Spinel forming refractory compositions, their method of production and use thereof

Abstract

A particulate composition for use in alumina-magnesia spinel forming dry vibratable mixtures may include, based on the total weight of the particulate composition, from 95 to 99.9 wt % of a mixture of particulate Al.sub.2O.sub.3 and particulate MgO, and from 0.1 to 5 wt % binding agent, wherein at least a portion of the particles of said mixture of particulate Al.sub.2O.sub.3 and particulate MgO is present in the particulate composition as a coating of particles on the surface of other particles. Methods of producing and using the particulate composition are also described.

Claims

1. A particulate composition for use in an alumina-magnesia spinel forming dry vibratable mix, the composition comprising, based on the total weight of the particulate composition: from 95 to 99.9 wt % of a mixture of particulate Al.sub.2O.sub.3 and particulate MgO; and from 0.1 to 5 wt % binding agent, wherein at least a portion of the particles of said mixture of particulate Al.sub.2O.sub.3 and particulate MgO is present in the particulate composition as a coating of particles on the surface of other particles, further comprising mineraliser particles as part of said coating, said mineraliser particles selected from the group consisting of B.sub.2O.sub.3, V.sub.2O.sub.5, TiO.sub.2, Y.sub.2O.sub.3, Fe.sub.2O.sub.3, CaO, NaCl, AlCl.sub.3, MgCl, LiF, ZnF.sub.2, BaF.sub.2, and CaF.sub.2.

2. The particulate composition of claim 1, wherein the particulate composition has the following particle size distribution: from 35 to 65 wt % of the particulate composition are particles having a particle size of 1 mm or above; from 45 to 75 wt % of the particulate composition are particles having a particle size of 0.5 mm or above; from 65 to 95 wt % of the particulate composition are particles having a particle size of 0.045 mm or above; and binding agent, wherein at least a portion of the particulate composition are first metal oxide particles selected from Al.sub.2O.sub.3, MgO, or both, and having a particle diameter of 0.25 mm or below, which are present in the particulate composition as part of a coating on second metal oxide particles selected from Al.sub.2O.sub.3 and MgO and having a particle diameter of 0.5 mm or above.

3. The particulate composition of claim 1, wherein substantially all of the particles present as part of the coating of metal oxide particles on the surface of other particles have a particle diameter of 0.25 mm or below and the other particles have a particle diameter of 0.5 mm or above.

4. The particulate composition of claim 2, wherein some or all of said particles having a particle size of 0.25 mm or below are particles having a particle diameter of 0.045 mm or below.

5. The particulate composition of claim 2, wherein said first metal oxide particles are Al.sub.2O.sub.3 or a mixture of Al.sub.2O.sub.3 and MgO, and said second metal oxide particles are MgO.

6. The particulate composition of claim 2, wherein said first metal oxide particles are MgO or a mixture of MgO and Al.sub.2O.sub.3, and said second metal oxide particles are Al.sub.2O.sub.3.

7. The particulate composition of claim 1, wherein the binding agent is selected from solid thermoplastic binding agents, thermohardening binding agents and liquid, thermo setting binding agents and liquid, and reacting binding agents.

8. A particulate composition for use in an alumina-magnesia spinel forming dry vibratable mix, the composition comprising, based on the total weight of the particulate composition: from 95 to 99.9 wt % of a mixture of particulate Al.sub.2O.sub.3 and particulate MgO; and from 0.1 to 5 wt % binding agent, wherein at least a portion of the particles of said mixture of particulate Al.sub.2O.sub.3 and particulate MgO is present in the particulate composition as a coating of particles on the surface of other particles, wherein the Al.sub.2O.sub.3 comprises white fused alumina.

9. A method of producing the particulate composition of claim 1, the method comprising the steps of, in order: (i) providing (a) from 95 to 99 parts by weight of a mixture of particulate Al.sub.2O.sub.3 and particulate MgO, and (b) from 1 to 5 parts by weight of a thermoplastic or thermosetting binding agent; (ii) introducing particles from a portion of the provided mixture of particulate Al.sub.2O.sub.3 and particulate MgO to be coated into a mixer; (iii) starting the mixing operation; (iv) introducing the provided binding agent into the mixer; (v) introducing particles from another portion of the provided mixture of particulate Al.sub.2O.sub.3 and particulate MgO to form part of a coating into the mixer; and (vi) gradually reducing temperature in the mixer to below the melting or transition temperature of the thermoplastic or thermosetting binding agent, wherein, prior to step (vi) the mixing was carried out at a temperature above the said melting or transition temperature.

