Patent Abstract:
A process for producing brick involves calcining electric arc furnace dust and subsequently mixing the dust with a brick raw material prior to shaping the material into bricks and firing. The precalcination of the EAF dust greatly increases the bulk density of the dust and provides a brick having a much higher compressive strength when compared to bricks incorporating raw dust which has not been precalcined. Volatilized and recondensed heavy metals produced during the calcination process are collected and refined. As a result of the separation and collection of the volatilized heavy metals during the calcination process, substantially no heavy metals are volatilized during the brick firing process.

Full Description:
FIELD OF INVENTION 
     The present invention relates to the use of electric arc furnace dust as a raw material in a brick making process. 
     BACKGROUND OF THE INVENTION 
     Electric arc furnace (EAF) dust is the dust collected from an electric arc furnace for melting scrap in a steel making process. EAF dust is classified as a hazardous waste by the Environmental Protection Agency and is designated the identification K061. EAF dust has a large heavy metals content, particularly lead, which is the main reason why it is classified as a hazardous material. 
     Tables I and II below show chemical analysis of EAF dust produced over a four month period during the manufacture of carbon steel (Table 1) and stainless steel (Table 2). 
     
                       TABLE I______________________________________Carbon SteelTotal (%)    Jan.   Feb.       Mar. Apr.______________________________________C            20.0   20.0       20.0 20.0Ca           19.60  21.00      24.50                               23.10Fe           28.8   25.50      17.10                               17.40Zn           16.40  14.30      9.85 11.90Mn           2.20   2.05       2.25 2.15Pb           2.55   1.70       1.48 1.21Na           0.76   0.67       0.91 0.63Si           0.13   0.33       0.29 0.31Al           0.41   0.39       0.37 0.26Mg           0.75   0.55       0.48 1.05Cr           0.17   0.25       0.33 0.42Cu           0.21   0.12       0.12 0.09Ni           0.13   0.08       0.12 0.02Cd           0.04   0.02       0.02 0.02Mo           0.02   0.02       0.06 0.05K            1.50   1.00       1.35 1.07______________________________________ 
    
     
                       TABLE II______________________________________Stainless SteelTotal (%)    Jan.   Feb.       Mar. Apr______________________________________C            20.0   20.0       20.0 20.0Ca           2.50   25.2       16.0 13.0Fe           30.0   26.1       21.6 24.3Zn           8.10   10.5       5.60 5.35Mn           1.90   1.95       1.75 1.65Pb           0.90   1.10       1.01 0.71Na           0.48   0.53       0.72 0.44Si           0.06   0.38       0.30 0.26Al           0.28   0.40       0.45 0.23Mg           0.85   0.48       0.62 0.88Cr           2.27   0.30       1.90 1.90Cu           0.16   0.10       0.11 0.09Ni           0.11   0.07       0.10 0.04Cd           0.02   0.02       0.01 0.01Mo           0.45   0.02       0.04 0.06K            1.10   0.90       0.95 0.60______________________________________ 
    
     Tables III and IV show parts by weight of different components found in the leachate of the same carbon steel samples shown in Table I and the leachate of the same stainless steel samples shown in Table II. In Tables III and IV, hexavalent chrome is identified as Cr(6). 
     
                       TABLE III______________________________________Carbon-Leachate (mg/l)Total     Jan.    Feb.       Mar.  Apr.______________________________________Cr(6)     0.01    0.41       1.48  0.44Cr        0.22    0.46       1.67  0.45Pb        145.00  110.00     39.00 76.80Cd        0.02    0.02       0.02  0.02Ag        0.04    0.04       0.05  0.06Ba        2.50    1.60       2.40  1.10Hg        0.0022  0.0054     0.0011                              0.0046______________________________________ 
    
     
                       TABLE IV______________________________________Stainless-Leachate (mg/l)Total     Jan.    Feb.       Mar.  Apr.______________________________________Cr(6)     4.30    0.68       2.63  6.30Cr        6.40    0.83       2.63  6.62Pb        0.26    60.5       64.0  37.2Cd        1.93    0.02       0.02  0.02Ag        0.04    0.06       0.05  0.06Ba        1.2     1.40       1.0   1.0Hg        0.0019  0.0047     0.0010                              0.0005______________________________________ 
    
