Several mill companies claim the superiority of their basket mill, and believe that immersion mill technology is ideal for easy-to-grind products, low-viscosity and small-batch processing.
Hockmeyer’s immersion mill makes use of circulation-milling technology. This technology is the same concept used in the new high-circulation horizontal mills. This allows for rapid pumping of slurry through the media field several more times and is highly effective when compared to any other mill, including vertical or horizontal mills.
Figure 1. Mill Basket
Disadvantages of Previous Immersion Mils
It is important to note that any vertical or horizontal external mill, depending on the same tank or tank-to-tank recirculation, is not continuous but a batch process. The reason why a number of basket mill manufacturers have added second and third mixing shafts to their basket design is because their basket designs feature low exit velocities, which is a by-product of poor output. Without using a bottom suction device below the basket the slurry passes by the media field bottom, which is a high energy zone following the least resistance path via the side screen.
Without this any product higher than a few hundred centipoise in viscosity will be motionless within a few inches of the basket, and the result is milling process stratification. A similar phenomenon called short cycling takes place in horizontal mills when the slurry follows the grinding chamber wall to its discharge instead of working against the media’s centrifugal force. Due to this the time needed to attain product quality is increased and results in a broad particle size distribution in the slurry.
The adding of agitation along the basket enhances flow surrounding the basket by the creation of side vortices in the tank. However, this pulls slurry away from the mill entry and into the side vortexes creating a rapid batch turnover illusion over the mill.
Even though the side vortexes compensate partly for the poor flow characteristics of a bottom only or a side only screen discharge, they do very little for improvement of performance or milling time. The agitation blades positioned at the sides produce back and suction pressure combined against the discharge area revoking any potential benefit in flow via the screen.
One solution to this issue is a dual draft tube (HEC Patent) top and bottom having a bottom screen in addition to the side screen and a pumper blade or a suction device positioned in the bottom draft tube below the bottom screen.
The lower draft tube blade is focused on slurry suction exclusively from the upper level of the mill. Bypass through the side screen is considerably reduced as a higher slurry percentage is drawn deeper into the high energy zone or the bead field before reaching the bottom or side screen discharges.
The batch throughput range is increased by 50% due to the uniform, rapid slurry flow drawn via the top and outwardly toward the mill bottom. There is a 20% increase in performance when the top propeller is mounted in the draft tube above the bead field in order to force feed the mill in cases where viscosities are above 15,000cps. Hence the need for additional shafts, motors for driving supplementary agitators is also eliminated.
The Hockmeyer HSD Immersion Mill
The Hockmeyer HSD Immersion Mill is flexible, efficient and can operate in batch sizes from 750mL to 5,300L. Its design can surpass vertical, horizontal or other basket-type media milling systems by 30–300%.
The basic machine includes a water-cooled, highly polished, submersible basket with bottom and side screens and lower and upper draft tubes. The basket includes a rotating hub with pegs and a removable (HEC Patent) counter-peg assembly.
The dispersion flow through the bead field in the basket is improved by the combined working of top and bottom pumping impellers. For accommodating a range of viscosities, three lower impeller styles are offered for flow acceleration inside and outside the basket namely mild, moderate and aggressive. There is a need for a bottom support bushing for the design in order to accommodate the lower concentric pumper blade.
The bushing is made of a PTFE-based thermoplastic having a high pressure velocity rating and which is resistant to most solvents, Rulon or it may have a ceramic composition based on the application. Replacement is required just once a year in most applications and the change takes below an hour to be completed. If required, in that same time period, it is possible to totally assemble or disassemble the mill with a fresh charge of media.
There is a drastic performance difference observed and this immersion mill may be the ideal one with the following benefits:
- Rapid mixing and milling of raw materials are done at the same time in a single tank engaged for the complete cycle. In certain cases there is no premixing benefit on a disperser (mostly viscosity and application dependent). The machine is developed to be used in multiple tanks with various capacities.
- Multiple tank positions with 360° rotation.
- Reduction of solvent loss, product loss, contamination and clean-up
- Increase in gloss durability, color strength and yield
- Reduction of raw material costs by an average of 5%.
- Reduction of process time by an average of 30–300%.
- Highly efficient use of small quantities of media
- Use media as small as .25mm and as large as 2mm.
- Reliable and predictable performance.
- Various construction materials available
PLC-Based Auto Process II Control Center
The PLC based Auto Process II control center allows the operator to run the machine on manual control or autopilot. APCII enables grind time and process temperature limits to be set on a case-by-case basis. Batch temperature is monitored and the mill is regulated for heat control in the automatic mode. There is a warning horn sounded in case product temperature is more than the lower limit set point. In case it is unanswered, the mill is shut down by the control at the upper limit set point.
The system is user-friendly, makes use of a simple push-button and selector switch combination. Three temperature parameters can be set- alarm, shut-down temperature and alarm and also batch time.
About Hockmeyer Equipment Corporation
Hockmeyer Equipment Corporation has been providing process solutions for over 70 years. It is a leader in the wet grinding and dispersion industry and offers custom designed process equipment for mixing, blending, dispersing and particle size reduction as fine as 25 nano-meters. The Hockmeyer Team is dedicated to building and designing processing equipment that is the highest quality available. Our goal is to increase the profitability of our customers through continuous innovations in process technology.
We are committed to learning our customer’s process so that we can provide the best possible equipment for the application. We will build equipment that is as efficient as possible to reduce the energy requirements, and therefore, the environmental impact of the industries we serve. Hockmeyer offers a state of the art customer service laboratory for product testing and process development.
“We don’t sell machines, we sell solutions." Herman Hockmeyer.
This information has been sourced, reviewed and adapted from materials provided by Hockmeyer Equipment Corporation.
For more information on this source, please visit Hockmeyer Equipment Corporation.