Reverse Thinking to Obtain Gypsum from Scheelite Slag

Because tungsten can form soluble heteropoly acid (such as [PW₁₂O₄₀]₃^-) with impurities such as phosphorus, arsenic and silicon at the ratio of 1:6 to 1:12, a small amount of phosphorus can cause a large amount of tungsten dispersion and loss in the process of hydrochloric acid decomposition, so hydrochloric acid decomposition process mainly deals with high-grade Scheelite Concentrate (requiring low impurities such as phosphorus and arsenic).

In order to solve the problem of tungstic acid encapsulation, scheelite can be decomposed by phosphoric acid with large excess coefficient. As phosphoric acid complexes calcium ions, the solubility of calcium sulfate increases gradually with the increase of phosphoric acid concentration in solution, reaching the maximum at about 20% of P₂O₅. Therefore, appropriate high concentration of phosphoric acid is also conducive to reducing the supersaturation of calcium sulfate in tungsten ore decomposition, which may also reduce the spontaneous nucleation rate of calcium sulfate, thus promoting the formation of coarse crystals; and phosphoric acid has low corrosiveness, and there is no volatilization problem like hydrochloric acid. A certain amount of SO₄^{2 -} was added to the leaching solution to bind calcium ions, so that calcium precipitated in the form of calcium sulfate and regenerated to form phosphoric acid, and gypsum-type (CaSO_4·nH₂O) decomposition residue was obtained.

The decomposition slag is difficult to be used directly, and if it is dumped, the resources will be wasted. For this reason, some scholars have made use of the gypsum decomposition residue, and the process is as follows:

A phosphoric acid solution containing 15% P₂O₅ was prepared from Scheelite (containing 70.6% WO₃) at 1 kg. The concentration of SO₄^2-was stabilized at 0.03 g/ml, the liquid-solid ratio was 6:1 ml/g, the reaction temperature was 60 C, and the reaction time was 4 hours. The leaching rate of tungsten was 94.3%, and the content of P₂O₅ in the filter residue was 5.3%. Then the slurry was transferred into the crystallization reaction tank for crystallization conversion. The amount of hemihydrate gypsum seed was 50g, the temperature was raised to 110 ℃, the crystallization conversion time was 6h, and the tungsten leaching rate was 99.6%. The filter slurry is made of a 200 mm Brinell funnel. The filtration time is 8 minutes. Then the cake is washed three times with pure water. The water consumption is 300 ml each time. After washing, the content of P₂O₅ in the cake is reduced to 0.1%. Tungsten was extracted from filtrate with primary amine alkaline extractant. The extraction rate of tungsten was 99.5%. The total loss of phosphorus in the form of phosphotungstic heteropoly acid and entrainment is 2.7%. The mother liquor is leached back after adding phosphoric acid and sulfuric acid.

In the recrystallization transformation process after scheelite decomposition, a small amount of undissolved scheelite can be further decomposed, and its decomposition rate is over 98%. The loss of intercrystalline P₂O₅ can also be released, which reduces the loss of P₂O₅ in slag to less than 0.5%, and reduces the leaching cost. In addition, due to the re-dissolution of impurities such as iron, aluminium and magnesium in gypsum during recrystallization and transformation, the quality of gypsum is further improved. The quality of the by-product hemihydrate gypsum obtained is very pure, even comparable to that of natural gypsum. It provides high-quality raw materials for the production of gypsum board, retarder for cement production, wall powder and so on. The production process realizes no waste residue discharge and improves the process.