GB2033369A

GB2033369A - Alumina Cement

Info

Publication number: GB2033369A
Application number: GB7936596A
Authority: GB
Original assignee: Didier Werke AG
Current assignee: Didier Werke AG
Priority date: 1978-10-23
Filing date: 1979-10-22
Publication date: 1980-05-21
Also published as: BE879583A; FR2439756B1; DE2846131B2; DE2846131C3; GB2033369B; FR2439756A1; DE2846131A1; NL7907467A

Abstract

An alumina cement comprises ground alumina cement clinker and ground alumina in which the alumina cement clinker is produced by reacting calcium oxide or a material which produces calcium oxide at the operational temperature with alumina in a fluidised bed at a temperature of between 1400 DEG C and 1450 DEG C, the alumina in the reaction comprising at least 50% by weight of ???-alumina and having an average particle size of less than 10 mu m, and the alumina of the cement comprises a mixture of 25 to 50% by weight of silting alumina and 75 to 50% by weight of Bayer alumina having a particle size of less than 44 mu m.

Description

SPECIFICATION Alumina Cement The invention relates to an alumina cement containing ground alumina cement clinker, ground alumina and optionally bauxite, sillimanite, kerphalite and/or other known alumina-carrying materials in ground form and optionally also conventional secondary constituents.

Alumina cement in its pure form corresponds to the empirical formula: CaO .2, 5A12O3. The socalled CA phase, i.e. CaO . Al203, is important for the formation of a hydraulic bond. In the production of a high-grade alumina cement, therefore, the alumina cement clinker is first produced by sintering or smelting, is finely ground and is then mixed with further alumina, bauxite, sillimanite, kerphalite or another alumina carrier in order to produce the general composition referred to above. It is also possible to grind the alumina cement clinker together with the alumina or the other additives. Alumina carriers are, for example, aluminas with at least 35% by weight of Al2O3.

Conventional secondary constituents are, for example, liquefiers or surface-active materials.

Depending upon the starting materials used the finished alumina cement has an Al2O3 content of 65 to 80% by weight for these types which have good refractory characteristics.

In the conventional method of production of alumina cement clinker the starting materials are produced at temperatures between 1200 and 1 6000C by sintering in a rotary kiln or by smelting the crude mixture which can consist for example of bauxite, alumina on the one hand and limestone on the other hand. Furthermore it is known to use other additives, e.g. MgO, in small quantities in the production of alumina cement clinker, see, for example, German Auslegeschrift No.1150612.

In a parallel application No. by the applicant of the same date entitled "Process for the production of alumina cement clinker" (Case 3031), the production of an alumina cement clinker is described which is particularly suitable for the production of a special alumina cement with a high Al2O3 content of up to approximately 80% by weight. This alumina cement requires particularly small quantities of water when used in the preparation of refractory cement mixtures. In addition, this type of alumina cement also makes it possible to produce refractory cements with good physical properties when both hot and cold.

According to the present invention an alumina cement comprises ground alumina cement clinker and ground alumina in which the alumina cement clinker is produced by a process in which calcium oxide or a material which produces calcium oxide at the operational temperature is reacted with alumina in a fluidised bed at a temperature of between 1 4000C and 1 4500C, the alumina comprising at least 50% by weight of alumina and having an average particle size of less than 10 Mm, and the alumina comprises a mixture of 25 to 50% by weight of silting alumina and 75 to 50% by weight of Bayer alumina having a particle size of less than 44 ym.

A preferred alumina cement contains an alumina cement clinker which is produced using silting alumina. The terms silting alumina or pouring alumina are to be understood to mean an alumina of which 90 parts by weight mixed with 10 parts by weight of water produce a silt, i.e. a workable clay sludge, which can be poured and does not settle, in the form of a fairly stable sol.

In a further preferred embodiment 20 to 80%, and preferably 40 to 60%, by weight of the alumina used in the production of the alumina cement clinker consists of particles with an average size of 2.5 to 6 zbm, and 80 to 20%, and preferably 60 to 40% by weight consists of particles with an average size of 1 Mm, this mixture having preferably 90% by weight of particles under 8 ym. The alumina used is preferably a calcined alumina which is low in sodium and produced by the Bayer process.

