Yu Zhang

The pyrolysis separation of calcium and magnesium from phosphate tailings is an important process due to its high-value resource utilization. In this paper, aiming to address the problems of high energy consumption, a slow decomposition rate and the low activity of decomposition products in the high-temperature pyrolysis of phosphate tailings, the medium-temperature pyrolysis of phosphate tailings under a H₂O atmosphere was carried out, and the phase reconstruction and activation of pyrolysis process were discussed. The results showed that compared with N₂, air and CO₂ atmospheres, the pyrolysis process of phosphate tailings in a H₂O atmosphere was changed from two stages to one stage, the starting decomposition temperature was reduced to 500 °C and the decomposition time was shortened to 30 min. The order of the influence of each factor on the pyrolysis of phosphate tailings was temperature > H₂O pressure > holding time. Under the optimized pyrolysis conditions, the yield of CaMg(CO₃)₂ decomposition of phosphate tailings into MgO and CaO was 97.3% and 98.1%, respectively, and the reactivity of MgO was 31.6%. The distribution of Ca and Mg elements in the phosphate tailings after pyrolysis showed a negative correlation, and both of them no longer formed associated compounds; Ca mainly existed in the form of Ca(OH)₂, Ca₅(PO₄)₃F, CaSiO₃ and CaF₂, and Mg mainly existed in the form of MgO, MgF₂ and Mg(OH)₂.

Electrolytic manganese residue (EMR) is a solid filter residue obtained from manganese carbonate ore during the production of metal manganese. A potential avenue towards large-scale utilisation of EMR is its use in cement preparation. However, the preparation of cement materials using EMR requires high-temperature calcination. In this study, the thermal properties and pyrolysis kinetics of belite–calcium sulfoaluminate cement raw meal were systematically studied using a multiple-heating-rate method based on thermogravimetric analysis and a kinetic model. The kinetic and thermodynamic parameters was studied using non-isothermal Flynn–Wall–Ozawa (FWO), Kissinger–Akahira–Sunose (KAS), Friedman and Kissinger methods. The results showed that from 30 to 1300°C, the pyrolysis reaction of cement raw meal was mainly divided into four steps: the crystalline water removal from calcium sulphate dihydrate and bauxite, the ammonia nitrogen removal from ammonium salts and the calcium sulphate crystal transformation; the decomposition of calcium carbonate and carbon-containing organic matter; the sulphate and carbonate substance decomposition and the clinker mineral phase formation. The average activation energies calculated when using the non-isothermal FWO, KAS, Friedman and Kissinger methods were 244.49, 240.7, 239.24 and 380.60 kJ/mol and the average pre-exponential factors were 1.75 × 10²⁰, 3.65 × 10²⁰, 7.11 × 10²¹ and 1.55 × 10¹³ s⁻¹, respectively. Herein, the pyrolysis kinetics of the cement raw meal was divided into two main stages: In stage 1 (α: 0.15–0.8, 524°C–754°C), the mechanism of P2/3 accelerated nucleation in the Mampel Power rule, and the reaction mechanism function was G(α)=α^3/2. In stage 2 (α: 0.80–0.95, 754°C–1165°C), during the local conversion of α = 0.2–0.8, when α was 0.5, a random nucleation and subsequent growth mechanism of A6 was noted and the mechanism function was G(α) = [−ln(1 − α)]^2/3.

The application of α-hemihydrate gypsum (α-HH) is limited by several factors, such as a rapid hydration rate, short setting time, poor water resistance, and high cost. Especially because of the high production cost, although α-HH has excellent mechanical strength, it is rarely used in the field of building materials. In this study, based on the composition characteristics of new industrial solid waste (phosphate-rock acid-insoluble residue, PAIR) and to meet the needs of resource utilization, gypsum matrix composites were prepared by adding PAIR to α-HH to solve the problems of short setting time and poor water resistance of gypsum matrix composites and improve the comprehensive properties of α-HH products. The results show that when the different types of pores formed during the hydration of α-HH were filled with inert substances, such as silicon dioxide, insoluble phosphate, and calcium fluoride from PAIR, the proportion of mesopores in the composite products increased, whereas that of harmful macropores decreased. The compressive and flexural softening coefficients of the PAIR/α-HH system with 23% PAIR were the highest at 57.25 and 60.125%, respectively, and the water resistance of the system was improved; when the content of PAIR reaches 35%, the strength of the composite products decreased from 58.125 to 43.8 MPa. The HPO₄²⁻ in PAIR partially replaces the SO₄²⁻ ion in the dihydrate gypsum (DH) lattice to form a Ca(SO₄, HPO₄)•2H₂O double salt, leading to the production of eutectic phosphorus. Soluble F⁻, Al³⁺, Mg²⁺, and phosphorus-containing substances in PAIR form a variety of complex ions in PAIR/α-HH aqueous solution, which are adsorbed onto the surface of the new DH phase; the crystal morphology changed from thick, long columns to clusters, thin rods, and plates, inhibiting the nucleation and growth of DH and changing its crystal growth rate and crystallization behavior. Therefore, the setting time of gypsum is prolonged; when 35% PAIR was added, the induced nucleation period of the PAIR/α-HH system was prolonged from 40 to 265 min, and the final setting time was from 12 to 360 min. By mixing solid waste PAIR, while the setting time of α-HH is prolonged, its water resistance is improved, and its mechanical strength is not significantly reduced, reducing the cost. From the perspective of economy and environmental protection, this study is a way to α-HH is widely used in the field of building materials. Graphical abstract