GB1598370A - Refractory linings for furnaces - Google Patents

Description

(54) NEW REFRACTORY LININGS FOR FURNACES (71) We, SOCIETE DES ELECTRODES ET REFRACTAIRES SAVOIE of 15 rue du Rocher 75008 Paris, France, a body corporate organised under the laws of France and SOCIETE EUROPEENNE DES PROTUITS REFRACTAIRES of 67 Boulevard du Chateau 92200 Neuilly-sur-Seine, France, a body corporate organised under the laws of France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a new refractory lining for furnaces.

The new type of refractory lining according to the present invention may generally be applied to industrial furnaces in which high temperatures, generally above 500"C or higher, prevail. This type of lining is particularly advantageous in cases where it is desired to avoid over-heating of the walls of the furnaces in certain zones and, in particular, in zones to which access is difficult. This is the case, for example, with fixed furnaces of large dimensions arranged directly on a supporting slab, generally of concrete, which is itself embedded in the ground.

With furnaces of this type, it is often very difficult to prevent the supporting slab from being heated at its centre to temperatures which, in the long term, are in danger of causing it to deform or to disintegrate.

This problem is particularly difficult to solve in the case of modern blast furnaces of very high capacity.

The new refractory lining according to the invention is suitable for the construction of numerous types of furnace and, in particular, for the construction of fixed furnaces of large dimensions.

It is characterised by an assemblage consisting of several successive alternate layers of low conductivity refractory and high conductivity refractory. The high conductivity refractory may be graphite or semigraphite and the low conductivity refractory a silicoaluminous refractory.

This new lining is applicable in particular to blast furnaces.

According to the present invention, we provide a refractory furnace lining comprising at least two materials of which one has a higher thermal conductivity than the other, in which the lining is formed at least partly by a sandwich structure oriented in such a way as to intersect the lines of thermal flux (as herein defined), in which the layers of materials of more or less high conductivity are alternately disposed, and in which at least one layer of material(s) of lower conductivity is comprised between at least two layers of material(s) of higher conductivity, the conductivity ratio between the two categories of materials being equal to or greater than 5:1, whereby lines of thermal flux entering the lining are deflected to the side. A "line of thermal flux" is herein defined as the direction in which heat flows and is perpendicular to isotherms.

The problem involved and the nature of the solution provided by the new refractory lining according to the invention will be better understood from the accompanying drawings, in which: Figure I shows the lower part of a blast furnace with a hearth 7 metres in diameter provided with a refractory silicoaluminous lining; Figure 2 shows the lower part of a blast furnace with a hearth 7 metres in diameter comprising a hearth lined with blocks of carbon; Figure 3 shows the lower part of a blast furnace with a hearth 7 metres in diameter provided with a lining containing graphite; Figure 4 shows the lower part of a blast furnace with a hearth 7 metres in diameter comprising a lining according to the invention; and Figure 5 shows the lower part of a blast furnace with a hearth 14 metres in diameter comprising a lining according to the invention.

Figure 1 diagrammatically illustrates the lower part of a blast furnace of conventional design. This blast furnace, which has an external diameter of approximately 9.5 metres, comprises a refractory silicoaluminous hearth 1 with an internal diameter of approximately 7 metres in which the molten iron collects after flowing from the upper part of the furnace at a temperature of substantially 1450"C. One or more casting channels, such as the channels 2, enable the melt which has collected to be cast at more or less close intervals. This hearth is itself supported by numerous rows 3 of refractory silicoaluminous bricks. At the base of the furnace, there is a baseplate 4 followed by the solid mass 5 which supports the furnace as a whole. The lateral wall of the furnace is formed by a steel plate or shield 6.The thickness of refractory material between the base of the hearth 7 and the baseplate is 5 metres.

It is possible to calculate the temperature distribution within this refractory mass and at different points of the baseplate when the furnace is in operation. This calculation is made by the method described in "Circulaire d'Informaton Technique du Centre de documenta don Siderurgique" No. 1, 1973, pages 183-217. If it is assumed that the thermal conductivity of this refractory mass is approximately 1.2 Kcal/m. C.h, it can be calculated that the maximum temperature of the baseplate in the vicinity of its axis is substantially 450"C.

When the furnace is in operation, the refractory mass is progressively worn by the melt and the level of the base of the hearth gradually sinks to a considerable extent.

