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#14 Additives for ceramic mixtures: an overview


  1. 1. Introduction
  2. 2. From the raw material to the raw tile
  3. 3. Ceramic mixture and main families of additives
            a) Mechanical resistance and temporary binders
            b) Organic additives and dispersant
            c) Sanitizing and preservatives



1. Introduction

Ceramic mixtures used for tile production, both for floor and wall, are largely made up of:

  1. Inorganic raw materials (clays, feldspars, sands, etc.
  2. Milling water
  3. Chemicals

2. From the raw material to the raw tile

The following are the phases in sequence to which the ceramic mixture undergoes:


  1. 1. GRINDING
  2. 2. SIEVING
  3. 3. STOCKING

The ceramic mixture, before the forming phase (during which the atomized powders are pressed or compacted), undergoes a wet grinding process by using ground water (recovery water and groundwater). During this step, the ground water and the inorganic raw materials thus mixed, form the slurry also called barbottina.
The milling process, instead, takes place by means of Alsing type mills; after that the barbottina is sieved and stored in tanks within which it remains under constant stirring so as to avoid any gelling and/or sedimentation phenomena. 
After the storage phase – whose duration can range according to manufacturer’s needs - we proceed with the atomization phase by means of spray dryers (also called atomizers). Inside the atomizer the barbottina is nebulised thanks to the use of very high temperatures, able to evaporate water in a rather violent way. This evaporation "transforms" the droplets of the barbottina into tiny hollow spheres, which together form the atomized powder, ready for the pressing/forming phase.



Despite the evaporation process that takes place inside the atomizers, the atomized powder retains a residual moisture content (usually 5/7%) that is useful for giving the granules the right plasticity value to guarantee their deformation during the pressing/forming process. Once pressed, the raw tile is, in fact, also called green, precisely because it still contains a certain percentage of residual moisture (just like freshly cut wood).


3. Ceramic mixture and main families of additives

Generally speaking, in all process decribed above the use of organic and inorganic chemical additives is very important  for the correct production of ceramic material.




While residual moisture gives the atomised powder the right degree of plasticity, it is also one of the variables responsible for the low mechanical strength of the green tile. In fact, it is only after the piece has been dried in the dryers that the raw material gains a higher mechanical resistance thanks to the definitive expulsion of residual moisture.

In particular, when water leaves both the surface of inorganic particles and the surface of organic binders molecules, inorganic molecules (and in particular clays) approach and interact by means of molecular attractions therefore acquiring greater hardness. It’s exactly this type of action that helps to give the raw tile a greater mechanical resistance.

If we also consider the aesthetic evolutions that ceramic materials have undergone in recent years - exponential increase in sizes and progressive, albeit partial, reduction in thickness - the tensions and mechanical resistance of raw tiles have become an important topic of discussion and work for all those who have the task of preserving the raw material from cracks and/or breakage.
In this respect, numerous studies are currently underway on organic raw materials and additives for ceramic mixture capable of acting on the resistance of the material. Compared to the past, in fact, the formulation of TEMPORARY BINDERS has to take into consideration mechanical stresses that are completely different from those present in small tiles, which were widely used in the past.  In general, the predominant mechanical tension of the latter was, in fact, substantially linked to the compression of the screen printing rollers: in the past it was usual to evaluate only the modulus of flexural strength.
Today, however, it is more important to evaluate other phenomena, such as resistance to deformation, resistance to impact or the cohesion of the product (especially if it undergoes grinding and/or raw cutting processes).


As a rule, to increase the mechanical strength of the ceramic dried piece, temporary binders should be used. They are so called due to the fact they bind the raw tile only during the pre-cooking phase so as to preserve it from possible damage. They differ from the binders tout court that, once added to the mixture, they definitively increase the mechanical resistance of the post-firing tile.



Binders are usually organic molecules able to promote a binding action with regard to the raw materials of the mixture, especially with clays.
What happens from a chemical point of view?
A molecule of temporary binder contains functional groups (parts of the molecule able to chemically interact with other materials) that create a binding effect both between the micelles and the hard inorganic raw materials. It basically links together the inorganic particles producing a crosslink net that increases the resistance of the tiles after drying. The temporary binder is usually added inside the mills during the raw materials’ wet grinding process or directly inside the slurry (the mix of clays, raw materials and water).




In general grinding a ceramic mixture without using appropriate DISPERSANT AGENT, in addition to several other critical issues, mainly prevents to obtain a barbottina with the proper rheology and a high-density value, foundamental industrial productivity requirements.

Let’s go step by step and start from the process water used for grinding, a decisive factor for a proper fluidization.



The grinding water, which is essential for a high-density and rheologically correct ceramic mixture, is generally composed of well water and process water generally deriving from the polishing and glazing department, and possibly from the cutting and sanding departments [the percentage of well water compared to that of process water is normally different for each ceramic company and changes according to the parameters in use].
In any case, the milling water plays a decisive role for the proper development of the processes and for obtaining high-performance results in terms of finished ceramic product. They must, however, be characterized by specific features without which the whole process could be compromised, requiring the intervention of targeted additions.
What features the grinding water must have? 
In general, it is important that the composition and quality of the water is constant over time so as to avoid possible fluctuations in the milling parameters that would force the companies to continuous adaptations and modifications of the kind of production already identified. For this reason they must be subjected to constant controls and monitoring, especially in terms of pH and electrical conductivity which essentially indicates the amount of ions (cations and anions) present in the water.

