Conventional methods of refining lead bullion containing antimony have included the use of refining reagents such as air, oxygen-enriched air, lead oxides, nitre and caustic, and aluminum, at high temperatures in excess of 1100.degree. F. (600.degree. C.). In such methods, gaseous oxidizing reagents such as air and oxygen-enriched air are pumped into the agitated bullion where the oxygen present in these gaseous reagents reacts with the antimony to form antimony oxides. When using solid oxidizing reagents, such as lead oxides or nitre and caustic, the reagents are added while the bullion is vigorously stirred. At the high operating temperatures involved, the antimony is oxidized and forms a dross which rises to the surface. In such processes, a large amount of lead is also either oxidized along with the antimony to form a lead oxide dross or is physically entrapped in the dross which rises to the surface as a high lead-content dross.
In using aluminum for antimony removal, the refining process is carried out at a temperature of 1300.degree. F. The aluminum, upon addition, melts and is mixed into the bullion to react with the antimony to form an aluminum-antimony intermetallic which rises and is separated from the bullion as a dross. In this process, a considerable amount of lead is also physically entrapped in the dross.
In U.S. Pat. No. 1,786,908 to Hanak, alkali metals and caustic alkalies are used to separate antimony from lead and/or tin. He teaches that sodium will combine with antimony to remove any part of the antimony, by adding only that portion of sodium corresponding to the quantity of antimony to be removed. Hanak heats the metal or alloy to approximately 200.degree. C. (approximately 360.degree. F.) or more above its melting point and stirs a predetermined quantity of sodium into the metal. After mixing well, the temperature of the bath is somewhat lowered and sufficient caustic alkali is stirred in to dissolve the antimony-sodium alloy. The amount of caustic soda used is approximately four times the weight of the antimony extracted. The caustic sodium antimony alloy melt is run off, allowed to cool, and then treated by a water wash to dissolve the caustic soda and to react with sodium in the sodium-antimony compound to form the hydroxide with antimony being liberated in the metallic state.
Thus, in the Hanak process, the caustic soda added is four times the weight of the antimony extracted or about eight to ten times the weight of alkali metal added, and produces a molten caustic soda-sodium antimony slag which is difficult to handle with conventional batch refining equipment in a smelter and, in addition, the operating temperature of the process is 200.degree. C. or more above the melting point of the lead-antimony bullion, thereby increasing the required fuel cost and significantly decreasing the overall efficiency of his process.
U.S. Pat. No. 3,607,232 to Lebedeff teaches a process for refining lead, particularly lead requiring softening and desilverizing, which comprises adding an alkali metal from the group consisting of sodium and potassium and mixtures thereof to a pool of the molten lead, agitating the molten pool to remove tellurium in the slag. The tellurium is removed from the molten lead at an extremely rapid rate at a temperature between the melting point of the lead and the temperature of the lead from the conventional pyrometallurgical lead softening process, i.e. at temperatures which ordinarily are below 1200.degree. F. (649.degree. C.) and which, depending on the softening process employed, may be as low as 850.degree. F. (454.degree. C.). The preferred operating temperature for carrying out the refining process is below 1200.degree. F. and above the melting point of the lead to be detellurized, preferably at a temperature above 650.degree. F. (343.degree. C.). The amount of sodium or potassium or mixtures thereof employed is equivalent to 0.3 to 0.8 pounds of sodium per pound of tellurium to be removed from the lead, with 1 pound of sodium being equivalent to 1.7 pounds of potassium. He also contemplates antimony removal during the refining process to remove most of the antimony due to the fact that where arsenic and antimony content of the lead is above about 0.02% by weight, relatively large quantities of antimony as well as arsenic are incorporated into the tellurium during the detellurizing procedure. The sodium or potassium or mixtures thereof are incorporated in the lead by adding them as a master alloy of lead which may be added in a liquid or solid form during agitation of the melt. The agitation required to reduce the tellurium in the lead to a given amount will vary inversely with the vigor of the agitation, i.e. with more vigorous agitation a short agitation period can be used and vice versa. The Lebedeff process is practiced at a temperature above 650.degree. F. and below 1200.degree. F. and most preferably in the range 800.degree. to 850.degree. F. for the removal of tellurium. Lebedeff also utilizes his process to soften bullion containing copper, bismuth, antimony, arsenic, selenium, silver and gold; however, the addition of sodium hydroxide and metallic sodium for the removal of tellurium from the bullion results in a bullion which retains the original percentage of copper, bismuth, arsenic, silver and gold although significant reductions are realized in the content of antimony and selenium retained in the bullion.
Thus, the Lebedeff process must be carefully controlled in order to remove the desired quantity of tellurium from the bullion and will also remove certain amounts of antimony and selenium, along with the tellurium; however, subsequent processing is required in order to further refine the bullion to remove copper, bismuth and other metal values.
All of the above prior art processes have shortcomings in that they are inefficient either in materials utilization, length of time to process the bullion, and/or energy consumption and fail to adequately remove other metal values in the initial processing, thereby necessitating extensive further softening procedures. In contrast, the process of the present invention refines the bullion within a controlled temperature range by the addition of sodium to remove antimony and substantial amounts of other common alloying metals and elements, such as arsenic, cadmium, copper, nickel, silver, sulfur, selenium, tellurium and zinc present in the lead bullion, which react with the sodium to form intermetallics and compounds which are recovered in the dross.