Description of background, problem and objectives
Environmental problem/issue addressed
The environmental problem targeted in this project regards the emissions deriving from the ceramic waste manufacturing. Regulations have allowed, in the past, to progressively decrease harmful gaseous emissions, and technology advancement allowed to reduce water and energy consumption. However, the manufacturing of porcelanized stoneware tiles still presents improvement possibilities mainly related to the high temperature firing step and to the finishing operations, which are increasingly required by the market. Such processes are energy intensive and generate large quantities of solid waste.
As a matter of fact, cutting on fired tiles currently leads to water consumption (0,8 mc/mq) and generation of sludge (0,9 kg/mq), debris and scraps (2,5 kg/mq).
Hypothesis demonstrated/verified by the project
The project objective was the demonstration of an innovative zero-solid and liquid waste emission approach to ceramic tiles manufacturing, while using recycled raw materials to create optimized large surface tiles for walls and floor coverings. Such product, identified as NW-Tiles (No- Waste Tiles), can be manufactured pursuing also the minimum energy consumption and CO2 emissions, thanks to the lower firing and milder finishing operations required.
Description of the technical/methodological solution
The core of the proposed innovation lies in the identification of a low firing ceramic body composition, containing up to 70% recycled glass and ceramic based materials, which will allow to conduct firing operations at lower temperature as well as to use lower grade glazes for the tile decoration before firing. Moreover, the extensive presence of glass formers in the NW-tile composition allows to use an innovative surface polishing process which makes the tile surface smooth before firing occurs.
The solution is represented by a process for tile manufacturing made possible by the composition using recycled waste with a high percentage of glass formers. The better thermal properties of the composition (lower processing temperature, ability of achieving a shiny surface after firing) allows to conduct all the finishing operations on unfired tiles and hence generating waste easily reintroduced in the manufacturing process, avoiding the use of water and minimizing the energy consumption. Such benefits are extended to the firing stage: lower sintering temperature, lower energy consumption, minimization of defects (typical of high temperature processes). Hence benefits in terms of reduction of defective parts and complete recyclability (previous milling) of the final product. The new tiles can also be recycled at the end of life thank to the high content of glass former.
Results and environmental benefits
The results obtained can be summarized as follows:
- No solid or liquid waste emitted by the new process and complete internal recycling of the produced waste. This will correspond to a reduction of 0.9 kg/m2 of tiles of cutting and polishing sludge (on a dry base) and of approx 2.5 kg/m2 of tiles of fired scraps (cutting, defective tiles).
- No water use in finishing operations (cutting, polishing), leading to savings of 0.8 cubic meters of water per square meter of tiles.
- Lower energy consumption in NW-tiles manufacturing: less than 50% energy consumption in cutting and polishing, with at least 40% longer diamond wheel life; reduction of 20% of natural gas consumption during firing; no specific energy consumption increase during forming (pressing).
- Use of up to 70% of recycled raw materials, as important glass formers lowering the firing temperature.
- Complete recyclability of the final product and its reinsertion in the same NW-tile manufacturing process.
- Full recycling, as it is, of the powder produced during cutting and polishing, by simple re-mixing and repressing in the base material.
- Productivity increase (30% expected) in cutting and polishing operations, due to the lower wear resistance of the pressed tiles with respect to the sintered ones.
Long term results
The project has field of application and interest of European significance because water issues transcend national boundaries, as well as those relating to the exploitation of resources and energy consumption. In addition, the production of ceramic tiles is distributed throughout the European territory.
For this reason the results of the project will continue to be developed, as well as the demonstration plant will continue its evolution in search of further improvements, taking into account the indications of the market, trying a mutual interaction to develop sustainable products that can be appreciated by the final users.
At the same time, through our dissemination plan "after LIFE", we will continue to encourage the spreading of the idea and of the environmental inspiration with consequent increase of the environmental benefits.
Activities carried out during the project’s implementation
Ceramic body composition and spray dried powders for NW tiles
The purpose of the action was to identify the mixtures containing up to 70% recycled material, and to develop the methods to prepare them (grinding and atomization), in order to make them suitable to use as mixtures in the subsequent processing steps.
To achieve this we proceeded as follows:
- identification of suitable waste sites rich in recycled glass formers and selection of those that minimize transportation and provide a more stable oxide content and greater daily production. First we identified the types of waste most suitable for subsequent use, then we identified potential suppliers of such waste, which could ensure a constant supply in terms of quantity and quality. The types of waste identified are as follows:
a) glass waste;
b) chamotte. This type of waste is made of rejects of fired ceramic products, dried and then ground. Part of this waste comes from our production;
c) unfired waste. This type of waste is mainly produced in-house;
- adaptation of wet milling conditions (water; milling media to product ratio, rotating speed of mills and duration) to process the new ceramic body composition, using firstly e jar rotating mixer, in the company’s laboratory, to simulate the mixing of the different components and subsequently a discontinuous mill with a production capacity of 1 ton/h.
This allowed us to obtain the optimal grinding parameters, the correct dry product/water ratio and the type and quantity of the most suitable fluidifier.
