The aesthetic problems that most frequently worry manufacturers of precious extruded materials such as covers, “facing bricks”, floor tiles and urban coverings are fundamentally two: efflorescence and “calcium carbonate inclusions”.
With regard to the efflorescence, an immense bibliography exists on their origin and the effects that derive from them and the interventions that can limit the manifestation. This type of defect does not in fact derive from the raw materials but from other factors, for example from the waters used in the fabrication of the product or mortars if dealing with fired products that require cement anchorage. Often it deals with a temporary disadvantage and not an easy objective to evaluate, so much so that even artificial techniques have been proposed that prescind the human eye.
The origin of calcium carbonate inclusions, on the contrary, is exclusively caused by the raw materials and the consequences are much more serious: in fact this could cause a permanent declassification of a large part of the production, disqualification of material in the eyes of the customers, drag companies through legal procedures sometimes very onerous, cause image problems leading to consistent damages rendering it difficult to be reabsorbed.
Sometimes such situations have led to choices that turn out to be fatally wrong, such as, the case of a roof-tile Factory that, so obsessed with the “calcium carbonate inclusions” problem, adopted a wet grinding system with spray drying of slurry and successive rewetting of powders. Apart from the economical increase represented by the new plant and additional management costs, the system has never worked due to the fact that the fineness of the spray dried constituent particles was such that it prevented the control of the extruded mars and all successive working processes.
Nor should it be forgotten that in cases where there is an extremely high presence of “calcium carbonate inclusions” with distribution in all of the mass, it might even cause the destruction of products with fragmentation in many pieces. Naturally in these cases the inconvenience goes well beyond the aesthetic defect and can be noted on any type of extruded material.
The purpose of this document is to highlight the problem of calcium carbonate inclusions towards the attention of “people in charge”, analyze the origin, verify the behaviour and propose effective solutions.
Traditionally it is believed that they derive from calcareous fragments (CaCO3), and even more probable that they derive also from Dolomite (Calcium carbonate & Magnesium) and, if we take as a reference the effect that is caused on the surfaces of the pieces, even chalk (calcium sulfate) falls into the category of suspect minerals.
The classic calcareous "calcium carbonate inclusions" derive from the fragments present in the clayey and sandy raw materials such as:
The conditions that favour the chemical precipitation of the carbonate nodules are:
It deals with the same conditions that preside at the formation of limestone and dolomite rocks (where there is a partial substitution of calcium with magnesium present in the marine solutions), however in this case the sedimentary contributions are marginal and not fundamental. With regard to chalk, this can be present, dispersed in the clayey ground in well formed crystals and also with large dimensions or chalk constitutes real levels of separation (of some millimetres) between successive beds.
Carbonatic shell fragments (CSH) |
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Thermo-ponderal analyses (TGA)
Carbonatic shell fragments (CSH) |
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Chalk fragments (G) |
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| All curves, with the sole exception of 1/G (dried sample) do not register weight loss and superimpose perfectly to the referred line 21/G. |
OBSERVATIONS
LIMESTONE (CC)
DTA & TGA of the raw materials in dried condition (at 110°C) present the classic progress with endothermic reactions and weight losses that started towards 800°C and concluded at approx 950°C.
The sample fired at 900°C is successively always protected with a permanence in the drier at 110°C and shows a further small endothermic reaction (DTA) between 450° & 500°C that corresponds to a modest weight loss on the TGA.
The pyrolysis remains well visible (almost like the sample) which indicates that at the firing at 900°C it only has a marginal part interested in this process and that the reaction was very far from the conclusion. The same sample exposed to humid air shows a net increase of intensity of the reaction between 450° & 500°C. The dissociation flex is similar of the carbonates even if the weight loss is slightly inferior.
The material fired at 1000°C shows the same reaction between 450°C & 500°C and an almost imperceptible sign at the pyrolysis of eventual carbonates residual. The same left in humid air presents a TGA with a high weight loss between 450° & 500°C accompanied by a very intense endothermic flex on the DTA.
The pyrolytic phenomena of an even higher temperature are reduced in respect to the sample but always visible. The DTA also shows a very small exothermic flex towards the 350°C which was only just noticeable on the fired sample at 900°C and exposed to air, leading to the crystalline phase.
