Enhancement of Recycling of Steelworks Remnants by the SCARABAEUS®

Most metallurgical processes are characterized by a gas flow stream through a material bed. Thus fine particles are carried away and have to be filtered after the metallurgical devise. Every modern metallurgical plant, no matter if based on the 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, the dust and sludges had been landfilled. For 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 steelwork remnants. Therefore, agglomeration is necessary to improve transport, storage, and utilization properties, especially the reintroduce into the metallurgical process. The pelletizing disc SCARABAEUS® is perfectly suitable and in multiple uses for the transmission of fine-grained particles into larger agglomerates with defined particle size distribution with a 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 tons of electrical arc furnace dust are produced annually. The dust contains 1,600,000 tons per year (tpy)of zinc. About 35 Waelz kilns with an average capacity of 75,000 tons per year are installed worldwide. They process 3,400,000 tons of electric arc furnace dust[2] every year.

In zinc recycling, zinc-containing residues are reduced by carbon carriers in rotary kilns. Zinc and lead sublimate and are re-oxidized in the airflow 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. Before the Waelz process, the zinc-containing components dust and sludge (approximately 74 weight percent), 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 utilization 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 steelwork’s remnants pellets are a pre-requisite for Waelz kiln optimization. Typically the pellets have a moisture content of approximately 14 wt. %. The target size of the pellets is mostly approximately four to eight mm. The capacity of the SCARABAEUS® in the recycling sector is approx. 50,000 to 150,000 tpy.

As early as 2010 Haver Engineering GmbH developed a pelletizing process, which was adapted 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 to realize the 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 the higher permeability of the pellets). It was possible to increase the zinc output significantly using the combination of a mixer and a pelletizing disc with a capacity of approximately 17 tph (tons per hour) instead of only using the vertical intensive mixer as they did 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 achieve 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 relatively high energy input into the material by the agitators and a more coarse particle size distribution compared to the roll agglomeration in a SCARABAEUS®, which is characterized by its segregation 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‘s 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 occur 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®. If necessary, a changeable rim height could be used to increase the tightness of particle size distribution.

Due to the porosity of the pellets and the tight particle size distribution what leads to a high pellet bed/bulk permeability, the capacity of the kiln and the zinc recovery increase.

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

Haver & Boecker Niagara supports the customers in developing and optimizing their processes, machines, and plants. In the test facility of Haver Engineering are various test equipments for pelletizing available. The objective of Haver Process Engineering is to meet customers’ technical requirements and economic benefits. Therefore, it is necessary to concentrate on the whole pelletizing process, starting with raw materials preparation, dosing, 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

 

(GR32020, Page 38, Advertorial, Photo: HAVER NIAGARA GmbH)