GB2042499A - Stabilised dead-burned dolomite - Google Patents

Abstract

A refractory material is obtained by pelletising and-dead-burning dolomite and optionally magnesium oxide, or a precursor therefor, in the presence of a stabilising agent which is an oxide or precursor therefor which reacts in the dead-burning with the calcium oxide of the calcined dolomite to produce a product less susceptible to reaction with water. Suitable oxides are chromium oxide, zirconia or silica, or a magnesium silicate such as talc, serpentine or olivine. Where the reaction produces dicalcium silicate the mixture should include an inhibitor for the inversion of beta to gamma dicalcium silicate e.g. a chromate, phosphate, manganate or borate.

Description

SPECIFICATION Stabilised dead-burned dolomite The present invention relates to refractory materials based on dolomite.

Dolomite is a naturally occurring mineral containing both calcium carbonate and magnesium carbonate.

When this is dead-burned it is useful as a refractory material. However dead-burned dolomite has serious shortcomings as a refractory material since it has a low resistance to chemical attack by metallurgical slags and the calcium oxide component has a marked tendency to hydrate thereby seriously degrading the refractory properties of the dead-burned dolomite.

It has now been found that if dolomite is stabilised in a particular way and optionally appropriately treated with magnesia its refractory properties are much improved.

Accordingly the present invention provides a method for preparing a dead-burned refractory material which method comprises pelletising a mixture containing dolomite and a stabilising agent (as hereinafter defined), and optionally magnesia or a magnesia precursor, and dead-burning the pelletised mixture.

A "stabilizing agent" in this specification means on oxide, or a material which forms an oxide on dead-burning in admixture with dolomite, which reacts in the dead-burning with the calcium oxide produced from the dolomite and thereby decreases reactivity of the dead-burned product with water, provided that where the said reaction produces dicalcium silicate, the mixture also includes an inhibitor for the inversion of beta dicalcium silicate to gamma dicalcium silicate upon cooling.

"Stabilised calcium oxide" as used in this specification, means the reaction product of calcium oxide and a stabilising agent as defined above. Suitable stabilising agents include zirconia, chromium oxide, silica or a magnesium silicate such as talc, serpentine or olivine. In the case of silica-containing stabilising agents, the lime component is converted into di- and/or tricalcium silicate (2CaO, SiO2 and 3CaO, SiO2). Effectively therefore the amount of silica-type stabilising agent which should be added is preferably such that the weight ratio of lime to silica in the mixture after dead-burning is in the range 1.87 to 2.95, preferably in the range 2.55 to 2.75.

The purpose of the inhibitor included in certain of the stabilising agents is to inhibit the inversion of beta dicalcium silicate to gamma dicalcium silicate during cooling of the dead-burnt pelletised mixture. Suitable inhibitors include compounds which provide ions such as chromate, phosphate, manganate and borate.

Preferably boric acid is used. Since inhibitors such as boric acid adversely affect refractory properties it is obviously desirable to use the minimum quantity which is effective.

In order to obtain strong pellets during the pelletisation step and to minimise pellet breakdown during firing, the mixture preferably also contains a carbonisable binding agent such as sulphite lye or other lignin sulphonate. The quantity of binding agent is desirably in the range 0.1% to 5% by weight based on the weight of the mixture for example in the range 0.25% to 2% by weight.

The magnesia when used in the present invention may be caustic or active magnesia (i.e. lightly calcined) or dead-burned magnesia. The magnesia precursor may be magnesium carbonate, natural magnesite or magnesium hydroxide. In all these cases the silica content of the magnesia or the magnesia precursor, which may be significant in relation to natural magnesites, will have to be taken into account in calculating the amount of stabilising agent, including inhibitor, required.