SHELLS(CSH)
The DTA of the sample presented a big endothermic flex that manifests just before 800°C up to 900°C and beyond. In correspondence the TGA shows a great weight loss whilst the one below 800°C is almost negligible, probably due to a minimal presence of vegetable substances.
The DTA of the sample fired at 900°C and then protected shows a very reduced pyroloysis of carbonates and a second small endothermic flex towards 450°C. The TGA signals a loss of weight in correspondence to the dissociation of the carbonate residuals.
In the case of the fired material at 900°C & left exposed, the DTA presents a large endothermic flex of average temperature, which culminates around 500°C, and that corresponds to an important weight loss on the TGA; that of the CaCO3 dissociation which is reduced & moves by approx 100°C towards a lower temperature.
The DTA of the fired sample at 1000°C & then protected shows only two endothermic flexes, almost unnoticeable, at around 450°C and 700°C. The TGA almost does not signal weight loss.
The DTA & the TGA of the fired sample at 1000°C, and unprotected, are a repetition, though more accentuated, that those fired at 900°C and left exposed to air.
DOLOMITE (D)
The DTA of the sample shows a typical double endothermic flex between 780°C & 930°C. The TGA reveals a large weight loss (subdivided into two times) that starts before 500°C and concludes before 900°C.
The samples fired at 900°C and non re-exposed to the air, signal two endothermic reactions at 400°/450°C and 750°/780°C.
The same exposed to air have very similar curves. The sample fired at 1000°C (protected) practically only has the low temperature reactions.
The sample exposed to air 32/D has a state which superimposes that of 22/D and are visible, even in these cases, small exothermic peaks between 300° & 350°C bound by a crystalline phase passage.
CHALK (G)
On the DTA, the sample has an important endothermic reaction around 200°C with a high weight loss (transformation in hemihydrated chalk) and an exothermic around 400°C (transformation in anhydride).
On the TGA, all the weight loss is developed within 200°C.
The TGA & DTA curves of all the other samples (fired, protected & not) have a flat state.
EFFECTS ON THE PRODUCTION OF EXTRUDED PRODUCTS
The notorious and typical classic “calcium carbonate inclusion effect” is the flaking of the fired clay layer that separates the surface of the product.
In order for the phenomena to occur it would be necessary to have realized the following conditions:
- That the fragment of the original carbonate has been transformed (at least partially) into CAO.
- That its dimensions are higher than 0,5 mm
- That casually, they end up close to the fired surface but covered by not more than 1 – 3 mm of sintered clay.
The calcium oxide in contact with the atmospheric humidity passes to Ca(OH)2 with a reaction that causes just less than the doubling of the initial volume of the corpuscle, or rather that it separates it from the closest external surface .
The force exercised is proportional to the square of its radius ; therefore a fragment of 2 mm has an effect 16 times higher than that of 0,5 mm.
On numerous texts it says that to reduce the calcium carbonate effect it is a good rule to sinter the material by firing it at a higher temperature with longer cycles. This advice evidently comes from the consideration that a stronger sintering favours the reactions of the calcium oxide with the silica present causing the formation of new minerals (such as Wollastonite) with the result of eliminating the calcium oxide and englobing it into a new element.
Unfortunately, if it were true for the carbonates present in a very fine form, therefore, very reactive , the same does not happen for the corpuscles of considerable dimensions which are in fact the calcium carbonate inclusions. On the contrary, as it is clearly observable from the thermal heat analysis of the shells, and dolomite (and even though in reduced measurements) , the fired samples at 1000°C left in the humid air show endothermic reactions and weight losses of 500°C which is more important than the tests that were fired at only 900°C with very incomplete carbonate pyrolysis.
This reaction seeing as it is connected to the transformation of Ca(OH)2 in to CaO has therefore a quantitative impact on the presence of hydrated lime and therefore in the case in which the general characteristics of the fired permit them (above all mechanical) it would be opportune to fire them at the lowest temperature possible.
So as not to fall into the calcium carbonate effect it would be necessary to avoid reaching the thermal level that initiates the pyrolysis of the carbonates.
The importance of this behaviour is certainly superior to that supposed decrease in reactivity of the calcium carbonate inclusions that derives from a stronger sintering and therefore with more dense corpuscles.
