TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The drying process comprises dehydrating the water content from the wet cassava slag, scattering into powdered wet material, introducing the powdered wet material into a inner cavity of the interlayer dryer to perform the primary gas-solid separation type drying, performing cyclone separation, drying the material with the secondary gas-material mixed hot air until the water content reaches 10-15%, and again performing cyclone separation to obtain the dry slag finished product. The water content of the wet cassava slag after dehydrating the water content is controlled to less than or equal to 71% and hot air flow is introduced into the inner cavity of the interlayer dryer. The primary gas-solid separation drying process and the secondary gas material mixed hot air drying process refer to the course of negative pressure drying and at least one hot air outlet is arranged in the polishing type pre-drying section. The primary gas-solid separation drying process comprises two interlayer dryers. The inner cavity of dryer is provided with a material inlet and a material outlet. The inner layer is provided with a hot air inlet and a hot air outlet. The primary gas-solid separation drying process comprises (i) mixing the powdered wet material from the polishing type pre-drying section with the hot air, then introducing negative pressure into the inner cavity of the primary inner layer dryer, where the interlayer hot air inlet of the primary interlayer dryer is connected with the inner layer hot air inlet of secondary interlayer dryer; (ii) introducing the material negative pressure from the primary interlayer dryer inner cavity into the primary cyclone separator to perform the primary cyclone separation, and discharging the separated wet air through the fan; (iii) mixing the material obtained from the first cyclone separation with the hot air supplied by the polishing type pre-drying section, introducing the negative pressure into the inner cavity of the secondary interlayer dryer inner cavity, again performing cyclone separation by introducing the material negative pressure from the secondary interlayer dryer inner cavity into a secondary cyclone separator, discharging the separated wet air through a fan, mixing the material obtained by performing cyclone separation with the hot air of the interlayer outlet of the secondary interlayer dryer, and then introducing the negative pressure into the secondary gas-material mixing hot air drying process. The whole course negative pressure is achieved through a low-noise centrifugal fan. The polishing type pre-drying is performed by introducing wet material at 300-420 degrees C. The secondary gas-material mixed hot air drying is completed with one or multiple pulse air flow dryers connected in series. The interlayer is maintained at higher temperature than that of the hot air in the corresponding to inner cavity. Preferred Components: An air lock valve is arranged in the material outlet of each cyclone separator.