Thorium Removal, Recovery and Recycling: A Membrane Challenge for Urban Mining

Abstract

Although only a slightly radioactive element, thorium is considered extremely toxic because its various species, which reach the environment, can constitute an important problem for the health of the population. The present paper aims to expand the possibilities of using membrane processes in the removal, recovery and recycling of thorium from industrial residues reaching municipal waste-processing platforms. The paper includes a short introduction on the interest shown in this element, a weak radioactive metal, followed by highlighting some common (domestic) uses. In a distinct but concise section, the bio-medical impact of thorium is presented. The classic technologies for obtaining thorium are concentrated in a single schema, and the speciation of thorium is presented with an emphasis on the formation of hydroxo-complexes and complexes with common organic reagents. The determination of thorium is highlighted on the basis of its radioactivity, but especially through methods that call for extraction followed by an established electrochemical, spectral or chromatographic method. Membrane processes are presented based on the electrochemical potential difference, including barro-membrane processes, electrodialysis, liquid membranes and hybrid processes. A separate sub-chapter is devoted to proposals and recommendations for the use of membranes in order to achieve some progress in urban mining for the valorization of thorium.

Keywords: thorium determination; thorium membrane concentration; thorium membrane separation; thorium recovery; thorium recycling; thorium removal; thorium separation; thorium separation processes; thorium transport.

Figure 1

Domestic applications of thorium and thorium dioxide, along with the alleged use in generating energy in nuclear power plants (U–Th cycle).

Figure 2

Simplified flowcharts for obtaining thorium from monazite: (a) acid digestion; (b) alkaline fusion.

Figure 2

Simplified flowcharts for obtaining thorium from monazite: (a) acid digestion; (b) alkaline fusion.

Figure 3

Hypothetical stability diagram of thorium hydroxo-complexes in aqueous medium.

Figure 4

Common organic reagents involved in thorium ion complexation and/or extraction. The colored groups (in red) interact with the thorium ion (in yellow).

Figure 5

Multicomponent system bordered by a selective window, including ions, small molecules, macromolecules, nanoparticles, microparticles, microorganisms and viruses as suspended particles: (a) system in equilibrium; (b) system subject to an electrochemical potential difference (Δµ). The meaning of shapes and symbols in Figure 5 is as follows.

Figure 6

Membrane separation processes under pressure difference: (a) obtaining drinking water through reverse osmosis; (b) piston type (dead-end filtration); (c) tangential flow; (d) tangential flow through large sections; (e) flow through tubes.

Figure 7

Advanced hollow-fiber filtration module.

Figure 8

Common types of carriers: macrocyclic compounds, modified classical complexant agents and nano-species [194].

Figure 9

Scheme of an electrolysis cell for the concentration of a salt by electrodialysis with two ion exchange membranes.

Figure 10

Schematic presentation of membrane systems with an organic solvent: denser (a) or less dense (b) than aqueous phases. Legend: M = membranes; SP = source phase; RP = receiving phase; A = chemical species of interest for separation [214].

Figure 11

Schematic presentation of extraction and membrane systems with organic solvent: (a) water 1 (W1)–organic solvent (OS)–water extraction (W2); (b) liquid membranes (LMs); (c) bulk liquid membranes (BLMs); (d) supported liquid membranes (SLMs); (e) emulsion liquid membranes (ELMs). Legend: M = Membrane; SP = Source Phase; RP = Receiving Phase [214,216].

Figure 12

Schematic presentation of the permeation module with dispersed phases: (a) front view; (b) cross-section detail. Legend: SP—source phase; RP—receiving phase; M—organic solvent membrane; m_np—magnetic nanoparticles; str—stirrer with magnetic rods [216,217].

Figure 13

Schematic presentation of the transport mechanism by liquid membranes (C—carrier, X—anion complexant): (a) physical “simple” shipping; (b) transport with carrier; (c) coupled transport; (d) counter-transport.

Figure 14

Scheme of proposals for the separation, recovery and recycling of thorium from waste of municipal waste management platforms: (a) valorization of thorium from unsorted waste; (b) recovery of thorium from electrodes and light bulb filaments; (c) valorization of thorium from magnesium or aluminum alloys.

Figure 14

Scheme of proposals for the separation, recovery and recycling of thorium from waste of municipal waste management platforms: (a) valorization of thorium from unsorted waste; (b) recovery of thorium from electrodes and light bulb filaments; (c) valorization of thorium from magnesium or aluminum alloys.