To gain a deeper understanding of the state of the art in the production of ceramic bodies for porcelain tile (or vitrified stoneware), we must first take at least a brief look at the productive issues affecting this important area of the ceramic tile market.
The different formulation of porcelain tile bodies (which have a higher percentage of hard materials, feldspar and sand, than conventional white-body single firing bodies) results in longer grinding times.
However it is the required grinding methods that are chiefly responsible for the drastic reduction in
productivity of the wet grinding mills (both continuous and discontinuous).
When producing a material such as porcelain tile where uniform composition is a fundamental requisite and some very precise characteristics are required (structural compactness, mechanical strength, chemical and abrasion resistance (hardness), frost resistance and stain resistance), it is inevitable that very extreme grinding cycles have to be used with the wet process.
This results in a falloff in productivity due, as we have said, to the higher content of hard materials in the body and the need to obtain a semi-finished product (at the press) capable of yielding a fully vitrified finished product (at the kiln exit) with zero water absorption.
The need to keep particle size below 50 µm (and the majority of the material between 30 and 40 µm) makes the wet grinding process fairly unprofitable (in both production and economic terms).
In a normal continuous wet mill sized to produce 30 ton/h of “monocottura” body, production capacity with a porcelain tile body is no higher than 10- 12 ton/h.
This means it is necessary to grind the hard materials separately using dry grinding methods, whose chemical and physical properties make them much more effective and specific.
The wet mills can then be used solely for mixing the microns feldspars with the plastic clay to bring the productivity of these machines at least back to their initial values.
To avoid even this last stage and completely bypass the horizontal mills, the mixture of plastic clays (provided they are sufficiently clean) can be amalgamated with the micronised feldspars inside normal ( fig. 1 turbine ) dissolvers.
This avoids the considerable cost involved in management of horizontal wet mills.
THE HARD MATERIALS MILLING “MICRO-DRY” LINE
One of the most innovative projects carried out in recent years by Manfredini & Schianchi’s Technological Sector is an in-depth study of the hard materials contained in porcelain tile bodies.
The new Micro-Dry lines have a production capacity ranging from 5 to more than 35 ton/h and have a central core consisting of a dryer/mill machine and a latest generation dynamic separator.
An important result both from the point of view of the plant (more compact line, lower management costs, etc.) and of technology (total lack of pollution) is the possibility of using raw materials with a specific moisture content of 8- 10% and a particle size of 0-5 mm (but with the possibility of using even coarser material), feeding them directly into the mill and thereby avoiding the problems associated with conventional drying systems.
Although the mill is the heart of the system, the true novelty of this technology is the precision that con be obtained with the latest generation dynamic separator. It is the perfect sizing of these two machines that makes it possible to have such a fine (below 50 µm), precise and consistent particle classification, thereby delivering the quality and structural compactness that porcelain tile requires.
The know-how behind the design of the entire line of the plant is in complete harmony with this priority, ie. to obtain clear separations with residue percentages close to zero.
The productive continuity of the mill and the precision of the high-separation MS dynamic separators give the line the technological potential it requires .
The entire plant, which is naturally complemented by the mixture preparation equipment and the storage and tank truck loading equipment, represents a step forward with respect to current technology.
It brings major improvements in terms of technology and process ensuring continuity and repeatability of the mixes. Raw materials preparation is based on the use of continuous extraction and weighing systems with a tolerance of just 1 % and fully controlled by electronic supervision.
The design of the Micro-Dry lines started out with an indepth analysis of the raw materials in the laboratory, followed by testing on a pilot plant which proved to be essential for correct sizing of the plant.
The dryer/mill/separator system is sized on a case-by-case basis according to the grind ability results and production requirements.
On-going collaboration between our engineers and the people in charge of technical and process aspects at the companies using this technology allows for further technical and process refinement.
As regards the project’s economics, we should note that the cost of transforming one ton of raw material (ie. drying, grinding, maintenance and labour costs) is well under 12 €/ton, including depreciation.
The investment also makes it possible to bring the productivity of the wet grinding lines back to the previous values.
Dimensions of dryer-mill group
~ 15000 x 8000 h 14000
Humidity at dryer-mill entrance
< 50 μm di cui 80% <20 μm
Power installed on mill
Power absorbed by mill
Power absorbed for each ton of raw material ground
~ 40 kW
Kg of Alubit necessary to integrate each ton of raw material ground
M3 of methane required to dry one ton of material
Labour required to check & carry out maintenance on the whole plant line over a period of 24 hours
3 + 1
Table shows that the total management cost inclusive of labour, electrical and thermal energy and cost of operative personnel (ie. the processing cost) is 8,80 €/ton
It should also be noted that for a 10 ton/h line (10 ton/h x 22 h/day x 300 days/year = 66,000 ton/year), depreciation can be calculated as about 3 €/t which, when added to the previous figure, gives a total processing cost of 11,80 €/t
TECHNOLOGICAL ADVANTAGES ON THE FINAL PRODUCT
A study conducted by the building materials Engineering faculty of the University in Trento (Italy) pursued an investigation over the reasons of Porcelain stoneware related to smudges.
Such a flaw especially on polished tiles compared to non treated ones lacks a clear regulation on standards to enforce.
The study fostered markers on microns by comparing the data on absorption of the body surface pores on a range between 1 and 10 μm with larger ones, it was tested that the latter do present higher water absorption levels on both unpolished and polished tiles.
A parallel study on the chemical analyses with porosity levels showed that the chemical analysis does not give any sufficient understanding on the reasons of the different affection to stains.
Other means to avoid the problem could be :
-Increase of firing temp and cycle
-Increase of milling cycle of the overall body
Both procedures as above are not considered convenient due to the processing costs involved.
The study showed further that several bodies with a presence of feldspar or other hard raw materials with a grain size lower than 45 μm, originate tiles with a far diminished open surface porosity.
Moreover the study determined that the addition of ultrafine feldspar particles lower than 45 μm to Porcelain stoneware enables the total elimination of stains by keeping same traditional processing cycle and at the same time a substantial reduction of wet milling time.
Thus the increase of firing cycle temp and reduction of grain size in wet milling to 45 μm are not required due to the abovementioned high running costs.