Synthesis of high purity ammonium alum crystallized from solutions containing impurities

•Synthesis of high purity ammonium alum by improved crystallization was carried out.•Crystallization parameters had a great effect on the purity of ammonium alum crystal.•Impurities were removed more effectively by non-agitation crystallization than by agitation crystallization.•K+ ions were more easily incorporated into ammonium alum crystal than other impurities.•High purity ammonium alum was successfully synthesized in non-agitation crystallization.