Red mud is the polluting waste residue discharged from the production of alumina by the Bayer process (currently 95% of alumina is produced by the Bayer process). Generally, for every ton of alumina produced, 1.0 to 2.0 tons of red mud is produced incidentally. Generally, it contains a large amount of iron oxide, and its appearance is similar to that of red soil, hence the name.

Red mud contains a variety of trace elements, and the radioactivity mainly comes from radium, thorium and potassium. Generally, the whiteness index of internal and external illumination is above 2.0.

1. How is red mud produced?

After the bauxite is dissolved, a mixed slurry of red mud and sodium aluminate is formed. The slurry must be diluted to settle or filter to separate the red mud and sodium aluminate solution. After separation, alumina and red mud are produced from the sodium aluminate solution. Washing is needed to reduce the loss of Na2O and Al2O3 through the red mud attachment.

The size and normal operation of the production efficiency of this process have a crucial impact on product quality, production costs and economic benefits. The separation of red mud generally goes through the following steps:

Red mud slurry dilution → sedimentation separation → red mud reverse washing → coarse liquid control filtration

2. Factors affecting the sedimentation and separation of red mud

form of minerals

The composition and chemical composition of bauxite are the main factors affecting the settlement and compressibility of red mud slurry. The inclusion of pyrite, pyrite, goethite, kaolinite and other minerals in bauxite can reduce the sedimentation rate of red mud because they contain more Al(OH)4-, OH-, Na+ and water molecules; However, the red mud generated by hematite, siderite, magnetite, water green vanadium, etc. has less Al(OH)4-, OH-, Na+ and water molecules adsorbed, so it is conducive to sedimentation.

Dilution concentration of dissolution slurry

At a certain temperature, the sodium aluminate solution with the same caustic ratio has high stability when the alumina concentration is lower than 25g/L or higher than 250g/L, while the medium concentration (70~200g/L) has high stability. Alumina solutions are less stable. Due to their stability, they cannot be decomposed directly.

Therefore, the red mud washing liquid of the previous cycle is generally used for dilution. After dilution, the stability of the solution decreases and the decomposition speed is accelerated, and the alkali and alumina in the red mud washing liquid can be recovered to achieve a higher decomposition rate. Improve the cycle efficiency of Bayer process production. However, if the solution is diluted too much, its stability will drop sharply, resulting in the hydrolysis of sodium aluminate solution, and the loss of alumina in red mud will increase.

temperature of the diluted slurry

When the temperature of the diluted slurry increases, its viscosity and density decrease, so the sedimentation rate of red mud increases. The temperature at which the slurry is diluted greatly affects the stability of the sodium aluminate solution, resulting in changes in the amount of alumina lost in the red mud. As can be seen from the figure below, in order to maintain the stability of the sodium aluminate solution with a lower concentration and a low caustic ratio after dilution, the solution temperature must be increased to above 94 °C.

Use of flocculants

During the sedimentation and separation of red mud, it is generally necessary to add flocculants to improve the overflow quality of the sedimentation tank and the consistency of the bottom flow, increase the sedimentation speed, increase the production capacity of the sedimentation tank and reduce the production cost. Adding flocculants is a commonly used and effective method to accelerate red mud sedimentation in alumina production.

3. Comprehensive utilization of red mud

At present, the research on comprehensive utilization of red mud mainly includes the following three aspects:
(1) Extraction and recovery of valuable metals, such as leaching and precipitation to extract aluminum, magnetization roasting to select iron, and acid leaching to extract rare metals such as scandium, titanium, and vanadium;
(2) It is the preparation of building materials, such as the preparation of unburned bricks and cement;
(3) It is the preparation of adsorbent materials, which is mainly used in wastewater treatment.