Process of calcining limestone or hydrated lime in a rotary kiln
BACKGROUND OF THE INVENTION 
     This invention relates to a process of calcining limestone or hydrated lime by a thermal treatment with hot gases in an oxidizing atmosphere in a rotary kiln through which the kiln atmosphere and the charge are countercurrently conducted and which is fed with fuel through shell burners. 
     DISCUSSION OF THE PRIOR ART 
     The rotary kiln is increasingly used to produce quicklime of high reactivity from limestone or hydrated lime because lime of higher quality is better produced therein than in a shaft kiln. On the other hand, the rotary kiln has a higher heat consumption. Such lime is often used in steel production and should have only a low sulfur content. The sulfur content is mainly due to the sulfur content of the fuels employed. The sulfur content of quicklime is minimized mainly by the use of fuels having a low sulfur content (Zement-Kalk-Gips, No. 2, 1969, pp. 75-81). Because fuels having a low sulfur content are more expensive than fuels having a high sulfur content, the use of the former adds to the costs of the process, particularly owing to the higher heat consumption of the rotary kiln. From French Pat. No. 1,487,240; U.S. Pat. No. 2,941,791; and German Pat. No. 618,872 it is known to supply fuels into the kiln through shell burners or shell nozzles. It is also known to minimize the sulfur content of the quicklime in that the latter is calcined in a neutral or reducing atmosphere (German Pat. No. 1,108,603). In such process, the central burner arranged at the discharge end of the kiln can be operated with an oxygen deficiency if part of the oxygen is supplied to the kiln at a succeeding point through shell openings (Czechoslovakian Pat. No. 127,978). On the other hand, a calcination in a neutral or reducing atmosphere involves a risk of temperature fluctuations and local overheating if the oxygen supply varies. For this reason the kiln must be very well sealed to prevent an inleak of air, but even expensive sealing means cannot completely prevent such air inleak. 
     It is an object of the invention to produce a quicklime which has a high reactivity and a low sulfur content in a process which involves low fuel costs and can be carried out with inexpensive equipment. 
     This object is accomplished according to the invention in that different fuels which serve to produce hot gases and which release SO 2  and SO 3  in different quantities per unit of calorific value are introduced into the rotary kiln in such a manner that the fuel which releases a larger quantity of SO 2  and SO 3  per unit of calorific value is supplied to the upper portion of the rotary kiln and the fuel which releases a smaller quantity of SO 2  and SO 3  per unit of calorific value is supplied to the lower portion of the rotary kiln. The upper portion of the rotary kiln extends from the charging end toward the discharge end. The lower portion extends from the discharge end toward the charging end. The fuel which is supplied to the upper portion of the rotary kiln through shell burners may be gaseous. Alternatively, solid fuel in the form of dust may be supplied through the shell burners. Additionally, a portion of the fuel may be charged as solid fuel into the charging end of the rotary kiln if that fuel releases SO 2  and SO 3  only in small quantities, e.g. less than 0.25 mg S/kJ (milligram sulfur per kilo Joule). The lower portion of the rotary kiln may be supplied with fuel through shell burners and/or a central burner., That fuel can also consist of a gas, liquid or dust. Part or all of the fuel to be supplied to the lower portion of the rotary kiln may be supplied in lump form and may be pneumatically or mechanically thrown into the kiln from the discharge end and may thus be distributed on the charge, provided that such fuel releases only small quantities of SO 2  and SO 3 . All of the oxygen-containing gases required to burn the carbonaceous fuels generally air can be supplied through the shell burners and/or the central burner. Alternatively, part of the oxygen-containing gases may be supplied through shell tubes, particularly when solid fuel is supplied to the charge of the rotary kiln. 
     The permissible sulfur content of the fuels will depend on the requirements to be met regarding the sulfur content of the quicklime, and on the quantities in which SO 2  and SO 3  are released from the fuels. Where solid fuels are used, the quantities in which SO 2  and SO 3  are released depend on the basicity of the ash which is present, i.e., on the ratio of CaO to SiO 2  in said ash. A higher ratio of CaO to SiO 2  results in a release of SO 2  and SO 3  in smaller quantities and vice versa. That behavior may be influenced in the desired sense by addition of basic (alkaline) components. The difference between the quantities in which SO 2  and SO 3  are released by the fuels supplied to the upper and lower portions of the rotary kiln depends also on the quantity of sulfir which enters the lime and the desired sulfur content of the lime. The terms &#34;shell burner&#34; and &#34;shell tubes&#34; describe radially extending burners and tubes, respectively, which extend radially through the shell of the kiln and have outlet openings disposed approximately at the center of the kiln and extending approximately parallel to the longitudinal axis of the kiln. 
