Metal Mining Research Articles

Analysis of influence of deep processing of metallic mineral materials on electromagnetic wave absorption performance

In this paper, the effects of deep processing of metallic mineral materials on electromagnetic wave absorption properties were studied. Firstly, the background and significance of the research are introduced, and the importance of metallic mineral materials in the field of electromagnetic wave absorption is expounded. Then the basic principle of electromagnetic wave absorption and the characteristics and influencing factors of metal mineral materials are discussed. In the section of deep processing technology, different processing methods and their applications are listed. Through experimental design and method, the electromagnetic wave absorption properties of metal mineral materials after deep processing are tested and analyzed. The experimental results are obtained and discussed. Finally, the application prospect of metallic mineral materials in electromagnetic wave absorption field is prospected, and the technical improvement and future development direction are put forward. This study provides an important reference for deepening the understanding of electromagnetic wave absorption properties of metallic mineral materials, and provides a reference for further research in related fields.

The application of “transfer learning” in optical microscopy: the petrographic classification of metallic minerals

Abstract Analysis of optical microscopic image data is crucial for the identification and characterization of mineral phases, and thus directly relevant to the subsequent methodology selections of further detailed petrological exploration. Here we present a novel application of Swin Transformer, a deep learning algorithm to classify metal mineral phases such as arsenopyrite, chalcopyrite, gold, pyrite, and stibnite, in images captured by optical microscopy. To speed up the training process and improve the generalization capabilities of the investigated model, we adopt the “transfer learning” paradigm by pretraining the algorithm using a large, general-purpose, image dataset named ImageNet-1k. Further, we compare the performances of the Swin Transformer with those of two well-established Convolutional Neural Networks (CNNs) named MobileNetv2 and ResNet50, respectively. Our results highlight a maximum accuracy of 0.92 for the Swin Transformer, outperforming the CNNs. To provide an interpretation of the trained models, we apply the so-called Class Activation Map (CAM), which points to a strong global feature extraction ability of the Swin Transformer metal mineral classifier that focuses on distinctive (e.g., colors) and microstructural (e.g., edge shapes) features. The results demonstrate that the deep learning approach can accurately extract all available attributes, which reveals the potential to assist in data exploration and provides an opportunity to carry out spatial quantization at a large scale (cm-mm). Simultaneously, boosting the learning processes with pre-trained weights can accurately capture relevant attributes in mineral classification, revealing the potential for application in mineralogy and petrology, as well as enabling its use in resource explorations.

Study on the Movement of Overlying Rock Strata and Surface Movement in Mine Goaf under Different Treatment Methods Based on PS-InSAR Technology

The goaf treatment of underground metal mines is an important link in mining, and it is particularly important to master the laws of overlying rock strata and surface movement of goaf. In this paper, Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) technology is used to monitor the surface subsidence of the Taibao lead-zinc mine, and the surface subsidence laws of goaf-closure, partial-filling, and full-filling treatments are analyzed by the time-series method. The findings indicate that the surface subsidence of the closed goaf is solely governed by the pillars, with the quality of these pillars playing a pivotal role in controlling such subsidence. Factors like stope span also influence the surface subsidence of partially filled goaf. Prior to compaction, it is primarily the pillars that control surface subsidence; however, after compaction, filling and pillars jointly regulate this phenomenon. Notably, in filled goaf, the quality of both roof and pillars significantly impacts surface subsidence. Before compaction occurs, control over surface subsidence is not evident, yet post-compaction, the filling is effective and tends to stabilize this process. The research findings are significant in enhancing goaf’s treatment efficacy, mitigating surface damage and minimizing ecological environmental impact.

