Metal mining focusing on zinc from waste fly ash by means of acidic process water

Syftet med projektet är att behandla flygaskan med hjälp av surt processvatten för att utvinna olika metaller, som zink, så att det sedan kan deponeras på ett enklare och billigare sätt, samt även at utveckla en metod för att utvinna mer zink med minsta möjliga andel surt processvatten (5 % HCl). Olika metaller med fokus på zink studerades i labbskala på Högskolan i Borås. Flygaskan och det sura processvattnet som har använts i denna studie är från Renova ABs anläggning. Resultatet visar att vid behandling av flygaskan med surt processvatten var den maximala mängden zink som utvanns 88% under de olika försöken som gjordes i labbet. Det finns olika faktorer bakom resultatet, såsom halten av surt processvatten, pH, blandningstiden och askans elementsammansättning, vilka kan påverka halten utvunnen zink.Sweden produces large amount of fly ash from waste combustion annually. Combustion the waste generates two types of ashes, bottom ash/slag, and fly ash. Bottom ash is considered to be more environmentally friendly and has a wide area of application such as road constructions on landfills. While the fly ash has high level of heavy metals and for instance dioxines which are harmful for the environment. This type of ash are deposited in a landfill and gets classified as a hazardous waste which is expensive given that the deposit fee is high. The fact that fly ash contains valuable metals as zinc, methods of recovering it are being developed such as acid leaching. This is a promising method as the leachate used are acid process water making the usage of the chemicals substantially lower which makes it financially viable. Waste incinerators in Sweden produce approximately 200 000 tons of fly ash annually and the majority of it is transfered to Norway to get treated and put on a landfill. Some waste management companies, e.g. Renova, are using another method called “the Bamberg method” where fly ash is mixed with sludge to form a cake and put in the company’s own landfill. The projects goal is to leach the fly ash by using acidic process water to obtain the metallic substance particularly zinc making it easier and cheaper to landfill the ash and also to optimize this method to get the most zinc out of the ash using minimum amount of the acidic process water (5% HCI). The laboratory work took place in the University of Borås. Fly ash and the acid process water which were used under the laboratory work was obtained from RenovaAB. The results shows that leaching the fly ash with acidic process water gave different release of zinc but was 88% at most. The variation in the results depends on a few factors such as amount of acidic process water, pH, time, blending time and the ashes content. This project took environment and access to acidic water in consideration, which optimized the method of using less amount of acid process water to get the most zinc as possible. The results shows that leaching fly ash with acid process water is cost efficient and easy way to recover zinc, which satisfy the goals of the project