for the CLG experiments system. Before experiments, the PG was dried in a drying box in air at 105 °C for 24 h and then was defined as fresh oxygen carrier and its chemical composition was analyzed by using a ZSX100e single-channel scanning X-ray fluorescence spectrometer (XRF). In order to determine the fluorine and phosphorus distributions in the present PG sample, the content of F was determined by XRF empirical coefficient

Results and discussion

The experimental results were presented in three different sections. The first section investigated the reaction performances, including gaseous productions distribution, XPS patterns of fluorine and phosphorus in solid phase after reduction experiments. Pollution prediction by FactSage7.1 equilib module and experimental verification of the value of fluorine and phosphorus in the gas and solid phase after reduction experiments also were presented in this section. In the second section,

Conclusions

This paper concentrates on the application of CLG with PG as an oxygen carrier, which is characterized by fluorine and phosphorus content, investigated the gasification performance and fluorine and phosphorus chemical behavior. Main conclusions are as follows:

  • (1)

    During the reducing period, fluorine element transforms from water-soluble NaF into gas phase HF and a small amount of solid phase CaF2 adhered to the oxygen carrier surface not participation in the reaction process.

  • (2)

    During the reducing

Declaration of competing interest

The authors declare they have no conflict of interest.

Acknowledgments

1.Financial support for this project was provided by National Natural Science Foundation of China (No. 21666016), which is greatly acknowledged.

2. Financial support for this project also was provided by National Natural Science Foundation of China (No.21868014), which is greatly acknowledged.

References (34)

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