Flue dust leaching

Production. Indium is recovered from fumes, dusts, slags, residues, and alloys from zinc or lead—zinc smelting. The source material itself, a reduction bullion, flue dust, or electrolytic slime intermediate, is leached with sulfuric or hydrochloric acid, the solutions are concentrated, if necessary, and cmde indium is recovered as 99+% metal. This impure indium is then refined to 99.99%, 99.999%, 99.9999%, or higher grades by a variety of classical chemical and electrochemical processes. 

Production and Economic Aspects. Thallium is obtained commercially as a by-product in the roasting of zinc, copper, and lead ores. The thallium is collected in the flue dust in the form of oxide or sulfate with other by-product metals, eg, cadmium, indium, germanium, selenium, and tellurium. The thallium content of the flue dust is low and further enrichment steps are required. If the thallium compounds present are soluble, ie, as oxides or sulfates, direct leaching with water or dilute acid separates them from the other insoluble metals. Otherwise, the thallium compound is solubilized with oxidizing roasts, by sulfatization, or by treatment with alkaU. The thallium precipitates from these solutions as thaUium(I) chloride [7791 -12-0]. Electrolysis of the thaUium(I) sulfate [7446-18-6] solution affords thallium metal in high purity (5,6). The sulfate solution must be acidified with sulfuric acid to avoid cathodic separation of zinc and anodic deposition of thaUium(III) oxide [1314-32-5]. The metal deposited on the cathode is removed, kneaded into lumps, and dried. It is then compressed into blocks, melted under hydrogen, and cast into sticks. 

Another reported example with two metals in solution is the recovery of copper and zinc from brass mill flue dust [11]. The material contains very little iron. The solid material is leached with sulfuric acid to produce a weak acid... 

The process flow sheet was first tested for direct leaching of steel mill flue dust and production of zinc metal by electrowinning. The tests were performed in a continuously operating pilot plant, producing 10-20 kg/day zinc metal. The same pilot plant was then used for treating copper/zinc-rich brass mill flue dust in a closed loop operation, recycling all the zinc solvent extraction raffinate to the copper circuit leach section. In the zinc circuit leach section, only the amount of zinc rich dust necessary for neutralization of the copper solvent extraction raffinate was used. The results obtained from the pilot plant tests indicated contamination problems within the solvent extraction loops. The estimation of economic data showed a weak return on the assets compared with the alkali route, and sensitivity toward the raw material price. 

The secondary flue dust from the converter, containing mainly zinc and lead, is leached with the ammoniacal raffinate solution from the Cu/Ni extraction. Lead is left in the residue. Finally, zinc carbonate is precipitated by addition of CO2 followed by thermal stripping of ammonia. The zinc carbonate is calcined to zinc oxide and ammonia is recycled to leaching. 

C Extraction. The major source of T1 is flue dusts from the roasting of Zn, Cu, and Pb ores. Many T1 compounds are much more volatile that those of other elements, and as a result, they collect in the flues, particularly TI2O and TI2SO4. Since both of these compounds are soluble in water, they may be leached out of the dusts to give solutions of TlOH and TI2SO4, leaving behind most impurites as insoluble residue. 

Many different chemical treatment systems have been developed to reduce the leachability of lead and cadmium compounds in flue dust. These systems usually rely on stabilization/solidification, adsorption, chemical reduction, or pH control. Chemical reduction employing the use of metallic iron has been successful in reducing the leachability of lead to below EP-Toxicity levels. Adding a 5 percent by weight dose of iron filings to cupola furnace emissions control sludge, for instance, reduced lead leaching from 28.6 mg/1 to less than 0.1 mg/1 (Stephens 1984). 

Versatic acid has been used in Japan to recover indium and gallium from solutions obtained from the leaching of bauxites, zinc minerals, coal ash and flue dusts.40 41-63 Extraction is carried out at a pH value of 2.5 to 4.0 some coextraction of tin(II), iron(III) and aluminum(III) occurs if these metals are present. In the extraction of indium(III) by n-hexanoic acid,64 the predominant species in the organic phase was found to be InA3(HA)3 whereas in the extraction by n-decanoic acid65 the existence of trimeric (InA3-HA)3 and hexameric [InA2(OH)]g species was also postulated. 

More complex antimony ores can be treated by leaching with alkali hydroxide or sulfide and by electrolysis of the resulting solution of sodium thioantimonate, Na3SbS4. Elemental antimony can also be recovered from the flue dust of lead smelters. 

The common sources of indium are the minerals dark sphalerite, christophite, and marmatite. Indium is also found in small amounts in manganese, tungsten, zinc, and tin ores. Rarely found as a free element, indium is commonly associated with gallium in tin and zinc ores. The main commercial source for indium is from zinc smelter flue dusts (Smith etal. 1977). Enrichment of indium from zinc residues is performed by acid leaching followed by chemical separation processes. Aqueous electrolysis of indium salts yields a final metal of 99.9% purity. Canada has the greatest resources of indium with approximately 27% of the world s reserves (based on estimated indium content of zinc reserves) and the United States has about 12% of the world reserves (Brown 2000). In recent years, there have been major improvements in the recovery, refining and recycling of... 

The furnace and boiler calcines are combined. Then the mixture is conveyed to a ball mill for grinding to a particle size of 50 % below 40 pm. The ground calcine, cyclone and electrostatic precipitator flue dust are combined and fed to calcine leaching. A typical calcine contains 0.2-0.3 percent sulfide and 1.5 percent sulfate, see Figure 2. 

Chlorine from PVC separators forms volatile lead chloride, which vaporises in the lower shaft and condenses in the upper shaft, forming a recycle loop as well as wall accretions. Some lead chloride reports to flue dusts, again causing accretions in the gas handling system, and collected dusts must be leached for removal of chlorine before recycling to the furnace feed. 

The metallic arsenic is obtained primarily from its mineral, arsenopyrite. The mineral is smelted at 650 to 700°C in the absence of air. However, the most common method of production of the metal involves reduction of arsenic trioxide, AsOs with charcoal. Arsenic trioxide is produced by oxidation of arsenic present in the lead and copper concentrates during smelting of such concentrates. The trioxide so formed, readily volatilizes and is collected in a dust flue system where further treatment and roasting can upgrade the trioxide content. The trioxide vapors are then condensed and further purified by pressure leaching and recrystallization techniques. It is then reduced with charcoal to give metallic arsenic. 

In the early days of the lithium industry considerable attention was paid to the recovery of lithium from moderately high-lithium Clay. Lien (1985) noted that in laboratory tests some clays could have as high as an 80% lithium extraction with a simple sulfuric acid leach, but that most required a more complex process. In brief tests a roast at 750°C with two parts of clay and one part limestone, followed by a leach with an excess of 20% hydrochloric acid gave a 70% lithium yield. In a second series of tests five parts of clay, three parts of gypsum and three parts of limestone were roasted at 900°C. A water leach resulted in an 80% recovery of lithium as lithium sulfate. In the later process the raw materials were first groimd together to a -100 mesh size and then formed into 6.5 mm pellets before being roasted. The pellets reduced the dust loss and increased the particles contact with the flue gas. 

Other proposed sources of calcium chloride include the thermal decomposition (at 270-280°C) of scrap PVC, with the flue gas being absorbed in a Ume or limestone scmbber (Aoki et al, 2002). Others have suggested the washing of incinerator or fly ashes to recover calcium chloride, or the leaching of blast furnace slag with acids. Dust from scrap steel shredders could also yield calcium chloride in the absorbed incinerator flue gas. 

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