World of Metallurgy ERZMETALL 68 2015 No 3 161 Paul B Queneau et al Recycling Lead and Zinc in the United States rich scale on the retort walls is minimized by blending galvanizer dross up to 5 Fe with diecast scrap up to 8 Al preferably also containing copper The aluminum isolates iron as FeAl3 intermetallic Excessive addition of diecast scrap can result in aluminum foaming as well as crusting over the melt The retort residue is poured and raked while it is still hot immediately after comple tion of the heat Energy consumption is on the order of 14 000 000 Btu ton 3 900 000 kcal mt of Zn product 3 2 5 The Larvik furnace The Larvik furnace is characterized by improved zinc re covery of higher purity as compared to a muffle furnace and by high thermal efficiency it is heated electrically Figure 7 The Larviks such as those operated by ZCA Monaca are particularly suited for treating lower grade iron rich residues containing zinc metallics e g FeZn3 bot tom dross Heat is provided by graphite resistors located in the back half of the furnace Power consumption is 1400 to 2000 kWh ton zinc produced the actual consumption depending upon the type of feedstock being processed The capacity of a typical Larvik treating iron rich dross is 1000 to 1500 t a of zinc depending on the zinc content of the feedstocks processed A Larvik furnace uses energy almost twice as efficiently as a muffle furnace However heating by electric power normally costs so much more than by natural gas that the energy cost to operate a Larvik is usually substantially higher than that for a muffle Vapor superheated by the globars travels countercurrent to the liquid zinc cooling as it heats the melt to near its boiling point 907 C Melt circulates between the melting and condensing chambers by natural convection or with assistance from a pump to transfer heat to the incoming charge Vapor exits through a vertical column in which refluxing occurs Lead condenses and runs down the column back into the furnace where it settles to the bottom and is periodically tapped Given a suitably tall column no trays are necessary Skimmings other than FeZn3 intermetallics which sink are blocked by a curtain wall between the charging cham ber and the condensing chamber When the objective is to recover zinc from the FeZn3 Al containing feedstocks such as diecast scrap are not added to avoid forming FeA13 top dross The latter would report to the skimmings along with an unacceptably high quantity of entrained zinc High boiling point impurities flowing under the curtain become more concentrated as they flow toward the tap hole at the far end of the furnace Here a flux such as P2O5 is added to alloy the accumulated iron with phosphorus which lowers its melting point to about 1250 C for tapping The discard iron alloy up to 10 P assays about 2 Zn 3 2 6 The Sweat furnace Additional charge material to a retort muffle furnace or distillation column may consist of metal upgraded in a sweat furnace A sweat furnace whether it be a rever beratory furnace or a rotary kiln is used to process badly contaminated scrap Selective melting sweating of scrap separates zinc from attachments of lead and metals having higher melting points than zinc as well as from nonmetallic residues Relatively clean scrap can be melted in an induc tion furnace kettle or crucible To efficiently sweat zinc from mixed metal scrap the material can be continuously fed to an indirectly fired stainless steel kiln lined with a ceramic coating Zinc and aluminum based alloys are sweated and reclaimed The temperature profile along the length of the kiln is accurate ly controlled to maximize metal separation efficiency En ergy consumption is about 650 000 Btu ton of feed 180 000 kcal tonne feed when sweating lead and zinc and about 1 100 000 Btu ton 300 000 kcal tonne when sweating alu minum Spiral flights within the kiln control the retention time The rotation assists in separating molten zinc from unmeltables while minimizing hot spots Sweating sequen tially separates lead zinc and aluminum with melting points of 327 C 420 C and 660 C respectively from Fig 7 Larvik boiler and stripping furnac es processing iron poor plant dross and iron rich metallic dross 31

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