During steelmaking by means of the oxygen blowing process (LD process), approximately 900,000 tons slag are produced in Austria per year. The mineral processing represents one utilization concept. The metals remain in their oxidic state; however, due to the separation of undesired elements such as phosphorous and mineral slag formers, in principal, the metal fraction can be reused in the steelmaking process. The generally known disadvantage of mineral LD slag processing routes lies in an incomplete separation of the metal fraction due to fine-grained intercrystal growths.
Beside this, LD slag treatment under reducing conditions possesses a higher potential regarding a more complete metal recovery and separation of the mineral fraction respectively. The proof-of-principle for the generation of a feed material for the cement industry with latent hydraulic properties was demonstrated in the first funding period of the K1-MET programme. However, some open questions remain for further necessary research work to realize a mineral product matrix with a maximum possible glass content to finally reach a certain strength level of the hardened mineral product.
Beside LD slag, recycling possibilities for secondary metallurgical slags are explored. A large part of these slags (e.g. ladle furnace slag) is currently landfilled. In literature, there are several possible utilization routes for secondary metallurgical slags e.g. in the construction industry. Similar considerations regarding slag utilization exist in nonferrous metallurgy. Slag from copper refining contain several valuable metals beside copper such as nickel, lead, tin and zinc. A separation of these metals can enable a utilization of the residual mineral fraction. Slags from copper metallurgy are currently used as construction material or as blasting agent for the surface treatment of steel products. Increasing stringent legislation makes the utilization of nonferrous slags more complicated and demand for an efficient and sustainable slag treatment.
Objectives and Motivation
- Concept of a mineral LD slag processing route for a maximum separation of phosphorous from the iron rich fraction
- Energy efficient treatment of LD slag under reducing conditions for a nearly complete separation and transfer of the phosphorous into the gas phase
- Optimized mineral product from the reducing LD slag treatment and mineral product with a positive market value
- Possible recycling routes for ladle furnace slag and the slag from the copper refining process
A pressurized addition of CO2 to LD slag enforces the formation of carbonates and a subsequent separation of the phosphorous-rich from the iron-rich fraction. The optimum process parameters are not known yet, and furthermore, a process concept is not investigated and evaluated yet. Beside the addition of pure CO2, alternative CO2-carriers should be tested as carbonate triggers to improve the intergrowth patterns.
During the LD slag treatment under reducing conditions, open questions must be clarified regarding the optimum composition of the mineral product to establish a certain glass content with a minimum addition of slag formers for basicity adjustment. Regarding the second product, the metal phase, a low-phosphorous product is expected to generate a positive market value. The InduRed process developed in the first funding period (2015-2019, inductively heated graphite bed reactor, called InduCarb reactor) avoids the generally known disadvantage of an oxidizing treatment of the generated metal product following the reducing slag treatment. This oxidizing refining step of the metal product always leads to a loss of valuable metals and to the generation of a residual slag respectively. The potential of the InduRed process lies in a treatment of this residual refining slag being enriched with manganese, chrome and phosphorous. Experimental trials should define the optimum process parameters for a transfer of the phosphorous from the refining slag into the gas phase. With a post-processing gas scrubber, phosphoric acid can be generated.
Beside LD slag, the current Project 1.2 considers other metallurgical slags from the ferrous and nonferrous production routes for a nearly complete closure of material cycles. Alternative utilization routes for ladle furnace slag and for slag from the copper refining process should be defined and evaluated. During copper refining, secondary raw materials (residues and waste materials) are used leading to certain contents of different heavy metals in the slag. Oftenly, a direct utilization of the copper slags is not possible.
Results and application
At the end of Project 1.2, different concepts for the treatment of metallurgical slags should be generated, which are evaluated technically and economically. Beside this, another goal is to increase the knowledge, if products generated from the slag treatment can be sold to markets in a sustainable way.
A mineral processing concept for LD slag should be defined as an alternative to the reducing route comprising a wet-based route with a low energy demand to establish a maximum possible separation of the iron-rich fraction. This product should have a minimum phosphorous content.
Regarding the reducing LD slag treatment, a nearly complete transfer of the phosphorous into the gas phase should be reached using the InduRed plant developed in the first funding period. The focus is on a treatment of the refining slag coming from the oxidizing treatment of the metal product (= product from a conventional LD slag reduction), whereas special emphasis is put on the generation of a low-phosphorous metal alloy. This demands for a maximum phosphorous transfer rate into the gas phase.
The second product, the mineral phase, should have an optimum chemical composition leading to a glass content with satisfying hydraulic properties to be suitable as additive for the cement clinker process.
Furthermore, for secondary metallurgical slags, Project 1.2 should define possible recycling routes for ladle furnace slag. Finally, application fields for copper refining process slags in road construction or construction industry should be defined. Building material related properties as well as environmental and physical properties should evaluate, if the mineral fraction from nonferrous refining slags can be introduced to the market as high-quality secondary material.