This phenomenon, which is inevitable in practice, only ceases when the hearth reaches an equilibrium profile. The reason for this is that, as the level of the base sinks, the temperature of the melt at the bottom of the hearth decreases in the absence of puddling, resulting in the formation of a dormant layer of melt which is denser and less hot.

The erosion process stops when the temperature of the melt in contact with the hearth becomes equal to its solidification temperature. Throughout this process, the temperatures of the baseplate and of the solid mass by which it is supported rise even further. The initial temperature of 450"C is already unacceptable for direct contact between the plate and a solid mass of conventional hydraulic concrete. For this reason, it is generally necessary to introduce either additional rows of refractory bricks or a refractory concrete between the baseplate and the slab of hydraulic concrete. Accordingly, it will be appreciated that this conventional solution to the problem of lining resulted in the construction of a refractory lining which was very thick in relation to the diameter of the hearth to provide for operating periods of sufficient duration.

In order to be able to construct hearths of the same or larger dimensions with maximum safety, it was proposed to replace the refractory materials normally used for constructing the hearth by blocks of carbon which are easy to machine to the required shapes and to assemble with considerable precision.

Figure 2 shows the lower part of a blast furnace having the same dimensions as that illustrated in Figure 1, in which the hearth 8 is formed by carbon blocks 9 of large dimensions. The joints between the blocks, such as 10, are made in the usual way by means of a carbon-containing paste and the gap 11 between the blocks and the wall plate of the blast furnace 12 is filled with a conductive lining which is also formed by a carboncontaining paste. The wall plate is vigorously cooled from the outside by a water sprinkling system (not shown). Below the carbon blocks, a conventional refractory silicoaluminous lining 13 is arranged in steps down to the baseplate 14 which itself rests on a solid support 15. An arrangement such as this promotes the lateral flow of heat towards the wall plate 12 which itself is cooled to around 50"C by external sprinkling. Along the axis of the furnace, the total thickness of the lining between the base of the hearth and the baseplate is 4.5 metres, of which 2.5 metres is made up of blocks of carbon. When this blast furnace is in operation, calculation shows that the temperature of the baseplate reaches approximately C. This calculation is made, with the necessary interpolations, on the basis of the conductivity values of the carbon blocks and the silicoaluminous refractory given in the following Table: THERMAL CONDUCTIVITY OF REFRACTORY MATERIALS IN KcaUm. C.h Temperature in "C 100 500 1200 Graphite 120 67 42 Semi-graphite 35 29 22.5 Carbon 6.6 8.5 11.5 Silicoaluminous refractory 1.2 1.2 1.2 In the same way as the silicoaluminous refractories, the carbon blocks are worn down until the equilibrium profile is reached.Since the thermal conductivity of the carbon blocks, having a temperature in the range from 1200 to 14000C., is higher than that of conventional refractories, and is of the same order of magnitude as that of the non-stirred molten iron, the downward movement of the isotherms during the wear of the hearth does not exceed a depth of approximately 1 metre. Under these conditions, there is no significant increase in the temperature of the baseplate in relation to its initial value.

The construction of blast furnaces with much larger dimensions than those which have just been described has however presented serious problems. That is because the diameters of the hearths of very large blast furnaces are currently reaching values of the order of 14 metres and values of 20 metres are even being envisaged. Calculation shows that extrapolation of the thickness of refractories and carbon blocks would lead to virtually unacceptable heights of refractories. Various solutions have hitherto been proposed with a view to reducing these heights. Generally, these solutions comprise providing a certain thickness of graphite below the blocks of amorphous carbon.By virtue of its high thermal conductivity, graphite tends to homogenise the temperature prevailing in the refractory and, hence, to reduce the temperature in the vicinity of the axis by removing more heat through the sides.

One embodiment of blast furnace linings following this principle is shown in Figure 3 in the case of a blast furnace again having a hearth diameter of 7 metres.

Starting from the top and proceeding downwards from the base of the hearth 16 to the baseplate 17, this lining comprises three rows of carbon blocks 18 with a total thickness of 1.75 metres, then two rows of graphite blocks 19 with a total thickness of 0.75 metre, two rows of refractory bricks 20 with a total thickness of 10 cm and a layer of corundum concrete 21 with a thickness of 5 cm in contact with the baseplate. The total thickness of this refractory is therefore 2.65 metres. Below the baseplate is the solid concrete mass 22 which supports the whole.

