World of Metallurgy ERZMETALL 68 2015 No 3154 Paul B Queneau et al Recycling Lead and Zinc in the United States temperature also results in lower flux consumption decreasing slag volume by about one third Recovery of lead antimony and particularly tin is therefore at higher concentration The gain is somewhat offset in greater difficulty handling the slag when produced due to the high temperature and associated retention time required for heat dissipation The baghouse dust is continuously returned to the reverberatory furnace as shown in Figure 3 Reverberatory furnace efficiencies have been improved through the addition of air oxy fuel and oxyfuel burner systems Greater heat values and more directed flame en velopes are produced by these systems as opposed to con ventional burners Sulfur dioxide in furnace off gases has been further reduced through spray tower scrubbing Re cently further furnace efficiency improvements have been made through revisions to the furnace charging process to prevent the infiltration of additional air from furnace ventilation The use of short rotary furnaces in North America has been expanded due to new smelting capacity principally owned by Johnson Controls Inc 11 In order to produce quantities in excess of 100 000 tons these facilities require 6 or more furnaces In the past short rotary furnace instal lations were generally found in areas where feed supply issues subjected the facility to operational interruption Past short rotary furnace facilities were not constructed to the operational scale currently used in the US secondary lead industry and as a result suffered a competitive disad vantage The recent trend to significantly higher capacities through multiple furnaces represents a process change The jury is still out as to whether this will be accepted as the re placement to reverberatory blast combinations in the US While a number of attempts have been made to replace the pyrometallurgical process with a leach electrowin op eration RSR MA Industries Engitech none of the processes have reached commercialization Most recently Doe Run in conjunction with Engitech s p a operated a pi lot scale plant to convert PbS concentrate 12 All of these attempts have been unable to economically replace the current smelting process due to its improved efficiencies and recycling of materials back to the battery manufactur ing process 2 4 Environmental improvements While each secondary lead smelter has site specific con straints and environmental limitations the industry as a whole is tightly regulated due to the toxicity of lead Both federal and state regulations limit the amounts of allowable emissions in all media including air water and solid waste The secondary lead industry has reduced its environmental impact significantly by recycling various components back to the manufacture of new batteries significantly reducing the volume of hazardous emissions over the past 15 years Most recently on December 14 2010 the Environmental Protection Agency EPA revised the ambient monitor ing requirements for measuring lead Pb in the air The new standard reduced the allowable level of lead from 1 5 micrograms per cubic meter μg m3 to 0 15 μg m3 mea sured as total suspended particles TSP 13 On January 5 2012 the EPA issued a final rule for National Emissions Standards for Hazardous Air Pollutants NESHAP from secondary lead smelting which included a facility wide flow weighted average lead emissions limit from stacks of 0 20 mg dscm dry standard cubic meter and an individual stack lead emissions limit of 1 0 mg dscm for each stack at existing sources Also emission limitations were finalized for organic hazardous air pollutants and controls required for fugitive emissions 14 These regulations have influenced the process design and facility modifications of the industry with an emphasis on the elimination of organic compounds from stack emis sions the control of fugitive emissions and the removal of additional heavy metals beyond lead Smelters have improved their control of organic emis sions both through temperature reliability and gas reten tion time of the thermal oxidizers afterburners located between the furnace and the off gas cooling system Sig nificant additional sanitary and process fugitive ventilation has been added along with increased enclosure resulting in buildings with continuous negative pressure The use of PTFE polytetrafluoroethylene as both a filtration mem brane and as filter bag media has resulted in improvement in stack emission levels When this is coupled with second ary filtration in the form of HEPA filters or secondary scrubbing the emission levels are significantly reduced RSR Corporation has implemented the use of WESP wet electrostatic precipitation in series after the process bag house in order to reduce the amounts of metal particulates less than 1 μm in size 15 16 The process is particularly effective in removing not only lead but other heavy metals also present in stack emissions Waste water treatment has been improved through the use of ferric iron co precipitation In addition East Penn has coupled the process to reverse osmosis and the crystalliza tion of sodium sulfate in order to create a process whereby waste water is purified dissolved solids are removed and the water can be returned to the battery manufacturing process 17 Technical grade sodium sulfate is sold to the markets Excess liquid is consumed used for evaporative cooling and through the ammonia scrubbing process in the smelter This has resulted in a significant decrease in the amounts of water discharged almost to the point of zero discharge 3 Secondary zinc Although zinc and lead often occur together in natural sul fide mineral deposits the two have important differences in chemistry When the oxides are leached with sulfuric acid ZnSO4 dissolves while PbSO4 remains insoluble The melt ing points of Zn 420 C and Pb 327 C are low both can be refined in gas fired kettles Zinc also has a low boiling point 907 C whereas Pb boils at 1744 C Consequently Zn unlike Pb is economically refined by distillation Lead is considerably more resistant to oxidation and thus can be efficiently separated from aluminum and iron by slagging off the latter two metals

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