## 1 Introduction
The first improvement consists of optimizing the leaching conditions and phase separation in the first stages 1-4 shown in Figure 2. We found that, after leaching at elevated temperatures, two stages of such separation should be carried out: hot filtration at a temperature above 90 \({}^{\circ}\)C and cold filtration at room temperature. During hot filtration, undecomposed residues are separated. Subsequent cooling leads to the selective and quantitative separation of ammonium alum with a small impurity of iron. Figure 1: Principal flowchart of the Buchner circular process. The second change in the Buchner method consists of the redissolution of alum in pure water (stage 5) and the implementation of the sorption process of deep purification of the resulting solution from traces of iron. Finally, another addition to the Buechner process is the use of the acid retardation method (Step 6) to separate the sulfuric acid and acidic ammonium salt remaining after cold filtration from the neutral salt [19,20]. This allows the use of an excess solution of the leaching agent--ammonium bisulfate, but at the same time, do not consume superequivalent amounts of ammonia at the stage of cleaning the solution from iron, which is returned to the head of the process. The actual process is more complex, for example, before crystallization at stage 11, and the solution containing an admixture of soluble alkali metal sulfates is cooled after evaporation to insulate potassium and sodium sulfates, and only then it is evaporated to dryness [7,18]. To maintain sulfate balance in a closed circuit, consumable sulfuric acid (around 1%) is additionally supplied to the leaching agent. In addition, water washing operations are used at the separation stages. For simplicity of presentation, these details are omitted, since they do not affect the fundamental issues discussed below. This article is devoted to the study of the possibilities for further improvement of the Buechner method, in particular, on the example of the processing of nepheline concentrate. Nepheline is a by-product of critical raw materials mining from alkaline rocks massits, like apatite (source of P\({}_{2}\)O\({}_{5}\) and REE\({}_{2}\)O\({}_{3}\)), loparite (source of Nb\({}_{2}\)O\({}_{5}\), REE\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), Ta\({}_{2}\)O\({}_{5}\)) and, highly likely in the nearest future,--eudialyte (source of ZrO\({}_{2}\) and REE\({}_{2}\)O\({}_{3}\)). For this purpose, the following tasks were set: (a) to study in detail the distribution of various macro- and meso-components of leaching between phases at the stage of isolation of ammonium alum depending on varied conditions; (b) to explore the possibilities of deeper purification of aluminum double salts from iron impurities in the processes of their  
Figure 2: Principal flowchart of the closed-loop process for alumina production from low-grade (high-silica) raw materials by the hydrosulfate method. crystallization, as well as to understand the effect of various chemical additives on these processes; and (c) to find out methods for isolating purer final products. All stages of the bisulfate processing nepheline, for which the experimental and theoretical studies were carried out in order to modify these stages, are highlighted in a different color in the diagram in Figure 2.