The alumina cement clinker is preferably produced at a temperature of 1415 to 1 4350C, and is preferably reacted for 10 minutes to 2 hours, in particular 20 minutes to 1 hour.

By regulating the rate of cooling after production of the alumina cement clinker in the fluidised bed it is possible to influence the setting properties of an alumina cement made from such an alumina cement clinker. In the preferred cement according to the invention the alumina cement clinker is cooled after the reaction at a rate of 400 to 7000C per hour from its production temperature to 3000 C.

As already stated, the rate of cooling of the alumina cement clinker produced according to the invention has an influence on the rate of setting of the cement. Thus it has been found for example that at a rate of cooling of 2000C/h of the temperature in the fluidised bed of 1 4300C to a temperature of 3000C an alumina cement made from this alumina cement clinker after grinding and addition of further alumina had a setting time of 20 to 40 minutes, whilst at a cooling rate of 6000C/h of the temperature in the fluidised bed from 1 4300C to 3000C the setting time was 30 to 60 minutes.

Preferably the alumina cement according to the invention contains a clinker in the production of which a molar ratio in the mixture of Al2O3 to CaO of approximately 1 was used.

In the production of the alumina cement clinker calcium oxide, or calcium carbonate which forms calcium oxide at the utilised temperature, is used with a particle size of up to 250 4m, advantageously up to 1 50 ym, preferably 50 to 250 Mm, especially 100 to 1 50 sswm.

If calcium carbonate is used as the starting material, the alumina and the calcium carbonate are used in the same proportions by weight as this corresponds to the desired phases. However, mixtures with 36 to 42 parts by weight of CaCO3 and 38 to 51 parts by weight of Al203 are also possible, but in this case greater proportions of phases other than CA may be formed. If starting materials which are not very pure are used then the impurities in them, particularly iron or SiO2, should naturally be taken into account in the quantities by weight of the individual constituents.

Furthermore, in the production of the clinker used to make the cement in accordance with the invention, MgO or a material such as MAC03 which forms MgO at the utilised temperatures, can also advantageously be added in a proportion of 0.5 to 1.5% by weight, advantageously 0.7 to 1.2% by weight. This MgO or MgCO3 is added in a finely ground form, advantageously with a particle size of less than 10 ym.

Advantageously the alumina cement according to the invention contains approximateiy equal proportions by weight of alumina cement clinker and of alumina.

In a prepared composition, the alumina in the cement comprises a mixture of 25 to 35% by weight of silting alumina and 75 to 65% by weight of conventional Bayer alumina with a particle size of less than 44 ssm.

The silting or pouring alumina used in accordance with preferred features of the invention both in the production of the alumina cement clinker and in the production of the alumina cement can be obtained by known processes. Aluminas or alumina mixtures are described in U.S. Patents Nos. 3432313 and 3516840 which can be used as such silting aluminas. In the production thereof, i.e. the grinding of the agglomerates of the crystallites of aluminium oxide to the desired particle size, it is preferable to use the method of U.S. Patent No.

3358937. Such aluminas are commercially available.

Using the production method given above for the alumina cement clinker it is possible to produce a product with the following phases: 30 to 80% by weight of the CA phase 20 to 65% by weight of the C,2A, phase 1 to 10% by weight of the CA2 phase in which other different phases occur only in smaller proportions of less than 1 to 2% by weight.

A cement clinker with the following phase ratios: 63 to 65% by weight of the CA phase 36 to 33% by weight of the C,2A, phase 1 to 2% by weight of the CA2 phase has proved especially advantageous as regards its setting characteristics since after mixing with further alumina to produce an alumina cement containing 80% by weight of Al203, and preparation of a refractory material with this cement, it showed a particularly small reduction in strength in the so-called weak zone occurring between about 6000C and about 11 000C. The formation of the individual phases given above can be easily determined by X-ray diffraction.

The parameters for the production of different ratios of the phases are in particular the period of dwell in the fluidised bed or sintering zone and the sintering temperature and to a lesser extent the rate of cooling Thus it is possible, on the basis of simple preliminary tests, to establish the individual parameters for a given starting mixture of a specific composition and of specific particle sizes in order to produce an alumina cement clinker with specific proportions of the phases.

As already stated, the alumina cement clinker is finely ground in a conventional manner after burning, e.g. to an average Blaine value of 8000 cm2/g.

