An in-depth exploration of the performance and influence of rapid dewatering filter aid for red mud slurry

Abstract

Red mud is a kind of industrial waste residue produced in the process of alumina production, which has strong suspension and is difficult to precipitate and filter. This study compared the effects of 4 kinds of filter aids, including CaCl2, polymerized ferrous sulfate (PFS), steel slag (SS), and Portland cement (PC), on the filtration rate, filter cake moisture content, and Na2O content of red mud slurry. At a dosage of 10 g·L‒1, the filtration effects were in the following order: PFS > CaCl2 > SS > PC. Under the combination of 5 g·L‒1 SS and 5 g·L‒1 PC, the better filtration effect was achieved with a filtration time of 205.17 s, which was reduced by 58.52% compared to the original red mud. The combined use of SS and PC exhibits better advantages in terms of cost and filtration effect. This study provides a data foundation for the rapid filtration of red mud slurry. The use of SS and PC as filter aids for red mud holds broad application prospects.

1. Introduction

Red mud is an insoluble strong alkaline waste produced by fine grinding and strong alkali dissolution of bauxite at high temperature in the process of alumina production [1‒3]. China is a major producer of alumina, and also the world’s largest producer of red mud. In recent years, while alumina production has been increasing [4‒5], the grade of bauxite has been decreasing. This has resulted in a growing amount of red mud produced for every ton of alumina, with accumulated stockpiles of red mud estimated to reach hundreds of millions of tons [6‒7]. Common storage methods for red mud in China mainly include wet storage and dry storage. Due to its strong alkalinity, fine particle size and complex composition, red mud can cause very serious environmental harm if not treated properly. This may lead to pollution of ecosystems such as soil and groundwater [8‒11], thereby presenting a significant challenge for the disposal of red mud.

In the Bayer process for alumina production, to minimize the adhesion loss of Al2O3 and Na2O, red mud needs to undergo multiple washings, sedimentations, and filtrations to remove free alkali and achieve alkali solution circulation. This process aims to enhance the quality of alumina products and production efficiency. The sedimentation and filtration processes for red mud are intricate and time-consuming. To prevent secondary reaction losses of alumina, it is essential to rapidly accomplish the solid–liquid separation of red mud. However, the red mud contains a large number of fine particles after high-pressure leaching, with the particle size mainly distributed between 0.1 μm and 100 μm, resulting in poor settling performance [12‒13]. Kumar and Kumar [14] determined the characteristic particle sizes (D10, D50, D90) of red mud to be 0.37, 1.57, and 56.88 μm, indicating an extremely fine particle size distribution and a high content of fine particles. The abundance of fine particles makes it difficult to achieve effective separation through mechanical filtration, often leading to clogging of filtration equipment and slow filtration rates. Red mud is an alkaline slurry with a pH typically ranging from 10 to 12 [15]. At high pH levels, the particles in red mud generally carry a negative surface charge and exhibit strong electrostatic repulsion [16‒17], which adversely affects settling and filtration processes [18]. These properties such as the fine particle size and strong alkalinity result in red mud having a high degree of suspension, making natural settling difficult and posing challenges for solid-liquid separation. The high specific surface area of red mud leads to strong adsorption [13], resulting in a higher moisture content in the filter cake after filtration. This also presents significant challenges for other production operations. Therefore, it is important to strengthen the liquid-solid separation process of red mud slurry, promote the rapid dehydration of red mud, and realize the lye circulation.