GB2051032A - Manufacturing refractory items - Google Patents

Abstract

An item such as a crucible or pipe is moulded from a composition containing 65-72% by weight of aluminium oxide and 28-35% by weight of magnesium oxide, and is then dried using a specified temperature regime in vacuum at the temperature of a heater, which is a component element of a mould being used, approximating 1000 DEG C; the item then being fired in an inert atmosphere by heating to a temperature of about 1850 DEG C at a rate of at least 100 DEG C/min; and the item is then fired in vacuum at the same temperature.

Description

SPECIFICATION Manufacturing highly refractory spalling-resistant ceramic items The present invention relates to methods of manufacturing highly refractory spalling-resistant ceramic items employed in metallurgical practice.

The invention can be used, for example, in the manufacture of crucibles employed for melting and pouring refractory alloys, for example: nickel-based alloys containing such alloying components as chromium, tungsten, molybdenum, niobium, tantalum, aluminium, titanium, zirconium, and rareearth elements, which require heating to a temperature of 16500C in finishing and pouring; or steels containing such alloying elements as nickel, chromium, molybdenum, vanadium, aluminium, and titanium, which require heating to a temperature of up to 17500C in finishing and pouring. The invention can also find application in the manufacture of conduits for metals employed in pouring such alloys into casting moulds.

There is known a method for manufacturing refractory thermal shock resistant ceramic items for synthesised alumino-magnesia spinel, including two stages. In the first stage of this method, an alumino-magnesia spinel is synthesised by firing, at a temperature of 1750 C, briquettes pressed from a mixture of 65-70% by weight of aluminium oxide and of 35'30% by weight of magnesium oxide, or synethesising is accomplished by fusing the said mixture of oxides. The fired briquettes are cooled, crushed, and ground, the resultant grains being sized and used to prepare lining masses thereafter.

These masses go to form pressed green items, which are dried and finally fired in tunnel or gas (fired) furnaces at a temperature of 1700 C.

The rate of temperature rise is limited by the admissible temperature gradient across the body of a pressed item, as it determines the intensity of vapour formation and the magnitude of thermal stresses in the body of a pressed item. Therefore, the rate of temperature rise in burning of pressed items is in the order of several tens of degrees per hour, and the full burning cycle takes 3.5 to 4 days (see, for example, pp.225-230, 132-134,Khimicheskaya tekhnologiya keramiki i ogneuporov [Chemical Technology of Ceramics and Refractoriesj, ed. by Academician of the Academy of Sciences of the Ukrainian Soviet Socialist Republic, P.P. Butnikov, and Dr. of Technical Sciences, Prof, D.N. Poluboyarinov; Building Literature Publishers, Moscow, 1972).

The above method suffers from a number of disadvantages. In the first place, there are a high labour consumption in charge preparation and duration of firing of green items.

There is also a method of manufacturing refractory ceramic items, such as crucibles, wherein use is made of a synthesised alumino-magnesia spinel, by melting a mass consisting of the following components: -fused magnesite containing 90-96% magnesium oxide, 70% by weight; - electrolytically produced corundum containing 99% aluminium oxide, 25% by weight; -zirconium dioxide, 3.5% by weight; -titanium diodixe, 1.5% by weight.

In the second stage of the manufacture of items by this method, as lumps obtained by fusion are crushed, ground, and sized, the resultant powders are mixed in required size fraction proportions. A crucible is rammed in an inductor using a hollow metallic ramming form and is dried, first, naturally for 20-24 hours, then with the aid of an inserted electric hater, for 8-10 hours at a temperature of 650-700"C. Next, the crucible is subjected to a burn (firing) effected for 3-4 hours by raising the temperature on the heater to 1350-1400 C, and then fired by introducing a charge of cast iron into the form, melting and holding it at a temperature of 1450-1 500"C for 15-20 minutes, pouring cast iron into moulds, subsequently melting in the crucible a charge of an alloy to be melted in working heats with a view to washing out contaminating elements (contained in the materials of the ramming form and cast iron) off crucible walls, and finally pouring the alloy into moulds. The rammed crucible thus obtained has a structure consisting of a weakly fired working surface of the crucible, several millimetres thick, and an underlying layer of non-fired spinel grains.

