Investigation of the potential for mineral carbonation of PGM tailings in South Africa

Increasing atmospheric CO₂ concentration is currently of considerable concern in terms of global warming. A possible technology that can contribute to the reduction of CO₂ emissions is its sequestration by mineral carbonation. In this study, tailings from several different platinum mines in South Africa will be mineralogically characterised and their potential for mineral carbonation reviewed. Mg and Ca-rich minerals (plagioclase, olivine, orthopyroxene, clinopyroxene) present in the tailings are good candidates for mineral carbonation, which mimics natural weathering processes in which these minerals react with gaseous CO₂ to form Ca or Mg carbonates. Since the reaction is influenced by particle surface area, the ultra fine grained nature of the PGM tailings provides another reason for the promise of PGM tailings for mineral carbonation. A preliminary ranking of the tailings samples and their efficacy for mineral carbonation has been developed according to whether the samples showed harzburgtic (e.g. Northam Platinum mine), pyroxenetic (e.g. BRPM) or noritic mineral assemblages. This information and understanding will assist in identifying opportunities and guiding the development of engineered facilities for the sequestration of CO₂ by means of mineral carbonation.     

Emulation of ambient carbon dioxide diffusion and carbonation within nickel mining residues

The dynamic evolution of CO₂ sequestration using nickel mining residue (NiMR) was studied using a specially-designed dual-compartment differential fixed-bed diffusion–carbonation cell. Reactivity of NiMR material in terms of carbonation uptake was monitored as a function of time, initial CO₂ composition and liquid saturation. Dynamic tests exposed a positive correlation between CO₂ breakthrough time across the NiMR layer and liquid saturation. Low carbonation rates were observed under both dry and completely saturated conditions. The latter suffered from slow CO₂ diffusion in water whereas the former, despite the facilitated transport of gaseous CO₂ across NiMR samples, lacked a sufficient supply of leached magnesium and dissolved CO₂. However, the CO₂ uptake was substantially stimulated when partially saturated NiMR layers were carbonated. The carbonation kinetics was rationalized in terms of reaction medium and CO₂ dissolved species. Under partially saturated conditions, rapid gaseous CO₂ diffusion and dissolution in interstitial water along with leaching of magnesium were key processes in carbonation.     

Magnesium chloride as a leaching and aragonite-promoting self-regenerative additive for the mineral carbonation of calcium-rich materials

Two approaches for the intensification of the mineral carbonation reaction are combined and studied in this work, namely: (i) the calcium leaching and aragonite promoting effects of magnesium chloride (MgCl₂), and (ii) the passivating layer abrasion effect of sonication. The alkaline materials subjected to leaching and carbonation tests included lime, wollastonite, steel slags, and air pollution control (APC) residue. Batch leaching tests were conducted with varying concentrations of additives to determine extraction efficiency, and with varying solids-to-liquid ratios to determine solubility limitations. Aqueous mineral carbonation tests, with and without the use of ultrasound, were conducted applying varying concentrations of magnesium chloride and varying durations to assess CO₂ uptake improvement and characterize the formed carbonate phases. The leaching of calcium from lime with the use of MgCl₂ was found to be atom-efficient (1 mol Ca extracted for every mole Mg added), but the extraction efficiency from slags and APC residue was limited to 26–35% due to mineralogical and microstructural constraints. The addition of MgCl₂ notably improved argon oxygen decarburization (AOD) slag carbonation extent under sonication, where higher additive dosage resulted in higher CO₂ uptake. Without ultrasound, however, carbonation extent was reduced with MgCl₂ addition. The benefit of MgCl₂ under sonication can be linked to the preferential formation of aragonite (85 wt% of formed carbonates), which precipitates on the slag particles in the form of acicular crystals with low packing density, thus becoming more susceptible to the surface erosion effect of sonication, as evidenced by the significantly reduced carbonated slag particle size.     

