Waelz Oxide Research Articles

Investigation of the Waelz oxide calcination process in tubular rotary kilns

Investigation of the Waelz oxide calcination process in tubular rotary kilns

Production increase of high-grade crude zinc oxide pellets at Shisaka Smelting Co., Ltd.

Shisaka Smelting Co., Ltd. has recovered crude zinc oxide pellets from an electric arc furnace dust (EAF dust) by the Waelz kiln process since 1977. The EAF dust processing line consists of a reduction roasting process, a Waelz oxide washing process, and a drying/calcination process. In late years, about 90,000 tons of EAF dust is processed annually at Shisaka. The recovered zinc is sold to zinc smelters as crude zinc oxide pellets, and the amount of production of the pellets is about 40,000 tons per year. Two types of zinc oxide pellets are produced, and the product for electro-refinery requires high quality, especially in fluorine reduction. Most of fluorine are distributed to the clinker in a reduction roasting process, and others are distributed to the Waelz oxide. Conventionally, high-grade crude zinc oxide pellets were produced by washing the Waelz oxide and by volatilizing and separating fluorine under high temperature operation in a drying rotary kiln. However, formation of accretion in the kiln and damage of the shell and refractories of the kiln often occurred under the influence of high temperature operation, and stable consecutive operation of the kiln was difficult.To improve this situation, the fluorine input was reduced by compounding management of EAF dust in the reduction roasting process, and the material residence time in the drying rotary kiln was extended by reducing rotary speed. These improvements resulted in stable consecutive operation under low temperature conditions and increased production of high-grade crude zinc oxide pellets. To improve this situation, the fluorine input was reduced by compounding management of EAF dust in the reduction roasting process, and the material residence time in the drying rotary kiln was extended by reducing rotary speed. These improvements resulted in stable consecutive operation under low temperature conditions and increased production of high-grade crude zinc oxide pellets.

Evaluation of recycled galvanised steel as a resource for Zn production

The high reduction potential of zinc has enabled its extensive use in the steel industry as a protective galvanising coating. Galvanising, which consumes approximately 50-60% of the global production of zinc, significantly increases the life span of steel products and contributes towards addressing the costly global problem of corrosion. While there does not appear to be a readily available and agreed-upon annual quantity of galvanised steel produced, it has been estimated by the authors to be 320-390 Mt/a (equating to 15-20% of the annual steel production). Due to the various methods employed to galvanise steel and different steel shapes which are coated, there is no clarity on the average percentage of zinc which galvanised steel is comprised of. However, a zinc content of between 1.5-2.5% appears to correlate with the estimated annual galvanised steel production.The reprocessing of secondary zinc streams is becoming increasingly important due to legislation changes and increased consumer demand for recycled and low carbon metals. Within the steel industry this will include the recycling of galvanised scrap steel, using primary process routes, as well as the re-processing of waste streams, such as steel dusts, using additional technologies such as the Waelz kiln. Zinc associated with galvanised scrap steel would exclusively report to these dusts, and thus represents a key resource for recycled zinc.Although there are two main steel process routes preceding the galvanising process, the focus here is on the Electric Arc Furnace (EAF) process route due to its ability to incorporate high levels of secondary (zinc containing) steel feeds, and the dust produced typically contains economically recoverable levels of zinc. EAF dust production is estimated between 5-10 Mt/a at a zinc content of 10-36%. Failure to recover this zinc would represent a loss of 0.5-3.6 Mt Zn/a to waste. The Waelz kiln is the most prominent technology utilised to recover Zn from EAF dust. However, this process requires additional energy, and thus the associated carbon footprint needs to be included in the evaluation of the potential of Waelz oxide as a secondary Zn resource. To this end, model analysis of a simplified Waelz kiln using the HSC simulation software shows that the carbon footprint associated with the processing of EAF dust ranges from 4-12 kg CO2/kg Zn dust at 10 wt.% Zn in the dust to 1-4 kg CO2/kg Zn at 35 wt.% Zn in the dust.

