NO313667B1 - Process for making a light, strong and heat-insulating clay-based ceramic - Google Patents

Description

The invention relates to a method for the production of a light, strong and heat-insulating clay-based ceramic, which is made according to the pattern of, among other things, brick, brick, insulating stone, etc., using clay and lime, and subsequent moistening and exposure in carbon dioxide.

BACKGROUND OF THE INVENTION

Previously known technique for the production of building stone, e.g. brick, concerns the firing of only clay, but it is known that in some cases small amounts of lime have been mixed in to adjust the color of the stone, and the firing then takes place at 1100- 1200°C. This results in high energy consumption. Traditional manufacture of stone, e.g. brick, means that the firing causes sintering processes in the clay, which in turn means a strong shrinkage of the stone during firing. The traditional brick has a high volume weight (i.e. it is heavy) and has relatively poor thermal insulation.

Related methods in the field are known from, among other things, DE patent document no. 898 269, DE explanatory document no. 1 571 301 and DE 40 21 028 A1, which include burning different mixtures of clay and lime.

DE patent no. 898 269 describes a method for the production of ceramic heat insulation pieces with a bulk density of less than 1.3, where a clay-containing mass is added to alkaline earth oxide-containing raw materials in finely divided form, to increase their natural porosity, and the firing temperature is in the range of 1000- 1100°C.

DE laying-out document no. 1 571 301 describes a method for the production of partially refractory insulating stones, i.e. stones and other shaped pieces which can withstand temperatures up to 1200°C, using a calcium carbonate-containing clay mixture, which after the addition of water and organic fillers, which ensures the porosity necessary to achieve insulating properties, is shaped into stones and burned.

DE 40 21 028 Al describes a method which can produce cellular ceramic building and construction elements of large format and/or of large volume with great accuracy. Burnt lime is added to a starting mixture of clay, water and a foaming agent, thereby forming calcium hydroxide which in a carbon dioxide atmosphere forms a crystal structure of calcium carbonate when carbonized in clay pot shards. This calcium carbonate is mechanically stable and prevents shrinkage and cracking during drying.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a clay-based

ceramic, such as a stone that can be used for building purposes, and especially where it is expedient to use a light (low volume weight) and porous stone with good heat-insulating ability and which has a bending tensile strength that is comparable to traditional stone, e.g. brick. The purpose is to be able to use the stone in a similar way to cement stabilized light clinker stone, but which has greater strength than this. To achieve this, the stone is fired at a lower temperature than previously known to avoid sintering and shrinkage of the stone, and to preserve the porosity. The lower burning temperature also leads to a significantly lower energy consumption than for the previously known methods and that the post-hardening of the stone leads to a lot of carbon dioxide being tied up in the stone, which brings about an environmental benefit.

According to the present invention, a method is provided for the production of a light, strong and heat-insulating clay-based ceramic, which is characterized by the fact that it includes the steps

- mixture of clay, lime and water to obtain a moist and mouldable

mixture,

- firing the mixture to obtain a ceramic, the firing being carried out using a temperature of 900°C and higher, and preferably at 1000°C,

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

- adding carbon dioxide to the moistened ceramic.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows volume weight as a result of calcite content and firing temperature.

Figure 2 shows open porosity as a result of calcite content and firing temperature

Figure 3 shows the relationship between the amount of unreacted CaO after firing at 1000°C and the amount of calcite in unfired samples.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a new type of clay-based ceramic such as brick, brick, insulating stone, etc., which has a lower volume weight and better insulating ability than traditional stone. The ceramic obtained by the method has high porosity (over 50% by volume) and a volume weight (density) of less than 1.3 g/cm<3>, preferably down to approx. 1.2 g/cm<3>. Furthermore, the ceramic's bending strength (strength) is comparable to, for example, brick.

