#20 Ceramic production, chemicals & environmental impact
- 1. Introduction
- 2. Step one - Grinding of ceramic mixture
- 3. Step two - Glaze's application
- 4. Step three - Digital printing process
- 5. Step four - Grit application
- 6. Step five - Firing cycle
This dissertation aims to refocus, from a special and provocative perspective, the role of chemicals used in ceramics: from the beginning to the end of the entire process, that is from the grinding of the clays that form the basis of the ceramic mixture up to the firing cycle. We will use a specific and clear filter, that is chemicals and environmental impact. We will try to break the preconception about the word chemistry, that often is related to the ideas of pollution, contamination, infection.
We do not want to belittle or deny the wrong use of potentially dangerous products by some industrial producers but, for sure, we would like to fight against another word – chemophobia – that is the irrational fear or idea that all chemicals are dangerous and harmful.
If we looked at chemicals for ceramics production with a magnifying glass, we would discover that their alleged danger does not make any sense. On the contrary, sometimes they can promote some environmental improvements that should not be underestimated or even disclaimed. We will proceed along the production line, highlighting the strength about chemicals and sustainability by showing their positive contribution.
Is this a provocation? Probably.
Is this food for thoughts? Surely.
2. Step one - Grinding of ceramic mixture
Ceramic mixtures are basically made of clays (more or less plastic) and hard materials such as feldspars and sands. Once they have been selected, raw materials are milled in the grinding mills within which a proper amount of water is added, to properly develop the process. Raw materials together with the water forms the slurry (or the so-called barbottina). Even if slurries can be very different one from each other, the usually contain the same water amount that is usually about 35%. A percentage that ceramic producers always try to reduce with different action, to increase the solid amount inside the suspension.
After the milling process, the slurry enters the atomizer to lose the liquid part of the suspension (except for a low amount of residual humidity that is about 5/7%), giving back only the atomized ceramic powder that, under the weight of the press, will form the raw ceramic tile. The water elimination/evaporation takes place thanks to the use of strong, high, and so energy-using heat forces: the higher the water content, the greater the energy to be used with a consequent greater environmental impact as well as a cost increasing. This is the reason why producers always try to work with high-density slurries, marked by high solid and low water contents.
Together with this, the combo more solid and less water allows you to produce in the atomizers higher amount of atomized powder (for the same volume of slurry). To put it shortly, the lower water amount decreases the environmental impact thanks to the reduction of gas/energy inside the atomizers and at the same time it increases the productivity by producing greater amounts of atomized powder.
How can chemicals involved at this stage of the production process promote an environmental impact reduction?
The use of proper DISPERSANTS – that are specifically studied to provide the slurry with the right viscosity values – allows you to correctly develop the production process and to reduce the water amount in the suspension: their fluidifying action, in other words, is synonymous with energy reduction. The latter can also be improved or intensified by using TEMPORARY BINDERS.
The tiles’ mechanical resistance almost entirely comes from the presence of plastic clays that, however, can be primarily responsible for a difficult fluidization of the ceramic mixture. In fact, since they absorb significant amount of water, plastic clays force the producers to increase the water amount inside the system (barbottina) to improve the flow (the movement) of clay micelles. The excessive reduction in the use of plastic clays could be useful to increase the slurry’s density values but it is surely not recommended if you do not want to negatively affect the raw tiles’ mechanical resistance values. Nevertheless, temporary binders can rebalance most of the values, also reducing the environmental impact. How do they do it?
- Restoration of the mechanical features so to avoid breakage of the raw ceramic tile
- Reduction in the use of plastic clays
- Increase in the density values (resulting in the reduction in the use of water
- That turns into a reduction in energy consumption (and into a productivity increasement)
3. Step two - Glaze applications
Ceramic glazes can be applied by means of several systems, but the airless spray application is surely the most common and widespread. This system is excellent and usually easier to manage, but it can be critical in terms of material waste and maintenance by line operators.
In what way?
The spray cabin must be provided with very powerful suction systems, capable of collecting and recovering the so-called over spray that could spread in the working environment as well as inside the cabin, where it could encrust or improperly drip on the tile. It is estimated that the amount of material collected by the suction machines (discarded and non-recoverable glaze) can range from 15% to 30% of the glaze (or engobe) applied. This percentage also includes the wasted glaze inside the cabin that is normally (and optimistically) washed at each work shift. Waste material is obviously synonymous with environmental impact.
Together with these topics that are all about waste, the airless application is also marked by other interesting criticalities that, of course, can be solved but carefully checked to promote a proper application process and produce high performing tiles. The whole machine management and the spray nozzle deterioration have from this perspective an important role. Nozzles, in fact, tend to widen their diameter over time, increasing both the weight of glaze applied on the tile and the material waste. Since the glaze is improperly applied, the tile can be easily compromised. Moreover, their temporary obstruction can affect the industrial yield, forcing producers to discard the defective material that have been produced.
How can we handle these problems and, overall, what has this to do with the environment?
