Some factors affecting sieving performance and efficiency

Sieving or screening has been the oldest yet most important unit operation for industrial separation of solid particles or as a laboratory method in size analysis. A stack of sieves with decreasing mesh size is usually used. Alternatively, particles can be sifted in a fine to coarse order by multiple sieving steps with each step using a single sieve. The latter is referred to as reverse sieve method. This study compared the two methods for sieving performance and efficiency using flours made from soft white and hard white wheat, hulless barley and medium grain rice. Additional factors, including milling method (impact vs. abrasive), flour moisture (7% vs. 11%), duration of sieving (60 vs. 120 min), and tapping (percussion during sieving), were also investigated. Mass frequency and protein content of oversize fractions were measured. Results show that all the variables and their interactions had significant effects on sieving performance and efficiency. Among them, tapping was most important, followed by sieving duration, sieving method, milling method, flour type, and flour moisture. When other conditions were equal, the reverse sieve method always gave improved sieving efficiency over the stacked sieve method. The observation can be attributed to the beneficial effect of oversized particles on reducing sieve blinding by near or sub-sieve sized particles. Furthermore, the reverse sieve method also expanded the difference in protein content among sieved fractions. Because of its practical significance, this so far unreported effect would bear further confirmation of other sieving and screening conditions.