     According to a preferred feature of the invention, the fuel supplied to the lower portion of the rotary kiln releases at least 30% less SO 2  and SO 3  per unit of calorific value than the fuel supplied to the upper portion of the rotary kiln. This practice results in a lime having a low sulfur content and permits of the supply of fuels which release SO 2  and SO 3  in relatively large quantities to the upper portion of the rotary kiln. 
     According to a preferred feature, the upper portion of the rotary kiln includes the length portion from the charging end to that point at which 50-90%, preferably 80-90% of the CO 2  content or the water of hydration content of the charge have been driven off. As a result, only a small quantity of sulfur from the larger quantities of SO 2  and SO 3  released by the fuel per unit of the calorific value enters the lime. 
     According to a preferred feature, the oxygen-containing gases required for burning the fuels are supplied through shell tubes and/or shell burners at controlled rates in such a manner that the charge is maintained throughout the length of the calcining zone at a temperature of 900°-1100° C., preferably 950°-1050° C. This results in a quicklime which has a high reactivity and a low sulfur content. 
     According to a preferred feature, the length of the calcining zone is at least 50% and preferably 55-70% of the kiln length. Particularly good results are produced with a calcining zone having that length. The calcining zone may have a length of about 90% of the length of the kiln if the latter is fed with a highly preheated charge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The attached drawing shows schematically a typical rotary kiln which can be used in the process of this invention. According to the drawing there is shown a rotary furnace 1 which is suspended on curved rails 2 for rotary movement. Curved rails 2 are supported by rail supports 3. The rotary furnace 1 is rotated by a driving mechanism 4 consisting of a motor and a ring gear. The charge 5 is charged through supply conduit 6 into the charging end of the kiln 1. The bed 7 of the charge travels through the kiln 1 and is discharged from the discharge end of the kiln and head 8. Shell burners 9 are disposed along the length of the kiln, which are fed through conduit 10 with fuel and through conduit 11 with oxygen containing gas (only shown for one shell burner 9). The kiln atmosphere moves in the direction of arrow 12 and the waste gas 13 leaves the kiln through head 14. 
     The shell burners 9 in the upper portion U of the kiln 1 are fed with a fuel which releases a larger quantity of SO 2  and SO 3  per unit of calorific value and the shell burners in the lower portion L are fed with a fuel which releases a higher quantity of SO 2  and SO 3 . The length of the upper portion U is adjusted such that 50% to 90% of the CO 2  content or the water of hydration content of the charge are driven off in this portion. The calcining zone C has a temperature of the charge from 900° to 1100° C. and a length of 55-70% of the kiln length and can have a length of about 90% if the charge is fed in a highly preheated condition. The beginning of the calcining zone C can about coincidence with the beginning of the lower zone L if the length of the upper zone U is chosen such that a low percentage of the SO 2  content or the water of hydration content of the charge is driven off in zone U. If a high percentage is driven off in zone U, than zone C overlaps the last part of zone U. 
     The FIGURE is not shown in true size and the positions and number of shell burners is also shown only schematically. 
     The advantages of the invention reside mainly in that fuels which release SO 2  and SO 3  in relatively large quantities may be used to produce a quicklime which has a high reactivity and a low sulfur content in a process which involves low fuel costs and a low structural expenditure. 
     EXAMPLE 
     Limestone containing 53% CaO was treated in a pilot rotary kiln in oxidizing atmosphere with hot gas which was produced by combustion of fuel oil. The calcining temperature was 980° C. 
     In a first run the total thermal treatment was performed with hot gases produced by combustion of fuel oil which contained 5% sulfur. The calcined product contained 0.5% CO 2  and 0.17% S. 
     In a second run the limestone was first treated with the same gas until 80% of its CO 2  -content were driven off. Then the treatment was performed with hot gas produced by combustion of fuel oil containing 0.5% sulfur. The calcined product contained 0.5% CO 2  and the highest sulfur content was 0.05%. 
     In other runs it was found that the fuel oil used in the lower portion of the rotary kiln should have not more than 0.25 mg S/kJ in order to obtain a calcined product having low contents of CO 2  and S, whereas the fuel used in the upper portion of the kiln can have above 0.8 mg S/kJ.