In this article we will explore the production cycle of the ceramic industry. This sector is made up of industrial companies active in the production of:
- floor and covering tiles;
- refractory and technical materials;
- porcelain and tableware.
In our country, the production cycle in this field-branch involves around 280 companies, which employ more than 27,500 people and develop an annual turnover of 6.5 billion euros (2019 data).
The Italian ceramic industry represents a global excellence. In the year 2020 the sector suffered a semester of loss. Then it recovered in the second half of the year, giving more than positive expectations for 2021. Thanks to technological innovation and product evolution, our ceramics industry is able to satisfy an increasingly demanding market.
The main challenge for the sector is sustainability, namely the reduction of the environmental impact of the production cycle. Later we will see what the ceramic industry does to fit into a circular economy model.
Preparation of raw materials
The preparation of raw materials represents the beginning of the production cycle of the ceramic industry. It aims to obtain a mixture as homogeneous as possible, with a particle size distribution and a shape of the grains appropriate and functional to the final product.
The word “ceramic” comes from the ancient Greek κέραμος, kéramos, which means “potter’s clay”, “pottery”. It is an inorganic, non-metallic material which is very ductile in its natural state and becomes rigid after the firing phase.
As a rule, ceramics are composed of several materials: mainly clays, but also feldspar (sodium, potassium or both), silica sand, iron oxides, alumina and quartz. Before they can be processed, it is necessary to resort to a preparation of the raw materials. The phases of this process can be different, depending on the result to be aimed at and the type of production (artisanal or industrial) that will be carried out.
As for the clay, in the preparation phase of the raw materials in an industrial production, it has to be selected according to the production to be carried out.
The most common types of clay are:
- sandy clay, characterized by thin granulometry and high plasticity;
- refractory clays, particularly fire resistant;
- kaolin, white, used for the production of porcelain and characterized by refractoriness, low plasticity and low drying power.
Once chosen the most suitable one, the clay must be cleaned of impurities (curing phase), and then subjected to washing, by dissolving it in water to disperse the soluble salts. Further purification allows you to eliminate residual impurities and refine the compound, removing the coarser granules.
The next step in this phase of preparing the raw materials is the processing of the clay, which is kneaded to eliminate any air bubbles and make it compact. Sometimes a powder is added to the compound obtained from the grinding of previously fired ceramic scraps (chamotte), that serves to make the product more resistant to sudden temperature leap.
This process of preparing raw materials, within the production cycle of the ceramic industry, can vary, depending on the product to be obtained, e.g.:
- porous-paste ceramics (terracotta, earthenware, majolica);
- compact-paste ceramics (stoneware, klinker, porcelain).
What differentiates terracotta from other types of ceramics is the presence of salts or iron oxides: they are responsible for the yellow to red-brown color that terracotta assume after being fired. Iron oxides contribute to the vitrification of this type of ceramic, improving its mechanical resistance and reducing its porosity.
Firing takes place at a temperature of 980-990 degrees centigrade.
Terracotta is particularly versatile and can be used with surface coating or not. In the first case, especially for the production of kitchenware (plates, cups, slow cooking pots, etc.), in the second as a structural and ornamental material for vases, jugs, bricks and more.
In the form of brick, terracotta is the material more used in buildings. Compared to real terracotta, brick is distinguished by a less accurate purification of clays.
Porcelain stoneware is obtained through a sintering process of raw materials (ceramic clays, feldspar, kaolins and sand) which are first grinded and transformed into slip, then finely atomized, until a granulometrically uniform powder is obtained, suitable for pressing.
In fact, stoneware is mainly used to produce tiles for bathrooms and kitchens. Firing takes place at a temperature between 1200 and 1350 degrees centigrade, and the colors vary according to the raw material used.
Two types of porcelain stoneware can be distinguished:
- natural, known as technical stoneware, similar to natural marble and maintains the technical characteristics of porcelain stoneware;
- glazed, or colored during mixing with the addition of kaolinite clays, fluxes and inert materials.
The klinker is a brick material subjected to a particular curing and drying process that ends with a firing at 1250 degrees centigrade for more than 30 hours. This complex procedure makes possible the solidification of the mixture and the disposal of moisture, making the klinker particularly resistant and water-proof.
With a similar appearance to terracotta, due to its technical characteristics, klinker is one of the most used supports for outdoor flooring, facade cladding, swimming pools and street furniture. Its versatility is also leading it to establish itself in residential architecture, as interior flooring.
Porcelain is a vitrified ceramic with a white, fine-grained, and usually translucent body. It is considered the “noble” ceramic par excellence: it was invented in China around the eighth century, and its main ingredient is aluminum hydro-silicate kaolin, a particular white clay.
The firing of porcelain can take place at different temperatures between 1200 and 1400 degrees centigrade, depending on whether you want to obtain soft porcelain (1200 degrees), mainly used for the creation of figures, or hard porcelain (from 1280 degrees upwards. ). The porcelain named Bone China, is harder than normal porcelain and has been created in Great Britain adding a certain part of calcined bone.
Hard porcelain, that in Europe and in the world has originated various historical productions (Meissen, Ginori, Capodimonte, Limoges, etc.), can be covered with colored glaze, usually composed of silicon, feldspar and alumina. The glazing is applied at a high temperature (800-1250 degrees centigrade).
Grinding and atomization
The production cycle of the ceramic industry continues with the grinding and atomization phases.
The purpose of the grinding and atomization is to reduce the original and coars sizes, into particles of ideal diameter and particle size distribution for the final product to be obtained.
Grinding can be:
Dry grinding is generally used when the raw materials are already very homogeneous in terms of morphology and hardness, and the final product does not require a very high quality.
