## 1 Introduction  
Aluminum is the second most used metal in the word and it is becoming extremely depleted: the global production of bauxite amounted to 390 Mt and annually rises at 4.7%, meaning that 30 Bt of world-total reserves of bauxites and even 55-75 Bt of global bauxite resources [1] will be completely exhausted from 2052-2070. The interest of specialists from many countries in acid and salt methods of processing alumina-containing raw materials is related to a gradual decrease in the availability of high-quality bauxites, which can be easily treated by standard alkaline methods, in particular, the Bayer process. A distinctive feature of acid methods is the possibility of processing high-silica raw materials by separating silica straightway at the leaching stage. This makes it possible to significantly expand the range of raw materials potentially of interest for industrial processing. At the same time, the disadvantage of acid and salt methods is the need to use additional technological stages for deep purification from iron which leaches into productive solutions along with aluminum. Other limitations of acid methods, especially with using sulfuric or nitric acids, have difficulties creating closed-loop processes with the recovery of reagents and using them in repetitive technological cycles. Taking this into account, various options of hydrochloride [2; 3; 4; 5] and hydrosulfate [6; 7; 8; 9] methods can be considered as the most promising today. A detailed review has recently been published on acidic and hydrosulfate methods for the processing of alumina containing wastes of coal fly ash [10]. The ammonium hydrosulfate (bisulfate) process was invented by [PERSON] ([PERSON]) in Hannover-Klefeld in 1921 [11] and tested in Germany in the 1920s and in Oregon in 1944. Now, this process is included in the list of named ones in chemical technology and is called \"Aloton\" or \"Buchner\" [12]. The process consists of the stages shown in Figure 1. The main stages of the process: (1) treating aluminum-containing raw materials by the hot concentrated solution of ammonium bisulfate in an autoclave; (2) separation of insoluble residues, for example, by filtering with the obtaining of the solution of ammoniumium; (3) precipitation of aluminum hydroxide from this solution with the use of ammonia; (4) separation of this semi-product from the remained solution of ammonium sulfate; (5) evaporation of this solution with obtaining water, crystallization and drying ammonium sulfate; and (6) thermal decomposition of (NH4)2 SO4 with obtaining ammonia and NH4 HSO4 to dissolve in water and using at the head of the process. The variants of the Buechner method exist with the first stage carried out not by the \"wet\" method, but by sintering ammonium sulfate with raw materials [13; 14; 15]. It can be assumed that at the time of [PERSON] with good access to high-quality bauxite raw materials, his process could not compete with [PERSON]'s one. Moreover, it turned out that the method is more suitable for the processing of easily degradable clay materials and other aluminosilicates than for bauxites [16]. In the last decade, studies by Chinese specialists in the production of alumina from fly ash [10; 17] have shown the possibility of industrial implementation of combined technologies based on the first stage of sintering ash with ammonium sulfate or hydrosulfate. Today, in the new conditions, it seems appropriate to return to the classical Buechner process to analyze its weaknesses and assess prospects. In recent years, we have been intensively studying the Buechner method, taking into account its weakest points, and have proposed some innovations for more efficient processing of low-grade alumina-containing raw materials [7; 8; 18].