Study on Composite Fracture Characteristics and Hydraulic Fracturing Behavior of Hard Rock

To investigate the influence of non-singular terms (T stress) in the stress field on the composite fractures of hard rock Type I–II, such as rock splitting failure and hydraulic fracture propagation, this study focused on hard rocks in metallic mines. Through splitting tests and hydraulic fracturing experiments, the impact of T stress on the characteristics of Type I–II composite fractures in hard rocks was analyzed. Utilizing the generalized maximum tangential (GMTS) stress criterion considering T stress, the stress intensity factors of hard rock Type I–II composite fractures with different pre-existing crack angles were predicted. The critical fracture pressure expression for hard rocks was derived based on the maximum tangential stress (MTS) criterion. The results indicate that the GMTS criterion, considering T stress, is more suitable for describing the characteristics of Type I–II composite fractures under rock-splitting loads. However, under hydraulic fracturing, T stress has a minor influence on the fracture characteristics of hard rock hydraulic fractures. Therefore, when predicting the critical fracture pressure of hydraulic fractures, T stress can be disregarded. This study provides a scientific basis and guidance for hard rock hydraulic fracturing engineering.

Organic Materials Promote Rhododendron simsii Growth and Rhizosphere Soil Properties in a Lead-Zinc Mining Wasteland.

The mining of metal minerals generates considerable mining wasteland areas, which are characterized by poor soil properties that hinder plant growth. In this study, a field plot experiment was carried out in the mining wasteland of the Lanping lead-zinc mine in Yunnan Province to study the effects of applying three organic materials-biochar (B), organic fertilizer (OF), and sludge (S)-at concentrations of 1% (mass fraction), on promoting the soil of mining wasteland and the growth of two plant varieties (Huolieniao and Yingshanhong). The results showed that the amount of available nutrients in the surface soil of a mining wasteland could be considerably increased by S and OF compared to the control check (CK). In the rhizosphere soils of two Rhododendron simsii varieties, the application of S increased the available phosphorus (P) content by 66.4% to 108.8% and the alkali-hydrolyzed nitrogen (N) content by 61.7% to 295.5%. However, the contents of available cadmium (Cd) and available lead (Pb) were reduced by 17.1% to 32.0% and 14.8% to 19.0%, respectively. Moreover, three organic materials increased the photosynthetic rate and biomass of two R. simsii varieties. Specifically, OF and S were found to significantly increase the biomass of R. simsii. Organic materials have direct impacts on the increased plant height and biomass of R. simsii. Additionally, organic materials indirectly contribute to the growth of R. simsii by reducing the content of available Cd and available Pb in rhizosphere soil while increasing the content of available nutrients according to the structural equation model (SEM). Overall, S can stabilize Cd and Pb, increase soil nutrient contents, and promote the growth of R. simsii effectively, and has great potential in the vegetation reconstruction of mining wasteland.

Multiple-pulse magmatic intrusion and fluid metasomatism in Mesozoic Qianlishan rare metal granite, South China: Records from apatite geochemistry

South China block contains abundant rare metal resources that have widely been considered to be associated with highly evolved granitic magmatism and related fluid metasomatism. Here we perform detailed geochemical analyses on apatite and bulk rock from the ore-bearing granites (equigranular zinnwaldite granite) and ore-barren granites (porphyritic biotite granite and granitic dyke) at Qianlishan pluton, South China, to decipher the respective roles of magmatic and fluid processes and further understand the petrogenesis of rare metal granite (RMG). Apatites from both granite types are F-rich, displaying heterogeneous textures and large compositional variations in Cl, Sr, rare earth elements (REEs), Ga, Th, and U. Relative to the ore-barren granite, the apatites from ore-bearing granite contain lower Sr but higher ∑REE, Th + U and Ga, reflecting more evolved features. The oscillatory compositional zonation and bimodal O-Nd isotopic compositions in apatite from the ore-barren granite indicate multiple-pulse intrusion instead of other open-system processes such as magma mixing and/or crustal assimilation during magmatic evolution, while the additional occurrence of monazite and lower δ18O (< 8‰) in half of the apatite crystals from the ore-bearing granite require further influence of hydrothermal metasomatism. In combination with their high W + Sn concentrations and La tetrad effect in chondrite-normalized REE patterns, the ore-bearing granite experienced extensive metasomatism, during which the fluids in equilibrium with the metasomatic apatite were likely magmatic in origin. We therefore suggest that multiple-pulse magmatic intrusion and fluid metasomatism are two predominant factors during the formation of highly evolved RMG, such as the Qianlishan pluton in South China. Our results demonstrates that apatite geochemistry can be a potential approach to monitoring the magmatic evolution and fluid metasomatism during rare metal mineralization.