According to the invention, clay, lime and water are mixed to obtain a moist and malleable mixture. Calcite or other carbonate rocks can be used as lime. The clay useful in the invention may be any clay. Examples of suitable clays include, but are not limited to, illite, chlorite, kaolinite, smectites, sepiolites and mixtures thereof. The clay used according to the present invention can e.g. be a Norwegian glacimarine clay, with main constituents illite, chlorite, feldspar and quartz.

The sintering properties of the clay used according to the invention can be measured in a dilatometer with a heating rate of 120°C/hour. The clay behaves in such a way that it begins to sinter at around 850°C. The maximum sintering rate is around 1080°C. At 1150°C, the clay has a significant melting phase.

An additive of fine-grained clay ("Ball Clay") of the Hypure Vector type from English China Clay (ECC) can also be added to improve the plasticity/formability of the mass. Up to 5% by weight of said additive can be used, calculated from the total mixture. The mass is then molded or extruded, and some drying can be carried out.

When clay and calcite are fired, a product is obtained which consists of various silicates plus a glass phase. CaO reacts relatively quickly with clay minerals when the temperature rises above 800°C and forms various calcium silicates or calcium aluminum silicates, such as e.g. can be wollastonite CaOSi02, anorthite CaOAl203-2Si02 or other minerals. When burning, it can e.g. a standard oven of the type Entech rapid oven with air access is used.

To calcine the carbonate rock, the temperature must be above 800°C, but not particularly above 1000°C, because then the clay sinters and shrinks. 1000°C therefore seems to be an optimal upper limit when using calcite as carbonate rock. At 1000°C, full calcination of the calcite is achieved, and at this temperature the sintering of the clay has just begun, i.e. the shrinkage of the clay is less than 1%. This means that the stone retains its shape during firing. During firing, most of the calcined calcite will react with the silicate minerals to form slag minerals. The amount of calcite in the stone must therefore be high enough so that after firing there is still free calcium oxide in the stone, and this is critical for post-hardening. At least 40% by weight of lime of the total mixture must be added to obtain CaO residues after the first firing. In a preferred embodiment, at least 50% by weight of lime, and more preferably up to 60% by weight, of the total mixture is used.

According to the present invention, after burning, unreacted CaO can be converted to CaCO3 by exposure to CO2. During the burning (calcination), calcite (CaCOs) will be converted into CaO and CO2 and so on. in the presence of water, hydration to calcium hydroxide (Ca(OH)2) occurs.

Post-hardening takes place by moistening the cooled stone by immersing it in water for 3-5 seconds and putting it under pressure in a carbon dioxide atmosphere in an autoclave for a certain number of hours. Carbonation occurs when calcium hydroxide reacts with carbon dioxide so that calcium carbonate and water are formed. The time for the carbon dioxide exposure depends on the carbon dioxide pressure and the size of the stone. With a pressure of 10 bar and a stone with dimensions of, for example, 155mm x 12mm x 11mm, 18 hours is sufficient. Optimal exposure time must be determined in relation to the size of the stone and the carbon dioxide pressure used, and can e.g. have a duration of one day or more.

Post-hardening causes a lot of carbon dioxide to be tied up in the stone as newly formed carbonate cement.

The following specific examples are intended to illustrate the advantages of the present invention, and are not intended to unduly limit the scope of the invention.

Examples

Sample series 1: clay + calcite

A series of samples of clay and increasing amounts of calcite were made. The purpose was to find how much calcite addition was needed in order to obtain unreacted CaO in the samples after firing. You need to have some CaO present to be able to get it. back reaction with CO2 so that CaC03 is formed in the pore structure of the material.

Clay and calcite were mixed with water and the water filtered off so that a plastic mass was obtained. This was rolled out into a flat sheet which was cut into smaller pieces for burning. The samples were heated at 150°C/hour, held for 2 hours at a peak temperature that varied between 800 and 1000°C, and cooled at 300°C/hour. After firing, the volume weight and porosity (ISO 5017) were measured on selected series. The results are shown in table 1 and figures 1 and 2.

