Waelz Kiln Research Articles

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.

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.

Drying Kinetics of Microwave-Assisted Drying of Leaching Residues from Hydrometallurgy of Zinc.

In the hydrometallurgical process of zinc production, the residue from the leaching stage is an important intermediate product and is treated in a Waelz kiln to recover valuable metals. To ensure optimal results during the Waelz kiln process, it is necessary to pre-treat the residues by drying them first due to their higher water content. This work studies the residue's drying process using microwave technology. The study results indicate that microwave technology better removes the residue's oxygen functional groups and moisture. The dehydration process's effective diffusion coefficient increases as the microwave's heating power, the initial moisture content, and the initial mass increase. The Page model is appropriate for imitating the drying process, and the activation energy of the drying process for the residues is -13.11217 g/W. These results indicate that microwave technology efficiently dries the residues from the leaching stage. Furthermore, this study provides a theoretical basis and experimental data for the industrial application of microwave drying.

The Optimal Physical Treatment Process for Production of High-grade Iron Concentrate from Waelz Kiln Slag

The aim of this study was to develop a physical treatment method for obtaining iron concentrate with a grade higher than 60% from slag generated in the Waelz Kiln (WK) process. The size of the WK slag used in this work was several millimeters, with T-Fe and ZnO contents of 40.24% and 1.03%, respectively. The slag samples were pulverized in a laboratory rod mill for various grinding times, followed by a magnetic separation test under a low magnetic intensity of 0.05 T initially. The results showed that the highest iron grade was obtained from samples with a grinding time of 10 min. Additionally, for a grinding time of 20 min, the highest grade and recovery ratios were achieved at a magnetic intensity of 0.8 T. Based on these findings, the optimal physical treatment process proposed in this study resulted in iron concentrates with a grade of 61.34% and an enrichment ratio of 1.52.

Life Cycle Impacts of Recycling of Black Mass Obtained from End-of-Life Zn-C and Alkaline Batteries Using Waelz Kiln

The utilization of end-of-life batteries (including Zn-C and alkaline batteries) is one of the areas that need to be perfected in order to provide environmental and human safety as well as to contribute to closing the material loop, as described in the EU Green Deal. The presented study shows the environmental impacts of the two selected pyrometallurgical technologies (processing of the black mass from waste Zn-C and alkaline batteries as an additive to an existing process of the recycling of steelmaking dust and treatment of the black mass as the primary waste material, both processes performed in a Waelz kiln). The presented LCA-based study of the recycling of end-of-life Zn-C and alkaline batteries focused on terrestrial ecotoxicity can be a useful tool in the process of the development of a circular economy in Europe, as it provides a multi-disciplinary overview of the most important environmental loads associated with the described recycling technologies. Therefore, the goal of the presented study was to compare the environmental performance (utilizing LCA) of two different metallurgical processes of black mass utilization, i.e., the conventional method utilizing black mass as a co-substrate and the newly developed method utilizing black mass as a primary substrate.

Efficient Corrosion/Erosion-Resistant Linings for Rotary Kilns

We studied the service conditions for the lining of the Waelz kilns at the Chelyabinsk Electrolytic Zinc Plant and the Ust’ Kamenogorsk Lead and Zinc Plant, and identified the primary sources of wear on the refractories. Refractory wear inWaelz kilns primarily occurs via a chemical reaction between the refractories and the components of the charge, which is then followed by abrasive wear. PKhPP [periclase-chromite fused grains] and KhPT [chromite-periclase thermal shock resistant] refractories were found to be the most stable under the service conditions in Waelz-kiln linings. The refractories were impregnated with coal-tar asphalt for improved durability. A test batch was produced and tested in the lining of a Waelz kiln. High temperature glue — for Waelz-furnace linings at various non-ferrous metallurgical plants — and mortar — for ceramic joints with better physical and chemical properties throughout the entire lining — were developed. The shape, optimum dimensions, and design of the ceramic products, and brickwork designs based on these products were all studied. The brickwork designs were tested inWaelz kilns at several ferrous and non-ferrous metallurgical plants.

