| The sludge indirect thermal drying technology is widely applied in sludge treatment engineering. The sludge drying process in the indirect heat transfer driers composed of mixing, heat transfer, mass transfer and many other complex processes. To enhance the mixture of sludge within the indirect heat transfer driers, the research results on materials mixing at home and abroad were reviewed; a cold mixing test system and a hot sludge drying test system were built; a method for evaluating the mixing quality of materials in blender based on image processing technology was established and a series of experimental studies were conducted.The mixing characteristics of solid particles in blender and the influence factors were studied by using the cold mixing test system and the visual mixing quality evaluation method. The mixing process of solid particles in blender was divided into three periods:the convective mixing period, the convection and shear mixing period and the diffusion mixing period. The speed of stirring shafts mainly effect on convective mixing stage, the higher the speed, the particle mixing degree is higher. Excessive and insufficient fill rates of materials go against the mixing of materials; to obtain the best mixing effect, appropriate filling rate was needed. Focus on the problem of mixing blind angle in the blender, some improvements for the blender structure were tried. The material mixing degree was improved by increasing the size of wedge blade and using the modified elliptic blade for their larger stirring range.On the basis of the cold experiments, the modified elliptic blade was applied to the hot sludge drying test system. The drying characteristics of Henggang sludge samples and the influence factors in the elliptic paddle dryer were studied and compared with that in the traditional wedge paddle dryer. The test results indicated that the sludge drying process was divided into four periods:the heating period, the first falling rate period, the drying sticky period and the second falling rate period. To some extent, the sludge mixture was enhanced and the drying rate was improved by increasing the speed of stirring shafts in the drying sticky period and the second falling rate period. Sludge can be stirred more adequately by blade with higher filling rate within a reasonable range. The sludge mixing effect and drying rate is effectively improved in elliptic paddle dryer for its strong self-cleaning ability playing an important role in scraping, shearing and breaking the sludge layer. |
