WO2002081409A1

WO2002081409A1 - Method for manufacturing a light, strong and heat-insulating clay-based ceramic

Info

Publication number: WO2002081409A1
Application number: PCT/NO2002/000133
Authority: WO
Inventors: Håkon RUESLÅTTEN; Ole Sivert Hembre; Ola Torstensen
Original assignee: Statoil Asa; Nord-Trøndelag Fylkeskommune; Nordland Fylkeskommune
Priority date: 2001-04-09
Filing date: 2002-04-05
Publication date: 2002-10-17
Also published as: NO20011807L; NO20011807D0; NO313667B1

Abstract

The invention relates to a method for manufacturing a light, strong and heat-insulating clay-based ceramic, which is designed for bricks, insulating blocks, and the like, by means of clay and lime, and subsequent re-moistening and exposure in carbon dioxide.

Description

Method for manufacturing a light, strong and heat-insulating clay-based ceramic

The invention relates to a method for manufacturing a light, strong and heat- insulating clay-based ceramic, which is designed for bricks, insulating blocks and the like, with the use of clay and lime, with subsequent re-moistening and exposure in carbon dioxide.

BACKGROUND OF THE INVENTION

In the prior art, the manufacture of building blocks, e.g. bricks, involves the baking of clay alone, but in some cases it is known that small amounts of lime have been added in order to adjust the colour of the block, in which case the baking is carried out at 1100-1200°C. This involves a high energy, which results in sintering processes in the clay, which in turn leads to severe shrinkage of the block during baking. The traditional brick has a high volumetric weight (i.e. it is heavy) and has relatively poor thermal insulating properties.

Related prior art methods are known, e.g., from DE PS 898 269, DE OS 1 571 301 and DE 40 21 028 Al, which involve baking of different mixtures of clay and lime.

DE PS 898 269 discloses a method for manufacturing ceramic heat insulating blocks with a bulk density of less than 1.3, wherein a mass containing clay is added to raw materials in finely divided form containing alkaline- earth oxide in order to increase their natural porosity, and the baking temperature is in the range of 1000- 1100°C.

DE OS 1 571 301 discloses a method for manufacturing partly fireproof insulating blocks, i.e. blocks and other moulded items that can withstand temperatures up to 1200°C, by employing a clay mixture containing calcium carbonate, which, after the admixture of water and organic filler materials that provide the porosity necessary to ensure insulating capacity, are moulded into blocks and baked.

DE 40 21 028 Al discloses a method for high-precision manufacture of cellular ceramic building and construction elements of large format and/or large volume. Baked lime is added to an initial mixture of clay, water and a foamer, thereby forming calcium hydroxide, which in a carbon dioxide atmosphere by means of carbonisation in clay potsherd forms a crystalline structure of calcium carbonate. This calcium carbonate is mechanically stable and prevents the occurrence of shrinkage and cracking during the drying process.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a clay-based ceramic, such as a block that can be employed for building purposes, and particularly where it is suitable for the purpose to have a light (low volumetric weight) and porous block with good heat-insulating properties and with a tensile strength in bending that is comparable with traditional blocks, e.g. bricks. The object is to be able to employ the blocks in a similar way to cement-stabilised light clinker blocks, although it is a stronger type of block. In order to achieve this, the block is baked at a lower temperature than that previously known in order to avoid sintering and shrinking of the block, and to maintain the porosity. The lower baking temperature also results in a substantially lower energy consumption than for the previously known methods, in addition to which the post-hardening of the block leads to a great deal of carbon dioxide being taken up by the block, thus providing environmental benefits.

According to the present invention a method is provided for manufacturing a light, strong and heat-insulating clay-based ceramic, which is characterised in that it comprises the following steps:

- mixing clay, lime and water in order to obtain a moist and ductile mixture,

- baking the mixture in order to obtain a ceramic, the baking being performed by means of a temperature of 900°C and higher, and preferably at 1000°C,

- adding water to the ceramic after the ceramic has been cooled, and

- adding carbon dioxide to the moistened ceramic.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 illustrates volumetric weight as a result of calcite content and baking temperature.

Figure 2 illustrates open porosity as a result of calcite content and baking temperature. Figure 3 illustrates the correlation between the amount of unreacted CaO after baking at 1000°C and the amount of calcite in unbaked samples.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a new type of clay-based ceramic such as bricks, insulating blocks and the like, which have lower volumetric weight and better insulating properties than traditional blocks. The ceramic obtained by means of the method has a high degree of porosity (over 50 vol-%) and a volumetric weight (density) of less than 1.3 g/cm , preferably down towards about 1.2 g/cm . Additionally, the bending strength of the ceramic is comparable with, e.g., that of bricks.