From the thermal analysis of the curves it can be further noticed that even the samples accurately protected from humidity after firing caused the registration of a reduced endothermic reaction towards 450°C. This behaviour underlines the easiness and rapidity that characterize the hydration of the calcium oxide.
With regard to the dolomite, it is highly evident that the thermal reaction of medium temperature is clearly inferior to that of the carbonates. This is consistent with the behaviour of the magnesium oxide that results as much less hydratable that the calcium ones. Seeing the dangerousness of the carbonate calcium effect , the eventual presence of dolomite is preferable to that of calcite.
Finally, the behavior of chalk is clearly different to that of the carbonates; there is no re-absorption phenomena therefore it can be excluded from causing the typical “flaking” of calcium carbonate inclusions. In cases where it is found right on the visual surface of the manufactured product it can however, cause a noticeable aesthetic damage to the colour, usually being much whiter, which stands out quite evidently.
INTERVENTIONS
The most classic treatment consists in sinking the product into water as soon as it has been fired so a partial solubilisation of the calcium carbonate inclusions is obtained with the creation of a supplementary space with a sufficient thickness to contain, without causing damage, the increase of volume that causes the hydration. In this case the water should be replaced with a certain frequency seeing as an eventual saturation in lime would inhibit the solubilisation.
This intervention is typically adopted by Manufacturers of Cotto Toscano who employ the flakey clay of Chianti so as to avoid substantial damage to the structure of the manufactured products that, otherwise, risk the transformation into a cluster of shapeless fragments.
The same operation, on an artisan scale, is carried out also by the most scrupulous masons during construction of walls with facing bricks. In this case, the immersion in a container of water intends to speed up the hydration and therefore the eventual flaking of fragments on the surface. The damaged pieces are eliminated and the replacement is usually at the Manufacturers’ expense.
Another system used during the drying process is the addition of a contained quantity (2-5%) of sodium chloride which is deposited on the surface of the product. In this way, the salt during firing is decomposed reacting with the clay & carbonates forming a silicate composition that joins the “calcium carbonate inclusions” on the surface of the pieces present in the presses.
Both treatments however have contraindications : the first increases the efflorescent phenomena whilst the second provokes an aggression on the walls of the kilns.
Upstream of these interventions carried out on the fired products, others exist that can radically be resolutery, among which the replacement of raw materials that cause the calcium carbonate inclusions or direct intervention in the quarry eliminating the levels that contain polluted materials, unfortunately however, it is not always possible to put into practice these options and often not there are no real alternatives to the use of the polluted raw materials.
One plant solution that is effective and really resolutive is to drastically reduce the carbonatic fragments to dimensions not higher than 0,2 mm.
PLANTS SOLUTIONS
A good part of Heavy Clay Industries, including those that produce products with a greater added value, are equipped with traditional plants that foresee the utilization of a refiner roller mill with the function of a finishing mill.
The limit of the dimensions of the tenacious fragments that go through this machine is theoretically represented by the distance between the rollers, and therefore by the space left that cannot anyhow be lower than 0,6-0,8 mm.
It would deal with a dimension that is still not sufficient to repair the calcium carbonate inclusion problem, but certainly useful to reduce it considerably.
Unfortunately this minimum space results as being only theoretical due to the following reasons:
- Particles of a flat shape, for example, fragments from shells or carbonatic depositions, mother rock fractures that can pass undamaged, even if they are of enormously greater dimensions.
- The rollers, especially if dealing with raw materials that contain rough quartz , are subject to very quick wear and tear which causes the formation of grooves with a strong increase in the dimensions of the space.
- The same rollers frequently have a protection system that, in case there is a presence of tenacious lithoids, operates an instantaneous removal allowing the passage of fragments that, in a contrary case, would remain embedded causing damage to the roller mill.
Another limit of this milling machine is represented by the lack of a particle size control, which, given the high humidity of the milled material, operates in all cases with very modest results.
In front of the calcium carbonate inclusion problem, this solution presents remarkable limits of reliability.
It can be derived from these considerations, the importance of using a dry grinding system that represents above all, a process that controls the results and it is important to underline that this plant can treat raw materials with initial humidity levels of even 30% using a dryer upstream that is able to reduce the humidity levels of the clays up to 18% and permits the MOLOMAX mill to grind with maximum throughput.
