Cheng Meng， Liu Xiangjie， Wei Yifei， Cao Wujie， Wu Changliang， Xia Xiao， Tian Yeshun， Duan Guangbin
School of Materials Science and Engineering， University of Jinan， Jinan 250022， China
Abstract
Objective This study aims to develop a high-value utilization pathway for flue gas desulfurization gypsum （FGD gypsum） by converting its sulfate component into calcium sulfide （CaS） through biomass reduction. FGD gypsum is an abundant solid waste from wet flue gas desulfurization， but its conventional use in building materials and agriculture is limited by regional demand and low added value. CaS is a calcium- and sulfur-containing intermediate for sulfur recovery， environmental treatment， and chemical conversion. Therefore， selective conversion from FGD gypsum to CaS is significant. Coconut shell biomass serves as both a solid carbon source and a precursor of reducing gases. The conversion behavior， key process variables， product characteristics， and technical limitations are clarified to provide a basis for the synergistic utilization of industrial gypsum waste and agricultural biomass residues.
Methods FGD gypsum and coconut shell powder were used as raw materials. FGD gypsum was dried at 105 ℃ for 24 h， and the coconut shell was dried， crushed， and sieved before use. The elemental composition and proximate characteristics of the coconut shell were determined， and the chemical composition， phase composition， and short-term air stability of FGD gypsum were characterized by XRF and XRD. For reduction tests， the mixed sample was loaded into a tubular reactor， purged with N2， heated in a vertical tube furnace， maintained for 60 min， and cooled under N2 protection. The effects of reaction temperature， N2 carrier gas flow rate， and coconut shell-to-gypsum mass ratio were investigated. An L9 orthogonal experimental design was first adopted to identify the relative importance of the variables. Single-factor experiments were then carried out， and response surface methodology was applied to establish a regression model for the CaS mass fraction. Thermodynamic equilibrium calculations for the CaSO4-C system were performed using FactSage. Products obtained under the optimized conditions were characterized by XRD， Jade-based indexing， SEM-EDS， micro-area XRF， and XPS.
Results and Discussion It was shown that the original FGD gypsum contained SO3 and CaO as the main oxide components， with minor amounts of SiO2， MgO， Al2O3， Fe2O3， K2O， Cl， and loss on ignition. Its crystalline phases mainly consisted of CaSO4·2H2O， CaSO4·0.5H2O， and CaSO4， and no obvious new crystalline phase was produced after short-term exposure to air， indicating good stability under ambient conditions. The orthogonal experiment indicated that the effects of process variables on CaS mass fraction ranked as follows： reaction temperature>mass ratio>carrier gas volumetric flow rate， and temperature was therefore the dominant factor. A clear temperature-dependent trend was demonstrated by single-factor tests. At 700 ℃， the CaS mass fraction was only 52.35%， and residual CaSO4 remained evident. At 800 ℃， the CaS mass fraction increased sharply to 98.42%， whereas CaSO4 decreased to 1.58%. At 900 ℃， CaS was the only obvious crystalline product identified by XRD semi-quantification analysis， and the CaS mass fraction approached 100%. When the temperature increased to 1 000 and 1 100 ℃， small amounts of CaO appeared， and the CaS mass fraction decreased slightly， suggesting that excessively high temperature promoted side reactions or partial decomposition. Carrier gas volumetric flow rate affected gas residence time， transfer of reducing gases， and atmosphere stability. The best comprehensive performance was obtained at 150 mL/min. The mass ratio controlled the availability of fixed carbon and biomass-derived volatiles， and 1：1.0 provided sufficient reducing capacity without excessive carbonaceous residue. The response surface model showed good fitting performance， with an R2 of 99.73%， adjusted R2 of 99.25%， and predicted R2 of 95.74%， indicating reliable prediction within the studied range. The optimized conditions were approximately 900 ℃， 150 mL/min N2， and a gypsum-to-coconut shell mass ratio of 1：1.0. Under these conditions， the product mainly consisted of cubic CaS with space group Fm-3m， and the refined lattice constant was close to the standard CaS value. Surface sulfur was mainly present as low-valence S2- according to XPS， while minor high-valence sulfur species were attributed to surface oxidation or hydrolysis. Rough and porous particles were observed， the distributions of Ca and S showed good spatial overlap， and the Ca：S atomic ratio was 0.992.
Conclusion It is concluded that the conversion of FGD gypsum to CaS is effectively promoted by coconut shell biomass through the synergistic effects of solid carbon reduction and gas-phase reduction. Almost complete conversion of crystalline sulfate phases identified by XRD into CaS is achieved under the optimized laboratory conditions， and the product is a CaS-rich intermediate with potential applicability in sulfur resource recovery and environmental treatment. An experimental basis is provided for co-utilization of industrial gypsum waste and biomass residues. However， the current evidence is not sufficient to define the product as commercial high-purity CaS or to demonstrate its economic feasibility for industrial-scale application. Further research is still required to investigate chemical CaS content， effective sulfide release， impurity control， energy consumption， material balance， techno-economic evaluation， and pilot-scale validation.
Keywords： flue gas desulfurization gypsum； biomass reduction； calcium sulfide； orthogonal experiment
Get Citation:Cheng Meng， Liu Xiangjie， Wei Yifei， et al. Process optimization for calcium sulfide preparation from flue gas desulfurization gypsum through biomass reduction［J］. China Powder Science and Technology， 2027， 33（1）： 47-63.
Received:2026-07-24， Revised： 2026-09-06， Online： 2026-10-08。
Funding： The research was supported by the National Natural Science Foundation of China (Grant No. 52400166) and the Natural Science Foundation of Shandong Province (Grant Nos. ZR2024QE143 and ZR2024QE346).
DOI：10.13732/j.issn.1008-5548.2027.01.004
CLC No.:X705； TB4
Type Code:A
Serial No.:1008-5548（2027）01-0047-17