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.

Thermo-environmental evaluation of a modified Waelz process for hazardous waste treatment

Electric arc furnace dust is a waste by-product of electric arc furnaces in steel production. This material is hazardous to the environment due to the presence of metals, such as chromium, lead, and cadmium; in addition to these, there is zinc in its composition. One of the main ways of recovering zinc from the electric arc furnace dust is carried out in rotary kilns called Waelz kilns. This work aims to develop a model of a Waelz kiln and evaluate the impact of processing sludge from an industrial effluent treatment plant as a feed along with electric arc furnace dust, with and without the inclusion of a drying system for the sludge and a thermal integration system with the Waelz slag. The model, validated against industrial data plant, revealed an optimum operational point, resulting in a zinc recovery of 98.3 %, which is equivalent to an increase in the production by 6.2 %. The scenario with direct sludge feed into the kiln resulted in a maximum zinc recovery rate of 89 %. The results from the sludge drying scenario imply a zinc recovery of 94 % and represent a viable alternative for reducing the amount of solid waste generated in the zinc production plants that use the Waelz kiln by 1100 t/year per tonne of sludge treated. Furthermore, it also increased zinc production to 1300 t/year.

Development of energy-efficient method for processing industrial waste

An energy-saving method for processing technogenic waste has been developed — a smelt layer with inversion phase as a combination of “ideal” mixing and “ideal” displacement regimes. On its basis, a new generation of melting unit was created - the “reactor inversion phase - rotary kiln”. Experimental data show that in the inversion phase layer the specific fuel consumption for processing the “poor” on zinc and “rich” on zinc slags is approximately the same. The latter provision contradicts the prevailing opinion of metallurgists that the processing of slag with a zinc concentration of less than 5% is unprofitable. Сalculation results demonstrate that in case of implementation of an industrial sample of “reactor inversion phase - rotary kiln for processing “poor” slag, compared to the Waelz kiln processing “rich” slag, the specific consumption of fuel will be reduced by 1.5-1.7 times and specific productivity will increase 1.4-1.5 times. The industrial realization of “reactor inversion phase -rotary kiln” would allow cost-effective processing of fuming slag dumps, Waelz clinker, “poor” zinc ores, enrichment tails and other non-ferrous metal wastes.

Vanadium-rich slags from the historical processing of Zn–Pb–V ores at Berg Aukas (Namibia): Mineralogy and environmental stability

The historical mining and processing of Zn–Pb–V ores at Berg Aukas in northern Namibia left large amounts of various wastes. This study focuses on the mineralogical and geochemical investigation of the V-rich slags issued from the processing of the local ores in the Waelz kiln, which was operational between 1968 and 1980 and left ca. 500 kt of slag deposited on the adjacent dump. A combination of mineralogical methods, bulk chemistry, leaching tests and speciation-solubility modeling was used to understand the binding of the major contaminants (Zn, Pb, V) in the solid phase and their potential release under the changing environmental conditions. The average concentrations of the metal(loid) contaminants in the slags are 3.78 wt% Zn, 3370 mg/kg Pb, 5880 mg/kg V, 767 mg/kg Cu, 578 mg/kg As and 92 mg/kg Sb. The mineralogy is dominated by high-temperature silicates (clinopyroxene, melilite, olivine-family phases) and Zn-bearing phases (willemite, zincite). All the primary silicates and oxides are Zn-rich, but vanadium is mainly concentrated in clinopyroxene (up to 5 wt% V2O3). Metallic Fe inclusions, formed under highly reducing conditions in the kiln, are highly weathered. Secondary Fe(III) (hydr)oxides, corresponding to the main weathering products in the slag, efficiently sequester the metal(loid)s (mainly As and Sb). The EU regulatory leaching tests indicated that the release of the metal(loid) contaminants is quite low at the natural pH (deionized water extract: 8.5–10.4) obtained by extraction in the deionized water and only Sb in all the slag samples exceeds the EU limits for the landfilling of inert waste. The pH-static leaching tests revealed up to 5 orders of magnitude higher release of Pb and Zn under acidic conditions (up to 38% and 63% of their total concentration, respectively), compared to the natural pH. In contrast, V exhibits relatively flat pH-dependent leaching patterns with only <1.6% of the total V leached. Using the slag re-processing costs by acidic (bio)leaching and the current metal prices, the recovery of V, being the most important critical metal in the Berg Aukas slags, seems to be non-economical.

