ADDITIVES IN EXTRUDED HEAVY CLAY PRODUCTS AND BRICKS
Mixtures are normally corrected as required by using additives with low malleability. Contrary to what is generally believed, some of these additives, for example fired rejects in small quantities (510%), do not actually increase wear and tear on the milling parts but actually improve the efficiency of the mills; when introduced together with the clayey material, they in fact facilitate the treatment and milling of the so-called soapy clays by improving the milling operation and the flow of the product inside the milling machines.
In addition, the use of chamotte has a positive effect on all the production process leading to the following advantages:
It is a lean component which stabilises the mixture without reducing its malleability
It increases mechanical resistance of the dry product by up to 10% 15%
It helps drying and firing as it improves the total porosity of the humid and dried mixture
It performs a binding function, resulting in better amalgamation of the clay particles, and therefore of all the mixture.
As well as these technical and productive characteristics, the use of chamotte in different particle sizes creates special aesthetic effects such as the dotted black and white surface of the renowned Cotto Fiorentino.
Compared to other types of lean additives, chamotte maintains in the fired product the characteristics of the base raw materials and amalgamates with them, whereas sand,for example, remains on the surface of the final product.
When the additives are already available in the required quantity and grain size, they are added to the clay powder directly into the dry mixers using volumetric or weighed batching devices. These have internal devices which guarantee perfect amalgamation between the components even if their bulk density is much different.
ADDITIVES IN EXTRUDED HEAVY CLAY PRODUCTS AND BRICKS, PROCESSING PLANTS FOR EXTRUDED HEAVY CLAY PRODUCTS AND BRICKS
SOME COMMENTS ON ASPECTS OF CERAMIC TECHNOLOGY
The dry preparation of raw materials has always been completely misunderstood. It is a common opinion that milling is synonymous with pulverising and that therefore effective milling should produce the maximum possible quantity of uniform fine powder.
The exclusive use of very finely ground powders creates the following problems:
Weak cohesion of the mixture being shaped
Low mechanical resistance of the shaped and dried product
Higher drying and firing cycles, with the risk of a faulty end product
Thanks to our vast and specific experience in the field of ceramics Manfredini & Schianchi have transferred this fundamental aspect of the technology of preparation and treatment of raw materials to the sector of extruded products.
Basically, the secret of an efficient preparation and milling process lies in the ability to maintain a balanced granulometry curve, with an appropriate distribution of powders in order to form a skeleton capable to guarantee aesthetic and technical results to the final products.
So as obtain that, it is necessary to consider the different qualities of powder which are produced by the milling process.
The Hammer Mill type PIG transforms the raw materials into irregularly shaped polyhedral powder with a complete granulometry distribution from the maximum grain size to the micronized particles.
On the other hand, the powders produced by the Pendular Mills type MS basically have a different morphology, being flat, very fine and homogeneous as a result of the friction of the rollers and/or of pendulums on the track.
The mixing and amalgamation of these two types of powder, in appropriate established percentages results in a mixture with an optimal structure providing:
Strong cohesion of the mixture during shaping
High mechanical resistance of the dry product
Homogeneous distribution of the apparent porosity
The basic idea is therefore to carry out a selective milling both regarding quantity and quality, obtaining very fine powders only when strictly necessary, and to make suitable selections of particle sizes using highly efficient grading methods.
An equally important issue concerning the application of technology and experience in the field of ceramics is the mixing process, the amalgamation of the clays and additives, and the subsequent preparation of the mixture.
Manfredini & Schianchi have developed their own dry mixing methods and has introduced the process of powder moistening into the extruded products sector using modified and improved machines from the ceramic industry with high output and efficiency characteristics.
Humidifying milled powders (Wetting Machine MS2000), before the bathing and mixing process (Mixing-Wetting Machine MS), has led to a significant development in the preparation of the body with the following results:
The total deflocculation of the body with a hydrosphere effect on the humidified particles
The complete absorption of the water during the subsequent humidifying stage
Measurement and continuous automatic control of the humidity content in the powder (Moisture Meter)
The main advantages of such a stage are:
Perfect amalgamation of the powder with partial conversion into pellets leading to a high
level of fluidity and facilitating treatment in the downstream stages.
