Abstract
A hearth for a metallurgical furnace, in particular for a blast furnace, the hearth including a wall lining and a bottom lining of refractory material for containing a molten metal bath, the bottom lining including a lower region and an upper region that is arranged to cover the top of the lower region and that is built of ceramic elements, the ceramic elements of the upper region being made of microporous ceramic material including a granular phase made of a silico-aluminous high alumina content granular material and a binding phase for binding grains of said granular material, said microporous ceramic material having thus an maintaining permanently a thermal conductivity lower than 7 W/m.° K.
Claims
1. A hearth for a metallurgical furnace comprising: a wall lining and a bottom lining that are made of refractory material for containing a bath comprising molten metal; said bottom lining having a lower region comprising a carbon refractory layer and an upper region that comprises a layer of ceramic elements arranged to cover said lower region wherein said ceramic elements of said upper region are made of microporous ceramic material comprising a granular phase made of a silico-aluminous high alumina content granular material and a binding phase for binding grains of said granular material, said microporous ceramic material having a thermal conductivity lower than 7 W/m.° K; a permeability ≦2 nanoperms and a mean pore width ≦2 μm, wherein said wall lining delimits a substantially horizontal top surface of said lower region and said layer of ceramic elements is an assembly that comprises bricks or blocks and that completely covers said top surface, wherein the ceramic elements are large-size blocks, measuring more than 200×400×500 mm, having a first part made of ceramic material baked in nitrogen atmosphere, said first part having an upper side and a lower side and comprising at least one conical blind hole disposed in the blocks, the at least one blind hole being open into said lower side, and a second part made of a refractory material rammed in said blind holes, the blind holes being arranged so that any point located in the ceramic material of the first part is at a distance from a surface of said first part lower than a maximum penetration depth of impermeation achievable by a baking process used for producing said blocks.
2. The hearth as claimed in claim 1, wherein said microporous ceramic material has a permeability ≦1 nanoperm.
3. The hearth as claimed in claim 1, wherein said microporous ceramic material has a mean pore width ≦1 μm.
4. The hearth as claimed in claim 1, wherein the granular phase comprises one or more of the followings: andalusite, chamotte, corundum, synthetic mullite.
5. The hearth as claimed in claim 4, wherein the granular phase comprises granular andalusite with an Al.sub.2O.sub.3 content of 55-65 wt %.
6. The hearth as claimed in claim 1, wherein the binding phase comprise a nitrided bond.
7. The hearth as claimed in claim 6, wherein the binding phase is based on silicon, aluminum, oxygen and nitrogen, in a range of ratii configured to generate a SiAlON bond.
8. The hearth as claimed in claim 1, wherein the ceramic elements are large-size ceramic blocks, measuring more than 200×400×500 mm, disposed in a herringbone pattern.
9. The hearth as claimed in claim 8, wherein the wall lining comprise, at the same level as said upper region, refractory blocks matching with said large-size ceramic blocks in said herringbone pattern, each alignment or group of alignments of ceramic blocks prolonging toward the periphery of the wall lining by one said refractory block.
10. The hearth as claimed in claim 8, wherein the wall lining comprise, at the same level as said upper region, a first annular row of refractory blocks disposed circumferentially side by side, and a second annular row of microporous ceramic blocks disposed circumferentially side by side is disposed between the first annular row of refractory blocks and the large-size ceramic blocks disposed in a herringbone pattern.
11. The hearth as claimed in claim 1, wherein the wall lining comprise, at the same level as said upper region, an first annular row of refractory blocks disposed circumferentially side by side, and the ceramic elements are large-size ceramic blocks disposed in concentric annular rows wherein each of said annular rows is constituted of microporous ceramic blocks disposed circumferentially side by side, the outer annular row of ceramic blocks being joined to the first annular row by a ramming material.
