Waste Recycling in an Integrated Melting Furnace for Stone Wool Production

Abstract

Stone wool is a fibrous material made from the spinning of a 1500 °C melt into fibers. The material is mainly known as an insulation material with excellent fire properties. As there is a growing global focus on limiting energy consumption and the release of greenhouse gasses, the use of building insulation material has been growing. As the use of stone wool grows, so does the amount of stone wool waste generated either during production and installation or during the renovation and demolition of buildings. Stone wool waste can be recycled back into the stone wool production process indefinetly without loss of product quality. The melt needed for the stone wool production is made in a melting furnace. The ROCKWOOL Group currently utilizes three different melting technologies: traditional cupola furnaces, electrical furnaces and a newly invented cyclone-based technology known as the integrated melting furnace (IMF). The IMF comprises two pre-heater cyclones and a melting cyclone. In the melting cyclone combustion of coal and natural gas takes place, and the pre-heated materials are fully molten and exits the cyclone as a 1500 °C melt. The IMF is able to handle both a conventional mixture of raw materials, known as the charge, as well as recycled stone wool waste. The only pre-processing needed for recycling of the stone wool waste is a coarse milling. The purpose of this study is to investigate the effect of introducing stone wool waste into the IMF. Special focus is given to the influence on the overall energy consumption and the material efficiency of the IMF cyclones at high recycling rates of stone wool waste. An experimental laboratory study is carried out to investigate the fundamental material differences between the conventional charge and the stone wool waste. The stone wool waste proves to initiate melting at lower temperatures than the conventional charge. It is also shown, that the stone wool waste requires around 20% less energy for heating and melting than the conventional charge, due mainly to the fact that the conventional charge experiences an energy consuming release of gasses during heating, whih is not seen in the stone wool waste. The lower energy for heating and melting makes stone wool waste a desired material in the production for both environmental and energy efficiency purposes. In cyclone technologies, ideally all material exits the cyclone as melt, however some fraction of material escapes entrained in the flue gas. The entrained material flux is thus a measure of the material efficiency of the melting cyclone. For the IMF the entrained material flux out of the melting cyclone has shown to cause problems with build-up due to the sticky nature of the material. The introduction of high rates of stone wool waste into the IMF has been experienced to increase the risk of problematic build-up. In order to investigate the influence of stone wool waste recycling on the material efficiency of the IMF, a series of measurements are carried out at an industrial IMF plant while in operation. Different probes are used in order to quantify the amount of material that escapes the melting cyclone entrained in the flue gas. The material efficiency of the IMF melting cyclone is studied under various process settings and levels of wool waste recycling. The wool waste recycling has not been seen to effect the material efficiency of the melting cyclone. Finally, a mathematical model is constructed to model the IMF mass streams. The model predicts the rate and particle size distribution of the particles entrained in the IMF flue gas. The model points to the material properties of the feed to be a key parameter regarding the material efficiency of the IMF cyclones, whereas little sensitivity towards other process parameters such as gas flow rates and material load is seen. The main driver of the carry-over is seen to be the particle size distribution of the solid materials. The model has not been able to replicate the experimental data. When modeling the experimentally investigated IMF process settings, a vast under-prediction of the material efficiency of the IMF melting cyclone at high stone wool waste dosing rates. This is due to the fine size distribution of the stone wool waste, and a main conclusion of this work is therefore, that the stone wool waste acts with an effective size distribution inside the IMF system, that is larger than what is expected from the size distribution of the stone wool waste fibers themselves.