# Sintering with Sodium Carbonate and Leaching of Wolframite Cakes
## 3 Results and Discussion
During water leaching of cakes for firing temperatures of 800 and 850 \({}^{\circ}\)C, similar dependences of the degree of tungsten extraction into solution (in terms of WO\({}_{3}\)) are obtained, shown in Table 6.  
_Balance experiments for cake leaching._ Experiments were performed with 100 g weighed cake samples and an LS ratio of 2:1 to assess the material balance of cake leaching. An approximate calculated balance of leaching of cake obtained at a charge firing temperature of 750 \({}^{\circ}\)C is specified in Table 7. \begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Factor Influencing Leaching, Interval** & **Function Type** & **Approximation Coefficient** \\ \hline \multicolumn{4}{c}{Leaching dependences for sinter obtained at 800 \({}^{\circ}\)C} \\ \hline
1 & Temperature, 75–95 \({}^{\circ}\)C & y = \(-\)0.0024x\({}^{2}\) + 0.5477x + 68.054 & R\({}^{2}\) = 0.9998 \\ \hline
2 & Duration, 30–75 min & y = \(-\)0.0013x\({}^{2}\) + 0.1941x + 89.952 & R\({}^{2}\) = 0.98 \\ \hline
3 & Particle size, 100–815 microns & y = \(-\)9 \(\times\) 10\({}^{-6}\)x\({}^{2}\) + 0.0047x + 96.578 & R\({}^{2}\) = 0.9927 \\ \hline
4 & Stirrer speed, 240–420 rpm & y = \(-\)8 \(\times\) 10\({}^{-5}\)x\({}^{2}\) + 0.0672x + 82.523 & R\({}^{2}\) = 0.9999 \\ \hline
5 & Excess sodium carbonate in the charge, 5–20\% (wt.) & y = \(-\)0.0060x\({}^{2}\) + 0.3242x + 93.1631 & R\({}^{2}\) = 1.0000 \\ \hline \multicolumn{4}{c}{Leaching dependences for sinter obtained at 800 \({}^{\circ}\)C} \\ \hline
1 & Temperature, 75–95 \({}^{\circ}\)C & y = \(-\)0.0026x\({}^{2}\) + 0.5895x + 66.102 & R\({}^{2}\) = 0.9993 \\ \hline
2 & Duration, 30–75 min & y = \(-\)0.001x\({}^{2}\) + 0.1419x + 91.802 & R\({}^{2}\) = 0.9947 \\ \hline
3 & Particle size, 100–815 microns & y = \(-\)3 \(\times\) 10\({}^{-6}\)x\({}^{2}\) + 0.0003x + 96.918 & R\({}^{2}\) = 0.9772 \\ \hline
4 & Stirrer speed, 240–420 rpm & y = \(-\)6 \(\times\) 10\({}^{-5}\)x\({}^{2}\) + 0.0437x + 88.139 & R\({}^{2}\) = 0.9944 \\ \hline
5 & Excess sodium carbonate in the charge, 5–20\% (wt.) & y = \(-\)0.0124x\({}^{2}\) + 0.412x + 93.424 & R\({}^{2}\) = 0.9926 \\ \hline \end{tabular}
\end{table} 
Table 6: Paired dependences of tungsten extraction into solution. Figure 1: Paired dependences of solution on factors: temperature (**a**), duration (**b**), particle size (**c**), stirrer speed (**d**), and sodium carbonate excess coefficient (**e**). According to the balance, the extraction of WO3 into leaching products was, in cake, 5.45%; in the productive solution, 85.58%; in wash water, 8.95%. In the traditional industrial scheme, the concentrate is mixed with a screw mixer with recycled materials from cake leaching, air separator dust, and sodium carbonate  
The WO3 content in the charge is reduced to 20-22% due to the turnover of leaching tailings to avoid melting of the charge and the formation of deposits in the drum furnace [3,7,8]. The proposal is to sinter the granulated charge in a vertical shaft-type reactor with the supply of hot air from a heater instead of roasting in a rotary kiln. No recycled materials are added to the charge, and the leaching cake is processed in a separate scheme. The possibility of electrodialysis of sodium tungstate solutions was reported in [20, 21, 22]. The process intended to remove sodium ions from sodium tungstate solutions with the use of membrane electrodialysis processes allows one to reduce the use of mineral acids, as well as prevent the formation of salt waste. The productive solution is purified from impurities using known reagents, with the exception of the consumption of inorganic acids for neutralization operations and the precipitation of tungstic acid. In this work, preliminary experiments demonstrated the feasibility of using electrodialysis in a two-chamber apparatus, which separates the anode and cathode spaces with a cation exchange membrane, for purifying sodium tungstate solutions from impurities and isolating commercial sodium tungstate at the final stage.