Abstract:
A method for recovery of metals or metal compounds from metal-bearing solid particles comprising volatile compounds comprises supplying the solid particles ( 50 ) through a flame ( 25 ) of a burner ( 20 ). The volatile compounds are then evaporated by means of heat from the flame ( 25 ) without melting the solid particles, thereby providing a raw material product ( 34 ). The raw material product ( 34 ) is then recovered. By feeding the solid particles directly to the flame, a compact and efficient plant is provided. The melting process can be controlled in a satisfying way, avoiding melting of the metal-bearing solid particles supplied through the burner. The use of a burner is also described.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates generally to a method for calcining of metal-bearing solid particles or dust. The invention also relates to the use of an apparatus for performing such a method.  
       BACKGROUND  
       [0002]     Large amounts of environmentally troublesome industrial wastes are generated in different processes in metallurgic industry. Such wastes include but are not limited to electric arc furnace (EAF) steel dusts and foundry wastes as well as zinc-bearing feeds, including foundry dusts and sludges.  
         [0003]     A prior art plant for metals recycling from steelmaking and foundry wastes is described in the article “Metals recycling from steelmaking and foundry wastes by Horsehead resource development” published in 1992 Electric furnace conference proceedings, pages 145-157. This prior art plant is shown in  FIG. 1 . The plant disclosed in this article comprises rotary kilns, which are arranged to heat the process materials and thereby provide a recycling process. This process includes two major steps. In a first step the starting material, such as a mixture of zinc bearing feed and coal or coke is fed to a first kiln and the starting material is divided into iron-rich material and dust. The dust, which is collected in a filter, comprises zinc oxide and other compounds and elements, such as salts, cadmium and lead. These other compounds and elements are unwanted and the dust is fed in a second step to a second kiln and is there divided into zinc oxide and dust comprising the other compounds and elements, for example salts, cadmium and lead. This second step is referred to as the calcining step. By calcining is generally meant the conversion of the physical or chemical properties of a substance by the application of heat.  
         [0004]     An alternative process route to the prior art plant described above consists of liquid slag fuming as the first step and calcining in a rotary kiln as the second step.  
         [0005]     In both the first and the second step of the first described process route and the in the second step of the alternative process route, hot air and possibly oxygen is added to the processes. Also, in a rotary kiln a large part of the heating energy is transferred to the material from the lining of the rotary kiln, i.e., as indirect heating.  
         [0006]     A major drawback with the described recycling process is the fact that the rotary kilns are large and cumbersome to operate. For example, a rotary kiln described in the above mentioned article could be of a length of up to 100 meters and have a diameter of up to three meters. It is appreciated that a plant including rotary kilns of this kind is expensive to operate.  
         [0007]     Another drawback is the fact that the process time is rather long; the processed material is transported by means of gravity as the kiln rotates. This relatively long process creates a bottleneck in the total recovery process.  
         [0008]     A further drawback is the fact that toxic compounds if present, such as dioxins, are evaporated by the process heat without being broken down. This is due to the limited process temperature of the counter current kiln based calcining process. Vaporized dioxins are then directed to the gas cleaning section of the plant or even the ambient atmosphere, thereby constituting a hazard to the environment.  
         [0009]     It is thus appreciated that the handling and recovery of metal-bearing dusts and solid particles from such processes are often difficult and costly problems. The pressure from both public authorities and customers to find new solutions is constantly increasing.  
         [0010]     Up to now no feasible alternative to the use of rotary kilns has been found for the process of calcining metal-bearing dusts wherein the recovered metal is obtained in solid form, i.e., wherein the recovered metal is not melted. It is however very difficult to combine an acceptable metallurgical result with an energy efficient and environmentally safe kiln process. Furthermore, due to the inherent low weight dust is easily carried away by the gas to be mixed with the exhaust from the furnace, thus lowering the yield and constituting an environmental hazard.  
         [0011]     The international publication WO01/86011 A1 discloses a method for recovery of metals, wherein metallic fines are supplied to a flame of a burner ( 20 ) and the fines are brought to melt and to agglomerate. The agglomerated product is then recovered.  
       SUMMARY OF THE INVENTION  
       [0012]     An object of the present invention is to provide a method for recovery of metals and metal compounds from metal-bearing solid particles in which the above-mentioned drawbacks with known techniques are avoided or at least mitigated. Another object is to provide a use of an apparatus for recovery of metals and metal compounds from metal-bearing solid particles.  
         [0013]     The invention is based on the realization that metal-bearing solid particles can be fed directly through the flame of an oxy-fuel burner without melting the metal or metal compound to be recovered, i.e., the original shape and the state of aggregate of the solid particles are maintained. The use of a burner in combination with feeding the solid particles directly into and through the burner flame makes it possible to control the heating process in such a way that the solid particles are left unmelted after having passed the burner flame.  
         [0014]     According to a first aspect of the present invention there is provided a method for recovering of metals and metal compounds as defined in claim  1 .  