Let us proceed step by step also broadening the perspective.
In recent times, several machine manufacturers have entered the market with new types of glazing machines based on a highly innovative application system. Among the several characteristics, the most significant is all about the possibility (or ability) to discharge the glaze exclusively on the ceramic support, drastically reducing waste and offering at once an automated management of the machine thanks to the presence of control and automation systems (as it happens with digital printing machines).
THE ROLE OF CHEMICALS
As has happened in the past with traditional application systems, even with this new technology it is important to define in advance the parameters that the glaze must have to develop the best performance according to the manufacturer’s line layout. Depending on the glaze’s discharging system, it is very important a detailed study of all aspects related to rheology, surface tension, pH, etc., and in this respect the role of additives - whether they are surfactants, rheological modifiers, wetting agents, lubricants, or dispersants - it is certainly decisive. Of course, this is quite clear, but it may be less obvious to underline the environmental implications that all the chemicals involved at this stage of the ceramic production process can promote. The reliability of the new glazing technology – together with the proper values of the glaze, achieved by adding the proper chemicals – promotes, in fact, a productive continuity, allowing you to apply on the raw tile a minimum amount of glaze, avoiding waste of material and reducing the maintenance of machine (waste of water). Both parameters are evidently related to environmental impact. The tools' washing times together with water consumption, in fact, can negatively impact on both productivity and ecosystem.
REDUCTION OF WATER AMOUNT USED ALONG THE GLAZING LINE
How can chemicals applied along the glazing line reduce the amount of water-consumption, decreasing therefore the environmental impact?
The ceramic glaze application, since we are talking about watery suspensions, obviously involve a significant use of water that, once it has been discharged on the raw tile, it is partially absorbed by the support and partially evaporate due to the temperature of the tile that previously was inside the dryer. A temperature that is important not only for the proper development of the evaporation process but also for the proper leveling of the glaze on the support. The water absorbed by the support must be expelled by evaporation in the preheating phase during the firing cycle inside the kilns. Even this process (like it happens inside the dryers) implies an important use of energy (evaporation = energy). Normally, the greater amount of water is absorbed by the support, the longer will be the time of the ceramic material in the preheating stage, producing a greater consumption of energy (that is gas) to evaporate the residual moisture with a gradual and calibrated water release, avoiding breakage of the material. That said, we can also add that if we decreased the amount of water along the glazing line, we could also potentially reduce the temperature of the tiles leaving the dryer (first reduction of energy consumption) and the energy released inside the kiln (second reduction of energy consumption).
Going backward, what actions can be taken to reduce the water-amount for glazes and engobes?
To reach the same application performance, the reduction of water must go hand in hand with an increase in the density of the glaze (or engobes) and to make this happen it is necessary to previously study inside the lab all chemicals to be used both in the grinding and application phase. During the grinding phase DISPERSANT and BINDERS must be added to get a high-density value during the application along the glazing: it is therefore important be aware of what will happen during the process. Once we have taken the proper decision during the grinding stage, we can also act (to reduce the amount of water that is usually added along the glazing line) by using LEVELING AGENTS or other kind of chemicals able to act on glaze’s drying and drainage time. In short, the action promoted by these chemicals affect the glaze performance, causing good result even with glazes marked by very high-density values.
4. Step three - Digital printing process
For this section, please read the text (Did you know that?) or listen to the episode of the podcast: What benefit can you get by using water-based digital glues? In this episode all benefits, even those about environmental impact, are pointed out.
5. Step four - Grit application
Let’s start with a long introduction to frame the topic.
In the last decades, the rapid technology development of ceramic industry has completely changed the scenario, expanding the aesthetic and technical offer. Just think of the new large size now available or of what the digital technology has introduced in terms of aesthetic possibilities such as, for example, the reproduction of woods and marbles.
For example, the production of these marbles - that must be lapped after firing to be completely like the original and natural material - involves the formation a glass surface marked by a significant thickness (often more than 300 microns) that can be partially removed to develop and reproduce the mirror effect of real marbles. The creation of this kind of glass - noble, hard, transparent, and resistant - takes place using grits or ground frits suspended in water (slurry) and with the addition of appropriate ORGANIC ADDITIVES that allow a correct application by means of airless systems: correct nebulization, appropriate drying process on the raw ceramic support, levelling action and binding power. Just to mention the most important.
THE WATER’S ROLE
To ensure a proper wettability of the grits, in addition to the use of suitable DISPERSANT and WETTING AGENTS, it is important to use significant water-amount.
There is a limit, beyond which a low water content, in fact, does not promote an adequate circulation of the grit in suspension, both inside the conveyor systems (the pipes that connect the tanks containing the slurry to the sprayer) and during the glaze discharge from the nozzles. Of course, a significant water-amount is also important to reach low-viscosity levels that in turns are required for a proper nebulization. For example, giving indicative numbers, we can say that an adequate amount of slurry (in preparation for a following lapping phase) should be about 1kg per m2 (of which 500/600 grams of grit and 500/400 grams of water and chemicals). This amount cannot be increased because an excessive water amount con lead to problems during the pre-heating phase such as the tiles explosion or increased drying time inside the kilns. And, of course, this is a very bad option in terms of industrial productivity.