Assessment of environmental radiological impact in former metallic mines in Extremadura (Spain): A case study

Metal mining in the Extremadura region was very important in the 19th and 20th centuries. However, due to different reasons the great majority of mines ceased operations, leading to plenty of abandoned mining sites, most of them with on-site waste dumps. Although metal extraction is not radioactive per se, it is considered a NORM activity. In this study, three former mining sites, in which Pb-V-Zn-Ag, Pb-Ag, and Pb-Zn were extracted, were selected to assess the radiological impact on the population and the environment. The external γ exposure was estimated by determining the effective dose and elaborating isodose maps of the sites. The presence of the mining sites increased up to 0.41 mSv/y the effective dose over the surrounding background, which is below the reference value of 1 mSv/y. In only one mining site, the uranium and radium activity concentration of waste dumps were higher than the surrounding soil. The soil to plant (wild grass) transfer factors were similar to other reported values without the influence of NORM activities. So, no enhanced transfer of radionuclides was observed. The radiological impact on the environment was assessed by the risk to non-human biota using the tiered approach developed in ERICA Tool. The sum of the risk quotients of all considered radionuclides in the most conservative Tier 1 was below 1. Total dose rates for several terrestrial Reference Animal and Plants (RAPs) were estimated using Tier 3, obtaining values below 40 µGy/h. Therefore, the impact on non-human biota can be considered as negligible.

Tourmaline chemical composition and boron isotopic composition at the Longmenshan pegmatite in Dahongliutan area, West Kunlun: Implication for rare-metal Li-Be mineralization

Tourmaline is a common mineral in rocks. It can record geological information during its formation and is a geochemical tracer. Its in-situ element and boron isotope tests provide a broader application perspective. Therefore, the chemical and boron isotope characteristics of tourmaline can be used to trace the mineralization of pegmatite-type rare metals. West Kunlun is an important Li-Be metallogenic belt, and many important Li-Be deposits have been developed. Longmenshan is one of the recently discovered pegmatite-type Li-Be deposits in the Dahongliutan area. There are barren and spodumene-bearing pegmatites that all contain tourmaline in Longmenshan. According to their occurrence, crystal form, and chemical composition, the tourmalines in Longmenshan are divided into two types: Tur-I tourmaline in the barren pegmatite close to the granite and Tur-II tourmaline from the spodumene-bearing pegmatite. Compositionally belongs to the alkali group and schorl-dravite solid solution series, with Tur-I being mainly schorl and Tur-II being schorl-dravite. Tur-I tourmalines are rich in Li, Sn, V, Cr, Zn, and Mn, while Tur-II tourmalines are rich in Be, Sr, and Sc. The δ11B of the Tur-I tourmalines hosted in the barren pegmatite and Tur-II from the spodumene-bearing pegmatite ranges from −8.38‰ to −6.81‰ and from −10.00‰ to −6.41‰, respectively. These characteristics indicate that the formation fluid of tourmaline in Longmenshan is mainly derived from magma, and other fluids are rarely involved. The large range of B isotope variation indicates that the Tur-II tourmalines are more obviously affected by fluid activity.