We have however assessed the possibility of foreseeing a preliminary dry grinding process of the glass waste to ensure an optimum particle size: this will further improve the environmental impact of the process.
- Tests of coordinated operation of raw material preparation, milling and spray drying subsystems, which will constitute the raw material preparation stage of the demonstrative line.
Forming and green machining with powders and scraps recovery
The main purpose was to obtain the forming system and green machining of the new doughs, with full recovery of dust and scraps, together with the system for reuse as such in the same production cycle.
In order to start the first experiments, with the use of the new mixture, necessary to identify the various process parameters for the construction of a demonstration line in the final conformation, it was first equipped a prototype, preparatory to the next phase of upscaling for the obtaining of the large plates.
On the prototype were carried out the first tests of a two-stage stratification, with pressing tape, as well as the first tests of cutting of the raw product, using formats and knead very simple.
The samples so obtained, smaller than those provided in the application (20x20, 120x120 against planned), were re-pressed in laboratory press and cooked in the oven, to simulate the behavior of the line and verify the criticalities of the demonstrative plant.
In the next phase of the demonstrative plant has been realized in its final configuration.
Starting also from components already existing inside the company, we have created the new line that consists in:
- Multiple loading system with different feeding stages: the first one spread the base that will compose the higher layer and the body of the tile. It contains most of the waste and glass formers and is one that most allows lowering of the sintering temperature. The second and the third are dedicated most of all to the decoration in mass: they also will spread even waste materials and the last will also contain flakes resulting from the grinding of pressed swarfs: these will give particular aesthetic effects to new products.
- Belt press for the pre-compaction of the continuous sheet: this system was already present but it was necessary to adjust the pressure conditions in order to adapt the use of the new mixture of raw materials.
- Two finishing units, one for lateral cutting of the sheet and one for the transverse cut. The combined action of the two systems allows the creation of 120 cm x 120 cm sheets.
The last step consists in the final pressing with the uniaxial traditional press with high pressure, 400Kg/cm2, for the final compaction, that lends strength to the semi-finished product which can now be treated on the raw surface.
After pressing, the tile undergoes cutting to size, for example cut from 120x120 in 2 pieces of 60x120. This operation generates a new and similar type of side trimming (that we call “TPD trimmings”), harder because more pressed, but still rich in moisture. The TPD trimmings are chopped in a more coarse way and redistributed on the powders during the decoration on tape, for aesthetical effects (as scales). In addition to the two lateral trimmings insert in the mass, the reatomized powders arising from raw grinding are inserted in the mixture that forms the layer to be compacted. The dusts from sanding in raw and from raw cuts are sent, via suction, filtering and pneumatic transport to a turbo dissolver.
This system dissolves the dusts in water, turning it into a new slip and then in a new atomized.
The individual components of the recipe that feeds the press (atomized, re atomized powders, "soft" and "hard" lateral trimmings) return to the top of the forming step, with the recipe from technological tower.
The different systems have been developed or modified individually and then have been installed sequentially and connected to the transportation system through the activities of the action B.4 started in advance, as will be explained below.
Thermal processing adaptation (drying, firing)
The first step of the action was the set up of the drying parameters. This passage is very important and strategic for the good results in the firing phase, even if not so complicated. The dryer curve in fact is simple and the problem is only to determinate the final moisture level wanted. Too much water inside the piece to be fired should translate in a instantaneous vaporization inside the kiln and therefore a broken of the tile. Too less water inside the piece should determine fragility in the fired tile not well sintered.
Starting from the recipes of raw materials developed under Action B.1 and from new processes applied under Action B.2, we have obtained a number of results with raw pieces that have been tested in a small furnace. Got the possibility of risk-free using the industrial furnace, we gradually increased the size of the tiles, starting from 20x20 up to the 60x120.
Setup of demonstrative line (closed loop of solid and liquid waste)
We assembled the loading systems, the two stage press, the cutting tools, the dryer and the raw finishing station. We connected the whole line with the kiln and we started with tests.
End of life procedures, energy and mass balance and LCA of the NW-Tiles
This action, launched after the realization of the demonstrative line, aimed at performing a mass and energy balance and at quantifying the environmental impacts associated with the Life process and the conventional process using a streamlined Life Cycle Assessment (LCA) methodology.
The first activity was the identification of the aim of the study and the scope, namely the function of the systems to be analyzed and the related functional unit, which represented the reference unit to calculate the environmental impacts and the energy and mass balance. Successively, the systems boundaries were developed thanks to the information provided by the project partner. For the both systems analyzed the main process stages of the life cycles were included in the assessment: from the extraction of raw materials, the manufacturing of the products, transportation, end-of-life of manufacturing scraps and of the final product, in order to obtain a “cradle to grave” application. Then, ad-hoc questionnaires were prepared to be filled in by projects partners and dealing with mass and energy input and output of the process stages included in the system boundaries.
After the collection of all of the main relevant data, a model able to represent the entire life cycle of the systems under analysis was developed using a specific software. Input and output flows were linked each other and to the different process stages in order to create a balanced model. Finally, the model created was processed with the utilization of a method for the characterization of the environmental impacts. Through these activities, it was possible to quantify the environmental impacts and to compare the environmental profile of the systems under analysis.