The electrical conductivity of the ground water must be low or, even better, very low in order to facilitate the fluidization process.


What does that mean (and why)?


To simplify, it is possible to declare that water with low electrical conductivity is a water containing a low concentration of ions. An important quantity of multivalent anions and cations (such as calcium 2+ or magnesium 2+) can create significant interferences to the fluidization process, reducing the thickness of the DIFFUSE LAYER on the surface of the clayey micelles, that is the distribution and thickness of anions and cations on the surface of the clay micella. This reduction results in a general higher viscosity of the system (barbottina) due to the appearance of electrostatic attraction phenomena. We could say, a bit simplistically, that the solid parts of the suspension (the micelles) attract by increasing the viscosity of the suspension.

In other words: an excessive presence of ions (whether monovalent or multivalent) inside a barbottina with a water low content can give rise to problems of increased viscosity: the aforementioned ions begin to strongly interfere between the diffuse layers of clay micelles and the only way to end this mechanism (and thus the problem) is to add water to the system so as to lower the ionic concentration level. 

In electrochemistry, the diffuse double layer (or electric double layer) consists of a structure that originates at the interface solid-liquid, at which a transfer of electric charge occurs, along with half-reactions of oxidation-reduction. From a chemical point of view, under the same condition of electrical conductivity, the presence of ions in the milling water (especially monovalentcations) facilitates the milling process compared to the same water containing cations or multivalent ions.


On the contrary, the low quantity of ions (both monovalent and multivalent) in the grinding water allows the micelles to fluidly glide one over the other, avoiding phenomena of attraction and therefore of viscosity increasing. (A significant amount of ions - and therefore a water with high conductivity - is therefore able to reduce the “availability of water” within the system, worsening its fluidity).
It is clear why the little water of the barbottina must have a low conductivity, even more if we consider that the challenge of the ceramic industry today is to increase the slip density as much as possible (by reducing the water content as much as possible) in order to increase productivity and decrease, at the same time, the energy costs necessary for the water evaporation process during the atomization step.




Improving defects resulting from the high conductivity of water through the use of additives is very complex. However, the use (during the milling process) of suitable dispersants that act with a sequestrant action against bivalent or multivalent cations can certainly mitigate possible defects and facilitate the processes. Dispersant product for ceramic mixture are all ionic, sodium salt, and they are usually formulated to carry out all the following fluidization mechanisms:


  1. Sequestrant action
  2. Cationic exchange
  3. Steric action


The ceramic mixture is an extremely heterogeneous and complex system, and this is why during a fluidization’s study it is important to evaluate different types of molecules in order to optimize both the cost of fluidification and the barbottina’s working density.

Generally speaking, a ceramic mixture’s fluidization is ideal when the rheology of the system provides, during milling, an average low viscosity that can reduce energy consumption and milling time. All additives used must make sure to make a barbottina that is not too fluid yet not overly viscous: both scenarios would impact on the Alsing grinding system so as to make it not fully performing. As an example, a too high viscosity value would be a problem for the proper barbottina’s flow inside the pipes that lead to the spray nozzles of the atomizer. 
It is important to highlight that all these aspects related to the rheology of the barbottina - from grinding to storage - are simulated and reproduced within the labs by the use of appropriate programs provided by the rheometers.




1. Microbiologic degradations:  increasing of the electrical conductivity and therefore of the system’s viscosity 


The trend of electrical conductivity can also affect on possible microbiological degradation phenomena of process waters which always contain more or less high concentrations of organic material. Microbiological degradation of organic molecules, which involves the formation of many metabolites (waste produced by micro-organisms), can sometimes increase the electrical conductivity value. This increase in turn leads to an increase in the ionic charge of the barbottina which, together with other elements, leads the system to VISCOSIZE.

2. Monitoring


The importance of constant monitoring of process and milling water in order to prevent any possible complications is quite clear. Especially in the warmer and wetter seasons, it would be useful to carry out in-depth studies of the water cycle within the production plant, identifying the most sensitive storage points and intervening with appropriate periodic sanitisation procedures and dosage (constantly over time) of appropriate preservatives.


3. Additives for process waters 


Even in these cases, the only possible action is the addition of water to restore mobility to the system. However, it is also possible to take preventive actions to avoid degradation phenomena using specific chemicals capable of sanitizing and preserving water: products capable of killing micro-organisms and/or avoiding their proliferation of time. We are talking especially about SANITIZERS (usually oxidizing products) when these products have a rather fast action but with short-term effects on microorganisms: Microorganisms are eradicated but then bacteria can reappear in a short time. We speak of PRESERVATIVES in the case of molecules that can react over time within the water, ensuring its health.




In any case, it is important to intervene in the preliminary phases of the entire process in order to avoid an increase in conductivity due to degradation. Adding these chemicals only in the milling phase would be late and therefore ineffective.


Why are sanitizers and preservatives usually used in combination?
The preservative guarantees in advance that the water without bacteria is not subject to degradation: any bacterial colony (or spore) that may occur is, in fact, neutralized.
The sanitizer guarantees, in case of uncontrolled proliferation of microorganisms, the complete removal in a short time.

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