If one really needs NH
4Cl, one could dissolve the (NH
4)
2SO
4 in water, add an equimolar amount of CaCl
2 dissolved in water. Weakly soluble CaSO
4 aka gypsum precipitates leaving behind a solution of NH
4Cl with minor calsium and sulfate contamination, both of which can be minimized by making sure the amounts are measured correctly and the solutions are as concentrated as possible. One could even concentrate the solution by evaporation to make sure there is even less gypsum relative to sal ammoniac present... and then sedimentation, decantation and filtration followed by complete evaporation to get crystalline NH
4Cl. Theoretically:
2NH
4+(aq) + SO
42-(aq) + Ca
2+(aq) + 2Cl
-(aq) ---> 2NH4
+(aq) + 2Cl
-(aq) + CaSO
4(s)
Read your chemistry!