Carbonation and cementation of ultramafic mine wastes
Authors: Lockhart Justin , Power Ian , Paulo Carlos , Stubbs Amanda , McDonald Duncan 
Published: March 2022

&lt;p&gt;Ultramafic (Mg-rich) mine wastes are produced in vast quantities, are of no economic value, and their storage impoundments can be susceptible to catastrophic failure.&lt;sup&gt;1&lt;/sup&gt; Carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) mineralization of these wastes to form carbonate cement can reduce greenhouse gas emissions&lt;sup&gt;2&lt;/sup&gt; and assist in de-risking storage impoundments through physical stabilization.&lt;sup&gt;3&lt;/sup&gt; CO&lt;sub&gt;2&lt;/sub&gt; mineralization and cementation have been documented at asbestos,&lt;sup&gt;4&lt;/sup&gt; nickel,&lt;sup&gt;2&lt;/sup&gt; and diamond mines, occurring unintentionally over long periods (e.g., decades).&lt;sup&gt;2,4&lt;/sup&gt; This research aimed to 1) better understand carbonation and cementation processes by examining historic kimberlite mine wastes from diamond mines and 2) accelerate these processes in experiments using brucite-bearing serpentinite mine wastes.&amp;#160;&lt;/p&gt;&lt;p&gt;We collected physical, mineralogical, and geochemical data of cemented historic kimberlite wastes 70 to &gt;110 years old. Analysis of cemented fine- and coarse-grained residues from the Voorspoed and Cullinan diamond mines (South Africa) revealed the presence of a fine-grained (&lt;63 &amp;#181;m) cement matrix with greater total inorganic carbon (TIC; +0.08&amp;#8211;0.34% relative to clasts), secondary clays (e.g., Mg-Al silicates), and some minor carbonates (e.g., calcite). Unconfined compressive strength varied considerably between fine- and coarse-grained wastes (UCS; 0.13&amp;#8211;4.45 MPa). Furthermore, kimberlite clasts and cements were isotopically distinct, suggesting that mineral weathering by meteoric water drove cementation over decades after the deposition of these wastes.&amp;#160;&lt;/p&gt;&lt;p&gt;In experiments, coupling organic and inorganic carbon cycling accelerated carbonation of synthetic tailings that contained brucite [Mg(OH)&lt;sub&gt;2&lt;/sub&gt;], a minor yet reactive mineral. In cylindrical test experiments (2.5 &amp;#215; 5 cm; 40 weeks), waste organics were either mixed (0&amp;#8211;10 wt.%) or kept separate from brucite-bearing serpentinite mine wastes to provide an additional source of CO&lt;sub&gt;2&lt;/sub&gt;. In the mixed cylinders, brucite consumption ranged from 3&amp;#8211;30% and was limited by CO&lt;sub&gt;2&lt;/sub&gt; generation, as evidenced by minor increases in TIC (+0.02&amp;#8211;0.22%). Compressive strengths amongst the cylinders reached 0.51 MPa with few cylinders becoming sufficiently stabilized; however, in experiments that exposed cylinders to CO&lt;sub&gt;2&lt;/sub&gt; generated from organics separate from cylinders, brucite carbonation (64&amp;#8211;84% consumption) and compressive strengths were substantial (0.4&amp;#8211;6.9 MPa).&lt;sup&gt;3&lt;/sup&gt; Our research demonstrates the role of long-term weathering for sequestering CO&lt;sub&gt;2&lt;/sub&gt; within ultramafic mine wastes, and how coupling organic and inorganic carbon cycling can accelerate CO&lt;sub&gt;2&lt;/sub&gt; sequestration and physically stabilize these wastes.&amp;#160;&lt;/p&gt;&lt;p&gt;References:&amp;#160;&lt;/p&gt;&lt;p&gt;1. Rourke and Luppnow (2015), Tailings Mine Waste Manag., 225&amp;#8211;230.&amp;#160;&lt;br&gt;2. Wilson et al. (2014), Int. J. Greenh. Gas Control &lt;strong&gt;25&lt;/strong&gt;, 121&amp;#8211;140.&amp;#160;&lt;br&gt;3. Power et al. (2021), Environ. Sci. Technol. &lt;strong&gt;55&lt;/strong&gt;, 10056&amp;#8211;10066.&amp;#160;&lt;br&gt;4. Wilson et al. (2009), Econ. Geol. &lt;strong&gt;104&lt;/strong&gt;, 95&amp;#8211;112.&lt;/p&gt;