Burned refractories with low water soluble chromium(VI)oxide content

Abstract: A burned chrome-containing refractory and method which provides a refractory having less than 5 pp, total soluble chromium. Such product is obtained by adding to the starting mix less than 10 wt. % (often less than 3 wt. %) zirconia, silica, zircon, or molybdic oxide either individually or in combination.

Description: BACKGROUND OF THE INVENTION This invention relates to chromium containing refractories and, more particularly, it concerns an improved burned refractory with low levels of soluble chromium. Chromium in its various oxide forms is a useful constituent of refractories because of its high melting point and its relative inertness when in contact with corrosive slags, gases, and molten metals. The main forms of chromium used in refractories are the mineral chromite -- a complex oxide consisting of about 30-60% Cr₂O₃ -- and pigment-grade chromium oxide (Cr₂O₃). Under certain conditions, the acid form of chromium, chromic acid (CrO₃), can form from these materials. After use, routine disposal of these refractories is of concern since CrO₃ (chromium VI) can be dissolved by water. Simple leaching of used chrome-bearing refractories by rain water or ground water may dissolve CrO₃ present in the refractory. The U.S Environmental Protection Agency has issued test methods to determine the level of water-soluble chromium from various materials and has specified a current (Yr. 1990) maximum allowable level of total water-soluble chromium (all valence states of chromium) to be no more than 5 mg/l (or 5 ppm). The test methods used to determine soluble chromium have evolved from the earliest method, the EP Method (Extraction Procedure), to the present method, the TCLP Method (Toxicity Characteristic Leaching Procedure - Method 1311). Since the EPA guidelines were announced, producers and consumers of chrome-bearing materials have been involved with determining the level of soluble chromium in their products. It is now apparent that different products containing similar levels of total chromium can differ in their soluble chromium levels. Therefore, a good correlation between total chromium and soluble chromium is not immediately apparent. Japanese Patent Publication JP-A-50133208 discloses a chromium oxide-magnesia refractory in which has been included molybdenum trioxide in order to improve slag resistance of the refractory. The refractory exemplified contained 1.0% molybdenum trioxide, 76% magnesia, 10.6% chromium oxide, 6.8% alumina, 4.2% iron oxide, 1.0% lime and 1.1% silica. That is, this refractory had a lime/silica ratio of less than 1. In light of the foregoing, there is a need for an improved refractory and method having a decreased level of soluble chromium, and in particular, a burned refractory having a level of soluble chromium reduced to below 5 ppm. SUMMARY OF THE INVENTION Accordingly, a principal object of the present invention is to provide a burned refractory with low soluble chromium. Another and more specific object of the invention is the provision of a method of reducing the soluble chromium levels of burned refractories. Other objects and further scope of applicability of the present invention will become apparent from the detailed description to follow in conjunction with the accompanying drawings and tables. In accordance with the present invention, it has been unexpectedly discovered that minor additions of fine zirconia, silica, zircon and/or molybdic oxide to burned chrome-containing refractories are effective in lowering the soluble chromium level of these compositions to acceptable values. This discovery is noteworthy, since it allows control of soluble chromium in a variety of chrome-bearing products. For example, products containing unacceptable levels of soluble chromium may now be made to contain low, acceptable levels of soluble chromium. Thus, in accordance with this invention, an additive is used which is selected from one or more of zirconia, silica, zircon, molybdic oxide, siliceous clay and a siliceous additive to lower the water soluble chromium content of a burned chromium-containing refractory, whose base mix has a lime/silica weight ratio greater than 1.1, to less than 5 ppm total water soluble chromium as defined by the Environmental Protection Agency's Test Method 1311. In accordance with a particularly preferred embodiment of the invention, the additive is one or more of zirconia, silica, zircon and molybdic oxide. Therefore in accordance with a particularly preferred embodiment of the invention an additive is used which is fine zirconia, silica, molybdic oxide and/or zircon to lower the soluble chromium content of a burned chromium-containing refractory to less than 5 ppm total soluble chromium, the additive being used in amounts of up to 10 wt. %. The additive may even be used in amounts less than 3 wt. %, or even less than 2%. Generally the additive will be used in amounts of at least 0.05 wt. %. The additive is preferably one or more of fine zirconia, zircon and molybdic oxide, most preferably being zirconia and/or molybdic oxide. The refractory may be any refractory composition but is preferably a magnesite-chrome or chrome-magnesite refractory. The invention also includes a method for lowering the water soluble chromium content of used chromium-containing refractories whose base mix has a lime/silica weight ratio greater than 1.1 to less than 5 ppm total water soluble chromium as defined by the Environmental Protection Agency's Test Method 1311 comprising crushing the used refractory, adding to the crushed material an additive selected from one or more of zirconia, silica, zircon, molybdic oxide, siliceous clay and a siliceous additive blending the crushed material and firing to a sufficient temperature to react the above additive with the water soluble