Process design and optimization for small-scale coal based direct reduction of iron ores.

Abstract

The iron and steelmaking industry, a cornerstone of construction, transportation, and manufacturing, faces significant challenges due to the depletion of coking coal reserves and the environmental impacts associated with traditional blast furnace processes. This project aims to design a direct reduction process for producing sponge iron using coal, presenting a sustainable and economical alternative. The study is contextualized within Uganda, a country endowed with substantial iron ore deposits, particularly hematite, which is preferred for its high iron content and minimal impurities. The project explores the coal-based direct reduction processes, focusing on the use of a rotary kiln due to its flexibility, lower capital investment, and environmental benefits. A comprehensive literature review identified critical design parameters for optimizing the direct reduction process, including reduction temperature, residence time, material flow rates, and feedstock composition. For instance, the reduction temperature, which ranges from 800℃ to 1,100℃, significantly influences the rate and extent of iron ore reduction, while the residence time, typically 6 to 8 hours, affects the process throughput and efficiency. Key raw materials for the process include hematite ore, non-coking coal, and dolomite. Hematite ore from southwestern Uganda, with an iron content of 55% to 68%, is selected for its high quality. Non-coking coal from Tanzania, with a high carbon content (40%-60%) and moderate volatile matter (25%-35%), serves as both the heat source and reducing agent. Dolomite acts as a fluxing agent, improving the quality of the direct reduced iron (DRI) by absorbing sulfur impurities. The design and simulation of the process were conducted using Aspen Plus V10, a chemical process simulator. The simulation environment incorporated blocks representing key process steps: gasification in a fixed bed gasifier, syngas cleaning using a cyclone separator, and reduction in a rotary kiln. The model validated against experimental data from literature showed an output of 11,320.6 kg/hr of sponge iron, with a metallization rate of 87.3%, aligning with typical industry standards of 85%-95%. Sensitivity analyses indicated that process performance is highly dependent on reduction temperature and hematite flow rates. Optimal temperature range was found to be 940℃ to 1,100℃, enhancing throughput and metallization rate. Additionally, the process was optimized for hematite flow rates, showing a peak efficiency at 20,000 kg/hr, beyond which the reducing gas became insufficient, decreasing the metallization rate. In conclusion, this project provides a detailed design and evaluation of a coal-based direct reduction process for iron ore, offering a sustainable and costeffective alternative to traditional methods. It addresses the challenges faced by the iron and steelmaking industry, particularly in regions like Uganda, contributing to the advancement of direct reduction technologies and promoting local resource utilization