Research on recovery/purification of ZnO from the Waelz oxide by NH4Cl leaching-CaO precipitation method

Zinc recovery from secondary sources has become a key issue for sustainable development of zinc metallurgy with the shortage of zinc concentrate supply. As the main intermediate product of the Waelz process, Waelz-fume has always been the focus of research. In this paper, a new hydrometallurgical process to recovery ZnO from Waelz oxides is developed. The main steps include leaching Waelz oxide with NH4Cl solution, purifying the leachate by zinc cementation, and precipitating zinc as ZnO by CaO addition. The zinc extraction can reach 87.9 % through NH4Cl leaching, and more than 99.0 % of Pb and Cd are from the leaching solution removed by the zinc cementation. Under optimal precipitation conditions, 96.6 % of the zinc in the leachate is precipitated as ZnO with high purity. The preliminary estimation shows that the process provides a cost-effective and environmentally feasible scheme for high purity ZnO preparation from Waelz oxide.

Research on Recovery of Zinc as Zno from Waelz Oxide by Leaching with Nh4cl Solutions

Research on Recovery of Zinc as Zno from Waelz Oxide by Leaching with Nh4cl Solutions

Investigation of dielectric heating method of zinc-containing materials for chlorides removal

The technological development of the world community makes it necessary to resolve the issues of the produced industrial residues utilization. Worn-out galvanized metal cars, ceilings and other metal structures require orderly utilization. Utilization of galvanized scrap may be carried out in Electric Arc Furnaces (EAF) to obtain intermediate steel and EAF dust. Electric Arc Furnaces dus contains zinc, so it is used as a secondary raw material for the production of metallic zinc. Waelz oxides are formed in Waelz kilns, after the primary processing of EAF dust. Waelz oxide contains halides that must be removed first. Halides may be removed by heating to temperatures of about 1000°C. Dielectric (microwave) heating is a promising and environmentally friendly method for material processing. Microwave heating is carried out without burning natural gas, which leads to decrease of waste gases volume. The work experimentally confirmed the possibility of zinc-containing materials heating at electromagnetic emission exposure. The duration of products heating up to the temperature of 1000°C was 128 - 188 s. The residual content of chloride ions in the calcined products is less than 0.05 wt%.

Purification of Zinc Oxide from Chlorides Using Microwave Radiation

Waelz oxide is a secondary zinc raw material and a product of Electric Arc Furnaces (EAF) dust, copper smelters dust and zinc production residues. The use of Waelz oxide in the traditional RLE (Roasting-Leaching-Electrowinning) zinc production scheme requires the removal of halogens (fluorides and chlorides). Waelz oxide is mainly composed of zinc oxide, also contains zinc chloride. Zinc chloride is removed into the gas phase at heating. Microwave heating is one of the promising methods. Test experiments of microwave heating of a ZnO-ZnCl2 mixture were carried out. It was shown that zinc chloride absorbs microwave radiation; zinc oxide does not absorb microwave radiation. The degree of zinc chloride removal from ZnO-ZnCl2 mixture was 100%.

The Study of the Melting of Waelz Oxide with an Increase in the Temperature of the Calcination Process

Waelz-oxide is a raw material for the production of metallic zinc. Waelz-oxide contains impurities of zinc and lead chlorides and fluorides. Halides have a negative effect on the process of zinc electrolysis. Halides have a relatively low boiling point; therefore, they are removed into the gas phase by calcining Waelz-oxide at 800–850 °С. To intensify the process, calcination is sometimes carried out at elevated temperatures of 1100–1250 °С. However, an increase in temperature leads to partial melting and granulation of the calcined product. In the present work, the chemical and phase composition of the initial and calcined Waelz-oxide was studied. Thermodynamic modeling of phase and chemical transformations of Waelz-oxide components during heating has been performed. Experiments on calcination of Waelz-oxide in laboratory conditions at temperatures of 600–1250 °C were carried out. It was found that partial melting and granulation of Waelz-oxide is the result of the formation of fusible eutectics containing lead oxide. Lead oxide is formed as a result of decomposition of lead sulfate when heated above 1100 °C. A similar effect is not observed at a standard calcination temperature of 850 °C.

