Mineral carbonation of PGM mine tailings for CO2 storage in South Africa: A case study

Abstract The ultra-fine milled tailings generated during the processing of PGM ores in South Africa have a theoretical potential to sequester significant amounts of CO 2 (∼14 Mt per annum) through mineral carbonation. Mg-bearing orthopyroxene is the major sequestrable mineral in these tailings, which also contains significant quantities of Ca-bearing plagioclase, as well as minor quantities of clinopyroxene, olivine, serpentine and hornblende. In this study, the feasibility of using PGM tailings to sequester CO 2 has been investigated empirically using the two-step, pH swing method. The rates and extents of cation (Ca, Mg and Fe) extraction and subsequent carbonation were determined and compared. Both organic (oxalic and EDTA) and HCl solutions were utilised in the cation extraction step, which was conducted at time periods up to 8 h and at a temperature of 70 °C. The extents of cation dissolution were relatively low under all experimental conditions investigated, particularly for the case of Mg (between 3.3% and 5.0% extraction). A comparison of the extents of leaching with the mineralogical composition of the tailings indicated that the extracted Mg originated primarily from clinopyroxene, with the orthopyroxene remaining relatively inert under the experimental conditions. Subsequent carbonation of the acid leach solution after pH adjustment with NaOH resulted in the rapid formation of a number of carbonate minerals, including gaylussite (Na 2 Ca(CO 3 ) 2 ·5(H 2 O)), magnesite (MgCO 3 ), hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O), dolomite (CaMg(CO 3 ) 2 ), ankerite (Ca(Fe,Mg)(CO 3 ) 2 ), and siderite (FeCO 3 ). On the basis of these findings, further studies will be focused on developing a better understanding of the factors affecting the dissolution of Mg-bearing orthopyroxene minerals, and on exploring alternative leach reagents and conditions, with a view to developing a more effective process for the accelerated carbonation of PGM tailings.

[1]  Liang-Shih Fan,et al.  CO2 Mineral Sequestration: Chemically Enhanced Aqueous Carbonation of Serpentine , 2008 .

[2]  M. Becker,et al.  Investigation of the potential for mineral carbonation of PGM tailings in South Africa , 2011 .

[3]  R. Wogelius,et al.  Olivine dissolution at 25°C: Effects of pH, CO2, and organic acids , 1991 .

[4]  S. S. Goldich.  A Study in Rock-Weathering , 1938, The Journal of Geology.

[5]  Klaus S. Lackner,et al.  Carbon dioxide disposal in carbonate minerals , 1995 .

[6]  M. Shopska,et al.  The influence of attrition milling on carbon dioxide sequestration on magnesium–iron silicate , 2010 .

[7]  I. Martinez,et al.  Experimental study of Mg-rich silicates carbonation at 400 and 500 °C and 1 kbar , 2009 .

[8]  Klaus S. Lackner,et al.  CARBONATE CHEMISTRY FOR SEQUESTERING FOSSIL CARBON , 2003 .

[9]  W. D. Keller,et al.  Dissolution of rock-forming silicate minerals in organic acids: Simulated first-stage weathering of fresh mineral surfaces , 1970 .

[10]  A. D. Surridge,et al.  Carbon capture and storage in South Africa , 2009 .

[11]  J. Drever,et al.  The effect of oxalate on the dissolution rates of oligoclase and tremolite , 1987 .

[12]  M. Maroto-Valer,et al.  Evaluation of reaction variables in the dissolution of serpentine for mineral carbonation , 2007 .

[13]  S. Welch,et al.  The effect of organic acids on plagioclase dissolution rates and stoichiometry , 1993 .

[14]  O. Levenspiel.  Chemical Reaction Engineering , 1972 .

[15]  Klaus S. Lackner,et al.  Enhancing process kinetics for mineral carbon sequestration , 2009 .

[16]  F. Saito,et al.  Enhancement of acid extraction of magnesium and silicon from serpentine by mechanochemical treatment , 1997 .

[17]  G. A. Parks,et al.  Dissolution kinetics of magnesium silicates , 1972 .

[18]  R. Zevenhoven,et al.  Dissolution of natural serpentinite in mineral and organic acids , 2007 .

[19]  S. Brantley,et al.  Dissolution of forsteritic olivine at 65°C and 2 , 2000 .

[20]  R. Kuusik,et al.  Production of magnesium carbonates from serpentinite for long-term storage of CO2 , 2007 .

[21]  I. Munz,et al.  Investigating dissolution of mechanically activated olivine for carbonation purposes , 2010 .

[22]  R. Berner,et al.  Mechanism of pyroxene and amphibole weathering; II, Observations of soil grains , 1982 .

[23]  Liang-Shih Fan,et al.  CO2 mineral sequestration: physically activated dissolution of serpentine and pH swing process , 2004 .

[24]  D. Grandstaff.  Changes in surface area and morphology and the mechanism of forsterite dissolution , 1978 .

[25]  G. Furrer,et al.  The coordination chemistry of weathering: I. Dissolution kinetics of δ-Al2O3 and BeO , 1986 .

[26]  G. E. Rush,et al.  Mineral carbonation: energy costs of pretreatment options and insights gained from flow loop reaction studies , 2004 .

[27]  S. Banwart,et al.  Carbon dioxide mediated dissolution of Ca-feldspar: implications for silicate weathering , 2000 .

[28]  J. Kubicki,et al.  Kinetics of water-rock interaction , 2008 .

[29]  J. J. Morgan,et al.  Dissolution kinetics of chrysotile at pH 7 to 10 , 1985 .

[30]  S. Gerdemann,et al.  Ex situ aqueous mineral carbonation. , 2007, Environmental science & technology.

[31]  R.C.L. Jonckbloedt,et al.  Olivine dissolution in sulphuric acid at elevated temperatures—implications for the olivine process, an alternative waste acid neutralizing process , 1998 .

[32]  A. Dogan Paktunc,et al.  Characterization of Mine Wastes for Prediction of Acid Mine Drainage , 1999 .