CERAMIC SLURRIES: RHEOLOGICAL ANOMALIES AND PRODUCTION PROBLEMS
Atomization and wet grinding systems as well as big continuous systems, work best when slurry’s chemical and physical features are consistent throughout the entire process.
In other words, the process is completely efficient only when the key parameters of the atomised product – such as grain size, density and viscosity – vary within very small ranges.
In theory, grinding mills produce always almost identical slurries only when production conditions do not change over time: raw materials, formulas of ceramic mixture, waters, deflocculant and milling times.
However, because of the many variables that must be controlled, some sudden and significant variations of the slyurry’s rheological properties may occur during the production process.
With a view to simplification, we could reduce these variations in two:
extremely high or extremely low viscosity.
EXTREMELY HIGH VISCOSITY
An extremely high viscosity can cause serious troubles, especially when it is combined with a high flow limit as well as a high thixotropic value.
Following here the most significant ones:
- Poor milling result
- Long discharging times of the mills. In extreme situations, the discharging could be incomplete
- Hard sieving process
- Gel and crusts formation in the tanks and/or in the pipe system
- Clogging of the pumps
- Partial or complete break-down of the production plant
EXTREMELY LOW VISCOSITY
A very low viscosity can similarly lead to major issues:
- Poor milling result and high residual content
- High consumption of the grinding tools
- Sedimentation of the inert matter in the tanks during production stops*
*This latter inconvenience can over time easily lead to ceramic mixture’s atomization always made up from different ingredients. It can also cause, on the base of the tanks, a sediment compaction whose removal is very difficult.
SO, WHAT ARE THE MAIN INGREDIENTS THAT CAN AFFECT AND CRACK THE CORRECT SLURRY'S RHEOLOGY?
1 - MAIN RAW MATERIALS
2 - MINOR AND ADDED RAW MATERIALS
3 - GRINDING WATERS
4 - DEFLOCCULANT
5 - GRAIN SIZE
6 - TEMPERATURE
7 - MILLED MATERIALS AND ADDITIONAL VARIABLES
Let’s go through them.
1 – MAIN RAW MATERIALS
Even though they come from rigorous and methodical excavations, raw materials such as clays, are natural products subject to variations that are sometimes hard to control. Especially considering the frequency with which they are restored in the warehouse.
A variation in the plasticity of the clayey part may lead, for example, to an alteration of the rheological properties of the ceramic mixture. Sensitive fluctuations in humidity can also change the viscosity, acting at the same time on density’s properties.
When the chemical or mineralogical composition of one or more raw materials changes, it is very hard to trace the trigger by chemical or diffractometric analysis, that in several cases do not fully clarify the problem.
2 – MINOR AND ADDED RAW MATERIALS
The addition or variation of additives, both organic and inorganic, can significantly modify the rheological behaviour of the ceramic mixture.
For this reason, their use must undoubtedly be preceded by strict laboratory tests.
This is true also in case of addition/variation of sludge, glazes and materials recovered from other production departments: these materials should also be evaluated and measured in advance.
3 – GRINDING WATERS
Grinding waters are usually real unknown for most of technicians.
The current trend is, in fact, to recover all the industrial waste in the waters, sometimes adding also waste waters deriving from other companies.
This attitude on the one hand offers a reasonable solution to the environmental problems to which the ceramics must rightly pay attention, but on the other hand facilitates possible flocculation phenomena in the ceramic mixture.
Laboratory and industrial studies show that the very high conductivity of both flocculating metal ions and organic substances (as well as an acidic pH) may significantly worsen the viscosity and the flow conditions of the slurry.
These side effects are nearly always repairable by varying the amount or the quality of deflocculants in use.
The real and most significant problems come from all waters that get to the milling department. For this reason some counter-measures should be adopted:
- Storing the water in large stirred tank so to better act on fluctuations
- Check the water’s density in order to avoid evident consequences on slurry’s density
- Control the water’s pH and electrical conductivity, modifying them when necessary
- Carefully check waters coming from purifiers. Even they seems transparent and clean they can lead to important issues (for example including them a significant amount of flocculant agents)
- Verify if bacterial contaminations are in place: they could increase the water conductivity
4 – DEFLOCCULANTS
The correct rheology of slurries can be affected due to a wrong deflocculant’s dosage (both manual or mechanical).
Each deflocculant, depending on the conditions and on its own characteristics, has to be added properly so to reach the requested rheological parameters.
It is obvious that a wrong dosage surely modifies the viscosity. Even when the dosage is mechanical, it is a good habit to periodically check the quantities actually added.
5 – GRAIN SIZE
The grain size of slurries is certainly related to rheological parameters.
All other things being equal, a high residual matter is synonymous of low viscosity, and vice versa.
It is therefore clear, for example, that if the viscosity is too high there is no point in continuing grinding without taking other collateral actions.
Random or deliberate additions of fine or hyperfine particle size fractions (deriving for example from industrial presses) have both an important influence on slurries’ final rheological behaviour.
6 – TEMPERATURE
The temperature reached by a slurry during the milling process has to be taken in consideration in order to get an optimal dispersion of the suspended particles.
In fact, it impact on the deflocculants’ efficiency during the milling and dispersing processes.
Medium high temperatures usually promote the milling process. On the other side, excessively high or excessively low temperatures could reduce production yield.
7 – MILLED MATERIALS AND ADDITIONAL VARIABLES
The amount and the grain size of the milled materials, just like the good status of the mill’s internal coating, are the foundation of a correct grinding process.
Periodic control of these parameters is certainly useful or even necessary.
Nevertheless, a spare attention to these latter issues, do not usually modify slurries’ rheology: a progressive worsening of the milling process merely occur.
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