Search

Developing a low-cost and eco-friendly alternative to cement is of great significance for reducing CO 2 emissions. Mine tailings (MTs) rich in Si and Al can be served as a promising precursor for geompolymer preparation only when the severe defect in the low reactivity is overcame. Alkali-hydrothermal activation technology was used to enhance the reactivity of MTs, and then activated tailings (ATs) and slag served as the precursors for the one-part geopolymer (OPG) preparation. Activation mechanism of MTs, workability, compressive strength, and hydration reactions of OPG samples were studied. The crystalline mineral phases in the MTs reacted with NaOH to form the amorphous sodium aluminosilicate, thus increasing the amorphization degree. High NaOH content favored the MTs activation, and the ATs with high NaOH content tended to create a higher initial pH and release more active species (Si and Al) for the slag hydration. The increase of NaOH content shortened the setting time, decreased the fluidity, and increased the strength of OPG samples. This was because the duration of the induction period was shortened and the hydration rate in the acceleration period of OPG samples was improved with increasing NaOH content, which generated large amounts of calcium–sodium aluminosilicate hydrate (C(N)-A-S-H) gel and yield the compact microstructure. However, excessive NaOH content resulted in a poor microstructure, and it is likely responsible for the reduction in strength. [ABSTRACT FROM AUTHOR]

Extremely large-scale multiple-input-multiple-output (XL-MIMO) is promising to meet the high rate requirements for future 6G. To realize efficient precoding, accurate channel state information is essential. Existing channel estimation algorithms with low pilot overhead heavily rely on the channel sparsity in the angular domain, which is achieved by the classical far-field planar-wavefront assumption. However, due to the non-negligible near-field spherical-wavefront property in XL-MIMO, this channel sparsity in the angular domain is not achievable. Therefore, existing far-field channel estimation schemes will suffer from severe performance loss. To address this problem, in this paper, we study the near-field channel estimation by exploiting the polar-domain sparsity. Specifically, unlike the classical angular-domain representation that only considers the angular information, we propose a polar-domain representation, which simultaneously accounts for both the angular and distance information. In this way, the near-field channel also exhibits sparsity in the polar domain, based on which, we propose on-grid and off-grid near-field XL-MIMO channel estimation schemes. Firstly, an on-grid polar-domain simultaneous orthogonal matching pursuit (P-SOMP) algorithm is proposed to efficiently estimate the near-field channel. Furthermore, an off-grid polar-domain simultaneous iterative gridless weighted (P-SIGW) algorithm is proposed to improve the estimation accuracy. Finally, simulations are provided to verify the effectiveness of our schemes. [ABSTRACT FROM AUTHOR]

Gold extraction from ores using non-cyanide lixiviants is currently a significant topic worldwide. Herein, we report the synthesis of an eco-friendly lixiviant that not only is much less toxic than sodium cyanide (NaCN) but also shows excellent effect in the leaching of gold. The lixiviant can be synthesized from roasting of the well-mixed potassium hexacyanoferrate(II) trihydrate (K 4 Fe(CN) 6 ·3H 2 O), urea (CO(NH 2) 2) and sodium carbonate (Na 2 CO 3) with a mass ratio of 1:6:2 under the conditions of heating rate 10–15 °C/min, roasting temperature 700 °C, and temperature holding time 1 h. X-ray diffraction (XRD) analysis suggested that the synthetic lixiviant contained a new phase that is efficient in leaching gold playing the role of the gold lixiviant. Gold leaching results from a gold concentrate showed that the synthetic lixiviant not only has a gold leaching efficacy comparable to NaCN but also achieves faster leaching kinetics than NaCN. Under the optimal leaching conditions of lixiviant concentration 0.3 wt%, initial pH 10–11, liquid to solid ratio 2.5:1, and agitation speed 600 rpm, the synthetic lixiviant could achieve a gold extraction of 87.0% in 8 h and 97.5% in 24 h on the gold concentrate, reducing the leaching time by 12 h compared with cyanidation (97.1% gold extraction in 36 h). In addition, the method of activated carbon adsorption was shown to efficiently adsorb almost all the gold (> 99.0%) from the leach solutions, and the barren leach solution could be reused back to the leaching stage to extract gold. • An efficient gold lixiviant is synthesized by a simple roasting process. • The synthetic lixiviant has a gold leaching efficacy comparable to NaCN. • The synthetic lixiviant achieves faster gold leaching kinetics than NaCN. • The lixiviant can leach 97.5% of gold from a gold concentrate in 24 h. • Activated carbon adsorption efficiently recovers gold (>99.0%) from leach solutions. [ABSTRACT FROM AUTHOR]

