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We have prepared Ca 3 Co 3.85 M 0.15 O 9 + δ (M = Ti-Zn, Mo, W, Pb, Bi) solid solutions, investigated their crystal structure and microstructure, assessed their thermal stability in air, and measured their thermal expansion, electrical conductivity, and thermoelectric power in air at temperatures from 300 to 1100 K. The results demonstrate that the Ca 3 Co 3.85 M 0.15 O 9 + δ cobaltites are p type semiconductors and that their unit cell parameters decrease with an increase in the number of d electrons in the 3d transition metal ion and with an increase in the average oxidation state of the cobalt. Their thermoelectric power increases with increasing temperature, reaching the highest value in the Ca 3 Co 3.85 Pb 0.15 O 9 + δ solid solution: 380 μV/K at a tempera ture of 1100 K. The Ca 3 Co 3.85 Bi 0.15 O 9 + δ solid solution has the largest thermoelectric power factor among the materials studied, 206 μW/(m K 2 ) at a temperature of 1100 K, which is twice the power factor of the unsub stituted calcium cobaltite Ca 3 Co 4 O 9 + δ .

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BACKGROUND At present, the volumes of use of electric arc furnace (EAF) dust are extremely insignificant. The volumes of their formation amount to thousands of tons per month. During storage, they have a negative impact on the environment. At the same time they are a material that has a valuable composition (oxides of iron, non‐ferrous metals, etc.) and is promising for recycling. However, the secondary use of zinc‐containing EAF dust in ferrous metallurgy leads to the accumulation of zinc in the lining of blast furnaces and frequent malfunctions. RESULTS It has been established that zinc oxide is mainly in the bound state in the composition of the franklinite phase (Zn,Mn,Fe)(Fe,Mn)2O4. The processes of solid‐phase reduction of EAF dust with the production of zinc concentrate and sponge iron, which are in demand as raw materials for non‐ferrous and ferrous metallurgy, have been studied. The optimal conditions for the reduction process were determined and the physicochemical characteristics, granulometric and phase composition of the initial materials, and reduction products were investigated. CONCLUSIONS It has been established that the use of a combination of reducing agents of different nature (coal coke and H2) during heat treatment at 1100 °C and cooling in a reducing medium (Ar/H2, 5 vol%) makes it possible to obtain sponge iron with impurities (the highest degree of iron metallization was 97.5%), and zinc oxide with a basic substance content of about 98.4 wt.%. © 2022 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).