Recent Advances in the Development of a New pH Swing Method Based on a Regenerable Precipitant-Solvent System for Metals Recovery and Greenhouse Gas Control

This present submission describes a new resource engineering process for metals recovery and greenhouse gas control, and details recent advances in understanding the chemistry of suitable amine and acid components for its regenerable solvent-precipitant system.  The work forms part of a series of examinations aimed at developing a potentially low energy means to perform pH-swing, such that CO₂ mineralisation could be performed by either precipitating magnesium carbonates (i.e, nesquehonite, MgCO₃∙3H₂O; hydromagnesite, Mg₅(CO₃)₄(OH)₂∙4H₂O) via contacting the alkalised leachate  with CO₂ or simply precipitating brucite, Mg(OH)₂ for use in subsequent direct gas capture-mineralisation.  The latter option allows for the direct combined capture and storage of CO₂ from point sources, where Mg(OH)₂ can be brought on-site and contacted with flue gases, converting them into magnesium-based minerals.  Furthermore, brucite production and deployment for mineralising other flue gases besides CO₂ (i.e., SO_x and NO_x) could pave way towards a wider utilisation in various industries and applications including those in the transport (i.e., mineralising tail-pipe emissions), agricultural (i.e., passive CO₂-absorber, soil pH modification), industrial (i.e., nickel, cement, steel, aluminium production) and in various energy generation sectors.  In addition, it could be used for air capture mineralisation, where brucite could be spread over specific areas allowing for passive carbonation over time.  This passive mineralisation capability could in the future facilitate for a market-based framework for carbon trading, bridging the gap between the CO₂ generators and mitigators. _­

Central to this process is a regenerable solvent-precipitant system that enables pH-swing via a change in operational temperatures.  The regenerable system consists of an aliphatic tertiary amine (R₃N) and an acid.  The process may use either magnesium sulphate tailings (MgSO₄∙xH₂O) or Mg-silicate minerals as feedstocks.  If commencing with the silicate feedstock, the silicate is contacted with an acidic solution that has been produced from the regeneration stage.  In the dissolution stage, proton exchange with the hydroxides and oxides in the feedstock would cause the liberation of Mg²⁺ and other metals ions, producing a leachate which can then be separated from the solid product (i.e., amorphous silica).  The leachate pH is then raised via a controlled R₃N addition to precipitate, firstly metal hydroxides (Al, Fe) at pH ~4, and then either MgCO₃ at pH 8 via concomitant CO₂ injection or in the absence of CO₂ to produce Mg(OH)₂ at pH > 10.  These hydroxide precipitates and carbonates are sequentially separated from the solution, allowing for the ease of product separation in downstream refinement stages.  The resultant solution after the precipitation stage, laden with the protonated amines (R₃N∙H⁺), is then heated to re-generate both the acid and R₃N components for recycling.  If commencing with tailings (i.e., from a nickel processing plant), the aqueous MgSO₄∙xH₂O-rich tailings are contacted with R₃N to precipitate MgCO₃ or Mg(OH)₂ which can be, firstly, used as the base needed to neutralise excess acid used in nickel processing (to replace the calcrete or other base that is currently used) and, secondly, either stored in the case of MgCO₃ or sold as a CO₂ mineralising compound in the case of Mg(OH)₂.  The protonated tertiary amine (R₃N∙H⁺) produced then goes to the regeneration stage where R₃N and acid are thermally regenerated.  When considering the tailings as the feedstock, the process is essentially a retrofit which re-generates the acid and base required for the process.    

We cover in this contribution the examination of various tertiary alkylamines to adjust the pH of an acidic solution to >8 and therefore enable MgCO₃ and Mg(OH)₂ precipitation, and the possibility of thermally recovering both the amine and acid back from the protonated amine.  Particular focus is on the liquid-phase, aliphatic tertiary amines and various types of acids, in order to identify the combination that is suitable for: (i.) precipitating metal hydroxides, (ii.) dissolving silicate feedstocks and (iii.) regenerable within the desired operational conditions.  A suitable amine-acid combination for this regenerable solvent-precipitant system must cover the pH regimes of interest (i.e., pH ≥ 10 for product precipitation and pH ≤ 2 for feedstock dissolution) within the desired operational constraints (T < 110 °C, P < 1 bar, t < 2h).  Within the desired operational constraints, we found the triethylamine–sulphuric acid aqueous system (Et₃N–H₂SO₄–H₂O) has sufficiently covered the pH regimes of interest, affording a pH swing (at 25 °C) between 10.5 and 1.9.  All other R₃Ns that were tested were neither alkalising nor regenerable via volatilisation.  Triethylamine (Et₃N) provided the highest acid neutralising capacity of 7 mol H⁺ L^-1 Et₃N, capable of aqueous alkalisation to pH > 10.5  (25 °C, 1 bar), and recoverable at ≤ 105 °C (1.0 bar) via fractional distillation.  As for the acid component, H₂SO₄ allows for the formation of an acidic aqueous solution (pH 1.9, 25 °C, 1 bar) resulting from the partial liberation of Et₃N from the acid-bound Et₃N∙H⁺ and HSO₄^- counter anion dissociation. Sulphuric acid, when paired to Et₃N, allows for the partial removal of the amine from a fully neutralised Et₃N-H₂SO₄–H₂O system, yielding a low pH (< 2).  Based on the reaction time, residual volume and resultant pH, we found that fractional distillation of the Et₃N-H₂SO₄–H₂O system under ambient conditions best provides high Et₃N recovery whilst minimising the co-evaporation of water.