Indirect determination of the melting temperature in aluminum scrap furnaces

Abstract

Based on the analysis of literary sources, it is shown that the production of secondary non-ferrous metals, and aluminum in particular, is very efficient since the costs of raw materials and materials for their production are much lower than for the production of primary metals. For the production of 1 ton of secondary aluminum, of which about 100,000 tons are produced in Ukraine, about 2,000 kWh of electrical energy is consumed, which is 7-9 times less than for the production of aluminum by the electrolytic method. At the same time, emissions of hydrogen fluoride and resinous substances into the atmosphere, which occur during the production of primary aluminum, are virtually eliminated. Furnaces heated by the burning of gaseous or liquid fuel, as well as furnaces with electric heating, are used for melting aluminum scrap, curing and processing the obtained aluminum. Previous authors’ research concerned the study of temperature and thermal conditions of operation of a stationary furnace for remelting aluminum scrap with electric and gas heating. The working space of the furnace is made in the form of a lined rectangular chamber with a floor, side and end walls, and a vault. The chamber has two zones: the lower one, where the aluminum is located after melting the loaded scrap, and the upper one (without melt). According to the technology, aluminum scrap is first loaded into the furnace through open prechambers, then external heating devices are turned on and the solid material is heated and melted, and then the molten metal is brought to the required temperature, after which steel and other impurities are removed from the working space through prechambers and perform casting of products. After the aluminum casting is complete, the cycle is repeated. Measurement of the melt temperature is usually carried out by thermocouples placed in stainless steel covers or ceramic covers, which, after melting the aluminum, are inserted into the working space of the furnace through holes in the vault. It is noted that the service life of such thermocouples is quite limited due to the destruction of the stainless steel covers upon contact with the melt, and the ceramic covers are quite fragile. That's why a method of an indirect determining the temperature of the melt, which will allow constant control of this temperature and save money on the manufacture of thermocouples in covers, was developed. The results of calculation studies on the determination of the heat flux density and the temperature of the outer surfaces of the side and end walls of the furnace for remelting aluminum scrap in the melt temperature range of 670-750 ºС that were performed using the software developed by the authors, are given. The results of calculations for all options with the relevant specified materials and thicknesses of individual layers of fences, heat transfer surface area and melt temperature tm included the heat flow density q, W / m2; wall temperature of the outer surface of the enclosure tw, ºС; temperature difference Δ tw = tm - tw, ºС; coefficient of heat transfer from the aluminum melt to the outer surface of the enclosure km = (q / Δ tw), W / (m2∙K). On the basis of calculated data, an approximating dependence q = f (tw) was obtained, the maximum deviation of which does not exceed 4,6%. The dependence of the wall temperature of the outer surface of the enclosure on the heat transfer coefficient was established, the dependence km = f (tw) was built, and the maximum deviation of the calculated data does not exceed 5,6%. Practical recommendations are provided regarding the operational determination of the temperature of the melt in the furnace in the cycle of melting aluminum scrap, according to which, after melting the aluminum, it is necessary to measure the temperature of the outer surface of the wall tw, ºС, at two or three points, for example, using chromel-alumel thermocouples for this purpose. In the future, it is necessary to determine the heat flow density q, W/m2 and the heat transfer coefficient km, W / (m2∙K) according to the corresponding graphs or the approximate dependencies shown on them. The temperature of the melt in the furnace, ºС, is determined by the equation tm = tw + (q / km). Before pouring aluminum melt from the furnace, it is possible to measure its temperature for a short time with a standard thermocouple in the cover.