National-scale Geochemical Survey: Distribution of chemical elements in stream sediment of South and Central Asia

This work was carried out as the Pakistani, Uzbekistani, Tajikistani, and Kyrgyzstani contribution to the National-scale Geochemical Survey of South and Central Asia (NGSSCA) project, the objective of which was to document and study the amounts and distribution of chemical elements in stream sediment of South and Central Asia where such national-scale geochemical mapping is launched for the first time. In the framework of the NGSSCA project deployed in Pakistan, Uzbekistan, Tajikistan, and Kyrgyzstan, 9237 stream sediment samples (<2 mm grain-size fraction in alpine desert and mountainous area) were collected at an average density of 1 sample site/100 km2, on the basis of a common stream sediment sampling protocol. The resultant compositional data sets and cartographic products will vastly benefit future mineral exploration activity, surficial (and even solid Earth) geochemical processes studies (e.g., chemical weathering), and environmental evaluation. Eight elements of economic interests (e.g., Ag, Sb, W, Mo, Li, Be, Cu, and Co) are selected to demonstrate their distribution pattern in stream sediment and the main controlling factors. It's concluded that the anomalies for the selected elements in the NGSSCA project can thus be directly linked to different geogenic sources, e.g., underlying bedrock, soil type, mineralization or ore deposits, and large-scale fault systems. Based on the results and the integrated anomaly maps, some new target areas for the corresponding metallic mineralization are predicted, which will provide a basis for further mineral exploration.

A comprehensive risk assessment method for hot work in underground mines based on G1-EWM and unascertained measure theory

A risk assessment method for hot work based on G1-EWM and unascertained measurement theory was proposed to prevent hot work accidents in underground mines. Firstly, based on the risk influencing factors and classification criteria for underground hot work operations in mines, a single indicator measurement matrix was constructed using unascertained measurement theory; Secondly, a risk assessment index system for mine underground hot work operations was established. The combination weight coefficient of each index was determined using the order relationship analysis method (G1) and entropy weight method (EWM) and coupled with the single index measurement evaluation vector to calculate the multi-index comprehensive evaluation vector of the evaluation object; Finally, the model was validated and examined using engineering examples, and the evaluation level was determined using confidence identification criteria. The results showed that the proposed method, when used to evaluate the risk of hot work operations in tunnels and vertical shafts in metal mines, produces risk levels that are in line with reality III (Moderate Risk) for the vertical shaft and IV (High Risk) for the tunnels. The evaluation model results are consistent with the risk evaluation results the whole process of on-site hot work, which verifies the model feasibility. A unique strategy and method for risk management in hot work operations in underground mines is provided by the combination of weighting and unascertained measure models, which has theoretical and practical value. Future research could focus on refineing this model by exploring the applicability in diverse mining environments and integrating advanced analytical techniques to enhance the predictive accuracy and operational efficiency.

Characteristics of resistivity variation in deep granite and in-situ detection applications

During the construction of deep vertical shafts, water inrush and flooding accidents are prone to occur, which seriously affect construction safety. Accurately determining the groundwater conditions is a prerequisite for effectively controlling water hazards and conducting risk management. In order to ensure the accuracy of the resistivity method in deep vertical well water exploration construction, a combination of indoor rock physics, mechanical testing, and on-site engineering measurements was used to analyze the influencing factors of granite resistivity. The corresponding relationship between resistivity and formation integrity was revealed, and water exploration experiments were conducted in the working face of deep underground mines. The results show that: (1) Rock resistivity is influenced by metallic minerals, saturation, temperature, ion content of fracture water, and joints. Regarding deep subsurface detection issues, the main factors affecting the detection results are water content and rock integrity. (2) During the loading process, rock resistivity exhibits significant stage response characteristics, which are closely related to rock integrity and damage accumulation. (3) A degradation model for aquifer zoning based on resistivity benchmark line was established. When the formation resistivity is higher than the benchmark line, it indicates a well-integrated formation with low water content. (4) Resistivity cloud maps and zoning degradation models can be used to visually determine and evaluate the occurrence status of formations and the effectiveness of grouting.

