NO159376B - CEMENTOUS MATERIAL AND USE OF THIS. - Google Patents

Description

The invention relates to basic, refractory, cementitious materials and their use.

In the application, refractory parts are produced from the cementitious material of the type that is exposed to chemical attack, wear and erosion due to the impact of molten metals, such as steel.

Refractory components for valves and refractory nozzles for various purposes in metal casting have generally been produced by pressing and firing at high temperatures. Expensive high-purity materials such as zirconium dioxide and refractories based on

85-9 5% A^O-j, have been considered necessary due to the extremely harsh conditions of use to which the components are exposed. The energy that goes into producing components by pressing and firing is very large, as firing temperatures normally above 1500°C must be created and maintained throughout the firing process. Energy costs contribute strongly to the price per unit of components manufactured from such fired refractories. Despite the use of high-burnt refractories in metal casting, it is usually necessary to frequently replace such items as valve plates with consequent large costs.

More recently, chemically bound concretes have been proposed, e.g. for sliding gate valve plates. In a similar way to fired refractory sheets, it is unlikely that sheets of chemically bonded concrete will be able to withstand repeated thermal shocks. The use of such plates in valves for casting blocks is therefore expected to be disrupted by unwanted downtime for replacement of the plates.

It has now been found that certain hydraulically bound, basic, cement-like . materials are surprisingly able to withstand heat shock very well, and it is particularly easy to manufacture components from these materials.

The invention thus relates to a hydraulic, refractory, cementitious material for the production of cast, refractory parts which are resistant to molten metals, comprising a mixture of fused or sintered magnesium oxide, aluminum oxide and hydraulic cement with a high aluminum content. oxide content, and the material is characterized by the fact that: it essentially consists of 70-86 wt% magnesium oxide containing at least 94 wt% MgO, 5-15 wt% aluminum oxide containing at least 98 wt% Al2O3 and 9-15 wt% cement containing at least 45 wt% Al2O3.

Cast refractory parts made from the material according to the invention are distinctive in that they have a compressive strength in the cold state after having been exposed to temperatures of 110°C, 1000°C and 1500°C, of respectively 492, 517 and 868 kp/cm<2> and a room density after they have been exposed to these temperatures, of respectively 2.83, 2.78 and 2.85 g/cm 3. Such refractory parts can be produced by vibration casting of the part in a mold from hydraulic concrete produced by adding water to the material according to the invention, followed by hardening the casting and drying it .

The material according to the invention essentially comprises three components, i.e. magnesium oxide, aluminum oxide and aluminium-containing hydraulic cement. Smaller amounts of other components can possibly be added for special purposes, such as plasticizing compounds, wetting agents or carbon-containing materials, such as tar or pitch. The latter are usually used in valve plates and nozzles to prevent slag adhering to such components.

The first two components are preferably of very high purity to achieve the best results. The magnesium oxide component must therefore have an MgO content of at least 94% by weight

and the aluminum oxide component an A^O^ content of at least 98% by weight. The aluminum oxide can be sintered, melted or preferably calcined.

The cement component can in principle1! is made up of any cement with a high aluminum oxide content (an A^O^ content of over 45% by weight of the cement). The aluminum-containing cement component. however, preferably has an A^O-j content of not less than 75% by weight of the cement component.

The magnesium oxide component must be present in an amount of i

of 70-86% by weight of the total weight of the three components.

On the same percentage basis, the aluminum oxide component is present in an amount of 5-15%, and the cement component in an amount of 9-15%, preferably 9-12%.

The present mixture is prepared from size-graded particulate materials. The cement component should preferably have a particle size of 75 µm or less. It can be tolerated that some cement particles are larger, but preferably at least 90% of the cement should have a particle size of 75 µm or less. Suitable mixtures can be selected as desired from the data given in the table below:

The mixtures are mixed with water in a sufficient quantity to obtain a workable mixture. Such a mixture may contain e.g. 7% water by weight based on the mixture. The mixture is self-hardening at room temperature. It is unnecessary to add heat, although moderate heating may be permissible to accelerate the hardening of castings. Without heating, however, hardening can take place to such an extent that the castings can be removed from the mold within approx.

1 hour. A high production capacity can thus be achieved.

The present hydraulically binding materials offer significant advantages compared to chemically bound systems. A problem that is always present with chemically bonded systems is that as they heat cure and dry, the binder tends to migrate towards exposed surfaces. A migration of the bond and the resulting lack of uniformity in the construction of the castings does not occur with the materials according to the invention. In contrast, a rigid hardening takes place, so that the castings can be handled without the risk of internal stress cracks being introduced. It is not excluded in connection with chemically bonded castings that these will be very adversely affected by handling.

The present materials exhibit a surprising

good resistance to thermal shock. It can therefore be expected that they will find use as parts for sliding gate valves and associated castings that are used for periodic casting of molten metals.