chromium compounds present in the used refractory. The invention also includes the novel chromium-containing refractories obtainable by this process and having reduced total water soluble chromium content of less than 5 ppm as defined by the EPA's Test Method 1311. In accordance with an exemplary embodiment of the present invention, a burned 60% magnesite/40% chrome ore brick, which otherwise could have soluble chrome levels well in excess of 5 ppm, has a reduced soluble chromium level below the mandated 5 ppm maximum by either adding less than 2 wt. % zirconia, less than 0.5 wt. % silica, or less than 5 wt. % molybdic oxide. These amounts are meant to be illustrative rather than restrictive since it is readily apparent that compositions with higher amounts of soluble chromium may require larger amounts of these additives in order to reduce the level of soluble chromium to acceptable levels. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graphical representation of the influence of lime/silica ratio on soluble chromium in a burned 60% magnesite/40% chrome ore brick. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Some commercially available magnesite-chrome bricks have levels of soluble chromium that exceed the EPA guideline of 5 ppm or less. In accordance with the present invention, it is possible to produce such a refractory with an acceptable level of soluble chromium by adding minor amounts of zirconia, silica, zircon and/or molybdic oxide to the base mix. As shown in Table I, as little as 0.5% zirconia was quite effective in preventing formation of soluble chromium in a shape containing 1.3% CaO+SiO₂. An addition of 2% zirconia was sufficient to decrease the level of the soluble chromium to under 1 ppm in a shape containing 2% CaO+SiO₂. Table II illustrates the effect of 0 to 3% fine zirconia in a burned 60/40 magnesite-chrome shape. Increasing the zirconia level to amounts greater than 1% had a dramatic effect on decreasing the level of soluble chromium. The relative fineness of the zirconia addition did not appear to be as important as the quantity of zirconia. Table III shows the effect of fine silica additions to the base mix on the level of soluble chromium. Small additions of silica in amounts less than 0.5% were effective in significantly decreasing the amount of soluble chromium. The influence of the lime/silica ratio on the level of soluble chromium is diagrammed in Figure 1. To determine if other oxides, such as molybdic oxide would produce a similar effect, additions of fine MoO₃ were made to the base mix ranging from 0 to 20% (Table IV). It is apparent that molybdic oxide was also effective in decreasing the level of soluble chromium. Two percent MoO₃ was required to decrease soluble chromium to less than 1 ppm. The above descriptions clearly illustrate the pronounced effect that minor additions of silica, zirconia, and molybdic oxide have on the soluble chromium content of fired magnesite-chrome refractories. Since both zirconia and silica work to lower soluble chromium levels, then zircon (ZrO₂·SiO₂) would also be effective at low levels. It is also obvious that these additions would be effective in other burned chrome-bearing refractories such as chrome-magnesite shapes. It is conjectured that chromium 3+ (the most stable form of chromium) may convert to Cr 6+ in a high temperature environment, such as in a kiln. This conversion appears to be accelerated in the presence of an alkali or an alkaline earth compound. The above examples clearly show that additions of zirconia in accordance with the present invention tend to decrease the amount of soluble chromium. Possibly, zirconia has a stronger affinity for alkaline earth compounds (such as CaO) than chromium. Therefore, chromium in the presence of lime and zirconia tends to stay in the stable insoluble Cr₂O₃ form. In accordance with another embodiment of the present invention, molybdic oxide was also found to be effective in decreasing the level of soluble chromium. MoO₃ appears to be effective because it is stable in the +6 state, it is chemically similar to CrO₃, and it may have a higher affinity for alkalies or alkaline earth compounds than chromium. Furthermore, additions of, for example, zirconia, silica and molybdic oxide can be used not only to lower the soluble chromium content of unused refractories, but are also useful in decreasing the soluble chromium content of used refractories. For example, used chromium-bearing refractories could be crushed, mixed with the aforementioned additives, and burned to reduce their soluble chromium content. In accordance with an exemplary embodiment of the present invention, the soluble chromium content of used chromium containing refractories is lowered to less than 5 ppm total soluble chromium as defined by the Environmental Protection Agency's Test Method 1311 by crushing the used refractory, adding to the crushed material appropriate amounts of fine zirconia, silica, siliceous clay, siliceous additive, molybdic oxide, and/or zircon, blending and/or pressing the crushed material, and firing the material to a sufficient temperature to react to the above additions with soluble chromium compounds present in the used refractory. Although it is not required, the above-mentioned additions are preferably sized minus 200µm (-65 mesh and even more preferably minus 40µm (-325 mesh). Based upon these and other considerations, various forms of other elements theoretically could be used to decrease the levels of water-soluble chromium. These elements are: S, W, U, Ba, V, Nb, Ta, Fe, Eu, Mn, Tc, N, Cl, Se, Br, I, Re, Ru, Os, B, D, P, As, Sb, Te and Bi. Thus, it will be appreciated that as a result of the present invention, a magnesite-chrome brick having acceptable levels of soluble chromium is provided by which the principal object and others are completely fulfilled.

