These equipments have changed with the years. They were used in the first years of the last century with the name of Humpreys Spirals and were made of cast iron. Nowadays they are fabricated with plastic material and the water addition is not a very special requirement. In general spiral can be considered single or double helical wrapped around a central column with a series of concentrate take-off ports placed at specific intervals along the spiral. Also, there are splitters located at the bottom of the unit so that the concentrate quality can be regulated. In order to increase capacity, two or more units are assembled around one central column.
Schematic section of the spiral
There are several manufacturers of spirals and the equipment has been accepted in many operations in Australia and South Africa. An interesting change in the design has been the modified cross section and only one concentrate take off port that is located at the bottom of the spiral. The next figure shows several profiles of spiral concentrators. The main difference of these spirals is the possibility of obtaining better concentrates grades due to the separation zone is different and the selectivity between light and heavy particles can be improved.
Different spiral concentrator profiles
Spiral usually operates at 40-50% solids and the design can consider employing a hydrocyclone as the first separation stage. In this way the underflow can be directed to the spiral. Once the slurry is in the spiral, light and heavy mineral are classified according to their densities. In general, heavy particles have a high rate settling and form a slow moving fluid film. The flow stream in divided vertically in two parts, one with heavy particles and the other with light particles and most of water. With the splitters is possible to selected the quality of these products and can be included a stream composed by heavy a light particles called middlings. The slow moving fluid film reduces its velocity due to friction forces. In general, separation is performed by the differences in centrifugal forces. Spirals can be employed as rougher and cleaner equipment. One of the key operative factors is the wash water addition and the difference between the different minerals.
Gravimetric circuit with Spiral and Shaking Table