Rammed crucibles manufactured according to the above procedure have a high refractoriness of the working surface of the crucible and a relatively high thermal shock resistance. However, this method also has some disadvantages, such as high labour requirements for charge preparation; weak firing of the working surface of the crucible due to low firing temperature and, as a consequence, poor resistance to attack by slag; shutdown of melting-and-pouring furnaces during the replacement of the crucible (for knocking out worn crucible, ramming, and firing in a fresh crucible); and loss of costly working alloys for wash heats.

What is desired is a method which will make it possible to manufacture highly refractory ceramic items in shorter time and which will permit the manufacturing costs to be cut down, while enhancing the thermal shock resistance to the resultant highly refractory ceramic items.

The present invention provides a method for manufacturing highly refractory ceramic items from aluminium and magnesium oxides, consisting in introducing and compacting the above starting oxides in a mould with a heater mounted in the centre thereof, drying and subsequently burning moulded items, wherein drying is performed by raising the temperature of the heater at a rate of 30-60"C/min to 400"C-450"C and at a rate of 30-100"C/min to 950-10500C in vacuum, whereas burning of items is effected in an inert gas atmosphere by raising the temperature of the heater at a rate equal to or higherthan 100"C/min to 1800-1850"C, the items then being held, first, at a temperature of 1800-1850 C for a time sufficient to enable free separation of the above heater from the items being burned, the holding said items in vacuum at the same temperature.

The effect of this method is that the time of manufacture of ceramic items drops from 5-7 days to 2-4 hours, the working surface of the items is a wellburned solid structure of ceramic material having a refractoriness of up to 1850"C, high chemical inertness and resistance to dynamic action of refractory alloys and high-alloy steels in melting and pouring, and a high service thermal shock resistance (due to the structure ofthe ceramic material being formed through unilaterally directed firing) amounting to 60-70 air thermal cycles (one thermal cycle involving a variation ofthetemperature between 1600 and 20"C).

The invention will now be described in more details, by way of example only.

Masses for manufacturing refractory items possessing a high chemical inertness with respect to melts of refractory alloys based on nickel and of high-alloy steels are generally fused magnesium and aluminium oxides.

In addition to simple oxides, the binary system MgO-AI2O3 has a single chemical compound MgAl2O4, an alumino-magnesia spinel, formed through interaction of MgO and Al203 and containing 71.7% by weight of aluminium oxide and 28.3% by weight of magnesium oxide. The spinel has a melting point of 2135"C and forms with magnesium oxide and eutectic mixture containing 32.5% (mol.) aluminium oxide, whose melting point is 1995"C; with aluminium oxide the spinel forms an eutectic mixture containing 95.5% (mol.) MgO, whose melting point is 19200C.

Alumino-magnesia spinel has a greater (as compared with magnesium oxide and with aluminium oxide) inertness with respect to melts of the above alloys, but even attemperatures higherthan 17500C the firing of spinel remains unsatisfactory. To improve the firing of grains of alumino-magnesia spinel, an excess amount of one of the components, magnesium or aluminium oxide, is added.

Refractory ceramic items based on aluminomagnesia spinel can be obtained by using a mixture of oxides containing 28-35% by weight of commercial-purity magnesium oxide, upto 4% by weight of various impurities, such as CaO and SiO2, and 72-65% by weight of commercial-purity aluminium oxide (not more than 1% by weight of impurities). Naturally occurring alumino-magnesia spinel is contaminated with impurities lowering its refractoriness and chemical resistance with respect to alloys, for example those based on nickel or iron.

This explains why spinel intended for the manufacture of refractories is obtained by synthesis. The most effective procedure for synthesising spinel is firing, which involves a solid phase heterogeneous diffusion reaction between magnesium and aluminium oxides.

When the synthesis of the spinel is combined with thermal processing of manufactured items, the growth in volume of the items may attain 20-30%.

This is because the magnesia spinel has a less compact crystalline structure of the cubic lattice as compared to a hexagonal lattice of corundum manufactured electrolytically and to a hexacyclical lattice of periclase, the densityofspinel being a mere 3.27 g/cm3, whereas that of corundum is 3.8 glcm3, and that of periclase 3.58 g/cm3. Combination of the synthesis of spinel with the thermal processing of items being manufactured results (because of a temperature gradient across items) in different rates ofthe processes of internal synthesis and shrinkage and, therefore, in different speeds of variation of these volumes, this underlying the deformation and cracking of items during firing.Refractory items, which are highly thermal shock resistant and capable of withstanding the dynamic action of melts, are generally ceramic items having a thin well-fired layer forming the working surface of the refractory and a porous weakly-fired internal structure.