Comparative Life Cycle Impact Assessment between the Productions of Zinc from Conventional Concentrates versus Waelz Oxides Obtained from Slags

(1) Background: The importance of Zinc in today’s world can hardly be exaggerated—from anticorrosion properties, to its durability, aesthetic, and even medicinal uses—zinc is ever-present in our daily lives ever since its discovery in ancient times. The natural, essential, durable, and recyclable features of zinc make it a prized material with uses in many applications across a wide array of fields. The purpose of this study was to compare two life cycle impact assessments of zinc production by using two different main raw materials: (A) zinc concentrates (sulfide ore) and (B) Waelz oxides (obtained through recycling existing imperial smelting process furnace slags). The Waelz oxide scenario was based on a case study regarding the existing slag deposit located in Copsa Mica town, Sibiu county, Romania. (2) Methods: consequential life cycle impact assessment methods were applied to each built system, with real process data obtained from the case study enterprise. (3) Results: Overall, the use of slags in the Waelz kiln to produce zinc oxides for use in the production of zinc metal is beneficial to the environment in some areas (acidification, water, and terrestrial eutrophication), whereas in other areas it has a slightly larger impact (climate change, photochemical ozone formation, and ozone depletion). (4) Conclusions: The use of slags (considered a waste) is encouraged to produce zinc metal, where available. The results are not absolute, suggesting the further need for fine-tuning the input data and other process parameters.

Experimental Study on the Dissolution Behavior of Calcium Fluoride

The presence of halogens has an adverse effect on the zinc extraction process through electrowinning, the last phase of the RLE (Roasting, Leaching and Electrowinning) zinc extraction route. Fluoride (F−) may be present as calcium fluoride (CaF2) and this is, for example, the case in double leached Waelz oxide (DLWO). Efficient removal of F− from primary and secondary raw materials for zinc extraction results in a simplified process and increases flexibility in the selection of raw materials. Understanding of the solubility behavior of pure CaF2 can give valuable information on treatment for maximized halogen removal. Dissolution of CaF2 was studied with the addition of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3). Dissolution studies were combined with thermodynamic calculations to understand the solubility behavior of CaF2 under different conditions. Results from the experiments and the thermodynamic calculations show that Na2CO3 and NaHCO3 have similar behavior if the pH is controlled at the same value. The available carbonate (CO32−) ion in the system limits the concentration of calcium (Ca2+) ion by precipitation of CaCO3, which enhances the dissolution of CaF2. At higher temperatures and pH, calcite, vaterite, and aragonite were formed and co-precipitation of CaF2 along with calcium carbonate (CaCO3) was observed. At lower temperatures and lower pH levels, only calcite and vaterite were formed and a coating by CaCO3 on CaF2 was found to hinder complete dissolution reaction. The results of this study indicate that the temperature along with the reagents used for the dissolution tests have a significant impact on the CaCO3 polymorph mixture (calcite, vaterite and aragonite) formation.

Waelz oxide calcination process adopted by Chelyabinsk Zinc Plant

Waelz oxide calcination process adopted by Chelyabinsk Zinc Plant

A kinetic study to recover valuables from hazardous ISF slag

Abstract The present paper investigates a reduction process using a carbon-saturated iron melt to treat an imerpial smelting furnace (ISF) slag. The process allows for the removal of heavy metals that are known to negatively impact the surrounding environment (Zn, Pb and As). The work includes a short characterization of the slag, describes the reductive metal bath process itself and finally, investigates the reaction kinetics of the process. Experiments show an almost complete removal of harmful elements from the slag and the possibility of the generation of marketable materials: A Zn- and Pb-rich flue dust, a utilizable slag, and an iron alloy. The produced flue dust has a similar composition as double washed Waelz oxide. The concentration of Zn, Pb, and As in the final slag phase was below 100 ppm corresponding to a recovery rate of > 99.9 % A kinetic model was developed describing the reaction rates of the carbothermic reduction reaction for FeO and ZnO. The temperature dependency of the reaction rate was found to follow the Arrhenius equation, the activation energies were determined to be 216 kJ/mol for ZnO and 191 kJ/mol for the FeO reduction reaction. Finally, mass transfer coefficients were calculated and visualized for both reactions.