10. A method of producing the particulate composition of claim 1, the comprising the steps of, in order: (i) providing (a) from 95 to 99.9 parts by weight of a mixture of particulate Al.sub.2O.sub.3 and particulate MgO, and (b) from 0.1 to 5 parts by weight of a thermoplastic or thermosetting two-component binding agent; (ii) introducing particles from a portion of the provided mixture of particulate Al.sub.2O.sub.3 and particulate MgO to be coated into a mixer; (iii) starting the mixing operation; (iv) introducing a first component of the two-component binding agent into the mixer; (v) introducing particles from another portion of the provided mixture of particulate Al.sub.2O.sub.3 and particulate MgO to form part of a coating into the mixer; and (vi) introducing a second component of said two-component binding agent into the mixer.

11. A method of producing the particulate composition of claim 1, the method comprising the steps of, in order: (i) providing (a) a mixture of particulate Al.sub.2O.sub.3 and particulate MgO, and (b) a water-based suspension comprising from 50 to 89.9 wt % particles to form part of a coating, from 0.1 to 5 wt % of a binding agent and from 10 to 50 wt % water; (ii) introducing particles from a portion of the provided mixture of particulate Al.sub.2O.sub.3 and particulate MgO to be coated into a mixer; (iii) starting the mixing operation; (iv) introducing the water-based suspension into the mixer; and (v) gradually reducing the temperature in the mixer to below 100 C.

12. The method according to claim 9, wherein: from 35 to 65 wt % of the total provided particulate composition are particles having a particle size of 1 mm or above; from 45 to 75 wt % of the total provided particulate composition are particles having a particle size of 0.5 mm or above; from 65 to 95 wt % of the total provided particulate composition are particles having a particle size of 0.045 mm or above; and such that at least a portion of the particulate composition formed are first metal oxide particles selected from Al.sub.2O.sub.3, MgO, or both, and having a particle diameter of 0.25 mm or below, which are present in the particulate composition as part of a coating on second metal oxide particles selected from Al.sub.2O.sub.3 and MgO and having a particle diameter of 0.5 mm or above.

13. The method of claim 12, wherein said first metal oxide particles are Al.sub.2O.sub.3, and said second metal oxide particles are MgO.

14. The method of claim 12, wherein said first metal oxide particles are MgO, and said second metal oxide particles are Al.sub.2O.sub.3.

15. The particulate composition of claim 8, wherein the particulate composition has the following particle size distribution: from 35 to 65 wt % of the particulate composition are particles having a particle size of 1 mm or above; from 45 to 75 wt % of the particulate composition are particles having a particle size of 0.5 mm or above; from 65 to 95 wt % of the particulate composition are particles having a particle size of 0.045 mm or above; and binding agent, wherein at least a portion of the particulate composition are first metal oxide particles selected from Al.sub.2O.sub.3, MgO, or both, and having a particle diameter of 0.25 mm or below, which are present in the particulate composition as part of a coating on second metal oxide particles selected from Al.sub.2O.sub.3 and MgO and having a particle diameter of 0.5 mm or above.

16. The particulate composition of claim 8, wherein substantially all of the particles present as part of the coating of metal oxide particles on the surface of other particles have a particle diameter of 0.25 mm or below and the other particles have a particle diameter of 0.5 mm or above.

17. The particulate composition of claim 15, wherein some or all of said particles having a particle size of 0.25 mm or below are particles having a particle diameter of 0.045 mm or below.

18. The particulate composition of claim 15, wherein said first metal oxide particles are MgO or a mixture of MgO and Al.sub.2O.sub.3, and said second metal oxide particles are Al.sub.2O.sub.3.

19. The particulate composition of claim 8, wherein the binding agent is selected from solid thermoplastic binding agents, thermohardening binding agents and liquid, thermo setting binding agents and liquid, and reacting binding agents.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) As described in the present disclosure, any particles described as forming part of a particle composition may be present in said particle composition in the shape of free flowing particles, or as forming part of an aggregate of particles, such as part of a coating on another particle.

