A review on the demineralisation of pre- and post-pyrolysis biomass and tyre wastes

•Demineralisation in combination with pyrolysis to valorise biomass and tyre waste.•Biomass to be demineralised before pyrolysis to improve the target products.•Tyre char to be demineralised after pyrolysis to improve char value.•Water is the best solvent for weakly bonded metals: cheapest and easiest disposal.•Strong acid (or base) to remove strongly attached minerals can affect the structure. Pyrolysis is an attractive technology to convert low-cost carbonaceous waste materials into fuels, energy and other value added products goods. During pyrolysis, the inorganic minerals present in the feedstock can cause problems to the equipment and give side reactions. Besides, the minerals present in the chars can hinder their possible applications. Therefore, it seems necessary to eliminate said contaminants in order to valorise the aforementioned goods. Demineralisation is a process widely used for purifying materials that are contaminated with inorganic matter. Although this technique is commonly used with waste materials that will undergo pyrolysis, or the products obtained from it, the studies analysing this practise are rather scattered. The aim of this paper was to compile and review the current literature concerning the demineralisation of carbonaceous waste (tyres and lignocellulosic biomass) materials. The chemistry involved, feedstock type and the effect of performing the purifying step before or after pyrolysis were addressed in this work. The review revealed that biomass samples should be demineralised before pyrolysis in order to affect not only the char but also the bio-oil quality. Depending on the form in which the minerals are linked to the structure, the solvent chosen will vary (from water to strong acids). However, water is the most popular option due to its price and easy disposal. In tyres, demineralisation should be performed after pyrolysis using strong acid and subsequently base. Due to the crosslinked chemical structure, rubber is highly resistant to chemicals thus the pre-treatment has to be avoided.