Elimination of lumps.
Precise weighed addition of water used for humidifying and mixing, which is guaranteed to remain constant as a percentage of the final mixture
Uniform pressure of the materials during extrusion and elimination of tensions within the mixture which facilitate the drying process and reduce the vacuum pressure required by the pugmill
Shaping with smaller quantities of water in the body (1 to 3% according to the chemical and mineral characteristics of the clays) with consequent savings in energy during the subsequent drying process.
The positive effects on the downstream stages are as follows:
HANDLING Better cohesion of the green moulded product which facilitates the handling of the material and makes it possible to simplify the plant engineering concerned.
DRYING Homogenous distribution of the apparent porosity which helps to eliminate water at the drying stage thus reducing the production cycles and the percentage of waste materials.
TREATMENTS Better mechanical resistance of the dried products and therefore safer handling of the material, reducing microfractures, and granting better results in the finishing and surface treatment and in the quality of the end product.
FIRING The skeleton structure of the mixture resulting from a correct distribution of the size and morphology of the particles, thanks to its apparent porosity, improves preheating, firing and cooling because the organic substances are expelled more quickly.
In addition, the perfect amalgamation of the raw materials which make up the mixture helps to obtain better fusibility and a more efficient vitrification, thereby improving the technical characteristics of the end product.
DESCRIPTION OF PLANT MODELS
In this chapter we will illustrate two types of processing plants which use the two main criteria as regards the ceramic technology applied to the engineering of dry processing.
The plants are also equipped with all the necessary devices for the maximum quality control of all the stages.
The technical data tables will show the two options which are as follows:
A) Technological line for milling by primary hammer mills and finishing hammer mills the
following particle sizes:
1) coarse, 2) semi-coarse, 3) semi-fine
suitable for the production of:
Common structural brick
Innovative structural brick
Covering materials
Extruded red body floor and wall tiles with minimum absorption of 5%
B) Technological line for milling by primary hammer mills and pendulum finishing mills to obtain the following particle sizes:
4) medium fine, 5) fine, 6) very fine
suitable for the production of :
Special covering materials
Extruded floors and tilings with maximum absorption of 3%
Technical products, clinker, extruded white body porcelain stoneware with maximum absorption of 0.5%
Terracotta garden pot
TECHNOLOGICAL LINE FOR EXTRUDED HEAVY CLAY PRODUCTS AND BRICKS TYPE A
Mainly consisting of:
1) Loading hoppers, suitable for receiving material directly from the quarry or from previously prepared heaps using a clay loading shovel able to handle pieces of considerable size (0 250 mm)
2) At the base of the hoppers there are extraction devices with double connecting rods, particularly
suitable for the pre-crushing of the bigger blocks and reducing them to 200 mm maximum
3) Alternatively a vibrating duct is used when the pieces received are already reduced to 150 mm max.
4) The material to be ground is off loaded onto conveyor belts, which can also weigh the material, should it be necessary to make up the mixture with a maximum variation of 1%
5) Primary hammer mill which produces between 60% and 80% of powders at the first milling and screening passage. It consists of a thick sheet steel casing lined internally with circular and lateral wearproof plating; a rotor on which the grinding hammers are mounted; lump breakers, a static sizing grid at the bottom and a set of electrical resistances for humid raw materials are also present.
6) Bucket elevator for vertical transport of solid material, powder or pieces of various sizes. Made of a strong sheet steel structure and treated internally with easy flow material. The buckets are made of polyethylene or pressed steel, sturdily bolted to the conveyor belt made of special three-layer rubber. This device is driven by a motor and a pendular gearmotor.
7) High-efficiency inclined mechanical screens, totally static and without oscillations in the net frame and in the support structure. These are used to separate the powders of required granulometry from those needing further finishing.
