Magnesite-carbon refractories

Abstract: A mix for forming a carbon-bonded refractory shape comprising from about 70 to 92 wt.% of a high purity magnesite and about 8 to 30 wt.% of a high purity graphite, and for each 100 wt.% of said magnesite and graphite, from about 1.5 to 6 wt.% of a carbon-containing bonding agent; said graphite containing at least about 98% carbon and having at least a majority by weight -65 mesh or finer particles and said magnesite containing at least about 98% MgO, and containing less than about 0.03% boron oxide, less than about 0.3% silica, and having a CaO/SiO₂ weight ratio above about 2, and the resultant carbon-bonded refractory shapes and liners for metal processing equipment, especially basic oxygen furnaces, made from such shapes and the method of increasing the life of liners for such equipment and especially basic oxygen furnaces.

Description: The present invention relates to carbon-containing refractories and in particular magnesite-carbon refractory brick suitable for use in metal processing equipment such as electric arc steel making furnaces, steel ladles, ladle furnaces and especially basic oxygen furnaces where high hot strength and high slag resistance of refractory linings are required. Carbon-containing basic refractory brick have been used in basic oxygen furnaces for many years. In the U.S.A., for example, pitch bonded and burned pitch impregnated magnesite brick have been used since the early 1960's. In other areas of the world, different practices have been used. For instance, pitch bonded and burned pitch impregnated dolomite brick have been used in Europe. In Japan, burned pitch impregnated basic brick with relatively high levels of lime (e.g. 30% CaO and 70% MgO) have been used. In the late 1970's and early 1980's, a major advance in the technology of BOF refractories was realized with the introduction of magnesite-carbon brick. Unlike the brick that had traditionally been used, these brick contained graphite. Flake graphite was used most often, although in some cases Sri Lankan vein graphite was employed. These magnesite-carbon brick generally had high carbon contents ranging from about 8 to 30%, while the traditional brick contained a maximum of 5% carbon as a rule. While use of the magnesite-carbon brick in basic oxygen furnaces did result in longer times between furnace relines, it was perceived in early trials that the performance of these brick could be improved by increasing their hot strength and oxidation resistance. As a result, magnesite-carbon brick that contained powdered metals were developed. These brick have been described in U.S. Patent 4,306,030. The metals used in these brick were aluminum, silicon and magnesium. The metals increased oxidation resistance by lowering the permeability of the brick and by consuming oxygen that would otherwise have oxidized carbon. The term "magnesite-carbon brick" as generally used in this industry, and as used herein, refers to brick that contain graphite in combination with deadburned magnesite or magnesia, with the brick containing in excess of 8% by weight carbon. While the introduction of metal-containing brick represented a significant improvement in BOF refractory technology, it has now become apparent that additional improvements are necessary. It is the object of this invention to provide an improved brick to replace metal-containing magnesite-carbon brick in metal processng equipment especially basic oxygen furnaces. These improved brick are more refractory and less subject to slag attack than the metal-containing brick. Briefly, the present invention comprises a mix for forming a metal-free refractory comprising from about 70 to 92 wt.% of a high purity magnesite and, correspondingly, about 8 to 30 wt.% of a high purity graphite, and for each 100 wt.% of said magnesite and graphite from about 1.5 to 6 wt.% of a carbonaceous bonding agent; said magnesite containing at least about 98% MgO, and preferably less than about 0.03% boron oxide, less than about 0.3% silica, and a CaO/SiO 2 weight ratio above about 2, and said graphite containing at least about 98% carbon as measured by loss on ignition (LOI) and at least 75 wt% of the particles are <212 µm (-65 mesh). The invention also comprises the resultant refractory shapes and particularly brick and liners for basic oxygen furnaces having an increased life using said brick as hereinafter set forth. FIGURE 1 is a graph depicting the effect of boron oxide content of the magnesite, and FIGURE 2 is a graph depicting the effect of the CaO/SiO 2 ratio of the magnesite. The essential elements of the instant invention are the high purity magnesite and the high purity graphite. It is essential