World of Metallurgy ERZMETALL 68 2015 No 3160 Paul B Queneau et al Recycling Lead and Zinc in the United States while indirect heat is applied from the outside of the barrel allows particulate zinc to melt and agglomerate with max imum protection from excessive oxidation Liquid metal collects at the base of the process barrel ready for tapping The dross floats on top of the metal during processing The dross can be either discarded per typical dross procedures or retained in situ and further processed to produce a residue valued by zinc chemical producers 3 2 3 The muffle furnace U S Zinc at Clarksville and Millington and by 2016 Zinc Oxide LLC at Dickson TN utilize muffle furnaces to upgrade dross and zinc metallics see Table 8 A partition wall extending downward into the melt divides the muffle furnace into two compartments one for melting and the other for vaporizing Figure 5 Zinc scrap is fed to the melting unit which can be a reverberatory furnace Skim mings are raked from the surface of the melt and later milled and screened for metal recovery As the zinc melts it flows under the partition wall into the vaporizing unit This underflow melt is low in iron content because iron reacts with the aluminum in the diecast scrap to form the FeAl3 intermetallic which floats The skimmings typically assay 60 to 65 Zn and 2 to 3 Pb The vaporizing unit is fired in an upper chamber separat ed from the vaporizing metal below by an arch muffle constructed of SiC brick Combustion gas from the upper chamber is exhausted into the melting unit Zinc vaporized below is either condensed or burned to oxide Cadmium follows the zinc Aluminum copper and lead accumulate in the melt which is typically about 40 tons About once a week up to ten tons of this metal is tapped into a ladle The resulting aluminum rich alloy high aluminum high zinc slab is sold to secondary aluminum smelters It is used as a source of copper and zinc to prepare aluminum diecast alloys The chemical analysis of this slab is variable 10 to 30 Cu 15 to 50 Zn the balance primarily aluminum Chromium should be low preferably 0 1 Up to about 1 5 Pb is acceptable 0 4 is preferred Note A demonstration plant is under construction us ing hydrometallurgy Metsol Process to produce rub ber grade ZnO from Waelz CZO galvanizer ash or oxide ore Ammonia ammonium chloride leaching cementation is followed by ammonia stripping crystallization and dry ing to generate HP and UHP ZnO 30 3 2 4 The zinc retort The bottle retort Figure 6 utilized is typically made of silicon carbide or clay graphite The larger retorts are ap proximately 3 ft inside diameter by 4 ft tall hold 2 4 tons and produce about 85 tons of zinc per month Feedstocks are milled and classified to recover metallics for retorting melted bottom dross can be fed directly to the retort Ox ides and halides report to the fines fraction e g minus 20 mesh Although one could retort zinc oxides plus a carbo naceous reductant economics dictate processing metallics to produce a specialty product such as zinc dust Zinc recovery as salable dust is about 90 Zinc vapor generated in the batch operated retort is typ ically converted to zinc dust One cycle takes about one day Metal is condensed in a removable refractory lined steel shell Preferably zinc dust is manufactured in a sur face condenser constructed of uninsulated thin steel sheet Dust production is on the order of 60 lb of zinc dust per cubic foot of condenser volume incorporating water cool ing tubes increases condenser capacity to about 600 lb ft3 Cooling can also be provided by addition of nitrogen Bottle retorts can economically generate a discard bottom residue containing as little as 5 Zn Deposition of iron Fig 4 Pyrotek s MZR Zinc Recovery Furnace 29 Fig 5 Muffle furnace for ZnO production U S Zinc Fig 6 Bottle retort for zinc distillation Ceramic retort Zinc containing metallics Vapors to condenser Grout seal Burner

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