As the magnitude of thermal stresses in a material is proportional to the gradient oftemperatures therein, both during firing and in service, the stresses occurring in a loose, porous, weakly fired part are relaxed through disintegration of bonds in the structure, so that only the layer of well-fired ceramic material remains in stressed state. The thinner the layer, the lower are the stresses caused by temperature variations occurring under service conditions.

It has been found that such a structure can be obtained through a sided unilaterally directed firing by providing a suitable temperature gradient across items being fired.

With the aim of combining the thermal processing of items being manufactured with the synthesis of alumino-magnesia spinel and the provision of a thermal shock resistance structure in the items, a structure capable of withstanding the dynamic action of melts inducing the failure of the surface of the items, the following conditions are provided for drying the firing green items: 1. Green items should be fired from one side and in a direction from the axis to the periphery.

2. Particles of the material of the green items should be bonded one to anotherthrough structural and mechanical cohesion only, i.e. green items should have no mechanical strength or minimal mechanical strength.

3. The structure of the fired items must consist of two or three layers, and the internal layer (forming the working surface) should be as thin as is practicable, possessing high strength and high density acquired through deep firing; the subsequent peripheral layers should have a lower strength and a loose structure. Such a structure of the items ensures minimum tensile and compressive stresses in the internal (working) well-fired layer, during both firing and service of items undertemperature varia- tions, since the stresses induced in weak loose layers materialize in cracking and are dampened in pores of poorly fired parts of the ceramic items. In addition; the solid, strong, and thin internal (working) layer will have a minimum temperature gradient because of its high thermal conductivity and because ofthe low thermal conductivity of the poorly fired loose peripheral layers.

(The above structure of green items compensates for an increase in the volume thereof during the layer-by-layer unilaterally directed firing, simultaneous with the synthesis of alumino-magnesia spinel.) 4. Firing conditions should rule out any chemical interaction of the source of heat and of the surrounding medium with the starting oxides and with the spinel resulting from the firing of the items.

The above conditions are provided by a unilaterally directed firing of a mixture of magnesium and aluminium oxides, placed in a mould cooled on the outside and defining the external surface of an item being fired with the aid of a heater located in the centre of the mould and defining the internal working surface of the items, under conditions specified for the method according to the present invention.

In practice, the starting material for obtaining a ceramic item is a mixture consisting of 65-72% by weight of aluminium oxide and 28-35% by weight of magnesium oxide. Once this mixture has been introduced into a mould, for example by ramming, it is dried by raising the temperature of a heater, located in the central zone of the mould, first in the open atmosphere at a rate of 30-60"C/min to 400-450"C, then at a rate of 30-100"C/min to 950-10500C in a vacuum of 5 x 10-' to 5x102 mm Hg (70 to 7 Pa).The subsequent firing of the item is effected in an inert atmosphere, for example argon or helium, at an increasing temperature of the heater proceeding at a rate equal to or higher than 100"C/min to a temperature of 1800-1850"C, subjecting the item to firing at a temperature of approximately 1850"C for a time sufficient to enable free separation from the heater, and finally in vacuo at the above-mentioned temperature of 1800-1850 C.

The processes which occur in firing and which explain the results achieved by the method will now be considered.

When an item is heated from a starting temperature (tS) to a temperature of onset of firing (to), the itenì is a uniformly packed body of a mixture of grains of the oxides Al2O3 and MgO. The heating of the item in an oxidizing atmosphere to 400"C and in vacuum from 400"C to to is accompanied by the following processes and phenomena:: - removal of fixed moisture in the range from the starting temperature of 120-150"C and of chemisorbed moisture effected at substantially higher temperatures, the reaction proceeding as follows Mg(OH)2 ~ MgO + H20; - reduction by carbon (of the heater) of several oxides (impurities), including K2O, Na2O, Fe2O3, according to the reaction: 2MeO + C 2Me + CO2; - increases in the volume of gases inside the green items because of heating and because of a drop in pressure as a vacuum is built up inside the firing chamber; - build-up of a difference in pressure inside the firing chamber and the item proper upon build-up of vacuum:: - increase in the volume of the item due to heat ing; - appearance of stresses in the item proper and across the "item-heater" and "item-mould" boundaries.