Characterization of Double Leached Waelz Oxide for Identification of Fluoride Mineral

Double leached Waelz oxide (DLWO), with 76% zinc, is a secondary zinc containing raw materials obtained by the treatment of electric arc furnace dust. The content of fluoride in DLWO is still too high for direct leaching, as fluoride has a detrimental effect on electrowinning for zinc production. Knowledge of the characteristics of DLWO, and especially on how a fluoride mineral might exist, can contribute to further improvement of the selective leaching for the removal of fluoride. In this study, DLWO was characterized using analytical techniques, such as inductively coupled plasma-optical emission spectroscopy (ICP-OES), 19F liquid-state nuclear magnetic resonance (19F LS NMR), X-ray powder diffraction analysis (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) and 19F solid-state nuclear magnetic resonance (19F SS NMR). This study showed that DLWO mainly consisted of zincite (ZnO), cerussite (PbCO3) and a spinel containing zinc, iron and manganese. The fluoride mineral identified was calcium fluoride (CaF2). In SEM analysis, fluorine was found in larger grains together with calcium and oxygen, which was possibly calcium carbonate.

Recovery of Metals from Secondary Raw Materials by Coupled Electroleaching and Electrodeposition in Aqueous or Ionic Liquid Media

This paper presents recent views on a hybrid process for beneficiation of secondary raw materials by combined electroleaching of targeted metals and electrodeposition. On the basis of several case studies with aqueous solutions or in ionic liquid media, the paper describes the potential and the limits of the novel, hybrid technique, together with the methodology employed, combining determination of speciation, physical chemistry, electrochemistry, and chemical engineering. On one hand, the case of electroleaching/electrodeposition (E/E) process in aqueous media, although often investigated at the bench scale, appears nevertheless relatively mature, because of the developed methodology, and the appreciable current density allowed, and so it can be used to successfully treat electrode materials of spent Zn/MnO2 batteries or Ni/Cd accumulators and Waelz oxide. On the other hand, the use of ionic liquids as promising media for the recovery of various metals can be considered for other types of wastes, as shown here for the case of electrodes of aged fuel cells. The combined (E/E) technique could be successfully used for the above waste, in particular by the tricky selection of ionic liquid media. Nevertheless, further investigations in physical chemistry and chemical engineering appear necessary for possible developments of larger-scale processes for the recovery of these strategic resources.

Development of a combined solid and liquid wastes treatment integrated into a high purity ZnO hydrometallurgical production process from Waelz oxide

Electric Arc Furnace Dust (EAFD) is an important hazardous waste from the steel production industry, due to its heavy metals content. It is recycled through a pyrometallurgical process to generate a Zn and Pb concentrate called Waelz oxide. This concentrate can be processed hydrometallurgically in order to obtain high purity ZnO. But this hydrometallurgical process also generates as wastes different solid residues with high content of heavy metals and a liquid waste stream with significant concentrations of ammonia and heavy metals. This is why the objective of this work is to design a process that can simultaneously treat these solid and liquid wastes. In the developed process, an alkaline solution is used to leach solid residues so as to produce an inert final waste. After filtration, the leaching liquor is mixed with the liquid waste in order to increase its pH and to enhance the ammonia evaporation by air stripping. This stripped stream can be recycled to the leaching stage of the Waelz oxide treatment process. Furthermore, the ammonia stripping precipitates the dissolved heavy metals as a zinc concentrate that can be commercialized as a raw material for the metallic zinc production industry. Therefore, the developed process reduces by more than half the wastes generation, increases the Zn recycling yield >5% and recovers >75% of the needed ammonia for the fresh leaching liquor.