Claims: Use of an additive which is selected from one or more of zirconia, silica, molybdic oxide, zircon, siliceous clay and a siliceous additive to lower the water soluble chromium content of a burned chromium-containing refractory, whose base mix has a lime/silica weight ratio greater than 1.1, to less than 5 ppm total water soluble chromium. Use of an additive according to claim 1 in which the additive has a fine particle size and is used in amounts of up to 10 wt. %. Use according to claim 1 or claim 2 in which the additive is zirconia, silica, molybdic oxide and/or zircon, the additive being used in amounts of up to 10 wt. %. Use according to any preceding claim wherein the additive is used in amounts less than 3 wt. %, preferably less than 2 wt. %. Use according to any preceding claim wherein the additive has particle size -65 mesh (200µm). Use according to claim 5 wherein the particle size of the additive is -325 mesh (40µm). Use according to any preceding claim wherein the additive is zirconia, preferably used in amounts below 2 wt. %. Use according to any of claims 1 to 7 wherein the additive is silica, preferably used in amounts below 0.5 wt. %. Use according to any of claims 1 to 7 wherein the additive is molybdic oxide, preferably used in amounts below 3 wt. %. Use according to any of claims 1 to 7 wherein the additive is zircon, preferably used in amounts below 5 wt. %. Use according to claim 3 in which the additive comprises a mixture of fine zirconia, zircon and molybdic oxide. Use according to claim 3 wherein the additive is any combination of fine zirconia and molybdic oxide. Use according to any preceding claim in which the refractory is a magnesite-chromium or chromium-magnesite composition. A method of lowering the water soluble chromium content of a used chromium-containing refractory whose base mix has a lime/silica weight ratio greater than 1.1, to less than 5 ppm total water soluble chromium as defined by the Environmental Protection Agency's Test Method 1311 comprising crushing the used refractory, adding to the crushed base material an additive selected from fine zirconia, silica, molybdic oxide, zircon, siliceous clay and/or a siliceous additive, blending the crushed material and firing to a sufficient temperature to react the additive with the water soluble chromium compounds present in the used refractory. A method according to claim 14 in which the additive is added to the crushed base material in an amount up to 10 wt. %. A method according to claim 14 or claim 15 wherein the additive is fine zirconia, silica, molybdic oxide and/or zircon. A method according to any of claims 14 to 16 wherein the additive has a particle size -65 mesh (200 µm). A method according to claim 17 wherein the particle size of the additive is -325 mesh (40 µm). A method according to any of claims 14 to 18 wherein the additive is used in amounts above 0.05% and less than 3 wt. %, preferably less than 2 wt. %. A method according to any of claims 14 to 19 in which prior to firing, the crushed, used refractory and additive are blended and pressed. A chromium-containing refractory obtainable by a process according to any of claims 14 to 20. A chromium-containing refractory obtainable by a process comprising crushing a used chromium-containing refractory, adding to the crushed material an additive in an amount up to 10 wt. % of fine zirconia, silica, molybdic oxide and/or zircon, blending the crushed material and firing to a sufficient temperature to react the additive with the water soluble chromium compound present in the used refractory and wherein the refractory has a total water soluble chromium content of less than 5 ppm as defined by the Environmental Protection Agency's Test Method 1311.

Inventors: MCGOWAN KENNETH A, MICHAEL DAVID J, RENKEY ALBERT L, MCGOWAN, KENNETH A., MICHAEL, DAVID J., RENKEY, ALBERT L.
Applicants: INDRESCO INC, INDRESCO INC.
Application year: 1992
Main classification: C04B35
IPC codes: C04B35