2.1. Leaching tests
The leaching tests were carried out in a conical flask with HCl
solution, and temperature was thermostatically controlled by an
electro-thermostatic water cabinet (DK-8D, Shanghai Medical
Thermostatic Manufacture, PR China). Agitation was conducted by
employing a mechanical stirrer (Jy2002, Hangzhou Instrument
Electrics Co., Ltd, PR China). After leaching tests, the leaching solution
was obtained through filtration, and the content of Al was
determined using visible spectrophotometric analysis.