A spiral is a category of mineral processing equipment that is mainly used to separate particles based on size or specific gravity - for either size classification or concentration unit operations. Essentially, the equipment is an upright screw with hollow threads. As feed (usually particles + liquid) is fed in a the top, it flows down the threads and centrifugal forces lift the lighter/finer particles out toward the edge of the threads; while the heavier/larger particles sink quickly toward the center of the thread.

When used in a mill or processing facility, the spiral is generally referred to as a classification spiral, a spiral separator or a spiral concentrator. The main design purpose for spiral classifiers is for separation of fine particles (and liquid) from coarse particles, or for separation of light particles (and liquid) from heavy particles. These classifiers are extremely useful in aggregate production mills, and in coal preparation, as well as processing of "heavy sands" to extract minerals such as titanium, gold, diamonds, and uranium.