Wet grinding, instead, is suitable for minimizing the particle size of the mixtures used, and for making them as homogeneous as possible. With this process, the grinded mixtures are then dispersed in a solution of solids in water (slip), with the aim to further reduce the natural particles.
Wet grinding is also used when clays contain polluting impurities that must be eliminated from the mixture. In this case, careful sieving of the slip is added to the process.
The atomization phase reduces the ceramic slip into a powder. Due to its particle size and humidity,slip will be suitable to be pressed
The process, that the production cycle of the ceramic industry has acquired by the pharmaceutical industry and food industry, involves the use of large tower-shaped containers (atomizers) where a jet of slip is injected at high pressure and sprayed upward. This is hit by a flow of hot air produced by a heat generator: the slip is finely pulverized, and the particles dry. The powder falls towards the lower part of the atomizer, is collected and sent to the storage tanks for a rest period (at least 24 hours).
The next phase of the ceramic industry production cycle is named pressing.
This process is used to shape the raw material, in powder or granular form, until it gets an almost definitive shape and above all a consistency that allows it to withstand the subsequent processing phases without breaking or deforming.
The basic operations of this step are 3:
- forming, during which the semi-manufactured product acquires a well-defined geometry;
- compaction of powders, which gives the product its mechanical characteristics;
- consolidation of the powders, to limit the voids in the pressed product.
Depending on the percentage of water contained in the raw material, pressing can lead to different conditions:
- plastic state: the mixture contains 20-25% of residual water;
- semi-dry: the residual moisture in the powders is 10-15%;
- medium dry: the humidity of the powders is between 3% and 7%.
The production cycle continues with firing, preceded by a drying phase, which is essential for the object to lose residual moisture and plasticity, and then fix it in its final shape.
Firing takes place in special ovens that operate at temperatures between 800 and 2000 degrees centigrade and can last many hours. At the end of the process, which can also include two phases, the product will have a reduction in volume.
The firing temperature is decisive for the final result. A few examples:
- terracotta is obtained between 960 and 1030 degrees centigrade;
- stoneware between 1200 and 1350 degrees;
- klinker at 1250 degrees, with a firing that can last 30 hours;
- soft porcelain between 1200 and 1300 degrees centigrade;
- hard porcelain between 1300 and 1400 degrees;
- high-tech ceramics are obtained between 1400 and 1700 degrees, and require the addition of substances like kaolin and alumina.
Enamelling represents the last phase of the ceramic production cycle, and has both an aesthetic and a practical purpose. In fact, glazed ceramic is waterproof and has good thermal insulation.
Enamelling consists in the application of crystalline glaze, (available in the form of powder or liquid) or of a covering glaze on the ceramic, between the first and second firing phases, or before the single firing.
The basic composition of the glazes involves the presence of four fundamental elements:
- glass (silicon, boron);
- alkaline fluxes (usually sodium, potassium and lithium);
- alkaline earth fluxes (e.g.: calcium, magnesium, barium, zinc);
- stabilizer (alumina).
In order to lower the firing point and consequently reduce the time and cost of work, the ceramic production cycle has involved the addition of lead oxides for a long time. Due to their toxicity, the latter have been declared out of rule in the production of ceramic containers that can be used for food use (regardless of their original destination, for example furniture or decoration).
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Ceramic industry and circular economy
The production cycle of the ceramic industry is particularly suitable to be included in a circular economy model.
The latter is a consolidated point of reference in the policies of the European Union and of individual countries, including Italy.
The most commonly mentioned definition of circular economy is that proposed by the Ellen MacArthur Foundation: it is “an economy designed to be able to regenerate itself”, that is a model of production and consumption that aims to minimize waste, extending the life cycle of the products as much as possible.
By its nature, the ceramic product is extremely long-lasting. Characterized by resistance to extreme atmospheric conditions, chemicals, humidity, sudden changes in temperature and UV rays, ceramics are a material with an estimated life cycle of more than 50 years, easily recyclable in waste reuse processes.
In fact, the production cycle of the ceramic industry is able to reuse most of the waste and residues that are generated during production.
Waste tiles (raw or fired), mud from washing lines or from the grinding and polishing phases, grinding residues and exhausted lime are some of the materials that can be included in the production cycle of the ceramic industry, replacing raw materials. This makes it possible to avoid the extraction, transport and use of thousands of tons of raw materials of natural origin (sands, feldspar, aluminia, clays, etc.), with the related environmental and economic impact.
According to sector data, the production cycle of the ceramic industry allows the reuse of 99.5% of production and purification waste, which covers 8.5% of the raw material requirement. Thanks to this capacity of the sector, in Italy alone the extraction in nature of over 600,000 tons of clays, feldspar and other raw materials is avoided.
A further reduction of raw materials stolen from nature is due to the inclusion, in the production cycle of the ceramic industry, of residues from other production sectors, such as glass waste, sludge from the textile industry and more.
The consolidated processes of reuse of waste water also contribute to fully inserting this industrial sector in a context of a circular economy. In fact, during the production cycle of the ceramic industry, the consumption of water is a very important aspect, especially in the phases of grinding of raw materials, glazing and finishing of fired tiles. Thanks to the reuse processes activated for decades in ceramic companies, almost all of the wastewater is recycled during the grinding process.
The ceramic industry and the circular economy are a combination that also works in terms of reducing the consumption of fossil fuels and the production of greenhouse gases. In fact, the internal reuse of materials limits the movement of heavy vehicles used for the transportation of raw materials, and it has a positive effect on the management of the waste cycle, and on the quantity of the latter.