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In this study metallurgical processing of two different kinds of zinc-bearing residues have been performed: Zinc A and Zinc B. These residues have been stored for over 15 years in Rotterdam Harbor. The chemical compositions of the residues have been determined and showed that zinc ferrite is a major phase present. Zinc ferrite is not soluble under normal alkaline and acidic conditions and is not recovered by the Waelz-process, which is commonly employed for such zinc-bearing residues. An innovative flowsheet for processing zinc ferrite-bearing residues has been developed during a pre-feasibility study, with goal to selectively recover zinc, including zinc from zinc ferrite. The innovative flowsheet consists of the following steps: (1) Water pre-washing: removing water soluble salts, in particular the chlorides in Zinc A (~9%). (2) 1st step alkaline leaching with caustic soda (NaOH): dissolving free ZnO into solution, for both water-washed Zinc A and original (unwashed) Zinc B. (3) Roasting of the first leach residue in the presence a suitable reagent: decomposing the zinc ferrite to free ZnO. (4) 2nd step alkaline leaching with NaOH: dissolving all free ZnO into solution. (5) Solution purification by cementation: removing impurities in particular lead and copper, by using zinc powder. (6) Electrowinning of zinc in NaOH solution: the purified zinc bearing solutions are subsequently precipitated to the final product of Zn metal. Optimal operating conditions for the processes are deduced from a literature review in which similar residues are processed. Additionally, optimal operating conditions for the conversion of zinc ferrite into zinc oxide has been investigated using synthetic zinc ferrite with addition of Mg(OH)2, Ca(OH)2, NaOH, or Na2CO3. Finally, Na2CO3 has been chosen as reagent and used in experiments with real zinc-bearing residues. Zinc A is water washed to remove the chlorides present. Then both the water washed residue of Zinc A, and Zinc B, are leached in an alkaline solution of 5M NaOH at 90˚C for 1 hour. Both zinc and lead are selectively extracted, leaving iron oxides and zinc ferrite in the residue. The filtercake is fused with Na2CO3 at 950˚C for 2 hours to convert zinc ferrite into zinc oxide. The calcined product is leached in fresh alkaline solution of 5M NaOH to recover zinc. The final residue is then water washed to remove residual sodium. The filtrates from the first and second leaching step are purified, with use of zinc dust, or directly used for electrowinning experiments. The removal efficiency of chloride, sodium and potassium during water washing of Zinc A were 62%, 41% and 71% respectively. Overall dissolution yields for Zinc A and Zinc B of zinc and lead were 82%, 80% and 64%, 78% respectively. Cementation of impurities (Pb, Cu, Cr) with zinc dust followed by an electrowinning step achieving a grade zinc deposit of 94%. Finally, it can be concluded that the combined hydro -and pyrometallurgical flowsheet is technically feasible. Furthermore, results can be improved further by optimization of major operating steps.

This thesis studies the zinc extraction from spinels through hydro and pyrometallurgical processing. Two zinc-bearing spinels are covered: zinc ferrite ZnO·Fe2O3 to a limited extent and gahnite ZnO·Al2O3 as the main subject, compounds which are found naturally on the Earth’s crust as well as in industrial residues from the zinc industry, steel industry and others. Zinc ferrite contains 27 % of Zn and 33 % as ZnO; resource recovery from ferrite has been studied already in the past. On the other hand, the processing of gahnite, containing 35 % of Zn and 44 % as ZnO, is studied more extensively since research in the field of extractive metallurgy is effectively non-existent. Hence, the main objective of the present thesis is finding routes of treatment for this spinel. Zinc ferrite was produced synthetically at the CiTG/3mE labs by mixing equimolar amounts of ZnO and Fe2O3 at 1100 °C. Gahnite was produced by an analog method, a mixture of equimolar amounts of ZnO and Al2O3. The first approach was hydrometallurgical. Atmospheric hot acid leaching (4 M, 95 °C, 120 min, L/S 40) was performed with H2SO4, HCl and HNO3, resulting in a non-detected dissolution of the compound. Pressure leaching (90 min, L/S 40) was carried out in an autoclave with H2SO4 and HNO3, resulting in a low (2.9 %; 0.75 M, 140 °C, 3.6 bar) and a moderate extraction (22.2 %; 4.0 M, 250 °C, 39.7 bar) respectively. The second approach was pyrometallurgical processing (60 min dwell, 10 °C/min heating rate), divided into two sub-routes. A series of carbothermic tests (1:1.25 stoichiometric ratio) successfully led to a full reduction of the spinel at 1300 °C (99.90 % extraction of zinc). Aluminothermic tests (1.5:2 stoichiometric ratio) successfully resulted in a 99.98 % zinc extraction at 1200 °C. The mix of gahnite and ferrite with carbon at 1300 °C produced a 99.65 % extraction of the metal. Addition of ZnO to the previous mixture resulted in a 100 % extraction, at 1300 °C. Further experiments with gahnite at 1200 °C by adding SiO2, first with carbon and later with aluminium, resulted in a moderate 23.14 % and a low 4.69 % extraction correspondingly. Trials with CaO at 1400 °C created a glass residue and a slag, in each case. It is thus possible to establish the zinc extraction from gahnite ZnO·Al2O3 as follows: Route / Zinc extraction Atmospheric acidic leaching / Non-detected Pressure leaching / Low – Moderate Reduction with aluminium and silica / Low Reduction with carbon and silica / Moderate Carbothermic reduction / Full Aluminothermic reduction / Full

