Precious Metals in Municipal Solid Waste Incinerat

Authors: Lenka Muchova Erwin Bakker Peter Rem

Publish Date: 2008/12/05

Volume: 9, Issue: 1-2, Pages: 107-116

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Abstract

Municipal solid waste incineration MSWI bottom ash contains economically significant levels of silver and gold Bottom ashes from incinerators at Amsterdam and Ludwigshafen were sampled processed and analyzed to determine the composition size and mass distribution of the precious metals In order to establish accurate statistics of the gold particles a sample of heavy nonferrous metals produced from 15 tons of wet processed Amsterdam ash was analyzed by a new technology called magnetic density separation MDS Amsterdam’s bottom ash contains approximately 10 ppm of silver and 04 ppm of gold which was found in particulate form in all size fractions below 20 mm The sample from Ludwigshafen was too small to give accurate values on the gold content but the silver content was found to be identical to the value measured for the Amsterdam ash Precious metal value in particles smaller than 2 mm seems to derive mainly from waste of electrical and electronic equipment WEEE whereas larger precious metal particles are from jewelry and constitute the major part of the economic value Economical analysis shows that separation of precious metals from the ash may be viable with the presently high prices of nonferrous metals In order to recover the precious metals bottom ash must first be classified into different size fractions Then the heavy nonferrous HNF metals should be concentrated by physical separation eddy current separation density separation etc Finally MDS can separate gold from the other HNF metals copper zinc Goldenriched concentrates can be sold to the precious metal smelter and the copperzinc fraction to a brass or copper smelterThe samples of bottom ash investigated during this research were obtained from the incinerator of Amsterdam and from the incinerator of Ludwigshafen The Amsterdam bottom ash was separated partly by a wet pilot plant at the site of the incinerator and partly in the laboratory resulting in three heavy nonferrous concentrates 2 2–6 and 6–20 mm Two of the concentrates 2 and 2–6 mm were analyzed for precious metal content The 6–20 mm HNF fraction was handpicked but the number of precious metal particles found was too small to give statistically significant results The raw bottom ash sample from Ludwigshafen was treated completely in the laboratory using a process that closely mimics the process executed on the Amsterdam sample For this sample only the 2–6 mm HNF concentrate was analyzedAt an early phase of the wet process development two batches of 2–6 mm HNF concentrate together about 1600 kg were smelted and the metal product was analyzed by Xray fluorescence spectroscopy XRF Based on these preliminary experiments it was expected to find one or two 2–6 mm goldcontaining particles per ton of bottom ash Bakker 2007 In order to reliably characterize the gold in this fraction of the Amsterdam bottom ash 15 tons was wet screened and concentrated for heavy metals by kinetic gravity separation and eddy current separation resulting in 75 kg of 2–6 mm HNF concentrate The 2–6 mm HNF concentrate contains 20–30 of large pieces of glass and stone while the rest is made up of copper zinc and lead with traces of iron tin silver and gold The 2–6 mm fraction contains about 45 kg of heavy nonferrous metal per ton of dry bottom ashStatistical analysis using Gy’s formula Gy 1999 shows that if the gold concentration of the 2 mm fraction is comparable to that of the 2–6 mm fraction less than 500 kg of bottom ash is needed for the same relative accuracy On this basis the heavy nonferrous in 145 dry kg of the 2 mm fraction of bottom ash obtained by wet screening and cyclones was concentrated by jigging The HNF concentrate produced after jigging had a poor nonferrous grade so it was upgraded by removing the steel and coarse sand by LIMS magnetic separation and tabling/sink–float in a heavy liquid The resulting mixture of heavy nonferrous metals was smelted producing 06 kg of solid HNF metal This smelt was analyzed by XRF and microprobe The 2 mm fraction yields about 14 kg of heavy nonferrous metal per ton of dry bottom ash The amount of bottom ash collected from the incinerator of Ludwigshafen was only 57 kg resulting in 023 kg of 2–6 mm heavy nonferrous metal after wet physical separation Since the sample was too small to get a reliable value for gold only the silver content was measuredThe batch of 75 kg of the 2–6 mm HNF concentrate was separated by MDS at a cutdensity of approximately 10000 kg/m3 The light and heavy products of this separation were each further processed to obtain fractions that could be smelted and analyzed The heavy MDS fraction 63 kg was first separated magnetically to remove the steel 105 kg and then the remaining nonmagnetic heavy material was treated with HCl to change the color of the brass particles from yellow to red Finally the nonmagnetic heavy fraction was handpicked to separate the yellow goldcontaining alloys from the copper–alloys and the lead and silver Each of the potential gold particles was analyzed by XRF to determine the alloy and gold mass The rest of the sample was smelted and analyzed by instrumental neutron activation analysis MDS tests on a parallel sample of the 0–2 mm HNF concentrate with the aim to further concentrate the precious metals were not very successful indicating that most of the precious metal in this fraction is not made up of solid gold–alloys but of material with a density below or equal to that of copperThe basic principle of magnetic density separation is to use magnetic liquids as the separation medium Such liquids have a material density which is comparable to that of water but in a gradient magnetic field the force on the volume of the liquid is the sum of gravity and the magnetic force By a clever arrangement of the magnetic induction it is possible to make the liquid artificially light or heavyMany designs of magnetic density separators are known from the literature Kaiser 1969 Reimers 1974 Vlasov 1988 Svoboda 1998 The most regular type of separator consists of a cavity between two curved polar pieces of an electromagnet in which the field lines run mainly horizontal and the concentration of field lines the magnetic induction increases toward the bottom of the cavity If the induction could be made to depend perfectly linear in the vertical direction and the magnetization of the magnetic liquid is a constant which is nearly so for ferrofluids the effective density of the medium would be the same in the entire cavity In reality Maxwell’s equations do not allow this and therefore the density is not entirely homogeneous in the cavity The particles will converge to the middle of the cavity and this will lower the capacity Another important point is the relatively complex geometry of the cavity Iron particles are present in most waste streams and such iron will be collected at the surface of the magnet With the geometry of the cavity it is difficult to remove any iron present at the surface of the magnet The complex geometry makes it also difficult to scale the separator to an industrial size

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