12. The hearth as claimed in claim 9, wherein said refractory blocks are carbon blocks.
13. The hearth as claimed in claim 11 wherein the junction surfaces between adjacent ceramic blocks are progressively more globally inclined from the center toward the periphery of the bottom lining, so that any block is partially surmounting a block inwardly adjacent.
14. The hearth as claimed in claim 11 wherein the junction surfaces are flat inclined surfaces or curved slopped surfaces or stepped surfaces.
15. The hearth as claimed in claim 1, wherein the ceramic elements are large-size ceramic blocks, measuring more than 200×400×500 mm, determining therebetween joints filed with ceramic mortar, a joint between any adjacent blocks having a width of 0.7 to 1.5% of the adjacent blocks dimension taken in the direction perpendicular to that of the joint.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:
(2) FIG. 1 is a vertical cross sectional view of a blast furnace hearth illustrating a bottom lining, wherein the ceramic elements of the upper region comprise microporous bricks or comparatively small blocks;
(3) FIG. 2 is a vertical cross sectional view of a blast furnace hearth illustrating a bottom lining, wherein the ceramic elements of the upper region comprise microporous large-size blocks;
(4) FIG. 3A-3B show a large-size refractory block in bottom view and vertical sectional view respectively, which block is specially adapted for the manufacture of large-size blocks as used in the embodiment of FIG. 2;
(5) FIG. 4 is a plan view of a first embodiment of the bottom lining, made of large ceramic blocks disposed in concentric rings;
(6) FIG. 5 is a plan view of a second embodiment of the bottom lining, made of large ceramic blocks disposed in herringbone design, the blocks of the wall lining being disposed in a ring;
(7) FIG. 6 is a plan view of a third embodiment of the bottom lining made of large ceramic blocks disposed in herringbone design, the blocks of the wall lining being disposed in a matching stepped design;
(8) FIG. 7 is a radial cross sectional view of the bottom lining of FIG. 4, sharing different examples of vertical joints between ceramic blocks.
DETAILED DESCRIPTION WITH RESPECT TO THE DRAWINGS
(9) FIG. 1 illustrates a generally cylindrical hearth 10 of a blast furnace (not fully shown), more specifically the lower hearth region below the tuyeres (not shown). The hearth 10 comprises a lateral wall lining 12 and a lower bottom lining 14 which are made of refractory material that resists to very high temperatures >1500° C. to contain the bath of molten pig iron produced by the blast furnace process. The wall lining 12 comprises an innermost additional lining 16. In typical manner, a surrounding outer shell 18, e.g. of cylindrical shell, is made of steel to contain and mechanically maintain the wall lining 12 and the bottom lining 14. The wall lining 12 and the bottom lining 14 respectively form the lateral boundary and the lower boundary of the useful volume of the hearth 10. As further illustrated in FIG. 1, the bottom lining 14 comprises a lower region 20 and an upper region 22 that is arranged to cover the top of the lower region 20. When made of ceramic material, the upper region 22 is often called “ceramic pad”.
(10) Although not illustrated in detail in FIG. 1, the lower region 20 comprises any conventional carbon based construction. The lower region 20 may for example be built of, starting from the bottom plate of the bottom lining, a ramming mass, a safety graphite layer, which is about 100 to 200 mm thick, and a carbon layer, which is about 1 m thick, of two or three superposed courses of thermally conductive carbonaceous refractory blocks.
