Original Article

Carbothermic reduction of hot rolling sludge by rice husk char with microwave heating

¹ Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
² Yamato Kogyo Co., Ltd., 380 Yoshimi, Otsu-ku, Himeji, Hyogo 671-1132, Japan

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 September 2025
Accepted: 7 January 2026

Abstract

The scale generated in the hot rolling process is a high-grade iron source. Fine scales are separated in a thickener and discharged as sludge in a slurry state, which makes them difficult to use. This study focused on sludge recycling through carbothermic reduction using microwave heating. Microwave heating occurs when an object absorbs microwaves. Compared with conductive heat transfer, microwave heating allows for internal heating, which makes it effective for heating fine powders with low thermal conductivity. To achieve carbon neutrality, the carbothermic reduction behavior was investigated using biomass-derived carbon. For the experiments, industrial sludge, rice husk char as biomass and reagent-grade graphite were used. The sludge had a composition of T. Fe = 64.1 wt.% and FeO = 36.2 wt.%. The biomass was obtained by pyrolyzing rice husks at 500 °C, containing 43.2 wt.% carbon with 47.7% SiO₂ as the main impurity. The biomass or graphite and sludge were mixed in a molar ratio the carbon and oxygen content in the sludge matched C/O = 1. Microwave heating was carried out with 2.45 GHz multimode irradiation with magnetron under a nitrogen atmosphere. The total heating time was 20 min, with a maximum output intensity of 1050 W. The samples were heated from room temperature to 1200 °C and then maintained at this temperature. After heating, the formation of metallic iron was confirmed in both samples. During microwave heating, the sample using biomass took longer to stabilize at 1200 °C compared with graphite. It seems due to the SiO₂ in the biomass not absorbing microwaves. SEM-EDS analysis of the reduced samples confirmed the presence of metallic iron and residual FeO in both samples. Additionally, in the biomass sample, a large amount of silicate slag containing iron oxides was observed.