         [0015]     According to a second aspect of the present invention there is provided a use of an apparatus for recovering of metals and metal compounds as defined in claim  11 .  
         [0016]     With the method according to the invention, the problems of prior art are overcome or at least mitigated.  
         [0017]     By feeding the solid particles directly to the flame, a compact and efficient plant is provided. The heating process can be controlled in a satisfying way, avoiding melting of the metal-bearing solid particles supplied through the burner.  
         [0018]     In a preferred embodiment, by using an oxygen-enriched gas with a burner, the control of the process is further enhanced and the volume of the exhaust gas is minimized. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]     The invention is now described, by way of example, with reference to the accompanying drawings, in which:  
         [0020]      FIG. 1  is a schematic diagram of a prior art plant for recovery of metals from metal-bearing dust;  
         [0021]      FIG. 2  is a schematic diagram of an apparatus used with the inventive method;  
         [0022]      FIG. 3  is a sectional view of a burner used with the method according to the invention;  
         [0023]      FIG. 4  is a cross-sectional view of the burner shown in  FIG. 2 ; and  
         [0024]      FIG. 5  shows an alternative embodiment of an apparatus used with the inventive method. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     In the following, a detailed description of the method and the apparatus according to the invention will be given. The expressions dust and solid particles will be used alternately in this description. By these expressions are to be understood particles in the solid state and having an overall diameter of approximately 5 millimeters or below. Agglomerated products, i.e., sintered or compacted aggregates of fines, are here not included.  
         [0026]     A prior art plant and a method of operating it have already been described with reference to  FIG. 1 . In the first kiln, the starting material, such as a mixture of zinc bearing feed and coal or coke is divided into iron-rich material and zinc rich but contaminated dust.  
         [0027]     In the second kiln is the zinc rich dust calcined to a zinc rich product with low contents of impurities.  
         [0028]     Starting with  FIG. 2 , an overall diagram of a plant for recovery and upgrading of metals and metal compounds, generally designated  10 , is shown. In this context, the term recovery is intended to encompass recovery with or without any further treatment of the resulting raw material. The plant  10  is essentially identical to the plant described in the international patent publication WO 01/86011. The plant is built around a burner  20  installed in a sidewall of a furnace  30 . The burner is a so-called oxy-fuel burner and is thus supplied with fuel, such as fuel oil, propane, natural gas, or butane through a first feeding line  21  and with oxygen through a second feeding line  22 . By oxygen is in this context meant a gas with an O 2  content exceeding 21% and preferably so-called technical oxygen having an O 2  content of 90-99.5%.  
         [0029]     Metal-bearing dust or solid particles are supplied through a third feeding line  23 . From the following description, it is clear that the inventive method is applicable to electric arc furnace (EAF) steel dusts, Waelz kiln dusts, slag fuming furnace dusts, lead and copper plant dusts and foundry wastes as well as zinc-bearing feeds, including foundry dusts. Thus, both fines, normally meaning products resulting from crushing and sintering, and dust, normally meaning products collected in filters, are possible raw material for use with the inventive method as well as other equivalent materials, such as powder.  
         [0030]     The metal-bearing solid particles also contain unwanted compounds, such as cadmium, lead, different salts, sodium chloride, potassium chloride, oxides, fluorides etc. It will be appreciated that the expression compounds are to include elements.  
         [0031]     The burner  20  will be described in more detail below with reference to  FIGS. 3 and 4 .  
         [0032]     The third feeding line  23  is also connected to a feeder, generally designated  40 . The feeder  40  comprises a silo  42 , into which solid particles are fed. The solid particles are directed from the silo  42  to a pressure vessel  44 , from which they are further directed to the third feeding line  23  connected to the burner  20 . By means of this arrangement, a desired supply rate of solid particles to the burner  20  is ensured.  
         [0033]     In an alternative embodiment, the solid particles are intermixed in a fluid acting as bearer, thus creating a slurry that is fed to the burner  20 .  
         [0034]     In the bottom of the furnace there is gathered a charge  34  resulting from the material supplied to the furnace  30 .  
         [0035]     The burner  20  will now be described in detail with reference to  FIGS. 3 and 4 . The burner  20  comprises a main portion  24 , to which the three supply lines  21 - 23  shown in  FIG. 2  are connected. The portion  24  is provided with an essentially circular cross-section, see  FIG. 4 , in which the configuration of the supply lines  21 - 23  appears in more detail. Fuel is supplied through the first supply line  21  in the form of six equidistant pipes  21   a - f  placed at a constant distance from the center axis of the main portion  24 . Oxygen is supplied through an annular outer portion  22  and thus surrounds the fuel supplied through the pipes  21   a - f . Finally, solid particles are supplied through the pipe  23 , which is co-axially placed in the burner.  
         [0036]     As already mentioned, the burner  20  is mounted in the sidewall of the furnace  30 . In the preferred embodiment, the burner can be tilted, i.e., can be positioned in different angles relative to the horizontal and the vertical. The different orientations can be used for obtaining desired characteristics for the calcining process.  
