Research On Pneumatic Separation And Rheological Characteristic Under Compression Molding Of Corn Straw’ Main Components

There are about 0.7 billion tons of various crop straws produced in China every year, and corn straw accounts for 1/3. But corn straw’s industrial utilization is relatively low. The significant difference in physical and chemical characteristics of corn straw’s components is one of the difficulties in utilization. If the leaf, skin and pith of corn straw can be separated and used as different raw materials respectively, not only can the quality and value of raw materials be significantly improved, but also its industrial utilization can be realized. This will promote corn straw’s utilization process. Based on studying basic properties, separation properties and compression characteristics of main components of corn straw, pneumatic separation and rheological characteristic under compression molding on corn straw’s main components were studied deeply. The main research contents and conclusions are as follows:(1) The compositions and their characteristics of main components of corn straw were studied. The basic parameters, such as mass fraction and volume fraction of corn straw’s main components, thickness of skin, density and form of fiber, and their change regularity were obtained. Main components of corn straw are leaf, stem and other components, whose proportion in mass were 40.2%, 48.9% and 10.9%, respectively. In the stem of corn straw, mass ratio of skin to pith was 2.29, and volume ratio was 0.33. The content of skin increased with the increasing of stem diameter slightly, while the content of pith reduced accordingly. In the main components, the form of skin fiber was best, but other components’forms were obviously poorer. This study provides basic data for separating main components.(2) The crush properties and compression characteristics of chopped corn straw were studied. The particle size distributions of crushed materials under different sieves, mass fraction of main components, bulk density and these indexes’change regularity were obtained. A component mass index was put forward to evaluate particle size distribution of main components of crushed material. This index illustrates the effects of particle size distribution on separation of main components. The results shown that larger debris was helpful to main components’separation. Compression test on pith debris showed that the initial compression stage was longer and the stress value (1.2 MPa) was lower at the turn point in middle stage. This feature presented that pith debris was suitable for molding some products with big depth ratio and abnormity plane.(3) The aerodynamic characteristics of corn straw’s main components were studied. The samples with different components and different specifications were made, and their floating speed range and the relationship with each other were measured and analyzed. Within the test range, the suspending velocities of skin, pith and leaf were 1.60~3.22 m/s, 0.65~2.75 m/s, and 0.75~ 1.25 m/s, respectively. The main obstacle in separating main components of corn straw with airflow was suspending velocity’s intercross of different particles. Consequently, critical separation particle size and critical suspending velocity were put forward, and the lighter fractions’separation kinetics curve corresponding to different minimum skin size was obtained. These provides theoretical basis for separating chopped corn straw with airflow.(4) The separation device for stem debris with vertical and horizontal airflow were designed and developed. Its feasibility was demonstrated. Horizontal airflow separation test showed that the separated materials were made up of skin debris, pith debris and mixed debris samples between skin and pith. But the range of mixed samples was relatively broader and separation rate was difficult to reach production requirements. Separation with vertical airflow could greatly increase isolation rate and separation rate. Moreover, skin and pith debris could be blown off separately. This test showed that the vertical airflow separation had excellent adaptability to chopped corn straw.(5) The separation device for stem debris with vertical airflow were designed and developed. The device’s separation rates were tested using design scheme of quadratic-rotation-orthogonality. The optimum separation conditions were 0.85m/s of airflow speed, 14kg/min of feed quantity and 0.75m of feed position. The skin isolation rate was 99.21%, pith isolation rate was 99.13%, skin separation rate was 98.45%, and pith separation rate was 98.67%. The results demonstrated that the device ran well and had a good separation rate. It provides a stable and reliable industrial separation method and equipment for separating corn straw’s main components.(6) The rheological properties of straw debris with typical abnormity shape under compression molding were studied through simulation and practical tests. The rheological properties of concave molding and convex molding and the effects of main factors were clarified. To deal with the problem of non-uniformity in material-filling, a molding method with variable lower plane was put forward and demonstrated with experiments. The results shown that the uniformity in material-filling can be effectively improved by this method.(7) A material-filling uniformity index was put forward to evaluate uniformity in material-filling. Based on this, the effects of main molding parameters on material-filling uniformity were analyzed deeply. The relationships between the height of molding-filling plane, molding-angle and the index of material-filling uniformity were established. It was found that the relationship between molding-angle and the index of material-filling uniformity followed secαcurve. The filling condition became worse rapidly if molding-angle above 70°. This test provides important theoretical basis for revealing the effects of molding-angle on molding process.