(2) The present invention according to the appended claims provides particulate compositions for use in dry vibratable mixtures to form alumina-magnesia spinel containing refractory compositions, such as refractory monolithics for use as linings in metallurgical ladles. The particulate compositions according to the present invention may form DVMs in their own right, or they may be part of a DVM comprising further constituents.

(3) DVMs are used in the formation of refractory linings in industrial applications, such as in metallurgical ladles (coreless induction furnaces) for melting and/or holding metals. Commonly, the DVM is installed along the inner walls of a metallurgical ladle, compacted mechanically, for example by using a mechanical vibrating tool. The compacted DVM is sufficiently stable for forming a green ceramic lining, which is subsequently sintered during use of the ladle, i.e. by introduction of, for example, molten iron or steel at a temperature of 1600 C. or higher. The sintering of the ceramic lining in order to form the sintered refractory lining may therefore be carried out during normal operation of the ladle.

(4) The use and formation of spinels in refractory linings is limited in practice by several practical constraints:

(5) In order for a DVM to be suitable for the formation of a refractory composition, the density of the compacted dry mix must be as high as possible. In other words, the compactness of the mix must be high and intergranular porosity low. Achieving a high compactness after compaction or vibration requires an optimised particle size distribution of the DVM, including obtaining a correct balance between fine or ultra-fine powders and larger aggregates. However, the total volume of fine powders needs to be limited, since an excessive proportion of particles having a particle diameter of 0.1 mm or below renders placing and compacting of the DVM very long and inefficient.

(6) In order to obtain a monolithic refractory lining having good thermal cycling resistance after compacting and heating/sintering of the spinel forming DVM, the MgO particles used may not be too coarse. MgO has a higher thermal expansion coefficient than Al.sub.2O.sub.3, and upon heating the presence of larger MgO particles in the DVM would cause cracks to form around larger magnesia grains that expand and generate stresses in the surrounding refractory microstructure, which it cannot accommodate.

(7) The presence of free unreacted MgO particles in the fired/sintered refractory lining is detrimental (a) due to their thermal properties mentioned above, but (b) also because free magnesia is easily chemically attacked by slags, in particular silica containing slags, compared to free Al.sub.2O.sub.3 or spinel. It is therefore advantageous that on one hand, the proportion of MgO in the particulate composition is not excessive (which would lead to unreacted MgO being present in the fired product), and on the other hand that spinel formation occurs at lowest temperature and as quickly as possible during the start of the firing/sintering operation of the metallurgical hardware lined with spinel forming DVM, before any unreacted particulate MgO can be attacked by slag.

(8) Finally, MgO has a strong affinity to water, in liquid or gaseous form, and reacts with water to form brucite. The MgO present in a spinel forming DVM may undergo hydration before installation of the DVM, or during service. The presence of hydrated MgO has a strong detrimental impact on the final performance of the DVM. If hydration occurs before installation, the flaky structure of magnesia hydrate crystals formed around hydrated magnesia particles can form lumps in the DVM, making installation impossible, or to a lower extend reduce the level of compactness reached after compaction or vibration. If hydration of MgO occurs after installation of the DVM, this can result in cracking of the lining, since the transformation of MgO into brucite takes place with a high volume expansion. Furthermore, transformation of MgO into brucite also causes a strong increase in the specific surface area of magnesia particles, which can affect spinel formation kinetics, which can thus become too rapid or brutal, resulting in cracking of the lining during spinel formation.

(9) For these reasons, conventional DVM formulations for alumina-magnesia spinel formation are alumina based mixes comprising about 15 wt.-% MgO having a particle size distribution such that all the MgO particles have a diameter of 2 mm or less. However, the stoechiometric ratio for forming alumina-magnesia spinel is 71.8 wt.-% Al.sub.2O.sub.3 to 28.2 wt.-% MgO.

(10) According to the present invention, it is possible to provide particulate compositions for use as an alumina-magnesia spinel forming DVMs having a stoechiometric or near-stoechiometric ratio of Al.sub.2O.sub.3 to MgO, hence making it possible to form refractory monolithic having a higher spinel content than the state of the art.

