Natural cassiterite on account of its crystalline nature is extremely difficult to dissolve in acids or alkalies. Artificial cassiterite on the other hand is amorphous and soluble in acids. Cassiterite can easily be reduced however to Sn, which soluble in acids or alkalies. Pressure leaching has been applied to cassiterite concentrates using Na2S and NaOH solution at 4000C Tin is recovered from the leach solution by exchange with Na amalgam whereby Sn enters the amalgam phase and sodium sulphide is regenerated.
Wolfram and Scheelite can be digested with excess concentrated Hydrochloric acid as follows; the digested mass being washed with water to remove iron and manganese chloride, the residue being then dissolved in hot ammonium hydroxide. Ammonium wolframate is crystallized from the solution by evaporation.
WO3. FeO + 2HC1 ^ FeC12 + H2WO4 WO3. CaO + 2HC1 ^ CaC12 + H2WO4
Leaching of wolframite with concentrated NaOH at high temperature in an autoclave yields a solution of sodium wolframate, while iron and manganese are precipitated as hydroxides:
WO3. FeO + 2NaOH ^ Na2WO2 + Fe(OH) 2
The solution of sodium wolframate is purified by precipitation with HC1: Na2WO4 + 2HC1 ^ 2NaC1 + H2WO4
Alternatively, it is possible to leach calcium tungstate with a solution of NH4F and NH4OH. Tungsten goes into solution as (NH4)2 WO4, while calcium is precipitated as Ca F2. Increasing the temperature, time or concentration of ammonium fluoride increases the rate of extraction. Tungsten from Scheelite can be extracted by leaching with excess sodium carbonate solution (30-50 g/1) at 200oC, the addition of small amounts of NaOH improving recovery.