The Manfredini & Schianchi DRY-TECH HEAVYCLAY Technological Process fundamentally foresees the use of a finishing Pendular Mill type MOLOMAX complete with static or dynamic separator.
In some cases it is possible to insert, as primary grinding, a P.I.G. type Hammer Mill, obtaining this way and already in this phase a percentage of powder passing between 60% & 80%.
In the successive phase the material are put inside the grinding chamber of the Pendular Mill type MOLOMAX by means of a dosing screw conveyor or weighing belt, and the grinding procedure occurs exclusively by crushing the product between rotating rollers against a fixed circular track.
The particles are therefore dragged upwards towards the higher part of the mill by an air flux generated by a centrifugal fan.
The particle air separation can take place with two different methods:
- By means of a cyclone, determining a “closed” circuit (mill-cyclone-fan)
- By means of a sleeve filter, with an “open” circuit (mill-filter-fan)
The functioning technological characteristic of the innovative system with an “open” circuit is to move the material just ground with a strong and immediate suction action. This solution permits a drastic reduction of time the material stays inside the grinding chamber, with consequent electrical savings, less wear & tear of grinding organs and a noticeable increase of the hourly production capacity (even by 100 % !!)
The Manfredini & Schianchi DRY-TECH HEAVYCLAY Technological Process can manage the drying of raw materials with a very efficient thermal consumption that does not ever rise above 650 Kcal/h of evaporated water, producing a humidity reduction from 18% to 8%. In this way such a line can serve Users with serious humidity problems in the raw materials without the installation of a preliminary drying phase and avoiding the consequent increase of investment & utilization costs.
The separator installed on the MOLOMAX Pendular Mill remarkably performs the particle size classification function of the powders making it sometimes non necessary to implement the dedicated screening system and, even in this case, significantly reduces costs.
The powder obtained this way is then sent to the moisture treatment; the machine that is used for this specific application is type MS/2000. The quantity of water is regulated by means of a mechanic or hydraulic system, with the possibility of automatic correction on the humidity level reading of the powder thanks to the electronic instrument MS/MU 7685. The nebulization of the water is obtained through a rotating disc set in action by a closed & ventilated motor. A set of rotating asps in steel remix the moistened powders in order to obtain perfect homogenization.
The percentage of water added to the product can vary from 1% to 3% with reference to the physical chemical characteristics of the raw material and necessity.
The mixture can be successively sent to storage in the metallic silos. The storage of the powder obtained through dry processing and successively moistened does not require long storage times and can be sent, after 24 or 48 hours, to production; therefore the silos also carry out the simple function of a raw materials buffer for a maximum of one or two production days.
The final phase foresees a mixer-wetting system duly studied for dry grinded raw materials. The apparatus, equipped with air-tight throttle valve and a high pressure water addition system, perfectly homogenizes the mixture and adds the necessary quantity of water that easily permeates the composition without forming lumps or large grains.
As well as total elimination of the calcium carbonate inclusion problem, the choice of the DRY-TECH HEAVY CLAY solution also foresees the recuperation in the production line of green, dried & fired rejects.
The maximum consumption is 15kW/h per ton of product ready for shaping.
CONCLUSIONS
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DRY-TECH HEAVYCLAY System |
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The Manfredini & Schianchi DRY-TECH HEAVY CLAY dry grinding system is considered avant-garde for the important innovative elements that characterize them under various profiles:
- Quality: Results are reached that are not obtainable with the most sophisticated rollers and the presence of carbon calcium inclusions is neutralized with a severe particle size reduction. Also, the extruded products mixture results more homogeneous and the surface of the products are presented perfectly smooth. This allows a lot of productive units to manufacture glazed bricks and roof files with excellent quality.
- Grinding & chamotte recuperation: the dry grinding plant is able to treat and recycle fired ceramic rejects, general inert components and clays with a high presence of impurities, without compromising the final quality of the products.
- Economization: The investment cost as per production costs decisively result in being competitive in relation to the tradition preparation with roller mills.
- Technology: The high automation of the system, the electronic control devices that characterize them, united to the possibility, in the successive phases, to use the roller kiln firing system, individualize in the DRY-TECH a system that is advanced and congenial to the requirements of modern production units.
