An example of the innovative approach that Extracthive can propose is the successful partnership with the French alumina producer, Alteo. Alteo used to discharge red mud, the final residue of the Bayer process, directly into the sea. In the Bayer process, strip-mined bauxite is treated with hot caustic soda, which selectively extracts aluminium from a range of other mineralised metals. The problem is that for every ton of alumina extracted, more than a ton of red mud is produced.
The huge amounts of red mud disposed into the sea slowly changed the marine environment of the region. Since 2015, Alteo has implemented two filtration steps at the end of the process in order to separate the solid fraction of the red mud, so called Bauxaline®, from the liquid part, which is still released to the sea. Despite these efforts, the final effluent still has a high alkalinity (pH > 12) and a high concentration of heavy metals, both above the limits set by French environmental regulations. The French administration thus gave the company until 2021 to find a solution to enhance the quality of its waste water.
Extracthive thus proposed a bio-inspired process to obtain a neutral metal-free effluent. According to the designer of the process, Quentin Ricoux, “the idea is to
2[Al(OH)4]−(aq) + 8OH−(aq) + 6Mg2+ + CO32−(aq) → Mg6Al2(CO3)(OH)16.4H2O(s)
Hydrotalcite is separated using a flocculation-decantation step, and is further centrifuged to increase the solid content of the final mud to 12%. After a successful development at labscale, this process was tested in pilot scale in September 2016, treating 1 m³ per day of effluent resulting from red mud filtration. Chemical analysis of the effluent after treatment shows a reduction of 99% of the aluminium content and 85% of the arsenic content. The pH was reduced to 9, thanks to the extraction of aluminates. Both the pH and the residual metal content are acceptable, according to French environmental regulations.
After achieving this promising result, the efforts are now in finding a market for the valorisation of the hydrotalcite extracted from the effluent. For that, Extracthive is working on a process to extract heavy metals trapped in the crystal matrix. Hydrotalcite is commonly used as a stabilizer for Polyvinyl chloride (PVC) as it can adsorb the chlorides generated during the thermal degradation of PVC. Other applications also include the use of Hydrotalcite as catalysts.
In order to seal the partnership with Alteo, Extracthive is also working on the valorisation of the filtered red mud. This mud is mainly composed by hematite, representing approximately 50% in mass of the total composition. The goal of Extracthive is to develop a process to recover electrolytic iron from hematite, which has a high value in the market. The innovative process is based on ancient methods to extract iron using electrowinning, but with a novel design of the electrolytic cell that simplifies the extraction from the highly alkaline red mud. The first laboratory tests have shown encouraging results, where pure iron could be extracted from red mud residue. This process is the subject of a doctoral thesis at Extracthive, in which the process is expected to develop up to pilot scale in a couple of years. If successful, this process can be expanded for the recovery of metals in other types of muds.
Vanessa Amaral de Oliveira, Research Engineer at Extracthive
For more information: http://www.extracthive.com/