     Tables III and IV indicate that a significant amount of lead and other metals in the dust are prone to leaching out. For this reason, EAF dust cannot be buried in the ground for disposal. Instead, some current methods require combining the dust with a heavy steel material to form a waste product which will not leach. Discarding EAF dust in this manner is an expensive process. EAF dust further poses a health risk due to heavy metal pollution by airborne fumes. 
     In a typical steel manufacturing process, EAF dust is produced in amounts of between about 0.7 and 1.6% based upon the total amount of steel produced. A need exists for a method of discarding heavy metal dust which method is both inexpensive and safe. 
     Japanese Publication No. 53-127511 discloses a method of using EAF dust in the production of bricks. The dust is mixed with a brick raw material, formed into bricks, and fired thereby incorporating the hazardous dust into the bricks. According to the Japanese publication, volatilized and recondensed metals produced during the brick firing process are collected in a baghouse on the kiln exhaust. 
     According to the Japanese publication, raw EAF dust is added in quantities from 30 to 50 weight percent to a normal clay brick mix, and fired in a tunnel kiln at between 630° and 830° C. in a normal manner. Volatilized and recondensed heavy metals are collected in a baghouse on the kiln exhaust. In a reproduction of the process taught in the Japanese publication, EAF dust was added in an amount of 40% by weight to a normal brick body and fired in a similar manner to that described. The resultant bricks were very friable and exhibited compressive strengths which averaged only 690 psi. Table V shows the results of the compressive strength test on five samples made according to the reproduction method. The bricks were tested flatwise. ASTM rated bricks in the United States are required to have a minimum allowable compressive strength of 1500 psi. Thus, a reproduction of the Japanese process failed to produce brick strong enough for U.S. standards. Further, it is expected that a significant amount of heavy metals may leach out of these weak bricks. 
     
                                           TABLE V__________________________________________________________________________40% BY WEIGHT RAW EAF DUST        5 Hour   24 Hour Compressive        Submersion                 Submersion                         Maximum Strength psi        in Boiling Water                 in Cold Water                         SaturationSample # (Gross Area)        % absorption                 % absorption                         Coefficient__________________________________________________________________________1     710    29.9     23.3    0.782     780    29.5     22.8    0.783     620    30.2     23.7    0.794     630    30.1     23.6    0.785     720    29.6     23.1    0.78Average 690    29.9     23.3    0.78__________________________________________________________________________ 
    
