Abstract:
A burned chrome-containing refractory and method which provides a refractory having less than 5 ppm 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:
This application is a continuation application of application Ser. No. 07/722,496, filed Jun. 27, 1991, now abandoned. 
    
    
     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% Cr 2  O 3  --and pigment-grade chromium oxide (Cr 2  O 3 ). Under certain conditions, the acid form of chromium, chromic acid (CrO 3 ), can form from these materials. After use, routine disposal of these refractories is of concern since CrO 3  (chromium VI) can be dissolved by water. Simple leaching of used chrome-bearing refractories by rain water or ground water may dissolve CrO 3  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. 
     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 
     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. 
     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. 
     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 taken in conjunction with the accompanying drawings and tables. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical representation of the influence of lime/silica ratio on soluble chromium. 
    
    
     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 molybdio 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+SiO 2 . An addition of 2% zirconia was sufficient to decrease the level of soluble chromium to under 1 ppm in a shape containing 2% CaO+SiO 2 . 
     
                                           TABLE I__________________________________________________________________________Effect of Zirconia Additions on Soluble ChromiumBrick Type:      Burned 60/40 Magnesite-Chrome                                 Comparative DataMix:             A  B  C  D  E  F  G  1990 EPA Guidelines__________________________________________________________________________Lime Content of Mix, %            0.94               0.91                  0.87                     1.38                        1.38                           1.45                              1.40Silica Content of Mix, %            0.41               0.41                  0.47                     0.60                        0.63                           0.68                              0.74Lime/Silica Ratio of Mix:            2.3               2.2                  1.9                     2.3                        2.1                           2.1                              1.9Total Cr.sub.2 O.sub.3 Content of Mix:            19.8               20.3                  20.0                     20.2                        18.6                           21.5                              20.0Addition to Mix:Zirconia, -325 mesh, %            0.0               0.5                  1.0                     0  0.5                           1.0                              2.0TLCP Toxicity TestHexavalent Chromium (Cr), mg/l            5.2               2.5                  0.5                     5.0                        3.1                           1.5                              0.4Total Chromium (Cr), mg/l            5.4               2.5                  0.6                     8.3                        3.4                           1.7                              0.4                                 5.0 (max.)__________________________________________________________________________ 
    
     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% has 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 II__________________________________________________________________________Effect of Zirconia Additions on Soluble ChromiumBrick Type:      Burned 60/40 Magnesite-Chrome                            Comparative DataMix:             A  B  C   D  E  1990 EPA Guideline__________________________________________________________________________Lime Content of Mix, %            0.9               0.9                  0.8 0.8                         0.8Silica Content of Mix, %            0.4               0.4                  0.4 0.4                         0.4Lime/Silica Ratio of Mix:            2.3               2.2                  2.2 2.1                         2.2Total Cr.sub.2 O.sub.3 Content of Mix, %            17.9               21.0                  19.0                      20.2                         18.7Addition to Mix:Zirconia, -44 microns            0.0               1.0                  2.0 3.0                         --Zirconia, -20 microns            -- -- --  -- 3.0TLCP Toxicity TestHexavalent Chromium (Cr), mg/l            4.8               0.4                  0.2 0.2                         0.3Total Chromium (Cr), mg/l            4.9               0.5                  0.7 0.2                         0.3                            5.0 (max.)__________________________________________________________________________ 
    
     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. 
     
                                           TABLE III__________________________________________________________________________Effect of Silica Additions on Soluble ChromiumBrick Type:      Burned 60/40 Magnesite-ChromeMix:             A  B  C  D  E  F  G  H  I  J  K__________________________________________________________________________Lime Content of Mix, %            1.53               1.47                  1.42                     1.45                        1.53                           0.97                              0.94                                 0.91                                    0.93                                       0.93                                          0.91Silica Content of Mix, %            0.73               0.77                  0.85                     0.97                        1.17                           0.42                              0.47                                 0.50                                    0.61                                       0.75                                          0.84Lime/Silica Ratio of Mix:            2.1               1.9                  1.7                     1.5                        1.3                           2.3                              2.0                                 1.8                                    1.5                                       1.2                                          1.1Total Cr.sub.2 O.sub.3 Content of Mix, %            21.8               20.9                  19.1                     19.3                        22.4                           21.3                              21.6                                 20.4                                    21.0                                       21.4                                          20.5Addition to Mix:Sub-Micron Silica            0.0               0.1                  0.2                     0.35                        0.5                           0.0                              0.05                                 0.1                                    0.2                                       0.35                                          0.5TLCP Toxicity TestHexavalent Chromium (Cr), mg/l            7.5               5.3                  2.3                     0.8                        0.3                           5.0                              3.7                                 2.8                                    0.7                                       0.2                                          0.2Total Chromium (Cr), mg/l            8.5               5.4                  3.0                     1.1                        0.7                           5.3                              3.7                                 3.4                                    0.9                                       0.5                                          0.4__________________________________________________________________________ 
    
     The influence of the lime/silica ratio on the level of soluble chromium is diagrammed in FIG. 1. 
     To determine if other oxides, such as molybdic oxide would produce a similar effect, additions of fine MoO 3  were made to the base mix ranging from 0 to 2% (Table IV). It is apparent that molybdic oxide was also effective in decreasing the level of soluble chromium. Two percent MoO 3  was required to decrease soluble chromium to less than 1 ppm. 
     
                                           TABLE IV__________________________________________________________________________Effect of Molybdic Oxide (MoO.sub.3) Additions on Soluble ChromiumBrick Type:      Burned 60/40 Magnesite-Chrome                                 Comparative DataMix:             A  B  C  D  E  F  G  1990 EPA Guidelines__________________________________________________________________________Lime Content of Mix, %            0.97               0.91                  0.87                     0.80                        1.36                           1.36                              1.33Silica Content of Mix, %            0.42               0.44                  0.42                     0.40                        0.68                           0.68                              0.68Lime/Silica Ratio of Mix            2.3               2.1                  2.1                     2.0                        2.0                           2.0                              2.0Total Cr.sub.2 O.sub.3 Content of Mix, %            21.3               20.0                  19.4                     19.2                        19.8                           20.8                              20.8Addition to Mix:Molybdic Oxide, -325 mesh, %            0.0               0.5                  1.0                     2.0                        0.5                           1.0                              2.0TLCP Toxicity TestHexavalent Chromium (Cr), mg/l            5.2               3.0                  1.3                     0.4                        5.5                           2.6                              0.5Total Chromium (Cr), mg/l            5.3               3.1                  1.5                     0.4                        5.8                           2.6                              0.5                                 5.0 (max.)__________________________________________________________________________ 
    
     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 (ZrO 2  ·SiO 2 ) 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 Cr 2  O 3  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. MoO 3  appears to be effective because it is stable in the +6 state, it is chemically similar to CrO 2 , 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&#39;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 -65 mesh, and even more preferably minus -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. It is contemplated and will be apparent to those skilled in the art from the foregoing description and accompanying drawing and tables that variations and/or modifications of the disclosed embodiment may be made without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawing and tables are illustrative of a preferred embodiment only, not limiting, and that the true spirit and scope of the present invention be determined by reference to the appended claims.