In this paper, the brightness of fly ash is improved by carbon removal by heating and iron-containing oxides removal by acid treatment using a two-step method to realize the application of fly ash as filler or coating in the future, which not only increases the range of resource utilization of fly ash, but also reduces dust pollution. The modification results show that the brightness of fly ash reaches the maximum value of 38.27% ISO (Brightness unit) after decarburization by heating at 600 °C. On this basis, the Box–Benhnken design scheme is adopted to optimize the brightening process for removing iron-containing oxides in fly ash. Finally, when the concentration of hydrochloric acid is 15%, the acid leaching time is 0.75 h, the reaction temperature is 74 °C, and the brightness of fly ash can eventually increase to 43.92% ISO.

Improving the utilization rate of fly ash (FA) and realizing its resource utilization has always been a popular research topic in the application field of solid waste recycling. This paper aims to improve the basic brightness of fly ash (FA) before calcium carbonate coating, and realize the filling application of fly ash in medium and high-end paper. Taking advantage of the high brightness of molecular sieves (MS) (brightness ≥ 90 %ISO), the alkali melting hydrothermal synthesis (AM-HS) method was used to generate high brightness MS on the surface of fly ash. The results showed that the optimum conditions of alkali melting hydrothermal synthesis (AM-HS) modification were as follows: the mass ratio of sodium hydroxide to fly ash was 1:1, the time of hydrothermal modification was 6 h, the temperature of hydrothermal modification was 80 °C, and the brightness reached to 64.54 %ISO; The results of SEM, particle size analysis and ultrasonic vibration showed that the surface of fly ash is covered by MS and has strong binding effect.

In order to solve the environmental problems caused by landfilling and incineration of Fenton sludge, realize the recycling of papermaking solid waste. In this paper, we explored the formation of a layer of calcium carbonate coating on the surface of Fenton sludge by crystallization coating in the Fenton sludge-[Ca(OH)2/H2O]-CO2 three-phase reaction system to improve the brightness of Fenton sludge and apply it to the filling of paper. The effects of particle size, temperature, stirring rate and calcium hydroxide addition on crystallization coating effect were studied. The advantages and disadvantages of the coating effect are mainly judged by the brightness and dustiness of the powder material. The research results show that when the particle size is 38–48 μm, the temperature is 25 °C, the stirring speed is 600 r/min, and the ratio of calcium hydroxide to Fenton sludge is 2:1, the brightness of the composite can be increased from 3.56 % ISO to 64.24 % ISO.