Speaking of costs and environmental impact, the massive contribution of water to the tile surface can lead to the following problems:
- Maintaining of tile’s high temperature along the entire production line to promote a better water evaporation process and to reduce the water-absorption by the support ► increased energy spent on temperature maintenance
- Presence inside the kilns of material with a high water-amount that turns into an increased energy consumptionto properly develop the evaporation process
- Increased production times during the pre-heating stage
Is it possible to improve this scenario, also decreasing the environmental impact, by using proper chemicals?
THE ROLE OF CHEMICALS
In general, as already underline, even at this stage of the production process, the use of CHEMICALS FOR GRITS, LEVELING AGENTS, and DISPERSANT positively acts on the density of the slurry and, therefore, on the water-content.
What do they do?
In simple words: while some compounds or rheological chemicals in formulation promote a non-associative behavior in respect of grits and other inorganic materials, the formula is also provided with other substances capable of increasing the wettability of the system (that is, the grit suspension) even in case of water shortage. This double action, which results in a lower interaction of the suspended particles, is important to make grits free to properly move. Glass particles, in fact, are electrically active and this characteristic exposes them to a form of attraction / interaction that reduces their mobility in suspension, compromising the process. Chemicals, among other properties, can screen or isolate particles by limiting interactions during suspension and promoting grit applications with low water content and therefore with reduced environmental impact.
As we know, grit applications can also be developed by means of dry technology that involved a previous application of glue (both digital or analogical) and, when necessary, a final low-weight application of glue on the already applied grit.
A three-steps process:
- Liquid glue application
- Dry grit application
- Low-weight application of glue
These application systems promote, by their very nature, not only technical benefits but also some interesting environmental improvements. The most significant is all about the important water-amount decrease:
- The grit is not in suspension and the liquid part does not exist (we are not talking about slurries)
- The amount of the applied glue (natural derivative product in water) usually ranges from 200 and 300 gr/mq (low quantities)
These is reflected, as we have already seen, in a lower energy consumption and therefore in a lower environmental impact.
THE ROLE OF CHEMICALS
If dry applications by their very nature promote a lower environmental impact (than that promoted by wet processes), It is also true that the proper formulation of chemicals involved in the process can be very useful to further reduce the impact on the environment.
Through the study and development of glue formulations that consider their impact in the combustion phase. This occurs to a large extent with the maximum reduction in the use of organic matter and in the appropriate choice of chemical compounds. In this perspective, the formulation of the glue must be based on molecules that develop an adequate binding power and simultaneously promote an optimal combustion process. Therefore, in this step of the production cycle, the conditions are created to reduce in the following one the serious impact originated by ceramic kilns.
6. Step five - The firing cycle
The firing phase is certainly the most energy-intensive of the entire ceramic production process and at the same time it is the one most involved in the emissions of pollutants and of CO2. All actions we have been talking about (at least most of them) are basically intended to decrease the emissions and to reduce the energy consumption that usually takes place in this last and important stage of the ceramic production process.
What are the issues to be addressed?
- PERCENTAGE OF MOISTURE OR RESIDUAL WATER IN THE TILES ENTERING THE KILN
It must be as low as possible. The water in the raw tile when entering the kiln leads to two different kinds of problem. If the temperature gradient is not calibrated and constant, pressure phenomena, expansion and finally bursting of the tile could develop within the material (check episode #01). To ensure that the water evaporates gradually leaving the ceramic body, it is therefore essential to properly manage the times of the firing cycle that obviously affect energy consumption and industrial productivity.
- AMOUNT AND KIND OF ORGAIC MATTER INSIDE THE RAW CERAMIC TILE
Depending on the course of the process, different kind of gases (organic molecules) may be produced during firing and pre-firing stages. This means that not only chemical components but also the parameters and the set-up of the kilns are decisive in obtaining the best conditions and balance between energy and emissions. What does that mean?
Just as in the case of older generation cars, older generation kilns (or even the new ones when improperly managed) are responsible for increased energy consumption and an inaccurate combustion that turns in an increased amount of organic matter and CO2 in the atmosphere. That said, the use of CHEMISTRY that minimizes emissions of pollutants and odors in the atmosphere is a cornerstone. This kind of chemistry must be developed considering the process that develop inside the kilns.
Just to make an example, there are organic molecules (organic chemicals) that, due to their structural conformation, can thermally break down during the pre-firing phase only producing CO2, small amounts of aldehydes, organic acids and molecules with a high olfactory threshold (perceivable only at high concentrations). In an eco-sustainable setting, it would be advisable to abolish, or at least reduce, molecules that during degradation give rise to other strongly harmful molecules such as, for example, aromatic compounds or benzene derivatives.
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