Development of the Metallurgical Waste Recovery Process

We have developed the process that enables receiving pellets from zinc-containing dust and recovering them in a Waelz kiln with practically total zinc distillation and its capture in bag filters. As this takes place, metallic iron, still present in the pellets, can be used in metallurgical treatment. We have elaborated the process instructions to obtain pellets from zinc-containing dust with an addition of solid fuel, magnesite, and bentonite. To simulate recovery of the carbon-containing pellets in production, bench tests have been performed. These allowed us to determine principal operating conditions of the technological procedure of metallurgical zinc-containing dust recovery and validate the unit for recovery of this dust. The data obtained from the production tests prove feasibility of implementation of this process and allow assessing economic efficiency of zinc-containing dust recovery.

Development of Charge Pelletizing Technology Based on Electric Arc Furnace Dust for Pyrometallurgical Processing in Rotary Kilns

New technology is developed for Waelz treatment of electric arc furnace dust with the addition of calcium oxide to a charge. In this case, lead and halides are transferred into distillates. Calcium ferrite and zinc oxide remain in clinker. Clinker is sent for leaching zinc followed by zinc electrolysis from solution. The insoluble residue of calcium ferrite is used in ferrous metallurgy. The efficiency of this technology is determined by the process of charge preparation for loading into a Waelz kiln. In order to provide complete reaction of zinc ferrite with calcium oxide, it is necessary to provide effective component mixing; in order to provide selectivity of halide and lead liberation, reducing distillate volume, a powder mixture is pelletized. Exploratory research is conducted for preparing pellets of a mixture of electric arc furnace dust with lime, and the effect of some binder reagents on pellet strength is demonstrated. The following optimum parameters are determined for charge pelletizing from electric arc furnace dust and calcium-containing flux under conditions of a plate-like granulator: pelletizing duration 15 min; granule moisture content 19–20%. With the use of an intense type mixer-pelletizer, the optimum moisture content is 13–14%, which is connected with the improved conditions for pellet compaction. Comparison of various inorganic and organic additives for mixture granulation shows that the best are additives containing sodium hydrate sulfate, although their optimum consumption is unacceptable for the technology of material pyrometallurgical treatment. The presence in the production mixture of calcium-containing flux makes it possible to use it also as a binding additive for pelletizing calcium hydroxide in the form of an aqueous suspension. Feeding a pelletized mixture of electric arc furnace dust with lime into a Waelz kiln makes it possible to provide effective reaction of charge components in the kiln and to reduce to reduce escape of dust.

Trouble-Free Construction of a Large Diameter Waelz Kiln Lining with Increased Life

Reasons for formation of large accretions are studied from results of work. A number of high-temperature refractories of magnesia composition are studied, exhibiting the least impregnation by melt reagents, high service temperature, and abrasion resistance. It is established that the most reaction resistant refractory is periclase-carbon fuzed refractory with an increased Cr2O3 content (PKhPP). A zone is selected for using PKhPP and combination of sections of PKhPP and PKhS refractories is established. PKhS and PKhPP refractory thermal conductivity is studied. Tests demonstrate absence of accretion formation in the reaction zone, and an increase in Waelz kiln lining life in the Ust-Kamenogorsk Lead and Zinc, and Leninogorsk Polymetal Combines by a factor of 1.6 – 1.8.