that both have the purities discussed below. Also, all mesh sizes set forth herein are Tyler mesh sizes. With the magnesite, it is necessary that it contain at least about 98% MgO and contain less than about 0.03% boron oxide, less than about 0.3% silica, and have a CaO/SiO 2 weight ratio above about 2. As to the graphite, it should have a loss on ignition (LOI) of 98% carbon or greater and particles, at least 75 wt% of which are 212 µm (-65 mesh) or finer; preferably about 95% by weight. It is preferred to utilize flake graphite or a vein graphite such as Sri Lankan graphite fines. With respect to the purity of the materials, this is essential since it is necessary to minimize the amount of boron and silica present in the mix because of its effect on the resultant product. The low boron content is required in order to enable the magnesite grain to be stable in the presence of carbon at high temperatures; particularly the high temperatures present in basic oxygen furnaces. While not completely understood, it is believed that the increased thermal stability of the brick of the present invention is due also to the fact that there is a limited amount of silica which can react with the carbon to form SiO gas and CO gas which vaporize and weaken the resultant brick. The noted CaO/SiO 2 ratio is also critical in ensuring the stability of the magnesite grains in the presence of carbon at high temperatures. With respect to the proportions of materials, there should be utilized from about 70 to 92 wt.% of the high purity magnesite as defined above and, correspondingly, about 8 to 30 wt.% of the high purity graphite. The particle size or the graded size of the magnesite is not critical; it should be that conventionally used in making this type of brick. Also, with respect to the graphite, it must have a particle size noted above. Although it is known that the oxidation resistance of graphite decreases as graphite gets finer and that the thermal conductivity of magnesite-carbon brick decreases as the particle size of the graphite decreases, it has surprisingly been found that the increased purity of the graphite and its combination with the high purity magnesite overcomes these prior problems to give brick of increased thermal stability. Also included in the mix must be a carbonaceous bonding agent that yields high levels of carbon on pyrolysis; i.e. over about 25% by weight carbon. Examples are any novolak or resol resin, tar, pitch or mixtures thereof, and the like conventionally used for bonding brick. At the temperatures at which these bricks are used, these materials are decomposed and the carbon derived acts to bind the brick. The amounts thereof are not critical but it is desired to avoid high binder levels in order to avoid difficulties in brick formation during processing. Ordinarily about 1.5 to 6% by weight; preferably 2.5 to 4%, of such bonding agent is added for each 100% by weight of the mix. The method of forming the brick is not critical in that the components noted above can simply be admixed, pressed into shape in the usual brick-making presses, and then baked at the conventional temperatures; i.e., about 121 to 288°C (250 to 550°F) to form the unburned brick which are then used as noted above, particularly as linings for basic oxygen furnaces. In the use, the bricks become burned at high temperatures in such furnaces to form carbon-bonded brick of high hot strength and improved slag resistance. The bricks of the present invention are particularly suitable as linings metal processing equipment, especially for basic oxygen furnaces where their increased hot strength, slag resistance, and stability at high temperatures results in longer times between furnace relines. The bricks of the present invention are made to the size and shape required to form the entire lining or portions of linings of any shape of basic oxygen furnace, for example, the linings are formed in the conventional manner by forming courses, or rings, of the proper size brick about the interior of the furnace. Other construction techniques are also commonly used and, of course, in areas where brick cannot be used, refractory ramming mixes are utilized. The invention will be further described in connection with the following examples which are set forth for purposes of illustration only. EXAMPLES 1 TO 3 It was known that aluminum in magnesite-carbon brick will oxidize during service in basic oxygen furnaces by several mechanisms: (1) Reaction with atmospheric oxygen; (2) Reaction with highly reducible oxides in the slag such as FeO and SiO 2 ; and/or (3) Reaction with easily reducible oxides in the brick such as SiO 2 and FeO. In basic oxygen furnaces, oxidation of