The dolomite and optionally the magnesia or magnesia precursor are preferably used in such proportions in the mixture that the amount of MgO in the product is 40% to 90% desirably 55% or even 60% to 85% by weight based on the weight of the dead-burned product. The proportion of lime component of the dolomite to magnesia is therefore preferably approximately in the range 60:40 to 10:90 by weight. It is thought that the provision of magnesia or a precursor therefor in the mixture increases the resistance of the product to attack by metallic slags.

The mixture may be formed by merely blending together in a suitable mixer the various ingredients of the mixture. However it is preferred to form the mixture in an apparatus which combines the functions of mixing and size reducing.

Desirably the mixing should be intimate, so as to produce a substantially uniform distribution of particles of dolomite and particles of magnesia or magnesia precursor.

The dolomite and the stabilising agent are preferably first intimately mixed dry by any means which involves size reduction while mixing although this does not mean that the materials to be mixed cannot be in an already finely divided condition. In general, tumbling mills are very suitable such mills including ball, pebble, rod, tube and compartment mills.

It has been found that the particle size of the components of the blend after mixing is an important feature in ensuring that, after dead burning, a product is obtained having a microstructure in which lime-containing grains are surrounded by grains of magnesia. After mixing, the size range should preferably be such that 100% passes through a 72 BS sieve, preferably 100% passes through a 100 BS mesh sieve.

Preferably the other components of the mixture are incorporated after mixing the dolomite and the stabilising agent. Preferably the magnesium hydroxide is added as filter cake containing 5% to 45% by weight of water.

It is preferred to use magnesia or magnesia precursor the impurity content of which, discounting lime, is a minimum, for example which is less than 6% by weight preferably less than 2.5% by weight. Typical impurities in magnesia are silica, iron, alumina, manganese and boron.

The pelletising of the ground mixture may be carried out in a mechanical or hydraulic press or pelletising apparatus to form shapes or pieces, for example, cubes, prisms, ovoids, or cylinders having e.g. diameters of the order of 0.25 to 1.25 inches. The pressure employed must be sufficient to enable the pressed material to pass through the subsequent dead-burning process without appreciable breakdown. A minimum pressure of 5 tons per square inch is necessary to obtain a low porosity in the final product. Pressures in the range 1 to 40 tons per square inch may be employed but it is preferred of from 15 to 40 tons per square inch. A wet mixture may be extruded.

The dead-burning of the pressed mixture may be carried out for example in a rotary kiln, a travelling grate kiln or a shaft kiln. The dead-burning temperature may be in the range 1350"C to 2000"C but is preferably in the range 1400into 1900"C.

It is to be understood that the present invention also includes a dead-burned refractory material whenever prepared by the process as hereinbefore described.

The appropriate treatment of dolomite in the preferred manner described above results in an improved refractory material the improvement in which it is believed can be attributed to the fact that the original lime component of the dolomite is converted completely into non-hydratable calcium silicates the particles of which are further to a great extent enveloped in a coating of magnesia. The present invention includes a refractory dead-burned material having a micro-structure in which particles of stabilised lime (as herein defined) are at least partially enveloped in or coated with magnesia.

The preferred products of the present invention are hydration resistant materials which are densified easily, are resistant to iron-containing slags and find application for a wide variety of refractory purposes such as fired or unfired bricks, and in slinging, gunning, ramming, casting and moulding mixes.

The following Examples in which parts and percentages are by weight is given to illustrate the present invention.

Example 1 62.6 parts of raw dolomite, 16.7 parts of olivine, 74 parts of magnesium hydroxide, 0.15 parts of boric acid and 1.5 parts of dry sulphite lye were fed to a ball mill and co-milled to -200 BS mesh.

The dolomite, olivine and magnesium hydroxide analysed as follows: SiO2 Fe2O3 Al203 CaO MgO Ign loss Dolomite 0.40 0.43 0.15 30.70 21.74 46.59 Olivine 41.55 7.86 0.80 0.38 .46.89 1.77 Magnesium Hydroxide 0.60 1.30 0.40 1.00 96.7 The blend obtained by the milling step was pressed by means of a Komarek-Greaves double roll press at 20 tons per square inch into pellets about an inch in width. These pellets were fired in a rotary kiln designed to hold the pellets at 1800"C for 3 hours.

