## 1 Introduction  
The primary method for processing tungsten raw materials in countries such as China [1, 2], Russia [3, 4], the Democratic Republic of the Congo, Canada, and the USA [5] is the roasting of concentrates with Na\({}_{2}\)CO\({}_{3}\) followed by the leaching of the calcined product. Another widely used method is the autoclave leaching of the concentrate with an alkaline solution to produce soluble sodium tungstate (Na\({}_{2}\)WO\({}_{4}\)) [6]. Further processing of the sodium tungstate solution [7; 8; 9] involves the sequential removal of impurities through the formation of insoluble compounds, which are subsequently separated by filtration. In China and Russia, this method [2; 8] is the main approach used for extracting tungsten from ores containing molybdenum and other metals. The conventional scheme for processing sodium tungstate solutions [10; 11] includes the following stages:  
* Sequential removal of sodium salt impurities, such as silicon, phosphorus, molybdenum, arsenic, and additional silicon;
* Precipitation of calcium tungstate (CaWO\({}_{4}\), synthetic scheelite) [12], followed by acid decomposition of the precipitate to produce H\({}_{2}\)WO\({}_{4}\);
* Dissolution of tungstic acid in aqueous ammonia [7; 13], evaporation of the resulting solution, and crystallization of ammonium paratungstate (APT) [14; 15; 16];
* The final stage of the process involves the calcination of ammonium paratungstate to obtain high-purity tungsten trioxide. Ammonium paratungstate can also be directly recovered from sodium tungstate solutions via extraction [17] or ion-exchange conversion of Na\({}_{2}\)WO\({}_{4}\) solutions into (NH\({}_{4}\))\({}_{2}\)WO\({}_{4}\). In [18], the authors present results on the processing of sodium tungstate solutions obtained by the autoclave leaching of wolframite concentrate with soda. The study investigated parameters such as solution acidity, type of mineral acid, and the presence of complexing and seeding agents. The sodium tungstate solution was acidified with concentrated nitric acid in a molar ratio of HNO\({}_{3}\):W = 12:1. The use of nitric acid enables a high degree of tungsten precipitation in the temperature range of 75-95 \({}^{\circ}\)C.  
In [19], a technology for processing tungsten-containing waste with subsequent production of sodium tungstate solutions is presented. The waste underwent mechanical treatment and was leached with the addition of sodium hydroxide. The productive solution was then directed to the impurity removal stage. The sorption process was carried out using the anion exchanger MP-62, followed by washing with nitric acid, while extraction purification was performed using an ammonia solution. The study [20] describes a method for investigating the sorption recovery of tungsten from tungsten-containing solutions using a flocculant (polyacrylamideamide) and the anion exchanger AV-17-8 in a ratio of 1:8 (ion exchanger to solution) in pressure sorption columns. In the work by the authors [21; 22], a method is described for obtaining sodium tungstate by smelting a charge composed of wolframite, Na\({}_{2}\)CO\({}_{3}\), and SiO\({}_{2}\) at a temperature of 1100-1300 \({}^{\circ}\)C. After smelting, sodium tungstate was separated from the slag, which contained excess sodium oxide, flux, and unreacted wolframite. The slag was additionally subjected to leaching to extract the remaining sodium tungstate. The resulting product contained 78% WO\({}_{3}\) and was presumed to be subsequently directed for electrolysis. In industrial processing, tungsten concentrates are sintered with sodium carbonate (Na\({}_{2}\)CO\({}_{3}\)), followed by the aqueous leaching of the sintered mass. This yields alkaline sodium tungstate (Na\({}_{2}\)WO\({}_{4}\)) solutions containing 110-150 g/L WO\({}_{3}\) and exhibiting a high pH in the range of 12-14 [11].