Salt recycling from fly ash: Process design and integration with a waste-to-energy plant

Abstract

Fly ash is a by-product from waste-to-energy plants and is classified as hazardous waste due to its content of chlorides and heavy metals. Most of the generated fly ash is disposed of in special landfills for hazardous waste, which is expensive, requires transportation over long distances and is not sustainable in terms of a circular economy. Therefore, more sustainable ways of handling fly ash are being requested on the market. HaloSep AB, part of the Stena Metall Group, has developed an on-site fly ash recycling process that converts fly ash into a non-hazardous product. After treatment in several wet-chemical steps, chlorides from the fly ash end up in a brine as dissolved chloride salts, mainly CaCl2, NaCl and KCl. Plants located close to a saltwater recipient can release the brine into the environment, but it is not possible for inland plants. An alternative way of handling the brine is therefore needed for inland plants. The purpose of this thesis is to evaluate different salt recycling processes for recycling salt in the HaloSep brine. A literature study is performed where existing salt recycling methods and applications of salt are reviewed. Based on the results from the literature study, a salt recycling process is designed and simulated in Aspen Plus. Further on, heat-integration with an existing waste-to-energy plant is performed and the net energy loss in the plant is evaluated. A sensitivity analysis is performed to see how the concentration and composition of the brine affect the outcome of the process. A brief economic analysis is also done to determine when the process is motivated. The findings in the literature study indicate that an evaporation-based process is the most suitable process to recycle the salts in the brine. The proposed design has two evaporation stages operating below atmospheric pressure, 0.65 bar and 0.14 bar, and results in three product streams that could be used as road salt for de-icing, dust suppression, or be further processed into purer salts. The net energy loss in the waste-to-energy plant is between 0.9-1.4% in electricity and 0-2.4% in heat, depending on the design conditions. The process is financially profitable considering the combination of revenue from energy costs and income generated from salt products.