By John Sneeringer*
Mechanisms of shear force
The reduction of the particle size of agglomerates occurs in fine media mills due to the action of moving media relative to each other. There are only two possible mechanisms of dispersion or attrition in media mills: impact and shear force.
There are two possible impact mechanisms, one frontal or one forward. These require a mass of particles to be placed between two media surfaces at the time of impact. Statistically, this is quite unlikely to happen. As discussed above, the high volume of media and liquid flow patterns lead to the conclusion that media tend to be layered and flow in the same direction. The amount of countercurrent or intermingled flow between the shear force zones is relatively small. This leads to the conclusion that frontal impacts represent a very small part of the work done. Since media generally moves in the same direction, there is a greater likelihood of an advance impact between moving media at different speeds.
The shear force or shear mechanism occurs when two media are exceeded while both go in the same direction, establishing a shear zone between the media. In this zone, the degree of shear is determined by the differential velocity of the media, the distance between them and the viscosity of the liquid. Extensive evidence has confirmed and reconfirmed that shear force is the dominant dispersion mechanism in any type of fine media mill.
Dispersion of shear force
As the beads of the rotating media are exceeded, a degree of shear force is established in the space between them due to differences in their linear velocities. The degree of shear increases by the directional rotation of the opposing media. When a particulate mass enters the space between the media, a shear stress is imposed on the particle.
The magnitude of this shear stress increases with the size of the mass. If the applied stress is greater than the associative forces holding the mass together, then the mass will be divided into smaller parts. This division process continues until the masses of particulates become so small that the reduced shear stress is less than the forces holding the particle mass together. This is what is called the point of diminishing returns; no additional work will be done until some physical aspect of the process has been changed.
At this point, the small mass passes through the media space undisturbed and no further dispersions or reductions in particle size occur unless the degree of shear stress increases. This can be achieved by making changes to the mill configuration, such as increases in the rotational speed of the disc, or a reduction in the size of the media (a reduction in the spaces of the fluidized media). The degree of shear stress can also be increased by an increase in viscosity or solid concentration in the liquid-solid formulation being pumped into the horizontal media mill.
Process variables: premix formulation
The process variables associated with the composition and preparation of the "premix" of the liquid-solid formulation have a great effect on the operation and productivity of the horizontal media mill. The "premix" variables relate to three areas: formulation, preparation procedures, and equipment configuration.
The variables of the premix formulation include, but are not limited to:
· Viscosity and rheology of the pre-dispersed liquid-solid formulation (rheology is the study of the deformation and flow of materials by the effect of external forces)
· Particle size of solids in the pre-dispersed formulation
· Shape and hardness of solids
· Specific gravity of the liquid-solid formulation (weight per gallon)
Variables of the premix preparation procedure may include:
· Proportions of ingredients
· Speed of addition of ingredients
· Addition sequence
· Temperature
· Time cycle or premix stage
Premix configuration variables can include:
· Type of dispersion mechanism.
The total quality of the pre-dispersed liquid-solid formulation will be affected by all these possible variables. Quality has to do mainly with the degree of wetting, reduction of the initial particle size and stability of the liquid-solid formulation.
A common flaw among users of horizontal media mills is that they do not attempt to optimize the premixing process to produce the highest quality in the practical premix. As a result, the initial quality can vary significantly between batches or between laboratory and plant lots.
In a test model, a premix dispersed at 50% of the norm required a residence time of 54 minutes in the horizontal mill to achieve a standard quality of 100%. The same formulation, dispersed to 70% of the norm in the premix, required 38 minutes of residence in the horizontal mill to reach a standard quality of 100%.
This indicates that optimizing the premix process (with less sophisticated devices) can increase productivity substantially. From the point of view of operations in the plant, it is desirable to have a reproducible initial quality, since when using the same grinding configuration and the same operating conditions, the residence time required to match the standard quality can be predicted by logical mechanisms.
Despite its vital importance for successful operation and low maintenance, the preparation and optimization of the premix is a variable over which the mill manufacturer has little or no control.
Process variables in the horizontal mill
The process configuration of the horizontal media mill includes the following variables, independent of the liquid-solid formulation and the final particle size required.
Mill-independent variables
Grinding media: size (0.5 mm to 2.5 mm commonly)
specific gravity
percentage of charge in the camera of
grinding (75 to 90% usually)
Grinding shaft configuration:
disc design
rotating surface area
spacer design
Grinding shaft speed: 2000 to 3000 ft/min (common)
(disc tip speed)
Product performance: variable speed, positive displacement, pumping system (by gears, progressive cavity, with air diaphragm, peristaltic)
Any change to any of the mill's "independent" variables will generate a change or limit any of the process"dependent" variables listed below:
Dependent variables
• Product temperature
• Pressure
• Quality
• Performance
• Electricity consumption
Optimization Guidelines
To optimize the horizontal media mill for any specific liquid-solid formulation, a reliable "benchmark" of the processing data must be recorded.
Normal processing parameters, which provide the information required for the evaluation and optimization of mill performance include:
· Product speed (pumping speed in gallons/hour or gallons/minute)
· Product quality (particle size after grinding)
· Product pressure in the grinding chamber
· Product temperature (inlet and outlet)
· Electrical consumption of the main motor
· Media size and density
· Media load percentage
· Grinding disc speed
· Grinding shaft configuration
· Cooling water temperature (inlet and outlet)
This viscosity and rheology of liquid-solid formation must be taken into account after dispersion or grinding is completed. It is quite common for the viscosity to rise significantly in a liquid suspension after grinding, basically because of the larger surface area of the solids present. This increase in surface area creates an absorption of the available liquid from the formulation.
Examples of cause and effect
in the grinding variables
Any change in any of the variables will produce a change in one or more of the other variables in the process. Here are some common examples of cause and effect to understand how the total horizontal grinding process works. These examples are presented only as an introduction to the optimization guidelines and only a brief mention of this topic will be made.
• The increase in the product performance index usually manifests itself in:
- Increased product pressure
- Increase in product temperature
- Increased electricity consumption
- Reduction of product quality
• The decrease in the product performance index usually produces:
- Increase in product quality (thanks to a longer residence time)
- Reduction of product pressure (if viscosity is kept constant)
- Reduction of product temperature
- Reduction of electricity consumption
• The increase in the speed of the grinding disc usually generates:
- Increased product quality
- Increase in product temperature
- Increased electricity consumption
- Increase or reduction of product pressure
• The increase in the number of grinding discs (addition of the rotating surface area):
- Increases product quality
- Increases the temperature of the product
- Increases electricity consumption
- Can increase product pressure
• The increase in the percentage of media load usually generates:
- Increased product quality
- Increase in product temperature
- Increased electricity consumption
- Increased product pressure
• The reduction of the percentage of media load usually generates:
- Reduction of product quality
- Reduction of product temperature
- Reduction of electricity consumption
- Reduction of product pressure
• Reducing the size of the media usually produces:
- Increased product quality
- Increase in product temperature
- Increased product pressure
- Increased electricity consumption
• The increase in media size usually manifests itself in:
- Reduction of product quality
- Reduction of product temperature
- Reduction of electricity consumption
- Reduction of product pressure
*Technical Sales for Premier Mill.