aluminum by reaction mechanism (1) is considered to be of least importance because the furnace atmosphere is highly reducing during each heat and between heats the brick are coated with slag, preventing direct contact between the brick and the atmosphere. Oxidation of aluminum by mechanisms (2) and (3) is more prevalent in basic oxygen furnaces. There was concern that the oxidized aluminum (alumina) would form low melting phases by reaction with the silicate minerals of high lime-to-silica ratio that are contained in the magnesite grain used in magnesite-carbon brick. Although formation of these low melting alumina-silica-lime phases is a well known phenomenon in burned magnesite brick, it was not obvious that such phases would form in magnesite-carbon brick because the carbon in the brick would be expected to inhibit reaction between the alumina and the magnesite grain. Examples 1, 2 and 3, listed in Table I, were made to determine if alumina additions would lower the refractoriness of a magnesite-carbon brick, as measured by hot strength. Three mixes were formed. Brick was made by pressing each mix in a conventional press at 124 MPa, with pauses at 17.2 and 51.7 MPa, and curing at a baking schedule of 37.8°C/hr to 177°C with a 3-hour hold. The bricks were tested, with the results shown in Table I. As the data in Table I indicate, there was a decrease in the crushing strength at 1093°C when alumina was added to the magnesite-carbon brick. These Examples are comparative in that they illustrate the decrease in refractoriness of magnesite-carbon brick with alumina additions, as measured by hot strength. These results indicated that alumina formed from oxidation of aluminium in service would decrease the refractoriness of an aluminium-containing magnesite-carbon brick. Another reason for not wanting to have aluminum metal in a magnesite-carbon brick is that it can increase the rate of slag attack on the brick. It is well known that alumina acts as a slag fluidiser with the limey slags that are used in basic oxygen furnaces. Thus, at the hot face of an aluminium-containing magnesite-carbon brick where the slag constituents have oxidised the aluminum metal, the slag will be more fluid, causing a greater amount of erosion than would have occurred had the aluminium not been present. EXAMPLES 4 to 7 The Examples are comparative in that they show the decrease in hot crushing strength with silicon addition. In magnesite-carbon brick that contains silicon, the silicon would oxidise in serve in a BOF, by mechanisms similar to those that cause aluminium to oxidise. The silica that would result from such oxidation is a contaminant in basic refractory systems and can lower refractoriness. In addition, silica is not very stable in the presence of carbon and can be reduced, resulting in a loss of carbon from the brick at high temperatures. The data in Table I show the damaging effect of adding silicon to magnesite-carbon brick, as illustrated by the decrease in the hot crushing strength associated with the silicon additions. Brick was formed as in Examples 1 to 3. EXAMPLES 8 to 10 Unlike oxidised aluminium and silicon, oxidised magnesium is not chemically detrimental to magnesite-carbon brick. However, magnesium easily vaporises as magnesite-carbon brick is heated, resulting in brick with porous textures. Porous textures reduce the slag resistance of the brick. Even if the magnesium reacts with the carbon in the brick to form magnesium carbides, the magnesium easily volatilises from the brick during heating, because the carbides themselves are not stable and decompose according to the following equations: 2MgC 2 600°C / Mg 2 C 3 +C Mg 2 C 3 749°C / 2Mg (gas) +3C This evidence led to the conclusion that, in applications like the BOF where a principal mode of wear was not oxidation by the furnace atmosphere, but was rather slag attack, with mechanical abuse sometimes playing a role, the addition of powdered aluminium, silicon and/or magnesium would increase the rate of wear of the magnesite-carbon brick. Thus, it was necessary to develop magnesite-carbon brick that contained no powdered metals, in order to maximise slag resistance, while at the same time retaining enough hot strength to withstand mechanical abuse that the brick might encounter in service. With this goal in mind, a series of three tests were run to examine the effect of graphite purity and particle size on the crushing strength at 1538°C. The particulars of the mixes used (brick was formed as in Examples 1 to 3), and the test results are set forth in Table I. The 99% LOI and 96.5% LOI graphites had the following screen analyses (Tyler mesh sizes): Mesh Size 99% 96.5% -10 +28 0 - Trace 0 - 