     It is therefore desirable to provide a method of incorporating EAF dust into a raw brick material and producing a brick which exhibits a high compressive strength and no significant leaching of heavy metals. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems presented in the prior art by providing a process wherein electric arc furnace dust is first heated so as to be calcined and subsequently mixed with a brick raw material such as a normal clay and/or shale mixture prior to shaping the material into bricks and firing. The precalcination of the EAF dust greatly increases the bulk density of the dust and provides a brick having a much higher compressive strength when compared to the incorporation of raw dust according to the method of the Japanese publication mentioned above. According to the present invention, volatilized and recondensed heavy metals produced during the calcination process are collected and refined. As a result of the collection of the volatilized heavy metal dust during the calcination process, substantially no heavy metals are volatilized during the brick firing process. Therefore, the hot gases present and produced during the brick firing process can be used for other heating requirements during the brick making process without the need for a gas cleaning step to remove volatilized components. 
     Bricks produced in accordance with the present invention exhibit very high compressive strength and no significant leaching of heavy metals. The process provides a safe and inexpensive method of forming a harmless product from a hazardous waste material. 
     According to an embodiment of the present invention a method is provided which comprises the steps of calcining electric arc furnace dust at a temperature of about 1040° C. or higher, mixing the calcined dust with a brick raw material to form a mixture comprising up to about 60%, preferably from about 20 to about 60% by weight calcined dust, forming the mixture into bricks, and firing the bricks at a temperature of between about 900° and about 1370° C. 
     According to another embodiment of the present invention a halogen or reducing agent is added in an amount of up to about 20% by weight to the electric arc furnace dust prior to calcining. The halogen or reducing agent chemically mixes with the heavy metals, lead in particular, to form new metal compounds. These lead compounds are thus formed which have lower volatilization temperatures enabling lead to be almost totally removed from the brick end product by heating at low temperatures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention may be more fully understood with reference to the accompanying drawings, wherein: 
     FIG. 1 is a flow diagram showing a process according to an embodiment of the present invention for producing brick; and 
     FIG. 2 is a graph showing the percentage of weight change versus temperature of an EAF dust sample during calcination. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention utilizes electric arc furnace dust as an additive for a brick manufacturing process. The EAF dust is first calcined by heating it to a temperature of about 1040° C. or higher and then mixed with a brick raw material such as a normal clay and/or shale mixture. The mixture may contain up to about 60% by weight calcined EAF dust. Preferably, the brick raw material is present in an amount of between about 40 and about 80 percent by weight. The raw material incorporating the calcined EAF dust is then processed in a substantially standard manner. One example of a method for processing brick according to the present invention is shown in FIG. 1. In the process shown in FIG. 1, the EAF dust is mixed with a reducing agent and calcined. The exhaust from the calcination is then sent through a pollution control and recovery process. After the calcined dust is mixed with shale in the crusher, the rest of the brick making process continues in a standard manner depicted within the dotted lines. The resulting bricks pass all ASTM compression and absorption tests and all EPA leachate tests including EP toxicity and TCLP. 
     In a preferred embodiment, a rotary, indirect fired calciner is used. By precalcining the EAF dust, the products of combustion do not come in contact with the brick raw material but are instead cooled as discussed above enabling the heavy metals in the combustion product to condense in a particulate form. 
     The EAF dust is calcined at a temperature of about 1040° C. or higher so as to substantially increase the bulk density of the dust and produce a calcined EAF dust. In one preferred embodiment, the dust is calcined at a temperature at 1120° C. or higher. Temperatures of greater than 1260° C. are not necessary since an almost complete calcination of the dust occurs at a temperature of between about 1175° and about 1205° C. 
     During the calcining process of the EAF dust, products of combustion and volatilized heavy metals are cooled to about 200° C. At or above this temperature the heavy metals condense in a particulate form that is easily captured in a baghouse. The captured heavy metals and oxides thereof are sent to secondary metal refineries for further processing to useable elements such as lead for automotive batteries. 
     After the calcined dust is mixed with standard brick raw material, the mixture is then formed into bricks and fired at a temperature of between about 900° and 1370° C. for a period of about 6 to about 48 hours, depending primarily on the brick raw material and the kiln used. A more preferred range of temperatures for firing the brick is from about 980° to about 1100° C. A more preferred firing period lasts at least 8 hours. 
     According to the present invention, it has been determined that precalcining the EAF dust prior to its addition to a brick raw material produces bricks upon firing which exhibit numerous significant advantages. The present invention provides a process for an environmentally safe and economical method of recycling a hazardous waste material of which over 800,000 tons are produced annually in the United States. The process also preserves natural resources by replacing clay and shale raw materials with otherwise useless EAF dust. The present invention also improves the quality of bricks produced by increasing the density of the brick. Unique and desirable colors may also be achieved according to the present invention which are substantially uniform throughout the brick body, not just on the surface. In an oxidizing atmosphere, bricks can be produced which have a golden-brown color. In a reducing atmosphere, brick having a dark, chocolate brown color may be produced. 
     The bricks according to the present invention pass ASTM requirements for compressive strength, cold and boiling water absorption, and saturation coefficient. As can be seen in Tables VIa-VId, bricks made according to the present invention containing from 30% to 60% by weight calcined EAF dust exhibit excellent strength and absorption characteristics. 
     