In this study a industrial survey was performed on the secondary products generated in the Dutch metal industry. From this industrial survey it was concluded that the Dutch metal industry is already far ahead with closing the materials cycle as for almost every product is a processing route to recover its metal content. The residues generated at LDM BV, a secondary brass producer located in Drunen, are currently being processed by a different company; however in their goal to have a zero footprint on the environment LDM BV (hereafter referred to as LDM) is looking for opportunities to process the secondary products on site. Two different kinds of products are generated at LDM; a filter dust containing around 80% ZnO, 8% C, 4% Cu and 1.5% Pb and a slag containing various amounts of copper ~45% and zinc ~40% in metallic and oxide form. A study was performed on the processing techniques available to process zinc containing residues and from this study several treatment options were designed and tested in the lab. The main focus was put on the recovery of zinc from the LDM filter dust as this is the first goal the company has set itself, however since the nature of the two materials is similar a combined treatment of both materials was investigated as well. First the pyrometallurgical reduction by utilizing the carbon contained in the LDM filter dust was tested at 1200oC. It was found that a maximum of 97% of the zinc could be recovered; a higher recovery at 1200oC was not possible as the remaining zinc was bound to gahnite (ZnAl2O4). The residue still contained 15% ZnO and did not form a slag. A combined reduction of both the filter dust and melting slag resulted in the recovery of 96.8% of the copper, 99.9% of the zinc (as ZnO) and 64.9% of the lead. This test was performed at 1400oC and it was found that at this temperature the gahnite would be reduced. Also by addition of lime and silica a suitable slag was formed which contained 2.18% CuO and 0.16% ZnO. Both the copper and zinc oxide produced had a purity of over 98%. Hydrometallurgical treatment was tested by using two different leaching agents, NaOH and H2SO4. It was found that the best leaching conditions for NaOH at a liquid solid ratio of 10 were 90oC using 320 g/L NaOH. After 15 minutes the maximum amount of zinc was already leached and total of 86% was leached. Lead was leached for 68% and Copper for 5%. Using 200 g/L H2SO4 at 65oC resulted in 96% of all the zinc to be leached and 90.5% of all the copper. Both residues contained mainly gahnite which is impossible to leach under atmospheric conditions. After an iron removal step for the acidic solution both leach liquors were purified with Zn dust cementation. A zinc product of 99.7% purity was produced through electrowinning for both solutions; however the power consumption for the NaOH solution was significantly lower at 2.2 kWh/kg opposed to 3.04 kWh/kg zinc produced Finally a conceptual flow sheet is given for both a pyrometallurgical and a (sulfuric acid) hydrometallurgical processing facility in a brass smelting plant.

