2 APPLIED PROCESSES AND TECHNIQUES
CERAMIC TILE MANUFACTURING PROCESS
The ceramic tile manufacturing process consists of a ries of successive stages, which can be summarid as follows:
§Raw materials preparation
§Pressing and drying of the green body
§Firing, with or without glazing
§Additional treatments
§Sorting and packing
Depending on whether the product to be made is glazed or not, and whether single fire, twice fire or third fire is involved, the tile will or will not be glazed in a given process, or the order of the glazing and firing stages will be suitably rearranged. (Figure 1).
Figure 1. Diagram of the manufacturing process considered.
Raw materials preparation – Wet milling – Spray drying - Pressing - Drying - (Firing) - Glazing - Firing (Variable without glazing and with or without polishing) (Variable with cogeneration). Raw materials preparation – Dry milling - Pressing - (Firing) - Glazing - Firing.
Raw materials preparation - Mixing - Extrusion - (Glazing) - Firing.
Raw materials prep aration
The ceramic process starts by lecting the raw materials required for the body composition, which are mainly clays, feldspars, sands, carbonates and kaolins.
In the traditional ceramic industry, the raw materials are generally ud as-mined or after some minor treatment. As natural raw materials are involved, preliminary homogenisation is required in most cas to ensure consistent characteristics.
奶牛美女Dry or wet milling
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After a first mixing of the body components, the mixture is usually dry milled (hammer or pendulum mills) or wet milled (continuous or batch ball mills).
The resulting milled material exhibits different characteristics depending on whether dry or wet milling is ud. In dry milling, fragmentation occurs and particle aggregates and agglomerates remain, with a larger particle size (there are particles larger than 300 microns) than by the wet method (all particles are smaller than 200 microns). A decisive factor in lecting the type of milling to be ud is the capital outlay required in each ca.
Wet milling and spray drying
Wet milling and subquent spray drying are currently the most widely implemented methods in ceramic floor and wall tile manufacture by the single-fire process, owing to the important technical improvements they provi de. (Figure 2).
Figure 2.
In wet milling, the raw materials can be wholly or partially fed into the ball mills, which is normally the ca, or they can be directly disperd.
Part of the water contained in the resulting suspension (slip) is removed by spray drying to obtain a product with the required moisture for each process stage. Spray drying is the most widely implemented drying method in tile manufacture.
In this drying process, the fine drops of sprayed suspension come into contact with hot air to yield a solid with a low water content.
The moisture content of the body slip usually ranges from 0.0-0.45 kg water/kg dry solid. The spray-drying process reduces the water content to 0.05-0.07 kg water/kg dry solid.
Spray drying takes place according to the scheme shown in Figure 3:
§Pumping and spraying the slip.
§Hot gas generation and feed.
§Drying by contact of hot gas-slip drops.
§Separation of spray dried powder from the gas.
Figure 3. Schematic illustration of the spray-drying process.
The spray-drying operation is as follows. The slip from the milling facility storage tanks, with a
60-70 % solids content and appropriate viscosity (around 1000cp.), is fed into the spray dryer by
reciprocating pumps.
The slip is sprayed as a fine mist, which dries on coming into contact with the hot gas stream.
The gas come from a conventional air-natural gas burner or are exhaust gas from a
cogeneration turbine.
The granulate, with a moisture content of between 5,5 and 7%, is discharged onto a conveyor
belt and conveyed to the silos for subquent pressing.
The stream of gas ud to dry the slip and produce the powder are exhausted through the top
of the spray dryer. The gas have a high water content and very fine suspended dust particles. The u of the spray-drying process to obtain the raw material for the body (spray-dried
powder) provides important advantages that favour the development of subquent
manufacturing process stages. One of the most important advantages is producing highly
uniform, more or less spherical hollow granules that provide the spray-dried powder with high
flowability and facilitate press die filling and the pressing of large-size tiles.
Another advantage worth mentioning is that it allows performing two operations, namely drying
and granulation, simultaneously with same facilities. On the other hand, control of process
variables is very simple, although the considerable rigidity of the operating boundary conditions impod by the facility’s geometry and constructive characteristic, need to be taken into
account. Further to be noted is the continuous character of the process, which allows process
automation.
The energy costs of the drying process are quite high, but energy efficiency can be raid by
heat recovery from the gas and electricity generation by installing cogeneration turbines.步的组词
Mixing
In this body preparation stage, the water and raw materials making up the body composition are
cloly mixed to a consistent paste that is readily mouldable by extrusion.
Tile forming
全国抗击新冠肺炎疫情表彰大会Dry pressing
Dry pressing (at 5-7% moisture content) with hydraulic press is the most common tile forming method. Forming takes place by mechanically compressing the paste in the die and is one of the most cost-efficient forming methods for making ceramic ware with a regular geometry.