                                           TABLE VIa__________________________________________________________________________30% BY WEIGHT CALCINED EAF DUST        5 Hour   24 Hour Compressive        Submersion                 Submersion                         Maximum Strength psi        Boiling Water                 Cold Water                         SaturationSample # (Gross Area)        % absorption                 % absorption                         Coefficient__________________________________________________________________________1     5600   14.4     10.3    0.712     5380   14.3     10.3    0.723     6310   14.3     10.3    0.724     6230   14.2     10.3    0.735     7050   13.2      9.4    0.71Average 6110   14.1     10.1    0.72__________________________________________________________________________ 
    
     
                                           TABLE VIb__________________________________________________________________________40% BY WEIGHT CALCINED EAF DUST        5 Hour   24 Hour Compressive        Submersion                 Submersion                         Maximum Strength psi        Boiling Water                 Cold Water                         SaturationSample # (Gross Area)        % absorption                 % absorption                         Coefficient__________________________________________________________________________1     5390   16.0     11.5    0.722     5890   15.8     11.4    0.723     5140   16.4     11.9    0.724     4670   16.3     12.0    0.735     4990   16.0     11.7    0.73Average 5220   16.1     11.7    0.72__________________________________________________________________________ 
    
     
                                           TABLE VIc__________________________________________________________________________50% BY WEIGHT CALCINED EAF DUST        5 Hour   24 Hour Compressive        Submersion                 Submersion                         Maximum Strength psi        Boiling Water                 Cold Water                         SaturationSample # (Gross Area)        % absorption                 % absorption                         Coefficient__________________________________________________________________________1     3440   19.4     14.4    0.742     3470   19.9     14.9    0.753     3370   19.3     14.4    0.754     3500   19.3     14.5    0.755     3780   19.0     13.8    0.73Average 3510   19.4     14.4    0.74__________________________________________________________________________ 
    
     
                                           TABLE VId__________________________________________________________________________60% BY WEIGHT CALCINED EAF DUST        5 Hour   24 Hour Compressive        Submersion                 Submersion                         Maximum Strength psi        Boiling Water                 Cold Water                         SaturationSample # (Gross Area)        % absorption                 % absorption                         Coefficient__________________________________________________________________________1     1660   24.1     18.9    0.782     1860   23.5     18.4    0.783     1660   23.9     19.0    0.804     1620   24.1     19.2    0.805     1730   23.4     18.9    0.81Average 1710   23.8     18.9    0.79__________________________________________________________________________ 
    
     Thermal gravimetric analysis (TGA) and lab furnace calcining tests, shown in FIG. 2 and Table VII, indicate that upon heating to 1120° C., EAF dusts loses about 17% of its weight. The dust also shrinks or agglomerates such that its bulk density increases by a factor of greater than 4. The shrinkage of the EAF dust accounts for the high percentage of voids and the extremely low compressive strength in the brick made according to the method of the Japanese publication mentioned above. Two samples comprising raw EAF dust were put into clay crucibles (1.2 in. sample depth). The crucibles were placed into a preheated electric kiln set at 1121° C. (2050° F.). One crucible was taken out after 0.5 hr. and the other after 1.0 hr. The powder had a considerable shrinkage and became semi-fused into small cones. The resultant properties of the dust are shown in Table VII below. 
     
                       TABLE VII______________________________________WEIGHT LOSS:      0.5 hr. sample                  17.10%      1.0 hr. sample                  17.12%BULK       Green       33.0 lb/ft.sup.3DENSITY:   0.5 hr.    129.3 lb/ft.sup.3      1.0 hr.    139.5 lb/ft.sup.3COLOR:     Green      Medium brown      0.5 hr.    Medium brown w/green tones      1.0 hr.    Medium brown w/green tones______________________________________ 
    
