Hockmeyer Equipment offers a unique and patented immersion mill that does not work like conventional vertical and horizontal media mills.
The Hockmeyer immersion mill removes the requirement for mechanical seals, valves, transfer pumps, hoes and secondary holding tanks and thus reduces clean up time, processing time and waste.
The machine is capable of producing the required results in considerably less time, with a universal homogeneous batch and a very narrow particle size distribution, thus further reducing the processing time.
Thanks to the high throughput of the jacketed tanks and cooling dome of the immersion mill, a high rate of heat exchange allows for precise temperature control. The mill can even be closed to reduce or prevent emissions and also to protect the product from contamination during processing.
The Hockmeyer immersion mill has an exclusive but limited feature that makes it possible to incorporate dry materials and still create a homogeneous batch. The resemblance between the immersion mill and conventional vertical and horizontal mills is in their usage of media to obtain fine grinds of pre-dispersed materials.
In order to achieve optimum results, the selection and matching of the media to the mill base is very important. Given that the immersion mill does not experience the same issues as standard mills, such as floating of the charge, hydraulic media packing, and seal features, it will show greater performance, more flexibility, low maintenance and reduced mill times.
In contrast to conventional media mills, the immersion mill has a specific media load irrespective of the media or product type. For each size machine, a specific volume has been determined and this volume is pre-measured. Efficiency, performance and the final product will be affected by over or under-filling of the basket.
Although under-filling the basket may promote greater throughput, it will enable the media to form a larger area of expanded beads. This in turn would result in pigment dispersion through impact rather than shear, and thus reduce the purity of the color development and the sheen of the product. On the other hand, over-filling of the basket may increase milling times, reduce throughput, draw more amperage, and increase wear on both the media and mill.
Conditioning/Polishing of Media
A large number of media need preparation before it is used in a batch. New media usually comprises undersized or fragile beads. The fragile beads may comprise beads that are poorly manufactured or beads that include air pockets. Such beads will break during initial usage and can sometimes clog the screens.
In the majority of cases, the Hockmeyer immersion mill will not allow the fractured beads to remain in the screens for long periods of time. The broken beads will be worn smaller and are ultimately forced via the screens. Next, the particles together with the remaining batch will be cycled via the basket a number of times, ground as fine as all other particles and finally become a part of the end product.
When there is a need to polish the new media, the mill should be operated at the lowest possible speed for the shortest possible time. This will prevent untimely wearing of both the media and mill. New media should be polished using solvent and resin to remove metal or ceramic dust.
Media Selection and Formula Optimization
When selecting the media, a number of pertinent factors should be taken into account. These include media size, media density, formulation, mill base viscosity, solids loading, raw particle/agglomerate size, type of pigments and final grind or particle size needed.
When the batch viscosity has been matched properly to the type and size of media, the immersion mill will surpass present techniques and may generate results that far exceed the present standards such as gloss, color development, grind, transparency, particle size distribution and so on. The immersion mill can produce maximum color development and thus reduce the quantity of pigment needed to create a given color strength.
Media Size and Density in Relation to Mill Base Viscosity
It is important to maintain the viscosity of the mill base within a specified range for a given density and size of media. Normal milling operations can be applied to media in the 1mm range.
Larger sized media enable greater throughput and will also aid in reaching lesser grind requirements more rapidly. Higher densities and larger sizes also work better with higher viscosities. Though smaller sizes will take longer to rough out the grind, it will reach sub-micron dispersion more quickly. For these media, the optimum mill base viscosities will differ but as a frame of reference anything less than 1000 cps is considered to be low and anything above 3000 cps is believed to be high.
It should be noted that the mill base viscosities are pertinent only during standard immersion mill operating conditions. Viscosities before or after milling will not affect the dispersion process. Heating or cooling can be used to tune the mill base viscosity to the optimum range. Color and gloss development largely depends on the particle size and distribution of the finished dispersion.
The immersion mill is capable of maximizing shear when the media/viscosity relationship is improved. Viscosities less than the optimum range will enable media to contact each other sans a film of dispersion between them. This will wear the media quickly. While a grind can still be achieved, mass tone color and undertone, particularly in high oil absorbing organic pigments, will not be optimized.
Other alternative media comprise a range of glass and plastic, all of which are developed for lower viscosity aqueous dispersions. Spherical steel media can be used to disperse high oil absorption pigments like carbon black. While the steel media will need more horsepower, it will produce generate maximum heat and impart maximum shear to the mill base. For smaller sized media, a basket screen opening would be required which may be too narrow to facilitate proper flow of a high viscosity mill base. For 0.8mm steel media, mill base viscosities of greater than 1000 cps can be used.
Batch Size, Portable Tank Selection
Using a portable tank for the immersion mill can impact the performance of the mill. The mill will work best when the top draft tube is positioned 1 diameter beneath the surface of the batch, and the bottom prop is placed 1 diameter off the bottom of the tank.
If the mill is kept too close to the base of the tank, it will reduce throughput, foul the prop and may cause back pressure inside the mill. Although this set up will not create optimum results with respect to the processing time, it will still generate the required results.
If the top draft tube is placed too closely to the surface of the batch, it will limit the amount of fluid available to the mill and thus reduce throughput. Processing time will again increase, however the desired results can still be obtained.
If a short batch had to be run, the mill off center must be located to reduce vortexing. Batch size increases as the tank diameters increase, and hence the effect of the mill on the product nearest the side walls is reduced to a great extent. In case the diameter of the tank is too great for the mill to create a proper flow, it will need more milling time or lead to a greater particle size distribution. Media size, operating viscosity of the batch, and screen slot opening will determine the maximum batch size.
Mill Wear and Media Life
The Hockmeyer immersion mill is manufactured from standard materials, including hardened stainless steel and stainless steel. The wear should be kept to a minimum. In order to reduce wear and extend media life, the media must properly match with the mill base viscosity. Maintaining shaft speeds and clean up times will significantly reduce wear on the mill and thus extend media life.
Introduction of Dry Materials
Though only pre-dispersed materials should be utilized with the immersion mill, small amounts of dry material can be integrated into the mill. Batch volume seems to be a limiting factor. In case the fluid level is not adequate to cover the mill properly it is not easy to produce the proper flow needed to disperse dry materials into the batch.
Most commercial dispersions need pigment loadings of 30% to 50%, and hence the initial volume will not cover the mill. If users want to experiment this option, they should consider the possible consequences. If dry pigments are loaded too rapidly, it may lead to partial or complete clogging of the screens and consequently result in partial or complete loss of media into the batch.
The Hockmeyer immersion mill is a unique, patented machine and produces results that far exceed existing standards such as color development, gloss, particle size distribution, grind and transparency. The ability of the immersion mill to produce maximum color development reduces the amount of pigment needed to create a given color strength. Unlike standard media mills, the immersion mill has a specific media load irrespective of the product or media type.
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.
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