Meitan xuebao (Jul 2023)
Reaction characteristics of carbon fixation and alkali reduction in high calcium fly ash and new way of large-scale utilization in coal mine
Abstract
High calcium fly ash slurry has a high pH value, which is prone to contaminating groundwater when it is directly used for fire prevention and extinguishing in gob. To study the reaction characteristics of carbon fixation and alkali reduction in the production of underground fire extinguishing materials using fly ash by CO2 mineralization, the fly ash from a coal and electricity integrated enterprise in northern Shaanxi is used to analyze its composition and phase structure through XRF and XRD. At the same time, using a mixture of N2 and CO2, laboratory experiments on CO2 mineralization is conducted to analyze the variation law of pH value during the carbon fixation and alkali reduction reaction process of fly ash, and the influence of key parameters such as CO2 concentration, gas flow rate, solid-liquid ratio, temperature and stirring method on the mineralization reaction process. Also, the key technologies that need to be addressed for a large-scale underground application are proposed from the characteristics of fly ash carbon fixation and alkali reduction, development of mineralization reaction equipment, diffusion law of slurry in gob, and the entire preparation process. The research results indicate that fly ash is rich in alkali metal elements such as Ca2+, Mg2+, K+, Al3+, with CaO content of up to 17.8%, belonging to high calcium ash. CO2 can be reacted with Ca2+, Mg2+ in the fly ash and CO2 is attached to the surface of spherical particles of fly ash with calcium carbonate type after the reaction, indicating the feasibility of carbon sequestration and alkali reduction of high calcium fly ash. During the mineralization reaction process, the pH value of the slurry decreases in three stages: slow−fast−slow. When the pH value of the fly ash slurry drops to 7.0−7.5, the pH value return will not exceed 9, meeting the requirements for fire prevention and extinguishing materials for coal mine gob. The optimal mineralization reaction parameters are that CO2 concentration being of 10%−15%, solid-liquid ratio being of 300 g/L, gas flow rate being of 1.0 L/min per liter of slurry, and temperature being not higher than 55 ℃. Taking into account factors such as carbon sequestration, alkali reduction and return rate, and reaction scale, a two-stage coupled reaction system consisting of a vortex reactor and a propeller reactor is proposed.
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