(11) In the particulate compositions according to the present invention, at least a portion of the fine particles are present as part of coatings on the surface of coarser particles and aggregates. For example, at least a portion of the MgO and Al.sub.2O.sub.3 fine particles are present as part of coatings on the surface of coarser Al.sub.2O.sub.3 particles and aggregates This solves the problem of too many free small particles present in the DVM, which would render installation and compaction inefficient. In fact, according to the present invention, it is rendered possible to include larger amounts of fine particles in alumina-magnesia spinel forming DVMs without suffering the conventional detrimental effects of overall particle size distribution of the DVMs as a whole.

(12) Furthermore, the fine MgO particles, normally prone to hydration due to their high specific surface areas, when they are part of coatings they are partially or entirely covered by the binding agent and may no longer react as easily with water from the air to form brucite, therefore rendering the DVMs according to the present invention more durable and reliable, improving storage capacity, making easier the production, handling and transport of the materials and therefore reducing the overall cost of forming alumina-magnesia spinel refractories.

(13) The presence of fine MgO and/or Al.sub.2O.sub.3 particles as part of coatings on the surface of coarse Al.sub.2O.sub.3 and/or MgO particles also brings the particles in close contact before spinel formation occurs, therefore promoting the spinel forming reaction upon heating. Improved efficiency of the heating/sintering step in the formation of the final alumina-magnesia refractory product is thus achieved in the case of coarse Al.sub.2O.sub.3-particles coated with fine MgO and Al.sub.2O.sub.3-particles. It furthermore renders possible and safe the use of coarse MgO particles, which was considered so far not possible due to the unfavourable kinetics of the spinel forming reaction and the risk of reaction between unreacted MgO particles with slag. In fact, the overall kinetics of the spinel forming reaction is improved according to the present invention, on the one hand because of the close contact created between the coarse MgO-particles and the finer particles, and on the other hand because of a faster formation of spinel during the sintering step, effectively shielding coarse MgO-particles in the refractory lining even early on during the sintering process from slag, therefore preventing contact between MgO-particles and slag. In the case of refractory linings for metallurgical ladles, the formation of the desired spinel phase upon introduction of molten metal is accelerated, therefore reducing the risk of slag reacting with MgO particles prior to formation of the finished product.

(14) The Al.sub.2O.sub.3-particles as disclosed herein can be any type of alumina known to the skilled person to be suitable in spinel formation. In one embodiment, the Al.sub.2O.sub.3-particles to be coated selected from alumina cement clinker/calcium aluminate aggregates having at least 60 wt.-% alumina, and/or aluminasilicate particles having at least 70 wt.-% alumina.

(15) A wide range of binding agents known to the skilled person in the art may be used according to the present invention. The binding agent may be selected from solid thermoplastic binding agents, thermohardening (thermo setting) binding agents and liquid, optionally multi-component, reacting binding agents. The binding agent may, for example, be selected from the group comprising cellulose, cellulose butyrate acetate, alkylds, phenolic binders, polyester binders (such as polycaprolactone or polyethylene terephthalate), vinyl-polymers (such as polybutadiene, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, styrene-butadiene-acrylonitrile, polyethyelene or polypropylene), polyurethane binders, epoxy-type binders, polycarbonate binders, acrylics (such as PMMA), polyamides, linear hydrocarbons having 20 or more carbon atoms, aromatic alkanes, glycols (such as PEG 1000), polylactic acids or polyimides. The liquid, optionally multi-component, binding agents may, for example, be selected from the group comprising alkylds (with possible addition of cobalt-derived catalysts for adjustment of reticulation speed), phenolics (with possible addition of catalysts), polyesters (with possible addition of catalysts), polyurethanes (polyisocianate reticulating due to the presence of moisture, or reticulating due to the addition of a second liquid component such as polyol and presence of a catalysts such as amine) or epoxy (reticulating due to the presence of a second liquid component such as amine). Alternative binding agents are calcium aluminate cement, polysaccharides, alkaline earth metal oxides and hydroxides, silica, cellulose and derivatives thereof, starch, polycarboxylates, magnesium or sodium lignosulfonate, magnesium salts such as magnesium sulphate, and mixtures thereof.

(16) Particularly preferred binding agents for use in the present invention are alkylds, linear hydrocarbons having 20 or more carbon atoms, glycols, such as PEG 1000, phenolic binders and epoxy binders.

