Experimental and mechanistic study on iron extraction from high-iron red mud under multiple physical field coupling conditions

Abstract

Red mud is a solid waste discharged in the process of alumina production, and how to realize the efficient recovery of its iron is an urgent problem to be solved. In this study, the iron extraction test and mechanism study of high-iron red mud were carried out under the coupling conditions of multiple physical field (microwave field, gas-solid flow field and temperature field) with biomass as the reducing agent. The test results showed that under the optimal conditions, an iron concentrate with a yield of 78.4%, an iron grade of 59.23%, and a recovery rate of 86.65% was obtained. The analyses of XRD, XPS, TEM, and SEM-EDS showed that during the roasting process, the hematite in the high-iron red mud was completely converted to magnetite, and the biomass produced the reductant that provided the magnetization reaction; A large number of cracks and pores appeared in the surface of the hematite reduction product particles, which helped to induce iron minerals to undergo effective mineral phase transformation. The above study provides ideas for the phase transformation and efficient recovery of iron minerals in red mud.

1. Introduction

Red mud is the main waste residue discharged in the process of alumina production, which is generally reddish-brown in colour, extremely fine in particle size, complex in mineral composition, and strongly alkaline and corrosive. Currently, the global stockpile of red mud has exceeded 4 billion tons, and is growing at a rate of up to 175.5 million tons per year [1−3]. The global production of alumina continues to run at a high level, the emission of red mud is increasing, and the environmental pollution, land resources occupation, ecosystem damage and other problems brought about by common stockpile disposal methods are becoming more and more prominent.

Red mud contains a large number of valuable metal elements, such as iron, aluminium, titanium, vanadium and rare earth metals. These useful components are not recycled, resulting in a serious waste of resources. Especially, high-iron red mud can be regarded as a kind of iron resource with complex composition and difficult selection due to its high iron grade. Usually, high-iron red mud is mainly used to recover iron. Comprehensively speaking, the methods of iron recovery from high-iron red mud can be mainly summarized as physical separation, pyrometallurgy, wet acid leaching and combined process [4−12]. In summary, although the physical method has a simple process and low cost, the obtained product has low iron grade and recovery rate, and high impurity content. The pyrometallurgical method can obtain high-quality iron concentrate, but it has high cost and energy consumption. The acid leaching method has the advantages of high dissolution rate and simultaneous leaching of multiple metals. However, due to the high alkalinity of red mud, acid leaching requires a large amount of acid to neutralize the alkali in the red mud, which makes this method suffer from the problems of high acid consumption and high acidity of leaching residue. The fluidization roasting technology utilizes a fluidized bed reactor to convert weakly magnetic refractory iron ores (such as hematite, siderite, and limonite) into strongly magnetic iron oxides through chemical reactions, followed by magnetic separation to obtain high-grade iron concentrates [13−17].