(11) The upper region 22 of the bottom lining 14 however has a specific configuration in accordance with the present invention. As seen in FIG. 1, the upper region 22 comprises an uninterrupted horizontal layer of a plurality of ceramic elements 24 that completely covers the top surface 26 of the conventionally configured lower region 20, i.e. the top surface 26 that would be exposed to the bath in the heart 10 in the absence of the upper region 22. Accordingly, the surface covered by the upper region 22 corresponds to the disc-shaped area that is circumferentially delimited by the wall lining 12 in the lower region 20. In the embodiment of FIG. 1, the layer of ceramic elements 24 is built of a masoned pavement-like assembly made mostly of comparatively small blocks, e.g. bricks or blocks having dimensions exceeding 100×250×500 mm, with the block being typically arranged with their lengthwise axis oriented in vertical direction. In the border region adjacent the wall lining 12, smaller elements can be used. More specifically, the upper region 20 comprises two superposed horizontal courses 28, 30 (i.e. planar strata) of blocks in staggered arrangement. The geometrical layout of the elements 24 into courses 28, 30 is of any known suitable type, e.g. a conventional “herring bone” layout. Besides the ceramic elements 24 as such, the upper region 22 comprises cement based vertical joints 34, 36 between the elements 24 of conventional material and configuration and horizontal cement joints in between the courses 28, 30 and between the lower course 30 and the lower region 20. Staggering the elements 24 of the course 28 with respect to those of the course 30 enables a more stable assembly and increases the tightness against molten pig iron. As will be understood from the foregoing, the upper region 22 forms a coherent uninterrupted barrier or separation between the bath to be contained in the hearth 10 and the conventionally configured lower region 20. Accordingly, the upper region 22 warrants a durably maintained position of the pig iron solidification isotherm in the upper region 22 (i.e. within the pad). In addition, the ceramic barrier of the upper region 22 provides additional protection against carburization dissolution of carbon refractory in the lower region 20, especially in case the bath in the hearth 10 is not saturated in carbon (e.g. in view of reducing carbon oxide emissions).
(12) As will be appreciated, each of the ceramic elements 24 are based on microporous ceramic material, i.e. material having a permeability ≦2 nanoperms, preferably ≦1 nanoperm (metric—measured using a method according to ISO 8841:1991 “Dense, shaped refractory products—Determination of permeability to gases”). More preferably, the ceramic elements 24 essentially comprise microporous material and have a mean pore with mean pore width ≦2 μm (measured using a method according to DIN 66133: “Determination of pore volume distribution and specific surface area of solids by mercury intrusion”).
(13) The protective layer of refractory elements 24 enable long-term maintenance of the level of the pig iron solidification isotherm (e.g. at 1150° C.), ideally within the upper region 22 during the entire furnace campaign. Moreover, and as will be appreciated, compared to protective layers made of conventional ceramics, the proposed upper region 22 with the covering layer of microporous ceramic material provides a durably raised level of the mentioned solidification isotherm as set out hereinabove. In addition, it is theorized that microporous refractory elements 24 will be less subject to wear and thus have longer service life due to improved resistance, e.g. to chemical attack by alkalies. As a consequence, the service life of the lower region 20 is significantly increased by virtue of microporous elements 24 in the upper region 22 in accordance with the invention.
(14) As further seen in FIG. 1, the wall lining 12 is equipped with an innermost additional assembly of ceramic elements 38 which may also be made of microporous ceramics. Together with the ceramic elements 24, the ceramic elements 38 can form a ceramic cup 32 providing an “artificial high-quality skull” protecting the main refractory construction of both the wall lining 12 and of the bottom 14 of the hearth 10. It is to be noted that ceramic materials also minimize heat losses in comparison with conventional refractories, such that more energy-efficient operation is possible when providing a ceramic cup 32. The microporous quality of the ceramic elements 24 is expected to significantly decrease at long term thermal conductivity compared to conventional ceramic refractories.
(15) Suitable microporous ceramic elements 24 of low-permeability can be produced using any known method, e.g. conventional hydraulic binding of preformed cast blocks based on granular andalusite (aluminum nesosilicate mineral Al.sub.2SiO.sub.5) or synthetic mullite.
(16) Preferably however, ceramic elements 24 of low thermal conductivity as well as thermally stable very low permeability, e.g. <1 nanoperm, are obtained by baking in nitrogen atmosphere.
