Fly ash is a mineral waste material generated during the combustion of coal, which constitutes the largest percentage of energy waste. Fly ash is used for many purposes, primarily in ceramics, construction, road construction, agriculture, as well as in mining (backfilling), etc. Annually, 750,000,000 Mg of ash is produced in the world, in Poland it is about 4 million Mg. Only less than 30% of this amount is used.

A very important component of fly ash are the so-called Rare Earth Elements (REE - Rare Earth Elements). They mainly belong to the group of lanthanides - 15 elements. Additionally, they include scandium and yttrium. These 17 elements are widely used in everyday life. Computers and cell phones would not exist without them. Their deposits on Earth are very irregularly distributed. In the twentieth century, they were mined mainly in California and Australia. As of today, over 90% of world demand is covered by China. Due to the development of technology and, consequently, the demand for REE, which increases every year, new sources of their acquisition should be sought. The lack of universal availability also affects the prices of rare earth elements, which are often subject to very sharp increases.

Taking into consideration China's monopoly, shrinking REE deposits and their growing use in modern technologies, there is a need to find alternative sources of these elements. It has been found that fly ash from coal combustion can become a very interesting secondary source of REE.

Although there are ores more abundant in REE than fly ash all over the world, they often contain a small amount of the most valuable elements such as neodymium and scandium. This, in turn, is a sufficient condition for the process to be financially attractive. Obtaining REE from Polish ashes is profitable because they contain a high concentration of the most valuable elements.