Rolling process
As a rolling process or rolling process , short rolling one is called metallurgical process in a rotary kiln . The rolling process is currently primarily used for processing zinc-containing residues (mainly zinc-containing steel mill dust). A basic distinction is made here between the acidic and the basic rolling process. The main product of the process is what is known as Waelz oxide, which essentially consists of zinc oxide (ZnO) and is used to extract zinc .
History of the rolling process
The beginnings of the rolling process known today go back to the 19th century. As early as 1881 George Drue proposed the vaporization of zinc for the purpose of its extraction. Originally, the rotary kiln was primarily used to concentrate lead and zinc ores , first patented in Germany in 1913. But it was also used as a roasting unit ( Debuch oven ) for desulphurising sulfidic ores. With the strong increase in zinc and lead production at the beginning of the last century, the amount of residual and waste materials generated in primary production inevitably increased . These include, for example, zinc-containing slag and clearing ash from Imperial smelting or muffle furnaces or the New Jersey process . Almost all of these were initially deposited, but the value of these materials was quickly recognized. High proportions of carbon and metal contents , which were sometimes above those of the primary ores, justified the effort to subject these substances to a recovery . Later, more and more environmental technological aspects were added, which in many places led to the processing of landfills that had accumulated over decades . Simple function and high throughput helped the rolling process to be widely used. Due to ongoing process improvements in the zinc primary smelter, however, the amount of residues and their content of recyclable materials decreased continuously. The economic basis of the rolling process was no longer given in many places, which is why the search for new input materials began. Parallel to this development, an increasing number of small blown and electric steelworks ( EAF = Electric Arc Furnace ) was put into operation. These plants already had dedusting systems in which the increasing processing of scrap containing zinc resulted in dust with sometimes high metal contents. Since steel mill fly ash was used to a small extent in the rolling process in the past, it made sense to use this new source of raw materials. The first rolling plant that mainly processed EAF dust went into operation in Duisburg in Germany in the 1970s . After that, the rolling process quickly developed into the most important process in the processing of steel mill dust and has remained so to this day. Around 80% of the total amount of processed steel mill fly ash, which corresponds to around 1.6 million tons (in 2006), goes through the rolling process every year.
Structure and functionality
The rolling work is one of a number of processes in which the oxidic constituents to be extracted are enriched via the formation of an intermediate metal phase with subsequent volatilization and reoxidation in the gas flow. Most of the unwanted residues remain in a highly viscous residue.
The steel mill dust is pelletized along with other zinc carriers such as electroplating sludge or cupola dust together with slag formers and a carbon carrier (mostly petroleum coke ) and fed into the inlet end of the slowly rotating Waelz furnace. Although there is no process-related minimum content with regard to the zinc content of the charged dust, in practice a zinc content in the preliminary run of around 20% represents the economic lower limit in most cases. A zinc content of around 50% is the upper limit, otherwise the furnace would get too hot. Sand (= acidic process ) or lime (= basic process ) are mainly used as the starting material for the slag . In the 1980s and 1990s, attempts were made with clearing ash (zinc slag from the muffle process) as a carbon carrier (20 to 45% residual carbon, as well as zinc and lead in a total of 5 to 15%). This has proven to be uneconomical. In today's systems, the rotary tube is 40 to 50 m long and 2.6 - 5 m in diameter. Due to the rotation of about 1 / min and a 2-3% inclination to the horizontal, the fixed load is gradually moved towards the discharge end against the gas flow. The system therefore works on the so-called countercurrent principle. Depending on the lining, the length and the speed of rotation of the furnace, the residence time of the feedstock in the rolling tube is between 4 and 12 hours. The material passes through the following four zones:
- Drying zone
- Heating zone with coke combustion
- Pre-reaction zone
- Main reaction zone
Four zones
Drying zone
Immediately after loading, the cool feed material encounters the hot furnace atmosphere at 500 to 1000 ° C. As a result, free and bound water is evaporated and the batch dries. This drying process takes place within the first 6 m. The feedstock heats up to around 150 ° C.
Heating zone
The temperature of the feed continues to rise to up to 500 ° C and coke combustion begins. The finest carbon and volatile hydrocarbon fractions in the coke burn depending on the residual oxygen content of the furnace atmosphere with the formation of carbon monoxide CO or carbon dioxide CO 2 . The combustion process takes place here exclusively in the furnace atmosphere or at the contact surface between the furnace atmosphere and the batch.