1% -28 +65 0 - 3% 27 - 52% -65 97 - 100% 48 - 73% -150 53 - 79% 15 - 36% -325 7 - 10% 2 - 6% Example 8 is representative of prior art metal-free magnesite-carbon brick, while Example 10 is representative of a typical aluminium-containing magnesite-carbon brick. The brick of Example 9 was made with high purity graphite having a loss on ignition (LOI) of 99%. These data illustrate that a substantial increase in the crushing strength at 1538°C resulted when the high purity flake graphite was used. Though not as strong as the aluminium metal-containing brick of Example 10, the brick of Example 9 is more refractory and would be expected to have a greater resistance to slag attack than the aluminium-containing brick. EXAMPLES 11 to 19 Experiments were conducted to determine what type of magnesite would be appropriate for use in metal-free brick to be applied in basic oxygen furnaces. This was important because at high temperatures at which basic oxygen furnaces often operate (above 1650°C), the magnesite in the magnesite-carbon brick can react with the carbon in the brick in the following manner. MgO+C → CO (gas) +Mg (gas) Reduction of magnesia (magnesite) by this mechanism would be expected to lower the hot strength of a magnesite-carbon brick. In the tests, 80% 600 to 850 µm (-20+28 mesh) magnesites of various chemistries (see Table II; magnesite A was used in Example 11, B in 12 etc, to I in 19) were mixed with 20% carbon black, plus additions of a phenolic resin binder (3%) and isopropyl alcohol (5%). The resulting mixture was pressed into pellets and baked at 177°C, to cure the resin. The pellets were then coked to convert the resin to carbon. The pellets were then heated to 1605°C in an argon atmosphere and measured for weight loss, with respect to boron oxide and lime/silica. The results are shown in Figs. 1 and 2. The amount of weight loss was taken as a measure of the stability of the magnesite grain in the presence of carbon. Comparison of Examples 11 and 12, for example, shows that generally higher purity magnesites were more stable than lower purity magnesites against reduction by carbon at high temperatures. However, the data also illustrated that purity level by itself did not dictate the stability of the magnesite grain in the presence of carbon. For example, Example 13 contained a relatively pure magnesite (magnesite C), but it still had a relatively high weight loss of 11.2%. This was attributed to magnesite C having a relatively low lime-to-silica ratio. Thus, a high lime-to-silica ratio was desirable. In addition, Example 17 contained a magnesite (magnesite G) of higher purity than that of Example 11. However, even though magnesite G had a high lime-to-silica ratio, Example 17 had a very high weight loss of 23.4%. This high weight loss was attributed to magnesite G having a high boron oxide content. Therefore, in addition to having high purity and a high lime-to-silica ratio, it was necessary for a magnesite grain to have a low boron oxide content in order to be stable in the presence of carbon at high temperatures. It will be understood that, as used herein, the term "metal-free" means those mixes and refractories that contain no metals and those that contain very minor amounts, so long as those minor amounts do not have any adverse effect on the desired properties of the refractories of the invention.

Claims: A substantially metal-free composition comprising a magnesite-graphite mixture consisting of 70 to 92 wt % of a high purity magnesite and 30 to 8 wt % of a high purity graphite; and, based on the weight of the mixture, 1.5 to 6 wt % of a carbonaceous bonding agent; wherein the magnesite contains at least 98% MgO, less than 0.03% boron oxide, and less than 0.3% silica, and has a CaO/SiO 2 weight ratio above 2, and wherein the graphite has a loss on ignition of at least 98% and at least 75% by weight of 212 µm (-65 mesh) or finer particles. A composition according to claim 1, wherein the magnesite contains at least 99% MgO, and the carbonaceous bonding agent is a novolak resin, a resol resin, tar, pitch, or a mixture thereof. A composition according to claim 1 or claim 2, wherein the graphite is a flake graphite or a vein graphite. A substantially metal-free unburned refractory shape comprising a baked composition according to any preceding claim. A shape according to claim 4, which is a brick. Metal processing equipment having a refractory lining of brick according to claim 5.

Inventors: BROOKS LEIGH F, KIRK DAVID ALAN, MICHAEL DAVID JAMES, BROOKS, LEIGH F., KIRK, DAVID ALAN, MICHAEL, DAVID JAMES
Applicants: DRESSER IND, DRESSER INDUSTRIES, INC.
Application year: 1989
Main classification: C04B35
IPC codes: C04B35, B22D41