Abstract:
A method for refining non-ferrous metal wherein a stream of oxygen containing gas is provided from a lance into the headspace of a refining vessel for passage to the molten metal bath within the refining vessel, and a flame envelope is provided around and along the oxygen containing gas stream for a portion of its length, wherein the flame envelope simultaneously serves to keep accretions from forming on the lance face and serves to maintain the oxygen-containing gas stream coherent.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to refining non-ferrous metal using oxygen to oxidize impurities in the molten metal.  
       BACKGROUND ART  
       [0002]     A problem which arises in the refining of non-ferrous metal using oxygen to oxidize impurities in the molten metal is the formation of accretions on the surface of the lance from which the oxygen is injected into the refining vessel. These accretions comprise solidified material from the headspace of the refining vessel which solidify on the face of the lance due to the relatively cold temperature of the lance which results from water-cooling and the oxygen passing through the lance. These accretions disturb the flow of oxygen from the lance causing some of the oxygen to be deflected away from the bath. This has three very detrimental effects. First a significant portion of the oxygen is not delivered to the target area of the molten metal bath resulting in inefficient oxygen usage. Second, some of the oxygen is deflected to such a degree that it impacts the vessel wall thus reducing the life of the refractory lining of the wall. Third, the lance must undergo more frequent maintenance and replacement. All of these problems increase the cost of the refining process.  
       SUMMARY OF THE INVENTION  
       [0003]     A method for refining non-ferrous metal comprising:  
         [0004]     (A) providing a refining vessel containing a bath of non-ferrous metal and having a headspace above the bath of non-ferrous metal;  
         [0005]     (B) passing a stream of oxygen containing gas from a lance at a velocity of not more than 3 Mach into the headspace, and passing the oxygen containing gas stream through the headspace from the lance to the non-ferrous metal bath;  
         [0006]     (C) providing a flame envelope around the oxygen containing gas stream for a portion of the distance from the lance to the bath; and  
         [0007]     (D) reacting oxygen from the oxygen containing gas stream with material in the bath to oxidize said material.  
         [0008]     As used herein the term “oxygen containing gas” means a gaseous fluid having an oxygen concentration of at least 25 mole percent.  
         [0009]     As used herein the term “flame envelope” means a combusting flow around and along one or more gas streams.  
         [0010]     As used herein the term “coherent jet” means a gas stream which has little or no increase in diameter in its flow direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]     The sole Figure is a cross-sectional end view of a non-ferrous metal refining vessel in operation with one preferred embodiment of the refining method of this invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]     In the practice of this invention oxygen containing gas is passed from a lance into the headspace of the refining vessel at a velocity which may be subsonic, sonic or supersonic but is not more than 3 Mach, preferably not more than 1.5 Mach and is most preferably within the range of from 0.835 Mach to 1.13 Mach.  
         [0013]     In addition, in the practice of the method of this invention which enables oxygen refining practice of non-ferrous metal with reduced accretion formation, there is employed a flame envelope around the gas stream proximate the lance face. The flame envelope serves to melt solidified material and/or to keep material from solidifying on the lance face and thus aids in the attainment of the beneficial results of this invention, i.e. avoidance of detrimental effects of solidified material buildup on the oxygen lance in oxygen refining practice. The flame envelope is formed preferably by providing fuel, such as natural gas or other hydrogen containing fuel, and oxidant, such as oxygen containing gas, from the lance into the vessel headspace. Most preferably the fuel and oxidant are provided respectively from two concentric rings of ports on the lance face around the central nozzle from which the refining oxygen containing gas is provided into the headspace, wherein the fuel is provided from the inner ring with respect to the nozzle and the oxidant is provided from the outer ring. A single ring design may also be used.  
         [0014]     In addition to contributing to the attainment of the beneficial result of reduced accretion formation, the flame envelope provides for a second beneficial effect. The flame envelope forms a barrier around and along the oxygen containing gas stream for a portion of the oxygen containing gas stream from the lance to the bath. This barrier keeps refining vessel gases in the headspace from passing into the oxygen containing gas stream. Thus the oxygen containing gas stream forms a coherent jet for at least a portion of the distance from the lance to the top surface of the molten metal bath. This enables the oxygen containing gas to impact the bath with greater force and purity than would otherwise be the case and this results in improved contact of the oxygen containing gas with the bath which in turn enables more efficient oxygen reaction with the bath constituents and better overall refining results. In addition, the application of the flame envelope or flame shroud can allow the oxygen lance to be operated at greater lance to bath distances than would otherwise be the case.  
         [0015]     The method of this invention may be employed to refine many non-ferrous or base metals among which are copper, nickel, lead, zinc and tin. It is understood that there may be small amounts of ferrous metal in the bath of non-ferrous metal refined in the method of this invention.  
         [0016]     The invention is particularly useful for the refining of copper wherein oxygen is employed to react with sulfur in the molten copper to produce sulfur dioxide which is then removed from the copper. It is in conjunction with this particularly preferred application and also with reference to the Drawing that the invention will be further described in detail.  
         [0017]     Referring now to the Figure there is shown refining vessel  1  which has a refractory lining  4  and which contains a bath  2  of copper and has a headspace  3  above the bath.  
         [0018]     At least one oxygen lance  10  is employed to provide oxygen containing gas into the headspace. Oxygen containing gas within the requisite velocity range is passed out of the lance into headspace  3  to form oxygen containing gas stream  12 . A flame envelope, as illustrated by flame envelope  13 , surrounds each oxygen containing gas stream for a portion of the distance from the lance to the top surface of bath  2 . The oxygen from the oxygen containing gas stream reacts with material in the bath to oxidize that material. In particular, the oxygen reacts with sulfur in the molten copper bath to form sulfur dioxide which then bubbles out from the bath and is removed from the refining vessel.  
         [0019]     Preferably, such as is illustrated in the Figure, the molten bath is agitated through the injection of a gas  15  from below the surface of the bath through one or more injection devices  14 . Among the suitable gases which may be employed as mixing gas  15  one can name oxygen, nitrogen, argon, steam and mixtures thereof. The injection device  14  may be any suitable injection device such as a tuyere or a porous plug. The inert gas flows upward from the injection device in a bubble plume  16  and serves to mix the molten metal bath to counteract stratification and to enhance the efficiency of the refining operation.  
         [0020]     The mixing gas which rises through the molten metal bath may form a continuous eye of freshly exposed bath material composed of solidified or semi-solidified material  17  on the surface of the bath above the injection device from which the mixing gas was provided into the bath. In a particularly preferred embodiment of the invention such as is illustrated in the Figure, one or more oxygen containing gas lances are positioned such that the oxygen containing gas stream from that lance is directed toward and impacts the agitated area of the bath such as at the eye. As a result of the bottom injected mixing gas, the coherent jet of oxygen containing gas is not required to penetrate deeply into the bath for improved contact and reaction with the bath and therefore can operate efficiently at low Mach number supply conditions.  
         [0021]     Although the invention has been described in detail with reference to a certain preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims.