The HCPS model can mill materials with viscosities up to 400,000cps. The HCPS Immersion Mill (Figure 1) can be fixed with two different sweep blades: the Anchor Helix Scraper blade for heavier, tackier materials and the Sidewall Scraper blade for thixotropic materials. The HCPS mill can also be integrated with an auger that extends out of the upper draft tube of the mill. This allows pulling the material above the upper draft tube and loading it into the media field for grinding.
Figure 1. HCPS Immersion Mill.
Side Wall Scraper
The Sidewall Scraper (Figure 2) comprises a single rod connected to a concentric hub that revolves around the mill assembly. Teflon scrapers are mounted on top of each other in a continuous row, thus covering the entire sidewall of the tank. The scrapers are pressed against the tank by the material being milled when the concentric hub rotates clockwise or counterclockwise.
Figure 2. The Sidewall Scraper.
The new design of the HCPS mill allows using the batch itself to press the scrapers against the sidewall, thus avoiding the use of springs as in the case of conventional type scrapers. The purpose is to facilitate flow control for lower viscosity, highly thixotropic materials. The single rod scraper support enables easy cleaning and changing of the Sidewall Scrapers. Convection, Radiation and Conduction are the three heat transfer conditions of these scrapers.
The Anchor Helix Scraper
The Anchor Helix Scraper (Figure 3) has a helical design featuring only one riser and a bottom leg crossing the entire tank bottom. The tank is scraped across the bottom and on its sidewall. The Anchor Helix Scraper is engineered for products with higher density and viscosity. It provides supplementary agitation to avoid hot spots and stratification. The Anchor Helix Scraper performs scraping in the counterclockwise direction. It is easier to clean and change and provides more efficient milling.
Figure 3. The Anchor Helix Scraper.
The Anchor Helix Scraper is an ideal cooling tool for products with high viscosity and density. It transfers heat more effectively than the Sidewall scraper under these conditions. The Sidewall Scraper forms a Forced Conduction Zone and a Forced Convection Zone during the milling process. The Anchor Helix Scraper can form these zones devoid of the mill in operation. The efficiency to this tool is further increased when forced radiation is considered due to the homogenous top to bottom turnover of the batch delivered by the helix.
Figure 4. Auger outside the intake upper draft tube of the mill.
The roles of the auger (Figure 4) in the Immersion Mill are as follows:
- Loads highly viscous material into the grind chamber
- Serves as a controller of feed rate into the grind chamber
- Serves as a seal to avoid floating or ‘burping’ of media as mill is immersed
- Entrapped air leaves the bead field through the screens
- Avoids film formation within upper draft tube, improving cooling
- The presence of auger makes the mill more effective.
Engaging the Issues of Scale-up Cooling
Special measures need to be taken for scaling up from lab size equipment to production size equipment in order to keep the batch cool. The available cooling surface area of a production tank is very small per gallon of product when compared to that of a laboratory tank. The way to achieve optimum heat control is by creating a forced conduction zone, a forced convection zone and a forced radiation zone in the tank concurrently.
This is achieved by physically scraping cool material from the tank walls, adding the hot material with the cool material, and exposing the hot material to the atmosphere above the batch. Using internal tank cooling baffles or coils would influence flow during high viscosity milling and be counterproductive. So, they are not considered. Although it is not possible to regain the lost surface area per gallon, efficiency can be significantly increased through the use of scraped surface agitation.
Other Variables to be Considered in Heat Transfer
Conductivity of the material being milled: If the batch has poor or non-conductive ingredients, temperature can build quickly and become difficult to remove.
- Cooling Water Temperature: Must be adjustable to ensure predictable temperature control
- Shear: may increase temperature when viscosity rises
- Viscosity: Batch temperature increases with viscosity increase, while there is no change in equipment and cooling variables
- Solids Content: Increasing solids content may increase shear and temperature.
The Immersion Mill becomes more effective through the integration of the auger, Sidewall Scraper, and Anchor Helix Scraper, allowing handling viscosity ranges that were previously beyond its limit. Companies can run most of their formulations that are adjustable on the new HCPS High Viscosity Immersion Mill.
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.
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