(17) The particulate composition of the present invention may further comprise fine mineraliser particles as part of the coatings. These mineralisers are intended to speed up the spinel formation during sintering of the compacted DVM mixture after installation and/or to reduce the required sintering temperature thereof. Suitable mineralisers are known to the skilled person and may be selected from the group consisting of B.sub.2O.sub.3, V.sub.2O.sub.5, TiO.sub.2, Y.sub.2O.sub.3, Fe.sub.2O.sub.3, CaO, NaCl, AlCl.sub.3, MgCl, LiF, ZnF.sub.2, BaF.sub.2, CaF.sub.2.

(18) The coated particles comprised in the particulate compositions according to the present invention can be produced as follows:

(19) The general concept is based on the agglomeration of the finer particles aimed to form the coating around the larger particles to be coated, using a liquid binder. The liquid binder will solidify during the coating process, permanently maintaining the fine particles coated around the larger particles to make them attrition resistant. It is thus important that the binder is liquid at the beginning of the mixing process and solid at the end of the mixing process.

(20) In the case that thermoplastic polymers or thermohardening (thermosetting) polymers are used as the binder, the conversion from liquid to solid is achieved by cooling down, below melting temperature or transition temperature of the binder, during mixing. In the case that reacting binder components are used (two-component binding agent), the conversion from liquid to solid is achieved by polymerization reaction of the binder components.

(21) According to one possibility, particles to be coated are introduced into a mixer permitting homogeneous mixing of particles of 0.5 to 15 mm, followed by heating, addition of the binding agent, addition of the particles to form part of the coating of the finished coated particles, and gradual cooling down during mixing. Alternatively, the particles to be coated and/or the binding agent may be pre-heated upon introduction into the mixer and no additional heating is provided during the mixing such that the mixture cools down naturally. The mixing is continued until hardening of the binding agent, due to temperature drop into the mixer, and the coated particles are obtained.

(22) According to one possibility, particles to be coated are introduced into a mixer permitting homogeneous mixing of particles of 0.5 to 15 mm, followed by addition of the first component of a two-component binding agent (or one or several components of a multi-component binding agent). Mixing is started, then the particles to form part of the coating of the finished coated particles are added during mixing, together with the second component of the two-component binding agent (or all remaining components of a multi-component binding agent). The mixing is continued until hardening of the binding agent, due to reaction of the components, and the coated particles are obtained

(23) The coated particles thus obtained also form part of the present invention.

(24) In the production of the finished DVM product, particles having the required nature (uncoated Al.sub.2O.sub.3/MgO, coated Al.sub.2O.sub.3/MgO) and particle sizes are combined. Furthermore, when the particulate compositions according to the present invention are used in the formation of the finished DVM product, further additives may be included. These further additives may be mineraliser particles, selected from the group consisting of B.sub.2O.sub.3, V.sub.2O.sub.5, TiO.sub.2, Y.sub.2O.sub.3, Fe.sub.2O.sub.3, CaO, NaCl, AlCl.sub.3, MgCl, LiF, ZnF.sub.2, BaF.sub.2, CaF.sub.2 may be added to the particulate composition. Also as further additives may be included drying agents, having a high affinity for moisture, such as for example anhydrous magnesium sulphate (MgSO.sub.4), boron oxide (B.sub.2O.sub.3), amorphous silica gel (SiO.sub.2), xanthan gum, acrylic or metacrylic acids. Also as further additives may be included a further binding agent, present in the DVM as free particles. In this embodiment, the further binding agents do not form part of the coatings on the particles in the DVM, and are aimed at increasing the mechanical strength of the compacted DVM, prior to sintering. The further binding agents may be of the same nature as the binding agents used for the formation of the particulate coatings in the particulate compositions according to the present invention. The further binding agents may also be selected from B.sub.2O.sub.3, boric acid B(OH).sub.3, or phosphate mixtures.