A commonly used test to determine thermal shock resistance is the burner test developed by the United States Steel Corporation's Research Laboratories. In this test, an oxygen-propane torch flame is passed slowly across a refractory material being examined, at a speed of 1.7 mm/s, and the torch is held at a distance of 6.4 mm from the surface of the refractory material.<1>

Normally pressed and burnt magnesium oxide valve sheets cannot ordinarily withstand even one passage of the oxygen-propane flame without significant damage occurring both on the surface and internally. Known chemically bonded magnesium oxides

in

valve plates are better able to resist the flame, but tests have shown moderate degradation after one pass..

In contrast, valve plates made from the present materials have been shown to be able to resist

stand repeated passes, e.g. twelve, without significant degradation of the surface. This indicates the ability of such valve plates to withstand the temperature variations encountered with repeated valve throttling, and opening/closing operations with valves will show a clear improvement compared to the use of burnt or chemically bonded plates.

As indicated above, the present materials can be used for casting valve plates for sliding gate valves and also as nozzles, as collectors, and tap pipe extensions connected therewith. Ladle wells and discharge nozzles can also be made from the available materials, and other applications of these will also be apparent.

Articles that have been cast from the materials present will usually be supplied in a hydraulically hardened state. Nevertheless, it may occasionally be desirable to deliver

they cast objects in a pre-burnt state rather than burning these during use. Pre-combustion may be applicable, e.g. for such items as replaceable wear- and erosion-resistant sleeves or liners for discharge nozzles.

Example

The material described here contained the following relative amounts of the following components. The stated percentages are again based on the weight of the combined weight of the magnesium oxide, aluminum oxide and cement components.

The cement had an A^O-^ content of over 75% by weight of the cement, and at least 90% by weight of the cement had a particle size of less than 75 µm. The magnesium oxide and the aluminum oxide respectively contained MgO and Al203 in an amount of 94% by weight and 98% by weight of these components.

The material produced a workable and sufficiently flowable concrete that it could be cast after it had been mixed with water in an amount of 7% of its weight. The mold filling can be facilitated by vibration, and an example of a usable vibration frequency is 3000 Hz.

Vibration-cast concrete samples that were prepared as described above showed the following properties at the indicated temperatures after curing and drying:

The above properties are considered to be completely suitable for the production of sliding gate valve components which, if desired, can be delivered in an afterburned condition.

Claims (6)

1. Hydraulic, refractory, cementitious material for the production of cast, refractory parts that are resistant to molten metals, comprising a mixture of fused or sintered magnesium oxide, aluminum oxide and hydraulic cement with a high aluminum oxide content, characterized in that it essentially consists of 70- 86 wt% magnesium oxide containing at least 94 wt% MgO, 5-15 wt% aluminum oxide containing at least 98 wt% Al^O^ and 9-15 wt% cement containing at least 45 wt% <A1>2<0>3. 2. Material according to claim 1, characterized in that 20-40% by weight of the total weight of the three components is made up of magnesium oxide with particle sizes in the range from 1 mm to 5 mm, 15-35% by weight, on the same basis, is made up of magnesium oxide with particle sizes in the range from 0 .3 mm to 1 mm, and 25-40% by weight, on the same basis, is magnesium oxide with particle sizes of 0.3 mm or less. 3. Material according to claim 1 or 2, characterized in that 0-15% by weight of the total weight of the three components is made up of aluminum oxide with particle sizes of 0.3 mm or less, and 1-15% by weight, on the same basis, consists of aluminum oxide with particle sizes of 45 µm or less. 4. Material according to claims 1-3, characterized in that, based on the combined weight of the three components, it comprises 26% by weight magnesium oxide with particle sizes in the range from 1 mm to 3 mm, 25% by weight magnesium oxide with particle sizes in the range from 0.3 mm to 1 mm and 3<*> % by weight of magnesium oxide with particle sizes below 0.3 mm, 6% by weight of calcined aluminum oxide with particle sizes below 4 5 µm and 9% by weight hydraulic aluminum oxide cement of which 90% by weight has particle sizes below 75 µm. 5. Use of the hydraulic, refractory, cementitious material according to claims 1-4, for the production of a refractory part which will be exposed to wear or erosion due to the impact of molten metals, by vibration casting of the part in a mold made from hydraulic concrete by adding water to the material according to requirements 1-4, followed by hardening the casting and drying it. 6. Use according to claim 5 in the production of cast, refractory parts with a cold compressive strength after they have been exposed to temperatures of 110°C, 1000°C and 1500°C, of respectively 2 in 492, 517 and 868 kg/cm and with a room density after they have been exposed to these temperatures, of; respectively 2.83, 2.78 and 3 2.85 g/cm .