According to the invention, clay, lime and water are mixed to obtain a moist and ductile mixture. Calcite or other carbonate rocks may be employed as lime. Any type of clay may be used according to the invention. Examples of suitable clays include, but are not limited to, illite, chlorite, kaolinite, smectites, sepiolites and mixtures thereof. The clay employed according to the present invention may, e.g., be Norwegian glacimarine clay with main components consisting of illite, chlorite, feldspars and quartz.

The sintering properties of the clay used according to the invention may be measured in a dilatometer at a heating rate of 120°C/hour. The clay behaves in such a manner that it begins to sinter at about 850°C. The maximum sintering rate occurs at about 1080°C. At 1150°C the clay is well into the smelting phase.

In addition, an additive of fine-grained clay ("Ball Clay") of the Hypure Vector type from English China Clay (ECC) may be added in order to improve the plasticity/ductility of the material. Up to 5 weight-% of the said addition may be used calculated from the total mixture. The mass is then moulded or extruded, and some degree of drying may be performed.

Baking of clay and calcite gives a product consisting of different silicates plus glass phase. CaO reacts relatively quickly with clay minerals when the temperature exceeds 800°C and forms different calcium silicates or calcium aluminium silicates, which may, e.g., be wollastonite CaO-Si0₂, anorthite CaO-Al₂0₃-2Si0₂ or other minerals. For the baking process a standard kiln of the Entech high-speed type kiln with air supply may, e.g., be employed. In order to calcine the carbonate rock the temperature must be over 800°C, but not much over 1000°C, since this causes the clay to sinter and shrink. 1000°C therefore seems to be an optimum upper limit when using calcite as carbonate rock. At 1000°C complete calcinations of the calcite is obtained, and at this temperature the sintering of the clay has only just begun, i.e. the shrinkage of the clay is less than 1%. This means that the block retains its shape during the baking process. During baking most of the calcined calcite will react with the silicate minerals to form slag minerals. The amount of calcite in the block must therefore be sufficiently high to ensure that after the baking process there is still free calcium oxide in the block, and this is critical for the post-hardening process. At least 40 weight-% of lime of the total mixture must be added in order to obtain CaO residue after the first baking. In a preferred embodiment at least 50 weight-% of lime is employed, and more preferred up to 60 weight-% of the total mixture.

According to the present invention, after baking it will be possible to convert unreacted CaO to CaC0₃ by exposure to C0₂- By means of the baking (calcination) calcite (CaC0₃) will be converted to CaO and C0₂, and with water present a hydration to calcium hydroxide (Ca(OH)₂) occurs.

The post-hardening process is carried out by the cooled block being moistened by being immersed for 3-5 seconds into water and placed under pressure in a carbon dioxide atmosphere in an autoclave for a certain period of time (hours). Carbonisation takes place when calcium hydroxide reacts with carbon dioxide, thus forming calcium carbonate and water. The carbon dioxide exposure time depends on the carbon dioxide pressure and the size of the block. With a pressure of 10 bar and a block whose dimensions are, for example, 155mm x 12mm x 11mm, 18 hours is sufficient. The optimal exposure time must be determined in relation to the size of the block and the carbon dioxide pressure employed, and may, e.g., have a duration of 24 hours or more.

The post-hardening process causes a great deal of carbon dioxide to be taken up by the block as newly-created carbonate cement.

The following specific examples are intended to illustrate the advantages of the present invention, and are not intended to limit the scope of the invention in an undue manner. Examples

Sample series 1 : clay + calcite

A series of samples was made of clay and increasing amounts of calcite. The object was to find how large a calcite additive it was necessary to have in order to obtain unreacted CaO in the samples after baking. The presence of some CaO is necessary in order to be able to obtain a reaction with C0₂, thus forming CaC0₃ in the pore structure of the material.

Clay and calcite were mixed with water and the water was filtered off, leaving a plastic mass. This was rolled out to form a flat lump that was cut into smaller bits for baking. The samples were heated up at 150°C/hour, kept for 2 hours at the top temperature, which varied between 800 and 1000°C, and cooled at 300°C/hour. After baking, volumetric weight and porosity (ISO 5017) were measured for selected series. The results are shown in table 1 and figures 1 and 2.

