Associate Professor Tom Honeyands

An experimental and modelling program has been conducted by BHP Billiton to study the rate and mechanism of melting of hot briquetted iron (HBI) during steelmaking. Single briquettes melt quickly relative to scrap, due to vigorous stirring from CO evolution caused by internal reaction of C and residual iron oxides. The melting rate is determined by the bath carbon level, with briquette carbon only important in a low carbon bath ( < 0.1 wt%). This information can be used to optimise the HBI continuous feeding rate for steelmaking, or the batch addition profile.

The conditions necessary for the optimal use of Hot Briquetted Iron (HBI) in scrap pre-heating systems have been determined by experiment on a laboratory and pilot scale. The development of a process model has allowed prediction of the pre-heat temperature that is achievable in shaft type systems, and the consequent electrical energy savings and productivity improvements possible for an electric arc furnace (EAF). The behaviour of HBI during pre-heating involves a complex series of chemical reactions, as shown in Figure 5. Single briquette experiments have demonstrated that gains in HBI metallisation can be realised during pre-heating. HBI was successfully heated to 1000°C in an atmosphere containing < 5% oxygen in pilot scale studies (2 tonne batches). Metallisation gains of approximately 0.5 to 1% were measured for batches of FIOR/FINMET HBI, confirming the laboratory scale work. Models have been developed for an EAF and a generalised pre-heating system. The EAF model is a versatile heat and mass balance model for heating, melting and chemical reactions. Key operating parameters such as electrical energy, oxygen and flux consumption, off-gas temperature and composition canbe calculated. The off-gas conditions are used as an input to the pre-heater model, which calculates the gas and HBI temperature distributions and HBI metallisation along the pre-heat system. Model predictions for a shaft pre-heater suggest that a charge of scrap and HBI has a heat capture efficiency up to 25% higher than an all-scrap charge. Optimum conditions for pre-heating require the HBI to be in a layer near the bottom of the charge or uniformly dispersed with scrap throughout the charge. Continuous charging and discharge of the pre-heat shaft would improve the overall performance. The challenge that remains is to confirm the model predictions at full scale and to develop operating practices, such as the optimum layering strategy for a mixed charge.

Hot briquetting reduces the porosity and surfaces to volume ratio of direct reduced iron (DRI). The usual air oxidation of iron is slow at ambient temperatures and becomes prominent only at temperatures above 500 °C. Briquettes passivated in air have a much slower corrosion rate in air at room temperature than unpassivated hot briquetted iron (HBI). Briquettes exhibit surface cracking, suggesting oxidation is not limited to the surface of the briquettes, but occurs also within the interior.

At present we are witnessing large investments in the iron ore industry, fuelled by demand from Asia. At the same time, there is a changing landscape in pricing of iron ores, with the recent demise of the benchmark system and the evolution of market based index pricing systems. From a customer perspective, it is the behaviour of iron ores in downstream processing that gives them their value; their impact on the sintering or pelletising process and subsequently blast furnace ironmaking. It is therefore important to consider this value when developing projects, making mine planning/cut-off grade decisions, and in setting quality price differentials. This paper describes the use of the Marx value in use (VIU) model to quantify the downstream value of iron ores. The Marx model consists of heat and mass balance modules for sintering, pelletising and a rigorous two-stage heat and mass balance model of blast furnace ironmaking. Mass balance and cost models are applied for steelmaking, casting and rolling. The use of a heat and mass balance allows accurate comparison of the impact of raw material properties on blast furnace operation. The impact of minor elements, such as alumina, silica and phosphorus, and metallurgical properties on ironmaking is described, and examples given for the relative value of haematite, Marra Mamba, and channel iron deposit (CID) ores.

Excellent chemistry and metallurgical properties compared to dense lump ores and most China domestic pellets suggests that BHP Billiton MAC lump is a good BF direct charge material. Blast furnace performance at Baosteel's Stainless Steel Company has proven this to be the case. In 2008, the operation at the No.1 BF achieved very good performance with MAC lump ratio in the burden averaging 17.3% for the year and reaching a high of 22% for one month; All BF operational parameters remained stable and the BF economic index was maintained. This paper discusses in detail the results of the joint research into the properties of MAC lump as determined from laboratory tests and the blast furnace operational performance.

The experiments conducted and the techniques used to analyze the melting rate before discussing the implications on the briquette melting rate were described. The experimental results show that the cylinders of hot briquetted iron (HBI) melt much faster than steel cylinders, due to the higher C and FeO content of the briquettes. It was observed that the higher C content tended to increase the driving force for melting, could lead to grains of DRI detaching from the bulk of the briquette during melting, and led to greatly enhanced heat transfer due to formation of CO gas. The inverse heat transfer analysis had been completed and had shown that the bath carbon content was important in determining the overall melting time.