Impact of silica on cadmium cementation by zinc dust in a zinc SX/EW plant

Horsehead Metal Products is a zinc refinery in Mooresboro, NC, treating crude Waelz oxide (WOX) to produce special high-grade zinc by a hydrometallurgical process comprising leaching, zinc solvent extraction/electrowinning, melting and casting and bleed treatment. Cadmium removal by zinc-dust cementation plays an essential role in the bleed treatment circuit to prevent cadmium buildup in the circuit. Silica, though present at a low level of approximately 0.3 percent in the WOX feed material, had a significant impact on the efficiency of cadmium cementation with zinc dust, most likely due to the passivation of zinc dust particles with silica gel. Solid/liquid separation efficiency by filter press was also affected. To identify the source of the silica issues, silica levels were mapped throughout the circuit, and adjustments, including using pregnant leach solution as the feed to cementation, and pH control in the WOX leaching and neutralization were introduced to mitigate the issues created by silica gel in the circuit.

Fluoride removal from Double Leached Waelz Oxide leach solutions as alternative feeds to Zinc Calcine leaching liquors in the electrolytic zinc production process

The most important limitation of leached Waelz Oxide from the pyrometallurgical treatment of Electric Arc Furnace Dust (EAFD) is its high fluorine content. This work deals with the reduction of the harmful effects of the Double Leached Waelz Oxide fluorine content in the hydrometallurgical process for electrolytic zinc production process when it is used as raw material directly in the leaching stage. The resulting process is a purification treatment of Double Leached Waelz Oxide leach solution to remove the dissolved fluoride in this unpurified zinc liquor. Leached fluorides from this liquor are precipitated in order to reduce their concentration until the specified values, avoiding cathode corrosion in the electrowinning cells. In this paper a selective precipitation process to maximize the fluoride removal and to minimize the zinc losses is presented, achieving the industrially specified solution composition when the Zinc Calcine is the used raw material. The formation of aluminium-fluoride complexes and the variation of their solubilities with the pH evolution are the basis of the proposed treatment. In addition, this fluoride removal has also been improved and optimized in order to be integrated in the overall industrial hydrometallurgical process and the obtained results are presented and discussed proving the viability of this improved stage.

State of the Art in Steel Mill Dust Recycling

AbstractThe Waelz technology still represents the main process for treating electric arc furnace dust (EAFD). Interesting developments within this process and some new approaches transform the product into a competitor for the primary metallurgical industry by producing high grade zinc from the Waelz oxide. Another progress constitutes the utilization of the slag, which turns the Waelz technology into a zero‐waste process. An outlook regarding the future role of low zinc‐containing EAFD from basic oxygen furnaces and an overview concerning the world‐wide generation of EAFD and possible future solutions are described as well.

Investigation into the kinetic regularities of impurity transfer during the sulfuric acid leaching of the zinc-containing middlings of two-stage Waelz process of zinc cakes: Part II. regularities of sulfuric acid leaching the industrial samples of calcined Waelz oxide of zinc production

Kinetic regularities of leaching the actual industrial sample of calcined Waelz oxide of zinc production are determined. The basic reaction mechanism is established. The dissolution of solid-phase particles can be conditionally differentiated into two stages. In the initial instant, the reaction is developed in the external diffusion mode following the topochemical mechanism. The process mechanism changes at the second leaching stage, which is evidenced by the limiting character of the time dependence of recovery of zinc, which is caused by the formation of dissolving parts of reaction products forming the poorly permeable films on the surface. The internal diffusion is the limiting stage in this case.

Investigation into the kinetic regularities of impurity transfer during the sulfuric acid leaching of the zinc-containing intermediate product of two-stage Waelz process of zinc cakes: Part I. leaching kinetics of individual model oxides

Kinetic regularities of impurity transfer into the solution when leaching the calcined Waelz oxide of zinc production are determined by leaching the individual iron, copper, and lead oxides. The escape of impurities into the solution during when the model iron and copper oxides is limited by the induction developmental period of the reaction, which is attributed to the selective adsorption of protons on the oxide surface and separate active centers with the subsequent diffusion of protons into the oxide phase bulk. The chemical interaction itself is the slowest stage at the degree of recovery α < 0.5, and the reaction runs in the kinetic mode.