Preparation of magnesium hydroxide from serpentinite by sulfuric acid leaching for CO2 mineral carbonation

Carbon capture and storage (CCS) by mineral carbonation is a promising way for CO₂ emissions mitigation that has been under studied for decades. In this work, the preparation of magnesium hydroxide from Finnish serpentinite using sulfuric acid leaching as the first step of a CO₂ mineral carbonation process was studied. Some details of leaching behavior of the ore were revealed and a valuable metal was recovered in this study. It was found that leaching yield of magnesium increased with sulfuric acid dosage, limited by a product layer formed on the ore particles, resulting in incomplete serpentinite decomposition. Agitation and ultrasonication were demonstrated to be effective in controlling the thickness of product layer. About 95% of iron was recovered from the leachate and leaching residues and valuable Fe-rich substances were obtained as by-products. After the iron extraction, a fine Mg(OH)₂-rich powder could be prepared from the Mg-rich solution by precipitation using sodium hydroxide solution.     

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

The ultra-fine milled tailings generated during the processing of PGM ores in South Africa have a theoretical potential to sequester significant amounts of CO₂ (∼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 CO₂ 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 (Na₂Ca(CO₃)₂·5(H₂O)), magnesite (MgCO₃), hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), dolomite (CaMg(CO₃)₂), ankerite (Ca(Fe,Mg)(CO₃)₂), and siderite (FeCO₃). 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.     

Effective CO2-specific sequestration capacity of steel slags and variability in their leaching behaviour in view of industrial mineral carbonation

Industrial mineral carbonation of alkaline wastes, an increasingly promising component of carbon capture and storage, may play an important role as a CO₂ mitigation strategy in the context of climate change. Steelmaking slags are of particular interest owing to their high content of calcium. The cumulated ‘effective’ CO₂-specific sequestration capacity (calculated on the basis of calcium and magnesium extracted to a 0.5 M HNO₃ solution) of three basic oxygen and one electric arc furnace slags generated at steel mills in South Africa was 253 kt CO₂ per annum, which was 25.2% lower than their cumulated ‘theoretical’ capacity (estimated on the basis of total calcium and magnesium content in slags). The mineralogical composition and solubility characteristics of slags conferred very distinct leaching behaviours to the slags, including differences in: (i) the amount of heat generated during their dissolution, (ii) their buffering capacity, (iii) the rate and extent of calcium and magnesium extraction from the slags, and (iv) the mineralogical composition of the non-dissolved residues. These findings suggest that separate leaching processes may need to be developed for slags with largely distinct mineralogical compositions and structural features.     

Effect of cyanobacteria Synechococcus PCC 7942 on carbonation kinetics of olivine at 20 °C

By accelerating the naturally-occurring carbonation of magnesian silicates, it would be possible to sequester some of the anthropogenic excess of CO₂ in more geologically-stable solid magnesium carbonates. Reaction rates can be accelerated by decreasing the particle size, raising the reaction temperature, increasing the pressure, using a catalyst, and hypothetically, by bacterial addition. We aimed here at assessing quantitatively the added value of photosynthetic microbial activity on the efficiency of Mg-silicates carbonation processes. Synechococcus PCC 7942 (freshwater cyanobacteria) was selected for this study. Two magnesian silicate minerals (substrates) were chosen: a synthetic forsterite with nanometer-sized grains and an industrial ultramafic slag (scoria). All tests were performed at 20 ± 1 °C in closed and sterile 1L Schott® glass bottle reactors. With the aim to elucidate the interaction between mineral phases and bacteria, we used pH and concentration measurements, scanning and transmission electron microscopy along with Raman spectroscopy. The results show that, at ambient temperature, cyanobacteria Synechococcus can accelerate silicate dissolution (i.e. Mg²⁺ release) and then magnesium carbonate nucleation and precipitation by adsorption on the produced exopolymeric substances and local pH increase during photosynthesis, respectively.     

Calcium leaching from waste steelmaking slag: Significance of leachate chemistry and effects on slag grain mineralogy