Pyrolysis Gas as a Renewable Reducing Agent for the Recycling of Zinc- and Lead-Bearing Residues: A Status Report

The topic “Zero Waste” has been in existence for several years in the industry, and the metallurgical industry has also made efforts to reduce the amounts of residues occurring and have started several investigations to cut down on metallurgical by-products which have to be landfilled. Especially, the additional costs for CO2 emissions in different metallurgical steps have led to investigations into alternative carbon carriers. Charcoal has been identified to serve as an ideal substitute due its CO2-neutrality. For the applications of this renewable carbon carrier in metallurgical processes, charcoal production by means of a carbonization process needs to be optimized. As a by-product during the heating of agricultural wastes or wood by excluding air, pyrolysis gas occurs. Due to the existence of combustible compounds in this gas, an application as a reduction agent instead of fossil carbon carriers in metallurgy is possible. Based on the prevention of dumping metallurgical by-products, an investigation has been developed to treat zinc- and lead-containing materials. To realize this, a dedicated process concept has been designed and developed. As the main focuses, the usage of the pyrolysis gas from charcoal production for the Waelz kiln process and the recycling of zinc- and lead-containing Waelz slag, resulting from the processing of steel mill dust in a vertical retort, have to be mentioned. Within this research, the process concept was executed from laboratory-scale up to pilot-scale testing, described in this article.

Silver recovery from zinc metallurgical sludge – analysis of solutions

During the hydrometallurgical process of zinc production, conducted in the ZGH “Boleslaw” S.A. in Bukowno [Mine and Metallurgical Plant], about 40,000 tons of sludge is generated. After dehydration in the Larox filter presses, sludge contains ca. 16-18% of Zn, 20-25% of Fe, and 200-300 ppm of Ag. Next, sludge is transported to the Olkusz concentrator for flotation to obtain concentrate enriched with Ag (1,000-1,500 ppm). The concentrate is then sent to the HC “Miasteczko Śląskie” [zinc smelter], while the flotation tailings are subjected to recycling in waelz kiln in Bukowno to regain mainly Zn and Pb, in the form of oxides (also sent later to the HC “Miasteczko Śląskie”).

Silver recovery from zinc metallurgical sludge – analysis of solutions

During the hydrometallurgical process of zinc production, conducted in the ZGH “Bolesław” S.A. in Bukowno [Mine and Metallurgical Plant], about 40,000 tons of sludge is generated. After dehydration in the Larox filter presses, sludge contains ca. 16-18% of Zn, 20-25% of Fe, and 200-300 ppm of Ag. Next, sludge is transported to the Olkusz concentrator for flotation to obtain concentrate enriched with Ag (1,000-1,500 ppm). The concentrate is then sent to the HC “Miasteczko Śląskie” [zinc smelter], while the flotation tailings are subjected to recycling in waelz kiln in Bukowno to regain mainly Zn and Pb, in the form of oxides (also sent later to the HC “Miasteczko Śląskie”).

Improvement of the Waelz Process for Slag Components Based on Solid-Fuel Combustion Mechanisms