They have electromagnetic heads mounted in line on support decks which transmit high frequency oscillating energy; this energy can be regulated and modified for each single head and is applied directly to the screening net at a number of points all over the net. A calibrated choice of inclination of the screening surface and of the net with the most appropriate mesh makes it possible to obtain an exact classification of the powders for a high rate of material processing.
There can be one, two or three grading surfaces in order to obtain different particle sizes from the same machine at the same time.
The screens are also equipped with a self cleaning system with brushes and electrical resistances to cope with humidity levels in the raw materials.
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Netcloth control device, positioned after the screens which carries out continuous analysis of the screened product. If unwanted particle size is produced owing to breakage of the net, an optical and acoustic alarm is given.
9) Finishing hammer mill, which is built and operates in a similar way to the primary mill, but has been suitably modified for an effective treatment of the powders from the first screening passage. The powders leaving the finishing mill are unloaded onto the previous elevator and taken back to the screening control.
10) The powders obtained in this way are then taken to the humidifying treatment; the machine used for this specific application is a Circular Humidifier MS2000, made of steel sheet with inspection hatches. The quantity of water is regulated by a mechanical or hydraulic system, the level of humidity of the powders exiting can be corrected automatically thanks to the electronic MS/MU 7685 reading device.
The nebulization of the water is obtained by a rotating disc activated by a closed and ventilated motor.
A series of steel rotating blades remix the moistened powders in order to obtain perfect homogenisation. The percentage of water added to the product can vary from 1% to 3% according to the chemical and physical characteristics of the raw materials and the production requirement.
11) The body may be then stored in circular or square metal silos of different storing capacity as necessary. These are made of prefabricated panels, pre-painted with anti-rust, coated internally with rubber and bolted to support structures of variable capacity as required. Normally, storage of powders obtained through dry milling and by moistening do not need the so-called seasoning but can be sent immediately to production. Therefore, the silos are used simply to store the raw material for a maximum of one or two days production.
12) Manfredini & Schianchi ends the technological proposal with a mixing wetting equipment duly studied for dry processed bodies. The system equipped with airtight throttle valves and a high pressure water addition system perfectly homogenizes the body and adds the necessary amount of water which easily permeates the composition without formation of lumps or big grains.
TECHNOLOGICAL LINE FOR EXTRUDED HEAVY CLAY PRODUCTS AND BRICKS TYPE B
This second line mainly consists of:
1) Loading hoppers as above
2) Extraction devices with double connecting rod as above or extracting belts
3) Conveyor belts as above
4) Hammer Mill as above or suitable primary mills
5) Pendular Finishing Mill with separators (static or dynamic, necessary to obtain the required values of grain sizes). The material is inserted into the milling chamber by a batching screw or a weighing belt, the milling process is carried out by rotating rollers which squeeze it against a fixed circular sector track. The particles are carried to the upper part of the mill by a flow of air generated by a centrifugal ventilator or collected by a process filter. Operations can take place in two ways:
- By means of a cyclone creating a closed mill-cyclone-fan circuit
- By means of a sleeve filter with an open mill-filter-fan circuit
The technical-operational characteristic of the innovative open circuit is that the material which has just been ground can be handled by powerful and immediate suction. This solution drastically reduces the retention time of the material inside the milling chamber thus substantially reducing wear and tear on the grinding parts and notably increasing the hourly production capacity by as much as 100 % compared to traditional technologies.
The pendular mills can operate drying of raw materials with a very efficient thermal consumption which never exceed 450 Kcal/Hr. of evaporated water.
With such a system,we can operate a drying stage with a humidity reduction from 18% to 8% therefore such a line could serve customers with serious problems of humidity in the raw materials without installing a preliminary drying stage which increases the capital and running
costs accordingly.
The process ends with stages 10,11 and 12 as shown above.
CONCLUSIONS
Of course like all industrial processes involving raw materials , tests and trials on the Components to handle are essential to guarantee low processing costs and quality consistency in the final products, therefore Manfredini & Schianchi are equipped with skilled technicians, laboratories and pilot plants to conduct all the necessary ceramic evaluations.
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