The sintered pellets had a lime/silica ratio of 2.6 and a magnesia content of 70%. A study of the microstructure showed it to be made up of grains of lime surrounded by grains of magnesia. The pellets showed no signs of deterioration by the action of moisture upon storage for 6 weeks.

Example 2 The procedure of Example 1 was repeated using the following mixture: 67 parts Dolimite 11 parts Silica - stabilising agent 0.12 parts Calsium Phosphate - inhibitor 0.15 parts Sulphite lye - binder After calcination, the microstructure of the product and its stability to storage was found to be similar to the product of Example 1.

Claims (20)

1. A method for preparing a dead-burned refractory material which method comprises pelletising a mixture containing dolomite and a stabilising agent (as hereinbefore defined), and optionally magnesia or a magnesia precursor, and dead-burning the pelletised mixture.

2. A method as claimed in claim 1 wherein the mixture contains dolomite and optionally magnesia or precursor therefor which is present in the mixture in an amount such that the amount of MgO in the product is from 40 to 95% by weight.

3. A method as claimed in claim 2 wherein the mixture contains dolomite and optionally magnesia or precursor therefor which is present in the mixture in an amount such that the amount of MgO in the product is from 60 to 85% by weight.

4. A method as claimed in claim 1 wherein the mixture contains magnesia, magnesium carbonate, natural magnesite, or magnesium hydroxide.

5. A method as claimed in any one of claims 1 to 4 wherein the mixture contains magnesia or a precursor therefor and the impurity content of the magnesia or magnesia precursor, discounting lime, is less than 6% by weight.

6. A method as claimed in any one of claims 1 to 5 wherein the impurity content of the magnesia or magnesia precursor, discounting lime, is less than 2.5% by weight.

7. A method as claimed in any preceding claim wherein the stabilising agent is zirconia or chromium oxide or silica or a magnesium silicate used with an inhibitor for the inversion of beta dicalcium silicate to gamma-dicalcium silicate.

8. A method as claimed in claim 7 wherein an amount of silica or a magnesium silicate is present in the mixture such as to provide a weight ratio of lime to silica in the range of from 1.87:1 to 2.95:1.

9. A method as claimed in claim 8 wherein an amount of silica or a magnesium silicate is present in the mixture such as to provide a weight ratio of lime to silica in the range of from 2.55:1 to 2.75:1.

10. A method as claimed in any preceding claim wherein the stabilising agent includes an inhibitor for the inversion of beta-dicalcium silicate to gamma dicalcium silicate which is a chromate, phosphate, manganate or borate.

11. A method as claimed in any one of claims 1 to 9 wherein the stabilising agent includes an inhibitor for the inversion of beta-dicalcium silicate to gamma dicalcium silicate which is a boric acid.

12. A method as claimed in any preceding claim wherein the mixture also contains a carbonisable binding agent.

13. A method as claimed in claim 12 wherein the binding agent is sulphite lye or lignin sulphonate.

14. A method as claimed in claim 12 or claim 13 wherein the quantity of binding agent present is from 0.1 to 5% by weight of the mixture.

15. A method as claimed in any preceding claim wherein the dolomite and stabilising agent are simultaneously mixed and reduced in particle size before being pelletised.

16. A method as claimed in any preceding claim wherein the particle size of the components ofthe mixture is such that the mixture will pass a 72 BS sieve.

17. A method as claimed in claim 16 wherein the particle size of the components of the mixture is such that the mixture will pass a 100 BS sieve.

18. A method for preparing a dead-burned refractory material substantially as herein before described in Example 1 or Example 2.

19. A refractory material produced by a method as claimed in any preceding claim.

20. A refractory dead-burned material having a micro-structure in which particles of stabilised lime (as hereinbefore defined) are at least partially enveloped in or coated with magnesia.