Pyrohydrolysis of CaCl₂ Waste for the Recovery of HCl Acid upon the Synergistic Effects from MgCl₂ and Silica

Abstract

In this paper, an efficient HCl acid recovery method from the mixture of alkaline-earth-metal chlorides waste was demonstrated via co-pyrohydrolysis in a laboratory-scale horizontal furnace in a temperature range of 700−1000 °C, and fixed additions of SiO 2 and steam. The synergistic effect of MgCl 2 on the HCl recovery from CaCl 2 was explored intensively. A double-sided effect was revealed. For the reaction temperatures below 1000 °C, the MgCl 2 addition delayed the HCl release through competing with CaCl 2 for the inclusion into silica matrix. In contrast, once the chloride mixtures were subjected to 1000 °C with a noticeable residence time (e.g., 2 h) and at a minimum molar ratio of 0.5 of MgCl 2 to CaCl 2 , the MgCl 2 addition promoted the HCl release remarkably, via promoting the conversion of Ca 3 (SiO 4 )Cl 2 into Ca 8 Mg(SiO 4 ) 4 Cl 2 . A portion of Mg 2+ derived from the early decomposition of MgCl 2 substituted the Ca(I) site in Ca 3 (SiO 4 )Cl 2 , thereby resulting in the formation of weak Mg−Cl bond that is in favor of the HCl release. Additionally, the remaining Mg 2+ consumed the excessive SiO 2 so as to cause the skeleton of [SiO 4 ] 4− to be fully affiliated and balanced by cations to form Ca 8 Mg(SiO 4 ) 4 Cl 2 in which the weaker ionic polarization between Ca 2+ and adjacent anions further enhanced the breakage of the Ca−Cl bonds.

Can CO2 and Steam React in the Absence of Electrolysis at High Temperatures?

“…The use of MgCl 2 was merely to ease the collection of the reaction residue, which would otherwise melt and stick firmly on the corundum boat at high temperatures. Compared to CaCl 2 • 4H 2 O, MgCl 2 • 6H 2 O decomposed quickly into MgO below 500°C, [11] which then served as an inert substance to dilute the CaCl 2 melt. Additionally, to rule out any effect of MgCl 2 addition, its amount was varied purposely from 1 to 4 moles per mole of CaCl 2 .…”

“…An open system in the FactSage was used to simulate the continuous flow of the gas. [11] Ca(OH)Cl was synthesized as follows. Around 10 g of CaCl 2 • 4H 2 O and CaO at a molar ratio of 2 : 1 was added to about 10 mL Milli-Q water and then stirred for around 12 h in a thermostatically controlled water bath at 70°C.…”

“…azeotropic HCl acid to be reused. To date, numerous CaCl 2containing wastes are being generated from many industrial processes, such as the HCl-leaching of E-waste, [11] refinement of heavy oils, [12] and soda-ash production utilizing the Solvay process that generates more than 10 million tons of CaCl 2 -rich waste annually. [13] However, the recovery of HCl from CaCl 2 via pyrohydrolysis (Equation 3) is still unproven, which was believed to be thermodynamically infeasible in the operating temperature window of a commercial pyrohydrolyser (i. e., 600-1000°C).…”

Critical Review on the Chemical Reaction Pathways Underpinning the Primary Decomposition Behavior of Chlorine-Bearing Compounds under Simulated Municipal Solid Waste Incineration Conditions

“…Zhou et al 66 revealed a double-sided effect of MgCl 2 on the pyrohydrolysis of CaCl 2 at 700−1000 °C, under a fixed molar ratio of 1 for SiO 2 /CaCl 2 in argon containing 70 vol % steam. In the initial heating stage, the MgCl 2 addition delayed the pyrohydrolysis of CaCl 2 through competing with CaCl 2 for the inclusion into the silica matrix.…”