Eutectic freeze crystallization for K2Cr2O7 recovery in the treatment of Cr-bearing wastewater

A M.Sc. Thesis Project in Chemical Engineering at the Division of Resource Recovery

Background:

Currently, the extensive use of Cr involves serious disposal problems and detrimental environmental impact. Large quantities of Cr-bearing wastewater are generated each year, from chrome tanning, electroplating and steel industry [1]. The soluble hexavalent chromium Cr (VI) is an environmental contaminant widely recognized to act as a carcinogen, mutagen and teratogen towards humans and animals [2]. Environmental protection and transition to a circular economy is attracting increased attention, so it is of great significance to recover and reuse Cr from Cr-bearing wastewater. Conventional treatment of Cr-bearing wastewater cannot achieve the recovery and recycling of Cr. Some new methods, such as selective precipitation of Cr(VI), adsorption using nanomaterials, electrochemical treatment, have been developed to separate Cr from the Cr-bearing wastewater [3]. After separation and purification unit, evaporative crystallization (EC) is usually used to produce Cr salts of high purity . However, EC is not energy-efficient and may cause equipment corrosion when processing strong oxidation of chromates. An alternative method to evaporative crystallization could be eutectic freeze crystallization (EFC). This technique is based on cooling a concentrated aqueous salt stream down to the eutectic point of the respective salt–water system, where salt crystals and ice crystals are formed simultaneously [4].

K₂Cr₂O₇ is an oxidizing agent for a variety of reactions in laboratories and industry. For example, it is used in the leather industry for chrome tanning by acting as a precursor for potassium chrome alum, and is also used in dyeing and calico printing [5]. A reduction in temperature can appreciably decrease the solubility of K₂Cr₂O₇ in an aqueous solution, which means low-temperature crystallization method is suitable for its isolation from the solution.

The project

Our research group develops novel processes and techniques for resource recovery from wastewater and primary or secondary raw materials. A project on Cr recovery from wastewater and a project on EFC for recovery of pure salts and water have been granted in our research group this year [6]. Previous experiments have demonstrated that EFC can be used to recover Ni, Co sulfates from aqueous solutions.

This degree project aims to recover K₂Cr₂O₇ in the treatment of Cr-bearing wastewater using EFC. Knowledge about the thermodynamics and kinetics of the crystallization of the solid phases (nucleation and growth) and the effect of impurities are imperative when designing an efficient process.

The main content of the master project is:

1. Literature studies on EFC.
2. Determination/ modeling of relevant solubility.
3. Performance of batch EFC experiments for K₂Cr₂O₇ reocvery.
4. Investigation of optimum parameters, such as pH, cooling rate, time.
5. Characterization of products.
6. Report writing.

The project is supervised by Drs. Yiqian Ma, Michael Svärd and Kerstin Forsberg, and carried out in collaboration with both industrial and academic partners. For questions or to express your interest, please contact:

Yiqian Ma (yiqianm@kth.se) or Kerstin Forsberg (kerstino@kth.se)

Reference:

[1] Owlad, M., Aroua, M.K., Daud, W.A.W., Baroutian, S. (2009). Removal of hexavalent chromium-contaminated water and wastewater: a review. Water, Air, and Soil Pollution, 200 (1), 59-77.

[2] Coetzee, J.J., Bansal, N., Chirwa, E.M. (2020). Chromium in environment, its toxic effect from chromite-mining and ferrochrome industries, and its possible bioremediation. Exposure and health, 12 (1), 51-62.

[3] Peng, H., Guo, J. (2020). Removal of chromium from wastewater by membrane filtration, chemical
precipitation, ion exchange, adsorption electrocoagulation, electrochemical reduction, electrodialysis,
electrodeionization, photocatalysis and nanotechnology: a review. Environmental Chemistry Letters, 1-14.

[4] Van der Ham, F., Witkamp, G. J., De Graauw, J., & Van Rosmalen, G. M. (1998). Eutectic freeze crystallization: Application to process streams and waste water purification. Chemical Engineering and Processing: process intensification, 37(2), 207-213.

[5] Basavaiah, K., & Swamy, J. M. (2001). Application of potassium dichromate and iron–thiocyanate in the spectrophotometric investigations of phenothiazines. Il Farmaco, 56(8), 579-585.

[6] https://www.kth.se/ket/resource-recovery/forskning-1.702492