(17) The ceramic elements 24 are preferably manufactured using suitable fine additives that, after baking in nitrogen atmosphere (“nitrogen firing” or “nitride hardening”) provide a high-temperature resistant permanent microporosity. In addition to decreasing the mean free width of the pores and thereby “impermeating” the material, this treatment can provide ceramic material, in particular SiAlON ceramics, with a better resistance to chemical attack, e.g. by alkaline substances, than non-nitrided ceramic materials. Large microporous ceramic elements 24 are preferred and obtained by baking in nitrogen atmosphere of prefabricated blocks. Suitable prefabricated (green) blocks can be based on high alumina content granular material. More preferably however, in view of reduced cost and reduced thermal conductivity, the blocks can be based on andalusite, synthetic mullite or chamotte granular material, e.g. chamotte with an Al.sub.2O.sub.3 content of 55-65% by weight, in particular 60-63% by weight. These three alternatives are considered to confer microporosity that remains reliably stable at high temperatures in excess of 1400° C. as may occur in the hearth. Preferably, the prefabricated blocks are composed so as to obtain a microporous SiAlON bonded ceramic, i.e. a sort of matrix (bonding phase) made of “ceramic alloy” based on the elements silicon (Si), aluminum (Al), oxygen (O) and nitrogen (N), introduced adequately into the grog (initial mix before baking), which is subsequently baked in nitrogen atmosphere. Whereas SiAlON bonded ceramics are known for their resistance to wetting or corrosion by molten non-ferrous metals, they have also been found beneficial in case of ferrous metal, e.g. in a pig iron producing blast furnace.
(18) In FIG. 1, the ceramic elements 24 are for instance made of pre-fabricated andalusite based blocks, with ca. 55-65, in particular 60-63, wt. % Al.sub.2O.sub.3 content, that have been impermeated by baking in nitrogen atmosphere, i.e. by surrounding the grains of the granular material with a SiAlON bonding phase.
(19) FIG. 2 shows an alternative embodiment of a hearth 210, in which only the configuration of the upper region 222 of the bottom lining 214 differs from the above-described hearth. In FIG. 2, the lower region 220 comprises any conventional carbon based construction, and the ceramic elements 224 are made of pre-fabricated blocks based e.g. on granular andalusite, on chamotte or on corindon also transformed into a microporous SiAlON bonded ceramic, by baking in nitrogen atmosphere. Permeability measurements also revealed permeability of <2 nanoperm.
(20) As will be appreciated, the layer of refractory elements 224 schematically shown in FIG. 2 is made of two courses, built essentially of relatively large-sized blocks having a volume typically in excess of 20 dm.sup.3 and, typically dimensions of at least 400×200×500 mm (height×width×length), however with at least one dimension significantly exceeding 200 mm. Typically, the layer 224 is made of two courses of blocks arranged with a 400 mm vertical extent, or even two courses of 500 mm vertical extent. Taking into account that the recommendation is to have a total thickness greater than 500 mm, the layer of refractory may also be made of only one course of large blocks.
(21) Independently of the foregoing, the present disclosure also proposes a configuration and impermeation method for producing large-size blocks 224 with highly homogenous microporosity throughout the constituent material.
(22) FIG. 3A-B illustrate a suitable non-baked (green) block 300, e.g. based on granular andalusite shaped by ramming or vibration molding. With respect to its orientation when installed, the generally parallelepiped block 300 has an upper side 302 and an opposite lower side 304 (base). As seen in the cross-section of FIG. 3A, the block 300 is molded so to have blind holes 306, which are preferably slightly conical for molding purposes. The blind holes 306 open into the lower side 304 and stop short of its upper side 302 at a distance d. Moreover, as seen in rear view of FIG. 3B, the large-size blocks have four (or any other suitable number depending on the size and shape) blind holes 306, which have a diameter of e.g. 10-50 mm, typically about 20 mm. The blind holes 306 are regularly arranged so as to be separated at a regular maximum distance d (e.g. on the diagonal of the rectangular lower side 304) from each other and from the outer faces. The distance d is chosen slightly smaller than twice the maximum achievable penetration depth of the chosen impermeation process. When using nitride hardening, d is typically 100-200 mm. Thanks to the blind holes 306, homogeneous baking in nitrogen atmosphere of large-size blocks is possible. After baking in nitrogen atmosphere of the large-size blocks 300, the slightly conical blind holes 306 are preferably closed by ramming. As preferred ramming mass, a granular mass similar to the ceramic material of the non-baked block, preferably suitable for phosphatic hardening (hardening due to a phosphate reaction with a matrix constituent), is used. Such ramming mass confers high temperature resistance and durability. Lifting holes, well known in the prior art, made on the upper side of the blocks, ca also participate to an efficient nitride hardening.