Electrolytic zinc plants need to take measures to control the magnesium content in their process liquors, because the natural magnesium bleed does not balance the input from concentrates. Presently used methods are environmentally unfriendly (due to the production of large amounts of waste gypsum) or expensive. Therefore, an alternative process route is explored in which magnesium is removed from zinc electrolyte by selective precipitation of magnesium fluoride (sellaite). As standard applications for bulk amounts of magnesium fluoride could not be identified, a method was developed to convert the magnesium fluoride into a useful product. Magnesium fluoride can be converted into magnesium hydroxide (brucite) by contacting it with sodium hydroxide. Magnesium hydroxide, if pure enough, has a wide range of (industrial) applications. With a modified electrodialysis reactor, the sodium fluoride solution obtained from the conversion process can be converted into reagents, i.e. zinc fluoride and sodium hydroxide, which are required for previous process steps. After magnesium precipitation, it is necessary to remove residual fluoride from the zinc sulphate solution. In addition, the magnesium hydroxide appeared to contain some fluoride, which could limit the application possibilities of magnesium hydroxide. However, it proved to be possible to calcine the fluoride-containing magnesium hydroxide at high temperature (> 1000 d.C.) to sufficiently pure periclase (magnesium oxide). A rough economic evaluation indicates that the process may compete with the conventional process if international dumping fees for the dumping of gypsum become similar to the Dutch dumping fee for gypsum disposal in C2-deponies.

With the aims of understanding the protective mechanism of chromate conversion coatings and developing alternatives to chromate treatments, the physical natures and corrosion properties of Cr(VI) and Cr(III) treated zinc have been investigated in this work. The Cr(VI) treatments were carried out in dichromate and sulfuric acid solution with different dipping times. The Cr(III) treatments were carried out in two commercial solutions (A and B). The thickness of the coatings was measured using ellipsometry. The morphologies and the compositions of the treated zinc have been studied by means of SEM, AFM, AES, FTIR and XPS. The drying temperature influence on the corrosion performance of the Cr(VI)âtreated zinc has been investigated. The Volta potential in treated and untreated areas has been measured using scanning Kelvin probe (SKP) and SKPFM. The corrosion behavior of the Cr(VI) and Cr(III) treated zinc has been investigated using polarization, electrochemical impedance measurements (EIS), and salt spray tests. Both Cr(VI) and Cr(III) species were detected by XPS in the outermost layer of the Cr(VI) coatings, while no Cr(VI) species was found in the Cr(III) coatings. AES depth profile results show that chromium oxides are the main components in the Cr(VI) coatings. Zinc oxide is mainly located at the chromium oxides / zinc interface. The Cr(III) coating is a mixture of chromium oxides and zinc oxide. Both the Cr(VI) and the Cr(III) treatments can supply corrosion protection to zinc. The corrosion resistance of the Cr(III)-B coating is greater than that of the Cr(III)-A coating. However, the inhibition of the corrosion of zinc by Cr(VI) coating is more effective than by the Cr(III) coatings. The inhibition of the corrosion of zinc by the Cr(VI) and the Cr(III) treatments is discussed, and future research topics are suggested.

The basis for this research project was that copper is lost in the Leaching Plant of the Nyrstar Budel zinc smelter. In the section of the Leaching Plant copper is lost, iron and aluminium are precipitated. Sampling of this process showed that the removal of copper from the solution is a function of pH and is mainly linked to the removal of aluminium. Tests with process fluids at stable pH levels showed that ferric ion and aluminium are mainly removed as jarosites at pH = 2 and pH =2,5. Copper is also removed at those pH values and is thus precipitated in jarosite form. This is the actual loss of copper. At pH = 3 and 3,5 more aluminium was removed, but now also in the form of aluminium hydroxides. It was found that copper is adsorbed onto these hydroxides and hence more copper is removed from the solution. At these pH values also jarosites are formed which permanently remove copper, but it is assumed that the adsorption onto aluminium hydroxides prevails. The result of the adsorption of copper onto these hydroxides is that it eventually returns to the FeP in which it has another chance to get lost to jarosites. Hence the only way to prevent copper from being lost is to prevent copper from reaching the FeP. One way to prevent copper from reaching the FeP is not using calcine to neutralise the acid from the SiHALO, but with alternative neutralising agents which do not contain copper. Another way is to prevent copper from leaving the SiHAL. Copper cementation is a method which can totally remove the copper dissolved in the SiHALO. Copper cementation with iron was found to yield the highest recovery and is thus also the most cost effective. Key words: Copper incorporation, jarosite, alunite, copper cementation

The major problem during laser welding of zinc coated sheet steel in an overlap configuration is the zinc vapour produced at the interface between two sheets. The vapour tends to evacuate through the keyhole and melt pool, particularly when no gap is present between the overlapped sheets. This causes process instabilities and results in the formation of pores and severe undercut. The aim of this work is to build an understanding of the material behaviour, particularly the zinc behaviour when welding zinc coated steels in overlap configuration without a pre-set gap. The interaction between zinc vapour and welded materials was investigated mainly by experimental approaches. Questions concerning when and how stable welds can be obtained are answered by introducing a mechanism describing the dynamic balance between the zinc vapour, the keyhole and the weld pool.

