Utilization of phosphogypsum waste through a temperature swing recyclable acid process and its application for transesterification

Abstract

In this paper, a biodiesel catalyst was prepared from the phosphogypsum (PG) by a hybrid approach through temperature swing acid leaching/crystallization steps followed by the subsequent fluidized bed calcination. The impacts of acid leaching and crystallization were extensively analyzed via a supervised machine learning approach using a limited number of experimental runs to find out the optimal condition. The determined optimal conditions are X₁-95 (ºC), X₂-30 (min), X₃-30 (wt%-H₂SO₄), and corresponding validation experimental result (at the optimal condition setting) shows± 5% uncertainties. The prepared catalyst predominately contains CaSO₄ (98 wt%) with the impurities less than 0.3 wt% (i.e., P₂O₅^- and F^-). The numbers of acid leaching cycles (up to 10 cycles) were investigated, and result indicates a good contaminates recovery (P₂O₅:1.8 g/100 g PG, Mg²⁺: 0.3 g/100 g PG, Al³⁺: 0.3 g/100 g PG, Fe³⁺: 0.1 g/100 g PG) in the leachate through the downstream solvent extraction. The catalytic conversion reaches about 50% with approximately± 5% deactivation when catalyst was reused at the same transesterification condition. The decreased binding energies of S2p (169.6 eV) and O1st (533 eV) in used catalyst indicate the deactivation of surface catalytic sites. The key properties of the prepared biodiesel are comparable to the American society for testing and materials (ASTM) standards.

Introduction

Phosphogypsum (PG) is a waste from phosphoric acid production, which is regarded as a by-product of the fertilizer industry. The accompanying 4–6 tons of PG will be generated when one ton of phosphoric acid is produced (Wang et al., 2021c). The global generation of this by-product is estimated about 280 million tons per year (Yang et al., 2009, El Zrelli et al., 2018), and about 10 million tons produced per year in China (Zeng et al., 2021). The status quo of the majority (> 85%) of the generated PG lies in stockpiling in open space without any treatment (Bisone et al., 2017). The significant potential hazards derived from liberations of chemicals (i.e. phosphor-P, fluoride-F) by the exposure of the natural weathering, especially in Yangzi River regions, have persistently jeopardized the wellbeing of local residents and sustainable development of economy in eastern coastal areas (Bisone et al., 2017). Apparently, from resources recycling and energy cascade utilization perspective, this mega amount of PG needed to be reused or high value converted to other useful products. Its utilization not only achieves environmental protection but also minimizes stacking cost (Canovas et al., 2018). Although the progresses of utilizing waste PG have been made during the last two decades, big gap needed to be appreciably narrowed orchestrating with ambitious implementation of carbon neutrality and peak carbon dioxide emissions in the next two decades or so (Kumar et al., 2020, Liu et al., 2021, Lian et al., 2021). The current popular utilization of PG includes soil improvers, construction materials, or raw material to manufacture building materials, chemical additions for CO₂ sequestrations (Kuttah and Sato, 2015, Lachehab et al., 2020). Both industry and academia call for the bold steps of creatively utilizing this industrial waste (Alcordo and Rechcigl, 1993, Wang, 2020, Zhao et al., 2021). Apart from utilizing PG as materials and supplementary additives, efforts have been made to use PG as precursor to prepare the solid acidic catalyst that possesses several appealing advantages: i) its inherent features of acidity and rich functional surface groups (Ping, 2019), ii) renewable energies become more attractive as paradigm of energy generation shifts from fossil fuel to renewable energies (Sun et al., 2021, Wang et al., 2021a, Wang et al., 2021b), and biodiesel has recently attracted tremendous attentions in alternative fuel research because it has several superior characteristics over fossil derived diesel including its renewability, carbon neutrality, biodegradability, non-toxicity, higher flash point, and cleaner emission profile (Silitonga et al., 2019, Sun et al., 2017, Sun et al., 2019, Usmani et al., 2020, Wang et al., 2020). The current conventional approach of solid acidic catalyst for transesterification mainly focusing on using the precursors such as eggshells, cockle shells, oyster shells, and Angel Wing Shell through simple calcination approach (Syazwani et al., 2019, Zuhaimi et al., 2015, Okoye et al., 2020), very little efforts have been made to use PG to catalyze the transesterification reaction (Wang et al., 2021c). From our recent achievement of producing acid/base from salt via high temperature steam hydrolysis process, the economical acid and base could be simultaneously produced on the large scale (Sun et al., 2014). This makes the recyclable acid processing using PG as raw material to produce the high-quality solid catalyst possible in a practical and cost-effective manner (Sun et al., 2012). Regarding the biodiesel conversion, the general practice is to use the fatty acid oil and methanol as reagents for transesterification reaction (Silitonga et al., 2019). From cost-effectiveness perspective, scholars have paid more attention to deploy the used fatty acid oil as resources for biodiesel productions (Silitonga et al., 2019, Zhao et al., 2018, Ideris et al., 2021). Therefore, this work aims to deploy recyclable acid process to prepare solid catalyst from PG for biodiesel conversion using waste oil. The efforts of preparation the transesterification (using waste fatty acid oil) catalyst using the waste PG generated from phosphoric acid production to the best of authors’ knowledge, has not been published before.