Research PaperA clean process for phosphorus recovery and gallium enrichment from phosphorus flue dust by sodium carbonate roasting
Graphical Abstract
Introduction
Phosphorus (P) is present in all the cells of the human body and is an indispensable element for life (Cordell et al., 2009). P is also widely used in agriculture and food production (Van Vuuren et al., 2010), and approximately 90% of the mined P is used to make fertilizer and animal feed (Sarvajayakesavalu et al., 2018, Gleason, 2007). Moreover, P is an essential raw material for many chemical products, such as medicine, oil, detergent, etc (Cieślik and Konieczka, 2017). Therefore, P is an important element that is closely linked to the future development of mankind. Phosphate rock (apatite, etc.), which is the main mineral that P is extracted from, is a non-renewable resource (Vaccari et al., 2019, Van Vuuren et al., 2010). However, with the rapid growth of the global population the demand for P is increasing and it is estimated that current reserves of phosphate rock may be depleted within 100 years (Kong et al., 2020). This has forced researchers to develop alternative methods for recovering P from different P-containing wastes (Senthilkumar et al., 2014), including fertilizer industrial wastewater (Hutnik et al., 2013), alkaline phosphate concentrate (Hermassi et al., 2015), municipal-wastewater sludge (Lahav et al., 2013), aquaculture wastewater (Zhang et al., 2016), etc.
In the industrial production of P, the electric furnace method of extraction is most common (Wang et al., 2011, Geng et al., 2017), but this process consumes large amounts of electricity and also generates various solid waste products, such as phosphorous slag and phosphorous flue dust (PFD) (Ji et al., 2021a). China is currently the world’s largest producer of P and generates a large amount of solid waste every year (Sattari et al., 2014). These solid wastes are difficult to dispose of, cause damage to the environment, and severely restrict the sustainable development of the P industry. In light of the unsustainability of the P industry, research into the reuse and treatment of these wastes has gained great importance (Li et al., 2019). Among these wastes, PFD is the dust collected by electrostatic precipitator during the P production process, and its elemental composition is complicated, making it difficult to treat (Ji et al., 2021b). Furthermore, in addition to being a source of P, PFD is enriched with a certain amount of gallium (Ga), which makes it a potential secondary source for the recovery of Ga (Lu et al., 2017). Previously, studies have successfully used spent sulfuric acid to leach Ga from PFD, achieving leaching rates of about 90%, and finally Ga was recovered in the form of precipitation (Xu et al., 2007). Some studies have also shown that the vacuum carbothermal reduction method can effectively extract Ga from PFD, and its mechanism is that the reductant carbon reacts with Ga2O3 to generate volatile low-valence Ga, thus enriching Ga in the condensation zone (Ji et al., 2020). In addition to Ga, PFD usually contains 25–45% P2O5, which has a high recovery value (Xu et al., 2007, Ji et al., 2020, Charlton et al., 1978). However, despite its value, research has generally focused on the recovery of Ga and ignored the recovery of P. Subsequently, methods for the recovery of P from PFD have not been well developed.
Recently, the recovery of P from various wastewater (Cieślik and Konieczka, 2017) or municipal sewage sludge (Chen et al., 2020) has gained interest among researchers, and many methods have been developed to recover P, including biological processes, crystallization, chemical precipitation, adsorption and ion exchange processes, etc (Peng et al., 2018). Furthermore, researchers have also developed methods for P recovery from MSW (municipal solid waste) incineration fly ash, mainly using HCl leaching-NaOH precipitation method or acid-alkali combined leaching method to recover P (Kalmykova and Fedje, 2013), however, most of these methods consume large amounts of acid or alkali and require additions of different chemical reagents (MgCl2, MgO, etc.), making the whole technical process relatively complex. Therefore, seeking an efficient and green method to recover P from PFD is an important environmental issue.
In this study, in view of the high P content of PFD, a new technological process for recovering P and enriching Ga from PFD using Na2CO3 roasting and water leaching was proposed. The effects of various experimental parameters (Na2CO3 dosage, temperature, time, and liquid-solid ratio) on the P leaching rate and Ga loss rate were investigated and the optimal experimental conditions were obtained. The phase transformations of P-containing compounds in the roasting process were studied and the reaction mechanism of the Na2CO3 roasting process was discussed. Furthermore, an attempt was made to recover PO43- and Na+ from the water leaching solution using evaporative crystallization. The whole process not only realized the recovery of P and Na (Na in Na2CO3), but also effectively enriched Ga, which made it a feasible method with high potential application value.
Section snippets
Materials
A phosphorus plant in Yunnan Province, China, supplied the PFD used as the raw material for this experiment. Before the experiment, the PFD was ground and dried resulting in a grayish black powder, as shown in Fig. 1. The analytical grade pure sodium carbonate (Na2CO3) used in the roasting process was from Tianjin FengChuan chemical reagent Technologies Co., Ltd. The water used in the experiment was deionized. Table 1 shows the chemical composition of the raw materials, the content of P in the
Thermodynamic analysis
According to the results in Table 1 and Fig. 1, PFD contains a complex composition of elements, but P is the main component and is primarily found in P-containing compounds. The possible reactions between the P-containing compounds in PFD and Na2CO3 are shown below.
The feasibility of these reactions can be determined based
Conclusions
In this study, a new technical process for recovering P and enriching Ga from PFD by Na2CO3 roasting and water leaching was proposed and tested. The P-containing compounds in PFD were converted to water-soluble Na3PO4, which was dissolved into aqueous solution, while Ga was enriched in the residue. The effects of roasting and leaching conditions on P leaching rate and Ga loss rate were investigated in detail and the optimal experimental parameters were obtained. Under the roasting conditions of
CRediT authorship contribution statement
Wentao Ji: Conceptualization, Methodology, Software, Validation, Writing-Original Draft, Writing-Review and Editing. Keqiang Xie: Funding acquisition, Project administration, Supervision. Shiyu Yan: Software, Investigation. Xiaolei Yuan: Investigation. Zhixiang Wang: Investigation. Yang Li: Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51764033).
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