Pre-reaction zone and main reaction zone
In this sector, oxygen in the furnace atmosphere begins to react more intensely with the carbon in the charge to form CO 2 . After the Boudouard reaction , CO 2 reacts with further carbon to form CO, which is then available for reducing the metal oxides in the feed material. In the pre-reaction zone, the feed reaches up to 900 ° C, with the first reduction reactions taking place, such as those of cadmium and cupric oxide . Iron oxides with a high oxygen content are also pre-reduced. As the temperature rises, the reduction potential increases and leads increasingly to the decomposition of other metal compounds such as zinc oxide and zinc ferrite . At a maximum temperature of around 1300 ° C to 1400 ° C, lead , chlorine , fluorine and alkalis are volatilized in addition to zinc . These follow the zinc in the process gas flow from the furnace.
The rate of zinc volatilization varies over the length of the furnace, but reaches its maximum approximately at the same time as the zone of highest temperature (see diagram). The total zinc output of a rolling plant is now up to 96%.
In addition to lead (II) chloride and lead (II) oxide , lead is primarily volatilized as lead (II) sulfide , as it has a high affinity for sulfur. A small amount of sulfur added to the feed can therefore improve lead recovery. Lead in the slag is undesirable as it has a negative effect on the eluate values of the slag. It is characteristic of the Waelz process that reducing conditions arise in the charge, but oxidizing conditions in the furnace atmosphere (see figure). As a result, the reduced, vaporous zinc is spontaneously reoxidized in the furnace atmosphere and any existing CO is burned to CO 2 .
ZnO reduction in the feed:
Zn reoxidation in the furnace atmosphere:
Since the zinc or iron reduction is an endothermic process , the amount of carbon added in the Waelz process does not depend on the amount stoichiometrically required for the Zn reduction, but on the heat requirement of the process, which is why carbon is added significantly overstoichiometrically. Due to the counter-current principle, cold, oxygen-rich air meets a hot charge at the end of the furnace, which heats the air. At the same time, carbon combustion begins. In addition to this, the CO post-combustion and reoxidation of the gaseous zinc also provide thermal energy, which is why the process gas continues to heat up. At the same time, the furnace atmosphere is already depleted of free oxygen . Finally, hot exhaust gas meets the cold charge at the furnace inlet and heats it up before the combustion reactions begin in the charge.
In addition to its function as an energy carrier and reducing agent , the coke also has the task of aerating the softening slag. This allows gaseous Zn and CO to escape better from the slag. Rolling supports this effect by constantly renewing the contact zone between the charge and the furnace atmosphere. In order to maintain this process, the slag must not melt. Due to this, aggregates are brought in to prevent melting. Depending on which slag formers are added, a distinction is made between the acidic and the basic waelz process. The acidic process, in which silicon dioxide SiO 2 is charged mostly in the form of quartz sand , has a basicity between 0.2 and 0.5. The basic process, with the addition of lime , limestone or quick lime, is operated with a basicity between 1.5 and 4. When we talk about slag forming, we mean the partial softening of the slag in the acidic process. This softening is beneficial to a certain extent as it binds the fine fraction of the slag (such as sand). With an alkaline operation, the slag does not soften. Forming the slag is therefore not possible, but also not necessary, since the lime is also pelleted in anyway. The decision to operate a roller tube furnace in a basic or acidic manner is essentially made by the local situation with regard to the available input materials and taking into account the sales opportunities for the roller slag. Operation between the two above-mentioned basicity ranges is avoided in practice, as it is difficult to control and usually leads to problems such as increased build-up of deposits on the furnace refractory lining.
The rolling slag occurs as a by-product when processing steel mill dust. After the feed material has passed through the rotary kiln and the volatile components have been separated off, the feed leaves the furnace as rolling slag. While still hot, at over 1000 ° C, it falls into a water bath and cools down quickly there. A magnetic separation device makes it possible to recover some of the carbon contained in the slag before the residual slag is removed from the process. Depending on the selected rolling process, the slag can be acidic or basic, whereby it essentially only differs in terms of its SiO 2 and CaO content. While the basic slag has CaO values of 15-25 % and only 8-9% SiO 2 , the CaO content of the acidic slag is lower at 6-12% and the SiO 2 content is correspondingly higher at 35-37%. The zinc and lead contents are usually up to 2% each. Since dumping the slag is associated with high costs, attempts are being made to sell it as building material (e.g. for road construction). The proportion of the elutable components (should be as low as possible) in the slag plays a central role.