Drying, heating, and combustion of lumps of coke in a Waelz kiln produces gases released with active combustion of carbon in the oxidizing zone in a counterflow regime with air blast entering through the kiln discharge end. Under conditions of a charged layer of lumps of coke and materials with occurrence of them at a surface from the direction of air stream supply, there is initially development of oxidation processes with significant release of heat and an increase in charge temperature. After charging, particles of solid components descend into a lower layer and a new portion of movable material is charged. There is almost no gas movement, and oxygen concentration is close to zero. As a result of this, oxidation processes within a layer slow down considerably with a reduction in heat liberated, and under conditions of a steady-state temperature regime, there is temperature equalization throughout the cross section of lumps of charge. With a limited capacity for pelletizing starting materials due to a shortage of factions less than 0.4 mm, the pelletizing time under conditions of a drum pelletizer 3 m in diameter and 6 m long is 7.4 min, with an overall path covered by particles of 103 m, and the degree of development of this process does not exceed 17.41% with a change in average granule diameter from 10.64 to 11.06 mm. With introduction into a charge composition of peat, there is an improvement of material pelletizing capacity with increased intensity of heat transfer to fired granules within a kiln and an improvement in strength properties. There is an increase in intensity of heat exchange processes in a charge layer with improvement of contacts between ore and carbon particles. On heating of ore-peat briquettes to 1000–1050°C, there is internal heating and there is an excess concentration of CO within exhaust gases in the range up to 300–320°C. The presence within the composition of heated materials of metal oxides provides development of reduction in a stream of carbon monoxide followed by oxidation (after burning carbon with gas phase oxygen). The use of these measures in implementing the Waelz process will make it possible to increase the yield of zinc oxide by 5–10% with an increase in unit productivity by 10–15% and a reduction in specific coke consumption by 5–7%.

Fused Refractory Oxides Increasing Nonferrous Metallurgy Unit Lining Life

Technology is developed for preparing periclase-chromite composition refractories based on fused grains of increased thermal-shock resistance. Technology is created for preparing periclase composition refractories based on fused grains exhibiting a high service temperature (up to 1900°C) and improved thermal shock resistance. Refractory material SHS-technology is developed on whose basis fused refractory objects, brickwork mortars, and torcrete mixes for hot torcreting are created. Refractory materials are turned out on an industrial scale in OAO Kombinat Magnezit. They are used for the most worn parts of linings such as the tuyere zone in horizontal converters, the slag belt of ore-roasting furnaces (ORF), in the reaction zone of a Waelz kiln, ORF outlet assemblies, Kivcet unit arches, and Vanyukov furnaces. Lining life is increased by a factor of 1.5 – 2.5.

New hydrometallurgical approaches for stainless steel dust treatment

In production of stainless steel 10–70 kg of dusts per tonne of steel is produced. The final rolling and pickling stages produce significant amounts of metal-rich wastes and acid sludge. The dusts are usually treated by pyrometallurgical methods such as plasma processing or Waelz kiln. Hydrometallurgical methods have been developed especially for the recovery of zinc from electric arc furnace (EAF) dusts, focusing on unalloyed steel EAF dust. A new concept has been developed to selectively leach out zinc from stainless steel production dusts enabling recycling the remaining iron, nickel and chromium compounds back to melting. Zinc can be dissolved using either a strong alkaline solution or strong acid solution. Factors affecting the leaching efficiency have been determined. Mineralogical factors limiting the zinc dissolution have been determined and pre-treatment methods have been developed. Using the optimal conditions the zinc level in the treated dust has been low enough allowing recycling of the dust to melting.

INDUTEC®/EZINEX® Integrate Process on Secondary Zinc-Bearing Materials

Steel used for many different applications is more and more often coated with zinc for corrosion protection purposes. The growing amount of Electric Arc Furnace Dust (EAFD) generated during the production of steel requires treatment that is able to convert this dust into usable materials. The current state of the art for the treatment of these wastes is based on the use of a Waelz Kiln, or any other zinc fuming system, where the zinc contained in the EAFD is concentrated in Crude Zinc Oxide (C.Z.O.), which is then fed to sulfate electrowinning or to the Imperial Smelting Process to produce zinc metal. These two processes require that the C.Z.O. be pre-treated, or at least washed, to meet the requirements of the processes. Engitec developed the EZINEX® Process that is able to convert C.Z.O. and other zinc oxides directly to metallic zinc cathodes. Engitec also studied the INDUTEC® Process, a pyrotechnology competitive to the Waelz Kiln. The INDUTEC®/EZINEX® Integrate Process will be described.