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Enhancement of recycling of steelworks remnants by the SCARABAEUS®

The experts from HAVER & BOECKER NIAGARA: Project & Sales Manager Dr. Metodi Zlatev and pelletizing expert Dr. Jan Lampke about recycling of steelworks remnants by the SCARABAEUS®


 

Most metallurgical processes are characterized by a gas flow through the material bed. Thus fine particles are carried away and have to been filtert after the metallurgical devise. Every modern metallurgical plant, no matter if based on blast furnace process, oxygen steelmaking process, electric arc furnace or direct reduction process consists of filters and wet separation for the exhaust gases. In the past these dusts and sludges had been landfilled. Since several decades different extraction processes for special reasons are in industrial use and only little amounts of the remains were discarded as dumps. Most recycling-processes are faced with very small particle sizes of the steel work remnants. Thus agglomeration is necessary in order to improve transport, storage and utilisation properties, especially the reintroduce into the metallurgical process.

The pelletizing disc SCARABAEUS® is perfectly suitable and in multible use for the transmission of fine-grained particles into larger agglomerates with defined particle size distribution with smooth surface. [1]

As an example of the use of the SCARABAEUS® in the recycling of steelworks remnants, such as blast furnace dust and sludge, converter dust and sludge, electric arc furnace dust, metallurgical debris, mill scale, mill scale sludge and various slags will be focused in the following. Since these metallurgical waste materials contain impurities such as zinc and lead, the reuse of these residual materials in the blast furnace or direct reduction plant may be problematic. Some of these materials are landfilled, anyhow the very most proportion is recycled to zinc, nickel, molybdenum or chromium.

Worldwide approximately 6,750,000 t of electrical arc furnace dust are produced annually. These dusts contain 1.600.000 tpy of zinc. About 35 Waelz kilns with an average capacity of 75,000 tpy are installed worldwide. They process every year 3.400.000 tons of electric arc furnace dust [2].

In zinc recycling, zinc-containing residues are reduced by carbon carriers in rotary kilns. Zinc and lead sublimate and are re-oxidized in the air flow in counterflow and are transported and separated in downstream waste gas purification plants. In the production of Waelz oxide, lime is used for slag formation. Prior to the Waelz process, the zinc-containing components dust and sludge (approx. 74 wt.%), coal (approx. 19 wt.%) and lime (approx. 7 wt.%) are homogenized and pelletized.  In opposite to the dust, the pellets have got improved transport and storage properties and most important significantly enhanced utilisation properties.

As the Waelz kiln works in counterflow, the risk of discharging unprocessed materials and contamination of the product is high. Defined particle sizes of the steelworks remants pellets are a pre-requisite for Waelz kiln optimization. Typically the pellets have a moisture content of approx. 14 wt.%. The target size of the pellets is mostly approx. 4 to 8 mm. The capacity of the SCARABAEUS® in the recycling sector is approx. 50,000 to 150,000 t/a.

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Scheme of Waelz kiln [3]

As early as 2010 HAVER ENGINEERING GmbH developed a pelletizing prozess, wich was attapted to the individual particle size requirements of the Waelz kiln process [4]. One year later HAVER NIAGARA GmbH delivered a SCARABAEUS SC 4200 for this process optimizing project in Germany. The plant was running with a vertical intensive mixer to blend the feed materials and charge them directly into the rotary kiln. To optimize the kiln capacity and the Waelz oxide output, the customer decided to produce agglomerates in the SCARABAEUS (better reaction with the process gas in the kiln due to higher permeability of the pellets). It was possible to increase the zinc output significant using the combination of a mixer and a pelletizing disc with a capacity of approx. 17 tph instead of only using the vertical intensive mixer as they done before.

The target of the project was to increase the total throughput and optimize the Zinc recovery. Due to the pellet production the customer could not only achive the set targets they exceed them.

The basic idea of the customer was using an agglomerate with high porosity to have a fast gas diffusion through the agglomerates into the inner core. Comparing the nature of agglomerates generated in a mixer and in a disc a general difference is the density of the agglomerates. Usually pellets produced in a mixing process have a higher density due to relative high energy input into the material by the agitators and a more coarse particle size distribuion compared to the roll agglomeration in a SCARABAEUS® which is characterized by its segreation effect.

The process chamber of the SCARABAEUS® is made by an inclined, rotating, flat cylindrical pan. Due to rotation of the pan, the fine material is pulled along to the disc uppermost point and then rolling down onto a material bed. Caused by the special movement of the material itself and the addition of water onto the material, nucleation and growing occurs as a result of the settings of the pelletizing disc. Typically, final pellets with a very tight particle size distribution are discharged continuously over the board of the SCARABAEUS®.

Due to the good porosity of the pellets and the teight particle size distribution what leads to a high pellet bed / bulk permeabilty the capacity of the kiln and the Zinc recovery increase.

 

In the following years that process was adapted on similar or compareble processes. Pelletizing disc with diameters from 2.2 m to 5.2 m are to be found nowadays in recycling of steelworks remnants.

HAVER & BOECKER NIAGARA supports the customers to develop and optimize their processes, machines and plants. In the test facility of HAVER ENGINEERING are various tests equipment for pelletizing available. The objective of HAVER Process Engineering is to meet customers technical requirements and economic benefits. To achieve this, it is necessary to concentrate on the whole pelletizing process, starting with raw materials preparation, dosing and mixing and binder treatment. The core-components of pelletizing are focused in detail to minimize the efforts of final product classification, material circulation as well as de-dusting, safety and environmental protection. Applying recognized technical norms and standards with tailor-made processing leads to a production process that saves energy and resources.


Sources:
[1] Eberhard Gock et al; Verwertung von deponierten eisenreichen Filterstäuben der Stahlindustrie durch Pelletierung. Recycling und Rohstoffe. 2013
[2] Noël Masson, Pascal Briol, PBSIM & BFS Consulting Engineering; Zinc One Resources Inc.: A Brief Summary of Zinc Oxide Processing Methods Available for the Bongará Deposit. 2017
[3] Eckhard von Billerbeck et al; Verarbeitung von Filterstäuben aus der Elektrostahlerzeugung im Wälzprozess. Mineralische Nebenprodukte und Abfälle. 2014
 [4] Jan Lampke et al; Solid and compact. Aufbereitungstechnik 03/2011. 2011