The exhaust gas cleaning systems of rolling systems have to meet significantly higher requirements than those of the rest of the metallurgical industry. This is because the actual product, namely the Waelz oxide, is also discharged with the exhaust gas. Coarse particles are caught in a dust settling chamber immediately after leaving the furnace and fed back into the process. The process gas is then cooled and cleaned in several stages. Nowadays, the raw zinc oxide obtained is processed almost exclusively hydrometallurgically in a zinc production electrolysis. Since the relatively high alkali and halide content (generally fluorine and chlorine) would cause problems in this, the crude oxide is nowadays cleaned of these via a two or three-stage leaching. The resulting solid residues are returned to the roller tube for processing. The double leached Waelz oxide achieves halogen values of <0.1% chlorine and <0.15% fluorine and can therefore usually be used directly in zinc electrolysis.
The SDHL process
The rolling process is still considered the best available technology to recycle zinc and lead-containing steel mill dust. One parameter of this process to be assessed critically is its high energy consumption. This can be explained simply by the composition of the rolling slag, which has a high proportion of residual coke and metallic iron. The SDHL process (named after the inventors: Saage, Dittrich, Hasche and Langbein) is a further development that drastically reduces the energy consumption of the rolling system and at the same time improves throughput and zinc output.
While in the conventional Waelz process the coke is added in excess of stoichiometry in order to cover the heat requirement, in the SDHL process only about 70% of the carbon required for metal oxide reduction is used. This is possible because in this process the previously reduced iron is reoxidized again through targeted exposure to fresh air at the end of the furnace, so that the heat released is available to the process. The residual carbon content of the SDHL slag drops to <1% and the metalized iron content to <10%. The released energetic potential of the slag is so high that there is no need for continuous additional heating with natural gas burners. In addition, the zinc output increases from around 90% to up to 96%. Despite a 20% increase in furnace throughput, CO 2 emissions are reduced by around 40%.
If no iron is recovered from the rolling slag, the SDHL process is currently (2009) the economically and economically best variant of the rolling process. The process can also be integrated into existing systems with relatively little technical effort.
Rolling systems in Germany
- Harz-Metall GmbH in Goslar / Oker
- Befesa Zinc Duisburg GmbH (formerly Berzelius Umwelt Service ) in Duisburg
- Befesa Zinc Freiberg GmbH in Freiberg (Saxony)
literature
- Kozlov, PA, The Waelz Process, Ore and metals - publishing house, Moscow, 2003.
- Meurer, U., Extraction of Zinc Oxide from Secondary Raw Materials - New Developments in the Waelz Process, Issue 87 of the GDMB series of publications (2000), 183–196.
- Rütten, J., Application of Pyro-Metallurgical-Processes on Resource Recycling of Steel Mill Dust, 55th session of the zinc technical committee of the GDMB, Balen, Flanders, Belgium, (2007).
- Matl, B., Processing zinc-containing dusts from secondary metallurgy, diploma thesis, Montanuniversität Leoben, 2004.
- Zechner, M., Reduction of aggregates in the rolling process while avoiding buildup, Diploma thesis, Montanuniversität Leoben, 2009
- Tafel, V. and K. Wagenmann, Textbook of Metallurgy, Volume II, 2nd Edition, S. Hirzel Verlagbuchhandlung Leipzig, Leipzig, 1953.
- Czernecki, J., E. Stós and J. Botor, Technology of EAF Dust Treatment in Rotary Furnaces, Proc. of EMC 2003, Hanover, Lower Saxony, Germany (2003), 465-479.
- Zunkel, AD, EAF dust as an electrolytic zinc resource, Proc. Third International Symposium on Recycling of Metals and Engineered Materials, Point Clear, Alabama, USA, (1995), 579-587.
- Franz Pawlek: Metallurgy . Walter de Gruyter & Co., Berlin 1982, ISBN 3-11-007458-3 .
Individual evidence
- ↑ Matthias Becker: Metal balance for the rolling work in the rotary kiln of Harz-Metall GmbH - study thesis . Unpublished, Goslar 1993.