• Gold extraction by novel eco-friendly synthetic gold lixiviants (NESGLs) was reviewed. • Leaching efficacy, synthesis and properties of NESGLs were summarized and discussed. • NESGLs have achieved some industrial application due to a series of advantages. • Future research efforts for promoting the application of NESGLs were suggested. Cyanide is currently the predominant lixiviant for gold extraction. However, cyanide is highly toxic, causing potential environmental hazards and high detoxification costs. This has initiated extensive research aimed at seeking and developing eco-friendly substitutes for cyanide. Although significant research has been undertaken, most, if not all, of the traditional non-cyanide lixiviants such as thiosulfate, thiourea, halide and thiocyanate have proven to be difficult to achieve widespread adoption at gold mines because of their inherent limitations. In recent years, a range of novel eco-friendly synthetic gold lixiviants (NESGLs) such as "Jinchan", one of the most representative products, have been developed successfully. Compared with cyanide, these NESGLs are much less or non-toxic, and have achieved comparable gold recoveries from ores without changing the original cyanidation process and equipment. The successful development of NESGLs appears to be making significant inroads into the dominance of cyanide, as evidenced by a growing number of their industrial applications at gold mines worldwide. This paper reviews the current status of gold extraction from its ores using the NESGLs with details focused on their leaching efficacy, synthesis, and composition as well as possible leaching mechanisms. Their future perspectives and developing trends of considerable concern are also discussed. [ABSTRACT FROM AUTHOR]

• Electrochemical behavior of Co(III)-catalyzed gold thiosulfate leaching was studied. • Co(NH 3) 6 3+ showed higher coulombic efficiencies of gold dissolution than Cu(NH 3) 4 2+. • Co(NH 3) 6 3+ achieved continuous gold leaching with much reduced S 2 O 3 2− consumption. • Stable mixed ligand complexes of Co2+ with ammonia and thiosulfate were formed. • The cycle between Co(NH 3) 6 3+ and Co(NH 3) x (S 2 O 3)0 (x = 4, 5) played the catalytic role. Thiosulfate is a green gold lixiviant that has promise to replace cyanide at the gold mining industry. Hexaamminecobalt(III) (Co(NH 3) 6 3+) is effective in catalyzing the thiosulfate leaching of gold from ores, but the electrochemical behavior and catalytic leaching mechanisms remain unclear. This paper reports an in-depth investigation into the electrochemical behavior of gold leaching in ammoniacal thiosulfate solutions catalyzed by Co(NH 3) 6 3+. The electrochemical results suggested that Co(NH 3) 6 3+ is more efficient than Cu(NH 3) 4 2+ in catalyzing the ammoniacal thiosulfate leaching of gold. Compared with Cu(NH 3) 4 2+, the coulombic efficiency of gold dissolution (Q Au / Q T) under the Co(NH 3) 6 3+ catalysis is much higher. In the presence of Co(NH 3) 6 3+, the Q Au / Q T was nearly 100% at 150–250 mV and higher than 78.9% at 250–350 mV; at 250 mV, the Q Au / Q T was approximately 100% in the initial electrolysis of 300 s and remained basically unchanged at around 75% after 1200 s. Leaching results further showed that the catalysis of Co(NH 3) 6 3+ not only achieved a continuous leaching of gold with time, but also significantly decreased the consumption of thiosulfate from 36.5% (for Cu(NH 3) 4 2+) to 20.9% after leaching 24 h. In the ammoniacal thiosulfate solution, Co(NH 3) 6 3+ is likely reduced to mixed ligand Co(II) complexes of Co(NH 3) x (S 2 O 3)0 (x = 4, 5), which are readily oxidized back to Co(NH 3) 6 3+. Thus, an improved catalytic mechanism of gold leaching is proposed that the redox cycle between Co(NH 3) 6 3+ and Co(NH 3) x (S 2 O 3)0 (x = 4, 5) catalyzes the leaching of gold with ammoniacal thiosulfate. [Display omitted] [ABSTRACT FROM AUTHOR]

Environmental and economic issues caused by several tailings storages have attracted worldwide attention; therefore, these urgently need to be solved cost-effectively. In this study, gold mine tailings rich in Si and Al oxides were activated by an alkaline hydrothermal method for use as precursors to produce a one-part geopolymer. The results indicated that the crystalline mineral phases in tailings were partially or completely decomposed by alkaline hydrothermal activation. However, this decomposition depended heavily on the alkali content, temperature, and time. The decomposed tailings were converted to the amorphous sodium aluminosilicate reactant, thereby making tailings a reactive starting material. Subsequently, the one-part geopolymer was prepared from the activated tailings and slag by just adding water without adding any extra alkaline activator. By adding 60 wt% of slag, the one-part geopolymer with an optimum compressive strength value of 33.5 MPa at 28 days was obtained. This was much higher than that of the control sample containing raw tailings without any pretreatment. The high strength could have been because of the amorphous sodium aluminosilicate formed in the activated tailings that could dissolve in water to create alkaline conditions, which could effectively activate the pozzolanic activity of the slag. The main geopolymerization product was the glassy calcium-sodium aluminosilicate hydrate (C(N)-A-S-H) gel, which acted as a binder to closely connect the residual solid particles and form a hardened geopolymer material. [Display omitted] • NaOH hydrothermally activated tailings were used as a reactive precursor. • Mineral phases in tailings were converted into the amorphous sodium aluminosilicate. • Soluble activation product could create alkaline conditions for the slag activation. • Hardened one-part geopolymer was prepared due to the formation of C(N)-A-S-H gel. [ABSTRACT FROM AUTHOR]