Figure 3 shows the isotherms A, B, C, D respectively corresponding to the temperatures of 1180, 900, 600 and 300"C, as determined by calculation. It is possible to see the characteristic influence which the layer of graphite has upon the shape of the isotherm D passing through it in a direction close to the vertical. Since it is known that the flow of heat is perpendicular to the isotherms, it can be seen that the effect of the graphite is to deflect the flow of heat to a significant extent towards the side wall. As a result, the 1180"C isotherm is flattened, in other words the depth to which the molten iron penetrates in the vicinity of the axis will be smaller than in the absence of graphite.However, the temperature reached by the baseplate, which is more than 400"C in the vicinity of the axis, is still far too high because it is desirable not to exceed a temperature of approximately 200"C in order to guarantee adequate stability of the concrete foundation. It should also be pointed out that the total thickness of refractory has been limited to 2.65 metres whereas the arrangement described in Figure 2 required a total thickness of refractory of approximately 4.5 metres. It will be appreciated that, by further increasing the thickness of graphite, it is possible further to reduce the temperature in the vicinity of the axis.

However, an approach such as this would hardly be economical because graphite is a very expensive material. In addition, calculation shows that if, for example, the thickness of the graphite is increased from 0.7 to 1.0 metre, the temperature of the baseplate decreases by only 50"C, which is still inadequate for the baseplate to rest directly on a hydraulic concrete.

In order sufficiently to reduce the temperature of the baseplates of modern blast furnaces, it has been proposed to cool them down by the circulation of fluid and this is in fact the case in a certain number of blast furnaces. This solution requires a number of precautions for avoiding the dangers of scaling, corrosion or supply failures which can occur during the very long service life of a blast furnace. Sufficient thermal inertia and natural cooling which enable overheating to be avoided up to catastrophic levels are therefore safety factors which have to be taken into consideration. In other words, even in cases where the baseplate is cooled by fluid circulation, a considerable thickness of refractory bricks is still provided above the baseplate.

The new type of lining according to the invention has proved to be an extremely effective means of reducing the thickness of the refractories in terms of relative value in blast furnaces of large diameter whilst, at the same time, eliminating the need to use cooling fluids for the baseplate or, at least, making their use entirely optional. This new type of lining is based on the entirely unexpected idea of forming alternate or sandwich layers of refractories having a high thermal conductivity and refractories having a low thermal conductivity. This new type of lining comprises at least one layer of low-conductivity refractory comprised between two layers of refractory having a higher conductivity.Since the thermal conductivity of the materials varies with the temperature, it is extremely difficult to specify with any degree of precision the conductivity limits which, according to the invention, divide the two categories of refractories. Quite generally, it is desired to have as high a conductivity ratio as possible between the two categories of refractories. A ratio of 5:1 may be considered as a minimum and ratios of from 10:1 to 20:1 or even greater are commonly used.

From the point of view of the absolute values, the low thermal conductivity refractories used are generally refractories having a conductivity of less than 4 Kcallm.0C.h, whilst the high thermal conductivity refractories used are refractories the thermal conductivity of which is equal to, or greater than, 15 KcaUm. C.h.

Among the high thermal conductivity refractories, it is preferred to use graphite or semi-graphite. Semi-graphite is formed by particles of graphite bonded by a hydrocarbon binder, the whole being then fired up to coking of the binder.

The low thermal conductivity refractories used include any of the refractories commonly used for the construction of furnaces, such as the silicoaluminous refractories which have a thermal conductivity of less than 4 KcaUm. C.h. In the least hot zones of the furnace lining, it is even possible to use heat-insulating materials showing a high thermal resistance and relatively poor refractory properties provided these heat-insulating materials have sufficient mechanical strengths to withstand the pressure of the materials. In these same zones, it is also possible to use materials having a very high thermal conductivity where the temperature conditions permit. At all events, these materials which are not really refractory and which can only be used in the least hot zones can only be employed in association with the refractories defined above.