Accelerated carbonation of alkaline wastes such as steelmaking slag offers the potential to combine waste valorisation with climate change mitigation by utilising carbon dioxide (CO₂). One method of achieving this is through an indirect carbonation process to produce a marketable precipitated calcium carbonate (PCC), using ammonium salts to selectively extract calcium from steelmaking slag. Two unaddressed design parameters for a slag based plant differing from that of a traditional PCC plant are the effect of mineralogy on extraction efficiency when using a multicomponent, heterogeneous feed such as slag and the challenges raised by the resulting leachate chemistry. This paper presents petrographic textural observations on the effect of calcium leaching via ammonium chloride on individual grains of dicalcium silicate in three different widely unutilised steelmaking slags. These observations are then interpreted in conjunction with measured changes in solution leachate chemistry. The results indicate that although silica enriched regions form at the reaction front, the reaction continues into the core of the particle due to fracture propagation caused by volume reduction as calcium is extracted. Co-leaching of sulfur alongside calcium and the formation of precipitate in the leachate highlights potential engineering challenges when the process is scaled up due to fouling of process equipment. The main mineral phases in all untreated slags were found to be calcium silicates, predominantly dicalcium silicate (Ca₂SiO₄). This was followed by a complex mixture of calcium/magnesium-wustite (CaFeMnMg)O type phases and srebrodolskite (Ca₂Fe₂O₅(Ti, V)). Results indicate that calcium silicate is the more reactive component of BOS slag, while lime bound as (CaFeMnMg)O is most reactive in HMD and SS slags. Selectivity of the ammonium chloride solvent was high at 95–97% and efficiency of calcium extraction ranged between 25% and 39%.     

Effect of accelerated carbonation on the microstructure and physical properties of hybrid fiber-cement composites

Carbonation takes place in the fiber-cement composites through the diffusion of carbon dioxide (CO₂) through the unsaturated pores of the cement matrix, and through its reaction with the hydration products of the Portland cement (mainly calcium hydroxide and CSH phases). The use of this technology in the fiber-cement production consists of an interesting procedure to prematurely decrease the alkalinity of the cement matrix, which is potentially harmful to the cellulose fiber reinforcement. It is also an initiative to CO₂ sequestration and partial replacement of petroleum-based fibers. Therefore, the objective of the present work is to show the impact of accelerated carbonation on the microstructure and physical properties of fiber-cement composites reinforced with cellulose pulp and synthetic fibers. The effectiveness of the accelerated carbonation was confirmed by thermogravimetric (TG) analysis. Accelerated carbonation increased bulk density (BD) and decreased apparent porosity (AP). The SEM micrographs show that the calcium carbonate (CaCO₃) formed from the carbonation reaction is precipitated in the pore structure of the matrix. The interface between the cellulose fibers and the cement matrix in the carbonated composites was improved, decreasing the typical voids around the cellulose fibers that prejudice the fiber-cement performance at long term.     

Valorization of steel slag by a combined carbonation and granulation treatment

This work reports the results of a combined accelerated carbonation and wet granulation treatment applied to Basic Oxygen Furnace (BOF) steel slag with the aim of producing secondary aggregates for civil engineering applications and of storing CO₂ in a solid and thermodynamically stable form. The tests were carried out in a laboratory scale granulation device equipped with a lid and CO₂ feeding system. In each test, humidified slag (liquid/solid ratio of 0.12 l/kg) was treated for reaction times varying between 30 and 120 min under either atmospheric air or 100% CO₂. Under both conditions, the particle size of the treatment product was observed to increase progressively with reaction time; specifically, the d₅₀ values obtained for the products of the combined granulation and carbonation treatment increased from 0.4 mm to 4 mm after 30 min and to 10 mm after 120 min. Significant CO₂ uptake values (between 120 and 144 g CO₂/kg) were measured even after short reaction times for granules with diameters below 10 mm and for the coarser particle size fractions after reaction times of 90 min. The density, mineralogical composition and leaching behavior of the obtained granules were also investigated, showing that the combined granulation–carbonation process may be a promising option for BOF slag valorization, particularly in terms of decreasing the Ca hydroxide content of the slag. Another interesting finding was that the leaching behavior of the product of the combined treatment appeared to be significantly modified with respect to that of the untreated slag only for coarse uncrushed granules, an indication that the carbonation reaction occurs mainly on the outer layer of the formed granules.     

The effects of LiOH and NaOH on the carbonation of SrSO4 by dry high-energy milling

In this work, the effects of alkaline hydroxides (NaOH and LiOH) on the direct mechanochemical carbonation using dry high-energy milling of celestine (SrSO₄) under CO₂ atmosphere was investigated with X-ray diffraction (XRD), infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and carbon analyses. It was observed that SrSO₄ was not directly carbonated by dry high-energy milling under CO₂ atmosphere without LiOH or NaOH additives. Direct mechanochemical SrCO₃ formation was observed during milling of the SrSO₄ and NaOH mixture under CO₂ atmosphere, and it was shown that a minimum 50% of the initial SrSO₄ was mechanochemically carbonated in the presence of NaOH and CO₂ gas. LiOH does not stimulate the direct carbonation. However, it was observed that washing of the milled mixture with water resulted in strontium carbonate (SrCO₃) formation because of the enhanced dissolution of Li₂CO₃ and SrSO₄, promoted by the activation effect of high-energy milling. Depending on the alkaline hydroxide used during the milling, strontium carbonates with different space group settings were formed, which have different (split or single) main carbonate absorption bands.     