Geology and geochronology of the Yingqian granite-type uranium deposit in the Taoshan-Zhuguang Belt, South China

The Yingqian uranium deposit in the Taoshan-Zhuguang belt, South China is a typical granite-type uranium deposit. The geological characteristics and timing of mineralization are not clear, which hinders the understanding of the ore genesis of the Yingqian deposit as well as regional uranium metallogeny in the Taoshan-Zhuguang belt. These scientific problems are examined in this study through detailed field work and petrographic study, as well as geochemical and geochronological analyses on apatite, rutile and zircon. The orebodies of the Yingqian uranium deposit are mostly present in the medium–coarse grained biotite granite and fine grained two-mica granite, and controlled by the NE-trending faults. The principle U-bearing minerals are pitchblende and brannerite, with the presence of metal minerals like pyrite, hematite and minor galena and sphalerite. The predominant hydrothermal alterations are hematitization, fluoritization, illitization, silicification, chloritization and carbonatization. Gangue minerals are mainly quartz, fluorite, illite and calcite. Moreover, four hydrothermal stages can be recognized in the Yingqian deposit, and the second and third stages are the uraniferous ones. The alterations and mineral assemblages indicate ore fluids are acidic, oxidized, and rich in F, P, CO2, which is favorable for dissolving uranium. Abundant apatite and rutile are discovered in the uraniferous third-stage veins. Geochemical analyses indicate that rutile is plotted in the hydrothermal area in the Ti-100 (Fe + Cr + V)-1000 (W) ternary diagram, and apatite is classified as fluorapatite. In-situ LA-ICP-MS dating indicates that the ore-hosting medium–coarse grained biotite granite and fine grained two-mica granite obtain the zircon U-Pb ages of 160.0 ± 0.3 Ma and 164.1 ± 0.6 Ma, which are consistent with many Jurassic granites in the South China Block. The hydrothermal apatite and rutile yield the U-Pb ages of 96.4 ± 11.6 Ma and 85.6 ± 10.8 Ma, respectively, indicating that uranium mineralization is significantly younger than the ore-hosting granites and generally coeval with the regional mafic magmatism. Consequently, the uraniferous fluids in the Yingqian deposit are independent from the magmatic system. Combined with previous research, it is concluded that fluid convection promoted by mantle–crust interaction would leach uranium from various lithologies, and fluid-rock interactions, fluid boiling and cooling are the causes for uranium precipitation.

Can rare earth elements be recovered from abandoned mine tailings by means of electrokinetic-assisted phytoextraction?

Given the high impact of traditional mining, the recovery of rare earth elements (REEs) from hazardous waste materials could become an option for the future in accordance with the principles of the circular economy. In this work, the technical feasibility of REEs recovery from metal mine tailings has been explored using electrokinetic-assisted phytoremediation with ryegrass (Lolium perenne L.). Phytoextraction combined with both AC current and DC current with reversal polarity was applied (1 V cm−1, 8 h day−1) to real mine tailings containing a total concentration of REEs (Sc, Y, La, Ce, Pr, and Nd) of around 146 mg kg−1. Changes in REEs geochemical fractionation and their concentrations in the soil pore water showed the mobilization of REEs caused by plants and electric current; REE availability was increased to a higher extent for combined electrokinetic-assisted phytoextraction treatments showing the relevant role of plants in the process. Our results demonstrated the initial hypothesis that it is feasible to recover REEs from real metal mining waste by phytoextraction and that the performance of this technology can be significantly improved by applying electric current, especially of the AC type, which increased REE accumulation in ryegrass in the range 57–68% as compared to that of the treatment without electric field application.

Long-term changes in water quality downstream of three abandoned metal mines

Abandoned metal mines pollute rivers with both acidic and circumneutral metal-rich waters, sometimes from well-mapped mines, but often from long-abandoned adits with limited historical records. Changes in water quality over the monitored timescales were calculated for three mines across Wales (Nantymwyn, Parys Mountain and Frongoch), each abandoned since the early 20th century. Using all available spot sampling data provided by the environmental regulator, it was found that there was a sustained input of metals to the receiving watercourses with no or limited reduction in the concentrations from the mines without remediation, despite the passage of time and impacts of weathering. At Nantymwyn, comparison with five months’ sampling in 2019 showed no significant change (p > 0.05) in metal concentrations compared to the same months in 1985. Parys Mountain showed an increase in filtered zinc concentrations of 2 % between 2004 and 2020. At Frongoch, however, a substantial reduction in its environmental impact was observed after remediation, including a 90 % reduction in total lead concentrations. Realisation that the pollution from these sites does not reduce over the decades since abandonment should galvanize the resolve to understand the highest priority sites, and where possible remediate them.

