Due to the known limitations of economic nature, the global production complex continues to focus on the most accessible technologies of alumina production, leading to the accumulation of solid waste (red mud), which has created the well-known problem of their processing, and its scale and importance continue to grow in connection with the steady increase in the world volume of aluminum production and consumption. The priority of rational use of natural resources allows us to speak about the preference of principles of deep and waste-free processing of mineral raw materials, which are fully applicable to the processing of red mud, as accumulated raw materials of technogenic origin. At the same time, the key point of this approach becomes the hierarchical division of components into groups of technological products, taking into account the processes that ensure this selective-group division. In this regard, the use of carboxylic acids is of notable interest, as they have a known selectivity to the main components of red mud, which allows to separate low-soluble components (iron and silicon compounds) from light and rare-earth metals with minimum consumption rates. An experimental study has shown that flow-through leaching of thermochemically prepared red mud using formic acid provides high and acceptable recoveries of sodium, calcium, scandium, and rare earth metals. This makes it affordable to use the cheapest method of apparatus-free leaching when implementing this approach on an industrial scale. Experimentally determined the differences in the sequence of elution of sodium, calcium, aluminum and rare-earth metals formates during flow leaching, which creates favorable conditions for concentrating components and separate processing of sampled solutions. The regime parameters of the process that provides complete precipitation of aluminum, scandium and rare earth metals during the neutralization of formate solution with milk of lime, resulting in the necessary prerequisites for further concentration of rare earth metals to obtain technologically significant product are determined.

The paper determines the indicators of the developed process flow for the complex processing of red sludge via the formate method: recovery of components; yield and composition of products when processing a sample of formate solution obtained from red mud leaching. The conducted experiments used red mud generated in the production of alumina at the Urals Aluminium Smelter. The samples of formate solution obtained in the course of red mud leaching were analyzed using an Optima 8000 ICP-OES Spectrometer, a Sartorius MA-30 Moisture Analyzer to measure moisture content, as well as an ARL 9800 XRF Spectrometer to ascertain the mass fraction of elements in metal and nonmetal specimens found in one of three states (solid, liquid, or powder). These experiments were performed while continuously measuring and monitoring pH values by means of a pH meter having a thermal compensation function. The performed experiments involved the total recovery of valuable elements from formate solutions produced during red mud leaching. A concentrate containing Al, Sc, and rare earth elements (REEs) was processed to produce scandium oxide and rare earth metal concentrate (after dissolving aluminum in an alkali). Rare earth metals and scandium were shown to concentrate in the solid phase; scandium was then selectively leached with a sodium bicarbonate solution to form water-soluble carbonate complexes [Sc(CO3)4]5- having carbonate ions СО32- and НСО3-. When using the proposed technology, the overall recovery of scandium and REEs amounts to 98–99%, whereas that of aluminum, calcium formate, and sodium formate from the produced solution reaches 99%. The processing of formate solution yields the following end products: scandium oxide (99 wt% Sc2O3) and REE concentrate (content of 56.1%). The paper demonstrates the possibility in pri nciple to process solutions obtained from the flow-through leaching of red mud via the formate method.