Mineral carbon storage in pre-treated ultramafic ores

Mineral carbon sequestration (MCS) is a type of carbon storage based on natural rock weathering processes where CO₂, dissolved in rainwater, reacts with alkaline minerals to form solid carbonates. Although MCS has advantages over other carbon storage techniques, an economic MCS process has not yet been developed. Two approaches were taken in this work to attempt to reduce the cost of MCS. The first approach was to use a waste material, serpentine waste from ultramafic nickel ore processing, as a feedstock. The second approach was to develop pre-treatments to increase the carbon storage capacity of the feedstock. Two pre-treatments were investigated in this work, including microwave pre-treatment and leaching with ligands at neutral to alkaline pH. The carbon uptake of ultramafic ores was found to increase with increasing microwave pre-treatment after a threshold heating time of 4 min was surpassed. A maximum carbon uptake of 18.3 g CO₂/100 g ore (corresponding to a carbonate conversion of 36.6%) was observed for microwave pre-treated ore. The increase in carbon uptake was attributed primarily to the conversion of serpentine to olivine in ultramafic ores that occurs as result of microwave pre-treatment. The effect of five different ligands (catechol, citrate, EDTA, oxalate and tiron) on the carbon uptake of ultramafic ores was investigated. Of the ligands tested, only catechol and tiron were found to both improve the leaching of magnesium from the ores and the quantity of CO₂ stored. A maximum carbon uptake of 9.7 g/100 g ore (corresponding to a carbonate conversion of 19.3%) was observed for ultramafic ore pre-leached and carbonated in tiron solution at pH 10. This is the first time ligands have been reported to improve the carbon uptake of mineral carbon sequestration feedstock. Although process optimization work was not conducted, both microwave pre-treatment and leaching with ligands at neutral to alkaline pH show promise as ways to lower the cost of MCS.     

Carbonation of stainless steel slag in the context of in situ Brownfield remediation

The main aim of this work was to assess the potential of in situ carbonation as a treatment to modify the properties of alkaline materials such as industrial soil in terms of leaching behaviour and mineralogy and to store the CO₂ generated by specific treatments applied in the context of Brownfield regeneration. The process was investigated through lab-scale column carbonation experiments, in which 100% CO₂ was fed through humidified stainless steel slag under ambient temperature and pressure for set reaction times. The reaction kinetics and the maximum CO₂ uptake attained (5.5%), corresponding to a Ca conversion yield of 15.6%, after 4 h treatment proved slightly lower than those resulting from batch tests carried out on the same particle size fraction at enhanced operating conditions. The mineralogy of the material showed to be affected by column carbonation, exhibiting a higher calcite content and the decrease of Ca hydroxide and silicate phases. As a result of carbonation, the material showed a decrease in pH and Ca release as well as an increase in Si mobility. Furthermore, a reduction of Cr and Ba leaching, up to 63% and 96% respectively, was achieved after 2 h of reaction. However, carbonation was observed to lead to an increased leaching of V and Mo. The effects of carbonation on the leaching behaviour of the material were also investigated performing pH-dependence leaching tests and the results indicated that in situ carbonation appears to be a promising treatment to improve the properties of alkaline materials in view of their reuse on-site.     

Characterisation Leaching Tests and Associated Geochemical Speciation Modelling to Assess Long Term Release Behaviour from Extractive Wastes

A pH dependent leaching test (CEN/TS 14429) and a percolation leaching test (CEN/TS 14405) developed in CEN/TC 292 have been used for the first time to characterise the release behaviour of different sulphidic mining wastes. Geochemical speciation modeling using LeachXS Orchestra provides another type of partitioning between mineral and sorptive phases than is currently practised in the mining industry. Comparing new leaching test data for seven tailings and two waste rock samples with model predictions gives new insights into release behaviour. In leaching, mineral transformations on the surface of waste rock or tailings particles, rather than bulk mineral composition, dictate release, which implies that mineralogical examinations are not necessarily relevant from a leaching perspective. Kinetic aspects of release from sulphidic waste can be addressed by testing material in different stages of oxidation or exposure to atmospheric conditions.     