Financial Modelling of Metal and Mineral Mining Companies in Indonesia using Altman Z-scores

Predictions about a company's financial condition related to bankruptcy are important information for interested parties, such as creditors, investors, government, auditors, and the company's internal management. This research models the financial condition of metal and mineral mining companies through the Altman Z-Score model and panel data regression analysis. This study focuses on a sample of nine companies involved in metal and mineral mining from 2017 to 2022. In this study, the Altman method was used to determine a company's health category using the Z-score standard. Panel Data Regression modelling showed that the fixed model was the best. The partial test results indicated that WCTA and RETA did not affect the Altman Z-score. However, when considering all the ratios simultaneously (WCTA, RETA, MVBV, and STA), they contribute to predicting a company's financial condition. These results highlight the importance of understanding the variables that influence a company's health and progress. Notably, the variables of WCTA, RETA, and STA play a crucial role in determining the Altman Z-score and contribute to our understanding of a company's financial condition.

The rhizosphere microbiome reduces the uptake of arsenic and tungsten by Blechnum orientale by increasing nutrient cycling in historical tungsten mining area soils

The growth of pioneer plants in metal mining area soil is closely related to their minimal uptake of toxic elements. Pioneer plants can inhibit the uptake of toxic elements by increasing nutrient uptake. However, few studies have focused on the mechanisms by which the rhizosphere microbiome affect nutrient cycling and their impact on the uptake of toxic elements by pioneer plants. In this study, we selected Blechnum orientale to investigate the potential roles of the rhizosphere microbiome in nutrient cycling and plant growth in a historical tungsten (W) mining area. Our results showed that while the arsenic (As) and W contents in the soil were relatively high, the enrichment levels of As and W in the B. orientale were relatively low. Furthermore, we found that the As and W contents in plants were significantly negatively correlated with soil nutrients (S, P and Mo), suggesting that elevated levels of these soil nutrients could inhibit As and W uptake by B. orientale. Importantly, we found that these nutrients were also identified as the most important factors shaping rhizosphere microbial attributes, including microbial diversity, ecological clusters, and keystone OTUs. Moreover, the genera, keystone taxa and microbial functional genes enriched in the rhizosphere soils from mining areas played a key role in nutrient (S, P and Mo) bioavailability, which could further increase the nutrient uptake by B. orientale. Taken together, our results suggest that rhizosphere microorganisms can improve pioneer plant growth by inhibiting toxic element accumulation via the increase in nutrient cycling in former W mining areas.

Substance-flow analysis and emission-reduction strategies for thallium in the steel industry process

Thallium, which is a highly toxic and sparsely distributed element, is often found in various mineral compounds. Its release into the environment primarily occurs through the mining of metals, metallurgical processes, and the combustion of fossil fuels. These activities release thallium from minerals, concentrating it in ash, wastewater, and exhaust gases, thus contributing to thallium pollution. With over 600 active steelworks in China continuously producing steel, the risk of thallium pollution is significant and thus presents a major challenge. This research focuses on understanding the mechanisms of the physical phase evolution, migration pathways, and emission characteristics of thallium in steelworks. For the first time, a thallium flow network in steelworks is developed using substance flow analysis. This involves examining the input, output, waste discharge characteristics, and potential for reduction of thallium. The findings reveal that iron concentrate is the primary source of thallium in steelworks, with dust being the main form of thallium output. The quantities of thallium input, output, and circulation are found to be 425.44, 365.49, and 49.66 mg/t-CS, respectively. Implementing source control, endpoint governance, or a combination of both can reduce pollutant emissions in steelworks by 41.47%, 43.35%, and 65.63%, respectively. This study enhances our understanding of thallium pollution in steelworks.