(25) In an alternative method of formation of the particulate compositions according to the present invention, the particles to form part of the coating are pre-mixed with the binder in an aqueous suspension. In this case, the particles to be coated are introduced into a mixer and the mixing operation is started. A water-based suspension is prepared, comprising from 50 to 89.9 wt.-% particles to form part of a coating, ie. Al.sub.2O.sub.3 and/or MgO, from 0.1 to 5 wt.-% of a binding agent and from 10 to 50 wt.-% water. This suspension is added to the mixer and during mixing, the temperature in the mixer is gradually reduced. In this embodiment, a water-soluble or water-dispersable binding agent is required, which may be selected from the group consisting of calcium aluminate cement, polysaccharides, alkaline earth metal oxides and hydroxides, silica, cellulose and derivatives thereof, starch, polycarboxylates, magnesium or sodium lignosulfonate, magnesium salts such as magnesium sulphate, and mixtures thereof. In one embodiment, the temperature in the mixer prior to the addition of the water-based suspension may be between 150 C. and 500 C. Alternatively, the temperature in the mixer may be heated to between 150 C. and 500 C. after introduction of the water-based suspension. In a further embodiment, the temperature in the mixer may be gradually reduced to a temperature below 100 C. after addition of the water-based suspension.

(26) Finally, the present invention allows to provide alumina-magnesia forming DVMs having a higher MgO content than state of the art formulations, therefore allowing stoechiometric or near-stoechiometric ratio of components in the DVM which gives improved spinel formation in the finished sintered monolithic refractory product.

EXAMPLES

(27) Three particulate compositions for use in alumina-magnesia spinel forming dry vibratable mixtures according to the present invention were prepared and compared to a state of the art mixture. The compositions were prepared by mixing together components as shown in Table I, in order to obtain compositions according to the invention (Examples 1, 2 and 3) and two conventional composition (Comparative Examples 1 and 2):

(28) TABLE-US-00001 TABLE I Example No. 1 2 Comp. 1 3 Comp. 2 WFA (3 to 6 mm) 12 0 12 7 7 WFA (1 to 3 mm) 0 0 35 30 30 WFA (0.5 to 1 mm) 15 15 15 10 10 WFA(0.5 mm) 0 0 16 16 16 WFA (0.045 mm) 23 23 7 17 17 MgO (3 to 6 mm) 5 MgO (1 to 3 mm) 5 MgO (0.5 to 1 mm) 5 MgO (0.5 mm) 15 15 0 MgO (0.045 mm) 0 0 15 5 5 MgO-coated WFA 0 12 0 (3 to 6 mm) MgO-coated WFA 35 35 0 (1 to 3 mm) Al.sub.2O.sub.3-coated MgO 5 (3 to 6 mm) Al.sub.2O.sub.3-coated MgO 5 (1 to 3 mm) Al.sub.2O.sub.3-coated MgO 5 (0.5 to 1 mm) Total 100 100 100 100 100 Total MgO 21.72 24.02 15 16.43 20 Total Al.sub.2O.sub.3 76.95 74.19 85 82.85 80 Total binding agent 1.33 1.79 0 0.72 0 All the values are given in wt.-% of the total composition

(29) The MgO-coated Al.sub.2O.sub.3-particles of Examples 1 and 2 were obtained as follows. 40 kg Al.sub.2O.sub.3 (white fused alumina; particle size distribution=1 to 6 mm, d.sub.50=3 mm; obtained from Treibacher Schleifmittel) were introduced into a heating mixer of type Eirich R08VAC. The mixing was started, at heating to 120 C., and binding agent (1.97 kg polyethylene glycol obtained from Aldrich Chemicals) was added during mixing. The mixing continued at constant temperature until all of the binding agent was molten. Next, 9.97 kg MgO (sea water dead burned magnesia nedMag 99 having a particle size of 0 to 0.045 mm) was added during constant mixing. The mixing was maintained after the heating was shut down until the contents reached a temperature of 30 C., giving magnesia coated alumina grains. The obtained MgO-coated alumina particles finally comprised 77 wt.-% alumina, 19.2 wt.-% MgO and 3.8 wt.-% binding agent.

(30) The Al.sub.2O.sub.3-coated MgO-particles of Example 3 were obtained as follows. 40 kg MgO (sea water dead burned magnesia nedMag 99 having a particle size of 1 to 6 mm, d.sub.50=3 mm) were introduced into a heating mixer of type Eirich R08VAC. The mixing was started, at heating to 120 C., and binding agent (2.52 kg polyethylene glycol from Aldrich Chemicals) was added during mixing. The mixing continued at constant temperature until all of the binding agent was molten. Next, 9.97 kg Al.sub.2O.sub.3 (white fused alumina having a particle size of 0 to 0.045 mm, obtained from Treibacher Schleifmittel) was added during constant mixing. The mixing was maintained after the heating was shut down until the contents reached a temperature of 30 C., giving alumina coated magnesia grains. The obtained alumina-coated MgO-particles finally comprised 76.2 wt.-% magnesia, 19.0 wt.-% alumina and 4.8 wt.-% binding agent.