In pressing, the oil-hydraulic press system drives the rams into the powder bed in the die. The main hydraulic press characteristics are as follows: high compaction force, high productivity, easy adjustment and consistency in holding the t pressing cycle schedule.
The pressing facilities have developed significantly in the last few years with very sophisticated, easily adjustable and highly versatile programmers.球茎茴香
Extrusion
Tile forming by extrusion process basically consists of putting the plastic body through a die that produces a constant tile cross ction.
The equipment involved is made up of three main parts: a driving system, the die and the cutter. The most common driving system is an auger.
Drying of the green ceramic bodies
After forming, the tile body is dried to reduce the moisture content (0.2-0.5 %) to appropriately low levels for the firing and eventual glazing stages.
In the dryers that are commonly ud in the ceramic industry, heat is transferred mainly by convection from hot gas to the tile surface, and also slightly by radiation from the gas and from the dryer walls to the tile surface.美国打叙利亚
Therefore, during the drying of ceramic bodies, a simultaneous and concutive displacement of the water takes place through the wet solid and the gas. The air ud must be sufficiently dry and hot, becau it not only rves to remove the water from the solid but also to provide energy in the form of heat to evaporate the water.
At prent, the bodies are dried in vertical or horizontal dryers. After shaping, the bodies are placed in the dryer where they face a hot gas countercurrent. The hot gas come from an air–natural gas b
urner or from the kiln cooling stack. The main heat transfer mechanism between the air and the bodies is convection.
In the vertical dryers, the pieces are fed into baskets consisting of veral decks of rollers. The groups of baskets move upward through the dryer, where they come into contact with the hot gas. The temperature in this type of dryer is normally less than 200ºC and the drying cycles range from 35-50 minutes.
The horizontal dryers are designed like the rollers kilns. The items are fed onto different decks inside the dryer, and conveyed horizonta lly on the rollers. Burners located on the sides of the kiln produce the hot drying air countercurrent. The maximum temperature in the types of facilities is usually higher than in the vertical dryers (around 350ºC) and the drying cycles are shorter, between 15 and 25 minutes.
Overall, horizontal dryers have a lower energy consumption compared to the vertical dryers due to a better arrangement of the items inside the dryer and a lower thermal mass.
The resulting emission from the drying stage is a gaous stream with a temperature of about 110ºC, with a very low concentration of suspended particulates from the tile surfaces being drawn al
ong in the exhaust stream.
Firing, with or without glazing
Unglazed products are fired after the drying stage. Similarly, in the ca of glazed twice-fire products, the green bodies are fired after drying.
Glazing
Glazing involves applying one or more coats of glaze with a total thickness of 75-500 microns onto the tile proper surface by different methods. Glazing is done to provide the fired product with a ries of technical and esthetical properties such as impermeability, cleanability, gloss, colour, surface texture, and chemical and mechanical resistance.
The nature of the resulting glaze coating is esntially v itreous, although in many cas the glaze structure contains crystalline elements.
Glazes and frits
The glaze, just like the ceramic body, is made up of a ries of inorganic raw materials. The major gl
aze component is silica (glass former), as well as other elements that act as fluxes (alkalis, alkaline earths, boron, zinc, etc.), opacifiers (zirconium, titanium, etc.), and as colouring agents (iron, chromium, cobalt, mangane, etc.).
A wide variety of glazes are formulated depending on the type of product, firing temperature, and the desired effects and properties of the finished product.
In other ceramic process (porcelain artware, sanitary ware), glazes are formulated that only contain crystalline, natural or synthetic raw materials, which contribute the necessary oxides. However, in ceramic floor and wall tile manufacture, raw materials of a glassy nature (frits) are ud. The are prepared from the same crystalline materials that have previously undergone heat treatment at high temperature.
Frits: Nature, advantages, composition and manufacture.
Frits are vitreous compounds, insoluble in water, made by melting at high temperature (1500ºC) followed by fast cooling of the raw materials mixture. Most of the glaze compositions ud in tile manufacture have a larger or smaller fritted part, which can consist of a single frit or blend of different types of frits.
For a given chemical composition using frits has certain advantages compared to using unfritted raw materials, such as:
Insolubility of certai n chemical elements.庸俗化
Lower toxicity; owing to its size and structure, the frit tends to form less ambient dust than the original raw materials, thus reducing the hazard associated with raw materials toxicity.
珍珠耳饰Wider glaze working temperature range, as they have no defined melting points.
The purpo of the frit production process, usually known as fritting, is to obtain a vitreous material that is insoluble in water by melting and subquent cooling of the mixture of different materials.