The above processes and phenomena limit the rate of the one-sided heating of the item, which is one of the main factors governing the gradient of temperature across the item, to a rate of 30 to 100 C/min.

Stresses induced in the item on heating within the above range oftemperatures improse no limitation upon the rate of rise of temperature of the heater, as the stresses are relaxed through mobility of grains in absence of strong bonds therebetween and availability of a sufficient free volume in the form of pores.

On further unilateral heating of the item from the temperature to to that of the end of firing (te = 18500C)the item should be considered not as a structurally homogeneous body, but as one composed of structurally different zones.

It is self-evident that a high rate of rise of temperature at the heater will result in a considerable gradient of temperature from the centre of the item to its periphery because the thermal conductivity of the item is low and the mould cools. The higherthe rate of heating, the greater is the temperature gradient in the material of the item, and the thinner will be the layer of the item within the range oftemperature of effective firing of the item (tf).This condition is one of frontal firing of the item from the centre of the periphery, under which the changes in volume occurring in the thin fired layer of the item through spinel formation are compensated for by structurally-mobile unbonded grains ofthe un-fired part of the item whose temperature at the boundary with the firing front is less than tf. In such a thin effectively fired elementary layer, stresses that are bound to occur will be relaxed by the presence of pores and of mobile grains in adjacent volumes.

Obviously, the temperature gradient across the item will even out with the course of time, and layers of the item adjacent the fired part will heat to tf, so that firing will begin. It is very important to conduct firing in such a manner as to avoid this phenomenon. The relevant firing parameters are, all other conditions being equal, the rate of rise of the temperature of the heater, the holding time at the firing temperature, and the residual pressure of gases during firing and holding.

Thus, as frontal firing proceeds, stresses in the elementary layer are dampened by adjacent loose poorly cohesive layers lacking in bonds between grains and featuring a considerable amount of pores. The thinner the layer of frontal firing, the more readily can the volume growth be compensated. Frontal firing is favoured by the recommended rate of rise of temperature at the heater.

The minimum rate of rise ofthe heater temperature is governed by the ratio of the time required to attain the temperature of the effective firing of the layer adjoining the heater to the time it takes for a rigid bond framework to appear in this layer. This ratio should always be greater than unity, otherwise the expanding heater will break up the framework formed of oxide grains, this inducting cracks in the layer of the item adjacent the heater.

It is common knowledge that the effectiveness of diffusion processes governing solid phase firing, which produces a solid and strong ceramic structure, is proportional to the temperature. Because ofthis, it is imperative to provide a maximum admissible temperature on the heater to fire the layer of oxides adjacent the heater. The factor which limits the firing temperature is the appearance during firing of a liquid phase, i.e. fusion ofthe refractory, the thermal shock resistance of such a ceramic.For oxides of the system MgO-AI203, the melting point of the eutectic containing 95.5% (mol.) AI2O3 is 19200C. For commercially-pure oxides the temperature of the eutectic transformation is lower by 50-60"C and the molten phase can be formed in firing at temperatures of approximately 1860-1880"C. In this connection, the maximum admissible temperature at the heater during the firing of oxides of the item in the method according to the present invention is limited to the range of 1800 to 1 8500C.

The material of the heater should have a high thermodynamic stability with respect to the oxides making up the green ceramic item underthe firing conditions above. The material of the heater must not change its composition and properties during heating to firing point and in the process of firing of the item and must admit of repetitive use. The use of some materials which react partially with magnesium and aluminium oxides and show no ten deny to interact with these oxides at high temperatures is often impracticable because of technical or economic considerations. However, there are a number of materials which react partially with the above oxides, forming a gaseous phase.In the gen eral case, such a section can be written thus: y(Mg, Al)nOnn + (x + m) My (Mg, Al) + m MxOy, where M is the material of the heater.

The equilibrium of such a reaction is characterised by a constant K:

where a is the activity of the respective component indicated in the suffix.

As a first approximation, the activity of magnesium and aluminium oxides, the activity of the material of the heater and the activity of the obtained metals may be assumed equal to unity, whereas the activity of the resultant gaseous oxide MgO can be expressed as the partial pressure P whence Pl\nxO,,whence

i.e. the rate of the reaction can be decreased by a deliberate rate increase of the partial pressure of the resulting gaseous phase.