Thin oxide interlayers are commonly added to the back reflector of thin-film silicon solar cells to increase their current. To gain more insight in the enhancement mechanism, we tested different back reflector designs consisting of aluminium-doped zinc oxide (ZnO:Al) and/or hydrogenated silicon oxide (SiOx:H) interlayers with different metals (silver, aluminium, and chromium) in standard p-i-n a-Si:H solar cells. We use a unique inverse modeling approach to show that in most back reflectors the internal metal reflectance is lower than expected theoretically. However, the metal reflectance is increased by the addition of an oxide interlayer. Our experiments demonstrate that SiOx:H forms an interesting alternative interlayer because unlike the more commonly used ZnO:Al it can be deposited by plasma-enhanced chemical vapour deposition and it does not reduce the fill factor. The largest efficiency enhancement is obtained with a double interlayer of SiOx:H and ZnO:Al.

Coimplantation of Zn and O ions into a single crystalline MgO and subsequent thermal annealing were applied in the synthesis of ZnO nanocrystals. Electron microscopy showed that rocksalt instead of wurtzite ZnO stabilizes for relatively large nanocrystals up to ~15 nm, resulting from its small lattice mismatch with MgO of ~1.7%. The vacancies initially created by implantation induce favorable nanocrystal growth kinetics and are effectively absorbed during the nucleation and growth processes. The optical band edge of the ZnO nanocrystals was detected at ~2.8 eV.

Document(en) uit de collectie Chemische Procestechnologie

Textured interfaces in thin-film silicon solar cells improve the efficiency by light scattering. A technique to get experimental access to the angular intensity distribution (AID) at textured interfaces of the transparent conductive oxide (TCO) and silicon is introduced. Measurements are performed on a sample with polished microcrystalline silicon layer deposited onto a rough TCO layer. The AID determined from the experiment is used to validate the AID obtained by a rigorous solution of Maxwell’s equations. Furthermore, the applicability of other theoretical approaches based on scalar scattering theory and ray tracing is discussed with respect to the solution of Maxwell’s equations.

We present an ultrafast all-optical gated amplifier, or transistor, consisting of a forest of ZnO nanowire lasers. A gate light pulse creates a dense electron-hole plasma and excites laser action inside the nanowires. Source light traversing the nanolaser forest is amplified, partly as it is guided through the nanowires, and partly as it propagates diffusively through the forest. We have measured transmission increases at the drain up to a factor 34 for 385-nm light. Time-resolved amplification measurements show that the lasing is rapidly self-quenching, yielding pulse responses as short as 1.2 ps.

We present a mathematical model that relates the surface morphology of randomly surface-textured thin films with the intensity distribution of scattered light. The model is based on the first order Born approximation [see e.g., M. Born and E. Wolf, Principles of Optics, 7th ed. (Cambridge University Press, Cambridge, England, 1999) ] and on Fraunhofer scattering. Scattering data of four transparent conductive oxide films with different surface textures were used to validate the model and good agreement between the experimental and calculated intensity distribution was obtained.

In this study, we successfully achieved a relatively high field-effect mobility of 37.7 cm2/Vs in an InZnO thin-film transistor (TFT) fabricated by excimer layer annealing (ELA). The ELA process allowed us to fabricate such a high-performance InZnO TFT at the substrate temperature less than 50 °C according to thermal calculation. Our analysis revealed that high-energy irradiation in ELA produced a mixed phase of InZnO and SiO2, leading to the deterioration of TFT characteristics.

A simulation of an optically pumped laser based on a ZnSe/Zn1−yCdySe double quantum well with a Zn1−xMnxSe diluted magnetic semiconductor barrier is presented. Giant Zeeman splitting in diluted magnetic semiconductors leads to splitting of electronic states, which in turn leads to tunability of laser wavelength by external magnetic field. Tunability is predicted throughout the wavelength range between 60 and 72 μm at low temperatures.