This finely ground alumina cement clinker can then be mixed with the other conventional additives for an alumina cement, for example in a ratio of 100 parts by weight of finely ground alumina cement clinker with 100 to 125 parts by weight of the alumina mixture.

A further advantage of an alumina cement according to the invention and having a high Awl203 content is that it has very high stability in the so-called weak zone between about 6000C and 11 000C when the hydraulic bond is transformed into a ceramic bond, i.e. the bonded water is eliminated.

The invention may be put into practice in various ways, and one example will now be given: A starting mixture of 24 parts by weight of aalumina with an average particle size of 5,um was sintered with 24 parts by weight of calcium carbonate with a particle size of 100 to 1 50 ym and 0.4 parts by weight of MgO with a particle size of less than 10,um in a fluidised bed at 1 4250C+50C. The average period of dwell was approximately 1 hour. The material removed from the rotating bed was ground after cooling to an average Blaine value of 8000 cm2/g.

The crystallographic analysis showed that this material consisted predominantly of the CA phase with only a small quantity of the CA2 phase, approximately 35% C,2A, phase, traces of acorundum and traces of the CA6 phase.

48.4 parts by weight of this finely ground alumina cement clinker were ground, i.e. mixed, together with 17.0 parts by weight of silting or pouring alumina with an average particle size of 5 Mm and 34.6 parts by weight of Bayer alumina with a particle size of less than 44 Mm, though there was no significant further reduction in the particle sizes of the ground materials.

In a comparative experiment at 11 00C the alumina cement obtained in this way had a cold bending strength and cold compression strength about 10% higher than a commercially available alumina cement with a comparable Awl203 content, and at 1 4000C to 1 8000C the results for the bending strength and the compression strength were about 20% higher than those of a comparable commercially available cement. In addition a refractory cement made with an alumina cement of this type did not show any reduction in stability in the so-called weak zone from about 6000C to about 11 000C.

Claims

1. An alumina cement comprising ground alumina cement clinker and ground alumina in which the alumina cement clinker is produced by a process in which calcium oxide or a material which produces calcium oxide at the operational temperature is reacted with alumina in a fluidised bed at a temperature of between 1 4000C and 1 4500C, the alumina comprising at least 50% by weight of a-alumina and having an average particle size of less than 10,us, and the alumina comprises a mixture of 25 to 50% by weight of silting alumina and 75 to 50% by weight of Bayer alumina having a particle size of less than 44 Mm.

2. A cement as claimed in Claim 1 containing one or more of bauxite, sillimanite and kerphalite.

3. A cement as claimed in Claim 1 or Claim 2 in which the alumina cement clinker was produced with a silting alumina.

4. A cement as claimed in Claim 1 or Claim 2, in which 20 to 80% by weight of the alumina used in the production of the alumina cement clinker consists of particles with an average size of 2.5 to 6 lim and 80 to 20% by weight consists of particles with an average size of 1 ym.

5. A cement as claimed in Claim 4 in which 40 to 60% by weight of the alumina used in the production of the alumina cement clinker consists of particles with an average size of 2.5 to 6 Mm and 40 to 60% by weight consists of particles with an average size of 1 Mm.

6. A cement as claimed in any one of the preceding claims in which cement clinker was produced at a temperature of between 141 5 and 1 4350C.

7. A cement as claimed in any of the preceding claims in which the alumina cement clinker was produced by a reaction in the fluidised bed lasting between ten minutes and two hours.

8. A cement as claimed in any one of the preceding claims in which the alumina cement clinker was cooled after reaction at a rate of between 400 and 7000C per hour.

9. A cement as claimed in any one of the preceding claims in which the alumina cement clinker was produced with Al2O3 and CaO in a molar ratio of substantially 1.

10. A cement as claimed in any one of the preceding claims which contains substantial equal amounts by weight of alumina cement clinker and alumina.

11. A cement as claimed in any one of the preceding claims in which the alumina comprises a mixture of 25 to 35% by weight of silting alumina and 75 to 65% by weight of Bayer alumina with a particle size of less than 44,us.

12. A cement as claimed in any one of the preceding claims in which the alumina cement clinker contains the following phase ratios: 63 to 65% by weight of the CA phase 36 to 33% by weight of the C,2A, phase 1 to 2% by weight of the CA2 phase

13. An alumina cement substantially as specifically herein described.