The temperature calculations carried out have shown that, in the linings according to the invention comprising alternate or sandwich structures in which at least three layers are formed, the inner layer being formed by a refractory of low conductivity and the two outer layers being formed by one or more refractories of high thermal conductivity, the following phenomenon is observed: A very significant proportion of the thermal flux which has entered this structure is deflected towards the sides thereof. This results in a considerable reduction in the flux issuing from the surface opposite the point of entry in the initial direction. For this structure to be effective, it must of course be arranged in such a way as to intersect the lines of flux of which it is proposed to control the flow.When the thermal flux is distributed substantially symmetrically about an axis, which is the case in that part of the blast furnace situated below the hearth, excellent results will be obtained by stacking the elements of the sandwich structure in the form of horizontal layers, i.e. layers perpendicular to the axis of the furnace which is also the axis of symmetry of the thermal flux. For each particular application, it will be possible to vary the thickness and the nature of the constituent materials of these sandwich structure and also the number of layers. Calculation shows that the greater the number of layers and the greater the conductivity ratio between the high conductivity refractory and the low conductivity refractory the better the results which were obtained.However, economic considerations have a direct bearing upon the choice of the materials and the number of layers. Thus, theoretical calculations show that, from the thermal point of view, optimum results can be obtained by means of layers of variable thickness the contact surfaces of which are not planar surfaces, but instead curved surfaces optionally having a symmetry of revolution. In practice, however, cost factors will generally result in the adoption of the simplest arrangements whose theoretical performance is slightly lower, but whose operating costs are also very much lower.

The following two non-limiting Examples describe two embodiments of the new type of refractory lining according to the invention.

Example I Figure 4 shows the lining of a blast furnace with a hearth diameter of 7 metres of the type already shown in Figure 3, in which the lining below the hearth comprises two rows of carbon blocks 23 with a total thickness of 1.40 metres.

Below the carbon blocks, the arrangement adopted in Figure 3 has been replaced by a sandwich structure which has the same total thickness as the structure which it replaces and in which two 25 cm layers 24 of silicoaluminous refractory are sandwiched between three layers of graphite 25 each having a thickness of 25 cm. It can be seen that the total height of the lining thus formed is 2.65 metres and is equal to that of the lining shown in Figure 3. In the space between the lateral plate 26 and the refractory, a conductive lining 27 has been rammed in, in the same way as in the case of Figure 2, and the cooling of this plate is also obtained in the same way by external sprinkling with water so as to limit its temperature to approximately 50"C.

Thermal calculations made on the basis of the figures presented in the Table have shown that, when the wear of the lining has stabilised, the arrangement of the isotherms is considerably modified in relation to the case of Figure 3. The temperature of the baseplate 28 in the vicinity of the axis is now approximately 1500C instead of more than 400"C in the preceding case. This shows that a blast furnace of this type may be directly disposed on a slab of hydraulic concrete without any risk of endangering the long-term life of the latter.

This spectacular result is obtained without modifying the total thickness of the lining.

Example 2 The new refractory lining according to the invention is of even greater interest when the diameter of the blast furnace which it is proposed to construct is larger.

Figure 5 is a half-section through the lower part of a blast furnace with a hearth diameter of 14 metres. This diameter is close to the maximum diameter currently in use. The hearth is lined in the usual way with carbon blocks. The layer below the hearth 29 has a thickness of 0.5 metre. A new lining according to the invention comprising the following nine layers (from the top downwards) is provided below these carbon blocks:: 1 35 cm of semigraphite 30, 2 25 cm of silicoaluinous refractory 31, 3 35 cm of semigraphite 32, (4) 25cm of silicoaluinous refractory 33, 5 35 cm of graphite 34, (6) 25 cm of silicoaluinous refractory 35, (7) 35 cm of graphite 36, 8 25 cm of silicoaluinous refractory 37, and finally 9 35 cm of graphite 38 in contact with the base plate 39 which rests directly on the solid mass of hydraulic concrete 40 without any cooling system. The use of two layers of semigraphite in the hottest zone is justified by the thermal conductivity of this material which, at elevated temperatures, is only slightly lower than that of graphite for a much lower price.

The calculations made show that the temperature of the base plate, by the time the wear of the base of the hearth reaches its equilibrium profile, will not exceed 200"C in the vicinity of the axis. In this case, as in the preceding case, the flow of heat is mostly deflected towards the lateral plate by the composite lining. Thermal contact between the lateral plate and the lining is ensured by a conductive filling carefully rammed into the gap, as in Example 1, and the dissipation of heat from this plate is obtained in the same way as in Example 1 by external sprinkling with water to an extent sufficient to keep its temperature around 50"C.