(23) FIG. 4 to 6 illustrates three alternatives design of bottom linings according to the invention; made of large-size ceramic blocks.
(24) In the first preferred design presented on FIG. 4, the ceramic blocks 224, having for instance a mean width of 500 mm in the circumferential direction, are designed in concentric rings parallel to the ring of surrounding carbon blocks 2 of the wall lining. The outer ring 4 of ceramic blocks, preferably of same composition, is designed for obtaining an adequate accommodation with the surrounding carbon blocks 2, by means of a thick joint 3 having a thickness of 50 mm for example.
(25) In the designs presented on FIGS. 5 and 6, the ceramic blocks 224a are lined up in two perpendicular directions. This design, often called the “herring bone design” advantageously allows to give identical rectangular shape and dimensions to many blocks, thus reducing the mound costs.
(26) When the surrounding carbon blocks 2 are of circular design, as shown on FIG. 5, an intermediary ring 5 of circular design is recommended between the “herring bone” blocks 224a and said carbon blocks. Only the blocks 224a′ situated at the periphery, adjacent the intermediary ring 5, need to be given a specific shape. Preferably, the ceramic blocks of ring 5 are of the same composition as the blocks 224a, or possibly better.
(27) On the contrary, when the carbon blocks 2a are designed, as shown on FIG. 6, according the so-called “stair shaped parallel beams”, then a direct accommodation with the carbon blocks, including the needed thick joint 3a can be used, providing that the width of the ceramic blocks 224a is adapted to the width of the carbon blocks. However, ceramic blocks having a different width, for example ceramic blocks 224b of half-width, can also be used if needed.
(28) Only the length of some ceramic blocks 224a″ needs to be adapted to ensure accommodation with the surrounding carbon blocks 2a, using there thick joints 3a.
(29) As already mentioned, a special attention is needed for the joints between the large-size ceramic blocks of the above examples. For example, in the case of the design of concentric rings of FIG. 4, the block length in the radial direction is 600 mm. Then, the joint thickness of the joint 234, 236 between 2 consecutive rings is 1% of said length, that is 6 mm.
(30) The junction surfaces of the joints can be either flat inclined surfaces (31a) or curved slopped surfaces (31c) or stepped surfaces (31b) as shown on FIG. 7. Preferably these joints are progressively more globally inclined joints from the center toward the periphery of the bottom lining, an important aspect being that the border of any block directed toward the axis A is surmounting the adjacent border of the adjacent block, so that a sort of arching effect, favorable to a better maintain of the blocs, is obtained by blocking successively the different rings from the center to the exterior ring. All the joints can have a same form as mentioned above. FIG. 7 show examples, in a non-limitative way, of the joints between the different rings of a lining in concentric rings, disposed above the lower region 20 of carbon lining. The axis A of the hearth is on the left side of the drawing. The progressive inclination of the joints is here obtained by a joint surface 31a between the blocks of the inner rings 4a substantially flat; the joint surface 31c between the blocks of the intermediary rings 4c gives an example of sloped curved; and the joint surface 31b between the blocks of the outer rings 4b gives an example of stepped interface. In practice, either slopped curved or stepped interfaces will be used, not both of them, in a given bottom.