         [0037]     In the following, the method for recovering metals and metal compounds will be described in detail.  
         [0038]     Initially, dust is supplied to the silo  42  of the feeder  40 . The dust used in the described process are metal-bearing solid particles. The solid particles making up the dust normally have an overall diameter of less than approximately 5 millimeters, and preferably less than approximately 1 millimeter.  
         [0039]     The dust fall from the silo and into the pressure vessel  44 , wherein the pressure is maintained by means of a gas also functioning as a carrying gas, such as compressed air, oxygen, nitrogen or argon. By means of the pressure in the pressure vessel  44 , the dust is then carried to the oxy-fuel burner  20  at a rate, which is determined by the pressure level in the vessel  44 , the amount of solid particles in the silo  42  etc.  
         [0040]     The operation of the oxy-fuel burner  20  is controlled by means of the amount of fuel and oxygen supplied through the first and second supply lines  21  and  22 , respectively. The supply lines are connected to sources of fuel and oxygen (not shown), as is conventional.  
         [0041]     The operation of the burner  20  will now be described in detail with reference to  FIGS. 3 and 4 . Dust is supplied through the central feeding pipe  23  at a rate that is controlled by the feeder. Fuel is supplied in the six fuel feeding pipes  21   a - f , see  FIG. 4 , while an envelope of oxygen is supplied through the annular feeding area  22 . The oxy-fuel mixture results in a flame  25  having properties, such as length, temperature etc., that are controlled by the supply rate of fuel and oxygen. The higher oxygen content, the higher temperature, resulting in a theoretical flame temperature as high as 1900-2500° C. and a flame velocity of 100 meters per second.  
         [0042]     Thus, the dust is injected into the central portion of the flame  25 . As is seen from  FIG. 3 , the dust injected into and through the flame  25  is left unmelted by the heat of the flame, i.e., the original shape and state of aggregate of the solid particles are maintained. This is made possible by the fact that the solid particles remain in the flame for a very short time and under strictly controlled conditions. For example, the solid particles can remain in the flame for less than one second and more preferably less than one half second. Thus, the heating of particles can be regulated so that—despite the high flame temperature—the particles are not melted but calcining is obtained.  
         [0043]     Basically, the process is controlled by the ratio of solid particles supplied into the flame and the amount of fuel burnt. However, the heating process is controlled by means of several parameters, of which can be mentioned: temperature and velocity of the flame  25 , energy content or density of the injected solid particles, stochiometry, i.e., the ratio oxidizing gas to added fuel, the oxygen content of the oxidizing gas, the supply rate of oxygen and added fuel, the rate of injection of the dust and their characteristics, the travel time of the solid particles in the flame, and burner characteristics and configuration, such as tilting. Thus the heating of particles can be regulated so that the calcined solid particles, such as zinc oxide particles, fall to the bottom of the furnace  30 , wherein they are added to the charge  34 . The particles can then be used as raw material for further processing. For example, the zinc oxide can be used as raw material for zinc.  
         [0044]     In the preferred embodiment, the process is run stochiometrically or sub-stochiometrically.  
         [0045]     The evaporated compounds leave the furnace  30  through one or more exhaust outlets (not shown) and are taken care of in some convenient way. It is believed that the inventive method using relatively high flame temperature breaks down some unwanted toxic compounds, such as dioxin, thereby preventing them from entering the ambient atmosphere.  
         [0046]     Preferred embodiments of the method and the use of an apparatus according to the invention have been described. The person skilled in the art realizes that these can be varied within the scope of the appended claims. Thus, although an oxy-fuel burner  20  has been shown, other equivalent burners, such as plasma burners, can be used as long as desired oxygen levels exceeding 21% are obtained. Also oxy-fuel burners of other configurations than the disclosed one can be used, such as a burner with a different number of fuel pipes than six.  
         [0047]     In the embodiment shown in  FIG. 2 , the burner is positioned in a sidewall of a furnace. However, it is realized that other suitable positions are possible, such as in the upper part of the furnace. Also, a configuration with more than one burner is also possible. In  FIG. 5 , yet an alternative embodiment is shown, wherein the burner  20  is provided in one end of a rotary kiln  30 ′. In this way, an existing plant can be retrofitted with a burner at the inlet end of the kiln, providing a co-current process instead of the prior art counter-current process. The solid particles are fed to the burner in the same way as in the embodiment described with reference to  FIG. 2  but are transported away by the rotation of the kiln  30 ′.  
         [0048]     In the described embodiments, the solid particles are fed to the furnace by means of a feeder. However, the particles supplied to the furnace could also be free-flowing, carried by means of a feed gear etc.  
         [0049]     A dry starting material has been shown in the figures. In the case the dust particles are intermixed in a liquid, such as water or sludge, a suitable feeding arrangement must be provided, comprising a feed screw, for example. Also, when arriving to the furnace, the wet part is vaporized by the high temperature of the flame, resulting in exhausts rising through the furnace  30  and subsequently leaving through an exhaust outlet (not shown).