Technically, it is not always possible to obtain a sufficient amount of the necessary gaseous phase. In order to shift the reaction toward the left, the space where firing is conducted can be filled to good advantage with an inert gas, this slowing down the diffusion of the gaseous phase resulting from the interaction, i.e. shifting the reaction from the kinetic to the diffusion range and so decreasing the rate of the interaction.

Once the oxides have been fired to produce a framework capable of withstanding the pressure of the mixture of oxides expanding beyond the fired layer of oxide mixtures it is useful, before this layer shrinks, to separate the heater from the item being fired and so provide a gap to avoid contact interaction of the heater material with the oxides of the item.

Firing must continue in vacuum so as to eliminate chemical interaction between the heater and the oxides and to prolong the time of frontal firing through a decrease in the thermal conductivity of the item being fired by eliminating convective heat exchange in the item and to hold back the levelling of the temperature gradient from the centre to the periphery of the item.

High temperature of the heater (within 18500C), heat transmission by radiation to the item, and drop in the heat conductivity of the item in vacuo provide favourable conditions under which the temperature gradient inside the item is retained for some time so as to ensure dead burning of the surface layer of the item directly adjacent the heater and differentiated firing of the underlying layers.The ratio of the thicknesses of these layers is determined, in the first place, by the rate of rise of the heatertemperature from approximately 950"C to 18500C in argon, the holding time in argon of helium before the heater is separated from the item, and the time of firing of the item in vacuo at a residual pressure of 5 x 1 -t to 5 X 1 0-2 mm Hg (70 to 7 Pa).

Therefore, to combine the synthesis of the magnesia spinel from the oxides MgO and Al203 with the firing of refractory items and to obtain a thermal shock resistant structure of refractories, it is essential: 1. To conform to the rate of rise of the heater temperature within the range described above.

2. To maintain the temperature of the heater, in the course of the firing of the oxides, at about 1 850"C.

3. To minimize the rate of the chemical interaction of the heater with magnesium and aluminium oxides through direct physical contact by conducting firing in argon and out of physical contact netween the heater and the item.

4. To complete the firing of the item in vacuo after the heater is separated from the item.

Firing and forming of the refractory item are carried out in a mould having a heater located at its centre. The formation of the internal working surface of the refractory item, and the drying and firing of oxides are performed with the aid of the heater. The heater should be shaped so as to enable it to be separated from the item in the course of firing, in orderto provide a gaptherebetween. The material of the heater should possess high refractoriness (of the order of 2000 C) and high thermal stability, should be heated in a medium-frequency electromagnetic field, and should form, on firing the items, no solid phases with magnesium and aluminium oxides.

These requirements are met by, for example, graphite.

The moes.ld ensures the shaping of a green item by packing of oxides and the regulation of the heat transmission from the heater to the mould during the burning of an item.

The method described above can be used to manufacture, for example, melting crucibles in capacities from 5 to 60 kg in terms of refractory alloys and steels.

To this end, a mixture of oxides MgO and Awl203 taken in a ratio of on the average 28-35% by weight and 72-65% by weight respectively, is packed in a mould with insertion of a heater made from graphite in its centre. The mould and the green crucible it holds are placed in an inductorofa firing chamber having means for building up an atmosphere of argon and for creating a vacuum in the chamber.

The heater is connected to the secondary circuit of inductor, and the thermal processing is carried out according to the following conditions: a rise of the temperature ofthe heater in an oxidizing atmosphere up to 400-4500C at a rate of 400Clmin, and from 400 to 1050"C in vacuo art a rate of 70"C/min. With the heater temperature equal to 950-1 0500C, the firing chamber is filled with argon to a pressure of 100-600 mm Hg (13-80 kPa), and the temperature of the heater is raised to 1800-1850 C at a rate of 1200C1min and held there until the heater is separated from the green crucible, firing of the crucible being subsequently continued at a temperature of 1800-1850 C in vacuo, the heater and the crucible being out of contact. Once the firing has been completed, the heater is disconnected from the secondary circuit of the inductor, and the crucible is cooled and taken out of the mould.

A crucible manufactured by the above-described method contains an internal layer, in contact with the melt, with up to 90% of alumino-magnesia spinel, has a solidly burned working surface, and is capable of withstanding 70 or more heats if proper care is taken.

