The critical moisture content of basic magnesium carbonate has been determined to be about 150 %（D.B） based on the characteristic curve for drying, obtained by both laboratory and pilot plant Tunnel Dryer methods.
Temperature, humidity, flow rate, size and thickness of the feed material, and direction of air flow had large effect on drying in the constant rate period, but had little effect in the falling rate period.
The optimum operation condition for drying with tunnel dryer had also been investigated.
The tunnel dryer designed in this work had the following characteristics : critical moisture content : Wc=1.47, equilibrium moisture content, We=0.01 ; initial moisture content, W₁=4.00 ; moisture content at time, W₂=0.16 （t=9 hrs） : inlet air humidity=50 % at 20 ℃ ; out let air humidity=60 % at 70 ℃ ; preheated air temp.=120 ℃, overall heat efficiency, over 60 %.