Application of an Immersion Milling System for the Production of Nanoparticles

By AZoNano.com Staff Writers

Topics Covered

Introduction
The Challenges
Solutions to the Challenges
     The Separation Device
     The Intake Upper Draft Tube
     The Support Bushing and Increasing Media Oscillation Frequency
     Avoidance of Cascade Milling (In Some Instances)
     No Stagnation
     Heat Generation and Removal
     Batch Turnover
Conclusion
About Hockmeyer Equipment Corporation

Introduction

The HCPN mill can grind particles into the nanoparticle range at shorter processing times. The new design integrates several changes in the internal milling system, but the basic 'easy to clean and use' aspect of the immersion mill remains the same.

The Challenges

The first thing was the ability to consistently produce a media/feedstock separation device with highly accurate flow through zones that avoid the very small media (0.3mm and below) to escape from the mill, while concurrently allowing high volume passage of the feedstock. In addition, it is necessary for the total open area to accommodate the rapid throughput from a hyperactive media field with no clogging.

The second step was to maintain the media in the grind chamber from getting escaped via the intake draft tube. Floatation as well as pressure increment in the screen area can cause expansion of the media field into the void volume area within the chamber, causing the media to escape via the upper intake draft tube. The third stage was to avoid the media leakage via the machined tolerances between the bushing that secures the shaft and the shaft itself as it crosses the bottom screen of the mill chamber and links to the suction propeller below it. The final stage involved significantly increasing the activity in the media field to facilitate rapid feedstock pass through while concurrently speeding up the count of contacts per unit of time.

Solutions to the Challenges

The Separation Device

The first challenge was addressed by the advent of the Hockmeyer contaminant wall featuring 43% porosity for feedstock flow with constant open area size. A technically improved and accurate production process at Hockmeyer avoided the use of typical screening equipment.

The Intake Upper Draft Tube

The second challenge was overcome by integrating an auger to the upper draft tube of the mill chamber. In the start-up process, the auger delivers a constant feed of material into the mill chamber and avoids floating of the media field during the initial immersion of the mill head into the batch. The auger holds large agglomerates in the milling chamber together with the media until they are pulverized into sizes small enough to leave the screen. The pumping rate of the auger controls the flow rate of over-sized agglomerates.

The Support Bushing and Increasing Media Oscillation Frequency

The development of the HCPN centrifugal peg hub addressed the third challenge by maintaining the media field away from the support bushing in the bottom of the screen through the generation of adequate centrifugal force. Fluidization of the bead field improves the ability of the slurry to flow through it. If the degree of fluidization is higher, the slurry passes through the mill more rapidly and returns for additional passes. The more passes means rapid dispersion. Hyperactivity of the media field enables aggressive fluidization by significantly increasing the count of contacts between the media and the feedstock per unit of time, thereby facilitating rapid pass through with high frequency contacts.

Avoidance of Cascade Milling (In Some Instances)

The new design developed by Hockmeyer enables the HCPN mill to perform a predispersion to a nano dispersion devoid of plugging the screen. The new HCPN can produce nano dispersions of Crude Blue, Phthalo Blue press cake, Ultra Transparent Yellow press cake, Rubine Red, Methol Violet, and other difficult to disperse pigments obtained directly from the high speed disperser pre-mix using 0.3mm media in a very short time. The new HCPN mill does not require larger media pre-milling.

No Stagnation

Besides containment, other issues needed to be addressed in handling media small enough to generate nano dispersions. For instance, stagnation of the media in the mill chamber must not take place in any area at any time. Larger conventional media (above 0.3mm) is not generally an issue, but smaller media poses several challenges. Hyperactivity of the flow field becomes vital when there is a decrease in media size.

Heat Generation and Removal

The increase in the amount of heat generated when there is an increase in the delivered HP/Liter of media at constant slurry volume becomes yet another barrier requiring a solution. Hockmeyer developed a highly efficient heat transfer device for cooling, placed underneath the lower discharge draft tube of the Immersion Mill. Chilled water passes through the device when the bottom propeller around the lower discharge tube of the mill drives the slurry discharge against, through and around it.

Batch Turnover

The Immersion Mill is capable of turning the batch over 12 to 16 times per minute and thousands of times during the batch cycle based on the feedstock and its cycle time. This yields tighter particle distribution bands and enhanced qualitative features such as gloss, transparency, and color development. The HCPN has demonstrated to be a significant step forward in processing technology by eliminating cascade milling in several instances through the combination of using this type of performance from the standard Immersion Mill and the ability to handle very small media.

Conclusion

Nano milling is increasingly gaining momentum. The ability to effectively use smaller and smaller media is a key aspect of this technique. The discoveries of efficient media/feedstock separation methods, the aggressive control of temperature, and the production intense and rapid energy exchange between media and feedstock are also equally important.

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.

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.

Date Added: Mar 11, 2014 | Updated: Mar 24, 2014
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