World of Metallurgy ERZMETALL 68 2015 No 3 159 Paul B Queneau et al Recycling Lead and Zinc in the United States Zinc production at Mooresboro began in May 2014 Zinc metal output rate by year end was 175 tpd 230 tons shipped per day covers cash operating cost and interest expense design capacity is 427 tons metal per day Issues during start up included impeller corrosion during leaching and difficulties with the subsequent solid liquid separation clarification operations prior to solvent extrac tion The facility is expected to produce 155 000 t a of SHG CGG and PW slab and jumbo ingot The plant is fed with a combination of crude zinc oxide CZO generated at Horsehead s EAF dust recycling plants see Table 8 and purchased milled and upgraded skimmings from Horse head s ThirtyOx joint venture The Mooresboro operation consists of two primary parts 1 leaching and ZINCEX solvent extraction licensed with Tecnicas Reunidas and 2 electrowinning melting and casting technology licensed with Asturiana de Zinc In the leach SX section CZO is first washed in water to remove soluble elements e g chlorine potassium and so dium then leached for zinc with sulfuric acid A follow up circuit recovers high grade lead carbonate byproduct en riched with silver The zinc rich pregnant leach solution is processed by sol vent extraction SX to selectively extract the zinc into an organic solvent 27 The organic is washed then stripped creating a purified zinc loaded electrolyte solution Impuri ties that build up in the system are bled to a neutralization cementation circuit to produce a clean effluent solution for discharge Zinc cathode sheets are recovered from the SX strip by electrowinning These sheets are bundled trans ported to an induction melting furnace then lowered into a bath of molten zinc The metal flows from the melting unit to alloying furnaces producing CGG and prime western Alternatively output can be cast directly as SHG in either slab or jumbo ingot molds This plant is expected to decrease Horsehead s production cost per ton of zinc produced by 35 to 40 3 2 Secondary zinc unit operations Classical secondary zinc plants see Table 8 melt selected and prepared secondaries followed by selective drossing and or upgrading by zinc vaporization There is usually a preliminary physical concentration step such as separating metallics from nonmetallics using an air swept hammer mill or a ball mill equipped with a trommel screen Liqua tion furnaces reject lead and iron from the metallics by taking advantage of the relatively low solubility of these impurities at temperatures slightly above the melting point of zinc Muffle furnaces are most efficient when processing zinc metallics especially diecast scrap Retorts are most efficient when processing zinc metallics particularly those derived from galvanizer dross Sweat furnaces are used to pre concentrate retort and muffle furnace feedstocks 3 2 1 Liquation Crude zinc containing lead and iron in amounts exceed ing those of Prime Western grade can be upgraded by liquation The metal is held just above its melting point ideally in an induction furnace Solubility limitations and differing densities result in formation of three strata 28 The lighter zinc rises the lower the zinc temperature the lower is its Pb and Fe content Insoluble lead sinks to the bottom the lower the lead temperature the lower its Zn content Excess iron concentrates at the zinc lead interface as a mushy FeZn3 intermetallic assaying 90 Zn due to entrained metal If aluminum is also present an FeA13 top dross forms also 90 Zn that floats on the zinc The liquation process can be carried out in a reverberatory furnace in a kettle or in a mold while the melt is waiting for additional treatment A typical liquated zinc contains 0 9 Pb and 0 02 Fe Gas fired iron or steel kettles and pots are used for liqua tion when the metal being remelted contains greater than 1 aluminum e g the scrap from zinc diecasting opera tions The presence of aluminum in the melt decreases zinc attack on the iron pot When steel pots are used for melting low aluminum zinc scrap such as that from hot dip galva nizing pot life is dramatically shortened and the zinc metal contains more iron Use of a Pinto furnace is an effective method for iron re moval from continuous galvanizer top and bottom dross as was practiced by U S Zinc s Coldwater MI plant The dross 0 5 Al and 0 1 Pb was first melted in a channel induction furnace Approximately two tons of metal were then transferred to one of two Pinto furnaces which are cy lindrical agitated vessels U S Patent 3 902 894 One add ed excess aluminum beyond that needed to form FeA13 which floated on the zinc as a low density 3 7 g cm3 high melting point 1150 C intermetallic One stirred for about 20 minutes at about 650 C zinc adhering to the dross was propelled outward Subsequent addition of air oxidized the surface of the FeA13 thus rendering the dross non wetta ble by zinc The refined zinc was poured into a reverbera tory holding furnace for alloying with aluminum to meet continuous galvanizing standards Dross ass retained in the Pinto furnace by an underflow weir The dross byproduct 8 to 12 each of Al and Fe plus 0 1 Pb was sold 3 2 2 Pyrotek MZR zinc recovery furnace Pyrotek s MZR System separates free zinc metal from mixtures generated during the zinc casting galvanizing and other molten metal processes Free zinc in the skimmings is recovered This system is widely applied for in house processing of hot dip galvanizing ashes and skimmings as well as for continuous galvanizing line top drosses Pyrotek MZR provides continuous galvanizers with the opportu nity to improve their position by processing their drosses on site to yield technically and commercially acceptable zinc for re introduction into their process baths Feedstocks are heated indirectly while being tumbled and stirred Melting starts and is encouraged by this continuous tumbling action under the sustained application of heat until liquid metal and dross segregate due to a density differential Figure 4 shows a loaded process barrel in po sition for placement in the heating chamber Slow rotation

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