Fluoroammonium Method for Processing Scheelite Concentrate

The mineral resource base of tungsten is mainly represented by minerals of the wolframite and scheelite groups, which are of industrial importance [1]. Classical methods of processing scheelite concentrates are based on sintering with soda and subsequent separation of tungsten chemical concentrate on ion-exchange resins [2]. We propose to investigate a new ammonium fluoride method for processing scheelite to reduce the cost of processing and increase the purity of the resulting tungsten product. Synthetic scheelite (CaWO₄) was used as a model mixture. The method of sintering scheelite with ammonium bifluoride (NH₄F*HF) has been proposed to separate tungsten and calcium. Sintering temperature 200 °C. Ammonium bifluoride was taken with an excess of 10% relative to stoichiometry. The sintering time of scheelite with ammonium bifluoride under laboratory conditions was 2 hours. The process is described by a chemical reaction with the formation of insoluble calcium fluoride.

2CaWO₄+ 7NH4F*HF = 2CaF₂ + 2(NH₄)₃WO₂F₅ + NH₃+ 4H₂O (1)

Calcium fluoride was separated by filtration, the tungsten-containing solution was studied by Infrared Spectroscopy (IRS) (Figure 1). Next, the tungsten-containing solution was evaporated, and the solid residue was subjected to thermal decomposition.

Figure 1:IR spectrum of the obtained sample of the sintering product - wave number 3180cm^-1 - NH4+ - wave number 480cm^-1 - W-F - wave number 790cm^-1 - W-O.

(NH₄)₃WO₂F₅ + H₂O = WO₃+ 5HF + 3NH₃ (2)

The solid stock was studied by IR spectroscopy (Figure 2). According to the results of the IR spectrum, the substance obtained as a result of the decomposition of (NH₄)₃WO₂F₅ is clearly tungsten trioxide (WO₃). The IR spectrum also indicates the absence of NH₄⁺, O₂^- and F- ions. Based on the results of laboratory experiments, a schematic diagram of scheelite processing was proposed [3], (Figure 3).

Figure 2:IR spectrum of the obtained sample of the sintering product.

Figure 3:Fluoroammonium scheme for processing scheelite.

a. It was possible to quantitatively decompose scheelite using ammonium bifluoride and separate solid CaF₂ from tungsten in the form of a (NH₄)₃WF₉ solution.
b. Thermal decomposition of (NH₄)₃WF₉ makes it possible to obtain pure WO₃.
c. The conducted experiments make it possible to start a laboratory study of natural scheelite concentrates and a study of the possibility of purifying a tungsten product from impurities.

1. (2020) U S Geological Survey, Mineral Commodity Summaries, USA, p. 178.
2. Zhao Z, Li J, Wang S, Li H, Liu M, et al. (2011) Extracting tungsten from scheelite concentrate with caustic soda by autoclaving process. Hydrometallurgy 108(1-2): 152-156.
3. Laptash NM, Melnichenko EI, Polyshchuk SA, Kaidalova TA (1992) The fluorination of scheelite with ammonium bifluoride. Journal of Thermal Analysis 38(10): 2335-2341.