A rotary siever is a mechanical device used to separate and classify materials based on particle size. It operates using a rotating cylindrical mesh or screen that allows finer particles to pass through while retaining larger particles. In the context of spice processing, a rotary siever ensures that ground spices are sifted to achieve uniform particle size, which can be crucial for maintaining quality and consistency. The rotating action of the siever helps to efficiently manage and process large quantities of material, making it an essential tool in both industrial and commercial spice production.