It appears from the results that all the samples became very porous and light. The porosity increases strongly with increasing addition of calcite, and varies little with the firing temperature. The samples with 55% calcite have a porosity of between 58 and 59% and a volume weight of 1.26-1.29 g/cm<3>.1 at the other end of the test series, one finds the samples with 10% calcite, which have a porosity of 38 .5% and a volume weight of 1.73 g/cm<3>.

Sample series 2: 50% clay + 50% calcite

The new sample series was made with a composition of 50% calcite, 5% additive of "Ball Clay" (ECC - Hypure Vector), and 45% clay. Said additive was added to increase the dry breaking strength of the samples. Staves were cast approx. 155 mm long and approx. 12 x 11 mm<2> in cross section. These were then dried and fired at 1000°C according to the same firing program as in test series 1. After firing, the samples were exposed to both water and CO2, and bending strength was measured in a 3-point bending test both before and after exposure. 10 rods were placed in water overnight before they were to be autoclaved, this so that the CaO had to be transferred to Ca(OH)2 before exposure to CO2. After the water treatment, they were dried at 110°C before being placed in an autoclave with 10 bar C02 pressure. After 18 hours in the autoclave, they were taken out and weighed again. Almost no weight gain was then recorded and it was concluded that no particular reaction had occurred. The rods were therefore dipped for 3-5 seconds in water and placed wet in an autoclave. This was from Friday afternoon to Monday morning and they were left in the autoclave at 10 bar C02 pressure. They were taken out and dried again. All the rods now showed a clear increase in weight, on average it was 4.27%. This meant that there had been a backlash. If one assumes that all free CaO had reacted to CaC03, one finds that the amount of CaO in burnt samples (1000°C) was 8.7% on average. 50% calcite addition thus gives an unconverted residue of 8.7% CaO. What was seen here was that the back reaction to CaC03 did not proceed as quickly in the samples with clay as for pure CaO. No particular reaction to Ca(OH)2 was obtained after a night in water, but by autoclaving wet samples a very good back reaction was obtained. This led to similar tests being carried out with the samples from the test series because there was now a method to indirectly measure how much unreacted CaO was in the samples after firing.

Weight gain after exposure to CO2 can be used as a measure of how much residual CaO there is in the samples after firing. It is assumed that the increase in weight is due to CaO having changed to CaC03. Of one gram of CaCO3, 0.56 g is CaO. You can therefore easily calculate the amount of unreacted CaO in the burnt samples when you measure the increase in weight after autoclaving in C02 gas. Such a series of measurements was carried out on samples FH10-FH55 which had been fired at 1000°C after they had been exposed to CO2 at 10 bar pressure over the weekend (Friday afternoon to Monday morning). Table 2 and Figure 3 show the results. It is found that the amount of unreacted CaO increases exponentially with the amount of calcite in the original sample. With 30% calcite addition, the amount unreacted after firing at 1000°C is approx. 1%. With 50% calcite addition, the amount unconverted after firing is approx. 8% and with 55% calcite addition, the amount unconverted is approx. 12%.

Converted to how much CO2 can be "stored" in a stone burnt at 1000°C, it is found that a stone made with 50% calcite can absorb approx. 64 kg CO2 per tonne of stone (equivalent to 82.5 kg CO2 per m3 stone).

Claims (5)

1. Method for producing a light, strong and heat-insulating clay-based ceramic, characterized in that it includes the steps - mixing clay, lime and water to obtain a moist and malleable mixture, - firing the mixture to obtain a ceramic, the firing being carried out using a temperature of 900°C and higher, and preferably at 1000°C, - adding water to the ceramic after the ceramic has cooled, and - adding carbon dioxide to the moistened ceramic. 2. Method as stated in claim 1, characterized in that said lime is calcite or other carbonate rocks. 3. Method as stated in claim 2, characterized 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. Method as stated in claim 1, characterized in that the moistened ceramic is exposed to carbon dioxide at a pressure of 1 to 10 bar, and preferably at a pressure of 10 bar for a duration of one day or more. 5. Method as stated in claim 1, characterized in that the said ceramic is brick, brick, insulating stone, etc.