Table 1: Volumetric weight and open porosity of samples based on clay and calcite used according to the invention

The results illustrate that all the samples became extremely porous and light. The porosity increases substantially as the calcite additive increases, and varies little with the baking temperature. The samples with 55%> calcite have a porosity of between 58 and 59%> and a volumetric weight of 1.26-1.29 g/cm . At the other end of the test series are the samples with 10% calcite that have a porosity of 38.5 % and a volumetric weight of 1.73 g/cm³.

Sample series 2: 50% clay + 50%> calcite

The new sample series was made with the composition 50% calcite, 5% additive of "Ball Clay" (ECC - Hypure Vector), and 45% clay. Said additive was added in order to increase the dry breaking strength of the samples. Rods were cast, which were about 155 mm long and about 12 x 11 mm² in cross section. These were then dried and baked at 1000°C according to the same baking program as in sample series 1. After baking, the samples were exposed to both water and C0₂, and flexural strength was measured in a 3 point bending test before and after exposure.

10 rods were placed in water overnight before being autoclaved, since CaO had to be converted to Ca(OH)₂ before exposure to C0₂. After the water treatment they were dried at 110°C before being placed in an autoclave at 10 bar C0₂ pressure. After 18 hours in the autoclave they were removed and weighed once more, whereupon almost no weight increase was registered, the conclusion being that no special reaction had taken place.

The rods were therefore immersed in water for 3-5 seconds and placed in the autoclave in a wet condition. This was from Friday afternoon until Monday morning and they remained in the autoclave at 10 bar C0₂ pressure. They were removed and dried once again. All the rods now showed a clear weight increase, which on average was of 4.21%. This meant that a reaction had been obtained. Assuming that all free CaO had reacted to CaC0₃, one finds that the amount of CaO in baked samples (1000°C) was 8.7% on average. 50%) calcite additive thus gives an unreacted residue of 8.7% CaO. What was demonstrated here was that the reaction to CaC0₃ did not occur so rapidly in the samples with clay as for pure CaO. No special reaction was obtained to Ca(OH)₂ after one night in water, but by autoclaving wet samples a very good reaction was obtained. This led to similar tests being conducted with the samples from the sample series, since a method was now available to measure indirectly how much unreacted CaO there was in the samples after baking. Weight increase after exposure in C0₂ can be used as a measure of how much residue CaO there is in the samples after baking. It is presumed that the weight increase is due to the fact that CaO has been converted to CaC0₃. Of one gram of CaC0₃, 0.56 g is CaO. It is therefore a simple task to calculate the amount of unreacted CaO in the baked samples by measuring the weight increase after autoclaving in C0₂ gas. A measurement series of this kind was carried out on the samples FH10-FH55, which were baked at 1000°C after being exposed to C0₂ at 10 bar pressure over the weekend (Friday afternoon to Monday morning). Table 2 and figure 3 illustrate the results.

It is found that the amount of unreacted CaO increases exponentially with the amount of calcite in the original specimen. With a 30% calcite additive the amount that is unreacted after baking at 1000°C is about 1%. With a 50% calcite additive the amount that is unreacted after baking is about 8% and with 55% calcite additive the amount that is unreacted is about 12%.

Reckoned in terms of how much C0₂ can be "stored" in a block baked at

1000°C, one finds that a block made with 50%> calcite may have an uptake of about 64 kg C0₂ per ton block (corresponding to 82.5 kg C0₂ per m block).

Table 2 Unreacted CaO in baked samples (1000 °C) as a result ofCaC0₃ amount in unbaked samples

Claims

PATENT CLAIMS

1. A method for manufacturing a light, strong and heat-insulating clay- based ceramic, characterised in that it comprises the following steps: - mixing clay, lime and water in order to obtain a moist and ductile mixture,

- adding water to the ceramic after the ceramic has been cooled, and - adding carbon dioxide to the moistened ceramic.

2. A method as indicated in claim 1, characterised in that said lime is calcite or other carbonate rocks.

3. A method as indicated in claim 2, characterised in that said lime is used in an amount of at least 40% by weight and preferably up to 60%> by weight of the total mixture.

4. A method as indicated in claim 1, characterised in that the moistened ceramic is exposed in carbon dioxide at a pressure of 1 to 10 bar, and preferably at a pressure of 10 bar for a duration of 24 hours or more.

5. A method as indicated in claim 1, characterised in that said ceramic is a brick, an insulating block, and the like.