It goes without saying that the above description of the method according to the present invention omitted all those process operations which are known to those skilled in the art of manufacturing shaped ceramic items. The invention will be further illustrated by the following Examples.

Example 1 Crucibles 5 kg in capacity (in terms of steel), intended for smelting steels and alloys, are manufactured from aluminium and magnesium oxides predried at a temperature of 200"C for 2 hours and taken in proportions of 70 and 30% by weight respectively.

The mixture of the oxides is packed into a ceramic mould, a graphite rod heater being placed at the centre of the mould. The temperature of the heater is raised in the air to 400"C at the rate of 60 C/min, and from 400 to 10000C in a vacuum of 5.10-' mm Hg or 70 Pa (with residual air pressure) at the rate of 100C/min. With the temperature of the heater being equal to 1000 C, the furnace is filled with argon, and the temperature of the heater is raised to 18500C at the rate of 200"C/min, the mould with the item then being held atthistemperature. After free separation of the heater from the crucible walls, the crucible is held atthetemperature of 1850 C in vacuum out of contact with the heater.

The overall thermal processing cycle takes not more than 80 min. The structure of the resultant crucibles can be described in terms of three layers: a first layer, solidly fired, 0.5 to 2 mm thick; a second layer, less fired, 5 to 15 mm thick; a third layer, fired only at the surfaces of contact of the grains and consisting practically of the starting oxides. The porosity of the crucibles increased from the first layer, defining the internal surface of the crubibles, to the third layer, defining the external surface of the crucibles.

The content of spinel in the first layer ranged between 60 and 90% by volume, depending on the firing time. The thermal shock resistance of the crucibles amounted to 60 thermal cycles.

Example 2 Crucibles 60 kg in capacity (in terms of steel) are manufactured from aluminium and magnesium oxides humidified during grinding and taken in proportions of 65 and 35% by weight respectively.

The mixture of the oxides is packed into a fireclay mould with a rod heater made from silicided graphite placed at the centre. After the oxides are packed, the mould is placed inside an induction fur- nace, and the heater is inserted in the secondary circuit of the inductor. The temperature of the heater is raised to 450 C in the air at the rate of 30"Clmin, and from 450 to 105Q"C in vacuum of 5.10-2 mm Hg (7 Pa) at the rate of 30 C/min. With the temperature of the heater equal to 1050 C, the furnace is filled with argon, and the temperature of the heater is raised to 1800"C at the rate of 100 C/min and held at that point.

Upon free separation of the heater from crucible walls, the crucible is held at the temperature of 1800"C in vacuum without any physical contact between the heater and the crucible. The overall thermal processing cycle is not more than 150 min.

The structure of the crucibles can be described in terms of three layers: a first layer, solidly fired, 1 to 3 mm thick; a second, less fired one, 10 to 20 mm thick, a third layer, which is fired only at the surfaces of contact of grains and consists practically of the starting oxides. The porosity of the crucibles increased from the first layer, defining the internal surface of the crucible, to the third layer, defining the external surface of the crucibles. The content of spinel in the first layer ranged between 55 and 90% by volume, depending on the firing time. The thermal I shock resistance of the crucibles amounted to 70 thermal cycles.

Example 3 Pipes to serve as conduits for metal in pouring of steel, having the average diameter to length ratio ranging between 2-to-1 and 10-to-1, are manufactured from magnesium and aluminium oxides humidified during grinding and dried naturally (at the temperature of 20-30"C), taken in proportions of 28.3 and 71.7% by weight respectively. The oxides are packed into a corundum mould, with a rod heater of graphite being placed at the centre of the mould.

Once the oxides have been packed, the mould is placed in an induction furnace, and the heater is inserted into the secondary circuit of the inductor.

The temperature of the heater is raised to 4500C in the air at the rate of 40"C/min and from 450 to 9500C in a vacuum of 5.10-' mm Hg (70 Pa) at the rate of 80"C/min. With the temperature of the heater equal to 950"C, the furnace is filled with argon, and the temperature of the heater is raised to 18500C at the rate of 120 Címin and held at this value.

