Understanding the Self-Sintering Process of BOS Filter Cake for Improving Its Recyclability

Abstract

Basic oxygen steelmaking (BOS) filter cake has been found to undergo a self-sintering process, improving its mechanical properties to allow easier recycling and utilization on plant. The aim of this study was to gain an understanding of the self-sintering of the BOS filter cake in terms of what reactions occurred, and how strength developed in the filter cake during self-sintering. The approach used was to characterize samples before reaction, and to measure the reactivity of the BOS filter cake during heating in air. Reacted samples were characterized and compared to self-sintered samples from the plant. The BOS filter cake consisted of very fine particles (200-500 nm) of metallic iron and wüstite. Upon heating in air from 100 to 1000 °C, the BOS filter cake underwent a sequence of drying, oxidation, and calcination events. The primary reactions in self-sintering were found to be the oxidation of metallic iron and wüstite to hematite and zinc ferrite, beginning at approximately 120 °C and were largely completed by 500-600 °C. These exothermic oxidation reactions at low temperatures were likely driven by the very fine particle size, and provided the energy required to heat the stockpiles and drive self-sintering. The strength required for recycling the BOS filter cake appeared to result from a network of particle-particle bonds that formed between the very fine iron oxide particles in the matrix during oxidation at elevated temperatures. Temperatures between 600 and 800 °C under oxidizing conditions are likely sufficient to form adequately strong material for transport and recycling in the BOS.

Self-sintering of BOS Filter Cake for Improved Recyclability

“…The BOS dust slurry may be dewatered using a tube press producing a low moisture filter cake. When stored in stockpiles prior to recycling, experience has shown that the BOS filter cake has a tendency to self-sinter, 7,8) which is likely a result of exothermic oxidation reactions. The self-sintered material is preferred for recycling as it has better handling/transport properties, with higher strength and larger particle size, when compared to the original filter cake.…”

<i>In situ</i> Phase Analysis during Self-sintering of BOS Filter Cake for Improved Recycling

“…Unfortunately, recycling of BOS filter cake can be problematic. 6,7) The high temperatures (well in excess of 1 873 K) realised in the BOS process cause the volatilisation of tramp elements (such as zinc) to the gas phase. As the gas phase is cooled in the extraction process, these volatilised metals can condense as separate particles or onto existing dust particles.…”

“…When stored in stockpiles prior to recycling, BlueScope BOS filter cake has been found to self-sinter. [6][7][8][9] The self-sintered material is preferred for recycling back through the BOS as it has better handling/ transport properties, with higher strength and larger particle size, when compared to the original filter cake. These improved properties offer opportunities for increased BOS filter cake recycling if the self-sintering process can be better understood and optimised.…”

“…The oxidation of the BOS filter cake was found to have a characteristic behaviour that appeared largely independent of sample size, reaction system or heating rate. 6,7) This characteristic behaviour of the BOS filter cake during heating in air was the result of a complex, multi-step process. The characteristic behaviour was divided into 5 steps.…”

“…Previous SEM characterisation of the BOS filer cake has shown that it is largely an agglomeration of extremely fine particles, with particle sizes significantly less than 1 µm. 6,7) This fine particulate nature of the material makes simple microscopy of the BOS filter cake to characterise the components of the non-crystalline content difficult. Identification of the com-ponents of the non-crystalline content within the BOS filter cake is an area that warrants further study.…”