Written by George J. Murphy, Updated by Mike Villardi
Several mill companies claim their basket mill to be superior. They play off of what they believe to be shortcomings in other “immersion”– or “basket”-type mills. They contend that immersion mill technology is best suited for small-batch processing and low-viscosity and easy-to-grind products.
Our immersion Mill uses rapid recirculation milling technology (the same concept as used in the high energy recirculation horizontal mills) by rapidly pumping the slurry through the media field many more times and more efficiently than any other mill, including horizontal or vertical mills. Remember that any external mill (horizontal or vertical) that relies upon tank-to-tank or same tank recirculation is not a continuous process but a batch process.
Several basket mill manufacturers have added 2nd and 3rd mixing shafts to their basket design because they have low exit velocities, a byproduct of poor throughput. Without using a bottom suction device directly below the basket, the slurry bypasses the bottom of the media field (the high energy zone) and follows the path of least resistance through the side screen. Any product higher than a few hundred centipoises in viscosity will be motionless within a few inches of the basket and result in stratification during the milling process. A similar phenomenon known as short cycling occurs in horizontal mills when the slurry follows the wall of the grinding chamber to its discharge, bypassing the high energy force of the media nearest the center shaft. This can double or sometimes triple the time required to reach product quality and result in a wide particle size distribution.
The addition of agitation alongside the basket improves the flow around the basket by creating side vortices in the tank. However, this simultaneously pulls slurry away from the mill entry and into the side vortexes, creating the illusion of rapid batch turnover through the mill. Although the side vortexes partially compensate for the poor flow characteristics of a side only or bottom only screen discharge, they do little to improve milling time or performance. The side-positioned agitation blades produce a combination of suction and back pressure against the discharge area, canceling any potential benefit inflow through the screen. The solution to the problem is an upper and lower draft tube with a side and bottom screen and a suction device (lower prop) positioned within the bottom draft tube below the bottom screen. The propeller in the lower draft tube focuses the suction of slurry exclusively from the mill’s upper level. This reduces bypass as a more significant percentage of slurry is drawn deeper into the bead field or high energy zone before reaching the side or bottom screen discharge. The rapid uniform flow of slurry drawn down through the top and outward toward the mill’s bottom increases batch throughput rate by as much as 50%. Additionally, performance is increased more when a top auger (HEC patent) is mounted within the upper draft tube above the bead field to govern the feed, especially when viscosities are above 15,000 cps.
The Hockmeyer HSD Immersion Mill is efficient and versatile and is available to operate in batch sizes from as little as 750ml in excess of 5,000 liters. Its proven design can out-perform horizontal, vertical, or other basket-type media milling systems with tighter particle distributions in half the time. The basic machine consists of a highly polished, water-cooled, submersible rack dome with side and bottom screens and upper and lower draft tubes. Within the basket is a rotating hub with pegs and a removable (HEC patent) counter-peg assembly. A top auger and bottom prop work together to enhance the dispersion flow through the bead field and out of the basket. To accommodate different viscosity ranges, three styles of lower impellers are available for the acceleration of the flow through the basket (mild/moderate/ aggressive). The design requires a bottom support bushing to accommodate the lower concentric propeller. The bushing is available in Rulon (a PTFE-based thermoplastic) or ceramic composition depending on the application. The bushing is simple, quick to replace, and inexpensive. The mill can be completely disassembled and reassembled with a fresh charge of media within that same time period when required.
The performance difference is so significant, the immersion mill is the “Ideal Mill Design” and offers the following advantages.
- The machine can be designed for use in multiple tanks with various capacities.
- Multiple tank positions with 360º rotation.
- Minimization of contamination, clean-up, solvent loss, and product loss.
- Maximization of color strength, gloss durability, and yield.
- Process time reduced by at least 50% in most trials.
- Reduction of raw material costs by an average of 5%.
- Highly efficient uses of small quantities of media.
- Consistent and predictable performance.
- Various materials of construction available.
The PLC based Auto Process II Control Center enables the operator to run the machine on autopilot or manual control. APCII allows process temperature and grind time limits to be set on a case-by-case basis. In the automatic mode, it monitors batch temperature and regulates the mill to control heat. A warning horn sounds if product temperature exceeds the operator’s lower limit set point. If left unanswered, the control will shut down the mill at the upper limit set point. The system is designed for ease of operator interface using a simple push-button and selector switch combination. The operator can set three temperature parameters (cooling water, alarm, shut-down temperatures) and batch time.