After free separation of the heater from crucible walls, holding is continued in vacuum without phys ical contact between the heater and the crucible. The overall crucible manufacturing time is not greater than 180 min, this bringing down the manufacturing cost. The structure of the pipe can be characterised in terms of three layers: a first layer, solidly fired, 0.5 to 3 mm thick; a second layer, less fired, 5 to 20 mm thick; a third layer, fired only at the surfaces of contact of grains and consisting practically of the start ing oxides. The porosity of the pipes increased from the first layer, defining the internal surfaces of the pipes, to the third layer, defining the external surface of the pipes; this structure is capable of withstanding 70 air thermal cycles.The content of spinel in the first layer range between 60 and 95% by volume, depending on the firing time, this making the pipes more inert (resistant) with respect to molten metals as compared with pipes previously known.

Example 4 Crucibles 10 kg in capacity (in terms of steel) are manufactured from MgO and Awl203 humidified in grinding and dried naturally and taken in proportions of 28.3 and 71.7% by weight, respectively. The mixture of the oxides is packed into a corundum mould, a rod heater of silicided graphite being placed at the centre.After the oxides have been packed, the mould is placed in an induction furnace, whereas the heater is connected to the secondary circuit of the inducton The temperature of the induc tor is raised in the airto 400"C at the rate of 30 C/min and from 400 to 10000C in vacuum of 5.10-2 mm Hg (7 Pa) at the rate of 60 C/min. With the temperature of the heater equal to 10000C, the furnace is filled with argon, and the temperature of the heater is raised to 18500C at the rate of 1200C/min and held at this value.After free separation ofthe heater from crucible walls, holding at this temperature is effected in a vacuum without physical contact between the heater and the crucible. The overall thermal processing cycle time is not more than 100 min.

The structure of the crucibles can be characterised in terms of three layers: a first layer, solidly fired, 0.5 to 2 mm thick; a second layer, less fired, 5 to 15 mm thick; a third layer, fired only at the surface of contact of grains and consisting practically of the starting oxides. The porosity of the crucible increased from the first layer, defining the internal surface, to the third layer, defining the external surface. The content of spinel in the first layer was as high as 95%. The thermal shock resistance of the crucible amounts to 65 thermal cycles.

Example 5 Pipes, which can be characterised by average diameter - to - length ratio of 1 to1 0, are manufactured from magnesium and aluminium oxides humidified in grinding and dried at a temperature of 200"C for 2 hours and taken in proportions of 35 and 65% by weight, respectively. These oxides are packed into a fireclay mould, with a rod heater of silicided graphite being placed at the centre.Once the oxides have been packed, the mould is placed in an induction furnace, and the heater is inserted into the secondary circuit of the inductor. The tempera ture of the heater is raised in the air to 420"C at the rate of 60 C/min and from 420"C to 1010"C in a vac uum of 5.10-' mm Hg (70 Pa) at the rate of 80"C/min.

With the temperature of the heater equal to 101 00C, the furnace is filled with argon, and the temperature of the heater is raised to 18200C at the rate of 300 C/min and held at this value. Upon free separation of the heater from crucible walls, the crucible is held in vacuum out of contact with the heateratthe above temperature ofthe heater.

The structure of the pipes can be characterised in terms of three layers: a first layer, solidly fired, 1 to 3.5 mm thick; a second layer, less fired, 10 to 25 mm thick; a third layer, fired only at the surfaces of contact of grains and consisting practically of the starting oxides. The pororisty of the pipes increased from the first layer, defining the internal surface of the crucible, to the third layer, defining the external surface of the crucible. The content of spinel in the first layer ranged from 60 to 90% by volume, depending on the firing time. The thermal shock resistance of the pipes is 70 thermal cycles.

Claims (2)

1. A method of manufacturing a ceramic item, comprising the sequential steps of introducing a mixture of aluminium and magnesium oxides into a mould with a heater in the central zone of the mould, the mixture being packed into the mould around the heater to form an item; drying the mixture by raising the temperature of the heater to a temperature of 400-450"C at a rate of 30-60"C/min in an oxidizing atmosphere and then to a temperature of 950-1050 C art a rate of 30-100 C/min in vacuo; firing the item in an inert atmosphere by raising the temperature of the heaterto a temperature of 1800-1850"C at the rate of at least 1000C/min and holding it at this temperature for a time sufficient to enable free separation of the above heater from the item; and firing the item in vacuo atthe same temperature.

2. A method of manufacturing a ceramic item, substantially as described in any of Examples 1 to 5.