The paper identifies the effect of temperature recovery on the iron slag output in the steel industry, the value of which is recommended to take in the range of 1600 – 1650 °C. Laboratory studies were carried out in an oven Tamman resistance. Studies have shown that an increase in the time delay interval in the molten slag 1300 – 1650 °C higher than the degree of metal removal. With the use of statistical processing dependences for determination of content of FeO content in the converter and open-hearth slag were received.