     As can be seen, the bulk density increased by a factor of greater than 4 in the 1.0 hour sample. Although the samples were agglomerated into cones, they could still be broken apart with prodding. All of the powder would go through a 140 mesh screen. The angle of repose was about 35 degrees. Calcination times generally range from about 0.5 to about 1 hour. Preferably, a calcination period of at least 0.5 hour is employed. 
     The weight loss is attributed to free water loss between 25° and 240° C., carbon oxidation between 240° and 440° C., and ZnCO 3  loss between 540° and 850° C. The weight loss between 920° and 1120° C. is unknown but believed to be from volatilized lead compounds. 
     Lead is the predominant heavy metal contaminant in EAF dust. Elemental lead boils at 1751° C. and lead oxide (PbO) boils at about 1472° C. These temperatures are much higher than typical 1100° C. firing temperatures for brick. Lead in the brick produced by the method set forth in the Japanese publication discussed above remains in the brick product and may possibly leach out in service. 
     According to an embodiment of the present invention, lead in the EAF dust is volatilized at low temperatures of approximately 1100° C. Under some conditions, the lead can be almost totally removed from the brick end product at these low temperatures. In order to volatilize the lead at these temperatures and even lower temperatures, such at 1040° C., small amounts of reducing agents or halogens are added to the lead to produce lower boiling lead compounds, e.g. lead chloride. Only a very small amount of reducing agent or halogen is necessary to produce the new lower boiling lead compounds. According to an embodiment, up to about 6% by weight halogen is added to the dust prior to calcining. 
     According to one embodiment of the present invention, a reducing agent is added in an amount of up to about 20% by weight based on the total weight of the electric arc furnace dust prior to calcining the dust. The reducing agent is preferably a scrap material or other cheap material which may itself contain heavy metals. One example is scrap wire insulation which, like EAF dust, is expensive to dispose of. Other reducing agents which may be used include, but are not limited to, coal, coke, sawdust and waste oils. As little as 1% reducing agent has a significant effect on the volatilization of lead from the EAF dust. In fact, as shown in Table VIII below, the addition of 1% coal yields even better results than the addition of 2 or 4% coal. By adding 1% coal to the EAF dust, the weight percent lead in the dust dramatically decreased from 7.408 weight percent to only 0.802 weight percent. Practically no reducing agent remains in the finished brick. 
     
                       TABLE VIII______________________________________               WEIGHT % Pb______________________________________Raw Electric Arc Furnace (EAF) dust                 7.408EAF dust calcined with 1% NaCl                 0.915EAF dust calcined with 2% NaCl                 0.565EAF dust calcined with 4% NaCl                 0.658EAF dust calcined with 1% Coal                 0.802EAF dust calcined with 2% Coal                 1.064EAF dust calcined with 4% Coal                 1.154______________________________________ 
    
     Any of the halogens may be added to produce a similar effect as that of the reducing agents. The preferred halogens are the most inexpensive ones, for example, chlorine from sodium chloride. As shown in Table VIII, the addition of 2% by weight sodium chloride based on the total amount of raw electric arc furnace dust gives the greatest reduction in total lead in the calcined product, specifically, a 92.4% reduction. Depending upon the composition of the halogen-containing additive, amounts between about 1 and about 6% by weight are preferred. Only a trace amount of the halogen remains in the finished brick. 
     While the range of calcination temperatures between 1037° and 1260° C. is preferred, it has been found that EAF dust containing greater amounts of calcium require the higher range of these temperatures. 
     Bricks produced according to the present invention also exhibit minimal leaching of heavy metals. The brick easily passes TCLP requirements for leaching. In one example, an EAF dust to which 4% coal was added was calcined at about 1120° C. The calcine was crushed and leach tested according to the EPA&#39;s well known TCLP test. Only 0.18 milligrams of lead per liter leached out of the reduced calcine. It is expected that leaching rates of about 0.2% or lower are possible in any of the calcines produced according to the present invention. The maximum allowable leach rate in the United States is 5.0 mg/l. 
     The low level of leaching provides distinct operational advantages for brick manufacturing in that the calcine may be stored without risk of it leaching into the environment. While the EAF dust is stored, other non-EAF dust brick may be manufactured at the plant. The prior art, on the other hand, requires that the dust be introduced directly into the brick raw material and not stored, thus limiting production of other types of brick at the plant. 
     The bricks produced according to the present invention exhibit compressive strengths and leaching properties which pass all requirements of the Environmental Protection Agency. The process takes advantage of an otherwise hazardous waste material while at the same time preserving natural resources. 
     Although the present invention has been described in connection with preferred embodiments, it will be appreciated by those skilled in the art that additions, modifications, substitutions and deletions not specifically described may be made without departing from the spirit and scope of the invention defined in the appended claims.

Technology Classification (CPC): 2