The preparation of the ceramic mixture for wet grinding offers two important benefits thanks to the raw materials involved:
As we know, the forementioned wet preparation which contains a high amount of solid, is called slurry (barbottina).
Before being used in the ceramic production process, the slurry has to be necessarily transformed in a humid dust suitable for pressing (atomized).
The process that turns the slurry in atomized takes place inside specific spray dryers which are capable to remove the water and leave a 6/7% of residual moisture: this is the essential precondition for keeping the dust plastic and suitable for pressing.
WHAT IS A SLURRY, CHEMICALLY SPEAKING?
The slurry is a suspension in water of plastic clays and other inorganic raw materials (such as feldspar).
WHICH ARE THE PROPERTIES OF THE CLAYS INCLUDED IN THE SLURRY?
Clays, due to their structural characteristics, tend to create a very high thixotropic effect that it is necessary to avoid in order to keep the slurry smooth and processable.
Clays are minerals consisting of octahedral and tetrahedral sheets of Al2O3 (alluminio oxide) and SiO2 (silica).
SHEETS OF CLAY STRUCTURE
The sheets can be disposed in many different ways according to the type of clay. Nevertheless various monovalent or multivalent cations are always present in the gaps between the sheets.
When the clays are dispersed in water, the octahedral and tetrahedral layers tend to expand due to the water that creeps between the cavities.
The distance in turn may cause the release within the suspension of the cations which are in the interstices.
The sheets of clay in water – also called clay micelles – have a positive electric charge on the edges and a negative charge on the two surfaces (top and bottom).
When there is not much water within the suspension, this particular morphology causes the electrostatic attraction between the two charges to generate a structural effect that increases the viscosity of the system (slurry) that can also reach the complete gelling.
To make the system fluid and smooth, micelles and suspended particles must be able to move freely in the suspension.
In order for that to happen, it is important to work on a few mechanism which are linked to deflocculation.
1. ELECTROSTATIC REPULSION FOR CATION EXCHANGE
The multivalent cations – with a double positive charge (such as calcium and magnesium) or with triple positive charge (such as iron or titanium) – have a very high charge and they are therefore able to break the strong negative charge that is on the micelle’s surfaces.
The negative charge is nevertheless required to keep the repulsion force between the micelles that ensure their flow (let us take the example of the repulsive force between two identical magnets).
The addition of monovalent cations within the system (such as sodium) allows the replacement of the multivalent positive charges with weak electric charge (CATION EXCHANGE).
This specific cationic exchange decreases the positive electric charge on the micelles without neutralizing the negative ones.
That’s the origin of the agglomeration’s reduction and of the decreasing of the system’s viscosity.
2. STERIC REPULSION
It is produced by using polymeric dispersants made of inactive molecular chains which contain functional group able to interact with clays and raw materials.
The edges of these molecules interact with the slurry’s suspended particles increasing their distance.
Polymeric dispersants, in other words, bind themselves together with particles (through the functional group) placing their “tail” at the edge: this position is functional to obtain the right distance needed to slide the particles one on the other with no electrostatic interaction.
This phenomena is responsible for the viscosity reduction.
Complexing agents are formed by particular chemical molecules that have functional groups containing atoms such as oxygen or nitrogen) that provide the system with a very negative electronic charge.
This charge ideally attracts multivalent instead of monovalent charges.
As soon as the complexing agents are added to the slurry, they release sodium (monovalent cation) attracting multivalent cations (such as calcium, magnesium, iron or titanium).
The final result of the process is the removal of the multivalent charges from the system and the recirculation of monovalent charges: this facilitates cationic exchange and so the increasing of the double electric layer and the viscosity’s reduction.
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