It will be noted that this remarkable result is obtained with a total thickness of refractory below the hearth of only 3.25 metres.

Accordingly, these two Examples show that, by virtue of the new lining according to the invention, it is possible to construct blast furnaces of larger diameter without cooling through the base which have a greatly reduced under-hearth height. This very significantly reduces the lining costs during construction or during repairs and also the running costs.

The operational safety of these blast furnaces is thus guaranteed by the absence of any cooling system at the level of the base plate.

As mentioned earlier on, the refractory lining according to the invention may be applied to many other types of furnaces in which it is desired to deflect the flow of heat to relatively remove points or in which it is desired to absorb local overheating or to level the temperatures. In the least hot zones of refractory sandwich linings such as these, it is possible to replace the refractories by materials which, although less heat-resistant, show higher or, on the other hand, lower thermal conductivity values. Thus, it is possible to replace the graphite with a metal of high thermal conductivity, such as copper or aluminium, and to replace the refractories of low thermal conductivity by insulating materials of the glass wool type or any other type, provided their mechanical and/or thermal stability is acceptable.In certain cases, it may be of advantage to form in a sandwich lining according to the invention one or more layers of high conductivity comprising in series in the same layer mixed arrangements the materials of which are selected according to the temperature conditions.

WHAT WE CLAIM IS: 1. A refractory furnace lining comprising at least two materials of which one has a higher thermal conductivity than the other, in which the lining is formed at least partly by a sandwich structure oriented in such a way as to intersect the lines of thermal flux (as herein defined), in which the layers of materials of more or less high conductivity are alternately disposed, and in which at least one layer of material(s) of lower conductivity is comprised between at least two layers of material(s) of higher conductivity, the conductivity ratio between the two categories of materials being equal to or greater than 5:1, whereby lines of thermal flux entering the lining are deflected to the side.

2. A lining as claimed in claim 1, wherein the material or materials of lower thermal conductivity have a conductivity of less than 4 Kcal/m."C.h and the material(s) of higher thermal conductivity have a conductivity greater than 15 Kcal/m. C.h.

3. A refractory lining as claimed in claim 1 or 2, wherein the refractory material(s) of high thermal conductivity are graphite or semigraphite, and wherein the low conductivity refractories are based on metallic oxides or mixtures thereof.

4. A refractory lining as claimed in any of claims 1 to 3, which comprises at least one metallic layer in the least hot zones of the lining.

5. A refractory lining as claimed in any of claims 1 to 4, which comprises in the least hot zones thereof at least one layer formed by a nonrefractory heat-insulating material of high thermal resistivity.

6. A refractory lining as claimed in claim 1, substantially as herein described with reference to Figure 4 or 5 of the accompanying drawings and in Example 1 or 2.

7. A furnace comprising a lining as claimed in any of claims 1 to 6.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. temperature conditions. WHAT WE CLAIM IS:

1. A refractory furnace lining comprising at least two materials of which one has a higher thermal conductivity than the other, in which the lining is formed at least partly by a sandwich structure oriented in such a way as to intersect the lines of thermal flux (as herein defined), in which the layers of materials of more or less high conductivity are alternately disposed, and in which at least one layer of material(s) of lower conductivity is comprised between at least two layers of material(s) of higher conductivity, the conductivity ratio between the two categories of materials being equal to or greater than 5:1, whereby lines of thermal flux entering the lining are deflected to the side.

2. A lining as claimed in claim 1, wherein the material or materials of lower thermal conductivity have a conductivity of less than 4 Kcal/m."C.h and the material(s) of higher thermal conductivity have a conductivity greater than 15 Kcal/m. C.h.

3. A refractory lining as claimed in claim 1 or 2, wherein the refractory material(s) of high thermal conductivity are graphite or semigraphite, and wherein the low conductivity refractories are based on metallic oxides or mixtures thereof.

4. A refractory lining as claimed in any of claims 1 to 3, which comprises at least one metallic layer in the least hot zones of the lining.

5. A refractory lining as claimed in any of claims 1 to 4, which comprises in the least hot zones thereof at least one layer formed by a nonrefractory heat-insulating material of high thermal resistivity.

6. A refractory lining as claimed in claim 1, substantially as herein described with reference to Figure 4 or 5 of the accompanying drawings and in Example 1 or 2.

7. A furnace comprising a lining as claimed in any of claims 1 to 6.