## 1 Introduction  
There is an extensive utilisation of unconventional raw materials in building industry. For instance, in brick production, phosphate sludge is used with rare earth elements (REE) added [1]. According to the data of the work [2], phosphate sludge containing fluorapatite, calcite, and clay, after temperature treatment from 900 to 1100 \({}^{\circ}\)C, is used for the production of fired bricks. Improved porosity, water absorption, and flexural strength are the resulting product features. Nanostructured composite systems of REE are used when forming cover of construction materials [3]. Obtained coatings which contain REE and transition elements radically change the properties of materials. This is due to photocatalytic and hydrophilic properties of coatings, as well as due to combined methods of their activation. The presence of REE in the form of nanoparticles in the composition of coatings improves the quality of welding, arc stability, weld hardness, and mechanical properties [4; 5]. When producing structural steel, alloying with REE (La, Ce, and Nb) after the smelling stage in the furnace makes it possible to increase the yield of the finished product and reduce the quantitative content of inclusions such as Al and Mg. Conducting extensive studies in target components flotation recovery is directly linked to the detailed analysis of resource-saving technologies' optimization techniques [6; 7; 8], the processing of technogenic hydromineral raw materials, concentration of diluted work solutions. Reception and application of rare earth element compounds, and the recovery of target components from heterogeneous systems, factors which all have a significant impact on flotation recovery of REE compounds from aqueous solutions [9; 10; 11]. To develop a resource-saving technology of lean rare earth raw materials and low concentration wastes of hydrometallurgical production processing [12; 13; 14], one should primary study the physical-chemical and technological properties of target components and studied processes, which was one of the tasks of the authors. An increase in role of high-technology production in the world [15; 16] permits predicting a significant increase in demand on rare earth raw materials, which in turn should cause the REE production increase. Today, it becomes profitable to invest in rare earth metals production. The largest sphere of rare earth metals consumption is the production of various types of catalysts: (1) catalytic reactors for vehicle exhaust gases, which are based on CeO\({}_{2}\) and have the capability of transforming carbon dioxide, unburned hydrocarbons, and nitrogen oxide to carbon dioxide, water, and nitrogen; and (2) industrial catalysts for oil processing and chemical industries [17; 18]. Permanent magnet production is the most dynamically developing rare earth metals consumption sphere. The application of NdFeB magnets is possible in motor, micromotor, and high-power motor production. Moreover, they are utilised in computers, pumps, compressors, and peripheral devices [19; 20]. The rare earth elements reserves in Russia are concentrated to a large degree in Murmansk Oblast (Region), Republic of Sakha (Yakutia), Krasnoyarsk Krai (Territory), and Irkutsk Oblast (Region). The following minerals are rich in rare earth elements: bastnaesite--Ln(CO\({}_{3}\))F (70-75%), monazite--LnPO\({}_{4}\) (55-60%), loparite--NaCaLn(TiO\({}_{3}\))\({}_{2}\)(NbO\({}_{3}\))\({}_{2}\) (30-35%), and xenotize--(Y, Eu, Gd)PO\({}_{4}\) (55-60%). As a rule, the average content of the mixture of rare earth elements is not to exceed 0.4%. Due to high production costs, rare earth elements are not recovered from the mined ones. In ore processing, plant ores are used to gain concentrates of target components. The type of concentrate will depend on the ore deposit and content of this component. REE remain in mill tailings, stockpiled in concentrating mill heaps [21; 22].