用充填技术促进矿山资源开发与环境保护协调发展

阐述了资源开发与环境保护的矛盾, 提出了矿山资源开发与环境保护协调发展的主要目标及途径。将充填技术应用于资源-经济-环境体系中, 根据矿山固体废弃物料的特点, 对不同的固体废弃物料采用废石充填、尾砂充填和矿碴灰充填等不同的充填工艺技术, 不仅提高了矿产资源的利用率, 而且降低了矿山废弃固体物料对环境的污染。     

我国矿山无废开采的现状

介绍我国矿山企业矿石开采中废料产出的现状及其对环境产生的负面影响，论述了无废开采与矿山资源综合利用和矿山可持续发展的密切关系及推行无废开采的必要性和意义，进而从矿产资源综合利用、溶浸采矿技术、尾砂和废石充填技术、矿山废料综合开发方面，例举了我国矿山近年来在无废开采方面的一些尝试。最后提出了进一步推进我国矿山无废开采应用的一些建议。     

Ranking and scoring potential environmental risks from solid mineral wastes

The overall environmental performance of minerals beneficiation operations, in addition to their significant energy demand, is largely a function of the success or failure of waste management strategies to minimise the long-term impacts associated with the disposal of vast volumes of solid waste. Whilst forecasting such impacts, particularly in the early stages of a project, is arguably the most important element in an environmental performance assessment, it remains the most challenging. This paper describes a generic methodology which has been developed for ranking and scoring solid waste constituents on the basis of their hazard-forming potential and chemical behaviour under disposal conditions. The approach is demonstrated for the case of typical porphyry-type copper sulfide tailings. The predictive approach developed here is consistent with the screening phase of risk-based environmental assessment protocols, and is considered to be a vital and integral part of reliably quantifying the environmental life cycle impacts of solid mineral wastes.     

The influence of attrition milling on carbon dioxide sequestration on magnesium–iron silicate

Present work evaluates the structural changes in olivine (Mg,Fe)₂SiO₄ (deposit Åheim, Norway) generated by mechanical treatment as well as its adsorption properties for carbon dioxide. The high-energy attrition milling was applied for mechanical activation. Identification of the mechanically-induced changes in the mineral was carried out applying various techniques: scanning electron microscopy, Mössbauer spectroscopy, specific surface measurements, carbon dioxide adsorption and Fourier-transformed infrared spectroscopy. The observed changes in the physico-chemical properties illustrate the possibility of the applied activation to modify the surface and/or bulk properties of olivine. Sensitivity for atmospheric CO₂ sequestration as well as sequestration of CO₂ by chemisorption can be derived from the obtained results.     

利用"数据挖掘"技术解决矿山资源危机的可行性研究

阐述了在解决老矿山资源危机的找矿勘探、矿产经济研究、尾矿和废石的利用等方面新采取的对策，应用数据挖掘技术的可行性，并就数据挖掘技术在矿山中的应用提出了一些建议。     

Alkaline digestion of dunite for Mg(OH)2 production: An investigation for indirect CO2 sequestration

The sequestration of CO₂ by carbonating natural minerals has a great potential for secure reduction of net CO₂ emissions. Feedstock Mg–silicate minerals are usually converted into Mg rich solutions or Mg(OH)₂ before the carbonation process, due to the slow reaction kinetic of direct carbonation. The present work studied the alkaline digestion of Mg–silicate minerals into Mg(OH)₂ for CO₂ sequestration. Powdered dunite containing ∼73 ± 2 wt% of forsterite (Mg₂SiO₄) was dissolved using highly concentrated NaOH aqueous systems at 90 and 180 °C with varied NaOH concentration and duration of reaction. Thermal analysis and Rietveld Refinement Quantitative Phase Analysis (QPA) confirmed that an effective digestion of dunite was possible at 180 °C achieving 80 wt% of Mg(OH)₂. It was found that NaOH concentration in solution, temperature and duration of reaction significantly influence the progress of digestion.