Direct Recovery Iron production
Stiff Vacuum Extrusion agglomeration technology of the fine anthropogenic and natural materials involved in the steel industry has gained over the past few years a reputation as a proven technology for the production of the excellent charge components – extrusion briquettes (BREX) - for the majority of the metallurgical furnaces (Blast furnaces, SEAF, RHF and etc.).
In 2013 A.M. Bizhanov with colleagues obtained convincing results of the industrial testing on the possibility to metallize BREX efficiently in the Direct Recovery Iron producing reactors. Experiments were conducted in industrial Midrex reactor (Qatar Steel Qatar, Doha). Qatar Steel Company annually imports 3.5 million tons of iron ore pellets for the manufacture of 2.35 million tons of HBI.
During unloading, warehousing and loading of pellets to the metallization reactors, unloading of the metallized pellets and their briquetting tens of thousands of tons of fine waste are formed every year. BREX chosen for testing were made of a mixture of pellet’s fines-55.6% (92% sizes no more than 6.3 mm), metallized sludge-27.8% (92% sizes no more than 6.3 mm) and mill scale-16.6% (99% sizes no more than 10 mm). The main objectives of the study were assessment of BREX metallurgical properties in the Midrex process and binder selection, ensuring their durability and maximum degree of metallization. Four compositions of BREX were used, differing only by the binder type.
Two series of the box tests of the BEX metallization in the industrial Midrex reactor were held. In the first series BREX (25-30 pcs.) were placed inside the hard steel baskets that were further downloaded into the reactor along with traditional indurated pellets, and after the reduction the baskets were extracted from the reactor. This allowed visually assess the condition of the recovered BREX, to study their composition and properties. In this case, the mechanical strength of BREX did not play a significant role, because they are not subjected to pressure of the charge column.
The stiffness of the basket completely eliminated pressure of the rest of the charge above on BREX and, consequently, possible deformation or destruction of BREX. In the second series of tests BREX were placed into soft and gas permeable steel packages, allowing to examine their behavior under conditions close to the conditions inside the layer pellets in a Midrex reactor.
After removal from the reactor BREX obtained by using cement binder have cracked, and the cracks turned more in BREX with higher content of binder. It is possible to draw a conclusion about the relationship of BREX hot strength with their crushing strength. BREX with lower levels of strength and density in cold conditions showed lower levels of hot strength, resulting in the formation of surface cracks that are consistent with the well-known results of the research of the hot strength of iron ore pellets. For pellets the negative dependence of hot strength on the ratio Al2O3/SiO2 has been observed.
These BREX with smaller hot strength values compared with BREX bonded by lime, had the highest values of this ratio due to the significant content of alumina in cement and, especially, in bentonite. Presence of lime and Hematite (pellets fines and mill scale) favors the formation of calcium Ferrites with a low melting point and softening during rapid heating in the Midrex reactor at temperatures above 400-500° C, what leads to strengthening of the BREX structure and improves their reducibility. Part of the cement-based BREX stick together during their reduction. Such phenomenon could have the same mechanism as that which leads to sticking of cement bonded non-indurated pellets.
After extracting the hard steel baskets with reduced BREX from reactor we have measured their chemical composition, total iron content, metallic iron content and degree of metallization. The maximum degree of metallization had the BREX lime bonded and minimum-BREX with 8% of the cement binder.
The results of the first series of testing the BREX made from a mixture of pellets fines, mill scale and disperse HBI production waste in the industrial Midrex reactor showed their high efficiency and ability to achieve high levels of metallization with conserved integrity.
In the second series of testing the BREX of similar composition using three different types of binders (Portland cement, lime and magnesium based new binder) were placed in deformable steel packages. Results of this testing showed that the best levels of hot strength and reducibility demonstrated the BREX bonded by the new magnesium based binder. Based on the results of this industrial testing Qatar Steel is considering the opportunity to build the briquetting factories for the production of BREX by the method of stiff extrusion.