Online analysis of iron ore slurry using PGNAA technology with artificial neural network

Real-time analysis of metallic mineral grade and slurry concentration is significant for improving flotation efficiency and product quality. This study proposes an online detection method of ore slurry combining the Prompt Gamma Neutron Activation Analysis (PGNAA) technology and artificial neural network (ANN), which can provide mineral information rapidly and accurately. Firstly, a PGNAA analyzer based on a D-T neutron generator and a BGO detector was used to obtain a gamma-ray spectrum dataset of ore slurry samples, which was used to construct and optimize the ANN model for adaptive analysis. The evaluation metrics calculated by leave-one-out cross-validation indicated that, compared with the weighted library least squares (WLLS) approach, ANN obtained more precise and stable results, with mean absolute percentage errors of 4.66% and 2.80% for Fe grade and slurry concentration, respectively, and the highest average standard deviation of only 0.0119. Meanwhile, the analytical errors of the samples most affected by matrix effects was reduced to 0.61 times and 0.56 times of the WLLS method, respectively.

A New Methodological Approach to Study the Interaction of Humic Acids, Minerals and Polyvalent Cations in Soil

ABSTRACT A new methodological approach of humic acid (HA) extraction is applied to study soil samples with contrasting management regimes: natural vegetation (NV) and agricultural (A). This innovative approach involves a modification of the Swift’s method, wherein several steps HCl/HF are replaced by a single-step HCl/HF. This modification enabled us to investigate the ability of HA to retain inorganic constituents. The carbon content was significantly higher in soils under NV (19.8–27.3 g C kg−1) than under A (11.2–13.4 g C kg−1). The same pattern was observed for the HA content. The new methodology revealed that HA from NV soils had a much higher capacity to retain inorganics (ash contents in HA from 12.3–28.3%) than in A soils (ash content in HA from 1.6–2.6%). Similar results were found for Ca+Al+Fe contents in the HA (3.7–2.5 meq g−1 in HA from NV soils; 2.2–.9 meq g−1 in HA from A soils). Some differences were observed in IR spectra, mainly an intense band at around 1050 cm−1 of HA from NV that reflects the high ash content of the HA and its capacity of retaining inorganic material. Ca binding isotherms of HA also showed that the HA differed significantly in their ability to retain inorganic components when extracted. Humic acid from NV was more efficient in retaining metal ions and minerals than HA from A soils. All results point to a better contact between organic and inorganic matter in NV soils, and this contact contributes to the stabilization of organic matter.

Field observations and interpretation of extensional fracture in hard rock surrounding deep underground openings

Extensional fracturing often occurs in hard rock masses during excavation at depths, for example, >1000 m below the ground surface. Surface-parallel fractures are created in the surrounding rock mass, which is typically subjected to stresses parallel to the free rock surfaces after excavation. These are called extensional fractures because the strains perpendicular to the fracture planes are extensional and the opposite surfaces of each fracture tend to separate from each other as soon as the fracture is created. These fractures predominantly propagate parallel to the maximum principal stress σ1 in the surrounding rock mass. This study analyses extensional fractures observed during excavations in cut-and-fill mining stopes in a deep metal mine. This analysis explores the process of extensional fracturing during excavation in an undisturbed rock mass. In general, intensive spalling occurred on the roof surfaces immediately after the excavation of the undisturbed rock mass. This spalling terminated after a certain depth of rock failure, while burst sounds intermediately emitted from the surrounding rock mass, indicating that rock fracturing was ongoing at depth. In the subsequent cutting slices, the spacing between the extensional fractures decreased with increasing mine-out space in the stope. An extensional fracturing criterion was proposed based on microscopic observations of microcrack development in the rock in response to applied stress. The crack initiation and extensional fracturing processes are associated with two critical extensional strains which are related to the secondary stress state in the position. In areas close to the free rock surface where σ3 = 0, the stress for crack initiation is (σ1+σ2) = 0.4σc, whereas the stress for extensional fracturing is (σ1+σ2) = 0.8σc.