Abstract <p>The existing technologies of processing low-grade iron ores by coke-free metallurgy include reduction roasting in a rotary kiln, crushing the resulting concentrate and magnetic separation to form a product suitable for steelmaking processes. The processing technology for siderites previously proposed by the authors of this work involves roasting lumps of ore along with a solid reductant in a rotary kiln, charging the resulting hot metallized concentrate (<i>t</i> &gt; 1000°C) into an electric furnace, and separation melting at 1600°C. This technology eliminates the need for grinding and magnetic separation operations. To form a liquid slag, calcined colemanite containing boron oxide is added to the charge. During melting, part of the boron transfers into the metallic melt. In the present paper, thermodynamic modelling is used to estimate the influence of the metallization of the calcined siderite concentrate (φ_Fe = 75–95%) and the fractions of colemanite (5, 10%) and residual carbon (0–6%) in the charge on the element distribution between the metal and the slag upon separation melting. It is shown that, when carbon is present in the charge, the final product of melting is a metallic alloy, which contains carbon, silicon, manganese and boron in addition to iron. The greater the amount of carbon fed to the furnace, the higher its content in the alloy. When a concentrate with 95% metallization and 5 or 10% colemanite in the charge is melted, up to 55% of boron transfers to the metal depending on the fraction of carbon, and its concentration in the metal increases from 0 to 1.0%. Such a metal can be used as a master alloy for producing boron-containing steel or cast iron. A decrease in the metallization of the concentrate to 75% allows the production of a metal containing less than 0.001% B at less than 2% carbon in the charge, and this metal is suitable for the direct production of boron-containing steels in a furnace–ladle unit. In other cases, the metal can be used as a master alloy.</p>