Like the horizontal ball mill, vertical ball mill slakers are best suited to applications where the lime quality is poor, water chemistry is poor, grit handling is costly or when the processing rate exceeds that achievable in a conventional slaker. Ball mill slakers operate on the same principle as detention slakers, with the added ability to grind inert materials (grit) contained in the quicklime and in general require little or no grit removal or disposal facilities. Vertical ball mill slakers are comparable to horizontal ball mills in cost, however, have much better turndown capability, and consume less energy. In addition, the temperature probe is located in the reaction zone for this type of mill, so a vertical ball mill can be optimized to maximize lime reactivity and reduce overall chemical consumption.
Vertical ball mill slakers are grinding mills commonly used in the mineral processing industry for fine grinding applications, modified to withstand the temperature of lime slaking. Vertical ball mills are simple and rugged and have the safety advantage over their horizontal equivalents of having no external moving parts. A typical ball mill slaker (vertical type) consists of a static vertical chamber with wear lining and a vertical screw. The screw agitates a grinding media (typically steel balls), and serves as a mixer for the lime slurry. The slaked lime slurry overflows into a separating chamber to allow the larger particles to be returned to the ball mill for further processing. In most cases the separating chamber and slurry recirculation provides sufficient size classification to eliminate the need for a hydrocyclone.
Vertical ball mill slakers are available with quicklime slaking capacities up to 50,000 kg/hr. As with conventional detention slakers, vertical ball mills operate at water to lime ratios (in the slaking compartment) between 3.5:1 to 6.0:1. The water to lime ratio in the mill is adjusted automatically to maintain a constant slaking temperature based on a temperature probe directly in the mill. Vertical ball mill slakers typically operate between 75 oC and 85 oC, and like conventional detention slakers they can, with good water temperature control and a stable lime supply, be run at temperatures in excess of 90 oC producing an extremely reactive hydrate (see case study on effects of slaking temperature).
The majority of vertical mills in operation in lime slaking service are Metso VERTIMILLS ®.
STT has partnered with Metso to integrate the VERTIMILL® in lime slaking applications. Learn more >>