(31) It should be noted that the formulation according to Comparative Example 2 does not represent a realistic choice, in view of the deficiencies of DVM compositions obtained from formulations comprising coarse MgO.

(32) The formulations thus obtained were used as DVMs. DVM compacted samples were prepared according to the method described in EN 1402 part 5, paragraph 6 preparation of test pieces from ramming materials, where liquid addition to DVM was 2% of liquid paraffin prior to compaction using a sand rammer. The compacted samples thus obtained were then tested. The results of these test are shown in Table II:

(33) TABLE-US-00002 TABLE II Example No. 1 2 Comp. 1 3 Comp. 2 Compaction speed (s) 42 43 48 42 47 Intergranular porosity (%) 21 20.5 20 20.6 20 Unreacted MgO (wt.-%) 12 13 9 9 14 Al.sub.2O.sub.3-MgO spinel (wt.-%) 40 48 20 40 21 Hydrated magnesia in DVM 0.2 0,3 1.5 0.3 0.7 (wt.-%) Ratio of MgO initially 1 1 10 1.5 3.5 present undergoing hydration (wt.-%) Level of cracks observed None None None medium V. high Maximum thickness of 15 13 23 19 29 worn-out refractory lining (mm) Worn-out lining surface 667 632 1049 882 1432 (mm.sup.2)

(34) The various properties of the finished DVM and monolithic lining were measured as follows:

(35) Compaction speed (s): vibrating time required to achieve 80% of final apparent density measured after 5 minutes compaction.

(36) Intergranular porosity (%): was obtained by vibrating the DVMs for 5 minutes (constant amplitude of 0.5 mm, frequency of 50 Hz and direction of vibration) and measuring the apparent density of the compacted DVMs. Intergranular porosity was then calculated by comparing the apparent density of the DVMs to the bulk density of the individual raw materials.

(37) Unreacted MgO (%) and Al.sub.2O.sub.3MgO spinel (%) were assessed by means of X-ray diffraction analysis of the compacted DVMs after having been fired for 5 hours at 1500 C.

(38) Hydrated magnesia in DVM (wt.-%) was assessed by calculation after measurement of moisture (weight loss after drying at 150 C. for 24 hours), then loss on ignition (weight loss after 1050 C. for 10 hours) of 200 g to 220 g of uncompacted DVM samples having been preliminarily submitted to following conditions: 48 hours at 60 C./90% relative humidity. From this, the ratio of hydrated MgO compared to originally present MgO was calculated.

(39) The level of cracks was assessed as follows: Compacted DVM samples were fired for 5 hours at 1500 C. in air atmosphere, according to the following heating schedule: room temperature to 1500 C. heating rate of 100 C./hour; dwell time at 1500 C. for 5 hours, cooling down to room temperature by natural cooling rate of the furnace (approx. 24 hours). This thermal treatment leaves the compacted DVM sample to expand freely. This means that the compacted DVM can thermally expand and shrink and also expand permanently due to spinel formation. When the DVM is installed in a furnace as a lining, it cannot expand freely, since it is in a geometrically constrained environment. The level of cracking of the samples is visually assessed and reported according following criteria: none: no cracks can be visually observed on samples; medium: some cracks can be visually observed on sample, but sample remains in one single piece; high: cracks are visible, and some pieces of original samples have detached from it; very high: sample is totally destroyed and has broken down into several pieces

(40) Maximal thickness of worn out refractory lining (mm) and worn-out lining surface (mm.sup.2): DVMs were used for lining a small scaled coreless induction furnace (capacity 15 kg) operating at 5 to 50 kHz and 10 to 25 kW for 10 hours at an average temperature of 1650 C., and melting steel and slag. The initial thickness of installed lining was 50 mm. Steel and slag were removed and replaced four times in two-hour intervals. After cooling down, the refractory lining was cut perpendicular to the heated (internal) surface, and the maximal thickness of the lost (worn out) lining was measured, as well as the global surface of worn out area.

(41) The results shown in Table II display marked improvements in all the assessed parameters.